Improvement of a novel anode material TeO2 by chlorine doping
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A simple and versatile method for the preparation of chlorine-doped TeO2 was developed via thermal decomposition of Te6O11Cl2 in situ. Te6O11Cl2 was prepared with TeCl4 and ethanol as reagents, while TeO2 was fabricated with water as a solvent. The morphology, surface, and electrochemical performances of the obtained materials were systematically studied. It was found that chlorine-doped TeO2 demonstrated the highest cycling efficiency and stability than Te6O11Cl2 and TeO2. The presence of Te–Cl bond is expected to contribute to the reversible capacity and Li inserting process.
KeywordsTeO2 Te6O11Cl2 Anode·lithium-ion battery Chlorine doping
The Te (IV)-containing compounds have attracted much interest, owing to various excellent properties related to macroscopic polarization and polarisability (dielectric, piezoelectric, optic, and electroacoustic), which are of great interest to fundamental science and technology . Among these compounds, amorphous TeO2 and alpha-TeO2 (paratellurite) have attracted much attention for their various important applications, such as in gamma-ray sensor  and temperature-stable SAW device . Also, TeO2-based glasses are potential materials for upconversion and thermometric , non-linear optical , and waveguide devices application .
But Te-containing compounds were seldom used as energy storage materials. Recently, the cathodic capacity of LiMn2O4 has been promoted via narrow range of nano-Te doping . Krins et al. investigated the relationship between the structural characteristics of the xLi2O (1 − x)(0.3 V2O5–0.7 TeO2) system and its electrical behavior . But single-phased tellurium oxide (TeO2) has not been reported to be tested as lithium-ion battery electrode material, as far as we know. It is well-known that TeO2 crystal contains cavities and tunnels . Furthermore, Te exhibits multiple covalent bonds. So it may be used as lithium-inserting electrode material. Until now, great attention has been drawn to prepare amorphous TeO2 film/nanoparticles [9, 10, 11, 12, 13] and tellurium oxides by various methods .
Herein, we develop a simple and versatile method for the preparation of TeO2 and study its electrochemical performance as anode material for lithium-ion battery. By careful adjustment of the synthesis parameters, Te6O11Cl2 and TeO2 microcrystals were controllably fabricated with ethanol and water as solvent, respectively. The corresponding chlorine-doped TeO2 was prepared via thermal decomposition of the precursor of Te6O11Cl2 in situ. When used as anode material for lithium-ion batteries, chlorine-doped TeO2 shows the best electrochemical properties than TeO2 and Te6O11Cl2. The possible reason is that the Te–Cl bond in TeO2Clx promotes formation of lithium–tellurium alloy. This facile method to fabricate chlorine-doped TeO2 would be of great significance to design other chlorine-doped lithium-ion battery electrode material with advanced functions.
Preparation and characterizations of materials
All chemicals are commercially available and used without further purification. In a typical procedure, 3 g of TeCl4 was dissolved in 30 ml absolute ethanol and stirred at room temperature for 30 min, and then the mixed solution was transferred into a 50-ml Teflon-lined stainless autoclave and kept at 200 °C for 24 h. After the reaction was finished, it was cooled to room temperature. The precursors were filtered, washed with absolute ethanol, and dried at 60 °C for 12 h. The dried precursor (denoted as Sample Tol-1) was calcined at 400 °C for 3 h with a heating rate of 5 °C/min to get Sample Tol-1c. When 30 ml deionized water was added to take the place of ethanol, sample Toh-2 was obtained under the identical condition, which was calcined at 400 °C for 3 h to get sample Toh-2c. TeCl4 is toxic and handle with utmost care.
The morphology of products was observed by Hitachi S-4800 field emission scanning electron microscope. X-ray diffraction (XRD) patterns were recorded on a diffractometer (Co Kα, PANalytical, X’Pert, data were convert into Cu Kα). X-ray photoelectron spectroscopy (XPS) measurements were performed with an Escalab 250 spectrometer.
The electrochemical properties
Te6O11Cl2 (Sample Tol-1), TeO2 (Sample Toh-2 and Toh-2c) and chloride-doped TeO2 (Sample Tol-1c) were used as anode materials for lithium-ion battery. The negative electrode was prepared via pasting slurries of active materials, acetylene black, and polyvinylidene fluoride with a weight ratio of 6:3:1 on a Cu foil circular flake. The flake was dried at 120 °C for 12 h under vacuum condition. The metallic lithium foil was used as the positive electrode. The electrolyte was 1 M LiPF6 in the mixed solvent of ethylene carbonate, dimethyl carbonate, and diethylene carbonate with a volume ratio of 1:1:1. All cells were assembled in an argon-filled glove box. Charge–discharge cycles were performed with a Land CT 2001A cycle life tester (Wuhan, China) at a current density of 20 mA g−1 in the voltage range between 0.05 and 3.0 V versus Li/Li+. Chloride-doped TeO2 (sample Tol-1c) was tested in the voltage from 0.05 to 3.0 V and successively discharged at the current density of 60, 120, 180, 240, 300, and 360 mA g−1. Cyclic voltammetry (CV) experiments were performed using a CHI660 and Zahner IM6 electrochemical work station at a scan rate of 1 mV s−1.
Results and discussion
Crystalline structure and morphologies of samples
Various lithium-ion battery cathode materials have been improved by chlorine doping for different reasons. The reversible capacity and cycle stability of LiNi0.7Co0.3O2 are improved for expanding the cell volume and decreasing the oxidation state of cobalt and nickel ions . Li1.06Mn2O4−zClz shows excellent cycle ability not only at ambient temperature but also at 55 °C for changing lattice parameter of spinel . Also, the particle-to-particle impedance can be decreased by Cl− substitution, resulting in greater reversibility for LiV3O7.90Cl0.10 . Cl− doped LiFePO4 also exhibits good electrochemical properties for the minor change of crystal structure and the increasing of Li+ diffusion and exchange current density [26, 27, 28]. Here, the cyclic stability of TeO2 anode material for lithium-ion battery is improved by chlorine doping, which promotes the formation of LixTe. The Te–Cl bond might play a great role in improving TeO2 electrochemical properties.
In summary, Te6O11Cl2 and TeO2 microcrystals were prepared via a hydrothermal method with ethanol and water as solvent, respectively. After calcinations, both Te6O11Cl2 and TeO2 were converted to TeO2Clx and TeO2, respectively. The cell made from TeO2Clx electrode material exhibits the highest discharge capacity and best cyclic stability than Te6O11Cl2 and other TeO2 due to Cl doping, which favors the formation of lithium–tellurium alloy and inhibits the formation of LixTeO2.
This work was supported by the funds (2010J05025, 2010-XY-5, and XRC-0926) and the open project in Key Lab Adv Energy Mat Chem (Nankai University) (KLAEMC-OP201201).
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