Surface Engineering through In Situ Construction of Co_xB-Spinel Dual Coating Layers for High-Voltage Stable Sodium-Ion Batteries

Transition metal layered oxides (NaxTMO2), boasting a high theoretical specific capacity and affordability, have emerged as prominent cathodes for sodium-ion batteries (SIBs). Their potential, however, is hindered when operating at higher voltage range (4.0-4.3 V) due to irreversible phase transition, heterogeneous surface reconstruction, and side reaction. Herein, using a straightforward room-temperature liquid-phase reductive method, a dual conformal protective layer is in situ constructed on the surface of NaNi1/3Fe1/3Mn1/3O2(NFM). This layer comprises both a spinel structure and an amorphous CoxB coating, thereby achieving a layered-spinel-CoxB configuration. The spinel structure provides 3D Na+ transport channels and works as a pillar to anchor the intrinsic layered structure. Simultaneously, the external CoxB layer effectively mitigates O loss, transition metal ion dissolution, and undesired side reactions on the surface. Benefiting from the synergistic effects on both the material's bulk and surface, the 1wt% CoxB coated cathode displays superior stability. After 300 cycles, the capacity retention is 79.6% between 2 and 4 V, significantly outperforming pristine-NFM's(p-NFM) 51.4%. When charged to 4.3 V, its capacity retention stands at 70%, much higher than that of p-NFM (51.2%). This work provides new insights into exploiting high-voltage stable cathode through constructing a dual conformal protective layer for high energy density SIBs.