Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was purchased from IoLiTec (Heilbronn, Germany), to prepare a range of electrolytes with concentrations from 0.5 to 20 mol•kg -1 . For calculating the physicochemical properties of the bulk solutions, the dynamic viscosity (η) was determined with a Modular Compact Rheometer following a standard protocol (MCR 502 SN 82231668 with a cone plate system, Anton Paar, Austria) at room temperature (25 ℃). The ionic conductivity, EDL capacitance, and other electrochemical properties were determined in a two-electrode (stainless steel) Swagelok cell using electrochemical impedance spectroscopy (EIS) at open circuit potential (OCP ≈ 30 mV). Cyclic voltammograms (CVs) of the electrolyte solutions were recorded in an electrochemical glass cell configured with three electrodes (glassy carbon as a working electrode) to estimate the differential capacitance. The detailed setup and methodology of EIS and CV are provided in Method S1-S2 and Figure (electronic supporting information). The ion association patterns in water-in-salt LiTFSI electrolytes were studied by Raman spectroscopy. Raman spectra of the bulk solutions were acquired with a LabRAM HR 800 spectrometer (Horiba, Kyoto, Japan) equipped with a 532 nm laser at a power of 0.5 mW and coupled with an Olympus BX41 microscope. A typical carbon electrode sheet prepared from multiporous Kuraray activated carbon powder YP80 F was used in the two-electrode symmetric Swagelok-type capacitor cells to investigate the evolution of the EDL structure of water-in-salt LiTFSI electrolytes in the carbon pores, as detailed in Method S3. The pore configuration of YP80 F was analyzed using nitrogen (N₂) adsorption and desorption at -196 °C with a Micromeritics ASAP 2020 surface area and porosity analyzer (Georgia, U.S.A). The sample was degassed at 140 ℃ for 12 h, before measuring the isotherm in the relative pressure range of 0.1 < P/P0 < 1.0.