Polymerized ionic liquids with guanidinium cations as host for gel polymer electrolytes in lithium metal batteries

Polymerized ionic liquids (PILs) having guanidinium cations with different counter-anions, such as PF ₆ ^- and N(CF ₃SO ₂) ₂ ^- (TFSI ^-), were synthesized by copolymerization of a guanidinium ionic liquid monomer with methyl acrylate followed by an anion exchange reaction. Furthermore, incorporating a guanidinium ionic liquid, LiTFSI salt and nano-size SiO ₂, a quaternary gel polymer electrolyte based on one of the PILs as the polymer host was prepared. The quaternary gel polymer electrolyte was chemically stable even at a higher temperature of 80 °C in contact with the lithium anode. In particular, the electrolyte exhibited high lithium ion conductivity, wide electrochemical stability window and good lithium stripping/plating performance. Li/LiFePO ₄ batteries with the quaternary gel polymer electrolyte at 80 °C had capacities of 140 and 130 mA h g ^-1 respectively at 0.1 and 0.2 C current rates.