Robust vanadium pentoxide electrodes for sodium and calcium ion batteries: thermodynamic and diffusion mechanical insights

It has long been a critical challenge to find suitable electrodes for rechargeable Na/Ca-ion batteries (NIBs/ CIBs) with superior electrochemical performance. Vanadium pentoxides offer the prospect of serving as cathodes in the development of high-capacity NIBs and CIBs. Here the concentration-dependent electrochemical characteristics of Na- and Ca-ions with alpha- and delta-V2O5 are examined using density functional theory with Hubbard U corrections. Multiple low energy configurations, stemming from the different ionic concentrations, are identified to evaluate the stability of alpha- and delta-V2O5 upon Na/Ca intercalation. It is computationally predicted that the a phase is more stable than the delta phase during both Na and Ca intercalation processes. Additionally, the energy barriers for Ca diffusion in alpha-V2O5 at high concentration are higher than that in delta-V2O5 (0.975-1.825 eV compared to 0.735-1.385 eV), which suggests that cycling V2O5 exclusively in the d phase may improve performance. More importantly, lower surface-to-bulk diffusion barriers of 0.498 and 0.846 eV are found for Na- and Ca-ion insertion at the (010) surface, which account for the improved electrochemical properties found in nanostructured V2O5 compared to their bulk counterparts. Our results provide crucial insights into the thermodynamic and electrochemical response of V2O5 to Na/Ca-ion intercalation, thus contributing to the design of high capacity NIBs/ CIBs.