Moderate Li+-solvent binding for gel polymer electrolytes with stable cycling toward lithium metal batteries

Solvation chemistry is crucial for gel polymer electrolytes (GPEs) due to the great impact on ionic conductivity and solid electrolyte interface (SEI) properties. However, its rational regulation to balance fast Li+ transport and stable SEI properties has not been well elucidated. Here, we design and synthesize three GPEs with high, moderate and low Li+-solvent binding by fluorinating solvents. The GPE with moderate Li+-solvent binding (MB-GPE) by incorporating fluorinated ethylene carbonate (FEC) and methyl 2,2,2-trifluoroethyl carbonate (FEMC) achieves an optimal balance between weak solvation modulation and lithium-ion transport performance. Simulations and characterizations reveal that the moderate Li+-solvent binding facilitates an anion-dominated solvation structure while maintaining sufficient lithium salt dissociation. Consequently, MB-GPE exhibits high ionic conductivity of 1.95 x 10-3 S cm-1 and robust LiF-rich SEI on the lithium metal anode, with stable cycling of over 3200 h for the lithium metal anode at a current density of 0.5 mA cm-2 and 80.1% capacity retention in Li & Vert;NCM811 cells after 400 cycles at 0.5C. In contrast, GPEs with excessively high and low Li+-solvent binding suffer from unstable SEI and reduced ionic conductivity, respectively. This work proposes a constructive route for the rational design of gel polymer electrolytes, with promising potential for energy storage applications.