Ankush Bhatia

Jiahui Xu

Jean-Pierre Pereira-Ramos

Gwenaelle Rousse

Rita Baddour- Hadjean

The '-V 2 O 5 Polymorph: a Genuine Zn Intercalation Material for Non-Aqueous Rechargeable Batteries

The electrochemical properties of the puckered layered '-V 2 O 5 polymorph as cathode material in a non-aqueous Zn-metal cell using acetonitrile-Zn(CF 3 SO 3 ) 2 electrolyte are investigated here for the first time. The typical galvanostatic profile in the 2 V-0.3 V vs. Zn 2+ /Zn voltage range shows Page 1 of 47 ACS Paragon Plus Environment Chemistry of Materials a sloping discharge curve involving a capacity of 130 mAh g -1 at C/20 in one single step centered at 0.9 V vs. Zn 2+ /Zn. The structural response of '-V 2 O 5 during the discharge-charge cycle is investigated by ex-situ X-ray diffraction (XRD) and Raman spectroscopy. Up to 0.41 Zn mol -1 can be accommodated between the '-V 2 O 5 layers, inducing only a moderate interlayer expansion of +6.4%, comparable to that found for Li + insertion. Remarkably, the insertion process is fully reversible in spite of the high charge density of Zn 2+ . Good cycle life can be achieved at moderate rate, with a stable capacity of nearly 130 mAh g -1 available at 0.9 V vs Zn 2+ /Zn over at least 60 cycles. The peculiar structural features of the new electroformed Zn 0.41 V 2 O 5 bronze highlight the interest of the '-V 2 O 5 polymorph to mitigate the expected large deformation upon electrochemical divalent Zn 2+ incorporation.

Introduction

Lithium Ion Batteries (LIBs) dominate the energy market of portable electronics due to their high energy density and long cycle life. However, their high costs, toxicity, the scarcity of lithium and safety issues are strong drawbacks that impede the use of LIBs for large-scale applications such as electric transportations and grid-scale energy storage. In search of alternative batteries, Page 2 of 47 ACS Paragon Plus Environment Chemistry of Materials aqueous rechargeable Zn-based batteries (ARZB) are quite attractive owing to the high volumetric and gravimetric capacities delivered by the Zn metal anode (5851 mAh mL -1 ; 820 mAh g -1 ), their low cost, safety and moderate toxicity [START_REF] Kundu | A High-Capacity and Long-Life Aqueous Rechargeable Zinc Battery Using a Metal Oxide Intercalation Cathode[END_REF][START_REF] Pan | Reversible Aqueous Zinc/Manganese Oxide Energy Storage from Conversion Reactions[END_REF][START_REF] Wan | Aqueous Rechargeable Zinc/Sodium Vanadate Batteries with Enhanced Performance from Simultaneous Insertion of Dual Carriers[END_REF][START_REF] Konarov | Present and Future Perspective on Electrode Materials for Rechargeable Zinc-Ion Batteries[END_REF] . The major problems of ARZBs are the limited energy density due to the narrow electrochemical window (1.2 V) as well as the spontaneous dissolution of MnO 2 -and V 2 O 5 -based cathodes in aqueous electrolytes [START_REF] Konarov | Present and Future Perspective on Electrode Materials for Rechargeable Zinc-Ion Batteries[END_REF] . The use of non-aqueous electrolytes would overcome these issues, providing a higher operating voltage of Zn-ion battery and avoiding dissolution. Various non-aqueous Zn-based electrolytes have recently been reported to work in a wide electrochemical window (3.8 V vs. Zn 2+ /Zn) such as acetonitrile (AN)-Zn(TFSI) 2 , AN-Zn(CF 3 SO 3 ) 2 , and propylene carbonate (PC)-Zn(TFSI) 2 electrolytes [START_REF] Han | Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes[END_REF] . The challenge is to find cathode materials affording a high working voltage combined with a high-specific capacity.

While many papers are devoted to cathode materials for ARZBs such as Prussian blue and analogues, manganese oxides and vanadium-based compound [START_REF] Ming | Zinc-Ion Batteries: Materials, Mechanisms, and Applications[END_REF][START_REF] Fang | Recent Advances in Aqueous Zinc-Ion Batteries[END_REF] , only a few candidates are reported for non-aqueous rechargeable Zn batteries (NARZBs) [START_REF] Gocke | Molybdenum Cluster Chalcogenides Mo 6 X 8 : Electrochemical Intercalation of Closed Shell Ions Zn 2+ , Cd 2+ , and Na[END_REF][START_REF] Arroyo Y De Dompablo | Novel superconductors obtained by electrochemical Zn intercalation of β-ZrNCl and related compounds[END_REF][START_REF] Guerfi | High Cycling Stability of Zinc-Anode/Conducting Polymer Rechargeable Battery with Non-Aqueous Electrolyte[END_REF][START_REF] Pan | ZnAl x Co 2-x O 4 Spinels as Cathode Materials for Non-Aqueous Zn Batteries with an Open Circuit Voltage of ≤2 V[END_REF][START_REF] Pan | ZnNi x Mn x Co 2-2x O 4 Spinel as a High-Voltage and High-Capacity Cathode Material for Nonaqueous Zn-Ion Batteries[END_REF][START_REF] Han | Mechanism of Zn Insertion into Nanostructured δ-MnO 2 : A Nonaqueous Rechargeable Zn Metal Battery[END_REF][START_REF] Senguttuvan | A High Power Rechargeable Nonaqueous Multivalent Zn/V 2 O 5 Battery[END_REF][START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF][START_REF] Huang | Hybrid Aqueous/Organic Electrolytes Enable the High-Performance Zn-Ion Batteries[END_REF][START_REF] Chen | Persistent Zinc-Ion Storage in Mass-Produced V 2 O 5 Architectures[END_REF] . From a pioneering work, the electrochemical Zn intercalation in Chevrel-type molybdenum cluster chalcogenides Mo 6 X 8 (X = S, Se) was found to occur at 0.5 V-0.3 V, using zinc perchlorate in PC or AN electrolyte [START_REF] Gocke | Molybdenum Cluster Chalcogenides Mo 6 X 8 : Electrochemical Intercalation of Closed Shell Ions Zn 2+ , Cd 2+ , and Na[END_REF] . In 2000, the reversible Zn intercalation in β-ZrNCl and related compounds was reported in 0.2 M solution of Zn(CF 3 SO 3 ) 2 in a mixture of dimethylsulfoxide (DMSO) and (PC) in a 1:4 volume ratio [START_REF] Arroyo Y De Dompablo | Novel superconductors obtained by electrochemical Zn intercalation of β-ZrNCl and related compounds[END_REF] . More recently, a Zn/PANi (polyaniline) cell was cycled in a non-aqueous PC-Zn(TFSI) 2 electrolyte [START_REF] Guerfi | High Cycling Stability of Zinc-Anode/Conducting Polymer Rechargeable Battery with Non-Aqueous Electrolyte[END_REF] .

Spinel cathodes ZnAl x Co 2-x O 4 and ZnNi x Mn x Co 2-2x O 4 investigated in AN-Zn(CF 3 SO 3 ) 2 [START_REF] Pan | ZnAl x Co 2-x O 4 Spinels as Cathode Materials for Non-Aqueous Zn Batteries with an Open Circuit Voltage of ≤2 V[END_REF][START_REF] Pan | ZnNi x Mn x Co 2-2x O 4 Spinel as a High-Voltage and High-Capacity Cathode Material for Nonaqueous Zn-Ion Batteries[END_REF] were shown to operate near 1.7-1.8 V, involving attractive capacities of 140-170 mAh g -1 . The reversible Zn insertion has been demonstrated near 1 V in a nanostructured hydrated layered-type birnessite -MnO 2 , leading to a capacity of 100 mAh g -1 without formation of ZnMn 2 O 4 spinel upon cycling [START_REF] Han | Mechanism of Zn Insertion into Nanostructured δ-MnO 2 : A Nonaqueous Rechargeable Zn Metal Battery[END_REF] . In spite of the richness of their chemical composition and structure, vanadium oxidesbased compounds have been little investigated in NARZBs [START_REF] Senguttuvan | A High Power Rechargeable Nonaqueous Multivalent Zn/V 2 O 5 Battery[END_REF][START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF][START_REF] Huang | Hybrid Aqueous/Organic Electrolytes Enable the High-Performance Zn-Ion Batteries[END_REF][START_REF] Chen | Persistent Zinc-Ion Storage in Mass-Produced V 2 O 5 Architectures[END_REF] . The hydrated layered forms V 3 O 7˙H2 O and V 2 O 5˙0 .67H 2 O have been reported to deliver interesting stable capacities in the range 150-200 mAh g -1 at 0.65 V in AN-Zn(CF 3 SO 3 ) 2 [START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF] and 0.9 V in AN-Zn(TFSI) 2 [START_REF] Senguttuvan | A High Power Rechargeable Nonaqueous Multivalent Zn/V 2 O 5 Battery[END_REF] , respectively. A freestanding vanadium oxide V 2 O 5˙0 .14H 2 O-carbon nanotube film has been recently explored with higher capacity values ( 300 mAh g -1 ) but working at a low potential, near 0.6 V [START_REF] Huang | Hybrid Aqueous/Organic Electrolytes Enable the High-Performance Zn-Ion Batteries[END_REF] . Indeed, water-containing structures constitute one strategy to promote reversible Zn 2+ storage [START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF] . The water molecules provide expanded interlayer spacing and are thought to buffer the charge density of inserted Zn 2+ cations. Finally, the most common and stable V 2 O 5 polymorph,  V 2 O 5 has been also reported, using a nanosizing strategy. Porous architectures made of 3D V 2 O 5 nanorods offer also a high reversible capacity of  300 mAh g -1 near 0.7 V [START_REF] Chen | Persistent Zinc-Ion Storage in Mass-Produced V 2 O 5 Architectures[END_REF] . However, except the study devoted to V 3 O 7 ˙H2 O [START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF] , the structural analysis of the cathode material upon dischargecharge is overlooked or absent.

Such capacity values prompted us to explore the interest of other V 2 O 5 polymorphs as cathode material in NARZB, especially the metastable 2D '-V 2 O 5 , that is the charge product of the -LiV 2 O 5 bronze formed upon electrochemical lithiation of -V 2 O 5 at 2.3 V vs. Li + /Li [START_REF] Cocciantelli | Crystal Chemistry of Electrochemically Inserted Li x V 2 O 5[END_REF] . This open intercalation host with larger interlayer spacing (≈ 5 Å vs. 4.37 Å for -V 2 O 5 ) and highly puckered V 2 O 5 stacked layers provides a vast abundance of interlayer interstitial sites. Furthermore, DFT studies have predicted lower diffusion barriers and higher operating voltage for such cation-free metastable phases [START_REF] Parija | Evaluation of Multivalent Cation Insertion in Single-and Double-Layered Polymorphs of V 2 O 5[END_REF][START_REF] Parija | Topochemically De-Intercalated Phases of V 2 O 5 as Cathode Materials for Multivalent Intercalation Batteries: A First-Principles Evaluation[END_REF] . Indeed, recent experimental studies in our group have demonstrated the superior electrochemical performance of '-V 2 O 5 toward Li +21,22 , Na +23-26 and K +27 insertion compared to -V 2 O 5 [START_REF] Bhatia | γ'-V 2 O 5 Polymorph as a Promising Host Structure for Potassium Storage: An Electrochemical and Structural Study[END_REF][START_REF] Muller-Bouvet | Electrochemically Formed α'-NaV 2 O 5 : A New Sodium Intercalation Compound[END_REF][START_REF] Baddour-Hadjean | The Richness of V 2 O 5 Polymorphs as Superior Cathode Materials for Sodium Insertion[END_REF] . The lithium insertion mechanism in '-V 2 O 5 was found to involve reversible and mitigated structural changes consisting in wide solid solution domains [START_REF] Baddour-Hadjean | Unraveling the Structural Mechanism of Li Insertion in γ'-V 2 O 5 and Its Effect on Cycling Properties[END_REF] which stand in contrast to the successive phase transitions required to accommodate Li-ions in the thermodynamically stable -V 2 O 5 polymorph [START_REF] Delmas | The Li x V 2 O 5 System: An Overview of the Structure Modifications Induced by the Lithium Intercalation[END_REF] . Also, '-V 2 O 5 was reported to insert almost 1 Na +23-26 and 0.78 K + per mole of oxide [START_REF] Bhatia | γ'-V 2 O 5 Polymorph as a Promising Host Structure for Potassium Storage: An Electrochemical and Structural Study[END_REF] , remarkably at close working voltage to that observed for Li + insertion (3.3 V vs. Na + /Na and 3.1 V vs K + /K, respectively).

These attractive results encouraged us to enrich the study of '-V 2 O 5 intercalation chemistry to the case of Zn 2+ insertion in a non-aqueous electrolyte. Han et al investigated a range of nonaqueous electrolytes and clearly indicated AN-Zn(CF 3 SO 3 ) 2 as one of the most suitable for reversible cycling of Zn metal anode, providing a wide electrochemical window up to 3.6 V vs. Zn 2+ /Zn 5 . In addition, the low viscosity (0.37 mPa s) of AN combined to a dielectric constant of 36.6 afford fast ionic mobility and high conductivity (17 mS cm -1 ).

Herein, we investigate for the first time the electrochemical properties of the '-V 2 O 5 polymorph in the 2.0 V  0.3 V voltage range in a Zn-metal cell using AN-Zn(CF 3 SO 3 ) 2 (Zn(OTf) 2 ) electrolyte. The cycling and rate capability properties are reported and the structural response of '-V 2 O 5 upon discharge and charge is explored using ex-situ X-ray diffraction (XRD) and Raman spectroscopy. We show that γ'-V 2 O 5 behaves as a genuine Zn intercalation compound, with the electrochemical formation of a new layered -type Zn 0.41 V 2 O 5 bronze and a fully reversible extraction accompanied by the complete restoration of the pristine oxide structure.

Experimental

'-V 2 O 5 was obtained by the chemical oxidation of -LiV 2 O 5 synthesized from the chemical lithiation of a fine powder of a α-V 2 O 5 precursor prepared by the polyol method [START_REF] Mjejri | Low-Cost and Facile Synthesis of the Vanadium Oxides V 2 O 3 , VO 2 , and V 2 O 5 and Their Magnetic, Thermochromic and Electrochromic Properties[END_REF] . Lithiation of α-V 2 O 5 is performed using an excess of lithium iodide in AN to obtain the intermediate -LiV 2 O 5 phase. A heat treatment at 300°C for 2 h under dynamic primary vacuum in a Büchi® furnace allows the phase transition toward -LiV 2 O 5 . The last step consists in quantitatively extracting Li from -LiV 2 O 5 by oxidation in a NO 2 BF 4 solution (solid Alfa Aesar 96%) in AN (V 2 O 5 /NO 2 BF 4 molar ratio 1/4) under stirring for 24 h at room temperature. After washing with AN and vacuum drying at 70°C for 24 h, the orange '-V 2 O 5 powder is obtained. Electrochemical titration using galvanostatic oxidation and chemical redox titration confirmed the 5+ oxidation state of vanadium in '-V 2 O 5 .

The pristine powder and the electrodes were characterized by scanning electron microscopy (SEM), (Zeiss, Merlin-type microscope). The elemental compositions were determined from electron dispersive X-ray spectroscopy (EDS) analysis together with SEM using an accelerating voltage of 10  15 kV. Crystal structures were characterized by XRD and Raman spectroscopy. XRD experiments were performed in Bragg-Brentano geometry using a Panalytical XPert pro apparatus equipped with a X'Celerator detector and Co Kα radiation ( Kα1 =1.78901 Å,  Kα2 = 1.79290 Å). The recorded patterns were refined using the Rietveld method as implemented in the FullProf program 32,[START_REF] Rodríguez-Carvajal | FullProf Suite Homepage[END_REF] .

The Raman spectra were recorded with a LaBRAM HR 800 (Jobin-Yvon, Horiba) Raman microspectrometer including edge filters and equipped for signal detection with a back illuminated charge-coupled device detector (Spex CCD) cooled by the Peltier effect to 200 K. A He-Ne laser (632.8 nm) was used as the excitation source. The spectra were measured in back-scattering geometry. To avoid local heating of the sample, the power of the laser beam was adjusted to 0.2  0.5 mW with neutral filters of various optical densities. The resolution was about 0.5 cm -1 . A 100 objective was used to focus the laser light on the sample surface to a spot size of 1 µm 2 .

Electrochemical experiments were carried out at 20°C using 2032 coin-type cells, which are assembled in an argon-filled inert atmosphere glovebox (<0.5 ppm of H 2 O and <0.5 ppm of O 2 ).

The positive electrode was prepared by mixing 80 wt % of active material (about 8 mg) with 7.5 wt % of acetylene black, 7.5 wt % graphite and 5 wt % of PTFE as the binder. Zn metal foil (Alfa Aesar, > 99% purity) was used as the negative electrode. The Zn metal surface was polished by ACS Paragon Plus Environment Chemistry of Materials using 600-grit sand paper followed by washing with ethanol to remove the surface oxide layer.

The two electrodes are separated by a Whatmann glass fiber separator soaked in the non-aqueous AN-Zn(OTf) 2 0.3 M electrolyte (AN from Sigma Aldrich > 99.8% purity). The Zn(OTf) 2 powder was dried overnight at 100°C under vacuum and the AN was dried by keeping it over regenerated molecular sieves before use. Galvanostatic cycling measurements were performed using a VMP3 Biologic apparatus in the 0.3 V-2 V vs. Zn 2+ /Zn voltage window at room temperature at different current densities (147 mA g -1 corresponds to 1C).

The structural investigation was carried out using two electrodes coin cells. The cells were discharged at C/20 rate up to different x composition in Zn x V 2 O 5 (0 ≤ x ≤ 0.41). The highest Zn content (x = 0.41) was attained on fifth discharge at C/20. After 2 h relaxation time, the cells were disassembled in the glove box. The positive electrodes were rinsed 3 times with AN in order to remove any electrolyte residue and placed in appropriate airtight sample holders to be further analyzed by XRD and Raman spectroscopy. Raman spectra were recorded on 10 different spots of each electrode. Very similar spectra were obtained, regardless of the illuminated spot, indicating a homogeneous electrochemical reaction in the chosen experimental conditions.

ACS Paragon Plus Environment

Chemistry of Materials

Results and discussion

Structural characterization

The solution technique carried out to synthesize γ'-V 2 O 5 leads to a nanosized powder composed of platelets of 500 nm long maximum and ~ 200 nm wide (Figure 1). The Rietveld refinement of the XRD pattern (Figure 2a) is performed with an initial structural model reported by J. M.

Cocciantelli [START_REF] Cocciantelli | On the Preparation and Characterization of a New Polymorph of V 2 O 5[END_REF] , and all atoms were freely refined. The final refinement shown in Figure 2a indicates that '-V 2 O 5 crystallizes in an orthorhombic unit cell (Pnma space group) with lattice parameter a = 9.94387(9) Å, b = 3.58389(3) Å, c = 10.03601(9) Å in good agreement with previous studies [START_REF] Cocciantelli | Crystal Chemistry of Electrochemically Inserted Li x V 2 O 5[END_REF][START_REF] Baddour-Hadjean | Unraveling the Structural Mechanism of Li Insertion in γ'-V 2 O 5 and Its Effect on Cycling Properties[END_REF][START_REF] Renard | γ'-V 2 O 5 : A New High Voltage Cathode Material for Sodium-Ion Battery[END_REF][START_REF] Emery | γ-Na 0.96 V 2 O 5 : A New Competitive Cathode Material for Sodium-Ion Batteries Synthesized by a Soft Chemistry Route[END_REF][START_REF] Cocciantelli | On the Preparation and Characterization of a New Polymorph of V 2 O 5[END_REF] . The structural parameters, including a bond-valence-sum (BVS) analysis [START_REF] Brown | Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database[END_REF] and V-O bond lengths are gathered in Tables S1 andS2.

The structure of γ'-V 2 O 5 (insert in Figure 2a) consists of infinite ribbons parallel to the b axis made of VO 5 edges-sharing distorted pyramids oriented alternatively up and down, leading to two non-equivalent vanadium environments. These chains are linked to each other along the a-direction by one pyramid corner oxygen, forming puckered layers perpendicular to the c-axis. The mean crystallite size estimated from diffraction peaks width is 90  100 nm.

From the local structure point of view, the Raman spectrum of γ'-V 2 O 5 (Figure 2b) is composed of 21 peaks in the 90-1050 cm -1 wavenumber range, as previously reported [START_REF] Baddour-Hadjean | The Raman Spectrum of the γ'-V 2 O 5 Polymorph: A Combined Experimental and DFT Study[END_REF][START_REF] Smirnov | Unraveling the Structure-Raman Spectra Relationships in V 2 O 5 Polymorphs via a Page 44 of 47 ACS Paragon Plus Environment Chemistry of Materials Comprehensive Experimental and DFT Study[END_REF] The galvanostatic study of γ′-V 2 O 5 has been performed at C/20 rate ( 8 mA g -1 ) in the same 0.3 V-2 V vs. Zn 2+ /Zn voltage range. As shown in Figure 3b, the working potential gradually decreases during the first discharge, from 1.37 V down to 0.75 V and then in a steeper way in the last third of the discharge, involving a total faradaic yield of 0.64 F mol -1 (specific capacity of 95 mAh g -1 ).

Electrochemical study

Upon subsequent charge of the cell, the potential increases abruptly to stabilize around 1.20 V, and then significantly increases from the mid-charge up to 2 V. The slightly higher capacity value recovered in charge is probably due to slight electrolyte decomposition [START_REF] Pan | ZnNi x Mn x Co 2-2x O 4 Spinel as a High-Voltage and High-Capacity Cathode Material for Nonaqueous Zn-Ion Batteries[END_REF][START_REF] Han | Mechanism of Zn Insertion into Nanostructured δ-MnO 2 : A Nonaqueous Rechargeable Zn Metal Battery[END_REF] that might be promoted when using acetylene black-graphite mixture in the composite electrode. Indeed, using another carbon matrix as carbon nanotubes (CNTs), we have checked this overcharge can be completely suppressed (Figure S1). From cycle 1 to cycle 5 (Figure 3b), the capacity continuously increases from 95 to 130 mAh g -1 without any change in the discharge profile. Such electrochemical phenomenon has already been described elsewhere. It could be attributed to a decrease in the charge transfer resistance at the negative electrode because of progressive electrochemical Page 12 of 47 ACS Paragon Plus Environment Chemistry of Materials polishing of the Zn surface [START_REF] Senguttuvan | A High Power Rechargeable Nonaqueous Multivalent Zn/V 2 O 5 Battery[END_REF][START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF] and/or gradual utilization of active material [START_REF] Chen | Persistent Zinc-Ion Storage in Mass-Produced V 2 O 5 Architectures[END_REF] . From the complete charged state, i.e. from 2 V, the voltage rapidly falls down to ~1.2 V upon discharge, then gives rise to a smooth slope involving the total charge capacity. After this short activation process of a few cycles, following charge-discharge curves completely superimpose, showing a moderate sloppy profile with a mid-discharge potential of 0.9 V (Figure 3c). Cycling experiments point to a remarkable retention of the capacity at C/20 over at least 60 cycles at a value of around 120 mAh g -1 combined with a 100% coulombic efficiency (Figure 3d). At 1C, the capacity moderately decreases by 10%, from 90 to 80 mAh g -1 after 100 cycles (insert of Figure 3d).

The maximum capacity of 130 mAh g -1 here achieved for γ′-V 2 O 5 is lower than that reported for the hydrated bilayered V 2 O 5˙0 .67H 2 O electrodeposited on carbon foam substrate, with ≈ 200 and 160 mAh g -1 at C/10 and 1C, respectively in 0.5 M Zn(TFSI) 2 -AN electrolyte [START_REF] Senguttuvan | A High Power Rechargeable Nonaqueous Multivalent Zn/V 2 O 5 Battery[END_REF] . Zn intercalation into BL(bilayered)-V 2 O 5 takes place practically at a slightly lower mid-discharge voltage ( ≈ 0.8 V). Such reaction seems to be more difficult in the hydrated V 3 O 7˙H2 O oxide investigated in the same AN-Zn(CF 3 SO 3 ) 2 0.25 M electrolyte [START_REF] Kundu | Nonaqueous Zn-Ion Batteries: Consequences of the Desolvation Penalty at the Interface[END_REF] . Indeed, this material delivers about the same capacity value of 150 mAh g -1 at near 0.65 V, but after at least 30 cycles at C/20, and the first discharge never exceeds 50 mAh g -1 . Remarkable values are exhibited by a freestanding hydrated V 2 O 5 -CNT electrode at 0.7C with  300 mAh g -1 in 1M Zn(ClO 4 ) 2 -ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte [START_REF] Huang | Hybrid Aqueous/Organic Electrolytes Enable the High-Performance Zn-Ion Batteries[END_REF] . However, this composite electrode rather consists in a model electrode since the carbon matrix is very large (33.7 wt % CNTs). In addition, in the absence of Zn perchlorate salt heat-treatment, huge capacities might be ascribed to the presence of residual water molecules in the electrolyte promoting Zn or Zn-proton insertion. A two-electron process is outlined in the case of 3D nanorods architectures of V 2 O 5 [START_REF] Chen | Persistent Zinc-Ion Storage in Mass-Produced V 2 O 5 Architectures[END_REF] after a quite long activation process of 80 cycles at C/3 leading to a capacity of 300 mAh g -1 . The authors claim this kind of -type nanosized vanadium pentoxide operates as a Zn intercalation compound, which seems doubtful according to the huge capacitive contribution as high as 30-60% observed in the coulombic charge.

The C-rate dependence on the galvanostatic cycles (Figure 4a) and the discharge capacity (Figure 4b) of '-V 2 O 5 have been investigated in the C/20-2C range. At C/20, the capacity stabilizes near 130 mAh g -1 after 5 cycles, indicating the maximum amount of Zn ions insertionextraction is close to 0.45 per mol of oxide. Increasing the C rate to C/10, C/5 and C/2 leads to a progressive capacity decrease to 113, 92 and 85 mAh g -1 , respectively, which corresponds to 87%, 71% and 65% of the maximum value, respectively. At 1C and 2C, the capacity nearly does not decline, with 80 mAh g -1 still available at 2C. The present rate capability performance is slightly Page 14 of 47 ACS Paragon Plus Environment Chemistry of Materials lower than that reported for the hydrated bilayered BL-V 2 O 5 cathode in 0.5 M Zn(TFSI) 2 -AN electrolyte with still 80% and 70% of the maximum capacity available at 1C and 5C, respectively [START_REF] Senguttuvan | A High Power Rechargeable Nonaqueous Multivalent Zn/V 2 O 5 Battery[END_REF] . However, as shown in Figure 4b, the cell completely recovers the initial capacity of 120 mAh g -1 when subsequently operated at C/20, which indicates the structure of the cathode has not been damaged by applying 2C rate. Finally, a high reversibility of the discharge-charge process and excellent capacity stability are achieved, regardless of the applied C rate.

The electrochemical behavior at room temperature of the conventional α-V 2 O 5 polymorph has been also investigated in the same AN-Zn(CF 3 SO 3 ) 2 0.3 M electrolyte for comparison. Figure S2a shows the cyclic voltammetric curves achieved for the two polymorphs. The coulombic charge involved in the case of α-V 2 O 5 is only  20% that achieved for '-V 2 O 5. From galvanostatic curves obtained at C/20 (Figure S2b), Zn insertion in α-V 2 O 5 takes place around 0.7 V vs. Zn 2+ /Zn V, i. e. 200 mV lower than '-V 2 O 5 , which indicates a much more difficult reaction. The major distinctive feature lies in a greatly lower capacity for α-V 2 O 5 , rapidly stabilizing around only 30 mAh g -1 (Figures S2b-S2c). The superior working voltage and higher capacity found for '-V 2 O 5 illustrate once again the advantage of this open structure toward Zn 2+ intercalation, as previously demonstrated in the case of monovalent alkali ions insertion [START_REF] Baddour-Hadjean | Unraveling the Structural Mechanism of Li Insertion in γ'-V 2 O 5 and Its Effect on Cycling Properties[END_REF][START_REF] Renard | γ'-V 2 O 5 : A New High Voltage Cathode Material for Sodium-Ion Battery[END_REF][START_REF] Bhatia | γ'-V 2 O 5 Polymorph as a Promising Host Structure for Potassium Storage: An Electrochemical and Structural Study[END_REF] .

Structural changes during the electrochemical process

In order to get insight into the Zn reaction mechanism, several γ′-V 2 O 5 electrodes have been prepared at various depths of discharge and charge (0.12 to 0.82 F mol -1 ) at C/20 rate and further analyzed by XRD and Raman spectroscopy. Table 3 summarizes the electrodes history and results of EDS analysis. Zn element is systematically detected by EDS, with a good accord between the expected Zn content and the faradaic yield, assuming the following intercalation process:

γ′-V 2 O 5 + 2 x e -+ x Zn 2+  Zn x V 2 O 5
Indeed, the Zn content in V 2 O 5 evaluated from EDS fits remarkably that calculated from coulombic charge (see Table 3). The negligible Zn amount of 0.02 achieved after the first charge is probably due to residual Zn traces in spite of the rinsing procedure. The ex-situ Raman spectra of the discharged Zn x V 2 O 5 electrodes (x = 0.06, 0.13, 0.19, 0.25, 0.32 and 0.41) are gathered in Figure 5b. One can see that the Raman spectra keep the original features of the pristine material, but with a loss of intensity and band broadening indicating the emergence of some disorder in the plane of the V 2 O 5 chains upon Zn insertion. In the high wavenumber range, the shift of the apical V=O stretching modes toward frequencies lower than 1000 cm -1 suggests a lengthening of these bonds upon V 5+ to V 4+ reduction. For the most reduced electrodes (x = 0.32 and 0.41), a new broad band at ≈884 cm -1 is observed, which can be assigned to the symmetric stretching vibration of the VO 3 group, as commonly reported for many vanadate minerals [START_REF] Frost | A Raman Spectroscopic Study of the Different Vanadate Groups in Solid-State Compounds-Model Case: Mineral Phases Vésigniéite[END_REF] .

Hence, the main effect of Zn intercalation in '-V 2 O 5 consists in an interlayer expansion by 6.4% (from 10.04 Å to 10.68 Å) while the a and b in-plane parameters undergo negligible changes. As a result, the unit cell volume increases by the same 6.4% factor, from 358 Å 3 for the pristine oxide to 381 Å 3 for the electroformed Zn 0 . 41 V 2 O 5 bronze (Figure 6). The moderate interlayer dilation of 6.4% induced by Zn accommodation is comparable to that observed upon Li + insertion (6%, see [START_REF] Baddour-Hadjean | Unraveling the Structural Mechanism of Li Insertion in γ'-V 2 O 5 and Its Effect on Cycling Properties[END_REF] , probably because of the small size of Zn 2+ (ionic radius of 0.74 Å vs. 0.76 Å for Li + ).

Differently, sodium insertion in '-V 2 O 5 was proved to induce a huge dilation of 18% [START_REF] Renard | γ'-V 2 O 5 : A New High Voltage Cathode Material for Sodium-Ion Battery[END_REF][START_REF] Safrany Renard | Kinetic Insight into the Electrochemical Sodium Insertion-Extraction Mechanism of the Puckered γ'-V 2 O 5 Polymorph[END_REF] while only a 10% expansion was reported for K + insertion 27 due to the important polarizability of potassium. Hence, it appears the size and polarizability of the guest cation in '-V 2 O 5 control the magnitude of the interlayer expansion. The V 2 O 5 layers puckering along the a axis is very limited in the case of Zn insertion (0.4%) compared to lithiation (2.6%) [START_REF] Baddour-Hadjean | Unraveling the Structural Mechanism of Li Insertion in γ'-V 2 O 5 and Its Effect on Cycling Properties[END_REF] and sodiation (1.7%) [START_REF] Renard | γ'-V 2 O 5 : A New High Voltage Cathode Material for Sodium-Ion Battery[END_REF] , suggesting that the charge of the guest cation would mitigate the puckering effect. Finally, the b parameter increases by 0.5%, which is in good accord with the previous evolution of 0.5% and 1% found for lithiation and sodiation, respectively. It is also consistent with a diffusion of Zn ions along the b direction.

The structural reversibility upon charge and cycling experiments has been examined by XRD and Raman spectroscopy. Comparison of the fingerprints of the pristine electrode and that after one cycle (Figure 7) clearly shows the typical XRD pattern and Raman spectrum of '-V 2 O 5 are recovered, indicating that highly reversible structural change takes place upon oxidation. After 60 cycles, the initial structure is recovered with however some intensity decrease and peaks broadening pointing to some disordering process.

Characterization of the electroformed Zn 0.41 V 2 O 5 bronze

The formation of the Zn 0.41 V 2 O 5 bronze from electrochemical reduction of '-V 2 O 5 at room temperature is reported here for the first time. Indeed, high temperature reaction of Zn on α-V 2 O 5 at 630°C leads to the poor Zn containing bronze Zn 0 . 03 V 2 O 5 whose structure is close to the parent oxide α-V 2 O 5 .Another vanadium bronze with a higher Zn content, -Zn 0 . 26-0.29 V 2 O 5 , has been reported without any structural determination [START_REF] Galy | Sur quelques nouvelles familles de composés non-stoechiométriques du vanadium[END_REF][START_REF] Hagenmuller | Recherches Recentes Sur Les Bronzes de Vanadium II. Elements d'insertion Divalents et Trivalents[END_REF] . The thermal treatment of an appropriate nitratesucrose mixture lead to the formation of a layered nanostructured Zn 0.29 V 2 O 5 [START_REF] Shreenivasa | An Introduction of New Nanostructured Zn 0.29 V 2 O 5 Cathode Material for Lithium Ion Battery: A Detailed Studies on Synthesis, Characterization and Lithium Uptake[END_REF] . The latter is also indicated as a discharge product of V 2 O 5 nanosheets on hierarchical TiN nanowire arrays in an aqueous rechargeable Zn-ion battery [START_REF] Li | Anchoring V 2 O 5 Nanosheets on Hierarchical Titanium Nitride Nanowire Arrays to Form Core-Shell Heterostructures as a Superior Cathode for High-Performance Wearable Aqueous Rechargeable Zinc-Ion Batteries[END_REF] . For higher Zn/V ratio values, for instance 0.5, 1 and 1.5, ZnV 2 O 6 , Zn 2 V 2 O 7 and Zn 3 V 2 O 8 vanadates with structural features very far from those found for the present Zn 0.41 V 2 O 5 bronze have been reported 43 .

The Rietveld refinement of the XRD pattern of Zn 0.41 V 2 O 5 is reported in Figure 8a, for which soft M-O (M = V, Zn) distance restraints were imposed. Crystallographic data and atomic positions of Zn 0.41 V 2 O 5 are gathered in Tables 1 and2, respectively. Even if caution should be taken regarding the accuracy of atomic positions due to the mean quality of the data obtained on composite electrodes, it is clear that Zn is placed in the interlayer space, in the same 4c position as Na in -Na 0.96 V 2 O 5 [START_REF] Emery | γ-Na 0.96 V 2 O 5 : A New Competitive Cathode Material for Sodium-Ion Batteries Synthesized by a Soft Chemistry Route[END_REF] and Li in -LiV 2 O 5 [START_REF] Galy | V 2 O 5 bronzes: structure of beta'-phase and refinement of gamma-phase structure[END_REF] . In Zn 0.41 V 2 O 5 , Zn is characterized by a distorted octahedral coordination with an average Zn-O distance of 2.238(3)Å. This is in agreement with the expected environment for CN 6 and mean bond length of 2.11 Å [START_REF] Gagné | Bond-Length Distributions for Ions Bonded to Oxygen: Results for the Transition Metals and Quantification of the Factors Underlying Bond-Length Variation in Inorganic Solids[END_REF] . The deconvolution of the Raman spectrum (Figure 8b) clearly shows the apical V=O stretching modes are seen at lower frequencies (957/993 cm -1 ) as a result of the lengthening of these bonds in the Zn bronze, as reported in Table 2. In the electroformed zinc bronze, the ZnO 6 octahedra share edges to form chains running along the [010] direction (Figure 9). The valence states of the vanadium ions have been estimated using the bond valence sum (BVS) method [START_REF] Brown | Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database[END_REF] (Table 1). The obtained values point to different oxidation states between the two inequivalent vanadium sites. Indeed, V a site is filled by V +5 ions (BVS value = 4.97) while V b site accommodates V +4 species (BVS value = 4.00), which suggests that the V b site is the redox center. This result reveals a localized character of electron, comparable to that also observed in vanadium bronzes with γ-type puckered layer structure such as γ-LiV 2 O 5 and γ-Na 0.96 V 2 O 5 [START_REF] Emery | γ-Na 0.96 V 2 O 5 : A New Competitive Cathode Material for Sodium-Ion Batteries Synthesized by a Soft Chemistry Route[END_REF] (see also values reported in Table 2).

Conclusions

In this work, the electrochemical properties of the '-V 2 O 5 polymorph as a cathode material for a rechargeable non-aqueous Zn-ion battery are reported for the first time. We demonstrate that despite its high charge density, Zn 2+ ions can be electrochemically accommodated in the '-V 2 O 5 host lattice through a solid solution process in line with the sloping discharge profile centered near 0.9 V vs Zn 2+ /Zn. The discharge product corresponds to a new zinc vanadate bronze with composition Zn 0.41 V 2 O 5 , whose refined structure reveals only modest structural changes. The present results highlight the flexibility of the puckered layered '-V 2 O 5 structure undergoing a limited interlayer expansion (6.4%) on Zn insertion, similar to that induced upon lithiation, along with minor impacts in the V 2 O 5 planes. A highly reversible extraction of Zn 2+ ions accompanied by a complete structural restoration of the pristine oxide during the charge process is found, revealing '-V 2 O 5 behaves as a genuine Zn intercalation compound. Such properties enable a long cycle life at moderate rate, with stable capacities around 130 mAh g -1 available at 0.9 V over at least 60 cycles. The unique intercalation properties offered by '-V 2 O 5 are definitively illustrated by two major remarkable findings. First, the peculiar '-V 2 O 5 structure promotes Zn insertion in comparison with the difficult insertion reaction found in α-V 2 O 5 , occurring at lower potential and involving a poor capacity limited to one quarter that of '-V 2 O 5 . More importantly is the astonishing high working voltage of 0.9 V vs Zn 2+ /Zn (≈ 3.18 V vs Li + /Li) exhibited by '-V 2 O 5 . Indeed, this potential value very close to that obtained for Li + , Na + and K + makes '-V 2 O 5 a unique host material in the field of intercalation chemistry with promising features as cathode material for "beyond Li" batteries.

Notes

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

ACKNOWLEDGMENT

One of the author wishes to thank the French ANR project "CASSIOPES" N°17-CE09 0016-03 for its financial support. All the authors would like to thank Rémy Pires Brazuna (ICMPE-CNRS) for his valuable contribution in SEM-EDS analysis.

  . The projection of the '-V 2 O 5 structure along the b-crystallographic direction showing all the VO contacts in the two vanadium environments (V a , V b ) is shown in insert of Figure 2b. All the vibrations of the chains can be identified: the stretching of the two shortest V a -O 1a and V b -O 1b bonds at 1037, 1021, and 1003 cm -1 , the bond stretching vibrations localized within the V a -O 3 -V b bridges at 752 and 603 cm -1 , and those of the V a -O 2a -V a and V b -O 2b -V b bridges forming the rails of the ladders at 722 and 694 cm -1 . Modes involving the longest V-O 2 inter-chains ladder step bonds are observed at 532 and 500 cm -1 , while lower-frequency modes located at 91, 126, 138, 153, 171, 190, 238, 266, 282, 299, 349, and 390 cm -1 are related to the complex distortions of the ladders 36 .

Figure

  Figure3ashows the cyclic voltammogram (CV) of '-V 2 O 5 recorded in the 0.3 V-2 V vs.

  The ex-situ XRD patterns of the Zn x V 2 O 5 discharged electrodes (x = 0.06, 0.13, 0.19, 0.25, 0.32 and 0.41) are gathered in Figure 5a. Upon Zn insertion, the '-V 2 O 5 peaks progressively decrease in intensity and shift toward lower angle peaks, indicating that Zn insertion occurs through a solid solution region in the whole 0 ≤ x ≤ 0.41 composition range, in good agreement with the electrochemical profile showing a characteristic sloping curve (Figure 3b). The diffraction pattern of the Zn 0.41 V 2 O 5 phase can be indexed with the same Pnma space group of the γ′-V 2 O 5 pristine material, with unit cell parameters of a = 9.9053(7) Å, b = 3.60501(20)Å, c = 10.6822(9) Å indicating an expansion of the c lattice parameter, which suggests Zn 2+ cations are likely placed between the γ'-V 2 O 5 layers.

Figure 1 .

 1 Figure 1. SEM micrographs of the as-prepared γ'-V 2 O 5 powder.

Figure 2 .Figure 6 .

 26 Figure 2. (a): Rietveld refinement of the XRD pattern of γ'-V 2 O 5 . The red crosses, black line, and green line represent the observed, calculated and difference patterns, respectively. The positions of the Bragg reflections are shown as vertical blue bars. Inset: crystal structure of γ'-V 2 O 5 (b) Raman spectrum of γ'-V 2 O 5 . Inset: projection of the structure of '-V 2 O 5 along the bcrystallographic direction showing all the VO contacts. Dashed lines show VO inter-chain ladder-step bonds.

1 . 3 a

 13 Crystallographic data and atomic positions of -Zn 0.41 V 2 O 5 deduced from the Rietveld refinement of its X-ray powder diffraction pattern. Results from Bond Valence Sum (BVS) analyses are listed in the last columns. * : not refined Space group P n m a R Bragg = 16.1 %  2 = 3.32 V = 381.45(8) Å = 9.9053(7) Å, b = 3.60501(20) Å, c = 10.6822(9) Å  =  =  =

Table 2 )

 2

Table 2 .

 2 V-O and Zn-O bond lengths in Zn 0.41 V 2 O 5 deduced from Rietveld refinement of its X-ray powder diffraction pattern. V-O bond lengths values in '-V 2 O 5 and -LiV 2 O 5 (from [25]) are also reported for comparison. LS: V-O inter-chain ladder-step bonds, indicated by dashed lines in insert

	/c	Occupancy BVS

Table 3 .

 3 Zn/V ratios in different electrodes from EDS analysis and electrochemistry (EC).

	Electrode history	F mol -1 x in Zn x V 2 O 5	Zn/V	x in Zn x V 2 O 5
			(EC)	(EDS)	(EDS)
	1 st partial discharge (2.25 h at C/20)	0.12	0.06	0.025	0.050
	1 st partial discharge (5 h at C/20)	0.25	0.13	0.062	0.124
	1 st partial discharge (10 h at C/20)	0.50	0.25	0.119	0.238
	1 st complete discharge at C/20	0.64	0.32	0.150	0.300
	5 th complete discharge at C/20	0.82	0.41	0.200	0.400
	After the first complete discharge-charge	0	0	0.010	0.020
	cycle at C/20

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