Shashwat Singh

Shubham Lochab

Lalit Sharma

Valérie Pralong

Dr Prabeer Barpanda
email: prabeer@iisc.ac.in

An overview of hydroxy-based polyanionic cathode insertion materials for metal-ion batteries

Keywords: batteries, cathodes, polyanions, hydroxy-based insertion materials, capacity

Rechargeable batteries based on Li-ion and post Li-ion chemistry have come a long way since their inception in early 1980s. The last four decades have witnessed steady development and discovery of myriads of cathode materials taking into account their processing, economy, performance along with ecological sustainability. Though oxides rule the battery sector with their high energy and power density, polyanionic insertion compounds work as gold mines for designing insertion compounds with rich structural diversity leading to tuneable redox potential coupled with high structural/ chemical/ thermal stability. The scope of polyanionic compounds can be taken a step further by combining two or different types of polyanions to get suites of mixed polyanionic materials. While most cathodes are built with metal polyhedra constituted by oxygen (MO m | XO m , M = 3d metals, X = P, S, Si, B, W etc., m = 3-6), in some cases, selected oxygen sites can form bonding with hydrogen to form Page 1 of 48 Physical Chemistry Chemical Physics OH/H 2 O ligands. It can lead to family of hydroxy-based mixed-polyanionic cathode materials. The presence of hydroxy components can affect the crystal structure, local chemical bonding, electronic, magnetic, diffusivity and electrochemical properties. Employing mineralogical survey, the current review renders a sneak peek on various hydroxy-based polyanionic cathode materials for Li-ion and post Li-ion batteries. Their crystal structure, and electrochemical properties have been overviewed to outline future research focus and scope for real-life application.

Introduction

The overdependence of global energy needs on fossil fuels and other non-renewable resources has triggered momentus effort to look for alternate choices. There is a growing demand for clean energy with the ever-increasing population and terawatt-scale energy consumption. 1 Renewable energy resources like solar, wind, geothermal, biothermal etc. are some of the viable alternatives in this regard. Storing energy obtained from renewable energy resources via electrochemical energy storage devices (EES) is an attractive and necessary prospect. Rechargeable metal-ion batteries, based on reversible intercalation of alkali ions into metal-anionic frameworks, are widely used EES for portable electronics. The intercalation principle involving an electronic transfer is a cleaner way of storing electrical energy for various applications. It was first put into practical applications with commercialisation of Liion batteries (circa 1991), which led to increased interest and steady development in EES devices for usages ranging from small-scale consumer electronics to large-scale automobiles/ grid storage. At this juncture, the ever-growing consumption of Li-based minerals has raised concerns about the availability of Li resources. [2][START_REF] Sonoc | Proc. CIRP[END_REF][START_REF] Olivetti | [END_REF] Moving beyond Li-ion, various other monovalent/ multivalent alkali chemistry (like Na + , K + , Mg +2 , Ca +2 , Al +3 etc.) have also been tested for EES devices based on similar intercalation mechanisms. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] On the basis of the constituent alkali ions, different types of electrochemical behaviour are observed. This is on the account of the different sizes of the alkali metal ions that leads to different properties of materials and stabilizing effects.

Among various intercalation-based cathode materials, two-dimensional (2D) layered oxides have conquered the commercial Li-ion battery sector. Different transition metal oxides like LiCoO 2 , NaMO 2 (M=Transition metal) etc. with stable structure have been explored as robust cathode materials. 11,[23][24][25] Oxides deliver high capacity with good reversibility, although with lower redox potential. To realize high redox potential, three-dimensional (3D) polyanionic materials can be put on anvil to develop structurally diverse cathode systems. Polyanionic materials have strong covalent bonding between the metal-ion redox centre and oxygen atoms, which provide a stable framework and durability for intercalation process. They offer wide range of three-dimensional structures possible with different geometries and linkages. The oxygen atoms present around the metal centres are held tightly in these 3D frameworks. This reduces the chances for release of structural metal/oxygen atoms while charging is done up to higher potentials, thereby increasing the overall safety. Based on inductive effect principle, 26 it is possible to tune the redox potential by choosing polyanionic groups with different electronegativity values. The iono-covalency of the metal-anion bond changes following the inductive effect of the polyanionic groups like phosphates (PO 4 -3 ), pyrophosphates (P 2 O 7 -4 ), fluorophosphates (PO 4 F -4 ), sulfates (SO 4 -2 ), silicates (SiO 4 -4 ), borates (BO 3 -3 ) etc. [27][28][29][30][31][32][33][34][35][36][37][38] Further, mixed polyanionic electrode materials involving different combination of polyanionic subunits [(XO 4 ) m -n , X = P, S, B, C, S, Si etc.] and/or single anion atoms (F -, Cl -, OH -, O -2 etc.) can be explored. 39 Such combinations can lead to variety of cathode materials having exciting new properties with multi electron redox system with superior electrochemical performance. [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] Among these combinations, triplite and tavorite LiFeSO 4 F with combination of sulfate group and fluoride ion exhibit high voltages of 3.9 V and 3.6 V, respectively. [56][57][58][59] Introduction of fluoride group increases the overall voltage due to inductive effect along with the stability from the polyanionic group. Although nowadays both fluorine-based electrolytes and binders are available, still the addition of fluorine makes the materials more hazardous and involves safety concerns too. Fluorine is hygroscopic in nature and could possibly form hydrogen fluoride (HF), which is hard to handle in large scale applications. Considering more sustainable alternatives, hydroxide OH -ion can replace the fluoride group to form hydroxy-based cathode materials. Replacing with OH -on one hand does slightly lower the redox potential owing to reduced inductive effect, but it can provide higher chemical and thermal stability to materials.

Presence of OH -group leads to additional structural durability along with enhanced resistance to external moisture attack. Keeping these advantages in mind, the current review provides an in-depth overview of various hydroxy (OH -) based battery electrode materials highlighting their uniqueness and versatility for usage in various electrochemical systems including (non)aqueous batteries. Their (low temperature) synthesis, structural features, electronic structure and electrochemical performance have been discussed along with possible future direction to realize hydroxy-based cathode materials. Three major class of electrodes, namely (i) hydroxyphosphates, (ii) hydroxysulfates and (iii) (non) jarosites, have been sketched below.

Hydroxyphosphates

Hydroxyphosphates based materials, with the general formulae LiMPO 4 .OH (M=V, Mn, Fe) combining both phosphate (PO 4 3-) polyanion and fluoride (F -) ions, form a niche class of battery cathode materials. Due to the structural versatility in polyanionic materials, they can adopt either two-dimensional (2D) or three-dimensional (3D) network structures depending on the constituent alkali and transition metals. The electronegativity of anion can tune the overall redox potential of transition metal as per the inductive effect. Here, hydroxyphosphate chemistry for Li-ion batteries has been explored (Table 1). Although there is no report, these hydroxyphosphates can form a potential family to design post Li-ion (Na/K-ion) batteries.

LiFePO 4 OH

Simonov et al. and Baur initially investigated the structure of tavorite mineral based

LiFePO 4 OH materials. 60,61 LiFePO 4 OH (LFPOH) was found to crystallize in triclinic structure (s.g. P-1), consisting of FeO 6 octahedra and PO 4 tetrahedra building blocks [Fig. 1(a)].

However, due to the small size of Li and H, their exact positions in the structure could not be identified. Marx et al. reported the precise position of Li-and H-atoms with the help of neutron diffraction study coupled with theoretical calculations [Fig. 1(b)]. 62 From Fourier difference maps, they claimed the location of Li atoms in the tunnel along c-axis, while H-atoms were found linked with oxygen atom shared by two adjacent FeO 6 octahedra units. From GGA+U method, two sites of lithium were predicted. The first Li site (Li1) that is in close proximity to H-atom was found to be relatively unstable due to electrostatic repulsive forces between them, whereas the second Li site (Li2) was found to be more stable.

Whittingham group first prepared LFPOH by hydrothermal synthesis in 2001, albeit with no electrochemical activity between 1.9 to 4.1 V. 63 Almost a decade later (in 2010), Marx et al. reported its electrochemical activity for the first time. The ball-milled LFPOH delivered a first discharge capacity of 105 mAh g -1 at C/50 rate when cycled between 2.0-4.5 V [Fig. 1(c)].

In the first cycle, an irreversible phase transition occurs at 2.35 V (vs. Li.Li + ), which is followed by a solid-solution based insertion mechanism in subsequent cycles. Later, Asl et al. reported a unique synthesis employing phosphorous acid as a source of phosphate leading to good quality single crystals. 64 During electrochemical analysis, a discharge plateau was observed at 2.37 V, with an increased additional voltage of 130 mV. 65 It was attributed to the formation of amorphous Li 2 FePO 4 OH phase upon first discharge and the upshift in the discharge plateau arose due to subsequent (dis)charge occurring from the amorphous phase. Moreover, due to electrode polarization, huge capacity fading was observed at higher C-rates partly due to its poor electronic and ionic conductivity. Recently, Sharma et al. revisited the electrochemistry of LFPOH by forming micrometric particles with sheet/rod like morphology. 66 In the voltage range of 2.2-4.2 V, LFPOH delivered a stable discharge capacity of 140 mAh g -1 (Q Th = 153 mAh g -1 ) at a current density of 1.0 mA cm -2 with an average cell potential of 2.6-2.7 V vs.

Li + /Li [Fig. 1(d)]. This voltage is slightly higher than the previous reported values, which can be attributed to the particle size reduction by ball-milling. Such increase in cell potential due to particle size reduction has also been reported for LiFePO 4 system. The cell showed excellent electrochemical properties with no significant capacity fading up to 60 cycles. Good rate kinetics was observed indicating improved ionic and electronic conductivity in the material due to particle downsizing and carbon coating. Reduced order model (ROM) computation was employed by varying several electrode parameters to gauge the suitability of LFPOH for grid storage. 67 Upon Li + /H + ion exchange in LFPOH, a new hydrated form of FePO 4 .nH 2 O with n=1 was formed. 68 Although both LFPOH and FePO 4 .H 2 O had similar structure, slight distortion in the polyhedra and the chains changed the symmetry so as to crystallize FePO 4 .H 2 O in a monoclinic (s.g. C2/c) structure. The magnetic properties of LFPOH were also studied where a Curie-Weiss type behavior was observed. Yang et al. first reported its lithium (de)intercalation in 2012. 71 LMPOH was synthesized by H + /Li + exchange reaction using hydrothermal method. When cycled between 2.0 to 4.6 V at C/100 rate, the lithium half-cell led to an initial discharge capacity of ~110 mAh g -1 with an average cell voltage of 3.4 V [Fig. 1(e)]. The first and subsequent (dis)charge curves exhibited different profiles with poor rate capability. At 1C current rate, only 50% of the initial capacity was obtained while no significant retention was observed even at C/100 rate. This poor rate performance can be attributed to the low electronic conductivity of the material, hence warrants suitable carbon coating. However, from ex-situ XRD measurements, no amorphization was observed upon (dis)charge. Irreversible phase transformation was observed after the first charge with the retention of overall structure. Kanno group further extended the work on hydroxyphosphates by reporting solid-solution series of LiMn 1-x Fe x PO 4 OH over the range 0 ≤ x ≤ 0.3 with triclinic structure [Fig. 1(f)]. 72 Materials with higher Fe content displayed better electrochemical properties in the voltage range of 2.0 to 4.6 V. LiMn 0.7 Fe 0.3 PO 4 OH delivered a discharge capacity of 150 mAh g -1 with low polarization. This improved performance was attributed to change in the local geometry due to Fe substitution in the structure. The synergic effect in the distortion due to Jahn-Teller effect of Mn 3+ decreased due to Fe 3+ substitution. A Curie-Weiss type magnetic behavior was noticed for LMPOH. 73

LiVPO 4 OH

Another interesting hydroxyphosphate material is LiVPO 4 OH (LVPOH). Unlike Fe and Mn-based materials with limited energy density, LVPOH can deliver high energy density powered by its high redox potential and occurrence of multiple oxidation states of vanadium.

Croguennec group reported a detailed study on LVPOH prepared by hydrothermal route. 74 Presence of hydroxyl group and small amount of vanadyl impurity was confirmed by infrared spectroscopy. Nuclear magnetic resonance spectroscopy highlighted the presence of hydroxyl group as the bridging anion between two octahedra [Fig. 2(a)]. The lithium diffusion pathways were studied by means of bond valence sum measurements, revealing one-dimensional Li + migration along c-axis [Fig. 2(b)]. An average cell potential of 3.95 V vs. Li/Li + was observed when cycled between 3.0-4.6 V [Fig. 2(c)]. A charge capacity of 280 mAh g -1 was observed which is double of the theoretical capacity, suggesting the extraction of both Li and H. However, during discharge a large irreversible capacity was observed indicating only one Li insertion into the system. Ex-situ analyses revealed the presence of oxyphosphate VPO 4 O phase at the end of charge and LiVPO 4 O phase at the end of discharge. A concerted mechanism was suggested as both Li and H are being extracted at same potential. When the material was cycled in the voltage window of 1.2 V to 3.0 V, a discharge capacity of 136 mAh g -1 was observed at a potential of 1.4 V vs. Li/Li + corresponding to one Li-insertion into LVPOH [Fig.

2(d)]. It again proves the advantage of employing vanadium chemistry as multiple electron

transfer can occur exploiting multiple oxidation states of vanadium.

Hydroxyphosphates in aqueous batteries

Hydroxyphosphates can also exhibit good electrochemical performance in aqueous electrolytes. Recently, Sharma et al. reported the (de)intercalation properties of LFPOH as an anode in aqueous electrolytes. 66 This was the first report of an Fe-based tavorite working as an anode for aqueous LIBs. As these water-based electrolytes are limited by the narrow working voltage window to prevent water splitting on either side, a water-in-bisalt configuration (21 m LiTFSi+7 m LiOTf) was chosen as electrolyte. The use of water-in-bisalt electrolytes enhances the operating voltage window and prevents the formation of SEI during charging. The electrochemistry was performed in a three-electrode setup with Ag/AgCl as reference electrode and Zn sheet as counter electrode. When cycled in the potential window of -1.0 to 1.0 V vs.

Ag/AgCl, a discharge capacity of 153 mAh g -1 was observed at a current density of 1.0 mA cm -2 [Fig. 2(e)], retaining ~81% of the initial discharge capacity at the end of 50 cycles. It exhibited a cell voltage of -0.3 to -0.4 V vs. Ag/AgCl, which lies in the anodic range of aqueous batteries. An all-phosphate full cell was assembled using LFPOH as anode and LiFePO 4 as cathode. A first discharge capacity of 120 mAh g -1 was observed at a cell voltage of 0.8 V with an energy density of 97 Wh kg -1 (w.r.t. anode weight) [Fig. 2(f)]. This work showed the possibility to implement hydroxyphosphates in aqueous batteries, which can be extended to other hydroxyphosphate chemistry in aqueous batteries.

Some other hydroxyphosphates

Apart from the traditional hydroxyphosphates discussed above, various materials with different stoichiometry of phosphate and hydroxide polyanion have been reported. They are basically found in various mineral forms across the globe. Although unexplored at this stage, there is a possibility of find electrochemical activity in some of these materials with potential application in rechargeable batteries. Other than the crystal structure, spectroscopic and magnetic properties of these minerals, there is a possibility to study the electrochemical properties of these minerals. A list of such unexplored hydroxyphosphate minerals is provided in Table 2 along with their chemical formulae.

Hydroxysulfates

Hydroxysulfates with the general formulae LiMSO 4 .OH (M= Mn, Fe, Co) provide another alternative to fluorosulfates and hydroxyphosphates (Table 3). The presence of highly electronegative sulfate polyanion instead of phosphate one provides more more ionicity (hence higher redox potential) to constituent transition metal species as per inductive effect. As the sulfate species are water soluble and prone to thermal decomposition, it restricts the possible synthesis routes. In hydroxysulfates, less electronegative OH -is present instead of F -species.

Consequently, it renders less ionic character of constituent M-X bonds, thereby reducing the metal redox potential (M n+1/n+ ) by ~0.3 V compared to fluorosulfates. On the other hand, the presence of OH -species offers enhanced moisture resistance and chemical stability. Reddy et al reported the first hydroxysulfate showcasing monoclinic FeSO 4 .OH compound (s.g. C2/c) as a Li + (de)intercalation host. 75 The target compound was prepared by annealing commercial FeSO 4 .7H 2 O precursor at 280 ⁰C for a week. Its crystal structure is composed of FeO 4 (OH) 2 octahedra (corner shared), with four equatorial oxygen bridged together by four sulfate (SO 4 - 2 ) tetrahedra [Fig. 3(a)]. These corner-shared octahedra form parallel chains along the a-axis with spaces for Li + insertion. It led to a reversible capacity ~150 mAh.g -1 involving an Fe 3+ /Fe 2+ redox activity centered at 3.2 V [Fig. 3(b)]. The electrochemical Li + insertion involved a biphasic mechanism with the formation of tavorite Li x FeSO 4 OH (x approaching 1) endproduct with triclinic (P-1) structure.

Inspired by the structural similarities present between FeSO 4 OH and FeSO 4 F, Ati et al.

prepared the solid-solution family of FeSO 4 F 1-y OH y (0 < y < 1) using Fe 2 (SO 4 ) 3 .nH 2 O and FeF 3 precursors. 76 The reaction was carried out at 300 ⁰C for 50 h and the structural water content was adjusted by varying the annealing time from 30 minutes to several hours. Overall, monoclinic frameworks (s.g. C2/c) were obtained consisting of FeO 4 (F,OH) 2 octahedra abridged together via either OH or F species. These chains were in turn connected by SO 4 2- polyanionic groups to form a 3D framework [Fig. 4(a,b)]. This one step synthesis led to the FeSO 4 F 1-y OH y phases with electrochemical performance ranging from 3.6 V to 3.2 V with decrease in the F/OH ratio. The lithium (de)insertion mechanism was also shown to be dependent on the F/OH ratio [Fig. 4(c)]. In-situ XRD and Mössbauer measurements revealed the formation of an intermediate Li 0.5 Fe 0.5 +3 Fe 0.5 +2 SO 4 OH during the (dis)charging step.

Possibility of using similar method for synthesizing metal-substituted Fe 1-x M x SO 4 F 1-y OH y (M = Ti, V) was also demonstrated.

Following, mesoporous LiFeSO 4 F 0.44 (OH) 0.56 was reported prepared by a two-step route. 77 This synthesis involved manual mixing of the precursors with the resultant paste kept at 45 ºC for 60 h. Finally, heating was done at 120 ºC to get the end product. Mesoporous morphology helped in longer durability and superior cyclability than previously reported fluorosulfate like cathodes. During galvanostatic cycling, a clear intermediate phase Li 0.5 Fe 0.5 +3 Fe 0.5 +2 SO 4 F 0.56 (OH) 0.44 was observed as reported by Ati et al (mentioned above), corresponding to the triclinic LiFeSO 4 F 0.56 (OH) 0.44 to monoclinic FeSO 4 F 0.56 (OH) 0.44 transition. 80 % capacity retention was observed upto 2800 cycles without any carbon coating or particle downsizing. Power density was found to be 5.9 kW kg -1 at 15C and 20.4 kW kg -1 at 30C. These values reported are one magnitude higher than commonly reported values for Liion batteries. Optimization of various synthesis processes can lead to improved electrochemical properties for the hydroxy-fluorosulfates LiFeSO 4 F y (OH) 1-y .

As it is difficult to prepare the 3. Layered LiFeSO 4 OH hydroxysulfate exhibits an operating voltage of 3.6 V for Li ion intercalation unlike its monoclinic analogue having 3.2 V activity [Fig. 3(e)]. This potential difference between two polymorphs is explained via the configuration of the polyhedra present in the crystal system. In case of layered LiFeSO 4 OH, FeO 6 octahedra building blocks are edge shared that leads to repulsion between the neighbouring Fe +3 ions in the charged state enhancing the overall Gibbs free energy of the system [Fig. 3(d eV than the tavorite one (≈0.7 eV), making it more suitable choice for cathode material. 79 The ion migration pathways for both polymorphs are illustrated in Fig. 3(c,f). Long range diffusion is possible in the case of layered polymorph owing to its lower activation barrier. Notably, while doing the theoretical studies, van der Waals interaction play a major role in finding accurate values for the data, i.e. crystal structure, cell voltage, interlayer spacing etc. Also, the hydrogen atom present was found to be immobile albeit with stationary vibrations. Defect properties of these hydroxysulfate cathodes were analyzed by computational tools. Exchange of Li + ion with Fe +2 ion (i.e. anti-site defects) was noticed in both LiFeSO 4 OH polymorphs.

However, the degree of anti-site defects was very low (lesser than LiFePO 4 ). Among all 3d-metal based hydroxysulfates, the Fe-analogue (LiFeSO 4 OH) was found to show efficient reversible Li + (de)intercalation with the insertion of 0.8 Li + ions per formula unit (pfu). On the other hand, LiCoSO 4 OH showed high-voltage redox activity at 4.8 V albeit with poor Li + insertion (i.e. 0.2 Li + ions pfu). LiMnSO 4 OH was found to be completely inactive [Fig. 5(d)]. 78 For layered LiFeSO 4 OH, in-situ XRD studies revealed a gradual and reversible shifts in peak positions implying a solid-solution (monophasic) redox mechanism [Fig. 5 Badrudin et al studied the effect of lithium (de)intercalation in layered (Li)FeSO 4 OH using first principle calculations using stable antiferromagnetic configuration. 80 Present Li + ions were found to be strongly ionic in nature, thereby limiting probability of complete delithiation of the material. Change in the Fe 3d orbital contribution in conduction and valence bands was observed according to the movement of Li. That in turn led to the conclusion that changing the transition element could pave a way for improving the battery performance. Subsequently, Badrudin et al utilised this concept to yield better electrochemical performance with vanadium (V) substitution in LiFeSO 4 OH. 81 LiFe 0.75 V 0.25 SO 4 OH was formed theoretically signifying a decrease in the overall band gap of the material with increase in the electronic conductivity. V substituion led to (~4%) expansion of unit cell volume that in turn ease the migration of lithium ions. Overall, vanadium substitution leads to increase in ionic as well as electronic conductivity. Enhanced battery stability can be obtained by the vanadium substitution while (dis)charging the battery.

Radha et al investigated the thermodynamic stability of the LiMSO 4 OH (M=Mn, Fe, Co) hydroxysulfate family using isothermal acid calorimetry. 82 A decrease in thermal stability was observed with increase in the ionic radius (i.e. from Co-to Mn-analogue) [Fig. 5(c)]. The tavorite LiFeSO 4 OH polymorph was found to be less stable than the layered one. It is due to the presence of corner sharing octahedra in tavorite case and edge sharing octahedra in the layered one. Distortions along with the sulfate group control the thermodynamic and electrochemical behaviour. As the symmetry changes from C 3v to C 2v , decrease in the electrochemical performance and redox potential was observed. Redox potential decreases with decrease in exothermic enthalpy of formation [Fig. 5(e)]. As we go from Mn to Co, the exothermic enthalpy increases and thus redox potential also increases. This can be correlated to the smaller size of Co than other analogues and thus stronger bonds. This study also provides an answer for the inactivity of Mn analogue (C 2v ), as its enthalpy of formation is really low.

Using DFT analysis of electronic structure along with optical properties of the layered LiMSO 4 OH family, 83 LiFeSO 4 OH was found to be a direct band gap semiconductor. After the analysis of the (non)polar covalent bonds and frequency dependent optical properties, LiFeSO 4 OH was found to have photoelectric properties with potential application in optoelectronic devices. In addition, thermoelectric property was noticed in heavily doped LiFeSO 4 OH at varying temperatures (300 to 750 K) with a large Seebeck coefficient value of around 400 μV/K.

The presence of OH-species render moisture resistance to hydroxysulfate vis-à-vis fluorosulfate. Zhang et al studied the effect of relative humidity on the structure and electrochemical properties of LiFeSO 4 F material. 84 Surprisingly, there was a partial degree of in-situ formation of LiFeSO 4 OH, when LiFeSO 4 F was cycled in the electrochemical cell due to the presence of humidity. Degradation of the tavorite-type LiFeSO 4 F to FeSO 4 .nH 2 O and LiF was observed with relative humidity of 62%, which then partially converts to LiFeSO 4 OH within the cell as evident from deviation in voltage profiles. This in-situ formation of LiFeSO 4 OH offers another approach to form metastable phases worth further exploration.

Jarosites

Moving on from lithium-based hydroxysulfates, we examined jarosite family of compounds having sodium and potassium based hydroxysulfate members. Jarosites belongs to the alunite super group of minerals, where original alunite is a hydroxylated aluminium based minerals with formula KAl 3 (SO 4 ) 2 (OH) 6 . While alunites can be composed of various moieties (PO 4 , AsO 4 etc), jarosite sub-group of minerals solely contain SO 4 -based moieties (Table 4).

These compounds have similar polyanions (i.e. SO 4 2-and OH -), but in different stoichiometry, thereby offering diversity in structural and electrochemical properties. Jarosite mineral, generally a hydrous sulfate of potassium and ferric (Fe 3+ ) iron [KFe 3 (SO 4 ) 2 (OH) 6 ], was discovered in 1852 by August Breithaupt. The mineral is named after the place of discovery, Barraco del Jaroso, Almeria, Spain. 85 The name is fitting as the mineral crystal has yellowish color (Jara, Spanish name of a yellow flower). As evidence of water on Mars, 86 Jarosite has been detected by three martian rovers: Curiosity, Spirit and Opportunity. [87][88][89][90] While jarosite is naturally formed by oxidation of pyrite (FeS 2 ) in presence of water, it can be obtained as an industrial byproduct during the purification and refining of zinc.

Crystal structure of Jarosite

Jarosite crystals stabilize in trigonal system with space group R m (#166-1). The and '9e' sites. 91 The presence of open space hexagonal ring in the corrugated layers is like that of hexagonal tungsten bronze (HTB) slabs and Kagomé-like structure [Fig. 6(b)]. The resilient crystal structure of Jarosite allows a range of substitution of (i) Fe 3+ ions by other metal ions in +3 oxidation states such as V 3+ , Cr 3+ , Al 3+ , Ga 3+ , In 3+ etc., (ii) K + ions by NH 4 + , Na + , K + , H 3 O + , Ag + , Tl + , ½Pb 2+ , ½Hg 2+ etc., and (iii) SO 4 2-polyanions by SeO 4 2-, CrO 4 2-etc. 92,93 The nature of transition metal, capping polyanionic unit and alkali ion in the interlayer region affect the bond distance among intralayer trimer units and interlayer spacing. Further, the stoichiometry of Jarosite compounds is highly dependent on the synthesis route, such as conventional rapid precipitation route 94 and kinetically slow redox-based hydrothermal method. 95

Magnetism in Jarosite compounds

Jarosites have long been studied for their magnetic property arising from magnetic frustration engendered by geometry of the lattice. Jarosites, a member of Kagomé lattice family, has the corner connected trimer units of [FeO 2 (OH) 4 ] octahedra capped by sulfate tetrahedra resulting in spin frustration at the Fe 3+ (d 5 , S=5/2) metal centres coupled by nearest neighbour Fe-O-Fe super-exchange interaction (antiferromagnetic interaction). [96][97][98][99] As jarosites offer structural flexibility by accommodating ion substitutions at K + , Fe 3+ and SO Possibly, spin frustration induced ground-state degeneracy is abolished by perturbations such as next-to-nearest neighbour exchange interactions, spin anisotropy and lattice disorder. In case of Fe-jarosite, the LRO was ascribed to weak single-ion anisotropy developed due to Dzyaloshinsky-Moriya interaction. 101 Later, experimental evidence of lattice disorder affecting LRO was confirmed as magnetic ordering temperature decreased for stoichiometrically pure V-jarosites prepared by novel redox-based hydrothermal route compared to non-stoichiometric compounds obtained using conventional precipitation route. 95 Interestingly, the extent of antiferromagnetic exchange interaction decreases with transition metal centers having lower number of electrons in d-orbital. 102 Finally, the V 3+ (d 2 ) jarosite shows ferromagnetic exchange within the Kagomé layer with interlayer antiferromagnetic coupling dependent on the interlayer spacing. 103 Monocation substitution has been used as a tool to change interlayer space of Jarosites and observe its effect on LRO temperature. Such investigation on Fe and Cr-jarosites did not reveal any trend in Néel-temperature, while a monotonic increase in Curie temperature with decreasing interlayer spacing was observed for monocation substituted V-jarosites. This is attributed to nature of super-exchange M-O-M interaction in Fe and Cr-jarosite, which is directly dependent on M-O bond covalency that remains unaffected to cationic substitution.

However, the bonding nature in transition metal polyhedra of KFe 3 (SO 4 ) 2 (OH) 6 , especially the Fe-O eq (equatorial oxygen), apart from interlayer space and interlayer H-bonding has been shown to change under external pressure. 104 With increase in applied pressure on single crystal of K-Fe-jarosite, first the interlayer spaces are reduced followed by increase in interlayer Hbonding and finally the shortening of Fe-O eq occurs resulting in an increase of metal-ligand covalency. The increased metal-ligand covalency and hence increased orbital overlapping induces stronger intra-layer antiferromagnetic ordering. Thus, K-Fe-jarosite exhibits a monotonic but non-linear increase in Néel temperature under applied pressure.

Jarosite as electrode for metal-ion batteries

Over the last decade, various jarosites and their derivatives have been examined as potential cathodes for metal-ion batteries. Some principal examples are summarised in ), which gradually disappears on ≤ continuous discharge to 1.0, 0.6 and 0.005 V giving way to the appearance of LiOH. Further, Li 2 S formation is detected in XPS which would stem from Li 2 SO 4 involving 8e -transfer for an additional theoretical capacity of 1948 mAh/g. On subsequent charge, new phases evolve: FeOOH, -Fe 2 O 3 , Li 2 SO 4 and LiKSO 4 [Fig. 9(d)]. Overall, the jarosite undergoes an irreversible conversion reaction during first cycle. Despite the complexity of phase evolution during conversion reaction, the reversibility is retained even after 600 cycles with capacity of 1350 mAh/g at 1 A/g [Fig. 9(e)]. In addition, the 2D nano-architecture was structurally stable after 100 cycles at 500 mA/g, owing to formation of a protective shell around nanobuilding blocks which can accommodate the volume change during conversion reaction cycle, apart from shielding the core from electrolyte corrosion [Fig. 9(f)]. Similar results were reported for 2D nanostructured natrojarosite.

Following another facile and scalable solution-phase oxidation process, a 2D jarosite@rGO composite was reported as suitable insertion host for Li-ion batteries. 110 The synthesis route was similar to the template-assisted route discussed earlier, however the singlelayer graphene oxide (GO) nanosheet as template was non-sacrificial, which was reduced in hydrazine hydrate solution at 25 °C to get rGO. Here, the rGO provides an edge in terms of high electrical conductivity, mechanical strength, and flexibility to Jarosite, which can enhance the capacity, rate capability and cycling stability. The jarosite@rGO composite delivered a first discharge and charge capacity of 143.7 and 146.8 mAh/g (~2.5 Li uptake per f.u.) respectively (vs. Li/Li + ). Even after 300 cycles at 10 C rate, the composite showed a capacity of 70.7 mAh/g. Replacing the Fe 3+ transition metal center in AFe 3 (SO 4 ) 2 (OH) 6 [A= Na + , K + , NH 4 + , H 3 O + ] with V 3+ leads to another member of Jarosite family, known as vanadium (V) jarosite.

Jarosite and V-jarosite (where A = Na, K and M = V, Fe) were synthesized in microwave hydrothermal reactor using iron (III) chloride and vanadium (III) chloride as the source of Fe and V metal, respectively. 111 Alkali (Na/K) sulfates were employed as alkali and sulfate source with concentrated sulphuric acid serving as pH controller crucial to product precipitation. The micrometric morphology of as-synthesized products exhibited dependance on composition.

Natrojarosite powders were faceted in contrast to the jarosite having micrometric aggregates of rounded particles. On the other hand, V-natrojarosite powders showed rose-like aggregates morphology, while V-jarosite powders were rounded and pill-shaped with smooth surfaces.

Additionally, compositional deviation from stoichiometric AM 3 (SO 4 ) 2 (OH) 6 formula was observed. The deviation can occur in two ways: (i) substitution of hydronium ion (H 3 O + ) at A site in the interlayer region resulting in deficiencies of A + , and (ii) M 3+ deficiencies triggering formation of H 2 O following protonation of interlayer hydroxyl group, under charge neutrality condition. 95 The Li + insertion chemistry of these samples were similar to that discussed above, albeit with poor cycling stability attributed to lower fraction of conducting carbon black in the electrode composite and larger particles size of pristine materials. With further optimization, V-natrojarosite and V-jarosites can be promising electrode candidates for Li-ion batteries.

Moving away from monovalent alkali ion-based jarosite, recently an isostructural divalent Pb-based jarosite having formula Pb 0.5 Fe 3 (SO 4 ) 2 (OH) 6 was reported as cathode material for Li-ion batteries. 112 There are two main advantages of Pb substitution in jarosite system. The aliovalent substitution of divalent Pb 2+ for monovalent Na + in parent jarosite structure can create vacancies and expedite facile alkali-ion migration during (de)insertion reaction. Further, the higher electronegativity of Pb (1.9) vis-à-vis Na (0.9) can induce secondary inductive effect in the system by drawing electron density away from coordinated oxygen ions, thereby decreasing the Fe-O covalency and eventually increasing the Fe 3+ /Fe 2+ redox potential.

The prepared Pb-jarosite was found to have ,

[𝑃𝑏 0.33 (𝐻 2 𝑂) 0.33 0.33]𝐹𝑒 3 (𝑆𝑂 4 ) 2 (𝑂𝐻) 6 composition corroborating the expected vacancy at A site. Cyclic voltammograms (at 0.1 mV/s) of Pb-jarosite revealed two reduction peaks at 2.3 V and 2.5 V, and one oxidation peak at 2.6 V (vs Li metal anode). Notably, at current rate of C/5, the Pb-jarosite exhibited first discharge capacity as 91.4 mAh/g (1.93 Li + ) which is higher than corresponding value for Najarosite (89.8 mAh/g, 1.62 Li + ). Compared to Na-jarosite, the average Li + diffusion coefficient was estimated to be 10 times larger for Pb-jarosite which was attributed to A site vacancy. Over prolonged cycling, the Pb-substituted jarosite showed more structural resilience to irreversible amorphous transformation than Na-jarosite, which becomes increasingly amorphous over 50 cycle. It can be ascribed to larger Pb 2+ cation pinning the layered crystal structure together.

Overall, Pb-jarosite opens avenues for further investigation of jarosites as economic cathode material for weight independent battery applications.

Non-jarosite hydroxy-sulfates

Exploring jarosite chemistry and its derivatives, recently K 2 Fe 3 (SO 4 ) 3 (OH) 2 (H 2 O) 2 was reported as a new hydroxysulfate cathode material with high-rate capability for all three alkali ions (Li + , Na + and K + ). 113 This non-jarosite layered hydroxysulfate compound is isomorphous to K 2 Co 3 (SO 4 ) 3 (OH) 2 (H 2 O) 2 assuming an orthorhombic crystal system with Cmc2 1 symmetry.

The compound was synthesized using hydrothermal route at 160 °C for 48 h, leading to hexagonal powder morphology (0.3-1 m) with exposed facets assigned to (100), ( 011) and (001) crystal planes. The cathode composite exhibited equally good insertion electrochemistry for Li, Na and K alkali ions. When tested in half-cell configuration against Li/Na/K metal anode, structural K + ion was removed at 3.0 V and corresponding alkali ions were reinserted below 3.0 V through a solid-solution mechanism. In case of Li half-cell, two reversible redox couples were observed at 2.5/2.2 V and 3.2/3.0 V attributed to Fe 2+ /Fe 3+ redox activity. For Na and K half-cells, the Fe 2+ /Fe 3+ redox couple was centered around 3.1/2.6 V and 3.3/2.8 V, respectively. Li + ions were predicted to migrate in a 2D path in the inter-layer region, whereas Na + and K + ions were structurally restricted to 1D migration. At 0.2 C rate, this cathode material exhibited a capacity of 120 mAh/g, 110 mAh/g and 100 mAh/g for Li + , Na + and K + insertion, respectively with excellent cycling stability. Even at high rate (10 C), K 2 Fe 3 (SO 4 ) 3 (OH) 2 (H 2 O) 2 material delivered capacities of 65 mAh/g, 55 mAh/g and 45 mAh/g respectively, thereby offering its candidacy as a high-rate performance cathode material. Non-jarosite derivatives warrants further exploration to unveil new batery insertion materials.

Perspectives and Summary

From synthesis perspectives, the hydroxyphosphates (A x MPO 4 OH) can be scalably prepared either via conventional solid-state or hydrothermal routes. While, the SO 4 -based candidates (hydroxysulfates and jarosites) can be produced involving energy-savvy low temperature (T < 400º C) routes. Caution must be exercised during heat treatment as the SO 4based compounds are prone to thermal decomposition evolving SO x gases. Nevertheless, all hydroxy-based cathode materials offer an advantage in terms of moisture resistance. While many oxides and polyanionic compounds (particularly electrodes with redox potential  3 V like LiFeBO 3 and fluorinated cathodes like FeF 3 , LiMSO 4 F, LiMPO 4 F) suffer from moisture attack, the presence of OH -species in hydroxy-based cathodes make them moisture-resistant.

The electrochemical performance of various hydroxy-based cathodes, namely (i) hydroxyphosphates, (ii) hydroxysulfates and (iii) (non)jarosites, have been summarised in Fig.

+ /Li 1.1/1.3 V 1.4/1.6 V V 3+ /V 2+ ~63/171 Na 1-x (H 3 O) x V 3 (SO 4 ) 2 (OH) 6
x = 0.048 microwave assisted hydrothermal reaction (e) Reproduced with permission. 71 Copyright 2012, Elsevier. (f) Reproduced with permission. 72 Copyright 2012, Elsevier. (e) Reproduced with permission. 78 Copyright 2013, American Chemical Society(c, f) Reproduced with permission. 79 Copyright 2014, American Chemical Society. Reproduced with permission. 108 Copyright 2015, American Chemical Society. With regards, we would like to submit a perspective article "An overview of hydroxy-based polyanionic cathode insertion materials for metal-ion batteries" authored by S. Singh, S. Lochab, L. Sharma, V. Pralong and myself, for your consideration to publish in Physical Chemistry Chemical Physics. We have prepared this article following your kind invitation last year.

Na + /Na V 3+ /V 2+ ~3 K 1-x (H 3 O) x V 3 (
High energy density polyanionic cathode materials form the cornerstone for next generation rechargeable batteries. They exhibit rich diversity in their structure, electrochemistry and various physical properties. One such sub-class of polyanionic material family is "hydroxybased insertion materials", having OH -moieties. The current article focuses on hydroxy-containing cathode materials with three broad types: (i) hydroxyphosphates, (ii) hydroxysulfates and (iii) (non)jarosites. The presence of one or more OH -units render structural diversity and diverse magnetic, electronic and electrochemical properties in these hydroxy-based cathode materials.

We strongly believe this article will attract the attention of battery as well as materials community to further explore hydroxy-based battery electrode materials. This is the first dedicated review on hydroxy-based insertion materials. We declare the content is original and is not under consideration elsewhere. All authors comply by the ethical standards of RSC Publications and have given their consent for submission. Once again, we sincerely thank you for your kind invitation and consideration of our manuscript for publication in Physical Chemistry Chemical Physics. We look forward to your kind response.

Sincerely yours, Prabeer Barpanda

69

 69

2. 2

 2 LiMnPO 4 OH Aranda et al. first studied the structure of LiMnPO 4 OH (LMPOH) by neutron diffraction and deuterium substitution of protons. Crystallizing in triclinic framework with P-1 space group, 70 lithium atoms were found to be located in the channel along [101] direction.

2 V

 2 LiFeSO 4 OH cathode via solution-based methods, Subban et al. employed mechanical-milling route, which led to the discovery of a new monoclinic polymorph. LiOH and FeSO 4 (anhydrous) were ball milled to get a mixture containing Li 2 SO 4 and a caminite mineral type intermediate phase Fe 3 (SO 4 ) 2 (OH) 2 . 78 Though this phase was electrochemically inactive, it formed LiFeSO 4 OH in the presence of excess LiOH and Li 2 SO 4 . Other isostructural analogues were also prepared by using MSO 4 (M = Co, Mn) and Li 2 SO 4 mixture and annealing at varying temperature range of 185-250 ºC.Interestingly, only the Fe analogue could be directly prepared form the caminite type mineral simply by mechanochemistry (ball milling), while other analogues (Co, Mn, Ni) required further annealing to yield the final products. Overall, the layered isostructural LiMSO 4 OH (M=Mn, Fe, Co) analogues [Fig.5(a)] can be formed either using a mechanochemical route or a ceramic one. The layered LiFeSO 4 OH phase obtained from the above-mentioned procedure had a monoclinic structure having a P2 1 /c symmetry [Fig.3(d)]. It consists of layers of FeO 4 (OH) 2 octahedra joined together in an edge sharing fashion. Zigzag chains are formed in [010] direction joined with the help of the FeO 6 octahedra vertices to form a layered structure with sulfate units present above and below it. Li + ions are present in the cavities formed between adjacent layers.

  )]. While the high-voltage monoclinic (layered) LiFeSO 4 OH underwent a solid solution (single-phase) behaviour, the lower-voltage polymorph exhibited a biphasic redox behaviour with flat voltage profile. Similar structureproperty relationship has earlier been noticed in case of LiFeSO 4 F fluorosulfate polymorphs. Theoretical studies involving DFT calculations and atomistic modeling on the layered (monoclinic) and tavorite (triclinic) LiFeSO 4 OH polymorphs show sinusoidal 2D Li-ion migration pathways. The layered polymorph shows a lesser activation energy barrier of ≈0.2

  (b)]. Although a split peak was observed in the derivative curve for LiFeSO 4 OH, no corresponding structure feature was confirmed by in-situ studies. During cathode cycling, a volume change of ~7% was observed (similar to LiFePO 4 ), while the tavorite LiFeSO 4 OH underwent ~10% volume change.

3

 3 structure is composed of 2D corrugated layers formed by linkage of transition metal octahedra [FeO 2 (OH) 4 ] and sulfate tetrahedra (SO 4 ), which is stacked along c direction. Each transition metal occupies crystallographically equivalent site (2/m) and forms octahedra having 4 [OH -] units attached in the equatorial plane and 2 O along the axial positions. Any two octahedra are connected along the equatorial corners, while metal octahedra and sulfate tetrahedra are connected along the axial oxygen. Each sulfate tetrahedra shares 3 corners with metal octahedra forming a unique trimer unit, while the fourth unshared oxygen corner points alternatively below and above 2D layer. These are present along the direction of hexagonal ring formed by three trimer-units [Fig.6(a)]. The alkali ion resides in the open spaces at '3a' in the layer containing sulfate tetrahedra. This arrangement leaves open space for guest alkali ion at '3d'

4 2 -

 2 sites, this family of compounds has become model systems for examining spin frustration and resultant bulk magnetic behaviour as a function of (i) interlayer spacing, (ii) interlayer alkali ions (iii) intra-layer metal ions, (iv) capping polyanion, and (v) M-O-M bond distance. The spin arrangement in the triangular Kagomé lattice of Jarosite results in infinite number of degenerate ground states [Fig.6(c)], therefore, ideally it should not exhibit long-range magnetic ordering (LRO) at temperature T > 0 K. However, jarosite compounds do exhibit LRO as AFe 3 (SO 4 ) 2 (OH) 6 : A= NH 4 + , Na + and K + undergo antiferromagnetic ordering at ~55 K, 53 K and 60 K respectively with each 2D-layer itself acting as compensated antiferromagnet.100

Figure 1 .

 1 Figure 1. (a) Crystal structure of LiFePO 4 OH. The iron octahedra (FeO 6 ) are shown in blue and brown colour while the phosphate tetrahedra (PO 4 ) are shown in pink color. (b) Fourier maps to determine the position of H and Li in LiFePO 4 OH. (c) First reported lithium (de)intercalation profile of LiFePO 4 OH. (d) The (dis)charge profile showing a stable discharge capacity of 140 mAh g -1 upto 60 cycles. (e) (Dis)charge profile of LiMnPO 4 OH as a cathode for lithium-ion batteries. f) Powder X-ray diffraction patterns recorded at different composition of LiMn 1-x Fe x PO 4 OH. (b, c) Reproduced with permission.62 Copyright 2010, Royal Society of Chemistry. (d) Reproduced with permission.66 Copyright 2019, Elsevier. (e) Reproduced with permission.71 Copyright 2012, Elsevier. (f) Reproduced with permission.72 Copyright 2012, Elsevier.

Figure 2 .

 2 Figure 2. (a) Crystal structure of LiVPO 4 OH showing the presence of O-H as the bridging bond between two octahedra. (b) Bond valence sum Li-ion diffusion pathway analysis. (c) Galavanostatic (dis)charge properties of LiVPO 4 OH when cycled between 3.0 to 4.6 V (vs. Li). (d) Galavanostatic (dis)charge properties of LiVPO 4 OH when cycled between 2.0 to 1.2 V (vs. Li) showing the utilization of multiple oxidation state of vanadium. (e) (Dis)charge profile of LiFePO 4 OH in 21m LiTFSi + 7m LiOTf aqueous electrolyte when cycled between -1.0 to 1.0 V vs. Ag/AgCl for 50 cycles. (f) First (dis)charge curve of LiFePO 4 |LiFePO 4 OH full cell in 21m LiTFSi + 7m LiOTf aqueous electrolyte having a discharge capacity of 120 mAh g -1 at 0.8 V. (a, b, c) Reproduced with permission. 74 Copyright 2016, Royal Society of Chemitsry. (d, e) Reproduced with permission. 66 Copyright 2019, Elsevier.

Figure 3 .

 3 Figure 3. Structural illustration of (a) tavorite and (d) layered polymorphs of LiFeSO 4 OH built from interconnected FeO 6 octahedra (blue and brown) and SO 4 tetrahedra (pink or grey). Galvanostatic charge-discharge profiles of (b) tavorite and (e) layered FeSO 4 OH polymorphs. Calculated low energy pathways (c) in tavorite LiFeSO 4 OH shown along b-and c-axis (activation energy ≤ 0.72 eV) and (f) in layered LiFeSO 4 OH shown along a-and c-axis (activation energy ≤ 0.19 eV) for long range Li + ion migration. (b) Reproduced with permission.75 Copyright 2009, Elsevier. (e) Reproduced with permission.78 Copyright 2013, American Chemical Society(c, f) Reproduced with permission.79 Copyright 2014, American Chemical Society.

Figure 4 .

 4 Figure 4. (a) Structure of FeSO 4 F 1-y OH y (i) View along [-110]. FeO 4 (F,OH) 2 octahedra (in red) are linked through F/OH atoms (in green) forming chains along the [001] direction. SO 4 tetrahedra (in blue) joining adjacent chains. (ii) View along [001] direction i.e. perpendicular to the chains. The adjacent octahedra splits away along the chains. (iii) View along the [201] direction. Traces of the (-2 0 4), (-1 1 2), and (-2 2 4) planes are represented. (iv) π-conjugation of the OH -lone pair (H atom light gray) with the d xz orbitals of Fe 3+ of the transition metal in the Fe-O(H)-Fe "trigonal" planar geometry. (b) Rietveld refinements of X-ray powder patterns of FeSO 4 F, FeSO 4 F 1-y OH y (with y = 0.55) and FeSO 4 OH, diffraction peaks shift shown in the inset and an example of SEM image is given in the bottom. (c) Electrochemical voltage profiles for Li/FeSO 4 F (top), Li/FeSO 4 F 1-y OH y (with y = 0.52) (middle) and Li/FeSO 4 OH cells (bottom) cycled at C/20 (left) along with the corresponding derivative curves (right). Reproduced with permission.76 Copyright 2012, American Chemical Society.

Figure 5 .

 5 Figure 5. Structure and electrochemical properties of layered hydroxysulfates LiMSO 4 OH (M=Fe, Mn, Co). (a) Structure built with MO 6 octahedra (maroon) and SO 4 tetrahedra (grey) delimiting voids for Li atoms (green spehere). (b) In-situ XRD patterns of layered LiFeSO 4 OH. (c) enthalpy of dissolution of LiMSO 4 OH compounds in 5 M HCl at 25⁰ C. (d) Derivative curves showing the respective redox potential. (e) enthalpies of formation (solid curve with square symbols) along with redox potentials (dashed lines with diamond symbols) as a function of cation radius. (b),(d) Reproduced with permission.78 Copyright 2013, American Chemical Society (c), (e) Reproduced with permission.82 Copyright 2014, The Royal Society of Chemistry.

Figure 6 .

 6 Figure 6. (a) Projected crystal structure of jarosite KFe 3 (SO 4 ) 2 (OH) 6 family along a-axis. Here, K site can accommodate monovalent Na + , Rb + , Tl + , NH 4 + , H 3 O + and divalent 0.5 Pb 2+ , 0.5 Hg 2+ and Fe site can be substituted by V 3+ , Cr 3+ , Al 3+ , Ga 3+ , In 3+ etc. The sulfate group can be replaced by SeO 4 2-, CrO 4 2-polyanions (SO 4 tetrahedra, grey; [Fe(OH) 4 O 2 ] octahedra, mustard yellow; Red sphere, Oxygen; Cyan sphere, Na/K atoms). (b) Illustration of 2D layers in jarosites, which are stacked along c-axis. The layers are also known as Kagomé lattice. (c) Schematic representation of Kagomé lattice in one of the many possible ground-state degenerate configurations. (a,b) Copyright 1937. Mineralogical Society of America 90 . (c)Copyright 2014, American Chemical Society.118

Figure 7 .

 7 Figure 7. (a) Rietveld refined XRD pattern of natrojarosite, NaFe 3 (SO 4 ) 2 (OH) 6 . (b) Galvanostatic charge-discharge curves recorded at current rate of C/20. Dramatic change can be noted between first discharge and first charge. (c) The PITT charge and discharge profile indicating change in the reaction mechanism from biphasic to topotactic in first cycle. (d) Ex situ XRD pattern comparison of pristine cathode, 2V (discharge), 3V (intermediate) and 4V (charge) state. (e) Comparative ex situ FTIR spectra of pristine cathode, 2V (discharge), 3V (intermediate) and 4V (charge). (f) Schematic representation of polyhedral distortion and defects generated at the end of first discharge. Reproduced with permission. 104 Copyright 2014, American Chemical Society.

Figure 8 .

 8 Figure 8. (a) Coefficient of diffusion D A + (A + =Li + , Na + ) calculated from GITT plot at different value of x [(Na,Li) x Fe 3 (SO 4 ) 2 (OH) 6 ] during 1 st (black sphere/circle) and 2 nd cycle (red sphere/circle). (b) Formation energy estimated using DFT calculations for different values of x in Na 1- x Li x Fe 3 (SO 4 ) 2 (OH) 6 . (c) Evidence of substitution of structural Na + with Li + is observed on comparison of EDS spectra of pristine NFSOH/C composite (see reference) and after 10 th charge. Minor peak at ~ 1 keV indicate low amount of Na atoms in NFSOH crystal structure. (d) Ex situ XRD pattern comparison during 1 st cycle, 2 nd charge, 3 rd charge, 4 th charge, 10 th charge and 30 th charge. (e) 57 Fe Mössbauer spectra of pristine natrojarosite, NFSOH (NaFe 3 (SO 4 ) 2 (OH) 6 ), NFSOH/C composite, NFSOH/C composite at 1 st discharge (2 V) and NFSOH/C composite at 1 st charge (4 V). A constant isomer shift value of 0.38 mm.s -1 is reported across four samples. Reproduced with permission. 106 Copyright 2020, Elsevier Ltd.

Figure 9 .

 9 Figure 9. (a) Schematic illustrations of KFe 3 (SO 4 ) 2 (OH) 6 jarosite nanosheets synthesized by a template-assisted redox coprecipitation method at room temperature. (b) XRD pattern of jarosite nanosheet matched with reference pattern corresponding to ICSD No. 71-1777. (c) Galvanostatic (Dis)charge profiles of jarosite acquired between 0.005 and 3.0 V at a high current rate of 100 mAg -1 . (d) Phase evolution during the initial discharge process for jarosite electrode from open circuit potential to 5 mV. (e) (Dis)charge curves of jarosite over prolonged cycling at a slower current rate of 1 Ag -1 . (f) Schematic illustration of the yolk-shell architecture of jarosite nanosheet believed to impart enhanced cycling stability.

Figure 10 .

 10 Figure 10. Comparative performance of various hydroxy-based cathodes (namely hydroxyphosphates, hydroxysulfates and jarosites) along with selected state-of-the-art cathodes (LiCoO 2 , LiFePO 4 etc.) are summarised emphasizing their operating voltage, specific capacity and energy density (dashed lines).

Table 5 .

 5 The first example of Jarosite family member as insertion cathode for Li-ion battery was reported by Gnanavel et al (circa 2014) focusing on Natrojarosite NaFe 3 (SO 4 ) 2 (OH)6 . the same reaction mechanism with reversible capacity of 120 mAh/g at a rate of C/20 with Fe 3+ /Fe 2+ redox couple centered ~2.72 V (vs. Na/Na + ).106 Plugging the gap of a rigorous mechanistic study of Li + intercalation in Natrojarosite of generation of FeO 5 pyramidal defects units during discharge, a constant presence of Fe 3+ in octahedral environment was observed during cathode cycling [Fig.8(e)].Thus, Li + insertion during first discharge strongly distort SO 4 and FeO 6 polyhedra resulting into disordering of the bulk structure and amorphization. Finally, the crystalline-amorphous transformation of natrojarosite host was found to be irreversible after 30 th cycle with appearance of Li 2 SO 4 .H 2 O and Fe 2 O 3 phase.The discovery of natrojarosite as promising cathode host for Li-and Na-ion batteries prompted further investigation of analogous compounds of jarosite family. In this pursuit, the original potassium-jarosite KFe 3 (SO 4 ) 2 (OH) 6 was fabricated in 2D nanostructure form at roomtemperature via facile, scalable and economic template-assisted redox precipitation route [Fig.+ diffusion and electron transport, higher surface area offering more reaction points for Li + (de)insertion and good structural integrity under lattice strain during galvanostatic cycling.When the 2D nanostructured jarosite was galvanostatically cycled in the voltage range of 0.005-3.0 V vs. Li + /Li, it showed promises to work as a conversion reaction-based anode material exhibiting high capacity, rate capability and cycling life [Fig.9(c)].109 At a current density of 100 mAg -1 , the as prepared jarosite exhibited first discharge and charge capacities as 1786.7 and 1300.8 mAh g -1 , respectively, which is more than thrice the theoretical capacity of 481 mAh/g estimated solely on complete reduction of Fe 3+ to Fe 0 involving 9 Li + /9e - reaction. This implied that the extra capacity stems from intermediate reactions. Synergistic analysis of ex-situ XRD and XPS measurements revealed the conversion reaction mechanism as follows. During first discharge to 1.5 V, Li insertion into KFe 3 (SO 4 ) 2 (OH) 6 takes place resulting into formation of Li x KFe 3 (SO 4 ) 2 (OH) 6 (x 3.7

	between crystalline to amorphous state [Fig. 7(c,d,e)]. The biphasic reaction during first
	discharge is perhaps due to the formation of defects beyond a critical lithium concentration in
	the structure in the form of FeO 5 pyramids from original FeO 6 octahedra, thereby, inducing
	strains in the 2D layers [Fe 3 (SO 4 ) 2 (OH) 6 ] of the host structure. It leads to the formation of

105

Solution drying route was used to quickly saturate and precipitate the titular product at 90 °C for 48 h (in air), followed by washing with distilled water and overnight drying at 110 °C.

Initially, the solution was prepared by dissolving Na 2 SO 4 and Fe 2 (SO 4 ) 3 .nH 2 O in molar ratio of 1:2 in 0.01 M H 2 SO 4 . Considering a reversible (de)insertion of 3 Li per f.u. of NaFe 3 (SO 4 ) 2 (OH) 6 , a theoretical capacity of 166 mAh/g was calculated. During galvanostatic cycling at C/20 rate, natrojarosite allows insertion of 2.1 Li + per f.u. during first discharge and deinsertion of 2 Li + during subsequent charging step, thus delivering a reversible capacity of 110 mAh/g at 2.82 V (vs. Li/Li + ) [Fig.

7(a,b)

]. Noteworthy is the flat shape of first discharge curve reminiscent of biphasic reaction mechanism, which is distinctly different from subsequent (dis)charge cycles having slopping profile due to topotactic reaction mechanism.

105

By synergizing ex situ XRD and TEM analysis, it was shown that the host cycles reversibly inorganic polymer from pristine host whose strains reversibly relaxe on Li + deinsertion forming original crystallized structure [Fig.

7(f)

]. The Na + (de)insertion in Natrojarosite is also governed by with conductive carbon to obtain the electrode composite. It exhibited bi-phasic crystallineamorphous transformation during first discharge on Li and Na insertion, in sync with earlier results. The structural rearrangement would occur to facilitate alkali ion migration which otherwise would not be allowed in the narrower ion conducting channels of natrojarosite. Change in intercalation mechanism from biphasic to solid solution during first cycle had a dramatic effect (two orders of magnitude) on the alkali ion diffusion coefficients of Li/Na ion [Fig. 8(a)]. After 10 cycles at C/20 current rate, a reversible capacity of 71 mAh/g was observed against Li metal anode with two redox peaks at (i) 2.71/2.63 V and (ii) 3.03/2.93 V attributed to Na and Li (de)intercalation phenomenon like that of sodium-iron carbonophosphate. The intensity of first redox peak diminished with further cycling indicating formation of a transformation after first discharge was observed. The structure breaks down completely after 30 th cycle evidently with appearance of Li 2 SO 4 .H 2 O and Fe 2 O 3 [Fig. 8(d)]. Second, in contrast to earlier assumption 9(a,b)]. 108 Nanosheets of -MnO 2 was used as both oxidant and self-sacrificing template. Stoichiometric amounts of FeSO 4 .7H 2 O and KNO 3 were added into a brown colloidal suspension of -MnO 2 nanosheets kept under magnetic stirring. The change in color of suspension from brown to orange in ~ 10 minutes signaled reaction completion. Following, the suspension was aged for 12 h at room temperature before washing with deionized water and ethanol, and vacuum drying. It led to 2D nanostructured morphology with 5-10 nm buiding blocks having thickness of ~13 nm and surface area of 71 m 2 g -1 . 2D nanosheets of natrojarosite could also be fabricated using the same template-assisted route. Like the Li + (de)intercalation reversible capacity of 75 mAh/g was recorded. Essentially, the enhanced Li storage performance of jarosite was attributed to its special 2D nanostructured morphology facilitating fast Li

10 .

 10 While majority of them work as voltage below 3 V, layered LiFeSO 4 OH (3.6 V) and tavorite LiVPO 4 OH (~4 V) offer high-voltage activity and thus high energy density comparable to commercialized LiFePO 4 . It is key to note several of these hydroxycompositions are capable of multiple-electron redox activity leading to high capacity. These cathodes can be further optimized using techniques like particle nanoscaling, homogenization, and coating by conductive carbon layers and/or ionic conducting ceramics (like Al 2 O 3 /ZrO 2 ).While these compounds are widely tested for metal-ion batteries, selected compositions (with emphasis on Co and Ni) can be exploited for electrocatalytic activity. As the hydroxybased compositions can be stable in water, they can work as electrocatalytic materials with oxygen evolution and oxygen reduction (OER and ORR) reactions. Selected compounds may demonstrate potential for usage in metal-air batteries.

	Polyanionic class of materials have long been the treasurehouse to unveil structurally
	diverse electrode materials with tuneable redox potential. On another note, earth's crust is rich
	with myriads of hydroxy-based (hydrated) mineral compositions. Exploiting crystallographic
	and mineralogical databases, hydroxy-based compositions and their synthetic derivatives can
	be tested to unravel new battery insertion materials. The current review attempts to summarize
	existing cathode materials exclusively containing hydroxy (OH -) moieties. The presence of
	OH -triggers variation in crystal structure, materials stability and magnetic properties. Among
	them, three best compounds were found to be (i) tavorite LiVPO 4 OH (4 V), (ii) LiFeSO 4 OH
	(3.6 V) and (iii) jarosite KFe 3 (SO 4 ) 2 (OH) 6 (2.58 V). These hydroxy-based insertion materials
	offer a fertile ground for materials discovery, crystallography, and fundamental research on
	electronic, magnetic and electrochemical properties.

Table 1 .

 1 Overview of hydroxyphosphate-based cathode materials for Li-ion batteries.

	Materials (Formulae)	Structure	Redox Species	Redox Potential	Rate	(Observed/ Theoretical) Capacity (mAh.g -1 )	References
			Fe +3 /Fe +2	2.6 V	1.0 mA cm -2
						140/ 153
	LiFePO 4 OH	Tavorite triclinic					66
		(P ) 1	Fe +3 /Fe +2	2.6-2.7 V	1.0 mA cm -2
						153/153
					(aqueous)
	LiMnPO 4 OH	Tavorite triclinic	Mn +4 /Mn +3	3.4 V	C/100	110/ 154	71
		(P ) 1
			V +4 /V +3	3.95 V	C/100
						280/ 158
	LiVPO 4 OH	(P ) 1 Tavorite triclinic	V +3 /V +2	1.4 V	C/100	136/ 158	74

Table 2 .

 2 Naturally occuring hydroxyphosphate-based minerals with potential application in design of battery insertion materials.[112][113][114][115][116]

	Mineral name	Chemical Formulae
	Whitetite	CaMn ++ Mg 2 Al 2 (PO 4 ) 4 (OH) 2 .8H 2 O
	Vantasselite	Al 4 (PO 4 ) 3 (OH) 3 .9H 2 O
	Apatite	Ca 5 (PO 4 ) 3 (F,OH,Cl)
	Leucophosphite	K 2 [Fe 4 (OH) 2 (H 2 O) 2 (PO 4 ) 4 ].2H 2 O
	Nordgauite	MnAl 2 (PO 4 ) 2 (F,OH) 2 .5H 2 O
	Skorpionite	Ca 3 Zn 2 (PO 4 ) 2 CO 3 (OH) 2 .H 2 O
	Metavivianite	Fe +2 Fe 2 +3 (PO 4 ) 2 (OH) 2 .6H 2 O
	Trolleite	Al 4 (OH 3 )[PO 4 ] 3
	Weinebeneite	CaBe 3 (PO 4 ) 2 (OH) 2 .4H 2 O
	Hentschelite	CuFe 2 (PO 4 ) 2 (OH) 2
	Kastningite	(Mn,Fe,Mg)(H 2 O) 4 [Al 2 (OH) 2 (H 2 O) 2 (PO 4 ) 2 ].2H 2 O
	Phosphuranylite	Ca(UO 2 )[(UO 2 ) 3 (OH) 2 (PO 4 ) 2 ] 2 .12H 2 O
	Plimerite	ZnFe 4 (PO 4 ) 3 (OH) 5
	Paravauxite	FeAl 2 (PO 4 ) 2 (OH) 2 .8H 2 O
	Tvrdyite	Fe +2 Fe 2 +3 Al 3 (PO 4 ) 4 (OH) 5 (OH) 5 (OH 2 ) 4 .2H 2 O
	Hydroxylpyromorphite	Pb 10 (PO 4 ) 6 (OH 2 )
	Rockbridgeite	Fe 4.32 Mn 0.62 Zn0 .06 (PO 4 ) 3 (OH) 5
	Lakebogaite	CaNaFe 2 +3 H(UO 2 ) 2 (PO 4 ) 4 (OH) 2 (H 2 O) 8
	Jahnsite	Mn +2 Mn +2 Fe +3 2 Fe +3 2 (PO 4 ) 4 (OH) 2 .8H 2 O
	Foggite	CaAl(PO 4 )(OH) 2 .H 2 O
	Vendidaite	Al 2 (SO 4 )(OH) 3 Cl(H 2 O) 6
	Girvasite	NaCa 2 Mg 3 (PO 4 ) 4 (CO 3 )(H 2 O) 6
	Meurigite	[Na(H 2 O) 2.5 ][Fe 3+ 8 (PO 4 ) 6 (OH) 7 (H 2 O) 4 ]
	Whitmoreite	CuFe 3+ 2 (PO 4 ) 2 (OH) 2 ⋅4H 2 O
	Eosphorite-Childrenite	(Mn,Fe)Al(PO 4 )(OH) 2 •H 2 O

Table 3 .

 3 Overview of hydroxysulfate-based cathode materials for Li-ion batteries.

	Materials			Redox	Rate	Capacity (mAh.g -1 )
	(Formulae)	Structure	Redox Species	Potential			References
						(Observed/ Theoretical)
	LiFeSO 4 OH	Layered	Fe +3 /Fe +2	3.6 V	C/20	110/159	78
		monoclinic (P2 1 /c)
	FeSO 4 OH	Tavorite monoclinic	Fe +3 /Fe +2	3.2 V	C/20	110/159	75
		(P2 1 /c)
	LiMnSO 4 OH	Layered	--	--	--	--/153.19	78
		monoclinic (P2 1 /c)
	LiCoSO 4 OH	Layered	Co +3 /Co +2	4.7 V	--	~30/149.77	78
		monoclinic (P2 1 /c)
	LiNiSO 4 OH	Layered	--	--	--	--/149.97	78
		monoclinic (P2 1 /c)

Table 4 .

 4 Alunite supergroup of minerals encompassing jarosites as a subclass of minerals.90

	Name	Composition
	Sulfates
	Alunite	KAl 3 (SO 4 ) 2 (OH) 6
	Natroalunite	NaAl 3 (SO 4 ) 2 (OH) 6
	Harttite	SrAl 3 (SO 4 ) 2 (OH) 6
	Jarosite	KFe 3 (SO 4 ) 2 (OH) 6
	Natrojarosite	NaFe 3 (SO 4 ) 2 (OH) 6
	Hydronium-jarosite	(H 3 O)Fe 3 (SO 4 ) 2 (OH) 6
	Argento-jarosite	AgFe 3 (SO 4 ) 2 (OH) 6
	Ammonio-jarosite	NH 4 Fe 3 (SO 4 ) 2 (OH) 6
	Plumbo-jarosite	Pb 1/2 Fe 3 (SO 4 ) 2 (OH) 6
	Beaverite	PbCu(FeAl) 2 (SO 4 ) 2 (OH) 6
	Phosphates
	Hamlinite	SrAl 3 (PO 4 ) 2 (OH) 5 .H 2 O
	Plumbogummite	PbAl 3 (PO 4 ) 2 (OH) 5 .H 2 O
	Gorceixite	BaAl 3 (PO 4 ) 2 (OH) 5 .H 2 O
	Florencite	Ce 5/6 Al 3 (PO 4 ) 2 (OH) 5 .H 2 O
	Mixed
	Beudantite	PbFe 3 (AsO 4 )(SO 4 )(OH) 6
	Corkite	PbFe 3 (PO 4 )(SO 4 )(OH) 6
	Savanbergite	SrAl 3 (AsO 4 )(SO 4 )(OH) 6
	Hinsdalite	PbAl 3 (PO 4 )(SO 4 )(OH) 6

Table 5 .

 5 Jarosites as electrode materials for metal-ion insertion batteries.

	Compounds	vs.	Activity	Redox Species	Rate	Capacity (mAh/g)	Reference
	as cathode
	NaFe 3 (SO 4 ) 2 (OH) 6	Li + /Li	2.82 V	Fe 3+ /Fe 2+	C/20	120/166	104
	Precipitation route	Na + /Na	2.72 V	Fe 3+ /Fe 2+	C/20	120/166	105
	Li + /Li Hydrothermal synthesis Na + /Na NaFe 3 (SO 4 ) 2 (OH) 6	2.71/2.63 V 3.03/2.93 V 2.55 V	Fe 3+ /Fe 2+ Fe 3+ /Fe 2+	C/20 C/20	71/166 80/166	106
	KFe 3 (SO 4 ) 2 (OH) 6 Template-assisted redox	Li + /Li	2.09/1.85 V 2.87/2.42 V	Fe 3+ /Fe 2+	C/5	117/160	107
	coprecipitation	Na + /Na		Fe 3+ /Fe 2+	C/10	~115/160
	KFe 3 (SO 4 ) 2 (OH) 6 /rGO composite	Li + /Li 2.31/2.58 V Fe 3+ /Fe 2+	1C	143.6/160	109
	NH 4 Fe 3 (SO 4 ) 2 (OH) 6 Hydrothermal route	Li + /Li		Fe 3+ /Fe 2+	C/20	~ 30/168	117
	NaFe 3 (SO 4 ) 2 (OH) 6-x F x
	x= 0.34	Li + /Li		Fe 3+ /Fe 2+	C/20	~ 105	117
	Precipitation route
		Li

Physical Chemistry Chemical Physics

Acknowledgements

We acknowledge the financial support from the Technology Mission Division (Department of Science and Technology, Government of India) under the aegis of Materials for Energy Storage (MES-2018) program (DST/TMD/MES/2K18/00217). SL thanks Dr. S. Adiga for hosting him at Samsung Research Institute for research internship. LS is grateful to the Electrochemical Society (ECS, USA) for the 2020 ECS F.M. Becket Summer Fellowship.

The crystal structures were illustrated with the help of VESTA software. 120