Surface modification of graphite materials can improve their surface structure and improve their electrical properties. The main methods are surface chemical treatment and surface coating. Surface chemical treatment includes surface oxidation, surface halogenation, etc., and surface coating can be divided into carbon coating, metal coating, metal oxide coating, polymer coating, etc. according to the composition of the coating layer. Other surface treatment methods include surface reduction and plasma treatment.
(1) Surface oxidation. Surface oxidation is divided into gas phase oxidation and liquid phase oxidation. Surface oxidation mainly has the following three functions: removing some active sites or defects in the graphitized carbon material, improving the surface structure; forming a low oxygen layer on the surface of the material, which can be used as an effective passivation layer; introduce some nanochannels or nanopores, the former facilitates the passage of lithium, and the latter can be used as lithium storage sites. The surface oxidized material can reduce its irreversible capacity on the one hand, and can also increase the reversible capacity of graphite, and on the other hand, it can improve its cycle performance. This is because the removal of an unstable surface and a highly reactive structure suppresses the decomposition of the electrolyte. At the same time, the presence of the surface oxide layer reduces the co-intercalation of solvent molecules and reduces the lithium consumption of the SEI film, which reduces the irreversible capacity and improves the cycle performance. The introduction of nanopores as lithium storage sites can increase the reversible capacity, and nanochannels are conducive to high-current discharge performance.
The gas phase oxidation method can use air as the oxidant, or use pure oxygen or CO2 to oxidize. Natural graphite is oxidized with air to increase its reversible capacity from 251mA·h/g to 350mA·h/g. The first charge and discharge efficiency is increased to more than 80%, and the capacity is not attenuated in the first 10 times. Someone used O2 and CO2 to oxidize the graphite surface respectively, which reduced the first irreversible capacity and improved the charging and discharging efficiency. However, when treated with O2 for a long time (above 60h), the surface area increases by 20%, and the capacity and efficiency are reduced.
The liquid-phase oxidation method uses a strong chemical oxidant solution such as (NH4)2S2O8, nitric acid, H2O2, Ce(SO4)2, etc. to react with graphite, which improves the electrochemical performance of the material. The standard oxidation potential of (NH4)2S2O8 solution is 2.08V, which has extremely strong oxidizing properties. With it as an oxidant, the reversible capacity of the treated graphite material is as high as 355mA·h/g; the oxidation of concentrated nitric acid is weaker than (NH4)2S2O8, its standard oxidation potential is 1.59V, and the capacity of carbon materials treated with it is also improved; the standard oxidation potentials of H2O2 and Ce(SO4)2 are between ((NH4)2S2O8 and concentrated nitric acid, 1.78V and 1.61V respectively; Ce(SO4)2 is a kind of salt, which has a corrosive effect on equipment.
In the gas phase oxidation method, the reaction occurs on the gas-solid interface, so the uniformity and reproducibility of the material is difficult to control, and it also generates gases such as CO and CO2, which are unfavorable to the environment. The reaction of the liquid-phase oxidation method takes place at the liquid-solid interface, so the contact is more sufficient and uniform, and the reaction is more uniform, thereby ensuring the uniformity of product quality.
(2) Surface halogenation. Chemical treatment on the surface of carbon materials, in addition to surface oxidation, there is also a halogenation treatment on the surface. Sanyo uses graphitized anode materials for surface fluorination, which can reduce self-discharge and increase the number of cycles. In addition, halogenation on the surface of the negative electrode material can reduce internal resistance, increase capacity, and improve charge and discharge performance. The American Vcarcarbon Technology company proposed that the modified graphite particles contain (2~50) 10-5 fluorine, chlorine or iodine.
(3) Carbon coated. As mentioned earlier, graphite materials and amorphous carbon materials have their own advantages and disadvantages. In order to make full use of the advantages of both and overcome their shortcomings, graphite can be coated with a layer of amorphous carbon material to form a core-shell structure (the shell content is generally less than 20%). In this way, it not only retains the characteristics of graphite's high capacity and low potential platform, but also has the characteristics of good compatibility of amorphous carbon materials with solvents and excellent high-current performance. A layer of amorphous carbon material is coated on the natural graphite, so that the amorphous carbon is in contact with the solvent, avoiding the direct contact between the natural graphite and the solvent, thus expanding the range of solvent selection; the layer spacing of amorphous carbon is larger than that of graphite, and the diffusion of lithium ions in it can proceed faster, which is equivalent to forming a buffer layer of lithium ions outside the graphite, which can improve the high-current performance of the material; the amorphous carbon shell layer formed outside the graphite can prevent the co-embedding of solvent molecules from damaging the graphite structure, so the phenomenon of layer exfoliation of the graphite material can also be greatly reduced. The key to this method is to form a complete coating on the outside of the graphite, otherwise it will not prevent the electrolyte from contacting the graphite.
The structure of the coating is generally: the inner core is a graphite material or a carbon material with high crystallinity, and the outer shell is an amorphous carbon material or a carbon material with low crystallinity. Coating structure type, the coated granular carbon can be graphite, carbon black, acetylene black or glassy carbon, and the coating layer can be polycarbodiimide, phenolic resin, pitch, tar, coke, furan resin, viscose fiber, dechlorinated acrylic, etc.
Coating methods include dipping, chemical vapor deposition and so on. The impregnation method is obtained by impregnating the graphite material in a liquid phase resin or other liquid phase carbon material precursor, and then pyrolyzing and carbonizing at a certain temperature. The impregnation method generally uses solid phase carbonization, and some resins must be dissolved in a solvent, and then volatilized after impregnation. The chemical vapor deposition method uses paraffins, olefins or aromatic compounds with 15-20 carbon atoms, and their boiling points are usually below 200°C. A layer of amorphous carbon is deposited on the surface of the graphite material by gas phase carbonization.
The graphite material is coated with resin, and the amorphous carbon formed by heat treatment can prevent the peeling of the graphite layer and improve the cycle performance of the graphite; at the same time, since the coating layer is beneficial to the migration of lithium, the high current discharge performance is also improved. The reversible capacity of graphite material coated with resin carbon has also increased, and it has good compatibility with PC-based electrolyte. The coating material also maintains the characteristics of the flat discharge voltage platform of graphite.
Using coal tar as the coating material and using vapor deposition to coat graphite, its discharge capacity and first charge and discharge efficiency have been greatly improved compared to the original non-core-shell structured carbon electrode, and there is almost no selectivity for PC-based electrolysis. The use of other amorphous carbon materials, such as PVC and coal tar pitch coated with graphite, has a similar effect. In addition, depositing a layer of BCx or CxN on the surface of natural graphite can also change its performance.
Although coating resin or other carbon materials on the graphite surface can effectively improve the charge and discharge performance of graphite, the liquid phase resin used to form the shell layer must be pulverized during the process of making the electrode due to the fusion and aggregation of the resin between the graphite particles during the baking and carbonization of the graphite particles. This will expose the active surface of the graphite again.
(4) Cladding metals and metal oxides. Many metals, such as Cu, Ag, Au, Bi, In, Pb, Sn, Pd, Ni, etc. can be coated on the surface of carbon materials. The coating metal layer can increase the diffusion coefficient of lithium ions in the material, thereby promoting the rate performance of the electrode, and the coating of the metal layer can also reduce the irreversible capacity of the material to a certain extent and improve the charge and discharge efficiency.
Depositing a layer of metal such as Ag on the surface of carbon materials such as graphite can form a stable solid electrolyte interface. When the silver content (mass fraction) is 5%, the capacity loss is only 12% after 1500 cycles. Lithium has a high diffusion coefficient in deposited silver and can move freely.
The graphite is coated with fine particles of Pd. Since Pd is deposited on the edge surface of the graphite, the diffusion and co-intercalation of solvated Li+ are inhibited, thereby reducing irreversible capacity loss. This is mainly for when Pd is less than 10%, and the efficiency is increased from 59% to 80.3%. When Pd is greater than 25%, the formation of Li-Pd alloy increases irreversible capacity loss and reduces efficiency.
The performance of graphite electrodes is improved by electroless plating, cobalt and other Group 1 metals on graphite. Use simple vacuum evaporation of metals, such as Ag, Au, Bi, In, Pb, Sn, and Zn on the carbon fiber (T=3100℃) which is coated on graphite, to form a complete covering layer on the surface of the carbon material, and the evaporated metal can improve the charging and discharging efficiency to a certain extent. Especially the effects of Ag, Sn and Zn are the most obvious. This is because the metal facilitates the movement of lithium in it. In addition, when Li and metal form an alloy, the activity of lithium in the metal is high, and the high-current performance is improved. If the selected metal has a high affinity for lithium, the diffusion rate of lithium in it is also fast, which is also conducive to the high current performance of the metal.
(5) Polymer coating. The polymers used for coating carbon materials can be divided into three categories. One type is polymers that have both electrical conductivity and electrochemical activity, such as polythiophene, polypyrrole (PPy) and polyaniline, etc.; the other is ionically conductive polymers that only have electrical conductivity, such as the polymer of lithium methacrylic acid sulfonate and acrylic wax; there is also a type of polymer that has neither electrochemical activity nor conductivity, such as gelatin, cellulose, polysiloxane, etc.
The first type of polymer, as a conductive agent, has good conductivity. The composite formed is a good conductive network, which provides a conductive backbone for the particles and reduces the contact resistance between the particles, and can greatly improve the conductivity of the electrode; secondly, because it is a polymer, it can be used as an adhesive, so it is not necessary to add an insulating fluorine-containing adhesive; moreover, this type of polymer has electrochemical activity, although its capacity is low (for example, polythiophene is about 44mA•h/g), it can also increase the reversible capacity of the composite to a certain extent. The polymer coating can also reduce the contact between graphite and electrolyte, reduce irreversible capacity, and improve Coulomb efficiency.
Coating polypyrrole on graphite (SFG10) found that polymerization can reduce the first irreversible capacity of graphite and improve charge and discharge efficiency. This is because the presence of the polymer reduces the thickness of the SEI layer, thereby reducing the lithium required to form the SEI layer. When the mass content of polypyrrole is 7.8%, the electrochemical performance is the best, and its Coulomb efficiency, high current discharge performance, and cycle performance are greatly improved. Figure 1 is a comparison chart of cycle performance before and after adding 7.8% polypyrrole. The use of polyaniline and polythiophene to coat carbon materials has similar effects and effects, which can improve the first charge and discharge efficiency and reduce the irreversible capacity.
Figure 1 - Cycle curve before and after coating with PPy
After a layer of ionic polymer film is coated on the surface of natural graphite, the irreversible capacity loss caused by the formation of a passivation film and the co-intercalation of solvated lithium ions can be suppressed and reduced, and its charge and discharge efficiency can be improved. At the same time, the elastic polymer film can adapt well to the changes in the volume and surface of the graphite particles during the cycle, prevent the passivation cracking and repair phenomenon caused by the process, and can improve the stability and elasticity of the passivation film, and improve the cycle performance of coated graphite.
Other polymers that are not electrochemically active include polymer electrolytes, such as gelatin, cellulose, and polysiloxane, which can also be coated on the surface of carbon materials.
