Power battery anode material in the battery cost, the anode material accounts for about 5%-15%, is one of the important raw materials of lithium ion battery. The global sales volume of lithium battery anode materials is about 100,000 tons, and will continue to grow in the future. At present, the world's lithium battery anode materials are still mainly natural/artificial graphite, new cathode materials such as meso-phase carbon microspheres (MCMB), lithium titanate, silicon-based anode, HC/SC, lithium metal are also growing rapidly.
An overview of
The anode material is the carrier of lithium ions and electrons in the battery charging process, and plays the role of energy storage and release. The anode material accounts for about 5%-15% of the battery cost and is one of the important raw materials for lithium-ion batteries.
The global sales volume of lithium battery anode materials is about 100,000 tons, mainly produced in China and Japan. According to the current growth trend of new energy vehicles, the demand for anode materials will also show a continuous growth state. At present, the world's lithium battery anode materials are still mainly natural/artificial graphite, new cathode materials such as meso-phase carbon microspheres (MCMB), lithium titanate, silicon-based anode, HC/SC, lithium metal are also growing rapidly.
As the carrier of lithium ion implantation, the anode material should meet the following requirements:
The REDOX potential of lithium ions inserted into the anode matrix is as low as possible, close to the potential of metal lithium, so that the input voltage of the battery is high.
A large amount of lithium can be inserted and de-intercalated reversibly in the matrix to obtain high capacity.
The main structure of the negative electrode has little or no change during insertion/de-embedding.
The REDOX potential should change as little as possible with the insertion and stripping of Li, so that the voltage of the battery will not change significantly, and a relatively stable charge and discharge can be maintained.
The inserted compound should have better electronic conductivity and ionic conductivity, which can reduce polarization and charge and discharge with high current.
The main material has good surface structure and can form good SEI with liquid electrolyte.
The inserted compounds have good chemical stability in the whole voltage range and do not react with electrolytes after forming SEI.
Lithium ion in the main material has a large diffusion coefficient, easy to charge and discharge quickly;
From the practical point of view, the material should have good economy and friendly to the environment.
Two carbon type anode material
The following figure shows the classification of common carbon anode materials.
2.1 Graphite negative electrode
Graphite, English name graphite, graphite is soft, smooth feeling, is a kind of non-metallic minerals, with high temperature resistance, oxidation resistance, corrosion resistance, thermal shock resistance, strength, good toughness, high self-lubrication strength, heat conduction, electrical conductivity and other unique physical and chemical properties.
Graphite has many excellent properties, so it is widely used in metallurgy, machinery, electrical, chemical, textile, national defense and other industrial sectors, such as graphite mold, graphite electrode, graphite refractory, graphite lubricating materials, graphite sealing materials and so on. China is the country with the most abundant graphite reserves in the world, as well as the largest producer and exporter, and occupies an important position in the world graphite industry.
The ideal graphite has a layered structure. The layer consists of SP2 carbon atoms forming a huge plane similar to benzene ring. The carbon atoms between the plane are connected by δ bonds with a bond length of 0.142nm and a bond Angle of 120°. The layers also have a large π bond that connects all the carbon atoms. The interlayer is 0.3354nm. Two crystal types: hexagonal system -2H (A) and rhombic system -3R(b) ø The two crystal types can be interchangeable: grinding and heating.
Lithium embedding mechanism of graphite
Theoretical capacity of graphite 372mAh/g, of course, only a very high degree of graphitization materials can achieve this value. However, all carbon materials have irreversible capacity loss due to side reaction when they are first charged and discharged. As the negative electrode potential decreases, it stops until the components in the electrolyte form a stable passivation film (SEI) on the negative electrode surface. Four voltage platforms appear in the first discharge (as shown below), where A is the formation of SEI, and most of the graphite capacity is in the range of 0.3~0.005V. In addition to A, different voltage platforms correspond to different lithium embedment states, which are respectively called fourth-order and third-order compounds... Finally, LiC6 was formed, with theoretical capacity of 372mAh/g and crystal plane spacing of 0.37. (From books and periodicals, don't spray if you don't understand!)
The arrangement pattern of LiC6 graphite ink sheet in the fully lithium inserted state changes (as shown in the figure below). Convert to AAAA... Arrangement. Some artificial graphite is difficult to change the arrangement and the capacity is low.
Graphite is mainly divided into natural graphite and artificial graphite, natural graphite needs to be treated in some ways to be used as the anode of lithium ion battery, such as our common oxidation treatment, mechanical grinding and so on. Artificial graphite is from organic matter (gaseous, liquid, solid) into graphite, the specific operation can be baidu.
So much, of course, because he used the broadest. Of course, as a cathode material, graphite also has many shortcomings, such as low potential of graphite, and electrolyte interface film, and easy to cause lithium; The ion migration rate is slow, so the charge and discharge rate is low. About 10% deformation of layered graphite will occur during lithium ion insertion and deembedding, which will affect the cycle life of the battery.
2.2 Non-graphite anode
As shown above, the non-graphite anode consists mainly of hard and soft carbons.
Soft carbon, also known as easily graphitized carbon, refers to the amorphous carbon that can be graphitized above 2000℃, with low crystallinity, small grain size, large crystal plane spacing and good compatibility with electrolyte. But the first charge and discharge irreversible capacity is high, the output voltage is low, because of his performance, generally not directly do anode materials, is the manufacture of natural graphite raw materials, common petroleum coke, needle coke and so on.
Hard carbon, also difficult to graphitization carbon, is the pyrolytic carbon of polymer, this kind of carbon is difficult to graphitization at 3000℃. Hard carbon resin carbon (such as phenolic resin, epoxy resin, polyfurfuryl alcohol, etc.), organic polymer pyrolytic carbon (PVA,PVC,PVDF,PAN, etc.), carbon black (acetylene black); It is conducive to lithium embedding without causing significant expansion of the structure, and has good charge-discharge cycle performance.
Hard carbon capacity is greater than the theoretical capacity of conventional carbon materials, high rate, cycling performance, safety performance is excellent, but the first effect is low, about 85%, voltage platform 3.6V is lower than graphite 3.7V, high cost. The improvement idea is mainly to improve the first effect (reduce specific surface area, form more regular hard carbon; Surface coating, control SEI formation); Improve material yield and reduce cost.
Compared with conventional graphite anode materials, HC has a more stable structure.
Trisilicon base anode material
As the cathode material with the highest theoretical gram capacity, silicon has a very broad prospect. Successful application will improve the energy density of batteries by an order of magnitude.
As can be seen from the figure above, the theoretical capacity of silicon is as high as 4200mAh/g, which is more than ten times that of graphite of 372mAh/g. The concept of this figure must be clear to everyone, and it will be possible to achieve 1000km on a charge.
The voltage platform of silicon is a little higher than that of graphite. The advantage of this is that the possibility of lithium separation when charging is not big, and the safety performance is much better than that of graphite. Silicon, one of the most abundant elements in the earth's crust, is widely available and inexpensive.
Friends, don't think we first said the current gram capacity of the highest anode material will not continue to see the back of the. This thing is so good, but it is not used on a large scale, must have its own special defects.
Before we talk about defects, let's talk about its charging and discharging mechanism:
The charge-discharge mechanism of silicon is different from that of graphite. Graphite is the embedding and de-embedding of lithium, while silicon is the alloying reaction.
Silicon's biggest drawback is that it expands in volume.
In the process of charge and discharge, the delithium reaction of silicon will be accompanied by a large volume change (>300%), resulting in the destruction of material structure and mechanical pulverization, resulting in the separation between electrode materials and electrode materials and collector fluid, thus losing electrical contact, resulting in rapid capacity attenuation and deterioration of cycle performance. Due to the intense volume effect, SEI film on silicon surface is in a dynamic process of destruction-reconstruction, which will cause continuous consumption of lithium ions and further affect the cycling performance.
It is also because of its 300% volume expansion, the current commercial application is limited. It is said that the solution to the problem is always accompanied by the emergence of the problem. Currently, the methods to solve the silicon charge and discharge expansion are nano-silicon, porous silicon and silicon matrix composite materials. The synergistic effect of various components of composite materials is utilized to achieve the purpose of complementary advantages. Silicon and carbon composite materials are an important research direction, including coating type, embedded type and dispersed type.
By preparing silicon nanowire, all silicon is utilized and expansion space is reserved, which can effectively improve the cycling performance. However, this method has high cost, complex process and difficulty in preparation.
Porous silicon also improves cycling performance by reserving space for silicon expansion. But the compacting density is small, the technological process is complex, the preparation is difficult. (It looks a little dense...)
Silicon/carbon composite materials, mainly carbon cladding, as shown in the figure below, although the expansion space is reserved and the cycling performance is improved, the compaction density is low and the industrialization is difficult.
Tetralithium metal anode material
Lithium metal, is one of the lowest density of metals, standard electrode potential -3.04V, theoretical specific capacity of 3860mAh/g, from this data, second only to silicon 4200mAh/g. Lithium sulfur battery (2600Wh /kg), lithium air battery (11680WH /kg), etc.
Lithium metal battery has a high capacity performance, but in use, due to the existence of lithium dendrite, negative electrode precipitation, negative electrode side reaction phenomenon, seriously affect the safety of the battery, so the present stage is in the conceptual stage.
The structure diagram and equation of lithium sulfur battery are as follows. Sulfur is also a very extensive element in nature. The high energy density of lithium sulfur battery (2600Wh /kg) may be the focus of the research and development of the next generation of lithium sulfur battery.
The structure diagram and reaction equation of lithium air battery are as follows. Lithium air battery has a high energy density (11680Wh /kg), which is close to the energy density of fuel oil. It is environmentally friendly and the reaction product is water.
Lithium pentatitanate anode material
Lithium titanate, spinel structure, potential platform 1.5V, THREE-DIMENSIONAL ion diffusion channel, stable lattice, theoretical capacity 176mAh/g. The material has the characteristics of high safety, high magnification and long life.
High safety, just as we said, voltage platform 1.5V, lithium free, overcharge and overdischarge resistance, high and low temperature performance is excellent.
High magnification, presumably graphite has a higher ionic diffusion coefficient, 25℃ lithium ion in lithium titanate diffusion coefficient (2*10^-8cm2/s) is an order of magnitude higher than graphite.
Long service life, due to its stable lattice, stable structure, zero strain, little volume change in the charging and discharging process, no SEI film formation, no negative impact caused by SEI film damage.
The preparation methods include solid phase reaction, sol-gel method and hydrothermal ion exchange method. By ball milling Li2CO3 and TiO2 in proportion (Li :Ti about 0.84) and doping Zr, the carbon black was added to improve the conductivity. The preparation temperature is about 800-1000℃. Generally, the longer the time, the more complete the lattice growth.
In fact, it can be seen that, although compared with graphite, it has a higher ionic diffusion rate, high safety and long life, but its poor conductivity requires carbon coating and doping modification; High potential, with high potential cathode material can only form 2.4-2.6V voltage, need to reduce the potential of lithium titanate (metal replace part of Ti); Theoretical capacity is low, 176mAh/g relative to graphite 372mAh/g, there is no advantage in capacity.
