KR20070034224A - Method of coating graphite on hard carbon and hard carbon coated graphite - Google Patents

Abstract

The present invention relates to a method for coating graphite on the surface of the hard carbon and to a hard carbon coated with the graphite in order to improve the initial charge and discharge efficiency of the hard carbon used as a negative electrode material of the secondary battery. According to the present invention, it is possible to efficiently coat hard carbon, which is not easy to coat conventionally.

Graphite, Hard Carbon, Coating, Agglomeration, Secondary Battery

Description

METHOD FOR COATING HARD-CARBON WITH GRAPHITE AND HARD-CARBON COATED WITH GRAPHITE}

1 is a SEM photograph showing the appearance of the hard carbon coated with graphite prepared in one embodiment of the present invention.

2 is a view showing the initial charge and discharge test results for a secondary battery using a hard carbon coated with graphite as a negative electrode material according to an embodiment of the present invention.

3 is a view showing the results of the initial charge and discharge test for the secondary battery using a non-coated hard carbon as a negative electrode material as a comparative example of the present invention.

The present invention relates to a method for coating graphite on the surface of the hard carbon and to a hard carbon coated graphite in order to improve the initial charge and discharge efficiency of the hard carbon used as a negative electrode material of the secondary battery.

Carbon materials such as graphite are widely used as electrode materials of secondary batteries.

Of these carbon materials, graphite has a crystal structure of carbon. Most of the crystal structures of graphite are hexagonal systems, in which carbon is connected in a hexagon like a benzene ring, and these hexagons form a plate-like body to form a continuous layer. Three of the free electrons of the carbon atom have strong covalent bonds in the plane, and one remaining electron is bonded to the upper or lower layer. The height of one layer on the hexagonal plate is 3.40Å, and the distance between the nearest carbons in the hexagonal ring is 1.42Å, and the distance between the upper and lower layers of the plate is much larger than the distance between two carbon atoms. In the case of graphite crystals, graphite has good electrical conductivity because electrons can move freely in the hexagonal plate like a metal. However, diamonds that are homogeneous with graphite are perfect insulators because all four free electrons of carbon have strong covalent bonds.

In general, when the graphitizing carbonaceous material is treated at a high temperature of 2000 ° C. or higher, the graphitized graphite is changed into a crystal structure. However, non-graphite amorphous carbon material that is not graphitized even after heat treatment at high temperature is called hard carbon. Such hard carbon may also be used as an electrode material of the secondary battery, in particular, a cathode material. Particularly, in the case of hard carbon, lithium ions can be stored in micropores having a diameter of 2 nm or less present in the particles, and thus have excellent capacity.

 The hard carbon used as a negative electrode material of the secondary battery, particularly a lithium secondary battery, may be manufactured by carbonizing materials such as resin, thermosetting polymer, wood, and the like. When the hard carbon is used as a negative electrode material for lithium secondary batteries, the reversible capacity is superior to 400 mAh / g due to the fine pores, but the initial efficiency is about 70% or less, so it is irreversibly consumed when used as an electrode of a lithium secondary battery. The disadvantage is that the amount of lithium is large.

This irreversible occurrence is due to the decomposition reaction of the electrolyte at the surface of the electrode during charging and the formation of a solid electrolyte interphase (SEI), which is a surface coating, and the discharge of lithium stored in the carbon particles during charging. Sometimes it is due to not being able to. More problematic than this is the former case, and the formation of the surface coating is known as a major irreversible cause.

Accordingly, various methods have been tried to suppress the formation of the surface coating.

However, although the surface modification method of graphite particles using pitch or resin has been widely used, studies on the surface modification method of hard carbon have been insignificant. For example, Korean Patent No. 275032 or U.S. Patent Publication No. 2005/0158550 disclose a method of heat treating graphite carbon with a carbonaceous material or pitch. In addition, Korean Laid-Open Patent Publication No. 2004-0086320 discloses a method for surface modification of a graphitic carbonaceous material by coating the carbonaceous material which can be graphitized by graphite or heat treatment with a carbon residue forming material and then heat-treating it. It is.

As such, methods known in the art are primarily directed to coating graphite or graphitic carbon materials.

As a method of modifying the surface of the hard carbon, there is a method of modifying the surface by reacting the hard carbon with a carbonaceous gas such as propane, ethane, or methane at a high temperature. However, this method has a disadvantage in that it is very difficult to uniformly coat a large amount of hard carbon by volume.

On the other hand, when the hard carbon coating using a pitch or a resin, the particles are stuck to each other by the pitch or the resin is entangled with each other, in order to overcome such a problem, the oxidative stabilization process (oxidative stabilization) process. However, this oxidation stabilization process has a disadvantage in that the coating process is complicated and the irreversible capacity is increased.

In addition, the hard carbon is manufactured by pulverizing the bulk material of the hard carbon present in a single phase (monolith) in the manufacturing process, in which case the surface is smooth, there is a difficulty in coating.

As described above, since it is not easy to coat the hard carbon, the surface of the hard carbon is substantially not modified or used by coating or the like. In particular, the method of coating the hard carbon with graphite and the hard carbon coated with the graphite is not known yet.

Thus, the present invention has been studied for a method for improving the initial charge and discharge efficiency of the hard carbon by modifying the surface of the hard carbon used as a negative electrode material of the secondary battery.

As a result, the present inventors have found a method of coating the surface of the hard carbon with graphite, and found that when the surface of the hard carbon is modified by coating the hard carbon with graphite, the initial charging and charging efficiency of the hard carbon can be improved. The present invention was completed.

Accordingly, the present invention provides a method of coating the surface of hard carbon with graphite. It is another object of the present invention to provide a hard carbon coated with graphite.

Another object of the present invention is to improve the initial charge and discharge efficiency of a secondary battery by using the graphite-coated hard carbon as an electrode material of a secondary battery.

The present invention provides a hard carbon coated surface with graphite. The hard carbon may be used as an electrode material. That is, according to the present invention, the hard carbon coated with graphite may be used as an electrode material of a secondary battery. Accordingly, the present invention provides a secondary battery using a hard carbon coated with graphite as an electrode.

The present invention also provides

Pretreating the surface of the hard carbon with an adhesive material;

Mixing the graphite in powder or paste state with the pretreated hard carbon to coat the hard carbon with graphite; And

It provides a method for producing a hard carbon coated with graphite, comprising; heat-treating the hard carbon coated with graphite.

The present invention also provides

Dispersing graphite in a tacky surfactant to prepare a graphite paste;

 Mixing the graphite paste with the hard carbon to coat the hard carbon with the graphite paste; And

It provides a method for producing a hard carbon coated with graphite, comprising; heat-treating the hard carbon coated with graphite.

According to the present invention, the initial charge and discharge efficiency of the secondary battery manufactured using hard carbon as an electrode material was about 70% before coating with graphite, but can be improved to about 80% by the graphite coating. As such, by coating the hard carbon with graphite, the irreversible capacity of the secondary battery manufactured using the same is reduced, and as a result, there is an advantage in that the positive electrode material is not used in excess.

Hereinafter, the present invention will be described in more detail.

The present invention provides a hard carbon coated with graphite. Preferably, the hard carbon is present in one or more particle states, and the surface of the hard carbon is coated with graphite particles. That is, in the hard carbon according to the present invention, a coating made of graphite is formed. The hard carbon coated with graphite may be used as an electrode material of the secondary battery, in particular, a cathode material. The graphite here serves to reduce the initial irreversible capacity.

In the present invention, the hard carbon refers to an amorphous non-graphite carbon material that is not graphitized even by high temperature heat treatment. Such hard carbon may be prepared by carbonizing resin, wood, fruit peel, coal material or thermosetting polymer that does not undergo a melting step. These materials maintain a macromolecular structure even at high temperature heat treatment, whereby small molecules are released (removed) to promote crosslinking. The site where these small molecules escape is changed into micropores.

In the present invention, the particle size of the hard carbon may be about 0.1 ~ 500㎛ average particle diameter. Preferably, an average particle diameter can be 1-10 micrometers. On the other hand, the particle size of the graphite for coating the hard carbon can be about 0.05 ~ 10㎛ average particle diameter smaller than the hard carbon. Preferably the average particle diameter can be 0.05-1 micrometer.

If the particle size of the hard carbon is too large, it is difficult to control the thickness at the time of forming the electrode, and if the particle size is too small, it is not easy to coat with graphite. On the other hand, graphite also has a problem that it is difficult to coat the hard carbon if the particles are too large, difficult to handle in the process if the particles are too small.

The amount of the coated graphite may be 1 to 50 parts by weight, preferably 2 to 20 parts by weight, and more preferably 5 to 10 parts by weight, based on 100 parts by weight of hard carbon. If the amount of graphite is too large, there is a problem that the hard carbon is not coated and separated, and if the content is too small, it is difficult to cover the hard carbon surface.

Meanwhile, in the coating process of the hard carbon, in addition to the graphite, a group consisting of silica (SiO ₂ ), zeolite (zeolite), alumina (alumina), ceria (CeO ₂ ), zirconia (ZrO ₂ ) and titania (TiO ₂ ). By further using at least one inorganic particles selected from, it is possible to prevent the particles from sticking to each other in the coating process of the hard carbon. Therefore, it is preferable that at least one inorganic particle of the inorganic particles is further coated on the surface of the hard carbon according to the present invention in addition to the graphite.

The particle size of the inorganic particles may be about 2 ~ 200nm average particle diameter. Preferably the average particle diameter may be 10 ~ 50nm. The inorganic particles are intended to assist graphite coating, but the content does not need to be excessively high. If the content is excessively large, there is a problem in that the capacity of the obtained carbon material is reduced.

The amount of the inorganic particles may be 0.1 to 20 parts by weight with respect to 100 parts by weight of hard carbon, preferably 0.1 to 5 parts by weight, and more preferably 0.2 to 1 part by weight.

Generally, coating the graphite material in powder form on the hard carbon surface in powder form is not an easy task as coating the powder with powder.

Therefore, in the present invention, an efficient coating can be achieved by the following coating method.

A method of producing hard carbon coated with graphite according to the present invention, comprising: pretreating the surface of a hard carbon with an adhesive material; Mixing the graphite in powder or paste state with the pretreated hard carbon to coat the hard carbon with graphite; And heat treating the graphite coated with the hard carbon.

By the above method, a hard carbon in a particulate state coated with graphite can be obtained.

In the above method, the pretreatment is to impart adhesion to the hard carbon so that the bond between the graphite and the hard carbon can be strengthened. The pretreatment may be applied without limitation as long as it can give a stickiness to the surface of the hard carbon. Pretreatment of the hard carbon, there is a method of removing the solvent after mixing the adhesive material, for example gelatin, pitch, resin or adhesive polymer resin dissolved in the solvent with the hard carbon.

The solvent depends on the tacky material used. For example, water or an organic solvent can be used. According to one embodiment of the present invention, in order to coat the hard carbon material with graphite, an aqueous solution in which coal tar or petroleum pitch, furan resin or phenol resin is dissolved in an organic solvent or gelatin is dissolved. Pretreatment may be performed to impart adhesion to the surface of the hard carbon.

The graphite is coated on the hard carbon provided with the tackiness. The graphite may be coated by mixing with the hard carbon in a powder state, or may be prepared in a paste state and then mixed with the hard carbon so that the graphite is coated on the hard carbon. Here, the method of making the paste is a method in which small artificial graphite particles having a diameter of 1 μm or less and a surfactant are added to water, stirred in a high viscosity state, and energy is added thereto using ultrasonic, milling, etc., so that uniform mixing occurs overall. There is this.

Meanwhile, in the coating of the hard carbon, at least one inorganic particle selected from the group consisting of silica (SiO ₂ ), zeolite, alumina, alumina, ceria (CeO ₂ ) and titania (TiO ₂ ) is further added. Can be added. The particle size of the inorganic particles may be about 2 ~ 100nm average particle diameter. Preferably, the average particle diameter may be 10 to 50 nm.

The inorganic particles serve to suppress the phenomenon that the particles adhere to each other by melting the molten material during the heat treatment of the hard carbon coated with graphite. In this case, entanglement of the particles can be prevented without a process such as oxidative stabilization, so that there is no need to go through an additional process such as oxidative stabilization conventionally performed before heat treatment. Typically, when the pitch material undergoes an oxidation stabilization process, crosslinking by oxygen occurs on the surface, thereby increasing the irreversible capacity.

The coating method may be a mechanical coating method using mechanofusion, a Henschel mixer, a z-mill, or the like. By this method it is possible to increase the efficiency of the coating and to make the coating even.

The heat treatment is to ensure that the coated graphite material remains stable on the hard carbon surface and is integrated with each other. The heat treatment may be carried out in an inert atmosphere. Heat treatment in an inert atmosphere is intended to prevent oxidation of the carbon material at high temperatures. The inert atmosphere is generally formed by, for example, nitrogen or argon gas, but may be formed by other inert gas.

The heat treatment temperature is suitable 400 ~ 1500 ℃. More preferably, it can be about 700-1200 degreeC. If the heat treatment temperature is too high, the fine pores of the hard carbon is blocked, there is a problem that the capacity is reduced, if the temperature is too low there is a problem that the heat treatment is not made properly, the irreversible capacity increases. As a result, the initial efficiency of the electrode material is not increased.

As another method for producing the graphite coated hard carbon according to the present invention, there is a method that does not undergo a pretreatment process. That is, dispersing graphite in a tacky surfactant to prepare a graphite paste; Mixing the hard carbon with the graphite paste to coat the hard carbon with the graphite paste; And a step of heat-treating the hard carbon coated with the graphite paste.

In the above method, a surfactant having an adhesive property was used to omit the pretreatment step. The graphite was dispersed in the surfactant to prepare a graphite paste, which is directly coated on hard carbon. That is, as a method for omitting the pretreatment step, the coating may be performed by mixing the hard carbon, which is not pretreated, with graphite in a paste state that is uniformly mixed with a surfactant having adhesive properties. According to an exemplary example of such a graphite paste, the amount of graphite may be 1 to 50 parts by weight, preferably 2 to 20 parts by weight, and more preferably 5 to 10 parts by weight with respect to 100 parts by weight of water. Can be. The amount of the surfactant may be 0.5 to 10 parts by weight based on 100 parts by weight of graphite, preferably 1 to 5 parts by weight.

As the surfactant, at least one selected from the group consisting of nonionic, cationic and anionic systems can be used. Surfactants that can be used include triton x-100, tergitol, etc., trialkyl ammonium bromide / chloride, which is a cationic surfactant, such as P123 and F127, which are ethylene oxide-propylene oxide-ethylene oxide block copolymer surfactants of Basf, or anions. Sodium dodecylsulfate, a surfactant. Of these, nonionic surfactants are more preferred.

In the coating of the hard carbon, at least one inorganic particle selected from the group consisting of silica (SiO ₂ ), zeolite, alumina, alumina, ceria (CeO ₂ ) and titania (TiO ₂ ) may be further added. Of course it can. In addition, the heat treatment is preferably carried out in an inert atmosphere.

According to the method according to the present invention, the hard carbon is prevented from agglomeration during the manufacturing process, so that it is not necessary to undergo a grinding process or a sieve after heat treatment, and thus the manufacturing is easy, and thus an electrode is used. It is easy to do

The present invention provides an electrode material including a hard carbon coated by the above method, and also provides a secondary battery including an electrode manufactured using the electrode material. Preferably, the electrode is a cathode. When manufacturing a secondary battery by applying the graphite-coated hard carbon prepared by the method according to the invention as a negative electrode material, it is possible to increase the initial efficiency of the secondary battery. For example, when the graphite-coated hard carbon is used as a negative electrode material of a lithium ion battery, the initial charging and discharging efficiency is excellent as 80% or more, and the advantage of not using an excessive amount of the positive electrode material when manufacturing a secondary battery is advantageous. have.

The method of manufacturing an electrode using the hard carbon coated with graphite according to the present invention as an electrode material and the method of manufacturing a secondary battery using the electrode may be applied to methods known in the art. Those skilled in the art can easily manufacture a secondary battery electrode and a secondary battery using the electrode material.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

<Example 1>

50 g of hard carbon (Kureha Chemical Industry) having a particle size of 10 μm was added to a solution prepared by dissolving 5 g of gelatin (gelatin) in 500 g of water, and pretreated by imparting adhesion to the surface of the hard carbon. After imparting tackiness, 5 g of powdered graphite (Timcal's KS4; average particle diameter: 2.4 μm) was mixed with mechanofusion by coating the hard carbon with graphite, and then heat-treated in a nitrogen atmosphere at 1000 ° C. to coat with graphite. 55 g of hard carbon was obtained.

1 is an electron scanning microscope (SEM) observation picture of the hard carbon coated with graphite. In Figure 1 it can be seen that small graphite particles are dispersed on the surface of the hard carbon.

<Example 2>

In the coating process of graphite, 55 g of hard carbon coated with graphite was obtained in the same manner as in Example 1, except that 5 g of titania was added as inorganic particles when the hard carbon and graphite were mixed.

<Example 3>

5 g of powdered graphite (Timcal's KS4) is mixed with 0.2 g of a surfactant P123 and then placed in 100 g of water. It was mixed with 50 g of hard carbon (Kureha Chemical Industry) and coated with hard carbon using mechanofusion. Subsequently, the coating was heat-treated under nitrogen atmosphere at 1000 ° C. to obtain 55 g of hard carbon coated with graphite.

<Example 4>

Except for adding 0.5 g of titania as an inorganic particle in the process of coating the graphite treated with a surfactant on the hard carbon, the same method as in Example 3 was carried out to obtain 55 g of graphite coated with hard carbon.

<Examples 5-8>

The graphite coated hard carbon prepared in Examples 1 to 4 was used as the anode, and the lithium metal foil was used as the cathode, and 1 mol of LiPF ₆ -EC: DMC (volume ratio 1: 2) was used as the electrolyte. Coin-type batteries were prepared using polypropylene membranes as separators, and examples 5 to 8 were used.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 4, using the same hard carbon of Kureha Chemical Industry as in Example 1, but using the hard carbon without graphite coating as a negative electrode material.

<Test Example 1>

Charge and discharge tests were performed using the secondary batteries prepared in Example 5 and Comparative Example 1. The charging and discharging conditions were charged and discharged at a constant current of 0.1 C, and constant voltage charging was performed at 5 mV during charging.

2 is a charge and discharge test result of the secondary battery manufactured in Example 5. Here, the charging and discharging experiment conditions were charged and discharged (CC; constant current) at a constant current of 0.1C from 0.05V to 2V, and a voltage was applied until a current of 0.005C was applied by applying a constant voltage at 0.05V (CV: constant voltage). As a result of the test, in the above case, the initial efficiency was about 79%, and the charging and discharging efficiency was improved as compared with the comparative example using the hard carbon without graphite coating, and the discharge capacity was about 350 mAh / g, similar to that before the coating.

Figure 3 shows the charge and discharge test results for the secondary battery prepared in Comparative Example 1. In the case of the comparative example, it can be seen that the initial efficiency is about 72%. In addition, it can be seen that the discharge capacity is about 350mAh / g.

When the graphite-coated hard carbon according to the present invention is applied as a negative electrode material of the secondary battery, the initial efficiency of the secondary battery may be increased. As a result, the irreversible capacity of the secondary battery is reduced and there is no need to use an excessive amount of the positive electrode material, which is very efficient for manufacturing a secondary battery.

Claims (20)

Hard carbon coated with graphite. 2. The hard carbon according to claim 1, wherein the hard carbon is in one or more particle states, and the surface of the hard carbon is coated with graphite particles. The hard carbon according to claim 1, wherein the hard carbon is for electrode material. The hard carbon according to claim 1, wherein the hard carbon is prepared by carbonizing a resin or a thermosetting polymer. The method of claim 1, wherein the hard carbon particle size of the hard carbon, characterized in that the average particle diameter of 0.1 to 500㎛. The method of claim 1, wherein the particle size of the graphite hard carbon, characterized in that the average particle diameter of 0.05 to 10㎛. The hard carbon according to claim 1, wherein the amount of the coated graphite is 1 to 50 parts by weight based on 100 parts by weight of hard carbon. The method of claim 1, wherein the hard carbon has at least one selected from the group consisting of silica (SiO ₂ ), zeolite, alumina, ceria (CeO ₂ ) and titania (TiO ₂ ) in addition to graphite. Hard carbon, characterized in that the inorganic particles are further coated. 9. The hard carbon according to claim 8, wherein the particle size of the inorganic particles is 2 to 200 nm in average particle diameter. The hard carbon according to claim 8, wherein the amount of the inorganic particles is 0.1 to 20 parts by weight based on 100 parts by weight of the hard carbon. Pretreating the surface of the hard carbon particles with an adhesive material; Mixing the graphite in powder or paste state with the pretreated hard carbon to coat the hard carbon with graphite; And Heat treating the graphite-coated hard carbon; Method for producing a hard carbon coated with graphite comprising a. The method of claim 11, wherein the pretreatment of the surface of the hard carbon comprises dissolving a gelatin, pitch, resin or adhesive polymer resin in a solvent and then mixing the hard carbon with the hard carbon. Way. The method of claim 11, wherein when the graphite is mixed with the hard carbon in the coating of the hard carbon with graphite, silica (SiO ₂ ), zeolite, alumina, ceria (CeO ₂ ) and titania (TiO) are added. ₂ ) at least one inorganic particle selected from the group consisting of. The method of claim 11, wherein the heat treatment is characterized in that carried out at a temperature of 400 ~ 1500 ℃. 12. The method of claim 11, wherein said heat treatment is performed under an inert atmosphere. Dispersing graphite in a tacky surfactant to prepare a graphite paste; Mixing the hard carbon with the graphite paste to coat the hard carbon with the graphite paste; And Heat-treating the hard carbon coated with the graphite paste; Method for producing a hard carbon coated with graphite comprising a. The method of claim 16, wherein when the hard carbon is mixed with the graphite paste in the coating of the hard carbon, silica (SiO ₂ ), zeolite, alumina, ceria (CeO ₂ ) and titania (TiO _{2) are added.} At least one inorganic particle selected from the group consisting of Hard carbon produced by the method according to any one of claims 11 to 17. A secondary battery comprising an electrode manufactured using the hard carbon according to any one of claims 1 to 10 as an electrode material. The secondary battery of claim 19, wherein the electrode is a negative electrode.

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