Effect of sodium content on the electrochemical performance of P2‐Na₂Ni₂TeO₆ layered oxide cathode for sodium‐ion batteries

Abstract Sodium‐ion batteries (SIBs) employ P2‐type layered transition metal oxides as promising cathode materials, primarily due to their abundant natural reserves and environmentally friendly characteristics. However, structural instability and complex phase transitions during electrochemical cycling pose significant challenges to their practical applications. Employing cation substitution serves as a straightforward yet effective strategy for stabilizing the structure and improving the kinetics of the active material. In this study, we introduce a Ni‐rich honeycomb‐layered Na_2+xNi₂TeO₆ (NNTO) cathode material with variable sodium content (x = 0, 0.03, 0.05, 0.10). Physicochemical characterizations reveal that excess sodium content at the atomic scale modifies the surface and suppresses phase transitions, while preserving the crystal structure. This results in enhanced cyclic performance and improved electrochemical kinetics at room temperature. Furthermore, we investigate the performance of the NNTO cathode material containing 10% excess sodium at a relatively high temperature of 60°C, where it exhibits 71.6% capacity retention compared to 60% for the pristine. Overall, our results confirm that a preconstructed surface layer (induced by excess sodium) effectively safeguards the Ni‐based cathode material from surface degradation and phase transitions during the electrochemical processes, thus exhibiting superior capacity retention relative to the pristine NNTO cathode. This study of the correlation between structure and performance can potentially be applied to the commercialization of SIBs.