This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 12741–12745 12741 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 12741–12745 Silicon core–hollow carbon shell nanocomposites with tunable buffer voids for high capacity anodes of lithium-ion batteriesw Shuru Chen, Mikhail L. Gordin, Ran Yi, Giles Howlett, Hiesang Sohn and Donghai Wang* Received 2nd July 2012, Accepted 27th July 2012 DOI: 10.1039/c2cp42231j Silicon core–hollow carbon shell nanocomposites with control- lable voids between silicon nanoparticles and hollow carbon shell were easily synthesized by a two-step coating method and exhibited different charge–discharge cyclability as anodes for lithium-ion batteries. The best capacity retention can be achieved with a void/Si volume ratio of approx. 3 due to its appropriate volume change tolerance and maintenance of good electrical contacts. Introduction Silicon has been considered as a promising alternative to graphite-based anodes for next generation lithium-ion batteries because of its natural abundance, environmental friendliness, and most importantly, low discharge potential and the high theoretical capacity (4200 mA h g 1 in Li 4.4 Si). 1,2 However, the practical application of Si anodes has thus far been mainly hindered by low electrical conductivity and low lithium diffusion rate of Si, 3 and by the enormous volume change (300–400%) experienced during the lithiation/delithia- tion process. 4 The volume change can cause bulk Si to be pulverized and lose electrical contact with the conductive additive or current collector, and can also lead to instability of the solid electrolyte interphase (SEI) resulting in continuous consumption of the Li-ion electrolyte for reformation of SEI layers, both of which therefore lead to fast capacity fading. In order to improve the cycling stability of silicon anodes, great efforts have been made to mitigate the pulverization of Si and improve the stability of the SEI layer. These efforts include the development of Si materials composed of nanostructures, 5–9 porous structures, 10–15 or nanocomposites, 16–22 the addition of coating layers, 23–25 and the application of electrolyte additives 26,27 and novel binders. 28–30 Among these efforts, a simple and widely applied strategy is to use Si–C nanocomposites. 20–25 However, the success of this approach is still limited because large Si volume change can only be tolerated to a limited degree, especially during deep charge–discharge processes. Suitable porosity in the Si–C nanocomposite is thus needed in order to further buffer the volume change of the Si. 31 Hollow micro-/nano-structured materials have been recognized as one type of promising material for applications in energy-related systems. 32 For example, Si–C nanocomposites in which Si nanoparticles are encapsulated in hollow carbon materials, such as Si-hollow carbon nanotubes 33 and Si-hollow carbon spheres, 34,35 can not only increase electrical conductivity and provide intimate electrical contact with Si nanoparticles, but also provide built-in buffer voids for Si nanoparticles to expand freely without damaging the carbon layer. However, Si-hollow car- bon nanotubes require a sophisticated binder-free fabrication process. 33 In comparison, direct synthesis of Si–C core–hollow shell nanocomposites in powder, reported by Iwamura S. et al. and Li X. et al., is promising as the materials can be fabricated using a conventional industrial coating procedure. 34,35 Both of these studies indicate the importance of carbon coating and void spaces in order to obtain good cycling performance from Si anodes. In their approaches, however, part of the Si is sacrificed to obtain buffer voids through its thermal conver- sion to SiO 2 and subsequent removal by etching, resulting in the loss of active materials. Moreover, the diameters of the Si cores vary based on different degree of oxidation during the formation of SiO 2 . As diameters of Si nanoparticles have a prominent influence on the cycling performance, 36 it is difficult to illustrate the direct relationship between cycling perfor- mance and various void/Si volume ratios in the nanocompo- site. Thus, it is desirable to develop a bottom-up synthesis approach to produce Si–hollow carbon nanocomposite materials with conformal carbon shells and tunable built-in buffer voids, in order to demonstrate the effectiveness of the Si–hollow carbon structure using a conventional electrode fabrication process and elucidate the relationship between void/Si volume ratio and electrochemical performance of the nanocomposites. Here we report a versatile solution growth method to synthesize silicon core–hollow carbon shell nanocomposites with controllable, built-in buffer voids between the silicon cores and the hollow carbon shells. A similar approach was reported to synthesize Si core–hollow carbon shell nanocomposites during preparation of the manuscript. 37 However, there have been no systematic studies on optimization of buffer voids for Si core–hollow carbon shell structures. As illustrated in Scheme 1, the synthesis process starts with solution growth Department of Mechanical & Nuclear Engineering, the Pennsylvania State University, University Park, Pennsylvania 16802, USA. E-mail: [email protected]w Electronic supplementary information (ESI) available: Additional XRD, TGA, TEM and electrochemical characterization. See DOI: 10.1039/c2cp42231j PCCP Dynamic Article Links www.rsc.org/pccp COMMUNICATION Published on 27 July 2012. Downloaded by Pennsylvania State University on 18/07/2013 21:07:43. View Article Online / Journal Homepage / Table of Contents for this issue
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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 12741–12745 12741
with controllable, built-in buffer voids between the silicon cores
and the hollow carbon shells. A similar approach was reported
to synthesize Si core–hollow carbon shell nanocomposites
during preparation of the manuscript.37 However, there have
been no systematic studies on optimization of buffer voids for
Si core–hollow carbon shell structures. As illustrated in
Scheme 1, the synthesis process starts with solution growth
Department of Mechanical & Nuclear Engineering, the PennsylvaniaState University, University Park, Pennsylvania 16802, USA.E-mail: [email protected] Electronic supplementary information (ESI) available: AdditionalXRD, TGA, TEM and electrochemical characterization. See DOI:10.1039/c2cp42231j
PCCP Dynamic Article Links
www.rsc.org/pccp COMMUNICATION
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View Article Online / Journal Homepage / Table of Contents for this issue
12744 Phys. Chem. Chem. Phys., 2012, 14, 12741–12745 This journal is c the Owner Societies 2012
The irreversible capacity loss may come from the irreversible
alloying of silicon with Li, the irreversible insertion of Li into
amorphous carbon, and the formation of SEI layers. The latter
two may significantly contribute to the loss because there is
around 40–60% of amorphous carbon in the composites and
the high-surface-area carbon shells directly contact the electro-
lyte. This is probably the reason why the first cycle CEs decrease
when the void volume increases; that in turn increases the
amount of carbon and the surface area of the nanocomposite.
Fig. 3b shows charge (delithiation process) capacities versus
cycle number at a rate of 0.05 C for the Si@HC nanocompo-
site anodes. Among these nanocomposites, Si@HC_3 delivers
the highest stable capacity (1625 mA h g�1) and best capacity
retention (69%), compared with 1165 mA h g�1 and 44% for
Si@HC_1.5 after 100 cycles, and 378 mA h g�1 and 28% for
Si@HC_6 after 80 cycles. The volume change of silicon during
cycling is known to be about 300–400%, corresponding to a
void/Si volume ratio of 2–3 for Si@HC nanocomposites.
Therefore, Si@HC_3 has buffer voids large enough to allow
the Si to expand freely without mechanical destruction of the
carbon shells. Si@HC_1.5 shows higher charge capacity in the
first 10 cycles but has more severe capacity fading compared
with Si@HC_3. This can be explained by the insufficient buffer
voids in Si@HC_1.5 resulting in destruction of the carbon
shell after a few cycles of volume change leading to loss of
electric contact between Si and carbon and the severe capacity
fading. In contrast, the charge capacity of Si@HC_6 decreases
dramatically as shown in Fig. 3b. This very poor cyclability
may be ascribed to the limited contact area between the Si core
and the hollow carbon shell and the resultant poor electrical
contact. Overall, the superior capacity retention of Si@HC_3
can be explained by it possessing buffer voids large enough to
tolerate the 300–400% volume expansion of full Si lithiation
(i.e. 4.4Li + Si - Li4.4Si) while still keeping the Si cores and
carbon hollow shells close enough to maintain good electrical
contact with each other (see Fig. S4 in ESIw).Fig. 2d shows a TEM image of fully lithiated Si@HC_3
taken after twelve charge–discharge cycles. The expanded
silicon nanoparticles are still well confined within the carbon
shells. The formation of SEI layers on carbon shells is also
observed and no signs of carbon shell destruction are found,
indicating their good stability owing to the free buffer volume.
The SEI formation on the carbon shell instead of on the Si
with its expanding/shrinking surfaces upon lithiation/delithiation
may improve the stability of the SEI, and in turn increase the
cyclability of the cell.
The nanocomposite of Si@HC_3 was then discharged and
charged at progressively higher rates. Fig. 3c illustrates that
the Si@HC_3 manifests a good rate capability. The discharge
capacity was 3600 mA h g�1 for the first cycle at 0.05 C which
faded gradually with continued cycling and increasing rate,
then stabilized to around 1700 mA h g�1 at a 0.5 C rate after
20 cycles, and 900 mA h g�1 at a 2.5 C rate after 25 cycles. The
cell could then operate at rates as high as 5 C, and still deliver a
capacity of approx. 500 mA h g�1. When the rate was reset
to 0.05 C after more than 35 cycles, the capacity was about
2100 mA h g�1. The cyclability of Si@HC_3 was further
investigated by charging and discharging it at 0.5 C after the
cell was charged–discharged at a 0.05 C rate for 3 cycles;
a capacity of 1100 mA h g�1 was obtained after 300 cycles,
which is about half of the initial capacity of 2250 mA h g�1 at
0.5 C (see Fig. S5 in ESIw).
Conclusions
In summary, silicon core–hollow carbon shell nanocomposites
with controlled, built-in buffer voids have been synthesized by
encapsulating commercial Si nanoparticles in hollow carbon
spheres via a solution coating process. The cycling capacity of
the nanocomposite can be greatly enhanced by optimizing the
buffer voids. The best performance of Si@HC nanocompo-
sites was achieved at a void/Si volume ratio of about 3. These
results suggest that core–hollow shell structure in the nano-
composites with both suitable buffer voids to tolerate volume
expansion and intimate contacts with Si can improve the
structural integrity and cycling performance of the commercial
silicon nanoparticles. Further optimization of the amount of
carbon, the first coulombic efficiency, total energy density
of the electrode, and cycling performance are still under
investigation.
Acknowledgements
This work was supported by the Assistant Secretary for
Energy Efficiency and Renewable Energy, Office of Vehicle
Technologies of the U.S. Department of Energy under
Contract No. DE-AC02-05CH11231, Subcontract NO.
6951378 under the Batteries for Advanced Transportation
Technologies (BATT) Program.
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