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Single layers of WS2 nanoplates embedded in nitrogen-doped carbon nanofibers as anode materials for lithium-ion batteries† Received 00th January 20xx, Accepted 00th January 20xx
Sunmoon Yu, Ji-Won Jung and Il-Doo Kim*
DOI: 10.1039/x0xx00000x www.rsc.org/nanoscale
Single layers of WS2 nanoplates are uniformly embedded in nitrogen-doped carbon nanofibers (WS2@NCNFs) via facile electrospinning method. Crystallization of the single-layer WS2 nanoplates and in situ nitrogen doping into the carbon nanofibers were simultaneously accomplished during a two-step heat treatment. The distinctive structure of the WS2@NCNFs enables outstanding electrochemical performances. Transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have recently attracted enormous interest due to their fascinating physicochemical properties, especially when they are confined to few layers or to a single layer.1,2 Such attention can be ascribed to the interesting phenomenon that a reduction in the dimensionality leads to a variety of exotic characteristics which differ from those of their bulk material counterparts. For example, single layers of TMDs are inherently flexible and mechanically strong, and their electronic structure can be significantly modified when TMDs are reduced to two dimensions.3-5 In an effort to take advantage of their unique properties, the two-dimensional (2-D) TMDs have been extensively studied, with results showing that they may be feasible in a variety of applications, such as high-performance electronics, hydrogen evolution reaction (HER) catalysts, and energy storage devices given their considerably enhanced performance compared to that of the three-dimensional (3-D) bulk TMDs.3,4,6-9 In particular, single-layer TMDs are regarded as promising anode materials for lithium-ion batteries because nano-sized, single-layered TMDs offer remarkable advantages in terms of additional interfacial lithium storage, a short lithium ion diffusion path, and great strain accommodation.10-13 However, while many approaches have been proposed including
mechanical cleavage,14-16 electrochemical exfoliation,17 and chemical vapor deposition (CVD),18 the preparation of single-layer TMDs requires complex and costly procedures. Therefore, a facile method to obtain single-layer TMDs is in great demand. WS2 has not been intensively explored for various applications, but a few studies have reported that it can be utilized in electrochemical devices such as supercapacitors,19 and lithium-ion batteries.19-25 When it is used as an anode material for lithium-ion batteries, given its larger theoretical specific capacity of 433 mAh g-1 compared to that of commercial graphite, it has been proved that lithium ions are stored in the form of lithium sulfide (Li2S) while electrons are stored in metallic tungsten (W) through a conversion reaction mechanism.11,19,20,26 Generally, TMDs show less of a volumetric expansion upon lithiation in comparison with Si and metal oxides, but they are also associated with a large volume change as well.11,27 In addition, although TMDs have higher electrical and ionic conductivities than transition metal oxides, they still possess semiconducting characteristics, which deteriorate their rate capabilities.11,28,29 Recently, some research efforts have been devoted to nanostructure engineering to handle the volume expansion issue; however, they could not effectively address the electrical conductivity problem, with the results showing poor rate capabilities.20,30,31 Several strategies have been suggested to simultaneously deal with the volume expansion problem and to provide facile electron pathways, incorporating WS2 into conductive carbonaceous materials such as graphene and carbon nanotubes (CNTs), though these processes are complicated.13,19,23,26,28,32 Hence, electrospinning method may be a good candidate as it is a facile and versatile technique which has the capability to fabricate onedimensional (1-D), interconnected carbon networks in which various types of nano-sized particles can be evenly embedded.33-36 These carbon networks are able to not only accommodate the volume change but also offer fast electron transport channels. Herein, we report a facile one-pot synthetic route by which the crystallization of WS2 single layers in electrospun, polyacrylonitrile (PAN) based carbon nanofibers, and in situ nitrogen (N) doping into the carbon nanofibers are concurrently achieved during a two-step heat treatment. The single layers of WS2 nanoplates are uniformly embedded into the N-doped carbon nanofibers (WS2@NCNFs) due
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to their confined growth. The distinctive design of the WS2@NCNFs enables outstanding electrochemical performances, including a large specific capacity (an initial charge capacity of 590.4 mAh g-1 at a current density of 0.1 A g-1), a high initial Coulombic efficiency (81.7%), excellent capacity retention (437.5 mAh g-1 after 200 cycles at a current density of 0.5 A g-1), and great rate capability (367.1 mAh g-1 at a current density of 2 A g-1). Fig. 1a shows a schematic illustration of the preparation procedure of WS2@NCNFs, and the atomic structure of single-layer WS2 nanoplates embedded at different angles in the NCNFs. The WS2@NCNFs were synthesized by means of the single-spinneret electrospinning of WS2 precursor@PAN nanofibers (NFs) followed by a two-step heat treatment under a reducing atmosphere (H2/N2, 5%/95%, v/v). The first heating step at 400 °C was conducted to stabilize the matrix PAN polymer and to thermally decompose the WS2 precursor as expressed in reaction (1) and (2), while the second heating step at 700 °C was carried out to carbonize the N-doped CNFs and to ensure the crystallization of the single phase WS2.36,37 (NH4)2WS4 → 2NH3 + H2S + WS3 WS3 → WS2 + S
(1) (2)
During the thermal decomposition step, NH3 gas is generated as expressed in reaction (1), participating in the in situ N-doping of the carbon nanofibers, further implemented by the flowing reducing gas.24 The morphology of the heat-treated WS2@NCNFs was more entangled and interconnected compared to the as-spun WS2 precursor@PAN NFs, which were smooth and straight, as shown in the scanning electron microscopy (SEM) images (Fig. 1b and c). The surface area of the WS2@NCNFs determined by Brunauer-EmmettTeller (BET) analysis was 6.84 m2 g-1. For comparison purposes, we also synthesized bulk WS2 powder using the same precursors through the same two-step heat treatment, grinding it for use as an anode material, the morphology of which are shown in Fig. S1.† Fig.
1d shows the thermogravimetry (TG) curve of the WS2@NCNFs. The weight loss of 6.5 wt% below 415 °C indicates the oxidation of WS2 into WO3, which is in good agreement with the calculated value.28 The weight loss of 22.4 wt% above 415 °C represents the decomposition of the NCNFs,35 which is also consistent with the carbon content of the WS2@NCNFs as determined in an elemental analysis (23.7 wt%). As shown in Fig. 1e, the X-ray diffraction (XRD) pattern of the WS2@NCNFs (P63/mmc space group, JCPDS no. 08-0237) is clearly distinguished from that of the WS2 powder, in that the (002) peak for the c-axis is absent whereas the (100) and (110) peaks for the ab-plane are present. Absence of the (002) peaks implies that the WS2 nanoplates are few layered or single layered.21,24,36 From the transmission electron microscopy (TEM) images of the WS2@NCNFs shown in Fig. 2a and b, we were able to confirm that single-layer WS2 nanoplates, with the thickness of ca. 0.31 nm and the lateral dimension of ca. 5 nm, were evenly dispersed in N-doped CNFs. In contrast, multiple layers (5-12 layers) of WS2 nanoplates with much longer lateral dimensions (ca. 25 nm) were observed for the WS2 powder (Fig. S2a†), indicating that single layers of WS2 nanoplates were obtained via the electrospinning method due to their confined growth within the NCNFs. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image shown in Fig. 2c more clearly reveals that the single-layer WS2 nanoplates are randomly oriented in the NCNFs. Fig. 2d shows a single-layer WS2 nanoplate embedded at a tilted angle with an interlayer distance of 0.27 nm in the (100) plane, while Fig. 2e shows the (002) plane of a single layered WS2 nanoplate from which the lattice fringes along the (100) and (110) planes can be observed. These HAADF-STEM images explicitly verify the good crystallinity of WS2. The lower insets in Fig. 2d and e present the atomic structures of the single-layer WS2 nanoplates in the dashed squares, and the upper inset in Fig. 2e shows the fast Fourier transformation (FFT) pattern of the dashed square indicated in Fig. 2e. The selected
Fig. 1 (a) Schematic illustration of the synthetic procedure of WS2@NCNFs, and top view (viewed along the c-axis) and side view (viewed along the a-axis) images of the atomic structures of the single-layer WS2 nanoplates embedded at different angles in the NCNFs. SEM images of (b) the as-spun WS2 precursor@PAN NFs and (c) the heat-treated WS2@NCNFs. (d) TG curve of the WS2@NCNFs. (e) XRD patterns of the WS2@NCNFs and the WS2 powder.
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Fig. 2 (a and b) TEM images and (c-e) HAADF-STEM images of WS2@NCNFs. The lower insets shown in (d and e) represent the atomic structures of single-layer WS2 nanoplates in the dashed squares. Green and yellow balls indicate W and S atoms, respectively. The upper inset shown in (e) is the FFT pattern of the dashed square marked in (e). (f) SAED patterns of the WS2@NCNFs. (g) STEM image and STEM-EDS mapping image of the WS2@NCNFs: W (green), S (yellow), C (red), and N (blue). area electron diffraction (SAED) pattern with the absence of the (002) plane in Fig. 2f further demonstrates that the embedded WS2 nanoplates are single layered, which is in good agreement with the XRD data in which the (002) plane peak is absent. In contrast, the SAED pattern of the WS2 powder in Fig. S2b† shows the existence of the (002) plane with more obvious diffraction rings, signifying that the WS2 powder is multilayered with better crystallinity. From the scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDS) mapping image (Fig. 2g), it was proved that the WS2 nanoplates were homogeneously distributed and that the N-doping was successfully accomplished. The Raman scattering spectra of the WS2@NCNFs shown in Fig. 3a confirms that the crystalline WS2 phase was successfully synthesized.12,24 The peak intensity of the out-of-plane vibrational mode (A1g) at 416 cm-1 was relatively low compared to that of the in-plane vibrational mode (E12g) at 352 cm-1 due to the few-layer effect as previously reported.20,38 For the WS2 powder, the peak intensity of the A1g mode was comparable to that of the E12g mode (Fig. S3†). The D-band related to the defects and disorder in the NCNFs was located at 1363 cm-1, while the G-band associated with the ordered graphitic crystallites in the NCNFs was observed at 1601 cm-1.26,35,39 The relative intensity ratio of the D-band to the Gband (ID/IG) was 0.97, suggesting that the synthesized NCNFs consist of mainly amorphous carbons with minor crystalline graphene sheets.35 The broad X-ray photoelectron spectroscopy (XPS) spectra in Fig. 3b confirm the presence of W, S, C, and N elements. The high resolution XPS spectra of W 4f and W 5p are shown in Fig. 3c. The peaks found at 32.9, 35.1 and 38.5 eV are assigned to W 4f7/2, W 4f5/2, and W 5p3/2, respectively, from which it can be concluded that the single layers of WS2 are not oxidized. In Fig. 3d, two S 2p peaks were identified at 162.2 and 163.3 eV,
Fig. 3 (a) Raman spectra of WS2@NCNFs. The in-plane vibrational mode (E12g) and out-of-plane vibrational mode (A1g) are illustrated in the inset, where green and yellow balls represent W and S atoms, respectively. XPS spectra of the WS2@NCNFs: (b) broad scan spectra, (c) W 4f, (d) S 2p, (e) C 1s, and (f) N 1s. corresponding to S 2p5/2 and S 2p3/2. The C 1s peak in Fig. 3e was deconvoluted into C-C, C-N, C=O, and CO-O in the NCNFs, which were located at 284.1, 285.1, 287, and 291.2 eV, respectively. Nitrogen atoms exist in three different forms as the N 1s peaks can be assigned to pyridinic-N, pyrrolic-N, and graphiticN. These were observed at 398.1, 400.2, and 403.1 eV, respectively (Fig. 3f).40 Both pyridinic N and graphitic N in the carbon host are sp2 hybridized; thus, they can improve the electrical conductivity of the carbon networks, leading to facile charge transfer, and consequently enhanced rate capability of the [email protected] Furthermore, the N-doping in the carbon nanofibers can create defects and vacancies in the composite structure, both of which can act as additional lithium storage sites, giving rise to a high specific capacity.40 All of the peak locations in the XPS spectra are in good agreement with those in former reports.19,26,32,40,42 The electrochemical properties of the WS2@NCNFs were investigated by cyclic voltammetry (CV) for the three initial cycles in a voltage range of 0.01-3.0 V at a scan rate of 0.1 mV s-1, as shown in Fig. 4a. In the first cycle, a reduction peak at 1.27 V was observed; it corresponds to lithium insertion into WS2 embedded in the NCNFs to form LixWS2.23,26 The following reduction peak at 0.57 V can be ascribed to the subsequent conversion reaction of Li with WS2, and electrolyte decomposition.25,35 After the first cycle, the reduction peak at 0.57 V disappeared, while the original
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Fig. 4 (a) CV curves of WS2@NCNFs over a voltage range of 0.01-1 3.0 V at a scanning rate of 0.1 mV s . (b) Discharge/Charge voltage -1 profile of the WS2@NCNFs at a current density of 0.1 A g . (c) Rate capabilities of the WS2@NCNFs and WS2 powder at current -1 densities of 0.1, 0.2, 0.5, 1, 2, and 0.1 A g . (d) Nyquist plots of the WS2@NCNFs and the WS2 powder with the inset showing an equivalent circuit model. The solid circles and solid curves indicate the experimental data and the fitted data, respectively. (e) Galvanostatic cycling performance of the WS2@NCNFs and the WS2 powder, and Coulombic efficiency of the WS2@NCNFs at a -1 current density of 0.5 A g . reduction peak at 1.27 V was shifted to 1.83 V.26 During anodic scans, oxidation peaks at 1.43 and 2.18 V were found to correspond to the lithium extraction processes.23,25 Fig. 4b shows the galvanostatic charge and discharge curves of the WS2@NCFNs at a constant current density of 0.1 A g-1. The CV curves and voltage profiles of the WS2 powder are shown in Fig. S4.† The sloping characteristic of the voltage profile of the WS2@NCNFs compared to that of the WS2 powder can be explained in part by the nano-size effects, which reduces the miscibility gap for the two-phase reaction.11,43,44 The anode electrode with the WS2@NCNFs delivered an initial discharge capacity of 722.3 mAh g-1 and a subsequent charge capacity of 590.4 mAh g-1. The measured specific capacity of the WS2@NCNFs was larger than the theoretical capacity of bulk WS2. This can be attributed to additional lithium storage sites, which include interfacial sites of single layered WS2 nanoplates.36 In addition, partially negatively charged W nanoparticles, which are formed after conversion reaction, can provide additional interfacial lithium storage sites.36,45,46 Since W does not alloy with lithium, lithium can be adsorbed on W nanoparticles with a partial charge transfer.36,45,46 The pseudo-capacitive behavior of transition metal dichalcogenides such MoS247,48 and WS219,20 has been reported previously. These
additional lithium storage sites offer possible explanations of the excess capacity. Fig. S5a† shows contributions of insertion, conversion, and interfacial reaction to the specific capacity of the composite anode in the initial discharge voltage profile. The schematic diagram in Fig. S5b† illustrates a single layered WS2 nanoplate undergoing insertion, conversion, and interfacial reaction consecutively during lithiation. Transition metal sulfides with conversion reaction generally show relatively low initial Coulombic efficiencies due to the irreversible capacity loss in the first cycle.11,27,35 It is important to note that the initial Coulombic efficiency of the WS2@NCNFs was as high as 81.7%, exhibiting excellent high capacity retention capability after the first cycle. Fig. 4c reveals tenth-cycle discharge capacities of 596.2, 580, 541, 496, and 367.1 mAh g-1 at current densities of 0.1, 0.2, 0.5, 1, and 2 A g1 , respectively. In addition, when the current density was reversed back to 0.1 A g-1, the WS2@NCNFs anode was able to recover to 628.9 mAh g-1. In contrast, the anode electrode using the WS2 powder exhibited poor rate performances at high current densities. At a current density of 2 A g-1, it delivered only 99.6 mAh g-1. The rate capabilities of the N-doped carbon nanofibers (NCNFs) and their galvanostatic cycling performance are shown in Fig. S6.† Also, the contribution of the NCNFs to the specific capacity of WS2@NCNFs was calculated in Table S1.† The result proves that the NCNFs can provide rapid pathways for electrons, allowing rapid charge transfers49,50 because it is widely accepted that Ndoping in carbon materials can effectively enhance its electrical conductivity.35,42 This is further supported by the electrochemical impedance spectroscopy (EIS) measurements. Nyquist plots after 50 cycles are shown in Fig. 4d. The second semicircular diameter in the medium-frequency region represents the charge-transfer resistance (Rct). The charge-transfer resistance value of the WS2@NCNFs (5.77 Ω) was much lower than that of the WS2 powder (33.2 Ω). Fig. 5e compares the cyclability of the WS2@NCNFs with that of the WS2 powder at a current density of 0.5 A g-1. The discharge capacities of the WS2@NCNFs for the second and 200th cycles were 502 and 437.5 mAh g-1, respectively. Thus, the corresponding capacity retention measured after the first cycle was 87.2% whereas the capacity of the WS2 powder severely decayed as it was cycled. This can be attributed to the N-doped carbon matrix, which effectively accommodates variations in the volume and isolates single-layer WS2 nanoplates from each other, preventing their restacking and the Ostwald ripening of the W nanoparticles formed by the phase separation of WS2, which can result in electrochemical pulverization.32,35,36 The morphologies of the WS2@NCNFs after cycling were well maintained as shown in Fig. S7.† The single layered WS2 nanoplates were not found after the first cycle in the high-resolution TEM images, which is indicative of irreversible conversion reaction of WS2. However, ultrasmall W nanoparticles were uniformly distributed in the carbon nanofibers, which were detected by STEM image and STEM-EDS mapping images as shown in Fig. S8† even though their lattice spacing could not be identified in the TEM images. In the initial cycle regions, the capacity of the WS2@NCNFs and the WS2 powder increases gradually and saturates to a certain capacity value in both cases. This activation behavior has been frequently observed in other transition metal sulfide anode materials.6,18,25 This can be ascribed to the degradation of crystallinity of the
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anodes as well as to the formation of a gel-like polymeric surface layer.11,19,23,32,35
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Conclusions
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In summary, we have demonstrated for the first time that single layers of WS2 nanoplates can be homogeneously embedded in N-doped CNFs via electrospinning method. They exhibited remarkably enhanced electrochemical performances, including a large specific capacity, a high initial Coulombic efficiency, excellent capacity retention, and great rate capability. These results can be attributed to several factors: i) the single layers of WS2 nanoplates can provide numerous interfacial lithium storage sites besides insertion and conversion reaction sites; ii) the carbon matrix can inhibit the restacking of WS2 single layers and the aggregation of W nanoparticles after cycling; iii) the 1-D interconnected carbon networks can buffer the volume expansion and facilitate charge transfers during lithiation/delithiation; iv) N-doping in CNFs can enhance the electrical conductivity of the carbon networks. Furthermore, due to the distinctive structural characteristics of the WS2@NCNFs, there are innumerable active sites for electrochemical reactions with facile electron transport. Thus, it is expected that WS2@NCNFs have the potential to be used in not only high capacity anode materials for lithium-ion batteries but also in other energy storage and/or conversion applications, such as supercapacitors, hydrogen evolution reaction (HER) catalysts, oxygen reduction reaction (ORR) catalysts, and counter electrode materials for dye-sensitized solar cells (DSSCs). This work was supported by a grant from the Korea CCS R&D Center (KCRC) funded by the Korean government (Ministry of Science, ICT & Future Planning) (no. NRF2014M1A8A1049303).
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39 C. Kim, S. -H. Park, J. -I. Cho, D. -Y. Lee, T. -J. Park, W. J. Lee and K. -S. Yang, J. Raman Spectrosc., 2004, 35, 928933. 40 B. Zhang, Y. Yu, Z. -L. Xu, S. Abouali, M. Akbari, Y. -B. He, F. Kang and J. -K. Kim, Adv. Energy Mater., 2014, 4, 1301448. 41 D. Nan, Z. -H. Huang, R. Lv, L. Yang, J. -G. Wang, W. Shen, Y. Lin, X. Yu, L. Ye, H. Sun and F. Kang, J. Mater. Chem. A, 2014, 2, 19678-19684. 42 Y. Qiu, J. Yu, T. Shi, X. Zhou, X. Bai and J. Y. Huang, J. Power Sources, 2011, 196, 9862-9867. 43 J. Maier, Nat. Mater., 2005, 4, 805-815. 44 W. -H. Ryu, J. -W. Jung, K. Park, S. -J. Kim and I. -D. Kim, Nanoscale, 2014, 6, 10975-10981. 45 J. Jamnik and J. Maier, Phys. Chem. Chem. Phys., 2003, 5, 5215-5220. 46 J. Maier, J. Power Sources, 2007, 174, 569-574. 47 X. Fang, X. Guo, Y. Mao, C. Hua, L. Shen, Y. Hu, Z. Wang, F. Wu and L. Chen, Chem. - Asian J., 2012, 7, 1013-1017. 48 X. Fang, C. Hua, X. Guo, Y. Hu, Z. Wang, X. Gao, F. Wu, J. Wang and L. Chen, Electrochim. Acta, 2012, 81, 155-160. 49 J. Jin, Z. -q. Shi and C. -y. Wang, Electrochim. Acta, 2014, 141, 302-310. 50 C. Kim, K. S. Yang, M. Kojima, K. Yoshida, Y. J. Kim, Y. A. Kim and M. Endo, Adv. Funct. Mater., 2006, 16, 23932397.
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Single layers of WS2 nanoplates embedded in nitrogen-doped carbon nanofibers as anode materials for lithium-ion batteries† Received 00th January 20xx, Accepted 00th January 20xx
Sunmoon Yu, Ji-Won Jung and Il-Doo Kim*
DOI: 10.1039/x0xx00000x www.rsc.org/nanoscale
Single layers of WS2 nanoplates are uniformly embedded in nitrogen-doped carbon nanofibers (WS2@NCNFs) via facile electrospinning method. Crystallization of the single-layer WS2 nanoplates and in situ nitrogen doping into the carbon nanofibers were simultaneously accomplished during a two-step heat treatment. The distinctive structure of the WS2@NCNFs enables outstanding electrochemical performances. Transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have recently attracted enormous interest due to their fascinating physicochemical properties, especially when they are confined to few layers or to a single layer.1,2 Such attention can be ascribed to the interesting phenomenon that a reduction in the dimensionality leads to a variety of exotic characteristics which differ from those of their bulk material counterparts. For example, single layers of TMDs are inherently flexible and mechanically strong, and their electronic structure can be significantly modified when TMDs are reduced to two dimensions.3-5 In an effort to take advantage of their unique properties, the two-dimensional (2-D) TMDs have been extensively studied, with results showing that they may be feasible in a variety of applications, such as high-performance electronics, hydrogen evolution reaction (HER) catalysts, and energy storage devices given their considerably enhanced performance compared to that of the three-dimensional (3-D) bulk TMDs.3,4,6-9 In particular, single-layer TMDs are regarded as promising anode materials for lithium-ion batteries because nano-sized, single-layered TMDs offer remarkable advantages in terms of additional interfacial lithium storage, a short lithium ion diffusion path, and great strain accommodation.10-13 However, while many approaches have been proposed including
mechanical cleavage,14-16 electrochemical exfoliation,17 and chemical vapor deposition (CVD),18 the preparation of single-layer TMDs requires complex and costly procedures. Therefore, a facile method to obtain single-layer TMDs is in great demand. WS2 has not been intensively explored for various applications, but a few studies have reported that it can be utilized in electrochemical devices such as supercapacitors,19 and lithium-ion batteries.19-25 When it is used as an anode material for lithium-ion batteries, given its larger theoretical specific capacity of 433 mAh g-1 compared to that of commercial graphite, it has been proved that lithium ions are stored in the form of lithium sulfide (Li2S) while electrons are stored in metallic tungsten (W) through a conversion reaction mechanism.11,19,20,26 Generally, TMDs show less of a volumetric expansion upon lithiation in comparison with Si and metal oxides, but they are also associated with a large volume change as well.11,27 In addition, although TMDs have higher electrical and ionic conductivities than transition metal oxides, they still possess semiconducting characteristics, which deteriorate their rate capabilities.11,28,29 Recently, some research efforts have been devoted to nanostructure engineering to handle the volume expansion issue; however, they could not effectively address the electrical conductivity problem, with the results showing poor rate capabilities.20,30,31 Several strategies have been suggested to simultaneously deal with the volume expansion problem and to provide facile electron pathways, incorporating WS2 into conductive carbonaceous materials such as graphene and carbon nanotubes (CNTs), though these processes are complicated.13,19,23,26,28,32 Hence, electrospinning method may be a good candidate as it is a facile and versatile technique which has the capability to fabricate onedimensional (1-D), interconnected carbon networks in which various types of nano-sized particles can be evenly embedded.33-36 These carbon networks are able to not only accommodate the volume change but also offer fast electron transport channels. Herein, we report a facile one-pot synthetic route by which the crystallization of WS2 single layers in electrospun, polyacrylonitrile (PAN) based carbon nanofibers, and in situ nitrogen (N) doping into the carbon nanofibers are concurrently achieved during a two-step heat treatment. The single layers of WS2 nanoplates are uniformly embedded into the N-doped carbon nanofibers (WS2@NCNFs) due
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to their confined growth. The distinctive design of the WS2@NCNFs enables outstanding electrochemical performances, including a large specific capacity (an initial charge capacity of 590.4 mAh g-1 at a current density of 0.1 A g-1), a high initial Coulombic efficiency (81.7%), excellent capacity retention (437.5 mAh g-1 after 200 cycles at a current density of 0.5 A g-1), and great rate capability (367.1 mAh g-1 at a current density of 2 A g-1). Fig. 1a shows a schematic illustration of the preparation procedure of WS2@NCNFs, and the atomic structure of single-layer WS2 nanoplates embedded at different angles in the NCNFs. The WS2@NCNFs were synthesized by means of the single-spinneret electrospinning of WS2 precursor@PAN nanofibers (NFs) followed by a two-step heat treatment under a reducing atmosphere (H2/N2, 5%/95%, v/v). The first heating step at 400 °C was conducted to stabilize the matrix PAN polymer and to thermally decompose the WS2 precursor as expressed in reaction (1) and (2), while the second heating step at 700 °C was carried out to carbonize the N-doped CNFs and to ensure the crystallization of the single phase WS2.36,37 (NH4)2WS4 → 2NH3 + H2S + WS3 WS3 → WS2 + S
(1) (2)
During the thermal decomposition step, NH3 gas is generated as expressed in reaction (1), participating in the in situ N-doping of the carbon nanofibers, further implemented by the flowing reducing gas.24 The morphology of the heat-treated WS2@NCNFs was more entangled and interconnected compared to the as-spun WS2 precursor@PAN NFs, which were smooth and straight, as shown in the scanning electron microscopy (SEM) images (Fig. 1b and c). The surface area of the WS2@NCNFs determined by Brunauer-EmmettTeller (BET) analysis was 6.84 m2 g-1. For comparison purposes, we also synthesized bulk WS2 powder using the same precursors through the same two-step heat treatment, grinding it for use as an anode material, the morphology of which are shown in Fig. S1.† Fig.
1d shows the thermogravimetry (TG) curve of the WS2@NCNFs. The weight loss of 6.5 wt% below 415 °C indicates the oxidation of WS2 into WO3, which is in good agreement with the calculated value.28 The weight loss of 22.4 wt% above 415 °C represents the decomposition of the NCNFs,35 which is also consistent with the carbon content of the WS2@NCNFs as determined in an elemental analysis (23.7 wt%). As shown in Fig. 1e, the X-ray diffraction (XRD) pattern of the WS2@NCNFs (P63/mmc space group, JCPDS no. 08-0237) is clearly distinguished from that of the WS2 powder, in that the (002) peak for the c-axis is absent whereas the (100) and (110) peaks for the ab-plane are present. Absence of the (002) peaks implies that the WS2 nanoplates are few layered or single layered.21,24,36 From the transmission electron microscopy (TEM) images of the WS2@NCNFs shown in Fig. 2a and b, we were able to confirm that single-layer WS2 nanoplates, with the thickness of ca. 0.31 nm and the lateral dimension of ca. 5 nm, were evenly dispersed in N-doped CNFs. In contrast, multiple layers (5-12 layers) of WS2 nanoplates with much longer lateral dimensions (ca. 25 nm) were observed for the WS2 powder (Fig. S2a†), indicating that single layers of WS2 nanoplates were obtained via the electrospinning method due to their confined growth within the NCNFs. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image shown in Fig. 2c more clearly reveals that the single-layer WS2 nanoplates are randomly oriented in the NCNFs. Fig. 2d shows a single-layer WS2 nanoplate embedded at a tilted angle with an interlayer distance of 0.27 nm in the (100) plane, while Fig. 2e shows the (002) plane of a single layered WS2 nanoplate from which the lattice fringes along the (100) and (110) planes can be observed. These HAADF-STEM images explicitly verify the good crystallinity of WS2. The lower insets in Fig. 2d and e present the atomic structures of the single-layer WS2 nanoplates in the dashed squares, and the upper inset in Fig. 2e shows the fast Fourier transformation (FFT) pattern of the dashed square indicated in Fig. 2e. The selected
Fig. 1 (a) Schematic illustration of the synthetic procedure of WS2@NCNFs, and top view (viewed along the c-axis) and side view (viewed along the a-axis) images of the atomic structures of the single-layer WS2 nanoplates embedded at different angles in the NCNFs. SEM images of (b) the as-spun WS2 precursor@PAN NFs and (c) the heat-treated WS2@NCNFs. (d) TG curve of the WS2@NCNFs. (e) XRD patterns of the WS2@NCNFs and the WS2 powder.
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Fig. 2 (a and b) TEM images and (c-e) HAADF-STEM images of WS2@NCNFs. The lower insets shown in (d and e) represent the atomic structures of single-layer WS2 nanoplates in the dashed squares. Green and yellow balls indicate W and S atoms, respectively. The upper inset shown in (e) is the FFT pattern of the dashed square marked in (e). (f) SAED patterns of the WS2@NCNFs. (g) STEM image and STEM-EDS mapping image of the WS2@NCNFs: W (green), S (yellow), C (red), and N (blue). area electron diffraction (SAED) pattern with the absence of the (002) plane in Fig. 2f further demonstrates that the embedded WS2 nanoplates are single layered, which is in good agreement with the XRD data in which the (002) plane peak is absent. In contrast, the SAED pattern of the WS2 powder in Fig. S2b† shows the existence of the (002) plane with more obvious diffraction rings, signifying that the WS2 powder is multilayered with better crystallinity. From the scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDS) mapping image (Fig. 2g), it was proved that the WS2 nanoplates were homogeneously distributed and that the N-doping was successfully accomplished. The Raman scattering spectra of the WS2@NCNFs shown in Fig. 3a confirms that the crystalline WS2 phase was successfully synthesized.12,24 The peak intensity of the out-of-plane vibrational mode (A1g) at 416 cm-1 was relatively low compared to that of the in-plane vibrational mode (E12g) at 352 cm-1 due to the few-layer effect as previously reported.20,38 For the WS2 powder, the peak intensity of the A1g mode was comparable to that of the E12g mode (Fig. S3†). The D-band related to the defects and disorder in the NCNFs was located at 1363 cm-1, while the G-band associated with the ordered graphitic crystallites in the NCNFs was observed at 1601 cm-1.26,35,39 The relative intensity ratio of the D-band to the Gband (ID/IG) was 0.97, suggesting that the synthesized NCNFs consist of mainly amorphous carbons with minor crystalline graphene sheets.35 The broad X-ray photoelectron spectroscopy (XPS) spectra in Fig. 3b confirm the presence of W, S, C, and N elements. The high resolution XPS spectra of W 4f and W 5p are shown in Fig. 3c. The peaks found at 32.9, 35.1 and 38.5 eV are assigned to W 4f7/2, W 4f5/2, and W 5p3/2, respectively, from which it can be concluded that the single layers of WS2 are not oxidized. In Fig. 3d, two S 2p peaks were identified at 162.2 and 163.3 eV,
Fig. 3 (a) Raman spectra of WS2@NCNFs. The in-plane vibrational mode (E12g) and out-of-plane vibrational mode (A1g) are illustrated in the inset, where green and yellow balls represent W and S atoms, respectively. XPS spectra of the WS2@NCNFs: (b) broad scan spectra, (c) W 4f, (d) S 2p, (e) C 1s, and (f) N 1s. corresponding to S 2p5/2 and S 2p3/2. The C 1s peak in Fig. 3e was deconvoluted into C-C, C-N, C=O, and CO-O in the NCNFs, which were located at 284.1, 285.1, 287, and 291.2 eV, respectively. Nitrogen atoms exist in three different forms as the N 1s peaks can be assigned to pyridinic-N, pyrrolic-N, and graphiticN. These were observed at 398.1, 400.2, and 403.1 eV, respectively (Fig. 3f).40 Both pyridinic N and graphitic N in the carbon host are sp2 hybridized; thus, they can improve the electrical conductivity of the carbon networks, leading to facile charge transfer, and consequently enhanced rate capability of the [email protected] Furthermore, the N-doping in the carbon nanofibers can create defects and vacancies in the composite structure, both of which can act as additional lithium storage sites, giving rise to a high specific capacity.40 All of the peak locations in the XPS spectra are in good agreement with those in former reports.19,26,32,40,42 The electrochemical properties of the WS2@NCNFs were investigated by cyclic voltammetry (CV) for the three initial cycles in a voltage range of 0.01-3.0 V at a scan rate of 0.1 mV s-1, as shown in Fig. 4a. In the first cycle, a reduction peak at 1.27 V was observed; it corresponds to lithium insertion into WS2 embedded in the NCNFs to form LixWS2.23,26 The following reduction peak at 0.57 V can be ascribed to the subsequent conversion reaction of Li with WS2, and electrolyte decomposition.25,35 After the first cycle, the reduction peak at 0.57 V disappeared, while the original
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Fig. 4 (a) CV curves of WS2@NCNFs over a voltage range of 0.01-1 3.0 V at a scanning rate of 0.1 mV s . (b) Discharge/Charge voltage -1 profile of the WS2@NCNFs at a current density of 0.1 A g . (c) Rate capabilities of the WS2@NCNFs and WS2 powder at current -1 densities of 0.1, 0.2, 0.5, 1, 2, and 0.1 A g . (d) Nyquist plots of the WS2@NCNFs and the WS2 powder with the inset showing an equivalent circuit model. The solid circles and solid curves indicate the experimental data and the fitted data, respectively. (e) Galvanostatic cycling performance of the WS2@NCNFs and the WS2 powder, and Coulombic efficiency of the WS2@NCNFs at a -1 current density of 0.5 A g . reduction peak at 1.27 V was shifted to 1.83 V.26 During anodic scans, oxidation peaks at 1.43 and 2.18 V were found to correspond to the lithium extraction processes.23,25 Fig. 4b shows the galvanostatic charge and discharge curves of the WS2@NCFNs at a constant current density of 0.1 A g-1. The CV curves and voltage profiles of the WS2 powder are shown in Fig. S4.† The sloping characteristic of the voltage profile of the WS2@NCNFs compared to that of the WS2 powder can be explained in part by the nano-size effects, which reduces the miscibility gap for the two-phase reaction.11,43,44 The anode electrode with the WS2@NCNFs delivered an initial discharge capacity of 722.3 mAh g-1 and a subsequent charge capacity of 590.4 mAh g-1. The measured specific capacity of the WS2@NCNFs was larger than the theoretical capacity of bulk WS2. This can be attributed to additional lithium storage sites, which include interfacial sites of single layered WS2 nanoplates.36 In addition, partially negatively charged W nanoparticles, which are formed after conversion reaction, can provide additional interfacial lithium storage sites.36,45,46 Since W does not alloy with lithium, lithium can be adsorbed on W nanoparticles with a partial charge transfer.36,45,46 The pseudo-capacitive behavior of transition metal dichalcogenides such MoS247,48 and WS219,20 has been reported previously. These
additional lithium storage sites offer possible explanations of the excess capacity. Fig. S5a† shows contributions of insertion, conversion, and interfacial reaction to the specific capacity of the composite anode in the initial discharge voltage profile. The schematic diagram in Fig. S5b† illustrates a single layered WS2 nanoplate undergoing insertion, conversion, and interfacial reaction consecutively during lithiation. Transition metal sulfides with conversion reaction generally show relatively low initial Coulombic efficiencies due to the irreversible capacity loss in the first cycle.11,27,35 It is important to note that the initial Coulombic efficiency of the WS2@NCNFs was as high as 81.7%, exhibiting excellent high capacity retention capability after the first cycle. Fig. 4c reveals tenth-cycle discharge capacities of 596.2, 580, 541, 496, and 367.1 mAh g-1 at current densities of 0.1, 0.2, 0.5, 1, and 2 A g1 , respectively. In addition, when the current density was reversed back to 0.1 A g-1, the WS2@NCNFs anode was able to recover to 628.9 mAh g-1. In contrast, the anode electrode using the WS2 powder exhibited poor rate performances at high current densities. At a current density of 2 A g-1, it delivered only 99.6 mAh g-1. The rate capabilities of the N-doped carbon nanofibers (NCNFs) and their galvanostatic cycling performance are shown in Fig. S6.† Also, the contribution of the NCNFs to the specific capacity of WS2@NCNFs was calculated in Table S1.† The result proves that the NCNFs can provide rapid pathways for electrons, allowing rapid charge transfers49,50 because it is widely accepted that Ndoping in carbon materials can effectively enhance its electrical conductivity.35,42 This is further supported by the electrochemical impedance spectroscopy (EIS) measurements. Nyquist plots after 50 cycles are shown in Fig. 4d. The second semicircular diameter in the medium-frequency region represents the charge-transfer resistance (Rct). The charge-transfer resistance value of the WS2@NCNFs (5.77 Ω) was much lower than that of the WS2 powder (33.2 Ω). Fig. 5e compares the cyclability of the WS2@NCNFs with that of the WS2 powder at a current density of 0.5 A g-1. The discharge capacities of the WS2@NCNFs for the second and 200th cycles were 502 and 437.5 mAh g-1, respectively. Thus, the corresponding capacity retention measured after the first cycle was 87.2% whereas the capacity of the WS2 powder severely decayed as it was cycled. This can be attributed to the N-doped carbon matrix, which effectively accommodates variations in the volume and isolates single-layer WS2 nanoplates from each other, preventing their restacking and the Ostwald ripening of the W nanoparticles formed by the phase separation of WS2, which can result in electrochemical pulverization.32,35,36 The morphologies of the WS2@NCNFs after cycling were well maintained as shown in Fig. S7.† The single layered WS2 nanoplates were not found after the first cycle in the high-resolution TEM images, which is indicative of irreversible conversion reaction of WS2. However, ultrasmall W nanoparticles were uniformly distributed in the carbon nanofibers, which were detected by STEM image and STEM-EDS mapping images as shown in Fig. S8† even though their lattice spacing could not be identified in the TEM images. In the initial cycle regions, the capacity of the WS2@NCNFs and the WS2 powder increases gradually and saturates to a certain capacity value in both cases. This activation behavior has been frequently observed in other transition metal sulfide anode materials.6,18,25 This can be ascribed to the degradation of crystallinity of the
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anodes as well as to the formation of a gel-like polymeric surface layer.11,19,23,32,35
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Conclusions
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In summary, we have demonstrated for the first time that single layers of WS2 nanoplates can be homogeneously embedded in N-doped CNFs via electrospinning method. They exhibited remarkably enhanced electrochemical performances, including a large specific capacity, a high initial Coulombic efficiency, excellent capacity retention, and great rate capability. These results can be attributed to several factors: i) the single layers of WS2 nanoplates can provide numerous interfacial lithium storage sites besides insertion and conversion reaction sites; ii) the carbon matrix can inhibit the restacking of WS2 single layers and the aggregation of W nanoparticles after cycling; iii) the 1-D interconnected carbon networks can buffer the volume expansion and facilitate charge transfers during lithiation/delithiation; iv) N-doping in CNFs can enhance the electrical conductivity of the carbon networks. Furthermore, due to the distinctive structural characteristics of the WS2@NCNFs, there are innumerable active sites for electrochemical reactions with facile electron transport. Thus, it is expected that WS2@NCNFs have the potential to be used in not only high capacity anode materials for lithium-ion batteries but also in other energy storage and/or conversion applications, such as supercapacitors, hydrogen evolution reaction (HER) catalysts, oxygen reduction reaction (ORR) catalysts, and counter electrode materials for dye-sensitized solar cells (DSSCs). This work was supported by a grant from the Korea CCS R&D Center (KCRC) funded by the Korean government (Ministry of Science, ICT & Future Planning) (no. NRF2014M1A8A1049303).
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