Gradient fluorination strategy to screen asymmetric ether for high-voltage and low-temperature lithium metal...

Fluorinated ethers have shown great potential as electrolyte solvent for lithium metal batteries (LMBs) due to their elevated oxidation stability and Li compatibility simultaneously. However, the influence of fluorination degree on electrolyte solvation and interfacial evolution has not been well clarified. Herein, we design and synthesize a family of asymmetric fluorinated 1,2-dimethoxymethane (FnDMM) as electrolyte solvent based on a gradient fluorination strategy, where the correlation of electrolyte solvation, ion transport, stability, interphase, and battery performance was systematically investigated. The combined experimental and theoretical studies reveal that the -CF₂CF₂H group in F4DMM solvent intensifies the H-F interaction that increases the viscosity and hinders ion migration. Meanwhile, the F3DMM with -CH₂CF₃ group exhibits insufficient oxidation stability while F7DMM with -CF₂CF₂CF₃ group suffers from poor salt solubility. In sharp contrast, the -CF₂CF₃ group in F5DMM solvent induces amphiphilic charge delocalization, which bestows efficient ion transport, robust interphase, and superior stability. Consequently, the F5DMM electrolyte not only bestows highly reversible Li plating/stripping Coulombic efficiency of 99.6% but also enables the stable cycling of Li||LiNi_0.8Mn_0.1Co_0.1O₂ cell for 650 cycles at room temperature and 300 cycles at -40 °C.