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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. 

Hybrid sulfide-polymer composite electrolytes are promising candidates to enable lithium metal batteries because of their high ionic conductivity and flexibility. These composite materials are primarily prepared through solution casting methods to obtain a homogenous distribution of polymer within the inorganic. However, little is known about the influence of the morphology of the polymer and the inorganic on the ionic conductivity and electrochemical behavior of these hybrid systems. In this study, we assess the impact of processing methodology, either solution processing or solvent-free ball milling, on overall performance of hybrid electrolytes containing amorphous Li₃PS₄(LPS) and non-reactive polyethylene (PE). We demonstrate that using even non-polar, non-reactive solvents can alter the LPS crystalline structure, leading to a lower ionic conductivity. Additionally, we show that ball milling leads to a non-homogenous distribution of polymer within the inorganic, which leads to a higher ionic conductivity than samples processed via solution casting. Our work demonstrates that the morphology of the polymer and the sulfide plays a key role in the ionic conductivity and subsequent electrochemical stability of these hybrid electrolytes.