The synthesis procedure of the highly porous Si electrode is
schematically illustrated in Figure 1 . First, submicrometer-sized silica particles were obtained via planetary milling from commercial
sand (quartz) particles. Then, pure Si with a hierarchical
structure composed of primary Si nanoparticles with ≈10 nm
diameter and larger-scale (≈1 μm) secondary aggregation structures
of the primary particles was obtained by magnesiothermic
reduction at a temperature of 680 °C. This temperature is much
lower than those of other synthesis methods such as carbothermal
or electrolytic reduction methods. [ 17 ] Finally, a conformal
carbonization reaction of aniline on the Si nanostructures was
carried out to obtain uniform carbon layers by diazotization
reaction. [ 18 ] The bottom of Figure 1 also shows an illustration of
the detailed reaction mechanism during the magnesiothermic
reduction under low pressure. We developed a highly effi cient
reduction method based on low-pressure conditions (pressure,
P ≈ 0.1 Torr), which can be easily reached by short (