Eliminating high-voltage phase transitions and oxygen loss in O3-type sodium layered cathodes via reversible...

O3-type sodium layered oxides have emerged as leading cathodes for practical sodium-ion batteries owing to their attractive theoretical capacities and facile scalability. Elevating the cut-off voltage above 4.0 V overcomes their practical capacity limitations but triggers severe structural degradation and electrochemical deactivation. In this study, an oxygen-active and solid-solution integrated concept is proposed to design a robust O3-NaNi_0.35Fe_0.2Mg_0.05Mn_0.3Ti_0.1O₂ cathode, which delivers an impressive capacity of 162.1 mAh g^-1 at 4.3 V. Reversible oxygen redox endows long-lasting electrochemical activity, while solid-solution reactions enable near-zero lattice strain and fast Na⁺ transport. As a result, the detrimental P3→O1 phase transition is eliminated. Compared to pristine NaNi_0.5Mn_0.5O₂, the modified cathode exhibits superior capacity retention (76.0% vs. 21.3% after 200 cycles) and excellent rate capability (95.6 vs. 39.3 mAh g^-1 at 5 C). This work pushes practical O3-type sodium batteries toward their theoretical capacity and provides guidance for the design of high-voltage-tolerant layered cathodes.