JPH10189052A - Nonaqueous electrolytic secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolytic secondary battery having the high level of safety, a load characteristic and a cycle characteristic by forming the battery out of a pressure molded negative electrode having the specific density of graphitized vapor phase growth carbon fiber, a positive electrode of a composite oxide containing lithium, and an electrolyte formed out of lithium salt dissolved in a cyclic and chain carbonate mixed solvent. SOLUTION: A nonaqueous electrolytic secondary battery is made of a negative electrode, a positive electrode, and an electrolyte formed out of lithium salt dissolved in a cyclic and chain carbonate mixed solvent. Furthermore, the negative electrode has a compact obtainable from the pressure molding of graphitized vapor phase growth carbon fiber having specific surface area equal to or less than 5m<2> /g and a mean aspect ratio between 2 and 30 to bulk density between 1.2 and 2.0g/cm<3> . The positive electrode has a composite oxide containing lithium, preferably a composite oxide containing 3B, 6A, 7A and 8A groups of metal, and lithium, particularly a lithium composite oxide expressed as LiNi1- XMXO2 , where M stands for Al, Mn, Cr, Co and Fe, and X is a value between 0 and 1, or LiMn2 O4 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery having improved safety and load characteristics.

[0002]

2. Description of the Related Art A vapor-grown carbon fiber is prepared by using a metal such as iron or nickel in the form of ultra-fine particles as a catalyst to form a carbon compound in an amount of 800 to 800 nm.
By heating to 1,300 ° C., it can be produced by thermal decomposition. This vapor grown carbon fiber has a feature that it can be easily converted to a graphite structure by heat treatment. For example, the graphitized vapor-grown carbon fiber heat-treated at 2,800 ° C. or more has few crystal defects, and has a carbon hexagonal lattice network developed in a tube shape around the fiber axis. On the outside of the surface, a similar reticular layer develops concentrically like an annual ring. Therefore, the graphitized vapor-grown carbon fiber has high strength and high elasticity, and has high thermal conductivity and high electrical conductivity.

As an application example utilizing the characteristics of the graphitized vapor grown carbon fiber, there is a nonaqueous electrolyte secondary battery using the graphitized vapor grown carbon fiber as an electrode active material.

Generally, this non-aqueous electrolyte secondary battery has a negative electrode, a separator, a positive electrode, and an electrolyte.

[0005] Materials used for the negative electrode include carbonaceous materials such as natural graphite, artificial graphite, hard-to-graphite carbon also called hard carbon, mesocarbon microbeads, and the like.
Pitch-based carbon fibers, vapor-grown carbon fibers, and the like can be given.

[0006] Materials used for the positive electrode include lithium-containing composite oxides such as lithium cobalt oxide (LiC).
oO ₂ ), lithium manganate (LiMn ₂ O ₄ , Li
MnO ₂ ) and lithium nickelate (LiNiO ₂ ).

[0007] Examples of the substance used for the electrolyte include a non-aqueous electrolyte obtained by mixing a lithium salt and an organic solvent. As the lithium salt, LiClO ₄ , LiP
Examples include F ₆ , LiBF ₄ , LiAsF ₆ , and LiCF ₃ SO ₃ . As the organic solvent, ethylene carbonate (hereinafter, sometimes referred to as EC), propylene carbonate (hereinafter, sometimes referred to as PC),
Examples thereof include dimethyl carbonate (hereinafter, sometimes referred to as DMC), diethyl carbonate (hereinafter, sometimes referred to as DEC), and methylethyl carbonate (hereinafter, sometimes referred to as MEC).

[0008]

In recent years, non-aqueous electrolyte secondary batteries having high energy density and excellent cycle characteristics have attracted attention as large batteries such as batteries for electric vehicles and home power storage systems. .

In general, in a lithium ion secondary battery, when overcharging or when a charging current is increased, lithium is needle-likely deposited on the surface of the negative electrode,
Since the separator interposed between the positive electrode and the negative electrode may be pierced, a short circuit is likely to occur. As a result, the lithium ion secondary battery may explode and ignite. Also, when the electrolyte is decomposed due to overcharging, the cycle life of the lithium ion secondary battery decreases. Conversely, when overdischarge occurs, the current collector coated with the electrode active material dissolves, and as a result, the cycle life of the lithium ion secondary battery is significantly reduced. In order to avoid such problems, the lithium ion secondary battery is provided with a safety device for preventing overcharge or overdischarge.

Conventionally, an organic solvent having a low-viscosity chain carbonate has been employed as the organic solvent for the purpose of enhancing low-temperature characteristics and cycle characteristics of a lithium ion secondary battery. Although a mixed solvent of a carbonate, a cyclic carbonate such as propylene carbonate, and a chain carbonate is employed, there is a problem of safety. The problem of safety is, for example, when the above-described short circuit occurs, or when a short circuit such as a nail penetration test occurs, a large current flows and the temperature of the lithium ion secondary battery increases, There is a problem that the solvent is decomposed to generate gas, and the battery is ruptured and further ignites. The cause of the problem is that the boiling point of the chain carbonate is low, the vapor pressure of the chain carbonate is high, and the part where the fault of the graphite crystal in the negative electrode having graphite is exposed (the fault active reaction part) To promote the decomposition reaction of the solvent, especially the gas generation electrolysis reaction of propylene carbonate, that is, that the fault active reaction section has a catalytic action on the decomposition reaction of the solvent, and that the design capacity of the negative electrode is insufficient. Factors such as deposition of lithium on the negative electrode side can be cited.

Furthermore, the conventional lithium ion secondary battery does not fully satisfy not only the cycle characteristics but also the conductivity of the electrode, and has excellent cycle characteristics and good stability against a higher load. There is a demand for a secondary battery, that is, a secondary battery that exhibits high capacity even when charged and discharged at a high current value.

In order to improve the conductivity of the electrode, particularly the negative electrode, a small amount of a conductive auxiliary material such as acetylene black has conventionally been added to the electrode. However, the addition of the conductive auxiliary material reduces the active material ratio in the electrode. Because of this, the capacity is reduced. Moreover, according to the findings of the present inventor,
The addition of a conductive auxiliary material to the electrode itself has increased the unsafety of the battery. The reason is that the specific surface area of the conductive auxiliary material is much larger than the specific surface area of the vapor grown carbon fiber. Therefore, even if the addition of the conductive auxiliary material is extremely small, the average specific surface area of the active material in the electrode is increased.

[0013] An object of the present invention is to provide a safety which would occur when a chain carbonate such as dimethyl carbonate (abbreviation; DMC) or diethyl carbonate (abbreviation 1; DEC) having a low boiling point is used as the mixed solvent. It is an object of the present invention to solve the problem and to provide a non-aqueous electrolyte secondary battery having excellent safety and load characteristics.

Another object of the present invention is to provide a non-aqueous electrolyte secondary battery having a long life.

An object of the present invention is to provide a highly safe non-aqueous electrolyte secondary battery by suppressing the decomposition reaction of a solvent.

An object of the present invention is to provide a highly safe non-aqueous electrolyte secondary battery which does not have a risk of rupture and ignition of the battery.

An object of the present invention is to provide a non-aqueous electrolyte secondary battery having high conductivity of electrodes, excellent cycle characteristics, and good stability against a higher load, that is, charging and discharging at a high current value. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery exhibiting a high capacity.

[0018]

Means for Solving the Problems A non-aqueous electrolyte secondary battery intended to solve the above-mentioned problems is: (1) The specific surface area is at most 5 m ² / g and the average aspect ratio is 2 Obtained by press-molding graphitized vapor-grown carbon fibers having a packing density of 1.2 to 2.0 g / cm ^3.
A negative electrode having a press-formed body formed by pressing, a positive electrode having a lithium-containing composite oxide, and an electrolytic solution obtained by dissolving a lithium salt in a mixed solvent of a cyclic carbonate and a chain carbonate (2) The lithium-containing composite oxide according to (1), wherein the lithium-containing composite oxide includes a Group 3B metal, a Group 6A metal, a Group 7A metal, and The nonaqueous electrolyte secondary battery according to the above (1), which is a composite oxide having at least one metal selected from the group consisting of Group 8 metals and lithium, (3) the (1) Wherein the lithium-containing composite oxide described in (1) is at least one selected from the group consisting of a lithium composite oxide represented by the following general formula (I) and LiMn ₂ O _4. electrolyte secondary battery, LiNi _1-X M X O ₂ ... (I) (wherein, M represents aluminum, manganese, chromium, cobalt, and iron, and X represents any number between 0, 1, and 0 to 1). (4) A negative electrode formed of graphitized vapor-grown carbon fiber having a specific surface area of at most 5 m ² / g and an average aspect ratio of 2 to 30; and a lithium-containing composite oxide. A non-aqueous electrolyte comprising: a positive electrode; and an electrolytic solution obtained by dissolving a lithium salt in a mixed solvent of a cyclic carbonate and a chain carbonate, wherein the designed capacity of the negative electrode is larger than the designed capacity of the positive electrode. Secondary batteries,
(5) The nonaqueous electrolyte secondary battery according to (4), wherein the design capacity of the negative electrode with respect to the design capacity of the positive electrode according to (4) is more than 1 and 1.6 or less. The lithium-containing composite oxide according to (4), wherein at least one metal selected from the group consisting of a Group 3B metal, a Group 6A metal, a Group 7A metal, and a Group 8 metal; The non-aqueous electrolyte secondary battery according to (4), which is a composite oxide having: and (7) the lithium-containing composite oxide according to (4), represented by the following general formula (I): The nonaqueous electrolyte secondary battery according to the above (4), which is at least one selected from the group consisting of a lithium composite oxide and LiMn ₂ O ₄ .

The _{LiNi 1-X M X O 2} ··· (I) ( where, in the formula, M is aluminum, showed manganese, chromium, cobalt, and iron, X is 0, 1, and 0 to 1 Indicates any number between.)

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

(1) Negative Electrode A preferable negative electrode in the non-aqueous electrolyte secondary battery according to the present invention has a press-formed body formed using graphitized vapor-grown carbon fiber.

The negative electrode in the nonaqueous electrolyte secondary battery according to the present invention has graphitized vapor grown carbon fibers. A preferable negative electrode is obtained from a press-molded body in which the surface of a current collector is coated with an active material, and the active material is laminated on the surface of the current collector to form an active material layer. The active material comprises graphitized vapor grown carbon fibers joined together by a binder.

(1-1) Graphitized vapor-grown carbon fiber The graphitized vapor-grown carbon fiber used for the negative electrode of the nonaqueous electrolyte secondary battery of the present invention has a specific surface area of at most 5 m ^2.
/ G, preferably at most 3 m ² / g,
More preferably, it is at most 2 m ² / g. When the specific surface area of the graphitized vapor-grown carbon fiber exceeds 5 m ² / g, the object of the present invention cannot be achieved,
The charge / discharge efficiency is reduced to a level not suitable for practical use, and the cycle life is shortened. In other words, the graphitized vapor grown carbon fiber is
When the specific surface area is smaller than the above value, it is advantageous as compared with spherical graphite such as plate graphite, mesocarbon microbeads and mesocarbon carbon fiber for the following reasons. That is, since the graphitized vapor-grown carbon fiber has a tubular mesh surface of a carbon hexagonal lattice laminated in an annual ring shape around the fiber axis, the graphitized vapor-grown carbon fiber cut to an aspect ratio of 2 to 30 is used. In this case, the exposed portion of the graphite crystal cross section is only the cut surfaces at both ends thereof, which causes a reduction in the catalytic action of the electrolysis of the solvent. On the other hand, in the case of plate graphite, the entire surface is exposed, and in the case of other graphites, most of its surface is exposed, so that the safety is poor.

More specifically, when the specific surface area of the graphitized vapor-grown carbon fiber is at most 5 m ² / g, the portion of the negative electrode where the fault of the graphite crystal is exposed (fault-active reaction portion). Acceleration of the decomposition reaction of the solvent, that is, the catalytic action on the decomposition reaction of the solvent by the tomographically active reaction section can be significantly suppressed.

If the specific surface area exceeds 5 m ² / g, when both electrodes of the battery are short-circuited (at the time of external short-circuit), a large amount of white smoke gushes from the battery and the positive electrode cap of the battery is ruptured. May be accompanied by moderate smoke.

Conversely, the specific surface area is at most 5 m ² /
When the value is g, the amount of the smoke can be significantly suppressed, and the burst can be prevented. When the electrode formed of the graphitized vapor-grown carbon fiber having a specific specific surface area is used as described above, the safety of the nonaqueous electrolyte secondary battery can be significantly improved.

The lower limit of the specific surface area is determined by the shape of the fiber. Assuming that the fiber has an ideal cylindrical shape, a theoretical value can be calculated from its average diameter, length, and density. Any one of the diameter and the length can be calculated even if the aspect ratio is changed. If the fibers have irregularities or holes, or if the fibers are mixed with crushed fragments or fine powder, the value will be larger than the theoretical specific surface area. It can be easily understood that it is theoretically impossible to make the specific surface area smaller than the theoretical specific surface area.

Further, when the graphitized vapor-grown carbon fiber is used, the conductivity or conductivity of the negative electrode is increased at a specific packing density, and a conductive auxiliary material such as acetylene black having a very high specific surface area is used. Since it is not necessary to add it to the negative electrode, the safety of the battery can be significantly improved.

The specific surface area can be measured by the BET method.

The graphitized vapor grown carbon fiber of the present invention has an average aspect ratio of 2 to 30, preferably 3 to 3.
20, more preferably 5 to 15. When the average aspect ratio of the graphitized vapor-grown carbon fiber is 2 to 30, the object of the present invention can be achieved well. In other words, when the average aspect ratio exceeds 30, it is possible to form a sheet-like electrode. And the packing density of the negative electrode is lowered, which causes problems in load characteristics and safety. Further, when the average aspect ratio is less than 2, there arises a disadvantage that the specific surface area becomes larger than 5 m ² / g.

When the average aspect ratio is from 2 to 30, the graphitized vapor-grown carbon fibers are in good contact with each other, and the conductivity of the electrode itself is increased. When the conductivity increases, even if a large current flows due to a large load,
The potential difference between the current collector of the electrode and the electrode surface does not increase, that is, charging and discharging can be performed uniformly from the core of the electrode to the surface of the electrode, and the charge and discharge capacity increases. Therefore, it is possible to provide a non-aqueous electrolyte secondary battery that exhibits high capacity even when charged and discharged at a high current value.

When the average aspect ratio is less than 2,
The contact resistance increases and the conductivity decreases. On the other hand, if the average aspect ratio exceeds 30, the packing density in the electrode decreases and the conductivity decreases. When the average aspect ratio is less than 2 or more than 30, the conductivity is reduced, and the battery performance is reduced. Conventionally, in order to increase the conductivity, the conductive auxiliary material had to be added. However, if the average aspect ratio is 2 to 30, the conductivity of the electrode itself increases, so the conductive auxiliary material is added. Thus, a safer non-aqueous electrolyte secondary battery can be provided.

The average diameter of the graphitized vapor-grown carbon fiber is usually 1 to 10 μm, preferably 2 to 5 μm.
When the average diameter of the graphitized vapor-grown carbon fiber is 1 to 10 μm, when the graphitized vapor-grown carbon fiber is dispersed in an organic solvent together with a binder to form a negative electrode, the dispersion is easily realized, The contact between the fibers is also facilitated. By facilitating the contact between the fibers, the conductivity or conductivity of the negative electrode formed at a specific packing density increases, and it is not only necessary to add a conductive auxiliary material such as carbon black, for example, acetylene black. The conductivity is higher than those obtained by adding a conductive auxiliary to other graphite materials, and the specific surface area is not increased by the addition of the conductive auxiliary. A battery can be provided.

The average aspect ratio of the graphitized vapor-grown carbon fiber is determined by taking an image of the graphitized vapor-grown carbon fiber with an electron microscope, observing the electron micrograph, and comparing the graphite in the electron micrograph. 1,0 from chemical vaporized carbon fiber
00 samples were randomly selected, and the length and diameter of the selected graphitized vapor grown carbon fibers were measured, assuming that the selected graphitized vapor grown carbon fibers were in the form of a tube. The aspect ratio of each graphitized vapor-grown carbon fiber is determined from the length and the diameter, and the average diameter of the graphitized vapor-grown carbon fibers is determined by averaging the aspect ratios of 1,000 samples. It is determined by measuring the diameter of the graphitized vapor grown carbon fiber and averaging the diameters of 1,000 samples.

The graphitized vapor-grown carbon fiber of the present invention has a highly developed graphite structure, and the distance between the graphite screens (d) is determined in view of the degree of development of the graphite screens in a number of six-membered ring patterns.
_oo2 ), i.e. the distance between adjacent mesh planes is usually at most 0.338 nm, preferably at most 0.337 n
m, more preferably 0.3355 to 0.3365 nm
It is.

The graphitized vapor-grown carbon fiber of the present invention has a thickness in which the graphite net surfaces are laminated, that is, a graphite crystallite thickness (L _c ) of usually at least 40 nm, preferably at least 60 nm, and more preferably at least 60 nm. Preferably at least 80 nm
It is.

In the measurement of L _c by X-ray diffraction, the value was 1
It is said that the measurement value of 00 nm or more is not reliable, and there is no other commonly used measurement method, so that the upper limit cannot be determined. However, it is easily understood that the value does not exceed the size of one carbon of interest, and in the case of a vapor growth carbon fiber having an annual ring structure, it cannot exceed the radius of the fiber.

Among the graphitized vapor-grown carbon fibers, if the distance between graphite mesh planes exceeds 0.338 nm or the thickness of graphite crystallites is less than 40 nm, in the case of a lithium ion secondary battery, lithium cation Intercalation is not sufficiently performed, and it may become inappropriate to use it as a negative electrode for a lithium ion secondary battery.

The distance between graphite mesh planes and the thickness of graphite crystallites were determined by the Japan Society for the Promotion of Science, described on page 55 of "Carbon Technology I" (published by Science and Technology Company, published in 1970). It can be measured by Gakushin method.

The graphitized vapor-grown carbon fiber of the present invention has a preferable spin density of at most 8 × 10 ¹⁸ spins / g, more preferably at most 7 × 10 ¹⁸ spins / g, as measured by the electron spin resonance absorption method. It is.

This spin density can be measured by an electron spin resonance absorption method.

The graphitized vapor-grown carbon fiber of the present invention can be suitably produced as follows.

That is, the graphitized vapor-grown carbon fiber of the present invention is obtained by breaking a graphitized vapor-grown carbon fiber having a predetermined form with a high impact force using a device such as a hybridizer. Alternatively, it can be manufactured by pressurizing and compressing using an isostatic pressing device.

Here, the graphitized vapor grown carbon fiber having the predetermined form can be obtained by graphitizing the vapor grown carbon fiber.

The vapor grown carbon fiber can be produced by a vapor growth method.

Specifically, Japanese Patent Application Laid-Open No. 52-107320
JP-A-57-117622, JP-A-58-156
No. 512, JP-A-58-180615 and JP-A-60-180
185818, JP-A-60-224815, JP-A-60-231821, JP-A-61-132630,
JP-A-61-13600, JP-A-61-13266
No. 3, JP-A-61-225319, JP-A-61-22
No. 5322, JP-A-61-225325, JP-A-61-225325
-225327, JP-A-61-225328, JP-A-61-275425, JP-A-61-282427
, A vapor-grown carbon fiber can be produced by the methods described in JP-A-5-222609.

In the graphitizing treatment, the vapor-grown carbon fiber is heated to 2,000 ° C. or more, preferably from 2,000 ° C. to 3.0 ° C.
The heat treatment is performed in the range of 00 ° C.

However, the vapor grown carbon fiber having a diameter of 70 nm or less may be sufficiently graphitized at the stage when it is formed under the above conditions. The above-mentioned vapor-grown carbon fiber can be used as a graphitized vapor-grown carbon fiber for the negative electrode without performing the above steps.

As an atmosphere for the heat treatment, an inert gas atmosphere is usually employed. The heat treatment time is usually 5 minutes or more.

(1-2) Binder Examples of the binder include fluorinated resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyolefins such as polyethylene and polypropylene, and copolymers thereof.

(1-3) Current Collector The current collector is preferably a material having a function as a support for a battery electrode, chemical resistance, electrochemical stability and conductivity. It is formed of a metal such as aluminum and iron. Preferred metals for the current collector are copper and aluminum. Usually, the more preferable metal for the current collector of the negative electrode is copper, and the more preferable metal for the current collector of the positive electrode is aluminum. The form of the current collector is appropriately determined according to the form of the secondary battery, but is usually a thin sheet.

(1-4) Method for Producing Negative Electrode The method for producing a negative electrode in the nonaqueous electrolyte secondary battery according to the present invention comprises the steps of: first, graphitized vapor-grown carbon fiber as a negative electrode active material, Disperse in solvent. Then, this dispersion is applied to the surface of the current collector. Then, after drying this, a pressure treatment is performed. In this manner, a compact obtained by forming the negative electrode active material layer on the surface of the current collector, that is, a press-formed body obtained by press-forming the graphitized vapor-grown carbon fiber is used as the negative electrode.

As the solvent, a polar organic solvent, particularly a non-aqueous polar solvent, is suitable.
Pyrrolidone is preferred. With this solvent, the viscosity of the dispersion is 20 to 70 dPa · s, preferably 25 to 6 dPa · s.
It is adjusted to be 0 dPa · s, more preferably 35 to 50 dPa · s.

When the dispersion is applied to the current collector,
The thickness and the application area of the active material layer are appropriately determined according to the scale of the secondary battery, and the method for applying the dispersion liquid employs appropriate means such as brush coating, dipping, coater coating, spray coating, and the like. Is done.

After applying the dispersion to the current collector, the coated surface is dried. At the time of this drying, the drying atmosphere is set to 100 ppm at the highest oxygen concentration, preferably 80 ppm at the highest.
It is preferable that the deoxidized atmosphere is adjusted to be at most 50 ppm, more preferably at most 50 ppm. The above deoxidized atmosphere is preferable in that the oxidation of the current collector can be suppressed even in drying at a high temperature. The drying time under the above deoxidized atmosphere is usually 5 to 60 minutes, preferably 10 to 40 minutes. The drying temperature is usually 100 to 180 ° C, preferably 120 to 1 ° C.
60 ° C.

The current collector coated with the dispersion and dried is further subjected to a pressure treatment. As the pressure processing device, for example, a device such as a press machine or a roll press machine is used. When a roll press machine is used as the pressure treatment device, the thickness of the negative electrode active material layer formed on the surface of the current collector is 40 to
It is better to press with a 60% reduction in clearance.

The packing density of the pressed body is 1.2 to
2.0 g / cm ³ , with a preferred packing density of 1.4 to
2.0 g / cm ³ , more preferably 1.5 to 1.
8 g / cm ³ . The packing density of the pressed body is 1.
When the amount is less than 2 g / cm ³ , the conductivity of the electrode is reduced, and the load characteristics and safety effects of the secondary battery of the present invention cannot be sufficiently exhibited. On the other hand, 1.2 to 2.0 g / c
With m ³ , the object of the present invention can be achieved well.

The negative electrode active material layer in the present invention contains the graphitized vapor grown carbon fibers bonded to each other by the binder. The content ratio of the graphitized vapor-grown carbon fibers in the negative electrode active material layer is generally 85 to 97% by weight, preferably 87%, with the total weight of the negative electrode active material layer being 100.
~ 95% by weight.

(2) Positive Electrode The positive electrode in the nonaqueous electrolyte secondary battery according to the present invention has a lithium-containing composite oxide. Preferred positive electrodes are
An active material layer formed by dispersing a lithium-containing composite oxide and a conductive inorganic substance in a binder is formed by covering the surface of the current collector.

The lithium-containing composite oxide includes at least one metal selected from the group consisting of a Group 3B metal, a Group 6A metal, a Group 7A metal and a Group 8 metal;
And a lithium-containing composite oxide having lithium. Suitable lithium-containing composite oxides include:
Lithium composite oxide and LiMn represented by the following general formula
_At least one selected from the group consisting of ₂ O ₄ can be mentioned.

LiNi _{1 -x} M _x O ₂ (wherein, M represents aluminum, manganese, chromium, cobalt and iron, and X represents any value between 0, 1 and 0-1. Further preferred lithium-containing composite oxides include, for example, lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and lithium nickelate (L
iNiO ₂ ). The lithium-containing composite oxides may be used alone or in a combination of two or more.

Examples of the conductive inorganic substance include acetylene black, artificial graphite, carbon black called Ketjen Black manufactured by Ketjen Black International, and vapor grown carbon fiber.

Examples of the binder and the current collector include the same ones as those used in forming the negative electrode. The type used for the positive electrode and the type used for the negative electrode may be different or the same.

In the method for producing a positive electrode of a non-aqueous electrolyte secondary battery according to the present invention, first, a lithium-containing composite oxide and a conductive inorganic substance as a positive electrode active material are dispersed in a solvent together with a binder. Then, this dispersion is applied to the surface of the current collector. Then, after drying this, a pressure treatment is performed. There is no particular limitation on the shape of the positive electrode.

Examples of the solvent include the same solvents as those used in forming the negative electrode. For example, N-methyl-2-pyrrolidone is preferred. In dispersing the lithium-containing composite oxide in such a solvent, the ratio of the lithium-containing composite oxide to the solvent is usually 50 to 70% by weight, preferably 55 to 6% by weight.
5%. The type used for the positive electrode and the type used for the negative electrode may be different or the same.

When the dispersion is applied to the current collector, the thickness and area of the active material layer, the method of applying the dispersion, the method of drying after application, the method of applying pressure, and the like are also applicable to the case where the negative electrode is formed. Is the same as

The active material layer of the positive electrode according to the present invention contains the lithium-containing composite oxide, the conductive inorganic substance, and the binder. When the total weight of the active material layer of the positive electrode is 100, the content ratio of the lithium-containing composite oxide is usually 80 to 95% by weight, preferably 85% by weight.
To 92% by weight, and the content ratio of the conductive inorganic substance is usually 15 to 3% by weight, preferably 8 to 4% by weight.

The current collector to which the dispersion is applied is usually a metal sheet, and preferably an aluminum sheet.

The drying method is not particularly limited.
An appropriate method can be adopted depending on the situation.

The packing density of the positive electrode of the present invention in the pressure treatment is 2.2 to 3.5 g / cm ³ , preferably 2.5 to 3.5 g / cm ³ .
It is desirable to perform a pressure molding treatment so as to obtain 3.3 g / cm ³ .

(3) Manufacturing Method of Secondary Battery The non-aqueous electrolyte secondary battery according to the present invention can be manufactured using a negative electrode, a positive electrode and a non-aqueous electrolyte.

The non-aqueous electrolyte contains a lithium salt. The concentration of the lithium salt is usually 0.8 to 2.0 mol / l, preferably 1 to 1.8 mol / l, and more preferably 1 to 1.6 mol / l. When the concentration of the lithium salt is within the above range, the object of the present invention can be achieved well, and there is an advantage that the cycle characteristics at high or low temperature are excellent.

As the lithium salt, LiClO ₄ ,
LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ S
O _{3 and the} like can be mentioned, and these can be used alone or in combination of two or more. Among these, LiP is preferable.
It is a F _6.

The solvent of the non-aqueous electrolyte is a mixed solvent of a cyclic carbonate and a chain carbonate.

Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and the like, and one kind or a mixture of two or more kinds can be used. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and the like, and one kind or a mixture of two or more kinds can be used. As a combination of two or more carbonates, for example, ethylene carbonate and dimethyl carbonate, ethylene carbonate and diethyl carbonate, ethylene carbonate and dimethyl carbonate and diethyl carbonate,
Examples include ethylene carbonate, propylene carbonate, and dimethyl carbonate, and ethylene carbonate, propylene carbonate, and diethyl carbonate.

It should be noted that other additives may be added to these mixed solvents as long as the object of the present invention is not impaired.

In the case of employing a mixed solvent consisting of ethylene carbonate, propylene carbonate and diethyl carbonate, the mixing ratio of ethylene carbonate: propylene carbonate: diethyl carbonate = 2 to 5: 0.5 to 3.0. : 2.5 to 7.5
(Volume ratio) is preferred. 3 consisting of ethylene carbonate, propylene carbonate and dimethyl carbonate
When a seed mixed solvent is employed, the mixing ratio thereof may be ethylene carbonate: propylene carbonate:
Dimethyl carbonate = 2-5: 1-3: 2-7 (volume ratio) is preferred.

When the solvent component of the non-aqueous electrolyte is three or more, the object of the present invention can be achieved well, and there is an advantage that ionic conductivity is high even at a low temperature.

In the non-aqueous electrolyte secondary battery according to the present invention, the design capacity of the negative electrode is adjusted to be larger than the design capacity of the positive electrode. The design capacity is determined, for example, from the initial charge capacity per unit weight of the active material in a predetermined charging voltage range and the amount of the active material applied to the current collector. The charge capacity can be determined, for example, by using a three-electrode cell in which both the counter electrode and the reference electrode are made of metallic lithium, and continuously recording a voltage that changes by flowing a constant current with time. The charge capacity is a charge amount until a predetermined voltage is reached. In the three-electrode cell, the voltage at the start when the vapor-grown carbon fiber negative electrode is used as the working electrode is 3 V, and the charging (current flows from the working electrode to the counter electrode) proceeds and decreases toward 0 V. I will do it. When the positive electrode is used as the working electrode, the voltage at the start is the potential difference between the metallic lithium and the positive electrode material, and the voltage increases with charging (current flows from the counter electrode to the working electrode). The predetermined voltage is 0 V for the vapor-grown carbon fiber negative electrode.
In the positive electrode, the range is set so that decomposition of the electrolytic solution does not occur and irreversible structural destruction of the positive electrode active material does not occur. For example, LiMn ₂ O ₄ has a voltage of 4.2 to 4.3 V, preferably 4.2 V, and LiCoO ₂ or LiNiO ₂ has a voltage of 4.1 to 4.2 V, preferably 4.1 V. In addition, when the graphitization of the negative electrode active material is complete, the design capacity of the negative electrode can be the theoretical capacity indicated by the capacity per unit weight of carbon in the negative electrode, that is, 372 mA / g. 10 mA if graphitization is not complete
The capacity is measured at a low current of not more than / g and a predetermined cutoff voltage to determine the designed capacity of the negative electrode.

Note that lithium nickelate (LiNiO)
_{When 2} ) is used as the positive electrode active material, the solvent of the nonaqueous electrolyte is a mixed solvent of a cyclic carbonate and a chain carbonate, so that the charge and discharge efficiency in the first cycle is reduced by 50%.
The non-aqueous electrolyte secondary battery can be suppressed to about 70% and the deterioration of the discharge capacity is small.

Further, in this non-aqueous electrolyte secondary battery, if the cut-off voltage during charging is limited to 4.1 V, a longer life non-aqueous electrolyte secondary battery can be obtained. The cutoff voltage during charging is the upper limit of the voltage during charging.

Examples of the non-aqueous electrolyte secondary battery according to the present invention include a button type secondary battery, a cylindrical type secondary battery, a square type secondary battery, a coin type secondary battery and the like.

The cylindrical secondary battery is manufactured as follows.

The negative electrode and the positive electrode are wound up in the form of a roll via a porous polypropylene sheet separator. The roll is placed in a cylindrical battery can and the negative electrode lead wire is welded to the bottom of the can. Next, a positive electrode lead wire is welded to a positive electrode cap having a rupture disk, a closing lid, and a gasket. The electrolytic solution is put into the battery can, and the positive electrode cap is caulked to the opening of the negative electrode can. This allows
A cylindrical secondary battery is obtained.

The prismatic secondary battery is formed by flattening the roll-shaped roll, which is the same as the cylindrical secondary battery, and storing the flat product in a prismatic can, or forming a positive electrode and a negative electrode having lead wires connected thereto. It can be manufactured by, for example, housing a laminate formed by sandwiching layers in a sandwich shape through a separator in a square can.

[0085]

【Example】

(Example 1) (1) Production of Graphitized Vapor-grown Carbon Fiber for Negative Electrode A vapor-grown carbon fiber having an average diameter of 2 μm and an average length of 50 μm was subjected to graphite at 2,800 ° C. for 30 minutes in an argon gas atmosphere. By performing the carbonization treatment, a graphitized vapor-grown carbon fiber was produced.

40 g of the graphitized vapor-grown carbon fiber was used as a hybridizer (NHS-1, manufactured by Nara Machinery Co., Ltd.).
And subjected to high impact treatment at 4,000 rpm (peripheral speed 50 m / s) for 2 minutes. The graphitized vapor-grown carbon fiber after cutting has a specific surface area of 1.4 m ² / g, an average aspect ratio of 12, an average diameter of 2 μm, and a distance between graphite mesh planes (d _oo2 ). Was 0.3361 nm, and the thickness (L _c ) of the graphite crystallite was 130 nm. The graphitized vapor-grown carbon fiber after cutting was the graphitized vapor-grown carbon fiber for the negative electrode of the present invention, and its specific surface area and aspect ratio are shown in Table 1.

(2) Cylindrical Battery A negative electrode was produced as follows. That is, 30 g of polyvinylidene fluoride (PVDF) was dissolved in 420 ml of N-methyl-2-pyrrolidone. 270 g of the graphitized vapor-grown carbon fiber for negative electrode of (1) above was added to the obtained solution.
And dispersed sufficiently with an ultrasonic dispersing machine. The obtained dispersion is applied to a copper sheet (thickness 10 μm, length 3 m, width 200 m)
m), and after drying, the electrode was pressure-formed with a press machine. After molding, cut into width 39mm, length 450mm,
When the packing density of the electrode was measured by measuring the thickness and the weight, it was 1.60 g / cm ³ . This was used as a negative electrode. Table 1 shows the packing density of the negative electrode.

A positive electrode was prepared as follows. That is, 20 g of PVDF was added to N-methyl-2-pyrrolidone 35
Dissolved in 0 ml to obtain a solution.

Next, 445 g of LiCoO ₂ , 20 g of artificial graphite, and 15 g of acetylene black were put in a ball mill and mixed to obtain a mixture.

The mixture and the solution were mixed and sufficiently dispersed by an ultrasonic disperser to obtain a dispersion.

[0091] The dispersion was applied to an aluminum sheet (thickness: 2).
0 μm). The application area at this time is 300 × 1
5 cm.

After the application, the electrode is pressed with a press machine,
A press-formed body was obtained. After molding, width 38mm, length 430m
m, and the packing density of the electrode was measured by measuring the thickness and the weight. The result was 3.1 g / cm ³ . This was used as a positive electrode.

A non-aqueous electrolyte secondary battery was manufactured as follows. The positive electrode and the negative electrode were wound up in a roll shape via a porous polypropylene sheet separator. This roll-shaped roll is 16 mm in diameter and 50 mm in height.
And the negative electrode lead wire was welded to the bottom of the can. Next, a positive electrode lead wire was welded to a positive electrode cap having a rupture disk, a closing lid, and a gasket. Wherein in the battery can, a mixed solvent (volume ratio of ethylene carbonate to become a LiPF ₆ 1.2 molar concentration (EC) and propylene carbonate (PC) and diethyl carbonate (DEC); EC / PC / DEC = 2/2), and the positive electrode cap was caulked to the opening of the negative electrode can. As a result, a cylindrical nonaqueous electrolyte secondary battery was obtained. The value of (designed negative electrode capacity / designed positive electrode capacity) of this nonaqueous electrolyte secondary battery was adjusted to 1.2, and the value is shown in Table 1.

[0094]

[Table 1]

(3) Nail Penetration Test of Cylindrical Secondary Battery A nail having a length of 35 mm and a diameter of 3 mm was placed on a side surface of a cylindrical secondary battery charged to a voltage of 4.1 V with a current of 800 mA by 50 m.
It was penetrated at a speed of m / min. The results are shown in Table 2.

[0096]

[Table 2]

(4) Charge / discharge test at each current value A charge / discharge voltage in the range of 2.5 to 4.1 V was set to 800
A charge / discharge test was performed at current values of mA, 1600 mA, 2400 mA, and 3200 mA. The results are shown in Table 3 and FIG.

[0098]

[Table 3]

(Example 2) (1) Production of graphitized vapor-grown carbon fiber for negative electrode 30 g of graphitized vapor-grown carbon fiber before cutting same as that used in Example 1 was isostatically pressed. 1,000 with pressure device
It was pressurized with a pressing force of kgf / cm ² .

The pressurized graphitized vapor-grown carbon fiber has a specific surface area of 2.4 m ² / g, an average aspect ratio of 8, an average diameter of 2 μm, and a distance between graphite mesh planes. (D _oo2 ) was 0.3361 nm, and the thickness (L _c ) of the graphite crystallite was 130 nm. The graphitized vapor-grown carbon fiber after pressurization is the graphitized vapor-grown carbon fiber for a negative electrode of the present invention, and its specific surface area and aspect ratio are shown in Table 1.

(2) Cylindrical Battery A negative electrode was manufactured in substantially the same manner as in Example 1. The packing density of the negative electrode was 1.80 g / cm ³ . Table 1 shows the packing density of the negative electrode.

A positive electrode was produced in the same manner as in Example 1 except that LiMn ₂ O ₄ was used instead of LiCoO ₂ and the packing density was 2.9 g / cm ³ .

Using the negative electrode, the positive electrode, and the same electrolytic solution as in Example 1, a cylindrical non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. The value of (designed negative electrode capacity / designed positive electrode capacity) of this nonaqueous electrolyte secondary battery was adjusted to 1.1, and the value is shown in Table 1.

A nail penetration test and a charge / discharge test were performed in the same manner as in Example 1. Tables 2 and 3 show the results.
And FIG.

Comparative Example 1 (1) Production of Graphitized Vapor-grown Carbon Fiber for Negative Electrode Vapor-grown carbon fiber having an average diameter of 4 μm and an average length of 50 μm was heated at 2,800 ° C. for 30 minutes in an argon gas atmosphere. The resulting mixture was subjected to a graphitization treatment to produce a graphitized vapor-grown carbon fiber.

40 g of the graphitized vapor-grown carbon fiber was used as a hybridizer (NHS-1, manufactured by Nara Machinery Co., Ltd.).
And subjected to high impact treatment at 8,000 rpm (peripheral speed 100 m / s) for 10 minutes.

The cut graphitized vapor-grown carbon fiber has a specific surface area of 7.0 m ² / g, an average aspect ratio of 1.2, an average diameter of 4 μm, and a graphite mesh surface. The distance (d _oo2 ) was 0.3370 nm, and the graphite crystallite thickness (L _c ) was 60 nm. Table 1 shows the specific surface area and aspect ratio of the graphitized vapor-grown carbon fibers after the cutting.

(2) Cylindrical Battery A negative electrode was manufactured in substantially the same manner as in Example 1. The packing density of the negative electrode was 1.1 g / cm ³ . Table 1 shows the packing density of the negative electrode.

Using the negative electrode, the same positive electrode and the same electrolyte as in Example 1, a cylindrical non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. The value of (designed negative electrode capacity / designed positive electrode capacity) of this nonaqueous electrolyte secondary battery was adjusted to 1.1, and the value is shown in Table 1.

A nail penetration test and a charge / discharge test were performed in the same manner as in Example 1. Tables 2 and 3 show the results.
And FIG.

(Comparative Example 2) The packing density of the negative electrode in Comparative Example 1 was 0.9 g / cm ^3, and the value of (negative electrode design capacity / positive electrode design capacity) of the nonaqueous electrolyte secondary battery was 0.6. A cylindrical non-aqueous electrolyte secondary battery was produced in the same manner as in Comparative Example 1 except that the adjustment was made as follows.

Table 1 shows the specific surface area and aspect ratio of the graphitized vapor-grown carbon fibers after cutting in Comparative Example 2.

Table 1 shows the packing density of the negative electrode and the value of (designed negative electrode capacity / designed positive electrode capacity) in Comparative Example 2.

A nail penetration test was performed in the same manner as in Comparative Example 1. The results are shown in Table 2.

Comparative Example 3 Graphitized mesocarbon microbeads having an average particle diameter of 6 μm (hereinafter sometimes referred to as MCMB) and acetylene black (hereinafter referred to as “MCMB”) were used instead of the graphitized vapor-grown carbon fibers in Example 1. , AB) (in terms of weight ratio, MCMB: AB = 80:
A cylindrical nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that 10) was used. The specific surface area of the mixture was 6.0 m ² / g, and the packing density of the negative electrode was 1.1 g / c.
m ^3, which is the negative electrode design capacity /
The value of the positive electrode design capacity) was 0.8.

Specific surface area of the mixture in Comparative Example 3,
Negative electrode packing density and (negative electrode design capacity / positive electrode design capacity)
Are shown in Table 1.

A nail penetration test was performed in the same manner as in Example 1. The results are shown in Table 2.

Comparative Example 4 Example 1 was repeated except that graphitized mesocarbon microbeads having an average particle size of 6 μm were used instead of the graphitized vapor-grown carbon fiber in Example 1.
Similarly to the above, a cylindrical non-aqueous electrolyte secondary battery was produced.

A charge / discharge test was performed in the same manner as in Example 1. The results are shown in Table 3 and FIG.

[0120]

According to the present invention, as the mixed solvent,
Since a decomposition reaction of a solvent which may occur when a chain carbonate such as dimethyl carbonate (abbreviation: DMC) or diethyl carbonate (abbreviation 1; DEC) having a low boiling point is employed is suppressed, the battery ruptures and ignites. There is provided a non-aqueous electrolyte-based secondary battery with high safety, which does not have a danger of performing.

Further, the non-aqueous electrolyte secondary battery of the present invention has high conductivity of the electrode itself, excellent cycle characteristics, exhibits high capacity even when charged and discharged at a higher current value, and has a long life. Is also long.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a current value and a discharge capacity in charge / discharge tests obtained in Examples 1, 2, 3 and 6.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI // D01F 9/127 D01F 9/127

Claims (7)

[Claims] 1. A graphitized vapor-grown carbon fiber having a specific surface area of at most 5 m ² / g and an average aspect ratio of 2 to 30 obtained by pressure molding, and having a packing density of 1.2. A negative electrode having a press-molded body formed to a pressure of 2.0 g / cm ³ , a positive electrode having a lithium-containing composite oxide, and a lithium salt in a mixed solvent of a cyclic carbonate and a chain carbonate. And a non-aqueous electrolyte secondary battery comprising: 2. The method according to claim 1, wherein the lithium-containing composite oxide is 3B
At least one metal selected from the group consisting of group metal, group 6A metal, group 7A metal, and group 8 metal;
2. The non-aqueous electrolyte secondary battery according to claim 1, which is a composite oxide containing lithium. 3. The lithium-containing composite oxide according to claim 1, wherein said lithium-containing composite oxide is represented by the following general formula (I):
Non-aqueous electrolyte secondary battery according to claim 1 is at least one selected from the group consisting of n ₂ O _{4. LiNi 1-X M X O 2} ··· (I) ( where, in the formula, M is aluminum, manganese, chromium, cobalt, and shows the iron, X is between 0,1 and 0-1 Show any number.) 4. A negative electrode formed of graphitized vapor-grown carbon fibers having a specific surface area of at most 5 m ² / g and an average aspect ratio of 2 to 30, and a lithium-containing composite oxide. Non-aqueous electrolysis comprising a positive electrode, and an electrolytic solution obtained by dissolving a lithium salt in a mixed solvent of a cyclic carbonate and a chain carbonate, wherein the designed capacity of the negative electrode is larger than the designed capacity of the positive electrode. Liquid secondary battery. 5. The non-aqueous electrolyte secondary battery according to claim 4, wherein a design capacity of the negative electrode with respect to a design capacity of the positive electrode exceeds 1 and is 1.6 or less. 6. The method according to claim 1, wherein the lithium-containing composite oxide is 3B
At least one metal selected from the group consisting of group metal, group 6A metal, group 7A metal, and group 8 metal;
5. The non-aqueous electrolyte secondary battery according to claim 4, which is a composite oxide containing lithium. 7. The lithium-containing composite oxide, wherein the lithium-containing composite oxide is represented by the following general formula (I):
Non-aqueous electrolyte secondary battery according to claim 4 is at least one selected from the group consisting of n ₂ O _{4. LiNi 1-X M X O 2} ··· (I) ( where, in the formula, M is aluminum, manganese, chromium, cobalt, and shows the iron, X is between 0,1 and 0-1 Show any number.)