Synthesis of Mesoporous ZnO Nanosheets via Facile Solvothermal Method as the Anode Materials for Lithium-ion Batteries
© Wang et al. 2016
Received: 6 September 2015
Accepted: 5 January 2016
Published: 27 January 2016
Mesoporous ZnO nanosheets are synthesized through a room temperature solvothermal method. Transmission and scanning electronic microscopy observations indicate that as-prepared ZnO hierarchical aggregates are composed and assembled by nanosheets with a length of 1–2 μm and a thickness of 10–20 nm, and interlaced ZnO nanosheets irregularly stack together, forming a three-dimensional network. Furthermore, large mesopores are embedded in the walls of ZnO nanosheets, confirmed by Brunauer-Emmett-Teller (BET) measurement. Accordingly, the resulting ZnO anode exhibits a high and stable specific discharge capacity of 421 mAh g−1 after 100 cycles at 200 mA g−1 and a good rate capability. Such electrochemical performance could be attributed to the multiple synergistic effects of its mesoporous nanosheet structure, which can not only provide a large specific surface area for lithium storage, but also favor the ion transport and electrolyte diffusion.
Lithium-ion batteries (LIBs) have received much attention in view of its applicability as popular power supplies in portable electronic devices . A new demand of ever-growing electric vehicles (EVs) requires much higher specific capacities for both anode and cathode materials . As a commercial anode material, graphite exhibits high coulombic efficiency and superior cyclability. However, owing to its relatively small specific capacity of 372 mAh g−1, intensive efforts are put on development of high-capacity anode materials with high efficiencies and long-term stability .
Among the various candidate materials, metal oxides have been considered as potential alternatives to replace commercial graphite as the anodes for LIBs due to their higher theoretical capacities . Among various metal oxides, ZnO has received a particular interest as one of the most promising anodes due to its several obvious advantages, such as its high theoretical capacities of 978 mAh g−1, low-cost, facile preparation, and high chemical stability . However, the practical use of ZnO as the anode for LIBs has been hindered by its large volume change during charge/discharge processes, which leads to high internal stress, electrode pulverization, and subsequent loss of electrical contact between the active material and current collector [6, 7].
Much effort has been dedicated to optimize and enhance the electrochemical performance of the ZnO anode in LIBs [5, 6]. Among them, nanostructured ZnO materials have been extensively explored with attempts to not only better accommodate large strains but also provide short diffusion paths for Li+ insertion/deinsertion . Furthermore, mesoporous ZnO materials have been reported to exhibit good electrochemical performance for their high surface area, good accessibility of the pores, and large amount of electroactive sites .
Our belief at this point is that the combination of a nanostructure and mesoporous structure could present synergistic or cooperative effects to successfully overcome the effect of volume change and improve the electrochemical performance with excellent cycling stability and rate capability. Herein, we reported the preparation of mesoporous ZnO nanosheets via the solvothermal method. The physical and electrochemical properties of as-prepared ZnO as the anode for LIBs have been investigated.
Mesoporous ZnO nanosheets were synthesized through a room temperature solvothermal method. The preparation techniques developed in this work allowed obtaining the ZnO via a low-cost and environment-friendly process, which takes place in aqueous media without the introduction of surfactants . The typical preparation procedure is as described in the literature with slight modifications : Firstly, 1.1 g Zn(AC)2 was dissolved into 80 mL DI water, and then 40 mL ethanol was added into the resultant Zn(AC)2 aqueous solution and stirred for 1 h. Then, 1 M of 80 mL NaOH aqueous solution was added with constant agitation at ambient temperature. After keeping stationary for 2 days, the precipitate was separated via filtration thoroughly with water and ethanol and dried at 60 °C for 12 h. Finally, the final product was obtained after aging at 300 °C for 30 h in air.
The crystalline phases of the as-prepared ZnO were characterized by X-ray diffraction (XRD, smart Lab, Rigaku Corporation, Cu Kα radiation). Nitrogen adsorption-desorption measurements were performed at 77 k by an Autosorb-iQ instrument (Quantachrome Instruments, USA). The specific surface area and the pore size distribution of the sample were evaluated via the Brunauer-Emmett-Teller (BET) method and Barret-Joyner-Halenda (BJH) method, respectively. Field emission scanning electron microscopy (SEM, S-4800, Hitachi Limited) and transmission electron microscopy (TEM, JEM-2100 F, JEOL) were applied to observe the surface morphology and the interior structure of the obtained products.
Coin-type cells (CR2025) with lithium metal as the counter and reference electrode were applied to investigate the electrochemical performance of samples at room temperature. The obtained ZnO samples were prepared as the working electrode. The ZnO powers were mixed with polyvinylidene fluoride (PVDF) as a binder and acetylene black conducting media of the weight ratio of 80:10:10 in 1-methyl-2-pyrrolidinone (NMP) solvent to prepare the electrode slurry. The slurry was uniformly coated onto nickel foam substrate with a doctor blade, and dried at 50 °C for 12 h. Circular disk electrodes 1 cm in diameter were prepared by punching under vacuum atmosphere. The active material loading in each electrode was about 2 mg cm−2. The separator was a polypropylene microporous film, and the electrolyte was 1 M LiPF6 dissolved in a mixed solution of dimethyl carbonate, diethyl carbonate, and ethylene carbonate (1:1:1 by volume). The cells were assembled in a glove box (MBraun) filled with argon atmosphere. Galvanostatic discharge/charge cycling tests were performed between 0.005 and 3 V at different current densities using a multichannel battery tester (BTS-5 V 5 mA, Neware). Between 0 and 3 V, cyclic voltammetry (CV) was carried out with a potentiostat (VMP3, Biologic) at a scanning rate of 0.5 mV s−1. All electrochemical measurements were performed at room temperature.
Results and Discussion
In this work, we developed mesoporous ZnO nanosheets via a room temperature solvothermal method, which possesses both the merits of nanometer-sized building blocks and micrometer-sized assemblies. As-prepared ZnO hierarchical aggregates are composed and assembled by mesoporous nanosheets with a length of 1–2 μm and interlaced ZnO nanosheets irregularly stacked together, forming a three-dimensional network. When tested as an anode material for lithium-ion batteries, the initial specific discharge capacity was 1523 mAh g−1 at 200 mA g−1, with 421 mAh g−1 maintaining after 100 cycles, and 325 mAh g−1 at 1000 mA g−1. The good electrochemical performance could be attributed to the hierarchical porous sheet-like structure of the ZnO architecture, which can not only provide a large specific surface area for lithium storage, but also favor the ion transport and electrolyte diffusion.
We acknowledge financial support from the Pearl River S&T Nova Program of Guangzhou (201506010045), the National Natural Science Foundation of China (21406052), and the Program for the Outstanding Young Talents of Hebei Province (BJ2014010).
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