Simultaneous Regulation of Organic and Inorganic Components in Interphase by Fiber Separator for High‐stable Sodium Metal Batteries

Uncontrollable side reactions at the metal interface have been identified as the root cause of the formation of a fragile solid electrolyte interphase, leading to irreversible sodium loss in sodium metal batteries. Here, we proposed an interface engineering strategy that employed a carboxyl functionalized cellulose separator to provide strong dipole moments and induce the cleavage of P–F bond to construct a SEI rich in NaF. In addition, we employed nuclear magnetic resonance technology confirmed that the separator with strong dipole moments prevented the reduction of organic solvents by attracting electrons, thereby inhibiting the formation of organic oligomers. SEI with high NaF content and few oligomers is smooth and robust, obviously decreasing the interface impedance of the Na anode. The symmetric Na||Na cells, equipped with the functionalized separator, efficiently operated for 1400 hours with a stable 72 mV overpotential at 0.25 mA cm−2, exhibiting low energy barrier and fast ion transport kinetics. The Na||Na3V2(PO4)3 cell also showed stable cycling performance, with the capacity remaining at 94.83% of the initial capacity after 1000 cycles at 1C. The proposed separator could control the formation and composition of SEI, paving the way for the development of long‐life sodium metal batteries.