JP7309650B2 - Separator for electrochemical device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a separator for electrochemical devices (hereinafter, "separator for electrochemical devices" may be referred to as "separator").

Principal members used in electrochemical devices such as lithium ion batteries and electric double layer capacitors are a positive electrode, a negative electrode, a separator for electrochemical devices, and an electrolytic solution. The separator separates the positive electrode and the negative electrode in the electrochemical element so that an internal short circuit in which the positive electrode and the negative electrode are in direct contact does not occur. In order to lower the internal resistance of the electrochemical device, the separator must have pores through which ions in the electrolyte can efficiently pass. Therefore, the separator should be porous.

A separator made of a wet-laid nonwoven fabric produced by a wet papermaking method, containing as essential components a solvent-spun cellulose fiber (fibrillated solvent-spun cellulose fiber) and a non-fibrillated synthetic fiber or rayon fiber as a separator. has been disclosed (see, for example, Patent Documents 1 to 3). In these separators, the degree of beating of the fibrillated solvent-spun cellulose and the fiber diameter of the non-fibrillated synthetic fibers or rayon fibers are set within a certain range, so that they have an appropriate denseness, resulting in a low internal resistance. The separator has excellent discharge capacity retention rate and internal short circuit defect rate.

In the trend of demand for higher performance electrochemical devices and higher energy density, there is a demand for thinner separators. There was a risk of pinholes being generated.

JP 2012-222266 A JP 2018-139283 A JP 2019-46776 A

SUMMARY OF THE INVENTION An object of the present invention is to provide a separator for an electrochemical device in which pinholes are less likely to occur even if it is thin.

The above problems have been solved by the following inventions.

In the electrochemical device separator containing fibrillated solvent-spun cellulose fibers and non-fibrillated fibers , the non-fibrillated fibers having a fiber length of 0.2 mm or less are contained, and the fiber length is The content ratio of non-fibrillated fibers with a fiber length of 0.2 mm or less, represented by the ratio of the total length of the non-fibrillated fibers of 0.2 mm or less, is 0.1% or more and less than 2.0%. A separator for an electrochemical device characterized by:

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain an electrochemical device separator that is less prone to pinholes even if it is thin.

1 is a fiber length distribution diagram of a non-fibrillated fiber slurry containing non-fibrillated fibers having a fiber length of 0.2 mm or less. FIG. 1 is a microscope photograph of non-fibrillated fibers, including non-fibrillated fibers with a fiber length of 0.2 mm or less.

The electrochemical device separator and the electrochemical device containing the electrochemical device separator of the present invention will be described in more detail.

<Separator for electrochemical device>
The electrochemical device separator of the present invention contains fibrillated solvent-spun cellulose fibers and non-fibrillated fibers, further contains non-fibrillated fibers having a fiber length of 0.2 mm or less, and the total length of all non-fibrillated fibers is The content ratio of non-fibrillated fibers with a fiber length of 0.2 mm or less, represented by the ratio of the total length of the non-fibrillated fibers with a fiber length of 0.2 mm or less to the thickness, is 0.1% or more and less than 2.0% The technical feature is that Due to this technical feature, in the process of forming a web on the papermaking wire of the wet papermaking method, the separation of the fine component of the fibrillated solvent-spun cellulose fibers is suppressed, and a separator that is less prone to pinholes even when thinned can be obtained. .

In this specification, "non-fibrillated fibers having a fiber length of 0.2 mm or less" are sometimes referred to as "non-fibrillated short fibers".

The electrochemical device separator of the present invention is preferably a wet-laid nonwoven fabric produced by a wet-laid papermaking method. Raw material fibers for producing a sheet by a wet papermaking method are fibrillated solvent-spun cellulose fibers and non-fibrillated fibers, and the non-fibrillated fibers include non-fibrillated short fibers.

FIG. 1 shows the fiber length of a non-fibrillated fiber slurry containing non-fibrillated short fibers according to "JIS P 8226-2: 2011, Pulp - Fiber length measurement method by optical automatic analysis method Part 2: Non-polarization method". based on OpTest Equipment Inc. Fig. 2 is a fiber length distribution diagram measured with a Fiber Quality Analyzer (FQA-360) manufactured by Co., Ltd.; In FIG. 1, the total length of non-fibrillated staple fibers was 0.5% of the total length of all non-fibrillated fibers.

FIG. 2 is a photograph of non-fibrillated fibers, including non-fibrillated short fibers, taken with a microscope at a magnification of 200. FIG. Fibers within circles are non-fibrillated short fibers.

Non-fibrillated fibers in the present invention include polyester, polyolefin, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyamide, acrylic, polyvinyl chloride, polyvinylidene chloride, polyvinyl ether, and polyvinyl. Ketone-based, polyether-based, polyvinyl alcohol-based, polyesteramide-based, polyetheretherketone-based, ethylene-vinyl alcohol copolymer-based, polyurethane-based synthetic fibers, rayon fibers, polynosic fibers, lyocell fibers, solvent-spun cellulose fibers, etc. regenerated cellulose fibers, wood-derived cellulose fibers, and non-wood-derived cellulose fibers such as hemp, cotton, and sugarcane. Among these fibers, polyester-based, acrylic-based, polyolefin-based synthetic fibers, or rayon fibers, which are easily made into small-diameter fibers, are preferably used in order to make the separator thinner.

The diameter of the non-fibrillated fibers is preferably 0.1 to 15 μm, more preferably 0.5 to 10 μm, even more preferably 1 to 8 μm. If the diameter of the fibrillated fibers is less than 0.1 μm, the strength of the separator may be insufficient. If the diameter is larger than 15 μm, it becomes difficult to make the separator thin.

The fiber length of the non-fibrillated fibers is preferably 20 mm or less, more preferably 10 mm or less, and even more preferably 8 mm or less. Moreover, as the non-fibrillated fibers, it is preferable to contain non-fibrillated fibers having a fiber length of 1 mm or more, and it is more preferable to contain non-fibrillated fibers having a fiber length of 3 mm or more.

The cross-sectional shape of the non-fibrillated fibers is not only circular and elliptical, but also polygonal such as triangles and squares, flat, Y-shaped, T-shaped, U-shaped and star-shaped. may In the wet papermaking method, the cross-sectional aspect ratio (longer diameter of cross section of fiber/shorter diameter of cross section of fiber) of the fiber before being dispersed in water is preferably 1.0 to less than 1.2. If the fiber cross-sectional aspect ratio is 1.2 or more, the dispersibility of the fibers may be lowered, or the uniformity of the separator may be adversely affected due to the entanglement or entanglement of the fibers.

In the present invention, the content ratio of non-fibrillated fibers is preferably 5 to 40% by mass, more preferably 10 to 35% by mass, more preferably 15 to 30% by mass, based on the total fiber components contained in the separator. % is more preferable. If the non-fibrillated fiber content is less than 5% by mass, it may be difficult to pick up the web from the papermaking wire of the wet papermaking method, and if it exceeds 40% by mass, the denseness of the separator will decrease and internal short circuit failure will occur. It may be a high efficiency separator. In the present invention, non-fibrillated short fibers are included in non-fibrillated fibers.

The non-fibrillated short fibers have a fiber length of 0.2 mm or less, preferably 0.05 to 0.2 mm, more preferably 0.1 to 0.2 mm. The cross-sectional shape of the non-fibrillated short fibers is not only circular and elliptical, but also polygons such as triangles and squares, flat shapes, Y-shapes, T-shapes, U-shapes, star-shapes, and other so-called irregular cross-section shapes. may

Methods for obtaining non-fibrillated staple fibers include cutting non-fibrillated fibers using a cutter. Also, in the wet papermaking process, non-fibrillated short fibers can be obtained by cutting the non-fibrillated fibers at the clearance between the stirring blades rotating in a water dispersion device such as a pulper and the container. Non-fibrillated short fibers can also be obtained by cutting the fibers into short pieces during the manufacturing process of the non-fibrillated fibers.

In the separator of the present invention, the content ratio of non-fibrillated short fibers is expressed by the ratio of the total length of non-fibrillated short fibers to the total length of all non-fibrillated short fibers. The content ratio of non-fibrillated short fibers is 0.1% or more and less than 2.0%, preferably 0.2% or more and less than 1.5%, more preferably 0.2% or more and less than 1.5%. less than 0%. If the content of the non-fibrillated short fibers is less than 0.1%, the effect of preventing the fine components of the fibrillated solvent-spun cellulose fibers from coming off is insufficient, and pinholes may occur. On the other hand, if it exceeds 2.0%, it may become difficult for the web to be picked up from the papermaking wire. In the present invention, the ratio of the total length of non-fibrillated short fibers to the total length of all non-fibrillated fibers is the fiber length of pulper-dispersed non-fibrillated fiber slurry, defined as "JIS P 8226-2:2011 , Pulp-Fiber length measurement method by optical automatic analysis method Part 2: Non-polarization method”, OpTest Equipment Inc. It is a value calculated based on length-weighted average fiber length data measured using Fiber Quality Analyzer (FQA-360) manufactured by Co., Ltd.

In the present invention, solvent-spun cellulose fibers are different from so-called regenerated cellulose fibers, such as conventional viscose rayon and cuprammonium rayon, in which cellulose is chemically converted into a cellulose derivative and then returned to cellulose again. It refers to a fiber in which cellulose is precipitated by dissolving cellulose in an amine oxide or ionic liquid and then dry-wet spinning the spinning stock solution in water without chemically changing it, and is also called "Lyocell fiber". Compared to natural cellulose fibers, bacterial cellulose fibers, and rayon fibers, solvent-spun cellulose fibers have highly aligned molecules along the long axis of the fibers. Since the fine fibers are easily fibrillated and thin and long fine fibers are generated, they are firmly entangled with the non-fibrillated fibers forming the main skeleton of the separator, so that a separator having excellent strength can be obtained.

In the present invention, fibrillated solvent-spun cellulose fibers are used, and the modified freeness, which indicates the fibrillation state, is preferably 50 to 250 ml, more preferably 75 to 220 ml, and more preferably 90 to 200 ml. is more preferred. If the modified freeness exceeds 250 ml, the denseness of the separator may decrease, resulting in a separator with a high internal short circuit defect rate. On the other hand, if the modified freeness is less than 50 ml, the separator may become excessively dense and have high resistance.

The modified freeness in the present invention is JIS P8121-2 except that 80 mesh metal with a linear 0.14 mm and mesh opening of 0.18 mm is used as the sieve plate, and the sample concentration is 0.1% by mass. It is the value measured according to the standards.

In the present invention, the length-weighted average fiber length of fibrillated solvent-spun cellulose fibers is preferably 0.2 to 2.5 mm, more preferably 0.3 to 2.0 mm, even more preferably 0.3 to 1.8 mm. If the length-weighted average fiber length is shorter than 0.2 mm, the separator surface may become fuzzy due to rubbing. On the other hand, when the length-weighted average fiber length is longer than 2.5 mm, the fibers tend to get tangled, and thickness unevenness may occur. The above length-weighted average fiber length was measured by OpTest Equipment Inc. based on "JIS P 8226-2: 2011, Pulp - Fiber length measurement method by optical automatic analysis method Part 2: Non-polarization method". It is a value measured using a Fiber Quality Analyzer (FQA-360) manufactured by Co., Ltd.

In the present invention, the method for producing fibrillated solvent-spun cellulose fibers includes a refiner, a beater, a mill, a grinding device, a rotary blade homogenizer that applies a shearing force with high-speed rotary blades, and a cylindrical inner blade that rotates at high speed. A double-cylinder high-speed homogenizer that generates a shearing force between the outer blade and a fixed outer blade, an ultrasonic crusher that makes it fine by impact with ultrasonic waves, and a pressure difference of at least 20 MPa to the fiber suspension to create a small diameter A method using a high-pressure homogenizer or the like is used in which the fibers are passed through an orifice at a high speed, collided and rapidly decelerated to apply a shearing force and a cutting force to the fibers. Among these methods, the method using a refiner is particularly preferable. By adjusting the type of beating/dispersing equipment and processing conditions (fiber concentration, temperature, pressure, rotation speed, shape of refiner blades, gap between refiner discs, number of processing times, etc.), the desired modified freeness and It is possible to achieve a length weighted average fiber length.

In the present invention, the content ratio of the fibrillated solvent-spun cellulose fibers is preferably 60 to 95% by mass, more preferably 65 to 90% by mass, more preferably 70 to 90% by mass, based on the total fiber components contained in the separator. More preferably, it is 85% by mass. If the content ratio of the fibrillated solvent-spun cellulose fibers is less than 60% by mass, the density of the separator may decrease, resulting in a separator with a high internal short-circuit defect rate. Moreover, when it exceeds 95 mass %, it may become difficult for the web to be picked up from the papermaking wire.

The separator of the present invention may further contain fibrillated natural cellulose fibers. The content ratio thereof is preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less relative to the total fiber components contained in the separator. Fibrillated natural cellulose fibers tend to be inferior to fibrillated solvent-spun cellulose fibers in the uniformity of the thickness of each fiber, but are characterized by strong physical entanglement and hydrogen bonding between fibers. Therefore, the inclusion of the fibrillated natural cellulose fibers has the effect of suppressing fluffing on the surface of the separator. However, if the content ratio of fibrillated natural cellulose exceeds 10% by mass based on the total fiber components contained in the separator, a film is formed on the surface of the separator, impairing the ionic conductivity, thereby reducing the internal resistance of the separator. may be higher.

In the present invention, fibrillated natural cellulose fibers include refiners, beaters, mills, grinder-type pulverizers, rotary-blade homogenizers that apply shearing force with high-speed rotary blades, and cylindrical inner blades that rotate at high speed. A double-cylinder high-speed homogenizer that generates a shear force between the outer blades, an ultrasonic crusher that makes the fiber finer by impact with ultrasonic waves, and a pressure difference of at least 20 MPa to the fiber suspension to pass through a small diameter orifice. It can be used after being treated with a high-pressure homogenizer, etc., which applies a shearing force and a cutting force to the fibers by colliding them to a high speed and rapidly decelerating them. Among these, fibrillated natural cellulose fibers treated with a high-pressure homogenizer are particularly preferable from the viewpoint of high productivity and uniformity of the fibrillated state.

In the present invention, raw materials for the fibrillated natural cellulose fibers include wood pulp such as softwood pulp and hardwood pulp, and non-wood pulp derived from cotton linter pulp, cotton pulp, hemp, bagasse, kenaf, bamboo, and straw. can be done.

The separator of the present invention may contain, as fibers other than non-fibrillated fibers, fibrillated solvent-spun cellulose fibers, and fibrillated natural cellulose fibers, synthetic fiber pulps and fibrillated materials.

The electrochemical device separator of the present invention is preferably a wet-laid nonwoven fabric obtained by a wet-laid papermaking method. A wet-laid nonwoven fabric is excellent in the uniform distribution of fibers, is less likely to cause internal short circuits, and is preferable as a method for producing a highly reliable separator for an electrochemical device.

In the wet papermaking method, first, the fibers are uniformly dispersed in water, and then the final fiber concentration is adjusted to 0.01 to 0.50% by mass through processes such as screening (removal of foreign matter, lumps, etc.). The resulting papermaking slurry is made by a paper machine to obtain wet paper. During the process, chemicals such as dispersants, antifoaming agents, hydrophilic agents, antistatic agents, polymer adhesives, release agents, antibacterial agents, and bactericides may be added.

As the papermaking method, for example, a fourdrinier, a cylinder, or an inclined wire method can be used. Using a paper machine having at least one papermaking system selected from these papermaking system groups, or a combination papermaking machine in which two or more of the same or different papermaking systems selected from these papermaking system groups are installed online can do. In the case of producing a nonwoven fabric having a multi-layered structure of two or more layers, a method of laminating wet paper made by each paper machine, or a method of forming one sheet and then placing the wet paper on the sheet. A method of casting a slurry in which fibers are dispersed can be used.

A sheet is obtained by drying wet paper produced by a paper machine with a Yankee dryer, an air dryer, a cylinder dryer, a suction drum dryer, an infrared dryer, or the like. When the wet paper is dried, it is brought into close contact with a hot roll such as a Yankee dryer and dried under heat and pressure, thereby improving the smoothness of the contacted surface. Hot-press drying means drying by pressing the wet paper against a hot roll with a touch roll or the like. The surface temperature of the heat roll is preferably 80 to 180°C, more preferably 90 to 160°C, even more preferably 100 to 160°C. The pressure is preferably 5-100 kN/m, more preferably 10-80 kN/m. Moreover, the separator of the present invention is subjected to calendering, heat calendering, or the like, if necessary.

The basis weight of the separator is preferably 5 to 25 g/m ² , more preferably 6 to 20 g/m ² and even more preferably 7 to 18 g/m ² . If the basis weight exceeds 25 g/m ² , it may be difficult to make the separator thin. If it is less than 5 g/m ² , it may be difficult to obtain sufficient strength. The basis weight is measured according to the method specified in JIS P 8124:2011 (Paper and paperboard - Basis weight measurement method).

The thickness of the separator is preferably 8 to 60 μm, more preferably 10 to 50 μm, even more preferably 20 to 45 μm. If the thickness is less than 8 μm, sufficient mechanical strength cannot be obtained, or the insulation between the positive electrode and the negative electrode is insufficient, resulting in a high internal short circuit defect rate and uneven discharge characteristics. The capacity retention rate and cycle characteristics may deteriorate. If the thickness is more than 60 μm, the internal resistance of the electrochemical device may increase, or the discharge characteristics may deteriorate.

<Electrochemical device>
A capacitor is suitable as the electrochemical element in the present invention. Examples of capacitors include electric double layer capacitors, hybrid capacitors, redox capacitors, and the like. Lithium secondary batteries are also suitable as electrochemical devices.

<Electric double layer capacitor>
An electric double layer capacitor (EDLC) is a capacitor that stores an electric charge in an electric double layer formed on the surfaces of a positive electrode and a negative electrode. By allowing more ions to be adsorbed on the surfaces of the positive and negative electrodes, an EDLC with a higher capacity can be obtained.

In order to allow more ions to be adsorbed on the surfaces of the positive and negative electrodes, the positive and negative electrodes need to have larger specific surface areas. Moreover, the positive electrode and the negative electrode of EDLC need not cause an electrochemical reaction. Activated carbon; graphite; nanocarbon such as carbon nanofiber and graphene are mainly used as materials that satisfy these conditions for the positive electrode and negative electrode of EDLC.

As the electrolytic solution, an aqueous solution of sulfuric acid, a solution obtained by dissolving a salt that does not electrochemically react within the working potential range in a polar organic solvent, an ionic liquid, or the like can be used. Salts that do not electrochemically react within the working potential range include a salt of tetraethylammonium and tetrafluoroborate (TEA.BF ₄ ), a salt of triethylmethylammonium and tetrafluoroborate (TEMA.BF ₄ ), Salts of 5-azoniaspiro[4.4]nonane and tetrafluoroboric acid (SBP.BF ₄ ) are exemplified. These may use 1 type or may use 2 or more types.

Examples of polar organic solvents include acetonitrile: γ-butyrolactone (GBL); carbonate esters such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC). . These may use 1 type or may use 2 or more types.

<Hybrid Capacitor>
A hybrid capacitor is a capacitor in which a battery reaction, that is, an electrochemical oxidation-reduction reaction occurs at either the positive electrode or the negative electrode, and charge is stored in an electric double layer formed on the surface of the other electrode. In the electric double layer capacitor, since the electric double layer of the positive electrode and the electric double layer of the negative electrode are connected in series, only half the capacitance of the electric double layer in each of the positive electrode and the negative electrode can be obtained. Hybrid capacitors, on the other hand, use an electrode that accumulates electric charges in the electric double layer as one of the electrodes. . As a hybrid capacitor, a lithium ion capacitor to be described later is exemplified.

<Lithium ion capacitor>
A lithium ion capacitor (LIC) is one type of hybrid capacitor. At the positive electrode of the LIC, electric charges are accumulated in an electric double layer like the EDLC, and at the negative electrode of the LIC, lithium ions are absorbed and released like the lithium secondary battery (LIB) described later. In LIC, the unipolar potential of the negative electrode is low like LIB, and the potential difference with the positive electrode is large. In other words, the voltage between the positive and negative electrodes in the LIC is high. Therefore, a higher voltage can be obtained with the LIC as compared with the EDLC.

Furthermore, although the same positive electrode material as EDLC is used, the capacitance of LIC, which is a kind of hybrid capacitor, is about twice that of EDLC. Due to the higher voltage and about twice the capacitance, the energy capacity that can be stored in LICs is very large compared to the energy capacity that can be stored in EDLCs.

As the positive electrode, activated carbon; graphite; nanocarbon such as carbon nanofiber and graphene are mainly used. A lithium-absorbing material is used as the negative electrode. Examples of lithium-absorbing substances include carbon-based materials, silicon-based materials, composite oxides of transition metals and lithium, and the like. A carbon-based material in which metallic lithium is pre-occluded is preferably used because of its low single-electrode potential.

As the electrolytic solution, a solution obtained by dissolving a lithium salt that does not electrochemically react within the usable potential range of the positive electrode in a polar solvent can be used. Lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), and the like are examples of salts that do not electrochemically react at the working potential of the positive electrode. Examples of polar solvents include carbonic acid esters such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and ethylmethyl carbonate (EMC); various ionic liquids. Each of the above salts and polar solvents may be used alone or in combination of two or more.

<Redox Capacitor>
A redox capacitor is a capacitor in which the electrode (solid phase) side of the electrode-electrolyte interface is charged by an oxidation-reduction reaction. Compared to EDLC, in which the charge on the electrode side depends on polarization, the charge density on the electrode surface is very high, so it has the characteristic of being able to obtain a high capacity. As the electrolytic solution, a solution obtained by dissolving a salt that does not electrochemically react within the working potential range in a polar solvent, or an ionic liquid can be used.

<Lithium secondary battery>
A lithium secondary battery is a secondary battery in which lithium ions move between positive and negative electrodes during charging and discharging. Lithium secondary batteries include lithium ion secondary batteries using a lithium absorbing material as a negative electrode active material and metallic lithium secondary batteries using metallic lithium as a negative electrode active material.

<Negative electrode of lithium ion secondary battery>
A lithium-absorbing material is used as a negative electrode active material for a lithium-ion secondary battery. Examples of lithium-absorbing substances include carbon-based materials, silicon-based materials, composite oxides of transition metals and lithium, and the like.

Carbon-based materials are preferably used because they have a good balance between the amount of lithium that can be absorbed per mass and the resistance to deterioration associated with the absorption and desorption of lithium. Examples of carbonaceous materials include graphite such as natural graphite and artificial graphite; amorphous carbon such as hard carbon, soft carbon and mesoporous carbon; and nanocarbon materials such as carbon nanotubes and graphene.

A silicon-based material is preferably used because it has a large amount of lithium that can be absorbed per mass. Examples of silicon-based materials include silicon, silicon monoxide (SiO), and silicon dioxide (SiO ₂ ).

Lithium titanate, which is one type of composite oxide of transition metal and lithium, has a relatively high single-electrode potential, and even if it comes into direct contact with the fibers used in the separator of the present invention, it is difficult to cause reduction deterioration of the fibers. , is preferably used.

An example of the negative electrode of a lithium ion secondary battery is an electrode in which a negative electrode material containing the negative electrode active material is coated on a metal foil. If necessary, the negative electrode material contains binders such as polyvinylidene fluoride (PVDF) and styrene-butadiene copolymer (SBR); conductive agents such as carbon black and nanocarbon materials; dispersants; Can be mixed. Copper, aluminum, etc. are illustrated as a metal used for metal foil.

<Positive electrode of lithium secondary battery>
Examples of positive electrode active materials for lithium secondary batteries include composite oxides of transition metals and lithium, composite salts of transition metals and lithium having an olivine structure, and sulfur.

Examples of composite oxides of transition metal and lithium include composite oxides of lithium and one or more transition metals selected from cobalt, nickel, and manganese. These composite oxides can be further compounded with typical metals such as aluminum and magnesium; transition metals such as titanium and chromium;

The composite salt having an olivine structure of a transition metal and lithium is exemplified by a composite salt having an olivine structure of at least one transition metal of iron and manganese and lithium.

As a positive electrode of a lithium secondary battery, an electrode obtained by coating a positive electrode material containing the positive electrode active material on a metal foil is exemplified. If necessary, the positive electrode material can be mixed with binders such as polyvinylidene fluoride and acrylic acid ester copolymers; conductive agents such as carbon black and nanocarbon materials; dispersants; thickeners and the like. Aluminum etc. are illustrated as a metal used for metal foil.

<Electrolyte solution for lithium secondary battery>
Examples of electrolyte solutions for lithium secondary batteries include a solution in which a lithium salt is dissolved in a polar solvent and a solution in which a lithium salt is dissolved in an ionic liquid.

Polar solvents used in the electrolyte of lithium secondary batteries include ester carbonates such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); ethyl acetate, propyl acetate, Fatty acid esters such as ethyl propionate are exemplified, and one type or two or more types may be used.

Lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ) are exemplified as lithium salts used in electrolytes of lithium secondary batteries. good too.

<Structure of Electrochemical Device>
An electrochemical device generally has a structure in which a positive electrode, a separator, and a negative electrode, which are members of the device, are laminated in this order. The positive electrode, the negative electrode, and the separator are each absorbed (impregnated) with an electrolytic solution. The types of laminated structures include a cylindrical type in which each member is laminated and then wound into a roll, and a rolled flat type in which two flat surfaces and curved ends are formed by crushing the cylindrical type (flat type) ), a zigzag type in which an electrode cut out into a sheet is inserted between the separators folded into a zigzag, a sheet in which a separator cut out into a sheet and an electrode cut out into a sheet are laminated A laminate type and the like are exemplified.

EXAMPLES The present invention will be specifically described below with reference to Examples, but the present invention is not limited to these Examples. All parts and percentages in the examples are by mass unless otherwise specified. Table 1 shows the slurry used in each example and comparative example, the content of non-fibrillated short fibers, and the content of each fiber contained in the separator.

(Example 1)
As non-fibrillated fibers, drawn polyester fibers with a diameter of 2.3 μm and a fiber length of 3 mm: 500 g of solid content are put into a 2 m ³ pulper (dispersion vessel) together with 1 m ³ of dispersion water, and dispersed for 10 minutes to obtain non-fibrillated fibers. A synthetic fiber slurry 1 was prepared. In the non-fibrillated fiber slurry 1, the content ratio of non-fibrillated short fibers was 1.5%.

Next, a fibrillated solvent-spun cellulose fiber having a diameter of 10.1 μm and a fiber length of 4 mm was fibrillated using a refiner, and the fibrillated solvent-spun cellulose fiber having a modified freeness of 90 ml: 1400 g of solid content and a linter using a high-pressure homogenizer. Fibrillated natural cellulose fibers obtained by fibrillating the above: 100 g of solid content were put into a pulper of 2 m ³ together with 1 m ³ of dispersing water, and dispersed for 3 minutes to prepare a fibrillated fiber slurry 1 . Non-fibrillated fiber slurry 1 and fibrillated fiber slurry 1 were mixed in a chest tank to obtain slurry 1 for papermaking.

Using the papermaking slurry 1, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer having a surface temperature of 130°C, and then calendered with metal rolls and elastic rolls to obtain a separator. .

(Example 2)
The non-fibrillated fiber slurry 1 prepared in the same manner as in Example 1 was filtered through a 18-mesh net to remove the dispersion water while leaving only the fibers, and 1 ^m3 of water was newly added to loosen the fibers. A non-fibrillated fiber slurry 2 was prepared. In the non-fibrillated fiber slurry 2, the content ratio of non-fibrillated short fibers was 0.6%. Next, non-fibrillated fiber slurry 2 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain papermaking slurry 2 .

Using the papermaking slurry 2, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer with a surface temperature of 130° C., and then subjected to calendering with a metal roll and an elastic roll to obtain a separator. .

(Example 3)
The non-fibrillated fiber slurry 2 prepared in the same manner as in Example 2 was again filtered through a 18-mesh net to remove the dispersed water while leaving only the fibers, and 1 ^m3 of water was newly added to loosen the fibers. to prepare a non-fibrillated fiber slurry 3. In the non-fibrillated fiber slurry 3, the content ratio of non-fibrillated short fibers was 0.2%. Next, non-fibrillated fiber slurry 3 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain slurry 3 for papermaking.

Using the papermaking slurry 3, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer with a surface temperature of 130° C., and then calendered with a metal roll and an elastic roll to obtain a separator. .

(Example 4)
As non-fibrillated fibers, drawn polyester fibers with a diameter of 2.3 μm and a fiber length of 3 mm: 497 g of solid content and drawn polyester fibers with a diameter of 2.3 μm and a fiber length of 0.2 mm: 3 g are put into a 2 m ³ pulper. A non-fibrillated fiber slurry 4 was prepared by adding 1 m ³ of dispersion water and dispersing for 5 minutes. In the non-fibrillated fiber slurry 4, the content ratio of non-fibrillated short fibers was 1.9%.

Next, non-fibrillated fiber slurry 4 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain papermaking slurry 4 .

Using the papermaking slurry 4, a wet paper was formed with an inclined wire paper machine, dried under hot pressure with a Yankee dryer with a surface temperature of 130° C., and then calendered with metal rolls and elastic rolls to obtain a separator. .

(Comparative example 1)
The non-fibrillated fiber slurry 3 prepared in the same manner as in Example 3 was filtered through a mesh of 18 mesh to remove the dispersion water while leaving the fibers, and the operation of adding new water was further performed twice. , a non-fibrillated fiber slurry 5 was prepared. In the non-fibrillated fiber slurry 5, the content ratio of non-fibrillated short fibers was 0.0%. Next, non-fibrillated fiber slurry 5 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain papermaking slurry 5 .

Using the papermaking slurry 5, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer having a surface temperature of 130°C, and then calendered with metal rolls and elastic rolls to obtain a separator. .

(Example 5)
As non-fibrillated fibers, rayon fibers with a diameter of 8.2 μm and a fiber length of 4 mm: 300 g of solid content are put into a 2 m ³ pulper together with 1 m ³ of dispersing water, and dispersed for 10 minutes to prepare a non-fibrillated fiber slurry 6. done. In the non-fibrillated fiber slurry 6, the content ratio of non-fibrillated short fibers was 1.2%.

Next, fibrillated solvent-spun cellulose fibers with a diameter of 10.1 μm and a fiber length of 4 mm were fibrillated using a refiner, and fibrillated solvent-spun cellulose fibers with a modified freeness of 90 ml: 1700 g of solid content was added to a pulper of 2 m ³ for 1 m The fibrillated fiber slurry 2 was prepared by adding the dispersion water of ³ and dispersing for 3 minutes. The non-fibrillated fiber slurry 6 and the fibrillated fiber slurry 2 were mixed in a chest tank to obtain a papermaking slurry 6 .

Using the papermaking slurry 6, a wet paper was formed with an inclined wire paper machine, and then dried under hot pressure with a Yankee dryer having a surface temperature of 130° C. to obtain a separator.

(Example 6)
The non-fibrillated fiber slurry 6 prepared in the same manner as in Example 5 was filtered through a mesh of 18 mesh to remove the dispersion water leaving only the fibers, and 1 ^m3 of water was newly added to loosen the fibers. A non-fibrillated fiber slurry 7 was prepared. In the non-fibrillated fiber slurry 7, the content ratio of non-fibrillated short fibers was 0.4%. Next, non-fibrillated fiber slurry 7 and fibrillated fiber slurry 2 prepared in the same manner as in Example 5 were mixed in a chest tank to obtain papermaking slurry 7 .

Using the papermaking slurry 7, a wet paper was formed with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 130° C. to obtain a separator.

(Example 7)
The non-fibrillated fiber slurry 7 prepared in the same manner as in Example 6 was filtered through a 18-mesh net to remove the dispersed water leaving only the fibers, and 1 ^m3 of water was newly added to loosen the fibers. A non-fibrillated fiber slurry 8 was prepared. In the non-fibrillated fiber slurry 8, the content ratio of non-fibrillated short fibers was 0.1%. Next, non-fibrillated fiber slurry 8 and fibrillated fiber slurry 2 prepared in the same manner as in Example 5 were mixed in a chest tank to obtain papermaking slurry 8 .

Using the papermaking slurry 8, a wet paper was formed with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 130° C. to obtain a separator.

(Comparative example 2)
The non-fibrillated fiber slurry 8 prepared in the same manner as in Example 7 was filtered through a mesh of 18 mesh to remove the dispersed water while leaving the fibers, and the operation of adding new water was further performed twice. , a non-fibrillated fiber slurry 9 was prepared. In non-fibrillated fiber slurry 9, the content ratio of non-fibrillated short fibers was 0.0%. Next, non-fibrillated fiber slurry 9 and fibrillated fiber slurry 2 prepared in the same manner as in Example 5 were mixed in a chest tank to obtain papermaking slurry 9 .

Using the papermaking slurry 9, a wet paper was formed with an inclined wire paper machine, and then hot-pressure dried with a Yankee dryer having a surface temperature of 130° C. to obtain a separator.

(Example 8)
As non-fibrillated fibers, acrylic fibers with a diameter of 2.9 μm and a fiber length of 3 mm: 500 g of solid content are put into a 2 m ³ pulper together with 1 m ³ of dispersing water, and dispersed for 10 minutes to prepare a non-fibrillated fiber slurry 10. done. In the non-fibrillated fiber slurry 10, the content ratio of non-fibrillated short fibers was 1.6%.

Next, non-fibrillated fiber slurry 10 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain papermaking slurry 10 .

Using the papermaking slurry 10, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer with a surface temperature of 130° C., and then calendered with a metal roll and an elastic roll to obtain a separator. .

(Example 9)
A non-fibrillated fiber slurry 10 prepared in the same manner as in Example 8 was filtered through a mesh of 18 mesh to remove the dispersed water leaving only the fibers, and 1 ^m3 of water was newly added to loosen the fibers. A non-fibrillated fiber slurry 11 was prepared. In the non-fibrillated fiber slurry 11, the content ratio of non-fibrillated short fibers was 0.5%. Next, non-fibrillated fiber slurry 11 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain papermaking slurry 11 .

Using the papermaking slurry 11, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer with a surface temperature of 130° C., and then subjected to calendering with metal rolls and elastic rolls to obtain a separator. .

(Example 10)
The non-fibrillated fiber slurry 11 prepared in the same manner as in Example 9 was again filtered through a 18-mesh net to remove the dispersion water leaving only the fibers, and 1 ^m3 of water was newly added to loosen the fibers. to prepare a non-fibrillated fiber slurry 12. In the non-fibrillated fiber slurry 12, the content ratio of non-fibrillated short fibers was 0.2%. Next, the non-fibrillated fiber slurry 12 and the fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain a papermaking slurry 12 .

Using the papermaking slurry 12, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer with a surface temperature of 130° C., and then calendered with metal rolls and elastic rolls to obtain a separator. .

(Comparative Example 3)
The non-fibrillated fiber slurry 12 prepared in the same manner as in Example 10 was filtered through a mesh of 18 mesh to remove the dispersed water while leaving the fibers, and the operation of adding new water was further performed twice. , a non-fibrillated fiber slurry 13 was prepared. In the non-fibrillated fiber slurry 13, the content ratio of non-fibrillated short fibers was 0.0%. Next, non-fibrillated fiber slurry 13 and fibrillated fiber slurry 1 prepared in the same manner as in Example 1 were mixed in a chest tank to obtain papermaking slurry 13 .

Using the papermaking slurry 13, a wet paper was formed with an inclined wire paper machine, dried under heat and pressure with a Yankee dryer with a surface temperature of 130° C., and then calendered with metal rolls and elastic rolls to obtain a separator. .

The basis weight and thickness of the separators obtained in Examples and Comparative Examples were measured, and pinhole evaluation was performed. Table 2 shows the results.

<Basis weight>
The basis weight of the substrate was measured according to JIS P8124:2011.

<Thickness>
Using an outer micrometer specified in JIS B7502:2016, the thickness was measured under a 5N load.

<Pinhole evaluation>
A 100 mm×100 mm area of the separator was scanned with a reflective flatbed scanner with a resolution of 2400 dpi and 256 gradations, and the scanned image was taken. Next, the peak luminance P in the histogram of the scanned image and the standard deviation of the luminance distribution were obtained. Next, the scanned image was binarized using the luminance P-6σ as a boundary value, and the number of black dots having 100 or more pixels was counted and evaluated according to the following criteria. Note that ImageJ, which is public domain software distributed by the National Institutes of Health, was used to perform operations after acquisition of the histogram of the scanned image.

A: The number of black dots with 100 or more pixels is 0. very good level.
B: The number of black dots with 100 or more pixels is 1 or more and 5 or less. good level.
C: The number of black dots with 100 or more pixels is more than 5 and 10 or less. available level.
D: More than 10 black dots with 100 or more pixels. Disabled level.

As shown in Table 2, the electrochemical device separators of Examples 1 to 10 contain fibrillated solvent-spun cellulose fibers and non-fibrillated fibers, and contain non-fibrillated fibers having a fiber length of 0.2 mm or less. showed good results in pinhole evaluation.

On the other hand, the separators of Comparative Examples 1 to 3, which did not contain non-fibrillated short fibers having a fiber length of 0.2 m or less, gave unusable results in pinhole evaluation.

The electrochemical device separator of the present invention can be used in electrochemical devices such as electric double layer capacitors, hybrid capacitors, redox capacitors, and lithium secondary batteries.

Claims (1)

In the electrochemical device separator containing fibrillated solvent-spun cellulose fibers and non-fibrillated fibers , the non-fibrillated fibers having a fiber length of 0.2 mm or less are contained, and the fiber length is The content ratio of non-fibrillated fibers with a fiber length of 0.2 mm or less, represented by the ratio of the total length of the non-fibrillated fibers of 0.2 mm or less, is 0.1% or more and less than 2.0%. A separator for an electrochemical device characterized by: