Carbon coated Li4Ti5O12 nanorods as superior anode material for high rate lithium ion batteries

Carbon coated Li4Ti5O12 nanorods as superior anode material for high rate lithium ion batteries

Journal of Alloys and Compounds 572 (2013) 37–42 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage...

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Journal of Alloys and Compounds 572 (2013) 37–42

Contents lists available at SciVerse ScienceDirect

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

Carbon coated Li4Ti5O12 nanorods as superior anode material for high rate lithium ion batteries Hongjun Luo, Laifa Shen, Kun Rui, Hongsen Li, Xiaogang Zhang ⇑ College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China

a r t i c l e

i n f o

Article history: Received 18 July 2012 Received in revised form 27 March 2013 Accepted 27 March 2013 Available online 6 April 2013 Keywords: Lithium ion batteries Li4Ti5O12 Carbon coating Nanorods

a b s t r a c t We describe a novel approach for the synthesis of carbon coated Li4Ti5O12 (Li4Ti5O12/C) nanorods for high rate lithium ion batteries. The carbon coated TiO2 nanotubes using the glucose as carbon source are first synthesized by hydrothermal treatment. The commercial anatase TiO2 powder is immersed in KOH sulotion and subsequently transforms into Li4Ti5O12/C in LiOH solution under hydrothermal condition. Fieldemission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, nitrogen adsorption/desorption and Raman spectra are performed to characterize their morphologies and structures. Compared with the pristine Li4Ti5O12, one-dimensional (1D) Li4Ti5O12/C nanostructures show much better rate capability and cycling stability. The 1D Li4Ti5O12/C architectures effectively restrict the particle growth and enhance their electronic conductivity, enabling fast ion and electron transport. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, lithium ion batteries (LIBs) have attracted considerable attention as the leading candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs) with the potential to save oil and decrease the environmental burden caused by the emission of CO2 [1–3]. Whereas the LIBs now dominate the portable electronic market, its implementation into electric transportation is constantly postponed due to low power density, high cost and, more importantly, safety issues [4–5]. The safety issues of conventional LIBs are mainly due to the graphite based anodic materials possessing intrinsic chemical instability at overcharged state [6– 8]. Also LIBs strongly suffers from the kinetic problems because the graphite based anodic material has slow Li-ion diffusion and increased resistance at the electrode/electrolyte interface at high rates. Spinel Li4Ti5O12 has been considered as an alternative anodic materials for LIBs due to its excellent Li-ion insertion/extraction reversibility with zero-strain characteristics and a higher Li-insertion process operating at about 1.55 V to entirely eliminate potential safety issues [9–11]. However, its low electronic conductivity (<1013 S cm1) is an impediment to realize high rate capability and a critical parameter for high power applications [12,13]. Several efforts have already been attempted to address this problem by surface conductive coatings [14–17], formation of nanocomposites with mesoporous carbon [18], carbon nanotubes [19], graphene [20], and, doping aliovalent metal ions [21–23]. There is a ⇑ Corresponding author. Tel.: +86 025 52112902; fax: +86 025 52112626. E-mail address: [email protected] (X. Zhang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.247

possibility to increase the electronic conductivity of Li4Ti5O12 is to substitute multiply charged ions on a Ti4+ site, which should lead to an increase in electron concentration [24], however, such a method usually leads to severe capacity degradation upon prolonged cycling. Carbon-based coating from carbonization of PAN [14], glucose [15], sucrose ⁄⁄[17], et al., has been widely used. On the other hand, fabricating nanostructured Li4Ti5O12 is expected to exhibit improved rate performance because of the shorter transport path lengths of lithium ions and electrons. Novel hybrid electrodes combining the above two approaches together could further improve the rate capability. Up to now, the most common and traditional method to manufacture Li4Ti5O12 is still using conventional synthesis routes (solidstate reaction or sol–gel methods), which require high temperature treatment to achieve phase-pure spinel lithium titanate. Recently, wet-chemical routes proved to be a feasible method to synthesize highly electrochemically nanostructured Li4Ti5O12. One-dimensional nanostructures, for instance, nanorods, exhibit remarkable electrochemical performance. The improved electrochemical performance may be attributed to good electronic conduction along the length of each rod, short Li+ insertion distances, high interfacial contact area with electrolyte, good electrical contact between each rod and the current collector [25–28]. Zhang et al. reported spinel Li4Ti5O12 nanowires by means of topochemical hydrothermal ion exchange reaction following a relatively low temperature treatment at 300 °C. The discharge capacity is less than 120 mAh g1 at low rate of 1 C [29]. Gao et al. prepared Li–Ti–O compound by simple ion exchange of titanate nanotubes and a following calcination procedure at different temperatures [30]. The tubular morphology was lost and BET specific surface area decreased sharply

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when annealed at 600 °C, which delivered bad rate capacity. Therefore, it still remains a great challenge to develop a facile approach to achieve morphology control to maintain 1D nanostructures and improved electronic conductivity simultaneously. In this work, we report a practical and efficient strategy for synthesizing Li4Ti5O12/C nanorods by a wet-chemical routes, yielding a high-performance anodic materials for high power LIBs. The carbon layer effectively improves the electronic conductivity of the hybrid materials and hinders the agglomeration and growth of Li4Ti5O12 to maintain 1D nanostructures, which reduces the transport path of lithium ions and electrons and provides high interfacial contact area with electrolyte. Therefore, the Li4Ti5O12/C nanorods possesses fast ionic and electronic conduction, enabling remarkable rate performance and long cycle life. 2. Experimental 2.1. Preparation of Li4Ti5O12/C nanorods All of the reactants were of analytical grade and used without further purification. The commercial anatase TiO2 powder (primary particle size: 5–10 nm, specific area: 210 ± 10 m2 g1) were purchased from Zhoushan Mingri Nanometer Material Co., Ltd. All aqueous solutions were freshly prepared using high purity water (18 MU cm resistivity) from an Ampeon 1810-B system (Jiangsu, China). The overall fabrication procedures of Li4Ti5O12/C nanorods are schematically illustrated in Fig. 1. In a typical synthesis procedure: 0.8 g anatase TiO2 powder and 1.0 g glucose were mixed with 50 mL KOH solution (10 M) by stirring for 0.5 h and sonicating for another 0.5 h, and then transferred to a Teflon-lined autoclave and maintained at 180 °C for 24 h. Following recovery via centrifugation, the product was washed with 0.1 M HCl solution and deionized water several times until a pH equal to 7 and drying at 80 °C generating desired H-titanate nanotubes. Then, the carbon coated H-titanate nanotubes was dispersed in a 0.2 M LiOH aqueous solution. After stirring for 30 min, the suspension was transferred into a Teflon-lined stainless steel autoclave and kept at 150 °C for 6 h. The resulting brown precipitate was separated by filtration, washed several times with deionized water to remove the excess hydroxides before drying at 80 °C for 10 h. Subsequently, the Li4Ti5O12 precursor (L–T–O) was calcinated at 600 °C for 2 h in a nitrogen atmosphere to obtain Li4Ti5O12/C nanorods. The pure Li4Ti5O12 nanoparticles was prepared in a similar manner to the preparation of Li4Ti5O12/C nanorods, except for the absence of glucose in the process. 2.2. Physicochemical characterization The crystal structure of the obtained samples was characterized by X-ray diffraction (XRD) (Bruker D8 advance) with Cu Ka radiation. Microstructural properties were obtained using transmission electron microscopy (TEM) (TEM, FEI, Tecnai-20, USA) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010), and field-emission scanning electron microscopy (FESEM, LEO 1430VP, Germany). The N2 adsorption/desorption were determined by Brunauer– Emmett–Teller (BET) measurements using an ASAP-2010 surface area analyzer. Raman spectra were collected using a Renishaw 2000 system with an argon ion laser (514.5 nm) and charge-coupled device detector. Thermogravimetric (TG) analyses were performed on a TG instrument (NETZSCH STA 409 PC) using a heating rate of 10 °C min1 in an air atmosphere from 30 °C to 700 °C. 2.3. Electrochemical tests

5 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) dissolved in Nmethyl pyrrolidinone (NMP), and was spread uniformly on an aluminium foil current collector. Finally, the electrode was dried under vacuum at 110 °C for 12 h. Test cells were assembled in an argon-filled glove box using Li foil as the counter electrode and polypropylene (PP) film as the separator. 1 mol L1 LiPF6 solution in a 1:1 (V:V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as electrolyte. Finally, the cells were then aged for 12 h before measurements. The cells were galvanostatically charged and discharged under different current densities between 1.0 and 2.5 V (vs. Li/Li+) using an Arbin Battery Tester BT-2000 (Arbin Instruments, College Station, Texas). Cyclic voltammetry (CV) studies were carried out between 1.0 and 2.5 V (vs. Li/Li+) on an electrochemical workstation (CH Instruments, Model 605B) The AC impedance spectrum was measured by using a Solatron 1260 Impedance Analyzer in the frequency range 102–106 Hz.

3. Results and discussion In order to reveal the composition of the pristine Li4Ti5O12 and Li4Ti5O12/C nanorods, powder X-ray diffraction (XRD) test was carried out. All the diffusion peaks of XRD patterns in Fig. 2 can be indexed as the face-centered cubic spinel phase Li4Ti5O12 (Fd3m space group, JCPDS Card No: 26-1198). Carbon, produced from the carbonization of glucose, left in the sample heated under argon, which can explain the gray color compared to the pristine white sample, where no glucose was add during the hydrothermal treatment. But no diffraction response of the carbon in XRD patterns (Fig. 2b) was observed most likely due to its low content and amorphous state. Thermogravimetric (TG) analyses were used to measure carbon content and the results were shown in Fig. 3. The TG curve exhibited appreciable weight loss, and the content of carbon in the Li4Ti5O12/C nanorods was estimated to be approximately 5 wt.%. The chemical composition of Li4Ti5O12/C nanorods was also confirmed by Raman spectroscopy. As shown in Fig. 4, five vibration peaks at 233, 274, 343, 427 and 673 cm1 can be clearly observed in the range of 150–700 cm1. The two stronger peaks at 99 and 438 cm1 can be assigned to the characteristic modes F2g and A1g. The band at 426 cm1 with a medium strength is associated with the Eg mode, while the other two weaker bands appeared at 275 and 343 cm1 are attributed to F2g. This is a Raman characteristic of the spinel structure Li4Ti5O12 [31–33], consistent with the results from XRD characterization. Besides, there are two peaks in the range of 1000–2000 cm1. The two major peaks located at around 1324 and 1591 cm1 could be attributed to the in-plane vibration of disordered amorphous carbon (D band) and crystalline graphic carbon (G band), respectively. The G band is associated with the allowed E2g optical modes of the Brillouin zone center of the crystalline graphite, while the D band is attributed to disorder-allowed phonon modes. The ratio between D and G intensities (ID/IG) is often used as an index of the degree of crystalline perfection of the graphite structure [34]. As the ID/IG ratio increases, the

The electrochemical characterization was carried out by galvanostatic cycling in a two-electrode electrochemical cell. The working electrodes were prepared by a slurry coating procedure. In brief, the slurry consisted of 85 wt.% active material,

Fig. 1. Schematic illustration for the synthesis of Li4Ti5O12 nanoparticles and Li4Ti5O12/C nanorods.

Fig. 2. XRD patterns of (a) Li4Ti5O12 nanoparticles, (b) Li4Ti5O12/C nanorods.

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Fig. 3. Thermogravimetric-differential scanning calorimetry (TG-DSC) curve for Li4Ti5O12/C nanorods at 10 °C min1 in an air atmosphere from 30 °C to 700 °C.

Fig. 4. Raman spectrum of Li4Ti5O12/C nanorods.

defect structure increases and the degree of graphitization becomes less. The value of the ID–IG ratio of 1.52 was calculated. It

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suggests that the carbon in Li4Ti5O12/C nanorods was mainly in an amorphous structure. Fig. 5 displays the FESEM images of the TiO2 nanotubes, pristine Li4Ti5O12 and Li4Ti5O12/C nanorods, respectively. The FESEM and TEM images of TiO2 nanotubes are shown in Fig. 5a and inset in Fig. 5a, respectively, revealing large quantity of tubular materials with narrow size distribution. The diameters of the tubular materials are almost uniform around 10–20 nm, and the lengths range from several tens to several hundreds of nanometer. In Fig. 5b, the pristine Li4Ti5O12 prepared in the absence of glucose consisted of only irregular nanoparticles aggregates with the size of 10– 100 nm, in which the morphology of the TiO2 nanotubes precursors has almost disappeared after calcined at 600 °C. The morphology of Li4Ti5O12/C nanorods is presented in Fig. 5c and d, which clearly demonstrates that the products still maintained the onedimensional morphology with some nanoparticles attached on the surface. It is reasonable that carbon layer serves as a physical barrier to prevent the aggregation of the nanocrystals. To further examine the architecture of the pristine Li4Ti5O12 and Li4Ti5O12/C nanorods, the samples were investigated by TEM and HRTEM (Fig. 6). Fig. 6a shows a TEM image of pristine Li4Ti5O12 prepared in the absence of glucose. It can be seen that the nanotubes morphology was found to be almost disappeared and completely converted to the particle shape in the range from 10 to 100 nm, which is consistent with the SEM result. After chemical lithiation and a short post-annealing, TiO2/C will lose their nanotubular morphology and convert to Li4Ti5O12/C nanorods at 600 °C because the nanorods structure is more stable than nanotube (Fig. 6b). Fig. 6c shows a thin carbon shell coating on the surface of Li4Ti5O12 core nanorods to form an effective core/shell onedimensional structure. The HRTEM image in Fig. 6d clearly presents that the carbon layer was coated on the surface of Li4Ti5O12 nanorods. The thickness of carbon layer was about 1–3 nm, which was favorable for lithium ion transport across the interface between Li4Ti5O12 and electrolyte. The distance of the interplanar

Fig. 5. (a) Typical FESEM images of TiO2 nanotubes (inset shows the TEM image of TiO2 nanotubes), (b) Li4Ti5O12 nanoparticles, (c and d) Li4Ti5O12/C nanorods.

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Fig. 6. (a) TEM image of the Li4Ti5O12 nanoparticles, (b and c) TEM and (d) HRTEM images of Li4Ti5O12/C nanorods.

spacing between adjacent lattice planes is 0.48 nm, which is in good agreement with the spacing of the (1 1 1) plane of spinel Li4Ti5O12, thus demonstrating the highly crystalline nature of the asprepared Li4Ti5O12 [35]. The carbon layer effectively prevents the Li4Ti5O12 agglomeration and growth during heat treatment process, providing a continuous channel favorable to electron transport along the length axis of the nanorods. As an approach to identify the surface area, pore size and the extent of porosity, nitrogen adsorption–desorption isotherm was recorded and the result is shown in Fig. 7. As shown in Fig. 7a, the isotherm for the pristine Li4Ti5O12 nanoparticles show a type IV isotherm and a significant physisorption/desorption activity at high pressures(p/p0 > 0.9), indicating the existence of mesopores

and macropores structure in the product. As summarized in Table 1, the calculated BET surface area is 21.62 m2 g1 and the average pore diameters is 40.48 nm. As shown in Fig. 7b, the Li4Ti5O12/C nanorods shows a distinct hysteresis in the larger range of ca. 0.7–1.0 P/P0, revealing the existence of abundant more smaller porous structures in the architectures. The BET surface of the carboncoated Li4Ti5O12 nanorods is 107.8 m2 g1, which is about five times higher than that of the pristine Li4Ti5O12 nanoparticles prepared in the absence of glucose. The corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curve (inset in Fig. 7b) shows that the pore size is not uniform and two porous central distributions is observed, one at 2 nm and another at 23 nm. Besides, the samples exhibit macropores with pore sizes of up to 100 nm.

Fig. 7. The gas (N2) adsorption–desorption isotherms of (a) Li4Ti5O12 nanoparticles, (b) Li4Ti5O12/C, inset shows the pore size distribution calculated from the adsorption branch of the isotherm, respectively.

Table 1 BET surface area, SBET, and pore volume of Li4Ti5O12 and Li4Ti5O12/C nanorods. Sample

SBET (m2 g1)

Pore volume (cm3 g1)

Average pore size (nm)

Li4Ti5O12 Li4Ti5O12/C nanorods

21.62 107.8

0.219 0.526

40.48 19.53

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Fig. 8. (a) Cyclic voltammograms (CVs) of Li4Ti5O12/C nanorods at a scan rate of 0.5 mV s1, (b) Galvanostatic charge–discharge curves for the Li4Ti5O12/C at different current densities, (c) Comparison of the rate capabilities of Li4Ti5O12/C nanorods and Li4Ti5O12 nanoparticles, (d) Cycle performance of the Li4Ti5O12/C nanorods and Li4Ti5O12 nanoparticles electrode at a high rate of 10 C.

Noting that the peak around 2 nm is too small, which may be derived from the carbon shell, core materials, or core–shell agglomerates in the Li4Ti5O12/C composite. Such a porous nature carbon will not obstruct the penetration of electrolytes, thus preserving the fast ionic conductivity within interior of the Li4Ti5O12 crystallites. The carbon coating suppresses the particle growth which helps to retain 1D nanostructures with high surface area and results in high electrode/electrolyte contact area. Such a porous structure provides efficient transport pathways for lithium ions to their interior voids, result in high rate capability. Electrochemical performance of Li4Ti5O12/C nanorods and pure Li4Ti5O12 as anode materials for LIBs has been evaluated and compared (Fig. 8). The lithium storage properties first examined using cyclic voltammetry (CV). Fig. 8a shows the CV profile of Li4Ti5O12/C nanorods electrode assembled in a coin-type half cell using lithium as both counter and reference electrodes. At a scan rate of 0.1 mV s1 between 2.5 and 1.0 V, a pairs of well defined redox peaks are observed at around 1.65/1.50 V, which could be attributed to the redox reaction of Ti4+/Ti3+, reaction mechanism of the following equation [36] þ

Li4 Ti5 O12 þ 3Li þ 3e

discharge

¢

charge

Li7 Ti5 O12

ð1Þ

These features suggest that Li+ insertion/deinsertion into the electrode fast and reversibly, which is consistent with that of typical spinel Li4Ti5O12, where tetrahedral sites are involved in the charge/discharge process. Fig. 8b shows constant current charge/ discharge profiles of the Li4Ti5O12/C nanorods electrode at different current rates from 0.2 to 30 C. At the initial lower rate of 0.2 C, the Li4Ti5O12/C nanorods give a discharge capacity of 168.4 mA h g1. The coulombic efficiency of the first cycle is ca. 95%, which might be ascribed to breakdown processes in the electrolyte solution, such as reduction of trace water, which is more pronounced for high-surface-area electrodes and for fresh electrodes during the initial cycles [35]. The specific discharge capacity was slightly reduced to 157.8, 146.4, 132.5 and 115.9 mA h g1 at the rates of 1 C, 5 C, 10 C and 20 C, respectively. The cell voltage decreased

with the increasing current density, 1.51 V at 1 C and 1.32 V at 20 C. Both of these are associated with the sluggish diffusion kinetics of the Li-ion at very high rates [37,38]. However, the pure Li4Ti5O12 samples synthesized in the absence of glucose presented a bad rate capability. Fig. 8c compares the rate capabilities of pristine Li4Ti5O12 nanoparticles and Li4Ti5O12/C nanorods electrodes at different rates. In sharp contrast, the discharge capacity of pristine Li4Ti5O12 nanoparticles drops significantly as the increase of discharge/charge rate. Even at a rate as high as 30 C, a reversible capacity of 92.7 mAh g1 can be achieved for Li4Ti5O12/C nanorods, whereas the value drops to only 46.3 mA h g1 for the pristine Li4Ti5O12 nanoparticles. The significantly improved rate capability of the Li4Ti5O12/C nanorods compared to that of the pristine Li4Ti5O12 nanoparticles is attributed to the carbon coated one-dimensional nanostructure. Most exceptionally, the Li4Ti5O12/C nanorods cell also exhibits excellent cyclability with no noticeable decrease compared to the pristine Li4Ti5O12 nanoparticles at extremely high current (10 C rate), as seen in Fig. 8d. After 100 cycles, the discharge capacity of the carbon-coated Li4Ti5O12 nanorods was 127.9 mA h g1, only with 3.6% capacity loss at the rate of 10 C, but for pristine Li4Ti5O12 nanoparticles, the corresponding values are 95.6 mA h g1 and 7.3%, respectively. Therefore, it is obvious that the Li4Ti5O12/C nanostructures demonstrate a much better lithium storage performance compared to pure Li4Ti5O12 nanoparticles. To understand the improved rate performance through carbon coating, we obtained electrochemical impedance spectroscopy (EIS) measurements on the pure Li4Ti5O12 and Li4Ti5O12/C. The EIS result shows that each curve consists of a depressed semicircle in the high-middle frequency region and an oblique straight line in the low frequency region (Fig. 9). In Table 2 the Li4Ti5O12/C composite electrodes exhibited much lower charge transfer resistance than that of the pure Li4Ti5O12 electrode. Furthermore, the exchange current densities (i0 = RT/nFRct) of the Li4Ti5O12/C cell were higher than the pure Li4Ti5O12 cell. Therefore, the greatly enhanced lithium storage properties of Li4Ti5O12/C nanorods may be attrib-

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References

Fig. 9. Electrochemical impedance spectra of the Li4Ti5O12 nanoparticles and Li4Ti5O12/C nanorods electrodes at the voltage of 1.55 V. Table 2 Impedance parameters of the pure Li4Ti5O12 and Li4Ti5O12/C nanorods electrodes. Sample

Rs (X)

Rct (X)

i0 (mA cm2)

Li4Ti5O12 Li4Ti5O12/C nanorods

3.988 3.901

118.3 68.16

0.217 0.377

uted to three aspects: First, the carbon coating shell serves as a physical barrier to prevent the aggregation of the nanocrystals to maintain the one-dimensional nanostructure, which have a good lithium ionic conductivity because the distance that Li+ diffuse is restricted to the radius direction. At the same time, the carbon shell on Li4Ti5O12 nanorods with good electronic conduction along the length of each rod, resulting in effectively improved the electronic conductivity of the hybrid materials. In addition, onedimensional Li4Ti5O12/C nanorods architecture with large surface area and hierarchically porous, providing high interfacial contact area with electrolyte to increase amount of reactive sites and facilitating ion transport within interior of the Li4Ti5O12 crystallites. 4. Conclusion In summary, we have successfully fabricated 1D Li4Ti5O12/C nanorods by a wet-chemical routes. The approach presented here can overcome the disadvantages of the solid state reaction method which required a long and high temperature treatment. It was found that carbon shell serves not only as a physical barrier to prevent the aggregation of the nanocrystal, but also provided fast electronic conduction along the length of each rod. Such Li4Ti5O12/C nanorods had abundant hierarchical pores and a specific surface area of 107.8 m2 g1, increasing the surface area for Li insertion/ extraction. Therefore the Li4Ti5O12/C nanorods with rapid ionic and electronic diffusion exhibit greatly enhanced lithium storage properties compared to the pure Li4Ti5O12 nanoparticles. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21173120), Natural Science Foundation of Jiangsu Province (No. BK2011030).

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