Annealing-Assisted Enhancement of Electrochemical Stability of Na-Preintercalated Bilayered Vanadium Oxide Electrodes in Na-Ion Batteries

Layered transition metal compounds with expanded interlayer regions, stabilized by structural water, often show high initial capacities but suffer from rapid capacity decay and poor rate capability in Na-ion batteries. High-temperature annealing, accompanied by phase transformation with the formation of more dense atomic structures, has been shown to improve electrochemical stability. However, the capacity of annealed materials decreases compared to their original forms. Here, we for the first time demonstrate that low-temperature annealing (260 °C under vacuum) can be used to achieve enhanced electrochemical stability of high capacity Na-preintercalated bilayered vanadium oxide (δ-NaxV2O5·nH2O) nanobelts, while preserving its open layered structure with expanded interlayer region available for insertion and diffusion of a large number of electrochemically cycled Na+ ions. Intriguingly, we demonstrate that using low-temperature vacuum annealing the interlayer water content can be varied in the 0.5 ≤ n ≤ 1.20 range without a significant change in the interlayer spacing. The mechanism of the thermally induced interlayer water loss is discussed. The improved capacity retention exhibited by low-temperature vacuum annealed δ-NaxV2O5·nH2O nanobelts is attributed to the partial removal of the structural water from the interlayer region, formation of additional bonds within the V–O bilayers, and increased stacking order of V–O bilayers. Low-temperature vacuum annealing is proposed as an efficient strategy to control interlayer water and advance electrochemical stability of the growing family of hydrated transition metal compounds used as electrodes in intercalation batteries.