JP2014182875A - Secondary battery separator and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery separator excellent in adhesion with an electrode and heat resistance with one porous layer.SOLUTION: The secondary battery separator is formed by laminating on at least one surface of a pours substrate a porous layer containing coat fine particles coated by a fluororesin of an average particle diameter of 0.15 to 10 μm, wherein a coat thickness of the fluororesin is 0.05 to 5 μm.

Description

  The present invention relates to a separator for a secondary battery and a secondary battery.

  Secondary batteries such as lithium-ion batteries are widely used in portable digital devices such as mobile phones, notebook computers, digital cameras, digital video cameras, and portable game machines. Recently, hybrid vehicles and electric vehicles have been used for automobiles. The use as a power source for plug-in hybrid vehicles is expanding.

In general, a lithium ion battery includes a secondary battery separator and an electrolyte interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. It has become.
As a separator for secondary batteries, a polyolefin-based porous substrate is used. The required characteristics include an electrolyte in the porous structure and the ability to move ions, and the lithium ion battery generates abnormal heat. In this case, the porous structure is closed by melting with heat, and the ion transfer is stopped to stop the power generation.
However, with the recent increase in capacity and output of lithium-ion batteries, not only the above characteristics but also repeated charge / discharge causes gaps between the electrode and the secondary battery separator, resulting in poor cycle characteristics. In order to prevent this, adhesiveness with an electrode and heat resistance to prevent a short circuit due to thermal contraction of a secondary battery separator accompanying an increase in temperature in the battery have been required.
In response to these requirements, Patent Documents 1 and 2 propose a separator for a secondary battery that has improved adhesion to an electrode by laminating an adhesive layer on a polyolefin-based porous substrate. Patent Documents 3, 4, and 5 propose secondary battery separators that have improved heat resistance by laminating a heat-resistant layer on a polyolefin-based porous substrate.

JP 2004-146190 A JP 2012-211741 A JP 2013-008481 A JP 2013-008690 A JP 2013-020818 A

  However, in Patent Documents 1 and 2, the adhesion to the electrode is improved, but it does not have heat resistance and there is a concern of short circuit at high temperature. In Patent Documents 3, 4, and 5, heat resistance is improved. However, since it does not have adhesiveness, the cycle characteristics deteriorate. In addition, simply laminating the adhesive layer on the heat-resistant layer causes problems such as peeling at the interface between the heat-resistant layer and the adhesive layer, causing deterioration of cycle characteristics, and insufficient porosity of the adhesive layer. Also, there is a problem that the cost is increased by laminating the two layers separately.

  Accordingly, an object of the present invention is to provide a secondary battery separator having adhesiveness and heat resistance with one functional porous layer in view of the above problems.

In order to solve the above problems, the secondary battery separator of the present invention has the following configuration.
(1) A porous layer containing fine particles coated with a fluororesin having an average particle diameter of 0.15 to 10 μm is laminated on at least one surface of a porous substrate, and the fluororesin coating A separator for a secondary battery having a thickness of 0.05 to 5 μm.
(2) The separator for a secondary battery as described in 1 above, wherein the porous layer has a thickness of 1 to 10 μm.
(3) The separator for a secondary battery according to any one of (1) and (2), wherein the fluororesin is a vinylidene fluoride resin.
(4) The secondary battery separator as described in any one of (1) to (3) above, wherein the core fine particles of the coated fine particles are inorganic oxide fine particles or organic fine particles.
(5) The separator for a secondary battery as described in any one of (1) to (4) above, wherein the porous layer is laminated on both surfaces of the porous substrate.
(6) The separator for a secondary battery as described in any one of (1) to (5) above, wherein the porous substrate is a polyolefin-based porous substrate.
(7) A secondary battery using the secondary battery separator as described in any one of 1 to 6 above.

  According to the present invention, compared to the conventional secondary battery separator having only one of the functions of adhesiveness and heat resistance, it has both functions, thereby improving cycle characteristics due to adhesion and short circuit due to heat resistance. A separator for a secondary battery that enables prevention can be provided. Moreover, manufacturing cost can also be suppressed by having adhesiveness and heat resistance with one functional porous layer. By using the separator for a secondary battery of the present invention, it is possible to provide a high capacity, high output lithium ion battery.

  The separator for a secondary battery of the present invention has a porous layer containing an average particle diameter of 0.15 to 10 μm and coated fine particles coated with a fluororesin on at least one surface of a porous substrate. The secondary battery separator is characterized in that the film thickness of the fluororesin is 0.05 to 2 μm. Hereinafter, the present invention will be described in detail.

[Porous layer]
(Core fine particles)
The core fine particles constituting the porous layer of the present invention are required to be electrically stable in the battery, to have electrical insulation, and to have heat resistance. The heat resistance in this case means that the thermal shrinkage at 150 ° C. is within 10%. If these characteristics are satisfied, inorganic fine particles, organic fine particles, or other fine particles can be used.

  Specifically, the inorganic fine particles include inorganic oxide fine particles such as aluminum oxide, silica, titanium oxide, zirconium oxide and iron oxide, inorganic nitride fine particles such as aluminum nitride and silicon nitride, calcium fluoride, barium fluoride and barium sulfate. Insoluble ion crystal particles such as may be used, and two or more types of combinations may be used. Among these, inorganic oxide fine particles are preferable, and aluminum oxide is more preferable.

  Examples of the organic fine particles include cross-linked polymethyl methacrylate, cross-linked polystyrene, methyl methacrylate-styrene copolymer, polyimide, melamine resin, phenol resin, polyacrylonitrile, and the like. . Moreover, a mixture, a modified body, a derivative, and a copolymer may be used for the resin constituting these organic fine particles.

  The average particle size of the core fine particles used is preferably 0.1 to 5 μm. If it is smaller than 0.1 μm, the internal resistance may increase due to the dense functional porous layer. Moreover, since the pore diameter is small, the impregnation property of the electrolytic solution is deteriorated, which is not preferable from the viewpoint of productivity. On the other hand, if the thickness is larger than 5 μm, the number of contacts with the electrode is decreased, and the adhesion may not be sufficient.

  Examples of the shape of the core fine particles to be used include a spherical shape, a plate shape, a needle shape, a rod shape, and an oval shape, and any shape may be used. Among these, the spherical shape is preferable from the viewpoints of surface modification, dispersibility, and coatability.

(Fluorine resin)
Examples of the fluororesin used in the present invention include homopolymers such as polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride, and polychlorotrifluoroethylene, ethylene / tetrafluoroethylene polymer, ethylene-chlorotrifluoroethylene polymer, And other copolymer systems. Moreover, the copolymer of a homopolymer type | system | group and tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, etc. are mentioned. As will be described later, since the resin used for the binder of the positive electrode and the negative electrode is polyvinylidene fluoride, among these fluorine resins, vinylidene fluoride resins such as polyvinylidene fluoride, and vinylidene fluoride and hexafluoropropylene are used. Copolymers are preferably used.

(Coating fine particles)
The coated fine particles of the present invention are coated with a fluorine-based resin, and the coated thickness is preferably in the range of 0.05 to 5 μm. More preferably, it is 0.5-4 micrometers, More preferably, it is 1-3 micrometers. When the coating thickness of the fluororesin is thinner than 0.05 μm, sufficient adhesion with the electrode may not be obtained. Moreover, when thicker than 5 micrometers, the air permeability of a functional porous layer may become large, and internal resistance may become high.

  The average particle diameter of the coated fine particles coated with the fluororesin is preferably in the range of 0.15 to 10 μm. More preferably, it is 0.5-8 micrometers, More preferably, it is 1-5 micrometers. If it is smaller than 0.15 μm, the internal resistance may increase due to the dense porous layer. On the other hand, if the thickness is larger than 10 μm, the number of contacts with the electrode is decreased, and the adhesion may not be sufficient.

  As a method for coating the core fine particles with the fluorine-based resin, a known example may be used, and examples thereof include a suspension polymerization method and an emulsion polymerization method, but are not particularly limited thereto.

(Formation of porous layer)
The separator for a secondary battery of the present invention can be obtained by laminating a porous layer containing coated fine particles coated with a fluororesin on at least one surface of a porous substrate. The method will be described below. To do.

  Coating fine particles coated with a fluorine-based resin are dispersed in a solvent to prepare a coating solution. Solvents used here include alcohols such as methanol, ethanol, isopropyl alcohol, 1-propanol and 1-butanol, ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone, esters such as ethyl acetate and butyl acetate, and water. Etc. are preferably used from the viewpoints of coating property and drying property. Examples of the dispersion method include a ball mill, a bead mill, a sand mill, a roll mill, a homogenizer, an ultrasonic homogenizer, a high-pressure homogenizer, an ultrasonic device, a paint shaker, and the like. Also good. It can disperse | distribute by the method suitable for the particle | grains to be used, a fluororesin, and a solvent. If necessary, an additive such as a dispersant or a binder such as an acrylic resin can be used as appropriate. The addition amount of the binder is preferably 20% by mass or less. More preferably, it is 10 mass% or less. When added in an amount of more than 20% by mass, the air permeability of the porous layer increases, and the internal resistance may increase.

  Next, the obtained coating liquid is applied onto a porous substrate, dried, and a porous layer is laminated. As a coating method, a known method may be used. For example, gravure coating, slit die coating, knife coating, kiss coating, roll coating, bar coating, spray coating, dip coating, spin coating, screen printing, inkjet Printing, pad printing, other types of printing, etc. can be used, but are not limited to these, and the coating method is selected according to the preferred conditions such as the particles used, the fluororesin, the solvent used, and the substrate. do it. When laminating a porous layer on both surfaces of a porous substrate, it may be coated and dried one by one, but it is preferable to apply both surfaces simultaneously and dry to improve productivity. Further, from the viewpoint of adhesiveness, it is preferable to laminate the porous layer on only one side because the adhesive property can be obtained on both sides of the positive electrode and the negative electrode because the cycle characteristics are excellent.

  The thickness of the porous layer is preferably 1 to 10 μm. More preferably, it is 2-6 micrometers. When the thickness of the porous layer is less than 1 μm, sufficient heat resistance may not be obtained, and sufficient adhesion may not be obtained. If it is thicker than 10 μm, the internal resistance of the porous layer may increase. Moreover, when it laminates | stacks only on one side, a curl may become remarkable. Moreover, when laminating | stacking on both surfaces, it is preferable that the thickness difference of the porous layer of each surface shall be 3 micrometers or less.

  Moreover, it is preferable that the porosity of a porous layer is 30 to 70%, More preferably, it is 40 to 60%. When the porosity of the porous layer is smaller than 30%, the internal resistance of the porous layer may increase. If it is larger than 70%, sufficient mechanical properties may not be obtained. The porosity in this case is calculated by calculating the sum for each component i using equation (1).

P = 100− (Σa _i ρ _i ) / (m / h) (1)
Here, P is the porosity, _{a i} is the ratio of the weight percent of component _i, [rho _i is the density of component ^{i (g / cm 3),} m is the mass per unit area of the porous layer (g / ^cm 2) , H is the thickness (cm) of the porous layer.

[Porous substrate]
In the present invention, the porous substrate is preferably composed of a resin that is electrically insulating, electrically stable, and stable to an electrolyte. The resin used from the viewpoint of providing a shutdown function is preferably a thermoplastic resin having a melting point of 200 ° C. or lower. Here, the shutdown function is a function to stop power generation by closing the porous structure by melting with heat and stopping ion movement when the lithium ion battery abnormally generates heat. Examples of the thermoplastic resin include a polyolefin-based resin, and the porous substrate is preferably a polyolefin-based porous substrate. Specific examples of the polyolefin-based resin include polyethylene, polypropylene, copolymers thereof, and mixtures thereof. For example, a single-layer porous base material mainly composed of polyethylene, and made of polyethylene and polypropylene. A multi-layered porous substrate can be used.

  As a method for producing a porous substrate, a method of making a polyolefin resin porous after being made into a sheet, or extracting a solvent after dissolving the polyolefin resin in a solvent such as liquid paraffin to form a sheet The method of making it porous is mentioned.

  The thickness of the porous substrate is preferably 5 to 50 μm, more preferably 10 to 30 μm. When the thickness of the porous substrate is greater than 50 μm, the internal resistance of the porous substrate may increase. In addition, when the thickness of the porous substrate is less than 5 μm, the production becomes difficult, and sufficient mechanical properties may not be obtained.

  The air permeability of the porous substrate is preferably 50 to 1,000 seconds / cc. More preferably, it is 50 to 500 seconds / cc. When the air permeability is higher than 1,000 seconds / cc, sufficient ion mobility cannot be obtained, and battery characteristics may be deteriorated. If it is less than 50 seconds / cc, sufficient mechanical properties may not be obtained.

[Secondary battery separator]
The separator for a secondary battery of the present invention is a separator for a secondary battery in which a porous layer containing coated fine particles coated with a fluororesin is laminated on at least one surface of a porous substrate as described above. . The laminated porous layer is preferably sufficiently porous to have ion permeability, and the air permeability of the secondary battery separator is 50 to 1,000 seconds / cc. preferable. More preferably, it is 50 to 500 seconds / cc. When the air permeability is higher than 1,000 seconds / cc, sufficient ion mobility cannot be obtained, and battery characteristics may be deteriorated. If it is less than 50 seconds / cc, sufficient mechanical properties may not be obtained.

[Secondary battery]
The separator for a secondary battery of the present invention can be suitably used for a secondary battery such as a lithium ion battery. A lithium ion battery has a configuration in which a secondary battery separator and an electrolyte are interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. Yes.

The positive electrode is obtained by laminating a positive electrode agent composed of an active material, a binder resin, and a conductive additive on a current collector. Examples of the active material include LiCoO ₂ , LiNiO ₂ , Li (NiCoMn) O ₂ , and the like. Examples thereof include lithium-containing transition metal oxides having a layered structure, spinel-type manganese oxides such as LiMn ₂ O ₄ , and iron-based compounds such as LiFePO ₄ . As the binder resin, a fluorine-based resin is generally used because of its high oxidation resistance, and specifically, a polyvinylidene fluoride-based resin is used. As the conductive assistant, carbon materials such as carbon black and graphite are used. As the current collector, a metal stay is suitable, and in particular, aluminum is often used.

The negative electrode is made by laminating a negative electrode agent consisting of an active material and a binder resin on a current collector. As the active material, carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon, tin, silicon, etc. Lithium metal materials such as Li, metal materials such as Li, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). As the binder resin, polyvinylidene fluoride resin, butylene-stadiene rubber, or the like is used. As the current collector, a metal stay is suitable, and in particular, a copper foil is often used.

The electrolytic solution is a place where ions are moved between the positive electrode and the negative electrode in the secondary battery, and the electrolyte is dissolved in an organic solvent. As the _electrolyte, LiPF 6, LiBF _4, and the like LiClO ₄ and the like, solubility in organic solvents, LiPF ₆ is preferably used in view of ion conductivity. Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma butyrolactone, sulfolane, and the like. Good.

  As a method for producing a secondary battery, first, an active material and a conductive additive are dispersed in a binder solution to prepare a coating solution for an electrode, and this coating solution is applied onto a current collector and the solvent is dried. Thus, a positive electrode and a negative electrode are obtained. The thickness of the coating film after drying is preferably 50 to 500 μm. A secondary battery separator is arranged between the positive electrode and the negative electrode so that the active material layers of the respective electrodes are in contact with each other, sealed in an exterior material such as an aluminum laminate film, and injected with an electrolyte after hot pressing. Or you may heat-press after inject | pouring electrolyte solution. Then, a negative electrode lead and a safety valve are installed, and the exterior material is sealed. The secondary battery thus obtained has good adhesion between the electrode and the separator for the secondary battery, and therefore has excellent cycle characteristics and high heat resistance, and therefore prevents short circuit due to abnormal heat generation of the secondary battery. be able to.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. The measurement method used in this example is shown below.

[Measuring method]
(1) Coated fine particles, average particle diameter of core fine particles and coating thickness of fluororesin The sample cross section was cut out with a microtome, and the cross section was subjected to an electrolytic emission scanning electron microscope (S-800 manufactured by Hitachi, Ltd., acceleration voltage 26 kV). ), The particle diameter of the coated fine particle having the largest particle diameter, the particle diameter of the core fine particle of the coated fine particle, and the coating thickness of the fluororesin were measured. Measurements were made at five arbitrary locations from a 100 mm × 100 mm sample and averaged.

(2) Thickness of porous layer A sample cross-section is cut out with a microtome, and the cross-section is observed with an electrolytic emission scanning electron microscope (S-800, manufactured by Hitachi, Ltd., acceleration voltage 26 kV). The highest point from the interface was measured as the thickness of the porous layer. Measurements were averaged at five arbitrary locations from a 100 mm × 100 mm sample.

(3) Air permeability Select one arbitrary location from each of 3 samples of 100 mm × 100 mm size, and use JIS using an Oken type air permeability measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.). P was measured in accordance with P 8117 (2009), and the average value was defined as the air permeability (seconds / 100 cc).

(4) Thermal contraction rate After a sample of 100 mm × 100 mm size was left in a thermostatic bath at 150 ° C. for 1 hour, the sample was taken out and allowed to cool to room temperature, and the size of four sides was measured. Using this value, a value calculated from (original size−size after test) × 100 / (original size) (%) was defined as the heat shrinkage rate.

(5) Adhesive strength The active material is LiCO ₂ , the binder is polyvinylidene fluoride, the conductive auxiliary agent is carbon black positive electrode 15 mm × 100 mm and the secondary battery separator 15 mm × 100 mm, and the active material and the functional porous layer are in contact with each other Then, heat pressing was performed at 2 MPa and 80 ° C. for 2 minutes using a hot press machine, and 180 ° peel strength was measured using a universal testing machine (AG-1S manufactured by Shimadzu Corporation). Similarly, the peel strength between the negative electrode and the secondary battery separator, in which the active material is graphite, the binder is polyvinylidene fluoride, and the conductive additive is carbon black, is measured, and the average value of each value is defined as the adhesive strength. Was evaluated with a relative value of 100.

(6) Charging / discharging cycle test The charging conditions were 1C, 4.2V constant current constant voltage charging, the discharging conditions were 1C, 3.0V constant current discharging, and the number of cycles was 100 times. The capacity retention rate at this time was evaluated as cycle characteristics.

Example 1
A coating solution was prepared by dispersing aluminum oxide (average particle diameter: 0.5 μm) coated with a copolymer of vinylidene fluoride and hexafluoropropylene (mass ratio 95: 5) with a coating thickness of 2 μm in water. . This coating solution is applied onto a polyethylene porous substrate (thickness 20 μm, air permeability 220 seconds / 100 cc) using a wire bar, and dried until the contained solvent volatilizes to form a porous layer. did. The same coating was performed on the opposite surface to form a porous layer on both sides of the polyethylene porous substrate to obtain a separator for a secondary battery of the present invention. Table 1 shows the measurement results of the thickness of the porous layer, the air permeability, the heat shrinkage rate, the adhesive strength, and the cycle characteristics of the obtained secondary battery separator. The measurement results are also shown in Table 1 for the following examples and comparative examples.

(Example 2)
A secondary battery separator of the present invention was obtained in the same manner as in Example 1 except that aluminum oxide having a coating thickness of 1.5 μm (average particle diameter of core fine particles: 0.3 μm) was used.

(Example 3)
A separator for a secondary battery of the present invention was obtained in the same manner as in Example 1 except that aluminum oxide having a coating thickness of 0.5 μm (average particle diameter of core fine particles: 0.5 μm) was used.

Example 4
Coated polymethyl methacrylate (average particle diameter of core fine particles: 1 μm) coated with a copolymer of vinylidene fluoride and hexafluoropropylene (mass ratio 95: 5) with a film thickness of 2 μm dispersed in methyl ethyl ketone A separator for a secondary battery of the present invention was obtained in the same manner as in Example 1 except that the working solution was used.

(Example 5)
Cross-linked polymethyl methacrylate having a coating thickness of 3 μm (average particle diameter of core fine particles: 1.5 μm) is used, and a polypropylene porous substrate (thickness 20 μm, air permeability 220 seconds / 100 cc) is used as the porous substrate. A separator for a secondary battery of the present invention was obtained in the same manner as in Example 4 except that.

(Comparative Example 1)
A secondary battery separator of the present invention was obtained in the same manner as in Example 1 except that silica having a coating thickness of 0.2 μm (average particle diameter of core fine particles: 0.05 μm) was used.

(Comparative Example 2)
A secondary battery separator of the present invention was obtained in the same manner as in Example 4 except that cross-linked polymethyl methacrylate having a coating thickness of 0.03 μm (average particle diameter of core fine particles: 1 μm) was used.

(Comparative Example 3)
A secondary battery separator of the present invention was obtained in the same manner as in Example 1 except that silica having a coating thickness of 0.03 μm (average particle diameter of core fine particles: 0.05 μm) was used.

  From Table 1, since Examples 1-5 of this invention are all in the range whose average particle diameter of film fine particle is 0.15-10 micrometers and the film thickness of fluororesin is 0.05-5 micrometers, it is heat resistant. Thus, a secondary battery separator excellent in both thermal shrinkage and cycle characteristics is obtained.

  On the other hand, in Comparative Example 1, since the average particle diameter of the coated fine particles is small, heat resistance cannot be obtained, and the heat shrinkage rate is large. In Comparative Example 2, the film thickness of the fluororesin is thin and sufficient adhesion cannot be obtained, and the cycle characteristics are deteriorated. In Comparative Example 3, the average particle diameter of the coating fine particles is small, the coating thickness of the fluororesin is also thin, and sufficient characteristics of heat resistance and adhesiveness are not obtained.

Claims (7)

  At least one surface of the porous substrate is laminated with a porous layer having an average particle diameter of 0.15 to 10 μm and containing coating fine particles coated with a fluorine resin, and the coating thickness of the fluorine resin is 0 A separator for a secondary battery characterized by having a thickness of 0.05 to 5 μm.   The secondary battery separator according to claim 1, wherein the porous layer has a thickness of 1 to 10 μm.   The separator for a secondary battery according to claim 1, wherein the fluororesin is a vinylidene fluoride resin.   The secondary battery separator according to any one of claims 1 to 3, wherein the core fine particles of the coated fine particles are inorganic oxide fine particles or organic fine particles.   The separator for a secondary battery according to any one of claims 1 to 4, wherein the porous layer is laminated on both surfaces of the porous substrate.   The separator for a secondary battery according to any one of claims 1 to 5, wherein the porous substrate is a polyolefin-based porous substrate.   A secondary battery using the secondary battery separator according to claim 1.