Summary:The key to the electrochemical performance of lithium-ion batteries is the performance of positive and negative electrode materials and electrolytes, in which n
The key to the electrochemical performance of lithium-ion batteries is the performance of positive and negative electrode materials and electrolytes, in which negative electrode 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 lithium battery anode material in the market.
Graphite materials are mainly divided into artificial graphite and natural graphite, among which flake graphite is used as raw material for 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. What are the specific operations, and what are their principles? The following is a brief analysis.
Spherification
As mentioned earlier, flake 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 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. The diffusion comparison of lithium ions before and after spheroidization can be seen in the figure below.
In the current graphite industry, the particle size (D50) 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 negative electrode 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 (including open pores and closed pores) during the spheroidization process, which affects the cycle life and rate performance of the negative electrode material to a certain extent. On the other hand, the curling, folding and close 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.
Coating
Just spheroidization is not enough, because after spheroidization, the flake edges of flake graphite are directly exposed on the surface of spherical graphite, which affects the stability of the negative electrode material. Therefore, it is also necessary to coat a layer of amorphous carbon material or metal on the surface of spherical graphite and The modified layer of its oxide is used to improve the compactness and stability of the solid electrolyte interfacial film (SEI). At present, the coating material (amorphous carbon precursor) is generally pitch. Asphalt is a mixture of complex components. Different components, contents of toluene insolubles and quinoline insolubles, the softening point, carbon residue rate, and microstructure of the coating layer after carbonization are quite different, which has great influence on the cycle performance. have a great impact. In addition, resin materials, sodium maleate, aluminum oxide, etc. can be used as coating materials.
Due to the large distance between the 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. The specific capacity, first cycle efficiency and cycle performance (capacity retention rate ≥ 80% after 500 cycles) of the natural graphite anode material modified by spheroidization and coating have been significantly improved. At this stage, it is mainly used in 3C digital and The field of small power electronic products.
Other Methods
At present, increasing the fast migration channel of lithium ions by building a pore structure on the graphite surface is also one of the effective means to improve the rate performance of graphite. The Ningde era announced the use of "channel optimization and 'fast ion ring' technology" to greatly increase the insertion speed of lithium ions in the graphite negative electrode, achieving the compatibility of fast charging performance and high energy density.
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 negative electrode materials.
In summary, for the electrochemical performance of graphite materials, the main point of electrochemistry is to expand graphite and increase the interlayer spacing to provide more active sites and spaces for lithium ion storage and expansion.
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