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-PDAD-MATFSI-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 degrees 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 degrees C to 35.8 MPa at 70 degrees 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 degrees C. Finally, an Na|NaFePO4 cell cycled with a specific capacity of 118 mAh g-1 at C-rate C/20 at 70 degrees C and a good Coulombic efficiency (98%), showing the promising potential of these solvent-free triblock copolymer electrolytes in Na metal batteries.