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 α- and δ-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 α- and δ-V₂O₅ upon Na/Ca intercalation. It is computationally predicted that the α phase is more stable than the δ phase during both Na and Ca intercalation processes. Additionally, the energy barriers for Ca diffusion in α-V₂O₅ at high concentration are higher than that in δ-V₂O₅ (0.975-1.825 eV compared to 0.735-1.385 eV), which suggests that cycling V₂O₅ exclusively in the δ 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.