Lithium-ion batteries (LIB) have a high capacity and are recycable. Most portable devices have LIB; however, their capacity is still too low to be used in some electrical vehicles. Metallic MoS₂ can serve as an anode in LIB due to its high conductivity, specific capacity, and large surface area which enable better intercalation of the incoming ions and enhance the battery’s stability and rate performance [73]. The chemical composition of metallic MoS₂ (1T MoS₂) was investigated using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and X-ray diffraction (XRD), and it was found to have binding energies of 228.8 and 231.9 eV corresponding to the 3d_5/2 and 3d_3/2 components, respectively, for Mo–S bonding. The S 2p components have binding energies of 161.5 eV and 162.5 eV, corresponding to S 2P_3/2 and 2P_1/2, respectively [43,62]. The bonding of both Mo and S is nearly 1 eV less than that of 2H MoS₂.

The research work in this area is based on two directions, namely, whether to enhance the stability of metallic MoS₂-based LIB or to enhance the conductivity of 2H MoS₂ to serve as an anode in LIB, since it has better stability. In a trial to suppress the high intrinsic electric conductivity of metallic MoS₂, it was alligned over graphene with a relatively large separation distance of 0.98 nm between them. The first cycle showed a high capacity of ≈1700 mA h g⁻¹ at a current density of 70 mA g⁻¹ and an initial coulombic efficiency of 70%. The battery had a high capacity of 666 mA g⁻¹ at a high current density of 3500 mA g⁻¹, with a reversible capacity of ≈1700 mA g⁻¹ at a low current density of 70 mA g⁻¹ [61]. MoS₂ was mounted on carbon fiber cloth to obtain a high reversible specific capacity of 1789 mA h g⁻¹ at 0.1 A g⁻¹ and a retained capacity of 853 mA h g⁻¹ after 140 cycles at 1 A g⁻¹ [63]. A composite of 1T-MoS₂ and conductive molybdate (NiMoO4) was used to obtain a coulombic efficiency of 99.5%, and it had stability after 750 cycle [74]. A pure metallic MoS₂ structure was developed in [62] to avoid stability problems in material stacking. The battery had a specific capacity of ≈935 mA h g⁻¹ for 200 cycles at 5 A g⁻¹ that can be increased to 1150 mA h g⁻¹. It had a high rate performance at the current density range from 0.2 to 20 A g⁻¹ and a reversible capacity of 589 mA g⁻¹. Table 1 summarizes some MoS₂-based LIB showing the used structure and phase of MoS₂ and its specifications.

Since NIB are less efficient than LIB, there is not much research work about the role of MoS₂ in Na-ion batteries; however, an early theoretical study in [82] showed that monolayer MoS₂ can have a higher Na adsorption when compared to bulk MoS₂. It is perfect as an anode electrode in Na-ion batteries, with a theoretical capacity of 335 mA h g⁻¹. The monolayer maintains a lower applicable voltage of 1.0 V when compared to the bulk (1.7–2.0 V). The low mobility of Na is overcome by the monolayer structure because when the dimensions decrease, the diffusion barrier decreases from 0.7 to 0.11 eV. A graphene sandwich of MoS₂, MoS₂-graphene-MoS₂, in [43] had a high capacity of 175 mA h g⁻¹ at a high current density of 2 A g⁻¹ and a reverse capacity of ≈313 mA h g⁻¹ at a low current density of 50 mA g⁻¹. It stabilized at a current density of 313 mA h g⁻¹ after 200 cycles. A dual phase of 2H and 1T MoS₂ was used to obtain a capacity of 300 mA h g⁻¹ after 200 cycles and 99% coulombic efficiency. The good interlayer spacing permitted a high reversibility of Na ion intercalation [75]. MoS₂ and amorphous carbon (C) microtubes (MTs) in [76] were used to improve the capacity to 563.5 mA h g⁻¹ at 0.2 A g⁻¹ and obtain 86.6% coulombic efficiency with cyclic stability of 484.9 mA h g⁻¹ at 2.0 A g⁻¹. Table 1 summarizes some MoS₂-based NIB showing the used structure and phase of MoS₂ and its specifications.

It is worth mentioning that, recently, multilayer intercalation of alkali metals (AM) (Li, K, Na) between bilayer graphene was possible and showed a higher storage capacity than the bulk structure [83]. A study in [84] compared the intercalation energetics of bilayer graphene and MoS₂ for a number of alkali metals (Li, Na, K, Rb, Cs). The weak van der Waal forces between MoS₂ layers enabled easy intercalation of Li ions without excess volume, and the Li storage capacity could reach 700 mA h g⁻¹. The study showed that the storage capacity of MoS₂ is significantly lower than graphene, but it can be increased through vertical van der Waals forces between graphene-MoS₂ heterostructures where it will benefit from the light weight of graphene and the low formation energy of MoS₂.

Supercapacitors are energy storage devices that have a lower energy density than batteries and a higher power density, meaning they can be used as a complementary device in electric vechiles beside batteries [73]. MoS₂ is a good capacitor since it is formed of layers (sheets) that provide a large area for charge storage, where ions are inserted between layers through intercalation. The layers are exfoliated and then restacked to form electrodes with improved electrochemical features [85]. Carbon-based supercapacitors are leading the market due to their fast charge–discharge, versatile synthesis, and stability [86], but MoS₂ can achieve extraordinary capacitances from 400 to 700 F cm⁻³ [85]. The charge storage mechanism of 1T MoS₂ was investigated in [87] for an interlayer spacing ranging from 0.615 to 1.615 nm in ionic liquids. It was found that the highest volumetric and gravimetric capacitances were 118 F cm⁻³ and 42 F g⁻¹, respectively, and occurred at a MoS₂ interlayer spacing of 1.115 nm. A micro-supercapacitor proposed in [77], developed through spraying MoS₂ nanosheets on a Si/SiO₂ chip followed by laser patterning, had excellent cyclic and electrochemical performance compared to graphene-based micro-supercapacitors. It had a high area capacitance of 8 mF cm⁻² and a volumetric capacitance of 178 F cm⁻³. The idea opens the door for portable and flexible micro-electronic devices. Some studies were directed towards the nano-MoS₂ structure, where it showed a better performance in energy storage. Metallic 1T phase MoS₂ nanosheets were found to efficiently intercalate ions such as H+, Li+, Na+, and K+ with capacitance values ranging from ∼400 to ∼700 F cm⁻³ in different aqueous electrolytes [85]. Their coulombic efficiencies were more than 95% and were stable until 5000 cycles. The MoS₂ flower-shaped nanostructure was paired with 3D graphene to develop a supercapacitor prototype with dimensions of 23.6 × 22.4 × 0.6 mm³ by stacking a MoS₂ nanoflower structure over 3D graphene over a graphite electrode [78]. The prototype had a high specific capacitance C_sp of 58 F g⁻¹, an energy density of 24.59 W h Kg⁻¹, and a power density of 8.8 W Kg⁻¹, with an operating window of nearly 2.7 V (−1.5 to +1.2 V). The study represents an inexpensive supercapacitor without the need for ionic liquid media. The nanostructures of MoS₂ showed excellent supercapacitance when grown on Ni foam through the hydrothermal process [79]. It was able to maintain 92% of its maximum capacitance after 9000 charge–discharge cycles at 5 A g⁻¹. The study confirmed that the high mass loading of MoS₂ nanostructures grown over conducting substrates corresponds to superior energy storage electrodes. A recent work studying the capacitance of MoS₂ quantum sheets (QSs) in [80] demonstrated that MoS₂ quantum sheets have a high capacitance of 162 F g⁻¹, which is very high if compared to typical MoS₂ supercapacitors. MoS₂ QSs have an energy density of 14.4 W h kg⁻¹ and a long cycle life. In [81], a 3D interlayer-expanded MoS₂/rGO nanocomposite (3D-IEMoS₂@G) was synthesized and experimented as an anode in lithium-ion and sodium-ion batteries. It was then modified by pairing it with nitrogen-doped hierarchically porous 3D graphene (N-3DG) to obtain sodium and lithium hybrid supercapacitors (HSCs). The Na-HSC showed an excellent performance of 140 W h kg⁻¹ at 630 W kg⁻¹, and 43 W h kg⁻¹ at an ultra-high power density of 103 kW kg⁻¹ (charge finished within 1.5 s). It can retain its capacitance even after 10,000 cycles. Table 1 summarizes some MoS₂-based supercapacitors showing the used structure and phase of MoS₂ and its specifications.

Hydrogen was recently studied to substitute fuel as a source of energy. It is not a source of energy by itself but rather a carrier of energy. It has to be manufactured as with electricity. It has to be manufactured from coal or natural gas; however, in both cases, carbon is released, and environmental pollution occurs. It is also generated from water, which represents a better environmental solution. It is not toxic, as opposed to fuel, has a high octane number, and does not cause ozone issues [88]. MoS₂ is a cheap catalyst in electrochemical HER [89] and water splitting reactions [90]. The large number of electrostatic active edges and high structural defects makes MoS₂ a good catalyst. 1T MoS₂ is known to be a better catalyst in HER than 2H MoS₂ because of its reactive basal planes, which gains its activity from the hydrogen binding affinity at the surface S sites [91]. Studies have been conducted to enhance the catalytic activity and stability of MoS₂ so that it can replace noble metals. The catalytic activity of MoS₂ mainly depends on the active edges or the cell vacancies. The work in [92], based on the first-principle density functional theory (DFT), studied different possible cell vacancies of MoS₂ and found that the best catalytic activity for MoS₂ occurs with Mo and MoS₂ cell vacancies. The efficiency of HER is enhanced when compared to platinum catalyst reactions. A later study conducted by the same researchers [90] focused on Mo defects on the inert basal plane of MoS₂ and showed its better performance in HER and water splitting reactions. The active sites of MoS₂ basal planes are restricted to edges and missing primitive cell vacancies. The weak van der Waal interactions between MoS₂ layers result in a hydrophobic characteristic which assigns more importance to layer defects [93]. A detailed study of five types of defects in MoS₂ layers was conducted [92]. The study investigated the effect of disulfur vacancy (VS₂), vacancy complex of Mo and nearby tri-sulfur (VMoS₃), Mo vacancy (VMo), nearby S tri-vacancy (VS₃), and VMoS₂, and it was found that VMo and VMoS₂ can activate inert basal planes and have a role in dissociating water in HER. The Gibbs energy for hydrogen adsorption (ΔG0H∗)  for VMo is −0.198 eV, and for VS₃, it is 0.06 eV, which is comparable to platinum, which has a value of −0.09 eV.

The effect of strain on Mo vacancies in single-layer MoS₂ was investigated in [94], where a biaxial compressive strain of 4.5%, carried out by modifying the Mo and S interaction around the vacancy, showed optimal catalytic properties, with Gibbs free energy between −0.03 and −0.04 eV at the active sites. A hybrid catalyst made by growing MoS₂ over cobalt diselenide (MoS₂/CoSe₂) approached the commercial platinum catalyst behavior [95]. The reaction in the acidic electrolyte had a Tafel slope of 36 mV dec⁻¹, onset potential of −11 mV, and exchange current density of 7.3 × 10⁻² mA cm⁻¹. A trial to increase the active sites of MoS₂ was introduced in [96] using cracked monolayers of 1T MoS₂. The monolayers were obtained through hydrothermal synthesis. 2H MoS₂ was ultrasonicated with lithium which facilitated the intercalation of MoS₂ layers, which were then exfoliated to obtain 1T MoS₂. The resulting MoS₂ had a favorable HER performance characteristic, with a low overpotential of 156 mV, at 10 mA cm⁻² in an acidic medium, and a low Tafel slope of 42.7 mV dec⁻¹. Table 2 summarizes some MoS₂ applications in HER.

MoS₂ acts as a catalyst in OER which is a step in water splitting. Few studies have been conducted related to the role of MoS₂ in OER. 1T MoS₂ with amorphous nickel–cobalt complexes was used as a catalyst in water splitting to generate hydrogen and oxygen [97]. The method represents a low-cost, easy, and stable way to perform water splitting instead of using expensive noble metal catalysts. Another hybrid nanocomposite made of MoS₂ microspheres over Ni foam was proposed in [98]. The study made use of the efficient catalytic activity of MoS₂ while increasing its conductivity by attaching it to the conductive Ni foam. The overpotential decreased rapidly (nearly by 290 mV) when compared with RuO₂ as a catalyst. MoS₂ quantum dots (MSQDs) in [99] were used as a catalyst for OER. The quantum dots were synthesized using (NH₄)₂MoS₄ as a precursor to produce MoS₂ quantum dots (MSQDs), and then activation of QDs was carried out using potential cycling under electrolyte conditions to produce the as-synthesized materials after cycling (MSQDs-AC). The catalytic current density increased as the number of potential cycles increased, and it reached its maximum after 50 potential cycles. The technique avoids using carbon that leaves carbon QDs behind which negatively interfere with the process. The resulting MSQDs-AC had the lowest Tafel slope (39 mV dec⁻¹) when compared to other state-of-the-art catalysts such as IrO₂/C, and they also had fast reaction kinetics. Table 2 summarizes some of the MoS₂ applications in OER.