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Carbon-Coated Silicon Material: an Ideal Anode Material for Lithium Batteries

Problems Facing Silicon Carbon Material System

Silicon offers a very high theoretical lithium insertion potential, around ten-times that of carbon, with the benefits of an easy to use charging and discharging interface, similar to graphite. However, the silicon’s poor electronic and/or ionic conductivity will cause large volume changes (>400%) in the lithium-deintercalation process. This can lead to the loss of contact with current collectors and conductive agents, pulverization and rapid capacity depletion. Additionally, silicon’s unstable solid electrolyte membrane interface (SEI) severely restricts its life span.

The process of releasing lithium causes the SEI layer on the silicon surface to expand and contract, creating new SEI layers. These SEI films can then be deformed, cracked and reformed, which will cause the SEI movie to thicken and accumulate on the surface. The selection of small-sized silicon particles may be able to reduce material powdering or decrease attenuation. However nanoparticles can easily agglomerate, and they have no apparent effect on the thickness of the SEI films. Therefore, its electrochemical performance must be enhanced. The current silicon anode tech focuses on solving two key problems during the charge-discharge process: “volume expansion” or “conductivity.” The current trend in anodes development is that carbon materials are essential for silicon anodes, both as buffer and conductive layers.

It is possible to improve the electrochemical performance by altering the manufacturing process as well as the morphology. The nanometerization process for the production of elemental silica anode can dramatically increase the material’s performance. To reduce production costs of nanosilicon materials and stabilize silicon’s SEI films, many materials have high intrinsic conductivity that can be compounded with them. Carbon materials have the ability to improve conductivity and stabilize the SEI film at the anode’s surface.

Modern electronic devices require both energy density and life expectancy to be met by one carbon or another silicon material. It is easier to mix the carbon and silicon materials through different ways, due to their similar chemical characteristics. A composite of silicon and carbon material is able to combine their strengths and make up for any deficiencies. This creates a new material with a significant increase in gram density and cycle life.

Additionally, the goal of increasing the ionic conductivity rather than electronic conductivity is achieved by reducing the particles of electrode materials. As the particles are smaller, the path of diffusion for lithium ions is shorter. The lithium ion will be more able to take part in the electrochemical reactions during charge and discharge. Two main methods of improving electronic conductivity are available. The first is coating with conductive materials. The second is doping by producing mixed valence state, which can improve intrinsic conductivity.

Carbon-Coated Silicon Material

Researchers have created a strategy to utilize carbon to cover silicon to create a negative electrode for lithium battery cells. Research has shown that carbon coated silicon increases its ability to withstand high temperatures. These methods include CVD, Hydrothermal Method and Coating various carbon precursors with silicon particles. They prepared the array by using silicon plates as a metal catalyst and coated them with carbon aerogel. The nanocomposite has a discharge potential of up to 3,344 mAh/g and a reversible power capacity of 1,326 mAh/g after 40 cycles. Excellent electrochemical performance is possible due to the excellent electronic contact between silicon-carbon material and conductivity. Also, the effect of carbon materials inhibiting the volume expansion silicon materials makes the material very conductive.

Development Prospects

A carbon-coated silicon material is ideal for lithium battery anodes because it combines high conductivity, stability, and silicon’s advantages with high capacities.

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