Stable High-Temperature Cycling of Na Metal Batteries Based on Na3V2(PO4)3 and Na2FeP2O7 Cathodes in NaFSI-Rich Organic Ionic Plastic Crystal Electrolytes.

Sodium batteries have emerged as a promising alternative for large-scale energy storage applications due to the low cost and high abundance of sodium. Sodium batteries require safe, high voltage and cost effective electrolytes and cathode materials for their practical applications to be realised. In the present study, Na metal cells with a mixed phase electrolyte comprising a high concentration of Na salt (NaFSI) in an organic ionic plastic crystal (OIPC) - namely triisobutylmethylphosphonium bis(fluorosulfonyl)imide are investigated - coupled with either a sodium vanadium phosphate-carbon composite (NVP/C) or a sodium iron pyrophosphate (NFpP) cathode. The performance of the Na/NVP/C and Na/NFpP cells are evaluated using cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic cycling at 60 °C and room temperature. The results reported herein indicate the performance improvement in terms of cycling stability, with high coulombic efficiency at 60 °C granted by the OIPC and ionic liquid mixtures, compared to a conventional organic solvent electrolyte.