Enhanced structural stability of P2-type Mn/Fe-based layered oxide by high entropy doping towards long-life sodium ion battery cathode

Mn/Fe-based layered oxides are supposed to be suitable as low-cost cathode materials for sodium ion batteries (SIBs) since they are of high specific capacity and wide availability of Mn and Fe sources in nature. However, these materials generally suffer from complex and irreversible phase transitions. The manufacture of high entropy materials can solve these matters. Herein, we rationally design and synthesized high entropy doped P2-Na0.62Ca0.03Mn0.58Fe0.23Cu0.085Mg0.01Ti0.015Li0.08O2 (NFM-HEO) material. As cathode for SIB, it exhibits enhanced long cycling stability and better rate capability compared to the undoped P2-Na0.65Mn0.6Fe0.4O2 material within 2.0 similar to 4.3 V. After 500 cycles at 1C, the capacity retention rate reaches 85.4 %, and it can provide a reversible specific capacity of 134 mAh g(-1) at 0.1C and 70 mAh g(-1) at 5C. In-situ XRD, diffusion kinetics results together with microstructure and phase characterization before and after cycling indicate that high entropy doping enhances structural stability of layered oxides, suppresses irreversible phase transitions, stabilizes Na+ diffusion paths, ensures reversible anionic redox reactions, inhibits crack generation after cycling, prevents transition metal dissolution, and improves humid air stability. This study is expected to offer guidance for the improvement of structural stability of layered cathode for SIBs.