Effect of buffer size around nanosilicon anode particles for lithium-ion batteries

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.