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C as anode material for lithium-ion batteries
Journal of Physics and Chemistry of Solids 72 (2011) 773–778
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis and electrochemical performances of Li4Ti4.95Al0.05O12/C as anode material for lithium-ion batteries Zhenhong Wang a,b, Gang Chen a,n, Jing Xu a, Zushun Lv a, Weiqi Yang c a
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China CNNC China Nuclear Power Engineering Co., Ltd., Zhengzhou Branch, Zhengzhou 450000, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China b
a r t i c l e i n f o
abstract
Article history: Received 29 April 2010 Received in revised form 29 August 2010 Accepted 17 March 2011 Available online 13 April 2011
Spinel compounds Li4Ti5 xAlxO12/C (x ¼ 0, 0.05) were synthesized via solid state reaction in an Ar atmosphere, and the electrochemical properties were investigated by means of electronic conductivity, cyclic voltammetry, and charge–discharge tests at different discharge voltage ranges (0–2.5 V and 1–2.5 V). The results indicated that Al3 þ doping of the compound did not affect the spinel structure but considerably improved the initial capacity and cycling performance, implying the spinel structure of Li4Ti5O12 was more stable when Ti4 þ was substituted by Al3 þ , and Al3 þ doping was beneficial to the reversible intercalation and deintercalation of Li þ . Al3 þ doping improved the reversible capacity and cycling performance effectively especially when it was discharged to 0 V. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction D. Electrochemical properties
1. Introduction Lithium titanate Li4Ti5O12 with a spinel structure has been studied as a promising candidate of negative electrode material for rechargeable lithium-ion batteries [1] due to its excellent cycling performance, safety, and low cost, especially the extreme tolerance to cycling because the volume of the cubic unit cell changes less than 1% [2,3]. It has been identified by Ohzuku et al. [4] as a zero-strain insertion material in the charge–discharge process, which displays a stable discharge plateau at about 1.55 V vs. lithium. However, Li4Ti5O12 with its spinel structure exhibits inadequate electronic and lithium ionic conductivities, which negatively impact its electrochemical performance [5]; therefore, it is necessary to improve its electrochemical properties by any material modifications for high current applications. Several effective ways have been proposed including the synthesis of nano-sized Li4Ti5O12 [6–8], coating a second phase with high electronic conductivity such as Ag, Cu, and carbon [9–13], and doping with alien-valent metal ions such as V5 þ , Ta5 þ , Zr4 þ , Fe3 þ , Ni2 þ , Zn2 þ , Mg2 þ , and Li þ in Li or Ti sites [14–22]. Developing of nanostructure electrode material is the research hotspot for electrode material; various methods have been used to prepare nanostructured electrode material, such as thermal
n
Corresponding author. Tel./fax: þ86 451 86413753. E-mail address: [email protected] (G. Chen).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.03.013
oxidation strategy [23] and Li4Ti5O12 nanorod preparation from hydrothermal treatment [8]. Liu and Xue [24] reviewed recent research activities on hollow nanostructured anode materials for Li-ion batteries, including carbon materials, metals, metal oxides, and their hybrid materials [25,26], which also showed high capacity, high rate capability, and excellent cycling stability. However, it is very significant for a simple synthesis route and low cost method to synthesize the final product of Li4Ti4.95 Al0.05O12/C. Although Huang et al. [27] have reported Al3 þ -doped Li4AlyTi5 yO12 (y¼0, 0.10, 0.15, and 0.25), and their discharge range is between 0.5–2.5 V with the largest reversible capacity under y¼0.15 in the first and second cycles of 195.6 and 173.6 mA h/g, respectively, to the best of our knowledge no investigation was systematically reported on the electrochemical properties of Al3 þ -doped Li4Ti5O12/C at (16d) sites discharged to 0 V. Different from the cathode material, it is valuable to study the electrochemical behaviors of the anode materials at low voltage because the capacity of anode materials at low voltage can offer a higher cell voltage and reversible capacity for lithium-ion batteries [14,28,29]. With these considerations, in this work, we first reported the electrochemical performances of Li4Ti5 xAlxO12/C (x¼ 0, 0.05) anode materials in a broad discharge range at 0.2 C charge– discharge rate. This anode material was synthesized via solid state method in which the formation of conductive carbons and cation-substitution were achieved simultaneously; also the low cost Al(NO3)3 9H2O selected as the Al3 þ source and the sucrose
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
(400)
(111)
40
3. Results and discussion 3.1. Structural characterization of Li4Ti5 xAlxO12/C The XRD patterns of the Li4Ti5 xAlxO12/C (x¼0, 0.05) materials are shown in Fig. 1. All the observed diffraction peaks are in good agreement with JCPDS card No. 26-1198. Any other phases such as TiO2 or Li2TiO3, the most possible impurities, were not detected among the diffraction peaks. The well-defined and sharp peaks in the XRD patterns of the synthesized Li4Ti5 xAlxO12/C (x ¼0, 0.05) samples suggested that they were both single-phase spinel compounds and well crystallized. Close inspection on the XRD patterns revealed that the peaks shifted to high degrees with the doping of Al3 þ . For a clear observation, the peak position variation of (1 1 1) plane was magnified and shown in Fig. 2; several values of 2y and d-value were listed in Table 1. The lattice parameters were calculated through the least square program method from the diffraction data of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C and are found to be ˚ respectively. The decrease of lattice para8.3597 and 8.3572 A, meter indicated that the aluminum-ion was successively substituted for titanium located at (16d) sites in the Li4Ti5O12/C host
60
(711)
80
2 /degree Fig. 1. XRD patterns of (a) pure Li4Ti5O12, (b) Li4Ti5O12/C, and (c) Li4Ti4.95Al0.05O12/C samples.
(111)
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
Intensity/a.u.
The electrodes were prepared by spreading the anode slurry (80 wt% of the active material, 10 wt% of polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone (NMP), and 10 wt% of acetylene black) onto a copper foil followed by drying in vacuum at 120 1C for 12 h. A typical electrode disk contained 2.0–2.5 mg active material. The cells (CR2025) were assembled in an argon filled glove-box using lithium metal foil as the counter electrode. The electrolyte was 1.0 mol dm 3 LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v).
(533) (622) (444)
(440)
(a)
20
2.3. Preparation of lithium-ion batteries
(c) (b)
2.2. Materials characterization The crystalline phase of Li4Ti4.95Al0.05O12/C was identified by X-ray diffraction (XRD, Rigaku D/MAX-RC) with CuKa radiation ˚ 45.0 kV, 50 mA). The cells were galvanostatically (l ¼1.5405 A, charged and discharged on a Neware Battery Testing System over a voltage range of 0–2.5 and 1.0–2.5 V at 0.2 C current rate. Cyclic voltammetry measurements were performed using a CHI604C electrochemical working station at a scanning rate of 0.2 mV s 1. The C rate was calculated from the weight and theoretical capacity of Li4Ti5O12.
(531)
(222)
Li4Ti4.95Al0.05O12/C sample was prepared using a solid state method with stoichiometric mixture of Al(NO3)3 9H2O, TiO2anatase, and Li2CO3 (mole ratio of Li:Ti¼4.3:5) as precursors; after the precursors were mixed in the mortar, adequate amount of sucrose was added, then the reactants were mixed in ethanol solvent and milled in a ball mill for 6 h. The dried mixture was placed in tubular furnace and heated at the rate of 5 1C min 1 to 800 1C in an Ar atmosphere, sintered at 800 1C for 12 h, and then cooled down to room temperature.
Intensity/a.u.
2.1. Materials preparation
(311)
2. Experimental
(511)
adopted as carbon source are expected to show a promising commercial application in lithium-ion batteries.
(331)
774
19
18
17
20
2 /degree Fig. 2. Enlarged XRD patterns of (1 1 1) peaks of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples.
Table 1 2y and d-value of synthesized Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples. (h k l)
(1 1 1) (3 1 1) (4 0 0) (4 4 0) (4 4 4) (7 1 1)
d-value
2y
Li4Ti5O12/C
Li4Ti4.95Al0.05O12/C
Li4Ti5O12/C
Li4Ti4.95Al0.05O12/C
4.8438 2.5238 2.0915 1.4784 1.2066 1.1705
4.8385 2.5224 2.0915 1.4780 1.2064 1.1701
18.300 35.540 43.220 62.800 79.340 82.300
18.320 35.560 43.220 62.820 79.360 82.340
structure [14]; this slight change was ascribable to the smaller ˚ than Ti4 þ ion (0.605 A) ˚ [22,30]. size of Al3 þ ion (0.535 A) Fig. 3 exhibits the SEM images of these materials. It is clear that all of the prepared powders have the uniform, nearly cubic structure morphology with narrow size distribution, which is less than 0.5 mm. Thus, during thermal decomposition it prevented phase separation and lead to the formation of homogeneous-sized
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
775
Fig. 3. SEM images of Li4Ti5 xAlxO12/C materials (a1, a2) x¼ 0; (b1, b2) x ¼0.05.
Table 2 Electronic conductivities of prepared samples.
Fig. 4. EDS for element analyses of Li4Ti4.95Al0.05O12/C.
particles. Very small particles were found to be highly dispersed on the coated Li4Ti4.95Al0.05O12 particles as shown in the SEM images, which strongly proved that the carbon was coated on the surface of the spinel particles. In order to further confirm the content and composition of the prepared Li4Ti4.95Al0.05O12/C, energy dispersive X-ray spectroscopy (EDS) for element analyses was performed and the results are shown in Fig. 4; the Al element could be clearly observed on the surface of the surface-tread Li4Ti4.95Al0.05O12/C. The intrinsic conductivity of semiconductor materials could be improved by introducing impurity ions, if the valence state of impurity ion was different with the substituted ion, extra charges
Sample
k (S cm 1)
Li4Ti5O12 Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
8.2 10 8 7.9 10 6 1.1 10 5
will be brought to the semiconductor material, and these charges should be balanced by other ions with the opposite charge or counteracted by formation of vacancy, in order to keep electroneutrality of the whole crystal. When Ti4 þ located at (16d) site was substituted by Al3 þ , oxygen vacancy will be produced 0 according to the charge neutrality condition 2½AlTi ¼ ½V::O , resulting in the distortion of crystal and lattice defects, therefore the increase in the conductivity of the crystal material. Electronic conductivities were employed to investigate the effects of Al3 þ substitution for Ti4 þ , the electronic conductivities of typical Li4Ti5O12, Li4Ti5O12/C, and Li4Ti4.95Al0.05O12/C powders were measured by the four-electrode method, and the data are listed in Table 2. The conductivity of the Li4Ti5O12 with a carbon coating on the surface can reach 7.9 10 6 S cm 1, about a factor of 100 higher than that of Li4Ti5O12 [22], Li4Ti4.95Al0.05O12/C showed the highest electronic conductivity among all samples, which was almost three orders of magnitude higher than that of pure sample, reaching to 1.1 10 5 S cm 1. These electronic conductivity results are consistent with the XRD results discussed above. Lithium insertion and extraction in Li4Ti5O12/C electrodes were accompanied by electron transfer not only on the particle surface but also inside the crystals. Because the substitute effect induced the increased semi-conductivity of Li4Ti5O12/C, which enhanced
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
3.2. Electrochemical characterization of Li4Ti5 xAlxO12/C Electrochemical performance of Li4Ti5 xAlxO12/C (x¼0, 0.05) was examined by charge–discharge tests. Fig. 5 shows the initial charge– discharge voltage profiles of bare Li4Ti5O12/C and substitution Li4Ti4.95Al0.05O12/C electrodes at 0.2 C between 0–2.5 and 1–2.5 V vs. Li/Li þ , and the cycling performances are plotted in Fig. 6. It is clear that Li4Ti4.95Al0.05O12/C had a higher initial capacity at different discharge voltage ranges, and a higher cycling performance than that of Li4Ti5O12/C, which could be ascribable to the presence of the doped Al3 þ . The discharge capacity reached 248 mA h g 1 after 50 cycles for the Li4Ti4.95Al0.05O12/C while it decreased to 226 mA h g 1 for the Li4Ti5O12/C discharged to 0 V. In addition, Li4Ti4.95Al0.05O12/C and Li4Ti5O12/C had nearly equivalent discharge capacity after 50 cycles at a voltage range of 1.0–2.0 V, indicating that the improving effect of discharge of Al3 þ -substituted at 1.0 V was not obvious. Al3 þ -substitution improved the reversible capacity and cycling performance effectively especially when it was discharged
3.0 0-2.5 V 2.5 (a)
(b)
Voltage/ V
2.0
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
300 280 260 240 220 0
10
30 20 Cycle number
230
40
50
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
220 210 200 190 180 170 160 150
1.5
140 0
1.0
0.0 0
50
100
150 200 250 Capacity/mAh⋅g-1
300
350
1.0-2.5 V 2.4 (b)
2.1
(a)
1.8 (b)
1.5 (a) 1.2
0
50
100 150 Capacity/mAh⋅g-1
10
20 30 Cycle number
40
50
Fig. 6. Cycling performance of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C discharged to (a) 0 V and (b) 1.0 V.
(b) (a)
0.5
Voltage/ V
320
Capacity/mAh⋅g-1
the electronic conductivity of the electrode materials on the crystal level, electron transfer in Al3 þ -substituted Li4Ti5O12/C would be more facilitated than that in bare Li4Ti5O12/C. Therefore, the substitution samples should demonstrate much better electrochemical performance compared to Li4Ti5O12/C, let alone pure Li4Ti5O12.
Capacity/mAh⋅g-1
776
200
250
Fig. 5. Initial charge–discharge curves of (a) Li4Ti5O12/C and (b) Li4Ti4.95Al0.05O12/C between 0–2.5 and 1.0–2.5 V vs. Li/Li þ .
at 0 V. All the results indicated that Al3 þ -substituted had significant improvement impact on the reversible capacity and cycling stability of the Li4Ti5O12/C material, and this could be attributed to the high electronic conductivity of Al3 þ -substituted Li4Ti5O12/C. The electrochemical behaviors of both samples prepared in this work were characterized by cyclic voltammograms with coin cells. As shown in Fig. 7, the voltage was scanned from 0 to 3 V and then back to 0 V at a rate of 0.2 mV s 1. It can be clearly found that there were two pairs of symmetric redox peaks for each sample in the voltage range of 1.0–2.0 V and below 0.6 V, implying that Al3 þ -doping did not change the electrochemical reaction process of Li4Ti5O12/C in the voltage range of 0–3 V. The sharp and symmetrical peaks appearing at around 1.45 and 1.75 V could be attributed to the redox of Ti4 þ /Ti3 þ . The reduction and oxidation peaks below 0.6 V could be proposed to another change of Ti4 þ to Ti3 þ , which suggested a multistep restoration of Ti4 þ during the discharge period [31]. In addition, values of the CV peaks are listed in Table 3. The potential differences between anodic and cathodic peaks for Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C were 226 and 210 mV, respectively, suggesting that the lower electrode polarization and higher lithium-ion diffusivity in solid state body of sample Li4Ti4.95Al0.05O12/C. These phenomena confirmed that the doping of Al3 þ was beneficial to the reversible
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
2.0
Current/ mA
1.5 1.0 0.5 0.0 -0.5 -1.0 0.5
0.0
1.0
1.5 Potential/ V
2.0
2.5
3.0
Fig. 7. Cyclic voltammograms of cells using Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples as the electrode materials. Table 3 Values of the CV peaks for Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples at the first cycle. Samples
jpa (V)
jpc (V)
Djp (mV)
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
1.701 1.699
1.475 1.489
226 210
-Z''/ohm
600
400
200
(b)
(a)
0 0
200
400
777
Table 4 Impedance parameters of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C electrodes. Sample
Rs (O)
Rct (O)
Zw (S1/2 cm 2)
CPE (mF cm 2)
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
3.189 2.942
372.0 166.5
0.00233 0.00295
5.87 18.48
the anode material of the so-called Warburg diffusion [14,32]. It can be seen clearly that Li4Ti4.95Al0.05O12/C composite displayed much lower charge-transfer resistance than that of Li4Ti5O12/C composite. The electrochemical impedance spectra were fitted using an equivalent circuit, which were depicted in Fig. 8. In this fitted equivalent circuit, Rs and Rct are the solution resistance and charge-transfer resistance, respectively; CPE is the constant phase-angle element, involving double layer capacitance and passivation film capacitance with a rough electrode surface, and Zw represents the Warburg impedance reflecting the solid state diffusion of Li-ions into the bulk of the anode material. The parameters of the equivalent circuit are shown in Table 4. From this table, it is obvious that Rct is much smaller for the Li4Ti4.95Al0.05O12/C than that of Li4Ti5O12/C electrode, which indicates that Al3 þ -substituted could enable the charge-transfer much easily at the electrode/electrolyte interface, and therefore decreases the overall battery internal resistance. The two values of Zw are almost equivalent, indicating that the Li þ diffusion processes in Li4Ti4.95Al0.05O12/C and Li4Ti5O12/C crystal were probably the same. Therefore, the Al3 þ -doping can facilitate the charge-transfer reaction of the Li4Ti5O12 electrodes, and the Al3 þ doped Li4Ti5O12/C had a higher capacity and better cycling stability at different voltage ranges. Compared with other nanostructured Li4Ti5O12 and other metal oxide anode materials such as Li4Ti5O12 nanorods prepared from hydrothermal treatment [8], double-shelled nanocapsules of V2O5 SnO2 composites assembled via a one-pot solution method [33], anisotropic column-shaped porous Co3O4 nanocapsules synthesized via a solvothermal route [34], and heterogeneous nanostructured Li4Ti5O12 synthesized through the solvothermal reaction [35], there were several superiorities of our synthesized material Li4Ti4.95Al0.05O12/C: one was its simple synthesis route and low cost comparable to the benefits of commercial manufacture; the second advantage was the perfect electrochemical performances such as the stable discharge plateau and the so-called zero-strain change. Thus, the synthesized Li4Ti4.95Al0.05O12/C material may find promising applications in lithium-ion batteries.
600
Z'/ohm Fig. 8. AC impedance spectra of (a) Li4Ti5O12/C and (b) Li4Ti4.95Al0.05O12/C electrodes measured at open-circuit potential 3.0 V vs. Li/Li þ .
intercalation and deintercalation of lithium ion in this synthesized anode material. The resistance and the electrochemical reaction properties of electrodes were further examined by AC impedance method. Fig. 8 shows the electrochemical impedance spectroscopy of Li4Ti5O12/C/Li and Al3 þ -doped Li4Ti5O12/C/Li cells at 0.2 C rate upon cycling. The open-circuit voltages were about 3.0 V when the cells were in the initial state. Each curve consisted of a depressed semicircle in high middle frequency region and an oblique straight line in low frequency region. The high middle frequency region of the semicircle represented the electrochemical reaction resistance and the double layer capacity of the electrode, while the low frequency region of the straight line corresponds to the diffusion of the lithium ions into the bulk of
4. Conclusions Well crystallized Li4Ti5 xAlxO12/C (x¼ 0, 0.05) compounds with spinel structure were prepared by a simple, low cost solid state method in which formation of conductive carbons and cation-substitution were achieved simultaneously. Al3 þ -doping of the compound did not affect the spinel structure but considerably improved the electronic conductivity, the initial capacity, and cycling performance; the discharge capacity reached 248 mA h g 1 after 50 cycles for the Li4Ti4.95Al0.05O12/C, while it decreased to 226 mA h g 1 for the Li4Ti5O12/C discharged to 0 V. Li4Ti4.95Al0.05O12/C and Li4Ti5O12/C had a nearly equivalent discharge capacity after 50 cycles at a voltage range of 1.0–2.0 V, indicating that the improving effect of Al3 þ -substituted discharged to 1.0 V was not obvious. Al3 þ -substituted improved the reversible capacity and cycling performance effectively especially when it was discharged to 0 V. The synthesized Li4Ti4.95Al0.05O12/C material may find promising applications in lithium-ion batteries due to its
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simple synthesis route, low cost, and excellent electrochemical performances.
Acknowledgments This work was supported by National Science Foundation of China (Project no. 20871036) and Development Program for Outstanding Young Teachers in Harbin Institute of Technology (HITQNJS. 2009.001). References [1] L. Aldon, P. Kubiak, M. Womes, J.C. Jumas, J. Olivier-Fourcade, J.L. Tirado, J.I. Corredor, C.Perez Vicente, Chem. Mater. 16 (2004) 5721–5725. [2] S.H. Huang, Z.Y. Wen, X.J. Zhu, Z.G. Gu, Electrochem. Commun. 6 (2004) 1093–1097. [3] C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Electrochem. Commun. 9 (2007) 1107–1112. [4] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431–1435. [5] S. Scharner, W. Weppner, P. Schmid-Beurmann, J. Electrochem. Soc. 146 (1999) 857–861. [6] C. Jiang, M. Ichihara, I. Honma, H. Zhou, Electrochim. Acta 52 (2007) 6470–6475. [7] Y.F. Tang, L. Yang, Z. Qiu, J.S. Huang, Electrochem. Commun. 10 (2008) 1513–1516. [8] Y. Li, G.L. Pan, J.W. Liu, X.P. Gao, J. Electrochem. Soc. 156 (2009) A495–A499. [9] S.H. Huang, Z.Y. Wen, J.C. Zhang, X.L. Yang, Electrochim. Acta 52 (2007) 3704–3708. [10] S.H. Huang, Z.Y. Wen, B. Lin, J.d. Han, X.G. Xu, J. Alloys Compd. 457 (2008) 400–403. [11] L.X. Yang, L.J Gao, J. Alloys Compd. 485 (2009) 93–97.
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Synthesis and electrochemical performances of Li4Ti4.95Al0.05O12/C as anode material for lithium-ion batteries Zhenhong Wang a,b, Gang Chen a,n, Jing Xu a, Zushun Lv a, Weiqi Yang c a
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China CNNC China Nuclear Power Engineering Co., Ltd., Zhengzhou Branch, Zhengzhou 450000, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China b
a r t i c l e i n f o
abstract
Article history: Received 29 April 2010 Received in revised form 29 August 2010 Accepted 17 March 2011 Available online 13 April 2011
Spinel compounds Li4Ti5 xAlxO12/C (x ¼ 0, 0.05) were synthesized via solid state reaction in an Ar atmosphere, and the electrochemical properties were investigated by means of electronic conductivity, cyclic voltammetry, and charge–discharge tests at different discharge voltage ranges (0–2.5 V and 1–2.5 V). The results indicated that Al3 þ doping of the compound did not affect the spinel structure but considerably improved the initial capacity and cycling performance, implying the spinel structure of Li4Ti5O12 was more stable when Ti4 þ was substituted by Al3 þ , and Al3 þ doping was beneficial to the reversible intercalation and deintercalation of Li þ . Al3 þ doping improved the reversible capacity and cycling performance effectively especially when it was discharged to 0 V. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. X-ray diffraction D. Electrochemical properties
1. Introduction Lithium titanate Li4Ti5O12 with a spinel structure has been studied as a promising candidate of negative electrode material for rechargeable lithium-ion batteries [1] due to its excellent cycling performance, safety, and low cost, especially the extreme tolerance to cycling because the volume of the cubic unit cell changes less than 1% [2,3]. It has been identified by Ohzuku et al. [4] as a zero-strain insertion material in the charge–discharge process, which displays a stable discharge plateau at about 1.55 V vs. lithium. However, Li4Ti5O12 with its spinel structure exhibits inadequate electronic and lithium ionic conductivities, which negatively impact its electrochemical performance [5]; therefore, it is necessary to improve its electrochemical properties by any material modifications for high current applications. Several effective ways have been proposed including the synthesis of nano-sized Li4Ti5O12 [6–8], coating a second phase with high electronic conductivity such as Ag, Cu, and carbon [9–13], and doping with alien-valent metal ions such as V5 þ , Ta5 þ , Zr4 þ , Fe3 þ , Ni2 þ , Zn2 þ , Mg2 þ , and Li þ in Li or Ti sites [14–22]. Developing of nanostructure electrode material is the research hotspot for electrode material; various methods have been used to prepare nanostructured electrode material, such as thermal
n
Corresponding author. Tel./fax: þ86 451 86413753. E-mail address: [email protected] (G. Chen).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.03.013
oxidation strategy [23] and Li4Ti5O12 nanorod preparation from hydrothermal treatment [8]. Liu and Xue [24] reviewed recent research activities on hollow nanostructured anode materials for Li-ion batteries, including carbon materials, metals, metal oxides, and their hybrid materials [25,26], which also showed high capacity, high rate capability, and excellent cycling stability. However, it is very significant for a simple synthesis route and low cost method to synthesize the final product of Li4Ti4.95 Al0.05O12/C. Although Huang et al. [27] have reported Al3 þ -doped Li4AlyTi5 yO12 (y¼0, 0.10, 0.15, and 0.25), and their discharge range is between 0.5–2.5 V with the largest reversible capacity under y¼0.15 in the first and second cycles of 195.6 and 173.6 mA h/g, respectively, to the best of our knowledge no investigation was systematically reported on the electrochemical properties of Al3 þ -doped Li4Ti5O12/C at (16d) sites discharged to 0 V. Different from the cathode material, it is valuable to study the electrochemical behaviors of the anode materials at low voltage because the capacity of anode materials at low voltage can offer a higher cell voltage and reversible capacity for lithium-ion batteries [14,28,29]. With these considerations, in this work, we first reported the electrochemical performances of Li4Ti5 xAlxO12/C (x¼ 0, 0.05) anode materials in a broad discharge range at 0.2 C charge– discharge rate. This anode material was synthesized via solid state method in which the formation of conductive carbons and cation-substitution were achieved simultaneously; also the low cost Al(NO3)3 9H2O selected as the Al3 þ source and the sucrose
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
(400)
(111)
40
3. Results and discussion 3.1. Structural characterization of Li4Ti5 xAlxO12/C The XRD patterns of the Li4Ti5 xAlxO12/C (x¼0, 0.05) materials are shown in Fig. 1. All the observed diffraction peaks are in good agreement with JCPDS card No. 26-1198. Any other phases such as TiO2 or Li2TiO3, the most possible impurities, were not detected among the diffraction peaks. The well-defined and sharp peaks in the XRD patterns of the synthesized Li4Ti5 xAlxO12/C (x ¼0, 0.05) samples suggested that they were both single-phase spinel compounds and well crystallized. Close inspection on the XRD patterns revealed that the peaks shifted to high degrees with the doping of Al3 þ . For a clear observation, the peak position variation of (1 1 1) plane was magnified and shown in Fig. 2; several values of 2y and d-value were listed in Table 1. The lattice parameters were calculated through the least square program method from the diffraction data of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C and are found to be ˚ respectively. The decrease of lattice para8.3597 and 8.3572 A, meter indicated that the aluminum-ion was successively substituted for titanium located at (16d) sites in the Li4Ti5O12/C host
60
(711)
80
2 /degree Fig. 1. XRD patterns of (a) pure Li4Ti5O12, (b) Li4Ti5O12/C, and (c) Li4Ti4.95Al0.05O12/C samples.
(111)
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
Intensity/a.u.
The electrodes were prepared by spreading the anode slurry (80 wt% of the active material, 10 wt% of polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone (NMP), and 10 wt% of acetylene black) onto a copper foil followed by drying in vacuum at 120 1C for 12 h. A typical electrode disk contained 2.0–2.5 mg active material. The cells (CR2025) were assembled in an argon filled glove-box using lithium metal foil as the counter electrode. The electrolyte was 1.0 mol dm 3 LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v).
(533) (622) (444)
(440)
(a)
20
2.3. Preparation of lithium-ion batteries
(c) (b)
2.2. Materials characterization The crystalline phase of Li4Ti4.95Al0.05O12/C was identified by X-ray diffraction (XRD, Rigaku D/MAX-RC) with CuKa radiation ˚ 45.0 kV, 50 mA). The cells were galvanostatically (l ¼1.5405 A, charged and discharged on a Neware Battery Testing System over a voltage range of 0–2.5 and 1.0–2.5 V at 0.2 C current rate. Cyclic voltammetry measurements were performed using a CHI604C electrochemical working station at a scanning rate of 0.2 mV s 1. The C rate was calculated from the weight and theoretical capacity of Li4Ti5O12.
(531)
(222)
Li4Ti4.95Al0.05O12/C sample was prepared using a solid state method with stoichiometric mixture of Al(NO3)3 9H2O, TiO2anatase, and Li2CO3 (mole ratio of Li:Ti¼4.3:5) as precursors; after the precursors were mixed in the mortar, adequate amount of sucrose was added, then the reactants were mixed in ethanol solvent and milled in a ball mill for 6 h. The dried mixture was placed in tubular furnace and heated at the rate of 5 1C min 1 to 800 1C in an Ar atmosphere, sintered at 800 1C for 12 h, and then cooled down to room temperature.
Intensity/a.u.
2.1. Materials preparation
(311)
2. Experimental
(511)
adopted as carbon source are expected to show a promising commercial application in lithium-ion batteries.
(331)
774
19
18
17
20
2 /degree Fig. 2. Enlarged XRD patterns of (1 1 1) peaks of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples.
Table 1 2y and d-value of synthesized Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples. (h k l)
(1 1 1) (3 1 1) (4 0 0) (4 4 0) (4 4 4) (7 1 1)
d-value
2y
Li4Ti5O12/C
Li4Ti4.95Al0.05O12/C
Li4Ti5O12/C
Li4Ti4.95Al0.05O12/C
4.8438 2.5238 2.0915 1.4784 1.2066 1.1705
4.8385 2.5224 2.0915 1.4780 1.2064 1.1701
18.300 35.540 43.220 62.800 79.340 82.300
18.320 35.560 43.220 62.820 79.360 82.340
structure [14]; this slight change was ascribable to the smaller ˚ than Ti4 þ ion (0.605 A) ˚ [22,30]. size of Al3 þ ion (0.535 A) Fig. 3 exhibits the SEM images of these materials. It is clear that all of the prepared powders have the uniform, nearly cubic structure morphology with narrow size distribution, which is less than 0.5 mm. Thus, during thermal decomposition it prevented phase separation and lead to the formation of homogeneous-sized
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
775
Fig. 3. SEM images of Li4Ti5 xAlxO12/C materials (a1, a2) x¼ 0; (b1, b2) x ¼0.05.
Table 2 Electronic conductivities of prepared samples.
Fig. 4. EDS for element analyses of Li4Ti4.95Al0.05O12/C.
particles. Very small particles were found to be highly dispersed on the coated Li4Ti4.95Al0.05O12 particles as shown in the SEM images, which strongly proved that the carbon was coated on the surface of the spinel particles. In order to further confirm the content and composition of the prepared Li4Ti4.95Al0.05O12/C, energy dispersive X-ray spectroscopy (EDS) for element analyses was performed and the results are shown in Fig. 4; the Al element could be clearly observed on the surface of the surface-tread Li4Ti4.95Al0.05O12/C. The intrinsic conductivity of semiconductor materials could be improved by introducing impurity ions, if the valence state of impurity ion was different with the substituted ion, extra charges
Sample
k (S cm 1)
Li4Ti5O12 Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
8.2 10 8 7.9 10 6 1.1 10 5
will be brought to the semiconductor material, and these charges should be balanced by other ions with the opposite charge or counteracted by formation of vacancy, in order to keep electroneutrality of the whole crystal. When Ti4 þ located at (16d) site was substituted by Al3 þ , oxygen vacancy will be produced 0 according to the charge neutrality condition 2½AlTi ¼ ½V::O , resulting in the distortion of crystal and lattice defects, therefore the increase in the conductivity of the crystal material. Electronic conductivities were employed to investigate the effects of Al3 þ substitution for Ti4 þ , the electronic conductivities of typical Li4Ti5O12, Li4Ti5O12/C, and Li4Ti4.95Al0.05O12/C powders were measured by the four-electrode method, and the data are listed in Table 2. The conductivity of the Li4Ti5O12 with a carbon coating on the surface can reach 7.9 10 6 S cm 1, about a factor of 100 higher than that of Li4Ti5O12 [22], Li4Ti4.95Al0.05O12/C showed the highest electronic conductivity among all samples, which was almost three orders of magnitude higher than that of pure sample, reaching to 1.1 10 5 S cm 1. These electronic conductivity results are consistent with the XRD results discussed above. Lithium insertion and extraction in Li4Ti5O12/C electrodes were accompanied by electron transfer not only on the particle surface but also inside the crystals. Because the substitute effect induced the increased semi-conductivity of Li4Ti5O12/C, which enhanced
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
3.2. Electrochemical characterization of Li4Ti5 xAlxO12/C Electrochemical performance of Li4Ti5 xAlxO12/C (x¼0, 0.05) was examined by charge–discharge tests. Fig. 5 shows the initial charge– discharge voltage profiles of bare Li4Ti5O12/C and substitution Li4Ti4.95Al0.05O12/C electrodes at 0.2 C between 0–2.5 and 1–2.5 V vs. Li/Li þ , and the cycling performances are plotted in Fig. 6. It is clear that Li4Ti4.95Al0.05O12/C had a higher initial capacity at different discharge voltage ranges, and a higher cycling performance than that of Li4Ti5O12/C, which could be ascribable to the presence of the doped Al3 þ . The discharge capacity reached 248 mA h g 1 after 50 cycles for the Li4Ti4.95Al0.05O12/C while it decreased to 226 mA h g 1 for the Li4Ti5O12/C discharged to 0 V. In addition, Li4Ti4.95Al0.05O12/C and Li4Ti5O12/C had nearly equivalent discharge capacity after 50 cycles at a voltage range of 1.0–2.0 V, indicating that the improving effect of discharge of Al3 þ -substituted at 1.0 V was not obvious. Al3 þ -substitution improved the reversible capacity and cycling performance effectively especially when it was discharged
3.0 0-2.5 V 2.5 (a)
(b)
Voltage/ V
2.0
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
300 280 260 240 220 0
10
30 20 Cycle number
230
40
50
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
220 210 200 190 180 170 160 150
1.5
140 0
1.0
0.0 0
50
100
150 200 250 Capacity/mAh⋅g-1
300
350
1.0-2.5 V 2.4 (b)
2.1
(a)
1.8 (b)
1.5 (a) 1.2
0
50
100 150 Capacity/mAh⋅g-1
10
20 30 Cycle number
40
50
Fig. 6. Cycling performance of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C discharged to (a) 0 V and (b) 1.0 V.
(b) (a)
0.5
Voltage/ V
320
Capacity/mAh⋅g-1
the electronic conductivity of the electrode materials on the crystal level, electron transfer in Al3 þ -substituted Li4Ti5O12/C would be more facilitated than that in bare Li4Ti5O12/C. Therefore, the substitution samples should demonstrate much better electrochemical performance compared to Li4Ti5O12/C, let alone pure Li4Ti5O12.
Capacity/mAh⋅g-1
776
200
250
Fig. 5. Initial charge–discharge curves of (a) Li4Ti5O12/C and (b) Li4Ti4.95Al0.05O12/C between 0–2.5 and 1.0–2.5 V vs. Li/Li þ .
at 0 V. All the results indicated that Al3 þ -substituted had significant improvement impact on the reversible capacity and cycling stability of the Li4Ti5O12/C material, and this could be attributed to the high electronic conductivity of Al3 þ -substituted Li4Ti5O12/C. The electrochemical behaviors of both samples prepared in this work were characterized by cyclic voltammograms with coin cells. As shown in Fig. 7, the voltage was scanned from 0 to 3 V and then back to 0 V at a rate of 0.2 mV s 1. It can be clearly found that there were two pairs of symmetric redox peaks for each sample in the voltage range of 1.0–2.0 V and below 0.6 V, implying that Al3 þ -doping did not change the electrochemical reaction process of Li4Ti5O12/C in the voltage range of 0–3 V. The sharp and symmetrical peaks appearing at around 1.45 and 1.75 V could be attributed to the redox of Ti4 þ /Ti3 þ . The reduction and oxidation peaks below 0.6 V could be proposed to another change of Ti4 þ to Ti3 þ , which suggested a multistep restoration of Ti4 þ during the discharge period [31]. In addition, values of the CV peaks are listed in Table 3. The potential differences between anodic and cathodic peaks for Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C were 226 and 210 mV, respectively, suggesting that the lower electrode polarization and higher lithium-ion diffusivity in solid state body of sample Li4Ti4.95Al0.05O12/C. These phenomena confirmed that the doping of Al3 þ was beneficial to the reversible
Z. Wang et al. / Journal of Physics and Chemistry of Solids 72 (2011) 773–778
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
2.0
Current/ mA
1.5 1.0 0.5 0.0 -0.5 -1.0 0.5
0.0
1.0
1.5 Potential/ V
2.0
2.5
3.0
Fig. 7. Cyclic voltammograms of cells using Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples as the electrode materials. Table 3 Values of the CV peaks for Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C samples at the first cycle. Samples
jpa (V)
jpc (V)
Djp (mV)
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
1.701 1.699
1.475 1.489
226 210
-Z''/ohm
600
400
200
(b)
(a)
0 0
200
400
777
Table 4 Impedance parameters of Li4Ti5O12/C and Li4Ti4.95Al0.05O12/C electrodes. Sample
Rs (O)
Rct (O)
Zw (S1/2 cm 2)
CPE (mF cm 2)
Li4Ti5O12/C Li4Ti4.95Al0.05O12/C
3.189 2.942
372.0 166.5
0.00233 0.00295
5.87 18.48
the anode material of the so-called Warburg diffusion [14,32]. It can be seen clearly that Li4Ti4.95Al0.05O12/C composite displayed much lower charge-transfer resistance than that of Li4Ti5O12/C composite. The electrochemical impedance spectra were fitted using an equivalent circuit, which were depicted in Fig. 8. In this fitted equivalent circuit, Rs and Rct are the solution resistance and charge-transfer resistance, respectively; CPE is the constant phase-angle element, involving double layer capacitance and passivation film capacitance with a rough electrode surface, and Zw represents the Warburg impedance reflecting the solid state diffusion of Li-ions into the bulk of the anode material. The parameters of the equivalent circuit are shown in Table 4. From this table, it is obvious that Rct is much smaller for the Li4Ti4.95Al0.05O12/C than that of Li4Ti5O12/C electrode, which indicates that Al3 þ -substituted could enable the charge-transfer much easily at the electrode/electrolyte interface, and therefore decreases the overall battery internal resistance. The two values of Zw are almost equivalent, indicating that the Li þ diffusion processes in Li4Ti4.95Al0.05O12/C and Li4Ti5O12/C crystal were probably the same. Therefore, the Al3 þ -doping can facilitate the charge-transfer reaction of the Li4Ti5O12 electrodes, and the Al3 þ doped Li4Ti5O12/C had a higher capacity and better cycling stability at different voltage ranges. Compared with other nanostructured Li4Ti5O12 and other metal oxide anode materials such as Li4Ti5O12 nanorods prepared from hydrothermal treatment [8], double-shelled nanocapsules of V2O5 SnO2 composites assembled via a one-pot solution method [33], anisotropic column-shaped porous Co3O4 nanocapsules synthesized via a solvothermal route [34], and heterogeneous nanostructured Li4Ti5O12 synthesized through the solvothermal reaction [35], there were several superiorities of our synthesized material Li4Ti4.95Al0.05O12/C: one was its simple synthesis route and low cost comparable to the benefits of commercial manufacture; the second advantage was the perfect electrochemical performances such as the stable discharge plateau and the so-called zero-strain change. Thus, the synthesized Li4Ti4.95Al0.05O12/C material may find promising applications in lithium-ion batteries.
600
Z'/ohm Fig. 8. AC impedance spectra of (a) Li4Ti5O12/C and (b) Li4Ti4.95Al0.05O12/C electrodes measured at open-circuit potential 3.0 V vs. Li/Li þ .
intercalation and deintercalation of lithium ion in this synthesized anode material. The resistance and the electrochemical reaction properties of electrodes were further examined by AC impedance method. Fig. 8 shows the electrochemical impedance spectroscopy of Li4Ti5O12/C/Li and Al3 þ -doped Li4Ti5O12/C/Li cells at 0.2 C rate upon cycling. The open-circuit voltages were about 3.0 V when the cells were in the initial state. Each curve consisted of a depressed semicircle in high middle frequency region and an oblique straight line in low frequency region. The high middle frequency region of the semicircle represented the electrochemical reaction resistance and the double layer capacity of the electrode, while the low frequency region of the straight line corresponds to the diffusion of the lithium ions into the bulk of
4. Conclusions Well crystallized Li4Ti5 xAlxO12/C (x¼ 0, 0.05) compounds with spinel structure were prepared by a simple, low cost solid state method in which formation of conductive carbons and cation-substitution were achieved simultaneously. Al3 þ -doping of the compound did not affect the spinel structure but considerably improved the electronic conductivity, the initial capacity, and cycling performance; the discharge capacity reached 248 mA h g 1 after 50 cycles for the Li4Ti4.95Al0.05O12/C, while it decreased to 226 mA h g 1 for the Li4Ti5O12/C discharged to 0 V. Li4Ti4.95Al0.05O12/C and Li4Ti5O12/C had a nearly equivalent discharge capacity after 50 cycles at a voltage range of 1.0–2.0 V, indicating that the improving effect of Al3 þ -substituted discharged to 1.0 V was not obvious. Al3 þ -substituted improved the reversible capacity and cycling performance effectively especially when it was discharged to 0 V. The synthesized Li4Ti4.95Al0.05O12/C material may find promising applications in lithium-ion batteries due to its
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simple synthesis route, low cost, and excellent electrochemical performances.
Acknowledgments This work was supported by National Science Foundation of China (Project no. 20871036) and Development Program for Outstanding Young Teachers in Harbin Institute of Technology (HITQNJS. 2009.001). References [1] L. Aldon, P. Kubiak, M. Womes, J.C. Jumas, J. Olivier-Fourcade, J.L. Tirado, J.I. Corredor, C.Perez Vicente, Chem. Mater. 16 (2004) 5721–5725. [2] S.H. Huang, Z.Y. Wen, X.J. Zhu, Z.G. Gu, Electrochem. Commun. 6 (2004) 1093–1097. [3] C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Electrochem. Commun. 9 (2007) 1107–1112. [4] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431–1435. [5] S. Scharner, W. Weppner, P. Schmid-Beurmann, J. Electrochem. Soc. 146 (1999) 857–861. [6] C. Jiang, M. Ichihara, I. Honma, H. Zhou, Electrochim. Acta 52 (2007) 6470–6475. [7] Y.F. Tang, L. Yang, Z. Qiu, J.S. Huang, Electrochem. Commun. 10 (2008) 1513–1516. [8] Y. Li, G.L. Pan, J.W. Liu, X.P. Gao, J. Electrochem. Soc. 156 (2009) A495–A499. [9] S.H. Huang, Z.Y. Wen, J.C. Zhang, X.L. Yang, Electrochim. Acta 52 (2007) 3704–3708. [10] S.H. Huang, Z.Y. Wen, B. Lin, J.d. Han, X.G. Xu, J. Alloys Compd. 457 (2008) 400–403. [11] L.X. Yang, L.J Gao, J. Alloys Compd. 485 (2009) 93–97.
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