An effective ‘salt in dimethyl sulfoxide/water’ electrolyte enables high-voltage supercapacitor operated at −50 °C

Compared to organic electrolytes, aqueous electrolytes exhibit significantly higher ionic conductivity and possess inherent safety features, showcasing unique advantages in supercapacitors. However, challenges remain for low-salt aqueous electrolytes operating at high voltage and low temperature. Herein, we report a low-salt (0.87 m) ‘salt in dimethyl sulfoxide/water’ hybrid electrolyte with non-flammability via hybridizing aqueous electrolyte with an organic co-solvent of dimethyl sulfoxide (hydrogen bond acceptor). As a result, the 0.87 m hybrid electrolyte exhibits enhanced electrochemical stability, a freezing temperature below -50 °C, and an outstanding ionic conductivity of 0.52 mS cm-1 at -50 °C. Dimethyl sulfoxide can anchor water molecules through intermolecular hydrogen bond interaction, effectively reinforcing the stability of water in the hybrid electrolyte. Furthermore, the interaction between dimethyl sulfoxide and water molecules diminishes the involvement of water in the generation of ordered ice crystals, finally facilitating the low-temperature performance of the hybrid electrolyte. When paired with the 0.87 m ‘salt in dimethyl sulfoxide/water’ hybrid electrolyte, the symmetric supercapacitor presents a 2.0 V high operating voltage at 25 °C, and can operate stably at -50 °C. Importantly, the suppressed electrochemical reaction of water at -50 °C further leads to the symmetric supercapacitor operated at a higher voltage of 2.6 V. This modification strategy opens an effective avenue to develop low-salt electrolytes for high-voltage and low-temperature aqueous supercapacitors.