Random copolymers of trimethylene carbonate (TMC) and epsilon-caprolactone (CL) were synthesized through bulk ring-opening polymerization for use as host materials for solid polymer electrolytes. Amorphous electrolytes were solution-cast from the copolymers together with LiTFSI salt and showed lower T-g and higher ionic conductivity as the CL content was increased. The best-performing electrolyte, with a TMC:CL ratio of 60:40 with 28 wt% of LiTFSI, was found to have a conductivity of 1.6 x 10(-5) S cm(-1) at 60 degrees C (7.9 x 10(-7) S cm(-1) at 25 degrees C) and a T-g of -26 degrees C. This electrolyte was used in all-solid-state LiFePO4 half-cells that showed high capacity and coulombic efficiency at rates up to and including C/5. (C) 2015 Elsevier Ltd. All rights reserved.