Solid State Ionics 292 (2016) 27–31
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Electrospun SiO2/C composite ﬁbers as durable anode materials for lithium ion batteries YuRong Ren a,b,⁎, Bo Yang a,b, HengMa Wei a,b, JianNing Ding a,b,c,⁎⁎ a b c
School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China Jiangsu Province Cultivation base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, Jiangsu, China Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
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Article history: Received 16 December 2015 Received in revised form 4 May 2016 Accepted 6 May 2016 Available online xxxx Keywords: Anode material Electrospun SiO2/C ﬁbers
a b s t r a c t Electrospun together with thermal treatment is proposed to synthetize reticular amorphous SiO2/C composite ﬁbers. The as-prepared ﬁbers show carbon-coated SiO2 nanoparticles and form a space network structure, which cannot only improve the electrical conductivity, but also buffer the volume change. The SiO2/C ﬁber anode displays excellent performance, with an enhanced reversible capacity of 465 mAh/g at a current density of 50 mA/g up to 50 cycles, which is much higher than that of a pure SiO2 anode even at the ﬁrst cycle (113.8 mAh/g). Favorable rate property (~240 mAh/g at a current density of 500 mA/g) also exhibits in the further electrochemical test. The excellent electrochemical properties are attributed to the carbon coat and the unique structure. These results suggest that the ﬁber materials can be used as an anode for rechargeable lithium-ion batteries. © 2016 Published by Elsevier B.V.
1. Introduction Recently, Li-ion batteries (LIBs) have attracted a great deal of attention because of the broad prospects using in portable electronics, electric vehicles, and aerospace applications [1,2]. To improve the performance of LIBs, it is important to explore new electrode materials with high-energy capacity and long cycle life . Graphite has been widely used as commercial anode material due to its advantages of long cycle life and low cost, but its low lithium-storage capacity has become a huge obstacle to apply in high power ﬁeld [4–6]. Compared to other anode candidates such as Sn-based [7,8] and Fe-based  materials, Si-based materials are more attractive because of its abundant reserves and high theoretical capacity (4200 mAh/g). However, it shows a substantial volume change (400%) during the process of delithiation and lithiation, which results in pulverization and fast capacity fading [10–13]. As an oxide of silicon, SiO2 has been considered as a promising material owning to its high theoretical capacity of 1965 mAh/g . Since the Li2O and/or Li4SiO4 generated during the initial lithiation helps buff the volume change, silicon oxide has good cycling stability [15, 16]. Nevertheless, the poor electrical conductivity and strong Si–O
⁎ Correspondence to: Y.R. Ren, School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China. ⁎⁎ Correspondence to: J.N. Ding, Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.ssi.2016.05.002 0167-2738/© 2016 Published by Elsevier B.V.
bond in bulk crystalline SiO2 prevent it from becoming an electrode material. Since now, much effort has been devoted to the practical use of silica, including ﬁlms [17,18], hollow nanospheres , amorphous states  and so on [21–23]. Favors et al.  prepared SiO2 nanotubes via a facile two step hard-template growth method, and they showed a highly stable reversible capacity of 1266 mAh/g after 100 cycles with minimal capacity fading. In addition, carbon coating is a cheap and effective way to improve the electrochemical performance [24–26]. Gong  succeeded in preparing SiO2/C composites via a diazotization reaction and carbonizing treatment, which show good electrochemical performance. More recently, electrospun has been recognized as a simple and lowcost method to produce 1D nanoﬁbers from a few microns to 100 nm [27,28]. Using the electrospinning technique, a nonwoven thin ﬁlm can be easily obtained by the deposition of nanoﬁbers, without complex equipment or special substrates . This has been adopted by a number of researchers for the development of electrode materials [30–33]. Carbon nanoﬁbers (CNFs) containing graphene-wrapped silicon nanoparticles  were fabricated by electrospinning and subsequent thermal treatment, showing stable capacity retention. Core–shell structured nanoﬁbers were reported by Hieu , which had a capacity of 1000 mAh/g and coulombic efﬁciency of 99% at the 100th cycle. If the Si nanoparticles are replaced by SiO2 nanoparticles, better cycling stability will thus be achieved. Herein, a combination of electrospun and carbonization at 700 °C under nitrogen was used for the fabrication of SiO2/C composite ﬁbers. In this way, the carbon shell can wrap tightly around the SiO2 core, thereby buff the volume change and improve the conductivity of SiO2.
Y. Ren et al. / Solid State Ionics 292 (2016) 27–31
The electrochemical measurements revealed that the SiO2/C ﬁbers had a good electrochemical performance with higher speciﬁc capacity and better cyclic performance. 2. Experimental 2.1. Preparation of precursor solution for electrospinning 2.0 g PVP (Mw = 1,300,000) was dissolved in 20 ml anhydrous ethanol to form a solution as the carbon ﬁber precursor. Then 0.3 g SiO2 nanoparticles were added into the solution with a 24 h magnetic stirring and a 1 h ultrasonic dispersion. We named it as SiO2/C ﬁbers. Pure SiO2 nanoparticles were used as contrast samples. 2.2. Preparation of SiO2/C composite ﬁbers The static spinning precursor solution was added into a 5 ml syringe, whose needle diameter was 0.8 mm, and the injection speed was 1 ml/h for electrospinning, as is shown in Scheme 1. The spinning voltage was 16 kV, and the receiving distance was 18 cm. The obtained spinning precursor was ﬁrstly pre-oxidated for 1 h at 250 °C in air, and then carbonized for 4 h at 700 °C in nitrogen to form the ﬁnal product of SiO2/C composite ﬁbers. 2.3. Material characterization Transform infrared (FTIR) spectra were recorded on Nicolet Avatar 370 using a KBr pellet technique. The phase structures of the synthesized samples were characterized by X-ray diffraction (XRD, D/max 2500 PC) with Cu Kα radiation (λ = 1.5406 Å, operating at 40 kV × 40 mA). The morphologies were observed by ﬁeld emission scanning electron microscopy (FESEM, SUPRA55). The microstructure and distribution of the composites were examined by using transmission electron microscopy (TEM, JEM-2100). Thermogravimeric analysis (TGA) measurement was performed with TG 209 F3 by heating in air to 850 °C at a rate of 10 °C/min. 2.4. Electrochemical measurements Electrochemical performances were measured using two-electrode 2032 coin-type cells. The working electrodes were prepared by mixing active material, carbon black (Super-P), and sodium carboxymethyl cellulose (CMC) at a weight ratio of 80:10:10 and then pasted on thick copper foil. Pure lithium foil was used as a counter electrode. The cells were assembled in an Ar ﬁlled glove box, with a 1 M LiPF6 solution in a mixture of ethyl carbonate (EC), dimethyl carbonate (DMC) and ethyl carbonate (EMC) (1:1:1, v/v) as the electrolyte. The cells were tested in the voltage range of 0.01–3 V on a LAND battery tester.
Scheme 1. The ﬁgure of electrostatic spinning device.
Fig. 1. FT-IR spectra of (a) pure SiO2, (b) SiO2/C ﬁbers.
3. Results and discussion Fig. 1 is the FTIR spectra of as-prepared samples. For pure SiO2 nanoparticles in Fig. 1(b), the peaks at 470 cm−1, 798 cm−1, 1095 cm−1 and 3430 cm−1 are assigned to O–Si–O bending vibration, Si–O–Si symmetric stretching vibration, Si–O–Si unsymmetric stretching vibrations and O–H stretching vibration, respectively. Compared to Fig. 1(b), SiO2/C ﬁbers in Fig. 1(a) show almost the same peaks at the same positions, but the intensity of the peaks has weakened a lot. Fig. 2(a) shows the XRD pattern of pure SiO2 nanoparticles. The broad band indicates that the structure of SiO2 is amorphous. Fig. 2(b) gives the XRD pattern of SiO2/C ﬁbers. The wide peaks at about 22° (0 0 2), 43° (1 0 0) are attributed to the carbon layer coated on pure SiO2 nanoparticles, which correspond to the PVP carbonation peak , and the composite ﬁbers also have an amorphous structure. No silicon peak can be found in the pattern, which proves that the SiO2 is not reduced after high temperature carbonization. It was reported that SiO2 cannot be reduced to glasslike compounds such as SiO2-δ or Si–C–O by carbon at 1000 °C . In order to conﬁrm the content of amorphous SiO2 nanoparticles in SiO2/ C ﬁbers, the TGA data of them is collected and shown in Fig. 3. The prominent weight loss between 420 °C and 700 °C is related to the oxidization of carbon. Based on the curve, the SiO2 content in them can be gotten out of 44.1 wt.%. There is no gradually regained mass above 700 °C,
Fig. 2. XRD pattern of (a) pure SiO2 and (b) SiO2/C ﬁbers.
Y. Ren et al. / Solid State Ionics 292 (2016) 27–31
Fig. 3. TGA analysis curves of SiO2/C ﬁbers.
Fig. 5. Charge–discharge potential curves vs. the capacities for the pure SiO2, CNFs and SiO2/C ﬁbers the in the ﬁrst cycle.
which proves that silicon is not existent in the composites. It is consistent of the result of XRD pattern. The FESEM images of SiO2/C ﬁbers in Fig. 4(a, b) reveal that the ﬁbers with diameter of about 500 nm are curved and form a reticular structure arranged in a crisscross pattern. The speciﬁc surface areas of the SiO2/C ﬁbers are 331.6 ± 0.3 m2/g from nitrogen adsorption measurements. There are also large gaps between the ﬁbers, providing a fast electronic and ionic pathway, thus enhancing the reaction kinetics. Meanwhile, SiO2 nanoparticles are tightly wrapped into carbon ﬁber frame. It can improve the electrical conductivity of silica, effectively. The SiO2/C ﬁbers (Fig. 4b) show a smooth morphology, illustrating that SiO2 nanoparticles are dispersed uniformly in the composites without agglomeration,
which has an important inﬂuence on their performance. The SiO2/C ﬁbers were further characterized by TEM (Fig. 4c, d). There are a lot of dark spots in the composite ﬁbers after carbonization, which are testiﬁed as SiO2. No bare nanoparticles can be observed from the images, as same as Fig. 4(b). The electrochemical properties of the SiO2/C ﬁbers anode, CNFs anode and the pure SiO2 anode were measured using a constant current density of 50 mA/g. As shown in Fig. 5, the SiO2/C ﬁber anode displays a gentle slope at about 0.6 V, which is related to the decomposition of electrolyte and formation of the solid dielectric layer (SEI). This process generates a capacity loss of approximately 100 mAh/g. Because the process of the electrochemical reaction mainly includes the reduction of
Fig. 4. SEM images (a, b) and TEM images (c, d) of SiO2/C ﬁbers.
Y. Ren et al. / Solid State Ionics 292 (2016) 27–31
Fig. 6. Cycling performance of pure SiO2, CNFs and SiO2/C ﬁbers at a current density of 50 mA/g.
SiO2 and the generation of Li2O and a series of silicate, it also results in irreversible capacity . The discharge and charge capacities of SiO2/C ﬁbers at the 1st cycle are 740.3 mAh/g and 503.7 mAh/g, respectively, with a good initial coulombic efﬁciency of 68%, which are higher than those of CNFs. Compared to them, the pure SiO2 only has a ﬁrst discharge capacity of 113.8 mAh/g. The huge difference mainly comes from the electrical contact among materials. Owing to the carbon layer, the electrical conductivity of SiO2/C ﬁbers has been improved, thus the electrochemical performance is enhanced. Cycling performance of SiO2/C ﬁbers, CNFs and pure SiO2 is shown in Fig. 6 with a cut off voltage of 0.01–3.0 V vs. Li+/Li. The SiO2/C ﬁbers exhibit excellent stability and the speciﬁc capacity of them is much higher than that of pure SiO2. It is found that the discharge capacity of pure SiO2 decreases rapidly and remains only 26% of the initial capacity at the second cycle, which can be attributed to the pulverization of SiO2. Though the CNFs are stable, the capacity of them is about half of the SiO2/C ﬁbers. In a comparison, the SiO2/C ﬁbers remain a rechargeable capacity of about 465 mAh/g for nearly 45 cycles. The reversible capacity retention of them is more than 63% and the coulombic efﬁciency (CE) has been kept over 99% since from the ﬁrst cycle. The rate capability of SiO2/C ﬁbers was tested at various rates and shown in Fig. 7. The cells
were ﬁrst cycled at 50 mA/g, and then switched to 100, 200, 500, 50 mA/g, successfully. They all show good rate capability and stability. At ﬁrst, the reversible capacity of the composites is kept at 450 mAh/g, and there is still 240 mAh/g left at current density of 500 mA/g, which is more valuable than that of pure SiO2 at 50 mA/g. When the current density returns to 50 mA/g, a reversible speciﬁc capacity over 460 mAh/g can be obtained. It is even a little higher than that at the ﬁrst 10 cycles. The good electrochemical properties are related to the unique structure of the composites. Impedance experiments were applied to explore the effect of carbon layer coated on SiO2 nanoparticles on the interfacial impedance of SiO2/ C ﬁbers. The semicircle at high frequency can be ascribed to the charge transfer resistance, which is linked to the electrochemical reaction between the particles or between the electrode and the electrolyte. The sloping line is related to lithium-ion diffusion in the active material. In Fig. 8(a), the impedance of SiO2/C ﬁbers is lower than that of pure SiO2, which means good electrical conductivity of SiO2/C ﬁbers. After one cycle, the resistance of the surface ﬁlm, Rf, decreased to about 40 Ω, because the electrode is activated. Fig. 8(b) exhibits SiO2/C ﬁbers after different cycles. There are no obvious changes about impedance from the 1st cycle to 50th cycle, which illustrates that SiO2/C ﬁbers can buff the volume change in cycles and keep stable. 4. Conclusion In summary, SiO2/C composite ﬁbers were successfully prepared via electrospun technique and thermal treatment. The macrostructure of the ﬁbers is composed of carbon-coated SiO2 nanoparticles. Compared to pure SiO2 nanoparticles, SiO2/C ﬁbers can prevent the agglomeration and improve the electrical conductivity of SiO2, in which the carbon layer plays a prominent role. Meanwhile, the unique structure can buff the volume change and shorten Li+ ion diffusion paths. Thus, high capacity, good stability and rate property are obtained. The synthesis process can also be used to improve the electrochemical properties of materials like SiO2. Acknowledgments
Fig. 7. Rate capability of SiO2/C ﬁber electrodes measured at different rates between 50 and 500 mA/g.
This study was supported by National Natural Science Foundation of China (21576030, 51304077 and 51374175), Science and Technology Department of Science and Technology of Project in Jiangsu Province (BY2014037-31), the Opening Project of State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (Grant
Y. Ren et al. / Solid State Ionics 292 (2016) 27–31
Fig. 8. EIS spectra of the (a) SiO2/C ﬁbers and pure SiO2 before cycling, (b) SiO2/C ﬁbers after different cycles.
No. LAPS15001). Material Corrosion and Protection Key Laboratory of Sichuan province (2014CL15) and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 15KJA150002) were also acknowledged. References                  
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