Electric double layer structure of water-in-salt electrolyte in a porous carbon electrode elaborated with Raman spectroscopy and electrochemical methods

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 dynamics in water-in-salt electrolytes within the nanopores of carbon electrodes are not fully understood. This study explores the ion association in water-in-salt LiTFSI electrolyte in more detail, classifying various ion pairs as a function of concentration. Based on Raman spectroscopy data of electrolyte and electrochemical investigations on non-porous electrodes, modification in the classical Gouy-Chapman-Stern (GCS) model has been proposed by incorporating ionicity to estimate Debye length. The modified model shows a sharp Debye length decrease as the concentration rises from 1 to 10 mol∙kg⁻¹ but an increase beyond 10 mol∙kg⁻¹ due to ion pairing. The modified model accurately reflects differential and experimental EDL capacitance values obtained from cyclic voltammetry and electrochemical impedance spectroscopy. The data obtained for non-porous electrodes was adjusted by dividing it with the MacMullin number of the carbon electrode to estimate the Debye length in pores. Further, introducing the MacMullin number into the Stokes-Einstein equation enabled the estimation of ionic radii within pores, which was subsequently utilized to calculate extent of ion desolvation/dehydration in micro- and mesopores. The concentration-dependent ionic association governs the Debye length trends in pores, which correlate with confined ionic radii, ion desolvation, and resulting EDL charging dynamics. Our findings highlight 5 mol∙kg⁻¹ LiTFSI as optimal for faster charging rates and 10 mol∙kg⁻¹ for higher energy density, providing critical insights for developing efficient electrolytes and porous carbon electrodes.