Lithium-ion battery anode material---related discovery of silicon

Silicon is currently the lithium ion battery anode material with the highest specific capacity (4200mAh/g), but due to its large volume effect (>300%), the silicon electrode material will be powdered during charging and discharging and peeled off from the current collector. The electrical contact between the active material and the active material, the active material and the current collector is lost, and a new solid electrolyte layer SEI is continuously formed, which eventually leads to 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 following is a small series to take a look at the research progress in this field, and propose further research directions and applications prospect.

Lithium removal mechanism and capacity decay mechanism of silicon

Silicon does not have a layered structure of graphite-based material, and its lithium storage mechanism is the same as that of other metals by alloying and de-alloying with lithium ions.

In the alloying and de-alloying process with lithium ions, the structure of silicon undergoes a series of changes, and the structural transformation and stability of the silicon-lithium alloy are directly related to the transport of electrons.

According to the lithium deintercalation mechanism of silicon, we can classify the capacity decay mechanism of silicon as follows: (1) In the first discharge process, as the voltage decreases, the two phases of intercalated lithium silicon and unintercalated lithium crystalline silicon are first formed core-shell structure. As the depth of lithium insertion increases, lithium ions react with the internal crystalline silicon to form a silicon-lithium alloy, which eventually exists as an alloy of Li15Si4. In this process, the volume of silicon is about 3 times larger than that of the original state. The huge volume effect causes the structure of the silicon electrode to be destroyed, and the active material and the current collector 'active material and the active material lose electrical contact, and the lithium ion is deintercalated. Cannot go smoothly, resulting in huge irreversible capacity. (2) The huge volume effect also affects the formation of SEI. As the process of deintercalation of lithium progresses, the SEI of the silicon surface will rupture and expand with volume expansion, making the SEI thicker and thicker. Since the formation of SEI consumes lithium ions, it causes a large irreversible capacity. At the same time, the poor conductivity of SEI will make the impedance of the electrode increase with the process of charge and discharge, 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, the thicker SEI also causes greater mechanical stress and further damage to the electrode structure. (3) The unstable SEI layer also causes silicon and silicon-lithium alloy to be in direct contact with the electrolyte to be lost, resulting in capacity loss.

Silicon material selection and structural design

1. Amorphous silicon and silicon oxides

(1) Amorphous silicon

Amorphous silicon has a higher capacity at low potentials, 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 alleviate particle breakage and chalking to a limited extent. The cycle stability still cannot meet the requirements as 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 (1200mAh/g or more), good cycle performance and low deintercalation of lithium potential, so it is also a highly promising high-capacity lithium ion battery anode material. . However, the difference in oxygen content of silicon oxide also affects its stability and reversible capacity: as the oxygen in the 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: since the formation process of Li2O and lithium silicate during the first lithium insertion process is irreversible, the first Coulomb efficiency is very low; at the same time, Li2O and lithium silicic acid The salt has poor conductivity, which makes the electrochemical kinetic performance poor, so its rate performance is poor. Compared with elemental silicon, silicon oxide as a negative electrode material has better cycle stability, but as the number of cycles continues to increase, its stability remains very poor.

2. Low-dimensional silicon material

Low-dimensional silicon materials have a larger surface area at the same mass, which facilitates sufficient contact of the material with the current collector and electrolyte, reduces stress and strain due to uneven diffusion of lithium ions, and improves the yield strength and powder resistance of the material. This allows the electrode to withstand greater stresses and deformation without comminution, resulting in 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 electrode material using nano-sized silicon has a significant improvement in electrochemical performance regardless of the first charge-discharge ratio capacity or cycle capacity.

Although the nano-silicon particles have better electrochemical properties than the micro-silicon particles, when the size is reduced to below 100 nm, the silicon active particles are prone to agglomeration during charge and discharge, and the capacity is accelerated, and the ratio is larger. The surface causes the silicon nanoparticles to make more contact with the electrolyte, forming more SEI so that its electrochemical performance is not fundamentally improved. Therefore, nano-silicon is often used in combination with other materials such as carbon materials for lithium ion battery anode materials.

(2) Silicon film

In the process of deintercalating lithium from a silicon film, lithium ions tend to proceed in a direction perpendicular to the film, and thus the volume expansion of the silicon film also proceeds mainly along the normal direction. Compared to bulk silicon, the use of a silicon 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 in 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 to micron-sized silicon films, nanoscale silicon thin film anode materials exhibit better electrochemical performance.

(3) Silicon nanowires and nanotubes

At present, methods for synthesizing silicon nanowires in large quantities have been mainly reported, including laser ablation, chemical vapor deposition, thermal evaporation, and direct growth of silicon substrates.

Silicon nanotubes have better electrochemical performance than silicon nanowires due to their unique hollow structure. Compared with silicon particles, silicon nanowires/nanotubes have no obvious lateral volume effect during deintercalation of lithium, and do not pulverize and lose electrical contact like nano-silicon particles, so cycle stability is better. Due to the small diameter, the deintercalation of lithium is faster and more thorough, and the reversible specific capacity is also high. The larger free surface inside and outside the silicon nanotubes is well adapted to radial volume expansion, resulting in a more stable SEI during charge and discharge, resulting in a higher coulombic efficiency.

3. Porous silicon and hollow structure silicon

(1) Porous silicon structure

The 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 charge and discharge, thereby improving the cycle stability. In the preparation of porous silicon materials, the addition of carbon materials can improve the electrical conductivity of silicon and maintain the electrode structure, further improving the electrochemical performance of the material. Common methods for preparing porous structure silicon include a template method, an etching method, and a magnesium thermal reduction method.

In recent years, the method of preparing silicon-based materials by magnesium thermal reduction of silicon oxide has attracted wide attention of researchers. In addition to using spherical silica as a precursor, silicalite molecular sieves are a commonly used method for preparing porous silicon materials because of their porous structure. Commonly used silicon oxide precursors are mainly SBA-15, MCM-41 and the like. 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. At present, the method for preparing hollow silicon is mainly a template method. Although the electrochemical performance of hollow silicon is excellent, its preparation cost is still 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 egg yolk and the eggshell, while effectively buffering the volume expansion of the silicon, the carbon as the eggshell can also improve the conductivity of the material, thus having an egg yolk eggshell. The structural carbon-silicon composite has better cycle stability and higher reversible capacity.

Preparation of silicon-based composite materials

1. Silicon metal composite

The metal is combined with silicon, and the metal can play a certain supporting role, preventing the volume expansion of the silicon and reducing the degree of pulverization during the insertion and removal of lithium ions. When the metal is alloyed with silicon, the free energy of lithium intercalation is lower, which makes the lithium intercalation process easier. At the same time, the excellent electrical conductivity of the metal can improve the dynamic properties of the silicon alloy material. Therefore, the combination of metal and silicon can effectively improve the electrochemical performance of the silicon-based composite.

Although the specific capacity of the Si-active metal is high, the active metal itself is also pulverized, and thus the cycle performance is poor. The inactive metal in the Si-inactive metal composite is an inert phase, which greatly reduces the reversible capacity of the silicon material, but the stability is 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 having high stability and high capacity.

2. Silicon carbon composite material

As a negative electrode material for lithium ion batteries, carbon materials have small volume change during charge and discharge, good cycle stability and excellent electrical conductivity, and are therefore often used for recombination with silicon. In the carbon-silicon composite anode material, it can be divided into two types according to the type of carbon material: silicon and conventional carbon materials and silicon and new carbon materials. Among them, 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

Graphite has excellent electrical conductivity, and when combined with silicon, it can improve the problem of poor conductivity of the silicon-based material itself. Under normal temperature conditions, silicon and graphite have strong chemical stability and it is difficult to generate strong force. Therefore, high energy ball milling and chemical vapor deposition are often used to prepare silicon graphite composites.

Mesophase carbon microspheres are micron-sized graphitized carbon materials formed by liquid phase thermal polycondensation and carbonization of asphaltic organic compounds. They have excellent electrochemical cycle characteristics and are now widely used in commercial lithium battery anodes material. Similar to graphite, the mesophase pitch carbon microspheres are combined with silicon to improve the electrochemical performance of the silicon electrode material.

(2) Silicon carbon black composite material

Carbon black has excellent electrical conductivity, and researchers have tried to combine carbon black with silicon for lithium ion battery anode materials. The scientists obtained the conductive network structure by treating the carbon black at a high temperature, depositing silicon and amorphous carbon successively, and then using a granulator to obtain a silicon-carbon composite material with a size of 15-30 μm high reversible capacity and good cycle stability.

(3) Silicon carbon nanotube/wire composite

One of the common methods for preparing carbon fibers is an electrospinning method, in which a silicon carbon fiber composite material is obtained by adding a silicon source to a selected precursor. Silicon carbon nanotube/wire composites can also be prepared by direct mixing or chemical synthesis. Carbon nanotubes/wires are often used as a second matrix to act as a conductive network.

In addition, chemical vapor deposition is a common method for preparing nanowires and nanotubes. The carbon fiber or carbon tube can be directly grown on the silicon surface by chemical vapor deposition, or the silicon can be directly deposited on the surface of the carbon fiber carbon tube.

(4) Silicon carbon gel composite

Carbon gel is a nanoporous carbon material prepared by a sol/gel method. The carbon gel maintains the nano-network structure of the organic aerogel before carbonization, and has abundant pores and a continuous three-dimensional conductive network, which acts to buffer 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 carbonization and is easily decomposed into Si and SiO2. The presence of SiO2 reduces the reversible capacity of the silicon-based material and affects the electrochemical properties of the material.

(5) Silicon graphene composite material

Graphene has the advantages of good flexibility, high aspect ratio, excellent electrical conductivity and stable chemical properties. The good flexibility makes the graphene easy to be combined with the active material to obtain a composite material having a coating or layer structure, and can effectively buffer the volume effect during charging and discharging. Compared to amorphous carbon, two-dimensional graphene has superior electrical conductivity, which can ensure good electrical contact between silicon and silicon, silicon and current collector. Graphene itself is also an excellent energy storage material. When 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 include simple mixing method, vacuum method, chemical vapor deposition method, lyophilization method, spray method and self-assembly method.

3. Other silicon-based composite materials

(1) Silicon compound type composite material

In the study of the silicon-compound type composite, as the matrix, there are mainly TiB2, TiN, TiC, SiC, TiO2, Si3N and the like. The commonly used preparation method for such composites is high-energy ball milling. Such silicon-based materials have better cycle stability than pure silicon anode materials, but the reversible capacity of such materials is generally low due to the absence of deintercalation of lithium in the matrix. .

(2) Silicon conductive polymer composite

The conductive polymer has the advantages of good electrical conductivity, good flexibility and easy structural design, which not only buffers the volume effect of the silicon-based material, but also maintains good electrical contact between the active material and the current collector. Commonly used conductive polymers are mainly polypyrrole, polyaniline and the like.

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, the researchers 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 silicon and current collector. In addition, PVDF is used as a binder, which is coated on the copper electrode with a certain ratio of nano-silicon. The carbon-coated silicon electrode can be directly obtained by rapid heat treatment at 900 °C for 20 min. The coulombic efficiency is high, the charge and discharge capacity is large, and the cycle performance is good. .

2. The choice of current collector

The large volume change of silicon causes self-pulverization, which causes the active material to fall off from the current collector, thus causing poor cycle stability. Maintaining good electrical contact by enhancing the force between the current collector and the silicon is also one of the methods of modification. The rough surface current collector works better with silicon, so the use of a porous metal current collector is an effective method for improving the electrochemical performance of silicon-based anode materials. In addition, the preparation of the film-like silicon and silicon-based composite material can save the current collector and directly be used for the negative electrode material of the lithium ion battery, thereby avoiding the problem that the silicon-based material loses electrical contact from the current collector due to the large volume effect.

3. Selection of bonding agent

When preparing a general lithium ion battery electrode material, a conductive agent such as an active material, a binder, and carbon black is usually mixed into slurry in a certain ratio and then applied to a current collector. Due to the large volume effect, the traditional cement PVDF does not fit well with silicon electrodes. Therefore, the electrochemical performance of the silicon-based material can be effectively improved by using a binder capable of adapting to the large volume effect of silicon. In recent years, researchers have done a lot of research on silicon-based material bonding agents. The commonly used silicon-based adhesives mainly include carboxymethyl cellulose, polyacrylic acid, alginic acid, and corresponding sodium salts. In addition, researchers have studied and designed polyamides, polyvinyl alcohols, polyfluorene polymers and adhesives with self-healing properties.

4. Choice of electrolyte

The composition of electrolyte affects the formation of SEI and the electrochemical properties of cathode materials. In order to form a uniform SEI, researchers improve the electrochemical properties of silicon-based materials by adding electrolyte additives. The additives used at present include lithium borate dioxalate, lithium borate difluoroxalate, propylene carbonate, succinic acid, vinylidene carbonate, vinylidene fluoride carbonate, etc., among which the best effect is vinylidene carbonate carbonate and vinylidene fluoride carbonate ester.

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