Bromine-triggered anion redox and strain buffering boost layered oxide cathodes for sodium-ion batteries

The pursuit of higher charge-cut-off potentials and greater capacities in cathode materials is essential for sodium-ion batteries (SIBs) to achieve higher energy densities, thereby expanding their application domains. However, for O3-type layered oxides, charging beyond 4.0 V often triggers irreversible oxygen redox and structural degradation, leading to rapid capacity decay. Here, we demonstrate an anion optimization strategy by designing Br-doped NaNi 1/3 Fe 1/3 Mn 1/3 O 2-x Br x (NFMBr) cathodes. The introduced Br, which has lower electronegativity, hybridizes its 4p orbital with the O 2p orbital, raising the O 2p band center toward the Fermi level and enhancing reversible anion redox activity. Concurrently, the weaker transition metal–Br bond buffers against lattice strain, suppressing detrimental phase transitions. Consequently, the Br-modified NFMBr cathode delivers 190.8 mAh g –1 at 0.1 C and retains 98.9 mAh g –1 at 10 C, compared to only 27.3 mAh g –1 for the unmodified NFM at 10 C, providing novel insights for high-performance layered oxide cathode design.