Electrochemical device separator and electric device

Abstract

To provide an electrochemical device separator that is mainly composed of beaten Lyocell fibers, is difficult to cause short circuits caused by coating layers that fall off from electrodes, and is thin and compact, and also provide an electrochemical device that is small and less prone to short-circuit failures.SOLUTION: An electrochemical device separator with a thickness of more than 15 μm and less than 40 μm and containing beaten Lyocell fiber as a main component, contains titanium oxide inside Lyocell fibers. An electrochemical device includes an electrode including a conductive coating layer. An electrochemical device includes the electrochemical device separator.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrochemical device separator and an electric device. [Background technology]

[0002] In various electrochemical devices such as electrolytic capacitors, electric double layer capacitors, nickel-metal hydride batteries, and lithium-ion batteries, a separator is placed between the positive and negative electrodes to prevent electrical contact between the electrodes. The separator has pores that allow ions to pass between the front and back.

[0003] Among these electrochemical devices, there are electrochemical devices whose electrodes include a conductive coating layer. Specifically, electrodes are used in which a conductive coating layer is provided on the surface of a current collector such as a metal foil. For example, lithium-ion batteries use positive electrodes formed by coating the surface of aluminum foil with a mixture containing an active material (a composite oxide of a transition metal and lithium), a conductive additive (e.g., carbon black), and a binder (e.g., poly(meth)acrylic acid ester or polyvinylidene fluoride). Negative electrodes are used in which the surface of copper foil is coated with a mixture containing an active material (e.g., natural graphite, artificial graphite, or hard carbon), a conductive additive (e.g., carbon black), and a binder (e.g., styrene-butadiene copolymer, poly(meth)acrylic acid ester, or polyvinylidene fluoride). Electric double-layer capacitors use electrodes formed by coating the surface of aluminum foil with a mixture containing an active material (e.g., activated carbon), a conductive additive (e.g., carbon black), and a binder (e.g., poly(meth)acrylic acid ester or polyvinylidene fluoride). These coating layers are generally electrically conductive so that the entire coating layer functions effectively.

[0004] The coating layer may partially fall off from such electrodes. If the size of the fallen coating layer piece (the length in the thickness direction of the separator) is larger than the thickness of the separator, the coating layer piece may cause electrical conduction between the front and back of the separator, resulting in a short circuit in the electrochemical device. However, the mechanical strength of electrode coating layers is generally low, and coating layer pieces that are too large often cannot maintain their shape and break off into smaller pieces.

[0005] In recent years, there has been a strong demand for miniaturization of electronic devices, and the electrochemical devices used in these devices are also required to be miniaturized. In electrochemical devices consisting of multiple stacked layers, such as lithium-ion batteries and electric double-layer capacitors, the thickness of the separator has a significant impact on the size of the element, so there is a demand for thinner separators. However, when the separator is made thinner, smaller coating layer pieces can cause electrical conduction between the front and back of the separator, making the electrochemical device more susceptible to short-circuiting. To address this issue, separators made primarily of beaten lyocell fibers, which are denser than conventional separators, have been proposed as an alternative to conventional separators made primarily of pulp such as bagasse or esparto (see, for example, Patent Documents 1 and 2). However, with conventional separators made primarily of beaten lyocell fibers, it has been difficult to prevent short-circuiting caused by coating layer pieces detached from the electrodes when the separator is thin. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-123279 [Patent Document 2] Japanese Patent Application Publication No. 2018-82132 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide an electrochemical device separator that is primarily composed of beaten lyocell fiber, that is thin and less likely to cause short circuits due to a coating layer that has fallen off an electrode, and a small electrochemical device that is less likely to cause short circuit defects. [Means for solving the problem]

[0008] The above problems can be solved by the following means.

[0009] (1) An electrochemical device separator having a thickness of more than 15 μm and less than 40 μm, the main component of which is beaten lyocell fiber, and characterized in that titanium oxide is contained inside the lyocell fiber.

[0010] (2) An electrochemical device in which an electrode includes a conductive coating layer, the electrochemical device including the electrochemical device separator of (1) above. [Effects of the Invention]

[0011] According to the present invention, a thin electrochemical device separator can be obtained that is less likely to cause short circuits caused by the peeling off of a coating layer contained in an electrode, and thus a small electrochemical device that is less likely to cause short circuits can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention provides an electrochemical device separator having a thickness of more than 15 μm and less than 40 μm, which is primarily composed of beaten lyocell fiber and contains titanium oxide inside the lyocell fiber. By containing titanium oxide inside the lyocell fiber, even a thin separator having a thickness of less than 40 μm can be made less susceptible to short circuits caused by the detachment of a coating layer contained in an electrode. Furthermore, the use of this separator has the advantageous effect of making it possible to obtain electrochemical devices that are less susceptible to short circuit defects, even when the electrochemical device contains a conductive coating layer on the electrode.

[0013] The thickness of the separator of the present invention is greater than 15 μm and less than 40 μm. Separators with a thickness of 40 μm or greater are less likely to develop short circuits caused by coating layers detached from electrodes, even when conventional technology is used, and there is little motivation to improve this. Separators with a thickness of 15 μm or less are less likely to develop short circuits caused by coating layers detached from electrodes, even when the lyocell fibers contain titanium oxide. From the perspective of reliably preventing short circuits caused by coating layers detached from electrodes and miniaturizing electrochemical devices, the thickness of the separator of the present invention is more preferably 20 μm to 35 μm, and even more preferably 20 μm to 30 μm. The thickness of the separator in the present invention is the thickness per sheet measured using an outside micrometer by folding a sample into 10 sheets according to the method specified in JIS C2300-2. The reason why the inclusion of titanium oxide inside the lyocell fibers reduces the risk of short circuits caused by detachment from electrodes has not been elucidated, but the inventors speculate as follows. In other words, the inclusion of titanium oxide in lyocell fiber reduces freeness, i.e., accelerates fibrillation of the lyocell fiber, and it is thought that the fibrillation process during beating differs between lyocell fiber containing titanium oxide internally and lyocell fiber without titanium oxide internally. It is presumed that this is because the microstructure of the fiber, such as the morphology of the fibrils, of lyocell fiber containing titanium oxide internally is more suitable for preventing the coating layer that has fallen off the electrode from penetrating through to the front and back of the separator, compared to lyocell fiber without titanium oxide internally.

[0014] In the present invention, the titanium oxide contained within the lyocell fiber is titanium(IV) oxide represented by the composition formula TiO2. From the viewpoint of achieving the effects of the present invention, its crystalline form may be either anatase or rutile. However, because anatase titanium oxide has photocatalytic activity and may easily cause photodegradation of the separator, rutile is preferred. The particle size of the titanium oxide is preferably 0.1 to 0.5 μm from the viewpoint of availability, and can be used without problems from the viewpoint of achieving the effects of the present invention. The titanium oxide content relative to the lyocell fiber is preferably 0.1 to 1 mass%, more preferably 0.2 to 0.8 mass%, because too little titanium oxide may not achieve the effects of the present invention, while too much may reduce the strength of the separator. In the present invention, the particle size of the titanium oxide is the geometric mean of the longest and shortest sides of the rectangle circumscribing the titanium oxide particle, as measured by observing the particle with a scanning electron microscope. When multiple primary particles come into contact with each other to form aggregated particles, the aggregated particles are considered to be a single particle for measurement. In the present invention, the particle size of titanium oxide is taken as the median value of 11 randomly selected particles, which is the representative value. Lyocell fibers containing titanium oxide therein are produced by, for example, mixing titanium oxide with a solution in which cellulose is dissolved in a mixed solvent of water and an amine oxide such as N-methylmorpholine oxide, and then ejecting the cellulose solution from a nozzle into water.

[0015] The separator of the present invention is preferably used in electrochemical devices whose electrodes include a coating layer. Specifically, an electrode formed by providing a coating layer on the surface of a current collector such as a metal foil is used. For example, electrodes for lithium ion batteries include positive electrodes formed by coating the surface of aluminum foil with a mixture containing an active material (a composite oxide of a transition metal and lithium), a conductive additive (e.g., carbon black), and a binder (e.g., poly(meth)acrylic acid ester or polyvinylidene fluoride). Negative electrodes are also used, which are formed by coating the surface of copper foil with a mixture containing an active material (e.g., natural graphite, artificial graphite, or hard carbon), a conductive additive (e.g., carbon black), and a binder (e.g., styrene-butadiene copolymer, poly(meth)acrylic acid ester, or polyvinylidene fluoride). Furthermore, electrodes for electric double layer capacitors include electrodes formed by coating the surface of aluminum foil with a mixture containing an active material (e.g., activated carbon), a conductive additive (e.g., carbon black), and a binder (e.g., poly(meth)acrylic acid ester or polyvinylidene fluoride). In particular, electric double layer capacitors are generally designed with loose bonds between the particles that make up the coating layer in order to obtain a large specific surface area, and the mechanical bonding strength between the constituent particles in the coating layer is low, making it easy for small pieces of the coating layer to fall off from the electrodes, which can easily cause short circuits, and therefore there is ample motivation for adopting the separator of the present invention.

[0016] In electrochemical devices that do not have a coating layer on the electrodes, such as electrolytic capacitors that use aluminum foil with an electrolytically oxidized surface as electrodes, and solid electrolytic capacitors that use a conductive polymer as the electrolyte, there is little motivation to use the separator of the present invention, since short circuits caused by a coating layer that falls off from the formed electrodes do not occur in the first place. However, there is no problem if the separator of the present invention is used in electrochemical devices that do not have a coating layer on the electrodes.

[0017] The separator of the present invention is a nonwoven fabric primarily composed of beaten lyocell fibers. Specifically, the separator contains beaten lyocell fibers in an amount of preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more, based on the total mass of the separator. The separator of the present invention may consist solely of beaten lyocell fibers, or may contain wood pulp such as kraft pulp or sulfite pulp, or non-wood pulp such as abaca, rice straw, esparto, kenaf, cotton, or bagasse. The separator may also contain one or more synthetic fibers such as acrylic fibers, aramid fibers, nylon fibers, polyolefin fibers such as polyethylene and polypropylene, polyester fibers such as polyethylene terephthalate and polybutylene terephthalate, and polyphenylene sulfide fibers, or recycled fibers such as viscose rayon and cuprammonium rayon. It is also possible to mix various inorganic particles such as alumina, kaolin, titanium oxide, silica, magnesium hydroxide, talc, calcium carbonate, and boehmite, and various organic particles such as (meth)acrylic acid ester copolymers and melamine formalin resins.

[0018] The separator of the present invention is primarily formed into a sheet by a wet papermaking method. More specifically, using a papermaking machine, beaten lyocell fibers and the other materials described above are mixed as needed, and the mixture is diluted with water to prepare a papermaking slurry. The mixture is spread on a wire, dewatered by suction from the opposite side of the wire or by gravity, and then dried and removed using a dryer such as an air dryer, cylinder dryer, or Yankee dryer to form a sheet. The sheeted product is optionally treated using a calendar. In the present invention, the type of papermaking machine used to form a sheet from beaten lyocell fibers by the wet papermaking method may be a Fourdrinier papermaking machine, a short wire papermaking machine, an inclined wire papermaking machine, a cylinder papermaking machine, or the like. Among these papermaking machines, inclined wire papermaking machines and cylinder papermaking machines are preferred because they can use a dispersion with a low solids concentration and therefore can produce a sheet with excellent uniformity. [Example]

[0019] The present invention will be described in more detail below with reference to examples, but is not limited to these examples. In these examples, unless otherwise specified, % and kg refer to mass % and mass of the solid content, respectively, and concentration refers to the concentration of lyocell relative to the dispersion.

[0020] Formulation 1 Water was added to titanium dioxide-containing lyocell fiber (1.7 dtex, 4 mm length) to a total weight of 10 kg, and the fiber was dispersed using a pulper to obtain a 4.5% lyocell fiber dispersion. This dispersion was passed through a single-disc refiner with a 1.0 mm gap between the rotating and fixed blades and a rotation speed of 3,000 rpm, where it was beaten while circulating. Beating was terminated when the modified Canadian Standard Freeness (ModCSD) measured as described below reached 120 ml.

[0021] Determination of modified Canadian standard freeness. The freeness (ml) of the lyocell fiber dispersion was measured according to JIS P8121-2:2012 (Canadian Standard Freeness Method) using an 80-mesh wire mesh with a wire diameter of 0.14 mm and openings of 0.18 mm as the sieve plate, with the exception that the sample concentration was 0.1%. The freeness of lyocell fiber measured by this method (modified Canadian Standard Freeness) is a good indicator of the degree of beating of the lyocell fiber, and the lower the freeness, the more the beating of the lyocell fiber has progressed.

[0022] Preparation of separator A 0.01% papermaking slurry consisting of 70% of the beaten lyocell fiber, 25% of polyethylene terephthalate fiber with a fineness of 0.1 dtex and an average fiber length of 3 mm, and 5% of beaten softwood kraft pulp (Canadian standard freeness 0 ml) was made into paper using an inclined screen papermaking machine, and the thickness was adjusted by calendering as necessary to obtain separators with different thicknesses and basis weights (conditions 1 to 8).

[0023] Formulation 2 Separators with different thicknesses and basis weights were manufactured in the same manner as in Blend 1, except that Lyocell fibers not containing titanium oxide inside (fineness 1.7 dtex, fiber length 4 mm) were used.

[0024] Formulation 3 Separators with different thicknesses and basis weights were manufactured in the same manner as in Blend 2, except that the composition of the papermaking slurry was 67% beaten lyocell fiber, 25% polyethylene terephthalate short fiber with a cut length of 3 mm and a fineness of 0.1 dtex, 5% beaten softwood kraft pulp, and 3% titanium oxide.

[0025] Evaluation of short circuit susceptibility A separator obtained under each formulation and condition was placed on aluminum foil (lower), and a strip of electrode coating layer, which had been removed by bending an electrode from an electric double layer capacitor, was placed on the top surface of the separator, with aluminum foil (upper) placed on top of that. A direct current of 10 V was applied between the aluminum foil (lower) and aluminum foil (upper). When pressure was applied with a finger to the area where the strip of electrode coating layer was placed and its surroundings, a short circuit was deemed to have occurred if a current of 10 μA or more flowed, even for just a moment. The test was performed 10 times, and the number of times a short circuit occurred was recorded.

[0026] Table 1 shows the evaluation results of the basis weight, thickness, and susceptibility to short-circuiting of the separators produced under each formulation and condition.

[0027] [Table 1]

[0028] As can be seen from Table 1, in formulation 1, in which titanium oxide is contained within the lyocell fiber, no short circuits occurred even under conditions 2 to 6 (within the present invention) where the separator thickness was thin, exceeding 15 μm and less than 40 μm. In contrast, in formulations 2 and 3 (outside the present invention), in which titanium oxide is not contained within the lyocell fiber, short circuits occurred under conditions 2 to 6, with the frequency of short circuits being particularly high under conditions 2 and 3, where the thickness was thin. In addition, in conditions 7 and 8 (outside the present invention), in which the separator thickness was 40 μm or greater, no short circuits occurred not only in formulation 1, in which titanium oxide is contained within the lyocell fiber, but also in formulations 2 and 3, in which titanium oxide is not contained within the lyocell fiber. However, using such thick separators makes it difficult to miniaturize electrochemical devices. In contrast, in condition 1 (outside the present invention), in which the separator thickness was 15 μm or less, short circuits occurred not only in formulations 2 and 3, in which titanium oxide is not contained within the lyocell fiber, but also in formulation 1, in which titanium oxide is contained within the lyocell fiber. [Industrial Applicability]

[0029] The electrochemical device separator of the present invention can be preferably used as a separator for electrochemical devices, particularly electric double layer capacitors and lithium ion batteries, and can provide small electrochemical devices that are less likely to short-circuit.

Claims

1. An electrochemical device separator having a thickness of more than 15 μm and less than 40 μm, the electrochemical device separator being composed primarily of beaten lyocell fibers, and characterized in that titanium oxide is contained inside the lyocell fibers.

2. 10. An electrochemical device comprising the electrochemical device separator of claim 1, wherein the electrodes comprise a conductive coating layer.

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