Effects of Transition-Metal Mixing on Na Ordering and Kinetics in Layered P2 Oxides

Layered P2 oxides are promising cathode materials for rechargeable sodium-ion batteries. In this work, we systematically investigate the effect of transition metal (TM) mixing on Na ordering and kinetics in the NaxCo1-yMnyO2 model system using density functional theory (DFT) calculations. The DFT predicted 0K stability diagrams indicate that Co-Mn mixing reduces the energetic differences between Na orderings, which may account for the reduction of the number of phase transformations observed during cycling of mixed TM P2 layered oxides compared to single TM. Using ab initio molecular dynamics simulations and nudged elastic band calculations, we show that the TM composition at the Na(1) (face-sharing) site has a strong influence on the Na site energies, which in turns impacts the kinetics of Na diffusion towards the end of charge. By employing a site percolation model, we establish theoretical upper and lower bounds for TM concentration based on their effect on Na(1) site energies, providing a framework to rationally tune mixed TM compositions for optimal Na diffusion.