Probing the Thermal Stability of the In Situ Ring-Opening Polymerized Solid Polymer Electrolytes

Ring-opening polymerization (ROP) is widely employed for the in situ construction of solid polymer electrolytes (SPEs) in batteries due to the absence of byproducts. However, the thermal stability of such SPEs has long been overlooked. Here, we systematically investigate the thermal stability of two types of the most popular in situ ROP-prepared SPEs, including ether-based poly(1,3-dioxolane) (PDOL) and poly(1,3,5-trioxane) (PTXE), as well as ester-based poly(valerolactone) (PVL) and poly(trimethylene carbonate) (PTMC). Given the essential role of Li-salts in SPEs and the impossibility of removing ROP catalysts during in situ polymerization in cells, the thermal stability of ROP-prepared SPEs was evaluated in the presence of both components. The in situ ROP-prepared PDOL- and PTXE-based SPEs undergo rapid degradation into gases at 100 °C. The ROP and pyrolysis of PDOL/PTXE are demonstrated to share the same chemical mechanism and coexist in a temperature-dependent equilibrium, in which ROP is dominant at room temperature, while pyrolysis prevails at elevated temperatures and produces gases. The decomposition temperature of in situ ROP-prepared PVL and PTMC is around 180 °C, which is, however, decreased to 110 °C in the presence of Li-metal. This work reveals the challenges to the thermal stability of ROP-prepared SPEs and brings attention to the development of safer SPEs in solid-state batteries.