Towards room temperature operation of all-solid-state Na-ion batteries through polyester–polycarbonate-based polymer electrolytes

Abstract

In an ambition to develop solid-state Na-ion batteries functional at ambient temperature, we here explore a novel electrolyte system. Polyester-polycarbonate (PCL–PTMC) copolymers were combined with sodium bis(fluorosulfonyl)imide salt (NaFSI) to form solid polymer electrolytes for Na-ion batteries. The PCL–PTMC:NaFSI system demonstrated glass transition temperatures ranging from −64 to −11 °C, increasing with increasing salt content from 0 to 35 wt%, and ionic conductivities ranging from 10−8 to 10−5 S cm−1 at 25 °C. The optimal salt concentration was clearly dependent on the level of crystallinity, which was largely determined by the CL content. At 70 and 80 mol% CL, the PCL–PTMC:NaFSI system was fully amorphous and exhibited high conductivities at lower salt concentrations. When the CL content was increased to 100 mol%, high ionic conductivities were instead observed at high salt concentrations. A decent transference number of ca. 0.5 at 80 °C was obtained for a polymer film containing 20 mol% CL units and 25 wt% NaFSI. Finally, a HC|80-2025|Na2−xFe(Fe(CN)6) all-solid-state polymer electrolyte full cell was assembled to demonstrate the practical application of the material and cycled for more than 120 cycles at ∼22 °C.