LIB Anode
Active Materials for Secondary Batteries are first mixed with binders and/or solutions and are then applied to the aluminum foil (cathode) or copper foil (anode) of the current collector. After the drying process, the materials are processed to increase the bulk density and finally become electrodes. In this section, the production process of an anode active material (materials which take in lithium ions during charging and releases electrons during electric discharge), specifically graphite materials, will be introduced. Graphite materials can be divided into two categories: Natural Graphite and artificially synthesized Artificial Graphite.
Negative electrode material, graphite
The active material for secondary batteries is first mixed with a binder or solution, and then coated onto aluminum foil (for the positive electrode) or copper foil (for the negative electrode) as the current collector. After a drying process, these materials undergo treatment to enhance their packing density, ultimately forming the electrode. This article will introduce the production process of negative electrode active materials (materials that absorb lithium ions during charging and release electrons during discharging), particularly focusing on graphite materials. Graphite materials can be categorized into two types: natural graphite and artificially synthesized artificial graphite.
Compared to artificial graphite, natural graphite has a lower price and a higher degree of graphitization. This allows natural graphite to store more lithium ions, which is beneficial for increasing battery capacity. However, most natural graphite has a flaky structure with low packing density. This low electrode density characteristic makes it difficult to truly increase battery capacity. In addition, natural graphite tends to form a planar flaky structure on the current collector, resulting in poor wettability. If these shortcomings of natural graphite can be addressed, its lower raw material cost compared to artificial graphite and potential to enhance lithium-ion battery capacity will be very beneficial to the automotive industry.
enhance electrode density and wettability, respectively. On the other hand, the large amount of energy required for the manufacturing of artificial graphite makes the material very expensive. Also, the degree of graphitization is lower compared to natural graphite. However, artificial graphite has the advantage of being able to control its composition (such as incorporating regions with different degrees of graphitization within the particle) and particle shape as well as inhibiting the decomposition of the electrolyte due to its low degree of graphitization. Based on these features, the use of artificial graphite can increase electrode density, leading to higher-capacity batteries. In the artificial graphite process, a high level of powder processing technology is needed in the processing stages before and after graphitization. In order to improve the performance of active materials, a process for controlling the crystallization of the particle surface is essential. Since graphite has extremely poor wettability with water, organic solvents must be used to produce slurries for coating onto current collectors. Although substituting the organic solvent with an aqueous solvents is possible, the mixing process is challenging. Therefore, processes to improve the aqueous wettability of graphite particles have become the norm
process
Grinding
For artificial graphite production, a grinding process is often required to obtain fine particles. In this case, the Rotoplex or Hammer Mill will first coarsely grind the material; then, a system consisting of the Mikro ACM Pulverizer and a Micron Separator will reduce the particle size to an average of several tens of microns.
Spheroidizing particles for increasing electron density
There are two methods to increase the packing dentity to enhance battery capacity. One method, commonly used for natural graphite, is to spheroidize the particles by rounding off the edges and classifying out the fine particles. Another method is to coat the surface of graphite particles with soft carbon-based materials such as pitch, followed by graphitizing the particles. When regerring to the spheroidizing process of graphite, this generally refers to the first method.
Shaving and fine particle separation for spheroidizing=increase packed bulk density (usually for natural graphite)
The surface of natural graphite particles consists of irregular surface protrusions. A unit that removes these protrusions and spheroidizes the particles to increase packing density is known as the Faculty F-S. The unit consists of a high-speed rotating hammer, which applies energy to the particles in the dispersing chamber, and a forced spiral-current classifier for fine particle removal. A discharge outlet located at the central portion of the casing wall to discharge the processed product. The fine particles pass through the classifying rotor and are collected by a bag filter, while the coarse particles that have been continuously impacted by the hammer for a set period are discharged from the central outlet and collected as the final product. Milled graphite with a packed bulk density of 0.5g/cc can be densified to 1.0g/cc by the Faculty F-s.
Spheroidizing and surface coating=increased packed bulk density (tipicallyapplied to artificial graphite and a section of spheroidized natural graphite)
While it is possible to spheroidize artificial graphite, due to its high cost compared to natural graphite, there is a need to minimize the generation of fine material. From these constraints, a process is performed in which compression and shearing forces are applied mechanically. Spheroidization of such materials is done through either solidifying ultra-fine graphite particles onto particle surfaces through shearing force generated between particles, or plastic deformation of the particle. Mechanofusion or Nobilta is utilized for this process. In the Mechanofusion, the raw material fed into the rotating vessel is fixed to the inner wall of the vessel by centrifugal force. The inner piece then continuously applies a strong compressing and shearing force to the particles. In the circulation type Mechanofusion, there is a slit in the wall of the rotor, through which the materials exit. Once the material exits to the outside of the rotor, the circulation blade forces the material to the upper portion of the rotor where it flows back into the interior of the rotor and has force applied by the inner piece once again. In the case of Nobilta, a rotor with a unique design rotates at peripheral speeds of over 30m/s in the horizontal mixing vessel. The structure is designed to uniformly apply impact, compaction, and shearing forces to each individual particle. Both units have integrated cooling jackets, allowing them to control increases in material temperature and apply high levels of energy to heat-sensitive materials.
Surface treatment
Coating for controlling Reactivity
The coating process is utilized for the enhancement of natural graphite and artificial graphite. Ground graphite particles have higher surface reactivity. To control the reactivity, materials such as pitch, other carbons, or carbon sources are coated onto the surface. This process utilizes the Mechanofusion and/or Nobilta.
Hydrophilization for application of water based solvents
Graphite has a hydrophobic characteristic, making water unusable for producing slurries for coating onto current collectors. Organic solvents can be used to produce slurries but pose the problem of handling and recovery. Therefore, oftentimes water-dispersion-based SBR materials are used for the hydrophilization of graphite particles. Another method for hydrophilization is changing the chemical state of the graphite’s surface. For example, by applying mechanical energy and creating a mechanochemical reaction on the graphite particle surface using the Nobilta, it can impact hydrophilic characteristics. For certain circumstances, the hydrophilization can be further enhanced by coating nanoparticles with hydrophilic properties such as silica or titania onto the surface using these units. For analysis of hydrophilization degrees, the wettability-analyzing unit Peneto Analyzer can be used.
For other surface treatment applications, these units can coat nanoparticles onto graphite particles to enhance charge/discharge capacities and/or irreversible capacity ratios, as well as applying PVDF-based binder particles to the graphite particle to increase the adhesion strength between particles and current collectors to maintain the capacity variation dependent on the discharge rate
Negative electrode material, silicon
Electrical charge and discharge of lithium-ion secondary batteries take place through the movement of lithium ions between the cathode and anode. Anode active materials receive lithium ions during charging, and release lithium ions 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 reducing and purifying silica, a material found extensively on the surface of the earth. The theoretical capacity of silicon is approximately 10 times the capacity of graphite, a widely used anode active material. Silicon is a material that is attracting attention for automotive use. Other than purified silicon, impure semiconductors (n or p type) such assilicon 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 electrolyte 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 extensive experience with providing inert atmosphere systems, even for silicon-based materials.
There are two major problems when using silicon as an anode material. The first is: when using pure silicon, due to its poor electrical conductivity, there is a possibility that the material cannot be used for batteries. Because of this, oftentimes the silicon is mixed with a conductive material (usually carbon-based materials). Given a fixed battery volume, if the ratio of conductive material is increased, it corresponds to a decrease in 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 gaps between conductive materials to promote contact.
The second problem is that through the exchange of lithium ions, the volume of the silicon particles fluctuate significantly. While this problem can be seen in graphite-based materials as well, the degree of volume fluctuation is much greater for silicon materials. This volume fluctuation can cause the destruction of the silicon particles, leading to the fragmentation of the conductive network and separation from the current collector.
To solve these two problems, we propose a process for coating all or part of the particle surface with a different particle.
process
Grinding
Generally, silicon particles with sizes ranging from a few microns to sub-micron sizes are utilized. These particles are produced by classifier-integrated jet mills or mixing-type ball mills. These grinding units have integrated centrifugal classifiers and achive high grinding efficiency through the rapid collection of ground particles. Grinding, classifying, and conveying processes are all operated in a closed-circuit intert gas circulation system (inert gases such as nitrogen and argon). Particles with sizes of several tens of microns are collected by the cyclone, while particles smaller than a few microns are collected by the bag filter. The particles are collected in a vessel without exposure to oxygen and are transferred via a glove box to the next process.
Applying conductive characteristics to silicon particles and suppressing volume fluctuations
When mixing carbon-based conductive materials and silicon particles in a dry process, the use of Mechanofusion and/or Nobilta is an effective methods for applying compressive, shearing and impact force to the materials. These units can disperse, fix, and coat various fine particles (often nanoparticles) onto the surface of the host particle. For example, these units can coat heat-sensitive alloy elements or carbon materials onto the surface of amorphized silicon-based alloys. By processing the material in this manner, we can suppress volume fluctuation, and even if fragmentation of particles occurs, the three- dimensional electrode conductive network can be maintained, preventing any performance degradation.
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