MoS₂ for beyond lithium-ion batteries

Efficient and economical energy storage is becoming significant because of the threat from global climate change, growing demand for energy, and depletion of fossil fuel reserves.^1,2 The lithium-ion battery has been quite successful commercially because of its high energy density, lightweight, wide operating voltage, and high efficiency.^3,4 The research on the lithium battery started more than forty years ago, when the group VI layered disulfides were not of interest for most scientists because of their poor rechargeability.⁵ Nevertheless, it was discovered that in molybdenite, the natural form of molybdenum disulfide (MoS₂), when the Mo coordination is changed from trigonal prismatic to octahedral, MoS₂ can be used as an electrode effectively. This system from Haering et al. sparked the foundation of a commercial battery.⁵ Since then, MoS₂ has attracted substantial attention and was investigated comprehensively as an electrochemical storage material,^6,7 catalyst,^8–11 and solid lubricant.^12–14 

MoS₂ as a typical transition-metal sulfide has a two-dimensional, layered S–Mo–S stacking structure and numerous active sulfur sites that offer numerous advantages for electrochemical energy storage.^15–17 Each of the Mo atoms in MoS₂ stacking layers is coordinated by six S atoms.¹⁸ In the layers stacked together, covalent bonds connect atoms within the MoS₂ layer, while van der Waals interactions bond together the individual layers.^19,20 The interlayer distance in MoS₂ is ∼0.62 nm, compared to the 0.34 nm spacing in graphite; this larger spacing between the S layers bestows fast diffusion of lithium without volume expansion.¹⁷ As a reversible intercalation host, it is considered as a desirable anode material for secondary batteries,^21,22 such as that used in lithium-ion batteries.^23,24 The theoretical capacity for MoS₂ through a complete conversion reaction, MoS₂ + 4Li⁺ + 4e⁻ → 2Li₂S + Mo, is 669 mA h/g.^25–27 This mechanism is comparable to the pioneering work by Jacobson et al. with respect to the electrochemical properties of amorphous Li/MoS₂.²⁸ However, the MoS₂ electrodes can provide a higher value of 1100 mA h/g at 0.5 A/g with an excellent retention rate, reversibility, and cyclic stability.²⁹ In recent years, advancement in the electrochemical performance of MoS₂ has been improved through the precise design and control of the nanosize and morphology,^30–34 and there are many reported charge storage capacities for the MoS₂ nanostructures higher than the MoS₂ theoretical value.^35,36 The reaction mechanism for the lithium intercalation in MoS₂ anodes is still under investigation despite decades of effort.¹⁶ One possible hypothesis is that the alkali sulfides will be oxidized and will generate S atoms, while the Mo atoms will remain inert electrochemically.^37,38 Another assumption is that the extra capacity results from the formation of a space charge layer with Li⁺ at the metal/lithium electrolyte interface, while the charge is counteracted by the electrons on the metal surface.³⁹ 

In situ/operando characterization methods, especially those sensitive to the electrode/electrolyte interface, have received more attention and been involved in scrutinizing the mechanism under working conditions.^17,32 For example, in situ/operando soft x-ray absorption spectroscopy (XAS) enables probing of such an electrode/electrolyte interface while an electrochemical bias is applied, shedding light on the facile solvation and desolvation at an electrified interface, which is critical for continuous operations. A comparison between the total electron yield (TEY) and total fluorescence yield (TFY) detection modes also indicates that the interface between the liquid electrolyte and the solid electrode can be different from the bulk when the electrochemical bias is applied.⁴⁰ The details of using such operando XAS in the probing of MoS₂ anode intercalation and conversion reactions at the interface in lithium-ion cells will be discussed in Sec. IV.

As a consequence of safety issues caused by dendrite formation, the depletion of lithium reserves, and the corresponding cost increase, an alternative energy storage system with large storage capacity and stability becomes growingly desirable.⁴ Additionally, considering the global climate change, the utilization of renewable energies, such as solar and wind power, requires efficient energy storage that levels their intermittent output, making it possible to integrate to the electric grid. The lithium-ion battery in this case is not the best candidate for large-scale electrochemical energy storage applications.^1,41,42 The beyond lithium-ion batteries such as Na-ion, Mg-ion, Zn-ion, and Ca-ion batteries are lower in terms of cost because of their higher abundance in nature while suffering from moderate energy density, low capacity, low cell voltage, and low energy density, respectively.⁴³ Meanwhile, Na-ion batteries are cost-efficient, sustainable, and safe; Mg-ion batteries are high in capacity, dendrite-free, and sustainable; Ca-ion batteries are similar in volumetric capacity compared to that of Li-ion batteries; and Zn-ion batteries offer potential in grid-scale related energy storage.^43–45 Starting from Sec. II, the use of MoS₂ in the Na-ion, Mg-ion, Zn-ion, Ca-ion, and Al-ion batteries will be briefly reviewed. Additional relevant applications of MoS₂ such as in the catalysis including hydrogen evolution, nitrogen reduction, and CO₂ reduction will be introduced. In Sec. IV, the experimental approaches involved in these studies with an emphasis on the use of in situ and operando characterization techniques will be presented. Finally, a brief summary of the MoS₂ in the advancing of beyond lithium-ion batteries and our opinion on the future prospects will be discussed.

MoS₂ has been studied extensively because of its wide applications, such as those in the electrochemical energy storage, and optical properties.^2,46 For a two-dimensional transition-metal dichalcogenide, various approaches have been used in the fabrication of the atomically thin film consisting of one Mo layer and two S layers (S–Mo–S). As shown in Fig. 1, typical fabrication methods include a top-down method by mechanical exfoliation,⁴⁷ high-energy sonication,² and a bottom-up approach through chemical vapor deposition (CVD), and wet-chemical synthesis, including hydrothermal and solvothermal methods. A typical top-down method starts from bulk MoS₂ powder, and exfoliation can be performed using mechanical force, e.g., the Scotch-tape method, which produces highest-quality monolayered samples, or using liquid, which is suitable for fundamental demonstrations in applications in which a large amount of materials are necessary.^47–49 Typical solvents used in the liquid sonication exfoliation are N-methyl-2-pyrrolidone and dimethylformamide.^50,51 The bottom-up approach, CVD, has been considered as a breakthrough that enabled the preparation of large area MoS₂ layers.^52,53 It involves a two-step thermolysis during which a dip-coating process in ammonium thiomolybdates [(NH₄)₂MoS₄] follows by annealing at 500 °C and sulfurization at 1000 °C in sulfur vapor.⁵³ During the thermolysis, MoS₂ layers formed through the reaction of (NH₄)₂MoS₄ + H₂ → 2NH₃ + 2H₂S + MoS₂. Wet chemistry is another bottom-up approach generally used in the fabrication of MoS₂ layers, including solvothermal and hydrothermal methods. Schematics of using (NH₄)₆Mo₇O₂₄·4H₂O + thiourea via the solvothermal method and Na₂MoO₄·2H₂O + NH₂CSNH₂ via the hydrothermal method are shown in Figs. 1(2c) and 1(2d), respectively. The different MoS₂ phases can be obtained by annealing at different temperatures, e.g., 1050 °C in an Ar atmosphere for 1T-phase and 400 °C for 2H-phase MoS₂ from hydrothermal fabrication.⁵⁴ 

In addition to the traditional methods mentioned above, in recent years, a novel atomic layer deposition (ALD) process as a precisely controlled bottom-up approach has also been developed and used in the fabrication of the MoS₂ film.⁴⁶ As one of the most rapidly developing thin film fabrication techniques, the ALD fabrication process can be precisely controlled because of its self-controlled mechanism in which the alternating growth of the film is dictated by two self-limiting gas–solid surface reactions.⁵⁸ Additionally, ALD produces conformal films, making it one of the very few techniques suitable for semiconductor industry where miniaturization is critical, even on very high aspect structures.⁵⁹ As shown in the schematic of ALD in Fig. 2(a), the ALD process generally involves binary reaction sequences during which two surface reactions take place and they are intrinsically self-limiting, which allows the two reactions to proceed sequentially to fabricate a thin film with atomic or monolayer level control. Although part of the surface will react with the precursor gas before some other parts, especially for some high aspect ratio surfaces, the precursors will also first desorb from the surface areas where the reaction is completed and proceed to react with other unreacted parts, making conformal deposition. For the ALD growth of MoS₂, binary reaction sequences occur through alternating exposure to molybdenum chloride (MoCl₅) and hydrogen disulphide (H₂S).⁴⁶ As shown in Fig. 2(b), one growth cycle starts with exposure to MoCl₅, dwelling, N₂ purge, H₂S exposure, dwelling, and N₂ purge. The reported reaction temperature for this ALD was 300 °C, and post-annealing at 800 °C was used to improve crystallinity. Faceted triangular crystals will be formed by the reconstruction of the as-deposited MoS₂ thin film on the sapphire. A cross-sectional transmission electron microscopy (TEM) image of the MoS₂ film in Fig. 2(c) shows that the interlayer spacing is 0.610 nm for the (002) planes. For 10 and 50 ALD cycles, the film thickness values are ∼1.5 and 9 nm, respectively.

MoS₂ of different geometrical structures and on different supporting substrates are being reported to promote their performance in catalysis and battery systems, including MoS₂ microspheres,^60,61 fullerene-like nanoparticles,²¹ MoS₂ supported by graphene sheets or reduced graphene oxide (rGO),^19,57 and MoS₂-PEO and other MoS₂ composites.^20,62,63 For example, in the fabrication of Mg batteries, the triple layered structure of MoS₂ makes it convenient for insertion and extraction of Mg²⁺ ions. The morphologies and nanostructures of MoS₂ have significant effects on the electrochemical properties of the electrode materials.⁶⁴ The formation of the sandwich-structured graphene-like MoS₂/C microspheres reported by Liu et al. is shown in Fig. 3(a), where the MoS₂ nanosheets were fabricated by a Na₂MoO₄ and NH₂CSNH₂ hydrothermal reaction.⁶¹ Glucose was added to the solution, during which it was dehydrated and polymerized to form oligosaccharides. Finally, intermolecular dehydration occurred between oligosaccharides, and then colloidal carbonaceous spheres formed through cross-linking and carbonization. Such a synthesis makes the MoS₂ nanosheets well dispersed in the colloidal carbonaceous spheres, and the carbonaceous materials also prevent MoS₂ from stacking together. The electrochemical performance was improved as a result of the better electronic conductivity of MoS₂ by carbon coating, the better accessibility for electrolyte because of the graphene-like structure, and the enhancement of Mg²⁺ insertion kinetics due to the enlarged interplanar spacing of MoS₂. In a separate work on rechargeable aluminum-ion batteries, MoS₂ microspheres were fabricated also using a hydrothermal method using (NH₄)₆Mo₇O₂₄·4H₂O as the Mo-source and (NH₂)₂CS as the S source.⁶⁰ A schematic of the hydrothermal process and SEM of the microspheres as-prepared are shown in Fig. 3(b). The battery system based on the MoS₂ microsphere cathode, ionic liquid electrolyte, and metal aluminum anode exhibits excellent electrochemical performance as a consequence of the Al³⁺ insertion and phase transformation in the electrode material. In another work, a novel hollow-cage fullerene-like nanoparticle was synthesized through the hydrothermal method at 180 °C using Na₂MoO₄ and a sulfurization reagent CH₃CHNH₂ under a pH < 1 acidic environment.²¹ As shown in Fig. 3(c), the TEM images of MoS₂ nanoparticles exhibit a polygonal shape with a diameter of around 150 nm. Such fullerene-like nanoparticles prepared have a relatively large amount of broken tips, which makes them suitable for ion intercalation.

MoS₂ was also fabricated on supporting materials such as graphene or rGO to form hybrids.^19,57 Typically, Na₂MoO₄·2H₂O, NH₂CSNH₂, and different amounts of GO were used in the hydrothermal process under acidic conditions (pH = 6.5) at 210 °C. The hybrids interlace between the MoS₂ and the rGO nanosheets to form sandwich-like MoS₂/rGO microspheres [Fig. 3(d)]. The stacking of MoS₂ was significantly inhibited by the supporting rGO, therefore providing favorable contribution to the rechargeable Mg batteries, e.g., higher discharge capacity and superior cycling performance. In the research of beyond lithium-ion batteries, one critical issue is to develop new intercalation hosts with different interlayer spacings to facilitate the diffusion of cations, and composites were involved in expanding such an interlayer to accommodate a Na ion with a radius of 1.06 Å compared to that of a lithium ion with a radius of 0.76 Å. As shown in Fig. 3(e), poly(ethylene oxide)-intercalated MoS₂ (PEO-MoS₂) composites were fabricated through an exfoliation-restacking method for sodium-ion batteries.⁶² The preparation of the restacked MoS₂ starts with commercially available MoS₂ and reaction with n-butyl-lithium in a Schlenk flask. After stirring overnight, the mixture was filtrated and washed with hexane and dried under vacuum to form LiMoS₂. The MoS₂ colloid will form through mixing of LiMoS₂ with deionized water and sonication. Composites can be fabricated by mixing the PEO aqueous solution with the MoS₂ colloid. PEO is cation-conducting and expands the lattice up to 160%, making the specific capacity more than twice that of the common MoS₂ under a current density of 50 mA/g as well as improving the cycling stability in sodium-ion batteries. Such a synthesis and strategy can also be applied to other host materials to provide high performance electrode materials for the storage of larger cations compared to lithium.

Efficient and economical energy storage is becoming significant because of the increasing electric vehicles and portable electronic devices. Among all the candidates, the lithium-ion battery is considered as the most promising energy storage device in portable applications.^4,65 As an anode material, the two dimensional MoS₂ exhibits a stable cycling stability and a high capacity of 1290 mA h/g,⁶⁶ benefiting from its large specific surface area and abundant voids or defects, which provide better lithium-ion diffusion pathway during the charging and discharging process.⁶⁷ The two dimensional MoS₂ reacts with the lithium-ion according to the conversion reaction of MoS₂ + 4Li⁺ + 4e⁻ → Mo + 2Li₂S with a theoretical capacity of 670 mA h/g.⁶⁸ The MoS₂ anodes still suffer from capacity fading and poor rate behavior because of their intrinsic low electronic conductivity and aggregation during cycling. A strategy to avoid this issue is using nanostructure engineering to build structures that are difficult to restack and hybrid with carbon materials to enhance the electron transport and promote the structural stability.^31,69 An example of a hierarchical porous MoS₂/C flower-like microsphere is shown in Figs. 4(a)–4(c), which is fabricated using a solvothermal method. The flower-like microsphere structure and carbon coating introduced a high interfacial contact area between the anode and the electrolyte, which favors the electron and lithium-ion transportation and prevents the active anode material aggregation. Used as an anode material in the lithium-ion battery, such a MoS₂/C flower-like microsphere provides a high specific capacity of 1125.9 mA h/g, good cycling stability, and enhanced rate performance.⁶⁹ Building hybrid nanostructures using graphene and rGO was also introduced to prevent the MoS₂-based anodes from rapid capacity fading.^19,70,71 Graphene stands out from the various kinds of carbon based materials because of its exceptional properties such as high electrical conductivity, excellent mechanical properties, thermodynamic stability, and a large specific surface area.^68,72 In a meticulously designed nanostructure with MoS₂ nanosheets [Fig. 4(d)] grown vertically on graphene sheets (MoS₂/G), the coupling of edge Mo of MoS₂ with the oxygen from GO functional groups (C–O–Mo bonds) is proposed.¹⁹ The graphene sheets disperse the active MoS₂ and improve the electrical conductivity of the composite. The interfacial interaction of the C–O–Mo bonds also increases the structural stability and electron transport. As an anode material for lithium-ion batteries, the MoS₂/G electrode exhibits excellent cycling life and rate capability. In a similar work using rGO, by employing the MoS₂/rGO composite as the anode and activated carbon as the cathode, the hybrid lithium-ion capacitor exhibits an ultrahigh specific energy density and long cycling stability.⁷⁰ The exfoliated MoS₂ has also been used in the all-solid-state lithium-ion batteries, in which it exhibited a high discharge capacity of 439 mA h/g.³ The mechanism of the consistently reported reversible charge storage capacities of MoS₂ higher than the theoretical value (670 mA h/g) has been investigated using TEM, Raman, and x-ray photoelectron spectroscopy (XPS) in a confined graphene nanoreactor.¹⁶ The contribution to the extra capacity in the system is attributed to the generation of Mo atoms and subsequent reversible reaction with lithium ions, producing Mo/Li_x. The atomically dispersed Mo precipitates in the Li₂S matrix, accommodating a large number of lithium ions. The reversible Mo → Li_xMo → Mo → 1T-MoS₂ reaction cycle was the critical foundation for the superior cycling stability and capacity.

Although the lightweight, wide operating voltage, and good efficiency dominate in the portable electronics, the safety concerns, depletion of lithium reserves, and corresponding cost increase have promoted the development of sodium-ion batteries as an alternative to lithium-ion batteries. The charge–discharge mechanism of Na/MoS₂ is a two-step process: the first step is xNa + MoS₂ → Na_xMoS₂ (x ≤ 0.5) and then the second step is 0.51Na + Na_xMoS₂ → Na_yMoS₂ (0.5 ≤ y < 1.1).⁷³ The MoS₂ structure does not change during the first step but distorts in the second one. Several stable and metastable intermediate phases in the sodium intercalation were identified, including Na_0.375MoS₂, Na_0.625MoS₂, Na_0.75MoS₂, Na_1.0MoS₂, and Na_1.75MoS₂. A phase transformation from 2H- to 1T-MoS₂ and the sodium ion ordering were confirmed when the sodium content was 0.375.⁷⁴ When forming a composite with carbon materials, a sodium ion battery with a high capacity of 400 mA h/g at 0.25 C and long cycling stability was achieved.⁶³ MoS₂ in confined yolk–shell nanospheres were also designed and fabricated, and as shown in the TEM in Fig. 4(e), the hollow mesoporous carbon nanospheres can effectively prohibit physical detachment or loss of active materials during the sodiation and desodiation process.⁷⁵ The coexistence of both the 1T- and 2H-phases can also promote the electrical conductivity and sodium ion diffusivity compared to the 2H-MoS₂ because of the expanded interlayer spacing.⁷⁶ The electrostatic potential built intrinsically between the 1T and 2H-phases can also attract the sodium ion to the 2H-MoS₂ part, thus promoting its migration, as shown in Fig. 4(f).

The dual-ion batteries also attracted significant attention due to their high energy density, environmental friendliness, and low cost.⁷⁷ Nanostructure engineering of the MoS₂/carbon composites is investigated for dual-ion storage for their applications as batteries and pseudocapacitors.^34,77 The first-principles calculations also showed that the strain can significantly increase the adsorption energy and narrow the n-doped semiconducting gap of MoS₂, which enhances the stability, implicating the promise of applying mechanical strain on the energy storage applications.⁷⁸ 

To briefly summarize the effort in lithium and sodium ion batteries, the interlayer structure engineering is critical and the introduction of carbonous material to form composites will remarkably enhance the lithium-ion storage. MoS₂ nanocomposites with different nanostructures can be a new opportunity for future design of energy storage devices involving MoS₂ anodes. The in situ/operando testing methods to tackle the evolution of the oxidation state and coordination environment and to probe interfacial structures, especially such buried interfaces, may be of great interest to employ and to further develop.^4,79

Multivalent batteries are promising candidates for the post-lithium-ion batteries in the development of cost-effective new energy storage. The higher theoretical volumetric capacity of the multivalent metal anode and restricted dendrite formation are the primary advantages of such post-lithium-ion multivalent batteries.⁸⁰ A significant amount of work in this area is focused on magnesium (Mg) because of its high theoretical specific capacity (2205 mA h/g and 3833 mA h/cm³) and relatively low electrode potential [−2.36 V vs standard hydrogen electrode (SHE)], material abundance, operational safety, and environmental friendliness.⁸¹ Early in 2004, novel structures of MoS₂, including fibrous floccus, spherical nanovesicles, platelets, nanorods, and hollow-cate fullerene-like particles, were synthesized through solution chemical reactions of Na₂MoO₄ and sulfurization reagents such as CH₃CSNH₂. Such precisely designed nanostructures provide better reversible intercalation/deintercalation cycles for Mg²⁺ ions.²¹ Using a similar approach as that used in the MoS₂ fabrication for lithium and sodium batteries, phase transformation from 2H to 1T of the microscale MoS₂ makes it capable of reversibly storing Mg²⁺ and Mg²⁺/Li⁺ in all-phenyl-complex electrolytes and with a high capacity (225 mA h/g) and long cycling stability.¹ MoS₂/C composites with different geometries such as sandwich-like or graphene-like have also been synthesized through hydrothermal and solvothermal routes, respectively.^61,64 As shown in Figs. 5(a) and 5(b), combining the graphene-like MoS₂ and ultrasmall Mg nanoparticles with an average diameter of 2.5 nm, a high operating voltage (1.8 V), a high discharge capacity (170 mA h/g), and long cycling stability was achieved. The reactions in this magnesium ion battery can be confirmed as 6MoS₂ + 4Mg → Mg₄Mo₆S₁₂. The sandwich-structured graphene-like MoS₂/C microspheres make the composite accessible for the electrolyte, expediting the transportation of Mg²⁺ ions from the electrolyte to the active surface of MoS₂. The larger spacing also facilitates the reversible Mg²⁺ ion insertion and extraction kinetics.⁶¹ In a separate work using exfoliated graphene-like MoS₂/graphene hybrid as the cathode [Fig. 5(c)], a notable capacity (115.9 mA h/g) and good cyclic stability (82.5 mA h/g after 50 cycles) have been realized.⁸¹ In this hybrid, the composite shows a three dimensional porous structure from exfoliated MoS₂ layers, and the graphene was inserted in the MoS₂, giving an expanded interlayer spacing of 0.98 nm, as shown in Fig. 5(d). The fast kinetics of Mg²⁺ ion intercalation were enabled by using solvated magnesium-ions ([Mg(DME)_x]²⁺) in nanostructured MoS₂@C porous nanorods (MoS₂@C-PNR).⁸² Representative SEM and TEM images of the MoS₂@C-PNR are shown in Figs. 5(e) and 5(f), respectively. The TEM shows an evenly distributed nanosheet array on the surface of the rod. Since the Mg²⁺ ion intercalation is limited by the low ion mobility because of the strong interaction between the Mg²⁺ ion and the host, the solvated [Mg(DME)₃]²⁺ has a much larger volume per cation; thus, it will introduce a lower energy barrier compared to that of the Mg²⁺ ion, as shown in Fig. 5(g). It is worthy to notify that the concept involved in this study regarding adjusting the solvation of ions can be a general approach to deal with the intercalation kinetics, not only for Mg²⁺ ions but also for other cations. Similar to that mentioned in the Li- and Na-ion batteries, hybrid dual-ion energy storage is also of great interest in the Mg²⁺/Li⁺ rechargeable batteries, combining the advantages of both Li- and Mg-electrochemistry.^1,83,84

The aqueous zinc-ion battery has also received massive attention because it is of low cost, environmental friendly, and safe to operate.⁸⁵ With a high 1T-phase content of about 70%, the MoS₂ is able to introduce an excellent specific capacity and outstanding cycling stability. With information derived from ex situ x-ray diffraction (XRD), the reaction on the cathode side is MoS₂ + xZn²⁺ + 2xe⁻ → Zn_xMoS₂ and that on the anode side is xZn²⁺ + 2xe⁻ → xZn. Density functional theory (DFT) calculations reveal that the 1T-phase MoS₂ has a lower Zn²⁺ diffusion energy barrier compared to that of the 2H-phase MoS₂. The nanostructure engineering of the MoS₂ microsphere cathode has also been used in the aluminum-ion batteries.⁶⁰ In the discharge process, the Al³⁺ ion intercalate into the cathode through MoS₂ + xAl³⁺ + 3xe⁻ → Al_xMoS₂, while the anode side reaction is xAl + 7xAlCl₄⁻ → 4xAl₂Cl₇⁻ + 3xe⁻. The discharge specific capacity is 253.6 mA h/g at a current density of 20 mA/g, and this system using the MoS₂ microsphere cathode also exhibits good cycling stability.

So far, various MoS₂-based electrode materials, including MoS₂ composites with different geometrical structures, have been reported in the literature for both lithium-ion and beyond lithium-ion batteries (Table I). The motivation for calcium-ion batteries was limited by the capability to plate and strip calcium at temperatures close to room temperature until the very recent work from Bruce et al.⁸⁹ Then, the solvation environment studies of Ca²⁺ ions have been performed using electrochemical and in situ/operando soft x-ray absorption spectroscopy (XAS).^40,90 As the research on calcium-ion batteries is still in its infancy, not much experimental effort has been reported for the Ca²⁺ ion intercalation in MoS₂.

Another important aspect we would like to discuss separately is the characterization methods involved, especially those in situ/operando characterization methods used in the study of MoS₂ applications in the lithium-ion and beyond lithium-ion batteries, including in situ/operando XAS, XRD, and TEM. Such an in situ/operando setup makes it possible to probe the transient state species that cannot be performed under ex situ conditions.

Zhang et al. used in situ/operando XAS in the probing of MoS₂ anode intercalation and conversion reactions in lithium-ion cells, through which the irreversible conversion reaction of MoS₂ and conversion of Li₂S to S in the first charge process are confirmed.¹⁷ As shown in Fig. 6(a), the in situ/operando sulfur K-edge XAS mapping shows no obvious change, and to visualize how the XAS spectra are developed, the intensity at 2471.5 eV is plotted as a function of specific capacity in Fig. 6(b). As summarized in Fig. 6(c), the intercalation of MoS₂ is reversible and associated with a phase transformation between 2H- and 1T-phases. The conversion reaction is not reversible and the discharge product Li₂S is converted to S in the first charge. The MoS₂ electrode also suppresses the shuttle effect in Li/S cells. The contrast between in situ and ex situ XAS measurements and valuable reaction dynamics information of MoS₂ are obtained.

In another interesting work of a Li–MoS₂ microbattery, a planar battery geometry was employed to allow in situ/operando TEM probing during the electrochemical charge and discharge processes.⁹¹ It was confirmed that in the first cycle of lithiation, the formation of a Mo conductive network in the Li₂S matrix results in boosted electrical conductivity and optical transmittance compared to the pristine MoS₂. A threefold capacity increase was realized after first cycle rapid lithiation compared to cells through a conventional constant-current discharge. In a separate work using in situ TEM, using a unique graphene@MoS₂ nanotube as the anode in the lithium-ion battery, a novel conversion reaction mechanism was confirmed.³² Along the [001] direction, lithium ions incorporate to the Li_xMoS₂ surface, and the exchange of lithium with the sulfur atom is the reaction pathway for Li₂S formation. As shown in Fig. 6(f), the conversion reaction starts with 1T-Li_xMoS₂ and then intercalates into mixed metallic Mo and Li₂S domains. Eventually, the composites of Mo clusters are captured in the Li₂S matrix in a fully lithiated fashion. These examples for probing such transient processes at non-equilibrium states shown here can only be achieved with the in situ/operando experimental methods, which is critical for a fundamental understanding of the electrochemical reactions, offering additional insights.

In summary, MoS₂ and composites containing MoS₂ have been proved to be promising candidates as electrode materials. The larger interlayer spacing makes them suitable for fast metallic ion diffusion, such as Na⁺, Zn²⁺, and Mg²⁺. The natural abundance and the corresponding low cost of these beyond lithium-ion batteries demand large scale production of MoS₂. However, dual-ion batteries often rely on lithium-ion electrolytes, hindering their long-term development due to the limited lithium resources. Additionally, the 1T-phase MoS₂ nanosheets are intrinsically unstable, and the fabrication of MoS₂ with controlled interlayer spacing requires more novel approaches. The intercalation of the metallic ion will also induce transformation of its layered structures, such as interlayer spacing, which results in a significant irreversible loss of capacities in the first cycle; thus, additional methods to improve the stability are required. As a typical transition-metal chalcogenide, MoS₂ plays an important part not only in the lithium-ion and beyond lithium-ion energy storage devices, such as batteries and supercapacitors, but also with applications in catalysis studies, such as the hydrogen evolution reaction (HER) process. The approaches used in the MoS₂ nanostructure synthesis, including the MoS₂ of different geometries, e.g., microspheres, nanorods, and hollow-interior vesicles, and on different supporting substrates, e.g., graphene and rGO, are reported. Possible strategies to promote the energy storage properties of MoS₂ might include adjusting the solvation of ions to modify the intercalation kinetics, precisely controlling the MoS₂ phases since the 1T-phase MoS₂ has a lower diffusion energy barrier compared to that of the 2H-phase MoS₂, and regulating the mechanical strain meticulously to alter the adsorption energy and semiconducting gap of MoS₂. With a strong motivation for calcium-ion energy storage and the fact that not much experimental effort has been made for the Ca²⁺ ion intercalation in MoS₂, the strategies mentioned above should be exercised to develop better calcium-ion batteries. To further extend the scope, the beyond MoS₂ analog, such as TiS₂, VS₂, MoSe₂, and MoS₃, and other in situ/operando probing techniques, such as Raman, XPS, and resonant inelastic x-ray scattering (RIXS), should be considered to provide more insight into deciphering the reaction mechanism. From the perspective of application, the use of MoS₂ in the nitrogen reduction reaction (NRR) catalysis, the carbon dioxide reduction reaction (CO₂RR) may also benefit from the unique properties of MoS₂ and its analog.

This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy. Soft x-ray spectroscopy experiments were carried out on the beamlines 5.3.1, 7.3.1, and 8.0.1 at the Advanced Light Source. This research used resources of the Advanced Light Source, a U.S. DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231.