Carbon-Coated Silicon Material: an Ideal Anode Material for Lithium Batteries
Problems Facing Silicon Carbon Material System
Silicon possesses an ultra-high theoretical capacity for lithium insertion, ten times greater than carbon materials. It has many advantages, including a low price, abundant sources, and a platform that is similar to graphite. Silicon will, however, produce volume changes of >400% during deintercalation, which can result in the material being pulverized, the loss of electrical connection with the current collector and conductive agents, and rapid degradation of capacity. The SEI membrane on the silicon surface is also a major factor in limiting its cycle life.
The lithium ions diffuse into the silicon particle, reducing the lithium insertion capability of the active materials. Selecting nano-scale silicon particle can also reduce material powdering. This will improve capacity. Nanoparticles, however, are easily agglomerated, and they have little effect on the thickening SEI films. In the current silicon anode technologies, they are mainly focused on solving two key problems: "volume expansion", and "conductivity", in the process of charge and discharge. As far as anodes are concerned, the carbon materials used in silicon anodes to form conductive and buffer layer are crucial.
The nanometerization process can enhance the performance of silicon material. To reduce the cost of manufacturing nano-silicon material and to stabilize the SEI film on the surface of silicon materials, a variety of materials with good intrinsic conductivity are used in compounding with silicon. Carbon materials can be used to improve the conductivity on silicon-based anodes and also stabilize the SEI films.
However, there is no carbon or silicon material that can simultaneously meet both the energy density and the cycle life requirements of electronic devices. The fact that carbon is a member of the same chemical group as silicon, and has similar properties to both, makes it easy to recombine them. The composite silicon-carbon can be used to complement both the benefits and shortcomings of each material. It also allows for a material with a much higher gram and cycle capacity.
By reducing the size of the particles in the electrode material, it is possible to increase ionic conductivity rather than electronic conductivity. As the particle size is reduced, the diffusion path of lithium ions is also shortened. This allows the lithium ion to quickly participate in electrochemical reactions, during charge and discharge. For the enhancement of electronic conductivity there are two methods. The first is to coat conductive material and the second is doping. This is done by producing mixed valences states.
Carbon-Coated Silicone Material
Scientists developed a plan for using carbon to wrap silicone as a negative electrolyte material in lithium batteries. They did this by synthesizing the electrochemical characteristics of carbon and silica. In experiments, scientists found that silicon coated with carbon can boost the material's performance. Preparation methods for this material include hydrothermal method CVD, and coating carbon precursors to silicon particles. The array of nanowires were prepared by metal catalytically etching the silicon plate. They then coated the surface with carbon using carbon aerogel and Pyrolysis. The initial discharge capacity of this nanocomposite was 3,344mAh/g. After 40 cycles, the capacity had been reduced to 1,326mAh/g. The material's excellent electrochemical performance is a result of its good electronic conductivity, contact between silicon and carbon materials and effective inhibition of volume expansion by the silicon materials.
The Development Prospects
The Carbon-Coated Silicon Material combines high conductivity, stability and silicon's advantages with high capacity. It is an ideal material for lithium batteries anodes.
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