Philippe Poizot

Joel Gaubicher

Steven Renault

Lionel Dubois

Yanliang Liang

Yan Yao

Alan G Macdiarmid

Alan J Heeger

Opportunities and Challenges for Organic Electrodes in Electrochemical Energy Storage

Keywords: Positioning Redox-Active Organic Species in Volumetric Capacity Aspect AK 82 Cost Aspect AL 83 8, 2, Hybrid Organic-Inorganic Batteries

As the world moves toward electromobility and a concomitant decarbonization of its electrical supply, modern society is also entering a so-called fourth industrial revolution marked by a boom of electronic devices and digital technologies. Consequently, battery demand has exploded along with the need for ores and metals to fabricate them. Starting from such a critical analysis and integrating robust structural data, this review aims at pointing out there is room to promote organic-based electrochemical energy storage. Combined with recycling solutions, redox-active organic species could decrease the pressure on inorganic compounds and offer valid options in terms of environmental footprint and possible disruptive chemistries to meet the energy storage needs of both today and tomorrow. We review state-of-the-art developments in organic batteries, current challenges, and prospects, and we discuss the fundamental principles that govern the reversible chemistry of organic structures. We provide a comprehensive overview of all reported cell configurations that involve electroactive organic compounds working either in the solid state or in solution for aqueous or nonaqueous electrolytes. These configurations include alkali (Li/Na/K) and multivalent (Mg, Zn)-based electrolytes for conventional "sealed" batteries and redox-flow systems. We also highlight the most promising systems based on such various chemistries relying on appropriate metrics such as operation voltage, specific capacity, specific energy, or cycle life to assess the performances of electrodes.

technologies and conversion solutions is now recog-125 nized to be particularly critical not only by specialists in the 126 field but also by ordinary consumers eager to use different nomadic electronic devices which make their life easier, safer, and more enjoyable. Representative devices of EES are rechargeable (or secondary) batteries and (super)capacitors.

Chemists, electrochemists, and materials science researchers helped by theoreticians (see for example the Materials Genome Project launched by G. Ceder 1 ) have already thoroughly screened the periodic table of the elements in the quest to find the best electrode associations essentially focused on increased gravimetric and volumetric energy densities while improving the safety, power, lifetime, and cost. Thus, decades of intensive and innovative research have enabled us to develop and to place on the market different kinds of primary and secondary batteries able to power an increasingly diverse kind of applications from microchips to the emerging large-scale application markets. 2 Without describing them one by one, it must be underlined that the pioneered lead-acid (PbA) battery devised by G. Plantéin 1859 is still in growing demand because it is unrivalled for microhybrid and internal combustion vehicles or large-scale power storage units (load-leveling applications, uninterrupted power supply (UPS) for entire cities) at this time; this technology is also robust, safe, and affordable associated with efficient recycling and disposal management programs notably to prevent lead emission. 3 Regarding Li-ion batteries (LIBs)the current flagship technology to get high energy densitiesthanks to substantial improvements and notably the discovery of new insertion positive electrode materials (e.g., LiFePO 4 , LFP; Li(Ni 1/2-x Mn 1/2-x Co 2x )O 2 , NMC; Li-rich layered oxides Li(Li x M 1-x )O 2 ), they have become essential to power the vast world of electronic equipment, robots, ongoing electric transportation technologies, and some stationary applications too. Consequently, nickel/metal hydride (Ni/ MH) rechargeable batteries, which have fully replaced nickel/ cadmium (Ni/Cd) cells, are struggling to compete with LIBs in the light of recent achieved progress including at the price level. For the moment, Ni-MH batteries still power more than 10 million hybrid electric vehicles, and companies like BASF-Ovonic maintain their R&D activities. 4 Redox flow batteries (RFBs) represent another promising choice for stationary energy storage because this particular cell configuration operating basically with redox-active solutions is more durable and scalable than conventional "sealed" battery systems working with solid state electrode materials. The major plants ever built to date are essentially based on the vanadium/ vanadium redox flow battery technology (VRFB) 5 first patented and developed by Skyllas-Kazacos in Australia in the mid 80s. [6][7][8] This rapid survey shows that all commercially available electrochemical storage solutions deal with redoxactive inorganic systems, which poses now more than ever certain problems in terms of metal resource constraints, production cost, and environmental footprint in view of the ever growing demand.

Rise of Organics for Electrochemical Energy Storage

For several reasons, which will be thoroughly explained later, it is now recognized that searching for organic matter-based electrodes could bring new chemical opportunities to further improve existing EES technologies while opening new playgrounds to create innovative cell configurations. Thus, over the last ten years, tremendous progress has been made to promote electroactive organic systems attracting much interest from the broad electrochemical storage community. This occurs to such an extent that today we are witnessing a considerable increase in the literature on the subject, ranging from Chemical Reviews pubs.acs.org/CR Review (PDTDA) and other related derivatives, but long-lasting cyclings were never attempted. [START_REF] Liu | Novel Solid Redox Polymerization Electrodes -4121 Electrochemical Properties[END_REF] Note that in the quest to develop lithium-sulfur (Li-S) batteries while limiting the polysulfide shuttle, high sulfur content organic materials have been recently investigated such as a new cross-linked disulfide material C 6 (SLi) 6 developed by Wudl's group but the restored specific capacity is still limited with 1/4 of the theoretical value. 74 A new class of polymers (nonconjugated) able to store electric energy and consisting of a stable organic polymeric chain bearing stabilized nitroxyl radicals such as 2,2,6,6tetramethylpiperidinyl-N-oxy (TEMPO) radicals emerged in the early 2000s thanks to joint efforts of NEC Corp. and Nishide's group in Japan. [START_REF] Preefer | High Sulfur Content Material with Stable Cycling in 4127 Lithium-Sulfur Batteries[END_REF][76][77] These studies have led to the development of the so-called organic radical batteries (ORBs) which are characterized by excellent rate performance, a flexible design, but moderate energy density values due to the adding of high amounts of conductive carbon in electrodes.

The achievement of robust 0.3 mm-thick ORB prototypes compatible with functional smart card and wearable devices was, however, announced by NEC Corp. as early as 2012. [START_REF] Nakahara | Organic Radical Battery 4137 Approaching Practical Use[END_REF] This innovative chemistry coupled with the emergence of promising high-capacity organic compounds characterized by multiple electroactive carbonyl (CO) functional groups [79][80][START_REF] Han | Aromatic 4144 Carbonyl Derivative Polymers as High-Performance Li-Ion Storage 4145 Materials[END_REF][82][START_REF] Chen | Lithium Salt of Tetrahydroxybenzoquinone: Toward the Development of a Sustainable Li-Ion Battery[END_REF] (a redox-active moiety encountered in the chemistry of life and numerous natural substances) enabled the publication of a broader review in 2011 focused this time on the perspectives of organic batteries in addressing some eco-development issues through the possibility of integrating the concept of "renewability" in electrode material design and the prospect of realizing greener and sustainable batteries. [START_REF] Poizot | Clean energy new deal for a sustainable world: from non-CO 2 generating energy sources to greener electrochemical storage devices[END_REF] The use of organics emerges also in RFBs with a first organic/inorganic flow battery reported in 2009 by Xu et al. [START_REF] Xu | Study on a single flow acid Cd-chloranil battery[END_REF] based on the Cdchloranil system operating in sulfuric acid aqueous medium.

Two years later, an all-organic redox flow battery (ORFB)

working in nonaqueous medium (NaClO 4 /acetonitrile) was reported by Li et al. [START_REF] Li | Electrochemical Properties of an All-Organic Redox Flow Battery Using 2,2,6,6-Tetramethyl-1-Piperidinyloxy and N-Methylphthalimide[END_REF] employing 2,2,6,6-tetramethyl-1-piperidinyloxy as the posolyte and N-methylphthalimide as the negolyte, respectively. Many other examples were then reported in the literature as developed later (section 9). The interest taken over time by organics in the field of electrochemical storage can simply be assessed thanks to common data analysis tools like Scopus with suitable query string (Figure S1). The histogram shows that the number of publications (including articles, conference papers, reviews, book chapters, conference reviews) focused on organic-based electrochemical storage devices and published by year from 1972 onward follows a clear increase over the past 10 years.

One can also observe two successive bumps ranging from 1980 to 2000 due to the investigations of conducting polymers then organodisulfide positive electrode materials followed by a larger increase thanks to the impetus given by ORBs.

Although beyond the scope herein, it seems instructive to briefly recall in this Introduction that the addition of electroactive molecules has also been shown to benefit carbonbased electrostatic double-layer capacitors (EDLCs) as well as the merging field of Li-ion capacitors. [START_REF] Anjos | Pseudocapacitance and performance stability of quinone-coated carbon onions[END_REF][START_REF] Assresahegn | Advances on the use of diazonium chemistry for functionalization of materials used in energy storage systems[END_REF][START_REF] Brousse | Grafting of Quinones on Carbons as Active Electrode Materials in Electrochemical Capacitors[END_REF][START_REF] Zeiger | Quinone-Decorated Onion-Like Carbon/Carbon Fiber Hybrid Electrodes for High-Rate Supercapacitor Applications[END_REF][START_REF] Madec | Covalent vs. non-covalent redox functionalization of C-LiFePO 4 based electrodes[END_REF][START_REF] Anothumakkool | Improved electro-grafting of nitropyrene onto onion-like carbon via in situ electrochemical reduction and polymerization: tailoring redox energy density of the supercapacitor positive electrode[END_REF][START_REF] Madec | In situ redox functionalization of composite electrodes for high power-high energy electrochemical storage systems via a non-covalent approach[END_REF][START_REF] Madec | Synergistic Effect in Carbon Coated LiFePO 4 for High Yield Spontaneous Grafting of Diazonium Salt. Structural Examination at the Grain Agglomerate Scale[END_REF][START_REF] Roldań | Towards a Further Generation of High-Energy Carbon-Based Capacitors by Using Redox-Active Electrolytes[END_REF][START_REF] Gorska | Redox active electrolytes in carbon/carbon electrochemical capacitors[END_REF][START_REF] Lebegue | Toward fully organic rechargeable charge storage devices based on carbon electrodes grafted with redox molecules[END_REF][START_REF] Naoi | Advanced Polymers as Active Materials and Electrolytes for Electrochemical Capacitors and Hybrid Capacitor Systems[END_REF][START_REF] Lang | Flexible, aqueous-electrolyte supercapacitors based on water-processable dioxythiophene polymer/carbon nanotube textile electrodes[END_REF][100][101][102] As early as 1983, Saga Sanyo was the first company to integrate highly conducting organic material (tetracyanoquinodimethane, TCNQ) in electrolytic capacitors. 69 Electroactive molecules are used to significantly improve storage performance by adding a reversible faradaic contribution (pseudocapacitance) to the double-layer capacitance at the carbon electrode surface; these devices, referred to as supercapacitors or ultracapacitors, can work both Chemical Reviews pubs.acs.org/CR Review in aqueous and nonaqueous electrolyte media. Different chemistries such as the functionalization of the carbon surface by self-assembly or grafting of the redox-active organic molecule can be used. [START_REF] Anjos | Pseudocapacitance and performance stability of quinone-coated carbon onions[END_REF][START_REF] Assresahegn | Advances on the use of diazonium chemistry for functionalization of materials used in energy storage systems[END_REF][START_REF] Brousse | Grafting of Quinones on Carbons as Active Electrode Materials in Electrochemical Capacitors[END_REF][START_REF] Zeiger | Quinone-Decorated Onion-Like Carbon/Carbon Fiber Hybrid Electrodes for High-Rate Supercapacitor Applications[END_REF][START_REF] Madec | Covalent vs. non-covalent redox functionalization of C-LiFePO 4 based electrodes[END_REF][START_REF] Anothumakkool | Improved electro-grafting of nitropyrene onto onion-like carbon via in situ electrochemical reduction and polymerization: tailoring redox energy density of the supercapacitor positive electrode[END_REF][START_REF] Madec | In situ redox functionalization of composite electrodes for high power-high energy electrochemical storage systems via a non-covalent approach[END_REF][START_REF] Madec | Synergistic Effect in Carbon Coated LiFePO 4 for High Yield Spontaneous Grafting of Diazonium Salt. Structural Examination at the Grain Agglomerate Scale[END_REF][START_REF] Roldań | Towards a Further Generation of High-Energy Carbon-Based Capacitors by Using Redox-Active Electrolytes[END_REF][START_REF] Gorska | Redox active electrolytes in carbon/carbon electrochemical capacitors[END_REF][START_REF] Lebegue | Toward fully organic rechargeable charge storage devices based on carbon electrodes grafted with redox molecules[END_REF] Although covalent anchoring of the carbon substrate via the diazonium chemistry [START_REF] Madec | Synergistic Effect in Carbon Coated LiFePO 4 for High Yield Spontaneous Grafting of Diazonium Salt. Structural Examination at the Grain Agglomerate Scale[END_REF] appears as the main approach, some authors have also reported the direct incorporation of the redox-active organic molecules into the electrolyte formulation referred to as "redox electrolyte" to improve the specific capacitance of carbon-based electrochemical capacitors. [START_REF] Madec | In situ redox functionalization of composite electrodes for high power-high energy electrochemical storage systems via a non-covalent approach[END_REF][START_REF] Roldań | Towards a Further Generation of High-Energy Carbon-Based Capacitors by Using Redox-Active Electrolytes[END_REF][START_REF] Gorska | Redox active electrolytes in carbon/carbon electrochemical capacitors[END_REF] A relevant example of "organic" electrochemical pseudocapacitor consisting of activated carbon powder electrodes modified with naphthalimide and 2,2,6,6tetramethylpiperidine-N-oxyl (TEMPO) was reported by Lebegue et al. [START_REF] Lebegue | Toward fully organic rechargeable charge storage devices based on carbon electrodes grafted with redox molecules[END_REF] It shows an increase in specific capacitance up to 51%, an extended operating voltage of 2.9 V in propylene carbonate, compared to 1.9 V for the unmodified system, and a power 2.5 times higher. Alternatively, redox-active polymer electrodes can be employed including, for instance, PPy, PAni, and PT derivatives which offer advantages for making lightweight and flexible (micro)supercapacitors while being compatible with aqueous electrolytes. [START_REF] Naoi | Advanced Polymers as Active Materials and Electrolytes for Electrochemical Capacitors and Hybrid Capacitor Systems[END_REF][START_REF] Lang | Flexible, aqueous-electrolyte supercapacitors based on water-processable dioxythiophene polymer/carbon nanotube textile electrodes[END_REF][100] Note that electroactive organic molecules have also recently been introduced as key materials to improve the sought-after "prelithiation" step of Li-ion capacitors as well as the sustainability while reducing the cost and complexity. 101,102 It should be underlined that the boundary between faradaic organic-adding for supercapacitors and capacitive carbon-adding for organic batteries is sometimes not so clear. For instance, Wang's group 103 have reported a home-made hierarchical porous carbon nanotubes (HPCNTs) decorated with anthraquinone (AQ) molecules exhibiting ultrahigh specific capacitance of 710 F g -1 (measured at 1 A g -1 ) when tested in 1 M H 2 SO 4 aqueous solution with the optimized mass ratio 7:5 indicating a larger specific organic loading. Otherwise, recent years have seen the emergence of so-called carbon-supported organic electrode materials for LIBs/SIBs, that actually mirrors a strategy to counteract common physical limitations of most low-weight (neutral) organic molecules: their high solubility in organic electrolytes and poor electrical conductivity. By mixing these small organic molecules with large amounts of carbon (generally by impregnation), a better stability can be expected on cycling especially at high rate thanks to the establishment of π-π stacking bonds with the surface of carbon particles (typical carbon loading: >55 wt %); this phenomenon being reinforced with extended aromatic cores. Note that biomolecules such as flavine 104 or dopamine 105 were reported. Chen and co-workers have reviewed this peculiar topic in 2015 [START_REF] Zhu | ReviewAdvanced Carbon-Supported Organic Electrode Materials for Lithium (Sodium)-Ion Batteries[END_REF] by questioning some relevant points for practical applications such as the uniformity/reproducibility at large scale production of carbonsupported organic electrodes or the poor as-obtained energy density values (especially in volumetric metrics). 1.3. Goal, Scope, and Organization of This Review Following these introductory elements, it is obvious that a consequent and growing amount of literature is now easily available on organic batteries after years of silence. However, it must be noted that because a certain disciplinary boundary naturally exists between inorganic and organic compounds and because the redox chemistry of organics is sometimes subtle (involving often reactive delocalized charges), reading research articles dealing with organic batteries (whatever the considered technology) could be somewhat challenging for nonspecialist readers. Therefore, the authors have thought it would be timely 1.3 billion today) 66% of which will reside in urban 475 areas in developing economies. 111 For comparison, the population was 700 million at the beginning of the first Industrial Revolution.

Although not yet fully accepted all over the world, such a mankind development still falls short in terms of sustainability and calls for a rapid and radical change in our current energy engineering together with a responsible behavior in our consuming fashion (pro-environmental behavior, "Nudge" theory, ... 113,114 ). It is interesting for example to read the study of Dong et al. 115 on the nexus among carbon dioxide emissions, economic and population growth, and renewable energy across regions. Their data allow us to underline that economic growth is highly emission intensive, and economic growth often means rising energy consumption and increasing CO 2 emissions with a proportional effect of the population size. Hence our entering this Fourth Industrial Revolution can be perceived as a threat but also as an opportunity to rethink the development of mankind as a whole. Positive initiatives are now under way, and some policy makers and important energy stakeholders are making things happen probably also pushed by the ostensibly large number of recent extreme weather events 116 such as Category 5 Katerina hurricane in 2005.

Basically, new political goals and innovative/disruptive economy models (like the "green growth" models, the circular economy governed by 3Rs, namely Reduce, Reuse, and Recycle) must also be formulated, notably in reference with CO 2 emission limits, in the quest for a long-term sustainability conjugating Ecology/Economy/Society. [The reader who would be interested in this exciting field could find relevant and very informative economic analyses in the specialized literature. 109,[117][118][119] For a more technological point of view, generation of decarbonized electricity and low-carbon transportation solutions are the two main levers (in association with better energy ef f iciency and conservation and the carbon capture utilization and storage 120 ) put traditionally forward to move toward a deep decarbonization of the energy system. 121 To better forecast the future in this regard, we recap below the main observed trends with supporting figures highlighting that rechargeable batteries are expected at the crossroads of several paths in the global demand pattern for electrical functionalities of today and tomorrow, some applications being to mitigate GHG emissions while others are probably less virtuous.

Decarbonizing the Power Supply and Its Related Storage Challenges

Regarding the future of electric grids, thanks to the efforts of worldwide researchers, engineers, and policy makers, technologies are currently being developed to upgrade existing electricity grid infrastructures that will enable so-called "smart grids", which are characterized by improved grid reliability and utilization, the synergies between the power electronics, control, and communication fields as well as the change from radial networks to mesh networks with the possibility to reconfigure and self-heal. 127,128 Thus, the role of IoT will be eminent with a significant reduction of costs associated with sensors, bandwidth, processing, and memory/storage. 129 Consequently, energy storage is increasingly seen as a valuable asset for electricity grids and one of the important tools of mitigation not only as a technical solution for network management, ensuring real-time load leveling, but it is also a means of better utilizing RESs by avoiding load shedding in times of overproduction. For the moment, the worldwide stationary electrical storage remains by far dominated by pumped storage hydropower (98% of the installed power) but the use of rechargeable batteries is emerging fast as underlined in 2011 by Dunn et al. 130 in a visionary paper; the wide use of batteries is now clearly included in the roadmap storage technologies. 131,132 Already widely used for load-leveling applications and UPS for entire cities (especially the PbA technology), the reduction of costs in the electrochemical storage technologies is attracting considerable interest for short-term storage (for a period of seconds to a few days) using "sealed" batteries and redox-flow batteries (RFBs) as shown in Figure 3. Thus, LIBs in various chemistries are even more seriously envisaged 132 because the cost of battery storage has declined fast in a few years with a drop of 73% between 2010 to 2016 to reach $273 kWh -1 as market value while a value of $73 kWh -1 is forecast in 2030. 133 Competing with gasfired peaking plants is made possible. For example, more than 18 000 LIB packs (400-MW peak hour battery) would replace a gas-fired power in California. 134 Interestingly, the analysis of 3 years of real usage of LIBs (1 MW/250 kWh using 384 modules connected in series) installed and in operation in Hawaii has been recently reported by Dubarry et al. 135 For such stationary applications, the capital invested and operational costs (maintenance, energy lost during cycling, Historically, the invention of ICE occurred in the middle of 644 the 19th century just like that of the electric motor but at a 645 time when oil and its derivatives were becoming cheap and 646 widely available whereas high energy batteries did not exist yet. 647 Gaston Plantéhad just invented the first rechargeable battery 648 with its PbA technology (then improved by Camille Faure in 649 1881), which enabled however the first automobile speed 650 record in 1899 (100 km h -1 but only on 2 km range) thanks to 651 the first BEV named "La jamais contente" powered by 750 kg of 652 Fulmen PbA batteries. In the early 1900s, 38% of the US 653 automobile market was captured by electric vehicles. Even after 654 this feat, ICEs would increasingly supplant the electric car for 655 the next century especially thanks to Ford's innovations with 656 the consequences that we are now witnessing. Now vehicle 657 emission regulations have been forcing the automotive industry 658 worldwide to reduce its carbon footprint especially in the EU, China, and India while governments around the world (like the Electric Vehicles Initiative (EVI)) 143 are providing incentives to the citizens for buying electrified vehicles (EVs). The French government is to take the initiative to propose an ambitious Euro 7 emission standard at European level and set the goal of ending the sale of cars emitting greenhouse gases in 2040! The Swedish automaker Volvo has announced that from 2019 all its new models would be either hybrid or 100% electric. Note that EVs include battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and fuel-cell electric vehicles (FCEVs). As a reminder, the key difference between BEVs and PHEVs is that FCEVs use a primary electrochemical cell (i.e., fuel cell) to power the electric motor; the recharging step of FCEVs needing hydrogen as a fuel instead of electricity.

Based on the current roadmap reported by the International Energy Agency (IEA) in the BLUE Map scenario 144 (Figure 4a), Today, consumer electronics (CE) mainly encompasses the 742 whole range of home electronic equipment, from audio systems, 743 home automation, home computing, and low-power electronics 744 to multimedia systems. In addition, IoT has nowadays gained 745 an incredible attraction 149,150 imparting networked connectiv-746 ity to everyday objects in the physical world with numerous 747 implications in logistics, manufacturing, retailing, environ-748 mental monitoring, healthcare monitoring, industrial monitor-749 ing, traffic monitoring, ... (as well as important monitoring in 750 the smart grid and EVs to relate to the previous paragraph).

In 2015, IBM predicted that 1 trillion devices would be connected to the Internet and the IoT. In 2017, a Cisco-revised forecast called for 50 billion devices to be connected by 2020 and, most recently, the GSMA Association predicted 25 billion IoT devices by 2025! Interesting data are reported by "The Shift Project" in several reports. 151 For example, the energy consumption of Information and Communication Technologies (ICT) is increasing by 9% every year while the share of ICT in GHG emissions has increased by half since 2013, rising from 2.5% to 3.7% in a few years.

Remarkable progress has also been made in many aspects of robotics for which 10 grand challenges have recently been pointed out in a Perspective article published in Science Robotics 152 including one related to power and energy sources to make the deployment of mobile robotics possible. In particular, "service robots" which smartly mix IoT and AI to assist human beingstypically by performing a job that is dirty, dull, distant, dangerous or repetitive, including household chores are on the verge of equipping our homes and some public places. Impressive statistics and forecast have been announced by The International Federation of Robotics in the Executive Summary World Robotics 2018 Service Robots reports. 153 For instance, it was estimated in 2017 that nearly 6.1 million robots for domestic tasks, including vacuum cleaning, lawn-mowing, window cleaning and other types, were sold which corresponds to a 31% increase compared to 2016. The market for elderly and handicap assistance is also expected to increase substantially within the next 20 years.

In the same vein of recent developments, benefiting of electric propulsion R&D, the market of drones (UAVs) is also expected to rise at unprecedented rates due to growing interest in monitoring applications by the military, researchers, farmers, hobbyists, and investors in e-business since drones offer superior abilities over their ground alternatives. It should be pointed out that using drones for commercial deliveries was something improbable ten years ago. Interestingly, Vaalma et al. 154 have very recently established a scenario-based supply and demand analysis concerning LIBs for 2016-2050 considering some selected applications including portable electronic devices and tools as well as several electrified road transportation modes and small stationary energy-storage devices (Figure 5). Clearly, if portable electronics dominates in terms of number of devices, they account for much less in terms of the total energy-storage demand if electric vehicles and residential energy-storage devices reach the high production numbers estimated in their scenarios. 154 However, several key applications such as the powering of domestic robots, drones, or IoT are not included in the scenario.

Summary

To sum up this overview, we are unambiguously facing a tremendous need for EES for a double reason, first as a key ingredient of the future energy engineering to fight global warming and, second, as a central hub for the emergence of disruptive technologies consecutive to the continuous modernization and development of mankind. The scale-up of industrial facilities for the production of rechargeable batteries, especially LIBs due to their high energy density values, is consequently an opened and sensitive question. The production is currently dominated by East-Asian competitors; with Panasonic (Japan) and LG Chem (South Korea), these leading manufacturers on the automotive market are closely followed by Samsung SDI (South Korea), CATL (China), and SK Innovation (South Korea). With its Gigafactory built in Nevada, Tesla (with Panasonic as a partner) is now on the road too on US soil whereas in the EU, things are moving forward to reduce its dependence on the Asia and US suppliers. Sweden's Northvolt has raised in June 2019 $1 billion in equity capital to complete funding for its future Gigafactory while France and Germany launched an Airbus-style €6 billion foray into the batterybuilding business. Considering a typical battery capacity range of 20-75 kWh, these factory capacities translate into a yearly Respective to the scope of this article, the relevant question 903 is therefore to establish if the current available and future 904 battery technologies depend on high material resource constraints 905 and at what cost in terms of environmental burden. A few 906 elements of an answer will be provided below.

Resource Constraints Forecast in Conventional

907 Material Supply 908 Based on the present state-of-the-art and whatever the con-909 sidered technologies, electrode reactions involve redox-active 910 inorganic compounds especially metal-based electroactive com-911 ponents, which is an historic heritage of the pioneered redox 912 chemistry (the voltaic pile followed by the PbA secondary cell) 913 as well as the material engineering that resulted (Figure 3). For 914 instance, if we consider the battery technology roadmap for 915 electrified vehicles (Figure 4c) it is expected that at least until 916 2030 matured and advanced insertion inorganic positive 917 electrode materials will be essentially based on the 3d-metal 918 redox chemistry. However, it is also widely acknowledged that 919 traditional Li-ion batteries are starting to approach their limits especially for long-range EVs. Beyond 2030 other battery chemistries are envisaged namely post-LIB systems. First, it is commonly forecast that conversion-based cathode reactions could be used (with O 2 and S) in Li-metal battery configuration (Figure 4c), which supposes however the achievement of consequent improvements. 168 Recent R&D trends also indicate an expected switch from liquid (organic) electrolytes to ceramic electrolytes (or solid state electrolytes, SEEs) notably following the recent discovery of lithium superionic conductors at room temperature by Kanno and co-workers (σ Li + = 25 mS cm -1 for Li Nevertheless, this ceramic electrolyte option could require again the consumption of more inorganic materials such as metal-based sulfides, oxides, or phosphates. 169,172 It is worth noting that the emerging field of Na and Na-ion batteries which are also considered in the post-LIB field logically follows the same trajectory in terms of materials choice and design as a sister material chemistry of Li-based batteries. 173,174 In addition, it should be remembered that major built RFBs also draw their chemical power from 3d-metals as well as already underlined above.

While the number of publications and other reports has greatly increased in recent years on the potential bottlenecks in material supplies due to the ramping up of LIBs, 146,155,154,[175][176][177] the resource issue questionings in the field of energy storage 193,194 Note that the majority of existing studies are focused on GHG emissions or energy demand; however, other Eco indicators such as human toxicity (HTP) might be even more important. 192,195 More importantly, such LCA studies are restricted at the manufacturing outlet ("cradle-to-gate analyses" and not "cradle-tograve analyses"), so they do not take into account both collection and recycling steps. Yet the results obtained show that battery manufacturing is energy-intensive because of the involvement of quite high flows of exergy mainly due to the chemical nature of the commonly used materials (inorganic compounds). 112,162 As a result, one common recommendation of current LCA analyses is the deployment of batteries with higher round-trip efficiency to extend the cycle life in operation but also the need for low-carbon innovations in future technologies while favoring the use of naturally abundant chemical elements of low toxicity. Within this background, one current trend in the post-LIB field is to reinvestigate Na-ion batteries as candidates for medium and large-scale stationary energy storage in light of possible concerns in terms of cost and abundancy of lithium. 174,196,197 3.3. Positioning Redox-Active Organic Species in the Battery Landscape

Within this background, which is particularly tense in many ways, the idea of taking advantage of organic chemistry for the electrochemical storage of energy can make sense. As stated in the Introduction, the idea to use organic electrode materials (OEMs) for rechargeable batteries is not new and goes back to the discovery of reversible redox-activity of conducting polymers in the late 70s following the discovery of PAc by Shirakawa [61][START_REF] Shirakawa | Conductivity in Doped Polyacetylene[END_REF][63][64] and its subsequent electrochemical activity (doping) both in oxidation and in reduction. 65,[START_REF] Nigrey | Lightweight Rechargeable Storage Batteries Using 4102 Polyacetylene, (CH) x as the Cathode-Active[END_REF] These findings led to the development of various conducting polymers as well as a first attempt to emerge in the marketplace before the boom of high-energy LIBs. 59,69 Today the motivations are different and authors agree that organics can be notably seen as a pathway to stabilize the pressure on the CRM for energy storage while seeking to improve the environmental footprint as well as finding innovative storage solutions. [START_REF] Liang | Positioning Organic Electrode Materials in the Battery Landscape[END_REF][START_REF] Poizot | Clean energy new deal for a sustainable world: from non-CO 2 generating energy sources to greener electrochemical storage devices[END_REF] Of course, OEMs have limitations especially when talking about volumetric energy density that can be achieved due to their intrinsic low volumetric mass densities (<2 g cm -3 ). Excluding ORFBs, this issue can be exacerbated by the need for large carbon addition when preparing composite electrodes. Moreover, their greater propensity to solubilize in liquids compared to inorganic compounds is an issue for solid electrodes but an asset for ORFBs.

The development of organic batteries is clearly in its early stages compared to 150 years of intensive research and innovations dedicated to conventional inorganic-based electrochemical storage devices explaining why some room is expected to be given to redox-active organic species in the battery landscape. To be convinced, Figure 7 reports an interesting assessment for the electrochemical storage of OEMs classified by families. 31 In fact, organics exhibit several interesting assets as recapped below.

First, organics make potential access to low cost and greener chemistry possible because they are mainly composed of C, H, O, N, and S, which are naturally abundant elements as well as the main constituents of biomass (Figure 6). 191 This situation Chemical Reviews pubs.acs.org/CR Review used to make chemicals [START_REF] Liang | Positioning Organic Electrode Materials in the Battery Landscape[END_REF] ). In terms of cost, it is difficult to make predictions for petrochemical reagents notably if in the future the demand for petroleum products used for combustion decreases; the access to new markets and applications can change the game. However, it is generally accepted that fluorinated derivatives are among the most expensive compounds.

Second, simplified recycling managements of organic spent batteries (Figure S7) can be expected notably with solid electrodes since organic structures are typical fuels that can be consumed by combustion at medium temperatures producing heat (energy recovery). Interestingly, if spent organic batteries were inadvertently not collected (spread out in the wild), the loss of scarce and costly metallic chemical is notably reduced compared to the current battery technologies. Moreover, the use of biodegradable materials can be envisaged too.

Last but not least, organic chemistry offers high structural designability providing great opportunities to find novel and innovative electrode materials with specific properties including the elaboration of multimodal systems such as electrochemical/chemical or electrochemical/photochemical rechargeability. [208][209][210] More importantly, physicochemical properties can be rationally tuned using well-established principles of organic chemistry and molecular engineering giving access to redox-active species able to work:

• from dissolved to solid state (including polymers) in aqueous or nonaqueous electrolytes making them versatile in terms of electrochemical storage devices including access to flexible devices; Therefore, to avoid confusions, the terms "positive" electrode or "negative" electrode are preferred whatever the considered cell reaction (i.e., charging or discharging), with the positive electrode always displaying the highest potential value. Note that a cell is the basic electrochemical unit. A battery is comg posed, strictly speaking, of two or more such cells connected in series or parallel. However, the term battery has evolved, especially in the language of the end user; therefore, the term "battery" will be used herein without this distinction for the sake of simplicity.

The use of two antagonist redox couples is then required with (e.g., Ox According to the IUPAC definition, 216 the OS of an atom is the charge of that atom after an ionic approximation of its heteroatomic bonds. The bonds between atoms of the same element are not replaced by ionic bonds; they are divided equally. In other words, to determine the OS of a given atom, it is therefore considered that all heteroatomic bonds in which it participates are 100% ionic and the electrons of each of these bonds are assigned to the most electronegative atom (Allen's scale).

Moreover, IUPAC recommends two closely related general algorithms for OS calculation in molecules, ions, and extended solids: 216

• Algorithm of assigning bonds. This algorithm works on structural Lewis formulas of molecules and ions which show all valence electrons.

• Algorithm of summing bond orders. This algorithm works on structural Lewis formulas and bond graphs for an extended solid, especially iono-covalent structures.

However, as thoroughly explained above, because the used redox chemistry in battery applications is related to the OS change of the 3d-metallic redox centers (e.g., Ti, V, Mn, Fe, Co, Ni) or elemental/diatomic substances (e.g., Li, Na, Mg, C, O 2 , Si, P, S, Sn), [START_REF] Song | Towards sustainable and versatile energy storage devices: an overview of organic electrode materials[END_REF] a set of more general rules for determining OS is more readily used (e.g., "the sum of OS for all atoms in the

∫ = • = • Δ Q I t dt I t ( ) t 0
which gives rise to Q = I•Δt whereas the energy, ε (in mWh), is obtained by multiplying Q with the voltage E I≠0 .

More interestingly for us are the redox processes for which no formation or rupture of electron-pair single bonds occurs after simple charge transfer reaction. In this latter case, stepwise transfer of single electrons must in principle be taken into account 219 giving potentially access to reversible organic redox couples like conventional third-kind electrode reactions (e.g., Ce 4+ + e -⇌ Ce 3+ ). However, caution should be exercised because one-electron oxidation or reduction of a neutral or ionic organic scaffold produces generally reactive ion radicals or radicals with possible subsequent reaction such as radical coupling. Consequently, some chemical tricks have to be used to ensure the production of stable (persistent) radical structures after charge transfer. 220,221 Thus, simple π-extension and substituents of resonance electron donating or withdrawing groups (mesomerism) make delocalization of spin and charge density possible which deeply reduces the reactivity of the odd electron (e.g., galvinoxyl or tris(pentachlorophenyl)methyl radicals). Aromatization after electron transfer and appropriate steric protection play also important role to protect a reactive radical center. Robust functional groups bearing an unpaired electron also exist such as nitroxide radicals (>N-O • ).

In this family, 2,2,6,6-tetramethylpiperidinyl-N-oxy (TEMPO) constitutes the most representative example. Beyond the realization of magnetic organic materials, 221 the nitroxide redox chemistry is at the origin of the rise of ORBs, 10,[START_REF] Preefer | High Sulfur Content Material with Stable Cycling in 4127 Lithium-Sulfur Batteries[END_REF][76][77] N-type OEMs with sufficiently low operating potential can be used as negative electrode materials as well. Many conductive polymers can be n-doped at <1 V vs Li + /Li, but the doped states usually lack stability, and the doping level is typically half of that for the p-doping of the same polymer. 259 Two other mechanisms are currently being actively researched: lithiation of π-conjugated carboxylate and "superlithiation" (see section 7.3) of π-conjugated systems. 260 A pioneering report on carboxylate negative electrodes concerned dilithium terephthalate 261 which discharged at 0.8 V vs Li + /Li (Figure 10I, Table 2, entry 41). [Note: For a negative electrode material tested in a Li cell, the charging potential is more relevant, however, discharging potential is used here for consistency.] Since this work, the family of π-conjugated carboxylates has been widely extended, and it has been shown that the working potential can be slightly adjusted by playing with the electronic effects on the organic skeleton notably by Lakraychi et al. 262,263 Replacing the phenyl group in dilithium terephthalate with biphenyl gives the dilithium 4,4′-biphenyl dicarboxylate (Li 2 BPDC, Table 2, entry 42) with a discharge potential of 0.7 V vs Li + /Li. 264 The lower potential compared with Li terephthalate was ascribed to the π-conjugation enhancement destabilizing the LUMO orbitals of the molecule. Further expansion of the π-conjugated system to diphenylacetylene 265 OEMs that undergo the peculiar "superlithiation" 1614 process (see section 7.3) exhibit sloping discharge profiles that 1615 start from >1.5 V and eventually approach 0 V vs Li + /Li and 1616 then charge at ≥1 V vs Li + /Li on average. Due to the unique-1617 ness of the reaction, these OEMs are to be separately covered 1618 in section 7.

Organic Electrode Materials with High Specific 1619 Capacity

1620 To gain high specific capacity, both formula weight and 1621 number of transferrable electrons are key considerations. 1622 Among the major classes of organic electrodes, organosulfur 1623 compounds offer the largest capacities. In particular, organo-1624 sulfur polymers bearing polysulfide bonds not only exhibit high 1625 capacities but also stable cycling. Poly(sulfur-random-1,3-1626 diisopropenylbenzene) (poly(S-r-DIB)) is one such example, 1627 which was copolymerized between molten S 8 and 1,3-diisopro-1628 penylbenzene (DIB) through inverse vulcanization. 266 The 1629 DIB feed ratios can be varied between 10-50 wt % during the 1630 synthesis. Galvanostatic voltage profiles of poly(S-r-DIB) with 1631 10 wt % DIB show distinct discharge plateaus at 2.3 and 2.1 V vs Li + /Li (Figure 11A, Table 2, entry 44). The initial discharge capacity is 1100 mAh g -1 . Sulfur-polyacrylonitrile (S-PAN) is another organosulfur polymer attracting significant attention due to the ease of synthesis and high performance. 267 S-PAN was formed by mixing and heating sulfur and polyacrylonitrile (PAN). It is interesting to note that S-PAN exhibits good capacity retention in a carbonate-based electrolyte, a behavior in sharp contrast with sulfur, which works well only in ethereal electrolytes. 268 S-PAN shows a single plateau at 2.1 V with a high specific capacity of 1200 mAh g -1 (Figure 11B, Table 2 11C). 82 The fact that Li 6 C 6 O 6 can be prepared from a renewable natural precursor opens up new pathways toward sustainable batteries for future clean energy economy. Recently, cyclohexanehexone (C 6 O 6 ) was claimed to be successively synthesized and surpass Li 2 C 6 O 6 in terms of specific capacity (Table 2, entry 47). 269 C 6 O 6 , a cyclic ketone composed of carbonyls without redundant mass, exhibits a higher capacity of 902 mAh g -1 (i.e., six-electron reaction per formula) (Figure 11D). Note that such an ultrahigh capacity was observed in an ionic liquid-based electrolyte measured at 70 °C. The compound exhibits sloping and polarized cycling curves, in contrast to the multiple plateaus observed for Li 2 C 6 O 6 . 82 After the C 6 O 6 motif, p-BQ offers the highest theoretical specific capacity among common n-type OEM building blocks. p-BQ itself can deliver a specific capacity of 429 mAh g -1 during the initial discharge, but the capacity retention was only 32% after 20 cycles due to its high solubility in organic solvents. 246 Due to the ready dissolution of p-BQ into nonaqueous electrolyte solutions, many OEMs containing the p-BQ unit have been developed to reduce dissolution while maintaining high capacity. They may be categorized into three types according to their molecular sizes. The first type includes molecules with one single p-BQ core modified with electron donating or withdrawing groups. Installation of methoxy functional groups on p-BQ results as DMBQ, which improves the stability compared to p-BQ albeit the discharge capacity is reduced to 320 mAh g -1 (Figure 11E, Table 2, entry 13). 270 Lawsone-Li is another example showing modification of naphthoquinone (NQ) with lithoxy results in lawsone-Li salt. Lawsone (2-hydroxy-1,4-naphthoquinone) is a nature-derived red-orange dye. Lithium cells based on lawsone-Li as positive electrode material displayed a capacity of 280 mAh g -1 and a cycle life of 1000 cycles (Table 2, entry 27). 271 The second type includes molecules with multiple p-BQ units. 2,2′-Bis-p-benzoquinone (BBQ), calix [4]quinone (C4Q), and pillar [5]quinone (P5Q) contain two, four, and five p-BQ units, respectively (Table 2, entries 18, 20, and 21). It seems that dissolution of oligomers could still be observed in liquid electrolytes; therefore, polymer electrolytes containing poly-(methacrylate) (PMA) and poly(ethylene glycol) (PEG) were used to increase capacity retention. Compared to BBQ (358 mAh g -1 ), P5Q and C4Q show higher specific capacity of 409 and 422 mAh g -1 (Figure 11F). 247,272,273 The third type includes polymers with (modified) p-BQ units. PBQS, PDBS, and Li 2 PDBS present three examples of p-BQ-based polymers when using sulfur as the linker (Table 2, entry 22-25). 246,249,274 Among the three polymers, PBQS shows the highest specific capacity of 275 mAh g -1 for 1000 cycles. Due to the addition of lithoxy and hydroxy groups on PBQS, Li 2 PDHBQS and PDBS exhibit slightly lower specific capacity of ∼250 mAh g -1 . Anthraquinone (AQ) can be considered as p-BQ with extended conjugation, which also has the solubility issue. Polymerization approach has been equally successful for AQ. P14AQ and poly-LiDHAQS are two polymers based on AQ. The specific capacities are 263 and 330 mAh g -1 , respectively (Figure 11G, Table 2, entries 29 and 30). 275,276 A closely related structure to p-BQ is pyrene-4,5,9,10tetraone (PTO) where two o-BQ units are connected with extended π-conjugated structure (Table 2, entry 39). PTO undergoes a four-electron reduction with a specific capacity of 360 mAh g -1 in EC/DMC and an average discharge voltage of 2.59 V vs Li + /Li (Figure 11H). 257 TCNQ can also be formed by replacing the oxygen atoms in p-BQ with more electron-withdrawing dicyanomethylene groups ( In parallel, many other new materials were reported in the literature as already exhaustively covered in recent reviews. [START_REF] Oltean | Sustainable Materials for Sustainable Energy Storage: Organic Na Electrodes[END_REF][START_REF] Xu | Organic materials for rechargeable sodium-ion batteries[END_REF][START_REF] Wang | Organic Carbonyl Compounds for Sodium-Ion Batteries: Recent Progress and Future Perspectives[END_REF]302 Considering that strategies known to affect the potential with inductive effects (electron-withdrawing/donating groups) or appropriate aromatic ring design will have similar effects in organic LIBs and SIBs, the only distinctions to keep in mind are the differences in electrochemical potential for the reference metal (-3.04 V vs SHE for the Li + /Li redox couple against -2.71 V vs SHE for the Na + /Na redox couple) and atomic differences. For instance, a sodium carboxylate is expected to insert a sodium ion at an average potential slightly lower than the potential of lithium ion insertion for the corresponding lithium carboxylate with the same aromatic system -0.33 V (e.g., 0.9 V vs Li + /Li average redox potential for dilithium terephthalate and 0.4 V vs Na + /Na for disodium terephthalate: 0.5 V difference instead of 0.33 V). Similarly, the strategies for improving the specific capacity or reducing the dissolution phenomenon of active species within the electrolyte system such as polymerization work usually as well for organic SIBs as they do for organic LIBs and hence will not be repeated in this section.

As a reminder, the use of sodium instead of lithium is also motivated by higher abundance and lower cost of the resource, with Na being widely distributed in the earth's crust and oceans (Figure 6) and nontoxic.

As previously stated, only the materials delivering the best performances in specific capacity, operating potential, and

AB

1881 cycling stability will be presented in the subsequent section 1882 (Table 3). carbon nanofibers (CNFs), CuTNCQ/CNFs composite displays reversible capacity of 137 mAh g -1 with 85% capacity retention after 300 cycles at a rate of 300 mA g -1 using the 3 redox couples. If restricted to a cutoff voltage of 2.5-4.1 V vs Na + /Na which corresponds to TNCQ -/TNCQ 0 and Cu + /Cu 0 redox couples, the cell exhibits a capacity retention of 78% after 1200 cycles for an average redox potential of 3.6 V vs Na + /Na. But in this case, the high redox potential is shifted up due to the contribution of a metal center, unlike p-type OEMs.

For low potential OEMs for SIBs, carboxylates (e.g., disodium terephthalate, disodium naphthalene dicarboxylate, 311 disodium pyridine-2,5-dicarboxylate, 312 disodium 4,4′biphenyldicarboxylate, 313 and so on) or Schiff bases [314][315][316] have already been covered in recent reviews. [START_REF] Oltean | Sustainable Materials for Sustainable Energy Storage: Organic Na Electrodes[END_REF][START_REF] Xu | Organic materials for rechargeable sodium-ion batteries[END_REF][START_REF] Wang | Organic Carbonyl Compounds for Sodium-Ion Batteries: Recent Progress and Future Perspectives[END_REF]302 The only recent example that will be covered here is the case of a metal organic framework (MOF) made of cobalt and 4,4′biphenyldicarboxylate ligands (Co-bpdc, Table 3, entry 5). 317 According to the authors, the cobalt ions do not undergo reduction during the electrochemical process, leaving the redox activity to the sole organic moiety. The reversible sodium insertion/deinsertion occurs at an average potential of 0.5 V vs Na + /Na and exhibits stable capacity of ca. 300 mAh g -1 with 90% capacity retention for 50 cycles at 20 mA g -1 or 264 mAh g -1 and 79% capacity retention for 1000 cycles at 100 mA g -1 (Figure 13D). The interest for low potential OEMs for SIBs started to rise when poor long-term stability or safety issues were reported with classical inorganic materials or soft/hard carbons, while positive inorganic electrodes gave promising results. 173,[318][319][320][321][322] The larger ion diffusion pathways in OEMs are believed to be appropriate for the large sodium ions (as compared to lithium ions) while more constrained ion diffusion pathways in inorganic materials might be too restricted for sodium intercalation at low potentials. 323 Ideally, a good compromise could be found with hybrid organic/inorganic sodium-ion batteries using inorganic materials as the positive electrode and OEMs as the negative electrode. One early example is the full sodiumion cell made of disodium terephthalate as negative electrode and Na 0.75 Mn 0.70 Ni 0.23 O 2 as positive electrode (Table 3, entry 6). 296 With 3.6 V as output voltage, this cell delivers 257 mAh g -1 initial capacity (anode limitation design) with limited capacity loss after 50 cycles (93% capacity retention, (Figure 13E). Interestingly, its stability is better for this full cell rather than the two half-cells using both electrodes and metallic sodium as counter electrode. However, this better stability for a hybrid organic/inorganic sodium-ion full cell is not systematic. In the case of a Na 3 V 2 O 2 (PO 4 ) 2 F/rGO// disodium naphthalene-2,6-dicarboxylate cell with an average redox potential of 3.3 V vs Na + /Na, a severe capacity decay is noticeable after merely 20 cycles (Table 3, entry 7), while the half-cells display a better capacity retention. 324 Other examples with acceptable stability include Na 4 Fe(CN) 6 //poly 1,4,5,8naphthalenetetracarboxylic dianhydride (PNTCD) and Na 3 V-(PO 4 ) 3 //PNTCD. 325 However, like most diimide compounds, the average redox potential of PNTCD is 2.1 V vs Na + /Na, which considerably restricts the average output voltage of the full cells to 1.2 V (Table 3,entry 8,9).

Examples of a hybrid organic/inorganic sodium-ion full cell using an OEM as the positive electrode are scarcer. One of the few cases is disodium rhodizonate which was cycled vs Napredoped hard carbon with improved cyclability as compared to cycling vs metallic sodium (Table 3, entry 10 and Figure 13F). 326 Comparably to its lithium counterpart, disodium rhodizonate is able to reversibly intercalate four sodium ions (Table 3, entry 11). 327 When nanosized, it delivers up to 484 mAh g -1 with several electrochemical features corresponding to the different sodium ion insertions. Disodium rhodizonate can also be cycled in a hybrid organic/inorganic full cell vs phosphorus encapsulated in a carbon scaffold (P@C) or in an all-organic configuration vs disodium terephthalate (Figure 13G).

All-organic Na-ion batteries is an attractive concept without sensitive metals (price, rarity, geopolitics) such as cobalt, nickel, or lithium. But merely some examples have been reported in the literature. In addition to previously mentioned cases (Table 3, entries 2 and 11), a cell using N,N′-diamino-3,4,9,10perylenetetracarboxylic polyimide (PI) as the positive electrode and disodium terephthalate as the negative electrode has been described (Table 3, entry 12, Figure 13H). 328 The limited output voltage of 1.6 V is here also connected to the choice of a polyimide whose average redox potential is 2.2 V vs Na + /Na.

There is still to date only one example of an all-organic rocking chair sodium-ion battery published by Chen's group (Table 3, entry 13 and Figure 13I). 329 The tetrasodium salt of 2,5-dihydroxyterephthalic acid (Na 4 DHTPA or Na 4 -p-DHT) is used in the same time as both positive and negative electrodes in a symmetrical cell as previously demonstrated in the case of the lithium chemistry. 251 With 2 V as output voltage, this cell is exhibits a remarkable stability at a very high rate of 50 A g -1 corresponding to a full charge in 7 s. After 10 000 cycles, it still maintains 97 mAh g -1 (Figure 14C).

Azobenzene-4,4′-dicarboxylic acid sodium salt (ADASS, Table 3, entry 25) is a material that can reversibly insert two sodium ions on its azo (-NN-) function at an average redox potential of 1.25 V vs Na + /Na. 335 It can be cycled at a high current of 10 or 20 C for 1000 and 2000 cycles, respectively, with moderate capacity losses (Figure 14D). The average redox potential of sodium carboxylates is rather low (usually in the 0.3-0.8 V range vs Na + /Na) which makes side reaction likely to occur during long time cycling. Still, the group of Palani Balaya succeeded to cycle trisodium-1,3,5benzene tricarboxylate (Table 3, entry 26) for 1500 cycles at a rate of 10 C with 75% capacity retention with a lower cutoff potential of 50 mV (Figure 14E). 336 Several covalent organic frameworks (COFs) have shown excellent cyclability. An early example from the literature is a p-type material (Table 3, entry 27) that reversibly inserts anion (ClO 4

-) at an average potential of 2.6 V vs Na + /Na and sustains 80% of its capacity after 7000 cycles at a rate of 1 A g -1 (Figure 14F). 337 Other triazine-based COFs have been used as n-type materials with the integration of carbonylcontaining moieties (Table 3, entry 28, 29). 343 After a stabilization period corresponding to 30 cycles, they sustain most of their capacities over 1000 cycles at a high rate of 5 A g -1 .

In addition to the previously mentioned tri-β-ketoenamine derivatives, which have already shown excellent stability (Table 3, entry 15, 16), the pentacenetetrone derivative 30 is able to maintain 95% of its capacity after 1400 cycles at a rate of 1 A g -1 . 344 One such example is the dipotassium terephthalate (Table 4, entry 1) which intercalates/deintercalates potassium ion at an average potential of 0.6 V vs K + /K with a well-defined plateau after the first cycle. 346 It exhibits reversible capacity up to 249 mAh g -1 , and good capacity retention of 95% over 500 cycles is obtained when cycled at a rate of 1000 mA g -1 (Figure 15A).

Quinone derivatives have already displayed their robustness in other EES devices. Poly(pentacenetetrone sulfide) (PPTS, Table 4, entry 2) can be cycled for 3000 cycles at a rate of 5 A g -1 with limited capacity decay (Figure 15B). 347 Interestingly, at such a high rate, the Coulombic efficiency is much higher as compared to cycling at a rate of 0.1 A/g (∼99% against ∼90%), which is due to the highly reactive metallic potassium counter electrode and dendrite formation according to the authors.

These two materials have been cycled in an all-organic full cell (thereby without metallic potassium) which delivers 254 mAh g -1 The first cycle is not taken into account. b Based on cathode material weight.

against 0.0017% for Li). 357,358 The current status is that few regular host inorganic electrode compounds, based on which Li-ion batteries are established, are able to deliver reasonable electrode performance for storing multivalent metal-ions. For Mg batteries, the flagship multivalent-metal batteries, Chevrel phase Mo 6 X 8 (X = S, Se), continue to be the only reliable positive electrode materials despite the limited specific energy of the Mo 6 X 8 -Mg systems (up to 140 Wh kg -1 ). 359,360 Recent discoveries of spinel Ti 2 S 4 and sulfur as Mg positive electrode materials in conjunction with the continuous development of non-nucleophilic chloride-free electrolytes greatly expanded the technology, but sluggish kinetics remain and stability issues are real. 361,362 This is where OEMs start to look interesting: some OEMs have been reported to show adequate kinetics even at room temperature and deliver higher specific energies than those achieved by inorganic intercalation compounds at elevated temperatures. Due to these early successes, OEMs are now regarded by some as a wild card to enable multivalentmetal batteries. [START_REF] Xie | Recent Progress in Multivalent Metal (Mg, Zn, Ca, and Al) and Metal-Ion Rechargeable Batteries with Organic Materials as Promising Electrodes[END_REF]358,363 The properties and cell performance of these materials are summarized for comprehensive comparison (Table 5).

The promises aside, it is necessary to emphasize that multivalent chemistries are more complicated than monovalent ones, and the ion storage mechanism has not received enough scrutinization for many OEMs. For example, Mg 2+ is notoriously difficult to be dissociated from electrolyte solution species such as Cl -and ethereal solvent molecules. 364,365 While most studies on Mg-organic batteries employed Cl --containing electrolytes, a very recent study showed that probably all n-type OEMs store MgCl + instead of the presumed Mg 2+ in these electrolytes. 366 This unintended storage mechanism would effectively make most Mg-organic batteries hybrid batteries, that is, the charge carrier ions at the positive and negative electrodes are different, and the electrolyte solution must contain enough of the necessary ions to maintain charge balance. Cell specific capacity will be heavily limited by the weight of the electrolyte solution as a result. Since unambiguous determination of the stored species in most previously reported Mg-organic batteries has been overlooked, it would be difficult to estimate the actual specific capacity of those systems. In fact, such negligence is not specific to Mg-organic batteries studies but Mg-ion storing positive electrode material research in general. In the future, this situation can be reverted by dedicated characterizations and rational selection of electrolyte solutions. This section mainly concerns the apparent specific capacity of OEMs and only discusses storage mechanism where sufficient experimental evidence of stored species is available.

Similar to nonaqueous Li-organic batteries, the highest discharge potentials for Mg-organic batteries have been reported for p-type OEMs. Conductive polymer polyaniline (PAni), nitrogen-centered PTPA, and oxygen radical-centered PTMA all discharge at >2 V vs Mg 2+ /Mg, with PTPA showing the highest discharge potential at 3.2 V vs Mg 2+ /Mg. [367][368][369] Note that none of these materials were tested in electrolyte solutions where reversible Mg plating/stripping is possible. Among n-type OEMs, p-BQ-based molecule DMBQ and polymer poly(hydrobenzoquinonyl-benzoquinonyl sulfide) (PHBQS) show the highest discharge potentials of 1.63 and 1.8 V vs Mg 2+ /Mg, respectively (Figure 16B,C). 370,371 Other carbonyl compounds based on anthraquinone and imides discharge at lower potentials, as they also do in Li batteries. 372 As far as specific capacity is concerned, the organosulfur compound carbyne polysulfide shows the highest 328 mAh g -1 (Figure 16A), with carbonyl compounds DMBQ (up to 260 mAh g -1 ) and P14AQ (180 mAh g -1 ) following (Figure 16C,D). 366,373,374 The change of charge carrier ion from Li + to Mg 2+ does not seem to alter the cycling stability generally observed for Li + -storing polymer OEMs. P(NDI2OD-T2) and PPDIE lose 13% and 11% of their initial capacities after 2500 and 200 deep cycles, respectively, which stability is rarely seen from inorganic positive electrode materials fused aromatic rings or to be kinetically limited, numerous examples have been described with a single 260 or no aromatic ring 384 and with excellent capacity at a high current rate, and there is still hope for further developments of this redox process. By nature, "superlithiated" materials are typically investigated as negative electrode (starting with a cation uptake).

The only case of a "superlithiated" positive electrode (starting with a cation release) is lithium carbide Li 2 C 2 , where up to one lithium ion can be released from the structure. 385,386 As previously mentioned, this review does not try to be exhaustive, and just a few examples will be presented here. The total amount of articles on materials which could be related to a "superlithiation" process has exceeded 100. For the sake of clarification, OEMs containing a transition metal such as metal-organic frameworks, coordination polymers, or organometallic compounds have been excluded, considering that their specific capacity has a contribution from the transition metal in addition to the "superlithiation" corresponding to the organic moiety.

Polyanthraquinone-triazine (PAT) is a COF using repeating units similar to COFs previously mentioned. However, unlike the other triazine-based and anthraquinone-based COFs, PAT undergoes a 17-electron reduction per repeating unit, corresponding to the reduction of 2 carbonyls (2 electrons), the triazine unit (3 electrons) and the 2 benzene rings of the anthraquinone moiety (12 electrons), leading to a maximum reversible capacity of 770 mAh g -1 (Table 6, entry 1, and Figure 17B). 387 Interestingly, the capacity decreases in the first 30 cycles before slowly increasing with time over 400 cycles, which has been assigned to a slow activation process. This behavior has been observed for several "superlithiated" materials, such as F-PDI-3-TC (Table 6, entry 2). 388 Its initial capacity is merely 95.7 mAh g -1 but reaches 783 mAh g -1 after 1000 cycles (Figure 17C). Other examples include poly-(benzobisimidazobenzophenanthroline) (BBL) and poly(1,6-

AL

it is therefore interesting to evaluate the effective chances of organics to realize user/market-acceptable aqueous batteries.

In addition, we believe that such a comparison would not be complete if another bottleneck of this technology that is the price of active materials is not compared. To address these points, the latest literature data associated with the most advanced and industry relevant inorganic material for molar range aqueous battery (as opposed to water-in-salt type ones that will be briefly discussed, vide inf ra) which is the carbon coated Na(Li)Ti 2 (PO 4 ) 3 (referred to as C@NTP or C@LTP) is considered below.

AM

quinone derivative also used on the negative side, the poly pyrene-4,5,9,10-tetraone (PPTO): 1.68 g cm -3 , 338 mAh cm -3 , 201 mAh g -1 cycled in Na + -based aqueous electrolyte. 395 Although the PPTO electrode contained 30 wt.% of carbon additive its electrode volumetric capacity (144 mAh cm -3 ) remains in the vicinity of that derived for the titanium phosphate ones: 121 mAh cm -3 , ref 393 and 152 mAh cm -3 , ref 394. It is also important to point out that the previous results were obtained from relatively thin electrodes with areal capacity in the vicinity or below 1 mAh cm -2 .

For thicker ones (close to 2 mAh cm -2 ), Y. M. Chiang's group 396 obtained 310 mAh cm -3 by volume of C@NTP at 0.6 C, which turns into 161 mAh cm -3 by volume of composite electrode. Industrially relevant aqueous batteries were demonstrated by Whitacre with impressive electrode loadings (between 20 and 450 mg•cm -2 ) using NTP derived from very cheap synthesis protocols. 397 These good results pair, however, with a lower volumetric capacity of the C@NTP material (182 mAh cm -3 in approximately 20 mAh cm -2 pouch cells and 159 mAh cm -3 in approximately 1 mAh cm -2 coin cells 397 ) which falls down to 83 and 77 mAh cm -3 in terms of electrode volume for pouch and coin cells, respectively, using the corresponding electrode chemistry. In comparison, the highest areal capacity demonstrated with organic materials lies in the vicinity of 4.5 mAh cm -2 . 232,398 Indeed, Perticarari et al. 232 achieved nearly 100 mAh cm -3 of material and 66 mAh cm -3 of electrode using a diblock oligomer and 25 wt.% of carbon additive for 4.5 mAh cm -2 while Nishide's group obtained 3 mAh cm -2 using on a carbon nanotube hybridized poly (2,2,6,6-tetramethylpiperidin-4-yl) acrylamide (PTAm) polymer. 398 To sum up, this short literature analysis of both inorganic and organic active materials for aqueous batteries near neutral pH highlights that some of the proposed organic electroactive materials have already surpassed inorganic ones in term of volumetric capacity in the case of thin electrodes and are not too far behind in the case of thick ones.

Cost Aspect

According to Whitacre,397 NTP can be produced with precursor materials cost of $4 per kg upon two synthesis steps, ball milling and calcination. Taking $5/kg/step, which is an average in the pigment industry, an estimate of the NTP price should be roughly 14$/kg. In comparison, competitive organic materials (in terms of volumetric capacity) in neutral molar aqueous media such as PNDI and PPTO are approximately $4-6/kg and $10-15/kg, respectively, highlighting similar or even lower prices can indeed be achieved for organics.

Lastly, it is noted that water-in-salt (WiSE) 399 or hydratemelt 400 electrolytes have emerged as interesting opportunities to significantly improve the energy density and the Coulombic efficiency and mitigate corrosion issues (materials, current collectors, ...) of these new classes of "water containing" batteries. A comprehensive and critical review of the scientific understanding as well as the electrochemical and physical properties of these new electrolytes has been recently published by Yamada et al. 401 It is noted that the ability of WiSE electrolytes to extend the electrochemical window can be significantly improved by using an immiscible electrolyte additive, the 1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether (HFE). The latter electrochemically decomposes at the surface of the materials and by virtue of its hydrophobicity expels water molecules from the inner-Helmholtz interface of the electrode thereby mitigating hydrogen formation at the electrolyte. 404 This system shows, however, a very poor capacity retention (-25% in 20 cycles) that was mainly ascribed to NaVPO 4 F (-30% loss of capacity in 20 cycles) compared to -17% for PNDIE. 404 Yao's group 395 slightly improved these cyclability results to nearly 80% of capacity retention after 80 cycles (150 h) by substituting Na 3 V 2 (PO 4 ) 3 for NaVPO 4 F and PPTO (208 mAh g -1 ) for PNDIE (160 mAh g -1 ). To date, ASIB hybrid aqueous batteries are therefore not competitive with corresponding ALIB as reported above especially in terms of capacity retention. For this reason, ASIB related research has mainly focused on the use of inorganic compounds 173 such as 7, entry 4). This assembly allowed to achieve 1000 cycles at 2 C rate while keeping approximately 88% of the initial capacity. Interestingly, Wang et al. 414 obtained nearly two times more energy density (62.4 Wh kg -1 per mass of materials) using Li 3 V 2 (PO 4 ) 3 , as the positive electrode and lighter diimide derivatives (polypyromellitic dianhydride), as the negative electrode (Table 7, entry 6).

Compared to the more often used naphthalene derivative, the smaller delocalization backbone of the polypyromellitic destabilizes the radical anion and dianions that form on reduction and push the potential to lower values. The cell shows indeed a high voltage of 1.9 V which can be realized by using a relatively concentrated electrolyte (4 M Mg(TFSI) 2 ). 414 This resulted in a promising capacity retention of nearly 87% after 1000 cycles at 2 C rate.

Ca 2+ . Ca 2+ is another interesting abundant ion to play with in aqueous media. Yao's group 415 recently showed that the diffusion of Ca 2+ is higher than that of Mg 2+ both in the solid state and in the aqueous electrolyte media. These results were ascribed to smaller size of the hydrated Ca 2+ complex and its more facile dehydration during the charge transfer process.

This group assembled a battery with a copper hexacyanoferrate reactions with many different cations (monovalent and divalent), anions, and even both simultaneously. This aspect, that is encountered for the Prussian blue family for cations, opens up a large panel of possibilities regarding the cell chemistry, with the additional advantage to be coupled to much larger storage capacity and, in some instance, with the possibility of supporting the oxygen cycle.

Although it still remains far from that achieved with some inorganics, 397 the technology readiness level (TRL) of organic based aqueous batteries has increased in the latest papers by the use of either highly loaded electrodes and/or pouch cell configurations. However, studies devoted to the impact of electrode formulation are scarce. As an example, the polytetrafluoroethylene (PTFE) binder that is predominantly used so far because of its readiness is presumably not the most appropriate for the wettability of composite electrodes, for the volume variations on cycling, as well as from an industrial production point of view.

Among the different cell chemistries that have been reviewed herein, three main topics attract research work: all organic systems, hybrid ones bearing an inorganic electroactive material, and water-in-salt electrolytes. Because of the prohibitive price incurred by the water-in-salt type electrolytes and although related energy densities can today approach that encountered for Li-ion batteries, we believe this strategy will remain confined to fundamental studies unless cheap salts are found to replace the capability of imide based ones to form stable SEI.

On the other hand, both all organic and hybrid organic batteries using molar electrolytes now can show promising capacity retention at the cell level. However, this aspect still remains to be confirmed in large format batteries, at different temper- decided to retain only studies on the most concentrated electrolytes. In solvent-based media, generally speaking, the performance is much weaker than in the case of aqueous electrolytes, so the selection we made was less drastic.

A RFB consists of three main components: an electrochemical conversion cell where electrochemical reactions take place, tanks to store redox-active fluids, and finally pumps to allow the circulation through the cell of the two electrolytes (posolyte/negolyte) containing the redox-actives species (Figure 21a). Behind this apparent simplicity lies a device that remains very difficult to optimize. Among the main parameters or components associated with the chemistry used, we can mention:

• the nature of the solvent that composes the electrolyte, on which depends the accessible potential window, as well as the power density (ionic conduction) of the conversion cell;

• the redox compounds solubility; it is important that the different redox states of each couple have the highest possible solubility. The number of electrons exchanged during the redox reaction must modulate this solubility; the important parameter is the quantity of electrons exchanged by the solution (mole electrons per liter);

• the electron transfer kinetics with the electrode surface; electron transfers must be fast and perfectly reversible to avoid losses due to overvoltage problems. In general, for a redox-flow conversion cell, faradic efficiencies are very good; main losses come from overvoltage problems at the electrodes (different potentials between charge and discharge);

• the chemistry itself which must obviously be stable enough without bringing corrosion problems that would shorten the battery life. RFBs must have a very long lifetime (>20 years) in order to make its components more cost-effective than in a regular "sealed" battery (membrane, pumps cost, etc, ...) since stationary storage applications are targeted as previously underlined; 130,136 • the membrane, which allows the two compartments to 2958 be chemically and electrically separated, while allowing 2959 the diffusion of ions that ensure the system electro-2960 neutrality without crossover.

conductivity of the electrolyte). The main redox-active moieties are nitroxide, viologen, perylene diimide, ferrocene, quinone, thio, amino, phenol, ... (Figure S8). In a general way, there is a strong lack of redox-active structures able to work at very low potential (as in the case of carboxylates in organic "sealed" batteries working with solid state compounds) in order to obtain possible to mix several solvents, even if this makes electrolyte development more complex. 427 This approach remains difficult to master because it is difficult to combine the advantages of solvents without also combining the disadvantages.

One way to get around this is to use hydrotropes. A hydrotrope is a concentrated aqueous solution (several molars) of a small organic molecule such as urea, para-toluene sulfonic acid, nicotinic acid, and so on. Organic molecules are generally much more soluble in a hydrotrope than in water and the electrochemical properties are preserved or improved. The use of urea, for example, increases the solubility of benzoquinone by a factor of 7. 428 Other strategies have been used to make redox Chemical Reviews pubs.acs.org/CR Review organic molecules "liquid". The use of ionic liquid was tested by following different paths: as a solvent, with an organic or aqueous cosolvent, and finally by making redox ionic liquids. The use of eutectics based on highly concentrated salts in a solvent has produced interesting results because they produce highly conductive solutions capable of dissolving highly polar redox organic molecules. 429 Finally, some redox molecules developed to be highly soluble have been found to be liquid due to their low solid-state cohesion. However, the addition of salt is necessary to make the liquid ionically conductive, resulting in a significant increase in viscosity. While these alternative strategies have proved to be relevant, they have not yet made possible the development of large-scale devices capable of competing with inorganic flow cells in terms of power density, energy density, or stability (cycling and calendar aging).

The use of ionized salt is necessary to enhance the solvent ionic conductivity and maintain electroneutrality during the electrochemical process. In aqueous media, acid (H 2 SO 4 ) or base (KOH) could be used if the redox molecule does not react with them. Salts like sodium or lithium associated with nitrate, and chloride or sulfate could be used in neutral pH. In organic medium, salts need to be highly soluble and dissociated, so lithium or tetra-alkylammonium cation associated with noncoordinating anion such as PF 6 -, BF 4

-, or TFSI -is mainly used, even if these salts are much more expensive than the ones used in aqueous solvents. Independently of the solvent, the choice of the supporting salt is very important on the electrochemical behavior of the redox molecule.

One of the main problems remaining for RFB concerns the fact that the electrolyte in a highly concentrated solution tends to be very viscous (both in inorganic or organic RFB). In organic RFB, the problem is more important in the way that organic molecules possess a higher molecular weight compared to inorganic ones (M(V) = 50.9 g mol -1 ). This means, for example, that an organic redox molecule with a molecular weight of 200 g mol -1 , typically nonfunctionalized anthraquinonoid derivative, at a concentration of 1 M corresponds to 200 g of molecule in 1 L of solution. In some cases, the solution became as viscous as honey, precluding their use in flowing cells, apart from using a lot of energy to power the pump. A high viscosity also lowers the molecule diffusion in solution, with a direct effect on the apparent electron transfer kinetic and ionic conductivity. As a result, highly concentrated electrolytes suffer from higher cell overvoltage either in charge or in discharge. 9.1.4. Favoring Highly Soluble Redox-Active Species. In order to make redox organic molecules highly soluble in the desired medium, it is necessary to functionalize them with appropriately selected groups. In aqueous media, ionic functions or functions with a large number of heteroelements (e.g., O or N) will be favored to increase the interactions between the redox compound and water molecules. The main ionic functions used are sulfonates, 430 phosphates, 431 carboxylates, 432 ammonium, 433 or hydroxo. 434 These ionic functions have also the advantage of increasing the ionic conductivity of the solution, allowing in some cases to eliminate the use of supporting salts. Neutral substituents such as PEGs are also used regularly. However, it is necessary to remain attentive to the positions chosen to graft these solubilizing groups, because the functionalization of redox molecules can completely change the electrochemical response and make it irreversible. The solubility of nonfunctionalized organic molecules is generally better in organic media. However, in most cases, functionalization is necessary to achieve the solubility necessary to

Aqueous Organic Redox-Flow Batteries

In all cases, it is necessary to develop sometimes complex strategies to avoid these adverse reactions for the battery operation and stability. In terms of solubility, by functionalization it is possible to achieve solubility values in about 2 M. In the case of quinones, the electrochemical and solubility properties are very strongly dependent on the number and substitution positions. 430 Thus 9,10-anthraquinone-2,7-disulfonic acid (AQDS) retains a high electrochemical reversibility while being highly soluble, which is not the case for other disulfonates. 439 Generally speaking, aqueous ORFBs have many advantages: nontoxic, low solvent and salt costs, highly conductive solution. However, they pose two important and very difficult problems: the small window of potential associated with this solvent and its reactivity with organic molecules. 9.2.2. Main Examples of Aqueous ORFB. Many works have been carried out over the last 5 years in the field of aqueous ORFBs, we have chosen a few didactic examples to show the diversity of approaches and the performances that can be obtained with this technology to date.

Aziz's group (Harvard University) has conducted numerous studies on the use of quinone for ORFBs, or combined organic/inorganic batteries (quinone -Br 2 , quinone ferrocyanide). 434,440 Their studies focus mainly on the use of sulfonated (AQDS acid medium) or hydroxylated (DHAQ basic medium) quinones that can achieve electron concentrations >1 M (Figure 22a). The output cell voltage obtained with the full system 0.5 M DHAQ/0.4 M ferrocyanide is about 1.2 V. The battery was tested for 100 cycles at a current density of 100 mA cm -2 and showed an energy efficiency greater than 80% and a capacity retention of 90%. The maximum power density is 400 mW•cm -2 . No significant degradation of the electrolyte seems to occur. With ADQS in sulfuric acid medium (1 M) as negative electrolyte, associated with 3.5 M hydrobromic acid and dibromine (0.5 or 2 M) the cell voltage is 0.8 V. Different types of carbon felt and membranes have been tested to optimize battery performances. With a 212 Nafion membrane and 2 M Br 2 , 3 M HBr, the battery reaches a power peak at 1 W cm -2 which is extremely high for a flow battery. It should be noted, however, that this was possible thanks to extremely high electrolyte flows (400 mL min -1 ) for an electrode surface area of 2 cm 2 , which is very demanding on the consumption of the pumps.

Schubert's group (University of Jena) has developed an approach combining methyl viologen derivatives (low potential) and nitroxide (high potential). 441,442 These organic compounds are reversible in electrochemistry and have very high electron transfer kinetics. The voltages of the cells reached are 1.4 V (Figure 22b). The TEMPTMA derivative has a solubility of 2.3 M in a solution of NaCl 1.5 M which corresponds to a theoretical capacity of 61 Ah. The flow battery combining MV and TEMPTMA (2 M each) addresses the maximum capacity of the electrolyte up to a current density of about 100 mA cm -2 . The energy efficiency is greater than 70%. At 80 mA cm -2 the battery is stable over 100 cycles without significant degradation of performance. To avoid using an ion exchange membrane, polymers have been developed from these molecular units. Due to the steric encumbrance of these polymers, a simple porous membrane is sufficient to prevent the two compartments from mixing. Ten Ah L -1 electrolytes are made by dissolving these polymers in 2 M NaCl solution. The battery thus formed is capable of cycling up to current densities of 50 mA cm -2 without significant loss of capacity. The properties are stable over 90 cycles with a loss of less than 10% in capacity. The The main problem with these mixed Li/organic molecules flow cell approaches comes from the very low current densities (often less than 1 mA cm -2 ) that can be passed through the device. This is particularly true if the separator between the two compartments is a ceramic membrane. When the separator is porous, it would appear that the SEI formed on the surface of the Li in the carbonates is beneficial. It would act as an ion exchange membrane while blocking electron transfers to the redox molecules. Thus, molecules in solution close to the negative electrode could not be reduced, which would explain the high Coulombic efficiency of these devices. On the other hand, at high concentration in redox molecules, the SEI is modified and/or no longer succeeds in blocking these electron transfers and the Coulombic efficiency falls strongly (Coulombic efficiency 99% at 0.1 M, 91% at 0.8 M, 85% at 1.5 M, and 84% at 2 M). 448 Finally, these devices do not allow the power and energy density to be separated; they are therefore more limited in terms of application. high-potential molecules in order to increase their solubility, and stability. 451 DBMMB offers the best compromise in terms of viscosity, stability and diffusion coefficient. Its redox potential is about 0.78 V vs Ag + /Ag, so it has been retained and implemented against several low-potential and highly soluble molecules: BzNSN and N-methylphthalimide (MePh). 452,453 MePh has a reduction potential of about -1.79 V vs Ag + /Ag in 1 M LiTFSI DME electrolyte. A major effort has been made to choose a porous membrane of the Celgard or Daramic-type that is effective in preventing species mixing and does not generate a too high ohmic drop. A Daramic 175 membrane was selected to make a complete redox-flow cell using an equimolar mixture of DBMMB and MePh at 0. An electrolyte containing an equimolar mixture of BzNSN and DBMMB (0.5 M) in 1 M LiTFSI ACN as electrolyte was cycled in a redox-flow cell composed of two carbon felt electrodes and a Daramic type separator (800 μm). Due to the viscosity of the solution and the problems of cross diffusion through the membrane, the current density applied is 10 mA•cm -2 . The energy yield is 72%, mainly due to ohmic drops. The energy density is in the order of 6-8 Wh L -1 . The stability of the cell is not very good (50 cycles) probably due to the reactivity at high concentration of the species that compose the electrolyte. These studies have shown that DBMMB derivatives are potentially relevant for the design of a fully organic redox-flow battery. They are extremely soluble in organic electrolytes (2-3 M) and the radicals resulting from their oxidation are stable in DME solvents. The two low-potential molecules tested, particularly BzNSN, are soluble up to 5 M, but their solutions are highly viscous precluding their use in flowing cell. Kentucky University has developed organically soluble phenothiazine derivatives. 454 MEEPT has been selected as the most efficient compound: it is liquid at room temperature, and its solubility in 0.5 M nBu 4 NBF 4 ACN is greater than 2 M. It is electrochemically stable, and it is possible to prepare both forms, neutral and oxidized, easily by chemical methods. The core of the conversion cell has been optimized to reduce both ohmic drop and losses related to fluid flowing through the system (channel electrodes). The optimized cell has a resistance of about 3.3 Ω cm -2 . The assembly was tested in a symmetrical configuration with an electrolyte composed of 0.5 M MEEPT at 50% state of charge with a theoretical capacity of 13.4 Ah L -1 . In constant current cycling at 50 and 100 mA•cm -2 , the measured capacity is respectively 97.3% and 60% of the theoretical capacity. Over 100 cycles (80.6 h in total) at 100 mA cm -2 , the capacity is maintained, and no significant loss is observed.

Columbia University, NY, has developed a flux battery based on perylene diimide derivatives (negative) and ferrocene tetramere (positive), both of which can achieve electron concentrations > 1 M. 455 The interest of this work is to demonstrate that In terms of solid electrode materials (for "sealed" batteries), electroactive molecules with simple structures such as simple quinones could offer the best balance between high discharge potential and high specific capacity, but stable cycling requires proper immobilization of these molecules which, until now, cannot be done without impacting potential and capacity. Two directions may directly counter this dilemma. The first is a chemical path revolving no-compromise immobilization. Rational connection of molecular building blocks complemented by suitable synthesis methods has been successful in preserving the full redox characteristics of active cores as seen in OEMs like PAQS and P(NDI2OD-T2). The second is to suit OEMs with solid electrolytes where dissolution is a nonissue. Solidstate batteries are emerging as an alternative to the traditional liquid electrolytes to enable safer and higher-energy batteries.

The nondissolving nature of solid electrolytes is an additional perk specifically useful for organic batteries. In addition, p-type materials could offer access to molecular (metal-free) batteries as well as possible assemblies with n-type systems to promote dual-ion cells.

Continued evolution of organic batteries will inevitably involve a better understanding of involved electrochemical mechanisms (especially in the solid state) as well as the development of new OEMs, which is both a strong suit for organic compounds from the fundamental study point of view and a potential uncertainty for practical applications. Organic compounds are known to have good tuning knobs to manipulate the molecular structures and many subsequent properties, but complicated functionalization can drive up synthesis cost. Two currently practiced approaches could lead to a balance between performance and cost. One scours through structures that can be synthesized from cheap raw materials via simple, few-step, and high-yield reactions, the other prioritizes function and performance with no specific focus on minimizing cost. At an early stage of research and development, the former approach rarely affords high-performance candidates due to the limited options, while the latter could end up with prohibitive costs.

Overtime, however, the building blocks found by the former and

BB

the design strategies established by the latter will merge the two approaches and strike the balance.

Beyond the direct using of redox-active organics as the main active electrode compounds, other functionalities can be used in an electrochemical cell such as the hybridization of the conventional inorganic active materials with organic redoxactive polymers giving rise to fast electrode kinetics. Thus let us recall that electrical properties of solid composite electrodes are critical to electrochemical performances, whatever the considered technology. This is especially the case for thick electrodes which are highly sought-after because they enable in principle simultaneous increase in volumetric and specific energy densities and decrease in price. It is estimated that 20 to 80% of power losses in the case of a thick electrode originate from insufficient electronic conduction. 458 The design of electronic conductivity at different scales is not easy, because it is currently based on the use of carbon-based conductive additives that are difficult to disperse homogeneously on the one hand, and on the other hand, that renders difficult the manufacturing of thick composite electrodes, because of the unstable nature of the electrode inks. In addition, the loss of intercluster and interparticle contacts due to volume variations of active materials is a major cause of the aging of battery electrodes. 459 It is therefore mandatory to add polymeric binders (insulators), with the downside of a difficult compromise between electronic conductivity and mechanical properties of the composite electrode. 460 The use of electron conducting binders or even redox binders ("smart binder") appears therefore quite attractive, since it would enable the design of electrodes with lower amount of carbon and binder additives. This objective was pursued in the particular case of LiMPO 4 (M = Fe, Mn). As early as 2006, Goodenough and co-workers showed that carbon-coated LiFePO 4 particles can be directly connected to the current collector using a matrix of p-doped PPy or PAni, [461][462][463] while Graëtzel was actually the first to connect non-carbon-coated LiMPO 4 (M = Fe, Mn) particles to the current collectors using molecular wires. 464 In the latter, the Fermi levels of the organic matter and the active material need to be adjusted while the adsorbed wires should be percolated to allow for cross-surface charge. This approach was also recently investigated by Nishide and Oyaizu using nonconjugated radical polymers (radicals are densely introduced as the pendant groups) that were specifically designed for fast charging of LiFePO 4 or LiCoO 2 . 465 Schougaard and co-workers also showed a poly (3,4-ethylenedioxythiophene) coating at the surface of bare LiFePO 4 particles advantageously replaced the carbon coating. 466 Targeting the contemporary concerns of industry-relevant electrodes and especially thicker ones, Gaubicher and Blanchard 467 used a short thiophene-based π-conjugated system as a molecular junction between uncoated LiFePO 4 and multiwall carbon nanotubes within undensified 2 mAh cm -2 electrodes. Electrochemical and electrical properties of such electrodes demonstrate the key role of molecular junctions to reach power and cyclability performances comparable to those of carboncoated LiFePO 4 electrodes.

To conclude, although this research activity is still in its infancy and much remains to be done to get attractive performances, redox-active organic compounds can be perceived today more than ever as an alternative chemical choice depending on the targeted application. We hope that this review will be a source of original and fresh ideas for our readers.

  photovoltaic (PV), and wind. Other emerging renewable technologies include wave and tidal energy conversion and biomass energy conversion. Therefore, some predictions seem to indicate that GHG emissions in the power sector could be drastically reduced thus becoming a major contributor to decarbonization (Figure2). Although the total electricity production is expected to more than double between 2010 and 2050 giving rise to the incredible value of more than 40 000 TWh of generated energy per year (notably to power the electromobility, see below), total emissions for the power sector could be divided by more than four according the Deep

Figure 1 .

 1 Figure 1. Forecast of the world's energy needs up to 2050. With the changing lifestyles of an increasing number of inhabitants, our energy rate demand will double from 14 TW (2010) to 28 TW (2050). TOE = ton of oil equivalent. Reproduced with permission from ref 112. Copyright 2015 Nature Publishing Group.

Figure 2 .

 2 Figure 2. (a, b) Decarbonization Wedges in the power sector extracted from the ANCRE's report 124 (with permission) elaborated in the DDPP and by considering the most ambitious scenario in each country; "worldwide scale" refers to only the 16 most emitting countries. These histograms show the emission trajectories for the electricity sector in the absence of any technological evolution and (in gray) the evolutions of the emissions within the framework of decarbonization scenarios; the difference between the two corresponds to reductions that allow different technologies (e.g., solar in yellow and orange, hydro in blue); CCS meaning carbon capture and sequestration. Further descriptions of the "Decarbonization Wedges" methodology can be found in ref 126.

Figure 3 .

 3 Figure 3. Comparison of discharge time and power rating for various electrical energy storage technologies highlighting the broad contribution of batteries with various chemistries; the latter being only based on inorganic electroactive materials, essentially metals. Reproduced with permission from ref 130. Copyright 2011 Science Publishing Group.

  rechargeable batteries are likely to remain the better choice to power light duty vehicles (LDV) in the next 2 decades whereas FCEVs are considered as the future vision for the global automotive industry beyond 2040-2050 because this technology and related hydrogen supply infrastructures are still not at the desirable level, 121,145 with the production of low-carbon H 2 being required too. For the present time progress made in recent years to improve battery performance and reduce costs 133 has already enabled the use of LIBs in numerous e-mobility applications such as two-wheelers, buses, taxis, shared cars, ride-hailing services, and the upcoming self-driving (or driverless) cars. Thus, sales of new electric cars worldwide surpassed 1 million units in 2017 (for 3 million EVs in circulation), a record volume which represents a growth in new electric car sales of 54% compared with 2016 (more than half of global sales of electric cars were in the People's Republic of China) whereas the EV30@30 scenario makes as projection 228 million EVs (excluding two-and three-wheelers) on the road by 2030 as shown in Figure 4b; 143 the EV30@30 campaign redefining the EVI ambition originally set at 20 million EVs on the road by 2020. More recently, Hache et al. 146 have reported a bottomup analysis using the Times Integrated Assessment Model (TIAM-IFPEN version) to forecast the diffusion of electrified road transportation modes by integrating two climate scenarios (4 and 2 °C) and two shapes of mobility (high/low mobility). The electric vehicles fleet could reach up to 1/3 of global fleet by 2050 in the 4 °C scenarios, while it could be up to 3/4 in the 2 °C scenarios both with high mobility, mostly located in Asian countries (China, India, and other developing countries in Asia) due to the large presence of 2-and 3-wheelers (Figure S2). Consequently, electric transportation is integrated into the smart grid/smart cities master plans 147 first because the electricity needs will boom but also as a means of flexibility since integrating BEVs/ PHEVs into the electric utility grid facilitates both vehicle-to-grid (V2G) and grid-to-vehicle applications owing to the bidirectional nature of the power flows between the BEVs/PHEVs and the grid. Last but not least, developing electric/hybrid aircraft is in progress too because electric propulsion has the potential to revolutionize aviation opening real opportunities for cleaner, quieter travel to completely new types of transportation models. In November 2017 Airbus announced the launching of E-Fan X with its partners Siemens and Rolls-Royce which is an ambitious technology demonstrator project: a hybrid civil aircraft, another emerging sector calling for high-performance batteries. Other equivalent projects exist such as the ZUNUM Chemical Reviews pubs.acs.org/CR Review

Figure 4 .

 4 Figure 4. (a) Annual light-duty vehicle (LDV) sales according to the BLUE Map scenario, 144 2000-2050 reported by IEA; combined EV/PHEV sales should share at least 50% of LDV sales worldwide by 2050. (b) Global EV stock for 2017-2030 excluding two-and three-wheelers by considering both New Policies Scenario and the EV30@30 Scenario reported by IEA 143 (PLDVs = passenger light duty vehicles; LCVs = light commercial vehicles; BEVs = battery electric vehicles; PHEV = plug-in hybrid electric vehicles). (c) Battery technology roadmap adapted from IEA 143 (HVS = high voltage spinel). Adapted with permission, copyrights of IEA.

Figure 6 .

 6 Figure 6. Periodic tables showing the abundance of elements in the Earth's crust (A), in the seawater (B), in the atmosphere (C), and in biomass (D); elements in gray indicate natural and/or radioactive elements. Courtesy of L. Simonin, CEA-Liten, adapted from ref 188 taking into account data from refs 189-191. This analysis highlights that among the naturally occurring elements only a few of them are abundant in each of these four compartments, which demonstrates the importance of developing recycling solutions too.

1 /

 1 Red 1 and Ox 2 /Red 2 characterized by standard potentials E°(Ox 1 /Red 1 ) > E°(Ox 2 /Red 2 )) to reach high electromotive force (emf = E I=0 , in V) at the cell level under zero current since emf depends on the difference E°(Ox 1 /Red 1 ) -E°(Ox 2 /Red 2 ) > 0 or more generally on the difference E(+) -E(-) > 0. In short, it depends on the thermodynamics of the chosen electrode chemistries because E I=0 is related to the g Gibbs free energy change of the galvanic cell reaction, Δ r G, as follows: Δ r G = -n•E I=0 •F (n being the number of electrons involved in the cell reaction and F is the Faraday constant, equal to 96,485 C mol -1 ). Under discharge current (I, in mA), for example, the operating voltage, E I≠0 , is lower than the E I=0

Figure 7 .

 7 Figure 7. Overview of fundamental properties of different types of organic electrode materials. Reproduced with permission from ref 38. Copyright 2018 Elsevier Ltd.

Figure 8 .

 8 Figure 8. General architecture of an electrochemical cell for energy storage whatever the considered technology is. Note that additional electrolyte can be stored externally and then pumped through the cell in the particular case of a flow battery (see Section 9). (a) Cell under discharge (namely galvanic cell) at 100% depth-of-discharge (DOD). (b) Cell under charge (namely electrolysis cell) at 100% state-of-charge (SOC); the electromotive force value (emf = E I=0 ) is, in principle, at the maximum value. The measured capacity, Q (in mAh), is experimentally obtained by integrating the operating time with current:

  a field particularly studied by Nishide, Oyaizu, and co-workers as previously underlined in the Introduction. Finally, the general structure principles that lead to multistage organic redox systems (expected reversible from a redox point of view) are owed to Deuchert and Hunig 219 reported in a comprehensive article published in 1978.

Figure

  Figure 9a recalls the three general key organic structures and their related charge transfer steps deriving from Hunig's classification together with representative examples of chemical compounds. Basically, two electrochemical storage mechanisms are accessible with organics characterized either by anion charge compensation (p-type system or System A) or by cation charge compensation (n-type system or System B); System C being a mixed assembly also named "bipolar" system. At otherwise constant conditions it is worth noting that redox-active p-type moiety exhibits as a rule higher formal potential values compared to n-type systems. 219 Figure 9b shows the corresponding cell configurations. For instance, developing Li-ion organic batteries requires two n-type compounds (Figure 9b-1). The p-type redox reactivity is rarely encountered with inorganic electrode materials. Except carbonaceous materials which intercalate ions at relatively high potential (use as positive electrode), 222-225 only a few conversion-type electrode materials have been reported to date (for F-ion batteries) exhibiting however poor electrochemical performances. 226 On the other hand, p-type organic compounds enable, in principle, the development of anionic rocking-chair cells 227,228 and potentially full "molecular" organic-ion batteries if the shuttling anion is metal free (Figure 9b-2). Nevertheless, the cell assembly most often encountered in the literature remains the dual-ion battery (Figure 9b-3) as recently review by Zhou et al.229 The use at the electrode level of advanced mixed p/n-type organic scaffolds that exchange simultaneously cations and anions can also be envisaged as recently shown in aqueous batteries with naphthalene diimide repeating units coupled to bipyridinium.[230][231][232] The molecular engineering can even allow cation shuttling in the electrolyte while using p-type electrodes as reported by Nishide, Oyaizu, and co-workers

Figure 9 .

 9 Figure 9. (a) Key reversible redox-active organic systems at the molecular level and their related charge transfer steps (adapted from refs 219 and 234) together with representative examples exhibiting the redox rocking from aromatic to quinoid form. X/Y could be N, O, S, P, π-systems but also carboxylate, anhydride, or amide functional groups; R, R′ being potentiality integrated within the same cyclic structure. Note that that p-and n-type structures correspond to system A and B, respectively, according to Hunig's classification. 219 (b) Corresponding cell configurations obtained by playing with both n-and p-type systems shown during the discharge process. Again, additional electrolyte can be stored externally and then pumped through the cell in the particular case of a flow battery (see Section 9). Adapted with permission from ref 36. Copyright 2018 Elsevier Ltd.

a

  PBQS: poly(benzoquinonyl sulfide); PDTTA: poly(5,8-dihydro-1H,4H-2,3,6,7-tetrathia-anthracene; ADALS: azobenzene-4,4′-dicarboxylic acid lithium salt; TCNQ: tetracyanoquinodimethane; PPy: polypyrrole; DBMMB: 2,5-di-tert-butyl-1-methoxy-4-[2′-methoxyethoxy]benzene; PT: polythiophene; PTMA: poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate). Note the redox-active nitroxide radical is bipolar, in practice, good kinetics are only attempted with the p-type activity.Chemical Reviewspubs.acs.org/CR Review (2,5-dilithium-oxy)-terephthalate (Mg(Li 2 )-p-DHT) shows the highest working potential (3.45 V vs Li + /Li) among n-type OEMs approaching the well-known LiFePO 4 electrode material (Table 2, entry 17). Note that an attempt to switch from the carboxylate functional groups present in Li 4 -p-DHT to the sulfonate substituent giving rise to the tetralithium salt of 2,5dihydroxy-1,4-benzenedisulfonate (Li 4 -p-DHBDS) 253 enables also a voltage gain (+650 mV) but with inferior electrochemical performance upon cycling. A closely related structure to p-BQ is tetracyanoquinodimethane (TCNQ), where the oxygen atoms in p-BQ are replaced by the even more electron-withdrawing dicyanomethylene groups (

Figure 10 .

 10 Figure 10. Voltage profiles of selected high-potential positive OEMs and low-potential negative OEMs measured vs Li including (A) pentakis-fused TTF (reproduced with permission from ref 238. Copyright 2014 The Royal Society of Chemistry), (B) PTMA (reproduced with permission from ref 75. Copyright 2002 Elsevier Ltd.), (C) DD (reproduced with permission from ref 241. Copyright 2019 Elsevier Ltd.), (D) BQ and PBQS (reproduced with permission from ref 246. Copyright 2015 John Wiley & Sons, Inc.), (E) Li 2 (Li 2 )-p-DHT and Mg(Li 2 )-p-DHT (reproduced from ref 252), (F) TCNQ, F-TCNQ, and F2-TCNQ (reproduced with permission from ref 255. Copyright 2013 John Wiley & Sons, Inc.), (G) 1,8-PhenQ and 4,5-PhenQ (reproduced with permission from ref 257. Copyright 2013 The Royal Society of Chemistry, (H) Li 4 -o-DHT (reproduced with permission from ref 234. Copyright 2014 American Chemical Society), and (I) Li terephthalate (reproduced with permission from ref 261. Copyright 2009 Nature Publishing group).

Figure 11 .

 11 Figure 11. Voltage profiles of selected high-capacity OEMs measured vs Li including (A) poly(S-r-DIB) (reproduced with permission from ref 266. Copyright 2013 Nature Publishing group), (B) S-PAN (reproduced with permission from ref 268. Copyright 2015 American Chemical Society), (C) Li 2 C 6 O 6 (reproduced with permission from ref 82. Copyright 2008 John Wiley & Sons, Inc.), (D) C 6 O 6 (reproduced with permission from ref 269. Copyright 2019 John Wiley & Sons, Inc.), (E) DMBQ (reproduced with permission from ref 270. Copyright 2010 Elsevier Ltd.), (F) P5Q (reproduced with permission from ref 273. Copyright 2014 American Chemical Society), (G) poly-LiDHAQS (reproduced with permission from ref 276. Copyright 2017 John Wiley & Sons, Inc.), (H) PTO (reproduced from ref 257. Copyright 2013 The Royal Society of Chemistry), and (I) 3Q (reproduced with permission from ref 277. Copyright 2013 Nature Publishing group).

  1743 that readily decay due to dissolution can be made stable when 1744 a sufficiently large amount of high-surface-area carbon adsor-1745 bent is included in the positive composite electrode as previ-1746 ously underlined in section 1.2. 13 Due to practical consid-1747 erations, cycling performances enabled by intricate adsorbents 1748 are not a priority for this review. 1749 The most stable OEMs for nonaqueous Li batteries have 1750 been polymers. Polymer OEMs containing varying types of redox 1751 centers, from p-type to n-type, from nitroxides (e.g., PTMA) 279 to 1752 aromatic amines (e.g., poly(3-vinyl-N-methylphenothiazine) or 1753 PVMPT) 280 to quinones (e.g., polydopamine) 105 to imides (e.g., 1754 poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboxi-1755 mide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} or P(NDI2OD-T2), 1756

  1761 difficulty may vary from type to type. With judicious selection 1762 of polymerization strategies, two-and three-dimensional 1763 polymer OEMs have been synthesized based on aromatic 1764 amines (e.g., polytriphenylamine or PTPA), 242 quinones (e.g.), 283 and imides (e.g., perylene diimide-1767 triptycene or PDI-Tc, Table 2, entry 52). 284 These additional 1768 architectures are proposed to further improve cycling stability. 1769 The advantage of polymer OEMs in cyclability does not 1770 preclude molecular OEMs from being important. Molecular 1771 materials can still be preferred for their well-defined structures, high specific capacity, flexibility in processing, and so on.

  1773Therefore, approaches to improve the cyclability of small 1774 molecules are still being developed. Considering the success of 1775 polymer OEMs, a widely practiced strategy is to increase the 1776 molecular weight of molecules or simply oligomerization. Both 1777 3Q and coronene are big planar molecules which benefit from 1778 relatively strong van de Waals interaction and effective π-π Chemical Reviews pubs.acs.org/CR Review inorganic electrode materials. Note that the specific capacities of batteries with p-type OEMs are impacted by the weight of the Li salt in the electrolyte (dual-ion cell configuration, Figure 9b-3). For comparison convenience, the calculation of all batteries with p-type OEMs considers LiBF 4 as the Li salt. Although p-type OEMs have comparable discharge potentials and specific capacities as those of inorganic electrode materials, batteries based on them fall short in specific energy where the weight of the electrolyte is included. Therefore, p-type OEMs are not yet contenders as high-energy battery cathode materials but may find applications in, for example, fast charging devices and wearables. Many quinone-based OEMs have surpassed inorganic electrode materials in terms of specific energy at the active material level (i.e., only the weight of active materials is included in calculation). In practice, however, most highenergy OEMs still require too much conductive agents in their electrode composites for decent performance, which decreases specific energies at the cell level. Another challenge for OEMs is to simultaneously achieve high specific energy and high cycling stability. Future development of OEMs for nonaqueous Li batteries demands deeper understandings of their electron and ion conduction mechanics as well as performance degra-dation mechanisms and strategies to address them.6. PERFORMANCES OF NONAQUEOUS SODIUM-ORGANIC BATTERIESFor decades, nonaqueous organic sodium-ion batteries have been overshadowed by other electrochemical energy systems (such as inorganic lithium-ion batteries, inorganic sodium-ion batteries, and later organic lithium-ion batteries) and have only been sporadically investigated. 293 However, in 2012, three different groups published their works on disodium terephthalate within a time frame of four months, marking then the beginning of the recent boom in this field.[294][295][296] Since then, disodium terephthalate has been regularly investigated as a model compound for composite electrode formulation.[297][298][299][300][301]

Figure 12 .

 12 Figure 12. Comparison of the discharge potentials and specific capacities of state-of-the-art OEMs and inorganic electrode materials for Li batteries. Specific capacity calculation considers the weight of the lithiated form of positive electrode materials and delithiated form of negative electrode materials (e.g., azo compounds, carboxylates, and graphite). The highest observed reversible capacities are used instead of theoretical values.

6. 1 .

 1 High/Low Voltage Organic Electrode Materials and 1883 Hybrid/All-Organic High Output Voltage Na-Ion Batteries 1884 The design of high and low voltage OEMs for SIBs has clearly 1885 taken inspiration from the lithium equivalents. "p-type" 1886 materials (i.e., anion insertion materials) are hence the most 1887 common compounds able to react at potential >3.0 V vs Na + / 1888 Na, while materials reacting at low potential <1.5 V vs Na + /Na 1889 belong to "n-type" (i.e., cation insertion materials). Conductive 1890 polymers are an example of a type of material which could belong to any or both of these categories. Early research on conductive polymers for SIBs has been carried out by Yang's group, who used polyaniline derivatives (p-type), 303 doped PPy 304,305 (p-type), or PT 306 (n-type) in some of the first articles in this field. Soon after, they reported the activity of polytriphenylamine (PTPAn, Table 3, entry 1) as p-type material with PF 6 -ingress. 307 Each triphenylamine unit can lose one electron and intercalate one anion upon oxidation at an average redox potential of 3.6 V vs Na + /Na forming then quinoneimine units (Figure 13A). PTPAn displays stable reversible capacity even at high C-rate with 88 mAh g -1 at aTable 3. continued a Based on cathode material weight. b The first cycle is not taken into account. c Based on anode material weight. d Based on cathode and anode.

Figure 13 .

 13 Figure 13. Voltage profiles of selected OEMs measured vs Na and all-organic Na-ion full cells including (A) PTPAn (reproduced from ref 307), (B) PDPPD (reproduced with permission from ref 308. Copyright 2019 The Royal Society of Chemistry), (C) CuTCNQ (reproduced with permission from ref 309. Copyright 2017 John Wiley & Sons, Inc.), (D) Co-bpdc (reproduced with permission from ref 317. Copyright 2018 The Royal Society of Chemistry), (E) Na 0.75 Mn 0.70 Ni 0.23 O 2 //Na 2 C 8 H 4 O 4 (from ref 296. Copyright 2012 Royal Society of Chemistry), (F) Na 2 C 6 O 6 // Na predoped carbon (reproduced with permission from ref 326. Copyright 2013 Elsevier Ltd.), (G) Na 2 C 6 O 6 //Na 2 C 8 H 4 O 4 (reproduced with permission from ref 327. Copyright 2017 Nature Publishing Group), (H) PI//Na 2 C 8 H 4 O 4 (reproduced with permission from ref 328. Copyright 2015 The Royal Society of Chemistry), and (I) Na 4 C 8 H 2 O 6 //Na 4 C 8 H 2 O 6 (reproduced with permission from ref 329. Copyright 2014 John Wiley & Sons, Inc.).

Figure 14 .

 14 Figure 14. Capacity retention of selected OEMs measured vs Na including (A) TSAQ (reproduced from ref 331), (B) DAAQ-COF (reproduced with permission from ref 332. Copyright 2019 American Chemical Society), (C) PPTS (reproduced with permission from ref 334. Copyright 2018 Elsevier Ltd.), (D) ADASS (reproduced with permission from ref 335. Copyright 2018 John Wiley & Sons, Inc.), (E) Na 3 C 9 H 3 O 6 (reproduced with permission from ref 336. Copyright 2018 John Wiley & Sons, Inc.), and (F) BPOE (reproduced from ref 337).

Figure 15 .

 15 Figure 15. Voltage profiles or capacity retention of selected OEMs measured vs K including (A) K 2 C 8 H 4 O 4 (reproduced with permission from ref 346. Copyright 2017 The Royal Society of Chemistry), (B) PPTS (reproduced with permission from ref 347. Copyright 2019 The Royal Society of Chemistry), (C) PPTS//K 2 C 8 H 4 O 4 (reproduced with permission from ref 347. Copyright 2019 The Royal Society of Chemistry), (D) PVK (reproduced with permission from ref 349. Copyright 2019 Elsevier Ltd.), (E) PDPPD (reproduced with permission from ref 308. Copyright 2019 The Royal Society of Chemistry), and (F) PAni (reproduced with permission from ref 350. Copyright 2018 John Wiley and Sons).

  indeed look promising as unique enablers for 2328 the otherwise quite problematic multivalent-metal batteries, 2329 even though detailed studies regarding the ion storage mech-2330 anism and performance improvement have only just begun.

7. 3 .

 3 Comments on The Peculiar Multiple Cation Insertion 2331 Phenomenon in Organics 2332 The electrochemical process coined as "superlithiation" 2333 corresponds to a two-electron reduction of a carbon-carbon Table 5. continued a Could be either SO 4 2-or CH 3 CH 2 SO 3 -. b Could be either Mg 2+ or MgCl + . c Only showed the initial "activation" cycles where capacity was increasing. d Could be either Al 3+ or AlCl 2+ or AlCl 2 + . e Abbreviations: EMIES = 1-ethyl-3-methylimidazolium ethyl sulfate; AN = acetonitrile; G1 = monoglyme; G2 = diglyme; G4 = tetraglyme; TFSI = bis(trifluoromethane)sulfonimide; DOL = dioxolane; THF = tetrahydrofuran; GBL = γ-lactone; EMIm = 1-ethyl-3-methylimidazolium; PC = propylene carbonate; DME = dimethoxyethane.

Figure 16 .

 16 Figure 16. Voltage profiles of selected OEMs studied measured vs Mg (A-E) and Al (F). (A) Carbyne polysulfide (reproduced from ref 373), (B) PHBQS (reproduced with permission from ref 371. Copyright 2016 Elsevier Ltd.), (C) DMBQ (reproduced with permission from ref 370. Copyright 2016 The Electrochemical Society), (D) P14AQ (reproduced with permission from ref 366. Copyright 2018 Elsevier Ltd.), (E) P(NDI2OD-T2) P14AQ (reproduced with permission from ref 366. Copyright 2018 Elsevier Ltd.), and (F) PQ-Δ (reproduced with permission from ref 380. Copyright 2016 Nature Publishing Group).

  Figure 17. (A) Typical voltage profile of a "superlithiated" material: Li 2 BDP (reproduced with permission from ref 260. Copyright 2016 American Chemical Society). (B-E) Capacity retention of selected OEMs: (B) PAT (reproduced with permission from from ref 387. Copyright 2018 American Chemical Society), (C) F-PDI-3-TC (reproduced with permission from ref 388. Copyright 2019 American Chemical Society), (D) BBL (reproduced with permission from ref 389. Copyright 2015 John Wiley and Sons), and (E) PQL (reproduced with permission from ref 390. Copyright 2015 John Wiley and Sons).

  2537expense of a more efficient SEI.402 Because, some articles 2538 commented in this review refer to these types of electrolytes, 2539 we feel that a few general comments should be recalled 2540 regarding this type of battery chemistry. First, such highly 2541 concentrated electrolytes have a volumetric density in the range 2542 of 1.6-1.9 g cm -3 , which partially offsets the gain in energy 2543 density. For instance the mass of a 700 cm 2 stack cell such as 2544 presented in ref 232 (taking 5 mAh cm -2 electrodes of 580 μm-2545 thick and 40% porosity) increases by 14% considering a WiSE 2546 electrolyte of 1.7 g cm -3 compared to a "salt in water" 1 M one. 2547 Second, the viscosity of these electrolytes is approximately ten 2548 times higher than molar range ones which considerably 2549 increases the wetting time of the electrodes and especially 2550 thick ones. 401 Last but not least, the price of these electrolytes 2551 is obviously the most challenging issue to be overcome before 2552 commercialization can be considered. Indeed, recent results 2553 point to the fact that expensive imide based anions (as 2554 opposed for instance to acetate ones) would have a significant 2555 role in the widening of the electrochemical window on the 2556 negative electrode side. 403 2557 Based on the previous comments, this section aims at providing 2558 the reader with a selection of relevant organic materials, that is to 2559 say those that allow performances approaching or higher than 2560 lead acid battery ones but with much extended cyclability. 2561 Accordingly, the lower limits were set to roughly 30 Wh kg -1 2562 per mass of the two materials and a capacity retention >80% 2563 after 500 cycles at 1 C rate or equivalent. The intention of the 2564 authors is to give a critical view of the selected papers and also 2565 to highlight advantages and drawbacks vs those associated 2566 with inorganic materials. Key performances are reported in 2567 (Table 7) along with materials structures, aqueous electrolyte 2568 formulations, cycling conditions and electrode loading. Note 2569 that this critical review is focused on neutral pH batteries 2570 because they inherently offer reduced production costs and 2571 mitigated corrosion issues. Results are subcategorized into 2572 hybrid cells, where only one of the electrodes contains an 2573 organic electroactive material and all organic cells.

8. 2 .

 2 Hybrid Organic-Inorganic Batteries 2574 8.2.1. Aqueous Lithium-Ion Batteries (ALIBs). Some of 2575 the most attractive organic materials for aqueous battery to date 2576 are among the polyimide derivatives. These materials indeed 2577 combine both a high capacity (typically 130-160 mAh g -1 , 2578 208-256 mAh cm -3 ) and a low price ($4-6/kg 395 ). To our 2579 knowledge such a dual advantage has not been reached by any 2580 inorganic materials used in the aqueous battery field. The 2581 electrochemical reduction/oxidation of a polyimide core is 2582 highly reversible in water at neutral pH. The redox centers 2583 have been identified as involving the aromatic-carbonyl system 2584 of the imide functional moiety following two redox steps. The 2585 first one corresponds to the formation of a radical-anion upon 2586 enolization of a carbonyl group by one electron, followed by a 2587 second electron reduction into the dianion quinoid form. 404 2588 Further reduction cannot be reached in molar range elec-2589 trolyte at neutral pH before water hydrolysis is triggered. The 2590 delocalization of excess electron density in the reduced states 2591 has been studied by FTIR and UV-vis spectroscopy. 405 Dong 2592 et al. 406 introduce a mixed liquid/solid cationic rocking chair 2593 aqueous battery using a I 3 -/I -based posolyte and a polyimide 8.2.2. Aqueous Sodium-Ion Batteries (ASIBs). As for 2636 Li-ion batteries, Na-based aqueous batteries must be 2637 investigated to counteract possible upcoming issues associated 2638 with geo-localized Li resources. One of the first instances of 2639 hybrid ASIBs bearing an organic electroactive material uses a polyimide derivative PNDI at the negative electrode and a NaVPO 4 F based positive electrode in a 5 M NaNO 3 aqueous

44 [

 44 Mn 1-x Ti x ]O 2 .410 We note however, that all these materials enable capacity values in the range of 40 to 60 mAh g -1 and energy density values in the range of 30-40 Wh kg -1 per mass of materials in molar range electrolyte which is the average value generally observed for most organic based aqueous cells.8.2.3. Aqueous Potassium-Ion Batteries (AKIBs). Full organic-inorganic hybrid AKIBs have not been reported until very recently owing to the scarcity of suitable electroactive materials and electrolytes. Hu's group 411 demonstrated an AKIB cell based on an Fe-substituted Mn-rich Prussian blueK x Fe y Mn 1 -y [Fe(CN) 6 ] w •zH 2O (KFeMnHCF) as the positive electrode and the 3,4,9,10-perylenetetracarboxylic diimide derivative as the negative one in a 22 M KCF 3 SO 3 water-insalt electrolyte (Table7, entry 3). The low water activity of the latter allowed not only to mitigate dissolution of both electrode materials but also to charge the positive electrode up to 1.2 V vs Ag/AgCl electrode, which allowed KFeMnHCF to reversibly reach 135 mAh g -1 at 0.5 C above 0 V vs SCE. In addition, thanks to the mitigation of phase transitions by Fe substitution, KFeMnHCF achieves 70% capacity retention at 100 C over 10 000 cycles. This pioneering AKIB system shows a high energy density of 80 Wh kg -1 by mass of the two electrodes at a power density of 41 W kg -1 (0.5 C) and 73% capacity retention over 2000 cycles at 4 C (Figure19A-C) which makes it one of the most attractive systems to date.Interestingly, authors have evaluated their system in pouch cell Table 7. continued a Based on anode material weight. b Based on cathode and anode materials weight. c Based on cathode material weight. d Based on the whole cell weight. Mg 2+ . Xia et al. 413 recently developed a 33 Wh kg -1 (1 V as output voltage) per mass of active materials considering Na 1.4 Ni 1.3 Fe(CN) 6.5 H 2 O paired with poly[N,N′-(ethane-1,2diyl)-1,4,5,8-naphthalene tetracarboximide (PNDIE) using 1 M MgSO 4 aqueous electrolyte (Table

Figure 18 .

 18 Figure 18. Characteristics of PPTO as an anode material for ALIBs. (A) Galvanostatic charge-discharge profiles for PPTO (280 mA g -1 ), LiTi 2 (PO 4 ) 3 (120 mA g -1 ), and LiMn 2 O 4 (140 mA g -1 ) in 2.5 M Li 2 SO 4 (pH 7). (B) Capacity retention of a LiMn 2 O 4 -PPTO cell during galvanostatic cycling at 1 C in 2.5 M Li 2 SO 4 (pH 7). (C) Schematic explaining the oxygen cycle in ALIBs: H 2 O is oxidized at the catalytic sites (*) on the cathode (for example, LiCoO 2 ) to generate O 2 and HC; the latter is then reduced by the charged anode (for example, PPTO-Li 4 ) to afford OH -. (D) Oxygen consumption by charged PPTO: a PPTO electrode is first electrochemically discharged (oxidized) and charged (reduced) under Ar for one cycle, then left to rest under O 2 , and finally put under Ar and charged again. Reproduced from ref 395. Copyright 2017 Nature Publishing Group.

Figure 19 .

 19 Figure 19. Performance of the K 1.85 Fe 0.33 Mn 0.67 [Fe(CN) 6 ] 0.98 •0.77H 2 O|22 M KCF 3 SO 3 |PTCDI full battery. (A) Charge-discharge curves for coin cells at 4 C from 0 to 2.6 V (1 C = 0.13 A g -1 ). (B) Comparison of average voltage, capacity based on total mass of both electrodes, lifespan, and energy density for the full battery with reported aqueous Na-ion full batteries. (C) Long-term cycling performance of the coin cell at 4 C. (D, E) Corresponding electrochemical performance measured in pouch cell at different rates (0.5/0.1 C) and temperatures (-20/-10/25/60 °C). Reproduced with permission from ref 411. Copyright 2019 Nature Publishing Group.

  atures and by including self-discharge tests. Regarding energy density and except for a few examples of hybrid cells, most results are in the range of 40-60 Wh kg -1 per mass of materials. However, as opposed to purely inorganic based aqueous batteries, the combination of attractive energy density, 80-90 Wh kg -1 per mass of materials, with promising cyclability (>3000 h) can be reached thanks to the large capacity of organics. In addition, owing to its large voltage and low price,

Figure 20 .

 20 Figure 20. Capacity retention on charge (oxidation of the material) and corresponding Coulombic efficiency curves for diblock-oligomers bearing p-type viologen and n-type naphthalene diimide moieties composite electrodes in (A) 2.5 M NaClO 4 aqueous electrolyte and (B) ocean water. Reproduced with permission from ref 231. Copyright 2019 John Wiley and Sons.

Figure 21 .

 21 Figure 21. (a) Redox-flow battery schematic with main relevant parameters referred to redox-active compounds. (b) Possible cell configurations.

  higher voltage systems. The main problem to address with redox molecule is stability. Since redox-active compounds are solubilized in the electrolyte, they are more subject to decomposition than solid electrodes. Thus, upon cycling, molecules can react together with solvent or electrolyte giving rise to poor performances. This especially at high concentrations (>0.5 M) which enhance decomposition kinetics.The main role of the solvent is to dissolve the redox molecule in order to make it transportable between the tanks and the conversion cell. It must also ensure the ionic conductivity necessary to achieve electroneutrality at all points of the solution and avoid polarization phenomena in the conversion cell. For this purpose, the best solvent is water; its high polarity combined with its ability to dissociate electrolytes results from the fact that aqueous solutions have very high ionic conductivities. Moreover, in terms of cost, water is the cheapest solvent and presents the fewest safety problems (nonflammable, nontoxic). However, aqueous electrolytes display low electrochemical stability window (1.23 V from thermodynamics up to 1.5 V for kinetics reason). Organic solvents have the advantage of having larger electrochemical stability windows (>4.5 V in the case of acetonitrile or carbonates), but electrolyte dissociation is less efficient, resulting in an ionic conductivity of about 100 times less than in aqueous solvent.Organic solvents are chemically unreactive, so they prevent degradation phenomena in solution and increase the lifetime of redox molecules. This also makes possible the stabilization of certain highly reactive redox molecules (e.g., radicals) and makes them relevant for redox-flow applications. The main solvents used are acetonitrile, carbonates, ethers, esters, and, more anecdotally, DMSO. Generally speaking, the solubility of organic molecules is not so different between aqueous and organic electrolytes, for two reasons: in organic media high concentrations of salts are used to increase the conductivity of solutions, with the consequence that the solubility of molecules decreases. An adapted functionalization renders the redox molecules highly soluble in water. The maximum concentrations achievable in a complete electrolyte (salt + solvent) are in the order of 2-3 M. As the conversion cells have not yet been really optimized for organic solvents, at the moment the solvent strongly conditions the type of batteries: high power density (0.1-0.3 W cm -2 ) in aqueous solution (low voltage, high conductivity) and high energy density (>100 Wh L -1 ) in organic solvent (high voltage, low conductivity). It is of course

  3146 develop an efficient flow battery. In organic media, neutral 3147 substituents are preferred, such as alkyl chains or PEGs. Alkyl 3148 chains are not necessarily the most efficient because of their 3149 low polarity, which is not optimal in polar solvents and which 3150 hinders the solubilization and dissociation of salts. PEG chains 3151 have the advantage of being more polar and their effect on 3152 solubility is more important. By choosing certain chain lengths, 3153 it is even possible to obtain liquid redox compounds. Finally, 3154 these PEG chains are capable of strongly complexing the alkaline 3155 cations contained in the supporting salt, thus improving its 3156 dissociation and increasing the conductivity of the solution. 3157 For the redox-flow system, the membrane is an important 3158 element; it allows the two compartments to be physically 3159 separated to avoid the mixing of species but must permit the 3160 passage of ions to ensure the electroneutrality of each of the 3161 compartments. In most cases, an ion exchange membrane is 3162 used, cationic in the case of a cationic rocking chair battery, 3163 anionic in the case of an anionic rocking chair battery, and one 3164 or the other in the case of a dual ion configuration. In aqueous 3165 environments, the most efficient membranes are mainly made 3166 of perfluoro sulfonated polymer. These membranes are very 3167 stable and have a high ionic conductivity. They have the dis-3168 advantage of not being as stable in organic media or they tend 3169 to swell and become porous. Alternative organic membranes 3170 have been developed, but unfortunately for the moment they 3171 are not as stable as the perfluoro one. To counter this, ceramic 3172 membranes have been used, particularly in the case of mixed 3173 devices using a lithium electrode. As the cost of such membranes 3174 is important, different strategies have been deployed to try to 3175 replace them with simpler separators. Size based separators 3176 (dialysis membranes) combined with redox molecules in the 3177 form of poly/oligomer to block diffusion from one compart-3178 ment to the other.

Figure 22 .

 22 Figure 22. (a) Performance of ORFB composed of AQDS and Br 2 showing both the cell voltage vs current density and the power density vs current density (reproduced with permission from ref 440. Copyright 2016 The Electrochemical Society). (b) Cycling capacity and efficiency of ORFB made of: MV and PEMPTMA (center); polymerized MV and TEMPO (left) (reproduced with permission from refs 441 and 442. Copyright 2015 John Wiley and Sons and Nature Publishing Group). (c) Cycling curve and aging behavior of the ORFB developed by Liu et coworkers. Concentration of redox molecule 1 M in NH 4 Br (reproduced with permission from ref 443. Copyright 2019 The Royal Society of Chemistry). (d) Capacity vs cycle number at different current densities for (A) FcNCl, (B) FcN 2 Br 2 at 0.5 M, and (C) cycling stability of FcNCl/ MV battery (0.5 M electrolyte) over 700 cycles (reproduced with permission from ref 444. Copyright 2017 American Chemical Society). (e) Capacity vs cycle number at different current densities for an HCNQ cell (0.5 M) in A. Corresponding voltage and power density vs current density in B (reproduced with permission from ref 432. Copyright 2018 American Chemical Society).

  The electrolytes were tested in static and flow cells. The flow cell is composed of a Li sheet, a ceramic separator (LAGP) and of the electrolyte circulating on a carbon paper. On the cycling curves (Figure23b), the electrolyte alone (0%) has a plateau at 3.55 V vs Li + /Li and a capacity of 7 Ah L -1 , which corresponds to 87% of the theoretical capacity. With 20% mixture, the cycling curves maintain the same behavior, but the capacity is increased to 27 Ah L -1 . The 50% mixture allows to reach a capacity of 55 Ah L -1 , which corresponds to an electron concentration of 2.23 M. The energy density of the 40% electrolyte is 190 Wh L -1 , one of the highest energy densities demonstrated to date. This is mainly due to the high cell voltage (3.55 V) even if the MPT/carbon Ketjenblack mixture significantly increases the capacity of the solution.

Figure 23 .

 23 Figure 23. (a) Cycling performance and rate capability reported by Wang and co-workers of the cell with 0.25 M 15D3GAQ as posolyte and lithium foil as anode (reproduced with permission from ref 447. Copyright 2012 The Royal Society of Chemistry). (b) Cycling performance of the Li/MPT flow cell reported by Lu and co-workers (reproduced with permission from ref 450. Copyright 2018 American Chemical Society). (c) Cycling performances of ORFBs based on MePh/DBMMB 0.3 M in DME-M LiTFSI (left) or BzNSN/DBMMB 0.5 M in ACN 1 M LiTFSI (right) reported by Wei and co-workers (reproduced with permission from refs 452 and 453. Copyright 2016 and 2017 American Chemical Society). (d) Symmetric flow cell characterization of MEEPT 0.5 M in ACN 0.5 M TEABF 4 reported by Milshtein and co-workers (reproduced with permission from ref 454. Copyright 2016 The Royal Society of Chemistry). Constant current cycling at 100 mA cm -2 (left), capacity vs cycle number at different current densities (right). (e) Molecule used by Sisto and co-workers to develop dialysis membrane-based ORFBs (left) together with the cycling performance (right), reproduced from ref 455.

  hydrodynamic radius, are very efficient to avoid 3521 mixing the two electrolytes. The authors also carried out studies 3522 on aging both in cycling and calendars (even if carried out in 3523 diluted solution). They were able to demonstrate that in 11 days 3524 in the charged state, no significant loss of capacity was noted. 3525 The cell could be cycled 450 times with an average capacity loss 3526 of 0.00614% per cycle. 3527 All organic redox-flow cell faces a lot of problems to develop 3528 a commercially viable system able to really compete with 3529 aqueous inorganic chemistries. This is mainly due to the fact 3530 that the molecular parameters are all interdependent. The fact 3531 of wanting high cell voltages implies the use of compounds 3532 where reduced (materials with very low potential) or oxidized 3533 (materials with very high potential) forms are poorly stabilized 3534 and therefore very reactive. From a solubility point of view, the 3535 use of highly concentrated solutions further increases decom-3536 position phenomena. This often results in poor cycling stability 3537 and is in addition to the fact that calendar aging is almost never 3538 achieved or under very diluted conditions that are more 3539 favorable. 456 Organic solutions that are highly concentrated 3540 generally become highly viscous, causing problems for flowing 3541 (pumps power consumption) and voltage efficiencies as these 3542 solutions become less conductive. The diffusion coefficients of 3543 redox species and ions decrease when viscosity increases with 3544 the result of lower apparent electron transfer kinetics. This 3545 results in high overvoltage values and low energy yields, often 3546 well below 60%. This probably explains why there are no flow 3547 cell studies at concentrations above 2 M in the literature, while 3548 several molecules have higher solubility values. The main 3549 interest of the studies in organic media is that due to the 3550 swelling of conventional ion exchange membranes and a high 3551 cross over, combined with low electrolyte conduction (high 3552 overvoltage), the community has been forced to develop 3553 membrane-free approaches, some of which have proven to be 3554 relevant.

  organic molecules to make electrolytes for flow 3556 batteries is now well described in the literature. Dozens of dif-3557 ferent molecules have been tested in various cell configurations. 3558 From a properties and behavior point of view, electrolytes based 3559 on organic molecules face the same problems as inorganic 3560 compounds: solubility, high viscosity of concentrated solutions 3561 with the result of a decrease in apparent electron transfer kinetics 3562 (overvoltage between charge and discharge), and a high power 3563 consumption by the pumps. Since the intrinsic stability of 3564 organic molecules in solution is lower than the stability of inorremains the key point to go further. 3568 Many parameters must be taken into account to understand 3569 the degradation of ORFB performance: molecules in solutions 3570 are much more reactive than in the solid state, in addition they 3571 are in contact with different materials (plastic, stainless steel, 3572 and so on) which can act as catalysts. Most of these com-3573 pounds are sensitive at least in one redox state to oxygen 3574 traces, which is difficult to prevent in the long term (leakage).nowadays mandatory in the general interest. In this context, global demand for batteries multiplies to store renewable energy, promote electromobility, and power the continued development of portable electronics and emerging technologies (mobile devices, IoT, AI, robotics, etc.). Seeking to develop innovative, efficient, and less polluting and energy consuming chemistries is therefore important today while jointly developing better recycling solutions (and second use).Depicted as a parallel and relatively recent research field in the history of rechargeable electrochemical storage systems, we have sequentially summarized the present state-of-the-art and highlighted important works that have contributed to the recent progress in organic-based rechargeable systems by covering all possible technologies and cell configurations to date. The as-obtained performance figures have been described without avoiding limitations. Some of the key performance metrics of organic electrode compounds such as specific energy, working potential, and cycling stability already look promising, and material design strategies specific to these metrics have been preliminarily established. Challenges remain for developing efficient storage solutions that simultaneously excel in all these metrics. It seems probable that the practical use of organics for the electrochemical storage in a near future could be through the development of ORFBs due to their attractive features of high power density and low cost for large-scale energy storage.

Table 2

 2

	plateaus at 2.9 and 2.5 V vs Li + /Li with equal capacities,
	averaging to a discharge potential of 2.7 V vs Li + /Li (Figure 10D, Table 2, entry 12). 246 OEMs resembling the p-BQ structure show
	similar potentials as p-BQ. BBQ, a dimer of p-BQ, and 1,4,5,8-
	phenanthrenediquinone (PADQ), a three-ring molecule incorpo-
	rating two p-BQ units show discharge potentials at 2.9 and 2.77 V
	vs Li

, entry 9) show competitive discharge potentials of ∼3.

6

V vs Li + /Li.

242

Many other aromatic amines have been studied as OEMs as well including small molecules

243,244

and organic salt forms such as dilithium 2,5-(dianilino)terephthalate (Li 2 DAnT), which are quite less soluble than non-salt molecules as further explained below (Table

2

, entry 10).

245

Compared with p-type OEMs, the n-type counterparts have attracted greater research interest because of giving access to the reversible storage of Li + and potentially to cationic "rocking-chair"-type batteries when n-type electrode materials are properly designed (Figure

9

). p-Benzoquinone (p-BQ) is one of the most studied n-type building blocks for OEMs (Table

1

). The basic p-BQ molecule shows two discharge + /Li, respectively

(Table 2, entry 18, 19)

.

247,248

The slightly higher potential than that of p-BQ may be related to the limited specific capacity (i.e., relatively small depth of discharge) rather than performance improvement by design, however. Polymers that incorporate the p-BQ structure without much modification to the building block preserve the discharge potential.

PBQS and poly(2,5-dihydro-p-benzoquinonyl sulfide) (PDBS)

Table 1 .

 1 A Few Electrochemical Storage Mechanisms in Redox-Active Organics Together with Examples a

Table 2

 2

	, entry 33). TCNQ itself is known

to four (F2-TCNQ), the average discharge potential increased from 3.1 to 3.15 V vs Li + /Li (Figure

10F

,

Table 2, entry 34, 35)

.

255

The ortho-regioisomer of p-BQ gives rise to higher discharge potentials in general. The simplest molecule, ortho-benzoquinone (o-BQ), has not been directly studied probably due to chemical stability issues, but o-BQ as a building block for OEMs is well documented. The pyridine rings-fused 1,8-diaza-Table

2

. continued a Specific energy considers the weight of both the cathode and anode materials. For p-type OEMs, LiBF 4 is considered as the Li salt due to its relatively light weight.

  Dilithium rhodizonate (Li 2 C 6 O 6 ) undergoes reversible four-electron reaction per C 6 O 6 ring (Table 2, entry 46).

	,
	entry 45).
	Carbonyl group undergoes reversible one-electron reduction
	to form a radical anion. When multiple carbonyls are con-
	jugatedly connected as in quinones, the uncoupled electrons
	generated during reduction could combine intramolecularly to
	form multivalent anions. The theoretical capacity of carbonyl-
	based electrodes is typically lower than that of organosulfur
	polymers. The observed initial discharge capacity of 580 mAh g -1 set the
	record capacity among carbonyl-based OEMs since its
	discovery in 2008 (Figure

Table 2 ,

 2 1719 entry 33). The discharge capacity of TCNQ is lowered to 1720 260 mAh g -1 albeit voltage is increased to 3.2 V. 254

	1721

Table 2 ,

 2 entry 48).277 When hybridized with graphene, 3Q When the discharge potential cutoff is set at 1.5 V vs 1732 Li + /Li, 6CN shows a specific capacity of 300 mAh g -1 .

	1727

1742

deliver stable cycling performance. Even small-molecule OEMs

Table 2 ,

 2 entry 50 and poly(ethylene-pyromellitic diimide) or

	1757

Table 3 .

 3 Performances of Selected Nonaqueous Na-Organic Batteries

	Chemical Reviews

pubs.acs.org/CR Review https://dx.doi.org/10.1021/acs.chemrev.9b00482 Chem. Rev. XXXX, XXX, XXX-XXX AA

Table 3 . continued

 3

	Chemical Reviews

pubs.acs.org/CR Review https://dx.doi.org/10.1021/acs.chemrev.9b00482 Chem. Rev. XXXX, XXX, XXX-XXX

Table 4 .

 4 Performances of Selected Nonaqueous K-Organic Batteries

	a

Table 6 .

 6

Performances of Selected OEMs Exhibiting the "Superlithiation" Phenomenon a Due to an activation process, the capacity increases with time.

Chemical Reviews pubs.acs.org/CR Review https://dx.doi.org/10.1021/acs.chemrev.9b00482 Chem. Rev. XXXX, XXX, XXX-XXX

  (during charging), respectively. An energy efficiency of 82% is measured; the cycling stability evaluated over 40 cycles shows a loss of about 0.8% in capacity per cycle. The measured energy density is about 25 Wh L -1 , with the low concentration used to make the electrolyte (0.5 M in electron) being compensated by the output cell voltage(2.3 V).

	Xu, Wang, and co-workers found that TEMPO was highly soluble in carbonate solvents. 448 Thus in an EC/PC/EMC
	mixture (4:1:5 w/w) 2.3 M LiPF 6 , TEMPO is soluble up to
	2 M. However, it should be noted that the solutions obtained
	are very viscous and difficult to circulate (67.1 cP at 2 M,
	4.02 cP at 0.1 M). The flow cell consists of a Li/graphite mixed
	negative electrode, a porous physical separator, and a carbon
	felt through which the electrolyte circulates. The energy density was measured with the cell at 2 M TEMPO and 126 Wh L -1 ,
	with an energy efficiency of 70%. Stability in concentrated
	condition does not seem very high, in dilute solution (0.1 M),
	capacity losses are about 0.2% per cycle. As there is no ceramic membrane, the cycling currents are higher, up to 5 mA cm -2 . Finally, Lu et al. 450 reported the use of 10-methylpheno-
	thiazine (MPT) melted, then mixed with carbon black
	(Ketjenblack EC-600JD), and finally ground when solidified.
	The electrolyte consists of a saturated solution of MPT (0.3 M)
	in an EC/DEC 1 M LiPF 6 mixture to which the ground solid
	is added. The MPT in solution acts as a redox shuttle to

electrochemically address the solid MPT. It should be noted that without carbon, solid MPT is not electroactive. Mixtures of 0%, 20%, and 40% (in vol.) solid and electrolyte were tested.

  3 M in 1 M LiTFSI DME electrolyte. Although tested only over about 50 cycles, the cell capacity is constant at about 3 Ah L -1 for an energy density of 9.3 Wh L -1 . This cell is able to work at current densities close to those used in aqueous ones: 50 mA•cm -2 with no significant degradation of its performance. At this current level, the energy efficiency remains above 60%. It is interesting to note that at low current the Coulombic efficiency is not very good due to the cross diffusion of species through the porous membrane while at high current density the voltage efficiency drops due to the ohmic drop. The redox potential of BzNSN is -1.58 V vs Ag + /Ag in 1 M LiTFSI ACN electrolyte, with its solubility being 2.1 M when considering 2.1 M LiTFSI ACN as the electrolyte.

https://dx.doi.org/10.1021/acs.chemrev.9b00482Chem. Rev. XXXX, XXX, XXX-XXX

https://dx.doi.org/10.1021/acs.chemrev.9b00482Chem. Rev. XXXX, XXX, XXX-XXX X

Poizot, P.; Tarascon, J.-M. Conjugated dicarboxylate anodes for Li-ion

ACKNOWLEDGMENTS

The authors express their sincere gratitude to economist colleagues E. Hache (IFP E ́nergies Nouvelles, Rueil-Malmaison, France) and P. Menanteau (GAEL, Grenoble, France) for helpful comments and discussions on this manuscript. The authors are also grateful to J. Zhang and X. Wang (UH, Texas, USA) for the help on Table 2 as well as to F. Dolhem (LG2A, Amiens, France) for insightful comments. The authors are thankful to P. Berrezig and C. Doutriaux for helpful assistance in editing the manuscript and the graphical abstract, respectively.