JP2006253157A - Lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-capacity lithium secondary battery in which the reduction in battery capacity is small during large current discharge, and the reduction of the battery capacity is small even if charging and discharging are repeated. <P>SOLUTION: In relation to a lithium secondary battery having an oval rolled electrode body 6 composed by spirally rolling a sheet-shaped positive electrode 1 and a sheet-shaped negative electrode 2 by interposing a separator 3, the sheet-shaped positive electrode is configured to form a coating film containing a positive electrode active material, a binder and an electron conduction auxiliary agent on at least one surface of a positive electrode collector body; the binder of the sheet-shaped positive electrode is configured to include acrylonitrile-butadiene-based rubber, and a polyvinylidene-fluoride-based polymer containing vinylidene fluoride as a main component monomer; the content in the coating film thereof is set not smaller than 0.2 % by mass and not greater than 2 % by mass; the content in the coating film of the electron conduction auxiliary agent for the sheet-shaped positive electrode is set to 1 to 25 times as much as that of the acrylonitrile-butadiene system rubber of the binder, and thus this lithium secondary battery is composed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a lithium secondary battery, and more particularly, to a high-capacity lithium secondary battery in which the battery capacity decreases little even with a large current discharge and the battery capacity decreases little even after repeated charge and discharge. Is.

  In general, a sheet-like positive electrode in which a coating material containing a positive electrode active material is formed by applying a paint prepared by adding a binder or a solvent to the positive electrode active material, dispersing and stirring the mixture on a conductive substrate, and drying it. In the same manner, a coating material containing a negative electrode active material is formed by applying a paint prepared by adding a binder, a solvent, etc. to the negative electrode active material, and dispersing and stirring the mixture on a conductive substrate and drying it. A lithium secondary battery produced by enclosing a laminated electrode body facing the negative electrode of the electrode with a separator together with an organic electrolyte in a battery case has an energy density per unit capacity or energy density per unit weight. It has the feature of being high.

  And as a binder used for sheet-like electrodes, such as the said sheet-like positive electrode and a sheet-like negative electrode, the polyvinylidene fluoride (PVdF) type polymer containing the vinylidene fluoride as a main component monomer is an organic electrolyte solution. On the other hand, since it is difficult to dissolve, the electrode coating film structure does not break during battery operation, and it has a fibrous structure and can suppress the increase in the internal resistance of the battery due to the insulating coating action on the active material, so it has been suitable so far Have been used as

  However, since the polyvinylidene fluoride-based polymer is a kind of fluororesin, its adhesive strength to a metal foil generally used as a conductive substrate is weak. Therefore, a polyvinylidene fluoride-based polymer is used as a binder. The formed electrode coating film has a weak adhesive force with the conductive substrate, and there was a problem that the battery capacity was reduced due to deterioration of the electrical connection between the electrode coating film and the conductive substrate during repeated charging and discharging. . Therefore, when a large amount of polyvinylidene fluoride polymer is used for the purpose of increasing the adhesive strength to the conductive substrate, the internal resistance of the battery increases due to the insulating coating action on the active material by the polyvinylidene fluoride polymer, and the active resistance is increased. There has been a problem that the filling amount of the substance is reduced and a high capacity cannot be obtained.

  On the other hand, it has been proposed that acrylonitrile-butadiene rubber (NBR) is used as a binder as an adhesive that can be obtained even in a relatively small amount.

  However, acrylonitrile-butadiene rubber has a strong insulating coating action on the active material particles, and since the swelling in the organic electrolyte is large, the coating film is loosened and the internal resistance of the battery is increased. There was a problem that the battery capacity at the time of current discharge decreased.

  Therefore, as a binder, a fluorine-based polymer such as a polyvinylidene fluoride-based polymer having a small insulating coating action on an active material, and acrylonitrile-butadiene rubber or hydrogenated acrylonitrile-butadiene rubber (H-NBR) excellent in adhesiveness. It has been proposed to improve the energy density and cycle characteristics of the battery by using in combination.

  However, also in the above case, in order to increase the adhesive strength to the extent that sufficient cycle characteristics can be obtained, the binder content in the coating film needs to be 2% or more, and acrylonitrile-butadiene rubber or hydrogenated acrylonitrile is required. -Insulating coating action on active material particles such as butadiene rubber could not be sufficiently suppressed, and accordingly, a decrease in battery capacity during large current discharge could not be sufficiently suppressed.

  In addition, lithium transition metal composite oxide is used as the positive electrode active material. However, since the positive electrode active material itself has low conductivity, when the positive electrode coating film is formed only with the positive electrode active material and the binder. There was a problem that the electric resistance of the coating film was increased, so that a desired discharge capacity could not be obtained. Therefore, it has been studied to simultaneously add a particulate material having good electrical conductivity as an electron conduction aid to increase the discharge capacity.

  As this electron conduction auxiliary agent, it is important that the electron conductivity of the material alone or a plurality of particles form a chain-shaped electron conduction network when the coating film is formed. For example, metal fine particles are preferable, and as an example of the latter, carbon-based powder is preferable.

  However, in the case of metal fine particles, it is necessary to break the barrier of the binder, which is an insulator, in order for a plurality of particles to form an electron conduction network in the form of a chain when a coating film is formed. As a result, there is a problem in that the amount of filling of the positive electrode active material is reduced and high capacity cannot be achieved. In contrast, carbon powder has the ability to form an electron conduction network in the form of a chain when the above coating film is formed, so it is not easily affected by the barrier of the binder. Since the mass (apparent specific gravity) is small, volume loss occurs, and the binder content must be increased to prevent a decrease in the mechanical strength of the coating film. There was a problem that the capacity could not be increased by lowering.

  Thus, carbon black is baked at 1800 to 3000 ° C. in an oxygen-blocking atmosphere, crystal growth is caused in the particles while maintaining the original particle diameter, and the structure is very close to the graphitized structure. It has been proposed to reduce the volume by increasing the specific gravity, thereby increasing the filling amount of the positive electrode active material.

  However, in this case as well, even if it is attempted to increase the filling amount of the positive electrode active material while keeping the electric resistance of the coating film below a certain value, the content of the binder and the electron conduction auxiliary agent cannot be sufficiently reduced, and sufficient No solution has been reached.

JP 58-48360 A Japanese Patent Laid-Open No. 9-63590 JP 2001-277771 A

  The present invention solves the problems in the conventional lithium secondary battery as described above, and the decrease in battery capacity is small even with large current discharge, and the decrease in battery capacity is small even after repeated charge and discharge. An object is to provide a lithium secondary battery.

  The present invention has been made as a result of various studies to solve the above-mentioned problems, and is an oval winding formed by winding a sheet-like positive electrode and a sheet-like negative electrode in a spiral shape via a separator. In the lithium secondary battery having an electrode body, the sheet-like positive electrode is formed by forming a coating film containing at least a positive electrode active material, a binder, and an electron conduction assistant on at least one surface of the positive electrode current collector. The sheet-like positive electrode binder comprises acrylonitrile-butadiene rubber and a polyvinylidene fluoride-based polymer containing vinylidene fluoride as a main component monomer, and the content in the coating film is 0.2% by mass or more and less than 2% by mass, and the content of the sheet-like positive electrode in the electron conduction aid in the coating film is acrylonitrile-butane as the binder. A high-capacity lithium secondary battery in which the content of the ene-based rubber is 1 to 25 times less, even when the current is discharged, the battery capacity is less decreased, and the battery capacity is less decreased even after repeated charging and discharging. Provided to solve the above problems.

  That is, the binder in the coating film of the sheet-like positive electrode of the present invention is composed of acrylonitrile-butadiene-based rubber and polyvinylidene fluoride-based polymer, but the former acrylonitrile-butadiene-based rubber is carbon powder, metal fine particles, etc. In the process of preparing the coating film for forming a positive electrode coating film, when strongly kneading with a relatively little solvent, a strong shear stress is generated by kneading due to rubber viscosity, Gaps between solid particles such as an electron conduction aid and a positive electrode active material before kneading can be reduced. In particular, even when a material having a small mass per unit volume (apparent specific gravity) is included, the voids can be greatly reduced.

  Therefore, a coating film formed by applying such a coating material for forming a positive electrode coating film to a positive electrode current collector and drying it has a high density and does not cause a volume loss. Furthermore, even when the binder content of the coating film is small, sufficient mechanical strength can be ensured.

  In addition, when acrylonitrile-butadiene rubber is added while strongly kneading solid particles such as carbon, it takes in solids several tens of times its volume to form a composite. And this mixture will reduce electrical resistance, if the taken-in solid has electronic conductivity. Therefore, the acrylonitrile-butadiene rubber contained in the coating film of the positive electrode works beneficially to form a flexible electronic conduction network while coating the active material to improve the adhesion, but rather to form a flexible electronic conduction network. A lithium secondary battery having a positive electrode can suppress a decrease in battery capacity due to deterioration of electrical bonding between the positive electrode coating film and the positive electrode current collector when charging and discharging are repeated.

  According to the present invention, it is possible to obtain a high-capacity lithium secondary battery that has a small decrease in battery capacity due to a large current discharge and a small decrease in battery capacity even after repeated charge and discharge.

  In the present invention, as the acrylonitrile-butadiene rubber used as a constituent component of the binder, for example, acrylonitrile-butadiene rubber, hydrogenated acrylonitrile-butadiene rubber and the like are preferably used.

  In the present invention, the polyvinylidene fluoride-based polymer used as a constituent component of the binder is preferably a polymer of a fluorine-containing monomer group containing 80% by mass or more of vinylidene fluoride as a main component monomer.

  Examples of the fluorine-containing monomer group containing 80% by mass or more of vinylidene fluoride as the main component monomer include vinylidene fluoride alone or a mixture of vinylidene fluoride and at least one other monomer. Examples of the other monomer include vinyl fluoride, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, hexafluoropropylene, and fluoroalkyl vinyl ether.

  By including this polyvinylidene fluoride polymer in the coating film of the positive electrode, even if it is immersed in the organic electrolyte for a long time, the adhesion structure of the coating film is not easily loosened, and stable battery characteristics are maintained. can do.

  In the present invention, the content of the binder in the coating film of the positive electrode needs to be 0.2% by mass or more and less than 2% by mass, and is 0.3% by mass or more and 1.8% by mass or less. preferable. When the content of the binder in the positive electrode coating film is less than 0.2% by mass, the mechanical strength of the coating film is insufficient, and the coating film peels off from the positive electrode current collector and repeats charging / discharging. When the content of the binder in the positive electrode coating film is 2% by mass or more, the filling rate of the positive electrode active material in the coating film decreases, and a high-capacity lithium secondary battery cannot be obtained.

  In the present invention, the mixing ratio of the acrylonitrile-butadiene rubber and the polyvinylidene fluoride polymer is preferably 3% by mass or more and 95% by mass or less based on the total amount of the acrylonitrile-butadiene rubber. When the ratio of acrylonitrile-butadiene rubber is less than the above range, there is a risk that the electrical connection between the positive electrode coating film and the positive electrode current collector deteriorates during repeated charging and discharging, and the battery capacity may be reduced. -When the ratio of the butadiene-based rubber is larger than the above range, the resistance to organic electrolytic solution is lowered and the internal resistance of the battery may be increased.

  In the present invention, as the electron conduction aid, for example, scaly graphite, carbon black, metal particles, and the like can be used. As described above, these electron conduction aids can form an electron conduction network in the coating film by kneading with acrylonitrile-butadiene rubber, and for that purpose, in the coating film of the electron conduction aid. The content of is required to be 1 to 25 times the content of the acrylonitrile-butadiene rubber of the binder. When the content of the electron conduction aid is less than the above range, the insulation of the acrylonitrile-butadiene rubber covering the positive electrode active material is increased, the internal resistance of the battery is increased, and the content of the electron conduction aid is increased. When the amount is more than the above range, the adhesiveness is lowered, and the electrical connection between the positive electrode coating film and the positive electrode current collector is deteriorated while repeating the charge and discharge, and the battery capacity is reduced.

  The electron conduction aid is preferably 0.1% by mass or more and 5% by mass or less in the coating film of the positive electrode. When the content of the electron conduction assistant in the positive electrode coating film is less than the above range, the internal resistance of the battery cannot be sufficiently reduced. When the content is higher than the above range, the positive electrode active material is filled in the coating film. The rate decreases and it becomes difficult to obtain a high-capacity lithium secondary battery.

  In order to increase the filling ratio of the positive electrode active material in the positive electrode coating film as much as possible, it is preferable to reduce the content of the electron conduction assistant in the coating film. In this case, in order to be able to suppress the increase in the internal resistance of the battery even with a small amount, as described above, when the coating film is formed, carbon black has a strong property of forming an electron conduction network in a chain shape. Is more preferable than metal fine particles.

  As this carbon black, for example, acetylene black, furnace black, thermal black and the like can be suitably used.

In the present invention, as the positive electrode active material, for example, lithium nickel oxide, lithium cobalt oxide, lithium manganese oxide (these are usually represented by LiNiO ₂ , LiCoO ₂ , LiMn ₂ O ₄ , Li and The ratio of Ni, the ratio of Li and Co, and the ratio of Li and Mn often deviate from the stoichiometric composition, but even if there is such a slight deviation, it can be used as a positive electrode active material. Are used alone or as a mixture of two or more thereof or as a solid solution thereof.

  The positive electrode includes, for example, the positive electrode active material, the electron conductive auxiliary agent, and a positive electrode coating film-forming coating material further containing a binder applied to one or both surfaces of the positive electrode current collector that also functions as a conductive substrate, By drying, forming a coating film containing at least a positive electrode active material, an electron conduction auxiliary agent, and a binder on at least one surface of the positive electrode current collector, and passing through a process of pressing and compressing as necessary. Produced.

  In the preparation of the coating material, a first step of mixing a binder, an electron conduction auxiliary agent and a solvent to obtain an electron conductive paste-like material, and a second step of mixing a positive electrode active material into the electron conductive paste-like material. It is preferable to prepare a coating film for forming a positive electrode coating film through these steps. This is because when the paint for forming a positive electrode coating film is prepared through such a process, the voids between the solid particles can be efficiently reduced by kneading the acrylonitrile-butadiene rubber and the electron conduction aid in the first process. Therefore, when this coating material is applied to at least one surface of the positive electrode current collector and dried to form a coating film, the resulting coating film has a high density and it is easy to achieve high capacity.

  As a solvent for the coating material for forming the sheet-like positive electrode coating film in the present invention, a solvent capable of dissolving both acrylonitrile-butadiene rubber and vinylidene fluoride polymer is preferable. As such a solvent, for example, N-methylpyrrolidone, dimethylacetamide, dimethylacetamide, tetrahydrofuran and the like can be used alone or in admixture of two or more.

  In the present invention, as described above, the sheet-like positive electrode is formed by applying the positive electrode coating film-forming coating material to at least one surface of the positive electrode current collector and drying to evaporate the solvent in the coating material. It is produced by forming a coating film and, if necessary, going through a step of pressurizing and compressing the coating film.

  In the present invention, as the negative electrode active material, for example, lithium metal or a lithium-containing compound is used, and examples of the lithium-containing compound include a lithium alloy and other materials. Examples of the lithium alloy include alloys of lithium and other metals such as lithium-aluminum, lithium-lead, lithium-bismuth, lithium-indium, lithium-gallium, and lithium-indium-gallium. Examples of the lithium-containing compound other than the lithium alloy include a carbon material having a turbulent layer structure and graphite. Some of these do not contain lithium at the time of production, but when acting as a negative electrode active material, they are in a state containing lithium by chemical means or electrochemical means.

  For example, the negative electrode is prepared by adding a binder and a solvent to the negative electrode active material and mixing them to prepare a negative electrode film-forming coating material, and at least one surface of the negative electrode current collector that also functions as a conductive substrate. It is prepared by applying to the substrate, drying, evaporating the solvent in the paint to form a negative electrode coating film, and pressing and compressing the coating film as necessary.

  In the present invention, the coating method for applying the positive electrode coating film-forming paint to the positive electrode current collector and the coating method for applying the negative electrode film-forming coating material to the negative electrode current collector include, for example, extrusion. Various coating methods such as a coater, reverse roller, doctor blade, applicator and the like can be adopted.

  Examples of the negative electrode binder that can be used include polyvinylidene fluoride (polyvinylidene fluoride), polytetrafluoroethylene, carboxymethylcellulose, and ethylcellulose.

  In the present invention, as the positive electrode current collector and the negative electrode current collector, for example, a metal conductive material such as aluminum, stainless steel, titanium, copper or the like is processed into a net, a punched metal, a foam metal, a plate, or the like. Used.

Examples of the electrolyte include 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate. An organic electrolyte prepared by dissolving, for example, an electrolyte such as LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiPF ₆ , LiBF ₄ , alone or in a mixture of two or more of Is used.

  As the separator, for example, a microporous polyethylene film or a microporous polyethylene film having a thickness of 10 to 50 μm and an open area ratio of 30 to 70% is preferably used.

  For example, a battery is a nickel-plated iron electrode obtained by winding a spiral electrode body wound in a spiral manner with a separator interposed between a sheet-like positive electrode and a sheet-like negative electrode produced as described above. It is manufactured through a process of inserting into a battery case made of or stainless steel, injecting an electrolyte, and sealing. The battery usually incorporates an explosion-proof mechanism that discharges gas generated inside the battery to a certain pressure to the outside of the battery to prevent the battery from bursting under high pressure.

  Next, the present invention will be described more specifically with reference to examples. However, this invention is not limited only to those Examples. In the following examples and the like,% indicating concentration is mass% unless otherwise noted.

Example 1
(1) Production of positive electrode First, as lithium cobalt oxide, scaly graphite as an electron conduction assistant, hydrogenated acrylonitrile-butadiene rubber as acrylonitrile-butadiene rubber constituting the binder, and polyvinylidene fluoride polymer A coating material for forming a positive electrode coating film was prepared using these polyvinylidene fluorides and N-methylpyrrolidone as a solvent in the following composition. Composition of positive electrode coating composition:
98 parts by mass of lithium nickel oxide [Nippon Chemical Industry Cell Seed C-10 (trade name)]
1 part by weight of acetylene black [Denka Black (trade name) manufactured by Showa Denko]
Hydrogenated acrylonitrile-butadiene rubber 0.2 parts by mass (solid content)
[Nippon Zeon BM720H (trade name) 8% N-methylpyrrolidone solution]
Polyvinylidene fluoride 0.8 parts by mass [KF Polymer (trade name) manufactured by Kureha Chemical]
25 parts by mass of N-methylpyrrolidone [including the solvent content of BM720H (supra) made by Nippon Zeon]

  The coating material was prepared as follows. First, polyvinylidene fluoride was dissolved in N-methylpyrrolidone to prepare a polyvinylidene fluoride solution having a concentration of 12%. Next, a hydrogenated acrylonitrile-butadiene rubber solution (BM720H manufactured by Nippon Zeon Co., Ltd.) and acetylene black were added to the polyvinylidene fluoride solution and mixed to prepare an electron conductive paste. . Next, a positive electrode coating film-forming coating material was prepared through a second step of adding and mixing the positive electrode active material and the remaining N-methylpyrrolidone to the electron conductive paste.

The coating material for forming a positive electrode coating film obtained as described above was applied to one surface of a positive electrode current collector made of aluminum having a thickness of 20 μm using an applicator, and dried at 110 ° C. to form a coating film. Similarly, the above-mentioned paint was applied to the back side of the positive electrode current collector, and vacuum dried at 110 ° C. for 8 hours to form a coating film having a positive electrode active material mass (single side) of 21.0 mg / cm ² per unit area of the positive electrode. . Then, the electrode body after the formation of the coating film was roll-pressed and compressed to obtain a double-coated sheet-type positive electrode having a coating thickness of 58 μm on one side and a total thickness of 136 μm.

(2) Production of negative electrode First, artificial graphite (average particle diameter 10 μm) synthesized at 2800 ° C. was used as the negative electrode active material, and the same polyvinylidene fluoride as that used for the positive electrode coating film-forming paint was used as the binder. A coating material for forming a negative electrode coating film containing them in the following ratio was prepared.
Composition of coating for forming negative electrode coating film:
Artificial graphite 95 parts by weight Polyvinylidene fluoride 5 parts by weight N-methylpyrrolidone 65.3 parts by weight

  The coating material was prepared as follows. First, polyvinylidene fluoride was dissolved in N-methylpyrrolidone to prepare a polyvinylidene fluoride solution having a concentration of 12%. Next, the negative electrode active material artificial graphite was added to this solution, and the remaining N-methylpyrrolidone from the above solution was added and mixed to prepare a negative electrode coating film-forming coating material.

Then, the obtained paint was applied to one surface of a negative electrode current collector made of copper foil having a thickness of 10 μm using an applicator and dried on a hot plate set at 110 ° C. for 20 minutes to form a negative electrode coating film. did. Similarly, the above-mentioned paint is applied to the back side of the negative electrode current collector, dried on a hot plate set at 110 ° C. for 20 minutes, and then vacuum dried at 100 ° C. for 8 hours to obtain the negative electrode active material mass per unit area of the negative electrode (One surface of the portion facing the positive electrode) A negative electrode coating film of 9.6 mg / cm ² was formed. And the electrode body after this coating film formation was roll-pressed and compressed, and the double-sided coating type sheet-like negative electrode whose one side coating film thickness is 58 micrometers and whose total thickness is 126 micrometers was obtained.

(3) Production of battery A sheet-like separator made of a microporous polyethylene film having a thickness of 15 μm and a porosity of 50% is interposed between the sheet-like positive electrode and the sheet-like negative electrode, and wound in a spiral shape. An oval wound electrode body was produced. Then, this oval wound electrode body is inserted into an aluminum rectangular battery case having a wall thickness of 0.3 mm, an outer diameter of 4 mm × 34 mm, and a depth of 48 mm, and a lead wire is welded to the positive terminal and the negative terminal. Then, an organic electrolyte prepared by dissolving 1 mol / L LiPF ₆ in a mixed solvent of ethylene carbonate and ethyl methyl carbonate (1: 1 by volume) was injected, sealed, and rectangular lithium secondary battery Was made.

  The battery shown in FIG. 1 will be described. The sheet-like positive electrode 1 and the sheet-like negative electrode 2 are wound in a spiral shape via a separator 3, and are formed into a rectangular battery case 4 as an oval wound electrode body 6. It is accommodated together with the electrolytic solution (not shown). However, in FIG. 1, in order to avoid complication, an electrode current collector such as a positive electrode current collector and a negative electrode current collector used in the production of the sheet-like positive electrode 1 and the sheet-like negative electrode 2, and the organic electrolyte solution described above Etc. are not shown. Further, the inner peripheral side portion of the oval wound electrode body is not cross-sectional.

  The battery case 4 is made of aluminum and is an outer case of the battery. The battery case 4 also serves as a positive electrode terminal. A can bottom insulator 5 made of a polytetrafluoroethylene sheet is disposed at the bottom of the battery case 4, and the positive electrode 1 and the negative electrode 2 are formed from the oval wound electrode body 6 made of the positive electrode 1, the negative electrode 2 and the separator 3. The positive electrode lead body 7 and the negative electrode lead body 8 connected to one end of each are drawn out. A stainless steel terminal 11 is attached to the aluminum lid plate 9 that seals the opening of the battery case 4 via an insulating packing 10 made of polypropylene, and the terminal 11 is made of stainless steel via an insulator 12. A steel lead body 13 is attached.

  And this cover plate 9 is inserted in the opening part of the said battery case 4, and the opening part of the battery case 4 is sealed by welding the junction part of both, and the inside of a battery is sealed.

  In the battery of Example 1, the battery case 4 and the cover plate 9 function as a positive electrode terminal by directly welding the positive electrode lead body 7 to the lid plate 9, and the negative electrode lead body 8 is welded to the lead body 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead body 13. However, depending on the material of the battery case 4, the sign may be reversed. There is also.

  FIG. 2 is a perspective view schematically showing a part of the battery of the first embodiment. FIG. 2 is shown for the purpose of showing that the battery of the first embodiment shown in FIG. 1 is a square battery. FIG. 2 schematically shows the battery, and only specific members of the battery constituent members are shown.

Example 2
In the coating composition for forming a positive electrode coating film of Example 1, 98 parts by mass of lithium nickel oxide was changed to 97.5 parts by mass, 0.2 parts by mass of hydrogenated acrylonitrile-butadiene rubber was changed to 0.3 parts by mass, Square lithium as in Example 1 except that 0.8 parts by weight of vinylidene fluoride was changed to 1.2 parts by weight, the thickness of the coating film on one side of the positive electrode after roll pressing was 59 μm, and the total thickness was 138 μm. A secondary battery was produced.

Example 3
In the coating composition for forming a positive electrode coating film of Example 1, 98 parts by mass of lithium nickel oxide was changed to 98.6 parts by mass, 0.8 parts by mass of polyvinylidene fluoride was changed to 0.2 parts by mass, and after roll press A rectangular lithium secondary battery was produced in the same manner as in Example 1 except that the coating thickness on one side of the positive electrode was 57 μm and the total thickness was 134 μm.

Example 4
In the coating composition for forming a positive electrode coating film of Example 1, 0.2 parts by mass of hydrogenated acrylonitrile-butadiene rubber was changed to 0.8 parts by mass, and 0.8 parts by mass of polyvinylidene fluoride was changed to 0.2 parts by mass. A rectangular lithium secondary battery was produced in the same manner as in Example 1 except that the thickness of the coating film on one side of the positive electrode after roll pressing was 58 μm and the total thickness was 136 μm.

Example 5
In the coating composition for forming a positive electrode coating film of Example 1, 98 parts by mass of lithium nickel oxide was changed to 97 parts by mass, 1 part by mass of acetylene black was changed to 2 parts by mass, and the coating film thickness on one side of the positive electrode after the roll press. A square lithium secondary battery was fabricated in the same manner as in Example 1 except that the thickness was 59 μm and the total thickness was 138 μm.

Example 6
In the coating composition for forming a positive electrode coating film of Example 1, 98 parts by mass of lithium nickel oxide was changed to 97 parts by mass, and 1 part by mass of acetylene black was scaled graphite [KS-15 (trade name) manufactured by Lonza Corporation] 2 Change to 0.2 parts by weight, change 0.2 parts by weight of hydrogenated acrylonitrile-butadiene rubber to 0.1 parts by weight, change 0.8 parts by weight of polyvinylidene fluoride to 0.9 parts by weight, one side of positive electrode after roll press A square lithium secondary battery was produced in the same manner as in Example 1 except that the coating thickness was set to 58 μm and the total thickness was set to 136 μm.

Example 7
In Example 1, when preparing a coating material for forming a positive electrode coating film, an electronically conductive paste-like material is obtained through a first step of adding and kneading a polyvinylidene fluoride solution, a hydrogenated acrylonitrile-butadiene rubber solution, and acetylene black. Instead of preparing and coating a coating material for forming a positive electrode coating film through a second step of mixing the positive electrode active material and the remaining N-methylpyrrolidone with this electron conductive paste, a polyvinylidene fluoride solution and A hydrogenated acrylonitrile-butadiene rubber solution, acetylene black, and a positive electrode active material are mixed together, and the remaining N-methylpyrrolidone is further mixed to prepare a coating material for forming a positive electrode coating film. A square lithium secondary battery was produced in the same manner as in Example 1 except that the film thickness was 60 μm and the total thickness was 140 μm.

Comparative Example 1
In the coating composition for forming a positive electrode coating film of Example 1, 0.2 parts by mass of hydrogenated acrylonitrile-butadiene rubber was changed to 1 part by mass, 0.8 parts by mass of polyvinylidene fluoride was changed to 0 parts by mass, and after roll press A rectangular lithium secondary battery was fabricated in the same manner as in Example 1 except that the thickness of the coating film on one side of the positive electrode was 58 μm and the total thickness was 136 μm.

Comparative Example 2
In the coating composition for forming a positive electrode coating film of Example 1, 0.2 parts by mass of hydrogenated acrylonitrile-butadiene rubber was changed to 0 parts by mass, and 0.8 parts by mass of polyvinylidene fluoride was changed to 1 part by mass. A rectangular lithium secondary battery was produced in the same manner as in Example 1 except that the coating thickness on one side of the positive electrode was 63 μm and the total thickness was 146 μm.

Comparative Example 3
In the coating composition for forming a positive electrode coating film of Example 1, 98 parts by weight of lithium nickel oxide was changed to 99.2 parts by weight, 0.2 parts by weight of hydrogenated acrylonitrile-butadiene rubber was changed to 0.09 parts by weight, Square lithium as in Example 1 except that 0.8 parts by weight of vinylidene fluoride was changed to 0.09 parts by weight, the coating thickness on one side of the positive electrode after roll pressing was 59 μm, and the total thickness was 136 μm. A secondary battery was produced.

Comparative Example 4
In the coating composition for forming the positive electrode coating film of Example 1, 98 parts by mass of lithium nickel oxide was changed to 97 parts by mass, 0.2 parts by mass of acetylene black was changed to 1 part by mass, and 0.8 parts by mass of polyvinylidene fluoride. Was changed to 1 part by mass, a square lithium secondary battery was prepared in the same manner as in Example 1 except that the thickness of the coating film on one side of the positive electrode after roll pressing was 59 μm and the total thickness was 138 μm.

Comparative Example 5
In the coating composition for forming a positive electrode coating film of Example 1, 98 parts by mass of lithium nickel oxide was changed to 97 parts by mass, 1 part by mass of acetylene black was changed to 2 parts by mass, and 0.2 parts by mass of hydrogenated acrylonitrile-butadiene rubber. Is changed to 0.07 parts by mass, 0.8 parts by mass of polyvinylidene fluoride is changed to 0.93 parts by mass, the coating thickness on one side of the positive electrode after roll pressing is 60 μm, and the total thickness is 140 μm. A rectangular lithium secondary battery was produced in the same manner as in Example 1.

  Regarding the batteries of Examples 1 to 7 and Comparative Examples 1 to 5 produced as described above, the battery capacity was measured, and the load characteristics and the cycle characteristics were examined. In the measurement of the battery capacity, when the charge / discharge current is expressed in C, 800 m is set to 1C, charging is performed at a constant voltage of 4.2V with a current limiting circuit of 1C, and discharging is provided with a current limiting circuit of 1C. The discharge capacity was measured until the voltage between the electrodes of the battery dropped to 3 V, and this was taken as the battery capacity. And when calculating | requiring load characteristics, charging / discharging current was changed into 0.2C and 2C, charging / discharging was carried out on the said conditions, discharge capacity was measured, and load characteristics were calculated | required from the following formula from those discharge capacities. The results are shown in Table 1 together with the battery capacity.

2C discharge capacity load characteristics = ──────────
0.2C discharge capacity

In addition, the cycle characteristics are examined for the change in battery capacity when the charge / discharge of 1C is repeated, and the discharge capacity of the 100th, 200th, 300th, 400th and 500th discharge capacities with respect to the first (first) discharge capacity. The retention rate was determined as the capacity retention rate from the following formula (where n is the number of times), and the result was evaluated. Their capacity retention rates (%) are shown in Table 2 and FIG.
nth discharge capacity capacity retention rate (%) = ─────────── × 100
Initial discharge capacity

  In FIG. 3, the characteristics of Example 2, Example 4 and Example 5 are the same because the characteristics do not change, and the characteristics of Examples 6 and 7 are also the same because the characteristics are almost the same. Since the characteristics of 4 and Comparative Example 5 are almost the same, the same symbols are used.

  As shown in Table 1, the batteries of Examples 1 to 7 have a large battery capacity and high capacity, and are superior in load characteristics as compared with the batteries of Comparative Examples 1 to 5, with large current discharge. There was little decrease in battery capacity.

  Moreover, as shown in Table 2 and FIG. 3, the batteries of Examples 1 to 7 have a high capacity retention rate when the number of charge / discharge cycles is increased, compared with the batteries of Comparative Examples 1 to 5, Even after repeated charge and discharge, the battery capacity decreased little and the cycle characteristics were excellent.

  That is, as a binder for the sheet-like positive electrode, using hydrogenated acrylonitrile-butadiene rubber as acrylonitrile-butadiene rubber and polyvinylidene fluoride as polyvinylidene fluoride-based polymer including vinylidene fluoride as a main component monomer, And the content rate of the binder in the coating film shall be 0.2 mass% or more and less than 2 mass%, and the content rate in the coating film of acetylene black of an electronic conduction auxiliary agent is 1 of the content rate of the said acrylonitrile-butadiene type rubber. The lithium secondary batteries of Examples 1 to 7 of the present invention within a range of ˜25 times have a small decrease in battery capacity due to a large current discharge, and a small decrease in battery capacity even after repeated charge and discharge. Moreover, the capacity was high.

  On the other hand, the battery of Comparative Example 1 using only acrylonitrile-butadiene rubber without using polyvinylidene fluoride as a binder has poor load characteristics compared to the batteries of Examples 1 to 7, and has a large current discharge. The battery capacity of the battery of Comparative Example 2 using only polyvinylidene fluoride without using acrylonitrile-butadiene rubber as the binder was poor in cycle characteristics as compared with the batteries of Examples 1-7. It was shown that the battery capacity was greatly reduced by repeated charging and discharging. And the battery of the comparative example 3 whose amount of binders is 0.18 mass% and has less binder amount than 0.2 mass% or more and less than 2 mass% prescribed | regulated by this invention is compared with the battery of Examples 1-7. The battery capacity was low, the load characteristics were poor, and the cycle characteristics were also poor. Further, the battery of Comparative Example 4 having a binder amount of 2% by mass and having more binder than 0.2% by mass to less than 2% by mass as defined in the present invention has a capacity as compared with the batteries of Examples 1 to 7. The battery of Comparative Example 5 is small and the content of acetylene black is about 28.6 times the content of the acrylonitrile-butadiene rubber and the content of acetylene black is more than 1 to 25 times defined in the present invention. Compared with the batteries of Examples 1 to 7, the cycle characteristics were particularly bad.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically an example of the lithium secondary battery of this invention, (a) is the top view, (b) is the fragmentary sectional view. FIG. 2 is a perspective view schematically showing the lithium secondary battery shown in FIG. 1 in a partially exploded state. It is a figure which shows the change of the capacity | capacitance retention rate with the increase in the repetition frequency of charging / discharging of the battery of Examples 1-7 and the battery of Comparative Examples 1-5.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Battery case 5 Can bottom insulator 6 Oval winding electrode body 7 Positive electrode lead body 8 Negative electrode lead body 9 Cover plate 10 Insulating annular packing 11 Terminal 12 Insulator 13 Lead body

Claims (8)

  A lithium secondary battery having an oval wound electrode body in which a sheet-like positive electrode and a sheet-like negative electrode are spirally wound via a separator, wherein the sheet-like positive electrode is a positive electrode current collector It is formed by forming a coating film containing at least a positive electrode active material, a binder, and an electron conduction auxiliary agent on at least one surface, and the binder of the sheet-like positive electrode includes acrylonitrile-butadiene rubber and a main component monomer. A coating film of an electron conduction assistant for the sheet-like positive electrode, comprising a polyvinylidene fluoride-based polymer containing vinylidene fluoride and having a content in the coating film of 0.2% by mass or more and less than 2% by mass. A lithium secondary battery characterized in that the content is 1 to 25 times the content of acrylonitrile-butadiene rubber as the binder.   2. The lithium secondary battery according to claim 1, wherein the electron conduction assistant of the sheet-like positive electrode is at least one selected from flaky graphite, carbon black, and metal particles.   The lithium secondary battery according to claim 1 or 2, wherein the sheet-like positive electrode electron conduction assistant contains carbon black.   The lithium secondary battery according to any one of claims 1 to 3, wherein the content of the electron conduction assistant in the coating film of the positive electrode is 0.1% by mass or more and 5% by mass or less.   The acrylonitrile-butadiene rubber is a hydrogenated acrylonitrile-butadiene rubber, and the content of the acrylonitrile-butadiene rubber in the coating film of the positive electrode is 0.1% by mass or more and 0.8% by mass or less. A lithium secondary battery according to any one of the above.   The lithium secondary battery according to any one of claims 1 to 5, wherein the lithium secondary battery is rectangular. In manufacturing the lithium secondary battery according to any one of claims 1 to 6,
The first step of mixing the binder and the electron conduction aid in the presence of a solvent to obtain an electron conductive paste, and the second step of adding a positive electrode active material to the electron conductive paste and mixing. To prepare a coating film for forming a positive electrode coating film, apply the coating composition for forming a positive electrode coating film on at least one surface of the positive electrode current collector, and dry the coating film to form a sheet-like positive electrode coating film A method for producing a lithium secondary battery.   The method for producing a lithium secondary battery according to claim 7, wherein the lithium secondary battery is rectangular.

Images