Negative electrode material: in-depth analysis of graphitization

Overview: Carbon atoms undergo rearrangement at high temperatures

Graphite has a layered structure, with sp2 hybridized orbitals of carbon atoms in each layer forming a 120 ° three coordinate plane hexagonal grid, covalently bonded, with a carbon atom spacing of 0.142nm. The hexagonal carbon atom plane grid is stacked in parallel, forming a network of hexagonal carbon atoms that are displaced from each other to the center of the hexagon. The configuration of carbon atoms in the third layer is the same as that in the first layer. Due to its high temperature resistance and conductivity, it is widely used in:

1) Conductive materials such as blast furnace electrodes, lithium negative electrodes, and other fields;

2) Refractory materials such as metallurgical crucibles and refractory bricks;

3) Wear resistant and lubricating materials, such as plug rings, sealing rings, and bearings.

Graphitization refers to the process in which non graphitic carbon is heated to over 2000 ℃ in a protective medium or isolated from air in a high-temperature electric furnace. Due to physical changes, the hexagonal carbon atom planar network layer stacking structure is perfected and developed, transforming into graphitic carbon with a three-dimensional regular ordered structure of graphite. Graphitization enhances the volumetric density, conductivity, thermal conductivity, corrosion resistance, and mechanical processing performance of the product. Graphitization is a key process in the production of artificial graphite negative electrodes.

Graphitization is mainly applied in the fields of lithium battery negative electrode artificial graphite, blast furnace/electrolytic aluminum electrode, etc. According to the prospectus of Xiangfenghua, some natural graphite will also undergo high-temperature treatment to further enhance its graphitization degree, thereby increasing its energy density.

The main factors affecting graphitization are raw materials, temperature, time, pressure, and catalyst.

1) Raw materials: Amorphous carbon that easily transforms into graphite at high temperatures is called easily graphitized carbon (or graphitizable carbon). Petroleum coke, needle coke, etc. belong to easily graphitized carbons. During the carbonization process, easily graphitized carbon generally undergoes a melting state, and the carbon molecular clusters in its structure are arranged in a nearly parallel manner.

2) The temperature determines the degree of graphitization: different carbon materials have different temperatures at which graphitization begins. Petroleum coke generally begins to undergo graphitization at 1700 ℃, while needle coke needs to enter the graphitization conversion stage at around 2000 ℃. The higher the heating temperature, the lower the resistivity, and the closer the distance between adjacent crystal layers to the ideal graphite crystal's 0.3354nm, resulting in a higher degree of graphitization.

3) Time: The degree of graphitization is also related to the residence time at high temperatures. The higher the graphitization temperature, the shorter the time required to enter a stable graphitization state, the longer the insulation time, the lower the resistivity, and the higher the degree of graphitization. In addition, pressure has a significant promoting effect on graphitization; The addition of catalysts under certain conditions can promote graphitization, such as boron, iron, tungsten, titanium, bonds, magnesium, and certain compounds.

Technological iteration: crucible furnace box furnace continuous furnace

The key step in graphitization process is furnace loading. The graphitization process mainly includes laying the furnace bottom, laying the furnace core, loading the negative electrode material precursor and insulation material into the furnace, power transmission, cooling, discharging the negative electrode material and by-products, packaging, and other steps. For the loading process, by continuously optimizing the loading method of processed materials inside the furnace, the efficiency of using the furnace space will continue to improve. In the graphite processing industry, enterprises charge based on the weight of processed materials, and improving the efficiency of furnace space utilization will enhance the profitability of enterprises.

The crucible loading method and the box loading method are currently widely used.

1) The crucible furnace loads the negative electrode material to be processed into a cylindrical graphite crucible, and then places the crucible inside the furnace for heating. At the same time, petroleum coke needs to be filled between the crucibles as a conductive material and insulation material to form a current circuit inside the furnace. This technology route is mature, with moderate requirements for the complexity of loading and suction processes, as well as the accuracy of placing crucibles in the furnace, and strong operability.

2) The box furnace process divides the entire furnace core space into several equal volume chambers, and the negative electrode material is directly placed in the box body surrounded by graphite sheets. After the box body is electrified, it generates heat on its own, serving as both a container and a heating element. The box furnace requires a high level of mastery and technical optimization of graphitization technology, high precision in box plate splicing, increased difficulty in loading and suction operations, and more precise control of the power transmission curve and temperature measurement during the heating process.

The continuous process has begun to be applied.

Continuous method refers to the process in which raw materials are continuously fed into a heating furnace without power interruption during production. Graphitized products need to go through a series of temperature zones and move through the furnace for heating, thereby achieving continuous graphitization. The crucible furnace method and the box furnace method require intermittent production, with a fixed amount of raw materials placed in the furnace each time, and discharged after high-temperature heating is completed. Taking the Acheson method of crucible loading as an example, a graphitization furnace has a production cycle of 12-14 days from furnace cleaning to product loading, electric heating, cooling, and unloading. Among them, electric heating only takes 2-3 days. Although each furnace group has 6-8 graphitization furnaces, each furnace can only rotate 2-2.5 times in a month, resulting in lower efficiency.

Various process methods have their own advantages and disadvantages, with the current dominant method being the Acheson method, and the continuous graphite furnace may be the future direction.

Crucible loading method: The Acheson method is widely used, which consumes a large amount of electricity, requires a large amount of resistance heating material and insulation covering material, has high internal transportation costs, and is difficult to collect exhaust gas due to open smelting. However, due to its high temperature retention, high capacity, high initial efficiency, and high graphitization degree, it is suitable for mid to high end products. The main difference between the internal series graphitization process and the Acheson graphitization process is that it does not require auxiliary materials to assist in heating. The product is placed in a crucible and connected in series, and is heated by electrodes from both ends. The product itself generates heat through resistance.

Box type furnace method: Compared with crucible method, it reduces production materials, lowers power consumption, reduces costs, and improves production efficiency. However, its capacity, initial efficiency, and degree of graphitization are slightly inferior to crucible furnace method.

Continuous method: It consumes the least amount of electricity, has no production materials, significantly reduces costs under continuous production, has high energy utilization efficiency, is environmentally friendly, and has good consistency. However, its capacity, first effect, and graphitization degree are relatively low, making it suitable for mid to low end products. If product quality can be improved, it is expected to become a future direction.

　　Different graphitization furnaces have their own applications depending on the product form and parameter requirements of the negative electrode material. Generally speaking, Acheson furnaces and internal series furnaces are suitable for high-end products due to their high heating temperature and graphitization rate; box furnaces are suitable for mid-range products; and continuous furnaces are currently suitable for low-end products.

　　Supply side: Under the dual carbon policy, long-term supply growth of graphitization will be limited.

　　Energy consumption dual control will effectively support the dual carbon policy. In April 2021, the National Energy Administration issued the "2021 Energy Work Guidance Opinion", proposing to reduce energy consumption per unit of GDP by about 3%. In September 2021, the National Development and Reform Commission issued the "Scheme for Improving the Dual Control System of Energy Consumption Intensity and Total Amount", proposing overall goals for 2025/2030/2035, resolutely controlling "dual high" projects, strengthening and improving the energy consumption dual control assessment mechanism, and promoting the marketization of energy consumption indicators transactions. This multi-faceted approach to energy consumption control will assist in achieving the goals of carbon peaking and carbon neutrality.

　　In August 2021, the National Development and Reform Commission released the "Weather Report on the Completion of Energy Consumption Dual Control Targets in Various Regions in the First Half of 2021." Compared with 20Q1-3, the completion of energy consumption targets in various provinces was significantly worse in 2021. Energy consumption intensity increased instead of decreased in 19 regions, including Qinghai, Ningxia, Guangdong, Guangxi, and Fujian; 13 other regions were also under Level 1 and 2 early warnings for energy consumption total control targets.

　　From the perspective of industrial categories, the non-metallic mineral products industry, to which graphitization belongs, is a high energy consumption ratio and high energy consumption intensity industry. Under the guidance of energy consumption dual control, it will be subject to key control. By the end of October 2021, the energy consumption ratio of the non-metallic mineral products industry was 7%. Excluding daily consumption, transportation, warehousing, and postal services, it ranked 4th among manufacturing industries in terms of energy consumption intensity. The non-metallic mineral manufacturing industry consumes 2.39 tons of standard coal per 10,000 yuan, second only to the ferrous metal smelting and rolling processing industry and the gas production and supply industry.

　　Graphitization is a major energy consumer among midstream materials and will receive key attention. Taking the most commonly used Acheson method as an example, the electricity consumption per ton is 10,000-12,000 kWh, higher than that of high energy-consuming ternary positive electrodes and copper foil.

　　The approval of new graphitization production capacity is becoming stricter, which may affect the expansion of graphitization production capacity that has not yet been approved. The National Development and Reform Commission stated that it will, together with relevant departments, urge various provinces (autonomous regions and municipalities) to establish a list of under-construction, planned, and existing "dual high" projects, implement classified disposal, and use an increase of 50,000 tons of standard coal as the boundary for national and local tiered management. Assuming that 320 grams of coal is consumed per kWh, 50,000 tons of standard coal consumes nearly 160 million kWh, corresponding to nearly 13,000 tons of graphitization projects. If the approval power is partially transferred to the National Development and Reform Commission, it will affect the subsequent expansion of graphitization production capacity that has not yet been approved.

　　Geographically, domestic negative electrode graphitization production capacity is mainly located in areas with lower electricity prices. Nearly 40% is concentrated in Inner Mongolia, and it is also distributed in Shanxi, Sichuan, and Hebei. Taking the 2021 10kV level large industrial electricity price as an example (yuan/kwh), the electricity prices in Inner Mongolia, Shanxi, Sichuan, and Hebei are 0.4489, 0.5082, 0.5774, and 0.5481 yuan/kWh respectively. These are relatively low-priced areas in the central and eastern regions with convenient transportation.

　　Negative electrode manufacturers' integrated expansion increases supply. Judging from the integrated production capacity planning announced by various manufacturers, the self-sufficiency rate of graphitization has been increasing year by year. It is estimated that companies focusing on artificial graphite will achieve a self-sufficiency rate of over 70% in 2023. However, the success rate and progress of approvals are tightening, and long-term supply growth of graphitization will be limited.