Effect of carbon coating on nano-Si embedded SiOx-Al2O3 composites as lithium storage materials

Abstract

Carbon-coated Si–SiOx–Al2O3 composite materials were prepared via a two-step process involving a high-energy mechanical milling and a pyrolysis reaction, and their electrochemical properties were subsequently examined for use as anode materials for Li-ion batteries. To synthesize the nanocrystalline Si embedded SiOx composite, amorphous SiO2 and metallic Al were chosen as the best pair of starting materials. First, a simple high-energy mechanical milling process was performed to partially reduce amorphous SiO2 using Al metal. Al was spontaneously oxidized to Al2O3; this resulted in a nanocrystalline Si dispersed amorphous SiOx–Al2O3 phase. Second, the composite was heat-treated in the presence of naphthalene as a carbon source. Through the pyrolysis reaction of naphthalene, an amorphous carbon coating layer was obtained; this layer stabilized the crystal structure of SiOx–Al2O3. The materials properties of the composites were investigated using various analytical tools. Electrochemical tests showed that the carbon-coated composite electrode exhibited good cycling stability up to 300 cycles. This performance can be attributed to the effect of the carbon coating, which both facilitated fast electronic and Li+ ionic transport and played a buffering role in the large volume change of the Si-based anode materials.