JP3309449B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a secondary battery which replaces a nickel-cadmium battery or the like. For example, a negative electrode is made of a material capable of doping and undoping lithium ions such as a carbon material, and a positive electrode is made of lithium. The present invention relates to a non-aqueous electrolyte secondary battery using a lithium composite oxide such as a cobalt composite oxide.

[0002]

[Prior art]

[0003] In recent years, with the advance of electronic technology, electronic devices have been improved in performance, downsized, and portable, and the demand for high energy density for secondary batteries used in these electronic devices has been increasing.

Conventionally, secondary batteries used in these electronic devices include nickel-cadmium batteries and lead batteries, but these batteries are low in discharge voltage and high in energy density. Not enough.

Therefore, recently, as a secondary battery which replaces the above-mentioned nickel-cadmium battery or the like, a material capable of doping and undoping lithium ions such as a carbon material is used for a negative electrode, and a lithium-cobalt composite oxide is used for a positive electrode. Research and development of non-aqueous electrolyte secondary batteries using lithium composite oxides such as materials have been actively conducted. This battery has a high battery voltage, a low self-discharge, and a high energy density.

Here, the principle of the lithium ion non-aqueous electrolyte secondary battery will be described. First, the positive electrode is LiXO ₂
Or, the anode 1 is made of LiX ₂ O ₄ (X is one or more kinds of transition metals such as Co, Ni, and Mn), while the anode 1 is made of carbon (graphite, easily graphitized carbon, hardly graphitized carbon, etc.). (See FIG. 5), in which lithium ions move between the positive electrode and the negative electrode 1. In the positive electrode and the negative electrode 1, lithium is intercalated. That is, it means that lithium ions enter between layers of the medium forming the positive electrode or the negative electrode 1.

[0007] On the other hand, the performance of the negative electrode material mainly deteriorates due to repeated charging and discharging. However, in the case of a positive electrode, deterioration can be reduced by stopping the extraction of Li up to about 0.6.

[0008]

However, in addition to the original deterioration of the negative electrode, when Li deposits on the surface of the current collector Cu foil or Ni foil and becomes electrochemically inactive Li, the capacity is reduced. Further accelerates deterioration.

That is, as shown in FIG. 6, the structure of the negative electrode is such that carbon particles are mixed with a binder such as PVDF and applied on a current collector 9, and an electrode lead is connected to the current collector 9. Is what is being done. When charging, see Fig. 7.
As shown in (1), lithium ions are extracted from the positive electrode and intercalated into the carbon particles, but actually, a part of the lithium ions is also attached to the surface of the current collector. This Li on Cu or Ni
Metal is a fine powder and reacts with an electrolytic solution to lose electrochemical activity. Therefore, there has been a problem that the effective Li ions are reduced as a whole, causing significant capacity deterioration.

The present invention has been made in view of such problems, and has as its object to provide a non-aqueous electrolyte secondary battery in which capacity deterioration due to charge / discharge cycles is improved.

[0011]

The non-aqueous electrolyte secondary battery of the present invention comprises, for example, as shown in FIG. 1, a positive electrode, particles of a carbon material capable of doping / dedoping lithium, and a binder. A negative electrode 1 having a carbon particle layer 16 provided on a current collector 9;
In a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte, the average particle size between the current collector 9 and the carbon particle layer 16 is 5
Carbon fine particle layer 1 composed of carbon fine particles of μm or less and a binder
5 is a non-aqueous electrolyte secondary battery.

In the nonaqueous electrolyte secondary battery of the present invention, for example, as shown in FIG.
It is a non-aqueous electrolyte secondary battery of the above-described configuration having a size of m to 50 μm.

[0013]

According to the non-aqueous electrolyte secondary battery of the present invention, the positive electrode and the carbon particle layer 16 composed of particles of a carbon material capable of doping / dedoping lithium and a binder are provided on the current collector 9. In the non-aqueous electrolyte secondary battery having the negative electrode 1 and the non-aqueous electrolyte, between the current collector 9 and the carbon particle layer 16, carbon fine particles having an average particle size of 5 μm or less and a binder By using a non-aqueous electrolyte secondary battery having the carbon fine particle layer 15 made of the above, capacity deterioration of the non-aqueous electrolyte secondary battery due to charge and discharge cycles can be improved.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the nonaqueous electrolyte secondary battery of the present invention will be described below with reference to FIGS. Here, samples A, B, and C were prepared.

Sample A First, the negative electrode 1 was manufactured as follows. A petroleum pitch was used as a starting material, and was calcined to obtain coarse-grained pitch coke. This coarse-grained pitch coke was pulverized to obtain fine particles and particles having an average particle size of 1.5 μm and an average particle size of 20 μm. Each of these is placed in an inert gas for 100
By baking at 0 ° C. to remove impurities, carbon fine particles having an average particle diameter of 1.5 μm and carbon particles of 20 μm of the coke material were obtained. The 20 μm carbon particles were further subjected to air classification to remove fine particles having a particle diameter of 3 μm or less contained therein.
The particle size range of the 20 μm carbon particles was 3 μm to 50 μm, and the distribution was a normal distribution.

The average particle size thus obtained is 1.5 μm
10% by weight of polyvinylidene fluoride (P
VDF) was added and mixed, and further dispersed in N-methylpyrrolidone as a solvent to obtain a carbon fine particle slurry.

The average particle diameter of 20 obtained as described above is used.
10% by weight of PVDF is added to and mixed with μm carbon particles,
Further, it was dispersed in N-methylpyrrolidone as a solvent to obtain a carbon particle slurry.

Next, the above-mentioned slurry of carbon fine particles (average particle size: 1.5 μm) is applied to both sides of a negative electrode current collector 9, which is a strip-shaped copper foil having a thickness of 10 μm, and the solvent is dried so that both sides have a thickness of 5 μm. the thickness of the formation of the fine carbon particles layer 1 5 (see FIG. 1).

Next, the above-mentioned carbon particle slurry (average particle diameter of 20 μm from which particles having a particle diameter of 3 μm or less are removed) is applied on the above-mentioned carbon fine particle layer 15 so as to be applied thereto.
Was formed, and the solvent was dried, and then compression-molded using a roller press to obtain a belt-shaped negative electrode 1.

The thickness of the carbon particle layer 16 after molding is as follows:
Both surfaces were 150 μm, and a carbon fine particle layer 15 having a thickness of about 5 μm was formed between the carbon particle layer 16 and the current collector. The width of the strip-shaped negative electrode 1 is 41.5 mm and the length is 5
It was 30 mm.

Next, a positive electrode was produced as follows. 0.5 mol of lithium carbonate and 1 mol of cobalt carbonate are mixed and calcined at 900 ° C. for 5 hours in air to obtain Li.
CoO ₂ was obtained. This LiCoO ₂ is used as a positive electrode active material,
91 parts by weight of this LiCoO ₂ , 6 parts by weight of graphite as a conductive agent, and 3 parts by weight of polyvinylidene fluoride as a binder were mixed to prepare a positive electrode mixture. This positive electrode mixture was dispersed in a solvent N-methylpyrrolidone to form a slurry (paste).

Next, this positive electrode mixture was uniformly applied on both sides of a positive electrode current collector, which is a 20-μm-thick strip-shaped aluminum foil, and dried. After drying, compression molding was performed to obtain a strip-shaped positive electrode. The film thickness of the mixture after molding was the same at 80 μm on both sides, and the width of the belt-shaped positive electrode was 39.5 mm and the length was 500
mm.

The strip-shaped negative electrode prepared as described above, a strip-shaped positive electrode, and a separator made of a microporous polypropylene film having a thickness of 25 μm and a width of 44 mm were laminated in the order of negative electrode 1, separator, positive electrode, and separator in the order of 4. A laminated electrode body having a layered structure is formed. The laminated electrode body is wound in a spiral shape many times along the length direction with the negative electrode 1 inside, and the final end of the outermost peripheral separator is fixed with tape. Was prepared.

The spiral electrode body manufactured as described above was housed in a nickel-plated iron battery can (see FIG. 3). Insulating plates are provided on the upper and lower surfaces of the spiral electrode body, and an aluminum positive electrode lead is led out from the positive electrode current collector to collect the negative electrode 1 and the positive electrode. It was taken out from the negative electrode current collector 9 and welded to the battery can.

Then, L was added to a mixed solvent of propylene carbonate and diethyl carbonate in an equal volume in a battery can.
5.5 g of a non-aqueous electrolyte in which iPF ₆ was dissolved at a ratio of 1 mol / l was injected, and impregnated in the spiral electrode body.

Then, the battery can was caulked through an insulating sealing gasket whose surface was coated with asphalt, thereby fixing the battery lid and maintaining the airtightness in the battery. As described above, a cylindrical nonaqueous electrolyte secondary battery having a diameter of 20 mm and a height of 50 mm was produced.

Sample B The negative electrode 1 was produced as follows. Sample A described above
The slurry of the carbon particles (average particle diameter of 20 μm obtained by removing particles having a particle diameter of 3 μm or less) used in the above was applied to both surfaces of the negative electrode current collector 9 which is a strip-shaped copper foil having a thickness of 10 μm, and the carbon particle layer 16 was formed. After forming and drying the solvent, compression molding was performed by a roller press machine to obtain a belt-shaped negative electrode 1. The thickness of the carbon particle layer 16 after molding is 150 μm on both sides. Other conditions are the same as those of the sample A.

Sample C The negative electrode 1 was produced as follows. As described in the sample A, the slurry of carbon particles having an average particle diameter of 20 μm (though particles having a particle diameter of 3 μm or less is not removed) obtained in the step of firing the pitch coke has a thickness of 10 μm.
coated on both sides of a negative electrode current collector 9 which is a μm band-shaped copper foil,
After forming the carbon particle layer 16 and drying the solvent, it was compression-molded by a roller press to obtain a strip-shaped negative electrode 1. The thickness of the carbon particle layer 16 after molding is 150 μm on both sides. Other conditions are the same as those of the sample A.

Using each of the ten non-aqueous electrolyte secondary batteries prepared as described above, charging and discharging were repeated at a charging rate of 0.2 C and a discharging rate of 0.2 C for each battery, and the battery capacity was charged and discharged. Frequency dependence was determined. The result is the following value, which is shown in FIG.

Number of charge / discharge cycles Sample A Sample B Sample C 300 cycles 94% 87% 90% 600 cycles 89% 76% 84% 1200 cycles 84% 60% 77%

As can be seen from the results, when the number of charge / discharge cycles was 1200, sample A was 84%
And the smallest capacity degradation, followed by 77% of sample C,
60% of sample B has the largest capacity deterioration.

However, sample C had a safety problem that the current flowed too much when the external short circuit occurred at full charge. To suppress this, increase the amount of binder (1
5%), the rate at which the surface of the carbon particles is covered with the binder increases, so that the initial capacity decreases (7).
%).

Here, a description will be given of the factors that can reduce the capacity deterioration due to the charge / discharge cycle in the sample A.

As described above, when a conventional non-aqueous electrolyte secondary battery is charged, lithium ions are extracted from the positive electrode and intercalated into carbon particles as shown in FIG. , A part of the surface also adheres to the current collector surface. The Li metal on Cu or Ni is a fine powder and reacts with the electrolyte to lose electrochemical activity. For this reason, the effective Li ions are reduced as a whole, which causes capacity deterioration.

In order to prevent this, the surface of the current collector may be filled with carbon particles. To do this,
Fine carbon particles may be used. However, when the amount of fine powder is large, a large amount of a binder is required to apply the carbon particles on the current collector, and the initial capacity is reduced and the safety is also deteriorated.

Therefore, it is considered to be a good method to apply fine carbon powder thinly on the current collector and then apply a predetermined amount of carbon particles of a predetermined size, that is, a so-called two-layer coating. (See FIG. 2).

When carbon particles having a particle size of 3 μm or less are cut, the specific surface area of the particles does not become unnecessarily large, that is, the surface area of the active carbonaceous material can be reduced. Become. Further, since the current collector surface is filled with carbon particles, there is no need to worry about fine powder of Li metal being precipitated. In the above-described embodiment, a result supporting this idea was obtained.

In the above embodiment, the negative electrode was made of pitch coke, but the material of the negative electrode is not limited to this. That is, a carbon material is used for the negative electrode,
For example, pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, and organic polymer compound fired bodies (phenol resin, furan resin, etc.) are fired at appropriate temperatures to carbonize. ), Carbon fiber, activated carbon and the like can be used.

The non-aqueous electrolyte used for the non-aqueous electrolyte secondary battery is not limited to the above-described embodiment. For example, an electrolyte obtained by using a lithium salt as an electrolyte and dissolving it in an organic solvent is used. Here, as the organic solvent, for example, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl carbonate, γ-butyrolactone, tetrahydrofuran,
A single solvent or a mixture of two or more solvents such as 1,3-dioxolan, 4-methyl-1,3-dioxolan, diethyl ether, sulfolane, methylsulfolane, acetonitrile, and propionitrile can be used. Can be used, and LiClO ₄ , Li
AsF ₆ , LiPF ₄ , LiBF ₄ , LiB (C
_{6 H 5) 4, LiCl,} LiBr, CH 3 SO 3 Li,
CF ₃ SO ₃ Li and the like.

The electrode is not limited to the two-layer coating, but may be a one-layer coating. Further, the present invention is not limited to a cylindrical battery using a spiral electrode, and can be applied to a flat battery such as a coin type (irrespective of the shape of the electrode).

As described above, according to this example, it is possible to improve the capacity deterioration of the nonaqueous electrolyte secondary battery due to the charge / discharge cycle.

It should be noted that the present invention is not limited to the above-described embodiment, but can adopt various other configurations without departing from the gist of the present invention.

[0043]

As described above, according to the present invention, there is obtained an advantage that capacity deterioration due to charge / discharge cycles of a nonaqueous electrolyte secondary battery can be improved.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a negative electrode of a nonaqueous electrolyte secondary battery of the present invention.

FIG. 2 is a sectional view showing a main part of a negative electrode of the non-aqueous electrolyte secondary battery of the present invention.

FIG. 3 is a cross-sectional view showing one embodiment of the non-aqueous electrolyte secondary battery of the present invention.

FIG. 4 is a diagram showing capacity deterioration due to charge / discharge cycles of the nonaqueous electrolyte secondary battery of the present invention.

FIG. 5 is a cross-sectional view showing a negative electrode of a conventional non-aqueous electrolyte secondary battery.

FIG. 6 is a cross-sectional view showing a main part of a negative electrode of a conventional non-aqueous electrolyte secondary battery.

FIG. 7 is a model diagram showing a main part of a negative electrode of a conventional nonaqueous electrolyte secondary battery.

[Explanation of symbols]

 Reference Signs List 1 negative electrode 9 negative electrode current collector 15 carbon fine particle layer 16 carbon particle layer

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 4/00-4/04 H01M 10/36-10/40

Claims (8)

(57) [Claims] 1. A non-aqueous electrolyte comprising a positive electrode, a negative electrode having carbon particle layers formed of carbon material particles capable of doping / dedoping lithium and a binder on both surfaces of a current collector, and a non-aqueous electrolyte. In the liquid secondary battery, the average particle diameter is 5 μm between the current collector and the carbon particle layer.
Intermediate carbon fine particle layer consisting of the following carbon fine particles and binder
A non-aqueous electrolyte secondary battery formed as a layer on both surfaces of a negative electrode . 2. The carbon fine particle layer has a thickness of 1 μm or more and 50 μm or more.
2. The non-aqueous electrolyte secondary battery according to claim 1, wherein m is equal to or less than m. 3. A positive electrode, doped with lithium and dedoped.
Collection of carbon particle layer consisting of particles of carbon material and binder
Non-aqueous electrolyte having a negative electrode provided on the body and a non-aqueous electrolyte
In solution secondary battery, non-aqueous electrolyte secondary batteries, characterized by the following. (A) carbon fine particles between the current collector and the carbon particle layer;
And a carbon fine particle layer made of a binder. (B) The average particle size of the carbon fine particles is the average particle size of the carbon material.
Smaller than the average particle size of the particles. 4. The carbon fine particles have an average particle size of about 1.5 μm.
4. The non-aqueous electrolyte secondary according to claim 3, wherein
battery. 5. The carbon particle layer removes particles having a small particle diameter.
3. A carbon material comprising:
Is a non-aqueous electrolyte secondary battery according to 4. 6. The carbon material in the carbon particle layer is 3 μm or less.
6. The particle having a particle size of (a) is removed.
The non-aqueous electrolyte secondary battery according to the above. 7. The thickness of the carbon particle layer is determined by the thickness of the carbon particle layer.
Claims 3, 4, 5, characterized in that it is thinner than
Is a non-aqueous electrolyte secondary battery according to 6. 8. The carbon fine particle layer has a thickness of about 5 μm.
The non-aqueous electrolyte secondary battery according to claim 7, wherein