Low-Temperature Properties of the Sodium-Ion Electrolytes Based on EC-DEC, EC-DMC, and EC-DME Binary Solvents

Sodium-ion batteries are a promising class of secondary power sources that can replace some of the lithium-ion, lead–acid, and other types of batteries in large-scale applications. One of the critical parameters for their potential use is high efficiency in a wide temperature range, particularly below 0 °C. This article analyzes the phase equilibria and electrochemical properties of sodium-ion battery electrolytes that are based on NaPF6 solutions in solvent mixtures of ethylene carbonate and diethyl carbonate (EC:DEC), dimethyl carbonate (EC:DMC), and 1,2-dimethoxyethane (EC:DME). All studied electrolytes demonstrate a decrease in conductivity at lower temperatures and transition to a quasi-solid state resembling “wet snow” at certain temperatures: EC:DEC at −8 °C, EC:DMC at −13 °C, and EC:DME at −21 °C for 1 M NaPF6 solutions. This phase transition affects their conductivity to a different degree. The impact is minimal in the case of EC:DEC, although it partially freezes at a higher temperature than other electrolytes. The EC:DMC-based electrolyte demonstrates the best efficiency at temperatures down to −20 °C. However, upon further cooling, 1 M NaPF6 in EC:DEC retains a higher conductivity and lower resistivity in symmetrical Na3V2(PO4)3-based cells. The temperature range from −20 to −40 °C is characterized by the strongest deterioration in the electrochemical properties of electrolytes: for 1 M NaPF6 in EC:DMC, the charge transfer resistance increased 36 times, and for 1 M NaPF6 in EC:DME, 450 times. For 1 M NaPF6 in EC:DEC, the growth of this parameter is much more modest and amounts to only 1.7 times. This allows us to consider the EC:DEC-based electrolyte as a promising basis for the further development of low-temperature sodium-ion batteries.