- Email: [email protected]
Anatase-TiO2 nanocoating of Li4Ti5O12 nanorod anode for lithium-ion batteries
Journal of Alloys and Compounds 601 (2014) 38–42
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Anatase-TiO2 nanocoating of Li4Ti5O12 nanorod anode for lithium-ion batteries Ming-ming Chen ⇑, Xin Sun, Zhi-jun Qiao, Qian-qian Ma, Cheng-yang Wang Key Laboratory for Green Chemical Technology of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 14 January 2014 Received in revised form 20 February 2014 Accepted 21 February 2014 Available online 28 February 2014 Keywords: Microemulsion-assisted hydrothermal Lithium titanate Anatase titanium dioxide Nanocoating layer Lithium-ion batteries
a b s t r a c t Li4Ti5O12 nanorod coated by anatase-TiO2 is in situ synthesized via a microemulsion-assisted hydrothermal method followed by heat treatment at 550 °C in air. Compared with pure Li4Ti5O12, Li4Ti5O12 nanorod coated by anatase-TiO2 presents much improved electrochemical characteristics in terms of high specific capacity, excellent rate capability and cyclic stability (96.0% of initial capacity at a current density of 1.75 A g1 up to 100 cycles). Acting as a perfect nanocoating layer, anatase-TiO2 contributes some capacity and gives an enhanced performance to the Li4Ti5O12 electrode. All the results suggest that Li4Ti5O12 nanorod coated by anatase-TiO2 could be suitable for use as a high-rate anode material for lithium-ion batteries. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Rechargeable Lithium-ion batteries (LIBs) are considered as one of the promising energy storage devices for the next generation of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) [1,2]. With respect to the potential anode materials of LIBs, spinel lithium titanate (Li4Ti5O12, LTO) has received tremendous interests owing largely to its great merit of zero-strain characteristic during lithium insertion/extraction [3–6]. Such unique feature of LTO allows it with excellent reversibility, structural stability and long cycle life. Furthermore, LTO offers a great improvement in safety, which exhibits a relatively high operating voltage (1.55 V vs. Li/Li+) avoiding the growth of lithium dendrites and the decomposition of electrolyte [7,8]. These features make it as a promising anode material with high safety for high-rate LIBs. However, LTO still suffers a low intrinsic electronic conductivity [9–11]. The large-scale application of LIBs made from LTO electrode is also limited due to severe gassing during energy storage [12,13]. A number of strategies have been devised to improve the rate performance of LTO anodes. Such as reducing particle size [14,15], doping with metal or non-metal ions [16,17] and coating with a conductive layer [18–20]. Among coating methods, carbon
⇑ Corresponding author at: Key Laboratory for Green Chemical Technology of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 22 2789048. E-mail address: [email protected] (M.-m. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.02.130 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
coating has been reported to be effective [21]. For example, carbon coating using amphiphilic carbonaceous material (ACM) as carbon precursor gives the LTO particles an enhanced performance with a discharge capacity of 137 mA h g1 at 20 C [22]. Besides carbon coating, TiO2 as a fast lithium insertion/extraction host can be a promising coating material. Actually, rutileTiO2 as a coating layer was ever evidenced to make LTO perform better than LTO itself [23]. However, as a coating layer, TiO2 with other polymorphs such as anatase has not been studied for any improvement of LTO. Herein, we introduce the water-in-oil microemulsion in the hydrothermal process to synthesize LTO nanorod coated by anatase-TiO2. Furthermore, the TiO2-coated LTO was characterized and tested as an anode material for lithium ion battery, focusing on reversible capacity, rate capability and cyclic stability.
2. Experimental 2.1. Synthesis All reagents were of analytical grade, purchased from Tianjin Guangfu Co., Ltd., PR China and used with no further purification. Typically, LiOHH2O was dissolved in distilled water to get a 0.6 M solution. 4.0 g of cetyltrimethylammonium bromide (CTAB), 25 ml of cyclohexane, 10 ml of n-butanol and 4 ml LiOH solution were mixed together under vigorous stirring to form a homogeneous and transparent microemulsion. Then 2.7 mmol of tetrabutyltitanate (Ti(C4H9O)4, TBT) was added dropwise to the as-prepared microemulsion. The mixture was kept stirring for 30 min and transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180 °C for different times. The precipitate was collected by
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
39
3. Results and discussion
Fig. 1. XRD patterns of calcined LTO samples at a definite Li/Ti ratio of 4.5:5 by hydrothermal reaction of 6 h (a), 10 h (b) and 14 h (c).
centrifugation, washed with ethanol several times till the solvents were completely removed, and then vacuum-dried at 100 °C. Subsequently, the powder was calcined at 550 °C in air for 6 h to form the final product. 2.2. Materials characterization
When the molar ratio of Li/Ti is 4.5:5, the samples from different hydrothermal treatment times were investigated by XRD. Fig. 1 shows the XRD patterns of three LTO samples different in hydrothermal duration (6 h, 10 h, 14 h). As shown in Fig. 1(a), a considerable amount of anatase-TiO2 [JCPDS Card No. 21-1272] can still be detected in the LTO sample of 6 h hydrothermal reaction (marked with *), indicating that TiO2 does not react completely with LiOH yet. When the hydrothermal duration is increased to 10 h (Fig. 1(b)), pure spinel-LTO [JCPDS Card No. 49-0207] is obtained which indicates that the TiO2 is completely converted into LTO after chemical lithiation. Any increase of reaction time do not change the XRD patterns any more, seen from Fig. 1(c). The morphologies of the three LTO samples from different hydrothermal treatment times were observed by FESEM. Fig. 2(a) shows that the sample is composed of irregular nanoparticles with average size of 40–70 nm, when the reaction time is just 6 h. With the reaction time prolonged to 10 h, some nanoparticles start to aggregate and align into rod (Fig. 2(b)). When the reaction time is further prolonged to 14 h, nanorods self-assemble to be of sheet-like morphology in an increased size (Fig. 2(c)). In order to get 1-D structure, hydrothermal reaction time of 10 h was preferred for the following experiments. When the molar ratio of Li/Ti was decreased to 4:5, no enough Li existed in the solution. Thus, even 10 h was adopted as the
The crystallographic structure of the samples was identified by the powder X-ray diffraction (XRD) using a Rigaku D/max 2500v/PC X-ray diffractometer equipped with Cu Ka radiation source (k = 1.5406 Å). Morphology and structure of the samples were characterized by using field-emission scanning electron microscopy (FE-SEM, S4800, ThermoFisher) and transmission electron microscopy (TEM, Tecnai G2F20, Philips). 2.3. Electrochemical measurements The electrochemical measurements were performed using CR2032 coin-type cells assembled in an argon-filled glove box. The coin cells were fabricated by using the final product as the cathode material, metallic lithium foil as the anode, and porous polypropylene film (Celgard 2400, Celgard Inc., USA) as the separator. The cathode electrode was made by pasting a slurry which was prepared by mixing the sample powder, acetylene black, and polyvinylidene fluoride (PVDF) binder at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent on a copper foil current collector. This prepared electrode sheet was then vacuum-dried at 120 °C for 12 h. The electrolyte employed was 1.0 M LiPF6 in the mixture of ethylene carbonate and diethyl carbonate (EC/DEC, 1:1, v/v) obtained from Tianjin Jinniu Power Sources Material Co., Ltd. Cyclic voltammogram (CV) test was carried out on a Princeton Parstat 2273 electrochemical system at a scan rate of 0.1 mV s1 between 1.0 V and 2.5 V. Galvanostatic charge–discharge performances were evaluated by a Land BT2000 battery test system (Wuhan, PR China) under different current densities in the potential range of 1.0–2.5 V. The electrochemical impedance spectrum (EIS) of the assembled cells was also measured using Princeton Parstat 2273 electrochemical system in the frequency range from 102 to 105 Hz with a potential perturbation at 5 mV.
Fig. 3. XRD patterns of the samples by hydrothermal reaction of 10 h at a Li/Ti ratio of 4:5 before (a) and after (b) calcination at 550 °C for 6 h.
Fig. 2. FESEM images of the three LTO samples at a definite Li/Ti ratio of 4.5:5 by hydrothermal reaction of 6 h (a), 10 h (b) and 14 h (c).
40
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
hydrothermal reaction time, still some anatase-TiO2 is found in spinel-LTO by XRD patterns in Fig. 3 no matter whether the sample was calcined or not. After the calcination, a well-crystallized composite of LTO@anatase-TiO2 is the formed. It is in rod-shape of 25 nm in diameter and 80–120 nm in length (shown in Fig. 4(a)). High-resolution TEM image of an individual nanorod in Fig. 4(b) displays clear lattice fringes. Two sets of different lattice fringes are detected which are indexed and calibrated to have a lattice spacing of d = 0.244 and 0.483 nm, corresponding to the (1 0 3) plane of anatase-TiO2 and (1 1 1) plane of LTO. In addition, by the image of high-resolution TEM in Fig. 4(b), anatase-TiO2 exists at the edge of LTO forming a perfect coating layer and almost no transitional areas between the two phases are detected. During the hydrothermal process, the Ti(OH)4 derived from the hydrolysis products of TBT changed into anatase-TiO2, and then part of them transformed into LTO through lithiation (Fig. 3(a)). After calcination, it can be seen from the XRD patterns of Fig. 3(b) that the sample almost converts to well-crystallized LTO and trace amounts of anatase-TiO2 still appears in final product. The chemical reactions for the synthesis of TiO2-coated LTO can be expressed as follows:
TiðC4 H9 OÞ4 þ 4H2 O ! TiðOHÞ4 þ 4C4 H9 OH
ð1Þ
TiðOHÞ4 ! TiO2 ðanataseÞ þ 2H2 O
ð2Þ
5TiO2 ðanataseÞ þ 4LiOH ! Li4 Ti5 O12 þ 2H2 O
ð3Þ
The formation of TiO2-coated LTO nanorod is a complicate process, in which chemical reactions and self-assembly occur simultaneously. The water-in-oil microemulsion plays a crucial role in the fabrication of the nanorod and a proposed mechanism is shown in Fig. 5. Firstly, nanoscale water pools (LiOH solution) disperse homogeneously in the continuous oil phase with the help of the surfactant (CTAB). When introduced stepwise into the microemulsion, TBT instantly dissolves into the oil phase. Under the driving force of hydrolysis reaction (happening following Eq. (1)), the TBT drops gradually diffuse into the water pools and then hydrolyze to form Ti(OH)4 which appears first close to the internal surface of the microemulsion ball and then keeps on accumulating towards the micelle center. Consequently, Li ions are wrapped by the Ti(OH)4. During the afterward hydrothermal process, Ti(OH)4 dehydrates to anatase-TiO2 (Eq. (2)), and then the innermost part changes to LTO through chemical lithiation (Eq. (3)). f Potential of new born LTO after hydrothermal treatment for 2 h is found to be negatively charged as 5.08 mV. Cationic Surfactant CTAB has positive charged head, leading to an assembly of surfactant molecules on the surface of the negatively charged particles. Such an assemblage is favorable the anisotropic growth of the particle during the intermicellar exchange. And in the earlier reports [24,25], nanorods have been obtained by the microemulsion route using
Fig. 4. TEM (a) and high-resolution TEM (b) images of TiO2-coated LTO at a Li/Ti ratio of 4:5.
Fig. 5. Schematic illustration for the growth mechanism of TiO2-coated LTO nanorod.
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
41
the lack of Li ions at the Li/Ti molar ratio of 4:5. Thus, a novel structure of LTO nanorod with a uniform anatase-TiO2 coating layer is finally formed. The electrochemical behaviour of the TiO2-coated LTO electrode was investigated by CV and the results of the first two cycles are shown in Fig. 6. Two pairs of cathodic/anodic peaks centered at 1.46/1.66 V and 1.72/2.0 V, corresponding to the lithium insertion/extraction in LTO and anatase-TiO2 lattice, respectively [26]. This indicates that both LTO and anatase-TiO2 phases possess the lithiation/delithiation activity and thereby anatase-TiO2 can contribute some electrochemical capacity to the electrode. The electrochemical reactions can be expressed by the following Eqs. (4) and (5), respectively. þ
Li4 Ti5 O12 þ xLi þ xe $ Li4þx Ti5 O12 ðx ¼ 0 3Þ þ
Fig. 6. CV profiles of TiO2-coated LTO electrode at the scan rate of 0.1 mV s1 between 1.0 and 2.5 V.
CTAB. Hence, at this stage the growth would be easier along the axis of the rod. Finally, during calcination LiOH continues to react with the surrounding anatase-TiO2 to form LTO (Eq. (3)). While the outmost layer of anatase-TiO2 could remain mainly due to
TiO2 þ xLi þ xe $ Lix TiO2 ðx ¼ 0:5 1Þ
ð4Þ
ð5Þ
Fig. 7(a) displays the galvanostatic charge and discharge behaviors of the TiO2-coated LTO at different current densities. Two pairs of voltage plateaus can be observed on charge–discharge curves at 0.0175 A g1. However, due to the polarization and unsaturated insertion and extrusion of Li+ at high current densities, the voltage plateaus of anatase-TiO2 disappear. The TiO2-coated LTO gives remarkably high discharge capacities of 233.8 mA h g1 at
Fig. 7. Electrochemical performances of TiO2-coated LTO and LTO: (a) 1st discharge–charge curves at different current densities; (b) rate capability; (c) cyclic stability at 1.75 A g1; (d) EIS profiles.
42
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
0.0175 A g1 and 202.4 mA h g1 at 0.175 A g1, which surpass the theoretical capacity of LTO (175 mA h g1) owing to the capacity contribution of anatase-TiO2 (336 mA h g1). High-rate performances of LTO based electrodes have been reported in the recent literatures [27–31]. In this paper, the TiO2-coated LTO also exhibits excellent rate performance. Fig. 7(b) compares rate capabilities of the TiO2-coated LTO and LTO at different current densities. The discharge capacities of TiO2-coated LTO decrease much more slowly than that of LTO from 0.175 A g1 to 7 A g1. Even at 7 A g1, the TiO2-coated LTO still delivers a capacity of 108.9 mA h g1, which is much higher than that of LTO (ca. 56 mA h g1). It is worth mentioning that a specific capacity of 206.1 mA h g1 (88.2% of that of the first cycle at 0.0175 A g1) is recovered when the current density is finally reduced back to 0.0175 A g1, which suggests good reversibility of the TiO2-coated LTO electrode. The satisfactory performances of the TiO2-coated LTO electrode could be attributed to the following two factors. First, within the voltage range of LTO lithiation, anatase-TiO2 is an active Li host with high theoretical capacity. Second, the electronic conductivity of the electrode can be improved by the anatase-TiO2 coating layer. Li ions could insert into TiO2 during the initial discharge process at 1.7 V, leading to the formation of LixTiO2. The in situ formed LixTiO2 would enhance the overall conductivity due to the existence of Ti3+ [32]. Compared to LTO, the TiO2-coated LTO exhibits higher reversible capacity and better cyclic stability, as seen in Fig. 7(c). After 100 cycles at 1.75 A g1, the TiO2-coated LTO retains 96.0% of its initial capacity, while LTO presents only 84.5%. These results demonstrate superior structural stability and electrochemical reversibility of this TiO2-coated LTO electrode. Electrochemical impedance spectroscopy (EIS) measurements of the TiO2-coated LTO and LTO electrodes were performed, and the results are shown in Fig. 7(d). Each plot consists of one semicircle at high frequency and a straight line at low frequency. The diameter of the semicircle is mainly related to the surface charge-transfer resistance which was calculated as about 32 X and 65 X for the TiO2-coated LTO and LTO, respectively, suggesting that the anatase-TiO2 coating layer could enable much easier charge transfer at the electrode and electrolyte interface. Thus, the in situ anatase-TiO2 coating is considered a key factor in improving the rate capability and cyclic stability of the TiO2-coated LTO. He et al. [12] ever reported that during charge–discharge cycles, bare LTO will gas caused by the interfacial reactions between bare LTO and its surrounding alkyl carbonate solvents. Upon this, it is a rational assumption that an inert barrier between LTO and alkyl carbonate electrolyte could make a difference. In He is research, when gassing stopped, a very thin layer of anatase-TiO2 was found on the outmost surface of LTO, indicating anatase-TiO2 maybe chemically-inert when facing with such kind of electrolyte. Therefore, we have reasons to believe that the anatase-TiO2 coating layer in our sample would improve its surface stability by preventing the electrode from gassing and simultaneously enhance the high-rate performance of LTO electrodes. Of course, more detailed confirmation is still needed. 4. Conclusion LTO nanorod coated by anatase-TiO2 has been in situ prepared via a microemulsion-assisted hydrothermal method. The exits of
water-in-oil microemulsion plays a key role in fabrication of the novel structure. Anatase-TiO2 acts as a well nanocoating layer, which can both contribute some capacity and give an enhanced performance to the Li4Ti5O12 electrode. Therefore, the TiO2-coated LTO exhibits high specific capacity, excellent rate capability and a significantly enhanced cyclic performance. This TiO2-coated LTO would be a promising anode material for highly efficient LIBs and other critical energy storages. Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC 51372168), the Natural Science Foundation of Tianjin City of China (Key Program, No. 12JCZDJC27000). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25] [26] [27] [28] [29] [30] [31] [32]
A. Manthiram, J. Phys. Chem. Lett. 2 (2011) 176–184. M. Armand, J.M. Tarascon, Nature 451 (2008) 652–657. S.W. Han, J.H. Ryu, J. Jeong, D.H. Yoon, J. Alloys Comp. 570 (2013) 144–149. A.S. Prakash, P. Manikandan, K. Ramesha, M. Sathiya, J.M. Tarascon, A.K. Shukla, Chem. Mater. 22 (2010) 2857–2863. X.F. Guo, C.Y. Wang, M.M. Chen, Mater. Lett. 83 (2012) 39–41. C.Y. Lin, J.G. Duh, J. Alloys Comp. 509 (2011) 3682–3685. L. Aldon, P. Kubiak, M. Womes, J.C. Jumas, J. Olivier-Fourcade, J.L. Tirado, J.I. Corredor, C.P. Vicente, Chem. Mater. 16 (2004) 5721–5725. A. Guerfi, P. Charest, K. Kinoshita, M. Perrier, K. Zaghib, J. Power Sources 126 (2004) 163–168. C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Electrochem. Commun. 9 (2007) 1107–1112. B.H. Li, C.P. Han, Y.B. He, C. Yang, H.D. Du, Q.H. Yang, F.Y. Kang, Energy Environ. Sci. 5 (2012) 9595–9602. J. Wang, L.F. Shen, H.S. Li, B. Ding, P. Nie, H. Dou, X.G. Zhang, J. Alloys Comp. 587 (2014) 171–176. Y.B. He, B.H. Li, M. Liu, C. Zhang, W. Lv, C. Yang, J. Li, H.D. Du, B. Zhang, Q.H. Yang, J.K. Kim, F.Y. Kang, Sci. Rep. 2 (2012) 913. K. Wu, J. Yang, Y. Liu, Y. Zhang, C.Y. Wang, J.M. Xu, F. Ning, D.Y. Wang, J. Power Sources 237 (2013) 285–290. Z.M. Liu, N.Q. Zhang, K.N. Sun, J. Mater. Chem. 22 (2012) 11688–11693. G.Y. Liu, H.Y. Wang, G.Q. Liu, Z.Z. Yang, B. Jin, Q.C. Jiang, J. Power Sources 220 (2012) 84–88. C.Y. Lin, Y.R. Jhan, J.G. Duh, J. Alloys Comp. 509 (2011) 6965–6968. Y. Ma, B. Ding, G. Ji, J.Y. Lee, ACS Nano 7 (2013) 10870–10878. N. Li, G.M. Zhou, F. Li, L. Wen, H.M. Cheng, Adv. Funct. Mater. 23 (2013) 5429– 5435. H.J. Luo, L.F. Shen, K. Rui, H.S. Li, X.G. Zhang, J. Alloys Comp. 572 (2013) 37– 42. C.T. Hsieh, B.S. Chang, J.Y. Lin, R.S. Juang, J. Alloys Comp. 513 (2012) 393– 398. M. Inagaki, Carbon 50 (2012) 3247–3266. X.F. Guo, C.Y. Wang, M.M. Chen, J.Z. Wang, J.M. Zheng, J. Power Sources 214 (2012) 107–112. Y.Q. Wang, L. Gu, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, J. Am. Chem. Soc. 134 (2012) 7874–7879. S. Vaidya, P. Rastogi, S. Agarwal, S.K. Gupta, T. Ahmad, A.M. Antonelli, K.V. Ramanujachary, S.E. Lofland, A.K. Ganguli, J. Phys. Chem. C 112 (2008) 12610– 12615. L.L. Li, Y. Chu, Y. Liu, L.H. Dong, L. Huo, F.Y. Yang, Mater. Lett. 60 (2006) 2138– 2142. M.M. Rahman, J.Z. Wang, M.F. Hassan, D. Wexler, H.K. Liu, Adv. Energy Mater. 1 (2011) 212–220. B. Zhang, Y.S. Liu, Z.D. Huang, S. Oh, Y. Yu, Y.W. Maib, J.K. Kim, J. Mater. Chem. 22 (2012) 12133–12140. X. Li, S.H. Tang, M.Z. Qu, P.X. Huang, W. Li, Z.L. Yu, J. Alloys Comp. 588 (2014) 17–24. G.Y. Liu, H.Y. Wang, G.Q. Liu, Z.Z. Yang, B. Jin, Q.C. Jiang, Electrochim. Acta 87 (2013) 218–223. H. Song, S.W. Yun, H.H. Chun, M.G. Kim, K.Y. Chung, H.S. Kim, B.W. Cho, Y.T. Kim, Energy Environ. Sci. 5 (2012) 9903–9913. B.F. Wang, J. Cao, Y. Liu, T. Zeng, L. Li, J. Alloys Comp. 587 (2014) 21–25. G.D. Du, N. Sharma, V.K. Peterson, J.A. Kimpton, D. Jia, Z. Guo, Adv. Funct. Mater. 21 (2011) 3990–3997.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Anatase-TiO2 nanocoating of Li4Ti5O12 nanorod anode for lithium-ion batteries Ming-ming Chen ⇑, Xin Sun, Zhi-jun Qiao, Qian-qian Ma, Cheng-yang Wang Key Laboratory for Green Chemical Technology of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 14 January 2014 Received in revised form 20 February 2014 Accepted 21 February 2014 Available online 28 February 2014 Keywords: Microemulsion-assisted hydrothermal Lithium titanate Anatase titanium dioxide Nanocoating layer Lithium-ion batteries
a b s t r a c t Li4Ti5O12 nanorod coated by anatase-TiO2 is in situ synthesized via a microemulsion-assisted hydrothermal method followed by heat treatment at 550 °C in air. Compared with pure Li4Ti5O12, Li4Ti5O12 nanorod coated by anatase-TiO2 presents much improved electrochemical characteristics in terms of high specific capacity, excellent rate capability and cyclic stability (96.0% of initial capacity at a current density of 1.75 A g1 up to 100 cycles). Acting as a perfect nanocoating layer, anatase-TiO2 contributes some capacity and gives an enhanced performance to the Li4Ti5O12 electrode. All the results suggest that Li4Ti5O12 nanorod coated by anatase-TiO2 could be suitable for use as a high-rate anode material for lithium-ion batteries. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Rechargeable Lithium-ion batteries (LIBs) are considered as one of the promising energy storage devices for the next generation of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) [1,2]. With respect to the potential anode materials of LIBs, spinel lithium titanate (Li4Ti5O12, LTO) has received tremendous interests owing largely to its great merit of zero-strain characteristic during lithium insertion/extraction [3–6]. Such unique feature of LTO allows it with excellent reversibility, structural stability and long cycle life. Furthermore, LTO offers a great improvement in safety, which exhibits a relatively high operating voltage (1.55 V vs. Li/Li+) avoiding the growth of lithium dendrites and the decomposition of electrolyte [7,8]. These features make it as a promising anode material with high safety for high-rate LIBs. However, LTO still suffers a low intrinsic electronic conductivity [9–11]. The large-scale application of LIBs made from LTO electrode is also limited due to severe gassing during energy storage [12,13]. A number of strategies have been devised to improve the rate performance of LTO anodes. Such as reducing particle size [14,15], doping with metal or non-metal ions [16,17] and coating with a conductive layer [18–20]. Among coating methods, carbon
⇑ Corresponding author at: Key Laboratory for Green Chemical Technology of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel./fax: +86 22 2789048. E-mail address: [email protected] (M.-m. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.02.130 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
coating has been reported to be effective [21]. For example, carbon coating using amphiphilic carbonaceous material (ACM) as carbon precursor gives the LTO particles an enhanced performance with a discharge capacity of 137 mA h g1 at 20 C [22]. Besides carbon coating, TiO2 as a fast lithium insertion/extraction host can be a promising coating material. Actually, rutileTiO2 as a coating layer was ever evidenced to make LTO perform better than LTO itself [23]. However, as a coating layer, TiO2 with other polymorphs such as anatase has not been studied for any improvement of LTO. Herein, we introduce the water-in-oil microemulsion in the hydrothermal process to synthesize LTO nanorod coated by anatase-TiO2. Furthermore, the TiO2-coated LTO was characterized and tested as an anode material for lithium ion battery, focusing on reversible capacity, rate capability and cyclic stability.
2. Experimental 2.1. Synthesis All reagents were of analytical grade, purchased from Tianjin Guangfu Co., Ltd., PR China and used with no further purification. Typically, LiOHH2O was dissolved in distilled water to get a 0.6 M solution. 4.0 g of cetyltrimethylammonium bromide (CTAB), 25 ml of cyclohexane, 10 ml of n-butanol and 4 ml LiOH solution were mixed together under vigorous stirring to form a homogeneous and transparent microemulsion. Then 2.7 mmol of tetrabutyltitanate (Ti(C4H9O)4, TBT) was added dropwise to the as-prepared microemulsion. The mixture was kept stirring for 30 min and transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180 °C for different times. The precipitate was collected by
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
39
3. Results and discussion
Fig. 1. XRD patterns of calcined LTO samples at a definite Li/Ti ratio of 4.5:5 by hydrothermal reaction of 6 h (a), 10 h (b) and 14 h (c).
centrifugation, washed with ethanol several times till the solvents were completely removed, and then vacuum-dried at 100 °C. Subsequently, the powder was calcined at 550 °C in air for 6 h to form the final product. 2.2. Materials characterization
When the molar ratio of Li/Ti is 4.5:5, the samples from different hydrothermal treatment times were investigated by XRD. Fig. 1 shows the XRD patterns of three LTO samples different in hydrothermal duration (6 h, 10 h, 14 h). As shown in Fig. 1(a), a considerable amount of anatase-TiO2 [JCPDS Card No. 21-1272] can still be detected in the LTO sample of 6 h hydrothermal reaction (marked with *), indicating that TiO2 does not react completely with LiOH yet. When the hydrothermal duration is increased to 10 h (Fig. 1(b)), pure spinel-LTO [JCPDS Card No. 49-0207] is obtained which indicates that the TiO2 is completely converted into LTO after chemical lithiation. Any increase of reaction time do not change the XRD patterns any more, seen from Fig. 1(c). The morphologies of the three LTO samples from different hydrothermal treatment times were observed by FESEM. Fig. 2(a) shows that the sample is composed of irregular nanoparticles with average size of 40–70 nm, when the reaction time is just 6 h. With the reaction time prolonged to 10 h, some nanoparticles start to aggregate and align into rod (Fig. 2(b)). When the reaction time is further prolonged to 14 h, nanorods self-assemble to be of sheet-like morphology in an increased size (Fig. 2(c)). In order to get 1-D structure, hydrothermal reaction time of 10 h was preferred for the following experiments. When the molar ratio of Li/Ti was decreased to 4:5, no enough Li existed in the solution. Thus, even 10 h was adopted as the
The crystallographic structure of the samples was identified by the powder X-ray diffraction (XRD) using a Rigaku D/max 2500v/PC X-ray diffractometer equipped with Cu Ka radiation source (k = 1.5406 Å). Morphology and structure of the samples were characterized by using field-emission scanning electron microscopy (FE-SEM, S4800, ThermoFisher) and transmission electron microscopy (TEM, Tecnai G2F20, Philips). 2.3. Electrochemical measurements The electrochemical measurements were performed using CR2032 coin-type cells assembled in an argon-filled glove box. The coin cells were fabricated by using the final product as the cathode material, metallic lithium foil as the anode, and porous polypropylene film (Celgard 2400, Celgard Inc., USA) as the separator. The cathode electrode was made by pasting a slurry which was prepared by mixing the sample powder, acetylene black, and polyvinylidene fluoride (PVDF) binder at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent on a copper foil current collector. This prepared electrode sheet was then vacuum-dried at 120 °C for 12 h. The electrolyte employed was 1.0 M LiPF6 in the mixture of ethylene carbonate and diethyl carbonate (EC/DEC, 1:1, v/v) obtained from Tianjin Jinniu Power Sources Material Co., Ltd. Cyclic voltammogram (CV) test was carried out on a Princeton Parstat 2273 electrochemical system at a scan rate of 0.1 mV s1 between 1.0 V and 2.5 V. Galvanostatic charge–discharge performances were evaluated by a Land BT2000 battery test system (Wuhan, PR China) under different current densities in the potential range of 1.0–2.5 V. The electrochemical impedance spectrum (EIS) of the assembled cells was also measured using Princeton Parstat 2273 electrochemical system in the frequency range from 102 to 105 Hz with a potential perturbation at 5 mV.
Fig. 3. XRD patterns of the samples by hydrothermal reaction of 10 h at a Li/Ti ratio of 4:5 before (a) and after (b) calcination at 550 °C for 6 h.
Fig. 2. FESEM images of the three LTO samples at a definite Li/Ti ratio of 4.5:5 by hydrothermal reaction of 6 h (a), 10 h (b) and 14 h (c).
40
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
hydrothermal reaction time, still some anatase-TiO2 is found in spinel-LTO by XRD patterns in Fig. 3 no matter whether the sample was calcined or not. After the calcination, a well-crystallized composite of LTO@anatase-TiO2 is the formed. It is in rod-shape of 25 nm in diameter and 80–120 nm in length (shown in Fig. 4(a)). High-resolution TEM image of an individual nanorod in Fig. 4(b) displays clear lattice fringes. Two sets of different lattice fringes are detected which are indexed and calibrated to have a lattice spacing of d = 0.244 and 0.483 nm, corresponding to the (1 0 3) plane of anatase-TiO2 and (1 1 1) plane of LTO. In addition, by the image of high-resolution TEM in Fig. 4(b), anatase-TiO2 exists at the edge of LTO forming a perfect coating layer and almost no transitional areas between the two phases are detected. During the hydrothermal process, the Ti(OH)4 derived from the hydrolysis products of TBT changed into anatase-TiO2, and then part of them transformed into LTO through lithiation (Fig. 3(a)). After calcination, it can be seen from the XRD patterns of Fig. 3(b) that the sample almost converts to well-crystallized LTO and trace amounts of anatase-TiO2 still appears in final product. The chemical reactions for the synthesis of TiO2-coated LTO can be expressed as follows:
TiðC4 H9 OÞ4 þ 4H2 O ! TiðOHÞ4 þ 4C4 H9 OH
ð1Þ
TiðOHÞ4 ! TiO2 ðanataseÞ þ 2H2 O
ð2Þ
5TiO2 ðanataseÞ þ 4LiOH ! Li4 Ti5 O12 þ 2H2 O
ð3Þ
The formation of TiO2-coated LTO nanorod is a complicate process, in which chemical reactions and self-assembly occur simultaneously. The water-in-oil microemulsion plays a crucial role in the fabrication of the nanorod and a proposed mechanism is shown in Fig. 5. Firstly, nanoscale water pools (LiOH solution) disperse homogeneously in the continuous oil phase with the help of the surfactant (CTAB). When introduced stepwise into the microemulsion, TBT instantly dissolves into the oil phase. Under the driving force of hydrolysis reaction (happening following Eq. (1)), the TBT drops gradually diffuse into the water pools and then hydrolyze to form Ti(OH)4 which appears first close to the internal surface of the microemulsion ball and then keeps on accumulating towards the micelle center. Consequently, Li ions are wrapped by the Ti(OH)4. During the afterward hydrothermal process, Ti(OH)4 dehydrates to anatase-TiO2 (Eq. (2)), and then the innermost part changes to LTO through chemical lithiation (Eq. (3)). f Potential of new born LTO after hydrothermal treatment for 2 h is found to be negatively charged as 5.08 mV. Cationic Surfactant CTAB has positive charged head, leading to an assembly of surfactant molecules on the surface of the negatively charged particles. Such an assemblage is favorable the anisotropic growth of the particle during the intermicellar exchange. And in the earlier reports [24,25], nanorods have been obtained by the microemulsion route using
Fig. 4. TEM (a) and high-resolution TEM (b) images of TiO2-coated LTO at a Li/Ti ratio of 4:5.
Fig. 5. Schematic illustration for the growth mechanism of TiO2-coated LTO nanorod.
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
41
the lack of Li ions at the Li/Ti molar ratio of 4:5. Thus, a novel structure of LTO nanorod with a uniform anatase-TiO2 coating layer is finally formed. The electrochemical behaviour of the TiO2-coated LTO electrode was investigated by CV and the results of the first two cycles are shown in Fig. 6. Two pairs of cathodic/anodic peaks centered at 1.46/1.66 V and 1.72/2.0 V, corresponding to the lithium insertion/extraction in LTO and anatase-TiO2 lattice, respectively [26]. This indicates that both LTO and anatase-TiO2 phases possess the lithiation/delithiation activity and thereby anatase-TiO2 can contribute some electrochemical capacity to the electrode. The electrochemical reactions can be expressed by the following Eqs. (4) and (5), respectively. þ
Li4 Ti5 O12 þ xLi þ xe $ Li4þx Ti5 O12 ðx ¼ 0 3Þ þ
Fig. 6. CV profiles of TiO2-coated LTO electrode at the scan rate of 0.1 mV s1 between 1.0 and 2.5 V.
CTAB. Hence, at this stage the growth would be easier along the axis of the rod. Finally, during calcination LiOH continues to react with the surrounding anatase-TiO2 to form LTO (Eq. (3)). While the outmost layer of anatase-TiO2 could remain mainly due to
TiO2 þ xLi þ xe $ Lix TiO2 ðx ¼ 0:5 1Þ
ð4Þ
ð5Þ
Fig. 7(a) displays the galvanostatic charge and discharge behaviors of the TiO2-coated LTO at different current densities. Two pairs of voltage plateaus can be observed on charge–discharge curves at 0.0175 A g1. However, due to the polarization and unsaturated insertion and extrusion of Li+ at high current densities, the voltage plateaus of anatase-TiO2 disappear. The TiO2-coated LTO gives remarkably high discharge capacities of 233.8 mA h g1 at
Fig. 7. Electrochemical performances of TiO2-coated LTO and LTO: (a) 1st discharge–charge curves at different current densities; (b) rate capability; (c) cyclic stability at 1.75 A g1; (d) EIS profiles.
42
M.-m. Chen et al. / Journal of Alloys and Compounds 601 (2014) 38–42
0.0175 A g1 and 202.4 mA h g1 at 0.175 A g1, which surpass the theoretical capacity of LTO (175 mA h g1) owing to the capacity contribution of anatase-TiO2 (336 mA h g1). High-rate performances of LTO based electrodes have been reported in the recent literatures [27–31]. In this paper, the TiO2-coated LTO also exhibits excellent rate performance. Fig. 7(b) compares rate capabilities of the TiO2-coated LTO and LTO at different current densities. The discharge capacities of TiO2-coated LTO decrease much more slowly than that of LTO from 0.175 A g1 to 7 A g1. Even at 7 A g1, the TiO2-coated LTO still delivers a capacity of 108.9 mA h g1, which is much higher than that of LTO (ca. 56 mA h g1). It is worth mentioning that a specific capacity of 206.1 mA h g1 (88.2% of that of the first cycle at 0.0175 A g1) is recovered when the current density is finally reduced back to 0.0175 A g1, which suggests good reversibility of the TiO2-coated LTO electrode. The satisfactory performances of the TiO2-coated LTO electrode could be attributed to the following two factors. First, within the voltage range of LTO lithiation, anatase-TiO2 is an active Li host with high theoretical capacity. Second, the electronic conductivity of the electrode can be improved by the anatase-TiO2 coating layer. Li ions could insert into TiO2 during the initial discharge process at 1.7 V, leading to the formation of LixTiO2. The in situ formed LixTiO2 would enhance the overall conductivity due to the existence of Ti3+ [32]. Compared to LTO, the TiO2-coated LTO exhibits higher reversible capacity and better cyclic stability, as seen in Fig. 7(c). After 100 cycles at 1.75 A g1, the TiO2-coated LTO retains 96.0% of its initial capacity, while LTO presents only 84.5%. These results demonstrate superior structural stability and electrochemical reversibility of this TiO2-coated LTO electrode. Electrochemical impedance spectroscopy (EIS) measurements of the TiO2-coated LTO and LTO electrodes were performed, and the results are shown in Fig. 7(d). Each plot consists of one semicircle at high frequency and a straight line at low frequency. The diameter of the semicircle is mainly related to the surface charge-transfer resistance which was calculated as about 32 X and 65 X for the TiO2-coated LTO and LTO, respectively, suggesting that the anatase-TiO2 coating layer could enable much easier charge transfer at the electrode and electrolyte interface. Thus, the in situ anatase-TiO2 coating is considered a key factor in improving the rate capability and cyclic stability of the TiO2-coated LTO. He et al. [12] ever reported that during charge–discharge cycles, bare LTO will gas caused by the interfacial reactions between bare LTO and its surrounding alkyl carbonate solvents. Upon this, it is a rational assumption that an inert barrier between LTO and alkyl carbonate electrolyte could make a difference. In He is research, when gassing stopped, a very thin layer of anatase-TiO2 was found on the outmost surface of LTO, indicating anatase-TiO2 maybe chemically-inert when facing with such kind of electrolyte. Therefore, we have reasons to believe that the anatase-TiO2 coating layer in our sample would improve its surface stability by preventing the electrode from gassing and simultaneously enhance the high-rate performance of LTO electrodes. Of course, more detailed confirmation is still needed. 4. Conclusion LTO nanorod coated by anatase-TiO2 has been in situ prepared via a microemulsion-assisted hydrothermal method. The exits of
water-in-oil microemulsion plays a key role in fabrication of the novel structure. Anatase-TiO2 acts as a well nanocoating layer, which can both contribute some capacity and give an enhanced performance to the Li4Ti5O12 electrode. Therefore, the TiO2-coated LTO exhibits high specific capacity, excellent rate capability and a significantly enhanced cyclic performance. This TiO2-coated LTO would be a promising anode material for highly efficient LIBs and other critical energy storages. Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC 51372168), the Natural Science Foundation of Tianjin City of China (Key Program, No. 12JCZDJC27000). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[25] [26] [27] [28] [29] [30] [31] [32]
A. Manthiram, J. Phys. Chem. Lett. 2 (2011) 176–184. M. Armand, J.M. Tarascon, Nature 451 (2008) 652–657. S.W. Han, J.H. Ryu, J. Jeong, D.H. Yoon, J. Alloys Comp. 570 (2013) 144–149. A.S. Prakash, P. Manikandan, K. Ramesha, M. Sathiya, J.M. Tarascon, A.K. Shukla, Chem. Mater. 22 (2010) 2857–2863. X.F. Guo, C.Y. Wang, M.M. Chen, Mater. Lett. 83 (2012) 39–41. C.Y. Lin, J.G. Duh, J. Alloys Comp. 509 (2011) 3682–3685. L. Aldon, P. Kubiak, M. Womes, J.C. Jumas, J. Olivier-Fourcade, J.L. Tirado, J.I. Corredor, C.P. Vicente, Chem. Mater. 16 (2004) 5721–5725. A. Guerfi, P. Charest, K. Kinoshita, M. Perrier, K. Zaghib, J. Power Sources 126 (2004) 163–168. C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Electrochem. Commun. 9 (2007) 1107–1112. B.H. Li, C.P. Han, Y.B. He, C. Yang, H.D. Du, Q.H. Yang, F.Y. Kang, Energy Environ. Sci. 5 (2012) 9595–9602. J. Wang, L.F. Shen, H.S. Li, B. Ding, P. Nie, H. Dou, X.G. Zhang, J. Alloys Comp. 587 (2014) 171–176. Y.B. He, B.H. Li, M. Liu, C. Zhang, W. Lv, C. Yang, J. Li, H.D. Du, B. Zhang, Q.H. Yang, J.K. Kim, F.Y. Kang, Sci. Rep. 2 (2012) 913. K. Wu, J. Yang, Y. Liu, Y. Zhang, C.Y. Wang, J.M. Xu, F. Ning, D.Y. Wang, J. Power Sources 237 (2013) 285–290. Z.M. Liu, N.Q. Zhang, K.N. Sun, J. Mater. Chem. 22 (2012) 11688–11693. G.Y. Liu, H.Y. Wang, G.Q. Liu, Z.Z. Yang, B. Jin, Q.C. Jiang, J. Power Sources 220 (2012) 84–88. C.Y. Lin, Y.R. Jhan, J.G. Duh, J. Alloys Comp. 509 (2011) 6965–6968. Y. Ma, B. Ding, G. Ji, J.Y. Lee, ACS Nano 7 (2013) 10870–10878. N. Li, G.M. Zhou, F. Li, L. Wen, H.M. Cheng, Adv. Funct. Mater. 23 (2013) 5429– 5435. H.J. Luo, L.F. Shen, K. Rui, H.S. Li, X.G. Zhang, J. Alloys Comp. 572 (2013) 37– 42. C.T. Hsieh, B.S. Chang, J.Y. Lin, R.S. Juang, J. Alloys Comp. 513 (2012) 393– 398. M. Inagaki, Carbon 50 (2012) 3247–3266. X.F. Guo, C.Y. Wang, M.M. Chen, J.Z. Wang, J.M. Zheng, J. Power Sources 214 (2012) 107–112. Y.Q. Wang, L. Gu, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, J. Am. Chem. Soc. 134 (2012) 7874–7879. S. Vaidya, P. Rastogi, S. Agarwal, S.K. Gupta, T. Ahmad, A.M. Antonelli, K.V. Ramanujachary, S.E. Lofland, A.K. Ganguli, J. Phys. Chem. C 112 (2008) 12610– 12615. L.L. Li, Y. Chu, Y. Liu, L.H. Dong, L. Huo, F.Y. Yang, Mater. Lett. 60 (2006) 2138– 2142. M.M. Rahman, J.Z. Wang, M.F. Hassan, D. Wexler, H.K. Liu, Adv. Energy Mater. 1 (2011) 212–220. B. Zhang, Y.S. Liu, Z.D. Huang, S. Oh, Y. Yu, Y.W. Maib, J.K. Kim, J. Mater. Chem. 22 (2012) 12133–12140. X. Li, S.H. Tang, M.Z. Qu, P.X. Huang, W. Li, Z.L. Yu, J. Alloys Comp. 588 (2014) 17–24. G.Y. Liu, H.Y. Wang, G.Q. Liu, Z.Z. Yang, B. Jin, Q.C. Jiang, Electrochim. Acta 87 (2013) 218–223. H. Song, S.W. Yun, H.H. Chun, M.G. Kim, K.Y. Chung, H.S. Kim, B.W. Cho, Y.T. Kim, Energy Environ. Sci. 5 (2012) 9903–9913. B.F. Wang, J. Cao, Y. Liu, T. Zeng, L. Li, J. Alloys Comp. 587 (2014) 21–25. G.D. Du, N. Sharma, V.K. Peterson, J.A. Kimpton, D. Jia, Z. Guo, Adv. Funct. Mater. 21 (2011) 3990–3997.