Stabilizing the Structure and Enhancing the Kinetics of O3‐Type Layered Cathodes for High‐Performance Sodium‐Ion Batteries via Targeted Multi‐Element Synergistic Doping

ABSTRACT 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 (Cu ²⁺ /Ti ⁴⁺ ) doping at the transition metal (TM) sites and Ca ²⁺ introduction into the Na sites of O3‐type NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ (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.