Interfacial behavior of water-in-salt electrolytes at porous electrodes and its effect on supercapacitor performance

Abstract

Though Water-in-salt (WIS) electrolytes offer a wide potential window for energy storage device operation in aqueous conditions that can deliver high energy density, their high viscosity and low ionic conductivity certainly limits the rate performance of the devices. Herein, we present electrical double layer behavior and nature of transportation of WIS ions at different porous electrode interfaces using in situ Raman spectroscopy and electrochemical impedance spectroscopy (EIS). The in situ Raman analysis shows that, in activated carbon electrodes (pore size > 30 nm), the WIS electrolyte ions diffuse into the pore network and charge/discharge potential dictates the ion dynamics at the interface. Whereas, these ions undergo adsorption phenomena on graphene electrodes (pore size < 3 nm), thus facilitating rapid sorption of ions during charge-discharge. EIS study presented herein, with detailed bode plot analysis elucidates the dependence of capacitance, rate capability of the supercapacitor on the nature of electrodes and their pore size. In a typical supercapacitor cell, graphene electrodes with WIS electrolyte delivered a very high specific energy of 55.3 Wh/kg at a specific power of 2.4 kW/kg, unparalleled to any of the existing WIS electrolyte reports. The current studies provide experimental insights into ion storage and their dynamic mechanism at the interface formed by WIS electrolytes that will assist in designing of suitable electrode materials.