High-performance hybrid supercapacitors enabled by protected lithium negative electrode and "water-in-salt" electrolyte

Aqueous supercapacitors are attracting increasing attention owing to their high power density, cyclability and environmental friendliness. However, the energy density of common aqueous supercapacitors are restricted by low cell voltage and narrow voltage window. Here we demonstrate that such limitations can be overcome with advanced hybrid supercapacitors using multi-layered water-stable protected lithium negative electrode combined with high concentration "water-in-salt" electrolyte. 4 V-class cell voltage with wide voltage window can be delivered for advanced hybrid supercapacitors with capacitive or pesudocapacitive positive electrodes. A typical advanced hybrid supercapacitor with MnO2 electrode shows a 4.4 V maximum cell voltage with a 1.5 V window, an energy density of 405 Wh kg(MnO2)(-1) at a power density of 0.88 kW kg(M)(O02)(-1)(-). A high energy density of 163 Wh k(gMO0)(-1)is maaintained at a power density of 16.7 kW k(MnO2)(-1). These advanced hybrid supercapacitors show acceptable cycle stability and good energy retentions (around 90% within 3000 cycles).