Ion dissociation in water-in-salt LiTFSI plays a key role in determining charge/discharge rates of nanoporous carbon electrode. Ionic association is influenced by the electrolyte concentration giving rise to various types of ion pairs which determine the EDL structure by influencing the Debye length. Thus, correct Debye Length estimation in water-in-salt electrolytes requires ionicity to be incorporated in Gouy-Chapman-Stern model, which accuracy is validated via similarity between models and experimental EDL capacitance values of non-porous and porous electrodes. Using the MacMullin number proved to be a straightforward approach, avoiding complex approximations for the Debye length in and outside of pores. Moreover, the use of imaginary conductivity-based EDL capacitance approximation from EIS provides reliable results without the need for additional models. Experimental CEDL values from imaginary conductivity, differential capacitance from CVs, and other theoretical CEDL values were higher at 10 mol•kg⁻¹, likely due to the higher desolvation number compared to 5 mol•kg⁻¹. Beyond 10 mol•kg⁻¹, the presence of intimate and aggregated ion pairs resulted in Bjerrum lengths of 2.5 nm and 3.5 nm for 15 mol•kg⁻¹ and 20 mol•kg⁻¹, respectively, exceeding the size of micropores (≤ 2 nm). These pairs stabilize partially desolvated states, as ions bound in pairs are less likely to disrupt their hydration shells, thereby limiting further desolvation in pores. This limitation leads to inefficient charge screening. From these findings, we discern that a concentration of 5 mol•kg⁻¹ LiTFSI is optimal for achieving fast charge/discharge rates and high-power density.