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Lithium ion batteries have the advantages of high energy density, long cycle life and less environmental pollution. They have become the focus of research around the world, and are widely used in computers, mobile phones and other portable electronic devices. However, with the rapid development of electric vehicles and advanced electronic equipment, the energy density of lithium ion batteries is required to be higher. How to improve the energy density of lithium ion battery depends on the improvement of electrode materials and performance. At present, the anode materials of commercial lithium-ion batteries are mainly graphite materials. Because of its low theoretical specific capacity (only 372mAh/g), and poor rate performance. Therefore, scientists are committed to the research of new high-capacity anode materials. Silicon has attracted much attention due to its high theoretical specific capacity (4200mAh/g). Its low de intercalation voltage platform (<0.5V), low reactivity with electrolyte, rich reserves in the crust, and low price. As the anode material of lithium ion batteries, silicon has broad development prospects. However, the volume of silicon changes greatly (>300%) in the process of lithium removal, leading to the rapid pulverization and shedding of active substances during the charge discharge cycle, which makes the electric contact between the electrode active substances and the collector lose. At the same time, due to the huge volume expansion of silicon material, the solid electrolyte interface facial mask cannot exist stably in the electrolyte, resulting in the reduction of cycle life and capacity loss. In addition, the low conductivity of silicon severely limits the full utilization of its capacity and the magnification performance of silicon electrode materials. At present, the methods to solve these problems include: nano, composite and other methods. Nanotechnology and silicon carbon composite technology are the research focus of scientists, and have made significant progress, improving the cycle performance and magnification performance of silicon anode materials.

This paper mainly summarized the research progress of silicon carbon composite technology, including silicon/graphite composite, silicon/amorphous carbon composite, silicon/carbon nanotube composite and silicon/graphene composite.

Carbon material is one of the preferred active matrices for silicon matrix composites, mainly because of its good conductivity and small volume change. In addition, carbon materials are light in weight and rich in sources. After the silicon material is coated with carbon, it can enhance the conductivity of the material, avoid the agglomeration of silicon nanoparticles and the volume expansion of the material, and a relatively stable and smooth solid electrolyte interface facial mask can be formed on the carbon surface, thus increasing the cycle life and improving the magnification performance.

1、 Silicon/graphite composite

As a structural buffer layer, graphite can accommodate huge volume changes during charging and discharging. Wu et al. prepared silicon graphite composites with special structure by high-energy mechanical milling. The silicon graphite composites showed excellent cycling performance. At 237mA/g current density, 0.03~1.5V electrochemical window, the first reversible capacity was 1592mAh/g, and had good magnification performance.

Su et al. prepared graphene coated silicon graphite composite by spray drying and heat treatment process. The composite has excellent electrochemical performance. Under the current density of 50mA/g, the first charge capacity is 820.7mAh/g, and the first coulomb efficiency is 77.98%; Under the condition of high current density of 500mA/g, the first reversible capacity is still as high as 766.2mAh/g, and shows excellent cycle and rate performance.

Zhang et al. prepared Si-Co-C composites by high-energy ball milling. Electrochemical tests showed that the first charge discharge capacities were 1068.8mAh/g and 1283.3mAh/g respectively, and the first coulomb efficiency was 83.3%. After 25 cycles, the reversible capacity is 620mAh/g, and after 50 cycles, the reversible capacity is still stable above 600mAh/g.

Jeong et al. synthesized carbon coated silicon graphite composite by hydrothermal carbonization method, showing excellent electrochemical performance, with a specific capacity of 878.6 mAh/g, and a capacity retention rate of 92.1% after 150 cycles. The carbon layer is conducive to electron transfer, and can be used as a buffer layer for silicon volume effect during charging and discharging.

Su et al. prepared carbon coated silicon graphite composites through liquid phase solidification and pyrolysis. The composites have preferred electrochemical properties and high first reversible capacity. The first coulomb efficiency is 73.82%. After 40 cycles, the capacity retention rate is still above 80%.

2、 Silicon/amorphous carbon composite

A thin layer of amorphous carbon film on the surface of nano silicon particles can improve the morphology of the solid electrolyte interface facial mask. Datta et al. have shown that the reversible capacity can be increased by 700mAh/g after a layer of carbon film is coated on the silicon surface at a current density of 0.25C and an electrochemical voltage window of 0.02-1.2V. At the current density of 0.3mA/mg, the capacity of carbon coated silicon composite can reach 1000mAh/g under constant current charging and discharging.

The results show that the nano silicon particles in the composite tend to agglomerate during the charge discharge process, and the agglomeration of silicon particles will lead to poor charge discharge dynamic performance. In order to improve silicon agglomeration during charging and discharging, Kwon et al. synthesized carbon coated silicon quantum dots. The initial charging capacity of this structural material is 1257mAh/g, and the coulomb efficiency is 71%. The uniform distribution of silicon quantum dots along the carbon layer is conducive to preventing agglomeration during charging and discharging.

Magasinski et al. used a layered bottom-up self-assembly technology to prepare dendritic carbon coated silicon nanoparticles. At 0.5C current, the reversible charging capacity reached 1950mAh/g. Dendritic carbon, as a network conductive structure, is conducive to the effective conduction of electrons, and provides suitable holes for the volume expansion of nano silicon.

In conclusion, after coating amorphous carbon on the silicon surface, the silicon carbon composite material has been significantly improved. This is because the carbon layer can enhance the conductivity of the material, avoid the agglomeration between silicon nanoparticles and the volume expansion of the material, and form a layer of stable and smooth solid electrolyte interface facial mask on the carbon surface, thus increasing the cycle life and improving the magnification performance.

3、 Silicon/carbon nanotube composites

Among all one-dimensional carbon materials, carbon nanotubes as additives to improve the electrochemical performance of silicon based materials have attracted much attention. The uniform distribution of nano silicon particles along the carbon nanotubes can optimize the electrochemical performance of silicon. Depositing 10 nm silicon particles on carbon nanotubes with a diameter of 5 nm, the reversible capacity of the composite is up to 3000 mAh/g (current density 1.3 C). Li et al. synthesized silicon/carbon nanotubes/carbon composites. The carbon matrix can alleviate the volume effect of silicon and provide a continuous path for charge transfer along the axis. Carbon nanotubes can improve the electronic conductivity and electrochemical properties of the composites.

Park et al. prepared a composite material of nano silicon ions coated by multilayer carbon nanotubes by chemical vapor deposition. A large number of silicon particles with a diameter of 50 nm are densely distributed in the pore space between the multilayer carbon nanotubes. The composite material has a high capacity and capacity retention rate. Under the current density of 840 mAh/g, after 10 and 100 cycles, the capacity is 2900 mAh/g and 1510 mAh/g respectively. In addition, the composites have excellent magnification properties. The composite materials have excellent electrochemical properties mainly because: in the process of lithium removal, multi-layer carbon nanotubes can provide an effective electron transport path, while mitigating the volume effect of nano silicon particles.

The research shows that attaching silicon nanoparticles to carbon nanowires can also significantly improve the electrochemical performance of silicon. During the carbonization process, silicon nanoparticles are anchored on carbon nanowires, and there is a strong interaction between silicon and carbon. Under the current density of 500mAh/g, this anode material has a specific capacity of 2500mAh/g, and after 50 times, it has a high capacity retention rate. The elasticity of carbon nanowire matrix is similar to that of polymer, which further reduces the stress caused by the change of silicon volume during the charge discharge process.

4、 Silicon/graphene composites

Graphene can be used in battery materials to improve the electrochemical performance of batteries because of its excellent conductivity. Silica graphene composites were prepared by ultrasonic method and magnesium thermal reaction. First, silicon dioxide particles are synthesized, and then deposited on the surface of graphene oxide by ultrasonic deposition. Then, silicon dioxide is reduced to nano silicon in situ by magnesium thermal reaction, and attached to the surface of graphene. By optimizing the proportion, the nano silicon particle size attached to the graphene surface was 30 nm. The first reversible capacity of silica graphene composite with 78% silicon content is 1100mAh/g, the charging current density increases from 100mAh/g to 2000mAh/g, and then returns to 100mAh/g, with only a small amount of capacity attenuation.

Ren et al. used chlorosilane as the silicon source to deposit silicon particles on the surface of graphene by chemical vapor deposition. In the process of charging and discharging, the silica graphene composite shows a high silicon utilization rate, and the capacity retention rate is 90% after 500 cycles. Li et al. have prepared graphene carbon coated nano silicon composites. Graphene and carbon layer can play a double layer protection role, thus improving the electrochemical performance of silicon in the charge discharge process. The reversible capacity of the composite still reaches 902mAh/g after 100 cycles at 300mA/g current density. This study provides a way to improve the electrochemical performance of lithium ions by using graphene as a scaffold to attach active substances, and using carbon layer as a protective layer.

Wen et al. used a spray dryer to prepare graphene coated silicon composites with special structure. Silicon is coated with graphene. At 0.1C current density, the specific charge capacity of the material is 2250mAh/g, and after 120 charges and discharges, the capacity retention rate is 85%. The defective graphene shell can quickly deinterinsert lithium, has excellent electronic conductivity, and prevents the agglomeration of nano silicon particles in the charge discharge process. Because graphene has good mechanical properties, the space in the graphene shell can effectively alleviate the volume expansion of silicon.

Sun et al. prepared silica graphene composites through discharge plasma assisted ball milling. The nano silicon particles are uniformly embedded in the graphene matrix. Fast heating plasma and mechanical ball milling make the nano silicon particles embedded in the graphene matrix in situ, which can effectively prevent the agglomeration of nano silicon and improve the electronic conductivity. The cycle stability and magnification properties of silica graphene composites have been improved, and the reversible capacity can be stabilized at 976mAh/g at 50mA/g current density.

Lee et al. synthesized a composite material with well dispersed silicon on three-dimensional reticulated graphene. Close contact between nano particles and graphene can improve the electrochemical performance. Silica graphene composites show high specific capacity and cycle stability. After 200 cycles, the reversible capacity is still greater than 1500mAh/g.

Wang et al. showed that graphene nano sheets can significantly improve the electrochemical performance of porous monocrystalline silicon nanowires. As a conductive additive, graphene sheets cover a large number of nanowires, providing a large number of locations for charge transfer. Interlaced graphene nanosheets can provide a large number of paths for electron and lithium ion transfer, thereby improving the conductivity and lithium ion diffusion rate. The initial charging capacity of the composite is 2347 mAh/g, and the capacity retention rate is 87% for 20 times. Sun et al. also showed that graphene nano sheet coated silicon nanocomposites have excellent cycle life and high capacity.

Although the above research has made progress, the core problem is the weak structural interface between carbon and silicon. In the process of lithium removal, the volume changes of carbon and silicon are inconsistent, which makes the composite easy to delaminate quickly, especially in the case of high charge discharge ratio.

5、 Conclusion

Due to its high theoretical specific capacity, silicon based materials can be used as cathode materials for lithium ion batteries. However, there are huge volume effect, low conductivity and unsatisfactory cycle life in the charge discharge process, which hinder its commercial application. However, it cannot be denied that this material has a great application prospect. It is the focus of scientists' research to minimize the first irreversible capacity, alleviate the volume expansion of materials, and thus improve the magnification and cycling performance. At present, silicon/carbon composites are the most effective and widely studied materials.

The author thinks that the future research on silicon based materials should be carried out in the following aspects: 1. Combining the nano silicon and silicon carbon composite to alleviate the volume expansion of silicon, improve the magnification and cycle stability; 2. Preparation of porous silicon/carbon composites, which use porous conductivity and network structure to alleviate volume effect and improve magnification and cycling performance; 3 Carry out theoretical calculation and simulation, and quantitatively analyze the volume expansion rate of silicon and the elasticity of carbon materials.

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