Synthesis of nano-TiO₂-decorated MoS₂ nanosheets for lithium ion batteries

Abstract

A facile process is developed to synthesize a TiO₂–MoS₂ nanohybrid via a one-pot hydrothermal route and post-annealing in an Ar atmosphere at 500 °C for 4 h. The precursor and target products were characterized by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM). TEM and FESEM analysis showed that TiO₂ nanoparticles with an average diameter of about 20 nm were uniformly distributed on MoS₂ nanosheets. Electrochemical measurements demonstrated that the nano-TiO₂-decorated MoS₂ nanosheets exhibited excellent cycling stability and rate performance, which delivered a capacity of 604 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. The TiO₂ is believed to act as a stabilizer to retain the MoS₂ structure upon prolonged cycling. This material can be a promising candidate for the lithium ion batteries (LIBs).

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Introduction

Rechargeable lithium ion batteries (LIBs) are considered as potential power sources for various applications.¹ They are becoming more and more popular in many applications in consumer products such as mobile phones, music players and laptops.² It is known that anode materials play an important role as crucial components.³ Focusing on general aspects of material technology for LIBs, graphite has been widely used as a major electrode material in commercial LIBs. However, graphite has a relatively low theoretical capacity (372 mA h g⁻¹),^4,5 which cannot fully meet the energy density requirement in electric vehicles (EVs) and the need for large-scale batteries in the future.⁶ Therefore, alternative anode materials with higher specific capacity and good cycling behavior are desirable for LIBs.

In recent years, layered transition-metal dichalcogenides are of great interest as the active materials for lithium storage based on their unique physical and chemical properties, such as relatively high energy density, long cycle life and design flexibility.^7,8 In particular, MoS₂ is one of the most stable and versatile members of this family, atoms within a layer are bound by strong ionic/covalent forces,⁹ while the individual layers are bound by weak van der Waals interactions, forming a sandwich structure.^9–11 Because of its layered structure and high theoretical capacity.¹² MoS₂ has been regarded as a potential candidate for electrode materials in lithium secondary batteries. Thus far, MoS₂ with different morphologies have been reported for lithium storage, such as nanorods,¹³ nanoplates,¹⁴ nanoflakes,¹⁵ nanotubes¹⁶ and nanoflowers.¹⁷ In spite of the theoretical capacity of MoS₂ being up to 670 mA h g⁻¹, it suffers from poor cycling stability and low rate capability.¹⁸ Additionally, MoS₂ nanosheets can be easily restacked together under the van der Waals interaction during charge–discharge processes. The restacked surface is hardly accessible to the electrolyte to penetrate. As a result, such a layered structure and their high active surface will be lost.¹⁹ These defects obstruct their practical applications as electrode materials of LIBs. To combat this problem, one doable strategy is to design hybrid nanostructures. A number of MoS₂-based hybrid nanostructures have been fabricated for LIBs such as MoS₂/carbon,⁸ MoS₂/Fe₃O₄,¹⁹ and MoS₂/graphene.²⁰

Titanium dioxide (TiO₂) has been proposed as a prospective candidate for the hybridization due to the low volume variation (<4%) during the lithiation–delithiation process, which can buffer the excessive volume change. Meanwhile, TiO₂ nanoparticles are introduced as spacers between MoS₂ nanosheets, thus making both faces of nanosheets accessible to electrolytes. TiO₂ acts as a spacer and can significantly stabilize the nanohybrid structure during the lithium insertion–removal process. Therefore, TiO₂ based electrode materials generally display favorable cycling stability compared to other transition metal oxides and sulfides.²¹ The MoS₂ nanosheets@TiO₂ nanotubes hybrid nanostructure has been synthesized through a template-assisted hydrothermal method by Xu et al. and it shows high reversible lithium storage capacity and superior rate capability.²² However, the complicated synthesis procedure might limit the practical applications of the composite.

Herein, we report a facile synthesis of MoS₂ nanosheets decorated with TiO₂ nanoparticles, as illustrated in Fig. 1. The current nano-TiO₂-decorated MoS₂ nanosheets that consist of single or few layers are successfully synthesized via a one-pot hydrothermal process by employing ammonium heptamolybdate, titanium tetrachloride and thiourea as starting materials and post-annealing in an Ar atmosphere at 500 °C for 4 h. In the hydrothermal synthesis process, TiCl₄ acts as the source of Ti, as well as providing hydrochloric acid. Electrochemical measurements demonstrate that the TiO₂–MoS₂ nanohybrid shows better cycling stability than MoS₂, which delivers a reversible capacity of 604 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. The results demonstrate that the TiO₂–MoS₂ nanohybrid is a promising material for LIB anodes.

Fig. 1 Schematic illustration of the synthesis route of the TiO₂–MoS₂ nanohybrid.

Experimental section

Materials preparation

All reagents were of analytical grade. A typical preparation process is described as follows: 0.1698 g of (NH₄)₆Mo₇O₂₄·4H₂O was dissolved in 20 mL of distilled water. The ethanol solution of titanium tetrachloride was added to (NH₄)₆Mo₇O₂₄·4H₂O aqueous solution under stirring in an ice water bath. Then 0.3662 g of thiourea was added. The mixtures were ultrasonicated for 10 min and stirred for 1 h to disperse them. The obtained solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was filled with distilled water up to 60% of the total volume, sealed and hydrothermally treated at 220 °C for 24 h. The autoclave was left to cool down to room temperature. The precursor was collected by centrifugation and washed with distilled water and absolute ethanol, dried at 60 °C for 12 h. In order to improve the crystallinity of the TiO₂–MoS₂ nanohybrid, the precursor was annealed at 500 °C at a heating rate of 10 °C min⁻¹ in an Ar atmosphere for 4 h. The desired TiO₂–MoS₂ nanohybrid was obtained. The preparation process of MoS₂ nanoclusters is similar to that of TiO₂–MoS₂, except for the addition of the ethanol solution of titanium tetrachloride and calcination.

Materials characterization

The product was characterized by X-ray diffraction (XRD) using a Rigaku D/max-ga X-ray diffractometer at a scan rate of 6° min⁻¹ in 2θ ranging from 10° to 80° with Cu Kα radiation (λ = 1.54178 Å). The elemental composition was determined using energy dispersive X-ray spectroscopy (EDX). Field-emission scanning electron microscopy (FESEM) was carried out on a Hitachi S-4800 electron microscope. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 T20 electron microscope operated at 200 kV with the software package for automated electron tomography. Thermogravimetric analysis (TGA) was carried out using a SDTQ600 at a heating rate of 10 °C min⁻¹.

Electrochemical measurements

The electrochemical tests were carried out in coin cells. The TiO₂–MoS₂ nanohybrid was used as a working electrode, lithium foil as counter and reference electrodes, a polypropylene film (Celgard-2400) as a separator, and 1 mol L⁻¹ of LiPF₆ dissolved in a mixture of ethylene carbonate and diethyl carbonate (EC–DEC, 1 : 1 in volume) as an electrolyte. The working electrodes were prepared by a slurry tape casting procedure. The slurry consisted of 70 wt% active materials, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone (NMP). The slurry was tape-cast on the copper foil, and then the coated electrodes were dried at 120 °C for 12 h in a vacuum. The electrode was assembled into coin in an argon-filled glove box. Cyclic voltammetry measurements were performed on an electrochemical work-station (CHI 660D) at a scan rate of 0.1 mV s⁻¹ over the potential range of 0.01–3.0 V (vs. Li⁺/Li). The galvanostatic charge–discharge tests were conducted on the battery measurement system (LAND CT2001A, China) at various current densities of 50–1000 mA g⁻¹ with a cut off voltage range of 0.01–3.0 V vs. Li/Li⁺ at room temperature. For the electrochemical impedance spectroscopy (EIS) measurement, the excitation amplitude applied to the cells was 5 mV. The activation energy measurements were collected between 0.01 Hz and 100 kHz at room temperatures.

Results and discussions

The synthesis of the TiO₂–MoS₂ nanohybrid was conducted through a facile hydrothermal and post-annealing method. Initially, the ethanol solution of titanium tetrachloride is added into (NH₄)₆Mo₇O₂₄·4H₂O aqueous solution to form TiO₂ nanoparticles through a fast nucleation process, then MoS₂ nanoparticles are gradually formed under thiourea-assisted hydrothermal conditions. MoS₂ nanoparticles grow into a sheet-like structure by oriented aggregation as time goes on. As a result, TiO₂ nanoparticles are in suit decorated in the layers of MoS₂.

Fig. 2a(III) shows the XRD patterns of the MoS₂ without adding titanium tetrachloride, all the diffraction peaks can be readily indexed to the hexagonal MoS₂ phase (JCPDS card No. 37-1492) and these results are in good agreement with those of previous reports.²³ The stacking peak at 14.2° from curve III implied that MoS₂ nanoclusters may contain a well stacked layered structure.

Fig. 2 (a) XRD patterns of the as-prepared TiO₂–MoS₂ nanohybrid after a hydrothermal process (I), TiO₂–MoS₂ nanohybrid after annealing at 500 °C in an Ar atmosphere (II), MoS2 nanoclusters (III). (b) EDX spectrum of the TiO₂–MoS₂ nanohybrid.

The TiO₂–MoS₂ nanohybrid was synthesized by a similar hydrothermal method and subsequent calcination. As shown in Fig. 2a(I) and (II), the annealed TiO₂–MoS₂ shows sharper peaks in comparison with the as-prepared TiO₂–MoS₂ nanohybrid, which demonstrates that the crystallinity of TiO₂–MoS₂ is slightly improved after annealing. It can be seen from Fig. 2a(II) that all the diffraction peaks can be readily indexed to the standard peak of hexagonal 2H-MoS₂ (JCPDS card No. 37-1492) and anatase TiO₂ (JCPDS card No. 21-1272). The absence of peak at 14.2° of MoS₂ indicates that stacking of the single layers has not taken place.²⁴Fig. 2b shows the EDX spectrum of the TiO₂–MoS₂ nanohybrid. The EDX spectrum indicates that the annealed sample only contains Mo, S, Ti and O elements.

TGA (Fig. 3) was carried out from room temperature to 700 °C in air flow to determine the amount of MoS₂ present in the TiO₂–MoS₂ nanohybrid. The TGA curve can be divided into two domains of 25–300 °C, 300–500 °C. The first weight loss was measured to be 3.58% and 3.39% for the TiO₂–MoS₂ nanohybrid and MoS₂ nanoclusters, respectively. The result is attributed to the evaporation of physisorbed water and the loss of chemisorbed water. The second weight loss occurs at approximately 300 °C, which may probably be attributed to the oxidation of MoS₂ to MoO₃. A common feature of both TGA curves is the large weight loss in the range of 300–500 °C. The mass fraction of MoS₂ in the TiO₂–MoS₂ nanohybrid can thus be estimated to be about 71.79 wt%, on the assumption that the remaining product after the TGA measurement was pure MoO₃.²⁵

Fig. 3 TGA curves of the TiO₂–MoS₂ nanohybrid and MoS₂ nanoclusters at a temperature ramp of 10 °C min⁻¹ in air.

Fig. 4a–d shows the SEM and TEM images of the as-prepared TiO₂–MoS₂ nanohybrid after a hydrothermal process and treatment at 500 °C in an Ar atmosphere. It reveals that the as-prepared TiO₂–MoS₂ nanohybrid has a large-scale uniform sheet structure (Fig. 4a). Fig. 4b indicates that MoS₂ nanosheets are almost transparent, showing the extremely small thickness of this layer structure. A large quantity of TiO₂ nanoparticles with a diameter of 20 nm are uniformly distributed on the surface of layered MoS₂ nanosheets. Fig. 4c shows the morphology of the TiO₂–MoS₂ nanohybrid after annealing at 500 °C in an Ar atmosphere and it maintains its sheet-like structure. The high-magnification FESEM image displays that the TiO₂ nanoparticles are distributed on the MoS₂ layers (Fig. 4d). Fig. 4e and f shows the TEM and high resolution transmission electron microscopy (HRTEM) images of the TiO₂–MoS₂ nanohybrid. It can be clearly seen that MoS₂ nanosheets retain the original morphology despite a slight shrinkage after calcination (Fig. 4e). And the inter-planar distances are measured to be around 0.62 nm and 0.35 nm, corresponding to the (002) plane of hexagonal MoS₂ and the (101) plane of anatase TiO₂. The inset of Fig. 4f corresponding to the FFT pattern demonstrates that the MoS₂ layers grow along the (002) direction.

Fig. 4 (a) SEM and (b) TEM images of the as-prepared TiO₂–MoS₂ nanohybrid synthesized after a hydrothermal process. (c) SEM and (d) high-magnification SEM images of the TiO₂–MoS₂ nanohybrid after annealing at 500 °C in an Ar atmosphere. (e) TEM and (f) HRTEM images of the TiO₂–MoS₂ nanohybrid and corresponding fast Fourier Transform (FFT) patterns obtained from the HRTEM images.

Fig. 5a shows the cyclic voltammograms (CVs) of the TiO₂–MoS₂ nanohybrid for the 1st, 2nd, and 3rd cycles in the potential window of 0.01–3 V vs. Li⁺/Li. During the 1st cycle, the reduction peak at 1.1 V suggests the presence of the lithium insertion mechanism, which is attributed to the insertion of lithium ions into the MoS₂ to form Li_xMoS₂. Another reduction peak at 0.57 V is attributed to the conversion of Li_xMoS₂ into metallic Mo and Li₂S. These peaks disappear in the 2nd and 3rd cycles resulting from few amorphous MoS₂ reformed after the first charge process.²² The oxidation peaks at 1.7 V and 2.3 V correspond to the lithium extraction process and the transformation of Mo to MoS₂, respectively. The peaks at 1.75 V in the cathodic sweep and 2.07 V in the anodic sweep can be ascribed to the discharge–charge process of TiO₂: TiO₂ + x (Li⁺ + e⁻) ↔ Li_xTiO₂ (0 < x < 1). The broad reduction peak at 1.8 V may be attributed to the insertion of lithium ions into the TiO₂. Fig. 5b shows the voltage profiles of the TiO₂–MoS₂ nanohybrid during the 1st, 2nd, and 50th cycles at a current density of 100 mA g⁻¹ at room temperature. In agreement with previous reports,²⁶ two voltage plateaus at around 1.1 V and 0.57 V are observed in the discharge process of the first cycle. The former was ascribed to the Li insertion reaction that led to the formation of Li_xMoS₂, however, the latter at 0.57 V was related to a reduction process, in which MoS₂ was reduced to Mo particles embedded in a LiS₂ matrix. The first discharge and charge capacities are 827 mA h g⁻¹ and 638.6 mA h g⁻¹, respectively, corresponding to a Coulombic efficiency of 74%, which may be due to the formation of a gel-like polymeric layer. The charge and discharge capacities in the second cycle are 643.6 mA h g⁻¹ and 674.4 mA h g⁻¹, respectively, showing a Coulombic efficiency of 95%. The capacity loss might arise from the irreversible reactions during the discharge/charge processes. As shown in Fig. 5c, the reversible capacity of TiO₂–MoS₂ retained at 604 mA h g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. From the second cycle onwards, the TiO₂–MoS₂ nanohybrid exhibited a Coulombic efficiency of approximately over 95%. The fresh electrode and the used electrode (after 100 cycles) are observed by SEM (Fig. S1, see ESI†). The MoS₂ nanosheets are difficult to be distinguished and many spherical nanoparticles are observed on the surface of the fresh electrode. It may be attributed to the existence of acetylene black and binder in the electrode.²⁷ Comparison of Fig. S1a and b (ESI†) reveals no obvious change in morphologies, indicating that the TiO₂–MoS₂ nanohybrid architecture is beneficial to the stable cycling performance. The initial capacity of TiO₂–MoS₂ nanohybrid is lower than that of pure MoS₂, which is mainly as a consequence of the influence of TiO₂. However, the cycling stability is preferable compared to that of pure MoS₂. Fig. 5d displays the rate performance at various densities. At a current density of 1000 mA g⁻¹, the capacity of the TiO₂–MoS₂ nanohybrid is 472.14 mA h g⁻¹. When the current density returns to 50 mA g⁻¹, the TiO₂–MoS₂ nanohybrid still delivers a capacity of 601.9 mA h g⁻¹.

Fig. 5 (a) Cyclic voltammograms (CVs) at a scan rate of 0.1 mV s⁻¹ for the 1st, 2nd, and 3rd cycle of the TiO₂–MoS₂ nanohybrid. (b) Charge–discharge voltage profiles at a current density of 100 mA g⁻¹ of the TiO₂–MoS₂ nanohybrid. (c) Cycling performance of the TiO₂–MoS₂ nanohybrid and MoS₂ electrodes, and Coulombic efficiency of the TiO₂–MoS₂ nanohybrid at a current density of 100 mA g⁻¹. (d) Rate behavior of the TiO₂–MoS₂ nanohybrid at different current densities.

Fig. 6 shows the EIS and the equivalent circuit model of the studied system. Re represents the resistance contribution from the electrolyte, electrode and the passive film between them. R_ct and CPE are associated with the charge-transfer resistance, Z_w is associated with the Warburg impedance. For the MoS₂–TiO₂ nanohybrid, the resistance Re and R_ct fitted by ZView software are 1.84 Ω and 133.7 Ω, which are significantly lower than those of MoS₂ (2.35 Ω and 162.3 Ω). This result further demonstrates that incorporation of TiO₂ can greatly enhance the conductivity of the MoS₂–TiO₂ nanohybrid electrode and greatly enhance rapid electron transport during the electrochemical lithium insertion–extraction reaction, resulting in significant improvement in the electrochemical performances.

Fig. 6 (a) Nyquist plots of the TiO₂–MoS₂ nanohybrid and MoS₂ electrodes obtained by applying a sine wave with an amplitude of 5.0 mV over the frequency range from 0.01 Hz and 100 kHz. (b) An equivalent circuit model of the studied system.

Conclusions

In summary, the TiO₂–MoS₂ nanohybrid has been successfully synthesized by a facile hydrothermal process and post-annealing in an Ar atmosphere at 500 °C for 4 h. Transmission electron microscopy and field-emission scanning electron microscope images demonstrate that TiO₂ particles are uniformly distributed on MoS₂ nanosheets. Electrochemical evaluation showed better cycling stability of TiO₂–MoS₂ than that of pure MoS₂ clusters, which may be due to the unique structure of the TiO₂–MoS₂ nanohybrid. The addition of anatase TiO₂ nanoparticles greatly facilitates electron and ionic transport and accommodates the volume change of MoS₂ during the charge–discharge process.^28,29 Besides, the excellent rate performance may be due to the large interfacial contact area with the electrolyte and shortened Li-ion insertion distances. The present results suggest that the TiO₂–MoS₂ nanohybrid is a promising candidate for anode material in LIBs.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21163020) and Excellent Talents in Xinjiang Province (2013721015).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nj01451k

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