Effect of Buffer Size around Nanosilicon Anode Particles for Lithium

Feb 18, 2012 - Indeed, the structure of the porous carbon matrix was well retained even after 20 charge–discharge cycles in the Si/carbon composite ...
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Effect of Buffer Size around Nanosilicon Anode Particles for Lithium-Ion Batteries Shinichiroh Iwamura, Hirotomo Nishihara,* and Takashi Kyotani Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan S Supporting Information *

ABSTRACT: Si nanoparticle/carbon composites in which each Si nanoparticle was embedded in a spherical nanospace were synthesized by a newly established hard-template pathway. A series of composites having different nanospace sizes were prepared, and their lithium insertion/extraction behaviors as an anode for lithium-ion batteries were examined. The nanospace which surrounds each Si nanoparticle can be a buffer against the Si expansion during its lithiation. By using the series of composites, the effect of the buffer size around nanosilicon was systematically investigated. The cyclability became better with increasing the buffer size up to about 3 times larger than the Si volume, i.e., the size which allows Si to expand up to 4 times larger than its original volume. Indeed, the structure of the porous carbon matrix was well retained even after 20 charge−discharge cycles in the Si/carbon composite with this appropriate buffer size, whereas a composite with a smaller buffer size was totally destroyed. The further increase of the buffer size, however, gave rise to the decline of the charge−discharge cyclability, probably because a larger buffer space makes it easier for Si nanoparticles to drop out from the carbon matrix during cycling. In addition, too large a buffer size in principle lowers the volumetric energy density of anode materials. It is thus concluded that the minimum necessary buffer size for Si is about 3 times larger than the Si volume, and this is also the best size to achieve a good cyclability as well as a high volumetric energy density.

1. INTRODUCTION Lithium-ion batteries (LIBs) are widely used for various mobile devices, such as cell phones and laptop computers, due to their higher energy density and lower self-discharge rate than other rechargeable batteries.1 As an anode material of LIBs, graphite is generally used because it has a good cyclability and is naturally abundant. The theoretical gravimetric and volumetric capacities of graphite are however restricted to 372 mAh/g and 837 mAh/cm3, respectively, and therefore, it is desirable to develop next-generation anode materials with much higher capacity.2 The total cell capacity of LIB is a function of anode and cathode capacity. Assuming the cathode capacity to be 140−200 mAh/g, the total cell capacity is saturated when the anode capacity reaches 1000−1200 mAh/g,3 which can be the target capacity for the anodes from a practical point of view. Silicon (Si) can be one of the promising candidates from its extremely large capacity (gravimetric, >3500 mAh/g; volumetric, >8200 mAh/cm3).3−7 A great deal of effort has been carried out thus far to utilize Si as an anode material for LIBs: a variety of Si-containing anodes have been prepared, and their performances have been examined, e.g., Si thin films,8−10 Si fine powders prepared by milling,11−13 Si nanoparticles formed by silane chemical vapor deposition (CVD),14,15 Si/graphite heterogeneous anodes,16,17 Si/C nanocomposites synthesized by chemical mixing approach,18 and Si−C core−shell particles.19 However, there are two serious problems for Si as © 2012 American Chemical Society

an anode material. The first drawback is a low rate capability because of the low electrical conductivity of Si and its low reaction rate with Li.20−24 The second one is a poor cyclability caused by a large volume expansion of Si up to 300−400% during lithiation.3,6 To solve the first problem, the size of Si should be down to nanosize, e.g., nanoparticles or nanofilms.25,26 For the second problem (poor cyclability), the introduction of buffer space around Si has been found to be effective.27,28 On the basis of the above-mentioned strategy and the concept of nanostructure designing, many researchers have recently fabricated various types of anode materials using nanosilicon-based materials, such as Si nanowires,29−32 hybrid Si/carbon nanotube heterostructure,33 inverse-opal structure,34,35 mesoporous Si on carbon core−shell nanowires,36 and C−Si nanocomposite granules.37 In these anodes, nanosized Si materials are attached to conductors, and more importantly, such Si nanomaterials are always surrounded by buffer nanospaces in which Si is allowed to expand/contract without any destruction of the electrode structures. These anodes indeed exhibit large capacities, excellent rate capabilities, and long cycle lives. Thus, the introduction of the buffer Received: September 28, 2011 Revised: February 17, 2012 Published: February 18, 2012 6004

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Figure 1. Preparation scheme of the Si/C composite. (a) Si nanoparticles originally have a thin SiO2 layer. They were oxidized with air for X min (X = 10, 90, 200, or 400) to form Si/SiO2 core−shell structure. Then, the particles were molded into a (b) Si/SiO2(X) disk, and carbon was introduced into the interparticle spaces inside the disk to obtain a (c) Si/SiO2(X)/C composite. Finally, the SiO2 shell was removed with HF washing, and an annealing treatment was performed. Thus, the (d) Si/(X)/C composite was obtained.

nanospace can therefore be precisely controlled by changing the thickness of the SiO2 shell. 2.2. Characterization. The cross sections of the Si/ SiO2(X) and Si/SiO2(X)/C disks were observed by a fieldemission scanning electron microscope (FE-SEM; Hitachi Ltd., Tokyo, Japan; S-4800). In addition, Si/SiO2(X) and Si/(X)/C samples were observed by using a transmission electron microscope (TEM; JEOL Ltd., Tokyo, Japan; JEM-2010) to compare the sizes of the SiO2 shell in Si/SiO2(X) with the corresponding buffer nanospace in Si/(X)/C. Powder XRD patterns of the samples were recorded at each preparation step with an XRD6100 diffractometer (Shimadzu Corporation, Kyoto, Japan) with Cu Kα radiation generated at 30 kV and 20 mA. The weight fractions of Si in Si/SiO2(X) (xSi [wt %]) and in Si/(X)/ C (XSi [wt %]) were estimated from the weight changes when the samples were completely oxidized in a thermogravimetric analyzer (Shimadzu, TGA-51H) at 1400 °C for 2 h under air flow. On the basis of xSi and true densities of Si and SiO2, the volume fractions of Si (vSi [vol %]) and SiO2 (vSiO2 [vol %]) in Si/SiO2(X) were calculated. Using vSi and vSiO2, the average SiO2 shell thickness (ts) in Si/SiO2(X) was calculated assuming that all the Si/SiO2 core−shell particles are spheres (see Supporting Information for the details of all calculations). The porosities of Si/(X)/C composites were measured by mercury intrusion porosimetry using an Autopore IV 9510 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA). 2.3. Electrochemical Measurement. The Si/(X)/C powder was mixed with a binder polymer solution (12 wt % poly(vinylidene fluoride) dissolved in N-methyl-2-pyrrolidone), and the resulting slurry was pasted onto a copper foil. After drying at 80 °C in air for 1 h, the film was cut into a circular shape (16 mm in diameter) to form a working electrode. The weight ratio of the Si/(X)/C composite to the binder was 4:1, and the amount of Si/(X)/C in the working electrode was ca. 7 mg. The thickness of the active material layer was 200 μm. The working electrode was again dried under vacuum at 120 °C for 6 h and was packed in a three-electrode cell together with a polypropylene separator (Celgard #2400; Celgard, LLC., Charlotte, NC, USA). A lithium foil was used as a counter and a reference electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 by volume), which was purchased from Tomiyama Pure Chemical Industries, Ltd., Tokyo, Japan. Cyclic voltammetry was performed with a potentiostat/galvanostat (BioLogic, Claix, France; VMP3) between 0.01 and 1.5 V (versus Li+/Li) at a scan rate of 0.1 mV/s. For obtaining Li insertion/extraction capacities, another three-electrode cell was prepared, and it was galvanostatically charged/discharged at a constant current of 200 mA/g between 0.01 and 1.5 V by using a battery charge/discharge unit

nanospace is one of the crucial factors to achieve the excellent performance of the Si anode. However, the presence of such space in principle makes the anode density significantly lower, resulting in the lower volumetric energy density of the battery. The size of the buffer nanospace therefore should be kept to the minimum necessary. To the best of our knowledge, there has been no report so far on the optimum size of the buffer space around nanosized Si. In this work, we propose a new methodology based on a hard-templating technique to control the size of buffer nanospace around each Si nanoparticle in Si/carbon (Si/C) composites. This method makes it possible to control the buffer size with the accuracy of 1 nm. By using a series of Si/C composites with different buffer sizes, we discuss the influence of the buffer nanospace on the performance of Sibased LIB anode materials. Moreover, we try to demonstrate the minimum necessary size of the buffer space around each Si nanoparticle.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Figure 1 shows a preparation scheme of the Si/C composites. Si nanoparticles (#0140KE) were purchased from Nanostructured & Amorphous Materials, Inc., Houston, TX, USA. The average particle size was measured to be 76 nm, from SEM examination. Each Si nanoparticle was originally covered with a thin SiO2 shell (Figure 1a) due to natural oxidation. The SiO2 shell was further thickened by an air oxidation at 900 °C for X min (X = 10, 90, 200, 300, or 400). As will be discussed later, the shell thickness can be precisely tuned by the oxidation time. Then, the Si/SiO2 core− shell particles were molded into a circular disk (12 mm in diameter and 1 mm in thickness) by pressing them under 700 MPa for 1 h. The resulting disk is referred to as Si/SiO2(X), where X is the air oxidation time (Figure 1b). Note that a disk made from the pristine Si nanoparticles was also prepared, and it is referred to as Si/SiO2(0). On a disk of Si/SiO2(X), 1.5 g of polyvinyl chloride powder was placed, and it was heat-treated under N2 flow at 300 °C for 2 h to transform polyvinyl chloride into a liquid pitch, which infiltrated into the interparticle cavities in the disk. The pitch thus introduced was then carbonized at 900 °C for 1 h under N2 flow. The resulting Si/ SiO2(X)/C composite disk sample (Figure 1c) was crushed into small fractions with a size of several micrometers and washed with 0.5 wt % HF for 90 min to remove the SiO2 shell. In this step, the SiO2 shell becomes the buffer nanospace. Finally, the sample was annealed at 900 °C for 2 h in N2 flow. The Si/C composite thus obtained is referred to as Si/(X)/C (Figure 1d). Note that “X” in Si/SiO2(X)/C and Si/(X)/C are the same as that in their parent Si/SiO2(X). In Si/(X)/C, each Si nanoparticle is surrounded by a buffer nanospace that is derived from the SiO2-shell template. The size of the buffer 6005

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corresponding Si/(X)/C composites were examined by TEM. Figure 3a, b, and c shows the TEM images of Si/SiO2(X) made from Si/SiO2 core−shell particles with different air-oxidation times: namely, as received Si nanoparticles (Figure 3a), after 10 min oxidation (Figure 3b), and after 200 min oxidation (Figure 3c). In Figure 3a, the pristine Si nanoparticle originally has a thin SiO2 shell. It is clearly seen that the SiO2 shell becomes thicker with increasing the air oxidation time (Figure 3b and c). The average SiO2 shell thicknesses, which were calculated from the SiO2/Si volume ratio, are shown in the second column of Table 1. The average values almost agree with those observed in Figure 3a−c. These results demonstrate that it is possible to control the SiO2 shell thickness just by changing the air-oxidation time. Figure 3d, e, and f shows the TEM images of Si/(X)/C composites prepared from the parent samples shown in Figure 3a, b, and c, respectively. All the samples have spherical nanospaces derived from the SiO2 shells, indicating that the present synthesis scheme shown in Figure 1 was successfully accomplished and the template morphology was well replicated with rigid carbon frameworks. We have confirmed that massive structural damage was not observed in each sample by TEM. Like the present results, similar replication processes using SiO2 colloid/opal template are known to provide fine carbon replicas having empty spherical nanospaces.41,42 Thus, each spherical Si nanoparticle is embedded in the spherical nanospace which is formed by the carbon matrix. Since X-ray diffraction patterns of the Si/(X)/C samples showed peaks only from Si and carbon (Supporting Information), the Si/(X)/C composites contain no crystalline impurities. The sizes of the Si nanoparticles in Figure 3d−f agree well with those of the Si cores in Figure 3a−c, indicating that the Si cores in Si/SiO2(X) composites remained unchanged during the template removal process using HF. In addition, the sizes of the nanospaces in Figures 3d−f are also in good agreement with those of the SiO2 shells in Figure 3a−c. We can thus conclude that the size of the buffer nanospace around each Si nanoparticle was successfully controlled by the present hard-template approach. The buffer nanospace of the Si/(X)/C composite was further analyzed by mercury intrusion porosimetry. Figure 4a shows the cumulative pore size distributions of Si/(0)/C and Si/ (200)/C composites. In Si/(200)/C, there are two steps around 20−35 and 60−130 nm. The former corresponds to the nanospaces around Si nanoparticles formed by the SiO2-shell template (Figure 4b), and the latter may be ascribed to blank nanospaces where no Si nanoparticles are present (Figure 4c). It is very likely that some of the Si nanoparticles fell out during the HF washing process. Considering the Si amount included in Si/(200)/C (21 wt %), which was determined by thermogravimetry of the Si/(200)/C composite (Table 1), it can be calculated that 30% of Si was lost on the HF washing of Si/ SiO2(200)/C. In other words, most of the Si nanoparticles still remain in the controlled nanospaces. The obtained composite actually meets our present purpose, i.e., the investigation of the buffer size effects around Si nanoparticles. As for Si/(0)/C, no apparent step is observed in Figure 4a because the nanospace of this sample is only about 10 nm (Figure 3d), which is too small to be detected by the present measurement. It is noteworthy that Si/(0)/C has no step around 60−130 nm, and this means that such blank nanospaces as observed in Si/(200)/C were not formed in this sample. These findings suggest that smaller Si nanoparticles tend to escape from the carbon matrix more easily and also suggest that

(Hokuto Denko Co., Tokyo, Japan; HJ1001). Hereafter, Li insertion and extraction are defined as “charge” and “discharge”, respectively. All the electrochemical measurements were performed at 25 °C.

3. RESULTS AND DISCUSSION 3.1. Morphology of Si/SiO2(X) and Si/SiO2(X)/C Disks. Figure 2 shows cross-sectional SEM images of Si/SiO2(200)

Figure 2. SEM images of the cross sections of (a) Si/SiO2(200) and (b) Si/SiO2(200)/C disks. Insets are their photographs.

and Si/SiO2(200)/C disks, together with their photographs (insets). The disk shape of Si/SiO2(200) is retained even after the carbon introduction. The cross-section SEM image of Si/ SiO2(200) (Figure 2a) shows that the disk is composed of nanoparticles with the size of 30−100 nm, and there are many interparticle cavities. The apparent density of the Si/SiO2(200) disk was 1.1 g/cm3, and the true density was determined to be 2.1 g/cm3 with using helium pycnometry. From these values, the cavity volume in the Si/SiO2(200) disk can be calculated as 0.44 cm3/g/disk; i.e., the porosity is 53%. In Figure 2b, it appears that the interparticle cavities are filled with carbon. Indeed, the carbon occupies as much as 76% of the original interparticle space of Si/SiO2(200) (see the Supporting Information for the calculation details). In addition, Figure 2b shows that carbon effectively covers all the particles. This is due to the nature of the liquid pitch, which generally has a high affinity for the metal oxide surface and very easily infiltrates even into small nanopores.38−40 After the removal of SiO2 shells by HF etching, the spaces occupied by the SiO2 shells become buffer nanospaces around Si particles. 3.2. SiO2-Shell Template and the Resulting Nanospace. To compare the shapes of the template (SiO2 shell) with the resulting buffer nanospace around Si nanoparticles, Si/SiO2(X) with different SiO2 shell thicknesses and the 6006

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Figure 3. TEM images of (a−c) Si/SiO2 core−shell particles and (d−f) the corresponding Si/C composites: (a) Si/SiO2(0), (b) Si/SiO2(10), (c) Si/SiO2(200), (d) Si/(0)/C, (e) Si/(10)/C, and (f) Si/(200)/C.

Table 1. Properties of Si/(X)/C Composites and the SiO2 Shell Thickness in the Corresponding Si/SiO2(X) Samples sample

tsa (nm)

εb (-)

XSic (wt %)

expected capacityd (mAh/g)

1st charge capacitance (mAh/g)

1st discharge capacitance (mAh/g)

Coulomb efficiency in the 1st cycle (%)

Si/(0)/C Si/(10)/C Si/(90)/C Si/(200)/C Si/(300)/C Si/(400)/C

5 7 11 14 15 15

1.6 2.0 3.1 4.1 5.0 5.1

45 40 24 21 17 16

2010 1550 1240 1140 1000 980

1350 1320 1200 980 780 700

790 900 860 690 530 500

54 68 72 70 68 71

The average SiO2 shell thickness in the Si/SiO2(X) samples corresponding to each Si/(X)/C composite. bε = (vSi + vSiO2)/vSi. cThe Si content. dExpected capacities of Si/(X)/C composites were calculated with assuming the capacities of carbon and Si as 372 and 4008 mAh/g, respectively.

a

3.3. Size Effect of the Buffer Nanospace on the Lithium Charge/Discharge Cycling. In this section, we examine the effect of the buffer nanospace size on anode performance of nanosilicon. To estimate the optimum size of the buffer nanospace around Si, we introduce a parameter, ε, which is defined as ε = (vSi + vSiO2)/vSi, where vSi [vol %] and vSiO2 [vol %] are the volume fractions of Si and SiO2 in Si/ SiO2(X), respectively, as shown in the third column of Table 1. The parameter (ε) is the volume ratio of a Si/SiO2 core−shell nanoparticle to its core Si nanoparticle in Si/SiO2(X) composites, and thus ε corresponds to the volume ratio of a spherical nanospace surrounded by a carbon matrix to a Si nanoparticle in such a carbon nanospace of Si/(X)/C composites. For example, ε = 4 means that the carbon nanospace is 4 times larger than the Si nanoparticle; i.e., the Si nanoparticle can expand up to 4 times larger than its original volume without any destruction. The ε value linearly increases with the average SiO2 shell thickness (Table 1). With referring ε, we can discuss how much buffer size is necessary for Si. Since it is well-known that Si expands to 3−4 times larger than its original volume during lithiation,3,6 we took the two

the fractions of Si escaped from the carbon matrices upon HF washing are less than 30% in Si/(X)/C (X < 200) samples.

Figure 4. (a) Cumulative pore size distributions of Si/(0)/C and Si/ (200)/C measured by mercury intrusion porosimetry and TEM images of Si/(200)/C for (b) Si in spherical carbon nanospaces and (c) blank nanospaces. 6007

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Figure 5. Cyclic voltammograms of (a) Si/(0)/C and (b) Si/(200)/C measured at 0.1 mV/s.

samples with and without enough buffer nanospace (Si/(0)/C (ε = 1.6) and Si/(200)/C (ε = 4.1)) to investigate the effect of the nanospace on Li insertion/extraction behavior. Figure 5 shows cyclic voltammograms of Si/(0)/C and Si/(200)/C. In Si/(0)/C (Figure 5a), the first cathodic scan shows a large peak around 0.01−0.15 V, corresponding to Li insertion into crystalline Si to form amorphous lithiated Si.43,44 Under an ideal process, the inserted Li is extracted stepwise around 0.3 and 0.5 V in an anodic scan, and amorphous Si is formed as a result.44 However, the first anodic scan of Si/(0)/C shows a broad peak around 0.1−0.7 V. This finding suggests that all the lithiated Si nanoparticles are not delithiated at the same time, but rather some of them were delithiated at higher potentials than 0.3 or 0.5 V. This may be due to the delay of the electrochemical response caused by the destruction of the carbon framework, as we will demonstrate later. In the second cathodic scan, a broad peak appears around 0.1−0.3 V, corresponding to Li insertion into the amorphous Si. This means that electrochemically active Si was generated on the first Li extraction, although the Li extraction peaks do not clearly appear. In the sixth cycle, however, Li insertion/extraction peaks became much weaker, indicating the degradation of the working electrode. On the other hand, Si/(200)/C shows two well-resolved peaks at 0.3 and 0.5 V in the first and the second anodic scans, and in addition, the peaks are still observed in the sixth cycle. This better cycle performance suggests that the structure destruction of the carbon framework in Si/(200)/C during the cycles is alleviated due to its sufficiently large buffer nanospace where Si nanoparticles reversibly expand and recover. Figure 6a shows the change of charge capacities of Si/(X)/C composites with cycle number. The Si/(0)/C composite with the smallest buffer nanospace has a very large charge capacity (up to 1350 mAh/g on the first cycle) since it contains the largest amount of Si (the fourth column of Table 1), thereby having the largest expected capacity (the fifth column of Table 1). However, its initial discharge capacity decreases to 790 mAh/g (Figure 6b); i.e., the Coulomb efficiency was 54%, and the capacity rapidly fades in the following cycles and becomes 0 mAh/g after only the fifth cycle. This very poor cyclability can be ascribed to the severe structural destruction because of the lack of the buffer nanospace around Si. The structural change on the charge/discharge cycling was directly analyzed by TEM observation. Figure 7 shows TEM images of Si/(0)/C and Si/ (200)/C after 20 charge−discharge cycles. The morphology of

Figure 6. (a) Charge capacities, (b) discharge capacities, and (c) the retention rate of discharge capacity for Si/(X)/C composites versus cycle number.

Si/(0)/C after the cycling (Figure 7a) is completely different from the original (see Figure 3d). The carbon matrix has lost its tailored spherical nanospaces, indicating that the Si expansion 6008

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Figure 7. TEM images of (a) Si/(0)/C and (b) Si/(200)/C after 20 charge−discharge cycles.

totally destroyed the carbon structure. This result is reasonable from its low ε value (as low as 1.6), which is not high enough for the large volume expansion of Si. With increasing ε value of the Si/(X)/C composites, i.e., with increasing the buffer size around Si, the expected capacity is decreased (the fifth column of Table 1), and therefore, the first charge capacities of Si/(X)/C composites are decreased in order (the sixth column of Table 1). However, the Coulombic efficiency in the first cycle is improved in all the Si/(X)/C samples having the buffer nanospace (the seventh and eighth columns of Table 1), indicating that the presence of buffer nanospace is more desirable to suppress the structure destruction of the carbon framework and enables better lithium extraction. Figure 6c shows the retention rate of discharged capacity (the ratio to the capacity in the first cycle) for Si/(X)/C composites over 20 cycles. Although Figure 6c represents only the early stage of the cycle lives rather than long-term trends, different tendencies can already be seen between the samples having different buffer sizes. The order of the better cyclability, i.e., the order of the capacity retention at the 20th cycle, is Si/ (200)/C > Si/(400)/C > Si/(90)/C > Si/(10)/C > Si/(0)/C. It is thus found that there is an optimum buffer size in the present system. Figure 7b shows a TEM image of the best sample, Si/(200)/C with an ε value of 4.1 after 20 cycles. Unlike the case of Si/(0)/C (Figure 7a), the carbon framework is not destroyed to any extent, and the spherical nanospaces can still be observed. These results clearly indicate that the introduction of a suitable-size buffer is essential to avoid structural destruction by Si volume expansion. It has been reported that, in some Si/C composites, rapid capacity fading is often observed after the 30−40th cycle.33 To examine whether such a phenomenon occurs or not in the present Si/(X)/C composites, Si/(300)/C was subjected to a longer-term cycling test (Figure 8). For comparison, the results of Si/(200)/C and Si/(400)/C shown in Figure 6c are also plotted in Figure 8. Up to the 20th cycle, the capacity of Si/ (300)/C gradually decreases like the other two samples, and such a continuous change is still observed after the 20th cycle up to the 100th cycle. Thus, Figure 8 clearly demonstrates that the present Si/(X)/C are free from the problem of the rapid capacity fading, at least during 100 cycles. This is probably due

Figure 8. Long-term examination for the retention rate of discharge capacity on Si/(300)/C. For comparison, the data of Si/(200)/C and Si(400)/C shown in Figure 6c were also plotted.

to the presence of the buffer nanospaces which avoid severe structure destruction as shown in Figure 7b. It is generally accepted that when fully lithiated Si expands up to 3−4 times larger than its original volume. This expectation mostly comes from a thermodynamic phase diagram of the binary Li−Si system: it consists of four crystalline lithium silicides, such as Li12Si7, Li7Si3, Li13Si4, and Li21Si5.6 The most Li-rich phase, Li21Si5, corresponds to the theoretical capacity of 4008 mAh/g, and its volume is 4.1 times larger than the original Si.4 However, at room temperature, none of these crystals are actually formed by electrochemical lithiation, but a lithiated amorphous silicide is formed instead.45 Then, further deep lithiation forms a metastable crystal phase, Li15Si4, which provides the theoretical capacity of 3572 mAh/g, and its volume is 3.7 times larger than the original Si.46 In the case of nanosized Si, only the metastable amorphous Li−Si alloy can be formed even by a deep lithiation.43 Although the capacity is highly dependent on the experimental environment, one of the highest capacities for nanosized Si reported so far is about 3800 mAh/g-Si (4.0 Li inserted per Si).47 As for Si/(200)/C in this work, we can calculate the first Li insertion capacity of Si to be 3050 mAh/g-Si (3.2 Li inserted per Si), considering the Si content in Si/(200)/C (21 wt %) and the first charge capacities of both Si/(200)/C (980 mAh/g) and the reference pure carbon (430 mAh/g, see the Supporting Information). If the density of the Li−Si alloy is proportional to the amount of Li up to 1.18 g/cm3 of Li15Si4, we can predict that Si in Si/(200)/C 6009

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ACKNOWLEDGMENTS We thank Kureha Co. for kindly supplying poly(vinylidene fluoride) binder. This research was supported by a NEDO Program, Development of High-Performance Battery System for Next-Generation Vehicles.

expanded up to 3.3 times its original volume. It is therefore very likely that the sizes of the buffer nanospaces in Si/(200)/C (ε = 4.2), Si/(300)/C (ε = 5.0), and Si/(400)/C (ε = 5.1) are large enough to allow the Si expansion without structural destruction of the carbon framework, whereas the buffer nanospaces in Si/ (0)/C, Si/(10)/C, and Si/(90)/C are insufficient. Accordingly, the improvement of the cyclability in the order of buffer size from Si/(0)/C to Si/(200)/C can be understood from the balance between the buffer and the expanded Si sizes. However, the capacitance retention of Si/(400)/C slightly declines compared to Si/(200)/C and Si/(300)/C. This may be because too small Si nanoparticles more easily drop out of the carbon matrix during the lithiation and/or delithiation, as was suggested from the results in Figure 4. In Figure 6c, even the best sample, Si/ (200)/C, shows gradual capacitance decline during the cycles. This can be ascribed to the structural change caused by the formation of the solid electrolyte interface (SEI). The electrolyte penetration may result in a loss of the electronic connection of Si and C after the SEI film formation on either Si particles or the carbon matrix. Though all of the Si/(X)/C samples may have this SEI problem, a clear difference is still observed in their charge−discharge behaviors (Figures 5 and 6) and the structure change after the cycling (Figure 7), depending on their buffer sizes. The present results indicate that the minimum necessary buffer space around nanosized Si is about 3 times larger than the Si volume (corresponding to the ε value of 4), and too large a buffer is not preferable to achieve better cyclability as well as high volumetric capacity.

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ASSOCIATED CONTENT

S Supporting Information *

Calculation methods for parameters, XRD patterns, and capacity of carbon framework. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS Si nanoparticles were calcined to form SiO2 shells with controlled thickness on the particle surfaces. After embedding the Si−SiO2 core−shell particles into a carbon matrix, the SiO2 shell was removed with HF. By this hard-template method, the Si/carbon composite which contains a buffer nanospace around each Si was successfully prepared. More importantly, the size of the buffer nanospace can be easily controlled in this method. A series of Si/C composites with different buffer/Si volume ratios were produced and used as model anode materials for lithiumion batteries to investigate the minimum necessary buffer size for a sufficient charge−discharge cycling of Si. It was found that the minimum necessary buffer size is about 3 times larger than the Si volume, i.e., the size which allows Si to expand up to 4 times larger than its original volume. Too large a buffer size (more than 3 times), however, gave rise to the decline of the charge−discharge cyclability, probably because such larger buffer space makes it easier for Si nanoparticles to drop out of the carbon matrix during the cycling.

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The authors declare no competing financial interest. 6010

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