Boosting the Structural Stability of High-Voltage Sodium Layered Cathodes via Site-Selective La/Ti Co-Doping

P2-type sodium layered oxide (Na_2/3Ni_1/3Mn_2/3O₂, NNMO) material is considered a potential cathode candidate for sodium-ion batteries (SIBs) due to their compact crystal structure, and high cationic and anionic redox at high voltage. However, their structural instability and capacity fading, due to P2-O2 phase transitions and lattice oxygen evolution, are significant challenges for their long-term cycling performance. To address these problems, co-doping is recognized as an effective technique. Here, a strategy to address some of the issues with the P2-type Na_2/3Ni_1/3Mn_2/3O₂ material following La/Ti co-doping at the Na and transition metal (TM) site is demonstrated. The modified electrode, LT-NNMO-0.01, exhibits remarkably improved electrochemical performance with high-voltage plateaus in the charging and discharging processes, indicative of a reversible oxygen redox reaction facilitated by enhanced TM─O bonding strength and suppressed oxygen migration. Systematic investigations including electrochemical, in situ X-ray diffraction (XRD) studies and Density functional theory (DFT) calculations demonstrate the effectiveness of this co-doping strategy in improving structural stability and minimizing volume change, allowing for reversible structural evolution of the material. The present study provides an effective pathway for designing layered metal oxide cathode materials toward advancing the energy density of sodium-ion batteries.