Introduction

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 [1]. Among these compounds, amorphous TeO2 and alpha-TeO2 (paratellurite) have attracted much attention for their various important applications, such as in gamma-ray sensor [2] and temperature-stable SAW device [3]. Also, TeO2-based glasses are potential materials for upconversion and thermometric [4], non-linear optical [5], and waveguide devices application [6].

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 [7]. 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 [8]. 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 [1]. 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 [9–13] and tellurium oxides by various methods [14].

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 TeO2Cl x 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.

Experimental

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

The structural information is provided by XRD, as shown in Fig. 1. It can be found that solvent has a dramatic effect on the crystalline structure of products. The precursor prepared with ethanol (sample Tol-1) is crystalline Te6O11Cl2 (JCPDS 85-2305) in Fig. 1a, while that obtained with water (sample Toh-1) is ascribed to Paratellurite TeO2 (JCPDS 11-0693), as shown in Fig. 1c. The possible reason might be ascribed to the formation of different tellurium precursors in different solvents. It was reported that TeCl4 could react with ethanol to form TeCl4-x (OC2H5) x [15], which was further transformed to Te6O11Cl2. It was also reported that Te6O11Cl2 could be prepared with TeO2, H2O, and HCl as reagents [16]. But no H2O or HCl was involved in our experimental procedure. After calcinations, both Te6O11Cl2 and TeO2 were converted to alpha-TeO2 (sample Toh-1c and Toh-2c) (JCPDS 84-1777), as shown in Fig. 1b, d, respectively.

Fig. 1
figure 1

XRD patterns of the as-synthesized products prepared with ethanol (sample Tol-1) (a), the calcining product of sample Tol-1 (sample Tol-1c) (b), the as-synthesized product prepared with water (sample Toh-2) (c) and the calcined product of Sample Toh-2 (Sample Toh-2c) (d)

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Scan electron microscopy (SEM) was performed to investigate the obtained four samples. The as-synthesized Te6O11Cl2 and TeO2 have similar morphologies as shown in Fig. 2a, b, which are all mono-dispersed irregular microcrystals about dozens of micrometers in size. After calcinations, the obtained TeO2 has a similar morphology to the as-synthesized Te6O11Cl2 and TeO2 in Fig. 2c, d, respectively.

Fig. 2
figure 2

SEM images of Te6O11Cl2 (sample Tol-1) (a), TeO2Cl x (sample Tol-1c) (b), the as-synthesized (sample Toh-2) and calcined TeO2 (sample Toh-2c) prepared with water (c, d), respectively

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Electrochemical performances

TeO2 (sample Toh-2 and Toh-2c) and Te6O11Cl2 (sample Tol-1) and chloride-doped TeO2 (TeO2Cl x , sample Tol-1c) were tested as anode materials for lithium-ion battery. The corresponding cyclic performance is shown in Fig. 3b. It can be clearly seen that Te6O11Cl2 and the as-synthesized and calcined TeO2 prepared with water as solvent exhibit relative low discharge capacity and bad cycling stability in Fig. 3b (marked with filled square, filled circle, and white circle, respectively). While chlorine-doped TeO2 (TeO2Cl x , Sample Tol-1c) exhibits high discharge capacity and good cycle stability in Fig. 3b (marked with white square). The corresponding discharge profiles at a discharge current of 20 mA g−1are shown in Fig. 3a. The initial, 1st, and 50th discharge capacity is 670.3, 336.1, and 294.2 mA h g−1, respectively. The corresponding discharge rate capability of chlorine-doped TeO2 (TeO2Cl x , sample Tol-1c) is tested at a current density of 60, 120, 180, 240, 300, and 360 mA g−1 (Fig. 3c), which exhibits stable discharge capacity at various discharge rates.

Fig. 3
figure 3

The discharge–charge curves of TeO2Cl x (sample Tol-1c) (a), the cycle performance of Te6O11Cl2 (filled square, sample Tol-1), TeO2Cl x (white square, sample Tol-1c), the as-synthesized TeO2 prepared with water (filled circle, sample Toh-2) and the calcined TeO2 prepared with water (white circle, sample Toh-2c) (b) and the evolution of the reversible capacity for TeO2Cl x (sample Tol-1c) at different current densities (c)

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Cyclic voltammetry (CV) was further performed to investigate electrochemical properties of Te6O11Cl2 and TeO2 prepared with different precursors, as shown in Fig. 4. The first CV cycle of Te6O11Cl2 (Sample Tol-1) exhibits one strong cathodic peak at 1.65 V and a weak one at 0.52 V (Fig. 4a), which is same to that of the as-synthesized (sample Toh-2) and calcined TeO2 (sample Toh-2c) prepared with water in Fig. 4c and d. But the first CV curve of TeO2 prepared with Te6O11Cl2 (sample Tol-1c) exhibits one strong cathodic peak at 0.52 and a weak one at 1.65 V (Fig. 4a). However, all the four samples exhibit same CV curves for the second, third, and fourth cycle with one weak cathodic peak at 1.65 V.

Fig. 4
figure 4

CV curves of Te6O11Cl2 (sample Tol-1) (a), the as-synthesized TeO2 prepared with water (sample Toh-2) (b), TeO2Cl x (sample Tol-1c) (c) TeO2 prepared with water after calcinations (sample Toh-2c) (d), respectively

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X-ray diffraction was further performed to investigate the crystalline structure of TeO2 prepared with Te6O11Cl2 (sample Tol-1c) discharged from 3 to 1.5 and 0.3 V.. The results show that the product discharged at 1.5 V is crystalline TeO2 (Fig. 5a), while that at 0.3 V is mixed-phase of TeO2 and Te (Fig. 5b). Therefore, the cathodic peak at 0.52 V can be ascribed to lithium–tellurium alloy (Li x Te), while that at 1.65 V might be ascribed to Li x TeO2. TeO2 prepared from Te6O11Cl2 (Sample Tol-1c)exhibits better electrochemical properties due to inhibit Li x TeO2 formation and promote to formation of lithium–tellurium alloy (Li x Te), the latter will favor good electrochemical performance.

Fig. 5
figure 5

XRD patterns of TeO2Cl x (sample Tol-1c) discharged at 0.3 (a) and 1.5 V (b), respectively

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XPS was performed to determine the state of the element on the surface of the obtainedTeO2 after calcinations. Figure 6 shows the high-resolution XPS spectra of Cl, O, and Te measured on the surface of the calcined samples prepared with ethanol (sample Tol-1c) and water (sample Toh-2c), respectively. The weak Cl 2p signal indicates there is trace of Cl. TeO2 prepared from Te6O11Cl2 shows two stronger Cl 2p signals than the calcined TeO2 prepared with water (Fig. 6), which implies there is trace Cl on the surface. The signal for Cl at 196.6 eV is ascribed to the ionic (Cl−) [17]. According to the references, the Cl signal of Pb–Cl and Cu–Cl were reported at 198.3 [18] and 198.7 eV [19, 20], respectively. So the strong peak at 198.6 eV is ascribed to the Cl of Te–Cl bond. Te 3d3/2 signals of TeO2 prepared from Te6O11Cl2 are at 529.2 and 574.9 eV, respectively. While the O 1s and Te 3d3/2 signals of TeO2 (Sample Toh-2c) are at 528.8 and 574.7 eV, respectively. The signal at 574.9 and 574.7 eV are ascribed to Te–O bond [21]. It can be seen that there is no considerable shift of the peaks of Te 3d at all, which may be because there is trace Te–Cl bond. The shift of O 1s signals might be ascribed to the oxygen contamination. It was reported that the shift of O 1s signals was presumably associated with the oxygen and CO2 adsorbed during the preparation of samples for XPS [22].

Fig. 6
figure 6

The Cl, Te, and O XPS spectra of TeO2Cl x (sample Tol-1c) (a) and TeO2 prepared with water after calcinations (b), respectively

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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 [23]. Li1.06Mn2O4−z Cl z shows excellent cycle ability not only at ambient temperature but also at 55 °C for changing lattice parameter of spinel [24]. Also, the particle-to-particle impedance can be decreased by Cl− substitution, resulting in greater reversibility for LiV3O7.90Cl0.10 [25]. 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 [26–28]. Here, the cyclic stability of TeO2 anode material for lithium-ion battery is improved by chlorine doping, which promotes the formation of Li x Te. The Te–Cl bond might play a great role in improving TeO2 electrochemical properties.

Conclusions

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 TeO2Cl x and TeO2, respectively. The cell made from TeO2Cl x 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 Li x TeO2.