Anion-regulated adsorption and solvation chemistry for high-voltage sodium-ion batteries

Constructing a robust cathode-electrolyte interphase (CEI) under high-voltage operation is essential for enabling high-energy-density sodium-ion batteries (SIBs). However, the underlying CEI formation mechanisms, governed by the complex interfacial interactions among electrolyte components, remain poorly understood. Herein, we report that the strongly electron-donating difluorophosphate (PO₂F₂^-) anion is highly effective in forming a stable CEI on the surface of layered O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathodes. This anion strengthens cation-anion interactions, reconstructs the primary solvation sheath, and promotes preferential interfacial adsorption. Its preferential decomposition yields a thin inorganic-rich CEI dominated by NaF and Na₃PO₄, which mitigates parasitic reactions and stabilizes the NFM structure. When incorporated into a carbonate-based electrolyte, the resulting NFM||Na cells deliver 80.0% capacity retention after 500 cycles, high-rate capability, and robust low-temperature performance. Furthermore, NFM||hard carbon (HC) pouch cells exhibit stable long-term cycling, retaining 88.0% of their capacity after 100 cycles. Notably, in situ X-ray diffraction performed directly on NFM||HC pouch cells confirms that this anion regulation significantly suppresses the detrimental phase evolution of the NFM cathode during high-voltage cycling. This work provides new insights into anion-regulated interfacial adsorption and solvation chemistry for the development of high-voltage SIBs.