Ionic association in highly concentrated electrolytes hinders the rate performance of batteries and supercapacitors. Herein, an ecofriendly water-in-salt electrolyte consisting of choline chloride is proposed. The diffusivity, ionic conductivity and dissociation of water-in-choline salt are compared with those of water-in-LiTFSI electrolytes, revealing a far better dissociation of the choline salt. The electrochemical studies reveal that ion penetration into the nanoporous carbon electrode is controlled by the hydration shell of the ions and by ion-pairing at high concentrations. Due to strong lithium hydration in water-in-LiTFSI, the positive and negative electrodes exhibit a disparity of the electric double-layer (EDL) capacitance. For water-in-choline chloride, on the contrary, better dissociation leads to facile EDL charging and a similar capacitance at two polarities. Further, battery-like electrodes are produced via electrodeposition of iodine in carbon electrode. The iodine-charged battery electrode is then coupled with a carbon-based EDL electrode in 25 mol kg−1 choline chloride to realize a hybrid capacitor. This device demonstrates constant energy efficiency for 20 000 galvanostatic charge/discharge cycles at a specific current of 1 A g−1 up to 1.6 V. Suppressed polyiodides shuttling due to a shortage of free water in the water-in-choline chloride makes it an electrolyte of choice for future hybrid energy storage.