Gel Polymer Electrolytes Based on Crosslinked Networks by the Introduction of an Ionic Liquid Crosslinker with Ethylene Oxide Arms

The volatility of liquid electrolytes is a major obstacle in the fabrication of efficient lithium-ion batteries that are safe. Solid-state electrolytes such as solid polymer electrolytes have been studied as a potential substitute for liquid electrolytes. However, their practical application is impeded owing to their low ionic conductivity and high interfacial resistance between the electrolyte and electrodes. Herein, we synthesize a novel ionic liquid crosslinker and use thermal crosslinking to prepare a gel polymer electrolyte (GPE), which shows a higher thermal stability than that of liquid electrolytes and a better ionic conductivity than that of solid electrolytes. The crosslinker, IL2, is designed to have a pyrrolidinium-bis(trifluoromethyl sulfonyl)amide structure and an acrylate terminal group with an ethylene oxide spacer connected between them. IL2-GPE, which is prepared by in situ thermal crosslinking, shows an ionic conductivity up to 5.37 mS cm–1 and high thermal and electrochemical stabilities. A cell with IL2-GPE sandwiched between a LiFePO4 cathode and lithium anode exhibits a capacity above 160 mA h g–1 and a high rate capability. By combining a crosslinker having four acrylate terminals with the IL2 crosslinker, we obtain HIL2-GPE, whose ionic conductivity is 20% higher than that of IL2-GPE. The HIL2-GPE cell exhibits capacities of 165 and 146 mA h g–1 at 0.1 and 1.0 C, respectively, thereby demonstrating better performance than that of the cell with IL2-GPE. We also prepared a cell using high-voltage cathode LiNi0.6Co0.2Mn0.2O2 (NCM622). The result suggested that the cell based on the GPEs maintained superior long-term stability even with high-voltage cathode materials over 100 cycles.