New polymerized ionic liquid (PIL) gel electrolyte membranes based on tetraalkylammonium cations for lithium ion batteries

New polymerized ionic liquids (PILs) based on tetraalkylammonium cations with N(CF3SO2)2− (TFSI−) anions are synthesized by homopolymerization and anion exchange reaction. Furthermore, a gel polymer electrolyte membrane based on the PILs as polymer host is prepared incorporating a tetraalkylammonium ionic liquid, LiTFSI salt and nano-size SiO2. The gel polymer electrolyte membranes show exceptional thermal and electrochemical properties. They are found to be thermally stable up to 336 °C. With the increase of the IL, Tg of the PIL-based electrolyte samples decreases obviously. When the IL concentration is 25 wt%, the ionic conductivity of the electrolytes reaches 4.9×10−4 S cm−1 at 60 °C. After 130 cycles, the Li/LiFePO4 battery with the gel polymer electrolyte membranes at 60 °C has discharge capacity of 122 mA h g−1 at 0.1 current rates.