Facile Synthesis of Porous Zn–Sn–O Nanocubes and Their Electrochemical Performances
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Porous Zn–Sn–O nanocubes with a uniform size were synthesized through a facile aqueous solution route combined with subsequent thermal treatment. The chemical composition, morphology, and microstructure of Zn–Sn–O nanocubes, which have significant effects on the lithium storage performances, were easily tuned by adjusting the calcination temperature in preparation processes of ZnSn(OH)6 solid nanocubes. Further studies revealed that porous Zn–Sn–O nanocubes prepared at 600 °C exhibited a good rate capability and a high reversible capacity of 700 mAh g−1 at a current density of 200 mA g−1 after 50 cycles, which may be a great potential as anode materials in Lithium-ion batteries.
KeywordsZn–Sn–O Nanocubes Porous Lithium-ion batteries
Lithium-ion batteries (LIBs), currently as versatile power sources for various portable electronics, are now considered for applications in electric or hybrid electric vehicles [1, 2, 3, 4, 5, 6]. Such applications require LIBs of high power, high energy density, long cycle life, excellent safety, low toxicity, and low cost [1, 7, 8]. To meet these requires, a great deal of efforts have been made to take a further step in anode electrode materials with superior performances [9, 10, 11, 12, 13, 14, 15]. Among various developed anode materials, Sn-based oxide compounds, including SnO2, ZnSnO3, CoSnO3, Zn2SnO4, and so on, have attracted an extensive attention as potential substitutes for graphite anodes because of their higher theoretical capacity [16, 17, 18, 19, 20, 21]. Unfortunately, the practical application of Sn-based oxide anode materials is usually hindered by drastic volume change of 300 % during Li+ insertion/extraction process, which results in very rapid capacity decay and pulverization of electrodes [7, 16, 22, 23]. Fabrication of porous nanostructures is one of the most effective methods to solve the problem and improve the cycle performance, because the local empty space in porous structures can partially accommodate the large volume change [24, 25]. In past few years, porous nanostructures based on Sn-based oxides were usually prepared through chemical vapor deposition, hydrothermal reaction, hard or soft template method, and so on [26, 27, 28, 29, 30].
Although the electrochemical performances of porous Sn-based oxide anodes have been improved a lot, the preparation procedures are usually complicated and costly. It is still a challenge to explore a facile approach for fabricating porous Sn-based oxide anode materials with controllable morphology and good electrochemical properties for practical applications. Herein, porous Zn–Sn–O nanocubes with different morphology and microstructure were synthesized by sintering ZnSn(OH)6 solid nanocubes at different temperatures. The obtained porous Zn–Sn–O nanocubes exhibited high reversible capacity and good rate capability.
2 Experimental Section
2.1 Synthesis of Porous Zn–Sn–O Nanocubes
In a typical procedure, 2.876 g of ZnSO4·7H2O was dissolved into 150 mL deionized water, and then 2.848 g of Na2SnO3·4H2O was added under continuous stirring at room temperature. After stirring for 5 h, the resulted white precipitation was collected by centrifuging, washing with deionized water for several times, and drying in air at 80 °C. Finally, porous Zn–Sn–O nanocubes were obtained by sintering the as-prepared white precursors at respective 500, 600, and 700 °C for 2 h in air with a heating rate of 1 °C min−1.
Morphology of the obtained products was characterized by field emission scanning electron microscope (FESEM, JSM-7401F) and transmission electron microscope (TEM, JEOL, JEM-2100). Powder X-ray diffraction (XRD) was recorded on a Shimadzu XRD-6000 with Cu-Kα radiation, in which the voltage and current of X-ray tube are of 40 kV and 30 mA, respectively. Thermogravimetric analysis (TGA) was performed on a SDT Q600 thermoanalyzer (DSC-TGA, TA, USA) in air. The specific surface area and pore size distribution were measured by a NOV A2200e analyser (Quantachrome, USA).
2.3 Electrode Fabrication
Working electrode was fabricated as follows: First, porous Zn–Sn–O nanocubes (70 wt%), Super-P carbon black (15 wt%), and sodium carboxymethyl cellulose (CMC, 15 wt%) were mixed in water to form a slurry. Then, the slurry was spread onto a Cu foil by a doctor blade method, followed by drying in vacuum at 80 °C for 8 h. A lithium foil acted as both the counter electrode and reference electrode, and a microporous polypropylene membrane (Celgard 2500) was used as separator. Then, CR2016 coin cells were assembled in an argon-filled glove box with moisture and oxygen contents below 1 ppm. The electrolyte was 1 M of LiPF6 in the mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 vol%). Charge–discharge cycles of cells were measured between 0.01 and 2.0 V versus Li+/Li using a battery test instrument (LAND CT2001A model, Wuhan Jinnuo Electronics, China) at room temperature. Cyclic voltammetry (CV) was conducted on the workstation at a scan rate of 0.1 mV s−1 in a potential range of 5 mV–2.0 V (vs. Li/Li+).
3 Results and Discussion
The theoretical weight loss based on the above formula is about 18.9 %, which is consistent with the experimental result. The SEM image (Fig. 1c) shows that the obtained ZnSn(OH)6 is composed of uniform and monodisperse nanocubes with side length of 80–120 nm. The TEM image suggests that the as-prepared ZnSn(OH)6 nanocubes are in solid morphology (Fig. 1d).
When sintering at 500 °C, ZnSn(OH)6 lost three water molecules to form amorphous ZnSnO3. At 600 °C, amorphous ZnSnO3 decomposed into amorphous Zn2SnO4 and crystalline SnO2. With calcination temperature further increasing to 700 °C, the state of Zn2SnO4 transferred from amorphous to crystalline.
In the first cycle for Zn–Sn–O-600 (Fig. 5a), two cathodic peaks, located at 0.53 and 0.05 V, correspond to the multistep electrochemical lithiation process with the decomposition of Zn2SnO4 and SnO2 (Eqs. 5 and 6) and the formation of alloys (Eqs. 7 and 8). Meanwhile, the anodic peak at 0.63 V corresponding to the dealloying reaction and a broad anodic peak at ~1.35 V are attributed to the oxidation of Sn (1.3 V) and Zn (1.5 V).
Figure 5b depicts the charge/discharge voltage profiles of the 1st, 2nd, 10th, 30th, and 50th cycle for the porous Zn–Sn–O-600 nanocubes cycled between 0.01 and 2.0 V at a current density of 200 mA g−1. The initial discharge and charge capacities are about 1290 and 860 mAh g−1, respectively. The large capacity loss in the first cycle is mainly attributed to the initial irreversible formation of Li2O and inevitable formation of a solid electrolyte interface (SEI) layer as well as additional reaction from conducting agent, which is common for most anode materials [35, 36, 37, 38]. Figure 5c shows the cycling performances of the as-prepared porous Zn–Sn–O nanocubes. The initial discharge capacities are about 1470 (Zn–Sn–O-500), 1290 (Zn–Sn–O-600), and 1230 (Zn–Sn–O-700) mAh g−1 at the current density of 200 mh g−1. However, the discharge capacity of Zn–Sn–O-500 exhibits a rapid decline, and Zn–Sn–O-700 just maintains 590 mAh g−1 after 50 cycles, while Zn–Sn–O-600 can keep around 700 mAh g−1. These results indicate that Zn–Sn–O-600 has a better capacity and cycling performance. To evaluate the rate capability of the obtained products, porous Zn–Sn–O-600 nanocubes are cycled at various current densities ranging from 200 to 1000 mA g−1. From Fig. 5d, one can see that only small capacity decreases as the current density increasing, and a stable reversible capacity of 538 mAh g−1 can be maintained even at a high current density of 1000 mA g−1. When the current density returns to 200 mA g−1, stable reversible capacity of 663 mAh g−1 can be restored for Zn–Sn–O-600, indicating the stability of the porous Zn–Sn–O anode materials.
In summary, a facile, low-cost, and scalable process was developed to synthesize porous Zn–Sn–O nanocubes. The chemical composition, morphology, and microstructure of Zn–Sn–O nanocubes were easily controlled by adjusting the calcination temperature. Electrochemical evaluation reveals that the porous Zn–Sn–O nanocubes prepared at 600 °C exhibited a good rate capability and high reversible capacity of 700 mAh g−1 at a current density of 200 mA g−1 after 50 cycles. Given the synthetic convenience, scalability for quantity production, and better lithium storage property, we believe that the as-prepared porous Zn–Sn–O nanocubes could elicit widespread interest in lithium-ion batteries or other significant applications.
This work is supported by the National Basic Research Program of China (2014CB239700), the Program of National Natural Science Foundation of China (21501120, 21371121 and 21331004), and Science and Technology Commission of Shanghai Municipality (14DZ1205700 and 14DZ2250800).
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