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Dispersing SnO2 nanocrystals in amorphous carbon as a cyclic durable anode material for lithium ion batteries
Journal of Energy Chemistry 23(2014)338–345
Dispersing SnO2 nanocrystals in amorphous carbon as a cyclic durable anode material for lithium ion batteries Renzong Hu, Wei Sun, Meiqin Zeng, Min Zhu∗ School of Materials Science and Engineering, South China University of Technology, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, Guangzhou 510640, Guangdong, China [ Manuscript received October 2, 2013; revised January 7, 2014 ]
Abstract We demonstrate a facile route for the massive production of SnO2 /carbon nanocomposite used as high-capacity anode materials of nextgeneration lithium-ion batteries. The nanocomposite had a unique structure of ultrafine SnO2 nanocrystals (∼5 nm, 80 wt%) homogeneously dispersed in amorphous carbon matrix. This structure design can well accommodate the volume change of Li+ insertion/desertion in SnO2 , and prevent the aggregation of the nanosized active materials during cycling, leading to superior cycle performance with stable reversible capacity of 400 mAh/g at a high current rate of 3.3 A/g. Key words lithium ion battery; anode; SnO2 nanocrystals; amorphous carbon; facile strategy
1. Introduction Lithium ion batteries (LIBs) have been considered as attractive power sources not only for portable consumer electronics but also for upcoming electric/hybrid vehicles and smart grids due to their potential for high power and high energy density [1,2]. To meet the challenges of LIBs with high energy density and long cycle life for use in electric/hybrid vehicles and so forth, great efforts are being made to explore new electrode materials with novel structures and higher specific capacities [3−5]. Accordingly, different kinds of materials, such as transition-metal oxides (Fe3 O4 , Co3 O4 , MnO2 , etc.) [6−8] and alloy-based materials (Si, Sn, Cu6 Sn5 , etc) [9−10] have been studied for the alternative anode materials. Among them, SnO2 -based materials are widely concerned because of their low toxicity, widespread availability, and especially their total theoretical capacity up to as high as 1494 mAh/g (including 711 mAh/g for the conversion reaction and 783 mAh/g for the alloying reaction), which is four times higher than that of the currently used graphite (372 mAh/g) [11−15]. However, the great challenge for practical application of SnO2 anodes is the severe pulverization and capacity fading problems caused by the extremely large volume change of Sn (about 250% for Li4.4 Sn formation) during cycling [16]. To overcome these
problems, many methods have been studied to reduce the absolute volume change and cracking of active materials by the nanosize effects, especially via nano-architectured materials, such as nanoparticles, nanowires and nanotubes [17−19]. However, limited cycleability improvement has been obtained by simply nano-modifying the pure SnO2 , because the agglomeration of nanosized SnO2 and/or Sn is still unavoidable unless there is an extra confining buffer [11−15, 17−19]. In contrast, the nanopainting of SnO2 with various carbonaceous materials has recently been found effective for improving the cycleability of SnO2 -based anodes [13,19−25], because the carbonaceous materials not only enhance the electrical conductivity but also act as buffering layers for the large volume change of SnO2 . Actually, carbonaceous materials have been widely investigated to enhance performances for different electrode materials for LIBs and other batteries [26−28]. And thus, the SnO2 -carbon composite anodes with various microstructures have been designed and prepared by different strategies. For example, many carbon nanotube-encapsulated SnO2 composites have been prepared using the wet chemical filling and rapid vacuum absorption [24,25,29]. Yu et al. reported the synthesis of SnO2 @carbon core-shell nanochains by carbonization of a SnO2 @carbonaceous polysaccharide precursor at a relatively high temperature using a new lowflow-rate inert atmosphere strategy [30]. Zhang et al. elevated
Corresponding author. Tel: +86-20-87113924; Fax: +86-20-87111317; E-mail: [email protected] This work was supported by the National Science Foundation of China (Grant No. 51201065 and No. 51231003), the Natural Science Foundation of Guangdong Province (S2012040008050) and the Doctorate Foundation of Ministry of Education (Grant No. 20120172120007 and No. 2014ZZ0002). ∗
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60156-X
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the electrochemical properties of the carbon-coated SnO2 /graphene nanosheets, whose synthesis processes included first exfoliation of graphene oxide nanosheets from graphite oxide, and then growth of SnO2 , and followed by heating after an addition of glucose [31]. Read et al. prepared a SnO2 -carbon composite by heating a mixture of colloidal SnO2 (average size of 15 nm) and sugar (sucrose) as carbon precursor, which was finally carbonized to form a hard carbon and acted as a diluent to prevent nanosized SnO2 agglomeration [32]. Lou et al. designed a novel anode material of coaxial SnO2 @carbon hollow nanospheres which showed a high capacity of about 460 mAh/g and little capacity fading after 100 cycles [23]. Although some properties of these complexly designed SnO2 -carbonaceous anodes far exceed those of the conventional graphitic anode materials and demonstrate very high capacity and good stability, overall performance (especially the output of materials, the electrode loading or the capacity per unit area), as well as those rather complicated production processes of the SnO2 -based anode materials are far from their practical applications. Therefore, it is still desirable to develop a simple and more efficient way to prepare composites with nanosized SnO2 stored in the carbon matrix, which not only preserve high-dispersion state of the active SnO2 but also achieve high capacity together with enhanced cyclic performance. In this work, we develop a facile strategy, which is cost-effective and has a good potential for large scale applications, to synthesize a SnO2 -based carbonaceous nanocomposite anodes for LIBs. And a unique structure with homogeneous dispersion of SnO2 nanocrystals (∼5 nm) in amorphous carbon matrix is achieved. The origin of the enhanced anode performance of the SnO2 /carbon nanocomposite has been analyzed by microstructure observations combining with electrochemical characterizations. 2. Experimental 2.1. Materials fabrication Generally, the SnO2 /carbon nanocomposite was fabricated by simply boiling and following heating of Sn together with sugar. In a typical experiment, an aqueous solution consisting of 20 g Sn powders (∼100 mesh, 99.9% purity), 30 g crystal sugar (edible grade) and 180 mL deionized water was put in a stainless steel pot and then boiled in air using an electric oven (2 kW). The solution was heating for 1 h until all the water boiled dry and a brown charred production was achieved (name as charred product). Then, the obtained charred product was transferred to an alumina crucible and kept in a tube furnace at 400 ◦ C (based on the DSC result shown in Figure 1) for 4 h in flowing Ar atmosphere. Finally, brown powders (named as SnO2 /carbon nanocomposite) were obtained and were tested as electrode materials after grinding in a bowl. Pure nanosized SnO2 (with average diameters of 50 nm) was bought and also characterized as electrode for comparison.
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2.2. Materials characterization The structure and morphology of the samples were characterized using a Philips X-ray diffractometer (XRD) with Cu-Kα radiation, a Carl Zeiss Supra 40 field emission scanning electron microscope (SEM) and a JEOL JEM-2100 transmission electron microscope (TEM) operating at 200 kV. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis were made under Ar and/or O2 on NETZSCH STA409 with a heating rate of 10 ◦ C/min. In order to retain the original morphology of the as-prepared products, the SnO2 /carbon nanocomposite powders were directly dispersed on Cu grids for TEM measurements. 2.3. Electrochemical measurements The discharge-charge cycling performance of the samples was investigated using a cell test system (LAND-CT2001A) with CR2016 coin-type cells assembled in an argon-filled glove box. The working electrode consisted of 80 wt% of the active material (SnO2 /carbon nanocomposite or pure nanosized SnO2 ), 10 wt% conductivity agent (Super-P) and 10 wt% binder polyvinyldifluoride. Lithium foil was used both as a counter electrode and as a reference electrode in the half cells. The electrolyte was LiPF6 (1 mol/L) in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC) with volume ratio of EC/DEC/EMC = 1/1/1 (Shanshan Tech Co., Ltd.). The cells were tested at various current rates between 0.01 and 1.5 V at room temperature. Cyclic voltammetry (CV) over the potential range of 0–2.0 V at a scan rate of 0.3 mV/s, as well as impedance spectroscopy at 5 mV amplitude signal in the 1 MHz to 0.05 Hz frequency range, was performed using a Gamry Interface 1000 Electrochemical System. 3. Results and discussions The heating conditions of the Sn-sugar composite were set according to the thermal analysis results. Figure 1(a) shows the DSC and TG curves for the charred product of SnSugar composite after boiling. It can be seen that during heating scanning under Ar, there are two endothermic peaks at about 100 ◦ C (peak 1) and 270 ◦ C (peak 2), respectively. In addition, two exothermic peaks appeared among 300∼500 ◦ C (peak 3) and around 750 ◦ C (peak 4). The endothermic peak 1 should be attributed to the evaporation of the residual water, while the sharp peak 2 was induced by the melting of the Sn, suggesting that the Sn particles remained their elemental state even boiling in water and mixing with the charred sugar. With respect to the two exothermic peaks, peak 3 was originated from the following Reaction 1, which involved the decomposition of the sugar and oxidation of Sn to form SnO2 , with weight loss of about 20% as indicated by the TG curve. Sn(liquid) + (C-H2 O)n (solid) → SnO2 (s) + C(s) + CO2 (gas ↑) + H2O(g ↑)
(1)
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In this step, the micro-sized Sn particles were expected to be refined as SnO2 nanocrystals with a high-dispersion state in an amorphous carbon matrix. For the other peak around 750 ◦ C, it should be related to the crystal growth of the previous formed nanosized SnO2 . According to the above results, the further heating treatment of the charred product of Sn-Sugar composite was set to be 400 ◦ C for 4 h. After heating treatment of the Sn-sugar composite to form a SnO2 /carbon nanocomposite, the amount of SnO2 and carbon in the nanocomposite was measured using TG analysis under O2 atmosphere with a heating rate of 10 ◦ C/min. As shown in Figure 1(b), the weight percentage of the carbon could be evaluated to be about 20%, and that of the SnO2 was about 80%.
completely disappeared, while only obvious broadened peaks of SnO2 were remained. These indicated that the Reaction 1 has happened as the charred product of Sn-sugar composite keeping heating at 400 ◦ C, leading to that the microsized Sn particles oxidized to form SnO2 nanocrystals. And meanwhile, the charred sugar had been carbonized to be amorphous carbon, which induced the obvious uplifted background in this XRD pattern in Figure 2(a).
Figure 2. XRD patterns of the pure Sn powders (a), charred product of Snsugar composite after boiling (b) and that after heating at 400 ◦ C for 4h (c)
Figure 1. (a) DSC and TG curves of the charred product of Sn-sugar composite under Ar atmosphere, with a heating rate of 10 ◦ C/min from 35 to 900 ◦ C, (b) TG curve of the nanocomposite under O2 atmosphere with a heating rate of 10 ◦ C/min
Figure 2 shows the XRD patterns of the charred product of Sn-sugar composite and that after heating at 400 ◦ C for 4 h, in comparison with that of the pure Sn powders. The sharp diffraction peaks of pure Sn shown in Figure 2(c) reveal their coarse particle/grain size nature in the pure Sn powder sample. After boiling with the sugar, it can be seen in Figure 2(b) that, in addition to the strong diffraction peaks of pure Sn, there are weak diffraction peaks of SnO2 , which is generated from oxidization of a small amount of Sn. Moreover, the raised background among 20o −35o should come from the amorphous charred sugar. However, surprisingly, as shown in Figure 2(a) that, after heating at 400 ◦ C for 4 h, the diffraction peaks of Sn
The microstructure evolutions of the Sn-sugar composites were investigated by SEM and TEM observations. Figure 3(a) shows the initial SEM morphology of the pure Sn powders, revealing that the spherical Sn particles have large size range from 5 to 50 µm. However, as shown in Figure 3(b), after boiling with the sugar for 1 h, the spherical morphology of the microsized Sn has been completely disappeared, although most of the elemental Sn remained as revealed by the XRD in Figure 2(b). Instead, anomalous particles of the Sn-sugar composite with size less than 5 µm presented, while the large magnification SEM image in Figure 3(c) further revealed the multi-scale (micro- and nano-) inside of the charred product, suggesting that the large Sn particles have been dramatically refined by simply boiling and burning with the sugar. These should be due to that the burning of the composite lead to the Sn particles melting and separating into smaller ones, which was stirred simultaneously by the boiling sugar. Figure 3(d) shows the morphology of the charred product after heating in Ar at 400 ◦ C for 4 h, in which there is no obvious change
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in comparison with that of the composite before heating as shown in Figure 3(c). Nevertheless, the TEM image shown in Figure 3(e) reveals that a micro-sized (1−2 µm) powder of the heat-treated composite is the aggregation of several nanosized (200−300 nm) particulates, which are composed of the SnO2 nanocrystals as indicated by the inserted (top right corner) selected area electron diffraction (SAED) pattern. This is in well consistance with the XRD result as shown in Figure 2(c). And furthermore, the HRTEM image in Figure 3(f) shows that the SnO2 nanocrystals are very fine (about 5 nm), and they are homogenously distributed in the amorphous carbonaceous matrix. So far, based on the above observations, we can conclude that a SnO2 /carbon nanocomposite, with the expected microstructure of ultrafine SnO2 nanocrystals dis-
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persing in amorphous carbon, has been successfully prepared by the simple facile strategies. We would like to emphasize that these materials and the preparation approaches are much more cost-effective and easy to scale up (recently, we have already achieved kg scale in lab) in comparison with the materials obtained by other chemical reactions [19−25, 29−32]. Therefore, it has a good potential to be used for large scale applications. And since the SnO2 phase is ultrafine and the amorphous carbon matrix is flexible and excellent conductive, it is expected that such a nanocomposite structure can accommodate the large volumetric changes of SnO2 and prevent their aggregation during discharge/charge cycling, leading to enhanced cycle stability and high-rate capability of the lithium storage electrode.
Figure 3. (a) SEM image of the pure Sn powders; (b) low magnification and (c) large magnification SEM images of the charred product obtained from Sn-sugar composites boiling and burning for 1 h; (d) SEM image, (e) TEM image and SAED pattern (insert), and (f) HRTEM image of the SnO2 /carbon nanocomposite
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The electrochemical performances of the SnO2 /carbon nanocomposite as anode were firstly investigated by a cyclic voltammogram (CV) scan. As the typical CV curves of a half cell shown in Figure 4(a), the CV curves almost overlap from the second to fifth cycle, indicating that the cycle performance is considerably stable. The difference between the first and latter cycles is partly ascribed to the formation of solid electrolyte interface (SEI) layer. Two reduction peaks at 0.8 and 0.05 V are observed during the first cathodic scan. The formed peak at 0.8 V, which is absent in the subsequently cycles, is ascribed to the formation of an SEI layer and the reduction of SnO2 to Sn [11]. The latter peak at 0.05 V is known to arise from the formation of Lix Sn alloys. However, the oxidation peaks appearing at 0.6 and 1.3 V are correspondingly attributed to the de-alloying reaction and the oxidation of Sn to SnO2 , respectively. The latter oxidation peak (1.3 V) is recently reported in some literatures [33,34], which implied that the inverse process with Li2 O transition back to the Li+ takes place as the ultrafine SnO2 nanocrystals (less than 10 nm) well contacted with the carbon matrix. Figure 4(b) shows the initial five charge/discharge profiles at 0.25 A/g for the SnO2 /carbon nanocomposite anode. It can be seen that the first discharge and charge capacities are 1832 and 795 mAh/g, respectively, corresponding to an initial coulumbic efficiency of 43.5%. The relative large irreversible capacity of 1037 mAh/g could be assigned to the formation of irreversible Li2 O, and especially electrolyte decomposition and SEI formation on the nanocom-
posite electrode surfaces, as well as exfoliation of some SnO2 particles attached outside the electrode, in the first anode reaction cycle. Nevertheless, from the second cycle, a reversible capacity of 819 mAh/g with a coulumbic efficiency more than 91% was achieved. And the voltage trend was retained for the all following cycles, confirming the good reversibility of the reactions at the SnO2 /carbon nanocomposite electrode. Figure 5(a) displays the charge capacity (Li extraction) versus cycle number for the SnO2 /carbon nanocomposite anode, in comparison with pure SnO2 nanoparticles anode, at a constant current density of 0.25 A/g between 0.01 and 1.5 V. As can be seen, the capacity of the pure SnO2 nanoparticle anode decreased very rapidly during cycling. It delivers a reversible capacity of 634 mAh/g at the first cycle, which was retained only 11% (72 mAh/g) after 50 cycles. In striking contrast, the SnO2 /carbon nanocomposite possesses much better cycle performance. Its initial reversible capacity (795 mAh/g) could be remained 58.7% (447 mAh/g) after 50 cycles, which was much superior to those of the pure SnO2 nanoparticle anode. The enhanced cycleability should be attributed to the unique features of this SnO2 /carbon nanocomposite, including the ultrafine SnO2 nanocrystals with high-dispersion state in the carbon matrix and stress absorption by the carbon matrix. In addition, the possible formation of Sn–O–C metastable phases in the boundary between nanosized SnO2 and carbon would also enhance the structure stability of the SnO2 phases [31].
Figure 4. Electrochemical performance of the SnO2 /carbon nanocomposite anode: initial five cyclic voltammogram curves with a scanning rate of 0.3 mV·s−1 (a), initial five discharge-charge profiles at a current density of 0.25 A/g between 0.01 and 1.5 V (b)
Figure 5(b) shows the long-term cyclic performance of the SnO2 /carbon nanocomposite with a voltage window of 0.01−1.5 V at different current rates. It can be seen that, the initial charge capacity of 961 mAh/g at a slower rate of 0.13 A/g is higher than that of 795 mAh/g at a rate of 0.25 A/g. However, lower reversible capacities of 351 mAh/g (36.5%) and 254 mAh/g (26.4%) were retained after 50 and 100 cycles
at 0.13 A/g, respectively, suggesting that this nanocomposite yields relatively inferior capacity retention as cycling at slow current rates (0.13 and 0.25 A/g). In contrast, the nanocomposite anodes have much better cycleability at faster rates, which could be revealed by that, 57.7% and 74.7% of their initial charge capacities were maintained after 100 chargedischarge cycles at 0.62 A/g and 3.3 A/g, respectively. And
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especially, at the high current rate of 3.3 A/g, a very stable reversible capacity of 400 mAh/g was remained from the 10th cycle to the 100th cycle. These results confirmed the good cycleability and high-rate capability of this nanocomposite,
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which is undoubtedly attributed to its unique microstructure, i.e. ultrafine SnO2 nanocrystals, high-dispersion state and good contact between SnO2 nanocrystals and carbon matrix (i.e. good electrical conductivity).
Figure 5. (a) Cyclic performance of the SnO2 /carbon nanocomposite in comparison with that of pure SnO2 nanoparticle anode, with a voltage window of 0.01−1.5 V at a current rate of 0.25 A/g, (b) cycle performances of the SnO2 /carbon nanocomposite taken at different current rates
In order to investigate the impedance variation of the half cells with SnO2 /carbon nanocomposite electrodes during charge-discharge cycling, electrochemical impedance spectroscopy (EIS) measurements were carried out when discharge to 0.01 V and recharge to 1.5 V, respectively, at different cycles with a current rate of 0.25 A/g. The impedance response of the pure nanosized SnO2 particle electrode before cycling was also recorded for comparison. The spectra evolutions at the desired cycles are summarized in Figure 6(a–c), showing that all the spectra (Nyquist plots) have a small semicircle at high frequency, a relative large semicircle at middle frequency and an inclining line at lower frequency. It is well known that, with respect to the impedance spectra of electrode in lithium ion battery, the high frequency semicircle can be attributed to the contact resistance caused by SEI film (Rf ), the medium-frequency semicircle is related to the charge-transfer resistance at the interface between the electrolyte and the electrode material (Rct ) and the inclined lines correspond to the Li+ diffusion process inside the electrode material (Rw ) [35]. And thus, the Nyquist plots could be fitted using the equivalent circuit as shown in Figure 6(d), where the Rs represents the solution resistance, and Cf and Cct are the intercalation capacitance along with the Rf and Rct , respectively. The fitting results indicated that the charge transfer resistance for the cells after discharge (Figure 6b) first increased and then decreased as prolonging the cycles, and at last even lower than that of the cell before the cycle tests (Figure 6a), indicating that an “in-situ activation” phenomenon occurred for the cell as discharge/charge beginning. Furthermore, the effect of carbon matrix on the electronic conductivity of the
composite electrode was generally demonstrated by the dramatic change in impedance of the SnO2 /carbon nanocomposite relative to that of the pure nanosized SnO2 anode as also shown in Figure 6(a). Such an improved performance can be attributed to the synergy between the amorphous carbon matrix and the SnO2 nanocrystals during cycles. With respect to the spectra obtained after recharge to 1.5 V (Figure 6c), they almost overlap from the first to the 20 th cycle, indicating that the SEI film and charge transfer resistances keep considerably stable in the recharged electrodes during cycling. It should be pointed out that, the higher impedances in the recharged electrodes, compared with those of the discharged electrodes, could be attributed to the inverse process with Li2 O transition back to the SnO2 and the volume shrinkage of the electrode during Li+ extraction, which however is beyond the scope of this paper, and will be further studied later together with tuning the microstructure and carbon content of the composites. And moreover, it should be noting that the as-prepared SnO2 nanoparticles with different sizes could be well controlled by adjusting the time of the heat treatment of the SnO2 /carbon nanocomposite. It was found that the SnO2 nanoparticles would grow to larger size as the nanocomposite heating at 400 ◦ C for more than 4 h. However, the influence of the particle size of the SnO2 crystals in carbon matrix on the electrode properties needs to be further investigated. Further, with respect to the SnO2 /carbon nanocomposite, the complete reversibility of SnO2 reacting with Li and keeping the SnO2 crystals in the carbon matrix could result in much more enhanced performances (higher capacity, better cyclability), which are the targets we pursue.
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Figure 6. Electrochemical impedance spectroscopy (EIS) measurement of the SnO2 /carbon nanocomposite half cells in comparison with that of pure SnO2 nanoparticle anode carried out with a 5 mV amplitude signal in the 1 MHz to 0.05 Hz frequency range. (a) Before cycling tests, (b) after discharge to 0.01 V at different cycles, (c) after recharge to 1.5 V at different cycles, (d) the corresponding equivalent circuit that was used to fit the impedance data
4. Conclusions In summary, SnO2 /carbon nanocomposite can be prepared via a facile strategy by simply boiling and following heating of Sn together with sugar. This nanocomposite anode exhibited a much higher Li storage capacity and markedly improved cyclic performance as compared to pure SnO2 nanoparticles. The unique structure of ultrafine SnO2 nanocrystals (∼5 nm) homogeneously dispersed in amorphous carbon matrix can well accommodate the volume change of Li+ insertion/extraction in SnO2 , and prevent the aggregation of the nanosized active materials during cycling. This is believed to be responsible for the superior cycle performance of this nanocomposite with stable reversible capacity of 400 mAh/g at a high current rate of 3.3 A/g. SnO2 /carbon nanocomposite and the preparation approaches are cost-effective and may be easy to scale up. Besides SnO2 , this method may be easily extended to other active oxide materials suitable for next-generation LIBs.
Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51201065 and No. 51231003). And it was also supported by the Natural Science Foundation of Guangdong Province (S2012040008050) and the Doctorate Foundation of Ministry of Education (Grant No. 20120172120007 and No. 2014ZZ0002).
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Dispersing SnO2 nanocrystals in amorphous carbon as a cyclic durable anode material for lithium ion batteries Renzong Hu, Wei Sun, Meiqin Zeng, Min Zhu∗ School of Materials Science and Engineering, South China University of Technology, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, Guangzhou 510640, Guangdong, China [ Manuscript received October 2, 2013; revised January 7, 2014 ]
Abstract We demonstrate a facile route for the massive production of SnO2 /carbon nanocomposite used as high-capacity anode materials of nextgeneration lithium-ion batteries. The nanocomposite had a unique structure of ultrafine SnO2 nanocrystals (∼5 nm, 80 wt%) homogeneously dispersed in amorphous carbon matrix. This structure design can well accommodate the volume change of Li+ insertion/desertion in SnO2 , and prevent the aggregation of the nanosized active materials during cycling, leading to superior cycle performance with stable reversible capacity of 400 mAh/g at a high current rate of 3.3 A/g. Key words lithium ion battery; anode; SnO2 nanocrystals; amorphous carbon; facile strategy
1. Introduction Lithium ion batteries (LIBs) have been considered as attractive power sources not only for portable consumer electronics but also for upcoming electric/hybrid vehicles and smart grids due to their potential for high power and high energy density [1,2]. To meet the challenges of LIBs with high energy density and long cycle life for use in electric/hybrid vehicles and so forth, great efforts are being made to explore new electrode materials with novel structures and higher specific capacities [3−5]. Accordingly, different kinds of materials, such as transition-metal oxides (Fe3 O4 , Co3 O4 , MnO2 , etc.) [6−8] and alloy-based materials (Si, Sn, Cu6 Sn5 , etc) [9−10] have been studied for the alternative anode materials. Among them, SnO2 -based materials are widely concerned because of their low toxicity, widespread availability, and especially their total theoretical capacity up to as high as 1494 mAh/g (including 711 mAh/g for the conversion reaction and 783 mAh/g for the alloying reaction), which is four times higher than that of the currently used graphite (372 mAh/g) [11−15]. However, the great challenge for practical application of SnO2 anodes is the severe pulverization and capacity fading problems caused by the extremely large volume change of Sn (about 250% for Li4.4 Sn formation) during cycling [16]. To overcome these
problems, many methods have been studied to reduce the absolute volume change and cracking of active materials by the nanosize effects, especially via nano-architectured materials, such as nanoparticles, nanowires and nanotubes [17−19]. However, limited cycleability improvement has been obtained by simply nano-modifying the pure SnO2 , because the agglomeration of nanosized SnO2 and/or Sn is still unavoidable unless there is an extra confining buffer [11−15, 17−19]. In contrast, the nanopainting of SnO2 with various carbonaceous materials has recently been found effective for improving the cycleability of SnO2 -based anodes [13,19−25], because the carbonaceous materials not only enhance the electrical conductivity but also act as buffering layers for the large volume change of SnO2 . Actually, carbonaceous materials have been widely investigated to enhance performances for different electrode materials for LIBs and other batteries [26−28]. And thus, the SnO2 -carbon composite anodes with various microstructures have been designed and prepared by different strategies. For example, many carbon nanotube-encapsulated SnO2 composites have been prepared using the wet chemical filling and rapid vacuum absorption [24,25,29]. Yu et al. reported the synthesis of SnO2 @carbon core-shell nanochains by carbonization of a SnO2 @carbonaceous polysaccharide precursor at a relatively high temperature using a new lowflow-rate inert atmosphere strategy [30]. Zhang et al. elevated
Corresponding author. Tel: +86-20-87113924; Fax: +86-20-87111317; E-mail: [email protected] This work was supported by the National Science Foundation of China (Grant No. 51201065 and No. 51231003), the Natural Science Foundation of Guangdong Province (S2012040008050) and the Doctorate Foundation of Ministry of Education (Grant No. 20120172120007 and No. 2014ZZ0002). ∗
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60156-X
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the electrochemical properties of the carbon-coated SnO2 /graphene nanosheets, whose synthesis processes included first exfoliation of graphene oxide nanosheets from graphite oxide, and then growth of SnO2 , and followed by heating after an addition of glucose [31]. Read et al. prepared a SnO2 -carbon composite by heating a mixture of colloidal SnO2 (average size of 15 nm) and sugar (sucrose) as carbon precursor, which was finally carbonized to form a hard carbon and acted as a diluent to prevent nanosized SnO2 agglomeration [32]. Lou et al. designed a novel anode material of coaxial SnO2 @carbon hollow nanospheres which showed a high capacity of about 460 mAh/g and little capacity fading after 100 cycles [23]. Although some properties of these complexly designed SnO2 -carbonaceous anodes far exceed those of the conventional graphitic anode materials and demonstrate very high capacity and good stability, overall performance (especially the output of materials, the electrode loading or the capacity per unit area), as well as those rather complicated production processes of the SnO2 -based anode materials are far from their practical applications. Therefore, it is still desirable to develop a simple and more efficient way to prepare composites with nanosized SnO2 stored in the carbon matrix, which not only preserve high-dispersion state of the active SnO2 but also achieve high capacity together with enhanced cyclic performance. In this work, we develop a facile strategy, which is cost-effective and has a good potential for large scale applications, to synthesize a SnO2 -based carbonaceous nanocomposite anodes for LIBs. And a unique structure with homogeneous dispersion of SnO2 nanocrystals (∼5 nm) in amorphous carbon matrix is achieved. The origin of the enhanced anode performance of the SnO2 /carbon nanocomposite has been analyzed by microstructure observations combining with electrochemical characterizations. 2. Experimental 2.1. Materials fabrication Generally, the SnO2 /carbon nanocomposite was fabricated by simply boiling and following heating of Sn together with sugar. In a typical experiment, an aqueous solution consisting of 20 g Sn powders (∼100 mesh, 99.9% purity), 30 g crystal sugar (edible grade) and 180 mL deionized water was put in a stainless steel pot and then boiled in air using an electric oven (2 kW). The solution was heating for 1 h until all the water boiled dry and a brown charred production was achieved (name as charred product). Then, the obtained charred product was transferred to an alumina crucible and kept in a tube furnace at 400 ◦ C (based on the DSC result shown in Figure 1) for 4 h in flowing Ar atmosphere. Finally, brown powders (named as SnO2 /carbon nanocomposite) were obtained and were tested as electrode materials after grinding in a bowl. Pure nanosized SnO2 (with average diameters of 50 nm) was bought and also characterized as electrode for comparison.
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2.2. Materials characterization The structure and morphology of the samples were characterized using a Philips X-ray diffractometer (XRD) with Cu-Kα radiation, a Carl Zeiss Supra 40 field emission scanning electron microscope (SEM) and a JEOL JEM-2100 transmission electron microscope (TEM) operating at 200 kV. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis were made under Ar and/or O2 on NETZSCH STA409 with a heating rate of 10 ◦ C/min. In order to retain the original morphology of the as-prepared products, the SnO2 /carbon nanocomposite powders were directly dispersed on Cu grids for TEM measurements. 2.3. Electrochemical measurements The discharge-charge cycling performance of the samples was investigated using a cell test system (LAND-CT2001A) with CR2016 coin-type cells assembled in an argon-filled glove box. The working electrode consisted of 80 wt% of the active material (SnO2 /carbon nanocomposite or pure nanosized SnO2 ), 10 wt% conductivity agent (Super-P) and 10 wt% binder polyvinyldifluoride. Lithium foil was used both as a counter electrode and as a reference electrode in the half cells. The electrolyte was LiPF6 (1 mol/L) in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC) with volume ratio of EC/DEC/EMC = 1/1/1 (Shanshan Tech Co., Ltd.). The cells were tested at various current rates between 0.01 and 1.5 V at room temperature. Cyclic voltammetry (CV) over the potential range of 0–2.0 V at a scan rate of 0.3 mV/s, as well as impedance spectroscopy at 5 mV amplitude signal in the 1 MHz to 0.05 Hz frequency range, was performed using a Gamry Interface 1000 Electrochemical System. 3. Results and discussions The heating conditions of the Sn-sugar composite were set according to the thermal analysis results. Figure 1(a) shows the DSC and TG curves for the charred product of SnSugar composite after boiling. It can be seen that during heating scanning under Ar, there are two endothermic peaks at about 100 ◦ C (peak 1) and 270 ◦ C (peak 2), respectively. In addition, two exothermic peaks appeared among 300∼500 ◦ C (peak 3) and around 750 ◦ C (peak 4). The endothermic peak 1 should be attributed to the evaporation of the residual water, while the sharp peak 2 was induced by the melting of the Sn, suggesting that the Sn particles remained their elemental state even boiling in water and mixing with the charred sugar. With respect to the two exothermic peaks, peak 3 was originated from the following Reaction 1, which involved the decomposition of the sugar and oxidation of Sn to form SnO2 , with weight loss of about 20% as indicated by the TG curve. Sn(liquid) + (C-H2 O)n (solid) → SnO2 (s) + C(s) + CO2 (gas ↑) + H2O(g ↑)
(1)
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In this step, the micro-sized Sn particles were expected to be refined as SnO2 nanocrystals with a high-dispersion state in an amorphous carbon matrix. For the other peak around 750 ◦ C, it should be related to the crystal growth of the previous formed nanosized SnO2 . According to the above results, the further heating treatment of the charred product of Sn-Sugar composite was set to be 400 ◦ C for 4 h. After heating treatment of the Sn-sugar composite to form a SnO2 /carbon nanocomposite, the amount of SnO2 and carbon in the nanocomposite was measured using TG analysis under O2 atmosphere with a heating rate of 10 ◦ C/min. As shown in Figure 1(b), the weight percentage of the carbon could be evaluated to be about 20%, and that of the SnO2 was about 80%.
completely disappeared, while only obvious broadened peaks of SnO2 were remained. These indicated that the Reaction 1 has happened as the charred product of Sn-sugar composite keeping heating at 400 ◦ C, leading to that the microsized Sn particles oxidized to form SnO2 nanocrystals. And meanwhile, the charred sugar had been carbonized to be amorphous carbon, which induced the obvious uplifted background in this XRD pattern in Figure 2(a).
Figure 2. XRD patterns of the pure Sn powders (a), charred product of Snsugar composite after boiling (b) and that after heating at 400 ◦ C for 4h (c)
Figure 1. (a) DSC and TG curves of the charred product of Sn-sugar composite under Ar atmosphere, with a heating rate of 10 ◦ C/min from 35 to 900 ◦ C, (b) TG curve of the nanocomposite under O2 atmosphere with a heating rate of 10 ◦ C/min
Figure 2 shows the XRD patterns of the charred product of Sn-sugar composite and that after heating at 400 ◦ C for 4 h, in comparison with that of the pure Sn powders. The sharp diffraction peaks of pure Sn shown in Figure 2(c) reveal their coarse particle/grain size nature in the pure Sn powder sample. After boiling with the sugar, it can be seen in Figure 2(b) that, in addition to the strong diffraction peaks of pure Sn, there are weak diffraction peaks of SnO2 , which is generated from oxidization of a small amount of Sn. Moreover, the raised background among 20o −35o should come from the amorphous charred sugar. However, surprisingly, as shown in Figure 2(a) that, after heating at 400 ◦ C for 4 h, the diffraction peaks of Sn
The microstructure evolutions of the Sn-sugar composites were investigated by SEM and TEM observations. Figure 3(a) shows the initial SEM morphology of the pure Sn powders, revealing that the spherical Sn particles have large size range from 5 to 50 µm. However, as shown in Figure 3(b), after boiling with the sugar for 1 h, the spherical morphology of the microsized Sn has been completely disappeared, although most of the elemental Sn remained as revealed by the XRD in Figure 2(b). Instead, anomalous particles of the Sn-sugar composite with size less than 5 µm presented, while the large magnification SEM image in Figure 3(c) further revealed the multi-scale (micro- and nano-) inside of the charred product, suggesting that the large Sn particles have been dramatically refined by simply boiling and burning with the sugar. These should be due to that the burning of the composite lead to the Sn particles melting and separating into smaller ones, which was stirred simultaneously by the boiling sugar. Figure 3(d) shows the morphology of the charred product after heating in Ar at 400 ◦ C for 4 h, in which there is no obvious change
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in comparison with that of the composite before heating as shown in Figure 3(c). Nevertheless, the TEM image shown in Figure 3(e) reveals that a micro-sized (1−2 µm) powder of the heat-treated composite is the aggregation of several nanosized (200−300 nm) particulates, which are composed of the SnO2 nanocrystals as indicated by the inserted (top right corner) selected area electron diffraction (SAED) pattern. This is in well consistance with the XRD result as shown in Figure 2(c). And furthermore, the HRTEM image in Figure 3(f) shows that the SnO2 nanocrystals are very fine (about 5 nm), and they are homogenously distributed in the amorphous carbonaceous matrix. So far, based on the above observations, we can conclude that a SnO2 /carbon nanocomposite, with the expected microstructure of ultrafine SnO2 nanocrystals dis-
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persing in amorphous carbon, has been successfully prepared by the simple facile strategies. We would like to emphasize that these materials and the preparation approaches are much more cost-effective and easy to scale up (recently, we have already achieved kg scale in lab) in comparison with the materials obtained by other chemical reactions [19−25, 29−32]. Therefore, it has a good potential to be used for large scale applications. And since the SnO2 phase is ultrafine and the amorphous carbon matrix is flexible and excellent conductive, it is expected that such a nanocomposite structure can accommodate the large volumetric changes of SnO2 and prevent their aggregation during discharge/charge cycling, leading to enhanced cycle stability and high-rate capability of the lithium storage electrode.
Figure 3. (a) SEM image of the pure Sn powders; (b) low magnification and (c) large magnification SEM images of the charred product obtained from Sn-sugar composites boiling and burning for 1 h; (d) SEM image, (e) TEM image and SAED pattern (insert), and (f) HRTEM image of the SnO2 /carbon nanocomposite
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The electrochemical performances of the SnO2 /carbon nanocomposite as anode were firstly investigated by a cyclic voltammogram (CV) scan. As the typical CV curves of a half cell shown in Figure 4(a), the CV curves almost overlap from the second to fifth cycle, indicating that the cycle performance is considerably stable. The difference between the first and latter cycles is partly ascribed to the formation of solid electrolyte interface (SEI) layer. Two reduction peaks at 0.8 and 0.05 V are observed during the first cathodic scan. The formed peak at 0.8 V, which is absent in the subsequently cycles, is ascribed to the formation of an SEI layer and the reduction of SnO2 to Sn [11]. The latter peak at 0.05 V is known to arise from the formation of Lix Sn alloys. However, the oxidation peaks appearing at 0.6 and 1.3 V are correspondingly attributed to the de-alloying reaction and the oxidation of Sn to SnO2 , respectively. The latter oxidation peak (1.3 V) is recently reported in some literatures [33,34], which implied that the inverse process with Li2 O transition back to the Li+ takes place as the ultrafine SnO2 nanocrystals (less than 10 nm) well contacted with the carbon matrix. Figure 4(b) shows the initial five charge/discharge profiles at 0.25 A/g for the SnO2 /carbon nanocomposite anode. It can be seen that the first discharge and charge capacities are 1832 and 795 mAh/g, respectively, corresponding to an initial coulumbic efficiency of 43.5%. The relative large irreversible capacity of 1037 mAh/g could be assigned to the formation of irreversible Li2 O, and especially electrolyte decomposition and SEI formation on the nanocom-
posite electrode surfaces, as well as exfoliation of some SnO2 particles attached outside the electrode, in the first anode reaction cycle. Nevertheless, from the second cycle, a reversible capacity of 819 mAh/g with a coulumbic efficiency more than 91% was achieved. And the voltage trend was retained for the all following cycles, confirming the good reversibility of the reactions at the SnO2 /carbon nanocomposite electrode. Figure 5(a) displays the charge capacity (Li extraction) versus cycle number for the SnO2 /carbon nanocomposite anode, in comparison with pure SnO2 nanoparticles anode, at a constant current density of 0.25 A/g between 0.01 and 1.5 V. As can be seen, the capacity of the pure SnO2 nanoparticle anode decreased very rapidly during cycling. It delivers a reversible capacity of 634 mAh/g at the first cycle, which was retained only 11% (72 mAh/g) after 50 cycles. In striking contrast, the SnO2 /carbon nanocomposite possesses much better cycle performance. Its initial reversible capacity (795 mAh/g) could be remained 58.7% (447 mAh/g) after 50 cycles, which was much superior to those of the pure SnO2 nanoparticle anode. The enhanced cycleability should be attributed to the unique features of this SnO2 /carbon nanocomposite, including the ultrafine SnO2 nanocrystals with high-dispersion state in the carbon matrix and stress absorption by the carbon matrix. In addition, the possible formation of Sn–O–C metastable phases in the boundary between nanosized SnO2 and carbon would also enhance the structure stability of the SnO2 phases [31].
Figure 4. Electrochemical performance of the SnO2 /carbon nanocomposite anode: initial five cyclic voltammogram curves with a scanning rate of 0.3 mV·s−1 (a), initial five discharge-charge profiles at a current density of 0.25 A/g between 0.01 and 1.5 V (b)
Figure 5(b) shows the long-term cyclic performance of the SnO2 /carbon nanocomposite with a voltage window of 0.01−1.5 V at different current rates. It can be seen that, the initial charge capacity of 961 mAh/g at a slower rate of 0.13 A/g is higher than that of 795 mAh/g at a rate of 0.25 A/g. However, lower reversible capacities of 351 mAh/g (36.5%) and 254 mAh/g (26.4%) were retained after 50 and 100 cycles
at 0.13 A/g, respectively, suggesting that this nanocomposite yields relatively inferior capacity retention as cycling at slow current rates (0.13 and 0.25 A/g). In contrast, the nanocomposite anodes have much better cycleability at faster rates, which could be revealed by that, 57.7% and 74.7% of their initial charge capacities were maintained after 100 chargedischarge cycles at 0.62 A/g and 3.3 A/g, respectively. And
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especially, at the high current rate of 3.3 A/g, a very stable reversible capacity of 400 mAh/g was remained from the 10th cycle to the 100th cycle. These results confirmed the good cycleability and high-rate capability of this nanocomposite,
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which is undoubtedly attributed to its unique microstructure, i.e. ultrafine SnO2 nanocrystals, high-dispersion state and good contact between SnO2 nanocrystals and carbon matrix (i.e. good electrical conductivity).
Figure 5. (a) Cyclic performance of the SnO2 /carbon nanocomposite in comparison with that of pure SnO2 nanoparticle anode, with a voltage window of 0.01−1.5 V at a current rate of 0.25 A/g, (b) cycle performances of the SnO2 /carbon nanocomposite taken at different current rates
In order to investigate the impedance variation of the half cells with SnO2 /carbon nanocomposite electrodes during charge-discharge cycling, electrochemical impedance spectroscopy (EIS) measurements were carried out when discharge to 0.01 V and recharge to 1.5 V, respectively, at different cycles with a current rate of 0.25 A/g. The impedance response of the pure nanosized SnO2 particle electrode before cycling was also recorded for comparison. The spectra evolutions at the desired cycles are summarized in Figure 6(a–c), showing that all the spectra (Nyquist plots) have a small semicircle at high frequency, a relative large semicircle at middle frequency and an inclining line at lower frequency. It is well known that, with respect to the impedance spectra of electrode in lithium ion battery, the high frequency semicircle can be attributed to the contact resistance caused by SEI film (Rf ), the medium-frequency semicircle is related to the charge-transfer resistance at the interface between the electrolyte and the electrode material (Rct ) and the inclined lines correspond to the Li+ diffusion process inside the electrode material (Rw ) [35]. And thus, the Nyquist plots could be fitted using the equivalent circuit as shown in Figure 6(d), where the Rs represents the solution resistance, and Cf and Cct are the intercalation capacitance along with the Rf and Rct , respectively. The fitting results indicated that the charge transfer resistance for the cells after discharge (Figure 6b) first increased and then decreased as prolonging the cycles, and at last even lower than that of the cell before the cycle tests (Figure 6a), indicating that an “in-situ activation” phenomenon occurred for the cell as discharge/charge beginning. Furthermore, the effect of carbon matrix on the electronic conductivity of the
composite electrode was generally demonstrated by the dramatic change in impedance of the SnO2 /carbon nanocomposite relative to that of the pure nanosized SnO2 anode as also shown in Figure 6(a). Such an improved performance can be attributed to the synergy between the amorphous carbon matrix and the SnO2 nanocrystals during cycles. With respect to the spectra obtained after recharge to 1.5 V (Figure 6c), they almost overlap from the first to the 20 th cycle, indicating that the SEI film and charge transfer resistances keep considerably stable in the recharged electrodes during cycling. It should be pointed out that, the higher impedances in the recharged electrodes, compared with those of the discharged electrodes, could be attributed to the inverse process with Li2 O transition back to the SnO2 and the volume shrinkage of the electrode during Li+ extraction, which however is beyond the scope of this paper, and will be further studied later together with tuning the microstructure and carbon content of the composites. And moreover, it should be noting that the as-prepared SnO2 nanoparticles with different sizes could be well controlled by adjusting the time of the heat treatment of the SnO2 /carbon nanocomposite. It was found that the SnO2 nanoparticles would grow to larger size as the nanocomposite heating at 400 ◦ C for more than 4 h. However, the influence of the particle size of the SnO2 crystals in carbon matrix on the electrode properties needs to be further investigated. Further, with respect to the SnO2 /carbon nanocomposite, the complete reversibility of SnO2 reacting with Li and keeping the SnO2 crystals in the carbon matrix could result in much more enhanced performances (higher capacity, better cyclability), which are the targets we pursue.
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Figure 6. Electrochemical impedance spectroscopy (EIS) measurement of the SnO2 /carbon nanocomposite half cells in comparison with that of pure SnO2 nanoparticle anode carried out with a 5 mV amplitude signal in the 1 MHz to 0.05 Hz frequency range. (a) Before cycling tests, (b) after discharge to 0.01 V at different cycles, (c) after recharge to 1.5 V at different cycles, (d) the corresponding equivalent circuit that was used to fit the impedance data
4. Conclusions In summary, SnO2 /carbon nanocomposite can be prepared via a facile strategy by simply boiling and following heating of Sn together with sugar. This nanocomposite anode exhibited a much higher Li storage capacity and markedly improved cyclic performance as compared to pure SnO2 nanoparticles. The unique structure of ultrafine SnO2 nanocrystals (∼5 nm) homogeneously dispersed in amorphous carbon matrix can well accommodate the volume change of Li+ insertion/extraction in SnO2 , and prevent the aggregation of the nanosized active materials during cycling. This is believed to be responsible for the superior cycle performance of this nanocomposite with stable reversible capacity of 400 mAh/g at a high current rate of 3.3 A/g. SnO2 /carbon nanocomposite and the preparation approaches are cost-effective and may be easy to scale up. Besides SnO2 , this method may be easily extended to other active oxide materials suitable for next-generation LIBs.
Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 51201065 and No. 51231003). And it was also supported by the Natural Science Foundation of Guangdong Province (S2012040008050) and the Doctorate Foundation of Ministry of Education (Grant No. 20120172120007 and No. 2014ZZ0002).
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