A Novel Si/C Composite as a High Capacity Anode Material for Lithium-ion Batteries

Silicon based electrode materials for Li-ion battery have attracted great attention due to the high theoretical capacity of 4212 mAh/g (3.75 moles of Li per mole of Si) compared to graphite (1 mol Li per 6 mol C) used in commercial LIBs. Unfortunately, practical applications of silicon as an anode material exhibit the huge volume change (>300%) during the alloying / dealloying process with Li, which would result in the loss of electrical conductivity, dramatic pulverization of silicon, and consequently a rapid capacity fading in the cycling. To alleviate this problem, silicon nanosized or nanostructures materials have been suggested, which can increase the transport of electron and lithium ion and reduce mechanical stress during cycling, resulting in improved electrochemical performance. Among these nanostructures, the combination with carbon is becoming attractive and important material because it improves the electric conductivity and plays a role of a buffer of volume expansion. In this research, a novel composite electrode material consisting of nanocrystalline silicon and porous graphite has been developed as an anode candidate for LIB. Nanocrystalline silicon was coated onto porous graphite via chemical vapor decomposition of SiH4 gas and subsequently pitch coating was carried out, which is known for a good method for preventing the natural oxidation of silicon layer and improving capacity. We analyzed electrochemical properties of blended anode with graphite and nanocrystalline silicon­-porous graphite composite material. The capacity retention of the blending anode for nanocrystalline silicon­-porous graphite is 96% after 50 cycles with high coulombic efficiency of 90.1%, which is improved electrochemical performance compared with silicon non-porous graphite composite anode. Especially, the nanocrystalline silicon­-porous graphite composite material has lower expansion of electrode, comparing with silicon non-porous graphite composite material. The expansion of the nanocrystalline silicon­-porous graphite composite electrode is 42% after 1 cycle, while the expansion of the silicon non-porous graphite composite is 62% after 1 cycle. These improvements are attributed to the porous carbon shell, which reduces the cell expansion, prevents rupture of electrical path, stabilize the cycle performance.