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Scalable product

aLaboratory of Advanced Materials (Ministr

Engineering, Tsinghua University, Beijing

tsinghua.edu.cn; xxm-dce@mail.tsinghua.e

10 62773607bHeilongjiang River Fishery Research Institu

Harbin 150070, ChinacAcademy of Fundamental and Interdisc

Technology, Harbin 150080, China. E-mail:

† Electronic supplementary information (the exfoliated 2D nanosheets. See DOI: 10

‡ These authors contributed equally to th

Cite this: RSC Adv., 2014, 4, 41543

Received 12th June 2014Accepted 27th August 2014

DOI: 10.1039/c4ra05640j

www.rsc.org/advances

This journal is © The Royal Society of C

ion of transition metal disulphide/graphite nanoflake composites for high-performance lithium storage†

Zhi-Qiang Duan,‡a Yan-Chun Sun,‡b Yi-Tao Liu,*a Xu-Ming Xie*a

and Xiao-Dong Zhu*c

A facile, industrially viable strategy is proposed for the scalable production of transition metal disulphide/

graphite nanoflake composites by a combination of ball milling and short-time sonication. The

experimental conditions are mild and energy efficient, and the yields are fairly high. This strategy can

produce larger MoS2 and WS2 nanoflakes with more lithium storage sites than the conventional, long-

time sonication method. Besides, the obtained graphite nanoflakes have a higher degree of lattice

integrity than reduced graphene oxide that is structurally permanently damaged, and can thus serve as a

high-efficiency conductive additive. A prominent synergy is witnessed between the excellent

electrochemical performances of the MoS2 and WS2 nanoflakes and the high electronic conductivity of

the graphite nanoflakes. The resulting MoS2 and WS2/graphite nanoflake composites exhibit superior

lithium storage capacities, cycling stabilities and rate capabilities, thus providing a basis for developing

high-performance anodes of next-generation lithium-ion batteries.

1. Introduction

Nowadays the global energy and environmental problems arebecoming increasingly obvious and serious. Therefore, how todevelop clean and renewable energy sources is an urgent task oftoday. The lithium-ion battery (LIB), among other candidates, isan ideal driving force for portable electronics due to its highenergy density, low self-discharging rate and long service life.1–4

However, the ever-increasing demands for large-size power tools,such as electric vehicles and power grids, set higher standardson the energy density and safety of the secondary power systems.As such, the present anode material of LIBs, i.e., graphiticcarbon, is not satisfactory due to its low theoretical capacity(372 mA h g�1). Moreover, the lithium dendrites generated onthe surface of the graphitic carbon anodes during the fastcharging processes raise serious safety concerns. In this sense,nding suitable anode substitutes with better electrochemical

y of Education), Department of Chemical

100084, China. E-mail: liu-yt03@mails.

du.cn; Fax: +86 10 62784550; Tel: +86

te, Chinese Academy of Fishery Sciences,

iplinary Sciences, Harbin Institute of

zxd9863@163.com

ESI) available: Supplementary gures of.1039/c4ra05640j

is work.

hemistry 2014

performances is a challenging yet promising competition inwhich researchers all over the world are involved.

Transition metal disulphides, such as MoS2 and WS2, aretypical members of a huge layered compound family.5,6 In thislayered structure, the transition metal and sulphur atoms aretightly bonded by strong covalent forces to form layers, whilethe adjacent layers are loosely bound by weak van der Waalsforces. When Li+ ions diffuse into the galleries between layers, aredox reaction occurs according to the following mechanism(M ¼ Mo or W):

1st discharge

MS2 + xLi+ + xe� / LixMS2 (1)

LixMS2 + (4 � x)Li+ + (4 � x)e� / M + 2Li2S (2)

1st charge

2Li2S 4 S + 2Li+ + 2e� (3)

Aer exfoliation, the resulting MoS2 and WS2 nanoakespossess larger gallery spacings and shorter diffusion path-ways, which translate into higher lithium storage capacitiesthan the bulk forms. Recently novel anode materials basedon MoS2 and WS2 nanoakes, as promising substitutes forgraphitic carbon, have been extensively exploited.7–17 It isworth noting, however, that although MoS2 and WS2 havesuperior ionic conductivity, their extremely low electronicconductivity inevitably leads to poor cycle and rate

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performances, which largely hamper their practical applica-tions in next-generation LIBs.

To overcome this embarrassment, researchers are endeav-oured to dope MoS2 and WS2 with conductive additives, therebyfabricating various conductive composites to improve theirelectronic conductivity. Carbonaceous materials, such as amor-phous carbon18–21 and carbon nanotubes,22–27 are themost widelyemployed conductive additives. Very recently attempts havebeen made on the in situ hydrothermal synthesis of conductivecomposites of MoS2 or WS2 and reduced graphene oxide(r-GO).28–40 It is found that the cycle stabilities and rate capabil-ities of the MoS2 or WS2/r-GO composites are enhanced becauser-GO can facilitate easy electron transport. However, the rig-ourous experimental conditions and low yields associated withthe in situ hydrothermal synthesis set limitations to the scalableproduction of the MoS2 or WS2/r-GO composite anodes. Toaddress this issue, an alternative method involving simplephysical mixing of pre-synthesised MoS2 or WS2 and r-GO hasbeen employed by ourselves41 and others.42 Whereas, the biggestproblem regarding the use of r-GO as a conductive additive liesin its irreversibly damaged structure due to the oxidation–reduction process. The reported electronic conductivity of r-GOvaries from 0.17–3.05 S cm�1 (reduced by NaBH4),43,44 20 S cm�1

(reduced by Al powder),45 26.9–77 S cm�1 (reduced by vitaminC),46 to 41–99 S cm�1 (reduced by hydrazine hydrate).46 Thesevalues are rather inadequate and non-uniform, making r-GO aninferior conductive additive to graphite nanoakes that have aperfect hexagonal symmetry.

Based on our experience on the top-down liquid-phaseexfoliation of graphite and transition metal disulphide nano-akes from their bulk forms,47–50 here we propose a facile,industrially viable strategy for the scalable production of MoS2and WS2/graphite nanoake composites by a combination ofball milling and short-time sonication. The experimentalconditions are mild and energy efficient, and the yields are fairlyhigh compared to the in situ hydrothermal synthesis. Thisstrategy can produce larger MoS2 and WS2 nanoakes withmore lithium storage sites than the conventional, long-timesonication method. Moreover, this strategy can also producegraphite nanoakes with a higher degree of lattice integritythan r-GO, and their electronic conductivity is up to 163 S cm�1

(tested on a pressed pellet by the standard four-probe tech-nique). The resulting MoS2 and WS2/graphite nanoakecomposites exhibit superior lithium storage capacities, cyclestabilities and rate capabilities when evaluated as anodematerials of LIBs, laying a basis for the industrial applicationsof high-performance anodes for next-generation LIBs.

2. Experimental section2.1. Materials and method

Natural graphite powder (particle sizes # 300 mesh) waspurchased from Sinopharm Chemical Reagent Co., Ltd. MoS2andWS2 powders (particle sizes¼�6 mm) were purchased fromSigma-Aldrich Co., LLC. N,N-Dimethylformide (DMF) andN-methylpyrrolidone (NMP) were purchased from BeijingChemical Works. Acetylene black (battery grade) was purchased

41544 | RSC Adv., 2014, 4, 41543–41550

from Shenzhen Luhua Industrial and Trading Co., Ltd. Poly-(vinylidene uoride) (PVDF, KYNAR 761) was purchased fromArkema Inc.

In a typical experiment, natural graphite, MoS2 or WS2powder 2 g and DMF 200 mL were added to a PA6 vial loadedwith ZrO2 balls (6 mm in diameter) 500 g, and milled in aplanetary ball mill (KQM-XH, Xianyang Jinhong GeneralMachinery Co., Ltd.) at 300 rpm for 24 h. The slurries were takenout, mixed at different wt ratios, sonicated at 300 W for 30 min,vacuum-ltered, and dried at 100 �C for 24 h. The resultingcomposites were then annealed at 800 �C in nitrogen for 2 h.

An anode was prepared by coating a copper foil with a slurrycontaining 70 wt% active material, 10 wt% acetylene black and20 wt% PVDF dissolved in NMP. The anode was dried at 120 �Cin vacuum for 12 h, and equipped in a half cell according to theconguration of (�) Li|electrolyte|anode (+) in a vacuum glovebox. The electrolyte is 1 M solution of LiPF6 in ethylenecarbonate–dimethyl carbonate at a vol ratio of 1/1. The sepa-rator is a microporous polypropylene membrane.

2.2. Sample characterisation

Transmission electron microscopy (TEM) and high-resolutionTEM (HRTEM) were performed by a JEOL JEM-2010 microscopeoperated at an accelerating voltage of 120 kV.

Scanning electron microscopy (SEM) was performed by aJEOL JSM-7401F microscope operated at an accelerating voltageof 3.0 kV.

Scanning probe microscopy (SPM) was performed by a Shi-madzu SPM-9700 microscope operated in the tapping mode.

Raman spectroscopy was performed by a Renishaw RM2000spectrometer (l ¼ 514 nm).

X-ray diffraction (XRD) spectroscopy was performed by aPANalytical X'Pert PRO spectrometer with Cu Ka radiation(l ¼ 1.5418 A).

UV/vis spectroscopy was performed by a Persee TU-1810spectrometer.

Brunauer–Emmett–Teller (BET) measurement was per-formed by a Quantachrome Autosorb-1 Canalyser (USA).

3. Results and discussion

The combination of ball milling and short-time sonication canrealise the scalable production of high-quality nanoakes atrelatively low energy and chemicals consumption. Recently,several groups have reported the successful application of ballmilling for exfoliating layered materials in differentsolvents.51–55 The difference lies in the rotary speed and time,which are key to the morphology and size of the nal product.In our case, the ball milling process is aimed at the preliminarythinning of the bulk powders, and the subsequent short-timesonication process is devoted to further breaking them intolarge-area nanoakes. Compared to the conventional, long-timesonication method, this strategy is industrially viable since theexperimental conditions are mild and energy efficient, and theyields are fairly high. A morphological observation of theresulting nanoakes is realised by TEM characterisation (Fig. 1).

This journal is © The Royal Society of Chemistry 2014

Fig. 1 TEM images of (a) graphite, (c) MoS2 and (e) WS2 nanoflakes andthe corresponding ED patterns (insert); HRTEM images of (b) graphite,(d) MoS2 and (f) WS2 nanoflakes.

Fig. 2 Raman spectra of (a) MoS2/graphite and (b) WS2/graphitenanoflake composites at different wt ratios.

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The nanoakes shown in the TEM images have large lateralsizes of 2–5 mm, an advantage over long-time sonication thatoen produces small pieces.5,47–50 As seen from the corre-sponding electron diffraction (ED) patterns, these nanoakesare highly crystalline, suggesting a high degree of latticeintegrity. The HRTEM images reveal that a substantial fractionof the obtained nanoakes are#10 layers. Themeasured galleryspacings of�0.33, 0.65 and 0.64 nm are in good agreement withthe (002) values reported for graphite, MoS2 and WS2, respec-tively.6,56 The atomically resolved HRTEM images conrm thatthe honeycomb structure of the nanoakes is largely preservedwith few, if any, disorders (Fig. S7–S9†). The nature of thenanoakes can be further claried by AFM analysis, as shown inFig. S10.† It can be seen that the nanoakes have a wide sizedistribution ranging from hundreds of nanometres to severalmicrometres. Note that most of the nanoakes, even thosewhose lateral sizes are on the micrometre scale, have thick-nesses#10 nm, which support the HRTEM results. These large,thin MoS2 andWS2 nanoakes are believed to be able to providemore lithium storage sites than the small ones (generally <100nm) produced by conventional, long-time sonication method.5

Raman spectroscopy can provide an insight into the crys-talline nature of the nanoakes and their composites. Fig. 2shows the Raman spectra of these composites at different wtratios. As seen from the gure, the Raman spectrum of the MoS2or WS2 nanoakes is featured by two sharp characteristic peaks,i.e., one at 378.2 or 351.1 cm�1 corresponding to the in-planeE2g vibration, and the other at 403.8 or 415.3 cm�1 corre-sponding to the out-of-plane A1g vibration.6 On the other hand,the graphite nanoakes have two well-known characteristicpeaks, i.e., peak A1g at 1354.0 cm�1 (also known as peak D)related to the symmetric vibration of the six-fold aromatic rings

This journal is © The Royal Society of Chemistry 2014

arising from the structural disorders, and peak E2g at 1580.9cm�1 (also known as peak G) assigned to the symmetric vibra-tion of the sp2-hybridized carbon atoms in the graphiticsheets.47 The integral intensity ratio of peak D to G, ID/IG, is usedas a measure of the structural disorders in a carbon product,that is, the smaller the ID/IG value, the better the lattice integrity.In our experiment, the ID/IG value of the graphite nanoakes isonly 0.185, which is signicantly lower than that of r-GO,41

indicating the former suffer from much fewer structural disor-ders and thus reasonably exhibit a much higher electronicconductivity (163 S cm�1). The Raman spectra of the graphitepowder and nanoakes are presented in Fig. S11.† The prom-inent peak occurring at 2708.6 cm�1, i.e., peak 2D, is the secondorder of peak D. The shape of peak 2D, an important indicatorof the number of layers, has been systematically studied for thegraphite powder or graphene with various layers, and found tobe considerably different from each other.57–59 When thenumber of layers is >10, the corresponding Raman spectrum isalmost identical to that of the graphite powder. A carefulcomparison informs us that the Raman spectrum of ourgraphite nanoakes, unlike that of the graphite powder, is inconsistent with those of 5–10 layers, verifying the successfulexfoliation of the graphite powder into few-layer nanoakes.The Raman result is in good agreement with the HRTEM andAFM data. As to the composites, the characteristic peaks of bothcomponents (MoS2 or WS2 and graphite nanoakes) can beclearly seen, conrming that the physical mixing processeffectively preserves the crystalline nature without introducingany new damage.

To understand the morphological features of the MoS2 andWS2/graphite nanoake composites, we present two typical SEMimages in Fig. 3. It is clearly seen that the lateral sizes of thenanoakes range from hundreds of nanometres to severalmicrometres, statistically larger than the ones produced byconventional, long-time sonication that oen results in thebreakage of the nanoakes into small pieces due to the strong

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Fig. 3 Typical SEM images of (a) MoS2/graphite and (b) WS2/graphitenanoflake composites at a wt ratio of 50/50.

Fig. 4 XRD patterns of (a) MoS2/graphite and (b) WS2/graphitenanoflake composites at different wt ratios.

Table 1 Specific surface area and pore volume of MoS2, WS2, graphitenanoflakes and their composites

Sample nameSpecic surfacearea (m2 g�1)

Pore volume(cm3 g�1)

MoS2 nanoakes 56.16 0.07WS2 nanoakes 25.08 0.06Graphite nanoakes 168.89 0.21MoS2/graphite 50/50 78.81 0.12WS2/graphite 50/50 36.74 0.07

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shearing forces.5,47–50 It is hard to distinguish the MoS2 or WS2nanoakes from the graphite nanoakes due to their similarmorphology.42 A closer observation on the edges of the nano-akes shows that the thicknesses are around 10 nm or below,implying that the nanoakes are mostly few-layer ones. It isworth noting that the nanosheets are uniformly mixed andloosely packed without obvious restacking, ensuring sufficientpores and voids that are desirable for creating pathways for Li+

ion and electron diffusion, and are also desirable for bufferingthe volume expansion during the repeated charging–discharg-ing processes. Therefore, these MoS2 and WS2/graphitenanoake composites are expected to exhibit excellent electro-chemical performances as will be proven below. The uniformmixing of the nanoakes can be further veried by HRTEMcharacterisation, as shown in Fig. S14.† The edges of the MoS2and graphite nanoakes can be clearly differentiated as indi-cated by arrows.

The XRD patterns of the MoS2 and WS2/graphite nanoakecomposites at different wt ratios are presented in Fig. 4. TheXRD patterns of the MoS2 and WS2 nanoakes show a series ofdiffraction peaks as resolved in the gure (JCPDS card nos37-1492 and 84-1398), which conrm the highly crystallinenature of the liquid-exfoliated MoS2 and WS2 nanoakescompared to the ion-intercalated6 or hydrothermally syn-thesised ones.28–40 The primary (002) diffraction peaks reside at14.42� (MoS2) and 14.37� (WS2), corresponding to d-spacings of0.613 and 0.616 nm, respectively. These values are in goodagreement with HRTEM observation (Fig. 1d and f), conrmingthat the nanoakes exist mainly in the form of few layers. TheXRD pattern of the graphite nanoakes has a primary (002)diffraction peak at 26.58�, which represents a d-spacing of 0.335nm.60 This value is also in good agreement with HRTEMobservation (Fig. 1b). Note that the crystallinity of the graphite

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nanoakes is much higher than that of r-GO as revealed by XRDcharacterisation, which reasonably explains the better struc-tural integrity and higher electronic conductivity of the former.Aer the liquid-phase mixing, the resulting MoS2 and WS2/graphite nanoake composites do not show obvious differencesin terms of peak shape and position, once again conrming thatthe physical mixing process does not cause structural disorders.

The BET data can provide us with sufficient information onthe specic surface areas and pore volumes of MoS2, WS2 andgraphite nanoakes as well as their composites, which aresummarized in Table 1. As discussed above, the combination ofball milling and short-time sonication can produce “gigantic”MoS2, WS2 and graphite nanoakes with relatively large specicsurface areas of 168.89, 56.16 and 25.08 m2 g�1, respectively.Introducing large-area graphite nanoakes can thus increasethe specic surface areas of the composites, and prevent theMoS2 and WS2 nanoakes from restacking. Therefore aer theliquid-phase mixing, the 50/50 MoS2 and WS2/graphite nano-ake composites have specic surface areas up to 78.81 and

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36.74 m2 g�1 (�1.5 times increase), which are much larger thanthe value reported for the conventional, long-time sonicationmethod,42 and are advantageous for Li+ ion and electronstorage. The larger specic surface area of the MoS2/graphitenanoake composite than its WS2 counterpart may partiallyexplain the better electrochemical performances of the former,as will be proven below. Moreover, the pore volumes of theMoS2and WS2/graphite nanoake composites, obtained by Barrett–Joyner–Halenda (BJH) calculation, are 0.12 and 0.07 cm3 g�1,respectively. The relatively large pore volumes, mainly contrib-uted by the large-area, exible graphite nanoakes, are desir-able for facilitating the electrolyte inltration as well asbuffering the volume expansion during the repeated charging–discharging processes.

Fig. 5 shows the rst three charging–discharging and cyclicvoltammetry (CV) curves of two representative composites, i.e.,the 50/50 MoS2 and WS2/graphite nanoake composites. Asseen from Fig. 5a, the rst discharging process of the MoS2/graphite nanoake composite is featured by three plateaus at�1.1, 0.5 and 0.1 V, indicating the formation of LixMoS2, theconversion of Mo4+ to Mo nanoparticles (embedded in a Li2Smatrix), and the intercalation of Li+ ions into the galleries of thegraphite nanoakes (Fig. S15†), respectively. The slope below�0.5 V is generally ascribed to the formation of a solid-elec-trolyte interface (SEI) layer originating from the electrochemi-cally driven electrolyte decomposition. During the seconddischarging process, a new plateau at �2.2 V occurs, corre-sponding to a reaction of Mo + 4Li+ + 2S + 4e� / 2Li2S + Mo(Mo is le to emphasise its inertness). Note that the actualpolarization between discharge and charge of the MoS2/graphite nanoake composite is �1.7 V, which is too high forthe oxidation of Mo to MoS2 (polarization ¼ �0.6 V).61 In thissense, the plateaus at �0.2 and 2.3 V during the rst chargingprocess should be attributed to the extraction of Li+ ions fromthe galleries of the graphite nanoakes (Fig. S15†) and the

Fig. 5 The first three charging–discharging (current density ¼ 100mA g�1) and CV (scan rate ¼ 0.1 mV s�1) curves of (a and b) MoS2/graphite and (c and d) WS2/graphite nanoflake composites at a wt ratioof 50/50.

This journal is © The Royal Society of Chemistry 2014

conversion of Li2S to S,8,14 respectively. During the secondcharging process, a new, inconspicuous plateau at �1.7 Voccurs, a proof of the partial oxidation of Mo to MoS2. In thecase of the WS2/graphite nanoake composite (Fig. 5c), thelithium storage mechanism is very similar. Basically, during therst discharging process the inconspicuous plateau at �1.5 V isattributed to the intercalation of Li+ ions and the formation ofLixWS2; the remarkable plateaus at�0.7 and 0.1 V are related toa reaction of WS2 + 4Li

+ + 4e� /W+ 2Li2S (accompanied by theirreversible electrolyte decomposition), and the insertion of Li+

ions into the galleries of the graphite nanoakes, respectively.During the subsequent discharging process two plateaus occurat �1.9 and 2.2 V, which are ascribed to the formation of a gel-like polymeric layer out of the dissolution of Li2S in the elec-trolyte.39 During the rst charging process, the plateaus at�2.3 and 0.2 V correspond to the extraction of Li+ ions, whichremain unchanged during the subsequent charging process.The charging–discharging results are in good agreement withthe CV curves (Fig. 5b and d). Note that the charging–dis-charging and CV curves of the second and third cycles are nearlysuperposed, indicating high cycle stabilities of the MoS2 andWS2/graphite nanoake composites.

Fig. 6 compares the cycle behaviours of MoS2, WS2 and theircomposites with the graphite nanoakes. The initial dischargecapacities of MoS2 and WS2 are 1223 and 830 mA h g�1, muchhigher than the theoretical values of their bulk forms because ofthe electrochemically driven electrolyte decomposition and theenlarged gallery spacings due to exfoliation. However, thecapacities fade rapidly to only 337 and 276 mA h g�1 at the 50thcycle, corresponding to retention rates of only 27.5% and33.3%, respectively. The drastic capacity losses can be ascribedto the poor intrinsic electronic conductivity of transition metaldisulphides. Aer being conductively doped, MoS2 and WS2have signicantly enhanced electrochemical performances. Inaddition, the large-area graphite nanoakes can also increasethe specic surface areas (Table 1), as well as prevent the MoS2and WS2 nanoakes from restacking, which are advantageousfor the Li+ ion and electron storage. The optimum compositionsare the 50/50 MoS2 and WS2/graphite nanoake composites,whose reversible capacities are as high as 951 and 782 mA h g�1

aer 50 cycles. Note that our MoS2/graphite nanoakecomposite even outperforms the CVD-grown MoS2/pristinegraphene composite (877 mA h g�1 @ 100 mA g�1) in terms of

Fig. 6 Cycle behaviours (current density ¼ 100 mA g�1) of (a)MoS2/graphite and (b) WS2/graphite nanoflake composites at differentwt ratios.

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capacity,62 thus exhibiting more splendid industrial potentialsespecially when one considers the mild experimental condi-tions and high yields of our strategy compared to CVD. The75/25 MoS2 and WS2/graphite nanoake composites havepoorer cycle stabilities, fading from 1186 and 1019 mA h g�1 to663 and 621 mA h g�1 aer 50 cycles, which can be ascribed tothe lower content of the graphite nanoakes leading to moredifficult electron transport. The 25/75 MoS2 and WS2/graphitenanoake composites show fairly good cycle performances,while their lower reversible capacities (538 and 485 mA h g�1)can be assigned to the higher content of the graphite nanoakeswhose lithium storage properties are inferior to MoS2 and WS2.It is shown that the reversible capacity of the graphite nano-akes is only 267 mA h g�1 aer 50 cycles (Fig. S16†), which iscomparable to the previous reports.63,64

The rate capabilities of the 50/50 MoS2 and WS2/graphitenanoake composites are also evaluated, as shown in Fig. 7.When the current density is raised to 500 mA g�1, the reversiblecapacity of the MoS2/graphite nanoake composite is above800 mA h g�1, still better than that of the CVD-grown MoS2/pristine graphene composite (665 mA h g�1 @ 500 mA g�1).62

Even at a very high current density of 2000 mA g�1, the revers-ible capacity is still 625 mA h g�1. Upon the recovery of thecurrent density to 100 mA g�1, the reversible capacity returns to938 mA h g�1, further proving the high cyclability of the MoS2/graphite nanoake composite. As to theWS2/graphite nanoakecomposite, the reversible capacities are 508, 492 and 488 mA hg�1 at current densities of 500, 1000 and 2000 mA g�1, respec-tively. The tremendous enhancement in the rate capabilitiesalso demonstrates the effectiveness of introducing the graphitenanoakes as a high-efficiency conductive additive for transi-tion metal disulphides. To elucidate the effect of the graphitenanoakes, we have conducted electrochemical impedancespectroscopy (EIS) on MoS2, WS2 and their composites with thegraphite nanoakes, and the results are shown in Fig. S17 andS18.† The diameters of the high- and medium-frequencysemicircles for MoS2 or WS2, corresponding to the SEI lmresistance (Rf) of 16.5 or 20.2 U and the charge-transfer resis-tance (Rct) of 57.1 or 109.2 U, are signicantly reduced to 9.7 or18.1 U and 15.5 or 13.6 U aer being conductively doped withthe graphite nanoakes. The partial electron transfer fromgraphite to the transition metal disulphide layers has beendemonstrated both theoretically and experimentally.65 It isinferred that the uniform mixing of the MoS2 or WS2 and

Fig. 7 Rate capabilities of (a) MoS2/graphite and (b) WS2/graphitenanoflake composites at a wt ratio of 50/50.

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graphite nanoakes can largely increase the electronicconductivity, which facilitates easy electron transport from thecurrent collector to the electrode. Therefore, the cycle and rateperformances of the MoS2 and WS2/graphite nanoakecomposites are signicantly enhanced compared to those ofneat MoS2 and WS2.

4. Conclusions

In summary, we have successfully proposed a facile, industriallyviable strategy for the scalable production of MoS2 and WS2/graphite nanoake composites by a combination of ball millingand short-time sonication. This strategy can produce largerMoS2 and WS2 nanoakes with more lithium storage sites thanthe conventional, long-time sonication method. Besides, theobtained graphite nanoakes have a higher degree of latticeintegrity than r-GO that is structurally permanently damaged,and can thus serve as a high-efficiency conductive additive. Aprominent synergy between the excellent electrochemicalperformances of the MoS2 and WS2 nanoakes and the highelectronic conductivity of the graphite nanoakes is witnessed.The MoS2 and WS2/graphite nanoake composites exhibitsuperior lithium storage capacities, cycle stabilities and ratecapabilities, thus laying a basis for developing high-perfor-mance anodes for next-generation LIBs.

Acknowledgements

This work was nancially supported by the National NaturalScience Foundation of China (nos 21304053 and 21274079) andthe Specialized Research Fund for the Doctoral Program ofHigher Education (no. 20120002130012). Y.-T. L. is grateful tothe China Postdoctoral Science Foundation (no. 2014T70077).X.-D. Z. is grateful to the Postdoctoral Science-Research Devel-opment Foundation of Heilongjiang Province (no. LBH-Q11130)and Natural Science Foundation of Heilongjiang Province (no.B201202).

Notes and references

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2 D. Kong, H. He, Q. Song, B. Wang, Q.-H. Yang and L. Zhi, RSCAdv., 2014, 4, 23372.

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