Elucidating Highly Activated Transition Metal Redox Chemistry in Low‐Nickel O3‐Type Layered Oxide Cathodes for Sodium‐Ion Batteries

Abstract Low‐nickel O3‐type layered oxides have emerged as cost‐effective cathode candidates for sodium‐ion batteries (SIBs). However, their practical viability is challenged by rapid capacity decay and insufficient redox activity within safe voltage windows. Here, we report a multi‐cationic compositional regulation strategy for Na 0.96 Ni 0.2 Mn 0.32 Fe 0.4 Mg 0.04 Cu 0.04 O 2 (NMFMC), which tunes the energy levels of transition metal (TM) 3d orbitals to enhance two‐electron Ni 2+ /Ni 4+ redox activity and facilitate cooperative Fe 3+ oxidation. This approach unlocks a 17% capacity enhancement (2.0–4.0 V) over conventional low‐Ni cathodes while maintaining structural integrity. Operando measurements and theoretical calculations demonstrate that the reinforced TM─O bonding upon Mg/Cu co‐doping mitigates structural distortion and suppresses multiphase transitions, thereby enabling superior cycling stability. A 2.65 Ah NMFMC||hard carbon pouch cell maintains 80% capacity after 1600 cycles at 1C and preserves 94% capacity when cycled from 0.5C to 4C, demonstrating practical potential for grid‐scale storage. By elucidating the interplay between orbital hybridization, redox chemistry, and structural evolution, this work establishes fundamental design principles for high‐energy, durable SIB's cathodes while advancing sustainable large‐scale energy storage solutions.