Highly Safe Ionic Liquid Electrolytes for Sodium-Ion Battery: Wide Electrochemical Window and Good Thermal Stability

Novel ionic liquid (IL) electrolytes are prepared by mixing 1-ethyl-3-methylimidazolium-bis-tetrafluoroborate (EMIBF4) with different concentrations of sodium salt (NaBF4). The as-prepared IL electrolytes display wide electrochemical windows of ∼4 V (1–5 V), which are consistent with the quantum chemical theoretical calculation. The IL electrolyte with 0.1 M NaBF4 shows excellent ionic conductivity, namely, 9.833 × 10–3 S cm–1 at 20 °C. In addition, nonflammability and good thermal stability are exhibited by combustion test and thermogravimetric analysis (TGA), which indicate the high safety of the IL electrolyte.