Li4Ti5O12 hollow microspheres assembled by nanoparticles as an anode material for lithium-ion batteries

Li4Ti5O12 hollow microspheres assembled by nanoparticles as an anode material for lithium-ion batteries

Materials Letters 181 (2016) 108–112 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet L...

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Materials Letters 181 (2016) 108–112

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Li4Ti5O12 hollow microspheres assembled by nanoparticles as an anode material for lithium-ion batteries Yonggang Lu, Feng Zhang n, Bin Zhai, Shangru Zhai, Zuoyi Xiao, Qingda An n, Changshun Yu, Shiping Gao School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 7 April 2016 Received in revised form 21 May 2016 Accepted 4 June 2016 Available online 7 June 2016

Li4Ti5O12 hollow microspheres assembled by nanoparticles are successfully prepared via a process employing hydrothermal method and subsquent calcination. The washing times, which can significantly influence the adsorbed amount of Li þ in the intermediate products, play an important role in the composition of the final hollow microspheres. The obtained hollow microspheres present a very stable reversible capacity of 173 mAh g  1 at 1 C. Furthermore, the Li4Ti5O12 hollow microspheres could remain relatively high reversible capacities even at higher charge/discharge rates. The reversible capacities of approximately 168.3, 163.0, 156.6 and 152.4 mAh g  1 are obtained after 10 cycles at 2 C, 5 C, 10 C and 15 C, respectively. & 2016 Elsevier B.V. All rights reserved.

Keywords: Hollow Mesoporous Nanoparticles Nanocomposites

1. Introduction In recent years, lithium-ion batteries are considered one of the most suitable energy storage devices for electric vehicles and hybrid electric vehicles due to their high energy and high power density [1]. Li4Ti5O12 has been usually considered as a promising anode material for lithium-ion batteries, since it possesses several advantages, such as high safety and excellent cycling stability [2–4]. However, the poor electrical conductivity and low Li þ diffusion coefficient results in the poor rate capability of Li4Ti5O12 [3,4]. Until now, many approaches have been developed to improve the rate capability of Li4Ti5O12. The modification of Li4Ti5O12 through conductive coating is usually considered one of the most common and useful strategies. Li4Ti5O12 materials modified by graphene, Au and other materials [5–8] with improved rate capability have been successfully prepared. However, achieving a uniform surface coating around the entire Li4Ti5O12 particle is difficult, significantly affecting the improved electrochemical properties. Recently, many nanostructures have been developed and significantly improve the electrochemical capability of Li4Ti5O12, for example, microsphere, nanorod, nanoclusters, nanotubes and so on [9–16]. Among these nanostructures, hollow microspheres have been given much attention because of their low density, delivering ability, and surface n

Corresponding authors. E-mail addresses: [email protected] (F. Zhang), [email protected] (Q. An). http://dx.doi.org/10.1016/j.matlet.2016.06.016 0167-577X/& 2016 Elsevier B.V. All rights reserved.

permeability [2–4,17]. However, the synthesis of Li4Ti5O12 hollow microspheres via facile methods remains a challenge. In this work, we report a facile approach to prepare Li4Ti5O12 hollow microspheres composed of nanoparticles. The amount of Li þ adsorbed by the intermediate products plays an important role in the composition of the final products. The obtained Li4Ti5O12 hollow microspheres possess superior electrochemical performance when used as anode material for lithium ion battery.

2. Experimental section The hollow TiO2 microspheres were firstly prepared according to the previous report [18], whereas the intermediate products were calcined at 450 °C for 3 h in air. Details of synthesis of Li4Ti5O12 hollow microspheres, characterization and electrochemical measurements are described in the Supplementary Information.

3. Results and discussion Fig. 1(a) and (c) show the low-magnification SEM and TEM images of the products. The materials are mainly composed of hollow microspheres with a diameter of approximately 3 mm. The SEM images with high magnification indicate that the hollow microspheres are assembled by the nanoparticles (Fig. 1(b)), most of which are octahedral. These nanoparticles are approximately 50–80 nm in diameter, cross-linked with one another irregularly.

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0.0012 0.0008 0.0004 0.0000

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Fig. 1. SEM images (a and b), TEM images (c and d) and XRD pattern (e) of hollow Li4Ti5O12 microspheres. (inset in c and d: SEAD pattern taken from the microsphere and HRTEM image from the nanoparticle) (f) N2 adsorption/desorption isotherms with pore size distributions for hollow Li4Ti5O12 microsphere.

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Fig. 2. XRD patterns of the obtained hollow microspheres with different washing times. a: 0 time b: 1 time; c: 4 times d: 5 times e: 6 times.

The SEAD rings (Fig. 1(c)) imply that the product is polycrystalline in nature. The structure of the nanoparticles is further investigated with the TEM images. Fig. 1(d) show that interestingly, the surface of the nanoparticles is not smooth and a considerable amount of mesopores with a diameter of about 6 nm is present in most of the

nanoparticles. Moreover, the interplanar spacing is measured as 0.48 nm, in accordance with the (111) plane of spinel Li4Ti5O12. To investigate the crystallographic structure of the obtained materials, XRD analysis was conducted (Fig. 1(e)). The XRD pattern shows that all diffraction peaks can be well indexed to spinel Li4Ti5O12 (JCPDS 26-1198) [19], a result indicating that no impurity was present in the final samples. The Li4Ti5O12 hollow microspheres is further studied by N2 adsorption/desorption isotherm with a corresponding pore size distribution. Fig. 1(f) shows the isotherm is a characteristic type-IV curve with a hysteresis loop, indicating the existence of mesopores in the Li4Ti5O12 hollow microspheres [20]. The pore size distribution presents that the majority of the mesopores have a diameter of about 6 nm, a result that is in agreement with the TEM studies. However, the mesopores larger than 10 nm also exist in the obtained products, and these mesopores may arise from the accumulation of the octahedral nanoparticles. The surface area and pore volume of the Li4Ti5O12 hollow microspheres are 10.7 m2 g  1 and 0.043 cm3 g  1, respectively. It should be noted that the washing times, which can significantly influence the adsorbed amount of Li þ in the intermediate products, play an important role in the composition of the

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Fig. 3. SEM images, XRD patter (a and b) and TEM images (c and d) of the intermediate products. (e) Schematic illustration for the formation of the Li4Ti5O12 hollow microspheres.

final products. It is obvious that the adsorbed amount of residual Li þ in the intermediate products decreases with the increase in washing times. The XRD patterns in Fig. 2 show that the products derived from intermediate products without washing with deionized water are mainly composed of Li2TiO3 (JCPDS 80249) [19], which possesses a high ratio of Li to Ti. As the washing times increase, the amounts of re Li in the intermediate products decrease; the phase of Li4Ti5O12, with a low ratio of Li to Ti, begins to present in the final products. When the intermediate products were washed with deionized water six times, only Li4Ti5O12 was present in the materials. This result indicates that the amount of used Li sources plays an important role in the composition of the Li-Ti-O compound. In the hydrothermal synthesis, the amount of used Li sources can be controlled through an adjustment in the washing times.

To investigate the formation mechanism of the Li4Ti5O12 hollow microspheres, the raw materials and intermediate products were characterized by SEM and XRD (Fig. S1 and 3). TiO2 hollow microspheres are mainly anatase and composed of many irregular nanospheres with a diameter of approximately 70 nm (Fig. S1). The SEM image shown in Fig. 3(a) indicates that the intermediate products remain in the hollow nanostructures after hydrothermal treatment. However, the SEM images with high magnification show that the irregular nanospheres in the hollow TiO2 microspheres transformed into regular octahedral nanoparticles. The crystallographic structures of the intermediate products are identified by XRD analysis, usually assigned to rock salt-type LiTiO2 (JCPDS 16-0223) [19]. Based on the above observations, a general formation mechanism for Li4Ti5O12 hollow microspheres is

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Fig. 4. (a) The charge/discharge behavior of Li4Ti5O12 hollow microspheres for several representative cycles at 1 C rate. (b) Cycle life of Li4Ti5O12 hollow microspheres at different charge/discharge rates from 1 to 15 C.

proposed (Fig. 3(e)). Under hydrothermal conditions, TiO2 powders completely react with LiOH to form LiTiO2 phase. During this process, the hollow nanostructures are completely reserved, whereas the irregular nanospheres transform into regular octahedral nanoparticles. Finally, LiTiO2 transforms into spinel Li4Ti5O12 hollow microspheres after calcination. The Li4Ti5O12 hollow microspheres have been investigated as anode materials for lithium-ion batteries. Cyclic voltammograms for Li4Ti5O12 hollow microspheres were performed (Fig. S2). It can be found that a couple of redox peaks at around 1.76 and 1.43 V, in accordance with the lithium ion intercalation and deintercalation of the spinel structure are present in the curve. Fig. 4(a) shows the galvanostatic discharge/charge curves for several representative cycles for Li4Ti5O12 hollow microspheres at 1 C rate. A pair of flat potential plateaus at approximately 1.51 and 1.60 V are clearly seen in the charge/discharge curves, and they arise from the twophase equilibrium between Li4Ti5O12 and Li7Ti5O12 [19]. Upon cycling, the difference between the potential of charge and discharge plateaus become small. After 50 cycles, the charge and discharge plateaus are at approximately 1.55 and 1.58 V, respectively. To study cycling behavior, a variation in charge/discharge capacities with the cycles for Li4Ti5O12 microspheres is performed at various charge/discharge rates from 1 C to 15 C (Fig. 4(b)). The discharge capacity and charge capacity were 195.3 and 176.8 mAh g  1 for the first cycle at 1 C, respectively. The specific capacity of the microsphere was higher than its theoretical value, a result that can be attributed to the hierarchical structure and/or pseudocapacitive effects [21]. A relatively large irreversible capacity was present in the first cycle, but the difference between the charge and discharge capacities became negligible in the course of the first several cycles, and thereafter, the coulombic efficiency reached approximately 100%. The Li4Ti5O12 hollow microspheres can give reversible capacities of 173.6 mAh g  1 after 50 cycles. This result indicates that Li4Ti5O12 hollow microspheres have excellent cycle performance and high reversible capacity over extended cycling. Furthermore, the Li4Ti5O12 hollow microspheres can retain relatively high reversible capacities even at high charge/discharge rates. A reversible capacity of approximately 168.3 mAh g  1 is obtained at 2 C after 10 cycles, and this value decreases to approximately 163 mAh g  1 at high rates of 5 C after 10 cycles. The reversible capacity of the Li4Ti5O12 hollow microspheres at 5 C can be compared with those of carbon-modified, graphene-modified, copper-modified and Au-doped Li4Ti5O12 nanocomposites, in which carbon, graphene copper and Au act as suitable electronic conductors to improve surface–surface interactions [5–8]. With a further increase in the charge/discharge rates, the reversible capacities of Li4Ti5O12 hollow microspheres remain at 156.6 and

152.4 mAh g  1 at high rates of 10 and 15 C. Subsequently, the charge/discharge rate decreases to 1 C, and the reversible capacity of Li4Ti5O12 hollow microspheres can also increase to 173 mAh g  1. The superior electrochemical performance of the Li4Ti5O12 hollow microspheres may be attributed to their efficient nanostructure. The hollow structure would facilitate electrolyte transportation and Li þ diffusion within the materials from both inside and outside. The large pores between the nanoparticles and small mesopores in the nanoparticles can further accelerate electrolyte transportation and Li þ diffusion. All these factors contribute to the excellent electrochemical performance of the Li4Ti5O12 hollow microspheres.

4. Conclusions Li4Ti5O12 hollow microspheres were successfully prepared via a process employing hydrothermal method and subsquent calcination. Our results show that the adsorbed amount of Li þ in the intermediate products could obviously influence the composition of final product. The superior electrochemical performance of the hollow Li4Ti5O12 microspheres is related to their efficient nanostructure.

Acknowledgements The authors are grateful to the financial aid from the National Natural Science Foundation of China (Grant no. 21401016, and 21546008), Science and Technology Research Project of the Education Department of Liaoning Province (No. L2015055), Open Project of State Key Laboratory of Inorganic Synthesis & Preparative Chemistry Jilin University (2016-23, 2014-33).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2016.06.016.

References [1] S. Goriparti, E. Miele, F.D. Angelis, E.D. Fabrizio, R.P. Zaccaria, C. Capiglia, J. Power Sources 257 (2014) 421–443. [2] Y.F. Tang, L. Yang, S.H. Fang, Z. Qiu, Electrochim. Acta 54 (2009) 6244–6249. [3] J. Liu, W. Liu, S.M. Ji, Y.L. Wan, H.Q. Yin, Y.C. Zhou, Eur. J. Inorg. Chem. (2014) 2073–2079.

112

Y. Lu et al. / Materials Letters 181 (2016) 108–112

[4] J. Cheng, R.C. Che, C.Y. Liang, J.W. Liu, M. Wang, J.J. Xu, Nano Res. 7 (2014) 1043–1053. [5] J.Y. Liu, Y. Shen, L. Chen, Y.G. Wang, Y.Y. Xia, Electrochim. Acta 156 (2015) 38–44. [6] C.L. Cheng, H.J. Liu, X. Xue, H. Cao, L.Y. Shi, Electrochim. Acta 120 (2014) 226–230. [7] J.W. Zhang, Y.R. Cai, J. Wu, J.M. Yao, Electrochim. Acta 165 (2015) 422–429. [8] C.C. Li, Q.H. Li, L.B. Chen, T.H. Wang, ACS Appl. Mater. Interfaces 4 (2012) 1233–1238. [9] Y.F. Tang, L. Yang, Z. Qiu, J.S. Huang, Electrochem. Commun. 10 (2008) 1513–1516. [10] N. Li, T. Mei, Y.C. Zhu, L.L. Wang, J.W. Liang, X. Zhang, Y.T. Qian, K.B. Tang, CrystEngComm 14 (2012) 6435–6440. [11] J. Liu, K.P. Song, P.A. van Aken, J. Maier, Y. Yu, Nano Lett. 14 (2014) 2597–2603. [12] L. Cheng, J. Yan, G.N. Zhu, J.Y. Luo, C.X. Wang, Y.Y. Xia, J. Mater. Chem. 20 (2010) 595–602.

[13] H.G. Jung, S.T. Myung, C.S. Yoon, S.B. Son, K.H. Oh, K. Amine, B. Scrosati, Y. K. Sun, Energy Environ. Sci. 4 (2011) 1345–1351. [14] H. Ge, L. Chen, W. Yuan, Y. Zhang, Q.Z. Fan, H. Osgood, D. Matera, X.M. Song, G. Wu, J. Power Sources 297 (2015) 436–441. [15] C. Lai, Z.Z. Wu, Y.X. Zhu, Q.D. Wu, L. Li, C. Wang, J. Power Sources 226 (2013) 71–74. [16] L. Sun, J.P. Wang, K.L. Jiang, S.S. Fan, J. Power Sources 248 (2014) 265–272. [17] Y.H. Yin, J.J. Xu, Z.X. Cao, H.Y. Yue, S.T. Yang, Mater. Lett. 108 (2013) 21–24. [18] F. Zhang, Y. Zhang, S.Y. Song, H.J. Zhang, J. Power Sources 196 (2011) 8618–8624. [19] L.F. Shen, C.Z. Yuan, H.J. Luo, X.G. Zhang, K. Xu, Y.Y. Xia, J. Mater. Chem. 20 (2010) 6998–7004. [20] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169–3183. [21] Y.J. Sha, B.T. Zhao, R. Ran, R. Cai, Z.P. Shao, J. Mater. Chem. A 1 (2013) 13233–13243.