JPH10270080A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Abstract] [Problem] Small capacity deterioration due to repeated charge and discharge,
Provided is a non-aqueous electrolyte secondary battery having a long charge-discharge cycle life and a large cycle accumulated capacity. SOLUTION: A non-aqueous electrolyte contains 5-300 Na ions.
It is contained at a concentration of 0 ppm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, remarkable progress in electronic technology has enabled electronic devices to become smaller and lighter one after another. Along with this, there is an increasing demand for a battery as a portable power supply to be smaller, lighter and have a higher energy density.

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

On the other hand, recently, research and development of a non-aqueous electrolyte secondary battery using lithium or a lithium alloy for a negative electrode has been actively conducted. This battery has a high energy density by using a Li-containing composite oxide typified by, for example, LiCoO ₂ for the positive electrode material, and has excellent characteristics such as low self-discharge and light weight.

However, a non-aqueous electrolyte secondary battery using such a lithium or lithium alloy for the negative electrode is:
As the charge / discharge cycle progresses, lithium may grow in a dendrite shape during charging, reach the positive electrode, and cause an internal short circuit. In addition, since the generation of dendrite is promoted, there is a problem that practical rapid charging and discharging cannot be performed. For this reason, practical use of a non-aqueous electrolyte secondary battery using this lithium or lithium alloy for the negative electrode has been distant.

In order to solve such a problem, a so-called rocking-chair type non-aqueous electrolyte secondary battery (which uses a layered compound such as an oxide or carbon into which lithium ions are incorporated) as a negative electrode material has been proposed. Lithium-ion secondary batteries) are attracting attention.

This non-aqueous electrolyte secondary battery utilizes the doping / dedoping of lithium ions between layers of such a layered compound for a negative electrode reaction. No dendrite-like precipitation is observed, showing good charge / discharge cycle characteristics.

[0008] By the way, there are various carbon materials which can be used as a negative electrode material of a non-aqueous electrolyte secondary battery, and those which were first put to practical use as a negative electrode material include hardly graphitizable carbon such as coke and glassy carbon. A material, that is, a carbon material with low crystallinity obtained by heat-treating an organic material at a relatively low temperature. Non-aqueous electrolyte secondary batteries in which a negative electrode composed of these non-graphitizable carbon materials and an electrolyte mainly containing propylene carbonate (PC) as a main solvent have been commercialized.

Further, recently, graphites having a developed crystal structure can be used as a negative electrode material. In the case of graphites, PC used as a main solvent is decomposed, and this has been an obstacle to use as a negative electrode material. However, highly stable ethylene carbonate (E
By using C) as the main solvent, such a problem is also solved.
It can be used as a negative electrode material.

[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 the non-graphitizable carbon material. Therefore, if the negative electrode is made of graphite, a high electrode filling property can be obtained,
The energy density of the battery will be increased. From this, it can be said that graphites are promising materials as negative electrode materials.

[0011]

The lithium ion secondary battery using a carbon material for the negative electrode has an improved cycle life as compared with a secondary battery using metal lithium or a lithium alloy for the negative electrode. .

However, recently, from the viewpoint of global environment and energy saving, the demand for secondary batteries has been further increased.

For example, in addition to a large capacity in one charge / discharge, an improvement in the accumulated energy during a battery cycle is also required. In particular, a secondary battery used for an automobile or the like is required to have a service life equal to or longer than that of a vehicle body, and further improvement in cycle life is desired.

In view of the above, the charge / discharge cycle life of the conventional lithium ion secondary battery cannot be said to be sufficient, and further improvement is required.

Therefore, the present invention has been proposed in view of such a conventional situation. By improving the charge / discharge cycle life, a single charge / discharge capacity is increased, and the cycle accumulated capacity is further increased. It is an object to provide a large non-aqueous electrolyte secondary battery.

[0016]

In order to achieve the above-mentioned object, a non-aqueous electrolyte secondary battery of the present invention comprises an anode, a cathode and a non-aqueous solvent capable of doping / dedoping lithium. Is a non-aqueous electrolyte secondary battery having a non-aqueous electrolyte in which is dissolved, wherein the non-aqueous electrolyte contains Na ions at a concentration of 5 to 3000 ppm. Things.

In a non-aqueous electrolyte secondary battery, when the non-aqueous electrolyte contains Na ions at a concentration of 5 to 3000 ppm, capacity deterioration due to repeated charge / discharge is suppressed, and the charge / discharge cycle life is prolonged. The accumulated capacity is improved.

[0018]

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 comprises a negative electrode capable of doping / dedoping lithium, a positive electrode, and a non-aqueous electrolyte in which an electrolyte is dissolved in a non-aqueous solvent. You.

In this non-aqueous electrolyte secondary battery, the non-aqueous electrolyte particularly contains Na ions at a concentration of 5 to 3000 ppm. At this concentration, N
When the a ion is contained, the charge / discharge cycle characteristics of the battery are improved. Although the detailed reason why the effect is obtained by Na ions is unknown, for example, when a carbon material is used for the negative electrode, it is considered as follows.

That is, in a negative electrode made of a carbon material, Li ions are doped between carbon layers of the negative electrode during charging to form a graphite intercalation compound or a carbon intercalation compound. At the time of discharge, the doped Li ions are extracted from the carbon layer. As described above, although the negative electrode is doped and undoped with lithium, a phenomenon in which the negative electrode expands and contracts during the doping and undoping of lithium is observed. The expansion and contraction of the negative electrode deteriorates the electrode structure and causes the charge / discharge cycle life of the battery to be shortened.

When Na ions are present in the electrolyte, Na is the same alkali metal as lithium, but does not form a graphite intercalation compound or a carbon intercalation compound, but exists at the entrance between the carbon layers of the negative electrode. It is considered that they become pillars and suppress the contraction of the negative electrode. This is presumed to improve the charge / discharge cycle of the battery.

However, it is necessary that the concentration of Na ions contained in the electrolyte is 5 ppm or more and 3000 ppm or less. If the Na ion concentration is less than 5 ppm, the effect is too small, and the charge / discharge cycle of the battery cannot be sufficiently improved. When the Na ion concentration is 30
When it is higher than 00 ppm, the charge / discharge reaction is inhibited by precipitation of metallic Na and the like. Note that a more preferable range of the Na ion concentration is 10 ppm or more and 2000 ppm or less.

To obtain an electrolyte containing Na ions,
An ionic crystal containing sodium may be added to a non-aqueous solution in which an electrolyte is dissolved to prepare an electrolyte.

As the ionic crystal, any sodium salt that can be dissolved in the electrolytic solution can be used. In particular,
NaCl, NaBr, NaClO ₄ , NaBF ₄ , NaA
sF ₆ , NaB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, CF ₃ S
O ₃ Na, NaN (CF ₃ SO ₂ ) ₂ , NaC (CF ₃ S
O ₂ ) _{3 and the} like, and NaPF ₆ is particularly preferable.
When NaPF ₆ is added to the electrolytic solution, when LiPF ₆ , which is widely used as an electrolyte, is used, the same effect as when the electrolyte concentration is increased by the amount of NaPF ₆ added can be obtained. These ionic crystals may be used alone or in combination of two or more.

As the Na ion source, metallic Na can also be used. Alkali metals can be dissolved in non-aqueous solvents, as well as metal Na. Therefore, by adding metal Na to the electrolytic solution, Na ions can be generated indirectly or directly in the electrolytic solution.

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

For example, as the non-aqueous solvent, ethers such as tetrahydrofuran, ethylene glycol diethyl ether, dimethyl ether and cyclic polyether, amines such as hexamethylphosphoramide, liquid ammonia and the like can be used.

Alternatively, a solvent mainly composed of a solvent having a relatively high dielectric constant as described below and a low-viscosity solvent added thereto may be used.

Examples of the solvent having a high dielectric constant include propylene carbonate (P) in addition to ethylene carbonate (EC).
C), butylene carbonate (BC), vinylene carbonate (VC), sulfolanoic acid, butyrolactonic acid, valerolactones and the like can be used.

Examples of the low-viscosity solvent include symmetric chain carbonates such as diethyl carbonate and dimethyl carbonate, asymmetric chain carbonates such as methyl ethyl carbonate and methyl propyl carbonate, and carboxylic acids such as methyl propionate and ethyl propionate. Esters, and phosphate esters such as trimethyl phosphate and triethyl phosphate can be used. One of these may be used, or two or more of them may be used in combination.

However, when a graphite material is used for the negative electrode,
It is preferable to use ethylene carbonate or a compound in which a hydrogen atom of ethylene carbonate is substituted with a halogen as the main solvent, since decomposition by graphite is harder than in other high dielectric constant solvents.

Even if the material is reactive with the graphite material, such as propylene carbonate, ethylene carbonate or a halide of ethylene carbonate is used as a main solvent, and is added as a second component solvent which replaces a part of this. If it is, you can use it.

The one used as the second component solvent is
Other than propylene carbonate, butylene carbonate,
Vinylene carbonate, 1,2-dimethoxyethane,
1,2-diethoxymethane, γ-butyrolactone, valerolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolan, 4-methyl-1,
3-dioxolan, sulfolane, methylsulfolane and the like. Among them, it is preferable to use a carbonate-based solvent such as propylene carbonate, butylene carbonate, and vinylene carbonate. In addition, the addition amount of these is preferably 40% by volume or less, more preferably 20% by volume or less.

As the electrolyte salt dissolved in the non-aqueous solvent, any of those used for the main battery can be used. Specifically, LiPF ₆ , LiClO ₄ , LiAsF
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 ,
LiC (CF ₃ SO ₂ ) ₃ , LiCl, LiBr and the like can be mentioned. Even if these electrolyte salts are used alone,
A plurality of types may be used in combination. When used in combination, it is desirable that LiPF ₆ be the main component.

In order to make the electrolyte contain Na ions, in addition to directly adding the Na ion source to the electrolyte as described above, an ion crystal of Na is attached to the components of the battery other than the electrolyte. In advance, this may be performed by eluting this into the electrolytic solution. The constituent elements of the battery containing the Na ion crystals include resin materials such as separators and insulators, metal materials such as battery cans and leads, and positive electrode materials mainly composed of a lithium transition metal composite oxide. .

As described above, in the present invention, an electrolytic solution containing Na ions is used, but the following are used as the negative electrode and the positive electrode.

First, as a negative electrode material, a carbon material capable of doping / dedoping lithium ions or a crystalline or amorphous metal chalcogenide is used.

Among these, as the carbon material, a graphitizable carbon material, a non-graphitizable carbon material, and a graphite material can be used.

As the non-graphitizable carbon material, (002)
Plane spacing 0.37 nm or more, true density 1.70 g / cm
Less than ³ , 7 in differential thermal analysis (DTA) in air
Materials having physical property parameters such as not having an exothermic peak above 00 ° C. are preferred.

Such a non-graphitizable carbon material can be obtained by heat-treating an organic material at a low temperature of, for example, about 1000 ° C.

Representative starting materials include furfuryl alcohol and furfural homopolymers and copolymers, and furan resins copolymerized with other resins.

Further, a phenol resin, an acrylic resin,
Vinyl halide resin, polyimide resin, polyamide imide resin, polyamide resin, polyacetylene, poly (p
Conjugated resins such as -phenylene), cellulose and derivatives thereof, and any organic polymer compounds can be used.

In addition, a functional group containing oxygen is introduced into a petroleum pitch having a specific H / C atomic ratio (so-called oxygen crosslinking).
The carbonized product (4
(00 ° C. or higher), and eventually becomes a non-graphitizable carbon material in a solid state.

The above-mentioned petroleum pitch includes tars obtained by high-temperature pyrolysis of coal tar, ethylene bottom oil, crude oil and the like,
It can be obtained from asphalt or the like by operations such as distillation (vacuum distillation, atmospheric distillation, and steam distillation), thermal polycondensation, extraction, and chemical polycondensation. At this time, the H / C atomic ratio of the petroleum pitch is important, and the H / C atomic ratio needs to be 0.6 to 0.8 in order to obtain non-graphitizable carbon.

The specific means for forming an oxygen bridge in these petroleum pitches is not limited.
Wet method using aqueous solution of mixed acid, sulfuric acid, hypochlorous acid, etc., dry method using oxidizing gas (air, oxygen), and reaction with solid reagents such as sulfur, ammonium nitrate, ammonium persulfate, ferric chloride, etc. Can be

Although the oxygen content is not particularly limited,
As described in JP-A-3-252553, it is preferably at least 3%, more preferably at least 5%.
This oxygen content affects the crystal structure of the finally manufactured carbon material, and when the oxygen content is within this range, the (002) plane spacing is 0.37 nm or more, and the air flow is In differential thermal analysis (DTA)
It has physical property parameters such as not having an exothermic peak at 0 ° C. or higher, and the negative electrode capacity is improved.

The starting material is not limited to these, and any other organic material can be used as long as it becomes a non-graphitizable carbon material through a solid phase carbonization process by an oxygen crosslinking treatment or the like. .

Further, in addition to the non-graphitizable carbon materials using the above-mentioned organic materials as starting materials,
The compound mainly containing phosphorus, oxygen, and carbon described in Japanese Patent Application Laid-Open Publication No. H10-284, also shows the same physical property parameters as the non-graphitizable carbon material, and is preferable as the material for the negative electrode.

The non-graphitizable carbon material can be obtained by carbonizing the above organic material by firing or the like. This firing is desirably performed by the following process.

That is, to synthesize a non-graphitizable carbon material, an organic material is carbonized at a temperature of 300 to 700 ° C.
Heating rate 1-100 ° C per minute, ultimate temperature 900-1300
The sintering is performed under the conditions of 0 ° C. and a holding time at the ultimate temperature of about 0 to 30 hours. In some cases, the carbonization operation may be omitted. The sintered body thus obtained is then pulverized and classified and provided to a negative electrode. This pulverization may be performed before or after carbonization, calcination, or high-temperature heat treatment during the heating process. You can go there.

Next, the graphite material has a true density of 2.1.
g / cm ³ or more, preferably 2.18 g / cm ³
More preferably. In order to obtain such a true density, the (002) plane spacing measured by X-ray diffraction is preferably less than 0.340 nm, more preferably 0.33 nm.
It is necessary to be 5 nm or more and 0.337 nm or less, and it is necessary that the C-axis crystallite thickness of the (002) plane is 14.0 nm or more.

In the graphite material, in addition to the above-described true density and crystal structure parameters, characteristics such as bulk density, average shape parameter x _ave , specific surface area, particle size distribution, and particle breaking strength are also important. Next, these characteristics will be described.

First, the bulk density is measured according to the method described in JIS K-1469. This measuring method will be described below.

<Method of Measuring Bulk Density> A measuring cylinder having a capacity of 100 cm ^3, whose mass has been measured in advance, is slanted, and 100 cm ³ of sample powder is gradually poured into the measuring cylinder using a spoon. Then, the total mass is measured at the minimum scale of 0.1 g, and the mass M of the sample powder is obtained by subtracting the mass of the measuring cylinder from the mass.

Next, the graduated cylinder into which the sample powder has been charged is cork-stopped, and the graduated cylinder in that state is dropped 50 times from a height of about 5 cm on 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 density (g / cm ³ ) is calculated by the following equation 1.

D = M / V Formula 1 D: Bulk density (g / cm ³ ) M: Mass of sample powder in graduated cylinder (g) V: Sample powder in graduated cylinder after falling 50 times Volume (cm ³ ) It is desirable to use a graphite material having a bulk density of 0.4 g / cm ³ or more. Since the graphite material has a scale-like shape, it is easily peeled off from the electrode, which causes a reduction in cycle life. However, with such a graphite material having a bulk density of 0.4 g / cm ³ or more, exfoliation is suppressed and cycle life is improved. In addition, a more preferable range of the bulk density is 0.5 g / cm ³ or more, and more preferably 0.6 g / cm ³ or more.

Next, the average shape parameter x _ave is obtained as follows.

<Average shape parameter x _ave : SEM method>
That is, the typical particle shape of the graphite material is a flat columnar shape or a rectangular parallelepiped shape as shown in the schematic diagram of FIG. 1 or FIG. When the thickness of the thinnest part of the graphite particles is T, the length of the longest major axis direction is L, and the length in the direction orthogonal to the major axis corresponding to the depth is W, L
And the product of W and T divided by T is the shape parameter x. The smaller the shape parameter x, the higher the height relative to the bottom area and the smaller the flatness.

X = (W / T) × (L / T) Formula 2 x: Shape parameter T: Thickness of the thinnest portion of powder L: Length of powder in the major axis direction W: Length in the direction orthogonal to the major axis of the powder To measure such a shape parameter x for an actual graphite powder, the shape of the graphite powder is observed using an SEM (scanning electron microscope), and the length is measured. Are selected such that the length of the long part is ± 30% of the average particle size. Then, for each of the selected ten powders, the shape parameters are calculated according to Equation 2, and the average is calculated. The calculated average value is the average shape parameter x _ave
It is.

The average shape parameter x _ave is desirably 125 or less. When a graphite powder having a low flatness such that the bulk density is within the above range and the average shape parameter x _ave thus obtained is 125 or less is used, the structure of the electrode is further improved, and the graphite powder is improved. Is difficult to peel off. As a result, the cycle life is further improved. Note that the average shape parameter x
The more preferable range of _ave is 2 or more and 115 or less, and further 2 or more and 100 or less.

Subsequently, the specific surface area of the graphite powder is determined by the nitrogen adsorption BET method, and is preferably 9 m ² / g or less. Bulk density, average shape parameter x
_ave satisfies the above conditions and the specific surface area is 9 m ²
The cycle life of the battery is further improved by using a graphite powder of not more than / g.

The reason why the regulation of the specific surface area has an effect on the cycle life of the battery is that this specific surface area reflects the adhesion of the fine particles to the graphite powder.

That is, fine particles of about submicron are often attached to the graphite powder, and it is considered that the attachment of the fine particles lowers the bulk density of the graphite material. Therefore, it is desirable that the adhesion of the fine particles to the graphite powder is as small as possible.

On the other hand, the specific surface area of the graphite powder increases as the number of fine particles increases, and decreases as the amount of fine particles decreases, as long as the particle size is the same. That is, the specific surface area is an index of the degree of adhesion of the fine particles, and the specific surface area is 9 m ² /
g means that adhesion of the fine particles is very small, a high bulk density is obtained, and a long cycle life is obtained. The specific surface area is
More preferably 7m ² / g or less, more preferably 5 m ²
/ G or less.

The particle size of the graphite powder is determined by a cumulative 10% particle size obtained from a particle size distribution diagram (horizontal axis: particle size, vertical axis: number of particles),
Optimized by cumulative 50% particle size, cumulative 90% particle size. This cumulative 10% particle size, cumulative 50% particle size, cumulative 90%
The particle size is the particle size when the area integrated from 0 μm is 10%, 50%, and 90% of the total area in the particle size distribution diagram.

Among them, the cumulative 10% particle size is 3 μm or more,
The cumulative 50% particle size is 10 μm or more, and the cumulative 90% particle size is 70 μm.
It is desirable that it is not more than μm. This is for the following reasons.

That is, in consideration of the electrode filling property, the particle size distribution of the graphite powder has a certain width on the horizontal axis (particle size). ,desirable.

However, if an abnormal situation such as overcharging occurs,
The battery can generate heat, and in such cases,
If the distribution bulk of the graphite powder having a small particle size is large, the heat generation temperature tends to increase.

On the other hand, when the battery is charged, the crystallites expand about 10% because lithium ions are inserted between the graphite layers. Then, the positive electrode and the separator are squeezed by this expansion, and an initial failure such as an internal short-circuit is likely to occur at the time of the first charge. Such a defect due to expansion becomes more remarkable as the distribution bulk of the graphite powder having a large particle size increases.

In other words, the graphite powder has a problem that there are too many particles having a small particle diameter or too many particles having a large particle diameter. Is desirable.

The above-described cumulative 10% particle size, cumulative 50% particle size,
The range of the 90% cumulative particle size is set by paying attention to these points, and the graphite material that satisfies these conditions is blended in a well-balanced manner from large to small particles. Therefore, heat generation of the battery is suppressed at the time of overcharging and the like, and initial failure is reduced, and high reliability is obtained. It is to be noted that among these cumulative particle diameters, particularly, the 90% cumulative particle diameter is desirably 60 μm or less from the viewpoint of preventing initial failure.

The particle size and the number of particles can be measured, for example, by scattering a laser beam using a Microtrac particle size analyzer.

Next, the breaking strength of the graphite powder is measured as follows. <Measurement method of average particle breaking strength> The breaking strength is measured by a Shimadzu micro compression tester (trade name: MCTM-, manufactured by Shimadzu Corporation).
500).

First, the graphite powder was observed with the attached optical microscope, and the length of the longest part was ± 1 of the average particle diameter.
Select 10 powders that are 0%. Then, a load is applied to each of the selected 10 powders to measure the breaking strength, and the average value is calculated. The calculated average value is the average particle breaking strength of the graphite powder.

In order to obtain sufficient heavy load characteristics for a practical battery, the average particle breaking strength of graphite powder is 6.0 kgf / mm.
^It is desirable that it be ² or more. Note that the following relationship exists between the breaking strength and the load characteristics.

First, the load characteristics are influenced by the ease of movement of ions during discharge.

Here, when the electrode material has many pores, the electrolyte is easily impregnated into the electrode, so that the ions can easily move and good load characteristics can be obtained. If the electrode material has few vacancies, it is difficult for ions to move, resulting in poor load characteristics.

On the other hand, a graphite material having a high crystallinity has a hexagonal network of graphite developed in the a-axis crystal direction, and a C-axis crystallite is formed by stacking. The bond between the carbon hexagonal mesh planes is a weak bond called van der Waals force, and is easily deformed by stress. For this reason, the graphite material is usually easily crushed when filling the electrode by compression molding, and it is difficult to secure holes for facilitating the movement of ions.

The breaking strength as described above is an index of the difficulty in crushing such a hole. Even if a graphite material is used with the breaking strength regulated to 6.0 kg / mm ² or more, pores are secured and good load characteristics can be obtained.

As the graphite material, those having the above-mentioned physical properties are selected and used. Even if this graphite material is natural graphite, it is an artificial material obtained by carbonizing an organic material and further performing high-temperature treatment. It may be graphite.

Coal and pitch are typical examples of organic materials used as starting materials for producing artificial graphite.

The pitch can be obtained by distillation (vacuum distillation, normal pressure distillation, steam distillation) from coal tar, ethylene bottom oil, crude oil, etc. obtained by high-temperature pyrolysis, asphalt, etc., thermal polycondensation, extraction, chemical weight There are also ones obtained by operations such as condensation, and other pitches formed during carbonization of wood materials.

Further, starting materials for forming the pitch include polyvinyl chloride resin, polyvinyl acetate, polyvinyl butyrate, 3,5-dimethylphenol resin and the like.

During the carbonization, these coals and pitches exist in a liquid state at a maximum of about 400 ° C., and when held at that temperature, the aromatic rings are condensed and polycyclic to form a laminated orientation. At the above temperature, a solid carbon precursor, that is, semi-coke is formed. Such a process is called a liquid phase carbonization process and is a typical production process of graphitizable carbon.

In addition, condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphene, pentacene, other derivatives (eg, carboxylic acids, carboxylic anhydrides, carboxylic imides, etc.) or mixtures thereof , Acenaphthylene, indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole,
Condensed heterocyclic compounds such as acridine, phenazine and phenanthridine, and their derivatives can also be used as raw materials.

In order to produce artificial graphite using the above organic material as a starting material, for example, the above organic material is carbonized in a stream of inert gas such as nitrogen at a temperature of 300 to 700 ° C., and then in a stream of inert gas. Calcination is carried out under the conditions of a heating rate of 1 to 100 ° C. per minute, a reached temperature of 900 to 1500 ° C., and a holding time at the reached temperature of about 0 to 30 hours. And heat treatment at a temperature of 2000 ° C. or higher, preferably 2500 ° C. or higher. Of course, in some cases, the carbonization and calcination operations may be omitted.

The produced graphite material is classified or pulverized.
The particles are classified and provided to the negative electrode material. The pulverization may be performed before or after carbonization or calcination, or during a temperature raising process before graphitization. In these cases, a heat treatment for graphitization is finally performed in a powder state.

However, in order to obtain a graphite powder having a high bulk density and a high breaking strength, a heat treatment is performed in the form of a raw material as a molded body.
It is desirable to pulverize and classify the obtained graphitized molded body.

That is, to produce a graphitized molded body,
A coke as a filler and a binder pitch as a molding agent or a sintering agent are mixed and molded. Then, after a heat treatment is performed on the molded body at a low temperature of 1000 ° C. or less, a pitch impregnation / firing step of impregnating with a molten binder pitch is repeated several times, and then a heat treatment is performed at a high temperature. The impregnated binder pitch is carbonized and graphitized in the above heat treatment process. Then, the obtained graphitized molded body is pulverized into graphite powder.

The pulverized powder of the graphitized molded body thus obtained has high bulk density and high breaking strength, and an electrode excellent in performance can be obtained.

Further, since filler (coke) and binder pitch are used as raw materials, the raw material is graphitized as a polycrystal, and sulfur and nitrogen contained in the raw materials are generated as a gas during heat treatment. Pores are formed. When the pores are formed, the reaction of the negative electrode, that is, the doping / dedoping reaction of lithium, easily proceeds.
In addition, there is an advantage that the processing efficiency is industrially high when the holes are open.

As the raw material of the molded body, a filler having moldability and sinterability itself may be used. In this case, it is not necessary to use a binder pitch.

As the negative electrode material, in addition to a carbon material, a metal oxide capable of doping / dedoping lithium ions can be used.

As the metal oxide, an oxide containing a transition metal is preferable. Specifically, iron oxide, ruthenium oxide, molybdenum oxide, tungsten oxide, titanium oxide,
A crystalline compound or an amorphous compound mainly composed of tin oxide, silicon oxide, or the like can be given. In addition, it is desirable to use one having a charge / discharge potential relatively close to metal Li.

As described above, the materials of the negative electrode include graphite,
A non-graphitizable carbon material, a metal oxide, or the like is used. Among them, expansion and contraction accompanying doping and undoping of lithium ions are remarkable in graphite. The effect of the present invention, that is, the effect of including Na ions in the electrolytic solution, is most exerted when such graphite is used.

Next, the material of the positive electrode will be described.

The cathode material preferably contains a sufficient amount of Li, for example, a general formula LiMO ₂ (where M represents at least one of Co, Ni, Mn, Fe, Al, V, and Ti) A composite metal oxide composed of lithium and a transition metal represented by the following formula, an intercalation compound containing Li, and the like are preferable.

In particular, in order to achieve a high capacity, the positive electrode must be in a steady state (for example, after repeating charge and discharge about five times).
It is necessary to include Li corresponding to a charge / discharge capacity of 300 mAh or more per gram of the negative electrode material, and 350 mAh
It is more preferable to include Li equivalent to the charge / discharge capacity described above.

It is not always necessary to supply all of the Li from the positive electrode, but it is essential that Li be present in the battery system in an amount corresponding to a charge / discharge capacity of 250 mAh or more per gram of the negative electrode material. The amount of Li in the battery system is determined by measuring the discharge capacity of the battery.

[0101]

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

Structure of Battery Produced FIG. 3 shows the structure of the battery fabricated in each of the examples described later.

As shown in FIG. 3, the nonaqueous electrolyte secondary battery has a negative electrode 1 formed by applying a negative electrode active material to a negative electrode current collector 10 and a positive electrode active material applied to a positive electrode current collector 11. The positive electrode 2 is wound with a separator 3 interposed therebetween, and is housed in a battery can 5 with an insulator 4 placed above and below the wound body.

A battery lid 7 is attached to the battery can 5 by caulking via 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. 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 battery cut-off thin plate 8, and the battery cover is connected to the battery cut-off thin plate 8 and the thermal resistance element (PTC element) 9. 7 is electrically connected.

In a battery having such a configuration,
When the battery temperature rises abnormally, the electric resistance of the PTC element 9 increases, and the current is cut off. Also, when the pressure inside the battery rises, the thin plate 8 such as the current interrupter is pushed up and deformed. Then, the positive electrode lead 13 is cut leaving a portion welded to the current interrupting thin plate 8, and the current is interrupted.

Example 1 First, a negative electrode active material was synthesized as follows.

Coal-based pitch as a binder was added to 100 parts by weight of coal-based coke as a filler.
After adding 0 parts by weight and mixing at a temperature of about 100 ° C., the mixture was compression-molded with a press to obtain a precursor of a carbon molded body.

Next, this precursor was heat-treated at a temperature of 1000 ° C. or less to produce a molded carbon material. Then, a pitch impregnation / firing step of impregnating the carbon material molded body with a binder pitch melted at a temperature of 200 ° C. or less and performing a heat treatment at a temperature of 1000 ° C. or less was repeatedly performed.

Thereafter, the carbon compact was heat-treated in an inert atmosphere at a temperature of 2700 ° C. to obtain a graphitized molded body, and pulverized and classified to prepare a graphite sample powder.

The physical properties of the graphite material obtained at this time are shown below.

(002) plane spacing: 0.337 nm C-axis crystallite thickness of (002) plane: 50.0 nm True density: 2.23 g / cm ³ Bulk density: 0.83 g / cm ³ Average shape parameter X _ave: 10 Specific surface area: 4.4 m ² / g Particle size: average particle size; 31.2 μm cumulative 10% particle size; 12.3 μm cumulative 50% particle size; 29.5 μm cumulative 90% particle size; 53.7 μm Average value of fracture strength: 7.1 kgf / mm ^{2 In} addition, the (002) plane spacing and the (002) plane C-axis crystallite thickness were measured by X-ray diffraction, the true density was Pycnometer method, the specific surface area was BET method, The particle size was measured from the particle size distribution by the laser diffraction method.

A negative electrode 1 was prepared using the graphite sample powder obtained as described above as a negative electrode active material.

First, 90 parts by weight of a graphite sample powder and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder are mixed to prepare a negative electrode mixture, which is dispersed in N-methylpyrrolidone as a solvent. A negative electrode mixture slurry (paste) was obtained.

Next, a negative electrode current collector 10 having a thickness of 10 μm was prepared.
The negative electrode current collector 10 was prepared by uniformly applying and drying the negative electrode mixture slurry on both surfaces of the negative electrode current collector 10, and then compression-molded at a constant pressure to prepare the negative electrode band 1.

On the other hand, the positive electrode active material was produced as follows.

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

This LiCoO ₂ was pulverized to obtain a LiCoO ₂ powder having a cumulative 50% particle size of 15 μm obtained by a laser diffraction method.

Then, 95 parts by weight of this LiCoO ₂ powder and 5 parts by weight of lithium carbonate powder were mixed, and 91 parts by weight of this mixture were used as a conductive agent, 6 parts by weight of flake graphite and polyvinylidene fluoride as a binder. The mixture was mixed with 3 parts by weight to prepare a positive electrode mixture, and dispersed in N-methylpyrrolidone to prepare a positive electrode mixture slurry (paste).

Next, as the positive electrode current collector 11, a 20 μm thick
m, and a positive electrode current collector 1 was prepared.
The positive electrode mixture slurry was uniformly applied to both surfaces of No. 1 and dried, followed by compression molding to produce a belt-shaped positive electrode 2.

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

The spiral electrode body thus manufactured is
It was stored in a nickel-plated iron battery can 5.

Then, insulating plates 4 are provided on the upper and lower surfaces of the spiral electrode.
The aluminum positive electrode lead 13 was led out from the positive electrode current collector 11 and was welded to the current interrupting thin plate 8, and the nickel negative electrode lead 12 was drawn out from the negative electrode current collector 10 and welded to the battery can 5.

On the other hand, 1 mol of LiPF ₆ was added to an equal volume mixed solvent of ethylene carbonate and dimethyl carbonate.
/ L, and NaPF ₆ was further added thereto at a concentration of 5 ppm in terms of Na to prepare an electrolytic solution. Then, the electrolytic solution was injected into the battery can 5.

Next, the battery can 5 is caulked via an insulating sealing gasket 6 coated with asphalt to fix the thin plate 8 for current cut-off, the PTC element 9 and the battery cover 7, thereby improving the airtightness in the battery. The cylindrical non-aqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced.

Examples 2 to 7 The procedure of Example 1 was repeated except that the amount of NaPF ₆ added to the electrolytic solution was set to the ppm concentration shown in Table 1 in terms of Na. A cylindrical non-aqueous electrolyte secondary battery having a height of 65 mm was produced.

Comparative Example 1 Example 1 was repeated except that NaPF ₆ was not added to the electrolyte.
A cylindrical non-aqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced in the same manner as described above.

Comparative Example 2 The amount of NaPF ₆ added to the electrolyte was 2 pp in terms of Na.
m, and a cylindrical non-aqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was prepared in the same manner as in Example 1 except that m was set to be m.

Comparative Example 3 The amount of NaPF ₆ added to the electrolyte was 500
A cylindrical non-aqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced in the same manner as in Example 1 except that the concentration was set to 0 ppm.

The non-aqueous electrolyte secondary battery produced as described above was subjected to a charge / discharge cycle test, and the capacity retention rate after 100 cycles with respect to the initial cycle [(100
(Capacity after cycle) / (initial capacity)] × 100%.

The charging and discharging were performed at a maximum charging voltage of 4.2 V,
After charging at a charging current of 1 A for 2.5 hours, a discharging current of 700
The test was performed under the condition that the battery was discharged at a constant current of 2.75 V at mA.

Table 1 shows the measurement results of the capacity retention ratio together with the Na ion concentration of the electrolytic solution. FIG. 4 shows the relationship between the Na ion concentration and the capacity retention ratio.

[0133]

[Table 1]

As shown in FIG. 4, the capacity retention rate changes depending on the Na ion concentration of the electrolytic solution, and it can be seen that the capacity retention rate of the battery can be increased by adding Na ions to the electrolytic solution.

However, the capacity retention ratio when the Na ion concentration is 2 ppm is the same as when no Na ion is contained in the electrolyte.

When the Na ion concentration is higher than 3000 ppm, the capacity retention ratio is lower than when no Na ion is contained in the electrolyte.

From this, it is appropriate that the concentration of Na ions contained in the electrolytic solution is 5 ppm to 3000 ppm.

Next, as other examples, the case where NaBF ₄ or NaCF ₃ SO ₃ was used as a sodium salt to be contained in the electrolytic solution and the case where a non-graphitizable carbon material was used as a negative electrode active material were examined. .

Example 8 As a sodium salt to be added to the electrolytic solution, NaBF ₄ was changed to N
Except that it was added so as to be 100 ppm in terms of a, the same as in Example 1 except that the diameter was 18 mm and the height was 65 mm.
Was manufactured.

Example 9 As a sodium salt to be added to the electrolytic solution, NaCF ₃ SO ₃
Of 18 mm in diameter and 65 m in height in the same manner as in Example 1 except that
m of a cylindrical nonaqueous electrolyte secondary battery.

Example 10 The same procedure as in Example 1 was carried out except that the non-graphitizable carbon material was used as the negative electrode active material, and that the amount of NaPF ₆ added to the electrolyte was set to 100 ppm in terms of sodium. Thus, a cylindrical nonaqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced.

The non-graphitizable carbon material was synthesized as follows.

A mixture of 0.5 parts by weight of 85% acetic acid and 10 parts by weight of water with respect to 100 parts by weight of furfuryl alcohol was heated on a hot water bath for 5 hours to obtain a viscous polymer (furfuryl alcohol resin). I got The remaining water and unreacted alcohol were removed by vacuum distillation.

Next, the obtained furfuryl alcohol resin (PFA) was carbonized in a nitrogen stream at a temperature of 500 ° C. for 5 hours, and then pulverized. Then, this pulverized material is heated at a temperature of 1
Heat treatment was performed at 200 ° C. for 1 hour, and further pulverized to obtain a carbon material powder having an average particle diameter of 20 μm (a non-graphitizable carbon material).

The physical properties of the non-graphitizable carbon material obtained at this time are shown below.

[002] Plane spacing of (002) plane: 0.383 nm True density: 1.52 g / cm ³ Exothermic oxidation peak temperature in differential thermal analysis in air stream:
634 ° C. The plane spacing of the (002) plane was measured by X-ray diffraction, and the true density was measured by a pycnometer method.

Comparative Example 4 Example 1 was repeated except that NaPF ₆ was not added to the electrolyte.
0, a cylindrical non-aqueous electrolyte secondary battery having a diameter of 18 mm and a height of 65 mm was produced.

For the battery fabricated as described above,
A charge / discharge cycle test was performed in the same manner as described above, and the capacity retention rate after 100 cycles with respect to the initial cycle was determined. Table 2 shows the results.

[0149]

[Table 2]

As shown in Table 2, in the batteries of Examples 8 and 9, NaBF ₄ and NaCF ₃ SO were used as electrolytes, respectively.
₃ was added, and in each case, a larger capacity retention ratio than that of Comparative Example 1 shown in Table 1 was obtained. That is, NaBF
Sodium salts such as ₄ and NaCF ₃ SO ₃ also have the effect of increasing the capacity retention of the battery, and it can be seen that this effect does not depend on the type of sodium salt.

However, when comparing the batteries of Example 4 with the Na ion concentration of 100 ppm and the batteries of Examples 8 and 9 using NaPF ₆ as the sodium salt,
Battery has a higher capacity retention rate. This indicates that NaPF ₆ is more effective than NaBF ₄ or NaCF ₃ SO ₃ .

Next, a comparison between Example 10 using the non-graphitizable carbon material as the negative electrode active material and Comparative Example 4 shows that the battery of Example 10 in which the electrolyte contained Na ions contained Na ions. A larger capacity retention ratio is obtained as compared with Comparative Example 4 in which no storage was performed.

From this, it can be seen that even when the non-graphitizable carbon material is used as the negative electrode active material, the capacity retention of the battery is increased by adding Na ions to the electrolyte.

However, when comparing the increase in the capacity retention ratio of Example 4 with respect to Comparative Example 1 and the increase in the capacity retention ratio of Example 10 with respect to Comparative Example 4, the former is larger than the latter in comparison with the latter. Is large. That is, it can be seen that the effect of including Na ions in the electrolytic solution is remarkably exhibited when graphite is used as the negative electrode active material.

[0155]

As is clear from the above description, in the non-aqueous electrolyte secondary battery of the present invention, since the electrolyte contains sodium ions at a concentration of 5 to 3000 ppm, the capacity accompanying the charge / discharge cycle is increased. Deterioration is kept small, and the charge / discharge cycle characteristics are excellent. Therefore, it is suitable as a power supply for an electric vehicle or the like which requires a large cycle accumulated capacity.

[Brief description of the drawings]

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

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

FIG. 3 is a longitudinal sectional view showing a configuration example of a nonaqueous electrolyte secondary battery to which the present invention is applied.

FIG. 4 is a characteristic diagram showing a relationship between a Na ion concentration of an electrolytic solution and a capacity retention ratio.

[Explanation of symbols]

 1 negative electrode, 2 positive electrode, 3 separator

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masayuki Nagamine 1-1-1 Shimosugishita, Takakura, Hiwada-cho, Koriyama-shi, Fukushima Prefecture Sony Energy Tech Co., Ltd.

Claims (7)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode capable of doping / dedoping lithium ions, a positive electrode, and a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent, The non-aqueous electrolyte contains 5 ppm to 3000 p of Na ions.
A non-aqueous electrolyte secondary battery characterized by being contained at a concentration of pm. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the Na ion source is NaPF ₆ . 3. The positive electrode is LiMO ₂ (where M is Co,
It represents at least one of Ni, Mn, Fe, Al, V, and Ti. The electrolyte solution is formed by dissolving a lithium salt in a non-aqueous solvent containing at least one of a cyclic carbonate and a chain carbonate. 2. The non-aqueous electrolyte secondary battery according to 1. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is made of a carbon material. 5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the carbon material is a graphite material. 6. The non-aqueous electrolyte secondary battery according to claim 4, wherein the carbon material is a non-graphitizable carbon material. 7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is made of a crystalline or amorphous metal chalcogenide.