Supercapacitors have been the key target as energy storage devices for modern technology that need fast charging. Although supercapacitors have large power density, modifications should be done to manufacture electrodes with high energy density, longer stability, and simple device structure. The polymorph MoS2 has been one of the targeted materials for supercapacitor electrodes. However, it was hard to tune its phase and stability to achieve the maximum possible efficiency. Herein, we demonstrate the effect of the three main phases of MoS2 (the stable semiconductor 2H, the metastable semiconductor 3R, and the metastable metallic 1T) on the capacitance performance. The effect of the cation intercalation on the capacitance performance was also studied in Li2SO4, Na2SO4, and K2SO4 electrolytes. The performance of the electrode containing the metallic 1T outperforms those of the 2H and 3R phases in all electrolytes, with the order 1T > 3R > 2H. The 1T/2H phase showed a maximum performance in the K2SO4 electrolyte with a specific capacitance of 590 F g–1 at a scan rate of 5 mV s–1. MoS2 showed a good performance in both positive and negative potential windows allowing the fabrication of symmetric supercapacitor devices. The 1T MoS2 symmetric device showed a power density of 225 W/kg with an energy density of 4.19 Wh/kg. The capacitance retention was 82% after 1000 cycles, which is an outstanding performance for the metastable 1T-containing electrode.

This chapter introduces the Hubbard model and its applicability as a corrective tool for accurate modeling of the electronic properties of various classes of systems. The attainment of a correct description of electronic structure is critical for predicting further electronic-related properties, including intermolecular interactions and formation energies. The chapter begins with an introduction to the formulation of density functional theory (DFT) functionals, while addressing the origin of bandgap problem with correlated materials. Then, the corrective approaches proposed to solve the DFT bandgap problem are reviewed, while comparing them in terms of accuracy and computational cost. The Hubbard model will then offer a simple approach to correctly describe the behavior of highly correlated materials, known as the Mott insulators. Based on Hubbard model, DFT+U scheme is built, which is computationally convenient for accurate calculations of electronic structures. Later in this chapter, the computational and semiempirical methods of optimizing the value of the Coulomb interaction potential (U) are discussed, while evaluating the conditions under which it can be most predictive. The chapter focuses on highlighting the use of U to correct the description of the physical properties, by reviewing the results of case studies presented in literature for various classes of materials.

A facile one-step method was demonstrated for the electrodeposition of manganese−nickel sulfide (Mn−Ni−S) 3D interconnected sheets on nickel foam substrates. The assynthesized materials were characterized using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) techniques. Upon their use as supercapacitor electrodes, the electrodeposited Mn−Ni−S showed exceptionally high specific capacitance (2849 and 1986 F/g at 1 and 5 A/g, respectively) and an excellent rate capability. Using Fe 3 O 4 -GR as the negative electrode and the Mn−Ni−S 3D interconnected sheets as the positive electrode to assemble an asymmetric supercapacitor device revealed high power density (800 W kg −1 ) and energy density (40.44 Wh kg −1 ) with 90% capacitance retention and a Columbic efficiency of 100% after 11 000 cycles, indicating the high potential of the fabricated materials for practical energy storage devices.

MoS 2 is a 2D material that has been widely used in supercapacitor applications because of its layered structure that provides a large surface area and allows for high electric double-layer charge storage. To enhance the cycling stability and capacitance of MoS 2 , it is usually mixed with carbon materials. However, the dependence of the charge storage mechanism on the structure of the carbon material is still unclear in literature. Herein, the effect of the structure of the carbon material on the charge storage mechanism in 2H flower-shaped MoS 2 is investigated in detail. Specifically, 2H MoS 2 was mixed with either 8 nm-diameter carbon nanotubes (CNTs) or graphene nanoflakes (GNFs) in different weight ratios. Also, a composite of MoS 2 , CNTs, and GNFs (1:1:1) was also studied. The charge storage mechanism was found to depend on the structure and content of the carbon material. Insights into the possible storage mechanism(s) were discussed. The MoS 2 /CNT/GNF composite showed a predominant pseudocapacitive charge storage mechanism where the diffusion current was ∼89%, with 88.31% of the resulted capacitance being due to faradic processes.

The intensive implementation of Li‐ion batteries in many markets makes it increasingly urgent to address the recycling of strategic materials from spent batteries. Batteries typically contain toxic chemicals and cannot be disposed of at will. In this study, Li−Ni−Mn−Co hydroxides are successfully recycled from spent Li‐ion batteries electrodeposited on nickel foam, and fully characterized using different techniques such as field emission scanning electron microscopy (FESEM), X‐ray diffraction (XRD), energy dispersive X‐ray spectroscopy (EDXS), inductively coupled plasma (ICP), and X‐ray photoelectron spectroscopy (XPS) techniques. The recycled nanostructured films are tested in a three‐electrode electrochemical system to investigate their capacitance behavior. The recycled electrodes show high capacitance of 951 F g−1 (specific capacity of 523.5 C g−1) at 1 A g−1. Moreover, the recycled materials are used as positive electrodes to construct asymmetric supercapacitor devices. The device shows a coulombic efficiency of 100 %, a capacitance retention reaching approximately 90 % with excellent cycling stability after 10 000 cycles as well as reasonable power and energy densities.