First-Principles Study of Layered Transition Metal Oxides As a Cathode Material for Ca-Ion Intercalating Batteries

Abstract

The Ca-based rechargeable batteries are a promising direction in multivalent energy storage that requires development of the intercalating cathode materials capable of high voltage and capacity. Given that monovalent alkaline metals (e.g. K, Na) with similar ionic radius of Ca can be reversely stored in layered transition metal (TM) oxides, we infer that divalent Ca-ion can be as well. Here, we demonstrate on the basis of first-principles calculations that layered TM oxides exhibit favorable thermodynamic and kinetic properties that enable topotactic Ca intercalating reactions. From the calculated phase diagram, the layered CaXCo2O4with space groups of P1 and P21/m are stable at multiple Ca concentrations x ranging from 0 to 1 and show a smooth voltage plateau higher than 3V up to x=0.5. The Ca migration barriers calculated by nudged elastic band method can be as low as 0.36 eV and 0.27 eV at the dilute and high vacancy concentration limits, respectively. At the intermediate vacancy concentration, the calculated migration barriers depend on the local vacancy environments of diffusing atom, and stochastic analysis of Ca hopping events performed by ab initio molecular dynamics (AIMD) simulation has shown that the migration barriers are still lower than 0.32 eV. Therefore, the Ca diffusivity at room temperature extrapolated from the AIMD results is comparable to Li diffusivity, suggesting feasibility of use of layered CaxCo2O4 as multivalent cathode materials. We will also discuss the trends in thermodynamic and kinetic properties as the TM in the layered structure is substituted with other TM species.