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Graphene oxide-confined synthesis of Li4Ti5O12 microspheres as high-performance anodes for lithium ion batteries
Electrochimica Acta 165 (2015) 422–429
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Graphene oxide-confined synthesis of Li4Ti5O12 microspheres as high-performance anodes for lithium ion batteries Jiawei Zhang a , Yurong Cai a, * , Jun Wu b , Juming Yao a a The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, College of Materials and Textiles, Zhejiang SciTech University, Hangzhou 310018, China b Institute of Electron Device & Application, Hangzhou Dianzi University, Hangzhou 310018, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 January 2015 Received in revised form 11 February 2015 Accepted 2 March 2015 Available online 5 March 2015
This paper reports a graphene oxide (GO) confined strategy to synthesize reduced GO-coated lithium titanate (Li4Ti5O12, LTO) microspheres using as-prepared TiO2 microspheres and GO as raw materials. The obtained samples are characterized by X-ray diffraction, field emission scanning electron microscopy and spectrophotometer. Results show that the spherical LTO is formed with approximate 1 mm diameter after hydrothermal reactions, which is due to a confined effect of GO on the surface of TiO2 spheres. Electrochemical tests reveal that the presence of rGO can increase the capacity and cycling stability of LTO anodes, especially at higher C rate. The 3 wt% rGO-coated LTO anodes present a higher reversible Li-ion storage with a specific discharge capacity of 131.6 mAh g1 at 5 C and 97% retention even after 500 cycles, which are more excellent than those of pristine LTO. The GO-confined method is anticipated to synthesize other electrode materials with high electrochemical performances. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Graphene oxide confined synthesis lithium titanate anode material capacity
1. Introduction Lithium ion batteries (LIBs) possess a higher volumetric and gravimetric energy density than lead acid, nickel-cadmium and nickel-metal hydride rechargeable batteries, which brings to their wide applications in portable electronics, such as cell phones, laptop computers, digital cameras and smart grid applications, etc [1]. Otherwise, LIBs can still not completely satisfy the requirement of electrified transportations, electric vehicle (EV) or hybrid electric vehicle (HEV), for example, mainly due to their sluggish charge/discharge speed, low capacity and poor cycling stability [2]. In commercial LIBs, carbonaceous materials, especially natural graphite, are generally served as anodes. However, the carbonaceous materials give rise to safety and life issues in a full cell configuration [3]. To overcome the common problems of this anode material, one of the approaches is to exploit new highcapacity and good-cycling-behavior materials for replacing the conventional carbon-based materials. Lately, spinel LTO has been paid a lot of attentions as a promising candidate of anode material due to its excellent Li-ion insertion/extraction reversibility with zero structural change and a relatively higher operating voltage (1.55 V vs. Li/Li+) to ensure safety of the battery by avoiding the
* Corresponding author. Tel.: +86 571 86843618; fax: +86 571 86843255 E-mail address: [email protected] (Y. Cai). http://dx.doi.org/10.1016/j.electacta.2015.03.016 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
trouble of lithium dendrites [3–6]. However, there are still some intrinsic barriers need to be resolved for the realistic applications of LTO anodes, for example, the poor electrical conductivity (<1013 S cm1) and sluggish lithium ion diffusion resulting in poor rate capability [7,8]. Great efforts have been made to improve the rate capability of LTO, including the synthesis of nanosized particles to shorten the Li+ diffusion path [9–13], the doping with metal or non-metal ions (such as Ca2+ [14], Na+ [15], Zn2+ [16], W6+ [4], Al3+ [17], Mn3+ [18], Zr4+ [19], V5+ [20], F [5] and Br [21]) in Li, Ti or O sites of LTO, and the coating LTO with conductive species (such as Ag [8], carbon [6], carbon nanotubes [22,23] and graphene [24–26] etc.). Among these methods, conductive coating has been considered as the most effective way. Graphene, a new two-dimensional macromolecular sheet of carbon family with a honeycomb structure, has superior electronic conductivity than graphite [27,28] and may be an ideal conductive additive for hybrid nanostructured electrodes [29]. For instance, Cao et al. fabricated graphene-embedded LTO nanofibers and obtained enhanced electrochemical properties [24], Kong et al. prepared a 3D dandelion-like LTO@graphene mcrosphere electrode with excellent rate capability and long cycle life [30]. However, it is difficult to achieve the homogeneous and complete graphene coating on the surface of the LTO particles. Herein, we report a graphene oxide (GO) confined strategy to synthesize rGO-coated LTO microspheres. The as-prepared samples are used as anode materials for LIBs. The electrochemical
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performance of pristine LTO and rGO-coated LTO microspheres is investigated in detail. The results demonstrated that the 3 wt% rGO-coated LTO can deliver a large capacity at a high chargedischarge rate with excellent cycling stability, which provides a new choice for the development of high performance anodes for lithium ion batteries.
The obtained materials were characterized by X-ray diffraction (XRD, Rigaku RU-200BVH with a Co-Ka source, with l = 0.1789 nm), field emission scanning electron microscopy (FE-SEM, Hitachi S4800), transmission electron microscopy (TEM, JEM-1230), thermo gravimetric analyzer (TG, Perkin-Elmer TGA), and Shimadzu UV 2550PC spectrophotometer.
2. Experimental
2.3. Electrochemical measurements
2.1. Materials
Electrochemical measurement was carried out using 2032 foil coin cells hardware. For preparing working electrodes, a uniform slurry was prepared by mixing the active material, acetylene black, and PVDF at a weight ratio of 80:10:10 via vigorously stirring for 3 h and then was pasted on the pure copper foil. The loading density of the total material was 2 mg cm2. Half cells were then assembled in an Ar-filled drybox (< 1 ppm H2O/O2) using pure lithium foil as counter electrode and Celgard 2500 separators saturated with LiPF6 electrolyte. The galvanostatic charge/discharge tests were performed using a NEWARE battery tester with a voltage window of 1.0-3.0 V vs. Li+/Li at room temperature. Cyclic voltammetry (CV) curves and the AC impedance measurements were measured on a CHI660E electrochemical workstation (CH Instrument, China). A 5 mV amplitude sinusoidal wave was applied in the tested frequency range of 0.1 Hz–100 kHz. The ZsimpWin version 3.1 was employed for complex nonlinear least square (CNLS) analyses of the EIS results.
Tetra-butyl ortho-titanate (TBOT), LiOHH2O, polyvinylpyrrolidone (PVP, Mw53,000) and ethanol were analytical grade and purchased from Hangzhou Mike Chemical Agents Company, China. Commerical graphite flake (average particle size is 1 mm), acetylene black, Poly(vinylidene fluoride) (PVDF) and N-methyl2-pyrrolidone (NMP, 99.5%) were obtained from Sigma–Aldrich Inc. Electrolyte of 1 M LiPF6 in 1:1 ethylene carbonate/dimethyl carbonate was provided from Ferro Corp. 2.2. Preparation of LTO and rGO-coated LTO composite Fig. 1 shows the schematic of the preparation process of LTO and rGO-coated LTO composite. TiO2 Powder was fabricated firstly via hydrolysis of TBOT. 10 ml solution of 2 ml TBOT dispersed in 8 ml ethanol was slowly added into 100 ml ethanol solution of PVP (0.02 wt%) containing 1 ml water. After stirring for 10 min at room temperature, the mixture solution was treated hydrothermally at 80 C for 2 h in a Teflon-lined autoclave. The obtained TiO2 precipitate was washed with ethanol and air-dried at 100 C for 24 h. And then, the TiO2 powder was added into a 36 ml LiOH solution composed of 12 ml ethanol and 24 ml of deionized water. The molar ratio of Ti to Li in the reaction system was kept as 5:4.5. After stirring for 30 min, the mixture of TiO2 and LiOH was treated hydrothermally at 180 C for 36 h and followed a thorough rinse and desiccation at 60 C for 48 h. Subsequently, the as-prepared sample was calcined at 600 C for 6 h under Ar atmosphere to obtain spinel LTO. For the GO-confined synthesis of LTO, the graphene oxide was prepared by a modified Hummers method [31,32]. The obtained colloidal solution of GO in deionized water was slowly added into the mixture of TiO2 and LiOH before the hydrothermal treatment and stirred subsequently for 1 h at room temperature. The system of GO, TiO2 and LiOH was further treated hydrothermally and calcined as above-mentioned method. The intended GO amounts are 1.0 and 3.0 wt% of the total GO/LTO composites, which were denoted as 1.0% and 3.0% rGO-coated LTO material respectively corresponding to their GO contents.
3. Results and discussion For the synthesis of rGO-coated LTO, we first prepare TiO2 particles via the hydrolysis of TBOT. As shown in Fig. 2a, the asreceived TiO2 are light yellow powder and have spherical morphology with a diameter of 12 mm. The surface of TiO2 sphere is coarse and porous, which is advantageous to the penetration of Li+ into its internal and the lithiation process. Subsequently, a thin GO layer is covered on the surface of TiO2 as a conductive coating. The image in Fig. 2b suggests that the intended GO layer is coated evenly on the surface of TiO2 microsphere, indicating that the scheme for fabricating GO coating is practicable. TEM has been used to further characterize the structure of GO and GO-coated TiO2 particles (Fig. 2cf). GO is thin sheet and obvious two-dimensional material (Fig. 2c). It could clearly observe that there are large amount of GO sheets around the TiO2 particles and the surface of TiO2 is coated by GO nanosheets completely, as shown in Fig. 2de. Fig. 3 presents the microstructure of pristine LTO, 1.0 wt% and 3.0 wt% rGO-coated LTO particles respectively. Obtained pristine
Fig. 1. Schematic of the synthesis process for the pristine LTO and rGO-coated LTO materials.
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Fig. 2. SEM images of (a) as-synthesized spherical TiO2, (b) GO-coated TiO2, and TEM images of (c) GO nanosheets, and (df) GO-coated TiO2 at different magnification.
LTO particles are irregular. They are composed of loose nanoplatelets. The size distribution of LTO particles is uneven, but all of them are bigger than the previous TiO2 microsphere, as shown in Fig. 3a. Conversely, both 1.0 wt% and 3.0 wt% rGO-coated LTO particles are spherical and have similar diameter to that of TiO2 microspheres (Fig. 3b and c). The reason is that the TiO2 microspheres were thoroughly coated by GO before hydrothermal synthesis, which provides a confined space for the formation of LTO crystal. Three samples are characterized further using TEM (Fig. 3df). It can be seen that the pristine LTO particle is irregular shape and the size is about 6 mm, which is corresponding with the SEM image (Fig. 3a). The particle is composed of loose nanoplates
based on TEM image of the sample at high magnification on the top-right corner. Fig. 3e and f shows the TEM images of 1.0 wt% and 3.0 wt% rGO-coated LTO particles respectively, both of them have similar microstructure. Compare with the pristine LTO, the shape of the 1.0 wt% and 3.0 wt% rGO-coated LTO are nearly spherical and just about 1.6 mm in diameter. In addition, the surface of 1.0 wt% and 3.0 wt% rGO-coated LTO particles show relatively compact due to the GO-confined synthesis. GO nanosheets can not be recognized in the images due to the shield of LTO nanoplates on the surface of the materials. The chemical composition and the crystallinity of LTO and rGO-coated LTO have been determined using X-ray diffraction. As
Fig. 3. SEM and TEM images of pristine LTO (a and d), 1.0 wt% rGO-coated LTO (b and e), and 3.0 wt% rGO-coated LTO (c and f).
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Fig. 4. XRD patterns of the pristine LTO and 3.0 wt% rGO-coated LTO.
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Fig. 6. TG curve of the 1.0 wt% and 3.0 wt% rGO-coated LTO sample measured from 50 to 800 Cwith a heating rate of 10 C min1under an air flow of 60 mL min1.
shown in Fig. 4, the diffraction peaks appearing at 18.4, 35.6, 43.3, 47.4, 57.2, 62.8, 66.1, 74.3, 75.4, and 79.4 can be ascribed to the spinel LTO phase (JCPDS card no. 49-0207). A tiny peak at 25.2 indicated the existence of a trace of anatase, which probably derived from residual TiO2 [7]. However, no obvious rGO peak is detected according to the XRD pattern of 3% rGO-coated LTO probably due to low rGO content in the composite. UV-vis absorption spectra of the GO, pristine LTO and 3.0 wt% rGO-coated LTO are investigated to further confirm the existence of rGO in composite samples and its status. As shown in Fig. 5, no absorption peaks could be observed for LTO sample. But obvious absorption peaks could be detected for both GO and 3.0 wt% rGO-coated LTO sample, which implies the introduction of GO into LTO. But the typical absorption peak shifted from 228 nm of GO to 268 nm of rGO-coated LTO. According to Loh et al's report, GO could be transferred into rGO after hydrothermal treatment, accompanying with a shift of absorption peak from 227 nm to 254 nm [27,28], which is consistence with our experimental results. So it can be deducted that the GO was reduced to rGO via hydrothermal treatment. In order to confirm the content of rGO, TG analysis was carried out (Fig. 6). The first stage of the weight loss is from room temperature to 180 C, and the preliminary weight loss is about 1.5 wt%, mainly due to the evaporation of the absorbed water on the surface of samples. In the second weight- loss stage from 180 to 500 C, the weight- loss content reach 1.8 and 3.1 wt% corresponding to the 1.0 and 3.0 wt% rGO-coated LTO, respectively. The weight
Fig. 5. UV-vis absorption spectra of GO, pristine LTO and 3.0 wt% rGO-coated LTO.
Fig. 7. The discharge capacity of pristine LTO, 1.0 and 3.0 wt% rGO-coated LTO and their corresponding coulombic efficiency at (a) 0.5 C, (b) 15 C and (c) different current rates.
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loss can be ascribed to the loss of rGO in the composites and the values are closed to their theoretical values. To investigate the role of rGO coating, coin cells (2032) with a metallic Li counter electrode are used to evaluate the electrochemical performance between the rGO-coated LTO and pristine LTO on copper coils. The capacity and cycle performance of all electrode samples are investigated by galvanostatic chargedischarge measurements. As shown in Fig. 7, at the standard discharge-charge rate of 0.5 C (1C = 175 mAh g1), the cycling ability of pristine LTO is rather unstable. The first discharge capacity is dropped from 168.1 mA h g1 of the first cycle to 138.5 mA h g1 after 100 cycles. In comparison, both 1.0 wt% and 3.0 wt% rGO-coated LTO exhibit higher capacity with substantially lower capacity fading in repeated cycling: their first discharge capacities are as high as 169.2 mA h g1 and 167.1 mA h g1, respectively; After 100 cycles, the specific capacities of 1.0 wt% rGO-coated LTO and 3.0 wt% rGO-coated LTO are still kept to be 157.5 mAh g1 and 162.1 mAh g1, respectively (Fig. 7a). To further investigate the effect of rGO coating, the cycling performances of samples at 15 C are detected (Fig. 7b). The discharge capacity of pristine LTO drop to 48.1 mA h g1 after 150 cycles at 15 C, however, the 1.0 and 3.0 wt% rGO-coated LTO samples maintain at 86.8 and 87.5 mA h g1 respectively at the same measurement condition. It is obvious that the presence of rGO helps LTO to increase the capacity retentions performance. In addition, there is no obvious distinction between the 1.0 and 3.0 wt% rGO-coated LTO samples because of the rGO coating is efficiently for improve the cycling performances of both samples at a similar level. Fig. 7c shows the discharge capacity vs. cycle number of pristine LTO, 1.0 wt% rGO-coated LTO and 3.0 wt% rGO-coated LTO cells at
different charge and discharge rates. The discharge capacity of pristine LTO drops dramatically with the increasing of C-rates and shows a very disappointing capacity of 91.4 mAh g1 at 10 C-rate. For both rGO-coated LTO samples, the charge/discharge performance at higher current density is better than that of pristine LTO. Their capacity retention at 10 C-rate is 78% and 77% for 1.0 and 3.0 wt% rGO, respectively. The discharge capacity of 3.0 wt% rGO-coated LTO is 160.1 mAh g1 at 0.5 C. The rate is then increased stepwisely to 1, 3, 5 and 10 C in succession, and the capacities of 154.5, 146.8, 138.6 and 119.1 mAh g1 were obtained, respectively, at corresponding rate. When the C-rate is finally returned to 0.5 C after a total of 50 cycles, the discharge capacity can still be recovered to 155.2 mAh g1 and maintained this value nearly without any loss for the next 10 cycles. Such demonstration of excellent rate capability makes rGO-coated LTO particularly suitable for energy storage system in EV or HEV [2]. The cycling stability of pristine LTO and 3.0 wt% rGO-coated LTO at high charge/discharge current density of 5 C is displayed in Fig. 8a. After an initial discharge capacity of 136.9 mAh g1 and 135.7 mAh g1, the pristine LTO and 3.0 wt% rGO-coated LTO achieve their capacity retentions of 83% and 97% at the end of 500 cycles, respectively. Especially for the 3.0 wt% rGO-coated LTO, the discharge capacity has almost no change and the coulombic efficiency remains at almost 100% after 500 cycles. The rGO-coated LTO sample exhibits very excellent rate capability and cycling stability, which are superior to those of many previously reported nanostructured LTO anodes [9,18,33]. In addition, as shown in Fig. 8b, the discharge voltage plateau of pristine LTO is unstable and drops to 1.38 V after 500 cycles. This is due to the increased polarization for the electrodes. On the other hand, the
Fig. 8. (a) The discharge capacity and corresponding coulombic efficiency of pristine LTO and 3.0 wt% rGO-coated LTO at the rate of 5 C. (b) The voltage-capacity curves after different cycles at 5 C.
Fig. 9. The voltage-capacity curves of (a) pristine LTO and (b) 3.0 wt% rGO-coated LTO electrodes at different C rates.
J. Zhang et al. / Electrochimica Acta 165 (2015) 422–429
charge-discharge profiles of the 3.0 wt% rGO-coated at increasing cycles exhibited well-overlapped and flat plateaus, demonstrating good stability and reversibility of the electrode. Fig. 9 shows the voltage-capacity profiles of pristine LTO and 3.0 wt% rGO-coated LTO electrodes cycled under various cycling rates from 0.5 to 10 C in 1.0 - 3.0 V voltage windows. The plotted profiles are taken from the 5th cycle of each C rate. It can be clearly seen that the discharge voltage plateau drops with the current rate increasing, suggesting an increased polarization for both electrodes. The decrease in capacity with the increase of C-rate is an innate material response, but the difference between the discharge capacities of two electrodes is magnified with the increase of the C-rate. It is evident that the 3.0 wt% rGO-coated LTO exhibits higher energy retention, and the reversible capacity of 167.1 mAh g1 at 0.5 C drops to 119.8 mAh g1 at 10 C. In contrast, the pristine LTO electrode exhibits a terrible rate capacity due to the relatively large potential difference during high current rate shown in Fig. 9a. The discharge capacities of pristine LTO are 138.2 mAh g1 at 0.5 C and 91.4 mAh g1 at 10 C, which indicates that rGO coating is particularly effective for improving the performance of LTO at high current densities. In addition, though the discharge voltage plateaus of both pristine LTO and rGO-coated LTO at 0.5 C are around 1.55 V, the voltage plateau of rGO-coated LTO is much longer and stable than that of pristine LTO, which is a competitive advantage for power sources to provide a high power density and stable electricity output.
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Fig. 10 shows the cyclic voltammograms (CVs) of pristine LTO (a) and 3.0 wt% rGO-coated LTO (b), respectively, for the first and second cycle within a potential window of 1-3 V (vs. Li/Li+) at a scan rate of 0.1 mV s1. In the first cycle, a pair of redox peaks appears at 1.47 V (cathodic scan)/1.66 V (anodic scan) in pristine LTO electrode, corresponding to 1.49 V/1.63 V in 3.0 wt% rGO-coated LTO electrode. These peaks are related to the Li+ insertion (reduction) and extraction (oxidation) processes [34,35]. The potential differences between anodic and cathodic peaks are 0.19 and 0.14 V for pristine LTO and 3.0 wt% rGO-coated LTO, which suggests that pristine LTO has a lower electrode polarization. To further investigate the influence of rGO coating on the kinetic performance of Li ion, the CVs of both samples are performed at different scan rate, as shown in Fig. 10c. The redox peaks of the pristine LTO have broadened with the increasing of scan rate, while those of rGO-coated LTO have retained a stable shape. Additionally, all the peak current densities of rGO-coated LTO are higher than those of pristine LTO at each scan rate. These results demonstrate that the surface structure of 3.0 wt% rGO-coated LTO has a very slight effect on the ion transferring between the electrolyte and the bulk LTO phase. Fig. 10d presents the correlation between the peak current densities and square root of the scan rate in the cathodic process for both pristine LTO and 3.0 wt% rGO-coated LTO, which matches the linear relationship very well. It indicates that the Li+ insertion/deinsertion in both materials belongs to the diffusioncontrolled transport. The diffusion coefficient can be estimated
Fig. 10. Cyclic voltammetry curves of (a) pristine LTO and (b) 3.0 wt% rGO-coated LTO electrodes in the first and second cycles at a scan rate of 0.1 mV s1; (c) Cyclic voltammetry curves of pristine LTO and 3.0 wt% rGOcoated LTO electrodes at at various scan rates; (d) Anodic peak current densities against square roots of scan rate of the pristine LTO and 3.0 wt% rGOcoated LTO electrodes.
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electrode has a higher electronic conductivity and exhibits the better electrochemical performance than pristine LTO. 4. Conclusions
Fig. 11. EIS of the pristine LTO and 3.0 wt% rGO-coated LTO electrodes after one charged–discharged cycle and at the stable voltage of 1.55 V (vs Li/Li+); the inserted images at the right-bottom corner is corresponding equivalent circuit.
according to the slope of the straight lines. Hence it can be deduced that the diffusion coefficient of rGO-coated LTO is slightly larger than that of pristine LTO. These results imply that the coating of rGO during the forming of LTO can effectively reduce the barrier for Li+ transportation and rendering relatively high Li+ mobility [8,17]. To demonstrate the effect of rGO coating on the electronic conductivity properties of LTO, the EIS measurements were carried out for both pristine LTO and 3.0 wt% rGO-coated LTO electrodes after the cells were charged–discharged by one cycle at the stable voltage of 1.55 V (vs. Li/Li+). Fig. 11 shows the corresponding Nyquist plots of the spectra and the equivalent circuit used to fit the EIS. Both EIS curves are composed of a depressed semicircle at the range of high and intermediate frequency, and there is a straight line at the lowest frequency region. The high-frequency semicircle usually corresponds to the charge transfer process (Rct), while the sloping line at the low frequency region indicates the Warburg impedance caused by a semi-infinite diffusion of Li+ ion in the electrode [15,36]. In the equivalent circuit, Rs is the ohmic resistance of electrolyte; Rct is the charge transfer resistance; CPE represent the double layer capacitance and passivation film capacitance; Zw represents the Warburg impedance [37,38]. The fitted parameters are listed in Table 1. It is found that the overall charge transfer resistance of the rGO-coated LTO is much smaller than that of the pristine LTO, indicating its higher conductivity and faster transferring of Li ion during charge-discharge process. Furthermore, the exchange current density (j) can be calculated according to the following equation [4]: j ¼ RT=nFRct
(1)
where the meaning of n is the number of electrons transferred in the half-reaction for the redox couple, R is the gas constant (8.314 J mol1 K1), T is the absolute temperature (298 K), F is the Faraday constant (96500 C mol1). As seen in Table 1, the exchange current density of the rGO-coated LTO is higher than that of the pristine LTO. Thus, the charge-transfer reaction of the rGO-coated LTO electrode took place more easily than pristine LTO electrode. The EIS results further demonstrate that the rGO-coated LTO Table 1 Fitted results of pristine LTO and 3.0 wt% rGO-coated LTO obtained by EIS. Samples
Rs(V)
Rct(V)
CPE-T (mF)
j(mA cm2)
Pristine LTO 3.0 wt% rGO-coated LTO
7.681 7.311
104.8 36.65
1.925 4.639
0.159 0.455
Spherical rGO-coated LTO composites are prepared by GO-confined hydrothermal reaction based on TiO2 and Li salt followed with calcinations treatment, which exhibits obviously improving specific capacity and rate capability compared with the pristine LTO. The discharge specific capacity of 3.0 wt% GO-coated LTO anodes reaches a higher value of 131.6 mAh g1 at 5 C and 97% retention of capacity has still been maintained even after 500 cycles, which are more excellent than those of pure pristine LTO. The enhanced electrochemical performances could be ascribed to significantly increase the electrical conductivity of LTO powders and improve the diffusion kinetic property of lithium ions by the uniform rGO coating on the surfaces of LTO spheres. We anticipate that this novel rGO coating approach can also be applied to the design and synthesis of other electrode materials with a high capacity and excellent cycling stability. Acknowledgment The Project is funded by the National Natural Science Foundation of China (Grant no. 61376005), Zhejiang Top Priority Discipline of Textile Science and Engineering of Open Foundation (2014KF07) and Young Researchers Foundation (2014YXQN02). References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] T.F. Yi, Y. Xie, Y.R. Zhu, R.S. Zhu, H.Y. Shen, Structural and thermodynamic stability of Li4Ti5O12 anode material for lithium-ion battery, J Power Sources 222 (2013) 448–454. [3] H.G. Jung, S.T. Myung, C.S. Yoon, S.B. Son, K.H. Oh, K. Amine, B. Scrosati, Y.-K. Sun, Microscale spherical carbon-coated Li4Ti5O12 as ultra high power anode material for lithium batteries, Energ Environ Sci 4 (2011) 1345–1351. [4] Q.Y. Zhang, C.L. Zhang, B. Li, D.D. Jiang, S.F. Kang, X. Li, Y.G. Wang, Preparation and characterization of W-doped Li4Ti5O12 anode material for enhancing the high rate performance, Electrochim Acta 107 (2013) 139–146. [5] Y. Ma, B. Ding, G. Ji, J.Y. Lee, Carbon-Encapsulated F-Doped Li4Ti5O12 as a High Rate Anode Material for Li+ Batteries, ACS Nano 7 (2013) 10870–10878. [6] N. Li, G.M. Zhou, F. Li, L. Wen, H.M. Cheng, A Self-Standing and Flexible Electrode of Li4Ti5O12 Nanosheets with a N-Doped Carbon Coating for High Rate Lithium Ion Batteries, Adv Funct Mater 23 (2013) 5429–5435. [7] G.N. Zhu, H.J. Liu, J.H. Zhuang, C.X. Wang, Y.G. Wang, Y.Y. Xia, Carbon-coated nano-sized Li4Ti5O12 nanoporous micro-sphere as anode material for highrate lithium-ion batteries, Energ Environ Sci 4 (2011) 4016–4022. [8] H.Q. Zhang, Q.J. Deng, C.X. Mou, Z.L. Huang, Y. Wang, A.J. Zhou, J.Z. Li, Surface structure and high-rate performance of spinel Li4Ti5O12 coated with N-doped carbon as anode material for lithium-ion batteries, J Power Sources 239 (2013) 538–545. [9] J. Liu, X.F. Li, J.L. Yang, D.S. Geng, Y.L. Li, D.N. Wang, R.Y. Li, X.L. Sun, M. Cai, M.W. Verbrugge, Microwave-assisted hydrothermal synthesis of nanostructured spinel Li4Ti5O12 as anode materials for lithium ion batteries, Electrochim Acta 63 (2012) 100–104. [10] J. Cheng, R.C. Che, C.Y. Liang, J.W. Liu, M. Wang, J.J. Xu, Hierarchical hollow Li4Ti5O12 urchin-like microspheres with ultra-high specific surface area for high rate lithium ion batteries, Nano Research 7 (2014) 1043–1053. [11] T.F. Yi, Z.K. Fang, Y. Xie, Y.R. Zhu, S.Y. Yang, Rapid Charge-Discharge Property of Li4Ti5O12-TiO2 Nanosheet and Nanotube Composites as Anode Material for Power Lithium-Ion Batteries, ACS appl mater inter 6 (2014) 20205–20213. [12] L. Yu, H.B. Wu, X.W.D. Lou, Mesoporous Li4Ti5O12 Hollow Spheres with Enhanced Lithium Storage Capability, Adv Mater 25 (2013) 2296–2300. [13] Y.X. Tang, Y.Y. Zhang, J.Y. Deng, D.P. Qi, W.R. Leow, J.Q. Wei, S.Y. Yin, Z.L. Dong, R. Yazami, Z. Chen, Unravelling the Correlation between the Aspect Ratio of Nanotubular Structures and Their Electrochemical Performance To Achieve High-Rate and Long-Life Lithium-Ion Batteries, Angew Chem 126 (2014) 1–6. [14] Q.Y. Zhang, C.L. Zhang, B. Li, S.F. Kang, X. Li, Y.G. Wang, Preparation and electrochemical properties of Ca-doped Li4Ti5O12 as anode materials in lithium-ion battery, Electrochim Acta 98 (2013) 146–152.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Graphene oxide-confined synthesis of Li4Ti5O12 microspheres as high-performance anodes for lithium ion batteries Jiawei Zhang a , Yurong Cai a, * , Jun Wu b , Juming Yao a a The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, College of Materials and Textiles, Zhejiang SciTech University, Hangzhou 310018, China b Institute of Electron Device & Application, Hangzhou Dianzi University, Hangzhou 310018, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 January 2015 Received in revised form 11 February 2015 Accepted 2 March 2015 Available online 5 March 2015
This paper reports a graphene oxide (GO) confined strategy to synthesize reduced GO-coated lithium titanate (Li4Ti5O12, LTO) microspheres using as-prepared TiO2 microspheres and GO as raw materials. The obtained samples are characterized by X-ray diffraction, field emission scanning electron microscopy and spectrophotometer. Results show that the spherical LTO is formed with approximate 1 mm diameter after hydrothermal reactions, which is due to a confined effect of GO on the surface of TiO2 spheres. Electrochemical tests reveal that the presence of rGO can increase the capacity and cycling stability of LTO anodes, especially at higher C rate. The 3 wt% rGO-coated LTO anodes present a higher reversible Li-ion storage with a specific discharge capacity of 131.6 mAh g1 at 5 C and 97% retention even after 500 cycles, which are more excellent than those of pristine LTO. The GO-confined method is anticipated to synthesize other electrode materials with high electrochemical performances. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Graphene oxide confined synthesis lithium titanate anode material capacity
1. Introduction Lithium ion batteries (LIBs) possess a higher volumetric and gravimetric energy density than lead acid, nickel-cadmium and nickel-metal hydride rechargeable batteries, which brings to their wide applications in portable electronics, such as cell phones, laptop computers, digital cameras and smart grid applications, etc [1]. Otherwise, LIBs can still not completely satisfy the requirement of electrified transportations, electric vehicle (EV) or hybrid electric vehicle (HEV), for example, mainly due to their sluggish charge/discharge speed, low capacity and poor cycling stability [2]. In commercial LIBs, carbonaceous materials, especially natural graphite, are generally served as anodes. However, the carbonaceous materials give rise to safety and life issues in a full cell configuration [3]. To overcome the common problems of this anode material, one of the approaches is to exploit new highcapacity and good-cycling-behavior materials for replacing the conventional carbon-based materials. Lately, spinel LTO has been paid a lot of attentions as a promising candidate of anode material due to its excellent Li-ion insertion/extraction reversibility with zero structural change and a relatively higher operating voltage (1.55 V vs. Li/Li+) to ensure safety of the battery by avoiding the
* Corresponding author. Tel.: +86 571 86843618; fax: +86 571 86843255 E-mail address: [email protected] (Y. Cai). http://dx.doi.org/10.1016/j.electacta.2015.03.016 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
trouble of lithium dendrites [3–6]. However, there are still some intrinsic barriers need to be resolved for the realistic applications of LTO anodes, for example, the poor electrical conductivity (<1013 S cm1) and sluggish lithium ion diffusion resulting in poor rate capability [7,8]. Great efforts have been made to improve the rate capability of LTO, including the synthesis of nanosized particles to shorten the Li+ diffusion path [9–13], the doping with metal or non-metal ions (such as Ca2+ [14], Na+ [15], Zn2+ [16], W6+ [4], Al3+ [17], Mn3+ [18], Zr4+ [19], V5+ [20], F [5] and Br [21]) in Li, Ti or O sites of LTO, and the coating LTO with conductive species (such as Ag [8], carbon [6], carbon nanotubes [22,23] and graphene [24–26] etc.). Among these methods, conductive coating has been considered as the most effective way. Graphene, a new two-dimensional macromolecular sheet of carbon family with a honeycomb structure, has superior electronic conductivity than graphite [27,28] and may be an ideal conductive additive for hybrid nanostructured electrodes [29]. For instance, Cao et al. fabricated graphene-embedded LTO nanofibers and obtained enhanced electrochemical properties [24], Kong et al. prepared a 3D dandelion-like LTO@graphene mcrosphere electrode with excellent rate capability and long cycle life [30]. However, it is difficult to achieve the homogeneous and complete graphene coating on the surface of the LTO particles. Herein, we report a graphene oxide (GO) confined strategy to synthesize rGO-coated LTO microspheres. The as-prepared samples are used as anode materials for LIBs. The electrochemical
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performance of pristine LTO and rGO-coated LTO microspheres is investigated in detail. The results demonstrated that the 3 wt% rGO-coated LTO can deliver a large capacity at a high chargedischarge rate with excellent cycling stability, which provides a new choice for the development of high performance anodes for lithium ion batteries.
The obtained materials were characterized by X-ray diffraction (XRD, Rigaku RU-200BVH with a Co-Ka source, with l = 0.1789 nm), field emission scanning electron microscopy (FE-SEM, Hitachi S4800), transmission electron microscopy (TEM, JEM-1230), thermo gravimetric analyzer (TG, Perkin-Elmer TGA), and Shimadzu UV 2550PC spectrophotometer.
2. Experimental
2.3. Electrochemical measurements
2.1. Materials
Electrochemical measurement was carried out using 2032 foil coin cells hardware. For preparing working electrodes, a uniform slurry was prepared by mixing the active material, acetylene black, and PVDF at a weight ratio of 80:10:10 via vigorously stirring for 3 h and then was pasted on the pure copper foil. The loading density of the total material was 2 mg cm2. Half cells were then assembled in an Ar-filled drybox (< 1 ppm H2O/O2) using pure lithium foil as counter electrode and Celgard 2500 separators saturated with LiPF6 electrolyte. The galvanostatic charge/discharge tests were performed using a NEWARE battery tester with a voltage window of 1.0-3.0 V vs. Li+/Li at room temperature. Cyclic voltammetry (CV) curves and the AC impedance measurements were measured on a CHI660E electrochemical workstation (CH Instrument, China). A 5 mV amplitude sinusoidal wave was applied in the tested frequency range of 0.1 Hz–100 kHz. The ZsimpWin version 3.1 was employed for complex nonlinear least square (CNLS) analyses of the EIS results.
Tetra-butyl ortho-titanate (TBOT), LiOHH2O, polyvinylpyrrolidone (PVP, Mw53,000) and ethanol were analytical grade and purchased from Hangzhou Mike Chemical Agents Company, China. Commerical graphite flake (average particle size is 1 mm), acetylene black, Poly(vinylidene fluoride) (PVDF) and N-methyl2-pyrrolidone (NMP, 99.5%) were obtained from Sigma–Aldrich Inc. Electrolyte of 1 M LiPF6 in 1:1 ethylene carbonate/dimethyl carbonate was provided from Ferro Corp. 2.2. Preparation of LTO and rGO-coated LTO composite Fig. 1 shows the schematic of the preparation process of LTO and rGO-coated LTO composite. TiO2 Powder was fabricated firstly via hydrolysis of TBOT. 10 ml solution of 2 ml TBOT dispersed in 8 ml ethanol was slowly added into 100 ml ethanol solution of PVP (0.02 wt%) containing 1 ml water. After stirring for 10 min at room temperature, the mixture solution was treated hydrothermally at 80 C for 2 h in a Teflon-lined autoclave. The obtained TiO2 precipitate was washed with ethanol and air-dried at 100 C for 24 h. And then, the TiO2 powder was added into a 36 ml LiOH solution composed of 12 ml ethanol and 24 ml of deionized water. The molar ratio of Ti to Li in the reaction system was kept as 5:4.5. After stirring for 30 min, the mixture of TiO2 and LiOH was treated hydrothermally at 180 C for 36 h and followed a thorough rinse and desiccation at 60 C for 48 h. Subsequently, the as-prepared sample was calcined at 600 C for 6 h under Ar atmosphere to obtain spinel LTO. For the GO-confined synthesis of LTO, the graphene oxide was prepared by a modified Hummers method [31,32]. The obtained colloidal solution of GO in deionized water was slowly added into the mixture of TiO2 and LiOH before the hydrothermal treatment and stirred subsequently for 1 h at room temperature. The system of GO, TiO2 and LiOH was further treated hydrothermally and calcined as above-mentioned method. The intended GO amounts are 1.0 and 3.0 wt% of the total GO/LTO composites, which were denoted as 1.0% and 3.0% rGO-coated LTO material respectively corresponding to their GO contents.
3. Results and discussion For the synthesis of rGO-coated LTO, we first prepare TiO2 particles via the hydrolysis of TBOT. As shown in Fig. 2a, the asreceived TiO2 are light yellow powder and have spherical morphology with a diameter of 12 mm. The surface of TiO2 sphere is coarse and porous, which is advantageous to the penetration of Li+ into its internal and the lithiation process. Subsequently, a thin GO layer is covered on the surface of TiO2 as a conductive coating. The image in Fig. 2b suggests that the intended GO layer is coated evenly on the surface of TiO2 microsphere, indicating that the scheme for fabricating GO coating is practicable. TEM has been used to further characterize the structure of GO and GO-coated TiO2 particles (Fig. 2cf). GO is thin sheet and obvious two-dimensional material (Fig. 2c). It could clearly observe that there are large amount of GO sheets around the TiO2 particles and the surface of TiO2 is coated by GO nanosheets completely, as shown in Fig. 2de. Fig. 3 presents the microstructure of pristine LTO, 1.0 wt% and 3.0 wt% rGO-coated LTO particles respectively. Obtained pristine
Fig. 1. Schematic of the synthesis process for the pristine LTO and rGO-coated LTO materials.
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Fig. 2. SEM images of (a) as-synthesized spherical TiO2, (b) GO-coated TiO2, and TEM images of (c) GO nanosheets, and (df) GO-coated TiO2 at different magnification.
LTO particles are irregular. They are composed of loose nanoplatelets. The size distribution of LTO particles is uneven, but all of them are bigger than the previous TiO2 microsphere, as shown in Fig. 3a. Conversely, both 1.0 wt% and 3.0 wt% rGO-coated LTO particles are spherical and have similar diameter to that of TiO2 microspheres (Fig. 3b and c). The reason is that the TiO2 microspheres were thoroughly coated by GO before hydrothermal synthesis, which provides a confined space for the formation of LTO crystal. Three samples are characterized further using TEM (Fig. 3df). It can be seen that the pristine LTO particle is irregular shape and the size is about 6 mm, which is corresponding with the SEM image (Fig. 3a). The particle is composed of loose nanoplates
based on TEM image of the sample at high magnification on the top-right corner. Fig. 3e and f shows the TEM images of 1.0 wt% and 3.0 wt% rGO-coated LTO particles respectively, both of them have similar microstructure. Compare with the pristine LTO, the shape of the 1.0 wt% and 3.0 wt% rGO-coated LTO are nearly spherical and just about 1.6 mm in diameter. In addition, the surface of 1.0 wt% and 3.0 wt% rGO-coated LTO particles show relatively compact due to the GO-confined synthesis. GO nanosheets can not be recognized in the images due to the shield of LTO nanoplates on the surface of the materials. The chemical composition and the crystallinity of LTO and rGO-coated LTO have been determined using X-ray diffraction. As
Fig. 3. SEM and TEM images of pristine LTO (a and d), 1.0 wt% rGO-coated LTO (b and e), and 3.0 wt% rGO-coated LTO (c and f).
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Fig. 4. XRD patterns of the pristine LTO and 3.0 wt% rGO-coated LTO.
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Fig. 6. TG curve of the 1.0 wt% and 3.0 wt% rGO-coated LTO sample measured from 50 to 800 Cwith a heating rate of 10 C min1under an air flow of 60 mL min1.
shown in Fig. 4, the diffraction peaks appearing at 18.4, 35.6, 43.3, 47.4, 57.2, 62.8, 66.1, 74.3, 75.4, and 79.4 can be ascribed to the spinel LTO phase (JCPDS card no. 49-0207). A tiny peak at 25.2 indicated the existence of a trace of anatase, which probably derived from residual TiO2 [7]. However, no obvious rGO peak is detected according to the XRD pattern of 3% rGO-coated LTO probably due to low rGO content in the composite. UV-vis absorption spectra of the GO, pristine LTO and 3.0 wt% rGO-coated LTO are investigated to further confirm the existence of rGO in composite samples and its status. As shown in Fig. 5, no absorption peaks could be observed for LTO sample. But obvious absorption peaks could be detected for both GO and 3.0 wt% rGO-coated LTO sample, which implies the introduction of GO into LTO. But the typical absorption peak shifted from 228 nm of GO to 268 nm of rGO-coated LTO. According to Loh et al's report, GO could be transferred into rGO after hydrothermal treatment, accompanying with a shift of absorption peak from 227 nm to 254 nm [27,28], which is consistence with our experimental results. So it can be deducted that the GO was reduced to rGO via hydrothermal treatment. In order to confirm the content of rGO, TG analysis was carried out (Fig. 6). The first stage of the weight loss is from room temperature to 180 C, and the preliminary weight loss is about 1.5 wt%, mainly due to the evaporation of the absorbed water on the surface of samples. In the second weight- loss stage from 180 to 500 C, the weight- loss content reach 1.8 and 3.1 wt% corresponding to the 1.0 and 3.0 wt% rGO-coated LTO, respectively. The weight
Fig. 5. UV-vis absorption spectra of GO, pristine LTO and 3.0 wt% rGO-coated LTO.
Fig. 7. The discharge capacity of pristine LTO, 1.0 and 3.0 wt% rGO-coated LTO and their corresponding coulombic efficiency at (a) 0.5 C, (b) 15 C and (c) different current rates.
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loss can be ascribed to the loss of rGO in the composites and the values are closed to their theoretical values. To investigate the role of rGO coating, coin cells (2032) with a metallic Li counter electrode are used to evaluate the electrochemical performance between the rGO-coated LTO and pristine LTO on copper coils. The capacity and cycle performance of all electrode samples are investigated by galvanostatic chargedischarge measurements. As shown in Fig. 7, at the standard discharge-charge rate of 0.5 C (1C = 175 mAh g1), the cycling ability of pristine LTO is rather unstable. The first discharge capacity is dropped from 168.1 mA h g1 of the first cycle to 138.5 mA h g1 after 100 cycles. In comparison, both 1.0 wt% and 3.0 wt% rGO-coated LTO exhibit higher capacity with substantially lower capacity fading in repeated cycling: their first discharge capacities are as high as 169.2 mA h g1 and 167.1 mA h g1, respectively; After 100 cycles, the specific capacities of 1.0 wt% rGO-coated LTO and 3.0 wt% rGO-coated LTO are still kept to be 157.5 mAh g1 and 162.1 mAh g1, respectively (Fig. 7a). To further investigate the effect of rGO coating, the cycling performances of samples at 15 C are detected (Fig. 7b). The discharge capacity of pristine LTO drop to 48.1 mA h g1 after 150 cycles at 15 C, however, the 1.0 and 3.0 wt% rGO-coated LTO samples maintain at 86.8 and 87.5 mA h g1 respectively at the same measurement condition. It is obvious that the presence of rGO helps LTO to increase the capacity retentions performance. In addition, there is no obvious distinction between the 1.0 and 3.0 wt% rGO-coated LTO samples because of the rGO coating is efficiently for improve the cycling performances of both samples at a similar level. Fig. 7c shows the discharge capacity vs. cycle number of pristine LTO, 1.0 wt% rGO-coated LTO and 3.0 wt% rGO-coated LTO cells at
different charge and discharge rates. The discharge capacity of pristine LTO drops dramatically with the increasing of C-rates and shows a very disappointing capacity of 91.4 mAh g1 at 10 C-rate. For both rGO-coated LTO samples, the charge/discharge performance at higher current density is better than that of pristine LTO. Their capacity retention at 10 C-rate is 78% and 77% for 1.0 and 3.0 wt% rGO, respectively. The discharge capacity of 3.0 wt% rGO-coated LTO is 160.1 mAh g1 at 0.5 C. The rate is then increased stepwisely to 1, 3, 5 and 10 C in succession, and the capacities of 154.5, 146.8, 138.6 and 119.1 mAh g1 were obtained, respectively, at corresponding rate. When the C-rate is finally returned to 0.5 C after a total of 50 cycles, the discharge capacity can still be recovered to 155.2 mAh g1 and maintained this value nearly without any loss for the next 10 cycles. Such demonstration of excellent rate capability makes rGO-coated LTO particularly suitable for energy storage system in EV or HEV [2]. The cycling stability of pristine LTO and 3.0 wt% rGO-coated LTO at high charge/discharge current density of 5 C is displayed in Fig. 8a. After an initial discharge capacity of 136.9 mAh g1 and 135.7 mAh g1, the pristine LTO and 3.0 wt% rGO-coated LTO achieve their capacity retentions of 83% and 97% at the end of 500 cycles, respectively. Especially for the 3.0 wt% rGO-coated LTO, the discharge capacity has almost no change and the coulombic efficiency remains at almost 100% after 500 cycles. The rGO-coated LTO sample exhibits very excellent rate capability and cycling stability, which are superior to those of many previously reported nanostructured LTO anodes [9,18,33]. In addition, as shown in Fig. 8b, the discharge voltage plateau of pristine LTO is unstable and drops to 1.38 V after 500 cycles. This is due to the increased polarization for the electrodes. On the other hand, the
Fig. 8. (a) The discharge capacity and corresponding coulombic efficiency of pristine LTO and 3.0 wt% rGO-coated LTO at the rate of 5 C. (b) The voltage-capacity curves after different cycles at 5 C.
Fig. 9. The voltage-capacity curves of (a) pristine LTO and (b) 3.0 wt% rGO-coated LTO electrodes at different C rates.
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charge-discharge profiles of the 3.0 wt% rGO-coated at increasing cycles exhibited well-overlapped and flat plateaus, demonstrating good stability and reversibility of the electrode. Fig. 9 shows the voltage-capacity profiles of pristine LTO and 3.0 wt% rGO-coated LTO electrodes cycled under various cycling rates from 0.5 to 10 C in 1.0 - 3.0 V voltage windows. The plotted profiles are taken from the 5th cycle of each C rate. It can be clearly seen that the discharge voltage plateau drops with the current rate increasing, suggesting an increased polarization for both electrodes. The decrease in capacity with the increase of C-rate is an innate material response, but the difference between the discharge capacities of two electrodes is magnified with the increase of the C-rate. It is evident that the 3.0 wt% rGO-coated LTO exhibits higher energy retention, and the reversible capacity of 167.1 mAh g1 at 0.5 C drops to 119.8 mAh g1 at 10 C. In contrast, the pristine LTO electrode exhibits a terrible rate capacity due to the relatively large potential difference during high current rate shown in Fig. 9a. The discharge capacities of pristine LTO are 138.2 mAh g1 at 0.5 C and 91.4 mAh g1 at 10 C, which indicates that rGO coating is particularly effective for improving the performance of LTO at high current densities. In addition, though the discharge voltage plateaus of both pristine LTO and rGO-coated LTO at 0.5 C are around 1.55 V, the voltage plateau of rGO-coated LTO is much longer and stable than that of pristine LTO, which is a competitive advantage for power sources to provide a high power density and stable electricity output.
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Fig. 10 shows the cyclic voltammograms (CVs) of pristine LTO (a) and 3.0 wt% rGO-coated LTO (b), respectively, for the first and second cycle within a potential window of 1-3 V (vs. Li/Li+) at a scan rate of 0.1 mV s1. In the first cycle, a pair of redox peaks appears at 1.47 V (cathodic scan)/1.66 V (anodic scan) in pristine LTO electrode, corresponding to 1.49 V/1.63 V in 3.0 wt% rGO-coated LTO electrode. These peaks are related to the Li+ insertion (reduction) and extraction (oxidation) processes [34,35]. The potential differences between anodic and cathodic peaks are 0.19 and 0.14 V for pristine LTO and 3.0 wt% rGO-coated LTO, which suggests that pristine LTO has a lower electrode polarization. To further investigate the influence of rGO coating on the kinetic performance of Li ion, the CVs of both samples are performed at different scan rate, as shown in Fig. 10c. The redox peaks of the pristine LTO have broadened with the increasing of scan rate, while those of rGO-coated LTO have retained a stable shape. Additionally, all the peak current densities of rGO-coated LTO are higher than those of pristine LTO at each scan rate. These results demonstrate that the surface structure of 3.0 wt% rGO-coated LTO has a very slight effect on the ion transferring between the electrolyte and the bulk LTO phase. Fig. 10d presents the correlation between the peak current densities and square root of the scan rate in the cathodic process for both pristine LTO and 3.0 wt% rGO-coated LTO, which matches the linear relationship very well. It indicates that the Li+ insertion/deinsertion in both materials belongs to the diffusioncontrolled transport. The diffusion coefficient can be estimated
Fig. 10. Cyclic voltammetry curves of (a) pristine LTO and (b) 3.0 wt% rGO-coated LTO electrodes in the first and second cycles at a scan rate of 0.1 mV s1; (c) Cyclic voltammetry curves of pristine LTO and 3.0 wt% rGOcoated LTO electrodes at at various scan rates; (d) Anodic peak current densities against square roots of scan rate of the pristine LTO and 3.0 wt% rGOcoated LTO electrodes.
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electrode has a higher electronic conductivity and exhibits the better electrochemical performance than pristine LTO. 4. Conclusions
Fig. 11. EIS of the pristine LTO and 3.0 wt% rGO-coated LTO electrodes after one charged–discharged cycle and at the stable voltage of 1.55 V (vs Li/Li+); the inserted images at the right-bottom corner is corresponding equivalent circuit.
according to the slope of the straight lines. Hence it can be deduced that the diffusion coefficient of rGO-coated LTO is slightly larger than that of pristine LTO. These results imply that the coating of rGO during the forming of LTO can effectively reduce the barrier for Li+ transportation and rendering relatively high Li+ mobility [8,17]. To demonstrate the effect of rGO coating on the electronic conductivity properties of LTO, the EIS measurements were carried out for both pristine LTO and 3.0 wt% rGO-coated LTO electrodes after the cells were charged–discharged by one cycle at the stable voltage of 1.55 V (vs. Li/Li+). Fig. 11 shows the corresponding Nyquist plots of the spectra and the equivalent circuit used to fit the EIS. Both EIS curves are composed of a depressed semicircle at the range of high and intermediate frequency, and there is a straight line at the lowest frequency region. The high-frequency semicircle usually corresponds to the charge transfer process (Rct), while the sloping line at the low frequency region indicates the Warburg impedance caused by a semi-infinite diffusion of Li+ ion in the electrode [15,36]. In the equivalent circuit, Rs is the ohmic resistance of electrolyte; Rct is the charge transfer resistance; CPE represent the double layer capacitance and passivation film capacitance; Zw represents the Warburg impedance [37,38]. The fitted parameters are listed in Table 1. It is found that the overall charge transfer resistance of the rGO-coated LTO is much smaller than that of the pristine LTO, indicating its higher conductivity and faster transferring of Li ion during charge-discharge process. Furthermore, the exchange current density (j) can be calculated according to the following equation [4]: j ¼ RT=nFRct
(1)
where the meaning of n is the number of electrons transferred in the half-reaction for the redox couple, R is the gas constant (8.314 J mol1 K1), T is the absolute temperature (298 K), F is the Faraday constant (96500 C mol1). As seen in Table 1, the exchange current density of the rGO-coated LTO is higher than that of the pristine LTO. Thus, the charge-transfer reaction of the rGO-coated LTO electrode took place more easily than pristine LTO electrode. The EIS results further demonstrate that the rGO-coated LTO Table 1 Fitted results of pristine LTO and 3.0 wt% rGO-coated LTO obtained by EIS. Samples
Rs(V)
Rct(V)
CPE-T (mF)
j(mA cm2)
Pristine LTO 3.0 wt% rGO-coated LTO
7.681 7.311
104.8 36.65
1.925 4.639
0.159 0.455
Spherical rGO-coated LTO composites are prepared by GO-confined hydrothermal reaction based on TiO2 and Li salt followed with calcinations treatment, which exhibits obviously improving specific capacity and rate capability compared with the pristine LTO. The discharge specific capacity of 3.0 wt% GO-coated LTO anodes reaches a higher value of 131.6 mAh g1 at 5 C and 97% retention of capacity has still been maintained even after 500 cycles, which are more excellent than those of pure pristine LTO. The enhanced electrochemical performances could be ascribed to significantly increase the electrical conductivity of LTO powders and improve the diffusion kinetic property of lithium ions by the uniform rGO coating on the surfaces of LTO spheres. We anticipate that this novel rGO coating approach can also be applied to the design and synthesis of other electrode materials with a high capacity and excellent cycling stability. Acknowledgment The Project is funded by the National Natural Science Foundation of China (Grant no. 61376005), Zhejiang Top Priority Discipline of Textile Science and Engineering of Open Foundation (2014KF07) and Young Researchers Foundation (2014YXQN02). References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] T.F. Yi, Y. Xie, Y.R. Zhu, R.S. Zhu, H.Y. Shen, Structural and thermodynamic stability of Li4Ti5O12 anode material for lithium-ion battery, J Power Sources 222 (2013) 448–454. [3] H.G. Jung, S.T. Myung, C.S. Yoon, S.B. Son, K.H. Oh, K. Amine, B. Scrosati, Y.-K. Sun, Microscale spherical carbon-coated Li4Ti5O12 as ultra high power anode material for lithium batteries, Energ Environ Sci 4 (2011) 1345–1351. [4] Q.Y. Zhang, C.L. Zhang, B. Li, D.D. Jiang, S.F. Kang, X. Li, Y.G. Wang, Preparation and characterization of W-doped Li4Ti5O12 anode material for enhancing the high rate performance, Electrochim Acta 107 (2013) 139–146. [5] Y. Ma, B. Ding, G. Ji, J.Y. Lee, Carbon-Encapsulated F-Doped Li4Ti5O12 as a High Rate Anode Material for Li+ Batteries, ACS Nano 7 (2013) 10870–10878. [6] N. Li, G.M. Zhou, F. Li, L. Wen, H.M. Cheng, A Self-Standing and Flexible Electrode of Li4Ti5O12 Nanosheets with a N-Doped Carbon Coating for High Rate Lithium Ion Batteries, Adv Funct Mater 23 (2013) 5429–5435. [7] G.N. Zhu, H.J. Liu, J.H. Zhuang, C.X. Wang, Y.G. Wang, Y.Y. Xia, Carbon-coated nano-sized Li4Ti5O12 nanoporous micro-sphere as anode material for highrate lithium-ion batteries, Energ Environ Sci 4 (2011) 4016–4022. [8] H.Q. Zhang, Q.J. Deng, C.X. Mou, Z.L. Huang, Y. Wang, A.J. Zhou, J.Z. Li, Surface structure and high-rate performance of spinel Li4Ti5O12 coated with N-doped carbon as anode material for lithium-ion batteries, J Power Sources 239 (2013) 538–545. [9] J. Liu, X.F. Li, J.L. Yang, D.S. Geng, Y.L. Li, D.N. Wang, R.Y. Li, X.L. Sun, M. Cai, M.W. Verbrugge, Microwave-assisted hydrothermal synthesis of nanostructured spinel Li4Ti5O12 as anode materials for lithium ion batteries, Electrochim Acta 63 (2012) 100–104. [10] J. Cheng, R.C. Che, C.Y. Liang, J.W. Liu, M. Wang, J.J. Xu, Hierarchical hollow Li4Ti5O12 urchin-like microspheres with ultra-high specific surface area for high rate lithium ion batteries, Nano Research 7 (2014) 1043–1053. [11] T.F. Yi, Z.K. Fang, Y. Xie, Y.R. Zhu, S.Y. Yang, Rapid Charge-Discharge Property of Li4Ti5O12-TiO2 Nanosheet and Nanotube Composites as Anode Material for Power Lithium-Ion Batteries, ACS appl mater inter 6 (2014) 20205–20213. [12] L. Yu, H.B. Wu, X.W.D. Lou, Mesoporous Li4Ti5O12 Hollow Spheres with Enhanced Lithium Storage Capability, Adv Mater 25 (2013) 2296–2300. [13] Y.X. Tang, Y.Y. Zhang, J.Y. Deng, D.P. Qi, W.R. Leow, J.Q. Wei, S.Y. Yin, Z.L. Dong, R. Yazami, Z. Chen, Unravelling the Correlation between the Aspect Ratio of Nanotubular Structures and Their Electrochemical Performance To Achieve High-Rate and Long-Life Lithium-Ion Batteries, Angew Chem 126 (2014) 1–6. [14] Q.Y. Zhang, C.L. Zhang, B. Li, S.F. Kang, X. Li, Y.G. Wang, Preparation and electrochemical properties of Ca-doped Li4Ti5O12 as anode materials in lithium-ion battery, Electrochim Acta 98 (2013) 146–152.
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