Competitive Li+ Coordination in Ionogel Electrolytes for Enhanced Li-Ion Transport Kinetics

Abstract

Developing ionogel electrolytes based on ionic liquid instead of volatile liquid in gel polymer electrolytes is regarded to be effective to diminish safety concerns in terms of overheating and fire. Herein, a zwitterion-based copolymer matrix based on the copolymerization of trimethylolpropane ethoxylate triacrylate (ETPTA) and 2-methacryloyloxyethylphosphorylcholine (MPC, one typical zwitterion) is developed. It is shown that introducing zwitterions into ionogel electrolytes can effectively optimize local lithium-ion (Li+) coordination environment to improve Li+ transport kinetics. The interactions between Li+ and bis(trifluoromethanesulfonyl)imide (TFSI−)/MPC lead to the formation of Li+ coordination shell jointly occupied by MPC and TFSI−. Benefiting from the competitive Li+ attraction of TFSI− and MPC, the energy barrier of Li+ desolvation is sharply decreased and thus the room-temperature ionic conductivity can reach a value of 4.4 × 10−4 S cm−1. Besides, the coulombic interaction between TFSI− and MPC can greatly decrease the reduction stability of TFSI−, boosting in situ derivation of LiF-enriched solid electrolyte interface layer on lithium metal surface. As expected, the assembled Li||LiFePO4 cells deliver a high reversible discharge capacity of 139 mAh g−1 at 0.5 C and good cycling stability. Besides, the pouch cells exhibit a steady open-circuit voltage and can operate normally under abuse testing (fold, cut), showing its outstanding safety performance.