Effect Of Heat Treatment Temperature On The Structure And Electrochemical Properties Of Petroleum Coke,Needle Coke And Microcrystalline Graphite

Carbonaceous materials as anode materials for lithium-ion batteries have attracted wide attention since they were first commercialized in 1991.Carbonaceous materials can generally be divided into three types according to their microstructure:graphite,soft carbon and hard carbon.Needle coke and petroleum coke have unique microstructure.Short-range ordered graphite is randomly arranged in the disordered carbon matrix.The microstructure is determined by the heat treatment temperature,and its electrochemical properties are directly affected.To elucidate the structure change rule and structure-activity relationship of petroleum coke and needle coke during the heat treatment process,and reveal the mechanism of lithium storage,which has an important guiding role for the production and application of petroleum coke as lithium ion anode materials.Microcrystalline graphite with high theoretical specific capacity（372 mah/g）,but its poor compatibility with electrolyte,cycle performance is poor,for the first time the coulomb efficiency is not high,hinder the actual commercial applications,therefore needs to be modified,looking for a high efficient value with the road of industrialization,and promote the development of new energy and related industries.For needle coke and petroleum coke,the effect of heat treatment temperature on their microstructure and electrochemical properties during the preparation process was mainly discussed.Under different heat treatment temperature stages,the mechanism of lithium intercalation was studied,and the structural characteristics and electrochemical properties of each stage were discussed.For microcrystalline ink,high temperature heat treatment was carried out after crushing and grading to find the best process conditions.In order to further improve its electrochemical performance,the high temperature asphalt solid phase coating treatment was carried out to make up for the defect of natural microcrystalline ink as anode material of lithium ion battery,and the coating material with excellent electrochemical performance was obtained.Specific research results are as follows:（1）The heat treatment of needle coke and petroleum coke was carried out at600℃-3000℃,and the changes of structure,lithium-embedded mechanism and electrochemical properties of needle coke and petroleum coke were analyzed under different heat treatment temperatures.The main structure of needle coke and petroleum coke of 600℃-1400℃heat treatment is transformed from amorphous carbon to chaotic layer structure carbon,and the corresponding lithium embedded mechanism is"lithium embedded at the edge of carbon layer"and"adsorption and storage of lithium on the surface".The discharge specific capacity decreases with the increase of heat treatment temperature.The main structure of needle coke and petroleum coke heat treated at 2200℃-3000℃changes from the graphite structure with lower three-dimensional order to the graphite structure with higher three-dimensional order.The mechanism of lithium intercalation is"lithium intercalation between graphite microcrystalline layers".The discharge specific capacity increases with the increase of heat treatment temperature.The lithium-embedded mechanism of the needle coke and petroleum coke treated at1400℃-2200℃is a comprehensive result of the above mechanisms.The specific capacitance of the needle coke and petroleum coke treated at 1800℃is the lowest at37.2mA/g current density,which is 160 mAh/g and 167 mAh/g.（2）The effects of heat treatment temperature and particle size on the structure and electrochemical properties of microcrystalline ink were studied.The microcrystalline ink was crushed and graded using a small stainless steel grinder,and the samples with three particle sizes of D₅₀of 22.2,14.87 and 6.91μm were obtained.The microcrystalline ink after grading was subjected to heat treatment at 2600℃and2800℃.The results show that the decrease of particle size will increase the lithium channels,and decrease the graphitization degree and the first coulomb efficiency,while the increase of temperature will increase the graphitization degree and reduce the defect sites.Finally,it is found that the composite electrochemical performance of the sample with D₅₀of 14.87μm is the best after heat treatment at 2600℃,and the first coulomb efficiency is 77.19%at the current density of 37.2mA/g.The reversible specific capacity was 362mAh/g.（3）The microcrystalline ink with the best comprehensive electrochemical performance obtained above was treated with high temperature asphalt solid phase coating to further improve its first coulomb efficiency,increase cycle stability and cycle life.The results show that with the increase of coating amount,the specific surface area decreases,and the surface defect sites increase.It is found that the coating amount of 6%-8%can better improve the first effect.After the microcrystalline ink coated with 6%high temperature asphalt,the first coulombic efficiency increases to 81.41%,and the discharge specific capacity increases to368.2mAh/g.After 8%high temperature asphalt coating,the first Coulomb efficiency is increased to 85.12%,and the discharge specific capacity is reduced to 359.5mAh/g.