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Since the invention of lithium-ion batteries in the 1990s, graphite based negative electrodes have firmly occupied the mainstream position as negative electrode materials for lithium-ion batteries. This is not only due to the excellent electrochemical performance of graphite based negative electrodes, but also to the extensive reserves and low price of graphite. Although new Si based negative electrodes have rapidly emerged in recent years, it is still difficult to shake the position of graphite based materials in the lithium-ion battery industry.

When the graphite negative electrode is used as the negative electrode of lithium-ion batteries, Li can be embedded inside the graphite crystal under the drive of an electric field, thereby avoiding the precipitation of metallic lithium and greatly improving the safety of lithium-ion batteries. The lithium insertion potential of graphite material is very low, very close to that of metallic lithium (graphite vs Li+/Li<0.1V), which greatly improves the energy density of lithium-ion batteries. Therefore, graphite negative electrodes have been widely used.

The main problem with graphite negative electrodes is their low capacity (372mAh/g), which limits the improvement of energy density in lithium-ion batteries. At the same time, the dynamic conditions of graphite negative electrodes are poor, resulting in a slow diffusion rate of Li+in the graphite negative electrode. Especially during low-temperature charging, due to the poor dynamic conditions of Li diffusion in graphite, Li is easily precipitated on the surface of the negative electrode. Relevant studies have shown that the C/5 charging rate at low temperatures can lead to a considerable amount of lithium precipitation phenomenon. After 20 hours of storage after charging, the lithium insertion degree of graphite increased by 17%, indicating that at least 17% of Li was precipitated on the surface of the graphite negative electrode in the form of metallic lithium during the charging process.

The dynamic characteristics of graphite negative electrodes are influenced by many factors, such as the morphology of graphite particles and surface coatings. Therefore, the dynamic characteristics and rate performance of different types of graphite materials vary greatly. Recently, S. from the Energy Technology Department of CSIRO (Commonwealth Scientific and Industrial Organization) in Australia R. Sivakkumar, J.Y. Nerkar, A.G. Pandolfo conducted a comparative study on the rate performance of different types of graphite materials. Research has shown that reducing the coating thickness on the surface of graphite can improve the rate performance of graphite to a certain extent. Lowering the particle size of graphite can improve the rate performance of graphite materials, but it can cause an irreversible increase in capacity. At the same time, research has also shown that although graphite materials can achieve high discharge rates, their charging rate performance is still poor and needs further improvement.

Seven negative electrode materials were evaluated for their rate performance in the experiment. The morphology of the seven materials is shown in the following figure, and the main information is presented in the table below. The electrode preparation used CMC as a binder and SP as a conductive agent, and the rate performance of the electrode was evaluated using a button half cell.

The following figure shows the evaluation of the coating thickness on the negative electrode surface using SLP30 material. The tests were conducted in two different ways. Figure a shows charging at C/10 and discharging at different rates, while Figure b shows charging and discharging at the same rate. As can be seen from the graph, with the increase of coating thickness, the rate performance of graphite material is significantly suppressed, especially under high current, the discharge capacity of the material rapidly decreases, mainly because the thick coating increases the diffusion impedance of electrons and ions. When charged and discharged at the same rate in Figure B, the discharge capacity of the material is significantly affected by the discharge rate, and it is almost impossible to complete charging and discharging at high rates. The above experiments also indicate that the discharge rate performance of graphite materials is good, but the charging rate performance of the materials still needs to be improved.

During the initial charging process of graphite materials, a porous inert SEI film is formed on their surface. Due to the larger interlayer spacing of carbon atoms in graphite crystals compared to Li, the diffusion barrier of Li is relatively small. Therefore, the main factors causing the decrease in the charging rate performance of graphite negative electrodes may be the SEI film and interface impedance. According to existing theories, Li+will first react with the solvent in the electrolyte to form a stable solvated Li+. However, in order for Li+to diffuse into the SEI film and graphite, it needs to first complete desolvation, which requires obtaining a certain amount of energy. Therefore, an invisible potential barrier is formed on the surface of the SEI film to hinder the diffusion of Li+into the graphite. However, the discharge process is exactly the opposite. When Li+diffuses into the electrolyte, the solvation process does not require additional energy, so solvation actually accelerates the diffusion of Li+. This also explains why the charging rate performance of graphite negative electrodes is significantly weaker than the discharging rate performance.

The following figure shows the rate performance test results of the seven different graphite materials mentioned above. Figure a shows C/10 charging and discharging at different rates, while Figure b shows charging and discharging at the same rate. From the data, it can be observed that materials with smaller particle sizes have better rate performance, mainly because smaller particle sizes reduce the diffusion distance of Li+. However, the small particle size also brings a problem of reversible capacity reduction and a decrease in volumetric energy density.

Due to the fact that volumetric energy density is also of great concern to us in commercial applications, S R. Sivakkumar also compared the discharge volume energy density of different materials at 20C. Figure a shows charging at C/10 and discharging at 20C, while Figure b shows charging and discharging at 20C. It can be seen that SFG materials, which have an advantage in rate discharge capacity, have lost their advantage in volumetric energy density, while materials such as SLP and SLC have an advantage in volumetric energy density.

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