Flake graphite based cathode materials for lithium batteries

Flake graphite is considered to be an ideal cathode material for lithium batteries because of its high stability, good conductivity, and wide source. However, the specific capacity and rate performance of natural graphite anode can not meet the needs of high-performance anode materials. In order to solve this problem, researchers have carried out a series of modification studies.

In this paper, the research progress of graphite negative electrode materials for lithium-ion batteries is described from the modification methods of graphite negative electrodes, and the advantages and disadvantages of various modification methods are pointed out. It is considered that collaborative modification through various methods is an effective method to comprehensively improve graphite negative electrode materials.

1.  Foreword

So far, graphitized carbon (natural flake graphite, graphitized mesophase carbon microspheres, etc.) and non-graphitized carbon (soft carbon, hard carbon, etc.) have been studied. Among them, graphite is considered to be an ideal cathode material for lithium-ion battery applications because of its low charge-discharge voltage platform, high cycle stability, and low cost. At present, the research on the modification of natural graphite has made some progress and has been commercialized.

Natural flake graphite is generally used as a graphite negative electrode, but it has the following disadvantages:

1. Flake graphite powder has a large specific surface area, which has a great influence on the first charge and discharges efficiency of the negative electrode;

2. The lamellar structure of graphite determines that Li + can only be embedded from the end face of the material and gradually diffuse into the particle. Due to the anisotropy of flake graphite, the diffusion path of Li + is long and uneven, resulting in its low specific capacity;

3. The layer spacing of graphite is small, which increases the diffusion resistance of Li +, and the rate performance is poor. During rapid charging, Li + is easy to deposit on the graphite surface to form lithium dendrites, resulting in serious potential safety hazards.

In order to solve the inherent shortcomings of the above flake graphite, it is necessary to modify the graphite and optimize the performance of the negative electrode material. At present, the modification methods mainly include spheroidization treatment, surface treatment, and doping modification.

2. Spheroidization treatment

In order to solve the problem of the low specific capacity of the negative electrode of the lithium-ion battery caused by the anisotropy of flake graphite, the morphology of flake graphite should be modified to achieve the effect of isotropy as much as possible.

The production of spherical graphite has been industrialized. In industrial production, a wind impact shaping machine is mainly used to spheroidize flake graphite. Among them, the airflow vortex mill is common equipment. This method has few impurities in the spheroidization process, but its equipment volume is large, the amount of graphite is large, and the yield is low, which is very limited in laboratory preparation.

In recent years, some scholars use small rotary impact mills for laboratory preparation. By analyzing the change of porosity in the spheroidization process, it is found that the increase of energy in the spheroidization process improves the opening rate of graphite particles and reduces their closed porosity, which will affect its electrochemical performance. In addition to the above dry grinding, some scholars also adopt the stirring mill wet grinding method, using water as the medium and adding carboxymethyl cellulose as the dispersant to prevent the agglomeration of graphite particles in water. This grinding method can effectively de angularize microcrystalline graphite particles; After the product is classified by cyclone and sedimentation, the particles with narrow particle size distribution are obtained. The research shows that after spheroidization and classification, the reversible capacity is significantly increased by about 20mah / g.

In addition to shaping the graphite particles themselves, the ultrafine graphite powder can also be bonded into a sphere through a binder. The graphite sphere prepared by this method has excellent isotropy. In recent years, some scholars have used glucose as amorphous carbon precursors and binders. Through spray drying, nano silicon particles and graphite particles effectively adhere together, and the ultrafine graphite particles are aggregated into regular spheres so that their specific capacity is above 600mAh/g. To a certain extent, the capacity loss of silicon in charge and discharge process is satisfied, and the capacity retention rate is over 90% after 100 cycles.

Wu et al. With the aid of spray drying, the superfine graphite powder can be dried into isotropic regular spherical particles by means of spray drying. The micropores of micro graphite increase their cycling stability. The specific capacity of the ultrafine graphite powder remains at 367mAh/g after 105 cycles, but the first efficiency is 77% because of the existence of micropores. After adding citric acid carbon coating, the efficiency was increased to 80% for the first time. This method has low requirements for the morphology of graphite raw materials, good isotropy of particles, more stable cycle performance than graphite powder, and closer to 372mah / g specific capacity.

The spheroidization of flake graphite can significantly improve the specific capacity (≥ 350mah / g), first cycle efficiency (≥ 85%), and cycle performance (capacity retention rate ≥ 80% after 500 cycles). As the cathode material of lithium-ion battery, its particle size D50 is 16 ~ 18 μ M is the most suitable. If the particles are too small, the specific surface area will be larger, so that the negative electrode will consume a lot of Li+ in the first cycle, thus forming a solid dielectric mask (SEI film), making the first charge and discharge efficiency low. If the particle size is too large, the specific surface area is small and the contact area with the electrolyte is small, which affects the specific capacity of the negative electrode.

3. Surface treatment

3.1 Change pore structure

The surface pore structure of graphite is an important factor determining the lithium intercalation capacity of batteries. The existence of micropores on the surface of graphite material can increase the diffusion channel of Li + and reduce the diffusion resistance of Li +, so as to effectively improve the rate performance of the material.

Cheng et al. Etched graphite in strong alkali (KOH) aqueous solution and annealed it at 800 ℃ in a nitrogen atmosphere to produce nanopores on its surface. These nanopores can be used as the inlet of Li +, so that Li + can not only enter from the graphite end face but also be embedded from the base surface, shortening the migration path. The capacity retention rate of graphite anode etched by KOH is 93%, which is higher than that of original graphite (85%); At the rate of 6C, the capacity retention rate of 74% can be achieved.

Shim et al. Compared the capacity retention rates of several negative electrode materials such as original graphite, Koh etched annealed graphite and KOH etched graphite at 80 ℃ and proved that the capacity retention rate of etched graphite at 80 ℃ is the best, followed by etched annealed graphite. The reason for this situation is that high-temperature annealing destroys the crystal structure. Through impedance analysis, after 50 cycles, the Li + diffusion resistance of etched graphite is only 60% of that of original graphite, which further explains the optimization of its magnification performance.

Some scholars also use vapor deposition to grow high conductivity carbon nanotubes on the surface of graphite in situ, so that the first charge-discharge efficiency of graphite is more than 95%, and the capacity retention rate after 528 cycles is more than 92%.

It can be seen that the optimization of pore structure on the surface of graphite 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.

3.2 Surface oxidation

Oxidation can eliminate the disordered carbon atoms on the surface of natural graphite so that the redox reaction on the surface of graphite can be carried out uniformly. At the same time, functional groups such as – COO – and – Oh are formed on the oxidized natural graphite surface. These functional groups are combined on the natural graphite surface in the form of a covalent bond. During the charge-discharge cycle, a chemically bonded stable SEI film is formed on the natural graphite surface, which improves the first charge-discharge efficiency of natural graphite and the cycle life of graphite. Oxidants are generally O2, HNO3, and H2O2.

The gas-phase oxidant is used for oxidation. Generally, high-temperature treatment is required to repair the surface defects of graphite particles. Shim et al. Oxidized natural graphite at 550 ℃ with air as an oxidant. It was found that the weight loss in the oxidation process was linear with the reduction of specific surface area; After oxidation, the surface area of pores with a surface diameter of 40 ~ 400A of natural graphite is significantly reduced, and its cycle performance and first charge-discharge efficiency are improved, but its reversible capacity and rate performance are not changed.

In addition, some inert gases with relatively weak oxidation properties such as H2O and CO2 are added to oxidize graphite at high temperatures. It is found that the introduction of Ni, Co, Fe, and other catalysts in the oxidation process can improve the oxidation treatment effect, and Li can also form alloys with metals used as oxidation catalysts. These alloys also help to improve the reversible capacity.

Graphite can be oxidized at low temperature by using liquid-phase reagents with strong oxidizability (such as H2O2, HNO3, etc.), generally micro oxidation or micro expansion treatment on the surface of graphite particles. Wu et al. Used a variety of oxidants (ammonium persulfate, H2O2, cerium sulfate, etc.) to oxidize the graphite cathode materials, and observed nanopores on the surface of graphite particles by high-resolution transmission electron microscopy (HRTEM), which provided a basis for the increase of reversible capacity of micro oxidized graphite.

Mao et al. Prepared micro graphite oxide with K2FeO4 as oxidant, eliminated the disordered part of the graphite surface, introduced nanopores and some Fe elements, and increased the reversible capacity of graphite from 244mah / g to 363mah / g.

In addition, some people used oxidants and intercalators to micro expand graphite, broaden the lithium intercalation channels, and improve the lithium intercalation capacity and rate performance. Zou et al. Prepared micro expanded graphite with H2O2 oxidant and concentrated sulfuric acid as intercalation agent; Then, phenolic resin was used as the precursor for carbon coating, so that the specific capacity of the negative electrode material reached 378mah / g, and the capacity retention rate was 100% after 100 charge-discharge cycles.

It can be seen that after micro expansion and carbon coating composite modification, the cyclic performance of the composite is greatly improved compared with natural flake graphite and coated natural flake graphite. The oxidation treatment of graphite is mainly to remove the disordered carbon atoms on the graphite surface or increase the nanopores, broaden the intercalation path of Li +, which can effectively improve the rate performance and cycle stability of the negative electrode material. The improvement effect of the contrast capacity is not great. This function is the same as changing the pore structure on the graphite surface.

3.3 Surface fluorination

Fluorinated graphite is prepared by fluorination on the surface of natural graphite. Through fluorination treatment, the C-F structure is formed on the surface of natural graphite, which can strengthen the structural stability of graphite and prevent the falling off of graphite sheets during circulation. At the same time, fluorination on the surface of natural graphite can also reduce the resistance in the process of Li + diffusion, improve the specific capacity, and improve its charge-discharge performance.

Wu et al. Fluorinated natural graphite with argon containing 5% fluorine at 550 ℃. After five cycles, the coulomb efficiency increased from 66% to 93%, and the specific capacity was also above the theoretical specific capacity of graphite. Matsumoto et al. Treated natural graphite with different particle sizes with clf3. After treatment, it was found that there were f and Cl elements on the graphite surface, and the specific surface of natural graphite with small particle size was reduced; Through the charge-discharge test, the first charge-discharge efficiency of all samples is increased by 5% ~ 26%.

Yin et al. Synthesized a series of polythiophene/graphite fluoride composites by in-situ polymerization of thiophene monomers on the surface of graphite fluoride. It was found that the PTH coating containing 22.94% can discharge at a high speed of 4C, and the energy density can reach 1707wh / kg, which is higher than that of natural graphite.

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

3.4 Coating modification

The coating modification takes graphite carbon material as the “core”, and a layer of an amorphous carbon material or metal and its oxide “shell” is coated on its surface to form particles similar to the “core-shell” structure. The precursors of amorphous carbon materials commonly used include low-temperature pyrolytic carbon materials such as phenolic resin, asphalt, and citric acid, and the metal materials are generally metal elements with good conductivity such as Ag and Cu.

The layer spacing of amorphous carbon material is larger than that of graphite, which can improve the diffusion performance of Li +, which is equivalent to forming a buffer layer of Li + on the outer surface of graphite, so as to improve the high current charge-discharge performance of graphite material; Metal elements can enhance the conductivity of negative electrode materials and their charge-discharge performance at low temperature. The method of asphalt as an amorphous carbon precursor has been relatively mature and has been mentioned many times in the dissertation.

In recent years, Han et al. Studied the effects of different components of coal tar pitch (CTP) (respectively dissolved in hexane, toluene, and tetrahydrofuran) and different softening points (20 ℃, 76 ℃, 145 ℃, and 196 ℃) on the electrochemical properties of graphite negative electrode. The results show that the specific capacity of 263mah / G can be maintained at 5C by charging and discharging at 5 ℃ and coating with hexane insoluble and toluene soluble in CTP; The higher the softening point of CTP, the higher the specific capacity of CTP material. The specific capacity of CTP material with a softening point of 196 ℃ can reach 278mah / g, and the charge transfer resistance decreases with the increase of softening point.

Wu et al. Mixed phenolic resin and spherical graphite in methanol evaporated the solvent and annealed at high temperature in an inert atmosphere; Through grinding and screening, the surface of graphite particles is smoother and its cycle stability is increased. After 5 cycles, its specific capacity is 172mah / g higher than that of graphite raw material. In addition to asphalt and phenolic resin, citric acid as an amorphous carbon precursor has been studied in recent years.

The composite of graphite, metal, and metal oxide is mainly realized by deposition on the surface of graphite. Metal coating can not only improve the electronic conductivity of graphite, such as Sn, its oxides, and alloys, but also act as the parent material of lithium storage, produce a synergistic effect with graphite, and further optimize the electrochemical performance of the negative electrode. When SnCl2 or SnCl4 is reduced in n-butanol with Nah, a layer of nano SN is deposited on the surface of graphite, and a stable specific capacity of 400 ~ 500mah / G can be obtained. Electroplating is generally used for the deposition of Ag, Cu, and other metals, and the resulting metal layer is smooth and uniform. In addition, silver mirror reaction is also a simple and effective method to form silver cladding.

Carbon coating is an effective method to optimize the electrochemical performance of graphite anode, but its optimization effect is limited, and it only has some optimization functions in cycle stability and first charge-discharge efficiency; Metal coating can only enhance the conductivity, cycle stability, and charge-discharge at low temperature. Therefore, carbon coating and metal coating can not solve the inherent disadvantage of the low specific capacity of graphite.

4. Doping modification

The doping modification method is flexible and has a variety of doped elements. At present, researchers are more active in the research of this method. Doping non-carbon elements into graphite can change the electronic state of graphite and make it easier to obtain electrons, so as to further increase the embedding amount of Li +.

Park et al. Successfully doped P and B on the surface of graphite by pyrolysis of H3PO4 and H3BO3 and formed chemical bonds with them, effectively improving the cycle stability and rate performance of graphite. Because Si and Sn have lithium storage capacity, there are many studies on the recombination of these two elements with graphite. Park et al. Added antimony containing tin oxide particles to the graphite negative electrode material. The antimony containing tin oxide particles are connected with the graphite particles through citric acid so that the specific capacity of the negative electrode material can be increased to 530mah / g, and the specific capacity can be maintained at 100% after 50 cycles.

Chen and other nano silicon particles, asphalt, and flake graphite were combined by spray drying to obtain the specific capacity of 1141mAh/g. At the same time, other researchers mixed graphite, amorphous carbon material precursors, and nano Si in organic solvents by ultrasound, stirring, or ball milling, and prepared composites by drying and annealing, which effectively improved the specific capacity of negative electrode materials and confirmed the synergistic effect of Si and graphite.

Doping different elements into graphite materials has different optimization effects on its electrochemical properties. Among them, the addition of elements (Si, Sn) with the same lithium storage capacity can significantly improve the specific capacity of graphite cathode material, but due to the limitation of the specific capacity of graphite itself, it still can not achieve the ideal effect.