Ca Cobaltites as Potential Cathode Materials for Rechargeable Ca-Ion Batteries: Theory and Experiment

Abstract

Rechargeable Ca-based batteries can potentially achieve higher energy capacity than Li-ion batteries. The development of Ca-ion batteries, however, remains in its infancy, especially due to the challenge of finding cathode materials with high reversible capacity. We investigate properties of Ca cobaltites as Ca-ion intercalation cathodes, by means of density functional theory calculations validated by synthesis and electrochemical measurements. Computationally, we accessed thermodynamic, diffusion properties, energy capacity, and the voltage profiles for four Ca cobaltite compounds of different stoichiometry and Co oxidation states; Ca3Co2O6, CaCo2O4, Ca2Co2O5, and (Ca2CoO3)(CoO2)1.62. We found good stability and relatively low migration barriers of some Ca cobaltites during cycling with the layered CaCo2O4 having the lowest Ca migration barrier of 0.7 eV and the highest theoretical capacity. To validate our calculations, we synthesized Ca3Co2O6, CaCo2O4, and (Ca2CoO3)(CoO2)1.62 with the Pechini method and, subsequently, to test the electrochemical extraction of Ca.