Stabilization of Silicon Anode for Li-Ion Batteries

Micrometer-sized Si particles with nanopore structures were investigated as anode material for Li-ion batteries. The porous structure of Si helps accommodate the large volume variations that occur during the Li insertion/extraction processes. To improve the electronic integrity of the Si-based anode, a two-step process was utilized. First, chemical vapor deposition CVD was used to enhance the electronic conductivity of individual Si particles by depositing a uniform carbon coating on both the exterior surfaces and the pores. Next, the electronic contact among silicon particles was improved by adding Ketjenblack KB carbon, which exhibits an elastic, chainlike structure that maintains a stable electronic contact among silicon particles during cycling. Using this approach, an anode with a reversible capacity of more than 1600 mAh/g after 30 cycles was obtained. The combination of the nanopore structure, CVD-coated carbon on the Si surface, and the elastic carbon KB among the silicon particles provides a cost-effective approach to utilize the large micrometer-sized Si particles in Li-ion batteries. Si-based anodes could significantly increase the energy density of Li-ion batteries. Compared with the capacity of conventional graphite anode material 372 mAh /g, Si exhibits a theoretical capacity of 4200 mAh/g, 1,2 which is more than 10 times that of graphite. However, the large volume changes that occur during the lithiation and delithiation processes cause severe cracking and pulverizing of the electrode, which in turn leads to a significant capacity fade during cycling. Many factors need to be investigated before Si anodes can be used in Li-ion batteries. The morphology of Si plays an important role in determining the capacity and cyclability. Nanosized Si particles exhibit better electrochemical properties than micrometersized Si particles because of the reduced mechanical stress. 3-6 The Si nanowires maintain an excellent cyclability because the large number of pores between the wires accommodate the expansion that occurs during cycling. 7 However, the weight ratio of Si nanowires to the substrate needs to be increased significantly for the practical applications of Si nanowires. Si nanofibers prepared by electrospinning can be used directly as an anode material without adding binders or carbon additives. 8 The initial capacity obtained from the use of these fibers is greater than 1000 mAh/g; however, fast fading, probably because of the organic residual left in the Si fibers, is still observed. Recently reported porous Si particles coated with carbon show superior cyclability because of the three-dimensional structure, 9 which facilitates Li transport and alleviates the large volume changes associated with Si anodes. Coating carbon on the Si surface by mechanical mixing, 10,11 sometimes combined with py