Is the cathode material of silicon-based lithium ion battery promising?

Silicon is given a known (4200 mAh/g) of the highest lithium ion batteries anode materials, but because of its huge volume effect (> 300%), silicon electrode materials in the process of charging and discharging will pulverization and peeling from the collection of fluid, make active substances and active substances, loss of electrical contact between the active materials, at the same time constantly forming new solid electrolyte layer SEI, eventually lead to the deterioration of the electrochemical properties. In recent years, the researchers have done a lot of research and exploration, and try to solve these problems and achieved some results, this paper stated the research progress in the field, and further research direction and application prospect is put forward.

Take off the intercalated-li mechanism of silicon and capacity attenuation mechanism

Silicon has no graphite base material layer structure, its lithium storage mechanism and other metals, is by alloying and alloying with lithium ion, the charge and discharge electrode reaction can write down the type:

Si + xLi++ xe -?LiXSi

In alloy with lithium ion and alloying process, the structure of silicon will through a series of changes, and silicon lithium alloy structural transformation and stability is directly related to the electronic delivery.

According to the principle of take off the intercalated-li of silicon, we can reduce capacity attenuation mechanism of silicon is as follows:

(1) in the process of discharge for the first time, as the voltage drop, forming first intercalated-li silicon embedded with lithium and amorphous silicon core-shell structure of two phase coexistence. With the increase of embedded depth of lithium, lithium lithium ions react with internal crystalline silicon to produce the silicon alloy, with a final Li15Si4 exists in the form of alloy. Compared to the original state in the process of the silicon volume change about 3 times, huge volume effect result in a silicon electrode structure damage, and active materials' loss of electrical contact between active material and the active material, lithium ion of embedded process cannot go smoothly, cause huge irreversible capacity.

(2) the huge volume effect will also affect the formation of the SEI, as the intercalated-li process, the surface of a silicon SEI will burst again with the volume expansion form, makes the SEI thicker.

Due to the formation of SEI consumes lithium ion, thus causing the large irreversible capacity. SEI poor conductivity but also makes the impedance of the electrode with the increasing of charging and discharging processes, hinder the collection of fluid and electrical contact of active material, increase the lithium ion diffusion distance, hinder the smooth lithium ion embedded, causing rapid attenuation of capacity. Thicker SEI also can cause large mechanical stress, cause further damage to the electrode structure.

(3) unstable SEI layer also makes the silicon and silicon lithium alloy and direct contact with the electrolyte loss, thereby causing loss to the capacity.

Silicon material selection and structure design

1. The amorphous silicon and silicon oxide

(1) the amorphous silicon

Amorphous silicon has higher capacity under low potential, as lithium ion battery anode materials "compared with graphite electrode material higher safety performance. But amorphous silicon can only in a limited extent of crushing and powder particles, the cycle stability still cannot meet the requirements of as high capacity battery cathode material.

(2) silicon oxide

As lithium ion battery cathode material, the SiO has high theoretical specific capacity (more than 1200 mAh/g), the cycle of good performance and low embedded lithium electricity and so is also a kind of potential high capacity lithium ion battery cathode materials. But different oxidation silicon oxygen can also affect the stability and the reversible capacity: with the improvement of oxygen in silicon oxide, cycle performance improvement, but the irreversible capacity.

In addition, the silicon oxide as lithium ion battery cathode material also has some problems: because the first intercalated-li Li2O and lithium silicate during the forming process is irreversible, makes the coulomb efficiency is very low for the first time;Li2O and lithium silicate conductivity difference at the same time, the electrochemical kinetics performance is poorer, therefore its performance ratio difference; Compared to the elemental silicon and silicon oxide as cathode material of cycle stability is better, but as cycles continue to increase, its stability is still very poor.

2. Low dimensional silicon materials

Low dimension of silicon material under the same quality have larger surface area, good materials and set a good contact with fluid and electrolyte, reduce caused by uneven lithium ion diffusion of stress and strain, improve the yield strength of material and ability to resist pulverization, enables the electrode to withstand greater stress and deformation and not crushed, and then get higher reversible capacity and cycle stability better. At the same time, the larger specific surface area can withstand the higher current density per unit area, so low dimensional silicon ratio performance is also better.

(1) the silica Nanoparticles

Compared with the micro silicon, using Nano particle size of silicon electrode materials, the electrochemical performance in both the first charge and discharge specific capacity circulation capacity, have been improved obviously.

Although Nano silicon particles relative to the micro electrochemical properties of silicon particles have better, but when have fallen to below 100 nm size, activity of silicon particle is easy to occur in the process of charging and discharging, and accelerate the capacity attenuation, and larger specific surface makes the silicon Nanoparticles have more contact with electrolyte, forming more SEI so its electrochemical properties have no fundamental improvement. So often Nanometer silicon and other materials (such as carbon materials) composite anode materials for lithium ion batteries.

(2) silicon thin film

In the process of the embedded on the take-off of the silicon thin film lithium, lithium ion tend to be along the direction perpendicular to the film, thus silicon thin film volume expansion is mainly along the normal direction. Compared to the massive tripoli, the use of silicon thin film can effectively restrain the silicon volume effect. Is different from other forms of silicon, silicon thin film don't need, the binder can be used as electrode in lithium ion batteries directly for testing. The thickness of the silicon thin films had a great influence on the electrochemical performance of electrode materials, with the increase of thickness, lithium ion of embedded process is restrained. Compared to the micron grade silicon thin film, Nanoscale silicon thin film cathode materials showed better electrochemical performance.

(3) silicon Nanowires, and Nanotubes

At present, have been reported to a large number of synthetic methods mainly include silicon Nanowires laser ablation method, chemical vapor deposition method, thermal evaporation method and the silicon substrate directly growth method, etc.

Silicon Nanotubes because of its unique hollow structure, compared to the silicon Nanowires have better electrochemical performance. Silicon Nanowires/Nanotubes compared to silicon particles, in the process of take off the intercalated-li transverse volume effect is not obvious, and will not happen like Nano silicon particles crushing loss of electrical contact, and cycle stability is better. Due to their small diameter, take off the intercalated-li faster and more thoroughly, and the reversible specific capacity is also very high. Silicon Nanometer tube outer surface of the larger freedom is a good way to meet the volume expansion of radial, formed in the process of charging and discharging of a more stable SEI, makes the material presents high coulomb efficiency.

3. Porous silicon and silicon hollow structure

(1) porous silicon structure

Appropriate pore structure can not only promote the lithium ion quick to take off the embedded in the material, increase the ratio of material performance, but also able to buffer the electrode in the process of charging and discharging volume effect, thus improve the cycle stability. In the preparation of porous silicon material, adding carbon materials can improve the conductivity of silicon and sustain electrode structure, further improve the electrochemical performance of materials. Preparation methods of porous silicon structure with template method, etching method and thermal reduction process of magnesium.

In recent years, the preparation of silicon-based materials magnesium thermal reduction oxidation silicon method has attracted wide attention of the researchers. In addition to using the spherical silicon oxide as the precursor in vitro, silica molecular sieve due to the porous structure, so it is a common method for preparation of porous silicon material. Commonly used silica precursors are mainly SBA - 15, MCM - 41, etc. Due to the conductivity of silicon is poor, often after the thermal reduction of magnesium in the surface of the porous silicon coated a layer of amorphous carbon.

(2) the hollow structure of silicon

Hollow structure is another way to effectively improve the electrochemical properties of silicon material via, the preparation of hollow silicon at present mainly as a template. Despite the hollow silicon electrochemical performance is excellent, but the preparation cost is still high at present, and there is a problem of poor conductivity were the same. By designing the egg yolk shell (yolk - shell) structure and control the size of the space between the egg yolk and egg, silicon in effective buffer volume expansion at the same time, as the shell of carbon can also increase the electrical conductivity of the material, thus has the egg yolk egg cycle stability of the structure of the silicon carbon composite material is better, the reversible capacity is also higher.

The preparation of silica based composite materials

1.Silicon metal composite materials

The metal and silicon compound, metal can play a supporting role, prevent the silicon in the process of lithium ions embedded out volume expansion reduce the degree of pulverization. Metal and silicon alloy after formation, lower free energy of the intercalated-li, thus make the intercalated-li process easier. Metal excellent electrical conductivity at the same time, can improve the dynamic performance of silicon alloy materials. So silicon metal and composite can effectively improve the electrochemical performance of silicon-based composite material.

Si - active metal high specific capacity, but due to the active metal itself also can appear pulverization phenomenon, thus cycle performance is poor. And Si - central African active metal inert metal composite material is inert phase, thus can greatly reduce the irreversible capacity of silicon, but stability will slightly improve accordingly. And when the Si and active metal and the active metal mixed together to form the complex, use of synergies, preparation can get good stability, and the silicon electrode materials of high capacity.

2. Silicon carbon composite material

Carbon materials for lithium ion battery cathode material in the process of charging and discharging volume change is small, has a good cycle stability performance and excellent electrical conductivity, therefore it is often used to composite with silicon. In silicon carbon composite anode materials, depending on the type of carbon materials can be divided into two categories: silicon with traditional silicon and new materials and carbon composite material. The traditional carbon materials mainly include graphite, carbon black, mesophase microspheres and amorphous carbon. New carbon materials including carbon Nanotubes and graphene, carbon Nanotubes, carbon gel, etc.

(1)/silica ink mesophase carbon microspheres composite materials

Graphite has excellent electrical conductivity, and silicon compound can improve the problem of poor conductivity of silicon material itself.Silicon and graphite under the condition of normal temperature, chemical stability strong, and it is difficult to have a strong reaction, and high energy ball mill and chemical vapor deposition is often is used for the preparation of silica composite materials.

Is pitch mesophase carbon microspheres organic compounds through thermal polycondensation reaction in liquid phase and carbonized form a micron grade the graphitization of carbon materials, the excellent electrochemical cycle characteristics, has been widely applied in the business lithium battery anode materials. Similar to graphite, mesophase pitch carbon microspheres with silicon compound can also improve the electrochemical performance of silicon material extremely.

(2) silicon carbon black composites

Carbon black have excellent electrical conductivity, the researchers will also attempt to carbon black and silica composite anode materials for lithium ion batteries. Scientists obtained by high temperature processing carbon black conductive network structure, successively deposited silicon and amorphous carbon, and then use comminutor get size in 15 ~ 30 microns silicon carbon composite material. The high irreversible capacity and good cycle stability.

(3) silicon carbon Nanotube composite materials/line

One of preparation methods of carbon fiber as the electrostatic spinning method, by adding silicon source to the selection of precursor, can get silicon carbon fiber composite materials. By direct mixing or chemical synthetic method can also get silicon carbon Nanotube composite materials/line. Carbon Nanotubes/line is often treated as the second substrate, conductive role as conductive network.

In addition, the chemical vapor deposition method is a kind of preparation methods of Nanowires and Nanotubes. Using chemical vapor deposition method can grow in the silicon surface direct carbon fiber or tubes, silicon can be directly deposit growth on the surface of carbon fiber tubes.

(4) silicon carbon composite gel

Carbon gel is a kind of through the sol/gel prepared Nano porous carbon materials. Before the carbonized carbon gel internal organic aerogels Nano network structure, has a wealth of holes and continuous three dimensional conductive network, have the effect of silicon buffer volume expansion. Therefore, because of the large specific surface area of carbon gel silicon carbon composite material for the first time the irreversible capacity is very large. While the Nano silicon in coking process of organic gel, amorphous SiOX and easily decomposed into Si and SiO2, the presence of SiO2 can reduce the irreversible capacity of silicon-based materials, influence the electrochemical properties of material.

(5) silica ink composite materials

Graphene has good flexible, high aspect ratio, excellent electrical conductivity and chemical advantages of stable performance. Good flexible make graphene is easy to composite with active substances with coated or laminated structure of composite material, and can effectively buffer the volume effect in the process of charging and discharging. Compared with amorphous carbon, two dimensional graphene has more excellent electrical conductivity, can guarantee between silicon and silicon, silicon fluid and set a good electrical contact. And graphene itself is also a kind of excellent energy storage material, with the silicon compound, can significantly improve the cycle stability of silicon-based materials and reversible capacity. The common method to preparation of silica ink olefin composites are mainly simple mixing method, extraction method, chemical vapor deposition method, operation freeze drying, spray method and self-assembly method, etc.

3. Other silicon-based composite materials

A composite material (1) silicon compounds

Research on silicon - compound type compound, as the main substrate of TiB2, TiN, TiC, SiC, TiO2, Si3N material, etc.This kind of compound method of commonly used preparation methods for high energy ball mill, this kind of silicon-based materials circulation stability is better than pure silicon anode materials, but because of not to take off the intercalated-li reaction matrix, and the reversible capacity of this kind of material is generally low.

(2) silicon conductive polymer composites

Conducting polymers due to their good electrical conductivity, flexible degree of good, and the advantages of easy for structural design, not only can the buffer volume effect of silicon material, also can maintain good electrical contact with the active materials. Commonly used of conductive polymer polypyrrole, polyaniline and so on.

The optimization of electrode preparation technology

1. Electrode processing

In addition to the mentioned above through the preparation of different morphological structure of silicon and silicon composite electrode to improve the stability of the silicon-based anode materials and reversible capacity, the researchers also through the study of the heat treatment of electrode achieve the same purpose.

Scientists using polyvinylidene fluoride as binder, found that heat treatment can make glue more uniform distribution in the electrode, and enhance the adhesion force between collectors and silicon. In addition, based on PVDF binder agent, with the Nanometer silicon with the copper electrode with a certain proportion, fast heat treatment under 900 ℃ for 20 min can be directly coated silicon carbon electrodes, coulomb efficiency is high, the charge and discharge capacity, cycle performance is good.

2. Set the choice of fluid

Silicon huge volume change caused itself, makes fell off from the active materials, thus causing poor circulation stability. By strengthening force between fluid and silicon, maintain its good electrical contact is one of the modification of the method. Rough set fluid and silicon on the surface of the effect is better, so using porous metal set fluid is a kind of effective way to improve electrochemical properties silicon-based anode materials. In addition, the preparation of thin film of silicon and silicon based composite materials can save set fluid, anode materials for lithium ion batteries directly, avoiding the silicon-based materials due to the huge volume effect from the collection of fluid loss of loss of electrical contact problem.

3. The choice of the binder

In the preparation of general lithium-ion battery electrode materials, usually to active substances, adhesives and carbon black conductive agent such as mixed according to certain proportion into the slurry spread on the collection of fluid. Due to the huge volume effect, the traditional adhesives PVDF and can better adapt to the silicon electrode. Therefore, through the use of can adapt to the silicon huge volume effect of the bonding agent can effectively improve silicon-based materials electrochemical performance. In recent years, researchers in silicon base material to do a lot of research on the binder, commonly used silica based adhesives are carboxymethyl cellulose, polyacrylic acid, sodium alginate, and the corresponding salt, etc.Researchers also of polyamide, polyvinyl alcohol, poly fluorene type polymer and has self-healing properties of adhesives are studied and designed.

4. The choice of the electrolyte

The composition of the electrolyte affects the formation of SEI, which affects the electrochemical performance of anode materials. In order to form a uniform and stable SEI researcher, add electrolyte additive to improve the electrochemical performance of silicon-based materials. Currently used additives have double oxalic acid boric acid lithium, two fluorine, propylene carbonate, succinic acid, oxalic acid boric acid lithium and ethylene carbonate, fluorinated ethylene carbonate, etc., which works best for the ethylene carbonate and fluoro ethylene carbonate.

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