Silicon As Anode For Lithium Ion Batteries

The graphite, as commercial anode material for lithium ion batteries, cannot meet the new requirement of high-performance lithium ion batteries because of its limited theoretical capacity of 372mAh/g. Kinds of new anode materials are being studied, such as silicon based material, metallic oxide. Tin-based material, and other composite materials. Silicon attracted much attention for its highest theoretical capacity of all,4200mAh/g. However, there are two big problems that limited the application of silicon as anode material. The first problem is that huge volume change during lithium insertion and extraction leads to material pulverization, resulting in severe capacity fade. The second is poor electrical conductivity of silicon. Literatures studies show that reducing silicon particles to nano size can relief the volume change. On the basis of this, we introduce an easy method of preparing silicon nanoparticles and optimize the structure of current collector to improve the cycle performance of lithium ion batteries.In this thesis, the following two aspects were mainly studied.1. Three kinds of particles with different sizes were prepared as anode materials: silicon nanoparticles with average size of 50 nm prepared by milling silicon nanowires, silicon nanoparticles with average size of 200 nm prepared by ball milling crystal silicon and silicon micron particles with average size of 2μm prepared by manually grinding crystal silicon. The smallest particles showed best cycle performance.2. Current collectors with different structures were fabricated. We take Ni foam as current collector and fabricate electrodes that contain 1 layer,2 layers and 3 layers of Ni foam, separately. Results indicate that 2 layers of Ni foam can enhance the cycle performance. Besides, we tried sputtering copper particles on Ni foam to further enhance the cycle performance by improving electrical conductivity.All in all, the lithium ion batteries’cycle performance were improved by reducing silicon particles to nano size and optimizing structure of current collector. Compared with conventional methods of preparing silicon nanoparticles, the method introduced here is simpler and requires no expensive equipments. The new way of improving the cycle performance by optimizing current collector can increase capacity by 20 percent and, what’s more, can be used on many other electrode materials.