One pot hydrothermal synthesis of graphene like MoS₂ nanosheets for application in high performance lithium ion batteries

Abstract

In this paper, we develop a simple one pot facile technique to synthesis few layer MoS₂ nanosheets through an aqueous hydrothermal method using L-cysteine as the sulphur source. The high resolution transmission electron microscopy shows that the as-synthesized MoS₂ nanosheets exhibit well crystallized few layer graphene like structures ten of nanometers in size. The experimental results indicate that few layer MoS₂ nanosheets exhibit the highest specific reversible capacity of 1097 mA h g⁻¹ at a current density of 50 mA g⁻¹ after 25 cycles. The excellent electrochemical properties are attributed to the synergistic effect of the few layer MoS₂ nanosheets.

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A critical topic in materials research is the design and the synthesis of novel materials for lithium ion batteries (LIBs) with potential application in vehicles. Commercial anode materials in LIBs are graphitic due to their structural stability and cost effective fabrication.¹ However, small theoretical specific capacity of graphitic materials (375 mA h g⁻¹) cannot fulfill the demanding high capacity storage.

Therefore alternative high specific capacity materials such as metals, metal oxides or metal sulphides should be substituted for graphitic materials. Metal sulphides are a suitable anode material due to their peculiar structural and high specific capacity.² Molybdenum disulphide (MoS₂) has a layered structure, which consists of covalently bound S–Mo–S trilayers separated by a relatively large vander Waals band gap.³ It has been widely studied as an anode material for rechargeable lithium batteries because of its higher capacity (>670 mA h g⁻¹). However it suffers from poor stability and rate capability because of its lithiation product Li₂S after the first cycle, which deteriorate the performance considerably. So, different strategies are proposed to avoid this problem in view of morphologies and synthesis method. There are several methods to synthesize single layered or few layered MoS₂, such as mechanical exfoliation, chemical exfoliation, hydrothermal synthesis and chemical vapor deposition. Coleman et al. proposed liquid phase exfoliation of bulk MoS₂ powders in an organic solvent followed by high energy ultrasonification to achieve monolayer thin film and its composite structures.⁴ And a micromechanical cleavage was reported to produce high quality monolayers with low yield.^5,6 Liquid-phase exfoliation is one of the widely used method to produce moderate yields. However, it involves hazardous organic solvents and severe ultrasonification. In addition to above methods, ion intercalation method is also effective in preparing MoS₂ monolayers, but effective strategy is necessary to remove ions which tends to comprise the metallic MoS₂ layers.⁷ Hydrothermal synthesis is the another effective and scalable method to synthesis MoS₂ nanosheets and nanoplates. Recently Hwang et al. reported a facile all solution route to synthesis a disordered graphene like MoS₂ nanoplates via solvothermal method.² Liang et al. synthesized MoS₂ nanosheets in flower like structure using PVP as surfactant, which was suitably applicable in lithium ion batteries.⁸ However, synthesis of pure free standing MoS₂ layers by hydrothermal method using MoO₃ powder as the source has not been reported yet up to our knowledge.^9–12

Herein, we propose a novel thinking of preparing graphene like MoS₂ few layer nanosheets via aqueous hydrothermal reaction. The graphene like nanosheets were synthesized through sulphurization of MoO₃ powder in aqueous medium. The synthesis method developed here is an efficient, scalable and reactive chemicals free strategy. The improved electrochemical performance of the few layer MoS₂ nanosheets were demonstrated for using as lithium ion battery anode material. In comparison with previous reports, MoS₂ few layer nanosheets manifest improved lithium storage capacity with better rate capability.

The XRD pattern in Fig. 1(a) shows crystalline and single phase MoS₂ nanosheets with hexagonal crystal structure without impurities (JCPDS-37-1492). The inset in Fig. 1(a) displayed the homogeneous solution of MoS₂ nanosheets dispersed in water solution. The characteristic peaks of MoS₂ nanosheets were observed at 33.69° and 59.51°, which were corresponding to (100) and (110) plane. The absence of (002) reflections related to bulk MoS₂ crystals indicates that the obtained products are single layer or few layer graphene like MoS₂.^2,12 FTIR spectroscopy was employed to confirm the composition of MoS₂ nanosheets. As shown in Fig. 1(b), the bands at 939 cm⁻¹ and 902 cm⁻¹ were assigned to Mo–O vibrations, indicating the presence of oxygen in similar coordination as in MoO₃ from the source material. The weak peak at about 471 cm⁻¹ is associated with Mo–S vibration.¹³ The other peak located at around 3432 cm⁻¹ was assigned to N–H stretching vibration. In middle frequency region, two peaks were observed at 1625 and 1397 cm⁻¹, which might be due to N–H bending and N–H plane vibration, respectively.¹⁴ The result of FTIR spectra shows the existence of amino groups on the surface of MoS₂ nanosheets originated from the source material.^15,16 The macroscopic view of the morphologies of MoS₂ nanosheets were observed by field emission SEM image. As shown in Fig. 1(c), the MoS₂ nanosheets were cross linked each other sharp edges. The inset of Fig. 1(c) displays a single nanosheet. The thickness of layers were approximately ∼10 nm indicates the layers were comprised together while dropcasted on the substrate. The crystal structure of the samples was further analyzed using HRTEM, as shown in Fig. 2. It is shown that MoS₂ nanosheets are densely arranged, and randomly oriented graphene like sheets. The high magnification image of MoS₂ nanosheets were depicted in Fig. 2(b)–(d), which evidently reveal that the as-synthesized MoS₂ nanosheets are well crystallized, and the interlayer distance between two layers are 0.68 nm (Fig. 2(b)). This is larger than the interlayer distance for the (002) plane of MoS₂ bulk counterpart (0.62 nm).¹⁷ The lattice d spacing is estimated to be 0.27 nm, which corresponds to (100) lattice plane of hexagonal MoS₂ phase and is in well agreement with XRD pattern results.

Fig. 1 (a) XRD pattern of as-synthesized MoS₂ nanosheets (inset shows the MoS₂ nanosheets in water solution); (b) FTIR spectra of MoS₂ nanosheets; (c) FESEM image of MoS₂ nanosheets. Inset: a single nanosheet.

Fig. 2 HRTEM images of the MoS₂ nanosheets.

To measure the thickness of the nanosheets, atomic force microscopy (AFM) measurement was conducted by dispersing the MoS₂ nanosheets on silicon substrates as shown in Fig. 3 (nanosheets with different sizes were indicated as (A) and (B)). Thickness was estimated to ∼3.5 nm and ∼2.5 nm respectively, which revealed that as-obtained MoS₂ sample was few layer (might be less than five layers) and it did not stacked together in solution (inset in Fig. 1(a)). It could be concluded that the as-synthesized product in our method was few layer MoS₂ nanosheets, free from aggregation in solution form. These graphene like nanosheets could decrease the diffusion length for mass transport and increase the specific surface area for fast performance of batteries.

Fig. 3 AFM images of single MoS₂ nanosheets (two different sizes indicated as (A) and (B)).

Fig. 4(a) shows the C–V profiles of few layer MoS₂ nanosheet electrodes in the potential range of 0.01–3.00 V (vs. Li⁺/Li). In the first cycle, the sharp peak observed between 0.40 V and 0.5 V in the discharge process may be attributed to the conversion of MoS₂ to Mo, the formation of amorphous Li₂S (MoS₂ + 4Li⁺ ↔ Mo + 2Li₂S) and the irreversible reaction with electrolyte.^18–20 The broad peak at about 1.50–2.20 V in the anodic process corresponds to the delithiation process. During the following cycles, both cathodic and anodic peaks are positively shifted due to the polarization of the electrode materials in the first cycle. It is observed that in successive cycles, the cathode peaks are diminished significantly (∼0.42) but anode peaks do not show considerable changes. The MoS₂ sample reported here, showed the broad peak at around 2 V instead of peaks observed by other groups. The reason might be due to peculiar properties of MoS₂ nanosheets synthesized in this work. Moreover, the C–V profiles are highly depending on the structure of the nanosheets, and the synthesis methods. Recently, much attention is devoted to explore the lithium storage mechanism of MoS₂/Li system.²¹ In the first step of discharge process, lithium ions are inserted into MoS₂ layer to form Li_xMoS₂ and with further discharging to 0.01 V, MoS₂ is reduced to Mo and Li₂S. Subsequently, Li₂S ↔ 2Li + S + 2e⁻ reaction occurred in the electrode. In overall, after the first cycle, the observed C–V curves are in good agreement with the previous results.²²

Fig. 4 Electrochemical characterization of MoS₂ (a) cyclic voltammogram curves of MoS₂ containing samples for 8 cycles (b) voltage profiles with specific capacity of MoS₂ electrodes at a current density of 50 mA g⁻¹. (c) Rate capability of MoS₂ nanosheets at different current densities (d) cycling stability of MoS₂ electrode at 50 mA g⁻¹.

Fig. 4(b) shows voltage–capacity curves at current density of 50 mA g⁻¹ with voltage window of 0.01–3.00 V. The first discharge (lithiation) and charge (delithiation) capacities are 1374 and 1024 mA h g⁻¹, respectively. The observed higher value is attributed to more lithium ions extraction in the first charge process due to its excellent electronic conductivity. After 25^th cycles the specific capacity in charging cycle decreases to 1040 mA h g⁻¹ and for discharge cycle to 985 mA h g⁻¹. The results of the charge–discharge curves are in accordance with the aforementioned C–V curves in Fig. 4(a).

The current density rate dependent capacity performance of few layer MoS₂ electrode was investigated at different current densities of 50, 100, 250, 500 and 50 mA g⁻¹, respectively, as shown in Fig. 4(c). The results demonstrate the good structural reversibility of the MoS₂ sample. Specifically, the sample delivers a reversible capacity of 962 mA h g⁻¹ after 10 cycles at the current density of 100 mA g⁻¹. When current density increased, only slight decrease of specific capacity occurred. A reversible capacity of 860 mA h g⁻¹ can be achieved even at a high current density of 500 mA g⁻¹ after 25 cycles. When the current density is changed to 50 mA g⁻¹, the capacity is recovered back to 1040 mA h g⁻¹ after 25 cycles, revealing the good rate capability of our MoS₂ samples. The results of higher reversible capacity of the sample can be attributed to the following reasons: first, the graphene like ultrathin MoS₂ nanosheets provide high specific surface area which could enhance the contact area with the electrolyte, in turn provide more active sites for Li interaction with electrode. Secondly, the introduction of carbon conductor to the MoS₂ layers would be the positive factor to improve the performance. In addition, few layer MoS₂ with interlayer distance of 0.68 nm provides more space for insertion of Li ions. Furthermore, the cyclic stability of MoS₂ electrodes were measured at current density of 50 mA g⁻¹ as shown in Fig. 4(d). For the first cycle the device shows the low capacity, which might be due to the following reason: the electrolyte is not fully wetting the electrode, which leads to slow insertion of Li ion into electrolyte–cathode interface. The capacity is 1015 mA h g⁻¹ after 50 cycles, which indicates excellent cyclic stability behavior of our MoS₂ electrodes. Our results indicate that few layer MoS₂ nanosheets holds promise as the anode material for high performance lithium ion batteries.

Conclusions

In conclusion, we presented a simple method to synthesize graphene like MoS₂ nanosheets via aqueous hydrothermal synthesis route using MoO₃ and L-cysteine as the starting material. Structural characterizations show that the MoS₂ samples composed of well crystallized few layer with interlayer distance of 0.68 nm. Because of these structural advantages, the few layer MoS₂ nanosheets exhibit greatly improved electrochemical performances. High specific discharge capacity of 1040 mA h g⁻¹ can be retained after 25 cycles at the current density of 50 mA g⁻¹. Even at high current density of 500 mA g⁻¹, the sample can deliver the specific capacity of 860 mA h g⁻¹. The results demonstrated the enhanced rate capability of MoS₂ nanosheets which is suitable for application in lithium ion batteries.

Experimental section

Synthesis of MoS₂ nanosheets

The MoS₂ nanosheets were synthesized as follows: 0.25 g MoO₃ powder was dissolved in 20 mL deionized (DI) water and subjected to ultrasonification for 20 min. And then 0.3 g L-cysteine was added with 50 mL DI water followed by violent stirring for about 30 min. HCl solution is added to adjust the pH value of the solution to 2.5. Subsequently, the mixture was transferred into a 75 mL Teflon-lined stainless steel autoclave and heated to 200 °C for 16 h. After cooling naturally, the black MoS₂ products were collected by filtration, washed with distilled water and absolute ethanol for several times, and then dried in vacuum at 45 °C for overnight. X-ray powder diffraction (XRD) pattern was operated on a Japan RigakuD/Maxr-A X-ray diffractometer equipped with graphite monochromatized high-intensity Cu Kα radiation (λ = 1.54178 Å). The sample morphologies were characterized by field emission scanning electron microscopy (FESEM, Hitachi, SU8010). The morphologies of the layers were recorded with a JEM 2100 high resolution transmission electron microscope (HRTEM).

Fabrication of electrode material & testing

The anode material was prepared by making slurry containing 70 wt% dried MoS₂ nanosheets, 15 wt% polyvinylidene fluoride binder and carbon conductor (15 wt%) in NMP solvent. The prepared slurry was pasted on copper foil. Each electrode contains 1.74 mg of MoS₂ active material. The electrodes were dried in a vacuum oven at 100 °C overnight. A lithium foil was used as a counter electrode. Electrolyte was composed of 1 M LiPF₆ salt dissolved in a solution consisting of ethylene carbonate and diethyl carbonate (1 : 1 volume). Cyclic-voltammetry (C–V) tests were performed on a versatile multichannel potentiostat of CH instruments Model 600D series at a scan rate of 0.5 mV s⁻¹ over a potential range of 0.01–3.0 V (vs. Li⁺/Li). Charge–discharge characteristics were tested galvanostatically in a voltage range of 0.01–3.0 V (vs. Li⁺/Li) at a desired current density using an 8 Dian-Landian battery test system.

Acknowledgements

This work was supported by the Natural Science Foundation of Fujian Province (2013J01233), National Natural Science Foundation of China (61377027), State Key Laboratory of Electronic Thin Films and Integrated Devices (Grant KFJJ201309), Basic Science Research Program through the National Research Foundation of Korea (NRF) (2013-016467).

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