Abstract
The recent intensive research for cathode materials beyond Li-ion batteries has revitalized interest in V2O5 due to its high reversible capacity. Among the various polymorphs, γ′-V2O5 exhibits a unique corrugated layered structure that promotes the insertion of guest species. However, when used as cathode material for SIB, this material suffers from a 50% first charge efficiency that prevents the full benefit of its high discharge capacity of 140 mAh g−1. Herein, we demonstrate and explain the effectiveness of a ball-milling approach to overcome this strong limitation. Several positive impacts of particle size reduction are highlighted: the charge efficiency is increased to 90%, allowing a 2-fold enhancement of the available capacity upon cycling (120 mAh g−1 after 50 cycles at C/2). The Na insertion mechanism investigated by XRD and Raman spectroscopy shows a peculiar behavior with wide solid solution domains at the expense of the diphasic region. The kinetics study reveals a faster diffusivity in the ball-milled material and enhanced Na diffusion in the single-phase region. Both structural and kinetic reversibility account for the high performance here achieved for γ′-V2O5: a high working voltage of 3.2 V vs Na+/Na, a high rate capability, excellent charge efficiency and good cycle life.
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