1. What is hard carbon?
Hard carbon refers to refractory graphitized carbon, which is a kind of pyrolytic carbon obtained by pyrolyzing high molecular polymers, petrochemical products, biomass, etc. Due to the presence of a large number of heteroatoms such as H, O, and N in the precursor, the formation of crystalline regions during heat treatment is hindered, resulting in difficult graphitization at high temperatures above 2500 °C.
According to the different pyrolytic carbonization temperature, hard carbon materials can be divided into high temperature pyrolytic carbon between 1000-1400 ℃ and low temperature pyrolytic carbon between 500-1000 ℃. According to different carbon sources, it can be divided into resin carbon (such as phenolic resin, epoxy resin, polyfurfuryl alcohol resin, etc.), organic polymer carbon (such as PVA, PVC, PVDF, PAN, etc.), carbon black (acetylene black prepared by CVD method) etc.), biomass carbon (such as plant residues and shells, etc.), etc.
Hard carbon facilitates the intercalation of lithium without causing significant structural expansion and has good charge-discharge cycle performance. Hard carbons used as negative electrodes of lithium-ion batteries are mainly prepared from precursors such as pitch-based, biomass-based, and resin-based.
2. What are the characteristics of hard carbon
During the pyrolysis process of hard carbon, the carbon layer tends to grow in a plane, but the cross-linked structure in the macromolecules hinders its plane growth. Therefore, the carbon layer in hard carbon cannot grow infinitely into a graphite-like sheet structure. The carbon layer stacking structure can only appear in the short-range, and the long-range is disordered. The hard carbon structure is dominated by the amorphous part. Due to the disordered accumulation of some carbon layers, defects and holes appear, while the other part of the carbon layer has a graphite microcrystalline structure. These graphite microcrystals have no orientation and are cross-linked with each other. Some researchers believe that the molecular weight of the precursor has an impact on the microstructure of hard carbon formed after pyrolysis. With the increase of molecular weight, the degree of graphitization of hard carbon gradually increases, and the specific surface area gradually increases.
Compared with graphite, hard carbon has larger interlayer spacing and more micropores, correspondingly more lithium ions intercalation and desorption active sites for lithium storage, and has a larger specific capacity. Moreover, hard carbon has better compatibility with PC electrolyte, and is more suitable for working at low temperature. In addition, hard carbon also has the advantages of good high-rate charge-discharge performance and long cycle life.
At present, there is no unified conclusion on the lithium storage mechanism of hard carbon. Some researchers have proposed a “house of cards” structural model to explain the reason why hard carbon has a higher lithium intercalation capacity. The researchers used epoxy resin as carbon source and phthalic anhydride as curing agent, and obtained a series of hard carbon materials by controlling pyrolysis treatment conditions. According to the characterization results of X-ray diffraction technology and electrochemical tests, the researchers believe that the hard carbon material contains many nano-pores surrounded by single-layer graphene sheets, the diameter of which is only about 1.5 nm. These single-layer graphene sheets are composed of In a house-of-card configuration, Li+ adsorbs on the surface of these single-layer graphene sheets and can enter and exit them reversibly. As the number of single-layer graphene sheets increases, their lithium intercalation capacity also increases. Other researchers have studied the lithium storage mechanism of hard carbon by HX-PES analysis. The study found that lithium exists between carbon layers and in micropores. When the charge state (SOC<70%), lithium ions are inserted between the carbon layers; when the charge state (SOC>50%), the lithium particle clusters enter the micropores and are in a semi-metallic state. The micropore size is not the same, and the particle cluster size is not uniform.
Although the special structure of hard carbon enables it to have high capacity and good rate capability, it also has some disadvantages, such as low first-week Coulomb efficiency, low tap density and severe voltage polarization.
3. How to prepare hard carbon
The precursors for preparing hard carbon include pitch, biomass, sugar, phenolic resin, organic polymer, etc. Hard carbon materials prepared from different substances show similar charge-discharge curves.
①Preparation of hard carbon from asphalt
Pitch-based precursors are good precursors for hard carbon preparation due to their high carbon residue rate, wide range of raw material sources, and low prices. However, the preparation of hard carbon from pitch requires pretreatment, because the pitch is easily graphitized to form a graphitic-like structure during the carbonization process. Asphalt pretreatment is usually to crosslink the pitch with a crosslinking agent, change its microstructure, hinder the growth of graphite crystallites during the pyrolytic carbonization process, and perform a solid-phase carbonization process to obtain hard carbon materials; another The method of preparing the asphalt is the pre-oxidation method, that is, using an oxidant to pre-oxidize the asphalt to obtain a pre-oxidized asphalt with a certain oxygen content. Due to the presence of oxygen heteroatoms, the pitch is not easy to form an ordered structure during the pyrolytic carbonization process, resulting in a hard carbon material with a relatively chaotic microstructure.
② Biomass preparation of hard carbon
Biomass has a wide range of sources, is green and environmentally friendly, and itself has abundant heteroatoms and unique microstructures, which can be used as precursors for the preparation of hard carbons. Some researchers have used grapefruit peel as a carbon source to prepare hard carbon materials. They believe that the excellent lithium intercalation performance of the prepared samples is closely related to the unique pore structure of the material. This structure helps the material to be in full contact with the electrolyte, providing channels and more lithium intercalation sites for the transport of Li+ inside the material. Some researchers have also used peanut shells as carbon source to prepare hard carbon materials. They believe that under the same experimental conditions, the rich pore structure of the material can help to improve the lithium intercalation capacity and cycle stability of the material. Other researchers used corn stalks as carbon source to prepare porous carbon materials through KOH activated carbonization, and showed excellent rate and cycle performance. They believe that the existence of micropores makes the material have a large number of defect sites, which provide active sites for the storage of lithium ions and improve the lithium storage capacity, while the existence of mesopores and macropores shortens the transport distance of lithium ions and improves the the mobility of lithium ions.
③Organic polymer to prepare hard carbon
Compared with biomass, the molecular structure of organic macromolecular polymers is relatively simple and controllable, and the relevant molecular structure can be designed according to needs. It is an excellent precursor for the preparation of hard carbon. Some researchers use phenolic resin as a carbon precursor and obtain resin-based hard carbon materials through pyrolysis and carbonization, which are used as anode materials for lithium-ion batteries and electrode materials for supercapacitors. The capacity of lithium-ion batteries can reach 526mAh g- 1. The first Coulombic efficiency can reach 80%. They believe that the higher porosity is one of the reasons for the higher lithium intercalation capacity. Other researchers have prepared graphene-coated hard carbon materials using epoxy resin and graphene oxide as carbon sources. The electrochemical performance characterization found that the obtained material has higher lithium intercalation capacity and cycle stability than the samples without graphene coating. They believe that since the coating of graphene enhances the electrical conductivity of the material, the transport efficiency of Li+ and electrons inside the material is improved, resulting in better rate and cycle stability performance of the material.
The above is an introduction to the characteristics of hard carbon and its preparation. Hard carbon has a large number of microporous structures and a layered structure with a larger interlayer spacing than graphite, which enables rapid de-intercalation of lithium ions and excellent rate performance. Some hard carbon materials have higher lithium storage performance than traditional graphite anode materials. Therefore, hard carbon is also regarded as a promising anode material. With the advancement of technology and in-depth research, it is believed that the application of hard carbon materials in lithium battery negative electrodes will also develop its own world.