Gas Generation in Anode-Free Li-Metal Batteries with Localized High-Concentration Electrolytes

The introduction of localized high-concentration electrolyte (LHCE), which inherits the advanced properties of concentrated electrolytes and exhibits lower viscosity and cost, is one of the important methods to improve the cycling stability of lithium metal anode. Although the similar strategy also has been proposed to extend the durable life of anode-free lithium metal batteries (AF-LMBs), few studies have focused on the electrolyte decomposition reaction and gas production. In this work, a typical LHCE system consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt, 1,2-dimethoxyethane (DME) solvent and tetrafluoroethyl tetrafluoropropyl ether (HFE) diluter were chosen, and Cu||NCM712 pouch cells were assembled to investigate the effects of LHCE concentration (0.7, 1.2, 1.7 and 2.3 mol/L), working temperature (25 and 45 degree celsius) and charging cutoff voltage (3.8 and 4.3 V) on the gas production of AF-LMB systems. Combined with gas chromatography, Raman spectroscopy and electrochemical tests, it is found that the concentration of lithium salt is a significant factor affecting the cycle life and gas production volume. The solvation structure gradually evolves and the number of free solvent molecules in LHCE reduces as the concentration of lithium salt increases, inhibiting the oxidation decomposition of electrolyte and gas production of battery. Moreover, the electrolyte decomposition is also dependent with the variety of cathode material. In contrast with lithium iron phosphate (LiFePO4) cathode, NCM712 cathode results in faster capacity decay of the full-cell and higher gas volume, which might be ascribed to the transition metal elements with the catalytic effect. Meanwhile, scanning electron microscope images indicate uniform and dense lithium deposition on the Cu foil, and X-ray photoelectron spectroscopy tests also reveal that more inorganic solid electrolyte interface (SEI) components are formed on the surface of lithium metal, which serves as the excellent protective layer and is beneficial to suppressing the side reaction. This work is expected to provide guidance for the gas production research of other ether-based LHCE systems.