Insights into the electrochemical behavior of layered oxides intercalated with Na or K

Layered transition-metal oxides, like those used in Li-ion batteries, remain popular candidate electrode materials for the emerging, low-cost Na- and K-ion battery technologies. However, shuttling the larger Na/K ions into and out of these compounds leads to two effects less commonly seen with Li: phase transitions between different stackings of the oxide layers and strong ion-vacancy orderings. Both phenomena are often detrimental, as they can result in mechanical degradation and sluggish ionic transport. To understand this behavior, we have performed first-principles studies of the layered Na_xCoO₂ and K_xCoO₂ cathode materials using configurational cluster expansions trained on density functional theory energies. We obtain excellent agreement with the experimentally observed voltage profiles and phase transitions upon cycling. In both systems, we predict several families of stable hierarchical orderings made up of antiphase boundaries between similar ordered regions. We find that K stabilizes unusual distorted phases at high concentrations due to strong K-K repulsion. Our results have important implications for diffusion and may be extended to other layered intercalation compounds.