Elucidating Highly Activated Transition Metal Redox Chemistry in Low-Nickel O3-Type Layered Oxide Cathodes for...

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_0.96Ni_0.2Mn_0.32Fe_0.4Mg_0.04Cu_0.04O₂ (NMFMC), which tunes the energy levels of transition metal (TM) 3d orbitals to enhance two-electron Ni²⁺/Ni⁴⁺ redox activity and facilitate cooperative Fe³⁺ 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.