Study on natural graphite powder for li-ion battery anode

There are abundant natural graphite resources in China. Modification of natural graphite for high-energy lithium-ion batteries is one of the effective ways to upgrade China's graphite industry. The high purity microcrystalline stone ink was plastic and coated with carbon film. The first cycle efficiency was increased to 89.9% and the cycle stability was improved obviously. The results show that microcrystalline ink coated on the surface is an excellent composite anode material for a lithium-ion secondary battery. The sub-micron-nano gap was formed in graphite particles by using h2SO4-GIC graphite interlaminar compound technology, which improved the discharge capacity, rapid charge-discharge capacity, and cycle life of graphite products, especially suitable for the development of high energy lithium-ion battery.

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Chinese graphite products can be divided into two categories: flake graphite and microcrystalline graphite. Flake graphite refers to the stone ink with crystal quality greater than 1μm and a well-developed lamellar structure, but the raw ore grade is low and the carbon content is generally less than 10%. Microcrystalline graphite is also known as amorphous graphite, implicit crystalline graphite, and soillike graphite. Its crystal quality is less than 1μm, and it is characterized by the agglomeration of small grains into polycrystalline. The raw ore has a high grade and generally contains more than 50% carbon, and the carbon content of lung ore in Chenzhou reaches more than 80%.

Microcrystalline graphite used as the anode material of lithium-ion battery has high lithium embedding capacity and cycling stability, and abundant resources, and low price. Modification of natural microcrystalline graphite to apply in a high-energy lithium-ion battery is one of the effective ways to upgrade the graphite industry in China. Similarly, flake graphite can be used as a cathode material for lithium-ion batteries, but the expansion and contraction of graphite during the storage process must be solved, otherwise, it will directly affect the battery life.

When using natural flake graphite as anode material, the project team found that the charge-discharge capacity of natural graphite was higher than that of man-made mesophase carbon microspheres (MCMB) due to its high degree of graphitization. The MCMB capacity is about 300 mA·h, while the flake graphite capacity is about 340 mA·h. However, when considering the cycle performance, the negative electrode of flake graphite is poor, and the capacity loss is large after multiple charging and discharging. The main reason is that the graphite crystal has about 10% expansion and contraction during charging and discharging. The multiple expansion and contraction of the flake graphite concentrated in one direction cause the damage of the negative film, resulting in the performance decline. To solve this problem, the principle of graphite interlaminar compounds (GICs) is proposed in this study. Micron-nano gaps are formed in graphite particles, and lattice expansion and contraction space are prefabricated to improve the cycling performance. The key to this technology is slow and orderly deintercalation so that the escape of the inserted gas can only create micron-nano pores in the graphite without significant volume expansion. H2so4-logic, McLx-gas, or other recipient GICs are usually used. The cathode material was prepared by deintercalation of the graphite powder at 100 ~ 300℃ for 12 ~ 72 h, and then surface modification and coating were carried out on the graphite powder. The resulting anode material has both the high capacity of flake graphite and good cycling properties (FIG. 2). At present, the product has been tested on the battery.

China's lithium-ion battery industry will still maintain an average annual growth rate of more than 30%. In 2005, the annual output of domestic small lithium-ion batteries exceeded 1 billion. The annual demand for graphite anode materials is 5000-10000 T, and the world demand is about 2× 104T. With the rapid development of electric vehicles, the demand for lithium battery anode materials will be more vigorous.

Given the abundant natural graphite resources, low price, and high capacity of lithium embedding, modification of natural microcrystalline graphite to apply in high-energy lithium-ion batteries is one of the effective ways to upgrade the domestic graphite industry. Considering cost and performance, natural graphite has the most development potential among anode materials of lithium-ion batteries, but there are some problems to be solved, such as irreversible capacity loss of the first cycle, cycle stability, and so on. The continuous development of natural graphite modification technology, including spherification treatment, surface coating resin, intercalation/de-intercalation micro-expansion treatment, improves the discharge capacity, rapid charge, and discharge capacity, cycle life of graphite products, modified natural graphite will become the preferred material of high energy lithium-ion battery anode.

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