JP7243716B2 - SECONDARY BATTERY SEPARATOR, THERMOPLASTIC RESIN GRANULE, SLURRY COMPOSITION, MANUFACTURING METHOD THEREOF, AND SECONDARY BATTERY - Google Patents

Description

TECHNICAL FIELD The present invention relates to a secondary battery separator, a thermoplastic resin granule, a slurry composition, methods for producing these, and a secondary battery.

Secondary batteries such as lithium ion batteries are widely used in portable digital equipment such as mobile phones, notebook computers, digital cameras, digital video cameras, and portable game machines. In recent years, as automobile applications, use as a power source for hybrid vehicles, electric vehicles, plug-in hybrid vehicles, and the like has been expanding.

Lithium ion batteries generally have a secondary battery separator and an electrolyte interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. have a configuration.

Polyolefin-based porous substrates are used as secondary battery separators. The characteristics required for secondary battery separators are the ability to contain the electrolyte in the porous structure and enable ion movement (ionic conductivity), and the ability to melt with heat when the lithium ion battery overheats. The porous structure is closed at , and ion migration is stopped, thereby stopping power generation (shutdown function). In addition, the secondary battery separator is required to have dimensional stability in order to prevent a short circuit due to contact between the positive electrode and the negative electrode that occurs when the secondary battery separator thermally shrinks at high temperatures.

In addition, with the recent increase in capacity and output of lithium-ion batteries, separators for secondary batteries are required to have not only the above characteristics but also further characteristics. For example, in order to prevent the cycle characteristics from deteriorating due to gaps between the electrodes and the secondary battery separator due to repeated charging and discharging, the secondary battery separator is impregnated with an electrolyte solution. adhesiveness (wet adhesiveness) is required.

In addition, in the manufacturing process of the secondary battery, the shape of the laminate is maintained when the laminate of the positive electrode, the separator, and the negative electrode is transported, and the wound laminate of the positive electrode, the separator, and the negative electrode is formed into a cylindrical shape. It is desired to improve productivity and battery performance by keeping the shape of the laminate from collapsing when it is pressed before being inserted into a mold such as a rectangular mold. From this point of view, the secondary battery separator is required to have adhesiveness (dry adhesiveness) between the separator and the electrode before being impregnated with the electrolytic solution. Furthermore, with the widespread use of secondary batteries, there is a demand for reducing the production cost of secondary batteries, and for secondary battery separators, there is a demand for cost reduction.

In Patent Document 1, a separator for a secondary battery is provided in which a fluorine-containing polymer and a large amount of inorganic particles are held on a porous substrate to reduce heat shrinkage and improve adhesion of the inorganic particles to the porous body. Proposed.

In Patent Document 2, by laminating a porous layer containing a fluororesin, inorganic particles, and crosslinked polymer particles on a microporous film made of polyolefin, there is little thermal shrinkage, and an electrode impregnated with an electrolytic solution. There has been proposed a secondary battery separator imparted with adhesiveness (wet adhesiveness) of .

WO2015/083790 Japanese Patent Application Laid-Open No. 2017-152268

In Patent Document 1, since the fluoropolymer is produced by an emulsion polymerization method, the particle size of the fluoropolymer is small, and the adhesiveness (dry adhesiveness) between the separator and the electrode before impregnation with the electrolytic solution, and There is a problem that the adhesiveness (wet adhesiveness) between the separator and the electrode after being impregnated with the electrolytic solution tends to be insufficient.

Moreover, in Patent Document 2, it is necessary to use an organic solvent capable of dissolving the fluororesin as a solvent for the paint, which has the problem of causing a high environmental load and a high cost.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a secondary battery separator capable of effectively suppressing environmental load and cost while maintaining excellent adhesiveness, that is, dry adhesiveness or wet adhesiveness.

The present inventors have found that by using a thermoplastic resin granule containing thermoplastic resin particles and a fused material in which a plurality of primary particles of thermoplastic resin particles are fused together, as a separator for a secondary battery, excellent adhesion can be achieved. The inventors have found that dry adhesion or wet adhesion can be obtained, and arrived at the present invention.

In order to solve the above problems, the present invention has the following configurations.
1. A separator comprising a porous layer laminated on at least one side of a porous substrate, wherein at least one surface layer of the porous layer contains thermoplastic resin granules, and the thermoplastic resin granules are thermoplastic A separator for a secondary battery, which is a granule containing resin particles, and contains a fused product in which primary particles of a plurality of thermoplastic resin particles are fused together.
2. 2. The secondary battery separator according to 1 above, wherein the thermoplastic resin granules have a median diameter of 0.5 μm or more and 30 μm or less.
3. The ratio (A) of the median diameter of the thermoplastic resin granules to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules represented by the following formula (1) is 10 or more and 250 or less. 3. The secondary battery separator according to 1 or 2 above, wherein:
(A)=median diameter of thermoplastic resin granules (μm)/median diameter of thermoplastic resin particles (μm) (1)
4. 4. The secondary battery separator according to any one of 1 to 3 above, wherein the thermoplastic resin particles contain thermoplastic resin particles having a glass transition point of 80° C. or lower.
5. 5. The secondary battery separator according to any one of 1 to 4 above, wherein the thermoplastic resin particles contain thermoplastic resin particles having a melting point of 50° C. or higher and 160° C. or lower.
6. 6. The secondary battery separator according to any one of 1 to 5 above, wherein the thermoplastic resin contains one or more of polyvinylidene fluoride, a copolymer thereof, or an acrylic resin.
7. The porous layer includes an inorganic particle layer containing inorganic particles, and at least a portion of the thermoplastic resin granules is partly embedded in the inorganic particle layer and the other part is the inorganic particles. 7. The secondary battery separator according to any one of 1 to 6 above, which protrudes outward from the surface of the layer.
8. 8. The secondary according to 7 above, wherein the ratio (D) of the thickness of the inorganic particle layer to the median diameter of the thermoplastic resin granules represented by the following formula (4) is 0.02 or more and 1 or less Battery separator.
(D)=thickness of inorganic particle layer (μm)/median diameter of thermoplastic resin granules (μm) (4)
9. The inorganic particles are alumina, detected by energy dispersive X-ray spectroscopy (EDX), represented by the following formula (5), the ratio of the number of pixels of F to the total number of pixels of F and Al (E ) is 0.1 or more and 0.5 or less.
(E)=number of pixels of F/(number of pixels of F+number of pixels of Al) (5)
10. 10. The secondary battery separator according to any one of 1 to 9 above, wherein the porous layer has a surface roughness of 0.4 μm or more and 15 μm or less.
11. A secondary battery using the secondary battery separator according to any one of 1 to 10 above.
12. 1. A granule containing thermoplastic resin particles, the thermoplastic resin granule comprising a fused material in which primary particles of a plurality of said thermoplastic resin particles are fused together.
13. The ratio (A) of the median diameter of the thermoplastic resin granules to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules represented by the following formula (1) is 10 or more and 250 or less. 13. The thermoplastic resin granule according to 12 above.
(A)=median diameter of thermoplastic resin granules (μm)/median diameter of thermoplastic resin particles (μm) (1)
14. 14. The thermoplastic resin granules according to 12 or 13 above, wherein the thermoplastic resin granules have a median diameter of 0.5 μm or more and 30 μm or less.
15. 15. The thermoplastic resin granule according to any one of 12 to 14 above, wherein the thermoplastic resin particles contain thermoplastic resin particles having a glass transition point of 80° C. or less.
16. 16. The thermoplastic resin granule according to any one of 12 to 15 above, wherein the thermoplastic resin particles contain thermoplastic resin particles having a melting point of 50° C. or higher and 160° C. or lower.
17. 17. The thermoplastic resin granule according to any one of 12 to 16 above, wherein the thermoplastic resin contains one or more of polyvinylidene fluoride or its copolymer, or acrylic resin.
18. 18. The thermoplastic resin granule according to any one of 12 to 17 above, which is used for a secondary battery separator.
19. 19. A slurry composition comprising a dispersion liquid in which at least the thermoplastic resin granules according to any one of 12 to 18 are dispersed in a solvent.
20. The dispersion is a dispersion in which at least inorganic particles and the thermoplastic resin granules are dispersed in a solvent, and is represented by the following formula (2): 20. The slurry composition according to 19 above, wherein the median diameter ratio (B) of the particles is 3 or more and 100 or less.
(B)=median diameter of thermoplastic resin granules (μm)/median diameter of inorganic particles (μm) (2)
21. 21. The slurry composition according to 19 or 20 above, containing 1 wt % or more and 50 wt % or less of the thermoplastic resin granules.
22. 22. The above 20 or 21, wherein the ratio (C) of the content of the thermoplastic resin granules to the content of the inorganic particles, represented by the following formula (3), is 0.05 or more and 0.7 or less. slurry composition.
(C) = content of thermoplastic resin granules (wt%) / content of inorganic particles (wt%) (3)
23. A method for producing thermoplastic resin granules, which comprises spray-drying and granulating thermoplastic resin particles dispersed in a solvent with a spray dryer under drying temperature conditions below the melting point and above the glass transition point of the thermoplastic resin particles.
24. 19. A method for producing a slurry composition, comprising mixing inorganic particles, a solvent, and the thermoplastic resin granules according to any one of 12 to 18 above.
25. A secondary slurry composition prepared by mixing the thermoplastic resin granules according to any one of 12 to 18, inorganic particles, and a solvent is coated on a porous substrate and dried. A method for manufacturing a battery separator.

INDUSTRIAL APPLICABILITY The secondary battery separator of the present invention can effectively reduce environmental load and cost while maintaining high adhesiveness, that is, dry adhesiveness or wet adhesiveness.

FIG. 1 is an SEM image of one embodiment of the secondary battery separator of the present invention. FIG. 2 is a front cross-sectional view schematically showing a method for measuring wet adhesive strength.

A secondary battery separator according to an embodiment of the present invention is a separator formed by laminating a porous layer on at least one side of a porous substrate, and a thermoplastic resin granulated on at least one surface layer of the porous layer The thermoplastic resin granules contain a fused material in which the primary particles of the thermoplastic resin particles are fused together.
A secondary battery separator according to an embodiment of the present invention includes a step of applying a slurry composition prepared by mixing a thermoplastic resin granule, inorganic particles, and a solvent on a porous substrate and drying it. It can be manufactured by a manufacturing method. Moreover, the secondary battery separator according to the embodiment of the present invention can be preferably used for a secondary battery.
An embodiment of the present invention will be described in detail below.

<Thermoplastic resin granule>
A thermoplastic resin granule according to an embodiment of the present invention is a granule containing thermoplastic resin particles, and includes a fused product in which primary particles of a plurality of thermoplastic resin particles are fused together.
The thermoplastic resin granules in the embodiment of the present invention are granules containing thermoplastic resin particles, and are particles in which a plurality of primary particles of the thermoplastic resin are fused together at adjacent portions. It is a thermoplastic resin granule with a shape. For this reason, the thermoplastic resin granules of the present invention can be used in a slurry composition dispersed by applying a shearing force in a solvent, and in a porous separator for a secondary battery formed by coating and drying the slurry composition. The particle size and shape can be maintained even in the layer without the granules collapsing, and high adhesiveness can be exhibited. As used herein, granules include secondary particles formed from a plurality of primary particles. A primary particle as referred to herein means a particle that can be considered a unit particle judging from its apparent geometrical form.

The thermoplastic resin granules according to the embodiments of the present invention can be preferably used for secondary battery separators. Since the thermoplastic resin granules according to the embodiment of the present invention are granulated from a plurality of thermoplastic resin particles, they have voids inside the granules. It has a relatively low specific gravity. Therefore, when a slurry composition containing the thermoplastic resin granules according to the embodiment of the present invention is applied on a porous substrate and dried, it is easier to migrate to the dry film surface than solid particles of the same particle size, and adhesion It is easy to express sexuality. In addition, since the voids are present, the amount of resin used can be suppressed and the cost can be reduced as compared with the case where solid particles having the same particle size are used. Furthermore, the thermoplastic resin granule according to the embodiment of the present invention has unevenness on the surface due to the plurality of constituent thermoplastic resin particles. This is preferable because it increases the bonding area.

In the thermoplastic resin granules according to the embodiment of the present invention, the thermoplastic resin granules with respect to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules, which is calculated using the following formula (1) is preferably 10 or more and 250 or less.
(A)=median diameter of thermoplastic resin granules (μm)/median diameter of thermoplastic resin particles (μm) (1)

When the ratio (A) of the median diameter of the thermoplastic resin granules to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules is 10 or more, the shape of the thermoplastic resin granules becomes irregular. It is possible to suppress the deterioration of the handling property due to the deterioration of the fluidity. Further, when the ratio is 250 or less, the proportion of coarse fused bodies is less likely to increase, and variations in physical properties can be suppressed. The ratio (A) of the median diameter of the thermoplastic resin granules to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules is more preferably 15 or more, still more preferably 20 or more, and particularly preferably 30 or more. Similarly, the ratio is more preferably 200 or less, more preferably 150 or less, and particularly preferably 100 or less.

The median diameter of the thermoplastic resin granules is preferably 0.5 μm or more and 30 μm or less. When the median diameter is 0.5 μm or more, the contact area with the adherend is less likely to decrease, and sufficient adhesion, ie, dry adhesion or wet adhesion can be obtained. On the other hand, when the median diameter is 30 μm or less, the decrease in dispersion stability is suppressed, and sedimentation in the slurry is less likely to occur. The median diameter of the thermoplastic resin granules is more preferably 1 μm or more, still more preferably 1.5 μm or more, even more preferably 2 μm or more, and particularly preferably 9 μm or more. Moreover, the median diameter is more preferably 25 μm or less, still more preferably 20 μm or less, and particularly preferably 15 μm or less.

The median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules according to the embodiment of the present invention is preferably 0.01 μm or more and 1 μm or less. The median diameter of the thermoplastic resin particles is more preferably 0.05 μm or more, more preferably 0.1 μm or more. Similarly, the median diameter of the thermoplastic resin particles is more preferably 0.7 μm or less, more preferably 0.4 μm or less. When the median diameter is 0.01 μm or more, the thermoplastic resin particles are less likely to agglomerate before granulation, and uniform granules are easily formed. When the median diameter is 1 μm or less, the contact area between the thermoplastic resin particles inside the thermoplastic resin granules is less likely to decrease, and fragility of the thermoplastic resin granules can be suppressed.

The thermoplastic resin used in the embodiment of the present invention preferably contains one or more thermoplastic resins having a low glass transition point in order to promote fusion bonding between thermoplastic resin particles. For the same reason, the thermoplastic resin is preferably a non-crosslinked thermoplastic resin rather than a crosslinked thermoplastic resin.
The thermoplastic resin having a low glass transition point is not particularly limited, but examples thereof include olefin resins such as polyethylene and polypropylene, acrylic resins, styrene-butadiene resins, methyl methacrylate-styrene copolymers, and polyamides. , polyacetal, polyurethane, ethylene-vinyl acetate copolymer, polyvinylidene fluoride (PVdF) or copolymers thereof. More preferably, the thermoplastic resin contains one or more of polyvinylidene fluoride (PVdF), a copolymer thereof, or an acrylic resin. More preferably, the thermoplastic resin is polyvinylidene fluoride (PVdF) or a copolymer thereof.

The copolymer component constituting the PVdF copolymer, that is, the fluorine-based monomer copolymerizable with vinylidene fluoride (VDF) is not particularly limited, but vinyl fluoride, trifluoroethylene, tetrafluoro Examples include ethylene, chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), and perfluoroalkyl vinyl ether represented by perfluoromethyl vinyl ether. Among these, copolymers of VDF and HFP are particularly preferred because of their high chemical and physical stability to non-aqueous electrolytes.

Two major industrial processes for producing PVdF and its copolymers are emulsion polymerization and suspension polymerization. Emulsion polymerization produces small particles with a median diameter of about 0.2 to 0.7 μm. Suspension polymerization produces large particles with a median diameter of about 50 to 300 μm. However, it is difficult to obtain particles having a median diameter of more than 0.7 μm and less than 50 μm, which is intermediate between the median diameters of the particles obtained by both production methods.
The thermoplastic resin granules according to the embodiment of the present invention are preferably granulated from PVdF having a small particle size synthesized by an emulsion polymerization method or a copolymer thereof. It is easy to obtain granules of less than

The glass transition point of the thermoplastic resin is preferably 80° C. or lower. The glass transition point of the thermoplastic resin is preferably −100° C. or higher, more preferably −80° C. or higher, and still more preferably −60° C. or higher. Similarly, the glass transition point of the thermoplastic resin is more preferably 50° C. or lower, more preferably 20° C. or lower. When the glass transition point is −100° C. or higher, it is possible to suppress the development of tackiness at room temperature and outside temperature, and prevent the occurrence of blocking, which makes separation difficult due to adhesion of separators during storage. When the glass transition point is 80° C. or lower, high-temperature treatment is not required for the development of adhesiveness, so handleability is improved. The preferred glass transition point range is the same for the thermoplastic resin granules and the thermoplastic resin particles. The glass transition point of the thermoplastic resin granules and thermoplastic resin particles can be measured by the method described in Examples.

Moreover, the melting point of the thermoplastic resin is preferably 50° C. or higher and 160° C. or lower.

From the viewpoint of dry adhesion, the melting point of the thermoplastic resin is preferably 50° C. or higher, more preferably 55° C. or higher, and even more preferably 60° C. or higher. Similarly, the melting point of the thermoplastic resin is preferably 150° C. or lower, more preferably 130° C. or lower, even more preferably 110° C. or lower, even more preferably 100° C. or lower, and particularly preferably 80° C. or lower. When the melting point is 50° C. or higher, the thermoplastic resin granules are less likely to fuse together during storage and handling, and the formation of coarse aggregates can be suppressed. When the melting point is 150° C. or less, high-temperature treatment is not required for developing dry adhesiveness, and handleability is improved.

From the viewpoint of wet adhesion, the melting point of the thermoplastic resin is preferably 120° C. or higher, more preferably 130° C. or higher, and even more preferably 140° C. or higher. Similarly, the melting point of the thermoplastic resin is preferably 160° C. or lower, more preferably 155° C. or lower, even more preferably 150° C. or lower, and particularly preferably 145° C. or lower. If the melting point is 120° C. or higher, the solubility in the electrolytic solution tends to be low, and the wet adhesiveness can be easily maintained for a long period of time. When the melting point is 160° C. or lower, the thermoplastic resin tends to swell easily in the electrolytic solution, and the swollen thermoplastic resin tends to exhibit an anchoring effect to the adherend. The preferred range of melting point is the same for the thermoplastic resin granules and the thermoplastic resin particles. The melting point of the thermoplastic resin granules and thermoplastic resin particles can be measured by the method described in Examples.

In the case of PVdF copolymer, the melting point of thermoplastic resin can be controlled by adjusting the ratio of copolymer components. As an example, in the case of a VDF-HFP copolymer, the melting point decreases as the proportion of the copolymer component (HFP) increases. The copolymer component ratio of the VDF-HFP copolymer is preferably 0.3 mol % or more and 20 mol % or less.

From the viewpoint of dry adhesion, the copolymer component ratio is preferably 9 mol % or more, more preferably 10 mol % or more, and even more preferably 12 mol % or more. Similarly, the copolymer component ratio is preferably 20 mol % or less, more preferably 18 mol % or less, even more preferably 16 mol % or less, and particularly preferably 14 mol % or less. When the copolymer component ratio is 9 mol % or more, it is likely to be thermally deformed, and sufficient dry adhesiveness is likely to be exhibited by treatment at a lower temperature. When the copolymer component ratio is 20 mol % or less, fusion to the wall surface of the granulator tends to be suppressed during granulation, which will be described later, and a high yield can be easily obtained.

From the viewpoint of wet adhesion, the copolymer component ratio is preferably 0.3 mol % or more, more preferably 1 mol % or more, even more preferably 2 mol % or more, and particularly preferably 3 mol % or more. Similarly, the copolymer component ratio is preferably 9 mol % or less, more preferably 8 mol % or less, even more preferably 6 mol % or less, particularly preferably 5 mol % or less, and even more preferably 4 mol % or less. When the copolymer component ratio is 0.3 mol % or more, the crystallinity of the polymer is suppressed, and the degree of swelling due to the electrolytic solution tends to increase, which facilitates wet adhesion. When the copolymer component ratio is 9 mol % or less, the solubility in the electrolytic solution tends to be low, and the wet adhesiveness can be easily maintained for a long period of time.

<Method for producing thermoplastic resin granules>
For the thermoplastic resin granules according to the embodiment of the present invention, as an example, thermoplastic resin particles dispersed in a solvent are preferably dried by a spray dryer under conditions of a drying temperature lower than the melting point of the thermoplastic resin particles and higher than the glass transition point. It can be manufactured by a manufacturing method including spray drying granulation. The granulation method is not particularly limited, and it can be prepared using a known production method such as a tumbling granulation method, a fluidized granulation method, a tumbling fluidized granulation method, a spray drying granulation method, and a mixed granulation method. . Among these, the spray-drying granulation method is preferable because it does not require additives such as flocculants that adversely affect battery characteristics. Although the spray drying granulation method is not limited to this, it is preferably carried out using a spray dryer. In the present specification, the spray drying granulation method using a spray dryer is hereinafter referred to as the spray drying method.

The method of drying and granulating by a spray drying granulation method is not particularly limited. For example, the thermoplastic resin particle dispersion is made into a fine mist, and is preferably jetted into hot air at 30° C. or higher and 160° C. or lower. Let it be. The temperature of the hot air is preferably 40°C or higher, more preferably 50°C or higher, and particularly preferably 60°C or higher. Similarly, the temperature of the hot air is preferably 130° C. or lower, more preferably 100° C. or lower, and particularly preferably 80° C. or lower. By setting the temperature of the hot air to 160° C. or less, it becomes easier to suppress the formation of coarse aggregates due to fusion between the thermoplastic resin granules. This makes it possible to obtain a thermoplastic resin granule containing a fused material in which primary particles of a plurality of thermoplastic resin particles are fused together. As a method for making the thermoplastic resin particle dispersion into a fine mist, various known methods such as centrifugal spraying using a rotating disk and pressurized spraying using a pressure nozzle can be used. can be adopted.

<Slurry composition>
A slurry composition according to an embodiment of the present invention is a dispersion liquid in which the thermoplastic resin granules described above are dispersed in a solvent. The slurry composition is preferably a dispersion in which inorganic particles and thermoplastic resin granules are dispersed in a solvent, and the thermoplastic resin granules to the median diameter of the inorganic particles are represented by the following formula (2): is preferably 3 or more and 100 or less.
(B)=median diameter of thermoplastic resin granules (μm)/median diameter of inorganic particles (μm) (2)

When the ratio (B) of the median diameter of the thermoplastic resin granules to the median diameter of the inorganic particles is 3 or more, the thermoplastic resin granules are hidden behind the inorganic particles, resulting in an exposed area on the outermost surface of the porous layer. can be suppressed from decreasing, and the adhesiveness is less likely to decrease. When the ratio (B) of the median diameter of the thermoplastic resin granules to the median diameter of the inorganic particles is 100 or less, the thermoplastic resin granules are less likely to block the porous structure formed by the inorganic particles, resulting in an increase in air permeability. can be suppressed. The ratio (B) of the median diameter of the thermoplastic resin granules to the median diameter of the inorganic particles is preferably 4 or more, still more preferably 9 or more, and particularly preferably 14 or more. Similarly, the ratio is more preferably 80 or less, more preferably 60 or less, and particularly preferably 40 or less.

Examples of the inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass filler, kaolin, talc, titanium dioxide, alumina, magnesia, magnesium hydroxide, silica-alumina composite oxide particles, barium sulfate, Calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, mica, boehmite and the like. One type of these inorganic particles may be used, or two or more types may be mixed and used.

The median diameter of the inorganic particles is preferably 0.05 μm or more, more preferably 0.20 μm or more, and even more preferably 0.30 μm or more. Similarly, the median diameter of the inorganic particles is preferably 5.0 μm or less, more preferably 3.0 μm or less, even more preferably 1.0 μm or less. When the median diameter of the inorganic particles is 0.05 μm or more, the porous layer is less likely to be dense and air permeability can be suppressed from increasing. In addition, it is possible to suppress the impregnation of the electrolytic solution from deteriorating due to the smaller pore diameter, thereby improving the productivity. When the median diameter of the inorganic particles is 5.0 μm or less, sufficient dimensional stability can be obtained, and an increase in the thickness of the porous layer can be suppressed, thereby improving battery characteristics.

The shape of the inorganic particles is not particularly limited, and may be any shape. .

The content of the thermoplastic resin granules in the slurry composition is preferably 1 wt % or more and 50 wt % or less. When the content of the thermoplastic resin granules is 1 wt % or more, the amount of the thermoplastic resin granules exposed to the surface of the porous layer is less likely to decrease, and a decrease in adhesiveness can be suppressed. When the content of the thermoplastic resin granules is 50 wt % or less, the porous structure is less likely to be blocked by the thermoplastic resin granules, and an increase in air permeability can be suppressed. The content of the thermoplastic resin granules is more preferably 3 wt % or more, more preferably 5 wt % or more, and particularly preferably 7 wt % or more. Similarly, the content of the thermoplastic resin granules is more preferably 30 wt% or less, more preferably 20 wt% or less, and particularly preferably 12 wt% or less.

In the slurry composition according to the embodiment of the present invention, the ratio (C) of the content of the thermoplastic resin granules to the content of the inorganic particles, represented by the following formula (3), is 0.05 or more and 0.05 or more. It is preferably 7 or less.
(C) = content of thermoplastic resin granules (wt%) / content of inorganic particles (wt%) (3)

When the ratio (C) of the content of the thermoplastic resin granules to the content of the inorganic particles is 0.05 or more, the thermoplastic resin granules are less likely to be embedded in a large amount of inorganic particles, and the content of the thermoplastic resin granules to the surface of the porous layer. It is possible to suppress deterioration in adhesiveness due to a decrease in the amount of exposure. Further, when the ratio is 0.7 or less, the inorganic particles necessary for maintaining the structure of the porous layer are less likely to run short, and an increase in thermal shrinkage can be suppressed. The ratio (C) of the content of the thermoplastic resin granules to the content of the inorganic particles (C) is more preferably 0.07 or more, still more preferably 0.1 or more, and particularly preferably 0.15 or more. Similarly, such a ratio is more preferably 0.5 or less, more preferably 0.4 or less, and particularly preferably 0.35 or less.

As the solvent used in the slurry composition according to the embodiment of the present invention, a solvent containing water as a main component is preferable from the viewpoint of suppressing impregnation of the solvent into the porous substrate. Here, the main component means a component contained in 50% by mass or more in 100% by mass of the solvent.

The ratio of water in the solvent containing water as a main component is preferably 50% by mass or more, more preferably 60% by mass or more, and still more preferably 70% by mass or more. When the proportion of water is 50% by mass or more, the substrate is less likely to be impregnated with the coating solution when applied to the porous substrate, and a desired porous layer can be easily formed. In addition, it is possible to suppress difficulty in transporting the porous substrate due to the impregnation of the coating liquid, and to prevent wrinkles from occurring during transport.

In addition, by using the solvent containing water as the main component in the slurry composition according to the embodiment of the present invention, compared to the case of using an organic solvent capable of dissolving the fluororesin, the environmental load and cost are effectively reduced. can be effectively suppressed.

An organic resin (binder) may be added to the slurry composition according to the embodiment of the present invention, if necessary. When the thermoplastic resin granules, the organic resin and the inorganic particles are dispersed in the slurry composition, a dispersant may be used if necessary.

The type of dispersant is not particularly limited, but examples include cationic surfactants such as alkylamine salts and quaternary ammonium salts, alkyl sulfate salts, polyoxyethylene alkyl ether sulfate salts, alkylbenzene sulfonates, Anionic surfactants such as fatty acid salts, nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenols, glycerin fatty acid esters, polyoxyethylene fatty acid esters, amphoteric surfactants such as alkylbetaines and alkylamine oxides , and the above-mentioned cationic, anionic, nonionic, amphoteric fluorine-based surfactants, silicon-based surfactants, and the like. Other types of dispersants include polymeric compounds such as polyvinylpyrrolidone, polycarboxylates, polysulfonates, and polyethers. These dispersants may be used singly or in combination of two or more.

The amount of the dispersant to be added is preferably 0.1 parts by mass or more and 40 parts by mass or less, more preferably 0.2 parts by mass, based on a total of 100 parts by mass of the thermoplastic resin granules, the organic resin, and the inorganic particles to be dispersed. It is at least 0.5 part by mass, more preferably at least 0.5 part by mass. Also, the amount of the dispersant added is preferably 30 parts by mass or less, more preferably 20 parts by mass or less. When the amount of the dispersant added is 40 parts by mass or less, it is possible to prevent the content of the thermoplastic resin granules in the porous layer from becoming small, thereby suppressing the decrease in adhesiveness.

As a method for dispersing the thermoplastic resin granules, the organic resin, and the inorganic particles, a known technique may be used. Examples include ball mills, bead mills, sand mills, roll mills, homogenizers, ultrasonic homogenizers, high-pressure homogenizers, ultrasonic devices, and paint shakers. A plurality of these mixing and dispersing machines may be combined for stepwise dispersion.

A slurry composition according to an embodiment of the present invention can be produced by a production method including mixing inorganic particles, a solvent, and the thermoplastic resin granules described above.

The slurry composition can be used as a coating liquid to be applied to a porous substrate for the purpose of being used for a secondary battery separator described later.

The order of preparing the coating liquid is not particularly limited. From the viewpoint of improving the efficiency of the dispersion process, a dispersant is added to and mixed with a solvent containing water as the main component, and thermoplastic resin granules, organic resins, and inorganic particles are added to the solution to obtain a coating liquid. is preferably prepared.

If necessary, an organic resin (binder) may be added to the coating liquid in order to bind the particles together or the particles and the porous base material. Moreover, if necessary, an antioxidant, a stabilizer, an antifoaming agent, a leveling agent, and the like may be added to the coating solution.

The type of leveling agent is not particularly limited, but examples include cationic surfactants such as alkylamine salts and quaternary ammonium salts, alkyl sulfate salts, polyoxyethylene alkyl ether sulfate salts, alkylbenzene sulfonates, Anionic surfactants such as fatty acid salts, nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenols, glycerin fatty acid esters, polyoxyethylene fatty acid esters, amphoteric surfactants such as alkylbetaines and alkylamine oxides , and polymer compounds such as cationic, anionic, nonionic, amphoteric fluorine-based surfactants, silicon-based surfactants, polyvinylpyrrolidone, polycarboxylates, polysulfonates, and polyethers. One type of these leveling agents may be used, or two or more types may be mixed and used.

The amount of the leveling agent to be added is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass with respect to a total of 100 parts by mass of the thermoplastic resin granules, the organic resin, and the inorganic particles. It is below. When the leveling agent is added in an amount of 20 parts by mass or less, it is possible to suppress the deterioration of the battery characteristics due to the deterioration of adhesion and the side reaction in the secondary battery.

<Separator for secondary battery>
A secondary battery separator according to an embodiment of the present invention is a separator formed by laminating a porous layer on at least one side of a porous substrate, and a thermoplastic resin granulated on at least one surface layer of the porous layer The thermoplastic resin granules are granules containing thermoplastic resin particles, and a separator for a secondary battery comprising a fused product in which primary particles of a plurality of thermoplastic resin particles are fused together be.
Further, a secondary battery separator according to an embodiment of the present invention is a separator obtained by laminating a porous layer on at least one side of a porous base material, wherein at least one surface layer of the porous layer is subjected to the above-described heat treatment. It preferably contains plastic resin granules. Further, in the secondary battery separator according to the embodiment of the present invention, the porous layer preferably includes an inorganic particle layer containing inorganic particles, and at least part of the thermoplastic resin granules is partially inorganic It is characterized by being buried inside the particle layer and protruding outside from the surface of the inorganic particle layer. The secondary battery separator according to the embodiment of the present invention can be suitably used for secondary batteries such as lithium ion batteries.

FIG. 1 is an SEM image of one embodiment of the secondary battery separator of the present invention. A secondary battery separator according to an embodiment of the present invention is formed by laminating a porous layer on at least one surface of a porous substrate 1 . The porous layer includes an inorganic particle layer 2 containing inorganic particles, and further includes thermoplastic resin granules 3 on at least one surface layer of the porous layer. The thermoplastic resin granules 3 need only be adhered to the surface of the inorganic particle layer 2 without being embedded in the inorganic particle layer 2 in order to exhibit adhesiveness to the electrode. In order to prevent the thermoplastic resin granules from falling off from the porous layer and to stably develop adhesiveness, at least a part of the thermoplastic resin granules 3 should have a portion inside the inorganic particle layer 2. is buried in the inorganic particle layer 2, and the other portion protrudes outward from the surface of the inorganic particle layer 2.

In the secondary battery separator according to the embodiment of the present invention, the ratio (D) of the thickness of the inorganic particle layer to the median diameter of the thermoplastic resin granules calculated from the following formula (4) is 0.02 or more and 1 The following are preferable.
(D)=thickness of inorganic particle layer (μm)/median diameter of thermoplastic resin granules (μm) (4)

If the ratio (D) of the thickness of the inorganic particle layer to the median diameter of the thermoplastic resin granules is 0.02 or more, the thermoplastic resin granules will not protrude greatly from the surface of the inorganic particle layer, and the separator will become porous during handling. It is possible to suppress falling off from the thin layer and obtain sufficient adhesiveness with the electrode. When the ratio (D) of the thickness of the inorganic particle layer to the median diameter of the thermoplastic resin granules is 1 or less, the thermoplastic resin granules are less likely to be embedded in the inorganic particle layer, are easily brought into contact with the electrode, and are sufficiently Adhesion is obtained. The ratio of the thickness of the inorganic particle layer to the median diameter of the thermoplastic resin granules is more preferably 0.05 or more, still more preferably 0.10 or more, and particularly preferably 0.15 or more. Similarly, such a ratio is more preferably 0.8 or less, more preferably 0.6 or less, and particularly preferably 0.4 or less.

The thickness of the inorganic particle layer is preferably 0.10 μm or more. More preferably, the thickness of the inorganic particle layer is 0.3 μm or more, and still more preferably 0.5 μm or more. Similarly, the thickness of the inorganic particle layer is preferably 5.0 μm or less, more preferably 4.0 μm or less, even more preferably 3.0 μm or less. When the thickness of the inorganic particle layer is 0.10 μm or more, the thermoplastic resin granules are less likely to fall off from the porous layer during handling of the separator, and sufficient adhesiveness to the electrodes can be obtained. Further, when the thickness is 5.0 μm or less, the increase in air permeability can be suppressed, and the adhesiveness is sufficient. In addition, if the inorganic particle layer is laminated on only one side, curling may become remarkable, so it is preferable to laminate the inorganic particle layer on both sides of the porous substrate. Moreover, when the inorganic particle layer is laminated on both sides, the difference in the thickness of the inorganic particle layer on each side is preferably 1 μm or less for the same reason.

The surface roughness of the porous layer of the secondary battery separator according to the embodiment of the present invention is preferably 0.4 μm or more and 15 μm or less. When the surface roughness is 0.4 μm or more, the protrusion of the thermoplastic resin granules from the surface of the inorganic particle layer is less likely to be small, and sufficient adhesiveness is obtained. Further, when the thickness is 15 μm or less, the embedding of the thermoplastic resin granules in the inorganic particle layer can be suppressed, and the thermoplastic resin granules can be prevented from falling off during handling of the separator. The surface roughness is more preferably 1.0 μm or more, more preferably 1.5 μm or more. Similarly, the surface roughness is more preferably 10 μm or less, more preferably 5 μm or less.

In the secondary battery separator according to the embodiment of the present invention, the inorganic particles are alumina, and fluorine (F) is measured by energy dispersive X-ray spectroscopy (EDX) and calculated from the following formula (5). The ratio (E) of the number of pixels of fluorine (F) to the total number of pixels of aluminum (Al) is preferably 0.1 or more and 0.5 or less.
(E)=number of pixels of F/(number of pixels of F+number of pixels of Al) (5)

When the ratio (E) of the number of pixels of fluorine to the total number of pixels of fluorine and aluminum is 0.1 or more, the distribution of the fluorine-containing thermoplastic resin granules on the surface layer of the inorganic particle layer is reduced. can be suppressed and the adhesion can be improved. Moreover, when it is 0.5 or less, the inorganic particles necessary for maintaining the structure of the inorganic particle layer are sufficient, and an increase in thermal shrinkage can be suppressed. The ratio (E) of the number of pixels of fluorine to the total number of pixels of fluorine and aluminum is preferably 0.12 or more, more preferably 0.14 or more, and particularly preferably 0.16 or more. Similarly, it is more preferably 0.4 or less, still more preferably 0.3 or less, and particularly preferably 0.25 or less.

In the secondary battery separator according to the embodiment of the present invention, a slurry composition prepared by mixing the above-described thermoplastic resin granules, inorganic particles, and a solvent is applied (coated) on a porous substrate. , can be produced by a production method including a step of drying. As a coating method, a known method may be used. For example, gravure coating, slit die coating, knife coating, kiss coating, roll coating, bar coating, spray coating, dip coating, spin coating, screen printing, inkjet printing, pad printing, other types of printing, and the like can be used. The coating method is not limited to these, and is appropriately selected according to preferable conditions such as thermoplastic resin granules, organic resins, inorganic particles, binders, dispersants, leveling agents, solvents to be used, and substrates. do it. In order to improve coatability, for example, the porous substrate may be subjected to surface treatment such as corona treatment or plasma treatment.

When the porous layer is laminated on both sides of the porous substrate, one side may be coated and dried, or both sides may be simultaneously coated and dried. Simultaneous coating and drying on both sides is preferred because of high productivity.

From the viewpoint of adhesiveness, lamination of porous layers on both sides is more preferable than lamination of porous layers on only one side. This is because by laminating the porous layers on both sides, adhesiveness can be obtained on both sides of the positive electrode and the negative electrode, so that process handleability is excellent.

[Porous substrate]
In an embodiment of the present invention, the porous substrate includes a porous membrane having pores inside, a nonwoven fabric, or a porous membrane sheet made of a fibrous material. The material constituting the porous substrate is preferably composed of a resin that is electrically insulating, electrically stable, and stable in an electrolytic solution. Moreover, from the viewpoint of imparting a shutdown function, the resin to be used is preferably a thermoplastic resin having a melting point of 200° C. or less. The shutdown function here is a function that, when the lithium-ion battery overheats abnormally, closes the porous structure by melting with heat, stops the movement of ions, and stops power generation.

Thermoplastic resins used for the porous substrate include, for example, polyolefin resins. The porous substrate is preferably a polyolefin porous substrate. Further, the polyolefin-based porous substrate is more preferably a polyolefin-based porous substrate having a melting point of 200° C. or less. Specific examples of polyolefin-based resins include polyethylene, polypropylene, copolymers thereof, and mixtures thereof. For example, a single-layer porous substrate containing 90% by mass or more of polyethylene, A multi-layered porous base material made of polypropylene and the like can be mentioned.

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

The thickness of the porous substrate is preferably 3 μm or more and 50 μm or less. When the thickness of the porous substrate is 50 µm or less, it is possible to suppress an increase in the internal resistance of the porous substrate. Moreover, when the thickness of the porous substrate is 3 μm or more, it is easy to manufacture, and sufficient mechanical properties can be obtained. More preferably, the thickness of the porous substrate is 5 μm or more. Similarly, the thickness of the porous substrate is more preferably 30 μm or less.

The air permeability of the porous substrate is preferably 50 sec/100 cc or more and 1000 sec/100 cc or less. More preferably, it is 500 seconds/100 cc or less. When the air permeability is 1000 sec/100 cc or less, sufficient ion mobility can be obtained, and deterioration of battery characteristics can be suppressed. Sufficient mechanical properties can be obtained when the air permeability of the porous substrate is 50 seconds/100 cc or more.

<Secondary battery>
A secondary battery according to an embodiment of the present invention is a secondary battery using the secondary battery separator described above. The secondary battery is preferably a secondary battery such as a lithium ion battery. Lithium ion batteries generally have a configuration in which a secondary battery separator and an electrolyte are interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. have.

The positive electrode is obtained by laminating a positive electrode material composed of an active material, a binder resin, a conductive aid, etc. on a current collector, and the active material includes LiCoO ₂ , LiNiO ₂ , Li(NiCoMn)O ₂ , and the like. spinel-type manganese oxides such as LiMn ₂ O ₄ , and iron-based compounds such as LiFePO ₄ . A resin having high oxidation resistance may be used as the binder resin. Specific examples include fluorine-based resins, acrylic-based resins, and styrene-butadiene-based resins. Carbon materials such as carbon black and graphite are used as the conductive aid. As the current collector, a metal foil is suitable, and aluminum is often used in particular.

The negative electrode is obtained by laminating a negative electrode material composed of an active material, a binder resin, a conductive aid, etc. on a current collector, and carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon are used as the active material. , lithium alloy materials such as tin and silicon, metal materials such as Li, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). A fluorine-based resin, an acrylic resin, a styrene-butadiene-based resin, or the like is used as the binder resin. Carbon black, graphite and the like are used as conductive aids. As the current collector, metal foil is suitable, and copper foil is often used in particular.

The electrolytic solution is a field for transferring ions between the positive electrode and the negative electrode in the secondary battery, and has a structure in which the electrolyte is dissolved in an organic solvent. Electrolytes include LiPF ₆ , LiBF ₄ , LiClO ₄ and the like. LiPF ₆ is preferably used as the electrolyte from the viewpoints of solubility in organic solvents and ion conductivity. Organic solvents include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, gamma-butyrolactone, sulfolane, and the like. These organic solvents may be used singly or in combination of two or more.

As a method for producing a secondary battery, first, an active material and a conductive agent are dispersed in a binder solution to prepare an electrode coating liquid, and this coating liquid is applied onto a current collector, and the solvent is dried. Thus, a positive electrode and a negative electrode are obtained. The film thickness of the coating film after drying is preferably 50 μm or more and 500 μm or less. A secondary battery separator is placed between the obtained positive electrode and negative electrode so as to be in contact with the active material layer of each electrode, sealed in an outer packaging material such as an aluminum laminate film, and hot-pressed after injecting an electrolytic solution. . After that, a negative electrode lead and a safety valve are installed, and the exterior material is sealed. The secondary battery obtained in this way has good adhesion between the electrode and the secondary battery separator, so that it has excellent cycle characteristics and excellent dimensional stability, and can be manufactured at low cost. .

[Measurement Example 1: Glass transition point]
In differential scanning calorimetry (DSC) according to the provisions of "JIS K7121: 2012 Plastic transition temperature measurement method", Shimadzu Corporation DSC-60 (differential scanning calorimeter), 2 to 3 mg in the measurement pan of the thermoplastic resin particles were put in and used as a sample for measurement, and the measurement was performed under the following conditions. The intersection of the observed baseline and the tangent line at the point of inflection was defined as the glass transition point (°C).
Heating rate: 10°C/min
Measurement temperature range: -100 to 200°C

[Measurement Example 2: Melting point]
In differential scanning calorimetry (DSC) according to the provisions of "JIS K7121: 2012 Plastic transition temperature measurement method", Shimadzu Corporation DSC-60 (differential scanning calorimeter), 2 to 3 mg in the measurement pan of the thermoplastic resin particles were put in and used as a sample for measurement, and the measurement was performed under the following conditions. The peak top temperature of the endothermic peak during the first temperature rise was taken as the melting point (°C).
Temperature rise, cooling rate: ±10°C/min
Measurement temperature range: -20 to 230°C

[Measurement Example 3: Median diameter]
The median diameter (volume basis) (μm) of the thermoplastic resin particles or thermoplastic resin granules was measured with a laser diffraction/scattering particle size distribution analyzer LA-920 manufactured by Horiba, Ltd. Measurement was started when ultrasonic waves were applied for 1 minute at a setting of intensity 6, and application of ultrasonic waves was continued until the end of the measurement. Water was used as the solvent in the apparatus, and the absolute refractive index of the PVdF copolymer was 1.41.

[Measurement Example 4: Weight Average Molecular Weight]
The weight average molecular weight of the thermoplastic resin particles was calculated using a gel permeation chromatography method by comparing with a polystyrene calibration curve.
Apparatus: LA-10A series manufactured by Shimadzu Corporation Column: KD-806M × 2 columns manufactured by Showa Denko K.K. Mobile phase: dimethylformamide Flow rate: 1.0 ml/min
Detection: Differential refractometer Column temperature: 40°C

[Measurement Example 5: Thickness of Inorganic Particle Layer]
A sample cross section of the secondary battery separator is cut out with an ion milling device IM-4000 manufactured by Hitachi High-Technologies Co., Ltd., and the cross section is observed with an ultra-high resolution field emission scanning electron microscope SU8010 manufactured by Hitachi High-Technologies Co., Ltd. In the observation area, the highest point from the interface with the porous substrate was selected for the portion composed of inorganic particles in which the thermoplastic resin granules did not exist, and the thickness (μm) of the inorganic particle layer was measured. As the thickness (μm) of the inorganic particle layer, a value obtained by observing, selecting, measuring, and averaging five arbitrary points from a 100 mm×100 mm size sample was used.

[Measurement Example 6: Median diameter of thermoplastic resin granules in porous layer]
The porous layer of the secondary battery separator was scraped off under a microscope, stirred and extracted in water, separated into soluble matter and insoluble matter by centrifugation, and the insoluble matter was further separated into inorganic particles and thermoplastic resin granules. . The median diameter (volume basis) (μm) of the thermoplastic resin granules was measured with a laser diffraction/scattering particle size distribution analyzer LA-920 manufactured by HORIBA, Ltd. Measurement was started when ultrasonic waves were applied for 1 minute at a setting of intensity 6, and application of ultrasonic waves was continued until the end of the measurement. Water was used as the solvent in the apparatus, and the absolute refractive index of the PVdF copolymer was 1.41.

[Measurement Example 7: Air Permeability]
Select an arbitrary point from each of three 100 mm × 100 mm size samples of a secondary battery separator, and measure according to JIS P 8117 (2009) using a Gurley densometer G-B3C manufactured by Toyo Seiki Seisakusho Co., Ltd. did The average value of the obtained values was defined as the air permeability (sec/100cc).

[Measurement Example 8: Dry Adhesion Strength]
A negative electrode of 30 mm × 100 mm and a separator of 40 mm × 150 mm for a secondary battery containing graphite as an active material, vinylidene fluoride resin (PVdF) as a binder, and carbon black as a conductive agent were placed so that the active material and the porous layer were in contact with each other. and a hot roll press at 0.5 MPa, 85° C., and 0.2 m/min. Subsequently, the dry adhesive strength (N/m) was measured by a peel method (180° peeling, peeling speed 100 mm/min) using a Tensilon universal tester RTG-1210 manufactured by A&D Co., Ltd. A TLU-5N-G (maximum load capacity of 5N) was mounted as a load cell, and the measurement was continuously performed for 100 mm from the start of measurement to the end of measurement. The average value of the load of 20 to 60 mm was calculated and converted into the value per 1 m of width to obtain the peel strength (dry adhesive strength). A part of the porous layer may remain on the side of the porous substrate at the peeling interface.

[Measurement Example 9: Wet Adhesion Strength]
In general, a fluororesin binder is used for the positive electrode, and when a porous layer containing the fluororesin is provided on the separator, mutual diffusion between the fluororesins tends to ensure adhesion. On the other hand, binders other than fluorine-based resins are generally used for the negative electrode, and diffusion of the fluorine-based resin is less likely to occur. Therefore, in this measurement, the wet adhesion strength described below was measured and evaluated as an index of the wet adhesion between the separator and the negative electrode. FIG. 2 is a front cross-sectional view schematically showing a method for measuring wet adhesive strength. Details are described below with reference to FIG.

(Preparation of negative electrode)
An aqueous solution containing 1.5 parts by mass of carboxymethyl cellulose as a thickener and a dispersant for the active material was added to 96.5 parts by mass of artificial graphite as the active material and mixed, and a binder of 2 parts by mass as a solid content was added. Styrene-butadiene latex was added and mixed to obtain a negative electrode mixture-containing slurry. This negative electrode mixture-containing slurry is evenly applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 8 μm and dried to form a negative electrode layer. A negative electrode 5 was prepared by setting the density of the negative electrode layer excluding the body to 1.5 g/cm ³ .

(Preparation of winding body for test)
The negative electrode 5 prepared above (161 mm in the machine direction × 30 mm in the width direction) and the prepared secondary battery separator 4 (160 mm in the machine direction × 34 mm in the width direction) were stacked, and a metal plate (length 300 mm, width 25 mm, thickness The secondary battery separator 4 and the negative electrode 5 were wound up so that the secondary battery separator 4 was on the inside, and the metal plate was pulled out to obtain the test wound body 7 . The test roll was about 34 mm long by about 28 mm wide.

(Method for measuring wet adhesive strength)
Two laminate films made of polypropylene (length 70 mm, width 65 mm, thickness 0.07 mm) were stacked, and three of the four sides were welded together. . 500 μL of an electrolytic solution prepared by dissolving LiPF ₆ at a ratio of 1 mol/L in a solvent obtained by mixing ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3:7 was injected from the opening of the laminate film 6 in a glove box, and a test roll was prepared. The circular body 7 was impregnated with it, and one side of the opening was sealed with a vacuum sealer.

Next, the test wound body 7 enclosed in the laminate film 6 is sandwiched between two gaskets (thickness 1 mm, 5 cm × 5 cm), and a precision heating and pressurizing device (CYPT-10, manufactured by Sintokogyo Co., Ltd.). was pressurized at 98° C. and 0.6 MPa for 2 minutes, and allowed to cool at room temperature. The wet adhesive strength of the test roll 7 was measured with a universal tester (AGS-J, manufactured by Shimadzu Corporation) while still enclosed in the laminate film 6 .

Two aluminum L-shaped angles 8 (thickness: 1 mm, 10 mm x 10 mm, length: 5 cm) are arranged parallel to each other with the 90° part facing upward, and the 90° part is used as the fulcrum. They were fixed so that the distance between them was 15 mm. Align the middle point of the side (about 28 mm) in the width direction of the test roll with the 7.5 mm point, which is the middle of the distance between the fulcrums of the two aluminum L-shaped angles 8, and the aluminum L-shaped angle 8 The test wound body 7 was arranged so as not to protrude from the longitudinal side of the .

Next, as an indenter, the lengthwise side (about 34mm) of the test roll protrudes from the lengthwise side of the aluminum L-shaped angle 9 (thickness 1mm, 10mm x 10mm, length 4cm). Align the 90° part of the aluminum L-shaped angle 9 with the midpoint of the side in the width direction of the test roll, and make the aluminum L-shaped angle so that the 90° part faces downward 9 was fixed to the load cell (load cell capacity 50N) of the universal testing machine. Three test rolls were measured at a load rate of 0.5 mm/min, and the average value of the obtained maximum test forces was taken as the wet adhesive strength (N).

[Measurement Example 10: EDX]
After sputtering the surface of the secondary battery separator obtained in each example and comparative example with an ion sputtering device (manufactured by JEOL Ltd., JEC-3000FC), a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, SU8010) It was measured with an energy dispersive X-ray spectrometer (EMAXEvolution X-MAXN, manufactured by Oxford Instruments Co., Ltd.) connected to . An observation field was determined at an accelerating voltage of 4.5 kV and a magnification of 1,000 times, and an electron beam image was captured at a resolution of 4,096, and then mapped at a resolution of 4,096. A phase analysis was performed on the mapping results to calculate the ratio of fluorine pixels to the sum of fluorine and aluminum pixels.

[Measurement Example 11: Surface Roughness]
The secondary battery separator obtained in each example and comparative example was observed with a 3D shape measuring machine (VR-3200 manufactured by Keyence Corporation), and the surface roughness of the separator surface was randomly measured at 10 points. The average value of the obtained values was obtained and taken as the surface roughness (μm).

[Example 1]
A 25 wt% aqueous dispersion of vinylidene fluoride-hexafluoropropylene copolymer (VDF/HFP = 88/12) particles prepared by emulsion polymerization was treated with a spray dryer MDL-050(C)M manufactured by Fujisaki Electric Co., Ltd. The mixture was spray-dried at a hot air temperature of 50° C. to obtain thermoplastic resin granules.

Alumina particles (median diameter: 0.5 µm) as inorganic particles were added to the above thermoplastic resin granules and mixed with water so that the mass ratio of the non-volatile solid content was 25:75. Further, 3.9 parts by mass of an acrylic resin is added as a binder with respect to a total of 100 parts by mass of the thermoplastic resin granules and inorganic particles, and a perfluoroalkyl compound, which is a surfactant, is added as a leveling agent to the thermoplastic resin granules. A slurry composition was prepared by adding 0.4 parts by mass to a total of 100 parts by mass of the solid and the inorganic particles. The solid content concentration of the obtained slurry composition was 45 wt %. This slurry composition was coated on both sides of a polyethylene porous substrate (thickness: 9 μm, air permeability: 175 sec/100 cc) using a bar coater so that the inorganic particle layer had a thickness of 0.7 μm per side. The coated slurry composition was dried in a hot air oven at 50° C. for 2 minutes to form a porous layer, thereby obtaining a secondary battery separator according to an embodiment of the present invention.

[Example 2]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 1, except that the slurry composition was applied using a bar coater so that the thickness of the inorganic particle layer was 1.5 μm per side. rice field.

[Example 3]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 1, except that the slurry composition was applied using a bar coater so that the thickness of the inorganic particle layer was 2.0 μm per side. rice field.

[Example 4]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 1, except that the slurry composition was applied using a bar coater so that the thickness of the inorganic particle layer was 3.0 μm per side. rice field.

[Example 5]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 3, except that the hot air temperature of the spray dryer was changed to 70°C.

[Example 6]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 3, except that the hot air temperature of the spray dryer was changed to 90°C.

[Example 7]
The second embodiment of the present invention was prepared in the same manner as in Example 3, except that the non-volatile solid content of the thermoplastic resin granules and the inorganic particles in the slurry composition was mixed so that the mass ratio was 10:90. A separator for a secondary battery was obtained.

[Example 8]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 3, except that the vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP=95/5 was used.

[Example 9]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 3, except that alumina particles having a median diameter of 0.05 μm were used as the inorganic particles.

[Example 10]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 1, except that a thermoplastic resin granule having a median diameter of 1 μm, which had been spray-dried with a spray dryer and recovered with a cyclone, was used.

[Example 11]
Using a thermoplastic resin granule having a median diameter of 2 μm and collected by a cyclone after spray drying with a spray dryer and alumina particles having a median diameter of 1 μm, a bar coater was used so that the thickness of the inorganic particle layer was 4.0 μm per side. A slurry composition was applied. A secondary battery separator according to the embodiment of the present invention was obtained in the same manner as in Example 1 except for the above.

[Example 12]
The second embodiment of the present invention was prepared in the same manner as in Example 3, except that the non-volatile solid content of the thermoplastic resin granules and the inorganic particles in the slurry composition was mixed so that the mass ratio was 3:97. A separator for a secondary battery was obtained.

[Example 13]
Using a thermoplastic resin granule having a median diameter of 2 μm collected by a cyclone after spray drying with a spray drier, the slurry composition was coated using a bar coater so that the thickness of the inorganic particle layer was 4.0 μm per side. A secondary battery separator according to the embodiment of the present invention was obtained in the same manner as in Example 1 except for the above.

[Example 14]
A separator for a secondary battery according to an embodiment of the present invention was obtained in the same manner as in Example 6, except that a spray dryer Model L-8i manufactured by Okawara Kakoki Co., Ltd. was used and the spraying method was changed to centrifugal spraying.

[Example 15]
A secondary battery separator according to an embodiment of the present invention was obtained in the same manner as in Example 14, except that the vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP=90/10 was used.

[Example 16]
The vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 95/8 was used, and the second embodiment of the present invention was prepared in the same manner as in Example 14, except that the hot air temperature of the spray dryer was changed to 100 ° C. A separator for a secondary battery was obtained.

[Example 17]
The vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 95/5 was used, and the second embodiment of the present invention was prepared in the same manner as in Example 14, except that the hot air temperature of the spray dryer was changed to 110 ° C. A separator for a secondary battery was obtained.

[Example 18]
The vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 85/15 was used, and the second embodiment of the present invention was prepared in the same manner as in Example 14, except that the hot air temperature of the spray dryer was changed to 50 ° C. A separator for a secondary battery was obtained.

[Example 19]
A vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 96/4 was used, and the temperature of the hot air of the spray dryer was changed to 120°C in the same manner as in Example 14. A separator for a secondary battery was obtained.

[Example 20]
A vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 97/3 was used, and the temperature of the hot air of the spray dryer was changed to 125°C in the same manner as in Example 14. A separator for a secondary battery was obtained.

[Example 21]
A vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 98/2 was used, and the hot air temperature of the spray dryer was changed to 135 ° C., in the same manner as in Example 14. A separator for a secondary battery was obtained.

[Example 22]
The vinylidene fluoride-hexafluoropropylene copolymer composition VDF/HFP = 100/0 was used, and the hot air temperature of the spray dryer was changed to 140 ° C., in the same manner as in Example 14. A separator for a secondary battery was obtained.

[Comparative Example 1]
Vinylidene fluoride-hexafluoropropylene copolymer (VDF/HFP=88/12) particles having a median diameter of 0.7 μm and produced by emulsion polymerization without granulation using a spray dryer were used in the slurry composition. A secondary battery separator was obtained in the same manner as in Example 3 except for the above.

[Comparative Example 2]
Vinylidene fluoride-hexafluoropropylene copolymer (VDF/HFP=95/5) particles having a median diameter of 0.7 μm and produced by emulsion polymerization without granulation using a spray dryer were used in the slurry composition. A secondary battery separator was obtained in the same manner as in Example 3 except for the above.

[Comparative Example 3]
Vinylidene fluoride-hexafluoropropylene copolymer (VDF/HFP=88/12) particles having a median diameter of 0.2 μm produced by an emulsion polymerization method without granulation using a spray dryer were used in the slurry composition. . A secondary battery separator was obtained in the same manner as in Example 3 except for the above.

[Comparative Example 4]
Vinylidene fluoride-hexafluoropropylene copolymer (VDF/HFP=88/12) particles having a median diameter of 100 μm prepared by suspension polymerization without granulation using a spray dryer were used in the slurry composition. A secondary battery separator was obtained in the same manner as in Example 3 except for the above. At the time of coating, a large amount of sediment thought to be vinylidene fluoride-hexafluoropropylene copolymer particles was observed in the slurry composition, and streaks dragged by the copolymer particles were observed in the coating film.

The glass transition point of the thermoplastic resin particles used in each example and comparative example, the melting point of the thermoplastic resin particles, the weight average molecular weight of the thermoplastic resin particles, the median diameter of the thermoplastic resin particles, the composition of the thermoplastic resin particles, the thermal Manufacturing method of plastic resin granules, median diameter of thermoplastic resin granules, ratio (A) of median diameter of thermoplastic resin granules to median diameter of thermoplastic resin particles, solvent for slurry composition, slurry composition median diameter of inorganic particles in the slurry composition, the ratio (B) of the median diameter of the thermoplastic resin granules to the median diameter of the inorganic particles in the slurry composition, the content of the thermoplastic resin granules in the slurry composition, the slurry composition Tables 1 to 3 for the mass ratio of the thermoplastic resin granules and the inorganic particles at the time of product production, and the ratio (C) of the content of the thermoplastic resin granules to the content of the inorganic particles in the slurry composition. shown in

Regarding the secondary battery separators obtained in each example and comparative example, the thickness of the inorganic particle layer (one side), the median diameter of the thermoplastic resin granules, and the thickness of the inorganic particle layer with respect to the median diameter of the thermoplastic resin granules. The thickness ratio (D), air permeability, surface roughness, dry adhesion strength, wet adhesion strength, and the ratio (E) of the number of pixels of F to the total number of pixels of F and Al detected by EDX were measured. . Tables 1 to 3 show the measurement results. Regarding Comparative Examples 1 to 4, each numerical value calculated using the median diameter of the thermoplastic resin granules was calculated using the median diameter of the thermoplastic resin particles.

As shown in Tables 1 to 3, Examples 1 to 22 show excellent dry adhesive strength or wet adhesive strength compared to Comparative Examples 1 to 4 by using thermoplastic resin granules. In addition, it showed surface roughness and air permeability equal to or higher than the same. FIG. 1 is an SEM image of the secondary battery separator of Example 1. FIG. As shown in FIG. 1, the secondary battery separator according to the embodiment of the present invention is formed by laminating a porous layer on at least one side of a porous substrate, and a thermoplastic resin on at least one surface layer of the porous layer. Contains granules. FIG. 2 is a cross-sectional front view schematically showing the method for measuring the wet adhesive strength shown in Measurement Example 9. FIG.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application (Japanese patent application 2019-063264) filed on March 28, 2019 and a Japanese patent application (Japanese patent application 2019-177758) filed on September 27, 2019, and the content is Incorporated here as a reference.

ADVANTAGE OF THE INVENTION According to this invention, the separator for secondary batteries which can control an environmental impact and cost effectively can be provided, maintaining excellent adhesiveness, ie, dry adhesiveness or wet adhesiveness.

1 Porous substrate 2 Inorganic particle layer 3 Thermoplastic resin granule 4 Secondary battery separator 5 Negative electrode 6 Laminate film 7 Test roll 8 Aluminum L-shaped angle 9 Aluminum L-shaped angle

Claims (24)

A separator comprising a porous layer laminated on at least one side of a porous substrate, wherein at least one surface layer of the porous layer contains thermoplastic resin granules, and the thermoplastic resin granules are thermoplastic A separator for a secondary battery, which is a granule containing resin particles, and contains a fused product in which primary particles of a plurality of thermoplastic resin particles are fused together. 2. The secondary battery separator according to claim 1, wherein said thermoplastic resin granules have a median diameter of 0.5 [mu]m or more and 30 [mu]m or less. The ratio (A) of the median diameter of the thermoplastic resin granules to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules represented by the following formula (1) is 10 or more and 250 or less. The secondary battery separator according to claim 1 or 2, wherein:
(A)=median diameter of thermoplastic resin granules (μm)/median diameter of thermoplastic resin particles (μm) (1) The secondary battery separator according to any one of claims 1 to 3, wherein the thermoplastic resin particles contain thermoplastic resin particles having a glass transition point of 80°C or less. The secondary battery separator according to any one of claims 1 to 4, wherein the thermoplastic resin particles contain thermoplastic resin particles having a melting point of 50°C or higher and 160°C or lower. The secondary battery separator according to any one of claims 1 to 5, wherein the thermoplastic resin contains one or more of polyvinylidene fluoride or its copolymer, or acrylic resin. The porous layer includes an inorganic particle layer containing inorganic particles, and at least a portion of the thermoplastic resin granules is partly embedded in the inorganic particle layer and the other part is the inorganic particles. The secondary battery separator according to any one of claims 1 to 6, which protrudes outward from the surface of the layer. 8. The two according to claim 7, wherein the ratio (D) of the thickness of the inorganic particle layer to the median diameter of the thermoplastic resin granules represented by the following formula (4) is 0.02 or more and 1 or less. Separator for secondary batteries.
(D)=thickness of inorganic particle layer (μm)/median diameter of thermoplastic resin granules (μm) (4) The inorganic particles are alumina, detected by energy dispersive X-ray spectroscopy (EDX), represented by the following formula (5), the ratio of the number of pixels of F to the total number of pixels of F and Al (E ) is from 0.1 to 0.5, the secondary battery separator according to claim 7 or 8.
(E)=number of pixels of F/(number of pixels of F+number of pixels of Al) (5) 10. The secondary battery separator according to claim 1, wherein said porous layer has a surface roughness of 0.4 μm or more and 15 μm or less. A secondary battery using the secondary battery separator according to any one of claims 1 to 10. A granule containing thermoplastic resin particles, which contains a fusion product in which primary particles of a plurality of thermoplastic resin particles are fused together, and the thermoplastic resin contains polyvinylidene fluoride or a copolymer thereof. Thermoplastic resin granules. The ratio (A) of the median diameter of the thermoplastic resin granules to the median diameter of the thermoplastic resin particles constituting the thermoplastic resin granules represented by the following formula (1) is 10 or more and 250 or less. The thermoplastic resin granule according to claim 12, which is
(A)=median diameter of thermoplastic resin granules (μm)/median diameter of thermoplastic resin particles (μm) (1) The thermoplastic resin granule according to claim 12 or 13, wherein the thermoplastic resin granule has a median diameter of 0.5 µm or more and 30 µm or less. The thermoplastic resin granule according to any one of claims 12 to 14, wherein the thermoplastic resin particles contain thermoplastic resin particles having a glass transition point of 80°C or less. The thermoplastic resin granule according to any one of claims 12 to 15, wherein the thermoplastic resin particles contain thermoplastic resin particles having a melting point of 50°C or higher and 160°C or lower. The thermoplastic resin granule according to any one of claims 12 to 16 , which is used as a secondary battery separator. A slurry composition containing at least a dispersion liquid in which the thermoplastic resin granules according to any one of claims 12 to 17 are dispersed in a solvent. The dispersion is a dispersion in which at least inorganic particles and the thermoplastic resin granules are dispersed in a solvent, and is represented by the following formula (2): 19. The slurry composition according to claim 18 , wherein the particles have a median diameter ratio (B) of 3 or more and 100 or less.
(B)=median diameter of thermoplastic resin granules (μm)/median diameter of inorganic particles (μm) (2) 20. The slurry composition according to claim 18 or 19 , comprising 1 wt% or more and 50 wt% or less of said thermoplastic resin granules. 21. The method according to claim 19 or 20 , wherein the ratio (C) of the content of the thermoplastic resin granules to the content of the inorganic particles, represented by the following formula (3), is 0.05 or more and 0.7 or less A slurry composition as described.
(C) = content of thermoplastic resin granules (wt%) / content of inorganic particles (wt%) (3) A thermoplastic resin for a secondary battery separator , comprising spray-drying and granulating thermoplastic resin particles dispersed in a solvent with a spray dryer under drying temperature conditions below the melting point of the thermoplastic resin particles and above the glass transition point. A method for producing granules. A method for producing a slurry composition, comprising mixing inorganic particles, a solvent, and the thermoplastic resin granules according to any one of claims 12 to 17 . A slurry composition prepared by mixing the thermoplastic resin granules, inorganic particles, and solvent according to any one of claims 12 to 17 is coated on a porous substrate and dried. A method for manufacturing a separator for a secondary battery.

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