2010: Preparation of Nanosilicon/carbon Composites via an Electrochemical Reduction of Nano-sized Silica Embedded in Carbon Matrices

Introduction As a new class of an ultra-high capacity anode material for Li-ion batteries, nano-Si/carbon composites have a considerable potential [1, 2]. In such composites, nano-Si can store lithium faster than bulk Si, and the carbon matrix serves nano-Si with an electrical path to compensate the poor electrical conductivity of Si. Though Si has an extremely high theoretical capacitance up to 4200 mAh/g, its cyclability is quite poor due to the significant volume expansion upon the Li insertion into Si. To overcome this problem, the introduction of an appropriate buffer space around Si is very effective [1, 2]. For the preparation of nano-Si/carbon composites, various methods have been proposed so far, e.g., a mechanical mixing of nano-Si and carbonaceous matrices [3], a formation of carbon around nano-Si [4, 5], silane-CVD onto the carbon [6], and a template assisted synthesis [1]. Herein, we present a new synthesis pathway of nanoSi/carbon composites with the buffer nanospace around nanoSi via an electrochemical reduction of nano sized SiO2 embedded in a carbon matrix, according to the synthesis scheme shown in Fig. 1. A nano-SiO2/carbon composite (Fig. 1a) is used as a starting material. The composite was immersed into a molten CaCl2 and then electrochemically reduced. During this step, SiO2 is reduced into Si at a threephase interface of SiO2 (reactant), carbon (electron path), and molten CaCl2 (solvent for a product, O), according to the following equation [8];