Honeycomb structures have been attracting attention from researchers mainly for their high strength-to-weight ratio. As one type of structure, honeycomb monoliths having microscopically dimensioned channels have recently gained many achievements since their emergence. Inspired by the microhoneycomb structure that occurs in natural tree xylems, we have been focusing on the assembly of such a structure by using the major component in tree xylem, cellulose, as the starting material. Through the path that finally led us to the successful reconstruction of tree xylems by the unidirectional freeze-drying (UDF) approach, we verified the function of cellulose nanofibers, toward forming xylem-like monoliths (XMs). The strong tendency of cellulose nanofibers to form XMs through the UDF approach was extensively confirmed with surface grafting or a combination of a variety of second components (or even a third component). The resulting composite XMs were thus imparted with extra properties, which extends the versatility of this kind of material. Particularly, we demonstrated in this paper that XMs containing reduced graphene oxide (denoted as XM/rGO) could be used as strain sensors, taking advantage of their penetrating microchannels and the bulk elasticity property. Our methodology is flexible in its processing and could be utilized to prepare various functional composite XMs.

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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.

Lithium-ion batteries (LIBs) are generally constructed by lithium-including positive electrode materials, such as LiCoO2, and lithium-free negative electrode materials, such as graphite. Recently, lithium-free positive electrode materials, such as sulfur, are gathering great attention from their very high capacities, thereby significantly increasing the energy density of LIBs. Though the lithium-free materials need to be combined with lithium-containing negative electrode materials, the latter has not been well developed yet. In this work, the feasibility of Li-rich Li-Si alloy is examined as a lithium-containing negative electrode material. Li-rich Li-Si alloy is prepared by the melt-solidification of Li and Si metals with the composition of Li21Si5. By repeating delithiation/lithiation cycles, Li-Si particles turn into porous structure, whereas the original particle size remains unchanged. Since Li-Si is free from severe constriction/expansion upon delithiation/lithiation, it shows much better cyclability than Si. The feasibility of the Li-Si alloy is further examined by constructing a full-cell together with a lithium-free positive electrode. Though Li-Si alloy is too active to be mixed with binder polymers, the coating with carbon-black powder by physical mixing is found to prevent the undesirable reactions of Li-Si alloy with binder polymers, and thus enables the construction of a more practical electrochemical cell.