Copolymers of trimethylene carbonate and ε-caprolactone as electrolytes for lithium-ion batteries

- Polycarbonate-co-polyester-LiTFSI solid polymer electrolytes are synthesized.
- Electrolytes show lower Tg and higher conductivity with increased ester content.
- LiFePO4 half-cells show stable cycling, with high capacity at up to C/5.

Random copolymers of trimethylene carbonate (TMC) and ε-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 Tg 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 × 10−5 S cm−1 at 60 °C (7.9 × 10−7 S cm−1 at 25 °C) and a Tg of −26 °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.