Revealing mechanism responsible for structural reversibility of single-crystal VO₂ nanorods upon lithiation/delithiation

A pure phase of VO₂(B) nanorods have been synthesized through an energy-efficient microwave hydrothermal reaction and used as cathode materials of lithium ion batteries, which exhibit promising specific capacity (e.g., 130 mA h g⁻¹ even after 100 charge/discharge cycles) and rate capacity (e.g., ~130 mA h g⁻¹ at a high current of 400 mA g⁻¹). The excellent cyclability originates from the structural reversibility of VO₂(B) upon lithiation/delithiation that is confirmed by the in situ high-energy synchrotron X-ray diffraction (HEXRD) and in situ x-ray adsorption near-edge spectroscopy (XANES) of the VO₂ nanorods in operating battery cells. The real-time results reveal that discharge forces lithium ions to insert firstly into the tunnels with the largest size along b direction followed by the second largest tunnels along c direction, which is completely reversible in the charge process.