Stabilizing the Structure and Enhancing the Kinetics of O3-Type Layered Cathodes for High-Performance Sodium-Ion...

Despite their high specific capacity, O3-type layered oxides face challenges of structural instability and sluggish kinetics as cathode materials for sodium-ion batteries (SIBs). Herein, we report a targeted synergistic strategy involving dual-element (Cu²⁺/Ti⁴⁺) doping at the transition metal (TM) sites and Ca²⁺ introduction into the Na sites of O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) to address their inherent defects. This multi-site modification effectively stabilizes the crystal structure, alleviates the phase transition amplitude, suppresses irreversible oxygen loss even at high voltage (up to 4.2 V vs. Na⁺/Na), and enhances Na⁺ migration by reducing the migration barrier and interfacial impedance. As a result, the optimized cathode (CCT) delivers a high reversible capacity of 125.3 mAh g^-1 with 76.9% capacity retention after 300 cycles (vs. 64.1% for NFM). Even at a high current density of 2000 mA g^-1 (∼13 C), the CCT cathode cycled at 4.2 V delivers a remarkable capacity of 94.3 mAh g^-1, demonstrating excellent rate capability. The CCT//HC full-cell demonstrates excellent performance, achieving a high initial capacity (123.5 mAh g^-1) and outstanding cycling stability (76.2% capacity retention) over 300 cycles. This work underscores the efficacy of multi-site synergistic doping strategy in designing high-performance layered oxide cathodes for practical SIBs.