JP2002324544A - Positive electrode for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium secondary battery, which can attain the lithium secondary battery showing an excellent discharging characteristic at a high rate discharging. SOLUTION: The positive electrode for the lithium secondary battery is made by forming a plied layer including an active material, a conductive material and an adhesive on a collector. The lithium secondary battery whose filling density in the layer of the piled layer is more than 3 g/cm<3> and whose impregnation ratio of an electrolyte as 6 to 20% is constituted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery.

[0002]

2. Description of the Related Art In general, a positive electrode of a lithium secondary battery generally has
(1) A layer of a mixture containing an active material, a conductive material and a binder is formed on a current collector made of a foil or the like. It is formed by applying and drying a slurry containing a material and a binder, and subjecting the obtained coating film to a rolling treatment. Here, as the active material, for example, a LiCo-based composite oxide such as LiCoO ₂ is used, and as the conductive material, for example, a granular carbon material is used.

[0003] When increasing the capacity of a battery, usually, the amount of the mixture on the current collector in the positive electrode (the coating amount of the slurry containing the active material, the conductive material, and the binder) is increased, and rolling is performed by rolling. And increase the packing density of the mixture in the layer of the mixture. However, if the packing density of the mixture is increased, the porosity of the layer is impaired, the electrolyte is not sufficiently impregnated, and the periphery of the active material is not sufficiently filled with the electrolyte. If the surroundings of such an active material are not sufficiently filled with the electrolytic solution, insertion and removal of lithium ions in the active material are prevented during high-rate discharge (particularly at low temperatures), and the discharge capacity is reduced (particularly at low temperatures. Capacity and discharge voltage).

[0004]

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a positive electrode for a lithium secondary battery capable of achieving a lithium secondary battery exhibiting excellent discharge characteristics during high-rate discharge. . Another object of the present invention is to provide a lithium secondary battery exhibiting excellent discharge characteristics during high-rate discharge.

[0005]

In order to achieve the above object, the present invention has the following features. (1) A positive electrode for a lithium secondary battery in which a layer of a mixture containing an active material, a conductive material, and a binder is formed on a current collector, and the packing density of the mixture in the layer of the mixture is Is 3g / cm
^A positive electrode for a lithium secondary battery, wherein the positive electrode has an impregnation ratio of 6 to 20% with an electrolyte solution of ³ or more. (2) A lithium secondary battery having the positive electrode according to (1).

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The positive electrode for a lithium secondary battery of the present invention (hereinafter, also simply referred to as a positive electrode) has a packing density of 3 g / cm ³ or more of a layer of a mixture containing an active material, a conductive material, and a binder on a current collector. Yes,
Further, it is characterized in that the impregnation rate of the electrolyte is 6 to 20%. That is, the positive electrode for a lithium secondary battery of the present invention,
Conventionally, it was possible to achieve both a high packing density of the mixture and a high impregnation rate of the electrolyte in the layer of the mixture containing the active material (hereinafter also referred to as “mixture layer” or “active material layer”), which was difficult in the past. By having a layer of the mixture having the specific packing density and the impregnation rate of the electrolyte on the current collector, the discharge capacity at the time of high-rate discharge of the lithium secondary battery is reduced (particularly, the discharge capacity at a low temperature). And a decrease in discharge voltage).

The packing density (g / cm ³ ) of the mixture layer
Is the amount of the mixture per unit area on the current collector (g / cm
² ) / is determined, and this is divided by the average thickness (cm) of the mixture layer. The impregnation rate (%) of the electrolytic solution is defined as the amount of the mixture per unit area on the current collector (g / cm ² ) being Wa and impregnating the positive electrode with the electrolytic solution until the cathode is saturated. Is a value obtained by calculating with the following formula, where Wb is the net weight increase before impregnation. Impregnation rate (%) = 100 × (Wb / Wa)

The active material used for the positive electrode of the present invention is L
Li represented by iCoO ₂ or Li _A Co _1-x Me _X O ₂
—Co-based composite oxide. The latter Li _A Co
In _1-X Me X O _2, the formula A 0.05 to 1.5
(Preferably 0.1 to 1.1), X is 0.01 to 0.5
(Preferably 0.02 to 0.2), and Me is Zr, V, C
r, Mo, Mn, Fe, Ni, B, Al, Ge, Pb,
One or more elements selected from Sn and Sb. When Me is two or more elements, X is the total amount of the two or more elements.

Among them, LiCoO ₂ (lithium cobaltate) is preferable, and among LiCoO ₂ (lithium cobaltate), the crystal size in the (003) plane direction is 800 Å or more and one cobalt atom More preferably, the coordination number of another cobalt atom with respect to is 5.7 or more. LiCoO ₂ having a crystallite size of 800 Å or more in the (003) plane direction of such a crystal and a coordination number of another cobalt atom to one cobalt atom of 5.7 or more.
(Lithium cobalt oxide) is relatively smooth to insert and remove lithium ions during high-rate discharge,
The effect of impregnating the electrolyte layer with a high impregnation rate in the mixture layer (by allowing the electrolyte solution to sufficiently exist around the active material) is sufficiently obtained, and more preferable results are obtained.

The ideal structure of lithium cobalt oxide is such that oxygen atoms (O) form a hexagonal close-packed structure, a layer of cobalt atoms (Co) and a layer of lithium atoms (Li) between oxygen atoms perpendicular to the C axis. Are alternately inserted (see FIG. 1). As an index indicating such crystal development (crystallinity), crystallite size is often used. The crystallite size in the above-mentioned LiCoO ₂ (lithium cobaltate) used in the present invention is a crystallite size as an index indicating the development (crystallinity) of such a crystal, and as shown in FIG. Means the size of the single crystal.

In the present invention, LiCoO ₂ (lithium cobaltate) having a crystallite size in the (003) plane direction of 800 Å or more and a coordination number of one cobalt atom to another cobalt atom of 5.7 or more is used.
Means that the crystallite size in the (003) plane direction of the crystal is 8
It is more preferably 50 Å or more, and the upper limit is not particularly limited.
00 angstrom or less, more preferably 1000
Angstrom or less. When the crystallite size in the (003) plane direction exceeds 10,000 angstroms, the movement distance of ions in the active material becomes long.
The movement of lithium ions is likely to be restricted, which is not preferable. The coordination number of one cobalt atom to another cobalt atom is preferably 5.8 or more, more preferably 5.9 or more, and the upper limit thereof is 6. When the coordination number of one cobalt atom to another cobalt atom exceeds 6, it means that the crystal structure is theoretically broken,
The capacity is reduced, which is not preferable.

The crystallite size in the (003) plane direction of LiCoO ₂ (lithium cobaltate) crystal can be measured, for example, by the following method.

First, high-purity X-ray standard silicon is pulverized in an agate mortar into a sieve of 350 mesh or less, uniformly filled in a sample plate, and subjected to an X-ray diffractometer (RINT2, manufactured by Rigaku Corporation).
000, X-ray source: CuKα)
1) Measure (220), (311) and (400) peaks. At this time, the tube voltage and the tube current of the X-ray source are fixed, and the counting time is adjusted so that the intensity of each peak becomes the same. The spread of the diffraction line of each obtained peak is represented by an integral width, which is expressed as (00
3) The extrapolation of the diffraction line due to the device is determined by extrapolating to the diffraction angle at which the peak is obtained.

Next, the (00)
3) The peak is measured under the same apparatus and under the same conditions as the above-mentioned standard substance, and the spread of the diffraction line caused by both the crystallite size and the apparatus is determined in the same manner as above. Further, assuming that the measured peak spread can be approximated by the Cauchy function, the spread of the diffraction line caused only by the crystallite size is obtained, and the crystallite size is calculated based on the Scherrer equation below. Is calculated.

[0015]

(Equation 1)

(D: crystallite size, K: Scherrer constant (= 1.05), λ: wavelength of X-ray, β: spread of diffraction line calculated from integrated width of peak, θ: diffraction angle)

In addition, 1 in lithium cobaltate crystals
The coordination number of one cobalt atom to another cobalt atom is based on the wide area X-ray absorption fine structure analysis method (EXAFS), and is performed by analyzing the CoK absorption edge. Specifically, the emitted light obtained by the 2.5 GeV-PF ring BL12 beam line of the Ministry of Education High Energy Accelerator Research Organization Synchrotron Radiation Research Facility is separated by a monochromator, and the obtained hard X-rays are irradiated on the sample, The transmitted X-ray absorption spectrum having an energy of 7200 to 8700 eV is detected by an ion chamber, and the Co-Co (interatomic distance = 2.81) of the kinematic structure function obtained by Fourier transform is obtained.
Angstroms) to calculate the coordination number.

LiCoO ₂ (lithium cobaltate) having a crystallite size of 800 Å or more in the (003) plane direction and a coordination number of one cobalt atom to another cobalt atom of 5.7 or more is as follows. It can be manufactured through a process. For example, lithium carbonate and cobalt oxide are mixed at an atomic ratio of lithium / cobalt of 0.99 to 1.10, and are mixed at 600 to 1100C, preferably 700 to 1000C.
Firing at a temperature of at least 2 hours, preferably 5 to 15 hours. 400-75 granules obtained by pulverizing the massive fired product
It can be produced by heat treatment at a high temperature of 0 ° C, preferably about 450 to 700 ° C for 0.5 to 50 hours, particularly about 1 to 20 hours. This heat treatment can be performed in the atmosphere, in a mixed gas of the atmosphere and carbon dioxide, or in an inert gas such as nitrogen or argon. Prior to this heat treatment, it is preferable to classify the crushed granules by sieving them.

The particle size of the Li—Co-based composite oxide used in the present invention is not particularly limited, but from the viewpoint of preventing abnormal battery reaction, the average particle size is preferably larger than 1 μm, more preferably 5 μm or more. More preferred. Further, from the viewpoint of reducing the resistance of the active material layer, the average particle size is preferably 25 μm or less, more preferably 23 μm or less.

The specific surface area is 0.1 to 0.3 m ² / g.
And more preferably 0.15 to 0.25 m ² / g. When the specific surface area is in such a preferable range, the discharge characteristics (particularly, high-rate discharge characteristics) of the battery are further improved, and the safety of the battery is improved (oxygen is not easily desorbed from the active material).

The average particle size of the Li-Co-based composite oxide is measured by the following method. First, the particulate matter to be measured is charged into an organic liquid such as water or ethanol, and the liquid is charged for 35 k.
A dispersion process is performed for about 2 minutes by applying an ultrasonic wave of about Hz to 40 kHz. Note that the amount of the particulate matter to be measured is such that the laser transmittance (the ratio of the output light amount to the incident light amount) of the dispersion liquid after the dispersion treatment is 70% to 95%. Next, this dispersion is applied to a Microtrac particle size analyzer, and the particle size (D ₁ , D ₂ , D ₃₎ of each particle is measured by scattering of laser light.
..) and the number of particles present for each particle size (N ₁ , N ₂ , N ₃
…)). Note that the microtrack particle size analyzer calculates the particle size distribution of the spherical particles that has the theoretical intensity closest to the observed scattering intensity distribution. That is, the particle is assumed to be a sphere having a cross-sectional circle having the same area as the projected image obtained by the irradiation of the laser light, and the diameter (equivalent sphere diameter) of this cross-sectional circle is measured as the particle diameter. Average particle size (μm)
Is calculated by the following equation (1) from the particle diameter (D) of each particle obtained above and the existing number (N) of each particle diameter. Average particle size (μm) = (ΣND ³ / ΣN) ^1/3 (Equation 1)

The measurement of the specific surface area of the Li—Co-based composite oxide is described in “Material Chemistry of Powder” [Yasuo Arai, First Edition, 9th Edition].
Publishing, Baifukan (Tokyo), 1995]
Among the adsorption methods described on page 184, a gas-phase adsorption method using nitrogen as an adsorbent (one-point method) can be used. The measurement of the specific surface area using such a gas-phase adsorption method using nitrogen as an adsorbent can be performed by using, for example, a specific surface area meter Monosorb (manufactured by Quantachrome).

As the conductive material, artificial or natural graphites; acetylene black, oil furnace black,
Granular carbon materials such as carbon black such as extra conductive furnace black; and the like ("granular" includes scaly, spherical, pseudo spherical, massive, whisker-like, etc., and is not particularly limited). The granular carbon material contains at least 1% by weight or more, preferably 1 to 40% by weight of particles having a particle diameter (primary particle diameter) of 1 μm or less.

The particle diameter of the granular carbon material is the diameter of the cross-sectional circle (sphere equivalent diameter) when the particles are assumed to be spherical, and those having a particle diameter (primary particle diameter) larger than 1 μm are as described above. Li
It can be measured using a Microtrac particle size analyzer as in the case of the -Co-based composite oxide. In addition, those having a particle size (primary particle size) of 1 μm or less can be measured using an electron microscope. Specifically, first, an electron micrograph is taken with the magnification set so that 20 or more particles enter the visual field, the area of the image of each particle shown in the photograph is calculated, and further, from the calculated area, Calculate the diameter of a circle with the same area. The average particle size is the number average of the measured number.

Examples of the binder include those conventionally used in the active material layer of the positive electrode of the lithium secondary battery, for example,
Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene, ethylene-propylene-diene copolymer (EPDM) and the like are used.

In the positive electrode of the present invention, the amount of the mixture (existing amount) on the current collector in the positive electrode of the present invention is usually from 1 to 100 mg as indicated by the amount of the active material (the amount of the active material present on one side of the current collector). / C
m ² . Further, the amount of the conductive material in the mixture is generally expressed as 1 by the weight ratio to the active material (active material: conductive material).
00: 0.1 to 100: 20. The amount of the binder is generally 0.1 to 2 with respect to the total amount of the active material and the conductive material.
0% by weight.

Examples of the current collector include a foil formed of a conductive metal such as aluminum, an aluminum alloy, and titanium, a perforated foil, and an expanded metal. When the current collector is a foil or a perforated foil, the thickness is usually 1 unit.
About 0 to 100 μm, preferably 15 to 50 μm
It is about. When the current collector is an expanded metal, its thickness is usually about 25 to 300 μm, preferably 30 to 300 μm.
It is about 150 μm.

The positive electrode of the present invention comprises a mixture layer (active) on a current collector.
The packing density of the mixture in the material layer) is 3 g / cm.^Threethat's all
And the impregnation rate of the electrolytic solution is 6 to 20%.
Packing density is 3-4g / cm^ThreeThe range is preferably
The impregnation rate of the liquid is preferably in the range of 10 to 15%. Electrolyte
If the impregnation rate is less than 6%, a battery using such a positive electrode
Is the discharge capacity and discharge capacity retention rate during high-rate discharge.
Discharge may not be possible, especially during low-temperature operation.
You. On the other hand, the upper limit of the impregnation rate of the electrolytic solution is 20%,
3g / cm packing density of mixture^ThreeIf the above is done,
Positive electrodes with a liquid impregnation rate of more than 20% are
Poor adhesion between the (active material layer) and the current collector (interfacial peeling) occurs
This is the result of the electrolyte being absorbed in that part
In some cases, the adhesion between such a mixture layer and the current collector
In the positive electrode where the failure (interface separation) has occurred, the discharge characteristics of the battery
It will drop significantly. In other words, the battery discharge characteristics
Only the effective electrolytic solution contributing to the above, the impregnation rate of the electrolytic solution is 2
Achieving cathodes greater than 0% is substantially difficult
You. Further, the packing density of the mixture material layer (active material layer) is 4 g / c.
m ^ThreeThe electrolyte solution to the mixture layer (active material layer)
Positive electrode whose impregnating property is significantly reduced and electrolyte solution is sufficiently impregnated
(A positive electrode having an impregnation ratio of 6% or more) cannot be obtained.

The positive electrode of the present invention is manufactured as follows. The process is as usual, including a step of preparing a slurry containing an active material, a conductive material and a binder, a step of coating the slurry on a current collector, a step of drying the slurry (coating), and
Although a rolling step is included, in preparing the slurry, the particle size (1
Uniform conductive material with a primary particle diameter of 1 μm or less (primary particle form)
And a slurry in which the conductive material having the particle size (primary particle size) of 1 μm or less is appropriately aggregated is obtained. For example, the active material, the conductive material and the binder are kneaded with a suitable solvent (for example, N-methylpyrrolidone or the like) to prepare a slurry. In this case, the kneading time is relatively short, and / or The preparation of the slurry is completed while the number of revolutions of the kneader is relatively low and the aggregate (secondary particles) of the conductive material having a particle size of 1 μm or less is appropriately left. Specifically, for example, when kneading with a planetary disperser kneading apparatus (manufactured by Asada Iron Works), the rotation speed of the planetary is about 10 to 70 rpm, and the rotation number of the disperser is 100 to 70 rpm.
5,000 rpm, and the dilution and stirring time is 10 minutes to 1
The kneading time (main dispersion time) is about 10 minutes to 1 hour. That is, a conductive material having a particle diameter (primary particle diameter) of 1 μm or less forms an aggregate at an initial stage of kneading (immediately after being charged into a solvent), but is crushed into primary particles as the kneading time elapses. Therefore, the kneading is terminated in a state where the aggregates remain moderately, without performing excessive kneading such that most of the aggregates are crushed. In addition, if the slurry after kneading is left undisturbed, the conductive material gradually dispersed re-aggregates, so that the amount of aggregates does not become excessive, it is applied to the current collector, and the coating film is dried and dried. It is necessary to perform rolling. As a result of the study by the present inventors, the slurry used in the present invention in which a conductive material having a particle diameter (primary particle diameter) of 1 μm or less is appropriately aggregated has a viscosity of approximately 1000 to 80,000.
mPa · s. The viscosity of the slurry is a value measured at 25 ° C. by a B-type viscometer at a rotation speed of 6 rpm.

As described above, the particle diameter (primary particle diameter) is 1 μm.
It is not clear why even if the packing density is increased by preparing a slurry in which the conductive material having a particle size of m or less is appropriately aggregated, a layer of the mixture (active material layer) having good permeability of the electrolyte can be obtained. The aggregate of the conductive material having a particle diameter (primary particle diameter) of 1 μm or less in the slurry is combined with a binder in the slurry coating and drying steps to form composite particles having a relatively large specific surface area. ,
Due to the presence of these composite particles, even if the coating film after coating and drying is rolled to increase the packing density, the coating film (mixture layer) has sufficient pores through which the electrolyte can penetrate. Is considered to be able to be impregnated. When most of the conductive material having a particle diameter (primary particle diameter) of 1 μm or less in the slurry is uniformly dispersed in the form of primary particles, the packing density of the mixture layer is 3 g / g.
If it is more than cm ³ , the impregnation rate of the electrolytic solution will be less than 6%. This means that the particle size dispersed in the form of primary particles is 1 μm
The following conductive materials are coated with a slurry, and in a drying step, they become a film covering the surface of the active material together with a binder. It is considered that the number of voids through which the electrolyte in the layer of the mixture can penetrate becomes extremely small.

When a lithium secondary battery is constructed using the positive electrode of the present invention, the components of the battery other than the negative electrode, such as the negative electrode, the electrolytic solution, and the separator, are not particularly limited, and known components are used according to a conventional method. be able to.

The negative electrode is formed by forming a layer of a composite material containing an active material and a binder (hereinafter, also referred to as a negative electrode active material layer) on a current collector. Granular carbon materials used as active materials for known negative electrodes of lithium secondary batteries, such as carbon black, amorphous carbon materials (hard carbon and soft carbon), and activated carbon can be used. Among these, graphitized carbon is preferable from the viewpoint of further improving the discharge characteristics of the battery. The particle shape of the granular carbon material is not particularly limited, and may be any of a flake, a sphere, a pseudo sphere, a lump, and a whisker.

As the graphitized carbon, a fibrous material other than a granular material can be used. In this case, the material may be linear or curled. The size of the fibrous graphitized carbon is not particularly limited, but the average fiber length is preferably 1 to 100 μm, and particularly preferably 3 to 50 μm. Further, the average fiber diameter is preferably from 0.5 to 15 μm, particularly preferably from 1 to 15 μm, particularly preferably from 5 to 10 μm. The aspect ratio (average fiber length / average fiber diameter) at this time is preferably 1 to 5, and 3 to 5 is preferable.
Is particularly preferred.

The size (fiber diameter, fiber length) of the fibrous graphitized carbon can be measured using an electron microscope. That is, it can be performed by setting a magnification so that 20 or more fibers are included in the visual field, taking an electron micrograph, and measuring the fiber diameter and the fiber length of each fiber in the photograph with a caliper or the like. The fiber length may be measured by measuring the shortest distance between one end and the other end if the fiber is linear. However, if the fiber is curled or the like, any two points on the fiber that are farthest apart from each other are taken, the distance between the two points is measured, and this is set as the fiber length. Note that the average fiber diameter and the average fiber length are the number average values of the measured number.

When graphitized carbon is used, it is preferable that the interplanar distance (d002) of the crystal lattice is 0.3380 nm or less (particularly preferably 0.3350 nm to 0.3370 nm). The interplanar distance (d002) of the crystal lattice of the graphitized carbon can be measured by the Japan Society for the Promotion of Science described below.

First, high-purity X-ray standard silicon is ground in an agate mortar to a size of 325 mesh standard sieve or less to prepare a standard substance, and this standard substance and the graphitized carbon of the sample to be measured are mixed in an agate mortar ( Mixing ratio: 10 parts by weight of standard substance per 100 parts by weight of graphitized carbon) to prepare an X-ray sample. This X-ray sample was prepared using an X-ray diffractometer (Rigaku Denki R
The sample plate of INT2000, X-ray source: CuKα ray) is uniformly filled. Next, the applied voltage to the X-ray tube was 40 kV,
The applied current was set to 50 mA, and the scanning range was 2θ = 2.
The 002 peak of carbon and the 111 peak of the standard substance are measured at 3.5 to 29.5 degrees and a scan speed of 0.25 degrees / min. Subsequently, from the obtained peak position and its half-value width, using the graphitization degree calculation software attached to the X-ray diffractometer, the interplanar distance (d
002) is calculated.

Examples of the binder include those conventionally used for the active material layer of the negative electrode of a lithium secondary battery, for example, fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF). , Ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (C
A polymer material such as MC) is used.

The amount of the mixture in the negative electrode is usually about 1 to 50 mg / cm ² , preferably 5 to 50 mg / cm ² , as indicated by the amount of the active material in the mixture (the amount of the active material present on one side of the current collector).
20 mg / cm is ² mm, the proportion of the active material and the binder in the cause material weight ratio: generally 80:20 (active material binder)
98: 2.

Examples of the Li salts used in the electrolyte include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAs
F ₆ , LiAlCl ₄ and Li (CF ₃ SO ₂ ) ₂ N
And these may be used alone or in combination of two or more. Further, as the organic solvent, for example,
Ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl sulfoxide, sulfolane,
γ-butyrolactone, 1,2-dimethoxyethane, N,
N-dimethylformamide, tetrahydrofuran, 1,
Examples thereof include 3-dioxolan, 2-methyltetrahydrofuran, diethyl ether, and the like, and these are used alone or in combination of two or more. Among them,
From the viewpoint of improving the charge / discharge cycle characteristics and high-rate discharge characteristics intended by the present invention, ethylene carbonate (E
Mixed solvent of C) and diethyl carbonate (DEC) (EC: DEC (volume ratio) is 40:60 to 60:40)
It is preferable to use In addition, the concentration of the Li salt in the electrolytic solution is generally about 0.1 mol / L to 2 mol / L, and from the viewpoint of charge / discharge cycle characteristics, the concentration is 0.1 mol / L.
It is preferably about 5 mol / l to 1.8 mol / l, and particularly preferably about 0.8 mol / l to 1.5 mol / l.

As the separator, a known separator, such as a polyolefin separator, conventionally used in lithium secondary batteries is used. Here, the separator may be a porous separator or a solid separator in which pores are not substantially formed. Further, the polyolefin separator may be a single polyethylene layer or a single polypropylene layer, but a type in which a polyethylene layer and a polypropylene layer are laminated is preferable, and PP / P is particularly preferable in terms of safety.
A three-layer type of E / PP is preferred.

The form of the battery is not particularly limited. Known ones conventionally used in lithium secondary batteries can be used, for example, cylindrical cans, square cans, button cans made of metal such as Fe, Fe (Ni plating), SUS, aluminum, and aluminum alloy. Alternatively, a sheet-like exterior material such as a laminate film is used. As a laminate film, copper, polyester on at least one side of a metal foil such as aluminum,
It is preferable that a thermoplastic resin laminate layer such as polypropylene is formed.

[0042]

The present invention will now be described more specifically with reference to examples and comparative examples. In addition, the viscosity of the slurry in the following Examples and Comparative Examples was measured at 25 ° C. with a B-type viscometer (DVM-B, manufactured by Tokyo Keiki Co., Ltd.) at a rotation speed of 6 rpm.

Example 1 [Positive electrode] Active material: LiCoO ₂ (crystallite size: 864)
(Angstrom), Co-Co coordination number: 5.9,
A mixture of 91 parts by weight of an average particle diameter of 20 μm and specific surface area of 0.12 m ² / g, 5 parts by weight of conductive material: spherical graphite (average particle diameter of 6 μm) and 1 part by weight of oil furnace black (average particle diameter of 40 nm) ( The content of particles having a particle size of 1 μm or less is 2
0% by weight), a binder: 3 parts by weight of polyvinylidene fluoride, and 50 parts by weight of N-methyl-2-pyrrolidone by a planetary disperser kneader (manufactured by Asada Iron Works) at a rotation of planetary 30 rpm and disperser 500 rpm. By number
The kneading time was 20 minutes for the main dispersion time + 30 minutes for the dilution and stirring time, and the mixture was kneaded to obtain a slurry. Next, the slurry was
After standing for 5 hours, the aluminum foil (thickness 20
μm), dried at 150 ° C., and rolled at a press pressure of 1 ton / cm ² to form an active material layer, thereby completing a positive electrode. The viscosity of the slurry immediately before coating is 7
000 mPa · s. The packing density of the active material layer (filling density of the mixture) in the positive electrode thus manufactured was 3.3 g / cm ² . Then, 1 mol / mol of LiPF ₆ was added to this positive electrode in an electrolytic solution (50 vol% of ethylene carbonate + 50 vol% of diethylene carbonate in a mixed solvent).
Liter dissolved) until a saturated state was reached, and the impregnation rate of the electrolytic solution was 14%.

[Negative electrode] Graphitized carbon fiber (average fiber diameter 8 μm, aspect ratio 3, d002 = 0.33) as active material
92 parts by weight), 8 parts by weight of polyvinylidene fluoride as a binder, and N-methyl-2-pyrrolidone 80
Parts by weight and mixed to form a slurry. This slurry is applied to both sides of a copper foil (thickness: 14 μm) serving as a current collector and dried.
Further, a rolling treatment was performed to complete a negative electrode.

[Assembly of Lithium Secondary Battery] The positive electrode and the negative electrode prepared above were wound through a porous polyethylene-polypropylene composite separator, and this was wound into a cylindrical battery can (18 mm in outer diameter, 17 mm in inner diameter). 5mm, height 65m
m). Further, the above-mentioned electrolytic solution was impregnated between the positive electrode and the negative electrode to form a lithium secondary battery (design capacity: 18
00 mAh).

[Evaluation] The lithium secondary battery obtained above was charged at room temperature. 3. At 1.8 A constant current.
After charging to 2V, charging was performed at a constant voltage of 4.2V, and charging was terminated when the total charging time reached 3 hours. Then, the lithium secondary battery is brought to room temperature (20 ° C).
Under the environment, discharge was performed at a cutoff of 0.2 C (360 mAh) /2.5 V. Also, after performing the same charge,
Discharge was performed at a cut-off of 2C (3.6 Ah) /2.5 V. After the same charge was performed, the battery was discharged at a cutoff of 1 C (1.8 Ah) /2.5 V in a -20 ° C environment.

In each discharge test, a discharge current value and a discharge time were measured, and a discharge capacity (mAh) was calculated.
The ratio of the discharge capacity to the 0.2% discharge capacity (20 ° C.) assuming 100% (capacity maintenance rate)
Was calculated. In a test in an environment of −20 ° C., a discharge voltage (V) was obtained from a value obtained by a discharge power capacity (Wh) / discharge current capacity (Ah).

Example 2 In accordance with Example 1, the slurry prepared in Example 1 was coated on both sides of a current collector (aluminum foil) immediately after the preparation, and the packing density of the active material layer (the total A positive electrode having a packing density of 3.3 g / cm ² was prepared. The viscosity immediately after preparation of the slurry (immediately before coating) was 14000 mPa · s. In the same manner as in Example 1, 1 m of LiPF ₆ was added to the thus prepared positive electrode in a mixed solvent of 50 vol% of ethylene carbonate + 50 vol% of diethylene carbonate.
ol / liter) was impregnated until a saturated state was reached, and the impregnation rate of the electrolytic solution was 13%. Next, a lithium secondary battery (design capacity: 1800 mAh) was prepared using the above-prepared positive electrode, and using the same negative electrode, separator, battery can, and electrolyte solution as in Example 1. The same evaluation test was performed.

Example 3 A slurry was prepared in the same manner as in Example 1 except that the kneading time was changed to 1 hour of main dispersion time + 30 minutes of dilution and stirring.
After the slurry was left for 15 hours, it was coated on both sides of a current collector (aluminum foil), and the packing density of the active material layer (packing density of the mixture) was 3.3 g / cm according to Example 1. ² positive electrodes were produced. The viscosity of the slurry immediately before coating is 25.
000 mPa · s. An electrolyte (ethylene carbonate 5) was applied to the positive electrode thus produced in the same manner as in Example 1.
1 mol / liter of LiPF ₆ dissolved in a mixed solvent of 0% by volume + 50% by volume of diethylene carbonate)
To a saturated state, and the impregnation ratio of the electrolytic solution was 12%. Next, using the positive electrode prepared above, a negative electrode, a separator, a battery can, and an electrolyte were prepared in Example 1.
Use the same lithium secondary battery (design capacity:
1800 mAh) and subjected to the same evaluation test as in Example 1.

Example 4 In place of LiCoO ₂ in Example 3, LiCoO ₂
(Crystallite size:> 1000 (angstrom), C
Immediately after the preparation of a slurry prepared in the same manner as in Example 1 except that the coordination number of o-Co: 5.8, the average particle diameter was 20 μm, and the specific surface area was 0.12 m ² / g, (Aluminum foil) on both sides, and others according to Example 1.
The packing density of the active material layer (packing density of the mixture) is 3.3 g / c.
to produce a positive electrode of the m ^2. The viscosity immediately after preparation of the slurry (immediately before coating) was 60000 mPa · s. The positive electrode thus prepared was impregnated with an electrolyte solution (a solution in which LiPF ₆ was dissolved at 1 mol / L in a mixed solvent of 50% by volume of ethylene carbonate and 50% by volume of diethylene carbonate) until the cathode was saturated as in Example 1. Then, the impregnation rate of the electrolytic solution was measured to be 11%. next,
A lithium secondary battery (design capacity: 1800 mAh) was produced using the above-prepared positive electrode and the same negative electrode, separator, battery can, and electrolyte as in Example 1, and the same evaluation test as in Example 1. Was served.

Example 5 A slurry was prepared in the same manner as in Example 4 except that the main dispersion was not performed during the kneading, and only the dilution and stirring were performed for 30 minutes. Immediately after this slurry was prepared, it was coated on both sides of a current collector (aluminum foil), and the packing density of the active material layer (the packing density of the mixture) was 3.3 g / cm ² according to Example 1. A positive electrode was produced. The viscosity immediately after preparation of the slurry (immediately before coating) is 30.
It was 00 mPa · s. An electrolyte (ethylene carbonate 50) was applied to the positive electrode thus prepared in the same manner as in Example 1.
(1 mol / liter of LiPF ₆ dissolved in a mixed solvent of 50% by volume + diethylene carbonate) was impregnated until a saturated state was reached, and the impregnation rate of the electrolytic solution was measured to be 15%. Next, using the prepared positive electrode, the same negative electrode, separator, battery can, and electrolyte as in Example 1 were used, and a lithium secondary battery (design capacity: 1) was used.
800 mAh) and subjected to the same evaluation test as in Example 1.

Example 6 Active material: 91 parts by weight of LiCoO ₂ (crystallite size: 955 (angstrom), coordination number of Co—Co: 5.8, average particle diameter: 20 μm, specific surface area: 0.12 m ² / g) When,
Conductive material: 6 parts by weight of flaky graphite (average particle size: 4 μm) (content of particles having a particle size of 1 μm or less is 3% by weight) and binder:
A kneading time of 3 parts by weight of polyvinylidene fluoride and 50 parts by weight of N-methyl-2-pyrrolidone were mixed with a planetary dispersing and kneading apparatus (manufactured by Asada Iron Works) at a rotational speed of planetary 30 rpm and a disperser 500 rpm, and the kneading time was 2 hours.
The mixture was kneaded with 0 minute + dilution stirring time of 50 minutes to obtain a slurry. Next, after leaving the above-mentioned slurry for 15 hours, the slurry was applied on both sides of an aluminum foil (thickness: 20 μm) serving as a current collector, dried at 150 ° C., and rolled at a press pressure of 1 ton / cm ² to obtain an active material layer. Was formed to complete the positive electrode. The viscosity of the slurry immediately before coating was 9000 mPa · s. The packing density of the active material layer (mixing material packing density) in the positive electrode thus manufactured was 3.6 g / cm ² . Then, the positive electrode is impregnated with an electrolyte solution (a solution in which LiPF ₆ is dissolved at 1 mol / liter in a mixed solvent of 50% by volume of ethylene carbonate and 50% by volume of diethylene carbonate) until a saturated state is reached, and the impregnation rate of the electrolyte solution is measured. As a result, it was 7.2%. Next, a lithium secondary battery (design capacity: 1800 mAh) was prepared using the above-prepared positive electrode, and using the same negative electrode, separator, battery can, and electrolyte solution as in Example 1. The same evaluation test was performed.

Example 7 A positive electrode having a filling density of the active material layer (packing density of the mixture) of 3.7 g / cm ² was produced in accordance with Example 6, with a slight change in conditions. The viscosity of the slurry immediately before coating is 900
It was 0 mPa · s. The positive electrode thus prepared was impregnated with an electrolyte solution (a solution in which LiPF ₆ was dissolved at 1 mol / L in a mixed solvent of 50% by volume of ethylene carbonate and 50% by volume of diethylene carbonate) until the cathode was saturated as in Example 1. Then, the impregnation rate of the electrolytic solution was measured, and it was 6.3%. Next, using the prepared positive electrode, the same negative electrode, separator, battery can, and electrolyte as in Example 1 were used, and a lithium secondary battery (design capacity: 1) was used.
800 mAh) and subjected to the same evaluation test as in Example 1.

Comparative Example 1 A slurry was prepared in the same manner as in Example 6, except that the kneading time was changed to 1 hour of main dispersion time + 40 minutes of dilution and stirring. Immediately after this slurry was prepared, it was coated on both surfaces of a current collector (aluminum foil), and a positive electrode having a filling density of the active material layer (filling density of the mixture) of 3.7 g / cm ² according to Example 6. Was prepared. The viscosity of the slurry immediately before coating was 1200.
It was 00 mPa · s. An electrolyte (ethylene carbonate 50) was applied to the positive electrode thus prepared in the same manner as in Example 1.
(1 mol / liter of LiPF ₆ dissolved in a mixed solvent of 50% by volume + diethylene carbonate) was impregnated until a saturated state was reached, and the impregnation rate of the electrolytic solution was 5.5%. Next, using the positive electrode prepared above, a negative electrode, a separator, a battery can, and an electrolyte were prepared in Example 1.
Use the same as the lithium secondary battery (design capacity:
1800 mAh) and subjected to the same evaluation test as in Example 1.

The evaluation results of Examples 1 to 7 and Comparative Example 1 are shown in Table 1 below.

[0056]

[Table 1]

From the table, it can be seen that in the lithium secondary battery (Example) using the positive electrode of the present invention, a decrease in discharge capacity during high-rate discharge is suppressed, and the capacity retention rate also shows a high retention rate. Also, it can be seen that particularly in high-rate discharge at a low temperature, a decrease in discharge voltage is also suppressed.

[0058]

As is apparent from the above description, according to the present invention, a mixture layer (a mixture containing an active material) having a high packing density and a high electrolyte impregnation rate, which could not be achieved conventionally, has been achieved. (Active material layer).
Therefore, by using such a positive electrode, the electrolyte is sufficiently present around the active material in the mixture layer, and insertion and removal of lithium ions in the active material during high-rate discharge (particularly, high-rate discharge at a low temperature). Separation is performed smoothly, and at the same time as increasing the capacity of the battery, it is possible to achieve an increase in the discharge capacity during a high rate discharge (particularly, a high rate discharge at a low temperature) and an increase in the capacity retention rate. Therefore, a battery with high capacity and good high-rate discharge characteristics can be obtained without increasing the size.

[Brief description of the drawings]

FIG. 1 is a diagram showing an ideal crystal structure of LiCoO ₂ (lithium cobaltate), in which the crystal lattice is halved in the C-axis direction.

FIG. 2 is a diagram illustrating a crystallite size.

Claims (2)

[Claims] 1. A positive electrode for a lithium secondary battery, comprising a current collector, and a layer of a mixture containing an active material, a conductive material, and a binder, wherein the positive electrode of the mixture in the layer of the mixture is formed. 3 packing density
g / cm ³ or more, and the impregnation rate of the electrolytic solution is 6 to 2
0% positive electrode for lithium secondary batteries. 2. A lithium secondary battery having the positive electrode according to claim 1.