Explore the main material of anode materials for lithium-ion batteries: graphite materials
As the negative electrode material of one of the four main materials of lithium ion battery, its specific capacity and working voltage directly determine the energy density and working voltage of the battery. Although the silicon material is gradually becoming industrialized, the current mainstream negative electrode material is still graphite. The negative electrode material has a lower lithium insertion potential during the reaction, and the lithium intercalation compound formed instead of the metal lithium negative electrode, thereby avoiding deposition of metallic lithium dendrites, so the safety is remarkably improved. As the last theme of the four main materials of lithium battery, it will have a systematic and intuitive understanding of the basic knowledge, production process, test methods and failure mode analysis of graphite materials. The basics of doing a brief introduction.
Graphite materials are mainly divided into artificial graphite and natural graphite. Artificial graphite is divided into MCMB (mesophase carbon microspheres), soft carbon and hard carbon according to the processing technology. The ideal graphite has a layered structure, each plane Similar to the benzene ring, the layers are connected by a large π bond; the 2H type hexagonal system and the 3R type rhombohedral crystal system.
For ideal graphite, the theoretical capacity is 372 mAh / g, but in the actual battery design process, the general negative electrode will be 5% -10% excess, while forming the SEI film during the first charging process to protect the surface of the negative electrode, preventing electrolysis Further reaction of the liquid and the negative electrode, and the quality of the film will directly affect the performance of the battery.
As the lithium ion in the graphite negative electrode is deeper and deeper (Stage-4-Stage-1), the surface color of the negative electrode gradually changes from black to cyan to dark yellow to golden, and the graphite negative electrode also completes C. -LiC12-LiC6 transition, thus completing the charging process.
From the above figure, we can see the difference in appearance between natural graphite and artificial ink. Natural graphite has different size and particle size, and the particle size distribution is wide. Untreated natural graphite cannot be used directly as a negative electrode material. The series can be used after processing, and the artificial graphite is much more consistent in shape and particle size distribution; it is generally believed that natural graphite has high capacity, high compaction density and relatively low price, but due to different particle sizes, the surface There are many defects, the compatibility with the electrolyte is relatively poor, and the side reactions are more; while the artificial graphite has relatively balanced performance, good cycle performance, good compatibility with the electrolyte, and the price is also more expensive.
For the negative electrode material, the concept of orientation degree is often heard, which is the so-called OI value. Its size will directly affect the electrolyte infiltration of the negative electrode, the impedance of the surface, the large rate charge and discharge performance, and also directly affect the negative electrode. Expansion during the cycle. The degree of orientation = I(004) / I(110) can be calculated from the XRD data.
As can be seen from the above figure, as the degree of orientation decreases, the ability to charge at a large rate is gradually increased to a stable value.
In addition, the morphology of the graphite negative 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 the contact of the irregular graphite particles, so the impedance is also larger, which is for the design of the material. In one direction, the matching of the particle size and the surface contact between the particles are ensured, the contact area is increased, and the contact resistance is lowered, thereby achieving the purpose of reducing the polarization.
The coating state of the material itself also affects the performance of the negative electrode, and generally covers some amorphous carbon materials, thereby improving the interface resistance of the negative electrode, improving the low temperature and the cycle performance.
As the energy density of the battery increases, the capacity utilization rate of the graphite negative electrode gradually approaches the theoretical value, and the compaction will be higher and higher, which requires the stability of the graphite negative electrode to be improved. Miscellaneous and coating are still a mainstream method of treatment. After modification, the structure and surface state of the graphite anode during the cycle can be protected, and the stability of the cycle is enhanced. In addition, the introduction of metal and non-metal elements can also be significant. Improve the performance of the negative electrode.