Electrochimica Acta 190 (2016) 556–565

Enhancing Electrochemical Performances of TiO2 Porous Microspheresthrough Hybridizing with FeTiO3 and Nanocarbon

Shimei Guoa,b, Jiurong Liua,**, Song Qiua, Yiran Wangc, Xingru Yanc, Nannan Wua,Shenyu Wanga, Zhanhu Guoc,*aKey Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education and School of Materials Science and Engineering,Shandong University, Jinan 250061, ChinabCollege of Physics and Electronic Engineering, Qujing Normal University, Qujing, Yunnan 655011, Chinac Integrated Composites Laboratory (ICL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA

A R T I C L E I N F O

Article history:Received 22 October 2015Received in revised form 28 November 2015Accepted 20 December 2015Available online 23 December 2015

A B S T R A C T

The carbon-coated TiO2 (TiO2@C) and FeTiO3 (TiO2/FeTiO3@C) hybrid porous microspheres with a specificsurface area of 35.1 m2g�1 have been successfully synthesized by carbonizing the mixture of pyrrole withlab-made TiO2/Fe2O3 porous microspheres. The as-prepared TiO2/FeTiO3@C porous electrode materialpossessed a reversible capacity of 441.5 mAh g�1 after 300 cycles at a current density of 100 mA g�1 and acoulombic efficiency of 99%. Comparing with TiO2, TiO2@C, and TiO2/Fe2O3 porous microspheres, theTiO2/FeTiO3@C porous composites exhibited superior cycling and rate performances, which are rooted inthe synergistic effect that generated by little volume variation of TiO2matrix, high capacity of FeTiO3 andgood electrical conductivity of carbon coating (with a thickness of 2–5 nm deposited on the surface andinner wall of pores) during the charge/discharge processes.

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1. Introduction

Rechargeable lithium ion batteries (LIBs) have been given greatexpectations to cope with energy crises and ease the environmen-tal deterioration resulting from the combustion of traditional fossilfuels. As is well known, LIBs have many advantages, such asavailable high voltage, high energy density, no memory effect andlong life-time, especially for the sustainable and renewablecharacteristics [1,2]. In general, the superior electrochemicalperformances of LIBs are largely dependent on their electrodematerials. Especially the anode active materials play an importantrole in the determination of energy density, safety and life cycle ofLIBs. Consequently, tremendous research efforts have beendevoted to investigate the electrochemical performances of avariety of active materials over the past few decades [3–6]. Amongthem, transition metal oxides have become of great interest forrechargeable LIBs due to their large theoretical capacity, reason-able cost, high power density, and abundant resource [7]. However,most of transition metal oxides including FexOy (Fe2O3 [8], Fe3O4

[9]), MnxOy (MnO2 [10], Mn2O3 [11], Mn3O4 [12], MnO [13]), CuO

* Corresponding author. Fax: +1 8659742933.** Corresponding author.

E-mail addresses: jrliu@sdu.edu.cn (J. Liu), zguo10@ut.edu (Z. Guo).

http://dx.doi.org/10.1016/j.electacta.2015.12.1350013-4686/ã 2015 Elsevier Ltd. All rights reserved.

[14], NiO [15], and Co3O4 [16], serving as anode materials for LIBsusually suffer from large volume expansion/shrinkage andagglomeration during the lithiation/delithiation processes, whichwill lead to the destruction of anode structure and hence rapiddegradation of electrochemical performances. Different from theaforementioned transition metal oxides, TiO2 possesses manyadvantages including high Li insertion potential (1.5–1.8 V vs Li/Li+), good capacity retention, low self–discharge, low cost,environmental friendliness, and chemical stability [17,18]. Espe-cially, less than 4% volume change (vs. ca. 10% for graphite) duringlithium ion insertion/extraction processes, intrinsically endowstitanium dioxide (TiO2) good structural stability and long cycle life[19,20]. Therefore, TiO2 has been regarded as one of the mostpromising candidate to compete with commercial graphite anodefor LIBs. However, so far, the intrinsic drawbacks of low electronicconductivity (�1 �10�12 to 1 �10�7 S cm�1), lithium ion diffusivity(�1 �10�15 to 1 �10�9 cm2 s�1), low theoretical capacity of 168–335 mAh g�1 (even lower than commercial graphite, 372 mAh g�1)[19–22], and poor rate capability are still the main obstacles for itsapplication in LIBs [23,24]. Strategies to overcome these intrinsiclimitations of TiO2 anode materials include fabricating onedimensional (1D) nanostructures (nanorod, nanofiber, nanotube,and core–shell nanowire) [25–28], constructing porous or threedimensional (3D) hierarchical architectures [29–32], hybridizingwith other conductive materials such as carbonaceous materials

S. Guo et al. / Electrochimica Acta 190 (2016) 556–565 557

(CNTs [33], graphene [34], mesoporous carbon [35]), noble metals(Au [36], Ag [37]), RuO2 [38], and/or doped with foreign atoms (N[39,40], Nb [41], N-Cr [42]). Utilizing the aforementionedstrategies, the rate capability of TiO2 anodes has been improvedby increasing the electrical conductivity and lithium ion diffusivity,but it is still a challenge to enhance the specific capacity of TiO2

anode. Therefore, TiO2 and transition metal oxides hybrid anodematerials, such as TiO2@Fe2O3 [43], TiO2/Co3O4 [20], and TiO2/MnO2 [44], have been studied since transition metal oxides havehigh theoretical capacity. In contrast, transition metal titanates,such as ferrous titanate (FeTiO3), manganese titanate (MnTiO3),and cobalt titanate (CoTiO3), are still an unheeded domain for LIBs.According to the lithiation reaction, MTiO3 þ 2 þ xð Þ Liþ þ3e� !M þ Li2O þ LixTiO2 (M = Mn, Fe, Co) [45,46], transition metaltitanates should have higher theoretical capacity comparing withTiO2 and graphite electrode [45–47]. Therefore, hybridizingtransition metal titanates with TiO2 will provide a feasibleapproach to enhance electrochemical performances for LIBs anodeby combining the high capacity of transition metal titanates withthe structural stability of TiO2. However, utilizing transition metaltitanate and TiO2 hybrid materials as LIBs anode has not beenreported yet.

In this work, we investigated the electrochemical performancesof carbon-coated TiO2 and FeTiO3 porous composites (TiO2/FeTiO3@C) as LIBs anode material, in which the porous TiO2

microspheres served as robust scaffold, the attached FeTiO3

nanoparticles as active materials with high capacity, and carboncoating as electrical conductive component. The electrochemicalmeasurements demonstrated that the as-prepared TiO2/FeTiO3@Cporous microspheres exhibited superior cycles and rate perform-ances.

2. Experimental

2.1. Materials

Tetrabutyl titanate (TBT, Ti(OC4H9)4, 99.0%), N,N-Dimethylfor-mamide (DMF, C5H9NO, 99.0%), isopropanol (IPA, C3H8O, 99.0%),Iron(III) chloride hexahydrate (FeCl3.6H2O, 99.0%), pyrrole (C4H5N,99.0%) and ammonia (NH3.H2O, 25%) were purchased fromSinopharm Chemical Reagent Co., Ltd. White Polyvinylidenefluoride (PVDF, (C2H2F2)n, MW 500 000-700 000) powders weredissolved in absolute N-methyl-2-pyrrolidinone (NMP, C5H9NO,99.0%) solvent in a weight ratio of 9:91. All the chemicals are ofanalytical grade and were used as received without furtherpurification.

2.2. Material preparation

2.2.1. Synthesis of porous TiO2 microspheresThe porous TiO2microspheres were prepared by a solvothermal

method reported in previous work [48]. In a typical synthesis,1.0 mL TBT was added to the mixed organic solvent comprising of10 mL DMF and 30 mL IPA without stirring. Then, the resultantsolution was quickly transferred into a 70 mL Teflon-lined stainlesssteel autoclave, and subsequently sealed and heated at 200 �C for20 h in an oven. After cooling to room temperature naturally, theresultant white precipitate was centrifuged and washed usingethanol for five times, and then dried in an oven at 60 �C for 12 h.The porous TiO2 microspheres were obtained by annealing thewhite precursor at 400 �C for 3 h in air.

2.2.2. Synthesis of TiO2 and Fe2O3 composite intermediateThe TiO2 and Fe2O3 (TiO2/Fe2O3) composites were prepared as

intermediates. Typically, 9.0 g FeCl3.6H2O was firstly dissolved in

10 mL deionized water to form a transparent solution undermagnetic stirring. Then, 1.6 g as-prepared porous TiO2 micro-spheres were dispersed into the above solution and immersed for4 h in vacuum. After filtering, the obtained sample was immersedin 10 mL ammonia solution for 2 h. The red brown precipitate wascentrifuged and washed using deionized water and ethanol forseveral times, and then dried in an oven at 60 �C for 12 h. Finally,the TiO2/Fe2O3 intermediate was obtained by annealing the redbrown precipitate at 500 �C for 1 h in air.

2.2.3. Synthesis of carbon coated TiO2 and FeTiO3 ternary hybridmicrospheres

1.5 g TiO2/Fe2O3 intermediate was mixed with 0.5 mL pyrroleand sealed in a 30 mL stainless steel autoclave. The carbonizationwas carried out in a furnace at 550 �C for 5 h to form carbon coating,accompanying with the formation of FeTiO3. For comparison, thecarbon coated TiO2 (TiO2@C) composites were also synthesized bycarbonizing the mixture of 1.5 g pure anatase TiO2 porousmicrospheres with 0.5 mL pyrrole in the same procedure.

2.3. Characterizations

The structure of the resultant products was determined byX-ray powder diffraction (XRD) on a Rigaku D/Max-RC X-raydiffractometer with Ni filtered Cu Ka radiation (l = 0.1542 nm,V = 40 kV, I = 50 mA) in the range of 10�80� at a scanning rate of 4�

min�1. The morphology and microstructure of the samples wereexamined by using a JSM–6700 F field emission scanning electronmicroscope (FE-SEM) at an accelerating voltage of 20 kV andelectric current of 1.0 � 10�10 A, and a JEOL JEM-2100 highresolution transmission electron microscope (HR-TEM) operatedat 200 kV. The element contents were examined by energy-dispersive X-ray spectroscopy (EDS) detector attached to the FE-SEM. The N2 adsorption/desorption isotherms of porous productswere measured at 77 K on a Quadrasorb-SI instrument. The specificsurface area was calculated with the Brunauer-Emmett-Teller(BET) model and the pore size distribution was determined usingthe Barrett-Joyner-Halenda (BJH) method. The composition wasdetermined by X-ray photoelectron spectroscopy (XPS) on a KratosAnalytical spectrometer, using Al Ka (hn = 1486.6 eV) radiation asthe excitation source under the anode voltage of 12 kV andemission current of 10 mA. Thermogravimetric analysis (TGA, TAInstruments SDT Q600) was performed in air from ambienttemperature to 800 �C at a heating rate of 5 �C min�1 using a SDTthermal-microbalance apparatus.

2.4. Electrochemical measurements

To prepare the working electrode, the active material, carbonblack, and polyvinylidene fluoride (PVDF) with a weight ratio of7:2:1 were mixed in N-methyl-2-pyrrolidinone (NMP) to form ahomogenous slurry, which was coated on a copper foil substrate,followed by drying in a vacuum oven at 120 �C for 12 h. TheCR2025-type cells were assembled using Li foil as counter andreference electrode, Celgard 2300 as separator, and 1 M LiPF6(dissolved in ethylene carbonate, dimethyl carbonate, and ethylenemethyl carbonate with a volume ratio of 1:1:1) as the electrolyte.The assembly was performed in a glove-box filled with argonatmosphere. The performance of the cells was evaluated galva-nostatically in the voltage range from 0.02 to 3 V at various currentdensities on a LAND CT2001A battery test system. The specificcapacity is calculated based on the total mass of the materials. Themass loading of the electrodes is around 1.5–2.0 mg cm�2. Cyclicvoltammogram (CV) was obtained by a PARSTAT 2273 electro-chemistry workstation at a scan rate of 0.3 mV s�1 and thepotential vs. Li/Li+ ranging from 0.01 to 3 V. Electrochemical

Fig. 1. XRD patterns of (a) atanase TiO2 precursor and (b) TiO2/FeTiO3@C.

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impedance spectra were tested on the same instrument with ACsignal amplitude of 10 mV in the frequency range from 100 kHz to0.01 Hz. The data were adopted to draw Nyquist plots using realpart Z0 as X-axis, and imaginary part Z00 as Y-axis.

Fig. 2. XPS spectra of (a) Fe 2p, (b) Ti 2p, (c) O 1s and

3. Results and discussion

3.1. Electrode materials preparation and characterizations

As shown in Fig. 1a, XRD pattern demonstrates the formation ofanatase TiO2 after the solvothermal and subsequent annealingprocesses. All diffraction peaks match well with the tetragonalanatase TiO2 (JCPDS no. 21–1272) without any other phase,suggesting the high purity of TiO2 [21,48]. After the porous TiO2

microspheres were successively immersed in FeCl3 and ammoniaaqueous solution, the TiO2/Fe2O3 porous composites wereobtained by annealing the dry impregnated precipitate as shownin Fig. S1b (see Supporting Information). When the carbonizationof coated pyrrole is completed, all XRD peaks of the final productare assigned to rhombohedral FeTiO3 (JCPDS no. 29–0733) andanatase TiO2 (JCPDS no. 21–1272) (Fig. 1b). No other diffractionpeak is detected, suggesting that Fe2O3 has been transformed intoFeTiO3 completely by reacting with carbon and TiO2. Meanwhile,the formed carbon coating on the TiO2/FeTiO3 surface isamorphous, consistent with the subsequent HR-TEM observation.In these procedures, the reactions for the formation of TiO2/Fe2O3

intermediate and TiO2/FeTiO3@C hybrid microspheres can beillustrated as follows:

Fe3þ þ 3OH� ! Fe OHð Þ3 # ð1Þ

(d) C 1s of TiO2/FeTiO3@C hybrid microspheres.

Fig. 3. FE-SEM images of the as-prepared (a) TiO2, (b) TiO2/Fe2O3, and (c) TiO2/

S. Guo et al. / Electrochimica Acta 190 (2016) 556–565 559

2Fe OHð Þ3 ! Fe2O3 þ 3H2O " ð2Þ

2Fe2O3 þ C þ 4TiO2 ! 4FeTiO3 þ CO2 " ð3ÞTo further characterize the composition and chemical bonding

of the resultant TiO2/FeTiO3@C hybrid microspheres, x-rayphotoelectron spectroscopy (XPS) was carried out. From thesurvey scan spectrum (Fig. S2a), the peaks of Fe 2p, Ti 2p, O 1s, N 1sand C 1s were detected, indicating the existence of Fe, Ti, O, C and Nelements in the as-prepared hybrid microspheres. Two peaks atthe binding energy of 724.6 and 710.6 eV (Fig. 2a) are assigned to Fe2p1/2 and Fe 2p3/2, respectively. The fingerprint shakeup satellitepeak of Fe2O3 at ca. 719 eV was not observed [49,50], suggestingthat Fe2O3 in TiO2/Fe2O3 intermediate has been converted toFeTiO3 during the carbonization process, consistent with XRDresult (Fig. 1b). In Fig. 2b, the Ti 2 p doublet centering at 458.3 and464.3 eV stems from Ti 2p3/2 and Ti 2p1/2, respectively [51]. Thesplitting binding energy between Ti 2p1/2 and Ti 2p3/2 core levelsis 6 eV, indicating a normal state of Ti4+ in the product [51,52].Three deconvoluted O 1s peaks (Fig. 2c) at 532.7, 531.2 and 529.6eV are assigned to O-C, Fe-O-Ti, and Ti-O-Ti bond, respectively,according to the successive decrease of C, Fe, and Ti electronega-tivity. The asymmetrical C 1s peak (Fig. 2d) could be deconvolutedinto four peaks centered at ca. 284.6, 285.9, 287.1, 288.9 eV, whichare corresponding to the sp2C-sp2C, N-sp2C, N-sp3C, and C-Obonds, respectively [53,54]. For the N 1s spectrum, two deconvo-luted peaks at 398.2 and 400.1 eV (Fig. S2b) are attributed to theformation of pyridinic N (398.3 eV) and pyrrolic N (400.0 eV) in thecarbon coating after the decomposing of pyrrole [55].

The morphology and structure of the as-synthesized TiO2

precursor, TiO2/Fe2O3 intermediate and TiO2/FeTiO3@C productwere investigated by a field emission scanning electron micro-scope (FE-SEM) and a high resolution transmission electronmicroscope (HR-TEM). As shown in Fig. S3, the low-magnificationSEM image indicates that the synthesized TiO2 precursors are well-defined microspheres with the diameter of ca. 0.6–1.5 mm andrough surface. A close inspection reveals that the microsphereshave a porous architecture, which is in fact built from the curvedtwo-dimensional nanoflakes (Fig. 3a). After being impregnated inFeCl3 and ammonia solution and annealed at 500 �C for 1 h in air,the size of microsphere has no obvious variation, but a lot of tinyparticles are observed adhering on the nanoflakes or filling in thepores (Fig. 3b). Following the carbon coating on the TiO2/Fe2O3

hybrid microspheres, the nanoflake structure has been trans-formed into nanoparticles due to the reaction of Fe2O3 with carbonand TiO2 as shown in Fig. 3c. Combining with HR-TEM observation(Fig. 4a), it can be confirmed that the as-prepared TiO2/FeTiO3@Chybrid microsphere has a loose architecture comprised ofnumerous nanoscale particles and pores. In the HR-TEM image(inset of Fig. 4b), two sets of lattice fringe spacings of 0.3488 and0.2689 nm are corresponding to the (101) plane of anatase TiO2 and(104) crystal plane of FeTiO3, respectively, suggesting that theadhered Fe2O3 nanoparticles on TiO2 nanoflake have been trans-formed into FeTiO3 after reaction with TiO2 and carbon. A carbonlayer with the thickness of 2–5 nm deposited on the surface ofnanoparticles is observed, further confirming the presence ofcarbon coating on the as-synthesized TiO2/FeTiO3@C hybridmicrospheres.

TGA test has been carried out to evaluate the carbon content inthe synthesized TiO2/FeTiO3@C porous microspheres. The weightloss can be separated into three steps, Fig. 5. In step (a), from roomtemperature to 150 �C, the weight loss of ca. 1.6% corresponds tothe evaporation of water absorbed in the porous microspheres. The

dominant weight loss of 5.14% between 150 and 400 �C in step (b) isattributed to the oxidation process of carbon to form volatilespecies, such as CO and CO2. After 400 �C, a slight bullish weightslope is observed, suggesting that the oxidation of Fe2+ to Fe3+

begins to occur. Some previous works have confirmed that FeTiO3

will be partially oxidized to Fe2Ti3O9 and Fe2O3 below 800 �C after aheat treatment in air, leading to a slight weight gain [56–58]. TheEDS spectrum of TiO2/FeTiO3@C microspheres (Fig. S4) shows thepresence of C, O, Ti, and Fe elements, and a small peak at ca. 2 KeV isassigned to Pt element resulting from the Pt plating of samplebefore FE-SEM operation. The atomic ratio of Ti to Fe in the TiO2/FeTiO3@C sample is 13.19: 2.04 achieved by the EDS quantitative

FeTiO3@C microspheres.

Fig. 4. HR-TEM images of TiO2/FeTiO3@C microspheres at different magnifications(a, b). The inset of (b) are enlarge images from the marked areas.

Fig. 5. TGA curve of TiO2/FeTiO3@C sample at a heating rate of 5 �C min�1 under airflux.

Fig. 6. N2 adsorption-desorption isotherms of porous (a) TiO2 and (b) TiO2/FeTiO @C microspheres. The insets are pore size distribution curves.

560 S. Guo et al. / Electrochimica Acta 190 (2016) 556–565

analysis without considering Pt. Combining the EDS with TGAresults, the mass fractions of TiO2, FeTiO3 and C in the hybridmicrospheres are calculated to be 70.4%, 24.46%, and 5.14%,respectively.

As LIBs anode material, porous structure can facilitate anefficient contact of the internal active materials with electrolyte,leading to a fast transportation of Li+ ions. Meanwhile, the highspecific surface area and porosity can alleviate the volume

variation during the Li+ insertion/extraction, resulting in arelatively high reversible capacity and cycling stability [21,59].Therefore, nitrogen absorption-desorption measurements wereperformed to investigate the porosity and specific surface area ofthe as-prepared TiO2 and TiO2/FeTiO3@C porous microspheres. Asshown in Fig. 6, the nitrogen adsorption/desorption isotherms ofthe two samples all exhibit the type IV nitrogen adsorption branchwith a type-H3 hysteresis loop, indicating the presence ofmesopores in the TiO2 and TiO2/FeTiO3@C microspheres [21,44].From the pore size distribution curve (inset of Fig. 6a), the pore sizeof TiO2 microspheres is nonuniform ranging from several to ca.100 nm, and the pores with a size of ca. 10 nm are dominant. TheBET surface area is 58.4 m2g�1, and the pore volume is calculated tobe 0.14 cm3 g�1. While as to the TiO2/FeTiO3@C sample, the BETsurface area and pore volume are 35.1 m2g�1 and 0.19 cm3 g�1,respectively. The pore size has a wider distribution (inset of Fig. 6b)than that of TiO2, and the dominant pore size is increased to ca.30 nm. The nitrogen adsorption–desorption measurements sug-gest the transformation of nanoflakes into nanoparticles, whichhas induced the variation of microstructure in the TiO2/FeTiO3@Csample, consistent with the SEM observation (Fig. 3), finallyresulting in an increased pore size. The reaction of TiO2 with Fe2O3

and carbon would consume TiO2 matrix and release CO2 gas,leading to the increase of pore volume during the carbonizationprocess. The decrease of BET surface area may be attributed to the

3

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formation of FeTiO3 and deposition of carbon coating intochannels.

3.2. Electrochemical performance of the products

For a better understanding of its applicability in LIBs, theelectrochemical performances were investigated. The workingelectrodes were fabricated from the as-synthesized porous TiO2/FeTiO3@C nanocomposites. In addition, the anatase TiO2, TiO2@Cand TiO2/Fe2O3 electrodes were also prepared for comparison.Fig. 7a&b show the cyclic voltammetry (CV) curves of anatase TiO2

and TiO2/FeTiO3@C samples in the initial three cycles, respectively.For the pure TiO2 porous microspheres (Fig. 7a), there are threeclear cathodic peaks appeared at ca. 0.4, 0.9 and 1.3 V in the firstdischarge process, where the peaks at 0.4 and 0.9 V disappearedfrom the second discharge process, suggesting the irreversiblereduction of electrolyte and the formation of amorphous Li2O andsolid electrolyte interphase (SEI) layer [23]. In the subsequentcharge process, only one anodic peak at about 2.3 V is observed.According to the previous works, the pair of cathodic/anodic peaksat 1.3 V (shifted to ca. 1.6 V from the second cycle) and 2.3 V arecharacteristics for Li+ insertion/extraction reaction in anatase TiO2,corresponding to the reversible biphasic transition betweenthe tetragonal anatase and orthorhombic LixTiO2

(TiO2 þ xLiþ þ xe�$LixTiO2) [23,60]. There is an observabledecrease of cathodic current in the second cycle than the first

Fig. 7. Cyclic voltammetry (CV) curves of porous (a) TiO2 and (b) TiO2/FeTiO3@Cmicrospheres at the scanning rate of 0.3 mV s�1 in the range of 0.01-3.0 V.

one for the TiO2 electrode, consistent with the previous report onTiO2 anode and arising from the irreversible lithium insertion/extraction reaction, indicating a large capacity loss during the firsttwo cycles [23,61]. As to the TiO2/FeTiO3@C sample, there are alsothree reduction peaks at ca. 0.5, 1.0 and 1.5 V observed in the firstcathodic process, but two oxidation peaks at ca. 1.7 and 2.3 Vappeared in the counterpart anodic process (Fig. 7b). Evidently, thecouple of 1.5/2.3 V is assigned to anatase TiO2, consistent with theabove result of pure anatase TiO2 porous microspheres. While thebroad cathodic peak at ca.1.0 V and the other cathodic peak from0.4 to 0.6 V are attributed to the initial reduction of FeTiO3

accompanying with a phase transformation from TiO2 to LixTiO2

(FeTiO3 þ 2 þ xð Þ Liþ þ3e� ! Fe þ Li2O þ LixTiO2) [45,46] and theformation of amorphous Li2O and irreversible solid electrolyteinterphase (SEI) layer [20,43,60]. In addition, the wide and weakanodic peak detected at ca. 1.7 V in the first charge process resultsfrom the oxidation of Fe� to Fe2+ [60,62]. After the second cycle, theCV curves tend to overlap, suggesting that the TiO2/FeTiO3@Celectrode exhibits a gradually enhanced cycling stability for theinsertion and extraction of lithium ions.

In order to evaluate the cycling performances of the electrodes,the discharge/charge measurements were evaluated by LANDCT2001A battery test system at a current density of 100 mA g�1.Fig. 8a&b show the discharge/charge curves of pure anatase TiO2

and TiO2/FeTiO3@C samples, respectively. In the cathodic process,Fig. 8a, a well-defined potential plateau at ca. 1.75 V is owing to theLi+ intercalation into the interstitial octahedral sites of anatase

Fig. 8. Galvanostatic discharge/charge curves of the 1st, 2nd, 3rd, 100th and 300thcycles for (a) porous TiO2 and (b) TiO2/FeTiO3@C microspheres.

562 S. Guo et al. / Electrochimica Acta 190 (2016) 556–565

TiO2, while the distinct potential plateau observed at ca. 2.0 V inthe anodic process corresponds to the Li+ de-intercalation ofanatase TiO2 [46,60]. When the first cycle is finished, the cellexhibits the initial discharge and charge capacities of 319.2 and130.6 mAh g�1, respectively, corresponding to the initial coulombicefficiency (the ratio of charge capacity to discharge capacity) ofonly 40.9%. The large capacity loss can be attributed to somelithium insertion into irreversible sites and poor intrinsic electricalconductivity of TiO2 [63]. For the TiO2/FeTiO3@C sample, as shownin Fig. 8c, the potential/capacity profiles show some distinctdifferences by comparing with those of pure anatase TiO2 sample.The potential plateau at ca. 1.7 and 2.0 V are obviously shortened inthe initial three cathodic/anodic processes. Furthermore, a shortpotential plateau is observed at ca. 0.75 V in the first dischargeprocess and an unconspicuous slope from 0.5 to 1.7 V is observed inthe subsequent charge process, corresponding to the change in Feoxidation states during lithium insertion and extraction [5,62].Evidently, these results match well with the above CV analysis.With decreasing the potential to 0.02 V in the first dischargeprocess, the cell exhibits an initial specific capacity of 565.4 mAhg�1, which is distinctly higher than the theoretical values ofanatase TiO2 (ca. 335 mAh g�1 for LiTiO2) [22], FeTiO3 (530 mAhg�1) [45], and graphite (372 mAh g�1). The reasons can beattributed to the decomposition of electrolyte and the formation ofSEI films [46]. Similar phenomena have also been found in otheranode materials [13,16,64]. When the first charge was completed, aspecific capacity of 297 mAh g�1 was obtained, and the initialcoulombic efficiency was 52.5%, which is obviously higher thanthat of TiO2 sample (40.9%). It is noted that, the specific capacity isincreased along with the cycling onward (Fig. 8b), the retainedcapacity of 441.5 mAh g�1 after 300 cycles is much higher than theinitial one (297mAh g�1), attributed to a gradual activation processof the electrode made of transition-metal oxide nanomaterials[23,60,64,65].

The cycling performances of the samples are depicted in Fig. 9.It can be found that the reversible capacities of the anatase TiO2,TiO2/Fe2O3, TiO2@C and TiO2/FeTiO3@C electrodes are 101, 136.7,236.3 and 336.4 mAh g�1 after 100 cycles, respectively. Based onthe mass ratio and theoretical capacity of each component in TiO2/FeTiO3@C sample, the theoretical capacity of TiO2/FeTiO3@C hybridstructure is calculated to be �385 mAh g�1 (see SupportingInformation). However, it is interesting that, the reversible capacityof TiO2/FeTiO3@C electrode exhibits an increasing tendency withthe cycling onward, even reaches 441.5 mAh g�1 after 300 cycles.The main reason could be attributed to the formation/dissolution

Fig. 9. Cycling performances of TiO2, TiO2/Fe2O3, TiO2@C, TiO2/FeTiO3@C and co

of polymeric gel-like films, where the newly generated Fe�

nanoparticles from FeTiO3 can promote/activate the reversibletransformation of some SEI components [65–67]. Meanwhile, thesynergetic effect of the hybrid materials was also the importantfactor for the increased capacity as for bare TiO2 and reportedFeTiO3 anodes [45,46], there were no capacity increase observed[65]. The higher reversible capacity of TiO2/FeTiO3@C than those ofTiO2, TiO2/Fe2O3, and TiO2@C demonstrates that the hybridizationof TiO2, FeTiO3 and carbon is an efficient way to improve its cyclingperformance and leads to a higher capacity. The cycling perfor-mance of TiO2/FeTiO3@C is also much better than those of thepreviously reported TiO2@C anodes (e.g., the reversible capacity of173 mA h g�1 after 100 cycles at the current density of 30 mA g�1

for nanoparticles [68], 182 mAh g�1 after 60 cycles at the currentdensity of 84 mA g�1 for mesoporous spheres [39], and 142.4 mAhg�1 after 170 cycles at the current density of 80 mA g�1 forparticulate structure [40]) or FeTiO3 anodes (e.g., 200 mAh g�1

after 50 cycles at the current density of 50 mA g�1 for nanoflowers[45]) in long cycling performance and reversible capacity.Moreover, the coulombic efficiency of TiO2/FeTiO3@C electrodequickly reaches 90% in the second cycle, even remains more than98% from three to 300 cycles. From the rate capability shown inFig. 10a, the pure anatase TiO2 electrode delivers the reversiblecapacities of 88.1, 66.0, 49.1, 30.1, and 20.9 mAh g�1 at the currentdensity of 100, 200, 400, 800 and 1600 mA g�1, respectively, whilethe TiO2/FeTiO3@C electrode exhibits higher reversible capacitiesof 287.3, 222.9,184.6, 139.3, and 75.1 mAh g�1 at the correspondingcurrent densities, indicating that TiO2/FeTiO3@C has more superiorrate capability than porous anatase TiO2 microsphere electrode.When the current density is returned to 100 mA g�1 after the rateperformance test, the discharge capacities of two samplesapproximately return back to their initial values, suggesting thatthe high current charge/discharge process did little to break downthe integrity of the electrodes [64,69]. The structure stability ofTiO2/FeTiO3@C sample was further investigated after 300 cycles ata current density of 100 mA g�1. As shown in Fig. S5, themicrosphere-like morphology of TiO2/FeTiO3@C has not beenchanged, suggesting that the structure integrity has maintainedwell upon cycling.

To further investigate the electrochemical performances of theTiO2/FeTiO3@C porous microspheres, electrochemical impedancespectroscopy (EIS) measurements were conducted to reveal thelithium ion diffusion and electron transfer. Fig. 10b shows theNyquist plots of the porous TiO2/FeTiO3@C and anatase TiO2

microspheres after 100 and 200 cycles at a current rate of

ulombic efficiency of TiO2/FeTiO3@C at the current density of 100 mAh g�1.

Fig. 10. (a) Rate capabilities of TiO2/FeTiO3@C and TiO2. (b) Experimental and fittedNyquist plots of TiO2 after 100 cycles, TiO2/FeTiO3@C after 100 cycles and TiO2/FeTiO3@C after 200 cycles. The inset of (b) is the corresponding equivalent circuit.

S. Guo et al. / Electrochimica Acta 190 (2016) 556–565 563

100 mA g�1. The Nyquist plot of each cell is comprised of arc in thehigh- and medium-frequency region, and an inclined line in thelow frequency region [69]. The diameter of the semicircle is indirect proportion to the impedance, which contains electrolyteresistance (Re), surface film resistance (Rsf) and charge transferresistance (Rct) [70,71]. The inclined line is related to the lithium-ion diffusion inside electrode materials corresponding to theWarburg impedance (Zw) [17,72,73]. The impedance spectra can befitted based on a reasonable equivalent circuit (inset of Fig. 10b), inaccordance with the Li-ion insertion/extraction mechanism inelectrode [72,73]. The fitted curves are in accordance with theexperimental data and CPE in this model expresses the doublelayer capacitance (inset of Fig. 10b). The impedance values of eachsamples obtained from the fitting results of EIS data aresummarized in Table S1. The charge transfer resistance (Rct) forTiO2/FeTiO3@C and pure TiO2 samples are ca. 165 and 196 V after100 cycles, respectively, indicating that the hybridization withcarbon coating and FeTiO3 enhanced the electrical conductivityand charge transfer of TiO2. Moreover, the TiO2/FeTiO3@C electrodeexhibits larger slope at low frequency than TiO2 (Fig. 10b),suggesting that the lithium ion diffusion in the electrode hasbeen also improved. This could be mainly attributed to the porevolume increase of TiO2/FeTiO3@C after the Fe2O3 doping andcarbonization (Fig. 6), which is more beneficial to lithium iondiffusion. Therefore, the addition of carbon and FeTiO3 in the

composites can not only provide good electrode electron contactand electrical conductivity, but also facilitate the lithium iondiffusion, in favor of the capacity enhancement of TiO2/FeTiO3@Canode in LIBS. Furthermore, it is noted that the Nyquist plot of TiO2/FeTiO3@C displays a smaller semicircle after 200 cycles than100 cycles, suggesting the gradual improvement of electricalconductivity with cycling. This experimental result provides areasonable evidence for increasing the capacity performance ofTiO2/FeTiO3@C electrode along with the cycling onward in Fig. 9.

For the LIBs anode material made from TiO2/FeTiO3@Ccomposite microspheres, TiO2 with little volume variation (lessthan 4%) serves as a matrix to relieve the stress and to remain thestructure stability of anode material during the charge/dischargeprocesses. In the TiO2/Fe2O3 intermediate, Fe2O3 was transformedinto FeTiO3 completely by the reaction with carbon and TiO2 afterthe carbonization process at 550 �C for 5 h (Fig. 1). The FeTiO3

nanoparticles hybridized with TiO2 (Fig. 3d) enhance the capacityof LIBs anode due to the higher theoretical capacity (530 mAh g�1)than that (335 mAh g�1) of anatase TiO2. The presence of carboncoating increases the electrical contact between FeTiO3 and TiO2

nanoparticles, avoiding effectively the electrical isolation of TiO2/FeTiO3@C anode. As expected, the TiO2/FeTiO3@C porous compo-sites exhibit higher cycling and rate performances than thecorresponding TiO2 porous microspheres.

4. Conclusions

The TiO2/FeTiO3@C ternary hybrid porous microspheres havebeen synthesized as anode material for Li-ion battery by a facileapproach. Comparing with the as-prepared TiO2, TiO2@C, and TiO2/Fe2O3 porous microspheres, TiO2/FeTiO3@C composites exhibitedsuperior cycling and rate performances by combining the structurestability of TiO2 matrix with high capacity of FeTiO3 and superiorelectrical conductivity of carbon coating during the charge/discharge processes. The hybridization of TiO2, FeTiO3 andconductive carbon coating is proved to be an efficient way toovercome the intrinsic drawbacks of TiO2 and to improve theelectrochemical performances of LIBs anode.

Acknowledgements

This work was supported by the Fundamental Research Fundsof Shandong University (2015JC016). The authors also acknowl-edge the financial supports from the Doctoral Program of HigherEducation of China (20130131110068), and Natural Science Fundfor Distinguished Young Scholars of Shandong (JQ201312). Z. Guoappreciates the start-up fund from University of TennesseeKnoxville.

Appendix A. Supplementary data

Supplementarydataassociatedwiththisarticlecanbefound, intheonline version, at http://dx.doi.org/10.1016/j.electacta.2015.12.135.

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