Carbon materials are still the protagonist, six types of lithium-ion battery carbon anode materials inventory

With the development of lithium-ion battery technology, many new materials have appeared in the field of lithium battery anode materials, such as silicon-based materials, tin-based materials, metal oxide/sulfide materials, etc. These new anode materials often have high energy density, but there is still a certain distance from large-scale commercial application. At present, carbon materials, especially graphite, are still the mainstream of anode materials for lithium-ion batteries.

There are many kinds of carbon anode materials, mainly including natural graphite, artificial graphite, hard carbon, soft carbon, carbon nanotubes, graphene, etc.

Natural graphite

Graphite is composed of parallel carbon hexagons and is a crystal with a lamellar structure. The atoms are sp2 hybridized to form bonds, and the molecular layers are connected by van der Waals forces, and each layer of electrons forms bonds to make it highly conductive. Due to the low lithium intercalation potential of lithium in graphite, the graphite interlayer spacing is much larger than the lithium ion radius, which promotes the intercalation of lithium ions into and out of the interlayer to form LixC6 compounds, so it becomes an ideal negative carbon material for lithium ion batteries.

Natural graphite can be divided into flake graphite and earthy graphite, and flake graphite is usually used as the anode material. The theoretical specific capacity of natural graphite is low, the uneven surface properties lead to the exfoliation of graphite sheets, the first cycle efficiency is low, and the charge-discharge cycle performance is poor. Therefore, it is necessary to modify natural graphite, and the common methods are: surface treatment method, surface coating method, doping modification method, etc.

Artificial graphite

Artificial graphite is a graphite material obtained by calcining easily graphitizable carbon (petroleum coke, needle coke, pitch, etc.) at a certain temperature, and then pulverizing, forming, classifying, and graphitizing at high temperature. Artificial graphite avoids the surface defects of natural graphite, but there are still problems such as poor rate performance, poor low temperature performance, and easy lithium precipitation due to crystal anisotropy. The modification method of artificial graphite is different from that of natural graphite. Generally, the purpose of reducing the degree of orientation of graphite grains (OI value) is achieved through the reorganization of particle structure. Usually, needle coke precursors with a diameter of 8-10 μm are selected, and easily graphitized materials such as pitch are used as the carbon source of the binder. After being treated in a drum furnace, several needle coke particles are bonded together, and the particle size D50 range is 14. -18μm secondary particles complete graphitization, effectively reducing the OI value of the material.

Mesocarbon microspheres (MCMB) and graphitized carbon fibers (GF) are typical artificial graphites.

①Mesocarbon microspheres

The microstructure of mesocarbon microspheres (MCMB) is spherical lamellar particles. When the pitch compounds are heat treated, thermal polycondensation occurs to form anisotropic mesophase spheres. The micron-scale spherical carbon material formed by separating the mesophase pellets from the pitch precursor is called mesophase carbon microspheres. The mesophase carbon microsphere anode has the performance advantages of high electrode compaction density and fast charge-discharge at large current in lithium-ion batteries. However, the repeated insertion and extraction of carbon atoms at the edge of mesocarbon microspheres by Li+ easily leads to the peeling and deformation of the carbon layer, which leads to capacity decay. The surface coating process can effectively suppress the peeling phenomenon. At present, most of the research on mesocarbon microspheres focuses on surface modification, compounding with other materials, and surface coating.

②Graphitized carbon fiber

Graphitized carbon fiber is mainly obtained by high temperature treatment of phenolic resin, polyacrylonitrile, mesophase pitch fiber, etc. This carbon material has good lithium storage reversibility, the first Coulombic efficiency is as high as 97%, and the lithium ion diffusion coefficient is about an order of magnitude higher than that of natural graphite, but its reversible capacity is lower than that of natural graphite. The diameter of graphitized carbon fibers is generally 200-500 nm, with a coaxial structure similar to tree rings, and the graphitized lamellae have high orientation. Some researchers prepared graphitized carbon fibers by heat treatment at 2200 °C. The specific capacity of lithium intercalation for the first time was 350.5mAh/g, but the irreversible capacity was as high as 202.4mAh/g for the first time. Such a high first irreversible capacity is mainly due to the large specific surface area of the fibers, which is caused by side reactions with the electrolyte. Compared with mesocarbon microspheres and natural graphite, the production cost of graphitized carbon fibers is high, and there are not many studies on anode materials for lithium-ion batteries. 

Hard carbon

Hard carbon is carbon that is difficult to graphitize, and is usually obtained by thermal cracking of polymer materials. Hard carbon has attracted great interest due to its high capacity, low cost and excellent cycle performance. Common hard carbons include resin carbon (such as phenolic resin, epoxy resin and polyfurfuryl alcohol PFA-C, etc.), organic polymer pyrolysis carbon (such as PFA, PVC, PVDF and PAN, etc.) and carbon black (acetylene black), etc. SONY company developed a lithium-ion battery in 1991 using hard carbon obtained by thermal cracking of polyfurfuryl alcohol (PFA) as a anode material. However, its irreversible capacity is too large, and the discharge voltage is too high, resulting in a hysteresis of the discharge-charge curve. Hard carbon materials have the advantages of high specific capacity, long service life, good rate performance and low production cost when used as the anode of lithium-ion batteries, but they also have  shortcomings like large irreversible capacity for the first time, obvious voltage hysteresis effect and low tap density, so the commercialization process is more difficult. 

Soft carbon

Soft carbon, also known as easily graphitizable carbon material, refers to an amorphous carbon material that can be graphitized at high temperatures above 2500°C. Generally speaking, according to the difference of the sintering temperature of the precursor, soft carbon will produce 3 different crystal structures, namely amorphous structure, turbulent layer disordered structure and graphite structure, which is also the common artificial graphite. Among them, the amorphous structure has received extensive attention due to its low crystallinity, large interlayer spacing, and good compatibility with electrolytes, so it has excellent low-temperature performance and good rate performance. Soft carbon has high irreversible capacity, low output voltage, and no obvious charge-discharge platform during the first charge and discharge. Therefore, it is generally not used as anode material independently, but is usually used as a anode coating material or component. 

Carbon nanotubes

Carbon nanotubes are a special carbon material with a relatively complete graphitized structure, which has the characteristics of unique structure (one-dimensional cylindrical tube of graphite sheet), low density, high rigidity, high tensile strength and high electrical conductivity. The reversible capacity of carbon nanotubes ranges from 300 to 600 mAh/g, which is higher than that of graphite. The morphology of carbon nanotubes allows it to replace graphite as an anode material for commercial lithium-ion batteries. Carbon nanotubes are materials "with a graphite-like layered structure" composed of single or multiple coaxial carbon sheets. The sp2 hybrid structure and high aspect ratio of carbon nanotubes bring a series of excellent properties. This microstructure enables lithium ions to have a small embedded depth, short stroke and many embedded positions (gap, holes, etc. in the tube and between layers), and at the same time, carbon nanotubes have excellent abilities like good electrical conductivity, good electron conduction and ion transport. It is suitable as a anode material for lithium ion batteries.

The use of carbon nanotubes directly as anode materials for lithium-ion batteries also has shortcomings. First, the first irreversible capacity is large, and the first charge and discharge efficiency is relatively low. Second, the carbon nanotube anode lacks a stable voltage platform. Third, there is potential hysteresis in carbon nanotubes. These problems restrict the application of carbon nanotubes in lithium-ion battery anode materials.

At present, the research of carbon nanotubes mainly focuses on the preparation of composite materials and their electrochemical properties, such as the composite of carbon nanotubes with silicon and metal oxides. In addition, as a new type of material, carbon nanotubes have many requirements for qualified production, such as diameter, number of layers, length, degree of defects and electronic properties are all important factors, and their production methods also need to be further improved. 

Graphene

Graphene is a rapidly rising star in the fields of materials science and condensed matter physics. This two-dimensional material has extremely high crystalline and electronic qualities, and although its development history is not long, it already has potential applications. Compared with other carbon-based anode materials, graphene can effectively adsorb lithium ions on both sides of the sheet, and expand the lithium storage capacity, which can reach twice that of graphite, and its irregularly arranged micropores can also enhance lithium storage capacity. Moreover, graphene has excellent mechanical strength, charge mobility, electrical conductivity and other properties. Its unique high flexibility and long diameter also make it potential as a anode for lithium-ion batteries.

However, the performance of graphene directly as a anode material for lithium-ion batteries is unstable, and there are some disadvantages: single-layer graphene sheets are easy to accumulate, and the reduction of specific surface area makes it lose part of the high lithium storage space; The first coulombic efficiency is low; Initial capacity decay Iin fast way; Voltage plateau and voltage lag, etc. Graphene can be composited with a variety of materials as a anode for lithium-ion batteries. For example, when it is composited with silicon-based and tin-based nanoparticle materials, it can play a complementary role and optimize the performance of anode materials.

Summary

Since the development of carbon materials for lithium-ion batteries, graphite materials have always been the mainstream anode materials due to their special microstructure, mature production and modification processes, and large raw material reserves, and will continue to be for a long period of time. However, with the iteration of technology and the in-depth research and development of high-energy-density anode materials, some new non-carbon anode materials are also advancing towards commercial application. In the future, the graphite-based pattern in the field of lithium battery anode materials may soon be broken, which is worth looking forward to.