Negative Enthalpy Doping Stabilizes P2-Type Oxides Cathode for High-Performance Sodium-Ion Batteries

P2-type Na_0.67Ni_0.33Mn_0.67O₂ (NNMO) as cathode material for sodium-ion batteries (SIBs) largely suffers from continuous accumulation of local stress caused by destructive structural evolution and irreversible oxygen loss upon cycling, leading to rapid capacity degradation. Herein, a strategy of negative enthalpy doping (NED), wherein transition metal (TM) sites are substituted with 0.01 mol each Sn, Sb, Cu, Ti, Mg, and Zn to increase the stability of the TM layers, is proposed. The robust structure of NED-NNMO significantly suppresses the P2 to O2 phase transition and improves the Na⁺ kinetics upon long-term cycling. Consequently, the NED-NNMO exhibits much smoothened voltage platforms and improved oxygen redox reversibility, thus considerably extended lifetime as compared with the pristine NNMO sample. The NED-NNMO delivers a high capacity of 138.9 mAh g⁻¹ with an operation voltage of 3.51 V under 0.1 C and prominent capacity retention of 94.6% after 100 cycles under 1 C, and 90.0% over 3000 cycles under ultra-high rate of 30 C, which is among the best over previous reports. Moreover, an ampere-hour scale pouch cell based on the NED-NNMO demonstrates an energy density of 139 Wh kg⁻¹. This work sheds light on a route of negative enthalpy doping to design high-performance sodium-ion batteries.