Electrical charge and discharge of lithium ion secondary batteries takes place through the movement of lithium ions between the cathode and anode. Anode active materials receive lithium ions during charging, and release electrodes during electrical discharge. The following passage will introduce the production process of silicon, which is one of the anode active materials.
Silicon is produced by restoring and purifying silica, a material found extensively on the surface of the earth. The theoretical capacity of silicone is approximately 10 times the capacity of graphite, a widely used anode active material.
Silicone is a material that is focused on for automotive use. Other than purified silicon, impure semiconductors (n or p type) such as silicon monoxide, silicon dioxide, and doped silicon to increase conductivity are used occasionally. To increase the exchange of lithium ions, increasing the surface area for contact with electrolytic solutions is one of the options. In this case, a grinding process to produce fine particles is utilized. When silicon particles become fine, they become highly reactive with the oxygen in the air, which could lead to combustion. In order to prevent this, the grinding, classifying, and product collection processes are conducted in an inert atmosphere. Hosokawa has many experiences with providing inert atmosphere systems, even for silicon based materials.
There are two major problems when using silicon as an anode material. One is : when using pure silicon, due to its low conductive characteristics, there is a possibility that the material cannot be used for batteries. Because of this fact, often times the silicon is mixed with a conductive material (usually carbon base materials). With a set battery volume, if the ratio of conductive material is increased, it corresponds to a decrease of active materials, meaning the battery capacity decreases as well. To avoid this problem, there is a need to coat all silicon particles with the conductive material, and at the same time construct a structure which minimizes the space between conductive materials to promote contact.
The second problem is that through the exchange of lithium ions, the volume of the silicon particles largely fluctuate. While this problem can be seen in graphite based materials as well, the fluctuation ratio is much greater for silicon materials. This size fluctuation can cause a destruction of the silicon particle, leading to a fragmentation of the conductive network and separation from the electric current collector.
To solve these two problems, we have offered procedures in coating all or a section of the particle surface with a different particle.
Grinding
Generally, silicon particles with sizes of a few microns to sub-microns are utilized. These particles are produced by classifier integrated jet mills or mixing type ball mills. These grinding units have integrated centrifugal force classifiers and have high grinding efficiency by fast collection of ground particles.
Grinding, classifying, and conveying processes are all operated in a closed circuit gas circulation system (Inert gases such as nitrogen and argon gas). Particles with sizes of a few ten microns are collected by the cyclone, while particle sizes of under a few microns are collected by the bag filter. The particles are collected by a vessel without oxygen exposure and are carried through a glove box to the following process.
Additionally, the product analysis needs to be performed without exposure to the atmosphere as well. Therefore, online or inline laser diffraction-scattering type particle size distribution analyzers are directly built into the grinding and classifying system. By utilizing this unit, the particle sizes can be measured in real-time. This allows the operator to view any changes in the particle distribution due to adhesions or lot differences, and adjust the operating condition of the grinder and/or classifier accordingly.
Applying conductive characteristics to silicon particles and maintaining volume fluctuations
When mixing conductive carbon based materials and silicon particles in a dry-process, utilizing the Mechanofusion and/or Nobilta are effective methods for applying a strong compressive shearing and impact force to the materials. These units can disperse, fix, and coat different small particles (often nano-particles) onto the surface of the main particle.
For example, these units can coat heat sensitive alloy elements or carbon materials onto the surface of an amorphized silicon based alloys. By processing the material in this manner, we can suppress the volume fluctuation, and even if fine particles are generated, the 3 dimensional electrode flowing network can be maintained, preventing any decrease in performance