JPH0927344A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery with a long cycle life by containing a mixture of flake graphite and fibrous carbon as a conductor in a positive electrode. SOLUTION: A nonaqueous electrolyte secondary battery is constituted with a negative electrode mainly comprising a carbon material (graphite material or the like) capable of doping/undoping lithium, a positive electrode, and a nonaqueous electrolyte prepared by dissolving an electrolyte in a nonaqueous solvent. The positive electrode contains a positive mix comprising a positive active material (lithium transition metal compound oxide), a mixture of flake graphite and fibrous carbon, and a binder (polyvinylidene fluoride or the like). The content of the mixture of flake graphite and fibrous carbon contained in the positive mix is 3-16wt.%, and the mixing ratio of flake graphite and fibrous carbon is 85:15 to 25:75. The nonaqueous electrolyte secondary battery with long cycle life and in which the electrode structure of the positive electrode is not broken even when the battery is used in heavy load discharge condition can be obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of a conductive agent used for a positive electrode.

[0002]

2. Description of the Related Art Recent remarkable advances in electronic technology have made electronic devices smaller and lighter one after another. Along with this, batteries, which are portable power sources, are required to be smaller and lighter and have higher energy density.

Conventionally, aqueous secondary batteries such as lead batteries and nickel-cadmium batteries have been mainly used as secondary batteries for general use. However, although these aqueous secondary batteries can satisfy cycle characteristics to some extent, they cannot be said to be sufficient in terms of battery weight and energy density.

On the other hand, recently, research on a non-aqueous electrolyte secondary battery using lithium or a lithium alloy as a negative electrode material
Development is active. In this battery, especially LiC
Li-containing composite oxides having a high discharge voltage such as oO ₂ are used as a positive electrode material, and have high energy density, little self-discharge, and light weight.

However, in this non-aqueous electrolyte secondary battery, lithium is dendrite-like crystal grown on the negative electrode at the time of charging as the charging / discharging cycle progresses, and finally reaches the positive electrode to cause an internal short circuit. There is a high possibility that Further, this dendrite-like crystal growth becomes more remarkable as the charge and discharge become faster, so it is necessary to avoid rapid charge and discharge as much as possible. For this reason, it cannot be said that the road to practical use is far away.

Therefore, a non-aqueous electrolyte secondary battery (so-called lithium ion secondary battery) using a carbon material as a negative electrode material has been attracting attention. This non-aqueous electrolyte secondary battery utilizes the fact that lithium is doped / dedoped between the carbon layers of the carbon material in the negative electrode reaction, and dendrite-like lithium is not formed on the negative electrode even if the charge / discharge cycle proceeds. No phenomenon such as precipitation is observed, it has a high energy density, is lightweight, and exhibits excellent charge / discharge cycle characteristics.

In such a non-aqueous electrolyte secondary battery,
There are various carbon materials that can be used as the negative electrode material. First, the carbon material that has been put into practical use as the negative electrode material is a non-graphitizable carbon material such as coke or glassy carbon, that is, an organic material is heat-treated at a relatively low temperature. It is a carbon material with low crystallinity obtained in. Non-aqueous electrolyte secondary batteries using an anode composed of these low crystalline carbon materials and an electrolyte containing propylene carbonate (PC) as a main solvent have already been commercialized.

More recently, graphites having a developed crystal structure have been used as a negative electrode material.

It has been considered difficult to use this graphite as a negative electrode material because it decomposes PC, which has been widely used as a main solvent of non-aqueous solvents. But PC
It was found that such inconvenience can be eliminated by using ethylene carbonate (EC) instead of, and it can be used for the negative electrode in the form of a combination with this EC.

[0010] Graphites are relatively easily available in the form of scales and have been widely used as conductive agents for alkaline batteries. These graphites have higher crystallinity and higher true density than low crystalline carbon materials. Therefore, if the negative electrode is composed of these graphites, a high electrode packing property is obtained and the energy density of the battery is increased. From this, it can be said that graphites are materials that are highly expected as negative electrode materials.

On the other hand, as a positive electrode material, a Li-containing composite oxide typified by LiCoO ₂ is mainly used. Here, when this Li-containing composite oxide is used as the positive electrode material, it is usual to use a conductive agent such as graphite in combination because the conductivity of the oxide itself is very small. That is, the positive electrode is
It is configured as a positive electrode mixture in which a conductive agent is further added to the Li-containing composite oxide and the binder.

[0012]

By the way, in a positive electrode using such a Li-containing composite oxide, the unit crystal lattice of this Li-containing composite oxide expands and contracts during charge and discharge, so that the maximum content is about 10%. The thickness changes. Also,
Also in the negative electrode, the crystallite of the carbon material expands and contracts during charge and discharge, so that the maximum thickness change is about 10%.

When such a thickness change occurs in the positive electrode and the negative electrode, the electrodes are pressed against each other, which may gradually destroy the electrode structure. When the electrode structure is destroyed, the performance is remarkably deteriorated particularly in the positive electrode in which the active material itself has low conductivity. In particular, such destruction of the electrode structure becomes noticeable under heavy load discharge conditions, and it is suitable for commercial video cameras for broadcasts, video cameras with liquid crystal, notebook type workstations, etc. where batteries are used under heavy load discharge conditions. There is a big problem when considering the use of.

Therefore, the present invention has been proposed in view of such conventional circumstances, and the electrode structure is not destroyed even when used under a heavy load discharge condition, the cycle life is long, and the reliability is high. It is an object to provide the obtained non-aqueous electrolyte secondary battery.

[0015]

[Means for Solving the Problems] In order to achieve the above-mentioned object, as a result of intensive studies by the present inventors, as a result of using a mixture of flake graphite and fibrous carbon as a conductive agent for the positive electrode, We have come to the knowledge that batteries with long cycle life can be obtained even under load discharge conditions.

The present invention has been completed based on these findings.

That is, according to the present invention,
A negative electrode mainly composed of a carbon material that can be dedoped,
It is applied to a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte solution in which an electrolyte is dissolved in a positive electrode and a non-aqueous solvent.

In such a non-aqueous electrolyte secondary battery, the positive electrode is composed of a positive electrode mixture composed of a positive electrode active material, a conductive agent and a binder. In the present invention, the conductive agent is scaly graphite. And a mixture of fibrous carbon.

When a mixture of flake graphite and fibrous carbon is used as a conductive agent for the positive electrode, the flake graphite and fibrous carbon impart conductivity to the positive electrode and the fibrous carbon retains the electrode structure of the positive electrode. Act as you do. Therefore, the destruction of the electrode structure caused by the expansion and contraction of the positive electrode active material accompanying charging and discharging is suppressed, and the performance of the positive electrode is not impaired even when charging and discharging under heavy load discharge conditions, and a long cycle life is obtained. .

Here, the mixing ratio of flake graphite and fibrous carbon used as the conductive agent is 85:15 by weight.
It is suitable to be set to 25:75. The content of the mixture of flake graphite and fibrous carbon in the whole positive electrode mixture is preferably 3 to 16% by weight.

The positive electrode active material of the positive electrode mixture is, for example, LiMO ₂ (where M is Co, Ni, Mn, Fe,
A lithium transition metal composite oxide represented by (representing at least one of Al, V, and Ti) is used.

On the other hand, a carbon material is used as the active material of the negative electrode. Examples of such carbon materials include graphite materials and non-graphitizable carbon materials.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

The non-aqueous electrolyte secondary battery of the present invention is a non-aqueous electrolyte in which an electrolyte is dissolved in a negative electrode, a positive electrode, and a non-aqueous solvent which are mainly composed of a carbon material capable of doping and dedoping lithium. Is configured.

The positive electrode is obtained by dispersing a positive electrode mixture prepared by mixing a positive electrode active material, a conductive agent and a binder in an organic solvent to prepare a positive electrode mixture slurry, and compression-molding the slurry.

In the present invention, a mixture of flake graphite and fibrous carbon is used as the conductive agent contained in the positive electrode mixture.

This flake graphite has been conventionally used as a conductive agent for imparting conductivity to oxides and the like, and has high crystallinity, and therefore has extremely high electron conductivity. However, in a positive electrode using this scaly graphite alone as a conductive agent, under heavy load discharge conditions, the electrode structure is destroyed due to expansion and contraction of the positive electrode active material accompanying charge and discharge, leading to performance deterioration. Here, when fibrous carbon is added to such a positive electrode, the fibrous carbon functions to retain the electrode structure, and the destruction of the electrode structure due to charge / discharge is suppressed. Further, although this fibrous carbon is slightly inferior to the flake graphite, it also has a function as a conductive agent that imparts conductivity. Therefore, in a positive electrode using a mixture of flake graphite and fibrous carbon as a conductive agent, while having the same performance as a conventional positive electrode, destruction of the electrode structure due to charging and discharging is suppressed, and it is greatly effective in extending the life of the battery. Will contribute.

Specific examples of the flake graphite include natural graphite produced as an ore and artificial graphite synthesized by carbonizing an organic material and further treating it at a high temperature.

Of these, natural graphite is produced in China, Madagascar, Ceylon, Mexico, Brazil and the like. In the state of ore, a large amount of inorganic impurities other than graphite are contained, and particularly when a metal element is mixed as an impurity, this is electrochemically eluted and adversely affects the battery. Therefore, it is necessary to dissolve these impurities in a solvent and wash away these impurities. As the solvent, hydrogen fluoride, an inorganic acidic aqueous solution containing hydrogen chloride or an aqueous solution containing an organic acid,
Further, an inorganic alkaline aqueous solution containing caustic soda or the like, an aqueous solution containing a basic organic substance, and an organic solvent can be used.

On the other hand, artificial graphite is produced by heat-treating an organic material. Typical examples of the organic material as the starting material are coal and pitch.

Pitch is distilled from tars, asphalt, etc. obtained by pyrolyzing coal tar, ethylene bottom oil, crude oil, etc. at high temperature (vacuum distillation, steam distillation,
Steam distillation), thermal polycondensation, extraction, chemical polycondensation and the like, and other pitches produced during carbon dry distillation.

Further, as the starting material for the pitch, there are polyvinyl chloride resin, polyvinyl acetate, polyvinyl butyrate, 3,5-dimethylphenol resin and the like.

These coals and pitches exist in a liquid state at a maximum temperature of about 400 ° C. during carbonization, and when kept at that temperature, the aromatic rings are condensed with each other to form a polycycle and become laminated orientation, and then about 500 ° C. At the above temperature, a solid carbon precursor, that is, a semi-coke is formed. Such a process is called a liquid-phase carbonization process and is a typical formation process of graphitizable carbon.

In addition, condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphene and pentacene, and other derivatives (for example, carboxylic acids, carboxylic acid anhydrides, carboxylic acid imides, etc.) thereof, Or mixture, acenaphthylene, indole, isoindole, quinoline,
Fused heterocyclic compounds such as isoquinoline, quinoxaline, phthalazine, carbazole, acridine, phenazine and phenanthridine, and derivatives thereof can also be used as the starting material for pitch.

In order to produce desired artificial graphite using the above organic materials as starting materials, for example, after carbonizing the above organic materials at 300 to 700 ° C. in a stream of an inert gas such as nitrogen,
In an inert gas stream, the temperature rising rate is 1 to 100 ° C. per minute, the reached temperature is 900 to 1500 ° C., and the holding time at the reached temperature is 0 to 30.
Calcination under the condition of time (it is a graphitizable carbon material that has gone through this process), and more than 2000 ℃,
Heat treatment is preferably performed at 2500 ° C. or higher. Of course, in some cases, the carbonization and calcination operations may be omitted.

The natural graphite or artificial graphite is made into flake graphite by pulverizing and classifying. At this time, in order to obtain an ideal flake shape, graphite has high crystallinity and is measured by X-ray diffraction analysis (002).
The C-axis crystallite thickness of the surface is preferably 100 nm or more. This is because in order to obtain a flatter powder, it is more convenient for the graphite to be crushed by peeling from the carbon hexagonal mesh surface weakly bonded by the Van der Waals force. High crystallinity is also advantageous in order to obtain high conductivity as a conductive agent.

In the present invention, fibrous carbon is used as a conductive agent in combination with such flake graphite.

The fibrous carbon is obtained by heat-treating a precursor composed of a polymer or pitch spun into a fibrous shape, and an organic vapor such as benzene is directly applied to a substrate heated to a temperature of about 1000 ° C. There is a vapor-grown carbon fiber obtained by growing a carbon crystal using iron fine particles as a catalyst.

In the case of obtaining fibrous carbon by heat treatment, polymer precursors include polyacrylonitrile (PAN) and rayon. Further, polyamide, lignin, polyvinyl alcohol and the like can also be used.

Pitch-based precursors include tars obtained by pyrolyzing coal tar, ethylene bottom oil, crude oil, etc. at high temperatures, asphalt, etc. (vacuum distillation, atmospheric distillation, steam distillation), thermal polycondensation , Those obtained by operations such as extraction and chemical polycondensation, and other pitches produced during carbonization of wood.

Further, as the starting material for the pitch, there are polyvinyl chloride resin, polyvinyl acetate, polyvinyl butyrate, 3,5-dimethylphenol resin and the like.

In addition, condensed polycyclic hydrogen compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphene and pentacene, and other derivatives (for example, carboxylic acid, carboxylic acid anhydride and carboxylic acid imide thereof). Or a mixture, acenaphthylene, indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole,
Fused heterocyclic compounds such as acridine, phenazine and phenanthridine, and derivatives thereof can also be used as a raw material for pitch.

Both the polymer type precursor and the pitch type precursor are infusibilized or stabilized, and then further heat treated at a high temperature to form fibrous carbon.

The infusibilizing or stabilizing step is a step of oxidizing the fiber surface with acid, oxygen, ozone or the like so that the polymer or the like does not melt or thermally decompose during carbonization. Is. At this time, the treatment method can be appropriately selected depending on the type of precursor. However, it is necessary to select the treatment temperature below the melting point of the precursor. Further, the treatment may be repeated a plurality of times as necessary so that the stabilization is sufficiently performed.

To obtain the fibrous carbon, the infusible or stabilized polymer precursor or pitch precursor is placed in a stream of an inert gas such as nitrogen.
After carbonization at a temperature of 300 to 700 ° C., in an inert gas stream, the temperature rising rate is 1 to 100 ° C. per minute, and the reached temperature is 900 to 15
It can be obtained by calcination under the conditions of a temperature of 00 ° C. and an ultimate temperature of 0 to 30 hours. Of course, carbonization may be omitted in some cases.

On the other hand, when the carbon fiber is obtained by the vapor growth method, the starting material may be any organic substance which can be in a gaseous state. For example, ethylene, propane, and the like that exist in a gaseous state at room temperature, benzene and the like that are vaporized by bubbling at room temperature, or organic substances that can be vaporized by heating at a temperature below the thermal decomposition temperature can be used. is there.

The vaporized organic substance is directly discharged onto a high-temperature substrate to grow crystals as fibrous carbon. The temperature at this time is preferably about 400 ° C. to 1500 ° C., and is appropriately selected depending on the kind of the organic material as the starting material. Further, the type of substrate is preferably quartz, nickel or the like, and is appropriately selected depending on the type of organic material which is also a starting material.

At this time, a catalyst may be used to accelerate the crystal growth. As the catalyst, fine particles of iron, nickel, or a mixture thereof can be used, and in addition, a metal called a graphitization catalyst or its oxide also functions as a catalyst. These catalysts can be appropriately selected depending on the type of organic material that is a starting material.

The obtained fibrous carbon was further heated in an inert gas stream at a temperature rising rate of 1 to 100 ° C./min and an ultimate temperature of 2
The graphitization treatment may be carried out under conditions of 000 ° C. or higher, preferably 2500 ° C. or higher, and holding time at the ultimate temperature of 0 to 30 hours. This increases electronic conductivity and improves the function as a conductive agent.

The obtained fibrous carbon may be crushed according to the thickness of the electrode and the particle size of the active material to be used as a conductive agent. Can be used as. The pulverization may be performed before or after carbonization or calcination, or during the temperature rising process before graphitization.

The conductive agent is constituted by mixing the scaly graphite and the fibrous carbon as described above, and the mixing ratio of the scaly graphite and the fibrous carbon is preferably 85:15 to 25:75,
0:20 to 30:70 is more preferable. If the ratio of flake graphite is less than this range, the conductivity may be insufficient, and if the ratio of fibrous carbon is less than this range, the effect of retaining the electrode structure cannot be sufficiently obtained. The amount of the conductive agent added to the positive electrode mixture is 3 to 16 with respect to the entire positive electrode mixture.
It is preferably wt%, more preferably 4 to 10 wt%. If the amount of the conductive agent added is less than this range, the effect of imparting conductivity and the function of maintaining the electrode structure cannot be sufficiently obtained. Further, when the amount of the conductive agent added is larger than this range, the proportion of the positive electrode active material decreases accordingly, leading to a decrease in capacity.

As the other materials used for the positive electrode mixture, that is, the positive electrode active material and the binder, any of those usually used in this type of non-aqueous electrolyte secondary battery can be used.

For example, as a positive electrode active material, a sufficient amount of L
It is preferable to include i, for example, the general formula LiMO
₂ (However, M is Co, Ni, Mn, Fe, Al, V, T
represents at least one of i. ), A composite metal oxide composed of lithium and a transition metal, an intercalation compound containing Li, and the like are preferable.

In particular, the present invention is aimed at achieving a high capacity, so that the positive electrode is 250 mAh or more per 1 g of the negative electrode carbonaceous material in a steady state (for example, after repeating charging and discharging about 5 times). It is necessary to include Li in an amount corresponding to the charging / discharging capacity of, and Li in an amount corresponding to the charging / discharging capacity of 300 mAh or more.
It is more preferred to include It is not always necessary to supply Li entirely from the positive electrode material, and the point is that Li corresponding to a charging / discharging capacity of 250 mAh or more per lg of carbonaceous material should be present in the battery system. Further, the amount of Li will be determined by measuring the discharge capacity of the battery.

Further, as the binder, a fluorine resin such as polyvinylidene fluoride is suitable because it has excellent solvent resistance.

On the other hand, the negative electrode is obtained by dispersing a negative electrode mixture prepared by mixing a negative electrode active material and a binder in an organic solvent to prepare a negative electrode mixture slurry, and compression-molding the slurry.

As the negative electrode active material, a carbon material that can be doped with lithium ions and dedoped can be used. As the carbon material, a graphite material, a graphitizable carbon material, or a non-graphitizable carbon material can be selected,
In particular, the present invention exerts a great effect when a graphite material is used for the negative electrode.

As the non-graphitizable carbon material, (002)
The surface spacing is 0.37 nm or more, and the true density is 1. 70 g /
Materials exhibiting physical property parameters of less than ³ cm ³ and having no exothermic peak at 700 ° C. or higher in differential thermal analysis (DTA) in air are suitable.

A typical non-graphitizable carbon material is a furan resin which is a homopolymer or copolymer of furfuryl alcohol or furfural, or a furan resin formed by copolymerization with another resin, which is carbonized by firing.

Further, as an organic material as a starting material, a conjugated resin such as phenol resin, acrylic resin, vinyl halide resin, polyimide resin, polyamideimide resin, polyamide resin, polyacetylene, poly (p-phenylene), or cellulose is used. And its derivative, and any organic polymer compound can be used.

Further, a petroleum pitch having a specific H / C atomic ratio with a functional group containing oxygen introduced (so-called oxygen cross-linking) is carbonized in the same manner as the furan resin (400).
It becomes a non-graphitizable carbon material in the solid state without melting at temperatures above ℃.

Here, the petroleum pitch is distilled (vacuum distillation, atmospheric distillation, distillation from tars, asphalt, etc. obtained by high-temperature thermal decomposition of coal tar, ethylene bottom oil, crude oil, etc.).
Steam distillation), thermal polycondensation, extraction, chemical polycondensation and other operations. At this time, the H / C atomic ratio of petroleum pitch is important, and in order to obtain non-graphitizable carbon, this H / C atomic ratio is important.
The atomic ratio must be 0.6 to 0.8.

The specific means for introducing a functional group containing oxygen into these petroleum pitches is not limited, but for example nitric acid,
Wet method using an aqueous solution of mixed acid, sulfuric acid, hypochlorous acid, etc., or dry method using oxidizing gas (air, oxygen), and reaction with solid reagents such as sulfur, ammonium nitrate, ammonium persulfate, ferric chloride, etc. Used. The oxygen content is not particularly limited, but as shown in JP-A-3-252053, it is preferably 3% or more, more preferably 5% or more. The oxygen content affects the crystal structure of the finally obtained carbon material, and when the oxygen content is within this range, the (002) plane spacing is 0.37.
The condition is that the exothermic peak does not occur at 700 ° C. or higher in DTA in air flow of 700 nm or more, and the negative electrode capacity becomes large.

Further, the compounds containing phosphorus, oxygen and carbon as the main components described in JP-A-3-137010 show the same physical parameters as those of the non-graphitizable carbon material and can be used as a negative electrode active material. is there.

Further, any other organic material can be used as a starting material as long as it becomes a non-graphitizable carbon through a solid phase carbonization process such as oxygen crosslinking treatment. The treatment method for performing the oxygen crosslinking is not limited.

To synthesize a carbon material using the above-mentioned organic materials as starting materials, for example, after carbonizing at a temperature of 300 to 700 ° C., the temperature rising rate is 1 to 100 ° C. per minute, and the reached temperature is 900.
The firing may be performed under the conditions of ˜1300 ° C. and holding time at the ultimate temperature of 0 to 30 hours. Of course, in some cases, the carbonization operation may be omitted.

The obtained carbon material is pulverized and classified to be used as a negative electrode material. This pulverization may be carried out before or after carbonization, calcination, high temperature heat treatment or during a temperature rising process.

Next, the graphite material used as the negative electrode will be described.

The graphite material has a true density of 2. lg / c
It is preferably m ³ or more, and 2. More preferably, it is 18 g / cm ³ or more.

In order to obtain such a true density, the spacing of (002) planes obtained by X-ray diffraction is less than 0.340 nm, more preferably 0.335 nm or more, 0.337.
nm or less and the C-axis crystallite thickness of the (002) plane is 1
It needs to be 4.0 nm or more.

Further, as a crystal structure parameter, G in the Raman spectrum, which is an index of microscopic structural defects, is obtained.
Value is also important. The G value is represented by the ratio of the area intensity of the signal derived from the graphite structure in the carbon material to the area intensity of the signal derived from the amorphous structure, and is an index of microscopic crystal structure defects. The G value is preferably 2.5 or more. If the G value is less than 2.5, 2. lg / cm
A true density of ³ or more may not be obtained.

In order to extend the cycle life of the battery, the bulk specific gravity of graphite, the average value of the shape parameter x (average shape parameter x _ave ) and the specific surface area are important.

That is, the bulk specific gravity of the graphite material is preferably 0.4 g / cm ³ or more from the viewpoint of achieving a long cycle life. When the negative electrode is formed by using a graphite material having a bulk specific gravity of 0.4 g / cm ³ or more, a negative electrode having a good electrode structure, in which the graphite material does not peel off from the negative electrode mixture layer, can be obtained, and the cycle life of the battery can be improved. Extend. A more preferable range of the bulk specific gravity is 0.5 g / cm ³ or more, and further 0.6 g / cm ³ or more.

Further, in order to obtain a longer cycle life, it is desirable to use graphite powder having a bulk specific gravity in this range and an average value of the shape parameter x represented by the following formula of 125 or less.

X = (W / T) × (L / T) x: shape parameter T: thickness of the thinnest part of the powder L: length of the powder in the major axis direction W: orthogonal to the major axis of the powder Length in Direction, That is, a typical shape of the graphite powder is a flat, substantially cylindrical shape or a substantially rectangular parallelepiped shape as shown in FIGS. 1 and 2.

When the thickness of the thinnest portion of this graphite powder is T, the length of the longest portion is L, and the length in the direction orthogonal to the long axis corresponding to the depth is W, L and W
The product of the values obtained by dividing each by T is the shape parameter x. The smaller the shape parameter x, the higher the height with respect to the bottom area and the smaller the flatness.

The average shape parameter x _ave is the average value of this shape parameter x, and is obtained as follows. First, the graphite sample powder was observed using an SEM (scanning electron microscope), and the longest part of the particles was ± 30 of the average particle size measured using a particle size distribution measuring device such as a laser diffraction method. Select 10 particles that are%. Then, the shape parameter x is calculated for each of the 10 selected powders, and the average value thereof is calculated.

The negative electrode composed of graphite powder having a bulk specific gravity within the above range and having an average shape parameter x _ave of 125 or less has a good electrode structure and a longer cycle life. To be A more preferable range of the average shape parameter x _ave is 2 or more and 1 15 or less,
It is more preferably 2 or more and 100 or less.

If graphite powder having a specific surface area of 9 m ² / g or less is used in addition to satisfying the above conditions, the cycle life of the battery is further extended. That is, in the graphite powder, fine particles of submicron level are usually attached, and it is considered that these fine particles influence the decrease in bulk specific gravity of the graphite powder. Here, the specific surface area of the graphite powder has a larger value as the number of adhered fine particles increases, and therefore serves as an index of the degree of adhesion of the fine particles. 9m ²
Graphite powder having a relatively small specific surface area, such as / g or less, has a small bulk of fine particles attached and a large bulk specific gravity.
Therefore, the cycle life of the battery is extended. The specific surface area is more preferably 7 m ² / g or less, further preferably 5 m ² / g or less.

Further, in order to obtain high safety and reliability as a practical battery, it is desirable that the particle size distribution of graphite powder is optimized. For example, in the particle size distribution of graphite powder obtained by the laser diffraction method, the cumulative 10% particle size is 3 μm.
m or more, the cumulative 50% particle size is 10 μm or more, and the cumulative 90% particle size is 70 μm or less. This is for the following reason.

First, when considering the packing density of the graphite powder, it is desirable that the particle size distribution has a width so that the graphite powder can be packed efficiently in the electrode. It is more advantageous that the particle size distribution is closer to the normal distribution.

However, when an abnormal situation such as overcharge occurs, heat generation tends to be accelerated if the negative electrode contains a large amount of graphite powder having a small particle size. Therefore, it is necessary to suppress the number of graphite powders having a small particle size to some extent. Further, when lithium ions are inserted between the graphite layers of the negative electrode, the crystallite expands by about 10% and presses the positive electrode and the separator in the battery. At this time, if the negative electrode contains a large amount of graphite powder having a large particle size, the influence of such crystallite expansion becomes large, and therefore initial defects such as internal short-circuiting tend to occur easily during initial charging. For this reason, the number of graphite powders having a large particle size must be suppressed to some extent in order to avoid initial failure.

The above-mentioned particle size distribution of the graphite powder is set in consideration of such influence of the particle size distribution on the battery, and the graphite powder having a large particle size to the small graphite powder is blended in a well-balanced manner. It has become. When graphite powder having such a particle size distribution is used as an active material for the negative electrode, high electrode packing property is obtained, heat is less likely to occur in an abnormal situation such as overcharge, and initial failure due to expansion of the electrode is prevented. Therefore, a battery having excellent safety and reliability can be obtained. Furthermore, when the cumulative 90% particle size of the graphite powder is 60 μm or less, the initial defects are greatly reduced, and the reliability of the battery is further improved.

In order to improve the heavy load characteristics of the battery, the average breaking strength of the graphite powder is 6.0 kgf / m.
m ² or more.

That is, the easiness of movement of electrolyte ions at the time of discharge affects the load characteristics of the battery. For example, if there are many holes in the electrode, the electrode is sufficiently impregnated with the electrolytic solution and the dynamics of the electrolyte ions are improved, so that the load characteristics are improved.

On the other hand, in the graphite material, a carbon hexagonal mesh plane is developed in the a-axis direction, and the stacking thereof forms a c-axis crystallite. Then, electrolyte ions are doped / undoped between the stacked layers.

The bond between the carbon hexagonal mesh planes of such a graphite material is a weak bond called van der Waals force, and is therefore easily deformed by stress. Therefore, the gap between the carbon hexagonal mesh planes is highly likely to be collapsed during compression molding in the process of manufacturing the negative electrode,
For this reason, it is usually more difficult for graphite materials to maintain pores than non-graphitizable carbon materials. Therefore, in the graphite material,
It is desirable to select and use those whose pores are difficult to be crushed by controlling the breaking strength. The above-mentioned average value of the breaking strength, that is, the range of 6 kgf / mm ² or more is set from this point, and by using graphite having such breaking strength, it is possible to maintain pores in the negative electrode. Therefore, a battery having excellent load characteristics can be obtained.

The average value of the breaking strength is measured as follows. First, the sample powder is observed with an optical microscope, and the length of the longest part is ± 10 of the average particle size.
Select 10 powders that are%. Then, a load is applied to each of the selected 10 sample powders to measure the breaking strength of the particles, and the average thereof is calculated.

The graphite material used as the negative electrode may be natural graphite produced as a mineral, as long as it satisfies the above conditions, and it is synthesized by carbonizing an organic material and further treating it at a high temperature. Artificial graphite may be used.

As the organic material used as a starting material for producing the above-mentioned artificial graphite, any of the organic materials exemplified above as the starting material of the flaky graphite used as a conductive agent can be used. Further, the heat treatment conditions for the organic material as the starting material may basically follow the above-mentioned conditions. However, the characteristics required for the negative electrode active material are different from the characteristics required for the conductive agent, and detailed conditions for producing graphite as the negative electrode active material are the crystallinity, the true density, the bulk specific gravity, and the shape parameter x as described above. , Specific surface area, particle size distribution, and particle breaking strength must be set. The obtained graphite material is used for the negative electrode in a crushed and classified form, but the crushing is performed before graphitization, such as before and after carbonization, calcination, or during the temperature rising process before graphitization. Is desirable.

However, when a graphite material is produced with an emphasis on bulk specific gravity and breaking strength, a carbon material molded body is prepared,
It is preferable to heat-treat this to graphitize it.

The carbon material molded body is obtained by mixing and molding coke serving as a filler and a binder pitch serving as a molding agent and a sintering agent, and heat-treating the molded body (precursor of the carbon material molded body). The binder pitch is carbonized. This carbon material molded body is impregnated with molten pitch to be carbonized, and further heat-treated to perform graphitization. Then, the obtained graphitized molded body is crushed and classified.

The graphite powder thus obtained has a high bulk specific gravity and a high breaking strength because it is obtained by crushing a graphite molding having high hardness. For this reason,
There is no crushing of pores during compression molding, and no peeling of the graphite powder from the negative electrode mixture during charge / discharge.

In addition, since coke which is a filler and binder pitch are used as starting materials in the process of producing this graphite, it is graphitized as a polycrystal, and further, sulfur and nitrogen contained in the raw material become gas during heat treatment. Then
The path eventually becomes a microscopic hole. In graphite generated with such microscopic holes, the holes can also serve as a storage place for lithium, so that the doping / dedoping reaction of lithium easily proceeds. Therefore, the negative electrode composed of such graphite powder exhibits extremely excellent performance.

Further, the production method of producing from such a carbon molded body has a high yield and is industrially advantageous.

The carbon molded body may be made of a raw material in which the filler itself has moldability and sinterability.

Next, as the electrolytic solution used in the non-aqueous electrolytic solution secondary battery, a non-aqueous electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent is used.

As the non-aqueous solvent, it is premised that a solvent having a relatively high dielectric constant such as ethylene carbonate (EC) is used as a main solvent, but a low viscosity is used for the purpose of enhancing the electrolyte ion transporting ability and low temperature characteristics. It is desirable to use a solvent together.

As the high dielectric constant solvent, propylene carbonate (PC), butylene carbonate, vinylene carbonate, sulfolanes, butyrolactones, valerolactones and the like are preferable in addition to EC.

As the low-viscosity solvent, symmetrical or asymmetrical chain ester carbonate such as diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate and methyl propyl carbonate is suitable. These low-viscosity solvents may be mixed alone in the high-dielectric constant solvent, or two or more kinds may be combined and mixed in the high-dielectric solvent.

Here, especially when a graphite material is used for the negative electrode, attention should be paid to the selection of the high dielectric constant solvent. That is, when a graphite material is used for the negative electrode, it is necessary to avoid using PC, for example, because PC has reactivity with the graphite material.
Etc. are preferably used as the main solvent. A compound having a structure in which a hydrogen atom of EC is replaced with a halogen element can also be used as a main solvent.

However, even if it is reactive with a graphite material such as PC, as a second component solvent, a part of a main solvent such as EC or a compound in which a hydrogen atom of EC is replaced by a halogen element is used. It can be used if it is used to replace a small amount of. The second component solvent is PC
In addition, butylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxymethane, γ-butyrolactone,
Valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl,
1,3-dioxolane, sulfolane, methylsulfolane and the like can be mentioned. The addition amount of these second component solvents is preferably suppressed to less than 10% by volume.

As the electrolyte soluble in the non-aqueous solvent, any of those usually used in this type of battery can be used.

Specifically, LiPF ₆ , LiClO ₄ , L
iAsF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO
₃ Li, CF ₃ SO ₃ Li, LiN (CF ₃ SO ₂ ) ₂ , Li
C (CF ₃ SO ₂ ) ₃ , LiCl, LiBr and the like are mentioned, and LiPF ₆ is particularly preferable. These electrolytes may be used alone or in combination of two or more.

[0105]

EXAMPLES Examples of the present invention will be described based on experimental results.

Structure of Battery Produced FIG. 3 shows the structure of the battery produced in each experimental example described later.

In this non-aqueous electrolyte secondary battery, as shown in FIG. 3, a negative electrode 1 formed by coating a negative electrode current collector 10 with a negative electrode active material and a positive electrode current collector 11 coated with a positive electrode active material. The positive electrode 2 and the positive electrode 2 are wound around the separator 3 and the insulator 4 is placed on the upper and lower sides of the wound body and housed in the battery can 5.

A battery lid 7 is attached to the battery can 5 by caulking it through a sealing gasket 6, and is electrically connected to the negative electrode 1 or the positive electrode 2 via a negative electrode lead 12 and a positive electrode lead 13, respectively. It is configured to function as a negative electrode or a positive electrode.

In the battery of this embodiment, the positive electrode lead 13 is welded and attached to the safety valve device 8 having a current cutoff mechanism, and electrically connected to the battery lid 7 via the safety valve device 8 and the PTC element 9. Connection is made.

In the battery having such a structure, when the pressure inside the battery rises, the safety valve device 8 is pushed up and deformed. Then, the positive electrode lead 13 is cut off leaving the portion welded to the safety valve device 8 and the current is cut off.

Experimental Example 1 In this experimental example, the effect of using a mixture of scaly graphite and fibrous graphite as the conductive agent of the positive electrode was examined when a non-graphitizable carbon material was used as the negative electrode active material.

First, the negative electrode active material of the negative electrode 1 was synthesized as follows.

0.5 parts by weight of 85% phosphoric acid and 10 parts by weight of water were mixed with 100 parts by weight of furfuryl alcohol.
A viscous polymer (furfuryl alcohol resin (PFA)) was obtained by heating this on a hot water bath for 5 hours. After the residual water and unreacted alcohol were removed by vacuum distillation, the furfuryl alcohol resin obtained was placed under a nitrogen stream at a temperature of 5
It was carbonized and crushed at 00 ° C. for 5 hours. Then, the pulverized product was heated to 1200 ° C., heat-treated for 1 hour, and further pulverized to obtain a carbon material powder (non-graphitizable carbon material) having an average particle diameter of 20 μm. As a result of X-ray diffraction measurement of the non-graphitizable carbon material, the (002) plane spacing was 0.383 nm. Further, the true specific gravity by Pycnometer method was 1.52 g / cm ³ , and the peak temperature of oxidation exothermic oxidation in differential thermal analysis in an air stream was 634 ° C.

Next, a negative electrode 1 was prepared using the non-graphitizable carbon powder thus obtained as a negative electrode active material.

First, 90 parts by weight of the above non-graphitizable carbon powder and polyvinylidene fluoride (PVDF) 1 as a binder.
A negative electrode mixture was prepared by mixing 0 parts by weight, and the negative electrode mixture was dispersed in N-methylpyrrolidone as a solvent to prepare a negative electrode mixture slurry (paste form).

Then, the negative electrode mixture slurry was applied on both sides of a strip-shaped copper foil having a thickness of 10 μm to be the negative electrode current collector 10, dried, and then compression-molded at a constant pressure to prepare a strip-shaped negative electrode 1. did.

On the other hand, a positive electrode active material for the positive electrode 2 was prepared as follows.

0.5 mol of lithium carbonate and 1 cobalt carbonate
The moles were mixed and calcined in air at a temperature of 900 ° C. for 5 hours. As a result of X-ray diffraction measurement of the obtained material, it was in good agreement with the peak of LiCoO ₂ registered in the JCPDS file.

LiCoO ₂ thus obtained was
It was pulverized so that the cumulative 50% particle diameter obtained by the laser diffraction method was 15 μm.

Next, using the obtained LiCoO ₂ powder as a positive electrode active material, a positive electrode 2 was prepared.

First, 95 parts by weight of LiCoO ₂ powder and 5 parts by weight of lithium carbonate powder were mixed. And 9 l parts by weight of this mixture and flake graphite (made by Lonza Co.
A positive electrode mixture was prepared by mixing 6 parts by weight of a mixture of product name KS-15) and fibrous carbon (product name VGCF manufactured by Showa Denko KK) and 3 parts by weight of polyvinylidene fluoride serving as a binder. In addition, the mixing amount of each of flake graphite and fibrous carbon was changed as shown in Table 1. Then, this positive electrode mixture was dispersed in N-methylpyrrolidone to obtain a positive electrode mixture slurry (paste form).

Then, the positive electrode mixture slurry was uniformly applied on both sides of a 20 μm-thick band-shaped aluminum foil which will be the positive electrode current collector 11, dried, and then compression molded to form the band-shaped positive electrode 2. .

The strip-shaped negative electrode 1 produced as described above,
As shown in FIG. 1, the strip-shaped positive electrode 2 is laminated in the order of the negative electrode 1, the separator 3, the positive electrode 2, and the separator 3 with a separator 3 made of a microporous polypropylene film having a thickness of 25 μm, and wound many times. A spiral electrode body having a diameter of 18 mm was produced.

Then, the produced spiral electrode body was housed in a nickel-plated iron battery can 5, and insulating plates 4 were provided on the upper and lower surfaces of the spiral electrode. Then, the aluminum positive electrode lead 13 was led out from the positive electrode current collector 11 and welded to the battery lid 7, and the nickel negative electrode lead 12 was led out from the negative electrode current collector 10 and welded to the battery can 5.

Thus, 1 mol / l of LiPF ₆ was mixed in a mixed solvent of equal volume of propylene carbonate and dimethyl carbonate in the battery can 5 containing the spirally wound electrode body.
The dissolved electrolyte was injected at a ratio of. And battery can 5
And a safety valve device 8 having a current cutoff mechanism, a PTC element 9
In addition, the battery lid 7 is fixed by caulking through the insulating sealing gasket 6 whose surface is coated with asphalt, to keep the airtightness inside the battery, diameter 18 mm, height 65 mm.
Was manufactured.

Regarding the battery manufactured as described above,
The charge / discharge cycle was repeated, and the ratio of the capacity at the 100th cycle to the capacity at the second cycle (capacity retention rate) was determined.

The charge / discharge cycle test was conducted under the ambient temperature 2
Maximum charge voltage 4.2V, charge current 1A 2.5 at 5 ℃
After charging for a time, the battery was discharged under a constant current of 2000 mA to 2.75 V.

The measured capacity retention rate is shown in Table 1 together with the mixing amount of the flake graphite and the fibrous graphite. FIG. 4 shows the results of plotting the ratio of the fibrous carbon to the entire conductive agent on the horizontal axis for the capacity retention rate.

[0129]

[Table 1]

As can be seen from FIG. 4, the battery using only the flake graphite as the conductive agent gave a capacity retention ratio of only 88%, and the battery using only fibrous carbon as the conductive agent gave 80%. Only the capacity maintenance rate can be obtained.

On the other hand, in the battery in which a part of the conductive agent is replaced by fibrous carbon, the capacity retention rate of 95% at the maximum can be obtained.

From this, it can be seen that using a mixture of flake graphite and fibrous carbon as the conductive agent is effective in improving the capacity retention rate of the battery.

However, the mixing ratio of fibrous carbon is 10% by weight.
When the mixing ratio of fibrous carbon is as large as 80% by weight, on the contrary, the capacity retention ratio is not so much improved as compared with the case where scaly graphite or fibrous carbon is used alone as the conductive agent. Absent.

From this, it is understood that the weight ratio of the flake graphite and the fibrous carbon is preferably 85:15 to 25:75.

Experimental Example 2 In this experimental example, the effect of using a mixture of scaly graphite and fibrous graphite as the positive electrode conductive agent was investigated in the case of using graphite as the negative electrode material.

A battery was produced in the same manner as in Experimental Example 1 except that the graphite powder produced as described below was used as the negative electrode active material. The mixing ratio of flake graphite and fibrous carbon in the conductive agent was changed as shown in Table 2.

To 100 parts by weight of coal-based coke as a filler, 30 parts by weight of coal tar-based pitch as a binder was added and mixed at a temperature of about 100 ° C. Then, this mixture was compression molded by a press to obtain a precursor of a carbon molded body.

Next, the precursor of this carbon molded body was set to 1000.
After heat treatment at a temperature of ℃ or less to form a carbon material molded body, 2
The binder pitch melted at a temperature of 00 ° C. or lower was impregnated, and heat treatment was performed at a temperature of 1000 ° C. or lower. Then, this pitch impregnation / firing process was repeated several times to obtain a carbon molded body. Then, this carbon molded body was heat-treated at a temperature of 2600 ° C. in an inert atmosphere to obtain a graphitized molded body, and crushed and classified to prepare graphite powder.

The graphite powder thus obtained was subjected to X-ray diffraction measurement, and as a result, the (002) plane spacing was 0.337.
nm, the C-axis crystallite thickness of the (002) plane was 50.0 nm. The true specific gravity by the pycnometer method is 2.23.
g / c ³ , bulk specific gravity of 0.83 g / cm ³ , average shape parameter x _ave of 10, specific surface area by BET method of 4.4 m ^2.
/ G. Furthermore, the particle size distribution by the laser diffraction method is
The average particle size is 3 l. 2 μm, cumulative 10% particle size is 12.3 μ
m, the cumulative 50% particle size is 29.5 μm, and the cumulative 90% particle size is 53.7 μm. The average value of the breaking strength is 7. l
kgf / mm ² . The bulk specific gravity, average shape parameter x _ave and breaking strength were measured as follows.

<Measurement Method of Bulk Specific Gravity> Bulk specific gravity is JISK-1
469. That is, a graduated cylinder having a capacity of 100 cm ³ whose mass has been measured in advance is tilted, and 100 cm ³ of the sample powder is gradually charged into the graduated cylinder. Then, the total mass of the graduated cylinder and the sample powder is measured with a minimum scale of 0.1 g, and the weight W of the sample powder is obtained by subtracting the mass of the graduated cylinder from the mass. Next, a cork stopper is put on the graduated cylinder in which the sample powder is put, and the graduated cylinder in that state is dropped 50 times from the height of about 5 cm with respect to the rubber plate. As a result, since the sample powder in the measuring cylinder is compressed, the volume V of the compressed sample powder is read. Then, the bulk specific gravity (g / cm ³ ) is calculated by the following formula.

D = W / V D: Bulk specific gravity (g / cm ³ ) W: Mass of sample powder in graduated cylinder (g) V: Volume of sample powder in graduated cylinder after dropping 50 times (cm ³ ) <Average shape parameter xave measuring method: SEM method> The sample powder is observed using an SEM (scanning electron microscope),
Ten powders are selected such that the length L of the longest part is ± 30% of the average particle size. And selected 10
For each of the sample powders, the thickness T of the thinnest portion, the length L of the longest portion, and the length W in the direction orthogonal to the long axis corresponding to the depth are measured, and based on the following equation: The shape parameter was obtained by calculating the average value. This average value was used as the average shape parameter x _ave .

X: (W / T) × (L / T) x: shape parameter T: thickness of the thinnest part of the powder L: length of the powder in the major axis direction W: orthogonal to the major axis of the powder Length in direction <Measurement method of average value of fracture strength> The fracture strength was measured using a micro compression tester (trade name: MCTM-500 manufactured by Shimadzu Corporation). First, a sample powder is observed with an optical microscope attached to a tester, and 10 powders are selected so that the length of the longest part is ± 10% of the average particle size. Then, a load is applied to each of the selected 10 sample powders to measure the breaking strength of the particles, and the average thereof is calculated. The calculated average value is used as the average value of breaking strength.

Regarding the battery manufactured as described above,
The charge / discharge cycle was repeated, and the ratio of the capacity at the 100th cycle to the capacity at the second cycle (capacity retention rate) was determined. The measurement results of the capacity retention rate are shown in Table 2 together with the mixing amount of the flake graphite and the fibrous graphite. FIG. 5 shows the result of plotting the ratio of the fibrous carbon to the entire conductive agent on the horizontal axis for the capacity retention rate.

[0144]

[Table 2]

As can be seen from FIG. 5, only 80% of the capacity maintenance ratio was obtained in the battery using only flake graphite as the conductive agent, and 70% in the battery using only fibrous carbon as the conductive agent. Only the capacity maintenance rate can be obtained.

On the other hand, in the battery in which a part of the conductive agent is replaced with fibrous carbon, the capacity retention rate of 91% at the maximum can be obtained.

From this, it is understood that the effect of using the mixture of flake graphite and fibrous carbon as the conductive agent is more remarkable when graphite is used as the negative electrode active material.

However, the mixing ratio of fibrous carbon is 10% by weight.
When the mixing ratio of fibrous carbon is as large as 80% by weight, on the contrary, the capacity retention ratio is not so much improved as compared with the case where scaly graphite or fibrous carbon is used alone as the conductive agent. Absent.

From this, it can be seen that even when graphite is used as the negative electrode material, the mixing ratio of flake graphite and fibrous carbon is 85:15 to 25:75 by weight.

Experimental Example 3 In this experimental example, the ratio of the conductive agent contained in the positive electrode mixture was examined.

Same as Experimental Example 2 except that the mixture amount of LiCoO ₂ powder and lithium carbonate powder, scaly graphite, fibrous carbon and polyvinylidene fluoride as the positive electrode mixture was changed as shown in Table 3. Then, a non-aqueous electrolyte secondary battery was produced.

The battery thus prepared was repeatedly charged and discharged, and the ratio of the capacity at the 100th cycle to the capacity at the second cycle (capacity retention rate) was determined. Table 3 shows the measurement results of the capacity and capacity retention ratio in the second cycle together with the composition of the positive electrode mixture. Regarding the capacity retention rate, the result of plotting the ratio of the conductive agent (scaly graphite and fibrous carbon) to the whole positive electrode mixture on the horizontal axis is shown in FIG. FIG. 7 shows the result of plotting the ratio of X-axis on the horizontal axis.

[0153]

[Table 3]

As can be seen from FIGS. 6 and 7, the capacity retention ratio and the capacity at the second cycle change depending on the ratio of the conductive agent to the entire positive electrode mixture. In any case, when the ratio of the conductive agent is lower than 3% by weight, or conversely exceeds 16% by weight, a sufficient value cannot be obtained. From this, it is understood that the proportion of the conductive agent to the whole positive electrode mixture is appropriately 3 to 16% by weight.

Experimental Example 4 In this Experimental Example, the effect of each of the fibrous carbons mixed in the conductive agent was changed and the effects were compared.

Experimental example except that the mixing amounts of the flake graphite and the fibrous carbon in the positive electrode mixture were fixed to 4% by weight and 2% by weight, respectively, and the type of the fibrous carbon was changed as shown in Table 4. A non-aqueous electrolyte secondary battery was produced in the same manner as in 2. The fibrous carbon used was a VGCF carbonized product (Showa Denko KK), a glass car carbonized product GWV-1A (manufactured by Nikkiso Co., Ltd.), a glass car graphitized product GWH-1A (manufactured by Nikkiso Co., Ltd.), T-. There are four types, 300 (manufactured by Toray).

The battery thus prepared was repeatedly charged and discharged, and the ratio of the capacity at the 100th cycle to the capacity at the second cycle (capacity retention rate) was determined. The measurement results of the capacity retention rate are shown in Table 4 together with the type of fibrous graphite.
In addition, as a comparison, Table 4 also shows the capacity retention ratios of the batteries produced using scaly graphite alone as the conductive agent.

[0158]

[Table 4]

As can be seen from Table 4, the batteries using the mixture of the flake graphite and the fibrous carbon as the conductive agent have a large capacity retention rate as compared with the battery using the flake graphite alone as the conductive agent. can get.

From this, it was confirmed that the effect of mixing the fibrous carbon with the conductive agent is exhibited regardless of the kind of the fibrous carbon.

[0161]

As is clear from the above description, in the non-aqueous electrolyte secondary battery of the present invention, since the mixture of flake graphite and fibrous carbon is used as the positive electrode conductive agent, the heavy load discharge condition is satisfied. The electrode structure of the positive electrode is not destroyed even when used in
Long cycle life is obtained. Therefore, the present invention can greatly contribute to improving the practicality of the non-aqueous electrolyte secondary battery.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of the particle shape of graphite powder.

FIG. 2 is a schematic view showing another example of the particle shape of graphite powder.

FIG. 3 is a vertical cross-sectional view showing an example of a non-aqueous electrolyte secondary battery to which the present invention is applied.

FIG. 4 is a characteristic diagram showing a relationship between a mixing ratio of fibrous carbon in a conductive agent and a capacity retention ratio of a battery when a non-graphitizable carbon material is used as a negative electrode active material.

FIG. 5 is a characteristic diagram showing a relationship between a mixing ratio of fibrous carbon in a conductive agent and a capacity retention ratio of a battery when a graphite material is used as a negative electrode active material.

FIG. 6 is a characteristic diagram showing the relationship between the content rate of the conductive agent and the capacity retention rate of the battery with respect to the entire positive electrode mixture.

FIG. 7 is a characteristic diagram showing the relationship between the content of a conductive agent in the whole positive electrode mixture and the capacity in the second cycle of the battery.

[Explanation of symbols]

 1 negative electrode 2 positive electrode

Claims (6)

[Claims] 1. A non-aqueous electrolyte solution 2 comprising a negative electrode mainly composed of a carbon material capable of doping and de-doping lithium, a positive electrode, and a non-aqueous electrolyte solution in which an electrolyte is dissolved in a non-aqueous solvent. A secondary battery, wherein the positive electrode contains a mixture of flake graphite and fibrous carbon as a conductive agent. 2. The non-aqueous electrolyte secondary according to claim 1, wherein the mixing ratio of the flake graphite and the fibrous carbon contained in the positive electrode is 85:15 to 25:75 by weight. battery. 3. The positive electrode comprises a positive electrode mixture comprising a positive electrode active material, a mixture of scaly graphite and fibrous carbon, and a binder, and the scaly graphite and fibrous carbon contained in the positive electrode mixture. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the mixture is 3 to 16% by weight. 4. The positive electrode is provided with LiMO as a positive electrode active material.
₂ (However, M is Co, Ni, Mn, Fe, Al, V, T
The lithium-transition metal composite oxide represented by (representing at least one kind of i) is contained, The non-aqueous electrolyte secondary battery according to claim 1. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon material of the negative electrode is a graphite material. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbon material of the negative electrode is a non-graphitizable carbon material.