discharge capability for lithium ion batteries

discharge capability for lithium ion batteries

Journal of Alloys and Compounds 671 (2016) 157e163 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 671 (2016) 157e163

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Nano-sized Li4Ti5O12/C anode material with ultrafast charge/discharge capability for lithium ion batteries Daobin Mu a, *, Yongjian Chen a, Borong Wu a, Rong Huang a, Ying Jiang a, Luyu Li b, Feng Wu a a

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, PR China Department of Electronic Engineering, Fudan University, Shanghai, China


a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 November 2015 Received in revised form 5 February 2016 Accepted 10 February 2016 Available online 12 February 2016

Nano-structured Li4Ti5O12 crystals coated with carbon layer are in situ synthesized via one-step liquid process taking advantage of low-cost sucrose as carbon source in this work. The as-prepared LTO/C particles present much larger specific surface area (58 m2 g1) relative to the value of pure LTO, with a size around 13 nm in average. Its electronic conductivity of 6.56  104 S cm1 is over three orders of magnitude higher than the pure one. The composite anode displays a distinguished electrochemical charge/discharge performance, especially, quite high rate capability along with a stable cyclability. It delivers the initial discharge specific capacities of 156.7 and 142.1 mA h g1 at 40C and 60C respectively, and remains the values of 114.2 and 98.1 mA h g1 after 200 cycles. Furthermore, a capacity of 132.8 mA h g1 is delivered even at an 80C rate and the value of 82.7 mA h g1 can be maintained after 200 cycles. The ultrafast charge/discharge capability may be attributed to the shorten Liþ transport path in the nanosized composite and the enlarged access area with electrolyte. Additionally, the carbon coating may provide an effective conductive network among the particles promoting charge transfer. © 2016 Elsevier B.V. All rights reserved.

Keywords: Lithium titanate Rate performance Carbon coating Anode Lithium ion batteries

1. Introduction Nowadays, with the fast development of electric vehicles (EV) and hybrid electric vehicles (HEV), the study on high-performance secondary batteries has attracted more and more attention. Lithium ion batteries (LIBs) are considered as the most promising candidates for their high energy density and long cyclic performance [1e4]. As a commonly-used commercial anode for LIBs, graphite material exhibits low electrode potential, high coulombic efficiency and high energy density and so on. But it also encounters several serious shortages when used in LIBs, such as poor rate performance, solid electrolyte interface (SEI) formation and lithium dendrite deposition during charge process [5e7]. Many efforts have been made to pursue high performance alternatives for the anode material [8,9]. Spinel lithium titanate (Li4Ti5O12, LTO) shows the potential as a candidate because of its excellent safety performance, outstanding reversibility, stable crystal structure [10e12]. The

* Corresponding author. E-mail address: [email protected] (D. Mu). http://dx.doi.org/10.1016/j.jallcom.2016.02.095 0925-8388/© 2016 Elsevier B.V. All rights reserved.

voltage plateau around 1.55 V (vs Li/Liþ) of LTO electrode is higher than the reduction potentials of electrolyte solvents in general, thus it is possible to prevent the formation of SEI due to solvent decomposition, and avoid lithium dendrite deposition on the surface of the anode material. In addition, as a kind of “zero-strain” material, the lattice parameters and the volume of LTO crystal almost have no change during the process of lithium insertion/ extraction [13e15]. In spite of the charming merits, LTO hasn't been utilized widely as the anode material for LIBs yet because of its low rate capability led by the poor electronic conductivity (109 S cm1). The factor tends to cause serious polarization so that the charge/discharge may be retarded when proceeded under a high current density [16e18]. In order to conquer the main obstacles and improve the power performance of the LTO anode, many approaches have been put forward [19e21]. One choice is to create suitable micronanostructure [22,23]. Sun et al. designed mesoporous LTO nanocluster material whose capacity could reach 132 mA h g1 at 5C, and its retention was about 86.3% after 100 cycles. Besides, doping with metal ions or metallic oxides is also an effective method to modify the LTO material [24,25]. Zhang et al. [26] reported a kind of


D. Mu et al. / Journal of Alloys and Compounds 671 (2016) 157e163

Gd-doped LTO composite, which could deliver a discharge capacity of 110.8 mA h g1 at 20C after 100 cycles. Another commonly-used method is to introduce conductive agents to LTO, especially conducting carbonaceous materials. Graphene is such a choice due to its good electronic conductivity and large specific surface area. Ni et al. [16] adopted reduced graphene oxide as incorporating conductive agent for LTO. The capacities of the electrode could reach 154 mA h g1 at 10C and 149 mA h g1 at 20C. Nevertheless, the preparation of graphene is generally complex and high cost in energy and time consuming [27], it hasn't been large-scale used as carbon source to modify LTO but limited in the laboratory [28e30]. In this work, sucrose was incorporated into LTO as carbon source not only because of its rich yield, low price, but also due to the wide application in the electrode materials for LIBs [31,32]. More importantly, the LTO/C composite could be in situ synthesized with a facile preparation process taking use of the sucrose as carbon source. Herein, one step liquid synthesis method was adopted to prepare target product under room temperature. In this process, no any additional heating was necessarily requested like hydrothermal process, as well as no any other additives except sucrose. The dispersed sucrose molecule in reaction solution might have effect on the nucleus formation and the growth of precursor products. Carbon coating was in situ attainable around LTO particles via subsequent high temperature calcination, during which the size of the crystals was constrained. As a result, a nanosized hybrid of LTO/ C was synthesized in an average size about 13 nm. Its structure and morphology were examined in detail, as well as the impact on the electrochemical performance. The composite electrode exhibited an outstanding lithium storage properties, in particular the ultrafast charging/discharging rate performance. The work may present a promising scale-up approach for the preparation of LTO/C anode material. 2. Experiment 2.1. Material preparation Initially, 12.8 g of tetrabutyl titanate (TBT) was added into 20 ml anhydrous ethanol at room temperature with stirring until it dissolved completely. The solution was marked as A. 12.3 g lithium hydroxide monohydrate (LiOH$H2O) and 1.7 g sucrose were added into 13 ml deionized water to make solution B under stirring for 30 min. Then, solution A was added dropwise to solution B by peristaltic pump (flow rate is about 300 ml$min1) accompanying with vigorous stirring. After the titration the stirring still lasted for 2 h, and then a cream-like mixture was obtained and kept in a vacuum oven at 100  C for 24 h to get precursor substance. Subsequently, the precursor product was calcined at the temperature of 800  C for 5 h at a heating rate of 5  C min1 under argon atmosphere, then cooled down to room temperature naturally. The synthesis procedures for LTO phase were identical to the steps above except the introduction of sucrose to solution B. 2.2. Material characterization The morphologies of the as-prepared LTO/C and LTO particles were characterized by field emitting scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Tecnai G12). The crystal structure of the sample was analyzed by X-ray powder diffraction (XRD, Rigaku UltimalV-185) with CuKa radiation at scanning rate of 8 min1, and the diffraction patterns were recorded in the angle (2q) range from 10 to 90 . Raman spectra were measured by a Renishaw invia Reflex Raman Microscope using laser excitation at 514.5 nm from an argon ion laser source. Thermogravimetric analysis (TGA) was performed on a

thermal analysis instrument (NETZSCH STA449F3) between 30  C and 900  Cat a heating rate of 10  C/min in air. BET surface area of the sample was estimated through nitrogen adsorption/desorption on a Gold APP V-sorb 2800P surface analyzer. As for electronic conductivity measurement, the material was pressed into an identical cylinder (with a pressure of 20 Mpa). Then, the resistance of the cylinder was measured based on the applied voltage and the corresponding current. Thus, the electronic conductivity was obtained to make the comparison between the samples. 2.3. Electrochemical measurement CR2025-type coin cells were assembled in an argon-filled glovebox in which both oxygen and water concentrations were less than 1 ppm. In the cell, pure lithium sheet acted as the reference and counter electrode. The working electrode was composed of active material, acetylene black as conducting agent, polyvinylidene fluoride (PVDF) as binder dissolved in N-methyl pyrrolidinone (NMP) and the mass ratio of the three matters was 80:10:10. The mixture was grinded in a carnelian mortar for 30 min to be a slurry substance. Then it was pasted on copper foil homogeneously and dried under vacuum at 80  C for 12 h, finally the electrode sheet was punched to disks in a diameter of 11 mm. The mass density of the active material in the electrode sheet was between 1.4 and 1.7 mg cm2. The electrolyte used in the cell was of 1 M LiPF6 in a mixing solvent of dimethyl carbonate (DMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) by the volume ratio of 1:1:1. The galvanostatic chargeedischarge measurement was conducted on Land battery test system (LAND CT2001A) and Arbin battery cycler (BT2000) under different current densities with the voltage range from 1 to 3 V at room temperature. Cyclic voltammogram was performed between 1 V and 3 V on CHI608C electrochemical workstation at various scan rates, the electrochemical impedance spectroscopy (EIS) was also recorded using the same electrochemical workstation in the frequency range of 0.01e105 Hz with perturbation signal amplitude of 5 mV. The potential/voltage mentioned in this study referred to Li/Liþ redox couple. 3. Results and discussion 3.1. Material characterization Scheme 1 shows the synthetic process of LTO/C composite. The TBT hydrolyzes and the corresponding resultants react with lithium ions to form precursor products during the process. The homogeneously dispersed sucrose in the solution may facilitate the nucleation and limit the dimensional growth of the precursor. It would be transformed into spinel structure LTO through subsequent high temperature calcination. Meanwhile, carbon coating may be formed via the pyrolysis of the sucrose surrounding the precursor. Fig. 1a exhibits the XRD patterns of the as-prepared LTO/C and LTO materials. As shown in the figure, all diffraction peaks of both samples correspond well with the standard pattern of Li4Ti5O12 (JCPDS card No. 49-0207), which proves the formation of LTO phase with high purity. But, no significant carbon peak could be detectable in the pattern. Herein, Raman spectrum of LTO/C has been collected to analyze the carbon species from sucrose decomposition, as shown in Fig. 1b. Two peaks evidently locating at 1350 and 1595 cm1 can be assigned to D-band and G-band, respectively, which are the extremely distinct symbols for carbon materials. As well known that G-band corresponds to the E2g mode (stretching vibrations) in the basal of the crystalline graphite, while D-band relates to structural disorder and the degree of anti-graphitization

D. Mu et al. / Journal of Alloys and Compounds 671 (2016) 157e163


Scheme 1. Schematic illustration of the preparation process of LTO/C composite.

Fig. 1. (a) XRD patterns, (c) TG analysis and (d) the nitrogen adsorptionedesorption isotherm of pure LTO and LTO/C samples, (b) Raman spectrum for LTO/C samples.

towards carbon [33,34]. For monocrystal graphite, G-band appears on its Raman spectrum without D-band. But with the disorder increase and the departure from graphitization, D-band appears and then becomes stronger [35]. In general, the peak intensity ratio of D-band and G-band (ID/IG) can offer an important index for comparing the degree of graphitization. The smaller the ratio value is, the higher the graphitization degree and the ordering of carbon will be. The ID/IG ratio of the as-prepared LTO/C is calculated to be about 0.92, indicating that the carbon species in the composite are partially graphitized. Thus, the carbon layer should take an important role in improving the electrical conductivity and rate performance of the LTO/C material [34,36]. Additionally, the crystallite size of LTO/C particle is calculated according to DebyeScherrer formula [37,38], using the full width at half maximum (FWHM) of the (111), (311), (400) and (440) planes in the XRD pattern,

D ¼ Kl=ðb cos qÞ where D means the crystal size, K is the shape factor (usually taken as 0.9), l is the wavelength of X-ray, b represents the FWHM and q is the diffraction Bragg angle. Here, the crystallite size of LTO/C particles is estimated as 13.4 nm in average. The thermogravimetric analysis (Fig. 1c) reveals the relationship

of mass loss with temperature increase. It is found that there's almost no weight loss in the case of the pristine Li4Ti5O12 within the whole temperature range. Thus, based on the remaining weight of the composite sample, the content of carbon is estimated about 8.6 wt%. Fig. 1d shows N2 adsorption/desorption isotherms of the two samples. The BET surface area of LTO/C composite is calculated to be 58 m2 g1, which is about 29 times larger than that of the pure LTO (2 m2 g1). The enlarged specific surface area originates from the nanosizing of the composite particles. It would provide a larger contact area with the electrolyte and more reaction sites during charge/discharge process. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images for pure LTO and LTO/C composites are shown in Fig. 2. It is seen that the particles of the composite display more uniform morphology and size distribution. The size is between 100 and 200 nm relative to the pure ones in the size of 200e500 nm. More clearly, the inset image shows much smaller particles stacking each other. TEM image makes an analysis further as shown in Fig. 2c, the size of the small particles is about 13 nm, which is well in accordance with the calculated value by Scherrer equation. These particles are distinguished from the carbon layer by virtue of the characteristic lattice fringe with the spacing of 0.21 nm as shown in Fig. 2c, The latter fringe can be


D. Mu et al. / Journal of Alloys and Compounds 671 (2016) 157e163

Fig. 2. SEM images of (a) pure LTO and (b) LTO/C, TEM images of (c) LTO/C composite samples. The inset (in b) is the partial enlarged figure.

indexed to the (400) plane of the spinel LTO. The formation of fine LTO particles is likely owing to the introduction of sucrose confining the precursor growth during one step liquid synthesis. It is also seen in Fig. 2c that a carbon layer is coated around LTO particles in the case of LTO/C sample. The thickness of the carbon layer is estimated in the range of 2e4 nm. It may be attributed to the incompletely uniform sucrose pasted on the precursor unevenly during drying and temperature rising process. Hence, an electronic transmission network is likely to be formed among LTO particles, which would make greatly a contribution to the improvement of electronic conductivity as well as the rate performance of LTO/C composite. As measured, the electronic conductivity of LTO/C material is 6.56  104 S cm1 while the conductivity of pure LTO is 1.55  107 S cm1. 3.2. Electrochemical performance The discharges specific at 5C, 10C, 20C, 40C, 60C and 80C (1C rate ¼ 175 mA g1, charge and discharge at the same rate) within the voltage range from 1 to 3 V are shown in Fig. 3a and b. Table 1 and Table 2 list the details about the rate performance of LTO/C and LTO electrodes. It is found that a reversible capacity of 150.7 mA h g1 is retained at 5C after 200 cycles for LTO/C composite electrode. With the rate increase, the initial capacity decreases, but the values of 156.7 mA h g1 at 40C and 142.1mAh g1 at 60C can still be obtainable. The retained capacities are 132, 123, 114.2 and 98.1 mA h g1, respectively, at the rates of 10C, 20C, 40C and 60C after 200 cycles, showing the outstanding high rate performance of the LTO/C electrode. Even at the ultrafast charge/ discharge of 80C, the capacity for the first cycle can reach 132.8 mA h g1, and it still remains 82.7 mA h g1 after 200 cycles. For pure LTO, the initial discharge specific capacities are just 123.1,

102.5 and 80.7 mA h g1 at 5C, 10C and 20C, respectively. Compared to pure LTO, both the discharge specific capacity and the retention of the LTO/C composite have been improved greatly at rate performance. Meanwhile, the coulombic efficiencies at various rates are more than 99% except the initial cycles. From Fig. 3a and b, it is also found that the discharge specific capacities under high rates decrease a bit and then rise again between the 1st to the 17th cycle. The detailed discussion would be given in below section. Fig. 3c displays the energy densities of LTO/C and pristine LTO electrodes at the 1st, 100th and 200th cycle under different rates. The initial energy density of LTO/C can reach 280 W h Kg1 at the rate of 5C while LTO's is about173.4 Wh Kg1. The value still remains 225.5 Wh Kg1 after 200 cycles, while it is just 146.3 Wh Kg1 in the case of LTO. Even at the quite high rate of 80C, the LTO/C electrode can also display a value of 142.2 Wh Kg1 after 100 cycles, its energy density still remains 104.4 Wh Kg1 over 200 cycles. The results are extremely marvelous for the ultrafast charge/ discharge processes. In contrast to the pure LTO, one of the reasons for the great promotion of LTO/C material performance is ascribed to its high electronic conductivity. In the case of the as-prepared LTO/C composite, it is of 6.56  104 S cm1, which is much higher than the pure LTO material's (1.55  107 S cm1). Another reason may be attributed to the nanosizing of the particles. As indicated, the particle size of the composite is about 13 nm, which is much smaller than the pure LTO's in hundreds of nanometers. The nanosized LTO particles effectively shortens lithium ion transport path and enlarges the access area with the electrolyte, which may promote the rate performance of the electrode. All these are benefited from introducing sucrose as carbon source for decorating LTO. Fig. 4a and b show the charge/discharge curves of LTO/C and pure LTO electrodes at the 1st, 100th and 200th circle, respectively.

D. Mu et al. / Journal of Alloys and Compounds 671 (2016) 157e163


Table 1 High rate performance of LTO/C sample.

Fig. 3. Cycle performance and coulombic efficiency of (a) LTO/C and (b) LTO. (c) The 1st, 100th and 200th cycle energy densities of LTO and LTO/C at different rates.

Almost all curves have long flat charge and discharge platforms, the plateaus in the case of LTO/C are much longer than the ones of pure LTO at the same condition. In addition, the largest voltage differences appear between the charge and the discharge plateaus at the 1st cycle irrespective of LTO/C and pure LTO electrodes, which indicates a most serious polarization at the initial cycle. Fig. 4c and d demonstrate the detailed charge/discharge curves of LTO/C electrode at 20C rate further. The voltage difference decreases to the smallest value from the 1st cycle to the 50th cycle, subsequently, it begins to increase with the accumulation of cycle


1st cycle capacity (mAh g1)

100th cycle capacity (mAh g1)

200th cycle capacity (mAh g1)

5C 10C 20C 40C 60C 80C

187.7 170.2 163.2 156.7 142.1 132.8

152.4 141.9 133.9 130.4 120.0 108.9

150.7 132 123.0 114.2 98.1 82.7

number again (as seen in the case of the 100th, 200th cycle). Fig. 4d shows the decreasing process from the 1st cycle to the 50th cycle clearly. It can be found that the charge and discharge curves nearly overlap before the 7th cycle, but the voltage differences decrease dramatically from the 10th to the 17th cycle. After that, the value remains unchanged, and the charge and discharge curves overlap again till the 50th cycle. The phenomenon suggests that an activation process takes place during the charge/discharge processes between the 1st and the 17th cycle. With the activation, the polarization decreases as well as the voltage difference. Actually, the phenomenon that the capacitits decrease at first and then increase at high rate cycling as mentioned above may also be ascribed to the process. In the initial cycles, the polarization is more significant under high rates, as shown in Fig. 4, the electrode shows larger irreversible capacities. So the capacity declines more obviously under high rates. With the activation effect and the decreased polarization, the irreversible capacity reduces and even disappears with cycling, the coulombic efficiency goes up near 100%. Apparently, the reversibility of electrochemical reaction increases, the capacity increases showing a rising tendency again. The cyclic voltammograms of the as-prepared LTO/C and LTO electrodes are shown in Fig. 5a and b. All plots demonstrate a pair of oxidation/reduction peaks similarly and symmetrically in shape, which is the symbol of lithium ion insertion and extraction accompanying with electrochemical reaction. Furthermore, with the increase of the scanning rate, the oxidation and reduction peaks of every curve move away from each other in opposite direction, along with broader peaks and large peak current. All these imply that the polarization becomes more seriously, which could deteriorate the reversibility and the cyclic performance of the electrode. But for the LTO/C composite, the peak shift displays a smaller increment compared to pure LTO's. This result shows that introducing carbon coating can decrease LTO electrode polarization effectively. Also, it is obvious that the plots in the case of the LTO/C composite are more symmetrical than the LTO's. It means that the modified material possesses better reversibility and electrochemical performance. Fig. 5c shows the EIS plots measured for LTO/C and pure LTO electrodes prior to cycling, and Fig. 5d displays the plots for LTO/C electrode before cycling and after 100 cycles at 20C. In this figure, a semicircle appears in high-middle frequency range while a sloped line emerges in low frequency range. The semicircle reflects the charge transfer resistance represented by Rct, which corresponds to the charge transfer process during electrochemical reaction. The

Table 2 High rate performance of pure LTO. Rate

1st cycle capacity (mAh g1)

100th cycle capacity (mAh g1)

200th cycle capacity (mAh g1)

5C 10C 20C

123.1 102.5 80.7

104.7 82.6 66.1

102 80.6 60.3


D. Mu et al. / Journal of Alloys and Compounds 671 (2016) 157e163

Fig. 4. The galvanostatic discharge/charge voltage profiles at 1st, 100th and 200th cycle respectively of LTO/C (a) and LTO (b) at different rate, and (c, d) LTO/C for 20C.

sloped line represents Warburg impedance (Rw), which is associated with the lithium ion diffusion in bulk phase of activated material. An equivalent circuit is seen as in the inset. Rs is on behalf of the resistance of electrolyte, electrical contact and separator. CPE represents the double-layer capacitance at the interface between

the electrolyte and the working electrode [39,40]. Here, the Rct for LTO/C composite electrode before cycling is 104.7 U, while the value for LTO electrode (178.8 U) is about 1.71 times larger than it. After cycled at 20C for 100 cycles, the Rct becomes 13 U for the LTO/C, which is just 12.4% of the value before cycling. This result can be

Fig. 5. Cyclic voltammograms of pure LTO (a) and LTO/C (b) at different scanning rates. Electrochemical impedance spectroscopy of LTO/C (c, d) and pure LTO (c).

D. Mu et al. / Journal of Alloys and Compounds 671 (2016) 157e163

attributed to the activation process that has been mentioned above in Fig. 4. With the continue cycling, the active materials contact the electrolyte more fully, the polarization also decreases, which is helpful for promoting the electrochemical reaction. Accordingly, the charge transfer resistance of the electrode decreases compared to the value before cycling. Evidently, the high electrical conductivity of LTO/C as well as its smaller size takes a significantly positive role in decreasing charge transfer resistance. This may also provide a support for the activation process mentioned above. 4. Conclusions One step process is adopted to in situ synthesize nanosized LTO/ C composite with the assistance of sucrose carbonation to obtain carbon coating around LTO crystals. The carbon layers will help construct an electronic transmission network among the particles. In comparison with pristine LTO phase, LTO/C composite possesses much higher electronic conductivity (6.56  104 S cm1) and smaller particle size (about 13 nm). The LTO/C electrode demonstrates outstanding ultrafast charge/discharge performance, low polarization and stable cycleability. At the high rates of 60C and 80C, the initial discharge specific capacities of LTO/C can reach 142.1 and 132.8 mA h g1, respectively. After 200 cycles the capacities still remain the values of 98.1 and 82.7 mA h g1. The superior performance is very likely to be attributable to the advanced electrode conductivity and the fine size of the hybrid material. With low cost, simple synthesis process and ultrafast charge/discharge performance, the LTO/C composite could be as an model for LIBs anode candidate in future scale-up production. Acknowledgments This project was financially supported by the National Key Basic Research Program of China (Contract No. 2015CB251100), the National Innovation Engineering Program for New Energy Vehicle Industry Technology of China (2012e2015), Beijing Key Laboratory of Environmental Science and Engineering (Grant no. 20131039031), Beijing Higher Institution Engineering Research Center for Power Battery and Chemical Energy Materials (Grant no. 20141039017). References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652e657. [2] J.B. Goodenough, Y. Kim, Challenges for rechargeable Li batteriesy, Chem. Mater. 22 (2010) 587e603. [3] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190e193. [4] Y.K. Sun, S.T. Myung, B.C. Park, J. Prakash, I. Belharouak, K. Amine, High-energy cathode material for long-life and safe lithium batteries, Nat. Mater. 8 (2009) 320e324. [5] J. Liu, K. Song, P.A. van Aken, J. Maier, Y. Yu, Self-supported Li4Ti5O12-C nanotube arrays as high-rate and long-life anode materials for flexible Liion batteries, Nano Lett. 14 (2014) 2597e2603. [6] Y. Ma, B. Ding, G. Ji, J.Y. Lee, Carbon-encapsulated F-doped Li4Ti5O12 as a high rate anode material for Liþ batteries, ACS Nano 7 (2013) 10870e10878. [7] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [8] P. Roy, S.K. Srivastava, Nanostructured anode materials for lithium ion batteries, J. Mater. Chem. A 3 (2015) 2454e2484. [9] L. Su, Y. Jing, Z. Zhou, Li ion battery materials with core-shell nanostructures, Nanoscale 3 (2011) 3967e3983. [10] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243. [11] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Virus-enabled synthesis and assembly of nanowires for lithium ion battery electrodes, Science 312 (2006) 885e888. [12] T. Liu, H. Ni, W.-L. Song, L.-Z. Fan, Enhanced electrochemical performance of Li4Ti5O12 as anode material for lithium-ion batteries with different carbons


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