Within this work we present novel approach for improving the cycle stability of silicon as anode material for lithium-ion batteries. It has been already shown that nanostructuring  or embedding  of silicon particles into a matrix can lead to significant improvements in cycling stability. In this context monodisperse porous silicon nanospheres (nSi) are synthesized via two different methods using magnesiothermic reduction and subsequently embedded into various matrix materials. In order to insure good electrical conductivity carbon or carbon-rich polymer-derived silicon oxycarbide ceramic (SiOC) are used as supporting matrix.
Monodisperse solid silica spheres are prepared via a modified Stober method  and used as silicon precursor. In order to reduce silica nanoparticles magnesiothermic reaction is employed via heat scavenger assisted route and two-chambers route. Significant increase in specific surface area (SSA, from 68 m2g-1 to 198 and 623 m2g-1 for two chambers and salt assisted route, respectively) is found after reduction of silica. This tremendous change in SSA is assigned to the evolution of the system from nonporous silica to mesoporous silicon, while the size and the shape of particles stayed unchanged (DLS, SEM and TEM investigation). Furthermore, apart from difference in SSA, obtained nSi is found to be nanocrystalline (two chambers method) or completely amorphous (salt assisted route) as proved by means of XRD. Completeness of magnesiothermic reduction and presence of silicon is further confirmed by Raman spectroscopy measurements (Fig. 1, Insert). Significantly improved stability compared to silicon based composites reported in the literature  is found. nSi dispersed within carbon matrix remains electroactive after 20 cycles with minor capacity fading of 24 mAhg-1 as shown by voltage vs. capacity curves (Fig. 1). In both, C and SiOC matrices, amorphous embedded nSi shows lower capacities but enhanced cycling stability with respect to crystalline one.
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|Manuskript||Figure 1||Figure 1: Voltage vs. capacity curve of nanostructured silicon embedded in carbon matrix; Insert: Raman spectra of nano structured silicon derived via salt assisted and two chambers route.||1 MB||Download|