JPWO2013187458A1 - Lithium ion battery separator - Google Patents

Abstract

In a lithium ion battery separator comprising a porous body mainly composed of at least inorganic particles, the shape of the inorganic particles is indefinite, and low internal resistance, pinholes and powder fall off are unlikely to occur. A separator for lithium-ion batteries that has low leakage current.

Description

  The present invention relates to a separator for a lithium ion battery (hereinafter sometimes abbreviated as “separator”).

  A lithium ion battery, which is one of electrochemical elements, is a secondary battery having a feature of high energy density. It is widely used as a power source for portable electric devices such as mobile phones, portable music players, and notebook personal computers. In addition, the use of lithium ion batteries is spreading in large equipment such as electric bicycles, hybrid cars, and electric cars. Therefore, lithium ion batteries are required to have high performance and charge / discharge characteristics at a large current. However, since a lithium ion battery is a non-aqueous battery, it is known that there is a higher risk of smoke, ignition, rupture and the like than an aqueous battery, and an improvement in safety is also required.

  In a lithium ion battery, the risk of fuming increases due to temperature rise due to external heat, overcharge, internal short circuit, external short circuit, and the like. These can be prevented to some extent by an external protection circuit. In addition, the polyolefin resin porous film used as a separator for lithium ion batteries melts at around 120 ° C, and the pores are blocked to block current and ion flow, thereby suppressing battery temperature rise. The This is called a shutdown function. However, when the temperature rises due to external heat or when a chemical reaction occurs inside the battery due to the temperature rise, the battery temperature further rises even if the shutdown function is activated. When the battery temperature reaches 150 ° C. or higher, the porous film contracts, an internal short circuit occurs, and ignition may occur.

  As described above, it is difficult to suppress the ignition of the battery by the shutdown function of the separator. In addition, with the increase in capacity of batteries, the increase in current during charging and discharging is also progressing, and in order to suppress the Joule heat generated at that time, it is also necessary to lower the resistance value of the separator impregnated with the electrolyte. It has become. Therefore, metal oxide particles were used for the purpose of suppressing the ignition of the battery by increasing the heat shrinkage temperature more than the polyolefin resin porous film, thereby preventing the internal short circuit and lowering the resistance value. Separator has been developed. In this separator, the pore diameter is controlled by the metal oxide particles, and internal short circuit can be suppressed, heat resistance can be improved, and resistance value can be reduced.

  For example, a separator having a microporous pseudoboehmite layer obtained as a porous film by mixing pseudoboehmite that is inorganic particles and a binder, drying and peeling after coating on a separately prepared film has been proposed. (For example, refer to Patent Document 1). However, although this separator has an improved heat shrinkage temperature, powder falling and cracking are likely to occur, so that it is difficult to take out as a rolled-up separator alone, and handling properties at the time of battery production are poor. there were.

  In addition, it has a porous inorganic coating on the nonwoven fabric and in the nonwoven fabric, and the inorganic coating has oxide particles of aluminum (Al), silicon (Si) and / or zirconium (Zr) which are inorganic particles. A separator has been proposed (see, for example, Patent Documents 2 and 3). Since this separator uses a non-woven fabric as a base material, handling properties are improved. However, because the silica particles produced by the sol-gel method keeps track of other inorganic particles, powder and cracks are likely to occur due to impact and deformation, which leads to the generation of pinholes, resulting in minute internal short circuits. This was a cause of leakage current, and it was difficult to be a useful separator.

  A separator provided with a porous body containing inorganic particles and an organic binder on and in a nonwoven fabric has also been proposed (see, for example, Patent Documents 4 and 5). Patent Document 4 proposes tabular boehmite particles as inorganic particles. However, when tabular boehmite particles are used, the effect of suppressing leakage current is improved, but the tabular inorganic particles block the pores of the separator, thereby preventing the passage of lithium ions and increasing the internal resistance. was there.

  Patent Document 5 proposes boehmite particles having a secondary particle structure in which primary particles are continuous as inorganic particles. In a lithium ion battery, in order to store more energy in a smaller battery volume, it is preferable to make a member such as a separator that does not directly contribute to power generation as thin as possible. However, in a separator in which such inorganic particles are coated on a substrate, pinholes may occur depending on the dispersion state of the inorganic particles and the coating method. Insulation between the electrode and the negative electrode material could not be sufficiently maintained, and the leakage current sometimes increased.

  Moreover, in the separator which provided the porous body which contains an inorganic particle and an organic binder on a nonwoven fabric and in a nonwoven fabric, the provision amount (henceforth "the coating amount" may be abbreviated as a "coating amount") of a porous body. When the amount is small, there is a problem that a battery having a small leakage current cannot be obtained. When the amount of coating is large, there is a problem that a separator having a small thickness and a low internal resistance cannot be obtained. In order to solve this problem, a separator formed by laminating two coating layers having different pore diameters has been proposed (see, for example, Patent Document 6), which has both high leakage current and high internal resistance. It wasn't.

JP-T-2001-527274 JP 2005-536658 Gazette Special table 2009-507353 JP 2007-157723 A International Publication No. 2008/114727 Pamphlet JP 2006-507635 A

  An object of the present invention is a lithium ion battery separator that contains at least inorganic particles and has high internal resistance, low internal resistance, resistance to pinholes and powder falling, and low leakage current. The object is to provide a battery separator.

  As a result of intensive studies to solve the above problems, the present inventors have found the following invention.

(1) A lithium ion battery separator comprising a porous body mainly composed of inorganic particles, wherein the inorganic particles have an irregular shape.
(2) The separator for lithium ion batteries according to (1), wherein the inorganic particles have a concave shape.

(3) The separator for lithium ion batteries according to (1) or (2), wherein the inorganic particles are alumina hydrate.

(4) In a lithium ion battery separator comprising a porous body mainly composed of at least inorganic particles, the inorganic particles have a 20 mass% aqueous dispersion having a pH of 7.0 or more and 8.3 or less, A separator for a lithium ion battery, wherein the aqueous dispersion is an alumina hydrate having a viscosity of 50 mPa · s to 2000 mPa · s.

(5) The lithium ion battery separator according to any one of (1) to (4), comprising a nonwoven fabric substrate.
(6) The separator for a lithium ion battery according to (5), wherein the fibers of the nonwoven fabric substrate are exposed on at least one surface.

(7) The first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less on the nonwoven fabric substrate and having an aggregated structure, and the shape is irregular And a second porous body mainly composed of inorganic particles having a dent shape, a dispersed particle diameter of less than 1.0 μm, and having no agglomerated structure. A separator for a lithium ion battery, wherein one side of the substrate is substantially covered with a second porous body, and fibers of the nonwoven fabric substrate are exposed on the opposite side.

  In the present invention, a lithium ion battery separator having high heat resistance, which contains at least inorganic particles, has a low internal resistance, a resistance to pinholes and powder falling off, and a low leakage current. A separator can be provided.

  In a lithium ion battery separator comprising a porous body mainly composed of at least inorganic particles, the lithium ion battery separator is characterized in that the shape of the inorganic particles is indefinite. The effect of being less can be achieved.

  In addition, since the inorganic particles have a concave shape, even when the inorganic particles are filled with the inorganic particles, voids are easily formed by the recesses, thereby achieving the effects of low leakage current and low internal resistance. Can do.

  When the inorganic particles are alumina hydrate, it is possible to achieve the effect that the heat resistance of the separator is increased and the life of the battery using the separator is also increased.

  In a lithium ion battery separator comprising a porous body mainly composed of at least inorganic particles, the inorganic particles have a pH of 7.0 to 8.3 in a 20% by mass aqueous dispersion, and the aqueous dispersion The lithium ion battery separator is characterized in that the product is an alumina hydrate having a viscosity of 50 mPa · s or more and 2000 mPa · s or less, thereby achieving an effect that there are few pinholes.

  In addition, in a lithium ion battery separator that contains at least a porous body mainly composed of inorganic particles, by further including a non-woven fabric base material, the handling property is excellent, the performance is uniform, and the tensile strength is high. The effect that a separator is obtained can be achieved.

  Moreover, the effect that the leakage current is small can be achieved by exposing the fibers of the nonwoven fabric substrate on at least one side of the separator containing the nonwoven fabric substrate.

  Further, in the nonwoven fabric base material, the dispersed particle diameter is 1.0 μm or more and 3.0 μm or less, the first porous body mainly composed of inorganic particles having an aggregated structure, and the shape is irregular, A second porous body mainly composed of inorganic particles having a dent shape, a dispersed particle size of less than 1.0 μm, and having no aggregate structure is laminated in this order, The lithium ion battery separator is characterized in that one surface is substantially covered with the second porous body, and the fibers of the nonwoven fabric substrate are exposed on the opposite surface. The effect of being thin and having low internal resistance can be achieved.

It is a scanning electron microscope (SEM) photograph of the inorganic particle whose shape is indefinite. It is a SEM photograph of the inorganic particle which has a rhombus column shape. It is a SEM photograph of inorganic particles which have a cube shape. It is a SEM photograph of the inorganic particle which has a shape with a dent. It is a SEM photograph of the inorganic particle which has flat form. It is a SEM photograph of inorganic particles having a columnar shape. It is a conceptual diagram which shows the cross-sectional structure of the separator for lithium ion batteries containing the nonwoven fabric base material and the porous body which has an inorganic particle as a main component. It is a conceptual diagram which shows the cross-sectional structure of the separator for lithium ion batteries containing the nonwoven fabric base material and the porous body which has an inorganic particle as a main component. It is a conceptual diagram which shows the cross-sectional structure of the separator for lithium ion batteries containing the nonwoven fabric base material and the porous body which has an inorganic particle as a main component. It is a conceptual diagram which shows the cross-sectional structure of the separator for lithium ion batteries containing the nonwoven fabric base material and the porous body which has an inorganic particle as a main component. It is a conceptual diagram which shows the cross-sectional structure of the separator for lithium ion batteries containing the nonwoven fabric base material and the porous body which has an inorganic particle as a main component.

  The separator of the present invention is a lithium ion battery separator containing a porous body mainly composed of at least inorganic particles. The porous body is a collection of at least a large number of inorganic particles. Examples thereof include a porous body in which only a large number of inorganic particles are aggregated, a porous body in which a large number of inorganic particles are aggregated together with at least one selected from an inorganic binder, an organic binder, and the like. In the present invention, “a porous body mainly composed of at least inorganic particles” means “a porous body in which 70% by volume or more of the portion other than the voids is composed of inorganic particles”.

  The separator (1) of the present invention is characterized in that the shape of the inorganic particles is indefinite. FIG. 1 is an SEM photograph of inorganic particles having an irregular shape. 2 and 3 are SEM photographs of regular inorganic particles. FIG. 2 shows diamond-shaped inorganic particles, and FIG. 3 shows cube-shaped inorganic particles. Although FIG. 1 shows tabular grains, the shape of each grain is not uniform and is indefinite. Due to the irregular shape of the inorganic particles, the inorganic particles are in a random state in the porous body, but are closely packed and have a complex arrangement, thus preventing powder falling and suppressing pinholes. The And a low leakage current can be realized.

  There is no particular limitation on the method for producing the inorganic particles having an irregular shape. For example, a method of forming an amorphous shape by manipulating the growth conditions at the stage of growing crystals of inorganic particles and a method of crushing and forming inorganic particles can be mentioned.

  Like the separator (2) of this invention, it is preferable that the inorganic particle whose shape is indefinite has a shape with a dent. FIG. 4 is an SEM photograph of inorganic particles having a concave shape. 2, 3, 5, and 6 are SEM photographs of inorganic particles having a shape without a dent. FIG. 5 shows irregular tabular inorganic particles. FIG. 6 shows regular cylindrical inorganic particles. FIG. 2 shows regular diamond-shaped columnar inorganic particles. FIG. 3 shows regular cube-shaped inorganic particles. FIG. 4 is an irregular tabular inorganic particle similar to FIG. 5, but has a recess at a part of its outer edge (white arrow portion). Due to the presence of the dent, voids are formed even when the inorganic particles are densely packed in the porous body. By densely filling, leakage current can be suppressed, and the internal resistance can be lowered by forming a gap by a dent.

  There is no particular limitation on the method for forming the recesses of the inorganic particles. For example, a method of forming a dent by manipulating growth conditions at the stage of growing crystals of inorganic particles, a method of crushing and forming inorganic particles, and the like can be mentioned.

  In the present invention, inorganic particles include alumina such as α-alumina, β-alumina and γ-alumina; alumina hydrate such as boehmite; oxide of alkaline earth metal such as magnesium oxide and calcium oxide; silica; Examples thereof include alkaline earth metal carbonates such as calcium and magnesium carbonate; complex oxides such as aluminum silicate. In particular, alumina or alumina hydrate is preferably used from the viewpoint of stability. Moreover, as for the separator (3) of this invention, it is more preferable that an inorganic particle is an alumina hydrate. Examples of the alumina hydrate include various crystal types such as gibbsite type, boehmite type, pseudoboehmite type, bayerite type, and diaspore type. In the present invention, synthetic boehmite is preferable from the viewpoint of obtaining a battery having high heat resistance and good cycle life.

  In the separator (4) of the present invention, the inorganic particles have a 20 mass% aqueous dispersion having a pH of 7.0 or more and 8.3 or less, and the viscosity of the aqueous dispersion is 50 mPa · s or more and 2000 mPa · s or less. It is characterized by being an alumina hydrate. The viscosity of the aqueous dispersion is more preferably 100 mPa · s or more and 500 mPa · s or less.

  The pH of the 20% by mass aqueous dispersion of inorganic particles was adjusted to a glass electrode pH at 25 ° C. in an aqueous dispersion of alumina hydrate particles prepared to 20% by mass using ion exchange water having a conductivity of 0.5 μS / cm or less. PH measured by a meter.

  According to JIS-Z8803, the viscosity of the 20 mass% aqueous dispersion of inorganic particles is 20 mass using ion-exchanged water having a conductivity of 0.5 μS / cm or less using a Brookfield viscometer (B-type viscometer). % Represents a measured value at 25 ° C. in an aqueous dispersion of alumina hydrate particles prepared in%.

  In the separator (4) of the present invention, examples of the alumina hydrate include various crystal types such as gibbsite type, boehmite type, pseudoboehmite type, bayerite type, and diaspore type. In the present invention, synthetic boehmite is preferable from the viewpoint of obtaining a battery having high heat resistance and good cycle life. Moreover, there is no restriction | limiting in particular about the shape of the particle | grains in an alumina hydrate, Granules, such as substantially spherical shape, rugby ball shape, and cube shape, scale shape, needle shape, plate shape, etc. may be sufficient. Moreover, the primary particles can be aggregated into secondary particles or non-aggregated particles can be used. However, the shape of the particles is preferably indeterminate, and more preferably has a concave shape.

  In the present invention, a binder may be contained in the porous body for the purpose of adhering inorganic particles. The binder is not particularly limited as long as it is electrochemically stable and stable with respect to the nonaqueous electrolytic solution, and an inorganic binder or an organic binder may be used.

  As the inorganic binder, for example, generally called a silane coupling agent, a 3-glycidyloxytrimethoxysilane, methacryloyloxypropyltrimethylsilane, which chemically bonds an inorganic oxide and an organic compound through a dehydration or dealcoholization reaction or the like. A mixture of a silicon compound having an organic functional group such as methoxysilane or 3-aminopropyltriethoxysilane and an inorganic oxide sol such as silica or zirconium oxide is preferable because of its excellent adhesive strength and heat resistance. It is not limited.

  Examples of the organic binder include water-insoluble water such as ethylene-vinyl acetate copolymer (EVA), acrylate copolymer, fluorine rubber, styrene butadiene rubber (SBR), acrylate copolymer, polyurethane, and the like. A binder is mentioned. A resin in which a crosslinked structure is introduced in order to prevent dissolution in a non-aqueous electrolyte can also be used for some of these resins. In addition, synthetic polymers such as polyvinyl alcohol and polyvinyl pyrrolidone; salts of carboxymethyl cellulose, cellulose derivatives such as hydroxymethyl cellulose; Can be used. These binders may be used individually by 1 type, and may use 2 or more types together. Among these, synthetic polymers such as SBR, acrylate copolymers, polyvinyl alcohol, and polyvinylpyrrolidone; salts of carboxymethyl cellulose and cellulose derivatives such as hydroxymethyl cellulose are particularly preferable.

  The addition amount of the binder needs to be less than 30% by volume of the volume of the porous body excluding the voids and from less than 20% by volume from the viewpoint of maintaining the permeability of ions required for the separator. preferable. Moreover, it is preferable that it is 3 volume% or more from a viewpoint of reducing the powder fall from a porous body.

  The separator of the present invention can be used alone as a porous body. However, from the viewpoint of providing the separator with the necessary strength, it may contain a substrate such as a porous film, a woven fabric, a nonwoven fabric, or a knitted fabric. preferable. Specifically, a separator having a porous body on a porous film, a separator having a porous body on a substrate made of a fibrous material such as a woven fabric, a nonwoven fabric, or a knitted fabric or in a pore inside the substrate, etc. Can be mentioned.

  As a method of forming a porous body, for example, a method of forming a porous body by coating and drying a coating solution containing inorganic particles on a film having peelability, etc., and for a lithium ion battery A method of forming a porous body by applying and drying a coating liquid containing inorganic particles on a positive electrode or a negative electrode, a coating containing inorganic particles on a substrate such as a woven fabric, a nonwoven fabric, a knitted fabric, or a porous film Examples thereof include a method of forming a porous body by applying and drying a working solution.

  The constituent material of the base material such as woven fabric, non-woven fabric, knitted fabric, and porous film is not particularly limited as long as it is electrochemically stable and stable to the non-aqueous electrolyte. For example, polyethylene terephthalate, polybutylene terephthalate and derivatives thereof, polyester such as aromatic polyester and wholly aromatic polyester; polyolefin; acrylic polymer; polyacetal; polycarbonate; polyketone such as aliphatic polyketone and aromatic polyketone; Polyamide, Polyamideimide, Polyphenylene sulfide, Polybenzimidazole, Polyetheretherketone, Polyethersulfone, Poly (para-phenylenebenzobisthiazole), Poly (para -Phenylene-2,6-benzobisoxazole); fluororesin such as polyvinylidene fluoride and polytetrafluoroethylene; Call; polyurethane; polyvinyl chloride, and the like. Two or more of these constituent materials may be used in combination. Of these, polyesters or aromatic polyamides are preferred because of their high melting points and high resistance to electrolytes used in batteries.

  There are no particular restrictions on the method of coating the inorganic particle coating solution, such as air doctor coater, blade coater, knife coater, rod coater, squeeze coater, impregnation coater, gravure coater, kiss roll coater, die coater, reverse roll. Examples thereof include a method using a coater, transfer roll coater, spray coater, rotor dampening and the like.

  In the present invention, the method of drying after coating is not particularly limited, but a method of drying by heating, such as a method of blowing hot air or a method of irradiating infrared rays, is preferably used with good productivity.

  In the above method, in order to produce a more uniform porous body, a thickener, an antifoaming agent, a wetting agent, a preservative, and the like can be appropriately used as necessary.

In this invention, 10.0-40.0 g / m < ² > is preferable and the basic weight of a separator has more preferable 15.0-37.5 g / m < ² >. Moreover, 10.0-40.0 micrometers is preferable and, as for the thickness of a separator, 15.0-35.0 micrometers is more preferable. Preferably 0.4~1.2g / cm ³ as the density of the separator, 0.6~1.0g / cm ³ is more preferable.

In the present invention, the amount of the porous body is preferably 1.0 to 20.0 g / m ² , more preferably 4.0 to 17.5 g / m ² in terms of dry solid content. When the amount of the porous material is less than 1.0 g / m ² , the pore diameter becomes large and a short circuit may occur, so that good battery characteristics may not be exhibited. On the other hand, if the amount of the porous body exceeds 20.0 g / m ² , it may be difficult to reduce the thickness of the separator.

  In the present invention, the surface of the separator may be smoothed by calendering or thermal calendering for the purpose of controlling the flatness and thickness of the porous body surface.

  When the separator of the present invention contains a substrate such as a porous film, woven fabric, non-woven fabric, or knitted fabric, it has excellent handling properties, can easily obtain a separator with uniform performance, can have high tensile strength, etc. For this reason, a separator (5) containing a nonwoven fabric base material is more preferable.

  As a nonwoven fabric base material, the nonwoven fabric base material manufactured by methods, such as a spun bond method, a melt blow method, other dry methods; wet methods; electrospinning method, can be used, for example.

  In the present invention, the nonwoven fabric substrate may be smoothed by calendaring or thermal calendaring for the purpose of controlling the surface and thickness of the nonwoven fabric substrate.

As a nonwoven fabric base material used for the separator for lithium ion batteries of this invention, it is preferable that a fabric weight is 5.0-20.0 g / m < ² >. When the basis weight exceeds 20.0 g / m ² , the nonwoven fabric base material alone occupies most of the separator, and the effect of the porous body may be difficult to obtain. If it is less than 5.0 g / m ² , it may be difficult to obtain uniformity as a nonwoven fabric substrate. As a fabric weight of a nonwoven fabric base material, 7.0-20.0 g / m < ² > is more preferable. The basis weight means the basis weight based on the method defined in JIS P 8124 (paper and paperboard—basis weight measurement method).

  When the separator of the present invention contains a nonwoven fabric substrate, the porous body may exist independently on the surface of the nonwoven fabric substrate, or penetrates into the nonwoven fabric substrate and mixes with the nonwoven fabric substrate. However, they may exist together. Further, a part of the porous body may penetrate into the inside of the nonwoven fabric base, and the remaining part of the porous body may exist independently on the surface of the nonwoven fabric base.

  When the separator of this invention contains a nonwoven fabric base material, it is preferable that it is a separator (6) by which the fiber of a nonwoven fabric base material is exposed to the surface at least in the 1 surface preferably. “The fibers of the nonwoven fabric substrate are exposed” means that 80% or less of the area of the observation field is covered with a porous body when observed with a scanning electron microscope. When both surfaces of the nonwoven fabric substrate are covered with a porous body, it may be difficult to reduce the leakage current. The reason for this phenomenon is unknown, but the leakage current is reduced because the pore diameter near the surface covered with the porous body is relatively small and the pore diameter near the opposite surface is relatively large. It is presumed to have some action to suppress. Even in the case of separators where the fibers of the nonwoven fabric substrate are exposed on both surfaces, in many cases there is a difference in the degree of exposure on both surfaces, so the difference in pore diameter on both surfaces suppresses leakage current. It is presumed to have some effect.

  FIGS. 7-11 is a conceptual diagram which shows the cross-section of the separator (5) formed by containing a nonwoven fabric base material. In FIG. 7A, the porous body 3 penetrates into the nonwoven fabric base material 1, and the nonwoven fabric base material 1 and the porous body 3 are mixed together. And the fiber of the nonwoven fabric base material 1 is exposed on both surfaces of the nonwoven fabric base material 1.

  In (B), the porous body 3 exists independently on one side of the nonwoven fabric substrate 1. In (C), a part of the porous body 3 penetrates into the inside of the nonwoven fabric substrate 1, and the remaining part of the porous body 3 exists independently on one side of the nonwoven fabric substrate 1. In (B) and (C), the fiber of the nonwoven fabric substrate 1 is exposed on the surface opposite to the surface on which the porous body 3 exists independently.

  In FIG. 8D, a part of the porous body 3 penetrates the entire inside of the nonwoven fabric substrate 1, and the remaining part of the porous body 3 exists independently on one side of the nonwoven fabric substrate 1. . In (D), the fibers of the nonwoven fabric substrate 1 are not exposed.

  In (E) of FIG. 9, a part of the porous body 3 permeates the entire inside of the nonwoven fabric substrate 1, and the remaining part of the porous body 3 exists independently on both surfaces of the nonwoven fabric substrate 1. . In (F), the porous body 3 exists independently on both surfaces of the nonwoven fabric substrate 1. In (G), a part of the porous body 3 penetrates into the inside of the nonwoven fabric substrate 1, and the remaining part of the porous body 3 exists independently on both surfaces of the nonwoven fabric substrate 1. In the central portion of the cross section of the nonwoven fabric substrate 1, there is a region where the porous body 3 has not penetrated.

  (H) to (M) in FIGS. 10 to 11 are separators containing another porous body 2 in addition to the porous body 3 and the nonwoven fabric substrate 1. Another porous body 2 (hereinafter sometimes abbreviated as “porous body 2”) is a porous body mainly composed of inorganic particles different from the inorganic particles in the separators (1) to (4).

  In (H), the porous body 2 permeates into the nonwoven fabric substrate 1 and is mixed with the nonwoven fabric substrate 1 and exists. Part of the porous body 3 penetrates into the nonwoven fabric substrate 1, and the remaining part of the porous body 3 exists independently on one side of the nonwoven fabric substrate 1. In (I), both the porous body 2 and the porous body 3 are present independently on one side of the nonwoven fabric substrate 1 in this order. In (J), a part of the porous body 2 penetrates into the inside of the nonwoven fabric substrate 1, and the remaining part of the porous body 2 exists independently on one side of the nonwoven fabric substrate 1. The porous body 3 exists independently on the porous body 2 on one side of the nonwoven fabric substrate 1. In (H) to (J), the fibers of the nonwoven fabric substrate 1 are exposed on the surface opposite to the surface on which the porous body 3 exists independently on the outermost surface.

  In (K), the porous body 3 penetrates into the inside of the nonwoven fabric substrate 1 and is mixed with the nonwoven fabric substrate 1 and exists. Part of the porous body 2 penetrates into the nonwoven fabric substrate 1, and the remaining part of the porous body 2 exists independently on one side of the nonwoven fabric substrate 1. In (L), both the porous body 3 and the porous body 2 exist independently on one side of the nonwoven fabric substrate 1 in this order. In (M), a part of the porous body 3 penetrates into the inside of the nonwoven fabric substrate 1, and the remaining part of the porous body 3 exists independently on one side of the nonwoven fabric substrate 1. The porous body 2 exists independently on the porous body 3 on one side of the nonwoven fabric substrate 1. In (K)-(M), the fiber of the nonwoven fabric base material 1 is exposed on the surface opposite to the surface where the porous body 2 exists independently on the outermost surface.

  The separator (7) of this invention which is a more preferable aspect in the separator formed by containing another porous body 2 in addition to the porous body 3 and the nonwoven fabric substrate 1 will be described in detail.

  In the separator (7), the nonwoven fabric substrate has a first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less and having an agglomerated structure. The second porous body mainly composed of inorganic particles having a regular shape, a concave shape, a dispersed particle diameter of less than 1.0 μm, and having no aggregate structure is laminated in this order, One surface of the nonwoven fabric substrate is substantially covered with the second porous body, and the fibers of the nonwoven fabric substrate are exposed on the opposite surface. The separator (7) can achieve the effects that the leakage current is small, the thickness is thin, and the internal resistance is low. The “first porous body” corresponds to “another porous body 2”, and the “second porous body” corresponds to the “porous body 3”.

  “One side of the nonwoven fabric substrate is substantially covered with the second porous body” means that 95% or more of the area of the observation field is the second porous body when observed with a scanning electron microscope. Say that it is covered with. “The fiber of the nonwoven fabric substrate is exposed” means that 80% or less of the area of the observation field is either the first porous body or the second porous body when observed with a scanning electron microscope. Say that it is covered with.

  The “dispersed particle size” of the inorganic particles is a 50% value of the coating liquid used for the formation of the porous body as measured by a laser diffraction type particle size distribution meter.

  The dispersed particle diameter of the inorganic particles in the second porous body is preferably less than 0.80 μm. Further, if the pore diameter is too small, a low internal resistance may not be obtained, and therefore it is preferably 0.10 μm or more.

  The presence / absence of the aggregate structure of the inorganic particles is defined as “having an aggregate structure” when the median of the distance between the diagonals of the inorganic particles observed with a scanning electron microscope is less than ½ of the dispersed particle diameter. The case of 2 or more is judged as “no aggregate structure”. FIG. 3 shows regular cube-shaped inorganic particles having a dispersed particle diameter of 2.3 μm and “with an agglomerated structure”. FIG. 4 shows irregular-shaped tabular inorganic particles having a dispersed particle diameter of 0.4 μm and “no aggregate structure”.

  In order to obtain the effect that can be achieved by the separator (7), the first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less and having an aggregated structure; A second porous body mainly composed of inorganic particles having an irregular shape, a concave shape, a dispersed particle diameter of less than 1.0 μm, and having no agglomerated structure is laminated in this order. It is essential to be made. When any one of the first porous body and the second porous body is used alone, when either the first porous body or the second porous body has a different configuration, lamination is performed. In the case where one side of the nonwoven fabric substrate is not substantially covered by the second porous body, or when the fibers of the nonwoven fabric substrate are not exposed on the opposite surface, etc. It is difficult to obtain the effect that can be achieved by the separator (7). Hereinafter, this point will be described in detail.

  When only the first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less and having an aggregate structure is provided on the nonwoven fabric substrate, the leakage current is reduced. Therefore, it is necessary to provide a first porous body having a thickness exceeding 10 μm. For this reason, it is difficult to manufacture a thin separator. In addition, the nonwoven fabric base material has an irregular shape, a dent shape, a dispersed particle diameter of less than 1.0 μm, and a second porous material mainly composed of inorganic particles having no aggregated structure. When only the body is provided, pinholes are likely to be generated in the second porous body. Therefore, in order to produce a separator with a small leakage current, for example, a fine fiber is contained as a nonwoven fabric base material. For example, it is necessary to select a nonwoven fabric base material that does not easily generate pinholes. Therefore, it becomes difficult to select an optimal nonwoven fabric base material from the viewpoint of cost, strength, etc. other than the difficulty of generating pinholes.

  The nonwoven fabric substrate has a first porous body mainly composed of inorganic particles having a dispersed particle diameter of less than 1.0 μm and an agglomerated structure, and an irregular shape and a shape with a dent. When the second porous body mainly composed of inorganic particles having a dispersed particle diameter of less than 1.0 μm and not having an aggregate structure is laminated in this order, a separator having a low internal resistance is manufactured. Is difficult.

  The first porous body mainly composed of inorganic particles having no aggregated structure on the nonwoven fabric base material, the shape is irregular, the shape is concave, and the dispersed particle size is less than 1.0 μm When the second porous body mainly composed of inorganic particles having no agglomerated structure is laminated in this order, pinholes are likely to occur in the first porous body, and it is difficult to generate pinholes. It becomes difficult to select an optimal nonwoven fabric base material from the viewpoint of cost, strength, etc.

  At the interface between the first porous body and the second porous body, the inorganic particles of the second porous body may enter the voids between the particles of the first porous body. The nonwoven fabric substrate has a first porous body mainly composed of inorganic particles having a dispersed particle diameter of more than 3.0 μm and an agglomerated structure, and an indefinite shape and a recessed shape. When the second porous body mainly composed of inorganic particles having a dispersed particle diameter of less than 1.0 μm and not having an agglomerated structure is laminated in this order, the second porous body enters the voids between the particles. The amount of inorganic particles in the porous body becomes too large and clogs, making it difficult to produce a separator with low internal resistance.

A first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less and having an agglomerated structure on a nonwoven fabric substrate, and a dispersed particle diameter of 1.0 μm or more When the second porous body mainly composed of inorganic particles is laminated in this order, a coating amount exceeding 10.0 g / m ² may be required to realize a small leakage current. This makes it difficult to produce a thin separator. A non-woven fabric base material having a dispersed particle size of 1.0 μm or more and 3.0 μm or less, mainly composed of inorganic particles having an aggregated structure and mainly inorganic particles having an aggregated structure Similarly, when the second porous body is laminated in this order, a coating amount exceeding 10.0 g / m ² may be required in order to realize a small leakage current, It becomes difficult to manufacture a thin separator.

A non-woven fabric base material having an irregular shape, a concave shape, a dispersed particle diameter of less than 1.0 μm, and a first porous body mainly composed of inorganic particles having no agglomerated structure; In the separator in which the dispersed particle diameter is 1.0 μm or more and 3.0 μm or less and the second porous body mainly composed of the inorganic particles having an aggregate structure is laminated in this order, Since it is easy to produce a pinhole in a porous body and in order to block this pinhole and to make a leakage current small, it is necessary to provide the 2nd porous body of the coating amount exceeding 10.0 g / m < ² >. This makes it difficult to produce a thin separator.

  In a separator in which one side of the nonwoven fabric substrate is not substantially covered with the second porous body, or in a separator in which the fibers of the nonwoven fabric substrate are not exposed on the opposite side, it is difficult to reduce the leakage current. The reason for this phenomenon is unknown, but the pore diameter in the vicinity of the surface substantially covered with the second porous body is relatively small, and the pore diameter in the vicinity of the opposite surface is relatively large. This is presumed to have some effect of suppressing the leakage current.

  The separator (7) is a nonwoven fabric base material having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less, and a first porous body coating liquid mainly composed of inorganic particles having an agglomerated structure; A coating liquid for a second porous body mainly composed of inorganic particles having an irregular shape, a concave shape, a dispersed particle diameter of less than 1.0 μm, and having no agglomerated structure It is manufactured by coating in this order. After the first porous body coating liquid is applied, the first porous body coating liquid is dried, the second porous body coating liquid is applied, and then the second porous body is coated. The material coating solution can also be dried. In addition, after the first porous body coating liquid is applied, the second porous body coating liquid is applied without drying the first porous body coating liquid. It is also possible to dry the coating liquid for the porous body and the coating liquid for the second porous body together. When the first porous body coating liquid and the second porous body coating liquid are mixed before drying, the first porous body may be clogged and the internal resistance may be increased. It is preferable to apply the coating liquid for the second porous body after at least partially removing the volatile components of the coating liquid for the first porous body by drying.

  In the separator (7), as a method of applying the first porous body and the second porous body coating liquid to the nonwoven fabric substrate, the above-described coating method of the inorganic particle coating liquid is used. Can do. A more preferable method is a method using a gravure coater, a die coater, a blade coater, a rod coater, a roll coater or the like. In particular, a method using a gravure coater or a die coater is preferable for coating the first porous body coating solution. This is because these two coating methods are unlikely to generate a dynamic pressure that causes the coating liquid to enter the nonwoven fabric substrate, and pinholes are unlikely to occur in the first porous body.

In the separator (7), the coating amount of the first porous body is preferably 3.0 g / m ² or more and 10.0 g / m ² or less in terms of dry solid content, and 4.0 g / m ² or more. More preferably, it is 8.0 g / m ² or less. When the coating amount of the first porous body is too small, pinholes may be generated and the leakage current may be increased. Moreover, when there is too much coating amount of a 1st porous body, the thickness of a separator may become thick and internal resistance may also become high.

In the separator (7), the coating amount of the second porous body is preferably 2.0 g / m ² or more and 8.0 g / m ² or less, and 3.0 g / m ² or more in terms of dry solid content. More preferably, it is 6.0 g / m ² or less. When the coating amount of the second porous body is too small, the leakage current may increase. Moreover, when there is too much coating amount of a 2nd porous body, the thickness of a separator may become thick and internal resistance may also become high.

  The thickness of the separator (7) is preferably less than 30 μm, more preferably less than 25 μm. About a separator thicker than this, even if it is not a structure like a separator (7), the selection of a nonwoven fabric base material is not restrict | limited remarkably, and a separator with small leakage current can be manufactured.

  The nonwoven fabric substrate used for the separator (7) preferably contains 50% by mass or more of fibers having a diameter of 3.5 μm or less. Thereby, it can prevent more reliably that a pinhole arises in a porous body. Moreover, the thickness of the nonwoven fabric base material used for a separator (7) becomes like this. Preferably it is 10 micrometers or more, More preferably, it is 15 micrometers or more. Thereby, it can prevent more reliably that a pinhole arises in a porous body. On the other hand, when the thickness of the nonwoven fabric substrate used for the separator (7) is too thick, the thickness of the nonwoven fabric substrate is preferably 30 μm or less and more preferably 25 μm or less because the thickness of the separator is too thick.

  Examples of the present invention will be described below. In the examples, all percentages (%) and parts are based on mass unless otherwise specified. When the number of parts is shown in (), the number of parts of the non-volatile content (solid content) in the liquid is shown. The coating amount is a dry coating amount.

≪First experiment≫
<Preparation of nonwoven fabric substrate 1>
45 parts of oriented crystallized polyethylene terephthalate (PET) short fibers with a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and a fiber length of 3 mm, and oriented crystals with a fineness of 0.1 dtex (average fiber diameter of 3.0 μm) and a fiber length of 3 mm Together 15 parts of PET-based short fibers and 40 parts of PET-based short fibers (softening point 120 ° C., melting point 230 ° C.) for single component binder with a fineness of 0.2 dtex (average fiber diameter 4.3 μm) and a fiber length of 3 mm The mixture was disaggregated in water with a pulper, and a uniform papermaking slurry having a concentration of 1% was prepared under stirring by an agitator. Using a circular paper machine, this papermaking slurry is made up by a wet method, and a PET-based short fiber for a binder is adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength, with a basis weight of 12.2 g / m ² . A non-woven fabric was used. Furthermore, this nonwoven fabric was heat-treated at a roll temperature of 185 ° C., a linear pressure of 740 N / cm, and a conveying speed of 20 m / min using a 1-nip thermal calendar composed of a metal roll and a metal roll, and a nonwoven fabric base having a thickness of 21 μm. Material 1 was produced.

Example 1
90 parts of amorphous particles having irregular shapes (FIG. 1, alumina hydrate) in terms of solids, and sodium carboxymethylcellulose (1% aqueous solution B viscosity 200 mPa · s, degree of etherification 0.65) in terms of solids 0.2 part was mixed and stirred with a homogenizer, then carboxymethylcellulose sodium salt (1% aqueous solution B viscosity 7000 mPa · s, etherification degree 0.7) was mixed and stirred with 0.2 part in terms of solid content. Next, 6 parts of styrene butadiene rubber latex in terms of solid content was mixed and stirred, and ion-exchanged water was further added to prepare a coating liquid for a porous material having a solid content concentration of 20%. The coating liquid is uniformly applied to one side of the nonwoven fabric substrate 1 and dried so as to have a dry solid content of 10.2 g / m ² by a gravure coater on the nonwoven fabric substrate 1 after the thermal calendar treatment. Thus, a separator having a thickness of 25.2 μm was obtained.

(Comparative Example 1)
The coating liquid was prepared, coated and dried in the same manner as in Example 1 except that regular rhomboid-shaped inorganic particles (FIG. 2, alumina hydrate) were used as the inorganic particles. A separator having a dry solid content of 10.6 g / m ² and a thickness of 25.8 μm was obtained.

(Comparative Example 2)
The coating liquid was prepared, applied and dried in the same manner as in Example 1 except that regular cube-shaped inorganic particles (FIG. 3, alumina hydrate) were used as the inorganic particles. A separator having a dry solid content of 10.1 g / m ² and a thickness of 25.2 μm was obtained.

<Evaluation>
The separators obtained in Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 1.

[Powder removal evaluation]
About the produced separator, it cuts into a strip shape of 50 mm width x 200 mm, the upper end is fixed with tape, and a black cloth on which a 50 g weight is placed is slid, and the black cloth and separator at that time are visually observed. The following degree was evaluated.

○: No adhesion of the porous body to the black cloth and no peeling of the porous body.
X: The porous body is attached to the surface of the black cloth, and peeling is seen in the porous body.

[Pinhole evaluation]
About the produced separator, the generation | occurrence | production state of the pinhole in a 10 cm x 10 cm separator was confirmed visually using the transmitted light, and it evaluated by the following degree.

○: No transmitted light is observed, and no pinholes are observed.
X: Generation | occurrence | production or nonuniformity of a pinhole is seen.

  In Example 1, in the separator containing at least a porous body mainly composed of inorganic particles, since the shape of the inorganic particles is indefinite, good results are obtained for both the pinhole evaluation and the powder fall evaluation. It was. In Comparative Examples 1 and 2, since the inorganic particles were not indefinite, poor results were obtained in the pinhole evaluation or the powder omission evaluation.

≪Second experiment≫
(Example 2)
A coating solution was prepared, coated and dried in the same manner as in Example 1 except that inorganic particles having an irregular shape, a flat plate shape, and dents (FIG. 4, alumina hydrate) were used as the inorganic particles. Thus, a separator having a dry solid content of 10.2 g / m ² and a thickness of 25.2 μm was obtained.

(Example 3)
A coating solution was prepared in the same manner as in Example 1 except that inorganic particles having an irregular shape, a flat shape, and no dents (FIG. 5, alumina hydrate) were used as the inorganic particles. The separator was dried to obtain a separator having a dry solid content of 10.5 g / m ² and a thickness of 26.0 μm.

(Comparative Example 3)
The coating liquid was prepared, coated and dried in the same manner as in Example 1 except that regular cylindrical inorganic particles (FIG. 6, alumina hydrate) were used as the inorganic particles, and the porous body was dried. A separator having a solid content of 10.2 g / m ² and a thickness of 25.4 μm was obtained.

<Evaluation>
The separators obtained in Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 2.

[Production of lithium ion secondary battery]
Lithium manganate as positive electrode active material, artificial graphite as negative electrode active material, ethylene carbonate / diethyl carbonate / dimethyl carbonate mixed solvent (volume ratio 1/1/1) solution of lithium hexafluorophosphate (LiPF ₆ ) as electrode solution (1 mol) / L) A pouch-type lithium ion battery having a design capacity of 30 mAh was assembled using each of the separators produced as described above, with the separator coated surface facing the negative electrode active material.

[Leakage current evaluation]
Using each of the lithium ion secondary batteries produced above, the charge capacity when performing constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V was measured, and the ratio with the design capacity was calculated and The degree of evaluation.

○: Less than 125% of the design capacity Δ: 125 to 150% of the design capacity ×: More than 150% of the design capacity

[Internal resistance evaluation]
Using each of the lithium ion secondary batteries produced above, aging (break-in / discharge) for 3 cycles at 1C, followed by constant current and constant voltage charge (1 / 10C cut) at 1C and 4.2V, Discharge was performed at 0.2 C and 1 C, and the internal resistance value (Ω) was calculated from the following formula (1).

Formula (1) Internal resistance value (Ω) = (A−B) / C

Discharge at A = 0.2C, voltage B = 90% of the discharge capacity at 1C, discharge B = 1C, voltage C = (current of 0.2C) at 90% of the discharge capacity at 1C Value)-(1C current value)

  The separator of Example 2 contains a porous body mainly composed of at least inorganic particles, and the inorganic particles have an indeterminate shape and a concave shape, so that both the leakage current and the evaluation of internal resistance are good. Results were obtained.

  Although the separator of Example 3 is an amorphous particle having an indeterminate shape, there is a tendency to increase the internal resistance as compared with the separator of Example 2 because of the non-dented inorganic particles. Good results were obtained. In Comparative Examples 1 to 3, since the inorganic particles had a regular shape with no dents, there was a tendency to deteriorate in the evaluation of leakage current. In the evaluation of internal resistance, the separators of Comparative Examples 1 and 3 had higher internal resistance values than the separator of Example 2.

≪Third experiment≫
<Preparation of Nonwoven Fabric Base 2>
45 parts of oriented crystallized PET short fibers with a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and fiber length of 3 mm, and short of oriented crystallized PET with a fineness of 0.1 dtex (average fiber diameter of 3.0 μm) and fiber length of 3 mm 15 parts of fiber and 40 parts of PET short fiber (softening point 120 ° C., melting point 230 ° C.) for single-component binder having a fineness of 0.2 dtex (average fiber diameter 4.3 μm) and a fiber length of 3 mm are mixed together, It was disaggregated in water with a pulper, and a uniform papermaking slurry having a concentration of 1% was prepared under stirring by an agitator. Using a circular paper machine, this papermaking slurry is made up by a wet method, and a PET-based short fiber for a binder is adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. The basis weight is 15.2 g / m ² . A non-woven fabric was used. Further, this nonwoven fabric was subjected to heat treatment at a roll temperature of 185 ° C., a linear pressure of 740 N / cm, and a conveyance speed of 20 m / min using a 1-nip thermal calendar composed of a metal roll and a metal roll, and a nonwoven fabric base having a thickness of 27 μm. Material 2 was produced.

Example 4
As inorganic particles, alumina hydrate having a pH of 7.8 and a viscosity of 348 mPa · s (boehmite, manufactured by Nabaltec Co., Ltd., trade name: APYRAL (registered trademark) -AOH100XP), 90 parts in terms of solid content, sodium carboxymethylcellulose Salt (1% aqueous solution B viscosity 200 mPa · s, etherification degree 0.65) 0.2 parts in terms of solid content was mixed and stirred with a homogenizer, then carboxymethylcellulose sodium salt (1% aqueous solution B viscosity 7000 mPa · s) In addition, 0.2 part of etherification degree 0.7) is mixed and stirred, and then 9 parts of styrene butadiene rubber latex is mixed and stirred, and ion-exchanged water is added. A coating solution having a solid content concentration of 20% was prepared. This coating solution is uniformly applied and dried on one side of the nonwoven fabric substrate 2 after the thermal calendering treatment with a gravure coater so that the dry solid content of the porous body is 10.2 g / m ^2. Thus, a separator having a thickness of 30.2 μm was obtained.

(Comparative Example 4)
As inorganic particles, except that alumina hydrate (boehmite, manufactured by Naval Tech, trade name: ACTILOX (registered trademark) -200SM) having a pH of 8.4 and a viscosity of 2750 mPa · s was used, A coating solution was prepared, coated and dried to obtain a separator having a dry solid content of 10.7 g / m ² and a thickness of 31.2 μm.

(Comparative Example 5)
As inorganic particles, except that alumina hydrate having a pH of 7.9 and a viscosity of 6 mPa · s (boehmite, manufactured by Naval Tech Co., Ltd., trade name: APYRAL (registered trademark) -AOH60) was used, the same as in Example 4, A coating solution was prepared, applied and dried to obtain a separator having a dry solid content of 10.5 g / m ² and a thickness of 31.0 μm.

(Comparative Example 6)
A coating solution was prepared in the same manner as in Example 4 except that alumina hydrate (boehmite, manufactured by Daimei Chemical Industry Co., Ltd., trade name: C20) having a pH of 8.4 and a viscosity of 47 mPa · s was used as the inorganic particles. Coating and drying were performed to obtain a separator having a dry solid content of 10.1 g / m ² and a thickness of 30.7 μm.

(Comparative Example 7)
A coating solution was prepared in the same manner as in Example 4 except that alumina hydrate having a pH of 9.3 and a viscosity of 1720 mPa · s (boehmite, manufactured by Daimei Chemical Co., Ltd., trade name: C06) was used as the inorganic particles. Coating and drying were performed to obtain a separator having a dry solid content of 10.4 g / m ² and a thickness of 31.0 μm.

(Comparative Example 8)
Example 4 was the same as Example 4 except that alumina hydrate (boehmite, manufactured by SASOL, trade name: DISPERAL (registered trademark) -8F4) having a pH of 4.0 and a viscosity of 9 mPa · s was used as the inorganic particles. Then, a coating solution was prepared, coated and dried to obtain a separator having a dry solid content of 10.7 g / m ² and a thickness of 31.2 μm.

<Evaluation>
The separators prepared in Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 3.

[Pinhole evaluation]
About the state of the pinhole of the produced separator, one A4 size sheet was visually confirmed using transmitted light, and evaluated according to the following degree.

○: No visual pinhole is observed.
(Triangle | delta): The part in which transmitted light is observed slightly exists.
X: Many transmitted light is clearly observed.

  In the separator of Example 4, the inorganic particles are alumina hydrate having a 20 mass% aqueous dispersion having a pH of 7.0 or more and 8.3 or less and a viscosity of 50 mPa · s or more and 2000 mPa · s or less. Good results with few pinholes were obtained.

  In the separator of Comparative Example 4 in which the pH of the aqueous dispersion exceeds 8.3 and the viscosity exceeds 2000 mPa · s, and in the separators of Comparative Examples 5 and 6 in which the viscosity is less than 50 mPa · s, the pinhole is the separator of Example 4. It was more than. Further, in the separator of Comparative Example 7 whose pH greatly exceeded 8.3, the separator of Comparative Example 8 whose pH was much lower than 7.0 and the viscosity was less than 50 mPa · s, the pinholes were remarkably increased.

≪Fourth experiment≫
<Preparation of Nonwoven Fabric Base 3>
For single component binder with fineness of 0.1 dtex (average fiber diameter of 3.0 μm), 60 parts of oriented crystallized PET short fibers with a fiber length of 3 mm, fineness of 0.2 dtex (average fiber diameter of 4.3 μm) and fiber length of 3 mm 40 parts of PET short fibers (softening point 120 ° C., melting point 230 ° C.) were dispersed in water by a pulper to prepare a uniform papermaking slurry having a concentration of 1%. This papermaking slurry is made by a wet method on a circular net type paper machine, and the intersection of PET short fibers for binders and the PET short fibers for binders and oriented crystallized PET systems by a cylinder dryer at 135 ° C. The intersection point of the short fibers was bonded to develop the strength of the nonwoven fabric, and a nonwoven fabric having a basis weight of 11 g / m ² was obtained. Furthermore, this nonwoven fabric was used under the conditions of a heat roll temperature of 200 ° C., a linear pressure of 100 kN / m, and a processing speed of 30 m / min using a 1-nip heat calender consisting of an induction heating roll (metal heat roll) and an elastic roll. The nonwoven fabric substrate 3 having a thickness of 15 μm was prepared by heat calendering.

<Preparation of first porous body coating liquid (first coating liquid)>
As inorganic particles, 100 parts of an alumina hydrate having a dispersed particle size of 2.3 μm and a specific surface area of 3 m ² / g was added to a 0.3% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. in a 1% aqueous solution (0 4 parts) and stirred well. Next, a 0.5% aqueous solution (0.3 parts) of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1% aqueous solution, and acrylic having a glass transition point of −18 ° C. and a dispersed particle size of 0.2 μm as a binder. An acid ester resin emulsion (solid content concentration 50%) (6 parts) was mixed and stirred to prepare a coating solution for the first porous body.

  FIG. 3 is a scanning electron micrograph of the used alumina hydrate. The median of the diagonal distance of the inorganic particles observed in this scanning electron micrograph is clearly smaller than ½ of the dispersed particle diameter, and this alumina hydrate is judged to have “aggregated structure”. The In Table 4, when the inorganic particles have “aggregation structure”, “aggregation” is described.

<Preparation of second porous body coating liquid (second coating liquid)>
As an inorganic particle, 100 parts of an alumina hydrate having an irregular shape, a dent, a dispersed particle diameter of 0.4 μm, and a specific surface area of 11 m ² / g is used. A 1% aqueous solution has a viscosity at 25 ° C. of 200 mPa · s. It mixed with 0.3% aqueous solution (0.4 part) of carboxymethylcellulose sodium salt, and stirred sufficiently. Next, a 0.5% aqueous solution (0.3 parts) of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1% aqueous solution, a glass transition point of −18 ° C., a non-volatile content concentration of 50%, and a dispersed particle size as a binder A 0.2 μm acrylate resin emulsion (6 parts) was mixed and stirred to prepare a coating solution for the second porous body.

  FIG. 4 is a scanning electron micrograph of the used alumina hydrate. The median of the diagonal distance of the inorganic particles observed in this scanning electron micrograph is clearly larger than ½ of the dispersed particle diameter, and this alumina hydrate has “no aggregate structure”. To be judged. In Table 4, when the inorganic particles have “no aggregate structure”, “non-aggregate” is described.

(Example 5)
Using a kiss reverse gravure coater, the first coating solution was applied onto the nonwoven fabric substrate 3 so that the coating amount was 6.0 g / m ² and dried with a hot air dryer. A first porous body was formed. Next, using a kiss reverse type gravure coater, the surface of the first porous body is coated with the second coating solution so that the coating amount is 4.0 g / m ² and dried with a hot air dryer. A separator having a thickness (measured with a micrometer) of 22 μm was prepared.

(Examples 6-9, Comparative Examples 9-18)
A separator was prepared in the same manner as in Example 5 except that the aggregated structure of inorganic particles, the dispersed particle diameter, and the coating amount of each porous material in the first coating liquid and the second coating liquid were changed as shown in Table 1. Was made. Table 4 also shows the thickness of each separator.

(Comparative Example 19)
A separator having a thickness of 20 μm was produced in the same manner as in Example 5 except that the first coating liquid and the second coating liquid were applied using a squeeze coater instead of the gravure coater.

(Comparative Example 20)
The second coating liquid is applied once more on the surface opposite to the surface coated with the second coating liquid of the separator of Example 5, so that the coating amount is 6.0 g / m ² . A separator having a thickness of 32 μm was produced in the same manner as in Example 5 except that the porous body 3 was formed.

(Comparative Example 21)
The surface of the separator of Example 5 opposite to the surface coated with the second coating solution is coated with the first coating solution once more so that the coating amount is 6.0 g / m ² . A separator having a thickness of 32 μm was produced in the same manner as in Example 5 except that the porous body 3 was formed.

<Evaluation>
[Microscopic observation]
As a result of observing the surface of each produced separator with a scanning electron microscope, for the separators of Examples 5 to 9 and Comparative Examples 9 to 18, one side of the nonwoven fabric substrate was covered with the second porous body, The fiber of the nonwoven fabric substrate was exposed on the opposite surface. For the separator of Comparative Example 19, the fibers of the nonwoven fabric substrate were exposed on either side. About the separator of Comparative Example 20, one surface is coated with the second porous body, and the opposite surface is coated with the third porous body (using the second coating liquid of Example 5). The fibers of the nonwoven fabric substrate were not exposed on this surface. About the separator of Comparative Example 21, one surface is coated with the second porous body, and the opposite surface is coated with the third porous body (using the first coating liquid of Example 5). The fibers of the nonwoven fabric substrate were not exposed on this surface.

[Production of evaluation battery]
Lithium manganate as the positive electrode active material, mesocarbon microbeads as the negative electrode active material, and 1 mol / L diethyl carbonate / ethylene carbonate (volume ratio 7/3) mixed solvent solution of lithium hexafluorophosphate (LiPF ₆ ) as the electrolyte As a separator, a pouch-type lithium ion battery having a design capacity of 30 mAh was produced by using each separator produced as described above with the second porous body facing the negative electrode.

[Evaluation of leakage current]
About each battery for evaluation, the charge capacity at the time of performing the first charge in the sequence of “30 mA constant current charge → 4.2 V constant voltage charge (end current 3 mA)” was measured. Each separator was classified into the following three levels according to the charge capacity. If the charging capacity greatly exceeds the designed capacity of 30 mAh, it means that a leakage current is generated inside the battery.

○: Initial charge capacity of less than 35 mAh Δ: Initial charge capacity of 35 mAh or more and less than 40 mAh ×: Initial charge capacity of 40 mAh or more

[Evaluation of internal resistance]
For each evaluation battery after leakage current evaluation, a cycle of “30 mA constant current charge → 4.2 V constant voltage charge (1 hour) → constant current discharge at 30 mA (end voltage 2.8 V)” (2 cycles of aging ( Break-in charge / discharge). Next, after charging with “30 mA constant current charging → 4.2 V constant voltage charging (1 hour)”, constant current discharging was performed at 90 mA. From the voltage E 480 seconds after the start of discharge (remaining charge rate 60% in calculation), the internal resistance was obtained by the following equation (2).

Formula (2) Internal resistance = (3.88V-E) / 90 mA

  In addition, 3.88V is the open circuit voltage of the battery when the remaining charge rate of the battery for this evaluation is 50%, and it has been confirmed that it is a constant value regardless of the separator.

○: Internal resistance less than 4.0Ω Δ: Internal resistance of 4.0Ω to less than 5.0Ω ×: Internal resistance of 5.0Ω or more

  As shown in Table 4, the nonwoven fabric substrate has a first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less and having an aggregated structure, A second porous body mainly composed of inorganic particles having an indeterminate shape, a dent shape, a dispersed particle diameter of less than 1.0 μm, and having no aggregate structure is laminated in this order. The separators of Examples 5 to 9, in which one side of the nonwoven fabric substrate is substantially covered with the second porous body and the fibers of the nonwoven fabric substrate are exposed on the opposite side, the leakage current is small, Thin thickness and low internal resistance.

  On the other hand, the separator of Comparative Example 9 in which the dispersed particle diameter is 1.0 μm or more and 3.0 μm or less and only the porous body mainly composed of inorganic particles having an aggregated structure is laminated has a leakage current. It was big. In the separator of Comparative Example 10 in which the dispersed particle size is 1.0 μm or more and 3.0 μm or less and the porous body mainly composed of inorganic particles having an aggregated structure is thickened and laminated, leakage occurs. Although the current could be suppressed, the thickness was thick and the internal resistance was high. In Comparative Example 11, the shape is irregular, has a dent, has a dispersed particle diameter of less than 1.0 μm, and is formed by laminating only porous bodies mainly composed of inorganic particles having no aggregated structure. The separator had a large leakage current.

  The separator of Comparative Example 12 in which the first porous body was mainly composed of inorganic particles having no agglomerated structure also had a large leakage current. The first porous body had a dispersed particle size of less than 1.0 μm, and the separator of Comparative Example 13 mainly composed of inorganic particles having an aggregated structure had high internal resistance. The separator of Comparative Example 14 in which the first porous body was mainly composed of inorganic particles having an agglomerated structure with a dispersed particle diameter exceeding 3.0 μm also had high internal resistance.

  The separator of Comparative Example 15 in which the second porous body was mainly composed of inorganic particles having an agglomerated structure had a large leakage current. The separator of Comparative Example 16 in which the second porous body mainly composed of inorganic particles having an agglomerated structure was thick could suppress the leakage current, but was thick and high in internal resistance.

  The first porous body has an irregular shape, a dent shape, a dispersed particle diameter of less than 1.0 μm, mainly composed of inorganic particles having no aggregated structure, and the second porous body. The separator of Comparative Example 17 mainly composed of inorganic particles having a dispersed particle size of 1.0 μm or more and 3.0 μm or less and having an aggregated structure had a large leakage current. The first porous body has an irregular shape, a dent shape, a dispersed particle diameter of less than 1.0 μm, mainly composed of inorganic particles having no aggregated structure, and the second porous body. The separator of Comparative Example 18 in which the porous body has a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less, mainly composed of inorganic particles having an agglomerated structure, and each porous body is thickened has a leakage current. Although it could be suppressed, the thickness was thick and the internal resistance was also high.

  The first porous body mainly composed of inorganic particles having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less and having an aggregated structure, the shape is irregular, and a shape having a dent, The second porous body mainly composed of inorganic particles having a dispersed particle diameter of less than 1.0 μm and not having an agglomerated structure is laminated in this order. The separator of Comparative Example 19 is not covered with the porous body 2 and the fibers are exposed on both sides of the separator, both sides of the separator are covered with the porous body, and the fibers of the nonwoven fabric substrate are exposed. With the separators 20 and 21, the leakage current was large.

  The lithium ion battery separator of the present invention is used for lithium ion battery applications, but besides this, a manganese dry battery, an alkaline manganese battery, a silver oxide battery, a lithium primary battery, a lead storage battery, a nickel-cadmium storage battery, Nickel-hydrogen battery, nickel-zinc battery, silver oxide-zinc battery, lithium polymer battery, various gel electrolyte batteries, zinc-air battery, iron-air battery, aluminum-air battery, fuel cell, solar battery, sodium sulfur battery It can be used for polyacene batteries, electrolytic capacitors, electric double layer capacitors, lithium ion capacitors and the like.

DESCRIPTION OF SYMBOLS 1 Nonwoven fabric base material 2 Another porous body 3 Porous body

Claims (7)

  A lithium ion battery separator comprising a porous body mainly composed of at least inorganic particles, wherein the inorganic particles have an indefinite shape.   The lithium ion battery separator according to claim 1, wherein the inorganic particles have a concave shape.   The separator for lithium ion batteries according to claim 1 or 2, wherein the inorganic particles are hydrated alumina.   In a lithium ion battery separator comprising a porous body mainly composed of at least inorganic particles, the inorganic particles have a pH of 7.0 to 8.3 in a 20% by mass aqueous dispersion, and the aqueous dispersion A separator for a lithium ion battery, wherein the separator is an alumina hydrate having a viscosity of 50 mPa · s to 2000 mPa · s.   The separator for lithium ion batteries according to any one of claims 1 to 4, comprising a nonwoven fabric substrate.   The separator for a lithium ion battery according to claim 5, wherein the fibers of the nonwoven fabric substrate are exposed on at least one side.   A non-woven fabric substrate having a dispersed particle diameter of 1.0 μm or more and 3.0 μm or less, a first porous body mainly composed of inorganic particles having an aggregated structure, an irregular shape, and a dent A second porous body mainly composed of inorganic particles having a certain shape and a dispersed particle diameter of less than 1.0 μm and having no aggregate structure is laminated in this order, and one side of the nonwoven fabric substrate is A separator for a lithium ion battery, which is substantially covered with a second porous body, and the fibers of the nonwoven fabric substrate are exposed on the opposite surface.