Effect of LiTFSI Solvation on Ionic Conductivity of Polyester-Based Solid Electrolytes

Carbonyl-containing polymers have been considered promising candidates as hosts in solid polymer electrolytes (SPEs) due to the reasonable chelating coordination with Li+, better antioxidation for high-voltage cathodes, and higher ion transference number compared with polyether SPEs. In this work, four polyesters of poly(octamethylene succinate), poly(hexmethylene succinate) (PHS), poly(butylene succinate), and polycaprolactone were investigated. In these SPEs with different −C═O/–CH2– ratios, PHS had the highest conductivity (σ) of 1.24 × 10–4 S/cm because of the excellent ability to deionize bis(trifluoromethane)sulfonimide (LiTFSI) up to 88.3 ± 3.2% at 70 °C and the lowest activation energy of Li+ ionic conduction. The effect of Li+/–C═O ratios (r) on the ionic conductivity can be clarified into low-, middle-, and high-concentrated regions. The decrease of PHS crystallinity due to LiTFSI solvation provided ion transport paths and mainly contributed to the improvement of ionic conductivity in the middle-concentrated region, while the solvation degree dominantly facilitated ionic conduction in the high-concentrated region and at higher temperatures. By combining the DFT simulation and polymer thermal analysis, we found the transition of Li+ coordination from multichain to single-chain bindings provided more flexible segment movement. It also proved that the sequence design of active groups in a polymer chain would be a promising strategy for stable and high-performance SPEs.