Bulk-Interface Dual Strategy for Enhancing the Cycling Stability of P2-Type Layered Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are attracting attention as cost-effective alternatives to lithium-ion batteries (LIBs) for large-scale energy storage. Among SIB cathodes, P2-Na_0.67Ni_0.33Mn_0.67O₂ delivers high capacity and rate capability but suffers from rapid capacity fading under high-voltage charging due to a detrimental P2-O2 phase transition and interfacial side reactions. Here, we demonstrate a dual-modification strategy combining Cu²⁺ doping and MgO surface coating to address these challenges. The dual-modified cathode (Na_0.67Ni_0.28Cu_0.05Mn_0.67O₂@MgO) delivers markedly improved performance: a high-capacity retention of 90.88% after 200 cycles at 1 C and significantly enhanced rate capability (95.23 mAh g^-1 at 10 C). Ex situ XRD analyses reveal that the P2-O2 phase transition is effectively suppressed, leading to minimal structural change during cycling. DFT calculations reveal that the Cu-MgO dual modification synergistically enhances the electronic conductivity of the electrode and suppresses transition-metal layer gliding. The results indicate that Cu²⁺ doping enhances structural stability by regulating Na⁺/vacancy ordering and suppressing the high-voltage phase transition, whereas the MgO coating alleviates electrolyte-induced surface degradation and enhances Na⁺ diffusion kinetics. This work offers a valuable reference for designing high-performance cathode materials in sodium-ion battery systems.