The latest research progress of silicon carbon anode materials

Since the beginning of the 21st century, the contradiction between the growing economic development and the energy environment has become increasingly acute. The traditional automotive technology using fossil energy has gradually failed to meet the requirements of environmental protection in the new century. Diversification of clean energy. Among the many clean energy sources, plug-in hybrid vehicles and pure electric bus vehicles can significantly reduce the emission of carbon dioxide and other harmful gases, while having the advantages of high efficiency, low emissions and even zero emissions. Plug-in and pure electric vehicles have become the mainstream of international automotive energy conservation and environmental protection development. Major automakers in the world, such as Volkswagen, Toyota, and Honda, have begun investing heavily in the development of new energy vehicles. And China's BYD has become the leader of new energy vehicles in China and the world. In the 21st century, the focus of the world's major auto companies is the new energy vehicles. The power technology revolution of new energy vehicles will completely change the face of the automotive industry and even the energy industry in the 21st century.

As the largest automobile producer and consumer market in the world, China has been committed to promoting the new energy automobile industry. In the second half of 2015, the Ministry of Science and Technology issued the “National Key R&D Plan New Energy Vehicle Key Specialized Implementation Plan (Draft for Comment)”, which clearly requires enterprises with declared projects to have a power battery cell energy density of not less than 300Wh/kg by the end of 2020. . Therefore, it is urgent to develop a new system of positive and negative materials to improve the energy density of power batteries.

One or three yuan NCA silicon carbon material system

Nickel-cobalt binary materials have many advantages such as high voltage, high energy density and relatively low cost, but their defects such as poor overcharge resistance, poor thermal stability, irreversible initial discharge, and high capacity greatly limit the nickel-cobalt binary materials in the power battery. Use in the field. The ternary nickel-cobalt-aluminum (NCA) material obtained by doping a small amount of aluminum in a nickel-cobalt binary material can significantly improve the stability and safety of the nickel-cobalt binary material, reduce the cost of the nickel-manganese binary material, and The high Ng capacity and high energy density of the NCA material make it a newcomer in the field of power batteries. Although the specific capacity of NCA is high, the energy density of the NCA is difficult to be greatly improved after it is assembled into a battery. It is found that a high battery capacity anode material and NCA ternary positive electrode can effectively match the monomer energy of the battery. The density is increased to more than 300Wh/kg.

Doping other elements or compounds in the graphite anode can significantly improve the capacity and electrochemical performance of the graphite anode. The theoretical lithium storage capacity of silicon is more than 10 times the graphite capacity, which can reach 4200 mAh/g, which is the additive for all graphite anodes. The highest theoretical capacity among heterogeneous elements. In addition, the safety performance of silicon is better than that of graphite anode material. This is because the voltage platform of silicon is higher than that of graphite, so the silicon surface is not easy to be decomposed during the charging and discharging process, thereby improving the safety of the battery. At the same time, as one of the most abundant elements in nature, silicon has a wide source and low production cost. The ternary NCA silicon carbon material system has attracted more and more attention from battery manufacturers and material researchers due to its unique advantages in energy density.

Second, the problems faced by the silicon carbon material system

Although the NCA silicon carbon system has the energy density advantages unmatched by other positive and negative systems, the rapid capacity reduction of the silicon material during the cycle seriously hinders the practical process. This is because after charging lithium, the volume of silicon atoms will expand to more than three times, thereby destroying the original solid electrolyte interface film (SEI) on the surface of the silicon atom, causing the active material to fall off from the current collector. Reduce battery capacity and cycle performance [7]. In addition, in the process of charge and discharge, the regenerative destruction of the SEI film is always accompanied, and the lithium ion and the film-forming additive are continuously consumed on the surface of the active material, and a stable SEI film cannot be formed, resulting in a decrease in charge and discharge efficiency and an increase in capacity attenuation. In addition, due to the low conductivity of silicon itself, it is necessary to add a conductive agent to improve the conductivity of the electrode [8]. How to solve the problems caused by the volume effect of silicon materials, and improve the cycle performance and conductivity of silicon materials is urgent.

Third, the nanomaterialization of silicon materials

The electrochemical properties of the silicon material can be improved by the fabrication process and morphology, and the nanotechnology of the elemental silicon anode material manufacturing process can significantly improve the performance of the silicon material. Nanocrystallization includes zero-dimensional, one-dimensional and two-dimensional nanocrystallization. The zero-dimensional nano-sized silicon material can refine the silicon nanoparticles and reduce the volume change of silicon in the process of delithiation and lithium intercalation. However, the nano-sized silicon material is too small to be easily formed into large particles and degraded. The capacity of the electrode; and the large specific surface area of ​​the silicon nanoparticles consumes a large amount of lithium ions and additives, resulting in an increase in battery side reactions, a decrease in coulombic efficiency, and ultimately a decrease in cycle performance.

One-dimensional nanocrystallization is mainly silicon nanowires and silicon nanotubes. Silicon nanowires can reduce radial volume expansion during cycling and provide a large amount of space and channels for the rapid transmission of axial lithium ions, thereby contributing extremely high capacity. And excellent cycle performance, but its cost is too high to limit the application of one-dimensional nano-silicon on the battery. The silicon nano-scale film has a high reversible capacity of 3500 mAh/g as a two-dimensional nano-sized negative electrode material, but the magnetron sputtering method used in the nano-film is not capable of mass production due to high production cost.

In order to reduce the fabrication cost of nano-silicon materials while stabilizing the surface SEI film of silicon materials, many materials with excellent intrinsic conductivity have been used to recombine with silicon materials. Among all of these materials, the carbon material not only improves the electrical conductivity of the silicon-based anode, but also stabilizes the SEI film on the anode surface. However, any single carbon material or silicon material cannot meet the requirements of modern electronic equipment for two important indicators of energy density and cycle life. Since silicon and carbon belong to the same main group, the chemical properties are similar, which makes it easier to combine the two through different routes. The composite silicon-carbon material can complement each other's advantages and make up for their respective shortcomings, resulting in a new composite material with significantly improved gram capacity and cycle density.

Fourth, the composite of silicon carbon materials

The composite methods of silicon material and carbon material mainly include: silicon/carbon hybrid grinding, silicon/carbon nanorod composite, silicon layer carbon structure, carbon layer silicon structure, silicon/carbon core shell system.

1. Silicon/carbon hybrid grinding

The high-energy ball milling method is a method in which a silicon-carbon mixed material is protected by an inert gas and then ball-milled at a high temperature, which is the first method widely proposed for making silicon-carbon nanomaterials and nanocomposites. The left and the like let the graphite and silicon particles be pyrolyzed in phenol formaldehyde and then polymerized, and the reversible capacity of the obtained silicon carbon graphite composite material can reach 700 mAh/g. Meanwhile, the silicon carbon graphene composite material is also studied after lithium insertion and lithium intercalation. The evolution of the structure and morphology of materials. Studies have shown that the graphite matrix plays a role in controlling the expansion of small-sized silicon particles and thus increases the mechanical stability of the material. Feng et al. recently reported the physicochemical reaction of tetrachlorosilane (SiCl4) and Li13Si4 under ball milling. They synthesized a series of porous silicon-carbon composites with excellent electrochemical properties. The optimal performance of the silicon-carbon composites was as high as 1413 mAh/g, and the cycle capacity was up to 91% at a current density of 100 mA/g. The open porous structure of the carbon layer and excellent electron ionic conductivity make the material have good electrochemical properties. The concept of composite silicon carbon nanoparticles has expanded into the graphene field by depositing silicon carbon nanoparticles on multilayer graphene with high specific surface area. Graphene obtained by flaking from natural graphite can coat the silicon material, and a thin carbon layer can reduce the oxidation of the silicon material.

2. Silicon carbon nanorod composite

Carbon nanorods have high electrical conductivity and high toughness and can withstand the volume expansion caused by charge and discharge of silicon materials. Therefore, researchers have grown silicon-carbon composites on carbon nanorods to improve the cycle performance of silicon-carbon materials. The main difference in these studies is the difference in the preparation of carbon nano-silicon systems. The micro-nano porous silicon-carbon composite structure has been industrially produced, and the silicon-carbon composite material is obtained by pyrolysis of silicon powder (average sizes of 0.7 mm, 4 mm, and 10 mm) with polyvinyl chloride or chlorinated polyethylene.

3. Silicon coated carbon material

Coating silicon nanoparticles (10-20 nm) on carbon materials by deposition method can significantly improve the electrochemical performance of carbon materials. Silicon nanoparticles are uniformly distributed on the surface of graphite particles by thermal decomposition of SiCl4 to form a new type. structure. The electrochemical properties of a silicon-carbon composite containing 7% ( mass fraction) of silicon show that lithium intercalation and lithium intercalation between silicon and carbon are independent, allowing the material to have a reversible capacity of up to 2500 mAh/g at the initial stage. Of course, in the past few years, there have also been researches on coating carbon nanotubes and carbon nano-petals. The conventional sputtering method is used to coat a carbon nano-petal to form a layer of amorphous 200-300 nm thick by slurry spreading method. Silicon layer. The coated silicon provides a conductive path and stress strain relaxation. The material has a specific capacity of up to 2000 mAh/g and the capacity retention after 100 cycles of cycling is also very high.

4. Carbon coated silicon material

Not only can silicon-coated carbon improve the electrochemical performance of the material, but carbon-coated silicon can also increase the material's capacity. Carbon-coated silicon is mainly hydrothermal, CVD, and coating various carbon precursors on silicon particles. Huang et al. prepared a nanowire array by metal-catalyzed etching on a silicon plate, and then coated the carbon on the surface of the silicon nanowire through carbon aerogel and pyrolysis. The mixed nanocomposite has a first discharge capacity of up to 3344 mAh/g, a reversible capacity of 1326 mAh/g after a 40-week cycle, good electronic contact and conductivity between silicon-carbon materials, and effective suppression of volume expansion of silicon by carbon materials. The material has excellent electrochemical performance.

5. Silicon carbon core shell structure

A layer of carbon material is uniformly coated on the outer surface of the silicon material to form a novel core-shell composite material. The silicon-carbon composite material of the core-shell structure can improve the electrical conductivity of silicon and suppress the volume of silicon material. Swell. Xu et al. disperse the nano-silicon material in a polyvinylidene fluoride solution and then heat-treat the mixed solution to obtain a core-shell structured silicon-carbon composite. The silicon core is coated with an amorphous carbon layer, which can increase the reversible capacity of the silicon material. The capacity of the material is as high as 450 mAh/g at a current density of up to 1 000 mAh/g. The presence of the amorphous carbon layer not only inhibits the aggregation of the silicon nanoparticles, but also inhibits the volume expansion of the silicon material during charge and discharge. Liu et al. obtained an egg-like core-shell structure by pyrolysis, and the silicon nanoparticles were encapsulated in the egg yolk by a thin polymer carbon nanolayer. The voids created in the carbon hollow spheres help prevent the carbon shell from rupturing during the volume expansion of the silicon, thereby improving the cycling stability of the material, resulting in a coulombic efficiency of up to 99% after 1 000 cycles.

Fifth, the application of silicon carbon material system

Silicon carbon materials are not only comprehensive in theoretical research, but also in practical applications. Japan's Maxell pioneered the development of practical wearable silicon carbon batteries. Maxell uses "ULSiON" technology to double the amount of electricity without changing the size of the battery. SiO-C (a composite material coated with a carbon coating on the surface of SiO) was used as a negative electrode active material. At the same time, the new battery can also be charged at different voltages. Hitachi expects the battery's discharge termination voltage to be reduced to 2.0V. However, this technology is used in wearable electronic devices, and the practicality on the power battery needs to be verified. Samsung Electronics successfully applied the coating of silicon microparticles with several layers of graphene to ensure the conductivity of the silicon-carbon negative electrode and to suppress electrode degradation and damage during expansion and contraction of the silicon-carbon negative electrode. When the silicon microparticles expand, the graphene expands the silicon storage capacity by sliding each layer in a state in which the silicon microparticles are encased, and thus does not fall off. The new negative electrode material has a capacity density of 2 500 mAh/cm 3 , while the conventional graphite negative electrode material is 550 mAh/cm 3 , and the new material is more than 4 times.

Major domestic battery manufacturers have begun research on ternary silicon carbon batteries. The capacity of the ternary high-nickel silicon-based battery made by Shenzhen BAK Battery Co., Ltd. is designed to be 3.5Ah and 4.0Ah respectively. The high and low temperature performance of these two different specifications of the battery is also very good, the battery can discharge 73% of the capacity at minus 20 ° C, 0.5C current, the discharge capacity of 60 ° C, 0.5C current discharge is as high as 107%; and the discharge of 1.5C current rate There is also 97%, the storage capacity is 92% at 45°C for 30 days, the capacity recovery is as high as 99%, and the capacity at 0.5C for 0.5C at room temperature is maintained at 89% (see Figure 1-2 for details). At the same time, Shenzhen Waterma Battery Co., Ltd., Zhuhai Guangyu Battery Co., Ltd., Guangdong Tianjin New Energy Technology Co., Ltd., Guangzhou Penghui Energy Technology Co., Ltd., Shenzhen Zhuoneng New Energy Co., Ltd., Hao Peng International Group, Zhaoqing Fenghua Lithium Battery Co., Ltd. and Zhongshan Tianmao Battery Co., Ltd. have started research on soft or cylindrical batteries of silicon carbon system.

Optimization of the electrolyte also improves the circulation of the silicon carbon negative electrode. The research on 18650 battery by Tianjin Lishen Battery Co., Ltd. shows that the capacity and volume expansion of the battery increase with the increase of the content of doped silicon. When doping 2% silicon in the graphite negative electrode, the capacity of the battery, Efficiency and volume expansion reach an optimum value. At the same time, they also studied the effect of fluoroethylene carbonate (FEC) content on the cycle performance of the battery. Compared with the FCC addition of 5% of the battery, the cycle life of FEC added 12% of the battery was greatly improved; and after 180 cycles, The morphology of the silicon carbon particles is still good, the pole pieces are also very complete, no powder and cracks appear, and the impedance of the battery does not change much. The increase in FEC content can significantly improve the cycle performance of the Si/C anode material.

At the same time, the R&D personnel of Zhuhai Saiwei Electronic Materials Co., Ltd. found that the silicon anode has better electrochemical performance in the mixed electrolyte than in the single electrolyte through the actual cooperation with the customer. Lithium hexafluorophosphate (LiPF6) was mixed with a certain amount of lithium bis(oxalate) borate (LiBOB), and vinylene carbonate (VC) was added. The addition of LiBOB and VC produced a good synergistic effect on the formation of a thick SEI layer. In addition, the addition of a novel lithium salt such as lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) to the electrolyte can also improve the cycle performance of the silicon carbon battery.

Conclusion

In summary, the silicon-carbon composite material combines the advantages of high conductivity and stability of the carbon material and high capacity of the silicon material, but the volume expansion problem caused by the charge and discharge process has not been fundamentally solved. By doping a small amount of silicon material in the carbon material, it is possible to not only suppress the thickness expansion of the battery within a controllable range, but also increase the energy density and cycle life of the battery, and add sufficient film-forming additives such as FEC to the electrolyte. It is also possible to significantly increase the cycle life of the silicon carbon negative electrode material. At the same time, by optimizing the material structure and manufacturing process, a new type of electrolyte with silicon-carbon negative electrode is developed, thereby improving the specific energy of the silicon-carbon composite material, improving the cycle life and safety of the silicon-carbon battery, and is a lithium-ion battery and the future. The focus of new energy research.