Unlocking Iron Redox Depth for High‐Energy Layered Sodium Oxide Cathodes

ABSTRACT High‐capacity O3‐type layered NiFeMn‐based oxides are promising cathodes for sodium‐ion batteries, though their practical deployment is constrained by the inherent limitations of Fe redox chemistry. Traditional designs generally enforcing stoichiometric symmetry (Ni ═ Mn) yield low Fe redox activity. Herein, we propose a valence engineering strategy that breaks conventional Ni/Mn stoichiometry to reconfigure Fe's local chemical environment and unlock unprecedented redox depth. Density functional theory (DFT) calculations reveal that the designed NaNi 0.35 Fe 0.225 Mn 0.425 O₂ cathode exhibits a reduced Bader charge on Fe (1.598 vs. 1.638 in NaNi 1/3 Fe 1/3 Mn 1/3 O 2 ) and elevated Fe 3 d orbital energy, signifying enhanced Fe redox activity. This configuration enables an exceptional Fe 2.60+ /Fe 3.88+ redox (1.28 e − per Fe), delivering a reversible capacity of 184.3 mAh g −1 within 2–4.2 V at 0.2 C, markedly exceeding the benchmark NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (161.3 mAh g −1 ) with low reaction depth of Fe 3.01+ /Fe 3.61+ . The intensified cationic redox reaction enables an ultrahigh energy density of 596 Wh kg −1 . The NaNi 0.35 Fe 0.225 Mn 0.425 O 2 cathode demonstrates robust performance over a broad temperature range from −15°C to 60°C. In situ and ex situ characterizations unveil a reversible O3 ↔ P3 ↔ OP2 phase transition with minimal volume change (1.88%) that circumvents detrimental deleterious O′3 intermediates and intragranular cracking. This work establishes valence engineering as a paradigm to consolidate cationic redox reaction in high‐energy layered sodium oxide cathodes.