Ionic liquid crystal with fast ion-conductive tunnels for potential application in solvent-free Li-ion batteries

Abstract

Insulated polymer host and lithium salt need to be separately dissolved into organic solvents prior to assembly of lithium batteries, implying a large usage of organic solvents and the consequent disadvantages of poor thermal stability, flammability, and explosibility of the end products. Herein, a new generation of thermotropic ionic liquid crystalline lithium salt (LiBIB) with potential bilayer fast ion-conductive tunnels, having functionalities of both lithium salt and polymer host or electrolyte, is successfully synthesized. Molecular structure, thermal stability, ionic conductivity, electrochemical stability, and ion-conductive mechanism of the LiBIB are extensively assessed. The obtained LiBIB shows typical behaviors of both ionic liquid and liquid crystal. Melt point at 43 °C, much lower than that of traditional lithium salts like LiClO4 and LiPF6, and clear point of the LiBIB at 83 °C are both detected. Solid LiBIB salt shows better ionic conductivity (3 × 10−3 S cm−1) and electrochemical stability at room temperature in comparison with those of traditional lithium salts. Time-resolved and perturbation-correlation moving window 2D-COR FTIR results indicate that the high ionic conductivity of the LiBIB is mainly originated from the formation of fast ion-conductive tunnels, implying a promising application in solvent-free Li-ion batteries.