Solutions of 1 M NaClO4 in mixtures of carbonates, including EC:PC 1:1 vol. (EP11), EC:DMC 1:1 vol. (ED11), EC:PC:DMC 1:1:3 vol. (EPD113) and EC:PC:DMC 3:1:1 vol. (EPD311), and 10–50% vol. sulfolane (SL) as co-solvent were investigated as flame- and oxidation-resistant electrolytes for sodium-ion battery. The dependence of important features, including thermal and fiery behavior, ionic conductivity and oxidative stability, on SL content as well as the compatibility with Na0.44MnO2 cathode materials are essentially reported. It was revealed that the presence of a moderate SL amount effectively promotes the electrolyte safety profile while maintaining acceptable conductivity. Amongst concerned samples, EPD311-based electrolytes with SL content within the optimized range (10–30% vol.) were found to exhibit the best balanced inherent properties. They also perform well when coupled with Na0.44MnO2 cathode in coin-type half-cells. Discharge capacity values of around 85–95 mAh·g−1 (C/10 rate), with almost 100% capacity retention after 20 cycles and good rate capability were demonstrated. Remarkably, EPD311 + 10% SL electrolyte was able to deliver the full theoretical capacity of Na0.44MnO2 material (120 mAh·g−1) at the C/20 rate.
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