Graphene composite silicon carbon anode material and its high energy density lithium-ion battery research

End products such as power batteries and consumer batteries have an increasing demand for high-energy-density lithium-ion batteries. At present, the industry mainly adopts the silicon-carbon composite route to improve the application level of silicon-based anodes. The silicon-carbon composite anode materials below 450mAh/g can basically meet the application requirements in terms of cyclability and rate capability, and silicon-based anodes above 450mAh/g. There are still technical difficulties in the application. The high specific capacity silicon carbon anode material has a huge volume expansion during the intercalation/delithiation process. During the cycle process, the active material will suffer structural failure, resulting in poor electrical contact, and the surface solid electrolyte membrane (SEI membrane) will be repeatedly ruptured/regenerated, resulting in rapid electrolyte consumption. The reversible capacity of lithium-ion batteries decays rapidly. Therefore, the development of silicon-carbon composite anode materials with high specific capacity and long cycle stability is full of challenges. In response to the volume expansion of silicon carbon anode materials, Liu Zhaoping's team from Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences started from the source and constructed a highly mechanically stable self-mechanically suppressed graphene composite silicon carbon anode material (Energy Storage Materials 2021, 35, 317 -326.). Since the reversible capacity of silicon-based anode materials has a linear relationship with volume expansion, the reversible capacity of silicon-based anode materials can be effectively controlled by mechanically limiting the volume expansion. Graphene is a known material with high mechanical properties, with an elastic modulus of up to 1 TPa, a yield strength of 130 GPa, a fatigue life of more than 109 times under a load of 71 GPa, and graphene exhibits elastic hardening during lithium intercalation. However, the weak adhesion between graphene sheets is weak, and the macroscopic mechanical strength of graphene is poor. Pitch cracked carbon can effectively stitch graphene and significantly improve the mechanical stability of graphene macroscopic body. Silicon oxide (SiOx) and graphene slurry are mixed uniformly in a liquid phase system, in which pitch is used as an additive to prepare spherical graphene/pitch cracked carbon through a series of processes such as spray drying, high temperature heat treatment and chemical vapor deposition Encapsulated silicon oxide composite anode material (SiOx/Graphene/C, SGC for short). The carbon atoms in graphene are arranged in a two-dimensional layered structure, which has good flexibility. At the same time, the van der Waals force between graphene layers is weak, and it is easy to slip under the force and has a certain elastic strain capacity; the asphalt cracking on the surface of graphene Carbon has an amorphous structure, and the rigid carbon-carbon chemical bonds have high tensile strength, which can maintain the integrity and mechanical stability of the graphene macrostructure. Studies have shown that the powder conductivity of the graphene-modified SGC negative electrode can be increased by more than 2 orders of magnitude; the graphene content≥At 7wt%, the lubricating effect of graphene is beneficial to improve the compaction density of the powder. The analysis of the intrinsic reversible delithiation capacity of SiOx and the relative content of LixSi show that the SGC composite anode can significantly control the amount of lithium uptake by SiOx, and the higher the graphene content, the lower the reversible capacity of SiOx. The intercalation and delithiation behavior of graphene is affected by the stress generated by the volume expansion of SiOx during charging and discharging. In conclusion, the SGC composite anode material can inhibit the lithium uptake of SiOx, reduce the volume expansion, and improve the cycle stability. As shown in Figure 2, the capacity of different graphene-modified silicon-carbon composite anodes and graphite composites is about 850mAh/g, and the reversible capacity of the soft pack battery is about 5.0mAh/cm2. Under the condition of 0.5C cycle and 80% capacity cut-off, the cycle life is from The 500-cycle capacity retention rate of the 353Wh/kg soft pack battery can still reach 78.1% even under the cell preparation process conditions. The research team is in the Science and Technology Service Network Program (STS) of the Chinese Academy of Sciences“Graphene industrial application technology research and development and application demonstration”With the support of the project (execution period from January 2019 to December 2020, as shown in Figure 3), the technical problem of large-scale preparation of graphene composite silicon carbon anode materials has been further solved. A 100-ton scale graphene composite silicon-carbon anode material pilot production line has been established. Using this high-performance graphene composite silicon carbon anode material, a series of new high-energy-density lithium-ion batteries with an energy density of 350-400 Wh/kg were further developed, which were awarded in the 2020 Future Energy Storage Technology Innovation Ideas Collection and Challenge Competition The first prize in the extreme challenge category of long cycle life and high energy density lithium-ion batteries; jointly developed with Kunshan Juxin Energy Technology Co., Ltd., a subsidiary of Baoneng Group, and realized the application demonstration of 310 Wh/kg power battery loading. Recently, the Science and Technology Promotion and Development Bureau of the Chinese Academy of Sciences organized the final acceptance of the STS project.