Graphite（Li-ion）

Lithium-ion batteries (LiBs) provide power for electric vehicles (EVs), and the anode plays a crucial role in their performance. Graphite materials, with excellent conductivity, thermal stability, and high performance, are the primary anode materials for lithium-ion batteries.

Graphite has become the earliest commercialized negative electrode material for lithium-ion batteries due to its advantages such as high electronic conductivity, large lithium ion diffusion coefficient, small volume change before and after lithium insertion in its layered structure, high lithium insertion capacity (theoretical capacity can reach 372mA·h/g), and low lithium insertion potential.

1.Overview of graphite anode materials

Since the development of lithium-ion batteries, a variety of positive electrode material systems have been studied, but the graphite-based negative electrode material system has been used to this day. Graphite materials have low lithium insertion potential and a layered structure suitable for lithium ion insertion/extraction. Currently, graphitized carbon (natural flake graphite, graphitized mesophase carbon microspheres, artificial graphite, etc.) and non-graphitized carbon (soft carbon, hard carbon, etc.) have been studied. In the context of the dual markets of power lithium batteries and consumer lithium batteries, artificial graphite and natural graphite negative electrode materials have become the mainstream of the market, occupying more than 90% of the market share for a long time.

Graphite negative electrode accounts for about 12% to 21% of lithium-ion batteries. Each pure electric vehicle contains about 50kg of graphite, and each hybrid vehicle requires about 10kg of graphite. Even though there is currently a surplus of supply, market demand will still be supported for a long time to come.

 

2.Electrochemical mechanism of lithium-ion batteries and graphite lithium insertion mechanism

Studying the working mechanism of lithium-ion batteries shows that during charging, lithium ions are smoothly released from the positive electrode LiCoO ₃ lattice, and then the lithium ions gradually diffuse into the electrolyte, and finally pass through the diaphragm and enter the graphite negative electrode layer. During the whole process, in order to fully ensure the balance between charges, an equal number of electrons will be released from the positive electrode and flow from the external current path to the graphite negative electrode, at which time a whole circuit will be constructed. During the discharge process, the lithium ions between the graphite layers of the negative electrode begin to slowly escape again, then pass through the electrolyte, and finally return and embed into the LiCoO ₃ lattice. At this time, the electrons will be transmitted to the positive electrode through the external current path, so that the charging and discharging cycle can be realized.

Graphite negative electrode charging process

 

3.Application of graphite negative electrode materials

(1) Artificial graphite negative electrode material
Artificial graphite is obtained by high-temperature graphitization of coke materials such as petroleum coke, asphalt coke, metallurgical coke, mesophase carbon microspheres, and needle coke. Needle coke, as a new type of carbon material, has a good graphite microcrystalline structure and a needle-like texture, and is an ideal carbon source for preparing negative electrode materials for lithium-ion batteries. Because of its advantages such as easy graphitization, high electrical conductivity, relatively low price, and low ash content, it also has a sufficiently high lithium embedding amount and good lithium de-embedding reversibility, which can ensure high voltage, large capacity, long cycle life, and current density. In short, the first coulombic efficiency, rate performance, and cycle performance of artificial graphite are better than those of natural graphite, and it is the mainstream product in the negative electrode material market of power lithium batteries in my country in recent years.

 

However, there are still some disadvantages in using needle coke to prepare artificial graphite negative electrode materials, such as the irreversible reaction with the electrolyte that causes the reduction of charge and discharge efficiency, the reduction of battery reversible capacity due to solvent co-embedding, the volume expansion of the material, and the poor cycle performance. These problems are bottlenecks that hinder the further development of artificial graphite. In addition, since the preparation process of artificial graphite requires extremely high temperatures (1900-3000℃) and a protective atmosphere, the preparation cost is much higher than that of natural graphite, so reducing the cost is also an important research topic.

 

(2)Natural graphite negative electrode material
Natural graphite negative electrode materials use flake graphite, microcrystalline graphite, etc. as raw materials. Due to the anisotropy and small interlayer spacing of flake graphite, the cycle and rate performance are poor when directly used as negative electrode materials. Generally, it undergoes a series of modification treatments such as spheroidization, purification, coating and carbonization. After natural flake graphite is shaped into a sphere, its specific surface area can be effectively reduced, the tap density can be increased, and the diffusion of lithium ions in the negative electrode material can be improved. Therefore, processing natural flake graphite into spherical graphite and preparing it into a negative electrode material can greatly improve the performance of the battery.

 

The spheroidization process of natural flake graphite is mainly divided into two methods: grinding method and air flow impact method. The spheroidization mechanism is basically the same. Both methods use high-quality high-carbon flake graphite as raw material. Through the mechanical action in the spheroidizer, a series of different forces such as collision, shearing, and friction are generated to break large particles of graphite and adsorb small particles of graphite to form spherical graphite with relatively uniform particle size. Usually, after full spheroidization, the yield is only 40%-50%, and the micro-powder waste generated in the process accounts for a large proportion, most of which can be used in the production of low-value-added products such as lubricating materials and sealing materials.

Scanning electron microscope images of flake graphite and spherical graphite anode materials

 

However, natural graphite negative electrode materials are mainly made of flake graphite, but they have the following disadvantages:
(1) The specific surface area of flake graphite is small, and the initial charge and discharge efficiency is low;

(2)It is anisotropic, which is not conducive to the diffusion of Li ⁺ inside it;

(3) Cracks will form between the layers due to the insertion and extraction of lithium ions, thereby increasing the Li ⁺ diffusion resistance.

 

(3)Amorphous carbon
Amorphous carbon materials are composed of amorphous carbon and graphite crystals. The material contains a large number of pores. During the charging and discharging process, these pores can become lithium storage sites, increasing the specific capacity of the material. Therefore, its theoretical specific capacity is also higher than the 372mAh/g of graphite. Amorphous carbon is also divided into soft carbon and hard carbon.

 

There are many graphite-like microcrystalline regions in soft carbon materials, which are arranged in a quasi-parallel manner and can be graphitized at high temperatures (above 2500°C). Soft carbon materials are generally obtained by heat treating coke at temperatures below 1500°C. The material has the characteristics of large interlayer spacing (>34nm), short-range order and long-range disorder, so it exhibits disadvantages such as low specific capacity (<300mAh/g), low first coulomb efficiency (<80%), and low compaction density. However, it also has advantages, showing excellent performance in rate performance and long cycle performance, so it has great potential for commercial applications. Relevant studies have shown that combining soft carbon with graphite can complement the advantages and disadvantages of the two and improve the rate performance and cycle performance of the battery.

Hard carbon materials refer to carbon materials that are difficult to graphitize even at temperatures above 3000°C. Their material structure contains many curved graphite sheets (also called pseudo-graphite regions), with larger interlayer spacing than graphite, and can be stacked in two to six layers. In addition to the graphite sheet structure, the hard carbon structure also contains many micropores, which will adsorb lithium ions embedded in the layered structure during charging and discharging, but these micropores will also lead to a decrease in the first coulomb efficiency.

 

(4)Graphene
In addition to the above materials, the carbon-graphite materials commonly used in the negative electrode of lithium-ion batteries include graphene, carbon fiber, etc. Zhang Tiange and others have shown that compared with traditional negative electrode materials, two-dimensional Fe ₃ O ₄ / graphene composite materials have good anti-polarization performance and conductivity, and good electrochemical performance; compared with two-dimensional Fe ₃ O ₄ graphene composite materials, three-dimensional graphene network Fe ₃ O ₄ / graphene composite materials have slow specific capacity decay, good cycle stability, better electrochemical performance, and great application potential.

 

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