Silicon is currently the highest known anode material for lithium-ion batteries, but due to its huge volume effect (>300%), it eventually leads to the deterioration of electrochemical performance. In recent years, researchers have done a lot of research and exploration, tried to solve these problems and achieved certain results.
Silicon is currently known as the anode material for lithium-ion batteries with the highest specific capacity (4200mAh/g), but due to its huge volume effect (>300%), the silicon electrode material will pulverize and peel off from the current collector during charging and discharging. , So that the active material loses electrical contact with the active material, the active material and the current collector, and at the same time, a new solid-phase electrolyte layer SEI is continuously formed, which ultimately leads to the deterioration of electrochemical performance. In recent years, researchers have done a lot of research and exploration, tried to solve these problems and achieved certain results. The editor will take you to look at the research progress in this field, and propose further research directions and applications. prospect.
Lithium deintercalation mechanism and capacity fading mechanism of silicon
Silicon does not have the layered structure of graphite-based materials. Its lithium storage mechanism is the same as that of other metals. It is carried out by alloying and dealloying with lithium ions. Its charge and discharge electrode reaction can be written as the following formula:
Si+xLi++xe-?LiXSi
In the process of alloying and dealloying with lithium ions, the structure of silicon undergoes a series of changes, and the structural transformation and stability of silicon-lithium alloys are directly related to the transport of electrons.
According to the mechanism of lithium deintercalation of silicon, we can summarize the capacity decay mechanism of silicon as follows: (1) During the first discharge, as the voltage drops, the two phases of lithium-intercalated silicon and non-lithium-intercalated crystalline silicon coexist first. Core shell structure. As the depth of lithium insertion increases, lithium ions react with internal crystalline silicon to form a silicon-lithium alloy, which eventually exists in the form of an alloy of Li15Si4. In this process, the volume of silicon becomes about 3 times compared with the original state. The huge volume effect leads to the destruction of the structure of the silicon electrode, the active material and the current collector lose electrical contact between the active material and the active material, and the lithium ion deintercalation process Can not proceed smoothly, resulting in huge irreversible capacity. (2) The huge volume effect will also affect the formation of SEI. As the process of deintercalating lithium proceeds, the SEI on the silicon surface will crack and re-form as the volume expands, making the SEI thicker and thicker. Since the formation of SEI consumes lithium ions, it causes a larger irreversible capacity. At the same time, the poor conductivity of SEI will make the impedance of the electrode continue to increase with the charge and discharge process, hinder the electrical contact between the current collector and the active material, increase the diffusion distance of lithium ions, and hinder the smooth deintercalation of lithium ions, resulting in capacity The rapid decay. At the same time, a thicker SEI will cause greater mechanical stress, causing further damage to the electrode structure. (3) The unstable SEI layer will also cause silicon and silicon-lithium alloy to directly contact the electrolyte and be lost, resulting in a loss of capacity.
Selection and structure design of silicon materials
1. Amorphous silicon and silicon oxide
(1) Amorphous silicon
Amorphous silicon has a higher capacity at low potential, and as a negative electrode material for lithium-ion batteries, it has higher safety performance than graphite electrode materials. However, amorphous silicon materials can only relieve particle crushing and pulverization to a limited extent. , Its cycle stability still cannot meet the requirements of being a negative electrode material for high-capacity batteries.
(2) Silicon oxide
As a negative electrode material for lithium-ion batteries, SiO has a high theoretical specific capacity (above 1200mAh/g), good cycle performance, and low lithium extraction potential, so it is also a potential high-capacity lithium-ion battery negative electrode material . However, the difference in oxygen content of silicon oxide will also affect its stability and reversible capacity: as the oxygen in silicon oxide increases, the cycle performance increases, but the reversible capacity decreases.
In addition, silicon oxide as a negative electrode material for lithium-ion batteries still has some problems: because the formation of Li2O and lithium silicate is irreversible during the first lithium insertion process, the first coulombic efficiency is very low; at the same time, Li2O and lithium silicate The poor conductivity of the salt makes the electrochemical kinetic performance poor, so its rate performance is poor; compared with elemental silicon, silicon oxide has better cycle stability as a negative electrode material, but as the number of cycles continues to increase, its stability remains Very bad.
2. Low-dimensional silicon materials
The low-dimensional silicon material has a larger surface area under the same mass, which is conducive to the full contact of the material with the current collector and the electrolyte, reduces the stress and strain caused by the uneven diffusion of lithium ions, and improves the yield strength and powder resistance of the material , So that the electrode can withstand greater stress and deformation without crushing, and then obtain a higher reversible capacity and better cycle stability. At the same time, the larger specific surface area can withstand higher current density per unit area, so the rate performance of low-dimensional silicon materials is also better.
(1) Silicon nanoparticles
Compared with micron silicon, the electrochemical performance of the electrode material using nano-sized silicon is significantly improved whether it is the first charge-discharge specific capacity or the cycle capacity.
Although nano-silicon particles have better electrochemical properties than micro-silicon particles, when the size is reduced to below 100nm, silicon active particles are prone to agglomeration during charge and discharge, which accelerates the attenuation of capacity and has a larger ratio. The surface allows the silicon nanoparticles to have more contact with the electrolyte, forming more SEI, so its electrochemical performance has not been fundamentally improved. Therefore, nano-silicon is often used in combination with other materials (such as carbon materials) as anode materials for lithium-ion batteries.
(2) Silicon film
In the process of deintercalating lithium in the silicon thin film, lithium ions tend to proceed along the direction perpendicular to the thin film, so the volume expansion of the silicon thin film also mainly proceeds along the normal direction. Compared with bulk silicon, the use of silicon thin films can effectively suppress the volume effect of silicon. Unlike other forms of silicon, thin-film silicon does not require a bonding agent and can be used as an electrode to be directly added to a lithium-ion battery for testing. The thickness of the silicon film has a great influence on the electrochemical performance of the electrode material. As the thickness increases, the deintercalation process of lithium ions is inhibited. Compared with micron-sized silicon thin films, nano-sized silicon thin-film anode materials exhibit better electrochemical performance.
(3) Silicon nanowires and nanotubes
At present, the methods that have been reported to be able to synthesize silicon nanowires in large quantities mainly include laser ablation, chemical vapor deposition, thermal evaporation, and direct growth on silicon substrates.
Due to its unique hollow structure, silicon nanotubes have better electrochemical performance than silicon nanowires. Compared with silicon particles, silicon nanowires/nanotubes have no obvious lateral volume effect in the process of lithium deintercalation, and they will not crush and lose electrical contact like nano silicon particles, so the cycle stability is better. Due to the small diameter, lithium is released faster and more thoroughly, and the reversible specific capacity is also high. The large free surface inside and outside the silicon nanotube can well adapt to the radial volume expansion, forming a more stable SEI during the charging and discharging process, so that the material exhibits a higher Coulomb efficiency.
3. Porous silicon and hollow structure silicon
(1) Porous structure silicon
A suitable pore structure can not only promote the rapid deintercalation of lithium ions in the material, improve the rate performance of the material, but also buffer the volume effect of the electrode during the charge and discharge process, thereby improving the cycle stability. In the preparation of porous silicon materials, the addition of carbon materials can improve the conductivity of silicon and maintain the electrode structure, further improving the electrochemical performance of the material. Common methods for preparing porous silicon include template method, etching method and magnesium thermal reduction method.
In recent years, the method of preparing silicon-based materials by thermally reducing silicon oxide by magnesium has attracted wide attention from researchers. In addition to using spherical silica as the precursor, silica molecular sieves are a commonly used method for preparing porous silicon materials due to their porous structure. Commonly used silicon oxide precursors mainly include SBA-15, MCM-41, etc. Due to the poor conductivity of silicon, a layer of amorphous carbon is often coated on the surface of porous silicon after magnesium thermal reduction.
(2) Hollow structure silicon
The hollow structure is another way to effectively improve the electrochemical performance of silicon-based materials. The current method for preparing hollow silicon is mainly the template method. Although hollow silicon has excellent electrochemical properties, its preparation cost is still very high, and there are also problems such as poor conductivity. By designing the yolk-shell structure and controlling the size of the space between the yolk and the eggshell, while effectively buffering the volume expansion of silicon, the carbon as the eggshell can also improve the conductivity of the material, so it has a yolk-shell The structured carbon-silicon composite material has better cycle stability and higher reversible capacity.
Preparation of silicon-based composite materials
1. Silicon metal composite
The metal is compounded with silicon, and the metal can play a supporting role to prevent the volume expansion of silicon during the insertion and extraction of lithium ions and reduce the degree of pulverization. After metal and silicon form an alloy, the free energy of lithium insertion is lower, which makes the process of lithium insertion easier. At the same time, the excellent electrical conductivity of metal can improve the dynamic performance of silicon alloy materials. Therefore, the combination of metal and silicon can effectively improve the electrochemical performance of silicon-based composites.
Although the specific capacity of Si-active metal is relatively high, the pulverization phenomenon also occurs in the active metal itself, so the cycle performance is poor. The inactive metal in the Si-inactive metal composite material is an inert phase, which will greatly reduce the reversible capacity of the silicon material, but the stability will be slightly improved accordingly. When Si is mixed with active metals and inactive metals to form a composite, a synergistic effect can be used to prepare a silicon-based electrode material with good stability and high capacity.
2. Silicon carbon composite material
As a negative electrode material for lithium-ion batteries, carbon materials have small volume changes during charging and discharging, have good cycle stability and excellent conductivity, so they are often used to composite with silicon. In carbon-silicon composite anode materials, according to the types of carbon materials, they can be divided into two categories: silicon and traditional carbon materials and silicon and new carbon materials. Among them, traditional carbon materials mainly include graphite, mesophase microspheres, carbon black and amorphous carbon. New types of carbon materials mainly include carbon nanotubes, carbon nanowires, carbon gels and graphene.
(1) Silicon graphite/mesocarbon microsphere composite material
Graphite has excellent electrical conductivity. After being compounded with silicon, it can improve the poor electrical conductivity of silicon-based materials. Under normal temperature conditions, the chemical stability of silicon and graphite is very strong, and it is difficult to produce a strong force. Therefore, high-energy ball milling and chemical vapor deposition are often used to prepare silicon-graphite composite materials.
The mesophase carbon microsphere is a micron-scale graphitized carbon material formed by the liquid phase thermal polycondensation reaction and carbonization of pitch-based organic compounds. It has excellent electrochemical cycling characteristics and has been widely used in commercial lithium battery anodes. material. Similar to graphite, compounding mesophase pitch carbon microspheres with silicon can also improve the electrochemical performance of the silicon electrode material.
(2) Silicon carbon black composite material
Carbon black has excellent electrical conductivity, and researchers have also tried to use carbon black and silicon as a negative electrode material for lithium-ion batteries. Scientists obtain a conductive network structure by treating carbon black at high temperature, depositing silicon and amorphous carbon successively, and then using a granulator to obtain a silicon-carbon composite material with a size of 15 to 30 μm. High reversible capacity and good cycle stability.
(3) Silicon carbon nanotube/wire composite material
One of the commonly used methods for preparing carbon fiber is the electrostatic spinning method. By adding a silicon source to the selected precursor, a silicon-carbon fiber composite material can be obtained. The silicon carbon nanotube/wire composite material can also be prepared by direct mixing or chemical synthesis. And carbon nanotubes/wires are often used as the second matrix, which acts as a conductive network to conduct electricity.
In addition, chemical vapor deposition is a common method for preparing nanowires and nanotubes. The chemical vapor deposition method can directly grow carbon fiber or carbon tube on the surface of silicon, or directly deposit silicon on the surface of carbon fiber carbon tube.
(4) Silicon carbon gel composite material
Carbon gel is a nanoporous carbon material prepared by sol/gel method. The carbon gel retains the nano-network structure of the organic aerogel before carbonization, has abundant pores and a continuous three-dimensional conductive network, and plays a role in buffering the volume expansion of silicon. Due to the large specific surface area of the carbon gel, the first irreversible capacity of the silicon-carbon gel composite is large. At the same time, the nano silicon in the organogel generates amorphous SiOX during the carbonization process and is easily decomposed into Si and SiO2. The presence of SiO2 will reduce the reversible capacity of silicon-based materials and affect the electrochemical performance of the materials.
(5) Silicon graphene composite material
Graphene has the advantages of good flexibility, high aspect ratio, excellent electrical conductivity and stable chemical properties. Good flexibility makes it easy for graphene to compound with active materials to obtain composite materials with a coated or layered structure, and can effectively buffer the volume effect during charge and discharge. Compared with amorphous carbon, two-dimensional graphene has better electrical conductivity, which can ensure good electrical contact between silicon and silicon, and between silicon and current collectors. Graphene itself is also an excellent energy storage material. After it is combined with silicon, it can significantly improve the cycle stability and reversible capacity of silicon-based materials. At present, the commonly used methods for preparing silicon-graphene composite materials mainly include simple mixing method, extraction method, chemical vapor deposition method, freeze-drying method, spray method and self-assembly method.
3. Other silicon-based composite materials
(1) Silicon compound type composite material
In the research of silicon-compound type composites, TiB2, TiN, TiC, SiC, TiO2, Si3N and other substances are mainly used as the matrix. The commonly used preparation method of this type of composite is high-energy ball milling. The cycle stability of this type of silicon-based material is better than that of pure silicon anode material. However, since the matrix does not undergo lithium-deintercalation reaction, the reversible capacity of this type of material is generally very low. .
(2) Silicon conductive polymer composite material
Conductive polymers have the advantages of good conductivity, good flexibility, and easy structural design. They can not only buffer the volume effect of silicon-based materials, but also maintain good electrical contact between the active material and the current collector. The commonly used conductive polymers mainly include polypyrrole and polyaniline.
Optimization of electrode preparation process
1. Electrode treatment
In addition to the above-mentioned preparation of silicon and silicon-based composite electrodes with different morphological structures to improve the stability and reversible capacity of silicon-based anode materials, researchers have also achieved the same goal by heat-treating the electrodes.
Scientists use polyvinylidene fluoride as a binder and found that heat treatment can make the binder more evenly distributed in the electrode and enhance the adhesion between the silicon and the current collector. In addition, using PVDF as a binder, coating it with nano-silicon in a certain proportion on the copper electrode, and rapid heat treatment at 900 ℃ for 20 minutes can directly obtain the carbon-coated silicon electrode, which has high coulombic efficiency, large charge and discharge capacity, and good cycle performance. .
2. Selection of current collector
The huge volume change of silicon causes itself to pulverize, which will cause the active material to fall off the current collector, resulting in poor cycle stability. By enhancing the force between the current collector and silicon, maintaining good electrical contact is also one of the modification methods. The rough surface current collector has a better effect on silicon, so the use of porous metal current collector is an effective method to improve the electrochemical performance of silicon-based anode materials. In addition, the preparation of thin-film silicon and silicon-based composite materials can save the current collector and be directly used as a negative electrode material for lithium-ion batteries, thereby avoiding the problem of silicon-based materials falling off from the current collector and losing electrical contact due to the huge volume effect.
3. Choice of bonding agent
When preparing general electrode materials for lithium-ion batteries, conductive agents such as active materials, binders, and carbon black are usually mixed in a certain proportion to form a slurry and then coated on the current collector. Due to the huge volume effect, the traditional adhesive PVDF is not suitable for silicon electrodes. Therefore, the electrochemical performance of silicon-based materials can be effectively improved by using a bonding agent that can adapt to the huge volume effect of silicon. In recent years, researchers have done a lot of research on silicon-based material adhesives. Commonly used silicon-based adhesives mainly include carboxymethyl cellulose, polyacrylic acid, alginic acid, and corresponding sodium salts. In addition, researchers have also conducted research and design on polyamide, polyvinyl alcohol, polyfluorene-type polymers and adhesives with self-healing properties.
4. Choice of electrolyte
The composition of the electrolyte affects the formation of SEI, which in turn affects the electrochemical performance of the negative electrode material. In order to form a uniform and stable SEI, researchers have improved the electrochemical performance of silicon-based materials by adding electrolyte additives. The additives currently used include lithium bisoxalate borate, lithium difluorooxalate, propylene carbonate, succinic acid, vinylene carbonate, fluoroethylene carbonate, etc., among which vinylene carbonate and fluoroethylene carbonate are the most effective. ester.
