In the previous article "The energy storage density of silicon is 10 times that of existing lithium battery negative electrode materials, why not replace it?", we mentioned that the volume expansion rate of silicon during charging can also reach more than 10 times that of graphite materials. For this reason, silicon cannot completely replace graphite at present, and can only be partially replaced to maintain the volume expansion rate within an acceptable range and prevent damage to the battery. In order to achieve partial substitution, the most important use is nanotechnology - this cool technology that has been in use for decades has also come into play here. People prepare nanoscale porous silicon carbon composite structures, leaving gaps inside the silicon carbon particles to allow the silicon material to occupy internal gaps for expansion during charging, while minimizing expansion into external space to prevent damage to the battery structure.

Today, we will outline several main research directions for the preparation of nano silicon carbon composite materials, as well as their respective advantages and disadvantages.
The most basic method is to wrap a carbon layer around silicon nanoparticles, which can be used to wrap a single silicon particle or a cluster of silicon particles. The following figure shows the use of electrospinning technology to wrap a cluster of silicon particles. The equipment used is a dual nozzle electrospinning machine, where silicon particles are dispersed in the adhesive solution and injected into the inner nozzle, while the polyacrylonitrile solution is injected into the outer nozzle. The long fibers formed by spraying are a composite structure with the outer layer of polyacrylonitrile wrapped around the inner layer of silicon particles. The outer layer of polyacrylonitrile is carbonized using high temperature to obtain the final product of carbon encapsulated silicon particles. The basic method of carbon coated silicon particles is relatively easy to achieve low-cost production, but there are not many gaps reserved inside the package. Silicon particles are prone to damage the outer carbon coating layer during charging and expansion, leading to battery failure.

There is an improved nanoparticle encapsulation method called the Yolk Shell structure, which is similar to a cooked egg. The protein part is extracted, leaving only the inner yolk (silicon particles) and outer eggshell (carbon outer layer). The gap between the two is left by the protein extraction, which is the internal space reserved for the expansion of silicon particles. The following figure shows the methods for preparing two types of yolk eggshell structures. Method A involves coating the outer layer of silicon particles with tetraethoxysilane, which is then converted into an organic silicon coating layer. Wrap organic matter outside the organic silicon layer and carbonize it into a carbon outer layer at high temperature. Finally, use hydrofluoric acid to dissolve the organic silicon layer between the silicon particles and the outer layer of carbon, forming voids. Method B is to prepare an egg yolk eggshell structure with two layers of carbon encapsulation. The difference from Method A is that before encapsulating the organic silicon layer, a layer of carbon encapsulation is added to the surface of the silicon particles. The advantage of two layers of carbon wrapping is that it can double protect the inner silicon particles from contact with the external electrolyte, but it also increases the resistance of electrons and lithium ions entering the silicon particles. The yolk eggshell structure can bring excellent and stable performance to batteries, but the cost of large-scale production is relatively high.


The yolk eggshell structure can be used not only for the morphology of single silicon particles, but also for the preparation of multi particle aggregation and encapsulation. As shown in the figure below, the nano silicon particles are first coated with a layer of silica, and then dispersed into the lotion system to form a micron sized microsphere. Wrap a layer of polymer outside the microspheres and convert the polymer layer into a carbon outer layer through high-temperature carbonization. Reusing hydrofluoric acid to dissolve the silica layer on the outer layer of the nano silicon particles, the entire micro cluster becomes a loosely aggregated structure of silicon particles, with sufficient voids to cope with the expansion of silicon particles. This multi particle aggregation encapsulated microsphere structure can increase the content of silicon material, thereby increasing the energy density of the negative electrode of the battery. However, because many silicon particles rely on the same carbon outer layer, their stability is not as good as that of a single particle structure.

In addition to particle encapsulation, porous structure dispersion can also be used to prepare silicon carbon composite structures. The figure below shows the dispersion of nano silicon particles attached to a porous carbon material, and the inherent porous structure of the carbon material provides space for the volume expansion of silicon particles. The preparation of this composite structure is relatively simple, and the cost of mass production is not high. However, the open porous structure means a very large surface area. During the first charge, all silicon carbon surfaces will generate electrode electrolyte interface facial mask, thus consuming a large number of electrolytes and reducing the energy storage capacity of the cell.

Porous structures can be used not only as carbon materials, but also as silicon materials. The following figure shows the method of preparing porous silicon materials and then wrapping them with a carbon layer. After acid etching a piece of aluminum silicon alloy and dissolving the aluminum metal, the remaining material is a porous silicon block. Use ball milling to break porous silicon blocks into micrometer particles, and finally add a carbon coating to obtain a carbon coated silicon porous structure material. The silicon carbon composite produced by this method has excellent and stable properties, but the preparation process is more material intensive, resulting in higher costs.

The above are several main methods for preparing silicon carbon composite electrode materials. Currently, companies that have been put into production of silicon carbon composite electrodes include Betray, Shanshan Co., Ltd., Putailai, Guoxuan High tech, Tianmu Pioneer, Zhengtuo Energy, Snow, etc. in China, Panasonic, Hitachi Chemical, GS Tangqian, etc. in Japan, Tesla, Amprius Technologies, Enoix, Enevate, NanotekInstruments, XGSciences, California Lithium Battery, Sila Nanotechnology, Group14 Technologies, etc. in the United States. We will conduct research and analysis on the products and technologies of some of these companies in subsequent articles.

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The energy storage density of silicon is 10 times that of existing lithium battery negative electrode materials, why not replace it?
Reference:
Research Progress of Silicon/Carbon AnodeMaterials for Lithium-Ion Batteries: Structure Design and Synthesis Method. ChemElectroChem 7, 4289–4302, (2020)
Author Introduction:
Dr. Xie Wei, Bachelor and Master of Materials Science from Tsinghua University, and Ph.D. in Chemical Engineering from the University of Texas (Austin) in the United States. Mainly engaged in the development of energy storage batteries, has held important positions in multinational corporations and startups, led multiple research and development projects funded by the US Department of Energy, and won the 2013 US Annual 100 Best Research and Development Technology Award. Published 17 papers in top journals in materials science and energy storage, served as a reviewer for 5 international journals, and has applied for 17 international invention patents.