Lithium-ion battery anode material main force: graphite material
As one of the four primary materials for lithium-ion batteries, the specific capacity and operating voltage directly determine the energy density and operating voltage of the battery. Although silicon materials have gradually begun to industrialize, the current mainstream anode materials are still graphite. The negative electrode material has a low lithium insertion potential during the reaction, and the lithium intercalation compound generated at the same time replaces the metal lithium negative electrode, thereby avoiding the deposition of metal lithium dendrites, so the safety is significantly improved. As the final theme of the four primary materials of lithium batteries, we will have a systematic and intuitive understanding of graphite materials through basic knowledge, production processes, test methods, and failure mode analysis. Make a brief introduction to the basics of graphite.
Graphite materials are mainly divided into artificial graphite and natural graphite, and artificial graphite will be divided into MCMB (mesophase carbon microspheres), soft carbon and hard carbon according to different processing techniques. Ideal graphite has a layered structure, each plane Similar to the benzene ring, the layers are connected by a large π bond; they have a 2H hexagonal crystal system and a 3R rhombohedral crystal system.
For ideal graphite, the theoretical capacity is 372mAh / g, but in the actual battery design process, the negative electrode will generally be excessive by 5% -10%, and at the same time, the SEI film is formed during the first charging process to protect the negative electrode surface and prevent electrolysis The further reaction of the liquid and the negative electrode, and the quality of this film will directly affect the performance of the battery.
With the deeper and deeper intercalation of lithium ions in the graphite anode, the surface color of the anode has gradually changed, from black to blue-black to dark yellow to finally golden, and the graphite anode has also completed C ----- LiC12 --- -Transformation of LiC6, thus completing the charging process.
Natural graphite has different size particles and a wide particle size distribution. Untreated natural graphite cannot be used directly as a negative electrode material. It needs to be used after a series of processing. Artificial graphite has the same morphology and particle size distribution There are many; it is generally believed that natural graphite has high capacity, high compaction density, and relatively low price, but due to different particle sizes, many surface defects, poor compatibility with electrolytes, and more side reactions; and Artificial graphite has more balanced performance, better cycle performance, better compatibility with electrolyte, and higher price.
For negative electrode materials, we often hear a concept of degree of orientation, that is, the so-called OI value. Its size will directly affect the electrolyte infiltration of the negative electrode, the surface resistance, and the charge and discharge performance at a significant rate. It also directly affects the negative electrode—expansion during cycling. Orientation = I (004) / I (110), which can be calculated by XRD data. As the degree of orientation decreases, the ability to charge at a high rate is gradually improved, reaching a stable value.
In addition, the shape of the negative graphite electrode also has a great influence on the performance of the battery. The contact between the spherical graphite particles is obviously not as good as that of the irregular graphite particles so that the impedance will be more significant. This is what the material design is. In one direction, the size of the particles is matched, and the surface contact between the particles is ensured, the contact area is increased, the contact resistance is reduced, and the purpose of reducing the polarization is achieved.
The coating state of the material itself also affects the performance of the negative electrode. Generally, some amorphous carbon materials are coated to improve the interface resistance of the negative electrode, improve low temperature and cycle performance.
With the improvement of battery energy density, the capacity utilization rate of graphite anodes is gradually approaching the theoretical value, and the compaction will become higher and higher, which requires the stability of graphite anodes to be improved accordingly. Miscellaneous and coating are still a mainstream method of treatment. After modification, the structure and surface state of the graphite anode during cycling can be protected, and the stability of the cycle is enhanced. In addition, the introduction of metallic and non-metallic elements can also be significant to improve the performance of the negative electrode.