JP5213158B2 - Multilayer porous membrane production method, lithium ion battery separator and lithium ion battery - Google Patents

Description

  The present invention relates to a method for producing a multilayer porous membrane suitable for a separator for an electrochemical device, a separator for an electrochemical device comprising the multilayer porous membrane, and an electrochemical device having the multilayer porous membrane as a separator. .

  Resin porous membranes based on thermoplastic resins are commonly used as separators to separate positive and negative electrodes in electrochemical devices such as lithium ion batteries, polymer lithium batteries, and electric double layer capacitors. Yes. In particular, a polyolefin-based separator is stable even in harsh redox atmospheres such as lithium batteries, and can provide a so-called shutdown characteristic that closes the pores of the porous membrane near the melting point of the resin. Therefore, it is widely used.

  However, on the other hand, the porous resin membrane has a capability of maintaining the membrane at a temperature equal to or higher than the melting point of the resin, so that so-called film breakage is likely to occur. There is a risk of occurrence.

  In order to improve the heat resistance stability of the resin porous membrane as described above, attempts have been made to form a layer on the surface of the resin porous membrane with a material having high heat resistance such as an inorganic oxide.

  For example, Patent Document 1 proposes a method for forming an inorganic thin film on the surface of a resin porous film by a sol-gel method. Patent Document 2 discloses a method for forming a surface protective layer having inorganic fine particles by applying a solvent in which inorganic fine particles are dispersed to a resin porous membrane. Furthermore, in Patent Document 3, when a heat-resistant resin solution is applied to a porous film to produce a separator comprising a heat-resistant resin layer and a porous film, an organic solvent is applied to the porous film before or after application of the heat-resistant resin solution. The method of impregnating is shown. Patent Document 4 discloses a step of mixing fine particles and a solvent binder to form a slurry, and applying the slurry to a first porous layer containing a thermoplastic resin as a main component from the first porous layer. A method for producing a separator by forming a second porous layer having high heat resistance is shown. In addition, Patent Document 5 discloses a method of forming an inorganic thin film on at least one surface of a porous film made of a heat-meltable polyolefin using a vacuum film forming method.

JP-A-10-172531 Japanese Patent Laid-Open No. 11-80395 Japanese Patent Laid-Open No. 2001-23602 JP 2006-32359 A JP 2001-35468 A

  As in Patent Documents 2 to 4, when forming a heat-resistant porous layer by applying a coating such as slurry on the surface of the resin porous membrane, a gravure coater, knife coater, reverse roll coater, die coater, etc. In order to apply the paint uniformly using the coating device, the wettability between the paint and the porous resin membrane as the base material is important. If the wettability is poor, apply the paint uniformly. It becomes difficult.

  Therefore, in order to improve the wettability between the resin porous film and the paint for forming the heat-resistant porous layer, for example, an organic solvent such as ketones (such as methyl ethyl ketone), furans (such as tetrahydrofuran), or alcohols is used. It is conceivable to use the paint that contains it, but in that case, the wettability is certainly improved, but because the base material (resin porous film) is porous, the paint and the solvent in the paint are not the coated surface. A so-called back-through phenomenon that penetrates to the opposite surface occurs, and a solvent or paint adheres to a roller used for a coater guide or the like, so that the paint cannot be applied satisfactorily. Therefore, it is difficult to produce a highly uniform multilayer porous film having a heat-resistant porous layer with good properties on the surface of the resin porous film with high productivity.

  In addition, when an inorganic film is formed on the surface of the resin porous film using the sol-gel method, it is necessary to heat, but since the resin porous film serving as a base material is composed of a thermoplastic resin, The resin porous membrane may be damaged by heating.

  Furthermore, when an inorganic film is formed on the surface of a porous resin film using a vacuum deposition method, there is a problem that the production rate of the inorganic film is slow and mass production is difficult. Therefore, it is difficult to increase the thickness of the inorganic film while suppressing damage to the resin porous film.

  The present invention has been made in view of the above circumstances, and its object is to provide a production method capable of producing a highly uniform multilayer porous membrane with high productivity, a separator for an electrochemical element comprising the multilayer porous membrane, and the above An object of the present invention is to provide an electrochemical element having a multilayer porous membrane as a separator.

The method for producing a multilayer porous membrane of the present invention that has achieved the above object includes a resin porous membrane mainly composed of a thermoplastic resin, and a heat resistant porous layer mainly composed of heat resistant fine particles and containing an organic binder. A surface-treating step of the resin porous membrane, and applying a heat-resistant porous layer forming composition to the surface of the resin porous membrane after the surface treatment to the heat and the porous layer and a step of forming at least a thickness of 1 [mu] m, the heat-resistant porous layer forming composition the surface tension (a) and before surface treatment of the resin porous membrane The relationship between the critical surface tension (B) at room temperature is (A) − (B) ≧ 10 mN / m, and the surface tension (A) and the critical surface tension at room temperature of the resin porous membrane after surface treatment ( B ′) is (A) − (B ′) ≦ 0 mN / m And

  Moreover, the separator for electrochemical devices of the present invention is composed of a multilayer porous membrane produced by the production method of the present invention.

  Furthermore, the electrochemical device of the present invention is characterized by having a multilayer porous membrane manufactured by the manufacturing method of the present invention as a separator.

  According to the present invention, a highly uniform multilayer porous membrane can be produced with high productivity. In addition, the electrochemical element separator of the present invention can constitute an electrochemical element excellent in safety during abnormal heat generation. Furthermore, the electrochemical device of the present invention is excellent in safety during abnormal heat generation.

  The multilayer porous film according to the present invention has a resin porous film mainly composed of a thermoplastic resin and a heat resistant porous layer mainly composed of heat resistant fine particles, and is suitable as a separator for an electrochemical element. It is.

  When the multilayer porous membrane according to the present invention is used as a separator for an electrochemical device, the resin porous membrane has the original function of the separator, that is, the function of preventing a short circuit mainly due to direct contact between the positive electrode and the negative electrode. It is what you have. In addition, when the electrochemical element such as a battery is exposed to a high temperature, the thermoplastic resin constituting the resin porous film imparts a so-called shutdown characteristic that softens the thermoplastic resin and closes the pores of the porous film. You can also

  However, since the resin porous membrane is mainly composed of a thermoplastic resin, it can be thermally shrunk at a high temperature in the same manner as a conventionally known polyolefin porous film separator.

  Therefore, the multilayer porous membrane according to the present invention is formed by forming a heat-resistant porous layer mainly composed of heat-resistant fine particles on the surface of the resin porous membrane so as to suppress the thermal shrinkage of the entire multilayer porous membrane. Yes. In the heat-resistant porous layer, since the amount of heat-resistant fine particles is large and they are densely present, even if the resin porous film shrinks at high temperatures, the heat-resistant fine particles in the heat-resistant porous layer collide with each other. This is considered to suppress the shrinkage of the entire multilayer porous membrane.

  Thus, in the multilayer porous membrane according to the present invention, the heat-resistant porous layer functions as a skeleton that retains the shape of the multilayer porous membrane, thereby suppressing thermal shrinkage of the multilayer porous membrane. Therefore, the electrochemical device using the multilayer porous membrane according to the present invention as a separator can ensure safety when abnormal heat generation occurs inside.

  In addition, when the multilayer porous film is provided with a shutdown characteristic by the thermoplastic resin constituting the resin porous film, the safety of the electrochemical element using such a multilayer porous film as a separator during abnormal heat generation is further enhanced. be able to.

  Considering the case of using a multilayer porous membrane as a separator for an electrochemical device, the resin porous membrane related to the multilayer porous membrane may be composed of a material usually used for the applied electrochemical device. Although there is no particular limitation, for example, when applied to an electrochemical element having a high potential and using an organic electrolyte, such as a lithium ion battery or a lithium polymer battery, from the viewpoint of stability in the element, It is preferable to use a porous film mainly composed of polyolefin (polyethylene, polypropylene, ethylene-propylene copolymer, etc.).

  In addition, when providing shutdown characteristics to a multilayer porous membrane, the temperature at which shutdown occurs due to the softening of the thermoplastic resin constituting the resin porous membrane is higher than the expected operating temperature range of the electrochemical element, The temperature is required to be lower than a lower limit temperature at which a risk of element ignition or the like is predicted, for example, a thermal runaway temperature in a lithium ion battery. Specifically, when a multilayer porous membrane is used as a separator for a lithium ion battery, the shutdown temperature (temperature at which shutdown occurs) of the multilayer porous membrane is preferably 100 to 140 ° C.

  Therefore, the resin porous membrane is made of a thermoplastic resin having a melting point, that is, a melting temperature measured using a differential scanning calorimeter (DSC) of 100 to 140 ° C., in accordance with JIS K 7121. More preferably, it is a single layer porous film mainly composed of polyethylene or a porous film composed mainly of polyethylene, such as a laminated porous film obtained by laminating 2 to 5 layers of polyethylene and polypropylene. More preferably, it is a laminated porous film as an element.

  As the resin porous membrane, for example, a porous membrane composed of the above-mentioned exemplified thermoplastic resin used in a conventionally known electrochemical element (such as a lithium ion battery), that is, a solvent extraction method, a dry type or a wet type An ion-permeable porous membrane prepared by a stretching (uniaxial or biaxial stretching) method or the like can be used.

  In the resin porous membrane, “having a thermoplastic resin as a main component” or “having a polyolefin as a main component” means a component (thermoplastic resin) as a main component among the components constituting the resin porous membrane. Or polyolefin) is 80% by mass or more.

  The pore diameter of the resin porous membrane is preferably 0.001 μm or more, and more preferably 0.01 μm or more, from the viewpoint of enabling good movement of ions within the electrochemical element. However, if the pore diameter of the resin porous membrane is too large, the effect of the treatment may reach the inside of the membrane during the surface treatment of the resin porous membrane, and the ion permeability is good, but the pore diameter relative to the thickness. The ratio of the pore diameter to the particle diameter of the active material used for the electrode of the electrochemical device becomes too large, so that the effect of isolating the positive electrode and the negative electrode and preventing a short circuit is reduced. There is a fear. Therefore, the pore diameter of the resin porous membrane is preferably 10 μm or less, and more preferably 5 μm or less.

  The pores of the resin porous membrane need to be “communication holes” that are connected from one surface of the resin porous membrane to the other surface. It is preferable that the hole is bent in the resin porous membrane, rather than the so-called “straight hole” that is linearly connected from one surface to the other surface. Since the pores of the resin porous membrane have flexibility, for example, in a lithium ion battery, it is possible to reduce the potential of internal short circuit due to the formation of lithium dendrite.

  The heat resistant porous layer in the multilayer porous membrane according to the present invention ensures heat resistance by containing heat resistant fine particles. As used herein, “heat resistance” means that a shape change such as deformation is not visually confirmed at least at 150 ° C. The heat resistance of the heat-resistant fine particles is preferably 200 ° C. or higher, more preferably 300 ° C. or higher, and further preferably 500 ° C. or higher.

The heat-resistant fine particles are preferably inorganic fine particles having electrical insulation properties, and specifically, inorganic oxides such as iron oxide, silica (SiO ₂ ), alumina (Al ₂ O ₃ ), TiO ₂ , and BaTiO _2. Inorganic fine particles such as aluminum nitride and silicon nitride; poorly soluble ionic crystal fine particles such as calcium fluoride, barium fluoride and barium sulfate; covalently bonded crystal fine particles such as silicon and diamond; clay fine particles such as montmorillonite And so on. Here, the inorganic oxide fine particles may be fine particles such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or a mineral resource-derived material or an artificial product thereof. In addition, the inorganic compound constituting these inorganic fine particles may be element-substituted or solid solution, if necessary, and the inorganic fine particles may be surface-treated. In addition, the inorganic fine particles electrically insulate the surface of a conductive material exemplified by metals, SnO ₂ , conductive oxides such as tin-indium oxide (ITO), carbonaceous materials such as carbon black and graphite. It is also possible to use particles that are made electrically insulating by coating with a material having the above (for example, the above-mentioned inorganic oxide).

  Organic fine particles can also be used as the heat-resistant fine particles. Specific examples of organic fine particles include polyimides, melamine resins, phenol resins, crosslinked polymethyl methacrylate (crosslinked PMMA), crosslinked polystyrene (crosslinked PS), polydivinylbenzene (PDVB), and high crosslinking amounts such as benzoguanamine-formaldehyde condensates. Molecular fine particles; heat-resistant polymer fine particles such as thermoplastic polyimide; The organic resin (polymer) constituting these organic fine particles is a mixture, modified body, derivative, copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) of the materials exemplified above. ) Or a crosslinked product (in the case of the heat-resistant polymer).

  As the heat-resistant fine particles, those exemplified above may be used alone, or two or more kinds may be used in combination.

  The average particle diameter of the heat-resistant fine particles is preferably 0.001 μm or more, more preferably 0.1 μm or more, preferably 15 μm or less, more preferably 1 μm or less. The average particle diameter of the heat-resistant fine particles is, for example, a number average particle diameter measured by dispersing the heat-resistant fine particles in a medium that does not dissolve using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA). Can be defined as

  The heat-resistant porous layer contains heat-resistant fine particles as a main component. The term “containing as a main component” as used herein means that the heat-resistant fine particles contain 70% by volume or more in the total volume of components of the heat-resistant porous layer. Means. The amount of the heat-resistant fine particles in the heat-resistant porous layer is preferably 80% by volume or more and more preferably 90% by volume or more in the total volume of the constituent components of the heat-resistant porous layer. By making the heat-resistant fine particles in the heat-resistant porous layer have a high content as described above, the heat shrinkage of the entire multilayer porous film can be satisfactorily suppressed. Further, the heat resistant porous layer preferably contains an organic binder in order to bind the heat resistant fine particles to each other or to bind the heat resistant porous layer and the resin porous film. The preferred upper limit of the amount of heat-resistant fine particles in the heat-resistant porous layer is, for example, 99% by volume in the total volume of the constituent components of the heat-resistant porous layer. If the amount of the heat-resistant fine particles in the heat-resistant porous layer is less than 70% by volume, for example, the amount of the organic binder in the heat-resistant porous layer needs to be increased. May be buried with an organic binder, for example, there is a risk of losing the function as a separator, and when porous using a pore-opening agent or the like, the interval between the heat-resistant fine particles becomes too large, There exists a possibility that the effect which suppresses heat shrink may fall.

  The organic binder used for the heat-resistant porous layer is a separator for electrochemical devices that can adhere well to heat-resistant fine particles or heat-resistant porous layer and resin porous membrane, is electrochemically stable, and is a multilayer porous membrane. In the case of using for the above, there is no particular limitation as long as it is stable to the organic electrolyte. Specifically, ethylene-vinyl acetate copolymer (EVA, vinyl acetate-derived structural unit is 20 to 35 mol%), ethylene-acrylic acid copolymer such as ethylene-ethyl acrylate copolymer, fluororesin [Polyvinylidene fluoride (PVDF), etc.], fluorinated rubber, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP) ), Poly N-vinylacetamide, crosslinked acrylic resin, polyurethane, epoxy resin and the like. These organic binders may be used alone or in combination of two or more.

  Among the organic binders exemplified above, a heat-resistant resin having a heat resistance of 150 ° C. or higher is preferable, and a highly flexible material such as an ethylene-acrylic acid copolymer, fluorine-based rubber, or SBR is particularly preferable. Specific examples include “Evaflex series (EVA, trade name)” manufactured by Mitsui DuPont Polychemical Co., Ltd., EVA manufactured by Nihon Unicar Co., Ltd., and “Evaflex-EEA Series (ethylene-acrylic) manufactured by Mitsui DuPont Polychemical Co., Ltd. Acid copolymer, trade name) ", EEA made by Nihon Unicar," Daiel Latex Series (fluoro rubber, trade name) "by Daikin Industries," TRD-2001 (SBR, trade name) by JSR "EM-400B (SBR, trade name)" manufactured by Zeon Corporation. A cross-linked acrylic resin (self-crosslinking acrylic resin) having a low glass transition temperature and having a structure in which butyl acrylate is a main component and is cross-linked is also preferable.

  In addition, when using these organic binders, they may be used in the form of an emulsion dissolved or dispersed in a medium (solvent) of a composition (slurry or the like) for forming a heat-resistant porous layer described later.

  The multilayer porous membrane is formed by applying a surface treatment to the resin porous membrane, and applying a heat-resistant porous layer forming composition such as a slurry to the surface of the resin porous membrane subjected to the surface treatment. Is formed by the manufacturing method of the present invention. In such a production method, the relationship between the surface tension (A) of the heat-resistant porous layer forming composition and the critical surface tension (B) at room temperature of the resin porous membrane before surface treatment is expressed as (A)-(B) ≧ The relationship between the surface tension (A) and the critical surface tension (B ′) at room temperature of the resin porous membrane after the surface treatment is (A) − (B ′) ≦ 0 mN / m.

  In the method of the present invention, the difference “(A) − (B)” between the surface tension (A) of the heat-resistant porous layer forming composition and the critical surface tension (B) at room temperature of the resin porous membrane before the surface treatment. Is set to 10 mN / m or more, the wettability between the heat-resistant porous layer forming composition and the resin porous membrane is suppressed to some extent, and the medium (solvent) used in the composition or the composition is reduced. In other words, so-called “back-through” that passes through the pores of the porous resin membrane and escapes to the surface opposite to the coated surface is prevented. Therefore, in the method of the present invention, contamination of a roll such as a backup roll of a coating apparatus for applying the heat-resistant porous layer forming composition due to the composition or a medium in the composition can be prevented, and the application of the composition The handling at the time is improved to increase the productivity of the multilayer porous membrane, and the composition can be applied to a desired coating thickness.

  However, in the state where the difference between the surface tension of the heat resistant porous layer forming composition and the critical surface tension of the resin porous membrane is large, even if the heat resistant porous layer forming composition is applied to the resin porous membrane surface, The wettability is low and uniform coating is difficult. Therefore, in the production method of the present invention, the surface of the resin porous membrane is treated before application of the heat-resistant porous layer forming composition, and the surface tension (A) and the criticality of the resin porous membrane after the surface treatment at room temperature are critical. The difference “(A) − (B ′)” from the surface tension (B ′) is as small as 0 mN / m or less, so that the heat-resistant porous layer forming composition and the resin porous membrane surface are wetted. It is possible to form a heat-resistant porous layer with good characteristics by improving the properties and allowing the composition to be applied more uniformly.

  In the method of the present invention, the treatment of the resin porous membrane before the application of the heat-resistant porous layer forming composition is performed only on the surface portion, and within the resin porous membrane, the surface tension difference “(A) − (B) "is maintained, so that the wettability with the heat-resistant porous layer forming composition is enhanced only on the surface portion of the resin porous membrane, while the composition and The medium in the composition can be prevented from falling through.

  In the method of the present invention, a highly uniform multilayer porous membrane can be produced with good productivity by the above mechanism.

  The surface tension difference “(A) − (B)” is more preferably 20 mN / m or more. The surface tension difference “(A) − (B)” is preferably 40 mN / m or less.

  On the other hand, the surface tension difference “(A) − (B ′)” is preferably as small as possible, and the relationship of (A) ≦ (B ′), that is, the surface tension of the heat-resistant porous layer forming composition. (A) is preferably equal to or smaller than the critical surface tension (B ′) at room temperature of the resin porous membrane after the surface treatment. The surface tension difference “(A) − (B ′)” is more preferably −10 mN / m or more.

  Conventionally known measurement methods can be applied to the measurement of the surface tension of the heat-resistant porous layer forming composition and the critical surface tension of the resin porous membrane. That is, the surface tension of the heat-resistant porous layer forming composition can be measured by a conventionally known method such as a plate method, a pendant drop method, or a maximum bubble pressure method. For the resin porous membrane, the contact angle (θ) between the resin porous membrane and the liquid is measured using several types of liquids having a known surface tension, cos θ is plotted against the surface tension, and cos θ = 1. From this point, the critical surface tension can be determined.

  The heat-resistant porous layer forming composition contains heat-resistant fine particles and, if necessary, an organic binder, etc., and these are dispersed in a medium such as water or an organic solvent (the organic binder may be dissolved in the medium). Slurry.

  The organic solvent used as a medium for the heat-resistant porous layer forming composition is one that does not damage the resin porous membrane by dissolving or swelling the resin porous membrane, When used, there is no particular limitation as long as the organic binder can be dissolved uniformly, but furans such as tetrahydrofuran (THF); ketones such as methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) And the like are preferred. Note that high boiling point organic solvents may damage the porous resin membrane by heat melting when the organic solvent is removed by drying after applying the heat-resistant porous layer forming composition to the porous resin membrane. Since there is a possibility of giving, it is not preferable. In addition, in order to control the surface tension, these organic solvents include polyhydric alcohols (ethylene glycol, triethylene glycol, etc.) and surfactants (linear alkylbenzene sulfonate, polyoxyethylene alkyl ether, polyoxyethyl alkylphenyl ether). Etc.) may be added as appropriate.

  In addition, water can be used as the medium for the heat-resistant porous layer forming composition, and also in this case, alcohol (such as ethanol or alcohol having 6 or less carbon atoms such as isopropanol) or a surfactant (for example, the above-mentioned The surface tension can be controlled by adding those exemplified as those that can be used in the heat-resistant porous layer-forming composition using the organic solvent as a medium.

  The surface tension (A) of the heat resistant porous layer forming composition is usually about 20 to 72.7 mN / m.

  In addition, the critical surface tension (B) at room temperature of the resin porous membrane is usually 18.5 mN / m or more, and preferably 50 mN / m or less. In the porous resin membrane mainly composed of polyolefin, the critical surface tension (B) at room temperature is 31 mN / m for polyethylene and 29 mN / m for polypropylene. Therefore, in this case, in order to satisfy the surface tension difference “(A) − (B)”, the medium of the heat-resistant porous layer forming composition contains water as a main component (for example, 90% by mass or more). It is preferable to use a material whose surface tension is adjusted by adding an alcohol or a surfactant as necessary.

  The said heat resistant porous layer forming composition is apply | coated to the surface of the resin porous membrane after surface treatment. Examples of the surface treatment of the resin porous membrane include those in which the effect of the treatment is limited only to the surface, such as ultraviolet treatment, corona discharge treatment, and plasma discharge treatment. When a treatment method that has the effect of treating the inside of the resin porous membrane is used, a “back-through” occurs in which the heat-resistant porous layer forming composition penetrates into the membrane and escapes to the back surface. This is not preferable because there is a possibility that it may be easily performed.

The condition of the surface treatment of the resin porous membrane varies depending on the device used and the critical surface tension of the resin porous membrane (critical surface tension before the surface treatment), etc., for example, a corona treatment device manufactured by Kasuga Electric Co., Ltd. Accordingly, when carried out using a metal electrode, the discharge amount is preferably set to 25~100W / min · ^{m 2.}

  As a method of applying the heat-resistant porous layer forming composition to the resin porous membrane after the surface treatment, for example, a method using a conventionally known coating apparatus such as a gravure coater, knife coater, reverse roll coater, die coater, etc. Can be mentioned.

  In FIG. 1, the schematic of an example of the coating apparatus applicable to the manufacturing method of this invention is shown. The coating apparatus shown in FIG. 1 includes a corona discharge device 2 as a device for performing a surface treatment on the resin porous membrane 1 before applying the heat-resistant porous layer forming composition.

  When producing a multilayer porous membrane using the coating apparatus shown in FIG. 1, first, the resin porous membrane 1 wound up in a roll shape is drawn out, and a corona discharge device 2 is subjected to a corona discharge treatment on the surface thereof. Subsequently, a heat resistant porous layer forming composition is applied to the surface of the treated resin porous membrane 1 by the die head 3. At this time, by applying the method of the present invention, the surface of the back roll 4 of the die head 3 or the resin porous membrane 1 after coating is formed by “back through” of the heat-resistant porous layer forming composition or its medium. The surface of the turn roll 6 to be conveyed can be prevented from being contaminated, and the heat-resistant porous layer forming composition can be applied uniformly. Thereafter, the coating film on the surface of the resin porous membrane 1 is dried in the drying zone 7 to obtain a multilayer porous membrane having the resin porous membrane and the heat-resistant porous layer.

  Although FIG. 1 shows an example of manufacturing a multilayer porous film in which a heat-resistant porous layer is formed only on one surface of the resin porous film 1, the multilayer porous film is thus made of a resin porous film. The structure which has only on one side of a porous film may be sufficient, and the structure which has a heat resistant porous layer on both surfaces of a resin porous film may be sufficient. In addition, the multilayer porous membrane may be configured to have not only a heat resistant porous layer but also a plurality of resin porous membranes. However, when using a multilayer porous membrane for a separator for an electrochemical element, increasing the number of layers may increase the thickness of the separator, leading to an increase in internal resistance and a decrease in energy density. Too much is not preferable, and the total number of layers (heat resistant porous layer and resin porous membrane) constituting the multilayer porous layer is preferably 5 layers or less, more preferably 2 layers.

  Further, in FIG. 1, a step of applying a surface treatment to the resin porous membrane, and a composition for forming a heat resistant porous layer is applied to the surface of the resin porous membrane after the surface treatment to form the heat resistant porous layer. However, these steps may be performed independently of each other.

  The thickness of the multilayer porous membrane thus produced is preferably 6 μm or more from the viewpoint of more reliably separating the positive electrode and the negative electrode when used for a separator for an electrochemical device. It is more preferable. On the other hand, if the multilayer porous membrane is too thick, the energy density when it is used as an electrochemical device may be lowered. Therefore, the thickness is preferably 50 μm or less, more preferably 30 μm or less. .

  Further, when the thickness of the resin porous membrane constituting the multilayer porous membrane is A (μm) and the thickness of the heat resistant porous layer is B (μm), the ratio A / B of A and B is 5 or less. Preferably, it is preferably 4 or less, more preferably 1 or more, and even more preferably 2 or more. In the multilayer porous membrane according to the present invention, as described above, even if the thickness ratio of the resin porous membrane is increased and the heat-resistant porous layer is thinned, it is possible to suppress the thermal shrinkage of the entire multilayer porous membrane. For example, when used for a separator for an electrochemical element, occurrence of a short circuit due to thermal contraction of the separator can be highly suppressed. In the multilayer porous membrane, when there are a plurality of resin porous membranes, the thickness A is the total thickness, and when there are a plurality of heat resistant porous layers, the thickness B is the total thickness.

  In terms of specific values, the thickness of the resin porous membrane (when there are a plurality of resin porous membranes, the total thickness) is preferably 5 μm or more, and 30 μm or less. Is preferred. The thickness of the heat resistant porous membrane (when there are a plurality of heat resistant porous layers, the total thickness) is preferably 1 μm or more, more preferably 2 μm or more, and 4 μm or more. More preferably, it is preferably 20 μm or less, more preferably 10 μm or less, and further preferably 6 μm or less. If the resin porous membrane is too thin, particularly when providing shutdown characteristics, such properties may be weakened, and if it is too thick, there is a risk of causing a decrease in the energy density of the electrochemical element, There is a possibility that the force of heat shrinkage becomes large and the effect of suppressing the heat shrinkage of the entire multilayer porous membrane becomes small. Further, if the heat-resistant porous layer is too thin, the effect of suppressing the thermal shrinkage of the entire multilayer porous membrane may be reduced, and if it is too thick, the thickness of the entire multilayer porous membrane is increased.

As for the porosity of the entire multilayer porous membrane, in view of application to the separator for electrochemical devices, it is 30 in a dry state from the viewpoint of ensuring the amount of electrolyte retained and improving ion permeability. % Or more is preferable. On the other hand, from the viewpoint of securing separator strength and preventing internal short circuit, the porosity of the multilayer porous membrane is preferably 70% or less in a dry state. The porosity of the multilayer porous membrane: P (%) is the sum of each component i using the following formula (1) from the thickness of the multilayer porous membrane, the mass per area, and the density of the constituent components. It can be calculated by finding it.
P = 100− (Σa _i / ρ _i ) × (m / t) (1)
Here, in the above formula, a _i : ratio of component i expressed by mass%, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of multilayer porous membrane (g / cm ² ), t: the thickness (cm) of the multilayer porous membrane.

Further, in the formula (1), m is the mass per unit area (g / cm ² ) of the resin porous membrane, and t is the thickness (cm) of the resin porous membrane. Can also be used to determine the porosity of the resin porous membrane: P (%). It is preferable that the porosity of the resin porous membrane calculated | required by this method is 30 to 70%.

Furthermore, in the above formula (1), m is the mass per unit area (g / cm ² ) of the heat resistant porous layer, and t is the thickness (cm) of the heat resistant porous layer. Can also be used to determine the porosity of the heat-resistant porous layer: P (%). It is preferable that the porosity of the heat resistant porous layer calculated | required by this method is 20 to 60%.

The multilayer porous membrane of the present invention is produced by a method according to JIS P 8117, and a Gurley value indicated by the number of seconds that 100 ml of air passes through the membrane under a pressure of 0.879 g / mm ² is 30 It is desirable that it is ˜300 sec. If the air permeability is too high, the ion permeability is reduced. On the other hand, if the air permeability is too low, the strength of the multilayer porous membrane may be reduced. Further, the strength of the multilayer porous membrane is desirably 50 g or more in terms of piercing strength using a needle having a diameter of 1 mm. If the piercing strength is too small, a short circuit may occur due to the breakthrough of the separator (multilayer porous film) when lithium dendrite crystals are generated. By adopting the above configuration, a multilayer porous film having the above air permeability and piercing strength can be obtained.

  The electrochemical device of the present invention is not particularly limited. In addition to lithium ion batteries (primary batteries and secondary batteries) using organic electrolytes, supercapacitors and the like that require safety at high temperatures. If there is, it can be preferably applied. That is, if the multilayer porous membrane according to the present invention is used as a separator (that is, if the separator for an electrochemical element of the present invention is used), the electrochemical device of the present invention has other configurations and structures. There is no particular limitation, and various configurations and structures provided in various electrochemical elements (lithium ion secondary batteries, lithium ion primary batteries, supercapacitors, etc.) having conventionally known organic electrolytes can be employed.

  Hereinafter, application to a lithium ion secondary battery will be described in detail as an example. Examples of the form of the lithium ion secondary battery include a cylindrical shape (such as a rectangular tube shape or a cylindrical shape) using a steel can or an aluminum can as an outer can. Moreover, it can also be set as the soft package battery which used the laminated film which vapor-deposited the metal as an exterior body.

The positive electrode is not particularly limited as long as it is a positive electrode used in a conventionally known lithium ion secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing Li ions. For example, as an active material, a lithium-containing transition metal oxide having a layered structure represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, etc.), LiMn It is possible to use spinel lithium manganese oxide in which ₂ O ₄ or a part of the element is substituted with another element, or an olivine type compound represented by LiMPO ₄ (M: Co, Ni, Mn, Fe, etc.) It is. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO ₂ and LiNi _1-x Co _xy Al _y O ₂ (0.1 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0. 2) and other oxides containing at least Co, Ni and Mn (LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , LiMn _5/12 Ni _5/12 Co _1/6 O ₂ , LiMn _{3 / 5} Ni _1/5 Co _1/5 O ₂ etc.).

  A carbon material such as carbon black is used as the conductive auxiliary agent, and a fluororesin such as polyvinylidene fluoride (PVDF) is used as the binder, and the positive electrode active material is mixed with a positive electrode mixture in which these materials and an active material are mixed. The substance-containing layer is formed on the current collector, for example.

  Further, as the positive electrode current collector, a metal foil such as aluminum, a punching metal, a net, an expanded metal, or the like can be used, but usually an aluminum foil having a thickness of 10 to 30 μm is preferably used.

  The lead portion on the positive electrode side is usually provided by forming an exposed portion of the current collector without forming the positive electrode active material-containing layer on a part of the current collector and forming the lead portion at the time of producing the positive electrode. However, the lead portion is not necessarily integrated with the current collector from the beginning, and may be provided by connecting an aluminum foil or the like to the current collector later.

  The negative electrode is not particularly limited as long as it is a negative electrode used in a conventionally known lithium ion secondary battery, that is, a negative electrode containing an active material capable of occluding and releasing Li ions. For example, carbon that can occlude and release lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers as active materials One type or a mixture of two or more types of system materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb, In and their alloys, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material. A negative electrode mixture obtained by appropriately adding a conductive additive (carbon material such as carbon black) or a binder such as PVDF to these negative electrode active materials is formed into a molded body (negative electrode active material-containing layer) using a current collector as a core material. A finished product, or one obtained by laminating the above-mentioned various alloys or lithium metal foils alone or on a current collector is used.

  When a current collector is used for the negative electrode, a copper or nickel foil, a punching metal, a net, an expanded metal, or the like can be used as the current collector, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is preferably 5 μm. Further, the lead portion on the negative electrode side may be formed in the same manner as the lead portion on the positive electrode side.

  The electrode can be used in the form of a laminate obtained by laminating the positive electrode and the negative electrode with the multilayer porous membrane according to the present invention as a separator, or an electrode wound body obtained by winding the laminate.

As the electrolytic solution (organic electrolytic solution), a solution in which a lithium salt is dissolved in an organic solvent is used. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and hardly causes side reactions such as decomposition in a voltage range used as a battery. For example, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ and other inorganic lithium salts, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] and the like can be used. .

  The organic solvent used in the electrolytic solution is not particularly limited as long as it dissolves the lithium salt and does not cause side reactions such as decomposition in the voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; Chain ethers such as dimethoxyethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; nitriles such as acetonitrile, propionitrile and methoxypropionitrile Sulfites such as ethylene glycol sulfite; etc., and these should be used as a mixture of two or more. It can be. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate. In addition, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexane, biphenyl, fluorobenzene, t- for the purpose of improving safety, charge / discharge cycleability, and high-temperature storage properties of these electrolytes. Additives such as butylbenzene can also be added as appropriate.

  The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  The electrochemical device of the present invention can be used for the same applications as conventionally known electrochemical devices.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

  In this example, the surface tension of the heat-resistant porous layer forming composition was measured using a surface tension meter “ESB-V” manufactured by Kyowa Chemical Co., Ltd. The critical surface tension of the resin porous membrane is measured by using several types of liquids with known surface tensions, measuring the contact angle (θ) between the resin porous membrane and the liquid, and plotting cos θ against the surface tension. It calculated | required from the point used as cos (theta) = 1. The measurement of the contact angle between the resin porous membrane and the liquid is performed by using a dynamic contact angle meter “1100DAT” manufactured by Fibro Co., Ltd. and determining the contact angle at 0.1 s after dropping the liquid on the resin porous membrane. It was.

Example 1
A self-crosslinking acrylic resin emulsion (solid content ratio: 40% by mass) as an organic binder: 200 g and water: 4000 g were placed in a container and stirred at room temperature until uniform. Further, alumina powder (average particle size 0.4 μm) as heat-resistant fine particles: 3000 g was added in four portions and stirred for 1 hour with a disper (2800 rpm) to form a uniform slurry (composition for forming a heat-resistant porous layer) Prepared). The surface tension (A) of this slurry was 42 mN / m.

The slurry is coated on a polyethylene porous film (thickness 12 μm) subjected to corona discharge treatment with a discharge amount of 50 W / min · m ² in advance with a gravure coater, and then dried to form a multilayer porous film having a thickness of 16 μm. Got. The corona discharge treatment was performed using a corona treatment apparatus manufactured by Kasuga Electric Co., Ltd., using a metal electrode. The polyethylene porous membrane had a critical surface tension (B) before corona discharge treatment of 31 mN / m, and a critical surface tension (B ′) after corona discharge treatment of 57 mN / m.

Comparative Example 1
A multilayer porous membrane was produced in the same manner as in Example 1 except that the slurry (heat-resistant porous layer forming composition) was applied to the polyethylene porous membrane without subjecting it to corona discharge treatment.

Comparative Example 2
An organic binder, vinylidene fluoride-6-propylene propylene copolymer (propylene hexafluoride ratio: 15 mol%): 90 g and acetone: 1510 g were placed in a container and stirred until evenly dissolved at room temperature. Further, alumina powder (average particle size 0.4 μm) as heat-resistant fine particles: 3000 g was added in four portions and stirred for 1 hour with a disper (2800 rpm) to form a uniform slurry (composition for forming a heat-resistant porous layer) Prepared). The surface tension (A) of this slurry was 28 mN / m.

  By applying the slurry to a polyethylene porous membrane (thickness 12 μm, the same porous membrane as used in Example 1 and Comparative Example 1) that has not been surface-treated with a knife coater, and then drying the slurry. A multilayer porous membrane having a thickness of 16 μm was obtained.

Comparative Example 3
A multilayer porous membrane was produced in the same manner as in Example 1 except that the surface treatment condition of the polyethylene porous membrane by the corona discharge treatment was changed to a discharge amount of 20 W / min · m ² . The critical surface tension (B ′) of the polyethylene porous membrane after the corona discharge treatment was 37 mN / m.

  The difference “(A) − (B)” between the surface tension (A) of the heat-resistant porous layer forming composition and the critical surface tension (B) of the polyethylene porous membrane is 10 mN / m or more (11 mN / m). The difference “(A) − (B ′)” between the surface tension (A) and the critical surface tension (B ′) of the polyethylene porous membrane before applying the heat-resistant porous layer forming composition is 0 mN In Example 1 which is set to / m or less (−15 mN / m), the heat-resistant porous layer-forming composition can be uniformly applied to the surface of the polyethylene porous membrane, and the heat-resistant porous material with high uniformity is obtained. A multilayer porous membrane having a porous layer could be produced with high productivity.

  On the other hand, in Comparative Example 1 in which the composition for forming a heat resistant porous layer was applied to a polyethylene porous film that was not subjected to surface treatment (corona discharge treatment), the heat resistant porous layer formed on the polyethylene porous film was formed. The composition for use was repelled and could not be applied uniformly. In addition, the difference between the surface tension (A) of the heat-resistant porous layer forming composition and the critical surface tension (B) of the polyethylene porous membrane using the organic solvent-based heat-resistant porous layer forming composition “( In Comparative Example 2 in which “A)-(B)” was −3 mN / m, when the composition was applied to the polyethylene porous membrane, the acetone solution containing the organic binder was removed from the back surface of the polyethylene porous membrane, Since it adhered to the back roll of the knife coater, the composition could not be continuously applied. Further, the difference “(A) − (B ′)” between the surface tension (A) of the heat-resistant porous film forming composition and the critical surface tension (B ′) of the polyethylene porous film after the surface treatment is 5 mN. Even in Comparative Example 3 that was / m, repelling occurred in the composition for forming a heat-resistant porous layer applied to a polyethylene porous membrane, and it could not be applied uniformly.

Example 2
<Preparation of positive electrode>
LiCoO _{2 as a} positive electrode active material: 90 parts by mass, acetylene black as a conductive additive: 7 parts by mass, and PVDF as a binder: 3 parts by mass uniformly using N-methyl-2-pyrrolidone (NMP) as a solvent A positive electrode mixture-containing paste was prepared by mixing. This paste is intermittently applied to both sides of an aluminum foil having a thickness of 15 μm as a current collector so that the coating length is 280 mm on the front surface and 210 mm on the back surface, dried, and calendered to a total thickness of 150 μm. Thus, the thickness of the positive electrode mixture layer was adjusted, and the positive electrode was produced by cutting to a width of 43 mm. Further, the exposed portion of the aluminum foil in this positive electrode was tabbed.

<Production of negative electrode>
A negative electrode active material graphite: 95 parts by mass and PVDF: 5 parts by mass were mixed so that NMP was used as a solvent to prepare a negative electrode mixture-containing paste, which was made of copper foil and had a thickness of 10 μm. After applying intermittently on both sides of the current collector so that the coating length is 290 mm on the front and 230 mm on the back, and drying, calendering is performed to adjust the thickness of the negative electrode mixture layer so that the total thickness is 142 μm. The negative electrode was produced by cutting to a width of 45 mm. Further, the exposed portion of the copper foil in this negative electrode was tabbed.

<Production of battery>
The positive electrode and the negative electrode obtained as described above are overlapped with the multilayer porous membrane prepared in Example 1 as a separator, and the heat-resistant porous layer of the separator is interposed so as to face the negative electrode, and wound in a spiral shape. A wound body was produced by turning. The obtained wound body was crushed into a flat shape, and then put into a laminate film outer package. Then, an electrolyte (LiPF ₆ was added to a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2 in a concentration of 1.2 M). After being injected, a battery was produced by vacuum sealing.

Comparative Example 4
A battery was produced in the same manner as in Example 2 except that the separator was changed to the multilayer porous membrane produced in Comparative Example 3.

  The batteries of Example 2 and Comparative Example 4 were evaluated for charge / discharge characteristics. These batteries were charged at a constant current at 25 ° C. and a current value of 150 mA, and when the voltage reached 4.2 V, the batteries were initially charged by constant current / constant voltage charging at a constant voltage of 4.2 V. The charging end time was 12 hours. About each battery after charge, the constant current discharge of the electric current value 150mA was performed continuously. Further, each of the batteries thereafter was subjected to constant current / constant voltage charging at a constant current of 500 mA at −5 ° C., and when the voltage reached 4.2 V, the battery was continuously charged at a constant voltage of 4.2 V. (Charge end time 2.5 hours).

  When each battery after charging was disassembled and the surface of the negative electrode was observed to determine the state of charge, the battery of Example 2 had almost no gray portion derived from the deposition of lithium metal and could be charged uniformly. On the other hand, in the battery of Comparative Example 4, many gray portions were observed, and it was confirmed that the state of charge was non-uniform due to non-uniformity of the heat-resistant porous layer related to the separator.

It is the schematic which shows an example of the coating apparatus applicable to the manufacturing method of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Resin porous membrane 2 Corona discharge device 3 Die head 7 Drying zone

Claims (9)

A method for producing a multilayer porous membrane for a separator of a lithium ion battery having a resin porous membrane comprising a thermoplastic resin as a main component and a heat resistant porous layer comprising a heat-resistant fine particle as a main component and containing an organic binder. There,
Surface treating the resin porous membrane;
Applying the heat-resistant porous layer forming composition to the surface of the resin porous membrane after the surface treatment, and forming the heat-resistant porous layer with a thickness of 1 μm or more,
The relationship between the surface tension (A) of the heat-resistant porous layer forming composition and the critical surface tension (B) at room temperature of the resin porous membrane before surface treatment is (A) − (B) ≧ 10 mN / m The relationship between the surface tension (A) and the critical surface tension (B ′) at room temperature of the resin porous membrane after the surface treatment is (A) − (B ′) ≦ 0 mN / m. A method for producing a multilayer porous membrane.   The method for producing a multilayer porous membrane according to claim 1, wherein the thermoplastic resin is polyolefin.   The method for producing a multilayer porous film according to claim 1 or 2, wherein the medium of the heat-resistant porous layer forming composition is mainly composed of water.   The method for producing a multilayer porous membrane according to any one of claims 1 to 3, wherein the surface treatment of the resin porous membrane is performed by ultraviolet irradiation, corona discharge treatment, or plasma treatment. A separator for a lithium ion battery comprising a multilayer porous membrane produced by the production method according to claim 1. The separator for a lithium ion battery according to claim 5, wherein the ratio of the heat-resistant fine particles is 80% by volume or more in the total volume of the constituent components of the heat-resistant porous layer. The lithium ion battery separator according to claim 5 or 6, wherein the heat-resistant porous layer has a thickness of 10 µm or less. The separator for a lithium ion battery according to any one of claims 5 to 7, wherein the porosity of the entire multilayer porous membrane is 30% or more and 70% or less. Lithium ion battery characterized by having a separator for a lithium ion battery according to any one of claims 5-8.

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