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Research progress and application prospects of silicon-based 18650 rechargeable battery lithium 3.7v 3500mah negative electrode materials

Silicon is the negative electrode material of lithium-ion batteries with the highest known specific capacity (4200mAh/g). However, due to its huge volume effect (>300%), the silicon electrode material will be pulverized and peeled off from the current collector during the charge and discharge process, causing the active materials to lose electrical contact with each other and between the active materials and the current collector. At the same time, a new solid electrolyte layer SEI is continuously formed, which eventually leads to the deterioration of electrochemical performance. In recent years, researchers have done a lot of research and exploration to try to solve these problems and have achieved certain results. This article describes the research progress in this field and proposes further research directions and application prospects.

Compared with traditional graphite negative electrodes, silicon has an ultra-high theoretical specific capacity (4200mAh/g) and a lower delithiation potential (<0.5V), and the voltage platform of silicon is slightly higher than that of graphite. It is difficult to cause surface lithium precipitation during charging, and has better safety performance. Silicon has become one of the potential choices for upgrading the carbon-based negative electrode of lithium-ion batteries.

However, silicon also has disadvantages as a negative electrode material for lithium-ion batteries. Silicon is a semiconductor material with low electrical conductivity. During the electrochemical cycle, the insertion and extraction of lithium ions will cause the volume of the material to expand and contract by more than 300%. The mechanical force generated will gradually pulverize the material, causing structural collapse, and eventually leading to the separation of the electrode active material from the current collector, loss of electrical contact, and greatly reduced battery cycle performance. In addition, due to this volume effect, it is difficult for silicon to form a stable solid electrolyte interface (SEI) film in the electrolyte. Along with the destruction of the electrode structure, a new SEI film is continuously formed on the exposed silicon surface, exacerbating the corrosion and capacity decay of silicon.

Silicon lithium deintercalation mechanism and capacity decay mechanism

Silicon does not have the layered structure of graphite-based materials. Its lithium storage mechanism is the same as other metals, which is carried out through alloying and dealloying with lithium ions. Its charge and discharge electrode reaction can be written as follows:

Si+xLi++xe-=LixSi

Research progress and application prospects of silicon-based 18650 rechargeable battery lithium 3.7v 3500mah negative electrode materials

Research progress and application prospects of silicon-based 18650 rechargeable battery lithium 3.7v 3500mah negative electrode materials

Figure 1 Schematic diagram of silicon-based lithium-ion battery: (a) charging; (b) discharging

During the process of alloying and dealloying with lithium ions, the structure of silicon will undergo a series of changes, and the structural transformation and stability of silicon-lithium alloys are directly related to the transport of electrons.

Based on the lithium deintercalation mechanism of silicon, we can summarize the capacity decay mechanism of silicon as follows:

(1) During the first discharge process, as the voltage decreases, a core-shell structure with lithium-intercalated silicon and unintercalated crystalline silicon coexisting is first formed. As the depth of lithium intercalation increases, lithium ions react with the internal crystalline silicon to form silicon-lithium alloys, which eventually exist in the form of Li15Si4 alloys. During this process, the volume of silicon increases by about 3 times compared to the original state. The huge volume effect causes the structural destruction of the silicon electrode, the loss of electrical contact between the active material and the current collector, and the loss of electrical contact between the active material and the active material. The lithium ion deintercalation process cannot proceed smoothly, resulting in a huge irreversible capacity.

(2) The huge volume effect will also affect the formation of SEI. As the lithium deintercalation process proceeds, the SEI on the silicon surface will break and re-form as the volume expands, 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 also cause the impedance of the electrode to increase continuously during the charging and discharging process, hindering the electrical contact between the current collector and the active material, increasing the diffusion distance of lithium ions, hindering the smooth deintercalation of lithium ions, and causing rapid capacity decay. At the same time, the thicker SEI will also 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 be directly in contact with the electrolyte and lose capacity.

In order to improve the cycle performance of silicon-based negative electrodes and enhance the structural stability of materials during the cycle, silicon materials are usually nano-sized and composited. At present, the main research directions of silicon material nano-size include: silicon nanoparticles (zero-dimensional nano-size), silicon nanowires/tubes (one-dimensional nano-size), silicon thin films (two-dimensional nano-size) and 3D porous silicon, hollow porous silicon (three-dimensional nano-size); the main research directions of silicon material composites include: silicon/metal composites, silicon/carbon composites and ternary composites (such as silicon/amorphous carbon/graphite ternary composite systems).

Selection and structural design of silicon materials

1. Amorphous silicon and silicon oxides

(1) Amorphous silicon

Amorphous silicon (a-Si), also known as non-crystalline silicon, is an allotrope of silicon. Crystalline silicon is usually arranged in a regular tetrahedron, with each silicon atom located at the vertex of the regular tetrahedron and tightly bound to the other four silicon atoms by covalent bonds.

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-based electrode materials. However, amorphous silicon materials can only alleviate particle breakage and pulverization to a limited extent, and their cycle stability still cannot meet the requirements of high-capacity battery negative electrode materials.

Research progress and application prospects of silicon-based negative electrode materials for lithium 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 insertion and extraction potential. Therefore, it is also a very promising high-capacity lithium-ion battery negative electrode material. However, the different oxygen content of silicon oxide will also affect its stability and reversible capacity: as the oxygen content in silicon oxide increases, the cycle performance improves, but the reversible capacity The amount is reduced.

Research progress and application prospects of silicon-based 18650 rechargeable battery lithium 3.7v 3500mah negative electrode materials

In addition, there are some problems with silicon oxide as a negative electrode material for lithium-ion batteries: since the formation process of Li2O and lithium silicate during the first lithium insertion process is irreversible, the initial coulomb efficiency is very low; at the same time, Li2O and lithium silicate have poor conductivity, resulting in poor electrochemical kinetics, and thus poor rate performance; 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 is still very poor.

2. Low-dimensional silicon materials

Low-dimensional silicon materials have a larger surface area at the same mass, which is conducive to the full contact between the material and the current collector and the electrolyte, reducing the stress and strain caused by the uneven diffusion of lithium ions, and improving the yield of the material The strength and anti-powdering ability enable the electrode to withstand greater stress and deformation without crushing, 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) Silicon nanoparticles

Both silicon nanoparticles and three-dimensional porous silicon can inhibit the volume effect of the material to a certain extent, while also reducing the diffusion distance of lithium ions and increasing the electrochemical reaction rate. However, their specific surface areas are large, which increases the direct contact with the electrolyte, resulting in increased side reactions and irreversible capacity, and reducing coulombic efficiency. In addition, silicon active particles are easy to agglomerate during the charge and discharge process, resulting in "electrochemical sintering" and accelerating capacity decay.

Compared with micron silicon, the use of nanoparticles The electrochemical performance of electrode materials made of silicon with a diameter of 100 nm has been significantly improved, both in terms of initial charge and discharge specific capacity and cycle capacity.

Although nano-silicon particles have better electrochemical properties than micro-silicon particles, when the size is reduced to below 100 nm, the silicon active particles are easily agglomerated during the charge and discharge process, which accelerates the attenuation of capacity. In addition, the larger specific surface area allows 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 combined with other materials (such as carbon materials) for lithium-ion battery negative electrode materials.

(2) Silicon film

During the lithium insertion and removal process of silicon film, lithium ions tend to proceed in a direction perpendicular to the film, so the volume expansion of silicon film also mainly proceeds in the normal direction. Compared with bulk silicon, the use of 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 added to lithium-ion batteries as an electrode 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 lithium ion insertion and removal process is suppressed. Compared with micron-scale silicon thin films, nanoscale silicon thin film negative electrode materials show better electrochemical performance.

(3) Silicon nanowires and nanotubes

Silicon nanowires/tubes can reduce the radial volume change during charge and discharge, achieve good cycle stability, and provide a fast lithium ion transmission channel in the axial direction. However, it will reduce the tap density of silicon materials, resulting in a decrease in the volume specific capacity of silicon negative electrodes. Silicon thin films can reduce the volume change in the direction perpendicular to the film and maintain the structural integrity of the electrode. However, after multiple cycles, silicon thin films are prone to breakage and detachment from the substrate, and the preparation cost of silicon thin films is high.

At present, the methods reported for synthesizing silicon nanowires in large quantities mainly include laser ablation, chemical vapor deposition, thermal evaporation, and direct growth on silicon substrates.

Due to their unique hollow structure, silicon nanotubes have better electrochemical performance than silicon nanowires. Compared with silicon particles, silicon nanowires/nanotubes do not have obvious lateral volume effects during lithium insertion and extraction, and they will not be crushed and lose electrical contact like nano silicon particles, so they have better cycle stability. Due to the small diameter, lithium can be inserted and removed more quickly and thoroughly, so 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 charge and discharge process, making the material present a higher Coulomb efficiency.

3. Porous silicon and hollow structured silicon

Research progress and application prospects of silicon-based 18650 rechargeable battery lithium 3.7v 3500mah negative electrode materials

(1) Porous structured silicon

The appropriate pore structure can not only promote the rapid insertion and removal of lithium ions in the material and 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 structures include template method, etching method and magnesium thermal reduction method.

In recent years, the method of preparing silicon-based materials by magnesium thermal reduction of silicon oxide has attracted widespread attention from researchers. In addition to using spherical silicon oxide as a precursor, silicon oxide molecular sieve is a commonly used method for preparing porous silicon materials because it has a porous structure. Commonly used silicon oxide precursors 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

Hollow structure is another effective way to improve the electrochemical performance of silicon-based materials. At present, the method for preparing hollow silicon is mainly the template method. Although hollow silicon has excellent electrochemical performance, its preparation cost is still very high, and it also has 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 silicon, the carbon as the eggshell can also improve the conductivity of the material. Therefore, the carbon-silicon composite material with yolk-shell structure has better cycle stability and higher reversible capacity.

Preparation of silicon-based composite materials

1. Silicon-metal composite materials

The metal component in the silicon/metal composite can improve the electronic conductivity of the material, reduce the polarization of the silicon material, and improve the rate performance of the silicon material. The ductility of metals can inhibit the volume effect of silicon materials to a certain extent and improve the cycle performance, but the silicon structural defects generated during the preparation process have high electrochemical activity, which will lead to an increase in irreversible capacity. Moreover, the composite of silicon and metal cannot avoid direct contact between active silicon and electrolyte, resulting in unstable SEI film, which leads to reduced battery cycle performance.

When metals are composited with silicon, metals can play a certain supporting role, preventing silicon volume expansion during the insertion and extraction of lithium ions and reducing the degree of pulverization. After metals and silicon form an alloy, the free energy of lithium insertion is lower, making the lithium insertion process easier. At the same time, the excellent conductivity of metals can improve the kinetic properties of silicon alloy materials. Therefore, the composite of metals and silicon can effectively improve the electrochemical properties of silicon-based composite materials.

Although Si-active metals have a higher specific capacity, the cycle performance is poor because the active metals themselves will also pulverize. In Si-inactive metal composites, inactive metals are inert phases, which greatly reduces the reversible capacity of silicon materials, but the stability will be slightly improved accordingly. When Si is mixed with active metals and inactive metals to form a composite, a silicon-based electrode material with good stability and high capacity can be prepared by utilizing the synergistic effect.

2. Silicon-carbon composite materials

In silicon/carbon composites, carbon materials have higher electronic and ionic conductivity, which can improve the rate performance of silicon-based materials and inhibit the volume effect of silicon during the cycle. In addition, carbon materials can block direct contact between silicon and electrolyte, reducing irreversible capacity. However, the disadvantage is that the interface contact between silicon and carbon materials is poor, and it is difficult to completely and uniformly coat the inner wall of the nanoscale pores of silicon materials with carbon.

As a negative electrode material for lithium-ion batteries, carbon materials have small volume changes during charging and discharging, good cycle stability and excellent conductivity, so they are often used to be composited with silicon. In carbon-silicon composite negative electrode 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. 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 gel and graphene.

(1) Silicon-graphite/mesophase carbon microsphere composites

Graphite has excellent electrical conductivity. When combined with silicon, it can improve the poor electrical conductivity of silicon-based materials. Under room temperature conditions, silicon and graphite are very chemically stable and it is difficult to generate strong forces. 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 asphalt-based organic compounds. They have excellent electrochemical cycle characteristics and are now widely used in commercial 18650 rechargeable battery lithium 3.7v 3500mah negative electrode materials. Similar to graphite, combining mesophase asphalt carbon microspheres with silicon can also improve the electrochemical performance of silicon-based materials.

(2) Silicon-carbon black composites

Carbon black has excellent electrical conductivity. Researchers have also tried to combine carbon black with silicon for lithium-ion battery negative electrode materials. Scientists obtained a conductive network structure by treating carbon black at high temperature, deposited silicon and amorphous carbon in turn, and then used a granulator to obtain a silicon-carbon composite material with a size of 15~30μm. It has high reversible capacity and good cycle stability.

(3) Silicon-carbon nanotube/wire composite material

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 composite materials 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. Using chemical vapor depositionCarbon 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 fibers and carbon tubes.

(4) Silicon-carbon gel composites

Carbon gel is a nanoporous carbon material prepared by the 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 carbon gel, the first irreversible capacity of the silicon-carbon gel composite is very large. At the same time, the nano-silicon in the organic gel 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 the silicon-based material and affect the electrochemical properties of the material.

(5) Silicon-graphene composites

Graphene has the advantages of good flexibility, high aspect ratio, excellent conductivity and stable chemical properties. Good flexibility makes graphene easy to compound with active substances to obtain composite materials with coated or layered structures, and can effectively buffer the volume effect during charging and discharging. Compared with amorphous carbon, two-dimensional graphene has better conductivity and can ensure good electrical contact between silicon and silicon, and between silicon and current collector. Graphene itself is also an excellent energy storage material. When it is combined with silicon, the cycle stability and reversible capacity of silicon-based materials can be significantly improved. Currently, the commonly used methods for preparing silicon-graphene composite materials 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 composite materials

In the research of silicon-compound composites, the main matrices include TIB2, TIN, TIC, SiC, TIO2, Si3N and other materials. The commonly used preparation method for 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 negative electrode material, but because the matrix does not undergo lithium insertion and extraction reaction, the reversible capacity of this type of material is generally very low.

(2) Silicon conductive polymer composites

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 active materials and current collectors. Commonly used conductive polymers include polypyrrole, polyaniline, etc.

Optimization of electrode preparation process

1. Electrode treatment

In addition to improving the stability and reversible capacity of silicon-based negative electrode materials by preparing silicon and silicon-based composite electrodes with different morphological structures as mentioned above, researchers also achieve the same goal by heat treating the electrodes.

Scientists use polyvinylidene fluoride as a binder and find that heat treatment can make the binder more evenly distributed in the electrode and enhance the adhesion between silicon and current collector. In addition, using PVDF as a binder, it is coated on a copper electrode with nano-silicon in a certain ratio. Rapid heat treatment at 900°C for 20 minutes can directly obtain a carbon-coated silicon electrode with high coulombic efficiency, large charge and discharge capacity, and good cycle performance.

2. Selection of current collector

The huge volume change of silicon causes self-crushing, which will cause the active material to fall off the current collector, resulting in poor cycle stability. One of the modification methods is to enhance the interaction between the current collector and silicon to maintain good electrical contact. The interaction between the current collector and silicon with a rough surface is better, so the use of porous metal current collectors is an effective way to improve the electrochemical performance of silicon-based negative electrode materials. In addition, the preparation of thin-film silicon and silicon-based composite materials can save the current collector and be directly used as lithium-ion battery negative electrode materials, thereby avoiding the problem of silicon-based materials falling off the current collector and losing electrical contact due to the huge volume effect.

3. Selection of binder

When preparing general lithium-ion battery electrode materials, active materials, binders and conductive agents such as carbon black are usually mixed into a slurry in a certain proportion and then applied to the current collector. Due to the huge volume effect, the traditional binder PVDF cannot adapt to silicon electrodes well. Therefore, the electrochemical performance of silicon-based materials can be effectively improved by using a binder that can adapt to the huge volume effect of silicon. In recent years, researchers have done a lot of research on silicon-based material binders. Commonly used silicon-based binders mainly include carboxymethyl cellulose, polyacrylic acid, alginate, and corresponding sodium salts. In addition, researchers have also studied and designed polyamide, polyvinyl alcohol, polyfluorene polymers and binders with self-healing properties.

4. Selection of electrolyte

The composition of the electrolyte affects the formation of SEI, and then affects the electrochemical properties of the negative electrode material. In order to form a uniform and stable SEI, researchers improve the electrochemical properties of silicon-based materials by adding electrolyte additives. The additives currently used include lithium bis(oxalatoborate), lithium difluorooxalatoborate, propylene carbonate, succinic acid, vinylene carbonate, fluoroethylene carbonate, etc., among which vinylene carbonate and fluoroethylene carbonate have the best effect.

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