WS2 nanoflowers on carbon nanotube vines with enhanced electrochemical performances for lithium and sodium-ion batteries

WS2 nanoflowers on carbon nanotube vines with enhanced electrochemical performances for lithium and sodium-ion batteries

Journal of Alloys and Compounds 766 (2018) 656e662 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 766 (2018) 656e662

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

WS2 nanoflowers on carbon nanotube vines with enhanced electrochemical performances for lithium and sodium-ion batteries Xin Li, Jinying Zhang*, Zechen Liu, Chengcheng Fu, Chunming Niu Center of Nanomaterials for Renewable Energy (CNRE), State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 January 2018 Received in revised form 19 June 2018 Accepted 1 July 2018 Available online 2 July 2018

Hybrids of tungsten disulfide (WS2, 74.7%) nanoflowers on multiewalled carbon nanotube veins have been produced and characterized. The petalelike WS2 nanosheets with diameters of 307 ± 52 nm are in situ grown on the carbon nanotubes to obtain the hybrid structure resembling to flowers (WS2 nanosheets) on vines (carbon nanotubes with diameters of 10 ± 3 nm), where the carbon nanotube vines are wrapped by WS2 petals through coreeshell growth, resulting in a strong interface connection. The electrochemical performances of the hybrids have been significantly enhanced, especially at high current densities, since the charges can be effective transported from WS2 to carbon nanotube conductive networks. The specific capacities of the hybrids are more than three times (seven times) of those of pristine WS2 and MWCNTs as anodes for lithium ion batteries (sodium ion batteries) at current rates of 2e3 A g1. The cycling capacities of WS2eMWCNTs at a current rate of 1 A g1 are comparable or higher than those reported for WS2 composites at current rates of 100 or 200 mA g1. © 2018 Elsevier B.V. All rights reserved.

Keywords: Tungsten disulfides MWCNT hybrids Nanoflowers LIBs SIBs

1. Introduction Transition metal dichalcogenides (TMDs) [1e4] have attracted much attention due to their layered structures. Layered structures are assumed to be effective reversible Liþ and Naþ storage hosts with high cycle stability because the insertion and extraction of Liþ and Naþ do not involve structural reorganization. The hybridizations of TMDs with electron conductive carbon networks, such as graphene [5e10] and carbon nanotubes [11e13], have been proved to be an effective path to obtain high electrochemical performances for lithium and sodium ion batteries (LIBs and SIBs). Various hybridizations of molybdenum sulfides (MoS2) with carbon nanomaterials have been investigated [7,11,13e15]. However, the reports of tungsten disulfides (WS2) as anodes for LIBs and SIBs are much less than those of MoS2. Several tungsten sulfide composites with different forms of carbon, including graphene [9,10,16e20], CMK3,21 carbon nanotubes (CNTs) [22], carbon nanofibers (CNFs) [23,24], carbon spheres [25], as anodes for LIBs and SIBs have been reported. CNTs have been proved to be effective conductive

* Corresponding author. E-mail address: [email protected]mail.xjtu.edu.cn (J. Zhang). https://doi.org/10.1016/j.jallcom.2018.07.008 0925-8388/© 2018 Elsevier B.V. All rights reserved.

networks for the improvement of electrochemical performances of active materials [26,27] due to their unique structural and electronic properties [28e32]. To date, composites of WS2 with carbon nanotubes are limited to the mixtures of exfoliated WS2 with singleewalled carbon nanotubes (SWCNTs) as electrodes for supercapacitor and LIBs [22]. The cycle performances are limited to less than 50 at low current density (100 mA g1) due to the weak connections between WS2 and conductive SWCNTs. The interactions between WS2 and CNTs were improved by in situ growth methods [33,34]. Few layers WS2 was grown on the outer surfaces of CNTs to improve their interface interactions. However, the electrochemical performances of the hybrids as anodes for LIBs and SIBs have not yet investigated. Herein, we report interesting WS2 hybrids with multiewalled CNTs (MWCNTs), where the WS2 sheets are in situ grown on the MWCNTs, resulting in a hybrid structure resembling to be flowers (WS2 nanosheets) on vines (MWCNTs). The MWCNT vines are wrapped by the petals of WS2 nanoflowers through coreeshell growth. The electrochemical performance of the hybrids has been significantly enhanced due to intimate bonding between WS2 nanoflowers and conductive MWCNT networks which results in a fast charge transportation between WS2 and MWCNTs.

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2. Experimental 2.1. Synthesis of WS2eMWCNTs 60 mg pristine MWCNTs from chemical vapor deposition were dispersed into 60 ml N,N-dimethylformamide (DMF, Sinopharm, analytical reagent) by ultra-sonication for 30 min. 340 mg (NH4)2WS4 powder (Sigma-Aldrich, 99.97% trace metal basis) was then dissolved to the MWCNT suspension. The mixture was stirred for 5 min to be homogeneous and then transferred into a teflonlined stainless steel autoclave (100 ml). The tightly closed autoclave was then heated in oven at 230  C for 20 h. The black precipitates were then filtered by a 0.45 mm membrane film and thoroughly rinsed by ethanol to get rid of residual reagents and DMF. The washed sample was then collected and dried in oven at 80  C overnight, and then heated at 600  C in an Ar atmosphere for 2 h with a heating rate of 1  C/min to give highly crystalline WS2MWCNT hybrid structures. 2.2. Material characterization Raman spectroscopy was taken in a backescattering geometry using a single monochromator with a microscope (Reinishaw inVia) equipped with CCD array detector (1024  256 pixels, cooled to 70  C) and an edge filter. The samples were excited by 514.5 nm Argon ion laser. The spectral resolution and reproducibility was determined to be better than 0.1 cm1. HRTEM images were acquired by JEOL JEM-2100 transmission electron microscopy (TEM; acceleration voltage: 200 kV). X-ray diffraction (XRD) patterns were obtained from a Rigaku SmartLab using Cu/Ka aradiation (l ¼ 1.5418 Å) at 40 kV and 30 mA. Thermogravimetric analysis (TGA) was carried out by METTLER-TOLEDO TGA/DSC 1 system under a flow of O2 (60 mL/min) and N2 (20 mL/min) at a heating rate of 5  C/min. 2.3. Electrochemical measurements The electrochemical performances of WS2 and MWCNT structures as anode materials were measured with a half-cell lithium (sodium) ion battery configuration. The CR2025 coin-type cells

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were assembled in an argon-filled glove-box with both moisture and oxygen level less than 0.1 ppm. A metallic Li (or Na) plate was used as the counter electrode and reference electrode. The work electrodes were prepared by mixing the active materials (80 wt%), super-P (10 wt%), and poly (vinyl difluoride) (PVDF, 10 wt%) pasting on a pure Cu foil for both LIB and SIB cell. The mass of active materials was ~1 mg. A Celgard polypropylene membrane (for LIB cell), or a Whatman glass fiber filter (for SIB cell) was used as a separator. The electrolyte used for LIBs was 1.0 M LiPF6 in a 50:50 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), while the electrolyte used for SIBs was 1.0 M NaClO4 in a 50:50:5 (v/ v/v) mixture of propylene carbonate (PC), ethylene carbonate (EC) and fluoroethylene carbonate (FEC). The galvanostatic charge and discharge experiments were performed on a NEWARE multiechannel battery test system in the voltage range between 0.01 and 3.0 V vs. Li/Liþ (or Na/Naþ) at 25  C. 3. Results and discussion The MWCNTs were prepared by chemical vapor deposition to have diameters of 10 ± 3 nm [13,33,35e37]. Low crystalline tungsten sulfides were introduced onto the surfaces of MWCNTs by the decomposition of (NH4)2WS4 in N, N-dimethylformamide (DMF) in a PTFE lined autoclave at 230  C for 23 h. The low crystalline tungsten sulfides were then annealed at 600  C under Ar atmosphere to crystallize and generate flowerelike WS2 nanostructures on MWCNT vines (WS2-MWCNTs). The WS2 nanoflowers with diameters of 307 ± 52 nm are tightly grown on the MWCNT vines (Fig. 1a). The MWCNT vines were wrapped by the WS2 nanoflower petals, as shown in the inset of Fig. 1a. The connection details between WS2 and MWCNT vines are shown in Fig. 1b. Some layers of the WS2 nanoflowers were grown on the outer walls of MWCNTs as coreeshell structures, where the interplanar distances of 6.2 Å and 3.4 Å are corresponding to {002} crystal plane fringe of 2HeWS2 and interelayer fringe of MWCNTs, respectively. The petals are usually composed of 5e15 layers of WS2. The hexagonal pattern of {001} crystal planes of 2HeWS2 are shown in the inset of Fig. 1c. Diffraction features corresponding to {102} (2.5 Å), {107} (1.5 Å), and {109} (1.2 Å) were clearly observed from the SAED pattern of WS2 with zone axis of [001] (Fig. 1d).

Fig. 1. HRTEM (aed) and SEM (eef) images of a) WS2 nanoflowers on MWCNT vines; b) connections between WS2 and MWCNTs; c) WS2 petal; d) SAED pattern of WS2; e) WS2MWCNT vines; f) WS2 plates without MWCNTs.

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Dense WS2 nanoflowers are well distributed on the MWCNT vines (Fig. 1a and e). The MWCNT plays a very important role to fabricate the WS2 nanoflowers. The WSx decomposed from (NH4)2WS4 during thermal treatment was nucleated on the defects of MWCNTs and further grown to be flower shapes. The WSx were further crystallized to be tightly connected to MWCNTs as flowers on vines. WS2 plates were produced when the reactions were performed without MWCNTs (Fig. 1f), further confirmed the controlling effect of MWCNTs on WS2 morphology. The XRD features of WS2 are extremely strong compared to MWCNTs in WS2-MWCNTs due to its high crystallinity, where the reflection corresponding to {002} of MWCNTs with 2q at 26 are negligible (Fig. 2a, black solid line). The reflection intensity distribution is distinctly different from that of commercial WS2 (Fig. 2a, red dotted line) due to preferred WS2 orientation induced during growth by MWCNTs. The XRD features of WS2-MWCNTs (Fig. 2a, black solid line) with 2q at 13.9 , 28.3 , 33.5 , 39.3 and 59.2 are attributed to {002}, {004}, {101}, {103}, and {110} reflections from hexagonal WS2 (2HeWS2, JCPDS card no.#08e0237, Fig. 2a, drop lines). The reflections from {002} and {004} of WS2 nanoflowers are downshifted 0.4e0.5 due to the less layered structure which induces slight increasing in interlayer distances. Both Raman features of MWCNTs and WS2 were clearly observed from WS2-MWCNTs. D and G band of MWCNTs were observed at 1352 and 1591 cm1 in addition to WS2 at 352 (2LA) and 417 cm1 (A1g). The content of WS2 was measured by TGA in air (Fig. 2c). Pure WS2 was also measured under the same condition to get the weight variation since WS2 is oxidized during heating in air. The first slope of WS2MWCNTs started from 200  C is due to the oxidation of WS2 and the second slope started from 500  C is due to the oxidation of MWCNTs. The remaining materials after TGA of WS2-MWCNTs are tungsten oxides. The calculated weight of WS2 is about 74.7%, further confirmed the dense distribution of WS2 as shown in Fig. 1a and e. The dense and well distributed WS2 nanoflowers with strong interface connections to MWCNT vines are highly potential active materials for hydrogenation, hydrodesulfurization, and batteries. The electrochemical performances of WS2-MWCNTs have been demonstrated as anodes for LIBs and SIBs using a half cell configuration (Figs. 3 and 4). The correlative potential plateaus shown in discharge/charge voltage profiles (Fig. 3a and b) agree well with the cyclic voltammograms (Fig. 3c and d). The dominant reduction potential at 0.6 V, attributed to the subsequent conversion reaction of Liþ with WS2 to form W nanoparticals embedded in the Li2S matrix and the formation of solid electrolyte interface (SEI) [20,21], and small band at 1.5 V, Liþ intercalation into the layers of WS2 to form LixWS2 [20,21], vs Li/Liþ disappeared after first cycle. The original reduction peaks are replaced by new peaks in the potential

range from 1.6 to 2.2 V, which are ascribed to the multistep conversion of S with Liþ to the formation of LixWS2 and Li2S [20,21]. The small oxidation potential at 1.7 V is due to the partial oxidation of W to WS2 [20,21]. The intense oxidation potential at 2.4 V is attributable to the extraction of Liþ and conversion of Li2S to S [20,21] (Fig. 3a&c). Three reduction potentials at 0.2, 0.6, and 1.2 V vs Na/Naþ were observed in the first reduction cycle (Fig. 3b and d). The weak reduction potential around 1.2 V is attributed to Naþ insertion into the layers of WS2 to form NaxWS2 [20,38]. The two sharp dominant peaks at 0.2 and 0.6 V are attributed the conversion reaction of WS2 with sodium ions into W metallic nanoparticles embedded into amorphous Na2S matrix and the formation of SEI [20,38]. The reduction peaks in the first cycle are replaced by new weak broad peaks in the potential range from 1.2 to 2.5 V, which could be ascribed to the multistep conversion of S with Naþ to the formation of NaxWS2 and Na2S [20,38]. Three oxidation potentials at 1.9, 2.3, and 2.6 V are attributed to the subsequent oxidation of W to WS2 during desodiation process [20,38] (Fig. 3b and d). The three oxidation peaks were slightly shifted to lower potentials after first cycle. Much higher specific capacities have been achieved for WS2eMWCNTs than pure WS2 and MWCNTs as anodes for both LIBs and SIBs, especially at high current rates (Fig. 4a and c). Specific capacities of 686 ± 8 (without the first cycle), 636 ± 8, 618 ± 4, 569 ± 6, and 512 ± 10 mA h g1 have been obtained for WS2eMWCNTs anodes for LIBs at the current rates of 100, 500, 1000, 2000, and 3000 mA g1, respectively. The rate capacities are 32% (70%), 64% (149%), 109% (198%), 168% (239%), and 237% (247%) higher than those of pristine WS2 (MWCNTs) at the current rates of 100, 500, 1000, 2000, and 3000 mA g1 (Fig. 4a), respectively. As for SIBs, specific capacities of 419 ± 5, 361 ± 3, 348 ± 4, 328 ± 5, and 293 ± 4 mA h g1 have been obtained for WS2eMWCNTs, which are 19% (245%), 88% (430%), 253% (535%), 613% (641%), and 1046% (627%) higher than those of pristine WS2 (MWCNTs) at the current rates of 100, 500, 1000, 2000, and 3000 mA g1 (Fig. 4c), respectively. The capacities returned to higher values when the current densities are adjusted back from 3 A g1 to 100 mA g1. The electrochemical performances of pristine WS2 are sensitive to the current applied. The specific capacities decrease significantly with increasing currents due to its low charge transportation characters. However, high specific capacities with high cycle stabilities have been achieved by the WS2eMWCNTs at high current rates due to the strong connections between WS2 nanoflowers and MWCNT vines which result in fast charge transportation from WS2 to MWCNT conducting networks. The specific capacities of WS2eMWCNTs are more than three times of those from pristine WS2 and MWCNTs as anodes for LIBs at the current rates of more than 2 A g1 (Fig. 4a). The specific capacities of WS2eMWCNTs as

Fig. 2. a) XRD b) Raman, and c) TGA spectra of WS2 nanoflowers on MWCNT vines (black solid) and pure WS2 (red dotted); drop lines corresponding to standard XRD patterns in JCPDS card no.#08e0237. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 3. Cyclic voltammograms of WS2-MWCNTs for a) LIBs and b) SIBs at a scan rate of 0.2 mV s1. Chargeedischarge voltage profiles of WS2-MWCNTs for c) LIBs and d) SIBs at a current rate of 100 mA g1.

Fig. 4. Rate capacities of WS2-MWCNTs (black circle), WS2 (red square), and pristine MWCNTs (blue triangle) for a) LIBs and c) SIBs; Cycling performances of WS2-MWCNTs for b) LIBs and d) SIBs at a current rate of 1 A g1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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anodes for SIBs are more than seven times of those from pristine WS2 and MWCNTs at the current rates of more than 2 A g1 (Fig. 4c). The electrochemical performances of WS2eMWCNTs are significantly enhanced due to the strong interaction between WS2 nanoflowers and MWCNT vines. Specific capacities about 600 mA h g1 were obtained for WS2eMWCNTs as anodes for LIBs at the first 50 cycles at a current rate of 1 A g1. The cycling capacities of WS2eMWCNTs reached the maximum at 40th cycle, slowly decreased to 450 mA h g1 at ~100th cycle and stabilized at this level. A capacity of 450 mA h g1 was still obtained after 150 cycles. High specific capacities with high cycle stability have been achieved for WS2eMWCNTs as anodes for LIBs at high current density (Fig. 4 b). High cycling capacities were also obtained from WS2eMWCNTs as anodes for SIBs at high current densities. Specific capacities of more than 300 mA h g1 were obtained for the first 50 cycles at a current rate of 1 A g1 The cycling capacities reached the maximum at 50 cycles and then tend to gradually degrade, which was also observed in the reported WS2 composties [20,21]. The cycling stability of WS2eMWCNTs as anodes for SIBs is not as good as that for LIBs. The cycling performances of WS2eMWCNTs as anodes for LIBs and SIBs are compared to the reported WS2 materials as shown in Table 1. The material structures, specific capacities, current rates and cycling numbers of the reported WS2 materials have been recorded in Table 1. Most of reported WS2 anodes are limited to current density of less than 100 mA g1 and low cycle numbers [38e42]. The WS2 nanowires has been reported to have high cycle stabilities under the current rates of 200 mA g1 for LIBs [43]. The specific capacities of WS2eMWCNTs at the current rate of 1000 mA g1 is even higher than those of the WS2 nanowires at the current rate of 200 mA g1. The electrochemical performances of [001] preferentially-oriented 2D WS2 nanosheets was also

measured under the current density of 1000 mA g1 [44]. However, the specific capacities of [001] preferentially-oriented 2D WS2 nanosheets is under 200 mA h g1, which is only 1/3 of those from WS2eMWCNTs. Doping [45] and Functionalization [46] were also introduced to WS2 structures to improve their electrochemical performances. However, there is no significant improvement achieved. Graphene [10,19], graphene oxides [47], and reduced graphene oxides (rGO) [18,48e50] have been added to WS2 structures to enhance their electrochemical performances. The specific capacities of 300e750 mA h g1 have been obtained for the WS2 composites with graphene, graphene oxides, and rGO at the current rates of 100e350 mA g1. The specific capacities of WS2eMWCNTs at the current rate of 1000 mA g1 are higher or comparable to the above composites at the much lower current rates of 100e350 mA g1 as anodes for LIBs. The specific capacities of WS2 nanocrystals anchored to graphene nanosheets [10] are about 329 mA h g1 at the current rate of 20 mA g1, which are comparable to those of WS2eMWCNTs at the current rates of 1000 mA g1 as anodes for SIBs. The rGO microspheres [17], 3D graphene networks [51], and N-doped graphene [16] have also been added to WS2 structures to improve their electrochemical performances. The electrochemical performances were slightly improved. However, the specific capacities at the current rates of 100e200 mA g1 are comparable to those of WS2eMWCNTs at the current rates of 1000 mA g1 as anodes for LIBs and SIBs. The WS2 structures were also embedded to different carbon structures [21,24,25,52e57] to improve their electrochemical performances at higher current rates. However, the specific capacities obtained from WS2-carbon composites are much lower than those from WS2eMWCNTs at the current rates of 1000 mA g1 as anodes for LIBs and SIBs. CNTs composites with WS2 were also investigated as anodes for LIBs. However, the electrochemical performances of WS2/SWCNTs [22] were only obtained at low current rate of 100 mA g1 due to the low

Table 1 Summary of the electrochemical performances of WS2 materials as anodes for LIBs and SIBs. Material structure

Specific capacity (mA$h$g1)

Current rate (mA$g1)

Cycling number

Battery type

WS2/N-graphene [16] WS2 sheets with rGO [18,48] Freeze-dried WS2/graphene nanosheets [19] porous WS2 nanosheets/SWCNTs [22] WS2/CNFs [24] Hollow N-carbon [email protected] nanosheets [25] WS2 nanoflakes [39] Graphene-like WS2 nanosheets [40] Ordered mesoporous WS2 [42] [001] preferentially-oriented 2D WS2 nanosheets [44] Co-doped WS2 nanorodes [45] Surface functionalized WS2 sheets [46] WS2/graphene oxide lamellar films [47] WS2 nanostructures embedded in rGO [49] Multi-slice structured WS2/rGO [50] WS2 nanoflakes/3D graphene networks [51] WS2 [email protected] porous carbon [54] WS2 [email protected] [55] WS2 [email protected] [56] Oleylamine (OLA)-coated WS2 nanosheets-graphene oxide [58] WS2/CNT-rGO aerogel [20] Porous WS2 in CMK-3 matrix [21] WS2 nanocrystals anchored on graphene nanosheets [10] WS2/rGO microspheres [17] WS2 nanowires [43] WS2 nanosheets with diverse crystal planes [38] Single crystalline 2D WS2 nanosheets [41] WS2 arrays/carbon lamellar hybrids [52] Nitrogen-doped conductive [email protected] nanosheets [53] Porous WS2/carbon composites [57] This work

825 400e450 647 861.6 458 677.1 680 590 805 195.7 380 118 697.7 730 565 748 282 322 437.5 486 556/252.9 720/333 329 334 330 ~300 460.8 180 ~360/~200 219 455/289

100 100 350 100 1000 500 47.5 43.2 100 1000 50 25 100 100 100 200 1000 200 500 500 200 100 20 200 200 100 100 1000 100/1000 1000 1000

75 50e100 80 50 100 100 20 70 100 1000 40 50 100 150 100 500 1220 100 200 200 100 100/70 500 200 1400 100 60 1200 100 300 140/60

LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs LIBs/SIBs LIBs/SIBs SIBs SIBs SIBs SIBs SIBs SIBs SIBs SIBs LIBs/SIBs

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interface connections between SWCNTs and exfoliated WS2 nanosheets. The electrochemical performances of WS2eMWCNTs are much higher than the composites of exfoliated WS2 and SWCNTs with different mass ratios [22]. The highest specific capacities obtained from the composites of exfoliated WS2 and SWCNTs are about 850 mA g1 at a current rate of 100 mA g1 and the cycling performances are limited to be less than 50 cycles due to the weak connections between WS2 and SWCNTs [22]. There is no flower like WS2 structures observed in the above composites. WS2 nanoflowers were also observed in the aerogel with CNTs and rGO [20]. However, there is no interface connections detected between WS2 structures with rGO or CNTs. The electrochemical performances were limited to low current rates. The specific capacities of WS2/ CNT-rGO aerogel at the current rates of 200 mA g1 are even lower than those of WS2eMWCNTs at the current rates of 1000 mA g1 as anodes for LIBs and SIBs. 4. Conclusions A new WS2eMWCNT hybrid has been synthesized and characterized. The flower petalelike WS2 nanostructures with diameters of 307 ± 52 nm have been in situ grown on MWCNTs, resulting in a hybrid structure resembling to be nanoflowers on vines. The MWCNT vines are wrapped by the petals of WS2 via coreeshell growth to generate a strong interface bonding between active WS2 and conductive MWCNT networks. The electrochemical performances of WS2eMWCNTs have been significantly enhanced, especially at high current densities, due to the strong interface connections between WS2 nanoflowers with conductive MWCNT networks. The specific capacities of WS2eMWCNTs are more than three times and seven times of those from pristine WS2 and MWCNTs as anodes for LIBs and SIBs at high current rates, respectively. The cycling capacities of WS2eMWCNTs at a current rate of 1 A g1 are comparable or higher than those reported for WS2 composites at current rates of 100 or 200 mA g1. The cycling capacities of the hybrids reached 600 mA h g1 at 50th cycle and then decreased and stabilized to 450 mA h g1 at a current rate of 1 A g1 for lithium ion batteries. The cycling capacities of the hybrids are about 300 mA h g1 at 50th cycle at a current rate of 1 A g1 for sodium ion batteries. Conflicts of interest There are no conflicts to declare. Acknowledgements The SEM and TEM works were done at International Center for Dielectric Research (ICDR), Xi'an Jiaotong University. The authors also thank Mr. C. Ma for his help in using TEM and Ms. Dai for her help in using SEM. This research was supported by the National Natural Science Foundation of China (21771143) and the Fundamental Research Funds for the Central Universities. J. Zhang was supported by the Cyrus Tang Foundation through Tang Scholar Program. References [1] G. Hong, Z. Tengfei, Z. Yang, Z. Qing, L. Yuqing, C. Jun, et al., CoS quantum dot nanoclusters for high-energy potassium-ion batteries, Adv. Funct. Mater. 27 (43) (2017), 1702634. [2] Y. Liu, X. Hua, C. Xiao, T. Zhou, P. Huang, Z. Guo, et al., Heterogeneous spin states in ultrathin nanosheets induce subtle lattice distortion to trigger efficient hydrogen evolution, J. Am. Chem. Soc. 138 (15) (2016) 5087e5092. [3] Z. Yang, Z. Tengfei, Z. Chaofeng, M. Jianfeng, L. Huakun, G. Zaiping, Boosted charge transfer in sns/SnO2 heterostructures: toward high rate capability for sodium-ion batteries, Angew. Chem. Int. Ed. 55 (10) (2016) 3408e3413.

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