Modulating Electrolyte Solvation Structure for High-Voltage and Low-Temperature Magnesium-Ion Supercapacitors

Aqueous supercapacitors have great potential in energy storage devices due to their high-power density and safety. However, due to the water hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the electrochemical stabilization window of aqueous electrolytes needs to be widened. Moreover, the application of aqueous electrolyte at low temperature is often limited by the freezing point of water. In this paper, we modulate the solvation structure of aqueous magnesium-ion supercapacitors by using sulfolane as a co-solvent. The modulated low salt concentration hybrid electrolyte extends the electrochemical stability window of the electrolyte to 2.9 V and enhances the stability of the electrolyte at extreme temperatures, as well as provides safe and non-flammable properties. Based on the hybrid electrolyte, the water-organic hybrid magnesium-ion supercapacitors (HMSCs) are able to operate within an enlarged voltage range of 0–2.2 V at a low temperature of −30 °C. The HMSC shows a specific capacitance of up to 58 F/g at room temperature and retains a specific capacitance of 39 F/g at a current density of 15 A/g, demonstrating a good rate performance. Furthermore, after 15,000 cycles at a charge/discharge current density of 5 A/g, the HMSC sustain 88 % specific capacitance. It also has an outstanding cycling performance at −30 °C, maintaining a specific capacitance of more than 92 % after 20,000 cycles. These findings suggest that modulating the solvation structure by organic solvent additive is an effective solutions enabling high-voltage and low-temperature aqueous supercapacitors.