Effect of salt concentration in water-in-salt electrolyte on supercapacitor applications

Electrical double-layer supercapacitors offer numerous advantages in the context of energy storage; however, their widespread use is hindered by the high unit energy cost and low specific energy. Recently, water-in-salt (WIS) electrolytes have garnered interest for use in energy storage devices. Nevertheless, their direct application in high-power devices is limited due to their high viscosity. In this study, we investigate the WIS Lithium bis(trifluoromethanesulfonyl)Imide (LiTFSI) electrolyte, revealing a high specific capacitance despite its elevated viscosity and restricted ionic conductivity. Our approach involves nuclear magnetic resonance (NMR) analysis alongside electrochemical analyses, highlighting the pronounced advantage of the WIS LiTFSI electrolyte over the WIS LiCl electrolyte at the molecular level. The NMR analysis shows that the LiTFSI electrolyte ions preferentially reside within the activated carbon pore network in the absence of an applied potential, in contrast to LiCl where the ions are more evenly distributed between the in-pore and ex-pore environments. This difference may contribute to the difference in capacitance between the two electrolytes observed during electrochemical cycling.