Preparation of Carbon-Encapsulated ZnO Tetrahedron as an Anode Material for Ultralong Cycle Life Performance Lithium-ion Batteries
Graphical abstract
Introduction
Lithium ion batteries have been widely studied for two decades due to the excellent advantages of high voltage, recyclable and long using lifetime [1], [2], [3]. In recent years, the development of new generation electric cars requires for better mileage and greater output power as well as lower prices. This means that the advanced lithium ion batteries (LIBs) are needed with larger energy densities, higher speed capabilities, longer cycle life and lower cost. Unfortunately, the energy density of the LIBs was limited by the low gravimetric capacity of current commercial graphite anode (372 mAhg−1). Accordingly, many efforts have been made on introducing a variety of new anode materials, such as silicon, intermetallic alloys and metal oxide [4], [5], [6], [7]. Among the numerous anode candidates, transition metal oxides are regarded as the most promising one because of their higher theoretical capacities, better safety and environmental friendliness [8], [9], [10]. Most of the transition metal oxides (Fe3O4, Co3O4, Mn3O4 and MoO3) are based on the “conversion reaction” in the discharge-charge cycles, while ZnO is based on the alloying-dealloying reactions [11], [12]. As a well-known functional material, ZnO has been used in various fields of gas sensors, optoelectronic devices and solar cells [13], [14], [15]. Recently, ZnO has also attracted much more attention as an alternative anode due to higher theoretical lithium storage capacity (987 mAhg−1), simpler preparation process and higher lithium-ion diffusion coefficient compared to other transition metal oxides [16]. ZnO will transform to Zn upon the redox reaction with Li+, followed by the alloying reaction forming LiZn, resulting the large volume expansion (∼228%) and poor capacity retention during cycling [17], [18]. Generally, the alloying process is accompanied with not only large volume change but also structural stress of the electrode, which result in the crack and pulverization of the anodes. Another drawback is the low electronic conductivity, which is also a critical problem limiting the application of the ZnO-based electrodes.
Downsizing the particles to nanoscale and/or decorating with a high conductive material on the surface of ZnO are effective approaches to improve the electrochemical properties [19], [20]. Particularly, the reduction of the ZnO particle size to nanometer dimensions is of great importance to improve the electrochemical performance, because the contact area between electrode and electrolyte is increased, the diffusion length for both Li ion and electron transport is shortened. Moreover, the nanoscale particles can minimize the mechanical strain-induced cracking. Various nanostructures have been employed to alleviate the volume expansion of ZnO, such as nanorods, porous nanosheet and some hierarchical structures [21], [22], [23]. But the nanoscale electrodes have also introduced new fundamental challenges, including high surface area, low tap density and generally poor electrical properties due to higher interparticle resistance [24]. The optimum design of the hollow, core shell and yolk-shell structures has received increasing attention because of their fast kinetics for transported Li-ion and electrons and a buffer against the volume expansion [25]. It is reported that the electrochemical properties of ZnO can be improved by (i) coating with NiO or TiO2 [26], [27] and (ii) incorporating with ZnAl2O4 or Ni3ZnC0.7 [28], [29]. However, complex preparation technologies are needed in preparation for these methods, most methods can only improve some of the drawbacks of ZnO, while the amorphous carbon has been regarded as an ideal layer material for ZnO because of its advantages of excellent conductive properties, good strain accommodation and easy preparation. In particular, construction of three dimension textures with amorphous carbon has been proven an effective architecture to tackle the pulverization and low conductivity of transition metal oxide electrodes [30], [31], [32], [33]. Porous carbon has been used as a dispersing medium for ZnO electrode, because of its merits like highly conductive network and good strain accommodation. Wang and his co-workers report a one-pot strategy to prepare the ZnO nanoparticles/conductive nanocarbon skeleton materials, which exhibit the superior electrochemical properties [34]. The ZnO/porous carbon nanocomposites synthesized by Shen et al. displays significantly enhanced lithium storage capacity and excellent cycling stability [35]. However, there is an inevitable issue that the usage of porous carbon or nanocarbon will lead to a high surface area of electrode as well as a relative low loading of active materials.
Herein, we develop a straightforward, high efficient and environmentally benign synthetic route to prepare ZnO tetrahedron crystals embedded in amorphous carbon matrix. The advantages of our method for the synthesis are numerous, such as atmospheric pressure, use of simple equipment, high reproducibility and scale-up prospects. We first synthesized the zinc-containing precursor via a simple internal-reflux method in argon. Then the ZnO@C composites were obtained by carbonization, where the surfactant was used as the carbon source. Accompanied with the construction of amorphous carbon matrix, ZnO tetrahedron is formed and dispersed uniformly into the carbon framework during the heating treatment, which results in a stronger adhesive force between carbon and ZnO crystals compared to other carbon-coating technology. The structure, morphology, and electrochemical performance of the ZnO@C composites are investigated in detail. The effect of the calcinations temperature on lithium-storage performance of the products is discussed. The SEM and TEM images show that the ZnO crystals morphology and size distribution of the ZnO@C composite calcined at 500°C (ZnO@C-5) are more uniform than that of the composite calcined at 700°C (ZnO@C-7). Moreover, the carbon skeleton of ZnO@C-5 exhibit higher degree of graphitization according to Raman spectrum, indicating the better electronic conductivity. The ZnO@C-5 electrode presents excellent electrochemical performance compared to ZnO@C-7 and/or pure ZnO when it was used as the LIBs anode materials.
Section snippets
Materials
Oleic acid (OA, 90%, Alfa Aesar), Oleylamine (OLA, 80-90%, Aladdin), Zinc nitrate hexanydrate (Zn(NO3)2·6H2O, Chemically Pure, Sinopharm Chemical Reagent), Ethanol (Chemically Pure, Sinopharm Chemical Reagent) were used as received without further purification.
Synthesis
The zinc-containing precursor was prepared by a simple internal-reflux method. In a typical synthesis process, OA (10 mL, 31.2 mmol), OLA (2 mL, 6.2 mmol) and Zn(NO3)2·6H2O (1.24 g, 4 mmol) were injected into a 100 mL three-neck flask, and the
Results and discussion
The ZnO@C composites were prepared by a simple internal-reflux method followed by heat treatment in argon at different temperature. For comparison, we also synthesized the pure ZnO crystals by calcining the zinc-containing precursor at 500°C in air. Fig. 1 shows the powder XRD patterns of the ZnO@C-5, ZnO@C-7 and pure ZnO. All the diffraction peaks of three samples can be indexed as the wurtzite phase of hexagonal ZnO (JCPDS No. 36-1451) without any other crystalline phase. In view of the sharp
Conclusions
In conclusion, we introduce a straightforward, high efficient and environmentally benign synthetic route to prepare ZnO tetrahedron crystals embedded in amorphous carbon matrix. Firstly, the precursor is synthesized via a simple internal-reflux method. Then the ZnO tetrahedron and carbon skeleton are formed after the precursor is calcined, in which the surfactant is used as carbon source. The calcination temperature is a key factor to affect the morphology and size distribution of the ZnO
Acknowledgment
The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (no. 51272231), and the Doctoral Fund of the Ministry of Education of China (no. 20100101110039).
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