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

Abstract β-V 2 O 5 , 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-V 2 O 5 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-V 2 O 5 //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 V 5+ ions. After 20 cycles, the discharge capacity retains 86% of the initial capacity. Concerning the reaction mechanism of high pressure β-V 2 O 5 upon sodium insertion/de-insertion, several phase transitions are inferred from the voltage-composition profile. Ex situ XRD reveals the appearance of several Na x V 2 O 5 phases in the 0 ≤ x ≤ 1 compositional range, which are closely related with the non-sodiated high pressure β-V 2 O 5 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-V 2 O 5 structure is not recovered upon full Na + extraction. Electrochemical performances are outstanding though.