Design of ionic liquid like monomers towards easy-accessible single-ion conducting polymer electrolytes

Abstract

The rational design of single-ion polymer electrolytes emerges as a primary strategy for enhancing the performance of lithium ion batteries. With the aim to increase ionic conductivity, four novel ionic liquid monomers were designed and synthesized in high purity. Such monomers differ from the previously reported systems by (1) the presence of a long and flexible spacer between the methacrylate group and chemically bonded anion or (2) by a long perfluorinated side chain. The investigation of their free radical copolymerization with poly(ethylene glycol) methyl ether methacrylate (PEGM) allowed to identify the impact of thei copolymer composition on thermal and ion conducting properties. The copolymer based on lithium 3-[4-(2-(methacryloyloxy)ethoxy)-4-oxobutanoyl)oxy) propylsulfonyl]-1-(trifluoromethylsulfonyl)imide showed the highest ionic conductivity (1.9 × 10−6 and 2 × 10−5 S cm−1 at 25 and 70 °C, respectively) at [EO]/[Li] = 61 ratio, along with a wide electrochemical stability (4.2 V vs. Li+/Li) and high lithium-ion transference number (0.91). The prepared copoly(ionic liquid)s (coPILs) were further applied for the assembly of Li/coPIL/LiFePO4 lithium-metal cells, which were capable to reversibly operate at 70 °C delivering relatively high specific capacity (up to 115 mAh g−1) at medium C/15 current rate.