Research progress on petroleum coke as anode material for CR2016 battery?

　　Lithium-ion battery is a recyclable energy storage device, also known as lithium-ion secondary battery, which consists of positive electrode, negative electrode, separator and electrolyte system. The characteristics of this battery are high energy density compared to other primary batteries, no memory effect and low self-discharge. Lithium-ion battery anode material aggregates are mainly divided into artificial graphite and natural graphite. Among them, the raw materials of artificial graphite are mainly oil-based and coal-based needle coke.

　　High-quality petroleum coke is used as anode material for lithium-ion batteries, which generally requires processes such as purification, crushing and particle size screening, graphitization, and surface modification. The whole process is relatively long, and there are many factors that affect the final result. The issues that receive the most attention are:

　　(1) The mechanism of carbon material structure changes with temperature;

　　(2) The relationship between the properties of negative electrode materials and the structure of carbon materials;

　　(3) Are there suitable carbon materials that meet the needs of negative electrode materials for power lithium-ion batteries?

　　This paper will review the research in these aspects, and finally discuss the structural characteristics of petroleum coke materials suitable for negative electrode materials and the development trend of petroleum coke negative electrode materials in the future.

　　1The influence of high-quality petroleum coke post-processing temperature on its performance

　　The post-heat treatment of high-quality petroleum coke is divided into two stages: calcination and high-temperature graphitization. Calcination refers to the calcination process below 1500°C, and high-temperature graphitization refers to the high-temperature treatment process close to 3000°C.

　　The research group then extended the maximum carbonization temperature to 2800°C and studied the changes in the graphite microcrystalline structure and its electrochemical properties during the heat treatment process. The paper points out that when the temperature reaches 2800°C, the processed needle petroleum coke sample is close to pure graphite. The battery charge and discharge experimental results show that the sample's stable lithium insertion capacity can reach 300mAh/g, and it has a stable charge and discharge platform. Different soft carbon structures and graphite microcrystalline structures change to different degrees with temperature.

　　2 Microstructure of high-quality petroleum coke and its lithium storage mechanism

　　Isao Mochida's research group proposed a carbon material structure model different from Franklin's, and put forward new perspectives on understanding easily graphitized and non-graphitized cokes. The principle is shown in Figure 2. They directly observed coke with a scanning tunneling electron microscope (STM) and found that regardless of easily graphitizable or difficult-graphitizable coke, the basic single microdomain size is about 2 to 5nm. The difference is that the easily graphitizable coke is relatively uniform across the wide area. Multiple micro-domains are closely connected, and the entire size increases to 20-70nm after graphitization; the phase of difficult-graphitizable coke is not uniform over a wide area, with mostly independent micro-domains and a few connected micro-domains, and the size does not increase significantly after graphitization. , 5~18nm.

　　Coke that is difficult to graphitize is believed to be caused by the existence of torsional stress between micro-regions, which makes it difficult for the micro-regions to connect, so the crystal size does not grow large. Therefore, poor quality coke will not obtain a higher crystalline form even at high temperatures, thus affecting its performance as an anode material.

　　There are two mechanisms for lithium storage in petroleum coke. The schematic diagram is shown in Figure 3:

　　(1) Represented by soft carbon, there are various lithium storage mechanisms, such as interlayer lithium storage of graphite microcrystals, lithium storage in nanopores or cracks inside soft carbon, and surface defects or residual functional groups of carbon materials reacting with Li+ to generate Solid electrolyte membrane (SEI) and so on.

　　(2) The second type, represented by artificial graphite, mainly stores lithium between the layers of graphite sheets, so the initial capacity will be smaller than that of soft carbon.

　　To sum up, the final result of the influence of graphitization temperature is the internal structure of carbon materials such as high-quality petroleum coke. If the internal structure of the material is more orderly and easier to graphitize, the final negative electrode will have a higher capacity and better cycle efficiency. However, although highly graphitized carbon materials have high capacity and stable charge and discharge platforms, their cycle performance and low-temperature performance are poor. This is because when Li+ is embedded in the graphite layer, it forms a graphite interlayer compound with the flake graphite, and the graphite layer expands; when Li+ is removed, the graphite returns to its original state; during repeated expansion and contraction, the graphite layer structure is easily damaged, and may cause solvent co-existence. embedded, thereby reducing the cycle performance of the negative electrode. Therefore, during the graphitization process of high-quality petroleum coke and other carbon materials, the degree of graphitization should be controlled. Some amorphous structures are needed between microcrystals to maintain a certain structural strength.

　　3Soft carbon as anode material for lithium-ion batteries

　　Power lithium-ion batteries have different negative electrode materials than ordinary lithium-ion batteries. They need higher rate performance to shorten charging time, good low-temperature performance to meet different working environments, large capacity to reduce battery size, and better stability to prevent safety issues.

　　As an anode material, soft carbon has low efficiency and no stable voltage platform for the first time. Regarding the low efficiency of the first cycle, Alcántara et al. provided two explanations:

　　(1) Irreversible reaction between Li+ and aliphatic hydrocarbons in coke at low temperature;

　　(2) Li+ combines with the graphite fragments existing on the exposed edges of the coke to cause irreversible formation. In addition to the low first cycle efficiency, due to the gaps between the sheets, the charge and discharge voltage will lag and the electrode will be unstable. However, the advantage of soft carbon negative electrode materials is that the working voltage is relatively high, which can prevent the precipitation of lithium metal from causing short circuits and other issues that affect safe use. Secondly, it is low in cost and does not require high-temperature graphitization.

　　Alcántara uses Fe2O3 to modify petroleum coke, which greatly improves the capacitance and cycle stability. He explained this phenomenon as the oxide can stabilize the soft carbon structure, reduce surface active sites, and form a stable protective layer on the surface.

　　In addition, Alcántara et al. also pointed out that soft carbon, used as anode material for sodium batteries, has higher capacitance and cycle efficiency than high-temperature graphitized coke. There is literature showing that soft carbon is also suitable for lithium-ion capacitors, which is safe and has excellent cycle performance. After pre-lithiation treatment, soft carbon shows better capacity and cycle stability, and has potential for application in long-cycle power batteries.

　　4 Conclusion and outlook

　　Petroleum-based coke suitable for use as anode material for lithium-ion batteries has a small content of S, O and other heteroatoms, is easily graphitized, and needs to have a suitable particle size distribution, a small surface area, etc. Calcined high-quality petroleum coke and other soft carbon materials have excellent low temperature and rate performance, making them more attractive in the field of anode materials for power lithium-ion batteries. However, cycle efficiency and stability issues still need to be solved.

　　The internal structure of high-quality petroleum coke materials can be changed through calcination and graphitization, thereby changing its electrochemical performance as an anode material. However, materials after graphitization still need to be upgraded and transformed using materials engineering methods so that they can exhibit good cycle, rate, and large-capacity performance.

　　There are three future development trends for petroleum coke anode materials:

　　(1) Have a deeper understanding of the coke structure and its influencing factors to achieve customized preparation for lithium-ion batteries with higher capacity and higher rate performance;

　　(2) Development and commercial application of new composite coke-based anode materials;

　　(3) The development of new petroleum coke-based anode materials, including the batch preparation of petroleum coke-based carbon nano-anode materials, and new coke anode and cathode materials that match the new battery system.

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