Figure 6.9 [102] shows the change in charge capacities for different nanocomposites on cycling.

Furthermore, nano-dispersed Si in carbon synthesized by chemical vapor deposition (CVD) had demonstrated a reversible capacity of 500 mAhg" 1 , However, the CVD approach produced SiC and the morphology of Si and C cannot be controlled [166] , Nanostructured thin-film form of Si electrode was investigated by some researchers and they reported a specific capacity of around of 1,100 mAh/g [167, 168] , Nano Si-C composite prepared by hand mixing has been reported to have a high reversible capacity of 1,700 mAh/g by Li et al. [148]. Also crystalline Si powders have been dispersed in sol-gel graphite [169] , in a TiN matrix [153] , and in synthetic graphite [170] , by ball milling. AH the Si-C composites mentioned, exhibited increased specific capacity compared to bare graphite, and improved cyclability compared to bare Si electrodes. Wang et al. [157] have reported the synthesis of nanostructured Si-C composites by dispersing nanocrystalline Si in carbon aerogel. A reversible capacity of 1,450 mAhg-1 for Si-C composite electrodes (Fig. 6.10a [157]) was reported. The good cyclability was attributed to the usage of nano-sized Si powders and their homogeneous distribution (Fig. 6.10b [157]) in an amorphous carbon matrix.

Fig. 6.10 (a) The discharge capacity vs. cycle number for Si-C electrode. (b) TEM image of nanocrystalline Si-C composites (Reprinted with permission from [171]. Copyright (2004) Elsevier)

A challenge in working with nanocomposites and nanostructured materials derives from difficulties in obtaining adequate structural characterization, especially, as they often lack long-range order. For example, although XRD can yield valuable information regarding structural changes that occur in the host material during Li+ insertion, the poor crystallinity of organic-inorganic hybrid nanocomposites limits the applicability of this technique. So the results obtained exclusively from this technique should be addressed in terms of their accuracy. Nuclear magnetic resonance technique is proved to be a better alternative for this purpose.

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