Stabilizing high-voltage operation of layered oxide cathodes through ionic interdiffusion-triggered surface reinforcement for sodium-ion batteries

O3-type Ni/Mn-based layered oxides have shown promise for application in sodium ion batteries (SIBs), due to their ability to achieve high energy density at increased upper cut-off voltages through the synergistic cationic/anionic redox reactions. However, a series of interfacial side reactions usually leads to rapid capacity/voltage decay. Herein, we propose a surface reinforcement approach to enable stable 4.5 V high-voltage cycling of O3-type layered oxides. Through thermally induced ionic interdiffusion, Zn(H 2 PO 4 ) 2 acts as a surface reinforcement initiator, enabling the simultaneous achievement of triple effects: elimination of residual alkali, construction of surface coating, and implementation of surface doping. Zn 2+ doping constructs a special Zn–O–Na/vacancy configuration, enhancing oxygen redox reaction reversibility by increasing non-bonding O 2 p states and improving ionic/electronic transport. Additionally, the cubic-structured Na 3−2 x Zn x PO 4 (NZP) nanolayer facilitates rapid Na + transfer, restrains lattice oxygen release, and alleviates electrolyte decomposition during cycling. This surface reinforcement treatment diminishes localized asynchronous structural evolution and simplifies the phase transition process during cycling. The surface-reinforced O3-type Na 0.9 Cu 0.12 Ni 0.33 Mn 0.4 Ti 0.15 O 2 exhibits superior cycling stability (76.8% vs. 48.4% capacity retention over 200 cycles at 100 mA g −1 ) and rate capability (88.2 vs. 43.1 mAh g −1 at 2000 mA g −1 ) compared to the pristine counterpart at 4.5 V. This work presents a new approach to fortify the surface structure for developing high-voltage stable layered oxide cathode materials for SIBs.