A sodium perchlorate-based hybrid electrolyte with high salt-to-water molar ratio for safe 2.5 V carbon-based supercapacitor

Expanding the electrochemical stability window (ESW) of aqueous electrolytes is receiving great interest because it directly determines the energy density of electrochemical energy storage devices. Super-concentrated aqueous electrolytes (i.e. "water in salt", WIS) with extremely high salt-to-water molar ratios enable substantially reduced water activity, thereby widening ESW. However, such aqueous electrolytes give rise to sluggish ionic transfer, leading to poor rate performance of devices. Here we report a water/organic hybrid electrolyte based on a low-cost sodium perchlorate (NaClO4) salt. The hybrid electrolyte with a salt-to-water-to-acetonitrile molar ratio of 1/1.5/2.4 shows a wide ESW, and exhibits high ionic conductivity, excellent wettability, moisture-tolerant and flame-retardant properties. Such advantages enable a safe carbon-based supercapacitor to operate at 2.5 V with high specific capacitance, and deliver a good stability over 15000 cycles.