Trace high-entropy doping enabling high-stability and high-energy-density sodium-ion batteries

Abstract

Layered oxides are regarded as promising cathode materials for sodium-ion batteries due to their high theoretical capacity and facile synthesis. Among these, the P2-type Na_2/3Ni_1/3Mn_2/3O₂ (NM) cathode has attracted significant interest due to its high theoretical capacity of approximately 173 mAh g⁻¹. However, when operated at high voltages, NM undergoes a severe irreversible phase transition from P2 to O2, accompanied by irreversible oxygen redox reactions, which leads to rapid capacity decay. To suppress this phase transition and enhanced reversibility of the oxygen redox reaction, this study introduces a synergistic doping strategy using Mg, Zn, and Zr, successfully preparing a high-performance cathode material, Na_0.67Ni_0.28Mg_0.025Zn_0.025Mn_0.62Zr_0.05O₂ (MZZ–NM). When assembled into a coin-type full cell with a hard carbon anode, the battery exhibits a high energy density of 321.6 Wh kg⁻¹ at 1C, with an energy retention of 82.6% after 100 cycles and an average discharge voltage maintained at 3.3336 V, showing negligible decay. Cyclic voltammetry and in situ XRD results collectively demonstrate that MZZ–NM possesses favorable cation/anion redox reversibility, excellent reaction kinetics, and structural stability. This study provides new insights into the design of high energy density cathode materials for sodium-ion batteries.