Straightforward Approach toward SiO2 Nanospheres and Their

Mar 17, 2014 - and it still remains at 876.7 mAh g. −1 even at a rate of 1 C over 500 cycles, showing a certain degree of increase. It can be consid...
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Straightforward Approach toward SiO2 Nanospheres and Their Superior Lithium Storage Performance Jiguo Tu, Yan Yuan, Pan Zhan, Handong Jiao, Xindong Wang, Hongmin Zhu, and Shuqiang Jiao* State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P. R. China ABSTRACT: SiO2 nanospheres were prepared according to a straightforward approach and evaluated as an anode material for lithium-ion batteries. The morphologies display that the obtained amorphous SiO2 powder has good uniform nanospheres with an average diameter of 400 nm. The SiO2 anode shows an initial charge capacity of 622.1 mAh g−1, and it still remains at 876.7 mAh g−1 even at a rate of 1 C over 500 cycles, showing a certain degree of increase. It can be considered that the superior performances are attributed to the partial generated Si, Li2O, and Li4SiO4 matrixes acting as a shielding layer, hence improving the lithium storage performance. In short, the discussed results suggest that SiO2 nanosphere is a promising anode material to improve the electrochemical performances for lithium-ion batteries.

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INTRODUCTION

meets the application demands for the high rate performance LIBs in our daily life. We herein adopt a straightforward approach to synthesize SiO2 hoping for a good cycle performance at a higher current density. In this study, different testing instruments were used to determine the composition and chemical structure of the asprepared SiO2, and the electrochemical reaction mechanism of SiO2 with Li ions was also investigated after different discharge/ charge states.

Lithium-ion batteries (LIBs) have been widely used in portable electronic devices, electric vehicles, and hybrid electric vehicles due to their superior properties such as high energy density, long cycle life, no memory effect, and environmental friendliness. To meet the demand for the scaled-up LIBs, the development of the higher performance electrode materials is necessary. As one of the higher capacity anode materials as an alternative to the traditional commercial graphite anode (theoretically 372 mAh g−1), Si has been paid more attention on account of the highest known theoretical capacity of 4200 mAh g−1 corresponding to Li22Si5.1−4 However, its severe volume expansion (>300%) has been the major disadvantage associated with the various phase transitions during the lithiation and delithiation processes, which causes the shrinkage, fracture, and pulverization of Si particles and loss of the electrical contact, thus leading to the rapid capacity fading and significantly limiting its commercial application for LIBs.5−8 So far, silicon dioxide (SiO2) has been suggested as an alternative anode material9−12 because of its improved cycling stability compared to Si. Besides, SiO2 is the major composition of sand and quartz and widely exists in nature. It can be noticed that the lithiation of SiO2 can result in the formation of Si, lithium oxide (Li2O), and lithium silicates, which play a buffer role in accommodating the volume change of LiSi alloy. Chang et al.13 demonstrated that a 24 h milled SiO2 exhibited a reversible capacity of ∼800 mAh g−1 at a current of 100 mAg−1 over 200 cycles, and Yan et al.14 later recommended that the hollow porous SiO2 nanocubes exhibited a reversible capacity of 919 mAh g−1 after 30 cycles at a low current density of 100 mAg−1. Despite these encouraging improvements, to the best of our knowledge, the above-mentioned current density hardly © 2014 American Chemical Society

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EXPERIMENTAL SECTION SiO2 nanospheres were prepared according to the following straightforward approach. Briefly, 1.86 g of tetraethyl orthosilicate (TEOS, 99%) was first dissolved in a mixture of 30 g of ethanol and 9 g of deionized water in a glass bottle. Afterward, 50 mL of concentrated ammonia solution (NH3· H2O, 28%) was added dropwise to the above solution under vigorous magnetic stirring within 10 min. The hydrolysis of TEOS and related condensation reactions lasted for another 4 h under continuous magnetic stirring. The SiO2 nanospheres were washed by centrifuge with ethanol and deionized water thoroughly and then dried at 60 °C overnight. Compositional analysis for the as-prepared sample was performed with Fourier transform infrared spectroscopy (FTIR, Nicolet, Nexus-470) and X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra DLD), respectively. The crystallographic structure and morphologies of the obtained SiO2 were characterized by X-ray diffraction (XRD, Rigaku, D/ max-RB), field emission scanning electron microscopy Received: January 31, 2014 Revised: March 15, 2014 Published: March 17, 2014 7357

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(FESEM, JEOL, JSM-6701F), and transmission electron microscopy (TEM, JEOL, JSM-2010). The all electrochemical measurements were performed using two-electrode coin half-cells (CR2032) with metallic lithium foil (45 μm in thickness) as the counter and reference electrode and Celgard 2325 microporous membrane as the separator. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) (1/1/ 1 in volume). SiO2 (active material), CNT (conductive agent), and PTFE emulsion (binder) in a weight ratio of SiO2/CNT/PTFE = 5:3:2 was an ultrasonic dispersion in ethanol above 20 min, forming the mixture slurry. Then the working electrode was fabricated by evenly coating the mixture slurry onto a copper foil current collector (10 μm in thickness), and further dried at 60 °C over 24 h. Then, the cells were assembled in a glovebox filled with Ar gas. The galvanostatic discharge−charge tests were measured with the cutoff potential window of 2.5 and 0.01 V (vs Li/Li+) (Neware BTS 5 V10 mA, Shenzhen). Cyclic voltammetry (CV, CHI 1140A) measurement was performed with a scan rate of 0.05 mV s−1 between 2.5 V and 1 mV (vs Li/Li+).

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RESULTS AND DISCUSSION SiO2 nanospheres can be synthesized by the hydrolysis and condensation reactions of TEOS. As known to us, under the slight alkaline condition, OH− can directly attack the Si atomic nucleus and take the place of hydrolysis (reaction 1). Then, the subsequent condensation reaction (reaction 2) occurs among the forming Si(OH)4 from the reaction 1. Finally, the condensation product was washed by centrifuge to obtain SiO2 nanospheres.

Figure 1. FTIR spectrum (a) and XPS spectrum of Si 2p (b) of the asprepared sample.

shown in Figure 2c, it can be further observed that SiO2 nanospheres are uniformly dispersed. The SAED image in Figure 2d also reveals that the as-synthesized SiO2 nanospheres are amorphous. Cyclic voltammetry was used to investigate the electrochemical process of the SiO2 anode. Figure 3a shows CV curves at a scan rate of 0.05 mV s−1 cycled between 2.5 V and 1 mV. It can be found that there is substantial difference between the first cycle and the subsequent cycles. In the first discharge scanning process, the broad reduction peak around 0.72 V can be observed, which is attributed to the formation of a solid electrolyte interphase (SEI) and Li2O (reaction 4). The cathodic peak below 0.4 V is due to the lithiation forming LiSi alloy. A prominent redox couple at about 0.1 V must be associated with the reversible alloy−dealloy reaction with Li ions (reaction 5), which makes the contribution to lithium storage capacity. During the subsequent scanning cycles, the CV curves become almost stable, which are indicative of the reversible behavior to some extent. A cathodic peak is present at around 0.68 V, which strongly indicates that an irreversible reaction of reaction 4 still takes place in the subsequent cycles. Moreover, it should be noted that an obvious anodic peak at around 1.2 V always appears during the initial five oxidation cycles. The Li−Si alloy/dealloy reaction only occurs below 1.0 V, which suggests that this is most likely due to the partial reversibility of Si to SiO2 (reaction 4). A similar phenomenon could be found in other previous literatures.19−22 On the basis of the above discussion, the reaction of SiO2 with Li can be expressed as follows:23

The total hydrolysis−condensation reaction of TEOS is as follows: Si(OC2H5)4 + 2H 2O → SiO2 + 4C2H5OH

(3)

The FTIR spectrum in Figure 1a shows the characteristic transmittance of the as-prepared powder. As one can see, the shoulder peak at 1162 cm−1 can be assigned to C−H in-plane bending mode, and the sharp peak at 1090 cm−1 to the Si−O− Si stretching band of SiO2. These results are in good agreement with previous spectroscopic characterizations of SiO2.15−17 XPS was employed for further confirming the composition of the asprepared powder, as shown in Figure 1b. The Si 2p spectrum shows that the peak is located at about 103.4 eV, which indicates that Si exists in the state of SiO2.18 The crystallographic structure of the obtained SiO2 was analyzed by XRD, as shown in Figure 2a. It can be seen that there is a weak broadening band at about 23°, which indicates the presence of amorphous SiO2. The morphology of the asprepared amorphous SiO2 was then characterized by FESEM and TEM. As shown in Figure 2b, it displays good uniform SiO2 nanospheres with an average diameter of 400 nm and shows a certain degree of aggregation. From the TEM image 7358

SiO2 + 4Li+ + 4e− → 2Li 2O + Si

(4)

Si + x Li+ + x e− ↔ LixSi

(5)

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Figure 2. XRD pattern (a), FESEM image (b), TEM image (c), and selected area electron diffraction (SAED) (d) of the as-prepared SiO2 nanospheres.

Figure 3b displays the discharge−charge behavior of the SiO2 anode for the first five cycles in the cutoff potential window of 2.5−0.01 V. The theoretical reversible special capacity of SiO2 is calculated based on the reactions 4 and 5 (supposing that Li22Si5 is the end product of discharge), i.e., 1965.4 mAh g−1. The specific capacity of the electrode is calculated on the basis of the weight of SiO2 in this work. It can be found that the discharge−charge curves are remarkably consistent with the above CV curves in Figure 3a. For the SiO2 anode, the initial discharge and charge capacities are 2420.7 and 1083.8 mAh g−1, respectively. The initial irreversible capacity loss is mainly due to the formation of the SEI layer and the irreversible reaction of SiO2 with Li ions. To further clarify the variation of the SiO2 anode in different state, XPS spectra were tested after being discharged to 0.01 V and after charged to 2.5 V, respectively, as shown in Figure 4. It can be found from Figure 4a that the binding energy of Si 2p decreases 0.8 eV after being discharged to 0.01 V compared to the pristine SiO2 (Figure 1b), which suggests that the nanoSiO2 is reduced to some extent. The remarkable broadening of the Si 2p peak indicates that the chemical state of Si is complicated rather than a single one. Upon fitting the Si 2p spectrum of the SiO2 anode after being discharged to 0.01 V demonstrates that Si exists in the state of SiO2 from the part that does not completely react: Li4SiO4 from reaction 6 and LiSi alloy.24 It can be also seen that the peak at 102.3 eV has remained largely unchanged after being charged to 2.5 V (Figure 4c), suggesting that the formation of Li4SiO4 is almost irreversible. Simultaneously, it can be observed from Figure 4b,d that Li is mainly in the form of Li2O except for Li4SiO4. Therefore, it is suggested that both Li2O (reaction 4) and

Figure 3. CV curves at a scan rate of 0.05 mV s−1 (a) and the discharge−charge profiles at a rate of 0.1 C (b) of the SiO2 anode.

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Figure 4. XPS spectra of Si 2p (a) and Li 1s (b) after being discharged to 0.01 V, and Si 2p (c) and Li 1s (d) after being charged to 2.5 V for the SiO2 anode.

Li4SiO4 (reaction 6) are coexistent in the electrochemical reaction between SiO2 and Li ions. 2SiO2 + 4Li+ + 4e− → Li4SiO4 + Si

(6)

The long-term cycling stability of the SiO2 anode was tested at a rate of 1 C for 500 cycles, as shown in Figure 5a. The SiO2 anode shows an initial discharge and charge capacity of 1134.5 mAh g−1 and 622.1 mAh g−1, with a Coulombic efficiency of 54.8%. Importantly, it can be found that the charge capacity still remains at 876.7 mAh g−1 even over 500 cycles, verifying the superior cycling stability at a higher rate. The as-prepared SiO2 nanospheres show significantly enhanced capacity and cycle stability with comparison to the previously reported hollow SiO2 nanospheres.12 In particular, it is important to observe that the capacity of the SiO2 anode can decrease for the first several cycles, then increase to a certain degree in the following cycles, which is suggested to result from the improved Li-ion diffusion kinetics by an activation and stabilization process during cycling. A similar phenomenon was also observed in other previous reports.25−28 Moreover, the formation of Li2O and Li4SiO4 matrixes via reactions 4 and 6 helps to improve the lithium storage performance by alleviating the volume expansion during cycling. Simultaneously, the conductive material CNT is also bound to contribute a part of the whole specific capacity and makes for the relatively stable capacity of the SiO2 anode. As a further important feature for LIBs, the rate capability of the SiO2 anode was also tested at various rates, as shown in Figure 5b. The reversible capacity of the SiO2 anode decreases from 1100.3 to 954.3 mAh g−1 over 20 cycles at a lower rate of 0.1 C. Upon increasing the current rate, the SiO2 anode can still achieve a capacity of 861.6, 821.1, 749.6, and 757.8 mAh g−1 at a rate of 0.2, 0.5, 1, and 2 C, respectively. Afterward, when the current rate returns to 0.1 C after cycling at the higher rate of 2 C, the capacity can increase to 1085.3 mAh g−1. Over another

Figure 5. Cycling performance at a rate of 1 C (a) and the rate capability at various rates (b) of the SiO2 anode.

20 cycles at a rate of 0.1 C, the capacity further increases to 1175.5 mAh g−1, showing a very excellent rate capability. To investigate the degradation mechanisms of SiO2 anode, the morphology of the as-prepared SiO2 electrode before and after 500 cycles was characterized by FESEM. As shown in Figure 6a, it displays uniformly dispersed SiO2 and CNT on the 7360

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generated Si, Li2O, and Li4SiO4 matrixes acting as a shielding layer, hence improving the lithium storage performance.

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AUTHOR INFORMATION

Corresponding Author

*(S.J.) Fax: +86-10-62334204. Tel: +86-10-62334204. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 51322402), the Program for New Century Excellent Talents in University (NCET-2011-0577), Ministry of Education of China, and the Fundamental Research Funds for the Central Universities (FRF-TP-12-002B and FRF-AS-11003A).

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Figure 6. FESEM images of SiO2 electrode before (a) and after (b) 500 cycles at a rate of 1 C.

surface of the pristine electrode. From the FESEM image shown in Figure 6b, it can be observed that SiO2 nanospheres are partially exposed after 500 cycles at a rate of 1 C, possessing an even morphology without any fracture. As a result, the uniform structure would play a beneficial role on improving the electrochemical properties. According to the above discussion, it can be suggested that SiO2 nanosphere is a promising anode material to improve the electrochemical performances for LIBs.

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CONCLUSIONS In summary, we have successfully synthesized SiO2 nanospheres according to a straightforward approach. It is confirmed that the obtained amorphous SiO2 powder has good uniform nanospheres with an average diameter of 400 nm. The SiO2 anode shows an initial charge capacity of 622.1 mAh g−1, and it still remains at 876.7 mAh g−1 even at a rate of 1 C over 500 cycles, showing a certain degree of increase. Moreover, the reversible capacity of the SiO2 anode decreases from 1100.3 to 954.3 mAh g−1 over 20 cycles at a lower rate of 0.1 C. Afterward, when the current rate returns to 0.1 C after cycling at various rates, the capacity can increase to 1085.3 mAh g−1, showing a very excellent rate capability. It can be considered that the superior performances are attributed to the partial 7361

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