The energy density of the lithium-ion capacitors are restricted by the low charge-storage capability of the capacitive electrodes. Fast redox reactions from the redox-active aqueous electrolytes can effectively boost the charge storage of the capacitive electrodes without sacrificing the power performance, but are limited by the narrow electrochemical stability window. The “water-in-salt” electrolytes can enlarge the electrochemical stability window by effectively passivating the electrodes, but on the other hand remarkably elevate the cost, impair the ion conductivity and sacrifice the functionality of free water. Herein, we energize the unexpected role of free water in the “water-in-salt” electrolytes as the solvent of high-potential redox-active species, combining with the lithium insertion/de-insertion reaction at the battery-electrode to constitute a bi-redox system for effectively boosting the energy density. In addition, the unexpected opposite shift of the bi-redox potentials resulted from the decreases of the electrolyte concentration leads to an increased discharging voltage plateau to 2.2 V, helping further increase energy density and reduce costs.