Understanding the Thermodynamic Role of Cation Substitution in P2-Na _0.67 Mn _0.8 M _0.2 O ₂ Cathodes via Atomistic Calculations

P2-type Na0.67MnO2 cathodes suffer from complex phase transitions during cycling, limiting their practical application. In this work, first-principles calculations are employed to systematically investigate the effects of partial cation substitution in Na0.67Mn0.8M0.2O2 (M0.2-NMO) on thermodynamics, structural evolution, and phase transition evaluation. Our results reveal that cation substitution exerts only a minor influence on the thermodynamic driving forces of phase transitions owing to the marginal formation energy differences among the P2, OP4, and O2 phases. However, cation substitution has a much larger effect on structure-related factors (octahedral distortion, bond-length distortion, and shear deformation) and the tetrahedral transition states arising from MO6 octahedral rotations and layer slipping, suggesting that structure regulation is a more effective strategy for enhancing phase stability. Our analysis of the P2-to-O2 transition reveals that the Mg- and Co-NMO systems exhibit significantly higher barriers than pure NMO, whereas nearly all other substituted systems result in increased slipping barriers. We also investigate the effect of cation substitution on the redox mechanism during desodiation and identify the activation of oxygen redox at high states of charge. Overall, this study provides mechanistic insights into how cation substitution governs structural evolution, phase stability, and electrochemical behavior of layered sodium cathodes, offering valuable guidance for the rational design of high-performance sodium-ion battery materials.