Transport Number Determination and Relevance for Lithium Metal Batteries Using Localized Highly Concentrated Electrolytes

The lithium transport number determination of fluorinated ether (1,2-(1,1,2,2-tetrafluoroethoxy) ethane, TFEE)-based localized highly concentrated electrolytes (LHCEs) with 1,2-dioxolane (DOL) and dimethoxyethane (DME) as solvents has been explored using molecular dynamics simulations, nuclear magnetic resonance spectroscopy, Bruce-Vincent's method, and low-frequency electrochemical impedance spectroscopy (EIS). We showcase that the TFEE-DOL LHCE has a as high as 0.65 but, on the other hand, exhibits low Coulombic efficiency (<90%) and poor stability <i>vs</i> Li metal anodes, <i>i.e.</i>, in a lithium metal battery (LMB) setting. In contrast, the TFEE-DME LHCE shows high Coulombic efficiency (98.9%) and stability, despite a much lower (0.25). A significant migration resistance through the porous solid electrolyte interphase (SEI) for the former is the likely explanation, as revealed by EIS and assisted by scanning electron microscopy and X-ray photoelectron spectroscopy experiments. We thus find the interfacial properties at the Li metal anode to be more crucial than the ionic transport through the bulk of the electrolyte for LMB performance. We therefore propose that the focus should be put on the full (<i>operando</i>) impedance spectra of Li metal anodes in contact with electrolytes, since it enables the characterization of the interphase layer(s), rather than solely determining the (bulk) of the electrolytes.