Sb Nanoparticles Anchored on Nitrogen-Doped Amorphous Carbon

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Sb Nanoparticles Anchored on Nitrogen Doped Amorphous Carbon Coated Ultrathin CoSx Nanosheets for Excellent Performance in Lithium-Ion Batteries Rencheng Jin, Hua Jiang, Qingyao Wang, Guihua Li, and Shanmin Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14280 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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ACS Applied Materials & Interfaces

Sb Nanoparticles Anchored on Nitrogen Doped Amorphous Carbon Coated Ultrathin CoSx Nanosheets for Excellent Performance in Lithium-Ion Batteries Rencheng Jin,* Hua Jiang, Qingyao Wang, Guihua Li, Shanmin Gao* School of Chemistry & Materials Science, Ludong University, Yantai 264025, P. R. China. *Corresponding authors. E-mail: [email protected], [email protected]

ABSTRACT: Compare to single component material, the hybrids materials with various components display superior electrochemical performance. In this work, two dimensional CoSx@NC@Sb nanosheets assembled by ultrathin CoSx nanosheets (~4 nm), thin layer of N-doped amorphous carbon (NC) combined with colloid like Sb nanoparticles are designed and synthesized via solvothermal route accompanied by carbonization and Sb deposition procedure. If applied in lithium-ion batteries, the hybrids exhibit the specific capacity of 960 mAh g-1 at the 100th cycle at 0.1 A g-1. And a reversible capacity still maintains at 494 mAh g-1 after 500 cycles at the high rate of 10 A g-1. All the enhanced electrochemical properties of the hybrids are attributed to the synergistic effect of the two components and the unique structure features, which can effectively increase the electrical conductivity, shorten the pathway of Li+ diffusion, accommodate the volume variation, and inhibit the aggregation and pulverization of the electrode. We believe that the current work can provide a new strategy for designing and fabricating the high-performance anode materials for LIBs. Key Words: cobalt sulfides; antimony; sandwich-like structure; hybrids, anodes; Li ion batteries

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1. INTRODUCTION Lithium ion batteries (LIBs), one of the promising energy storage devices, have attracted a wide attention for their application in portable electronic devices and electric vehicles.1, 2 With the rapid increasing development of these electronic devices, the commercial graphite/carbon anode material is extremely prohibited because of the inferior theoretical capacity (372 mAh g-1).3, 4 Compared with carbon materials, transition metal sulfides have attracted the attention of recent researchers and scientists for their higher theoretical capacity and low cost.5-11 Among them, cobalt sulfides with different stoichiometric compositions (CoS,12, 13 Co1-xS,14 Co9S8,15-17 Co3S418, 19 and CoS29, 20) are considered as the alternative candidates for LIBs due to their high theoretical capacity, good electrical conductivity and thermal stability. Unfortunately, hampering issues including poor cycling stability and rate performance originated from the larger volume changes can also be observed. To improve their performance, constructing nanocomposites of cobalt sulfide and carbon is an effective strategy. For instance, the graphene-wrapped CoS nanoparticles with the capacity of 749 mAh g-1 after 40 cycles at 62.5 mA g-1 have been fabricated via a solvothermal approach.12 Huang et al. synthesized the CoSx/graphene nanocomposites through one-pot solvothermal route. The obtained two dimensional hybrids displayed the improved capacity of 950 mAh g-1 at the current density of 100 mA g-1 after 50 cycles.21 Qian’s research groups have been prepared one-dimensional MWCNT@a-C@Co9S8 nanocomposite through a solvothermal method and a calcination treatment.22 The capacity was pushed up to 662 mAh g-1 even at high rate of 1 A g-1. However, the dramatic volume variation and structure change of these materials still existed. Therefore, exploiting the cobalt sulfides nanocomposites with satisfactory performance is a crucial issue.

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In the past decades, two dimensional (2D) nanostructure has been attracted much attention because of its unique structural properties and surface characteristics.23 Unfortunately, the surface of most 2D structures is smooth, which leads to the stacking or aggregation.24 Such overlapped nanosheets will lead to the reduction of active surface area and lengthen the diffusion paths. An effective strategy to prevent the agglomeration is to construct loosely stacked sandwich like architectures by introducing additional additives. Therefore, different sandwich like 2D structures were fabricated, which displayed excellent electrochemical performances because of their rapid diffusion of Li+ and electron transport.23, 25-28 However, most of sandwich like 2D architectures are limited to graphene nanosheets and the fabrication process is complex. Therefore, exploiting facile and effective synthetic routes to prepare the sandwich-like 2D nanostructures is extremely crucial. Due to the high theoretical capacity (660 mAh g-1 for Li3Sb), good electrical conductivity and flat discharge plateau, antimony based materials are consider as a promising candidate for LIBs.29-32 The antimony supported cobalt sulfides are expected to effectively improve the electrochemical behavior. Herein, CoSx ultrathin nanosheets (~4 nm) were firstly fabricated by a facile solvothermal procedure. And CoSx@NC@Sb hybrids were then synthesized by using CoSx@C as template. In this unique 2D sandwich-like structure, the N-doped amorphous carbon (NC) layer not only serves as binder to connect the CoSx and Sb, but also enhances the electrical conductivity and accommodates the volume variation. Combined with the synergistic effect between ultrathin CoSx nanosheets and Sb nanoparticles, such unique 2D sandwich-like CoSx@NC@Sb hybrids exhibit excellent electrochemical performance.

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2. RESULTS AND DISCUSSION 2.1 Characterization of the samples The phase and crystal structure of the currently synthesized CoSx and CoSx@NC@Sb hybrids are determined by XRD analysis. The XRD pattern of CoSx (Figure 1a) shows that the obtained products are composed of Co1-xS (JCPDS no. 42-0826) and Co9S8 (JCPDS no. 02-1459). The wide half peak width in the pattern indicates the small size of the products. Figure 1b corresponds to the XRD pattern of CoSx@NC@Sb hybrids, the presence of Co1-xS and Co9S8 with no change in peak position and intensity confirm that the crystallinity and phase still preserve after carbonization and Sb deposition process. Except the Co1-xS and Co9S8, some tiny peaks assigned to Sb (JCPDS no. 35-0732) can be observed. The exact composition of the obtained samples is determined by ICP (Table S1). The carbon content of the samples is determined by carbon and sulfur analyzer. The carbon content of CoSx@NC@Sb, CoSx@NC-Sb and CoSx@C is about 4.6%, 4.2%, and 5.1%, respectively. Morphology and size of the obtained CoSx are determined by FESEM. The low magnification FESEM image shows that the CoSx consists of numerous nanosheets (Figure 2a). The wrinkled nanosheets are similar to the graphene (Figure 2b). The transmission electron microscopy images (Figure 2c and 2d) present that these nanosheets are highly transparent to electron beam, implying the ultrathin nature of the nanosheets. The exact thickness of nanosheets is charaterzied by AFM. As can be seen in Figure S1, the thickness of the obtained CoSx nanosheet is ~4 nm. The TEM image of the CoSx@C in Figure S2 indicates that the morphology have no change after carbonization treatment using glucose as carbon source. More structural features of the CoSx@C nanosheets are determined by the HRTEM. The lattice spacings of 0.253 nm, 0.248 nm and 0.301 nm match well with the (101) plane of Co1-xS and 4


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(400) and (311) planes of Co9S8 (Figure 2e and 2f). Different lattice orientations imply that the ultrathin nanosheets consist of many tiny nanocrystals with two kinds of components. Furthermore, the HRTEM image in Figure 2f clearly indicates that the CoSx nanosheet is homogeneously wrapped by a thin layer of carbon. Figure 3a displays the FESEM image of the CoSx@NC@Sb, wherein the nanosheets still preserve and the surface becomes coarse. After solvothermal treatment in the presence of antimony potassium tartrate and ethanediamine (reductant), the Sb particles can be uniformly decorated on the surface of CoSx nanosheets (Figure 3b). The TEM images (Figure 3c and Figure 3d) further prove that colloid like Sb nanoparticles are well-distributed on the CoSx and these particles are inclined to be compact and connected to each other. It should mention that the colloid like Sb nanoparticles are firmly attached to CoSx nanosheets even after the ultrasonic treatment. In the HRTEM images (Figure 3e and 3f), two lattice fringes of 0.352 and 0.310 nm appear, corresponding to the (101) and (012) planes of rhombohedral Sb, respectively. The elemental mapping in Figure 4 indicates that the elements C, Co, S, Sb and N are homogeneously distributed. The surface elemental compositions of the CoSx@NC@Sb hybrids are further investigated by XPS. The survey spectrum confirms that five elements C, Co, S, Sb, and N coexist in the composites (Figure 5a). For the survey spectrum of the CoSx, only C, Co, S and O can be observed and no signals from N and Sb are detected (Figure S3), implying the N-doping in the amorphous carbon for CoSx@NC@Sb. The deconvoluted Co 2p spectrum is presented in Figure 5b. The peaks located at 780.3 and 795.4 eV are attributed to Co 2p3/2 and Co 2p1/2 of Co3+, respectively, while the peaks at 782.5 and 797.3 eV are ascribed to Co 2p3/2 and Co

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2p1/2 of Co2+ in CoSx.33-35 Meanwhile, two shake-up satellites related to Co 2p3/2 and Co 2p1/2 appear at 788.2 and 803.9 eV, respectively. For the S 2p spectra, the fitting peaks at 160.3 and 162.1 eV originate from S 2p3/2 and S 2p1/2, respectively.35 The binding energies at 161.1 and 163.2 derive from the sulfur ions in a low coordination state at the surface and the metal-sulfur bond in the material, respectively.35 As can be seen in Figure 5d, the binding energies of 531.3 and 539.1 eV match with the Sb 3d5/2 and Sb 3d3/2, respectively.29,

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Notably, the characteristic peak of oxygen (530.4 eV) in the Sb 3d spectrum can also be found, which probably originate from the absorbed water and carbon dioxide.29 The N1s XPS spectrum of the CoSx@NC@Sb hybrids appears at 399.4 and 398.2 eV, which correspond to C-N and C=N, respectively (Figure 5e).36, 37 The XPS measurement combined with the element mapping analysis determine that the N distributes uniformly in the amorphous carbon. Figure 5f presents the high resolution C1s spectrum, in which the peaks at 283.6 eV, 284.9 and 287.8 eV can be assigned to the C-C bond, C-OH and –COOH, respectively.38, 39 The existed C-OH and -COOH functional group can complex with metal cations and promote the growth of Sb onto the CoSx nanosheets. To evaluate the effect of the functional groups, some comparative experiments are conducted. If CoSx is used as template instead of CoSx@C, the Sb cannot be anchored on the surface of CoSx due to the existence of lattice mismatch. As a result, only randomly distributed nanoparticles and CoSx nanosheets can be obtained (Figure S3a). In the absence of surfactant (glucose), Sb nanoparticles are scarcely deposited on the CoSx@C nanosheets (CoSx@NC-Sb). The prepared samples consist of numerous microspheres and nanosheets, as shown in Figure S3b. The experimental result indicates that the presence of amorphous carbon and glucose has much effect on the final morphology of the products. As

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discussed above, the amorphous carbon possesses large amount of -OH and -COOH, which can coordinate with Sb3+ under the ultrasonic treatment. In the solvothermal reaction system, the adsorbed Sb3+ and the free Sb3+ can be reduced to Sb with the assistance of ethanediamine. The Sb nuclei on amorphous carbon serve as template for further growth. Because the fresh nuclei are metastable, the glucose with numerous -OH groups will adsorb onto the surface to reduce the surface energy. Combined with the hydrogen bond and electrostatic effects of the amorphous carbon and glucose, the capped Sb nuclei are anchored onto the CoSx@C quickly. As the reaction proceeded, more Sb nanoparticles will deposited on the surface of CoSx@C and the sandwich-like hybrids generate. In addition, ethanediamine can also serve as N source. Through a facile solvothermal procedure, the N atoms can be doped into porous carbon. Compared with the previous reported N-doped method (for example, N2, NH3 or PPy treatment of at high temperature),40-43 applying the solution approach to prepare N-doped porous carbon is very attractive. The porous structure of the CoSx, CoSx@C, and CoSx@NC@Sb are characterized by the N2 adsorption-desorption measurement. In Figure 6a, all the prepared samples display a typical IV isotherm with the H3 type of hysteresis loop within a relative pressure between 0.5 and 1.0 (P/P0), implying the mesoporous structure of the product. The pore size distribution further confirms the mesoporous nature of the obtained samples (Figure 6b). The calculated Brunauer-Emmett-Teller (BET) specific surface area for CoSx, CoSx@C, and CoSx@NC@Sb are 82.3, 97.5, and 93.7 m2 g-1, respectively. The high surface area of CoSx based electrodes with the meso- and macro-porous features can enhance the contact area and facilitate Li+ diffusion. 2.2 Electrochemical performance 7



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The obtained CoSx, CoSx@C, and CoSx@NC@Sb are assembled into CR2025 coin type cells to evaluate the performance in LIBs. Figure 7a demonstrates the cyclic voltammograms (CV) of the CoSx@C in the 1st, 2nd, 3rd, 5th, 10th, and 15th cycles at a scan rate of 0.2 mV s-1. In the first discharge step, the reduction peak located at 1.02 V is attributed to the intercalation of lithium into CoSx and then conversion into metallic Co and Li2S.44 The wide and weak peak at 0.52 V maybe come from the formation of a solid electrolyte interphase (SEI) film. In the subsequent anodic step, three oxidation peaks centered at 1.41, 2.08, and 2.38 V are ascribed to the phase transition, similar to the cobalt sulfide electrode in the previous report.45, 46 The reduction peaks shift to the high voltage range after the first cycle, which may originate from the irreversible structural changes during the repeated discharge/charge process. Different from CoSx@C, three reduction peaks at 0.94, 0.71, and 0.42 V can be observed in the CV profiles for sandwich-like CoSx@NC@Sb electrode (Figure 7c). The peak appeared at 0.94 V belongs to the conversion of CoSx into LixCoSx and then to metallic Co and Li2S, while the other two peaks are associated with the transformation of Sb to Li3Sb and formation of SEI film, respectively.29, 30 During the oxidation process, the delithiation reaction of Li3Sb to metallic Sb appears at 1.08 V.30, 32 And the peak centered at 2.06 V originates from the sulfuration of metallic Co. Furthermore, the CV profiles for the two electrodes after first cycle are almost coincided with each other, implying the stable cyclability. Moreover, the appearance of metallic Co in the electrode is ahead of the formation of Li3Sb. The existence of Co can directly contact with the electrolyte and improve conductivity of the hybrids, which effectively promote the conversion from Sb to Li3Sb. Meanwhile, the metallic Co can also facilitate the transformation of

Li3Sb

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Sb

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intercalation/deintercalation behavior of Li ion for Sb is behind the reduction reaction of CoSx to Co during the reduction process and ahead of the sulfuration reaction process, respectively. As a consequence, the conductivity is effectively enhanced and the charge transfer is facilitated through introducing the Sb into CoSx@NC@Sb electrode. Figure 7b and 7d present the charge and discharge voltage curves of the two electrodes at a current density of 0.1 A g-1 between 0.01 and 3.0 V (vs Li+/Li). Both of the discharge/charge voltage profiles are in accordance with the corresponding CV plots. As can be seen in Figure 7b, a rapid potential drop to 1.25 V demonstrates that the Li+ can be inserted into CoSx to form LixCoSx. And the voltage plateau at about 1.25 V matches with the conversion reaction of LixCoSx to Co and Li2S. The voltage plateau of sulfuration of metallic Co can be observed at about 2.0 V. While for CoSx@NC@Sb, two new voltage plateaus at about 0.5 V and 0.9 V correspond to formation of Li3Sb and conversion of Li3Sb to Sb can be found, respectively (Figure 7d). The initial capacities of the CoSx@C are 1306 and 1061 mAh g-1, and the coulombic efficiency reaches to 81.2%. The irreversible capacity fade may be attributed to the deactivation of the conversion product and formation of SEI film, as frequently observed in metal sulfides electrodes.47, 48 For CoSx@NC@Sb, a specific capacity of 1283 mAh g-1 and 1096 mAh g-1 is exhibited, leads to a coulombic efficiency of 85.4%. Such improved coulombic efficiency can be ascribed to the increased reversibility of the involved electrochemical reactions, which can be supported by CV results in Figure 7c. In this sandwich like structure, the interface of CoSx@NC and Sb@NC exists. Such unique interface can provide extra Li+ channel for accelerating the Li+ diffusion rate. In addition, the loosely stacked sandwich like architecture can inhibit the aggregation and pulverization the electrode during cycling. As a

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consequence, the reversibility of the electrode increased. Similar results can also be observed in β-MnO2/α-Fe2O3, Fe2O3/SnO2 and other metal oxide electrodes.49-51 The cycling performance of the sandwich-like CoSx@NC@Sb electrode along with those of CoSx@NC-Sb composites, ultrathin CoSx and CoSx@C electrodes are presented in Figure 7e at the current density of 0.1 A g-1. As expected, the sandwich-like CoSx@NC@Sb delivers a reversible capacity of 960 mAh g-1 after 100 cycles, which is higher than those of CoSx@NC-Sb (809 mAh g-1 at 100th cycle) and CoSx@C (746 mAh g-1 at 100th cycle). Fo CoSx@NC-Sb r the ultrathin CoSx nanosheets, the specific capacity decrease sharply to 576 mAh g-1 at the first 20 cycles and tends to stabilize to 543 mAh g-1 after 100 cycles. These values are better and comparable to the previous reported cobalt sulfides based electrodes, as summarized in Table 1.9, 13, 17, 19, 20, 22, 44-46, 52-54 The rate performance of the obtained CoSx@NC@Sb, CoSx@NC@Sb, CoSx@C and CoSx is also investigated. As shown in Figure 7f, the sandwich-like CoSx@NC@Sb exhibits the best rate performance. The reversible capacities of 1052, 925, 846, 788, 746, 660 mAh g-1 can be achieved at the current density of 0.1, 0.2, 0.5, 1, 2, and 5 A g-1, respectively. If the current density is back to 0.1 A-1, the specific capacity returns to 1008 mAh g-1, displaying the good rate capacity of the materials. The specific capacities for the ultrathin CoSx nanosheets are 722, 506, 420, 332, 253, and 193 mAh g-1 at the rate of 0.1, 0.2, 0.5, 1, 2, and 5 A g-1, respectively. As the rate is back to 0.1 A g-1, the specific recovers to the initial level. To further confirm the excellent cycling stability of the obtained sandwich-like CoSx@NC@Sb hybrids, the cells are detected at a high rate of 10 A g-1 up to 500 cycles. As indicated in Figure 7g, the capacity of CoSx@NC@Sb hybrids still maintains at 494 mAh g-1 after 500 cycles, which is higher than that of CoSx@C (257 mAh g-1). The cells after 500 cycles are decomposed and the morphology

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is characterized by TEM (Figure 8). Clearly, the sheets like structure still preserved even at high current rate, indicating the good structural stability during the long-term repeated discharge-charge process. Electrochemical impedance spectra (EIS) are applied to further investigate the superior performance of the sandwich-like CoSx@NC@Sb electrode. Figure 9a displays the Nyquist plots of the obtained CoSx@NC@Sb, CoSx@NC-Sb, ultrathin CoSx nanosheets and CoSx@C electrodes and the fitting results are listed in Table 2. In the equivalent circuit model (Figure 9b), Re represents the internal resistance of the cells, Rf and CPE-1 are attributed to SEI surface resistance and constant phase element of the SEI film on the surface of the electrodes. The fitting circuit model at the medium frequency region is associated with the charge transfer resistance (Rct) and double-layer capacitance across the surface (CPE-2) in the interface of electrode/electrolyte. And Zw is attributed to the Warburg resistance, which is related to the lithium diffusion process. In Figure 9a, the four plots with similar shape are composed of two semicircles in the high- and medium frequency region and a sloping line in the low-frequency range. The two semicircles are associated with Rf and Rct of the cells, while the sloping line corresponds to the solid-state diffusion Li+ into the active materials. The fitted impedance parameters in Table 2 indicate that the Rct value (70.35 Ω) of CoSx@NC@Sb electrode is much lower than those of CoSx@NC-Sb (90.37 Ω), ultrathin CoSx nanosheets (238 Ω) and CoSx@C (102.33 Ω) electrodes. The result demonstrates that the incorporation of NC and Sb can increase the electrical conductivity, leading to the improved electrochemical performance. According to the above analysis, the improved performance of the electrodes is due to the following factors. First, the ultrathin CoSx and the anchored colloid Sb nanoparticles can

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enhance the contact area and provide extra channel for Li+ diffusion. Second, the synergistic effect of the CoSx and Sb improves the reversibility and accommodates the volume change during cycles. As discussed above, the preferential formation of metallic Co increases the conductivity and facilitates the phase conversion of Sb to Li3Sb. In other words, Sb@NC serves as a matrix to inhibit the aggregation of metallic Co and buffer the volume variation induced by the reduction of CoSx. During the charge process, the voltage of sulfuration of Co to CoS appears higher than that of Li3Sb to Sb, which similarly increases the electrical conductivity. Third, the amorphous N-doped carbon between CoSx and Sb not only prevents aggregation and pulverization of the electrode for maintaining the structural integrity, but also promotes rapid electron transport. 3. CONCLUSIONS In summary, sandwich-like CoSx@NC@Sb nanosheets have been fabricated through three steps: Ultrathin CoSx nanosheets (~4 nm) have been firstly prepared by a facile solvothermal method. After hydrothermal treatment in the presence of glucose, the CoSx@C nanosheets are synthesized. By using CoSx@C nanosheets as template, the sandwich-like CoSx@NC@Sb nanosheets generate via a solvothermal procedure. In this unique structure, the amorphous carbon layer can serve as the binder to tightly connect the CoSx nanosheets with the Sb nanoparticles and inhibit the lattice mismatch of the two components. As the anodes material of LIBs, the sandwich-like structure not only enhances the electrical conductivity, but also buffers against the volume variation during Li+ intercalated and deintercalated process. Moreover, the advantages such as large surface area, synergistic effect of different components and good mechanical flexibility are also beneficial for maintaining the structural integrity and inhibiting

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the aggregation of the electrode during cycling. As a result, the sandwich-like CoSx@NC@Sb hybrids exhibits excellent electrochemical properties such as good stability, and high rate capability. After 500 cycles, a reversible capability of 494 mAh g-1 can be preserved even at a higher current density of 10 A g-1. This result shows that the sandwich-like CoSx@NC@Sb hybrids are a promising candidate for 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). ASSOCIATED CONTENT Supporting Information. The experimental section, ICP, AFM image of CoSx, TEM image of CoSx@C, and XPS spectrum of CoSx@C, are available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1.

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36. Yang, L.; Li, X.; He, S.; Du, G.; Yu, X.; Liu, J.; Gao, Q.; Hu, R.; Zhu, M., Mesoporous Mo2C/N-doped carbon heteronanowires as high-rate and long-life anode materials for Li-ion batteries J. Mater. Chem. A 2016, 4, 10842-10849. 37. Qie, L.; Chen, W.-M.; Wang, Z.-H.; Shao, Q.-G.; Li, X.; Yuan, L.-X.; Hu, X.-L.; Zhang, W.-X.; Huang, Y.-H., Nitrogen-Doped Porous Carbon Nanofi ber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv Mater 2012, 24, 2047-2050. 38. Yin, Y.; Liu, W.; Huo, N.; Yang, S., Synthesis of Vesicle-Like MgFe2O4/Graphene 3D Network Anode Material with Enhanced Lithium Storage Performance. ACS Sustainable Chem. Eng. 2017, 5, 563-570. 39. Li, D.; Zhou, J.; Chen, X.; Song, H., Amorphous Fe2O3/Graphene Composite Nanosheets with Enhanced Electrochemical Performance for Sodium-Ion Battery. ACS Appl Mater Interfaces 2016, 8, 30899-30907. 40. Lei, C.; Han, F.; Li, D.; Li, W.-C.; Sun, Q.; Zhang, X.-Q.; Lu, A.-H., Dopamine as the coating agent and carbon precursor for the fabrication of N-doped carbon coated Fe3O4 composites as superior lithium ion anodes Nanoscale 2013, 5, 1168-1175 41. Landi, B. J.; Ganter, M. J.; Cress, C. D.; DiLeo, R. A.; Raffaelle, R. P., Carbon nanotubes for lithium ion batteries Energy Environ. Sci. 2009, 2, 638-654. 42. Sevilla, M.; Mokay, R.; Fuertes, A. B., Ultrahigh surface area polypyrrole-based carbons with superior performance for hydrogen storage. Energy Environ. Sci. 2011, 4, 2930-2936 43. Su, F.; Poh, C. K.; Chen, J. S.; Xu, G.; Wang, D.; Li, Q.; Lin, J.; Lou, X. W., Nitrogen-containing microporous carbon nanospheres with improved capacitive properties Energy Environ. Sci. 2011, 4, 717-724 44. Qiu, W.; Jiao, J.; Xia, J.; Zhong, H.; Chen, L., A Self-Standing and Flexible Electrode of Yolk–Shell CoS2 Spheres Encapsulated with Nitrogen-Doped Graphene for HighPerformance Lithium-Ion Batteries. Chem. Eur. J. 2015, 21, 4359-4367

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(b)

(a)

Figure 1

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 2

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 3

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Co

S

Sb

N

C

Figure 4

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 5

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(b)

(a)

Figure 6

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(a)

(b)

(c)

(d)

(e) (f)

(g)

Figure 7

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(a)

(b)

Figure 8

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Table 1 Cobalt sulfide based anode materials

Current density (mA g-1)

Cycles

Capacity (mAh g-1)

references

Ultrathin CoSx nanosheets CoSx@C nanosheets CoSx@NC@Sb hybrids CoSx@NC@Sb hybrids worm-like CoS2 CoS2/RGO nanocomposites CoS2 nanocages/Graphene MWCNT@α-C@Co9S8 nanocomposite CoS-graphene nanocomposite Mesoporous Co9S8 hollow nanosphere Yolk-shell Co9S8 microspheres Carbon-coated Co9S8 nanodandelions CoS2/graphene nanoarchitecture Co3S4/graphene composites CoS2/N-doped porous carbon Yolk-Shell CoS2/N-doped graphene

100 100 100 10000 100 100 100 1000 0.1 C 200 1000 1000 50 0.2 C 100 100

100 100 100 500 100 50 150 120 80 100 100 50 40 100 50 150

543 746 960 494 883 640 800 662 898 1122 634 520 600 720 560 882

This work This work This work This work

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20 52 53 22 13 45 17 54 9 19 46 44


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Table 2 Electrodes

Re (Ω)

Rf (Ω)

Rct (Ω)

CPE-1 (×10-5)

CPE-2 (×10-5)

CoSx

3.84

8.54

238

11.84

494.9

CoSx/C

2.99

10.44

102.33

6.46

273.3

CoSx/NC-Sb

2.37

9.32

90.37

7.19

488.2

CoSx/NC@Sb

1.66

13.66

70.35

76.92

143

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(a)

(b)

Figure 9

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Figure captions Figure 1. XRD patterns of (a) CoSx and (b) CoSx@NC@Sb. Figure 2. (a, b) Typical FESEM images of the as-prepared CoSx, (c, d) TEM images of the as-prepared CoSx, (e, f) HRTEM images of the CoSx@C. Figure 3. (a, b) Typical FESEM images of the obtained CoSx@NC@Sb hybrids, (c, d) TEM images of the obtained CoSx@NC@Sb hybrids, and (e, f) HRTEM images of the CoSx@NC@Sb hybrids. Figure 4. STEM elemental mapping images of the CoSx@NC@Sb hybrids. Figure 5. (a) XPS survey spectrum, (b) Co 2p, (c) S 2p, (d) Sb 3d, (e) N 1s, and (f) C 1s of the CoSx@NC@Sb hybrids. Figure 6. (a) Nitrogen adsorption-desorption isotherms and (b) pore-size-distribution curves of CoSx, CoSx@C, and CoSx@NC@Sb hybrids Figure 7. (a) Cyclic voltammograms of CoSx@C at the scan rate of 0.2 mV s-1, (b) charge/discharge profiles of CoSx@C at a current density of 0.1 A g-1 cycled between 0.01 and 3.0 V, (c) Cyclic voltammograms of CoSx@NC@Sb at the scan rate of 0.2 mV s-1, (d) charge/discharge profiles of CoSx@NC@Sb at a current density of 0.1 A g-1, (e) cycle performance for CoSx, CoSx@C, CoSx@NC-Sb, and CoSx@NC@Sb electrodes at a current density of 0.1 A g-1, (f) rate performance against cycle number at various current densities, (g) long-term cycling performance of CoSx@C and CoSx@NC@Sb at the current density of 10 A g-1.

Figure 8. TEM images of the (a) CoSx@NC@Sb and (b) CoSx@C electrodes after 500 cycles at the current density of 10 A g-1. 30


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Table 1. Comparison of our product CoSx@NC@Sb hybrids with other cobalt sulfides based anode materials reported in the literature. Table 2. EIS fitting results of the different electrodes. Figure 9. (a) EIS spectra of CoSx, CoSx@C, CoSx@NC-Sb , and CoSx@NC@Sb electrodes, (b) corresponding equivalent circuit that is used to fit the experimental data.

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Table of Contents Graphic

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153x57mm (144 x 144 DPI)


Sb Nanoparticles Anchored on Nitrogen-Doped Amorphous Carbon

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