Phase Stability and Na Ordering in Layered NaxCoO2 Cathode Material
Jonas Kaufman, University of California, Santa Barbara
Sodium-ion batteries offer a cheaper, safer alternative to their lithium-ion counterparts. However, additional structural phase transitions upon cycling can lead to faster mechanical degradation in Na-ion electrode materials. Also, ordering of Na within a given structure greatly influences the voltage profile and ion transport. We have performed a first-principles statistical mechanics study of the layered NaxCoO2 cathode material, which exhibits several stable host structures related by gliding of CoO2 planes. Phase stability between these structures and their hybrids is presented, as well as intricate ground state Na-ordering phenomena. We find that the ground states tend to remain ordered at room temperature and we compare our calculated voltage profile to experiment. This study was conducted using CASM (A Clusters Approach to Statistical Mechanics), an open-source software package for modeling multi-component crystalline solids with almost arbitrary degrees of freedom. Formation energies were calculated with density functional theory and effective Hamiltonians were fit with a genetic algorithm. These Hamiltonians were then used to predict ground states and to determine finite temperature properties through grand canonical Monte Carlo simulations.
Abstract Author(s): Jonas Kaufman, Anton Van der Ven