[Hanergy Technology • Technology] depth analysis of silicon carbon negative material composite way
Lithium-ion battery with high energy density, high open circuit voltage, long cycle life, etc., are widely used in computers, mobile phones, EV and other portable electronic devices. At present, the commercialization of lithium batteries is relatively high. As one of the four main materials (cathode materials, anode materials, separators and electrolytes) of lithium batteries, the performance of anode materials has a key impact on battery performance. The types of anode materials are shown in Figure 1 As shown. Currently on the market, lithium battery manufacturers mainly choose graphite materials as the negative electrode material of lithium batteries, and graphite belongs to one of carbon negative electrode materials, including artificial graphite and natural graphite.
Figure 1. Lithium battery anode material types
Graphite is an ideal negative electrode material. It is widely used in lithium batteries due to its good cycling stability, excellent conductivity and good intercalation properties. With the continuous improvement of lithium battery performance in our country, the shortage of graphite as negative electrode material is also gradually revealed. For example, the gram capacity is low (372 mAh / g), the layered structure is easy to peel and peel off when there are many cycles, Than the energy and performance to further enhance. Researchers are committed to finding a material that can replace carbon negative materials.
Much attention has been drawn to the fact that silicon can form a binary alloy with lithium and has a high theoretical capacity (4200 mAh / g). In addition, silicon has a very low release lithium voltage platform (less than 0.5V vs Li / Li +), low reactivity with electrolyte, abundant reserves in the earth's crust, low price and other advantages. It is a very promising lithium battery Anode material.
Figure 2. Structure comparison of graphite and silicon
However, silicon as a lithium battery negative has a fatal flaw. During charging, lithium ions are released from the positive electrode material and intercalated into the internal crystal lattice of the silicon crystal, resulting in a large expansion (about 300%) to form a lithium-silicon alloy. Lithium ion discharge from the lattice between the prolapse, but also formed into a very large gap. The use of silicon crystal alone as a negative electrode material easily causes the following problems:
First, in the process of de-embedding, the volume of silicon crystal changes obviously. Such volume effect can easily lead to the silicon anode material being peeled off from the current collector, resulting in the electrochemical corrosion and short-circuit phenomenon caused by the foil dew foil. Affect the battery safety and service life.
Second, silicon and carbon are the same main group elements, and SEI is also coated on the silicon surface during the first charge and discharge. However, the exfoliation caused by the silicon volume effect can cause repeated damage and reconstruction of the SEI, thereby increasing the lithium Ion consumption, the final impact on battery capacity.
Combining the advantages and disadvantages of carbon materials and silicon materials, the two are often used in combination to maximize their usefulness. Composites are usually divided into two categories based on the type of carbon material: silicon-carbon conventional composites and silicon-carbon composites. The traditional composite material refers to the composite of silicon and graphite, MCMB and carbon black, and the new type of silicon-carbon composite material refers to the composite of silicon and carbon nanotubes, graphene and other novel carbon nanomaterials. Different materials will form a different combination of ways, silicon carbon materials, composite methods / structures are the following:
First, the walnut structure
Figure 3. Walnut structure silicon carbon composite
The walnut structure of the silicon-carbon composite material is made of porous silicon particles, and then the carbon material is filled into the porous silicon formed, as shown in Figure 3. This nanometer structure effectively solves the problems of charge and discharge of micron and nanometer silicon materials and shows excellent electrochemical performance. At a current density of 1 A / g, a reversible capacity of 1459 mAh / g can be maintained after 200 cycles of charging and discharging. At a current density of 12.8 A / g, there is still a reversible capacity of 700 mAh / g. The excellent performance of this material is due to the three-dimensionally interconnected pore network composed of nanoscale silicon particles and carbon.
Professor Ci Ci Jie from Shandong University combined with silicon and graphene successfully prepared a walnut-like porous Si / reduced graphene oxide (P-Si / rGO) material by in-situ reduction and degassing techniques and has excellent electrochemical performance ,As shown in Figure 4.
Figure 4. Walnut-shaped porous silicon / reduced graphene oxide
Second, the coating structure
Core-shell structure is a common type of composite, is the carbon material wrapped in the outer layer of silicon particles to form a composite material. After the carbon is coated on the surface of the silicon material, the conductivity of the material can be enhanced, the carbon material has a certain toughness, the agglomeration of the silicon particles and the volume expansion of the material during the de-intercalation of lithium are avoided, and the SEI film is formed on the surface of the carbon material, Electrolytic erosion of the negative electrode material damage, thereby increasing the cycle life and improve rate performance. Compared with the walnut-structured silicon-carbon material, the silicon-carbon material with a larger content of silicon in the cladding structure greatly increases the space for intercalation of lithium. In addition, the expansion and comminution of the silicon particles are also greatly reduced.
By cladding the silicon material with carbon, a core-shell structure can be constructed to help improve the cycling stability of the material. However, when the pyrolytic carbon in the silicon carbon core-shell structure covers the surface of the silicon particles without any void, the volume effect of the silicon nucleation process is too large, which causes the entire core-shell particles to expand and even cause the surface carbon layer to occur Ruptured, the composite structure collapsed, and the cyclic stability dropped rapidly. To solve this problem, some researchers have designed a double-shell structure by strengthening the mechanical properties of the shell, as shown in FIG. 5. Firstly, SiO2 is coated on the surface of the silicon particles, and then a layer of carbon material is coated on the surface of the composite particles. This can effectively alleviate the structural changes of the composite material and improve the cycle life of the lithium battery.
Figure 5. Double-clad structure
Three, three yuan embedded composite structure
Embedded silicon-carbon structures are often found on new silicon-carbon composites, such as silicon / CNTs, silicon / graphene composites. Figure 6 is a silicon, carbon materials and CNT composite structure of the three Schematic diagram, the first silicon particles coated with a layer of carbon film, and then carbon nanotubes attached to the surface, then these materials into a spherical shape. The surface of the silicon particles is coated with a carbon film, the thickness of the film is nanoscale (10-20 nm), on which carbon nanotubes are adhered. Such carbon nanotubes filled between the silicon particles, both play a conductive role, but also play a role in absorbing the volume expansion of silicon particles. Finally, these carbon nanotubes attached to the silicon and carbon composite material, spray-dried way to produce a pellet pellets, the pellets of about 10 microns in diameter, the composite particles under the scanning electron microscope shown in Figure 7 shows.
Figure 6. Ternary embedded composite structure
Figure 7. Ternary embedded composite structure silicon carbon anode material SEM
Four, three yuan cladding filled structure
Institute of Physics, Chinese Academy of Sciences has developed a watermelon structure of silicon carbon composites, as shown in Figure 8. Nano-silicon and graphite composite doping together, then wrapped in a layer of carbon material on its outer layer to form a similar watermelon structure of silicon carbon composites. This structure can effectively reduce the volumetric change and particle fragmentation under high electrode density. Based on the practical application, the prepared Si carbon negative electrode has an appropriate reversible capacity of 620 mA · h / g and exhibits a cycle stability of more than 500 cycles at a high surface area (2.54 mA · h / cm2) And excellent magnification performance.
Figure 8. Ternary cladding filled structure model
Preparation of silicon-carbon composite materials are ball milling method, pyrolysis method, chemical vapor deposition, sputtering deposition, evaporation and so on. Therefore, the structure of the silicon carbon material is varied, but all of them are designed with the idea of increasing the capacity of the lithium battery and reducing the defects of the silicon particles.
Regarding the market situation of silicon carbon anode, domestic manufacturers of anode materials such as Shanshan Co., Zichen Jiangxi and Shenzhen Bertre have already laid out the production of silicon carbon anode material. Several silicon carbon anode materials have been introduced and have certain Capacity; on the market part of the lithium production enterprises have adopted the silicon-carbon composite materials as lithium battery anode material in the domestic battery companies, Guoxuan Hi-Tech, BYD, CATL, Lishen, Wanxiang A123, micro- Carbon anode system research and development and trial production; in foreign enterprises, Tesla by adding 10% artificial graphite in the silicon-based materials, the Model 3 on the use of silicon carbon anode as a new battery material, the battery capacity of 550mAh / g above, the battery energy density up to 300wh / kg.
Japan GS Yuasa introduced Si-based anode material lithium batteries and successfully applied to Mitsubishi Motors; Hitachi Maxell announced that it has developed a high current capacity of silicon negative lithium batteries. For the production and utilization of silicon carbon anode are in full swing, I believe silicon carbon anode material in the 2018 lithium market will have a qualitative and quantitative leap.