An aqueous magnesium-ion hybrid supercapacitor operated at −50 °C

The recent advances in aqueous magnesium-ion hybrid supercapacitor (MHSC) have attracted great attention as it brings together the benefits of high energy density, high power density, and synchronously addresses cost and safety issues. However, the freeze of aqueous electrolytes discourages aqueous MHSC from operating at low-temperature conditions. Here, a low-concentration aqueous solution of 4 M Mg(ClO4)2 is devised for its low freezing point (−67 °C) and ultra-high ionic conductivity (3.37 mS cm−1 at −50 °C). Both physical characterizations and computational simulations revealed that the Mg(ClO4)2 can effectively disrupt the original hydrogen bond network among water molecules via transmuting the electrolyte structure, thus yielding a low freezing point. Thus, the Mg(ClO4)2 electrolytes endue aqueous MHSC with a wider temperature operation range (−50 °C–25 °C) and a higher energy density of 103.9 Wh kg−1 at 3.68 kW kg−1 over commonly used magnesium salts (i.e., MgSO4 and Mg(NO3)2) electrolytes. Furthermore, a quasi-solid-state MHSC based on polyacrylamide-based hydrogel electrolyte holds superior low-temperature performance, excellent flexibility, and high safety. This work pioneers a convenient, cheap, and eco-friendly tactic to procure low-temperature aqueous magnesium-ion energy storage device.