Fluorination promotes lithium salt dissolution in borate esters for lithium metal batteries

Abstract

Lithium metal batteries promise higher energy densities than current lithium-ion batteries but require novel electrolytes to extend their cycle life. Fluorinated solvents help stabilize the solid electrolyte interphase (SEI) with lithium metal, but are believed to have weaker solvation ability compared to their nonfluorinated counterparts and are deemed ‘poorer electrolytes’. In this work, we synthesize tris(2-fluoroethyl) borate (TFEB) as a new fluorinated borate ester solvent and show that TFEB unexpectedly has higher lithium salt solubility than its nonfluorinated counterpart (triethyl borate). Through experiments and simulations, we show that the partially fluorinated –CH₂F group acts as the primary coordination site that promotes lithium salt dissolution. TFEB electrolyte has a higher lithium transference number and better rate capability compared to methoxy polyethyleneglycol borate esters reported in the literature. In addition, TFEB supports compact lithium deposition morphology, high lithium metal Coulombic efficiency, and stable cycling of lithium metal/LiFePO₄ cells. This work ushers in a new electrolyte design paradigm where partially fluorinated moieties enable salt dissolution and can serve as primary ion coordination sites for next-generation electrolytes.