Facile synthesis and electrochemical properties of MoS₂ nanostructures with different lithium storage properties

Abstract

MoS₂ nanomaterials with different morphologies such as nanoplates, nanowalls, and 3D microspheres composed of ultrathin nanoflakes were synthesized via a simple solid-phase reaction process. The structure and morphology of these samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller analysis (BET). The electrochemical test behavior of the as-prepared MoS₂ nanostructure electrodes were also investigated and the results indicated that the 3D MoS₂ microsphere electrode exhibits a high discharge capacity of 850.9 mA h g⁻¹ at 100 mA g⁻¹ after 50 cycles, which displays higher specific capacity and cycling stability than other as-prepared samples. Moreover, the reversible capacity for the 3D MoS₂ microspheres can still be maintained at 783.5 mA g⁻¹ at 800 mA g⁻¹. The enhanced electrochemical performance of the 3D MoS₂ microspheres could be attributed to their spherical structure, the ultrathin nanoflakes, high specific surface area and their unique layered structure.

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1. Introduction

It is well known that Li-ion batteries (LIBs) are the main power sources for portable electronic devices, which can meet the ever-increasing energy demands and environmental concerns. However, their energy and power densities, cycle life and rate capability have to be improved to meet the more demanding specifications of power-intensive applications.¹ In addition, the performance of LIBs is largely dependent on the electrode materials such as graphite with a limited theoretical capacity of 372 mA h g⁻¹, so could not meet the increasing needs for lighter and thinner Li batteries.² Transition metal dichalcogenides MS₂ (M: Mo, W, Nb) have shown promise in that lithium insertion/extraction is possible during the charging–discharging process. They have unique structure and superior properties. The sandwich interlayer structure formed by the stacking of the S–M–S layers, which are loosely bound to each other only by van der Waals forces and are easily cleaved.³ Moreover, their electronic structure is such that band-edge excitation corresponds largely to a metal centred d–d transition. Owing to these features, laminar MS₂ materials have numerous applications such as solid lubricants, high-density batteries and efficient solar energy cells.^4–7

Molybdenum disulfide (MoS₂) is one of the transitionmetal dichalcogenide layered compounds with an analogous structure to graphite, have been paid close attention as anode materials for LIBs because they own high theoretical capacities, unusual electronic and physical properties.^8–13 To date, numerous synthetic methods have been developed to prepare novel MoS₂ nanostructures. For instance, gas-phase reactions, laser ablation, sonochemical process, hydrothermal synthesis and thermal decomposition.^14–18 Therefore, much effort has been devoted to the synthesis of various nanoscale MoS₂ with specific morphologies and unique properties. A large number of MoS₂ nanoparticles with different morphologies such as nanowires,¹⁹ nanotubes,^20,21 nanosheets,^22,23 nanorods,²⁴ and nanoflowers^25,26 have been prepared. The morphology could have an important impact on the surface area, active site and ion kinetics of materials, which could effectively influence its electrochemical performance.^27–29 However, these synthetic methods for obtaining MoS₂ are quite complex. For instance, 3D-flower like MoS₂ was prepared from a hydrothermal synthesis, assisted with an ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), via two-step procedures (hydrothermal treatment and annealing at 240 and 800 °C, respectively).³⁰

In order to use a MoS₂ electrode as a possible candidate in a lithium battery, a synthetic method should be simple for an easy scale-up. In addition, a high rate performance needs to be satisfied up to 10 C (>10 A g⁻¹). In this work, we report a simple, one-pot approach to prepare MoS₂ nanostructures with different morphologies and sizes via a facile solid-state reaction by controlling reaction temperature. MoS₂ nanostructures with nanoplates, nanowalls, and microspheres morphologies were successfully synthesized, and the electrochemical properties of three kinds of as-prepared MoS₂ samples were investigated as anode materials for LIBs. Among these samples, the 3D MoS₂ microspheres with high specific surface area and stable structure show the best cycling stability and rate performance.

2. Experimental

2.1. Synthesis and characterization of MoS₂ nanostructures

All chemical reagents (analytical purity) were purchased from SCRC Chemical Co. and used directly without further purification. The experiment was designed by three different preparation nanostructures of MoS₂ as the following; Sample A (nanoplates): 1 g of MoO₃ and 10.55 g of thiocarbamide, Sample B (nanowalls): 1 g of MoO₃ and 4.44 g of sulfur powder, Sample C (microspheres): 1 g of molybdenum powder and 6.67 g of sulfur powder. The three groups of mixture powders were subjected to planet ball-milling at 400 rpm (rotation per minute) in the presence of ethanol for 12 h in a planetary ball mill respectively. The ball-milled mixture was pressed in 15 mm-diameter discs and cold isostatically pressed under a 250 Mpa pressure. Then it was transferred into a temperature-controlled tube furnace, and heated to 800 °C at a rate of 10 °C min⁻¹ in a argon atmosphere for 2 h. Subsequently the reactor gradually cooled to room temperature, opened, and the black powders was obtained. The product was directly characterized without further processing by various analytic techniques.

The X-ray diffraction (XRD) patterns were recorded using a D8 advance (Bruker-AXS) diffractometer with Cu Kα radiation (λ = 0.1546 nm). The 2θ range used in the measurement was from 10 to 80° with a velocity of 5° min⁻¹. The morphologies and structures of the samples were characterized by scanning electron microscopy (SEM, JEOL JXA-840A) and transmission electron microscopy (TEM) with a Japan JEM-100CX II transmission electron microscope. The chemical composition of the MoS₂ was obtained by X-ray photoelectron spectroscopy (XPS, Theta Probe AR-XPS System). Nitrogen (N₂) adsorption–desorption measurements were performed by using Quantachrome instrument (Quabrasorb SI-3MP). Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used to calculate the surface areas and pore sizes of the samples.

2.2. Electrochemical measurements

The electrochemical characterizations were performed using coin cells. The working electrode was fabricated in this study by following process: the slurry consisting of 75 wt% active material, 15 wt% acetylene black and 10 wt% polyvinylidene fluoride was coated on a copper foil (15 mm in diameter, 20 μm in thickness, active material loading of about 1.2 mg), then the coated electrode was dried at 90 °C in a vacuum oven for 12 h and compressed. The two-electrode test cells were assembled in an argon-filled glove box using a lithium sheet as the counter electrode, a polypropylene film (Celgard-2300) separator, and an electrolyte of 1.0 M LiPF₆ solution in a 1 : 1 v/v mixture of ethylene carbonate(EC)/dimethyl carbonate(DMC). The charge and discharge measurements were carried out on an Arbin BT2000 system with the potential window of 0.01–3.0 V at galvanostatic density of 100 mA g⁻¹.

3. Results and discussion

3.1. Structure and morphology characterization

The crystalline structure and phase purity of three MoS₂ nanostructures were confirmed by XRD. As shown in Fig. 1a, All labelled diffraction peaks can be indexed to those of the pure hexagonal phase of MoS₂ with lattice constants a = 3.161, c = 12.84 Å, which are in good agreement with the values of standard card (JCPDS no. 37-1492). No characteristic peaks were detected from other impurities, indicating that the sample was of high purity. Moreover, the XRD patterns reveal wide and weak diffraction peaks, which is evidence of the formation of nanoparticles. Energy-dispersive X-ray Spectrometer (EDS) results were shown in Fig. 1b–d, which reveals that the samples consist of element Mo and S, no other element was observed.

Fig. 1 (a) XRD patterns and (b–d) EDS of the MoS₂ nanoplates (Sample A), nanowalls (Sample B), and microspheres (Sample C).

The size and morphologies of all MoS₂ nanostructures were primarily investigated by SEM and TEM measurement. Fig. 2a shows that the MoS₂ are irregular plate-like structures. Its well-multilayer structure can be clearly observed in Fig. 2b. Fig. 2c indicates that the MoS₂ nanowalls are composed of MoS₂ nanosheets and shows that the size of the nanosheets is about 50–100 nm in size. Fig. 2d shows a typical TEM image of MoS₂ nanowalls, which further confirms the as-prepared MoS₂ nanowalls consist of many MoS₂ nanosheets. More details for MoS₂ structure are illustrated by inserted HRTEM images in Fig. 2d, which indicates that the nanosheets consist of about 8–9 layers structures. As a mean value, the distance between the lattice fringes is 0.63 nm, which is coincidental with the theoretical spacing for (002) planes of the hexagonal MoS₂ structure. Fig. 2e clearly shows that the 3D MoS₂ microspheres are in diameters of 1 μm and composed of the sheet-like subunits. Further, the highly wrinkled surface and extruded lamella-like structure of the spherical aggregates could be obviously observed. The MoS₂ nanosheets could form nanoflakes and then nanoflakes aggregate to form the loose sphere-like architectures. As shown in Fig. 2f, few-layer MoS₂ sheets (5–7 layers) with d(002) of 0.63 nm bond together tightly by van der Waals force to form thin sheets which then curl up to form the MoS₂ microspheres.

Fig. 2 SEM, TEM and HRTEM images of (a and b) the MoS₂ nanoplates (Sample A), (c and d) nanowalls (Sample B) and (e and f) 3D microspheres (Sample C) composed of ultrathin nanoflakes.

In order to characterize the specific surface area of three MoS₂ nanostructures, the nitrogen adsorption–desorption analysis were performed using Brunauer–Emmett–Teller (BET) method. Fig. 3 shows a type IV isotherm of these samples, and the insets exhibit the corresponding pore size distribution by Barrett–Joyner–Halenda (BJH) method. The BET specific surface areas of three samples were calculated to 65.2, 89.3 and 105.6 m² g⁻¹ by the BET method for nanoplates, nanowalls, and microspheres, respectively. The pore size distribution of MoS₂ nanostructures calculated from the BJH method were given in the inset of Fig. 3, and it also can be clearly seen that all nanostructures possess the bimodal pore size distribution. The most pores of MoS₂ nanowalls are in mesoporous range with a peak centered at 3–5 nm and the MoS₂ microspheres are at 2–3nm, while MoS₂ nanoplates have more macropores (>15 nm).

Fig. 3 N₂ adsorption–desorption isotherms of (a) MoS₂ nanoplates, (b) MoS₂ nanowalls and (c) 3D MoS₂ microspheres. The inset shows the BJH pore size distribution of corresponding samples.

Based on the references and the above results, the precursors and reaction temperature played a very important role in determining the final structure of the products. The MoS₂ nanoplates are well-multilayer structure, the MoS₂ nanowalls consist of many MoS₂ nanosheets with 8–9 layers structures and the MoS₂ microspheres with highly wrinkled surface assembled by ultrathin nanosheets (5–7 layers) undergoes a series of clusters process. Based on the experimental results for the MoS₂ microspheres. At the initial stage of the reaction, Mo and S powders react and form MoS₂ nanoparticles. With increasing the reaction temperatures, growth of MoS₂ occurred in two directions only to form nanosheets. The monolayered or bilayered MoS₂ nanosheets were attached to each other by van der Waal interaction and finally self-assembled to form MoS₂ nano flake. Because of the relatively weak interaction between adjacent layers and the strong intralayer interaction, exfoliating MoS₂ to few layers. Meanwhile, the surface tension can help overcome the van der Waals force, which makes the exfoliation easier and more effective. MoS₂ nanoparticles with a large size will be subsequently obtained. Moreover, with large specific surface area and narrow pore size distribution, the MoS₂ microspheres are expected to have excellent performance in LIBs.

Furthermore, the composition of the MoS₂ microspheres was confirmed by XPS analysis. As shown in Fig. 4a, the predominant elements in the sample are Mo, S and C, and the atomic ratio of Mo to S is about 1 : 2. The core-level spectrum of MoS₂ (Fig. 4b) showed two strong peaks located at 232.05 and 229.0 eV, which are attributed to the doublet of Mo 3d5/2 and Mo 3d3/2 spin orbitals, respectively, and confirmed the formation of MoS₂.³¹

Fig. 4 XPS spectra of the MoS₂ microspheres: (a) survey scan, (b) Mo 3d.

3.2. Electrochemical properties of three MoS₂ nanostructures

The electrochemical property of prepared three samples were tested as anode materials of LIBs with the potential window of 0.01–3 V at the current density of 100 mA g⁻¹ and the scan rate of 0.5 mV s⁻¹ by cyclic voltammetry (CV) (Fig. 5a, c and e). As shown in Fig. 5a, in the first sweep, it exhibits two reduction peaks at about 1.1 V, 0.6 V and two corresponding oxidation peaks at about 2.32 V and 1.71 V, respectively. The peak at 1.1 V is due to the intercalation of Li⁺ in the interlayer spacing of MoS₂ (formation of Li_xMoS₂), whereas the peak at 0.6 V is the characteristics of Li_xMoS₂ to Li₂S and metallic Mo conversion reaction.^10,32 The two oxidation peaks at about 1.71 V and 2.32 V could be attributed to the oxidation of Li₂S.³³ In the following cathodic cycles, two new reduction peaks emerge at about 1.82 V and 1.16 V, respectively, while the two peaks locate at about 1.1 V and 0.6 V disappear, which agree with the previous lithiation/delithiation profiles. The two reduction peaks at about 1.82 V and 1.16 V are related to the insertion of lithium ions into the layered MoS₂. For the oxidation peaks, there is no significant change in the potentials during the 2^nd and 3^rd. For the CV curves of the MoS₂ nanowalls and microspheres shown in Fig. 5c and e, the slight shifts in reduction peak potentials or changes in reduction peak shape. The CV curves of the second and third charge process are nearly overlapping with each other, indicating the MoS₂ nanomaterial possess excellent cycle performances as anode material.

Fig. 5 The cyclic voltammograms of samples at the scan rate of 0.5 mV s⁻¹. (a) MoS₂ nanoplates, (c) MoS₂ nanowalls and (e) 3D MoS₂ microspheres. The galvanostatic charge and discharge curves of samples at a current density of 100 mA g⁻¹. (b) MoS₂ nanoplates, (d) MoS₂ nanowalls and (f) 3D MoS₂ microspheres.

Fig. 5b, d and f depicts the galvanostatic charge and discharge curves of initial three cycles of MoS₂ nanoplates, nanowalls and microspheres, respectively. As shown in Fig. 5b, the first voltage profile exhibits two discharge plateaus at 1.1 V and 0.6 V, which were coincident with the first cathodic peaks in the CV sweeps (Fig. 5a), respectively. In the subsequent voltage profiles, the plateaus 0.9 V and 0.4 V disappear in the first cycle, while the two potential plateaus at about 1.82 V and 1.16 V were observed. In the charging process, two potential plateaus at about 2.32 V and 1.71 V can be clearly identified for the first and subsequent cycles. Therefore, the charge and discharge curves of the MoS₂ nanoplates are agreement with the CV analysis. As can be seen from Fig. 5d and f, the MoS₂ nanowalls and MoS₂ microspheres electrodes display similar discharge–charge profiles to the MoS₂ nanoplates electrode but with a higher initial lithium storage capacity and better cycling stability. At the 1st cycle, the MoS₂ nanoplates, nanowalls and microspheres electrodes deliver the initial discharge capacity of 758.1, 1080.3 and 1231.5 mA h g⁻¹, respectively. The corresponding reversible capacity are 615.1, 890.2 and 1020.4 mA h g⁻¹ with the coulombic efficiency of 81.1%, 82.4% and 82.9%. The initial irreversible capacity loss for all the samples may be due to the formation of solid–electrolyte interface (SEI) film, decomposition of electrolyte, and some irreversible lithium trapping inside lattice.^10,34

As a generalization, the mechanism of Li storage for the MoS₂ during the discharge–charge process in lithium batteries can be described as follows:

MoS₂ + xLi → Li_xMoS₂ (1)

Li_xMoS₂ + (4 − x)Li → Mo + 2Li₂S (2)

Mo + 2Li₂S ↔ Mo + 2S + 4Li (3)

Intercalation of Li in MoS₂ happens in the first discharge process, followed by decomposition of Li_xMoS₂ to Li₂S and Mo after further discharging to 0.01 vs. Li⁺/Li. Then, Li₂S is converted to S after charging to 3.0 V while Mo remains inert. Thereafter, the reversible conversion reaction occurs between S and Li₂S during the subsequent cycles.

The cycling performances and rate behaviors of the three MoS₂ samples were presented in Fig. 6. As shown in Fig. 6a, the 3D MoS₂ microspheres exhibits much higher specific capacity with the excellent cycling stability in comparison with MoS₂ nanoplates and MoS₂ nanowall. From the second cycle, its reversible capacity decreases from 1020.4 mA h g⁻¹ to 850.9 mA h g⁻¹ after 50 cycles, which corresponds to capacity retention of 83.4%. Nevertheless, MoS₂ nanoplates suffer a rather rapid capacity fading and its reversible discharge capacity decreases from 758.1 to 390.9 mA h g⁻¹ after 50 cycles. It is evident that the 3D hierarchical structure of microspheres possesses long-term stable structure to accommodate volume change, large electrode–electrolyte contact area to offer more active sites for Li⁺ insertion/distraction as well as more diffusion paths for Li⁺ ions.^35–37 As shown in Fig. 6b, when the current density increase from 100 to 200, 400, and 800 mA g⁻¹, the discharge capacity of 3D MoS₂ microspheres electrode decreases more slowly than that of MoS₂ nanoplates and MoS₂ nanowall electrodes. In addition, the MoS₂ nanowall electrode also exhibits a high rate performance, but its capacities are slightly lower than those of 3D MoS₂ microspheres. When the current density returns back to 100 mA g⁻¹, the discharge capacity of 3D MoS₂ microspheres can restore to 808.7 mA h g⁻¹, comparing with 930.5 mA h g⁻¹ in the 10th cycle. Comparing with other studies on electrode materials,^38,39 the 3D spherical structure composed of nanosheets possesses a long term stable structure to accommodate volume change, large electrode–electrolyte contact area to offer more active sites for Li⁺ accommodation as well as more efficient diffusion paths for Li⁺ ions, and the restacked MoS₂ ultrathin nanosheets showed greatly enhanced cycling stability and rate capability.

Fig. 6 (a) Cycle performances of three MoS₂ nanostructures electrodes at a current density of 100 mA g⁻¹. (b) Comparison of rate performances of various MoS₂ nanostructures electrodes at current densities from 100 to 800 mA g⁻¹.

As a consequence, the excellent electrochemical performance of 3D MoS₂ microspheres with ultrathin nanosheets might be attributed to the following reasons. Firstly, dispersive 3D microspheres with nanosheets structure own lots of open spaces and larger specific surface area, which can increase reactive site and contact area between the active material and electrolyte, so endow the electrode with high specific capacity. Secondly, the nanometer size (5–7 nm) of sheet and its distinct layered structure could effectively buffer volume variation and store large quantity lithium ions during the charge and discharge process, so we can obtain cycling stability and rate performance.

4. Conclusions

In summary, we have successfully synthesized three MoS₂ nanostructures (nanoplates, nanowalls and microspheres) by a facile solid-state reaction. Both samples deliver excellent cycling and rate performance owing to its unique layered structure. Moreover, Uniform 3D MoS₂ microspheres assembled by nanosheets with 5–7 layers shows best capacity and excellent cycling performance as anode materials for LIBs, which can be attributed to the large specific surface area, the presence of ultrathin sheets and their layered structure that could provide a large number of reaction sites, and its unique 3D hierarchal structure that could not only reduce the diffusion length of Li⁺ ions, but also accommodate huge structural deformation or changes than other MoS₂ nanostructures. It is our hope that this simple synthetic route can provide more information about the influence of structure on lithium storage performance of other transition metal sulfide nanostructures.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (51302112), Jiangsu colleges and universities Nature Science research project (14KJB430009), Jiangsu Industry-University-Research Joint innovation Foundation (BY213065-05, Y213065-06) and Jiangsu Graduate student innovation project (CXZZ13_0669).

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