Lithium-ion batteries with fluorinated mesogen-based liquid-crystalline electrolytes: molecular design towards enhancing oxidation stability

Abstract

Two-dimensional (2D) nanostructured liquid crystals containing fluorinated cyclohexylphenyl and cyclic carbonate moieties have been developed as quasi-solid-state self-organized electrolytes for safe lithium-ion batteries. We have designed lithium ion-conductive liquid-crystalline (LC) materials with fluorine substituents on mesogens for improved oxidation stability. Computational studies suggest that the fluorination of mesogens lowers the highest occupied molecular orbital (HOMO) level of LC molecules and improves their oxidation resistance as electrolytes. The LC molecule complexed with lithium bis(trifluoromethanesulfonyl)imide exhibits smectic A LC phases with 2D ion transport pathways over wide temperature ranges. Cyclic voltammetry measurements of the fluorinated mesogen-based LC electrolytes indicate that they are electrochemically stable above 4.0 V vs. Li/Li⁺. Lithium half-cells composed of fluorinated LC electrolytes show higher discharge capacity and coulombic efficiency than those containing non-fluorinated analogous LC molecules. Combining molecular dynamics simulations with the experimental results, it is revealed that the fluorination of the mesogen effectively enhances the electrochemical stability of the LC electrolytes without significantly disrupting ionic conductivities and the LC order.