Decoupling bulk and interfacial contributions to performance in localized high concentration electrolytes for Li metal batteries

Localized high concentration electrolytes (LHCEs) are a promising class of electrolytes 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, in contrast to prior reports that suggest diluents are inert. Using a combination of solution 17O NMR and cyclic voltammetry (CV), we assess differences in solvation and electrochemical reduction in LHCEs and compare to low concentration electrolytes (LCEs). We find that BTFE diluents are chemically (rather than electrochemically) reduced during Li metal battery operation, which can be detected with operando NMR, but not conventional electrochemical methods. Solid-state NMR (SSNMR) and X-ray photoelectron spectroscopy (XPS) measurements confirm that LHCEs decompose to form a SEI on Li metal that contains organic BTFE reduction products as well as high quantities of lithium fluoride from both BTFE and FSI– reduction. Insight into the (electro)chemical reduction mechanisms underpinning SEI formation in LHCEs suggests that fluorinated ethers exhibit tunable reactivity that can be leveraged to control Li deposition behavior.