Synthesis by TEA sol–gel method and electrochemical properties of Li4Ti5O12 anode material for lithium-ion battery
Introduction
Spinel Li4Ti5O12 [1], [2], [3], [4], [5] is a promising anode material for lithium-ion batteries. The excellent cycling performance and long life of the spinel Li4Ti5O12 make it a candidate as an anode electrode especially for larger scale applications such as all solid-state lithium-ion batteries [6], [7], [8], [9] and hybrid supercapacitors [10], [11]. Spinel oxides Li1 + xTi2 − xO4, 0 ≤ x ≤ 1/3 (space group Fd3m), were described in 1971 by Deschanvers et al. [12] and electrochemically characterized in the early 1990s by Ohzuku et al., Colbow et al., Ferg et al.[13], [14], [15]. The material shows very flat discharge and charge curves at the potential of 1.55 V (vs. Li) which is above the potential range where most electrolytes or solvents are reduced. The material has a theoretical specific capacity of 175 mA h/g and exhibits a practical specific capacity of 150 ∽ 160 mA h/g. During the process of Li+ intercalation and de-intercalation, its cubic symmetry of the parent spinel is almost unaffected and the change in the unit cell volume is less than 1%, so it is regarded as a “zero strain” insertion material [1]. This is very important for solid-state lithium-ion batteries to avoid structural damage due to expansion and contraction of the electrode materials at the insertion material/solid electrolyte interfaces during the charge and discharge process. At the same time, the improved safety and reliability of the spinel compared with that of carbon electrodes make the lithium-ion batteries using Li4Ti5O12 material as anode suitable to electric vehicle (EV) and power storage batteries.
Synthesis process plays a major role in improving the physicochemical properties of the electrode materials. Li4Ti5O12 powders are usually synthesized by a solid-state reaction of lithium and titanium salts [1], [2], [3], [4], [5], [16], [17]. Because of the several disadvantages of this method such as inhomogeneity, irregular morphology, larger particle size, broader particle size distribution and a longer period of calcinations, a sol–gel method [18], [19], [20], [21] has been introduced for the synthesis of electrode materials for rechargeable lithium batteries. The latter method offers products with a homogeneous distribution of uniform, sub-micron size particles with good stoichiometry control.
Triethanolamine(TEA, N(C2H4OH)3) is an alkanolamine. It can complex with transition metal to form stable complex [22], [23], [24], [25], [26], [27] to avoid any rapid hydrolysis and condensation in existence of water. And it has been widely used in the sol–gel method to prepare oxides such as TiO2 [28], [29], [30], [31], Al2O3 [32], AgO [27], LiMn2O4 [33] and others. Furthermore, TEA can also use as surfactants [34] in many reactions to prepare powders with good dispersivity. In this paper, Li4Ti5O12 was prepared by a novel sol–gel route using triethanolamine (TEA) as a chelating agent under different molar ratio of TEA to Ti ions and calcination temperature. TEA complex with Ti to form a stable complex for preventing the excessively rapid hydrolysis of Ti4+ to form homogeneous Ti(OH)4 gel. After heating the gel, sub-micron Li4Ti5O12 powders with narrow particle size distribution and superior dispersion were achieved. The effect of varying the TEA to Ti ion ratio R on the particle size, morphology and electrochemical properties of the prepared compounds was investigated.
Section snippets
Preparation of Li4Ti5O12
The Li4Ti5O12 powder was synthesized by the sol–gel method using triethanolamine (TEA) as a chelating agent. Tetrabutyl titanate [Ti (OC4H9) 4] was dissolved in ethanol, and then TEA was added under stirring to obtain solution A. The TEA to Ti ions ratio R values were varied at 0.4, 0.5, 0.6, 0.7 and 0.8. When the R value was larger than 0.8, it is difficult to form gel because of the excessive TEA. Lithium acetate was dissolved in the mixture solutions of ethanol and deionize water to obtain
XRD analysis
The XRD patterns for the Li4Ti5O12 precursors calcined at different temperatures for 24 h in air after preheated at 600 °C for 5 h are presented in Fig. 1. The molar ratio of TEA to Ti ions is 0.8. As can be seen from Fig. 1(a), after calcining the precursor at 500 °C for 24 h, the intermediate phase TiO2 and less Li4Ti5O12 phase began to appear. And there are still some impurity substances observed. The diffraction peaks of Li4Ti5O12 phase and TiO2 phase gradually enhanced simultaneously after
Conclusion
Sub-micron Li4Ti5O12 powders with narrow size distribution and superior dispersion was successfully synthesized by a sol–gel method using TEA as a chelating agent. The SEM results indicated that the prepared Li–Ti–O powders had a uniform morphology with average particle size about 100 nm, which is smaller than those obtained by solid-state method. The content of TEA has important role on the electrochemical property of the prepared spinel Li4Ti5O12. The compound synthesized with R = 0.8 yielded
Acknowledgements
This work was supported by the Chemical and Engineering College of Central South University. The authors are grateful to Professor Kelong Huang and Professor Aidong Tang for their help in Electrochemical measurements.
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