Since people's requirements for the energy density of lithium batteries are getting higher and higher, silicon anode materials have been hotly debated. Although silicon lithium has an ultra-high specific capacity of 4200mAh / g, its huge dilatation coefficient has always restricted it. Step forward.
Silicon does not have a layered structure of graphite-based materials. Its lithium storage mechanism, like other metals, is carried out by alloying and de-alloying with lithium ions. Its charge-discharge electrode reaction can be written as: 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 silicon's lithium deintercalation, we can summarize the capacity attenuation mechanism of silicon as follows:
(1) During the first discharge, as the voltage decreases, a core-shell structure in which lithium-doped silicon and unlithium-doped crystalline silicon coexist is formed first. As the depth of lithium intercalation increases, lithium ions react with the internal crystalline silicon to form a silicon-lithium alloy, which ultimately exists as an alloy of Li15Si4. Compared with the original state, the volume of silicon is changed by about 3 times in this process. The huge volume effect causes the structure of the silicon electrode to be destroyed, the electrical contact between the active material and the current collector 'active material and the active material is lost, and the lithium ion de-intercalation process Cannot proceed smoothly, resulting in huge irreversible capacity.
(2) The huge volume effect will also affect the formation of SEI. As the lithium desorption process proceeds, the SEI on the silicon surface will rupture and re-form as the volume expands, making the SEI thicker and thicker. Because the formation of SEI consumes lithium ions, it results in a large irreversible capacity. At the same time, the poor conductivity of the SEI will also make the electrode impedance 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, hinder the smooth deintercalation of lithium ions, and cause capacity Fast decay. At the same time, thicker SEI will also cause greater mechanical stress and further damage the electrode structure.
(3) The unstable SEI layer will also cause the silicon and silicon-lithium alloy to be in direct contact with the electrolyte and be lost, resulting in capacity loss.
First, the choice of silicon material and structural design
1. Amorphous silicon and silicon oxides
(1) Amorphous silicon. Amorphous silicon has a higher capacity at low potential. As a negative electrode material for lithium ion batteries, it has higher safety performance than graphite electrode materials. However, amorphous silicon materials can only be used to a limited extent. Alleviate the breakage and pulverization of particles, and its cycle stability still cannot meet the requirements as a negative electrode material for high-capacity batteries.
(2) Silicon oxide, as the anode material of lithium-ion batteries, SiO has a high theoretical specific capacity (above 1200mAh / g), good cycle performance, and low deintercalation lithium potential, so it is also a potential Anode material for high capacity lithium ion batteries. But the different oxygen content of silicon oxide will also affect its stability and reversible capacity: with the increase of oxygen in silicon oxide, the cycle performance improves, but the reversible capacity decreases.
In addition, there are still some problems with silicon oxide as the anode material for lithium-ion batteries: the formation of Li2O and lithium silicate during the first lithium insertion process is irreversible, making the first Coulomb efficiency very low; at the same time, Li2O and lithium silicic acid The poor conductivity of salt results in poor electrochemical kinetics performance and therefore poor rate performance. Compared to elemental silicon, silicon oxide has better cycle stability as a negative electrode material, but its stability remains as the number of cycles continues to increase. Very poor.
2. Low-dimensional silicon materials
Low-dimensional silicon materials have a larger surface area under the same mass, which is conducive to the full contact between the material and the current collector and the electrolyte, reduces the stress and strain caused by the uneven diffusion of lithium ions, and improves the material's yield strength and resistance to chalking. , So that the electrode can withstand greater stress and deformation without smashing, thereby obtaining 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) Compared with micrometer silicon, silicon nanoparticle electrode materials using nanometer-sized silicon have significantly improved electrochemical performance in both the first charge-discharge specific capacity and cycle capacity.
Although the nano-silicon particles have better electrochemical properties than the micro-silicon particles, when the size drops below 100 nm, the silicon active particles are likely to agglomerate during the charge and discharge process, which accelerates the capacity decay and has a large ratio The surface makes the silicon nanoparticles more contact with the electrolyte and form more SEI, so its electrochemical performance has not been fundamentally improved. Therefore, nano-silicon is often compounded with other materials (such as carbon materials) for the anode material of lithium-ion batteries.
(2) Silicon thin film. In the process of deintercalating lithium into a silicon thin film, lithium ions tend to proceed in a direction perpendicular to the thin film, so the volume expansion of the silicon thin film also mainly proceeds in a normal direction. Compared with bulk silicon, using silicon thin film can effectively suppress the volume effect of silicon. Unlike other forms of silicon, thin film silicon does not require a binder, and can be directly used as an electrode for testing in lithium-ion batteries. The thickness of the silicon thin film has a great influence on the electrochemical performance of the electrode material. As the thickness increases, the deintercalation process of lithium ions is suppressed. Compared with micron-scale silicon thin films, nano-scale silicon thin film anode materials show better electrochemical performance.
(3) Silicon nanowires and nanotubes. At present, reported methods that can synthesize a large number of silicon nanowires mainly include laser ablation, chemical vapor deposition, thermal evaporation, and direct growth of silicon substrates. Because of its unique hollow structure, silicon nanotubes have better electrochemical performance than silicon nanowires. Compared with silicon particles, silicon nanowires / nanotubes have a small lateral volume effect during the lithium intercalation process, and they will not pulverize and lose electrical contact like nanosilicon particles, so the cycle stability is better. Due to the small diameter, the lithium insertion and release is faster and more thorough, so the reversible specific capacity is also high. The larger free surface inside and outside the silicon nanotube can well adapt to the radial volume expansion, and form a more stable SEI during the charge and discharge process, which makes the material exhibit a higher Coulomb efficiency.
3. Porous silicon and hollow structure silicon
(1) Porous structure silicon, 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 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, and further improve the electrochemical performance of the materials. Common methods for preparing porous structure silicon include template method, etching method and magnesium thermal reduction method.
In recent years, the method for preparing silicon-based materials by magnesium thermal reduction of silicon oxide has attracted widespread attention of researchers. In addition to using spherical silica as a precursor, silica molecular sieves are a commonly used method for preparing porous silicon materials because of their porous structure. The commonly used silica precursors are SBA-15, MCM-41, etc. Due to the poor conductivity of silicon, the surface of porous silicon is often coated with an amorphous carbon after magnesium thermal reduction.
(2) Hollow structure silicon. 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 the hollow silicon has excellent electrochemical performance, its preparation cost is still high at present, and there are also problems such as poor conductivity. By designing the yolk-shell structure and controlling the space between the yolk and the eggshell, while effectively buffering the volume expansion of the silicon, carbon as the eggshell can also improve the conductivity of the material, so it has an egg yolk eggshell Structured carbon-silicon composites have better cycle stability and higher reversible capacity.
Preparation of silicon-based composite materials
1. Silicon metal composite
By combining metal with silicon, metal can play a certain supporting role, prevent the volume expansion of silicon and reduce the degree of powdering during the process of lithium ion insertion and extraction. After the metal forms an alloy with silicon, the free energy of lithium insertion is lower, which makes the lithium insertion process easier. At the same time, the excellent electrical conductivity of metals can improve the dynamic properties of silicon alloy materials. Therefore, metal and silicon composite can effectively improve the electrochemical performance of silicon-based composites.
Although the Si-active metal has a high specific capacity, the active metal itself also has a pulverization phenomenon, so the cycle performance is poor. The inactive metal in the Si-inactive metal composite is an inert phase, which will greatly reduce the reversible capacity of the silicon material, but the stability will be slightly improved. When Si is mixed with an active metal and an inactive metal 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 of a lithium ion battery, carbon material has a small volume change during charging and discharging, has good cycle stability performance and excellent electrical conductivity, so it is often used for compounding with silicon. Among carbon-silicon composite anode materials, they can be divided into two categories according to the type of carbon material: silicon and traditional carbon materials and silicon and new carbon materials. The traditional carbon materials mainly include graphite, mesophase microspheres, carbon black and amorphous carbon. New carbon materials mainly include carbon nanotubes, carbon nanowires, carbon gels, and graphene.
(1) Silicon-graphite / mesophase carbon microsphere composite material, graphite has excellent conductivity, and compounding with silicon can improve the problem of poor conductivity of silicon-based materials. Under normal temperature conditions, silicon and graphite are very chemically stable, 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.
Mesophase carbon microspheres are micron-sized graphitized carbon materials formed by liquid-phase thermal polycondensation and carbonization of pitch-based organic compounds. They have excellent electrochemical cycling characteristics and have 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 silicon electrode materials.
(2) Silicon carbon black composite materials. Carbon black has excellent electrical conductivity. Researchers have also tried to composite carbon black with silicon for lithium ion battery anode materials. Scientists obtain the conductive network structure by processing carbon black at high temperature, deposit silicon and amorphous carbon in succession, and then use a pelletizer to obtain a silicon carbon composite material with a size of 15-30 μm. High reversible capacity and good cycle stability.
(3) Silicon carbon nanotube / wire composite materials. One of the common methods for preparing carbon fibers is electrospinning. By adding a silicon source to a selected precursor, a silicon carbon fiber composite material can be obtained. Silicon carbon nanotube / wire composites can also be prepared by direct mixing or chemical synthesis. Carbon nanotubes / wires are often used as a second substrate, which acts as a conductive network.
In addition, chemical vapor deposition is a common method for preparing nanowires and nanotubes. Using chemical vapor deposition, carbon fibers or carbon tubes can be grown directly on the surface of silicon, or silicon can be directly deposited and grown on the surface of carbon fiber carbon tubes.
(4) Silicon-carbon gel composite material. Carbon gel is a nano-porous carbon material prepared by a sol / gel method. The carbon gel maintains the nano-network structure of the organic aerogel before carbonization, with abundant pores and a continuous three-dimensional conductive network, which buffers the volume expansion of silicon. Because of 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 organic gel generates amorphous SiOX during carbonization and easily decomposes 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 materials. Graphene has the advantages of good flexibility, high aspect ratio, excellent electrical conductivity, and stable chemical properties. Good flexibility makes graphene easy to compound with active materials to obtain composite materials with a cladding or layered structure, and can effectively buffer the volume effect during charge and discharge. Compared to amorphous carbon, two-dimensional graphene has better electrical conductivity, which can ensure good electrical contact between silicon and silicon, and silicon and current collectors. Graphene itself is also an excellent energy storage material. After compounding it 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 include simple mixing method, extraction method, chemical vapor deposition method, lyophilization method, spray method and self-assembly method.
3. Other silicon-based composite materials
(1) Silicon compound-type composite materials. In the research of silicon-compound type composites, the main substrates are TiB2, TiN, TiC, SiC, TiO2, Si3N and other materials. The high-energy ball milling method is commonly used for the preparation of such composites. The cyclic stability of such silicon-based materials is better than that of pure silicon anode materials, but the reversible capacity of such materials is generally low because the substrate does not undergo a deintercalation lithium reaction. .
(2) Silicon conductive polymer composites. Due to their good electrical conductivity, flexibility, and ease of structural design, conductive polymers can not only buffer the volume effect of silicon-based materials, but also maintain active materials and current collectors. Good electrical contact. Commonly used conductive polymers are polypyrrole and polyaniline.
Optimization of electrode preparation process
Electrode treatment
In addition to improving the stability and reversible capacity of silicon-based anode materials by preparing silicon and silicon-based composite electrodes with different morphologies mentioned above, the researchers also achieved the same goal by heat-treating the electrodes. Scientists used polyvinylidene fluoride as a binder, and found that heat treatment can make the binder more uniformly distributed in the electrode, and enhance the adhesion between silicon and the current collector. In addition, PVDF is used as a binder, and it is coated on a copper electrode with nano-silicon at a certain ratio. The carbon-coated silicon electrode can be directly obtained by rapid heat treatment at 900 ° C for 20 minutes. The coulomb efficiency is high, the charge and discharge capacity is large, and the cycle performance is good. .
2.Selection of current collector
The huge volume change of silicon causes it 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 the silicon, maintaining its good electrical contact is also one of the methods of modification. The effect between the rough surface current collector and silicon is better, so using 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 can be directly used as a negative electrode material for lithium ion batteries, thereby avoiding the problem that the silicon-based material loses electrical contact from the current collector due to the huge volume effect.
3. Choice of adhesive
When preparing a general lithium ion battery electrode material, an active material, a binder, and a conductive agent such as carbon black are usually mixed into a slurry at a certain ratio and then coated on a current collector. Due to the huge volume effect, the traditional adhesive PVDF cannot adapt well to silicon electrodes. Therefore, by using a binder that can adapt to the large volume effect of silicon, the electrochemical performance of silicon-based materials can be effectively improved. In recent years, researchers have done a lot of research on silicon-based material adhesives. The commonly used silicon-based adhesives are mainly carboxymethyl cellulose, polyacrylic acid, alginic acid, and corresponding sodium salts. In addition, researchers have also studied and designed polyamides, polyvinyl alcohols, polyfluorene 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 anode 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 currently used additives are lithium dioxaborate, lithium difluorooxalato borate, propylene carbonate, succinic acid, vinylene carbonate, fluoroethylene carbonate, etc. Among them, the most effective are vinylene carbonate and fluoroethylene carbonate. ester.