JP2001143760A - Lithium ion secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary cell capable of obtaining a capacity that cannot be achieved by a conventional lithium ion secondary cell. SOLUTION: It manufactures a lithium ion secondary cell by employing as a positive electrode activating material a composite oxide of Li-Co system in the shape of particle where an average particle diameter is equal to or more than 10 μm and the numerical value obtained by dividing the multiplication of the average particle diameter with the specific surface area by 20 is 7-9, employing as a conductive material for a positive pole a mixture a large size conductive material with a small size conductive material, employing as a negative pole activating material a graphitization carbon where the specific surface area is equal to or more than 2.0 m2/g, a surface-to-surface distance of crystallization lattice is less than or equal to 0.3380 nm, and a crystallite numerical value in C axis direction is equal to or more than 30 nm, and employing as an electrolyte solvent a mixing solvent at least one kind selected from diethyl carbonate and ethyl methyl carbonate with ethylene carbonate, propylene carbonate and dimethyl carbonate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery.

[0002]

2. Description of the Related Art Generally, a lithium ion secondary battery has a structure in which a separator impregnated with an electrolyte is sandwiched between a sheet-like positive electrode and a sheet-like negative electrode. The positive electrode and the negative electrode are formed by providing a positive electrode active material layer or a negative electrode active material layer obtained by mixing a conductive material, a binder, and the like with a positive electrode active material or a negative electrode active material on a current collector such as a metal foil. Usually, a granular Li—Co-based composite oxide is used as the positive electrode active material, and a carbon material is used as the negative electrode active material.

[0003] A lithium ion secondary battery configured as described above can achieve a higher energy density and a higher voltage than a nickel-cadmium battery or the like. Therefore, in recent years, lithium-ion secondary batteries have been rapidly adopted as driving sources for portable devices such as mobile phones and notebook computers.
In addition, the scope of application is expected to expand in the future. Therefore, research and development on lithium ion secondary batteries have been actively conducted in order to improve battery performance.

[0004]

However, at present, a lithium ion secondary battery having satisfactory battery performance has not yet been obtained. For example, in a conventional lithium ion secondary battery, when trying to improve the charge / discharge cycle characteristics, the energy density may be reduced, which makes it difficult to obtain satisfactory cycle characteristics. Further, at extremely low temperatures of -20 ° C. or lower, the discharge capacity is significantly reduced, so that it is difficult to adopt them to equipment that is assumed to be used at low temperatures. Further, a decrease in discharge capacity due to spontaneous discharge is not negligible.

An object of the present invention is to provide a lithium ion secondary battery that can achieve performances that cannot be achieved by a conventional lithium ion secondary battery.

[0006]

The first aspect of the lithium ion secondary battery of the present invention has the following features. (1) The positive electrode active material is a granular Li-Co-based composite oxide, and the Li-Co-based composite oxide has an average particle size of 10
μm or more, and the value obtained by dividing 20 by the product of the average particle size and the specific surface area is 7 to 9. The conductive material used with the positive electrode active material is a granular conductive material having a particle size of 3 μm or more. A mixture of the material and a granular conductive material having a particle size of 2 μm or less;
The negative electrode active material is graphitized carbon having a specific surface area of 2.0 m ² / g or less, a crystal lattice plane distance of 0.3380 nm or less, and a crystallite size in the c-axis direction of 30 nm or more. A lithium ion secondary battery characterized by being a mixed solvent of at least one selected from the group consisting of diethyl carbonate, diethyl carbonate and ethyl methyl carbonate, ethylene carbonate, propylene carbonate and dimethyl carbonate.

Further, a second aspect of the lithium ion secondary battery of the present invention has the following features. (2) The positive electrode active material is a granular Li-Co-based composite oxide, and the Li-Co-based composite oxide has an average particle size of 10
μm or more, and the value obtained by dividing 20 by the product of the average particle size and the specific surface area is 7 to 9. The conductive material used with the positive electrode active material is a granular conductive material having a particle size of 3 μm or more. The material, the aspect ratio is 3 or more, and the fiber diameter is 2 μm
It is a mixture with the following fibrous conductive material, and the negative electrode active material has a specific surface area of 2.0 m ² / g or less, a crystal lattice face-to-face distance of 0.3380 nm or less, and a crystallite size in the c-axis direction of 3
0 nm or more graphitized carbon, the solvent of the electrolytic solution is at least one selected from diethyl carbonate and ethyl methyl carbonate, ethylene carbonate,
A lithium ion secondary battery comprising a mixed solvent of propylene carbonate and dimethyl carbonate.

Further, the first and second aspects of the lithium ion secondary battery of the present invention also have the following features. (3) The mixing ratio of at least one selected from diethyl carbonate and ethyl methyl carbonate is 25% by volume to 50% by volume, the mixing ratio of ethylene carbonate is 4% to 20% by volume, and the mixing ratio of propylene carbonate is 3 to 17% by volume, and the mixing ratio of dimethyl carbonate exceeds 40% by volume to 60% by volume.
The following lithium ion secondary battery according to the above (1) or (2).

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a lithium ion secondary battery of the present invention will be described in detail. In the lithium ion secondary battery of the present invention, the positive electrode active material has an average particle size of 10 μm.
m or more, and a value obtained by dividing 20 by the product of the average particle diameter and the specific surface area is 7 to 9, that is, a granular Li-Co-based composite oxide satisfying the following formula (1) is used. Can be 7 ≦ [20 / (specific surface area × average particle size)] ≦ 9 (1)

[0010] Examples of Li-Co-based composite oxide used in the positive electrode active material, and _{LiCoO 2, Li A Co 1-} X
Those represented by Me _X O ₂ and the like. In the latter, A is 0.05 to 1.5, particularly 0.1 to 1.1.
It is preferred that X is preferably 0.01 to 0.5, particularly preferably 0.02 to 0.2. Examples of the element Me include Group 3 to 10 elements of the periodic table such as Zr, V, Cr, Mo, Mn, Fe, and Ni, and B, Al, Ge, Pb, and S.
Group 13 to 15 elements such as n and Sb are exemplified.

The Li-Co-based composite oxide used for the positive electrode active material needs to satisfy the formula (1) when the value of 20 / (specific surface area × average particle size) is less than 7 or 9 If it is larger, the action of increasing the resistance component of the positive electrode active material itself acts, and the cycle characteristics, low-temperature characteristics, and storage characteristics are degraded.

In the present invention, the average particle diameter of the Li—Co-based composite oxide is set to 10 μm or more because the average particle diameter is 10 μm.
If it is less than m, an abnormal battery reaction is likely to occur, and safety may be impaired. When the average particle size exceeds 25 μm, the electric resistance increases, and the energy density per unit volume of the lithium ion secondary battery is reduced. Therefore, the average particle size of the Li—Co-based composite oxide is 10 μm to 25 μm. Preferably, especially 17 μm to 23
μm is preferred.

The specific surface area of the Li—Co-based composite oxide is 0.1.
1m ² /g~0.3m ² / g, especially 0.15m ² / g
It is preferably set to 0.25 m ² / g. If the specific surface area is less than 0.1 m ² / g, the resistance component increases, causing a decrease in charge / discharge capacity and a decrease in rate characteristics. On the other hand, if it exceeds 0.3 m ² / g, desorption of oxygen from the active material easily proceeds, which causes a problem in safety.

The average particle size of the Li—Co-based composite oxide can be measured by the following method. First, the particulate matter to be measured is put into an organic liquid such as water or ethanol, and a dispersion treatment is performed for about 2 minutes by applying ultrasonic waves of about 35 kHz 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 subjected to a Microtrac particle size analyzer, and the particle size (D
_{1, D 2, D 3 ··} ), and the presence the number of each particle each diameter (N
_{1, N 2, N 3 ···} ) to measure.

[0015] In the Microtrac particle size analyzer,
The particle size distribution of the spherical particle group having the theoretical intensity closest to the observed scattering intensity distribution is calculated. That is, the particle is assumed to be a sphere having a sectional circle having the same area as the projected image obtained by the irradiation of the laser beam, and the diameter of this sectional circle (equivalent sphere diameter)
Is measured as the particle size.

The average particle diameter (μm) is calculated from the particle diameter (D) of the individual particles obtained above and the number of particles present (N) for each particle using the following equation (2). You. Average particle size (μm) = (ΣND ³ / ΣN) ^1/3 (2)

The measurement of the specific surface area of the Li—Co-based composite oxide is described in “Powder Material Chemistry”, written by Yasuo Arai, First Edition, 9th edition, published by Baifukan (Tokyo), 1995, pp. 178-18.
Of the adsorption methods described on page 4, it can be carried out by a gas phase adsorption method (single point method) using nitrogen as an adsorbent.

Next, a method for producing a Li—Co-based composite oxide that can be used as a positive electrode active material in the present invention will be described below. In the present invention, the method for producing the Li—Co-based composite oxide is not limited to the following method. As one method, a lithium compound and a cobalt compound as starting materials are mixed so that the atomic ratio of cobalt to lithium is 1: 1 to 0.8: 1, and the mixture is heated at a temperature of 700 ° C to 1200 ° C. In an air atmosphere at a temperature of 3 ° C., the reaction is carried out by heating for 3 hours to 50 hours, etc., and the reaction product is further pulverized into granules, from which the average particle size is 10 μm or more and There is a method of collecting only those satisfying the expression (1).

As another method, a method of further heat-treating the above-obtained granules, for example, the above-obtained granules at 400 ° C. to 750 ° C., particularly 450 ° C.
A method of heating at a temperature of about 700C to about 700C for about 0.5 hours to about 50 hours, particularly about 1 hour to about 20 hours may be used.
At this time, it is preferable to use, as described above, the granular material having an average particle size in the range of 10 μm to 25 μm. As described above, when the granular material is subjected to the heat treatment, the specific surface area can be reduced without substantially changing the average particle size of the granular material.
An i-Co-based composite oxide can be easily obtained.

The heat treatment of the pulverized granular material can be performed in any atmosphere, for example, in an air atmosphere or an inert gas atmosphere such as nitrogen or argon. . However, if carbon dioxide gas is present in the atmosphere, lithium carbonate may be generated and the content of impurities may increase.
It is preferable to carry out in an atmosphere of about 0 mmHg or less.

Examples of the lithium compound as the starting material include lithium oxide, lithium hydroxide, lithium halide, lithium nitrate, lithium oxalate, lithium carbonate, and the like, and mixtures thereof. Examples of the cobalt compound include cobalt oxide, cobalt hydroxide, cobalt halide, cobalt nitrate, cobalt oxalate, cobalt carbonate and the like, and a mixture thereof. In addition, Li _A C
If o _1-X Me _X O of the producing Li-Co-based composite oxide represented by _2, the mixture of the lithium compound and a cobalt compound may be added required amount of a compound of the substituent elements.

In the lithium ion secondary battery of the present invention, as the conductive material used together with the positive electrode active material, two types of conductive materials having different sizes are used. In this case, the conductive material having a small size gathers on the surface of the particles of the positive electrode active material to make the surface conductive, and the conductive material having a large size enters between the conductive particles of the positive electrode active material and the space between the particles becomes electrically conductive. Connection. Therefore, sufficient electrical continuity between the surface and the inside of the positive electrode active material can be obtained, and the resistance component of the positive electrode itself can be reduced. Therefore, it is possible to suppress deterioration in cycle characteristics, low-temperature characteristics, and storage characteristics.

In the lithium ion secondary battery of the present invention, in both the first embodiment and the second embodiment, two types of conductive materials are used. The shape of the small conductive material is different. In the first aspect of the lithium ion secondary battery of the present invention, a conductive material having a large granular size (hereinafter, referred to as a “first conductive material”) and a conductive material having a small granular size (hereinafter, referred to as a “second conductive material”). And a conductive material of "."). In the second aspect of the lithium ion secondary battery of the present invention, a conductive material having a large granular size (hereinafter, referred to as a “first conductive material”) and a conductive material having a small fibrous size (hereinafter, referred to as a “first conductive material”). And "a three conductive material"). The term “granular” as used in the present invention includes, but is not particularly limited to, a scale, a sphere, a pseudo sphere, a lump, and a whisker.

As the first conductive material, a carbon material conventionally used in a lithium ion secondary battery can be used. Examples thereof include artificial or natural graphites; carbon blacks such as acetylene black, oil furnace black, and extra-conductive furnace black. Of these carbon materials,
In the first conductive material, graphites, particularly graphitized carbon having a crystal lattice spacing (D002) of 0.34 nm or less and a crystallite size (Lc) in the c-axis direction of 10 nm or more, are preferably used.

The interplanar distance (d002) of the crystal lattice and the crystallite size (Lc) in the c-axis direction can be measured by the Japan Society for the Promotion of Science. This will be specifically described below. First, a high-purity silicon for X-ray standard is pulverized in an agate mortar to a size less than 325 mesh standard sieve to prepare a standard substance. A standard substance is 10% by weight with respect to 100% by weight) to prepare an X-ray sample. This X-ray sample was prepared using an X-ray diffraction device (RINT2000, manufactured by Rigaku Corporation).
X-ray source: CuKα ray) is uniformly filled in a sample plate. Next, the applied voltage to the X-ray tube is 40 kV, and the applied current is 50 kV.
mA and the scanning range is 2θ = 23.5 degrees to 2
At 9.5 degrees and a scan speed of 0.25 degrees / min, the 002 peak of carbon and the 111 peak of the standard substance are measured. Subsequently, from the obtained peak position and its half-value width, using the software for calculating the degree of graphitization attached to the X-ray diffraction apparatus, the distance between the planes of the crystal lattice (d002) and the crystallite in the c-axis direction are used. Calculate the dimension (Lc).

The first conductive material is for improving the electrical connection between the particles of the positive electrode active material, and if it is too small, it is difficult to achieve this electrical connection. on the other hand,
If the first conductive material is too large, it will prevent close packing of the positive electrode active material. Therefore, as the first conductive material, the particle size is 3 μm.
m or more, and preferably 5 μm to 25 μm. Furthermore, the specific surface area, 20 m ² / g or less, particularly 1m ² / g~10m ² / g
It is preferred to use

The particle diameter of the first conductive material in the present invention refers to the diameter of a cross-sectional circle (equivalent sphere diameter) when the particles constituting the first conductive material are assumed to be spherical. Li-Co
It can be measured using a Microtrac particle size analyzer as in the case of the system composite oxide.

As the second conductive material, similarly to the first conductive material, artificial or natural graphites; carbon blacks such as acetylene black, oil furnace black, and extra conductive furnace black; Such a carbon material can be used.

The second conductive material is used to make the surface of the particles of the positive electrode active material conductive, and if it is too large, such an effect is poor. Therefore, as the second conductive material, a material having a particle size of 2 μm or less, preferably 1 μm or less, particularly preferably 0.5 μm to 0.001 μm is used. Further, the specific surface area is 10 m ² / g.
As described above, it is particularly preferable to use one having a surface area of 15 m ² / g or more.

The particle size of the second conductive material in the present invention is also the diameter of a cross-sectional circle (equivalent sphere diameter) assuming that the particles constituting the second conductive material are spherical. The particle size of the second conductive material can also be measured using a Microtrac particle size analyzer as in the case of the first conductive material. However, if the particle size is less than 1 μm, the particles tend to aggregate in the dispersion. Therefore, when the particle size is less than 1 μm, it is preferable to use an electron microscope. Specifically, first, 2 particles are in the field of view.
Electron micrographs are taken with the magnification set to zero or more. Next, calculate the area of the image of each particle in the photograph,
Further, the diameter of a circle having the same area is calculated from the calculated area. The particles constituting the second conductive material are assumed to be spheres having a cross section circle of this diameter, and this diameter is the particle size of the second conductive material.

The specific surface areas of the first conductive material and the second conductive material are measured by a gas-phase adsorption method using nitrogen as an adsorbent (one point) as in the case of the above-mentioned Li—Co-based composite oxide. It can be carried out.

Various carbon fibers can be used as the third conductive material. Specific examples include carbon fibers produced by a vapor growth method or the like, and graphitized carbon fibers such as mesophase-based graphitized carbon. The carbon fibers may be linear, or may be curled into a loop, spiral, or other shape.

The third conductive material, like the second conductive material,
This is for making the surface of the particles of the positive electrode active material conductive, and if it is too large, such an effect becomes poor. Therefore, as the third conductive material, one having an aspect ratio (fiber length / fiber diameter) of 3 or more, preferably 10 to 50, and a fiber diameter of 2 μm or less, preferably 1 μm or less is used.

The aspect ratio and fiber diameter of the third conductive material were measured in the same manner as the second conductive material having a particle size of 1 μm or less.
It can be performed using an electron microscope. In particular,
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 in the photograph are measured with a caliper or the like.
In addition, the measurement of the fiber length, if the fiber is linear,
It may be performed by measuring the shortest distance between one end and the other end. However, if the fiber is curled or the like, any two points that are most distant from each other on the fiber are taken, the distance between these two points is measured, and this is set as the fiber length.

If the mixing ratio of the first conductive material to the second conductive material or the third conductive material is too large or too small, a sharp discharge drop at the beginning of discharge may occur. May be encouraged. Therefore, in the present invention, the second conductive material or the third conductive material is used in an amount of 1 to 200 parts by weight, particularly 2 to 100 parts by weight, based on 100 parts by weight of the first conductive material. Is preferred. The amount is preferably 5 to 100 parts by weight, particularly preferably 10 to 50 parts by weight from the viewpoint of improving conductivity and safety.

The total amount of the first conductive material and the second conductive material or the third conductive material is, for example, Li-C
The amount may be about 3 to 15 parts by weight based on 100 parts by weight of the o-based composite oxide. However, in the present invention, since two kinds of conductive materials having different sizes are used in combination, the amount of use is smaller than that of the prior art, for example, Li-Co-based composite oxide 10
Sufficient electrical connection can be provided between the particles of the positive electrode active material at about 3 parts by weight to 10 parts by weight with respect to 0 parts by weight. Therefore, the amount of the Li—Co-based composite oxide can be increased, and the battery capacity can be increased.

As the binder for forming the positive electrode active material layer, the same binder as in the prior art can be used. For example, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, ethylene-propylene-diene-based polymer and the like can be mentioned. As the current collector, the same as the conventional one can be used. For example, aluminum,
Examples include foils and expanded metals formed of aluminum alloy, titanium, and the like.

[0038] In the lithium secondary battery of the present invention, as the negative electrode active material, the specific surface area of 2.0 m ² / g or less, particularly 0.5m ² /g~1.5m ² / g, between the surfaces of the crystal lattice The distance (d002) is 0.3380 nm or less, especially 0.33
Graphitized carbon having a crystallite size (Lc) of 55 nm to 0.3370 nm and a c-axis direction of 30 nm or more, particularly 40 nm to 70 nm, is preferably used. Examples of the graphitized carbon satisfying the above numerical range include mesophase-based graphitized carbon.

If the specific surface area is larger than 2.0 m ² / g, a decomposition reaction of polypropylene as an electrolyte component occurs during charging, and the battery capacity may be reduced. Also,
The interplanar distance (d002) of the crystal lattice exceeds 0.3380 nm, or the crystallite size (Lc) in the c-axis direction is 30 nm.
If it is lower than the lower limit, the potential of the negative electrode may increase and the average discharge potential of the battery may decrease.

In the present invention, the graphitized carbon is used in the form of particles similar to the usual graphite-based negative electrode active material. The shape of the particles constituting the graphitized carbon is not particularly limited, and may be a flake, fiber, sphere, pseudo sphere, lump, whisker, or the like. However, in the present invention, the graphitized carbon is preferably in a fibrous form in that the application to the negative electrode current collector is easy and the orientation of the particles after application can be controlled.

From these points, in the present invention, fibrous mesophase-based graphitized carbon, that is, mesophase-based graphitized carbon fiber is preferably used as the graphitized carbon serving as the negative electrode active material. An example of a method for producing a mesophase-based graphitized carbon fiber is described below. First, pitches such as petroleum pitch and coal tar pitch, particularly, mesophase pitch having a mesophase content of 70% by volume or more are spun into fibers having a length of about 200 μm to 300 μm by a melt blow method. Next, the fiber is carbonized at 800 to 1500 ° C.
Grind to about 00 μm and average fiber diameter of about 1 μm to 15 μm. Subsequently, the pulverized fiber is heated at 2500 ° C to 32 ° C.
By heating at 00 ° C., preferably at 2800 ° C. to 3200 ° C. to graphitize, mesophase-based graphitized carbon fibers can be obtained.

However, in order to improve the applicability of the negative electrode active material composition described later to the negative electrode current collector, the above-mentioned pulverization has an average fiber length of 1 μm to 100 μm, particularly 3 μm to 50 μm.
m, more preferably 2 μm to 25 μm, and the average fiber diameter is 0.5 μm to 15 μm, especially 1 μm to 15 μm,
Further, it is preferable to perform the process so that the thickness is 5 μm to 10 μm. At this time, the aspect ratio (the ratio of the average fiber length to the average fiber diameter) is preferably 1 to 5.

In the present invention, the measurement of the specific surface area of the graphitized carbon is performed in the same manner as the measurement of the specific surface area of the Li—Co-based composite oxide described above.
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. In the present invention, the interplanar distance (d002) of the crystal lattice of the graphitized carbon and the crystallite size (Lc) in the c-axis direction are measured by the Japan Society for the Promotion of Science in the same manner as in the case of the above-mentioned conductive material. Can be.

In the lithium ion secondary battery of the present invention, the binder used together with the negative electrode active material includes
As in the related art, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, ethylene-propylene-diene-based polymer, or the like can be used. Also,
In the present invention, a conductive material may be mixed in the negative electrode as necessary. In this case, the conductive material has an average particle size of 5
Examples include natural graphite, artificial graphite, carbon black and the like having a size of μm or less. As the negative electrode current collector, the same as the conventional one can be used, and examples thereof include a foil and an expanded metal formed of copper, nickel, silver, stainless steel and the like.

In the lithium ion secondary battery of the present invention, diethyl carbonate (D
A mixed solvent containing at least one selected from EC) and ethyl methyl carbonate (EMC), and further containing ethylene carbonate (EC), propylene carbonate (PC) and dimethyl carbonate (DMC) is used. The mixing ratio of each component constituting the mixed solvent is not particularly limited. However, from the viewpoint of improving the cycle characteristics of the lithium ion secondary battery, the content of ethylene carbonate (EC) is preferably 4% by volume to 20% by volume, and particularly preferably 6% by volume to 18% by volume. PC), it is preferably 3% by volume to 17% by volume, particularly preferably 5% by volume to 15% by volume. Also,
40% by volume in dimethyl carbonate (DMC)
Over 60% by volume, particularly preferably 45% by volume to 55% by volume. Furthermore, diethyl carbonate (DEC) and ethyl methyl carbonate (EMC)
At least one selected from the group consisting of
It is preferably 50% by volume, particularly preferably 30% by volume to 35% by volume.

In the case of ethylene carbonate (EC), if the mixing ratio is less than 4% by volume, it is difficult to form a stable film on the negative electrode surface, and there is a possibility that the cycle characteristics may be deteriorated. On the other hand, if it exceeds 20% by volume, the viscosity of the electrolytic solution increases, the internal resistance of the battery increases, and the cycle characteristics may decrease.

In propylene carbonate (PC), if the mixing ratio is less than 3% by volume, the effect of suppressing the increase in impedance due to the charge / discharge cycle becomes small, and the cycle characteristics may be degraded. On the other hand, when the content is less than 17% by volume, the viscosity of the electrolytic solution increases, the internal resistance of the battery increases, and the cycle characteristics may decrease.

In the case of dimethyl carbonate (DMC), if the mixing ratio is less than 40% by volume, the viscosity of the electrolyte increases, the internal resistance of the battery increases, and the cycle characteristics may decrease. On the other hand, if it exceeds 60% by volume, the freezing point of the electrolytic solution increases, and particularly at a low temperature of −20 ° C. or lower, the internal resistance of the battery may increase, and the cycle characteristics and low-temperature characteristics may decrease.

In diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), when the content is less than 25% by volume, the freezing point of the electrolytic solution is increased, and especially −2%.
At a low temperature of 0 ° C. or lower, the internal resistance of the battery may increase, and the cycle characteristics and the low-temperature characteristics may decrease. On the other hand, if it exceeds 50% by volume, the viscosity of the electrolyte increases, the internal resistance of the battery increases, and the cycle characteristics may decrease.

In the lithium ion secondary battery of the present invention, the electrolyte is LiCl
_{O 4, LiBF 4, LiPF 6} , LiAsF 6, LiA
One or two or more of lithium salts such as lCl ₄ and Li (CF ₃ SO ₂ ) ₂ N may be dissolved. The electrolyte had a Li salt concentration of 0.1 mol / mol.
It may be prepared so as to be L to 2 mol / L, preferably 0.5 mol / L to 1.8 mol / L. This is because if the concentration of the lithium salt is less than 0.1 mol / L, sufficient ionic conductivity as an electrolyte cannot be obtained, and the function as a battery is impaired. On the other hand, if it exceeds 2 mol / L, the viscosity of the electrolytic solution increases and the low-temperature characteristics and high-rate characteristics decrease.

[0051]

EXAMPLES The present invention will be specifically described below with reference to examples. Actually, a lithium ion secondary battery of the present invention was produced and evaluated.

Example 1 [Preparation of Positive Electrode] LiCoO _{2 serving as a} positive electrode active material (average particle size: 20 μm (measured by SALD-3000J manufactured by Shimadzu Corporation, same hereafter)), specific surface area: 0.12 m ² / g, 20 /
(Average particle diameter × specific surface area): 8.3) 91 parts by weight and a 6 μm particle diameter spheroidal graphitized carbon (MCMB6-
28) A slurry obtained by uniformly dispersing 5 parts by weight, 1 part by weight of Ketjen black ECP having a particle size of 0.01 μm, and 3 parts by weight of polyvinylidene fluoride (PVdF) as a binder in N-methyl-2-pyrrolidone And This slurry is applied to an aluminum foil (20 μm thick) serving as a current collector.
m) coated on both sides, dried, then rolled,
20 mg / cm ² LiC per side of aluminum foil
A positive electrode having oO ₂ was produced.

[Preparation of Negative Electrode] Graphitized carbon (Merbronmeld FM-14) as a negative electrode active material (specific surface area: 1.
32 m ² / g, distance between planes of crystal lattice: 0.3364 n
95 parts by weight of crystallite size in m and c axis directions: 50 nm)
Polyvinylidene fluoride (PVd
F) 5 parts by weight and 50 parts by weight of N-methyl-2-pyrrolidone were mixed to form a slurry, and this slurry was applied to both surfaces of a copper foil (14 μm in thickness) serving as a current collector and dried. Next, the copper foil was subjected to a rolling treatment to obtain a negative electrode.

[Preparation of Electrolyte Solution] Diethyl carbonate 4
LiPF ₆ in a mixed solvent of volume%, 29% by volume of ethyl methyl carbonate, 11% by volume of ethylene carbonate, 9% by volume of propylene carbonate, and 47% by volume of dimethyl carbonate at a concentration of 1.0 mol / L.
(To the prepared electrolyte solution) to prepare an electrolyte solution.

[Assembly of Lithium Ion 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 (outer diameter 18 mm, high 65 mm). Further, the electrolyte solution obtained above was impregnated in a separator to obtain a lithium ion secondary battery of the present invention.

Next, the lithium ion secondary battery obtained above was subjected to a cycle characteristic test, a low-temperature characteristic test, and a storage characteristic test according to the following procedures. Table 1 shows the results. [Cycle Characteristics Test] The lithium ion secondary battery obtained above was charged and discharged at 1 C / 1 C for 500 cycles at room temperature (20 ° C.), and the discharge current value and discharge time were measured for 1 cycle and 500 cycles. From the discharge capacity [m
A · H] is calculated. Next, the discharge capacity [mA · H] at the 500th cycle is changed to the discharge capacity [mA · H] at the first cycle.
And the discharge capacity change rate [%] is obtained.

[Low temperature characteristic test] After charging the lithium ion secondary battery obtained above at room temperature, it is left in an air atmosphere at -35 ° C for 24 hours. In addition,
Charging is performed at a constant current of 1 C (1600 mA) and a voltage of 4.2 V.
And then charge the battery for a total time of 2.5
The current was passed at a constant voltage of 4.2 V until the time was reached. Then, 0.5C (8
Discharge is performed at a cutoff of 00 mAh) /2.5 V, and the discharge capacity [mA · H] at that time is determined. At room temperature (20
C.), charging and discharging are performed under the same conditions, and the discharge capacity [mA · H] is obtained. Further, the discharge capacity at -35 ° C. is divided by the discharge capacity at room temperature to obtain a discharge capacity change rate [%].
Is shown in Table 1.

[Storage Characteristics Test] After charging the lithium ion secondary battery obtained above at room temperature, it is left in an air atmosphere at 60 ° C. for 40 days. The charging was performed by supplying a current at a constant current of 1 C (1600 mA) until the voltage became 4.2 V, and then supplying a current at a constant voltage of 4.2 V until the total charging time became 2.5 hours. Was.
Next, it was left in an air atmosphere at −5 ° C. for 24 hours.
1C (1600 mAh) /2.5 in an air atmosphere of 5 ° C.
Discharge is performed at V cutoff, and discharge capacity at that time [mA ·
H]. Further, this discharge capacity was divided by the RT discharge capacity (1C (1600 mAh) / discharge at 2.5 V cutoff) to obtain a discharge capacity change rate [%].
Shown in The RT discharge capacity here is 1600
After a current was passed at a constant current of mA until the voltage reached 4.2 V, the voltage was then increased to 4.2 V until the total charge time reached 2.5 hours.
The discharge capacity [mA · H] obtained by charging the battery by flowing a current at a constant voltage and then discharging the battery at 800 mA in a 20 ° C. atmosphere until the voltage becomes 2.5 V.

Example 2 A lithium ion secondary battery was fabricated in the same manner as in Example 1, except that LiCoO ₂ having an average particle size of 16 μm and a specific surface area of 0.17 m ² / g was used as a positive electrode active material. .
Next, a cycle characteristic test, a low-temperature characteristic test, and a storage characteristic test were performed on the lithium ion secondary battery in the same manner as in Example 1. Table 1 shows the results.

Example 3 A lithium ion secondary battery was produced in the same manner as in Example 1 except that spherical graphitized carbon having a particle size of 4 μm and Ketjen black having a particle size of 0.05 μm were used as conductive materials. Was prepared. Next, a cycle characteristic test, a low-temperature characteristic test, and a storage characteristic test were performed on the lithium ion secondary battery in the same manner as in Example 1. Table 1 shows the results.

Example 4 As a solvent for the electrolytic solution, 6% by volume of diethyl carbonate was used.
27% by volume of ethyl methyl carbonate, 9% by volume of ethylene carbonate and 10% by volume of propylene carbonate.
A lithium ion secondary battery was produced in the same manner as in Example 1, except that a mixed solvent of volume% and dimethyl carbonate 48 volume% was used. Next, for this lithium ion secondary battery, a cycle characteristic test was performed in the same manner as in Example 1.
A low-temperature characteristic test and a storage characteristic test were performed. Table 1 shows the results.

Comparative Example 1 LiCoO ₂ (average particle size 18 μm, specific surface area 0.19 m ² / g, 20 / (average particle size × specific surface area): 5.8) was used as a positive electrode active material, and particle size was used as a conductive material. Only 3 μm spherical graphitized carbon (MCMB 6-28) was used (6 parts by weight), and a mixed solvent of 30% by volume of ethylene carbonate, 30% by volume of propylene carbonate, and 40% by volume of dimethyl carbonate was used as a solvent for the electrolytic solution. Except for the above, a lithium ion secondary battery was manufactured in the same manner as in Example 1. Next, a cycle characteristic test, a low-temperature characteristic test, and a storage characteristic test were performed on the lithium ion secondary battery in the same manner as in Example 1. Table 1 shows the results.

Comparative Example 2 LiCoO ₂ (average particle size: 19 μm, specific surface area: 0.10 m ² / g, 20 / (average particle size × specific surface area): 10.5) was used as a positive electrode active material, and particle size was used as a conductive material. 0.01μ
m (6 parts by weight), and 20% by volume of ethylene carbonate, 20% by volume of propylene carbonate, and dimethyl carbonate 6
A lithium ion secondary battery was produced in the same manner as in Example 1 except that a mixed solvent with 0% by volume was used. Next, a cycle characteristic test, a low-temperature characteristic test, and a storage characteristic test were performed on the lithium ion secondary battery in the same manner as in Example 1. Table 1 shows the results.

Comparative Example 3 LiCoO ₂ (average particle diameter 7 μm, specific surface area 0.32 m ² / g, 20 / (average particle diameter × specific surface area): 8.9) was used as the positive electrode active material, and the particle size was used as the conductive material. Lithium ion was obtained in the same manner as in Example 1 except that only the flake graphite material of 25 μm was used (6 parts by weight), and a mixed solvent of 50% by volume of ethylene carbonate and 50% by volume of dimethyl carbonate was used as a solvent for the electrolytic solution. A secondary battery was manufactured. Next, this lithium ion secondary battery is also described in Example 1.
A cycle characteristic test, a low-temperature characteristic test, and a storage characteristic test were performed in the same manner as described above. Table 1 shows the results.

[0065]

[Table 1]

From the above Examples 1 to 4 and Comparative Examples 1 to 3, the use of the lithium ion secondary battery of the present invention makes it possible to improve all of the cycle characteristics, low temperature characteristics and storage characteristics as compared with the conventional one. You can see that you can do it.

[0067]

From the above description, it can be seen that the use of the present invention can provide a lithium ion secondary battery having a performance not achieved conventionally. For example, the life of the battery itself can be extended by improving the cycle characteristics. In addition, the improved low-temperature characteristics allow the device to be used as a drive source for a device that is expected to be used at low temperatures. By improving the storage characteristics, it is possible to suppress the deterioration of performance due to being left in a fully charged state, which is a particular problem in the conventional lithium ion secondary battery.

Continued on the front page (72) Inventor Kenichi Kizu 4-3-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Mitsuhiro Asano 4-3-1 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries Itami Inside the factory (72) Inventor Ken Takeshi 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd.Itami Works (72) Inventor Kazuyuki Tateishi 4-3-1, Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd.Itami Works (72) Inventor Toshi 4-3 Ikejiri, Itami-shi, Hyogo F-term in Mitsubishi Cable Industries, Ltd.Itami Works (Reference) AJ05 AK03 AL07 AM03 AM05 AM07 BJ02 BJ14 DJ08 DJ15 DJ16 DJ17 EJ04 HJ04 HJ05 HJ07 HJ13

Claims (3)

[Claims] 1. The positive electrode active material is a granular Li—Co-based composite oxide, wherein the Li—Co-based composite oxide has an average particle diameter of 10 μm or more, and a difference between the average particle diameter and the specific surface area. The value obtained by dividing 20 by the product is 7 to 9. The conductive material used together with the positive electrode active material is a mixture of a granular conductive material having a particle size of 3 μm or more and a granular conductive material having a particle size of 2 μm or less. Wherein the negative electrode active material has a specific surface area of 2.0 m ² / g or less,
Graphitized carbon having a crystal lattice spacing of 0.3380 nm or less and a crystallite size in the c-axis direction of 30 nm or more, wherein the solvent of the electrolytic solution is at least one selected from diethyl carbonate and ethyl methyl carbonate, and ethylene carbonate. And a mixed solvent of propylene carbonate and dimethyl carbonate. 2. The positive electrode active material is a granular Li—Co-based composite oxide, wherein the Li—Co-based composite oxide has an average particle diameter of 10 μm or more, and has an average particle diameter and a specific surface area. The value obtained by dividing 20 by the product is 7 to 9, and the conductive material used together with the positive electrode active material is a granular conductive material having a particle diameter of 3 μm or more, an aspect ratio of 3 or more, and a fiber diameter of 3 μm or more. Is a mixture with a fibrous conductive material having a particle size of 2 μm or less. The negative electrode active material has a specific surface area of 2.0 m ² / g or less, a distance between crystal lattice planes of 0.3380 nm or less, and a crystallite size in the c-axis direction. Is 30 nm or more graphitized carbon, and the solvent of the electrolytic solution is
A lithium ion secondary battery comprising a mixed solvent of at least one selected from diethyl carbonate and ethyl methyl carbonate, ethylene carbonate, propylene carbonate, and dimethyl carbonate. 3. The mixing ratio of at least one selected from diethyl carbonate and ethyl methyl carbonate is 25% by volume to 50% by volume, the mixing ratio of ethylene carbonate is 4% by volume to 20% by volume, and the mixing ratio of propylene carbonate is The ratio is 3% to 17% by volume, and the mixing ratio of dimethyl carbonate exceeds 60% by volume.
3. The lithium ion secondary battery according to claim 1, wherein the content is at most volume%. 4.