1 Introduction
As the global population expands and industrial processes accelerate， issues related to energy crises and environmental pollution have become increasingly prominent
[1-2]. In light of the "dual carbon" objectives， the transformation of energy structures and the development of efficient renewable energy sources have emerged as pivotal areas for scientific and technological advancement globally
[3]. Sustainable energy sources， such as solar， wind， and tidal power， are gaining widespread recognition for their low-carbon attributes. However， these energy sources are inherently intermittent and unstable， rendering their practical application highly sensitive to external factors， including geographical location and climatic conditions. Consequently， power fluctuations in the grid present a significant challenge
[4]. Thus， establishing reliable， durable， and environmentally friendly energy storage solutions is crucial for addressing the challenges associated with renewable energy sources. Among various energy storage technologies， electrochemical energy storage has attracted considerable interest due to its high energy conversion efficiency， rapid response times， and adaptable modular design. Batteries and supercapacitors are two prominent types of energy storage devices
[5]. While batteries offer high energy density， their relatively slow electron and ion transport rates lead to low power density. Additionally， under high current discharge conditions， batteries may generate substantial heat， posing safety risks
[6]. Conversely， supercapacitors have significant application potential in scenarios requiring short-term， high-power output
[7-8]. The electrochemical performance of supercapacitors is largely influenced by the properties of the electrode materials. Currently， mainstream electrode materials can be classified into three categories
[9]： carbon-based materials， conducting polymers
[10]， and transition metal oxides/sulfides
[11-12]. Although carbon-based systems demonstrate excellent charge transport properties and corrosion resistance， they primarily rely on electrostatic charge accumulation at interfaces and lack sufficient Faradaic reaction centers， which limits their energy density enhancement. Conducting polymers store energy through redox reactions but suffer from poor cycling and mechanical stability. Transition metal oxides， while exhibiting excellent pseudocapacitive properties， often face challenges such as limited transport efficiency and significant structural degradation with repeated use. Therefore， the search for novel 2D electrode materials that achieve a balance of high capacitance， rapid rate capability， and long cycling life has become a central focus in contemporary electrochemical energy storage
[13-14].
Despite notable advancements in the application of MXenes and graphene for supercapacitors， their further development encounters significant hurdles. MXenes are limited by a narrow electrochemical window and inadequate ion transport， complicating the simultaneous optimization of energy density and rate capability
[15]. Although graphene boasts exceptional electrical conductivity， its strong sp
2 interlayer bonding severely limits the dynamic adjustment of interlayer spacing， thereby constraining the enhancement of intercalation pseudocapacitance
[16]. In this context， transition metal dichalcogenides （TMDs） have rapidly gained attention as a prominent research area， owing to their tunable band structures and unique quantum properties that set them apart from bulk materials
[17]. These unique characteristics position TMDs as promising candidates for diverse applications， including electronics， optoelectronics， and energy storage
[18]. Among them， molybdenum disulfide （MoS
2） has emerged as a highly investigated representative， evidenced by a notable increase in research activity since 2013 and a consistent rise in scholarly output （
Fig. 1）. As a 2D semiconductor， MoS
2 exhibits a high theoretical specific capacitance， a tunable bandgap， and abundant edge-active sites， making it highly suitable for electrochemical energy storage
[19]. Notably， the presence of multiple polymorphs， including the semiconducting 2H phase and metallic 1T phase， imparts tunable electronic properties to MoS
2， enhancing its potential for use in electrochemical energy storage devices， such as Li/Na-ion cells and various catalytic systems. However， MoS
2 faces several critical challenges in practical applications. First， the natural 2H phase is semiconducting and has poor electrical conductivity， limiting its high-rate performance
[20-21]； Second， the layered structure maintained by van der Waals forces leads to significant restacking between layers， resulting in underutilization of active sites and hindering ion diffusion， thus diminishing both capacity and cycling stability. Additionally， multilayer MoS
2 often experiences structural collapse and volume expansion in electrolyte， further compromising its energy storage capabilities
[22-23]. Therefore， it is essential to enhance structural stability and optimize charge and ion mobility in MoS
2 while retaining its intrinsic layered advantages
[24-25]. Various strategies have emerged to improve the electrochemical performance of MoS
2， including heterostructure fabrication， surface functionalization， defect engineering， and phase transformation
[26-28].
Among the various strategies for structural optimization， interlayer spacing engineering has garnered significant attention
[29-30]. Increasing the interlayer distance in MoS
2 can markedly improve the kinetics of ion intercalation and deintercalation， accelerate diffusion rates， lower diffusion barriers， and create more accessible space for electrolyte ions， thus enhancing pseudocapacitive performance
[31-33]. Theoretical studies indicate that broader interlayer spacing effectively reduces the intercalation energy for multivalent ions， thereby improving both thermodynamic and kinetic characteristics of energy storage. Furthermore， expanded interlayer spacing helps prevent layer restacking， maintains sheet dispersion， increases surface area， and enhances electrolyte wettability， all of which facilitate charge transport and storage. Various methods have been developed to modulate the interlayer spacing of MoS
2， including the intercalation of guest species such as organic molecules， metal ions， and hydrated ions； heteroatom doping； and phase transformation. These approaches not only structurally increase the interlayer distance but also synergistically enhance electrical conductivity， structural integrity， and electrochemical activity
[34-36]. MoS
2-based supercapacitor electrodes engineered through interlayer spacing regulation have exhibited improved specific capacitance， enhanced rate capability， and superior cycling stability， demonstrating considerable potential for flexible electronics and micro-scale energy storage devices
[37-38].
MoS
2 is widely recognized as a promising candidate for supercapacitor electrodes due to its unique layered structure and remarkable energy retention capabilities. To address its inherent challenges， such as low electrical conductivity and limited ion diffusion， interlayer spacing engineering has emerged as a crucial strategy for synergistically enhancing both its structural and electrochemical performance
[39-41]. Chen et al.
[42] studied van der Waals gap engineering and discussed various strategies， including intercalation， doping， and heterostructures， but not establish a cohesive framework that connects structural modifications to performance outcomes. Moreover， their analysis lacked quantitative assessments of how adjustments in interlayer spacing influence ion diffusion barriers and charge redistribution at interfaces. Liu et al.
[43] primarily focused on MoS
2 but did not provide computational simulations to support findings related to intercalation kinetics or variations in electronic structure. Similarly， Zhang et al.
[44] offered a brief overview of fundamental mechanisms such as charge transfer and size effects during intercalation， neglecting practical issues like production stability and long-term performance. In this work， we analyze how integrating various approaches， such as combining intercalation with doping or heterostructures， can enhance performance. Through first-principles calculations， we demonstrate charge transfer during intercalation， changes in chemical bonds at interfaces， and the influence of material dimensions on outcomes. We emphasize the work mechanisms， their effects， and their implications for energy storage applications， while also addressing future challenges and directions in this field.
2 Overview of MoS2 as a fundamental electrode material for supercapacitors
In recent years， MoS
2 has attracted considerable interest in various fields such as optoelectronic devices， supercapacitors， catalysis， and lubrication
[45-47]. Structurally， MoS
2 is a prototypical 2D material characterized by a three-atom-thick "sandwich" configuration， comprising one layer of Mo atoms situated between two layers of S atoms， which are bonded by robust Mo-S covalent bonds. Adjacent trilayers are held together by weaker van der Waals forces
[19,48-49]. This layered architecture provides MoS
2 with excellent exfoliation capabilities and significant potential for interlayer modifications， making it suitable for electrochemical systems and next-generation electronics. In nature， MoS
2 predominantly exists in the 2H phase in bulk form， appearing as a black powder
[50]. Bulk MoS
2 can be exfoliated through mechanical or liquid-phase methods to yield few-layer （FL-MoS
2） or even single-layer （SL-MoS
2） nanosheets
[51-52]. Compared with bulk MoS
2， SL-MoS
2 exhibits higher electron mobility and a more direct bandgap structure. Specifically， SL-MoS
2 has a direct bandgap of 1.7~1.9 eV， whereas bulk MoS
2 exhibits an indirect bandgap of 1.1~1.3 eV， making SL-MoS
2 capable of visible-light excitation and well-suited for photocatalysis and optoelectronic applications
[53-56]. Additionally， SL-MoS
2 displays strong photoluminescence， which is absent in bulk material. At room temperature， SL-MoS
2 shows an on/off current ratio as high as 10
8 and an electron mobility of approximately 200 cm
2·V
-1·s
-1， positioning it as an excellent candidate for next-generation semiconductor technologies
[57].
MoS
2 displays several polymorphic structures， which are influenced by interlayer stacking modes and intralayer coordination， as illustrated in
Fig. 2. The five prevalent polymorphs are 1T， 1T′， 1H， 2H， and 3R， where the numbers denote the number of MoS
2 layers in a unit cell， and the letters “T”， “H”， and “R” refer to trigonal， hexagonal， and rhombohedral crystal systems， respectively
[58]. Among these， 2H and 3R are naturally occurring phases， while the 1T phase is a metastable structure. 2H-MoS
2 represents the most thermodynamically stable form， with each Mo atom coordinated by six S atoms in a trigonal prismatic arrangement， resulting in a hexagonally symmetric bilayer structure that exhibits typical semiconducting properties
[59-61]. Its bandgap originates from the energy level difference between the Mo d
z2 orbital and the
and
dxy orbitals. The 3R phase contains three Mo-S units per unit cell and is also considered metastable， whereas the 1T phase exhibits metallic characteristics with Mo atoms in octahedral coordination. Despite its high electrical conductivity， the 1T phase is unstable and tends to convert to the lower-energy 2H structure
[62-63].
It is important to highlight that the phase transitions among various polymorphs of MoS
2 can be reversible under specific conditions. For instance， the thermodynamically stable 2H-MoS
2 can be transformed into the metallic 1T-MoS
2 phase through lithium-ion intercalation combined with ultrasonic exfoliation. The resulting 1T phase can then revert to the 2H structure via laser irradiation or thermal annealing
[63]. Moreover， both the 3R and 1T phases are likely to convert to the energetically favorable 2H structure when subjected to elevated temperatures
[62]. Owing to the loose interlayer bonding in MoS
2， its layers can easily slide over each other， making it highly suitable for solid lubrication applications. Recently， interlayer engineering has emerged as an effective strategy to improve the electrochemical performance of MoS
2[65-66]. It is well established that the physical and catalytic properties of solids are influenced by their structural characteristics
[67-68]. Among the strategies for interlayer engineering， interlayer spacing is a critical parameter that can modify the electronic structure of MoS
2’s active sites and expose additional catalytic centers by adjusting the distance between layers
[69].
3 Synthesis strategies of MoS2
The synthesis of MoS2 is primarily divided into two strategies： bottom-up and top-down approaches. Bottom-up methods utilize atomic or molecular precursors， allowing for the direct growth of MoS2 nanostructures under controlled conditions. Conversely， top-down techniques begin with bulk MoS2 and employ mechanical or chemical processes to exfoliate it into few-layer or monolayer forms.
3.1 “Bottom-up” synthesis of MoS2
3.1.1 Hydrothermal method
The hydrothermal method is a widely employed technique for producing 2D MoS
2 nanosheets， noted for its simplicity， controllable reaction conditions， and eco-friendliness
[70]. The experimental procedure is illustrated in
Fig. 3a[71]. The resulting nanosheets typically consist of 5~6 stacked layers （
Fig. 3b）， providing MoS
2 with high adsorption capacity and a large surface area. By varying the hydrothermal reaction time， the morphology of MoS
2 can be tailored
[72]. Shorter reaction times yield dispersed nanosheets （
Fig. 3c， d）， whereas prolonged reaction times result in self-assembled， uniform flower-like structures with increased surface exposure.
3.1.2 Solvothermal method
The solvothermal method， primarily differs by using organic solvents or non-aqueous systems as the reaction medium. This approach offers notable advantages for synthesizing MoS
2 hybrids， including increased interfacial area， reduced aggregation， and lower energy requirements
[73]. Compared to conventional hydrothermal synthesis， the solvothermal method provides enhanced control over the morphology， crystal structure， and active surface sites of the resulting products
[74-75]. For example， one study employed a solvothermal method with organic solvents to synthesize MoS
2 （
Fig. 3e）
[76]， and combined it with polyvinylidene fluoride （PVDF） to create a 1T/2H-phase MoS
2-PVDF nanocomposite membrane. This membrane exhibited excellent structural uniformity， high catalytic reactivity， and remarkable stability， demonstrating outstanding reusability and pollutant degradation efficiency （
Fig. 3f）. Typically， the solvothermal process is a one-pot synthesis that facilitates high-pressure growth without requiring additional catalysts， thereby minimizing contamination risks and enhancing product uniformity.
3.1.3 Chemical vapor deposition
Chemical vapor deposition （CVD） is a widely utilized “bottom-up” synthesis technique for producing high-quality 2D MoS
2 films. This method typically employs molybdenum sources MoO
3[77]， Mo
[78] or （NH
4）
2MoS
4[79]， in combination with sulfur powder or H
2S gas as sulfur sources
[80]， under high temperatures ranging from 700~1000 ℃， these precursors react on substrates like Si/SiO
2[81] and sapphire
[82] to form MoS
2 films. Studies indicate that substrate type and processing temperature significantly influence film structure and integrity. Sitek et al.
[83] systematically studied substrate effects on morphology （
Fig. 3g~j）， finding that graphene substrates form thermodynamically stable triangular MoS
2， while SiO
2 substrates produce truncated triangular or circular domains. This highlights how substrate surface properties control nucleation behavior.
3.2 “Top-down” synthesis of MoS2
3.2.1 Liquid phase exfoliation method
Liquid-phase exfoliation （LPE） is a simple， cost-effective， and scalable method for preparing MoS
2 nanosheets. It dissociates bulk MoS
2 into single- or few-layer structures through physical/chemical processes. A typical LPE procedure comprises three stages： solvent dispersion， exfoliation， and product purification （
Fig. 4a）
[84]. The process of synthesizing MoS
2 using the LPE method is shown in
Fig. 4b[85]. The exfoliation mechanism relies on 2-MeIm intercalation between MoS
2 layers （
Fig. 4c）， with the nanosheets exhibiting aggregation-induced photoluminescence at 420~480 nm and semiconductor properties when formed into films.
3.2.2 Solid phase exfoliation
Solid-phase exfoliation is one of the earliest "top-down" methods for preparing high-quality 2D MoS
2 nanomaterials. Its principle involves applying external forces to directly peel bulk MoS
2 into few-layer or even monolayer nanosheets. This method is simple to operate， and the resulting materials exhibit high purity and good crystalline integrity， making it widely used for studying the intrinsic properties and device performance of MoS
2. Magda et al.
[86] developed an improved mechanical exfoliation method using chemically enhanced adhesion to produce large-area monolayer MoS
2， addressing limitations of traditional tape-based methods （e.g.， limited productivity and lateral sizes <10 μm）. By leveraging strong interactions between sulfur atoms and gold surfaces， they enhanced adhesion between the bottom MoS
2 layer and underlying gold film. This enabled efficient removal of upper multilayers while leaving monolayers firmly attached to the substrate. In
Fig. 4d， e： optical microscopy revealed large， darkest-contrast regions （marked by dashed lines） corresponding to monolayer MoS
2， surrounded by blue multilayer areas. This method extends beyond MoS
2 to other layered materials， generating monolayers spanning hundreds of micrometers.
3.2.3 Electrochemical exfoliation method
Electrochemical exfoliation is an efficient， low-cost， and scalable technique for synthesizing MoS
2 nanosheets. Nayak et al. employed cathodic exfoliation （
Fig. 4f）
[87]， where Na
2MoO
4 and thiourea were ultrasonically dispersed and adsorbed onto positively charged NiFe layered double hydroxide （LDH） surfaces. Hydrothermal treatment at 210 ℃ for 24 hours formed interwoven heterostructures. Electrostatic interactions built strongly coupled p-n junction interfaces， while simultaneous hydrothermal exfoliation increased surface area and exposed active sites， suppressing material restacking and promoting spatial separation of photogenerated charges. You et al.
[88] developed anodic exfoliation （
Fig. 4g） using bulk MoS
2 as the anode and platinum wire as the cathode in a sulfuric acid electrolyte. Initially， a +1 V voltage enabled hydroxyl radicals to attack crystal edges and form defects. Then， increasing voltage to +10 V drove SO
42-/OH
- intercalation between layers （
Fig. 4h）. Finally， high-pressure gas expansion from embedded ions achieved mechanical exfoliation， yielding mono-/few-layer nanosheets （
Table 1）.
4 Strategy of modulating MoS2 interlayer spacing and the effects on electrochemical properties
4.1 Interlayer agent induces interlayer spacing expansion
Intercalant-induced interlayer expansion is achieved by introducing external substances into the interlayer spaces of MoS2， disrupting the original van der Waals interactions and expanding the interlayer distance， thereby enhancing ion diffusion efficiency and the exposure of active sites. Based on the structural dimensionality and functional roles of the intercalants， this strategy can be classified into two typical types： ionic intercalants and polymer intercalants. These intercalants not only regulate the interlayer spacing but also influence the electronic structure， interfacial stability， and electrochemical reaction kinetics of MoS2. Several types of intercalants will be discussed in detail.
4.1.1 Ionic intercalation
Ionic intercalants are various cations or anions that penetrate the spaces between MoS
2 layers， primarily guided by electrostatic interactions with sulfur atom surfaces， resulting in the expansion of interlayer spacing. Sarkar et al.
[34] synthesized MoS
2/r-GO （reduced graphene oxide） composite nanosheets via a one-step hydrothermal method. The composite materials were directly grown on molybdenum foil， eliminating the need for binders and resulting in enhanced mechanical stability and electronic conductivity. In this process， in addition to acting as the sulfur precursor， thiourea decomposed during hydrolysis to generate NH
4+ ions， which acted as intercalants （
Fig. 5a）. The insertion of NH
4+ weakened the interlayer attractions in MoS
2， resulting in an expanded spacing of around 0.95 nm， markedly exceeding the initial ~0.63 nm， which markedly improved the insertion/extraction capability of Na⁺ ions and enhanced the pseudocapacitive reaction efficiency （
Fig. 5b）. The use of rGO further improved conductivity and electrochemical stability. As a result， MoS
2/r-GO//Fe
2O
3/MnO
2 asymmetric supercapacitor was constructed， maintaining more than 98% capacitance even after 20 000 charge-discharge cycles （
Fig. 5c）.
In addition to using single ions as intercalants， multi-ion intercalation strategies have also been explored. Wang et al.
[89] utilized thiourea as both a reactant and a precursor for intercalants. During the hydrothermal reaction， thiourea was transformed into ammonium thiocyanate （NH
4SCN）， which dissociated into NH
4+ and SCN
- ions. These ions were simultaneously intercalated into the interlayer spaces of MoS
2， significantly expanding the interlayer distance to 9.4 Å （
Fig. 5d）. This expanded structure enhanced the Na
+ adsorption capacity and diffusion rate， thereby markedly improving the electrochemical performance. The resulting E-MoS
2 exhibited excellent specific capacitance in a 3-electrode system and demonstrated good cycling stability in a symmetric supercapacitor， retaining 81.7% of its capacity after 3000 cycles. Density functional theory （DFT） calculations further revealed the stable adsorption behavior of NH
4+ and SCN
- on the MoS
2 surface and within its interlayers， as well as their role in lowering the diffusion energy barrier for Na
+ （
Fig. 5e， f）， ultimately leading to expanded interlayer spacing and enhanced capacitive performance （
Fig. 5g）.
4.1.2 Organic molecular intercalation
Organic molecular intercalation can also be employed. Zhang et al.
[90] significantly enhanced the electrochemical performance of 2H-MoS
2 in supercapacitors by introducing polypyrrole （PPy） as an organic intercalant and constructing a MoS
2-PPy@Ti
3C
2T
x MXene heterostructure （
Fig. 6a）. It demonstrated that PPy intercalation effectively increased the gap between MoS
2 from 6.5 to 10.3 Å （
Fig. 6b~g）， thereby enhancing ion diffusion pathways and exposing more active sites. Meanwhile， MXene served as a highly conductive substrate， further improving charge transfer efficiency and boosting the electrode’s energy storage performance and fast-charging responsiveness （
Fig.6h~l）. As a result， the synthesized MoS
2-PPy@Ti
3C
2T
x composite delivered a high charge storage capacity of 265 F·g
-1 along with stable long-term performance， maintaining 94.1% retention over 10 000 charge-discharge cycles （
Fig. 6k）. The enhanced performance of the composite stems from multiple synergistic effects. PPy intercalation directly expands the interlayer spacing of MoS
2， exposing more edge active sites. In addition， the PPy connects MoS
2 with MXene， forming continuous conductive pathways while providing additional redox-active sites. Furthermore， surface functional groups （—O， —OH） on MXene form chemical bonds with MoS
2， strengthening interfacial charge transfer.
Polymer intercalants refer to long-chain， flexible polymer molecules that can form stable network structures within the interlayers， enhancing the flexibility and electrochemical stability of the composite material. Upon intercalation， they increase electron/ion transport channels， inhibit the restacking of MoS
2 nanosheets， and improve mechanical stability； if the polymers are conductive， they can also contribute to pseudocapacitance. Lian et al.
[91] successfully constructed a multilayer three-dimensional PPy/MoS
2 intercalated composite by polymerizing pyrrole monomers under acidic conditions and subsequently reacting them hydrothermally with sodium molybdate and thiourea （
Fig.7a）. X-ray photoelectron spectroscopy （XPS） characterization confirmed that PPy molecules coordinated with the sulfur atoms of MoS
2 via nitrogen atoms and intercalated into the MoS
2 layers， forming a s
table 3D intercalated composite structure （
Fig.7b）. Experimental data revealed that this composite delivered a notable capacitance of 895.6 F·g
-1 under 1 A·g
-1， sustained an energy output of 3.774 Wh·kg
-1 even at a high power level of 252.8 kW·kg
-1， and preserved 98% of its original capacitance following 10 000 cycles-markedly surpassing the retention rates of pristine PPy （59.8%） and MoS
2 （74.8%） （
Fig. 7c）. This structure takes advantage of the interlayer ion transport channels and pseudocapacitive behavior of MoS
2， while the conductive network of PPy enhances charge transfer efficiency and suppresses volume deformation， thereby significantly improving cycling stability （
Fig. 7d）. Other polymer intercalants include polyaniline （PANI）. Wang et al. directly intercalated aniline monomers into exfoliated MoS
2 layers and doped them with dodecylbenzene sulfonic acid （DBSA）， successfully preparing a MoS
2/PANI intercalated composite （
Fig. 7e）
[92]. Experimental results showed that the intercalation interaction between MoS
2 and PANI significantly improved the composite’s conductivity and thermal stability. The MoS
2/PANI-38 sample achieved a conductivity of 2.38 S·cm
-1， and its maximum thermal decomposition rate occurred at 353 ℃ （
Fig.7f）. Electrochemical evaluation revealed that this composite achieved a capacitance value of 390 F·g
-1 and maintained 86% of its initial performance over 1000 charge-discharge cycles-substantially outperforming pure PANI electrodes， which only delivered 131 F·g
-1 with a 42% retention rate. （
Fig.7g， h）. This improvement stems from the combined contributions of MoS
2 electrostatic charge storage and PANI’s redox-based capacitance， alongside the homogeneous distribution and effective charge transport facilitated by the interlayer architecture. The study clearly applied polymer intercalation technology， embedding PANI into MoS
2 layers via in-situ polymerization， thereby regulating the interlayer spacing and microstructure of the composite for performance optimization.
When comparing the three organic ion intercalation methods， the MoS2/PANI system achieves enhanced conductivity of 2.38 S·cm-1 through dodecylbenzene sulfonic acid （DBSA） doping. However， this remains notably lower than the MXene-based composite approach， where Ti3C2Tx contributes ultrahigh conductivity （150 000 S·m-1） in the MoS2-PPy@Ti3C2Tx structure. For the PPy/MoS2 hydrothermal method， though conductivity wasnot directly measured， the uneven PPy coating thickness （25~66 nm） may cause localized resistance variations. These conductivity differences directly correlate with rate capability： the MXene composite maintains 57.6% capacity at 20 A·g-1， while the PPy/MoS2 system sustains high energy density （3.774 Wh· kg-1） even at 252.8 kW·kg-1 power density. Regarding structural stability， PPy/MoS2 exhibits superior cycling retention primarily due to covalent N—S bonding between polypyrrole’s nitrogen atoms and sulfur in MoS2 interlayers， coupled with PPy’s three-dimensional network effectively absorbing ion-insertion stress. Conversely， the MoS2/PANI system relies on weaker electrostatic interactions. Repeated expansion/contraction of PANI chains during lithiation/delithiation leads to only 86% capacity retention. The MXene composite demonstrates 94.1% retention but faces oxidation susceptibility of terminal functional groups during high-voltage cycling. In summary， while PPy intercalation offers optimal stability， inconsistent coating thickness risks localized stress concentration； the MXene strategy provides unmatched conductivity yet carries unquantified long-term corrosion risks at multi-phase interfaces； and though the PANI approach is cost-effective， electrolyte contamination from degradation byproducts remains a concern.
4.2 3D structure construction
The construction of 3D architectures is an important strategy aimed at preventing the restacking of layered materials through spatial structural design， thereby enhancing specific surface area and electrolyte accessibility
[26]. This approach effectively improves electron/ion transport and the exposure of active sites. Generally， 3D structural construction strategies can be categorized into two main types： self-assembled flower-like structures and carbon-based 3D framework composites
[93].
4.2.1 Self-assembling flower-like structure
By adjusting the reaction conditions， MoS
2 nanosheets can be microscopically arranged in a radial or layer-by-layer unfolding manner， ultimately forming flower-like 3D structures. These structures consist of numerous MoS
2 nanosheets with outward-facing edges， presenting an open and loose morphology. Compared to conventional dense layered stacks， this configuration provides increased accessible surface regions and a greater number of electrochemically active regions. For example， Chen et al.
[94] synthesized a 3D flower-like MoS
2/NiCo（OH）
2CO
3 （MoS
2/NiCoHC） composite and applied it to supercapacitors. In
Fig. 8a， utilizing MoS
2 nanospheres as initial templates through a solvothermal approach， NiCoHC thin layers spontaneously organized into a 3D flower-like architecture. The morphology was systematically characterized by SEM （
Fig.8b）. Experiments revealed that MoS
2 nanospheres acted as nucleation agents to induce the surface growth and assembly of NiCoHC nanosheets into 3D flower-like particles. This architecture notably expanded the accessible surface region， and a large number of surfactant sites are provided through NiCoHC nanosheets， and sulfur atoms at the edge of MoS
2 nanospheres act as additional redox sites， synergistically improving the reaction interface and improving the overall charge storage capacity. Under charge-discharge evaluation， the MoS
2/NiCoHC hybrid exhibited impressive storage capability， reaching 583 C·g
-1 （1296 F·g
-1） at 1 A·g
-1， and maintained 94.3% of its capacity after 2000 cycles （
Fig. 8c）. The resulting hybrid device further demonstrated outstanding energy storage capabilities. The authors further used kinetic analysis and impedance spectroscopy to demonstrate that the 3D flower-like structure promoted charge transport and ion diffusion while ensuring long-term cycling stability. Liu et al.
[95] investigated a 3D sandwich-structured MoS
2/C@RGO hybrid fabricated through a combination of molecular self-organization and hydrothermal treatment， where uniformly distributed MoS
2 layers were anchored onto reduced graphene oxide （RGO） along with in situ-formed carbon nanoparticles （
Fig.8d）. Among them， RGO provides a high-conductive network in-plane， greatly reducing charge transfer resistance， and MoS
2 provides pseudocapacitor active sites. Supramolecular self-assembly realizes the directional growth of MoS
2 on the RGO surface， forming strong interface bonding. This sandwich structure （RGO-MoS
2-carbon particles） forms a three-layer role of “conductive-active-buffering”. Such a distinctive three-dimensional framework suppressed MoS
2 and RGO aggregation， improved conductivity， enlarged electrolyte accessibility， and promoted ion transport and exposure of active electrochemical sites （
Fig.8e~g）. Wei et al.
[96] studied the electrochemical performance enhancement of self-assembled flower-like MoS
2 nanomaterials used as supercapacitor electrode materials. Through a simple one-pot hydrothermal reaction， 1T-phase MoS
2 nanosheets were self-assembled into 3D nanoflower structures. In
Fig. 8h， i， this nanoflower framework exhibited a more stable structure and superior charge transport channels compared to discrete nanosheets. In KCl electrolyte， it achieved a specific capacitance of 483 F·g
-1 at 0.5 A·g
-1， while in KOH electrolyte， the activated battery-type charge storage mechanism led to an outstanding capacitance of 1120 F·g
-1. After 2000 cycles， the nanoflower electrodes retained 94% and 96% of their capacities in KCl and KOH electrolytes， respectively， significantly outperforming the nanosheet electrodes （
Fig.8j， k）.
4.2.2 Carbon based 3D skeleton composite
Carbon-based 3D framework composites involve integrating layered materials such as MoS
2 with 3D porous carbon structures （e.g.， graphene aerogels， CNT networks， carbon foams， or rGO）. This strategy constructs a stable conductive support network that effectively suppresses the interlayer restacking of MoS
2 induced by van der Waals forces during intercalation. Simultaneously， it improves the overall architecture’s available active surface and ion transport pathways. Tiwari et al.
[97] developed MoS
2@CNT heterostructure by combining chemical vapor deposition and physical vapor deposition techniques， enabling precise control over the electrode’s microstructure. This heterostructure forms an open porous network through the synergy between the 3D conductive CNT scaffold and vertically grown MoS
2 nanosheets （
Fig. 9a）. Such architecture not only shortens the ion and electron diffusion paths but also broadens the MoS
2 gallery distance to 0.63 nm （
Fig. 9b）， providing more intercalation sites for Na
+ ions. The robust mechanical properties and excellent electrical performance of CNTs effectively suppress volume expansion and structural deformation of MoS
2 during charge/discharge cycles. The CNT network thus offers both mechanical support and improved electron transport for MoS
2. Contact angle measurements further revealed that MoS
2 coating alters the originally superhydrophobic CNT surface （154°，
Fig. 9c） to a hydrophilic one （49.2°，
Fig. 9d）， thereby enhancing electrolyte infiltration. Electrochemical tests demonstrated that the MoS
2@CNT electrode exhibits a high areal capacitance of 337 mF·cm
-2 at a scan rate of 5 mV·s
-1. In symmetric devices， a capacitance of 131 mF·cm
-2 and a cycling stability of 97.6% were achieved （
Fig. 9e，f）. These results confirm the critical role of 3D carbon frameworks in modulating structural stability， conductivity， and ion transport kinetics of 2D materials. Guan et al.
[98] investigated a core-shell structured MoS
2@C/CNTs composite as a supercapacitor electrode. In
Fig.9g， few-layer MoS
2 nanosheets were integrated with CNTs and carbon interlayers through a hydrothermal method followed by carbonization， forming a coaxial nanocable structure. CNTs served as the conductive backbone， coated with PEDOT：PSS to create a stable interface. Glucose and CTAB were introduced to facilitate in situ carbonization within the MoS
2 interlayers and on its surface， resulting in carbon intercalation structures. This design increased the gap between MoS
2 layers to 0.72 nm （
Fig. 9h，i）， enhancing both electron transport and ion diffusion pathways. Moreover， the carbon layers effectively suppressed MoS
2 volume expansion and improved structural integrity. Electrochemical tests revealed that the material delivered 335 F·g
-1 at 1 A·g
-1 and maintained 127% of its initial capacitance after 40 000 cycles， indicating outstanding durability over extended cycling （
Fig 9j~l）. CNT is the basis of core conductivity， PEDOT：PSS performs interface regulation， and carbon interpolation MoS
2 forms the electron transmission gradient. The three work together to break through the bottleneck that traditional materials cannot have both “high capacity-long life”.
4.3 Defect engineering
Defect engineering refers to the intentional and controllable introduction of structural imperfections into materials to modulate their electronic structure， lattice strain， and reaction activity， thereby enhancing their performance for specific applications
[99]. In layered transition metal dichalcogenides such as MoS
2， defect engineering is widely employed to improve electrochemical properties， demonstrating significant effects in fields like supercapacitors and battery electrode materials
[100-101]. The layered structure of MoS
2 consists of a sulfur-molybdenum-sulfur （S-Mo-S） trilayer configuration， held together by van der Waals forces. This architecture inherently exhibits low basal-plane chemical reactivity and limited electronic conductivity， which restricts its performance in high-rate charge/discharge and high-power output scenarios
[102]. By introducing sulfur vacancies， molybdenum vacancies， edge defects， interstitial defects， or even lattice dislocations， defect engineering can disrupt the original symmetric structure of MoS
2， activating otherwise inert basal planes and generating more electrochemically active sites
[103]. Moreover， such defects can alter the electronic density of states and tune the bandgap， thereby enhancing electronic conductivity and accelerating ion migration rates
[104-106].
4.3.1 Doped defects of MoS2
The electrochemical activity of MoS
2 primarily originates from the sulfur atoms located at the edge sites， while the basal planes exhibit low activity due to their intrinsically low surface reactivity and poor electrical conductivity. Metal doping has emerged as an effective strategy to improve the electrochemical performance of MoS
2， typically achieved by introducing a controlled number of dopants during the synthesis of precursors. The type and concentration of dopants can significantly influence the resulting electrochemical behavior. Among various metal doping approaches， transition metal doping is one of the most representative. Transition metals possess unfilled d orbitals， which can accept lone pair electrons， enabling them to alter the electronic structure of MoS
2. Introducing dopant elements from the transition metal group effectively boosts electrical transport properties， promotes the formation of additional reactive edge sites， and can also stimulate the activity of the inert basal planes， thus markedly enhancing the electrochemical behavior. It has been demonstrated that the incorporation of transition metal ions such as Cu， Ni， Mo， and Fe into the MoS
2 lattice effectively modulates its activity. Vikraman et al.
[107] investigated the regulation of active sites in MoS
2 nanostructures via transition metal doping to enhance their performance in hydrogen evolution reaction （HER） and supercapacitor applications. As shown in
Fig. 10a， Ni， Cu， and Fe were successfully doped into MoS
2 matrices through a chemical precipitation method to address issues such as low conductivity and limited active site availability. Structural characterization and electrochemical testing revealed that the doped MoS
2 nanostructures exhibited significant morphological changes： the introduction of transition metals expanded the interlayer spacing and facilitated the formation of vertically aligned nanosheet arrays， while concurrently altering the charge distribution to enhance the accessibility of catalytically active regions （
Fig. 10b，c）. Specifically， Cu-doped MoS
2 achieved an impressive capacitance value of 353 F·g
-1 and maintained 94% of its initial capacity over prolonged cycling， attributed to the enlarged active interface and the presence of a porous framework （
Fig. 10d， e）. Through Fe/Ni doping， MoS
2 is transformed from semiconductor phase （2H） to metal phase （1T）， and the proportion of 1T phase is increased： the conductivity of the metal phase is higher than that of the 2H phase， which accelerates charge transfer at the electrode interface， Fe doping exposes more edge sites， Cu doping triggers lattice distortion， and experiments combine atom doping with microstructure regulation to achieve a significant improvement in electrochemical performance.
The doping concentration is a crucial factor influencing the electrochemical performance of MoS
2. Singha et al.
[108] investigated the effect of various Mn doping levels on the nanoflower morphology of MoS
2 and its performance in supercapacitors. By adjusting the Mn doping concentration， four types of samples were synthesized： undoped MoS
2 （S-Ⅰ）， low Mn-doping （S-Ⅱ）， medium Mn-doping （S-Ⅲ）， and high Mn-doping （S-Ⅳ） （
Fig.10f）. The results demonstrate that an appropriate level of Mn doping in the S-Ⅱ sample significantly enhanced the electrochemical activity. Structural characterizations， such as XRD and TEM， revealed that Mn doping expanded the interlayer spacing of MoS
2 from 0.63 nm to 0.76 nm and increased the specific surface area and porosity. These changes provided more electrochemically active sites， facilitating enhanced adsorption and diffusion of electrolyte ions （
Fig.10g，h）. However， excessive Mn doping， as in the S-Ⅳ sample， resulted in a performance decline due to pore blockage and a reduced proportion of Mn
3+ species. Electrochemical testing showed that the S-Ⅱ sample exhibited the highest specific capacitance in 0.5 M Na
2SO
4 electrolyte and maintained excellent cycling stability， retaining 77% of its capacity after 5000 cycles （
Fig.10i~l）.
In addition to transition metals， noble metals and rare earth elements have also been doped into MoS
2 for supercapacitor applications. Shao et al.
[109]， utilized a sequential two-step solvothermal process to synthesize Pt-doped MoS
2 layered structures directly on the surface of carbon cloth （CC）. Compared with undoped pristine MoS
2， the Pt doping significantly enhanced the electrochemical performance. The synthesized Pt-MoS
2 delivered an impressive capacitance value of 250 F·g
-1 under 0.5 A·g
-1， and when operated at 10 A·g
-1， its capacitance doubled compared to unmodified MoS
2， indicating a 100% gain. This performance stemmed from the incorporation of Pt， which optimized the material's electronic and structural characteristics. which， as shown by TEM analyses， reduced the crystallinity of MoS
2 and created more active sites， thereby facilitating charge transport and electrolyte ion diffusion （
Fig. 11a~c）. To explore practical applications， an asymmetric flexible supercapacitor （ASC） device was constructed， employing Pt-MoS
2 as the cathode material and activated carbon （AC） as the anode. The configuration and corresponding electrochemical performance are illustrated in
Fig. 11d~f， the flexible device was tested under various bending angles， and CV curves remained nearly overlapping with negligible changes in specific capacitance， demonstrating excellent mechanical flexibility and bending stability. These properties make the device highly suitable for flexible electronic applications.
Exploiting the abundant f-orbitals and strong spin-orbit coupling inherent in trivalent rare earth （RE
3+） ions enables modulation of MoS
2 properties. Their incorporation does the lattice， generating abundant defect sites and expanding interlayer spacing， which widens ion transport pathways， reduces bandgap width， and enhances intrinsic electrical conductivity， all of which contribute to improved energy storage performance. Isacfranklin et al.
[110] investigated the application of Nd and Gd-doped MoS
2 electrode materials for supercapacitors. SEM analysis showed that pristine MoS
2 exhibited spherical nanoparticles， while rare earth doping-particularly with Gd-induced a significant morphological transformation into nanosheet structures （
Fig.11g~i）. The sheet-like structure enhanced surface exposure and reduced ion migration distances， leading to a significant improvement in electrochemical performance. Electrochemical testing confirmed that Gd-MoS
2 demonstrated the best performance. Electrochemical impedance spectroscopy （EIS） further revealed that Gd-MoS
2 exhibited the lowest solution resistance （0.292 Ω） and charge transfer resistance （0.796 Ω）， indicating superior conductivity. Moreover， Gd-MoS
2 retained 81.50% of its initial specific capacitance after 5000 charge-discharge cycles， demonstrating excellent cycling stability （
Fig.11j，k）. This study clearly demonstrated that rare earth element doping-especially with gadolinium-effectively modulated the morphology of MoS
2 （from spherical to sheet-like structures） and significantly improved its key supercapacitor performance metrics， including specific capacitance， conductivity， and cycle life （
Fig.11l）. These improvements are primarily attributed to the strong binding affinity between the rare earth 4f vacant orbitals and functional groups， leading to structural optimization. This offers crucial evidence for developing novel RE-modified metal sulfides as high-performance electrodes.
4.3.2 Intrinsic defects of MoS2
Although defect-rich MoS
2 retains locally ordered hexagonal arrangements of Mo atoms， it exhibits long-range structural disorder overall， primarily manifested as lattice distortion. This “order within disorder” structural characteristic leads to the presence of numerous intrinsic defects-such as sulfur vacancies， voids， and cracks-throughout the surface and bulk of the material in addition to naturally occurring edge sites. These defects provide extra electrochemically active sites， which contribute to enhanced pseudocapacitive reactions and ion intercalation capability. In particular， sulfur vacancies tailor the local atomic arrangement and electronic environment of MoS
2， promoting enhanced charge transfer and conductivity， thereby significantly augmenting its electrochemical reactivity and overall conductivity. Thus， rationally introducing and regulating such intrinsic defects represents a vital approach to enhancing the energy storage function of MoS
2 as an electrode material. For instance， Wu et al.
[111] synthesized ultrathin MoS
2 nanosheets with abundant structural defects by reacting ammonium heptamolybdate （（NH
4）
6Mo
7O
24·4H
2O） and L-cysteine in the presence of 1，6-hexanediamine. Physical characterization clearly confirmed the existence of numerous intrinsic defects， especially those related to sulfur atoms， such as S vacancies or uncoordinated sulfur atoms at the edges. These defects were formed intrinsically during synthesis due to the coordination agent （1，6-hexanediamine） modulating the crystal growth kinetics and inhibiting the perfect lattice ordering of MoS
2， rather than being induced by external dopants （
Fig. 12a）. The intrinsic defects exposed a high density of unsaturated sulfur active sites， significantly altering the electronic structure and surface properties of the material. In
Fig. 12b~d， this defect-rich MoS
2 exhibited excellent electrochemical performance when used as a supercapacitor electrode， delivering high specific capacitance and outstanding cycling stability. The presence of defects promoted the formation of electric double layers and accelerated ion diffusion， contributing to the superior energy storage behavior.
Defect concentration strongly determines the electrochemical performance of materials. Joseph et al. synthesized two types of MoS
2 samples-high defect density （HDD-MoS
2） and low defect density （LDD-MoS
2） by adjusting the amount of thiourea， and probed defect-mediated changes to lattice symmetry， charge transport， and interfacial reactivity
[112].Structural characterizations revealed that higher defect density led to lattice distortion and increased interlayer spacing， thereby enhancing the intercalation and diffusion of electrolyte ions.
The presence of S vacancies introduces additional electronic states adjacent to the Fermi edge and creates localized spin carriers， which contribute to activating the otherwise inert MoS
2 surface and enhancing its charge transport characteristics. Wang et al.
[113] introduced sulfur vacancies into 2H-phase MoS
2 nanosheets by annealing them at different temperatures under an Ar/H
2 atmosphere. The study demonstrated that incorporating sulfur vacancies markedly enhanced the charge transport efficiency and electrochemical storage capacity of MoS
2. As the annealing temperature increased， the concentration of sulfur vacancies rose， resulting in enlarged interlayer spacing， shortened ion diffusion paths， and enhanced electron mobility， all of which facilitated reversible redox reactions （
Fig. 12e~g）. MoS
2-x sample annealed at 700 ℃ exhibited the best performance， achieving a maximum specific capacitance of 142.3 F·g
-1 and excellent cycling stability with 87.1% capacitance retention after 5000 cycles （
Fig.12h）. DFT calculations further revealed that sulfur vacancies， particularly sulfur edge vacancies， made significant contributions to enhancing conductivity and capacitance （
Fig.12i）， by introducing new electronic states near the Fermi level， increasing carrier density， and strengthening electron transfer capability.
Although defect engineering can significantly improve the electrochemical performance of MoS2， excessively high defect concentrations can cause multiple structural stability problems. For example， too many sulfur vacancies distort the local lattice structure and create stress concentration. Additionally， defect sites allow electrolytes to penetrate more easily， worsening corrosion of the electrode material. Current methods to create defects in MoS2 require complex equipment and demanding reaction conditions， making large-scale production difficult. That’s why future research should develop advanced characterization tools that can monitor defect behavior during actual operation， providing real-time insights for designing industrial-grade electrodes.
Studies have shown that moderate interlayer spacing is critical to the capacitive performance of two-dimensional electrode materials. As summarized in
Table 2， excessively large interlayer distances can create inactive dead volume within the electrode， thereby reducing the effective specific surface area and ion accessibility. This， in turn， weakens the charge storage density and release efficiency， hinders fast ion transport， and limits the utilization of electrochemically active sites-ultimately leading to a decline in the overall energy storage capacity.
4.4 Other methods to regulate the interlayer spacing of MoS2
In addition to the strategies discussed above， various alternative approaches have also been explored to adjust the spacing between MoS
2 layers， aiming to enhance their electrochemical behavior. Thomas et al.
[114] investigated the phase transformation of hydrothermally synthesized MoS
2 under thermal treatment at different temperatures and its impact on supercapacitor performance. MoS
2 was synthesized at 200 ℃ using Na
2MoO
4·2H
2O and CH
4N
2S as precursors， followed by annealing in air at 250 ℃， 300 ℃， and 350 ℃， respectively. Annealing at 300 ℃ resulted in the development of a novel MoO
3-x/MoS
2-y hybrid structure， featuring the coexistence of flower-like MoS
2 and brick-like MoO
3 morphologies. This hybrid material exhibited superior structural and electrochemical properties compared to pure MoS
2 or MoO
3， as evidenced by its higher specific capacitance and excellent cycling stability. Unlike conventional methods that rely on intercalating agents， the modulation of MoS
2 interlayer spacing in this case was achieved through thermally induced formation of the MoO
3-x/MoS
2-y heterostructure. During the structural evolution process， phase transition and lattice distortion occurred， which indirectly resulted in the expansion of interlayer spacing and structural relaxation. Abdulhamid adopts a green and environmentally friendly physical exfoliation strategy， dispersing commercial bulk MoS
2 powder in a mixed solvent of acetone/deionized water， and preparing high-quality few layer MoS
2 by 550 W pulsed ultrasound in an ice bath environment. The mechanical shear force generated by the ultrasonic cavitation effect breaks the interlayer van der Waals force， causing the interlayer spacing of MoS
2 to increase from 0.61 nm in the bulk to 0.66 nm， and the number of layers decreases to 1~3， while maintaining the 2H phase structure. The stripped few layers of MoS
2 are directly loaded on the foam nickel electrode to avoid blocking of active sites by traditional binders and conductive carbon black. MoS
2 electrodes exhibit ultra-high specific capacitance far exceeding that of bulk MoS
2.
4.5 Theoretical understanding
Two-dimensional layered MoS
2 has emerged as an ideal electrode material for high-performance supercapacitors due to its exceptional ion intercalation capability and tunable electronic structure. The interlayer spacing， a critical structural parameter， directly governs ion diffusion kinetics and charge storage behavior. DFT calculations provide atomic-scale insights into the tuning mechanism， revealing that enhanced electrochemical performance stems primarily from optimized ion diffusion pathways and cooperative electronic structure regulation. For instance， Zhang et al.
[90] directly expanded the interlayer spacing of MoS
2 via polypyrrole （PPy） intercalation. DFT charge density difference analysis （
Fig.13a~c） demonstrated stronger sodium ion adsorption energy and increased charge transfer after spacing expansion， along with reduced ion diffusion barriers. Density of states （DOS） calculations （
Fig. 13d~f） indicated higher electron occupation near the Fermi level due to MXene incorporation， enhancing overall electrode polarity. Similarly， Cai et al.
[115] improved electrode performance by directly widening the MoS
2 interlayer spacing. DFT modeling at 9.0 Å spacing （
Fig. 13g） showed enlarged ion diffusion channels， and calculations revealed a low K⁺ adsorption energy of -2.16 eV （
Fig. 13j） in the expanded structure， significantly outperforming conventional configurations. DFT analysis （
Fig. 13h） confirmed bandgap narrowing in the 1T/2H hybrid phase， while DOS （
Fig. 13i） showed increased density near the Fermi level， improving electron mobility.
Alternatively， introducing active sites via elemental doping can also induce MoS
2 interlayer expansion to enhance supercapacitor performance. Prakash et al.
[116] expanded the MoS
2 interlayer spacing through Ni doping （
Fig.14a，b）. Total density of states （TDOS） plots （
Fig. 14c） revealed new electronic states near the Fermi level， attributed to hybridization between Ni-3d and Mo-4d/S-2p orbitals （
Fig. 14f）， thereby increasing electron density at the Fermi level. Ni doping reduced the MoS
2 bandgap from 1.73 eV to 1.24 eV （
Fig. 14e~g）. These narrower bandgaps enhanced electrode conductivity. Concurrently， Ni
2+/Ni
3+ redox reactions contributed additional pseudocapacitance， further boosting electrochemical properties. Similarly， Radhakrishna et al.
[117] developed MoS
2@Te heterostructures where Te nanorods formed Mo-O-Te interfacial bonds within MoS
2 layers （
Fig.14h）. This chemical bonding reorganization directly expanded the interlayer spacing. After expansion， the overall quantum capacitance （QC） increased significantly （
Fig. 14i）. DFT results confirmed electron transfer from Te-5p to Mo-4d orbitals， leading to heightened density of states at the Fermi level of MoS
2 and consequent performance enhancement （
Fig.14j~m）.
Fig.15 illustrates various strategies for modulating the interlayer spacing of MoS
2 to improve its electrochemical performance， encompassing strategic mechanisms， modifications to electronic structure， performance benefits， and associated limitations. Together， these elements set a coherent framework that enhances the understanding of how interlayer engineering influences MoS
2 properties. Regarding strategic mechanisms， it emphasizes three key approaches： （1） intercalation of ions （e.g.， Na⁺， K⁺） or other physical/chemical methods to expand interlayer spacing， which enhances ion adsorption and diffusion kinetics； （2） modification of the electronic structure through the introduction of sulfur vacancies or metal dopants， thereby improving conductivity and charge transfer efficiency； and （3） the formation of hybrid or mixed-phase structures （e.g.， 1T/2H）， which introduce new energy bands and active sites. These methods have been validated by DFT calculations， demonstrating significant impacts on the electronic behavior of MoS
2. At the electronic structure level， strategies like interlayer expansion and defect engineering modify valence states and Fermi-level occupancy， increase the density of empty orbitals， and enhance material polarity and metallicity， promoting rapid electron and ion transport as well as energy storage reactions. The introduction of defects generates new hybrid orbitals and electronic states， providing additional sites for pseudocapacitive reactions.
From an electrochemical performance perspective， these strategies often yield advantages such as high specific capacitance， improved rate capability， enhanced structural stability， and environmentally friendly synthesis pathways. Notably， 1T/2H mixed-phase configurations and MoS
2-based composites integrate conductivity， structural integrity， and compatibility with flexible electrode designs， balancing experimental feasibility with application potential. However， challenges persist， including complex processing， high-temperature annealing， and precise dopant control， which can hinder reproducibility and scalability. Excessive interlayer expansion may create electrochemically inactive voids， diminishing material density and energy efficiency. Additionally， phase transformation strategies can induce lattice distortions or phase separation， leading to increased interfacial resistance and reduced cycling stability. Overall，
Fig. 15 provides theoretical insights into the mechanisms and trade-offs of interlayer modulation strategies for MoS
2， assisting in the rational design of next-generation energy storage materials.
5 Summary and outlook
MoS
2， characterized by its unique layered structure and numerous electrochemically active sites， shows significant potential for supercapacitor applications. However， the inherently narrow interlayer spacing of MoS
2 often leads to layer stacking， which hinders the effective diffusion of electrolyte ions and limits the accessibility of active sites， thereby constraining its energy storage capabilities. Recently， engineering interlayer spacing has emerged as a vital strategy to enhance the electrochemical properties of MoS
2 electrodes， attracting considerable attention. Studies indicate that a moderate increase in the spacing between MoS
2 layers can significantly expand the accessible surface area， facilitate ion transport pathways， reduce diffusion barriers， and improve pseudocapacitive characteristics， thereby enhancing overall electrochemical performance （
Fig. 16）.
From a material design perspective， optimizing the electrochemical performance of MoS2 involves several key aspects， including ionic intercalation， heterostructure complementarity， and defect engineering. These adjustment strategies ultimately influence the diffusion and transport of ions and electrons between the electrode and electrolyte， making the interlayer environment of MoS2 crucial. However， expanding interlayer spacing necessitates a careful balance between improved ion diffusion and the potential for structural instability， as excessive spacing could reduce electrode density and compromise mechanical integrity. Future work should concentrate on developing controllable and reversible mechanisms for regulating interlayer spacing， gaining deeper insights into how structural changes between layers affect energy storage behavior， and investigating the synergistic effects of novel intercalants with MoS2 layers. These initiatives aim to achieve balanced energy and power outputs with sustained cycling stability in MoS2-based supercapacitor electrodes.
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