The key to the electrochemical performance of lithium-ion batteries is the performance of cathode and anode materials and electrolytes, in which anode materials have a great impact on the energy density, rate performance and cycle life of lithium-ion batteries. Due to its low cost, low price and high safety, graphite is still the mainstream anode material in the lithium ion battery anode material companies. Graphite materials are mainly divided into artificial graphite and natural graphite.
Among them, flake graphite is used as the raw material for the natural graphite anode material. Due to the anisotropy and small interlayer spacing of flake graphite, the cycle and rate performance are poor when directly used as anode materials. Therefore, a series of treatments are required to improve the diffusion of lithium ions in anode materials and finally be applied.
1. Definition and classification of natural graphite
Graphite is an allotrope of carbon. It is a gray-black, opaque solid with stable chemical properties and corrosion resistance. It is not easy to react with chemicals such as acids and alkalis. Natural graphite is graphite that is naturally formed in nature, and generally appears in ores such as graphite schist, graphite gneiss, graphite-containing schist and metamorphic shale.
According to its crystal form, natural graphite can be divided into two types: crystalline graphite (flaky graphite) and aphanitic graphite (earth graphite). Among them, crystalline graphite can be divided into high-purity graphite, high-carbon graphite, medium-carbon graphite and low-carbon graphite according to the fixed carbon content.
2. Natural graphite vs artificial graphite, which is better
Natural graphite particles vary in size and have a wide particle size distribution. The mined natural graphite ore needs to go through steps such as flotation, spheroidization, and surface coating to make natural graphite anode materials. The artificial graphite is relatively consistent in shape and particle size distribution. Natural graphite has high capacity, high compaction density, and relatively cheap price. However, due to the different particle sizes, there are many surface defects, the compatibility with the electrolyte is relatively poor, and there are many side reactions.
The performance of artificial graphite is relatively balanced, the cycle performance is good, and the compatibility with the electrolyte is also relatively good, so the price will be more expensive. Although natural graphite has cost and specific capacity advantages, its cycle life is low and its consistency is lower than that of artificial graphite. Compared with natural modified graphite, China's artificial graphite technology is more mature.
Natural graphite is mainly used in small lithium batteries and general-purpose lithium batteries for electronic products. Artificial graphite conforming to requirement of the car battery voltage is widely used in automotive power batteries and it is also used in high-end electronic products due to its excellent cycle performance, high-rate charge-discharge efficiency, and electrolyte compatibility.
At present, with the continuous expansion of the power battery market, the comprehensive requirements for factors such as material cost, processing performance, energy density, cycle life, and fast charging rate have increased, helping artificial graphite to become the most important material in China's anode materials. There is no clear distinction between artificial graphite and natural graphite technology routes, and more is based on the choice of one's own technical route.
3. Methods for improving anode performance of natural graphite
Flake natural graphite is anisotropic and the interlayer spacing is small, and these shortcomings can be improved by spheroidization. The spheroidization process is actually equivalent to the granulation process of flake natural graphite. The flake graphite collides, breaks, and curls under the impact of airflow to form a core, and the fine scales with smaller particle sizes adhere to the surface of the core to form spherical graphite. At present, in the graphite industry, the particle size of spherical graphite is mostly controlled at 8-23μm.
An excessively small particle size leads to an excessively large specific surface area, causing excessive side reactions in the formation process of the anode material, excessive consumption of lithium ions, and reduced initial charge and discharge efficiency. Conversely, if the particle size is too large, the contact area between the graphite particles and the electrolyte is small and the lithium ion diffusion distance is too large, which will affect its specific capacity.
However, flake graphite will produce certain pores during the spheroidization process, which affects the cycle life and rate performance of the anode material to a certain extent. On the other hand, the curling, folding and tight stacking of graphite flakes inside spherical graphite will cause a certain degree of stress concentration inside, which will intensify the dissociation and shedding of graphite flakes to a certain extent, thus causing abnormal lithium storage phenomenon. At present, the quality of spherical graphite is mainly judged by physical indicators such as tap density, particle size distribution, and specific surface area.
Spherification alone is not enough, because after spheroidization, the flake edges of flake natural graphite are directly exposed on the surface of spherical graphite, thus affecting the stability of anode materials. Therefore, it is also necessary to coat a modified layer of amorphous carbon material or metal and its oxide on the surface of spherical graphite to improve the compactness and stability of the solid electrolyte interfacial film (SEI).
At present, the cladding material is generally asphalt. Bitumen is a mixture of complex components, with different components, toluene insolubles and quinoline insolubles content. The softening point, carbon residue rate, and microstructure of the coating layer after carbonization are quite different, which has a great influence on the cycle performance. In addition, resin materials, sodium maleate, aluminum oxide, etc. can be used as coating materials.
Due to the large spacing between amorphous carbon layers and the relatively easy diffusion of lithium ions, this is equivalent to building a buffer layer for lithium ion diffusion on the surface of spherical graphite. After spheroidization and coating modification, the specific capacity, first cycle efficiency and cycle performance of the natural graphite anode material have been significantly improved. At this stage, it is mainly used in the field of 3C digital and small power electronic products.
At present, increasing the rapid migration channel of lithium ions by constructing a pore structure on the graphite surface is also one of the effective means to improve the rate performance of natural graphite. In addition, micro-expansion treatment is another common method to improve the rate performance of natural graphite, which reduces the diffusion resistance of lithium ions by regulating the interlayer spacing of graphite. At present, the most common process of micro-expansion treatment is chemical oxidation. In addition to the process, the selection of reagents and the optimization of operating process conditions are also one of the important directions to improve the rate performance of anode materials.
To improve the electrochemical performance of natural graphite anode materials, the key points are to expand the interlayer spacing of graphite, increase the number of micropores, enhance the migration rate of lithium ions, and promote the adsorption and diffusion reaction of lithium ions at the interface. Change the graphite microcrystalline structure to provide more active sites and spaces for lithium ion storage and diffusion.