MXenes and MXene-Based Composites: Preparation, Characteristics, Theoretical Investigations, and Application in Developing Sulfur Cathodes, Lithium Anodes, and Functional Separators for Lithium–Sulfur Batteries

Abstract

Lithium–sulfur batteries (LSBs) are favorable candidates for advanced energy storage, boasting a remarkable theoretical energy density of 2600 Wh kg⁻¹. Moreover, several challenges hinder their practical implementation, including sulfur’s intrinsic electrical insulation, the shuttle effect of lithium polysulfides (LiPSs), sluggish redox kinetics of Li₂S₂/Li₂S, and the uncontrolled growth of Li dendrites. These issues pose significant obstacles to the commercialization of LSBs. A viable strategy to address these challenges involves using MXene materials, 2D transition metal carbides, and nitrides (TMCs/TMNs) as hosts, functional separators, or interlayers. MXenes offer exceptional electronic conductivity, adjustable structural properties, and abundant polar functional groups, enabling strong interactions with both S cathodes and Li anodes. Despite their advantages, current MXene synthesis methods predominantly rely on acid etching, which is associated with environmental concerns, low production efficiency, and limited structural versatility, restricting their potential in LSBs. This review provides a comprehensive overview of traditional and environmentally sustainable MXene synthesis techniques, emphasizing their applications in developing S cathodes, Li anodes, and functional separators for LSBs. Additionally, it discusses the challenges and outlines future directions for advancing MXene-based solutions in LSBs technology.

1. Introduction

Conventional lithium-ion batteries (LIBs) face significant challenges owing to their limited energy conversion efficiency and high production costs, making them less capable of meeting the increasing demand for energy storage, especially in the electric vehicle sector [1,2]. In contrast, LSBs offer several advantages, such as an impressive energy density of 2600 Wh kg⁻¹, eco-friendliness, and the low cost of sulfur (S). These features make LSBs highly promising for advancing electric vehicles and smart grid technologies [3,4,5]. However, several inherent challenges hinder the commercialization of LSBs [6]. First, S and its discharge products, such as Li₂S, exhibit extremely poor electronic conductivity, significantly impeding redox reactions and reducing S utilization. For instance, the conductivities of S and Li₂S are as low as ~5 × 10⁻³⁰ and ~10⁻¹³ S cm⁻¹, respectively [7]. Second, LiPSs are highly soluble in the electrolyte during cycling, leading to the shuttle effect. This phenomenon accelerates capacity decay and lowers the battery’s coulombic efficiency (CE). LiPSs can penetrate the separator and reach the Li anode, where they react with Li, causing corrosion. Third, Li’s spontaneous reaction with the liquid electrolyte forms a fragile solid electrolyte interphase (SEI) on the anode, further reducing the CE. Lastly, uneven Li deposition can result in internal short circuits, posing significant safety risks to the battery.

To overcome these challenges, scientists have developed various approaches to enhance the activity of LSBs. For S cathodes, C-based hosts like porous C [8], CNTs [9], and hollow C spheres [10] have been engineered to entrap polysulfides within their nanopores. Nevertheless, the non-polar nature of C composites limits their ability to effectively immobilize polysulfides. To address this, polar host materials with strong chemisorption characteristics for LiPSs have garnered significant interest. These include heteroatom-doped carbons (for instance, B-doped [11] and N-doped [12]), metal oxides and sulfides [13,14], MXene [15], and GO [16], have gained significant attention. For Li anodes, various strategies have been explored to stabilize the Li/electrolyte interface and adjust Li deposition. These include the use of electrolyte additives, 3D current collectors, protective physical layers, and solid electrolytes [17].

MXenes are a versatile family of 2D TMCs and TMNs. The first discovered MXene, Ti₃C₂T_x (where T_x signifies surface-terminating groups like O, OH, and F), is synthesized using selectively etching of an A-layer from the parent MAX phase. The MAX phase has the common formula M_n+1AX; here, M is the early transition metals (TMs), A is typically an element from groups IIIA or IVA (e.g., Ti, V, Nb, or Mo), X represents C or N, and n varies from 1 to 3. The derived MXenes follow the formula M_n+1X_nT_x; here, M denotes the TMs site, X refers to the C or N atoms, n varies from 1 to 4, and T_x indicates the functional groups attached to the surface. Structurally, MXenes preserve a hexagonal close-packed (hcp) arrangement like the basal plane of their MAX phase. In this framework, the TM atoms (M) form a densely packed layer, while the X atoms occupy octahedral sites between these M atom layers.

MXene, characterized by outstanding conductivity, remarkable mechanical properties, and adjustable interlayer distance [18,19,20], presents significant promise for advancing LSBs (Figure 1). Its potential is evident in several key respects: (i) MXene’s superior conductivity facilitates efficient reduction of S at the cathode, thereby achieving high S consumption. (ii) The ample polar functional groups on its surface exhibit a robust affinity for LiPSs, efficiently minimizing their dissolution into the electrolyte and enhancing the CE of LSBs. Furthermore, these functional groups act as catalysts, accelerating the conversion of LiPSs into Li₂S₂ and Li₂S, thereby improving reaction kinetics. (iii) The polar functional groups also enable robust chemisorption of Li⁺, promoting uniform Li nucleation and deposition. (iv) MXene’s exceptional mechanical strength ensures electrode stability during cycling, while its customizable architecture makes it adaptable for various scenarios. Despite these benefits, the practical implementation of MXene in LSBs is constrained by environmentally detrimental preparation processes and suboptimal structural design.

In this work, we first explore the preparation approaches of MXene, transitioning from conventional techniques to eco-friendly alternatives. We thoroughly evaluate how these methods impact the cost, energy efficiency, environmental sustainability, and structural characteristics of the resulting MXene. Subsequently, we review recent progress in the application of MXene as a cathode and anode host, as well as a functional interlayer or separator in LSBs (Figure 2), with a focus on the relationship between MXene’s surface chemistry and the energy storage performance of LSBs. The studies highlighted in Figure 2 collectively showcase the evolution of MXenes in LSBs, beginning with the discovery of Ti₃C₂T_x MXene in 2011. Early work demonstrated MXene’s potential as a conductive S host (2015) and as a separator coating (2016) to suppress polysulfide shuttling. Subsequent research explored MXene as a Li host (2017) and incorporated N-doping (2018) and porous architecture (2019) to enhance active material utilization and ion transport. Hybrid structures such as MXene@CNF (2020) and vertically aligned MXenes (2022) improved Li plating stability and ionic conductivity. Recent innovations include multifunctional composites like TiS₂/TiO₂@MXene (2023) and 3D TiN-MXene-Co@CNT networks (2024), which synergistically address conductivity, polysulfide retention, and structural stability. These advancements have progressively established MXenes as versatile and high-performance components for next-generation LSBs. Lastly, we propose future directions for MXene synthesis and discuss its potential for real-world applications in LSBs.

2. Synthesis of MXene

MXene originates from the MAX phase (M_n+1AX_n), a ternary layered carbide or nitride, where “A” denotes elements from groups IIIA or IVA, for instance, Al, Si, or Ti. Due to the robust metallic M-A bonds with high bond energy [21], simple mechanical exfoliation cannot separate MXene sheets from the MAX phase. To overcome this, researchers have developed selective etching techniques to eliminate the A-layer from the MAX phase, thereby producing MXene. This etching process generates dangling bonds on the M atomic layer, which accelerates the attachment and coordination of various surface terminations. The synthesis method plays a critical role in determining MXene’s characteristics, including its surface chemistry, sheet size, and structure, as different etching techniques lead to distinct physicochemical characteristics. Traditional etching methods often involve F-containing compounds, which are highly detrimental to the environment and human health. Conversely, green synthesis methods eliminate the use of F, providing a safer and more eco-friendly alternative. This section offers a detailed review of both traditional and green synthesis approaches for MXene. Furthermore, a comparison of the differences between these methods is presented in

Table 1. The differences between traditional and green preparation routes [5].

Feature	Traditional Route	Green Route
Raw material	Containing HF	HF-free
Yield	Low	High
Preparation condition	Time consuming	Mild and efficient
By-product	HF gas	HF-free
Surface groups	Terminations limited to -F, -O, and -OH	Terminations with affinity for S and Li

.

2.1. Traditional Preparation Routes

2.1.1. HF Etching

In 2011, Gogotsi and colleagues achieved the successful delamination of Ti₃AlC₂ to synthesize Ti₃C₂ MXene for the first time using HF etching [22]. This process involves the selective elimination of A-layer from the MAX phase, as outlined below:

Ti₃AlC₂ + 3HF → AlF₃ + 3/2H₂ (g)↑ + Ti₃C₂

(1)

Ti₃C₂ + 2H₂O → Ti₃C₂(OH)₂ + H₂ (g)↑

(2)

Ti₃C₂ + 2HF → Ti₃C₂F₂ + H₂ (g)↑

(3)

HF removes Al atoms from Ti₃AlC₂ through a substitution reaction, releasing H₂ as a byproduct. The resulting Ti₃C₂ reacts further with H₂O and HF to yield Ti₃C₂T_x (T_x: -F, -OH). In 2015, Rosen et al. successfully synthesized Mo₂CT_x MXene by etching Mo₂Ga₂C MAX phase, which contains extra A-layer atoms, using HF acid as the etchant [23]. This approach was later applied to produce other MXenes, such as Nb₄C₃T_x [24] and Hf₃C₂T_x [25]. Naguib and colleagues immersed the MAX phase in a 50 wt% HF solution for 2 h, followed by washing, etching, and centrifugation, yielding a multi-layered, accordion-type MXene architecture that is difficult to replicate using other etching techniques. This distinctive structure results from the exothermic removal of Al and the release of H₂ during the process [26]. Since its introduction, the HF etching route has remained the most widely employed due to its simplicity and rapid effectiveness in etching the MAX phase. Moreover, HF’s extreme corrosiveness poses significant environmental and health hazards and can lead to the over-etching of MXene, adversely affecting its performance. Consequently, there is an urgent need for milder, less toxic, and more eco-friendly alternatives to HF etching technique.

2.1.2. In Situ HF Etching

To address the corrosion challenges associated with HF, Ghidiu and colleagues developed an in situ HF generation etching method. This technique includes the reaction of 6 M HCl with 1.98 g of LiF to produce in situ HF, which specifically removes the Al atomic layer from Ti₃AlC₂. The process yields a clay-type MXene with exceptional dispersion and hydrophilicity, suitable for direct use as a flexible thin-film electrode using a simple rolling technique [27]. Additionally, this etching process can be modified by substituting LiF and HCl with other fluoride salts (NaF, KF, CsF) and strong acids like H₂SO₄. Halim’s team suggested that NH₄HF₂ could serve as an alternative to HF for etching the Al layer from Ti₃AlC₂ [28]. Feng et al [29]. further elucidated the etching mechanism, presenting it through the following reaction formulae:

Ti₃AlC₂ + XHF₂ → X_aAlF_b + AlF₃ + H₂ (g)↑ + Ti₃C₂

(4)

AlF₃ + cH₂O → AlF₃. cH₂O

(5)

Ti₃C₂ + XHF₂ + H₂O → Ti₃C₂F_x(OH)_yX_z

(6)

In this context, “X” represents cations such as NH₄⁺, K⁺, and Na⁺; while a, b, c, x, y, and z denote the numerical values [29]. The equations above highlight that both H⁺ and F⁻ ions are essential for MXene production, suggesting that this method operates through a reaction process similar to HF etching.

Compared to conventional HF etching, this approach offers several advantages: (i) it involves safer and milder preparation processes; and (ii) cations in the solution, such as Li⁺ and NH₄⁺, intercalate between MXene layers, increasing interlayer spacing and reducing van der Waals forces, thereby simplifying synthesis, resulting in fewer -F terminations and more -O terminations, which minimizes side reactions. As a result, this method has become a preferred approach among researchers for synthesizing MXene. However, some challenges persist, such as the formation of unetched residual MAX phase biproducts, lower etching efficacy, and the lengthy nature of the process.

2.1.3. Molten Fluorine Salt Etching

Compared to carbide MAX precursors, nitride MAX precursors have stronger bonds between the A and M layer atoms, requiring greater energy to eliminate the A-layer. Furthermore, Ti_n+1N_n precursors are less stable and can easily dissolve in HF. To overcome these challenges, Urbankowski et al. introduced this etching route in 2016 for synthesizing nitride MXene [30]. In this approach, Ti₄AlN₃ was etched using a molten fluoride salt mixture (LiF, KF, and NaF in a mass ratio of 29:59:12 wt%) and calcined at 550 °C in an Ar atmosphere to eliminate the Al layer. After etching, fluoride impurities were eliminated with H₂SO₄, and Ti₄N₃T_x NSs were obtained through TBAOH intercalation and sonication. This method relies on precise temperature control and the use of an inert atmosphere to selectively etch the Al layer while maintaining the structural integrity of Ti-N bonds in Ti₄N₃. Nitride MXenes prepared using this method exhibit improved electronic conductivity and a higher DOS. Additionally, non-terminated Ti₄N₃ displays metallic and magnetic characteristics. Furthermore, Ti₄N₃T_x MXenes exhibit lower crystallinity compared to those produced by the previously mentioned etching methods, resulting in reduced flexibility.

2.1.4. Water-Free Etching

Acid-etching methods typically rely on water as a solvent, which can limit the potential applications of MXene. For instance, certain polymerization reactions cannot proceed in the presence of water, and quantum dot (0D) materials cannot be integrated with MXene under aqueous conditions. Moreover, even trace amounts of water in MXene materials can significantly degrade battery performance. As a result, MXene must undergo vacuum annealing before cell assembly. To address these limitations, Natu and colleagues introduced an innovative method that utilizes organic polar solvents like DMF, NMP, DMSO, and PC in combination with NH₄HF₂ to etch MAX precursors in the water-free atmosphere. This process produces Ti₃C₂T_x with a high-content of -F terminations (Figure 3a) [31]. MXene composite electrodes prepared using this approach demonstrated twice the anode capacity in SIBs compared to those etched in water. Moreover, this method requires significantly more time (196 h compared to 48 h for aqueous etching) and necessitates working in a glove box, adding operational complexity for researchers.

2.2. Green Synthesis

Traditional MXene synthesis methods suffer from several limitations, including high energy requirements, the use of toxic HF, and the production of hazardous waste, making them less environmentally sustainable and impractical for industrial-scale applications. As a result, there is an urgent demand for green, energy-efficient, controllable, and environmentally friendly synthesis approaches. Furthermore, conventional methods limit MXene surface terminations to -F and -OH, whereas green preparation approaches can create new terminations with stronger affinities for S and Li, enabling improved structural control of MXene. These methods also allow for electronic modulation by creating defect vacancies and single-atom doping, thereby enhancing MXene’s characteristics [36]. Exploring green preparation strategies is therefore essential for achieving greater structural diversity, advanced surface chemistry, precise synthesis, and broader potential advantages of MXene.

2.2.1. Electrochemical Etching

Etching with F-containing agents creates -F terminations on MXene, which can react with residual water to produce HF, potentially causing electrode corrosion. Hence, the development of F-free etching methods for MXene is crucial. Electrochemical etching offers a viable alternative, utilizing a cathode–anode system submerged in an electrolyte. By applying a voltage difference and carefully adjusting the voltage range (etching potential) within the reaction window between the A and M atomic layers, the A-layer in the MAX phase can be precisely removed [37]. In 2017, Sun et al. pioneered the preparation of MXene using an F-free solution through the electrochemical etching approach [38]. Building on this, Yang et al., in 2018, successfully etched Ti₃AlC₂ using a two-electrode electrochemical system [39]. In this method, a mixture of NH₄Cl and TMAOH was used as an electrolyte, with Ti₃AlC₂ acting as both the cathode and anode. The chemical reactions involved proceeded as below:

Ti₃AlC₂ − 3e⁻ + 3Cl⁻ → Ti₃C₂ + AlCl₃

(7)

Ti₃C₂ + 2OH⁻ − 2e⁻ → Ti₃C₂(OH)₂

(8)

Ti₃C₂ + 2H₂O → Ti₃C₂(OH)₂ + H₂ (g)↑

(9)

Shen and colleagues initiated a molten salt-assisted electrochemical etching technique. By subjecting the MAX precursor to a 2 V voltage in a LiCl-KCl molten salt environment, they successfully synthesized Ti₃C₂Cl₂ without introducing any metal impurities. To modify the surface terminations, they substituted -Cl with -O or -S by utilizing different salts, streamlining the process of functional group modification and broadening the variety of achievable functional groups (Figure 3b) [32]. This technique significantly accelerates MXene preparation due to the application of an electric field. Moreover, it offers a safer etching atmosphere, and the resulting MXene film features a large lateral size. However, the harsh etching conditions constrain the MXene yield, presenting challenges for scaling up production.

2.2.2. Salt-Templated Approach

Gogotsi and colleagues introduced a salt-templated technique to enable the large-scale synthesis of nitride MXene [33]. The process involved coating NaCl with MoO₃, achieved by calcining a Mo precursor deposited NaCl under an Ar atmosphere. This was followed by ammoniation to convert the coating into MoN. Lastly, the NaCl was dissolved through washing, successfully isolating MoN. By adjusting the precursor and synthesis parameters, it is possible to produce a broader range of nitride MXenes. The resulting product exhibits metallic characteristics and exceptional rate capabilities (Figure 3c–e). This approach offers significant promise for producing high-quality nitride MXenes without relying on etching processes.

2.2.3. Alkali Etching Approach

Aluminum’s amphoteric nature allows it to react with acids and bases, making NaOH a viable etchant for synthesizing Ti₃C₂T_x [40]. In this process, OH⁻ ions react with Al to form Al(OH)₄⁻, which interacts with Ti₃C₂ to yield Ti₃C₂T_x. However, the Ti₃C₂T_x structure is unstable in alkaline environments and prone to corrosion. To address this, Li and colleagues introduced an alkali-assisted hydrothermal technique, carefully controlling the NaOH concentration (27.5 M) and reaction temperature (270 °C) to prevent framework corrosion [41]. Despite its effectiveness, this method requires high temperatures and concentrated alkali, raising safety concerns and limiting scalability. As a safer alternative, Xuan et al. utilized organic bases such as TBAOH as etching agents to intercalate and delaminate Ti₃AlC₂, facilitating the formation of Al(OH)₄⁻ as the surface termination of Ti₃C₂ [42]. Although the alkali-assisted hydrothermal method yields MXene with great purity (92 wt%), the resulting material is often multilayered and requires additional delamination to achieve few-layer or monolayer MXene for broader applications. On the other hand, the TBAOH method necessitates HF pretreatment to remove surface oxides from Ti₃AlC₂, introducing safety risks.

2.2.4. Lewis Acidic Molten Salt Etching

Although many MAX phases contain non-Al atomic layers, studies on synthesizing MXene from the MAX phases without Al layers remain scarce. Li’s group demonstrated a process using high-temperature ZnCl₂ molten salts to process MAX phases with Al atomic layers. This approach enabled the formation of novel MAX precursors with Zn as the A-layer, such as Ti₃ZnC₂, V₂ZnC, and Ti₂ZnN [43]. Building on this work, Huang’s group introduced this approach. In this strategy, Lewis acid cations with greater redox potentials oxidize and selectively etch the A-layers of MAX phases with lesser redox potentials [44]. Using CuCl₂ molten salts at elevated temperatures, this method successfully etched MAX phases with A-layers made of elements like Si, Zn, and Ga, resulting in MXene materials rich in -Cl terminations. The details of the etching process are as follows:

Ti₃SiC₂ + 2CuCl₂ → Ti₃C₂ + SiCl₄ (g)↑ + 2Cu

(10)

Ti₃C₂ + CuCl₂ → Ti₃C₂Cl₂ + Cu

(11)

Huang and colleagues, in 2023, proposed a “chemical scissor” strategy for structural modification of MAX phases and MXene, paving the way for 3D assembly of 2D materials (Figure 3f) [34]. This innovative technique expands the variety of MAX phases that can be utilized and enables precise tuning of MXene’s surface functional groups and structural characteristics. Despite its potential, the application of this approach is restricted by the intrinsic characteristics of traditional Lewis acidic molten salts, such as their low redox potential and instability at high temperatures. Additionally, the chemical mechanisms underlying the intercalation process remain insufficiently understood.

2.2.5. UV Induced Selective Etching

UV irradiation has been reported to increase the surface reactivity of materials, with Mo₂Ga₂C standing out for its exceptional UV-absorbing characteristics. Mei and colleagues, in 2020, leveraged a UV-sensitive Mo₂Ga₂C precursor and employed UV irradiation under mild conditions in the presence of phosphoric acid to etch Ga atoms. This process yielded Mo₂C MXene NSs, which, after ultrasonic delamination, demonstrated potential as anodes for rechargeable batteries [45]. This approach allows for the synthesis of MXene within just a few hours, significantly dropping fabrication time. Moreover, the Mo₂C exhibits mesoporous architecture, making it highly favorable for electrochemical applications. Nevertheless, the technique may require specialized equipment and precise control of the reaction conditions, potentially necessitating advanced technical expertise. In summary, while this etching offers notable benefits in certain applications, its strengths and limitations should be carefully evaluated based on the intended use.

2.2.6. CVD

CVD is a versatile technique that enables gaseous precursors to react and form solid materials, which are then deposited onto substrates for fabricating thin films, and heterostructures. Xu and colleagues employed the CVD method to directly synthesize ultrathin α-Mo₂C 2D crystals with lateral dimensions exceeding 100 μm by employing methane as the C source and Cu/Mo foils as substrates at 1085 °C [46]. Later, Geng and colleagues utilized CVD to grow Mo₂C on graphene, achieving lateral sizes till the centimeter dimension [47]. Notably, the Mo₂C structures synthesized on graphene exhibit greater uniformity and possess hexagonal structures compared to pure Mo₂C crystals. Recently, Talapin and colleagues developed a “carpet-like” Ti₂CCl₂ structure by depositing a gas blend of CH₄ and TiCl₄ onto a surface of Ti at 950 °C. This MXene featured a unique spherical architecture formed through bending and unfolding, resulting in a highly accessible surface with exposed catalytically active edges, offering exceptional Li⁺ storage capacity (Figure 3g,h) [35]. CVD offers several advantages: (i) it enables the direct preparation of MXene, free of surface functional groups, enabling a detailed examination of its internal atomic layer structure; (ii) it produces MXene with greater lateral sizes, great surface quality, less defect density, and precise control; (iii) it eliminates the need for MAX phase precursors, significantly reducing preparation time. Despite these advantages, the range of MXenes that can be synthesized through CVD is currently limited. Furthermore, the great production costs and stringent reaction environments, such as extended durations, specific gas requirements, and elevated temperatures, pose significant challenges for large-scale purposes.

2.2.7. Thermal Reduction

In 2020, Sun and colleagues proposed a novel thermal reduction as a straightforward and eco-friendly approach for preparing MXene. This process involved heating the Ti₂SC MAX phase in an Ar and H₂ atmosphere at 800 °C to precisely remove S atoms. The process was followed by ultrasound-supported treatment, yielding free-standing 2D Ti₂C NSs [48]. This straightforward technique shows promise for large-scale industrial production. However, it is crucial to keep the temperature below 700 °C to prevent the nucleation of TiO₂, which can adversely impact MXene yield and quality.

2.2.8. In Situ Hydrothermal Process

The hydrothermal process involves conducting a reaction within a sealed container, utilizing water as the solvent at a controlled atmosphere inside the reactor [49,50,51]. Song and colleagues employed an in situ hydrothermal process to prepare amorphous MoS₂ integrated with F-free Mo₂CT_x. This was achieved using NH₄Cl as the etchant, DMSO as the S source, and Mo₂Ga₂C as the starting material, under conditions of 180 °C for five days [52]. This study offers fresh perspectives on developing novel heterogeneous MXenes beyond traditional types, broadening the scope of MXene research. Moreover, the incorporation of MoS₂ imparts a distinctive morphology and heterojunction structure to the heterogeneous Mo₂C, enhancing its structural stability. This modification adjusts the Li⁺ storage activity and facilitates ion movement. Additionally, the hetero-Mo₂C sample demonstrated outstanding cycle stability, retaining a capacity of 683.9 mAh g⁻¹ after 1200 cycles (Figure 4a–c).

2.2.9. Photo-Fenton (P.F.)

The P.F. is among the widely investigated advanced oxidation processes, recognized as a promising approach for pollutant removal through reactive oxygen groups in both homo- and heterogeneous iron-mediated techniques [54,55,56,57]. Xiao and colleagues introduced a low temperature “soft chemistry” technique depending on the P.F. process. In this process, Ti₃AlC₂ was immersed in a solution of Fe (III) and Na₂C₂O₄ under continuous stirring. H₂O₂ was then added, and the mixture was exposed to UV illumination while stirring for 10 h. After centrifugal drying, F-free Ti₃C₂ (Ff-Ti₃C₂) with a purity of up to 95% was obtained [53]. During the P.F. reaction, reactive oxygen species (HO* and O₂*⁻) are continuously generated, weakening the Ti-Al bond in the MAX phase and promoting the high concentration of OH⁻ formation, which facilitates the topochemical delamination of Al layer. The resulting Ff-Ti₃C₂ displays exceptional mechanical stability, enhanced wettability of the electrolyte, and robust electrocatalytic action for the decomposition of Li₂S, making it an excellent S cathode host for advanced LSBs (Figure 4d). However, the use of light sources and oxidative processes in the P.F. technique may result in extensive synthesis periods compared to further preparation techniques, potentially enhancing experimental complication and costs.

2.2.10. Physical Vacuum Distillation

Feng and colleagues introduced an acid- and F-free physical vacuum distillation technique for the direct synthesis of various MXenes [58]. This method began with the incorporation of low-boiling-point Zn into the MAX phase using ZnCl₂ molten salt. Later, the A-layer is removed through this technique, leading to the successful fabrication of a range of F-free MXenes with different TMs. These included Nb₂CT_x, Ti₃C₂T_x, Nb₄C₃T_x, Ta₂CT_x, and Ti₂NT_x, where T_x denotes -Cl and -O functional groups. The activity is governed by the following reactions:

Ti₃AlC₂ (s) + 1.5ZnCl₂ (s) → Ti₃C₂ (s) + 1.5 Zn (s) + AlCl₃ (g)↑

(12)

Ti₃C₂ (s) + ZnCl₂ (s) → Ti₃C₂Cl₂ (s) + 1.5 Zn (s)

(13)

Zn (s) → Zn (g)↑

(14)

The structure of MXene and specific surface area (SSA) can be customized by varying the temperature, making them suitable for diverse applications. This synthesis approach eliminates the need for acids or bases, and the metallic Zn can be reclaimed after evaporation, offering an eco-friendly, cost-efficient, and sustainable approach for producing various MXenes. Additionally, this technique shows great promise for large-scale production owing to its affordability, straightforwardness, and precise control. Furthermore, despite its capability for single-step MXene synthesis, the method operates at high temperatures. Therefore, developing alternative preparation techniques that work at room temperature or lower is crucial for enabling the industrial utilization of MXenes.

Therefore, MXenes can be produced through various synthesis techniques, including acid, and electrochemical etching, salt templating, and physical vacuum distillation. Traditional acid etching typically yields MXenes with surface functional groups like -OH, -O, and -F. In contrast, methods like electrochemical etching, salt templating, and vacuum distillation may present alternative functional groups, like S or Cl. These electronegative elements exhibit strong interactions with S and Li, enhancing electrochemical performance. By selecting and optimizing the preparation approach, the surface chemistry of MXenes may be effectively tailored to enhance the performance of LSBs.

Table 2. The characteristics, benefits, limitations, electrochemical characteristics, and impact on surface terminations associated with various MXene preparation techniques [5].

Technique	Feature	Merits	Demerits	Application	Role	Current Density (mA g−1)	Capacity (mAh g−1)	Cycle Number	Terminations	Ref.
HF etching	Accordion-type structure	Easy preparation process and relatively safe	Hazardous	LIBs	Anode	200	146	200	-F, -O, -OH	[26]
In Situ HF etching	Few -F terminations clay type MXene with great interlayer distance		Low yield and Time taking	LSBs	Separator	0.5 C	687	100	-F, -O, -OH, -Cl	[59]
Molten fluorine salt etching	Metallic and magnetic characteristics	High efficiency	High preparation temperature	-	-	-	-	-	-F, -O, -OH	[30]
Water-free etching	Accordion-type structure with many -F terminations	High anode capacity	Long etching time and Inconvenient operation	SIBs	Anode	100	160	700	-F, -O, -OH	[31]
Electrochemical etching	Large lateral size and Rich surface terminations	High safety and Low energy consuming	Low yield	SCs	-	-	-	-	-O, -OH -S, -Cl	[60]
Salt-Templated	NSs	High efficiency	High preparation temperature		-	-	-	-	-Cl	[32]
Alkali etching	Accordion-like structure	High purity	Certain safety hazards		-	-	-	-	-O, -OH Al(OH)4−	[41]
Lewis acidic molten salts etching		Wider etching precursor range	Limited applicability	LIHCs	-	-	-	-	-O, -Cl	[43]
UV-induced selective etching	Graphene-like structure	Great purity and short preparation time	Advanced instruments needed	LIBs	Anode	10	90	140	-O	[34]
CVD	Large lateral size and high surface quality	Controllable thickness and atom-economic preparation	High cost and high preparation temperature	Three electrode System	-	-	-	-	-	[47]
Thermal reduction	Graphene-like structure	Short preparation time and simple preparation process	High preparation temperature and relies on S-containing MAX phase	LIBs	Anode	2000	70	130	-O, -OH	[35]
In situ hydrothermal	NSs	Simple preparation and low synthesis temperature	Long preparation time			100	683.9	1200	-	[61]
Photo-Fenton	Exceptional hydrophilicity Accordion-type structure	Short preparation time and High purity	Complex preparation activity	LSBs	Cathode	1 C	585	500	-O, -OH	[54]
Physical vacuum distillation	Controllable structure and SSA	Low cost and easy process	High preparation temperature	SIBs	Anode	200	51.5	110	-O, -Cl	[58]

summarizes the preparation approaches, electrochemical performance, and characteristics of MXenes, while

Table 3. Guiding principles for utilizing MXene in various applications for LSBs [5].

Preferred Synthesis Methods	Preferred Structure	Feature	Function	Surface Groups
HF etching, salt-templated, hydrothermal	NSs or porous structure	S cathode host	Dispersing S and confining S	-F, -O, -OH, -S, -Cl et al.
Electrochemical etching, UV-induced selective etching	NSs	Separator	Blocking S and inhibiting dendrite
HF etching, salt-templated, hydrothermal	NSs or porous structure	Li anode host	Suppressing dendrite

outlines design recommendations for their application in S cathodes, separators, and Li anodes in LSBs.

3. The Characteristics of MXenes

3.1. Structural Characteristics

Atomic structure of MXenes is key to investigating their unique properties. As earlier discussed, MAX precursors are layered hexagonal materials classified under the P6₃/mmc space group, where alternating “MX” and “A” layers stack along the c-axis. When the A-layer is removed, MXenes retain the hexagonal close-packed structure of their parent MAX phases. This structural continuity means that the compositional and structural diversity of MAX precursors clearly translate into a wide range of MXenes. Researchers have identified numerous new MAX precursors to prepare unique MXenes. Depending on the choice of “M,” MXenes can be categorized into four distinct varieties [62], as depicted in Figure 5. The first and most common category is mono-TM MXenes. Examples include Ti₂C [63,64,65,66], Ti₂N [67,68,69,70], Nb₂C [71,72], V₂C [71,73], V₂N [74,75], Cr₂C [76,77], Ta₂C [78,79], Mo₂C [23,80,81], Ti₃C₂ [82,83,84], Zr₃C₂ [85], Hf₃C₂ [25], Ta₄C₃ [26,86], Nb₄C₃ [87,88], V₄C₃ [89,90], and Ti₄N₃ [30,91], among others. The second category consists of randomly arranged double-TM (double-M) MXenes, such as (Ti,Nb)₂C [26,92], (Cr,V)₃C₂ [26,93], (Nb,V)₂C [94], (Nb,Ti)₄C₃ [95], (Nb,Zr)₄C₃ [95], and (Mo,V)₄C₃ [96], among others. The third category includes ordered double-M MXenes, such as Mo₂TiC₂ [97], Cr₂TiC₂ [98], Mo₂Ti₂C₃ [97], and Mo₂ScC₂ [99]. The final category is ordered divacancy MXenes, for example, Mo_1.33C [100] and W_1.33C [101]. Newly, some high-entropy MXenes have also been investigated, including Ti_1.1V_0.7Cr_xNb_1.0Ta_0.6C₃T_z (T_z = -F, -O, -OH) [102], TiVNbMoC₃T_x [103], TiVCrMoC₃T_x [103], and (Ti_1/3V_1/6Zr_1/6Nb_1/6Ta_1/6)C_xN_1-xT_y [104].

MXenes can also be prepared from non-MAX precursors. For instance, layered carbides like Zr₃Al₃C₅ [85], Hf₃(AlSi)₄C₅ [25], and Mo₂Ga₂C [23] have been utilized to produce Zr₃C₂T_x, Hf₃C₂T_x, and Mo₂CT_x, respectively. Additionally, surface terminations are critical to the structural diversity and characteristics of MXenes, with their nature strongly influenced by the preparation process. These surface groups can be precisely tailored through post-treatment processes. For example, F terminations can be eliminated through high-temperature vacuum annealing [105] or alkali behavior [106]. O terminations can be introduced by exposing MXenes to O₂ [107], CO₂ [108], and Li₂O treatment [44], or by transforming OH terminations through vacuum annealing [105]. Conversely, O terminations can be eliminated under a H₂ atmosphere [105], while halide terminations can be eliminated using LiH [44]. Additionally, terminations of S, Se, and Te can be incorporated by treating MXenes with NaNH₂, Li₂S, Li₂Se, and Li₂Te, respectively [44]. However, conventional etching methods often lead to the oxidation and hydrolysis of resulting MXene moieties. Thus, it is crucial to establish a clear structure–property relationship for MXenes to better understand how to prevent their degradation. This will help improve their oxidative and aqueous-phase stability.

Figure 5. Structures of synthesized MXenes. Adapted from [109], Copyright 2021, Wiley-VCH.

Figure 5. Structures of synthesized MXenes. Adapted from [109], Copyright 2021, Wiley-VCH.

3.2. Electronic Characteristics

Theoretical studies have revealed that MXenes can display diverse electronic properties, ranging from metallic and semiconductor-like to topological insulating behaviors depending on their M and X atoms, as well as T surface terminations [110]. Interestingly, the electronic characteristics of MXenes are more significantly influenced by their external metal atomic layer than the internal metal layer [111]. While most MXenes exhibit a metallic nature, a subset has been predicted to function as semiconductors. Kawazoe and colleagues demonstrated through simulations that Sc₂CT₂ (T = F, OH, and O), Ti₂CO₂, Zr₂CO₂, and Hf₂CO₂ are semiconductors with band gaps spanning 0.24 to 1.8 eV [112]. The additional valence electrons contributed by N atoms make M_n+1C_nT_x generally more favorable as semiconductors, whereas M_n+1N_nT_x tends to exhibit stronger magnetic properties [112]. Surface terminations are also pivotal in shaping MXenes electronic characteristics. For instance, their work functions range widely from 1.6 to 8.0 eV [113] and can be finely tuned by modifying the surface termination groups [113]. Generally, OH terminations typically result in lower work functions owing to their effective surface dipole moment, while terminations of O and F yield greater work functions due to their great electronegativity [114].

To date, all experimentally synthesized MXenes have exhibited metallic or semi-metallic characteristics, with no semiconducting MXenes realized yet. Studies show that MXene electrical conductivity is significantly affected by factors like the precursor material, its size, etching techniques, and post-treatment processes [115]. For instance, post-annealing treatments can efficiently eliminate water, intercalants, and surface functional groups, resulting in enhanced conductivity (Figure 6a–c) [105]. Recently, Gogotsi and colleagues set a record for MXene conductivity, achieving approximately 20,000 S cm⁻¹ for Ti₃C₂T_x by optimizing the Ti₃AlC₂ MAX [115]. Surface terminations of MXenes, typically uncovered by synthesis methods, are often diverse and contribute to their wide-ranging properties [113]. For instance, the commonly studied Ti₃C₂T_x generally features terminations like -O, -OH, and -F. These surface groups can be modified through vacuum annealing, allowing adjustments in the work function from 4.1 to 4.8 eV (Figure 6d,e) [114]. Additional chemical treatments, for instance, grafting diazonium surface groups [113], or using ammonium hydroxide [116], N₂H₄, or UV-ozone [117], provide further control over the MXenes work function. Talapin and co-workers in 2020 reported superconducting transitions in Nb₂C with terminations such as -Cl₂, -S₂, -Se, and -NH at temperatures below 10 K [67]. Moreover, the functional groups present on MXenes facilitate their integration with a variety of organic materials and ligands, greatly enhancing their application potential [118].

3.3. Mechanical Characteristics

MXenes significantly enhance the performance of S cathodes through three main advantages: (1) enabling uniform and efficient interactions with numerous S moieties; (2) maintaining electrocatalytic activity to facilitate the conversion of polysulfides; and (3) accommodating the substantial volume changes of S during charging and discharging. These benefits stem from the exceptional mechanical stability of MXenes, which permits them to endure the significant volume expansions of S without structural degradation. This stability ensures consistent bonding with S, thereby enhancing the cathode’s performance. The mechanical characteristics of MXenes are closely linked to their surface terminations, thickness, and structural attributes. First-principles measurements by Sun and colleagues demonstrated that 2D Ti₂C can withstand considerable strain, enduring 9.5, 18, and 17% under bi- and uni-axial tension beside the x- and y-axes [119]. Additionally, functionalizing the surface with oxygen was found to prevent the failure of the Ti layer, allowing the material to stretch further to 20, 28, and 26.5% in the corresponding directions. This highlights the critical role of surface termination in enhancing the mechanical strength of MXenes [119]. Balke’s group investigated the mechanical properties of stacked Ti₃C₂ NSs with various surface terminations (-F, -O, and -OH) using DFT [120]. Their findings revealed that OH-terminated MXene layers had a minimal gap distance of 0.57 Å, whereas O- and F-terminated layers exhibited a gap distance of approximately 2.5 Å. The improved elastic constants of OH-terminated MXenes were attributed to the formation of hydrogen bonds between their hydroxyl groups. However, the presence of H₂O molecules in the interlayer spaces significantly weakens the mechanical strength of MXenes. Structurally, bare Ti₂C displayed greater stiffness and strength compared to bare Ti₃C₂ and Ti₄C₃. The Young’s modulus values for these materials were calculated using DFT and molecular dynamics (MD) simulations, yielding 597, 502, and 534 GPa, respectively [119]. Under high strain, the first crack in Ti₄C₃ was observed to originate at its center, whereas failure in Ti₃C₂ and Ti₂C initiated at their edges [119]. Experimentally, the Young’s modulus of Ti₃C₂T_x and Nb₄C₃T_x was evaluated at 0.33 [121] and 0.39 TPa [122], respectively. Compared to other 2D materials, MXenes displayed exceptional mechanical performance, underscoring their competitiveness in this regard.

Although monolayer MXenes exhibit excellent mechanical properties, widely used MXene films are typically composed of stacked multilayer NSs, resulting in significantly lower mechanical performance. For instance, Gogotsi et al. reported that pure Ti₃C₂T_x films exhibit a Young’s modulus of 3.52 GPa and a uniaxial tensile strain of 1%, significantly lower than the predicted values for monolayer NSs [123]. This disparity is primarily due to the inherent properties of Ti₃C₂T_x NSs, like their size, defects, imperfect edges, and weak interlayer forces. Low-quality MXene NSs can further compromise mechanical performance, as the interlayer forces in MXene films are primarily van der Waals interactions, which do not provide sufficient mechanical strength to bulk films. Strengthening the interlayer interactions has proven to be a highly efficient strategy for improving the MXenes performance. One common strategy involves introducing ionic or bonding agents. For example, incorporating polyvinyl alcohol (PVA) into Ti₃C₂T_x films significantly improved their strength [123]. Gogotsi and colleagues demonstrated that the Ti₃C₂T_x/PVA composite films achieved a tensile strength of 91 ± 10 MPa, almost four times greater than that of Ti₃C₂T_x films. Similarly, Kim and colleagues drew inspiration from mussel adhesive proteins to utilize dopamine for enhancing interlayer interactions and structural organization in Ti₃C₂T_x films [124]. By polymerizing dopamine in situ on the surfaces of Ti₃C₂T_x flakes, they created a thin adhesive layer, resulting in a greatly aligned and densely packed architecture. This modification improved the tensile strength by sevenfold and significantly enhanced the films elongation. Cheng and colleagues developed a high-performance Ti₃C₂T_x film using a two-step bridging strategy [125]. This process involved sequentially bridging Ti₃C₂T_x NSs with sodium alginate through hydrogen bonding and Ca²⁺ ions via ionic bonding. The resulting modified Ti₃C₂T_x film demonstrated exceptional mechanical properties, including an in-plane tensile strength of 436 MPa and a Young’s modulus of 14 GPa, representing approximately 6.9 and 2.5-fold improvements, respectively, compared to pure Ti₃C₂T_x NSs. Building on this work, the same team recently implemented an effective densification technique that combined sequential bridging with hydrogen and covalent bonding. This method utilized borate ions for hydrogen bonding and sodium carboxymethyl cellulose (CMC) for covalent bonding [126]. The resulting mixed Ti₃C₂T_x film achieved remarkable mechanical performance, with a tensile strength of 583 ± 16 MPa, a young’s modulus of 27.8 ± 2.8 GPa, and an impressive toughness of 15.9 ± 1.0 MJ m⁻³.

4. Principle of LSBs

As illustrated in Figure 7, LSBs are composed of four main components: the S cathode, metal anode, organic liquid electrolyte, and a porous polymer separator. Unlike conventional LIBs that operate through an “ion intercalation/deintercalation” process, LSBs function based on redox reactions among elemental S and the metal. In LSBs, S acts as the final charge moiety, so the operational cycle typically begins with the discharge activity. During discharge, the metal anode undergoes oxidation, releasing metal cations that migrate toward the cathode in the presence of electric field. At the cathode, these cations react with S to produce metal sulfides. The reverse reactions happen during the charging phase.

Figure 8 depicts the typical charge and discharge processes of LSBs in ether-related electrolytes. The entire reaction between Li and S is expressed as 2Li + S → Li₂S. However, the actual conversion process between S and metal sulfides is more complex. At room temperature, elemental S exists as crown-shaped S₈ rings, and the charge/discharge process implies the breaking and reforming of these S₈ molecules. During this process, multi-stage LiPSs (Li₂S_x, 1 ≤ x ≤ 8) are formed as intermediates [128]. Out of these intermediates, long-chain polysulfides (Li₂S_x, 4 ≤ x ≤ 8) are greatly soluble in ether-related electrolytes. This high solubility has both advantages and disadvantages. On the positive side, it enables faster reaction kinetics due to liquid-phase reactions. However, it also leads to the undesirable shuttle effect, where dissolved polysulfides migrate between the electrodes, causing premature battery degradation. In contrast, short-chain polysulfides (Li₂S_x, 1 ≤ x < 4) exhibit moderate solubility, reducing dissolution and shuttle-related issues. Nevertheless, their poor conductivity and slower kinetics, resulting from solid-phase reactions, hinder complete charge/discharge cycles. This limitation leads to reduced battery capacity, higher polarization, and diminished performance.

5. Challenges and Solutions of LSBs

5.1. Challenges of LSBs

The charge storage activity in LSBs is governed by the reversible redox process between S₈ and Li₂S, expressed as follows:

S₈ + 16Li⁺ + 16e⁻ ↔ 8Li₂S

(15)

During the discharge activity of LSBs, multiple intermediate reactions occur, forming various LiPSs (Li₂S_x, 1 ≤ x ≤ 8), as illustrated in Figure 8. Discharge curve features two distinct voltage plateaus. The first plateau, observed between 2.4 and 2.1 V, corresponds to the decrease in S₈ into long-chain LiPSs, providing a theoretical capacity of 418 mAh g⁻¹, which involves the transfer of 0.5 electrons/S atom. The second plateau, from 2.1 to 1.7 V, reflects the conversion of LiPSs into Li₂S₂/Li₂S, with a theoretical capacity of 1254 mAh g⁻¹, representing 1.5 electron transfers/S atom. Due to the great solubility of LiPSs in the electrolyte, the discharge method includes phase transitions: solid (S₈) → liquid (LiPSs) → solid (Li₂S₂/Li₂S). During charging, the reverse reaction takes place, where Li₂S₂/Li₂S is oxidized to LiPSs and eventually back to S₈ [8]. However, the charge activity is significantly influenced by electric fields and LiPSs concentration gradients. Some LiPSs dissolve in the electrolyte, diffuse across the separator to an anode, and are degraded to short-chain polysulfides Li₂S_n (2 ≤ n ≤ 4). These short-chain polysulfides then migrate back to the cathode where they are reoxidized to LiPSs. This repetitive cycle, called as the “shuttle effect,” causes active S loss, anode passivation, and several issues, including rapid capacity decay, reduced cycle life, minimal CE, and severe self-discharge [130]. Another challenge is the extremely low electrical conductivity of S (5 × 10⁻³⁰ S cm⁻¹ at 25 °C) and Li₂S (10⁻³⁰ S cm⁻¹ at 25 °C), which limits S utilization and reduces capacity. Furthermore, the substantial volume expansion (80%) during the conversion of S₈ (2.03 g cm⁻³) to Li₂S (1.66 g cm⁻³) can also damage the cathode’s structural integrity [131]. For the Li anode, dendrite formation poses a critical safety risk by potentially causing short circuits. Compared to LIBs, the LSBs are more complex due to direct interactions between LiPSs and the Li anode, leading to “dead Li” formation and material loss [132]. Moreover, volume changes during plating and stripping of Li can compromise the anode’s structural stability, eventually causing its failure [133].

5.2. Solutions of LSBs

For the commercial viability of LSBs, it is crucial to enhance their practical capacity and extend cycle life. Achieving these goals primarily relies on mitigating the shuttle effect, improving electrical conductivity, reducing volume changes, and preventing Li dendrite formation. Key approaches involve the strategic design of the cathode, incorporating an interlayer between the cathode and the separator, and optimizing anode structure.

5.2.1. Rational Construction of S Cathode

Since Nazar and colleagues presented the utilization of greatly ordered mesoporous C (CMK-3) for fabricating CMK-3/S composite cathodes in 2009 [134], the design of S cathodes has become a primary strategy for enhancing the electrochemical activity of LSBs. The conductive mesoporous C structure physically traps the S, improves its utilization, and accommodates the volume changes during the charge–discharge cycles, thereby significantly boosting capacity and cycle stability [135]. Since then, extensive research has been conducted on developing composite materials to encapsulate S, leading to notable advancements in the field [136]. Broadly, the materials used to host S in composite cathodes can be classified to three main categories.

The first category includes conductive C materials, including graphene [137], CNTs [138], C nanofibers (CNFs) [139], various porous C structures [140], and their hybrids [141,142]. These C materials form a conductive framework that facilitates rapid electron transport, enabling efficient utilization of S. Additionally, their porous architecture physically confines S and LiPSs, helping to mitigate the shuttle effect and assist significant volume changes during cycling. Moreover, the nonpolar conjugated C planes offer limited ability to strongly bind polar LiPSs, resulting in capacity declining in S/C composite cathodes. To address this issue, incorporating polar sites into the C matrix through heteroatom doping [143,144,145] introduces additional anchoring points that chemically bind LiPSs, further improving the cycle stability of LSBs [146].

The second category consists of polar inorganic compounds [147], including TM oxides (TMOs) (e.g., TiO₂ [148] and MnO₂ [149]), TM sulfides (e.g., TiS₂ [144] and MoS₂ [150]), TMNs (e.g., TiN [151] and VN [152]), and TMCs (e.g., Ti₃C₂T_x [153] and V₂CT_x [154]). These materials can chemically trap LiPSs within their frameworks, exhibiting significantly stronger binding energies (B.Es) with LiPSs compared to C materials. As a result, they more effectively prevent the shuttle effect and improve the cycling performance of LSBs. However, most inorganic compounds suffer from low conductivity. To address this, they are often combined with C materials to create S hosts, which concurrently suppress the shuttle effect, enhance conductivity, and accommodate the volume variations of LSBs [155]. This confinement can lead to the accumulation of LiPSs, slowing their redox kinetics, particularly in electrodes with great S loading or during prolonged cycling. Therefore, it is essential not only to adsorb LiPSs but also to enhance the reaction kinetics among LiPSs and Li₂S. This strategy helps minimize their accumulation, establish a dynamic balance of adsorption and conversion, and further mitigate the shuttle effect while improving S utilization [156]. Consequently, the third major category of S host materials centers on catalytic materials designed to accelerate the conversion between LiPSs and Li₂S. These include metals (e.g., Pt [157], Pt@Ni [158]), metal compounds (Fe₃O₄ [159], CoP [160], MoS₂ [161]), metal-free inorganic compounds (black phosphorous [162], C₃N₄ [163]), and their composites. To improve electrical conductivity further, these catalytic materials are often integrated with conductive C frameworks, including graphene [161] and porous C [158].

5.2.2. Rational Construction of Interlayer Between Cathode and Separator

Adding an interlayer among the cathode and separator offers an additional mechanism to mitigate the shuttle effect of LiPSs by providing physical confinement and/or chemical adsorption. Research on interlayers has primarily focused on the following categories: (i) C materials with porous architecture and high conductivity, enabling them to physically trap LiPSs while also acting as the upper current collector to enable electron transport and improve LiPSs utilization [164]; (ii) polar metal compounds [165] or heteroatom-doped C [166], which chemically bind LiPSs to prevent their shuttle; (iii) negatively charged materials for instance Nafion [167], which prevent LiPSs through electrostatic repulsion; and (iv) catalytic materials, including metal compounds [168] and metals [169], which accelerate redox kinetics by promoting the conversion of adsorbed LiPSs [170]. Interlayer materials can be applied as coatings or vacuum-filtered layers on the separator or exist as independent components. An ideal interlayer should demonstrate strong LiPSs adsorption, high catalytic activity for LiPSs conversion, and possess properties such as being thin, lightweight, and highly conductive to minimize its impact on the overall cell design.

5.2.3. Rational Construction of Li Anode

To enhance the structural stability of Li anodes in LSBs, several approaches have been exploited. These include designing suitable Li hosts [171], applying protective coatings [172], modifying electrolytes to improve the stability of the SEI [173], and transitioning from liquid to solid-state electrolytes [174]. Among these, the advanced preparation of Li hosts has proven particularly effective in suppressing Li dendrite growth and accommodating the volume variations of Li anodes. A novel Li host should possess the following characteristics: (i) light, durable, and porous to effectively accommodate Li; (ii) electrochemical stability during Li plating and stripping processes; (iii) high conductivity and a narrow Li⁺ diffusion energy barrier to facilitate fast electrochemical kinetics; and (iv) lithophilic properties to ensure efficient interaction with Li and promote uniform Li nucleation [175].

6. Possibility of MXenes for LSBs

Thanks to their outstanding characteristics, 2D MXenes have been widely investigated and utilized in LSBs, playing critical roles in the cathode, as an interlayer between the cathode and separator, and in the Li anode [176]. Their metallic conductivity promotes efficient electron transport, thereby improving S utilization [177]. The 2D structure offers abundant reaction sites for S conversion and can be engineered into 3D porous frameworks to support high S loading and accommodate volume expansion during cycling [178]. Additionally, MXenes exhibit strong B.E with LiPSs through the formation of metal-S bonds [179] and demonstrate catalytic mechanism to speed up the redox reaction kinetics between LiPSs and Li₂S [180]. Consequently, MXenes are considered promising candidates for constructing high-performance cathodes and interlayers in LSBs. In addition to their applications in cathodes and interlayers, MXenes also serve as excellent Li hosts. Their metallic conductivity, coupled with a minimal Li⁺ diffusion energy barrier, enables rapid Li⁺/electron transfer and supports fast electrochemical kinetics [181]. Additionally, the abundant surface terminations on MXenes promote homogeneous Li nucleation and growth [182], while their 2D structure offers significant opportunities to design advanced MXene/Li composite structures that effectively inhibit Li dendrite formation [183]. Although other 2D materials, such as graphene, TMOs, and layered double hydroxides (LDHs), have also been explored for LSBs, none of them combine all the advantages offered by MXenes. Graphene, for instance, exhibits excellent conductivity, structural stability, and adjustable surface functionality, making it a favorable S host. However, it lacks catalytic activity to accelerate LiPSs conversion, and its actual conductivity is often reduced due to the incomplete reduction of GO [184]. Similarly, some 2D TMOs and LDHs can adsorb LiPSs and catalyze their conversion, but their limited conductivity restricts S employment [185]. Therefore, MXenes stand out as highly promising materials for constructing high-performance LSBs, offering a unique combination of properties that address key challenges in battery design.

6.1. Conductivity

Since the pioneering discovery of MXene by Gogotsi in 2011 [22], researchers have successfully prepared more than 30 distinct stoichiometric phases of MXenes [62]. These materials are renowned for their exceptional electrical conductivity. For instance, a transparent Ti₂CT_x film, 100 nm thick and produced via the spin-coating method, exhibits a conductivity of 5.25 × 10⁵ S m⁻¹ [186]. Likewise, a Ti₃C₂T_x film with a thickness of 214 nm, fabricated through blade coating, achieves a remarkable conductivity of 1.51 × 10⁶ S m⁻¹ [187]. The conductivity of MXenes is primarily governed by interfacial interactions between MXene NSs and can be precisely controlled by altering their composition, structural arrangement, surface terminations, stacking sequence, and intercalants [67]. For example, by controlling the terminations of Nb₂C MXene to exclusively include -Cl groups, a remarkable superconductive transition is observed in Nb₂CCl₂ MXene at 6.3 K [67]. This tunability highlights the versatility and potential of MXenes in applications requiring tailored electrical properties. In LSBs, the metallic conductivity of MXenes enables rapid electron transfer, which supports efficient S utilization and contributes to high capacity and excellent rate performance [177]. Nazar’s group was the first to utilize Ti₂CT_x MXene NSs as a highly effective S host for LSBs [188]. The resulting S/Ti₂CT_x cathode, containing 70 wt.% S, achieved an impressive capacity of nearly 1200 mAh g⁻¹ at 0.2 C and 1000 mAh g⁻¹ at a 1 C rate. This demonstrated MXene’s ability to enhance electrical conductivity and electrochemical performance. Furthermore, a Ti₃C₂T_x interlayer, with a thickness of 522 nm, was vacuum filtrated onto a PP separator. Acting as an upper current collector, this interlayer facilitated electron transfer and enhanced LiPSs utilization. This configuration significantly boosted the capacity of the LSBs, highlighting the advantages of MXenes as interlayer materials for enhancing battery performance [189].

6.2. Structural Variety

The 2D structure of MXenes offers extensive surface contact with S. Moreover, Van der Waals forces and hydrogen bonding cause MXene NSs to stack together, which hinders Li-ion diffusion and limits the maximum utilization [190]. To overcome this, structural modifications can be implemented, such as transforming 2D NSs to 0D nanodots (NDs) or 1D nanoribbons (NRs) or assembling them into 3D porous networks. For instance, 0D Ti₃C₂T_x NDs were generated in situ and distributed within Ti₃C₂T_x NSs alongside NRs, which acted as reaction initiators to exfoliate the NSs into smaller NDs. This strategy effectively mitigated the restacking of NSs and self-agglomeration of NDs [191]. Similarly, delaminating 2D Ti₃C₂ MXenes through a shaking process in KOH solution produced MXene NRs, which facilitated high S loading and rapid Li⁺ diffusion [192]. Additionally, the abundant surface terminations of 2D MXenes allow their assembly into 3D networks, providing numerous active sites for S use and ample space for great S loading. For instance, a 3D flower type porous MXene structure was synthesized through hydrothermal treatment of Ti₃C₂T_x MXene in an ethylenediamine solution [193]. This structure exhibited an SSA of 132.8 m² g⁻¹ and a notable pore volume of 0.262 cm³ g⁻¹. When S was loaded at 61.5 wt.%, the resulting MXene/S cathode achieved an aerial capacity of 10.04 mAh cm⁻² with an S loading of 10.5 mg cm⁻². These versatile MXene architectures present significant opportunities for developing high-performance LSBs with enhanced S loading.

6.3. Adsorption of Soluble Polysulfides Through Chemical Interactions

MXene NSs produced through etching methods are characterized by abundant surface terminations, which play a crucial role in shaping their magnetic [194], electronic [195], and optoelectronic characteristics [196]. These terminations are influenced by the synthesis mechanism [197] or can be customized by post-treatment techniques [67]. Consequently, the characteristics of MXenes may be precisely tuned by modulating their surface terminations. In the context of LSBs, these terminations establish stable chemical interactions with LiPSs, effectively diminishing the shuttle effect and enhancing cycling stability. This functionality makes MXenes highly advantageous as S hosts or as interlayers placed between the cathode and separator [180]. Nazar’s team used XPS analysis to uncover the robust interaction between Ti-based MXenes (for instance Ti₂CT_x, Ti₃C₂T_x, and Ti₃CNT_x) and LiPSs [188]. Their findings suggest that the -OH terminations on MXenes are initially utilized by LiPSs, resulting in the development of thiosulfate and polythionate species. Once the -OH groups are depleted, some Ti atoms with unoccupied orbitals are exposed, allowing these Ti atoms to accept electrons from supplementary LiPSs and form Ti-S bonds through Lewis’s acid–base relations. This dual mechanism, comprising thiosulfate/polythionate transformation and Lewis’s acid–base interactions, enables MXenes to effectively trap LiPSs (Figure 9a). Additionally, DFT measurements validated that Ti-S bond formation is the primary interaction between MXenes and LiPSs [198]. However, the B.E between MXenes and LiPSs must be carefully optimized. Excessively high B.E may cause the breakdown of LiPSs, while too low B.E may fail to securely adsorb them [199]. Achieving an optimal B.E is essential for balancing the binding strength and ensuring the stability of LiPSs. DFT calculations indicate that bare Ti₂C free of surface terminations forms excessively strong Ti-S bonds with LiPSs. This interaction disrupts the S-S and Li-S bonds in LiPSs, thereby impeding the reversible reaction among LiPSs and Li₂S [198]. As a result, bare Ti₂C MXene is not an ideal S host. To address this issue, introducing surface terminations on MXenes is an effective strategy. These terminations facilitate additional reactions with LiPSs and reduce the strength of Ti-S interactions. The B.Es with LiPSs for terminated MXenes follow the order: Ti₂CF₂ < Ti₂CO₂ < Ti₂C(OH)₂ (Figure 9b). On Ti₂CO₂ and Ti₂CF₂ surfaces, LiPSs are physically adsorbed, while they bond more strongly to Ti₂C(OH)₂ surfaces. In this case, the H atoms in Ti₂C(OH)₂ lower the repulsive forces between negatively charged O and S atoms [198]. Other surface terminations, like -S, -N, and -Cl [200], as well as vacancy defects [201], can also be utilized to adjust MXenes. For instance, S-terminated Ti₂C MXene demonstrates a high B.E with LiPSs and a relatively less Li⁺ diffusion energy barrier, making it an efficient S host with excellent capability to suppress the shuttle effect [200].

In addition to Ti-based MXenes, the interactions of other MXenes with LiPSs have been investigated theoretically [201]. DFT measurements studied six O-terminated M₃C₂O₂ (M = Cr, V, Ti, Nb, Hf, and Zr) MXenes, all of which demonstrated the ability to trap soluble LiPSs (Figure 9c) [202]. Among these, Cr₃C₂O₂ presented the greatest anchoring effect. Furthermore, the study revealed that the anchoring effect for soluble LiPSs increases as the lattice constant of MXenes decreases, offering valuable insights for selecting and designing MXene-based S hosts. The composition, surface terminations, and vacancy defects of MXenes significantly influence their adsorption capabilities with LiPSs. Unlike theoretical MXene models that often assume a single type of termination, actual MXene NSs typically feature a mixture of terminations, leading to more complex interaction mechanisms with LiPSs. Consequently, the theoretical results utilized to predict LiPS adsorption need further refinement to enhance their accuracy. Guided by these theoretical insights, exploring MXenes with novel compositions and diverse terminations remains an essential area of research.

6.4. Catalytic Role in Accelerating the Conversion of Soluble Polysulfides

MXenes exhibit exceptional properties, including metallic conductivity, a high SSA, a tunable bandgap structure, and remarkable carrier anisotropic mobility, making them highly promising electrocatalysts with excellent activity, selectivity, and prolonged loading lifetimes [204]. As a result, MXenes and MXene-based nanocomposites have been successfully utilized in various electrocatalytic applications, such as the HER, OER, N₂ RR, O₂ RR, and CO₂ RR [205]. In the context of LSBs, MXenes have demonstrated their effectiveness as catalysts in facilitating the transformation of LiPSs to Li₂S. By lowering the redox reaction barrier and accelerating redox kinetics, MXenes significantly enhances the rate performance of LSBs [154].

The catalytic performance of MXenes was strongly influenced by surface terminations and vacancies. Using the climbing-image nudged elastic band technique, the decomposition barriers of Li₂S on functionalized Ti₃C₂T₂ MXenes were calculated in the following order: Ti₃C₂S₂ < Ti₃C₂O₂ < Ti₃C₂F₂ < Ti₃C₂N₂ < Ti₃C₂Cl₂. Notably, all these values are significantly lower than the natural decomposition barrier of Li₂S [180]. Among them, Ti₃C₂S₂ and Ti₃C₂O₂ exhibit the lowest decomposition barriers for Li₂S₆, highlighting their efficiency in accelerating the transformation between LiPSs and Li₂S. These two MXenes also have the shortest Li⁺ diffusion energy barriers, making them excellent candidates for S host materials. Furthermore, introducing suitable vacancies on the MXene can further boost their catalytic activity. For instance, Ti₃C₂Cl₂, with a 1/16 Cl deficiency, exhibits significantly reduced decomposition barriers for Li₂S and Li₂S₆, at 0.316 and 0.537 eV, respectively, compared to 1.625 and 1.532 eV for the non-deficient surface. This demonstrates a substantial improvement in catalytic performance [180].

Beyond modifying surface terminations, heteroatom doping, such as with N or S, is a powerful strategy to enhance the catalytic activity of MXenes in LSBs. For example, a porous N-doped Ti₃C₂ with active electrocatalytic properties was synthesized via a melamine-formaldehyde template method [206]. This porous structure, characterized by a high SSA and exceptional conductivity, offered numerous active sites for LiPSs adsorption and facilitated their transformation. The incorporation of N atoms further improved electron transfer and lowered the decomposition barrier of Li₂S, thereby accelerating LiPSs conversion and amplifying the catalytic mechanism. Additionally, a single Zn atom implantation into MXenes was explored as an S host, significantly reducing the energy barriers for the conversion of Li₂S₄ to Li₂S₂ and Li₂S. This improvement is attributed to the great electronegativity of Zn atoms (Figure 9d), enabling the LSBs to achieve outstanding rate performance, delivering a capacity of 640 mAh g⁻¹ at 6 C [203].

6.5. Prevention of Li Dendrite Growth

MXene NSs feature a distinctive 2D architecture, metallic conductivity, rapid Li⁺ diffusion, and abundant surface terminations, making them highly effective as Li hosts for suppressing dendrite formation and accommodating volume changes of Li during cycling [207]. First, the structural versatility and exceptional mechanical properties of MXenes offer significant opportunities for designing MXene/Li composite architectures [208]. Second, their metallic conductivity combined with a low Li⁺ diffusion energy barrier ensures efficient electron and Li⁺ passage, enabling greater electrochemical kinetics [208]. Third, the surface terminations of MXenes exhibit a strong affinity for Li, offering numerous nucleation sites that promote homogeneous Li deposition and inhibit dendrite formation [182]. For instance, a lamellar Ti₃C₂T_x-Li metal film was developed using a roll-to-roll process. In this structure, Li was evenly distributed between the MXene sheets, and the conductive Ti₃C₂T_x framework directed controlled Li dendrite growth within nanoscale gaps. This approach effectively suppressed vertical dendrite development, mitigating the risk of separator piercing [209].

MXenes hold significant potential for enhancing the performance of LSBs by serving as high-performance cathode materials and interlayers between the cathode and separator. Their metallic conductivity, diverse structures, robust chemical adsorption of LiPSs, and catalytic activity for the transformation of LiPSs to Li₂S make them ideal candidates for this application. Incorporating MXenes as S hosts or interlayer materials in LSBs can boost S consumption and capacity, mitigate the shuttle effect to improve cycle performance, and accelerate redox reaction kinetics, thereby enhancing rate performance. Additionally, MXenes exhibit a strong affinity for Li, promoting uniform Li nucleation and suppressing dendrite growth, which lowers the risk of short circuits. Consequently, MXenes are widely regarded as promising materials for S hosts in cathodes, effective interlayer components, and reliable matrices in Li anodes for LSBs.

7. Theoretical Investigations on MXene-Based LSBs

Recently, extensive research has focused on designing S hosts with robust chemical interactions with LiPSs and investigating the fundamental mechanisms underlying these materials [210]. With their interactive surfaces and excellent conductivity, MXenes have emerged as highly favorable anchoring materials for S cathodes [211]. Gaining a comprehensive understanding of their fundamental properties and potential applications is crucial for advancing both science and technology in this field. This section provides a concise summary of key theoretical insights into LSBs using MXenes with uniform or non-uniform functionalization (Figure 10).

7.1. MXenes with Uniform Functionalization

7.1.1. MXenes with OH-Functionalization

In 2015, Nazar’s group pioneered the use of the Ti₂CT_x MXene phase as an S host material [188]. They investigated the surface reactivity of Ti₃C₂ and Ti₃CN MXenes upon interaction with LiPSs through XPS and DFT analyses. Their study revealed that before the formation of Ti-S bonds through Lewis’s acid–base interactions, the terminal -OH groups on Ti₃C₂ and Ti₃CN are steadily utilized by trapped LiPSs, leading to the generation of thiosulfate and polythionate arbitrates. Subsequent research by Rao and collaborators highlighted the strong interactions between S in polysulfide chains and hydrogen in -OH groups, which facilitate the binding of LiPSs to OH-terminated MXenes. This interaction improves the retention of S species within the cathode area of LSBs [198]. Moreover, MXenes are typically prepared from the MAX phases by etching Al or Si using aqueous HF or HCl/LiF solutions, a process that introduces -F, -OH, and -O groups onto the MXene surface [213]. Identifying which specific functional groups dominate or whether they act synergistically remains a challenge. Understanding the anchoring mechanisms of these surface terminations toward LiPSs is therefore critical for advancing experimental research on MXenes.

7.1.2. MXenes with O/F-Functionalization

The surface terminations of MXenes are heavily influenced by etching conditions and post-synthetic treatments [213]. While there are discrepancies in quantifying termination group concentrations using characterizations like XPS and NMR, it is generally accepted that -F and -O are the dominant functional groups on MXene surfaces. Recent studies have explored the B.E, bond characteristics, and electron transfer associated with -O and -F uniformly functionalized MXenes [198,201]. Zhao and colleagues provided a detailed analysis of the anchoring mechanisms of O-terminated Ti₃C₂ and Ti₂C MXenes on LiPSs (Figure 11a) [201]. Their findings revealed that interactions between Li⁺ in LiPSs and O-atoms in Ti₂CO₂ and Ti₃C₂O₂ monolayers enable O-terminated MXenes to bind LiPSs with control binding strength, stabilizing soluble LiPSs. This behavior demonstrates the amphiphilic nature of MXenes, with lithiophilic properties stemming from Li-O bonding and sulfiphilic characteristics from Ti-S bonding. Fan and collaborators extended this investigation by examining the chemical interactions between LiPSs and six M₃C₂O₂-type MXenes, where M represents Cr, V, Ti, Nb, Hf, and Zr (Figure 11b). Their findings confirmed significant Li-O interactions between these MXenes and LiPSs, akin to the behavior observed with Ti₂CO₂. Additionally, they identified a correlation between B.Es and lattice constants of M₃C₂O₂ MXenes, where smaller lattice constants resulted in stronger binding with LiPSs [202]. In another work, Wei’s team explored the synergistic effects of LiPSs binding and the catalytic delithiation of Li₂S using Ti₃C₂ MXenes (Figure 11c) [180]. Their results demonstrated that O- and S-terminated Ti₃C₂ MXenes are highly effective as hosts for S cathodes, corroborating Zhao’s earlier findings [201].

Lin and colleagues investigated the properties of O/F-functionalized Ti₂N as substrates, demonstrating that Ti₂NO₂ and Ti₂NF₂ exhibit average adsorption energies for LiPSs (Figure 11d) [214]. They found that these MXenes strike an optimal balance between strong adsorption and preserving the structural integrity of LiPSs. Additionally, DOS analysis for S₈, Li₂S₈, Li₂S₆, Li₂S₄, Li₂S₂, and Li₂S adsorbed on Ti₂NO₂ validated that the metallic nature of Ti₂NO₂ and Ti₂NF₂ remains intact after adsorption. This retention of metallic properties allows trapped LiPSs species to be readily reduced, with their free electrons facilitating the redox reactions of S species. Similar behavior was observed for F/O-functionalized Ti₂C (Ti₂CF₂ and Ti₂CO₂) [215]. As a result, Ti₂NO₂, Ti₂NF₂, Ti₂CF₂, and Ti₂CO₂ are promising candidates for LSBs with improved electrochemical characteristics. Moreover, Rao and collaborators noted that an attraction between Ti and S atoms weakens due to the improved repulsive forces exerted by O/F-termination groups on S atoms [198]. This explains the lower B.Es of LiPSs on functionalized MXenes compared to bare MXenes, with B.E following the order: Ti₂CF₂ < Ti₂CO₂ < Ti₂C(OH)₂. Interestingly, introducing H atoms partially mitigates this repulsive force, as the positively charged H atoms enhance attraction to S atoms. This phenomenon is pictorially illustrated in Figure 11e.

Theoretical calculations reveal that after interacting with long-chain LiPSs, the -O groups on MXenes increase while the -OH groups decrease. This occurs because the protruding H atoms on the MXene surface are more readily replaced by Li, as the dissociation energy required for H removal is lower than that for -OH groups. As a result, the conductivity of MXenes remains largely unaffected by LiPSs adsorption, with their band gaps exhibiting minimal adjustment. Additionally, surface-sulfurized Ti₂C (Ti₂CS₂) have been investigated (Figure 11f) [200]. Compared to Ti₂CO₂ and Ti₂CF₂, Ti₂CS₂ demonstrates the robust affinity for LiPSs, with B.Es following the trend: Ti₂CF₂ < Ti₂CO₂ < Ti₂CS₂. This enhanced binding is attributed to Li’s tendency to interact with negatively charged atoms, for instance, O and F, while being repelled by positively charged Ti atoms. S-terminated MXenes strike a balance between attractive (F-Li) and repulsive (F-Ti) forces because of S’s lower electronegativity (S: 2.58; O: 3.44; F: 3.98), consistent with findings reported by Rao and colleagues [198]. Although S-terminated MXenes have not yet been experimentally synthesized, their robust affinity for polysulfides suggests significant potential for mitigating the LiPSs shuttle effect. Additionally, their metallic properties and low-energy barriers for Li diffusion facilitate the electrochemical reactions of S moieties. These features make S-terminated MXenes promising candidates for designing S host materials for high-performance LSBs.

7.2. MXenes with Non-Uniform Functionalization

Theoretical models of MXenes used in DFT measurements typically assume idealized structures with surfaces uniformly terminated by a single type of functional group. However, in reality, MXene surfaces are far from uniform and cannot be exclusively terminated with one specific functional group. Experimental findings consistently demonstrate that MXenes possess complex and diverse surface terminations [213]. Therefore, it is essential to investigate the interactions between non-uniformly functionalized MXene surfaces and LiPSs to enhance their performance as effective anchoring materials for LSBs. Chung and colleagues explored the anchoring behavior of LiPSs on F/O-functionalized Ti₂C employing spin-polarized DFT models [216]. Their study examined the effects of substitutional, vacancy, and S-trapped sites on the F/O-functionalized Ti₂C surfaces (Figure 12a–i). On the Ti₂CO₂ surface, F-substitutional sites weaken the interactions between Li and O atoms, thereby preventing the neutralization of S atoms. This suppression effect is influenced by the presence of -F functional groups, even during LiPSs adsorption. In contrast, O-substitutional sites showed robust interactions with Li sites than F sites, making them energetically more favorable for anchoring LiPSs. Additionally, the study revealed that surface vacancy sites, regions partially devoid of -F/-O groups, can stabilize S atoms but pose a risk of losing active material. This is due to the robust interactions between exposed Ti and S atoms, which can break the S-S covalent bonds in S species. Therefore, MXene preparation strategies should prioritize reducing functional group vacancies rather than aiming for uniform functionalization with a single type of group.

The widespread adoption of LSBs is still limited by the challenge of soluble LiPSs dissolving into the electrolyte. Although theoretical predictions about the anchoring mechanisms of LiPSs on MXenes may be difficult to replicate experimentally due to differences between theoretical models and real-world results, these investigations have offered valuable insights. They contribute to advancing MXene design and provide critical guidance for optimizing MXenes as anchoring materials to achieve high-performance LSBs.

8. Experimental Advances of MXenes-Based LSBs

LSBs have gathered significant interest for their exceptional energy storage capabilities. Moreover, several inherent drawbacks have hindered their commercial viability, with low conductivity, the shuttle effect, slow reaction kinetics at the S cathode, dendrite formation, and volume fluctuations in the Li anode [217]. Thanks to their distinctive characteristics, for instance, excellent metallic conductivity, efficient polysulfide trapping, rich lithiophilic surface groups, and a customizable structure, MXenes have been widely applied in various parts of LSBs, showing great promise for enhancing their electrochemical performance [218]. This section offers a comprehensive review of recent developments in the utilization of MXenes to the cathode, anode, and separator in LSBs.

8.1. MXene-Based S Cathode

First, the S cathode and its discharge moieties have extremely low conductivity (S: 5 × 10⁻³⁰; Li₂S: 10⁻¹³ S cm⁻¹), leading to significant overpotentials and reduced energy efficiency during cycling. During discharge, S forms an insulating, insoluble layer on the cathode surface, resulting in poor consumption of the active S material and sluggish kinetics. Second, the density variation between S and Li₂S (1.67 vs. 2.03 g cm⁻³) causes substantial volume change when S is converted to Li₂S during discharge, leading to detachment of S from the cathode structure and significant capacity loss. Lastly, the soluble LiPSs generated during discharge diffuse into the electrolyte. Driven by the concentration gradient, long-chain LiPSs migrate to Li anode, where they are diminished to short-chain LiPSs. During charging, these short-chain LiPSs move back to the cathode and are re-oxidized into higher-order LiPSs, creating a shuttle effect. This phenomenon lowers the CE and ultimately causes battery failure [219]. To address these challenges, MXene-based S hosts have been developed. To optimize the performance of LSBs, MXene-based S hosts should possess the following key characteristics: (i) ample surface terminations combined with great conductivity to efficiently capture polysulfides and facilitate fast electron transport; (ii) a well-controlled structure with strong mechanical characteristics and a high SSA to deliver numerous reaction sites for S conversion and accommodate volume fluctuation; and (iii) abundant active sites that reduce redox energy barriers, improving the conversion kinetics of polysulfides.

8.1.1. Direct Use of MXenes as S Host

Electrochemical characteristics of LSBs can be greatly improved by directly incorporating S into MXene NSs. For instance, an accordion-like Ti₃C₂, fabricated via HF etching, was infused with S using a melt-diffusion approach, resulting in a S/Ti₃C₂ composite containing 57.6 wt.% S. This composite delivered an initial capacity of 1291 mAh g⁻¹ at 0.2 A g⁻¹ and retained 970 mAh g⁻¹ after 100 cycles [220]. Similarly, a S@MXene composite was prepared through a hydrothermal treatment, where S nanoparticles (NPs) were uniformly distributed within the interlayer space and deposited on the accordion-type Ti₃C₂T_x surface (Figure 13a,b). Featuring a great areal S loading of 4 mg cm⁻² and 67.1 wt.% S content, the S@MXene composite demonstrated an initial capacity of 1477.2 mAh g⁻¹ at 0.2 C, retained 82% of its capacity after 100 cycles (Figure 13c), and achieved a capacity of 860.2 mAh g⁻¹ at a rate of 2 C [221].

Using DMSO intercalation treatment, accordion-like Ti₂CT_x MXene was delaminated into thinner, layered NSs with increased SSA (Figure 13d). These NSs were then combined with S through a melt-diffusion process to form a uniform Ti₂CT_x/S composite [188]. When coated onto a C paper current collector, the resulting Ti₂CT_x/S electrode, containing 70 wt.% S, demonstrated a capacity of 1090 mAh g⁻¹ at 0.5 C and exhibited exceptional cycle performance over 650 cycles, with a low-capacity decay of just 0.05% per cycle (Figure 13e). Further, the behavior of S/MXene composites can be enhanced by adding a protective coating. For example, a S/delaminated Ti₃C₂T_x composite was encapsulated with a PDA layer, which efficiently mitigated the LiPSs shuttle effect and accommodated S’s volume growth [222]. The S/Ti₃C₂T_x@PDA electrode, with a S content of 60 wt.%, achieved a capacity of 1044 mAh g⁻¹ after 150 cycles at 0.2 C, a high-rate capacity of 624 mAh g⁻¹ at 6 C, and retained 556 mAh g⁻¹ after 330 cycles at 0.5 C, even with an S loading of 4.4 mg cm⁻². Single-layered MXene NSs synthesized using a one-step LiF/HCl etching approach have proven to be excellent conductive hosts for S. Using this method, Ti₃C₂T_x NSs (Figure 13f) with a wide range of flake sizes were produced and combined with S via a melt-diffusion technique to form a Ti₃C₂T_x/S composite (Figure 13g). By incorporating a single-walled CNTs (SWCNTs) interlayer between the cathode and separator, the Ti₃C₂T_x/S cathode with 50 wt.% S achieved a capacity of 1458 mAh g⁻¹ at 0.1 A g⁻¹, retained 608 mAh g⁻¹ at 8.2 A g⁻¹, and demonstrated remarkable stability with a capacity decay of just 0.04% per cycle at 0.8 A g⁻¹ over 1500 cycles. Even with an increased S content of 80 wt.% and an S loading of 3.5 mg cm⁻², the cathode exhibited an impressive initial capacity of 675.2 mAh g⁻¹ at 1.5 A g⁻¹ and maintained 92.6% retention in capacity after 600 cycles (Figure 13h) [223].

Thanks to its 2D morphology and exceptional mechanical characteristics, MXenes are well-suited as flexible substrates for S electrodes. The negatively charged surfaces of single-layered MXene NSs, resulting from abundant functional groups, allow them to disperse uniformly in aqueous solutions, forming stable colloidal suspensions. By introducing Na₂S_x and HCOOH into a Ti₃C₂T_x colloidal suspension, S NPs are synthesized in situ, producing a homogeneous Ti₃C₂T_x/S ink (Figure 13i,j) [224]. A vacuum filtration applied to a separator transforms this ink into a freestanding S@Ti₃C₂T_x film (Figure 13j,k). This film, containing 70 wt.% S, achieved capacities of 1184 and 1044 mAh g⁻¹ at 0.2 and 2 C, respectively, and retained 724 mAh g⁻¹ after 800 cycles at 0.2 C (Figure 13l). Furthermore, a pouch cell was constructed, employing the S@Ti₃C₂T_x as the cathode and a Li ribbon as the anode, housed within a commercial plastic casing. This setup delivered an initial capacity of 1263 mAh g⁻¹ at 0.5 C and retained 1119 mAh g⁻¹ after five cycles under bending conditions, outperforming its performance under flat conditions (initial capacity of 1124 mAh g⁻¹ at 0.5 C with 903 mAh g⁻¹ retention after 5 cycles) [224]. Additionally, a flexible Ti₃C₂T_x/S film was fabricated using a straightforward vapor deposition technique to insert S onto a robust freestanding Ti₃C₂T_x paper. Containing 30 wt.% S, this Ti₃C₂T_x/S paper delivered capacities of 1383 and 1075 mAh g⁻¹ at 0.1 and 2 C and exhibited an exceptionally low capacity decay of just 0.014% per cycle over 1500 cycles at 1 C. A prototype full cell, utilizing the Ti₃C₂T_x/S paper as the cathode and a pre-lithiated germanium anode, achieved an initial capacity of 483 mAh g⁻¹ at 0.34 A g⁻¹ following precycling, maintained 351 mAh g⁻¹ at 1.68 A g⁻¹, and retained 226 mAh g⁻¹ after 186 cycles at 1.68 A g⁻¹ [225]. These findings underscore the remarkable improvements in energy storage performance when MXene NSs are directly utilized as S hosts for LSBs. However, a persistent challenge remains: the restacking of MXene NSs, which hinders Li-ion diffusion and prevents MXenes from reaching their full potential [190]. To overcome this problem, researchers have explored various strategies, including chemical modification, structural control of MXenes, and the development of MXene-based composites.

Figure 13. (a) Pictorial illustration, (b) SEM micrograph, and (c) cycle activity of S@accordion-type Ti₃C₂T_x composite. Adapted from [221], Copyright 2020, De Gruyter. (d) SEM micrograph and (e) cycling stability of the delaminated Ti₂C/S composite. Adapted from [188], Copyright 2015, Wiley-VCH. (f) Schematic, (g) SEM micrograph, and (h) cycling stability of the S/Ti₃C₂T_x electrode. Adapted from [223], Copyright 2018, Royal Society of Chemistry. (i) Diagram illustrating the preparation of S@Ti₃C₂T_x ink. (j) Optical image, (k) SEM micrograph, and (l) cycling stability of S@Ti₃C₂T_x film electrode. Adapted from [224], Copyright 2018, Wiley-VCH.

Figure 13. (a) Pictorial illustration, (b) SEM micrograph, and (c) cycle activity of S@accordion-type Ti₃C₂T_x composite. Adapted from [221], Copyright 2020, De Gruyter. (d) SEM micrograph and (e) cycling stability of the delaminated Ti₂C/S composite. Adapted from [188], Copyright 2015, Wiley-VCH. (f) Schematic, (g) SEM micrograph, and (h) cycling stability of the S/Ti₃C₂T_x electrode. Adapted from [223], Copyright 2018, Royal Society of Chemistry. (i) Diagram illustrating the preparation of S@Ti₃C₂T_x ink. (j) Optical image, (k) SEM micrograph, and (l) cycling stability of S@Ti₃C₂T_x film electrode. Adapted from [224], Copyright 2018, Wiley-VCH.

8.1.2. Chemical Adjustment of MXenes as S Host

Chemical adjustment is a viable approach to enhance the performance of MXenes. Studies have shown that doping heteroatoms like N, S, or metal atoms into conductive hosts can greatly improve their catalytic properties [226]. In LSBs, heteroatom-doped MXenes show improved chemical adsorption of LiPSs and improved catalytic activity [203]. For example, single Zn atom-inserted Ti₃C₂T_x layers were prepared by etching Al layers from the Ti₃AlC₂ MAX phase using molten ZnCl₂, resulting in Zn atoms being embedded at Ti sites [203]. The Zn inserted MXene revealed robust interactions with LiPSs due to the great electronegativity of Zn, facilitating rapid conversion between LiPSs and Li₂S₂/Li₂S by accelerating the nucleation of Li₂S₂/Li₂S. After S incorporation, the Zn inserted MXene NSs were covered around S balls (Figure 14a,b). The resulting S@Zn-inserted MXene electrode, with an S content of 89 wt.%, achieved a capacity of 1136 mAh g⁻¹ at 0.2 C and 517 mAh g⁻¹ at 6 C, retaining 706 mAh g⁻¹ after 400 cycles at 1 C (Figure 14c), surpassing the performance of S@MXene. Additionally, further adjustment with 1,3-diisopropenylbenzene improved structural stability, increasing the electrode’s capacity to 1210 mAh g⁻¹ at 0.2 C and 640 mAh g⁻¹ at 6 C.

Doping MXenes with heteroatoms like N and S has been shown to significantly improve their performance in LSBs [227]. For example, a 3D crumpled N-doped Ti₃C₂T_x network was prepared through heat treatment of negatively charged Ti₃C₂T_x flakes and positively charged melamine (Figure 14d) [228]. N doping improved the electrical conductivity of MXene, strengthened its interaction with LiPSs, and enhanced its catalytic behavior. As a result, the crumpled N-doped MXene/S electrode, containing 73.85 wt.% S and an S loading of 1.5 mg cm⁻², delivered a capacity of 1144 mAh g⁻¹ at 0.2 C and demonstrated excellent cycling performance, retaining 610 mAh g⁻¹ at 2 C after 1000 cycles (Figure 14e). Even with a greater S loading of 5.1 mg cm⁻², the electrode achieved an initial capacity of 765 mAh g⁻¹ at 0.2 C and retained 588 mAh g⁻¹ after 500 cycles (Figure 14f). Additionally, a porous N-doped Ti₃C₂ with a SSA of 171.5 m² g⁻¹ was synthesized using a melamine-formaldehyde template method [206]. The porous architecture, combined with N doping, enhanced the MXene’s ability to adsorb LiPSs and its catalytic activity. The N-doped MXene/S electrode, containing 64 wt.% S, revealed a capacity of 1072 mAh g⁻¹ at 0.5 C, excellent rate capability with 729 mAh g⁻¹ at 3 C, and outstanding cycling performance, showing only 0.033% capacity decay per cycle over 1200 cycles at 2 C. Even at a high S loading of 8.2 mg cm⁻², the electrode achieved a remarkable areal capacity of 9 mAh cm⁻². When incorporated into a pouch cell with an S loading of 1.5 mg cm⁻², it retained 65.3 and 79.1% of its capacity at 0.1 C and 0.2 C after 100 cycles, respectively.

Heteroatom doping has been proven to significantly increase the adsorption capability and catalytic behavior of MXenes toward LiPSs. Consequently, further research is warranted to explore the introduction of various dopant atoms, for instance, S, P, Ni, and Co. Additionally, in LSBs, surface engineering of MXenes remains largely confined to theoretical computational studies, with limited experimental validation. Given the significant impact of MXene terminations on their adsorption B.E and the breakdown barrier of LiPSs, it is essential to study MXenes with well-defined terminations to achieve optimal performance in LSBs.

Figure 14. (a) Diagram illustration the preparation, (b) SEM micrograph, and (c) cycling stability of S electrode coated with single atom Zn inserted MXene (S@SA-Zn-MXene). Adapted from [203], Copyright 2020, Wiley-VCH. (d) Illustration of synthesis. Cycling stability of crumpled N-doped Ti₃C₂T_x/S electrode at (e) 2 C with an S loading of 1.5 mg cm⁻² and (f) 0.2 C with an S loading of 5.1 mg cm⁻². Adapted from [228], Copyright 2018, Wiley-VCH.

Figure 14. (a) Diagram illustration the preparation, (b) SEM micrograph, and (c) cycling stability of S electrode coated with single atom Zn inserted MXene (S@SA-Zn-MXene). Adapted from [203], Copyright 2020, Wiley-VCH. (d) Illustration of synthesis. Cycling stability of crumpled N-doped Ti₃C₂T_x/S electrode at (e) 2 C with an S loading of 1.5 mg cm⁻² and (f) 0.2 C with an S loading of 5.1 mg cm⁻². Adapted from [228], Copyright 2018, Wiley-VCH.

8.1.3. Structure Engineering of MXenes for S Host

Structural engineering, focused on constructing open configurations for MXenes, is essential for minimizing restacking and maximizing exposed SSA, thereby enhancing their utilization [229]. Different approaches have been devised to achieve this, including expanding the interlayer spacing [230] and creating open structures such as 0D [191], 1D [192], and 3D frameworks [193]. A widely adopted approach to expanding the interlayer spacing of MXenes involves introducing molecular or ionic spacers between the 2D MXene NSs. For instance, Li⁺ intercalation enhanced the interlayer distance of V₂C MXene from 0.736 to 0.939 nm (Figure 15a), facilitating Li⁺ movement within the S electrode [230]. By employing a rGO/CNT composite as a flexible scaffold, a freestanding S@Li⁺-intercalated V₂C/rGO-CNT electrode with 70 wt% S content was constructed. This electrode demonstrated excellent stability, with minimal capacity decay of 0.053 and 0.051% per cycle over 500 cycles at 1 C and 2 C, respectively (Figure 15b). Another strategy to mitigate restacking and create open pathways for rapid Li⁺ movement involves transforming 2D MXene NSs into 0D NDs. Using sodium alginate as a reaction initiator, 2D Ti₃C₂T_x NSs were hydrothermally treated to produce 0D Ti₃C₂T_x NDs that were interspersed in situ among the Ti₃C₂T_x NSs [191]. Sodium alginate facilitated the exfoliation process by interacting strongly with Ti sites on Ti₃C₂T_x, triggering C-Ti bond cleavage and layer separation. This yielded a composite of Ti₃C₂T_x NDs and NSs that effectively prevented restacking of NSs and aggregation of NDs, enabling spatial immobilization of LiPSs and catalytic transformation of high-loaded S moieties. The resulting cathode, containing 67.6 wt% S and 1.8 mg cm⁻² S loading, achieved an impressive capacity of 1609 mAh g⁻¹ at 0.05 C and 882 mAh g⁻¹ at 3 C. Furthermore, with a high S loading of 13.8 mg cm⁻², it delivered exceptional areal and volumetric capacities of 13.7 mAh cm⁻² and 1957 mAh cm⁻³, respectively, at 0.05 C.

Transforming 2D NSs into 1D NRs offers another efficient approach to preventing MXene NSs from restacking. Wang and colleagues developed Ti₃C₂ MXene NRs by subjecting multilayer Ti₃C₂ MXene to a shaking treatment in a KOH solution. This process created numerous open macropores, which facilitated high S loading and rapid Li⁺ diffusion (Figure 15c) [192]. The delaminated Ti₃C₂ NSs were vacuum-filtrated onto a PP separator, serving as the interlayer to physically and chemically suppress the shuttle effect. Consequently, a slurry of Ti₃C₂ NRs/S was deposited onto the modified separator, forming an all-MXene-based integrated electrode (Figure 15d). With an S content of 68 wt%, this electrode demonstrated impressive performance, achieving capacities of 1062 mAh g⁻¹ at 0.2 C and 288 mAh g⁻¹ at 10 C (Figure 15e).

Developing 3D MXene networks with organized porous structures from 2D MXene NSs is a highly effective approach to enhance Li-ion movement [193]. These frameworks provide physical confinement and chemical adsorption of LiPSs, suppress the shuttle effect, support high S loading, and accommodate the volumetric variation of S, thereby significantly improving the behavior of MXene-based LSBs. For instance, a 3D flower type porous Ti₃C₂T_x network was synthesized using ethane diamine as an aid during a hydrothermal treatment process, which served as an S host (Figure 15f) [193]. Within this network, the conductive Ti₃C₂T_x NSs were arranged at various angles, facilitating rapid electron and Li⁺ transport while supporting a great tap density after S loading (Figure 15g,h). This structure achieved an outstanding areal capacity of 10.04 mAh cm⁻², even with an S loading of 10.5 mg cm⁻² (Figure 15i). Additionally, a 3D Ti₃C₂T_x foam was fabricated using a hydrazine-induced foaming technique, resulting in a well-arranged porous architecture with a SSA of 182.9 m² g⁻¹ and a pore volume of 0.461 cm³ g⁻¹ [153]. As an S host, the freestanding Ti₃C₂T_x foam/S electrode, containing 71.1 wt% S, recorded an initial capacity of 1226.4 mAh g⁻¹ at 0.2 C, a capacity of 711.0 mAh g⁻¹ at 5 C, and an exceptionally low-capacity decay of approximately 0.025% per cycle over 1000 cycles at 1 C.

Strategic structural optimization of MXenes is essential for maximizing the exposed SSA of MXene NSs, enhancing Li-ion diffusion, and preserving their metallic conductivity. Advanced MXene architecture provides numerous active sites for the chemical adsorption of LiPSs and facilitates their transformation to Li₂S. Whether employed as S hosts or separator coatings, such structured MXenes significantly boost the electrochemical performance of LSBs. For instance, MXenes with expanded interlayer spacing exhibit rapid Li⁺ diffusion and improved S utilization [230]. Meanwhile, 0D and 1D MXenes effectively mitigate the restacking challenges associated with 2D NSs [191,192]. Furthermore, 3D porous MXene frameworks demonstrate excellent areal and volumetric capacities, even at high S loadings [193]. Overall, the structural optimization of MXenes opens up new possibilities and presents exciting opportunities for advancing their applications in LSBs.

8.1.4. Development of MXene-Based Composites as S Host

C materials and TM compounds have traditionally been considered effective S hosts [12]. C materials, with their highly porous structures, physically trap LiPSs, provide ample space for increased S loading, and accommodate volume expansion [231]. TM compounds, on the other hand, chemically anchor LiPSs, thereby effectively mitigating the shuttle effect [232]. Leveraging their unique 2D structure and exceptional conductivity, MXenes have emerged as ideal candidates for creating composite materials with C materials or TM compounds. These composites capitalize on the complementary benefits of both components for LSBs. In such systems, MXenes create a continuous conductive framework that supports efficient charge transfer while exhibiting strong LiPSs adsorption and catalytic properties. Simultaneously, the secondary materials contribute their inherent functions while acting as spacers to prevent MXene restacking and facilitate Li⁺ transport [233]. As a result, MXene-based composites demonstrate excellent electrochemical performance as S hosts, highlighting a promising pathway for MXene applications in LSBs.

MXene/C composites: C materials, known for their high conductivity, adjustable porous structures, and low cost, have been extensively utilized as S hosts in LSBs [234]. Combining MXenes with C materials in composite structures leverages the inherent benefits of both components. MXene NSs improve the electrical conductivity of the composite, enabling efficient electron transfer, while C materials act as spacers, preventing MXene restacking and facilitating rapid ion transport. Moreover, C materials, particularly 1D CNTs and 2D graphene, open up significant possibilities for advanced structural designs when integrated with MXene NSs [235]. This synergistic combination results in MXene/C composite-based S cathodes that deliver excellent electrochemical performance, even under conditions of high S loading.

MXene/CNT composites are renowned for their exceptional conductivity and open structures, enabling high S loading and superior performance [179]. By simply combining 1D CNTs with 2D MXene NSs, 3D CNT/MXene composite frameworks were developed and utilized as S hosts. Combining 1D CNTs with 2D MXene NSs has led to the development of 3D CNT/MXene networks, which have been effectively employed as S hosts. Examples include CNTs/Ti₂C (83 wt% S content), CNTs/Ti₃C₂ (79 wt% S content), and CNTs/Ti₃CN (83 wt% S content), all of which delivered initial capacities above 1200 mAh g⁻¹ at 0.05 C and retained ≈ 450 mAh g⁻¹ after 1200 cycles at 0.5 C [179]. Mo₂C MXene, known for its strong interaction with LiPSs, efficiently suppresses the shuttle effect. Integrating CNTs with Mo₂C MXene produced a Mo₂C-CNT composite with an increased SSA (116.8 cm² g⁻¹) and great conductivity, which was used as an S host [236]. The Mo₂C-CNT/S electrode, containing 87.1 wt% S, exhibited an initial capacity of 1235 mAh g⁻¹ with a retention of 74.9% after 250 cycles at 0.1 C and a capacity of 519 mAh g⁻¹ at 5 C. At a great S loading of 5.6 mg cm⁻², the electrode achieved an initial capacity of 959 mAh g⁻¹. Additionally, positively charged polyethyleneimine (PEI) modification enabled the electrostatic self-assembly of CNTs with Ti₃C₂T_x, forming a Ti₃C₂T_x@PEI-CNT composite (Figure 16a) featuring a highly porous 3D architecture (Figure 16b,c) [237]. This composite is utilized as both an S host and an interlayer. The resulting LSBs provided a capacity of 980 mAh g⁻¹ at 1 C after 500 cycles with an S loading of 2.6 mg cm⁻² (Figure 16d). Even at an S loading of 5.8 mg cm⁻², the batteries achieved an impressive areal capacity of 7.1 mAh cm⁻² at 2.4 mA cm⁻² and retained a capacity above 4 mAh cm⁻² after 100 cycles at 9.7 mA cm⁻².

Characterized by their µm scale diameters and excellent conductivity, 1D C fibers are ideal candidates for integration with MXenes. This combination forms uniform MXene@C fiber composites with well-dispersed MXene NSs (Figure 16e) [238]. Incorporating a C/S slurry into this framework enables the MXene NSs to serve multiple purposes, including suppressing the shuttle effect, facilitating rapid electron transport, and enhancing the flexibility of C fibers to accommodate S’s volume extension. Consequently, the Ti₃C₂@C fiber-S cathode, featuring a uniform S loading of 4 mg cm⁻², achieved an initial capacity of 1058.4 mAh g⁻¹ and maintained 59.1% retention in capacity after 1000 cycles at 1 C (Figure 16f). Incorporating 2D graphene and rGO with MXenes has also proven effective in creating high-performance S cathodes for LSBs [239]. Various 3D MXene/graphene composites have been designed to function as S hosts, offering enhanced ion transport and S loading capabilities [190]. For instance, a 3D porous Ti₃C₂T_x/rGO hybrid aerogel was synthesized through hydrothermal treatment of Ti₃C₂T_x and GO, followed by freeze-drying. This aerogel served as a freestanding cathode to accommodate Li₂S₆ in LSBs (Figure 16g,h) [239]. The 3D interconnected MXene/rGO framework (Figure 16h) facilitated rapid Li-ion diffusion and electron transport, delivering a capacity of 1270 mAh g⁻¹ at 0.1 C. It also demonstrated a low-capacity decay of 0.07% over 500 cycles at 1 C (Figure 16i) and an aerial capacity of 5.27 mAh cm⁻² with an S loading of 6 mg cm⁻². In another example, a freestanding 3D porous MXene/graphene hybrid aerogel was fabricated using a straightforward liquid infiltration–evaporation process and employed as a host for Li₂S [240]. The resulting Li₂S@MXene/graphene cathode, with a great Li₂S loading till 9 mg cm⁻², achieved 545 mAh g⁻¹, retained 88% capacity over 100 cycles at 0.2 C, and delivered an aerial capacity of 3.42 mAh cm⁻² at 2.0 C. Furthermore, a 3D layered Ti₃C₂T_x MXene/rGO hybrid was assembled utilizing a straightforward liquid-phase impregnation technique and subsequently loaded with S. This Ti₃C₂T_x/rGO/S cathode, with an S content of 70.4 wt%, exhibited a capacity of 1144.2 mAh g⁻¹ at 0.5 C and retained 878.4 mAh g⁻¹ after 300 cycles [190].

Porous C materials deliver additional space for S loading and help address the issue of volume extension during cycling [241], making MXene/porous C composites highly effective as hosts for high S content. One example is the synthesis of a 3D robust Ti₃C₂T_x@mesoporous C hybrid network by carbonizing an MXene@MOF-5 network [242]. After S loading, the MXene@mesoporous C-S cathode, containing 72.88 wt% S and an S loading of 2 mg cm⁻², delivered an impressive initial capacity of 1225.8 mAh g⁻¹ and retained 704.6 mAh g⁻¹ after 300 cycles at 0.5 C. Similarly, hollow porous C spheres were combined with Ti₃C₂ MXene to create a sandwiched composite [243]. The resulting cathode, with an S content of 76.5 wt%, achieved an initial capacity of 1397.5 mAh g⁻¹ at 0.05 C and sustained 494.7 mAh g⁻¹ after 500 cycles at 1 C.

Additionally, porous C have been adjusted with heteroatom doping before being combined with MXene. This doping enhances polarity, which strengthens the affinity for LiPSs to mitigate shuttle effect. It also delivers high conductivity, ensuring efficient S utilization [244]. For instance, a composite of N-doped porous C and Ti₃C₂ was created with a 3D sponge-like architecture and plentiful mesopores (417.6 m² g⁻¹). This composite was formed by carbonizing a composite of ZIF-67 and Ti₃C₂. After loading with 80 wt% S, the cathode achieved a capacity of 759 mAh g⁻¹ at 1 C and demonstrated exceptional cycling stability over 800 cycles, with a capacity decay of just 0.04% per cycle [245]. Moreover, another composite of MXene and Co/N-co-doped porous C was prepared through the in situ self-assembly of bimetallic ZIFs on Ti₃C₂T_x NSs, followed by calcination and etching [246]. This composite exhibited a high SSA of 726.6 m² g⁻¹ (Figure 16j) and a pore volume of 0.71 cm³ g⁻¹ (Figure 16k), with an 81.9 wt% S content as a host. Coupled with an MXene-modified PP separator, the cathode with an S loading of 1.5 mg cm⁻² delivered capacities of 1333.9 and 579.2 mAh g⁻¹ at 0.2 and 7 C rates (Figure 16l). Even at a higher S loading of 5.2 mg cm⁻², the cathode exhibited the initial capacity of 924.7 mAh g⁻¹ at 1 C and maintained 616.7 mAh g⁻¹ after 1000 cycles [246].

Figure 16. (a) Illustration of synthesis, (b) N₂ adsorption–desorption isotherms, and (c) SEM micrograph of Ti₃C₂T_x@PEI-CNTs. (d) Cycling performance of LSBs using a Ti₃C₂T_x@PEI-CNTs-S cathode paired with a Ti₃C₂T_x@PEI-CNTs interlayer. Adapted from [237], Copyright 2019, Elsevier B.V. (e) Pictorial representation and (f) cycling stability of Ti₃C₂@C fiber-S cathode. Adapted from [238], Copyright 2020, Royal Society of Chemistry. (g) Diagram illustrating the fabrication, (h) TEM micrograph, and (i) cycling performance of MXene/graphene aerogel electrode. Adapted from [239], Copyright 2019, Royal Society of Chemistry. (j) N₂ adsorption–desorption isotherms and (k) pore size distribution of the MXene/Co,N-codoped porous C. (l) Rate capability of LSBs with MXene/Co,N-codoped porous C/S cathode, and MXene-modified PP separator. Adapted from [246], Copyright 2019, American Chemical Society.

Figure 16. (a) Illustration of synthesis, (b) N₂ adsorption–desorption isotherms, and (c) SEM micrograph of Ti₃C₂T_x@PEI-CNTs. (d) Cycling performance of LSBs using a Ti₃C₂T_x@PEI-CNTs-S cathode paired with a Ti₃C₂T_x@PEI-CNTs interlayer. Adapted from [237], Copyright 2019, Elsevier B.V. (e) Pictorial representation and (f) cycling stability of Ti₃C₂@C fiber-S cathode. Adapted from [238], Copyright 2020, Royal Society of Chemistry. (g) Diagram illustrating the fabrication, (h) TEM micrograph, and (i) cycling performance of MXene/graphene aerogel electrode. Adapted from [239], Copyright 2019, Royal Society of Chemistry. (j) N₂ adsorption–desorption isotherms and (k) pore size distribution of the MXene/Co,N-codoped porous C. (l) Rate capability of LSBs with MXene/Co,N-codoped porous C/S cathode, and MXene-modified PP separator. Adapted from [246], Copyright 2019, American Chemical Society.

MXene/TM compound composites: TM compounds, such as TMOs [247] and TMSs [248], exhibit intense chemical interactions with LiPSs. When combined with MXenes, these compounds help prevent the restacking of MXene NSs and increase the chemical adsorption of LiPSs, effectively reducing the shuttle effect. While some metal compounds have limited electrical conductivity [247], MXenes’ metallic conductivity compensates for this drawback. Consequently, composites of MXenes and TM compounds are considered promising S hosts for LSBs as they efficiently suppress the shuttle effect and provide great conductivity for improved S utilization.

The in situ growth of TM compounds on MXene NSs is a commonly employed approach for producing composites. For instance, a 3D hierarchical porous MXene/1T-2H MoS₂-C was created through a hydrothermal technique involving ammonium molybdate and thiourea with MXene and glucose, followed by annealing in a N₂ atmosphere and then in ammonia at 600 °C (Figure 17a,b) [249]. The NH₃ treatment introduced N-doping into the MoS₂ NSs, generated S vacancy defects, and facilitated a phase transition in MoS₂ from the 2H to the 1T phase. As an S host in LSBs, the S vacancy defects and the 1T MoS₂ phase demonstrated strong LiPSs trapping and accelerated redox kinetics. Additionally, high electrical conductivity of the 1T MoS₂, MXene, and N-doped C enabled rapid electron transport. The 3D porous network (153.3 m² g⁻¹) provided ample space for great S loading and accommodated volume variation. Consequently, the MXene/1T-2H MoS₂-C/S cathode, containing 79.6 wt% S, achieved an initial capacity of 1194.7 mAh g⁻¹ at 0.1 C and maintained 799.3 mAh g⁻¹ after 300 cycles at 0.5 C (Figure 17c). A pouch cell using this cathode exhibited an initial capacity of 915.2 mAh g⁻¹ at 0.2 C and maintained stable cycling performance for 40 cycles, showcasing the electrode’s advantages [249]. Similarly, a VO₂-V₂C hybrid was synthesized through the in situ growth of VO₂ rod clusters on 2D V₂C NSs via hydrothermal treatment of a VO₂ precursor in the presence of V₂C [154]. Both the VO₂ NRs and V₂C MXene demonstrated robust adsorption capabilities and elevated catalytic activity for LiPSs. As a result, the VO₂-V₂C/S cathode, with 72.7 wt% S content, exhibited a capacity of 1250 mAh g⁻¹ at 0.2 C, 585 mAh g⁻¹ at 4 C, and retained 69.1% of its capacity after 500 cycles at 2 C. Even with an S loading of 10.2 mg cm⁻², the cathode achieved an aerial capacity of 9.3 mAh cm⁻² at 0.2 C and maintained 6.5 mAh cm⁻² after 200 cycles, demonstrating excellent performance.

Interestingly, TMOs can be directly generated on MXene NSs through partial surface oxidation, leading to the formation of TMOs/MXene composites [3]. Following this approach, an accordion-type TiO₂/Ti₃C₂ heterostructure was fabricated using a simple flash heat treatment of Ti₃C₂ in air (Figure 17d,e) [250]. The TiO₂ layer effectively inhibited the restacking of MXene NSs, improved LiPSs adsorption, and boosted catalytic activity for LiPSs conversion. Simultaneously, MXene component provided excellent conductivity, while the accordion-type architecture allowed for high S loading and accommodated significant volume variations during cycling. Consequently, the TiO₂-Ti₃C₂ heterostructure, with an S content of 80 wt%, achieved an initial capacity of 1567 mAh g⁻¹ at 0.5 C and maintained 803 mAh g⁻¹ after 1000 cycles (Figure 17f).

Ex situ assembly offers an efficient method for fabricating composites, enabling precise control over the ratio of MXene and TM compounds. Using an electrostatic self-assembly process with CTAB-functionalized MnO₂ NSs and delaminated Ti₃C₂ MXene, a 3D mesoporous MnO₂@Ti₃C₂ aerogel framework was developed as an S host (Figure 17g,h) [251]. The polar MnO₂ and Ti₃C₂ MXene exhibited strong LiPSs adsorption capabilities, enabling the MnO₂@Ti₃C₂ aerogel, with an S loading of 70 wt%, to deliver an initial capacity of 1140 mAh g⁻¹ at 0.05 C. The aerogel maintained 91% of its capacity after 500 cycles at 1 C. Even at an S loading of 3.7 mg cm⁻², the aerogel retained a capacity of 474 mAh g⁻¹ after 500 cycles at 1 C, which was comparable to the 491 mAh g⁻¹ achieved with an S loading of 2.1 mg cm⁻² (Figure 17i,j).

MXene/covalent organic frameworks (COFs) composites: Two-dimensional COFs, characterized by their well-developed porous structures and adjustable pore sizes, offer precisely designed structures and heteroatom compositions for controlled chemical interactions with LiPSs, positioning them as a promising class of S host materials for LSBs. Moreover, COFs generally suffer from poor electrical conductivity [252]. This limitation can be effectively addressed by combining COFs with MXenes. An MXene/COFs composite was recently developed as an S host to harness their synergistic properties [253]. Through the in situ polymerization of 1,4-dicyanobenzene, COFs with an ordered porous structure were anchored onto Ti₃C₂ NSs. This composite featured hierarchical micro- and meso-porous architecture with an SSA of 318 m² g⁻¹. The lithophilic N sites within the covalent triazine frameworks and the sulfurophilic Ti sites in MXene provided robust chemical adsorption of LiPSs, efficiently suppressing the shuttle effect. Consequently, the S@covalent triazine frameworks/Ti₃C₂ cathode, containing 76 wt% S and an S loading of 1.5 mg cm⁻², delivered an initial capacity of 1441 mAh g⁻¹ at 0.2 C and exhibited excellent cycling performance, with a capacity decay as low as 0.014% per cycle over 1000 cycles at 1 C. Even at an S loading of 5.6 mg cm⁻², the cathode retained a capacity of 816 mAh g⁻¹ after 100 cycles at 0.2 C, corresponding to 94% retention [253].

8.1.5. MXene Employed as a Conductive Binder for S Cathodes

Beyond their role as S hosts, 2D MXene NSs, recognized for their excellent mechanical characteristics, can also function as conductive binders for fabricating flexible and freestanding electrodes [254]. Traditional electrode preparation typically involves blending active materials with a conductive additive and an insulating polymer binder (PTFE or PVDF) to create a slurry, which is then deposited onto a metal current collector. However, using polymer binders often compromises the electrode’s conductivity and rate capability. By incorporating 2D MXenes as conductive binders, a continuous MXene network is formed, offering exceptional conductivity and flexibility. Moreover, MXenes’ inherent ability to chemically adsorb and catalytically interact with LiPSs makes them particularly advantageous as conductive binders for S cathodes in LSBs. This dual functionality enhances both the structural stability and electrochemical behavior of the electrodes.

Using small S molecules (S_2-4) as active materials instead of traditional S₈ offers a fundamental solution to the shuttle effect by preventing the formation of LiPSs. Xu and collaborators developed ultramicroporous C (UMC) with a pore size of 0.55 nm to encapsulate S_2-4, achieving exceptional cycling stability [255]. They further designed a freestanding, flexible electrode by combining MXene as a conductive binder and substrate with S_2-4/UMC (Figure 18a,b) [254]. In this design, MXene NSs created a conductive framework that facilitated rapid electron transport, significantly enhancing the rate capability of S_2-4/UMC electrode (Figure 18c). Consequently, the MXene-bonded S_2-4/UMC electrode achieved capacities of 1029.7 mAh g⁻¹ at 0.1 C and 502.3 mAh g⁻¹ at 2 C (Figure 18d), along with exceptional cycling performance, retaining 91.9% of its capacity after 200 cycles at 0.1 C. These results markedly surpassed that of traditional S_2-4/UMC electrodes using a PVDF binder and Al current collector. Additionally, this MXene-based electrode fabrication approach has been successfully applied to SCs [256] and Li/Na/K-ion batteries [257,258], demonstrating its versatility for developing high-performance energy storage devices.

MXene NSs can also function as conductive additives to improve the conductivity of S electrodes. For instance, incorporating Ti₂C MXene into an S@TiO₂ composite resulted in the S@TiO₂/Ti₂C electrode achieving an initial capacity of 1408.6 mAh g⁻¹ at 0.2 C [259]. Similarly, integrating Ti₃C₂T_x into polyethyleneimine (PEI)-modified Ketjen black/S NPs, along with a Ketjen black@Ti₃C₂T_x-deposited PP separator, allowed for an LSB with an aerial S loading of 5.6 mg cm⁻² to deliver an aerial capacity of 4.5 mAh cm⁻² at 0.2 C while maintaining 74% retention capacity over 100 cycles [260].

As summarized in

Table 4. Electrochemical characteristics of MXene-based S cathode in LSBs [6].

Materials	S Loading (mg cm−2)	S Content (%)	Initial Capacity (mAh g−1)/Current Density	Capacity Decay (%) @Cycles/Current Density	Rate Capacity (mAh g−1)/Current Density	Ref.
S@Ti3C2Tx ink	4	67.1	1477.2/0.2 C	0.18@100/0.2 C	860.2/2 C	[221]
Accordion-like layered Ti3C2/S	-	57.6	1291/200 mA g−1	0.25@100/200 mAg−1	620/3.2 A g−1	[220]
S@MXe@PDA	1.7	60	1439/0.2 C	0.183@150/0.2 C	624/6 C	[222]
	4.4	-	1034.1/0.2 C	0.128@140/0.2 C	735/1 C
Delaminated-Ti2C/S	1.0	70	1090/0.5 C	0.05@650/0.5 C	660/4 C	[188]
Ti3C2Tx “clay”/S and SWCNT interlayer	1.0–1.2	50	1458/0.1 A g−1	0.04@1500/0.8 A g−1	608/4.9 C	[223]
	3.5	80	675.2/1.5 A g−1	0.012@600/1.5 A g−1	-
Robust Ti3C2Tx paper/S	-	30	1383/0.1 C	0.014@1500/1 C	1075/2 C	[225]
V2C-Li/rGO-CNTs/S	3	70	1140/0.1 C	0.053@500/1 C	400/5 C	[230]
S@Ti3C2Tx ink	-	50	1350/0.1 C	0.048@800/0.2 C	1004/2 C	[224]
		70	1244/0.1 C	0.035@175/2 C	1161/2 C
Zn-Ti3C2Tx/S	1.7	89	1136/0.2 C	0.03@400/1 C	517/6 C	[203]
Ti3C2 nanoribbon/S and delaminated-Ti3C2 interlayer	0.7–1	68	1062/0.2 C	0.2516@200/0.5 C	373/6 C	[192]
Ti3C2Tx nanodots-Ti3C2Tx NSs/S	1.8	67.6	1609/0.05 C	0.057@400/2 C	-	[191]
	9.2	-	1827 mAh cm−3/0.05 C	0.02@100/0.05 C
	13.8		1957 mAh cm−3/0.05 C	0.02@100/0.05 C
Flower-like porous Ti3C2Tx/S	4.2	61.5	1547 mAh cm−3/0.033 C	0.366@75/0.033 C	-	[193]
	6.8	-	1814 mAh cm−3/0.033 C	0.395@75/0.033 C
	10.5		2009 mAh cm−3/0.033 C	0.294@75/0.033 C
Porous N-doped Ti3C2Tx/S	1.4–1.6	64	1072/0.5 C	0.033@1200/2 C	792/3 C	[206]
	3.6	-	993/0.2 C	0.062@50/0.2 C	-
	8.2		9 mAh cm−2/0.1 C	0.722@20/0.1 C
Crumpled N-doped Ti3C2Tx/S	1.5	73.85	1609/0.05 C	0.026@1000/2 C	770/2 C	[228]
	5.1	-	765/0.2 C	0.046@500/0.2 C	-
Ti2CTx/CNTs/S	1.5	83	1240/0.05 C	0.043@1200/0.5 C	-	[179]
Ti3C2Tx/CNTs/S		79	1216/0.05 C
Ti3CNTx/CNTs/S		83	1263/0.05 C
Ti3C2Tx foam/S	1.5	71.1	1226.4/0.2 C	0.025@1000/1 C	711.0/5 C	[153]
Mo2C/CNTs/S	0.8	87.1	1438/0.1 C	0.1@250/0.1 C	519@5 C	[236]
	1.5	-	1314/0.1 C	-	-
	3.5		1068/0.1 C
	5.6		959/0.1 C
Ti3C2@Carbon Fibers/S	4	-	1175.2/0.5 C	0.042%@1000/1 C	-	[238]
Ti3C2Tx/graphene/Li2S	3	62	710/0.2 C	0.133@100/0.2 C	550/2 C	[240]
	6	-	590/0.2 C	-	520/1 C
	9		545/0.2 C		380/1 C
Ti3C2Tx@PEI-CNTs/S	2.6	70.2	1110/0.5 C	0.02@1500/1 C	950/2.5 C	[237]
Ti3C2Tx@PEI-CNTs interlayer	5.8	-	1184/0.25 C	0.286@100/0.25 C	-
3D Ti3C2Tx/rGO aerogel/S	1.57	45	1270/0.1 C	0.07@500/1 C	977/1 C	[239]
	6	-	879/0.1 C	-	-
Ti3C2Tx/rGO/S	1.5	70.4	1144.2/0.5 C	0.0774@300/0.5 C	750/5 C	[190]
Hollow porous carbon spheres@d-Ti3C2Tx/S	1	76.5	1397.5/0.05 C	0.069@500/1 C	398.9/2 C	[243]
Ti3C2Tx/Co, N-codoped C/S and MXene interlayer	1.5	-	1340.2/0.2 C	0.016@1000/1 C	579.2/7 C	[246]
	5.2		924.7/1 C	0.033@1000/1 C	-
Ti3C2Tx@mesoporous C/S	2	-	1225.8/0.5 C	0.142@300/0.5 C	544.3/4 C	[242]
Ti3C2Tx/N-doped C/S	-	80	1064/0.1 C	0.04%@800/1 C	595/2 C	[245]
Ketjen black/S@Ti3C2Tx cathode and Ketjen black@Ti3C2Tx interlayer	4.5	82	920/0.05 C	0.158@100/0.2 C	517/2 C	[260]
	5.6	-	1137/0.05 C 810/0.2 C	0.25@100/0.2 C	-
Covalent triazine/Ti3C2	1.5	76	1441/0.2 C	0.014@1000/1 C	-	[253]
	5.6	-	868.1/0.2 C	0.06@100/0.2 C
VO2(p)-V2C/S	-	72.7	1250/0.2 C	0.0618@500/2 C	585/2 C	[154]
S@TiO2/Ti2C	1.8–2.0	78.4	1408.6/0.2 C	0.2036@200/2 C	317.7/5 C	[259]
Ti3C2Tx-1T-2H MoS2-C/S	1	79.6	1194.7/0.1 C	0.07@300/0.5 C	677.2/2 C	[249]
Ti3C2Tx-MnO2/S	1.2	70	1140/0.05 C	0.06@500/1 C	615/2 C	[251]
Ti3C2Tx/S2-4/UMC	1	37.2	1029.7/0.1 C	0.0405@200/0.1	502.3/2 C	[254]
Ti3C2Tx-TiO2/S	-	60	1417/1 C	0.053@1000/1 C	367/10 C	[250]

, MXenes demonstrate exceptional electrochemical performance when applied in the S cathodes of LSBs. This performance stems from their metallic conductivity, versatile structures, robust chemical interactions with LiPSs, and effective catalytic activity for LiPSs conversion. To address the issue of restacking commonly observed in 2D MXene NSs, advanced MXene architectures have been developed, including structures with expanded interlayer spacing and 3D porous designs. These innovations optimize the benefits of MXenes by fully exposing their surfaces to enhance S utilization, LiPSs adsorption, and catalytic activity. Moreover, the increased structural space supports high S loading and accommodates the volume expansion of S. In addition, the creation of MXene-based composites generates synergistic effects, preventing NSs restacking while improving their chemical adsorption and catalytic capabilities for LiPSs. Such enhancements significantly boost the performance of LSBs, even under high S loading conditions, which is a critical requirement for their practical application.

Despite significant progress, the application of MXenes in S cathodes remains in its infancy. Current research has predominantly centered on Ti-based MXenes, particularly Ti₃C₂T_x, with limited attention given to other MXene types. However, the potential of MXenes beyond Ti₃C₂T_x can be anticipated based on their unique properties and theoretical predictions of their interactions with LiPSs. For instance, DFT calculations suggest that Cr₃C₂O₂ exhibits the robust interaction with LiPSs among six oxygen-terminated M₃C₂O₂ (M = Cr, V, Ti, Nb, Hf, and Zr) MXenes [202]. Similarly, Nb₂C MXenes, depending on their surface terminations, demonstrate intriguing superconducting transitions at low temperatures [67], indicating their potential as promising candidates for LSBs and warranting further experimental exploration. The surface terminations of MXenes significantly influence their chemical adsorption and catalytic properties toward LiPSs, yet experimental investigations into surface engineering of MXenes as S hosts remain unexplored. Strategies such as heteroatom doping and the introduction of vacancies could further optimize their performance and are promising areas for future research. These challenges also present exciting opportunities to advance MXenes for high-performance S cathodes in LSBs. For practical LSB applications, achieving high S loading in MXene-based cathodes is essential, but additional factors such as reducing the E/S ratio and enhancing the tap density of electrode must also be addressed. Investigating pouch-cell-based configurations with optimized MXene utilization, active material loading, and electrolyte management could yield deeper insights, enabling enhanced energy density and prolonged cycling life for LSBs.

8.2. MXene-Based Interlayer Between Cathode and Separator in LSBs

Beyond advanced designs for S cathodes, incorporating an interlayer between the cathode and separator is a highly efficient approach to boost the performance of LSBs. This interlayer, which can either be freestanding or deposited onto the separator, plays a dual role: it acts as a barrier to retain LiPSs within the cathode region and serves as an additional current collector to boost the application of LiPSs (Figure 19a) [189].

8.2.1. Separator Modified with Pristine MXene

A straightforward vacuum-filtration method allows for the efficient assembly of 2D MXene NSs onto the separator in LSBs. This approach creates a conformal Ti₃C₂T_x layer with a mass loading of just 0.1 mg cm⁻² and a thickness of 522 nm, applied to PP membrane, forming a Ti₃C₂T_x-modified separator free of noticeable pores or gaps (Figure 19a) [189]. Leveraging the elevated conductivity and strong polysulfide adsorption capabilities of Ti₃C₂T_x layer, LSBs equipped with a sulfur/super P cathode (68% S content) and this modified separator demonstrated a capacity of 550 mAh g⁻¹ after 500 cycles, with a low-capacity decay rate of 0.062% per cycle at 0.5 C (Figure 19b). Thinner Ti₃C₂T_x coatings allow for rapid Li⁺ diffusion (Figure 19c), and even a coating as thin as 100 nm, with a mass loading of just 0.016 mg cm⁻², effectively limits polysulfide diffusion [261]. To further enhance electrical conductivity and electrolyte wettability, 10 wt% CNTs were incorporated to the 0.016 mg cm⁻² Ti₃C₂T_x coating. The resulting Ti₃C₂T_x/CNTs-10–functionalized PP separator enabled LSBs with a S/CNT cathode (70% S content) to deliver a capacity of 640 mAh g⁻¹ at 2 C and retain this capacity after 200 cycles at 1 C. Glass fiber membranes, which are known for their high porosity and excellent electrolyte uptake, have also been utilized as separators in LSBs. When modified with a Ti₃C₂T_x layer via vacuum-filtration, these separators allowed LSBs with an S/super P cathode (70% S content) to achieve capacities of 1366 and 802 mAh g⁻¹ at 0.5 and 2 A g⁻¹ [262]. Additionally, MXenes can be deposited onto separators using binders. For instance, Ti₃C₂T_x MXene debris, obtained through hydrothermal treatment of MXene, were coated onto a biodegradable eggshell membrane to fabricate a modified separator for LSBs [263]. The MXene debris provided strong chemical confinement of polysulfides, while the eggshell membrane, rich in hydroxyl and amine functional groups, improved electrolyte infiltration and enhanced polysulfide affinity. Consequently, LSBs employing a S/Ketjen black cathode (67% S content) and this MXene/eggshell membrane separator demonstrated excellent rate performance, achieving a capacity of 948 mAh g⁻¹ at 1 C and retaining 74% of the capacity after 250 cycles at 0.5 C.

8.2.2. Separator Enhanced with MXene-Based Composites

MXene-based composites, similar to their role as S hosts, can also modify separators in LSBs to suppress the shuttle effect and improve Li-ion diffusion through synergistic interactions between MXenes and complementary components. C materials are particularly popular for creating such composites. For example, a 3D Ti₃C₂T_x/CNT network with a mass loading of 0.16 mg cm⁻² was fabricated by vacuum-filtrating a mixed solution of PVP-treated CNTs and Ti₃C₂T_x NSs onto a PP separator. When paired with a S/CNT cathode (70% S content), LSBs using this modified separator delivered an initial capacity of 1415 mAh g⁻¹ at 0.1 C, maintaining a capacity decay as low as 0.06% per cycle over 600 cycles at 1 C [264]. Additionally, an N-doped Ti₃C₂/C (N-Ti₃C₂/C) composite was fabricated by calcining ZIF-67-decorated Ti₃C₂ NSs, followed by blending the product with porous C and a binder to create a slurry. This slurry was then deposited on PP separator, producing a modified separator with a mass loading of 0.6 mg cm⁻² [265]. The N doping in Ti₃C₂ enhanced its chemical affinity for LiPSs and introduced lithiophilic sites to assist efficient Li-ion diffusion. Simultaneously, the porous C provided robust physical adsorption of LiPSs and lowered local current density, which helped delay the formation of Li dendrites. The N-Ti₃C₂/C interlayer efficiently mitigated the shuttle effect and suppressed dendrite growth in Li anodes (Figure 19d). LSBs using the N-Ti₃C₂/C@PP separator and a S/CNT cathode (79% S content) delivered excellent performance, achieving capacities of 1332 mAh g⁻¹ at 0.1 C and 675 mAh g⁻¹ at 2 C. The LSBs also exhibited a low-capacity decay rate of 0.07% per cycle over 500 cycles at 0.5 C with an S loading of 3.4 mg cm⁻². Even at a high S loading of 10.3 mg cm⁻², the LSBs maintained a constant areal capacity of 6.3 mAh cm⁻² after 50 cycles at 0.1 C (Figure 19d). In another study, a Ti₃C₂T_x/GO membrane with excellent mechanical strength was fabricated through vacuum filtration of a mixed solution containing Ti₃C₂T_x and GO [266]. This freestanding membrane, positioned between the cathode and separator, served as a dual-function barrier, providing both physical and chemical confinement to block LiPSs (Figure 19e). When implemented in LSBs with a S/CNT cathode (70.5% S content), the Ti₃C₂T_x/GO interlayer significantly boosted battery performance, achieving an initial capacity of 1621.5 mAh g⁻¹ at 0.1 C and retaining 833.2 mAh g⁻¹ over 200 cycles. The batteries also demonstrated an excellent rate capacity of 640 mAh g⁻¹ at 5 C (Figure 19f).

TMOs with a robust chemical affinity for LiPSs represent a promising class of materials to complement Ti₃C₂T_x. By partially oxidizing Ti₃C₂T_x via hydrothermal method, TiO₂ NPs were synthesized in situ on Ti₃C₂T_x NSs, resulting in the formation of a TiO₂-Ti₃C₂T_x heterostructure [267]. This heterostructure was then integrated with graphene and applied onto a PP separator using vacuum filtration, creating a compact and thin coating with a mass loading of 0.15 mg cm⁻². In this configuration, the TiO₂ NPs acted as adsorption centers to capture LiPSs, while the heterointerface facilitated rapid diffusion of LiPSs from TiO₂ to Ti₃C₂T_x. The Ti₃C₂T_x contributed a great catalytic mechanism for efficient LiPSs conversion, and the graphene formed a conductive structure that physically restricted LiPSs migration (Figure 19g,h). Consequently, LSBs featuring a CMK-3/S cathode (70% S content, 1.2 mg cm⁻² S loading) and the TiO₂-MXene-graphene-coated separator delivered a capacity of 800 mAh g⁻¹, with a minimal capacity decay of 0.028% per cycle over 1000 cycles at 2 C (Figure 19i). Even at a higher S loading of 5.1 mg cm⁻² in the S/CNT cathode (75% S content), the battery retained 93% of its capacity after 200 cycles at 0.5 C [267].

Ti₃C₂T_x/polymer heterostructures have also been employed to enhance separators in LSBs. A notable example is the creation of a Ti₃C₂T_x-Nafion interlayer, achieved by vacuum-filtering a solution of Ti₃C₂T_x and Nafion onto a PP separator [268]. In this composite interlayer, Nafion was evenly distributed among the Ti₃C₂T_x NSs, promoting efficient Li⁺ transport while electrostatically repelling LiPSs. Consequently, LSBs with a S/C black cathode (74.1% S content) and the Ti₃C₂T_x/Nafion adjusted PP separator demonstrated an initial capacity of 1234 mAh g⁻¹ at 0.2 C, a capacity of 794 mAh g⁻¹ at 3 C, and a minimal capacity decay of 0.03% per cycle over 1000 cycles at 1 C.

Figure 19. (a) Diagram illustrating Li-S cells equipped with PP and PP separators functionalized with Ti₃C₂T_x. (b) Cycling performance of Li-S cell featuring the Ti₃C₂T_x-functionalized PP separator. Adapted from [189], Copyright 2016, American Chemical Society. (c) Illustration of Li-ion diffusion through Ti₃C₂T_x-modified PP separators with varying mass loadings in LSBs. Adapted from [261], Copyright 2019, Elsevier B.V. (d) Diagram showing the preparation of N-doped Ti₃C₂T_x/C composite and its role in preventing Li dendrites formation when utilized as an interlayer. Additionally, it highlights the cycling stability of Li-S cells with high S loading, featuring N-doped Ti₃C₂T_x/C-modified PP separator. Adapted from [265], Copyright 2019, Elsevier B.V. (e) Optical image of freestanding Ti₃C₂T_x/GO membrane. (f) Rate performance comparison of Li-S cells using the Ti₃C₂T_x/GO@PP and standard PP separators. Adapted from [266], Copyright 2020, American Chemical Society. (g) Illustration of TiO₂-Ti₃C₂T_x heterostructures demonstrating their role in trapping LiPSs and facilitating the conversion mechanism. (h) TEM micrograph of the TiO₂-Ti₃C₂T_x heterostructures, with the corresponding SAED pattern shown in the inset. (i) Cycling stability of Li-S cell equipped with an oxidized Ti₃C₂T_x/graphene interlayer. Adapted from [267], Copyright 2019, Wiley-VCH.

Figure 19. (a) Diagram illustrating Li-S cells equipped with PP and PP separators functionalized with Ti₃C₂T_x. (b) Cycling performance of Li-S cell featuring the Ti₃C₂T_x-functionalized PP separator. Adapted from [189], Copyright 2016, American Chemical Society. (c) Illustration of Li-ion diffusion through Ti₃C₂T_x-modified PP separators with varying mass loadings in LSBs. Adapted from [261], Copyright 2019, Elsevier B.V. (d) Diagram showing the preparation of N-doped Ti₃C₂T_x/C composite and its role in preventing Li dendrites formation when utilized as an interlayer. Additionally, it highlights the cycling stability of Li-S cells with high S loading, featuring N-doped Ti₃C₂T_x/C-modified PP separator. Adapted from [265], Copyright 2019, Elsevier B.V. (e) Optical image of freestanding Ti₃C₂T_x/GO membrane. (f) Rate performance comparison of Li-S cells using the Ti₃C₂T_x/GO@PP and standard PP separators. Adapted from [266], Copyright 2020, American Chemical Society. (g) Illustration of TiO₂-Ti₃C₂T_x heterostructures demonstrating their role in trapping LiPSs and facilitating the conversion mechanism. (h) TEM micrograph of the TiO₂-Ti₃C₂T_x heterostructures, with the corresponding SAED pattern shown in the inset. (i) Cycling stability of Li-S cell equipped with an oxidized Ti₃C₂T_x/graphene interlayer. Adapted from [267], Copyright 2019, Wiley-VCH.

Inserting an interlayer between the cathode and separator offers a straightforward yet efficient approach to mitigating the shuttle effect in LSBs. Both pristine MXenes and MXene-based composites have been utilized to construct interlayers, leading to substantial improvements in the electrochemical performance of LSBs (

Table 5. Electrochemical characteristics of LSBs with MXene-based interlayer between cathode and separator [6].

Materials	Cathode	S Loading (mg cm−2)	S Content (%)	Initial Capacity (mAh g−1)/Current Density	Capacity Decay (%)@Cycles/Current Density	Rate Capacity (mAh g−1)/Current Density	Ref.
Ti3C2Tx/glass fiber	S/super P	1.9	70	1462/0.1 A g−1	0.22@100/0.5 A g−1	802/2 A g−1	[262]
Ti3C2Tx	S/super P	1.2	68	1046.9/0.2 C	0.062@500/0.5 C	743.7/1 C	[189]
Ti3C2Tx/eggshell membrane	S/Ketjen black	2.07	67	1003/0.5 C	0.104@250/0.5 C	948/1 C	[263]
N-Ti3C2Tx/C	S/CNTs	3.4	79	1332/0.1 C	0.07@500/0.5 C	675/2 C	[265]
Ti3C2Tx/TiO2	S/CMK-3	1.2	70	800/2 C	0.028@1000/2 C	663/2 C	[267]
	S/CNTs	5.1	75	712/0.5 C	0.035@200/0.5 C	-
	-	7.3	-	-	0.2115@200/0.5 C	-
3D Ti3C2Tx/CNTs	S/CNTs	0.8	70	1415/0.1 C	0.06@600/1 C	728/2 C	[264]
Ti3C2Tx/10% CNTs	S/CNTs	1.2	70	≈1100/0.1 C	0.086@200/1 C	640/2 C	[261]
Ti3C2Tx/Nafion	S/carbon black	2	74.1	1234/0.2 C	0.03@1000/1 C	794/3 C	[268]
Ti3C2Tx/GO	S/CNTs	3-4	70.5	1621.5/0.1 C	0.103@300/1 C	640/5 C	[266]

). Research findings suggest that an MXene-based interlayer with a thickness of just 100 nm is sufficient to suppress LiPSs. However, as an additional component in LSBs, it is essential to develop interlayers that are thinner and lighter while maintaining their effectiveness in restricting the shuttle effect. Moreover, exploring other MXene types beyond Ti₃C₂T_x, particularly those with enhanced LiPSs adsorption capabilities and superior catalytic efficiency, could further advance interlayer technology for LSBs.

8.3. MXene-Based Li Metal Anode

Effectively addressing the issue of dendrite formation in Li anodes can be achieved by constructing an appropriate host for Li metal [269]. MXene materials have shown great potential as Li hosts, offering a promising approach to control Li dendrite formation and growth, thereby enabling stable Li anodes. The surface terminations of MXenes demonstrate a strong affinity for Li, promoting controlled nucleation [182]. Their metallic conductivity and high Li-ion diffusion rates support fast electrochemical kinetics [181]. Furthermore, the structural versatility and exceptional mechanical strength provide numerous possibilities for designing advanced MXene/Li composite architectures [270]. Consequently, MXenes have gained considerable attention for improving the performance of Li anodes, playing a key role in advancing high-performance LSBs.

8.3.1. Lamellar MXene-Li Anode

A lamellar-structured Ti₃C₂T_x-Li metal film was created using a roll-to-roll mechanical technique, taking advantage of the inherent ductility of Li and the lubricating properties of atomic layers (Figure 20a) [209]. The Ti₃C₂T_x facilitated controlled nanoscale development of Li dendrites within the interlayer spaces, efficiently suppressing their vertical expansion. This Ti₃C₂T_x-Li film exhibited stable voltage curves with a minor overpotential of 32 mV at a 1 mA cm⁻². LSBs with this Ti₃C₂T_x-Li anode and an S/C cathode achieved an energy density of 656 Wh kg⁻¹ after 100 cycles at 0.5 mA cm⁻². Furthermore, lamellar Ti₃C₂T_x layers were fabricated via self-assembly process at a water–air interface and applied to the Li anode surface using a rolling technique. This process formed a smooth, dense protective layer on the Li anode [270]. Li preferred to grow horizontally along the aligned NSs of MXene, with nucleation sites forming between the sheets to encourage uniform Li distribution during cycling (Figure 20b). Consequently, the Ti₃C₂T_x-Li anode maintained a smooth surface after 100 cycles, achieved a cycle life of 900 h at 1 mA cm⁻² with a stripping/plating capacity of 0.5 mAh cm⁻², and delivered a high stripping/plating capacity of 35 mAh cm⁻² (Figure 20c).

A layered rGO/Ti₃C₂T_x film was prepared through vacuum filtration and then infused with molten Li metal to create a Li-rGO/Ti₃C₂T_x anode [271]. DFT measurements revealed that the interface between Ti₃C₂T_x and rGO exhibited robust Li adsorption, acting as preferential sites to guide Li nucleation and growth. Additionally, the rGO/Ti₃C₂T_x composite demonstrated a short energy barrier for Li-ion diffusion, allowing the Li-rGO/Ti₃C₂T_x anode to maintain stable cycling for 1000 h at 10 mA cm⁻². The composite anode also showed exceptional resistance to corrosion from LiPSs. When coupled with an S/C cathode, LSBs employing the Li-rGO/Ti₃C₂T_x anode exhibited an initial capacity of 1070 mAh g⁻¹ at 0.2 C and sustained 409 mAh g⁻¹ after 300 cycles at 1 C, resulting in a capacity retention of 64.5%.

8.3.2. Perpendicular MXene-Li Anode

Compared to lamellar Ti₃C₂T_x, perpendicular Ti₃C₂T_x structures offer greater advantages in promoting Li-ion transfer. Using a straightforward rolling and cutting technique, perpendicular Ti₃C₂T_x-Li arrays with dual periodic interspaces were created. These structures significantly enhanced rapid Li⁺ transport and effectively minimized dendrite formation during Li stripping and plating processes [181]. During deep Li stripping, the perpendicular Ti₃C₂T_x arrays preserved their periodic interspaces. Upon Li plating, Li preferentially deposited within the interspaces of the MXene arrays, guided by the surface terminations of the MXene (Figure 21a). This mechanism effectively suppressed Li dendrite growth and accommodated the volume changes of Li (Figure 21b). In comparison, perpendicular rGO and Cu arrays were also developed, but Li managed to nucleate and grow on the top of these arrays rather than within the gaps (Figure 21c–e), highlighting the distinctive advantage of Ti₃C₂T_x arrays in preventing dendrite growth. Consequently, the perpendicular Ti₃C₂T_x-Li anode achieved an overpotential of 25 mV, a high capacity of 2056 mAh g⁻¹, and stable cycling for 1700 h (Figure 21f).

Another approach involved arranging Ti₃C₂T_x stacks perpendicularly and randomly dispersing them on a Li anode by pressing the stacks onto Li foils [272]. This configuration created ample interlayer space for Li storage, while the Ti₃C₂T_x stacks guided Li growth in the perpendicular direction. As a result, the structure maintained a less overpotential of 15 to 20 mV over 1125 cycles at 3 mA cm⁻². Additionally, in a Li/Li symmetric cell, the structure exhibited an overpotential of 120 mV after 1050 cycles at 10 mA cm⁻² with a capacity of 10 mAh cm⁻².

8.3.3. 3D MXene-Li Anode

The development of a 3D MXene network as a Li host effectively suppresses Li dendrite growth and minimizes volume changes during the Li stripping and plating activities. A 3D porous Ti₃C₂T_x/rGO aerogel (Figure 22a) was created through hydrothermal treatment of a mixed Ti₃C₂T_x and rGO solution [208]. In this structure, the Ti₃C₂T_x NSs promoted homogeneous Li nucleation, while rGO contributed to enhanced mechanical stability. The 3D conductive structure enabled rapid electron and Li-ion transport, leading to uniform Li deposition. Furthermore, the interconnected pores provided a stable framework for great Li loading and helped mitigate volume fluctuations. Consequently, the Ti₃C₂T_x/rGO aerogel exhibited remarkable cycling stability over 350 cycles and maintained a low overpotential of 42 mV at a current density of 10 mA cm⁻² (Figure 22b). Additionally, a Ti₃C₂T_x/graphene network was created by freeze-drying a mixed suspension of Ti₃C₂T_x and GO, followed by a spark reduction process to transform GO into graphene (Figure 22c). This framework possessed a SSA of 259 m² g⁻¹ with an interconnected porous architecture (Figure 22d) [273]. After infusion with molten Li (Figure 22c), the 3D Ti₃C₂T_x/graphene network achieved a great Li mass loading of approximately 92%. The resulting Ti₃C₂T_x/graphene-Li anode demonstrated exceptional cycling efficiency (~99%), an extended lifespan of 2700 h (Figure 22e), and outstanding stability over 230 cycles, even at a current density of 20 mA cm⁻².

A Ti₃C₂T_x@cellulose NFs paper was developed as a Li host using spin-steaming technology by assembling Ti₃C₂T_x dispersions surrounding trace amounts of cellulose NFs [182]. Initially, Ti₃C₂T_x NSs bonded with cellulose NFs through intermolecular hydrogen bonding, forming layered micelle structures. These micelles then curled layer by layer into spherical shapes via drum granulation, resulting in Ti₃C₂T_x@cellulose NFs microspheres distributed among the Ti₃C₂T_x NSs (Figure 22f). In the fabricated Ti₃C₂T_x@cellulose NFs paper, the Ti₃C₂T_x NSs enabled uniform Li nucleation, effectively preventing dendrite formation. At the same time, the Ti₃C₂T_x@cellulose NFs microspheres, with their expanded interspaces, enhanced reaction kinetics, supporting great Li stripping/plating capacities and excellent rate performance. Consequently, the Ti₃C₂T_x@cellulose-Li anode displayed a stable overpotential of 60 mV for more than 250 cycles at 1 mA cm⁻². Additionally, it exhibited exceptional cycling stability, maintaining an overpotential of 47 mV for over 1300 h at 0.5 mA cm⁻² (Figure 22g).

8.3.4. Alteration of MXene-Based Li Anode

Efficient Li nucleation is crucial for mitigating Li dendrite formation as it promotes uniform Li growth. Conductive substrates with dispersed metal atoms have been identified as effective nucleation agents [274]. This makes MXenes combined with metal atoms a promising strategy for achieving dendrite-free Li electrodes. For instance, etching the Ti₃AlC₂ MAX phase with molten ZnCl₂ salts immobilized Zn atoms at Ti sites in the resulting Ti₃C₂Cl_x MXene (Zn-MXene), which guided Li nucleation and growth [275]. In pure MXene layers, Li typically formed a flat layer across the surface. However, in Zn-MXene layers, the existence of Zn atoms enabled Li nucleation at lower plating levels due to the formation of a Zn-Li alloy phase. This process promoted vertical Li growth along the edges of Zn-MXene layers, resulting in the formation of Li spheres and a bowl-type Li structure devoid of dendrites. The Li-Zn-MXene electrode demonstrated exceptional performance, including a remarkably low overpotential of 11.3 mV, a cycle life of 1200 h at 1 mAh cm⁻², and a high stripping/plating capacity of 40 mAh cm⁻².

Beyond immobilizing metal atoms within MXenes, directly modifying the MXene surface with metal atoms can also effectively promote uniform Li growth. For instance, coating a Ti₃C₂T_x substrate with an amorphous liquid metal (GaInSnZn, with a melting point of 3 °C) served as nucleation sites, enabling homogeneous Li deposition [276]. In symmetric cells, the liquid metal-Ti₃C₂T_x/Li electrode exhibited stable cycling for over 300 h at 0.5 mA cm⁻² and a Li deposition capacity of 0.5 mAh cm⁻².

A 2D lamellar Ti₃C₂T_x has proven to be highly effective for constructing dendrite-free Li anodes. Advanced Ti₃C₂T_x/Li architectures, including lamellar/perpendicular Ti₃C₂T_x-Li, and 3D/modified Ti₃C₂T_x-Li configurations, have significantly alleviated the dendrite issue and enhanced electrochemical performance. However, most research has focused on Ti₃C₂T_x MXene. Future studies should investigate other MXene types, particularly those with great conductivity, low Li-ion diffusion energy barriers, and excellent stability. Additionally, chemical modification of MXenes holds promise for further improving Li nucleation efficiency. Developing MXene-Li superlattice structures, where MXene NSs and Li layers alternate at the atomic level, represents an exciting and valuable avenue for innovation.

9. Summary and Prospects

MXenes are highly promising materials for building high-performance LSBs due to their metallic conductivity, structural diversity, strong chemical adsorption of LiPSs, effective catalytic activity for LiPS conversion, and ability to enable homogeneous Li deposition. The metallic conductivity of MXenes improves S utilization, their diverse structures enable high S loading, and their robust chemical adsorption significantly mitigates the shuttle effect. Additionally, their catalytic activity enhances electrochemical kinetics. By leveraging these properties, MXenes used as S cathodes or as interlayers between the cathode and separator contribute to achieving great capacity, long cycling life, and excellent rate capability in LSBs. Further, the electrochemical behavior of MXenes can be enhanced through chemical modifications, structural engineering, and hybridization with complementary components such as C and TMOs. Furthermore, MXenes with advanced structures and tailored modifications hold promise as effective hosts for Li metal anodes, addressing the critical challenge of dendrite formation. Despite significant progress, the development of MXene-based LSBs is still in its infancy, requiring further research and innovation to fully unlock their potential.

A key challenge in utilizing MXenes for LSBs lies in developing a deeper understanding of their mechanisms for chemically adsorbing LiPSs and catalyzing their conversion. Although some studies have emphasized the critical influence of surface terminations on MXenes, the complexity of the intermediate species formed during the charge/discharge activities in LSBs complicates the full elucidation of their interactions with MXenes. Advancing fundamental research in this area remains both essential and challenging. To address this, a combination of computational and experimental methods is crucial for uncovering these mechanisms in greater detail.

Surface engineering and chemical modifications, such as tailoring terminations, creating vacancies, and introducing doped atoms, significantly impact the characteristics of MXenes, including their conductivity, adsorption capacity, catalytic activity, and Li affinity. These factors, in turn, have a profound impact on the characteristics of MXene-based LSBs. Despite their importance, relatively little attention has been given to optimizing MXenes for use as S hosts, interlayers between the cathode and separator, or Li hosts. Expanding research in this area is critical to fully harness the potential of MXenes. Insights from progress in other fields, particularly advancements in tailoring MXene terminations, vacancies, and doping, can serve as valuable references to accelerate this effort. Recently, significant strides have been made in controlling MXene terminations beyond the conventional -O, -OH, and -F groups. Exploring the effects of these novel terminations on MXene-based LSBs represents a promising and necessary direction for future research.

MXenes represents the largest family of 2D materials, with more than 100 stoichiometric phases theoretically identified. Moreover, only about 30 types have been successfully prepared, and research has mainly focused on Ti-based MXenes, particularly Ti₃C₂T_x. Since Ti₃C₂T_x is susceptible to oxidation when exposed to water and air, improving its stability through chemical modifications is essential. Moreover, the extensive variety within the MXene family, along with their customizable surface terminations, offers significant potential for the development of high-performance LSBs. The identification of new MXenes with boosted stability and electrochemical properties remains a hopeful research direction that will require substantial effort. DFT calculations are proving to be an effective tool to guide the design of MXene compositions and surface terminations.

For practical LSBs, it is crucial to address challenges related to both the S cathode and Li anode. In addition to these, factors such as S content, S loading, areal capacity, E/S ratio, and Li excess need to be optimized to accomplish great energy density and longer cycle life. Exploring the application of MXenes in pouch cells is especially valuable as it can offer important insights and speed up their practical adoption in LSBs.

In conclusion, MXenes, with their distinctive advantages and rapidly advancing research, show great potential for LSBs. Key future directions include gaining a deeper understanding of their mechanisms for adsorbing and catalyzing LiPSs, advancing chemical modifications and surface engineering to improve characteristics, and discovering new MXene species with enhanced stability and electrochemical properties. These advancements will unlock the full potential of MXenes and pave the way for the construction of high-performance LSBs with broad practical applications.