Boric Acid-Assisted Thermal Restoration of an Air-Degraded O3-Type Sodium Layered Oxide Cathode

Abstract Air-induced degradation of O3-type sodium layered oxide cathodes compromises their structural integrity and electrochemical performance, presenting a major obstacle to practical handling and deployment. Here, we develop an H3BO3-assisted thermal restoration strategy for air-degraded O3-NaNi1/3Fe1/3Mn1/3O2. Thermal annealing restores bulk lattice ordering, whereas H3BO3 converts surface alkaline residues into B2O3-rich surface layer that limits Na loss and forms a protective surface layer. Experimental characterization and density functional theory calculations further suggest that a fraction of B occupies Na–O interstitial environments, modulating local coordination and strengthening TM–O orbital hybridization to facilitate Na+ transport and stabilize the layered framework. During cycling, the restored surface is associated with the formation of a thinner and more uniform cathode–electrolyte interphase. Upon renewed air exposure, the B2O3-rich surface layer reversibly hydrates to H3BO3, which preferentially interacts with H2O through hydrogen bonding and serves as a dynamic moisture buffer. Consequently, R-NFM@5%B delivers 133 mA h g–1 at 0.1 C, corresponding to 108% of the pristine NFM capacity, and retains 88% of its capacity after 400 cycles at 1 C. Even after 30 days of air exposure, it retains 87% of its capacity after 200 cycles at 1 C. This work demonstrates that chemically assisted thermal restoration can recover degraded layered oxide cathodes while imparting resistance to subsequent environmental degradation.