US20040227264A1

US20040227264A1 - Methods for fabricating improved graphite granules

Info

Publication number: US20040227264A1
Application number: US10/794,020
Authority: US
Inventors: Caisong Zou; Chuanfu Wang; Junqing Dong
Original assignee: BYD America Corp
Current assignee: BYD America Corp
Priority date: 2003-05-16
Filing date: 2004-03-04
Publication date: 2004-11-18
Also published as: US20040229125A1; US20040229041A1; US8133612B2

Abstract

This invention relates to a method for the fabrication of improved graphite granules from unimproved graphite granules, where a polymer solution is uniformly sprayed onto a fluidized bed of unimproved graphite granules that are being stirred three dimensionally in high speed. The granules are then heat to dry to obtain coated graphite granules. To further improve the electrochemical properties of the final improved graphite granules, the coated graphite granules are immersed in a surface modifier, filtered, heat to dry and sifted. Then, they are solidified and carbonized in an inert environment to obtain final improved graphite granules. This invention is simple, easy to implement, and easy to implement for industrial production. The improved graphite granules made using embodiments of this invention, when used as the material for the negative electrode of a rechargeable battery produces a battery that has high initial charge/discharge efficiency, high reversible specific capacity and good cycle ability.

Description

CROSS REFERENCE

This application claims priority from the following Chinese patent applications: “A Type of Improved Graphite and Its Method of Fabrication” filed on Aug. 16, 2003, having a Chinese Application No. 03140199.6; “The Fabrication Method of A Type of Improved Graphite” filed on May 16, 2003, having a Chinese Application No. 03126631.2; “Method for the Fabrication of a Carbon Containing Material for the Negative Electrode of A Type Lithium Ion Rechargeable Battery” filed on Jun. 20, 2003, having a Chinese Application No. 03 1 26961.3; and “A Type of Lithium Ion Rechargeable Battery” filed on Jul. 22, 2003, having a Chinese Application No. of 03132939.X. All of the above applications are incorporated herein by reference. [0001]
This application is a continuation-in-part of a U.S. patent application entitled “Improved Graphite Granules and Their Method of Fabrication” filed on Feb. 2, 2004, having a U.S. application Ser. No. ______ yet-to-be-assigned ______. This application is also a continuation-in-part of a U.S. patent application entitled “Negative Electrodes for Rechargeable Batteries” filed on Feb. 2, 2004, having a U.S. application Ser. No. ______yet-to-be-assigned ______. [0002]

FIELD OF INVENTION

This invention relates to a method for fabricating improved graphite granules. More particularly, it relates to a fabrication method for a type of improved graphite granules, which when used as the material for the negative electrode of lithium ion rechargeable batteries, it produces a battery with high initial charge/discharge efficiency and good cycle characteristics

BACKGROUND

The material for the negative electrode of a lithium ion rechargeable battery greatly affects the battery's capacity and its cycle characteristics. The materials that are being used for the negative electrode of lithium ion battery include forms of carbon such as graphite, soft carbon, and hard carbon, oxides of metal, sulfides of metal, and other materials. Among these, the technology for the fabrication of graphite materials is the most mature. This material has excellent overall properties. It is cheap and easily available, and the cost of fabrication is comparatively low. Other materials for the negative electrodes either have certain problems in their practical application or are still in the research stage.

There are two types of graphite, man-made graphite and natural graphite. There are certain limitations if graphite without any improvement or modification is used as negative electrodes of rechargeable batteries. The reversible specific capacity of batteries with negative electrodes made with natural graphite is higher; theoretically, the capacity can almost reach 372 mAh/g for some natural graphite with a high degree of graphitization. However, when natural graphite is used directly in lithium ion batteries, its cycle ability is comparatively poor and cannot satisfy market demand. Moreover, there exist certain variations on its properties that are dependent on its source of origin and actual structure. Some man-made graphite has better cycle characteristics. However, the reversible specific capacity ratio for man-made graphite is lower and it is difficult to satisfy the market demand for portable equipment due to increasing miniaturization and long battery life.

During the initial charging and discharging process of the lithium ion battery using graphite type material as its negative electrode, a compact SEI (solid electrolyte interface) membrane is formed on the surface of the graphite. Two critical factors affect the properties of the lithium ion battery using carbon material as its negative electrode: the degree of graphitization and the density of the SEI layer that is formed as described above. For natural graphite and unmodified/unimproved man-made graphite, the degree of graphitization and the density of the SEI layer often cannot be both taken into account to improve battery characteristics. In natural graphite, the degree of graphitization and its reversible specific capacity are higher but its compatibility with the electrolyte solution is worse. Between the edges and surfaces of its crystallite, there are bigger differences that are not favorable to the formation of a dense SEI thus affecting the rate for the initial charge/discharge efficiency and the cycle characteristics. Also, the higher the degree of graphitization, the smaller is its d ₀₀₂value and the closer it is to 0.3354 nm. In the process of multiple charging and discharging, solvent molecules will be attached. Since the Van de Waal force is weaker between the graphite layers, this can easily cause crystallite interlayer spacing, d₀₀₂, to expand, even to the extent that the layers appear to peel off. This results in the destruction of the graphite structure thus seriously affecting the cycle properties of the battery.

Therefore, there is a need to modify graphite in order to ensure its initial charge/discharge efficiency and cycle characteristics. Often seen methods for modification include oxidation, coating, CVD deposition, and doping. Among these, the coating method has better results and is easier to implement for industrial production.

In general, amorphous carbon obtained from the decomposition of the polymer is more compatible to the organic electrolyte, with better cycle stability. However, its irreversible specific capacity is higher; its charging and discharging platform is also not as ideal as the graphite. Therefore, many researchers have attempted combine the excellent qualities of the graphite and polymer by coating the graphite with a layer of polymer and carbonizing it under certain temperature to obtain a “shell-nucleus” structure of the graphite compound. By doing so, they are attempting to retain the special properties of the graphite, the high reversible specific capacity and the better charging and discharging platform, and also incorporate the better compatibility of the thermally decomposed polymer with the organic electrolyte to obtain a battery with longer cycle life and stable characteristics. Researchers in both China and other countries have done extensive related work.

An article published in the Battery publication, [Battery. 2002.32(1): 13-15], describes the method to use a multi-step modification process to modify the characteristics of crystalline flake natural graphite from China with the use of epoxy resin and thermally decomposed carbon. This changed the form of the graphite, lowering the directional property of the crystalline flake graphite, improving the surface morphology and the compatibility of the graphite with the electrolyte and enabling a battery made with this modified graphite to better raise its cycle capacity. However, modified graphite using this method has lower initial charge/discharge efficiency and cannot satisfy existing manufacturing demand.

Patent CN1304187 discloses a type of lithium ion battery using graphite with multiple modifications as the material for the negative electrode. It uses graphite granules as the core, thermally decomposed carbon as the outer coating material (1 wt % to 50 wt %). In the coating process, electrically conducting agent (0.01 wt % to 10 wt %) is doped into the thermally decomposed carbon to obtain a graphite compound with 0.1 m ²/g to 20 m²/g of specific surface area. The patent indicates that the negative electrode made from the graphite compound has a reversible specific capacity of about 350 mAh/g, and its initial charge/discharge efficiency can reach 85% to 90%. However, this patent only shows a 20 cycles diagram and cannot adequately explain whether the cycle results can satisfy the demands of practical applications. Also, the technology uses a very powerful mechanical crushing process. This not only increases the investment of equipment, but the crushing process can very easily cause the “shell-core” structure formed by the coating to peel, affecting its effectiveness.

Japanese Patent JP2000-106182 discloses an invention for a method using chemical vapor deposition (CVD) to coat a carbon layer on the surface of the graphite. That method takes 2% to 50% concentrated coal gas or organic matter and pass through special CVD furnaces at 900° C. to 1200° C., gradually depositing a layer of carbon with comparatively good crystalline structure on the surface of the graphite. Deposition time is between 0.5 hours to 4 hours. The resulting coated graphite has a reversible specific capacity that reaches 350 mAh/g to 370 mAh/g. The initial charge/discharge efficiency is 88% to 92%. However, that invention did not provide the cycle data. Also, the equipment for the CVD method is complex and expensive, thus increasing the cost of production. Besides, the efficiency of the CVD method is lower, the production rate per unit time is lower, and energy consumption is higher. Japanese Patent JP2001-283848A also discloses an invention utilizing the CVD method to modify the properties of graphite. This invention has certain results but it also presents similar problems during industrialization.

Above stated existing technology all have some problems. Either the cycle characteristics cannot satisfy the needs of practical applications or the technology or equipment is complicated, resulting in higher cost and higher difficulty in the manufacturing process.

Due to the limitations of the prior art, it is therefore desirable to have novel methods of fabricating improved graphite granules with good electrochemical properties such that batteries with negative electrodes made with these improved graphite granules can have better performance characteristics. Moreover, novel methods for the fabrication of such improved graphite granules are needed in order to produce said improved graphite granules using a process that is simple, low cost, and easy to implement for industrial production.

SUMMARY OF INVENTION

The object of this invention is to disclose a method for fabricating improved graphite granules that is simple, with low cost of fabrication and no secondary effects that may affect the effectiveness of the electrochemical properties of the improved graphite granules, and is easy to implement for industrial production.

The present invention relates to a method of fabricating a type of improved graphite granules from unimproved graphite granules by coating the granules in a layer of carbon material. The unimproved graphite granules are natural, man-made, or otherwise modified graphite whose electrochemical properties need to be improved. To fabricate said improved graphite granules, the method steps are as follows: dissolving a polymer, the material that will form the coating for the improved graphite granules in organic solvent; spraying the polymer in organic solvent uniformly onto a fluidized bed of the unimproved graphite granules that are being stirred three dimensionally in high speed; dry said sprayed graphite granules; sifting and solidifying; then carbonizing the sifted graphite granules to form the final improved graphite granules.

The advantages of this invention, a method for fabrication of improved graphite granules, are:

(1) The fabrication method of this invention does not require any procedure using mechanical pulverization, thus eliminating the concern for the peeling of a “shell-core” improved graphite granules that is made from a process using mechanical pulverization.

(2) The fabrication method of this invention is simple and easy, the cost of production is inexpensive, and it is easy to implement for industrial production; and

(3) The improved graphite granules made with this invention, when used as material for the negative electrode of a rechargeable battery produces a battery with a high initial charge/discharge efficiency, high reversible specific capacity, and stable cycle characteristics—all characteristics that are needed to fulfill the ever-increasing demands on rechargeable batteries for electronic products.

DESCRIPTION OF DRAWINGS

The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of preferred embodiments of this invention when taken in conjunction with the accompanying drawings in which: [0020]
FIG. 1 is the scanning electron micrograph of the improved graphite granules. [0021]
FIG. 2 is a second scanning electron micrograph of the improved graphite granules made by embodiment 4. [0022]
FIG. 3 is the graph of cycle characteristics of the battery whose negative electrode is made from improved graphite granules of embodiment 4. [0023]
FIG. 4 is the graph of the cycle characteristics of the battery whose negative electrode is made from improved graphite granules of embodiment 6. [0024]
FIG. 5 is the graph of the cycle characteristics of the battery whose negative electrode is made from improved graphite granules of comparison example 1.[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of this invention to fabricate improved graphite granules from unimproved graphite granules by coating them in a layer of carbon material includes the following steps: [0026]
1. Dissolving the polymer that is the material for the coating in organic solvent to obtain a predetermined concentration for the polymer solution; [0027]
2. Uniformly spraying said polymer solution from step 1 onto said unimproved graphite granules. To ensure that the surfaces of the unimproved graphite granules are adequately and uniformly coated with a layer of the polymer solution, the unimproved graphite granules are fluidized by stirring with a specially designed blades at between 500 to 4000 rpm both parallel and perpendicular to the direction of flow of the polymer solution. [0028]
3. Drying the sprayed graphite granules, first by evaporating the solvent from the surface and then sifting. [0029]
4. In an inert environment, solidifying and carbonizing the sifted graphite granules to finally obtain the improved graphite granules. [0030]
Embodiments of this invention uses a “dynamic dry process for coating” to improve and coat natural or man-made graphite granules. That is, the polymer solution that is used for the coating layer is uniformly sprayed onto the fluidized unimproved graphite granules that are being stirred at high speed causing a layer of polymer solution to adequately and uniformly coat the unimproved graphite granules. After heating to evaporate the solvent, a layer of polymer solution uniformly coats the surface of the dried graphite granules. [0031]
Different embodiments of this invention can be constructed by varying the specifications of the above stated method as follows: [0032]
Either the natural or man-made graphite granules, with average diameter between 5 μm to 40 μm, can be used as the material for the unimproved graphite granules. The preferred specification for the average granule diameter of said unimproved graphite granules is between 8 μm and 25 μm. The average granule diameter is the D[0033] ₅₀measured by using a particle size analyzer. If the average granule diameter is too small, then the surface ratio is too big such that it is not beneficial in increasing the reversible specific capacity of a battery made from such improved graphite granules. If the average granule diameter is too big, then it does not allow the Li⁺ to fully attach and detach easily. In addition, it is difficult to manufacture the electrode plate from these improved graphite granules with large granule diameters thus making such improved graphite granules unsuitable as material for making electrodes.
The polymer for the coating is one or more of the following: epoxy resin, phenol formaldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar. These materials can decompose and carbonize at high temperatures to form a layer of amorphous carbon with fixed structure that coats the surface of the unimproved graphite granules. Corresponding organic solvents for the polymer in step (1) is one of the following: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane. The different polymers may be dissolved using corresponding solvents. For example, acetone can be used to dissolve epoxy resin, and N-pyrrolidone or N, N-dimethyl formamide can be used to dissolve polyacrylonitrile etc. [0034]
The weight ratio between the polymer solution and the unimproved graphite granules is between 1:5 and 1:1. If too much polymer solution is used, the material becomes too wet and easily forms lumps after heat dry, making it difficult to pulverize and sift, thus affecting the technology. The properties of the improved graphite granules can also be negatively affected if the thermally decomposed carbon polymer content is too high. Improved graphite granules fabricated under such conditions, when used as material for the negative electrode, produces a rechargeable battery with a high irreversible specific capacity. If too little polymer solution is used, the layer of coating material is too thin, not completely coating the surface of the graphite granules such that the effect of the coating would not be apparent and the cycling ability of a battery fabricated with such improved graphite granules would not be increased. [0035]
The improved graphite granule fabricated by using embodiments of this invention has a “nucleus-shell” structure where the nucleus is the unimproved graphite granule and the shell is the coating added. The ratio of the weight of the shell or coating layer material and the core material, the unimproved graphite granule, is between 5:95 and 40:60. A better range is between 7:93 and 20:80. The shape of the improved graphite granule is spherical or potato shaped, XRD (X-ray diffractometer) picture spherical shaped index I[0036] ₁₁₀/I₀₀₄is bigger than 0.1. It is better to be as big as 0.27. The specific surface area is lower, between 1.76 m²/g and 5.73 m²/g. The crystallite interlayer spacing, d₀₀₂, is between 0.3354 nm and 0.3420 nm. The average granule diameter for the improved graphite granules is between 8 μm and 50 μm, better between 10 μm and 30 μm.
In the above description, the specific surface area is obtained using the single-point BET method. The crystallite interlayer spacing, d[0037] ₀₀₂, is measured using an X-ray diffractometer. The average granule diameter is the D₅₀measured using a particle size analyzer.
The concentration of said polymer solution is between 1% and 30%. The preferred range is between 5% and 25%. [0038]
The quantity and concentration of the polymer solution used can be adjusted to ensure that the amount of coating reaches predetermined requirements. The concentration of the polymer solution is between 1% and 30%. For better results, it is between 5% and 25%. The adhesion of the resulting coating is increased if the concentration of the polymer solution is too high. After they are heated dry, the graphite granules will easily stick together, making sifting difficult. If the concentration of the solution is too low, then the effect is not apparent. [0039]
In the embodiments of the fabrication method of this invention to modify graphite granules, after the polymer solution has uniformly coated the unimproved graphite granules, dry heat is needed to evaporate the solvent in the polymer solution on the surface of the graphite granules. The time and temperature for the evaporation depends on the properties of the solvent such as the boiling point, volatility, and safety properties. For example the boiling point of anhydrous ethanol is 78° C., therefore the evaporation temperature can be set at 70° C., paying attention to the ventilation to avoid explosions. [0040]
The improved graphite granules that have been heat dried have to be sifted before proceeding to the next step of the process. The embodiments of this invention obtain improved graphite granules without the need for any pulverization process and can easily pass though a sifter with the corresponding mesh number of holes. [0041]
The temperature, the holding time, and the rate of increase and decrease of temperature for solidification and carbonization can be controlled to optimize above stated processes. [0042]
In said solidification process, the solidification temperature is held between 100° C. and 600° C. for 0.2 hours to 12 hours. Preferably, the temperature is held between 200° C. and 500° C. for 0.5 hours to 3 hours. [0043]
The rate of increase of temperature for said solidification process to reach the temperature for solidification is between 5° C./min and 35° C./min. The preferred range for the increase in temperature is between 5°/C. to 20° C./min. [0044]
The temperature for said carbonization process is held between 750° C. and 1300° C. for 1 hour to 24 hours. The preferred temperature range is 800° C. to 1100° C. and that temperature is held for 2 hours to 10 hours. [0045]
The rate of increase of temperature for said carbonization process is between 0.1° C./min to 30° C./min. The preferred range is between 3° C./min to 20° C./min. [0046]
The temperature is lowered after carbonization. The rate of decrease in temperature for said step 4 of the carbonization process is between 1° C./min and 20° C./min. A preferred rate of decrease in temperature is between 5° C./min and 15°/min. The temperature can also be lowered naturally. [0047]
Box-style electrical resistance furnaces, tubular furnaces, push-pull tunnel furnaces, and rotary tunnel furnaces etc can be used to solidify and carbonize in the embodiments of this invention. The only requirement is the capability to reach the required temperature, to be sealed and to aerate the inert environment. Said inert environment can use one or more of the following gases: argon, helium and nitrogen. [0048]
A further embodiment of this invention can be added to improve the electrochemical properties of the improved graphite granules by adding the following step between step 3 and step 4 of the above stated methodology. [0049]
Immerse the sifted graphite granules from step (3) in a polymer surface modifier that includes a polymer that has been dissolved and diluted in a corresponding solvent. Then filter, heat dry, and sift the immersed graphite granules before the solidification and carbonization processes. The sifting process in this additional step does not need any mechanical crushing equipment. [0050]
The polymer in said polymer surface modifier is of one or more of the following: coal pitch, coal tar, petroleum pitch, coke, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin. [0051]
The solvent in said polymer surface modifier is one of the following: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane. [0052]
The embodiments discussed below provide further details about this invention. To test the electrochemical properties of the improved graphite granules made by the embodiments discussed below, the improved graphite granules is made into the active ingredient of the negative electrode of lithium ion rechargeable batteries and the performances of these batteries are tested against batteries with negative electrodes made from the unimproved or partially improved graphite granules of two comparison examples discussed below. [0053]
To make the negative electrodes from the improved or unimproved graphite granules in the embodiments and comparison examples discussed below, add binder and de-ionized water to said improved or unimproved graphite granules. Stir the resultant mixture, then coat, heat to dry and compress to form the negative electrode plate. A battery is then made with existing technology using the negative electrode slice made from said method, the positive electrode that is made with LiCoO[0054] ₂, compatible conducting agent, and binder agent, and corresponding electrical conducting agent.
The following are the definitions and explanations of the charging and discharging and cycling characteristics of the batteries made from the improved or unimproved graphite granules in the embodiments and comparison examples below that are tested and measured: [0055]
The initial charge/discharge efficiency=(initial discharging capacity/initial charging capacity)*100%;
Where initial charging capacity is the charging capacity to initially charge to 4.2 volts with 0.1 C of electricity; [0056]
Initial discharging capacity is the discharging capacity to initially discharge from 4.2 volts to 3.0 volts with 0.1 C of electricity; [0057]
Reversible specific capacity is the discharging capacity of 1C of electricity from 4.2 volts to 3.0 volts divided by the weight of the active ingredient of the negative electrode; and [0058]
One cycle means the charging to 4.2 volts with 1C of electricity and followed by the discharge to 3.0 volts with 1C of electricity. After repeated cycles, a particular cycle's capacity is the discharge capacity of that cycle. [0059]

Embodiment 1

The unimproved graphite granules are potato shaped natural graphite. The material for the coating layer uses thermoset phenol formaldehyde resin as the polymer and anhydrous ethanol as its solvent. The polymer material used for the coating is 5%. [0060]
The fabrication process of this embodiment is as follows: [0061]
(1) Dissolve 5 g of thermoset phenol formaldehyde resin in anhydrous ethanol to form 50 ml of a 10% polymer solution; [0062]
(2) Prepare 200 ml of a 5% polymer surface modifier by dissolving 10 g of coal pitch in tetrahydrofuran; [0063]
(3) Use 100 g of the unimproved natural graphite granules and 50 ml of the polymer solution. Using a custom made sprayer head to spray 50 ml of the polymer solution uniformly on 100 g of the fluidized unimproved graphite granules that are being stirred at high speed. Heat to dry and sift; [0064]
(4) Immerse the sifted graphite granules in the polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules and heat dry. Sift with 300 mesh; and [0065]
(5) Put sifted graphite granules into sealed tubular high temperature furnace. Pass highly pure nitrogen at 10 liter/min of flow volume and increase the temperature at 15°/min to 400°. Hold the temperature for 1 hour. Then raise the temperature at 10° C./min to 1000° C. and hold the temperature for 3 hours before naturally lowering to room temperature. Remove the improved graphite granules from the furnace. [0066]
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0067] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.80 g.
The initial charge/discharge efficiency of the battery made by this embodiment is 89.1%. Its reversible specific capacity is 347 mAh/g and it retains 80% of its capacity after 235 cycles. [0068]

Embodiment 2

The unimproved graphite granules are potato shaped natural graphite granules. The material of the coating layer uses polyacrylonitrile as the polymer and N,N-dimethyl formamide as its solvent. The polymer material used for the coating is 7%. [0069]
The fabrication process of this embodiment is as follows: [0070]
(1) Dissolve 7 g of polyacrylonitrile in N,N-dimethyl formamide to form 140 ml of 5% polymer solution; [0071]
(2) Dissolve 15 g of coke in tetrahydrofuran to formulate 200 ml of 7.5% of polymer surface modifier; [0072]
(3) Use 100 g of the unimproved natural graphite and 70 ml of the polymer solution. Using a custom made sprayer head, spray 70 ml of the polymer solution uniformly on 100 g of the fluidized unimproved graphite granules that are being stirred at high speed rolling in high speed. Heat to dry and sift. Repeat once; [0073]
(4) Immerse the sifted graphite granules in the polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules. Heat to dry. Sift with 300 mesh; and [0074]
(5) Put sifted graphite granules into sealed tubular high temperature furnace. Pass highly pure argon at 7 liter/min of flow volume and increase the temperature at 20° C./min to 500° C. and hold the temperature for 40 minutes. Then raise the temperature at 15° C./min to 1100° C. and hold the temperature for two hours before naturally lowering to room temperature. Remove the improved graphite granules from the furnace. [0075]
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0076] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.83 g.
The initial charge/discharge efficiency of the battery made by this embodiment is 88.0%. Its reversible specific capacity is 345 mAh/g and it retains 80% of its capacity after 520 cycles. [0077]

Embodiment 3

The unimproved graphite granules are crystalline flake natural graphite. The material for the coating uses thermoset phenol formaldehyde resin as the polymer and anhydrous ethanol as its solvent. The polymer material used for the coating is 10%. [0078]
The fabrication process is as follow: [0079]
(1) Dissolve 10 g of thermoset phenol formaldehyde resin in anhydrous ethanol to formulate 100 ml of 10% polymer solution; [0080]
(2) Dissolve 10 g of petroleum pitch in tetrahydrofuran to formulate to form 200 ml of 5% of polymer surface modifier; [0081]
(3) Use 100 g of natural graphite and 50 ml of the polymer solution. Using a custom made sprayer head, spray 50 ml of the polymer solution uniformly on 100 g of the fluidized unimproved graphite granules that are being stirred at high speed. Heat to dry and sift. Then repeat once; [0082]
(4) Immerse the sifted graphite granules in the polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules. Heat to dry. Sift with 300 mesh; and [0083]
(5) Put sifted graphite granules into sealed tubular high temperature furnace. Pass highly pure nitrogen at 8 liter/min of flow volume and increase the temperature at 5° C./min to 300° C. and hold the temperature for 0.5 hour. Then raise the temperature at 5° C./min to 800° C. and hold the temperature for two hours before lowering the temperature at 5° C./min to 10° C./min to room temperature. Remove the improved graphite granules from the furnace. [0084]
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0085] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.80 g.
The initial charge/discharge efficiency of a product made by this embodiment is 87.7%. Its reversible specific capacity is 341 mAh/g and it retains 80% of its capacity after 258 cycles. [0086]

Embodiment 4

The unimproved graphite granules are potato shaped natural graphite granules. The material for the coating layer uses thermoset phenol formaldehyde resin as the polymer and anhydrous ethanol as its solvent. The polymer material used for the coating is 15%. [0087]
The fabrication process is as follows: [0088]
(1) Dissolve 15 g of thermoset phenol formaldehyde resin in anhydrous ethanol to formulate 150 ml of 10% polymer solution; [0089]
(2) Dissolve 9 g of coal pitch in tetrahyrdrofuran to formulate for [0090] use 300 ml of 3% of polymer surface modifier;
(3) Use 100 g natural graphite and 50 ml of the polymer solution. Using a custom made sprayer head, spray 50 m. of the polymer solution uniformly on 100 g of the fluidized unimproved graphite granules that are being stirred at high speed. Heat to dry and sift. Repeat twice; [0091]
(4) Immerse the sifted graphite granules in polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules and heat dry. Sift at 300 mesh; and [0092]
(5) Put sifted graphite granules into sealed tubular high temperature furnace. Pass highly pure nitrogen at 10 liter/min of flow volume and increase the temperature at 15° C./min to 400° C. and hold the temperature for 1 hour. Then raise the temperature at 10° C./min to 1000° C. and hold the temperature for four hours before lowering the temperature naturally to room temperature. Remove the improved graphite granules from the furnace. [0093]
The, lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0094] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.75 g.
FIG. 3 is the graph of the cycle characteristics of the battery whose negative electrode is made by improved graphite granules of this embodiment. The initial charge/discharge efficiency is 91.2%. Its reversible specific capacity is 357 mAh/g and it retains 80% of its capacity after 443 cycles. [0095]

Embodiment 5

The unimproved graphite granules are manmade graphite granules. The material for the coating layer uses thermoset phenol formaldehyde resin as the polymer and anhydrous ethanol as its solvent. The polymer material used for the coating is 15%. [0096]
The fabrication process is as follows: [0097]
(1) Dissolve 15 g of thermoset phenol formaldehyde resin in anhydrous ethanol to formulate 100 ml of 15% polymer solution; [0098]
(2) Dissolve 10 g of coke in tetrahydrofuran to form 200 ml of 5% of polymer surface modifier; [0099]
(3) Use 100 g manmade unimproved graphite granules and 50 ml of the polymer solution. Using a custom made sprayer head, spray 50 ml of the polymer solution uniformly on 100 g of the fluidized unimproved graphite granules that are being stirred in high speed. Heat to dry and sift. Repeat once; [0100]
(4) Immerse the sifted graphite granules in the polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules and heat dry. Sift at 300 mesh; and [0101]
(5) Put sifted graphite granules into sealed tubular high temperature furnace. Pass highly [0102] pure nitrogen 10 liter/min of flow volume and increase the temperature at 20° C./min to 350° C. and hold the temperature for 1.5 hours. Then raise the temperature at 10° C./min to 900° C. and hold the temperature for 3 hours before lowering the temperature naturally to room temperature. Remove the improved graphite granules from the furnace.
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0103] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.85 g.
The initial charge/discharge efficiency of a product made by this embodiment is 85.5%. Its reversible specific capacity is 329 mAh/g and it retains 80% of its capacity after 627 cycles. [0104]

Embodiment 6

The unimproved graphite granules are potato shaped natural graphite granules. The material for the coating layer uses epoxy resin as the polymer and acetone as its solvent. The polymer material for the coating is 20%. [0105]
The fabrication process is as follows: [0106]
(1) Dissolve 20 g of epoxy resin in acetone with a predetermined ratio of solidifier in ethanol to formulate 100 ml of 20% polymer solution; [0107]
(2) Dissolve 9 g of petroleum pitch in tetrahydrofuran to formulate for [0108] use 300 ml of 3% of polymer surface modifier;
(3) Use 100 g natural graphite and 50 ml of the polymer solution. Using a custom made sprayer head, [0109] spray 500 ml. of the polymer solution uniformly onto 100 g of the fluidized unimproved graphite granules that are being stirred at high speed. Heat to dry and sift. Repeat one more time. Leave at room temperature for 24 hours;
(4) Immerse the sifted graphite granules in the polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules and heat dry. Sift with 300 mesh; and [0110]
(5) Put sifted graphite granules into sealed tubular high temperature furnace. Pass highly pure nitrogen at 10litre/min of flow volume and increase the temperature at 10° C./min to 850° C. and hold the temperature for 5 hour. Then lower the temperature naturally to room temperature. Remove the improved graphite granules from the furnace. [0111]
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0112] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.88 g.
FIG. 4 is the graph of cycle characteristics of the battery whose negative electrode is made from improved graphite granules of this embodiment. The initial charge/discharge efficiency is 86.1%. Its reversible specific capacity is 338 mAh/g and it retains 80% of its capacity after 204 cycles. [0113]

Embodiment 7

The unimproved graphite granules are potato shaped natural graphite granules. The material for the coating layer uses thermoset phenol formaldehyde resin as the polymer and anhydrous ethanol as its solvent. The polymer material for the coating is 40%. [0114]
The fabrication process is as follows: [0115]
(1) Dissolve 40 g of thermoset phenol formaldehyde resin in anhydrous ethanol to formulate 200 ml of 20% polymer solution; [0116]
(2) Dissolve 10 g of coal pitch in tetrahydrofuran to formulate for [0117] use 200 ml of 5% of polymer surface modifier;
(3) Use 100 g natural graphite and 50 ml of the polymer solution. Using a custom made sprayer head, spray 50 ml of the polymer solution uniformly onto 100 g of the fluidized unimproved graphite granules that are being stirred at high speed. Heat dry and pulverize. Repeat three times; [0118]
(4) Immerse the pulverized graphite granules in the polymer surface modifier. Stir adequately, then filter to obtain filtered graphite granules and heat dry. Sift with 300 mesh; and [0119]
(5) Put the sifted graphite granules into sealed tubular high temperature furnace. Pass highly pure nitrogen at 10 liter/min of flow volume and increase the temperature at 15° C./min to 400° C. and hold the temperature for 1 hour. Then raise the temperature at 10° C./min to 1000° C. and hold the temperature for 4 hours before lowering the temperature naturally to room temperature. Remove the improved graphite granules from the furnace. [0120]
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this embodiment, LiCoO[0121] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.70 g.
The initial charge/discharge efficiency of a product made by this embodiment is 86.9%. Its reversible specific capacity is 381 mAh/g and it retains 80% of its capacity after 280 cycles. [0122]

COMPARISON EXAMPLE 1

The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from potato shaped natural graphite granules from China, LiCoO[0123] ₂as the positive electrode and LiPF₆as the organic electrolyte. The average weight of the negative electrode material is 1.80 g.
FIG. 5 is the graph of the cycle characteristics of the battery whose negative electrode is made from improved graphite granules of this comparison example. The initial charge/discharge efficiency is 88.5%. Its reversible specific capacity is 350 mAh/g and it retains 80% of its capacity after only 34 cycles. [0124]

COMPARISON EXAMPLE 2

The process for the fabrication of the improved graphite granules of this example is identical to Embodiment 4 except it does not use the polymer surface modifier step to treat the material. [0125]
The lithium ion rechargeable battery whose cycle characteristics are tested uses a negative electrode fabricated from the improved graphite granules of this comparison example, LiCoO[0126] ₂as the positive electrode and LiPF₆as the organic electrolyte.
The initial charge/discharge efficiency of the battery made from the product of this comparison example is 83.8%. Its reversible specific capacity is 312 mAh/g and it retains 80% of its capacity after 421 cycles. [0127]
FIGS. 1 and 2 are the scanning electron microscope (using JEOL Company's JSM-5160 model) micrograph of the improved graphite granules. The micrograph shows that the improved graphite granules fabricated using an embodiment of this invention is either potato or spherical shaped. [0128]
The results from the different embodiments and the comparison examples and FIGS. 1-3 show that, the lithium ion battery using the improved graphite granules fabricated by the embodiments of this invention has high initial charge/discharge efficiency, high reversible specific capacity and stable cycle characteristics and can satisfy the demands of practical applications. The fabrication methods of embodiments of this invention are simple and easy to implement, have low costs, and easy to implement for industrial production. [0129]
While the present invention has been described with reference to certain preferred embodiments, it is to be understood that the present invention is not limited to such specific embodiments. Rather, it is the inventor's contention that the invention be understood and construed in its broadest meaning as reflected by the following claims. Thus, these claims are to be understood as incorporating and not only the preferred embodiments described herein but all those other and further alterations and modifications as would be apparent to those of ordinary skilled in the art.[0130]

Claims

We claim:

1. A method for fabricating improved graphite granules from unimproved graphite granules, comprising the steps of:

dissolving a first polymer in a first solvent to form a polymer solution;

fluidizing said unimproved graphite granules;

spraying said polymer solution to evenly coat said fluidized unimproved graphite granules;

drying said sprayed graphite granules of excessive polymer solution;

sifting said dried graphite granules;

solidifying said sifted graphite granules; and

carbonizing said solidified graphite granules to form said improved graphite granules.

2. The method of claim 1 wherein said first polymer is one or more polymers selected from the group consisting of: epoxy resin, phenolic aldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar.

3. The method of claim 1 wherein said first solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane.

4. The method of claim 1 wherein the concentration of said first polymer in said polymer solution is between 1% and 30%.

5. The method of claim 1 wherein said unimproved graphite granules have an average diameter between 5 μm and 40 μm and the weight ratio between said polymer solution and said unimproved graphite granules is between 1:5 and 1:1.

6. The method of claim 1 wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours.

7. The method of claim 6 wherein in said solidification step, the rate for increasing temperature to the first holding temperature is between 5° C./min and 20° C./min.

8. The method of claim 1 wherein in said carbonizing step, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours.

9. The method of claim 8 wherein in said carbonizing step, the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min.

10. The method of claim 8 wherein in said carbonizing step, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.

11. The method of claim 8 wherein in said spraying step, said fluidized graphite granules are being stirred at between 500 rpm and 4000 rpm in three dimensions and in directions that are parallel and perpendicular to the flow of said sprayed polymer solution.

12. The method of claim 9 wherein in said carbonizing step, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.

13. The method of claim 1 wherein after said sifting step and before said solidification step, the following steps are added:

immersing said sifted graphite granules in a polymer surface modifier comprising of a second polymer and a second solvent;

filtering said immersed graphite granules from said polymer surface modifier;

drying said filtered graphite granules with heat; and

sifting said dried graphite granules.

14. The method of claim 13 wherein said second polymer is one or more polymers selected from the group consisting of: coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin.

15. The method of claim 13 wherein said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.

16. The method of claim 14 wherein said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.

17. The method of claim 4 wherein said first polymer is one or more polymers selected from the group consisting of: epoxy resin, phenolic aldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar and said first solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane.

18. The method of claim 17 wherein said unimproved graphite granules have an average diameter between 5 μm and 40 μm and the weight ratio between said polymer solution and said unimproved graphite granules is between 1:5 and 1:1.

19. The method of claim 12 wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours; and the rate for increasing temperature to the first holding temperature is between 5° C./min and 20° C./min.

20. The method of claim 18 wherein after said sifting step and before said solidification step, the following steps are added:

filtering said immersed graphite granules from said polymer surface modifier;

drying said filtered graphite granules with heat; and

sifting said dried graphite granules;

wherein said second polymer is one or more polymers selected from the group consisting of: coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin; and said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane.

21. The method of claim 18 wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours and the rate for increasing temperature to said first holding temperature is between 5° C./min and 20° C./min; and in said carbonizing step, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours and the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min; and, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.

22. The method of claim 18 wherein in said spraying step, said fluidized graphite granules are being stirred at between 500 rpm and 4000 rpm in three dimensions and in directions that are parallel and perpendicular to the flow of said sprayed polymer solution.

23. The method of claim 22 wherein in said solidification step, said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours and the rate for increasing temperature to said first holding temperature is between 5° C./min and 20° C./min; and in said carbonizing step, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours and the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min; and, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.

24. A method for fabricating improved graphite granules from unimproved graphite granules, comprising the steps of:

dissolving a first polymer in a first solvent to form a polymer solution wherein said first polymer is one or more polymers selected from the group consisting of: epoxy resin, phenolic aldehyde resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, coal pitch, petroleum pitch, and coal tar, said first solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, N,N-dimethyl formamide, benzene, toluene, chloroform, and cyclohexane, and, the concentration of said first polymer in said polymer solution is between 1% and 30%;

fluidizing said unimproved graphite granules wherein said unimproved graphite granules have an average diameter between 5 μm and 40 μm and the weight ratio between said polymer solution and said unimproved graphite granules is between 1:5 and 1:1;

spraying said polymer solution to evenly coat said fluidized unimproved graphite granules wherein said fluidized graphite granules are being stirred at between 500 rpm and 4000 rpm in three dimensions and in directions that are parallel and perpendicular to the flow of said sprayed polymer solution;

drying said sprayed graphite granules of excessive polymer solution;

sifting said dried graphite granules;

immersing said sifted graphite granules in a polymer surface modifier comprising of a second polymer and a second solvent wherein said second polymer is one or more polymers selected from the group consisting of: coal pitch, coal tar, petroleum pitch, petroleum tar, benzene, naphthalene, copolymers of benzene and naphthalene, petroleum wax, and petroleum resin, and said second solvent is one or more solvent selected from the group consisting of: acetone, anhydrous ethanol, N-methyl pyrrolidone, chloroform, tetrahydrofuran, carbon tetrachloride, and cyclohexane;

filtering said immersed graphite granules from said polymer surface modifier;

drying said filtered graphite granules with heat;

sifting said dried graphite granules;

solidifying said sifted graphite granules wherein said sifted graphite granules are held in an inert environment at a first holding temperature of between 200° C. and 500° C. for 0.5 hours to 3 hours and the rate for increasing temperature to the first holding temperature is between 5° C./min and 20° C./min; and

carbonizing said solidified graphite granules to form said improved graphite granules wherein, said solidified graphite granules are held in an inert environment at a second holding temperature of between 800° C. and 1100° C. for 2 hours to 10 hours; the rate for increasing temperature to said second holding temperature is 3° C./min to 20° C./min; and, after the solidified graphite granules have been held at said second holding temperature for said predetermined period of time, the rate for decreasing the temperature is 5° C./min to 15° C./min.