Advanced Progress of Non-Stoichiometric Transition Metal Sulfides for Sensing, Catalysis, and Energy Storage

Abstract

Beyond the extensively studied two-dimensional transition metal dichalcogenides, a wide range of non-stoichiometric transition metal sulfides, such as molybdenum sulfides and tungsten sulfides (Mo₂S₃, W₂S₃, Mo₆S₈, Mo₆S₆, NiMo₃S₄), have attracted significant attention for their promising applications in sensing, catalysis, and energy storage. It is necessary to review the current advanced progress of non-stoichiometric transition metal sulfides for various applications. Here, we systematically summarize the synthesis strategies of the non-stoichiometric transition metal sulfides, encompassing methods such as the molten salt synthesis method, high-metal-content growth strategy, and others. Particular emphasis is placed on how variations in the metal-to-sulfur ratio give rise to distinct crystal structures and electronic properties, and how these features influence their conductivity, stability, and performance. This review will deepen the understanding of the state of the art of non-stoichiometric transition metal sulfides, including the synthesis, characterization, modification, and various applications.

1. Introduction

Two-dimensional transition metal dichalcogenides (TMDs) possess tunable electronic structures, excellent catalytic properties, and strong excitonic effects, which have been widely applied in various fields [1,2,3,4,5,6,7,8]. Beyond the TMDs, a wide range of non-stoichiometric transition metal sulfides (NSTMSs) have also been applied in the field of sensing, catalysis, and energy storage due to their unique physicochemical properties. A large number of NSTMSs, such as Mo₂S₃, W₂S₃, Mo₆S₈, Mo₆S₆, NiMo₃S₄, have been explored [9,10,11,12,13,14,15]. Their electronic structures can be readily tailored through intrinsic defects, intercalated atoms, or elemental doping [16,17,18,19,20]. Compared to their stoichiometric counterparts, these NSTMSs also exhibit superior performance in sensing, catalysis, and energy storage applications. Despite their non-stoichiometric composition, these transition metal chalcogenides retain the layered crystal structure and weak interlayer van der Waals interactions characteristic of their stoichiometric counterparts, enabling them to be similarly processed into nanosheets through standard exfoliation or chemical methods. These unique properties have made NSTMSs a research hotspot in materials science, driving the development of novel conceptual devices [21,22,23,24,25].

Advanced progress of NSTMSs for sensing, catalysis, and energy storage has been made in recent years. Zhao et al. fabricated Mo₂S₃ nanowire grids via liquid–liquid interfacial self-assembly, which exhibited uniform morphology and exceptional flexibility, demonstrating outstanding performance in flexible strain sensors and high-energy-density full-cell systems [26]. Tan et al. proposed replacing Mo₆S₈ synthesized by traditional methods with Mo₆S₈ encapsulated in carbon fiber (Mo₆S₈@CF). This approach solves the problem of Mo₆S_8′s tendency to aggregate and sinter at high temperatures, which reduces catalytic efficiency, while significantly improving its performance in nitroaromatic hydrocarbon electroreduction [27]. Liu et al. constructed Mo₆S₆ nanostructures through electron beam lithography. Though not yet practically applied, it exhibits theoretically high catalytic potential in electrocatalytic hydrogen evolution due to its unique electronic states and surface-active site distribution [28]. The applicability of NSTMSs further extends to magnetic systems and photonic platforms [29]. Advancements in novel synthesis techniques have provided fresh insights for structural optimization and performance enhancement [30,31,32,33,34,35,36,37]. These innovative synthesis approaches not only address the critical challenge of preparing NSTMSs but also establish a material foundation for performance optimization across diverse application scenarios [38,39,40,41,42,43,44].

In this work, we systematically summarize and analyze the synthesis strategies, structural diversity, and functional applications of non-stoichiometric transition metal sulfides. First, we provide a comprehensive overview of the synthetic approaches and applications of M₂S₃ (M = Mo, W) and its based composites, highlighting their promising potential in sensor technologies and catalysis technologies. Second, we further explored M₆S₈ compounds, with a particular focus on Mo₆S₈ due to its high conductivity and reversible electrochemical intercalation, demonstrating significant potential for applications in lithium-ion batteries. Finally, we selected Mo₆S₆ as a representative of the important transition metal sulfide M₆S₆ family, providing a detailed discussion on its structural formation mechanisms and synthesis methods, while highlighting its promising properties. By summarizing the synthetic approaches and functional applications of these transition metal sulfides, this work contributes to a deeper understanding of their broad prospects in sensing, catalysis, and energy storage.

2. M₂S₃ and Their Applications

M₂S₃ (M = Mo, W) has shown broad application prospects in various fields such as sensing, catalysis, electrochemical energy storage, and electronic devices. The structural characteristics and functional applications of different forms of M₂S₃ and M₂S₃ matrix composites are systematically analyzed, with special attention to their advantages and future directions in sensor, catalyst, and energy storage applications [45]. Notably, nanowire-networked Mo₂S₃ exhibits superior sensitivity and selectivity in energy storage and biosensing applications due to its high conductivity and open framework, which facilitates efficient electron transport pathways. Layered Mo₂S₃ nanosheets with adjustable layer spacing have been widely used in humidity sensors. At the same time, existing studies have shown that W₂S₃ has been widely used as a high-efficiency catalyst in hydrogen evolution reactions. In order to further broaden the horizons of hydrogen evolution reaction catalysts, we note that Mo₂S₃ matrix composites have abundant surface-active sites and show great potential in hydrogen evolution reactions (HER) and oxygen evolution reactions (OER), making them promising candidates for advanced catalytic applications [46]. Looking forward, the synthesis of these functional materials could be revolutionized by emerging machine learning-assisted techniques, such as Bayesian optimization for reaction parameter screening, which may enable precise control over morphology and defect engineering to further enhance their catalytic and sensing performance.

2.1. Ultrathin Mo₂S₃ Nanowire Network

At present, Mo₂S₃ is being widely used in the field of sensing and energy storage [47,48,49,50,51]. In order to synthesize Mo₂S₃, a molten salt reaction method was employed to insert conductive MoS bonds into semiconducting MoS₂ (Figure 1a). This process led to the formation of numerous MoS clusters within the interlayers of MoS₂. Fermi level analysis revealed that, prior to insertion, the interstitial gaps of MoS₂ exhibited Fermi levels close to zero. However, after the incorporation of MoS bonds, a pronounced peak in the Mo-d Fermi level was observed, confirming the successful intercalation and the formation of Mo₂S₃. For the fabrication of large-area, ultrathin Mo₂S₃ nanowire networks, a liquid–liquid interfacial self-assembly technique was adopted (Figure 1b). A biphasic mixture of water and n-hexane was used, into which a pre-prepared Mo₂S₃/ethanol solution was injected at the interface. The diffusion of ethanol into the aqueous phase generated a Marangoni force, and by controlling the injection volume, the Mo₂S₃ nanowires were uniformly distributed along the interface. Subsequent evaporation of n-hexane resulted in the nanowire network floating on the water surface, from which it could be transferred onto various substrates. Additionally, the incorporation of silica gel into the n-hexane phase was found to enhance the stability of the assembly. The pressure-sensing performance of the Mo₂S₃ nanowires was systematically evaluated. As the applied pressure was increased from 0 to 1000 Pa, the relative resistance of the nanowires increased proportionally (Figure 1c), while the sensitivity decreased linearly with pressure (Figure 1d). At the lower static response limit, a relative resistance change exceeding 0.3% enabled the detection of pressure variations as small as 0.08 Pa, with a response time of 96 ms and a recovery time of 110 ms (Figure 1e). These results indicate that the nanowires can rapidly respond to and recover from subtle external pressure changes. Owing to their high adaptability and sensitivity, Mo₂S₃ nanowires have been applied in monitoring applications for Parkinson’s disease. The sensing mechanism is based on detecting the electrical current fluctuations caused by high-frequency oscillations superimposed during the grasping of objects. Specifically, the current profile becomes irregular during gripping and gradually returns to a smooth baseline upon cessation of oscillatory activity (Figure 1f,g). Thus, these nanowires demonstrate significant potential for real-time monitoring of motor fluctuations in Parkinson’s patients. Mo₂S₃ nanowires also exhibit broad applicability in the field of flexible electronics. By fabricating Mo₂S₃ nanofilms for use in full-cell systems, the galvanostatic charge–discharge (GCD) curves (Figure 1h) and cycling performance (Figure 1i) were analyzed. The results demonstrate that the full cell exhibits excellent electrochemical performance, enabling stable charge–discharge cycles. Moreover, even after extensive cycling, the device maintains a high capacitance retention, highlighting its promising potential for energy storage applications.

2.2. Layered Mo₂S₃ Nanosheets

While Mo₂S₃ nanowires have demonstrated exceptional electronic transport properties and catalytic activity owing to their distinctive one-dimensional architecture, recent research efforts have increasingly focused on two-dimensional nanonets to overcome inherent limitations in specific surface area and active site exposure. This paradigm shift is driven by the superior structural advantages of Mo₂S₃ nanonets—their highly porous, interconnected networks not only optimize charge transport pathways but also maximize electrolyte permeability, collectively elevating their catalytic efficiency and energy storage capabilities to new heights [52,53,54,55,56,57].

The crystal structure of MoS₂ consists of S–Mo–S layers arranged in a three-atomic-layer configuration, where the interlayer spacing is relatively large, and the interlayer interactions are weak (Figure 2a). In contrast, Mo₂S₃ features independent metallic bonds in its crystalline structure, with Mo–Mo bonding along the b-axis, rendering it significantly more stable than MoS₂ (Figure 2b). Mo₂S₃ can be regarded as MoS₂ with inserted Mo atoms, leading to a quasi-layered structure in which adjacent layers are held together by weak van der Waals interactions. Previous studies have investigated the influence of different surface terminations and thicknesses on the relative stability of Mo₂S₃ nanosheets (Figure 2c). Three types of Mo₂S₃ films were considered: (001) non-stoichiometric film, (001) stoichiometric film, and (101) film. The results indicate that Mo₂S₃ nanosheets with (101) surface termination exhibit the highest stability among all configurations, with their stability remaining unaffected by stoichiometry and thickness. The relative energy of all three nanosheet types increases with thickness, rapidly approaching the value of surface formation energy. The colloidal dispersions formed by ultrasonic treatment of Mo₂S₃ in organic media have been explored, with a series of solvents commonly used for MoS₂ liquid-phase exfoliation tested to evaluate the suitability of Mo₂S₃ for similar processing. The solvent-based dispersions exhibited a grayish appearance and displayed the Tyndall effect, confirming the presence of colloidal particles in the liquid phase (Figure 2d). Dynamic light scattering (DLS) measurements of the dispersions revealed a relatively broad particle size distribution, suggesting a uniform colloidal state (Figure 2e). These findings clearly demonstrate that bulk quasi-layered Mo₂S₃ can be successfully exfoliated via ultrasonic treatment. To further characterize the material, thermogravimetric and mass spectrometric analyses were conducted using simultaneous thermogravimetric–differential scanning calorimetry (TG-DSC) and evolved gas analysis–mass spectrometry (EGA-MS). These analyses confirmed the presence of solvent molecules in the solid phase obtained from dimethyl sulfoxide (DMSO)-based colloidal dispersions (Figure 2f). The mass loss and corresponding ion current increase indicate that DMSO molecules are the primary gas-phase products released during this stage. The observed decomposition temperature aligns well with the boiling point of DMSO (189 °C), suggesting that Mo atoms generated upon Mo–S bond cleavage may be stabilized through complexation with solvent molecules or their residual species. Finally, Mo₂S₃ nanosheets were tested for their electrical resistance response to humidity variations (Figure 2g). A sensing device fabricated using Mo₂S₃ exhibited a significant increase in resistance upon exposure to water vapor, which rapidly returned to its initial value upon removal from the testing chamber. The humidity sensing performance of Mo₂S₃ nanosheets surpasses that of several other nanomaterial-based sensors, including those based on two-dimensional MoS₂, ZnO nanowires, and graphene oxide. While these materials demonstrate certain sensitivity to humidity changes, they often suffer from slower response times or limited stability. In contrast, Mo₂S₃ exhibits a unique quasi-layered structure and enhanced stability, resulting in faster response and recovery times, as well as improved repeatability. Owing to its unique quasi-layered structure, high stability, and rapid humidity response, Mo₂S₃ represents a promising candidate for next-generation sensing materials. Future research should focus on surface functionalization to enhance solvent exfoliation efficiency, explore heterostructural integration with other two-dimensional materials, and expand its applications in flexible electronics and environmental monitoring.

Moreover, the unique quasi-layered structure and high stability exhibited by Mo₂S₃ nanosheets not only contribute to their outstanding humidity sensing performance but also provide a robust foundation for energy storage applications. Compared with conventional MoS₂, which often suffers from limited electrical conductivity and relatively sluggish ion diffusion in its semiconducting 2H phase, Mo₂S₃ offers a higher density of active sites and more efficient electron transport pathways owing to its non-stoichiometric composition and defect-rich structure [58]. In energy storage devices, critical factors such as electrical conductivity, interfacial reaction activity, and structural stability strongly influence electrochemical performance. Mo₂S₃ nanosheets demonstrate significant promise for high-performance applications in lithium-ion batteries, supercapacitors, and related systems, enabling both efficient charge storage and rapid charge–discharge behavior. Furthermore, the rapid response and recovery observed in humidity sensing reflect the material’s excellent structural reversibility and chemical stability in electrolyte environments, implying favorable durability and cycling stability under electrochemical conditions. These characteristics highlight the advantages of Mo₂S₃ over traditional disulfide-based materials and underscore the potential of integrating sensing and energy storage functionalities in next-generation Mo₂S₃-based nanodevices.

2.3. W₂S₃ Nanosheets

W₂S₃ has emerged as a promising candidate owing to its unique crystal structure, rich defect chemistry, and favorable electrical conductivity. These features suggest strong potential for W₂S₃ in catalytic and energy storage applications. The following section highlights recent developments in the synthesis and functional exploration of W₂S₃, aiming to provide insights into its advantages over conventional layered dichalcogenides.

WS₂ is a common catalyst with decent performance in HER, but its catalytic activity and conductivity are limited. In contrast, W₂S₃ features a unique non-centrosymmetric structure that induces spontaneous polarization and metallic conductivity, significantly enhancing its catalytic performance. Experimental results show that W₂S₃ nanosheets exhibit lower overpotentials and higher catalytic activity than WS₂ in both acidic and alkaline media, even outperforming commercial Pt/C catalysts under alkaline conditions. Additionally, W₂S₃ demonstrates excellent stability in practical electrolyzers, maintaining performance during long-term high-current operation. Theoretical calculations further confirm that W₂S₃ has an optimal hydrogen adsorption free energy, supporting its efficient catalytic mechanism.

W₂S₃ has been extensively utilized as a catalyst in HER, significantly enhancing reaction efficiency. To synthesize W₂S₃, researchers initially obtained metastable 1T-phase WS₂ (octahedral coordination) and stable 2H-phase WS₂ (trigonal prismatic coordination) through shear exfoliation and rearrangement of WS₂ layers [59,60,61,62,63,64]. Subsequently, monolayer WS fragments were covalently bonded with 2H- and 1T-WS₂ layers to form α’-W₂S₃ and β’-W₂S₃ derivatives (Figure 3a). To fabricate W₂S₃ nanosheets (NSs), a high-metal-content growth strategy was employed, wherein the chemical ratio of tungsten to sulfur was precisely controlled to facilitate the crystallization of W₂S₃. The obtained W₂S₃ NSs were characterized by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) to evaluate surface potential variations. The potential mapping revealed highly distinct spherical domains with significant potential differences (Figure 3b). This surface potential disparity primarily arises from spontaneous polarization induced by the novel non-centrosymmetric structure of W₂S₃. Furthermore, temperature-dependent electrical measurements demonstrated that the resistance of W₂S₃ increased with rising temperature, indicating its metallic nature (Figure 3c). To evaluate the HER performance of W₂S₃ NSs in both acidic and alkaline media, a three-electrode system was utilized in 0.5 M H₂SO₄ and 1 M KOH solutions. The catalytic activity of W₂S₃ NSs was benchmarked against exfoliated WS₂, 2H-WS₂, and commercial Pt/C (20 wt%). In acidic solutions, W₂S₃ NSs exhibited significantly lower overpotentials and superior catalytic activity compared to WS₂ and 2H-WS₂, approaching the performance of commercial Pt/C (20 wt%) (Figure 3d). Remarkably, in alkaline solutions, W₂S₃ NSs outperformed commercial Pt/C (20 wt%) under high current densities (Figure 3e). To further elucidate the origin of the electrocatalytic activity of W₂S₃ NSs and investigate the HER catalytic mechanism, density functional theory (DFT) calculations were conducted. The Gibbs free energy of hydrogen adsorption (ΔGH*) serves as a crucial parameter for evaluating HER catalytic activity, where a ΔGH* value closer to zero indicates optimal adsorption–desorption equilibrium and enhanced HER performance. Among all examined sites, β-W₂S₃ exhibited the lowest ΔGH* values, suggesting its superior HER catalytic activity (Figure 3f). To assess the feasibility of large-scale HER applications, W₂S₃ NSs were employed as the cathode in a flow-type anion exchange membrane (AEM) electrolyzer, with commercial IrO₂ as the anode. Under 30% KOH at 60 °C, the W₂S₃-based electrolyzer demonstrated excellent operational stability, maintaining a steady voltage at a current density of 1.0 A cm⁻² for 360 h without significant degradation (Figure 3g). By leveraging a stoichiometrically controlled growth strategy coupled with theoretical structural predictions, W₂S₃ has been successfully developed as a highly efficient HER catalyst. Its outstanding performance in both acidic and alkaline conditions highlights its potential as a promising cathode material for large-scale hydrogen production.

2.4. Mo₂S₃ Based Composites

Through the research of the existing M₂S₃ in the field of catalysis, people have gradually turned their attention to the matrix composites composed of M₂S₃. Building upon the intrinsic properties of Mo₂S₃ nanostructures, recent studies have focused on the design and synthesis of Mo₂S₃-based composite materials to further enhance functional performance [65,66]. Among these, NiMo₃S₄ composites have attracted significant attention due to their synergistic effects, combining the advantageous characteristics of both Mo₂S₃ and nickel sulfides. This composite not only improves electrical conductivity and structural stability but also introduces additional active sites, thereby boosting catalytic and electrochemical activities.

Electrochemical water splitting, encompassing the hydrogen evolution reaction and oxygen evolution reaction, represents a sustainable approach for continuous hydrogen production [67,68,69,70,71,72]. However, traditional serial fabrication methods face challenges in achieving durable and rapid semiconductor heterostructures for electrode preparation. To address this issue, Mo-modulated molybdenum disulfide (MoS₂) phases can be introduced to construct metal sulfide supports, facilitating the epitaxial growth of metal heterostructures (Figure 4a). This strategy enables the formation of Mo₂S₃@NiMo₃S₄ heterostructures, where each component plays a complementary catalytic role. The Mo₂S₃ phase primarily enhances HER, whereas the NiMo₃S₄ phase mainly promotes OER. The incorporation of Ni, which possesses asymmetric 3d orbitals, can reconfigure with the vacant 4d orbitals of Mo in the sulfide framework, driving electron transfer from the OER site to the HER site and providing abundant active sites, thereby accelerating hydrogen generation. Mo₂S₃ is synthesized via the reaction of MoS₂ with Mo, while Mo₂S₃@NiMo₃S₄ is fabricated through the in situ growth of NiMo₃S₄ nanosheets on Mo₂S₃ nanorods. To synthesize Mo₂S₃@NiMo₃S₄ nanorods, an excess of Mo was first introduced to transform the semiconductor MoS₂ into metallic Mo₂S₃ (Figure 4b). Subsequently, NiMo₃S₄ nanosheets were grown in situ on the Mo₂S₃ nanorod surface via a hydrothermal process, forming the Mo₂S₃@NiMo₃S₄ metal heterostructure (Figure 4c). The high Fermi-level metallic Mo₂S₃ serves as an efficient charge transport medium at the interface. Moreover, compared to pure metallic Mo, Mo₂S₃ features shorter Mo–Mo bonds, which generate delocalized electrons and electronic states, thereby facilitating HER. The electronic band structures of Mo₂S₃, NiMo₃S₄, and Mo₂S₃@NiMo₃S₄ demonstrate that the formation of the heterointerface promotes favorable electron migration from NiMo₃S₄ to Mo₂S₃ (Figure 4d). The crystal structures of the synthesized samples were further characterized using X-ray diffraction (XRD) analysis. The characteristic peaks of the Mo₂S₃ phase confirm its successful formation, while the distinctive peaks of the NiMo₃S₄ phase validate the presence of NiMo₃S₄ (Figure 4e). These observations unequivocally confirm the successful synthesis of Mo₂S₃@NiMo₃S₄. Furthermore, the electrical conductivity of the samples was investigated via temperature-dependent resistivity measurements (Figure 4f). At 298 K, Mo₂S₃@NiMo₃S₄ exhibits significantly lower resistivity than Mo₂S₃ and NiMo₃S₄, indicating superior electrical conductivity. Additionally, the resistivity of both Mo₂S₃ and Mo₂S₃@NiMo₃S₄ decreases with decreasing temperature, confirming their metallic nature. Ultraviolet photoelectron spectroscopy (UPS) analysis was conducted to further validate the low-energy electronic features of Mo₂S₃, NiMo₃S₄, and Mo₂S₃@NiMo₃S₄, substantiating the metallic characteristics of Mo₂S₃ and Mo₂S₃@NiMo₃S₄ (Figure 4g). The catalytic performance of Mo₂S₃, NiMo₃S₄, and Mo₂S₃@NiMo₃S₄ was subsequently evaluated. The overpotentials for OER and HER at 10, 100, and 1000 mA·cm⁻² were measured in a 1.0 M KOH solution saturated with N₂ at room temperature, using Mo₂S₃, NiMo₃S₄, and commercial Pt/C as reference samples (Figure 4h). The synthesized Mo₂S₃@NiMo₃S₄ catalyst exhibits lower overpotentials for both OER and HER, outperforming commercial RuO₂, Pt/C, and recently reported electrocatalysts (Figure 4i).

Mo₂S₃@NiMo₃S₄ demonstrates exceptional OER and HER electrocatalytic performance in alkaline media, primarily due to the high activity and electrical conductivity provided by interfacial electronic engineering within its metal heterostructure [73]. This study addresses the limitations of traditional catalysts in terms of conductivity and activity through interface electronic modulation and metallic heterostructure design, offering a novel strategy for developing highly efficient and stable bifunctional electrocatalysts.

3. M₆S₈ and Their Applications

While M₂S₃ and its composite materials have demonstrated promising properties in sensing, catalysis, and energy storage, M₆S₈ has also attracted widespread attention. In this context, Mo₆S₈, a representative member of the Chevrel-phase family, has emerged as a particularly intriguing candidate [74,75,76,77,78]. Its unique cluster-based structure enables efficient ion intercalation, excellent structural stability, and multi-electron redox activity, making it highly attractive for applications in advanced batteries and electrocatalysis. Recent advances in AI-driven characterization (e.g., convolutional neural network-based defect analysis) are expected to provide deeper insights into the structure-activity relationships of these Chevrel-phase materials, particularly in understanding their exceptional ion transport mechanisms at atomic scales. The following section discusses recent advances in the use of M₆S₈ for energy-related applications, highlighting its performance advantages and underlying mechanisms.

Chevrel-phase Mo₆S₈ has been widely studied as an anode material due to its unique crystal structure. The traditional preparation method involves first synthesizing Cu₂Mo₆S₈ and then removing Cu to obtain Mo₆S₈. To obtain Cu₂Mo₆S₈, CuS, MoS₂, and elemental Mo were mixed in a stoichiometric ratio and heated at 1000 °C for 24 h, forming Cu₂Mo₆S₈. The X-ray diffraction (XRD) patterns of Cu₂Mo₆S₈ and Mo₆S₈ (Figure 5a) show that the diffraction peaks of Cu₂Mo₆S₈ are highly consistent with the standard Cu₂Mo₆S₈ pattern (ICSD#158985). Furthermore, the strong peak at 13.8° and the minimal peak intensity at 14.4°, corresponding to MoS₂, indicate the high purity of the synthesized Cu₂Mo₆S₈. The obtained Cu₂Mo₆S₈ was then leached in 6 M HCl, yielding Chevrel-phase Mo₆S₈. When used as an anode material for lithium-ion batteries, Mo₆S₈ exhibits excellent electrochemical performance during the lithiation/delithiation process. Its cyclic voltammetry (CV) curves (Figure 5b) show a small peak separation between the anodic and cathodic reactions, suggesting fast lithium-ion insertion kinetics. The overpotential is significantly lower than that of conventional anode materials, such as VO₂(B), V₂O₅, and LiTi₂(PO₄)₃. This outstanding performance is primarily attributed to the nanoscale morphology of Mo₆S₈, which shortens the lithium-ion diffusion path and enhances kinetic performance. The rate capability and cycling stability of Mo₆S₈ were further evaluated by assembling Mo₆S₈-based coin cells with LiMn₂O₄ as the cathode and conducting charge–discharge tests at different rates (Figure 5c). The results indicate that the specific capacity decreases at higher charge–discharge rates, mainly due to the limited lithium-ion diffusion rate rather than electrode degradation. This finding suggests that Mo₆S₈ exhibits excellent structural stability. In the rate recovery test, when the charge–discharge rate was restored to its initial value, the battery capacity nearly returned to its first-cycle level, demonstrating outstanding high-rate stability. To gain deeper insight into the structural evolution of Mo₆S₈ during lithium-ion insertion/extraction, in situ XRD characterization was performed on cycled electrodes (Figure 5d). The XRD results reveal that during the charging process, Mo₆S₈ undergoes a phase transformation, forming LixMo₆S₈ (0 < x < 4). However, in the fully discharged state, the reappearance of a distinct diffraction peak at 18° indicates that the original structure of Mo₆S₈ is restored, confirming the reversibility of this phase transition. In summary, Chevrel-phase Mo₆S₈ exhibits exceptional lithium-ion insertion kinetics, superior rate performance, and high structural stability as an anode material for lithium-ion batteries. Its unique nanoscale morphology shortens the lithium-ion diffusion path, enhances kinetic response, and maintains excellent cycling stability even at high charge–discharge rates. Additionally, the reversible phase transition further ensures structural integrity during long-term cycling, making Mo₆S₈ a highly promising high-performance anode material.

However, conventional synthesis methods often lead to severe aggregation and sintering of the active materials at high temperatures, which significantly reduces the catalytic efficiency [79,80]. Therefore, Mo₆S₈ nanoparticles wrapped in carbon fiber (Mo₆S₈@CF) were selected as an alternative to overcome these limitations. Mo₆S₈@CF was synthesized through a combination of electrospinning and high-temperature sulfurization. Initially, a homogeneous electrospinning precursor solution was prepared by dissolving polyacrylonitrile (PAN) and copper ammonium thiomolybdate (Cu (NH₄) MoS₄) (Figure 6a). The solution was then subjected to electrospinning under a high-voltage electric field to form nanofibers, effectively confining the Cu (NH₄) MoS₄ within the PAN matrix. Subsequently, the obtained composite fibers were annealed in a 5% H₂/Ar atmosphere at 865 °C for 5 h, during which CuxS and CuxMo₆S₈ intermediates were formed. Finally, these intermediates were treated with 1.0 M hydrochloric acid (HCl) to selectively remove the copper-containing phases, yielding phase-pure Mo₆S₈@CF. To evaluate the catalytic performance of Mo₆S₈@CF for the electroreduction of nitroarenes, the production efficiencies of 4-vinylaniline (4-VA) and 4-ethylaniline (4-EA) at −0.45 V were compared under identical conditions (Figure 6b). Mo₆S₈@CF exhibits significantly higher catalytic efficiency than other tested catalysts. The Mo₆S₈ structure contains cavities within its nanofibers that are capable of hosting metal ions. To verify the possible intercalation of metal ions, X-ray diffraction (XRD) analysis was conducted on the samples after the electrochemical reduction process (Figure 6c). A distinct shift in the (101) diffraction peak of Mo₆S₈ was observed, indicating that Li⁺ ions were successfully inserted into the Mo₆S₈ lattice. This intercalation leads to a noticeable lattice expansion, as evidenced by the peak shift. The electrocatalytic reduction performance of Mo₆S₈@CF electrodes was compared in LiClO₄ and NaClO₄ electrolytes, as illustrated in Figure 6d. It is evident from the figure that the reaction rate in LiClO₄ solution is significantly higher than that in NaClO₄. Moreover, in NaClO₄ solution, the catalytic rate of Li-Mo₆S₈@CF is also markedly superior to that of Mo₆S₈@CF. These results fully verify the possibility of using Mo₆S₈@CF instead of Mo₆S₈ as a high-performance lithium-ion battery cathode.

With the strong demand for increased energy density in batteries, Mg is widely used as a metal anode due to its high volumetric specific capacity, low cost, high natural abundance, and sustainability [81,82]. The use of a chlorine-containing adsorption layer on the surface of Mo₆S₈ on the cathode can reduce its activation energy, thereby effectively transferring Mg²⁺. However, chloride tends to corrode the metal on the anode surface, so the use of LiBxOy as the CEI type improves the stability of the cathode. The XRD pattern of the Mo₆S₈//Mg cell was observed by performing a charge and discharge reaction (Figure 7a). When discharged, MgMo₆S₈ is formed as an intermediate product, and finally Mg₂Mo₆S₈ is generated. However, when charging, Mg₂Mo₆S₈ is converted into MgMo₆S₈, but the Mg²⁺ in it cannot continue to obtain electrons to generate Mg, because at low Mg content, Mg²⁺ has a unique ring with low potential energy, so the diffusion in MgMo₆S₈ is slower, so there will be coexistence of MgMo₆S₈ and Mo₆S₈ at the cathode. X-ray photoelectron spectroscopy (XPS) was performed on the Mo₆S₈ surface at 2.0 eV (Figure 7b). A strong B-O peak was found at around 192.5 eV, corresponding to the MgBxOy species. At the same time, it was found that the peak of the signal continued to decrease over time. This phenomenon illustrates the formation of a homogeneous CEI on the surface of the cathode made of Mo₆S₈. The electrochemical resistance of Mo₆S₈//Mg cells after different cycles was tested at voltages of 2.0 V (Figure 7c) and 2.6 V (Figure 7d), respectively. It is found that the interfacial impedance is significantly lower than that of 2.0 V at 2.6 V. And as the number of cycles increases, the interface impedance gradually decreases. Therefore, at higher voltages, Mg²⁺ is easier to transfer. Based on the above experimental process, the surface CEI working mechanism of Mo₆S₈ was predicted. [B(hfip)4]⁻ decompose first to form CEI and Mg²⁺ at high voltage, CEI reduces its activation energy and accelerates the movement of Mg²⁺, while Mg²⁺ on the surface of Mo₆S₈ without CEI is difficult to pass. Therefore, the storage performance of Mg²⁺ in the cathode can be improved (Figure 7e). Under high voltage, [B(hfip)4]⁻ decomposes to form the CEI layer, which reduces the activation energy of Mg²⁺, accelerates its migration, and improves the magnesium storage performance of Mo₆S₈. This study demonstrates that interfacial chemistry optimization significantly enhances the magnesium storage performance of the Mo₆S₈ cathode, advancing the practical development of magnesium batteries.

4. Other Non-Stoichiometric Metal Sulfides

Nanowires are typical 1D systems that hold potential for applications spanning from flexible nanodevices to ultrasmall functional interconnects. Among them, nanowires composed of transition metals and chalcogens, especially M_6×6 nanowires (M = transition metal; X = chalcogen), have potential applications in 1D electron channels, spintronics, optoelectronics, and catalysis due to their authentic sub-nanometer width (<1 nm) and intrinsic metallicity [83]. However, there are still many challenges in the fabrication of M_6×6 nanowires, especially single nanowires. Traditional methods, such as physical vapor deposition, can make the purification of individual nanowires difficult. At present, the latest research shows that the treatment of trimethyl molybdenum disulfide (TMDC) monolayer by electron beam irradiation technology is a clean and efficient preparation method [84,85,86].

Firstly, MoS₂ is a layered crystal, in which Mo atoms are sandwiched between two layers of S atoms, and when irradiated by a high-energy electron beam, they will preferentially excite and sputter S atoms with lower binding energy, resulting in the formation of S vacancies and defects in local areas. This initial etching disrupts the original lattice equilibrium. Then, due to the bond breakage and the uneven distribution of local energy in the defect region, the remaining Mo atoms and some S atoms began to diffuse and migrate, undergoing atomic rearrangement in the defect edge region. This atomic migration and self-organization effect prompts the atoms to be reconstructed in a new arrangement, transforming from a two-dimensional structure to a narrow nanoribbon. With the continuous action of the electron beam and the contribution of the local heat shock effect, the atoms inside the nanoribbon are further adjusted, and finally form a one-dimensional Mo₆S₆ nanowire with a new crystal structure, which is not only stable, but may also aggregate and fuse in the local region and evolve into a thicker wire-like structure (Figure 8).

This study reveals the unexplored mechanism of M_6×6 single nanowire formation, which provides an important basis for further understanding the narrowing process of TMDC nanoribbons under electron irradiation [87]. Though not yet practically applied, it exhibits theoretically high catalytic potential in electrocatalytic hydrogen evolution due to its unique electronic states and surface-active site distribution.

5. Conclusions and Prospects

Scalable synthesis methods for non-stoichiometric transition metal sulfides have been explored to meet industrial demands in sensing, catalysis, and energy storage. These methods provide viable routes for translating laboratory-scale materials into industrial applications. Despite significant advances in their application for sensing, catalysis, and energy storage, several challenges and opportunities remain to be addressed to fully realize their potential. First, the precise correlation between crystal structure, defect engineering, and catalytic activity warrants deeper exploration. Tailoring the active sites via controlled sulfur vacancies, phase engineering, or heteroatom doping could further enhance HER performance while improving material stability under operational conditions. Studies by Denis Gentili et al. have demonstrated that defect engineering in transition metal dichalcogenides can achieve precise spatial control through advanced techniques such as electrochemical nanopatterning [88]. Second, these non-stoichiometric transition metal sulfides exhibit multifunctional characteristics. Owing to their exceptional electron transport properties and tunable surface-active sites, these materials simultaneously achieve the trifecta of high-precision sensing, efficient catalysis, and high-capacity energy storage. This unique combination opens new avenues for developing integrated or multifunctional energy devices. Future investigations should focus on elucidating the fundamental mechanisms governing this multifunctionality, thereby enabling the rational design of materials that seamlessly combine rapid charge-transfer kinetics with robust ion-intercalation capabilities. From an application perspective, scaling synthesis methods while maintaining precise structural control remains a critical hurdle. Additionally, integrating these materials into practical device architectures—such as flexible electrodes, hybrid electrolyzers, or tandem energy storage–conversion units—requires multidisciplinary approaches bridging materials science, engineering, and system design.

In summary, this work systematically reviews the synthesis strategies, structural diversity, and functional applications of NSTMS, highlighting their unique advantages over stoichiometric counterparts in terms of sensing, catalysis, and energy storage. The M₂S₃, M₆S₈, and M₆S₆ series deliver superior performance in flexible sensors, lithium-ion batteries, and electrocatalysis through defect engineering, intercalation, or doping of custom electronic structures. Advancements in synthesis techniques, such as liquid–liquid interface self-assembly, address key challenges in material preparation while opening up new opportunities for structural optimization. The article highlights the importance of several advanced characterization techniques, such as X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, which play a crucial role in uncovering the crystal structure, surface morphology, and defect distribution of these materials. Furthermore, density functional theory calculations have become indispensable tools for predicting the electronic properties and catalytic behavior of these sulfides, providing valuable theoretical support and guidance. With these calculations, material design and optimization can be more precisely targeted to meet specific application needs. To harness the full potential of these materials, the article further explores their applications in sensors, ion batteries, and other energy storage devices, highlighting their advantages in improving device efficiency, extending lifespan, and enhancing stability. This review highlights the critical role of NSTMS in driving innovation in next-generation devices. Future research should focus on scalable synthesis, mechanistic studies of structure-property relationships, and integration into actual systems to maximize their potential. This work provides a basic perspective for the further exploration of NSTMS in functional materials science.