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[Omar Falyouna](https://orcid.org/0000-0003-4236-6433), [Toshihiko Mandai](https://orcid.org/0000-0002-2403-7794)

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[Molybdenum disulfide (MoS_₂) −based cathode materials for magnesium (Mg2+) and Mg2+/Li+ hybrid-ion batteries: Progress and perspective](https://mdr.nims.go.jp/datasets/59eb6972-4335-4790-96e2-4135f9714184)

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Numerical modeling of nitrate reduction using of low cost organic carbon source九州大学学術情報リポジトリKyushu University Institutional RepositoryMolybdenum disulfide (MoS_₂) −based cathodematerials for magnesium (Mg2+) and Mg2+/Li+hybrid-ion batteries: Progress and perspectiveFalyouna, OmarCenter for Advanced Battery Collaboration, Research Center for Energy and EnvironmentalMaterials, National Institute for Materials ScienceMandai, ToshihikoCenter for Advanced Battery Collaboration, Research Center for Energy and EnvironmentalMaterials, National Institute for Materials Sciencehttps://doi.org/10.5109/7157941出版情報：Proceedings of International Exchange and Innovation Conference on Engineering &Sciences (IEICES). 9, pp.34-42, 2023-10-19. 九州大学大学院総合理工学府バージョン：権利関係：Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 InternationalProceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES) 34  Molybdenum disulfide (MoS₂)−based cathode materials for magnesium (Mg2+) and Mg2+/Li+ hybrid-ion batteries: Progress and perspective Omar Falyouna1* and Toshihiko Mandai1* 1 Center for Advanced Battery Collaboration, Research Center for Energy and Environmental Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan *Corresponding author email: FALYOUNA.Omar@nims.go.jp & MANDAI.Toshihiko@nims.go.jp   Abstract: Rechargeable magnesium batteries (RMBs) are gaining a great deal of attention as a promising alternative for lithium-ion batteries (LIBs) because of the distinctive properties of Mg such as abundant sources in nature, large theoretical capacity, stability of bulk metal against the ambient atmosphere, etc. However, the development of RMBs is still in the early stages, especially on the cathode side. RMBs with intercalation-type cathodes often suffer from sluggish kinetics due to the high polarization of the divalent Mg2+. Molybdenum disulfide (MoS2) is a layered transition metal dichalcogenide and serves as a prospective base material for Mg2+ intercalation. This mini-review summarizes the state-of-the-art in the development of MoS2−based cathode materials for RMBs and Mg2+/Li+ hybrid ion batteries (MLIBs).      Keywords: Rechargeable magnesium battery (RMBs); Hybrid Mg2+/Li+ ion battery; Molybdenum disulfide (MoS₂).    1. INTRODUCTION  Lithium-ion batteries (LIBs) are widely utilized in the market as excellent energy storage technologies, especially for portable electronic devices, because of their high energy density and exceptional durability [1]. However, the scarcity of lithium resources (i.e., 0.0017% by weight in the earth’s crust) significantly increases the cost of LIBs [2]. Moreover, the utilization of flammable electrolytes and the formation of dendritic depositions impose many serious operational safety issues which hinder their further development, particularly for industrial−scale applications (e.g., e−transportation, energy grid storage systems, etc.) [3].  On the other hand, rechargeable magnesium batteries (RMBs) are promising alternatives for energy storage and conversion applications owing to their unique electrochemical characteristics such as remarkable theoretical capacity (i.e., 2205 mAh g⁻1 and 3833 mAh cm⁻³), low electrode potential (−2.36 V vs. standard hydrogen electrode), the high elemental abundance of magnesium in the earth’s crust (24.31% by weight), reasonable cost, and environmentally friendliness [2], [4]. In addition, unlike Li anodes, Mg metal anodes are chemically stable and don’t form dendrites under normal electrochemical reactions which remarkably improves the operational safety of RMBs [5].  Nevertheless, the development of RMBs is still facing several challenges and issues. For instance, Mg2+ intercalation/deintercalation into/from the interlayer spacing of various cathode materials is kinetically sluggish due to the high energy barrier of Mg2+ diffusion [2], [6]. Moreover, the Mg metal anode and conventional carbonate-based electrolytes are incompatible because of the strong polarization nature of Mg2+ cations [7], [8]. These shortcomings severely deteriorate the capacity, rate capability, and cycling ability of RMBs. Hence, it is crucial to develop a cathode material with high affinity to easily insert/extract Mg2+, an anode with less passivation, and a highly stable electrolyte to promote the electrochemical performance of RMBs.   Fig. 1. Two-dimensional structure of MoS2 [9]. Over the past years, researchers have made great efforts to propose suitable cathode materials for RMBs such as titanium disulfide nanotubes (TiS2) [10], layered manganese dioxide (MnO2) [11], vanadium(V) oxide nanotubes (V2O5) [12], MXenes [13], cobalt sulfide (CoS) spheres [14], and so forth. Among these materials is molybdenum disulfide (MoS2), a well-known layered transition metal dichalcogenide (TMD) with a graphene−like structure [15]. It is an ideal candidate as an active cathode material for RMBs because of its unique structure where two sulfur (S) layers are separated by a molybdenum (Mo) layer and stacked all together by van der Waals forces (FVW) as shown in Fig. 1 [9], [15]. The flexibility of MoS2 allows the smooth insertion and extraction of Mg2+ cations during the discharge and charge processes [16]. This review is dedicated to shedding light on the recent development of MoS2−based composites as cathode materials in RMBs and Mg2+/Li+ hybrid-ion batteries (MLIBs). The electrochemical performance of MoS2−based composites was summarized based on the initial specific capacity, cyclic capacity, and number of cycles under different battery configurations as presented in Table 1 and 2. 2. WORKING PRINCIPLE OF RMBs RMBs are storage devices that transform chemical energy into electrical energy through an electrochemical reaction [2]. They consist of three main parts, namely anode, cathode, and electrolyte (Fig. 2) [17]. The anode composes of pure Mg or Mg alloys and acts as a reductant (Eq. 1). While the cathode acts as an oxidant and comprises a chemically stable material that is capable of a reversible insertion/extraction of Mg2+ such mailto:FALYOUNA.Omar@nims.go.jpmailto:MANDAI.Toshihiko@nims.go.jpProceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES)  35  as MoS2 (Eq. 2). Also, as displayed in Fig. 2, the cathode and anode are separated by a permeable membrane to prevent their direct contact and only permits the free movement of Mg2+ [6]. Anode 4Mg ↔ 4Mg2+ + 8e− (1) Cathode 6MoS2 + 4Mg2+ ↔ Mg4Mo6S12 (2) Overall 4Mg + 6MoS2 ↔ Mg4Mo6S12 (3) During the discharge Mg metal will undergo an oxidation reaction to release Mg2+ and electrons (Eq. 1) [4], [17]. Then, Mg2+ will diffuse in the electrolyte and migrate to the cathode side. After reaching the cathode, Mg2+ will intercalate in the interlayer spacing of MoS2 (Fig. 2 & Eq.2). On the contrary, on charging, Mg2+ will be deintercalated from MoS2 and moves back toward the anode (Fig. 2) [6]. Throughout the discharge/charge cycles, electrons will transfer via the external circuit to balance the charge between the electrodes (Fig. 2) [2].   Fig. 2. Schematic diagram of RMB cell. 3. RMB with MoS2−based cathode  The literature survey revealed that sixteen research articles, with a focus on the application of MoS2−based materials as a cathode material for RMBs, were published between 2004 and 2023 (Fig. 3.a). MoS2 is used to be synthesized by utilizing many reductants, for example, H2S, NaBH4, SO2, etc. at high temperatures (e.g., 850 ℃) [18]. In contrast, Li et al. demonstrated that MoS2 nanostructures could be prepared by using sulfur compounds as reductants at moderate temperatures [18]. They emphasized that sulfurization compounds will decompose at temperatures higher than 150 ℃ in acidic solutions to form H2S which is used to reduce Mo(VI) (Eq. 4).  Na2S + 2HCL → 2NaCl + H2S (4) Using several sulfurization compounds, e.g., CS2, KSCN, CH3CSNH2, Na2S, CSN2H4, in a hydrothermal process at 180 ℃, they managed to obtain various nanostructures of MoS2 namely, spherical nanovesicles, hollow-cage fullerene-like structures, and fibrous floccus [18]. They investigated the Mg2+ intercalation/deintercalation ability of as-prepared MoS2 and they found that these materials could deliver a cell capacity ranging between 2 and 25 mAh g⁻1 at 0.02 mA g⁻1 (Table 1). Liang et al. followed the same procedures to synthesize graphene-like MoS2 (G−MoS2) via a solvothermal reaction between MoO3 and thioacetamide while being mixed in pyridine at 150 ℃ [4]. The as-synthesized G−MoS2 was then used as a cathode material and magnesium nanoparticles (N−Mg) as an anode in RMBs. The fabricated [G-MoS2|Mg(AlCl₃Bu)₂/THF|N−Mg] cell exhibited an excellent reversible specific capacity of 170 mAh g⁻1 at 20 mA g⁻1 (Table 1). 95% of the initial specific capacity of G−MoS2 was obtained after 50 cycles whereas the Columbic efficiency was approximately 99% (Table 1). These results clearly state that simple modification in the synthesis process will substantially affect the electrochemical activity of MoS2 in RMBs. Yang et al. estimated the maximum theoretical capacity of zigzag MoS2 nanoribbon via density functional theory (DFT) calculations [16]. They declared that a maximum capacity of 223.2 mAh g⁻1 could be theoretically achieved due to the double-side Mg adsorptions. Since the maximum theoretical capacity of MoS2 hadn’t been reached yet, various enhancement techniques have been applied to improve the electrochemical performance of MoS2 such as carbon coating, expansion of interlayer spacing, ion introduction, defect engineering, crystal water intercalation, and so on [2], [6]. 3.1 Carbon Coating/Supportation It has been reported that carbon coating is an effective method to improve the electrochemical performance of MoS2 by not only improving the electric conductivity and accelerating the transfer of charge but also by providing a framework that helps in reducing the volume change throughout the discharge/charge reactions [2], [19]. For example, Liu et al. coated MoS2 with a carbon material by adding different weights of glucose, as a carbon source, during the hydrothermal process to formulate MoS2/C microspheres [19]. MoS2/C, with a carbon content of 46 wt%, showed a remarkable specific capacity of 213 mAh g⁻1 with a cycling capacity of 84.3 mAh g⁻1 after 50 cycles (Table 1). This improvement was ascribed to the enhancement in the electrical conductivity of the MoS2/C cathode as the electrochemical impedance spectra measurements indicated that the overall resistance of the battery significantly reduced after incorporating carbonaceous materials in MoS2. On another two occasions, Liu et al. replaced the carbonaceous material, which resulted from glucose carbonization, with reduced graphene oxide (MoS₂−rGO) [20] and graphene (MoS2/G) [8]  to support MoS2 and promote the electrochemical performance of RMBs. They manifested that the existence of a graphene layer within the MoS2 structure not only expanded the interlayer spacing (i.e., from 0.62 to 0.98 nm in the case of MoS2/G [8]) but also prevented the stacking of MoS2 layers and provided a good platform for MoS2 to grow with less aggregation [8], [20]. Although graphene intercalation in MoS2 facilitated the interfacial charge transfer and enhanced the electric conductivity, MoS2−rGO and MoS2/G−15 (i.e., 15 mg of graphene was added during the hydrothermal process) provided a moderate specific capacity of 104.2 and 115.9 mAh g−1, respectively (Table 1). Conversely, Wu et al. presented a promising technique to synthesize van der Waals’ heterostructures (vdWHs) by sandwiching a graphene layer (GR) between two layers of MoS2 to synthesize MoS2/GR [21]. The outcomes of the electrochemical analysis declared that the MoS2/GR delivered an outstanding specific capacity of 210 mAh g−1 with an excellent cycling capacity of 182.7 mAh g−1 after 300 cycles at 20 mA g−1 (Table 1). The excellent performance of MoS2/GR, compared to MoS2−rGO and MoS2/G−15 (Fig. 3.a), was attributed to the successful intercalation of graphene between MoS2 layers which notably expanded the interlayer distance from 0.62 to 1.16 nm. Proceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES)  36  This layer expansion greatly enhanced the diffusivity of Mg2+ in MoS2/GR (i.e., 3.24×10−9 cm2 S−1) eleven times more than that of bulk MoS2 (i.e., 10−20 cm2 S−1) and reduced the diffusion energy barrier to 0.4 eV. 3.2 Interlayer Expansion Previous reports indicated that the interlayer spacing between the layers of MoS2 is approximately 0.62 nm (Fig. 1) [8], [21], [22]. As stated in the previous section, expanding the interlayer spacing of MoS2 can improve the insertion/extraction of Mg2+ by reducing the diffusion energy barrier [6], [22]. Instead of using graphene, Liang et al. adopted a different technique to expand the interlayer spacing by inserting certain amounts of poly(ethylene oxide) (PEO) between the layers of MoS2 (PEO−MoS2) [22]. Their technique managed to enlarge the interlayer spacing from 0.62 to 1.45 nm (i.e., more than MoS2/GR [21]) by intercalating PEO with a MoS2:PEO molar ratio of 1:1 (peo2−MoS2). Although the huge interlayer expansion by PEO is beneficial to accelerate the diffusion kinetics of Mg2+ in MoS2, only less than 75 mAh g−1 by peo2−MoS2 could be delivered definitely due to the inclusion of large amounts of inactive components (Table 1). Wu et al. obtained an interlayer spacing of 0.97 nm by inserting polyvinylpyrrolidone (PVP) into the layered structure of MoS2 (PVP−MoS2) [5]. The diffusivity coefficient of Mg2+ in PVP−MoS2 considerably increased by 51 orders of magnitude compared with the bulk MoS2. PVP−MoS2 exhibited a good initial specific capacity of 143.4 mAh g−1 at 20 mA g−1 (Table 1). Also, the cycling experiments proved that PVP helped MoS2 to maintain the layered structure during the cycling process as 92% of the initial capacity was achieved after 100 cycles. 3.3 Ion Introduction The sluggish diffusion of Mg2+ in the interlayer spacing of MoS2 is mainly caused by the divalent nature of Mg2+ [23], [24]. However, the pre-introduction of ion/ions  (e.g., cations or anions) could enhance the diffusion kinetics of Mg2+ by shielding the effect of the strong charge of Mg2+ during the intercalation/deintercalation in MoS2 [2], [15], [25]. Venkateswarlu et al. demonstrated that fluorination of MoS2 (F−MoS₂) enhanced the electrochemical performance of bulk MoS2 (B−MoS2) from approximately 39 to nearly 55 mAh g−1 at 15 mA g−1 (Table 1) [26]. The electrochemical impedance spectroscopy (EIS) of B−MoS2 and F−MoS2 indicated that the electrochemical enhancement resulted from increasing the conductivity of F−MoS2|Mg cell after F−doing (i.e., RSEI: 500 Ω → 400 Ω and Rct: 1400 Ω → 1100 Ω). Also, Zhuo et al. reported that Cu−MoS2 achieved a better specific capacity of 150 mAh g−1, compared to pristine MoS2 (i.e., less than 100 mAh g−1 at 50 mA g−1) [25]. Unfortunately, the electrochemical efficiency of Cu−MoS2 gradually decreased as the cycling number increased to finally achieve around 91 mAh g−1 after 200 cycles. On the other hand, when Cu−MoS2 was supported by hydrogen-substituted graphdiyne nanotubes (HsGDY) to form Cu−MoS2@HsGDY nano−capsules, the specific capacity of Cu−MoS2@HsGDY was preserved even after 200 cycles (i.e., 150 mAh g−1 declined to 148.5 mAh g−1 after 200 cycles) (Table 1). The expanded interlayer spacing and the abundant active sites of Cu−MoS2 facilitated the diffusion of Mg2+. While the HsGDY prevented the restacking and aggregation of Cu−MoS2. The mutual effect of the structure’s components uniquely promoted the electrochemical performance of Cu−MoS2@HsGDY in RMBs. The findings of Zhuo et al. [25] reflect the importance of the supporting material to enhance the cycling ability of MoS2−based materials. 3.4 Defect Engineering/Modification Defect engineering is another strategy to modify the surface properties and improve the electrochemical performance of MoS2 in multivalent rechargeable batteries [2], [15]. There are two types of defects: intrinsic and non-intrinsic [15].  Intrinsic defects, e.g., Schottky defects, are internal defects generated via thermal vibration of the atomic lattice without changing the composition of the crystal [15]. In contrast, non-intrinsic defects, also known as heteroatomic defects, are created by inserting ions in the interlayer spacing of MoS2 [15]. Zhu et al. tried to create a defective MoS2 structure by changing the Mo:S ratio, such as 1:2, 1:4, and 1:7, during the thermal synthesis of MoS2 nanosheets [27]. MoS2 nanosheets with a Mo:S ratio of 1:7 exhibited a neglectable electrochemical performance and were excluded from the comparison. Whereas MoS2 with Mo:S of 1:2 and 1:4 showed a poor initial specific capacity of approximately 20 and 25 mAh g−1, respectively. However, MoS2 with Mo:S of 1:2 and 1:4 needed 141 and 41 cycles, respectively, to reach their maximum specific capacity of 67 and 152 mAh g−1 (Table 1). After reaching their maximum specific capacity, the specific capacity of both defective MoS2 nanosheets decreased to approximately 75 and 20 mAh g−1, after increasing the number of cycles to 200 and 100, respectively. It is hard to control the formation of defects during the synthesis process and the misapplication of this technique may cause serious failures in the structure of MoS2 [15]. Therefore, further research on the proper use of defect engineering for MoS2−based materials is needed. 3.5 Crystal water incorporation The insertion of H2O molecules into the interlayer spacing of MoS2 facilitates the diffusion of Mg2+ by shielding the charge effect of the divalent Mg2+ [2], [28]. Wu et al. investigated the impact of H2O intercalation on the electrochemical activity of the synthesized oxygen−doped MoS2 (O−MoS2) in RMBs [29]. They stated that O−MoS2 was in the 1T phase with an enlarged interlayer spacing of 0.94 nm. The special structure of O−MoS2, compared with the pristine MoS2, favors the intercalation of Mg2+ with an initial specific capacity of 130.2 mAh g−1. This excellent performance was further promoted to 190.3 mAh g−1 after MoS2 hydration (H−MoS2) via an electrochemically assisted procedure (Table 1). XRD and SEM analysis confirm that MoS2 hydration further enlarged the interlayer spacing from 0.92 nm to 1.03 nm. Also, the EIS analysis and the galvanostatic intermittent titration technique (GITT) proved that water intercalation accelerated the diffusivity of Mg2+ in H−MoS2 (i.e., 1.41×10−9 cm2 S−1), four and eleven times faster than that of O−MoS2 and pristine MoS2 (i.e., 10−20 cm2 S−1), respectively. Also, the cyclic voltammetry analysis indicated that MoS2 hydration not only boosted the electrochemical performance of O−MoS2 but also promoted the capacity retention from 44% (O−MoS2) to nearly 91% (H−MoS2) after 300 cycles (Table 1). Proceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES)  37  Table 1. Electrochemical performance of various MoS2−based cathode materials under different configurations of RMBs. Battery Configuration Electrochemical performance Ref. Cathode Material Anode Material Electrolyte Separator Current density (mA g−1) Temp. (℃) Voltage window (V vs. Mg/Mg2+) Initial specific capacity                 (mAh g−1) Cyclic capacity (mAh g−1) No. of cycles  MoS2 Mg flakes Mg(AlCl₃Bu)₂/THF Cellgard 2400 0.02 Room Temp. 0.6−2.8 25 − − [18] MoS₂  (G−MoS₂) Mg nanoparticles Mg(AlCl₃Bu)₂/THF Celgard  20 − 0.0−3.0 170 168.3 50 [4] MoS₂ Nanoribbon − − − − − − 223.2 − − [16] MoS₂/C microspheres (Carbon content: 46 wt%) AZ31 Mg alloy 0.4 M Mg₂Cl₃+·AlPh2Cl₂−/THF Celgard 2320  50 Room Temp. 0.0−2.2 213 84.3 50 [19] MoS₂−rGO  AZ31 Mg alloy 0.4 M Mg₂Cl₃+·AlPh2Cl₂−/THF Celgard 2320  20 Room Temp. 0.0−2.2 104.2 74.1 50 [20] MoS₂−PEO (peo2−MoS2) Polished Mg foil 0.25 M APC/THF − 5 Room Temp. 0.2−2.0 75 70 30 [22] MoS₂/G−15 AZ31 Mg alloy 0.4 M (PhMgCl)₂ −AlCl3/THF Celgard 2320  20 25 0.0−2.2 115.9 82.5 50 [8] MoS₂/MXene  Mg disc APC electrolyte Polypropylene membranes 50 − 0.01−2.0 165 108 50 [7] F−MoS₂ Mg powder 0.5M Mg(ClO4)2 in [bim][Br] Celgard 15 − 0.3−2.5 55 40 50 [26] PVP−incorporated MoS2 Mg metal 0.25 M APC/ THF Celgard 2400 20 − 0.2−2.0 143.4 131.9 100 [5] MoS₂-infilled microcapsule AZ31 Mg alloy 0.4 M APC/THF Glass fiber filter 50 − 0.01−2.2 161 100 100 [30] Defective MoS2 nanosheets Mg tablets [Mg2Cl3]+⋅[AlPh2Cl2] −/THF (APC) Whatman GF/B 20 − 0.2−2.2 30 75 100 [27] MoS2@carbon@ polyaniline AZ31 Mg alloy 0.4 M (PhMgCl)2-AlCl3/THF − 100 − 0.01−2.0 127 114 100 [31] MoS2/GR Mg metal 0.25 M APC/THF Celgard 2400 20 − 0.2−2.0 210 182.7 300 [21] Hydrous MoS2 (H−MoS2) Mg metal 0.25 M of APC electrolyte/THF Celgard 2400 20 − 0.2−2.2 190.3 173.55 300 [29] Cu−MoS2@HsGDY  Mg metal foil 0.25 M of MgCl2 and AlCl3 (1:2 mol ratio) in DME Whatman glass fibers 50 − 0.1−2.2 148.5 148.5 200 [25] Proceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES)  38   Fig. 3 Comparison between the electrochemical capacity of MoS2−based cathode materials in (a) RMBs and (b) MLIB.4. Mg2+/Li+ HYBRID-ION BATTERY (MLIB) Fig. 3.a epitomizes the electrochemical competence of sixteen different MoS2−based materials in RMBs. Despite the appreciated efforts to promote the electrochemical ability of MoS2, Fig. 3.a  displays that few materials were able to nearly achieve the maximum theoretical capacity of MoS2 (223.2 mAh g⁻1) such as MoS2/C (213 mAh g⁻1), MoS2/GR (210 mAh g⁻1), and hydrous MoS2 (190.3 mAh g⁻1). In addition, the excellent performance of these materials faded as the number of discharge/charge cycles increased. These conclusions confirm the difficult and sluggish intercalation/deintercalation of Mg2+ in the interlayer spacing of MoS2 even after the application of certain improvement techniques.  A group of researchers adopted a new approach to tackle the sluggish insertion of Mg2+ by fabricating Mg2+/Li+ hybrid ion batteries (MLIBs) [1], [3], [40], [41], [32]–[39]. The components of MLIBs are the same as RMBs except for the utilization of dual-salt electrolytes (Mg2+/Li+) [33]. The unique configuration of MLIBs combines the benefit of using the dendrite-free Mg anode and the fast intercalation kinetics of Li+ which greatly enhances the reaction kinetics and magnifies the electrochemical performance of MLIBs. There are two types of MLIBs: Daniell−type battery and rocking−chair battery [1], [38]. In Daniell batteries, Li+ ions are solely intercalating in the interlayer spacing of the cathode materials [39]. Conversely, in rocking−chair batteries, both Mg2+ and Li+ are simultaneously inserted/extracted in/from the interlayer spacing of the cathode material during the discharge/charge cycles [1]. Table 2 and Fig. 3.b summarize the electrochemical efficiency of different MoS2−based cathode materials in MLIBs. Fig. 3 (a & b) highlights the superiority of MLIBS over RMBs as Fig. 3.b shows that most of the applied MoS2−based cathodes in MLIBS exhibited exceptional specific capacities higher than the maximum theoretical capacity of MoS2 in RMBs. For example, when Yanming Ju et al. employed MoS2 nanoflowers as a cathode material in a dual−salt electrolyte of 1 M LiCl + 0.4 M APC/THF, the battery cell achieved a good initial specific capacity of 243 mAh g−1 (Table 2) [1].   Proceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES)  39  Table 2. Electrochemical performance of various MoS2−based cathode materials under different configurations of MLIBs. a The cyclic capacity of 96.4 mAh g−1 was attained at 1000 mA g−1.  Battery Configuration Electrochemical performance Ref. Cathode Material Anode Material Electrolyte Separator Current density (mA g−1) Temp. (℃) Voltage Window  (V vs. Mg/Mg2+) Initial specific capacity                 (mAh g−1) Cyclic capacity (mAh g−1) No. of cycles  MoS2 nanoflowers Mg foil 1 M LiCl + 0.4 M APC/THF − 20 − 0.01−2.0 243 96.4a 2300 [1] MoS2/graphene Mg foil 0.5 M LiCl + APC/THF  Glassy fiber membrane 25 25 0.1−1.8 225 160 200 [32] MoS2/Graphene foam   Metallic Mg ribbon 0.25 M LiCl + 0.25 M APC/THF Celgard 3501 20 − 0.01−2.0 320 290 120 [33] Exfoliated MoS2 Mg metal 0.4 M LiCl + 0.4 M APC Microporous polypropylene film 20 − 0.2−2.2 135 81 10 [34] N−CNFs@MoS2 Mg foil 84 mg LiCl + APC/THF Whatman glass fiber paper 200 Room Temp. 0.01−1.8 290 131 120 [35] HONF−MoS2−CuS−EG Mg foil  0.4 M LiCl + 0.4 M APC/THF − 50 − 0.01−2.0 221.55 73.5 200 [3] v−MoSSe/G Mg plate 1.696 g LiCl + APC/THF Celgard 2400 20 − 0.2−2.0 350.8 299.2 400 [37] MoS2/G VH Mg plate 1.696 g LiCl + APC/THF Celgard 2400 20 − 0.2−2.0 348.3 260.8 200 [42] O−MoS2/G Mg plate 1.696 g LiCl + APC/THF Celgard 2400 20 − 0.2−2.0 298.1 239.4 150 [38] HCL-MoS2 Nanoflowers Mg metal 1 M LiCl + 0.4 M APC/THF Glass fiber 100 − 0.01−2.0 321 192.8 100 [39] Exfoliated MoS2 (E−MoS2) Mg foil 0.8 M LiCl + APC/THF Polypropylene film 100 Room Temp. 0.01−2.0 125 150 300 [40] SnS2−MoS2 heterostructure Mg ribbon 0.25 M LiCl + 0.25 MAPC/THF Celgard 3501 50 − 0.01−2.0 1009 450 500 [41] MoS2-CuS−EG/carbon cloth Mg foil 0.4 M LiCl + 0.4 M APC/THF − 50 − 0.01−2.0 261.1 222.9 200 [36] Proceeding of International Exchange and Innovation Conference on Engineering & Sciences (IEICES)  40  Xiong et al. further improved the electrochemical competence of MoS2 by adding hydrochloric acid (HCL) to control the pH of the mixture of ammonium molybdate tetrahydrate and thiourea at 0.9 before starting the hydrothermal treatment at 220 ℃ [39]. The HCL−assisted synthesized MoS2 attained a remarkable specific capacity of 321 mAh g−1 with a cyclic capacity of 192.8 mAh g−1 after 100 cycles (Table 2). On the contrary, the exfoliated MoS2 proposed by Truong et al. [34] and Li et al. [40] showed poor electrochemical performance in MLIBs with s specific capacity of 135 and 125 mAh g−1, respectively (Table 2 & Fig. 3.b). Similar to RMBs, many MoS2−based composites were developed and applied as cathode materials to boost the electrochemical performance of MLIBs (Table 2). For instance, Fan et al. reported that the interlayer enlarged MoS2/graphene composite (E−MG) could deliver an initial specific capacity of 320 mAh g−1 with a good cycling capacity of 290 mAh g−1 after 120 cycles [33]. Also, a comparable electrochemical performance was reported by Yu et al. when they applied various vdWHs in MLIBs namely, MoSSe/graphene (v−MoSSe/G) (350.8 mAh g−1) [37] and MoS2/graphene (MoS2/G VH) (348.3 mAh g−1) [42]. Among the reported MoS2−composites, tin(IV) sulfide−molybdenum disulfide heterostructures (SnS2−MoS2) stand out as superior cathode material for MLIBs with an initial specific capacity of 1009 mAh g−1 as displayed in Fig. 3.b [41]. Unfortunately, the exceptional specific capacity of SnS2−MoS2 sharply decreased to 600 mAh g−1 in the second cycle and stabilized at 450 mAh g−1 after 500 cycles. The excellent performance of SnS2−MoS2 was ascribed to the collaborative interaction between SnS2 and MoS2 in the heterostructure through multiple intercalation, conversion, and alloying reactions [41]. More specifically, MoS2 played an important role in improving the structural integrity and cycling stability of SnS2−MoS2 by (I) providing adsorption sites for polysulfide byproducts, (II) allowing the rapid ion diffusion through its 1T−phase, and (III) preventing the aggregation of SnS2 nanoparticles by permitting them to dispersedly grow on its nanosheets [41].  Many theoretical calculations elucidated that the Mg2+ intercalation in the interlayer spacing of MoS2 requires a large activation energy of 2.61 eV [1]. In contrast, Li+ ions need a lower activation energy of 0.49 eV [38]. Moreover, the charge density of Li+ (54 C mm−3) is much lower than that of Mg2+ (120 C mm−3 [41]. These features of Li+ will favor the easy and fast insertion/extraction of Li+ ions in/out of the interlayer spacing of MoS2. Consequently, the reaction kinetics on the cathode side will greatly be enhanced. In addition, most of the articles tabulated in Table 2 mentioned that the Li+ intercalation improves the electrochemical properties of MoS2 by phase transformation of MoS2 from 2H−phase to 1T−phase [1], [35], [40]. 1T−MoS2 has higher electric conductivity (10−100 S cm−1) than that of 2H−MoS2 [41]. Thus, the phase transformation of MoS2 during Li+ intercalation will also enhance the electric conductivity of the electrode and reduce the charge transfer resistance [32], [33]. Furthermore, the pre−intercalation of Li+ improves the ion diffusivity by deforming the layered structure of MoS2, consequently, opening more channels for ion insertion, particularly Mg2+ [40]. 5. CONCLUSIONS This review article comprehensively discussed the development of various MoS2−based cathode materials for RMBs and MLIBs. The literature review survey showed that MoS2 prepared by Liang et al. [4] was the best pristine MoS2 applied in RMBs with a specific capacity of 170 mAh g−1. Also, several MoS2−based composites were proposed in the literature as active cathode materials for RMBs. However, few composites exhibited a good electrochemical performance; for example, MoS2/C [19], MoS2/GR [21], and hydrous MoS2 [29]. 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