The 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 (<i>C</i>_<i>Q</i>) 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 C_Q of the material. In addition, the absence of Van der Waals (<i>VdW</i>) interactions overestimates the bandgap and <i>C</i>_<i>Q</i>, emphasizing the importance of the VdW correction in the C_Q calculations. As MoS₂ usually stores charges <i>via</i> the intercalation of cations, the effect of H⁺, Na⁺, Li⁺, and K⁺ intercalation on the <i>C</i>_<i>Q</i> 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 Li⁺ as revealed from the estimated binding energies. To this end, herein, we demonstrate the co-intercalation of Li⁺/Na⁺ ions as a strategy to enhance the overall capacitance performance. The Li⁺/Na⁺ co-intercalation shows a C_Q 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, K⁺ intercalation is recommended to achieve the highest C_Q and the Li⁺/Na⁺ co-intercalation for the best overall performance.