Dry Synthesis of Sulfur‐Terminated MXene as Multifunctional Catalyst for Stable Lithium–Sulfur Batteries

Abstract

A multifunctional catalyst with enhanced polysulfide adsorption, rapid lithium diffusion, and exceptional catalytic activity is crucial for accelerating redox kinetics and effectively suppressing the shuttle effect in lithium–sulfur (Li–S) batteries. However, developing an efficient synthesis method for such catalysts remains challenging. Here, a sustainable, sulfur‐terminated MXene is introduced via a completely dry molten salt process, which avoids the need for harsh acid treatment, byproduct removal, and extensive rinsing, typical in MXene syntheses. Theoretical calculations and electrochemical data confirm that this sulfur‐terminated MXene serves as a powerful multifunctional catalyst, promoting rapid lithium diffusion, effective polysulfide adsorption, and superior catalytic performance, making it highly suitable for advanced separators in Li–S batteries. As a result, Li–S cells incorporating sulfur‐terminated MXene separators demonstrate a high capacity of 665 mAh g⁻¹ after 500 cycles at 1 C, with a remarkably low‐capacity decay rate of 0.05% per cycle. This study underscores the potential of precise surface termination control in MXenes to drive further advancements in Li–S battery technology.

This study presents Ti₃C₂S₂ MXene synthesized via a dry molten salt method, offering enhanced polysulfide adsorption, fast lithium diffusion, and superior catalytic performance. Applied as a separator coating, it effectively inhibits the shuttle effect, enabling Li–S batteries to achieve high capacity, excellent rate performance, and improved cycle stability.

1. Introduction

Rechargeable lithium–sulfur (Li–S) batteries have attracted substantial attention as advanced energy storage solutions due to their low production cost, high theoretical specific capacity of 1675 mAh g⁻¹, and impressive energy density of 2600 Wh kg⁻¹.^{[ 1} , ² , ³ , ⁴ , ^{5 ]} However, their commercial viability is significantly hindered by critical challenges, including poor sulfur conductivity, substantial volume expansion, slow reaction kinetics, and the well‐known polysulfide shuttle effect.^{[ 6} , ^{7 ]} These issues result in rapid capacity fading, increased polarization, and limited cycling stability.^{[ 8 ]} To address these obstacles, extensive research has explored various approaches, such as sulfur‐host cathode design,^{[ 9} , ¹⁰ , ^{11 ]} lithium anode protection,^{[ 12} , ^{13 ]} electrolyte optimization,^{[ 14} , ¹⁵ , ^{16 ]} and functional interlayers/separators.^{[ 17} , ¹⁸ , ^{19 ]} Among these, the development of functional separators stands out as a key strategy for enhancing polysulfide conversion kinetics and mitigating the shuttle effect, offering a promising route to overcome current bottlenecks in Li–S battery technology.^{[ 18 , 20 ]}

MXenes, a new class of 2D materials derived from transition metal carbides and nitrides, have gained significant attention due to their excellent electrical conductivity and tunable surface chemistry, making them highly versatile candidates for diverse applications, including Li–S batteries.^{[ 21} , ²² , ²³ , ²⁴ , ^{25 ]} Traditionally, MXenes are synthesized by selectively etching the A‐layer from MAX precursors using highly corrosive hydrofluoric acid (HF) or in situ HF approaches, resulting in F‐terminated MXenes.^{[ 26} , ²⁷ , ^{28 ]} While these approaches are effective in producing MXenes, the reliance on HF raises significant environmental and safety concerns due to its toxicity and corrosiveness. Moreover, the resulting F‐terminated MXenes exhibit weak interactions with polysulfides, limiting their effectiveness in Li–S batteries.^{[ 29} , ^{30 ]}

To address these issues, Lewis‐acid molten salt synthesis has emerged as a greener and safer alternative, producing MXenes with Cl‐terminations (Ti₃C₂Cl₂, MX‐Cl).^{[ 23} , ³¹ , ³² , ^{33 ]} However, despite the reduced environmental risks, the synthesis process still involves multiple treatment steps, such as purification, rinsing, and drying, that limit scalability. Additionally, Cl‐terminated MXenes, like their F‐terminated counterparts, display weak polysulfide adsorption capabilities, further hindering their performance in Li–S batteries.^{[ 29} , ^{30 ]} In response to these limitations, researchers have explored various MXene heterostructures as advanced separators for Li–S batteries, such as Co₃Fe₇‐MXene,^{[ 30 ]} MXene/MoS₂/SnS@C,^{[ 34 ]} MXene@CoS₂,^{[ 35 ]} g‐C₃N₄@MXene,^{[ 36 ]} and Ti₃C₂T_x@CoSe₂.^{[ 37 ]} While these heterostructures have demonstrated improved performance, their complex structures might induce phase separation and interfacial resistance, potentially complicating their feasibility for practical applications.

Recent theoretical studies have suggested that MXenes with S‐ and O‐terminations can offer moderate polysulfide adsorption and enhanced electrocatalytic activity, making them more effective in addressing the polysulfide shuttle effect.^{[ 29 ]} Despite the promising advantages, the synthesis of sulfur‐terminated MXene (Ti₃C₂S₂, MX‐S) has remained a significant challenge, involving intricate processes that limit production scalability. To date, only Talapin et al. have successfully synthesized MX‐S using a molten salt method with sulfur substitution, followed by multi‐stepwise wet purification in an inert atmosphere.^{[ 23 ]} These limitations underscore the inevitable need for a simpler, more sustainable approach that can precisely control MXene surface terminations while being scalable for practical applications.

Here, we introduce a scalable eco‐friendly dry molten salt strategy to produce MX‐S, specifically designed for Li–S batteries. This approach eliminates the need for hazardous acid treatments and tedious purification steps, typically associated with MXene synthesis. It also minimizes the risk of introducing unwanted contaminants during processing, preserving the desirable surface terminations and properties of the material. ​Our theoretical and experimental analyses demonstrate that MX‐S functions as an efficient multifunctional catalyst, offering fast lithium‐ion diffusion, high polysulfide adsorption capacity, and superior conversion kinetics. As a result, Li–S cells incorporating MX‐S separators achieve a high capacity of 665 mAh g⁻¹ after 500 cycles at 1 C, with a notably low‐capacity decay rate of 0.050% per cycle.

2. Results and Discussion

MX‐S can serve as an ideal multifunctional catalyst for advanced separators in Li–S batteries, due to its moderate polysulfide adsorption strength, high electrocatalytic activity, and rapid Li⁺ ion diffusion (Figure 1a). To assess these properties, theoretical calculations using density functional theory (DFT) are conducted to evaluate the polysulfide anchoring effect, Li⁺ ion diffusivity, and catalytic activity of Li₂S decomposition for both MX‐Cl and MX‐S. Optimal adsorption configurations for polysulfides on MX‐Cl and MX‐S surfaces are presented in Figures S1 and S2 (Supporting Information), respectively. The anchoring effect of MXene is quantified by calculating the adsorption energies (E_ad) for polysulfides as follows:

$${{\mathrm{E}}_{\mathrm{a}}}_{\mathrm{d}}={\mathrm{E}}_{\mathrm{MX}}+{\mathrm{E}}_{\mathrm{PS}}-{{\mathrm{E}}_{\mathrm{MX}}}_{+\mathrm{PS}}$$ (1)

where E_MX, E_PS, and E_MX+PS are the energy of MXene, polysulfide, and the combined MXene and polysulfide system, respectively. A positive E_ad indicates favored adsorption. The calculated binding energies of S₈, Li₂S₈, Li₂S₆, Li₂S₄, Li₂S₂, and Li₂S on MX‐S are calculated to be 0.51, 1.33, 1.64, 2.35, 3.27, and 4.31 eV, respectively, which are much higher than those on MX‐Cl surface (0.41, 0.64, 0.72, 0.56, 1.12, and 1.11 eV) (Figure 1b). These results demonstrate that MX‐S has an exceptional adsorption capacity for polysulfide species, which is essential for mitigating the shuttle effect.

Figure 1.

Ti₃C₂S₂ MXene (MX‐S) as a multifunctional catalyst for advanced separators in Li–S batteries. a) Schematics showing the advantages of MX‐S. b) Theoretically calculated adsorption energies of polysulfides on MX‐Cl and MX‐S. c) (Left) electronic band structure and (right) integrated density of states of MX‐S. d) Energy barrier of Li⁺ ion diffusion on (002) facet of MX‐Cl and MX‐S. e) Decomposition energy of Li₂S on (002) facet of MX‐Cl and MX‐S.

Since Li₂S, the final discharge product of Li–S batteries, has low electronic conductivity, poor Li⁺ ion diffusivity, and high decomposition energy, it generally leads to high overpotentials and low‐rate capability.^{[ 38} , ^{39 ]} To address this, the decomposition kinetics of Li₂S and Li⁺ ion diffusion on the MXene surface are analyzed. The electronic band structures of MX‐S show metallic characteristics with a finite density of states at the Fermi level, suggesting its high electrical conductivity (Figure 1c). The Li⁺ ion diffusivity on MXene is studied by calculating energy barriers for Li⁺ ion diffusion (Figure S3, Supporting Information). The energy profiles show a lower diffusion barrier of 0.15 eV on MX‐S compared to 0.20 eV on MX‐Cl, suggesting faster Li⁺ ion diffusion on MX‐S (Figure 1d). Li₂S decomposition is further examined by considering its dissociation into a LiS cluster and a Li⁺ ion on the MXene surface (Figure S4, Supporting Information):

$$\mathrm{L}{\mathrm{i}}_2\mathrm{S}\to \mathrm{LiS}+\mathrm{L}{\mathrm{i}}^{+}+{\mathrm{e}}^{-}$$ (2)

MX‐Cl shows a high decomposition barrier of 1.71 eV (Figure 1e), likely due to weak interactions between Li₂S and the Ti₃C₂Cl₂ structure, limiting the bond breaking of Li–S. In contrast, MX‐S exhibits a much lower decomposition barrier at 0.57 eV, demonstrating its superior catalytic activity in facilitating Li₂S decomposition, thus improving electrode kinetics and performance in Li–S batteries.

To address the synthetic challenge of MX‐S, we have developed a fully dry synthesis approach, utilizing a modified molten salt method without the need for acid treatments or wet purification, thereby simplifying the process while enhancing scalability. In the initial step, Ti₃AlC₂ MAX precursor is etched by ZnCl₂ at 550 °C in a molten state to produce MX‐Cl (Figure 2a). During this process, the weakly bonded Al layer in the MAX precursor is selectively oxidized by the Lewis acidic Zn²⁺ cation and forms AlCl₃, which evaporates rapidly at the reaction temperature.^{[ 32 ]} Concurrently, Zn²⁺ is reduced to metallic Zn, while charge compensation is achieved through the participation of Cl⁻ anion, resulting in the formation of MX‐Cl.^{[ 31 ]} Excess ZnCl₂ and metallic Zn nanoparticles are then eliminated through thermal evaporation at 850 °C,^{[ 40 ]} leaving behind purified MX‐Cl. MX‐S is achieved through a surface‐termination substitution from Cl to S by heat treatment at 900 °C with Li₂S. This substitution is facilitated by the proximity of the reaction temperature to the melting point of Li₂S at 938 °C. The entire synthesis process, from MAX to MX‐Cl, and finally to MX‐S, is carried out under dry conditions, eliminating the need for harsh acid washing, byproduct removal, and extensive rinsing, typically required in conventional MXene synthesis. The key reactions involved in this process are as follows:

$$\mathrm{T}{\mathrm{i}}_3\mathrm{Al}{\mathrm{C}}_2+1.5\mathrm{ZnC}{\mathrm{l}}_2\to \mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2+\mathrm{AlC}{\mathrm{l}}_3+1.5\mathrm{Zn}\uparrow$$ (3)

$$\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2+\mathrm{ZnC}{\mathrm{l}}_2\to \mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2\mathrm{C}{\mathrm{l}}_2+\mathrm{Zn}\uparrow$$ (4)

$$\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2\mathrm{C}{\mathrm{l}}_2+{\mathrm{S}}^{2-}\to \mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{S}}_2+2\mathrm{C}{\mathrm{l}}^{-}$$ (5)

Figure 2.

Preparation and structural characteristics of MX‐S. a) Schematics showing the fully dry synthesis of MX‐S from Ti₃AlC₂ MAX precursor. b–d) SEM images of MAX precursor, MX‐Cl, and MX‐S, respectively. e–g) TEM image, HRTEM image, and SAED pattern of MX‐S, respectively.

The detailed morphological, microstructural, and elemental characteristics of MAX, MX‐Cl, and MX‐S are examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution TEM (HRTEM), and energy dispersive X‐ray spectroscopy (EDS). The MAX precursor exhibits a densely packed layered structure (Figure 2b). After etching the MAX precursor with molten ZnCl₂ for 3 h, the tightly stacked architecture is largely retained, with some layers removed (Figure S5, Supporting Information). When the exfoliation time is extended to 6 h, the product exhibits an exfoliated layer structure with a small layer distance. After 10 h of etching, a characteristic accordion‐like structure forms, confirming the successful synthesis of MX‐Cl (Figure 2c). Microstructural analysis of MX‐Cl reveals sheet‐like structures with a lattice spacing of 2.67 Å, corresponding to the (100) plane of Ti₃C₂Cl₂ (Figure S6, Supporting Information).^{[ 31} , ^{41 ]} The accordion structure of MX‐Cl remains unchanged after the sulfur substitution process, resulting in MX‐S (Figure 2d). This suggests the comparable porosity of both MX‐Cl and MX‐S with a specific surface area of ≈2 m² g⁻¹ (Figure S7, Supporting Information). The microstructural analysis of MX‐S reveals a multilayered sheet structure (Figure 2e). HRTEM image manifests the lattice spacings of 2.35 and 2.14 Å, which correspond to (105) and (107) crystal planes of Ti₃C₂S₂ (Figure 2f).^{[ 23 ]} The selected area electron diffraction (SAED) pattern presents four diffraction planes of (103), (105), (106), and (107) (Figure 2g), further supporting the multilayered structure of MX‐S.^{[ 23 ]} EDS elemental mapping shows clear changes in elemental composition from MAX to MX‐Cl, and MX‐S, highlighting the loss of Al and the uniform distribution of Cl and S in MX‐Cl and MX‐S, respectively (Figures S8–S10, Supporting Information). This confirms the successful synthesis of Cl‐ and S‐terminated MXenes. The elemental composition data are provided in Table S1 (Supporting Information).

The crystallinity of the samples is investigated by X‐ray diffraction (XRD) (Figure 3a). The MAX precursor shows a strong peak at 9.5°, corresponding to the (002) basal plane reflection, and peaks at 39° and 41.7°, representing the (104) and (105) planes of the hexagonal crystal structure of Ti₃AlC₂, consistent with the literature.^{[ 42} , ^{43 ]} For MX‐Cl, the sharp (002) peak shifts to a lower value of 7.9°, indicating an increase in the c‐lattice parameter due to the removal of the Al layers and the intercalation of Cl between the Ti₃C₂ layers.^{[ 32} , ^{44 ]} The broadening and shift of the (104) peak to 38.5° confirms the successful etching of the Al from the MAX phase and the formation of the Ti₃C₂Cl₂ MXene structure.^{[ 44 ]} No peaks of Zn metal are observed, indicating complete evaporation of Zn during the heat treatment at 850 °C. For MX‐S, the (002) peak further broadens and shifts to a lower value of 6°, indicating a larger interlayer spacing due to the larger size of substituted S atoms.^{[ 23 ]} Peaks observed at 33°, 34.7°, 37.9°, 40°, and 42.3° correspond to the (101), (103), (105), (106), and (107) planes of multilayer Ti₃C₂S₂ MXene.^{[ 23 ]}

Figure 3.

Phase structure and spectral analysis. a) XRD spectra. b) Raman spectra. c) XPS spectra. d–i) Deconvoluted XPS spectra of Ti2p, C1s, Cl2, S2p, O1s, and Li1s in MX‐S.

The vibrational modes of the samples are characterized by Raman spectroscopy (Figure 3b). In the Raman spectrum of the MAX precursor, major peaks are observed at 201 and 425 cm⁻¹, corresponding to the vibrations of Al atoms.^{[ 45} , ⁴⁶ , ^{47 ]} A peak at 272 cm⁻¹ is associated with the Ti─C vibrations, while peaks at 617 and 660 cm⁻¹ are related to the vibrations of C atoms within the Ti₃AlC₂ structure.^{[ 21} , ⁴⁵ , ^{46 ]} After Al etching, the disappearance of Al‐related peaks confirms the successful removal of Al atoms from the structure. A new peak emerging at 151 cm⁻¹ is attributed to Ti─Cl vibrations in MX‐Cl, together with a peak at 400 cm⁻¹ representing in‐plane vibrations of Ti─C and a peak at 630 cm⁻¹ corresponding to C vibrations in the Ti₃C₂Cl₂ structure.^{[ 23 ]} MX‐S exhibits similarity with distinct differences compared to MX‐Cl due to the replacement of Cl with S. A peak at 156 cm⁻¹ is attributed to Ti─S vibrations, while peaks at 390 and 600 cm⁻¹ are related to C vibrations influenced by S terminations.^{[ 23 ]} The broadening of the carbon D and G bands from the MAX phase to MX‐Cl, and MX‐S reflects a decrease in long‐range order and an increase in structural disorder, consistent with the etching process and the substitution of S terminations.

The chemical composition and valance states of the samples are characterized by X‐ray photoelectron spectroscopy (XPS) (Figure 3c). The MAX precursor is composed of four elements of Ti, C, Al, and O. After Al etching and S substitution, the detection of Cl and S confirms the successful synthesis of Cl‐ and S‐terminated MXenes. The high‐resolution Ti2p spectrum of MX‐S can be interpreted into Ti─C (455.3/461.3 eV), Ti─Cl (457/463 eV), Ti─S (458.5/464.2 eV), and Ti─O (459.7/465.1 eV) (Figure 3d). The existence of Ti─C bonds is attributed to the core TiC₆ framework of Ti₃C₂T_x MXene, as further supported by the C─Ti bond at 281.3 eV in the C1s spectrum (Figure 3e).^{[ 30 ]} The Ti─Cl bond is derived from the Cl termination in MX‐Cl, formed during the molten salt etching process, as indicated by the Cl─Ti bond at 198.7/200.3 eV (Figure 3f).^{[ 32} , ^{37 ]} In the S2p spectrum, peaks at 160.4/161 and 161.7/162.9 eV correspond to Ti─S and C─Ti─S bonds, confirming the successful preparation of S terminations in MX‐S (Figure 3g).^{[ 48} , ^{49 ]} The Ti─O bond likely results from inherent impurity in the MAX precursor. The O1s spectrum shows peaks at 529.2, 530.9, and 532.3 eV, corresponding to Ti─O, C─Ti─O, and C─O bonds, respectively (Figure 3h).^{[ 30} , ^{37 ]} In addition, a small Li1s peak at 55.3 eV is likely derived from LiCl, a byproduct of the S substitution process using Li₂S. In general, all analyses so far have confirmed that multilayer MXene with S terminations is successfully prepared using an eco‐friendly dry molten salt synthesis and S substitution, offering the potential for enhanced performance in Li–S batteries.

To intuitively evaluate the adsorption capabilities of MXene toward polysulfides, a visual adsorption test is conducted by adding equal amounts of different MXenes into a 2.5 mm Li₂S₆ solution. After aging overnight, the solution containing MX‐S appeared almost colorless, while the solution with MX‐Cl retained a slight yellow hue (Figure 4a). This result suggests that MX‐S exhibits superior polysulfide adsorption capability, which aligns with the DFT calculations presented in Figure 1b. The corresponding UV–vis absorption spectra further support this finding, as the solution containing MX‐S exhibits the lowest absorbance (Figure 4b), implying the lowest concentration of polysulfides in the solution. This suggests that MX‐S can effectively suppress the diffusion of soluble polysulfides in the electrolyte. To further investigate the conversion kinetics of polysulfides on the MXene surfaces, symmetric cells using MXenes as electrodes are assembled with electrolytes, both with and without Li₂S₆. Cyclic voltammetry (CV) profiles in a voltage window of −1.0 to 1.0 V at 1 mV s⁻¹ show distinct current responses for the cells with Li₂S₆, in contrast to the nearly zero current for the cell without Li₂S₆ (Figure 4c). Notably, the MX‐S symmetric cell with Li₂S₆ displays the highest current response, with four distinct redox peaks at −0.41, −0.13, 0.13, and 0.41 V. The reduction peaks at −0.13 and −0.41 V are attributed to the electrochemical reduction of S₈ to Li₂S₆ and Li₂S₆ to Li₂S, respectively.^{[ 30} , ^{37 ]} Likewise, the oxidation peaks at 0.13 and 0.41 V are assigned to the reverse conversion of Li₂S to Li₂S₆ and Li₂S₆ to S₈, respectively. Even at a high scan rate of 10 mV s⁻¹, the redox peaks are still evident (Figure S11, Supporting Information), highlighting the critical role of S terminations in facilitating the polysulfide conversion kinetics. The catalytic function of MX‐S for the polysulfide conversion is further validated through potentiostatic discharge measurements. Specifically, MX‐S exhibits a higher discharging current at 1.75 mA and a larger Li₂S precipitation capacity of 178 mAh g_S ⁻¹, compared to MX‐Cl, which shows a discharge current of 1.25 mA and a capacity of 129 mAh g_S ⁻¹ (Figure 4d,e). These results suggest that MX‐S can substantially reduce the nucleation barrier of Li₂S and accelerate its formation kinetics. Altogether, these findings confirm that MX‐S can serve as an advanced separator, enhancing the polysulfide conversion reactions in Li–S batteries and improving overall battery performance.

Figure 4.

Polysulfide adsorption and redox kinetics analysis. a) Digital photo of visualized Li₂S₆ adsorption tests. b) UV–vis absorbance spectra of Li₂S₆ solution after adsorption test. c) CV profiles of symmetric cells with and without the Li₂S₆ electrolyte at a scan rate of 1 mV s⁻¹. Potentiostatic discharge curves of d) MX‐Cl and e) MX‐S for evaluating the Li₂S nucleation. f) Comparative CV profiles of Li–S cells using PP/MX‐Cl and PP/MX‐S separators at 0.1 mV s⁻¹. g) CV profiles of a representative Li–S cell using the PP/MX‐S separator at different scan rates. h) Plot of the reduction peak R2 versus the square root of scan rates showing Li⁺ diffusion coefficients (D_Li) for the PP/MX‐Cl and PP/MX‐S cells. i) Tafel plots calculated for the reduction peak R2.

The flexibility and mechanical stability of the modified separator, produced by casting MXenes on commercial Celgard 2400 (PP) films, are thoroughly investigated. The cast MXene layer displays structural uniformity with a thickness of ≈15 µm (Figure S12, Supporting Information). This thickness was carefully optimized to balance mechanical stability, polysulfide management, and effective ion transport. A thinner coating may provide inadequate performance due to its limited ability to effectively block polysulfide diffusion and suppress the shuttle effect. Conversely, an excessively thick coating could lead to large polarization effects, restricting lithium‐ion diffusion, increasing internal resistance, and reducing the rate capability.^{[ 18 ]} The resulting MX‐S separator (PP/MX‐S) shows excellent adhesion and robust mechanical strength withstanding bending‐induced stress without any signs of peeling or delamination (Figure S13, Supporting Information). Wettability is a critical property that influences sulfur utilization and Li⁺ ion transfer in battery separators. The unmodified PP separator shows a contact angle of 36°, indicating poor electrolyte affinity. In contrast, the PP/MX‐S separator exhibits outstanding wettability with a contact angle of 0° (Figure S14, Supporting Information). This remarkable improvement is attributed to the polar sulfur terminations of MX‐S, which significantly enhance its affinity for the electrolyte, promoting faster Li⁺ ion diffusion and improved electrochemical performance.^{[ 50 ]}

CV profiles of Li–S cells using MX‐Cl and MX‐S modified separators are performed in the voltage range of 1.7–2.7 V at a scan rate of 0.1 mV s⁻¹ (Figure 4f). The cell with PP/MX‐S separator displays two distinct reduction peaks: R1 at 2.33 V, corresponding to the reduction reactions of S₈ to long‐chain polysulfides Li₂S_n (4 ≤ n ≤ 8), and R2 at 2.00 V, associated with the subsequence reduction of short‐chain polysulfides Li₂S_n (1 ≤ n ≤ 4) to Li₂S.^{[ 51} , ^{52 ]} In addition, two oxidation peaks, O1 at 2.35 V and O2 at 2.43 V, are attributed to the reversible oxidation process from Li₂S to Li₂S_n (4 ≤ n ≤ 8), and finally to S₈.^{[ 53} , ^{54 ]} Notably, the PP/MX‐S separator exhibits a larger current response at these redox peaks compared to the PP/MX‐Cl, highlighting superior reaction kinetics for polysulfide conversion. Moreover, the reduction peaks in the PP/MX‐S are shifted toward more positive voltages than those in the PP/MX‐Cl, indicating smaller potential polarization and enhanced reversibility.^{[ 35} , ^{51 ]} To further assess the impact of the modified separators on the redox kinetics and Li⁺ ion diffusion, CV measurements are conducted at increasing scan rates ranging from 0.1 to 0.7 mV s⁻¹ (Figure 4g; Figure S15, Supporting Information). Using the Randles–Sevcik equation,^{[ 55} , ^{56 ]} the Li⁺ ion diffusion coefficient (D_Li) can be calculated from the slope of the peak current (I_p) versus the square root of the scan rate (v):

$${\mathrm{I}}_{\mathrm{p}}=2.69\times {10}^5{\mathrm{n}}^{1.5}\mathrm{A}{{\mathrm{D}}_{\mathrm{Li}}}^{0.5}{\mathrm{C}}_{\mathrm{Li}}{v}^{0.5}$$ (6)

where n, A, and C_Li are the number of electrons transferred, electrode area, and Li⁺ ion concentration, respectively. The higher D_Li value of 2.03 × 10⁻¹⁰ cm² s⁻¹ for the PP/MX‐S cell suggests enhanced reaction kinetics, compared to the PP/MX‐Cl cell with a smaller D_Li value of 1.63 × 10⁻¹⁰ cm² s⁻¹ (Figure 4h). The polysulfide conversion kinetics are further examined using Tafel plots for the Li₂S_n to Li₂S conversion at the R2 reduction peak. The PP/MX‐S cell shows lower Tafel slopes of 155 and 132 mV dec⁻¹, compared to 240 and 151 mV dec⁻¹ for the PP/MX‐Cl cell (Figure 4i). These lower Tafel slopes confirm that MX‐S significantly enhances the polysulfide conversion kinetics, facilitating faster and more efficient electrochemical reactions.

To demonstrate the advantage of MX‐S in improving electrochemical properties, coin‐type Li–S batteries are assembled using S‐loaded Ketjen black carbon as the cathode (with ≈1.2 mg cm⁻² sulfur loading and ≈61.5 wt.% sulfur content, Figure S16, Supporting Information), a Li chip as the anode, and the PP/MX‐S separator. The sulfur reduction reactions during discharge occur in four distinct stages (Figure 5a).^{[ 8 ]} The process begins at ≈2.3 V, where S₈ is reduced to Li₂S₈, forming the first voltage plateau. This is followed by a voltage drop from 2.3 to 2.05 V, attributed to the reduction of Li₂S₈ into Li₂S₄. Subsequently, a long plateau observed at 2.05 V occurs as soluble Li₂S₄ is reduced further to form insoluble Li₂S₂ and Li₂S. Finally, the voltage drops again from 2.05 to 1.7 V, corresponding to the solid‐solid transformation of Li₂S₂ into Li₂S. The cell using the PP/MX‐S separator delivers a high capacity of 1215 mAh g⁻¹ at 0.2 C, which surpasses the capacities of cells using the PP and PP/MX‐Cl separators (1040 and 1105 mAh g⁻¹, respectively). The PP/MX‐S cell exhibits the lowest polarization voltage of ≈0.2 V, indicating the most efficient redox kinetics for polysulfide conversion kinetics compared to the cells with PP and PP/MX‐Cl separators (≈0.21 V for both). In terms of rate capability, the Li–S cell using the PP separator shows relatively poor performance, delivering capabilities of 1040, 881, 669, and 479 mA h g⁻¹ at 0.2, 0.5, 1, and 2 C, respectively (Figure 5b; Figure S17, Supporting Information). By contrast, the PP/MX‐Cl cell achieves higher capacities of 1105, 956, 802, and 632 mA h g⁻¹ at the same rates. The PP/MX‐S cell outperforms both, delivering the highest rate capacities of 1215, 1117, 995, and 878 mA h g⁻¹. This superior rate performance is attributed to the MX‐S's ability to accelerate polysulfide conversion kinetics. Upon reverting to 0.2 C, the PP/MX‐S cell retains the highest specific capacity of 1177 mAh g⁻¹, further proving its exceptional rate performance. Even at a high rate of 2 C, the charge‐discharge curves retained their characteristic plateaus, indicating sustained rapid polysulfide conversion reactions (Figure 5c). Nyquist plots from electrochemical impedance spectroscopy show the PP/MX‐S cell has the lowest charge‐transfer resistance of 27.5 Ω, compared to 35.5 Ω for the PP/MX‐Cl cell and 40.7 Ω for the PP cell, indicating enhanced Li⁺ ion diffusion for superior electrochemical performance (Figure 5d). Additionally, cycling stability tests at 0.2 C over 200 cycles revealed that the PP/MX‐S cell retains a high discharge capacity of 960 mAh g⁻¹, with 77.5% capacity retention. This is significantly better than the cells with PP and PP/MX‐Cl separators, which retain 362 mAh g⁻¹ (37%) and 614 mAh g⁻¹ (53%), respectively (Figure 5e). Post‐cycling SEM analysis of the Li anode shows that the PP and PP/MX‐Cl cells develop rough, corroded surfaces with dendrite formation (Figure S18, Supporting Information). This suggests significant degradation caused by inadequate polysulfide management at the separators. In contrast, the Li anode in the PP/MX‐S cell displays a smoother surface with minimal corrosion. This improved performance is attributed to the strong polysulfide anchoring capability of MX‐S and its effective suppression of the shuttle effect. By serving as a robust physical and chemical barrier, the MX‐S separator significantly reduces polysulfide migration and its subsequent deposition on the anode. These results highlight the multifunctional role of MX‐S in alleviating degradation mechanisms and preserving the structural and electrochemical integrity of Li–S cells. Furthermore, long‐term cycling tests at a high rate of 1 C demonstrate that even after 500 cycles, the PP/MX‐S cell maintains a high reversible capacity of 665 mAh g⁻¹, with a low‐capacity decay rate of 0.050% per cycle (Figure 5f). This decay rate is smaller than that of the PP/MX‐Cl cell (0.055% per cycle) and more than 2.5 times lower than that of the PP cell (0.135% per cycle). Moreover, the PP/MX‐S cell demonstrates a high energy density of 230 Wh kg⁻¹ at 0.2 C, with a superior retention of 68.5% as the C‐rate increases to 2 C (Figure S19, Supporting Information). This retention is significantly higher than that of the PP cell (44%) and PP/MX‐Cl (52.6%). These results indicate that the PP/MX‐S cell offers enhanced electrochemical properties, comparable to other reported modified separators (Table S2, Supporting Information) and MXene‐based materials (Table S3, Supporting Information).^{[ 25} , ⁴³ , ⁵² , ⁵⁴ , ^{57 ]} Overall, these results confirm the robust anchoring capability and exceptional catalytic activity of the S‐terminated MXene, which effectively mitigates the shuttle effect and enhances the reaction kinetics of polysulfide conversion, enabling long cycling life for Li–S batteries.

Figure 5.

Electrochemical performance of Li‐S cells using different separators. a) Comparison of typical charge–discharge profiles of various cells at 0.2 C. b) Rate capabilities of the Li‐S cells at various current densities. c) Charge‐discharge profiles of the cell using PP/MX‐S separator at various current rates. d) Nyquist plots of the Li‐S cells. e,f) Long‐term cycling stability of Li–S cells at 0.2 and 1 C, respectively.

3. Conclusion

In summary, sulfur‐terminated MXene was successfully synthesized through a facile and eco‐friendly dry molten salt method with sulfur substitution. This innovative approach bypasses the need for harsh acid treatments and intricate procedures typically required in current MXene synthesis methods. The MX‐S modified separator efficiently acts as a barrier, significantly reducing the shuttle effect while accelerating polysulfide conversion. Extensive electrochemical testing and theoretical calculations demonstrate that MX‐S exhibits exceptional adsorptive capacity and remarkable catalytic activity toward polysulfides. Consequently, Li–S cells equipped with the MX‐S separator achieve a high capacity of 665 mAh g⁻¹ after 500 cycles at 1 C, with an impressively low‐capacity decay rate of 0.050% per cycle. Although the MXene synthesis process is still in its early stages, advancements in recycling salt, scaling up batch sizes, and implementing continuous synthesis methods hold significant potential to improve cost‐effectiveness and scalability. This research not only demonstrates the feasibility of utilizing functionalized MXene in Li–S batteries but also highlights the long‐term benefits of these materials for future applications in sustainable energy technologies.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

D.V.L. designed the entire set of experiments, analyzed the data, and wrote the draft of the paper. V.H.N. and H.J. purchased chemicals and tools, characterized the materials, and revised the manuscript. D.T.D., S.A.S., J.H, W.O., S.M.L. characterized the materials and revised the manuscript. I.K.O. supervised the research at all stages and wrote the paper. All authors discussed the results and commented on the paper.

Supporting information

Supporting Information

Lam D. V., Nguyen V. H., Yoo H., Dung D. T., Syed S. A., Ha J., Oh W., Lee S.‐M., Oh I.‐K., Dry Synthesis of Sulfur‐Terminated MXene as Multifunctional Catalyst for Stable Lithium–Sulfur Batteries. Small 2025, 21, 2411668. 10.1002/smll.202411668

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.