JP2015056304A - Electrode for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for lithium secondary battery having excellent adhesiveness to a separator, and when it is laminated integrally with the separator, ion permeability of the separator is not impaired.SOLUTION: 1) An electrode for lithium secondary battery having an ion permeable adhesive layer, composed of a polyolefin-based resin, on the outer surface of an active material layer. 2)An electrode for lithium secondary battery to be use while being laminated integrally with a separator. Preferably, the polyolefin resin is a polyolefin-based resin exhibiting adhesiveness in thermocompression of 70-120°C.

Description

The present invention relates to an electrode for a lithium secondary battery.
This electrode is preferably used by being laminated and integrated with the separator.

Along with the reduction in size and weight of electronic devices, lithium secondary batteries in which the exterior of the non-aqueous secondary battery is an aluminum laminate film have been put into practical use. When an aluminum laminate film is used as an exterior, since the exterior is soft, a gap may be formed between the electrode and the separator along with charge / discharge, resulting in a problem that cycle characteristics are deteriorated.

Therefore, as a method for solving this problem, a method of laminating and integrating electrodes (positive electrode and negative electrode) and a separator made of polyolefin resin has been proposed. For example, Japanese Patent No. 3225864 proposes a method in which an adhesive layer made of PVDF is formed on the surface of the separator and laminated with an electrode. Japanese Patent No. 3447610 proposes a method of laminating and integrating with an electrode using a polymer cross-linked product disposed on the separator surface as an adhesive layer. Furthermore, Japanese Patent Application Laid-Open No. 2013-137743 proposes a method of stacking and integrating the electrode and the separator by performing high pressure treatment at a low temperature of 40 ° C. to 70 ° C. without using an adhesive layer. Yes.

On the other hand, in a lithium secondary battery, the insulation of the separator in contact with the electrode may be destroyed due to scratches or irregularities on the electrode surface, and an internal short circuit may occur. In order to prevent this internal short circuit, Japanese Patent Application Laid-Open No. 9-147916 and Japanese Patent No. 5071056 propose to provide a protective layer made of an insulating porous film on the electrode surface.

Japanese Patent No. 3225864 Japanese Patent No. 3447610 JP 2013-137934 A JP-A-9-147916 Japanese Patent No. 5071056

In the conventionally proposed methods of providing an adhesive layer on a separator or a method of providing a protective layer on an electrode, it has been difficult to ensure the adhesion between the electrode and the separator while sufficiently ensuring the ion permeability of the separator.

Accordingly, the present invention solves the above-mentioned problems, and has an excellent adhesive property with the separator, and the lithium ion battery electrode does not impair the ion permeability of the separator when laminated and integrated with the separator. The purpose is to provide.

As a result of intensive studies to solve the above problems, the present inventors have found that the above problems can be solved by providing a specific adhesive layer on the outer surface of the electrode active material layer, and have reached the present invention.

That is, the present invention has the following gist.
<1> An electrode for a lithium secondary battery in which an ion-permeable adhesive layer made of a polyolefin resin is formed on the outer surface of the active material layer.
<2> The electrode for a lithium secondary battery according to <1>, wherein the electrode is laminated and integrated with a separator.

According to the present invention, it is possible to provide a lithium secondary battery electrode that has good adhesion to the separator and that does not impair the ion permeability of the separator when laminated and integrated with the separator.

Hereinafter, the present invention will be described in detail.
The electrode for a lithium secondary battery of the present invention is one in which an adhesive layer made of a polyolefin-based resin (hereinafter sometimes simply referred to as “adhesive layer”) is formed on the outer surface of the active material layer.
Here, the electrode for the lithium secondary battery is an electrode constituting the lithium secondary battery, and the positive electrode formed by joining the positive electrode active material layer to the positive electrode current collector or the negative electrode active material layer as the negative electrode current collector. The negative electrode formed by bonding to

The positive electrode active material layer is a layer obtained by binding positive electrode active material particles with a resin binder. The material used as the positive electrode active material particles, preferably those of lithium ion capable of occluding stored, generally include those used as the positive electrode active material for lithium secondary batteries, such as lithium manganate, LiCoO _2, LiNiO _2, and Examples thereof include lithium composite oxides such as Li _x V ₂ O ₅ (0 <x <2), and polymer compounds such as polyaniline and polythiophene. Among these, lithium manganate such as LiMn ₂ O ₄ , LiCoO ₂ , and LiNiO ₂ are preferable.

The negative electrode active material layer is a layer obtained by binding negative electrode active material particles with a resin binder. As the material used as the negative electrode active material particles, those capable of occluding and storing lithium ions generally used as the negative electrode active material of the lithium secondary battery are preferable. For example, graphite particles, amorphous carbon particles, silicon-based particles, tin-based particles and the like are used. Can be mentioned. Among these, graphite particles and silicon-based particles are preferable. Examples of the silicon-based particles include particles of silicon alone, a silicon alloy, a silicon-silicon dioxide composite, and the like. Among these silicon-based particles, particles of silicon alone (hereinafter abbreviated as “silicon particles”). Are preferred). Here, the silicon simple substance means crystalline or amorphous silicon having a purity of 95% by mass or more.

The particle diameter of the active material particles is preferably 50 μm or less, and more preferably 10 μm or less, for both the positive electrode and the negative electrode. Moreover, since it becomes difficult to bind with a resin binder even if the particle size is too small, it is usually 0.1 μm or more, preferably 0.5 μm or more.

The porosity of the active material layer is preferably 5 to 50% by volume and more preferably 10 to 40% by volume in both the positive electrode and the negative electrode. Moreover, as the layer thickness, it is about 20-200 micrometers normally.

Examples of the resin binder include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene / butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, and polyimide. Etc. Among these, polyvinylidene fluoride, styrene / butadiene copolymer rubber and polyimide are particularly preferable.

Examples of the current collector include metal foil and metal mesh. In the case of the positive electrode, an aluminum foil is preferable, and in the case of the negative electrode, a copper foil is preferable, and the thickness is usually about 5 to 50 μm.

The electrode of the present invention is obtained by forming an adhesive layer made of an ion-permeable polyolefin resin on the outer surface of the active material layer. The thickness of the adhesive layer is preferably 0.1 to 5 μm, and more preferably 0.5 to 2.5 μm.

The adhesive polyolefin resin is not limited as long as it is a polyolefin-based resin exhibiting adhesiveness, but it is preferable to use a polyolefin-based resin that exhibits adhesiveness by thermocompression bonding at 70 to 120 ° C. The polyolefin-based resin exhibiting adhesiveness has a structure in which an unsaturated carboxylic acid such as maleic anhydride or acrylic acid is copolymerized with a mixture of one or more polymers having a polymer skeleton of polyolefin. A modified polyolefin resin is preferably used. The modified polyolefin resin preferably has a melting point or softening point of 70 to 120 ° C. Specific examples of the polymer whose skeleton is polyolefin include LDPE, L-LDPE, HDPE, EVA, homopolypropylene, random polypropylene, ethylene-butene-1 copolymer, ethylene-propylene copolymer, polybutadiene, and the like. be able to.

The adhesive layer has ion permeability. Although there is no restriction | limiting in the method of providing ion permeability to an adhesion layer, The method of mix | blending a filler with the said olefin resin and making an adhesion layer porous structure and ensuring ion permeability is preferable.

As the filler, an inorganic or organic filler can be used. Specific examples of the organic filler include styrene, vinyl ketone, acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate, and methyl acrylate, or a copolymer of two or more kinds; polytetrafluoroethylene, 4 fluorine Fluorine resins such as fluorinated ethylene-6-propylene-propylene copolymer, tetrafluoroethylene-ethylene copolymer, polyvinylidene fluoride; melamine resin; urea resin; polyethylene; polypropylene; filler made of organic matter such as polymethacrylate Specific examples of inorganic fillers include calcium carbonate, talc, clay, kaolin, silica, hydrotalcite, diatomaceous earth, magnesium carbonate, barium carbonate, calcium sulfate, magnesium sulfate, barium sulfate, Aluminum oxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, zeolite, and a filler made of an inorganic substance such as glass. In addition, these fillers can be used individually or in mixture of 2 or more types. Among these, from the viewpoint of heat resistance and chemical stability, the filler is preferably an inorganic filler, more preferably an inorganic oxide filler, and particularly preferably an alumina filler. Alumina has many crystal forms such as α-alumina, β-alumina, γ-alumina, θ-alumina, etc., and α-alumina is most preferable because of its particularly high thermal and chemical stability.

The average particle size of the filler is preferably 3 μm or less, and more preferably 1 μm or less. Here, the average particle diameter refers to, for example, a volume-based average particle diameter measured with a laser diffraction particle size distribution measuring apparatus. This average particle diameter can also be confirmed from the SEM image on the surface of the adhesive layer. Moreover, it is preferable that content of a filler is 70 mass% or more of the whole contact bonding layer, and it is more preferable that it is 80 mass% or more. By doing in this way, the space | gap formed by contact of fillers increases and ion permeability can be kept favorable. In addition, there is no restriction | limiting in the particle shape of the said filler, Any shapes, such as an indefinite shape, spherical shape, and fibrous shape, may be sufficient.

As a method of forming an adhesive layer on the outer surface of the active material layer, the polyolefin resin is used as a solution or an emulsion to form an adhesive layer forming coating liquid (hereinafter sometimes simply referred to as “coating liquid”). I can do it. After applying this coating liquid to the outer surface of the polyolefin-based active material layer, the electrode of the present invention can be obtained by drying to remove the solvent or the dispersion medium. The polyolefin resin is preferably an emulsion containing water as a main dispersion medium from the viewpoint of environmental adaptability, and a commercially available product can be used. As a commercial item, the product numbers SA-1200, SB-1200, SE-1200, SB-1010 etc. of "Arrow base" (brand name) by Unitika Ltd. can be illustrated. These are aqueous emulsions of modified polyolefin resins that exhibit adhesion by thermocompression bonding at 70 to 120 ° C. In addition, what is necessary is just to add a filler to the said solution or emulsion, and to mix uniformly, when mix | blending a filler with polyolefin resin.

The electrode of the present invention in which an adhesive layer is formed on the outer surface of the active material layer obtained as described above (hereinafter sometimes abbreviated as “adhesive electrode”) has ion permeability, The presence or absence can be determined by performing the following measurement. That is, 5 μL of a mixed solvent (volume ratio 1: 1: 1) of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate at 20 ° C. was dropped on the surface of the conductive porous layer, and the time for which this completely penetrated was visually measured. To do. If this solvent absorption time is 600 seconds or less, it is determined to have ion permeability. In the electrode of the present invention, the solvent absorption time is preferably 300 seconds or shorter, more preferably 150 seconds or shorter.

The adhesive electrode obtained as described above can be used by being laminated and integrated with a separator. Here, as a separator, it is preferable to use the porous film which consists of polyolefin normally used as a separator for lithium secondary batteries. This porous film has a structure having pores connected to the inside thereof, and is a film through which gas or liquid can pass from one surface to the other surface. Here, examples of the polyolefin include homopolymers or copolymers obtained by polymerizing olefins such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, and 1-hexene. Among these, polyethylene obtained by homopolymerizing ethylene is preferable, and high molecular weight polyethylene having a weight average molecular weight of 1,000,000 or more is more preferable. Polypropylene obtained by homopolymerizing propylene is also preferable as the polyolefin.

The air permeability of the separator is preferably 600 seconds / 100 cc or less, and more preferably 400 seconds / 100 cc or less in terms of Gurley value (JIS standard P8117). When the air permeability is in the above range, sufficient ion permeability can be obtained when used as a separator. The porosity is preferably 20 to 80% by volume, more preferably 30 to 75% by volume from the viewpoint of increasing the amount of electrolyte retained and ensuring a shutdown function, and the pore diameter is preferably 3 μm or less, preferably 1 μm. The following is more preferable. Here, the shutdown function means that if an internal short circuit or external short circuit occurs due to damage to the lithium secondary battery, etc., a large current may flow and cause abnormal heat generation. In this abnormal heat generation, it refers to a function of preventing heat generation by blocking the passage of ions between the electrodes.

The film thickness of the separator is preferably 8 to 50 μm, more preferably 10 to 30 μm, from the viewpoint of ensuring insulation by the shutdown. Further, the structure may be a single-layer structure composed of only one layer or a multilayer structure composed of two or more layers. Examples of the multilayer structure include a structure in which a polyolefin layer made of another polyolefin is laminated on at least one surface of a polyolefin layer made of a certain polyolefin. A structure in which a polypropylene layer mainly composed of polypropylene is laminated (polypropylene layer / polyethylene layer / polypropylene layer) is preferable.

A commercial item can be used for the separator. Examples of commercially available products include polyethylene porous films from SK and Foshan, and polypropylene porous films from Celgard. These commercially available products have a thickness of 9 to 40 μm and have a shutdown function.

In order to stack and integrate the adhesive electrode and the separator, the adhesive electrode and the separator may be stacked and heated and pressed. That is, the adhesion electrode and the separator are stacked, 50 to 130 ° C., preferably at temperatures of 70 to 120 ° C., 1 to 30 kg / cm ^2, preferably hot-pressed at a pressure of 5~15kg / cm ^2. By doing in this way, the laminated integrated product (henceforth abbreviate | omitted to a "laminate") by which the favorable adhesiveness between layers was ensured can be obtained. The laminated structure may be a two-layer structure such as an adhesive positive electrode / separator or an adhesive negative electrode / separator, or a three-layer structure of adhesive positive electrode / separator / adhesive negative electrode.

As described above, the electrode of the present invention has an ion-permeable adhesive layer made of a polyolefin-based resin on its outer surface, and since the adhesive layer has good adhesiveness with the separator, It can be easily laminated and integrated with the separator.

The present invention will be described more specifically with reference to the following examples. The present invention is not limited to the examples.

The electrodes (positive electrode and negative electrode) used in Examples and Comparative Examples were produced as follows.
(Positive electrode)
86 parts by mass of lithium cobaltate powder as a positive electrode active material, 8 parts by mass of graphite powder as a conductive additive, and 6 parts by mass of polyvinylidene fluoride as a binder resin are uniformly dispersed in N-methylpyrrolidone and paste for positive electrode Was made. This positive electrode paste was applied to a 20 μm-thick aluminum foil as a positive electrode current collector, and the obtained coating film was dried and hot pressed to produce a positive electrode having a positive electrode active material layer having a thickness of 60 μm.
(Negative electrode)
A negative electrode paste was prepared by uniformly dispersing 95 parts by mass of graphite powder as a negative electrode active material and 5 parts by mass of polyvinylidene fluoride as a binder resin in N-methylpyrrolidone. This negative electrode paste was applied to a 10 μm-thick copper foil as a negative electrode current collector, and the obtained coating film was dried and hot pressed to prepare a negative electrode having a negative electrode active material layer having a thickness of 50 μm.

The characteristics of the electrodes obtained in the examples and comparative examples were evaluated by the following methods.
(1) Ion permeability
5 μL of a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (volume ratio 1: 1: 1) at 20 ° C. was dropped onto the electrode surface, and the time during which this completely penetrated was visually measured.
(2) Judgment was made from the tension when the separator was peeled 180 degrees in the opposite direction from the laminated integrated product of the adhesive separator and the adhesive electrode sheet.
○: Good adhesion between electrode sheet and separator ×: Poor adhesion between electrode sheet and separator

<Example 1>
[Coating solution]
A product number SB-1200 of “Arrow Base” (trade name) manufactured by Unitika Ltd., which is an aqueous emulsion of a modified polyolefin resin, was prepared. A coating liquid was prepared by adding α-alumina powder having an average particle size of 0.9 μm to this aqueous emulsion and stirring to obtain a uniform dispersion. Here, the mass ratio of the modified polyolefin resin and the α-alumina powder was 1.5 / 8.5 (modified polyolefin resin / α-alumina powder).
[Adhesive electrode]
The coating liquid diluted moderately was applied to the positive electrode and then dried at 80 ° C. to obtain an adhesive electrode. The thickness of the adhesive layer of this electrode was 1.2 μm, the solvent absorption time was 35 seconds, and the ion permeability was good.
[Laminate]
As a porous film made of polyolefin, a polypropylene porous film made by Celgard (product number 2500, thickness 25 μm) was prepared. This and the adhesive electrode were laminated and integrated by thermocompression bonding at a temperature of 90 ° C. and a pressure of 8 kg / cm ² . The adhesiveness of this laminated integrated product was good with good. The solvent absorption time was 25 seconds, and the ion permeability was good.

<Example 2>
The adhesive electrode and the laminated integrated product were obtained in the same manner as in Example 1 except that the coating liquid without adding α-alumina powder was used as the coating liquid, and the thickness of the adhesive layer was 0.5 μm. Obtained. The solvent absorption time of the adhesive electrode was 51 seconds. Moreover, the adhesiveness of the laminated integrated product was good with ○, the solvent absorption time was 28 seconds, and the ion permeability was good.

<Example 3>
An adhesive electrode and a laminated integrated product were obtained in the same manner as in Example 1 except that the negative electrode was used as an electrode. The thickness of the adhesive layer of the adhesive electrode was 1.4 μm, and the solvent absorption time was 42 seconds. Moreover, the adhesiveness of the laminated integrated product was good with ○, the solvent absorption time was 18 seconds, and the ion permeability was good.

<Comparative Example 1>
An adhesive electrode was obtained in the same manner as in Example 1 except that the thickness of the adhesive layer was 5.5 μm. The solvent absorption time of the adhesive electrode exceeded 600 seconds, and the ion permeability was poor.

<Comparative Example 2>
Except that the adhesive layer was not formed, a laminate was obtained in the same manner as in Example 1, and the adhesiveness was evaluated. However, it was easily peeled off by hand, and the adhesiveness was x.

As described above, as shown in Examples and Comparative Examples, the electrode for a lithium secondary battery of the present invention is excellent in ion permeability and adhesiveness, and can be suitably used by stacking and integrating with a separator.

Claims (2)

An electrode for a lithium secondary battery in which an ion-permeable adhesive layer made of a polyolefin resin is formed on the outer surface of an active material layer. 2. The electrode for a lithium secondary battery according to claim 1, wherein the electrode is laminated and integrated with a separator.