Abstract: The growing interest in the Na-ion batteries is justified by cost and availability of sodium resources combined with an insertion chemistry close to the lithium one. While α-V 2 O 5 was one of the first example of Li intercalation compounds only a few works report Na insertion into that layered host lattice. We recently reported sodium insertion into the so called α-V 2 O 5 with a capacity of 120 mAh/g at a low voltage of 1.6V [1]. In this work a new sodium insertion compound is prepared by the chemical oxidation of the γ-LiV 2 O 5 using NO 2 BF 4 as oxidizing agent. One sodium ion per mole of γ’-V 2 O 5 can be reversibly inserted at a remarkably high potential of 3.3V against 1.6V in the usual α-V 2 O 5 (figure 1). The γ’-V 2 O 5 electrode can deliver a reversible and stable capacity of 145 mAh/g at C/60 and 50°C and 125 mAh/g at C/20. An excellent capacity retention is also demonstrated at RT with 80 mAh/g recovered after 100 cycles at C/20. A two phases mechanism involving the γ’-V 2 O 5 / γ-NaV 2 O 5 system is evidenced from XRD and Raman spectroscopy experiments. The structural features of the fully sodiated γ-NaV 2 O 5 phase with an usual expansion of the interlayer spacing (+2Å/compared to γ’-V 2 O 5 ) are solved. These results reveal that the γ’-V 2 O 5 constitutes a new competitive cathode material for the reversible intercalation of sodium ions.  Reference [1] D. Muller-Bouvet, R. Baddour-Hadjean, M. Tanabe, L.T.N. Huynh, M.L.P. Le, J.P. Pereira-Ramos Electrochemically formed α’-NaV2O5 : A new sodium intercalation compound Electrochimica Acta 2015, Vol. 176 (2015) 586-593 doi : 10.1016/j.electacta.2015.07.030 Figure 1

Abstract: Due to the cost and low availability of Li sources, Na-ion batteries (NIBs) are attracting considerable interest as tomorrow’s world batteries. Compared to LIBs, the number of electrode materials for NIBs are limited but progress in Na intercalation grows very rapidly [1]. Layered materials with Van der Waals interlayer spacing constitute ideal frameworks for intercalation reactions of guest cationic species from which high discharge-charge rate and minimum structural distortions can be expected. While orthorhombic α-V 2 O 5 was identified in the 70’s as a promising cathode material for secondary Li batteries [2], it is only very recently that Na insertion was addressed in this oxide at room temperature [3] . An alternative way to identify new attractive stable V 2 O 5 polymorphs consists in considering the chemical removal of metallic species from vanadium pentoxide bronzes M x V 2 O 5 . Such approach allows taking advantage of the availability of new types of structure with various original layer stacking. This strategy was successfully applied in the 90s’ to obtain the puckered layer γ’-V 2 O 5 polymorph synthesized from the topotactic chemical removal of Li from γ-LiV 2 O 5 by means of strong oxidizing agent [4]. The electrochemical behavior of γ’-V 2 O 5 was shown to explain the enhancement of the cell potential of Li//V 2 O 5 [4] and a recent work has demonstrated the potentialities of this polymorph toward Li insertion [5] . Here we report the interesting capability of γ’-V 2 O 5 toward electrochemical sodium insertion. Nearly one Na + / mole involving the V 5 + /V 4 + redox couple can be inserted between the puckered layers of the γ’-V 2 O 5 structure (see inset in Figure 1 ). As a result, an attractive initial discharge capacity of 145 mAh g -1 is obtained at a high working potential of 3.3 V vs. Na + /Na ( Fig.1 curve a ). Nevertheless, strong kinetic limitations are evidenced during the first charge process, with a 50% efficiency at RT ( Fig.1 curve a ). Further cycles exhibit an excellent capacity retention (stable value of 70 mAh g − 1 available after 70 cycles at C/10) [6]. To solve the charge efficiency issue, a downsizing approach was performed using planetary ball milling on the as prepared γ’-V 2 O 5 platelets-like powder. While the platelet morphology is kept, the mean crystallite size is reduced by a factor 3 (90 nm for the as prepared sample vs. 35 nm for the ball-milled γ’-V 2 O 5 -BM). As shown in Fig.1 curve b , a strong effect of the crystallite size reduction is observed both on the shape of the voltage-composition curve and on the first charge efficiency that is strongly improved for the ball-milled powder. Indeed, a charge capacity of 127 mAh g -1 corresponding to a 90% efficiency is recovered for γ’-V 2 O 5 -BM. A detailed structural study by X-ray diffraction (XRD) and Raman spectroscopy reveals a greatly modified phase diagram by reducing the dimensions of the particle. The nanosize effect promotes a wide single phase domain at the expense of diphasic region and is also responsible for an easier sodium extraction process in the ball milled compound, leading to a genuine electrochemical and structural reversibility. A mastering of the particle morphology has also been conducted : γ’-V 2 O 5 was synthesized from a home-made α-V 2 O 5 precursor obtained through polyol process, leading to pure, fine and coral-like porous powders with homogeneous grain size distribution [7]. This peculiar morphology drastically increases the available surface for sodium diffusion, leading to a quantitative charge process at a high working voltage of 3.4 V vs. Na + /Na ( Fig.1 curve c ). Furthermore, enhanced rate capability performance and excellent cycle life are achieved, with 130 mAh g -1 still available after 60 cycles at C/10. Raman and XRD measurements demonstrate the high structural reversibility of the sodium insertion-extraction reaction in γ’-V 2 O 5 . References 1- N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 114 (2014) 11636. 2- M.S. Whitthingham, Chem. Rev. 104 (2004) 4271. 3- D. Muller, R. Baddour-Hadjean, M. Tanabe, L.T.N. Huynh, M.L.P. Le, J-P. Pereira-Ramos, Electrochim. Acta 176 (2015) 586. 4- J. M. Cocciantelli, P. Gravereau, J-P. Doumerc, M. Pouchard, P. Hagenmuller, J. Solid State Chem., 93 (1991) 497. 5- R. Baddour-Hadjean, M. Safrany Renard, J-P. Pereira-Ramos, Acta Mater. 165 (2019) 183 6- M. Safrany Renard, N. Emery, R. Baddour-Hadjean, J-P. Pereira-Ramos, Electrochim. Acta 252C (2017) 4. 7- N. Emery, R. Baddour-Hadjean, D. Batyrbekuly, B. Laïk, Z. Bakenov, J-P. Pereira-Ramos, Chem. Mater 30 (2018) 5305. Figure 1. First discharge-charge curve of a γ’-V 2 O 5 composite electrode made of the following active material: (a) as-prepared platelet-like powder/ crystallite size 90 nm (b) ball-milled powder / crystallite size 30 nm (c) coral-like powder. C/10 rate. NaClO 4 1M/PC electrolyte, 2% vol. FEC. Inset: crystal structure of γ’-V 2 O 5 . Figure 1

Abstract: A new sodium insertion compound, γ'-V 2 O 5 , was prepared by the chemical oxidation of γ-LiV 2 O 5 phase using NO 2 BF 4 as oxidizing agent. One sodium ion per mole of γ'-V 2 O 5 can be inserted in γ'-V 2 O 5 involving a high V 5 + /V 4 + redox potential of 3.5 V vs. Na + /Na. A fully reversible capacity of 145 mAh g − 1 was obtained at C/60, at 50°C (figure 1). The γ'-V 2 O 5 electrode can deliver a high discharge capacity of 125 mAh g − 1 at C/20 rate at 50°C, stable over 50 cycles. An excellent retention capacity is also demonstrated over 90 cycles at RT: capacities in the range 60-80 mAh. g -1 in the C/5-C/20 range against 100-125 mAh g -1 at 50°C were recovered. A discharge capacity of 80 mAh g − 1 was still available after 90 cycles at C/20 rate. A two phases mechanism involving the γ'-V 2 O 5 / γ-NaV 2 O 5 system was evidenced from X-ray diffraction and Raman spectroscopy measurements. The structural features of the fully sodiated γ-NaV 2 O 5 phase, with an unusual expansion along the c axis (+2 Å/ γ'-V 2 O 5 ) were solved (inset in figure 1). These results revealed that the γ'-V 2 O 5 forms a new competitive cathode for the reversible intercalation of sodium ions. Figure 1

Abstract: Sodium-ion batteries (SIBs) have entailed an increasing interest as attractive alternatives to Lithium-ion batteries (LIBs) because of the lower cost, high abundance and large worldwide availability of Na sources, especially in the context of large-scale storage applications. The major challenge in SIBs is to propose good Na-host materials with optimal electrochemical properties and most SIBs cathode materials are either imitating or duplicating existing lithium analogues. Recently, our group reported the potential interest of various V 2 O 5 polymorphs towards Na insertion with promising performance revealed for the γ’-V 2 O 5 polymorph in terms of specific capacities and cycle life [1]. γ’-V 2 O 5 is prepared by chemical oxidation of the γ-LiV 2 O 5 bronze synthesized through a carbothermal route. Topotactic lithium removal is observed, that keeps the original puckered layer stacking of the bronze precursor and leads to micrometric γ’-V 2 O 5 platelets. γ’-V 2 O 5 is able to accommodate nearly 1 Na ion leading to an attractive capacity of 145 mAh g -1 , surprisingly at the same working potential than Li insertion (3.3 V vs Na + /Na). However, this cathode material suffers from a poor first charge efficiency that never exceeds 50% at moderate rate. A detailed structural study has established the discharge-charge mechanism for 0 We show in this work that an appropriate ball-milling protocol can be successfully applied to enhance the electrochemical performance of : a significant increase in the first cycle efficiency (from 50 to 90%) is achieved ( Fig. 1 ). The rate capability of the charge capacity is greatly improved with for instance more than 100 mAh g -1 available at 1C for the ball milled material vs. only 40 mAh g -1 for as prepared γ’-V 2 O 5 . While the structure of γ’-V 2 O 5 is not altered upon ball milling , a lower particle size combined with a decrease by a factor 3 of the crystallite size is observed as well as a modification of the discharge-charge profile, quite different from that of as-prepared γ’-V 2 O 5 ( Fig. 1 ). Such results suggest peculiar structural mechanism and kinetic features upon sodiation of the ball-milled material. The present work aims at understanding the origin of the enhanced electrochemical properties of ball-milled γ’-V 2 O 5 as cathode material for NIB. A detailed structural study as a function of sodium uptake will enlighten the typical electrochemical profile of ball-milled γ’-V 2 O 5 . Special attention will also be paid to the crucial kinetic parameters of electrochemical Na insertion reaction in ball-milled γ’-V 2 O 5 as a function of x in γ-Na x V 2 O 5 (0 ≤ x 〈 1), using ac impedance measurements. A comparative study of structure modifications (phase diagram) and kinetic parameters (double-layer capacity, charge transfer kinetics, Na diffusion, electrode impedance) for the balled-milled and un-milled materials allows explaining the promoting impact of crystallite size on electrochemical sodium insertion properties of γ’-V 2 O 5 . References 1- M. Safrany Renard, N. Emery, R. Baddour-Hadjean, J. P. Pereira-Ramos, γ’-V 2 O 5 : A new high voltage cathode material for sodium-ion battery, Electrochim. Acta 252 (2017) 4-11. 2- M. Safrany Renard, R. Baddour-Hadjean, J. P. Pereira-Ramos, Kinetic insight into the electrochemical sodium insertion-extraction mechanism of the puckered γ’-V 2 O 5 polymorph, Electrochim. Acta 322 (2019) 134670 Figure 1 . First discharge-charge galvanostatic cycle of g’-V 2 O 5 . (a) as-prepared powder; (b) ball-milled powder. C/10 rate. Electrolyte 1M NaClO 4 /PC, 2% Vol. FEC Figure 1