# MoWS2 Nanosheet Composite with MXene as Lithium-Sulfur Battery Cathode Material

Authors: Wangjun Feng; Zhiqiang Zhao; Ziru Lei; Li Zhang
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

Due to their superior theoretical specific capacity and energy density, lithium-sulfur (L‒S) batteries are gaining popularity in order to achieve the growing terms for more power generation. However, drawbacks such as low electrical conductivity of the active ingredient sulfur, severe volume expansion and shuttle effect of polysulfides, rapidly decaying battery capacity, and short battery life have hampered their development. A MoWS2@MXene@CNT composite material is used as the main cathode material for L-S batteries in this study. MoWS2 can improve the electrochemical reaction rate by accelerating polysulfide conversion, whereas MXene can suppress electrode volume expansion. Furthermore, the addition of carbon nanotubes (CNT) with high electrical conductivity improves the rate of the electrochemical reaction. Therefore, the MoWS2@MXene@CNT composites have good capacity and versatility as cathode materials and enhance the behavior of L-S batteries.


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## Body

## 1. Introduction

Owing to energy shortages and environmental degradation, the demand for sustainable high-performance electronic devices, such as smartphones and electric vehicles, is increasing [1, 2]. However, commonly used energy storage devices, for instance, lithium-ion batteries, are no longer able to fulfill the growing energy needs of consumers [3–5]. Moreover, lithium-ion batteries for daily use have several disadvantages, such as environmental hazards, high manufacturing costs, and insufficient theoretical capacity [6, 7]. Thus, the exploration of a new generation of energy storage devices is ongoing. In recent years, the development of lithium-sulfur (L-S) batteries has rapidly progressed, positioning them as interchangeable energy materials owing to their environmentally friendliness and inexpensive raw materials [8–10]. However, L-S batteries have disadvantages, such as low electrical conductivity of the active material, shuttle effect of the intermediate product polysulfide, and partial conversion of Li2Sn (4 ≤ n ≤ 8) into the last discharge products, thereby limiting their commercialization [11–13].To overcome these issues, researchers have designed composite materials that can rapidly convert polysulfides and retard the shuttle effect [14, 15]. For example, carbon nanotubes [16], porous carbon [17], and carbon cloths [18] can be used as physical frameworks to accelerate the reaction kinetics and thus mitigate the shuttle effects. Transition metal oxides [19], sulfides [20], and carbides [21] can be used to accelerate the conversion of polysulfides. However, the force between polar materials and polar polysulfides is important in inhibiting the shuttle effect [22–24].Herein, we synthesized MoWS2@MXene@CNT composites. We selected the molybdenum-based metal sulfide MoWS2 as the cathode material in this study [25, 26] because the strong interactions between the polar material MoWS2 and polar polysulfide can significantly suppress the shuttle effect. Because of their excellent electrical conductivity, carbon nanotubes (CNT) were chosen as the carbon material, which aids in accelerating the conversion of polysulfide [27]. Furthermore, the layered structure of the transition metal carbide MXene [28, 29] can limit volume expansion [30, 31] when singlet sulfur is used as the electrode.


## 2. Materials and Methods

### 2.1. Synthesis of MoWS2

First, Na2MoO4·2H2O and Na2WO4·2H2O in a molar mass ratio of 4 : 1 were dissolved in approximately 40 mL of deionized water. To this mixture, CH4N2S and C2H2O4·2H2O in a molar mass ratio of 5 : 1 were subsequently added. As the precursor solution transformed into a clear liquid, it was transferred to a Teflon reactor at 200°C for 24 hours. The contents were then repeatedly cleaned with distilled water before being vacuum dried.


### 2.2. Synthesis of MoWS2@CNT

MoWS2@CNT was created using a hydrothermal method as follows: first, 60 ml of deionized water was mixed with MoWS2 and CNT in a mass ratio of 1 : 1. Following ultrasonication, the mixture was transferred to a Teflon reactor and the hydrothermal reaction was allowed to run for 12 hours at 200°C. Finally, the contents were cleaned with distilled water before being vacuum dried.


### 2.3. Synthesis of MXene and MoWS2@MXene@CNT

First, 20 mL of 40% HF was added to a 100 mL Teflon reactor, followed by 10 minutes of slowly adding 1 g of Ti3AlC2. The mixture then continuously reacted at a rate of approximately 500 rpm at 40°C for 8 hours to produce MXene. Following etching, the contents were repeatedly cleaned with distilled water and ethanol to remove impurities until a pH of greater than 6 was achieved and then vacuum dried.The MoWS2@MXene@CNT composites were similarly synthesized. Specifically, the added CNT was converted into the MXene@CNT complex. The complete procedure is shown in Figure 1.Figure 1
Preparation flowchart of MoWS2@MXene@CNT.


### 2.4. Synthesis of MoWS2@CNT/S and MoWS2@MXene@CNT/S

Grinding for 10 minutes combined the sulfur powder with the MoWS2@CNT and MoWS2@MXene@CNT powders in a mass ratio of 7 : 3. For 20 hours, the reaction occurred continuously in a Teflon-lined reactor containing argon gas at a temperature of 155°C. Finally, the compound materials MoWS2@CNT/S and MoWS2@MXene@CNT/S were obtained.


### 2.5. Materials Characterization

X-ray diffraction (XRD, Bruker D8 Advance, Cu-K radiation) was utilized to ascertain the crystallinity of the samples in the 5°–90° test range. Scanning electron microscopy was applied to examine the dimensions and shape of the samples (SEM, JSM-6700F). The internal structure and element distribution of the material were evaluated utilizing transmission electron microscopy (TEM, JEM-2100F). Multifunctional X-ray photoelectron spectroscopy was used to determine the composition and chemical state of the elements in the materials (XPS, SCALAB250Xi). The sulfur content in the materials was proven using thermogravimetric analysis (TGA) (Perkin Elmer TG47).


### 2.6. Electrochemical Test

The preprepared electrode sheet was cut into 0.6 cm diameter disks with an average surface sulfur loading of 1.5 mg/cm2. The average thickness of the cathode was 0.1 nm. The components were then gathered, along with the Celgard 2400 septum, unoxidized lithium sheet cathode, Celgard 2025 button cell cathode housing, and lithium-sulfur cell electrolysis. The rate and long cycle assays were conducted on an electrochemical workstation using just a cell test system (CT2001A) with a test voltage range of 1.7–2.8 V. The tests included cyclic voltammetry (CV) and electrochemical impedance spectroscopy (Corrtest 350). Electrochemical impedance spectroscopy (EIS) was tested with a frequency band of 0.01–100 kHz and a voltage amplitude of 5 mV.


## 2.1. Synthesis of MoWS2

First, Na2MoO4·2H2O and Na2WO4·2H2O in a molar mass ratio of 4 : 1 were dissolved in approximately 40 mL of deionized water. To this mixture, CH4N2S and C2H2O4·2H2O in a molar mass ratio of 5 : 1 were subsequently added. As the precursor solution transformed into a clear liquid, it was transferred to a Teflon reactor at 200°C for 24 hours. The contents were then repeatedly cleaned with distilled water before being vacuum dried.


## 2.2. Synthesis of MoWS2@CNT

MoWS2@CNT was created using a hydrothermal method as follows: first, 60 ml of deionized water was mixed with MoWS2 and CNT in a mass ratio of 1 : 1. Following ultrasonication, the mixture was transferred to a Teflon reactor and the hydrothermal reaction was allowed to run for 12 hours at 200°C. Finally, the contents were cleaned with distilled water before being vacuum dried.


## 2.3. Synthesis of MXene and MoWS2@MXene@CNT

First, 20 mL of 40% HF was added to a 100 mL Teflon reactor, followed by 10 minutes of slowly adding 1 g of Ti3AlC2. The mixture then continuously reacted at a rate of approximately 500 rpm at 40°C for 8 hours to produce MXene. Following etching, the contents were repeatedly cleaned with distilled water and ethanol to remove impurities until a pH of greater than 6 was achieved and then vacuum dried.The MoWS2@MXene@CNT composites were similarly synthesized. Specifically, the added CNT was converted into the MXene@CNT complex. The complete procedure is shown in Figure 1.Figure 1
Preparation flowchart of MoWS2@MXene@CNT.


## 2.4. Synthesis of MoWS2@CNT/S and MoWS2@MXene@CNT/S

Grinding for 10 minutes combined the sulfur powder with the MoWS2@CNT and MoWS2@MXene@CNT powders in a mass ratio of 7 : 3. For 20 hours, the reaction occurred continuously in a Teflon-lined reactor containing argon gas at a temperature of 155°C. Finally, the compound materials MoWS2@CNT/S and MoWS2@MXene@CNT/S were obtained.


## 2.5. Materials Characterization

X-ray diffraction (XRD, Bruker D8 Advance, Cu-K radiation) was utilized to ascertain the crystallinity of the samples in the 5°–90° test range. Scanning electron microscopy was applied to examine the dimensions and shape of the samples (SEM, JSM-6700F). The internal structure and element distribution of the material were evaluated utilizing transmission electron microscopy (TEM, JEM-2100F). Multifunctional X-ray photoelectron spectroscopy was used to determine the composition and chemical state of the elements in the materials (XPS, SCALAB250Xi). The sulfur content in the materials was proven using thermogravimetric analysis (TGA) (Perkin Elmer TG47).


## 2.6. Electrochemical Test

The preprepared electrode sheet was cut into 0.6 cm diameter disks with an average surface sulfur loading of 1.5 mg/cm2. The average thickness of the cathode was 0.1 nm. The components were then gathered, along with the Celgard 2400 septum, unoxidized lithium sheet cathode, Celgard 2025 button cell cathode housing, and lithium-sulfur cell electrolysis. The rate and long cycle assays were conducted on an electrochemical workstation using just a cell test system (CT2001A) with a test voltage range of 1.7–2.8 V. The tests included cyclic voltammetry (CV) and electrochemical impedance spectroscopy (Corrtest 350). Electrochemical impedance spectroscopy (EIS) was tested with a frequency band of 0.01–100 kHz and a voltage amplitude of 5 mV.


## 3. Results and Discussion

The preparation flowchart of MoWS2@MXene@CNT composite is depicted in Figure 1. In Figure 2(a), the main characteristic peaks of MXene and MXene@CNT are located at 8.7°, 18.3°, 27.4°, 35.2°, and 60.6°, which correspond to the (002), (004), (006), (008), and (110) crystal planes of MXene [32]. Those of MXene@CNT composites are approximately at 26°, which corresponds to the carbon nanotube’s (002) crystal plane. Furthermore, Figure 2(a) shows that the characteristic peaks of MXene are sharp, indicating that MXene has excellent crystallinity. The XRD patterns of MoWS2, MoWS2@CNT, and MoWS2@MXene@CNT composites are shown in Figure 2(b). A diffraction peak at 14.1° can be seen in the figure. The shift of the peak at the (002) crystal plane indicates the result of increased layer spacing when compared to the peaks of MoS2 and WS2in the literature (JCPDS card numbers: 75-1539 and 08-0237) [25].Figure 2
(a) XRD patterns of MXene (Ti3C2Tx) and MXene@CNT; (b) XRD patterns of MoWS2, MoWS2@CNT, and MoWS2@MXene@CNT; (c) TGA curves of MoWS2@MXene@CNT/S and MoWS2@CNT/S.
(a)(b)(c)Both the MoWS2@CNT and MoWS2@MXene@CNT composites show a minimal change because their overall crystallinity was inferior to that of the original MoWS2 crystals; moreover, the characteristic peaks of MoWS2 and the standard characteristic peaks of CNT dominate these two samples, which could better accelerate the reaction kinetics. MoWS2 composites show obvious sharp (002) diffraction peaks, indicating that the hydrothermally prepared samples possess a high level of crystallinity. Meanwhile, for the composites, the evident diffraction peaks similarly indicate a higher crystallinity for materials prepared by the hydrothermal method, with standard cards at 14.38° and 14.32°, corresponding to the (002) crystal plane of MoS2 and WS2, respectively. By comparison, the diffraction peaks of the synthetic MoWS2 (002) are smaller than the fixed peaks, indicating the presence of strain, stress, and a large layer spacing between layers in the structure [26]. In Figure 2(b), the characteristic peak of MXene is significantly lower, indicating that MoWS2 is successfully modified on the Ti3C2Tx surface; furthermore, the peak of (002) shifts to the left, indicating increased layer spacing.TGA was performed on the composites, and the TGA curves of the MoWS2@CNT/S and MoWS2 @MXene@CNT/S composites are presented in Figure 2(c). Meanwhile, the MoWS2@CNT/S and MoWS2 @MXene@CNT/S materials experience rapid mass loss in the 150–300°C range; this mass loss is primarily attributed to sulfur sublimation in the composite. The presence of sulfur is confirmed by the presence of a sublimed sulfur diffraction pattern in the material. The mass loss in MoWS2@CNT/S and MoWS2@MXene@CNT/S is 72.63 and 68.73 wt.% sulfur mass fractions, respectively, as shown in Figure 2(c).After etching of the MAX phase precursor Ti3AlC2, the multilayer Ti3C2Tx obtained is shown in Figures 3(a)–3(c). The figures show a sparse accordion structure. In Figure 3(d), for the prepared MXene@CNT composites, long strips of CNT are densely compounded on MXene.Figure 3
SEM images of (a), (b), and (c) MXene, (d) MXene@CNT, (e), (f), and (g) MoWS2, (h) WS2@CNT, and (i) MoWS2@MXene@CNT.
(a)(b)(c)(d)(e)(f)(g)(h)(i)In the SEM images of MoWS2 and MoWS2@CNT shown in Figures 3(e), 3(g), and 3(h), the hydrothermally synthesized MoWS2 has a three-dimensional nano-microflower structure and is intertwined in long strips of carbon nanotubes that can provide a good channel for ion transport [25].For the MoWS2@MXene@CNT composites (Figure 3(i)), the hinged CNT and the nano-microflower MoWS2 are evidently interwoven and born together, and the CNT is densely grown on the surface of MXene. This forms an overall structure with a large and stable specific surface area, which indicates that MoWS2@MXene@CNT was successfully prepared. These morphological and surface analyses confirm that MoWS2@MXene@CNT composites can promote rapid insertion/deinsertion of ions between flowers.The structure and morphology of the MoWS2@MXene@CNT material were studied using TEM. Many long strips of carbon nanotubes are interwoven with black MoWS2 particles to form a hinge structure (Figure 4(a)) that grow over the MXene to form the MoWS2@MXene@CNT composite material (Figure 4(b)). The (002) lattice plane of CNT is distinctly visible in Figure 4(c).Figure 4
(a) and (b) TEM and (c) HRTEM images of MoWS2@MXene@CNT.
(a)(b)(c)Figures5(a) and 5(b) similarly exhibit that the lamellar MXene is interspersed with hinge-like carbon nanotubes and black MoWS2 nanoparticles. Figure 5(c) shows an evident lattice diffraction ring, indicating the good crystallinity of MoWS2@MXene@CNT, which correlates directly to the spacing observed in the XRD plots, and HRTEM analysis confirms the XRD results. All of these results show that MoWS2@MXene@CNT composites can be successfully prepared and have improved electrochemical activity due to their unique structure [25]. The elemental mapping images in Figure 5(d) obtained through the EDX spectroscopy confirm that Ti, C, Mo, S, and W have indeed been successfully coated onto MoWS2@MXene@CNT.Figure 5
(a) and (b) TEM image of MoWS2@MXene@CNT, (c) SAED pattern of MoWS2@MXene@CNT, and (d) mapping of Ti, C, Mo, S, and W of MoWS2@MXene@CNT.The sample was analyzed by XPS, and the results are shown in Figure6. The presence of Ti, C, S, Mo, and W in the composites is confirmed by the XPS data.Figure 6
XPS spectra for the MoWS2@MXene@CNT: (a) survey, (b) Ti 2p, (c) C 1s, (d) S 2p, (e) Mo 3d, and (f) W 4f.
(a)(b)(c)(d)(e)(f)Figure6 depicts the composite’s entire XPS profile. Figure 6(a) depicts the survey of the underside. The full spectrum shows the peaks of each element, proving that the MoWS2@MXene@CNT composites were synthesized. Figures 6(b)–6(f) show the XPS spectra of Ti, C, Mo, S, and W.In Figure6(b), the three major diffraction peaks of Ti 2p are observed at 459.6, 465.3, and 469.6 eV, which correspond to the Ti-C (Ti 2p3/2), Ti-X (Ti 2p3/2), and TiO2 (Ti 2p3/2) bonds, respectively [33].The C 1s spectrum is depicted in Figure 6(c), with three peaks at 284.8, 285.7, and 290.5 eV, corresponding to the C-C/C=C, C-O, and C=O bonds, respectively [34].In Figure6(d), two peaks near 162.5 and 163.7 eV, corresponding to S 2p3/2 and S 2p1/2, respectively, demonstrate the presence of sulfur in the MoWS2@MXene@CNT composites [35].The Mo 3d spectrum is depicted in Figure 6(e). The Mo 3d peak was deconvoluted into two additional peaks at 229.8 and 232.8 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively, as well as a S 2s peak at 226.7 eV. The Mo 3d peak points to the Mo4+ [25].The high-resolution W 4f XPS profile of the MoWS2@MXene@CNT samples is displayed in Figure 6(f). The existence of W4+ is strongly recommended by the two peaks of W 4f7/2 and W 4f5/2 at 37.1 and 39.0 eV, respectively [25, 36].The composite was used as the cathode for CR2025 button cell assemblies to evaluate MoWS2@MXene@CNT/S and its electrochemical performance as a cathode material for L-S batteries. The charge/discharge curves, rate performance, and CV diagrams of the MoWS2@MXene@CNT/S and MoWS2@CNT/S composite materials are presented in Figure 7. Figure 7(a) depicts the charge/discharge curve of the MoWS2@MXene@CNT/S. Meanwhile, the discharge profile exhibits a L-S double discharge platform. At approximately 2.3 V, the first plateau is observed, which corresponds to a transition from the elemental sulfur state to the soluble high-valent polysulfides. And the other one is observed at about 2.1 V, corresponding to the transition from Li2Sn (4 ≤ n ≤ 8) to Li2S2 and Li2S. Figure 7(b) depicts the composites’ rapid loss of discharge capacity as the charging and discharging rates of MoWS2@CNT/S increase. The large capacity loss could be thought to be due to the polysulfide, which ultimately resulted in the continuous loss of active material sulfur as well as the rapid decay of discharge capacity. The discharge capacity of the MoWS2@MXene@CNT/S electrode material is 1319.4 mAh/g, which is clearly superior to that of the MoWS2@CNT/S, which is 996.0 mAh/g. Figure 7(c) shows a rate performance graph of MoWS2@CNT/S and MoWS2@MXene@CNT/S with a discharge rate increasing from 0.1 C to 2 C. The rate performance of MoWS2@MXene@CNT/S electrode is superior to that of MoWS2@CNT/S electrode material. The CV curves were determined to better understand the electrochemical properties of MoWS2@MXene@CNT/S. A split oxidation peak and two reduction peaks are obtained in Figure 7(d). The low and high potential peaks in the reduction peaks are approximately 2.1 and 2.3 V, respectively, corresponding to the insoluble Li2S2 and Li2S and the conversion of the S8 to high-order polysulfides (Li2Sn, 4 ≤ n ≤ 8). Because of the occurrence of the reverse reaction from Li2S to Li2Sn to S8 [36], the split oxidation peak is observed at about 2.4 V. Meanwhile, two reduction peaks of MoWS2@MXene@CNT/S have such a greater area than MoWS2@CNT/S, indicating that the addition of MXene can improve polysulfide redox kinetics [37, 38].Figure 7
(a) and (b) First charge/discharge profiles of the MoWS2@MXene@CNT/S and the MoWS2@CNT/S, (c) rate curves of the MoWS2@MXene@CNT/S and MoWS2@CNT/S, and (d) C-V curves of the MoWS2@MXene@CNT/S and the MoWS2@CNT/S.
(a)(b)(c)(d)The cathode materials MoWS2@MXene@CNT/S and MoWS2@CNT/S have different capacities after 200 cycles at 0.2 C in Figure 8. The MoWS2@MXene@CNT/S cathode material outperforms the MoWS2@CNT/S cathode material in terms of discharge capacity and cycling performance. As a result of electrode material activation, the final capacity of the MoWS2@MXene@CNT/S cathode material slightly increases, which can provide additional lithium interfacial storage at low potentials via a mechanism known as pseudocapacitance [39, 40].Figure 8
Cycle performance of the MoWS2@MXene@CNT/S and MoWS2@CNT/S cathodes for 200 cycles at 0.2 C.To further evaluate the good electrochemical properties of the MoWS2@MXene@CNT/S composites, the EIS of both electrode materials was investigated separately. In Figure 9, the EIS of both materials have a comparison after and before cycling. A semicircle and a diagonal line comprise the impedance diagram [41, 42]. The semicircle denotes the impedance of charge transfer between the electrode material and the electrolyte, and the diagonal line reflects the diffusion of lithium I in the electrode material [43]. MoWS2@MXene@CNT has the smallest half-circle diameter and total electrochemical resistance before and after the multiplier cycle of the composites, indicating that it has the lowest charge transfer impedance. Table 1 displays the specific fitted values prior to the multiplicative cycle.Figure 9
(a) EIS of MoWS2@MXene@CNT/S and MoWS2@CNT/S (a) after and (b) before the multiplication cycle.
(a)(b)Table 1
Composite impedance fitting values (in Ω).
MoWS2@CNTMoWS2@MXene@CNTRs2.0294.405Rct188.1121.8Rtotal190.129126.205


## 4. Conclusion

In this article, MoWS2, MXene, MoWS2@CNT, and MoWS2@MXene@CNT complexes were prepared by hydrothermal and etching methods. Additionally, the electrode materials after sulfur loading were prepared. These materials were characterized using XRD, TGA, SEM, TEM, and XPS and evaluated for their electrochemical performance. In the MoWS2@MXene@CNT composite, all three substances are simultaneously present in the electrode material. Furthermore, the polar substance MoWS2 can accelerate the conversion of polysulfide, and the highly conductive CNT can accelerate the electron transfer rate. Meanwhile, MXene suppresses the volume expansion of the cell during discharge and enhances the electron conductivity. The final electrochemical tests showed that the MoWS2@MXene@CNT/S electrode material exhibited a good rate and cycling stability compared to the MoWS2@CNT/S electrode material, which provides a reference for transition metal sulfide and transition metal carbide composites as lithium-sulfur battery cathode materials.



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*Source: 6211780-2023-01-25.xml*