The lithium-ion battery has the advantages of high energy density, long cycle life and low environmental pollution, and has become the focus of various countries in the world, and has been 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 puts higher requirements.
How to improve the energy density of lithium-ion batteries, the key lies in the improvement and performance of electrode materials. At present, the negative electrode material of commercial lithium-ion batteries is mainly graphite material, since its theoretical is lower than the capacity (only 372mAh / g), and has poient performance. Therefore, scientists are committed to studying new high-capacity negative electrode materials, and silicon has received much attention due to high theoretical specific capacity (4200mAh / g), and its deinterlidation lithium voltage platform is low (<0.
5V), and electrolyte reactions Low activity, rich in reserves in the crust, and low price, as a lithium-ion battery negative material, has a wide development prospect. However, the volume of silicon occurs huge (300%) during the deinterlaxing, resulting in a sharp powder from the active substance during the charge and discharge cycle, so that the electrode active material and the current fluid lose electrical contact..
At the same time, since the mass expansion of the silicon material is expanded, the solid electrolyte membrane cannot be stably present in the electrolyte, resulting in reduced circulating life and capacity loss.. In addition, the low electrical conductivity of silicon has severely limiting the full use of the capacity and the magnification performance of the silicon electrode material.
. Currently, methods for solving these problems include: nanofed, composite and other methods. Namized and silicone composite technology is the focus of scientists, and significant progress has improved the cycle performance and magnification performance of silicon negative materials.
. This paper summarizes the research progress of silicon / graphite composite technology, including silicon / graphite composite, silicon / unsettled carbon composite, silicon / carbon nanotube composite and silicon / graphene composites 4 aspects..
Carbon materials are one of the preferred active matrices preferred by silicon-based composites, because the conductive performance of the carbon material is good, the volume change is small, in addition, the quality of the carbon material is light, and the source is rich. After the silicon material coated carbon, the conductive properties of the material can be enhanced to prevent agglomeration between silicon nanoparticles and the volume expansion of the material, and the carbon surface can form a relatively stable, smooth solid electrolyte membrane, thereby rising circulating life. Improve multiplier performance.
First, the silicon / graphite composite graphite as a structural buffer layer, and graphite can accommodate huge volume changes during charge and discharge.. WU et al.
 prepared a special structure of silica graphite composite materials using high energy mechanical ball mills, and silica graphite composites show excellent circulating properties, at 237 mA / g current densities, 0.03 to 1.5V electrochemical windows, first reversible capacity 1592mAh / g, and have good magnification performance.
Su et al., The suplason graphite composite material is prepared by spray drying and heat treatment process, which has excellent electrochemical properties, at 50 mA / g current density, the first charging capacity is 820.7mAh / g, the first Kurlen efficiency is 77.
98%; under high current density 500mA / g, the first reversible capacity is still up to 766.2mAh / g, and excellent cycle and magnification performance are displayed..
ZHANG et al, uses high-energy ball mills, electrochemical tests show that the first charge and discharge capacity is 1068.8mAh / g and 1283.3mAh / g, respectively, the first Kurlen efficiency is 83.
3%.. After 25 cycles, the reversible capacity is 620mAh / g.
After 50 cycles, the reversible capacity is still stable in 600mAh / g above.. Jeong et al, synthesized carbon coated silicon graphite composite materials, exhibited excellent electrochemical properties, with more than 878.
6mAh / g, over 150 cycles, with a capacity retention rate of 92.1%..
The carbon layer facilitates the transfer of electrons, and can be used as a buffer layer of silicon volume effect during charge and discharge.. SU et al.
Et al. Through liquid phase solidification and pyrolysis, the compound has priority electrochemical properties, high first reversible capacity, and the first Cullen efficiency is 73.82%, after 40 cycles, capacity retention rate Still more than 80%.
Second, the silicon / unsettled carbon composite material plated a thin layer of amorphous carbon film on the surface of nanola particles, improve the morphology of solid electrolysis, DATTA and other studies have shown that at 0.02 ~ 1.2V electrochemical voltage window.
Under. At the current density of 0.3 mA / mg, the current charge is charged, and the capacity of the carbon coated silicon composite can reach 1000 mAh / g.
. Studies have shown that in the charging and discharge process, nano silicon particles in the composite material tend to reunite, and the group of silicon particles lead to poor charging and discharging power..
In order to improve the agglomeration of silicon during the charge and discharge, KWON et al. Synthesized the amount of carbon coated silicon dots. The first charge capacity of this structural material was 1257mAh / g, and Kurlen efficiency was 71%.
. Silicon quantum dots are uniformly distributed along the carbon layer facilitate agglomeration in the charging and discharge process..
Magasinski et al, a layered self-assembly technique is prepared from dendrimeric carbon coating silicon nanoparticles, under 0.5c current, reversible charging capacity reaches 1950mAh / g. Dendritic carbon as a mesh conductive structure facilitates electronically efficient conduction, and supplies suitable voids for nano-silicon volume expansion.
In summary, after the silicon surface is covered with a layer of amorphous carbon, silicone composites have been significantly improved, which is due to the electrical conductivity of the material to enhance the conductivity of the material and prevent aggregation between silicon nanoparticles and materials. The volume expansion, and the carbon surface can form a layer of stable, smooth solid electrolyte membrane, thereby rising circulating life while increasing the magnification performance..
Third, the silicon / carbon nanotube composite material is used in all one-dimensional carbonaceous materials, and the carbon nanotube is used as an additive to improve the electrochemical properties of the silicon-based material.. The nano-silicon particles can optimize the electrochemical properties of silicon along the carbon nanotube.
. The 10 nm silicon particles were deposited on carbon nanotubes having a diameter of 5 nm, and the obtained composite can be reversible with up to 3000 mAh / g (current density 1.3c).
Li et al. Synthesized silicon / carbon nanotube / carbon composite, and the carbon matrix can alleviate the volume effect of silicon, along the axial direction to supply continuous paths..
Carbon nanotubes can improve electronic conductivity and electrochemical properties of composites. PARK et al, prepared a multi-layer carbon nanotube coated nano-silicon ion composite by chemical vapor deposition, a large amount of 50 nm silicon particles inset the pore space between the multilayer carbon nanotubes, and the composite material has a high Capacity and capacity retention ratio, at a current density of 840 mAh / g, after 10 and 100 cycles, capacity is 2900 mAh / g and 1510 mAh / g, respectively..
In addition, the composite material has excellent magnification performance. The composite material has excellent electrochemical properties. It is important that multi-layer carbon nanotubes can supply effective electron transport paths while deinterlasis.
. Studies have shown that silicon nanoparticles can also significantly improve silicon nanowires on carbon nanowires. During carbonization, silicon nanoparticles are anchored on carbon nanowires, silicon and carbon have strong inter alia At 500 mAh / g current densities, the negative electrode material has a specific capacity of 2500 mAh / g, 50 times, has a high capacity retention ratio.
. Since the carbon nanowire matrix has an elasticity similar to the polymer, this further reduces the stress that occurred due to silicon volume during charging and discharge process..
Fourth, silicon / graphene composite graphene can be applied to battery materials due to its excellent conductivity, improve battery electrochemical performance. Silicon graphene composite materials by ultrasonic methods and magnesium heat reactions. The silica particles are first synthesized, and then the surface of the oxide oxide is deposited by ultrasonic oxide, and then the silica has a nano silicon using magnesium thermally reactive in situ, and attached to the surface of the graphene.
By optimizing the proportion, the nano silicon particle diameter adhering to the surface of the graphene is 30 nm. The first reversible capacity of silicon graphene composite of 78% is 1100 mAh / g, and the charging current density increases from 100mAh / g to 2000mAh / g, and then returns to 100 mAh / g, only a small amount of capacity attenuation. Ren et al.
Use a silicone source to deposit silicon particles in a graphene surface using a chemical vapor deposition process.. During the charge and discharge process, the silicon graphene composite shows high silicon utilization, 500 cycles, and the capacity retention ratio is 90%.
. Li et al., The graphene carbon is prepared, the graphene and the carbon layer can function as a double-layer protection, thereby improving electrochemical properties of silicon in the charging and discharge process.
. The composite material is still 902 mAh / g after 100 cycles at 300 mA / g..
The study supplied a way to improve lithium ion electrochemical properties, by using graphene as a stent attached active substance, and as a protective layer in a carbon layer. Wen et al, a special structure is prepared using a spray dryer to prepare a special structure..
Silicon is coated with graphene, at 0.1C current density, the material charge ratio capacity is 2250 mAh / g, and the capacity retention ratio is 85% after 120 times..
It has a defective graphene shell to quickly deinterolize lithium, with excellent electron conductivity, and prevent reunion of nano-silica particles during charge and discharge process.. Since graphene has better mechanical properties, the space in the graphenege envelope can effectively relieve the volume expansion of silicon.
. SUN et al, prepared a silicon graphene composite material by discharging plasma auxiliary ball milling, and nano-particle uniformly embedded graphene matrix, rapid heating plasma and mechanical ball milling causes nano-silica particles in situ in situ in graphene matrix, which can effectively block Nano silicon reunion improves electron conductivity. The circulating stability and magnification performance of the silicon graphene composite is improved, and the reversible capacity can be stabilized at 976 mAh / g at 50 mA / g current densities.
. LEE et al. Synthesized a composite material having good dispersible silicon on three-dimensional mesh graphene, and the nanoparticles and graphene contact can improve electrochemical properties.
. Silicon graphene composites show high specific capacity and circulatory stability, after 200 cycles, reversible capacity is still greater than 1500mAh / g. Wang et al.
Suggests that the graphene nanoflake can significantly improve the electrochemical performance of porous single crystal silicon nanowires.. Gradenene is used as a conductive additive, and the nano sheet covers a large number of nanowires, supplies a large number of positions for charge metastasis.
. Interlaced graphene nanofina can supply a large number of paths for electron and lithium ions, thereby improving conductivity and lithium ion diffusion rates..
The first charging capacity of the silicon graphene composite is 2347 mAh / g, and the 20-time capacity retention ratio is 87%.. SUN et al.
Also indicated that the graphene nanoflake coated silicon nanocomposite has excellent cycle life and high capacity.. Although the above studies have made progress, the core problem is the weaker structure interface of carbon and silicon.
In the process of deion, the volume variation of carbon and silicon is inconsistent, which makes the composite material easy to lay back, especially in high In the case of charge and discharge. V. V.
Since the conclusion of silicon-based materials can be used as a lithium ion battery negative electrode material due to its theoretical specific capacity, but there is a huge volume effect, low conductivity, low conductivity and low cycle life, hinder its commercial application. However, it is undeniable that the material has a lot of application prospects..
Maximize the first irreversible capacity, relieve the volume expansion of the material, thereby improving the multivation and cycle performance is the focus of scientists. At present, the most effective and research is the most extensive is silicon / carbon composite..
The author believes that the study of silicon-based materials in the future should be carried out from the following aspects: 1 binding silicon nanochemistry and silicone compounds to alleviate the volume expansion of silicon, increase the magnification and cycle stability; 2 Preparation of porous silicon / carbon composite materials, Using porous conductivity and mesh structures, mitigates volume effect, improves magnification and cycle performance; 3 to carry out theoretical calculations and simulation, quantitatively analyze the volume expansion ratio of silicon and the elasticity of carbonaceous materials, etc.