Recent progress in the research of silicon-carbon composite negative electrode materials for CR1620 battery

Lithium-ion batteries have the advantages of high energy density, long cycle life and less environmental pollution. They have become the focus of research in countries around the world and have been widely used in computers, mobile phones and other portable electronic devices. However, with the rapid development of electric vehicles and advanced electronic devices, higher requirements are placed on the energy density of lithium-ion batteries. The key to improving the energy density of lithium-ion batteries lies in the improvement of electrode materials and the improvement of performance. At present, the negative electrode materials of commercial lithium-ion batteries are mainly graphite materials. Due to their low theoretical specific capacity (specific capacity is only 372mAh/g) and poor rate performance. Therefore, scientists are committed to studying new high-capacity negative electrode materials. Silicon has attracted much attention due to its high theoretical specific capacity (4200mAh/g), low lithium insertion and extraction voltage platform (<0.5V), low reaction activity with electrolyte, abundant reserves in the earth's crust, and low price. As a negative electrode material for lithium-ion batteries, it has broad development prospects. However, the volume of silicon changes greatly (>300%) during the process of lithium insertion and extraction, which causes the active material to pulverize and fall off rapidly during the charge and discharge cycle, resulting in the loss of electrical contact between the electrode active material and the current collector. At the same time, due to the huge volume expansion of silicon materials, the solid electrolyte interface film cannot exist stably in the electrolyte, resulting in reduced cycle life and capacity loss. In addition, the low conductivity of silicon severely limits the full utilization of its capacity and the rate performance of silicon electrode materials. At present, the methods to solve these problems include: nano-sizing, composite and other methods. Nano-sizing and silicon-carbon composite technology are the research focus of scientists, and have made significant progress, improving the cycle performance and rate performance of silicon negative electrode materials.

This article mainly summarizes the research progress of silicon-carbon composite technology, including four aspects: silicon/graphite composite materials, silicon/amorphous carbon composite materials, silicon/carbon nanotube composite materials and silicon/graphene composite materials.

Carbon materials are one of the preferred active matrices for silicon-based composite materials, mainly because carbon materials have good conductivity and small volume change. In addition, carbon materials are light in weight and abundant in sources. After silicon material is coated with carbon, the conductivity of the material can be enhanced, the agglomeration of silicon nanoparticles and the volume expansion of the material can be avoided, and a relatively stable and smooth solid electrolyte interface film can be formed on the carbon surface, thereby increasing the cycle life and improving the rate performance.

1. Silicon/graphite composite materials

Graphite is used as a structural buffer layer, and graphite can accommodate huge volume changes during the charging and discharging process. Wu et al. prepared a silicon-graphite composite material with a special structure by high-energy mechanical ball milling. The silicon-graphite composite material showed excellent cycle performance. At a current density of 237mA/g, the electrochemical window of 0.03-1.5V, the first reversible capacity was 1592mAh/g, and it had good rate performance.

Su et al. prepared graphene-coated silicon-graphite composite materials by spray drying and heat treatment process. The composite materials have excellent electrochemical properties. At a current density of 50mA/g, the first charge capacity is 820.7mAh/g and the first coulombic 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 it shows excellent cycle and rate performance.

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

Jeong et al. synthesized carbon-coated silicon-graphite composites by hydrothermal carbonization, showing excellent electrochemical performance, with a specific capacity of up to 878.6 mAh/g. After 150 cycles, the capacity retention rate was 92.1%. The carbon layer is conducive to the transfer of electrons, and at the same time, it can serve as a buffer layer for the silicon volume effect during the charge and discharge process.

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

2. Silicon/amorphous carbon composites

Coating a very thin amorphous carbon film on the surface of nano-silicon particles can improve the morphology of the solid electrolyte interface film. Datta et al. showed that in the electrochemical voltage window of 0.02-1.2 V, at a current density of 0.25C, after coating a carbon film on the silicon surface, the reversible capacity can increase by 700 mAh/g. Under constant current charge and discharge at a current density of 0.3mA/mg, the capacity of the carbon-coated silicon composite material can reach 1000mAh/g.

Studies have shown that during the charge and discharge process, the nano-silicon particles in the composite material tend to agglomerate, and the agglomeration of silicon particles will lead to poor charge and discharge kinetics. In order to improve the agglomeration of silicon during the charge and discharge process, Kwon et al. synthesized carbon-coated silicon quantum dots. The first charge capacity of this structural material was 1257mAh/g and the coulombic efficiency was 71%. The uniform distribution of silicon quantum dots along the carbon layer is conducive to preventing agglomeration during the charge and discharge process.

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

In summary, after a layer of amorphous carbon is coated on the surface of silicon, the silicon-carbon composite material is significantly improved. This is because the carbon layer can enhance the conductivity of the material, avoid the agglomeration of silicon nanoparticles and the volume expansion of the material, and the carbon surface can form a relatively stable and smooth solid electrolyte interface film, thereby increasing the cycle life and improving the rate performance.

3. Silicon/carbon nanotube composite materials

Among all one-dimensional carbonaceous materials, carbon nanotubes have attracted much attention as additives for improving the electrochemical properties of silicon-based materials. The uniform distribution of nanosilicon particles along carbon nanotubes can optimize the electrochemical properties of silicon. The reversible capacity of the composite material obtained by depositing 10nm silicon particles on carbon nanotubes with a diameter of 5nm is as high as 3000mAh/g (current density 1.3C). Li et al. synthesized a silicon/carbon nanotube/carbon composite material. The carbon matrix can alleviate the volume effect of silicon and provide a continuous path for charge transfer along the axial direction. Carbon nanotubes can improve the electronic conductivity and electrochemical properties of the composite material.

Park et al. prepared a multilayer carbon nanotube-coated nano-silicon ion composite material by chemical vapor deposition. A large number of silicon particles with a particle size of 50nm were densely distributed in the pore space between the multilayer carbon nanotubes. The composite material has a high capacity and capacity retention rate. At a current density of 840mAh/g, after 10 and 100 cycles, the capacity is 2900mAh/g and 1510mAh/g, respectively. In addition, the composite material has excellent rate performance. The composite material has excellent electrochemical properties mainly because: in the process of lithium insertion and extraction, multilayer carbon nanotubes can provide an effective electron transmission path and alleviate the volume effect of nano-silicon particles.

Studies have shown that attaching silicon nanoparticles to carbon nanowires can also significantly improve the electrochemical properties of silicon. During the carbonization process, silicon nanoparticles are anchored on carbon nanowires, and there is a strong interaction between silicon and carbon. At a current density of 500mAh/g, the negative electrode material has a specific capacity of 2500mAh/g, and after 50 times, it has a high capacity retention rate. Since the carbon nanowire matrix has elasticity similar to that of polymers, this further reduces the stress caused by the volume change of silicon during the charging and discharging process.

IV. Silicon/graphene composite materials

Due to its excellent conductivity, graphene can be used in battery materials to improve the electrochemical performance of batteries. Silicon-graphene composite materials are prepared by ultrasonic method and magnesium thermal reaction. First, silicon dioxide particles are synthesized and then ultrasonically deposited on the surface of graphene oxide. Then, magnesium thermal reaction is used to in-situ reduce silicon dioxide to nano-silicon and attach it to the graphene surface. By optimizing the ratio, the nano-silicon particle size attached to the graphene surface is 30nm. The first reversible capacity of the silicon-graphene composite material with a silicon content of 78% 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 decay.

Ren et al. used chlorosilane as a silicon source and deposited silicon particles on the graphene surface using a chemical vapor deposition process. During the charge and discharge process, the silicon-graphene composite material showed a high silicon utilization rate, and the capacity retention rate was 90% after 500 cycles. Li et al. prepared a graphene-carbon-coated nano-silicon composite material. The graphene and carbon layers can play a double-layer protection role, thereby improving the electrochemical performance of silicon during the charge and discharge process. The composite material still has a reversible capacity of 902mAh/g after 100 cycles at a current density of 300mA/g. This study provides a way to improve the electrochemical performance of lithium ions by using graphene as a scaffold to attach active substances and a carbon layer as a protective layer.

Wen et al. prepared a graphene-coated silicon composite material with a special structure using a spray dryer. Silicon is coated with graphene. At a current density of 0.1C, the material has a charge specific capacity of 2250mAh/g, and a capacity retention rate of 85% after 120 charge and discharge cycles. The defective graphene shell can quickly deintercalate lithium, has excellent electronic conductivity, and prevents the agglomeration of nano-silicon particles during the charge and discharge process. Since graphene has good mechanical properties, the space inside the graphene shell can effectively alleviate the volume expansion of silicon.

Sun et al. prepared silicon-graphene composites by discharge plasma-assisted ball milling. Nano-silicon particles were uniformly embedded in the graphene matrix. Rapid heating of plasma and mechanical ball milling allowed nano-silicon particles to be in situ embedded in the graphene matrix, which can effectively prevent the agglomeration of nano-silicon and improve electronic conductivity. The cycle stability and rate performance of silicon-graphene composites have been improved. At a current density of 50mA/g, the reversible capacity can be stabilized at 976mAh/g.

Lee et al. synthesized a composite material with well-dispersed silicon on a three-dimensional mesh graphene. The close contact between nanoparticles and graphene can improve the electrochemical performance. The silicon-graphene composite material exhibits high specific capacity and cycle stability. After 200 cycles, the reversible capacity is still greater than 1500mAh/g.

Wang et al.'s research shows that graphene nanosheets can significantly improve the electrochemical properties of porous single-crystalline silicon nanowires. As a conductive additive, graphene nanosheets cover a large number of nanowires, providing a large number of locations for charge transfer. The interlaced graphene nanosheets can provide a large number of paths for the transfer of electrons and lithium ions, thereby improving conductivity and lithium ion diffusion rate. The first charge capacity of the silicon-graphene composite material is 2347mAh/g, and the capacity retention rate is 87% after 20 times. The research of Sun et al. also shows that the graphene nanosheet-coated silicon nanocomposite has 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. During the process of lithium insertion and extraction, the volume changes of carbon and silicon are inconsistent, which makes the composite material easy to delaminate quickly, especially at high charge and discharge rates.

V. Conclusion

Si-based materials can be used as negative electrode materials for lithium-ion batteries due to their high theoretical specific capacity, but there are huge volume effects, low conductivity and unsatisfactory cycle life during the charge and discharge process, which hinders its commercial application, but it is undeniable that the material has great application prospects. Minimizing the first irreversible capacity, alleviating the volume expansion of the material, and thus improving the rate and cycle performance are the focus of scientists' research. At present, the most effective and widely studied are silicon/carbon composite materials.

The author believes that future research on silicon-based materials should be carried out from the following aspects: 1. Combining silicon nano-sizing and silicon-carbon composites to alleviate silicon volume expansion and improve rate and cycle stability; 2. Prepare porous silicon/carbon composite materials, use porous conductivity and mesh structure to alleviate volume effects, improve rate and cycle performance; 3. Carry out theoretical calculations and simulations to quantitatively analyze the volume expansion rate of silicon and the elasticity of carbonaceous materials.

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