Research progress of natural graphite anode materials

Lithium-ion battery is a new type of high-energy battery with lithium intercalation compound as the positive and negative electrode materials. Because of its high specific capacity, high working voltage, low self-discharge, good safety performance, and long charge-discharge cycle life, it has a series of advantages. Consumers’ favor has now been applied in large-scale energy storage, power tools, and portable electronic devices. At present, in terms of investment or production and application, the development speed of lithium batteries has surpassed traditional rechargeable batteries such as lead-acid batteries.

The rapid development of lithium-ion batteries is mainly due to the contribution of electrode materials, especially the research of carbon anode materials. There are many types of carbon anode materials. According to different types of raw materials and preparation processes, their structure, lithium ion intercalation and deintercalation mechanism and electrochemical performance will also be quite different.

The carbon anode materials that have been studied so far include graphitized carbon (natural flake graphite, graphitized mesophase carbon microspheres, etc.) and non-graphitized carbon (soft carbon, hard carbon, etc.). Among them, graphite has the advantages of low charge and discharge voltage platform, high cycle stability and low cost, and is considered to be an ideal negative electrode material in current lithium-ion battery applications. At present, the modification research of natural graphite has made certain progress and has been commercialized.

Graphite negative electrodes generally use natural flake graphite, but there are several shortcomings:

(1) The flake graphite powder has a large specific surface area, which has a greater impact on the first charge and discharge efficiency of the negative electrode;

(2) The layer structure of graphite determines that Li+ can only be embedded from the end surface of the material and gradually diffuse into the inside of the particles. Due to the anisotropy of flake graphite, the Li+ diffusion path is long and uneven, resulting in a low specific capacity;

(3) The layer spacing of graphite is small, which increases the diffusion resistance of Li+, and the rate performance is poor. Li+ is easy to deposit and form lithium dendrites on the graphite surface during fast charging, causing serious safety hazards.

In order to solve the above inherent shortcomings of flake graphite, spheroidization, surface treatment and doping modification of graphite are required.

1 Sphericalization

Aiming at the problem of low specific capacity of the negative electrode of lithium ion battery caused by the anisotropy of flake graphite, the morphology of flake graphite should be modified to achieve the isotropic effect as much as possible.

The production of spherical graphite has been industrialized. In industrial production, wind impact shaping machines are mainly used to spheroidize flake graphite. Among them, the airflow vortex pulverizer is a commonly used equipment. This method has less impurities during the spheroidization process, but its equipment is large in size, the amount of graphite is large, and the yield is low, which is very limited in laboratory preparation. In recent years, some scholars have used small rotary impact mills for laboratory preparation. By analyzing the changes in porosity during the spheroidization process, they found that the increase in energy during the spheroidization process increased the open porosity of graphite particles and reduced their closed porosity. , Which will affect its electrochemical performance.

In addition to the above-mentioned dry grinding, some scholars also use agitated grinding wet grinding method, using water as a medium, adding carboxymethyl cellulose as a dispersant to prevent graphite particles from agglomerating in the water. Graphite particles are effectively de-angularized; after the product is classified by cyclone and sedimentation, particles with a narrow size distribution are obtained. Research shows that after spheroidization and classification, its reversible capacity is significantly increased by about 20 mAh/g.

In addition to shaping the graphite particles themselves, the ultra-fine graphite powder can also be bonded into a spherical shape through a binder. The graphite balls prepared by this method have excellent isotropy. In recent years, some scholars have used glucose as an amorphous carbon precursor and binder, and spray-dried to effectively adhere nano-silicon particles and graphite particles together, and agglomerate ultrafine graphite particles into regular spheres, so that the specific capacity can reach 600mAh/ Above g, the capacity loss of silicon during charging and discharging is overcome to a certain extent, and the capacity retention rate after 100 cycles is ≥90%.

Wu et al. used the viscosity of polyvinyl alcohol to bond and dry the ultrafine graphite powder into isotropic regular spherical particles by spray drying. Due to the tiny pores between the fine graphite, the cycle stability was increased. After 105 cycles After the specific capacity is still maintained at 367mAh/g, but also due to the existence of micropores, the first efficiency is lower than 77%; after adding carbon citrate coating, the first efficiency is increased to 80%. This method does not have high requirements on the morphology of the graphite raw material, and the isotropy of the formed particles is good. It has a more stable cycle performance than graphite powder and a specific capacity closer to 372mAh/g.

By spheroidizing flake graphite, the specific capacity (≥350mAh/g), first cycle efficiency (≥85%) and cycle performance of the negative electrode material can be significantly improved. As a negative electrode material for lithium-ion batteries, the particle size d50 is most suitable between 16 and 18 μm. If the particle size is too small, the specific surface area is larger, causing the negative electrode to consume a large amount of Li+ during the first cycle, thereby forming a solid dielectric interfacial film, making the first charge and discharge efficiency low; The small liquid contact area affects the specific capacity of the negative electrode.

2 Surface treatment

2.1 Change the pore structure

The surface pore structure of graphite is an important factor that determines the ability of batteries to insert lithium. The presence of micropores on the surface of the graphite material can increase the diffusion channel of Li+ and reduce the diffusion resistance of Li+, thereby effectively improving the rate performance of the material.

Chang et al. placed the graphite in a strong alkali (KOH) aqueous solution for etching, and then annealed it at 800°C in a nitrogen atmosphere to produce nanopores on the surface. These nano-pores can be used as the entrance of Li+, so that Li+ can not only enter from the end surface of the graphite, but also can be embedded from the base surface, shortening the migration path. After testing, charging and discharging at a rate of 3C, the KOH-etched graphite anode has a capacity retention rate of 93%, which is higher than that of the original graphite (85%); at a rate of 6C, a capacity retention rate of 74% can be achieved.

Shim et al. compared the capacity retention rates of raw graphite, KOH etched-annealed graphite, and KOH etched graphite at 80°C, and proved that the capacity retention rate of etched graphite at 80°C is the best. The reason for this is that high-temperature annealing destroys the crystal structure. Through impedance analysis, after 50 cycles, the Li+ diffusion resistance of the etched graphite is only 60% of the original graphite, which further explains the optimization of its rate performance. Some scholars also use vapor deposition to grow high-conductivity carbon nanotubes on the graphite surface in situ, so that the initial charge and discharge efficiency of graphite is> 95%, and the capacity retention rate after 528 cycles is> 92%. It can be seen that the optimization of the pore structure of the graphite surface can increase the diffusion channel of Li+ and reduce the diffusion resistance of Li+, which is an effective means to improve the rate performance and cycle stability of graphite.

2. 2 Surface oxidation

Oxidation can eliminate the disordered carbon atoms on the surface of natural graphite, so that the oxidation-reduction reaction on the graphite surface can proceed uniformly. At the same time, functional groups such as -COO- and -OH are formed on the surface of oxidized natural graphite. These functional groups are bound to the surface of natural graphite in the form of covalent bonds. During charge and discharge cycles, a chemically bonded and stable SEI film is formed on the surface of natural graphite, thereby improving The first charge-discharge efficiency of natural graphite and the cycle life of graphite are improved. The oxidant generally chooses O2, HNO3 and H2O2.

Oxidation using gas phase oxidant generally requires high temperature treatment to repair the surface defects of graphite particles. Shim et al. used air as the oxidant to oxidize natural graphite at 550°C. The study found that the weight loss during the oxidation process is linearly related to the reduction in specific surface area; after oxidation, the surface diameter of natural graphite is 40～400A. The surface area is significantly reduced, and its cycle performance and first charge-discharge efficiency are improved, but its reversible capacity and rate performance remain unchanged. In addition, there are also relatively weak oxidizing gases such as H2O and CO2 added to inert gases to oxidize graphite at high temperatures. Experiments have found that the introduction of Ni, Co, Fe and other catalysts in the oxidation process can improve the effect of oxidation treatment, and Li can also form alloys with metals used as oxidation catalysts, and these alloys can also help increase reversible capacity.

The use of strong oxidizing liquid reagents can oxidize graphite at a lower temperature. Generally, graphite particles are subjected to surface micro-oxidation or micro-swelling. Wu et al. used a variety of oxidants (ammonium persulfate, H2O2, cerium sulfate, etc.) to oxidize the graphite anode materials, and observed nano-pores on the surface of graphite particles through high-resolution transmission electron microscopy, which is a reversible increase in the capacity of micro-oxide graphite Provide a basis.

Mao et al. prepared micro-oxidized graphite with K2FeO4 as the oxidant, which eliminated the disordered part of the graphite surface, and introduced nanopores and some Fe elements to increase the reversible capacity of graphite from 244mAh/g to 363mAh/g. In addition, some people use oxidizers and intercalants to micro-expand the graphite, which widens the lithium-intercalation pores and improves the lithium-intercalation capacity and rate performance.

Zuou et al. used H2O2 oxidant and concentrated sulfuric acid as intercalating agent to prepare micro-expanded graphite; then phenolic resin was used as a precursor for carbon coating, so that the specific capacity of the negative electrode material reached 378 mAh/g, and after 100 cycles of charge and discharge , The capacity retention rate is 100%. It can be seen that after micro-expansion and carbon-coated composite modification treatment, the cycle performance of the composite material is greatly improved compared with natural flake graphite and coated natural flake graphite.

The oxidation treatment of graphite is mainly to remove disordered carbon atoms on the surface of graphite or increase nanopores, broaden the path of Li+ insertion and release, which can effectively improve the rate performance and cycle stability of the negative electrode material, and the effect of improving the contrast capacity is not large. This function is the same Changing the pore structure of the graphite surface is the same.

2. 3 Surface fluorination

Fluorinated graphite is prepared by fluorinating the surface of natural graphite. Through fluorination treatment, a CF structure is formed on the surface of natural graphite, which can strengthen the structural stability of graphite and prevent the graphite flakes from falling off during the cycle. At the same time, the surface fluorination of natural graphite can also reduce the resistance in the Li+ diffusion process, increase the specific capacity, and improve its charge and discharge performance.

Wu et al. used argon gas containing 5% fluorine gas to fluorinate natural graphite at 550°C. After five cycles, the coulombic efficiency increased from 66% to 93%, and the specific capacity was also above the theoretical specific capacity of graphite. Matsumotto et al. used ClF3 to process natural graphite with different particle sizes. After the treatment, it was found that there were F and Cl elements on the graphite surface, and the smaller particle size of natural graphite was smaller than the surface. Through the charge and discharge test, the first charge and discharge efficiency of all samples was Increased by 5% to 26%.

Yin et al. synthesized a series of polythiophene/graphite fluoride composite materials by polymerizing thiophene monomer in situ on the surface of fluorinated graphite as raw material, and found that the PTh coating containing 22.94% can discharge at a high rate of 4C with energy The density can reach 1707Wh/kg, which is higher than that of natural graphite materials.

Through the fluorination treatment of graphite, the rate performance and cycle performance are effectively improved, but the specific capacity is not greatly improved; after the fluorinated graphite is modified again, the specific capacity can be effectively improved.

2. 4 Coating modification

The coating modification uses graphite-like carbon material as the "core", and a layer of amorphous carbon material or a "shell" of metal and its oxide is coated on its surface to form particles with a "core-shell" structure. The precursors of commonly used amorphous carbon materials include low-temperature pyrolytic carbon materials such as phenolic resin, pitch, and citric acid. The metal materials are generally metal elements with good conductivity such as Ag and Cu. The layer spacing of amorphous carbon materials is larger than that of graphite, which can improve the diffusion performance of Li+, which is equivalent to forming a Li+ buffer layer on the outer surface of graphite, thereby improving the high-current charge and discharge performance of graphite materials; metal elements can be enhanced The conductivity of the negative electrode material enhances its charge and discharge performance at low temperatures.

The method of using pitch as the precursor of amorphous carbon has been relatively mature and has been mentioned in the thesis many times. In recent years, Han et al. studied the influence of different components of coal tar pitch (CTP) and different softening points on the electrochemical performance of graphite anodes. Studies have shown that charging and discharging at 5°C, coated with hexane insolubles and toluene solubles in CTP, can maintain a specific capacity of 263mAh/g at 5C; and the higher the CTP softening point, the higher the specific capacity of the material. The specific capacity of the CTP-material with a softening point of 196°C can reach 278 mAh/g, and the charge transfer resistance also decreases as the softening point increases.

Wu et al. mixed the phenolic resin and spherical graphite in methanol, evaporated the solvent and annealed at a high temperature in an inert atmosphere; through grinding and sieving, the surface of the graphite particles obtained was smoother, which increased its cycle stability, and after 5 cycles Its specific capacity is 172 mAh/g higher than that of graphite raw material. In addition to pitch and phenolic resin, some scholars have also done research on citric acid as an amorphous carbon precursor in recent years.

The composite of graphite, metal and metal oxide is mainly realized by depositing on the surface of graphite. The metal coating layer can not only improve the electronic conductivity of graphite, but also Sn and its oxides and alloys can be used as a matrix material for lithium storage, which has a synergistic effect with graphite to further optimize the electrochemical performance of the anode. Using NaH to reduce SnCl2 or SnCl4 in n-butanol to deposit a layer of nano-Sn on the surface of graphite, a stable specific capacity of 400-500 mAh/g can be obtained. The deposition of metals such as Ag and Cu generally uses electroplating, and the resulting metal layer is smooth and uniform. In addition, the silver mirror reaction is also a simple and effective method of forming a silver coating.

Carbon coating is an effective method for optimizing the electrochemical performance of graphite anodes, but its optimization effect is limited. It only has a partial optimization function in terms of cycle stability and first charge and discharge efficiency; metal coating only improves the conductivity and cycle stability of the anode material It has an enhanced effect on the charging and discharging performance at low temperature. Therefore, the two methods of carbon coating and metal coating cannot solve the inherent disadvantage of low specific capacity of graphite.

3 Doping modification

The doping modification method is more flexible and the doping elements are diverse. At present, researchers are more active in this method. Doping of non-carbon elements into graphite can change the electronic state of graphite, making it easier to get electrons, thereby further increasing the amount of Li+ embedded.

By pyrolyzing H3PO4 and H3BO3, Park et al. successfully doped P and B onto the graphite surface and formed chemical bonds with them, which effectively improved the cycle stability and rate performance of graphite. Because Si and Sn have the ability to store lithium, there have been more studies on the compound of these two elements with graphite. Park et al. added antimony-containing tin oxide particles to the graphite anode material. The antimony-containing tin oxide particles and graphite particles were connected by citric acid to increase the specific capacity of the anode material to 530 mAh/g. The specific capacity can be maintained after 50 cycles. 100%.

Chen et al. combined nano-silicon particles, pitch, and flake graphite through spray drying to obtain a specific capacity of 1141 mAh/g. At the same time, other researchers have mixed graphite, amorphous carbon material precursors and nano-Si in an organic solvent through ultrasound, stirring or ball milling, and then dried and annealed composite materials, which effectively increased the specific capacity of the negative electrode material. It confirms the synergy of Si and graphite.

Doping different elements in graphite materials has different optimization effects on its electrochemical performance. Among them, the addition of elements (Si, Sn) that also have the ability to store lithium has a significant effect on increasing the specific capacity of graphite anode materials, but due to the limitation of the specific capacity of graphite itself, the desired effect is still not achieved.

4 Conclusion

Spheroidization, changes in pore structure, oxidation modification, fluorination modification and coating modification can improve the initial charge and discharge efficiency of graphite-based anode materials, increase the diffusion rate of Li+ in anode materials, and optimize the rate performance of anode materials. The effect is significant in terms of cycle stability, but there is no obvious optimization effect in improving the specific capacity. Doping modification can fully combine materials with different lithium storage capabilities, exert their respective advantages, and significantly increase the specific capacity of the negative electrode material, but its rate performance and cycle stability will be reduced to a certain extent. Therefore, synergistic modification through a variety of methods, effective composite of graphite and Si or Sn elements, and solving the defects of poor cycle stability of composite materials, will become the focus of future research.

Source: New Chemical Materials

Author: When Zanghao Yu Jie Liu Lvxian Jun