Capacitive Performance of Water-in-Salt Electrolytes in Supercapacitors: A Simulation Study

Water-in-salts are a new family of electrolytes with very promising electrochemical properties for energy storage applications. Despite several studies involving them inside Li-ion batteries and supercapacitors, their interfacial properties remain largely unknown. Here, we simulate the interface between electrified graphite electrodes and a highly concentrated water-in-salt[where the salt is bis-(trifluoromethane)sulfonimide lithium, LiTFSI] using constant applied potential molecular dynamics. We show that the capacitance differs markedly between the positive and the negative electrodes, due to the large asymmetry in size (and shape) between the ions. By using importance sampling, we further investigate the changes in the structure of the salt at the interface, and we observe large variations that are at the origin of a series of peaks in the differential capacitance.