JP2003086185A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having improved productivity and an excellent cycle characteristic by enhancing adhesion between a negative electrode material and a current collector. SOLUTION: The lithium ion secondary battery comprises a negative electrode containing a negative electrode active material enabling insertion and desorption of lithium ions, a positive electrode, and a nonaqueous electrolyte. The negative electrode contains an imidazole silane compound.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having improved battery manufacturing productivity and battery cycle characteristics.

[0002]

2. Description of the Related Art Lithium ion secondary batteries have higher electromotive force than lead / acid storage batteries and nickel / hydrogen secondary batteries, and are capable of higher energy density. Manufactured as a power source for
In addition, materials and processes are being actively developed with a view to mounting them on electric vehicles in the future.

In a lithium ion secondary battery, a lithium-containing transition metal oxide such as lithium cobalt oxide or lithium manganate is used as a positive electrode active material, and lithium, a lithium alloy or a carbon material capable of inserting and desorbing lithium ions is used as a negative electrode active material. This is a secondary battery in which a nonaqueous electrolyte is provided with a solution of a substance, which is a lithium salt having a complex mainly composed of a group V element and fluorine as an anion, and a mixed solvent of a cyclic carbonic acid ester and a chain carbonic acid ester. Especially when a carbon material is used as the negative electrode active material,
Since the lithium intercalation compound is formed on the negative electrode active material during charging, dendrite is not generated unlike the case where lithium is used as the negative electrode, and high safety and cycle characteristics are achieved.

By the way, in order to fully exhibit the above-mentioned advantages of the lithium ion secondary battery, the characteristics such as the capacity of the positive and negative electrode active materials are important. However, including the electrodes, especially the negative electrode active material, the binder and the current collector. The structure of the negative electrode becomes important.

Conventionally, as a carbon material for a negative electrode active material of a lithium ion secondary battery, a carbon raw material derived from petroleum, coal or the residue of a chemical substance in a reaction tank is calcined at 800 ° C to 1,500 ° C.
Crushed and classified carbon powder or carbon raw material 2,800-3,000 ℃
Artificial graphite or natural graphite that has been graphitized in 1. is used. A laminate in which a negative electrode material obtained by mixing these negative electrode active materials with a binder or the like is formed into a film on a current collector is used as a negative electrode. In the negative electrode, when the battery is charged, lithium ions are inserted into the active material to form an intercalation compound, and the potential of the current collector decreases to almost the reduction potential of lithium. Since the binder is used in such a reducing atmosphere, the binder is limited to a specific material such as polyvinylidene fluoride or styrene-butadiene polymer. Conversely, when the battery is discharged, lithium ions are desorbed from the active material, the potential of the current collector rises, and reaches about 1.5 V based on lithium at the end of discharge.
The current collector that can be used in such an oxidizing atmosphere is limited to copper foil except for expensive precious metals, and the plating treatment of dissimilar metals that is applied to copper foil for printed circuit boards etc. to improve the adhesive strength is also required. It cannot be given. Further, the copper foil for the current collector is limited to a rolled copper foil having a low profile in order to keep the surface of the negative electrode material laminated on the surface flat. On the other hand, due to the demand for higher energy density of the battery, the content of the binder is required to be as small as possible.

Under the above conditions, the adhesiveness between the negative electrode material and the current collector is inevitably lowered, and the negative electrode material exhibits a hard and brittle property.
This results in at least two problems. One is peeling between the negative electrode material and the current collector during the battery manufacturing process. During handling in coating and battery assembly or winding with a positive electrode, bending stress is generated at the interface between the negative electrode material and the current collector, so that the negative electrode material peels from the current collector and detaches from the surface. In this case, not only the battery itself of the peeled laminate becomes a defective product, but in the worst case,
The inside of the coating machine is contaminated, making continuous production impossible.
The other is peeling during the cycle of using the secondary battery.
The volume of the carbon-based negative electrode active material is about 10% when the battery is charged.
It has the property of increasing and returning to the original volume when the battery is discharged.
Therefore, the binder repeats expansion and contraction following the volume change of the negative electrode active material, and stress is generated at the interface between the negative electrode material and the current collector to cause peeling. Peeling of the negative electrode material inside the battery
To break the electrical connection between the negative electrode active material and the current collector,
This increases the internal resistance of the battery and deteriorates the cycle characteristics. Since the increase in the resistance value is partial, in the worst case, the lithium dendrite is generated in the normal part and the internal short circuit is caused, which causes the battery to burst.

[0007]

SUMMARY OF THE INVENTION The present invention has been proposed in view of the above circumstances, and aims to improve the adhesion between the negative electrode material and the current collector to improve the productivity and the lithium having excellent cycle characteristics. An object is to provide an ion secondary battery.

[0008]

The present invention has solved the above problems, and a lithium ion secondary battery of the present invention comprises:
A lithium ion secondary battery comprising a negative electrode containing a negative electrode active material capable of inserting and releasing lithium ions, a positive electrode and a non-aqueous electrolyte solution, wherein the negative electrode contains an imidazole silane compound. The negative electrode preferably further includes a current collector made of copper or a copper alloy. The negative electrode containing the imidazole silane compound can be produced by adding the imidazole silane compound to the negative electrode material or by coating the copper foil as the negative electrode current collector with the imidazole silane compound.

[Operation] In a lithium secondary battery having a negative electrode using a carbon material capable of inserting and releasing lithium ions as an active material, a positive electrode and a non-aqueous electrolyte, an imidazolesilane compound is added to the negative electrode material. Alternatively, or by coating the negative electrode current collector with an imidazole silane compound, a siloxane network derived from the imidazole silane compound can be formed on the current collector to improve the adhesion between the negative electrode and the current collector. Further, as a result, the cycle characteristics of the battery are significantly improved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The imidazole silane compound used in the present invention is a compound having an imidazole group and a silanol group in one molecule, and is represented by the following general formula (1): imidazole or an imidazole derivative (hereinafter, imidazole). It can be obtained as a reaction product of a compound and a so-called silane coupling compound (hereinafter referred to as a silane compound).

[Chemical 5] In the formula, R ¹ , R ² and R ³ are hydrogen, an alkyl group having 1 to 20 carbon atoms, a vinyl group, a phenyl group or a benzyl group.

When all of R ¹ , R ² and R ³ are hydrogen, it is an imidazole, and the other compounds represented by the above formula (1) are imidazole derivatives. Preferred imidazole compounds are imidazole, 2-alkylimidazole, 2,4-dialkylimidazole, 4-vinylimidazole and the like. Of these, particularly preferable are imidazole; 2-methylimidazole, 2-methylimidazole, 2-ethylimidazole, 2-undecylimidazole, 2-heptadecylimidazole; and 2,4-dialkylimidazole. 2-ethyl-4-methylimidazole and the like can be mentioned.

The silane coupling compound is generally an organosilicon compound represented by the following general formula (2).

Embedded image (R ⁴ ) _n —Si—X _4-n (2) In the formula, R ⁴ has an organic functional group such as a vinyl group, a glycidoxy group, a methacryl group, an amino group or a mercapto group as a terminal group. Is a group, X is mainly a halogen and an alkyl group, an alkoxy group, etc., and n is an integer of 1 to 3.

Specific examples of the silane coupling compound that can be preferably used in the present invention include vinyltrichlorosilane and vinyltris (β-methoxyethoxy).
Silane, vinyltriethoxysilane, vinyltrimethoxysilane, γ- (methacryloxypropyl) trimethoxysilane, β- (3,4-epoxycyclohexyl) ethyltritrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ- Glycidoxymethyldiethoxysilane, N-β- (aminoethyl) γ-aminopropyltrimethoxysilane, N-β- (aminoethyl)-
Examples include γ-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, and γ-chloropropyltrimethoxysilane.

The imidazole silane compound is a compound obtained by synthesizing the above imidazole compound and a silane coupling compound. The synthesis is described in JP-A-5-18647.
It can be carried out by the method described in Japanese Patent Publication No. 9 or other known methods. For example, if the imidazole compound is 0.
It can be obtained by reacting for about 5 minutes to 2 hours while dropwise adding 1 to 10 molar times of the silane compound. This reaction does not require a solvent in particular, but an organic solvent such as chloroform, dioxane, methanol or ethanol may be used as a reaction solvent. However, since this reaction does not like water, it is preferable to carry out this reaction under an atmosphere of a gas containing no water such as dry nitrogen or argon so as not to mix water. The imidazolesilane compound is obtained in the form of a mixture, but it is not necessary to be isolated as long as the unreacted substances and the solvent are removed. When used in the lithium ion secondary battery of the present invention, it is convenient and preferable to use the obtained imidazole silane compound as a mixture. If necessary, the mixture can be isolated and purified by a known method such as a method utilizing a difference in solubility or column chromatography.

The imidazole silane compound has an imidazole group in the molecule and has a high affinity with the copper foil, and the alkoxysilyl group in the silane coupling compound portion can easily form a siloxane network. It is possible to form a dense coating of alkoxysilyl groups on the foil surface. This film increases the affinity with the polyvinylidene fluoride or the styrene-butadiene polymer used as the binder and increases the adhesive strength. On the other hand, in the vicinity of the current collector, when the battery is charged, its potential drops to near the reduction potential of lithium ions, so that the primer is reduced by normal primer treatment,
At this time, lithium ions are inactivated and the capacity is lowered, which is a problem. However, the imidazolesilane compound is electrochemically stable even under such a reducing atmosphere, and can be applied to a battery.

In the present invention, an imidazole silane compound represented by the following general formula (3) obtained from the synthesis of an imidazole compound and a silane coupling compound having a methacryloxyalkyl-based substituent, or a silane coupling compound An imidazole silane compound represented by the general formula (4) obtained by synthesis with a silane coupling compound having a glycidoxyalkyl-based substituent can be preferably used as

[0017]

[Chemical 7] In the above general formula (3), R ¹ , R ² and R ³ are hydrogen, an alkyl group having 1 to 20 carbon atoms, a vinyl group, a phenyl group or a benzyl group, and R ⁵ is hydrogen and having 1 to 3 carbon atoms. R ⁶ and R ⁷ are alkyl groups having 1 to 5 carbon atoms, m is 1 to 10 and n is an integer of 1 to 3. However, R ² and R ³ may combine to form a benzene ring.

[0018]

[Chemical 8] In the above general formula (4), R ¹ , R ² , R ³ , R ⁶ , and R ⁷ , and m and n have the same definitions as in the above formula (3).

As described above, in this synthetic reaction, the product is obtained as a mixture of various kinds of imidazolesilane compounds. For example, in the latter reaction above, in addition to the compound of the general formula (4), a compound represented by the following general formula (5) cyclized in the molecule, or two compounds of the above formula (4) are crosslinked. Although the compounds represented by the general formula (6) that are chemically bonded are also produced, these imidazole silane compounds can also be preferably used in the present invention.

[0020]

[Chemical 9]

[Chemical 10] In the above general formulas (5) and (6), R ¹ , R ² ,
R ³ , R ⁶ and R ⁷ , and m and n have the same definitions as those in the above formula (3).

The present invention is a lithium ion secondary battery comprising a negative electrode using a carbon material capable of inserting and releasing lithium ions as an active material, a positive electrode and a non-aqueous electrolyte, and bonding the negative electrode material and a current collector. It is intended to improve productivity, and thereby improve productivity and cycle characteristics. Specific means for improving the adhesive strength is to add an imidazole silane compound to the negative electrode material or to apply the imidazole silane compound to a copper foil which is a negative electrode current collector. By these treatments, it becomes possible to increase the adhesive strength between the negative electrode material and the copper foil by forming a siloxane network derived from the imidazole silane compound on the current collector. Particularly preferable imidazole silane compounds that enable such strong adhesion are the above-mentioned general formulas (3) to (6).
Is a compound represented by.

The optimum amount of the imidazole silane compound added to the negative electrode material or applied to the copper foil is the minimum amount necessary for forming a dense siloxane network. When added to the negative electrode material, 0.01% to 10% of the weight of the negative electrode active material,
Preferably 0.5% to 5%, and when applied to copper foil, the film thickness on the copper foil surface is 0.01 μm to 1 μm, preferably 0.05 μm
To 0.2 μm. If it is more than these values, the internal resistance of the battery increases, and if it is too small, the adhesive strength does not improve.

The addition of the imidazolesilane compound to the negative electrode material is preferably carried out in the process of producing the negative electrode. Specifically, the imidazole silane compound is added when the active material, the binder and other auxiliary materials are dispersed in water or a solvent. Alternatively, the negative electrode material laminated on the current collector in a film shape may be impregnated with a solution of an imidazole silane compound. On the other hand, the application to the copper foil can be carried out by preparing an aqueous solution of an imidazole silane compound or a solution with an organic solvent and applying the solution to the copper foil by a coating method or the like. Furthermore, addition to the negative electrode material and application to the copper foil can be used together.

The laminate of the negative electrode material and the copper foil treated with the imidazole silane compound as described above is heated before the battery is assembled to promote the formation of a siloxane network, and a special effect for improving the adhesive strength is obtained. The heating conditions are preferably 80 ° C. to 120 ° C. and 5 minutes to 30 hours. If the temperature is higher or longer than this, the copper foil is oxidized, which causes an increase in internal resistance of the battery. In addition, when the temperature is lower than this or the time is shorter than that, the formation of the siloxane network is insufficient.

Any negative electrode active material may be used as long as it is capable of inserting and desorbing lithium ions, but the carbon residue of the petroleum coke or ethylene reaction tank is 800 to 1,5.
Carbon materials such as carbon fired at 00 ° C and pitch-based carbon graphitized at 2,800 to 3,000 ° C such as artificial graphite or natural graphite can be preferably used. As the binder that collects the active material on the film and coats it stably on the current collector, polyvinylidene fluoride, styrene-butadiene polymer, and the like, which are commonly used in the production of batteries, can be preferably used. As the copper foil used for the current collector, rolled copper foil or the like used in ordinary battery production can be preferably used. The electrolyte contains cyclic carbonic acid ester,
A solution of a chain carbonic acid ester or a mixture thereof and a lithium salt dissociable in these organic solvents can be preferably used.

[0026]

EXAMPLES Preferred examples of the present invention will be described below based on experimental results. The present invention is not limited to the embodiments. (Example 1) 86% of carbon (calcining temperature 1000 ° C, average particle size 20μ) using the residue of the ethylene reaction tank as a raw material, polyvinylidene difluoride 9%, imidazole and γ-glycidoxypropyl as imidazole silane compounds -N-methyl-2-pyrrolidinone was added to 5% equimolar reaction of trimethoxysilane (92% of composition weight) and mixed well with a small mixer. This mixture was coated on a rolled copper foil (thickness: 35 μm) with a SUS screen having a mesh size of 25 mm × 60 mm and a thickness of 100 μm and dried under reduced pressure at 120 ° C. for 16 hours. The coating film after dry baking was pressed at a pressure of 1 ton / cm ² . This time,
The coating thickness was about 60 microns. The evaluation score of the cross-cut test (JISK5400) of the coating film was 8 to 10.

A coating film was formed into a size of 20 mm × 20 mm, and a porous polypropylene film (openness of about 50%) impregnated with a lithium battery electrolyte (1M LiPF ₆ EC: DMC = 1: 1 vol) was inserted between them. Then, it was laminated with metallic lithium and enclosed in a SUS container. The test cell was subjected to 100 cycles under the conditions of a temperature of 25 ° C., a current of 0.1 mA / cm ² , and a cutoff voltage of 0 V on the discharge side and 1.5 V on the charge side. The initial discharge capacity (capacity associated with the release of lithium) and efficiency were 206mAh / g, 81%, respectively, and the discharge capacity after 100 cycles was 206mAh / g, the ratio to the initial discharge capacity was 10%.
It became 0%. After 100 cycles, the coating film was taken out from the SUS container and a cross-cut test was conducted. The evaluation score was 8
It was a point. When the test cell was subjected to 300 cycles under the same conditions, the discharge capacity after 300 cycles was 186 mAh / g, and the ratio to the initial discharge capacity was 90%. The evaluation results of the cross cut test and the cycle characteristic test are shown in Examples 2 to 18 below.
Table 1 shows the results of Comparative Examples 1 to 3.

Comparative Example 1 Example 1 except that the composition of the negative electrode material was 91% carbon and 9% polyvinylidene difluoride.
The test was conducted in exactly the same manner as in. Cross-cut test of coating film (JISK54
The evaluation score of 00) was 0 to 4 points. The initial discharge capacity (capacity associated with lithium release) and efficiency are each 204
mAh / g, 79%, discharge capacity after 100 cycles is 184 mA
The ratio (maintenance rate) to h / g and the initial discharge capacity was 90%. When the coating film was taken out from the SUS container after the cycle test, the coating film was easily peeled off from the copper foil, and the cross-cut test could not be carried out. When the test cell was subjected to 300 cycles under the same conditions, a rapid decrease in discharge capacity was observed after 150 cycles, and after 300 cycles, the discharge capacity was almost 0 mAh / g.

Example 2 A test was conducted in exactly the same manner as in Example 1 except that the negative electrode material composition was changed to 90.5% of carbon of Example 1, 9% of polyvinylidene difluoride and 0.5% of the imidazolesilane compound of Example 1. went. Coating cross-cut test (JI
The evaluation score of SK5400) was 8-10 points. The initial discharge capacity (the capacity associated with the release of lithium) and efficiency are
213 mAh / g, 79%, discharge capacity after 100 cycles is 20
4 mAh / g, ratio to initial discharge capacity was 96%. 100
The score of the cross-cut test after the cycle was 8 points. 300
The discharge capacity after the cycle was 190 mAh / g, and the ratio to the initial discharge capacity was 89%.

(Examples 3 and 4) The negative electrode active material was changed to petroleum pitch coke (calcining temperature 900 ° C, average particle size 20μ), and the same tests as in Examples 1 and 2 were conducted. (Comparative Example 2) The negative electrode active material was changed to petroleum pitch-based coke (calcining temperature 900 ° C, average particle size 10 µ), and the same test as in Comparative Example 1 was conducted.

(Examples 5 and 6) The negative electrode active material was changed to carbon fiber graphitized at 3,000 ° C. with a naphthalene firing pitch,
The test was conducted in exactly the same manner as in Examples 1 and 2. (Comparative Example 3) Naphthalene firing pitch of the negative electrode active material was 3,000
A test was conducted in exactly the same manner as in Comparative Example 1 except that the carbon fiber graphitized at 0 ° C. was used.

Example 7 A 4% ethanol solution of the imidazolesilane compound of Example 1 was applied to a rolled copper foil (thickness: 35 μm) and dried under reduced pressure at 120 ° C. for 24 hours. 91% carbon (calcining temperature 1000 ° C, average particle size 20μ) made from the residue of the ethylene reactor in this copper foil, 9% polyvinylidene difluoride and N-methyl-2-pyrrolidinone (92% of the weight of the composition) Was added in the same manner as in Example 1, dried at 120 ° C. for 2 hours under reduced pressure, and subjected to a cross-cut test and a cycle test.

(Examples 8 and 9) The negative electrode active material was changed to petroleum pitch-based coke (calcining temperature 900 ° C, average particle size 50μ) and carbon fiber graphitized at 3,000 ° C in the naphthalene baked pitch. The test was conducted in exactly the same manner as in.

(Examples 10 to 18) The imidazole silane compound was changed to an equimolar reaction product of imidazole and γ-methacryloxypropyl-trimethoxysilane.
The test was conducted in exactly the same manner as in No. 9.

[0035]

[Table 1]

[0036]

As is apparent from the above description, in the present invention, in a lithium ion secondary battery comprising a negative electrode containing a negative electrode active material capable of lithium insertion and desorption, a positive electrode and a non-aqueous electrolyte, By adding an imidazole silane compound to the negative electrode material or applying an imidazole silane compound on the current collector, the adhesion between the negative electrode material and the current collector is improved, improving the productivity and cycle characteristics of battery manufacturing. Has a special effect against.

   ─────────────────────────────────────────────────── ─── Continued front page    (71) Applicant 301054313             Petka Materials Co., Ltd.             2-10-1 Toranomon, Minato-ku, Tokyo (72) Inventor Ryoichi Tajima             Stock exchange, 4 Towada, Kamisu-cho, Kashima-gun, Ibaraki             Company Petka Materials (72) Inventor Masashi Kumagai             187-4 Usuba, Hwagawa Town, Kitaibaraki City, Ibaraki Prefecture             Company Nikko Materials Isohara factory F term (reference) 5H029 AJ05 AJ14 AL06 AL07 AL08                       AM03 AM05 AM07 EJ12 HJ02                 5H050 AA07 AA19 BA17 CB07 CB08                       CB09 DA09 EA23 HA02

Claims (2)

[Claims] 1. A lithium ion secondary battery comprising a negative electrode containing a negative electrode active material capable of inserting and releasing lithium ions, a positive electrode and a non-aqueous electrolyte, wherein the negative electrode contains an imidazole silane compound. And a lithium-ion secondary battery. 2. The imidazole silane compound is one of the following:
A compound represented by any one of formulas (3) to (6) or
The lithium carbonate according to claim 1, which is a mixture of two or more thereof.
Muion secondary battery. [Chemical 1] [Chemical 2] [Chemical 3] [Chemical 4] (In the above general formulas (3) to (6), each formula is
Independently, R^l, R², R ³Is hydrogen, having 1 to 20 carbon atoms
Alkyl group, vinyl group, phenyl group or benzyl group
R^FiveIs hydrogen or an alkyl group having 1 to 3 carbon atoms,
R⁶, R⁷Is an alkyl group having 1 to 5 carbon atoms, and m is 1
To 10 and n is an integer of 1 to 3. However, R², R³
May combine to form a benzene ring. )