Structural Regulation of P2-Type Layered Oxide with Anion/Cation Codoping Strategy for Sodium-Ion Batteries

P2-type layered transition metal oxides are potential cathodes for sodium-ion batteries (SIBs), but they commonly suffer from severe capacity degradation owing to multiple phase transitions and Na+/vacancy ordering during the extraction/insertion process. An anionic/cationic co-doping strategy at high sodium contents is proposed to effectively achieve high-rate and long-term stability of P2-Na0.67Ni0.33Mn0.67O2. The resulting Na0.75Mg0.1Ni0.23Mn0.67O1.95F0.05 (NMNMOF) cathode delivers a reversible capacity of 116 mAh g-1 at 75 mA g-1 and maintains an initial capacity of 73% at 1500 mA g-1 after 1000 cycles. The Mg/F anionic/cationic co-doping strategy impacts the local environment of the surrounding transition metal and oxygen, regulates the electron distribution, and modifies the initial diffusion state of Na sites, enhancing the diffusion ability of Na+. Moreover, the phase transition of P2-O2 is well suppressed and the decrease in Mn3+ content greatly alleviates the Jahn-Teller effect to enhance structural stability. The full-cell devices with NMNMOF cathode and hard carbon anode demonstrate a high capacity of 80 mAh g-1 at 10 C and an excellent cycle life of over 500 cycles for applications. The anionic/cationic co-doping strategy will inspire the rational design of P2-type layered oxides and provide a new perspective for advanced SIBs.