Amorphous carbon coated multiwalled carbon nanotubes@transition metal sulfides composites as high performance anode materials for lithium ion batteries

Electrochimica Acta 257 (2017) 20–30

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Amorphous carbon coated multiwalled carbon nanotubes@transition metal sulfides composites as high performance anode materials for lithium ion batteries Rencheng Jina,* , Yitian Jiangb , Guihua Lia , Yanfeng Menga a b

School of Chemistry & Materials Science, Ludong University, Yantai 264025, PR China Shanghai Institute of Space Power Source, Shanghai 200245, PR China

A R T I C L E I N F O

Article history: Received 12 June 2017 Received in revised form 11 October 2017 Accepted 12 October 2017 Available online 13 October 2017 Keywords: Transition metal sulfides Carbon nanotubes Anodes Lithium-ion batteries Electrochemical reaction mechanism

A B S T R A C T

Transition metal sulfides as anodes for lithium-ion batteries (LIBs) have attracted much attention because of their large Li+ storage capacity. However, the lower electrical conductivity and rapid capacity fading during the charge/discharge process strictly prohibit their practical applications. Here, a new strategy is adopted to accommodate the volume change and enhance the electrical conductivity by anchoring Co1-xS and NiS nanocrystals on amorphous carbon coated multiwalled carbon nanotubes (CNTs). Benefiting from the unique structure, the Co1-xS and NiS anodes present excellent electrochemical performances including remarkable cyclability and outstanding rate property. Furthermore, the electrochemical reaction process of the anodes is investigated by high-resolution transmission electron microscope and X-ray photoelectron spectroscopy technique. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Lithium-ion batteries (LIBs) have been applied in energy storage devices due to the advantages such as high energy density and long cycle life [1–5]. However, the commercial graphite anodes with low theoretical capacity (372 mAh g
1) cannot meet the pressing requirements of future electric vehicles and portable electronics [6]. Thus, searching alternative materials to replace the graphite carbon becomes of extreme importance. Transition metal sulfides with high theoretical capacity have been considered as promising alternative anode materials [7–16]. Unfortunately, the practical application of these metal sulfides is still prohibited by large volume change during lithium ion insertion and extraction process [17–19]. In addition, the electrical conductivity of these metal sulfides is comparatively lower, which leads to the poor rate capability [20,21]. To solve these issues, enhancing their electrical conductivity and structure stability during cycling becomes crucial strategies. Therefore, different size and morphologies of metals sulfides such as nanorods [22–24], nanowires [25], nanobelts [26], nanosheets [27,28], and hierarchical structures [29–32] have been prepared to buffer the volume variation and reduce the lithium ion migration distance. Constructing nanocrystals with conductive

* Corresponding author. E-mail address: [email protected] (R. Jin). https://doi.org/10.1016/j.electacta.2017.10.078 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

additives is another procedure to enhance the lithium storage and cycling performance, which not only accommodates the volume change, but also increases the electrical conductivity of electrode during cycling [33–40]. Among these conductive substrates, multiwalled carbon nanotubes (CNTs) have been attracted much attention due to its superior electrical conductivity and good chemical stability. Therefore, the combination of CNTs and metals sulfides is considered to be an effective method to improve the performance of the nanocomposites. Among these metal sulfides for Li ion storage, NiSx and CoSx have attracted great attention. In order to improve their performances, NiS hierarchical hollow spheres [41], CoS2 hollow spheres [42], rose-like Co9S8 [43], and yolk-shell Co9S8 microspheres[44] have been fabricated and evaluated as anodes to show their potential in LIBs. In addition, hybridizing nanostructured NiSx and CoSx with carbon can prevent the aggregation and pulverization of the electrodes and enhance the electrical conductivity, which further improves their electrochemical properties. For instance, b-NiS nanoparticles embedded in porous carbon matrices deliver a specific capacity of 300 mAh g
1 after 100 cycles at 60 mA g
1 [45]. NiS nanorod-assembled nanoflowers grown on graphene exhibit a reversible capacity of 887 mA h g
1 after 60 cycles at 59 mA g
1 [46]. Graphene-wrapped CoS nanoparticles display the reversible capacity of 749 mA h g
1 at 62.5 mA g
1 [37]. CoS/CNTs nanocomposites are prepared by an effective solvothermal method, which deliver the capacity of 780 mA h g
1 after 50 cycles at the

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current density of 100 mA g
1 [47]. The reversible capacity is promoted to 950 mAh g
1 after 150 cycles at 200 mA g
1 by CoSx/ graphene nanocomposites [48]. The capacity of MWCNT@aC@Co9S8 is pushed up to 662 mAh g
1 after 120 cycles even at high rate of 1 A g
1 [40]. Although some studies have been focused on the synthesis of transition metal sulfides/carbon hybrids for improving the electrochemical properties, the construction of nanostructured metal sulfides on conductive substrate is still appealing. Here in this work, Co1-xS nanoparticles and NiS nanosheets anchored on amorphous carbon coated CNTs (denoted as CNTs@C@Co1-xS and CNTs@C@NiS, respectively) have been synthesized through a hydrothermal/solvothermal process accompanied by a high-temperature calcination treatment using amorphous carbon coated CNTs (CNTs@C) as template. Due to the unique structure, the CNTs@C@Co1-xS and CNTs@C@NiS nanocomposites display high reversible capacity, excellent cycling stability and rate capacity when evaluated as anode materials in LIBs. Furthermore, the electrochemical reaction mechanism is investigated by ex situ HRTEM and XPS analysis.

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washed with distilled water and ethanol for several times and dried under vacuum at 80  C for 12 h. After that, the dried products were sintered at 400  C for 4 h with the protection of argon. For comparison, the CNTs@Co1-xS and CNTs@NiS were fabricated by using pure CNTs as template instead of CNTs@C without changing other reaction conditions. 2.3. Materials Characterization Powder X-ray diffraction (XRD) pattern of the samples was obtained by Rigaku D/Max–2550 pc X-ray diffractometer in the 2u range of 10
80 . The morphology and structure features of the samples were detected on FEI Quanta 200F field-emission scanning electron microscope and Tecnai G2 S-Twin transmission electron microscope. A PHI-5702 multifunctional X-ray photoelectron spectrometer was applied to analyze the chemical state and composition of the samples. The exact composition of the products was characterized by a NexION 300 inductively coupled plasma (ICP, USA). The content of the Co1-xS and NiS was characterized by thermal gravimetric analysis (TGA, Mettler, TGA/DSC 1/1600HT) with a heating rate of 10  C min
1 in flowing air atmosphere.

2. Experimental section 2.4. Electrochemical Measurements 2.1. Synthesis of CNTs@C All reagents used in this work were purchased from Sinopharm Chemical Reagent Co., Ltd. The multiwalled carbon nanotubes were provided by Shenzhen Nanotech Port Co., Ltd. In the typical synthesis, 50 mg of CNTs and 0.15 g of glucose were dispersed in 15 mL of distilled water. Then, the mixed solution was sealed into a 25 mL of Teflon-lined autoclave and heated at 180  C for 6 h. The products were collected and washed with distilled water and 95% ethanol for several times. 2.2. Synthesis of CNTs@C@Co1-xS and CNTs@C@NiS The CNTs@C@NiS was prepared via a solvothermal process using CNTs@C as template. 50 mg of the pretreated CNTs@C, 0.236 g of NiCl2.6H2O, 0.3 g of thiourea and 0.2 g glucose were dissolved into 10 mL of distilled water and constantly stirred for 10 min. Then, 10 mL of ethylene glycol was added into above solution and ultrasonically treated for 30 min. Subsequently, the mixture was sealed and held at 180  C for 12 h. The CNTs@C@Co1-xS was synthesized by a hydrothermal method. 50 mg of CNTs@C, 0.252 g of CoCl2.6H2O, 0.4 g of thioacetamide combined with 0.2 g of glucose were ultrasonically dispersed in 20 mL of distilled water for 30 min. The suspension was transferred into Teflon-lined autoclave (80% degree of filling) and then heated at 180  C for 12 h. The obtained CNTs@C@Co1-xS and CNTs@C@NiS were collected,

To prepare the working electrode, the obtained sample (active materials), carbon black, and polyvinylidene fluoride (PVDF) with the weight ratio of 80:10:10 were mixed in N-methyl-2pyrrolidinone (NMP) and constantly stirred for 8 h to form homogenous slurry. The slurry was spread onto copper foil and dried at 120  C for 12 h. The mass of active material for each electrode was 1 mg. The CR2025 coin cells were assembled in argon-filled glovebox, in which lithium metal foil was applied as the counter and reference electrode, Celgard 2400 membrane was used as the separator. The electrolyte was composed of 1 mol L
1 of LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) with the volume ratio of 1:1. Charge/discharge measurements were evaluated on a NEWARE BTS-3008 (Neware Co., Ltd, China) between 0.01 V and 3.0 V. Cyclic voltammetry (CV) measurements were tested on CHI660E electrochemical workstation at a scan rate of 0.2 mV s
1 in the potential rang of 0.01-3.0 V. Electrochemical impedance spectra (EIS) were performed on the same electrochemical workstation between the frequency range of 10 kHz to 0.1 Hz. 3. Results and discussion Fig. 1a shows the XRD pattern of the CNTs@C@Co1-xS after hydrothermal treatment at 180  C. The diffraction peaks at 25.9 and 42.7 are ascribed to the (002) and (100) planes of carbon

Fig. 1. XRD patterns of the synthesized (a) CNTs@C@Co1-xS, and (b) CNTs@C@NiS.

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(JCPDS card no. 41-1487). While all other peaks match well with hexagonal Co1-xS (JCPDS card no. 42-0826). The ICP analysis indicates that the exact molar ration of Co and S is about 0.96:1. The morphology of the obtained CNTs@C@Co1-xS is investigated by field-emission scanning electron microscopy (SEM). As shown in Fig. 2a–b, the wire-like nanostructure can be preserved. Compare with the CNTs@C (Fig. S1), the coarse surface of the sample implies that the CNTs@C is surrounded by numerous Co1-xS nanoparticles. The microstructure of CNTs@C@Co1-xS is further confirmed by transmission electron microscopy (TEM). It is interesting to find that some small nanocrystals are uniformly dispersed on the CNTs@C, in accordance with the SEM observation (Fig. 2c–d). And the nanocrystals are wrapped by a thin layer of amorphous carbon. Furthermore, the pores between the Co1-xS nanocrystals can be

observed clearly (Fig. 2d), which can effectively accommodate the volume change of Co1-xS during lithium ion insertion and extraction process. The high-resolution TEM image of the CNTs@C@Co1-xS displays the clear lattice fringes of 0.254 nm and 0.290 nm (Fig. 2e), corresponding to the (101) and (100) planes of hexagonal Co1-xS. The different lattice fringes orientations of the nanocrystals indicates the polycrystalline nature of the sample. The selected-area electron diffraction pattern (SAED, Fig. 2f) with four diffraction rings further proves the polycrystalline CNTs@C@Co1-xS. Fig. 2g shows the EDX mapping of the obtained CNTs@C@Co1-xS. As expected, the Co and S elements are homogeneously distributed around the CNTs@C. For comparison, when no CNTs@C is introduced in reaction system or the CNTs@C is substituted by CNTs, only Co1-xS nanoparticles or dispersed

Fig. 2. (a, b) SEM images of the synthesized CNTs@C@Co1-xS, (c) Low magnitude TEM image of CNTs@C@Co1-xS, (d) High magnitude TEM image of CNTs@C@Co1-xS, (e) HRTEM image of CNTs@C@Co1-xS, (f) corresponding selected area electron diffraction (SAED) pattern. (g) EDS mapping of CNTs@C@Co1-xS.

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nanoparticles and CNTs can be observed, which can be seen in Figs. S2a and S2b. The CNTs@C@NiS is fabricated by using CNTs@C as hard template in the mixed solution of distilled water and ethylene glycol. Fig. 1b shows the XRD pattern of the obtained sample. Except the carbon (JCPDS card no. 41-1487), all the peaks can be perfectly indexed as the hexagonal phase of NiS (JCPDS card no. 021280). The exact molar ratio of Ni and S is equal to 1:1.02. Fig. 3a and b show the SEM images of the obtained CNTs@C@NiS. As expected, the CNTs@C@NiS presents the wire-like shape with the diameter of 20–50 nm. Compare with the SEM observation of CNTs@C (Fig. S1), less change can be observed in the morphology. However, the surface of the CNTs@C@NiS becomes coarse (Fig. 3b),

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indicating that the NiS nanocrystallites are successfully anchored on the CNTs@C. The TEM image in Fig. 3c reveals that the ultrathin NiS nanosheets are well dispersed on the surface of CNTs@C. From the high-magnification TEM image, it can be found that the thickness of the NiS nanosheets is about 1.5-3.0 nm. The HRTEM images (Fig. 3e and f) depict that the NiS nanosheets are composed of many smaller nanocrystallites. And the nanocrystallites are surrounded by amorphous carbon. The clear lattice fringe with dspacings of 0.218 nm corresponds to the (211) plane of hexagonal NiS. The element mapping of the CNTs@C@NiS further demonstrates that the NiS nanosheets are tightly anchored on CNTs@C (Fig. 3g). The SEM images of the pure phase NiS and CNTs@NiS are presented in Fig. S3. When no CNTs@C is added, the NiS nanosheets

Fig. 3. (a, b) SEM images of the synthesized CNTs@C@NiS, (c) Low magnitude TEM image of CNTs@C@NiS, (d) High magnitude TEM image of CNTs@C@NiS, (e, f) HRTEM image of CNTs@C@NiS. (g) EDS mapping of CNTs@C@ NiS.

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as well as nanoparticles can be fabricated (Fig. S3a). If the CNTs are used as template instead of CNTs@C, the randomly dispersed NiS nanosheets and the aggregated CNTs generate (Fig. S3b). The chemical composition and chemical valence states are investigated by X-ray photoelectron spectroscopy (XPS). The survey spectrum in Fig. 4a presents the four element signals of Co, S, C, and O. Fig. 4b displays the high-resolution XPS spectra of Co 2p. Two fitted spin-orbit doublets located at 780.1 and 795.1 eV, 781.9 and 796.5 eV can be observed, which correspond to Co 2p1/2 and Co 2p3/2, respectively. The values can be assigned to Co3+ and Co2+, respectively [49]. In S 2p spectrum, the peaks centered at 160.1 and 162.2 eV are in accordance with the binding energies of S 2p1/2 and S 2p3/2, respectively (Fig. 4c). The peak appeared at 162.7 eV is ascribed to the metal-sulfur bond in the material, whereas the peak of 161.1 eV is derived from the sulfur ions in a low coordination state at the surface [50]. As presented in Fig. 4d, the C 1s spectrum can be deconvoluted into four peaks. The peaks at 282.9 and 283.6 eV can be attributed to non-oxygenated C (C¼C and C

C) [51,52]. And the peaks centered at 285.6 and 289.9 eV can be mainly assigned to C

OH and O¼C

OH groups, respectively [51,53]. For comparison purpose, the C 1s spectrum of pure CNTs is characterized (Fig. S4). The result indicates that less carboxyl and hydroxyl groups exist on the surface of CNTs. The strong chemical bonding existed in the CNTs@C@Co1-xS can effectively strengthen the adhesion between carbon matrix and Co1-xS nanocrytals. Fig. 5 shows the X-ray photoelectron spectroscopy spectrum of CNTs@C@NiS. As shown in Fig. 5a, four elements of C, O, Ni, and S are clearly observed. Ni 2p spectra of CNTs@C@NiS depict that two strong peaks at 854.4 and 872.5 eV can be observed, in accordance with the Ni 2p3/2 and Ni 2p1/2, respectively (Fig. 5b). The binding energies at 860.3 and 878.7 eV are attributed to the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively. Fig. 5c shows the fitted spectra of S 2p, four deconvoluted peaks with the binding energies at 159.9, 160.9, 162.1 and 163.2 eV can be attributed to S 2p3/2, sulfur ions, S 2p1/2, and metal-sulfur bond, respectively, similar to

the S 2p spectra of CNTs@C@Co1-xS. The C1s spectra demonstrate that carboxyl and hydroxyl groups exist in the CNTs@C@NiS (Fig. 5d). Fig. S5 displays the TG profiles of CNTs@C@Co1-xS and CNTs@C@NiS hybrids in air. The mass loss below 180  C can be attributed to the evaporation of absorbed water. The weight loss at 180–700  C corresponds to the oxidation of Co1-xS and NiS and the burning out of CNTs@C, respectively. According to the TGA analysis of the hybrids, the mass loading of Co1-xS and NiS in the hybrids is calculated to be 58.4% and 62.6%, respectively. Fig. 6 demonstrates the electrochemical performances of CNTs@C@Co1-xS. The cyclic voltammetry (CV) curves for the first three cycles are detected at a scan rate of 0.2 mV s
1 in the voltage rang of 0.01-3.0 V (Fig. 6a). In the first cathodic sweep, the wide peak appeared at 0.96 V corresponds to the Li+ insertion process and the reduction of Co1-xS to metallic Co and Li2S [4]. The wide and weak peak at 0.5 V is assigned to the decomposition of the electrolyte and the formation of a solid electrolyte interphase (SEI). In the subsequent cycles, these peaks shift to the higher voltage, indicating the reduced electrode polarization. Similar shift can be observed in other cobalt sulfide electrodes [40,54]. Meanwhile, the peak assigned to the Li+ insertion process can be observed at 1.71 V. The peak assigned to formation of SEI can be observed in the subsequent cycles, indicating that the SEI film continually generates at the initial cycles. During the first oxidation scan, the peaks located at 2.08 V and 2.35 V represent the conversion of metallic Co to CoS and Li extraction process [40]. Additionally, the CV curves after first cycle can be superimposed on each other, implying the excellent cycling stability of the electrode. Fig. 6b displays the first three charge-discharge capacities of CNTs@C@Co1-xS at the current density of 0.1 A g
1. The specific capacities are calculated based on the mass of the CNTs@C and Co1
1 is presented in xS together. The capacity of the CNTs@C at 0.1 A g Fig. S6a. The CNTs@C@Co1-xS electrode delivers the first discharge and charge capacities of 1249 and 992 mAh g
1, leads to a Coulombic efficiency of 79.4%. The high capacity fading is mainly

Fig. 4. (a) XPS survey spectra of CNTs@C@Co1-xS, High-resolution XPS spectra of the deconvoluted (b) Co2p, (c) S2p, and (d) C1s of CNTs@C@Co1-xS.

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Fig. 5. (a) XPS survey spectra of CNTs@C@NiS, High-resolution XPS spectra of the deconvoluted (b) Ni2p, (c) S2p, and (d) C1s of CNTs@C@NiS.

Fig. 6. Electrochemical performance of CNTs@C@Co1-xS: (a) CV curves for the first three cycles, (b) Discharge-charge curves at a current density of 0.1 A g
1, (c) cycling performance at a current density of 0.1 A g
1, (d) rate performance.

related to electrolyte decomposition and the formation of the SEI layer on the electrode surface. It should mention that the discharge and charge profiles are almost overlapped after first cycle, indicating good reversibility of the CNTs@C@Co1-xS electrode. The cycle performances of the CNTs@C@Co1-xS, CNTs@Co1-xS and

pure phase Co1-xS are evaluated over 0.01-3.0 V at a current density of 0.1 A g
1. As depicted in Fig. 6c, the CNTs@C@Co1-xS exhibits a reversible capacity of 875 mAh g
1 after 100 cycles, leads to the capacity retention of 91.7% after the second cycle. In contrast, the capacities of the CNTs@Co1-xS and pure phase Co1-xS decrease

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sharply at the first 20 cycles and reach about 468 and 254 mAh g
1 after 100 cycles, respectively. The cycle performance of the CNTs@C is also investigated. As shown in Fig. S6b, a capacity of 363 mAh g
1 can be achieved after 50 cycles. Fig. 6d demonstrates the rate performances of CNTs@C@Co1-xS, CNTs@Co1-xS and Co1-xS electrodes. The CNTs@C@Co1-xS nanocomposite displays the excellent rate performance compared with CNTs@Co1-xS and Co1-xS. At the rate of 0.1, 0.2, 0.5 1, 2, and 5 A g
1, the CNTs@C@Co1-xS delivers the reversible capacities of 897, 785, 715, 668, 627, and 570 mAh g
1, respectively. If the current density returns to 0.1 A g
1, the reversible capacity can be recovered to 878 mAh g
1 after 80 cycles, demonstrating excellent reversibility of the electrode. The rate performance of CNTs@C@Co1-xS is much better than those of the previous reports [4,40,42,44,48,55–59]. For example, hollow nanospheres of mesoporous Co9S8 display the specific capacity of 426 mAh g
1 after 60 cycles at 5 A g
1 [4]. The capacity of MWCNT@a-C@Co9S8 nanocomposites decreases sharply to 492 mAh g
1 at 5 A g
1 [40]. Only 306 mAh g
1 of capacity is delivered by the three-dimensional CoS2/RGO at 4 A g
1 [56]. The electrochemical impedance spectroscopy (EIS) is carried out to further investigate the excellent rate performance of CNTs@C@Co1-xS. Fig. 7a shows the Nyquist plots of the electrode collected after certain cycles at 0.1 A g
1 (fresh cell, 20th, 100th). The Nyquist plots are similar to each other in the shape, in which the depressed semicircle in the high and medium frequency region represents the charge-transfer resistance (Rct), and the inclined line in the low-frequency range is ascribed to the Warburg impedance (Zw). From Fig. 8a, one can observe that the Rct value of the CNTs@C@Co1-xS obviously decreases after 20th cycle. Then the resistance has no significant change in the subsequent cycles. The result indicates that the lithium ions and electrons can transfer more freely after cycling. Moreover, the reduced resistance also displays the well-maintained structural integrity of the CNTs@C@Co1-xS electrode, which can be seen in other anode materials. To reveal the structural integrity of the electrode and electrochemical reaction mechanism, TEM, HRTEM and XPS analysis are carried out. The TEM image shown in Fig. 8a indicates that the one dimensional morphology of CNTs@C@Co1-xS electrode is still maintained even after 100 cycles at current density of 0.1 A g
1. Fig. 8b shows the HRTEM image of the electrode after being discharged to 0.01 V. It is interesting that electrode materials become seriously amorphous when discharged to 0.01 V, whereas some crystalline regions can still be recognized. The lattice fringes of 0.255, 0.201 and 0.206 nm are in accordance with the (101), (220) and (111) planes of Co1-xS (JCPDS card no. 42-0826), Li2S (JCPDS card no. 26-1188) and Co (JCPDS card no. 15-0806), respectively. When the electrode is recharged to 3.0 V, no obvious

change can be observed on the shape (not show here). The lattice spacing of 0.297 nm agrees well with the (100) plane of CoS (JCPDS card no. 70-2864) (Fig. 8c). To further investigate the electrochemical reaction mechanism of the electrode, the XPS tests are performed with the cutoff voltage of 0.01 and 3 V after the 100th cycle. Compare with the Co 2p XPS spectra, the binding energies at 777.1 and 792.2 eV correspond to metallic Co can be observed when discharged to 0.01 V [60,61]. According to the above analysis, the electrochemical reaction mechanism of CNTs@C@Co1-xS can be summarized as the following two reaction processes. Co1-xS + 2Li+ + 2e
! 1-xCo + Li2S

(1)

Co + Li2S $ CoS +2Li+ + 2e


(2)

The electrochemical performance of CNTs@C@NiS is also detected and the results are shown in Fig. 9. Fig. 9a represents the CV curves of CNTs@C@NiS composites at a scan rate of 0.2 mV s
1. In the first cathodic scan, two reduction peak centered at 1.31 V and 1.15 V can be attributed to transformation from NiS to Ni3S2 and then to metallic Ni, respectively [45,62]. While the wide and weak peak at 0.49 V can also be observed but disappears in the subsequent cycles, indicating the formation of SEI layer during the first cycle [9,62]. In the first anodic scan, the oxidation peak at 1.0 V corresponds to the active nature of CNTs towards reversible Li+ insertion [9]. And another oxidation peak at 2.01 V represents the conversion of metallic Ni to NiS [45]. Meanwhile, the CV curves after first cycle are well overlapped, indicating the excellent cycling stability of the CNTs@C@NiS electrode. Fig. 9b displays the charge and discharge profiles of the CNTs@C@NiS at a current density of 0.1 A g
1 in the potential range of 0.01-3.0 V. The CNTs@C@NiS electrode exhibits initial discharge and charge capacities of 860 and 770 mAh g
1, with the Coulombic efficiency of 89.5%. In the subsequent two cycles, the discharge/charge curves are almost overlapped, demonstrating that the CNTs@C@NiS nanocomposite possesses a high reversibility and good stability. The cycle performance of the CNTs@C@NiS at 0.1 A g
1 is demonstrated in Fig. 9c. Compare with the CNTs@NiS and bare NiS, the CNTs@C@NiS electrode displays the best cyclability. After 100 cycles, CNTs@C@NiS maintains a reversible capacity of 649 mAh g
1, whereas the CNTs@NiS and bare NiS exhibit the lower capacities of 315 and 126 mAh g
1, respectively. The rate performances of the CNTs@C@NiS, CNTs@NiS and bare NiS in LIBs are also evaluated. As presented in Fig. 9d, the reversible capacities of CNTs@C@NiS are 687, 626, 571, 500, and 429 mAh g
1 when the current densities are 0.1, 0.2, 0.5, 1, and 2 A g
1, respectively. Even at higher current density of 5 A g
1, the CNTs@C@NiS nanocomposite still exhibits the capacity of 377 mAh g
1, which is higher than those of the

Fig. 7. (a) Nyquist plots of the CNTs@C@Co1-xS electrode before and after certain cycles at 0.1 A g
1, inset shows the enlarged depressed semicircle, (b) Nyquist plots of the CNTs@C@NiS electrode before and after certain cycles at 0.1 A g
1.

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Fig. 8. (a) TEM image of CNTs@C@Co1-xS after 100 cycles at 0.1 A g
1. (b) HRTEM images of the lithiated electrode (discharged to 0.01 V), (c) HRTEM images of the delithiated electrode (charged to 3.0 V), (d) Co 2p spectra of the lithiated electrode (discharged to 0.01 V) and delithiated electrode (charged to 3.0 V) in the 100th cycle at the current density of 0.1 A g
1.

Fig. 9. Electrochemical performance of CNTs@C@NiS: (a) CV curves for the first three cycles, (b) Discharge-charge curves at a current density of 0.1 A g
1, (c) cycling performance at a current density of 0.1 A g
1, (d) rate performance.

previously reported NiS hierarchical hollow microspheres [41], NiS/porous carbon hybids [45], NiS/graphene composites [46], and Ni3S4@C spheres [62]. When the current density is reduced to 0.1 A g
1, the capacity returns to its initial value, implying the excellent reversibility of the electrode. For the CNTs@NiS electrode,

only 89 mAh g
1 of capacity can be maintained at the rate of 5 A g
1. And the capacity is closed to 0 mAh g
1 when the current density is increased to 2 A g
1 for the bare NiS electrode. To further investigate the excellent cycle performance of the CNTs@C@NiS, the electrochemical impedance spectra of the

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electrode for the fresh cell and after certain cycles (20th and 100th) are performed. Clearly, the Rct values of the electrodes after 20th and 100th cycle are126.6 and 124.3 V, respectively (Fig. 8b). The values are smaller than that of fresh cell (276.9 V), indicating that the charge transfer resistance decreases with cycling. The smaller charge transfer resistance represents the higher ionic and electronic conductivity, which is beneficial for the electrochemical performance. The cell after 100 cycles at the current density of 0.1 A g
1 is decomposed and the morphology is determined by TEM. As shown in Fig. 10a, the nanosheets anchored on CNTs@C are well preserved, demonstrating the good structural stability during the repeated discharge-charge process. To understand the reaction process of the electrode, The HRTEM and XPS analysis of the electrode being discharged to 0.01 V and charged to 3.0 V are carried out. Fig. 10b shows the HRTEM image of the electrode materials after discharged to 0.01 V (100th). The nanosheets become amorphous and few nanocrystals can be observed. The lattice distances of 0.232 nm and 0.216 nm can be assigned with the (010) and (002) planes of Ni (JCPDS card no. 45-1027). After 100 times charge (cutoff voltage: 3.0 V), the conversion of Ni to NiS occurs. The lattice fringes of 0.218 nm corresponds to the (211) plane of NiS (Fig. 10c). XPS spectra of the electrode materials collected at the cutoff of 0.01 V and 3.0 V are depicted in Fig. 10d. From Ni2p spectra, one can find that binding energies of Ni appear partially in lower oxidation states after discharged to 0.01 V. Meanwhile, new peaks at 851.5 eV and 867.9 eV can be attributed to metallic Ni [63,64]. Based on the above results and previous reports [45,46], the electrochemical reactions between NiS and lithium can be illustrated as the following equations. 3NiS + 2Li+ + 2e
! Ni3S2 + Li2S

(3)

Ni3S2 + 4Li+ + 4e
! 3Ni + 2Li2S

(4)

Ni + Li2S ! NiS + 2Li+ + 2e


(5)

The above experimental results indicate that the electrochemical full lithiation reactions of the Co1-xS and NiS consist of three main stages: (1) lithium insertion in Co1-xS and NiS, (2) reduction reaction of Co1-xS and NiS forming amorphous nano-Co, Ni and Li2S, and (3) the nano-Co and Ni dispersed in Li2S matrix convert to CoS and NiS. In this work, the electrochemical performance of Co1-xS and NiS can be effectively improved by combining the CNTs@C with the Co1-xS and NiS. The comparison of these materials is shown in Table S1. The excellent Li-storage performances are attributed to the following reasons. First, the Co1-xS nanocrystals and ultrathin NiS nanosheet are confined on the CNTs, which is crucial for buffering the volume variation during charge-discharge process. And the ultrafine Co1-xS and NiS nanocrystals can effectively reduce the diffusion path of lithium-ion and increase the contact area between the active materials and electrolyte. Second, the amorphous carbon layer on the CNTs or the Co1-xS and NiS nanocrystals can further accommodate the large volume change and prevent the aggregation and pulverization. Third, the strong chemical bonding between carboxyl and hydroxyl groups and Co1xS/NiS nanocrystals can strengthen the adhesion between CNTs and Co1-xS/NiS nanocrystals and maintain the structural integrity over the repeated discharge/charge process. Finally, the one dimensional nanostructures can promote electron and ion transport throughout the electrode. All these factors lead to the excellent rate capability and superior cycling performance of CNTs@C@Co1-xS and CNTs@C@NiS towards high-performance Li storage.

Fig. 10. (a) TEM image of CNTs@C@NiS after 100 cycles at 0.1 A g
1. (b) HRTEM images of the lithiated electrode (discharged to 0.01 V), (c) HRTEM images of the delithiated electrode (charged to 3.0 V), (d) Ni 2p spectra of the lithiated electrode (discharged to 0.01 V) and delithiated electrode (charged to 3.0 V) in the 100th cycle at the current density of 0.1 A g
1.

R. Jin et al. / Electrochimica Acta 257 (2017) 20–30

4. Conclusions In summary, CNTs@C@Co1-xS and CNTs@C@NiS hybrids are designed and fabricated by a facile hydrothermal/solvothermal strategy using CNTs@C as template. Together with their nanoscale size, amorphous carbon layer and one dimensional nanostructure, the CNTs@C@Co1-xS and CNTs@C@NiS hybrids display outstanding electrochemical properties in lithium-ion batteries. The CNTs@C@Co1-xS delivers the reversible capacity of 875 mAh g
1 after 100 cycles at 0.1 A g
1, and keeps at 570 mAh g
1 at rate of 5 A g
1 over 60 cycles. For the CNTs@C@NiS electrode, high capacities of 649 and 377 mAh g
1 are still maintained at 0.1 A g
1 (100th cycle) and 5 A g
1 (60th cycle), respectively. All these data demonstrate that the CNTs@C@Co1-xS and CNTs@C@NiS are promising next generation anode material for LIBs. Acknowledgements The authors are grateful for the financial support of the Natural Science Foundation of China (Project no. 21301086), and Natural Science Foundation of Shandong Province (Project no. ZR2013BQ008). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta.2017.10.078. References [1] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652– 657. [2] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190–193. [3] R. Jin, H. Jiang, Y. Sun, Y. Ma, H. Li, G. Chen, Fabrication of NiFe2O4/C hollow spheres constructed by mesoporous nanospheres for high-performance lithium-ion batteries, Chem Eng J 303 (2016) 501–510. [4] Y. Zhou, D. Yan, H. Xu, J. Feng, X. Jiang, J. Yue, J. Yang, Y. Qian, Hollow nanospheres of mesoporous Co9S8 as a high-capacity and long-life anode for advanced lithium ion batteries, Nano Energy 12 (2015) 528–537. [5] L. Xia, S. Wang, G. Liu, L. Ding, D. Li, H. Wang, S. Qiao, Flexible SnO2, /N-Doped Carbon Nanofi ber Films as Integrated Electrodes for Lithium-Ion Batteries with Superior Rate Capacity and Long Cycle Life, Small 12 (2016) 853–859. [6] M. Winter, R.J. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors, Chem Rev 104 (2004) 4245–4270. [7] Y. Zhao, T. Liu, H. Xia, L. Zhang, J. Jiang, M. Shen, J. Ni, L. Gao, Branch-structured Bi2S3-CNT hybrids with improved lithium storage capability, J. Mater. Chem. A 2 (2014) 13854–13858. [8] J. Xiao, D. Choi, L. Cosimbescu, P. Koech, J. Liu, J.P. Lemmon, Exfoliated MoS2 Nanocomposite as an Anode Material for Lithium Ion Batteries, Chem Mater 22 (2010) 4522–4524. [9] N. Mahmood, C. Zhang, Y. Hou, Nickel Sulfide/Nitrogen-Doped Graphene Composites: Phase-Controlled Synthesis and High Performance Anode Materials for Lithium Ion Batteries, Small 9 (2013) 1321–1328. [10] X. Xu, Z. Fan, S. Ding, D. Yu, Y. Du, Fabrication of MoS2 nanosheet@TiO2 nanotube hybrid nanostructures for lithium storage, Nanoscale 6 (2014) 5245– 5250. [11] R. Jin, Y. Xu, G. Li, J. Liu, G. Chen, Hierarchical chlorophytum-like Bi2S3 architectures with high electrochemical performance, Int J Hydrogen Energ 38 (2013) 9137–9144. [12] C. Xu, Y. Zeng, X. Rui, N. Xiao, J. Zhu, W. Zhang, J. Chen, W. Liu, H. Tan, H.H. Hng, Q. Yan, Controlled Soft-Template Synthesis of Ultrathin C@FeS Nanosheets with High-Li-Storage Performance, ACS Nano 6 (2012) 4713–4721. [13] L. Li, M. Cabán-Acevedo, S.N. Girard, S. Jin, High-purity iron pyrite (FeS2) nanowires as high-capacity nanostructured cathodes for lithium-ion batteries, Nanoscale 6 (2014) 2112–2118. [14] B. Luo, Y. Fang, B. Wang, J. Zhou, H. Song, L. Zhi, Two dimensional grapheneSnS2 hybrids with superior rate capability for lithium ion storage, Energy Environ Sci 5 (2012) 5226–5230. [15] N. Mahmood, C. Zhang, J. Jiang, F. Liu, Y. Hou, Multifu nctional Co3S4/Graphene Composites for Lithium Ion Batteries and Oxygen Reduction Reaction, Chem Eur J 19 (2013) 5183–5190. [16] H. Liu, D. Su, G. Wang, S.Z. Qiao, An ordered mesoporous WS2 anode material with superior electrochemical performance for lithium ion batteries, J Mater Chem 22 (2012) 17437–17440.

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