Impact of the Cyclic-to-Linear Carbonate Ratio in EC-DEC-DMC Electrolytes on the Low-Temperature Performance of Sodium-Ion Batteries Comprising Metallic Sodium and Intercalation-Type Electrodes

The low-temperature properties of sodium-ion electrolytes based on 1 M NaPF 6 solutions in EC-DEC-DMC (2:1:1 and 1:1:1 by volume) were studied. The data on phase transitions obtained from visual experiments and DSC studies were compared with the results of electrochemical experiments using steel (St), metallic sodium (Na), and NASICON-type active materials (NVP, NTP) electrodes. The specific conductivity of the “2:1:1” electrolyte, although initially slightly higher than that of the “1:1:1,” decreases sharply upon cooling and at −40 °C is ≈0.5 mS cm −1 vs ≈1.5 mS cm −1 for the “1:1:1.” However, the capacities of NVP-Na half-cells in both electrolytes are almost identical down to −40 °C. Studies of symmetrical cells showed that the main contributor to polarization loss is the sodium-electrolyte interface. Both electrolytes behave similarly with respect to metallic sodium. However, in the case of NVP-NVP cells, the charge transfer resistance in case of “1:1:1” electrolyte was lower on cooling than for the “2:1:1.” NVP-NTP full cells additionally demonstrated an advantage of the “1:1:1” system compared to “2:1:1” at low temperatures. At −40 °С, the cell with “1:1:1” electrolyte demonstrated retention of 75% of the room-temperature capacity, while in the case of “2:1:1” this value was slightly more than 50%.