Lithium Storage Performance of Hollow and Core/Shell TiO2Microspheres Containing Carbon☆

Songgyun Ri,Honggui Deng,Lihui Zhou,Jun Hu*,Honglai Liu,Ying Hu

Energy,Resources and Environmental Technology

Lithium Storage Performance of Hollow and Core/Shell TiO2Microspheres Containing Carbon☆

Songgyun Ri,Honggui Deng,Lihui Zhou,Jun Hu*,Honglai Liu,Ying Hu

State Key Laboratory of Chemical Engineering,Department of Chemistry,East China University of Science and Technology,Shanghai 200237,China

A R T I C L EI N F O

Article history:

Lithium ion battery

Titanium dioxide microsphere

Hollow

Core/shell

Electrochemical properties

TiO2microspherescontainingcarbonhavebeensynthesizedviaaone-pothydrothermalprocessusingCTABasthe mesoporous template and nanoparticle stabilizer and Ti(SO4)2and sucrose as titanium and carbon precursors, respectively.Through well designed calcinations,TiO2microspheres with various amounts of carbon-residue, such as core/shell C@TiO2,hollow neat H-TiO2,and hollow C/TiO2composites,are obtained.When these microspheres are used as anode materials for lithium ion batteries,the lithium storage performance is signif i cantly inf l uenced by the structure and carbon-residue.With a thin shell of TiO2nanoparticles and carbon-residue,the capacity of hollow C/TiO2composites maintains at 143.3 mA·h·g?1at 0.5 C(83.5 mA·g?1)after 100 cycles. Moreover,afterhighratecharge/dischargecyclesfrom0.2Cto20Candbackto0.2Cagain,thereversiblecapacity recovers atas high as 195.1 mA·h·g?1with respect to its initial value of 205.0 mA·h·g?1.The results of cycle voltammograms and electrochemical impedance spectroscopy further reveal that Li+insertion/extraction processes are reversible,and the diffusion coeff i cient of Li+in the hollow C/TiO2composites is much higher than those of others,because the hollow structure can act as the ion-buffering reservoir and facilitate Li+transfer from both sides of the shell,and the carbon-residue in the shell improves the conductivity as well.

?2014TheChemicalIndustryandEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

TiO2has been applied as a promising anode material for high rate charge/discharge lithium ion batteries because it avoids the formation of solid electrolyte interphase,which is generally carbon anode materials[1-3].With the development of new nanomaterial technology and the application of novel materials,the performance of lithium ion batteries has been greatly improved.The nanostructures provide a large surface area for active points to contact with electrolyte and a shorter distance for insertion/extraction of eff i cient lithium ions[4-6]. However,aggregation of nanoparticles is inevitable in the lithium ion battery with long time of operation[7].An effective approach to avoid theaggregationisusingsoftorhardtemplatestoseparatenanoparticles into mesoporous networks[8-12].Mesoporous TiO2nanocomposites are synthesized as a high-power anode material by using TiCl4and triblock copolymer Pluronic P123[13].Ordered mesoporous rutile TiO2and anatase TiO2are prepared by using mesoporous silica SBA-15 as the hard template[14].It is proved that TiO2hollow spheres are a highperformancelithiumstoragematerialbecauseoftheirgoodsurface permeability,adjustablediameterandshellthickness[15].Thetemplate method is a facile and reproducible approach to fabricate the hollow structure.WithCu2Opolyhedraasthetemplate,anisotropicTiO2hollow nanostructures with tunable sizes of 100-1000 nm can be obtained through hydrothermal treatment of TiF4aqueous solution[16].More generally,a sacrif i cial spherical template is synthesized and then the metal oxide shell is developed based on the template core;after the removal of the sacrif i cial template by calcination or extraction method, hollow spheres can be obtained[17].Wang et al.[18]and Titirici et al.[19]have synthesized monodispersed carbon microspheres as the sacrif i cial template by hydrothermal treatment of the carbon precursor, andsuccessfullydevelopedhollowmetaloxidemicrospheres.However, the low intrinsic electrical conductivity of pure TiO2particles could cause a sluggish electrode kinetic of lithium ion insertion/extraction especially at high charge/discharge rate.Incorporating conductive materials such as carbon into TiO2is a feasible method[20].Das et al.[21] have synthesized mesoporous microspheres of anatase TiO2/carbon composites via a one-pot optimized solvothermal process.This percolationnetworkofcarbonaroundtheTiO2nanoparticlesshowsarelatively higher storage capacity and better cyclic life.Although the TiO2/C composite is proved as a promising anode material,controllable fabrication of ordered mesostructure or regular morphology is still a great challenge,since titanium precursors,such as titanium chloride(TiCl4) and titanium alkoxide[Ti(OR)4],have a strong hydrolysis reactivity.Moreover,theunderstandingontherelationshipbetweenstructureand lithium storage performance is far from enough.

In this work,we report a simple one-pot synthesis method to fabricate TiO2microspheres.The as-synthesized composites possess a thin shell of TiO2nanoparticles and a carbon precursor core,which permit us to fabricate various structures of TiO2-carbon composites.Core/ shellC@TiO2,hollowC/TiO2composites,andhollowneatH-TiO2microspheres are prepared,in which the TiO2nanocrystalline shell would facilitate the transfer of Li+,and the remained carbon core or shell would improve the conductivity and inhibit the aggregation of TiO2nanocrystallines during the calcination.The electrochemical properties of TiO2-carbon composites as anodic materials for lithium ion batteries are examined.The relationship between structures and electronic properties is proposed.

2.Experimental

2.1.Synthesis of TiO2microspheres

A simple one-pot hydrothermal synthesis method was adopted by using Ti(SO4)2(9.1 g)as the titanium precursor,CTAB(4.4 g)as the mesoporousstructure-directingagent,andsucrose(7.6g)asthecarbon template precursor.With the molar composition of Ti(SO4)2:CTAB:sucrose:H2O:ethanol=1:0.3:0.5:48:18.6,all the reactants were mixed togetherina150mlbeaker.Afterstirringat45°Cfor2h,aclearsolution was obtained.The solution was transferred into a Te fl on-lined stainless steelautoclaveandagedat150°Cfor24h.Thebrownsolidproductwas fi ltered and washed with deionized water and ethanol separately for several times until the pH of the fi ltrate became neutral.The assynthesized products were calcinated in a tubular furnace under different conditions to obtain three types of TiO2microspheres.In the calcination at 500°C in N2fl ow,the sucrose in the core was carbonized and the obtained core/shell composite was denoted as C@TiO2.When the calcination was carried out at 600°C in air,the sucrose burned off and the obtained hollow microsphere of pure nanocrystalline TiO2was denoted as H-TiO2.While in the calcinations at different temperaturesinair,hollowC/TiO2compositeswithdifferentamountsofcarbonresidue were obtained.They were denoted as C/TiO2-x,in which x represents the calcination temperature.For example,C/TiO2-400 refers to the as-synthesized sample calcinated at 400°C in air for 1 h.The heating rates for all calcinations were 1°C·min?1.

2.2.Characterization

X-ray powder diffraction(XRD)patterns were collected on a Rigaku D/max 2550 VB/PC diffractometer with Cu Kα radiation at 40 kV and 50 mA.The micron-morphology was observed by scanning electron microscopy(SEM)on JEOL/JSM-6360.High-resolution transmission electron microscopy(HRTEM)images were obtained on a JEOL JEM-2010 microscope.Samples for HRTEM investigations were prepared by dispersing particles in ethanol under ultrasonication and then dropping on a copper grid doped with a holey carbon fi lm.Energy dispersive X-ray spectroscopy(EDS)measurements were performed using EDAX/Genesis.Nitrogen adsorption-desorption isotherms were determined on an ASAP-2020 analyzer at?196°C. Prior to the measurement,the samples were degassed at 100°C under vacuum foratleast 6 h.The surface areas and sizedistributions were calculated by the Brunauer-Emmett-Teller(BET)method and the Barrett-Joyner-Halenda(BJH)model,respectively.Thermogravimetric analysis(TGA)and differential scanning calorimetry(DSC) were carried out on a NETZSCHSTA 499F3 thermal analyzer with a heating rate of 5°C·min?1.The protection gas was nitrogen or a mixed gas of nitrogen(80%)and oxygen(20%)with a fl ow rate of 100 ml·min?1.

2.3.Electrochemical characterization

Theworkingelectrodewaspreparedbyamixtureofsamplepowders, carbonblack conductingagent andpolyvinylidene f l uoridebinder,witha mass ratio of 75:15:10 in n-methyl-2-pyrrolidinone solvents,The slurry was pasted on a copper foil.Electrochemical test cells were assembled in an argon-f i lled glove box,using coin-type half-cells with lithium foil as a counter electrode.The electrolyte was 1.0 mol·L-1LiPF6in a mixture of ethylene carbonate and dimethyl carbonate(volume ratio of 1:1).The cells were galvanostatically charged and discharged in a voltage range from 1.0 to 3.0 V vs.Li/Li+.For the rate performance measurement,the current was varied from 0.2 C(33.4 mA·g?1)to 20 C(3340 mA·g?1). The cycle performance for 100 cycles was recorded at the rate of 0.5 C. Thecapacitywascalculatedbasedon1gofsample.Thecyclicvoltammogram(CV)tests were conducted on an Arbin BT2000 battery test system at scan rates of 0.1,0.2,0.5,and 1 mV·s?1in the voltage range of 1.0-3.0 V.Electrochemical impedance spectroscopy(EIS)analyses were also carried out using a Gamry Instrument.All the electrochemical measurements were carried out at room temperature.

3.Results and Discussion

Fig.1.SEM images of mesoporous TiO2microspheres.(a)Synthesized TiO2microspheres. (b)Hollow H-TiO2microspheres by calcination at 600°C in air.

3.1.Structure and morphology of TiO2microspheres

Fig.1 shows the SEM images of TiO2microsphere samples.The assynthesizedmicrospherespresentalitchi-likemorphologywithadiameter of 3-5 μm.The rough surface of the microsphere is composed of TiO2granule aggregations with an average size of less than 100 nm.However,some hemispheres with smooth surface,as if extruded from the core of the TiO2microspheres,can be observed.EDS analysis confi rms that they are sucrose spheres.It indicates that TiO2nanoparticles may grow up around the spherical sucrose core through the functional groups of-OH in hydrolyzed Ti(SO4)2and-C=O in sucrose to form the core/shell structure[19].After the calcination at 600°C in air,the sucrosecoreburnsoff.AsshowninFig.1(b),somebrokenmicrospheres reveal the formation of hollow TiO2microspheres,and the product indeed has a core/shell structure.

Fig.2 shows the DSC curves of the as-synthesized sample in pure N2fl ow and in air,which help us to design an accurate routine to prepare desired structures of TiO2microspheres.In the calcination under N2fl ow,there is a transition plateau at about 270°C in the DSC curve, attributed to the decomposition of CTAB and the partial carbonization of sucrose into carbon.The continual carbonization and crystallization of TiO2make the DSC curve decline.Another broad exothermic peak occurs at about 450°C,suggesting complete carbonization.For the calcination in air,there is a signi fi cant heat release in the range of 200-500°C.Thepeakatabout300°Cisattributedtothedecomposition of CTAB and sucrose,while that at 430°C is mainly attributed to the crystallization of TiO2.

Fig.3 shows the proposed routines and corresponding formation mechanism.In the hydrothermal process,the Ti(SO4)2precursor is slowly hydrolyzed into titanyl sulfate(TiOSO4),which is relatively stable compared with TiO(OH)2hydrolyzed from TiCl4or Ti(OR)4. And then,TiOSO4produces TiO2nanobuilding blocks slowly to selfassemble into mesoporous nanoparticles induced by CTAB micelles. Because of the high density of-OH and-C=O groups on the surface of polysaccharide microspheres[19],TiO2nanoparticles aggregate easily around the polysaccharide spherical surface to form a shell through hydrogenbondingandelectricinteraction.Basedonthethermalproperties of the as-synthesized composite sample,the following calcinations are carefully controlled to obtain different types of TiO2composites.In pureN2f l owat500°C,thepolysaccharidecoreiscompletelycarbonized, which results in the core/shell composite of C@TiO2.The TEM image in Fig.4(a)shows that TiO2nanoparticles aggregate closely on the surface ofthemicrosphere.ThecenterdarknessoftheC@TiO2sphereisuniform, suggestingitssolidsphericalstructure.Intheairf l ow,thepolysaccharide core burns off partially at 400-500°C and completely at 600°C[22],so that hollow C/TiO2composite microspheres with different amounts of carbon-residue and the hollow microspheresof H-TiO2without carbonresidue can be produced.As shown in Fig.4(b),the black dots of carbon embedded in the packing-pore of TiO2nanoparticles can be clearly observed in C/TiO2-400.With thehindrance of the tightly aggregated TiO2shell,instead of completely burning to CO2,some of the polysaccharide core is carbonized,followed by inside-out Ostwald ripening effect[23], andthecarbonthatremainedinthestacking-poreofTiO2nanoparticles forms the blend hollow structure.As the calcination temperature increases further,as shown in Fig.4(c),the hollow H-TiO2microspheres, withmuchdarkeredges alongthesphericalboundary,canbeobserved.

Fig.4(d)shows the HRTEM image of TiO2crystals on the surface of hollow H-TiO2spheres.TiO2nanocrystals have irregular shapes with anaveragesizeof about10 nm.Theinset reveals that thelattice spacing of the TiO2crystal is 0.38 nm.Only a few of mesopores can be observed, and most of them will disappear with the growth of TiO2nanocrystals. According to the EDAX analysis(Fig.5),the hollow H-TiO2spheres aremadeofneatTiO2.Onlyatracecarbon(lessthan2.5%)wasdetected, which could be attributed to the sample carrier of carbon f i lm coating on the copper grid.

Fig.2.DSC curves of as-synthesized composite sample in different gas f l ows.1—80% N2+20%O2;2—100%N2.

Fig.3.The proposed synthesis process and mechanism for three types of TiO2microspheres.

The WAXRD patterns of the as-synthesized TiO2composite,C/TiO2-400,C/TiO2-500,C@TiO2,and H-TiO2in Fig.6 show that all samples can be indexed as a typical anatase TiO2(JCPDS:21-1272),without any other polymorph or carbon crystals coexisted.As listed in Table 1, the crystallite size of TiO2in each sample,calculated by Scherrer equation,is in the sequence of H-TiO2(10.1 nm)＞C@TiO2(9.1 nm)＞assynthesized TiO2composite(8.8 nm),suggesting that the TiO2crystal grows larger at higher calcination temperature.It is worth mentioning that,unlike the report[20],the as-synthesized TiO2composite already possesses distinct characteristic peaks of anatase,which reveals that the crystallization occurs even during the hydrothermal process.

AsshowninFig.7,TGAcurvesrevealthecompositionofvariousTiO2microspheres through the mass loss in air f l ow.For the as-synthesized composite,there is a signif i cant mass loss of 33.3%in the temperaturerange of 240-470°C,whichis attributedtothe burningof CTAB andsucrose.In the curve of core/shell C@TiO2,since most of CTAB and sucrose are carbonized into carbon,the remained amount of carbon is about 25%.For the hollow C/TiO2-400 composite,with only a part of polymerized sucrose burned off,7.5%carbon remains.After 500°C,the TGA curve of the as-synthesized TiO2composite already approaches a platform,so the difference between the TGA curves of C/TiO2-500 and H-TiO2(calcinated at 500 and 600°C,respectively)is little,where the carbon amount of the former is 2.5%,and almost no carbon is left in the later.

Fig.4.TEM and HRTEM images.(a)Core/shell C@TiO2.(b)Hollow C/TiO2composite.(c)Hollow neat H-TiO2.(d)TiO2nanoparticles on the surface of H-TiO2spheres.

Fig.5.EDAX analysis of hollow H-TiO2spheres and its chemical element distribution(including the copper grid with a carbon f i lm).

As estimated by N2adsorption/desorption isotherms of various TiO2microspheres[Fig.8(a)],the BET surface areas of the as-synthesized product,C@TiO2,C/TiO2-400,C/TiO2-500 and H-TiO2,as listed in Table 1,are 48.67,130.87,99.56,82.68 and 53.02 m2·g?1,respectively. Except for the as-synthesized sample,they are all larger than those of TiO2microspheresreported[21,24].Combiningtheporesizedistributions calculated by the BJH method[Fig.8(b)]and the results listed in Table 1,we can see that larger surface areas are mainly caused by the existence of mesopores in C-TiO2composites and macropores among the packing nanoparticles.With the increase of calcination temperature,the mesopores collapse due to the growth of TiO2crystals and the burning of carbon and,accordingly,the surface area decreases.

Fig.6.WAXRD patterns of various TiO2samples.

3.2.Electrochemical properties

Fig.9 shows the galvanostatic charge/discharge cyclic performance of TiO2microspheres at 0.5 C,used as anodic materials.The hollow C/TiO2-400,C/TiO2-500,C@TiO2and H-TiO2at the 1st,2nd,5th and 100th cycles are presented.All samples show stable charge and dischargepotentialplateausat2.0and1.7V.However,thereisahysteresis effect at 1.4 V for each sample,indicating the irreversible capacity loss. For the f i rst cycle,there is an irreversible capacity loss[25],with the coulombic eff i ciency of 76.5%,66.7%,82.3%and 76.8%for C/TiO2-400, C/TiO2-500,C@TiO2,and H-TiO2microspheres,respectively.After that, the capacities are soon stabilized at 2nd or 3rd cycle,with coulombic eff i ciency of about 100%.

Fig.7.TGA curves of different TiO2microspheres in air f l ow.1—As synthesized(O2); 2—C@TiO2;3—C/TiO2-400;4—C/TiO2-500;5—H-TiO2.

The reversible capacities during 100 cycles at 0.5 C for various TiO2microspheres are further revealed in Fig.10.The initial reversible capacitiesofC/TiO2-400,C/TiO2-500,C@TiO2andH-TiO2compositesare188.8, 178.9,115.1 and 126.1 mA·h·g?1,respectively.After 100 cycles,they decrease to 143.3,126.1,108.9 and 75.2 mA·h·g?1,corresponding to the capacity retention of 75.9%,70.5%,94.6%and 59.6%,respectively. For the hollow structure,the capacity is in the sequence of C/TiO2-400＞C/TiO2-500＞H-TiO2,indicating that the higher the calcination temperature,the fewer the amount of carbon-residue,the lower the surface area,the larger the TiO2crystal and,consequently,the lower the capacity.Core/shell C@TiO2exhibits relatively stable retention of reversible capacity,maintaining as high as 95%of the initial capacity after100 cycles.However,theoverallreversiblecapacityislow.Itisbecauseof the specif i c core/shell C@TiO2structure.During the hydrothermal process,there are many CTAB micelle structures and self-assembled blocks in the core,as shown in Fig.3.When annealing to 500°C,micelle surfactantsareburnedoffinN2gasf l owandleavemesoporouscarbonin thecore,correspondingtorelativelyhighBETsurfaceareafortheC@TiO2sample.Therefore,the25%ofcarbonthatremainedintheC@TiO2sample improvestheconductivityofTiO2,whichnotonlyisref l ectedbyrelatively low fading,but also results in low capacity.

Fig.9.Galvanostatic charge(Li extraction)/discharge(Li insertion)prof i les duringthe 1st,2nd,5th and100th cyclesat0.5 C.(a)Hollow C/TiO2-400.(b)Hollow C/TiO2-500.(c)Core/shell C@TiO2.(d)Hollow neat TiO2.

Fig.10.Cycling performance during 100 cycles at 0.5 C.▽hollow C/TiO2-400;□hollow C/TiO2-500;△core/shell C@TiO2;○hollow neat TiO2.

Fig.11.High rate discharge-charge performance plots at different current densities for various TiO2microspheres.▽hollow C/TiO2-400;□hollow C/TiO2-500;△core/shell C@TiO2;○hollow neat TiO2.

Fig.11 further displays the high rate discharge/charge performance of various TiO2composite microspheres at different densities in the potentialrangeof1.0-3.0 V.At0.2C,reversiblecharge/dischargecapacities of C/TiO2-400,C/TiO2-500,C@TiO2and H-TiO2are high,about 205.0,183.5,145.8 and 112.5 mA·h·g?1,respectively.Thereafter, their capacities gradually drop as the current density increases.At 20 C,the discharge capacities are79.1,70.4,15.4 and 44.4 mA·h·g?1, respectively.When the current density returns to 0.2 C,all samples almost recover their initial reversible capacities,with the values of 195.1,160.3,129.9 and 105.2 mA·h·g?1for C/TiO2-400,C/TiO2-500, C@TiO2and H-TiO2,respectively,suggesting that all TiO2microspheres are stable after high rate charge/discharge cycles.Among them,thehollow C/TiO2-400 composite microspheres show superior high-rate performance and capacity,and it is also better than the recent reported workofTiO2nanocageswithorwithoutinteriormetalfunctionalization (80-150 mA·h·g?1at 0.5-10 C)[16].

Fig.12 shows the CV curves of the hollow C/TiO2-400,C/TiO2-500, C@TiO2and H-TiO2microspheres at scan rates of 0.1,0.2,0.5,and 1 mV·s?1in the voltage range of 1.0-3.0 V.Since the area under the CV curvesrepresents thetotalamount of transferred charge,the reversibility is usually estimated by comparing the areas of the anodic to cathodic peaks during the electrochemical redox process.As shown in Fig.12,the area ratios of the anodic to cathodic peaks of all samples almost approach one,and only show a little variation with the increase of scan rate,suggesting the good reversibility of Li+charge/discharge processes in all samples.As shown in Fig.13,plotting the cathodic peak current(ipc)against the square root of scan rate(ν1/2),straight lines can be obtained for various TiO2microspheres.For the reversible process,these linear relationships can be described as

Fig.12.Cyclevoltammogramsoffreshelectrodesatscanrates of 0.1,0.2,0.5,and 1 mV·s?1(voltage range:1.0-3.0 V).(a)Hollow C/TiO2-400.(b)Hollow C/TiO2-500.(c)Core/shell C@TiO2.(d)The hollow neat TiO2.

Fig.13.Plots of linear relationship between cathodic current(ipc)and square root of scan rate(ν1/2)for various TiO2microspheres.■hollow C/TiO2-400;●hollow C/TiO2-500;▲core/shell C@TiO2;?the hollow neat TiO2.

Table 2Electrochemical electrode parameters of hollow and core/shell microspheres from EIS

where C′is the bulk concentration(10?3mol·cm?3),n is the number oftransferelectrons(1forLi+),andAisthesurfaceareaoftheelectrode material(0.5 cm2).The diffusion coef fi cients,Da,of various TiO2microspheres can be calculated by the slopes of the straight lines.As listed in Table 2,the values of Daare in the range from 2.33×10?7to 1.39×10?6cm2·s?1.Among them,the Daof H-TiO2is more than f i ve times larger than that of C@TiO2due to an easier transfer through both sides of the TiO2shell of the hollow structure.With carbon-residue,the hollow C/TiO2composites still show much larger Dathan C@TiO2.

EIS was conducted to investigate the electrical kinetic properties of variousmicrospheres.Fig.14showstheNyquist-typeplotsofelectrodes and theimage above thegraph is theequivalentcircuit model,where Rsrepresents the bulk resistance,R1-6represent resistances related to the surface electrolyte interface and various mesoporous structures,Rctis related to the charge transfer resistance at the active material interface, W is the Warburg impedance caused by a semi-inf i nite diffusion of the Li+ionin theelectrode,and Cdlrepresents thedouble-layer capacitance of the electrode-electrolyte interface.Each parameter in the equivalent circuit is f i tted by the Zview2 software and listed in Table 2.The exchange current density can be calculated by

wheren=1 fortheelectrontransferoftheTi4+/Ti3+redoxcouple.The diffusioncoeff i cientofLi+intheelectrodecanbecalculatedthroughthe Nyquist-type plot in the low-frequency region as follows

where σWis the Warburg factor,Z′is the real resistance of the complex impedance plane,and ω is the frequency.Different from theDadiscussed above,Dsrepresents the diffusion coeff i cient of the Li+ion in a semi-inf i nite diffusion process at the electrode-electrolyte interface.Plotting Z′against ω?1/2,σWcan be obtained by the slope of the linear relationship described inEq.(4)[26].Substituting σWinto Eq.(3),the diffusion coeff i cient of Li+,Ds,can be obtained.As listed in Table 2,the values of the diffusion coeff i cient of all TiO2microspheres are～10?9cm2·s?1,signif i cantly higher than those for neat TiO2crystal (～10?13cm2·s?1)or anatase f i lms(10?10-10?17cm2·s?1)[27]. Among them,the hollow C/TiO2-400 composite again shows signif icantly better performance than the others.

Combining the electrochemical properties with their structures,the improvements of the hollow C-TiO2composite are considered to arise from good contact between carbon and TiO2in the shell and short Li+diffusion distance in nanocrystalline TiO2with small size and large surface area.In addition,the hollow structure can act as the ion-buffering reservoir and facilitate Li+transfer from both sides of the shell.As the calcination temperature increases,the combustion of carbon and the increase of crystalline TiO2size lead to poorer conductivity and lower Li+diffusioneff i ciencyand,consequently,toalowerperformanceforhollow neat H-TiO2.For core/shell C@TiO2,the large surface area is mainly caused by the mesoporous carbon core,which shows only little contribution to Li+transfer.Moreover,the distance between the carbon core and TiO2shell is too close to hold enough LiPF6electrolytes,so Li+can only transfer freely from the outside of the shell.Consequently,the high rate performance of the core/shell structure is inferior to that of samples with hollow structure.Overall,the hollow C-TiO2composite microspheres could be a promising anodic material for the lithium ion battery.

Fig.14.Equivalent circuit diagram and Nyquist-type plots of electrodes for various TiO2microspheres(frequency range:0.01-100 kHz).□hollow C/TiO2-400;○hollow C/TiO2-500;△core/shell C@TiO2;?hollow neat TiO2.

4.Conclusions

Core/shell C@TiO2,hollow C/TiO2composites and hollow neat TiO2microspheres have been synthesized via a facile one-pot hydrothermal process followed by different calcinations.From SEM and HRTEM images,it is found that when calcinated in N2f l ow,the core/shell C@TiO2microspheres can be obtained.When calcinated in air at 400°C or 600°C,the hollow microspheres with(C/TiO2-400)or without(H-TiO2)carbon-residue can be produced.All samples show quite good lithium storage performance.Among them,hollow C/TiO2-400 composite microspheres signif i cantly improve the conductivity and the diffusion of Li+.After 100 cycles at 0.5 C,the discharge capacity is as high as 143.3 mA·h·g?1.Moreover,after the processes of high rate charge/discharge cycles with the current density from 0.2 C to 20 C, the capacity(195.1 mA·h·g?1)almost recovers to its initial value (205.0 mA·h·g?1)when the current density changes back to 0.2 C.The CV results show that Li+insertion/extraction processes are reversible, and the diffusion coeff i cient Daof the hollow C/TiO2composite with carbon-residue is much larger than that of C@TiO2.The EIS analysis conf i rms that the hollow C/TiO2-400 composite microspheres have the largest Li+diffusion coeff i cient.Therefore,the hollow C/TiO2-400 composite microspheres provide a potentially promising anodic material for the high-rate lithium ion battery.

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15 May 2013

☆Supported by the National Natural Science Foundation of China(21176066),the 111 Project of the Ministry of Education of China(B08021)and the Fundamental Research Funds for the Central Universities.

*Corresponding author.

E-mail address:junhu@ecust.edu.cn(J.Hu).

http://dx.doi.org/10.1016/j.cjche.2014.09.005

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 18 June 2013

Accepted 15 August 2013

Available online 16 September 2014