W₂C/WS₂ Alloy Nanoflowers as Anode Materials for Lithium-Ion Storage

Abstract

Recently, composites of MXenes and two-dimensional transition metal dichalcogenides have emerged as promising materials for energy storage applications. In this study, W₂C/WS₂ alloy nanoflowers (NFs) were prepared by a facile hydrothermal method. The alloy NFs showed a particle size of 200 nm–1 μm, which could be controlled. The electrochemical performance of the as-prepared alloy NFs was investigated to evaluate their potential for application as lithium-ion battery (LIB) anodes. The incorporation of W₂C in the WS₂ NFs improved their electronic properties. Among them, the W₂C/WS₂_4h NF electrode showed the best electrochemical performance with an initial discharge capacity of 1040 mAh g⁻¹ and excellent cyclability corresponding to a reversible capacity of 500 mAh g⁻¹ after 100 cycles compared to that of the pure WS₂ NF electrode. Therefore, the incorporation of W₂C is a promising approach to improve the performance of LIB anode materials.

1. Introduction

Two-dimensional (2D) materials such as graphene and transition metal chalcogenides show great potential for energy storage and conversion applications owing to their large surface area, high conductivity, and good physical and chemical stability [1,2,3,4,5,6,7,8,9,10,11,12]. Among these 2D materials, MXenes, which have been discovered recently, have been extensively investigated for energy storage applications. It should be noted that MXenes are the transition metal carbides/nitride, which have graphene-like structure, possessing many advantages of 2D materials such as high conductivity, flexibility, easy processing, and so on [13]. Mashtalir et al. synthesized intercalated Ti₃C₂ flakes with high stability and the charging rate for application as an anode material for lithium-ion batteries (LIBs) [14]. Naguib et al. prepared niobium and vanadium carbide LIB anodes with high rate capacity [15]. The MXene anodes exhibited excellent electrochemical performance at high currents because of their low diffusion barrier [16]. In addition, Fe₃O₄@Ti₂C₃ LIB anodes exhibited an ultrahigh capacity of approximately 747 mAh g⁻¹ at 1 C for 1000 cycles. Moreover, these materials exhibited a capacity of approximately 278 mAh g⁻¹ at the high rate of 5 C [17]. Zhang et al. have demonstrated the use of MXenes as conductive binders for viscous aqueous inks of silicon materials, which were used as high-capacity anode materials for LIBs [18]. In these materials, MXenes not only acted as a conductive network for Si particles, but also improved the mechanical stability of the material.

Tungsten metal compounds such as oxides, chalcogenides, and carbides are used in a wide range of applications such as catalysis, energy conversion, and energy storage [19,20,21,22,23,24,25,26,27]. Feng et al. prepared WS₂ nanoflakes with a high reversible capacity of 680 mAh g⁻¹ for 20 cycles as anode materials for LIBs [27]. Srinivaas et al. have prepared highly rich 1T WS₂ phase in few layered nanoflowers (NFs) with stable electrochemical performance and a high initial capacity of approximately 890 mAh g⁻¹ as anode materials for LIBs [28]. W₂C is a good catalyst. When grown on carbon nanotubes, it exhibits high photo/electrocatalytic performance for the hydrogen evolution reaction [26,29]. Simulation results have shown that W₂C can exhibit an ultra-fast loading of lithium ions owing to its low diffusion barrier of about 0.045–0.13 eV [30]. Thus, the combination of WS₂ and W₂C is expected to yield a promising LIB anode material.

In this study, we prepared a W₂C/WS₂ NF composite via a facile hydrothermal process. By controlling the reaction time, alloy NFs with different sizes could be prepared. The structures, chemical compositions, and binding states of the as-prepared NFs were investigated. Finally, the potential of the W₂C/WS₂ NFs for LIB anode applications was investigated. The NFs showed promising electrochemical performance and excellent Li storage.

2. Materials and Methods

2.1. Chemical Materials

Tungsten (VI) chloride (WCl₆, 99.9% trace metals basis, Sigma-Aldrich Inc., St. Louis, MO, USA), thioacetamide (C₂H₅NS, 99%, Sigma-Aldrich, Sigma-Aldrich Inc., St. Louis, MO, USA), amorphous carbon black Super-P (C, approximately 40 nm 99.99%, Alfa Aesar Inc., MA, USA), absolute ethanol (C₂H₅OH, Alpha Aesar Inc., MA, USA), and polyvinylidene fluoride (PVDF, 534,000 M_W, Sigma-Aldrich, Sigma-Aldrich Inc., St. Louis, MO, USA) were used as received without any treatment.

2.2. Synthesis of W₂C/WS₂ NFs

The W₂C/WS₂ alloy NFs were prepared according to a previously reported procedure [31]. Briefly, 0.6 g of WCl₆ was dispersed as the W source in a 20 mL tube with 4 mL of absolute ethanol. For the sulfur and carbon source, 1.2 g of thioacetamide was dispersed in 4 mL of absolute ethanol under stirring. Then, the WCl₆ solution was quickly added to the thioacetamide solution, and the resulting solution was stirred for 5 min. Then, 30 mL of deionized (DI) water was added, and the solution was stirred for 1 h. Then, the reaction mixture was transferred to a 50 mL polypropylene-lined autoclave and heated in an oven at 250 °C for 2, 4, and 12 h to obtain the W₂C/WS₂_2h, W₂C/WS₂_4h, and W₂C/WS₂_12h precipitates, respectively. These precipitates were washed four times with ethanol and DI water and were then dried in an oven at 60 °C to obtain the W₂C/WS₂ powders.

2.3. Characterization

X-ray diffraction (XRD) (D/MAX-2200 Rigaku, Tokyo, Japan) was used to analyze the structure of the samples. The XRD patterns of the samples were recorded over the 2θ range of 5–70°. The morphologies, structures, and sizes of the samples were investigated by scanning electron microscopy (SEM) (Hitachi S4700, Tokyo, Japan) and transmission electron microscopy (TEM) (TECNAI G2F30, FEI corp., OR, USA). The Raman spectra of the samples were acquired using a Raman spectrometer (Lab RAM HR, Horiba JobinYvon, Horiba Ltd., Kyoto, Japan, 532 nm laser excitation). X-ray photoelectron spectroscopy (XPS) (Axis Ultra DLD, Kratos Analytical Ltd, Kyoto, Japan) under a high vacuum of 1.6 × 10⁻¹⁰ mbar with a monochromatic Al Kα line was used to investigate the chemical compositions and atomic binding of the samples.

2.4. Electrochemical Measurements

The LIBs were assembled using coin-type cells (CR 2032, Rotech Inc., Gwangju, Korea). The working electrode was prepared by casting a slurry of 70% active material (W₂C/WS₂ alloy NFs), 15% conductive carbon black (Super-P), and 15% PVDF in N-methyl-2-pyrrolidinone on a copper foil by doctor blading. After drying in a vacuum oven at 70 °C for 12 h, the electrodes were punched into circular discs with a diameter of 12 mm. The battery half-cell structures were assembled under an Ar₂ atmosphere in a glovebox. A lithium foil, polyethylene, and 1M LiPF₆ in ethylene carbonate/diethylene carbonate (1:1 in volume) were employed as the reference electrode, separator, and electrolyte, respectively. Galvanostatic electrochemical charge–discharge measurements were carried out using a battery cycle tester (WBCS3000, WonAtech, Seocho-gu, Seoul) over the voltage range of 0.1–3.0 V versus Li/Li⁺. Cyclic voltammetry (CV) tests were carried out using ZIVE MP1 (WonAtech, Seocho-gu, Seoul) over the voltage range of 0.1–3.0 V at a scanning rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopic (EIS) measurements were carried out using a ZIVE MP1 (WonAtech, Seocho-gu, Seoul) over the frequency range of 100 kHz–0.1 Hz.

3. Results

For the synthesis of the W₂C/WS₂ NFs, the WCl₆ precursor was pre-mixed with absolute ethanol (WCl₆ + xC₂H₅OH → WCl_6-x(OC₂H₅)_x + xHCl) before mixing with thioacetamide to prevent its unexpected reaction with moisture (WCl₆ + xH₂O → WCl_6-x(OH)_x + xHCl) [31,32]. The hydrolysis of thioacetamide produced hydrosulfide, acetic acid, and ammonia (2C₃H₅NS + 6H₂O → 2H₂S + 3CH₃COOH + 2NH₃). The introduction of hydrosulfide, acetic acid, and ethanol, respectively, acted as the sulfur and carbon sources for the formation of the W₂C/WS₂ alloys. Figure 1a–c show the SEM images of the W₂C/WS₂_2h, W₂C/WS₂_4h, and W₂C/WS₂_12h NFs, respectively. The particle sizes of the W₂C/WS₂_2h, W₂C/WS₂_4h, and W₂C/WS₂_12h NFs were 200, 400, and 1000 nm, respectively. The NFs showed a highly uniform structure, indicating that the synthesis procedure was highly reproducible. Each NF consisted of several leaves of W₂C nanosheets and WS₂ nanocrystals. Meanwhile, the WS₂ NFs synthesized without ethanol showed a complex structure because of the non-uniform dispersion of W (Figure 1d).

To further examine the structure of the W₂C/WS₂ NFs, powder XRD measurements were carried out over the 2θ range of 5–70° (Figure 2a). The XRD peaks of all the samples could be indexed to the hexagonal structure of W₂C and WS₂ [32,33,34,35]. It should be noted that the W₂C/WS₂_2h sample showed weak WS₂ peaks. Moreover, this sample showed broad W₂C peaks, indicating the small crystallite size of W₂C. The W₂C/WS₂_4h and W₂C/WS₂_12h samples showed clear W₂C and WS₂ peaks, indicating the co-existence of these hexagonal-structured materials. For comparison, a WS₂ sample was prepared using the same procedure as that used for preparing the W₂C/WS₂ NFs but without the addition of ethanol. This sample showed peaks corresponding to WS₂ only. Therefore, the addition of ethanol during the synthesis not only prevented the oxidation of WCl₆ but also contributed to the formation of carbide from the thioacetamide source. The structures of W₂C and WS₂ in the alloy NFs were examined by Raman spectroscopy (Figure 2b). The samples for Raman spectroscopy measurements were prepared on SiO₂/Si substrates, and a 532-nm wavelength laser source was used. The samples showed two peaks at 351.5 and 415.2 cm⁻¹ corresponding to the in-plane mode vibration E¹_2g and out-plane mode vibration A_1g of WS₂, respectively [2,36]. Moreover, the peaks detected at 700 and 805 cm⁻¹ can be attributed to the stretching modes of W−C [26,37]. The D- and G-bands corresponding to the sp³ and sp² carbon atoms in W₂C were observed at 1200–1700 cm⁻¹. These results indicate the co-existence of W₂C and WS₂ in the alloy NFs.

The structure of the W₂C/WS₂ alloy NFs was further examined using TEM. As can be observed from Figure 3a, the samples showed a uniform NF structure with many leaves. Some Moiré patterns with a size of about 0.9–1.0 nm could be observed in the TEM images of the samples (Figure 3b). This can be attributed to the formation of the W₂C/WS₂ interlayer spacing [31]. The lattice spacings of W₂C and WS₂ in the samples were also measured from their high-resolution TEM (HRTEM) image shown in Figure 3c. The distance spacings of the WS₂ (100) and W₂C (101) planes were measured to be 0.27 and 0.23 nm, respectively. These spacings are characteristic of these materials. Moreover, the selected-area electron diffraction (SAED) pattern shown in Figure 3d indicates that W₂C/WS₂ alloy NFs with a highly crystalline hexagonal structure were successfully fabricated.

The chemical compositions and atomic binding energies of the W₂C/WS₂ alloys were analyzed by XPS (Figure 4a–d). The survey scan XPS profiles of the alloys showed clear and sharp peaks corresponding to W, S, O, C, and Si (from substrate) (Figure 4a). No impurity peak was detected, indicating the high purity of the W₂C/WS₂ alloys. The high-resolution W 4f, S 2p, and C 1s spectra of the alloys are shown in Figure 4b–d, respectively. The W 4f peak consisted of doublet peaks corresponding to the W−C, W−S, and W−O bonds. The contribution of W−O bonding can be attributed to the slight oxidation on the surface of the alloys, which always occurs during the preparation or natural oxidation of a material in air [29]. The W 4f7/2 and W 4f5/2 peaks of the W−C doublet were observed at 31.9 and 34.1 eV, respectively, while those of the W−S doublet were observed at 32.4 and 34.6 eV, respectively. The W 4f7/2 and W 4f5/2 peaks of the W−O doublet were observed at 36.0 and 38.2 eV, respectively. The S 2p peak of the samples could be deconvoluted into the S 2p3/2 and S 2p1/2 peaks, which were observed at 161.4 and 162.6 eV, respectively. Furthermore, the slight oxidation of the surface of the alloys during the preparation resulted in the appearance of the S−O bond peak at about 169 eV. Finally, the C 1s peak of the samples could be deconvoluted into those corresponding to C−W bonding at 284.7 eV, C−O bonding at 286.2 eV, and C−OH bonding at 288.5 eV. This is consistent with the binding energy of C in carbide compounds [38].

To evaluate the electrochemical performance of the alloys for application as LIB anode materials, their CV tests were carried out for three cycles over the voltage range of 0.1–3.0 V (Figure 5a–d). In the first cycle, the as-prepared WS₂NFs showed lithiation at 0.5, 1.2, and 1.4 V attributing to the reduction of WS₂ to Li₂S through multiple steps, including the formation of Li_xWS₂ and Li₂S [28]. The peak observed at 0.5 V corresponds to the conversion reaction (4Li⁺ + WS₂ + 4e⁻ → 2Li₂S + W) as well as the decomposition of the non-aqueous electrolyte to form the solid electrolyte interface (SEI) layer [28,39]. In the second cycle, the peak at 0.5 V disappeared because of the formation of the SEI and gel-like polymeric layers by the dissolution of Li₂S into the electrolyte, leading to its degradation [40]. The peaks at 1.2 and 1.4 V shifted to 1.27 V. An additional peak was observed at 1.9 V, indicating the insertion of Li into Li_xWS₂ [28]. Figure 5b–d show the cyclic voltammograms of the W₂C/WS₂_2h, W₂C/WS₂_4h, and W₂C/WS₂_12h samples, respectively. In the first cycle, the W₂C/WS₂_2h sample showed a small peak at about 1.1 V. In the second and third cycles, the sample showed reduction peaks at 1.25 and 1.9 V attributing to the multi-step lithiation of WS₂. Interestingly, the W₂C/WS₂_4h and W₂C/WS₂_12h samples with large NFs showed only a broad peak at 1.3–1.5 V in the first discharge cycle. This phenomenon can be attributed to the change in the work function of WS₂ by the addition of W₂C [31]. The changes in the work function of the alloys corresponded to the changes in their reduction potentials. The work function of the W₂C/WS₂ NFs increased from 4.31 to 4.7 eV with an increase in the reaction time from 2 to 12 h. The work function of the pure WS₂ sample was 4.95 eV. The bare WS₂ electrode showed lithiation peaks at 1.4 and 1.9 V. On the other hand, the W₂C/WS₂_2h and W₂C/WS₂_4h electrodes showed lithiation peaks at lower potentials at around 1.2–1.5 V. These samples did not show any lithiation peak at 1.9 V. Hence, it can be stated that the decrease in the work function resulted in a decrease in the lithiation potential of the samples. As shown by the XRD patterns, the W₂C/WS₂_2h sample showed broader W₂C peaks than the other samples, indicating that the W₂C crystallite size of this sample was smaller than those of the other samples. Therefore, the change in the second lithiation potential of the W₂C/WS₂_2h electrode was comparable to the initial potential of the WS₂ electrode because of the instability of its smaller W₂C crystals. The W₂C/WS₂_4h and W₂C/WS₂_12h electrodes only showed reduction peaks at about 1.2–1.5 V. This indicates that with an increase in the reaction time, the bonding between W₂C and WS₂ became stronger. The W₂C/WS₂ and WS₂ NFs showed similar oxidation peaks because of the restoration of the WS₂ structure at about 1.7–1.9 V and the oxidation of Li₂S (Li₂S → 2Li+S) at about 2.3–2.5 V [39]. The W₂C/WS₂ NFs showed an oxidation peak at approximately 1.2 V, which was attributed to the delithiation of W₂C.

The initial discharge and charge voltage profiles of the W₂C/WS₂ and WS₂ NFs were obtained over the voltage range of 0.1–3.0 V at a scan rate of 100 mA g⁻¹, as shown in Figure 6a. The WS₂ NFs showed the charge and discharge capacities of 504.0 and 656.6 mAh g⁻¹, respectively. The W₂C/WS₂_2h, W₂C/WS₂_4h, and W₂C/WS₂_12h NFs showed the charge and discharge capacities of 595.2 and 935.2 mAh g⁻¹, 751.8 and 1040.5 mAh g⁻¹, 717.7 and 953.5 mAh g⁻¹, respectively. It has been reported that carbide materials are promising candidates for energy storage applications [41]. In this study, the addition of W₂C to the WS₂ NF sample improved its storage capacity. The cyclic performances of the W₂C/WS₂ and WS₂ NFs were evaluated over 100 cycles, as shown in Figure 6b. The alloys NF electrodes exhibited different electrochemical properties depending on the reaction time. For instance, after 10 cycles, the capacities of the W₂C/WS₂_2h and W₂C/WS₂_12h electrodes decayed rapidly by about 55%. After 30–40 cycles, the samples showed low stable capacities of 100–200 mAh g⁻¹ with a high Coulombic efficiency of approximately 95–99%. On the other hand, the W₂C/WS₂_4h electrode showed excellent cyclic stability with a high capacity of 500 mAh g⁻¹ after 100 cycles as compared to the other electrodes. The reasons why the W₂C/WS₂_4h electrode demonstrated the best performance are the stable binding of W₂C to WS₂ and the lowering of reduction potential in addition to the low resistance (discussed later). This sample showed a Coulombic efficiency of 97–98%. Meanwhile, the WS₂ NFs showed a Coulombic efficiency of 93–94% and a continuous decrease in the cyclic capacity to approximately 145 mAh g⁻¹ during the 100th cycle.

To further investigate the performance of the NFs, their EIS measurements were carried out over the frequency range of 100 kHz–0.1 Hz at a voltage of 3.0 V. Figure 7 shows the Nyquist plots of the WS₂ and W₂C/WS₂ NF electrodes with the equivalent circuit as an inset containing the constant phase elements (CPE1, 2), series resistance (R_s), charge transfer resistance (R_ct), solid electrolyte resistance (R_SEI), and diffusion Warburg element (W). The semicircular arc corresponds to their charge transfer resistances between the interface of electrode and electrolyte [42]. The resistance values are summarized in

Table 1. Comparison of resistance values extracted from equivalent circuit for the electrochemical impedance spectroscopic (EIS) measurement.

Sample	Rs	Rct	RSEI
WS2 NFs	2.12	627	430
WC/WS2_2h	3.04	381	367
WC/WS2_4h	1.24	312	350
WC/WS2_12h	5.41	620	422

. The WS₂ NF anode showed the highest charge transfer resistance among all the anodes investigated. The charge transfer resistance of the WS₂ NFs decreased with the addition of W₂C in the case of the W₂C/WS₂_2h and W₂C/WS₂_4h electrodes. However, it increased again for the W₂C/WS₂_12h electrode. The difference in the performance of the as-prepared W₂C/WS₂ electrodes could be due to the different amount of W₂C in the alloys. As shown by the Raman spectrum in Figure 2b, the peak intensities of the D- and G-band from the carbon atoms in W₂C decreased when the reaction time increased, illustrating an increase in the amount of WS₂. In the case of the W₂C/WS₂_12h sample, it could have an excess amount of H₂S at 250 °C, and the applied high pressure sulfurized the weak bonding of W−C, leading to the lower amount of C, which was confirmed by Raman spectra representing the decrease of the D-band and G-band. Moreover, the flower size of the W₂C/WS₂_12h sample is about 1 μm, which is even bigger than the random size of WS₂ NFs. It should be noted that the increase in size could lead to the decrease in the conductivity [28]. When the amount and size of WS₂ increase, the resistance of the electrodes increases, as discussed in the EIS results. Specifically, the W₂C/WS₂_2h and 4h electrodes showed lower resistances compared to that of the W₂C/WS₂_12h electrode. Even though the W₂C/WS₂_2h electrode showed the lowest resistance value, the cycling performance was not stable due to the weak binding of the W₂C crystal to WS₂, leading to the gradual capacity decay, as discussed earlier. Thus, it can be concluded that the control of reaction time for the preparation of W₂C/WS₂ electrodes is crucial to optimize the overall electrochemical properties, where the W₂C/WS₂4h alloy is the best electrode. Computational calculations have revealed that W₂C materials are promising candidates for Li storage applications [30]. Li ions tend to adsorb on W₂C materials to form a metal cluster. Moreover, MXenes nanosheets materials exhibit high conductivity [43]. In this study, the addition of W₂C reduced the charge transfer resistance of the WS₂ NFs, and the resulting W₂C/WS₂ alloy NFs showed high storage capacity and stability for LIB applications.

4. Conclusions

W₂C/WS₂ NFs were successfully fabricated via a facile hydrothermal method at low temperature. The particle size of the NFs could be controlled (200 nm–1 μm). The obtained NFs exhibited high purity and well-defined hexagonal structures of W₂C and WS₂. The NF alloys were employed as anode materials for LIBs. The W₂C/WS₂_4h sample showed a high initial discharge capacity of 1040 mAh g⁻¹. The alloy electrodes showed a low charge transfer resistance of 200–600 Ω, indicating that the W₂C/WS₂ alloy electrodes were highly conductive as compared to the WS₂ NF electrode. Among all the electrodes investigated, the W₂C/WS₂_4h electrode exhibited the highest stable capacity of up to approximately 500 mAh g⁻¹ over 100 cycles, which could be attributed to the optimized W₂C in WS₂ NFs, Therefore, the W₂C/WS₂ alloy NFs prepared in this study showed a potential for energy storage and conversion applications.