Facile synthesis of hierarchical MoS₂ microspheres composed of few-layered nanosheets and their lithium storage properties

Abstract

In this work, we demonstrate an interesting polystyrene microsphere-assisted synthesis of hierarchical MoS₂ spheres composed of ultrathin nanosheets. The as-prepared sample exhibits promising lithium storage properties with improved cyclic capacity retention and rate capability.

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As a typical two-dimensional (2D) nanostructure with a large lateral size and a small thickness, nanosheets (NSs) have attracted increasing research interest due to their high surface area, controlled exposed crystal facets,^1–7 diverse compositions such as TiO₂,^1–12SnO₂^9,13–17 and SnO^18,19 and wide application potentials in many fields such as lithium-ion batteries (LIBs),^{1,8–11,13,14,20} gas sensing,²¹glucose sensing¹² and catalysis.^3,5,7,16

MoS₂ has a similar layered structure with graphite, in which molybdenum atoms are sandwiched between two layers of sulfur atoms. MoS₂ has been found attractive in many applications, such as lubricants,²²catalysts,²³ and transistors.²⁴ This layered structure is also beneficial for the intercalation of guest ions, such as Li⁺ ions^25–30 and Mg²⁺ ions.^31,32 As a result, many nanostructures of MoS₂ such as nanoflakes, nanotubes and nanoflowers have been reported as electrode materials for LIBs.^29,33,34 Although these MoS₂ nanostructures exhibit high capacities up to 1000 mA h g⁻¹, their cycling stability cannot meet the requirement for practical use in LIBs. Some methods have been proposed to improve the cycling performance of MoS₂ by integrating MoS₂ with different carbonaceous materials, like amorphous carbon, graphene^25,26 and carbon nanotubes (CNTs).^22,35

Herein, we report a facile synthesis of uniform hierarchically structured MoS₂ microspheres composed of NSs (designated as MoS₂-NS microspheres thereafter), in which polystyrene (PS) microspheres are added to facilitate the assembly of MoS₂ NSs to form uniform microspheres. After an annealing treatment in inert atmosphere, the PS microspheres are decomposed and uniform MoS₂-NS microspheres are obtained. The high surface area resulting from the assembly of nanosheets should help store more lithium ions and the large void space between nanosheets will buffer the volume variation accompanying the charge–discharge process, thus leading to improved cycling stability. As expected, in comparison to MoS₂ flakes, these MoS₂-NS microspheres manifest improved lithium storage properties with better cycling performance and higher capacity.

Fig. 1 shows the morphology of the as-prepared products. From the scanning electron microscopy (SEM; Fig. 1A) image, it is very obvious that the product is composed of PS microspheres and MoS₂-NS microspheres with sizes of ∼1.5 μm and ∼1 μm, respectively. After being treated in the atmosphere of H₂/N₂ mixture at 800 °C, PS microspheres are decomposed and the MoS₂-NS microspheres are preserved. Fig. 1B shows the morphology of the MoS₂-NS microspheres. It can be seen that these MoS₂-NS microspheres are composed of sheet-like subunits and the size is slightly reduced to 700–900 nm. With a closer examination with transmission electron microscopy (TEM) (Fig. 1C), the nanosheets with a lateral size of around 500 nm aggregate to form relatively loose microspheres. The high resolution TEM (HRTEM) image (Fig. 1D) shows that the interlayer distance is about 0.62 nm, which is assigned to the (002) plane of MoS₂. The nanosheets are composed of 3–5 layers of MoS₂ molecular sheets.

Fig. 1 (A) Scanning electron microscopy image (SEM) of the as-synthesized material; (B) SEM and (C) transmission electron microscopy (TEM) images of the MoS₂-NS microspheres; (D) a high-resolution TEM image of several MoS₂ NSs; the inset shows a HRTEM image of a single MoS₂ nanosheet.

The crystallinity of the samples is measured by X-ray diffraction (XRD), and the XRD pattern of MoS₂-NS microspheres is shown in Fig. 2A. It is clear that the diffraction peaks at 2θ = 14, 33 and 59° can be assigned to (002), (100) and (110) planes of MoS₂ (JCPDS 37-1492) respectively. The surface structure of the MoS₂-NS microspheres is further studied by the gas sorption measurement. From the N₂ adsorption–desorption isotherm (Fig. 2B), a distinct hysteresis loop is identified at a relative pressure of 0.1–0.9, indicating the presence of a mesoporous structure, which leads to a Brunauer–Emmett–Teller (BET) surface area of 36 m² g⁻¹. The pore size distribution is calculated by the Barrett–Joyner–Halenda (BJH) method. The plots (inset of Fig. 2B) show that most pores are in the mesoporous range with a peak centered at 3–10 nm. These pores are formed from the stacking of MoS₂ nanosheets.

Fig. 2 (A) XRD pattern of MoS₂-NS microspheres; (B) N₂ adsorption–desorption of MoS₂-NS microspheres; the inset shows the pore size distributions calculated from both the adsorption and desorption branches.

We have studied the effect of PS on the morphologies of the products. The morphology of the MoS₂ sample prepared by the same hydrothermal route without adding PS is shown in Fig. 3. As can be seen, the MoS₂ product obtained without adding PS is composed of micrometre-sized flakes, which are formed by dense stacking of MoS₂ nanosheets. By comparing the morphologies of MoS₂-NS microspheres and MoS₂ flakes, it is evident that the PS microspheres have played a key role on the formation of MoS₂-NS microspheres. The exact mechanism for the function of PS microspheres is unclear at this moment. It is hypothesized that in the presence of PS microspheres in the solution, nanosheets with smaller thickness and lateral size are formed easily initiated by the functional groups on the PS surface. At the same time, the PS microspheres may act as physical confinement to effectively prevent the further growth of MoS₂ NSs into thick flakes and facilitate the assembly of MoS₂ NSs into microspheres.

Fig. 3 (A) SEM and (B) TEM images of the MoS₂ flakes obtained without adding PS microspheres.

Next, we investigate the electrochemical properties of MoS₂-NS microspheres as an anode material for LIBs. Fig. 4A shows the representative cyclic voltammograms (CVs) of MoS₂-NS microspheres. As can be seen, the CV behavior is generally consistent with those reported previously.^25–27,35 In the first cathodic sweep, the peak at 0.74 V is attributed to the intercalation of lithium ions into the MoS₂ lattice which transforms the triangular prism (coordination of Mo by six S atoms) into an octahedral structure.^25,36 This peak disappears in the second cathodic sweep because only nearly amorphous MoS₂ is reformed after the charge process (lithium extraction) in the first cycle. The other peak at 0.1 V is attributed to the complete reduction process:^25–27

MoS₂ + 4Li ↔ Mo + 2Li₂S (1)

This peak is shifted to 0.16 V in the second cycle and the current intensity is decreased. In the following cathodic sweeps, two new peaks at 1.8 V and 1.0 V appear, perhaps suggesting the presence of a multi-step lithium insertion mechanism. In the anodic sweeps only one peak at 2.5 V is observed, corresponding to the lithium extraction process and the oxidation of Mo to MoS₂. Fig. 4B shows the charge–discharge voltage profiles of the sample for the first 3 cycles. In agreement with the above CV study, two plateaus at 1.1 and 0.6 V are observed in the first discharge process. The first plateau at 1.1 V is attributed to the formation of Li_xMoS₂, and the plateau at 0.6 V corresponds to the conversion reaction process, in which Li_xMoS₂ is completely decomposed into Mo nanoparticles embedded in a Li₂S matrix and a gel-like polymeric layer is formed resulting from electrochemically driven electrolyte degradation.^25,36 In the second and third discharge sweeps, the MoS₂-NS microspheres electrode shows two potential plateaus at 1.9 and 1.1 V, and the potential plateau at 0.6 V in the first discharge disappears; in the charge (delithiation) process, the MoS₂-NS microspheres electrode exhibits a conspicuous potential plateau at 2.2–2.4 V, which also agrees with the previous CV curves.

Fig. 4 (A) Representative CVs at a scan rate of 0.5 mV s⁻¹ for the first, second, and fifth cycles of MoS₂-NS microspheres. (B) Charge–discharge voltage profiles at a current density of 100 mA g⁻¹ of MoS₂-NS microspheres. (C) Comparative cycling performance of MoS₂ flakes (I) and MoS₂-NS microspheres (II) at a current density of 100 mA g⁻¹. (D) Cycling performance of MoS₂ flakes (I) and MoS₂-NS microspheres (II) at different current densities indicated (mA g⁻¹).

The initial discharge and charge capacities of MoS₂-NS microspheres are found to be 1160 and 791 mA h g⁻¹. Such a high initial lithium storage capacity might be associated with the unique hierarchical structure of the stacked MoS₂ nanosheets. The irreversible capacity loss of around 31.8% may be mainly attributed to the irreversible processes, such as formation of solid–electrolyte interface (SEI) layer and trapping of some lithium in the lattice. The discharge and charge capacities in the second cycle are 873 and 822 mA h g⁻¹, respectively, giving rise to a Coulombic efficiency of 94.2% and this value further increases to 95.4% in the third cycle. Fig. 4C shows the cycling performance at a current density of 100 mA g⁻¹. After 50 discharge–charge cycles, the MoS₂-NS microspheres can still deliver a capacity of 672 mA h g⁻¹, while the MoS₂ flakes only deliver 390 mA h g⁻¹. After 70 cycles, the capacities of MoS₂-NS microspheres and MoS₂ flakes further decrease to 585 mA h g⁻¹ and 163 mA h g⁻¹, respectively. Furthermore, the MoS₂-NS microspheres also exhibit better performance compared to the nanocomposite composed of several layered MoS₂ nanosheets on CNTs backbone, whose capacity is about 350 mA h g⁻¹ after 50 cycles.³⁷Fig. 4D further displays the comparative cycling performance of MoS₂-NS microspheres and MoS₂ flakes at various current densities. At current densities of 200, 500 and 1000 mA g⁻¹, the capacities of MoS₂-NS microspheres are 726, 581 and 353 mA h g⁻¹, respectively as compared to 311, 207 and 134 mA h g⁻¹ only for pure MoS₂ flakes. It is thus evident that the MoS₂-NS microspheres exhibit much improved lithium storage performance compared to the pure MoS₂ flakes. The enhanced lithium storage properties might be attributed to the unique hierarchical structure of MoS₂-NS microspheres. Specifically, the large surface area offered by the ultrathin NSs endows the MoS₂-NS microspheres with high specific capacity as a result of increased reactive sites and interface between the active material and electrolyte. Moreover, the presence of sufficient void space between the organized MoS₂ nanosheets not only allows fast lithium ion diffusion but also effectively buffers the mechanical stress and volume variation accompanying the lithium inserting/de-inserting process. Furthermore, the flexible nature of such ultrathin MoS₂ NSs enhances the robustness of the electrode structure.

In summary, we have prepared hierarchical spheres composed of ultrathin MoS₂ nanosheetsvia a facile PS microsphere-assisted hydrothermal method. It is found that the presence of PS microspheres is critical for the formation of such interesting hierarchical spheres. In the absence of PS microspheres, only MoS₂ flakes are formed. Both MoS₂ hierarchical microspheres and flakes have been evaluated as anode materials for lithium-ion batteries. The results show that these unique MoS₂-NS microspheres exhibit greatly enhanced lithium storage properties compared to the MoS₂ flakes.

Experimental section

Materials preparation

In a typical synthesis of MoS₂-NS microspheres, 50 mg of PS microspheres were dispersed into 40 mL of de-ionized water by ultrasonication for 5 minutes. 0.25 g sodium molybdate (Na₂MoO₄·2H₂O, Sigma-Aldrich) was then added to the above solution. After stirring for 5 minutes, 0.5 g of thiourea was added. After stirring for another 5 minutes, the reaction solution was then transferred to a 60 mL Teflon-lined stainless steel autoclave and kept in an electric oven at 200 °C for 24 h. The autoclave was then left to cool down to room temperature in the oven. The black precipitate was collected by centrifugation, washed thoroughly with ethanol, and dried at 80 °C for 12 h. The as-prepared MoS₂-NS microspheres were further treated at 800 °C in the atmosphere of 5% H₂ balanced by N₂ for 2 h with a heating rate of 1 °C min⁻¹ to obtain the highly crystalline MoS₂-NS microspheres. The preparation process of MoS₂ flakes is similar to the one for MoS₂-NS microspheres, except the addition of PS.

Materials characterization

The product morphology was examined using a field-emission scanning electron microscope (FESEM; JEOL, JSM-6700F, 5 kV) and a transmission electron microscope (TEM; JEOL, JEM-2100F, 200 kV). Crystallographic information of the samples was collected using powder X-ray diffraction (XRD; Bruker, D8 Advance X-ray diffractometer, Cu Kα radiation, λ = 1.5406 Å). The nitrogen sorption measurement was performed using a Quantachrome Instrument (Autosorb AS-6B).

Electrochemical measurements

The electrochemical measurements were carried out using two-electrode Swagelok-type cells (X2 Labwares, Singapore) with pure lithium metal as both the counter and the reference electrodes at room temperature. The working electrode consisted of the active material, a conductive agent (carbon black, Super-P-Li), and a polymer binder (poly(vinylidene difluoride), PVDF, Aldrich) in a 70 : 20 : 10 weight ratio. The electrolyte used was 1.0 M LiPF₆ in a 50 : 50 (w/w) mixture of ethylene carbonate and diethyl carbonate. Cell assembly was carried out in an Ar-filled glovebox with concentrations of moisture and oxygen below 1.0 ppm. Cyclic voltammetry (0.005–3.0 V, 0.5 mV s⁻¹) was performed using an electrochemical workstation (CHI 660C). The charge/discharge tests were performed using a NEWARE battery tester at different current densities with a voltage window of 0.01–3.0 V.

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