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Enhanced reversible lithium ion storage in stable 1T@2H WS2 nanosheet arrays anchored on carbon fiber
Electrochimica Acta 259 (2018) 1e8
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced reversible lithium ion storage in stable 1T@2H WS2 nanosheet arrays anchored on carbon fiber Tailin Wang, Changlong Sun, Mingzhi Yang, Lei Zhang, Yongliang Shao, Yongzhong Wu, Xiaopeng Hao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2017 Received in revised form 12 October 2017 Accepted 24 October 2017 Available online 26 October 2017
Recent years, as a competitive anode material for lithium ion batteries (LIBs), tungsten disulfide (WS2) has attracted significant attention for its high theoretical capacity, abundant resources and low cost, but how to improve the cycle stability and reversible capacity is still an important research subject. In this work, stable metallic WS2 anchored on carbon fiber cloth was fabricated as flexible anode for LIBs. The transversal displacement of S atoms in WS2 was induced by incorporation of N atoms, and the stability of metallic phase WS2 was ensured by the formation of N-W covalent bonds. The results of electrochemical measurement and dynamics analysis demonstrate that the incorporation of metallic phase could not only reduce the initial discharge capacity loss, but also improve the reversible capacity than traditional semi-conducting phase WS2. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Nitrogen doping Metallic WS2 Carbon fiber Flexible bind-free anode
1. Introduction As prospective graphene-like two-dimensional (2D) materials, transitional metal sulfides (TMDs) have attracted substantial interest and investigation due to the unique morphology structure and excellent performance in a range of fields [1e6]. Among these 2D TMDs, tungsten disulfide (WS2) has achieved increasing attention due to its excellent thermal, mechanical, optical and electric performances [7e10]. Laminar WS2 is composed of sandwich construction stacked through the weak van der Waals interaction, while strong covalent bonds in-plane between S atoms and W atoms, which is beneficial for lithium ion storage (theoretical capacity ¼ 433 mAh g1) [11]. But the low electrical conductivity and unreliable stability during long cycles of WS2 in nature limits its potential application in LIBs [12e14]. To improve the electrochemical properties, two methods are commonly employed. One is combined with carbon-based materials, which were reported widely in the previous literature [15e17], another effective method is realizing the phase-transformation of WS2 from semi-conducting phase to metallic phase. The metallic (1T) phase WS2 (octahedral coordination) exhibits inequable performances and microstructure compared with
* Corresponding author. E-mail address: [email protected] (X. Hao). https://doi.org/10.1016/j.electacta.2017.10.154 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
semiconducting (2H) phase WS2 (trigonal prismatic coordination), and can be obtained by the transversal displacement of one of S atomic layer. However, relatively few literature have been reported in the fabrication and application of 1T WS2. Similar to MoS2 [18,19], the synthesis path and method for 1T WS2 mainly include rare metal doping [20], alkali metal and butyllithium intercalation [21e23], ammonia ions intercalation [24], pulsed laser deposition [25]. Liu [24]et al. synthesized 1T-WS2 nanoribbon by the intercalation of ammonia ions and 0.75 electrons per NHþ 4 are transferred to WS2. Similarly, Re, Tc, and Mn acting as electron donors can induce the phase transformation of WS2, and the formation of covalent bonds increases the stability of metallic WS2 greatly [20]. However, some shortcomings limit the application of these methods, such as complex preparation process, risk of safety or not suitable for large-scale preparation. Therefore to find a simple and efficient method obtaining 1T phase WS2 is the key to improve its electrochemical performances. In this study, we propose a novel and universal method for the fabrication of stable 1T phase WS2 via covalent bond between nitrogen and WS2. The introduction of nitrogen induces the phase transformation from 2H to 1T and the strong N-W bonds improve the stability of 1T phase. In particularly, 1T@2H WS2 anchored on the interlinked carbon fiber cloth (CFC) was used as flexible anode without any binder and current collector for lithium ion batteries (LIBs). The electrochemical measurement show that 1T phase
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WS2@CFC (1T@2H WS2@CFC) possess a smaller initial discharge capacity loss and a higher reversible capacity than pure 2H phase WS2@CFC (2H WS2@CFC), which is critical to the improvement of electrochemical performances. 2. Experimental section 2.1. Preparation of the carbon fiber cloth The hollow carbon fiber cloth was fabricated by the carbonization of degreasing cotton according to reference [14]. The detailed process is as follows: degreasing cotton was rinsed with ethanol for several times and fully dried before using. A level and smooth degreasing cotton was placed onto an aluminium oxide panel, meanwhile another aluminium oxide panel was placed on the cotton to exert a pressure on it. Then the degreasing cotton was transferred into a tubular furnace and heated to 1000 C for 2 h under an argon flow with a heating rate of 10 C min1, and the sample was cooled down to room temperature naturally after carbonization. The black, thinner and denser CFC with good flexibility was obtained. 2.2. Synthesis of 1T@2H WS2 nanosheets@CFC The WS2 nanosheets were synthesized by a solvothermal method. 1.4 g sodium tungstate (Na2WO4$2H2O, AR) and 1.50 g thiourea (CS(NH2)2, AR) were weighed and dissolved in 35 mL deionized (DI) water under vigorous stirring to form a homogeneous solution. Then the solution was carefully transferred into a 50 mL PPL-lined stainless-steel autoclave, maintained at 265 C for 24 h, and naturally cooled down to room temperature. The obtained black product was washed several times by DI water and dried at 80 C for the characterization of structure and performance. The WS2 nanosheets@CFC was obtained through a similar process except the denser CFC being placed in the PPL-lined. Then the composite was put into the tubular furnace for 2 h under argon or ammonia flow at 800 C and cooled down slowly, the final product was named 2H WS2 nanosheets@CFC (2H WS2@CFC) and 1T@2H WS2 nanosheets@CFC (1T@2H WS2@CFC) respectively. 2.3. Materials characterizations X-ray diffraction (XRD) was conducted using a Bruker D8 Advance X-ray diffraction with Cu Ka radiation (l ¼ 0.154178 nm). Raman spectra were acquired on a LabRam HR system from Horiba Jobin Yvon with 532 nm excitation. Scanning electron microscopy (SEM) images were recorded by Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 5.0 kV and energy-dispersive spectrometer (EDS) at 15.0 kV. Highresolution transmission electron microscopy (HRTEM) images were performed using a JEM-2100F high-resolution transmission electron microscope at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) measurement were carried out using a monochromatized XPS spectrometer (a ThermoFisher ESCALAB 250) with Al (Ka) radiation as the probe. The specific surface area were studied using Brurauer-Emmerr-Teller (BET) Procedure and pore distributions were analyzed. 2.4. Electrochemical measurements The electrochemical behaviors of the product were measured by CR2032-type coin cells, which were assembled in an argon-filled glove box. The 3D 1T@2H WS2@CFC was used as working electrode directly without any extra conductive additives and binder, meanwhile lithium foil was used as counter electrode and reference
electrode, and the electrolyte was 1 mol L1 LiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbonate/dimethyl carbonate (1:1:1 vol%). The separator was a polypropylene film (Celgard 2320). Electrochemical measurements were conducted by galvanostatic charge-discharge testing between 0.001 V and 3.0 V on a battery testing system BTS (Newarel Electronic Co., Ltd., China) at ambient temperature. The A.C. impedance spectra were tested on a CHI660D electrochemical workstation in the frequency range from 0.01 to 100 kHz and the cyclic voltammetry (CV) curves were performed between 0.001 V and 3.0 V (vs. Liþ/Li). 3. Results and discussion Fig. 1 illustrates the fabrication process of 1T@2H WS2@CFC. Flexible CFC was fabricated by the carbonization of degreasing cotton [14], and 2H WS2 nanosheets@CFC was obtained by a mild hydrothermal method, and then the composite was heated to 800 C for 2 h under an ammonia flow. The black product was then washed with distilled water and ethanol and then dried under vacuum at room temperature. Fig. 2a shows the X-ray diffraction patterns of WS2 nanosheets, as-prepared 2H WS2@CFC and 1T@2H WS2@CFC. It was observed that both WS2 nanosheets and 2H WS2@CFC show similar peaks [26], this means the WS2 nanostructure was maintained after being incorporated with CFC. However, the (002) peak of 1T@2H WS2@CFC was shifted left compared with 2H WS2@CFC, the difference was ascribed to the reaction of NH3 and WS2, which could expand the interlayer distance [27e29]. To further understand the structure of the 1T@2H WS2@CFC, Raman scattering was performed to investigate the transformation before and after the ammoniation process. As shown in Fig. 2b, there are two apparent peaks at 350 and 416 cm1 in 2H WS2@CFC, which are attributed to the in-plane E12g mode and out of plane A1g mode of the 2H phase WS2, respectively [30]. After being ammoniated, three additional Raman peaks emerge at low frequency region (130, 260 and 293 cm1), which are associated with the 2H WS2 contained 1T phase [31,32]. N dopant, as the donor atom, could induce the phase transition of MoSe2 by means of thermal treatment in NH3 atmosphere [33]. Enyashin [20] has pointed that the covalently bound lattice of the 1T-phase was more stable compared with the lattice of the alkali intercalated compound. This was because of the additional electrons from a donor atom could occupy the Mo 4dxy, Mo 4dyz, Mo 4dxz orbitals and increase the stability of the 1T-phase. The specific surface area was measured to investigate the porosity of composite as shown in Fig. 2c and d, the nitrogen adsorption-desorption isotherm is shown in Fig. 2c, and the BET surface is calculated to be as high as 212.5 m2 g1 (BET surface of 2H WS2@CFC is 196.4 m2 g1 (Fig. S1)), which is advantageous to promote the speedy delivery of lithium ion and electron between electrode and electrolyte [34e36]. Fig. 3a shows the SEM image of carbon fiber cloth, carbon fiber with a uniform diameter (ca. 5 mm) can be clearly observed, and the length is beyond several hundred micrometers. The evenly distributed carbon fibers are closely interwoven, which could not only serve as the carbon matrix but also benefit the speedy delivery of lithium ion during the process of charge/discharge. The SEM images of 2H WS2@CFC are shown in Fig. 3b and c, WS2 nanosheets are uniformly distributed on CFC, the transverse size of nanosheet is 200 nm and the thickness is about 10 nm. After being ammoniated, the morphology and structure of WS2 nanosheets remain stable (Fig. 3d), and the digital photo (inset in Fig. 3d) indicates that the composite remains good toughness. The elementary composition of 1T@2H WS2@CFC is confirmed by energy dispersive X-ray spectroscopy (EDS) in Fig. 3eef. The SEM image and corresponding elemental mappings reveal that nitrogen atoms in the WS2
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Fig. 1. Schematic illustration of the fabrication process for 1T@2H WS2@CFC and the schematic illustration of composite for LIBs.
Fig. 2. (a) XRD patterns of WS2 nanosheets, as-prepared 2H WS2@CFC and 1T@2H WS2@CFC. (b) Raman spectra of as-prepared 2H WS2@CFC and 1T@2H WS2@CFC. (c) Nitrogen adsorptionedesorption isotherms and (d) pore size distributions of 1T@2H WS2@CFC.
nanosheets distribute well. Fig. 3g shows that 1T@2H WS2@CFC contains only nitrogen, sulfur, and tungsten elements, indicating there is no impurity elements in the sample, and the mass ratio of 1T@2H WS2 is 90.06 wt% according to the TG results (Fig. S2). The typical mass of 1T@2H WS2@CFC with diameter of 12 mm was z1.6 mg, and the mass of 1T@2H WS2 was about 1.44 mg after calculation. Fig. 3h and i shows the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of 1T@2H WS2@CFC, the lamellar thickness of WS2 nanosheets is small, and 1T phase (trigonal lattice area, octahedral coordination) [22,30] is co-existed with 2H phase (honeycomb lattice area, trigonal prismatic coordination) [37]. The corresponding ball-and-stick models of 1T and 2H MoS2 were shown in inset images of Fig. 3i, and 1T MoS2 could be viewed as transversal displacement of one of S atomic layer in 2H MoS2, the lattice plane spacing of S atomic layer in 1T phase in keeping with 2H phase.
The high-resolution X-ray photoelectron spectra (XPS) of 1T@2H WS2@CFC and 2H WS2@CFC are shown in Fig. 4. Fig. 4aeb depict the HR-XPS spectra of W 4f and W 5p, the signal of the W 4f 7/2 and W 4f 5/2 is essential to distinguishing the discrepancy between the 1T and 2H structures. It can be clearly seen that the curve exhibits two apparent peaks of W 4f 7/2 and W 4f 5/2 at 32.7 eV and 35.1 eV in Fig. 4a, which are corresponding to the known 2H phase WS2 [38]. Besides the original peaks, two new peaks shift about 0.4 eV to lower binding energy than 2H phase WS2 after being ammoniated (Fig. 4b), which confirms the presence of 1T phase [21,25,37]. Fig. 4c shows the N-W bonds at 398.2 eV [39,40], and the peak located at 400.8 eV can be typically identified as the quaternary graphitic N [41], which have been proven to be conducive to improve performance [42]. Meanwhile the peaks at about 284.6 eV, 285.9 eV and 287.7 eV are corresponding to the bonds of C-C, C-N and C]N [39,43] in Fig. 4d. These results demonstrate that the
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Fig. 3. SEM images of (a) carbon fiber cloth, (b, c) 2H WS2@CFC and (d) 1T@2H WS2@CFC (inset of (d) is photographs of 1T@2H WS2@CFC). (e, f) The selected SEM image and corresponding element mapping profiles of 1T@2H WS2@CFC. (g) The EDS microanalysis on selected areas. (h, i) The TEM and HRTEM images of 1T@2H WS2@CFC, inset images of (i) are simulated of 1T and 2H MoS2 (S, blue; Mo, red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nitrogen are merged into WS2 nanosheets and carbon fiber cloth, and the obtained metallic phase WS2 and N-CFC are very conducive to improve the electrical conductivity of composites. The electrochemical properties of the 1T@2H WS2@CFC were investigated by galvanostatic charge/discharge and cyclic voltammetry (CV). Selected galvanostatic discharge/charge profiles of the composite electrode at a current density of 0.1 A g1 in the voltage range of 0.001e3.0 V vs Liþ/Li are displayed in Fig. 5a, the initial cycle curve exhibits two main discharge platforms at about 1.18 V and 0.7 V and two charge platforms at 1.95 V and 2.45 V. The charge and discharge capacities are 1280 mAh g1and 1456 mAh g1, respectively. What is the most important is that the second discharge capacities is 1272 mAh g1 and the initial discharge capacity loss is 12.64%, which are lower than the 21.6% of 2H WS2@CFC. The smaller initial discharge capacity loss can be ascribed to the incorporation of 1T phase [20] and the initial phase conversion before the electrochemical intercalation allows a more homogeneous phase conversion and uniform charge distribution compared to the 2H WS2 [28]. Fig. 5b shows CV curves of the first four cycles from 0.001 to 3.0 V vs Liþ/Li at the scan rate of 0.1 mV s1. Two main reduction peaks at 1.28 V and 0.14 V, which is corresponded to the lithium ion insertion into WS2 embedded in the CFC to form LixWS2, conversion reaction of Li with WS2, and electrolyte decomposition, respectively. Two oxidation peaks at 1.85 V and 2.49 V are found to correspond to the lithium extraction processes. From the second cycle, the reduction peak at 2.49 V disappears while new reduction peak appears in the potential range from 1.5 to 2.2 V [17], which can be assigned to the formation of a gel-like polymer layer formed out of the dissolution of the Li2S in the electrolyte, in accordance with previously reported characteristics of the WS2 electrode [11,44]. It's worth noting that no
obvious changes can be observed for the next two cycles, suggesting good architectural stability cycle stability of 1T@2H WS2@CFC electrode. The cyclic voltammetry profiles match well with the results of charge-discharge tests. Fig. 5c reveals that the discharge capacities are stable from the second cycle and a capacity of 1130 mAh g1 is achieved after 200 cycles at 0.1 A g1. For comparison, N-doped carbon fiber cloth electrode and 2H WS2@CFC is tested under the same current density (Figs. S3 and S4). Besides the achieved high specific capacity and superior cycling stability, the 1T@2H WS2@CFC electrode shows an excellent rate capability as well. The galvanostatic charge/ discharge profiles of sample at different current density are shown in Fig. 5d, the tenth-cycle discharge capacities of about 1251, 1015, 877, 706, 557 mAh g1 at the current densities of 0.1, 0.2, 0.5, 1, 2 A g1. Besides, the specific capacity of composite anode could recover to 1180 mAh g1 when the current density is recovered to 0.1 A g1. Fig. 5e reveals the long cycle performance of 1T@2H WS2@CFC electrode at high current density of 2 A g1. The discharge capacity can be stable at 510 mAh g1 after 800 cycles, which exhibits stable cycling performance and higher specific capacities than the 2H WS2@CFC (Fig. S4). Moreover, the HRTEM and XRD results of 1T@2H WS2@CFC electrode after 200 cycles at 0.1 A g1 (Fig. S5) are corresponds to the results of previous analysis. Electrochemical impedance spectroscopy (EIS) is effective for studying the mechanism and kinetics of electrode reaction. The Nyquist plots diagrams of 1T@2H WS2@CFC and 2H WS2@CFC electrode are shown in Fig. 6a, both impedance spectra consist of a semicircle in high frequency area and a straight line in low frequency area. According to the previous reports, the high-frequency semicircle is attributed to the charge-transfer impedance and the
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Fig. 4. XPS survey spectrums of W 4f and W 5p (a) 2H WS2@CFC and (b) 1T@2H WS2@CFC. XPS spectra of (c) N 1s and (d) C 1s of 1T@2H WS2@CFC sample.
Fig. 5. Electrochemical performance measurement: (a) Galvanostatic chargeedischarge profiles in the 0.001e3.0 V window (versus Liþ/Li) for the 1st, 2nd, 3rd, 50th, 100th and 200th cycles at 0.1 A g1; (b) Cyclic voltammetry of 1T@2H WS2@CFC electrode at a scan rate of 0.1 mV s1 between 0.001 and 3.0 V at the room temperature. Charge-discharge cycling and rate capabilities profiles of the 1T@2H WS2@CFC electrode at (c) 0.1 A g1, (d) various current densities and (e) 2 A g1.
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Fig. 6. (a) Nyquist plots and fitted curve for the 1T@2H WS2@CFC electrode. (b) Cyclic voltammetry (CV) curves from 0.1 to 10 mV s1 of 1T@2H WS2@CFC electrode. (c) Linear relationship of the peak current (ip) and the square root of the scan rate (v1/2). (d) Capacitive (gray) contribution of 1T@2H WS2@CFC to the total charge storage at 0.5 mV s1.
low-frequency line in correspond to the Warburg impedance (Zw) of electrode [45,46]. The impedance data are fitted by the equivalent circuit (inset of Fig. 6a), and the diffusion coefficient of lithium ion could be calculated by means of equations (1) and (2) from the plots in the low frequency region:
D¼
R2 T 2 2A2 n4 F 4 C 2 s2
Zreal ¼ Re þ Rct þ su1=2
(1)
(2)
where R represents the gas constant, F represents the Faraday’ s constant, A denotes the area of the electrode and C denotes the molar concentration of lithium ions. 1T@2H WS2@CFC electrode exhibits a lower impedance value (110 U) and higher diffusion coefficient of lithium ion (3.8 1015 cm2 s1) than 2H WS2@CFC (200 U, 1.4 1015 cm2 s1). It is deduced that the existence of 1T phase could reduce the impedances and improve the impedances of composite. Cyclic voltammetry (CV) has been proven to be a powerful technique to evaluate the electrochemical kinetics of electrode materials towards lithium ion battery. Here, kinetic analysis based on CV test was carried out to gain further insight into the electrochemistry of 1T@2H WS2@CFC electrode. Fig. 6b shows the CV curves at the scan rates of 0.1, 0.5, 1, 5 and 10 mV s1, respectively. The CV curves reveal two reductive peaks and two oxidation peaks, the relationship between the measured current (i) and the scan rate (v) can be described by equations (3) and (4): i ¼ avb
(3)
logi ¼ blogv þ loga
(4)
where i is the peak current, v is scan rate, and a, b are adjustable parameters. The b value can be derived from the slope of the log(v)log(i) curve, which reflects the amount of charge stored by capacitive and intercalation processes. If the b value is 0.5, the current density of the insertion reaction is proportional to the square root of the scan rate; if b ¼ 1, the current density of the capacitive contribution is proportional to the scanning rates. The log(v)-log(i) plots for the 1T@2H WS2@CFC electrode is shown in Fig. 6c, the calculated b values are 0.67, 0.77, 0.70 and 0.72, which is similar to the case of MoS2 [47]. Fig. 6d reveals the capacitive and diffusion-controlled charge storage contributions for 1T@2H WS2@CFC electrode at 0.5 mV s1, which can be calculated using equation (5): i(V) ¼ k1v þ k2v1/2
(5)
where n is the sweep rate, k1n and k2n1/2 represent current due to surface capacitive effects and current due to diffusion-controlled lithium ion insertion process, respectively. By determining the value of k1 and k2, the fraction of the current arising from Liþ diffusion can be distinguished from capacitive processes. The light gray area in Fig. 6d represents the capacitive contribution, total integrated capacitive charge storage is about 72%, in good agreement with the calculated b-values. The results of dynamics analysis demonstrate that the incorporation of 1T phase is conducive to promoting the electrochemical performances of WS2@CFC.
4. Conclusion To summarize, we demonstrate a versatile and simple route to obtain stable metallic WS2 nanosheets@carbon fiber cloth composite with large surface area and great potential for constructing
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high-performance flexible LIBs anode. The formation of strong N-W covalent bonds improves the stability of 1T WS2 and increases the conductivity of the composite electrode materials. Owing to the introduction of metallic phase WS2, the 1T@2H WS2@CFC electrode shows a higher specific capacity (1130 mAh g1 after 200 cycles at 0.1 A g1), excellent rate capability (510 mAh g1 after 800 cycles at 2 A g1), and a smaller initial discharge capacity loss (12.64%). Moreover, the integrated capacitive charge storage in the 1T@2H WS2@CFC electrode was investigated through in-depth research, which is beneficial to fast charge storage and long-term cyclability. This work paves a new simple and scalable approach to fabricate metallic TMDs and could be readily applicable to other TMDs and TMDs-based energy storage and conversion applications.
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Acknowledgment
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This work was supported by the National Natural Science Foundation of China (Contract No. 51572153, 51321091).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2017.10.154.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced reversible lithium ion storage in stable 1T@2H WS2 nanosheet arrays anchored on carbon fiber Tailin Wang, Changlong Sun, Mingzhi Yang, Lei Zhang, Yongliang Shao, Yongzhong Wu, Xiaopeng Hao* State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 August 2017 Received in revised form 12 October 2017 Accepted 24 October 2017 Available online 26 October 2017
Recent years, as a competitive anode material for lithium ion batteries (LIBs), tungsten disulfide (WS2) has attracted significant attention for its high theoretical capacity, abundant resources and low cost, but how to improve the cycle stability and reversible capacity is still an important research subject. In this work, stable metallic WS2 anchored on carbon fiber cloth was fabricated as flexible anode for LIBs. The transversal displacement of S atoms in WS2 was induced by incorporation of N atoms, and the stability of metallic phase WS2 was ensured by the formation of N-W covalent bonds. The results of electrochemical measurement and dynamics analysis demonstrate that the incorporation of metallic phase could not only reduce the initial discharge capacity loss, but also improve the reversible capacity than traditional semi-conducting phase WS2. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Nitrogen doping Metallic WS2 Carbon fiber Flexible bind-free anode
1. Introduction As prospective graphene-like two-dimensional (2D) materials, transitional metal sulfides (TMDs) have attracted substantial interest and investigation due to the unique morphology structure and excellent performance in a range of fields [1e6]. Among these 2D TMDs, tungsten disulfide (WS2) has achieved increasing attention due to its excellent thermal, mechanical, optical and electric performances [7e10]. Laminar WS2 is composed of sandwich construction stacked through the weak van der Waals interaction, while strong covalent bonds in-plane between S atoms and W atoms, which is beneficial for lithium ion storage (theoretical capacity ¼ 433 mAh g1) [11]. But the low electrical conductivity and unreliable stability during long cycles of WS2 in nature limits its potential application in LIBs [12e14]. To improve the electrochemical properties, two methods are commonly employed. One is combined with carbon-based materials, which were reported widely in the previous literature [15e17], another effective method is realizing the phase-transformation of WS2 from semi-conducting phase to metallic phase. The metallic (1T) phase WS2 (octahedral coordination) exhibits inequable performances and microstructure compared with
* Corresponding author. E-mail address: [email protected] (X. Hao). https://doi.org/10.1016/j.electacta.2017.10.154 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
semiconducting (2H) phase WS2 (trigonal prismatic coordination), and can be obtained by the transversal displacement of one of S atomic layer. However, relatively few literature have been reported in the fabrication and application of 1T WS2. Similar to MoS2 [18,19], the synthesis path and method for 1T WS2 mainly include rare metal doping [20], alkali metal and butyllithium intercalation [21e23], ammonia ions intercalation [24], pulsed laser deposition [25]. Liu [24]et al. synthesized 1T-WS2 nanoribbon by the intercalation of ammonia ions and 0.75 electrons per NHþ 4 are transferred to WS2. Similarly, Re, Tc, and Mn acting as electron donors can induce the phase transformation of WS2, and the formation of covalent bonds increases the stability of metallic WS2 greatly [20]. However, some shortcomings limit the application of these methods, such as complex preparation process, risk of safety or not suitable for large-scale preparation. Therefore to find a simple and efficient method obtaining 1T phase WS2 is the key to improve its electrochemical performances. In this study, we propose a novel and universal method for the fabrication of stable 1T phase WS2 via covalent bond between nitrogen and WS2. The introduction of nitrogen induces the phase transformation from 2H to 1T and the strong N-W bonds improve the stability of 1T phase. In particularly, 1T@2H WS2 anchored on the interlinked carbon fiber cloth (CFC) was used as flexible anode without any binder and current collector for lithium ion batteries (LIBs). The electrochemical measurement show that 1T phase
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WS2@CFC (1T@2H WS2@CFC) possess a smaller initial discharge capacity loss and a higher reversible capacity than pure 2H phase WS2@CFC (2H WS2@CFC), which is critical to the improvement of electrochemical performances. 2. Experimental section 2.1. Preparation of the carbon fiber cloth The hollow carbon fiber cloth was fabricated by the carbonization of degreasing cotton according to reference [14]. The detailed process is as follows: degreasing cotton was rinsed with ethanol for several times and fully dried before using. A level and smooth degreasing cotton was placed onto an aluminium oxide panel, meanwhile another aluminium oxide panel was placed on the cotton to exert a pressure on it. Then the degreasing cotton was transferred into a tubular furnace and heated to 1000 C for 2 h under an argon flow with a heating rate of 10 C min1, and the sample was cooled down to room temperature naturally after carbonization. The black, thinner and denser CFC with good flexibility was obtained. 2.2. Synthesis of 1T@2H WS2 nanosheets@CFC The WS2 nanosheets were synthesized by a solvothermal method. 1.4 g sodium tungstate (Na2WO4$2H2O, AR) and 1.50 g thiourea (CS(NH2)2, AR) were weighed and dissolved in 35 mL deionized (DI) water under vigorous stirring to form a homogeneous solution. Then the solution was carefully transferred into a 50 mL PPL-lined stainless-steel autoclave, maintained at 265 C for 24 h, and naturally cooled down to room temperature. The obtained black product was washed several times by DI water and dried at 80 C for the characterization of structure and performance. The WS2 nanosheets@CFC was obtained through a similar process except the denser CFC being placed in the PPL-lined. Then the composite was put into the tubular furnace for 2 h under argon or ammonia flow at 800 C and cooled down slowly, the final product was named 2H WS2 nanosheets@CFC (2H WS2@CFC) and 1T@2H WS2 nanosheets@CFC (1T@2H WS2@CFC) respectively. 2.3. Materials characterizations X-ray diffraction (XRD) was conducted using a Bruker D8 Advance X-ray diffraction with Cu Ka radiation (l ¼ 0.154178 nm). Raman spectra were acquired on a LabRam HR system from Horiba Jobin Yvon with 532 nm excitation. Scanning electron microscopy (SEM) images were recorded by Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 5.0 kV and energy-dispersive spectrometer (EDS) at 15.0 kV. Highresolution transmission electron microscopy (HRTEM) images were performed using a JEM-2100F high-resolution transmission electron microscope at an accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) measurement were carried out using a monochromatized XPS spectrometer (a ThermoFisher ESCALAB 250) with Al (Ka) radiation as the probe. The specific surface area were studied using Brurauer-Emmerr-Teller (BET) Procedure and pore distributions were analyzed. 2.4. Electrochemical measurements The electrochemical behaviors of the product were measured by CR2032-type coin cells, which were assembled in an argon-filled glove box. The 3D 1T@2H WS2@CFC was used as working electrode directly without any extra conductive additives and binder, meanwhile lithium foil was used as counter electrode and reference
electrode, and the electrolyte was 1 mol L1 LiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbonate/dimethyl carbonate (1:1:1 vol%). The separator was a polypropylene film (Celgard 2320). Electrochemical measurements were conducted by galvanostatic charge-discharge testing between 0.001 V and 3.0 V on a battery testing system BTS (Newarel Electronic Co., Ltd., China) at ambient temperature. The A.C. impedance spectra were tested on a CHI660D electrochemical workstation in the frequency range from 0.01 to 100 kHz and the cyclic voltammetry (CV) curves were performed between 0.001 V and 3.0 V (vs. Liþ/Li). 3. Results and discussion Fig. 1 illustrates the fabrication process of 1T@2H WS2@CFC. Flexible CFC was fabricated by the carbonization of degreasing cotton [14], and 2H WS2 nanosheets@CFC was obtained by a mild hydrothermal method, and then the composite was heated to 800 C for 2 h under an ammonia flow. The black product was then washed with distilled water and ethanol and then dried under vacuum at room temperature. Fig. 2a shows the X-ray diffraction patterns of WS2 nanosheets, as-prepared 2H WS2@CFC and 1T@2H WS2@CFC. It was observed that both WS2 nanosheets and 2H WS2@CFC show similar peaks [26], this means the WS2 nanostructure was maintained after being incorporated with CFC. However, the (002) peak of 1T@2H WS2@CFC was shifted left compared with 2H WS2@CFC, the difference was ascribed to the reaction of NH3 and WS2, which could expand the interlayer distance [27e29]. To further understand the structure of the 1T@2H WS2@CFC, Raman scattering was performed to investigate the transformation before and after the ammoniation process. As shown in Fig. 2b, there are two apparent peaks at 350 and 416 cm1 in 2H WS2@CFC, which are attributed to the in-plane E12g mode and out of plane A1g mode of the 2H phase WS2, respectively [30]. After being ammoniated, three additional Raman peaks emerge at low frequency region (130, 260 and 293 cm1), which are associated with the 2H WS2 contained 1T phase [31,32]. N dopant, as the donor atom, could induce the phase transition of MoSe2 by means of thermal treatment in NH3 atmosphere [33]. Enyashin [20] has pointed that the covalently bound lattice of the 1T-phase was more stable compared with the lattice of the alkali intercalated compound. This was because of the additional electrons from a donor atom could occupy the Mo 4dxy, Mo 4dyz, Mo 4dxz orbitals and increase the stability of the 1T-phase. The specific surface area was measured to investigate the porosity of composite as shown in Fig. 2c and d, the nitrogen adsorption-desorption isotherm is shown in Fig. 2c, and the BET surface is calculated to be as high as 212.5 m2 g1 (BET surface of 2H WS2@CFC is 196.4 m2 g1 (Fig. S1)), which is advantageous to promote the speedy delivery of lithium ion and electron between electrode and electrolyte [34e36]. Fig. 3a shows the SEM image of carbon fiber cloth, carbon fiber with a uniform diameter (ca. 5 mm) can be clearly observed, and the length is beyond several hundred micrometers. The evenly distributed carbon fibers are closely interwoven, which could not only serve as the carbon matrix but also benefit the speedy delivery of lithium ion during the process of charge/discharge. The SEM images of 2H WS2@CFC are shown in Fig. 3b and c, WS2 nanosheets are uniformly distributed on CFC, the transverse size of nanosheet is 200 nm and the thickness is about 10 nm. After being ammoniated, the morphology and structure of WS2 nanosheets remain stable (Fig. 3d), and the digital photo (inset in Fig. 3d) indicates that the composite remains good toughness. The elementary composition of 1T@2H WS2@CFC is confirmed by energy dispersive X-ray spectroscopy (EDS) in Fig. 3eef. The SEM image and corresponding elemental mappings reveal that nitrogen atoms in the WS2
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Fig. 1. Schematic illustration of the fabrication process for 1T@2H WS2@CFC and the schematic illustration of composite for LIBs.
Fig. 2. (a) XRD patterns of WS2 nanosheets, as-prepared 2H WS2@CFC and 1T@2H WS2@CFC. (b) Raman spectra of as-prepared 2H WS2@CFC and 1T@2H WS2@CFC. (c) Nitrogen adsorptionedesorption isotherms and (d) pore size distributions of 1T@2H WS2@CFC.
nanosheets distribute well. Fig. 3g shows that 1T@2H WS2@CFC contains only nitrogen, sulfur, and tungsten elements, indicating there is no impurity elements in the sample, and the mass ratio of 1T@2H WS2 is 90.06 wt% according to the TG results (Fig. S2). The typical mass of 1T@2H WS2@CFC with diameter of 12 mm was z1.6 mg, and the mass of 1T@2H WS2 was about 1.44 mg after calculation. Fig. 3h and i shows the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of 1T@2H WS2@CFC, the lamellar thickness of WS2 nanosheets is small, and 1T phase (trigonal lattice area, octahedral coordination) [22,30] is co-existed with 2H phase (honeycomb lattice area, trigonal prismatic coordination) [37]. The corresponding ball-and-stick models of 1T and 2H MoS2 were shown in inset images of Fig. 3i, and 1T MoS2 could be viewed as transversal displacement of one of S atomic layer in 2H MoS2, the lattice plane spacing of S atomic layer in 1T phase in keeping with 2H phase.
The high-resolution X-ray photoelectron spectra (XPS) of 1T@2H WS2@CFC and 2H WS2@CFC are shown in Fig. 4. Fig. 4aeb depict the HR-XPS spectra of W 4f and W 5p, the signal of the W 4f 7/2 and W 4f 5/2 is essential to distinguishing the discrepancy between the 1T and 2H structures. It can be clearly seen that the curve exhibits two apparent peaks of W 4f 7/2 and W 4f 5/2 at 32.7 eV and 35.1 eV in Fig. 4a, which are corresponding to the known 2H phase WS2 [38]. Besides the original peaks, two new peaks shift about 0.4 eV to lower binding energy than 2H phase WS2 after being ammoniated (Fig. 4b), which confirms the presence of 1T phase [21,25,37]. Fig. 4c shows the N-W bonds at 398.2 eV [39,40], and the peak located at 400.8 eV can be typically identified as the quaternary graphitic N [41], which have been proven to be conducive to improve performance [42]. Meanwhile the peaks at about 284.6 eV, 285.9 eV and 287.7 eV are corresponding to the bonds of C-C, C-N and C]N [39,43] in Fig. 4d. These results demonstrate that the
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Fig. 3. SEM images of (a) carbon fiber cloth, (b, c) 2H WS2@CFC and (d) 1T@2H WS2@CFC (inset of (d) is photographs of 1T@2H WS2@CFC). (e, f) The selected SEM image and corresponding element mapping profiles of 1T@2H WS2@CFC. (g) The EDS microanalysis on selected areas. (h, i) The TEM and HRTEM images of 1T@2H WS2@CFC, inset images of (i) are simulated of 1T and 2H MoS2 (S, blue; Mo, red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nitrogen are merged into WS2 nanosheets and carbon fiber cloth, and the obtained metallic phase WS2 and N-CFC are very conducive to improve the electrical conductivity of composites. The electrochemical properties of the 1T@2H WS2@CFC were investigated by galvanostatic charge/discharge and cyclic voltammetry (CV). Selected galvanostatic discharge/charge profiles of the composite electrode at a current density of 0.1 A g1 in the voltage range of 0.001e3.0 V vs Liþ/Li are displayed in Fig. 5a, the initial cycle curve exhibits two main discharge platforms at about 1.18 V and 0.7 V and two charge platforms at 1.95 V and 2.45 V. The charge and discharge capacities are 1280 mAh g1and 1456 mAh g1, respectively. What is the most important is that the second discharge capacities is 1272 mAh g1 and the initial discharge capacity loss is 12.64%, which are lower than the 21.6% of 2H WS2@CFC. The smaller initial discharge capacity loss can be ascribed to the incorporation of 1T phase [20] and the initial phase conversion before the electrochemical intercalation allows a more homogeneous phase conversion and uniform charge distribution compared to the 2H WS2 [28]. Fig. 5b shows CV curves of the first four cycles from 0.001 to 3.0 V vs Liþ/Li at the scan rate of 0.1 mV s1. Two main reduction peaks at 1.28 V and 0.14 V, which is corresponded to the lithium ion insertion into WS2 embedded in the CFC to form LixWS2, conversion reaction of Li with WS2, and electrolyte decomposition, respectively. Two oxidation peaks at 1.85 V and 2.49 V are found to correspond to the lithium extraction processes. From the second cycle, the reduction peak at 2.49 V disappears while new reduction peak appears in the potential range from 1.5 to 2.2 V [17], which can be assigned to the formation of a gel-like polymer layer formed out of the dissolution of the Li2S in the electrolyte, in accordance with previously reported characteristics of the WS2 electrode [11,44]. It's worth noting that no
obvious changes can be observed for the next two cycles, suggesting good architectural stability cycle stability of 1T@2H WS2@CFC electrode. The cyclic voltammetry profiles match well with the results of charge-discharge tests. Fig. 5c reveals that the discharge capacities are stable from the second cycle and a capacity of 1130 mAh g1 is achieved after 200 cycles at 0.1 A g1. For comparison, N-doped carbon fiber cloth electrode and 2H WS2@CFC is tested under the same current density (Figs. S3 and S4). Besides the achieved high specific capacity and superior cycling stability, the 1T@2H WS2@CFC electrode shows an excellent rate capability as well. The galvanostatic charge/ discharge profiles of sample at different current density are shown in Fig. 5d, the tenth-cycle discharge capacities of about 1251, 1015, 877, 706, 557 mAh g1 at the current densities of 0.1, 0.2, 0.5, 1, 2 A g1. Besides, the specific capacity of composite anode could recover to 1180 mAh g1 when the current density is recovered to 0.1 A g1. Fig. 5e reveals the long cycle performance of 1T@2H WS2@CFC electrode at high current density of 2 A g1. The discharge capacity can be stable at 510 mAh g1 after 800 cycles, which exhibits stable cycling performance and higher specific capacities than the 2H WS2@CFC (Fig. S4). Moreover, the HRTEM and XRD results of 1T@2H WS2@CFC electrode after 200 cycles at 0.1 A g1 (Fig. S5) are corresponds to the results of previous analysis. Electrochemical impedance spectroscopy (EIS) is effective for studying the mechanism and kinetics of electrode reaction. The Nyquist plots diagrams of 1T@2H WS2@CFC and 2H WS2@CFC electrode are shown in Fig. 6a, both impedance spectra consist of a semicircle in high frequency area and a straight line in low frequency area. According to the previous reports, the high-frequency semicircle is attributed to the charge-transfer impedance and the
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Fig. 4. XPS survey spectrums of W 4f and W 5p (a) 2H WS2@CFC and (b) 1T@2H WS2@CFC. XPS spectra of (c) N 1s and (d) C 1s of 1T@2H WS2@CFC sample.
Fig. 5. Electrochemical performance measurement: (a) Galvanostatic chargeedischarge profiles in the 0.001e3.0 V window (versus Liþ/Li) for the 1st, 2nd, 3rd, 50th, 100th and 200th cycles at 0.1 A g1; (b) Cyclic voltammetry of 1T@2H WS2@CFC electrode at a scan rate of 0.1 mV s1 between 0.001 and 3.0 V at the room temperature. Charge-discharge cycling and rate capabilities profiles of the 1T@2H WS2@CFC electrode at (c) 0.1 A g1, (d) various current densities and (e) 2 A g1.
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Fig. 6. (a) Nyquist plots and fitted curve for the 1T@2H WS2@CFC electrode. (b) Cyclic voltammetry (CV) curves from 0.1 to 10 mV s1 of 1T@2H WS2@CFC electrode. (c) Linear relationship of the peak current (ip) and the square root of the scan rate (v1/2). (d) Capacitive (gray) contribution of 1T@2H WS2@CFC to the total charge storage at 0.5 mV s1.
low-frequency line in correspond to the Warburg impedance (Zw) of electrode [45,46]. The impedance data are fitted by the equivalent circuit (inset of Fig. 6a), and the diffusion coefficient of lithium ion could be calculated by means of equations (1) and (2) from the plots in the low frequency region:
D¼
R2 T 2 2A2 n4 F 4 C 2 s2
Zreal ¼ Re þ Rct þ su1=2
(1)
(2)
where R represents the gas constant, F represents the Faraday’ s constant, A denotes the area of the electrode and C denotes the molar concentration of lithium ions. 1T@2H WS2@CFC electrode exhibits a lower impedance value (110 U) and higher diffusion coefficient of lithium ion (3.8 1015 cm2 s1) than 2H WS2@CFC (200 U, 1.4 1015 cm2 s1). It is deduced that the existence of 1T phase could reduce the impedances and improve the impedances of composite. Cyclic voltammetry (CV) has been proven to be a powerful technique to evaluate the electrochemical kinetics of electrode materials towards lithium ion battery. Here, kinetic analysis based on CV test was carried out to gain further insight into the electrochemistry of 1T@2H WS2@CFC electrode. Fig. 6b shows the CV curves at the scan rates of 0.1, 0.5, 1, 5 and 10 mV s1, respectively. The CV curves reveal two reductive peaks and two oxidation peaks, the relationship between the measured current (i) and the scan rate (v) can be described by equations (3) and (4): i ¼ avb
(3)
logi ¼ blogv þ loga
(4)
where i is the peak current, v is scan rate, and a, b are adjustable parameters. The b value can be derived from the slope of the log(v)log(i) curve, which reflects the amount of charge stored by capacitive and intercalation processes. If the b value is 0.5, the current density of the insertion reaction is proportional to the square root of the scan rate; if b ¼ 1, the current density of the capacitive contribution is proportional to the scanning rates. The log(v)-log(i) plots for the 1T@2H WS2@CFC electrode is shown in Fig. 6c, the calculated b values are 0.67, 0.77, 0.70 and 0.72, which is similar to the case of MoS2 [47]. Fig. 6d reveals the capacitive and diffusion-controlled charge storage contributions for 1T@2H WS2@CFC electrode at 0.5 mV s1, which can be calculated using equation (5): i(V) ¼ k1v þ k2v1/2
(5)
where n is the sweep rate, k1n and k2n1/2 represent current due to surface capacitive effects and current due to diffusion-controlled lithium ion insertion process, respectively. By determining the value of k1 and k2, the fraction of the current arising from Liþ diffusion can be distinguished from capacitive processes. The light gray area in Fig. 6d represents the capacitive contribution, total integrated capacitive charge storage is about 72%, in good agreement with the calculated b-values. The results of dynamics analysis demonstrate that the incorporation of 1T phase is conducive to promoting the electrochemical performances of WS2@CFC.
4. Conclusion To summarize, we demonstrate a versatile and simple route to obtain stable metallic WS2 nanosheets@carbon fiber cloth composite with large surface area and great potential for constructing
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high-performance flexible LIBs anode. The formation of strong N-W covalent bonds improves the stability of 1T WS2 and increases the conductivity of the composite electrode materials. Owing to the introduction of metallic phase WS2, the 1T@2H WS2@CFC electrode shows a higher specific capacity (1130 mAh g1 after 200 cycles at 0.1 A g1), excellent rate capability (510 mAh g1 after 800 cycles at 2 A g1), and a smaller initial discharge capacity loss (12.64%). Moreover, the integrated capacitive charge storage in the 1T@2H WS2@CFC electrode was investigated through in-depth research, which is beneficial to fast charge storage and long-term cyclability. This work paves a new simple and scalable approach to fabricate metallic TMDs and could be readily applicable to other TMDs and TMDs-based energy storage and conversion applications.
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Acknowledgment
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This work was supported by the National Natural Science Foundation of China (Contract No. 51572153, 51321091).
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2017.10.154.
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