A 2.4-V asymmetric supercapacitor based on cation-intercalated manganese oxide nanosheets in a low-cost “water-in-salt” electrolyte

The main challenge for aqueous asymmetric supercapacitors (ASCs) is the relatively low voltage, which significantly diminishes the energy density of the device. Here, cation-intercalated manganese oxide nanosheets Na0.55Mn2O4·1.5H2O (NaMnO) are synthesized via a facile molten salts method. We find that the electrode potential window for NaMnO nanosheets can be extended to 1.2 V using a low-cost and safe superconcentrated sodium perchlorate (NaClO4) “water-in-salt” (WIS) electrolyte. To construct the asymmetric supercapacitor, the as-prepared NaMnO nanosheets and activated carbon (AC) with a potential window of − 1.2–0 V are used as the positive and negative electrode, respectively. A high-voltage 2.4-V NaMnO//AC aqueous ASC in the concentrated NaClO4 WIS electrolyte is successfully assembled, which exhibited excellent rate performance (the highest power density of 24.0 kW kg−1) as well as good cycling stability (94.9% capacitance retention over 20,000 cycles at 5 A g−1). This low-cost WIS electrolyte provides new opportunities for developing high-voltage aqueous ASCs with high energy and high power densities.