JP2005302634A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery suppressing temperature rising of the battery caused by internal short circuit without substantially decreasing battery characteristics. <P>SOLUTION: The nonaqueous electrolyte secondary battery has an electrode group formed by winding a positive electrode and a negative electrode through a separator, the positive electrode is formed by applying a mix containing an active material to the surface of a current collector, and a current collector exposed part of the positive electrode and the mix applied part of the negative electrode are faced through the separator. A heat resistant layer is formed in a part of the surface of the separator, and the heat resistant layer is positioned in the faced part of the current collector exposed part of the positive electrode and the mix applied part of the negative electrode. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to the form of a separator and a wound electrode group.

Lithium composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ etc. are used for the positive electrode, and alloys and carbon materials capable of occluding and releasing lithium and composites thereof are used for the negative electrode. A nonaqueous electrolyte secondary battery using an electrolyte solution in which a lithium salt is dissolved has a high voltage and a high energy density, and has been widely used as a power source for portable devices in recent years.

  In a nonaqueous electrolyte secondary battery, the battery temperature may increase when an excessive current flows due to an internal short circuit or an external short circuit, or when overcharge is performed. A porous polyolefin thin film is generally used as a separator for non-aqueous electrolyte secondary batteries, but when the temperature rises as described above, the porous polyolefin softens and becomes non-porous, blocking the current. It has a so-called shutdown function.

  If the battery temperature rises due to external short circuit or overcharge, it is considered that safety is ensured by shutting off the current by the shutdown function, but if excessive current flows due to internal short circuit, the short circuit part The temperature is locally and instantaneously high, and there is a possibility that the separator locally melts before the temperature rise is stopped by the shutdown function. If it does so, a big hole will open in a separator and a positive electrode and a negative electrode will short-circuit.

Therefore, many separators composed of a composite film of a porous polyolefin layer and a heat-resistant porous layer have been proposed in order to prevent the separator from melting to a higher temperature while maintaining the shutdown function. For example, there are a laminated film of a polyolefin porous body and a polyphenylene sulfide porous body (for example, see Patent Document 1), a laminated film of an aramid porous film and a polyolefin porous film (for example, see Patent Document 2), and the like. .
Japanese Patent Application Laid-Open No. 08-87995 JP 2000-100408 A

  Although it is conceivable that the safety of the battery is improved by the above means, there is a problem that it is difficult to achieve both the battery characteristics and the safety. In a separator composed of a composite film of a porous polyolefin layer and a heat-resistant porous layer, the heat-resistant porous layer is interposed on the entire surface where the positive and negative electrode mixture layers face each other. Not only significantly affects the rate characteristics of the battery, but also increases the thickness of the separator as compared with the case of the porous polyolefin thin film alone, and the capacity density and energy density per volume of the battery are greatly reduced.

When the internal short circuit is examined in detail, one of the causes may be that foreign matter or the like is mixed in the electrode plate group. At that time, if the foreign material is in contact with the mixture layer, even if pressure or expansion of the electrode plate occurs, there is a possibility that the foreign material is buried in the mixture layer and the separator is not damaged, and the internal short circuit does not occur. If the foreign substance is in contact with the exposed portion of the current collector where the mixture is not applied, the foreign substance is pressed against the separator, and an internal short circuit due to breakage of the separator is likely to occur. In addition, in the portion where the positive and negative current collectors face each other, even if an internal short circuit occurs due to the inclusion of the foreign matter and an excess current flows, the resistance is small and the amount of heat generation is small, which is not a problem. Considering the fact that the negative electrode mixture application part is usually longer than the positive electrode mixture application part, the above may cause an internal short circuit with a large calorific value to melt the separator. It can be said that the portion where the current is high is a portion where the current collector exposed portion of the positive electrode and the mixture application portion of the negative electrode face each other.

  In particular, since the non-aqueous electrolyte secondary battery including the positive and negative electrodes of the present invention has a high voltage and a high energy density, the amount of electricity flowing at the time of an internal short circuit is very large. Furthermore, since a material having a large specific surface area such as a carbon material is used for the negative electrode, the temperature rise is fast and large.

  In order to solve the above-described problems, a nonaqueous electrolyte secondary battery according to the present invention includes a positive electrode using a lithium composite oxide as an active material, an alloy capable of occluding and releasing lithium, and / or a carbon material as an active material. A positive electrode and a separator are wound on each other, and the positive electrode is formed by partially applying a mixture containing an active material on the surface of the current collector. In the non-aqueous electrolyte secondary battery having a configuration in which the mixture application portion of the non-aqueous electrolyte solution has a portion facing through the separator, the separator is provided with a heat-resistant layer on a part of the surface, and the current collector of the positive electrode The heat-resistant layer is located in a portion where the exposed portion and the negative electrode mixture application portion face each other.

  According to the above configuration, by covering the facing portion of the positive electrode current collector and the negative electrode mixture, which is highly likely to cause an internal short circuit with a large amount of heat generation, with a heat-resistant layer, an increase in battery temperature due to an internal short circuit with a large amount of heat generation is suppressed. be able to. At this time, since most of the portions of the positive and negative electrode mixtures facing each other are only interposed by the separator of the porous polyolefin thin film, the battery characteristics are not greatly deteriorated.

  When the heat-resistant layer is interposed in the portion where the current collector exposed portion of the positive electrode and the mixture application portion of the negative electrode face each other, try to completely cover the positive electrode current collector portion facing the negative electrode mixture Then, it is necessary to cover even the end of the positive electrode mixture layer that is continuous with the current collector exposed portion. For this reason, when sticking an adhesive film as a heat-resistant layer, the active material at the end of the positive electrode mixture cannot be charged and discharged, and strictly speaking, the capacity density and energy density of the battery are slightly reduced. Therefore, if the heat-resistant layer is porous, the active material at the end portion of the positive electrode mixture can be charged and discharged, and a more preferable effect can be obtained.

  The non-aqueous electrolyte secondary battery according to the present invention has the above-described configuration, and has an excellent effect of suppressing an increase in battery temperature due to an internal short circuit without causing a significant decrease in battery characteristics.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment)
As shown in FIG. 1, the separator for a non-aqueous electrolyte secondary battery in the present invention comprises a heat-resistant layer 1a and a porous polyolefin layer 2a, and the heat-resistant layer is a mixture of the current collector exposed portion of the positive electrode and the negative electrode. It has a structure formed on a porous polyolefin layer interposed in a portion 3 facing the coating portion.

  As the porous polyolefin layer, polyethylene, polypropylene, or a composite film thereof can be used. Considering the influence on the battery characteristics, the thickness is preferably 5 to 30 μm, the porosity is preferably 30 to 70%, and the shutdown temperature is preferably 100 to 150 ° C. in order to ensure the safety of the battery.

  The material of the heat-resistant layer is not limited as long as it does not cause a reaction with the electrolytic solution or the electrode plate and can form a thin film layer on the porous polyolefin thin film. In consideration of the above-mentioned influence on the strict battery characteristics, a porous thin film layer containing a ceramic powder, a porous thin film layer of a heat resistant resin, or a composite of a ceramic powder and a heat resistant resin is preferable. A porous thin film layer is more preferred. At that time, considering the influence on battery characteristics and safety, the thickness is preferably 1 to 20 μm and the porosity is preferably 30 to 70%.

As the ceramic powder used for the porous thin film layer, alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ), titania (TiO ₂ ), zirconia (ZrO ₂ ), or a mixture thereof should be used. The particle size is preferably 0.1 to 10 μm. This powder is dispersed in a solvent containing a binder to form a paste, which is applied to the porous polyolefin thin film and then subjected to a desolvation treatment to produce a porous thin film layer containing ceramic powder. As the binder, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyacrylic acid derivative rubber binder, or the like can be used.

  Examples of the heat resistant resin used for the porous thin film layer include polyimide, aramid, polyamideimide, polyethersulfone, polyetherimide, and polyphenylene ether. After preparing a solution in which these heat-resistant resins are dissolved in a good solvent, applying the solution to a porous polyolefin thin film, and then contacting the poor solvent, the heat-resistant resin is precipitated, and the heat-resistant resin is porous by removing the solvent. A thin film layer is obtained. Examples of good solvents include n-methyl-2-pyrrolidone (NMP), n, n-dimethylacetamide (DMA), dimethyl sulfoxide, toluene, xylene, and the like. Examples of poor solvents include water, ethanol, methanol, and the like. It is done.

  Moreover, a composite of ceramic powder and heat resistant resin is prepared by preparing a solution in which ceramic powder is mixed and dispersed in the above heat resistant resin coating solution and applying the same treatment to a porous polyolefin thin film. A porous thin film layer is obtained.

  The non-aqueous electrolyte secondary battery according to the present invention uses the separator described above, and, as shown in FIG. 2, the heat-resistant layer 1b at the portion where the current collector exposed portion 4 of the positive electrode and the mixture application portion 7 of the negative electrode face each other. Has an electrode plate group in which a positive electrode, a negative electrode, and a separator are overlapped and wound.

As the positive electrode active material, lithium composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ can be used, and as the negative electrode active material, alloys and carbon materials capable of occluding and releasing lithium, and composites thereof. Etc. can be used.

As the electrolyte, a lithium salt such as LiPF ₆ , LiClO ₄ , LiBF ₄ , or a mixture thereof can be used. As the non-aqueous solvent, ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC) is used. , Dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and the like can be used.

  A more specific embodiment of the present invention will be described.

(Example 1)
A separator for a non-aqueous electrolyte secondary battery having the configuration shown in FIG. 1 was prepared by the following procedure. As the heat-resistant layer, a porous thin film made of alumina powder and PVdF as a binder was used.

  100 g of alumina powder (Sumitomo Chemical Co., Ltd.) classified to an average particle size of 0.5 μm is classified into 5 g of PVdF (Kureha Chemical Co., Ltd. # 1320) dispersion (in terms of solid content) ), An appropriate amount of NMP was added as a solvent to prepare a paste for coating. This paste was thinly applied to one side of a 20 μm thick porous polyethylene thin film (E20MMS, manufactured by Tonen Chemical Co., Ltd.) with a bar coater. At this time, the paste was applied over the entire width in the short direction of the porous polyethylene thin film, and partially applied in the longitudinal direction so that the length of the paste application part was 2 cm in the longitudinal direction. This thin film was vacuum dried at 70 ° C. to obtain a separator for a non-aqueous electrolyte secondary battery. The thickness of the heat-resistant layer application part was 26 μm.

Disperse of LiCoO ₂ powder as a positive electrode active material, 10 g of acetylene black (AB) powder as a conductive agent and PVdF (manufactured by Kureha Chemical Co., Ltd., # 1320) as a conductive agent with respect to 100 g of the positive electrode active material. John 6g (in terms of solid content) is thoroughly mixed, then an appropriate amount of NMP is added, mixed thoroughly to form a paste, and corresponds to the outermost peripheral part of the electrode plate group on both sides of a 20 μm thick aluminum foil as a current collector It was applied so that an exposed portion having a length of 70 mm remained at the end portion to be dried, dried and rolled to obtain a positive electrode having a thickness of 200 μm.

  Water-soluble 15 g of AB powder as a conductive agent and SBR as a binder (BM-400B, manufactured by Nippon Zeon Co., Ltd.) with respect to 100 g of artificial graphite powder (manufactured by Timcal Corporation, KS-44) as a negative electrode active material 8 g (in terms of solid content) mixed well and made into a paste using water as a dispersion solvent on both sides of a 15 μm thick copper foil as the current collector, on the outermost periphery of the electrode plate group It was applied so that an exposed portion having a length of 80 mm remained at the corresponding end, and then dried and rolled at 100 ° C. to obtain a negative electrode having a thickness of 250 μm.

A positive electrode lead made of aluminum was joined to the end of the aluminum foil of the positive electrode current collector by ultrasonic welding, and a copper negative electrode lead was similarly joined to the end of the copper foil of the negative current collector by ultrasonic welding. The positive electrode, the negative electrode, and the separator produced in this manner were placed in such a manner that the heat-resistant layer coating portion of the separator was positioned at the portion where the current collector exposed portion of the positive electrode and the mixture application portion of the negative electrode faced each other. After that, a small piece of nickel (length: about 2 mm, width: about 0.2 mm, thickness: about 0.1 mm) is inserted and wound on the positive electrode side where the positive electrode current collector and negative electrode mixture face each other. The electrode group was used. Polypropylene insulating plates were placed on the upper and lower sides of the prepared electrode plate group, respectively, and inserted into a battery outer can having a diameter of 18 mm and a height of 65 mm. There, an equal volume mixed solution of EC and DEC in which 1 mol / l LiPF ₆ was dissolved was injected as a non-aqueous electrolyte. Thereafter, it was impregnated with a vacuum, a sealing plate was inserted, and sealed by mechanical caulking to obtain a cylindrical battery.

(Example 2)
As the ceramic powder in the porous thin film layer, titania powder (manufactured by Fuji Titanium Industry Co., Ltd.) classified to an average particle size of 0.5 μm was used, and a nonaqueous electrolyte secondary battery was obtained in the same manner as in Example 1. A separator was prepared. The thickness of the heat-resistant layer application part was 26 μm. A cylindrical battery was produced in the same manner as in Example 1 except that this separator was used.

  In addition, this invention is not limited only to these Examples, The same heat resistant porous thin film layer can be obtained also by using the other ceramic powder and binder mentioned above.

(Example 3)
A separator for a non-aqueous electrolyte secondary battery having the configuration of FIG. 1 was produced by the following procedure. As the heat-resistant layer, a porous thin film made of a polyimide resin prepared by the following procedure was used.

Biphenyltetracarboxylic dianhydride (manufactured by Ube Industries, Ltd.) as carboxylic acid anhydride
s-BPDA) using 4,4′-diaminodiphenyl ether (ODA, manufactured by Mitsui Chemicals, Inc.) as a diamine, and each of these equal amounts was dissolved in DMA as a solvent to obtain an approximately 25% raw material solution. Obtained. This was further diluted with DMA to prepare a 10% coating solution.

  The above coating solution was applied to a porous polyethylene thin film having a thickness of 20 μm (E20MMS, manufactured by Tonen Chemical Corporation) in the same manner as in Example 1. This thin film was immersed in methanol at room temperature to deposit polyimide, dried at room temperature, heat treated at 70 ° C., and vacuum dried at 70 ° C. to obtain a separator for a non-aqueous electrolyte secondary battery. The thickness of the heat-resistant layer application part was 26 μm.

  A cylindrical battery was produced in the same manner as in Example 1 except that this separator was used.

Example 4
A separator for a non-aqueous electrolyte secondary battery having the configuration of FIG. 1 was produced by the following procedure. As the heat-resistant layer, an aramid resin produced by the following procedure, specifically, a porous thin film made of polyparaphenylene terephthalamide (PPTA) was used.

  To 100 g of NMP, 6.5 g of calcium chloride powder was added and heated to completely dissolve. After returning this solution to room temperature, 3.2 g of paraphenylenediamine (PPD, manufactured by Mitsui Chemicals, Inc.) was added and completely dissolved. This solution was placed in a constant temperature bath at 20 ° C., and 5.8 g of terephthalic acid dichloride (TPC, manufactured by Mitsui Chemicals, Inc.) was added dropwise to obtain a PPTA solution. Furthermore, 50 g of this solution was diluted with 200 g of NMP to prepare a coating solution.

  The PPTA solution was applied to a porous polyethylene thin film having a thickness of 20 μm (E20MMS, manufactured by Tonen Chemical Corporation) in the same manner as in Example 1. This thin film was placed in an atmosphere with a humidity of 50% to deposit PPTA, washed thoroughly with ion exchange water, and then vacuum dried at 70 ° C. to obtain a separator for a non-aqueous electrolyte secondary battery. The thickness of the heat-resistant layer application part was 26 μm.

  A cylindrical battery was produced in the same manner as in Example 1 except that this separator was used. In addition, this invention is not limited only to these Examples, A heat-resistant porous thin film layer is similarly formed by using the combination of the other heat-resistant resin mentioned above, a good solvent, and a poor solvent. Can be obtained.

(Example 5)
As the heat-resistant layer, an adhesive film made of polyimide resin having a thickness of 25 μm and a width of 2 cm (No. 360UL, manufactured by Nitto Denko Corporation) was used and pasted at the same position as in Example 1, and the non-aqueous electrolyte secondary A battery separator was obtained. A cylindrical battery having the same configuration as in Example 1 was produced except that this separator was used.

(Comparative Example 1)
As a first comparative example, a cylindrical battery having the same configuration as that of Example 1 except that a 20 μm thick porous polyethylene thin film (E20MMS, manufactured by Tonen Chemical Co., Ltd.) without a heat-resistant layer was used as a separator. Was made.

(Comparative Example 2)
As a second comparative example, a separator (total thickness 22 μm) in which a heat-resistant porous thin film layer made of polyimide resin was applied to all one side of a 16 μm thick porous polyethylene thin film (E16MMS manufactured by Tonen Chemical Co., Ltd.) was used. A cylindrical battery having the same configuration as in Example 3 was produced except that the volume of the electrode plate group was adjusted by shortening the mixture application portion of the positive and negative electrodes in the longitudinal direction.

  About these produced batteries, the safety verification test regarding an internal short circuit was done with the following method. Charge the battery at a constant current up to 4.2V at a test temperature of 20 ° C and 0.7C, disassemble the battery case and take out the electrode plate group. Then, attach a terminal for voltage monitoring to the lead of the electrode plate group. A thermocouple was attached to the test cell with an adhesive tape to obtain a test cell. An internal short circuit was generated by pressing the portion where the nickel piece was inserted from the outside of the cell. The occurrence of an internal short circuit was confirmed by the cell voltage dropping to 4.1 V or less. After the short circuit, the cell temperature was not increased to 80 ° C., and the cell safety was verified as acceptable, while the cell temperature increased to 80 ° C. or higher was rejected.

  Also, cylindrical batteries having the same structure as those of Examples 1 to 5, Comparative Example 1 and Comparative Example 2 were prepared without inserting nickel pieces, and the test temperature was 20 ° C., and the voltage was 4.2V to 3V at 0.2C and 2C. Constant current charging was performed, and the rate characteristics of each battery were confirmed by the ratio of the discharge capacity at 2C to the discharge capacity at 0.2C. Furthermore, when the discharge capacity at 0.2 C of the battery of Comparative Example 1 was set to 100, the battery capacity was confirmed by the ratio of the discharge capacity at 0.2 C of the other batteries to that.

  The above test results are shown in (Table 1).

  In the battery of Comparative Example 1, a temperature increase of 80 ° C. or more was observed in the internal short circuit test, but in the batteries of Examples 1 to 5 and Comparative Example 2, no temperature increase was observed up to 80 ° C. It has been confirmed that the safety in an internal short circuit is improved by the presence of.

  Compared to the rate characteristics and battery capacity of Comparative Example 1, the battery of Comparative Example 2 showed a decrease in battery characteristics, but the batteries of Examples 1 to 4 obtained battery characteristics almost equivalent to Comparative Example 1. . Thus, it was confirmed that when the heat-resistant porous layer is present on the entire surface where the positive and negative electrode mixture layers face each other, the battery characteristics are deteriorated. In addition, the battery of Example 5 showed a slight decrease in battery characteristics as compared with Examples 1 to 4.

  From the above results, in the batteries of Examples 1 to 4, the presence of the heat-resistant porous layer only in the short-circuit portion suppresses an increase in battery temperature due to an internal short-circuit without causing a decrease in battery characteristics. I understood.

  Since the nonaqueous electrolyte secondary battery according to the present invention can suppress an increase in battery temperature due to an internal short circuit, it is useful as a power source for portable devices and the like.

The figure which shows the structure of the separator for nonaqueous electrolyte secondary batteries in Example 1 of this invention Sectional drawing which shows the electrode group winding method in Example 1 of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a, 1b Heat-resistant layer 2a, 2b Porous polyolefin layer 3 Opposite part of positive electrode current collector and negative electrode mixture 4 Positive electrode current collector 5 Positive electrode mixture 6 Negative electrode current collector 7 Negative electrode mixture

Claims (3)

A positive electrode using a lithium composite oxide as an active material, an alloy capable of occluding and releasing lithium, and / or a negative electrode using a carbon material as an active material, and a separator, and a positive electrode group that is wound. A non-aqueous electrolyte having a structure in which a mixture containing an active material is partially applied to the body surface, and a positive electrode current collector exposed portion and a negative electrode mixture applying portion are opposed to each other via a separator In the secondary battery, the separator is provided with a heat-resistant layer on a part of the surface, and the heat-resistant layer is located at a portion where the current collector exposed portion of the positive electrode and the mixture application portion of the negative electrode face each other. A non-aqueous electrolyte secondary battery. 2. The nonaqueous electrolytic solution according to claim 1, wherein the heat resistant layer is a porous thin film layer containing ceramic powder, a porous thin film layer of a heat resistant resin, or a porous thin film layer made of a composite of ceramic powder and a heat resistant resin. Secondary battery. The nonaqueous electrolyte secondary battery according to claim 1, wherein the heat resistant layer is an adhesive film made of a heat resistant resin.

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