A novel approach to describe the electric double layer structure of water-in-salt electrolytes in porous carbon electrodes

Understanding the interplay between ion association, desolvation, and electric double layer (EDL) structure is crucial for designing high-performance energy storage devices with concentrated electrolytes. However, these ion dynamics in water-in-salt electrolytes within the nanopores of carbon electrodes are not fully understood. Based on Raman spectroscopy of electrolytes and electrochemical investigations of non-porous electrodes, the classical Gouy-Chapman-Stern model has been modified by incorporating ionicity to estimate the Debye length. The modified model shows a sharp Debye length decrease as the concentration rises from 1 to 10 mol kg ⁻¹, followed by an increase due to ion pairing above 10 mol kg ⁻¹. The modified model accurately reflects differential and EDL capacitance values obtained from cyclic voltammetry and electrochemical impedance spectroscopy. The data obtained for non-porous electrodes was divided by the MacMullin number (ratio of tortuosity to porosity) of the carbon electrode to estimate the Debye length in the pores. The introduction of the MacMullin number into the Stokes-Einstein equation allowed the estimation of ionic radii within pores, which was subsequently used to calculate the extent of ion desolvation/dehydration in micro- and mesopores. The concentration-dependent ion association governs the Debye length trends in pores, correlating radii of confined ions, ion desolvation, and EDL charging dynamics. Our findings suggest a concentration of 5 mol kg ⁻¹ LiTFSI as optimal for fastest charging rates and 10 mol kg ⁻¹ for highest energy density, providing critical insights for developing efficient electrolytes and porous carbon electrodes.