To improve the thermal stability of lithium- and sodium-ion batteries, the room-temperature molten salts LiBF4 /1-ethyl-3-methyl imidazolium tetrafluoroborate (EMIBF4) and NaBF4/EMIBF4 were used as ionic liquid (IL) electrolytes instead of flammable carbonate-type organic electrolyte solvents. To avoid cathodic decomposition of the IL electrolytes, a symmetric cell configuration with Na Superionic CONductor (NASICON)-type normalA3normalV2(PO4)3 (where A is Li or Na) as both cathode and anode was tried in a coin-type cell (type 2320). As a result, both the polyanionic-based Li3normalV2(PO4)3 (LVP) as well as Na3normalV2(PO4)3 (NVP) symmetric cells using organic electrolytes were found to operate as secondary batteries and exhibited satisfactory electrochemical performances. The substitution of the organic electrolytes by the appropriate IL electrolytes in both cases resulted in the reduction in the first discharge capacities. However, the IL-based cells revealed better cyclability and a more stable behavior at elevated temperatures. The obtained electrochemical behavior of the symmetric cells was confirmed by the complex impedance measurements at 25 and 80°C . In addition, the thermal stability of LVP and NVP with the IL electrolytes was also examined.

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