JP2022037399A - Manufacturing method for separator for electrochemical element - Google Patents

Abstract

To provide a manufacturing method for a separator for an electrochemical element, the method enabling suppression of a degradation in ion permeability even when a plurality of functional layers are used.SOLUTION: A manufacturing method for a separator for an electrochemical element including a first functional layer and a second functional layer on at least one side of a polyolefin porous membrane substrate is provided, the method includes a first coating step of coating the first functional layer, a second coating step of forming the second functional layer on the surface of the first functional layer before fixing the first coating film layer, and further includes a step of performing coating in such a manner that a region in which the second coating film layer is present and a region in which the second coating film layer is not present, thereby forming the second coating film layer, and fixing the first and second coating film layers at the same time.SELECTED DRAWING: None

Description

The present invention relates to a method for manufacturing a separator for an electrochemical element.

Electrochemical elements such as lithium-ion secondary batteries have made it possible to improve the performance of electronic devices such as mobile phones and notebook computers and to operate them for a long time as high-capacity batteries that can be repeatedly charged and discharged. Recently, electrochemical elements have been installed as driving batteries for environment-friendly vehicles such as electric vehicles and hybrid electric vehicles, and further improvement in performance is expected.

In order to improve the performance of electrochemical devices, various studies have been made on the improvement of various materials constituting the electrochemical devices. The separator, which is one of the materials, is arranged between the positive electrode and the negative electrode, and has a role of preventing a short circuit between the positive electrode and the negative electrode and impregnating the electrolytic solution to enable ion conduction between the electrodes. In addition to the above roles, the separator is required to have heat resistance from the viewpoint of ensuring safety at high temperatures.

Further, when manufacturing an electrochemical element, a dry heat press (a hot press treatment performed without impregnating the separator with an electrolytic solution) may be performed on a laminate in which a separator is arranged between a positive electrode and a negative electrode. If the separator and the electrode are well adhered by the dry heat press, it is possible to improve the manufacturing yield of the battery. Therefore, a separator having an excellent function of adhering to an electrode by a dry heat press (hereinafter, may be referred to as "dry adhesiveness") is desired.

Furthermore, lithium-ion batteries are expected to have improved battery characteristics such as longer life, higher capacity, and higher output. To extend the life, it is necessary to suppress the increase in internal resistance between the electrode and the separator. One method for controlling this is to make the current distribution between the electrode and the separator uniform, and by giving this function to the separator, it is possible to improve the life of the battery.

The separator is usually one in which a functional layer is laminated on at least one surface of a porous membrane base material, but one functional layer is imparted with a plurality of functions such as heat resistance, dry adhesiveness, and uniform current distribution. Is not easy.

Therefore, it has been proposed to add a plurality of functional layers to the separator.
For example, in Patent Document 1, an organic / inorganic composite coating layer is provided on at least one surface of a porous substrate, and the average diameter containing adhesive polymer particles on the surface thereof is 0.1 mm or less and is regularly separated from each other. A separator for a secondary battery is disclosed, which comprises providing a pattern coating layer having a dot shape arranged in a row.

Japanese Unexamined Patent Publication No. 2015-99776

However, according to the study by the present inventors, a pattern having a dot shape containing an adhesive polymer on the surface of the organic / inorganic composite coating layer provided on the surface of the porous substrate as described in Patent Document 1. When the coating layer is provided, in the thermocompression bonding process of joining the separator and the electrode, a slight gap is generated between the separator and the electrode due to the uneven shape of the dots, the uniformity of the internal resistance is lowered, and the battery life is impaired. There was a problem of being patented.

The present invention has been made in view of the above-mentioned conventional circumstances, and provides a method for manufacturing a separator for an electrochemical element that does not impair the life of a battery even when a dry adhesive functional layer is provided. It is an issue to be solved.

As a result of diligent studies to achieve the above object, the present inventors have found that the above problems can be solved by the following manufacturing method, and have completed the present invention.

That is, the present invention relates to the following <1> to <8>.
<1> A method for producing a separator for an electrochemical device having a first functional layer and a second functional layer on at least one surface of a porous film base material made of polyolefin.
A first coating step of coating the first functional layer and a second coating step of forming the second functional layer on the surface of the first functional layer before fixing the first coating film layer. Including,
A second coating film layer is formed by coating so as to form a region in which the second coating film layer is present and a region in which the second coating film layer is not present.
It is characterized by including a step of fixing the first and second coating film layers at the same time.
A method for manufacturing a separator for an electrochemical element.
<2> The method for producing a separator for a thermochemical element according to <1>, wherein the first coating liquid and the second coating liquid are compatible with each other.
<3> The separator for an electrochemical element according to <1> or <2>, wherein the difference in surface tension between the first coating liquid and the second coating liquid is 20 mN / m or more and less than 40 mN / m. Manufacturing method.
<4> The separator for an electrochemical element according to any one of <1> to <4>, which uses a spray coating method, a rotor dampening method, a die coating method, a curtain coating method or an inkjet method in the second coating step. Production method.
<5> The method for manufacturing a separator for an electrochemical element according to any one of <1> to <5>, which uses a gravure coat method, a chamber doctor method, and a DM coater method in the first coating step.
<6> The method for producing a separator for an electrochemical element according to any one of <1> to <6>, wherein the first coating liquid contains inorganic particles. <7> The second coating liquid is a fluororesin. The method for producing a separator for an electrochemical element according to any one of <1> to <7>, which comprises at least one resin selected from the group consisting of an acrylic resin and an olefin resin.

According to the manufacturing method of the present invention, it is possible to provide a separator for an electrochemical element that does not impair the life of the battery even when the dry adhesive functional layer is provided.

Hereinafter, the present invention will be described in detail, but these are examples of desirable embodiments, and the present invention is not specified in these contents.

The separator for an electrochemical device (hereinafter, may be referred to as "separator of the present invention") obtained by the production method of the present invention has a first functional layer and a second function on at least one surface of a porous membrane substrate. Has a layer.

[Porous membrane substrate]
Examples of the porous membrane base material include a porous membrane having pores inside, a non-woven fabric, and a porous membrane sheet made of a fibrous material. The porous membrane substrate is preferably composed of a resin that is electrically insulating, electrically stable, and stable to an electrolytic solution.

Further, from the viewpoint of imparting a shutdown function, the resin used is preferably a thermoplastic resin, and more preferably a thermoplastic resin having a melting point of 200 ° C. or lower. The shutdown function here is a function of closing the porous structure by melting with heat when the lithium ion battery heats up abnormally, stopping the ion movement, and stopping the power generation.

Examples of the thermoplastic resin include polyolefin resins. The porous membrane base material preferably contains a polyolefin-based resin, and more preferably contains a polyolefin-based resin having a melting point of 200 ° C. or lower.

Specific examples of the polyolefin-based resin include polyethylene, polypropylene, copolymers thereof, and mixtures thereof. Examples of the porous membrane substrate containing a polyolefin resin include a single-layer porous membrane substrate containing 90% by mass or more of polyethylene, a multilayer porous membrane substrate composed of polyethylene and polypropylene, and the like.

As a method for producing a porous film base material, for example, a method of forming a polyolefin resin into a sheet and then stretching it to make it porous, or a method of dissolving a polyolefin resin in a solvent such as liquid paraffin to form a sheet and then extracting the solvent. There is a method of making it porous.

The porous membrane substrate obtained by the above method may be surface-treated from the viewpoint of adhesion between the porous membrane substrate and the functional layer.

The thickness of the porous membrane substrate is preferably 3 to 50 μm, more preferably 5 to 30 μm. If the thickness of the porous membrane substrate is thicker than 50 μm, the internal resistance of the porous membrane substrate may increase. Further, if the thickness of the porous membrane substrate is thinner than 3 μm, it becomes difficult to manufacture, and sufficient mechanical properties may not be obtained.

The air permeability of the porous membrane substrate is preferably 50 to 1,000 seconds / 100 cc, more preferably 50 to 500 seconds / 100 cc. If the air permeability is larger than 1,000 seconds / 100 cc, sufficient ion mobility may not be obtained and the battery characteristics may deteriorate. If the air permeability is less than 50 seconds / 100 cc, sufficient mechanical properties may not be obtained.

[First functional layer]
The first functional layer can be obtained by performing the first coating step and the first film fixing step.

(1st coating process)
The first coating step is a step of applying the first coating liquid to at least one surface of the porous film substrate to form the first coating film layer.

The first coating liquid is for forming the first functional layer and contains a resin.
Examples of the resin contained in the first coating liquid include polyamide resin, polyamideimide resin, polyimide resin, polyvinyl alcohol resin, acrylic resin, fluororesin, polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylensulfide, and polyallylate. , Polyetherimide, polyetheretherketone, cellulose and derivatives thereof and the like. These may be used alone or in combination of two or more.

From the viewpoint of adhesiveness, the resin contained in the first coating liquid is preferably a fluororesin, and is a vinylidene fluoride homopolymer, a vinylidene fluoride-olefin fluorinated copolymer, or a vinyl fluoride single weight. Combines and fluorovinyl-fluoroolefin copolymers are more preferred, and vinylidene fluoride-hexafluoropropylene copolymers and polymers containing vinylidene fluoride units are even more preferred.

Examples of the aromatic polyamide resin include a meta-oriented aromatic polyamide resin and a para-oriented aromatic polyamide resin. Either of them may be used, but a para-oriented aromatic polyamide resin is preferable from the viewpoint of excellent battery characteristics and heat shrinkage of the electrochemical element.

Further, the first coating liquid preferably contains inorganic particles from the viewpoint of foreign matter resistance. Foreign matter resistance refers to resistance to foreign matter that has fallen off from the positive electrode or negative electrode and is mixed in during the manufacturing process of the electrochemical element.

Examples of the inorganic particles include inorganic oxide particles such as aluminum oxide, boehmite, silica, titanium oxide, zirconium oxide, iron oxide and magnesium oxide, inorganic nitride particles such as aluminum nitride and silicon nitride, calcium fluoride and fluoride. Examples thereof include sparingly soluble ionic crystal particles such as barium and barium sulfate. These particles may be used alone or in admixture of two or more.

The primary average particle size of the inorganic particles is preferably 0.10 μm to 5.0 μm, more preferably 0.10 μm to 2.5 μm, from the viewpoint of the strength and porosity of the first functional layer. By setting the lower limit of the primary average particle size to 0.10 μm, the first functional layer becomes dense, and the pores of the porous membrane substrate are closed, so that the air permeability is increased and the battery characteristics are improved. It can be prevented from getting worse. Further, by setting the upper limit of the primary average particle size to 5.0 μm, it is possible to prevent the first functional layer from having a non-uniform structure and not being able to obtain a sufficient heat shrinkage rate, and also to prevent the film of the first functional layer from being obtained. It is possible to prevent the thickness from increasing and the battery characteristics from deteriorating. The primary average particle size is the particle size when the volume-based integration rate calculated by the method described later is 50%.

Further, in order to achieve both the porosity and the strength of the first functional layer, it is preferable to contain two or more kinds of particles having different primary average particle sizes, and the primary average particle size is 0.30 μm to 2.5 μm. It is more preferable that the inorganic particles and the inorganic particles having a primary average particle size of 0.10 μm or more and less than 0.30 μm are contained.

Examples of the shape of the inorganic particles used include a spherical shape, a plate shape, a needle shape, a rod shape, an elliptical shape, and the like, and any shape may be used. Among them, it is preferable that it is spherical from the viewpoint of surface modifier, dispersibility, and coatability.

The content of the inorganic particles in the first coating liquid is preferably 50 to 98% by mass, more preferably 60 to 95% by mass, from the viewpoint of battery characteristics.

In addition to the resin and the inorganic particles, other components such as a dispersant, a thickener, a stabilizer, a defoaming agent, and a leveling agent may be added to the first coating liquid, if necessary. ..

The method for preparing the first coating liquid is not particularly limited, but from the viewpoint of uniformly dispersing the inorganic particles and making the primary average particle size of the inorganic particles in the coating liquid uniform, a resin is mixed with the solvent and used. A dissolved solution (resin solution) and a dispersion in which inorganic particles are dispersed in a solvent (inorganic particle dispersion) are mixed, and if necessary, other additives are added to the inorganic particle dispersion. It is preferable to prepare a coating liquid by dispersing the resin dissolution liquid in the coating liquid.

As the solvent in the first coating liquid, polar solvents such as water, glycerol, ethylene glycol, propylene glycol, dimethyl sulfoxide, dimethylformamide, acetonitrile, ethylene carbonate, flufuryl alcohol, and methanol can be used.

The method for dispersing the resin solution is not particularly limited, but from the viewpoint of heat resistance and wettability of the electrolytic solution, the inorganic particles in the coating liquid are uniformly dispersed, and the primary average particle size of the inorganic particles is uniform. It is important that the particles are dispersed in a solvent using a ball mill, a bead mill, a sand mill, a roll mill, or the like, and then the resin solution is dispersed in the solvent.

In particular, from the viewpoint of uniformity of the primary average particle size of the inorganic particles in the coating liquid, it is preferable to disperse using a bead mill, and the bead diameter used for the bead mill is preferably 0.1 to 1 mm, and the beads to be used. As the material of, it is preferable to use aluminum oxide, zirconium oxide, zirconia-reinforced alumina and the like.

Further, the dispersion step is preferably performed a plurality of times, and the peripheral speed of the mixing device used when mixing the resin with the solvent is higher than the speed at which the inorganic particles are dispersed in the solvent and is gradually increased. However, it is preferable from the viewpoint of the uniformity of the primary average particle size of the inorganic particles in the coating liquid.

The solid content concentration of the first coating liquid is not particularly limited as long as it can uniformly coat the first coating liquid, but is preferably 10 to 90% by mass, more preferably 20 to 80% by mass. If the solid content concentration is less than 10% by mass, the first functional layer may become brittle. Further, if the solid content concentration exceeds 80% by mass, the coatability may be deteriorated.

The viscosity of the first coating liquid is preferably 3 cP or more and 200 cP or less, more preferably 5 cP or more and 150 cP or less, and further preferably 10 cP or more and 100 cP or less. When the viscosity of the first coating liquid is 200 cP or less, high-speed coating is possible and good productivity can be obtained. If the coating liquid has a low viscosity, it is suitable for high-speed coating, so it is preferable. However, for example, from the viewpoint of stable coating film formation, it is preferable that the coating liquid does not fall below 3 cP. The viscosity of the first coating liquid can be controlled by the solid content concentration of the first coating liquid, the mixing ratio of the organic resin and the inorganic particles, and the molecular weight of the organic resin.

Examples of the coating method used when applying the obtained first coating liquid to at least one surface of the porous membrane substrate include dip coating, gravure coating, chamber doctor coating, slit die coating, and knife coating. Methods such as chamber coating, kiss coating, roll coating, bar coating, spray coating, dip coating, spin coating, screen printing, and pad printing can be mentioned. Among these, gravure coating, chamber doctor coating, and slit die coating are preferable from the viewpoint of mass productivity of the first coating film layer, and when the first coating liquid is applied to both surfaces of the porous polyolefin film. , Gravure coating, chamber doctor coating are particularly preferred.
As described above, the first coating film layer can be formed in the first coating step.

When the first coating step is continuously performed, the transport speed of the porous film substrate can be set in the range of, for example, 5 m / min to 200 m / min, and the coating can be applied from the viewpoint of productivity and uniformity of coating film thickness. It can be set as appropriate according to the construction method.

[Second functional layer]
The second functional layer can be obtained by performing the second coating step and the second film fixing step.

(Second coating process)
In the second coating step, the second coating film is applied to at least one surface of the first coating film layer, and the area where the second coating film layer formed by the second coating film is present (hereinafter, “existence”). It is coated so as to form a region "" and "existing area") and a region where the second second coating film layer does not exist (hereinafter, may be referred to as "non-existing region"). This is the step of forming the second coating film layer.

The first coating film layer and the second coating film layer have different functions, and the main functions are that the first coating film layer has heat resistance and ion permeability, and the second coating film layer. Is the adhesiveness with the electrode. By forming the non-existent region of the second coating film layer, both the heat resistance and ion permeability of the first coating film layer and the adhesiveness of the second coating film layer with the electrode are functions. It is possible to obtain a separator for an electrochemical element having a porous layer provided with the above.

The existing region and the non-existing region of the second coating film layer can be discriminated by the following method. That is, a small piece of 0.5 cm × 0.5 cm was cut out from the manufactured separator for an electrochemical device, and the separator surface SEM-EDX was measured in 10 fields at different locations under the conditions described later. The obtained SEM-EDX image is read by the image analysis software HALCON (Ver. 13.0, manufactured by MVtec), and the contour is enhanced (differential filter (emphasize, width of low-pass mask: 3 / height of low-pass mask). : 3 / Contrast enhancement intensity: 0.3), Edge enhancement filter (shock_filter, Time step length: 0.5 / Number of repetitions: 1 / Edge detection type: canny / Edge detection smoothing: 1) After processing), binarization was performed. Regarding binarization, the lower limit of the threshold value for the gray value was set to 180 and the upper limit was set to 255, and the portion of 180 or more was defined as the portion where alumina was present. Furthermore, the gray value in the region where the alumina is present is replaced with 255, the gray value in the other regions is replaced with 0, continuous pixels having the gray value 255 are connected to each other, and the region area where the alumina is present is integrated after extraction. Then, the area ratio of the fluorine-containing resin contained in the second coating film layer as the main component in the SEM-EDX image is calculated by dividing by the total number of pixels of the original image and subtracting from 100. The average value of these values was taken as the area ratio of the second coating film layer, that is, the second functional layer, to the entire film surface of the measured electrochemical element separator.

Further, in the present invention, in the second coating step, the first coating step and the first film fixing step are performed in a state where the solvent contained in the first coating film layer remains on the surface of the porous film substrate. It is characterized by being done in between. As a result, the first film fixing step and the second film fixing step described later can be simultaneously performed as a common fixing step, and the manufacturing cost and the manufacturing time of the separator of the present invention can be reduced. In addition, by performing the second coating step with the solvent contained in the first coating film layer remaining, the first functional layer and the second functional layer are made of a polyolefin porous film substrate via an interface. It is possible to form a state in which they coexist on the same film surface as above.

The second coating liquid is for forming the second functional layer, and preferably contains a resin.
Examples of the resin contained in the second coating liquid include fluororesins, acrylic resins, olefin resins, styrene-butadiene resins, crosslinked polystyrene resins, methylmethacrylate-styrene copolymers, polyimide resins, melamine resins, and phenol resins. Examples thereof include polyacrylonitrile resin, silicon resin, urethane resin, polycarbonate resin, and carboxymethyl cellulose resin. These may be used individually by 1 type, or may be used in combination of 2 or more type.

Among these, fluororesins, acrylic resins and olefin resins are preferable, and fluororesins and acrylic resins are even more preferable, from the viewpoint of electrical stability and oxidation resistance.

Examples of the fluororesin include homopolymers such as polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl fluoride and polychlorotrifluoroethylene, and copolymers such as ethylene-tetrafluoroethylene polymer and ethylene-chlorotrifluoroethylene polymer. .. Moreover, a copolymer of a homopolymer and tetrafluoroethylene, hexafluoropropylene, trifluoroethylene and the like can also be mentioned.

Among these, polyvinylidene fluoride is preferable from the viewpoint of electrical stability and oxidation resistance, and a resin made of a copolymer of vinylidene fluoride and hexafluoropropylene is more preferable.

The weight average molecular weight of the fluororesin is preferably 50,000 to 2,000,000, more preferably 100,000 to 1,500,000, and even more preferably 200,000 to 1,000,000. If the weight average molecular weight of the fluororesin is less than 50,000, sufficient adhesion to the electrode may not be obtained. Further, when the weight average molecular weight of the fluororesin is larger than 2 million, the handleability and coatability may be lowered due to the increase in viscosity.

Examples of the acrylic resin include poly (meth) acrylic acid and poly (meth) acrylic acid ester. Here, "(meth) acrylic" means acrylic or methacrylic.

Examples of the poly (meth) acrylic acid monomer include acrylic acid and methacrylic acid, and examples of the poly (meth) acrylic acid ester monomer include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, and butyl. Examples include methacrylate.

Examples of the olefin resin include polyethylene and polypropylene.

The content of the resin in the second coating liquid is preferably 3 to 15% by mass, more preferably 5 to 10% by mass, from the viewpoint of battery characteristics.

In addition to the resin, other components such as the above-mentioned inorganic particles, dispersants, thickeners, stabilizers, defoamers, leveling agents, etc. are added to the second coating liquid, if necessary. May be good.

The method for preparing the second coating liquid is not particularly limited, but it is prepared by adding other additives or the like to the dissolution liquid (resin dissolution liquid) in which the resin is mixed and dissolved in the solvent, if necessary. Can be done. As the solvent, the same solvent as that used in the first coating liquid can be used.

As the solvent in the second coating liquid, two kinds selected from polar solvents such as water, glycerol, ethylene glycol, propylene glycol, dimethyl sulfoxide, dimethylformamide, acetonitrile, ethylene carbonate, furfuryl alcohol, acetone, and methanol. The above solvents can be used.

The surface tension of the second coating liquid is preferably higher than the surface tension of the first coating liquid in order to form the existing region and the non-existing region. The surface tension difference between the first coating liquid and the second coating liquid is preferably 20 mN / m or more and less than 40 mN / m, and more preferably 25 mN / m or more and less than 35 mN / m. The surface tension of the second coating liquid can be controlled by the solid content concentration of the coating liquid and the mixing ratio of the polar solvent species.

When the surface tension difference between the first coating liquid and the second coating liquid is 20 mN / m or more, the solvent contained in the first coating film layer remains on the surface of the porous film substrate. When the two coating steps are performed, it is suppressed that the second coating film layer is diffused, wet and spread on the first coating film layer. When the diffusion wet spread becomes large, the region where the resin contained in the second coating film layer exists becomes large, the ion permeability decreases, and there is a high possibility that the battery characteristics are impaired. Further, if the diffusion wet spread is small, the desired dry adhesion function can be exhibited with a small amount of the second coating liquid, and the manufacturing cost can be reduced.

When the surface tension difference between the first coating liquid and the second coating liquid is less than 40 mN / m, the solvent contained in the first coating film layer remains on the surface of the porous film substrate. 2 Generated between the dry adhesive functional layer formed by the second coating film layer and the heat-resistant layer formed by the first coating film layer on the first coating film layer when the coating step is performed. It is possible to reduce the height difference. By reducing the height difference, in the thermocompression bonding process of joining the separator for the electrochemical element and the electrode, a gap due to the uneven shape derived from the second functional layer is generated between the surface of the separator for the electrochemical element and the surface of the electrode. It is difficult, the increase in internal resistance is suppressed, and the deterioration of battery characteristics can be reduced.

Therefore, when the surface tension difference between the first coating liquid and the second coating liquid is 20 mN / m or more and less than 40 mN / m, the minimum amount of the second coating liquid required to exhibit the dry adhesive function is used. By using this, it is possible to control the region where the resin having the dry adhesive function contained in the second coating liquid is present within the range that does not impair the ion permeability while exhibiting the dry adhesive function. In addition, in the thermocompression bonding step of joining the separator for an electrochemical element and the electrode, the height difference between the dry adhesive functional layer formed by the second coating film layer and the heat-resistant layer formed by the first coating film layer is determined. It can be made smaller. As a result, it becomes possible to suppress the gap generated between the surface of the separator for an electrochemical element and the surface of the electrode, which is one of the causes of the increase in internal resistance, and to form a separator for an electrochemical element that does not impair the battery characteristics. ..

The viscosity of the second coating liquid is preferably lower than the viscosity of the first coating liquid in order to form the existing region and the non-existing region. The viscosity difference between the first coating liquid and the second coating liquid is preferably 20 mPa · s or more, and more preferably 15 mPa · s or more. The viscosity of the second coating liquid can be controlled by the solid content concentration of the coating liquid, the content ratio of the resin, and the molecular weight of the resin.

The solid content concentration of the second coating liquid is preferably 3 to 15% by mass, more preferably 5 to 10% by mass from the viewpoint of battery characteristics.

As a coating method used when applying the obtained second coating liquid to at least one surface of the first coating film layer, for example, a method such as spray coating, die coating, curtain coating, inkjet, rotor dampening and the like. Can be mentioned. Among these, the spray coat and rotor dampening methods are preferable from the viewpoint of forming a structure in which the first functional layer and the second functional layer coexist.

For the existing region and the non-existent region, for example, the formation location of the existing region and the non-existent region is set in advance so that the decrease in ion permeability can be sufficiently suppressed, and the second existing region corresponds to the location of the set existing region. It can be formed by spraying a coating liquid.

The abundance ratio of the second functional layer and the first functional layer that coexist on the same film surface of the surface layer of the porous layer can be obtained when discriminating the existing region and the non-existing region by the above-mentioned method. can. The abundance area ratio of the second functional layer with respect to the entire film surface of the surface layer of the porous layer is preferably 5% or more and less than 55%, and more preferably 15% or more and less than 40%. Within the above range, the expected heat resistance and dry adhesiveness with the electrode can be obtained, and the expected battery characteristics can be obtained because ions can be uniformly transmitted.

When the spray coating method is used in the second coating step, it is preferable to use a two-fluid spray nozzle from the viewpoint of dealing with the second coating liquid having a wide range of viscosities, and from the viewpoint of uniform coating, the outside is used. It is preferable to use a mixing method. The diameter of the spray nozzle is preferably 3 to 20 mm, and is preferably 8 to 20 mm from the viewpoint of supply stability of the second coating liquid.

The coating amount of the second coating liquid is preferably 5 to 100 mL / m ² , and preferably 5 to 50 mL / m ² from the viewpoint of battery characteristics. When the external mixing method is used, the spray angle is preferably 30 to 70 °, and the air pressure is preferably 0.02 to 0.1 MPa.
As described above, the second coating film layer can be formed in the second coating step.

[Membrane fixing process]
(First film fixing step)
The first functional layer can be obtained by performing a first film fixing step of fixing the first coating film layer to form the first functional layer. Membrane fixing means removing the solvent contained in the first coating liquid by drying to obtain a first functional layer. The first functional layer is preferably a porous layer from the viewpoint of ion permeability, and can be made into a porous layer by the first membrane fixing step.
The first film fixing step is not limited as long as the solvent contained in the coating liquid volatilizes, and at least one selected from warm air drying, infrared drying, suction drying, vacuum drying, and microwave drying. It is preferable to include a step.

Drying can be performed, for example, with hot air of 100 ° C. or lower.

As described above, the first functional layer can be formed in the first film fixing step.
The total film thickness of the first functional layer per layer and the second functional layer per layer is preferably 0.4 to 5.0 μm, more preferably 1.5 to 4. It is 0 μm. When the total film thickness is 1.5 μm or more, the adhesiveness to the electrode is ensured, the porous membrane substrate is prevented from melting and shrinking at the melting point or higher, and the film breaking strength and the insulating property can be ensured. When the total film thickness is 5.0 μm or less, the winding bulk can be suppressed, which is suitable for increasing the capacity of the electrochemical element.

(Second film fixing process)
The second functional layer can be obtained by performing a second film fixing step of fixing the second coating film layer to form the second functional layer. Membrane fixing means removing the solvent contained in the second coating liquid by drying to obtain a second functional layer.

The second film fixing step is not limited as long as the solvent contained in the coating liquid is volatilized, as in the one film fixing step, and is not limited as long as it is hot air drying, infrared drying, suction drying, vacuum drying, and microwave drying. It preferably comprises at least one step selected from drying. The second functional layer is preferably a porous layer from the viewpoint of ion permeability, and can be made into a porous layer by a second film coating and fixing step.

The second film fixing step and the first film fixing step may be performed separately, but the internal resistance between the separator and the electrode created by the height difference between the first functional layer and the second functional layer in the separator of the present invention can be obtained. From the viewpoint of reducing the production cost and reducing the manufacturing cost and the time required for manufacturing, it is preferable to simultaneously perform the first film fixing step and the second film fixing step as a common fixing step.

[Separator for electrochemical device]
As described above, a separator for an electrochemical device having a first functional layer and a second functional layer on at least one surface of a porous membrane substrate can be manufactured.

The electrochemical element includes all elements that carry out an electrochemical reaction, and examples thereof include a primary battery, a secondary battery, a fuel cell, a solar cell, a capacitor such as a supercapacitor element, and the like. Among these, the separator of the present invention is preferably used for a secondary battery, and is more preferably a lithium secondary battery such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery. Used.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

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) Surface tension of coating liquid The surface tensions of the first coating liquid and the second coating liquid prepared in each Example and Comparative Example are automatically measured as a tensiometer using the pendant drop method. Using a contact angle meter (CA-V type manufactured by Kyowa Surface Science Co., Ltd.), measurements were taken in an environmental region where the liquid temperature and the measured room temperature were 25 ° C. and the relative humidity (RH) was 65%. In the pendant drop method, the interfacial tension decreases with the passage of time as soon as the drop is formed. Therefore, the measured value after the holding time of 1 second until the drop was formed and the drop was stabilized was taken as the surface tension of the coating liquid.

(2) Adhesion to electrodes The active material is natural graphite, the binder is vinylidene fluoride resin, the conductive additive is carbon black negative electrode 25 mm x 100 mm, the separator for electrochemical elements is used, and the active material and the second functional layer are. Install so that they are in contact with each other, perform hot pressing at 6.0 MPa, 60 ° C. for 1 second with a hot press machine, and fix the electrodes using Force Gauge ZP5N and MX2-500N (product name) manufactured by Imada Co., Ltd. Then, a peeling test was conducted at a peeling speed of 300 mm / min by a peeling method in which the end portion of the separator was gripped and the peeling accuracy was 180 degrees, and the peeling strength was examined. At this time, the average value of the peel strength in the peel test for a length of 60 mm performed under the above conditions was taken as the peel strength.
The adhesiveness with the electrode was evaluated in the following four stages.

Evaluation criteria S (best): 30N / 25mm or more A (good): 20N / 25mm or more and less than 30N / 25mm B (possible): 10N / 25mm or more and less than 20N / 25mm or more C (impossible): 5N / 25mm or more

(3) Fabrication of cycle characteristic battery The positive electrode sheet contains 92 parts by mass of Li (Ni _5/10 Mn _2/10 Co _3/10 ) O ₂ as the positive electrode active material, and 2.5 parts of acetylene black and graphite as the positive electrode conductive aid. A positive electrode slurry in which 3 parts by mass of polyvinylidene fluoride as a positive electrode binder was dispersed in N-methyl-2-pyrrolidone using a planetary mixer was applied, dried and rolled on an aluminum foil. It was prepared (applied grain: 9.5 mg / cm ² ).

This positive electrode sheet was cut out to a size of 40 mm × 40 mm. At this time, the tab adhesive portion for current collection without the active material layer was cut out so as to have a size of 5 mm × 5 mm on the outside of the active material surface. An aluminum tab having a width of 5 mm and a thickness of 0.1 mm was ultrasonically welded to the tab adhesive portion.

In the negative electrode sheet, 98 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, and 1 part by mass of a styrene-butadiene copolymer as a negative electrode binder are dispersed in water using a planetary mixer. The negative electrode slurry was coated, dried, and rolled on a copper foil (applied grain: 5.5 mg / cm ² ).

This negative electrode sheet was cut out to a size of 45 mm × 45 mm. At this time, the tab adhesive portion for current collection without the active material layer was cut out so as to have a size of 5 mm × 5 mm on the outside of the active material surface. A copper tab of the same size as the positive electrode tab was ultrasonically welded to the tab adhesive portion.

Next, the separator for an electrochemical element is cut out to a size of 55 mm × 55 mm, and the positive electrode and the negative electrode are laminated on both sides of the separator for an electrochemical element so that the active material layer separates the separator for the electrochemical element, and the positive electrode coating portion is entirely coated with the negative electrode. A group of electrodes was obtained by arranging them so as to face each other. The positive electrode / negative electrode / electrochemical element separator is sandwiched between one 90 mm × 200 mm aluminum laminated film, the long sides of the aluminum laminated film are folded, and the two long sides of the aluminum laminated film are heat-sealed to form a bag. did.

An electrolytic solution was prepared by dissolving LiPF ₆ as a solute in a mixed solvent of diethyl carbonate: diethyl carbonate = 1: 1 (volume ratio) so as to have a concentration of 1 mol / liter. 1.5 g of an electrolytic solution was injected into a bag-shaped aluminum laminated film, and the short sides of the aluminum laminated film were heat-sealed while impregnated under reduced pressure to form a laminated battery.

The discharge rate characteristics of the manufactured laminated battery were tested by the following procedure and evaluated by the discharge capacity retention rate.
Constant current charging with a charging condition of 3C and 4.25V and constant current discharging with a discharging condition of 3C and 2.7V were performed 500 times.

<Calculation of discharge capacity maintenance rate>
The discharge capacity retention rate was calculated by (discharge capacity after 500 times) / (discharge capacity of the first time) × 100. Five of the above laminated batteries were produced, and the average value was taken as the discharge capacity retention rate.
◯ (good): Discharge capacity retention rate was 70% or more, Δ (possible): Discharge capacity retention rate was 60% or more and less than 70%, × (impossible): Discharge capacity retention rate was less than 60%.

(4) Existence area ratio of the second functional layer with respect to the entire film surface of the surface layer of the porous layer The surface SEM-EDX image was measured by image processing. Details are given below.

<Measurement of porous layer surface SEM-EDX>
A small piece of 0.5 cm × 0.5 cm was cut out from the separator for an electrochemical element, and the separator surface SEM-EDX was measured in 10 different fields under atmospheric exposure under the conditions shown below.

<SEM-EDX measurement conditions>
-Measuring device SEM: Electron emission scanning electron microscope manufactured by Hitachi High Technology FE-SEM S-4800, EDX: QUANTAX FLAT QUAD System Xflash 5060FQ manufactured by Bruker AXS
・ SEM observation acceleration voltage: 1.5kV
・ EDX observation acceleration voltage: 3.5kV
・ Emission current: 10μA
・ Measurement magnification: 300 times ・ Electron beam incident angle: 0 °
・ X-ray extraction angle: 35 ° ~
・ Dead time: 1%
-Mapping element: Al
-Number of measurement pixels: 600 x 450 pixels-Measurement time: 600 sec.
-Brightness: Adjust the brightness and contrast so that there are no pixels that reach the maximum brightness and the average value of the brightness falls within the range of 40% to 60% brightness.

<Image processing>
The SEM-EDX image obtained above is read by the image analysis software HALCON (Ver. 13.0, manufactured by MVtec), and contour enhancement (differential filter (emphasize, low-pass mask width: 3 / low-pass mask width: 3 / low-pass mask). Height: 3 / Contrast enhancement intensity: 0.3), Edge enhancement filter (shock_filter, Time step length: 0.5 / Number of repetitions: 1 / Edge detection type: canny / Edge detection smoothing: 1)
After performing the processing in the order of), binarization was performed.
The differential filter "emphasize" used for contour enhancement and the edge enhancement filter "shock_filter" are image processing filters included in HALCON.

Regarding binarization, the lower limit of the threshold value for the gray value was set to 180 and the upper limit was set to 255, and the portion of 180 or more was defined as the portion where alumina was present. Furthermore, the gray value in the region where the alumina is present is replaced with 255, the gray value in the other regions is replaced with 0, continuous pixels having the gray value 255 are connected to each other, and the region area where the alumina is present is integrated after extraction. Then, the area ratio of the fluorine-containing resin contained in the second coating film layer as the main component in the SEM-EDX image is calculated by dividing by the total number of pixels of the original image and subtracting from 100.

By performing the above treatment on all 10 SEM-EDX images obtained, 10 abundant area ratios of fluorine-containing resins contained in the second coating film layer as a main component were obtained. The average value of these values was taken as the ratio of the abundance area of the second functional layer to the entire film surface of the measured electrochemical element separator.

(5) Thickness of Porous Layer A sample cross section is cut out from the separator with a microtome, the cross section is observed with a field emission scanning electron microscope, and the highest point in the porous layer is selected in the observation region. Then, the distance from the bottom surface of the porous layer (on the side of the porous film substrate made of polyethylene) to the highest point was measured as the film thickness of the porous layer. The film thickness of the porous layer was defined as the average value obtained by observing, selecting, and measuring each of five arbitrary locations from a sample having a size of 100 mm × 100 mm.
The measured film thickness of the porous layer is the total film thickness of the first functional layer per layer and the second functional layer per layer.

(6) Air permeability resistance Select one location randomly selected from each of the three porous composite film samples with a size of 100 mm x 100 mm, and use the Oken type air permeability measuring device (EG01-5, manufactured by Asahi Seiko Co., Ltd.). It was measured according to JIS P 8117 (2009) using 1MR), and the average value was taken as the air permeability (seconds / 100 cm ³ ) of the separator for an electrochemical element.

(7) Primary average particle size of inorganic particles The average particle size of inorganic particles is a volume-based integration rate using a laser diffraction type particle size distribution measuring device (LA-960V2, manufactured by Horiba Seisakusho Co., Ltd.) according to JISZ8825 (2013). The particle size D50 (μm) was measured when the particle size was 50%, and the obtained value was taken as the primary average particle size of the inorganic particles.

(Example 1)
The coating liquid of the first functional layer is 2.6 g of VDF-HFP copolymer (molar ratio of HFP monomer component 7%, weight average molecular weight 250,000), VDF-CTFE copolymer (CTFE monomer component). (20% by molar ratio, weight average molecular weight of 250,000) 0.8 was added to 54.0 g of pure water, and the mixture was stirred with a disper and dissolved. To this, 41.8 g of alumina (average particle size 0.5 μm) and 0.8 g of a dispersant were added, and the inorganic powder was crushed and dispersed by a ball mill method to produce a coating liquid (slurry).

The coating liquid for the second functional layer is a mixture of pure water and acetone at a ratio of 98% and 2%, respectively, so that the acrylic resin (Acrylic emulsion glass transition temperature 50 ° C. manufactured by Daicel FineChem) is 10% by mass. Then, it was dispersed and produced with a stirrer.

The coating liquid of the first functional layer was coated on one side of a polyethylene porous membrane substrate (thickness 5 μm, air permeability 120 seconds / 100 cc) with a gravure coat. The coating liquid of the second functional layer is sprayed on the surface of the coating liquid of the first functional layer containing pure water using an external mixing type two-fluid spray nozzle (nozzle diameter: 1 mm). It was applied to one side with a spray coat so that the angle was 60 ° and the air pressure was 0.04 MPa. Then, the pure water and the acetone contained in the coating liquid of the second functional layer were dried until volatilized to form the first functional layer and the second functional layer, and a separator for an electrochemical element was obtained. .. Table 1 shows the measurement results of the characteristics of the obtained separator for an electrochemical device.

(Comparative Example 1)
Except for the fact that the mixing ratios of pure water and acetone used for preparing the coating liquid for the second functional layer were 95% and 5%, respectively, and the coating liquid for the second functional layer was mixed and mixed, the same as in Example 1. Similarly, the coating liquid of the first functional layer and the coating liquid of the second functional layer are applied to one side of the polyethylene porous film substrate by using a gravure coat and a two-fluid spray nozzle, and pure water is applied. And by drying until the acetone volatilized, a first functional layer and a second functional layer were formed, and a separator for an electrochemical element was obtained. Table 1 shows the measurement results of the characteristics of the obtained separator for an electrochemical device.

(Comparative Example 2)
The coating liquid of the first functional layer and the coating liquid of the second functional layer obtained in Example 1 were subjected to a two-layer die coating to a polyethylene porous membrane substrate (thickness 5 μm, air permeability 120 seconds / 100 cc). ), Then dried until the pure water volatilized to form a first functional layer and a second functional layer, and a separator for an electrochemical element was obtained. Table 1 shows the measurement results of the characteristics of the obtained separator for an electrochemical device.

From Table 1, it was found that in Example 1, a separator for an electrochemical element suitable for a separator for a secondary battery having good adhesiveness to an electrode and a good cycle characteristic which is a guideline for battery life can be obtained.

Claims (7)

A method for producing a separator for an electrochemical device having a first functional layer and a second functional layer on at least one surface of a polyolefin porous membrane substrate.
A first coating step of coating the first functional layer and a second coating step of forming the second functional layer on the surface of the first functional layer before fixing the first coating film layer. Including,
A second coating film layer is formed by coating so as to form a region in which the second coating film layer is present and a region in which the second coating film layer is not present.
It is characterized by including a step of fixing the first and second coating film layers at the same time.
A method for manufacturing a separator for an electrochemical element. The method for producing a separator for a vapor chemical element according to claim 1, wherein the first coating liquid and the second coating liquid are compatible with each other. The manufacturing method for an electrochemical element according to claim 1 and 2, wherein the difference in surface tension between the first coating liquid and the second coating liquid is 20 mN / m or more and less than 40 mN / m. The method for manufacturing a separator for an electrochemical element according to any one of claims 1 to 4, wherein a spray coating method, a rotor dampening method, a die coating method, a curtain coating method or an inkjet method is used in the second coating step. The method for manufacturing a separator for an electrochemical element according to any one of claims 1 to 5, wherein a gravure coat method, a chamber doctor method, and a DM coater method are used in the first coating step. The method for producing a separator for an electrochemical device according to any one of claims 1 to 6, wherein the first coating liquid contains inorganic particles. The method for producing a separator for an electrochemical element according to any one of claims 1 to 7, wherein the second coating liquid contains at least one resin selected from the group consisting of a fluororesin, an acrylic resin and an olefin resin.