Interplay of quantum capacitance with Van der Waals forces, intercalation, co-intercalation, and the number of MoS2 layers

Polymorph MoS2 attracts great attention for supercapacitor and Li-ion battery applications. Although the capacitance origin is usually attributed to the intercalation process, MoS2 exhibits a well-defined electrical double layer (EDL) behavior. Nonetheless, the nature of the EDL behavior of MoS2 is yet to be revealed. Herein, we investigate the quantum capacitance (CQ) of the three main phases of MoS2 (the semiconductor 2H and 3R phases and the metallic 1T phase). Interestingly, the number of MoS2 layers greatly affects the CQ of the material. In addition, the absence of Van der Waals (VdW) interactions overestimates the bandgap and CQ, emphasizing the importance of the VdW correction in the CQ calculations. As MoS2 usually stores charges via the intercalation of cations, the effect of H+, N+, Li+, and K+ intercalation on the CQ of the three phases of MoS2 is thoroughly investigated. The intercalation of all studied alkali metal cations leads to the transformation of the 2H and 3R phases into the metallic 1T phase and increases its stability. Importantly, the K+ and Na+ intercalations show the greatest impact in enhancing the C-Q of the studied phases. However, the charging process of K+ and Na+ ions is not as reversible as that of Lithorn as revealed from the estimated binding energies. To this end, herein, we demonstrate the co-intercalation of Lithorn/Nathorn ions as a strategy to enhance the overall capacitance performance. The Li+ /Na+ co-intercalation shows a CQ of 3163 F/g with a more thermodynamically stable adsorption process. Finally, the study recommends the use of 2H-MoS2 only as a positive electrode and 1T-MoS2 as a negative and/or positive electrode in energy storage devices. Also, Kthorn intercalation is recommended to achieve the highest C-Q and the Li+/Na+ co-intercalation for the best overall performance. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the quantum capacitance of the three main phases of MoS2 and reveals that the number of MoS2 layers greatly affects the material's CQ. The absence of Van der Waals interactions overestimates the bandgap and CQ, emphasizing the importance of VdW correction in CQ calculations. Intercalation of alkali metal cations impacts the CQ of MoS2 phases, with K+ and Na+ intercalations showing the greatest effect on enhancing capacitance.