Fluorinated ether decomposition in localized high concentration electrolytes

Localized high concentration electrolytes (LHCEs) are a promising class of battery materials to enable stable cycling of the lithium metal anode. Here, we report the use of operando nuclear magnetic resonance (NMR) spectroscopy to observe electrolyte decomposition during Li stripping/plating and identify the influence of individual components in LHCEs on Li metal battery performance. Data from operando 19F solution NMR indicates that both bis(fluorosulfonyl)imide (FSI−) salt and bis(2,2,2-trifluoroethyl) ether (BTFE) diluent molecules play a key role in solid electrolyte interphase (SEI) formation. Three-electrode electrochemical impedance spectroscopy (EIS) of commercial pouch cells also shows differences in interfacial resistances between LHCE and standard high concentration electrolytes (HCEs) that may be explained by incorporation of BTFE and BTFE reaction products into the SEI. Based on solid-state NMR and X-ray photoelectron spectroscopic characterization, we find that BTFE diluents decompose to form CF2- and CF3-containing fragments within a LiF-rich SEI deposited on the anode surface. The CEI on the cathode (here, LiNi0.8Mn0.1Co0.1O2) side of the battery also contains higher quantities of LiF and trapped LiFSI after cycling in the LHCE compared to a HCE that are attributed to diluent decomposition and correlated with lower impedance at the cathode. Overall, this work provides a new framework to consider highly fluorinated ether molecules—instead of functioning purely as diluting agents in LHCEs, these fluorinated ethers exhibit tunable interfacial reactivity that can be leveraged to control Li deposition behavior.