Synergistic enhancement of oxygen redox activity and structural integrity through Li/F doping in layered oxide cathodes for sodium-ion batteries

Abstract

Utilizing oxygen redox in layered oxide cathodes offers a pathway to exceed the capacity limits of conventional cationic redox sodium-ion batteries, yet its poor reversibility and oxygen loss lead to severe capacity fading. Here, we design a Li and F co-doped P2-type cathode, Na0.7Li0.1Mg0.15Mn0.75O1.9F0.1 (NLMMOF), to simultaneously enhance and stabilize oxygen redox activity. Li substitution promotes oxygen redox via the Na–O–Li configuration, while F substitution stabilizes the oxygen redox by strengthening the metal-anion bonding. NLMMOF exhibits a high discharge capacity of 191.96 mAh g⁻¹ at 0.05 C (1.5–4.5 V) and retains 85.1 % of its capacity over 100 cycles at 0.5 C. Multiple analyses confirm that the enhanced electrochemical performance of NLMMOF is due to suppressed P2-O2 phase transition, minimal local structural distortion, and negligible oxygen evolution. DFT calculations further reveal that F substitution raises the Mn migration barrier at deep charge, mitigating Mn in-plane migration and under a coordinated oxygen lattice. This synergistic Li/F co-doping strategy provides a practical design principle for stabilizing oxygen redox in layered oxide cathodes, advancing the development of high-energy, long-life sodium-ion batteries.