Poly(vinyl chloride) Ionic Liquid Polymer Electrolyte Based on Bis(fluorosulfonyl)Amide for Sodium Secondary Batteries

Non-flammable electrolytes have been extensively studied to improve the safety of energy storage devices. In this study, a new ionic liquid polymer electrolyte (ILPE) prepared by a cast technique using poly(vinyl chloride) (PVC) as the host polymer was examined for sodium secondary batteries. The ILPE containing 50 wt% of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [C2C1im][FSA] ionic liquid showed an ionic conductivity of 5.6 mS cm−1 at 318 K. It remains stable up to 517 K based on 5 wt% loss. Stable sodium metal electrodeposition/dissolution was observed at the cathodic limit of the electrochemical window for the ILPE containing Na[FSA] ([C2C1im][FSA]:Na[FSA]:PVC = 2.0:1.0:1.0 w/w). The anodic limit was observed at around 4.7 V vs. Na+/Na on a Pt working electrode, while on an Al working electrode a higher anodic stability owing to passivation was observed. In the Na/[C2C1im][FSA]-Na[FSA]-PVC/-Na2FeP2O7-cell with the theoretical capacity of 97 mAh g−1, a reversible capacity of 92 mAh g−1 was obtained at 363 K for the Na2FeP2O7 positive electrode. These results suggest great potential of this PVC-based ILPE for constructing very safe sodium secondary batteries.