Development of high-voltage electrolytes with non-flammability is significantly important for future energy storage devices. Aqueous electrolytes are inherently non-flammable, easy to handle, and their electrochemical stability windows (ESWs) can be considerably expanded by increasing electrolyte concentrations. However, further breakthroughs of their ESWs encounter bottlenecks because of the limited salt solubility, leading to that most of the high-energy anode materials can hardly function reversibly in aqueous electrolytes. Here, by introducing a non-flammable ionic liquid as co-solvent in a lithium salt/water system, we develop a "water in salt/ionic liquid" (WiSIL) electrolyte with extremely low water content. In such WiSIL electrolyte, commercial niobium pentoxide (Nb2O5) material can operate at a low potential (−1.6 V versus Ag/AgCl) and contribute its full capacity. Consequently, the resultant Nb2O5-based aqueous lithium-ion capacitor is able to operate at a high voltage of 2.8 V along with long cycling stability over 3000 cycles, and displays comparable energy and power performance (51.9 Wh kg−1 at 0.37 kW kg−1 and 16.4 Wh kg−1 at 4.9 kW kg−1) to those using non-aqueous electrolytes but with improved safety performance and manufacturing efficiency. © 2020 Science China Press