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Rare Metals2018年第2期

Carbon and few-layer MoS₂ nanosheets co-modified TiO₂ nanosheets with enhanced electrochemical properties for lithium storage

Hui-Hui Lu Chun-Sheng Shi Nai-Qin Zhao En-Zuo Liu Chun-Nian He Fang He

Tianjin Key Laboratory of Composite and Functional Materials,Tianjin University

Collaborative Innovation Center of Chemical Science and Engineering

收稿日期：5 November 2016

基金：financially supported by the National Natural Science Foundation of China(No.51472177);the China-EU Science and Technology Cooperation Project (No.SQ2013ZOA100006);

Carbon and few-layer MoS₂ nanosheets co-modified TiO₂ nanosheets with enhanced electrochemical properties for lithium storage

Hui-Hui Lu Chun-Sheng Shi Nai-Qin Zhao En-Zuo Liu Chun-Nian He Fang He

Tianjin Key Laboratory of Composite and Functional Materials,Tianjin University

Collaborative Innovation Center of Chemical Science and Engineering

Abstract：

Carbon and few-layer MoS₂ nanosheets comodified TiO₂ nanocomposites(defined as MoS₂-C@TiO₂)were prepared through a facile one-step pyrolysis reaction technique. In this unique nanostructure, the TiO₂ nanosheets with stable structure serve as the backbones, and carbon coating and few-layer MoS₂ tightly adhere onto the surface of the TiO₂. It needs to be pointed out that the carbon coating improves the overall electronic conductivity and the few-layer MoS₂ facilitates the diffusion of lithium ions and offers more active sites for lithium-ion storage. As a result, when evaluated as lithium-ion battery anodes, the MoS₂-C@TiO₂ nanocomposites exhibit markedly enhanced lithium storage capability compared with pure TiO2. A high specific capacity of 180 mA·h·g^-1 has been achieved during the preliminary cycles, and the specific capacity can maintain 160 mA·h·g^-1 at a high current density of 1 C(1 C=167 mA·g^-1) even after 300 discharge/charge cycles, indicating the great potential of the MoS₂-C@TiO₂ on energy storage.

Keyword：

Few-layer MoS₂ nanosheets; Carbon; Co-modified; TiO₂ nanosheets; Lithium-ion battery anodes;

Author： Chun-Sheng Shi,e-mail:csshi@tju.edu.cn;

Received： 5 November 2016

1 Introduction

Rechargeable lithium-ion batteries (LIBs),which possess higher power density and longer cycle life,have been regarded as one of the most promising energy storage devices for electrical vehicles [ 1, 2, 3] .Although graphite has been widely used as commercial anode material of LIBs,it still exposed serious safety and cycling issues.The operating voltage of graphite is below 0.2 V (vs.Li⁺/Li),which is close to the lithium plating voltage,and that would cause serious safety issues [ 4, 5, 6] .In this regard,titanium dioxide(TiO₂),with a high operating voltage of 1.7 V (vs.Li⁺/Li),has been considered as a promising candidate for anode materials on account of avoiding the formation of lithium dendrites.Besides,the volume variation of TiO₂ during the discharge-charge process is quite low (<4%),which indicates the inherent structural stability of TiO₂.In addition,TiO₂ has the advantages of low cost,abundance and environmental friendliness.However,as a semiconductor material,the electronic and ionic conductivity of TiO₂(anatase) is relatively poor,thus leading to the low electronic transfer and ion diffusion efficiency,which would severely decrease the electrochemical properties for LIB s.

To overcome above obstacles,fabricating TiO₂-based nanocomposites is proved to be a common and efficient way via combining with various conductive additives,such as metals [ 7, 8, 9] ,metal oxides [ 10] and carbonaceous materials [ 11, 12, 13] .Among them,carbon is widely used due to its outstanding electronic conductivity and economical feasibility.Several carbon-added TiO₂ composites [ 13, 14, 15, 16] were reported and utilized as the anode materials of LIB s,and those carbon-added composites did exhibit an improved cycle retention and rate performance,which proved that the addition of carbon actually promotes the conductivity of the whole architecture.Furthermore,carbon can protect TiO₂ from the direct contact with electrolyte,which further improves the structure resistance to electrode pulverization and material invalidation.However,the relatively low specific capacity of anatase TiO₂(167 mA·h·g-1) is still a challenge to satisfy the requirements for high-power density and high-energy density LIBs.Up to now,numerous efforts have been made to solve this problem.

Molybdenum disulfide (MoS2),typical layered transition metal dichalcogenides (TMDs),has attracted great attention as a promising electrode material due to its high theoretical specific capacity and unique two-dimensional layered structure where hexagonal layers of Mo are stuck in two S layers and held together by strong covalent forces,while the MoS₂ lamella is bonded by weak van der Waals interactions [ 17] .The special structure can enable fast insertion/extraction kinetics of lithium ions.In addition,recent investigations revealed that the few-layer MoS₂ exhibits distinctively physical and chemical properties compared to the bulk materials [ 18, 19, 20, 21, 22] ,such as luminescence quantum efficiency [ 18] ,high on-off ratio [ 19] and strong photoluminescence [ 20] .When evaluated as the anode material for LIBs,the bulk MoS₂ would suffer huge volume expansion,which results in severe structural deformation and unexpected pulverization,thus causing a quite poor cycling performance,while the few-layer MoS₂ configuration can keep their original structure and become more stable during the discharge/charge cycles due to the ultrathin 2D flexible nanostructure.Therefore,it is believed that the incorporation of MoS₂ nanosheets into TiO₂ can remarkably improve its reversible capacity without sacrificing the cycle stability,if the microstructure of MoS₂ is confined to few layers.In fact,MoS₂/TiO₂ composites have been successfully synthesized by Zhuang et al. [ 22] ,which exhibited the high specific capacity and notable electrochemical performance.However,due to the lack of conductive additives,the specific capacity still suffers from a certain degree of fading.Therefore,it is quite meaningful to develop a facile and effective strategy to simultaneously improve the cycle stability and lithium storage capability of TiO₂.Besides,up to now,it is still difficult to fabricate controllable,uniform and scalable TiO₂-based composites due to the weak chemical interaction between chemically stable TiO₂ and other agents.To improve inherent chemical activity,the TiO₂skeleton was recently constructed into mesoporous or rough structure to provide more accessible active sites for the nucleation and growth of carbon and MoS₂ nanosheets [ 10, 23, 24, 25, 26] ,such as fluorinated TiO₂ (defined as F-TiO₂).

Herein,carbon and few-layer MoS₂ nanosheets co-modified F-TiO₂ nanosheets (defined as MoS₂-C@TiO₂) were prepared via a simple and facile one-step pyrolysis reaction technique.The microstructure and electrochemical characteristics of MoS₂-C@TiO₂ nanocomposites were systematically investigated.F-TiO₂ nanosheets provide a stable framework in the nanocomposites which allows the uniform dispersion of C and MoS₂ on their surface.Importantly,glucose serves as carbon source to improve the electrical conductivity and protect the TiO₂ electrode from the direct contact with electrolyte.Furthermore,glucose also is regarded as binder to help few-layer MoS₂ nanosheets grow on the surface of TiO₂.Besides,few-layer MoS₂ nanosheets facilitate a fast insertion/extraction of lithium ions and offer more active sites to improve the specific capacity.When evaluated as the anode material of lithium-ion batteries,the synthesized MoS₂-C@TiO₂ nanocomposites exhibit high specific capacity and excellent cyclic performance.

2 Experimental

2.1 Preparation of fluorinated TiO2 nanosheets

F-TiO₂ nanosheets were prepared using a hydrothermal method reported by Chen et al. [ 10] and Han et al [ 25] .Typically,12.5 ml tetrabutyl titanate coupled with 1.5 ml hydrofluoric acid solution (40 wt%) was sealed in a 50-ml Teflon-lined stainless steel autoclave and kept at 200℃for 24 h.After cooling down to room temperature,the obtained products were washed with distilled water and ethanol several times,and then dried at 60℃in the air.

2.2 Synthesis of MoS2-C@TiO2 nanocomposites

MoS2-C@TiO2 nanocomposites were prepared by using a facile one-step pyrolysis reaction technique.In a typical synthesis process,200 mg as-obtained F-TiO₂ nanosheets and 100 mg glucose (C₆H₁₂O₆) were dispersed uniformly into 200 ml deionized water by ultrasonication.Then,22.1 mg sodium molybdate (Na₂MoO₄·2H₂O) and 22.8 mg thiourea (CN₂H₄S) were added to the above liquid under magnetic stirring to obtain a homogeneous suspension solution.After removing the water of the suspension by freeze-drying technique,the product was ground into fine composite powder.Finally,the as-obtained composite powder was further subjected to calcination at 800℃under the protection of Ar with a heating rate of 10℃·min^-1 to synthesize MoS₂-C@TiO₂ nanocomposites.For comparison,under the same preparation conditions of MoS₂-C@TiO₂,MoS₂-TiO₂ and C-TiO₂ were also synthesized by calcining the freeze-dried precursor gel without the addition of C₆H₁₂O₆ and Na₂MoO₄·2H₂O-CN₂H₄S,respectively.

2.3 Characterizations

Morphologies of the samples were observed by field emission scanning electron microscope (FESEM,HITACHI S4800) and high-resolution transmission electron microscope (HRTEM,JEOL JEM-2100f).The energy-dispersive X-ray (EDX) mapping was performed along with TEM.X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Bruker,D8 Advanced).Thermogravimetric(TG) analysis was performed with a PerkinElmer (TA Instruments) up to 700℃at a heating rate of 10℃·min^-1 in air.X-ray photoelectron spectroscopy (XPS) analyses were carried out on a PHI-5000 Versa Probe.And the BrunauerEmmett-Teller (BET) surface areas were measured by nitrogen adsorption isotherms using an autosorb-iQ instrument (Quantachrome,USA).The pore volume,pore diameter and pore size distribution data were calculated using the Barrett-Joyner-Halenda (BJH) method.

2.4 Electrochemical measurements

The electrochemical tests were performed using two-electrode coin-type cells.The coin cells (CR2032) were assembled in an Ar-filled glove box,where lithium metal was used as cathode,Celgard 2400 polypropylene as separator and LiPF₆ (1 mol·L^-1) dissolved in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC,volume ratio of 1:1:1) as electrolyte.The working electrode slurry was prepared by mixing the active materials,conductive agent (carbon black) and binder(polyvinylidene fluoride,PVDF) in N-methyl-2-pyrrolidinone (NMP) with a mass ratio of 70:15:15.The homogeneous slurry was spread onto a copper foil and then dried in a vacuum oven at 80℃.Cyclic voltammetry (CV) measurement was taken by using a CHI660E electrochemical workstation in the voltage range of 0.5-3.0 V (vs.Li⁺/Li)at a scan rate of 0.1 mV·s^-1.The galvanostatic discharge and charge measurements (GDC) of the batteries were taken using a battery testing system (LAND CT 2001 A,China) at different current densities between 0.5-3.0 V (vs.Li⁺/Li) at room temperature.Electrochemical impedance spectroscopy (EIS) was recorded on a Gamry Interface1000 electrochemical workstation in the frequency range from 100 kHz to 100 mHz.All of the specific capacities were calculated on the basis of the total weight of MoS₂-C@TiO₂,MoS₂-TiO₂ and F-TiO₂.

3 Results and discussion

3.1 Morphology and structure

The fabrication procedure of the MoS₂-C@TiO₂nanocomposites is schematically depicted in Fig.1.Firstly,the F-TiO₂ nanosheets are homogeneously dispersed in the C₆H₁₂O_6-Na₂MoO_4-CN₂H₄S aqueous solution (Fig.1a),where F-TiO₂ preferentially adsorbed glucose molecules via F abundantly contained on the surface (Fig.1b).Then, was tightly adsorbed onto F-TiO₂ surface due to the abundant functional groups in adsorbed glucose (i.e.,aldehyde and hydroxyl groups)(Fig.lc) [ 10, 26, 27, 28, 29, 30, 31, 32] .During the pyrolysis process,MoO₄^2-is converted into MoS₂ by reaction with H₂S released from CN₂H₄S under the heat treatment.Meanwhile,C₆H₁₂O₆ is decomposed into C during the calcination process,thus forming C and few-layer MoS₂ co-modified TiO₂ nanosheets (Fig.1d).

Figure 2 exhibits XRD patterns,SEM and TEM images of the obtained F-TiO₂ and MoS₂-C@TiO₂ nanocomposites.Figure 2a shows that the diffraction peaks of F-TiO₂agree well with the standard XRD pattern of anatase TiO₂(JCPDS card No.21-1272).After the reaction,the MoS₂-C@TiO₂ nanocomposites do not exhibit typically diffraction peaks of C and MoS₂,although C and MoS₂ have been successfully prepared according to SEM and TEM results.This phenomenon is attributed to the small content of MoS₂ and C (seen in subsequent XPS results) [ 21] .The pristine F-TiO₂ displays a uniform sheet-like structure,where the length is in the range of 40-50 nm and the thickness is about 10-20 nm (Fig.2b,d).From TEM image of the F-TiO₂ nanosheets (Fig.2d),it can be observed that the F-TiO₂ nanosheets are highly crystallized.After the pyrolysis process,the as-prepared MoS₂-C@TiO₂nanocomposites still exhibit a pore configuration constructed by abundant nanosheets,which is similar to F-TiO₂ sample (Fig.2c).However,the surface of TiO₂nanosheets in the nanocomposites becomes rougher,indicating the successful growth of carbon and MoS₂ onto their surface.The fine structure of the as-prepared MoS₂-C@TiO₂ was further investigated by TEM (Fig.2e,f).The interfacial spacing value of 0.35 nm corresponds to (101)plane of anatase TiO₂,indicating well-defined crystal structure.Meanwhile,it can be clearly observed that the TiO₂ nanosheets are entirely coated by carbon and fewlayer MoS₂.Besides,the face-to-face contact between TiO₂and C as well as C and MoS₂ can effectively pin the fewlayer MoS₂ nanosheets and restrain aggregation,thus ensuring the integrity of the overall structure [ 10] .To further reveal the MoS₂ and C distributions in the MoS₂-C@TiO₂ nanocomposites,EDX element mappings were performed.Figure 2g shows dark-field scanning TEM(STEM) image of MoS₂-C@TiO₂.Figure 2h-1 represents the elemental distributions of C,Ti,O,Mo and S,respectively.Those elements are distributed uniformly within the whole matrix,confirming the formation of MoS₂and C on the surface of TiO₂ nanosheets.

For comparison,the morphologies of the samples C-TiO₂ and MoS₂-TiO₂ are shown in Fig.3.For C-TiO₂,the TiO₂ nanosheets are entirely covered by carbon according to HRTEM image (Fig.3a).For MoS₂-TiO₂composite (Fig.3b),the TiO₂ is separated from MoS₂nanosheets and the obtained MoS₂ nanosheets turn to aggregation.Therefore,the above results further confirm that glucose serves as the binder to help those few-layer MoS₂ nanosheets grow on its surface and prevent MoS₂nanosheets from aggregation.

Fig.1 Schematic illustration of procedure for one-step synthesis of MoS₂-C@TiO₂:a F-TiO₂ nano sheets dispersed in C₆H₁₂O₆-Na₂MoO₄-CN₂H₄S aqueous solution,b absorption of C₆H₁₂O₆ on surface of F-TiO₂ nanosheets,c absorption of Na₂MoO₄-CN₂H₄S precursor on surface of F-TiO₂@C₆H₁₂O₆ nanosheets and d few-layer MoS₂-C@TiO₂ nanocomposites after pyrolysis process

Fig.2 a XRD patterns of F-TiO₂ and MoS₂-C@TiO₂,b SEM images and d TEM image of F-TiO₂,c SEM image and e,f TEM images of MoS₂-C@TiO₂,g STEM image of MoS₂-C@TiO₂,and EDX mapping images of h C,i Ti,j O,k Mo and I S

In order to estimate the content of every component in the MoS₂-C@TiO₂ nanocomposites,TG analysis was carried out firstly (Fig.4).According to the literature reported previously [ 33] ,after the sample was heated to 700℃in air,the remaining products were considered as TiO₂ and MoO₃.Subsequently,the mixed powders were dissolved in0.5 mol·L^-1 NaOH solution to remove MoO₃.By weighing the mass of residual TiO₂,the contents of MoO₃ and TiO₂ can be c alculated.Therefore,it can be estimated that the contents of TiO₂,C and MoS₂ in MoS₂-C@TiO₂ nanocomposites are73.53 wt%,14.12 wt%and 10.72 wt%,respectively.

Nitrogen adsorption measurements were taken at 77 K to examine the surface area and pore size of the F-TiO₂nanosheets and MoS₂-C@TiO₂ nanocomposites (Fig.5a),and the results are summarized in Table 1.The pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method.F-TiO₂ nanosheets exhibit a broad range of pore sizes from 10 to 20 nm,indicating a typical mesoporous structure.After the pyrolysis process,the MoS₂-C@TiO₂ nanocomposites display two kinds of pore,with average pore size of 2 and 4 nm,respectively(Fig.5b).Based on TEM observation,these mesopores imply that the surface of TiO₂ nanosheets is decorated with produced C and MoS₂.Besides,the MoS₂-C@TiO₂nanocomposites exhibit a specific surface area of139.1 m²·g^-1,which is higher than that of F-TiO₂nanosheets (91.4 m²·g^-1).The increased specific surface area suggests more accessible contact with the electrolyte and promotes the transport kinetics of lithium ions in the MoS₂-C@TiO₂ nanocomposites.

XPS tests were carried out to investigate the chemical characteristics of the surfaces of F-TiO₂,C-TiO₂ and MoS₂-C@TiO₂ (Fig.6).The spectra show that F element is only presented in F-TiO₂ (Fig.6a),and F in TiO₂ would easily be removed by heat treatment [ 34, 35] .The Mo,S and C elements appear in MoS₂-C@TiO₂ along with Ti and O elements(Fig.6a),which agree well with STEM results above.The binding energies of Mo 3d_3/2,Mo 3d_5/2,S 2p_1/2 and S 2p_3/2peaks (Fig.6b,c) are located at 232.8,229.5,163.7 and162.4 eV,respectively,in agreement with those of the MoS₂/C and MoS₂/TiO₂ composites reported previously [ 10, 21, 22, 30, 36, 37] .Moreover,the peak positions of corelevel spectrum of O 1s (Fig.6d) are observed at 529.8,530.5and 531.6 eV corresponding to the Ti-O bond in TiO₂,Ti-O-C bond between TiO₂ and carbon,and C-O bond in carbon,respectively.Among them,the peak at 530.5 eV directly demonstrates the presence of Ti-O-C bond at the interface.Based on XPS analyses,it can be confirmed that strong interfacial bonding are formed between MoS₂ and C,as well as C and TiO₂ interface in the MoS_2-C@TiO₂ nanocomposites.The strong interfacial contact is considered to be beneficial for the rate performance,which enables fast transport of both electrons and ions in the overall electrode [ 10] .

Fig.4 TG analysis profile of MoS₂-C@TiO₂

3.2 Electrochemical performance

LIBs are the state-of-the-art energy storage devices for electric vehicles and hybrid electric vehicles.TiO₂ and its derived nanocomposites have been considered as the promising active materials for lithium-ion battery anodes,on account of their cycle stability and service safety.Therefore,the as-prepared MoS₂-C@TiO₂ nanocomposites have been performed for LIBs anodes as a proof-of-concept application.The lithium storage mechanism of the MoS₂-C@TiO₂ nanocomposites was explored by CV curves,as shown in Fig.7a.During the discharge process,there are three cathodic peaks at the initial cycle.The peak located at1.7 V is attributed to the intercalation of Li⁺into TiO₂,resulting in the formation of Li_xTiO₂ (Reaction 1) [ 24] .The peak located at 1.24 V corresponds to lithium ions into the interlayer space of MoS₂ (Reaction 2),while this peak disappears in the second and third discharge processes.A broad peak at around 0.79 V can be assigned to the intercalation of Li⁺in the expanded layer structure or in the defect sites of MoS₂ [ 38, 39, 40] .In the following process,the intermediate Li_xMoS₂ decomposes into metallic Mo nanoparticles embedded into the Li₂S matrix (Reaction (3)) [ 22, 41, 42, 43, 44] .The disappearance of the two peaks at 1.2 and0.79 V in the subsequent cycles suggests the irreversible reaction of MoS₂ upon full lithium storage.After undergoing the substantial structural and compositional variation,the following cycles remain stable and are almost overlapped.

Fig.3 TEM images of a C-TiO₂ and b MoS₂-TiO₂

Fig.5 a N₂ adsorption-desorption isotherms and b pore size distribution of F-TiO₂ and MoS₂-C@Ti0₂ (V_ads,volume adsorbed;p/p₀,relative pressure;V_p,pore volume;D_p,average pore diameter)

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Table 1 Specific surface area and pore parameters of F-Tio₂ and MoS₂-C@Tio₂

Fig.6 a XPS full-spectra of F-TiO₂,C-TiO₂ and MoS₂-C@TiO₂;high-resolution XPS spectra of b Mo 3d,c S 2p and d O 1s peak in MoS₂-C@TiO₂

Fig.7 a CV curves at a scan rate of 0.1 mV·s^-1 for the first,second and third cycles of MoS₂-C@TiO₂ over a voltage window of 0.5-3.0 V;b discharge-charge profiles of MoS₂-C@TiO₂ at a current density of 1.0C (1.0C=167 mA·g^-1);c cycling performance of TiO₂,C-TiO₂,MoS₂-TiO₂ and MoS₂-C@TiO₂ nanocomposites;d rate capability of electrode of MoS₂-C@TiO₂ nanocomposites;e capacity retention of MoS₂-C@TiO₂ electrode at current densities of 0.1,0.2 and 0.5 A·g^-1 for initial four cycles and then 1 A·g^-1 for subsequent 1200 cycles

In the charge process,the unconspicuous anodic peak at1.66 V corresponds to the partial oxidation of Mo,as a result of the small amount of MoS₂ in the nanocomposite.Such a phenomenon is in agreement with the previous reports about few-layer MoS₂-based anode materials [ 21] .The obvious anodic peak which appears at 2.08 V should be attributed to the extraction of Li⁺from the Li_xTiO₂,accompanying the formation of TiO₂ and the oxidation of Li₂S to S [ 21, 24] .Figure 7b displays the galvanostatic discharge-charge (GDC) profiles of MoS₂-C@TiO₂ for the different cycles at a current density of 1C(1C=167 mA·g-1).As can be seen,two dominant voltage plateaus appear at 1.70 and 2.08 V during the discharge and charge processes,which correspond to the lithium insertion and extraction in TiO₂,respectively,in good agreement with previous reports.In the discharge profile,the plateau at 1.24 V should be ascribed to lithium insertion into the interlayer space of MoS₂ to form Li_xMoS₂,while the plateau at 0.79 V is assigned to the decomposition of Li_xMoS₂ to Li₂S and Mo.All voltage plateaus are in good agreement with those of CV curves.Furthermore,it is noteworthy that GCD curves overlapped from the 2nd cycle to the 300th cycle,further demonstrating the excellent structural stability of the MoS₂-C@TiO₂ nanocomposites.

The cycle behavior of MoS₂-C@TiO₂ at a current density of 1C is presented in Fig.7c.The MoS₂-C@TiO₂nanocomposites exhibit a relatively high specific capacity coupled with an excellent cycling stability.Unexpectedly,a relatively high capacity of 292 mA·h·g^-1 can be achieved in the first discharge process,while it also delivers an irreversible capacity of 231 mA·h·g^-1 after the electrode was fully charged to 3 V at the initial cycle,revealing a high initial Coulombic efficiency (CE) calculated to be80%.The irreversible capacity loss is caused by the decomposition of electrolyte.In spite of the fact that the initial CE is 80%,it rapidly increases to above 95%from the fourth cycle and remains above 99%in the following cycles.Importantly,the MoS₂-C@TiO₂ still can deliver a high specific capacity of 160 mA·h·g^-1 even after 300 fast discharge/charge cycles,with a capacity retention of 89%compared with the stable capacity recorded from the second cycle,which indicates a quite low average capacity fading of 0.04%per cycle.In contrast,the cycle behaviors of TiO₂,C-TiO₂ and MoS₂-TiO₂ at a current density of 1C are also displayed.From the profiles,the specific capacity of pure TiO₂ nanosheets is about 90 mA·h·g^-1 after 300cycles and the specific capacity of C-TiO₂ does not show an obvious improvement.The C-TiO₂ exhibits an initial discharge and charge capacity of 137 and 100 mA·h·g^-1,while the reversible capacity fades to 115 mA·h·g^-1 after130 cycles.Besides,the capacity of MoS₂-TiO₂ composites also demonstrates a significant capacity decrease from 170to 100 mA·h·g^-1 after 300 cycles,with a capacity retention of only 58.9%(compared with the initial discharge capacity).It should be attributed to the agglomeration of MoS₂ nanosheets during the lithiation/delithiation processes,which could cause the remarkably decline of active sites and destruction of 2D microstructure,finally resulting in the severe electrode pulverization as well as material elimination.

The rate performance of MoS₂-C@TiO₂ nanocomposites was also measured,as presented in Fig.7d.When the current densities increase from 0.5C to 1.0C,2.0C,5.0C and 10C,the reversible specific capacities of the electrode of MoS₂-C@TiO₂ are 189,159,140,116 and101 mA·h·g^-1,respectively.Additionally,it is noteworthy that when the current density reverts to 1.0C from high rates,the capacity can still deliver a high level of150 mA·h·g^-1 and maintains stable in the subsequent 50cycles.The aforementioned experimental data give a proof of the fact that our MoS₂-C@TiO₂ nanostructures do exhibit an excellent rate capability.

To better elucidate the cycle stability at high current density,the MoS₂-C@TiO₂ nanocomposites were tested at1 A·g^-1as presented in Fig.7e.As can be seen,after the activation process of electrode,the MoS₂-C@TiO₂nanocomposites exhibit a reversible specific capacity of75 mA·h·g^-1 at the 4th cycle.Unexpectedly,the specific capacity is quite stable in the subsequent cycles.Even after1200 deep discharge/charge cycles,a specific capacity of73 mA·h·g^-1 still can be achieved,showing a capacity retention of near 100%when compared with the capacity of 75 mA·h·g-1 recorded at the 4th cycle.The above results adequately demonstrate that the MoS₂-C@TiO₂nanocomposites possess excellent structural stability.

Electrochemical impedance spectrum (EIS) analyses were performed to further investigate the reaction kinetics associated with TiO₂ and MoS₂-C@TiO₂ nanocomposites for lithium storage,as shown in Fig.8a.It is found that the MoS₂-C@TiO₂ electrode shows a much smaller radius of semicircle in the high-medium-frequency region compared to TiO₂ electrode,which suggests that MoS₂-C@TiO₂nanocomposites possess the faster diffusion efficiency of lithium ions together with the shorter transportation pathways of electrons.Moreover,the relationship curve between the real part of the impedance (Z’) and the inverse square root of angular frequency (ω^-1/2) in the low-frequency region describes the mass transfer process in electrode materials,and a low slope of this relationship curve indicates fast Li⁺diffusion kinetics in the electrode [ 45] .As shown in Fig.8b,MoS₂-C@TiO₂ electrode shows the more placid linear fitting slope than TiO₂.It is believed that the improved reaction kinetics is an important key factor that should be responsible for the enhanced electrochemical performance of MoS₂-C@TiO₂.

Fig.8 a EIS curves of F-TiO₂ nanosheets and MoS₂-C@TiO₂ nanocomposites and b linear fits of relationship between Z’andω^-1/2 in low-frequency region of F-TiO₂ nanosheets and MoS₂-C@TiO₂ nanocomposites (Z”,Z’andωdenote imaginary part,real part of the impedance and angular frequency,respectively)

According to the above results,it can be deduced that the co-modified synergistic effect and the structural advantages of the MoS₂-C@TiO₂ nanocomposites are contributed to their excellent electrochemical performance.On the one hand,TiO₂ nanosheets provide a stable framework to stabilize the whole nanostructure,while the carbon coating layer improves the electrical conductivity and prevents TiO₂ nanosheets from the direct contact with the electrolyte.On the other hand,the few-layer MoS₂nanosheets facilitate a fast insertion/extraction of lithium ions and offer more active sites to improve the specific capacity.The enhanced electronic conductivity and ionic conductivity should be originated from the co-modified effect of carbon and few-layer MoS₂ nanosheets.As a result,when evaluated as a lithium-ion battery anode,the MoS₂-C@TiO₂ nanocomposite electrodes exhibit a markedly enhanced lithium storage capability and excellent cycle performance.

4 Conclusion

In summary,a facile one-step pyrolysis reaction method was successfully developed to prepare carbon and fewlayer MoS₂ nanosheets co-modified TiO₂ nanocomposites,which are considered to be of great benefit to improve both the electronic conductivity and ionic conductivity of TiO₂.The as-prepared MoS₂-C@TiO₂ nanocomposites demonstrate a uniform double-coated structure with high surface areas and uniform mesopores.Importantly,when evaluated as a LIB anode,the nanocomposites exhibit improved specific capacity and cycle stability.A relatively high capacity of 292 mA·h·g^-1 can be achieved in the first discharge process,and it also delivers an irreversible capacity of 231 mA·h.g^-1 after the electrode was fully charged to 3 V at the initial cycle with a high initial CE of about 80%.The specific capacity decreases slightly from180 to 160 mA·h·g^-1 at the current density of 1.0C after300 cycles,revealing a superior cycle stability.The excellent electrochemical properties of MoS₂-C@TiO₂nanocomposites indicate their great potential in energy storage devices.