Formulation and Characterization of PS-Poly(ionic liquid) Triblock Electrolytes for Sodium Batteries

Solvent-free solid polymer electrolytes (SPE) are gaining more attention to develop postlithium battery technologies due to the safety and performance benefits of solid-state batteries. In this work, we present a new SPE for a sodium metal battery based on high salt concentration polymer electrolyte membranes comprising mixed anions, polymerized ionic liquid (PIL), block copolymer (BCP) polystyrene-b-poly(diallydimethylammonium)bis(trifluoromethanesulfonyl)imide-b-polystyrene (PS-b-PDADMATFSI-b-PS) and NaFSI salt. The maximum salt concentration incorporated was up to 1:2 mol ratio (PIL block: NaFSI). The ionic conductivity was 10–3 S cm–1 at 70 °C for 1:2 composition, and the anion diffusion as measured by 19F NMR decreased. FTIR measurement indicates that the ion coordination in the polymer–salt mixtures changes with composition. The storage modulus as measured by dynamic mechanical analysis (DMA) was observed in the range 300 MPa at −40 °C to 35.8 MPa at 70 °C. The optimized electrolyte (1:2 mol ratio) membrane was investigated for its long-term stability against Na metal cycling with Na/Na symmetrical cells demonstrating stable Na plating/stripping behavior at 0.2 mA cm–2 at 70 °C. Finally, an Na|NaFePO4 cell cycled with a specific capacity of 118 mAh g–1 at C-rate C/20 at 70 °C and a good Coulombic efficiency (98%), showing the promising potential of these solvent-free triblock copolymer electrolytes in Na metal batteries.