Revisiting Fluorobenzene as Diluents in Ether-Based Electrolytes for Lithium Metal Batteries

The localized high-concentration electrolyte based on the low-cost, low-density, low-viscosity, and low-fluorine-substitution fluorobenzene diluents and 1,2-dimethoxyethane solvents has been successfully demonstrated in high-performance lithium metal batteries. However, it requires high salt-to-solvent molar ratio, which causes high production costs and great environmental burden. Decreasing the salt-to-solvent molar ratio without sacrificing its electrochemical performance remains a challenge. Herein, we reveal that as the salt-to-solvent molar ratio is decreased, the compatibility of the fluorobenzene-diluted 1,2-dimethoxyethane-based electrolyte with lithium metals transitions from compatible to incompatible. We elucidate the degradation mechanism of the fluorobenzene-diluted 1,2-dimethoxyethane-based electrolyte undergoing severe side reactions with lithium metals. Inspired by these findings, we develop a fluorobenzene-diluted dimethyl acetal-based electrolyte, which enhances the stability of the electrolyte under a reduced lithium-salt concentration, making it show good compatibility with lithium metal (Coulombic efficiency: 99.43% at 25 °C, 97.74% at -40 °C). Moreover, the assembled Li | |SPAN battery displays a high capacity retention of 83% after cycling 500 cycles and can operate at -60 °C. Besides, a high specific energy of 334.53 Wh kg^-1 (excluding package) can be achieved for the Li | |SPAN pouch cell. This work prompts us to re-examine the applicability of fluorobenzene as diluents in ether-based electrolytes for lithium metal batteries.