JP2003077459A - Positive electrode active material and positive electrode for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a lithium secondary battery that can realize a lithium secondary battery that shows excellent cold temperature and rate characteristics and has a large capacity, and is superior in safety. SOLUTION: This is a positive electrode active material for a lithium secondary battery that is made of 60-95 wt.% of LiCoO2 having an average particle size in the range of 10-30 μm and 5-40 wt.% of LiNiO2 , LiNi1-x Cox O2 (0<x<1) or LiNi1-x-y Cox Aly O2 (0<x<1-y, 0<y<0.2) that has a smaller average particle size than the average particle size of LiCoO2 .

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a positive electrode active material and a positive electrode for a lithium secondary battery, and a lithium secondary battery.

[0002]

2. Description of the Related Art Lithium secondary batteries are excellent in electromotive force and battery capacity, and have many advantages such as higher energy density and higher voltage than nickel-cadmium batteries. . In particular, the adoption of lithium secondary batteries as a drive source for mobile devices such as mobile phones and laptop computers is rapidly advancing. For this reason, in the field, in order to develop higher performance products, researches on their constituent materials are actively conducted.

As an active material used for the positive electrode of a lithium secondary battery, LiCo is more stable than other lithium-transition metal composite oxides because it is chemically stable and easy to handle.
O ₂ is most practically used. Among them, LiCoO ₂ having a large particle size is considered to be preferable from the viewpoint of battery safety because it hardly causes an abnormal reaction. However, when the particle size of the active material is increased, the reaction area is decreased and the migration distance of ions within the active material is also increased. Therefore, the rate characteristic at low temperature of the battery (hereinafter, also referred to as low temperature / rate characteristic)
There is a problem in that Therefore, the inventors of the present invention heat-treat large LiCoO ₂ having an average particle size of 10 μm or more, improve the conductive material used with the LiCoO ₂ and improve the method for producing the positive electrode (electrode). Therefore, measures have been taken to improve the low temperature / rate characteristics. However, with the widespread use of lithium secondary batteries, the demand for higher capacity of batteries in recent years has never stopped, and therefore the packing density of the active material is increased (that is, the active material provided on the current collector, the conductive material, and The packing capacity of the positive electrode is increased by increasing the packing density of the layer of the mixture containing the binder). However, when the packing density of the positive electrode active material is increased,
Since the low temperature / rate characteristics deteriorate (especially, the phenomenon that the voltage drops sharply at the beginning of discharge at low temperatures), new measures are required to achieve both high battery capacity and low temperature / rate characteristics. It has become.

[0004]

In view of the above circumstances, the present invention has a high capacity, excellent safety, and excellent low temperature / temperature.
An object of the present invention is to provide a positive electrode active material for a lithium secondary battery and a positive electrode that can realize a lithium secondary battery exhibiting rate characteristics. It is another object of the present invention to provide a lithium secondary battery having a high capacity, excellent safety, and good low temperature / rate characteristics.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above object, and as a result, as a positive electrode active material,
LiCoO ₂ with a relatively large particle size and LiN with a relatively small particle size
iO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1) or Li
_{Ni 1-xy Co x Al y} O 2 (0 <x <1-y, 0 <y <
It was found that even if the positive electrode (battery) has a high capacity, the low temperature / rate characteristics are less likely to be deteriorated by adding 0.2) in a specific amount, and the present invention has been completed based on the finding. .

That is, the structure of the present invention is as follows.
It (1) LiC having an average particle size within the range of 10 to 30 μm
oO₂60 to 95 wt% and the LiCoO₂Average particle size of
LiNiO with smaller average particle size₂, LiNi _1-xCo
_xO₂(0 <x <1) or LiNi_1-xyCo_xAl_yO₂
(0 <x <1-y, 0 <y <0.2) 5 to 40% by weight
A positive electrode active material for a lithium secondary battery. (2) LiCoO₂Average particle size of LiNiO₂, LiN
i_1-xCo_xO₂(0 <x <1) or LiNi_1-xyCo
_xAl_yO₂Average of (0 <x <1-y, 0 <y <0.2)
Lithiu according to the above (1), wherein the difference from the particle size is 5 μm or more.
Positive electrode active material for secondary batteries. (3) LiNiO₂, LiNi_1-xCo_xO₂(0 <x <
1) or LiNi_1-xyCo_xAl_yO₂(0 <x <1-
The average particle size of y, 0 <y <0.2) is 0.1 μm or more.
For the lithium secondary battery according to (1) or (2) above
Positive electrode active material. (4) The positive electrode active material according to any one of (1) to (3) above.
Positive electrode for lithium secondary battery including quality. (5) Lithium secondary battery having the positive electrode described in (4) above.
pond.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
The positive electrode active material for a lithium secondary battery according to the present invention has an average particle diameter of 10 to 30 μm, LiCoO ₂ 60 to 9
5% by weight, the average particle size of which is smaller than that of LiCoO ₂ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0
<X <1) or _{LiNi 1-xy Co x Al y} O 2 (0 <x
<1-y, 0 <y <0.2) 5 to 40% by weight.

By incorporating a positive electrode using the positive electrode active material of the present invention having the above characteristics into a lithium secondary battery, a phenomenon in which the voltage suddenly drops at the beginning of discharge at a low temperature does not occur, and good results are obtained thereafter. A lithium secondary battery that can be continuously discharged is obtained. In addition, the battery thus obtained has excellent safety.

It has been found by many experiments by the present inventors that a lithium secondary battery exhibiting good low temperature / rate characteristics can be obtained by using the positive electrode active material of the present invention. The reason is as follows.

The positive electrode is usually a layer of a mixture material containing an active material, a conductive material and a binder on a current collector (hereinafter, also referred to as a mixture material layer).
Is formed by forming. In such a mixture layer, Li having an average particle size within the range of 10 to 30 μm
Since CoO ₂ has a relatively large particle size, the reaction area is small, and the migration distance of ions within the substance is long, so that when a large current flows, a concentration gradient of ions is formed and an apparent potential is increased. It will be low. However, at this time, around the LiCoO _{2 having} an average particle diameter of 10 to 30 μm, LiNi having an average particle diameter smaller than that of LiCoO ₂ is used.
O ₂ , LiNi _1-x Co _x O ₂ (0 <x <1) or LiN
_{i 1-xy Co x Al y} O 2 (0 <x <1-y, 0 <y <0.
2) is present, the LiNiO ₂ , L
iNi _1-x Co _x O ₂ or LiNi _1-xy Co _x Al _y O ₂
Since has a small particle size and has high reactivity by itself, it is possible to prevent the voltage drop of the positive electrode by actively inserting ions. After that, if discharge is performed for a long time, LiNiO ₂ , LiNi _1-x Co _x O ₂ or LiNi
_Although the potential drop of _1-xy Co _x Al _y O ₂ occurs,
At that time, heat generated by the internal resistance of the positive electrode caused electrolyte and L
Since the movement of ions in iCoO ₂ becomes active,
The discharge can be continued without any trouble. In addition, when the discharge is stopped midway, LiCoO ₂ , LiNiO ₂ , L
iNi _1-x Co _x O ₂ or LiNi _1-xy Co _x Al _y O ₂
Depending on the potential difference between the large particle size LiCoO _{2 and the} small particle size LiNiO ₂ , LiNi _1-x Co _x O ₂ or LiNi
By charging _1-xy Co _x Al _y O ₂ , LiN having such a small particle size is charged.
iO ₂ , LiNi _1-x Co _x O ₂ or LiNi _1-xy Co _x
Since the potential of Al _y O ₂ is restored, good low temperature / rate characteristics can be obtained even when the discharge is started again by the same mechanism as in the case of full charge.

In the positive electrode active material of the present invention, LiC
The use of oO ₂ having an average particle size within the range of 10 to 30 μm is to ensure the safety of the battery and improve the packing density of the positive electrode active material in the positive electrode (that is, per a certain volume of the positive electrode). This is because of the improvement in capacity). That is, if the average particle diameter is 10 μm or more, the safety of the battery is good, but if the average particle diameter is too large, the electric resistance of LiCoO ₂ becomes too large, and the rate characteristics deteriorate, especially at low temperature. The characteristics tend to deteriorate. The L
It is preferable to use iCoO ₂ having an average particle diameter in the range of 12.5 to 27.5 μm, and an average particle diameter of 15 to
It is particularly preferable to use one having a size in the range of 25 μm. The LiCoO ₂ used in the present invention contains 85 to 95% by volume of the whole particles within the particle size range of (average particle size × 0.25) to (average particle size × 2.5). .

On the other hand, LiNiO₂, LiNi_1-xCo_xO₂
Or LiNi_1-xyCo_xAl_yO₂Has an average particle size of
LiCoO₂Average particle size (selected from the range of 10 to 30 μm
However, the effect of the present invention is
In order for the fruits to appear remarkably, the average particle size should be LiCoO 2.₂of
It is preferable that the average particle size is 5 μm or more smaller than 10
It is more preferable that it is smaller than μm (that is, LiC
oO₂Average particle size of LiNiO₂, LiNi_1-xCo_xO
₂Or LiNi_1-xyCo_xAl_yO₂Difference from the average particle size
Is preferably 5 μm or more, and 10 μm or more
Is more preferable. ). Also, the average particle size is too small
And high reactivity during abnormal conditions such as overheating and short circuit,
Since the safety tends to decrease, LiNiO₂, Li
Ni_1-xCo _xO₂Or LiNi_1-xyCo_xAl_yO₂of
The average particle size is preferably 0.1 μm or more, and particularly preferably.
It is preferably 1 μm or more.

Incidentally, LiNiO ₂ and Li used in the present invention
Ni _1-x Co _x O ₂ or LiNi _{1-x- y} Co _x Al _y O
₂ is usually (average particle size x 0.25) to (average particle size x
Those having 85 to 95% by volume of the whole particles within the particle size range of 2.5) are used.

In the present invention, LiCoO ₂ and LiN
iO ₂ , LiNi _1-x Co _x O ₂ or LiNi _1-xy Co _x
The compounding ratio with Al _y O ₂ is 60 to 95% by weight [LiCoO 2.
₂ ]: 5 to 40% by weight [LiNiO ₂ , LiNi _1-x Co _x
O ₂ or _{LiNi 1-xy Co x Al y} O 2] a but, particularly preferred compounding ratio 85-70 wt% [LiCo
O ₂ ]: 15 to 30 wt% [LiNiO ₂ , LiNi _1-x
Co _x O ₂ or _{LiNi 1-x- y Co x Al} y O 2] it is.
When the proportion of LiCoO ₂ exceeds 95% by weight, the low temperature / rate characteristics tend to deteriorate, which is not preferable, and conversely LiNiO ₂ , LiNi _1-x Co _x O ₂ or LiNi.
_If the proportion of _1-xy Co _x Al _y O ₂ exceeds 40% by weight, the safety of the battery, the average discharge voltage, etc. tend to decrease, which is not preferable.

LiCoO ₂ and LiNi used in the present invention
O ₂ , LiNi _1-x Co _x O ₂ and LiNi _1-xy Co _x A
The average particle size of l _y O ₂ is measured by the following method. First, the granular material to be measured is put into an organic liquid such as water or ethanol, ultrasonic waves of about 35 kHz to 40 kHz are applied, and dispersion treatment is performed for about 2 minutes. The amount of the particulate matter to be measured is such that the laser transmittance (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 to measure the particle size (D1, D2, D3 ...) Of the individual particles by the scattering of laser light, and the number of existing particles (N1, N2, N3) for each particle size. ...) is measured. The Microtrac particle size analyzer calculates the particle size distribution of the spherical particle group that has the theoretical intensity closest to the observed scattering intensity distribution. That is, the particles are assumed to be spheres having a cross-sectional circle with the same area as the projected image obtained by irradiation with laser light, and the diameter of this cross-sectional circle (sphere equivalent diameter) is measured as the particle diameter.
The average particle size (μm) is calculated from the particle size (D) of the individual particles obtained above and the number of existing particles (N) for each particle size using the following formula (1). Average particle size (μm) = (ΣND ³ / ΣN) ^1/3 (1)

LiCoO ₂ used in the present invention is, for example,
It can be produced by the following method. That is, a lithium compound as a starting material and a cobalt compound are weighed and mixed so that the atomic ratio of cobalt and lithium is 1: 1 and the mixture is heated under an oxygen atmosphere at a temperature of 700 ° C to 1200 ° C. React by heating for 3 hours to 50 hours,
Further, the reaction product is pulverized into particles, and a group of particles having an intended average particle size is collected from the particles.

Further, LiNiO ₂ , LiNi _1-x Co _x O ₂
(0 <x <1) and _{LiNi 1-x- y Co x Al} y O 2 (0
The same applies to <x <1-y, 0 <y <0.2). In LiNiO ₂ , the lithium compound and the nickel compound, which are the starting materials, are mixed so that the atomic ratio of nickel and lithium is 1: 1. Are weighed and mixed with each other, and the mixture is mixed in an oxygen atmosphere at a temperature of 600 ° C to 900 ° C.
The reaction is carried out by heating for 3 hours to 50 hours, and the reaction product is pulverized into particles, and a group of particles having a desired average particle size is collected from the particles. LiNi _1-x Co _x
In O ₂ (0 <x <1), the atomic ratio (LI: (Ni + Co)) of the total amount of nickel and cobalt with respect to the lithium compound, the nickel compound, and the cobalt compound, which are the starting materials, is 1 with respect to lithium. Each of them is weighed and mixed so that the ratio becomes 1: 1, and the mixture is heated at a temperature of 600 ° C to 90
The reaction is carried out by heating for 3 hours to 30 hours in an oxygen atmosphere at 0 ° C., and the resulting reaction product is pulverized into granules. From the granules, a group of particles having an intended average particle diameter is obtained. To collect. In addition, LiNi _1-xy Co _x Al _y O ₂ (0 <x <
1-y, 0 <y <0.2), Li, Ni, and C
atomic ratio of o and Al 1: (1-x-y ): x: were weighed and mixed compounds as a raw material so that y, firing the mixture in the same manner as LiNiO _2, was pulverized, granular material To collect a group of particles having an intended average particle size.

As the raw material compound, hydroxides, nitrates, carbonates and the like of each metal can be used. Further, in order to obtain a small particle size in good yield, a water-soluble compound of a metal other than lithium is dissolved in water, co-precipitated by pH adjustment or the like, and then dispersed in water together with the lithium compound to form a slurry. Alternatively, a method of spray drying can be used.

The positive electrode active material of the present invention is made to exist in the positive electrode by providing a layer of a mixture containing the active material, a conductive material and a binder on a current collector. That is, the positive electrode for a lithium secondary battery of the present invention is configured by providing a layer of a mixture material containing the positive electrode active material of the present invention, a conductive material and a binder on a current collector. The preparation method is a conventional method, that is, a slurry containing an active material, a conductive material and a binder is prepared (for example, N-methylpyrrolidone is used as a solvent for preparing the slurry. ), And a method in which such a slurry is applied onto a current collector and dried, and the resulting coating film is subjected to a rolling treatment.

In the present invention, LiCoO ₂ and LiN
iO ₂ , LiNi _1-x Co _x O ₂ or LiNi _1-xy Co _x
Al _y O ₂ is may be simply kneaded (mixed) with conductive material and a binder or the like at the time of slurry preparation, both prior to preparation of the slurry may be pre-mixed in a mixer such as a ribbon blender.

As the conductive material used for the positive electrode of the present invention,
Artificial or natural graphite and granular carbonaceous materials such as acetylene black, oil furnace black, carbon black such as Ixtra conductive furnace black ("granular" means scaly, spherical, pseudo-spherical, massive,
The whiskers are included and are not particularly limited. ) Is mentioned. Further, as the binder, a binder that has been conventionally used in the active material layer of the positive electrode of a lithium secondary battery can be used without any problem, and examples thereof include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF). , Polyethylene, ethylene-propylene-diene copolymer (EPDM) and the like are used.

As the current collector, a conductive metal such as aluminum, aluminum alloy or titanium, having a thickness of 10 to 10 is used.
Foil or perforated foil of about 0 μm, especially about 15 to 50 μm,
Expanded metal or the like having a thickness of about 25 to 300 μm, particularly about 30 to 150 μm is preferable.

In the composite material layer provided on the current collector, the amount of the conductive material used is preferably about 1 to 10 parts by weight, and particularly preferably about 4 to 7 parts by weight, per 100 parts by weight of the active material. The amount of the binder used is about 1 to 10 parts by weight, preferably about 2 to 5 parts by weight, per 100 parts by weight of the active material.

In order to increase the capacity of the positive electrode, it is necessary to increase the packing density of the composite material layer provided on the current collector, the active material, the conductive material and the binder. That is, it is important to reduce the voids in the mixture layer. Therefore, when the composite material layer is produced,
A rolling process is performed at the final stage to reduce voids in the composite material layer. In the positive electrode of the present invention, the packing density of the mixture material in the mixture layer is preferably from 3.0~3.6g / cm ^3, particularly preferably 3.3~3.5g / cm ^3.
That is, if the packing density is less than 3.0 g / cm ³ ,
If the capacity of the positive electrode is not sufficiently secured, the capacity of the battery may not be sufficiently increased. If the capacity is more than 3.6 g / cm ³ , the positive electrode (composite material layer) may retain a sufficient amount of electrolytic solution. In some cases, the low temperature / rate characteristics may be deteriorated, and further, the capacity may be reduced together with the deterioration of the low temperature / rate characteristics.

Here, the packing density (g / cm ³ ) is the weight per unit volume of the mixture in the positive electrode, and the amount (g / cm ² ) of the mixture per unit area on the current collector is determined. , The existing amount (g / cm ² ) of the mixture is determined by the average thickness (c
It is calculated by dividing by m).

In the positive electrode of the present invention, the thickness of the composite material layer (after rolling treatment) is preferably 40 to 100 μm, particularly preferably 50 to 80 μm.

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 ones can be used according to a conventional method. be able to.

The negative electrode is constructed by forming a layer of a mixture material containing an active material and a binder on a current collector. As the active material, various graphite materials, carbon black and amorphous carbon materials ( Granular carbon materials such as hard carbon, soft carbon) and activated carbon, which are used as an active material for a negative electrode of a known lithium secondary battery, can be used. Of 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 scale-like, spherical, pseudo-spherical, lump-like, whisker-like, or the like.

As the graphitized carbon, fibrous ones may be used in addition to the granular ones, and in this case, straight or curled ones may be used. The size of the fibrous graphitized carbon is not particularly limited, but the average fiber length is preferably 1 to 100 μm, particularly preferably 3 to 50 μm. The average fiber diameter is preferably 0.5 to 15 μm, particularly preferably 1 to 15 μm, particularly preferably 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 particularly preferable.

The size (fiber diameter, fiber length) of the fibrous graphitized carbon can be measured by using an electron microscope. That is, the magnification can be set so that 20 or more fibers are included in the visual field, an electron micrograph is taken, and the fiber diameter and fiber length of each fiber shown in the photo can be measured 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 fibers are curled or the like, it is sufficient to take any two points on the fiber that are most distant from each other, measure the distance between these two points, and use this as the fiber length. The average fiber diameter and the average fiber length are number average values of the measured numbers.

The binder is a binder that has been conventionally used in the active material layer of the negative electrode of a lithium secondary battery, for example, a fluororesin such as polytetrafluoroethylene (PTFE) or 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 active material in the negative electrode mixture layer (the amount of the active material present per unit area on the current collector) is usually 3 to 30 m.
g / cm ² or so, preferably from 5 to 20 mg / cm ² or so. The ratio of the active material to the binder in the entire mixture layer is generally 80:20 to 98: 2 by weight ratio (active material: binder).

Solvents for the electrolytic solution include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane, N, N-dimethylformamide, tetrahydrofuran. , 1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and the like can be mentioned, and these can be used alone or in admixture of two or more. (Preferably 6 to 18% by volume), propylene carbonate 3 to 17% by volume (preferably 5 to 15% by volume), and at least one selected from diethyl carbonate and ethylmethyl carbonate 25 to 50% by volume. Preferably 30 to 35% by volume)
It is preferable to use a mixed solvent of 40 to 60% by volume (preferably 45 to 55% by volume) of dimethyl carbonate. By using the mixed solvent, it is possible to form an electrolytic solution having a low viscosity and being hard to coagulate at a low temperature,
The improved low temperature / rate characteristics aimed at by the present invention give more favorable results.

Mixing ratio of ethylene carbonate is 4% by volume
If it is less than the above range, dissociation of the lithium salt is unlikely to occur and the ionic conductivity tends to decrease, which is not preferable, and 20
When the content exceeds the volume%, the viscosity of the electrolytic solution increases and the ionic conductivity tends to decrease, which is not preferable. Also,
If the mixing ratio of propylene carbonate is less than 3% by volume, dissociation of the lithium salt is unlikely to occur and the ionic conductivity tends to decrease, which is not preferable, and if it exceeds 17% by volume, the viscosity of the electrolytic solution becomes high. However, the ionic conductivity tends to decrease, which is not preferable. Further, if the mixing ratio of at least one selected from diethyl carbonate and ethyl methyl carbonate is less than 25% by volume, the electrolytic solution is likely to freeze at low temperature, there is a possibility that the transfer of lithium ions may be inhibited, which is not preferable, 50% by volume
If it exceeds, the viscosity of the electrolytic solution tends to increase and the ionic conductivity tends to decrease, which is not preferable. If the mixing ratio of dimethyl carbonate is less than 40% by volume, the viscosity of the electrolytic solution tends to be high and the ionic conductivity tends to be low, which is not preferable. If it exceeds 60% by volume, the electrolytic solution will freeze at low temperatures. It is not preferable because it is easy to do so and migration of lithium ions may be hindered.

As the lithium salt to be dissolved in the electrolytic solution,
For example, LiClO ₄ , LiBF ₄ , LiPF ₆ , Li
AsF ₆ , LiAlCl ₄ , Li (CF ₃ SO ₂ ) ₂ N
And so on. Only one of these may be used, or two or more may be used. Of these, L has a large dissociation constant, high thermal stability, and low toxicity.
iPF ₆ is preferably used.

It can be said that increasing the amount of lithium salt dissolved in the electrolytic solution is effective in increasing the limiting current density at room temperature or higher. However, there is a limit to salt dissociation at low temperatures. Therefore, even if the amount of lithium salt is increased, it is not possible to expect an increase in the amount of lithium salt that is effective for carrying electric charges, and conversely,
This increases the viscosity of the electrolytic solution and slows the diffusion rate of lithium ions, resulting in deterioration of low temperature characteristics.
Therefore, it is preferable to prepare the electrolytic solution so that the concentration of the lithium salt is 0.5 mol / L to 1.5 mol / L, preferably 0.7 mol / L to 1.2 mol / L.

As the separator, a known separator such as a polyolefin separator which has been conventionally used in lithium secondary batteries is used. Here, the separator may be a porous separator or a solid separator in which substantially no pores are formed. The polyolefin separator may be a single polyethylene layer or a single polypropylene layer, but is preferably of a type in which a polyethylene layer and a polypropylene layer are laminated, and PP / P is particularly preferable in terms of safety.
A three-layer type of E / PP is preferable.

The form of the battery is not particularly limited. Known materials that have been used in lithium secondary batteries can be used, for example, cylindrical cans, rectangular cans, button cans, etc. made of metals such as Fe, Fe (Ni plating), SUS, aluminum, aluminum alloys. Alternatively, a sheet-shaped exterior material such as a laminated film is used. As the laminate film, copper, polyester on at least one surface of a metal foil such as aluminum,
Those having a laminated layer of thermoplastic resin such as polypropylene are preferred.

[0039]

EXAMPLES The present invention will be described in more detail with reference to examples.

Example 1 [Positive electrode] LiCoO 2 having an average particle size of 19 μm as an active material
₂ 85 parts by weight and 15 parts by weight of LiNiO _{2 having an} average particle size of 1 μm, a mixture of 1 part by weight of Ketjen black as a conductive material and 3 parts by weight of massive graphite, and 4 parts by weight of polyvinylidene fluoride as a binder, 50 parts by weight of N-methyl-2-pyrrolidone as a dispersion solvent was kneaded (mixed) to obtain a slurry. The width of this slurry, which is the collector, is 55m.
m and a length of 550 mm are applied on both sides of an aluminum foil, dried, and rolled to give a positive electrode having a total thickness of 14
A composite material layer was formed to have a thickness of 5 μm to complete the positive electrode. When the packing density of the composite material in the composite material layer was measured,
It was 3.5 g / cm ³ .

[Negative Electrode] 100 parts by weight of graphitized carbon fiber (average fiber diameter 8 μm) as an active material, 8 parts by weight of polyvinylidene fluoride as a binder, and N as a dispersion solvent.
-Methyl-2-pyrrolidone is mixed with 80 parts by weight to form a slurry, and the slurry is applied to both sides of a copper foil having a width of 57 mm and a length of 600 mm to be a current collector, dried, and further subjected to a rolling treatment, A composite material layer was formed so that the total thickness of the negative electrode was 165 μm, and the positive electrode was completed. The negative electrode was completed.

[Assembly of Lithium Secondary Battery] 10% by volume of ethylene carbonate, 10% by volume of propylene carbonate, 30% by volume of ethyl methyl carbonate, and 50% by volume of dimethyl carbonate were mixed with L solvent.
An electrolyte solution was prepared by dissolving 1 mol / L of iPF ₆ . Then, the positive electrode and the negative electrode prepared above were wound with a porous polyethylene-polypropylene composite separator interposed therebetween, and this was wound into a cylindrical battery can (outer diameter 18 mm, inner diameter 17.5 m).
m, height 65 mm), and then impregnated with an electrolytic solution between the positive electrode and the negative electrode to complete a lithium secondary battery.

Example 2 As the active material, 70 parts by weight of LiCoO ₂ having an average particle size of 19 μm and 30 parts by weight of LiNi _0.8 Co _0.2 O ₂ having an average particle size of 7 μm were used. Thickness is 1
A 48 μm positive electrode was prepared. The packing density of the composite material in the composite material layer was measured and found to be 3.4 g / cm ³ . A negative electrode having a total thickness of 162 μm was prepared in the same manner as in Example 1 except that only the coating amount of the slurry was changed. A battery was produced in the same manner as in Example 1, except that these positive electrode and negative electrode were used.

Example 3 As an active material, 75 parts by weight of LiCoO ₂ having an average particle size of 19 μm and LiNi _0.79 Co _0.2 Al _0.01 O ₂ having an average particle size of 9 μm were used.
A positive electrode having a total thickness of 148 μm was produced in the same manner as in Example 1 except that 25 parts by weight was used. The packing density of the composite material in the composite material layer was measured and found to be 3.4 g / cm ³ . A battery was prepared in the same manner as in Example 1, except that this positive electrode and the same negative electrode as the negative electrode prepared in Example 2 were used.

Comparative Example 1 100 parts by weight of LiCoO ₂ having an average particle size of 19 μm was used as the active material, and the other conditions were the same as in Example 1, and the total thickness was 1
A 50 μm positive electrode was prepared. When the packing density of the composite material in the composite material layer was measured, it was 3.3 g / cm ³ . Further, a negative electrode having a total thickness of 160 μm was produced in the same manner as in Example 1 except that only the coating amount of the slurry was changed. A battery was produced in the same manner as in Example 1 except that these positive electrode and negative electrode were used.

Comparative Example 2 100 parts by weight of LiNiO ₂ having an average particle size of 11 μm was used as the active material, and the other conditions were the same as in Example 1, and the total thickness was 1
A 56 μm positive electrode was prepared. The packing density of the composite material in the composite material layer was measured and found to be 2.7 g / cm ³ . A negative electrode having a total thickness of 154 μm was produced in the same manner as in Example 1 except that only the coating amount of the slurry was changed. A battery was produced in the same manner as in Example 1 except that these positive electrode and negative electrode were used.

Comparative Example 3 70 parts by weight of LiCoO ₂ having an average particle size of 5 μm and 30 parts by weight of LiNi _0.8 Co _0.2 O ₂ having an average particle size of 11 μm were used as active materials. Is 15
A 0 μm positive electrode was prepared. The packing density of the composite material in the composite material layer was measured and found to be 3.4 g / cm ³ . A battery was produced in the same manner as in Example 1, except that this positive electrode and the same negative electrode as the negative electrode produced in Comparative Example 1 were used.

The average particle diameters of LiCoO ₂ , LiNiO ₂ , LiNi _0.8 Co _0.2 O ₂ and LiNi _0.79 Co _0.2 Al _0.01 O ₂ used in the above-mentioned Examples and Comparative Examples are measured by Microtrack particle size analyzer (Shimadzu Corporation). Made, SALD
-3000J).

The lithium secondary batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to the following measurements and evaluation tests.

[Battery capacity] Constant current-constant voltage charging (1900 mA, 4.2 V, 2.5 hours) in an environment of 20 ° C.
After that, constant current discharge (950 mA, cutoff voltage: 2.5)
V) is performed, and the capacity is obtained from discharge time × current.

[Low temperature characteristic] Voltage drop immediately after the start of discharge Constant current-constant voltage charge (1900 mA,
After 4.2 V, 2.5 hours), a constant current discharge (1
900 mA, cut-off voltage: 2.5 V) and record the voltage change. After making a graph with the recorded voltage on the vertical axis and the time from the start of discharge on the horizontal axis, the minimum value observed immediately after the start of discharge is read and used as the dip voltage.
The higher the voltage due to this drop, the smaller the voltage drop, which is preferable.

Intermittent Discharge at Low Temperature This test assumes, for example, a case where a portable wireless device having an operating voltage of 3 V or more per battery is intermittently used in an environment of −20 ° C. -20 ℃ after constant current-constant voltage charge (1900mA, 4.2V, 2.5 hours) in the environment of 20 ℃
At this time, a constant current discharge (1900 mA) for 5 minutes and a rest for 1 hour are alternately performed and repeated until the voltage during discharge becomes 3 V or less, and the number of discharges up to the previous one is set as the number of times that intermittent discharge is possible.

[Nail piercing test] In a 20 ° C. environment, after constant current-constant voltage charging (1900 mA, 4.3 V, 2.5 hours),
Stick a nail into the center of the side of the battery can and observe if it bursts, smokes or ignites. The test was carried out on 10 batteries, and the number of batteries in which rupture, smoke emission or ignition occurred was divided by the total number, and the generation rate was obtained.

The above test results are shown in Table 1 below.

[0055]

[Table 1]

[0056]

As is apparent from the above description, according to the present invention, a lithium secondary battery capable of achieving a lithium secondary battery having a high capacity, excellent safety, and good low temperature / rate characteristics can be achieved. A positive electrode active material for a battery and a positive electrode can be provided. Also, by using these, high capacity,
It is possible to obtain a lithium secondary battery having excellent safety and exhibiting good low temperature / rate characteristics.

Continued front page    (72) Inventor Shunichiro Ose             4-3 Ikejiri, Itami City, Hyogo Prefecture Mitsubishi Electric Cable             Industrial Co., Ltd. Itami Works F-term (reference) 5H029 AJ03 AJ12 AK03 AL07 AM03                       AM04 AM05 AM07 BJ02 BJ14                       CJ28 DJ16 HJ02 HJ05                 5H050 AA06 AA08 AA15 BA17 CA08                       CA29 CB08 FA17 GA05 GA27                       HA02 HA05

Claims (5)

[Claims] 1. LiCoO ₂ having an average particle size in the range of 10 to 30 μm, 60 to 95% by weight, and LiNiO ₂ and LiN having an average particle size smaller than the average particle size of the LiCoO _2.
i _1-x Co _x O ₂ (0 <x <1) or LiNi _1-xy Co
_{x Al y O 2 (0 <} x <1-y, 0 <y <0.2) 5~4
A positive electrode active material for a lithium secondary battery, which comprises 0% by weight. _2. The average particle size of LiCoO ₂ and LiNi
O ₂ , LiNi _1-x Co _x O ₂ (0 <x <1) or LiN
_{i 1-xy Co x Al y} O 2 (0 <x <1-y, 0 <y <0.
The positive electrode active material for a lithium secondary battery according to claim 1, wherein the difference from the average particle size in 2) is 5 μm or more. 3. LiNiO ₂ , LiNi _1-x Co _x O ₂ (0
<X <1) or _{LiNi 1-xy Co x Al y} O 2 (0 <x
The positive electrode active material for a lithium secondary battery according to claim 1, wherein the average particle size of <1-y, 0 <y <0.2) is 0.1 μm or more. 4. A positive electrode for a lithium secondary battery, which comprises the positive electrode active material according to claim 1. 5. A lithium secondary battery having the positive electrode according to claim 4.