Design of Polymer Network and Li+ Solvation Enables Thermally and Oxidatively Stable, Mechanically Reliable, and Highly Conductive Polymer Gel Electrolyte for Lithium Batteries

Abstract

Herein, we demonstrate that design of polymer network and Li+-ion solvation enables the fabrication of thermally and oxidatively stable, mechanically reliable, and highly conductive polymer gel electrolytes for lithium batteries. Polymer gel electrolytes have been used for Li-ion batteries (LIB) due to their quasi-solid natures and flexible shapes. However, they frequently suffer from the high vapor pressures of the incorporated solvents, low oxidative stabilities, and poor mechanical properties. To overcome these drawbacks, we fabricated a tough gel electrolyte comprising a tetra-arm poly(ethylene glycol) (TPEG) homogeneous polymer network, in which a tetraglyme(G4)-based solvate ionic liquid (SIL) was incorporated. It was intriguing to find that the solvation of Li+ ion by oxygen atoms (within G4 and TPEG), represented as [O]/[Li], governed the thermal and oxidative stabilities of the gel electrolyte, while the homogeneous network contributed to the mechanical reliability and high ionic conductivity. At [O]/[Li] = 5, the TPEG-based gel electrolyte with no free solvent simultaneously exhibited high thermal (>200 °C) and oxidative stabilities (>4.4 V), high stretchability, and high ionic conductivity (∼1 mS cm−1 at 30 °C). These favorable properties of the gel electrolyte resulted in reversible charge/discharge of a 4-V-class high-voltage cathode (LiNi0.6Mn0.2Co0.2O2, NMC622).