Extending phase-variation voltage zones in P2-type sodium cathodes through high-entropy doping for enhanced cycling stability and rate capability

P2-type layered sodium oxides hold great promise for grid-scale energy storage applications owing to their low-cost merit, while their detrimental structural and chemical transition lead to low reversible capacities and poor cycling stability. Herein, we leverage a high-entropy (HE) doping strategy to stabilize the P2-type Na0.667Mn0.667Ni0.167Co0.167O2 (NMNC) cathode. A novel Na0.667Mn0.667Ni0.167Co0.117Ti0.01Mg0.01Cu0.01Mo0.01Nb0.01O2 (HE-NMNC) composite, with five equal content (1 at %) of cation substitution and exhibiting a pure P63/mmc space group structure, is proposed. Interestingly, a reversible P2-type phase variation is identified for HE-NMNC at high degrees of charge, but such phase variation occurs in a wider voltage region and is much slower than that in the NMNC baseline. It is demonstrated that the HE-NMNC delivers outstanding rate capability and cycling stability under a charging cutoff voltage of 4.5 V, achieving a reversible capacity of 111 mAh/g at 5 C and retaining around 130 mAh/g after 100 cycles at 1 C. Apart from the charge compensation of main transition metals (Ni, Co, and Mn), the reversible redox of lattice oxygen also contributes to the capacity of HE-NMNC upon cycling. The proposed high-entropy doping strategy and reaction mechanism may provide new insights for developing advanced cathode materials.