JP2005339938A - Manufacturing method of electrode for lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a lithium-ion secondary battery with a porous film with excellent heat resistance and resistance to swelling. <P>SOLUTION: The manufacturing method of the electrode contains a process of coating paste for a porous film consisting of an inorganic oxide filler and a binder with water as a dispersant on the surface of an electrode, and drying it to form a porous film. At least, one of the binders uses scaly silica. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a lithium ion secondary battery having excellent life characteristics and excellent safety such as short circuit resistance and heat resistance, and more particularly to a method for producing the electrode.

  As electronic devices become more portable and cordless, lithium-ion secondary batteries that are small, light, and have high energy density are attracting attention as power sources for driving them. In an electrochemical battery such as a lithium ion secondary battery, there is a separator between the positive electrode and the negative electrode that serves to electrically insulate both electrodes and hold an electrolyte solution. In a lithium ion secondary battery, a microporous thin film sheet mainly made of polyethylene is currently used as a separator.

  However, sheet-like separators made of these resins generally tend to shrink at a low temperature, so that when an internal short circuit occurs or a sharply shaped protrusion such as a nail penetrates the battery, the short-circuit reaction heat generated instantaneously The short-circuit portion is enlarged, and further, a large amount of reaction heat is generated, and abnormal overheating is promoted.

Therefore, in order to improve the safety including the above-described problems, a technique (Patent Document 1 and Patent Document 2) for forming a porous film containing inorganic particles on a sheet-like separator, or a resin having a low glass transition point. A technique for forming a porous film on the electrode (Patent Document 3) and a technique for forming a protective layer made of solid particles such as alumina and a water-soluble polymer on the electrode (Patent Document 4) have been proposed. Yes.
JP 2001-319634 A JP 2002-8730 A JP-A-11-144706 JP-A-9-147916

  However, Patent Documents 1 and 2 have a drawback that the porous film is contracted together with the sheet-like separator due to a large amount of heat generated in the internal short circuit portion because the porous film is formed on the sheet-like separator. In the first place, this technology is aimed at suppressing the growth of lithium dendrites and improving the high-rate discharge characteristics, and it is inevitable that the safety at the time of internal short circuit or nail penetration cannot be guaranteed.

  Regarding Patent Document 3, as in the case of conventional separators, the resin softens during short-circuit heat generation and exhibits a shutdown effect. For example, in a nail penetration test that is a substitute evaluation of internal short-circuit, depending on the test conditions, Since the exothermic temperature locally exceeds several hundred degrees Celsius, the nail penetrates the positive and negative electrodes due to deformation of the porous film due to softening or burning of the resin, which may cause abnormal overheating. Therefore, means using the resin shutdown effect cannot be an absolute safety mechanism against an internal short circuit. This also applies to “Combination with separator” described in Patent Document 3.

  Since Patent Document 4 includes inorganic solid particles having excellent heat resistance and water-soluble polymers such as polyacrylic acid derivatives and cellulose derivatives, it can be expected to suppress deformation of the film itself during short-circuit heat generation. However, in general, styrene-butadiene copolymer rubber particles and modified products thereof are generally used for the negative electrode of a lithium ion secondary battery. These may be added in a smaller amount than conventional binders such as polyvinylidene fluoride, and the lithium ion acceptability is improved. When using these binders, in order to apply | coat an active material layer to a core material normally, water-soluble cellulose resin is used together as a thickener. When the porous film of Patent Document 4 is applied to such a negative electrode, water, which is a solvent for the porous film paint, is immersed in the negative electrode to swell the negative electrode thickener, and deformation of the negative electrode frequently occurs after solvent volatilization. The problem that occurs. A negative electrode that is free from deformation is put to practical use, but is not suitable for practical use because the yield is greatly reduced.

  In order to avoid this problem, it is considered effective to use a water-insoluble binder. However, many of these use N-methyl-2-pyrrolidone whose physical properties are close to those of the electrolytic solution, which is the main material of the battery, as a solvent, and basically have high affinity with the electrolytic solution. Therefore, most water-insoluble binders take in the electrolyte solution during charge / discharge and swell, reducing the ionic conductivity between the electrode plates, resulting in deterioration of the discharge characteristics of the battery.

This invention solves the said subject, and it aims at providing the electrode for lithium ion secondary batteries which has a porous film excellent in heat resistance and swelling resistance.
Another object of the present invention is to provide a method for producing a porous film applicable to a negative electrode having high lithium ion acceptability.

  The present invention provides a positive electrode made of a composite lithium oxide, a negative electrode made of a material capable of reversibly occluding and releasing lithium, a separator, and at least one of the positive electrode and the negative electrode of a lithium ion secondary battery comprising a non-aqueous electrolyte. The present invention relates to an electrode manufacturing method applied to the above. The electrode manufacturing method of the present invention is a method for manufacturing an electrode in which a porous film paste comprising an inorganic oxide filler and a binder containing water as a dispersion medium is applied to the surface of the electrode and dried to form a porous film. In addition, phosphorus (scale) flake silica is used for at least one of the binders.

  In a binder dispersion in which flaky silica is dispersed in water, the silanol groups in the silica show hydrophilicity and become a collection of micelle bodies incorporating water, and the affinity with graphite as the negative electrode active material Is moderately reduced. For this reason, the problem that water is immersed in a negative electrode, the thickener of a negative electrode is swollen, and a deformation | transformation of a negative electrode occurs frequently after water volatilization can be avoided.

  When used in combination with flaky silica binder, and the emulsion membrane rubber-like polymer is included in the porous membrane binder, the flexibility of the porous membrane is further improved, and the electrode plate is produced by winding the electrode. Yield can be improved.

  According to the present invention, it is possible to provide an electrode that provides a lithium ion secondary battery having greatly improved safety while having discharge characteristics equivalent to those when only a conventional sheet-like separator is used. Become.

Although the preferable aspect of this invention is shown below, this invention is not limited to these.
The porous membrane, which is the gist of the present invention, must be bonded to any of the positive and negative electrodes described in detail below. First, when the adhesive is formed on the separator, the porous film is contracted together with the sheet-like separator due to a large amount of heat generated by the internal short-circuit portion regardless of its own heat resistance, as described in Patent Documents 1 and 2 described above. Has the disadvantages. In addition, when forming a sheet with a porous film alone, it is necessary to increase the thickness from the viewpoint of maintaining the sheet shape, and a large amount of binder is required. Is not preferable.

As at least one of the binders, flaky silica is used from the viewpoint of avoiding deformation of the electrode, particularly the negative electrode, when forming the porous film. Including an emulsion type rubbery polymer in combination with flake shaped silica is a preferred embodiment from the viewpoint of yield improvement when producing a wound electrode group.
The flaky silica has a thickness of 0.01 to 0.5 μm, a surface diameter of 2 to 5 μm, a large silanol group per specific surface area of 50 to 70 μmol / m ^2, and can be obtained as a slurry dispersed in water. For example, functional industrial silica manufactured by Dokai Chemical Industry Co., Ltd., sold under the name Sun Lovely, is preferred.

  An inorganic oxide is selected for use as a filler in the porous membrane. Various resin fine particles are also commonly used as fillers. However, as described above, heat resistance is required, and in addition, the resin must be electrochemically stable within the range of use of the lithium ion battery. An inorganic oxide is most preferable as a material suitable for coating. The inorganic oxide is most preferably α-alumina or titanium oxide from the viewpoint of electrochemical stability, and the content of the filler in the porous film is preferably 50% by weight or more and 99% by weight or less. When the binder is excessively less than 50% by weight, it becomes difficult to control the pore structure constituted by the gaps between the fillers. In addition, when the amount of the binder exceeds 99% by weight, the adhesion of the porous film is lowered, and thus a loss of function due to dropping off is caused. This inorganic oxide may be used by mixing or multilayering a plurality of types.

As for the positive electrode, lithium cobaltate and its modified products such as those obtained by eutectic aluminum or magnesium as the active material, lithium nickelate and its modified products such as those obtained by replacing some nickel with cobalt, etc. A composite oxide such as lithium and a modified product thereof can be given. As the binder, polytetrafluoroethylene, modified acrylonitrile rubber particle binder, such as Nippon Zeon Co., Ltd. BM-500B, etc., carboxymethyl cellulose (CMC), polyethylene oxide, soluble modified acrylonitrile rubber having a thickening effect, For example, you may combine with Nippon Zeon Co., Ltd. product BM-720H. Further, a single polyvinylidene fluoride (PVDF) having both binding properties and thickening properties and a modified product thereof may be used alone or in combination.
As the conductive agent, acetylene black, ketjen black, and various graphites may be used alone or in combination.

  For the negative electrode, various natural graphites and artificial graphites, silicon-based composite materials such as silicide, and various alloy materials can be used as the active material. As the binder, various binders including PVDF and modified products thereof can be used. From the viewpoint of improving lithium ion acceptability as described above, styrene-butadiene copolymer rubber or modified products thereof are used. It is more preferable to add a small amount in combination with a cellulose resin such as CMC.

For the non-aqueous electrolyte, ethylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate can be used alone or in combination as a solvent. In order to form a good film on the positive and negative electrodes and to ensure stability during overcharge, it is also possible to use vinylene carbonate, cyclohexylbenzene, and modified products thereof. As a salt dissolved in these solvents, various lithium salts such as LiPF ₆ and LiBF ₄ can be used.

  The separator is not particularly limited as long as it is a composition that can withstand the use range of the lithium ion battery, but a microporous film of an olefin resin such as polyethylene or polypropylene is generally used in a single or composite manner, Moreover, it is preferable as an aspect. Although the thickness of this separator is not particularly limited, from the viewpoint of maintaining the design capacity while demonstrating the effects of the porous membrane described above, the total of the thickness of the combined porous membrane is conventionally used (15-30 μm) and It is preferable that it is comparable.

Examples of the present invention will be described below.
<< Comparative Example 1 >>
3 arms of lithium cobaltate with 1 kg of polyvinylidene fluoride (PVDF # 1320, Kureha Chemical Co., Ltd., N-methyl-2-pyrrolidone (NMP) solution with a solid content of 12% by weight), 90 g of acetylene black and an appropriate amount of NMP The mixture was stirred with a type kneader to prepare a positive electrode paste. This paste was applied to a 15 μm thick aluminum foil, dried, rolled to a total thickness of 160 μm, and then slit into a width that could be inserted into a cylindrical mold 18650 to obtain a positive electrode hoop.

  On the other hand, 3 kg of artificial graphite was mixed with Nippon Zeon Co., Ltd. styrene-butadiene copolymer rubber particle binder BM-400B (solid content 40% by weight) 75 g, CMC 30 g and an appropriate amount of water in a double arm kneader. Stirring to prepare a negative electrode paste. This paste was applied to a 10 μm thick copper foil, dried, rolled to a total thickness of 180 μm, and then slit into a width that could be inserted into a cylindrical mold 18650 to obtain a negative electrode hoop.

<< Comparative Example 2 >>
Using NMP as a dispersion medium, based on 300 g of α-alumina with a median diameter of 0.3 μm, polyacrylonitrile modified rubber binder BM720H manufactured by Nippon Zeon Co., Ltd. as a binder and 4 wt. After adding so as to be a part, the mixture was stirred with a double-arm kneader to prepare a porous film paste. This paste was applied to the negative electrode hoop of Comparative Example 1 by 5 μm on one side and dried.

<< Comparative Example 3 >>
Comparative Example 2 except that water was used as a dispersion medium and polyacrylic acid manufactured by Wako Pharmaceutical Co., Ltd. was added as a solid membrane binder to a solid content of 4 parts by weight with respect to 100 parts by weight of alumina. Thus, a negative electrode was produced.

Example 1
Except that water is used as a dispersion medium, and functional industrial silica sun labry LFS HN050 manufactured by Dokai Chemical Industries, Ltd. is added as a binder for the porous membrane to a solid content of 4 parts by weight per 100 parts by weight of alumina Produced a negative electrode in the same manner as in Comparative Example 3.

<< Examples 2 and 3 >>
Using water as a dispersion medium, the same binder HN050 as in Example 1 as a binder for the porous membrane and styrene-butadiene rubber binder BM400B manufactured by Nippon Zeon Co., Ltd. as solids, respectively, with respect to 100 parts by weight of alumina A negative electrode was produced in the same manner as in Comparative Example 3 except that 2 parts by weight was added (Example 2). Further, a negative electrode was produced in the same manner by using an acrylic-modified rubber binder AD-211 instead of the styrene-butadiene rubber binder (Example 3).

Out of the negative electrodes of these examples and comparative examples, the non-defective external appearance was wound together with the positive electrode of comparative example 1 and a polyethylene microporous film separator having a thickness of 20 μm in a spiral shape, cut into a predetermined length, and then inside the battery case Inserted into. Next, 5.5 g of an electrolytic solution in which 1M of LiPF ₆ and 3% by weight of vinylene carbonate were dissolved in a solvent in which ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate were mixed at a volume ratio of 1: 1: 1 was injected. A cylindrical 18650 lithium ion battery with a design capacity of 2 Ah was produced.

  These batteries were evaluated by the following method. The results are shown in Table 1 together with the configuration conditions.

[Negative electrode appearance]
The state of the negative electrode immediately after forming the porous film by applying and drying the porous film paste on the negative electrode was visually observed.

[Porosity of porous membrane]
When the positive electrode, the negative electrode on which the porous film was formed, and the polyethylene microporous film separator were wound, the state of the porous film near the core was visually observed. Table 1 shows the number of work-in-process items that were chipped, cracked, or dropped out due to each of the 10 work-in-process items.

[Battery discharge characteristics]
The finished battery after sealing, that is, a non-defective product with no chipping, cracking, and dropping off, was charged and discharged twice and stored for 7 days in a 45 ° C. environment, and then charged and discharged as follows.

(1) Charging: Charge to 4.2 V with a constant current of 1400 mA, then charge until the current drops to 100 mA with a constant voltage of 4.2 V. Discharge: Discharge to a final voltage of 3 V with a current of 400 mA.
(2) Charging: Same as above. Discharge: Discharge to a final voltage of 3 V with a current of 4000 mA.
The ratio between the 4000 mA discharge capacity and the 400 mA discharge capacity at this time is shown in Table 1 as a measure of discharge characteristics.

[Nail penetration safety]
The battery after the charge / discharge characteristics of the battery were evaluated was charged to 4.25 V at a constant current of 1400 mA, and then charged until the current decreased to 100 mA at a constant voltage of 4.25 V. Regarding the battery after charging, the heat generation state was observed when a 2.7 mm diameter iron round nail was penetrated at a speed of 5 and 180 mm / sec in a 20 ° C. environment. Table 1 shows the temperature reached after 1 second and 90 seconds after the penetration of the battery.

The evaluation results are described below.
First, in the battery using the negative electrode of Comparative Example 1 having no porous film, remarkable overheating was observed particularly when a nail was pierced at a low speed. Since the conventional polyethylene microporous film separator is melted by heat generated at the time of nail penetration, a short-circuit reaction accompanied by overheating due to expansion of a short-circuit portion cannot be suppressed.

  In order to solve the overheating at the time of short circuit as described above, the negative electrode of Comparative Example 2 in which a porous film was applied and formed using an organic solvent-based binder had no problem in the production process, but the discharge characteristics were significantly deteriorated. This is presumably due to the fact that the affinity between the electrolyte and the binder in the porous membrane is high and the binder has swelled.

  On the other hand, the negative electrode of Comparative Example 3 in which a porous film is applied and formed using polyacrylic acid, which is a general water-soluble binder, is significantly deformed after drying. This is considered to have occurred because water, which is a dispersion medium for the coating material for the porous film, permeates into the negative electrode and swells CMC in the negative electrode mixture layer. Since a non-defective product suitable for the subsequent winding process could not be selected from the negative electrode of Comparative Example 3, the following evaluation of various characteristics of the battery was abandoned. The negative electrode other than Comparative Example 3 did not have a poor appearance.

In contrast to the comparative example described above, the negative electrode of Example 1 using a binder containing silica in the porous film is similarly freed from deformation while using water as a dispersion medium. As described above, it is considered that such an improvement was achieved as an effect that the silanol group in silica suppressed the penetration of water in the porous film coating material into the negative electrode. Further, in the negative electrodes of Examples 2 and 3 in which the rubber-like binder is used in combination, the deformation of the negative electrode can be avoided, and the porous film becomes soft as the flexibility is improved. Obtained. As shown in Table 1, the batteries comprising the negative electrodes of Examples 1 to 3 achieved both the securing of discharge characteristics and the improvement of nail penetration safety, which were the improvement targets of the present invention.
In the above embodiment, an example in which a porous film is formed on the surface of the negative electrode mixture has been shown, but the same effect can be obtained even if a porous film is formed on the surface of the positive electrode mixture.

The lithium ion secondary battery provided with the electrode plate according to the present invention has a discharge characteristic equivalent to that when only a conventional sheet separator is used, and the safety is remarkably improved. Therefore, it can be used for various applications including portable devices.

Claims (2)

  A method for producing an electrode comprising a step of applying a porous film paste comprising an inorganic oxide filler and a binder with water as a dispersion medium to the surface of the electrode and drying to form a porous film, the binder A method for producing an electrode for a lithium ion secondary battery, wherein at least one of the agents is flake silica. The method for producing an electrode for a lithium ion secondary battery according to claim 1, wherein the binder further contains a rubbery polymer.