Moderate active Fe³⁺ doping enables improved cationic and anionic redox reactions for wide-voltage-range sodium storage

Layered metal oxides are promising cathode materials for sodium-ion batteries (SIBs) due to their high theoretical specific capacity and wide Na⁺ diffusion channels. However, the irreversible phase transitions and cationic/anionic redoxes cause fast capacity decay. Herein, P2-type Na_0.67Mg_0.1Mn_0.8Fe_0.1O₂ (NMMF-1) cathode material with moderate active Fe³⁺ doping has been designed for sodium storage. Uneven Mn³⁺/Mn⁴⁺distribution is observed in NMMF-1 and the introduction of Fe³⁺ is beneficial for reducing the Mn³⁺ contents both at the surface and in the bulk to alleviate the Jahn–Teller effect. The moderate Fe³⁺/Fe⁴⁺ redox can realize the best tradeoff between capacity and cyclability. Therefore, the NMMF-1 demonstrates a high capacity (174.7 mAh g⁻¹ at 20 mA g⁻¹) and improved cyclability (78.5% over 100 cycles) in a wide-voltage range of 1.5–4.5 V (vs. Na⁺/Na). In-situ X-ray diffraction reveals a complete solid-solution reaction with a small volume change of 1.7% during charge/discharge processes and the charge compensation is disclosed in detail. This study will provide new insights into designing high-capacity and stable layered oxide cathode materials for SIBs.