Fluorinated Sodium Salt Functionalized Copolymer as Artificial Solid Electrolyte Interphase for High Performance Lithium Metal Batteries

Lithium metal batteries (LMBs) are promising for high-energy storage due to their high specific capacity (3860 mAh/g) and low density (0.534 g/cm3), but face major safety risks such as thermal runaway, electrolyte leakage, and lithium dendrite growth. To address these issues, an artificial solid electrolyte interphase (SEI) layer can shield the lithium negative electrodes from side reactions. Developing inorganic/organic hybrid SEI has become a particularly promising strategy because it combines high ionic conductivity, mechanical strength, and flexibility to enhance battery stability. Herein, a functional copolymer FFN, incorporating sodium salt, fluorinated groups, and ethylene carbonate units, is designed and applied as an artificial SEI. The molecular design of FFN innovatively integrates the charge regulation function of sulfonate groups, the interfacial stability characteristics of fluorinated segments, the high reactivity of carbonate side groups with lithium metal batteries. The results demonstrate that FFN can undergo a reaction with lithium metal to form an inorganic/organic composite SEI layer. The solid electrolyte interphase (SEI) layer induced on the lithium metal surface is multicomponent, primarily consisting of the organic flexible segments of the polymer and inorganic components such as lithium fluoride (LiF) and sodium fluoride (NaF). This multicomponent hybrid structure not only enhances the mechanical strength of the SEI layer but also significantly improves its ionic conductivity, promotes uniform lithium-ion deposition and provides a stable interfacial environment for the lithium metal anode. Carbonate-based electrolytes exhibit high ionic conductivity and excellent high-voltage stability in lithium metal batteries. Lithium metal symmetric batteries modified with FFN interphase can stably cycle for 800 h at 1 mA cm–2 and 1 mAh cm–2 in ester electrolytes. In addition, lithium metal full battery FFN@Li||LFP achieved a capacity retention rate of 75% after completing 800 cycles, demonstrating excellent cycling stability. This work provides an effective strategy for designing high-performance artificial SEI layers toward safe and durable LMBs.