As one of the four main materials of lithium-ion batteries, the negative electrode material, its specific capacity and operating voltage directly determine the energy density and operating voltage of the battery. Although silicon materials are gradually becoming industrialized, the current mainstream negative electrode materials are still graphite. The negative electrode material has a lower lithium intercalation potential during the reaction process, and the intercalated 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 last theme of the four main materials of lithium batteries, we will have a systematic and intuitive understanding of graphite materials through basic knowledge, production technology, testing methods, failure mode analysis, etc. A brief introduction to the basics of class materials.

Graphite materials are mainly divided into artificial graphite and natural graphite. Artificial graphite can be divided into MCMB (mesocarbon microspheres), soft carbon and hard carbon according to different processing techniques. The ideal graphite has a layered structure. Similar to the benzene ring, the layers are connected by large π bonds; it has a 2H-type hexagonal crystal system and a 3R-type rhombohedral crystal system.

For ideal graphite, its theoretical capacity is 372mAh/g, but in the actual battery design process, the negative electrode is generally excessive by 5%-10%. 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 between the liquid and the negative electrode, and the quality of this film will directly affect the performance of the battery.

As the lithium ion intercalation in the graphite negative electrode becomes deeper and deeper (Stage-4-Stage-1), the surface color of the negative electrode gradually changes, from black to blue-black to dark yellow and finally to golden yellow, and the graphite negative electrode also completes C -----LiC12----LiC6 transformation, thus completing the charging process.

The difference in morphology between natural graphite and artificial ink is that natural graphite has different particle sizes and wide particle size distribution. Untreated natural graphite cannot be used directly as a negative electrode material. It needs to be processed after a series of processes. However, artificial graphite is more consistent in morphology and particle size distribution; it is generally believed that natural graphite has high capacity, high compaction density and relatively cheap price, but due to different particle sizes, there are many surface defects, which are not compatible with electrolytes. The compatibility of graphite is relatively poor, and there are many side reactions; while artificial graphite has more balanced properties, good cycle performance, better compatibility with electrolyte, and higher price.

For negative electrode materials, we often hear the concept of degree of orientation, which is the so-called OI value. Its size will directly affect the electrolyte infiltration of the negative electrode, the impedance of the surface, and the high-rate charge-discharge performance. Expansion during cycling.
Degree of orientation=I(004)/I(110), which can be calculated from XRD data.

With the decrease of the degree of orientation, the ability of high-rate charging is gradually improved, reaching a stable value.

In addition, the morphology of the graphite negative electrode also has a great influence on the battery performance. The contact between spherical graphite particles is obviously not as good as that of irregular graphite particles, so the impedance will also be larger, which is important for material design. In one direction, matching the particle size and ensuring the surface contact between the particles increases the contact area and reduces the contact resistance, thereby achieving the purpose of reducing polarization.

The coating state of the material itself will also affect the performance of the negative electrode. Generally, some amorphous carbon materials are coated to improve the interface impedance of the negative electrode and improve the low temperature and cycle performance.

As the energy density of the battery increases, the capacity utilization rate of the graphite negative electrode is gradually approaching the theoretical value, and the compaction will be higher and higher, which requires the stability of the graphite negative electrode to be improved accordingly. Impurity and coating are still a mainstream method of treatment. After modification, the structure and surface state of the graphite anode can be protected during the cycle, which enhances the stability of the cycle. In addition, the introduction of metal and non-metal elements can also significantly to improve the performance of the negative electrode.

This time, I mainly introduce some basic knowledge of negative electrode. The next article will mainly introduce the detection of negative electrode related parameters, so stay tuned. "