Unveiling the role of base layer in the fluorinated cathode interface for robust single-crystal Na-layered oxide

Fluorinated interface engineering has emerged as a viable strategy for designing high-performance layered oxide cathodes and sodium-ion batteries (SIBs). Nevertheless, the rational approaches for the interface fluorination of oxide cathodes remain controversial. In particular, the influence exerted by various surfaces of Na-layered oxides as the base layer on the fluorinated interface remains an unresolved issue. Herein, the cathode surface fluorination engineering (CSFE) is proposed to modulate the state of the base layer and the fluorinated electrolyte additive engineering (FEAE) is adopted to synergistically construct fluorinated cathode interface on the single-crystal NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM). CSFE converts the surface Na₂CO₃ layer into a NaF layer in-situ with partial F surface doping, which gives rise to a NaF-rich cathode-electrolyte interface (CEI) with enhanced stability during cycling even in the fluorine-free carbonate-based electrolyte and makes surface fluorinated oxide (NFM@F) possess higher coulombic efficiency and better cycling stability than the oxide with Na₂CO₃ base layer. While in the electrolyte with fluorinated additive, a NaF-rich CEI that is thinner, more uniform and denser can be formed on the NaF base layer of NFM@F (NFM@F-FE) than on the Na₂CO₃ based layer, thereby significantly reducing side reactions, shielding the layered structure, mitigating structural change during charge-discharge, and manifesting exceptional structure stability. Consequently, NFM@F-FE exhibits a remarkable capacity retention of up to 94.26 % even after 200 cycles at 1 C.