Synthesis of hierarchical molybdenum disulfide microplates consisting of numerous crosslinked nanosheets for lithium-ion batteries

Journal of Alloys and Compounds 781 (2019) 174e185

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Synthesis of hierarchical molybdenum disulfide microplates consisting of numerous crosslinked nanosheets for lithium-ion batteries Jingjuan Zhao a, Haibo Ren a, Cuiping Gu a, ***, Wenmei Guan a, Xinjie Song b, **, Jiarui Huang a, * a

Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241002, PR China Department of Food Science and Technology, Yeungnam University, Gyeongsan, Gyeongbuk, 712749, South Korea

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 June 2018 Received in revised form 7 December 2018 Accepted 8 December 2018 Available online 10 December 2018

Hierarchical MoS2 microplates, consisting of numerous crosslinked nanosheets, are successfully synthesized using a self-sacrificial template method. MoO3 microplates (synthesized using a solution method) are used as the self-sacrificial template in the solvent thermal sulfidation process, resulting in the formation of MoS2 microplates. The resulting hierarchical structure possesses numerous active sites and a large surface area. As an anode material for LIBs, the hierarchical MoS2 microplates, both before and after the thermal treatment, demonstrate reversible and high capacity, and good rate capability. In addition, the energy storage performance of the MoS2 microplates is found to be significantly improved after thermal treatment. The annealed MoS2 microplates deliver a high specific capacity of 1220.8 mAh g
1 at a current density of 0.1 A g
1 and a high capacity retention of 84.8% (of the initial capacity) over 130 cycles. The high capacity and good rate capability suggest that the as-prepared hierarchical MoS2 microplates are a promising anode material for LIBs. © 2018 Elsevier B.V. All rights reserved.

Keywords: MoS2 microplates Hierarchical structure Self-sacrificial template Solvent thermal method Lithium-ion batteries

1. Introduction Recently, continuous and numerous efforts have been devoted to the development of energy storage devices with excellent performance, to reduce the gradual increase in environmental pollution and solve the global energy crisis [1]. Lithium-ion batteries (LIBs), with characteristics of environmental friendliness, safety, and high capacity have been applied in many fields, including portable electronics [2] and hybrid/electric vehicles [3,4]. Graphite, a traditional anode material, has been used in commercial LIBs in recent years because of its high conductivity [5], large surface area [6], and high thermal stability [7]. However, owing to its low theoretical specific capacity (372 mAh g
1) [8], it cannot be used to meet the energy storage requirements of modern society, which

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (C. Gu), [email protected] (X. Song), [email protected] (J. Huang). https://doi.org/10.1016/j.jallcom.2018.12.117 0925-8388/© 2018 Elsevier B.V. All rights reserved.

severely limits its large-scale practical application. Layered nanostructures of transition metal sulfides (TiS2, VS2, and SnS2) have drawn sustained interest because of their high specific capacities [9], safety [10], and cycling stability [11]. Consequently, they are regarded as effective anode materials for LIBs. Among the layered transition metal sulfides, molybdenum disulfide (MoS2) has attracted a significant amount of attention, and is expected to replace graphite as the anode material for LIBs because of its high theoretical specific capacity (670 mAh g
1). The favorable two-dimensional layer structure facilitates the insertion and extraction of Liþ, and MoS2 demonstrates a significantly higher theoretical specific capacity than graphite [12]. However, the two layers of MoS2 are linked by weak van der Waals forces, which can easily cause a significant volume change during the conversion reaction, and leading to low conductivity [13]. The low conductivity and severe volume change of MoS2 active materials can significantly decrease the specific capacity and cycling stability of LIBs. To overcome these drawbacks and improve the cycling stability of LIBs, various MoS2-based nanocomposites, i.e., MoS2/graphene [14e21], MoS2/carbon nanotubes [22,23], MoS2/carbon fiber paper

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[24], MoS2@C nanostructures [25e27], p-Ti3C2/MoS2 heterostructures [28], MoS2@TiO2 nanowires [29], MoS2/Co3O4 composites [30], and MoS2/TiN nanostructures [31], have been proposed as electrode materials. For example, Wang et al. prepared nitrogendoped 2D layered mesoporous MoS2/reduced graphene oxide hybrids via a nanocasting method, which exhibited a reversible capacity of 872.8 mAhg
1 after 50 cycles at a current density of 100 mAg
1 [16]. Chao et al. developed a MoS2/nitrogen-doped carbon nanocomposite via a simple hydrothermal process and subsequent annealing process, which exhibited ultrafast and stable charge/discharge capacities in LIB applications [25]. Another efficient approach toward high electrochemical properties is to construct nanostructured electrode materials that can provide short diffusion paths for lithium ions and a large electrode/ electrolyte contact area. Recently, MoS2 low-dimensional nanostructures with different morphologies, such as MoS2 nanobelts [32], small-sized MoS2 [33], MoS2 nanosheets [34], and MoS2 nanoribbons [35], have been developed to improve the cycling performance of LIBs. For instance, Tian et al. synthesized uniform small-sized MoS2 via a solution-based microwave-assisted method. As a LIB anode, it exhibited a remarkably large specific capacity of 1355 mA h g
1 at a low current density of 0.5 A g
1 [33]. The use of nanostructured MoS2-based electrodes can decrease the extent of significant volume changes and enhance the cycling stability of LIBs during Liþ insertion and extraction from MoS2 layers. To further mitigate the volume change, shorten the lithium-ion diffusion path, and improve the rate performance of anode materials, hierarchical MoS2 nanostructures, such as hierarchical MoS2 microspheres [36], hierarchical MoS2 nanotori [37], aligned MoS2 nanotubes [38], and 3D MoS2 nanomasks [39] have been used as anode materials for LIBs. They are considered excellent anode materials because the hierarchical structure not only possesses a large specific surface area but also better adapts to volume changes during lithium insertion/extraction, which promotes Liþ ion transport and shortens the Liþ-ion diffusion channel, leading to a significant improvement in the specific capacity and cycling stability of LIBs. To obtain appropriate MoS2 materials with hierarchical structures, typical synthetic methods have been normally employed, such as electrochemical deposition [40], anion exchange [41], (self-sacrificial) template methods [42e44], and hydrothermal methods [45]. Among these, the self-sacrificial template method is very effective for the synthesis of hierarchical nanostructures, because no steps are required to remove the template during the synthesis process. In this study, hierarchical MoS2 microplates were first synthesized using MoO3 microplates as a self-sacrificial template through a solvent thermal sulfidation process. To improve their crystallinity and electrochemical properties, the hierarchical MoS2 microplates were annealed in an H2/Ar atmosphere. The electrochemical properties of the hierarchical MoS2 microplates before and after thermal treatment were also studied. As an anode material for LIBs, the annealed hierarchical MoS2 microplates demonstrated a high specific capacity as well as excellent rate capability and cycling retention. 2. Experimental details All chemical reagents used in this study were of analytical grade, and they were used without further purification. 2.1. Synthesis of MoO3 precursor The MoO3 precursor was prepared as follows: first, 3.5 g of ammonium molybdate and 0.01 g of polyvinylpyrrolidone (PVP) were dissolved in 40 mL of deionized water under constant stirring to form a homogeneous solution. Then, 1 mL of acetic acid/HCl (1:1)

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acid mixture was added dropwise to the homogeneous solution under stirring for ~5 min. Thereafter, the resulting solution was heat-treated in a water bath at 90  C for 1 h to obtain a white precipitate. Finally, the white precipitate was washed several times with deionized water and absolute ethanol, and dried at 80  C for 12 h in a vacuum oven. The white MoO3 precursor was then collected. 2.2. Synthesis of hierarchical MoS2 microplates To synthesize the hierarchical MoS2 microplates, 0.13 g of thioacetamide (CH3CSNH2) and 0.2 g of urea (NH2CONH2) were dissolved in 15 mL ethanol under constant stirring to form a solution. The as-prepared white MoO3 powder (0.1 g) was then added to the solution under stirring for ~10 min, and the resulting solution was transferred into a 50 mL Teflon-lined autoclave and heated at 180  C for 24 h, to obtain a black precipitate. Thereafter, the black precipitate was washed several times with deionized water and absolute ethanol, and was vacuum-dried at 60  C for 12 h. The black hierarchical MoS2 microplates were then collected. To improve crystallinity, the MoS2 microplates were annealed at 700  C in H2/ Ar (5% H2 in volume) for 2 h. For the control experiment, the MoS2 products were prepared by a similar process, but thioacetamide was replaced by sodium sulfide (Na2S) or thiourea (NH2CSNH2). 2.3. Characterization The as-prepared products were characterized using fieldemission scanning electron microscopy (FE-SEM, Hitachi S-4800, operated at 5 kV), X-ray diffraction (XRD, Shimadzu XRD-6000, high-intensity Cu Ka radiation with a characteristic wavelength of 1.54178 Å), transmission electron microscopy (TEM, Hitachi H-800, operated at an acceleration voltage of 100 kV), high-resolution transmission electron microscopy (HRTEM; JEOL-2010, operated at an acceleration voltage of 200 kV; the microscope was equipped with an energy-dispersive X-ray spectrometer), and Raman spectroscopy (Brooke-Senterra, wavelength ¼ 532 nm). Nitrogen adsorption/desorption measurements were performed with a Nova 2000E instrument and the pore size distributions were determined using the BarretteJoynereHalenda (BJH) technique. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) analysis was performed to identify the valence state of the elements in the products. For thermogravimetric analysis (TGA, Setaram Labsys Evo SDT Q600), the samples were heated in air from room temperature to 600  C at a rate of 10  C min
1. Energy dispersive X-ray spectroscopy (EDS) was performed using a scanning electron microscope (FE-SEM, Hitachi S-4800, operated at 15 kV). The composition of the products was also investigated with an element analyzer (Vario EL Ⅲ, Elementar). 2.4. Electrochemical measurements Electrochemical measurements were conducted on the assynthesized hierarchical MoS2 microplates and the commercial MoS2 powder (the SEM image and XRD pattern are given in Figs. S1 and S2, respectively, of the Supporting Information) using a coincell system. The electrodes were prepared as follows: the hierarchical MoS2 microplates (75 wt%), sodium carboxymethyl cellulose binder (10 wt%), carbon black (10 wt%), and a few drops of styrene butadiene rubber (5 wt%) were mixed to form a highly uniform slurry. The slurry was then spread on a copper foil and vacuumdried at 80  C for 12 h. The mass loadings of the electrode and the active mass were approximately 3.65 and 2.74 mg cm
2, respectively. Cell assembly was performed inside a glove box filled with argon. The as-synthesized hierarchical MoS2 microplates, lithium

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foil, and 1 M LiPF6 in a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate (EC, DMC, and DEC, volume ratio ¼ 1:1:1) were used as the working electrode, counter/reference electrode, and electrolyte, respectively. Galvanostatic discharge/ charge cycling was performed using a Neware battery tester (Shenzhen Neware Technology Co., Ltd) to test the electrochemical performance of the products. An electrochemical workstation (CHI660E, Shanghai Chenhua Instruments Co., Ltd) was used to perform cyclic voltammetry (CV) in the voltage range 0.01e3.0 V at a rate of 0.5 mV s
1, and electrochemical impedance spectroscopy (EIS) in the frequency window 0.1e100 MHz. 3. Results and discussion 3.1. Structural and morphological characterization Fig. 1 presents a schematic of the preparation process of the hierarchical MoS2 microplates. During the hydrothermal reaction, the MoO3 microplate precursor is reduced to MoO2 seeds via ethanol reduction [42]. Moreover, at high temperature, thioacetamide hydrolyzed to produce H2S. Thereafter, at high temperature, S2
anions in the solution slowly replaced the O2
anions on the surface of MoO2 microplates to form MoS2 nanocrystals. As the sulfidation progressed, a large quantity of crosslinked MoS2 nanosheets began to grow vertically on the surface of the MoO2 microplates. Finally, the MoO2 template was completely consumed. Thus, hierarchical MoS2 microplates consisting of crosslinked nanosheets were obtained. The porous structure is attributed to the gaps among the numerous crosslinked MoS2 nanosheets. The crystalline phases of the products were investigated using XRD. Fig. 2a presents the XRD pattern of the MoO3 precursor. All diffraction peaks match those of orthorhombic MoO3 (JCPDS card no. 35-0609) [46], confirming that the precursor is crystalline MoO3. Fig. 2b presents the XRD patterns of the unannealed and annealed products. Only two weak and broad characteristic peaks at approximately 32.8 and 58.2 are observed in the pattern of the pre-heat-treatment sample; the peaks are attributed to the (100) and (110) diffraction planes of 2H-MoS2 (JCPDS card no. 37-1492), respectively. This suggests that before the thermal treatment, the product possessed a weak crystal structure [47]. After the thermal treatment, the XRD pattern of the final product revealed strong, intense diffraction peaks. As observed in Fig. 2b, the three characteristic peaks at approximately 13.9 , 39.6 , and 69.2 are assigned to the (002), (103), and (201) crystal planes of hexagonal MoS2, respectively. Thus, the XRD results confirm that the crystallinity of the MoS2 product is significantly improved after the thermal treatment. Fig. 3a presents the FE-SEM image of the MoO3 precursor. The precursor is composed of a significant amount of irregular nanosheets with lengths ranging from 200 to 500 nm. Fig. 3b and c presents the FE-SEM images of the final product before the thermal treatment. As seen in the low-magnification image (Fig. 3b), the microplate-like morphology of the MoO3 precursor is retained in the majority of the product. In addition, as observed in Fig. 3c, the

product consists of a large number of small crosslinked MoS2 nanosheets. Moreover, numerous pores exist in the hierarchical MoS2 microplates. Fig. 3d presents the TEM image of a single MoS2 microplate before the thermal treatment. It can be clearly seen that the microplate structure consists of numerous small crosslinked nanosheets, further confirming the porosity of the MoS2 microplates. In addition, an inter-layer spacing of 0.62 nm can be observed in the HRTEM image (Fig. 3d inset), which indicates a poorly-layered structure. Fig. 4a and b present SEM images of the as-synthesized hierarchical MoS2 microplates after the thermal treatment at 700  C in H2/Ar (5% H2 in volume) for 2 h; the structure and the morphology are unchanged. In addition, no significant change in shape is observed. However, the lattice fringes of the post-heat-treatment microplates appear clearer than those of the pre-heat-treatment microplates. As observed, the interlayer distance is ~0.66 nm, which is larger than that of the unannealed MoS2 microplates (0.62 nm). This further verifies the increased interlayer spacing [48]. Furthermore, an amorphous carbon layer is observed at the edge of the MoS2 nanosheets, which may be caused by carbonized polymer. To confirm the composition of the product, EDS elemental mapping was conducted (Fig. 4e and Fig. S3). As observed, the final product contains four elements: Mo, S, N, and C, which are distributed homogeneously and uniformly on the surface of the product. The elements Mo and S belong to MoS2 nanosheets. Nitrogen and carbon are probably derived from the carbonized polymer, which attached to the MoS2 nanosheets. During the hydrothermal reaction, ethanol is probably oxidized to acetaldehyde by Mo6þ. Subsequently, polymerization of the acetaldehyde with urea would occur on the microplates [49]. Later, the polymer attached to the microplates may have carbonized at high temperature (200  C), forming amorphous carbon. MoS2 products were also prepared by a similar process, in which thioacetamide was replaced by sodium sulfide or thiourea. Similar hierarchical MoS2 microplates were obtained when thiourea was used as the sulfidation agent (Fig. S4). However, only a few MoS2 nanosheets were obtained (Fig. S5). This result indicates that the sulfidation agent plays an important role in the formation of the hierarchical structure. The composition and electronic state of the elements present in the as-synthesized hierarchical MoS2 microplates after thermal treatment were analyzed using XPS (Fig. 5aec). Fig. 5a presents the X-ray photoelectron survey spectra in the binding energy range 0e1100 eV. As observed, the product contains five elements: Mo, S, N, C, and O. The detected Mo and S belong to MoS2 nanosheets. From the XPS analysis, the atomic ratio of Mo and S is calculated to be approximately 1:2, confirming the stoichiometric composition of MoS2. The detected nitrogen and carbon are probably derived from the carbonized polymer, attached to the MoS2 nanosheets. Moreover, the detected oxygen is possibly from the air to which the product was exposed. Fig. 5b presents the high-resolution Mo 3d core-level XPS profile. The three typical peaks observed at 232.8, 229.7, and 227.0 eV are assigned to Mo 3d3/2, Mo 3d5/2, and S 2s, respectively, confirming the existence of Mo4þ in MoS2. The S 2p

Fig. 1. Schematic illustration: preparation of hierarchical MoS2 microplates.

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Fig. 2. XRD patterns of (a) MoO3 precursor and (b) hierarchical MoS2 microplates before and after thermal treatment.

Fig. 3. SEM images of (a) MoO3 precursor, (b, c) unannealed hierarchical MoS2 microplates at low and high magnifications, and (d) TEM and HRTEM (inset) images of unannealed hierarchical MoS2 microplates.

spectrum of the MoS2 microplates is presented in Fig. 5c. The two characteristic peaks at 163.8 eV and 162.6 eV are assigned to S 2p1/2 and S 2p3/2, respectively, of S2
in the MoS2 material. The XPS analysis results thus confirm the formation of MoS2 [48,50]. In addition, the C 1s spectrum displayed in Fig. 5d exhibits two dominant peaks at 284.7 and 286.3 eV, which are assigned to sp3 CeC and the sp3 CeN, respectively [51]. The carbon content in the products plays an important role in determining their Li-storage performance; thus, the carbon content was calculated using TGA, performed between room temperature and 600  C in air. As shown in Fig. 6, the first weight-loss step observed in the region from room temperature to 200  C is due to the removal of adsorbed water. The second weight-loss step in the region 200e355  C is mainly attributed to the combustion of carbon; the weight loss is determined to be 14.1% and 4.3% for prethermal-treatment and post-thermal-treatment hierarchical MoS2 microplates, respectively. The third weight-loss step in the region 355e600  C is mainly due to the oxidation of MoS2 to form molybdenum trioxide [52]; the weight loss is determined to be 8.2% and 13.6% for the unannealed and annealed hierarchical MoS2 microplates, respectively. Furthermore, the carbon content in the

products was also determined by elemental analysis. Relative contents of carbon in the microplates before and after the thermal treatment are 13.42% and 5.63%, respectively, which are consistent with the EDS results. As shown, the carbon content decreases after the thermal treatment. Raman spectroscopy was performed to further investigate the structure and composition of the products. Fig. 7 presents the Raman spectra of hierarchical MoS2 microplates before and after the thermal treatment. The two typical peaks at approximately 376.9 cm
1 and 402.9 cm
1 are assigned to the E1 2g and A1g modes, respectively, in the MoS2 crystal. E1 2g and A1g are attributed to the in-plane displacements of Mo and S atoms, and the outof-plane symmetric displacements of S atoms along the c-axis [53], respectively. Thus, the Raman spectroscopy analysis results also confirm the formation of crystalline MoS2. In addition, the intensities of both the peaks for the post-thermal-treatment sample are higher than those for the pre-thermal-treatment sample, indicating that the crystallinity of hierarchical MoS2 microplates is significantly enhanced by the thermal treatment. The results are in good agreement with the XRD results and the HRTEM images. Moreover, the annealed as well unannealed microplates exhibit two peaks at approximately 1361 cm
1 and 1588 cm
1, which are characteristic of the breathing mode of aromatic rings (D band) and bond stretching of the sp2 carbon (G band), respectively. Fig. 7 indicates that the intensities of the D and G bands, especially the G band, increase after annealing in H2, suggesting an improvement in carbon crystallization during the heat treatment [54]. Overall, the results of XRD and Raman spectroscopy analyses suggest that annealing in a reducing atmosphere can cause an improvement in the crystallinity of inorganic carbon and compounds such as MoS2. To compare the internal structure and textural properties of the hierarchical MoS2 microplates before and after the thermal treatment, nitrogen adsorption/desorption measurements were carried out. Fig. 8 presents the nitrogen adsorption/desorption isotherms and pore-size distributions (inset) of the samples. The isotherms of the hierarchical MoS2 microplates form a fine hysteresis loop in the P/P0 range 0.44e0.98. This is attributed to the filling and emptying of the mesopores through capillary condensation, and it clearly suggests the existence of a large number of pores in the microplates. The pore sizes of the samples before and after the thermal treatment are determined to be in the ranges 3.3e96.4 nm and 3.3e101.8 nm, respectively, thus verifying that the products are indeed mesoporous, and that the average pore size increased by the thermal treatment. The specific surface areas of the hierarchical MoS2 microplates before and after the thermal treatment are calculated to be 45.1 m2 g
1 and 38.3 m2 g
1, respectively, which

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Fig. 4. (a, b) SEM images, (c) TEM image, (d) HRTEM image, and (e) TEM image and the associated elemental map of the calcined hierarchical MoS2 microplates.

are much higher than that of commercial MoS2 (9.2 m2 g
1). Because of their porous structure and large specific surface area, the hierarchical MoS2 microplates are thought to provide numerous Liþ-ion transport channels, facilitating the diffusion of Liþ ions; consequently, the transport time decreases and the transport efficiency of the Liþ ions improves. Thus, the hierarchical MoS2 microplates are considered excellent for LIB applications. Thermal treatment can cause a significant increase in the conductivity of the hierarchical MoS2 microplates. Therefore, we measured the electrical conductivity of the microplates before and after the thermal treatment. Before the thermal treatment, the electrical conductivity of the microplates was ca. 5.5  10
6 S m
1. After the thermal treatment, the conductivity increased, reaching up to 3.0  10
4 S m
1, which is ascribed to the high crystallinity of MoS2 and carbon. We believe that the conductivity increase facilitates fast movement of electrons through the electrode, enhancing the rate performance.

3.2. Electrochemical properties for lithium-ion storage The electrochemical properties of the hierarchical MoS2 microplates before and after thermal treatment, as an anode material for LIBs, were investigated by employing coin half cells. First, cyclic voltammetry (CV) tests were performed in the potential window 0.01e3.0 V at a scan rate of 0.5 mV s
1. Fig. 9 presents the

CV and discharge/charge curves of the samples. The first three CV curves of the unannealed and annealed microplates are shown in Fig. 9a and b, respectively. The first-cycle curve of the unannealed sample exhibits two reduction peaks at 1.29 V and 0.34 V (Fig. 9a) and that of the annealed sample exhibits reduction peaks at 0.99 V and 0.52 V (Fig. 9b). The reduction peaks at 1.29 V and 0.99 V are attributed to the formation of LixMoS2 (Equation (1)), caused by the intercalation of Liþ between two layers of MoS2 nanosheets and its absorption at the surface of the hierarchical MoS2 microplates. Subsequently, the MoS2 internal structure changes from trigonal prismatic to octahedral [55e57]. The absorption and intercalation of Liþ proceed more easily on the surface of amorphous MoS2 than on the surface of highly crystalline MoS2 owing to the presence of more kink sites [58,59]. Therefore, compared to the annealed sample, the unannealed sample exhibits a higher reduction peak potential (1.29 V). Moreover, the other two reduction peaks at 0.34 (Fig. 9a) and 0.52 V (Fig. 9b) are attributed to the conversion of lithium intercalates (LixMoS2, x < 1) to metallic Mo particles embedded in a Li2S matrix (Equation (2)), resulting in the formation of a solid electrolyte interface (SEI) [60]. In the highly crystalline MoS2 sample (annealed), the resulting intercalated phase of MoS2 is highly distorted, and therefore, it undergoes appreciable lattice expansion. Subsequently, intercalation-induced lattice strain in the basal plane causes the formation of dislocations. The dislocations fragment the lattice, possibly creating diffusion pathways for Liþ,

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Fig. 5. XPS spectra of annealed hierarchical MoS2 microplates: (a) broad-scan spectrum; (b) Mo 3d, (c) S 2p, and (d) C 1s spectra.

Fig. 6. TGA curves of hierarchical MoS2 microplates (a) before and (b) after thermal treatment in air from 25 to 600  C at a heating rate of 10  C min
1.

which allow them to easily penetrate further into the host material [58]. Furthermore, this fragmentation may lead in part to the disproportionation reaction associated with the formation of lithium sulfide (Li2S) and molybdenum metal particles [58]. Therefore, the annealed hierarchical MoS2 microplates exhibit a high reduction peak potential (0.52 V). In the anodic scan, dominant oxidation peaks are clearly observed at 2.38 V and 1.62 V (Fig. 9a) for the unannealed sample, and at 2.29 V and 1.70 V for the annealed sample (Fig. 9b). The oxidation peaks at 2.38 V and 2.29 V are attributed to the delithiation of Li2S (Equation (3)) [61,62], and those at 1.62 V and 1.70 V are attributed to the partial oxidation of Mo to form MoS2 [54]. In the following cycles, the intensity of the second reduction

peak (0.34 V) for the unannealed sample decreases, which is attributed to the formation of the SEI. Moreover, the third pair of reduction peaks at 1.78 (Fig. 9a) and 1.91 V (Fig. 9b) for the unannealed and annealed samples, respectively, correspond to the multi-step lithiation process [63,64]. Thus, the results confirm high cycling stability and reversibility, indicated by the well-overlapped CV curves, for the hierarchical MoS2 microplates. The relevant equations are:

  MoS2 þ xLiþ þ xe
/Lix MoS2 x < 1;  1:1VVS: Li=Liþ

(1)

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Fig. 7. Raman spectra of hierarchical MoS2 microplates (a) before (red) and (b) after (black) thermal treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

  Lix MoS2 þ ð4
xÞLiþ þ ð4
xÞe
/2Li2 S þ Mo  0:6VVS: Li=Liþ (2)   Li2 S/S þ 2Li  2:3VVS: Li=Liþ

(3)

Fig. 9c and d presents the first three discharge/charge curves (current density ¼ 100 mA g
1 and voltage range ¼ 0.01e3.0 V) of the unannealed and annealed hierarchical MoS2 microplates, respectively. The first discharge curves exhibit two discharge plateaus at 1.38 V and 0.78 V (Fig. 9c) for the unannealed sample and at 1.1 V and 0.78 V (Fig. 9d) for the annealed sample. The discharge plateaus are attributed to the intercalation of Liþ into MoS2 interlayers to form LixMoS2, the reduction of MoS2 to Mo metal particles and Li2S, and the formation of the SEI. The unannealed hierarchical MoS2 microplates exhibit a high discharge plateau potential (1.38 V) because the absorption and intercalation of Liþ can easily proceed on the surface of amorphous MoS2, as mentioned above. The results are consistent with those obtained from the CV curve analysis corresponding to the first cycle. In the

first discharge/charge cycle, the irreversible specific capacities of the hierarchical MoS2 microplates before and after thermal treatment are 1262.1 and 1438.5 mAh g
1, respectively, and the corresponding Coulombic efficiencies are 80.70% and 84.77%. During the cycle, the formation of the SEI and the collapse of the structure are the main causes for the specific capacity loss [65]. Compared to the unannealed sample, the annealed sample exhibits higher specific capacity and Coulombic efficiency; this can be attributed to the high crystallinity and conductivity, and to the low carbon content in the post-heat-treatment product, which were verified by the elemental analysis and TGA results. Because the theoretical capacity of MoS2 is higher than that of carbon, the theoretical capacity of the annealed hierarchical MoS2 microplate sample, with a lower amount of carbon, is higher than that of the unannealed sample. Furthermore, after the thermal treatment (which facilitates the transport of electrons), the hierarchical MoS2 microplates, with low carbon content, are found to possess enhanced crystallinity and high conductivity. After the first cycle, the Coulombic efficiencies of the unannealed and annealed microplates achieve ~98% of the original values, and they are maintained in the subsequent cycles, revealing the outstanding electrochemical stability and reversibility of the hierarchical MoS2 microplate electrodes. Furthermore, the reversible capacity of the synthesized samples is much higher than the theoretical capacity of MoS2. This is primarily attributed to the significant merits, including the short Liþ diffusion path, large electrode/electrolyte contact area, physical adsorption sites for the Li ions, and strain accommodation [58], provided by the hierarchical nanostructure of the materials. Fig. 9e presents the rate capabilities of the commercial MoS2 and the hierarchical MoS2 microplates before and after thermal treatment. The reversible discharge capacities of hierarchical MoS2 microplates after thermal treatment are 1315, 1219, 859, and 717 mAh g
1 at current densities of 100, 200, 500, 1000, and 2000 mA g
1, respectively. As these values indicate, the annealed hierarchical MoS2 microplates deliver a higher capacity than the commercial MoS2 and unannealed hierarchical MoS2 microplates at each current density. Notably, the specific capacity of the hierarchical MoS2 microplate sample at a high current density of 2000 mA g
1 greatly improves after the thermal treatment. The original morphology and porous structure of the hierarchical MoS2 microplates are retained even after the thermal treatment. In addition, compared to the unannealed sample, the annealed sample (with low carbon content) possesses higher crystallinity and conductivity, which facilitate the transport of electrons and Liþ ions. Therefore, the annealed microplates exhibit higher rate performance.

Fig. 8. Nitrogen adsorption and desorption isotherms of hierarchical MoS2 microplates (a) before and (b) after thermal treatment, and the corresponding pore-size distribution data (insets) calculated using the BJH method from the desorption isotherm.

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Fig. 9. Cyclic voltammetry curves of hierarchical MoS2 microplates (a) before and (b) after thermal treatment corresponding to the first three cycles at a scan rate of 0.5 mV s
1; discharge and charge curves of the hierarchical MoS2 microplates (c) before and (d) after thermal treatment at a current density of 100 mA g
1; (e) rate capability of the commercial MoS2 and hierarchical MoS2 microplates before and after thermal treatment; and (f) cycling performance of the commercial MoS2 and hierarchical MoS2 microplates before and after thermal treatment at a current density of 100 mA g
1.

Fig. 9f presents the cycling performances (current density ¼ 100 mA g
1) of commercial MoS2 and the hierarchical MoS2 microplates before and after thermal treatment. As seen, the capacity of both the annealed MoS2 microplate and the commercial MoS2 samples gradually decreases, while that of the unannealed MoS2 microplate sample continuously increases, which can be ascribed to the gradual activation of the electrode material with cycling. However, no activation occurs in hierarchical MoS2 microplates after heat treatment, which is perhaps due to the high crystallinity of the MoS2 nanosheets with large interlayer spacing. As we know, MoS2 is a representative 2D-layered material, in which Mo and S atoms are covalently bonded and the adjacent layers are connected via van der Waals interactions. Because the van der

Waals force is weak, the interlayer space between the SdMdS layers increases; consequently, rapid diffusion of Li ions occurs and they are accepted into the internal structure of the material [58,66]. Therefore, the annealed MoS2 microplates, because of the high crystallinity and large interlayer spacing, exhibit high specific capacities even without the activation process during the discharge/ charge cycles. The high cycling stability of the unannealed MoS2 microplate sample is due to the high content of amorphous carbon, which is beneficial for maintaining the structure of MoS2 microplates and preventing the nanosized particles from agglomerating. As shown, both the unannealed and annealed hierarchical MoS2 microplates exhibit high specific capacities even after 130 cycles; the values are much higher than for commercial MoS2 powder

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(99.5 mAh g
1). After 130 cycles, the annealed hierarchical MoS2 microplates deliver a high specific capacity of 1220.8 mAh g
1 and high capacity retention of 84.8%. As seen in Fig. 9f, the capacity of the annealed hierarchical MoS2 microplate sample abruptly drops at the 42nd cycle, but then stabilizes. This drop is perhaps due to the gradual destruction of the MoS2 nanosheet structure and a certain degree of agglomeration. Furthermore, after the thermal treatment, the specific capacity of the hierarchical MoS2 microplates increases by 11.8%. Moreover, compared with certain nanoscale MoS2 as well as MoS2 composites recently reported [42,67e71], the annealed hierarchical MoS2 microplates demonstrate superior electrochemical properties as electrodes for LIBs (see Table S1). This is mainly owing to the fluffier/fractured state and increased pore density of the product after thermal treatment, which could provide significantly more active sites for the storage of Liþ and shorten the transport pathways of Liþ. Moreover, after the thermal treatment at 700  C for 2 h, the hierarchical MoS2 microplates exhibit a higher conductivity, thus greatly promoting the diffusion of electrons. This unique hierarchical structure can offer a stable environment and buffer the volume change during the lithiation/de-lithiation processes [71]. For the unannealed hierarchical MoS2 microplates, the reversible capacities and trends determined at 0.1 A g
1 for the initial cycles from the rate capability (Fig. 9e) and cycling performance (Fig. 9f) measurements are significantly different. This is because the activation ability of the unannealed sample is not uniform, and thus, the electrochemical performance is not stable. Achieving long-term cycling stability at high rates (>1 A g
1) for a MoS2-based anode is a significant challenge because of the effects of its low electrical conductivity. The galvanostatic discharge/ charge cycles of the samples were measured at 1 and 2 A g
1. The annealed hierarchical MoS2 microplates maintain high reversible discharge capacities during the first 50 cycles (Fig. S6). Subsequently, the reversible discharge capacities drop to 413.2 mAh g
1 and 406.1 mAh g
1 at 1 A g
1 and 2 A g
1, respectively, and then gradually decrease to lower values. For instance, after 200 cycles, the reversible discharge capacity values were 153.9 mAh g
1 and 130.2 mAh g
1 at 1 A g
1 and 2 A g
1, respectively. Thus, the longterm cycling stability of the hierarchical MoS2 microplates is poor. Because the surface of the electrode active-material forms a high electric field gradient and concentration gradient, the surface structure of the active material can be easily damaged under the high current density [58]. Therefore, the structure of the microplates may be destroyed during lithiation/delithiation, and consequently, they exhibit low long-term cycling stability at high rates. However, as recently reported, MoS2@C hollow nanospheres deliver 926.3 mAh g
1 at a current density of 1 A g
1 over 500 cycles [27] and nanostructured MoS2-TiN delivers 600 mAh g
1 at a current density of 2.0 A g
1 over 100 cycles [31]. The high cycling stabilities exhibited by these MoS2-based nanocomposites at high current densities are ascribed to carbon layers and TiN, which improve the electrical conductivity of MoS2 and prevent the aggregation and restacking of MoS2 layers. Therefore, we believe that the long-term cycling stability of the as-prepared hierarchical MoS2 microplate sample can be improved by optimizing the microstructure of the nanocomposite. Fig. 10 presents the electrochemical impedance spectra of the commercial MoS2 powder and the unannealed and annealed hierarchical MoS2 microplates as active materials before and after cycling. The Nyquist plots with an equivalent electrical circuit model (inset) are presented in Fig. 10b. R1 represents the internal resistance of the test battery; R2 and CPE1 represent the resistance and constant phase element of the SEI, respectively; Rct and CPE2 represent the charge-transfer resistance and constant phase element of the electrode/electrolyte interface, respectively; and Wo

Fig. 10. (a) Nyquist plots of hierarchical MoS2 microplate electrodes and the commercial MoS2 powder before cycling, and (b) Nyquist plots of unannealed and annealed hierarchical MoS2 microplates after 130 cycles. The inset is the equivalent electrical circuit model.

represents the Warburg impedance corresponding to the lithiumdiffusion process [42]. The fitting results reveal that the chargetransfer resistance (Rct) of the annealed hierarchical MoS2 microplates (48.3 U) before cycling is the lowest in comparison with the commercial MoS2 powder (79.4 U) and the unannealed hierarchical MoS2 microplates (250.3 U). The high Rct for the unannealed hierarchical MoS2 microplates (with high carbon content) is ascribed to the low conductivity of amorphous carbon and MoS2. Furthermore, in the low-frequency region, the straight line of the plot represents typical Warburg behavior, which is related to the diffusion of lithium ions in the hierarchical MoS2 microplate elecþ trode. The diffusion co-efficient (Dþ Li) of Li -ions in LIBs can be calculated using EIS data, according to the following formula: 2 2 2 Dþ Li ¼ 0.5(RT/n F AsC) , where R is the gas constant, T is the absolute temperature, A is the electrode surface area, n is the number of electrons transferred per mole of the material involved in the electrode reaction, C is the concentration of lithium in the electrode, F is the Faraday constant, and s is the Warburg coefficient. The Liþ-ion diffusion coefficients of the hierarchical MoS2 microplates before and after thermal treatment were calculated to be 4.83  10
12 and 8.53  10
8 cm2 s
1, respectively. The high Liþ-ion diffusion coefficient of the annealed hierarchical MoS2 microplates is due to the high crystallinity and large interlayer spacing. Fig. 10b presents the Nyquist plots (130 cycles) of the unannealed and annealed hierarchical MoS2 microplates. It can be seen that the Rct

J. Zhao et al. / Journal of Alloys and Compounds 781 (2019) 174e185

of both samples increases after 130 cycles; however, as observed, the Rct of the annealed microplates is 142.5 U, which is lower than that of the unannealed microplates (357.2 U). These findings reveal an enhanced charge transfer characteristic in the annealed microplates. In addition, the Liþ-ion diffusion coefficient of the unannealed microplates increases to 4.69  10
10 cm2 s
1 from 4.83  10
12 cm2 s
1, while that of annealed microplates decreases to 3.33  10
10 cm2 s
1 from 8.53  10
8 cm2 s
1 after 130 cycles. As is known, the MoS2 nanosheets of both samples will transform into miniscule nanosized particles after the cycling test. As for the annealed microplates, the high crystallinity and large interlayer spacing of the MoS2 nanosheets will be destroyed during the cycling process, resulting in the decrease of the Liþ-ion diffusion coefficient. However, the unannealed microplates consist of amorphous MoS2 nanosheets, which will transform into miniscule nanosized particles after the cycling test, resulting in a large electrode/electrolyte contact area. Therefore, the Liþ-ion diffusion coefficient of the unannealed hierarchical MoS2 microplates will increase after the cycling test. As observed, the Liþ-ion diffusion coefficient of the unannealed sample is higher than that of the annealed sample after the cycling test. This is possibly due to the high amorphous carbon content in the unannealed MoS2 microplates, which facilitates Li-ion transport between the electrolyte and the electrode [72]. In addition, the high amorphous carbon content leads to high Liþ-ion accessibility and effectively prevents the nanosized particles from agglomerating [72e74]. Although the Liþ-ion diffusion coefficient of the unannealed sample is higher than that of the annealed sample after the cycling test, its capacity remains lower than the annealed sample. This is mainly attributed to the following reasons: First, the annealed hierarchical MoS2 microplate sample contains a high content of MoS2, which possesses higher theoretical capacity than carbon. Second, the annealed microplates exhibit high conductivity, thus greatly promoting the diffusion of electrons. Third, the annealed MoS2 microplates with high crystallinity and large interlayer spacing exhibit high specific capacities even without the activation process during the cycling test. To better understand the charge/discharge process in LIBs, the annealed hierarchical MoS2 microplates after 130 cycles were characterized using SEM, TEM, EDS, XRD, and XPS. Fig. S7a and S7b present the SEM and TEM images of the sample after 130 cycles. The original microplate-like morphology is found to be retained. However, some nanosheets are transformed into nanoparticles; the transformation is possibly caused by the volume change of MoS2 during lithiation/delithiation. A schematic illustration of the structure changes during lithiation/delithiation is shown in Fig. S8. The hierarchical porous structure of the microplates accommodates volume expansion, which can effectively preserve the structural integrity and prevent nanoparticles from detaching from the electrode. The EDS analysis presented in Fig. S7c indicates that the annealed hierarchical MoS2 microplates, after 130 cycles, contains five elements: Mo, S, C, N, and O. The Mo and S are likely derived from the active material of the hierarchical MoS2 microplates, and the carbon is mainly attributed to the carbon black, sodium carboxymethyl cellulose binder (CMC), and to the carbonized polymer attached to the MoS2 nanosheets. Moreover, the nitrogen is attributed to the carbonized polymer. Furthermore, the oxygen is attributed to oxygen species deposited after oxidation of the sample during post-treatment. Fig. S9a and S9b present the XRD patterns of the annealed hierarchical MoS2 microplates after 130 cycles. The XRD patterns further confirm the existence of MoS2 and carbon within the electrode materials after cycling. In addition, the XPS survey spectrum (Fig. S10a) of the electrode materials after the cycling test confirms the predominant presence of Mo and S. The C 1s and N 1s peaks are also observed in the survey spectrum. The

183

peak of O 1s can be attributed to the oxidation of the sample during post-treatment. As seen in Fig. S10b, the Mo 3d XPS peak is asymmetric. It can be Gaussian-divided into three symmetrical peaks at 235.5, 232.5, and 229.5 eV. The peak at 235.5 eV is assigned to Mo 3d3/2 of Mo6þ, and the other two peaks are assigned to Mo 3d3/2 and Mo 3d5/2 of Mo4þ. Furthermore, the presence of Mo6þ is attributed to the oxidation of the sample, consistent with the XRD and EDS results. The rough S 2p spectrum (Fig. S10c) shows the S2
S 2p1/2 and S 2p3/2 peaks at 163.6 eV and 162.2 eV, respectively, and the S6þ S 2p1/2 and S 2p3/2 peaks at 170.3 eV and 169.3 eV, respectively. The presence of S6þ is attributed to the oxidation of the sample. Moreover, the presence of S2
indicates the existence of MoS2 in the final electrode material, which is again consistent with the XRD analysis. The C 1s spectrum displayed in Fig. S10d exhibits two dominant peaks at 284.8 eV and 286.5 eV, which are assigned to sp3 CeC and the sp3 CeN. The peak at 289.8 eV is perhaps attributed to sp2 C from sodium carboxymethyl cellulose and styrene butadiene rubber. The N 1s peak at 398.5 eV (Fig. S10e) is assigned to the CeN bond, which confirms the existence of N in the final electrode material. 4. Conclusions Hierarchical MoS2 microplates consisting of numerous crosslinked nanosheets were synthesized using a facile self-sacrificial template method. The MoO3 microplate precursor served as the self-sacrificial template in the solvent thermal sulfidation process. As an anode material for LIBs, the energy storage performance of the hierarchical MoS2 microplates was studied, and the results demonstrated a significant performance improvement for the sample annealed in an H2/Ar atmosphere. The annealed hierarchical MoS2 microplates delivered a high specific capacity of 1220.8 mAh g
1 at a current density of 0.1 A g
1 and a high capacity retention of 84.8% (of the initial capacity) over 130 cycles. The results indicate that the unique hierarchical structure provides active sites for the storage of lithium ions, facilitates fast transport of lithium ions and electrons, and buffers the aggregation and collapse of MoS2 nanosheets during lithiation/delithiation processes, which led to the high specific capacity and outstanding rate capability. Thus, we believe that the excellent electrochemical performance makes the hierarchical MoS2 microplates a promising anode material for LIBs. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21471005 and 21105001), Science and Technology Major Project of Anhui Province (Grant No. 18030901093), and the Major Project of the Department of Education of Anhui Province (Grant No. KJ2018ZD034). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2018.12.117. References [1] B. Huang, Z.F. Pan, X.Y. Su, L. An, Recycling of lithium-ion batteries: recent advances and perspectives, J. Power Sources 399 (2018) 274e286. [2] P.P. Zhang, F.X. Wang, M.H. Yu, X.D. Zhuang, X.L. Feng, Two-dimensional materials for miniaturized energy storage devices: from individual devices to smart integrated systems, Chem. Soc. Rev. 47 (2018) 7426e7451. [3] M.G. Jeong, H.L. Du, M. Islam, J.K. Lee, Y.K. Sun, H.G. Jung, Self-rearrangement of silicon nanoparticles embedded in micron carbon sphere framework for high-energy and long-life lithium-ion batteries, Nano Lett. 17 (2017) 5600e5606.

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