High-Entropy Structural Modification Strategy for O3-type Layered Sodium-Ion Cathode Materials with Enhanced Electrochemical Performance

O3-type layered oxide materials are considered one of the most promising cathode materials for sodium-ion batteries due to their high capacity. However, the structural evolution of the O3-type layered oxide during cycling is more complex, exhibiting poorer structural reversibility. In order to achieve a high electrochemical performance of O3-type sodium-ion cathode materials, we adopted a high-entropy strategy to realize multielement codoping of Mg, Co, and Ti into the transition metal layer of O3-type NaNi0.4Fe0.2Mn0.4O2 and prepared a high-entropy cathode material of NaNi0.35Mg0.05Fe0.15Co0.05Mn0.35Ti0.05O2 (NMFCMT-2). The transition metal layer doped with multiple elements Mg, Co, and Ti not only stabilizes the crystal structure and widens the layer spacing but also weakens the Na–O bonding, thereby facilitating sodium-ion diffusion and mitigating volume changes during the charge–discharge process. In situ XRD results show that this strategy can delay the phase transition of O3–P3 and realize the phase transition of P3–O3 in advance during the discharge process so that NMFCMT-2 can maintain the O3 phase to a greater extent during the cycles. The results show that the capacity retention rate was 80.6% after 200 cycles at 1C and 88.6% after 200 cycles at 5C. In addition, its high voltage cycling stability is also significantly improved compared to the pristine NaNi0.4Fe0.2Mn0.4O2.