Oxygen-Configuration-Driven Anionic Redox Regulation for High-Voltage NaNi_1/3Fe_1/3Mn_1/3O₂

To boost the energy density of sodium-ion batteries (SIBs), elevating the operating voltage (≥4.2 V) of NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathodes is imperative, while inherently triggers irreversible anion redox reactions (ARR). Unfortunately, the activation mechanism of ARR in NFM has yet to be clearly elucidated, and the targeted regulation remains a significant challenge. Herein, the oxygen redox course in NFM is investigated, and the activation of ARR can be attributed to the generation of □-O-□ and □-O-Na configurations with high-energy unhybridized O 2p nonbonding orbitals, which origins from the Fe³⁺ migration from the transition metal layer to the Na layer. To mitigate the formation of ARR-activated oxygen configurations, a pre-occupation strategy in the Na layer is proposed to suppress Fe³⁺ migration, which significantly reduces O-O dimerization and oxygen evolution. Meanwhile, the activity and reversibility of Fe redox is markedly improved by the stabilized layered structure. Benefiting from the robust lattice oxygen, the modified sample exhibits excellent cycling performance with an ultrahigh capacity retention up to 70% after 1500 cycles at 5 C. This work establishes a direct link between oxygen configuration evolution and ARR irreversibility and provides a rational design principle for high-voltage NFM.