Elevating redox potential towards robust single-crystalline NaNi_1/3Fe_1/3Mn_1/3O₂ via chlorination engineering

The active surface chemistry of O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) results in poor air stability, leading to severe structural degradation upon exposure to air. Elevating the redox potential of the material can suppress the spontaneous deintercalation of Na⁺, thereby mitigating this issue; however, the underlying strategies and mechanisms for such enhancement remain unclear. In this study, we develop a chlorination treatment approach to elevate the redox potential of the single-crystalline NFM, and the modified sample (NFM-Cl) demonstrates enhanced electrochemical performance and air stability. In NFM-Cl, partial Cl⁻ is incorporated into O sites, expanding the Na⁺ channels, and the band gap between the transition metal (TM) 3d orbitals and the Fermi level is broadened to enhance the cationic redox potential of TM ions and reduce spontaneous sodium deintercalation. Simultaneously, the surface residual alkali of NFM-Cl is in situ transformed into a chlorinated interfacial layer, serving as a physical barrier to prevent direct contact between NFM-Cl and the electrolyte or humid air. Consequently, NFM-Cl exhibits a remarkable capacity retention of 89.1 % after 200 cycles at 1 C and 82.0 % after 15 days of exposure to a simulated air environment. This study presents a novel strategy for elevating the redox potential of cathode materials for sodium-ion batteries (SIBs) and elucidates the underlying mechanism, offering an air-stable oxide cathode material with high specific energy and long cycle life, along with an advanced SIB system. © 2025 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences