Silicon has an ultra-high theoretical lithium insertion capacity, about ten times that of carbon materials, and has the advantages of a charging and discharging platform similar to graphite, low price and abundant sources. However, in addition to poor electronic and ionic conductivity, silicon will produce serious volume changes (>400%) during the deintercalation of lithium, which will result in pulverization of the material, loss of electrical contact with the current collector and conductive agent, and rapid capacity degradation. In addition, the unstable solid electrolyte interface membrane (SEI membrane) on the silicon surface also severely limits its cycle life.
In the process of releasing lithium, with the expansion and contraction of silicon, the SEI film on the silicon surface is constantly deformed and cracked, and new SEI film will be formed on the exposed silicon surface, causing the SEI film to gradually accumulate and thicken, which greatly hinders The diffusion of lithium ions into the silicon particles reduces the lithium insertion capacity of the active material. In addition, the selection of nano-scale silicon particles can certainly suppress material powdering and reduce capacity attenuation, but nanoparticles are easy to agglomerate and have no obvious effect on inhibiting the thickening of SEI film, so its electrochemical performance needs to be improved. At present, the silicon anode technology focuses on solving the two core problems of "volume expansion" and "conductivity" during the charge and discharge process. However, as far as the current development trend of anodes is concerned, carbon materials are indispensable in silicon anodes as conductive and buffer layers.
The electrochemical performance of the silicon material can be improved through the manufacturing process and the morphology, and the nanometerization of the manufacturing process of the elemental silicon anode material can significantly improve the performance of the silicon material. In order to reduce the production cost of nano-silicon materials and stabilize the surface SEI film of silicon materials, many materials with excellent intrinsic conductivity have been used to compound with silicon materials. Among all these materials, carbon materials can not only improve the conductivity of silicon-based anodes, but also stabilize the SEI film on the anode surface.
However, no single carbon material or silicon material can simultaneously meet the requirements of modern electronic devices for the two important indicators of energy density and cycle life. In view of the fact that silicon and carbon belong to the same main group and have similar chemical properties, this makes it easier to recombine the two through different routes. The composite silicon-carbon material can complement the advantages of the two, make up for their respective shortcomings, and obtain a new composite material with significantly improved gram capacity and cycle density.
In addition, the purpose of reducing the particle size of the electrode material is to increase the ionic conductivity of the material rather than the electronic conductivity. Because the particle size becomes smaller, the lithium ion diffusion path is shortened, so that the lithium ion can quickly participate in the electrochemical reaction during the charge and discharge process. As for the improvement of electronic conductivity, there are two main ways, one is the coating of conductive materials, and the other is through doping, such as producing mixed valence states, so as to improve the intrinsic conductivity of the material.
Synthesizing the electrochemical properties of carbon and silicon, scientists have devised a plan to use carbon to wrap silicon as a negative electrode material for lithium batteries. Through experiments, researchers found that carbon-coated silicon can increase the capacity of the material. The preparation methods of this material mainly include hydrothermal method, CVD and coating various carbon precursors on silicon particles. The researchers prepared the array of silicon nanowires by metal catalytic etching on the silicon plate, and then coated the surface of the silicon nanowires with carbon through carbon aerogel and pyrolysis. The first discharge capacity of the nanocomposite is as high as 3,344mAh/g, and the reversible capacity after 40 cycles is 1,326mAh/g. The good electronic contact and conductivity between the silicon-carbon materials and the effective inhibition of the volume expansion of the silicon materials by the carbon materials make the material's electrochemical performance excellent.
Carbon-coated silicon material combines the high conductivity and stability of carbon and the advantages of silicon with high capacity, making it an ideal lithium battery anode material.
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