Stabilized O3-Type Layered Oxide Cathode via High-Entropy Engineering

O3-type layered transition metal oxides are promising cathodes for sodium-ion batteries due to their high initial sodium content and theoretical capacity. However, they suffer from irreversible phase transitions, sluggish Na⁺ diffusion, and lattice strain during cycling, leading to rapid capacity fading and poor rate performance. Herein, we report a high-entropy O3-type layered oxide cathode, NaNi_0.24Fe_0.24Mn_0.24Li_0.07Mg_0.07Ti_0.07Sb_0.07O₂ (HEO), designed by incorporating near-equiatomic redox-active Ni/Fe/Mn with low-content inactive Li/Mg/Ti/Sb dopants to maximize configurational entropy while maintaining charge neutrality and ionic radius matching. Compared to the benchmark NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM), HEO exhibits single-phase formation, expanded interlayer spacing, narrowed bandgap (from 1.35 to 0.63 eV), and reduced Na⁺ migration barriers. This enables superior rate capability (63.57 mAh g^-1 at 10 C with 49.0% retention) and exceptional cycling stability (80.71% retention after 300 cycles at 1 C; 80.09% after 1000 cycles at 5 C). In situ XRD reveals a highly reversible O3 ↔ P3 ↔ OP2 phase transition sequence, suppressing the irreversible O'3 phase in NFM. These improvements stem from entropy stabilization and synergistic multi-cation effects, demonstrating high-entropy engineering as an effective strategy for developing robust, long-lifespan sodium-ion battery cathodes.