JPH0714607A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To maintain charge/discharge reaction by using a mixture of methylethyl carbonate and dimethyl carbonate as a low viscosity solvent of an electrolyte in a nonaqueous electrolyte secondary battery using a carbon material capable of doping/undoping lithium in an negative electrode and a lithium transition composite oxide in a positive electrode. CONSTITUTION:A mixture of methylethyl carbonate and dimetyl carbonate is used as a low viscosity solvent of an electrolyte so as to obtain normal charge/discharge reaction in overcharging and even after leaving under high temperature in a charged state. When the total volume of a nonaqueous solvent is represented by T, the volume of methylethyl carbonate by M, and the volume of dimethyl carbonate by D, a mixing ratio of methylethyl carbonate and dimethyl carbonate to the total volume is specified to 2/10<=(M+D)/T<=8/10, and a mixing ratio methylethyl carbonate and dimethyl carbonate is specified to 1/9<=D/M<=8/2. If D/M is less than 1/9, effect on preventing capacity deterioration can not be obtained sufficiently, and If D/M exceeds 8/2, internal pressure may be increased when the battery is left at high temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a lithium ion system using a lithium transition metal composite oxide as a positive electrode and a lithium ion doping / dedoping carbon material as a negative electrode. The non-aqueous electrolyte secondary battery of.

[0002]

2. Description of the Related Art Conventionally, nickel-cadmium batteries, lead batteries and the like which use an aqueous electrolyte solution have been widely used as general secondary batteries. However, a camera-integrated VT
With the emergence of new portable electronic devices such as R, mobile phones, and laptop computers, etc. in recent years, even for secondary batteries, which are portable power sources that are portable to achieve further size reduction and weight reduction of these electronic devices. There is a demand for higher energy density, and the nickel cadmium battery and the lead battery are becoming insufficient. Further, cadmium and lead are not preferable from the viewpoint of protecting the global environment, and in some countries, the use of cadmium and lead is actually beginning to be regulated. Therefore, it is desired to develop a secondary battery using a material replacing them.

As an alternative to the above-mentioned nickel-cadmium battery and lead battery, a non-aqueous electrolyte battery using a non-aqueous electrolyte solution obtained by dissolving an electrolyte in a non-aqueous solvent has been attracting attention.

Here, as the non-aqueous electrolyte battery, one having a primary battery specification has been already developed. In the case of a primary battery, the negative electrode only discharges and does not need reversibility, and it can be said that the energy density of the battery is substantially determined by the characteristics of the positive electrode. Therefore, as the active material used for the positive electrode, a great many materials have been proposed and evaluated.

However, in order to develop the above non-aqueous electrolyte battery as a secondary battery specification, the characteristics of the negative electrode active material are very important in order to achieve good cycle characteristics. However, although many studies have been conducted from this perspective, the results have to be said to be very small.

For example, lithium metal is used as a negative electrode active material of a non-aqueous electrolyte battery of primary battery specifications. However, the problem in using this lithium metal as a negative electrode material of a secondary battery is from the initial stage of study. It has been pointed out.

That is, when lithium metal is used as a negative electrode active material of a secondary battery, lithium is deposited and dissolved in the negative electrode due to repeated charging and discharging, and lithium is deposited in a dendrite form. It penetrates to reach the positive electrode and an internal short circuit occurs.
Therefore, the battery life is short. In particular, such lithium deposition is remarkable in charging with a large current density (rapid charging).

[0008] Here, such lithium precipitation can be delayed in progress by performing gentle charging / discharging, whereby the cycle life can be lengthened somewhat.

However, when used as a practical battery,
Excellent safety performance is also an important requirement. From this point of view, when using lithium metal as a negative electrode material, finely divided active lithium is present on the negative electrode in the repeated process of dissolution and deposition even if gentle charging and discharging are performed, regardless of the current density. To generate. If an internal short circuit occurs in that state or if a shock such as accidentally deforming the battery occurs, the battery falls into a very dangerous state. That is, in the worst case, there is a report that ignition and rupture occur with a probability of about 0.4%. (Proceedings of the 1991 Electrochemical Society Autumn Meeting p127).

In order to solve such a problem, it has been considered to improve the non-aqueous electrolytic solution to try to improve the form of lithium deposition or to use an alloy such as lithium-aluminum as the negative electrode active material. However, with such a method, no great results have been obtained.For example, when an alloy is used as a negative electrode material, the cycle life during deep charge and discharge is inferior, and since the alloy is hard, it can be wound in a spiral shape. Instead of being used for small flat batteries such as coin type.

Therefore, based on the research results of so-called lithium-graphite intercalation compounds in which lithium ions can be doped between graphite layers and exist as a stable compound, it has been attempted to apply this lithium-graphite intercalation compound to a negative electrode material of a battery. ing. It has been clarified that various carbon materials can be electrochemically doped / undoped with lithium ions.

When such a carbon material is used for the negative electrode and a lithium composite oxide such as a lithium cobalt composite oxide is used for the positive electrode, lithium is deposited as a metal by simply moving between the positive and negative electrodes in an ionic state during charge and discharge. There is no such thing.
Therefore, it is possible to overcome the problems caused by the deposition of lithium metal, such as safety, cycle life, and rapid charging. Moreover, since the operating voltage of the negative electrode using this carbon material as the negative electrode active material is 0 to 1.5 V, the high operating voltage of 4 V or higher of the positive electrode using the lithium composite oxide as the positive electrode active material is not sacrificed, and the operating voltage is high. A lithium ion secondary battery having an energy density will be completed.

Further, in other non-aqueous electrolyte batteries of secondary battery specifications, a base metal oxide having a charge / discharge potential was used as a negative electrode active material, and a metal intercalation compound was used for both the positive electrode active material and the negative electrode active material. RC type (Rocking Chain) batteries have been proposed. When using a metal oxide whose charge / discharge potential is higher than that of a carbon material as the negative electrode active material, the energy density is lower than when using the above carbon material as the negative electrode active material, but problems such as safety can be cleared. It is regarded as a promising lithium-ion secondary battery system that does not require high voltage.

[0014]

By the way, various types of non-aqueous electrolyte batteries having secondary battery specifications, which are excellent in energy density and cycle life, have been proposed as described above, but they are more abnormal when used as a portable power source for consumer use. Modes, such as safety performance during overcharge and external short-circuit, and environmental resistance performance that is assumed to be left in a high temperature environment such as in a car during the summer, should also have no problems.

In particular, it is said that the temperature on the dashboard of a car in summer rises up to 100.degree. If the batteries were placed in such a place, they would be exposed to a high temperature of around 100 ° C. for 8 hours during the day. In this case, even if the battery itself becomes unusable, safety and reliability must be secured at least for the surrounding environment.

The safety performance and the environmental resistance performance against such overcharge, leaving in a high temperature environment, and the like are improved by, for example, selecting a non-aqueous solvent used for the electrolytic solution. That is, the non-aqueous solvent of the electrolytic solution is composed of a high-dielectric constant solvent and a low-viscosity solvent. Conventionally, propylene carbonate (P
C) uses dimethoxyethane (DME) as a low viscosity solvent. When a mixed solvent of propylene carbonate and diethyl carbonate using diethyl carbonate (DEC) is used as the non-aqueous solvent instead of this dimethoxyethane, the cycle at the time of high temperature use that occurs when the mixed solvent of propylene carbonate and dimethoxyethane is used. It is described in Japanese Patent Laid-Open No. 4-067998 that a large decrease in life is suppressed.

However, if a mixed solvent of propylene carbonate and diethyl carbonate is used, a large decrease in cycle life during use at high temperature can be suppressed, but when overcharged, the temperature greatly rises, for example, battery internal pressure response type current interruption. When the device is provided, even after the operation of the current interruption device, the temperature rise does not stop and continues further, and a trouble often occurs that the battery is damaged relatively quickly.

Although the cause of such trouble is not clear, diethyl carbonate and lithium metal are placed in a closed container.
When stored at a high temperature of about 0 ° C., diethyl carbonate reacts rapidly with lithium metal, causing the liquid to turn yellow. From the experimental fact that the reaction is further accelerated by the heat of reaction while gas is generated, and the liquid solidifies at the end, the precipitation amount exceeds the dopeable amount of diethyl carbonate and the carbon negative electrode in the process of temperature rise during overcharge. It is considered that this is due to the fact that a reaction has occurred with the lithium metal.

Further, in a non-aqueous electrolyte secondary battery using a mixed solvent of propylene carbonate and diethyl carbonate, when stored at a high temperature in a charged state, self-discharge occurs, the voltage is lowered, and the charge / discharge cycle is performed again. However, irreversible capacity deterioration that cannot be recovered may occur.

The reason for this is not clear, but the positive electrode, negative electrode, electrolytic solution, etc. of the battery after being stored at a high temperature in a charged state has a high impedance, which causes deterioration of the battery capacity. It is considered that the deterioration of

As described above, the conventional non-aqueous electrolyte secondary battery is superior to the nickel-cadmium battery and the lead battery in terms of energy density and environmental protection, but has insufficient safety performance and environmental resistance performance. It must be said that practical application is a long way off.

Therefore, the present invention has been proposed in view of such conventional circumstances, and it is a secondary non-aqueous electrolyte solution having high energy density, long cycle life, and excellent safety performance and environmental resistance performance. The purpose is to provide a battery.

[0023]

In order to achieve the above-mentioned object, the present inventors believe that it is necessary to use a low-viscosity solvent having low reactivity with lithium as the low-viscosity solvent,
As a result of a comprehensive search for such low-viscosity solvents, we have found a mixed solvent of methyl ethyl carbonate and dimethyl carbonate.

The present invention has been completed based on the above findings, and is a negative electrode using a carbon material capable of doping / dedoping lithium ions as a negative electrode active material, and a composite oxide of lithium and a transition metal. In a non-aqueous electrolyte secondary battery comprising a positive electrode having as a positive electrode active material and a non-aqueous electrolyte solution obtained by dissolving an electrolyte in a non-aqueous solvent, the non-aqueous solvent contains methyl ethyl carbonate and dimethyl carbonate. It is characterized by doing.

The negative electrode active material has a (002) plane spacing of 0.37 nm or more, a true density of 1.7 g / cm ³ or less, and an oxidation exothermic peak observed in a differential thermal analysis in an air stream. It is a carbon material at 700 ° C. or lower, and the non-aqueous solvent is characterized by containing propylene carbonate, methyl ethyl carbonate and dimethyl carbonate.

Further, the negative electrode active material has a (002) plane spacing of 0.340 nm or less and a crystallite thickness in the C-axis direction of 1
It is a carbon material having a true density of 2.1 g / cm ³ or more and 4.0 nm or more, and the non-aqueous solvent is characterized by containing ethylene carbonate, methyl ethyl carbonate and dimethyl carbonate. Further, the mixing ratio of the non-aqueous solvent methyl ethyl carbonate and dimethyl carbonate is 3/10 ≦ (M + D, where T is the total volume of the non-aqueous solvent, M is the methyl ethyl carbonate capacity, and D is the dimethyl carbonate capacity. ) / T ≦ 7/10.

The mixing ratio of non-aqueous solvent methyl ethyl carbonate and dimethyl carbonate is 1 / 9≤D / M≤8 / 2, where M is the methyl ethyl carbonate capacity and D is the dimethyl carbonate capacity. It is characterized by that. Further, it is characterized in that diethyl carbonate is added to the non-aqueous solvent at a ratio of 1 to 20% by volume.

The non-aqueous electrolyte secondary battery of the present invention comprises a negative electrode, a positive electrode and a positive electrode in a battery can. It contains a non-aqueous electrolyte.

As the negative electrode active material used for the negative electrode, any of those used in this type of secondary battery can be used, but the carbon materials listed below are particularly preferable.

First, a carbon material which is not graphitized even when heat-treated at about 3000 ° C., that is, non-graphitizable carbon is mentioned.

A furan resin composed of furfuryl alcohol or furfural homopolymer or copolymer is suitable as a starting material for producing such a non-graphitizable carbon material. The carbon material obtained by carbonizing the furan resin has a (002) plane spacing of 0.37 nm.
Above, the differential thermal analysis (DT) with a true density of 1.70 g / cc or less.
This is because in A), it has an oxidation exothermic peak at 700 ° C. or higher and exhibits very good characteristics as a negative electrode material for batteries.

As another starting material, an organic material obtained by introducing a functional group containing oxygen into petroleum pitch having a specific H / C atomic ratio (so-called oxygen cross-linking) is carbonized as in the furan resin. It can be used because it sometimes becomes a carbon material with excellent characteristics.

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

The specific means for introducing a functional group containing oxygen into these petroleum pitches is not limited, but for example, a wet method using an aqueous solution of nitric acid, mixed acid, sulfuric acid, hypochlorous acid or the like, or an oxidizing gas (air, oxygen). ), A reaction with a solid reagent such as sulfuric acid, ammonium nitrate, ammonium persulfate, and ferric chloride. For example, when a functional group containing oxygen is introduced into the petroleum pitch by the above method, the final carbon material is obtained in the solid state without melting during the carbonization process (about 400 ° C.), which is difficult to graphitize carbon. Similar to the generation process.

The petroleum pitch having oxygen-containing functional groups introduced therein is carbonized by the above-mentioned method to form a negative electrode material, but the conditions for carbonization are not particularly limited. The surface spacing of the (002) plane is 0.
If the carbonization conditions are set so that a carbon material satisfying the characteristics of 37 nm or more, true density of 1.70 g / cc or less, and having no oxidation exothermic peak at 700 ° C. or more by differential thermal analysis (DTA) is set, A negative electrode material having a large lithium doping amount per weight can be obtained. For example, by setting conditions such that the oxygen content of the oxygen-crosslinked precursor of petroleum pitch is 10% by weight or more, (00
2) The surface spacing can be 0.37 nm or more. Therefore, the oxygen content of the precursor is preferably 10% by weight or more, and practically in the range of 10 to 20% by weight.

The oxygen-crosslinking organic material may have an H / C atomic ratio of 0.6 to 0.8, and can be obtained by subjecting the following starting materials to preheat treatment such as pitching. It can be used.

Examples of such starting materials include organic polymer compounds such as phenol resin, acrylic resin, vinyl halide resin, polyimide resin, polyamideimide resin, polyamide resin, conjugated resin, cellulose and its derivatives, and the like. Condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphene, pentacene, other derivatives (for example, carboxylic acids thereof, carboxylic acid anhydrides, carboxylic acid imides, etc.), and mixtures of the above compounds Various pitches, acenaphthylene, indole,
It is a condensed heterocyclic compound such as isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole, acridine, phenazine and phenanthridine, and derivatives thereof.

Further, examples of the negative electrode active material include a carbon material which is graphitized when heat-treated at about 3000 ° C., that is, graphitizable carbon.

Typical examples of the organic material as a starting material for the graphitizable carbon include coal and pitch. Pitch can be used for operations such as distillation (vacuum distillation, atmospheric distillation, steam distillation), thermal polycondensation, extraction, chemical polycondensation, etc. from tars and asphalt obtained by high-temperature pyrolysis of coal tar, ethylene bottom oil, crude oil, etc. And the pitch produced during carbonization of wood.

As the polymer compound raw material, there are polyvinyl chloride resin, polyvinyl acetate, polyvinyl butyrate, 3,5-dimethylphenol resin and the like.

These starting materials have a maximum of 4 during the carbonization.
It exists in a liquid state at about 00 ° C, and when kept at that temperature, the aromatic rings are condensed, polycyclic, and become in a layered orientation, and at a temperature of about 500 ° C or higher, a solid carbon precursor, that is, semi-coke. To form. Such a process is called a liquid-phase carbonization process and is a typical formation process of graphitizable carbon.

The above-mentioned raw materials of coal, pitch and polymer compound are, of course, those which have undergone the above-mentioned liquid phase carbon process when carbonized. In addition, as a starting material, condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylene, pyrene, perylene, pentaphene, and pentacene, other derivatives (for example, carboxylic acids thereof, carboxylic acid anhydrides, carboxylic acid imides, etc.), It is also possible to use a mixture of the above compounds, a condensed heterocyclic compound such as acenaphthylene, indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole, acridine, phenazine and phenanthridine, and derivatives thereof.

When a carbon material is obtained using the above raw material organic material, for example, after carbonizing at 300 to 700 ° C. in a nitrogen stream, the temperature rising rate is 1 to 20 ° C. per minute in the nitrogen stream, and the reached temperature is 900 to The firing may be performed under the conditions of 1300 ° C. and a holding time of 0 to 5 hours at the ultimate temperature. Of course, in some cases, the carbonization operation may be omitted.

Further, as the negative electrode active material, (0
02) face spacing is 0.340 nm or less, C-axis crystallite thickness is 14.0 nm or more, and true density is 2.1 g / cm.
Graphite-based carbon materials of ³ or more also have excellent electrode filling properties, and high capacity batteries can be obtained.

As a typical carbon material exhibiting the above physical property parameters, natural graphite can be mentioned. Further, artificial graphite obtained by carbonizing an organic material and further treated at high temperature also exhibits the above-mentioned physical property parameters. To obtain artificial graphite, the easily graphitizable carbon material is used as a precursor and heat-treated at a high temperature of 2000 ° C. or higher.

The above carbon material is pulverized and classified to be used as a negative electrode material, but this pulverization may be performed before or after carbonization, calcination, high temperature heat treatment, or during the temperature rising process.

Furthermore, by adding a phosphorus compound to the precursor of the non-graphitizable carbon material or the easily graphitizable carbon material at the time of carbonization, a special compound having a large lithium doping amount can be obtained. It can be used as a negative electrode active material.

Examples of the phosphorus compound to be added include phosphorus oxides such as phosphorus pentoxide, oxo acids such as orthophosphoric acid and salts thereof, and the like. Phosphorus oxides and phosphoric acids are preferable from the viewpoint of ease of handling. Is. The phosphorus compound is added in an amount of 0.2 to 30% by weight, preferably 0.5 to 15% by weight, in terms of phosphorus, based on the raw material, that is, the organic material or the carbon material.
The proportion of phosphorus remaining in the negative electrode material is 0.2 to 9.0.
%, Preferably 0.3 to 5% by weight.

This compound is mainly composed of carbon, oxygen and phosphorus (hereinafter referred to as C--P--O compound), and the presence state of phosphorus is shown by ³¹ P nucleus-solid NMR spectrum in orthophosphoric acid ( 0ppm) ± 100p standard
In the X-ray photoelectron spectroscopy measurement, when the binding energy between carbon and carbon in the carbon atom 1s orbital spectrum is set to 284.6 eV, the phosphorus atom 2p orbital spectrum becomes 135.0 eV or less. Those having a peak show good characteristics.

When the phosphorus compound is added, basically, a C--P--O compound is formed even if it is added to a material that has already become a carbon material, but the amount of remaining phosphorus decreases, and as a result, lithium is added. Dope does not increase much. Therefore, it is preferable to add the phosphorus compound from the starting material as much as possible.

The C--P--O compound obtained by firing is ground and classified to be used as the negative electrode material. This grinding may be performed before or after firing or during the temperature rising process.

On the other hand, as the positive electrode active material used for the positive electrode,
Examples thereof include lithium composite metal oxides represented by the general formula Li _X MO ₂ (where M represents at least one of Co, Ni, and Mn), an intercalation compound containing Li, and the like. Among them, particularly when LiCoO ₂ is used, a high energy density is exhibited.

Since the non-aqueous electrolyte battery of the present invention is aimed at achieving a high capacity, the positive electrode is
In a steady state (for example, after repeating charging / discharging about 5 times), it is necessary to include Li corresponding to a charge / discharge capacity of 250 mAh or more per 1 g of the negative electrode active material, and to include Li equivalent to a charge / discharge capacity of 300 mAh or more. Desirably 350mAh
It is more preferable to include Li corresponding to the above charge / discharge capacity. In addition, it is not always necessary to supply Li entirely from the positive electrode.
It suffices if Li corresponding to the charge / discharge capacity of 0 mAh or more exists. The amount of Li can be converted by measuring the discharge capacity of the battery.

The above negative electrode and positive electrode perform a charge / discharge reaction by being housed in a battery can together with a non-aqueous electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent.

Here, the selection of the non-aqueous solvent to be used is important in order that the negative electrode and the positive electrode can maintain a normal charge / discharge reaction even after the battery is overcharged and left under high temperature. In other words, the non-aqueous solvent of the electrolytic solution is composed of a high-dielectric constant solvent and a low-viscosity solvent, but among those used as low-viscosity solvents such as diethyl carbonate, when the temperature rises due to overcharging, the This is because the deposited lithium reacts with the temperature rise to induce battery damage, or when the battery is left in a charged state in a high temperature environment, it deteriorates, resulting in deterioration of the battery capacity.

Therefore, in the present invention, in order to obtain a battery having excellent safety performance and environmental resistance, which can maintain a normal charge / discharge reaction even after being overcharged and left in a high temperature environment in a charged state, A mixed solvent of methyl ethyl carbonate and dimethyl carbonate is used as the low-viscosity solvent used for the electrolytic solution.

First, methyl ethyl carbonate is a non-aqueous solvent having extremely low reactivity with lithium metal. Therefore,
By using such methyl ethyl carbonate, battery damage caused by reaction between lithium metal and low-viscosity solvent due to temperature rise during overcharge can be prevented. However, when a non-aqueous solvent is composed only of a high-dielectric-constant solvent such as propylene carbonate and methyl ethyl carbonate, the voltage gradually decreases when the battery is left at a high temperature in the charged state, and the voltage recovers even if the charge / discharge cycle is performed again. Irreversible capacity deterioration that cannot be caused occurs.

Therefore, in the present invention, a second non-aqueous solvent is used.
Dimethyl carbonate is mixed as a low viscosity solvent. When non-aqueous solvent is mixed with high-dielectric constant solvent, methyl ethyl carbonate, and dimethyl carbonate, the voltage drop that occurs when the battery is left at high temperature in the above-mentioned charged state is suppressed, and overcharge, and even when left in a high-temperature environment in the charged state. It is possible to obtain a non-aqueous electrolyte secondary battery that can maintain a normal charge / discharge reaction even after passing through.

As the high dielectric constant solvent used for the non-aqueous solvent, propylene carbonate, ethylene carbonate or the like which is commonly used can be used, but it can be appropriately selected depending on the carbon material used for the negative electrode active material. preferable. For example,
When a graphite-based carbon material is used as the negative electrode active material, ethylene carbonate is preferably used as the high dielectric constant solvent because propylene carbonate is used as the high dielectric constant solvent because the solvent is decomposed. When a carbon material other than graphite is used as the negative electrode active material, propylene carbonate is suitable.

Here, the mixing ratio of methyl ethyl carbonate and dimethyl carbonate is 2/10 ≦ (M + D), where T is the total volume of the non-aqueous solvent, M is the methyl ethyl carbonate volume, and D is the dimethyl carbonate volume. / T ≦ 8/10 More desirably, it is preferable to set 3/10 ≦ (M + D) / T ≦ 7/10.

Further, the mixing ratio of methyl ethyl carbonate and dimethyl carbonate, which are low-viscosity solvents, is preferably set to 1 / 9≤D / M≤8 / 2. When D / M is less than 1/9, the capacity deterioration preventing effect by dimethyl carbonate cannot be sufficiently obtained, and when D / M exceeds 8/2 and the mixing ratio of dimethyl carbonate becomes too high, dimethyl carbonate Since the boiling point of is relatively low at 90 ° C., the internal pressure of the battery may increase when left in a high temperature environment.

Further, diethyl carbonate may be added to the non-aqueous solvent to improve the environmental resistance performance. That is, under the condition that the battery is left on the dashboard of the automobile in the summer, a mixed solvent of methyl ethyl carbonate (boiling point 108 ° C) and dimethyl carbonate (boiling point 90 ° C) having a low boiling point suppresses the rise in the internal pressure of the battery. I can't. However, the addition of diethyl carbonate having a higher boiling point (boiling point 126 ° C.) makes it possible to suppress the rise in the internal pressure of the battery.

For the mixed solvent of the high dielectric constant solvent, methyl ethyl carbonate and dimethyl carbonate, 1 to 1 of diethyl carbonate is added.
It is preferable to add 20% by volume, more preferably 3 to 15% by volume. If a large amount of diethyl carbonate is added outside the range specified in the present invention, the safety during overcharging will be impaired, and the capacity deterioration after leaving the battery in a charged state in a high temperature environment will increase. Therefore, the amount of diethyl carbonate added should be kept to the minimum necessary.

As the electrolyte dissolved in the above non-aqueous solvent, any electrolyte can be used as long as it is used in this type of battery, but LiPF ₆ is particularly preferable, and LiClO ₄ ,
LiAsF ₆ and LiBF ₄ can also be used. These electrolytes can be dissolved in the non-aqueous solvent at a concentration of 0.1 to 3 mol / l, but are preferably dissolved at a concentration of 0.5 to 2 mol / l.

[0065]

[Function] In a non-aqueous electrolyte secondary battery using a carbon material capable of doping / dedoping lithium as a negative electrode active material and a lithium transition metal composite oxide as a positive electrode, a high dielectric constant solvent is used as a non-aqueous solvent of the electrolytic solution. , When using a mixed solvent of methyl ethyl carbonate and dimethyl carbonate, since methyl ethyl carbonate has low reactivity with metallic lithium, it does not react with metallic lithium deposited on the negative electrode even when overcharged and the temperature rises, Battery damage caused by the reaction between the low-viscosity solvent and metallic lithium is prevented. In addition, the voltage drop that occurs when the battery is left in a charged state in a high temperature environment is suppressed by dimethyl carbonate, and an irreversible capacity drop that cannot be recovered even when a charge / discharge cycle is performed is prevented.

Further, when a certain amount of diethyl carbonate having a higher boiling point is added to the mixed solvent of the high dielectric constant solvent, methyl ethyl carbonate and dimethyl carbonate, a battery is left on the dashboard of an automobile in the summer season. Even when the battery is exposed to a considerably high temperature environment, the increase in the internal pressure of the battery is suppressed, and the reliability of the battery is improved.

[0067]

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

Structure of Battery Produced FIG. 1 shows the structure of the battery produced in each Example described later.

As shown in FIG. 1, this non-aqueous electrolyte secondary battery has a negative electrode 1 formed by coating a negative electrode current collector 9 with a negative electrode active material.
And a positive electrode 2 obtained by applying a positive electrode active material to the positive electrode current collector 10.
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 via a sealing gasket 6, and is electrically connected to the negative electrode 1 or the positive electrode 2 via a negative electrode lead 11 and a positive electrode lead 12, respectively. It is configured to function as a positive electrode.

In the battery of this embodiment, the positive electrode lead 12 is welded and attached to the battery breaking thin plate 8 and is electrically connected to the battery lid 7 through the current breaking thin plate 8. There is. In the battery having such a structure, when the pressure inside the battery rises, the thin plate 8 for current interruption is pushed up and deformed. Then, the positive electrode lead 1
2 is cut off except for the portion welded to the current breaking thin plate 8 to cut off the current.

Examples 1 to 5 First, the negative electrode 1 was produced as follows. After introducing 10 to 20% by weight of a functional group containing oxygen into a petroleum pitch as a starting material for oxygen crosslinking, carbonization was performed in an inert gas stream to obtain a carbon precursor. This carbon precursor was fired at a temperature of 1200 ° C. to produce a carbon material having properties close to those of glassy carbon. As a result of X-ray diffraction measurement of this carbon material, the (002) plane spacing was 0.381 nm, and the crystallite thickness in the C-axis direction was 1.2 nm. As a result of differential thermal analysis in an air stream, an exothermic peak was recognized at 659 ° C. Further, the true specific gravity measured by the pycnometer method was 1.54 g / cm ³ , and the 50% cumulative particle size measured by the laser diffraction method was 23.5 μm.

90 parts by weight of the powder of the carbon material thus obtained is used as a binder, polyvinylidene fluoride (PV).
10 parts by weight of DF) was mixed to prepare a negative electrode mixture, and the negative electrode mixture was dispersed in N-methylpyrrolidone as a solvent to prepare a negative electrode mixture slurry (paste form).

This negative electrode mixture slurry was applied to both surfaces of a strip-shaped negative electrode collector made of copper foil having a thickness of 10 μm, dried, and then compression molded to prepare strip-shaped negative electrode 1. The material thickness of the strip-shaped negative electrode 1 was 80 μm on both sides, and the width was 41.5 mm and the length was 700 mm.

The positive electrode 2 was manufactured as follows. 90 mol of lithium carbonate and 1 mol of cobalt carbonate are mixed,
LiCoO ₂ was obtained by firing in air at 0 ° C. for 5 hours. As a result of X-ray diffraction measurement of the obtained LiCoO ₂ , it was in good agreement with the peak of LiCoO ₂ registered in the JCPDS file. This material was pulverized to obtain LiCoO ₂ powder having a 50% cumulative particle size of 15 μm. This Li
91 parts by weight of a mixture of 95 parts by weight of CoO ₂ powder and 5 parts by weight of lithium carbonate powder, 6 parts by weight of graphite as a conductive material, and 3 parts by weight of polyvinylidene fluoride as a binder were mixed to form a positive electrode mixture. It was prepared and dispersed in N-methylpyrrolidone to prepare a positive electrode mixture slurry (paste form).

This positive electrode mixture slurry was uniformly applied to both sides of a 20 μm-thick belt-shaped positive electrode current collector made of aluminum foil, dried and then compression-molded to prepare a belt-shaped positive electrode 2. The mixture thickness of the strip-shaped positive electrode 2 is 80 μm on both sides.
The width of the electrode is 40.5 mm and the length is 650.
mm.

The strip-shaped negative electrode 1, the strip-shaped positive electrode 2, and the separator 3 made of a microporous polypropylene film having a thickness of 25 μm and a width of 44 mm, which were produced in this manner, were laminated in the order of the negative electrode, the separator, the positive electrode, and the separator, and then a large number of them were prepared. The spirally wound electrode was manufactured by winding the electrode in an outer diameter of 20 mm.

The spirally wound electrode body was housed in a nickel-plated iron battery can 5. The insulating plates 4 are arranged on the upper and lower surfaces of the spiral electrode, the aluminum positive electrode lead 12 is led out from the positive electrode current collector, and the nickel negative electrode lead 11 is led out from the negative electrode current collector to the battery. Welded to can 5.

Then, propylene carbonate (PC) and methylethyl carbonate (M
EC) and dimethyl carbonate (DMC) were mixed at various mixing ratios, and an electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1 mol / l was injected into a mixed solvent, and then the surface was coated with asphalt to form an insulating sealing gasket 6. Safety valve device 8 with current cutoff mechanism by caulking battery can 5
Further, the battery lid 7 was fixed, the airtightness inside the battery was maintained, and a cylindrical non-aqueous electrolyte secondary battery having a diameter of 20 mm and a height of 50 mm was produced.

Table 1 shows the volume mixing ratio of the non-aqueous solvent in the electrolytic solution injected into the battery can 5.

[0080]

[Table 1]

Comparative Example 1 PC and MEC were used as the non-aqueous solvent of the electrolytic solution PC: MEC =
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the mixed solvent mixed at a volume mixing ratio of 5: 5 was used.

Comparative Example 2 PC and DEC were used as the non-aqueous solvent of the electrolytic solution PC: DEC =
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the mixed solvent mixed at a volume mixing ratio of 5: 5 was used.

[Study of Reactivity of Solvent with Lithium Metal]
The following experiment was conducted in order to investigate the reactivity of the low-viscosity solvent used in the thus-prepared non-aqueous electrolyte secondary battery with lithium metal.

First, DEC, DMC, and MEC were placed in Teflon containers, and lithium metal pieces were placed in these solvents. Then, the Teflon container containing the solvent and the lithium metal piece was sealed so that water did not enter, and left in a high temperature tank at various temperatures. Table 2 shows the reaction status between the solvent and the lithium metal piece during standing.

[0085]

[Table 2]

Since the lithium metal piece has a natural oxide film on the surface, it does not react immediately, but DE
When it is put into C, the reaction with DEC starts in 10 minutes if the temperature is 80 ° C. and it is in a high temperature environment to some extent. Further, when stored in DEC at a temperature of 60 ° C. for 1 week, it reacts with DEC and disappears at the end. Further, the DEC charged with the lithium metal pieces solidifies to brown.

On the other hand, when the lithium metal pieces are put into the DMC and MEC, the reaction as in the case where the lithium metal pieces are put into the DEC does not occur.

From the above, DEC is unsuitable when used as a low-viscosity solvent for a non-aqueous electrolyte secondary battery, when the battery is overcharged, there is a high possibility that it will react with lithium metal deposited on the negative electrode. I found out.

[Study on Addition of DMC] Next, D was added to the non-aqueous solvent.
About the non-aqueous electrolyte secondary battery made to investigate the effect of mixing MC, the open circuit voltage, the internal resistance (AC 1 kHz) and the 11th cycle charge immediately after the 11th cycle charging when the charge / discharge cycle was repeated. Then, the open circuit voltage and the internal resistance (AC 1 kHz) were measured after the battery was stored in a charged state at a temperature of 90 ° C. for 40 hours. Further, the ratio of the battery internal pressure before storage and the battery internal pressure after storage, and the ratio of the capacity of the second charge / discharge cycle after storage to the capacity of the 10th charge / discharge cycle before storage (capacity recovery rate) were determined. These results are shown in Table 3,
The relationship between the DMC capacity mixing ratio in the electrolytic solution and the open circuit voltage after storage is shown in Fig. 2, the relationship between the DMC capacity mixing ratio and the internal resistance after storage is shown in Fig. 3, and the relationship between the DMC capacity mixing ratio and the capacity recovery ratio is shown. FIG. 4 shows the relationship between the DMC capacity mixing ratio and the battery internal pressure ratio before and after storage.

The charging / discharging cycle consists of a charging current of 1 A,
Constant current charging was performed under conditions of an upper limit voltage of 4.2 V (constant voltage), and then discharging was performed under conditions of a resistance of 6.2Ω and a final voltage of 2.75 V.

[0091]

[Table 3]

As can be seen from Table 3 and FIGS.
As the DMC capacity mixing ratio in the electrolytic solution increases, the internal resistance after storage decreases, the open circuit voltage increases, and the capacity recovery ratio increases. From this, it was found that the addition of DMC to the non-aqueous solvent prevents the capacity deterioration caused by leaving the battery in a charged state in a high temperature environment.

However, as can be seen from FIG. 5, when the mixing ratio of DMC in the electrolytic solution becomes too large, the boiling point of DMC is relatively low, and the internal pressure of the battery becomes high after being left in a high temperature environment. That is, DMC is 9/1 ≦ D / M ≦ 2, where MEC capacity in the electrolytic solution is M and DMC capacity is D.
It was found that it is preferable to add it so as to be / 8.

[Study on Addition of DEC] Next, D was added to the non-aqueous solvent.
To investigate the effect of adding EC, PC: MEC: D
Batteries were produced in the same manner as in Example 4 except that DEC was further added to the non-aqueous solvent having a composition of MC = 4: 3: 3 in the range of 1 to 30% by volume (Examples 4-A to Examples). 4-
F).

Assuming that the battery containing DEC, the battery of Example 4, and the battery of Comparative Example 2 were left on the dashboard of the automobile in the summer, the battery was charged and the temperature was 105 ° C. for 8 hours. The battery was stored, the increase in the battery internal pressure was measured, and the appearance was observed. The charging was constant current charging with a charging current of 1 A and an upper limit voltage of 4.2 V (constant voltage), and the charging was performed for 2.5 hours.

Further, when the charge / discharge cycle was repeated in the same manner as described above, the open circuit voltage immediately after the 11th cycle charging, the internal resistance (AC 1 kHz), and the 11th cycle charging, the temperature was 90 ° C. for 40 hours in the charged state. The open circuit voltage after storage and the internal resistance (AC 1 kHz) were measured. Also,
The ratio of the battery internal pressure before storage and the battery internal pressure after storage, and the ratio of the capacity of the second charge / discharge cycle after storage to the capacity of the 10th charge / discharge cycle before storage (capacity recovery rate) were determined.

The results of the storage test at a temperature of 105 ° C. for 8 hours are shown in Table 4.
Table 5 shows the results of the storage test at a temperature of 90 ° C. for 40 hours.

[0098]

[Table 4]

[0099]

[Table 5]

As can be seen from Table 4, the internal pressure of the battery after storage at a temperature of 105 ° C. for 8 hours becomes lower as the amount of DEC added to the non-aqueous solvent increases. From this, it is understood that adding DEC to the non-aqueous solvent is effective in suppressing gas generation during high temperature storage.

However, referring to Table 5, the temperature is 90 ° C. and the temperature is 40 ° C.
Regarding the battery characteristics after storage for a long time, when the amount of DEC added increases, the open circuit voltage decreases, the internal resistance increases, and the capacity recovery rate decreases. From these facts, it is understood that it is not preferable to add too much DEC, and it is appropriate to add it in the range of 1 to 20% by volume.

Example 6 Artificial graphite KS-75 (manufactured by Lonza Co., Ltd .: (002) plane spacing = 0.3358 nm, C-axis crystallite thickness = 25.4 n)
m, Raman spectrum G value = 8.82, true specific gravity = 2.2
EC, MEC, DMC = EC: MEC: DMC =
A non-aqueous electrolyte battery was produced in the same manner as in Example 1 except that a mixed solvent mixed at a volume mixing ratio of 5: 2: 3 was used as the non-aqueous solvent of the electrolytic solution.

Comparative Example 3 Nonaqueous electrolytic solution was prepared in the same manner as in Example 2 except that a mixed solvent prepared by mixing EC and MEC at a volume mixing ratio of EC: MEC = 5: 5 was used as the nonaqueous solvent of the electrolytic solution. A secondary battery was produced.

With respect to the non-aqueous electrolyte secondary battery thus manufactured, the open circuit voltage, the internal resistance (AC 1 kHz) and the internal resistance (AC 1 kHz) immediately after the 11th cycle when the charge and discharge cycle was repeated in the same manner as described above. After 11 cycles of charging, the open circuit voltage and the internal resistance (AC 1 kHz) were measured after the battery was stored in a charged state at a temperature of 90 ° C. for 40 hours. Also,
The ratio of the battery internal pressure before storage and the battery internal pressure after storage, and the ratio of the capacity of the second charge / discharge cycle after storage to the capacity of the 10th charge / discharge cycle before storage (capacity recovery rate) were determined. The results are shown in Table 6.

[0105]

[Table 6]

As can be seen from Table 6, the non-aqueous electrolyte secondary battery of Example 6 in which DMC was mixed in the electrolytic solution contained D in the electrolytic solution.
Compared with the non-aqueous electrolyte secondary battery of Comparative Example 3 in which MC was not mixed, the internal resistance after storage was low, the open circuit voltage and the capacity recovery rate were high. From this, even when a graphite-based carbon material is used for the negative electrode and EC is used as the high dielectric constant solvent, DMC
It has been found that the mixing of the above with the electrolytic solution prevents the capacity deterioration caused by leaving the battery in a charged state in a high temperature environment.

Specific examples to which the present invention is applied have been described above, but the present invention is not limited to these examples, and various modifications can be made without departing from the gist of the present invention. is there.

[0108]

As is apparent from the above description, in the present invention, a non-aqueous electrolyte solution using a carbon material capable of doping / dedoping lithium as a negative electrode active material and a lithium transition metal composite oxide as a positive electrode. In a secondary battery, a mixed solvent of methyl ethyl carbonate and dimethyl carbonate is used as a low-viscosity solvent for the electrolytic solution, so that a normal charge / discharge reaction can be maintained even after being overcharged and left in a high temperature environment in a charged state, and high energy consumption can be maintained. density,
It is possible to obtain a non-aqueous electrolyte secondary battery that has a long cycle life and excellent safety and environmental resistance.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing one structural example of a non-aqueous electrolyte secondary battery to which the present invention is applied.

FIG. 2 is a non-aqueous electrolyte secondary battery, DMC of the electrolyte
It is a characteristic view which shows the relationship between a mixing ratio and an open circuit voltage after leaving it in a charged state in a high temperature environment.

FIG. 3 is a non-aqueous electrolyte secondary battery, DMC of the electrolyte
It is a characteristic view which shows the relationship between a mixing rate and an internal resistance after leaving it in a charged state in a high temperature environment.

FIG. 4 is a non-aqueous electrolyte secondary battery, DMC of the electrolyte
It is a characteristic view which shows the relationship between a mixing rate and a capacity recovery rate.

FIG. 5: DMC of the electrolytic solution for the non-aqueous electrolytic solution secondary battery
FIG. 6 is a characteristic diagram showing a relationship between a mixing ratio and a battery internal pressure ratio before and after storage in a high temperature environment.

[Explanation of symbols]

 1 ... Negative electrode 2 ... Positive electrode 3 ... Separator 4 ... Insulating plate 5 ... Battery can 6 ... Sealing gasket 7 ... Battery lid 8 ... Safety plate 9 ... Negative electrode Current collector 10 ... Positive electrode current collector 11 ... Negative electrode lead 12 ... Positive electrode lead

Front page continued (72) Inventor Keiichi Yokoyama 580-2 32, Takuji Nagaura, Sodegaura-shi, Chiba Mitsui Petrochemical Industry Co., Ltd. Within Mitsui Petrochemical Industry Co., Ltd.

Claims (6)

[Claims] 1. A negative electrode using a carbon material capable of doping / dedoping lithium ions as a negative electrode active material, a positive electrode using a composite oxide of lithium and a transition metal as a positive electrode active material, and an electrolyte dissolved in a non-aqueous solvent. A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte solution obtained by, and the non-aqueous solvent containing methyl ethyl carbonate and dimethyl carbonate. 2. The negative electrode active material has a (002) plane spacing of 0.37 nm or more, a true density of 1.7 g / cm ³ or less, and an oxidation exothermic peak observed in a differential thermal analysis in an air stream. The non-aqueous electrolyte secondary battery according to claim 1, which is a carbon material at 700 ° C. or lower, and the non-aqueous solvent contains propylene carbonate, methyl ethyl carbonate, and dimethyl carbonate. 3. The negative electrode active material has a (002) plane spacing of 0.340 nm or less and a crystallite thickness in the C-axis direction of 14.0.
2. The non-aqueous electrolysis according to claim 1, wherein the non-aqueous solvent is a carbon material having a nanometer or more and a true density of 2.1 g / cm ³ or more, and the non-aqueous solvent contains ethylene carbonate, methyl ethyl carbonate and dimethyl carbonate. Liquid secondary battery. 4. The mixing ratio of methyl ethyl carbonate and dimethyl carbonate as the non-aqueous solvent is 3/10 ≦ (where T is the total volume of the non-aqueous solvent, M is the methyl ethyl carbonate volume, and D is the dimethyl carbonate volume. The non-aqueous electrolyte secondary battery according to claim 2, wherein M + D) / T ≦ 7/10. 5. The mixing ratio of non-aqueous solvent methyl ethyl carbonate and dimethyl carbonate is 1/9 ≦ D / M ≦ 8/2 where M is methyl ethyl carbonate capacity and D is dimethyl carbonate capacity. The non-aqueous electrolyte secondary battery according to claim 4, wherein 6. The non-aqueous electrolyte secondary battery according to claim 2, wherein diethyl carbonate is added to the non-aqueous solvent at a ratio of 1 to 20% by volume.