WO2016175539A1 - Anode active material and anode including same - Google Patents

Abstract

The present invention relates to an anode active material and a preparation method therefor and, particularly, to an anode active material comprising: a core including artificial graphite and hard carbon; and a shell encompassing the core and including natural graphite, wherein the shell is formed such that the natural graphite is stacked and structured therein so as to cover the surface of the core. The anode active material according to the present invention enables natural graphite to fully encompass artificial graphite and hard carbon, thereby exhibiting a high initial efficiency and lifespan since hard carbon, which exhibits a low initial efficiency and has high electrolyte consumption, is not exposed to the outside. In addition, since all of the natural graphite, artificial graphite, and hard carbon are used, a high output property can be exhibited due to a low diffusion resistance of lithium ions compared to when only the natural graphite is used.

Description

Negative active material and negative electrode comprising same

Cross Citation with Related Application (s)

This application claims the benefit of priority based on Korean Patent Application No. 10-2015-0060452 dated April 29, 2015 and Korean Patent Application No. 10-2016-0049966 dated April 25, 2016. All content disclosed in the literature is included as part of this specification.

Technical Field

The present invention relates to a negative electrode active material and a negative electrode including the same.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing, and the most actively researched fields are power generation and storage using electrochemical reactions.

A representative example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is gradually increasing. Recently, as the development and demand for portable devices such as portable computers, portable telephones, cameras, and the like, the demand for secondary batteries is rapidly increasing, and these secondary batteries exhibit high energy density and operating potential, and have a cycle life. Many researches have been conducted on this long, low self-discharge rate lithium battery and are commercially available and widely used.

In addition, as interest in environmental issues grows, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuel, such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. . As a power source of such electric vehicles and hybrid electric vehicles, nickel-metal hydride secondary batteries are mainly used, but researches using lithium secondary batteries with high energy density and discharge voltage have been actively conducted and some commercialization stages are in progress.

Conventionally, a typical lithium secondary battery uses graphite as a negative electrode active material, and charging and discharging are performed while repeating a process in which lithium ions of a positive electrode are inserted into and detached from a negative electrode. The theoretical capacity of the battery is different depending on the type of the electrode active material, but as the cycle progresses, the charge and discharge capacity is generally lowered.

On the other hand, as a prior art, spherical natural graphite particles are spherical natural graphite particles assembled in a cabbage phase and a random phase in the center portion of the surface; And spherical natural graphite modified composite particles comprising amorphous or semi-crystalline carbon, wherein a gap between the flaky natural graphite fragments by ultrasonication is present in the surface portion of the spherical natural graphite particles. Amorphous or semicrystalline carbon is coated on the surface of the spherical natural graphite particles, and the amorphous or semicrystalline carbon is present in the gap between the lithium secondary to maintain the gap between the spherical natural graphite particles. A negative electrode active material for a battery and a manufacturing method thereof have been proposed.

However, the anode active material has a problem in that the efficiency of the secondary battery decreases and the amount of electrolyte consumption increases because amorphous carbon appears to the outside.

Accordingly, development of a negative electrode active material which reduces electrolyte consumption and exhibits high initial efficiency and low diffusion resistance of lithium ions is required.

Prior art

Republic of Korea Patent No. 10-1430733

The first technical problem to be solved by the present invention is to provide a negative electrode active material and a method for producing the same, which exhibits high initial efficiency and low diffusion resistance of lithium ions while reducing electrolyte consumption.

The second technical problem to be solved of the present invention is to provide a negative electrode including the negative electrode active material.

The third technical problem to be solved of the present invention is to provide a secondary battery including the negative electrode, a battery module and a battery pack having the same.

In order to solve the above problems, an embodiment of the present invention includes a core comprising artificial graphite and hard carbon; And a shell surrounding the core and including natural graphite, wherein the shell provides a negative electrode active material formed so that the natural graphite is laminated and covered to cover the surface of the core.

In addition, in one embodiment of the present invention; Hard carbon; And spherical natural graphite having an average length of 20 to 30 times longer with respect to a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average particle diameter of the hard carbon. It provides a method for producing a negative electrode active material comprising the step.

In addition, an embodiment of the present invention provides a negative electrode on which a negative electrode mixture including the negative electrode active material is coated on a negative electrode current collector.

In addition, an embodiment of the present invention provides a secondary battery including a positive electrode and an electrolytic solution coated with a positive electrode mixture including the negative electrode and the positive electrode active material.

The present invention also provides a battery module and a battery pack including the secondary battery.

The negative electrode active material according to the present invention has a structure in which natural graphite completely surrounds artificial graphite and hard carbon, and thus exhibits low initial efficiency and high initial consumption and life characteristics because hard carbon having high electrolyte consumption is not exposed to the outside. Can be represented.

In addition, since both natural graphite, artificial graphite, and hard carbon are used as the negative electrode active material component, a negative electrode having high output characteristics may be manufactured because the diffusion resistance of lithium ions is lower than that of only natural graphite.

1 is a schematic diagram showing the structure of a negative electrode active material according to the present invention.

2 is a graph showing the initial efficiency measurement results according to Experimental Example 1 of the present invention.

Figure 3 is a graph of the results of measuring the irreversible capacity in the first charging process of the battery prepared in Examples 1, 2 and Comparative Examples 1 to 3 of the present invention.

Figure 4 is a graph of the results of measuring the life characteristics of the batteries prepared in Example 1 and Comparative Example 4 of the present invention.

5 is a graph showing the results of measuring the life characteristics of the batteries prepared in Example 1 and Comparative Example 2 of the present invention.

Explanation of the sign

1: artificial graphite

3: hard carbon

5: natural graphite

10: negative electrode active material

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

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

As used herein, the terms "comprise", "comprise" or "have" are intended to indicate that there is a feature, number, step, component, or combination thereof, that is, one or more other features, It should be understood that it does not exclude in advance the possibility of the presence or addition of numbers, steps, components, or combinations thereof.

Among the materials used as the graphite-based negative electrode material of the conventional secondary battery, natural graphite has a problem of high diffusion resistance of lithium ions due to the close distance between graphenes, and hard carbon has a low initial efficiency of the battery due to high electrolyte consumption. There is this.

Accordingly, the present invention is to provide a negative active material and a negative electrode and a secondary battery comprising a low carbon electrolyte and a low diffusion resistance of lithium ions while preventing the hard carbon from being directly exposed to the electrolyte, showing a high initial efficiency and low electrolyte consumption .

Specifically, a schematic diagram of the negative electrode active material according to the present invention is shown in Figure 1, with reference to this will be described in detail the negative electrode active material according to the present invention.

The negative electrode active material 10 according to the present invention,

A core comprising artificial graphite (1) and hard carbon (3); And

And a shell surrounding the core and comprising natural graphite (5),

The shell may be a negative electrode active material formed by stacking the natural graphite and covering the surface of the core.

First, in the negative electrode active material of the present invention, the artificial graphite contained in the core is crystalline carbon in which coke powder or the like is artificially developed crystals through a high temperature baking process. Examples of the artificial graphite are artificial graphite heat treated at 2800 ° C. or higher, graphitized MCMB obtained by heat treating MCMB (MesoCarbon MicroBeads) at 2000 ° C. or higher, or graphitized mesophase heat treated at 2000 ° C. or more with mesophase pitch-based carbon fibers. It may include a pitch-based carbon fiber, but is not limited thereto.

The artificial graphite may be in the shape of scales, such as pieces of scales, and the average length L ₅₀ of the long axis may be 5 μm to 7 μm.

Since the artificial graphite is heat treated at about 3,000 ° C. during the manufacturing process, all functional groups on the surface are removed. Therefore, the irreversible reaction due to side reactions is reduced, thereby exhibiting a high initial efficiency.

In the present invention, by using the artificial graphite as a core of the negative electrode active material, it is possible to supplement the disadvantage that the hard carbon and natural graphite shows a relatively low initial efficiency.

In addition, in the negative electrode active material of the present invention, the hard carbon contained in the core is sucrose (sucrose), phenol resin (phenol resin), furan resin (furan resin), furfuryl alcohol (furfuryl alcohol), polyacrylonitrile ( Polyacrylonitrile, polyimide, epoxy resin, cellulose and styrene may include carbonized one or more carbonaceous material selected from the group consisting of phenol resin plastic body, Polyacrylonitrile-based carbon fiber, pseudoisotropic carbon, and furfuryl alcohol resin fired body (PFA). In addition, the hard carbon may include amorphous carbon.

The hard carbon may have a spherical shape, and the average diameter (D ₅₀ ) of the hard carbon may be 4 μm to 6 μm.

In the present invention, by using the hard carbon as a core of the negative electrode active material, the problem of low solid diffusion of lithium ion migration due to high crystallinity of graphite can be overcome, and the buffering capacity against volume expansion is improved. Can be.

In the negative electrode active material according to an embodiment of the present invention, the average diameter (D ₅₀ ) of the core may be 9 ㎛ to 13 ㎛.

In addition, the core may include a total of about 2 to 4 artificial graphite and hard carbon, but is not limited thereto. Specifically, the weight ratio of the artificial graphite and hard carbon may be in the range of 1: 0.1 to 1.0, specifically 1: 0.66. By including the artificial graphite and hard carbon in the weight ratio, it is possible to implement a negative electrode active material having a high lithium ion diffusion degree, strong adhesion and buffering capacity against volume expansion.

In addition, the negative electrode active material according to an embodiment of the present invention may include a shell surrounding the core and containing natural graphite.

The shell may be formed in such a way that the natural graphite is stacked and formed to cover the surface of the core. Specifically, the shell is in the form of completely enclosing the artificial graphite and hard carbon inside the core, the plurality of long piece natural graphite in the shell is laminated in a random direction to cover all of the surface of the core, and consequently the core and shell The negative electrode active material including may be in the form of a sphere. In particular, in the negative electrode active material of the present invention, since the shell is formed in a form surrounding the entire core surface so that artificial graphite and hard carbon inside the core are not exposed to the outside of the shell, the hard carbon constituting the core is exposed to the electrolyte solution. Prevent it. Therefore, the secondary battery manufactured using the negative electrode active material of the present invention can realize the effect of showing a high initial efficiency.

In this case, the natural graphite contained in the shell may include crystalline carbon.

Particularly, the natural graphite has a long average length (L ₅₀ ) of 20 to 30 times that of a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average diameter of the hard carbon. Natural graphite. The long piece means a long shape, and specifically, an aspect ratio (length of a long axis / length of a short axis) of natural graphite of the long piece may be 25 to 100. In the case of using natural graphite of the above length, the surface of the core may be completely wrapped so that artificial graphite and hard carbon used as the core cannot come into contact with the outside. If the average length of the natural graphite is less than 20 times with respect to a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average particle diameter of the hard carbon, the core may not be completely wrapped. In the case where the hard carbon is exposed to the electrolyte and exhibits low initial efficiency, and has a length of more than 30 times, the hard carbon is bound to the electrolyte while the long carbons are united together instead of surrounding the hard carbon and the artificial graphite core. The exposure may cause a problem that the initial efficiency is lowered.

Specifically, in the case of the negative electrode active material of the present invention, the diameters of artificial graphite and hard carbon in the core may be about 4 μm to 7 μm, and the length of natural graphite in the shell may be 80 μm to 120 μm. At this time, the artificial graphite and hard carbon can be completely wrapped while several to several tens of pieces of natural graphite are stacked in a random direction on the surface of the core formed by several to several tens of artificial graphite and hard carbon.

In addition, in the negative electrode active material of the present invention, the thickness of the shell may be a distance from the inner portion of the natural graphite located closest to the center of the core to the outer portion of the natural graphite located farthest.

Specifically, the thickness of the shell may range from 5 μm to 12 μm. If the thickness of the shell is less than 5 μm, there is a high possibility that the hard carbon and electrolyte are in contact with a large amount of electrolyte consumption, and thus there may be a problem of low initial efficiency and a reversible capacity decrease of the battery, and the thickness of the shell is 12 μm. If it exceeds the problem that the diffusion resistance of lithium ions may increase.

The average diameter (D50) of the negative electrode active material of the present invention may be 14 ㎛ to 25 ㎛.

In the negative electrode active material of the present invention, the weight ratio of the core and the shell may be included in the range of 1: 0.5 to 1.5, specifically 1: 1. By satisfying the weight ratio, while exhibiting a high lithium ion diffusion degree of the core, the shell can surround the core and prevent the reaction with the electrolyte, thereby exhibiting excellent initial efficiency.

In addition, in one embodiment of the present invention

Artificial graphite; Hard carbon; And a long piece of natural graphite having an average length of 20 to 30 times longer with respect to a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average particle diameter of the hard carbon. It provides a method for producing a negative electrode active material comprising the step of.

As in the present invention, spherical natural graphite having a length of 20 to 30 times with respect to a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average particle diameter of the hard carbon is spherical. In this case, the long piece of natural graphite is formed in a form surrounding the artificial graphite and hard carbon, and specifically, the plurality of long piece of natural graphite is laminated in a random direction on the surfaces of the plurality of artificial graphite and hard carbon, A negative electrode active material of a core-shell structure in which the surface of is not exposed to the outside can be prepared.

On the other hand, the spheronization may be performed through a drum mixer, but dry tumbler, super mixer, Henschel mixer, flash mixer, air blender, flow jet mixer, ribocon mixer, pug mixer, Nauta mixer, ribbon mixer, spartan riser, A ready-mixed mixer, a planetary mixer, a device such as a screw-type kneader, a defoaming kneader, a paint shaker, a press kneader, a kneader such as two rolls, etc. may be used, but the apparatus for spheronization is limited thereto. It is not necessary to select a device capable of mixing two or more materials as appropriate.

In this case, the drum mixer may be rotated for 120 minutes to 150 minutes at a rotation speed of 700 rpm to 1,000 rpm.

If the rotation speed of the dream mixer is less than 700 rpm and less than 120 minutes, there is a problem that natural graphite is not spherical in the form of wrapping artificial graphite and hard carbon due to insufficient rotational force, and the rotation speed is 1,000 rpm. If it is performed in excess of 150 minutes, the area of artificial graphite and hard carbon that cannot be wrapped while the natural graphite of the long piece is broken due to excessive rotational force is increased, and thus there is a problem that the initial efficiency is lowered.

In addition, an embodiment of the present invention provides a negative electrode on which a negative electrode mixture including the negative electrode active material is coated on a negative electrode current collector.

Specifically, the negative electrode may be prepared by mixing a negative electrode mixture including the negative electrode active material of the present invention in an organic solvent to prepare a slurry, and then applying the same to a positive electrode current collector, followed by drying and rolling.

In this case, the negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like on the surface of may be used.

Since the negative electrode active material of the present invention completely encapsulates artificial graphite and hard carbon, natural graphite may exhibit low initial efficiency and high initial efficiency and lifetime characteristics because hard carbon having high electrolyte consumption is not exposed to the outside. In addition, since both natural graphite, artificial graphite, and hard carbon are used, the diffusion resistance of lithium ions is lower than that of natural graphite, resulting in high output characteristics.

Meanwhile, the negative electrode mixture of the present invention may further include at least one or more of a conductive material, a binder, and a filler in some cases.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The conductive material may typically be included in an amount of 1 to 30% by weight based on the total weight of the mixture including the negative electrode active material.

The binder is not particularly limited as long as the component assists in bonding the active material and the conductive material and bonding to the current collector, and is not particularly limited. For example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, and hydroxide Roxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluorine rubber, various airborne Coalescence, etc. are mentioned.

The binder may be typically included in an amount of 1 to 30% by weight based on the total weight of the mixture including the negative electrode active material.

The filler may be optionally used as a component for inhibiting the expansion of the electrode, and is not particularly limited as long as it is a fibrous material that does not cause chemical change in the battery, for example, an olefin polymer such as polyethylene, polypropylene; Fibrous materials, such as glass fiber and carbon fiber, can be used.

In addition, in one embodiment of the present invention

It includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, the negative electrode provides a lithium secondary battery comprising the negative electrode of the present invention.

The positive electrode may be prepared by a conventional method known in the art, for example, coating a positive electrode active material slurry on a positive electrode current collector, compressing the same, and drying the same.

In this case, the positive electrode active material slurry may further include a positive electrode active material, and optionally a conductive material, a binder and a filler.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, the positive electrode current collector may be formed of stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. Surface treated with nickel, titanium, silver, or the like may be used.

In addition, the positive electrode active material may use a lithium transition metal oxide as a specific example. Examples of the lithium transition metal oxide include Li.Co-based composite oxides such as LiCoO ₂ , Li.Ni.Co.Mn-based composite oxides such as LiNi _x Co _y Mn _z O ₂ , and Li.sub.2 such as LiNiO ₂ . Ni-based composite oxide may be mentioned, such as LiMn ₂ O ₄ of the Li-Mn composite oxide such, may be mixed alone or a plurality of them.

The conductive material, the binder and the filler may be the same as or different from those included in the negative electrode mixture.

The non-aqueous electrolyte may be composed of an electrolyte and a lithium salt, and a non-aqueous organic solvent may be used as the electrolyte.

As the non-aqueous organic solvent, for example, N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1,2-dime Methoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxoron, acetonitrile, nitromethane, methyl formate, Methyl acetate, phosphate triester, trimethoxy methane, dioxoron derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate Or an aprotic organic solvent such as ethyl propionate can be used.

The lithium salt is a material that is good to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, chloroborane lithium, lower aliphatic lithium carbonate, lithium phenyl borate, imide and the like can be used. .

In particular, when using propylene carbonate (PC) as the non-aqueous electrolyte of the conventional secondary battery, artificial graphite used in the negative electrode active material may cause an exfoliation problem. However, since the negative electrode active material of the present invention has a structure in which the surface of artificial graphite is surrounded by natural graphite as a core / shell structure, the phenomenon of artificial graphite being peeled off by the propylene carbonate electrolyte can be suppressed.

Further, according to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same are provided. Since the battery module and the battery pack include the secondary battery that is stable and exhibits excellent efficiency and output characteristics, a power tool, an electric vehicle (EV), a hybrid electric vehicle (HEV), And an electric vehicle including a plug-in hybrid electric vehicle (PHEV), or a power storage system.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

Example

Example  One.

(Step 1) 300 g of flaky artificial graphite having an average length (L ₅₀ ) of a long axis, 200 g of hard carbon having an average diameter (D ₅₀ ) of 4 μm, and a natural of 100 μm of an average length (L ₅₀ ) 500 g of graphite was added to a drum mixer, and the mixture was rotated at a speed of 800 rpm for 120 minutes to prepare a negative active material in which artificial graphite and hard carbon were wrapped in natural graphite.

(Step 2) After preparing the negative electrode mixture by the negative electrode active material prepared in step 2 to a weight ratio of negative electrode active material: SBR: CMC = 97.0: 1.5: 1.5, after coating the negative electrode mixture on a copper foil, Drying and rolling to produce a coin type half cell (half coin cell). In the cell, a counter electrode was used as lithium metal, and the electrolyte was an electrolyte in which 1 M of LiPF ₆ was dissolved in a carbonate solvent.

Example  2.

In Step 1 of Example 1, except that the average length (L ₅₀ ) of 90 ㎛ natural graphite was carried out in the same manner as in Example 1 to prepare a coin-type half cell.

Comparative example  One.

In Step 1 of Example 1, except that the average length (L ₅₀ ) of 50 ㎛ natural graphite was carried out in the same manner as in Example 1 to prepare a coin-type half cell.

Comparative example  2.

In Step 1 of Example 1, a coin-type half cell was prepared in the same manner as in Example 1 except that the average length (L ₅₀ ) was 20 μm of natural graphite.

Comparative example  3.

In step 1 of Example 1, a coin-type half cell was prepared in the same manner as in Example 1 except that the average length (L ₅₀ ) was 10 μm of natural graphite.

Comparative example  4.

In step 1 of Example 1, a coin-type half cell was prepared in the same manner as in Example 1, except that the average length (L ₅₀ ) of 128 μm of natural graphite was used.

Comparative example  5.

In Step 1 of Example 1, instead of adding the flaky artificial graphite and hard carbon, the same as in Example 1 except that only the natural graphite having an average length (L ₅₀ ) of 100 ㎛ in the drum mixer A coin-type half cell was prepared by the above procedure.

Experimental Example

Experimental Example  1. Observation of the form of the negative electrode active material

In order to observe the form of the negative electrode active material prepared in step 1 of Example 1, the negative electrode active material was dispersed in a antacid in a solution of the mixture of the prepared negative electrode active material in the ratio of 95% water, 5% negative electrode active material, and then It was observed with a particle size analyzer (device name: mastersizer3000, company: Malvern Instruments Ltd).

As a result, it can be seen that a spherical negative electrode active material having a diameter ranging from 16 μm to 20 μm is produced.

Experimental Example  2. Initial efficiency measurement

Initial efficiency of the coin-type half cells prepared in Examples 1 and 2 and Comparative Examples 1 to 4 was measured, and the results are shown in Table 1. Specifically, the initial efficiency, after charging the CC mode to 5 mV at a current density of 0.1 C at the time of charging, and maintained constant at 5 mV in CV mode to complete the charging when the current density is 0.005 C. The initial efficiency was obtained by completing the discharge in CC mode to 1.5 V with a current density of 0.1 C during discharge. The results are shown in Table 1 and FIG. 2.

	Hard Carbon Average Diameter (㎛)	Natural Graphite Average Length (㎛)	Average diameter of hard carbon: ratio of average length of natural graphite	Initial Efficiency (%)
Example 1	4	100	1:25	92.2
Example 2	4	90	1: 22.5	91.3
Comparative Example 1	4	50	1: 12.5	88.9
Comparative Example 2	4	20	1: 5	86.1
Comparative Example 3	4	10	1: 2.5	85.3
Comparative Example 4	4	128	1:32	87.8

As shown in Table 1, Examples 1 and 2 shows an initial efficiency of about 91 to 92%, whereas Comparative Examples 1 to 4 shows an initial efficiency of about 85 to 89%. In particular, it can be seen that Example 1 exhibits an initial efficiency of about 7% higher than that of Comparative Example 3.

Through this, when using a long piece of natural graphite having a length of 20 to 30 times longer than the diameter of a relatively small hard carbon as in Examples 1 and 2, the long piece having a length of less than 20 times the diameter of the hard carbon Unlike Comparative Examples 1 to 3 using natural graphite on the phase, since both artificial graphite and hard carbon inside the core can be wrapped, it can be seen that the efficiency is higher than 90%, which is the initial efficiency of conventional natural graphite. It can be seen that Examples 1 to 3 show a low initial efficiency of conventional hard carbon because hard carbon is directly exposed to the electrolyte because the core is not tightly wrapped using natural graphite of short length.

On the other hand, in the case of Comparative Example 4, by using a long piece of natural graphite having a length of more than 30 times the diameter of the hard carbon, it is hard to bundle the long piece of natural graphite to surround the hard carbon and artificial graphite core, It can be seen that carbon is exposed to the electrolyte and the initial efficiency decreases.

Experimental Example  3. Irreversible  Capacity measurement

The irreversible capacity during the first charging process of the coin-type half cells prepared in Examples 1 and 2 and Comparative Examples 1 to 3 was measured, and the results are shown in FIG. 3.

Specifically, after charging CC mode up to 5 mV at a current density of 0.1 C during charging, charging was continued at 5 mV in CV mode to complete charging when the current density became 0.005 C. The discharge was completed in the CC mode to 1.5 V at a current density of 0.1 C during the discharge to obtain a graph of the irreversible region.

As shown in FIG. 3, at 0.9 V, the voltage of the region corresponding to the irreversible reaction, the highest value of 0.020 dQ / dV in Comparative Example 2, 0.015 dQ / dV in Comparative Example 3, and 0.012 dQ / in Comparative Example 1 dV, Example 2 were found to be 0.009 dQ / dV, and Example 1 was found to be 0.008 dQ / dV. Therefore, in the case of Example 1, it can be seen that the numerical value is about 1/2 times less than the comparative example.

As a result, when using natural graphite having a length of 20 to 30 times longer than artificial graphite and hard carbon particles of the core, as in Examples 1 and 2, unlike the case of using natural graphite of the fragment, hard carbon inside the core Since it can be wrapped all, it can be seen that it can exhibit a high reversible capacity because it can prevent the reaction of the hard carbon and the electrolyte.

Experimental Example  4. Observe the life characteristics

The lifetime characteristics through charge and discharge of the coin-type half cells prepared in Example 1 and Comparative Examples 2 and 5 were measured, and the results are shown in FIGS. 4 and 5.

Specifically, after charging CC mode up to 5 mV at a current density of 0.1 C during charging, charging was continued at 5 mV in CV mode to complete charging when the current density became 0.005 C. The discharge was completed in CC mode to 1.5 V at a current density of 0.1 C during discharge. Thereafter, only the current density was changed to 0.5 C, and the remaining charge and discharge were repeated 160 times under the above conditions.

As shown in FIG. 4, the coin cell of Example 1, in which three carbon materials were mixed and used as a negative electrode active material, was shown to have about 10% higher discharge capacity retention characteristics than Comparative Example 5 using only natural graphite. Can be.

In addition, as shown in Figure 5, even if the same three kinds of carbon-based material is mixed, in the case of the coin cell of Example 1 using natural graphite having a length of 20 times or more than the hard carbon, the length of the fragment relatively It can be seen that the discharge capacity retention characteristics of about 5% are superior to those of the coin cell of Comparative Example 2 using natural graphite having.

As a result, when artificial graphite and hard carbon are further added than natural graphite only as a negative electrode active material, it can be seen that a secondary battery having improved life characteristics can be manufactured by lowering resistance to diffusion of lithium ions. Even when the three kinds of carbon-based materials are mixed and used, when the length of natural graphite is particularly long enough to completely wrap the hard carbon and the hard carbon, the electrolyte may be caused by the contact between the hard carbon and the electrolyte. Since the reaction can be reduced, it can be seen that it can exhibit better life characteristics than when using the natural graphite of the fragment.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of the invention.

Claims (18)

1. A core comprising artificial graphite and hard carbon; And

   And a shell surrounding the core and including natural graphite.

   The shell is a negative active material for a secondary battery formed so that the natural graphite is laminated and covered to cover the surface of the core.
2. The method according to claim 1,

   The natural graphite is a long piece of natural graphite having an average length of 20 to 30 times longer with respect to a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average particle diameter of the hard carbon. Anode active material for secondary battery.
3. The method according to claim 1,

   The hard carbon has a diameter of 4 μm to 7 μm, and the length of the natural graphite is 80 μm to 120 μm.
4. The method according to claim 1,

   The diameter of the core is 9 ㎛ to 13 ㎛, the thickness of the shell is 5 ㎛ to 12 ㎛ negative electrode active material.
5. The method according to claim 1,

   The negative electrode active material has a diameter of 14 μm to 25 μm.
6. The method according to claim 1,

   The hard carbon is at least one selected from the group consisting of phenol resin fired body, polyacrylonitrile-based carbon fiber, pseudoisotropic carbon, and furfuryl alcohol resin fired body (PFA).
7. The method according to claim 1,

   The artificial graphite and hard carbon are negative electrode active materials for secondary batteries having a weight ratio of 1: 0.1 to 1.0.
8. The method according to claim 1,

   The weight ratio of the core and the shell is 1: 0.5 to 1.5 negative electrode active material for secondary batteries.
9. Artificial graphite; Hard carbon; And a long piece of natural graphite having an average length of 20 to 30 times longer with respect to a material having a relatively smaller average length or average particle diameter among the average length of the artificial graphite or the average particle diameter of the hard carbon. Method of manufacturing a negative active material for a secondary battery of claim 1 comprising the step of.
10. The method according to claim 9,

    The spheronization is a method of manufacturing a negative active material for a secondary battery that is performed through a drum mixer.
11. The method according to claim 9,

    The drum mixer is a method for producing a negative active material for a secondary battery to spheroidized natural graphite by rotating for 120 minutes to 150 minutes at a rotation speed of 700 rpm to 1,000 rpm.
12. The negative electrode for secondary batteries which the negative electrode mixture containing the negative electrode active material of Claim 1 is apply | coated on the negative electrode collector.
13. Lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, the negative electrode comprises the negative electrode of claim 12.
14. The method according to claim 13,

    The non-aqueous electrolyte is a lithium secondary battery containing a non-aqueous organic solvent and a lithium salt.
15. The method according to claim 14,

    The non-aqueous organic solvent is a lithium secondary battery containing propylene carbonate.
16. A battery module comprising the lithium secondary battery of claim 13 as a unit cell.
17. A battery pack comprising the battery module of claim 16 and used as a power source for medium and large devices.
18. The method according to claim 17,

    The medium-to-large device is a battery pack that is selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

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