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carbon coated lithium sodium titanate as advanced anode material for lithium-ion batteries
Journal of Power Sources 259 (2014) 177e182
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Copper/carbon coated lithium sodium titanate as advanced anode material for lithium-ion batteries Kaiqiang Wu, Xiaoting Lin, Lianyi Shao, Miao Shui*, Nengbing Long, Yuanlong Ren, Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Li2Na2Ti6O14 is prepared by a solid state reaction. Cu/C layer is coated on Li2Na2Ti6O14 by a thermal decomposition. Cu/C coating layer improves the electrochemical properties of Li2Na2Ti6O14. Coreeshell Li2Na2Ti6O14@Cu/C shows a reversible capacity of 120.3 mAh g 1 .
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2014 Received in revised form 14 February 2014 Accepted 25 February 2014 Available online 12 March 2014
Coreeshell Li2Na2Ti6O14@Cu/C is prepared by a preliminary formation of Li2Na2Ti6O14 by solid state reaction and a following coating process with Cu/C layer by thermal decomposition. The amorphous Cu/C coating layer reveals a thickness of 5 nm on the surface of Li2Na2Ti6O14, which improves the electronic conductivity and charge transfer rate of active materials. As a result, Li2Na2Ti6O14@Cu/C shows lower electrochemical polarization and quicker kinetic behavior compared to bare Li2Na2Ti6O14. Cycled at 50 mA g1, Li2Na2Ti6O14@Cu/C can deliver a reversible capacity of 120.3 mAh g1 after 50 cycles, which is much higher than the value of 96.8 mAh g1 obtained by Li2Na2Ti6O14. Even kept at 400 mA g1, a reversible lithium storage capacity of 76.3 mAh g1 can be delivered by Li2Na2Ti6O14@Cu/C. The improved electrochemical properties of Li2Na2Ti6O14 are attributed to the electronic conductive Cu/C coating layer on the surface. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Lithium sodium titanate Coating Coreeshell structure Anode material Lithium-ion batteries
1. Introduction The eco-friendly lithium-ion batteries have been widely used in portable equipments to meet the demands for electrochemicalenergy-storage systems with high rate capacity and long cycling
* Corresponding authors. Tel.: þ86 574 87600787; fax: þ86 574 87609987. E-mail addresses: [email protected] (M. Shui), [email protected], [email protected] (J. Shu). http://dx.doi.org/10.1016/j.jpowsour.2014.02.097 0378-7753/Ó 2014 Elsevier B.V. All rights reserved.
life since they was firstly introduced by SONY in 1991 [1e5]. In the past decades, carbonaceous materials, such as graphite and hard carbon, have dominated the main market for anode materials in lithium-ion batteries [6e8]. However, the unsolvable safety problem about lithium dendrite growth on the surface of carbon anodes is still existed [9e13]. Therefore, it is necessary to develop new anode materials with high working potential, improved cycling performance and outstanding thermal stability to substitute conventional carbonaceous materials. In recent years, titanium-based compounds have been actively studied as a typically safe anode material in lithium-ion batteries
178
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[14e18]. TiO2 with low cost, environmentally friendliness and natural abundance has attracted increasing commercial interests [19]. However, the low working potential makes TiO2 suffer from the electrolyte irreversible decomposition during lithiation process, leads to poor cycle life and then hinder further applications for lithium-ion batteries [20e22]. Spinel Li4Ti5O12 is a promising anode material with high safety due to the absence of solid electrolyte interphase (SEI) film [23e27]. Furthermore, its discharge platform at about 1.55 V, which is higher than the reduction potential of most organic electrolytes, can restrain the origination of SEI film and avoid the formation of metallic lithium dendrite. However, bare Li4Ti5O12 suffering from low electronic conductivity (1013 S cm1) shows poor lithium storage properties at high rates, which has huge negative effect on its commercial applications [28,29]. In order to overcome this problem, plenty of improvements have been focused on improving the electronic conductivity and electrochemical properties of Li4Ti5O12 by developing nanostructures, using various doping and coating techniques [30e39]. Li2Na2Ti6O14 with its low discharge platform (1.3 V) has been regarded as a novel promising anode material to replace Li4Ti5O12 in recent years. Unfortunately, it also suffers from low electronic conductivity which hinders its further research. In this work, a new composite Li2Na2Ti6O14@Cu/C is synthesized by a preliminary formation of Li2Na2Ti6O14 by solid state reaction and a following coating process with Cu/C layer by thermal decomposition. Electrochemical results show that the coating of Cu/C composite layer can enhance the electronic conductivity and contribute greatly to the reversible specific capacity, cycling calendar life and rate performance of Li2Na2Ti6O14. 2. Experimental Li2Na2Ti6O14 was synthesized by a solid state method. Stoichiometric amounts of CH3COOLi$2H2O (Aladdin Chemistry), CH3COONa$3H2O (Aladdin Chemistry) and TiO2 (Aladdin Chemistry) were mixed with oxalic acid in the preparation process. The final molar ratio of Li:Na:Ti:oxalic acid was 1:1:3:1 and pretreated by planetary ball milling in ethanol for 15 h. The obtained precursor slurry was dried at 80 C for 24 h and then calcined at 600 C for 10 h. Li2Na2Ti6O14@Cu/C composite was prepared by the following steps as shown in Fig. 1. 500 mg of Li2Na2Ti6O14 was dispersed in 100 mL deionized water and then 16 mg of Cu(CH3COO)2$H2O (Aladdin Chemistry) was dissolved in this suspension under vigorous stirring at 80 C. In the next step, 250 mg of glucose
Fig. 1. Schematic procedure of Li2Na2Ti6O14@Cu/C.
(C6H6O6) was added into the mixed solution and then kept stirring for 2 h. As deionized water evaporated, the solution was dried at 120 C in a vacuum oven and the dried powder was then calcined at 400 C for 2 h in N2 atmosphere to get the Li2Na2Ti6O14@Cu/C sample. The identification of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C phases was performed by Bruker AXS D8 Focus X-ray diffraction (XRD) instrument with Cu Ka radiation (l ¼ 0.15406 nm). Samples were observed with a scan angle range from 10 to 80 . The surface morphologies, element composition and crystal structure of Li2Na2Ti6O14 particles before and after coating were evaluated by Hitachi S3400 scanning electron microscopy (SEM), Oxford Inca energy dispersive spectrometry (EDS) and JEOL JEM-2010 high resolution transmission electron microscopy (HRTEM). For electrochemical test, the working electrode was prepared by dispersing a mixture of as-prepared active material, carbon black and polyvinyldifluoride binder with a weight ratio of 8:1:1 in Nmethyl-2-pyrrolidinone to form homogeneous slurry and then pasting the slurry on copper foil to form a thin film. This film was dried at 120 C in a vacuum oven for 12 h and then cut into discs with a diameter of 15 mm. In the coin-type cells, metallic lithium foils were used as the counter and reference electrodes, electrolyte was 1 mol L1 solution of LiPF6 dissolved in ethylene carbonatedimethyl carbonate (1:1 in volume), and Whatman glass fiber was used as separator. Chargeedischarge properties of as-obtained samples were measured with a current density of 50 mA g1 by multi-channel Land battery test system. Cyclic voltammograms (CVs) were carried out between 1.0 and 3.0 V by CHI 1000B electrochemical workstation. Electrochemical impedance spectroscopy (EIS) analysis was carried out by CHI 660D electrochemical workstation with the frequency range of 105e102 Hz. 3. Results and discussion The XRD patterns of as-prepared Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C are shown in Fig. 2. All the diffraction peaks of XRD patterns can be indexed to the orthorhombic structure of Li2Na2Ti6O14 (JCPDS card No. 52-0690), which are in accordance with the previous results [40,41]. No impurity can be observed in the XRD patterns. It indicates that copper and carbon coating does not change the structure of Li2Na2Ti6O14. The lattice parameters of Li2Na2Ti6O14 are a ¼ 16.473, b ¼ 5.736 and c ¼ 11.228 A. The carbonization of glucose and the reduction formation of copper from copper acetate result in the color of Li2Na2Ti6O14 changing from white to gray as shown in Fig. 2. However, no featured diffraction peaks of copper and carbon in the XRD pattern of Li2Na2Ti6O14@Cu/C are observed due to their low content and
Fig. 2. XRD patterns of (a) Li2Na2Ti6O14 and (b) Li2Na2Ti6O14@Cu/C.
K. Wu et al. / Journal of Power Sources 259 (2014) 177e182
179
Fig. 3. (a, b) SEM images of Li2Na2Ti6O14; (c, d) SEM images and corresponding EDS pattern of Li2Na2Ti6O14@Cu/C.
amorphous state, which is similar to the carbon or copper coating layer on Li4Ti5O12 [31,34,38,39]. SEM is performed to provide the evidence for the change of surface morphology before and after copper and carbon coating.
Fig. 3(a) and (b) shows the SEM images of bare Li2Na2Ti6O14. The pristine Li2Na2Ti6O14 powders display irregular particle shape with average particle size of 0.2e0.4 mm. Moreover, the surface of bare Li2Na2Ti6O14 particles is smooth before coating. Fig. 3(c) and (d)
Fig. 4. TEM images of (a, c) Li2Na2Ti6O14 and (b, d) Li2Na2Ti6O14@Cu/C.
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Fig. 5. (a) Cyclic voltammograms, (b) chargeedischarge curves, (c) cycling property and (d) EIS patterns of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C.
shows the surface morphology of Li2Na2Ti6O14@Cu/C. After copper and carbon coating, the surface of Li2Na2Ti6O14@Cu/C becomes rough. Furthermore, EDS observation confirms that this rough layer is Cu/C coating film as the image inserted in Fig. 3(c). Beside, the low relative intensities of copper and carbon elements demonstrate the low contents in as-prepared Li2Na2Ti6O14@Cu/C, which can also explain the disappearance of diffraction peaks for copper and carbon in the XRD pattern after coating. In order to further investigate the morphology evolution of Li2Na2Ti6O14 before and after Cu/C coating, TEM images of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C are shown in Fig. 4. It can be clearly seen that the bare Li2Na2Ti6O14 particles show quite smooth surface as shown in Fig. 4(a). For comparison, a rough thin layer can be observed on the surface of Li2Na2Ti6O14@Cu/C (Fig. 4(b)), which is consistent with the analysis result of SEM and EDS images. Moreover, the HRTEM images in Fig. 4(c) and (d) make a clear view that an amorphous Cu/C layer with thickness of about 5 nm is coated on the surface of Li2Na2Ti6O14 particles. The existence of Cu/C coating layer provides a conductive three-dimensional network between Li2Na2Ti6O14 particles, which can effectively improve the rate property of Li2Na2Ti6O14. Fig. 5(a) expresses the initial three cycles of cyclic voltammograms for bare Li2Na2Ti6O14 (full line) and Li2Na2Ti6O14@Cu/C (dotted line) with a scan rate of 0.1 mV s1. It can be clearly observed that a pair of characteristic redox peaks can be observed at 1.18 and 1.37 V for bare Li2Na2Ti6O14. In contrast, Li2Na2Ti6O14@Cu/C reveals the reduction peak at 1.20 V and the oxidization peak at 1.35 V. It indicates that Li2Na2Ti6O14@Cu/C shows lower polarization (0.15 V) than that (0.19 V) of bare Li2Na2Ti6O14. Furthermore, Li2Na2Ti6O14@Cu/C shows higher peak current of about 0.3 mA compared to that (only 0.17 mA) of bare Li2Na2Ti6O14. It suggests that Li2Na2Ti6O14@Cu/C has better kinetic characteristics. The galvanostatic chargeedischarge curves of bare Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C at the current density of 50 mA g1 between 1.0 and 3.0 V are shown in Fig. 5(b). Two samples show
similar long dischargeecharge plateaus. For bare Li2Na2Ti6O14, the discharge and charge plateaus can be observed at around 1.26 and 1.32 V, respectively. For comparison, the working plateaus appear at 1.27 and 1.31 V for Li2Na2Ti6O14@Cu/C. Thus, Li2Na2Ti6O14@Cu/C shows lower chargeedischarge polarization, which is consistent with the result of CVs. Moreover, Li2Na2Ti6O14@Cu/C expresses higher discharge and charge specific capacities (140.1 and 122.5 mAh g1) than bare Li2Na2Ti6O14 (only 111 and 96.5 mAh g1). Besides, a comparison of reversible capacity and cycling calendar life before and after Cu/C coating is shown in Fig. 5(c). After 50 cycles, Li2Na2Ti6O14@Cu/C can deliver a reversible charge capacity of 120.3 mAh g1. In contrast, only 96.8 mAh g1 can be obtained for the bare Li2Na2Ti6O14 after 50 cycles. This further confirms the advantage of Cu/C coating for Li2Na2Ti6O14. In order to understand the improved electrochemical performance through Cu/C coating, EIS measurements on bare Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C are investigated. As revealed in Fig. 5(d), both EIS patterns show that each curve consists of a depressed semicircle in the high frequency region and a straight line in the low frequency region. By using the equivalent circuit as inserted in Fig. 5(d), EIS spectra are simulated and the calculated data are listed in Table 1. In the equivalent circuit, Rs is solution/ contact resistance, Rct denotes charge transfer resistance, CPEct is constant phase element and W means Warburg diffusion impedance. It is obvious that Li2Na2Ti6O14@Cu/C not only shows lower solution/contact resistance (19.195 U) but also exhibits lower charge transfer resistance (142.814 U) than those of the pristine Table 1 The electrochemical parameters calculated from EIS patterns using an equivalent circuit. Sample
Rs (U)
Rct (U)
CPE (mF)
W ( U)
Li2Na2Ti6O14 Li2Na2Ti6O14@Cu/C
22.992 19.195
311.223 142.814
0.00182 0.00212
0.918 0.929
K. Wu et al. / Journal of Power Sources 259 (2014) 177e182
181
Fig. 6. (a) Cycling property and (bed) corresponding chargeedischarge curves of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C at different current densities.
Li2Na2Ti6O14. It suggests that Cu/C coating layer improves the electronic conductivity and charge transfer rate of Li2Na2Ti6O14. Similar results have been reported for carbon or copper coated Li4Ti5O12 [31,34,38,39,42]. Therefore, the greatly enhanced lithium storage properties of Li2Na2Ti6O14@Cu/C should be attributed to the Cu/C coating shell on Li2Na2Ti6O14 particles. Fig. 6(a) shows the rate properties of bare Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C. Obvious improvement of rate performance for Li2Na2Ti6O14@Cu/C can be observed with reversible charge capacities of 103.6 mAh g1 at a current density of 100 mA g1, 92.0 mAh g1 at a current density of 200 mA g1 and 76.3 mAh g1 at a current density of 400 mA g1. Viewed from Fig. 6(b)e(d), lower chargeedischarge polarizations for Li2Na2Ti6O14@Cu/C at different current densities (100, 200 and 400 mA g1) result in the improved electrochemical properties of Li2Na2Ti6O14, which is also attributed to the Cu/C coating layer on the surface of Li2Na2Ti6O14. 4. Conclusions In this study, Cu/C composite coated Li2Na2Ti6O14 is synthesized by thermal decomposition of a mixture of Cu(CH3COO)2$H2O and C6H6O6 on Li2Na2Ti6O14. SEM, EDS and TEM images demonstrate an amorphous Cu/C layer with a thickness of 5 nm coating on the surface of Li2Na2Ti6O14. This Cu/C coating layer effectively improves the electronic conductivity and charge transfer rate of Li2Na2Ti6O14. As a result, Li2Na2Ti6O14@Cu/C shows lower electrochemical polarization (0.15 V) and higher peak current (0.3 mA) than those (0.19 V, 0.17 mA) of bare Li2Na2Ti6O14 in the CVs. Chargeedischarge results also show that Li2Na2Ti6O14@Cu/C delivers higher lithium storage capacity than that of bare Li2Na2Ti6O14. Li2Na2Ti6O14@Cu/C reveals the reversible capacities of 120.3 mAh g1 at 50 mA g1, 103.6 mAh g1 at 100 mA g1, 92.0 mAh g1 at 200 mA g1 and 76.3 mAh g1 at 400 mA g1. In contrast, bare Li2Na2Ti6O14 only delivers the charge capacities of 96.8 mAh g1 at 50 mA g1, 84.5 mAh g1 at 100 mA g1, 77.0 mAh g1 at 200 mA g1 and
66.2 mAh g1 at 400 mA g1. The enhanced lithium storage behaviors are contributed to the introduction of Cu/C coating layer on the surface of Li2Na2Ti6O14. Acknowledgments This work is sponsored by National Natural Science Foundation of China (No. 51104092) and National 863 Program (2013AA050901). The work is also supported by K. C. Wong Magna Fund in Ningbo University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Copper/carbon coated lithium sodium titanate as advanced anode material for lithium-ion batteries Kaiqiang Wu, Xiaoting Lin, Lianyi Shao, Miao Shui*, Nengbing Long, Yuanlong Ren, Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Zhejiang Province, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Li2Na2Ti6O14 is prepared by a solid state reaction. Cu/C layer is coated on Li2Na2Ti6O14 by a thermal decomposition. Cu/C coating layer improves the electrochemical properties of Li2Na2Ti6O14. Coreeshell Li2Na2Ti6O14@Cu/C shows a reversible capacity of 120.3 mAh g 1 .
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 January 2014 Received in revised form 14 February 2014 Accepted 25 February 2014 Available online 12 March 2014
Coreeshell Li2Na2Ti6O14@Cu/C is prepared by a preliminary formation of Li2Na2Ti6O14 by solid state reaction and a following coating process with Cu/C layer by thermal decomposition. The amorphous Cu/C coating layer reveals a thickness of 5 nm on the surface of Li2Na2Ti6O14, which improves the electronic conductivity and charge transfer rate of active materials. As a result, Li2Na2Ti6O14@Cu/C shows lower electrochemical polarization and quicker kinetic behavior compared to bare Li2Na2Ti6O14. Cycled at 50 mA g1, Li2Na2Ti6O14@Cu/C can deliver a reversible capacity of 120.3 mAh g1 after 50 cycles, which is much higher than the value of 96.8 mAh g1 obtained by Li2Na2Ti6O14. Even kept at 400 mA g1, a reversible lithium storage capacity of 76.3 mAh g1 can be delivered by Li2Na2Ti6O14@Cu/C. The improved electrochemical properties of Li2Na2Ti6O14 are attributed to the electronic conductive Cu/C coating layer on the surface. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Lithium sodium titanate Coating Coreeshell structure Anode material Lithium-ion batteries
1. Introduction The eco-friendly lithium-ion batteries have been widely used in portable equipments to meet the demands for electrochemicalenergy-storage systems with high rate capacity and long cycling
* Corresponding authors. Tel.: þ86 574 87600787; fax: þ86 574 87609987. E-mail addresses: [email protected] (M. Shui), [email protected], [email protected] (J. Shu). http://dx.doi.org/10.1016/j.jpowsour.2014.02.097 0378-7753/Ó 2014 Elsevier B.V. All rights reserved.
life since they was firstly introduced by SONY in 1991 [1e5]. In the past decades, carbonaceous materials, such as graphite and hard carbon, have dominated the main market for anode materials in lithium-ion batteries [6e8]. However, the unsolvable safety problem about lithium dendrite growth on the surface of carbon anodes is still existed [9e13]. Therefore, it is necessary to develop new anode materials with high working potential, improved cycling performance and outstanding thermal stability to substitute conventional carbonaceous materials. In recent years, titanium-based compounds have been actively studied as a typically safe anode material in lithium-ion batteries
178
K. Wu et al. / Journal of Power Sources 259 (2014) 177e182
[14e18]. TiO2 with low cost, environmentally friendliness and natural abundance has attracted increasing commercial interests [19]. However, the low working potential makes TiO2 suffer from the electrolyte irreversible decomposition during lithiation process, leads to poor cycle life and then hinder further applications for lithium-ion batteries [20e22]. Spinel Li4Ti5O12 is a promising anode material with high safety due to the absence of solid electrolyte interphase (SEI) film [23e27]. Furthermore, its discharge platform at about 1.55 V, which is higher than the reduction potential of most organic electrolytes, can restrain the origination of SEI film and avoid the formation of metallic lithium dendrite. However, bare Li4Ti5O12 suffering from low electronic conductivity (1013 S cm1) shows poor lithium storage properties at high rates, which has huge negative effect on its commercial applications [28,29]. In order to overcome this problem, plenty of improvements have been focused on improving the electronic conductivity and electrochemical properties of Li4Ti5O12 by developing nanostructures, using various doping and coating techniques [30e39]. Li2Na2Ti6O14 with its low discharge platform (1.3 V) has been regarded as a novel promising anode material to replace Li4Ti5O12 in recent years. Unfortunately, it also suffers from low electronic conductivity which hinders its further research. In this work, a new composite Li2Na2Ti6O14@Cu/C is synthesized by a preliminary formation of Li2Na2Ti6O14 by solid state reaction and a following coating process with Cu/C layer by thermal decomposition. Electrochemical results show that the coating of Cu/C composite layer can enhance the electronic conductivity and contribute greatly to the reversible specific capacity, cycling calendar life and rate performance of Li2Na2Ti6O14. 2. Experimental Li2Na2Ti6O14 was synthesized by a solid state method. Stoichiometric amounts of CH3COOLi$2H2O (Aladdin Chemistry), CH3COONa$3H2O (Aladdin Chemistry) and TiO2 (Aladdin Chemistry) were mixed with oxalic acid in the preparation process. The final molar ratio of Li:Na:Ti:oxalic acid was 1:1:3:1 and pretreated by planetary ball milling in ethanol for 15 h. The obtained precursor slurry was dried at 80 C for 24 h and then calcined at 600 C for 10 h. Li2Na2Ti6O14@Cu/C composite was prepared by the following steps as shown in Fig. 1. 500 mg of Li2Na2Ti6O14 was dispersed in 100 mL deionized water and then 16 mg of Cu(CH3COO)2$H2O (Aladdin Chemistry) was dissolved in this suspension under vigorous stirring at 80 C. In the next step, 250 mg of glucose
Fig. 1. Schematic procedure of Li2Na2Ti6O14@Cu/C.
(C6H6O6) was added into the mixed solution and then kept stirring for 2 h. As deionized water evaporated, the solution was dried at 120 C in a vacuum oven and the dried powder was then calcined at 400 C for 2 h in N2 atmosphere to get the Li2Na2Ti6O14@Cu/C sample. The identification of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C phases was performed by Bruker AXS D8 Focus X-ray diffraction (XRD) instrument with Cu Ka radiation (l ¼ 0.15406 nm). Samples were observed with a scan angle range from 10 to 80 . The surface morphologies, element composition and crystal structure of Li2Na2Ti6O14 particles before and after coating were evaluated by Hitachi S3400 scanning electron microscopy (SEM), Oxford Inca energy dispersive spectrometry (EDS) and JEOL JEM-2010 high resolution transmission electron microscopy (HRTEM). For electrochemical test, the working electrode was prepared by dispersing a mixture of as-prepared active material, carbon black and polyvinyldifluoride binder with a weight ratio of 8:1:1 in Nmethyl-2-pyrrolidinone to form homogeneous slurry and then pasting the slurry on copper foil to form a thin film. This film was dried at 120 C in a vacuum oven for 12 h and then cut into discs with a diameter of 15 mm. In the coin-type cells, metallic lithium foils were used as the counter and reference electrodes, electrolyte was 1 mol L1 solution of LiPF6 dissolved in ethylene carbonatedimethyl carbonate (1:1 in volume), and Whatman glass fiber was used as separator. Chargeedischarge properties of as-obtained samples were measured with a current density of 50 mA g1 by multi-channel Land battery test system. Cyclic voltammograms (CVs) were carried out between 1.0 and 3.0 V by CHI 1000B electrochemical workstation. Electrochemical impedance spectroscopy (EIS) analysis was carried out by CHI 660D electrochemical workstation with the frequency range of 105e102 Hz. 3. Results and discussion The XRD patterns of as-prepared Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C are shown in Fig. 2. All the diffraction peaks of XRD patterns can be indexed to the orthorhombic structure of Li2Na2Ti6O14 (JCPDS card No. 52-0690), which are in accordance with the previous results [40,41]. No impurity can be observed in the XRD patterns. It indicates that copper and carbon coating does not change the structure of Li2Na2Ti6O14. The lattice parameters of Li2Na2Ti6O14 are a ¼ 16.473, b ¼ 5.736 and c ¼ 11.228 A. The carbonization of glucose and the reduction formation of copper from copper acetate result in the color of Li2Na2Ti6O14 changing from white to gray as shown in Fig. 2. However, no featured diffraction peaks of copper and carbon in the XRD pattern of Li2Na2Ti6O14@Cu/C are observed due to their low content and
Fig. 2. XRD patterns of (a) Li2Na2Ti6O14 and (b) Li2Na2Ti6O14@Cu/C.
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Fig. 3. (a, b) SEM images of Li2Na2Ti6O14; (c, d) SEM images and corresponding EDS pattern of Li2Na2Ti6O14@Cu/C.
amorphous state, which is similar to the carbon or copper coating layer on Li4Ti5O12 [31,34,38,39]. SEM is performed to provide the evidence for the change of surface morphology before and after copper and carbon coating.
Fig. 3(a) and (b) shows the SEM images of bare Li2Na2Ti6O14. The pristine Li2Na2Ti6O14 powders display irregular particle shape with average particle size of 0.2e0.4 mm. Moreover, the surface of bare Li2Na2Ti6O14 particles is smooth before coating. Fig. 3(c) and (d)
Fig. 4. TEM images of (a, c) Li2Na2Ti6O14 and (b, d) Li2Na2Ti6O14@Cu/C.
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Fig. 5. (a) Cyclic voltammograms, (b) chargeedischarge curves, (c) cycling property and (d) EIS patterns of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C.
shows the surface morphology of Li2Na2Ti6O14@Cu/C. After copper and carbon coating, the surface of Li2Na2Ti6O14@Cu/C becomes rough. Furthermore, EDS observation confirms that this rough layer is Cu/C coating film as the image inserted in Fig. 3(c). Beside, the low relative intensities of copper and carbon elements demonstrate the low contents in as-prepared Li2Na2Ti6O14@Cu/C, which can also explain the disappearance of diffraction peaks for copper and carbon in the XRD pattern after coating. In order to further investigate the morphology evolution of Li2Na2Ti6O14 before and after Cu/C coating, TEM images of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C are shown in Fig. 4. It can be clearly seen that the bare Li2Na2Ti6O14 particles show quite smooth surface as shown in Fig. 4(a). For comparison, a rough thin layer can be observed on the surface of Li2Na2Ti6O14@Cu/C (Fig. 4(b)), which is consistent with the analysis result of SEM and EDS images. Moreover, the HRTEM images in Fig. 4(c) and (d) make a clear view that an amorphous Cu/C layer with thickness of about 5 nm is coated on the surface of Li2Na2Ti6O14 particles. The existence of Cu/C coating layer provides a conductive three-dimensional network between Li2Na2Ti6O14 particles, which can effectively improve the rate property of Li2Na2Ti6O14. Fig. 5(a) expresses the initial three cycles of cyclic voltammograms for bare Li2Na2Ti6O14 (full line) and Li2Na2Ti6O14@Cu/C (dotted line) with a scan rate of 0.1 mV s1. It can be clearly observed that a pair of characteristic redox peaks can be observed at 1.18 and 1.37 V for bare Li2Na2Ti6O14. In contrast, Li2Na2Ti6O14@Cu/C reveals the reduction peak at 1.20 V and the oxidization peak at 1.35 V. It indicates that Li2Na2Ti6O14@Cu/C shows lower polarization (0.15 V) than that (0.19 V) of bare Li2Na2Ti6O14. Furthermore, Li2Na2Ti6O14@Cu/C shows higher peak current of about 0.3 mA compared to that (only 0.17 mA) of bare Li2Na2Ti6O14. It suggests that Li2Na2Ti6O14@Cu/C has better kinetic characteristics. The galvanostatic chargeedischarge curves of bare Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C at the current density of 50 mA g1 between 1.0 and 3.0 V are shown in Fig. 5(b). Two samples show
similar long dischargeecharge plateaus. For bare Li2Na2Ti6O14, the discharge and charge plateaus can be observed at around 1.26 and 1.32 V, respectively. For comparison, the working plateaus appear at 1.27 and 1.31 V for Li2Na2Ti6O14@Cu/C. Thus, Li2Na2Ti6O14@Cu/C shows lower chargeedischarge polarization, which is consistent with the result of CVs. Moreover, Li2Na2Ti6O14@Cu/C expresses higher discharge and charge specific capacities (140.1 and 122.5 mAh g1) than bare Li2Na2Ti6O14 (only 111 and 96.5 mAh g1). Besides, a comparison of reversible capacity and cycling calendar life before and after Cu/C coating is shown in Fig. 5(c). After 50 cycles, Li2Na2Ti6O14@Cu/C can deliver a reversible charge capacity of 120.3 mAh g1. In contrast, only 96.8 mAh g1 can be obtained for the bare Li2Na2Ti6O14 after 50 cycles. This further confirms the advantage of Cu/C coating for Li2Na2Ti6O14. In order to understand the improved electrochemical performance through Cu/C coating, EIS measurements on bare Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C are investigated. As revealed in Fig. 5(d), both EIS patterns show that each curve consists of a depressed semicircle in the high frequency region and a straight line in the low frequency region. By using the equivalent circuit as inserted in Fig. 5(d), EIS spectra are simulated and the calculated data are listed in Table 1. In the equivalent circuit, Rs is solution/ contact resistance, Rct denotes charge transfer resistance, CPEct is constant phase element and W means Warburg diffusion impedance. It is obvious that Li2Na2Ti6O14@Cu/C not only shows lower solution/contact resistance (19.195 U) but also exhibits lower charge transfer resistance (142.814 U) than those of the pristine Table 1 The electrochemical parameters calculated from EIS patterns using an equivalent circuit. Sample
Rs (U)
Rct (U)
CPE (mF)
W ( U)
Li2Na2Ti6O14 Li2Na2Ti6O14@Cu/C
22.992 19.195
311.223 142.814
0.00182 0.00212
0.918 0.929
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Fig. 6. (a) Cycling property and (bed) corresponding chargeedischarge curves of Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C at different current densities.
Li2Na2Ti6O14. It suggests that Cu/C coating layer improves the electronic conductivity and charge transfer rate of Li2Na2Ti6O14. Similar results have been reported for carbon or copper coated Li4Ti5O12 [31,34,38,39,42]. Therefore, the greatly enhanced lithium storage properties of Li2Na2Ti6O14@Cu/C should be attributed to the Cu/C coating shell on Li2Na2Ti6O14 particles. Fig. 6(a) shows the rate properties of bare Li2Na2Ti6O14 and Li2Na2Ti6O14@Cu/C. Obvious improvement of rate performance for Li2Na2Ti6O14@Cu/C can be observed with reversible charge capacities of 103.6 mAh g1 at a current density of 100 mA g1, 92.0 mAh g1 at a current density of 200 mA g1 and 76.3 mAh g1 at a current density of 400 mA g1. Viewed from Fig. 6(b)e(d), lower chargeedischarge polarizations for Li2Na2Ti6O14@Cu/C at different current densities (100, 200 and 400 mA g1) result in the improved electrochemical properties of Li2Na2Ti6O14, which is also attributed to the Cu/C coating layer on the surface of Li2Na2Ti6O14. 4. Conclusions In this study, Cu/C composite coated Li2Na2Ti6O14 is synthesized by thermal decomposition of a mixture of Cu(CH3COO)2$H2O and C6H6O6 on Li2Na2Ti6O14. SEM, EDS and TEM images demonstrate an amorphous Cu/C layer with a thickness of 5 nm coating on the surface of Li2Na2Ti6O14. This Cu/C coating layer effectively improves the electronic conductivity and charge transfer rate of Li2Na2Ti6O14. As a result, Li2Na2Ti6O14@Cu/C shows lower electrochemical polarization (0.15 V) and higher peak current (0.3 mA) than those (0.19 V, 0.17 mA) of bare Li2Na2Ti6O14 in the CVs. Chargeedischarge results also show that Li2Na2Ti6O14@Cu/C delivers higher lithium storage capacity than that of bare Li2Na2Ti6O14. Li2Na2Ti6O14@Cu/C reveals the reversible capacities of 120.3 mAh g1 at 50 mA g1, 103.6 mAh g1 at 100 mA g1, 92.0 mAh g1 at 200 mA g1 and 76.3 mAh g1 at 400 mA g1. In contrast, bare Li2Na2Ti6O14 only delivers the charge capacities of 96.8 mAh g1 at 50 mA g1, 84.5 mAh g1 at 100 mA g1, 77.0 mAh g1 at 200 mA g1 and
66.2 mAh g1 at 400 mA g1. The enhanced lithium storage behaviors are contributed to the introduction of Cu/C coating layer on the surface of Li2Na2Ti6O14. Acknowledgments This work is sponsored by National Natural Science Foundation of China (No. 51104092) and National 863 Program (2013AA050901). The work is also supported by K. C. Wong Magna Fund in Ningbo University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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