Stable Cycling of Sodium Metal All-Solid-State Batteries with Polycarbonate-Based Polymer Electrolytes

Solid polymer electrolytes based on high-molecular-weight poly(trimethylene carbonate) (PTMC) in combination with NaFSI salt were investigated for application in sodium batteries. The polycarbonate host material proved to be able to dissolve large amounts of salt, at least up to a carbonate:Na+ ratio of 1:1. Combined DSC, conductivity, and FTIR data indicated the formation of a percolating network of salt clusters along with the transition to a percolation-type ion transport mechanism at the highest salt concentrations. While the highest total ionic conductivities were seen at the highest salt concentrations (up to a remarkable 5 × 10–5 S cm–1 at 25 °C at a 1:1 carbonate:Na+ ratio), the most stable battery performance was seen at a more moderate salt loading of 5:1 carbonate:Na+, reaching >80 cycles at a stable capacity of ∼90 mAh g–1 at 60 °C in a sodium metal/Prussian blue cell. The results highlight the importance of the choice of salt and salt concentration on electrolyte performance as well as demonstrate the potential of utilizing polycarbonate-based electrolytes in sodium-based energy storage systems.