carbon hybrid nanowebs as anode material for lithium-ion batteries

Energy 55 (2013) 925e932

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Highly reversible lithium storage in uniform Li4Ti5O12/carbon hybrid nanowebs as anode material for lithium-ion batteries Zunxian Yang a, *, Qing Meng b, Zaiping Guo b, c, **, Xuebin Yu d, ***, Tailiang Guo a, Rong Zeng e a

National & Local United Engineering Laboratory of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, PR China Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia School of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, NSW 2522, Australia d Department of Materials Science, Fudan University, Shanghai 200433, PR China e School of Computing, Engineering and Mathematics, University of Western Sydney, NSW 2751, Australia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 December 2012 Received in revised form 21 January 2013 Accepted 22 January 2013 Available online 5 March 2013

Very large area, uniform Li4Ti5O12/carbon composite nanowebs consisting of interconnected nanofibers were synthesized by a simple method based on thermal pyrolysis and oxidation of a composite of electrospun lithiumetitanium/polyacrylonitrile nanowebs in argon atmosphere. This novel composite is characterized by the encapsulation of highly uniform nanoscale Li4Ti5O12 crystals in the porous cottonlike carbon matrix. This unique structure, consisting of ultra-small crystals in carbon core/shell architecture, is also characterized by high porosity, with many nanopores and mesopores in the composite, and this, together with the high conductive carbon matrix, would facilitate the excellent electrochemical performance of Li4Ti5O12/carbon composite nanoweb electrode. The Li4Ti5O12/carbon hybrid nanoweb electrodes display a reversible capacity of approximately 160.8 mAhg
1 at a current density of 30 mAg
1 and excellent cycling stability. The Li4Ti5O12/carbon hybrid nanoweb electrodes also exhibit excellent rate performance, delivering a discharge capacity of over 87 mAhg
1 at a current density of 3000 mAg
1. These results indicate that the composite is a promising anode candidate for lithium ion batteries. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Li4Ti5O12/carbon nanowebs Electrospinning Lithium-ion battery

1. Introduction Spinel Li4Ti5O12, as one of the most stable phases of the Li2OTiO2 system, has attracted particular attention for lithium ion battery (LIB) application, mainly due to its nearly zero-strain characteristics [1e5], its high lithium mobility [6], attributable to the particular mechanism of phase transition between spinel and the NaCl structure, and further, the absence of surface lithium plating and dendrite formation [7] during lithium insertion/extraction, unlike other anode materials [2]. Spinel Li4Ti5O12 is thus theoretically expected to be characterized by good capacity, excellent cyclability, and high-rate capability, all very important qualities that are needed for the safe LIB anode that is required for hybrid electric vehicles (HEVs) [1,8e17]. * Corresponding author. ** Corresponding author. Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia. Tel.: þ61 2 4221 5225; fax: þ61 2 4221 5731. *** Corresponding author. E-mail addresses: [email protected] (Z. Yang), [email protected] (Z. Guo), [email protected] (X. Yu). 0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.01.055

Recently, a variety of Li4Ti5O12 nanomaterials including nanoparticles [18], especially one-dimensional (1D) Li4Ti5O12 nanomaterials [19], have been prepared and used as electrode materials in lithium ion batteries because they offer shorter paths for Liþ transport and higher surface area, resulting in more side reactions with the electrolyte than with bulk Li4Ti5O12. However, certain problems, stemming from the relatively poor natural electronic conductivity of Li4Ti5O12, still challenge its further practical application in high energy lithium ion batteries. In order to improve the conductivity of Li4Ti5O12 materials, various strategies such as surface coating with conductive materials, e.g., Ag nanoparticles [20e 22], or dispersion of such nanoparticles into a carbon matrix [6,23,24], preparation of submicron or nanosized Li4Ti5O12 [18] aiming to significantly shorten the diffusion length for lithium ions, etc., have been developed. In addition, one possible way to effectively improve the electrochemical lithium storage properties of 1D Li4Ti5O12-based nanomaterials is to effectively enhance the contact area between the electrode and the electrolyte and to improve the diffusion of both electrolyte and lithium ions during lithium insertion/extraction by forming three-dimensional (3D) nanoweb architectures for LIBs, based on the cross-connection of

Z. Yang et al. / Energy 55 (2013) 925e932

3. Results and discussion 3.1. Characterization Fig. 1 shows the X-ray diffraction patterns of the as-prepared Li4Ti5O12/TiO2 nanowebs and Li4Ti5O12/carbon nanowebs produced via heat treatment in air or in argon atmosphere, respectively. When post-treated in air, the as-collected lithium-titanium composite/PAN nanowebs were transformed into Li4Ti5O12/TiO2 hybrid nanowebs consisting of anatase TiO2 (JCPDS 21-1272) and Li4Ti5O12 (JCPDS 49-0207) (see Fig. 1(a)). However, when heat treated in argon, the lithium-titanium/PAN nanoweb composite

Li4Ti5O12/TiO2 nanowebs

(a)

(215)A

A (440)(204) L

(211)A (105)A

(200)A

JCPDS49-0207(L))

Li4Ti5O12/Carbon nanowebs

(b)

(JCPDS49-0207(L)) (440)L

(400)L

(311)L (103) (112)AA (004)A (400)L

(JCPDS21-1272(A),

(311)L

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The procedures for preparing the electrospinning solution are similar to those described previously [27e29]. Simply, 0.7 g polyacrylonitrile (PAN, MW ¼ 150,000, Aldrich) was dissolved in 5.6 g N,N-dimethylformamide (DMF, 99.8%, Aldrich) at 80  C with vigorous stirring for 2 h (solution No. 1), and then, 1.0 g titanium (IV) isopropoxide (97%, Aldrich) and 0.286 g CH3COOLi (Aldrich) were mixed with 2.4 g anhydrous ethanol and 3.1 g acetic acid (solution No. 2). Afterwards, the No. 2 solution was added dropwise to the No. 1 solution at 80  C with vigorous stirring. The mixed solution was then stirred at room temperature for 3 h again. The polymer solution was transferred into a 10 ml syringe with a capillary tip (0.8 mm diameter). For spinning, the set-up was similar to those reported [25]. Typically, the collector was placed 9.5 cm from the spinneret to collect the nanofibers. A high voltage of 13.3 kV was applied between the spinneret and the collector by a direct-current power supply (DW-P303-5ACCD, Tianjin Dongwen High Voltage Power Supply Co., China.). The solution was pushed out of the spinneret by a syringe pump (TS2-60, Baoding Lange Constant Flux Pump Co., China) at the rate of 0.3 ml/h. The collector was kept at 180  C during the electrospinning process to evaporate the solvent. After spinning for more than 20 h, the nanofiber films were easily peeled off. The electrospun nanofibers were slowly decarbonized at 500  C for 2 h in an air environment, or carbonized at 500  C for 2 h in argon atmosphere (heating rate: less than 1  C min
1), respectively. Finally, a white film (Li4Ti5O12/TiO2 nanowebs) and a black film (Li4Ti5O12/ carbon nanowebs) were obtained.

Electrochemical properties were measured on electrodes prepared by compressing a mixture of as-prepared Li4Ti5O12/carbon hybrid nanowebs or Li4Ti5O12/TiO2 nanowebs, carbon black (Super P, MMM, Belgium), and poly(vinyl difluoride) (PVDF) binder in a weight ratio of 70:15:15 and pasting the mixture on copper foil. Pure lithium metal foil was used as the counter and reference electrode. The electrolyte was LiPF6 (1 M) in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v; MERCK KgaA, Germany). Coin cells were assembled in a high-purity argon-filled glove box (Mbraun, Unilab, Germany). The galvanostatic method was used to measure the electrochemical capacity of the electrodes at room temperature with a LAND-CT2001A instrument. The cut-off potentials of both the Li4Ti5O12/carbon hybrid nanoweb and the Li4Ti5O12/TiO2 nanoweb electrodes for charge and discharge were set at 3.0 and 1.0 V versus Liþ/Li, respectively. Cyclic voltammetry (CV) was performed on a ChI660B electrochemical workstation.

(110)L

2.1. Synthesis of Li4Ti5O12/carbon and Li4Ti5O12/TiO2 composite nanowebs

2.3. Electrochemical characterization

(111)L

2. Experimental

Li4Ti5O12/carbon nanowebs were obtained by X-ray diffraction (XRD) analysis (MMA, GBC, Australia). Energy dispersive X-ray spectroscopy (EDX) analysis was carried out on the JEOL 7500FA analytical electron microscope as well. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a VG Scientific ESCALAB 220IXL instrument using aluminum Ka X-ray radiation during XPS analysis.

(111)L

one-dimensional Li4Ti5O12-based nanomaterials during their synthesis process. Electrospinning is a simple method to form continuous onedimensional nanofibers under the electrostatic force of the charges on the surface of a liquid droplet in a sufficiently high electric field, which is applied between the capillary nozzle and the metal collector [25,26]. Electrodes with 3D nanoweb architectures can be readily prepared by electrospinning via controlling the configuration of the collector or adjusting the ingredients of the spun composite and the thermal treatment temperature in argon or in air atmosphere. Herein, this paper presents a relatively simple and low-cost approach to prepare Li4Ti5O12/carbon hybrid nanowebs consisting of interconnected nanofibers by a combination of electrospinning and subsequent thermal treatments. The asprepared Li4Ti5O12/carbon nanowebs have more unique advantages, such as nanoporosity, mesoporosity, three-dimensional nanoweb architecture, and large surface-to-volume ratio. These Li4Ti5O12/carbon nanowebs have been investigated for potential use as an anode material for the lithium ion battery and have exhibited excellent cycling stability and rate capability.

Intensity ( a.u.)

926

2.2. Materials characterization Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the samples containing carbon were carried out with a TGA/DSC1 type instrument (METTLER TOLEDO, Switzerland) at a heating rate of 10  C min
1 from 25 to 1000  C in air. The morphology was evaluated using a JEOL 7500FA field emission scanning electron microscope (FE-SEM, JEOL, Tokyo, Japan). Transmission electron microscope (TEM) investigations were performed using a JEOL 2011F analytical electron microscope (JEOL, Tokyo, Japan) operating at 200 kV. The composition and crystal structures of the Li4Ti5O12/TiO2 nanowebs and the

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Fig. 1. X-ray diffraction patterns of as-prepared Li4Ti5O12/TiO2 nanowebs and Li4Ti5O12/carbon nanowebs: (a) Li4Ti5O12/TiO2 hybrid nanowebs consisting of Li4Ti5O12 with cubic structure (JCPDS 49-0207) and anatase with tetragonal structure (JCPDS 211272); (b) Li4Ti5O12/carbon nanowebs with cubic structure (JCPDS 49-0207), as indexed in the patterns. Subscripts ‘A’ and ‘L’ denote anatase and Li4Ti5O12, respectively.

Z. Yang et al. / Energy 55 (2013) 925e932

was transformed into Li4Ti5O12/carbon composite nanowebs, in which one main obvious phase except for carbon, Li4Ti5O12 with a cubic structure (JCPDS 49-0207) (see Fig. 1(b)), can be detected from its relatively weak diffraction peaks. This is mainly attributable to the encapsulation effect of the high carbon content on the titanium composite (carbon percent ¼ 17.6wt%), according to the TGA results (see Supporting Information Fig. S1). The encapsulation interferes with the X-ray diffraction of the crystalline Li4Ti5O12 in the composite and to some extent prevents lithium evaporation during heat treatment in argon. Therefore, the post heat treatment of as-spun lithium-titanium/PAN composite in argon atmosphere facilitates the formation of Li4Ti5O12 phase as compared with the post heat treatment in air. The as-collected lithium-titanium/PAN nanoweb composite, Li4Ti5O12/carbon nanowebs, and Li4Ti5O12/TiO2 nanowebs display web-like morphology owing to the conglutination of randomlyaligned nanofibers after high-temperature sintering, as shown in the FE-SEM images (Fig. 2 and Supporting Information Fig. S2). The FE-SEM images clearly reveal an overview of the uniform lithiumtitanium/PAN nanoweb composite consisting of randomly-aligned nanofibers with diameters of 200e300 nm and lengths extending to several tens of millimeters (see Fig. 2(a)). Observation of the lithium-titanium/PAN nanoweb composite also clearly shows the randomly-aligned nanofibers with slightly curved morphology, which is possibly attributable to the strain aroused by the solvent evaporation during the synthesis process. After calcination in argon at 500  C for 2 h, the uniform lithiumetitanium/PAN nanoweb composite has been transformed to fully carbonized Li4Ti5O12/carbon nanowebs (Fig. 1(b)), consisting of conglutinated Li4Ti5O12/ carbon hybrid nanofibers with diameters of 80e120 nm (Fig. S2ae b), and Fig. 2(b) and its inset) possibly owing to the encapsulation of carbon matrix, which effectively prevents lithium evaporation from the composite during the low temperature heat treatment.

927

Those conglutinated Li4Ti5O12/carbon composite nanofibers were much more curved than the lithium-titanium/PAN composite nanofibers, which can probably be ascribed to the thermal strain between the two constituents arising from the difference in swelling coefficient during carbonization. However, when heated in air at 500  C for 2 h, as shown in Fig. 2(c) and its inset, and Fig. S2(ced), it was not pure Li4Ti5O12 nanowebs but uniform Li4Ti5O12/TiO2 hybrid nanowebs (see Fig. 1(a)) that were obtained after removing the carbon composite from the lithiumetitanium/PAN composite hybrid nanowebs, mainly owing to the greater tendency to form pure Li4Ti5O12 phase at higher temperature [30,31] in air, which is different from the behavior in argon. Additionally, the porous cotton-like carbon matrix with many holes/voids arising from the pyrolysis of PAN is clearly observed on the surface of the Li4Ti5O12/carbon hybrid nanofibers (see Fig. 2(d) and Supporting Information Fig. S2), and some mesoporosities between conglutinated nanofibers both in the Li4Ti5O12/carbon and in the Li4Ti5O12/ TiO2 nanowebs are revealed from Fig. S2, and Fig. 2(bed) and its insets. Those holes/voids and mesoporosities formed in these nanowebs would ensure a high electrode-electrolyte contact area in the hybrid nanowebs, so there is more access to accommodate lithium ions without any remarkable degradation of the structure during charge/discharge cycling, which is favorable to both the Liþ diffusion and further the lithium ion storage. Transmission electron microscope (TEM) observations of the pyrolyzed Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanowebs provide worthwhile structural and chemical information (Fig. 3 and Supporting Information Fig. S3). Fig. 3(a) shows a high resolution TEM (HRTEM) bright-field image of a section of the pyrolyzed Li4Ti5O12/carbon nanofiber. No obvious crystal fringes exist, only some particle-like patterns displayed in Fig. 3(a) and Fig. S3(a). This is possibly because the crystals in the nanofiber are too small to be imaged under our TEM condition, which is further confirmed by the

Fig. 2. FE-SEM images of as-collected lithiumetitanium composite/PAN nanowebs, Li4Ti5O12/carbon nanowebs, and Li4Ti5O12/TiO2 nanowebs: (a) lithiumetitanium/PAN composite nanowebs, (b) as-pyrolyzed Li4Ti5O12/carbon nanowebs with a high magnification image (inset), (c) Li4Ti5O12/TiO2 nanowebs with a high magnification image (inset), (d) single Li4Ti5O12/carbon composite nanofiber with porous surface structure.

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Fig. 3. (a) HRTEM image of a section of a Li4Ti5O12/carbon composite nanofiber, (b) low-magnification TEM image and SAED pattern (inset) of Li4Ti5O12/carbon composite nanowebs, (c) HRTEM image of a section of a Li4Ti5O12/TiO2 composite nanofiber, (d) low-magnification TEM image and SAED pattern (inset) of Li4Ti5O12/TiO2 hybrid nanowebs. The “L” and “A” subscripts represent “Li4Ti5O12” and “Anatase TiO2”, respectively.

weak selected area electron diffraction (SAED) pattern (Fig. 3(b) inset), revealing the fine microstructure of the Li4Ti5O12/carbon nanofibers. The ring-shaped SAED pattern indicates that the Li4Ti5O12 in the Li4Ti5O12/carbon nanowebs is in polycrystalline form, similar to what has been reported previously [9]. The spotted diffraction rings from inside to outside can be indexed to the (111)L and (400)L planes of Li4Ti5O12, respectively (Fig. 3(b) inset). Fig. 3(b) further demonstrates that the Li4Ti5O12/carbon nanoweb consists of cross-linked Li4Ti5O12/carbon nanofibers, which finally form a few mesoporosities among them. This is advantageous for the lithium ion and electrolyte diffusion, leading to the enhancement of electrochemical properties in Li4Ti5O12/carbon nanoweb electrode. The Li4Ti5O12/TiO2 sample in Fig. 3(c), which was calcined in air, displays a mixed phase crystal structure consisting of the tetragonal anatase TiO2 as the main phase, with interplanar spacing of w0.35 nm between its neighboring (101)A planes, and some cubic Li4Ti5O12 as the minor phase, with interplanar spacing of approximately 0.48 nm between its neighboring (111)L planes and a crystal size of w20e 30 nm (see Supporting Information Fig. S3(b) and its inset), much larger than for the Li4Ti5O12/carbon hybrid nanowebs, which is possibly owing to carbon interference with the growth of Li4Ti5O12 crystals in the hybrid nanowebs. Similarly, Fig. 3(d) and Fig. S3(b) (see Supporting Information) display the polycrystalline form of these Li4Ti5O12/TiO2 nanowebs, which is further confirmed by the corresponding selected-area electronic diffraction (SAED) pattern (Fig. 3(d) inset), but only in that there are more obviously spotted diffraction rings. The rings from inside to outside can be indexed to the (111)L, (101)A, (311)L, (411)L, and (200)A planes of Li4Ti5O12 and anatase TiO2, respectively, which are in good agreement with the XRD results described above. Energy-dispersive X-ray spectroscopy (EDX) analysis of the Li4Ti5O12/carbon (Supporting Information Fig. S3(c)) reveals that only the elements Ti, O, and C can be detected, but no Li, mainly owing to its nature of a light element like the boron element in the LaB6 electron gun material used for FESEM, while the EDX analysis of the Li4Ti5O12/TiO2 nanowebs (Supporting

Information Fig. S3(d))reveals the presence of a little C, as well as the elements Ti and O, which is possibly because part of the tape was not covered by the sample during EDX testing. Furthermore, the carbon content in the Li4Ti5O12/carbon nanofibers is about 17.8% (weight percent) according to the EDX spectrum (see Fig. S3(c)), which is in good accordance with the TGA results (Fig. S1). X-ray photoelectron spectroscopy (XPS) can provide much useful information concerning the elemental composition of the compound and the oxidation states of the elements. In order to fully understand the chemical state of the as-synthesized Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanowebs, X-ray photoelectron spectroscopy (XPS) of the Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanowebs was conducted from 0 to 1100 eV. Obvious Ti2p, Li1s, O1s, and C1s peaks were detected for the Li4Ti5O12/carbon nanowebs, and their highresolution spectra are shown in Fig. 4(aed), respectively, while the Ti2p, Li1s, and O1s peaks for the Li4Ti5O12/TiO2 nanowebs were also obtained, and their high-resolution spectra are displayed in Supporting Information Fig. S4(aec), respectively. The Ti2p spectrum (Fig. 4(a)) for the Li4Ti5O12/carbon nanowebs consists of two symmetrical peaks with binding energies (BEs) of 458.81 eV and 464.50 eV, which are ascribed to Ti2p3/2 and Ti2p1/2, respectively, and are slightly larger than those of the Li4Ti5O12/TiO2 nanoweb sample (Supporting Information Fig. S4(a)). The separation between these two peaks is 5.69 eV, slightly larger than the energy splitting reported for TiO2 [32] and also larger than that for the Li4Ti5O12/TiO2 nanowebs. The cause is possibly the encapsulation of the Li4Ti5O12 in the carbon matrix, which, to some extent, prevents the X-ray photons from reaching the Li4Ti5O12 core effectively and also hinders the excitation of Ti2p electrons, with a consequent influence on the BEs of the Ti2p electrons. This is similar to the case of TiO2 with hydrocarbon decomposition on its surface due to UV irradiation [33]. Similarly, the binding energy of the Li1s spectrum (Fig. 4(b)) for the Li4Ti5O12/carbon nanowebs, i.e. 55.29 eV, is also somewhat larger than that for the Li4Ti5O12/TiO2 nanowebs (Supporting Information Fig. S4(b)), which is similarly attributable to the

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Z. Yang et al. / Energy 55 (2013) 925e932

4 0

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Fig. 4. XPS high-resolution spectra of the (a) Ti2p, (b) Li1s, (c) O1s, and (d) C1s regions of as-prepared Li4Ti5O12/carbon nanowebs.

encapsulation of Li4Ti5O12 in the carbon matrix for Li4Ti5O12/carbon nanowebs. As shown in Fig. 4(c) and Fig. S4(c), the main portion of the response for the O element could come from TieO in the Li4Ti5O12/carbon nanowebs or TieO in the Li4Ti5O12/TiO2 nanowebs, as evidenced by the O1s binding energy (BE) peak at w530.27 eV (Fig. 4(c)) and 529.90 eV (Supporting Information Fig. S4(c)) [33], while the peaks at 532.06 eV (Fig. 4(c)) and 531.41 eV (Supporting Information Fig. S4(c)) are possibly attributable to the OH
radical, adsorbed oxygen, or carbonyl [33,34]. As for the high BE peaks at 533.08 eV (Fig. 4(c)) and 532.30 eV (Supporting Information Fig. S4(c)), they possibly originate from adsorbed H2O [34]. Similarly, the encapsulation of Li4Ti5O12 in the carbon matrix possibly also leads to the three fitted O1s peaks being slightly larger for the Li4Ti5O12/carbon than for the Li4Ti5O12/TiO2 nanowebs. As for the high resolution spectrum of the C1s region of the as-prepared Li4Ti5O12/carbon nanowebs, as presented in Fig. 4(d), there are five fitted peaks, including the large peak at 285.00 eV, which is attributed to CeC or CeH bonding in un-oxidized graphitic carbon [34,35]. The peak at 286.52 eV possibly results from disordered carbon or oxidant carbon, such as from the bonds between C and O i.e. CeO [33e35], which is in good accordance with the fitted O1s peaks. The remaining three small peaks at 288.03 eV, 289.21 eV, and 290.68 eV possibly come from a trace amount of carboxyl in the hybrid sample or some absorbed CO2, such as from C]O bonding [34,35]. From a combination of the XRD, TGA, FE-SEM, TEM, EDX, and XPS results, it is concluded that both Li4Ti5O12 and Li4Ti5O12/ TiO2 in the hybrid nanowebs appear in form of polycrystalline Li4Ti5O12 or in Li4Ti5O12/anatase TiO2 form, where each particle is one small crystal because of the low diffusion capability of TiO2 in the nanowebs at the pyrolysis temperature [36]. Additionally, the

nanosized Li4Ti5O12 crystals in the Li4Ti5O12/carbon nanowebs are uniformly dispersed into and encapsulated in the carbon matrix while the carbon in those nanowebs exists as a porous cotton-like carbon matrix with many holes/voids. Furthermore, there are many mesoporosities formed between conglutinated nanofibers in those hybrid nanowebs. Such Li4Ti5O12/carbon nanowebs consisting of conglutinated nanofibers would exhibit many unique advantages in lithium ion battery application, such as high conductivity, mainly owing to the encapsulation of nanoscale Li4Ti5O12 crystals in the abundant porous carbon matrix, improved Liþ and electrolyte transport in the hybrid nanowebs because of both hole/void nanopores existing in the porous cotton-like carbon matrix and mesoporosities in those nanowebs, etc., all of which finally favor the enhancement of the electrochemical performance of the electrode as compared with the Li4Ti5O12/TiO2 hybrid nanowebs. 3.2. Electrochemical analysis The electrochemical performance of Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanoweb electrodes has been investigated by galvanostatic discharge-charge cycling and cyclic voltammetry (Fig. 5 and Fig. S5), respectively. Cyclic voltammograms (CV) of Li4Ti5O12/ carbon electrode are presented in Fig. 5(a) while those of the Li4Ti5O12/TiO2 electrode are shown in Fig. S5(a) for comparison. The curves of the initial cycles for both electrodes are somewhat different from the following ones, which is possibly owing to the gradual formation of an inactive solid/electrolyte interphase (SEI) on the surface of the active materials. From the third cycle on, both electrodes revealed highly reversility. As for the Li4Ti5O12/carbon electrode, in the first cycle, the cathodic/anodic peak pair at 0.015 V

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Fig. 5. Electrochemical performance of Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanoweb electrodes: (a) Cyclic voltammograms of Li4Ti5O12/carbon nanoweb electrode from the first cycle to the fifth cycle at a scan rate of 0.1 mVs
1 in the voltage range of 0.01e3.0 V. (b) Voltage profiles for the first chargeedischarge cycles of Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanoweb composite electrodes at the current density of 150 mAg
1 (1 C) cycled between 1.0 and 3.0 V vs. Liþ/Li. (c) Cycling performance from the first cycle to the 100th cycle for the Li4Ti5O12/carbon composite nanoweb at the current density of 30 mAg
1 cycled between 1.0 and 3.0 V vs. Liþ/Li. (d) Chargeedischarge capacity for the Li4Ti5O12/carbon and Li4Ti5O12/TiO2 composite nanoweb electrodes at different rates (0.1, 0.5, 1, 2, 5, 10, and 20 C). (Note: here, 1 C ¼ 150 mAg
1; the LTO/T and LTO/C in on the figure are the abbreviations of Li4Ti5O12/TiO2 and Li4Ti5O12/carbon, respectively.)

and 0.13 V should be ascribed to the lithium ion insertion into/ extraction out of the carbon matrix, while another little peak pair at 0.69 V and 1.35 V possibly originates from the decomposition of electrolyte. Those decomposition products of the electrolyte finally form a SEI layer on the surface of the active materials [37], which ensures highly reversible cycling performance of the electrode. In addition, one obvious cathodic/anodic peak pair at 1.504 V and 1.631 V in the first cycle, stabilizing quickly at 1.536 V and 1.681 V in the following cycles, corresponding to the lithium ion insertion into/extraction out of the Li4Ti5O12, while another small cathodic/ anodic peak pair at 1.635 V and 1.901 V is probably due to the lithium ion reaction with trace amount of anatase TiO2 in the hybrid nanowebs. The anatase TiO2 in the Li4Ti5O12/carbon sample cannot be found in the XRD result due to its trace content. As for Li4Ti5O12/ TiO2 electrode, except for the main cathodic/anodic peak pair at 1.693 V and 2.039 V, attributable to the main phase anatase TiO2 in the hybrid nanowebs, there is another obvious cathodic/anodic peak pair at 1.519 V and 1.595 V, which is ascribed to the Li4Ti5O12 in the hybrid nanowebs. However, there are unknown peaks at 2.32 V (Fig. 5(a)) and at 2.305 V (Fig. S5(a)) in the first cycle, which disappear gradually in the next few cycles. The reason or corresponding reaction for those peaks is still unclear. Further work may be conducted in order to clarify this in the future. The anodic performance of Li4Ti5O12/carbon and Li4Ti5O12/TiO2 nanowebs were tested in the potential range from 1.0 to 3.0 V (versus Li/Liþ), respectively. As shown in Fig. 5(b), the initial discharge/charge of Li4Ti5O12/carbon composite electrode at the current density of 150 mAg
1 (1C), delivered a specific capacity of 134.61 and 133.78 mAhg
1, respectively, obviously higher than that of the Li4Ti5O12/TiO2 nanoweb electrode (103.1 and 101.9 mAhg
1). There are two obvious discharge/charge voltage plateau in the voltage profile of the Li4Ti5O12/TiO2 electrode, while only one

obvious discharge/charge voltage plateau appears in the voltage profiles of the Li4Ti5O12/carbon nanoweb electrode, which is in good accordance with their XRD results (Fig. 1). Fig. 5(c) and Fig. S5(c) show the cycling performance of the Li4Ti5O12/carbon with that of Li4Ti5O12/TiO2 electrodes at 0.2 C (30 mAg
1). The Li4Ti5O12/carbon electrode delivered an initial discharge/charge specific capacity above 160 mAhg
1with excellent cyclability. Fig. 5(d) shows the rate performance of the Li4Ti5O12/TiO2 and the Li4Ti5O12/carbon electrodes. Compared with the Li4Ti5O12/TiO2 electrode, the Li4Ti5O12/carbon electrode exhibited excellent rate performance, and it delivered a discharge capacity of over 165 mAhg
1 at a current density of 15 mAg
1, 140 mAhg
1 at 75 mAg
1, 134 mAhg
1 at 150 mAg
1, 125 mAhg
1 at 300 mAg
1, 114 mAhg
1 at 750 mAg
1, 105 mAhg
1 at 1500 mAg
1, and 87 mAhg
1 at 3000 mAg
1, respectively. We believe the higher electronic conductivity, together with higher specific surface area of the Li4Ti5O12/Carbon hybrid nanowebs owing to the porous carbon matrix, contribute to the improvement of the rate performance of Li4Ti5O12/Carbon electrode. The excellent performance of the Li4Ti5O12/carbon electrode including high initial dischargee charge capacity, high reversible discharge capacity, and rate capability could be ascribed to the short Liþ diffusion paths and easy access of the electrolyte through interconnected open mesopores or nanopores, as well as the intricate three-dimensional network established in the nanoweb architecture. In particular, the abundant porous carbon matrix (Supporting Information Fig. S1 and Fig. S3(c)), able to encapsulate nanoscale Li4Ti5O12 crystals in the Li4Ti5O12/carbon nanoweb sample, could dramatically enhance the electronic conductivity and the specific surface area to promote charge transfer reactions and shorten the electron and lithium ion diffusion paths, especially at high rates, as compared with those of Li4Ti5O12/TiO2 nanoweb electrode.

Z. Yang et al. / Energy 55 (2013) 925e932

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3.3. Mechanism discussion

References

We now discuss the charge diffusion mechanism of the Li4Ti5O12/carbon nanowebs and the Li4Ti5O12/TiO2 nanowebs during charge/discharge processes. As demonstrated in Scheme S1 (Supporting Information), nanosized Li4Ti5O12 crystals were uniformly dispersed into the porous cotton-like carbon matrix to form conglutinated Li4Ti5O12/carbon nanofibers, where there are many hole/void nanopores owing to the pyrolysis of PAN during the heat treatment, as well as mesopores between the conglutinated Li4Ti5O12/carbon nanofibers. As for the Li4Ti5O12/TiO2 nanowebs, although there are also many mesopores between the conglutinated Li4Ti5O12/TiO2 nanofibers, it is the lower conductivity that leads to their much lower initial discharge-charge capacity, reversible discharge capacity, and rate capability. Therefore, it is both the nanoscale Li4Ti5O12 crystals and the highly porous carbon matrix in the Li4Ti5O12/carbon nanowebs that ensure good electrode-electrolyte contact, good electronic conductivity, and short lithium ion diffusion pathways during discharge/charge cycling, and ultimately make a great contribution to the enhanced lithium ion storage and high coulombic efficiency of the electrodes.

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4. Conclusions In summary, very large surface area, uniform Li4Ti5O12/carbon nanowebs and Li4Ti5O12/TiO2 nanowebs have been prepared by thermal pyrolysis and oxidation of lithium-titanium/PAN nanoweb composites in argon or in air, respectively. Many hole/void nanopores, owing to pyrolysis of PAN, and mesopores formed by the conglutinated Li4Ti5O12/carbon nanofibers, as well as the abundant porous carbon matrix in Li4Ti5O12/carbon nanowebs greatly improve their lithium storage, especially its rate capability, by means of enlarging the electrode-electrolyte contact area, shortening lithium ion diffusion pathways, and enhancing the lithium ion and electrolyte diffusion of the active materials during charge/ discharge processes. These Li4Ti5O12/carbon hybrid nanoweb electrodes display a high initial capacity over 160 mAhg
1 at the current density of 30 mAg
1 and maintain a reversible capacity of approximately 152 mAhg
1 up to 100 cycles, with high coulombic efficiency of nearly 100%, much higher than that of the Li4Ti5O12/ TiO2 nanoweb electrode. Furthermore, the Li4Ti5O12/carbon hybrid nanoweb electrode exhibited excellent rate performance delivering a reversible capacity of 87 mAhg
1 when the charge/ discharge current is up to 3000 mAg
1. Therefore, the composite is promising as a potential anode material for LIBs, even though the composition and structure of these materials require further improvement. Acknowledgments Part of the work was funded by the Australian Research Council (ARC) through a Discovery project (DP1094261), the Natural Science Foundation Program of Fujian Province (2010J01332), the Science Development Foundation of Fuzhou University (2012-XQ40) and the Foundation Program of the Ministry of Education for Returned Exchange Personnel (LXKQ201101). The authors also would like to thank Dr. Tania Silver at the University of Wollongong for critical reading of the manuscript and Mr. Darren Attard for his great contribution. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.energy.2013.01.055.

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