KR20010054904A - Negative active material for lithium secondary battery and method of preparing same - Google Patents

Abstract

PURPOSE: Provided is a cathode active material for a lithium secondary battery, which contains a mesophase carbon microbeads core and an amorphous carbon layer formed on the core and is excellent in charging and discharging capacity and efficiency. CONSTITUTION: The cathode active material is produced by a process comprising the steps of: heat-treating coal- or petroleum-based pitch to form the mesophase carbon microbeads; adding amorphous hard carbon or a precursor thereof to the mesophase carbon microbeads when the mesophase carbon microbeads having a size of more than 1 micrometer is more than 10wt% based on the total mesophase carbon microbeads; separating the mesophase carbon microbeads; carbonizing the mesophase carbon microbeads; graphitizing the carbonized mesophase carbon microbeads. The thickness of the amorphous carbon layer is 0.1-20 micrometer.

Description

Negative active material for lithium secondary battery and manufacturing method thereof {NEGATIVE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY AND METHOD OF PREPARING SAME}

[Industrial use]

The present invention relates to a negative electrode active material for a lithium secondary battery and a manufacturing method thereof, and more particularly, to a negative electrode active material for a lithium secondary battery excellent in charge and discharge performance.

[Prior art]

The carbon material used as a negative electrode material of a lithium secondary battery can be roughly classified into amorphous carbon and crystalline graphite according to crystallinity. Among them, crystalline graphite generally used can be classified into artificial graphite and natural graphite. Typical examples of artificial graphite include mesophase carbon microbeads, mesocarbon fibers, and the like. All of them are negative electrode materials used in lithium ion secondary batteries. Mesophase carbon microbeads are propylene which has a strong electrolytic decomposition reaction due to damage to the onion structure of the surface layer due to the negative active material having excellent shape and graphitization degree, or strong organic solvent during mesophase carbon microbead extraction during the manufacturing process. In the carbonate electrolyte, the battery performance is deteriorated.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a negative electrode active material for a lithium secondary battery in which the electrolyte solution decomposition reaction of the edge surface is suppressed.

Another object of the present invention is to provide a method for producing the negative electrode active material for a lithium secondary battery.

In order to achieve the above object, the present invention is a mesophase carbon microbead core; And to provide a negative electrode active material for a lithium secondary battery comprising an amorphous carbon layer formed on the core.

The invention also comprises the steps of heat treating coal-based or petroleum-based pitch to form mesophase carbon microbeads; Separating the prepared mesophase carbon microbeads from the heat-treated material; Carbonizing the mesophase carbon microbeads; And graphitizing the carbonized mesophase carbon microbeads, wherein the mesophase carbon microbeads having a size of 1 μm or more are 10% by weight of the total mesophase carbon microbeads. The above provides a method for producing a negative active material for a lithium secondary battery, further comprising the step of adding amorphous hard carbon or a precursor thereof to mesophase carbon microbeads.

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

The negative electrode active material for lithium secondary batteries of the present invention is a mesophase carbon microbead negative electrode active material. The negative active material of the present invention includes mesophase carbon microbeads as a core, and includes an amorphous carbon layer formed on the core.

As the amorphous carbon layer is formed on the surface of the mesophase carbon microbead of the present invention, the decomposition reaction by the electrolyte solution is suppressed, thereby improving the performance of the battery, and the electrolyte solution can be used without any kind.

The manufacturing method of the negative electrode active material of this invention is as follows.

Coal or petroleum pitch is heat treated to form mesophase carbon microbeads. The heat treatment temperature is carried out at 300 to 450 ℃. When mesophase carbon microbeads having a size of 1 μm or more become 10 wt% or more of the total weight of mesophase carbon microbeads, an amorphous hard carbon or a precursor thereof is added. In the present invention, the amorphous hard carbon or its precursor is added after the mesophase carbon microbeads are formed, and thus the amorphous hard carbon or its precursor cannot be penetrated into the very dense and very compact mesophase carbon microbeads. In contrast, if amorphous hard carbon or a precursor thereof is mixed with a pitch, and the pitch is treated with an organic solvent, then the organic solvent soluble pitch is heat-treated to form mesophase carbon microbeads, the amorphous hard carbon or precursor thereof is It penetrates into the formed mesophase carbon microbeads and prevents an increase in crystallinity of the final active material. Therefore, the final active material shows both amorphous and crystalline properties, and in particular, the capacity can be increased as the amorphous properties are strongly shown, but the irreversible capacity is increased and the cycle life characteristics are lowered.

Precursors of amorphous hard carbon may be polymer resins such as phenol resins, naphthalene resins, polyvinyl alcohol resins, urethane resins, polyimide resins, furan resins, cellulose resins, epoxy resins, and polystyrene resins. As amorphous hard carbon, all amorphous hard carbon prepared using such a precursor can be used, and carbon black can also be used. Preferably, carbon black may be used, and carbon black generally has a very small size of submicron and its surface area is very large, thereby increasing the degree of graphitization. Amorphous hard carbon or its precursor may be added as it is, or may be pulverized to add amorphous hard carbon or its precursor having a submicron particle size.

When the amount of the amorphous hard carbon or its precursor is added in an amount of 0.1 to 35% by weight, and when the amount of the amorphous hard carbon or its precursor is added in an amount of less than 0.1% by weight, the amorphous carbon layer is hardly formed on the surface of the mesophase carbon microbeads. When it exceeds weight%, the amorphous carbon layer is formed too thick, and lithium ion charging and discharging does not proceed well.

The prepared mesophase carbon microbeads are separated from the heat-treated material by using an organic solvent. In general, pyridine, quinoline, or tetrahydrofuran may be used as the organic solvent. The mesophase carbon microbeads are carbonized at 300 to 450 ° C. under an inert atmosphere such as nitrogen. The carbonized mesophase carbon microbeads are graphitized at 2000 to 3000 ° C.

Conventionally, in order to control the size of mesophase carbon microbeads, a method of adding carbon black or needle cokes during the process of manufacturing mesophase carbon microbeads has been used (Nineth biennial Conference on Carbon, June 25-30 1989, 124-125). This method suppresses the coalescence growth of mesophase carbon microbeads by mixing heat treatment by mixing isotropic pitch with a minimum of carbon black to prevent the microbeads from growing and sticking more than a certain size. . In contrast, the present invention is a method of adding sufficient amorphous hard carbon at a time when mesophase carbon microbeads are formed to some extent. That is, according to the method of the present invention, the amorphous hard carbon may be formed to surround the surface of the mesophase sphere to form a coating, and through the final heat treatment, a mesophase carbon microbead having a naturally amorphous coating may be prepared, and the amorphous formed on the surface Due to the coating, it is possible to suppress the electrolyte decomposition reaction of the edge surface. Accordingly, propylene carbonate, which is excellent in low temperature characteristics as an electrolyte solution but is difficult to use in the past due to severe electrolyte solution decomposition reaction, can also be used.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only one preferred embodiment of the present invention and the present invention is not limited to the following examples.

(Example 1)

The tetrahydrofuran water-soluble pitch which melt | dissolves in tetrahydrofuran was heated up at 5 degree-C / min, and it maintained at 370 degreeC. Mesophase carbon microbeads were formed at this time. When mesophase carbon microbeads having a particle diameter of 8 µm or more occupy 50% or more of the total weight of mesophase carbon microbeads, carbon black having an average particle size of 40 nm is added to the mesophase carbon microbeads and stirred to uniformly disperse the carbon black. Dispersed. The addition amount of carbon black was 5 weight% of the weight of tetrahydrofuran water soluble pitch. When the prepared mesophase carbon microbead having a 30㎛ or more was extracted using pyridine. The extracted mesophase carbon microbeads were subjected to an air stabilization process, carbonized at 1000 ° C. in a nitrogen atmosphere, and then graphitized at 2800 ° C. for 2 hours to obtain mesophase carbon microbead graphitized bodies.

(Example 2)

The same procedure as in Example 1 was carried out except that 10% of carbon black was added.

(Example 3)

The same procedure as in Example 1 was carried out except that 20% of carbon black was added.

(Example 4)

The same process as in Example 1 was conducted except that 10% of the submicron phenolic amorphous hard carbon prepared by grinding was added.

(Comparative Example 1)

The same procedure as in Example 1 was carried out except that carbon black was not added.

(Comparative Example 2)

Natural graphite was used as the negative electrode active material.

(Comparative Example 3)

10 wt% of carbon black and 90 wt% of pitch were mixed and the mixture was treated with tetrahydrofuran to prepare tetrahydrofuran water soluble pitch. This pitch was heated up at 5 degree-C / min, and was maintained at 370 degreeC. Mesophase carbon microbeads were formed at this time. When the prepared mesophase carbon microbead having a 30㎛ or more was extracted using pyridine. The extracted mesophase carbon microbeads were carbonized in a nitrogen atmosphere at 1000 ° C. in an air stabilization process and then graphitized at 2800 ° C. for 2 hours to obtain mesophase carbon microbead graphitized bodies.

(Comparative Example 4)

The tetrahydrofuran water-soluble pitch which melt | dissolves in tetrahydrofuran was heated up at 5 degree-C / min, and it maintained at 370 degreeC. Mesophase carbon microbeads were formed at this time. When the prepared mesophase carbon microbead having a 30㎛ or more was extracted using pyridine. 10 wt% of carbon black was added to 90 wt% of the extracted mesophase carbon microbeads. The mixture was subjected to an air stabilization step, carbonized at 1000 ° C. in a nitrogen atmosphere, and then graphitized at 2800 ° C. for 2 hours to obtain a mesophase carbon microbead graphitized body.

After preparing the slurry using 90% by weight of the polyvinylidene fluoride as a binder and N-methylpyrrolidone as a solvent to 90% by weight of the negative electrode active material prepared by the methods of Examples 1 to 4 and Comparative Examples 1 to 4 It was coated on a Cu foil as a current collector, dried and rolled to prepare a negative electrode plate. A coin-type battery was manufactured by using a lithium metal as a counter electrode and a lithium metal as a counter electrode, and using an electrolyte solution to which LiPF ₆ 1M was added. At this time, as the electrolyte solution, ethylene carbonate (EC), dimethylene carbonate (DMC), diethylene carbonate (DEC) or propylene carbonate (PC) was mixed and used as shown in Table 1 below. Capacity and efficiency after charging and discharging the prepared battery at 0.2C was evaluated, and the results are shown in Table 1 below.

Capacity [mAh / g] / charge and discharge efficiency [%] EC / DMC / DEC EC / EMC / DEC EC / DMC / DEC / PC Example 1 325 / 93.8 326 / 94.0 323 / 93.5 Example 2 325 / 92.8 323 / 93.3 324 / 93.8 Example 3 326 / 92.5 326 / 93.1 325/94 Example 4 324/93 326 / 92.8 324 / 93.2 Comparative Example 1 323 / 93.8 322 / 93.5 312/85 Comparative Example 2 344 / 90.1 342 / 90.6 255/52 Comparative Example 3 330/91 328 / 90.5 321/90 Comparative Example 4 320 / 93.5 324 / 93.3 310/83

Comparative Example 1 is an active material that does not form an amorphous coating film as carbon black in the preparation of mesophase carbon microbeads, and shows a decrease in efficiency due to electrolyte decomposition reaction in a specific electrolyte solution. In Comparative Example 2, it can be seen that the irreversible capacity was increased by the electrolyte decomposition reaction on the open edge as the negative electrode active material. In addition, Comparative Example 3 is an active material in which mesophase carbon microbeads are used using pitches to which carbon black is initially added, and carbon black penetrates into some mesophase carbon microbeads during mesophase carbon growth. As such, the impregnated carbon black interferes with the degree of crystallinity, and thus the formed active material is formed in a mixture of amorphous and crystalline, and thus exhibits some charge and discharge characteristics of amorphous carbon. As a result, it can be seen that the capacity is increased, but the irreversible capacity is increased to lower the charge and discharge efficiency. In Comparative Example 4, as the carbon black was added after the growth of the mesophase carbon microbeads, even if the carbon black did not form an amorphous film and was present on the surface of the mesophase carbon microbeads or formed an amorphous film, the amount was very small. The trend is almost similar to materials without carbon black at all.

On the contrary, as in Examples 1 to 4, when carbon black was added during the preparation of mesophase carbon microbeads, the etching of the edge surface generated in the extraction process using an organic solvent while forming an amorphous coating of carbon black was observed. By suppressing, it can be seen that the decomposition reaction is suppressed in the electrolyte solution, thereby preventing an increase in irreversible capacity.

As described above, the negative electrode active material for the lithium secondary battery of the present invention has an amorphous film formed on the surface thereof to suppress decomposition reaction by the electrolyte solution and has excellent charge and discharge capacity and charge and discharge efficiency.

Claims (5)

Mesophase carbon microbead cores; And An amorphous carbon layer formed on the core A negative electrode active material for a lithium secondary battery comprising a. The negative active material of claim 1, wherein the amorphous carbon layer has a thickness of 0.1 μm to 20 μm. Heat treating coal-based or petroleum-based pitches to form mesophase carbon microbeads; Separating the prepared mesophase carbon microbeads from the heat-treated material; Carbonizing the mesophase carbon microbeads; And Graphitizing the carbonized mesophase carbon microbeads A method for producing a negative electrode active material for a lithium secondary battery comprising a, wherein when the mesophase carbon microbeads having a size of 1 μm or more becomes 10% by weight or more of the total weight of the mesophase carbon microbeads, the amorphous hard carbon or A method for producing a negative electrode active material for a lithium secondary battery, further comprising the step of adding a precursor thereof. The method of claim 3, wherein the amorphous hard carbon is carbon black, the polymer resin carbide, and the precursor of the amorphous hard carbon is a polymer resin. The method according to claim 3, wherein the amount of the amorphous hard carbon or its precursor is added in an amount of 0.1 to 35 wt%.