JP2003031218A - Negative active material for lithium secondary battery and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative active material constituting a lithium secondary battery with high storage characteristics, especially the high storage characteristics under high temperature, and its manufacturing method. SOLUTION: The negative active material is made of powdery carbon material, the carbon material is such a material that three parameters obtained by a powder X-ray diffraction method using CuKα line satisfy the following three conditions: (A) the ratio I110 /I002 of the intensity I110 of diffraction line from the (110) face and the intensity I002 of diffraction line from the (002) face; I110 /I002 <=0.015, (B) the half-value width W( deg.) of the peak in the (002) face; W>=0.2, and (C) the interlay distance d002 (nm); d002 >=0.337. The manufacturing method contains a raw material preparation process preparing a powdery raw carbon material satisfying prescribed conditions, and a crystallinity adjusting process for making the carbon material to satisfy the conditions (A) to (C) by changing the crystallinity of the carbon material.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to storage of lithium
The present invention relates to a negative electrode active material used in a lithium secondary battery utilizing a desorption phenomenon and a method for manufacturing the same.

[0002]

2. Description of the Related Art With the miniaturization of personal computers, video cameras, mobile phones, etc., in the field of information-related equipment and communication equipment, lithium secondary batteries are used as a power source for these equipment because of their high energy density. Has been put into practical use and has become widespread. On the other hand, also in the field of automobiles, the development of electric vehicles has been rushed due to environmental problems and resource problems, and lithium secondary batteries are also being considered as a power source for these electric vehicles.

As described above, the lithium secondary battery has been demanded in a wide variety of fields, but its price is high, so that it is required to have a longer life than other secondary batteries. As one of the requirements for long life, when the lithium secondary battery is stored in a state where the charge rate is kept high, it is required that the so-called storage characteristics are good, for example, the internal resistance of the battery does not increase. To be done. In particular, since the battery reaction is activated at a high temperature and the internal resistance increases greatly, for example, when it is assumed that the lithium secondary battery is used for a power source for an electric vehicle that may be left outdoors, One of the important characteristics is that the storage characteristics at high temperature are good.

At present, a lithium secondary battery uses a carbon material as a negative electrode active material and a lithium transition metal composite oxide as a positive electrode active material for safety reasons such as no dendrite deposition on the negative electrode surface. Is the mainstream. In the carbon material used as the negative electrode active material, graphite is preferably used as a material that can form a battery with high energy density because of its high crystallinity and high density.

[0005]

However, such a lithium secondary battery has a large increase in internal resistance of the battery when stored in a state where the charge rate is kept high, and storage characteristics, particularly storage at high temperature. There was a problem with the characteristics.

The present invention has been made in order to solve the problem of an increase in internal resistance during high temperature storage that a lithium secondary battery using a carbon material as a negative electrode active material has, and has storage characteristics, particularly storage characteristics at high temperatures. It is an object of the present invention to provide a negative electrode active material which can form a good lithium secondary battery.

[0007]

The negative electrode active material for a lithium secondary battery of the present invention comprises a powdery carbonaceous material, which is obtained by a powder X-ray diffraction method using Cu Kα rays. The three parameters of are the following conditions: (A) Intensity I ₁₁₀ of the diffraction line on the (110) plane and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.015 (B) Half-width W (°) of peak of (002) plane; W
≧ 0.2 (C) interlayer distance d ₀₀₂ (nm); d ₀₀₂ ≧ 0.337 are satisfied.

Among the carbon materials used as the negative electrode active material, for example, artificial graphite is a highly crystallized material obtained by firing the carbon material at a high temperature of 2800 ° C. or higher. Therefore, it is considered that when the temperature is returned to room temperature after firing, strain is generated inside the graphite particles and a large stress is generated inside the particles. The present inventors have found that the increase in internal resistance due to storage of a lithium secondary battery is
It was considered that this was due to a change in the crystallinity of the negative electrode active material, graphite. That is, when the lithium secondary battery is stored for a long time,
It is considered that the internal stress of graphite, which is the negative electrode active material, significantly changes the crystallinity, and as a result, the internal resistance of the battery increases. Therefore, it was found that the internal stress of graphite can be relaxed and the change in crystallinity can be suppressed to suppress an increase in internal resistance of the lithium secondary battery due to long-term storage.

The negative electrode active material for a lithium secondary battery of the present invention comprises a powdery carbonaceous material, and the above parameters satisfy the respective conditions. Where (110)
The ratio I ₁₁₀ / I ₀₀₂ of the intensity I ₁₁₀ of the diffraction line of the plane to the intensity I ₀₀₂ of the diffraction line of the (002) plane is a parameter representing the degree of graphitization, and the smaller the value of I ₁₁₀ / I ₀₀₂ is, the smaller the graphite is. It shows that the structure is well developed. With ordinary graphite having high crystallinity, the value of I ₁₁₀ / I ₀₀₂ is 0.01 or less. Also,
The half-width W (°) of the peak of the (002) plane is a parameter indicating the orientation of crystallites, etc., and the smaller the value of W, the higher the orientation of crystallites and the uniform interlayer distance. ing. For graphite, W is typically less than 0.2 °. Further, the inter-layer distance d ₀₀₂ (nm) is the inter-layer distance of the hexagonal carbon network, and in an ideal graphite structure, the value of d ₀₀₂ is 0.33.
It becomes 48 nm. Therefore, the value of d ₀₀₂ is 0.334.
The closer it is to 8 nm, the higher the degree of graphitization and the higher the crystallinity.

On the other hand, for example, by lowering the firing temperature, it is possible to produce graphite with low crystallinity in which the graphite structure is not well developed. In this way, graphite having originally low crystallinity (hereinafter referred to as “low crystalline graphite” in the present specification) has a large I ₁₁₀ / I ₀₀₂ value of about 0.05 and a d ₀₀₂ value of 0. It is about 339 nm.

The negative electrode active material for lithium secondary batteries of the present invention
The fee is I₁₁₀/ I₀₀₂Value of 0.015 or less, and d
₀₀₂Is 0.337 nm or more. I₁₁₀
/ I_{00 2}Since the value of is as small as 0.015 or less, I₁₁₀
/ I₀₀₂Is low crystalline graphite with a large value of about 0.05
to differ greatly. In addition, I of the negative electrode active material of the present invention₁₁₀
/ I₀₀₂It is desirable that the value of is 0.008 or more.
Yes. In addition, d of the negative electrode active material of the present invention₀₀₂The value of
Greater than the value (0.3348 nm) in the ideal graphite structure
It does not increase in width, but is 0.337 nm or more.
It is about 340 nm or less. Therefore, the book
The negative electrode active material for a lithium secondary battery of the invention has a ratio of graphite to graphite.
Although the graphitization degree is slightly reduced by comparison, the graphite structure
It can be said that it is a fully developed negative electrode active material.

On the other hand, the negative electrode active material of the present invention has a greatly different half-width value as compared with graphite. The value of W of graphite is usually less than 0.2 °, which is the same for the above low crystalline graphite. On the other hand, the value of W of the negative electrode active material of the present invention is 0.2 ° or more. Despite the high degree of graphitization, (00
2) The peak of the plane becomes broad and the value of full width at half maximum is large because the crystallinity of individual crystallites is high, but they are not regularly oriented, and the interlayer distance of the hexagonal carbon network is not uniform. It is possible that there is no reason. The value of W is preferably 0.4 or less. As described above, the crystallinity of each particle is high, but the crystallinity of the crystallite is low, so that the crystallinity of the entire particle is different from that of graphite.

Therefore, the negative electrode active material for a lithium secondary battery of the present invention has a different crystallite orientation from that of graphite, so that the internal stress of the material is relieved. Therefore, the negative electrode active material for a lithium secondary battery of the present invention has a small change in crystallinity even when stored for a long period of time in a highly charged state, and has a small increase in internal resistance due to storage and good storage characteristics. Secondary battery can be configured.

Further, in the method for producing a negative electrode active material for a lithium secondary battery of the present invention, the following three parameters obtained by the powder X-ray diffraction method using Cu Kα rays are as follows: (a) Diffraction line intensity I _{110 of} (110) plane and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.01 (b) Half-width of peak of (002) plane W (°); W
<0.2 (c) interlayer distance d ₀₀₂ (nm); a raw material preparing step of preparing a powdery raw material carbon material satisfying d ₀₀₂ <0.337; and the raw material carbon material The following three parameters obtained by the powder X-ray diffraction method using the Cu Kα ray by changing the crystallinity of the above-mentioned materials have the following conditions: (A) the intensity I ₁₁₀ of the diffraction line on the (110) plane and (002) )
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.015 (B) Half-width W (°) of peak of (002) plane; W
≧ 0.2 (C) interlayer distance d ₀₀₂ (nm); and d ₀₀₂ ≧ 0.337.

That is, the method for producing a negative electrode active material for a lithium secondary battery according to the present invention uses as a raw material a carbon material having high crystallinity such that the above parameters satisfy the respective conditions (a) to (c). , By changing its crystallinity,
This is a method for producing the above-mentioned negative electrode active material material for a lithium secondary battery of the present invention, which is different from a carbon material having low crystallinity.

According to the method for producing a negative electrode active material for a lithium secondary battery of the present invention, the internal stress is relieved by changing the crystallinity of a carbon material having a high crystallinity to reduce the internal stress. A negative electrode active material material capable of forming a suppressed secondary battery can be easily manufactured.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The negative electrode active material material for a lithium secondary battery of the present invention and the method for producing the same will be described below, followed by a usage form of the negative electrode active material for a lithium secondary battery of the present invention. The lithium secondary battery will be described.

<Negative Electrode Active Material Material for Lithium Secondary Battery> The negative electrode active material material for lithium secondary battery of the present invention comprises a powdery carbon material, and the carbon material is a powder X-ray diffraction method using Cu Kα ray. The following three parameters obtained by the following conditions are as follows: (A) Intensity I ₁₁₀ of the diffraction line on the (110) plane and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.015 (B) Half-width W (°) of peak of (002) plane; W
≧ 0.2 (C) interlayer distance d ₀₀₂ (nm); d ₀₀₂ ≧ 0.337 is satisfied.

The negative electrode active material of the present invention comprises a powdery carbon material. Examples of the carbon substance include natural graphite and artificial graphite. As will be described later in detail in the production method, for example, the negative electrode active material of the present invention can be produced by changing the crystallinity of the powdery carbonaceous material having high crystallinity as a raw material. The conditions to be satisfied by the above parameters regarding the crystallinity of the negative electrode active material of the present invention have been described above, and will not be repeated here.

The particle diameter of the particles constituting the powder is not particularly limited, but the volume average particle diameter D ₅₀ is preferably 2 μm or more and 30 μm or less. When D ₅₀ is less than 2 μm, the proportion of fine particles having low crystallinity increases,
This is because there is a risk of promoting deterioration of the battery.
This is because if it exceeds m, it becomes difficult to manufacture the electrode. Here, the volume average particle diameter D ₅₀ is obtained by converting the number average value of the volume into the particle diameter.

Most of the particles constituting the powder are 1
It is desirable that the particles have a particle size of μm or more.
As will be described later in detail, particles having low crystallinity, which are amorphous components, exist as fine particles of less than 1 μm. Therefore, by reducing the proportion of such fine particles as much as possible, the deterioration of the battery can be further suppressed. Specifically, it is desirable that 99% or more of the total weight of the particles constituting the powder have a particle diameter of 1 μm or more. <Method for producing negative electrode active material for lithium secondary battery>
A method for producing a negative electrode active material for a lithium secondary battery of the present invention is a method for producing a negative electrode active material for a lithium secondary battery comprising a powdery carbon material, which comprises a raw material preparing step and a crystallinity adjusting step. It is configured to include. Both steps will be described below.

(1) Raw Material Preparation Step In this step, the following three parameters obtained by the powder X-ray diffraction method using Cu Kα rays are as follows: (a) Intensity of diffraction line of (110) plane I ₁₁₀ and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.01 (b) Half-width of peak of (002) plane W (°); W
<0.2 (c) interlayer distance d ₀₀₂ (nm); a step of preparing a powdery raw material carbon material satisfying d ₀₀₂ <0.337.

The raw carbon material is not particularly limited as long as it can satisfy the above conditions (a) to (c). That is, the raw material carbon material is a carbon material having high crystallinity, and for example, natural graphite, artificial graphite obtained by mixing natural graphite and coke and heat-treated at 2800 ° C or higher, or graphitizable carbon at 2800 ° C or higher. Artificial graphite heat-treated at a high temperature is used. The raw carbon material may be prepared and prepared by the manufacturer, or may be purchased and prepared.

Here, the easily graphitizable carbon includes coke,
Optically anisotropic small spheres (mesocarbon microbeads: MCM) obtained by heating pitches at around 400 ° C
B) etc. When artificial graphite is used as the raw material carbon material, it is desirable to use graphitized mesocarbon microbeads (graphitized MCMB) obtained by graphitizing the above mesocarbon microbeads. This graphitized MCMB is characterized in that it has a spherical shape, has a small specific surface area, can minimize the decomposition of the electrolytic solution, and can contribute to the improvement of the packing density. Therefore, the secondary battery using the negative electrode active material material of the present invention manufactured by using graphitized MCMB as a raw material carbon material as a negative electrode active material has a higher energy density. Also, the crystallites are arranged in a lamella shape,
Because the crystallite end face is exposed on the particle surface, graphitized MC
A secondary battery using the negative electrode active material of the present invention manufactured by using MB as a raw material carbon material has a smooth lithium occlusion / release at the time of charging / discharging and an excellent output characteristic. Become.

(2) Crystallinity adjusting step In this step, the crystallinity of the raw carbon material prepared in the raw material preparing step is changed to obtain the following powder X-ray diffraction method using Cu Kα rays. The three parameters are as follows: (A) Diffraction line intensity I _{110 on the} (110) plane and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.015 (B) Half-width W (°) of peak of (002) plane; W
≧ 0.2 (C) An interlayer distance d ₀₀₂ (nm); d ₀₀₂ ≧ 0.337.

The method of changing the crystallinity is not particularly limited. An example thereof is a method in which a raw material carbon material is dispersed in an organic solvent containing nitrogen.
When the raw material carbon material is dispersed in an organic solvent containing nitrogen, the nitrogen atoms in the solvent, which tend to have a positive charge, and the carbon atoms, which tend to have a negative charge, attract each other, and the solvent penetrates into the carbon material. The crystallinity of the raw carbon material can be easily changed.

In this case, as the nitrogen-containing organic solvent, for example, N-methyl-2-pyrrolidone, N, N-dimethylacetamide, triethylamine, pyridine or the like can be used. In particular, because of its high affinity with polyvinylidene fluoride used as a binder, N-
It is desirable to use methyl-2-pyrrolidone. When the raw carbon material is dispersed in an organic solvent containing nitrogen, it is desirable to stir it. In addition, the time for dispersion is about 24 hours to several days, and the solvent temperature is room temperature to
It may be about 80 ° C.

When particles of the raw material carbon material or carbon material obtained by changing the crystallinity contain fine particles having low crystallinity, it is desirable to remove the fine particles. As described above, the particles having low crystallinity exist as fine particles of less than 1 μm. Therefore, by reducing the proportion of such fine particles as much as possible, the deterioration of the battery can be further suppressed. In this case,
This crystallinity adjusting step can be configured to include a fine particle removing step of removing fine particles of the raw material carbon material or fine particles of the carbon material. The fine particles may be removed before the crystallinity of the raw material carbon material is changed or after the crystallinity is changed. Furthermore, it may be performed while changing the crystallinity of the raw material carbon material. The number of times the fine particles are removed is not particularly limited and may be about 1 to 10 times.

The method for removing the fine particles is not particularly limited, and a general method used for classifying the particles may be used. For example, gas classification, liquid classification and the like can be used. Above all, it is desirable to use the liquid classification because the fine particles having a smaller particle size can be surely removed. In this case, the fine particle removing step includes a step of dispersing the raw material carbon material or the carbon material in the classification liquid, and then allowing the material to settle by allowing it to stand, and removing fine particles suspended in the supernatant liquid without settling. can do.

The classification liquid is not particularly limited,
The viscosity, specific gravity and the like may be taken into consideration according to the particle size of the fine particles to be removed and the like, and may be appropriately selected. For example, in the crystallinity adjusting step, when a method of dispersing the raw material carbon material in an organic solvent containing nitrogen is adopted as a method of changing the crystallinity of the raw material carbon material, the organic solvent containing nitrogen is classified. It can be a liquid. When this embodiment is adopted, it is not necessary to newly prepare a classifying liquid, and the raw material carbon material is dispersed in an organic solvent containing nitrogen and then allowed to stand to classify the fine particles. That is, since the fine particles can be removed at the same time as the crystallinity is changed, the crystallinity adjusting step and the fine particle removing step can be substantially one step.

<Lithium Secondary Battery> The lithium secondary battery of the present invention uses the above-mentioned negative electrode active material material for lithium secondary batteries of the present invention as a negative electrode active material. The main configuration of the lithium secondary battery will be described below. Generally, a lithium secondary battery includes a positive electrode and a negative electrode that absorbs and releases lithium ions, a separator that is sandwiched between the positive electrode and the negative electrode, and a non-aqueous electrolyte that moves lithium ions between the positive electrode and the negative electrode. Composed of. Since the secondary battery of this embodiment also follows this configuration, the following description will be given for each of these components.

For the positive electrode, a positive electrode active material capable of occluding and releasing lithium ions is mixed with a conductive material and a binder, and a suitable solvent is added if necessary to obtain a paste-like positive electrode mixture. Apply to the surface of the metal foil collector of
It can be formed by drying and then increasing the density of the active material by pressing.

The positive electrode active material has a basic composition of LiCoO 2 from the viewpoint that a secondary battery of 4 V class can be constructed.
₂ , LiNiO ₂ , LiMnO _2, etc., layered rock salt structure lithium transition metal composite oxide, basic composition of which is LiMn ₂ O ₄
A lithium transition metal composite oxide having a spinel structure such as Among them, the basic composition is LiNiO
_The layered rock salt structure lithium transition metal composite oxide of 2 is preferable because it is cheaper than the lithium transition metal composite oxide having Co as a central metal and can form a secondary battery having a large discharge capacity per unit weight. is there.

The basic composition means a typical composition of each of the above composite oxides, and in addition to the one represented by the above composition formula, for example, a lithium site or a transition metal site may be used as another one or two. It also includes compositions such as those partially substituted with one or more elements. Further, the composition is not necessarily limited to the stoichiometric composition, and for example, a non-stoichiometric composition in which a cation element such as Li or Ni, which is inevitably produced in production, is deficient or an oxygen atom is deficient. Also includes things. Further, one kind of lithium-transition metal composite oxide may be used, or two or more kinds thereof may be mixed and used.

The conductive material used for the positive electrode is for ensuring the electric conductivity of the positive electrode active material layer, and one or a mixture of two or more carbon materials such as carbon black, acetylene black and graphite is used. Can be used. The binder plays a role of binding the active material particles, and may be a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. As a solvent for dispersing the active material, the conductive material, and the binder, N-
An organic solvent such as methyl-2-pyrrolidone can be used.

The negative electrode is formed by mixing a negative electrode active material with a binder, adding a suitable solvent to form a paste of the negative electrode mixture, and applying and drying the mixture on the surface of a metal foil current collector such as copper. You can

As the negative electrode active material, the negative electrode active material material for lithium secondary batteries of the present invention is used. The negative electrode active material of the present invention has various materials depending on its crystal structure, particle size, and the like. Therefore, one of them may be used as the negative electrode active material, or two or more thereof may be mixed and used. Furthermore, it is also possible to adopt a configuration in which the negative electrode active material of the present invention is mixed with a known negative electrode active material to obtain a negative electrode active material. As with the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent.

The separator sandwiched between the positive electrode and the negative electrode is for separating the positive electrode and the negative electrode, holding the electrolytic solution and allowing ions to pass therethrough, and uses a thin microporous membrane such as polyethylene or polypropylene. You can

The non-aqueous electrolytic solution is obtained by dissolving an electrolyte in an organic solvent, and as the organic solvent, an aprotic organic solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate,
It is possible to use one kind of γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, dioxolane, methylene chloride, etc., or a mixed solution of two or more kinds thereof. Further, as the electrolyte to be dissolved, LiI and LiCl which generate lithium ions when dissolved
O ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ or the like can be used.

Instead of the separator and the non-aqueous electrolyte solution, a polymer solid electrolyte using a high molecular weight polymer such as polyethylene oxide and a lithium salt such as LiClO ₄ or LiN (CF ₃ SO ₂ ) ₂ is used. It is also possible to use a gel electrolyte in which the above nonaqueous electrolytic solution is trapped in a solid polymer matrix such as polyacrylonitrile.

The lithium secondary battery composed of the above-mentioned ones can have various shapes such as a coin type, a laminated type and a cylindrical type. Regardless of which shape is adopted, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the positive electrode terminal and the negative electrode terminal that communicate with the outside are electrically connected to each other by the electrode body. The battery can be completed by hermetically sealing the battery case with the non-aqueous electrolyte.

<Acceptance of Other Embodiments> The embodiments of the negative electrode active material for a lithium secondary battery, the method for producing the same, and the lithium secondary battery of the present invention described above are merely examples, and the present invention is not limited thereto. A negative electrode active material for lithium secondary batteries and a method for producing the same, and a lithium secondary battery using the negative active material for lithium secondary batteries of the present invention as a negative active material,
Starting from the above-described embodiment, various modifications and improvements can be made based on the knowledge of those skilled in the art.

[0043]

EXAMPLE Two types of negative electrode active material materials for lithium secondary batteries of the present invention were manufactured based on the above-described embodiment. For comparison, ordinary graphite having low crystallinity and low-crystallinity graphite were prepared, and a lithium secondary battery using each of these materials as a negative electrode active material was prepared to evaluate the storage characteristics.

The production of a negative electrode active material for a lithium secondary battery, the production of a lithium secondary battery and the evaluation of storage characteristics will be described below.

<Manufacture of Negative Electrode Active Material for Lithium Secondary Battery> (1) Powdery (average particle diameter 25 μm) graphitized mesocarbon microbeads (graphitized) as the raw material carbon material for the negative electrode active material of Example 1 MCMB:
A firing temperature of 2800 ° C.) was prepared. In addition, graphitized MCM
B was analyzed by a powder X-ray diffraction method using Cu Kα rays,
From the three parameter values shown below, it was confirmed that the crystallinity of graphitized MCMB was high. I ₁₁₀ / I ₀₀₂ = 0.00
7, W = 0.165 (°), d ₀₀₂ = 0.335 (n
m).

This graphitized MCMB was dispersed in N-methyl-2-pyrrolidone, which is an organic solvent containing nitrogen,
It was left for one week. Then, it dried under reduced pressure at 200 degreeC and the powdery carbon material was obtained. The obtained carbon material was used as the negative electrode active material material of Example 1.

(2) Negative Electrode Active Material Material of Example 2 A negative electrode active material material of Example 1 was manufactured in the same manner as in the above (1) except that a step of removing fine particles of carbon material was added. . That is, graphitized MCMB
Was dispersed in N-methyl-2-pyrrolidone and allowed to stand for 3 days, and then the supernatant was discarded to remove fine particles of carbonaceous material suspended in the supernatant. Further, N-methyl-2-pyrrolidone was added as a classification liquid, the carbon substance was dispersed again, and the mixture was allowed to stand for 4 days to similarly remove fine particles of the carbon substance suspended in the supernatant. Then, it dried under reduced pressure at 200 degreeC and the powdery carbon material was obtained. The obtained carbon material was used as the negative electrode active material material of Example 2.

(3) Analysis by powder X-ray diffraction method using Cu Kα ray and SEM observation Regarding the negative electrode active material of Examples 1 and 2, Cu Kα α
The powder X-ray diffraction analysis using the X-ray was performed. Figure 1
In addition, the X-ray diffraction patterns of the negative electrode active material of Example 2 and the graphitized MCMB, which is a raw material carbon material, dried under reduced pressure at 200 ° C. (hereinafter, referred to as “carbon material of Comparative Example 1”). Show. Table 1 shows the values of the three parameters obtained for the negative electrode active material materials of Examples 1 and 2 and the carbon material of Comparative Example 1. For comparison, 2
20 pieces of graphitized MCMB with low crystallinity, which was baked at 300 ° C
The values of the parameters for the one dried under reduced pressure at 0 ° C. (hereinafter, referred to as “carbon material of Comparative Example 2”) are also shown.

[0049]

[Table 1]

From Table 1, the negative electrode active material materials of Examples 1 and 2 have slightly higher I ₁₁₀ / I ₀₀₂ and d ₀₀₂ values than the carbon material of Comparative Example 1. That is, it can be understood that the negative electrode active material materials of Examples 1 and 2 have a degree of graphitization slightly lower than that of the raw material carbon material. On the other hand, the values of I ₁₁₀ / I ₀₀₂ and d ₀₀₂ of the negative electrode active material materials of Examples 1 and 2 are smaller than that of the carbon material of Comparative Example 2 which is low crystalline graphite. That is, it can be seen that the negative electrode active material materials of Examples 1 and 2 have a higher degree of graphitization than carbon materials having originally low crystallinity.

Further, the value of the full width at half maximum W is less than 0.2 ° in the carbon substances of Comparative Examples 1 and 2, whereas in Examples 1 and 2
In the case of the negative electrode active material, the angle is 0.2 ° or more. This is because the crystallinity of the carbon material having high crystallinity was changed, and it is considered that the crystallinity of the crystallite is low and the interlayer distance is not uniform. Therefore, the negative electrode active material materials of Examples 1 and 2 which are negative electrode active material materials of the present invention have a slightly lower graphitization degree than the carbon material having high crystallinity, but the carbon material originally has low crystallinity. It was confirmed to be different from.

When the negative electrode active material material of Example 1 and the negative electrode active material materials of Examples 2 and 2 are compared, the negative electrode active material material of Example 2 has a crystallinity closer to that of the carbon material of Comparative Example 1. It turns out that It is considered that this is because the negative electrode active material of Example 2 was prepared by removing fine particles of a carbon material having low crystallinity.

A photograph of the negative electrode active material of Example 1 taken with a scanning electron microscope (SEM) is shown in FIG.
The photographs of the negative electrode active material of Example 2 taken by SEM are shown in FIG. 3, respectively. From the photograph of FIG. 2, in the negative electrode active material of Example 1, many particles having a particle diameter of less than 1 μm were observed in addition to the particles having a particle diameter of about 10 μm to 30 μm.
On the other hand, from the photograph of FIG. 3, in the negative electrode active material of Example 2, most of the particles have a particle size of 1 μm or more, and fine particles of less than 1 μm are hardly observed.
Therefore, in the negative electrode active material of Example 2 in which the liquid classification was performed, most of the particles forming the powder had a particle size of 1 μm or more, and particles having lower crystallinity than those not subjected to the classification were It was confirmed to be small.

For reference, FIG. 4 shows a photograph taken by SEM of the fine particles removed when the negative electrode active material of Example 2 was manufactured. From the photograph of FIG. 4, it is understood that the removed fine particles are amorphous fine particles having a particle diameter of less than 1 μm. Although not shown here, the X-ray diffraction pattern confirmed that the fine particles were an amorphous component.

Further, N- in which the graphitized MCMB used in the production of the negative electrode active material of Examples 1 and 2 was dispersed.
Methyl-2-pyrrolidone was recovered, concentrated to a weight of about 1 / 100,000, and measured by an infrared absorption method (IR method). As a result, the recovered N-methyl-2-pyrrolidone showed almost the same absorption spectrum as the unused N-methyl-2-pyrrolidone. From this, graphitized MCM
Even if B is dispersed in N-methyl-2-pyrrolidone, it is considered that the components of graphitized MCMB are not eluted. That is, the negative electrode active material materials of Examples 1 and 2 were the graphitized MCMB.
It is considered that the composition of the is not changed.

<Preparation of Lithium Secondary Battery and Evaluation of Storage Characteristics> (1) Preparation of Lithium Secondary Battery Negative electrode active material of Examples 1 and 2 and Comparative Example 1,
A lithium secondary battery was prepared by using the carbon materials of No. 2 as the negative electrode active material.

First, 5 parts by weight of polyvinylidene fluoride as a binder was mixed with 95 parts by weight of each material to be the negative electrode active material, and an appropriate amount of N-methyl-2-pyrrolidone was added as a solvent to prepare a paste. A negative electrode mixture was prepared.
Next, this paste-like negative electrode mixture is applied to both sides of a copper foil current collector having a thickness of 10 μm, dried, and then compressed by a roll press to form a sheet-like negative electrode mixture having a thickness of 30 μm per side. The thing was produced. This sheet-shaped negative electrode is 56
It was used after being cut into a size of mm × 500 mm.

The positive electrodes facing each other have a composition formula of LiNi _0.8 C.
A lithium nickel composite oxide represented by _0.15 Al _0.05 O ₂ was used as an active material. First, 85 parts by weight of a lithium nickel composite oxide serving as a positive electrode active material is mixed with 10 parts by weight of carbon black as a conductive material and 5 parts by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl as a solvent is mixed. 2-Pyrrolidone was added to prepare a paste-like positive electrode mixture. Next, this paste-like positive electrode mixture is applied to both sides of an aluminum foil current collector having a thickness of 20 μm, dried, and then compressed by a roll press to obtain a sheet-like sheet having a thickness of 40 μm per side of the positive electrode mixture. The thing was produced. This sheet-shaped positive electrode was used after being cut into a size of 54 mm × 450 mm.

The positive electrode and the negative electrode were wound by sandwiching a polyethylene separator having a thickness of 25 μm and a width of 58 mm between them to form a roll-shaped electrode body. Then, the electrode body was inserted into a 18650 type cylindrical battery case (outer diameter 18 mmφ, length 65 mm), a nonaqueous electrolytic solution was injected, and the battery case was sealed to produce a cylindrical lithium secondary battery. . The non-aqueous electrolyte used was a mixture of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 with LiPF ₆ dissolved at a concentration of 1M.

The lithium secondary battery using the negative electrode active material of Example 1 as a negative electrode active material was used as the lithium secondary battery of Example 1, and the negative electrode active material of Example 2 was used as the negative electrode active material. The lithium secondary battery was used as the lithium secondary battery of Example 2, the lithium secondary battery using the carbon material of Comparative Example 1 as the negative electrode active material was used as the lithium secondary battery of Comparative Example 1, and the carbon material of Comparative Example 2 was used. The lithium secondary battery used as the negative electrode active material was used as the lithium secondary battery of Comparative Example 2.

(2) Evaluation of storage characteristics The storage characteristics of the lithium secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated. First, as conditioning, at a temperature of 20 ° C., a constant current of 0.2 mA / cm ² was charged to 4.1 V, and then a constant current of 0.2 mA / cm ² was discharged to 3.0 V. went. After conditioning, in order to measure the initial capacity, charging / discharging was performed for 3 cycles at a temperature of 20 ° C. The charging / discharging condition is a current density of 0.1 mA /
The charging was performed at a constant current of cm ^{2 up to} the charging upper limit voltage of 4.1 V, and the charging was continued at a constant voltage of 4.1 V for 2 hours, and thereafter, the discharging lower limit voltage was 3. at a constant current of 0.1 mA / cm ² . The charging / discharging for discharging to 0 V is one cycle. The discharge capacity of the third cycle of this charge / discharge is set to 2
It was defined as the initial discharge capacity per unit weight of the positive electrode active material at 0 ° C.

Next, in order to calculate the initial internal resistance, the input / output power was measured and the internal resistance at the time of input / output was calculated. The input / output power was measured under the following conditions. First, in a state where each lithium secondary battery was charged to 50% of the initial capacity (SOC 50%), the lithium secondary battery was discharged at a current of 1 A for 10 seconds, and the voltage at the 10th second was measured. After recharging to the state of SOC 50%, discharge it with a current of 3A for 10 seconds, and
The voltage at 0 second was measured. Further, after being charged to a state of SOC 50%, it was discharged at a current of 5 A for 10 seconds, and the voltage at the 10th second was measured. Then, the current dependency of the voltage was obtained, and the slope of the current-voltage line was used as the internal resistance at the time of output.
In addition, charging was performed in the same procedure, the voltage at each 10th second was measured, and the internal resistance at the time of input was determined from the slope of the current-voltage line. The average value of the obtained internal resistances at the time of input / output was used as the initial internal resistance.

Next, a storage test was conducted. In the storage test, the battery was charged at a constant current with a current density of 0.1 mA / cm ² until the voltage reached 4.1 V, and further charged at a constant voltage of 4.1 V for 2
By continuing to charge for a time, each secondary battery can
After the state of 0%, it was stored in a constant temperature bath at 60 ° C. for 1 month. Then, after storage, the internal resistance at the time of input / output was determined in the same manner as above, and the average value was used as the internal resistance after storage. Then, from the value of the internal resistance before and after the storage test, the formula [{(internal resistance after storage / initial internal resistance) -1} ×
100 (%)] was used to calculate the internal resistance increase rate (%).

Further, as a reference, a charge / discharge cycle test was performed on each lithium secondary battery. The charge / discharge cycle test is 60 ° C, which is regarded as the upper limit of the actual temperature range of the battery.
Under the temperature condition of ^2. , the battery was charged to a charging upper limit voltage of 4.1 V with a constant current of a current density of ² mA / cm.
Charge and discharge for discharging at a constant current of mA / cm ^{2 to} a discharge lower limit voltage of 3.0 V is defined as one cycle, and this cycle is a total of 5
It was supposed to perform 00 cycles. Then, the discharge capacity at the 500th cycle of each lithium secondary battery was measured, and the post-cycle discharge capacity per unit weight of the positive electrode active material was calculated.
Then, the formula [discharge capacity after cycle / initial discharge capacity × 10
0 (%)] to capacity retention rate (%) of each lithium secondary battery
I asked. Table 2 shows the values of initial internal resistance (mΩ), internal resistance after storage (mΩ), internal resistance increase rate (%), initial discharge capacity (mAh / g), and capacity retention rate (%) of each lithium secondary battery. Indicates.

[0065]

[Table 2]

From Table 2, there is no great difference in the initial internal resistance value among the secondary batteries, but the internal resistance values after storage are the same as those of the secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2. It can be seen that it is significantly different from the secondary battery. That is, in the secondary batteries of Examples 1 and 2, the internal resistance increase rates after storage were about 14% and about 10%, respectively.
The secondary batteries of No. 2 had large values of about 43% and about 81%. In particular, in the secondary battery of Comparative Example 2 in which low crystalline graphite was used as the negative electrode active material, the internal resistance was significantly increased. That is, it can be seen that the negative electrode active material of the present invention is not substantially uniform in crystallite orientation and has a slightly low graphitization degree, but is significantly different from a carbon material having originally low crystallinity. It can be seen that the change in crystallinity during storage is sufficiently suppressed due to the difference in crystallite orientation and the like.

Therefore, it was confirmed that the use of the negative electrode active material of the present invention as the negative electrode active material effectively suppressed the increase in internal resistance due to the storage of the battery. In particular, the secondary battery of Example 2 using the negative electrode active material in which the fine particles were removed had a small increase rate of internal resistance. This shows that the deterioration of the battery is further suppressed by removing the fine particles which are the amorphous component contained in the negative electrode active material.

Regarding the capacity retention ratio after the cycle test, no significant difference was observed between the secondary batteries of Examples 1 and 2 and the secondary battery of Comparative Example 1. On the other hand, in the secondary battery of Comparative Example 2 which originally used a carbon material having low crystallinity, the capacity was greatly reduced after the cycle test, and the cycle characteristics were not good. From this, it was confirmed that the secondary battery using the negative electrode active material of the present invention had cycle characteristics equivalent to those of the secondary battery using the carbon material having high crystallinity.

From the above, the negative electrode active material for a lithium secondary battery of the present invention has a large storage capacity and a storage characteristic that does not increase the internal resistance even when stored for a long period of time in a highly charged state. It was confirmed that a good secondary battery could be constructed.

[0070]

The negative electrode active material for a lithium secondary battery of the present invention is made of a powdery carbon material, and the internal stress of the carbon material particles is relaxed by changing the crystallinity. By using the negative electrode active material for a lithium secondary battery of the present invention as a negative electrode active material, a secondary battery having good storage characteristics with little increase in internal resistance even when stored for a long period of time in a highly charged state. Can be configured. Further, according to the method for producing a negative electrode active material for a lithium secondary battery of the present invention, the negative electrode active material of the present invention can be easily produced.

[Brief description of drawings]

FIG. 1 shows X-ray diffraction patterns of a negative electrode active material of Example 2 and a carbon material of Comparative Example 1.

FIG. 2 shows an SEM photograph of the negative electrode active material of Example 1.

FIG. 3 shows an SEM photograph of the negative electrode active material of Example 2.

FIG. 4 shows an SEM photograph of fine particles removed when the negative electrode active material of Example 2 was manufactured.

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[Submission date] July 17, 2001 (2001.7.1)
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   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideyuki Nakano             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Yoshio Ukyo             Aichi Prefecture Nagachite Town Aichi District             Local 1 Toyota Central Research Institute Co., Ltd. F-term (reference) 4G046 CA07 CB02 CB09 CC10                 5H029 AJ04 AK03 AL06 AL07 AM03                       AM04 AM05 AM07 CJ12 CJ28                       DJ16 DJ17 HJ05 HJ13                 5H050 AA09 AA12 BA17 CA07 CA08                       CA09 CB07 CB08 CB09 FA17                       FA19 GA12 GA27 HA05 HA13

Claims (10)

[Claims] 1. A carbonaceous substance in powder form, which is obtained by a powder X-ray diffraction method using Cu Kα rays. The following three parameters are defined as follows: (A) (110) plane Diffraction line intensities I ₁₁₀ and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.015 (B) Half-width W (°) of peak of (002) plane; W
≧ 0.2 (C) Interlayer distance d ₀₀₂ (nm); d ₀₀₂ ≧ 0.337 is satisfied, and a negative electrode active material material for a lithium secondary battery. 2. A volume average particle diameter D of particles constituting the powder.
_The negative electrode active material for a lithium secondary battery according to claim 1, wherein ₅₀ is 2 μm or more and 30 μm or less. 3. Most of the particles constituting the powder are 1 μm.
The material for a negative electrode active material for a lithium secondary battery according to claim 1, which has the above particle size. 4. A lithium secondary battery using the negative electrode active material according to any one of claims 1 to 3 as a negative electrode active material. 5. A method for producing a negative electrode active material for a lithium secondary battery, comprising a powdery carbonaceous material, wherein the following three parameters obtained by a powder X-ray diffraction method using Cu Kα rays are as follows: (A) Intensity I ₁₁₀ of the diffraction line on the (110) plane and (002)
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.01 (b) Half-width of peak of (002) plane W (°); W
<0.2 (c) Interlayer distance d ₀₀₂ (nm); Raw material preparing step of preparing a powdery raw material carbon material satisfying d ₀₀₂ <0.337; The following three parameters obtained by the powder X-ray diffraction method using the Cu Kα ray by changing the crystallinity of the above-mentioned materials have the following conditions: (A) the intensity I ₁₁₀ of the diffraction line on the (110) plane and (002) )
Ratio of intensity of diffraction line I ₀₀₂ to plane I ₁₁₀ / I ₀₀₂ ; I ₁₁₀
/ I ₀₀₂ ≦ 0.015 (B) Half-width W (°) of peak of (002) plane; W
≧ 0.2 (C) interlayer distance d ₀₀₂ (nm); a crystallinity adjusting step of using a carbon material satisfying d ₀₀₂ ≧ 0.337, and a negative electrode active material for a lithium secondary battery, comprising: Production method. 6. The lithium secondary battery according to claim 5, wherein the crystallinity adjusting step includes a step of changing the crystallinity of the raw material carbon material by dispersing the raw material carbon material in an organic solvent containing nitrogen. For manufacturing negative electrode active material. 7. The method for producing a negative electrode active material for a lithium secondary battery according to claim 6, wherein the organic solvent containing nitrogen is N-methyl-2-pyrrolidone. 8. The lithium secondary according to claim 5, wherein the crystallinity adjusting step includes a fine particle removing step of removing fine particles of the raw carbon material or fine particles of the carbon material. A method for manufacturing a negative electrode active material for a battery. 9. In the step of removing fine particles, the raw material carbon material or the carbon material is dispersed in a classification liquid, and then allowed to stand to settle, and fine particles floating in the supernatant liquid without settling are removed. The method for producing a negative electrode active material for a lithium secondary battery according to claim 8, which includes a step. 10. The crystallinity adjusting step includes a step of changing the crystallinity of the raw material carbon material by dispersing the raw material carbon material in a nitrogen-containing organic solvent. The method for producing a negative electrode active material for a lithium secondary battery according to claim 9, wherein the fine particles are removed as the classification liquid in the fine particle removing step.