The Nyquist impedance plot in Figure demonstrates an initial decrease in electrolyte resistance with increasing salt concentration, reaching a minimum at 5 mol•kg -1 . Beyond this concentration, the resistance increases again, becoming progressively higher. This trend indicates that the ionic conductivity of water-in-LiTFSI electrolytes is the result of a specific balance between the number of charge carriers and their mobility. The increasing resistivity observed at salt concentrations above 5 mol•kg⁻¹ directly indicates ion association, which is discussed in detail in the following section. The low-frequency region is primarily explored here to outline the structure of the interface of a non-porous stainless electrode in water-in-LiTFSI and expressed through a set of electrochemical parameters, which are calculated using equations S1 to S11 as detailed in Method S4. The observed impedance behavior, with a tail-like Nyquist plot and the absence of maxima in the plots of imaginary impedance (Z′′) (Figure ), real permittivity (ε′) (Figure ), and imaginary capacitance (C′′) vs. frequency (Figure ), indicate the water-in-LiTFSI systems do not have a distinct dielectric relaxation between 1 MHz and 1 mHz, where the capacitive or permittivity properties would significantly change at open circuit potential. The real part of the capacitance of the non-porous electrode also gives no reasonable information (Figure ). Such a phenomenon is typical also for other reported homogeneous diluted electrolytes including KCl and NaCl, where the double-layer capacitance is the primary contributor to the impedance response across the measured frequency range. In such a case, representations of conductance (equation S8) and susceptance (equation S9) would be insightful , as the major focus of this study is the EDL response. These two quantities are also often termed as real (Y′) and imaginary (Y′′) parts of admittance (Figure ). Khademi et al. compared both equivalent circuit models and admittance for determining the EDL capacitance in NaCl/surfactant mixtures and found similar values. Hence, they recommended utilizing admittance for accurate and direct measurement of capacitance, eliminating the necessity for additional modeling assumptions. However, for broad reader interest, we further normalized the real and imaginary part of admittance with the cell constant to obtain real (σ′) and imaginary (σ′′) ionic conductivity from equations S10 and S11, respectively. Notably, according to these equations, the entire measurement cell is treated as a single capacitor.