Abstract
Carbon-coated Zn2SnO4 nanomaterials have been synthesized by a facile hydrothermal method in which as-prepared Zn2SnO4 was used as the precursor and glucose as the carbon source. The structural, morphological, and electrochemical properties were investigated by means of X-ray (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical measurement. The first discharge/charge capacity of carbon-coated Zn2SnO4 was about 1248.8 mAh/g and 873.2 mAh/g at a current density of 200 mA/g in the voltage range of 0.05 V–3.0 V, respectively, corresponding to Coulombic efficiency of 69.92%. After 40 cycles, the capacity retained 400 mAh/g, which is much better than bare Zn2SnO4.
1. Introduction
Tin-based oxide materials have been considered as promising anode materials in lithium-ion batteries (LIBs) due to their large lithium-ion capacities (over 600 mAh/g [1]) at low potentials and high densities [2–5], which is greater than that of commercialized graphite anodes (372 mAh/g) [6]. However, the practical usage of these anodes is hindered by their poor capacity retention during cycling as well as the large irreversible capacity during the first discharge/charge cycle [7–10]. This is mainly caused by a large volume change during the lithium insertion and extraction, which is leading to the pulverization of electrodes and rapid capacity fading [11–14]. To overcome the large volume change, numerous efforts have been made to develop anode materials with high energy density and excellent cycling performance, one of which is to synthesize nanosized materials with different shapes and structures. It was also found that nanosized tin-based oxides exhibit especially good electrochemical performance on account of the large specific area and high Li-ion conductivity [15–18]. Another is to prepare carbonaceous materials because carbon materials have high electronic conductivity and favorable electrochemical performance, which not only provides greatly improved electric conductivity, but also helps buffer the severe volume change [19–21].
As an important semiconductor material, Zn2SnO4 has attracted much more attention owing to its applications in gas sensors [22, 23], photocatalysts [24, 25], and solar cells [26, 27]. Recently, nanosized Zn2SnO4 has been successfully synthesized by various methods and applied in lithium-ion batteries with good electrochemical properties. For instance, Zhu et al. [28] synthesized inverse-spinel by hydrothermal method. After 20 cycles, the reversible capacity decayed to 664.7 mAh/g at a current density of 50 mA/g, with an average capacity fade rate of 3.3% per cycle. Feng et al. [29] synthesized single crystal Zn2SnO4 cubes with reversible capacity of 775 mAh/g after 20 cycles at a current density of 50 mA/g. Rong et al. [30] synthesized cube-shaped particles with the reversible capacity of 580 mAh/g and a slowly fading capacity after 50 cycles at a current density of 50 mA/g. To the best of our knowledge, little attention has been paid to carbon-coated Zn2SnO4 as anode materials for lithium-ion batteries at a bigger current density. In this study, carbon-coated Zn2SnO4 materials were made by a facile hydrothermal method using the Zn2SnO4 octahedron as the precursor and glucose as the carbon source. The electrochemical properties were investigated, and the electrochemical tests showed that the carbon-coated Zn2SnO4 exhibits high rate capability and long cycle life compared with bare Zn2SnO4, indicating its promising application as anode for lithium-ion batteries.
2. Experimental
2.1. Preparation of Bare and Carbon-Coated Zn2SnO4
In a typical synthesis, 0.025 M and 0.05 M were dissolved in 60 mL distilled water, and then 1 M NaOH was added dropwise to the mixture under magnetic stirring. After 20 min, the slurry was transferred to a 100 mL Teflon autoclave, heated at 200°C in an electric oven for 24 h, and then cooled down to room temperature naturally. After the reaction, the precipitate was centrifuged and washed with distilled water and ethanol and then dried at 60°C for 3 h. The bare Zn2SnO4 was obtained by carbonizing precursor at 450°C for 3 h under air.
Then, 0.2 g obtained-bare Zn2SnO4 was dispersed in mixed solution of distilled and ethanol by ultrasonication, and then 1 g glucose was dissolved in the mixed solution by magnetic stirring for 30 min. The mixed solution was placed in a Teflon autoclave, and then it was placed in an electric oven at 180°C for 16 h. After cooling down in air, the carbon precursor-coated Zn2SnO4 was obtained by washing with distilled water and ethanol and dried in a vacuum oven at 60°C for 3 h. Hereafter, the as-prepared product was carbonized at 450°C for 3 h under nitrogen flow to obtain carbon-coated Zn2SnO4.
2.2. Sample Characterizations
The morphology of the as-prepared product was characterized by field-emission scanning electron microscopy (FESEM, JEOL, and JSM-6700F). The crystalline structure of the product was analyzed by an X-ray diffractometer (XRD, Y-2000) with Cu radiation at a scan rate of 0.04°s−1. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were carried out on a JEOL JEM-2010 instrument in bright field.
2.3. Electrochemical Test
Electrochemical performance evaluations were performed with coin cells using Li foil as counter electrode. The working electrodes were prepared by mixing the bare Zn2SnO4 or carbon-coated Zn2SnO4, carbon black and polyvinyldifluoride (PVDF) at a weight ratio of 70 : 15 : 15. The mixture was dissolved in 1-methyl-2-pyrrolidinone (NMP) forming a slurry and was uniformly cast on a thick copper foil. The electrolyte consists of 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 by volume). Electrochemical cells were assembled in an argon-filled glovebox. The charge-discharge cycling was performed by using a multichannel battery tester (model SCN, Bitrode, USA). Discharge/charge measurements were performed in the voltage range 0.05 V–3.00 V at a current density of 200 mA/g.
3. Results and Discussion
As reported in the literatures, glucose-derived carbon precursor (PS) can be readily integrated onto nanostructure in solution and furthermore carbonized at a temperature as low as 400°C [31]. In this work, throughout a complex polymerization of glucose at 180°C in the autoclave, the color of sample turned from white to puce, indicating that organic carbonaceous layers exist. After heat treatment at 450°C for 3 h under nitrogen atmosphere, the sample became black, indicating existing carbon layer.
All the diffraction peaks of the two samples were consistent with the data of pure reverse spinel Zn2SnO4 (JPCDS No. 74-2184), indicating that the glucose did not affect the configuration of sample in the process of hydrothermal synthesis. From the XRD patterns, no other peaks are detected, accounting for the low carbon content or forming amorphous carbon. Figure 1(b) shows the TGA curve of carbon-coated Zn2SnO4 in air between 30°C and 800°C. The weight loss before 100°C is due to the loss of water. The continuous weight loss between 400°C and 600°C signifies the removal of the combustion of the carbon layers into carbon oxides. The carbon content in carbon-coated Zn2SnO4 is estimated to be 14.3% based on the TGA analysis.
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Representative SEM images of as-obtained products are observed in Figure 2. Figures 2(a) and 2(b) show a typical image of the bare Zn2SnO4 and carbon-coated Zn2SnO4, respectively. As can be seen, the sample of bare Zn2SnO4 is dispersive quasi-octahedron shape with an average diameter of about 300 nm and nanoparticles on its surface. From Figure 2(b), it can be seen that the nanoparticles on the surface disappear and the morphology of carbon-coated Zn2SnO4 shows quasisphere. All of the quasi-octahedron Zn2SnO4 is uniformly coated by amorphous carbon layer and the sample is well dispersive as before after coating. The decoration of carbon on the surface of Zn2SnO4 is further investigated by TEM in Figures 2(c) and 2(d), from which carbon layer coated on the surface of quasi-octahedron samples is markedly observed. It is noteworthy that introducing the amorphous carbon layer does not affect the other features of Zn2SnO4. From HRTEM image in Figure 2(d), it can be confirmed that the average thickness of carbon layer is about 35 nm and the carbon layer closely sticks to the Zn2SnO4 surface without vacant space between the carbon “shell” and Zn2SnO4 “core.” It is expected to provide good electric conductivity and help buffer the volume change of anode materials.
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Recently, different morphology and structures of Zn2SnO4 electrodes have been demonstrated to improve the cycling capability, but the electrochemical performance of dispersive carbon-coated Zn2SnO4 nanomaterials at higher current density was rarely investigated. To demonstrate the potential use of the carbon-coated Zn2SnO4 as an anode for LIBs, we carried out a comparison of the electrochemical performance of the bare Zn2SnO4 and carbon-coated Zn2SnO4. Galvanostatic charge-discharge test was carried out at a current density of 200 mA/g between 0.05 and 3.00 V. The bare Zn2SnO4 and carbon-coated Zn2SnO4 electrodes show similar voltage profiles which agree well with that previously reported Zn2SnO4 materials. As reported, the electrochemical reaction mechanism of Li with Zn2SnO4 can be described in (1)–(3). The theoretical maximum capacity of Zn2SnO4 is about 547 mAh/g based on (2) and (3) according to the reaction mechanism proposed by Rong et al. [30]: Figures 3(a) and 3(b) show the first, second, and twentieth charge-discharge curves of the two different samples. As can be seen, there is a wide steady discharging plateau around 0.5–1.0 V (versus Li+/Li) for the lithium reaction with Zn2SnO4 and subsequent metallic Sn or Zn between 0.05–0.5 V. From the second cycle afterward, the potential plateaus shift upward to 1.0 V gradually and display more inclined shape. The initial charge and discharge capacity is 1215.9 mAh/g and 812.1 mAh/g for bare Zn2SnO4 and 1248.8 mAh/g and 873.2 mAh/g for carbon-coated Zn2SnO4, respectively. The extra capacity resulted from the formation of solid electrolyte interphase film and possibly interfacial lithium storage during the first discharging process [32]. The initial Coulombic efficiency for the first cycle is 66.79% and 66.92%, which is higher than many corresponding reports. However, for both materials a large first irreversible capacity is observed, due to the decomposition of the electrolyte and the formation of the solid electrolyte interphase (SEI) [30].
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The comparison of cycling stability between bare Zn2SnO4 and carbon-coated Zn2SnO4 is shown in Figures 3(c) and 3(d). It can be seen that the capacity retain rate of carbon-coated Zn2SnO4 is obviously better than that of bare Zn2SnO4. The charge capacity of bare Zn2SnO4 and carbon-coated Zn2SnO4 retains 479.7 and 533.2 mAh/g after 20 cycles at 200 mA/g, which is higher than the theoretical capacity of graphite. However, the charge capacity of bare Zn2SnO4 drops rapidly after 30 cycles. For carbon-coated Zn2SnO4, the charge capacity fades slowly and a charge capacity of 341 mAh/g can still be retained after 45 cycles at 200 mA/g. The better electrochemical properties of carbon-coated Zn2SnO4 can be ascribed to the carbon shell. First, carbon has enough mechanical strength to buffer volume expansion [33]. Thus, the presence of carbon shell on the Zn2SnO4 surface would effectively alleviate the strains caused by the volume variation of Zn2SnO4 and prevent the disintegration. For carbon-coated Zn2SnO4, the preservation of structure during the Li+ insertion/extraction processes helps to keep physical contact between the anode materials and the current collector, leading to better cycling stability. Second, carbon itself is an electronic conductor [34], which ensures good electrical contact of Zn2SnO4 with the current collector and ensures the charge transfer/Li+ transport. Because of uniform and full coating of carbon, electrons can easily reach all the positions where Li+ ion intercalation takes place [35]. Thirdly, carbon is a very stable electroactive anode material for LIBs. The solid electrolyte interphase (SEI) film on carbon surface was reported to be relatively stable compared with that on bare transition metal oxides [36]. The stable SEI film will also be useful for the maintenance of inner Zn2SnO4 structure. Therefore, all of these advantages contribute to the improved battery performance for carbon-coated Zn2SnO4.
4. Conclusions
In summary, dispersive carbon-coated Zn2SnO4 was synthesized by a hydrothermal method using bare Zn2SnO4 as the precursor and glucose as the carbon source. The as-made electrodes of carbon-coated Zn2SnO4 have shown high reversible capacity, good cycle stability, and enhanced rate capability as anode material in LIBs. The carbon introducing can help prevent crack and pulverization, maintain the integrity of electrodes, and provide good electric conductivity, eventually leading to better electrochemical performance than bare Zn2SnO4. The good electrochemical performance of carbon-coated Zn2SnO4 makes it a promising anode material for LIBs.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was supported by the Natural Science Foundation of Henan Province (nos. 122300410214 and 132102210048) and the Natural Science Foundation of China (no. 61306007). The authors would like to thank Professor X. T. Huang for his guide and help at the Central China Normal University.