Layered Hierarchical Modification Strategy in P2-Type Oxide Cathodes Enables High-Rate Capability and Long-Term Stability for Sodium-Ion Batteries

Abstract

P2–Na_0.67Ni_0.33Mn_0.67O₂ cathodes have been widely applied in sodium-ion batteries (SIBs). However, this material faces three inherent critical challenges: irreversible P2–O2 phase transitions caused by excessive desodiation above 4.2 V, unfavorable Na⁺/vacancy ordering at specific sodium concentrations, and irreversible anion redox reaction during high-voltage operation. In this work, a layered hierarchical modification strategy is well-designed to solve all the above problems. F substitution at oxygen sites significantly enhances the reversibility of anionic redox reactions at high voltages. Li incorporation into transition metal sites promotes cationic disorder within the TM layer, effectively inhibiting Na⁺/vacancy ordering. Meanwhile, the copresence of Li and F can strengthen cation–anion interactions and increase the TM–O bond strength, further enhancing the structural stability of the P2-NaNM material. The substitution of sodium sites by partial Mg mitigates repulsive forces between adjacent oxygen layers under high-voltage conditions, and enhances the O–Na–O electrostatic cohesion between adjacent TM-O layers, thereby impeding irreversible P2–O2 phase transitions. As a result, the optimized P2–Na_0.67Ni_0.25Li_0.08Mn_0.57Mg_0.10O_1.93F_0.07 (Mg-NaNLMF) cathode exhibits exceptional electrochemical performance, delivering a capacity retention of 98.07% over 60 cycles at 0.1C. More impressively, it maintains 81.72% of the initial capacity after ultrafast charge/discharge processes of 1000 cycles at 10C. This work establishes a new paradigm for designing high-performance layered oxides in sodium-ion batteries through a layered hierarchical modification strategy.