Preparation and characterization of nanocrystalline Li4Ti5O12 by sol–gel method

Materials Chemistry and Physics 78 (2002) 437–441

Preparation and characterization of nanocrystalline Li4 Ti5O12 by sol–gel method Cheng-min Shen, Xiao-gang Zhang, Ying-ke Zhou, Hu-lin Li∗ Department of Chemistry, Lanzhou University, 730000 Lanzhou, PR China Received 21 December 2000; received in revised form 18 April 2002; accepted 20 May 2002

Abstract Li4 Ti5 O12 based spinel-framework structures are of great interest as negative electrode materials for lithium-ion batteries. We describe here the first synthesis of nano-scale Li4 Ti5 O12 material using sol–gel method, by which excellent phase purity and good stoichiometric inorganic oxides were obtained. According to the X-ray diffraction and transmission electron microscopy analysis, uniformly distributed Li4 Ti5 O12 particles with grain sizes of 100 nm were synthesized. Lithium cells, consisting of Li4 Ti5 O12 cathodes and lithium anodes, showed the initial capacity of 272 mAh g−1 in the range of 1.0–2.5 V. Furthermore, the crystalline structure of Li4 Ti5 O12 didn’t transform during the lithium intercalation and deintercalation process. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Sol–gel growth; Insertion compounds; Electrochemical properties

1. Introduction The lithium-ion rechargeable batteries have been widely used in portable electronic devices such as laptop computers, mobile phones, and camcorders. It is also very promising to use these batteries as power sources for electric vehicles. Despite lithium batteries have obvious advantages over nickel–cadmium and lead–acid cells, i.e. superior energy density and lower toxicity, there are still some safety concerns with the carbon anode and a safer alternative which can offer the same performance would be preferable [1–3]. Future electronic circuitry requests that the power voltage to be progressively reduced from 4 to 3 V or even to a lower range, which may somewhat eliminate the need of high voltage power source and the restriction of choosing low voltage anode. On the other hand, this may open the chance to consider alternative materials operating within the stability window of the electrolyte and with important advantages in terms of cycliability and safety [4]. The lithium transition metal oxides are such interesting examples. The [B2 ]X4 framework of these spinel A[B2 ]X4 compounds is a stable structure and the capacities are very stable with cyclability [5,6]. However, the voltages are rather high for utilization as the cathode electrode. The best performance and lower ∗ Corresponding author. Tel.: +86-931-8912517; fax: +86-931-8912582. E-mail address: [email protected] (H.-l. Li).

voltage is obtained with Li4 Ti5 O12 . Insertion/extraction of Li into/from Li4 Ti5 O12 is known to proceed with little change of lattice dimensions and it is considered as a zero strain insertion compound [5–10]. The corresponding electrochemical processes are: Li4 Ti5 O12 + 3Li+ + 3e

Discharge




Charge

Li7 Ti5 O12

E∼ = −1.5 V

In most studies, Li4 Ti5 O12 has been synthesized using solid-state reaction that involves the mechanical mixing of oxides and/or carbonates, firing at higher temperature (about 800–1100 ◦ C), and then further grinding [5,6,11]. These synthetic conditions (for example, sintering time and temperature), which require long-range diffusion of the reactants, may result in non-homogeneity, irregular morphology, large particle sizes, broad particle size distribution, and poor control of stoichiomety. In order to achieve good efficiency of Li utilization at high current rates and reliability of lithium secondary batteries, a sol–gel method has been introduced. It is a desirable method to obtain anode or cathode materials with good homogeneity, uniform morphology, and narrow size distribution [12]. However, studies on the preparation of Li4 Ti5 O12 nanocrystalline by solution–chemical method are still scarce. In this paper, we report the first synthesis of Li4 Ti5 O12 nanocrystalline using the sol–gel method to obtain nano-scale particles (100 nm). Structure, morphology, and electrochemical behaviors of the product are also discussed.

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 2 2 5 - 0

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2. Experimental

2.3. Measurement of structure and morphology

2.1. Synthesis of Li4 Ti5 O12

The structure and morphology properties of the precursors and oxide powder were characterized by several techniques. Powder X-ray diffraction (XRD) data were collected using a Rigaku D/MAX 2400 diffractometer with Cu–Kα radiation (λ = 1.5418 Å). The transmission electron microscope (TEM, Hitachi 600, Japan) was used to observe the morphology and degree of agglomeration. The thermogravimetric and differential thermal analysis (TG-DTA, Setarm TGDTA92A) were performed with ␣-Al2 O3 as the reference substance at a heating rate of 10 ◦ C min−1 .

A flow chart of the synthesis procedure is shown in Fig. 1. Tetrabutyl titanate [Ti(OC4 H9 )4 ] and isopropyl alcohol were first mixed with mole ratio of 1:60 (A solution). Lithium acetate was dissolved into the mixture solutions of isopropyl alcohol/deionize water/acetate acid (B solution). With vigorous stirring, B solution was gradually dropped into A solution and then a clear solution was produced. After aging for 3 h, a milk white gel formed. The resulting gelatin was heated at 80 ◦ C to extract out excess isopropyl alcohol and yield an organic precursor. Fine white powders were obtained by calcining the precursors in air at various temperatures (400–800 ◦ C) for 4 h.

3. Results and discussion 3.1. Structure analysis

2.2. Measurement of electrochemical properties The electrochemical cells consisted of a Li4 Ti5 O12 based composite as the positive electrode, lithium metal as the reference electrode, and 1.0 M LiClO4 in propylene carbonate (PC) as the electrolyte. Microporous polypropylene sheet was used as the cell separator. The cathode was a mixture of 80% (weight percent) Li4 Ti5 O12 active material, 15% acetylene black, and 5% Teflon binder. The cell was assembled in an argon-filled dry box. All the electrochemical tests were carried out at room temperature. Cyclic voltammetry scans were recorded from 0.0 to 2.6 V at a scan rate of 10 mV s−1 , with the Li4 Ti5 O12 as working electrode, Li metal disk as both counter and reference electrodes using CHI660A electrochemical work station system (Covarda). Charge–discharge cycle tests were performed at a constant current density of 0.3 mA cm−2 with the cutoff voltage of 2.5–1.0 V. The specific capacity values were calculated from the value of the current, the mass of active material in the cathode, and the elapsed time.

Fig. 1. Flow chart of the synthesis procedure.

3.1.1. Thermal Analysis Fig. 2 showed the TG-DTA curve (from 20 to 1000 ◦ C) of Li4 Ti5 O12 precursor prepared by the sol–gel method. Two distinct steps of weight loss were observed on the TG curve of the gel. The first step is obviously due to the vaporization of water and free acetic acid, and the liberation of isopropyl alcohol and acetic acid from the alkoxide and acetate group, and their successive combustion. They correspond to two endothermic peaks around 95 and 320 ◦ C on the DTA curve, respectively. The weight loss is about 40% in this temperature range. This process meant the decomposition of the organics and the primary formation of Li4 Ti5 O12 phase. The second step between 575 and 790 ◦ C also accompanies a broad endothermic peak. The TG curve shows that the sample has not weight loss in this temperature range. We thought that the phase-change reaction and formation of Li4 Ti5 O12 crystalline started during this temperature range, i.e. Li4 Ti5 O12 crystalline phase can be obtained by controlling the sintering temperature (between 590 and 800 ◦ C) and the sintering time. For example, the precursor was fired at a lower temperature (>600 ◦ C) and for a long time (10–16 h). 3.1.2. XRD Analysis The formation of crystalline phase can be seen from the XRD patterns of Li4 Ti5 O12 fired at different temperatures (Fig. 3). The polymer precursor shows an amorphous structure. At 400o C, diffraction peaks corresponding to the intermediate phase TiO2 and less Li4 Ti5 O12 phase began to appear. At the same time, some impurity substances were still observed. The diffraction peaks of Li4 Ti5 O12 phase gradually enhanced and the peaks of TiO2 phase sharply decreased in the mixtures after fired at 600 ◦ C. On the other hand, less carbonates were observed in the sample after fired at 600 ◦ C. The diffraction peaks of Li4 Ti5 O12 gradually sharpened with the increasing of temperature, which indicates an increase of crystallinity as may occur from the growth of grain size, ordering of local structure, and/or release of lattice strain. According to the DTA analysis, the

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Fig. 2. TG and DTA curve of dry gel.

Li4 Ti5 O12 crystal phase began to form at 600 ◦ C and the phase of Li4 Ti5 O12 can be obtained at 800o C. The XRD pattern of Fig. 3 (800 ◦ C) correlates well with the report that Li4 Ti5 O12 has a defective spinel-framework structure (α = 8.367 Å) [7]. The weak diffraction peak appeared at 2θ = 27.4◦ in Fig. 3 (800 ◦ C) indicated that Li-Ti-O phase may contain smaller amount of rutile phase TiO2 . The reason derived from the loss of Li in the precursor fired at 800 ◦ C. The lattice parameter α was 8.368 Å by the lease square method using 10 diffraction lines. Ohzuku et al. [7] reported α = 8.367 Å and Colbow et al. [8] also reported α = 8.365 Å, which are in good agreement with our value α = 8.368 Å. Moreover, both our sample (α = 8.368 Å) and that of Robertson et al. [3] (α = 8.367 Å) are white in color (electronic insulator).

The dimensions of the Li4 Ti5 O12 nanocrystallines were estimated from the widths of the major diffraction peaks observed in Fig. 3 (800 ◦ C), using the Scherre formula: Dhkl =

βλ B cos θ

where Dhkl is the linear dimension of the coherent diffracting domain along a direction normal to the diffraction plane (hkl), λ X-ray wavelength (1.5418 Å), β the crystal shape constant (0.89), θ the reflection angle of the peak, and B is the corrected full width at half maximum (FWHM) of the peak in radians. Calculated from the widths of the XRD reflection peaks in Fig. 3 (800 ◦ C), the dimensions of Li4 Ti5 O12 nanocrystallines are 98 nm (111), 102 nm (311), 105 nm (400), and 136 nm (440), respectively. 3.1.3. TEM analysis TEM images of Li4 Ti5 O12 crystallites are shown in Fig. 4. It is apparent that Li4 Ti5 O12 crystallines are regular nanoparticles with good dispersivity, and the average size of these nanoparticles is about 100 nm, which is in good agreement with the XRD result. Fig. 4(b) is an electron diffraction (ED) picture and the measurement results show that the Li4 Ti5 O12 crystallines are excellent crystallinity. These electron diffraction spots correspond to the (111), (311), (400), (511), and (440) diffraction planes of spinel Li4 Ti5 O12 crystal. 3.2. Electrochemical analysis

Fig. 3. X-ray diffraction patterns of Li4 Ti5 O12 system fired at different temperatures for 4 h.

3.2.1. Cyclic voltammetry The electrochemical behavior of various Li4 Ti5 O12 nanocrystalline samples was characterized by cyclic voltammogram as shown in Fig. 5. For all the voltammograms, the voltage was initially scanned from 0.0 to 2.6 V, and then back to 0.0 V at a scan rate of 10 mV s−1 . A negligible background current was observed when the cell only used

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Fig. 4. TEM and ED image of Li4 Ti5 O12 fired at 800 ◦ C for 4 h; (a) TEM image; (b) ED image.

a nickel gauze as the cathode (Fig. 5a). The fifth cycle in cyclic voltammetry curve of Li4 Ti5 O12 fired at 600, 700 and 800 ◦ C are shown in Fig. 5b-d, respectively. In these three systems, the oxidation peaks and reduction peaks are different. The oxidation peaks located at around 2.3, 2.1, and 1.9 V, and the corresponding reduction peaks around 0.9,

1.0, and 1.5 V for the samples fired at 600, 700, and 800 ◦ C, respectively. These results indicate that lithium extraction and insertion become easy with the increasing firing temperature, because higher degree of crystallinity gives rise to a more reversible Li transfer in three-dimensional framework of Li4 Ti5 O12 . One electrochemical peak appeared

Fig. 5. Cyclic voltammograms of Li4 Ti5 O12 system fired at different temperatures: (a) back-ground scan; (b) Li4 Ti5 O12 fired at 600 ◦ C; (c) Li4 Ti5 O12 fired at 700 ◦ C; (d) Li4 Ti5 O12 fired at 800 ◦ C; scan rate: 10 mV/s

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phase (TiO2 ) and non-crystalline phase in samples. The first discharge special capacity of Li4 Ti5 O12 fired at 800 ◦ C is 272 mAh g−1 , which is higher than previous reports [6,7]. The increase of cell capacity derived from the Li4 Ti5 O12 samples with more uniform particle distribution, smaller size (100 nm), and larger surface area. These characteristics are beneficial to the Li+ ion migration in the Li4 Ti5 O12 host and can improve reversibility and capacity of the cell. The cycling tests of the cell were carried out in our laboratory.

4. Conclusion

Fig. 6. Discharge curve of the first cycle of Li4 Ti5 O12 fired at different temperatures: (a) 600 ◦ C; (b) 700 ◦ C; (c) 800 ◦ C. Current density: 0.3 mA cm−2 .

for both oxidation and reduction process indicating that both the insertion and extraction of lithium ions occur in one stage, which is in agreement with the previous reports by other methods [3,7]. At 800 ◦ C, peaks of oxidation and reduction become clearly sharp, indicating the high crystalline degree of the material. For Li4 Ti5 O12 fired at 600 ◦ C, the peak is much broader and this reflects a profile typical to a disordered electrochemical system [13]. 3.2.2. Constant current charge–discharge of Li4 Ti5 O12 The specific capacity of various samples was determined by discharge test at a constant current density of 0.3 mA cm−2 between the cut-off voltages of 2.5 and 1.0 V. In order to give electrical contact to all the particles and also to supply electrolyte inside the cathode, acetylene black and Teflon binder were used to prepare the cathode. Fig. 6 shows the first discharge character of Li4 Ti5 O12 fired at different temperatures. The initial open-circuit voltage was 2.9 V for Li4 Ti5 O12 fired at 800 ◦ C. The first discharge test was carried out between 2.5 and 1.0 V and the voltage dropped quickly down to below 2 V and decreased as the reaction proceeded until the voltage reached about 1.51 V (initial 4.8% of discharge capacity), after which the voltage was almost independent upon the degree of reduction (initial 53.9% of discharge capacity) [3,6,9]. The voltage fell quickly below 1.51 V (initial 15.1% of discharge capacity). As the firing temperature decreased, capacity of the cell quickly drowned at the first discharge. For Li4 Ti5 O12 fired at 600 and 700 ◦ C, the initial capacities were 77.7 and 85.6 mAh g−1 , respectively. The reason for the initial capacity difference results from the coexistence of impure

Li4 Ti5 O12 has been successfully synthesized by sol–gel method and phase purity and good stoichiometric product was obtained. Nanocrystalline samples exhibited excellent cycliability and the first capacity was 272 mAh g−1 . These nanocrystalline samples showed good electrochemical performance. This work demonstrated that Li4 Ti5 O12 with nanostructure and good crystallinity was a promising negative electrode host.

Acknowledgements This work is supported by the National Science Foundation of China (No. 69871013). References [1] T. Nagaura, K. Tozawak, Prog. Batts. Sol. Cells 9 (1990) 209. [2] S. Passerini, J.M. Rosolen, B. Scrosati, J. Power Soruce 45 (1993) 331. [3] A.D. Robertson, H. Tukamoto, J.T.S. Irvine, J. Electrochem. Soc. 146 (1999) 3958. [4] B. Scrosati, Electrochim. Acta 45 (2000) 2461–2466. [5] M.M. Thackeray, P.J. Johnson, L.A. De Picciott, et al., Mater. Res. Bull. 19 (1984) 179. [6] E. Ferg, R.J. Gummow, A. De Kock, M.M. Thackeray, J. Electrochem. Soc. 141 (1994) L147. [7] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431. [8] K.M. Colbow, J.R. Dahn, R.R. Haering, J. Power Source 26 (1989) 397. [9] D. Peramunage, K.M. Abraham, J. Electrochem. Soc. 145 (1998) 2609. [10] D. Peramunage, K.M. Abraham, J. Electrochem. Soc. 145 (1998) 2615. [11] K. Zaghib, M. Armand, M. Gauthier, J. Electrochem. Soc. 145 (1998) 3135. [12] Y.-K. Sun, S.-H. Jin, J. Mater. Chem. 8 (1998) 2399–2404. [13] W. Liu, G.C. Farrington, F. Chaput, B. Dunn, J. Electrochem. Soc. 143 (1996) 879.