Origin of the Enhanced Electrochemical Performance of Ball-Milled g’-V2O5 Puckered Layer Polymorph for Na-Ion Battery: A Kinetic and Structural Study
Authors: Baddour-Hadjean Rita , Safrany-Renard Marianne , Pereira-Ramos Jean-Pierre 
Published: May 2020

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 V2O5 polymorphs towards Na insertion with promising performance revealed for the γ’-V2O5 polymorph in terms of specific capacities and cycle life [1]. γ’-V2O5 is prepared by chemical oxidation of the γ-LiV2O5 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’-V2O5 platelets. γ’-V2O5 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’-V2O5. While the structure of γ’-V2O5 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’-V2O5 (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’-V2O5 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’-V2O5. Special attention will also be paid to the crucial kinetic parameters of electrochemical Na insertion reaction in ball-milled γ’-V2O5 as a function of x in γ-NaxV2O5 (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’-V2O5.

References
1- M. Safrany Renard, N. Emery, R. Baddour-Hadjean, J. P. Pereira-Ramos, γ’-V2O5: 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’-V2O5 polymorph, Electrochim. Acta 322 (2019) 134670

Figure 1. First discharge-charge galvanostatic cycle of g’-V2O5. (a) as-prepared powder; (b) ball-milled powder. C/10 rate. Electrolyte 1M NaClO4/PC, 2% Vol. FEC

Figure 1