Solid State Ionics 232 (2013) 13–18
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Sr-doped Li4Ti5O12 as the anode material for lithium-ion batteries Hongbin Wu a, Sha Chang a, Xiuling Liu a, Liqiu Yu a, Guiling Wang a, Dianxue Cao a,⁎, Yiming Zhang b, Baofeng Yang b, Peiliang She b a Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b Shuangdeng Institute of Science and Technology, Nanjing Jiangning Development Zone Zhuangpai Road #109, Nanjing, 211000, China
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Article history: Received 17 July 2012 Received in revised form 24 September 2012 Accepted 25 October 2012 Available online 23 December 2012 Keywords: Lithium titanate Strontium doping Anode material Lithium-ion batteries
a b s t r a c t Sr-doped Li4Ti5O12 composites are prepared by a solid-state reaction method and characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy. Their electrochemical properties as the anode material of lithium ion batteries are investigated by galvanostatic charge and discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy. The results show that Sr2+ doping increases the lattice parameter, reduces the particle size, decreases the charge transfer resistance, and signiﬁcantly enhances the rate capability of Li4Ti5O12. The Sr-doped Li4Ti5O12 exhibits a speciﬁc discharge capacity of about 1.62 times that of pristine Li4Ti5O12 at 5 C charge/discharge rate. Sr doping introduces a small amount of SrLi2Ti6O14 to the composites, which also makes a contribution to the speciﬁc capacity of Li4Ti5O12 at low charge/discharge potentials. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The spinel Li4Ti5O12 is a zero-strain material for Li ion intercalation/ deintercalation with a theoretical speciﬁc capacity of 175 mAh g−1 and a discharge plateau of about 1.55 V versus Li/Li+. It is regarded as one of the most promising anode materials for lithium ion batteries (LIBs) because of its high charge/discharge rate capability, high columbic efﬁciency, excellent cycling stability, and safety [1–5]. However, Li4Ti5O12 has a poor electronic conductivity, which largely limited its practical applications. Therefore, several methods have been investigated to improve the conductivity of Li4Ti5O12, which include surface coating with carbon, substitution of Li, Ti or O with cations and anions and surface modiﬁcation with metals . So far, a variety of cations (e.g. Mo6+/ Mo4+, V 5+, Ta5+, Nb5+, Zr4+, Sn4+, Mn3+, Cr3+, Ni3+, Fe 3+, Al3+, Ga 3+, Co3+, Cu2+, Zn2+, Mg2+, and Ag +) have been doped into Li4Ti5O12. These dopants resulted in improved electrochemical performances of Li4Ti5O12 [6–17]. Sr2+ doping has been reported for LiMn2O4, LiCoO2 and LiNi0.8Co0.2O2. For example, Subramania et al.  found that the substitution of Sr for Mn leads to an improvement of the cycling stability of LiMn2O4. Valanarasu et al.  demonstrated that the substitution of Sr for Co increased the electronic conductivity and stabilized the layered rhombohedral phase of LiCoO2. Fey et al.  reported that the substitution of Sr for Li enhanced both the speciﬁc capacity and cycling performance of LiNi0.8Co0.2O2. Recently, Dambournet et al.  reported the synthesis of high packing density SrLi2Ti6O14 for use as the anode material of ⁎ Corresponding author. Tel./fax: +86 451 82589036. E-mail address: [email protected]
(D. Cao). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2012.10.027
LIBs. They found that a reversible insertion of three lithium ions per formula unit was achieved at different voltage plateaus below 1.3 V, and a capacity of 102 mAh g−1 was retained after 100 cycles at ~1 C. Johnson et al.  studied the charge/discharge behavior of SrTiO3 as the anode material of LIBs, and found that SrTiO3 has a much lower lithiation/ de-lithiation potential than Li4Ti5O12. The capacity of SrTiO3 reached approximately 135 mAh g−1. These literature results indicated that the incorporation of Sr into Li intercalation/deintercalation compounds can modify their electrochemical performance. In this study, we reported on the preparation of Sr-doped Li4Ti5O12 composites and the investigation of their electrochemical properties as the anode material of LIBs. The cation doping elements of Li4Ti5O12 usually have an ionic radius close to Li+ (0.076 nm) or Ti4+ (0.0605 nm), for example, Mo6+ (0.059 nm), Ni3+ (0.060 nm), Al3+ (0.0535 nm), and Mg2+ (0.072 nm) . Sr2+, on the other hand, has a much larger ionic radius (0.118 nm) than Li+ and Ti4+, which makes the doping of Li4Ti5O12 with Sr difﬁcult. However, if Sr2+ can enter the Li4Ti5O12 lattice, it will expand the channels for lithium ion intercalation and deintercalation, and thus improve the rate performance of Li4Ti5O12. In this study, we demonstrated that Sr can substitute cations of Li4Ti5O12 and enhance its high rate performance. 2. Experimental 2.1. Preparation and characterization of Sr-doped Li4Ti5O12 composites Li4Ti5O12 and Sr-doped Li4Ti5O12 were prepared by solid-state reaction. Li2CO3 (99%), TiO2 (rutile, ~20 nm) and Sr(NO3)2 were dispersed in anhydrous ethanol by sonication (1 h) and ball milling
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(10 h). The mixture was slowly dried at 60 °C to evaporate ethanol. The obtained powders were ground and calcined at 600 °C for 6 h, 850 °C for 18 h, 600 °C for another 6 h, and then slowly cooled down to ambient temperature in argon atmosphere to obtain the ﬁnal products, which were grounded and dried at 80 °C for 10 h in vacuum oven prior to use. Three Sr-doped Li4Ti5O12 composites with different content of Sr were prepared and denoted as 0.01Sr-LTO, 0.02Sr-LTO and 0.03Sr-LTO. The number in front of Sr-LTO indicates the mole ratio of Sr to Ti in the composite. Un-doped Li4Ti5O12 was denoted as LTO. The crystal structure analysis was performed by powder X-ray diffraction spectrometer (XRD, RigakuD/max-2550/PC) using Cu Kα radiation (λ = 0.1506 nm) with 2θ ranging from 10° to 90° at a scan rate of 10°min−1 and a step width of 0.02°. The measurements of morphology and particle size were carried out using a scanning electron microscope (SEM, JEOL JSM-6480) equipped with an energy dispersive spectroscopy and transmission electron microscope (TEM, FEI Teccai G2 S-Twin, Philips). 2.2. Electrochemical measurements Electrochemical tests were carried out using model CR2032 coin-type cells with Li foil as the reference and counter electrode. The working electrodes contain 85 wt.% Sr-doped or un-doped Li4Ti5O12, 8 wt.% acetylene black and 7 wt.% binder (LA132, Indigo, China). To make the working electrode, the required amount of active material, acetylene black and binder were blended in distilled water to form a slurry, which was coated onto an aluminum foil, dried at 80 °C for 24 h in a vacuum oven, and pressed at 10 MPa. The cells were assembled in a glove box using Celgard 2400 ﬁlm as the separator and 1 mol L −1 LiPF6 as the electrolyte, which was dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethylene methyl carbonate (EMC) with a volume ratio of EC:DMC:EMC to be 1:1:1. Galvanostatic charge/discharge tests were performed on a Land Battery Test System (model CT2001A, China) in the potential rage of 1.0 to 2.5 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (VMP3/Z, Bio-Logic). The EIS tests were carried out in a frequency range of 1 Hz to 100 kHz at the open circuit potential with the potential amplitude of ±5 mV. 3. Results and discussion 3.1. Characterization of Sr-doped Li4Ti5O12 composites Fig. 1 shows the X-ray diffraction patterns of Sr-doped and un-doped Li4Ti5O12. It can be seen that the main diffraction peaks of all the samples can be well indexed to the cubic spinel Li4Ti5O12 (PDF No.49-0207) in terms of both peak position and intensity. The XRD peak intensities decrease with the doping amount of Sr, suggesting the relatively poor crystallinity of Sr-doped Li4Ti5O12. Lattice distortion of Sr-doped Li4Ti5O12 has been observed. A close inspection revealed that all the peaks corresponding to Li4Ti5O12 shifted to lower 2θ values for the Sr-doped Li4Ti5O12. For a clear observation, the peak position variation of (111) plane is magniﬁed and shown in Fig. 1b. Vegard's law is an approximate empirical rule which holds that a linear relationship exists between the crystal lattice parameter of an alloy and the concentrations of the constituent elements at constant temperature. The formula can be described as: a ¼ x1 a1 þ x2 a2 where, a is the lattice parameter of the complex. x1, x2, a1, and a2 refer to the percentage contents and lattice constants of two elementary substance or compounds, respectively. According to Vegard's law, if the lattice parameter (a) of Sr-doped Li4Ti5O12 increases with the doping amount of Sr, then Sr 2+ enters the lattices of Li4Ti5O12 and causes lattice
Fig. 1. XRD patterns of the Sr-doped and un-doped Li4Ti5O12 (a) and the partially enlarged drawing (b).
expansion because ionic radius of Sr2+ (0.118 nm) is larger than that of Li+ (0.076 nm) and Ti4+ (0.0605 nm). The Rietveld reﬁnement of the XRD data clearly indicated that the lattice parameter increases with the doping amount of Sr (Table 1). So it can be concluded that Sr entered the lattice of Li4Ti5O12. The study on Mg (in the same group with Sr) doping of Li4Ti5O12 showed that Mg2+ substituted Li+ and occupy the tetrahedron 8a site resulting in an enhancement in the electronic conductivity of Li4Ti5O12 [14,24]. The divalent Sr2+ might also replace Li+ and occupied the tetrahedron 8a site. As a result, the lattice parameter increased with the doping amount of Sr2+ ions. There are several weak impurity peaks for the Sr-doped Li4Ti5O12 and their intensity increased with the content of Sr. These peaks are well indexed to the single crystal SrLi2Ti6O14 according to literatures [21,25,26]. So it can be concluded that Sr-doped Li4Ti5O12 is composed of Li4Ti5O12 with some Li+ replaced by Sr2+ and a small amount of SrLi2Ti6O14. Fig. 2 shows the typical SEM and TEM images of Sr-doped and un-doped Li4Ti5O12. Clearly, Sr-doping reduced the mean particle size Table 1 Lattice parameters of Sr-doped Li4Ti5O12 obtained by the Rietveld reﬁnement of the XRD data based on standard Li4Ti5O12 structure. Sample
LTO 0.01Sr-LTO 0.02Sr-LTO 0.03Sr-LTO
0.83514 0.83521 0.83536 0.83554
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Fig. 2. SEM images of LTO (a), 0.01Sr-LTO (b), 0.02Sr-LTO(c), and 0.03Sr-LTO (d). The insert is the corresponding TEM images.
Fig. 3. Energy dispersive analysis and element mapping of 0.02Sr-LTO.
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Fig. 4. The charge/discharge curves of the Sr-doped Li4Ti5O12 at 0.5 C.
but widened the particle size distribution of Li4Ti5O12. The particle size is as large as approximately 700 nm for LTO and approximately 200 nm for 0.02Sr-LTO (TEM image). Reducing the particle size of Li4Ti5O12 can shorten the distance for Li+ transport within Li4Ti5O12 and thus increase the high rate performance. The distribution and content of Sr in 0.02Sr-LTO was investigated by energy dispersive spectroscopy and the results are shown in Fig. 3. The average atomic ratio of Sr to Ti is about 0.019 measured from three selected areas, which is in good agreement with the theoretical value of 0.02. The elemental mapping shows that Sr was uniformally distributed within the 0.02Sr-LTO. 3.2. Electrochemical properties of Sr-doped Li4Ti5O12 composites Fig. 4 shows the charge/discharge curves of Sr-doped and un-doped Li4Ti5O12 at 0.5 C rate in the potential rage of 1.0 to 2.5 V. The charge/ discharge curves of the pure Li4Ti5O12 exhibited the typical single ﬂat plateau at around 1.55 V. While, the charge/discharge behavior of Sr-doped Li4Ti5O12 is quite different from that of Li4Ti5O12. Two plateaus at around 1.35 V and 1.51 V were observed. The plateaus near 1.51 V can be attributed to the two phase transformation between Li4Ti5O12 and Li7Ti5O12 induced by the conversion between the tetrahedral 8a and octahedral 16c sites of the host spinel. The charge/discharge plateaus near 1.35 V can be ascribed to the intercalation/deintercalation of Li+ within SrLi2Ti6O14 [21,25,26]. This is because, ﬁrst, we observed the existence of SrLi2Ti6O14 (Fig. 1a), and secondly, Belharouak et al.  and Dambournet et al.  have reported that SrLi2Ti6O14 has an operating potential of around 1.3–1.4 V. So, the charge/discharge tests further conﬁrmed the presence of SrLi2Ti6O14 in the Sr-doped Li4Ti5O12 and this impurity also makes a contribution to the capacity of the samples. The charge/discharge speciﬁc capacity corresponding to SrLi2Ti6O14 increased with the increase of Sr content (around 10, 20 and 25 mAh g−1 for 0.01Sr-LTO, 0.02Sr-LTO and 0.03Sr-LTO, respevtively). This implied that more SrLi2Ti6O14 was formed when more Sr was introduced. Among the three Sr-doped Li4Ti5O12 samples, 0.02Sr-LTO displayed the largest overall speciﬁc capacity (158.5 mAh g−1), which is higher than that of LTO (145.3 mAh g−1). 0.03Sr-LTO shows the worst performance in terms of both speciﬁc capacity and charge/discharge potentials. So Sr doping can improve the charge/discharge speciﬁc capacity of Li4Ti5O12. Since 0.02Sr-LTO exhibited higher speciﬁc capacity than LTO, we further investigated the rate performance of 0.02Sr-LTO and compared with that of LTO. Fig. 5 shows the inﬂuence of charge/discharge rate on the speciﬁc capacity of 0.02Sr-LTO and LTO. It can be seen that the speciﬁc capacity of 0.02Sr-LTO is higher than that of the pure Li4Ti5O12 phase at all charge/discharge rate between 0.5 C and 5 C. At 5 C, the
Fig. 5. The charge/discharge curves of LTO (a) and 0.02Sr-LTO (b) at different charge/ discharge rates.
discharge speciﬁc capacity of 0.02Sr-LTO is 104.1 mAh g−1, which is about 1.62 times that of LTO (64.3 mAh g−1). This clearly demonstrated that Sr-doping can signiﬁcantly enhance the rate performance of Li4Ti5O12. Fig. 6 shows the cycling performance of 0.02Sr-LTO at different charge/discharge rates from 0.5 C to 5 C. The speciﬁc capacity loss after being cycled at 0.5 C, 1 C, 2 C, and 5 C each for 100 times is 6.9%, 7.0%, 8.0% and 8.8%, respectively. These values are relatively
Fig. 6. The cycling performance of the 0.02Sr-LTO at different charge/discharge rates.
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Fig. 7. The cyclic voltammograms of the Sr-doped Li4Ti5O12 at a scan rate of 0.1 mV s−1.
high meaning that the cycling stability of Sr-doped Li4Ti5O12 needs to be improved. Charge/discharge efﬁciency of 0.02Sr-LTO was maintained at ~ 100% at all cycling rates. The loss of speciﬁc capacity might be attributed to the presence of SrLi2Ti6O14 which has a poor cycling stability. Belharouak et al.  have demonstrated that the speciﬁc capacity loss of SrLi2Ti6O14 after 40 cycles at C/7 is at least 9.7%. The charge/discharge behavior of Sr-doped Li4Ti5O12 was further investigated by cyclic voltammetry and Fig. 7 shows the CV of Sr-doped and un-doped Li4Ti5O12. Li4Ti5O12 displayed one couple of redox peaks (peaks 1 and 2) corresponding to the Li+ intercalation/deintercalation. Two couples of redox peaks (peaks 1 and 2, and peaks 3 and 4) were observed for the Sr-doped Li4Ti5O12 composite. Peaks 1 and 2 occur in the same position with Li4Ti5O12 and thus corresponds to Li+ deintercalation and intercalation within Sr-doped Li4Ti5O12, respectively. Peak 3 and 4 can be assigned to the Li+ deintercalation and intercalation within SrLi2Ti6O14, respectively . The potential differences between peaks 1 and 2 for 0.02Sr-doped Li4Ti5O12 (0.121 V) is smaller than that for Li4Ti5O12 (0.132 V), 0.01Sr-LTO (0.200 V) and 0.03Sr-LTO (0.168 V). This indicated that 0.02Sr-LTO has a better rate capability than LTO . So CV further demonstrated that there exists of a small amount of SrLi2Ti6O14 impurity in the Sr-doped Li4Ti5O12 composites and appropriate content of Sr dopant can improve the rate performance of Li4Ti5O12. Fig. 8 shows the Nyquist plots of the Sr-doped and un-doped Li4Ti5O12. The impedance spectrum of Li4Ti5O12 displays a depressed semicircle in the high frequency region and a straight line in the low frequency region, which is in good agreements with the literature
Fig. 8. Nyquist plots of the Sr-doped Li4Ti5O12.
results . The semicircle corresponds to the charge transfer resistance, which is around 30 Ω. The straight line is attributed to the diffusion of the lithium ions within the bulk of Li4Ti5O12. 0.01Sr-LTO displayed a clearly different impedance spectrum with Li4Ti5O12. It shows a semicircle, an incomplete semicircle and a straight line in the high, medium and the low frequency regions, respectively. This implies that there might be two charge transfer processes for 0.01Sr-LTO. This result is in good agreement with the cyclic voltammogram of Sr-doped Li4Ti5O12 (Fig. 7), in which two couples of redox peaks were observed, and are ascribed to Li+ deintercalation and intercalation within Sr-doped Li4Ti5O12 and SrLi2Ti6O14. The total resistance of the two semicircles for 0.01Sr-LTO is approximately equal to the charge transfer resistance of Li4Ti5O12. When the content of Sr dopant was increased (e.g., 0.02Sr-LTO and 0.03Sr-LTO), the two semicircles merged to a single depressed semicircles, and the charge transfer resistance of Sr-doped Li4Ti5O12 (e.g. 7.24 Ω for 0.02Sr-LTO) is much smaller than that of the pure phase Li4Ti5O12. This suggested that Sr doping can signiﬁcantly increase the charge-transfer rate of Li4Ti5O12, and consequently improve the rate capability. Similar results have been reported for the Mo-doped Li4Ti5O12 . 4. Conclusions This work successfully demonstrated that Sr-doping can effectively improve the high rate performance of Li4Ti5O12. The high rate performance can be attributed to the reduction of particle size, the decrease of charge transfer resistance and the enlargement of lattice parameter caused by the introduction of Sr to Li4Ti5O12 lattice. The Sr content has a signiﬁcant effect on the performance of Sr-doped Li4Ti5O12 and the optimum Sr content was found to be at the atomic ratio of Sr to Ti of 0.02. A small amount of SrLi2Ti6O14 was formed in the Sr-doped Li4Ti5O12, which however has a positive effect on the charge/discharge speciﬁc capacity of the doped Li4Ti5O12 because Li+ can intercalate/deintercalate within SrLi2Ti6O14 at lower charge/discharge potentials than Li4Ti5O12. Acknowledgments We gratefully acknowledge the ﬁnancial support of this research by Harbin Science and Technology Innovation Fund for Excellent Academic Leaders (2012RFXXG103), Fundamental Research Funds for the Central Universities (HEUCFT1205) and the Science & Technology Pillar Program of Jiangsu Province (BE2012152). References  T.F. Yi, L.J. Jiang, J. Shu, C.B. Yue, R.S. Zhu, H.B. Qiao, J. Phys. Chem. Solids 71 (2010) 1236.  A.S. Prakash, P. Manikandan, K. Ramesha, M. Sathiya, J.M. Tarascon, A.K. Shukla, Chem. Mater. 22 (2010) 2857.  A. Guerﬁ, S. Sévigny, M. Lagacé, P. Hovington, K. Kinoshita, K. Zaghib, J. Power Sources 119–121 (2003) 88.  T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431.  K. Zaghib, M. Simoneau, M. Armand, M. Gauthier, J. Power Sources 81–82 (1999) 300.  T.F. Yi, J. Shu, Y.R. Zhu, X.D. Zhu, C.B. Yue, A.N. Zhou, R.S. Zhu, Electrochim. Acta 54 (2009) 7464.  J. Wolfenstine, J.L. Allen, J. Power Sources 180 (2008) 582.  B. Tian, H. Xiang, L. Zhang, H. Wang, J. Solid State Electrochem. 16 (2012) 205.  B. Zhang, Z.D. Huang, S.W. Oh, J.K. Kim, J. Power Sources 196 (2011) 10692.  Z. Zhong, Electrochem. Solid-State Lett. 10 (2007) A267.  D. Capsoni, M. Bini, V. Massarotti, P. Mustarelli, S. Ferrari, G. Chiodelli, M.C. Mozzati, P. Galinetto, J. Phys. Chem. C 113 (2009) 19664.  H. Zhao, Y. Li, Z. Zhu, J. Lin, Z. Tian, R. Wang, Electrochim. Acta 53 (2008) 7079.  B. Zhang, H. Du, B. Li, F. Kang, Electrochem. Solid-State Lett. 13 (2010) A36.  C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, T. Goacher, M.M. Thackeray, J. Electrochem. Soc. 148 (2001) A102.  S. Huang, Z. Wen, X. Zhu, Z. Lin, J. Power Sources 165 (2007) 408.  S. Huang, Z. Wen, J. Zhang, X. Yang, Electrochim. Acta 52 (2007) 3704.  S. Huang, Z. Wen, Z. Gu, X. Zhu, Electrochim. Acta 50 (2005) 4057.  A. Subramania, N. Angayarkanni, T. Vasudevan, J. Power Sources 158 (2006) 1410.  S. Valanarasu, R. Chandramohan, J. Alloys Compd. 494 (2010) 434.  G.T.K. Fey, V. Subramanian, J.G. Chen, Mater. Lett. 52 (2002) 197.
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