Water-in-salt electrolytes exhibit high ionic conductivity, and non-flammability, positioning them as promising sustainable alternatives to organic electrolytes for next-generation energy storage devices such as capacitors and batteries. One of the key challenges in harnessing the potential of water-in-salt electrolytes, however, lies in optimizing their concentration. While low electrolyte concentrations may lead to insufficient ion availability and diminished charge storage capacity, excessively high concentrations can result in hindered ion transport, compromising device performance. Sluggish ion transport significantly influences access to subnanometer-size pores, affecting charge/discharge kinetics and power delivery in electrochemical capacitors. As concentration increases, various types of ion pairs emerge, limiting ionicity and charge carrier concentration, which are crucial for achieving high energy densities. Additionally, the compatibility of ionic radii and carbon pore size in electrochemical capacitor electrodes governs ion desolvation, influencing the electrochemical double layer (EDL) structure. There is a lack of fundamental knowledge regarding ion electrosorption of water-in-salt electrolytes and their corresponding EDL structure within nanoporous carbon electrodes.