JP5260851B2 - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery having high reliability and good workability by suppressing occurrence of electrode scraps and separation of the electrode active material at the time of manufacturing the lithium-ion secondary battery. <P>SOLUTION: The lithium-ion secondary battery is equipped with a positive electrode and a negative electrode having an electrode mixture layer containing an electrode active material 1 and a binder 2 formed on a current collector 4, an electrolyte, and a separator. A binder layer 5 is provided on the surface of the electrode mixture layer 3 of at least one of the positive electrode or the negative electrode. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to an electrode structure.

  Conventionally, in a lithium ion secondary battery, a positive electrode slurry containing a positive electrode active material capable of occluding and releasing lithium ions is applied onto a positive electrode current collector made of a metal such as an aluminum foil, dried, and compressed to form a positive electrode mixture layer. The formed positive electrode and a negative electrode slurry containing a negative electrode active material that absorbs and releases lithium ions on a negative electrode current collector made of a metal such as copper foil, and dried and compressed to form a negative electrode mixture layer. The electrolyte solution, a non-woven fabric containing the electrolyte, and a separator made of a polyolefin microporous membrane are arranged to be opposed to each other, and these are sealed in a battery case.

  In the laminated lithium ion secondary battery, the positive electrode is dissolved in lithium cobaltate as a positive electrode active material, carbon as a conductive material and PVDF (polyvinylidene fluoride) as a binder are dissolved in NMP (N-methyl 2-pyrrolidone), The mixed positive electrode slurry was applied to an aluminum foil serving as a positive electrode current collector, dried and compressed to form a positive electrode mixture layer on the positive electrode current collector. Also, the negative electrode is made of graphite as a negative electrode active material, carbon as a conductive material and PVDF as a binder are dissolved in NMP, and the mixed negative electrode slurry is applied to a copper foil as a negative electrode current collector, dried and compressed to obtain a negative electrode current collector. It was manufactured by forming a negative electrode mixture layer on the electric body.

  These positive electrode and negative electrode are cut to a predetermined size and laminated via a separator, and then the positive electrode and the negative electrode are welded to external lead tabs, respectively, to produce an electrode assembly, which is then housed in an exterior body made of a laminate material. Then, after injecting the electrolyte, it was sealed and then charged to produce a laminated lithium ion secondary battery.

  When manufacturing conventional electrodes such as a positive electrode and a negative electrode, PVDF as a binder inside the electrode is usually uniformly dispersed. When the PVDF is low, the bonding between the active material particles of the electrode is weak and the electrode is easily peeled off. When the PVDF is high, the density of the active material is low and the capacity of the battery cannot be increased.

  In the electrode manufacturing process, electrode debris occurs due to electrode peeling when the electrode is cut, and contact between the electrodes that occurs mainly during assembly of the electrode assembly, peeling of the electrode active material due to rubbing, etc. occur. It was.

  In Patent Document 1, the concentration of the binder in the electrode mixture layer is increased near the current collector to improve adhesion with the current collector, and the binder concentration on the surface of the electrode mixture layer is also increased to increase the surface. Proposals have been made to prevent the electrode material from falling off. Further, Patent Document 2 proposes to regulate the distribution state of the binder in the negative electrode mixture layer in order to suppress peeling of the electrode active material from the current collector in the negative electrode. Further, proposals for changing the binder concentration in the electrode mixture layer are described in Patent Documents 3 and 4.

JP-A-10-270013 Japanese Patent Laid-Open No. 10-32235 JP-A-11-265708 JP 2004-95538 A

  However, by changing the binder concentration in the electrode mixture layer as in the conventional patent documents 1 to 4, the surface of the electrode mixture layer is not sufficiently protected, and electrode scraping and electrode active material peeling occur. There was a fear.

  It is an object of the present invention to provide a lithium ion secondary battery that has good workability and high reliability by suppressing generation of electrode debris and peeling of an electrode active material when producing a lithium ion secondary battery. .

The present invention has been made as a result of studying the structure of an electrode in order to solve the above problems. The lithium ion secondary battery of the present invention is a lithium ion secondary battery comprising a positive electrode and a negative electrode on which an electrode mixture layer containing an electrode active material and a binder is formed on a current collector, an electrolyte, and a separator. consisting only of polyvinylidene fluoride on a surface of at least one of the electrode mixture layer of the negative electrode have a focal binder layer spider, 0.005 times to 0 of the average particle diameter of the thickness of the binder layer is the electrode active material .1 Baidea Rukoto and said. Preferably it has a binder concentration near the surface portion of the front Symbol electrode mixture layer or higher than the internal binder concentration of the electrode mixture layer, the binder concentration of the near-surface portion and the current collector vicinity of the electrode mixture layer Is preferably higher than the binder concentration inside the electrode mixture layer, the binder concentration in the vicinity of the surface of the electrode mixture layer and in the vicinity of the current collector is 10 to 20% by mass, and the inside of the electrode mixture layer It is preferable that the binder density | concentration of 1-5 mass%.

  According to the present invention, it is possible to prevent the electrode active material from falling off from the surface of the electrode mixture layer by forming a binder, preferably PVDF, layer of about 0.1 to 2 μm on the surface of the electrode mixture layer. In addition, by controlling the binder concentration inside the electrode mixture layer, it is possible to provide a lithium ion secondary battery that is difficult to peel off and has a high capacity. Furthermore, by increasing the binder concentration in the vicinity of the surface of the electrode mixture layer and in the vicinity of the current collector and making the binder concentration inside the electrode the same as in the past, it is possible to prevent electrode scraping and peeling due to load during product rubbing, handling, and cutting. It becomes possible to prevent.

  According to the present invention, by forming a binder layer on the surface of the electrode mixture layer, generation of electrode debris is suppressed, mixing into the battery is eliminated, and the reliability of the lithium ion secondary battery with a low defect rate is improved. Is effective.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an electrode of a lithium ion secondary battery of the present invention, FIG. 2 is an explanatory diagram of an electrode mixture layer coating and drying process, and FIG. 3 is an explanation of a binder layer coating and drying process. FIG.

  A lithium ion secondary battery becomes an anode, an electrode mixture layer containing an electrode active material and a binder that can occlude and release lithium ions formed on a current collector made of a metal such as an aluminum foil that serves as a cathode, and an anode. An electrode active material that occludes and releases lithium ions formed on a current collector made of a metal such as copper foil, and an electrode mixture layer that contains a binder, an electrolyte solution, a nonwoven fabric containing the electrolyte, and a polyolefin microporous membrane It arrange | positions and is comprised facing through the separator which consists of.

First, manufacture of the positive electrode will be described with reference to FIGS. 1 and 2. The electrode 6 is a complex oxide represented by LiMO ₂ (wherein M represents at least one kind of transition metal), for example, LiCoO ₂ , LiNiO ₂ or LiMn ₂ O An electrode mixture slurry 7 prepared by dispersing and mixing an electrode active material 1 composed of ₄ and the like, a conductive material such as carbon black, and a binder 2 such as polyvinylidene fluoride (PVDF) in a solvent such as NMP is applied and dried by a coating apparatus. The mixture layer 3 is formed on the current collector 4 so as to be 50 to 150 μm after drying. After the electrode mixture layer 3 was formed on one side of the current collector 4 to form the electrode 6a, the electrode mixture slurry 7 was similarly applied to the opposite surface of the current collector 4 to form the electrode mixture layer 3 on both sides. Let it be an electrode 6a. Then, after compressing in order to improve the packing density of the electrode active material, it is cut into a predetermined size.

  When applying and drying the electrode mixture slurry 7, as shown in FIGS. 1 and 2, heat 8 is applied to the opposite side of the surface on which the current collector 4 is applied, for example, with warm air at 120 to 150 ° C. The solvent such as NMP in the current collector vicinity 3c of the electrode mixture layer 3 is dried. By this drying method, after a binder such as PVDF is first formed on the current collector 4, evaporation of a solvent such as NMP proceeds, so that the adhesion between the current collector 4 and the electrode mixture layer 3 is improved. If heat is applied simultaneously from above and below and dried, a binder layer such as PVDF is formed on the surface of the electrode mixture layer, and evaporation of a solvent such as NMP is inhibited, and at the same time, the current collector of the electrode mixture layer 3 The amount of the binder made of PVDF or the like in the vicinity 3c is reduced, and the adhesion is weakened. As for the binder density | concentration of the electrical power collector vicinity part 3c of the electrode mixture layer 3, 10-20 mass% is desirable. If it is less than 10% by mass, the adhesion between the electrode and the metal foil is deteriorated.

  Thereafter, as shown in FIGS. 1 and 3, for example, a binder solution 9 such as a 0.5 to 5 mass% PVDF solution (NMP 95 to 99.5 mass%) is applied to the surface of the electrode mixture layer 3 and dried. The electrode 6 is provided with the binder layer 5 formed thereon. By changing the concentration of the binder solution, the coating thickness, and the drying time, the binder concentration distribution of PVDF or the like inside the electrode, that is, the thickness of the electrode mixture layer 3 near the surface 3a and the inside 3b and the electrode mixture layer 3 The thickness of the binder layer 5 such as PVDF on the surface changes. The binder solution 9 may be applied and dried after the electrode mixture layer 3 is applied and dried, or may be applied, dried and further compressed. The thickness of the binder layer is preferably 0.005 to 0.1 times the average particle diameter of the electrode active material. If it is less than 0.005 times, the active material peeling prevention function is reduced, and if it is more than 0.1 times, there is a problem of deterioration of battery characteristics, particularly an increase in impedance.

  As for the negative electrode, an electrode mixture slurry in which a current collector made of a metal foil such as a flat copper foil is mixed with an electrode active material made of graphite and, if necessary, a conductive material and a binder in a solvent is mixed as in the production of the positive electrode. And the electrode mixture layer is formed on the current collector. Thereafter, a binder layer is formed as in the case of the positive electrode. Note that the binder layer may be formed by only one of the positive electrode and the negative electrode.

  After forming the positive electrode and the negative electrode, the electrodes are cut to a predetermined size by a known method, laminated via a separator, and then the positive electrode and the negative electrode are welded to the external lead tabs to produce an electrode assembly. This is housed in an outer package made of a laminate material, injected with an electrolytic solution, sealed, and then charged to produce a laminated lithium ion secondary battery.

  Hereinafter, the present invention will be specifically described based on examples.

  First, the production of the positive electrode will be described. An electrode mixture slurry was produced by dispersing and mixing 96% by mass of lithium cobaltate having an average particle size of 25 μm, a conductive material composed of 1% by mass of carbon black, and 3% by mass of PVDF in NMP. The electrode mixture slurry was applied to a current collector made of aluminum foil so that the thickness after drying was 200 μm. As shown in FIG. 2, the drying was performed by applying heat with warm air of 150 ° C. from the opposite side of the surface on which the current collector was applied. Then, after forming an electrode also on the opposite surface and compressing the electrode, an electrode electrode peeling test was performed on this electrode. That is, when the limit of the electrode mixture layer peeling from the current collector was measured by rubbing the sheet-like electrode plate produced on a metal round bar having a different diameter every 1 mm, the electrode was not removed even with a round bar of Φ3.0 mm. There was no peeling off. At this time, the binder concentration of the electrode mixture layer at 2 μm from the current collector was 10 mass%.

  For comparison, the same electrode mixture slurry as in Example was used, and drying was performed by applying heat from both the upper and lower sides of the current collector application surface with hot air, and then the electrode was compressed, and then an electrode electrode peeling test was performed. However, the electrode peeled off with a round bar of Φ4.0 mm. At this time, the binder concentration of the electrode mixture layer at 2 μm from the current collector was 3 mass%. It was confirmed that the adhesion between the electrode mixture layer and the current collector is stronger when the heat is applied from the current collector side than when the heat is applied simultaneously from the top and bottom.

Thereafter, a binder solution of 5% by mass PVDF solution (NMP 95% by mass) is applied to the surface of the electrode mixture layer and dried with hot air at 150 ° C. When an amount of 17.5 g / m ² is applied to a 5% by mass solution, a PVDF film of about 0.5 μm is formed on the electrode surface. Is formed. By forming the binder layer in the form of a spider web, the ingress of the electrolytic solution and the ingress of lithium ions are not prevented, and the active material particles do not fall off.

  Next, production of the negative electrode will be described. 94% by mass of graphite having an average particle diameter of 20 μm, 1% by mass of a conductive material (carbon black) and 5% by mass of PVDF were dispersed and mixed in NMP to produce an electrode mixture slurry. The electrode mixture slurry is applied to a current collector made of copper foil so as to have a thickness of 170 μm after drying, and heated by applying hot air from the opposite side of the surface on which the current collector is applied to dry the electrode mixture layer Formed. Similarly, after the electrode mixture layer was formed on the current collector on the opposite side, it was compressed with a metal roller so that the thickness after compression was 110 μm in order to improve the electrode density.

Thereafter, a binder solution of 5% by mass PVDF solution (NMP 95% by mass) is applied to the surface of the electrode mixture layer 3 and dried. A PVDF film of about 0.5 μm is formed on the electrode surface by applying 17.5 g / m ^{2 of} 5 mass% solution. Actually, a spider web-like binder film is formed on an active material having a particle diameter of about 20 μm. By forming the binder film in the form of a spider web, the ingress of the electrolytic solution and the ingress of lithium ions are not prevented, and the active material particles do not fall off.

  About this electrode, the adhesiveness of the surface of an electrode mixture layer was investigated by the peeling test using an adhesive tape. Peeling was done by attaching Nichiban LP-15, 15 mm wide adhesive tape to the electrode, pressing it on a 15 mm square area with a load of 100 g, and then peeling the tape for about 5 seconds. When about 0.5 μm of PVDF film was calculated on the surface of the electrode mixture layer and the amount of negative PVDF was 5% by mass, the electrode did not adhere to the adhesive surface of the tape in the peeling test.

  As a comparison, when the binder layer was not formed on the surface of the electrode mixture layer and the PVDF content in the electrode mixture layer was 5% by mass, the electrode adhered to 90% of the adhesive surface of the tape. Moreover, when the PVDF amount in the electrode mixture layer was 10% by mass without forming the binder layer, the electrode adhered to 50% of the adhesive surface of the tape.

  From this result, it has been found that electrode dropping can be reduced by forming a PVDF film on the electrode surface. That is, when the binder layer is not formed on the surface of the electrode mixture layer, the electrode powder adheres to the adhesive surface of the tape, and when the electrode is handled during the manufacturing process, electrode debris is generated and mixed into the battery, resulting in a short circuit failure. The electrode of the present invention may cause a cause, but even if a tape is applied to the electrode surface and peeled, the electrode does not adhere to the tape, and the generation of electrode scraps is reduced in the work area. There was no electrode waste. Therefore, the defect rate and the reliability regarding the short circuit of the product are improved.

The cross-sectional schematic diagram of the electrode of the lithium ion secondary battery of this invention. Explanatory drawing of the application | coating mixture layer of the lithium ion secondary battery of this invention, and a drying process. Explanatory drawing of the application | coating of the binder layer of the lithium ion secondary battery of this invention, and a drying process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrode active material 2 Binder 3 Electrode mixture layer 3a (electrode mixture layer) surface vicinity part 3b (electrode mixture layer) inside 3c (electrode mixture layer) current collector vicinity part 4 Current collector 5 Binder Layer 6 (binder layer formed) electrode 6a (electrode mixture layer formed on current collector) electrode 7 electrode mixture slurry 8 heat 9 binder solution

Claims (4)

In a lithium ion secondary battery comprising a positive electrode and a negative electrode on which an electrode mixture layer containing an electrode active material and a binder is formed on a current collector, an electrolyte, and a separator, at least one electrode mixture layer of the positive electrode or the negative electrode the have a focal binder layer spider composed only polyvinylidene fluoride on the surface, characterized in 0.005 times to 0.1 Baidea Rukoto an average particle size of the thickness of the binder layer is the electrode active material Lithium ion secondary battery. The lithium ion secondary battery according to claim 1, wherein the binder concentration in the vicinity of the surface of the electrode mixture layer is higher than the binder concentration inside the electrode mixture layer. Lithium-ion secondary battery according to claim 1 or 2, wherein the near-surface portion and the binder concentration of the current collector vicinity of the electrode mixture layer is higher than the internal binder concentration of the electrode mixture layer. The binder concentration in the vicinity of the surface of the electrode mixture layer and the vicinity of the current collector is 10 to 20% by mass, and the binder concentration in the electrode mixture layer is 1 to 5% by mass. The lithium ion secondary battery according to any one of 1 to 3 .

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