Sodium insertion in high pressure β-V2O5: A new high capacity cathode material for sodium ion batteries

Abstract

β-V2O5, obtained by a high-temperature high-pressure method, exhibits a layered structure that favours the insertion of Na+. In this work, we report the electrochemical insertion of sodium in high pressure β-V2O5 and its performance as cathode material for sodium-ion batteries. The material shows a first discharge capacity of 132 mAh g−1 in the 3.6–2.0 V range at a C/20 current density and a maximum capacity of 147 mAh g−1 under equilibrium conditions, corresponding to the insertion of 1 Na+ ion per formula unit. The β-V2O5//Na cell delivers a specific energy as high as 370–410 Wh kg−1. The amount of inserted sodium points to the reduction of 50% of the available V5+ ions. After 20 cycles, the discharge capacity retains 86% of the initial capacity. Concerning the reaction mechanism of high pressure β-V2O5 upon sodium insertion/de-insertion, several phase transitions are inferred from the voltage-composition profile. Ex situ XRD reveals the appearance of several NaxV2O5 phases in the 0≤x≤1 compositional range, which are closely related with the non-sodiated high pressure β-V2O5 structure. An irreversible structural transformation occurs during the very first inserted sodium, at the beginning of the first discharge, and the original high pressure β-V2O5 structure is not recovered upon full Na+ extraction. Electrochemical performances are outstanding though.