Improving Cycling Performance of Lithium Metal Batteries by Balancing Solvation Structure and Interphase Chemistry with Strong and Weak Co-Solvents

Abstract The cycling performance of lithium (Li)-metal batteries (LMBs) is improved by incorporating co-solvents that strongly and weakly coordinate with Li ions in localized high-concentration electrolytes. The synergistic interactions of these solvents facilitate the inclusion of contact ion pairs within the primary solvation sheath and promote the development of fluorine-rich interphase layers on both the Li-metal anode and the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode surfaces. The optimized electrolyte comprises Li bis(fluorosulfonyl)imide dissolved in a co-solvent mixture consisting of the strongly coordinating solvent 1,2-dimethoxyethane (DME) and the weakly coordinating solvent 1,2-bis(2,2,2-trifluoroethoxy)ethane (F6DEE), along with the diluent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether at a molar ratio of 1.0:(1.0:0.2):3.0. The Li||NMC811 cells with this electrolyte can retain 80% of the initial capacity after 375 cycles at elevated charge/discharge rates of 0.2C/0.5C and 260 cycles at 0.33C/1C. Combined molecular dynamics simulations, nuclear magnetic resonance measurements, microscopy, and X-ray photoelectron spectroscopy indicate that partial replacement of DME with F6DEE increases anion participation in the local Li+ coordination environment and promotes more inorganic-rich interphases on both electrodes. As a result, interphase chemistry and cycling stability in LMBs can be largely improved. Similar approaches can also be used to improve the performance of other alkali-metal batteries.