Tuning Na Occupation and Annealing Temperature on Na Diffusion Pathways in P2-Type Na_<i>x</i>Ni_0.25Mn_0.75O₂ for Sodium-Ion Batteries by Ab Initio Calculations

The Na_<i>x</i>Ni_0.25Mn_0.75O₂ cathode material draws ongoing interest owing to its considerable specific capacity along with its elevated average operating voltage. However, its application is limited by weak rate performance and quick capacity fading. In this study, a series of Na_<i>x</i>Ni_0.25Mn_0.75O₂ (<i>x</i> = 0.65, 0.70, 0.75, 0.80) cathode materials are prepared by the solid-phase method based on an orthogonal experiment. The optimum preparation process is investigated by optimizing factors such as Na content, calcination temperature, calcination time, and heating rate. The Na_e/Na_f ratio is adjusted, and the Na⁺ at different sites are rearranged to reduce the Na⁺/vacancy ordering by changing the Na content on the basis of process optimization. Owing to the fast migration kinetics of Na_e sites, experimental results indicate that the P2-Na_0.75Ni_0.25Mn_0.75O₂ cathode material, which has Na⁺ occupying more thermodynamically stable Na_e sites, demonstrates superior battery performance. High initial discharge specific capacity (157.2 mAh g^-1) along with favorable cycle performance of the Na_0.75Ni_0.25Mn_0.75O₂ cathode material can be achieved by modifying the Na_e site occupancy and optimizing the experimental conditions. Together with the microscopic control of Na⁺ occupancy, this novel orthogonal experiment design offers a fresh perspective and approach to a thorough comprehension of cathode materials for sodium-ion batteries.