Adaptive lattice breathing enabled by Cu/Mg co-doping for stable anionic redox chemistry in sodium layered oxides

Advancing the energy density of sodium-ion batteries requires layered oxide cathodes with higher specific capacity, necessitating redox chemistry beyond conventional cations. Oxygen anionic redox offers a pathway but presents inherent challenges, including irreversible structural degradation, such as Jahn-Teller distortion. Here, we report that cooperative Cu/Mg co-doping triggers an adaptive lattice respiration mechanism that concurrently suppresses structural distortion and unlocks highly reversible anionic redox. Through in situ spectroscopy, we visualize that this dynamic process involves the oxidation of Cu²⁺ to Jahn-Teller inactive Cu³⁺, which induces a predictable lattice distortion, while Mg²⁺ orchestrates a compensatory symmetric breathing of the oxygen framework. This respiration effectively mitigates structural strain and preserves the layered integrity. Consequently, the P3-Na_0.67Mn_0.9Mg_0.05Cu_0.05O₂ enables a remarkable reversible capacity of 258.1 mAh g^-1. It retains 75.3% capacity after 80 cycles at 5.0C, demonstrating that adaptive lattice respiration is a viable strategy for achieving stable anionic redox chemistry.