JP2002279983A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery, having superior storage characteristics by improving a negative electrode using carbon material in a lithium secondary battery, having a positive electrode, the negative electrode using the carbon material, and a non-aqueous electrolyte. SOLUTION: This lithium secondary battery comprises a positive electrode 1, a negative electrode 2 using carbon material, and a nonaqueous electrolyte. The first surface of the carbon material, forming a core part which does not contain fluorine covered with the second carbon material, having a crystalline lower than that of the first carbon material forming the core part and containing fluorine, is used as the carbon material of the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery having a positive electrode, a negative electrode using a carbon material, and a non-aqueous electrolyte.
The feature is that the storage characteristics of the lithium secondary battery are improved.

[0002]

2. Description of the Related Art In recent years, as a new type of secondary battery having a high output and a high energy density, a non-aqueous electrolyte has been used to oxidize lithium.
High-electromotive lithium secondary batteries utilizing reduction have come to be used.

[0003] In such a lithium secondary battery, carbon materials such as graphite and coke capable of occluding and releasing lithium ions are widely used as a material for the negative electrode. When graphite having high crystallinity is used as a material, there is an advantage that a very low discharge potential is exhibited in the vicinity of a lithium metal potential, and a lithium secondary battery having a high energy density can be obtained.

However, in the case of a lithium secondary battery using graphite having high crystallinity as a negative electrode, when stored in a charged state, lithium in the graphite reacts with a solvent in a non-aqueous electrolyte and becomes self-reactive. There is a problem that the battery is discharged and the battery capacity is reduced.

In the prior art, Japanese Patent Laid-Open No. 8-314
As disclosed in Japanese Patent Application Publication No. 04-2004, it has been proposed to use a carbon material whose surface has been treated with fluorine for a negative electrode of a lithium secondary battery to improve the charge / discharge characteristics of the lithium secondary battery.

However, if the surface of the carbon material made of graphite having high crystallinity as described above is treated so as to contain fluorine, the crystal structure of the graphite is disturbed, and the graphite still has a problem during storage in a charged state. In addition, there is a problem that lithium in graphite reacts with a solvent or the like in a non-aqueous electrolyte and battery capacity is reduced.

[0007]

SUMMARY OF THE INVENTION The present invention provides a positive electrode,
An object of the present invention is to solve the above-described problems in a lithium secondary battery including a negative electrode using a carbon material and a non-aqueous electrolyte. It is an object of the present invention to obtain a lithium secondary battery having excellent characteristics.

[0008]

In order to solve the above-mentioned problems, a lithium secondary battery according to the present invention has a lithium secondary battery including a positive electrode, a negative electrode using a carbon material, and a non-aqueous electrolyte. In the battery, as the carbon material in the negative electrode, the surface of the first carbon material serving as a core containing no fluorine is lower in crystallinity and contains fluorine than the first carbon material serving as the core. Thus, a material coated with the second carbon material is used.

Then, as in the lithium secondary battery of the present invention, the surface of the first carbon material which does not contain fluorine as the carbon material of the negative electrode is changed to the first carbon material which becomes the core. When a material coated with a second carbon material having lower crystallinity than the material and containing fluorine is used, the crystal structure of the first carbon material serving as a core is not disturbed by fluorine, and the initial charge time is reduced. In this case, a stable lithium fluoride film having excellent lithium ion permeability and being formed on the surface of the fluorine-containing second carbon material that covers the surface of the first carbon material serving as the core is formed. The reaction of lithium in the first carbon material serving as the core with the solvent or the like in the non-aqueous electrolyte is suppressed, and the battery capacity is prevented from lowering. Kicking storage characteristics can be improved.

Here, in the carbon material used for the negative electrode,
As the first carbon material serving as the core portion containing no fluorine, it is preferable to use a graphite-based carbon material having high crystallinity in order to obtain a lithium secondary battery with high energy density. It is preferable to use natural graphite, artificial graphite, graphitized pitch-based carbon fibers, and the like. In particular, those having a (002) plane spacing d ₀₀₂ measured by X-ray wide-angle diffraction in the range of 0.335 to 0.338 nm and a crystallite size Lc in the c-axis direction of 30 nm or more are used. Preferably, more preferably,
The (002) plane distance d ₀₀₂ is 0.335 to 0.336.
nm, and the size Lc of the crystallite in the c-axis direction.
Is 100 nm or more. When a granular material is used as the first carbon material,
Its average particle size is 1 to 80 µm, more preferably 5 to 40
Use a μm one.

On the other hand, as the second carbon material containing fluorine which coats the first carbon material serving as the core,
In order to suppress the reaction of lithium in the first carbon material with a solvent or the like in the non-aqueous electrolyte, the plane distance d ₀₀₂ of the (002) plane measured by X-ray wide-angle diffraction is 0.33.
It is preferable to use one having a thickness in the range of 9 to 0.390 nm.

When the second carbon material containing fluorine is provided on the surface of the first carbon material serving as the core, for example, the first carbon material serving as the core can be carbonized. Immersed in an organic compound and taken out, and then taken out at a temperature of about 500 to 1800 ° C., preferably 700 to 1800 ° C.
After carbonizing by heat treatment at a temperature of about 1400 ° C., it is fluorinated.

The organic compound used for providing the second carbon material may be, for example, pitch, tar, phenolformaldehyde resin, furfuryl alcohol resin, carbon black, polyvinylidene chloride, cellulose, etc. Or a solution in which these are dissolved in an organic solvent such as methanol, ethanol, benzene, acetone, and toluene.

As a method of fluorinating the carbonized material as described above, for example, a method of reacting with fluorine gas at a high temperature can be used, and the temperature of reacting with fluorine gas is 50 to 400 ° C. The temperature is preferably in the range of 100 to 250 ° C.

Here, as described above, the first core serving as the core is formed.
In providing the fluorine-containing second carbon material on the surface of the carbon material of (1), if the amount of the fluorine-containing second carbon material is small, the surface of the first carbon material serving as the core will While not sufficiently covered by the second carbon material, it is not possible to sufficiently suppress the reaction of lithium in the first carbon material with a solvent or the like in the non-aqueous electrolyte, while the second carbon containing the fluorine is not sufficiently covered. When the amount of the carbon material is too large, the first carbon material serving as the core cannot be sufficiently used. For this reason, the weight ratio of the above-mentioned fluorine-containing second carbon material in the total carbon material of the negative electrode is preferably in the range of 1 to 20% by weight, more preferably in the range of 5 to 15% by weight. To be.

In adjusting the weight ratio of the second carbon material containing fluorine in the total carbon material of the negative electrode, the first carbon material serving as the core is immersed in the organic compound as described above. Can be controlled by adjusting the immersion time and the number of immersions.

When the surface of the first carbon material serving as the core is coated with the above-mentioned second carbon material containing fluorine, if the amount of fluorine in the second carbon material is small, the initial During the charging, the surface of the second carbon material has excellent lithium ion permeability and a stable lithium fluoride film is not formed.
If the amount of fluorine in the carbon material becomes too large,
The fluorine is also contained in the first carbon material serving as the core, so that the crystal structure of the first carbon material serving as the core is disturbed. For this reason, the atomic ratio of fluorine to carbon (F / C) on the surface of the fluorine-containing second carbon material determined by X-ray photoelectron spectroscopy is in the range of 0.01 to 0.2. Is more preferable, and more preferably in the range of 0.05 to 0.1.

Further, when sulfur is contained in the second carbon material before fluorine is contained, a defect is formed in the CC bond of the second carbon material, and the second carbon material is fluorinated as described above to contain fluorine. In this case, a C—F bond is easily formed.

Here, the lithium secondary battery of the present invention is characterized in that the above-mentioned carbon material is used for the negative electrode, and the type of the positive electrode and the non-aqueous electrolyte used in the lithium secondary battery is as follows. There is no particular limitation on the like, and known ones conventionally used generally can be used.

In the lithium secondary battery of the present invention, a known positive electrode material capable of inserting and extracting lithium ions can be used as the positive electrode material of the positive electrode. For example, a lithium cobalt oxide LiCoO ₂ , Lithium nickel oxide LiNi
Chalcogen compounds of lithium metal oxides such as O ₂ and lithium manganese oxide LiMn ₂ O ₄ , metal oxides such as chromium oxide, titanium oxide, cobalt oxide and vanadium pentoxide, and transition metals such as titanium sulfide and molybdenum sulfide Etc. can be used.

In the lithium secondary battery according to the present invention, as the non-aqueous electrolyte, a known non-aqueous electrolytic solution or the like in which an electrolyte is dissolved in a non-aqueous solvent can be used.

The non-aqueous solvent used for the non-aqueous electrolyte is, for example, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, cyclopentanone, sulfolane, dimethyl sulfolane, 3-methyl-1,3 -Oxazolidine-2
-One, γ-butyrolactone, dimethyl carbonate,
Diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, butyl methyl carbonate, ethyl propyl carbonate, butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, methyl acetate, Organic solvents such as ethyl acetate can be used alone or in combination of two or more.

In this non-aqueous electrolyte, the electrolyte to be dissolved in the non-aqueous solvent is, for example, Li
PF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ S
O ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , Li
A lithium compound such as OSO ₂ (CF ₂ ) ₃ CF ₃ can be used, and it is particularly preferable to use a fluorine-containing electrolyte such as LiPF ₆ .

[0024]

EXAMPLES Hereinafter, the lithium secondary battery according to the present invention will be described in detail with reference to examples, and the lithium secondary battery according to this example will be described as having excellent storage characteristics. I will clarify it. The lithium secondary battery according to the present invention is not limited to those shown in the following examples, but can be appropriately modified and implemented without changing the gist of the invention.

Example 1 In Example 1, a positive electrode and a negative electrode prepared as described below were used, and a non-aqueous electrolyte prepared as described below was used.
A cylindrical lithium secondary battery as shown in FIG. 1 having a height of 2 mm and a height of 50 mm was produced.

[Preparation of Positive Electrode] In preparing the positive electrode, a lithium cobalt oxide LiCo
Using O ₂ powder, this LiCoO ₂ powder, carbon powder as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 90: 5: 5, and N-
Methyl-2-pyrrolidone was added to form a slurry, and this slurry was applied to both sides of an aluminum foil as a positive electrode current collector by a doctor blade method, dried and then rolled and cut into a predetermined width. A positive electrode was produced.

[Preparation of Negative Electrode] In preparing the negative electrode, as the first carbon material serving as the core, the (002) plane spacing d ₀₀₂ measured by X-ray wide-angle diffraction was 0.33.
At 56 nm, the crystallite size Lc in the c-axis direction is 100 n
m or more, and after immersing the natural graphite powder in a molten pitch, taking out and drying the powder to obtain a natural graphite powder whose surface is covered with the pitch. Later, this is carried out in an inert atmosphere for 1100
After baking at 2 ° C. for 2 hours, the surface of the above-mentioned natural graphite powder to be a core was covered with a carbon material having a lower crystallinity than the natural graphite.

Then, the surface of the natural graphite powder serving as the core, which is coated with a low-crystalline carbon material as described above, is reacted with fluorine gas at a temperature of 100 ° C. for 10 minutes to obtain the above-mentioned crystallinity. Low-carbon material containing fluorine,
A carbon material was obtained in which the surface of the natural graphite powder serving as the core was coated with a second carbon material having a lower crystallinity than the natural graphite and containing fluorine.

Here, when the weight ratio of the above-mentioned fluorine-containing second carbon material to the total carbon material of the negative electrode including the natural graphite was determined, the weight ratio was 10% by weight. The plane distance d ₀₀₂ of the (002) plane in the second carbon material containing fluorine measured by X-ray wide-angle diffraction was 0.342 nm.

The atomic ratio of fluorine to carbon (F / C) on the surface of the fluorine-containing second carbon material was determined by X-ray photoelectron spectroscopy. Was 0.05.

Then, the carbon material obtained as described above and polyvinylidene fluoride as a binder are mixed at a weight ratio of 90:10, and N-methyl-2-pyrrolidone is added to the mixture to form a slurry. This slurry is applied to both sides of a copper foil as a negative electrode current collector by a doctor blade method,
After drying, this was rolled and cut into a predetermined width to produce a negative electrode.

[Preparation of Non-Aqueous Electrolyte] In preparing a non-aqueous electrolyte, a mixed solvent of ethylene carbonate and diethyl carbonate mixed at a volume ratio of 1: 1 was used as a solvent. Lithium hexafluorophosphate LiPF ₆ was dissolved as an electrolyte to a concentration of 1 mol / l to prepare a non-aqueous electrolyte.

[Preparation of Battery] In preparing a battery, as shown in FIG. 1, a separator 3 made of lithium ion-permeable polypropylene was placed between the positive electrode 1 and the negative electrode 2 prepared as described above. After a microporous membrane is interposed and wound in a spiral shape and accommodated in the battery can 4, the non-aqueous electrolyte prepared as described above is injected into the battery can 4 and sealed. 1 is connected to a positive electrode external terminal 6 via a positive electrode lead 5, and the negative electrode 2 is connected to a battery can 4 via a negative electrode lead 7, and the positive external terminal 6 and the battery can 4 are electrically separated by an insulating packing 8. I let it.

(Comparative Example 1) In Comparative Example 1, the same carbon material as that of the first carbon material in Example 1 was used as the carbon material in the negative electrode in the lithium secondary battery of Example 1 described above. (002) Surface distance d
₀₀₂ is 0.3356 nm, and the size Lc of the crystallite in the c-axis direction is 100 nm or more.
The surface of the natural graphite powder was not coated.

Then, the natural graphite powder was used for the negative electrode as it was, and in the other respects, a negative electrode was prepared in the same manner as in Example 1 described above, and the lithium of Comparative Example 1 was used using the negative electrode. A secondary battery was manufactured.

(Comparative Example 2) In Comparative Example 2, in the production of the negative electrode in the lithium secondary battery of Example 1 described above, the (002) plane distance d ₀₀₂ is 0.3356 nm, and the size Lc of the crystallite in the c-axis direction is 100 nm or more. After the surface of the first carbon material made of natural graphite powder is coated at a pitch, this is coated in an inert atmosphere. By baking at 1100 ° C. for 2 hours, the surface of the first carbon material made of the above natural graphite powder is coated with a carbon material having a lower crystallinity than the first carbon material, and this is reacted with fluorine gas. Instead, the carbon material having low crystallinity was not allowed to contain fluorine.

Then, the surface of the first carbon material composed of natural graphite powder as the core as described above coated with the second carbon material having a low crystallinity containing no fluorine is used for the negative electrode. Other than that, in the above-described first embodiment,
In the same manner as in the above case, a negative electrode was produced, and a lithium secondary battery of Comparative Example 2 was produced using the negative electrode.

(Comparative Example 3) In Comparative Example 3, the same method as in the first carbon material in Example 1 described above was used to fabricate the negative electrode in the lithium secondary battery of Example 1 described above.
The (002) plane spacing d ₀₀₂ is 0.3356 nm, and c
A natural graphite powder having a crystallite size Lc of 100 nm or more in the axial direction is reacted with fluorine gas at a temperature of 100 ° C. for 10 minutes so that the surface of the natural graphite powder contains fluorine. did.

The powder of natural graphite thus containing fluorine on the surface is used for the negative electrode, and otherwise the same procedure as in Example 1 is carried out to produce the negative electrode. Using this negative electrode, a lithium secondary battery of Comparative Example 3 was produced.

Comparative Example 4 In Comparative Example 4, in the production of the negative electrode in the lithium secondary battery of Example 1 described above, the first carbon material serving as the core was measured by X-ray wide-angle diffraction. 002) The plane spacing d ₀₀₂ is 0.34
4 nm, the crystallite size Lc in the c-axis direction is 3.2 nm
The surface of the core portion made of the coke powder was coated with a second carbon material containing fluorine in the same manner as in Example 1 except that the coke powder was used. .

Here, in the carbon material obtained as described above, the fluorine-containing second carbon material covering this surface is more crystalline than the core coke powder. Was higher.

The core having the surface made of the coke powder coated with the second carbon material containing fluorine is used for the negative electrode as described above. A negative electrode was produced in the same manner as in Example 1, and a lithium secondary battery of Comparative Example 4 was produced using the negative electrode.

Next, Example 1 manufactured as described above was used.
For each of the lithium secondary batteries of Comparative Examples 1 to 4, the battery was charged at a constant current of 75 mA until the battery voltage reached 4.2 V, and then discharged at a constant current of 75 mA until the battery voltage reached 2.7 V. The discharge capacity Qo before storage was measured.

Next, each of the above lithium secondary batteries was charged at a constant current of 75 mA until the battery voltage reached 4.2 V, stored in a thermostat at 25 ° C. for 30 days, and then charged at a constant current of 75 mA. To discharge the battery voltage to 2.7 V, measure the discharge capacity Qa after storage, and calculate the self-discharge rate in each of the lithium secondary batteries of Example 1 and Comparative Examples 1 to 4 by the following equation. And the results are shown in Table 1 below.
It was shown to.

Self-discharge rate (%) = [(Qo−Qa) / Qo] × 100

[0046]

[Table 1]

As is apparent from the results, the surface of the first carbon material, which is a core using a powder of natural graphite having high crystallinity, has lower crystallinity than the first carbon material and contains fluorine. The lithium secondary battery of Example 1 using the negative electrode coated with the obtained second carbon material, the lithium secondary battery of Comparative Example 1 using natural graphite powder having high crystallinity as the negative electrode, Comparative example in which a surface of a first carbon material serving as a core portion using a powder of natural graphite having high crystallinity and coated with a low-crystallinity second carbon material containing no fluorine was used as a negative electrode. The lithium secondary battery of Comparative Example 3 and the lithium secondary battery of Comparative Example 3 in which the surface of natural graphite powder having high crystallinity and fluorine were used as the negative electrode, and the powder of coke having low crystallinity were used. The surface of the first carbon material serving as the core is In comparison with the lithium secondary battery of Comparative Example 4 in which a negative electrode was coated with a second carbon material having higher crystallinity than that of the first carbon material and containing fluorine, the self-discharge rate was significantly reduced. Thus, the storage characteristics were greatly improved.

(Examples 2 to 4) In Examples 2 to 4, only the type of the first carbon material used for the core of the negative electrode in the lithium secondary battery of Example 1 was changed. I did it.

Here, as shown in Table 2 below, as the first carbon material used for the core portion, in Example 2, the plane distance d ₀₀₂ was 0.3360 nm, and the crystallite size in the c-axis direction was An artificial graphite powder having an Lc of 60 nm was prepared in Example 3
In this example, artificial graphite powder having a plane spacing d ₀₀₂ of 0.3378 nm and a crystallite size Lc in the c-axis direction of 30 nm was obtained.
In Example 4, an artificial graphite powder having a plane distance d ₀₀₂ of 0.3388 nm and a crystallite size Lc in the c-axis direction of 20 nm was used.

Thereafter, in the same manner as in the first embodiment, the surface of the first carbon material is made to have a lower crystallinity and a second fluorine-containing second carbon material. Each of the negative electrodes was coated with a carbon material, and each of the negative electrodes was manufactured using such a carbon material, and each of the lithium secondary batteries of Examples 2 to 4 was manufactured using each of the negative electrodes thus manufactured.

Example 2 produced as described above
In each of the lithium secondary batteries of Examples 1 to 4,
In the same manner as in the case of the lithium secondary battery described above, the discharge capacity Qo before storage and the discharge capacity Qa after storage were measured to determine the self-discharge rate, and the discharge capacity Qo before storage and the self-discharge rate were calculated as shown in Table 2 below. It was shown to.

[0052]

[Table 2]

As is apparent from the results, as the first carbon material of the core in the negative electrode, the plane distance d ₀₀₂ of the (002) plane is in the range of 0.335 nm to 0.338 nm, and the c-axis direction is In each of the lithium secondary batteries of Examples 1 to 3 using a crystallite having a crystallite size Lc of 30 nm or more, as the first carbon material of the core portion, the (002) plane spacing d ₀₀₂ was 0. In comparison with the lithium secondary battery of Example 4 using a battery having a size of 3388 nm and a crystallite size Lc of 20 nm in the c-axis direction, the discharge capacity was increased, the self-discharge rate was reduced, and the storage characteristics were low. Had improved.
In particular, as the first carbon material of the core in the negative electrode, (0
02) surface plane spacing d ₀₀₂ of 0.3356 nm, in a lithium secondary battery of Example 1 was used as the size Lc in the c-axis direction of the crystallite is equal to or greater than 100 nm, the discharge capacity is further increased On the other hand, the self-discharge rate was further reduced, a sufficient discharge capacity was obtained, and the storage characteristics were excellent.

(Examples 5 to 9) In Examples 5 to 9, in the production of the negative electrode in the lithium secondary battery of Example 1 described above, the first carbon material used for the core was used as the first carbon material. As in the case of No. 1, a natural graphite powder having a (002) plane spacing d ₀₀₂ of 0.3356 nm and a crystallite size Lc in the c-axis direction of 100 nm or more is used. The second carbon material, which has lower crystallinity than the first carbon material covering the surface of the first carbon material and contains fluorine, is used.

Here, in Example 5, as in Example 1 described above, the natural graphite powder was immersed in a pitch in a molten state, taken out and dried, and the surface was pitched. After obtaining the coated natural graphite powder, it is calcined at 2800 ° C. for 2 hours in an inert atmosphere, so that the surface of the natural graphite powder as a core part is a carbon material having a lower crystallinity than natural graphite. After that, in the same manner as in Example 1 described above, the carbon material having low crystallinity is made to contain fluorine, and the surface of the natural graphite powder serving as the core is crystallized from the natural graphite. Thus, a carbon material coated with a second carbon material having a low property and containing fluorine was obtained. Note that the plane distance d ₀₀₂ of the (002) plane of the second carbon material containing fluorine was 0.3358 nm.

In Example 6, as in Example 1, the natural graphite powder was immersed in a molten pitch, taken out and dried, and the surface was coated with the pitch. After obtaining the obtained natural graphite powder, it is calcined at 2400 ° C. for 2 hours in an inert atmosphere,
The surface of the above-mentioned natural graphite powder to be the core is coated with a carbon material having lower crystallinity than natural graphite, and thereafter, in the same manner as in Example 1 described above, the carbon material having lower crystallinity is coated. A carbon material was obtained in which fluorine was contained and the surface of a natural graphite powder serving as a core was coated with a second carbon material having a lower crystallinity than the natural graphite and containing fluorine. In addition,
In the above-mentioned second carbon material containing fluorine, (0
Surface spacing d ₀₀₂ of 02) plane was supposed to 0.3372nm.

In Example 7, as in Example 1 above, the natural graphite powder was immersed in a molten pitch, taken out and dried, and the surface was coated with the pitch. After obtaining the obtained natural graphite powder, it is calcined at 2000 ° C. for 2 hours in an inert atmosphere,
The surface of the above-mentioned natural graphite powder to be the core is coated with a carbon material having lower crystallinity than natural graphite, and thereafter, in the same manner as in Example 1 described above, the carbon material having lower crystallinity is coated. A carbon material was obtained in which fluorine was contained and the surface of a natural graphite powder serving as a core was coated with a second carbon material having a lower crystallinity than the natural graphite and containing fluorine. In addition,
In the above-mentioned second carbon material containing fluorine, (0
02) The plane spacing d ₀₀₂ was 0.3391 nm.

In Example 8, the above natural graphite powder was immersed in a molten furfuryl alcohol resin, and then taken out and dried to coat the surface with the furfuryl alcohol resin. After the obtained natural graphite powder is obtained, it is fired in an inert atmosphere at 1100 ° C. for 2 hours, and the surface of the above-mentioned natural graphite powder to be a core is coated with a low-crystalline carbon material. In the same manner as in Example 1 described above, the carbon material having low crystallinity is made to contain fluorine, and the surface of the natural graphite powder serving as the core is made to have a lower crystallinity and a lower fluorine content than that of the natural graphite. A carbon material coated with the second carbon material containing is obtained. Note that the plane distance d ₀₀₂ of the (002) plane of the second carbon material containing fluorine was 0.386 nm.

In Example 9, the above natural graphite powder was immersed in a furfuryl alcohol resin in a molten state, and then taken out and dried to coat the surface with the furfuryl alcohol resin. After obtaining the natural graphite powder, it is heated at 700 ° C. for 2 hours in an inert atmosphere.
After baking for a time, the surface of the natural graphite powder to be the core is coated with a low-crystalline carbon material, and thereafter, the low-crystalline carbon powder is treated in the same manner as in Example 1 above. A carbon material was obtained in which the material contained fluorine, and the surface of a natural graphite powder serving as a core was coated with a second carbon material having lower crystallinity than the natural graphite and containing fluorine. The spacing d ₀₀₂ of the (002) plane of the second carbon material containing fluorine was 0.398 nm.

Then, each of the carbon materials obtained as described above was used, and other than that, each negative electrode was prepared in the same manner as in Example 1 above, and each of the negative electrodes thus prepared was used. Thus, each lithium secondary battery of Examples 5 to 9 was produced.

Next, with respect to each of the lithium secondary batteries of Examples 5 to 9 produced as described above, similarly to the case of the lithium secondary battery of Example 1 described above, the discharge capacity Qo before storage and the discharge capacity Qo before storage were obtained. The self-discharge rate was determined by measuring the discharge capacity Qa after storage, and the results are shown in Table 3 below.

[0062]

[Table 3]

As is clear from the results, in the negative electrode, as the second carbon material having low crystallinity and containing fluorine, which covers the surface of the first carbon material serving as the core,
The (002) plane distance d ₀₀₂ is 0.339 nm or more.
In each of the lithium secondary batteries of Examples 1, 7, and 8 using a battery having a range of 0.390 nm, the (002) plane spacing d ₀₀₂ of the second carbon material was 0.3.
Each of the lithium secondary batteries of Examples 5 and 6 using a battery having a thickness of less than 39 nm, and a second carbon material having a (002) plane spacing d ₀₀₂ exceeding 0.390 nm. The self-discharge rate was lower than that of each of the lithium secondary batteries of Example 9 used, and the storage characteristics of the lithium secondary battery were improved. Particularly, as the second carbon material, the (002) plane spacing d ₀₀₂ is 0.3420 n.
In the lithium secondary battery of Example 1 using the battery having the value of m, the self-discharge rate was further reduced, and the storage characteristics were further improved.

(Examples 10 to 15) In Examples 10 to 15, in the production of the negative electrode in the lithium secondary battery of Example 1 described above, the first carbon material used for the core was used as the first carbon material. As in the case of No. 1, a powder of natural graphite having a (002) plane spacing d ₀₀₂ of 0.3356 nm and a crystallite size Lc in the c-axis direction of 100 nm or more is used. In coating the surface of the carbon material with a second carbon material having a lower crystallinity than that of the first carbon material and containing fluorine, the time for immersing the natural graphite powder in a pitch in a molten state Was changed to change the weight ratio of the second carbon material containing fluorine to the total carbon material of the negative electrode including the natural graphite.

In Examples 10 to 15, the weight ratio of the second carbon material containing fluorine to the total carbon material of the negative electrode including natural graphite was determined as shown in Table 4 below. 0.5 wt% in Example 10, 1 wt% in Example 11, 5 wt% in Example 12, 1 wt% in Example 13.
5% by weight, 20% by weight in Example 14, and 25% by weight in Example 15, except that the surface of the first carbon material serving as the core portion was the same as in Example 1 described above. Each carbon material coated with a fluorine-containing second carbon material having low crystallinity was produced.

Then, each of the carbon materials produced in this manner was used, and in the other respects, each negative electrode was produced in the same manner as in Example 1 above, and each of the negative electrodes produced in this manner was used. Thus, each lithium secondary battery of Examples 10 to 15 was produced.

Next, with respect to each of the lithium secondary batteries of Examples 10 to 15 manufactured as described above, similarly to the case of the lithium secondary battery of Example 1, the discharge capacity Qo before storage and the discharge capacity Qo before storage were obtained. The self-discharge rate was determined by measuring the discharge capacity Qa after storage, and the results are shown in Table 4 below.

[0068]

[Table 4]

As is apparent from the results, in the negative electrode, the weight ratio of the second carbon material containing fluorine to the total carbon material of the negative electrode including the first carbon material made of natural graphite was 1 to 20% by weight. %, The weight ratio of the second carbon material containing fluorine to the total carbon material of the negative electrode was 0.5% by weight. And the weight ratio of the second carbon material containing fluorine to the total carbon material of the negative electrode was 2
The self-discharge rate was lower than that of the lithium secondary battery of Example 15 using 5% by weight, and the storage characteristics of the lithium secondary battery were improved. In particular, in each of the lithium secondary batteries of Examples 1, 12, and 13 in which the weight ratio of the second carbon material containing fluorine to the total carbon material of the negative electrode was in the range of 5 to 15% by weight. Is
The self-discharge rate was further reduced, and the storage characteristics were further improved.

(Examples 16 to 20) In Examples 16 to 20, in the production of the negative electrode in the lithium secondary battery of Example 1 described above, the above-described Example was used as the first carbon material used for the core. As in the case of Example 1 above, a natural graphite powder having a (002) plane spacing d ₀₀₂ of 0.3356 nm and a crystallite size Lc in the c-axis direction of 100 nm or more was used. In the same manner as described above, the surface of the natural graphite powder serving as the core is coated with a carbon material having a lower crystallinity than the natural graphite, and then is reacted with fluorine gas to form a fluorine on the carbon material having a low crystallinity. Is changed only in the same conditions as in Example 1 except that the first material composed of natural graphite as a core portion is changed.
The surface of the carbon material was coated with a second carbon material having a lower crystallinity than that of the natural graphite and containing fluorine.

Then, as described above, the surface of the natural graphite powder serving as the core is coated with a carbon material having a lower crystallinity than that of the natural graphite and reacted with fluorine gas to form a carbon material having a low crystallinity. Example 1 for containing fluorine
In Example 6, the reaction was performed at 50 ° C. for 5 minutes with fluorine gas. In Example 17, the reaction was performed at 50 ° C. for 10 minutes with fluorine gas. In Example 18, the reaction was performed at 250 ° C. for 10 minutes with fluorine gas. 400 in Example 19
It was made to react with fluorine gas at 10 ° C. for 10 minutes, and in Example 20, it was made to react with fluorine gas at 500 ° C. for 10 minutes.

Here, after reacting with the fluorine gas as described above to make the carbon material having low crystallinity contain fluorine,
When the atomic ratio of fluorine to carbon (F / C) on the surface of the second carbon material containing fluorine was determined by X-ray photoelectron spectroscopy, as shown in Table 5 below, The value of the ratio (F / C) was determined in Example 16.
0.005 in Example 17, 0.01 in Example 17, Example 18
Was 0.1 in Example 19, 0.2 in Example 19, and 0.5 in Example 20.

Then, each of the carbon materials prepared as described above was used, and other than that, each negative electrode was prepared in the same manner as in Example 1 above, and each of the negative electrodes thus prepared was used. Thus, each lithium secondary battery of Examples 16 to 20 was produced.

Next, with respect to each of the lithium secondary batteries of Examples 16 to 20 manufactured as described above, similarly to the case of the lithium secondary battery of Example 1, the discharge capacity Qo before storage and the discharge capacity Qo before storage were obtained. The self-discharge rate was determined by measuring the discharge capacity Qa after storage, and the results are shown in Table 5 below.

[0075]

[Table 5]

As is apparent from these results, in the negative electrode, the atomic ratio of fluorine to carbon (F / C) on the surface of the second carbon material covering the surface of the first carbon material serving as the core is 0.1. Each of the lithium secondary batteries of Examples 1 and 17 to 19 using the ones in the range of 01 to 0.2,
The lithium secondary battery of Example 16 using a material having an atomic ratio of fluorine to carbon (F / C) of 0.005 on the surface of the second carbon material of 0.005 or the second carbon material Example 2 using a material having an atomic ratio of fluorine to carbon (F / C) of 0.5 on the surface of the material
0, the self-discharge rate was lower than that of the lithium secondary battery, and the storage characteristics of the lithium secondary battery were improved. In particular, in each of the lithium secondary batteries of Examples 1 and 18, using the second carbon material having an atomic ratio of fluorine to carbon (F / C) in the surface range of 0.05 to 0.1 was used. In the battery, the self-discharge rate was further reduced, and the storage characteristics were further improved.

(Example 21) In Example 21, in the production of the negative electrode in the lithium secondary battery of Example 1 described above, the first carbon material used for the core was the same as that of Example 1 described above. Same (002) plane spacing d ₀₀₂
Is 0.3356 nm, the size Lc of the crystallite in the c-axis direction is 100 nm or more. Natural graphite powder is used, and the natural graphite powder is immersed in a pitch in a molten state to which sulfur is added. After being taken out and dried, it was baked at 1100 ° C. for 2 hours in an inert atmosphere, so that the surface of the above-mentioned natural graphite powder to be a core had lower crystallinity than this natural graphite and contained sulfur. Coated with carbon material.

Then, the surface of the natural graphite powder, which is to be the core part, was coated with the carbon material having a low crystallinity and containing sulfur as described above at 100 ° C. in the same manner as in Example 1 above. Reacting with fluorine gas for 10 minutes under the temperature conditions described above to cause the carbon material having a low crystallinity and containing sulfur to contain fluorine, and the surface of the natural graphite powder serving as the core is more crystalline than the natural graphite. A low carbon material coated with a second carbon material containing fluorine and sulfur was obtained.

Then, the carbon material produced in this manner was used for a negative electrode. Except for the other points, a negative electrode was produced in the same manner as in Example 1 described above, and the negative electrode of Example 21 was produced using this negative electrode. A lithium secondary battery was manufactured.

Next, with respect to the lithium secondary battery of Example 21 manufactured as described above, similarly to the case of the lithium secondary battery of Example 1 described above, the discharge capacity Qo before storage and the discharge capacity after storage were The self-discharge rate was determined by measuring the discharge capacity Qa, and the results are shown in Table 6 below.

[0081]

[Table 6]

As is apparent from the results, the surface of the natural graphite powder as the core in the negative electrode was made to have a lower crystallinity than that of the natural graphite and a second powder containing fluorine and sulfur.
Example 21 using a carbon material coated with a carbon material
In the lithium secondary battery of Example 1, the self-discharge rate was further reduced as compared with the lithium secondary battery of Example 1 in which no sulfur was contained in the second carbon material, and excellent storage characteristics were obtained.

In each of the above embodiments, a cylindrical lithium secondary battery having a diameter of 14.2 mm and a height of 50 mm has been described. However, the shape and size of the lithium secondary battery are not limited. There is no particular limitation, and similar effects can be obtained in lithium secondary batteries having various shapes such as a flat coin shape and a square shape.

[0084]

As described above in detail, in the lithium secondary battery according to the present invention, the surface of the first carbon material which does not contain fluorine is used as the carbon material in the negative electrode. Since the first carbon material is coated with a second carbon material having lower crystallinity and containing fluorine than the first carbon material, the first carbon material coated with the first carbon material at the time of initial charging is used. The second carbon material, which has low crystallinity and contains fluorine, has a stable lithium fluoride film having excellent lithium ion permeability and a high crystallinity. Since no carbon material contained fluorine, the crystal structure of the first carbon material was not disturbed.

As a result, in the lithium secondary battery of the present invention, when stored in a charged state, the lithium in the first carbon material having high crystallinity reacts with the solvent in the non-aqueous electrolyte. And the battery capacity was prevented from lowering, and the storage characteristics of the lithium secondary battery were greatly improved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a lithium secondary battery produced in an example of the present invention and a comparative example.

[Explanation of symbols]

 1 Positive electrode 2 Negative electrode

Continued on the front page (72) Inventor Yoshinori Kida 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Inside Sanyo Electric Co., Ltd. (72) Inventor Atsuhiro Funabashi 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Inside Sanyo Electric Co., Ltd. (72) Inventor Toshiyuki Noma 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Prefecture Inside Sanyo Electric Co., Ltd. (72) Inventor Ikuo Yonezu 2-chome, Keihanhondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. F term (reference) 5H029 AJ04 AK02 AK03 AK05 AL06 AM03 AM04 AM05 AM07 DJ16 HJ01 HJ02 HJ13 5H050 AA09 BA17 CA01 CA02 CA08 CA09 CB07 DA03 EA24 FA17 FA18 HA01 HA02 HA13

Claims (6)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a negative electrode using a carbon material, and a non-aqueous electrolyte, wherein the carbon material in the negative electrode is a first core that does not contain fluorine. A lithium secondary battery, wherein a surface of a carbon material is coated with a second carbon material having lower crystallinity than the first carbon material serving as a core and containing fluorine. 2. The lithium secondary battery according to claim 1, wherein (0) in the first carbon material serving as the core portion.
02) The plane spacing d ₀₀₂ is 0.335 to 0.338 nm
And the size Lc of the crystallite in the c-axis direction is 3
0 nm or more. 3. The lithium secondary battery according to claim 1 or 2, wherein the fluorine-containing second carbon material has a (002) plane spacing d ₀₀₂ of 0.339 to 0.339.
The range is 0.390 nm. 4. The lithium secondary battery according to claim 1, wherein the weight ratio of the fluorine-containing second carbon material in the total carbon material of the negative electrode is 1 to 20. % By weight. 5. The lithium secondary battery according to claim 1, wherein fluorine is determined by X-ray photoelectron spectroscopy with respect to carbon on the surface of the fluorine-containing second carbon material. Has an atomic ratio (F / C) of 0.01 to 0.2. 6. The lithium secondary battery according to claim 1, wherein the fluorine-containing second carbon material further contains sulfur.