JP6094805B2 - Secondary battery - Google Patents

Description

  The present invention relates to a secondary battery. Specifically, the present invention relates to a secondary battery including a separator having a porous insulating layer formed on the surface of a resin base material.

  In recent years, lithium secondary batteries, nickel metal hydride batteries and other secondary batteries (storage batteries) have become increasingly important as power sources for mounting on vehicles or as power sources for personal computers and portable terminals. In particular, a lithium secondary battery that is lightweight and has a high energy density is preferably used as a high-output power source for mounting on a vehicle. One typical configuration of this type of lithium secondary battery includes a positive electrode, a negative electrode, and a porous separator interposed between the positive electrode and the negative electrode. The separator plays a role of forming an ion conduction path between both electrodes by preventing a short circuit due to contact between the positive electrode and the negative electrode and impregnating the electrolyte in the pores of the separator. Patent documents 1 and 2 are indicated as conventional technology about this kind of separator.

  Conventionally, a porous resin substrate made of polyethylene (PE), polypropylene (PP), or the like has been used as a separator. Since the separator is porous, heat shrinkage occurs when the temperature increases. Using this, the shutdown function works. However, if the degree of thermal contraction is large, a local short circuit due to a membrane breakage or the like may occur, and the short circuit may further expand therefrom. Therefore, it has been proposed to form a porous insulating layer on the surface of the resin base material in order to prevent direct contact between the positive electrode and the negative electrode even when the resin base material is thermally contracted. The porous insulating layer is formed, for example, by applying a water-based paint in which an inorganic filler such as alumina and a binder are dispersed in an appropriate medium to the surface of the resin substrate, and passing it through a hot air dryer or the like to dry. Has been.

International Publication No. 2009/038229 JP 2001-068088 A

  The resin base material on which the porous insulating layer is formed generally has hydrophobicity because it does not substantially have a polar functional group, and the surface thereof exhibits water repellent properties (that is, water repellency). When a water-based paint is applied to such a hydrophobic resin substrate, the wettability of the resin substrate with respect to the water-based paint is low, so the thickness of the applied water-based paint (and thus a porous insulating layer obtained after drying) Tends to be non-uniform depending on the location. In addition, since the water-based paint is difficult to adapt to the resin base material, the adhesion of the porous insulating layer obtained after drying to the resin base material is insufficient, and the porous insulating layer partially lifts from the resin base material. Tends to cause inconvenience such as being easily peeled off.

  In order to cope with such a problem, the present inventor is considering performing a treatment (modification treatment) for increasing the hydrophilicity of the resin base material, and then applying a water-based paint. That is, the wettability with respect to the water-based coating material of the resin base material is improved by performing a treatment for increasing the hydrophilicity (for example, corona discharge treatment) on the resin base material. As a result, the paint can be uniformly applied on the resin substrate, and a porous insulating layer having good adhesion (binding property) to the resin substrate can be formed. However, if the treatment to increase the hydrophilicity is performed on the resin base material, the fiber part constituting the resin base material is damaged, so that a minute defect occurs in the resin base material or the void of the resin base material is enlarged. It may be. Such an increase in the degree of opening can be a factor that reduces the liquid retention of the resin base material and, consequently, the battery performance. The present invention solves the above problems.

  According to the present invention, a method for producing a secondary battery (for example, a non-aqueous electrolyte secondary battery represented by a lithium secondary battery such as a lithium ion secondary battery) is provided. The method includes preparing (manufacturing, purchasing, etc.) a porous resin substrate. Further, the surface of the resin base material is subjected to a hydrophilic treatment for improving hydrophilicity by modifying the surface of the resin base material (typically, a polar functional group containing oxygen (O) such as a hydroxyl group or a carboxyl group) A treatment for introducing a group into the resin base material). Furthermore, a coating containing an inorganic filler, a fibrillated polymer (hereinafter also abbreviated as “fibrillated polymer”), and an aqueous medium is applied (applied, dried, etc.) to the hydrophilic surface of the resin substrate. To obtain a separator having a porous insulating layer formed on the surface of the resin substrate. The manufacturing method also includes constructing a secondary battery using the separator.

  In the present specification, the “secondary battery” generally refers to a power storage device that can be repeatedly charged, and is a so-called lithium secondary battery (typically a lithium ion secondary battery), a nickel hydride battery, a nickel cadmium battery, or the like. It is a term encompassing power storage elements such as storage batteries and electric double layer capacitors. Moreover, the above-mentioned “hydrophilic treatment” means that the hydrophilicity of the resin base material is increased by performing the treatment as compared with that before the treatment.

  In the production method according to the present invention, after preparing the resin base material, the resin base material is subjected to a hydrophilic treatment to improve the wettability of the resin base material with respect to the water-based paint, and then the hydrophilicity of the resin base material. A water-based paint is applied to the resin base material (hydrophilic surface) that has been subjected to the chemical treatment. By this, the separator provided with the porous insulating layer with favorable adhesiveness with respect to the resin base material can be obtained. Further, since the paint contains a fibrillated polymer, when the paint is applied to the resin base material, at least a part of the paint soaks into the resin base material, and the fibrillated polymer contained in the paint is The surface of the fiber part which comprises a resin base material is coat | covered. This makes it possible to fill minute defects in the resin base material caused by the hydrophilization treatment and prevent the voids in the resin base material from being enlarged. Therefore, according to the present invention, it is possible to improve the wettability of the resin base material with respect to the water-based paint while suppressing the expansion of the voids of the resin base material, and thus the electrolyte retention is high and the porous insulating layer is peeled off. It is difficult to obtain a separator having excellent quality stability. A secondary battery constructed using such a separator is, for example, less susceptible to capacity deterioration and excellent in durability.

  In a preferred embodiment of the production method disclosed herein, the hydrophilic treatment is performed by adding a hydrophilic functional group (for example, a polar functional group containing oxygen (O) such as a hydroxyl group or a carboxyl group) to the resin substrate. It is a process to introduce. By this, the hydrophilic property of the said resin base material can be improved more effectively.

  In a preferred embodiment of the production method disclosed herein, the hydrophilic treatment is a corona discharge treatment. This makes it possible to introduce more efficiently (for example, at a higher density) polar functional groups such as a hydroxyl group and a carboxyl group that have a high hydrophilicity-improving effect. Therefore, the hydrophilicity of the resin base material can be improved more effectively.

  According to the present invention, there is also provided a secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The separator includes a porous resin base material and a porous insulating layer formed on the surface of the resin base material. The resin substrate has a hydrophilic surface on which a hydrophilic functional group (for example, a polar functional group containing an oxygen atom) exists. The porous insulating layer contains an inorganic filler and a fibrillated polymer. Due to the presence of the hydrophilic functional group, it is possible to impart hydrophilicity to the surface of a resin substrate generally having water repellency. As a result, when the water-based paint for forming the porous insulating layer (typically a slurry-like paint in which a powdery inorganic filler or the like is dispersed in an aqueous medium) is applied to the resin base material, the water-based paint becomes a resin. It becomes easy to become familiar with the base material. Therefore, it is possible to avoid a situation in which the porous insulating layer obtained after drying is lifted or peeled off from the resin base material. Furthermore, since the coating material contains a fibrillated polymer, it can be filled with minute defects in the resin base material resulting from the hydrophilization treatment, and expansion of the voids in the resin base material can be prevented. Therefore, according to the present invention, it is possible to provide a secondary battery including a separator having good adhesion (bonding strength) between the porous insulating layer and the resin base material while maintaining high input / output characteristics.

  In a preferred aspect of the secondary battery disclosed herein, the fibrillated polymer constitutes a massive aggregate in which cellulose fibers having a fiber length of 1 μm to 10 μm are aggregated, and is formed on the outer surface of the aggregate. The fibril fiber is protruding. According to this configuration, it is possible to more efficiently fill minute defects in the resin base material caused by the hydrophilic treatment.

  In a preferred embodiment of the secondary battery disclosed herein, the average fiber diameter of the fibril fibers protruding from the outer surface of the aggregate is 0.05 μm to 0.5 μm. When the average fiber diameter of the fibril fibers is within the above range, a suitable porous insulating layer free from pinholes and aggregates (dama) can be formed. In addition, when the water-based paint is applied to the resin base material, the fibrillated polymer is likely to penetrate into the resin base material, so that defects in the resin base material can be reliably compensated.

  In a preferred embodiment of the secondary battery disclosed herein, the content of the fibrillated polymer is 0.3% by mass to 5% by mass when the total mass of the porous insulating layer is 100% by mass. It is. When the content of the fibrillated polymer is within the above range, a suitable porous insulating layer free from pinholes and aggregates (dama) can be formed. In addition, when the water-based paint is applied to the resin base material, the fibrillated polymer is likely to penetrate into the resin base material, so that defects in the resin base material can be reliably compensated.

  In a preferred aspect of the secondary battery disclosed herein, the porous insulating layer includes a facing portion that faces the positive electrode or the negative electrode, and a non-facing portion that does not face the positive electrode or the negative electrode. Part. And the said fibrillated polymer is selectively arrange | positioned in the whole region of the said non-opposing part among the said porous insulating layers. In this case, the input / output characteristics of the secondary battery can be further improved.

FIG. 1 is a diagram showing a manufacturing flow of a separator according to an embodiment of the present invention. FIG. 2 is a diagram schematically showing a fibrillated polymer. FIG. 3 is a diagram schematically showing a cross section of a separator according to an embodiment of the present invention. FIG. 4 is an external view showing a lithium ion secondary battery according to an embodiment of the present invention. FIG. 5 is a cross-sectional view showing a VV cross section of FIG. 4. FIG. 6 is a view showing a wound electrode body of a lithium ion secondary battery. FIG. 7 is a cross-sectional view showing a VII-VII cross section in FIG. 6. FIG. 8 is a cross-sectional view for explaining a region where the fibrillated CMC of the sample 15 is arranged. FIG. 9 is a cross-sectional view for explaining the arrangement region of the fibrillated CMC of the sample 16. FIG. 10 is a diagram showing a vehicle equipped with a secondary battery.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than the matters specifically mentioned in the present specification and necessary for carrying out the present invention (for example, a method for producing an electrode active material, a method for preparing a composition for forming an electrode mixture layer, an electrolyte The configuration and manufacturing method, lithium secondary battery and other general techniques related to the construction of batteries, etc.) can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  If the separator manufacturing method disclosed here provides the water-based coating material containing an inorganic filler to the surface of the porous resin base material with which the hydrophilic treatment was performed, if a porous insulating layer is formed There are no particular restrictions on the composition (material) and shape of the resin substrate used. As said resin base material, polyolefin resin, such as polyethylene (PE) and polypropylene (PP), can be used suitably, for example. As the PE resin, a homopolymer of ethylene is preferably used. The PE resin is a resin containing 50% by mass or more of a repeating unit derived from ethylene, and is a copolymer obtained by polymerizing an α-olefin copolymerizable with ethylene, or at least copolymerizable with ethylene. It may be a copolymer obtained by polymerizing one kind of monomer. Examples of the α-olefin include propylene. Examples of other monomers include conjugated dienes (for example, butadiene) and acrylic acid.

  Moreover, it is preferable that a resin base material is comprised from PE whose shutdown temperature is about 120 to 140 degreeC (typically 125 to 135 degreeC). The shutdown temperature is sufficiently lower than the heat resistant temperature of the battery (for example, about 200 ° C. or higher). Examples of such PE include polyolefins generally referred to as high-density polyethylene or linear (linear) low-density polyethylene. Alternatively, various types of branched polyethylene having medium density and low density may be used. Moreover, additives, such as various plasticizers and antioxidants, can also be contained as needed.

  The structure of the resin substrate may be a single layer structure or a multilayer structure. For example, it is preferable to use a resin base material having a three-layer structure of a PP layer, a PE layer laminated on the PP layer, and a PP layer laminated on the PE layer. In this case, the porous insulating layer can be laminated on the PP layer that appears on the surface of the resin substrate. The number of layers of the resin sheet having a multilayer structure is not limited to 3, and may be 2 or 4 or more.

  The separator manufactured by any of the methods disclosed herein can be preferably used as a separator provided in various types of secondary batteries (for example, lithium secondary batteries). Since the shape of the resin base material used in the separator manufacturing method can vary depending on the shape (for example, rectangular parallelepiped shape, flat shape, cylindrical shape) of the secondary battery constructed using the obtained separator, there is no particular limitation. For example, it may be in various forms such as a sheet shape and a plate shape. As a preferable embodiment of the secondary battery constructed using the separator obtained by the method of the present invention, a secondary battery (for example, a lithium secondary battery) including a wound electrode body can be given. In this embodiment, a long sheet-like resin base material can be preferably used. The base material itself having various shapes can be manufactured in the same manner as a conventionally known current collector in the field of manufacturing a secondary battery, and does not characterize the present invention.

  Although not intended to be particularly limited, a lithium secondary battery (typically a lithium ion secondary battery) using a sheet-shaped resin base material mainly having a three-layer structure of a PP layer, a PE layer, and a PP layer will be described below. The present invention will be described in detail with reference to the flowchart shown in FIG.

  As shown in FIG. 1, in the method for manufacturing a separator for a lithium secondary battery disclosed herein, a resin sheet having a thickness of about 10 μm to 30 μm is prepared as a resin base material (step S10). Such a resin base material may be manufactured, or a commercially available resin base material (an existing product) may be purchased and used.

  Although the thickness of a resin base material is not specifically limited, For example, it is preferable that it is about 10 micrometers-30 micrometers, and it is more preferable that it is about 16 micrometers-20 micrometers. If the thickness of the resin substrate is too large, the ion conductivity of the separator may be reduced. On the other hand, if the thickness of the resin base material is too small, there is a risk of film breakage. The thickness of the resin substrate can be obtained by image analysis of an image photographed by SEM.

Further, the porosity of the resin base material is preferably about 30% to 70%, and more preferably about 40% to 60%, for example. When the porosity of the resin substrate is too large, the strength is insufficient, and film breakage may occur easily during overcharge. On the other hand, if the porosity of the resin substrate is too small, the amount of electrolyte solution that can be held on the resin substrate decreases, and the input / output characteristics of a battery constructed using this separator may deteriorate. In addition, the porosity of the resin base material is defined as V ₁ is the apparent volume of the resin base material, W ₁ is the mass of the resin base material, and the true density of the material constituting the resin base material When (value obtained by dividing the mass W ₁ ) is ρ ₁ , it can be grasped by (1−W ₁ / ρ ₁ V ₁ ) × 100.

  Such a resin substrate is generally hydrophobic because it does not substantially have a polar functional group, and its surface exhibits a water repellent property (that is, water repellency). When a water-based paint (paint for forming a porous insulating layer) is applied to such a hydrophobic resin sheet, the wettability of the resin sheet with respect to the water-based paint is low. (Thus, the thickness of the porous insulating layer obtained after drying) tends to be non-uniform depending on the location. In addition, since the water-based paint is difficult to adjust to the resin sheet, the adhesion of the porous insulating layer obtained after drying to the resin sheet is insufficient, and the porous insulating layer is likely to partially lift or peel off from the resin sheet. Sometimes. According to the production method of the present invention, a treatment (modification treatment) for increasing the hydrophilicity of such a hydrophobic resin base material (typically at least the surface of the resin base material) is performed, and then an aqueous paint is applied. By doing so, a separator for a lithium secondary battery provided with a porous insulating layer that is uniform and has good adhesion (high bonding strength) can be obtained. According to such a separator, a lithium secondary battery with more stable performance can be constructed.

That is, as shown in FIG. 1, in the manufacturing method according to the present invention, the surface of the prepared resin base material is subjected to a hydrophilic treatment for improving the hydrophilicity by modifying the surface of the resin base material (step S20). . As a typical example of such a hydrophilic treatment, a treatment for introducing a polar functional group into the resin substrate can be preferably employed. Preferable examples of the polar functional group include polar functional groups containing an oxygen atom (O) (hydroxyl group, carboxyl group, aldehyde group, keto group, etc.). In particular, it is effective to perform a hydrophilization treatment capable of introducing at least one selected from a hydroxyl group and a carboxyl group. The hydrophilization treatment for introducing the polar functional group containing an oxygen atom includes chemical species containing an oxygen atom (for example, oxygen gas (O ₂ ), ozone (O ₃ ), oxide ion (O ²⁻ ), oxygen radical, The treatment may include supplying oxygen plasma or the like to the resin base material. Such chemical species can be obtained by, for example, applying a corona discharge treatment, a plasma treatment, an ultraviolet irradiation treatment, or the like to the resin substrate (typically, performing the treatment in an atmosphere containing oxygen). The material can be supplied. Moreover, in the technique disclosed here, the process (ozone process) which supplies ozone to the said resin base material can be employ | adopted preferably as a process which introduce | transduces the polar functional group containing the said oxygen atom. Such supply of ozone can be preferably performed, for example, by generating corona discharge treatment in an atmosphere containing oxygen gas (O ₂ ) to generate O ₃ from O ₂ .

  In the treatment for increasing the hydrophilicity with respect to the resin substrate described above (for example, corona discharge treatment), a large load is applied to the resin substrate itself, so that the fiber part constituting the resin substrate is damaged and the resin substrate is damaged. There may be a case where a minute defect occurs in the surface of the resin base material or a void of the resin base material is enlarged. If such a resin base material with expanded voids is used as a separator as it is, the electrolyte solution retainability of the separator is lowered, and the characteristics of a battery constructed using the separator tend to deteriorate. Therefore, according to the production method of the present invention, a fibrillated polymer that functions as a filler for filling minute defects in the resin base material is added to the water-based paint, and then the paint is applied.

  That is, in the manufacturing method according to the present invention, a paint containing an inorganic filler, a fibrillated polymer, and an aqueous medium is applied to the hydrophilic surface of the resin base material (step S30). Thereby, the separator by which the porous insulating layer was formed on the surface of a resin base material is obtained (step S40).

  The fibrillated polymer is not particularly limited as long as it is a fibrous polymer that functions as a thickener for the paint and can be fibrillated. For example, cellulose polymer fibers such as carboxymethylcellulose (CMC), methylcellulose (MC), hydroxypropylmethylcellulose (HPMC), and hydroxyethylmethylcellulose (HEMC) can be preferably used. Alternatively, polymer fibers such as polyvinyl alcohol (PVA) and ethylene-vinyl alcohol copolymer (EVOH) may be used. The polymer fibers described above can be used alone or in appropriate combination.

  As a preferred example of the fibrillated polymer 10 disclosed herein, as shown in FIG. 2, a mass aggregate (basic part) 14 in which cellulose fibers having a fiber length of 1 μm to 10 μm are aggregated is formed. The thing which the fibril fiber 12 protrudes in 14 outer surface parts is mentioned. The average fiber diameter d of the fibril fibers 12 protruding from the outer surface portion of the aggregate 14 is generally 0.05 μm or more, preferably 0.1 μm or more, more preferably 0.2 μm or more. If the fiber diameter d of the fibril fiber 12 is too small, bubbles mixed in the paint cannot be completely removed until drying, which may cause pinholes in the porous insulating layer obtained after drying. On the other hand, when the fiber diameter d of the fibril fiber 12 is too large, cellulosic fibers are aggregated to easily cause lumps. Moreover, since it becomes difficult for the fibrillated polymer to soak into the resin base material, there is a possibility that defects in the resin base material cannot be reliably compensated. From the viewpoint of reliably filling the defects, the average fiber diameter d of the fibril fibers 12 is appropriately 0.5 μm or less, preferably 0.4 μm or less, more preferably 0.3 μm or less. The fiber diameter d of the fibril fiber 12 is obtained by, for example, extracting at least 10 or more fibrillated polymers arbitrarily selected from an SEM (Scanning Electron Microscope) image of the porous insulating layer, The average value (arithmetic average value) should be evaluated.

  Although not particularly limited, it is appropriate that the size of the aggregate 14 (basic portion) of the fibrillated polymer 10 is about 3 μm to 20 μm, for example, 3 μm to 10 μm (for example, about 5 μm). It is more preferable. Moreover, as a fiber length of the cellulose fiber which comprises the aggregate | assembly 14, it is suitable that it is about 0.5 micrometer-10 micrometers generally, For example, it is more preferable to set it as 1 micrometer-5 micrometers. For example, the aggregate 14 having a size of about 3 μm to 20 μm and a fiber length of about 0.5 μm to 10 μm can be suitably used.

  Examples of the method for fibrillating the polymer include a method of pulverizing polymer agglomerates (lumps) using a jet mill, a rotary mill, an attritor (for example, a mass collider), a refiner, a high-pressure homogenizer, or the like. By using these to apply mechanical shearing force and compressive force to the polymer, the polymer is mainly torn in a direction parallel to the fiber axis and loosened to develop fine fibers (fibril fibers) on the surface. The conditions for the pulverization may vary depending on the apparatus to be used. For example, when a jet mill is used, it is preferable to employ conditions of a pulverization pressure of 0.1 MPa to 2 MPa and a pulverization frequency of 1 to 5 times. Such pulverization may be carried out dry or wet.

  As a material constituting the inorganic filler, it is preferable to use an inorganic material having a melting point higher than that of the resin substrate used. Furthermore, it is preferably an inorganic material that has high electrical insulation and is electrochemically stable (not oxidized / reduced) even when used in contact with an electrode. One or more inorganic materials satisfying such conditions can be used without particular limitation. Examples of such inorganic materials include Group 2 of the periodic table (alkaline earth metals such as magnesium and calcium), Group 4 (transition metals such as titanium), Group 5 (transition metals such as vanadium and niobium), Group 8 (transition metals such as iron and ruthenium), Group 9 (transition metals such as cobalt), Group 12 (base metals such as zinc), Group 13 (base metals such as aluminum and indium) and Group 14 (metalloid elements) The metal oxide of any metal belonging to (a base metal such as silicon or tin) can be used. Typical examples include alumina, boehmite, magnesia, titania, niobium oxide, vanadium oxide, iron oxide, ruthenium oxide, cobalt oxide, zinc oxide, indium oxide, silica, mullite, tin oxide, and calcium oxide. Is done. Since these metal oxides have a melting point higher than that of the resin substrate used and are electrochemically stable, they can be suitably used as inorganic fillers suitable for the purpose of the present invention. These inorganic fillers may be used individually by 1 type, and may be used in combination of 2 or more types.

  The shape (outer shape) of the inorganic filler is not particularly limited. From the viewpoints of mechanical strength, manufacturability, etc., generally spherical inorganic fillers can be preferably used. Moreover, it is preferable that the size (average particle diameter: D50 diameter) of an inorganic filler is larger than the average pore diameter of a resin base material. For example, it is preferable to use an inorganic filler having an average particle diameter of about 0.1 μm to 3 μm, more preferably about 0.2 μm to 2 μm, and particularly preferably 0.2 μm to 1.8 μm. The average particle diameter of the inorganic filler can be determined by a method known in the art, for example, measurement based on a laser diffraction scattering method. Such a preparation method of the inorganic filler powder itself does not characterize the present invention at all, and thus a detailed description thereof is omitted.

  The coating material can be prepared by mixing the fibrillated polymer and inorganic filler powder as described above with other porous insulating layer forming components used as necessary in an aqueous medium. As a result, an aqueous coating material in which the fibrillated polymer and the inorganic filler powder are dispersed in the aqueous medium and the aqueous medium containing other porous insulating layer forming components used as necessary is obtained. Here, the “aqueous medium” is a concept indicating water or a mixed medium mainly composed of water. As the solvent other than water constituting the mixed medium, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, it is preferable to use an aqueous solvent in which 80% by mass or more (more preferably 90% by mass or more, more preferably 95% by mass or more) of the aqueous solvent is water. A particularly preferred example is an aqueous solvent substantially consisting of water.

  In addition to the fibrillated polymer, the inorganic filler powder, and the aqueous medium, the coating material may contain one or more materials (the “other porous insulating layer forming component”) as necessary. A representative example of such a material is a binder. As the binder, a polymer that is dispersed or dissolved in an aqueous medium can be used. Examples of the polymer that is dispersed or dissolved in the aqueous medium include acrylic resins. As the acrylic resin, a homopolymer obtained by polymerizing monomers such as acrylic acid, methacrylic acid, acrylamide, methacrylamide, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, methyl methacrylate, ethylhexyl acrylate and butyl acrylate. Is preferably used. The acrylic resin may be a copolymer obtained by polymerizing two or more of the above monomers. Further, a mixture of two or more of the above homopolymers and copolymers may be used. In addition to the acrylic resins described above, polyolefin resins such as styrene butadiene rubber (SBR) and polyethylene (PE), polytetrafluoroethylene (PTFE), and the like can be used. These polymers can be used alone or in combination of two or more. Among these, it is preferable to use an acrylic resin. The form of the binder is not particularly limited, and a particulate (powdered) form may be used as it is, or a solution prepared in the form of a solution or an emulsion may be used. Two or more kinds of binders may be used in different forms.

  Although it does not specifically limit, the solid content concentration (nonvolatile content, ie, the ratio of the porous insulating layer forming component) of the coating material may be, for example, about 10% by mass to about 60% by mass. The content of the inorganic filler in the solid content (that is, the porous insulating layer forming component) is preferably at least about 80% by mass, and may be, for example, about 85% to 99% by mass. Usually, it is appropriate to set this ratio to about 90% by mass to 98% by mass (preferably 95% by mass to 98% by mass). The content of the fibrillated polymer in the solid content (that is, the porous insulating layer forming component) (when the total mass of the porous insulating layer is 100% by mass) is 0.3% by mass or more. It is suitable, preferably 1% by mass or more, more preferably 2% by mass or more. If the content of the fibrillated polymer is too small, the void filling effect due to the addition of the fibrillated polymer may not be sufficiently obtained. On the other hand, if the content of the fibrillated polymer is too large, cellulosic fibers are aggregated to cause lumps, the surface quality of the porous insulating layer is lowered, and the excess fiber content acts as resistance, resulting in capacity deterioration. May occur. The content of the fibrillated polymer is suitably 5% by mass or less, preferably 4% by mass or less, more preferably 3% by mass or less. In the composition containing a binder, it can be set as the composition containing a binder in the ratio of 1 mass%-5 mass%, for example.

  The operation of mixing the inorganic filler, the fibrillated polymer and the binder with the aqueous medium includes ball mill, homodisper, Robomix (registered trademark), dispermill (registered trademark), Claremix (registered trademark), fillmix (registered trademark), A suitable kneader such as an ultrasonic disperser may be used.

The operation of applying such a paint to the surface of the resin substrate can be used without any particular limitation on conventional general application means. For example, using a suitable coating device (gravure coater, slit coater, die coater, comma coater, dip coat, etc.), a predetermined amount of paint is coated on the hydrophilic surface of the resin substrate to a uniform thickness. Can be applied. The coating amount of the coating material (the coating amount per unit area of the resin base material) is not particularly limited, and may be appropriately changed depending on the shape and application of the separator and the battery. For example, when producing a sheet-like separator used for the construction of a lithium secondary battery provided with a wound-type electrode body (rolled electrode body) as described above, on the surface of the sheet-like resin substrate the coating material, the coating amount of the solid basis (i.e., weight after drying) may be applied such that a for instance, about 0.2mg ^/ cm ² ~0.5mg ^/ cm 2 approximately. Thereafter, the solvent in the paint may be removed by drying the coated material by a suitable drying means (typically a temperature lower than the melting point of the resin substrate, for example, 110 ° C. or lower, for example, 30 to 80 ° C.). Thereby, the separator by which the porous insulating layer was formed on the surface of the resin base material is obtained.

  Although not particularly limited, the thickness of the porous insulating layer is usually 0.5 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 15 μm. Further, the porosity of the porous insulating layer is usually 40% or more, and particularly preferably 50% or more. The upper limit of the porosity is not particularly limited, but is generally 70% or less, preferably 65% or less.

  FIG. 3 schematically shows a cross-sectional structure of a sheet-like separator for a lithium secondary battery that is preferably manufactured by applying the separator manufacturing method disclosed herein. The separator sheet 70 includes a porous resin base material 72 and a porous insulating layer 74 formed on the surface of the resin base material 72. The resin base material 72 has a hydrophilic surface 72a on which a hydrophilic functional group exists. The porous insulating layer 74 contains an inorganic filler and a fibrillated polymer. The porous insulating layer 74 is formed by applying an aqueous paint containing an inorganic filler, a fibrillated polymer, and an aqueous medium to the hydrophilic surface 72a of the resin substrate 72 and drying it.

  Here, the resin base material 72 has been subjected to a hydrophilization treatment (for example, a modification treatment for introducing a highly hydrophilic polar functional group such as a hydroxyl group or a carboxyl group). Improved wettability to paint. As a result, the coating material can be uniformly applied on the resin base material 72, and the porous insulating layer 74 having good adhesion (binding property) to the resin base material 72 can be formed. Further, since the coating material contains a fibrillated polymer, when the coating material is applied to the resin base material 72, at least a part of the coating material soaks into the resin base material 72, and the fibrillation contained in the coating material is performed. The polymer coats the surface of the fiber part (particularly damaged part) constituting the resin base material 72. As a result, it is possible to fill minute defects in the resin base material 72 caused by the hydrophilic treatment, and to prevent the voids of the resin base material 72 from expanding. Therefore, according to the present invention, it is possible to improve the wettability of the resin base material 72 with respect to the water-based paint while suppressing the expansion of the voids of the resin base material 72, thereby having a high electrolyte solution retaining property and a porous insulating layer. Thus, it is possible to obtain a separator 70 in which 74 is hardly peeled off and excellent in quality stability.

  Since the separator provided by the method of the present invention has high electrolyte solution retention and excellent quality stability as described above, the constituent elements of various types of secondary batteries or electrode bodies incorporated in the batteries are used. It can be preferably used as a component. For example, it can be preferably used as a component of a lithium secondary battery comprising a separator produced by any of the methods disclosed herein, and a positive electrode and a negative electrode arranged to face each other with the separator interposed therebetween. . Structure (for example, metal casing or laminate film structure) and size of an outer container constituting such a battery, or structure of an electrode body (for example, a wound structure or a laminated structure) having a positive / negative electrode current collector as a main component There is no particular restriction on the etc.

  Hereinafter, one embodiment of a lithium ion secondary battery constructed using a sheet-like separator 70 (see FIG. 3) manufactured by applying the method of the present invention will be described with reference to schematic diagrams shown in FIGS. However, it will be explained. FIG. 4 is a perspective view schematically showing the lithium ion secondary battery 100 according to the present embodiment. FIG. 5 is a cross-sectional view taken along line VV in FIG. FIG. 6 is a diagram for explaining the configuration of the wound electrode body 20. 7 is a cross-sectional view taken along line VII-VII in FIG.

  As shown in FIGS. 4 to 7, the lithium ion secondary battery 100 according to the present embodiment is essentially a positive electrode sheet (positive electrode) 30 including a positive electrode active material layer 34 on a long positive electrode current collector 32. A negative electrode sheet (negative electrode) 50 having a negative electrode active material layer 54 on a long negative electrode current collector 52, and two separators 70 interposed between the positive electrode sheet 30 and the negative electrode sheet 50, A wound electrode body 20 is provided which is wound by overlapping. The lithium ion secondary battery 100 includes a battery case 80 (typically a flat rectangular parallelepiped shape) that accommodates the electrode body 20 and a suitable nonaqueous electrolyte solution.

≪Battery case 80≫
The battery case 80 is provided on a box-shaped (ie, bottomed rectangular parallelepiped) case body 84 having an opening at one end (corresponding to an upper end portion in a normal use state of the battery 100), and an upper surface in the vertical direction. It is comprised from the cover body 82 which consists of a rectangular plate member attached to the opening part and plugging up the opening part.

  The material of the battery case 80 is not particularly limited as long as it is the same as that used in the conventional sealed battery. A battery case 80 mainly composed of a lightweight and highly heat conductive metal material is preferable, and examples of such a metal material include aluminum, stainless steel, and nickel-plated steel. The battery case 80 (the case main body 84 and the lid body 82) according to the present embodiment is made of aluminum or an alloy mainly composed of aluminum.

  As shown in FIG. 4, a positive electrode terminal 40 and a negative electrode terminal 60 for external connection are formed on the lid 82. A thin safety valve 88 configured to release the internal pressure when the internal pressure of the battery case 80 rises above a predetermined level and a liquid injection port 86 are formed between both terminals 40 and 60 of the lid 82. Has been. In FIG. 4, the liquid injection port 86 is sealed with a sealing material 87 after liquid injection.

≪Winded electrode body 20≫
As shown in FIG. 6, the wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet 30) and a long sheet-like negative electrode (negative electrode sheet 50) similar to the positive electrode sheet 30 in total. Long sheet separators (separators 70, 70).

≪Positive electrode sheet 30≫
As shown in FIGS. 6 and 7, the positive electrode sheet 30 includes a strip-shaped positive electrode current collector 32 and a positive electrode active material layer 34. For the positive electrode current collector 32, for example, a metal foil suitable for the positive electrode can be suitably used. In this embodiment, a strip-like aluminum foil having a thickness of about 15 μm is used as the positive electrode current collector 32. The positive electrode sheet 30 has an uncoated portion where the positive electrode current collector 32 is exposed without forming the positive electrode active material layer 34 at one end in the width direction orthogonal to the longitudinal direction of the positive electrode current collector 32. 33 is provided. Further, a positive electrode active material layer 34 is formed so that the uncoated portion 33 is not substantially provided at the other end portion. In the illustrated example, the positive electrode active material layer 34 is held on both surfaces of the positive electrode current collector 32 except for the uncoated portion 33 set on the positive electrode current collector 32. The positive electrode active material layer 34 includes a positive electrode active material, a conductive material, and a binder.

As the positive electrode active material, a material used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of positive electrode active materials include LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide), LiNiO ₂ (lithium nickelate), LiCoO ₂ (lithium cobaltate), LiMn ₂ O ₄ (lithium manganate), LiFePO ₄ ( Lithium transition metal oxides such as lithium iron phosphate). Here, LiMn ₂ O ₄ has, for example, a spinel structure. LiNiO ₂ and LiCoO ₂ have a layered rock salt structure. LiFePO ₄ has, for example, an olivine structure. LiFePO ₄ having an olivine structure includes, for example, nanometer order particles. Moreover, LiFePO _{4 having} an olivine structure can be further covered with a carbon film.

  For example, carbon black such as acetylene black (AB) and ketjen black and other powdery carbon materials (such as graphite) can be mixed with the positive electrode active material. In addition to the positive electrode active material and the conductive material, a binder such as polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR) can be added. A positive electrode mixture (paste) can be prepared by dispersing these in a suitable dispersion medium and kneading. The positive electrode active material layer 34 is formed by applying this positive electrode mixture to the positive electrode current collector 32, drying it, and pressing it to a predetermined thickness.

≪Negative electrode sheet 50≫
As shown in FIGS. 6 and 7, the negative electrode sheet 50 includes a strip-shaped negative electrode current collector 52 and a negative electrode active material layer 54. For the negative electrode current collector 52, for example, a metal foil suitable for the negative electrode can be suitably used. In this embodiment, a strip-shaped copper foil having a thickness of about 10 μm is used for the negative electrode current collector 52. The negative electrode sheet 50 has an uncoated portion in which the negative electrode current collector 52 is exposed without forming the negative electrode active material layer 54 at one end in the width direction orthogonal to the longitudinal direction of the negative electrode current collector 52. 53 is provided. A negative electrode active material layer 54 is formed on the other end so that the uncoated portion 53 is not substantially provided. The negative electrode active material layer 54 is held on both surfaces of the negative electrode current collector 52 except for the uncoated portion 53 set on the negative electrode current collector 52. The negative electrode active material layer 54 includes a negative electrode active material, a thickener, a binder, and the like.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium ion secondary batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon and amorphous carbon, lithium transition metal oxides, and lithium transition metal nitrides.

  In addition to the negative electrode active material, a binder such as polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR) can be added. Furthermore, in addition to the negative electrode active material and the binder, a thickener such as carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), ethylene-vinyl alcohol copolymer (EVOH) can be added. And like a positive electrode, a negative electrode mixture (paste) can be prepared by disperse | distributing and knead | mixing these negative electrode active material layer structural components in a suitable dispersion medium. The negative electrode active material layer 54 is formed by applying this negative electrode mixture to the negative electrode current collector 52, drying it, and pressing it to a predetermined thickness.

<< Separator 70 >>
As shown in FIGS. 5 to 7, the separator 70 is a member that separates the positive electrode sheet 30 and the negative electrode sheet 50. As described above, the separator 70 includes the porous resin base material 72 and the porous insulating layer 74 formed on the surface of the resin base material 72. In this example, the porous insulating layer 74 is formed on the side facing the negative electrode active material layer 54 on the front and back of the resin base material 72. In this example, as shown in FIG. 6, the width m of the negative electrode active material layer 54 is slightly wider than the width l of the positive electrode active material layer 34. Further, the width n of the separator 70 is slightly wider than the width m of the negative electrode active material layer 54.

≪Winded electrode body 20≫
The wound electrode body 20 is an electrode body in which the positive electrode sheet 30 and the negative electrode sheet 50 are overlapped and wound while the separator 70 is interposed between the positive electrode active material layer 34 and the negative electrode active material layer 54. In this embodiment, as shown in FIG. 3, the positive electrode sheet 30, the negative electrode sheet 50, and the two separators 70 are arranged in the order of the positive electrode sheet 30, the separator 70, the negative electrode sheet 50, and the separator 70. It is piled up. In this embodiment, although the separator 70 is interposed, the negative electrode active material layer 54 is overlaid so as to cover the positive electrode active material layer 34. Further, the negative electrode current collector 52 and the positive electrode current collector 32 are formed so that the uncoated portions 33 and 53 of the negative electrode current collector 52 and the positive electrode current collector 32 protrude to the opposite side in the width direction of the wound electrode body 20. , Are piled up. The stacked sheet material (for example, the positive electrode sheet 30) is wound around a winding axis set in the width direction.

  The wound electrode body 20 is attached to the positive terminal 40 and the negative terminal 60 attached to the battery case 80 (in this example, the lid 82). The wound electrode body 20 is housed in the battery case 80 in a state of being flatly pushed and bent in one direction perpendicular to the winding axis. The wound electrode body 20 is housed in the battery case 80 so that the winding axis direction is the horizontal direction. In the wound electrode body 20, the uncoated portion 33 of the positive electrode sheet 30 and the uncoated portion 53 of the negative electrode sheet 50 protrude on the opposite sides in the width direction of the separator 70. Among these, one positive electrode terminal 40 is fixed to the uncoated part 33 of the positive electrode current collector 32, and the other negative electrode terminal 60 is fixed to the uncoated part 53 of the negative electrode current collector 52. . The wound electrode body 20 is accommodated in the flat internal space of the case main body 84. The case body 84 is closed by the lid 82 after the wound electrode body 20 is accommodated.

≪Nonaqueous electrolyte≫
As the non-aqueous electrolyte, those similar to the non-aqueous electrolyte conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,3-dioxolane, and the like. One kind or two or more kinds selected from the group can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _{3 and the} like. Lithium salts can be used. As an example, a nonaqueous electrolytic solution in which LiPF ₆ is contained in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mass ratio of 1: 1) at a concentration of about 1 mol / L can be given.

≪Test evaluation≫
The inventor conducted a test to evaluate the function and effect of the separator sheet 70. Hereinafter, although the test example regarding this invention is demonstrated, it is not intending to limit this invention to what is shown to the following test examples.

≪Fibrylated CMC≫
Aggregates (lumps) of carboxymethyl cellulose (CMC: fiber length 1.2 μm) were made into a PJM jet mill machine (manufactured by Nippon Pneumatic Kogyo Co., Ltd .; crushing pressure 0.9 MPa, crushing frequency once) and a super mass collider machine (Masuyuki Sangyo A fibrillated CMC was produced by pulverization using a product manufactured by Co., Ltd .; a rotational speed of 3000 rpm and a clearance between grinding wheels of 5 μm. At that time, the shear force and the compressive force applied to the CMC are appropriately changed and pulverized, classified by an MDS-1 type airflow classifier (manufactured by Nippon Pneumatic Industry Co., Ltd.), and the fiber diameter d of the fibril fiber (FIG. 2). Different fibrillated CMCs were obtained.

≪Separator≫
Alumina powder (D50 diameter: 0.2 μm) as an inorganic filler, the fibrillated CMC, and an acrylic resin as a binder so that the mass ratio of these materials is 96: 1: 3 in solid content ratio. The mixture was mixed with water to prepare a slurry paint. In addition, a porous resin sheet (PP / PE / PP three-layer structure having a thickness of 20 μm was used) was prepared, and a general corona discharge treatment apparatus was used on the surface of the resin sheet in the atmosphere. The corona discharge was generated at, and the resin sheet was hydrophilized. On the surface of the resin sheet after the hydrophilic treatment, the paint was applied with a gravure roll and dried to obtain a separator having a porous insulating layer formed on the surface of the resin sheet.

≪Samples 1-6≫
Samples 1-6 differ in the fibril fiber diameter d (FIG. 2) of the fibrillated CMC. Samples 1 to 6 have the same configuration except for the fibril fiber diameter d.

<< Samples 7 to 10 >>
Samples 1 and 7 to 10 have different fibrillated CMC contents. Samples 1 and 7 to 10 have the same configuration except for the content of the fibrillated CMC.

<< Samples 11-14 >>
In samples 1, 11 to 14, the thickness of the porous insulating layer is different. Samples 1 and 11 to 14 have the same configuration except for the thickness of the porous insulating layer.

<< Samples 15 and 16 >>
Samples 1, 15, and 16 have different fibrillated CMC arrangement regions. Samples 1, 15, and 16 have the same configuration except for the arrangement region of fibrillated CMC. Specifically, as illustrated in FIGS. 8 and 9, the porous insulating layer 74 has a facing portion that faces the negative electrode sheet 50 and a non-facing portion that does not face the negative electrode sheet. In Sample 1, the fibrillated CMC was disposed throughout the porous insulating layer 74. On the other hand, in the sample 15, as shown in FIG. 8, the fibrillated CMC was selectively disposed in the entire region R1 of the portion (non-opposing portion) of the porous insulating layer 74 not facing the negative electrode sheet 50. In Sample 16, as shown in FIG. 9, the fibrillated CMC was selectively disposed in a part R2 of a portion (non-opposing portion) of the porous insulating layer 74 not facing the negative electrode sheet 50. The difference between the width of R1 and the width of R2 was 1 mm.

<< Sample 17 >>
In sample 17, a separator was prepared without performing corona discharge treatment on the resin sheet. Otherwise, the configuration was the same as Sample 2.

«Sample 18»
In sample 18, a separator was prepared using a paint to which non-fibrillated CMC (non-fibrillated CMC) was added. Otherwise, the configuration was the same as Sample 2.

≪Samples 19-21≫
In samples 19 to 21, separators were prepared using a coating material to which CMC that was not fibrillated (non-fibrillated CMC) was added, and the resin sheet was not subjected to corona discharge treatment. In addition, in order to improve the wettability of the paint to the resin sheet, Leocoal (manufactured by Lion Corporation) as a dispersant was added to the paint. Otherwise, the configuration was the same as Sample 2.

≪Measurement of pinhole≫
For each sample separator, the surface of the porous insulation was observed with an SEM (Scanning Electron Microscope), and the number of aggregates formed per 1000 cm ^{2 of} separator and the number of pinholes having a diameter of 0.5 mm or more was measured. did.

≪Positive electrode sheet≫
LiNi _1/3 Co _1/3 Mn _1/3 O ₂ powder (D50 diameter: 5.7 μm), AB (conductive material), and PVdF (binder) as a positive electrode active material have a mass ratio of these materials of 100 : 5: 2 was mixed with N-methylpyrrolidone (NMP) to prepare a positive electrode active material layer forming paste. The positive electrode active material layer forming paste is formed on both surfaces of the positive electrode current collector by applying the paste for forming the positive electrode active material layer on both surfaces of a 15 μm-thick aluminum foil (positive electrode current collector) and drying it. The obtained positive electrode sheet was produced. The coating amount of the positive electrode active material layer forming paste was adjusted so as to be about 15 mg / cm ² (solid content basis) per side. The thickness of the positive electrode sheet was 170 μm, the length was 4500 mm, and the coating width of the positive electrode active material layer was 94 mm.

≪Negative electrode sheet≫
Graphite powder (D50 diameter: 25 μm) as a negative electrode active material, CMC (thickening agent), and SBR (binder) are mixed with water so that the mass ratio of these materials is 100: 1: 1. Thus, a negative electrode active material layer forming paste was prepared. This negative electrode active material layer forming paste was applied to both sides of a 14 μm thick copper foil (negative electrode current collector) and dried to provide a negative electrode active material layer on both sides of the negative electrode current collector. A negative electrode sheet was produced. The thickness of the negative electrode sheet was 154 μm, the length was 4700 mm, and the coating width of the negative electrode active material layer was 100 mm.

≪Lithium ion secondary battery≫
A wound body is produced by winding the positive electrode sheet and the negative electrode sheet through two separators, and the wound body is pressed from a lateral direction with a flat plate at a pressure of 4 kN / cm ² for 2 minutes. A flat wound electrode body was produced by crushing. At that time, the separator sandwiched between the positive electrode sheet and the negative electrode sheet was disposed so that the porous insulating layer formed on one side of the separator faced the negative electrode sheet. The wound electrode body thus obtained was housed in a battery case (a square shape was used here) together with a non-aqueous electrolyte (non-aqueous electrolyte), and the opening of the battery case was hermetically sealed. As the non-aqueous electrolyte, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 4: 3, and 1 mol of LiPF ₆ was dissolved. used. To the electrolytic solution, 1% by mass of cyclohexylbenzene (CHB) and 1% by mass of biphenyl (BP) were added as gas generating agents that generate gas during overcharge.

≪Measurement of initial capacity≫
Each of the test lithium ion secondary batteries constructed using the separator of each sample was charged to a voltage of 4.1 V at a current value of 1 C under a temperature condition of 25 ° C. After 5 minutes of rest, the charged battery was discharged to a voltage of 3.0 V at a current value of 1 C at 25 ° C. Then, after a pause of 5 minutes, the battery was charged at a current value of 1 C to a voltage of 4.1 V, and then charged by a constant voltage method until the current value was reduced to 0.1 C. The charged battery was discharged at 25 ° C. at a current value of 1 C to a voltage of 3.0 V, and then discharged by a constant voltage method until the current value was reduced to 0.1 C. The discharge capacity at this time was defined as the initial capacity (rated capacity). Here, the initial capacity of the test lithium ion secondary battery was 24 Ah.

≪Capacity maintenance ratio after cycle≫
A charge / discharge pattern that repeats CC charge / discharge at 2C was applied to each test lithium ion secondary battery of each sample, and a cycle test was performed. Specifically, in an environment of about 50 ° C., a charging / discharging cycle in which charging is performed up to 4.1 V by constant current charging at 2 C and discharging is performed up to 3.0 V by constant current discharging at 2 C is repeated 1000 times continuously. It was. Then, the capacity retention rate was calculated from the capacity after the charge / discharge cycle test and the initial capacity. Here, the capacity after the charge / discharge cycle test was performed in the same manner as in “Measurement of initial capacity”. The capacity retention rate was determined by “battery capacity after charge / discharge cycle test / initial capacity” × 100. Here, the above test was performed on five batteries of each sample, and the average value was calculated.

≪Capacity maintenance ratio after 0 ℃ pulse cycle≫
For each test lithium ion secondary battery of each sample, after adjusting to a charged state (SOC 50%) of about 50% of the initial capacity, CC discharge for 10 seconds at 20C and 20C under the temperature condition of 0 ° C The charging / discharging cycle which performs CC charge for 10 seconds was repeated 50000 times continuously. Then, the battery capacity after the 0 ° C. pulse cycle is measured under the same conditions as the initial capacity. From [(battery capacity after 0 ° C. pulse cycle) / (initial capacity)] × 100 (%), after 0 ° C. pulse cycle The capacity maintenance rate was obtained.

≪High temperature storage test≫
Each sample lithium ion secondary battery was adjusted to a charged state (SOC 100%) of about 100% of the initial capacity, and then housed in a constant temperature bath at 60 ° C. and subjected to high temperature aging for 100 days. . The battery capacity after high-temperature storage was measured under the same conditions as the initial capacity, and the capacity retention rate after high-temperature storage was determined from [(battery capacity after high-temperature storage) / (initial capacity)] × 100 (%). . Here, the above test was performed on 50 batteries of each sample, and the average value was calculated.

≪Overcharge test≫
An overcharge test was performed on each of the test lithium ion secondary batteries. In the overcharge test, the battery was charged at a constant current of 48 A (equivalent to 2 C) in a room temperature (about 25 ° C.) environment atmosphere until the charge upper limit voltage was 20 V. At that time, a thermocouple was attached to the side surface of the battery case to measure the battery surface temperature and the battery voltage. When the separator was shut down during the charging and became unable to be energized, the behavior of the battery for the next 5 minutes was observed.

≪Evaluation of each sample≫
The results of the above test are shown in Table 1 and Table 2 for each sample. Table 1 summarizes the configuration of each sample. Table 2 is a table summarizing the evaluation for each sample.

  As shown in Table 1 and Table 2, in Samples 1 to 16, the porous insulating layer contains fibrillated CMC. About these samples 1-16, the capacity | capacitance maintenance factor after the high temperature storage test preserve | saved at 60 degreeC for 100 days is as favorable as about 70% or more, and is extremely high durability compared with the sample 18 using non-fibrillated CMC. Showed performance. As described above, by including the fibrillated CMC in the porous insulating layer, the capacity maintenance rate after high-temperature storage of the lithium ion secondary battery is remarkably improved.

  In Samples 1 to 5, the average fiber diameter d (FIG. 2) of the fibrils of the fibrillated CMC contained in the porous insulating layer is 0.5 μm or less. In these samples 1 to 5, the capacity maintenance ratio after the high-temperature storage test was 97% or more, and a much better result was obtained, which is superior to the sample 6 (average fiber diameter d: 0.7 μm). there were. In Sample 6, since the fibril fiber diameter d is large, it is understood that the fiber filling of the resin base material of the separator does not proceed and the battery characteristics are deteriorated. In sample 6, since the fibril fiber diameter d was large, CMC was not dispersed well, causing aggregation of CMCs, resulting in a decrease in surface quality (increase in the number of aggregates and pinholes). From these results, the fibril fiber diameter d of the fibrillated CMC is preferably 0.5 μm or less, more preferably 0.4 μm or less, and particularly preferably 0.3 μm or less.

  In Samples 2 to 5, the average fiber diameter d of the fibrils of the fibrillated CMC contained in the porous insulating layer is 0.05 μm or more. In these samples 2 to 5, the number of aggregates and pinholes could be further reduced as compared with sample 1 (average fiber diameter d: 0.03 μm). In sample 1, since the fibril fiber diameter d is small, it is considered that the air bubbles mixed in the paint cannot be completely removed until drying, resulting in pinholes and deterioration in surface quality. From this result, the fibril fiber diameter d of the fibrillated CMC is preferably 0.05 μm or more, and particularly preferably 0.08 μm or more.

  In Samples 2 and 8 to 10, the amount of fibrillated CMC contained in the porous insulating layer is set to 0.3% by mass or more. Samples 2 and 8 to 10 have a capacity retention rate of 97% or more after the high-temperature storage test, which is much better than that of sample 7 (addition amount of fibrillated CMC: 0.2% by mass). It was excellent in properties. In sample 7, since the amount of fibrillated CMC added is small, it is understood that the performance improvement effect by using the fibrillated CMC was lowered. From this result, it is appropriate that the amount of fibrillated CMC added is 0.3% by mass or more, preferably 0.5% by mass or more (more preferably 1% by mass or more).

  In Samples 2 and 7 to 9, the amount of fibrillated CMC contained in the porous insulating layer is set to 5% by mass or less. Samples 2 and 7 to 9 have a capacity retention rate of 90% or more after the cycle, which is much better than that of sample 10 (addition amount of fibrillated CMC: 7% by mass). there were. In Sample 10, since the amount of fibrillated CMC added is large, CMC does not disperse well, agglomeration of CMCs occurs, surface quality deteriorates, and excess fiber content acts as resistance, so that the battery capacity after cycling It is understood that this led to a decline in From this result, the amount of fibrillated CMC added is suitably 5% by mass or less, preferably 3% by mass or less (more preferably 2% by mass or less).

  In samples 2 and 12 to 14, the thickness of the porous insulating layer containing the fibrillated CMC is set to 0.8 μm or more. Samples 2 and 12 to 14 have a capacity retention rate of 97% or more after the high-temperature storage test, which is much better than that of sample 11 (porous insulating layer thickness: 0.5 μm). It was excellent. In sample 11, since the thickness of the porous insulating layer containing fibrillated CMC is small, it is understood that the performance improvement effect by using the fibrillated CMC is reduced. From this result, it is appropriate that the thickness of the porous insulating layer is 0.8 μm or more, preferably 1 μm or more (more preferably 3 μm or more, particularly 5 μm or more).

  In samples 2, 11 to 13, the thickness of the porous insulating layer is set to 15 μm or less. Samples 2 and 11 to 13 had a capacity retention rate after cycling of 90% or more, which was much better than that of sample 14 (porous insulating layer thickness: 17 μm). . In sample 14, since the thickness of the porous insulating layer is large, it is understood that the fiber content of fibrillated CMC becomes excessive, resulting in a decrease in battery capacity after cycling. From this result, the thickness of the porous insulating layer is suitably 15 μm or less, preferably 12 μm or less (more preferably 10 μm or less, especially 8 μm or less).

  In the sample 15, the fibrillated CMC is selectively disposed in the entire region R1 (FIG. 8) of the portion (non-opposing portion) that does not face the negative electrode sheet of the porous insulating layer. The sample 15 has a capacity retention rate of 97% or more after high-temperature storage, which is much better than that of the sample 2 in which the fibrillated CMC is arranged in the entire porous insulating layer. Met. On the other hand, in the sample 16, the fibrillated CMC is selectively disposed in a part R2 (FIG. 9) of a portion (non-opposing portion) that does not face the negative electrode sheet of the porous insulating layer. The sample 16 had a lower capacity retention rate after storage at a higher temperature than the sample 15. From this result, it is preferable to arrange the fibrillated CMC at least over the entire region of the porous insulating layer not facing the negative electrode sheet (non-facing portion).

  In Sample 17, the resin sheet is not subjected to corona discharge treatment. In Sample 17, the wettability between the resin sheet and the paint was poor, and unevenness and repellency occurred after application. Therefore, the number of pinholes was significantly increased as compared with Samples 1 to 16 subjected to corona discharge treatment. Also, the battery reaction became non-uniform, leading to a decrease in battery characteristics (capacity retention after cycling). In Samples 19 to 21, a dispersant is added to the paint instead of performing the corona discharge treatment on the resin sheet. In these samples 19 to 20, the capacity retention rate after high-temperature storage and the 0 ° C. capacity retention rate tended to decrease. In Samples 19-20, the wettability between the resin sheet and the paint was improved by adding a dispersant instead of performing a corona discharge treatment, but a reaction with moisture occurred due to the influence of the hydrophilic group of the dispersant. As a result, it is presumed that the battery characteristics were deteriorated.

  Although detailed data is omitted here, samples 2 to 5, 8, 9, 12, 13, and 15 in which the porous insulating layer contains fibrillated CMC reach 20 V in the overcharge test. Since the separator was shut down before it was turned on, it became impossible to energize, so the overcharge test was terminated here. In these batteries, the battery temperature gradually decreased when the energization was disabled. On the other hand, for the sample 18 in which the porous insulating layer contains non-fibrillated CMC, in the above overcharge test, the separator was shut down before reaching 20 V, and the current could not be supplied. Ended. However, the battery temperature of the sample 18 when the shutdown occurred was higher than the battery temperature when the shutdown of the samples 2 to 5, 8, 9, 12, 13, and 15 occurred. In sample 18, the battery temperature continued to rise even after the energization was disabled due to the shutdown. From the above result, it was confirmed that the shutdown function of the separator at the time of overcharge can be exhibited appropriately by including fibrillated CMC in the porous insulating layer.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

  Up to this point, a lithium ion secondary battery has been described as a typical example of a secondary battery, but the present invention is not limited to this form of secondary battery. For example, it may be a non-aqueous electrolyte secondary battery using a metal ion other than lithium ion (for example, sodium ion) as a charge carrier, a nickel hydride battery, a nickel cadmium battery, or a lithium battery equipped with the electrode body described above. It may be an electric double layer capacitor (physical battery) such as an ion capacitor.

  As described above, the secondary battery provided by the technology disclosed herein improves the capacity maintenance ratio after high-temperature storage, and is particularly suitable as a power source for a motor (electric motor) mounted on a vehicle such as an automobile. Can be used. Therefore, as schematically shown in FIG. 10, the present invention provides a vehicle 1 (typically an automobile, particularly a hybrid automobile) provided with such a secondary battery 100 (typically, a battery pack formed by connecting a plurality of batteries in series) as a power source. , Automobiles equipped with electric motors such as electric vehicles and fuel cell vehicles).

1 vehicle 10 fibrillated polymer 12 fibril fiber 14 lump aggregate 20 wound electrode body 30 positive electrode sheet 32 positive electrode current collector 34 positive electrode active material layer 50 negative electrode sheet 52 negative electrode current collector 54 negative electrode active material layer 70 separator 72 resin base material 72a Hydrophilic surface 74 Porous insulating layer 100 Secondary battery

Claims (8)

A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,
The separator includes a porous resin base material and a porous insulating layer formed on the surface of the resin base material,
The resin substrate has a hydrophilic surface on which a hydrophilic functional group exists,
The porous insulating layer contains an inorganic filler and a fibrillated polymer ,
The fibrillated polymer constitutes a mass aggregate in which cellulosic fibers having a fiber length of 1 μm to 10 μm are aggregated, and the fibril fibers protrude from the outer surface of the aggregate.
The average fiber diameter of the fibril fibers protruding from the outer surface of the aggregate is 0.05 μm to 0.5 μm,
A secondary battery in which the content of the fibrillated polymer is 0.3% by mass to 5% by mass when the total mass of the porous insulating layer is 100% by mass . The secondary battery according to claim 1, wherein the hydrophilic functional group is a polar functional group containing an oxygen atom. The porous insulating layer has a facing portion that faces the positive electrode or the negative electrode, and a non-facing portion that does not face either the positive electrode or the negative electrode,
It said fibrillated polymer, wherein one of the porous insulating layer, wherein is selectively disposed on the entire of the non-opposing portions, the secondary battery according to claim 1 or 2. Providing a porous resin substrate;
Applying a hydrophilic treatment to the surface of the resin base material to improve the hydrophilicity by modifying the surface of the resin base material;
A separator having a porous insulating layer formed on the surface of the resin substrate is obtained by applying a coating containing an inorganic filler, a fibrillated polymer, and an aqueous medium to the hydrophilic surface of the resin substrate. ;and,
Constructing a secondary battery using the separator;
It encompasses,
The fibrillated polymer constitutes a mass aggregate in which cellulosic fibers having a fiber length of 1 μm to 10 μm are aggregated, and the fibril fibers protrude from the outer surface of the aggregate.
The average fiber diameter of the fibril fibers protruding from the outer surface of the aggregate is 0.05 μm to 0.5 μm,
The manufacturing method of a secondary battery , wherein the content of the fibrillated polymer is 0.3% by mass to 5% by mass when the total mass of the porous insulating layer is 100% by mass . The said hydrophilization process is a manufacturing method of Claim 4 which is a process which introduce | transduces a hydrophilic functional group into the said resin base material. The said hydrophilization process is a manufacturing method of Claim 5 which is a process which introduce | transduces the polar functional group containing an oxygen atom into the said resin base material. The hydrophilic treatment is a corona discharge treatment method according to any one of claims 4-6. A porous resin substrate;
A porous insulating layer formed on the surface of the resin substrate,
The resin substrate has a hydrophilic surface on which a hydrophilic functional group exists,
The porous insulating layer contains an inorganic filler and a fibrillated polymer ,
The fibrillated polymer constitutes a mass aggregate in which cellulosic fibers having a fiber length of 1 μm to 10 μm are aggregated, and the fibril fibers protrude from the outer surface of the aggregate.
The average fiber diameter of the fibril fibers protruding from the outer surface of the aggregate is 0.05 μm to 0.5 μm,
The separator for secondary batteries whose content of the said fibrillated polymer is 0.3 mass%-5 mass% when the total mass of the said porous insulating layer is 100 mass% .

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