JP2003187863A - Secondary battery using organic electrolyte - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a secondary battery using an organic electrolyte, which has superior safety by using a material capable of trapping gasses induced in overcharging operation. <P>SOLUTION: This secondary battery using an organic electrolyte comprises a positive and negative electrodes, and the organic electrolyte contains a mild acidic organic lithium salt, in particular, at least one component selected from a lithium acetate, lithium benzoate or a lithium phenoxide. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolyte secondary battery, and more particularly to an organic electrolyte secondary battery having excellent safety.

[0002]

2. Description of the Related Art An organic electrolyte secondary battery uses an organic solvent as a solvent for an electrolyte and has a large capacity, high voltage, high energy density, and high output. It is in. As the organic solvent, a cyclic ester such as ethylene carbonate and a chain ester such as dimethyl carbonate, diethyl carbonate and methyl propionate have been mixed and used so far.

In such an organic electrolyte secondary battery, a protection circuit or the like in the charger is usually used to prevent overcharging and prevent an internal short circuit. The fever just heats up, and it doesn't lead to an abnormal situation. However, assuming that the charger has failed, especially if the protection circuit has failed, it is desirable to confirm the safety by conducting a test under more dangerous conditions.

[0004]

When a voltage higher than the allowable voltage is applied to the battery during charging, the battery is overcharged and the gas produced by an abnormal chemical reaction in the battery raises the internal pressure, which results in , The battery may explode. Therefore, in order to improve the safety of the battery, it is essential to prevent the battery from bursting due to an increase in internal pressure during overcharging.

In order to suppress such an increase in internal pressure, it is necessary to reduce the amount of gas generated during overcharge, and as a method therefor, a substance capable of trapping the gas generated during overcharge is used in the battery. The method of adding to is considered. However, so far, no particularly suitable substance of this kind has been found.

In view of such circumstances, the present invention seeks a substance suitable for trapping a gas generated during overcharge, thereby obtaining an organic electrolyte secondary battery having excellent safety. I am trying.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies in order to achieve the above object. As a result, a weakly acidic organic lithium salt was used as a trapping agent, and the weakly acidic organic lithium salt was contained in the organic electrolytic solution. However, the anion of the dissociated weakly acidic organic lithium salt and the proton are bound to trap the proton, which is the hydrogen gas source, so that the amount of gas generated during overcharge is greatly reduced, which reduces the risk of rupture. The present invention has been completed, knowing that an organic electrolyte secondary battery having excellent safety can be obtained.

That is, the present invention is an organic electrolyte secondary battery having a positive electrode, a negative electrode and an organic electrolyte, wherein the organic electrolyte contains a weakly acidic organic lithium salt. In particular, the weakly acidic organic lithium salt is at least one selected from lithium acetate, lithium benzoate, or lithium phenoxide, and the organic electrolyte secondary battery having the above-mentioned constitution, The present invention relates to the organic electrolyte secondary battery having the above-mentioned configuration, wherein the acidic organic lithium salt is 0.05 parts by weight or more and 5 parts by weight or less per 100 parts by weight of the organic electrolyte solution. The present invention also provides an organic electrolyte secondary battery having the above-mentioned configuration, in which the positive electrode is a lithium metal composite compound as a positive electrode active material, and the negative electrode is a carbon material capable of inserting and extracting lithium as a negative electrode active material, and the negative electrode further contains lithium. The present invention relates to the organic electrolyte secondary battery having the above-mentioned configuration, which uses the composite nitride as the negative electrode active material.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied in detail the effect of adding a weakly acidic organic lithium salt to an organic electrolyte solution on the safety of a battery. To explain this in detail, the present inventors first conducted an overcharge test of a lithium ion battery assuming an internal short circuit and the like,
The risk is low with ordinary commercial lithium-ion batteries,
It has been found that the danger increases as the energy density of the battery increases.

A compound such as a carbon material capable of deintercalating lithium is usually used for the negative electrode of these batteries. However, when the negative electrode is overcharged and a little of the lithium is electrodeposited, it is treated with an electrolytic solution from about 100 ° C. An exothermic reaction occurs between the electrodeposited lithium and the carbon material in which lithium is inserted. On the other hand, the desorption of lithium in the positive electrode lowers the reaction start temperature with the electrolytic solution, and when the temperature rises to the thermal runaway temperature of the positive electrode due to the reaction heat of the negative electrode, the battery undergoes an abnormal reaction and gas is generated. However, the internal pressure of the battery rises.

Further, the larger the volume per unit volume,
Since the amount of reactants at the time of overcharge increases, the amount of gas produced increases, and the internal pressure rises significantly, it is necessary to suppress the gas generation at the time of overcharge. Also, when the battery size is large, the amount of reactants increases, so it is necessary to suppress gas generation during overcharge.

In order to improve the safety of the battery, it is known to add a non-flammable solvent, dissolve a polymer, or add an aromatic compound to the electrolytic solution. However, the effect cannot be said to be sufficient by these means. On the other hand, as in the present invention, when a weakly acidic organolithium salt is added to an organic electrolytic solution, particularly an organic electrolytic solution containing a chain ester as a main solvent, a markedly excellent effect for improving safety is obtained. It was found that The reason for this is considered as follows.

When a negative electrode is made of a compound capable of deintercalating lithium such as a carbon material, the reactivity between the electrolytic solution and the negative electrode at high temperature is suppressed more than when lithium is used, but the negative electrode is charged and discharged. The increase in the possible volume increases the reactivity with the electrolytic solution, increases the amount of gas generated when the reaction occurs, increases the internal pressure, and is likely to burst. However, even in such a battery configuration, when the weakly acidic organic lithium salt is added to the organic electrolytic solution, the anion and the proton of the weakly acidic organic lithium salt dissociated during overcharge are combined to trap the hydrogen gas source, It was found that the increase in internal pressure was suppressed, the battery was prevented from bursting, and safety was greatly improved.

In the present invention, the weakly acidic organolithium salt exhibiting such an effect is obtained by converting an organic substance obtained by substituting H of Li of this salt into an aqueous solution of 1 mol / liter.
A solution having a PKa within the range of 1 ≦ PKa ≦ 12 is used. Preferred examples of such weakly acidic organic lithium salt include lithium acetate, lithium benzoate, lithium phenoxide and the like.

The amount of such weakly acidic organic lithium salt used is preferably 0.05 parts by weight or more, more preferably 0.1 parts by weight or more, and 0% by weight per 100 parts by weight of the organic electrolyte. Most preferably, the amount is not less than 2 parts by weight. Further, it is preferably 5 parts by weight or less, more preferably 2 parts by weight or less, and most preferably 1 part by weight or less. If it is less than the above, safety cannot be sufficiently improved, and if it is more than the above, the cycle characteristics and load characteristics of the battery may deteriorate.

In the present invention, as the organic electrolytic solution to which such a weakly acidic organic lithium salt is added, one prepared by dissolving an electrolyte in an organic solvent containing a chain ester as a main solvent is used. The concentration of the electrolyte in the organic electrolytic solution is not particularly limited, but if it is 1 mol / l or more, the safety is improved, and it is more preferably 1.2 mol / l or more. Also, 1.
When the amount is 7 mol / liter or less, good electric characteristics are maintained, which is desirable, and the amount is more preferably 1.5 mol / liter or less.

The chain ester used as the main solvent is a chain CO such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate or methyl propionate.
It is an organic solvent having an O-bond. The main solvent is
It means that the chain ester exceeds 50% by volume in the whole organic solvent containing these chain esters. When the chain ester exceeds 65% by volume in the whole organic solvent, the safety of the battery in the nailing test tends to be lowered, and the effect of adding the weakly acidic organic lithium salt of the present invention becomes large. In addition, when the chain ester has a methyl group, the safety of the battery tends to decrease, and the effect of adding the weakly acidic organic lithium salt of the present invention becomes remarkable.

It is desirable to use an ester having a high dielectric constant (dielectric constant of 30 or more) together with the above-mentioned chain ester because the cycle characteristics and the load characteristics of the battery are improved as compared with the chain ester alone. Examples of the ester having a high dielectric constant include propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, and ethylene glycol sulfite. Those having a cyclic structure are particularly preferable, and cyclic carbonate is particularly preferable, and ethylene carbonate is most preferable.

The amount of the ester having such a high dielectric constant to be used is preferably less than 40% by volume, more preferably 30% by volume or less, in the whole organic solvent.
It is more preferably 25% by volume or less. When these esters with high dielectric constant are used together, the safety of the battery is improved as compared with the case of using the chain ester alone, but this improvement effect is that the ester with high dielectric constant is 10 volume in the whole organic solvent. %, It becomes remarkable, and when it reaches 20% by volume, it becomes more remarkable.

Examples of the organic solvent which can be used in combination with the chain ester include, in addition to the above-mentioned ester having a high dielectric constant,
1,2-dimethoxyethane, 1,3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran,
Examples thereof include diethyl ether. Other,
Amine imide-based organic solvent or C = O C of carbonic acid ester
It is also possible to use an organic solvent in which is substituted with S, an organic solvent in which a part of hydrogen atoms of the carbonic acid ester is substituted with fluorine, and the like.

As the electrolyte to be dissolved in the above organic solvent, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiA
sF ₆ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F
₉ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (S
O ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃
SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN
(Rf ₃ OSO ₂ ) ₂ (where Rf is a fluoroalkyl group) and the like are used alone or in combination of two or more. Among these, especially LiPF ₆ and LiC ₄ F
₉ SO _{3 and the} like are desirable because they have good charge and discharge characteristics.

In the present invention, for the positive electrode, a mixture of a positive electrode active material to which a conductive additive or a binder such as polyvinylidene fluoride is appropriately added is formed into a molded body using a current collecting material such as aluminum foil as a core material. The finished one is used. The positive electrode active material is a lithium cobalt compound such as LiCoO ₂ ,
Lithium nickel compounds such as LiNiO _2, LiMn
Lithium manganese compounds such as ₂ O ₄ and layered lithium manganese compounds such as LiMnO ₂ are used. In addition to these, metal oxides such as manganese dioxide, vanadium pentoxide, and chromium oxide, and metal sulfides such as titanium disulfide and molybdenum disulfide are also used.

Among the above positive electrode active materials, LiCo
It is preferable to use a lithium metal composite oxide such as O ₂ , LiNiO ₂ or LiMn ₂ O _{4 which} has an open circuit voltage at the time of charging of 4 V or more based on Li, because a high energy density can be obtained. In particular, charged LiCoO ₂ and LiN
In the case of iO ₂ , the reaction start temperature with the electrolytic solution is LiMn ₂ O
_{Since the} temperature is lower than _{4, and} the thermal runaway temperature of the positive electrode is easily reached due to heat generation of the negative electrode, the effect of adding the weakly acidic organic lithium salt of the present invention is more remarkably exhibited.

In the present invention, a mixture of a negative electrode active material to which a conductive additive, a binder such as polyvinylidene fluoride, etc. is appropriately added is formed into a molded body using a current collecting material such as copper foil as a core material. The finished one is used. The negative electrode active material may be any material capable of inserting and extracting lithium (material capable of doping and dedoping lithium ions), and examples thereof include graphite, pyrolytic carbons, cokes, glassy carbons, and organic polymer compounds. Fired body, mesocarbon microbeads,
Carbon materials such as carbon fiber and activated carbon, and LiαMβN
(0.2 ≦ α ≦ 2.9, 0.1 ≦ β ≦ 0.8, M: one or more elements containing a transition metal element), such as a lithium-containing composite nitride is used. . Also, S
An alloy of i, Sn, In, or the like and lithium, or an oxide of Si, Sn, In, or the like that can be charged / discharged at a low potential close to Li may be used.

When a carbon material is used as the negative electrode active material,
This carbon material preferably has the following characteristics. First, the lower limit value of the interlayer distance d ₀₀₂ of the (002) plane is preferably 3.2 Å or more, more preferably 3.25 Å or more, and further preferably 3.3 Å or more. Also, d
The upper limit of ₀₀₂ is preferably 3.5 Å or less.
45 Å or less is more desirable, and 3.4 Å or less is even more desirable. Further, the lower limit of the crystallite size Lc in the c-axis direction is preferably 30 Å or more, more preferably 80 Å or more, and further preferably 250 Å or more. The upper limit of Lc is preferably 20,000 Å or less,
It is more preferably 10,000 Å or less, still more preferably 5,000 Å or less. The average particle size is preferably 8 to 15 μm, particularly 10 to 13 μm. The purity is 99.
9% or more is desirable.

[0026]

EXAMPLES Next, examples of the present invention will be described to more specifically describe. However, the present invention is not limited to the following examples. In addition, in the following, "parts" means "parts by weight".

Example 1 Methyl ethyl carbonate and ethylene carbonate were mixed in a volume ratio of 67:33, and LiP was used as an electrolyte in the mixture.
F ₆ was dissolved in 1.2 mol / liter to prepare an organic electrolytic solution. To this, 0.1 part of lithium acetate as a weakly acidic organic lithium salt was added per 100 parts of this organic electrolytic solution and mixed well.

[0028] To LiCoO ₂ as the positive electrode active material, scaly graphite as a conductive additive was added and mixed at a weight ratio of 100: 5, and this mixture was mixed with a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone. And made into a slurry. This positive electrode mixture slurry was passed through a 100-mesh filter to remove large ones, and then uniformly applied on both sides of a positive electrode current collector made of an aluminum foil having a thickness of 15 μm and dried, and then roller press. A belt-shaped positive electrode was produced by compression molding with a machine.

As a negative electrode active material, a graphite-based carbon material (interlayer distance d ₀₀₂ : 3.37Å, crystallite size Lc in the c-axis direction Lc: 950Å, average particle diameter: 10 μm, purity: 99.
90% of 9%) and a solution of 10 parts of polyvinylidene fluoride in N-methyl-2-pyrrolidone were mixed to form a slurry. This negative electrode mixture slurry was passed through a 100-mesh filter to remove large ones, and then uniformly coated on both surfaces of a negative electrode current collector made of strip-shaped copper foil having a thickness of 10 μm and dried, and then, A band-shaped negative electrode was produced by compression molding with a roller press.

The above positive electrode has a thickness of 2 as a separator.
It was placed on the above negative electrode through a 5 μm microporous polyethylene film, wound, and then housed in an outer can (size: 5 mm × 30 mm × 48 mm) as a battery container. The positive electrode / negative electrode weight ratio of the active material-containing mixture per unit volume is 2.13.
Met. In this battery container, the above organic electrolyte was injected, and after the organic electrolyte had sufficiently penetrated into the separator, etc., it was sealed, precharged and aged to produce a square organic electrolyte secondary battery. .

Example 2 The same as Example 1 except that 0.1 part of lithium benzoate was added to the organic electrolytic solution as the weakly acidic organic lithium salt instead of 0.1 part of lithium acetate. Then, a prismatic organic electrolyte secondary battery was produced.

Example 3 As Example 1 except that 0.1 part of lithium phenoxide was added to the organic electrolytic solution as the weakly acidic organic lithium salt instead of 0.1 part of lithium acetate. Similarly, a prismatic organic electrolyte secondary battery was produced.

Example 4 Lithium-containing composite nitride (Li _2.6
Co _0.4 N) with carbon black as a conduction aid at a weight ratio of 70:30 and mixed, and this mixture and a solution of a thermoplastic resin (styrene-butadiene rubber) dissolved in toluene are mixed to form a slurry. I chose This negative electrode mixture slurry was passed through a 100-mesh filter to remove large ones, and then uniformly coated on both surfaces of a negative electrode current collector made of strip-shaped copper foil having a thickness of 10 μm and dried, and then, A band-shaped negative electrode was produced by compression molding with a roller press. A rectangular organic electrolyte secondary battery was produced in the same manner as in Example 1 except that this negative electrode was used. The positive electrode / negative electrode weight ratio of the active material-containing mixture per unit volume in this battery was 2.13, which was the same as in Example 1.

Comparative Example 1 89.2 parts of a graphite-based carbon material (the same as in Example 1) as a negative electrode active material, 0.9 part of lithium phosphate, 9.9 parts of polyvinylidene fluoride were added to N-methyl-2-. The solutions dissolved in pyrrolidone were mixed into a slurry. This negative electrode mixture slurry was passed through a 100-mesh filter to remove large ones, and then uniformly coated on both surfaces of a negative electrode current collector made of strip-shaped copper foil having a thickness of 10 μm and dried, and then, A band-shaped negative electrode was produced by compression molding with a roller press. A rectangular organic electrolyte secondary battery was prepared in the same manner as in Example 1 except that this negative electrode was used and 0.1 part of lithium acetate as a weakly acidic organic lithium salt was not added to the organic electrolyte. It was made. The positive electrode / negative electrode weight ratio of the active material-containing mixture per unit volume in this battery was 2.13, which was the same as in Example 1.

Comparative Example 2 The same as Example 1 except that 0.1 part of lithium carbonate was added to the organic electrolyte instead of 0.1 part of lithium acetate as the weakly acidic organic lithium salt. Thus, a prismatic organic electrolyte secondary battery was produced.

Comparative Example 3 Secondary organic electrolyte secondary electrolyte solution was prepared in the same manner as in Example 1 except that 0.1 part of lithium acetate as a weakly acidic organic lithium salt was omitted from the organic electrolyte solution. A battery was made.

With respect to each of the organic electrolyte secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3 described above, after discharging to 3 V at 600 mA, charging to 600 mA and then 4.2 V after reaching 4.2 V. The battery was charged for 2 hours and 30 minutes under the condition that the constant voltage was maintained. Then, the battery was subjected to an overcharge test under the conditions of 600 mA and 12 V, and after the overcharge test was completed, the gas in the battery was extracted, and the gas amount was measured and the gas composition was analyzed.
The hydrogen gas amount is calculated from the gas composition analysis result. The results are shown in Table 1.

[0038]

As is clear from the results of Table 1 above, when a weakly acidic organic lithium salt was added as a gas trap agent to the organic electrolyte as in Examples 1 to 4, it was compared with Comparative Example 3 in which it was not added. Then, the amount of gas generated during overcharge can be reduced, especially hydrogen gas can be reduced, and the addition of lithium acetate as a weakly acidic organic lithium salt as in Examples 1 and 4 makes the above effect remarkable. I understand. On the other hand, when the gas trapping agent different from the present invention is added to the organic electrolytic solution or the negative electrode as in Comparative Examples 1 and 2, it is clear that the above effect is poor.

[0040]

INDUSTRIAL APPLICABILITY As described above, the present invention provides an organic electrolyte secondary battery having a positive electrode, a negative electrode and an organic electrolyte solution.
By including a weakly acidic organic lithium salt in the organic electrolytic solution, the safety of the battery can be improved, and particularly by including lithium acetate as the weakly acidic organic lithium salt, the safety of the battery can be significantly improved.

Continued front page    (72) Inventor Shuichi Wada             Hitachima, 1-88, Torora, Ibaraki City, Osaka Prefecture             Within Kucsel Co., Ltd. F-term (reference) 5H029 AJ12 AK03 AL01 AL06 AM03                       AM05 AM07 BJ02 BJ14 DJ16                       HJ01                 5H050 AA15 BA17 CA04 CA07 CB01                       CB07 CB08 HA01

Claims (5)

[Claims] 1. An organic electrolytic solution secondary battery having a positive electrode, a negative electrode and an organic electrolytic solution, wherein the organic electrolytic solution contains a weakly acidic organic lithium salt. 2. The organic electrolyte secondary battery according to claim 1, wherein the weakly acidic organic lithium salt is at least one selected from lithium acetate, lithium benzoate, and lithium phenoxide. 3. The weakly acidic organic lithium salt is used as the organic electrolytic solution 1.
The organic electrolyte secondary battery according to claim 1, wherein the amount is 0.05 parts by weight or more and 5 parts by weight or less per 00 parts by weight. 4. The organic electrolyte solution according to claim 1, wherein the positive electrode uses a lithium metal composite compound as a positive electrode active material, and the negative electrode uses a carbon material capable of inserting and extracting lithium as a negative electrode active material. Next battery. 5. The organic electrolyte secondary battery according to claim 1, wherein the negative electrode uses a lithium-containing composite nitride as a negative electrode active material.