A "Flexible" Solvent Molecule Enabling High-Performance Lithium Metal Batteries.

Abstract

Electrolyte chemistries are crucial for achieving high cycling performance and high energy density in lithium metal batteries. The localized high-concentration electrolytes (LHCEs) exhibit good performance in lithium metal batteries. However, understanding how the intermolecular interactions between solvents and diluents in the electrolyte regulate the solvation structure and interfacial layer structure remains limited. Here, we reported a new LHCE in which strong hydrogen bonding between diluents and solvents alters the conformation and polarity of "flexible" solvent molecules, thereby effectively regulating the solvation structure of Li⁺ ion and promoting the formation of robust electrode interfaces. The endpoint H of the "flexible" chain O-CH-CH₃ of the 2,5-dimethyltetrahydrofuran (2,5-THF) solvent and the F of the benzotrifluoride (BTF) diluent can form strong hydrogen bonds, which expand the maximum bond angle of the 2,5-THF molecule from 119° to 123°. The expanded bond angle increases the steric hindrance of the 2,5-THF molecule and decreases its polarity. This leads to an increase in the anion content within the solvation structure, which in turn enhances the performance of both the lithium metal anode and the sulfurized polyacrylonitrile (SPAN) cathode. As a result, the lithium metal anode shows a Coulombic efficiency (CE) of as high as 99.4 %. The assembled Li||SPAN battery based on our developed LHCE exhibits impressive stability with an average CE of 99.8 % over 700 cycles. Moreover, the Li||SPAN pouch cell can be stably cycled with a high energy density of 301.4 Wh kg^-1. This molecular-level understanding of the correlation between molecular interactions and solvation structures provides new insights into the design of advanced LHCEs for high-performance lithium metal batteries.