Metallic MoS2 Nanosheets As Electrodes with High Volumetric Energy and Power Densities

In the past few years, considerable effort has been devoted to the study of graphene as energy storage material for supercapacitors. More recently other 2D materials have emerged such as MXenes and transition metal dichalcogenides. In this study, chemically exfoliated and restacked nanosheets of metallic 1T phase MoS₂ has been introduced as a supercapacitor electrode and its electrochemical charge storage mechanism has been investigated in aqueous and organic electrolytes. High volumetric capacitance values (650F/cm³) have been obtained with 1T phase MoS₂ even at high scan rates. High volumetric energy density (0.1Wh/cm³) and power density (51W/cm³) values were measured with organic electrolytes. During chemical exfoliation and restacking process, structural, chemical and electronic properties of MoS₂ change via phase transformation from semiconducting 2H MoS₂ to metallic 1T MoS₂. High charge storage performances can be explained with the distinct structural and electronic properties of metallic 1T phase MoS₂.  The good electrical conductivity of the 1T phase enables thicker and binder free film fabrication. Interestingly the capacitance obtained from 1T MoS₂ is surprisingly high comparing the modest surface area of MoS₂. In order to understand the charge storage mechanism, ex-situ XRD measurements were conducted. Our results demonstrate that cations (Proton, Li⁺, Na⁺, K⁺, TEA⁺) intercalate to the two dimensional MoS₂ flakes and enhance the accessible surfaces for the charge storage. Charge storage performance is stable over 5000cycles (97% capacitance retention). Our study provides insight into the electrochemical behavior of the phase engineered exfoliated and restacked nanosheets.