C composite as high performance anode material for lithium ion batteries

C composite as high performance anode material for lithium ion batteries

Journal of Alloys and Compounds 728 (2017) 1139e1145 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: htt...

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Journal of Alloys and Compounds 728 (2017) 1139e1145

Contents lists available at ScienceDirect

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

Scalable synthesis of Sb/MoS2/C composite as high performance anode material for lithium ion batteries Youguo Huang a, Cheng Ji a, Qichang Pan a, Xiaohui Zhang a, c, Jiujun Zhang b, Hongqiang Wang a, *, Tao Liao a, Qingyu Li b, ** a b c

School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin, 541004, China Guangxi Key Laboratory of Low Carbon Energy Materials, Guangxi Normal University, Guilin, 541004, China College of Materials and Environmental Engineering, Hezhou University, Hezhou, 542899, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 June 2017 Received in revised form 8 September 2017 Accepted 9 September 2017 Available online 12 September 2017

Metal Sb is considered as alternative candidate anode material for lithium-ion batteries (LIBs) due to its high capacity, but the significant volume expansion during the charge and discharge process cause poor stability seriously influencing on the practical application in batteries. Herein, a facile and scalable method is developed for preparation of Sb/MoS2/C composite using the cubic NaCl particles as template. The Sb nanoparticles embedded deeply in carbon nanosheets buffer the internal stress caused by the volume expansion of Sb and avoid the direct exposure of electrolyte to the Sb nanoparticles. The MoS2 nanoparticles can provide more active sites to store more Liþ and speed up the migration rate of Liþ. Tested as an anode material, the Sb/MoS2/C composite shows outstanding rate capability (763 mA h g1 at 0.2 A g1, 642.2 mA h g1 at 0.5 A g1, 544 mA h g1 at 1.0 A g1, 459 mA h g1 at 2.0 A g1 and 353 mA h g1 at 5.0 A g1) and long cycling stability (a high capacity of 679.5 mA h g1 is achieved at 0.2 A g1 after 250 cycles). © 2017 Published by Elsevier B.V.

Keywords: Antimony Molybdenum disulfide Carbon nanosheets Anode Lithium ion batteries

1. Introduction Lithium ion batteries (LIBs) play a crucial and irreplaceable role in our lives, which extensively application in a wide range such as mobile phones, and electric vehicle due to their high power/energy density, and environmentally friendly as well as excellent security [1e4]. However, the current commercial anode material, graphite, is far from meeting the future power tools, portable electronics and renewable energies owing to its relatively small theoretical specific capacity (372 mA h g1) [5e9]. Therefore, developing an anode electrode material with long cycle life and high capacity for LIBs to meet the increasingly large consumer electronics are urgently necessary. For the anode materials, there are a series of candidates that can replace to graphite for LIBs [10,11]. Recently, alloy-based materials (such as tin, antimony, phosphorus or silicon) can manifest wonderful energy performances on the basis of alloying reactions

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected] (Q. Li). http://dx.doi.org/10.1016/j.jallcom.2017.09.101 0925-8388/© 2017 Published by Elsevier B.V.

towards lithium among the elements in the periodic table [12e17]. Among all of these alloy-based materials, Sb has been widely studied as alternative material to graphite, owing to its natural abundance and high theoretical capacity (660 mA h g1) accompanied with the transformation of Sb to Li3Sb [18,19]. However, the practical application of Sb has been limited owing to the dramatic expansion and contraction during the lithiation/delithiation process lead to pulverization and cracking of electrodes, which cause the loss of contact in the electrode and/or separation of the electrode materials from the current collector, resulting in poor cycling stability [20,21]. Therefore, in order to solve these issues, substantial and useful efforts have been made to suppress the volume expansion to improve the structure stability [19,21,22]. Designed composite material composed of nano-sized Sb and carbon is one of the effective method [23]. On one hand, the carbon can offer space to cushion the internal stress which caused by volume changes. On the other hand, the carbon can not only avoid the direct contact between electrolyte and metal but also enhance the electrical conductivity of the electrode, which significantly improve cycling performance and rate capability [24]. Recently, MoS2 attract great interest due to it as anode material for LIBs with high capacity [25e27]. MoS2 is one of the typical

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transition metal sulfides, it has a hierarchical structure that is grouped orderly by weak van der Waals forces, which is similar to graphite [28]. Due to the layered structure of MoS2, it promotes Liþ intercalation/extraction. Therefore, many compound systems based on MoS2 nanostructure, such as Fe2O3/MoS2 [29], SnO2/MoS2 [30], Fe3O4/MoS2 [31], and [email protected] [32], have been studied as high performance anode materials for LIBs. In this study, a facile and extensible method has been developed to manufacture Sb/MoS2/C composite as anode material for LIBs, which based on our previous works [33,34]. In such a constructed architecture, the Sb nanoparticles embedded deeply in carbon nanosheets buffer the internal stress caused by the volume expansion of Sb and avoid the direct exposure of electrolyte to the Sb nanoparticles. In addition, the MoS2 nanoparticles provide more active sites to store more Liþ and speed up the migration rate of Liþ, resulting in higher capacity and good rate capability. The Sb/MoS2/C composite exhibits superior cycling stability and outstanding rate capability when tested as anode material for LIBs. 2. Experimental 2.1. Material synthesis 2.1.1. Synthesis of the MoS2 The MoS2 was prepared by a simple hydrothermal method. Firstly, 1.8 g of Na2MoO4$2H2O were dissolved in 300 ml of mixed solution (deionized water-ethanol solution, 1:1, v), and then 2.4 g NH2CSNH2 was added. After stirring until it completely dissolved to form a transparent mixture. The solution transformed into a 500 ml Teflon-lined stainless steel autoclave and maintained at 200  C for 24 h to yield the black turbid liquid. Subsequently, filter the liquid and dried in a vacuum oven at 60  C overnight to get the MoS2 powder. 2.2. Synthesis of the Sb/MoS2/C composite 0.5 g Sb (99.90% purity, yfnano), 0.2 g MoS2, 3 g citric acid (AR), 8 g H2O and 25 g NaCl and 25 g stainless-steel balls were put into a stainless-steel ball milling vial. Then, the ball mill was sealed and with a repeated rotation speed of 500 rpm for 20 h. After the end of the ball milling, the obtained slurry was dried at oven. Subsequently, the hybrids were heated to 750  C under an Ar atmosphere with a heating rate of 2  C min1 for 2 h. The resulting black powder was soaked with deionized water for one hour to obtain a black turbid solution. The black solid was treated with deionized water to ensure complete removal of NaCl. Finally, the resulting black powder was dried in an 80  C oven. In addition, the Sb/C composite was also prepared without adding MoS2 under the same conditions. Finally, three different samples was also prepared to obtain the optimal proportion at the same condition, the weight ratio of Sb:MoS2 was varied as 5:3, 5:2 and 6:1. Also, the samples were named as SMC-532, SMC-523, SMC-613, respectively. 2.3. Materials characterization The Sb/MoS2/C and Sb/C material were characterized by using XRD, which were taken on a Rigaku D/max 2500 with Cu KR radiation at 40 kV and 40 mA from 10 -90 at 10 min1 with the step length of 0.02 . TEM and SEM were performed on a JEOL 2011 and Philips FEI Quanta 200 FEG, respectively, to explore the microstructure and morphology of the Sb/MoS2/C composite. TGA analysis was performed to determine the content of carbon with a Perkin Elmer (TA Apparatus), which the temperature was raised to 600  C at rate of 5  C min1 under air atmosphere. X-Ray photoelectron spectroscopy (XPS) spectra were performed to study the

Sb/MoS2/C composite by using an AXIS ULTRA DLD system. 2.4. Electrochemical measurements The performance testing of all as-prepared materials were conducted in the form of coin test cells cell (CR2025) with lithium foil was used as the counter electrode, the electrolyte composed with LiPF6 (1 M), ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:2, v/v). During the preparation of working electrodes, active materials (Sb/MoS2/C composite or Sb/C composite), Super-P (SP), and polymer binder (PVDF) with mass percent ratio of 80:10:10 were added into the N-methyl-pyrollidone (NMP). The resulting slurry was coated onto pure Cu foil and dried in a vacuum oven at 80  C overnight to drying the solvent completely. The cycle and rate performance of the cells was tested by using a Land battery testing system (BT2013A) with the voltage from 0.01 V to 3.0 V. The cyclic voltammogram (CV) of the Sb/MoS2/C composite electrodes was tested by using an Electrochemical Workstation (IM6, Germany) with the potential range from 0.005 Ve3 V. This instrument is also used to test the electrochemical impedance of the Sb/MoS2/C composite electrodes by using an AC voltage of 5 mV with frequencies form 0.1 Hz to 100 kHz. 3. Results and discussion The preparation process of the Sb/MoS2/C composite is illustrated in Fig. 1. As described in the experimental section, the addition of 3D NaCl cube facilitates the dispersion of Sb, prevents the agglomeration and provides a template for the formation of nanosheet. First, the NaCl, Sb, MoS2, distilled water and sodium citrate were added in a stainless-steel ball milling vial, and homogeneous slurry was obtained after ball milling. In the process, the Sb, MoS2, and sodium citrate was coated on the smooth surface of NaCl evenly and the particle size of the Sb is reduced. After drying the slurry, the black solid was heated at 750  C under Ar, in which the sodium citrate transformed into carbon. Finally, the flaky structure of Sb/MoS2/C composite was obtained after removing the NaCl particle with distilled water. To ensure the crystalline phases of as-prepared samples, XRD analysis was carried out. As shown in Fig. S1, four strong diffraction peaks at 2q ¼ 23.69 , 28.69 , 40.08 and 41.95 for both Sb/MoS2/C and Sb/C samples, which are attributed to the (003), (012), (104) and (110) planes for metallic antimony (JCPDS card No.37-1492), respectively. Moreover, there are some slight peaks can be assigned to the crystalline planes of the hexagonal MoS2 phase (PDF-#35-0732) only appeared in Sb/MoS2/C. And a peak at 2q ¼ 14.18 is attributed to the (002) crystalline plane of MoS2, which indicates the ordered stacking of S-Mo-S layers [35]. In addition, there are no peak of carbon observed because of the annealing temperature of 750  C is much lower than the graphitization temperature of 3000  C; so the carbon in the composite should be amorphous. Thermo-gravimetric analysis (TGA) of the Sb/MoS2/C was tested to gain the chemical composition, as shown in Fig. S2. The weight loss at about 100e350  C can be ascribed to the loss of water while the weight loss approximately after 350  C is attributed to oxidize Sb to Sb2O4 and MoS2 to MoO3 as well as the decomposition of amorphous carbon in air. According to TGA result, the mass fraction of Sb, MoS2 and C in the Sb/MoS2/C composite can be calculated to be around 54.37, 21.75 and 23.88 wt%, respectively. The Sb/MoS2/C composite is further confirmed by XPS analysis. Fig. 2A shows the survey XPS spectrum of the Sb/MoS2/C composite, indicating that the presence of Sb, Mo, S and C elements. The high resolution spectrum of Sb 3d is shown in Fig. 2. B, two peaks located at 531.2 and 540.5 eV are observed, corresponding to the Sb 3d5/2 and Sb 3d3/2 which are attributed to antimony metal [19].

Y. Huang et al. / Journal of Alloys and Compounds 728 (2017) 1139e1145

Fig. 1. Schematic illustration of the synthesis process for Sb/MoS2/C composite.

Fig. 2. (A) Survey XPS spectrum of Sb/MoS2/C composite, (B, C and D) High-resolution spectra of Sn 3d, Mo 3d and S 2p, respectively.

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Fig. 3. SEM images of Sb/MoS2/C composite (A, C), and (B, D) Sb/C composite.

Furthermore, from Fig. 2C and D, the Sb/MoS2/C composite represent the Mo 3d3/2; 3d5/2 and S 2p3/2; 2p1/2 satellite peaks at 232.5 eV; 229.4 eV and 162.2eV; 163.6 eV, respectively. Such Mo peaks corresponded to 4þ oxidation state and the peaks of S corresponded to 2- restore state, which confirm the MoS2 is successfully synthesized [27,28]. On the basis of the XRD and XPS analyses, the composite was confirmed to be composed of Sb, MoS2, and carbon. The morphology and microstructure of the all samples are characterized by SEM. The SEM images of as-prepared MoS2 and

commercial Sb powders as shown in Fig. S3, the MoS2 exhibits a sphere-like structure with consisting of nanosheets (Fig. S3A), and the Sb powders show bulk materials with big particles (Fig. S3B). Moreover, as shown in Fig. 3A and C, the Sb/MoS2/C exhibit a sheetlike structure, obviously. Moreover, some Sb particles are embedded in nanosheets. Nevertheless, the surface become slightly rougher and the Sb particles turn to be smaller. It is due to the force caused by the ball milling during the milling process, and the Sb particles are reduced and embedded in the carbon nanosheets. In addition, the Sb/C composite also exhibits nanosheets structure

Fig. 4. TEM images (A, B) and (C, D) HRTEM images of Sb/MoS2/C composite.

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from Fig. 3B and D. However, it is not observed clearly that Sb nanoparticles are embedded in the carbon layer, which can be attributed to the Sb/C composite have a thicker carbon layer because of the Sb/C with higher carbon content than Sb/MoS2/C composite. EDS analysis of Sb/MoS2/C composite reveals the existence of Sb, C, Mo, and S as shown in Fig. S4. The EDS mapping images show that Sb nanoparticles and MoS2 are distributed evenly in the carbon nanosheets.

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TEM was further employed to investigate the structure of the Sb/ MoS2/C composite. As shown in Fig. 4A and B, a continuous Sb nanoparticles with an average size of around 15 nm are evenly embedded in carbon nanosheets can be observed. In addition, the lattice fringe orientations in the HRTEM images (Fig. 4C and D) demonstrate clear lattice fringes with d-spacings of 0.31 nm and 0.27 nm, corresponding to the (012) plane of Sb crystal and the (100) plane of MoS2 crystal respectively.

Fig. 5. The CV curves of the (A) Sb/MoS2/C composite and (B) Sb/C composite at the same scan rate of 0.1 mV s1. (C) Cycle performance of SMC-523, SMC-532 and SMC-613 composite at current density of 0.2 A g1. (D) Rate performance of the Sb/MoS2/C and Sb/C composite at different current densities. (E) Cycle performance of MoS2, Sb and the Sb/MoS2/C and Sb/C composite at current density of 0.2 A g1.

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The electrochemical performances of the Sb/C and Sb/MoS2/C were investigated and the results are shown in Fig. 5. CV measurement was conducted to understand the electrochemical behavior. Fig. 5A and B shows the CV curves of Sb/MoS2/C and Sb/C composites for the first three cycles respectively. It is obvious that both of the two CV curves for the charge and discharge branch in the first cycle is different from the subsequent cycles, due to the formation of an irreversible SEI layer on the surface of the materials [20]. Then, Fig. 5A shows two reduction peaks at 1.12 and 0.56 V and an oxidation peak at 2.28 V. As previously reported [28], these two reduction peaks at 1.12 and 0.56 V during the first cathodic sweep, corresponding to the insertion of Li ions into the MoS2 lattice to form LixMoS2 and then the reduction of LixMoS2 to Li2S and Mo, respectively. Also, the oxidation peak at about 2.28 V is owing to the oxidation of Li2S into S and lithium ions [28]. In addition, the voltage plateaus of Sb are observed at around 0.75 and 1.1 V in Fig. 5A and B, corresponding to the formation of Li3Sb, which are contributed to the alloying reaction between Sb and lithium ions and the reduction of Li3Sb to Sb, respectively [19]. The charge/discharge profiles of Sb/MoS2/C composite and Sb/C composite are shown in Fig. S5. The Sb/MoS2/C composite shows a charge and discharge capacity of 1046.7 and 1450 mA h g1, respectively. However, the Sb/C composite displays a lower charge capacity and discharge capacity of 677.5 and 852.7 mAh g1. Compared with Sb/MoS2/C composite, the Sb/MoS2/C composite shows a higher reversible capacity than the Sb/C composite due to the addition of MoS2 providing more active sites for hosting Liþ. During the charge process of Sb/MoS2/C composite, a plateau appears at around 2.3 V, according with the previous CV curves. Finally, the two smooth voltage platform at around 0.75 and 1.1 V in both two figures are also in agreement with the above CV study. To obtain the optimal electrochemical performances, the cycle performance of three different weight ratio of Sb:MoS2 are compared as shown in Fig. 5C. At the same current density of 0.2 A g1, the Sb/MoS2/C composite with a ratio of 5: 2: 3 displays the most excellent cycle performance in the three samples. Although the cycling performance of the SMC-613 composite is relatively as stable as SMC-523 composite, but the reversible capacity is much lower than the SMC-523 composite after 250 cycles at the same current density of 0.2 A g1. More MoS2 content in the SMC-523 composite provide more active sites to host lithium ions. On the other hand, the SMC-532 composite delivers the highest discharge capacity in the first few cycles, but it shows a rapid decay

after around 60 cycles, indicating the destroyed structure of the material. Fig. 5D shows the rate capability of the Sb/C and Sb/MoS2/ C composites. The Sb/MoS2/C displays a high reversible capacity of 763, 642.2, 544, 459, and 353 mA h g1 at 0.2, 0.5, 1, 2, and 5 A g1, respectively. More importantly, when the current density comes back to 0.2 A g1 even after cycling at high rate of 5.0 A g1, a higher reversible capacity of 598 mA h g1 can be kept, suggesting the outstanding rate capability. However, Sb/C composite exhibits lower capacity of 414, 318, 260, 216, and 161 mA h g1 at the same current densities. Moreover, the Sb/C composite only gives a reversible capacity of 277 mA h g1 when the current density comes back to 0.2 A g1, which proves superior rate capability the of the Sb/MoS2/C compared to Sb/C composite. The rate capability of the Sb/MoS2/C electrode is improved because of the cooperating action of MoS2, which offers more active sites for hosting lithium ions, resulting in higher capacity and enables fast transport of both electrons and ions. In addition, Sb/MoS2/C electrode also shows stable cycling performance. After 250 cycles, a high capacity of 679.5 mA h g1 can be achieved at 0.2 A g1 (Fig. 5E). In contrast, as for the Sb/C composite, a lower capacity of 409 mA h g1 was obtained under the same current density. However, the capacity of MoS2 and pristine Sb decay fast, a discharge capacity of 154.0 and 158.6 mA h g1 after several cycles at the same rate, which may be caused by the destroyed structure during the charge/discharge process. Finally, ascribing to the strong synergistic effect between MoS2 and carbon nanosheets, the cycle performance of the Sb/MoS2/C composite is improved than the Sb/C composite [30,36e39]. The carbon nanosheets can not only supply enough space to cushion the internal stress which caused by the volume change of Sb nanoparticles but also restrain agglomeration of the Sb nanoparticles to larger particles during the continuous charge/discharge process, resulting in structural stability of the Sb/MoS2/C composite. In addition, the MoS2 can provide more active sites to store lithium ions and shorter the diffusion distance of both electrons and Liþ [31]. The long-term cycling performance of the Sb/MoS2/C and Sb/C composite at a high current of 0.5 A g1 has also been carried out, and the result is shown in Fig. S6. The Sb/MoS2/C and Sb/C composites exhibit the capacity of 387.9 and 266.3 mA h g1 at 0.5 A g1 after 250 cycles, respectively. It is proved that the conductivity of lithium ions and the ability to insertion/extraction of lithium ions have been greatly improved in our Sb/MoS2/C electrode. EIS analysis was performed to understand the mechanism for

Fig. 6. Nyquist plots of the AC impedance spectra for the Sb/MoS2/C (A) and Sb/C composites (B) before and after 250th cycle at 0.2 A g1.

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the enhanced electrochemical performance of Sb/MoS2/C and Sb/C composites. Fig. 6 shows the Nyquist plots of the two composites before and after 250 cycles at 0.2 A g1. It can be seen that both of the curves in CV are composed of a depressed semicircle in the high frequency region and a inclined straight line at low frequencies [32]. It is observed that the ohmic resistance after 250th cycle is much lower than the one before cycling, suggesting a greatly reduced charge transfer resistance at the electrode/electrode interface. It is mainly due to the lack of electrolyte wetting in the electrode materials before cycling. Nevertheless, the ohmic resistance of the Sb/MoS2/C composite is much smaller than Sb/C composite after 250 cycles, indicting the addition of MoS2 is benefit to form a more stable SEI film, which speed up the transmission of lithium ions effectively [40,41]. It also suggests the superior rate and cycle performances of the Sb/MoS2/C electrode. 4. Conclusion In conclusion, a facile and scalable method have been developed with help of the self-assembly cube NaCl particles as template to fabricate a novel Sb/MoS2/C composite. Due to the synergistic influence of Sb/C nanosheets and MoS2, Sb/MoS2/C composite exhibited higher capacity, more stable cycling performance and excellent rate capability compare to than the Sb/C composite. Hence, this novel composite shows outstanding rate performance (763 mA h g1 at 0.2 A g1, 642.2 mA h g1 at 0.5 A g1, 544 mA h g1 at 1 A g1, 459 mA h g1 at 2.0 A g1 and 353 mA h g1 at 5.0 A g1) as well as extremely cycling stability (a reversible capacity of 679.5 mA h g1 is observed at 0.2 A g1 after 250 cycles). All of these strongly verified that the Sb/MoS2/C is a promising anode material for next generation high-performance LIBs. Acknowledgements This research was supported by National Science Foundation of China (U1401246, 21473042, 51364004, 51474110 and 51474077), Natural Science Foundation of Guangxi (2016GXNSFDA380023) and Scientific Research and Technology Development Plan of Guangxi (20150106-3 and 1598008-14). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.09.101. References [1] D.T. Ngo, H.T.T. Le, C. Kim, J.-Y. Lee, J.G. Fisher, I.-D. Kim, C.-J. Park, Energy Environ. Sci. 8 (2015) 3577e3588. [2] H.-Q. Wang, G.-H. Yang, L.-S. Cui, Z.-S. Li, Z.-X. Yan, X.-H. Zhang, Y.-G. Huang, Q.-Y. Li, J. Mater. Chem. A 3 (2015) 21298e21307.

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