Capacitive Charging Mechanisms at the Molybdenum Disulfide/Ionic Liquid Interface

Abstract

Molybdenum disulfide (MoS2) is a 2D material of the transition metal dichalcogenide family which has recently gained significant attention for use in supercapacitors. MoS2 can exist as several polymorphs such as the 2H and 1T which are semiconducting and semi-metallic, respectively. The 2H phase, found in nature, is the most stable form of MoS2. The 1T phase is metastable and can be obtained by chemical methods such as via lithium intercalation. It shows a much higher conductivity (100 S/m). Furthermore, restacked single layers of 1T-MoS2 can be intercalated by smaller ions such as Na+, Li+, H+ and K+, in aqueous electrolyte, allowing the electrolyte to access their interlayer spacing and therefore providing impressively high volumetric capacitance of between 400 – 700 F/cm3. Hence, molybdenum disulfide is a promising material for supercapacitor application.While these high values are achieved in aqueous electrolyte, few studies have examined their capacitive behavior in room temperature ionic liquid (IL) electrolytes which exhibit much higher electrochemical stability windows > 3-4 V. The potential of coupling high capacitance materials with high voltage operation has the potential to lead to high energy densities due to a supercapacitor’s square dependence of the energy density on cell voltage. However, it is currently unknown whether these electrolytes can intercalate between restacked 1T-MoS2. Furthermore, the intrinsic capacitance of the 1T-MoS2/IL electrolyte remains largely unexplored along with the impact that defects and oxidation may have. To investigate the charging behavior in IL electrolytes, we examined the electrochemical behaviour of monolayer MoS2 deposited on highly oriented pyrolytic graphite (HOPG), using cyclic voltammetry and electrochemical impedance spectroscopy. The monolayer MoS2 is deposited using a Langmuir Blodgett (LB) deposition method. Multilayers are deposited by sequential layer-by-layer deposition. The negligible roughness and surface coverage of the monolayers are confirmed and estimated by atomic force and scanning electron microscopies. Highly purified 1-ethyl, 3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) is used as the electrolyte and the monolayers are electrochemically probed in custom-made O-ring sealed electrochemical cell. Using these analyses, we probe the frequency and potential-dependence of the intrinsic capacitance of the MoS2/IL interface and provide design criteria for building improved IL-based supercapacitors.