JP2011018589A - Slurry for insulating layer forming, separator for lithium ion secondary battery and manufacturing method thereof, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slurry in order to form an insulating layer suitable for composing a separator for lithium ion secondary battery, the slurry containing particulates and having a superior long-term storage property, to provide a separator for lithium ion secondary battery manufactured by using the slurry, to provide a manufacturing method of the separator, and to provide the lithium ion battery having the separator for lithium ion secondary battery.SOLUTION: The slurry used for forming the insulating layer, containing heat resistant particulates, a thickener, and a medium containing water as main component in which the numbers of fungi and living microbes contained in 1 mL of the medium determined by an aseptic test method described in a general test method of Japanese Pharmacopoeia are 50 or less, respectively, and having pH of 7 to 11, the separator for lithium ion secondary battery manufactured through a process of coating the slurry for the insulating layer formation on the base material, the manufacturing method of the separator, and the lithium ion secondary battery having the separator, are provided.

Description

  The present invention relates to a slurry for forming an insulating layer suitable for constituting a separator for a lithium ion secondary battery, a separator for a lithium ion secondary battery having an insulating layer formed using the slurry, and a method for producing the same. In addition, the present invention relates to a lithium ion secondary battery having the lithium ion secondary battery separator.

  A lithium ion secondary battery, which is a type of nonaqueous electrolyte battery, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density. As the performance of portable devices increases, the capacity of lithium ion secondary batteries tends to increase further, and ensuring safety is important.

  In current lithium ion secondary batteries, as a separator interposed between a positive electrode and a negative electrode, for example, a polyolefin-based microporous film having a thickness of about 20 to 30 μm is used. In addition, as separator material, the constituent resin of the separator is melted below the thermal runaway temperature of the battery to close the pores, thereby increasing the internal resistance of the battery and improving the safety of the battery in the event of a short circuit. In order to ensure the so-called shutdown effect, polyethylene having a low melting point may be applied.

  By the way, as such a separator, for example, a uniaxially stretched film or a biaxially stretched film is used for increasing the porosity and improving the strength. Since such a separator is supplied as a single film, a certain strength is required in terms of workability and the like, and this is secured by the stretching. However, with such a stretched film, the degree of crystallinity has increased, and the shutdown temperature has increased to a temperature close to the thermal runaway temperature of the battery. Therefore, it can be said that the margin for ensuring the safety of the battery is sufficient. hard.

  In addition, the film is distorted by the stretching, and there is a problem that when it is exposed to a high temperature, shrinkage occurs due to residual stress. The shrinkage temperature is very close to the melting point, ie the shutdown temperature. For this reason, when a polyolefin-based microporous film separator is used, if the battery temperature reaches the shutdown temperature in the case of abnormal charging, the current must be immediately reduced to prevent the battery temperature from rising. This is because if the pores are not sufficiently closed and the current cannot be reduced immediately, the battery temperature easily rises to the contraction temperature of the separator, and there is a risk of ignition due to an internal short circuit.

  As a technique for preventing such a short circuit due to thermal contraction of the separator and improving the reliability of the battery, for example, it has a porous substrate with good heat resistance, filler particles, and a resin component for ensuring a shutdown function It has been proposed that an electrochemical element such as a lithium ion secondary battery is constituted by a separator (Patent Document 1).

  According to the technique of Patent Document 1, it is possible to provide a lithium ion secondary battery excellent in safety in which thermal runaway hardly occurs even when abnormal overheating occurs.

International Publication No. 2006/62153

  Patent Document 1 also describes a method of manufacturing a separator through a process of using inorganic fine particles as a filler and applying a slurry in which the filler is dispersed to a substrate or the like.

  However, since the inorganic fine particles have a specific gravity larger than that of a medium such as water or an organic solvent, the fine particles are likely to settle in the slurry. In some cases, it is difficult to stably maintain the dispersion state of the fine particles in the slurry.

  If the dispersion state of the fine particles in the slurry cannot be kept stable, the fine particles are aggregated or settled during storage of the slurry. If a slurry in which fine particles aggregate or settle is applied to a substrate or the like, uneven coating tends to occur. In addition, when the dispersion state of the fine particles in the slurry is particularly unstable, the coated surface may become uneven due to aggregation or settling of the fine particles after application to the substrate and before drying. There is also.

  When uneven application of the slurry occurs, the uniformity of the formed separator is lowered, and uneven ion conductivity in the separator occurs in the lithium ion secondary battery, particularly during charging at a high current density. There is a risk that defects such as precipitation may occur, and when the precipitated lithium becomes a dendrite-like crystal, a short circuit may occur due to the dendrite.

  For this reason, in the slurry for separator production, in order to further stabilize the quality of the separator to be produced, it is preferable to improve the stability of the dispersion state of the fine particles. In this respect, there is still room for improvement.

  In addition, in the slurry in which the fine particles are dispersed, as a means for preventing sedimentation of the fine particles, for example, a synthetic polymer such as polyethylene glycol or a natural polysaccharide such as carboxymethyl cellulose is added as a thickener, and the viscosity of the slurry is increased. There is a known technique that makes it difficult for the fine particles to sink by increasing the height. Therefore, it is conceivable to apply such a technique to a slurry for manufacturing a separator.

  It is preferable to use water as the slurry medium for manufacturing the separator from the viewpoint of ease of medium recovery after coating and drying and environmental problems. When the slurry medium is water, it is common to use water as a thickener and add a suitable amount of a thickener dissolved or dispersed in water to the slurry to adjust the viscosity of the slurry. .

  However, when the thickener medium is water, the thickener may be decomposed by bacteria contained in the water. Since the decomposition reaction of the thickener proceeds with time, even when the viscosity is adjusted to a value suitable for preventing sedimentation of fine particles when the slurry is prepared, it is gradually reduced during the storage of the slurry. There is a possibility that the viscosity is lowered and the fine particles are settled.

  Therefore, in a slurry for producing a separator that may be stored for a long time after preparation, it is required to improve the long-term storage property so that the dispersion state of the fine particles can be stably maintained for a long time.

  The present invention has been made in view of the above circumstances, and an object thereof is a slurry for forming an insulating layer suitable for constituting a separator for a lithium ion secondary battery containing fine particles, which is excellent. Another object of the present invention is to provide a slurry having a long-term storage property, a separator for a lithium ion secondary battery produced using the slurry, a method for producing the same, and a lithium ion secondary battery having the lithium ion secondary battery separator. .

  The slurry for forming an insulating layer of the present invention capable of solving the above problems is a slurry for forming an insulating layer having ion permeability and heat resistance, which can be applied to a separator for a lithium ion secondary battery, , Containing heat-resistant fine particles, a thickener and a medium, wherein the number of fungi and viable bacteria determined by the sterility test method described in the general test method of the Japanese Pharmacopoeia is 50 or less per 1 mL, respectively. It has water as a main component and has a pH of 7-11.

  In the insulating layer forming slurry according to the present invention, a medium mainly composed of water having a small number of fungi and viable bacteria is used, and the pH of the slurry is adjusted as high as 7 to 11 to suppress the decomposition of the thickener. Thus, even when stored for a long time, the dispersion state of the heat-resistant fine particles can be maintained well.

  As used herein, “heat-resistant insulating layer” means that the heat-resistant temperature of the insulating layer is 150 ° C. or higher, that is, the insulating layer is not substantially deformed at least at 150 ° C. More specifically, it means that thermal contraction is not confirmed when the insulating layer heated to 150 ° C. is visually observed.

  The separator for lithium ion secondary batteries of the present invention (hereinafter sometimes simply referred to as “separator”) is a group consisting of a positive electrode for lithium ion secondary batteries, a negative electrode for lithium ion secondary batteries, and a porous substrate. It has a porous insulating layer formed through the process of apply | coating the slurry for insulating layer formation of this invention to the at least 1 sort (s) of base material selected from these.

  Furthermore, the method for producing a separator for a lithium ion secondary battery of the present invention includes at least one substrate selected from the group consisting of a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a porous substrate. The method further comprises a step of continuously applying the insulating layer forming slurry of the present invention with a coater and drying to form a porous insulating layer integrated with the substrate.

  The lithium ion secondary battery of the present invention is a lithium ion secondary battery having at least a positive electrode, a negative electrode, and a separator, wherein the separator is the separator for a lithium ion secondary battery of the present invention. Is.

  ADVANTAGE OF THE INVENTION According to this invention, the slurry for insulating layer formation which has the outstanding long-term storage property can be provided. The separator for a lithium ion secondary battery of the present invention produced using the slurry for forming an insulating layer of the present invention has excellent heat resistance, and the present invention has the separator for a lithium ion secondary battery. The lithium ion secondary battery has excellent reliability. Furthermore, according to the manufacturing method of the present invention, the separator for a lithium ion secondary battery of the present invention can be manufactured stably and continuously.

  The slurry for forming an insulating layer of the present invention is for forming a porous insulating layer capable of constituting a separator for a lithium ion secondary battery, and contains at least heat-resistant fine particles, a thickener and a medium. Yes.

  In the separator for lithium ion secondary batteries produced by the slurry for forming an insulating layer of the present invention, the heat resistant fine particles increase the heat resistance and improve the dimensional stability at high temperatures of the lithium ion secondary battery separator. It has an effect of suppressing the occurrence of a micro short circuit caused by lithium dendrite.

  As heat-resistant fine particles, it has electrical insulation properties, is electrochemically stable, and is stable in the organic electrolyte solution of lithium ion secondary batteries and the medium (solvent, dispersion medium) used for the slurry for forming the insulating layer. As long as it does not dissolve in the electrolyte solution at a high temperature, there is no particular limitation.

  As used herein, “stable heat-resistant fine particles with respect to an organic electrolyte” refers to deformation and chemical composition in an organic electrolyte (an organic electrolyte used as an electrolyte for a lithium ion secondary battery). It means heat-resistant fine particles that do not change. In addition, the “high temperature state” in the present specification specifically refers to a temperature of 150 ° C. or higher, and may be a stable particle that does not undergo deformation or chemical composition change in an organic electrolyte at such a temperature. (In other words, “heat resistance” of “heat-resistant fine particles” means that deformation and chemical composition change do not occur in the organic electrolyte at least at 150 ° C.). Furthermore, “electrochemically stable” as used in the present specification means that no chemical change occurs during charging and discharging of the lithium ion secondary battery.

Specific examples of the heat-resistant fine particles include, for example, oxide fine particles (metal oxide fine particles) such as iron oxide, Al ₂ O ₃ (alumina), SiO ₂ (silica), TiO ₂ , BaTiO ₃ , and ZrO ₂ ; aluminum nitride Nitride fine particles such as silicon nitride; hydroxide fine particles such as calcium hydroxide, magnesium hydroxide and aluminum hydroxide (metal hydroxide fine particles); sparingly soluble ions such as calcium fluoride, barium fluoride and barium sulfate Crystalline fine particles; Covalent crystalline fine particles such as silicon and diamond; Clay fine particles such as talc and montmorillonite; Substances derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, sericite, bentonite Thing; etc. are mentioned. Further, the surface of conductive fine particles such as metal fine particles; oxide fine particles such as SnO ₂ and tin-indium oxide (ITO); carbon fine particles such as carbon black and graphite; It may be fine particles that have been made electrically insulating by surface treatment with the above-mentioned materials constituting the electrically insulating heat-resistant fine particles. As the heat-resistant fine particles, those exemplified above may be used alone or in combination of two or more. Among these, metal oxide fine particles (particularly alumina and silica), metal hydroxide fine particles, and metal hydroxide oxide fine particles (boehmite, etc.) are preferable.

The form of the heat-resistant fine particles may be any form such as a spherical shape, a particle shape, or a plate shape, but is preferably a plate shape. Examples of the plate-like particles include various commercially available products, such as “Sun Lovely (trade name)” (SiO ₂ ) manufactured by Asahi Glass S Tech Co., Ltd., and pulverized products of “NST-B1 (trade name)” manufactured by Ishihara Sangyo Co., Ltd. TiO ₂ ), Sakai Chemical Industry's plate-like barium sulfate “H series (trade name)”, “HL series (trade name)”, Hayashi Kasei Co., Ltd. “micron white (trade name)” (talc), Hayashi Kasei "Bengel (trade name)" (bentonite), Kawai Lime "BMM (trade name)" and "BMT (trade name)" (Boehmite), Kawai Lime "Cerasur BMT-B (trade name)" [Alumina (Al ₂ O ₃ )], “Seraph (trade name)” (alumina) manufactured by Kinsei Matec Co., Ltd., “Yodogawa Mica Z-20 (trade name)” (sericite) manufactured by Yodogawa Mining Co., Ltd., etc. are available. is there. In addition, SiO ₂ , Al ₂ O ₃ , ZrO ₂ , and CeO ₂ can be produced by the method disclosed in Japanese Patent Laid-Open No. 2003-206475.

  In the case where the heat-resistant fine particles are plate-like, the occurrence of a short circuit can be suppressed more satisfactorily by orienting the heat-resistant fine particles in the separator so that the flat plate surface is substantially parallel to the surface of the separator. This is because the heat-resistant fine particles are oriented as described above so that the heat-resistant fine particles are arranged so as to overlap each other on a part of the flat plate surface. Therefore, there are voids (through holes) from one side of the separator to the other side. It is thought that it is formed in a bent shape instead of a straight line (that is, the curvature is increased), and this can prevent the lithium dendrite from penetrating the separator, so that the occurrence of a short circuit is suppressed better. Presumed to be.

  For example, the aspect ratio (the ratio of the maximum length in the plate-like particle to the thickness of the plate-like particle) is preferably 5 or more, more preferably 10 or more as the form when the heat-resistant fine particles are plate-like particles. Thus, it is preferably 100 or less, more preferably 50 or less. Moreover, the average value of the ratio of the major axis direction length to the minor axis direction length of the tabular surface of the grain is preferably 0.3 or more, more preferably 0.5 or more (ie, the major axis direction length and The length in the minor axis direction may be the same). When the plate-like heat-resistant fine particles have an aspect ratio as described above or an average value of the ratio of the major axis direction length to the minor axis direction length of the flat plate surface, the above-mentioned short-circuit prevention effect is more effectively exhibited. The

  In addition, when the heat-resistant fine particles are plate-like, the average value of the ratio of the long axis direction length to the short axis direction length of the flat plate surface is obtained by image analysis of an image taken with a scanning electron microscope (SEM). Is a value obtained by Further, the aspect ratio in the case where the heat-resistant fine particles are plate-like is also a value obtained by image analysis of an image taken by SEM.

The heat-resistant fine particles preferably include fine particles having a secondary particle structure in which primary particles are aggregated. Examples of such fine particles include “Boehmite C06 (trade name)”, “Boehmite C20 (trade name)” (boehmite) manufactured by Daimei Chemical Co., Ltd., “ED-1 (trade name)” manufactured by Yonesho Lime Industry Co., Ltd. ( CaCO ₃ ), J.M. M.M. Examples include “Zeolex 94HP (trade name)” (clay) manufactured by Huber.

  When fine particles having a secondary particle structure in which primary particles are aggregated are used as the heat-resistant fine particles, the aggregated secondary particles prevent fine packing between the particles, so that the pores of the insulating layer are made larger. Lithium ion secondary batteries composed of lithium ion secondary battery separators having such an insulating layer can be used for electric vehicles, hybrid vehicles, electric motorcycles, electric assist bicycles, electric tools, shavers, etc. Therefore, it is suitable for applications that require higher output.

  The average particle size of the heat-resistant fine particles is preferably 0.01 μm or more, more preferably 0.1 μm or more, more preferably 15 μm or less, and even more preferably 5 μm or less. The average particle diameter of the heat-resistant fine particles referred to in the present specification is determined by dissolving the heat-resistant fine particles or swelling the heat-resistant fine particles using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA). It is a number average particle diameter measured by dispersing heat-resistant fine particles in a non-destructive medium.

  A thickener is used for the slurry for forming an insulating layer of the present invention. By using the thickener, the viscosity of the insulating layer forming slurry can be adjusted, and the dispersion stability of the heat-resistant fine particles can be enhanced.

  The thickener may be any thickener that does not have side effects such as agglomeration of heat-resistant fine particles in the insulating layer forming slurry and can adjust the slurry to the required viscosity. What has an effect | action is preferable. Moreover, it is preferable that a thickener can melt | dissolve or disperse | distribute favorably with respect to the medium used for a slurry. If a large amount of undissolved matter or aggregates (so-called “mamako”) exist in the slurry, the dispersion of the heat-resistant fine particles becomes uneven, and the insulation is formed by applying the slurry to a substrate and drying it. There is a possibility that a portion having a low concentration of the heat-resistant fine particles is generated in the layer. In such a case, the effect of improving the heat resistance of the insulating layer due to the use of heat-resistant fine particles is reduced, and as a result, the reliability improvement effect and the heat resistance improvement effect of the lithium ion secondary battery may be reduced.

  In addition, as a standard of this content as it is in the insulating layer forming slurry, when the slurry is passed through a mesh filter having an opening of 30 μm, the residue remaining on the filter is 1 or less per 1 L of slurry. Preferably, it is 1 or less per 5 L of slurry.

  Specific examples of the thickener include synthetic polymers such as polyethylene glycol, polyacrylic acid, polyvinyl alcohol, vinyl methyl ether-maleic anhydride copolymer; xanthan gum, welan gum, gellan gum, guar gum, carrageenan, cellulose derivative (carboxymethylcellulose) , Hydroxyethyl cellulose, hydroxypropyl cellulose, etc.), natural polysaccharides such as starches such as dextrin and pregelatinized starch; clay minerals such as montmorillonite and hectorite; inorganic oxides such as fumed silica, fumed alumina and fumed titania And the like. These may be used alone or in combination of two or more. In the case of the above clay minerals and inorganic oxides, it is preferable to use a primary particle having a particle size smaller than that of the heat-resistant fine particles (for example, about several nm to several tens of nm). Those having a structure structure in which a large number of are connected (such as fumed silica) are preferred.

  Among the thickeners exemplified above, natural polysaccharides are more preferable because of their high solubility in water, which is a suitable medium for the insulating layer forming slurry, and high thickening effects in small amounts. Xanthan gum, welan gum, gellan gum Is more preferable, and xanthan gum is particularly preferable. In addition, when thixotropy is imparted to the insulating layer forming slurry, it is preferable to add inorganic oxides such as fumed silica, fumed alumina, fumed titania and the like.

  The content of the thickener in the insulating layer forming slurry varies depending on the viscosity of the insulating layer forming slurry to be set. For example, when using a thickener that does not volatilize in the drying step after slurry application. Since it remains in the insulating layer, it is not preferable to use it in a large amount. Specifically, it is 10% by volume in the total volume of solids in the slurry (components excluding the medium; the same applies hereinafter). Is preferably 5% by volume or less, more preferably 1% by volume or less. Moreover, it is preferable that content of the thickener in the slurry for insulating layer formation is 0.1 volume% or more in the total volume of the solid content in a slurry.

  As the medium for the insulating layer forming slurry, the sterility test method thioglycolic acid medium I was used as the medium by the membrane filter method of the sterility test method described in the Japanese Pharmacopoeia general test method, the culture temperature was 30 ° C., the culture time was 14 The number of fungi and viable bacteria determined under the conditions of the day is such that the main component is 50 or less water in 1 mL of water. By using a medium mainly composed of water having a small number of fungi and viable bacteria as described above, for example, even when a natural polysaccharide that is particularly easily decomposed is used as the thickener, the decomposition can be suppressed. Therefore, even if the slurry for forming an insulating layer of the present invention is stored for a long period of time, the viscosity fluctuation during that period is small, and the dispersion state of the heat-resistant fine particles is well maintained.

  The “medium” as used in the present invention refers to the remaining part of the insulating layer forming slurry excluding the solid content remaining upon drying during the formation of the insulating layer. In addition, the term “main component” for the water refers to the fact that 70% by mass or more of the water is contained among the constituent components in the medium. In addition, when using media other than the said water together, as this medium, the postscript alcohols etc. which are added for the interfacial tension control of the slurry for insulating layer formation are mentioned, for example. In particular, from the viewpoint of environmental protection, it is preferable to use a medium containing 100% by mass of water.

  Examples of the water having a small number of fungi and viable bacteria as described above include, for example, ion-exchanged water obtained by ion-exchange of well water, tap water, and purified water obtained by distillation treatment of these water, gamma rays, ethylene oxide gas, or ultraviolet light. The water sterilized by the above is mentioned, and the water sterilized by the purified water is particularly preferable.

  The slurry for forming an insulating layer of the present invention has a pH of 7 or higher, preferably 8 or higher. Since the decomposition of the thickener can also be suppressed by increasing the pH of the insulating layer forming slurry, it is possible to suppress a change in viscosity during long-term storage. However, if the pH of the slurry for forming the insulating layer is too high, for example, corrosion of the container for storing the slurry may occur, or danger may arise during handling. Therefore, the pH of the insulating layer forming slurry is 11 or less, and preferably 10 or less.

  The method for adjusting the pH of the insulating layer forming slurry is not particularly limited, and a method of adding a common alkaline agent, for example, an aqueous solution of metal hydroxide (sodium hydroxide, potassium hydroxide, etc.); When the heat-resistant fine particles and thickeners, and the binder described later, are alkaline, a method of diluting with water; when using heat-resistant fine particles that are likely to be mixed with alkaline impurities in the production process, And a method for preparing a slurry after washing the heat-resistant fine particles. As for the water used for pH adjustment, water having a small number of fungi and viable bacteria exemplified above as the water used for the insulating layer forming slurry (eg, ion-exchanged water or purified water is sterilized by the above method). Treated water) is preferred.

  In addition, pH of the slurry for insulating layer formation as used in this specification is a value measured using a zeta potential measuring device.

  In addition, in the insulating layer forming slurry, in the insulating layer (separator for lithium ion secondary battery), heat-resistant fine particles or other components (described later) constituting the heat-resistant fine particles and the insulating layer are bonded. In addition, a binder may be included for the purpose of bonding the insulating layer and a base material (a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, a porous base material, etc.).

  The binder has good adhesion between the heat-resistant fine particles and other components constituting the heat-resistant fine particles and the insulating layer, the insulating layer and the base material, and is electrochemically stable inside the lithium ion secondary battery. Any material that is stable with respect to the organic electrolyte solution of the ion secondary battery may be used. In addition, about the thing which also has a function as a binder among the said thickeners, it can also be used as a binder.

  Specific examples of the binder include ethylene-acrylic acid copolymer such as ethylene-vinyl acetate copolymer (EVA, vinyl acetate-derived structural unit is 20 to 35 mol%), ethylene-ethyl acrylate copolymer, etc. Combined, fluorinated rubber, styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), poly N-vinylacetamide, crosslinked acrylic resin, polyurethane, epoxy resin, etc. are used. . These binders may be used individually by 1 type, and may use 2 or more types together.

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

  The content of the binder in the slurry for forming the insulating layer is preferably 1% or more when the volume of the heat-resistant fine particles is 100% from the viewpoint of more effectively exerting the action due to the use of the binder. More preferably, it is more preferably 10% or more. In addition, if the amount of the binder in the insulating layer forming slurry is too large, the pores are filled in the insulating layer to be formed and the ion permeability is lowered, which may adversely affect the characteristics of the lithium ion secondary battery. Therefore, the content is preferably 30% or less, more preferably 20% or less, when the volume of the heat-resistant fine particles is 100%.

  Further, in order to further improve long-term storage properties, the insulating layer forming slurry may be added with a preservative or a bactericidal agent to suppress the decomposition of the thickener. Specific examples of preservatives and fungicides include benzoic acid, parahydroxybenzoic acid esters, alcohols (ethanol, methanol, etc.), chlorines (sodium hypochlorite, etc.), hydrogen peroxide, acids (boric acid, acetic acid) Etc.) and alkalis (sodium hydroxide, potassium hydroxide, etc.).

  In addition, when the insulating layer forming slurry easily foams and affects the coating property, an antifoaming agent can be used as appropriate. As the antifoaming agent, various defoaming agents of mineral oil type, silicon type, acrylic type and polyether type can be used. Specific examples of antifoaming agents include “Formrex (trade name)” manufactured by Nikka Chemical Co., Ltd., “Surfinol (trade name) series” manufactured by Nissin Chemical Co., Ltd., and “Awazero (trade name) series” manufactured by Sugawara Engineering Co., Ltd. And “SN deformer (trade name) series” manufactured by San Nopco.

  Furthermore, in the slurry for forming the insulating layer, a dispersant can be appropriately used for the purpose of further enhancing the dispersibility of the heat-resistant fine particles and preventing the heat-resistant fine particles from agglomerating with each other. Specific examples of the dispersant include various anionic, cationic, and nonionic surfactants; polymer dispersants such as polyacrylic acid and polyacrylate; and the like. More specifically, ADEKA “Adekator (trade name) series”, “Adekanol (trade name) series”, San Nopco “SN Dispersant (trade name) series”, Lion “Politi (trade name)” "Series", "Armin (trade name) series", "Duomin (trade name) series", "Homogenol (trade name) series", "Leodoll (trade name) series", "Amate (trade name) series" , “Falback (trade name) series”, “Ceramisole (trade name) series”, “Polystar (trade name) series” manufactured by NOF Corporation, “Ajisper (trade name) series” manufactured by Ajinomoto Fine Techno Co., Ltd., Toagosei Co., Ltd. "Aron Dispersant (trade name) series" manufactured by the company.

  The content of the dispersant in the insulating layer forming slurry is preferably 0.1 parts by mass or more with respect to 100 parts by mass of the heat-resistant fine particles from the viewpoint of more effectively exerting the action. If the amount of the dispersant in the insulating layer forming slurry is too large, not only will the effect be saturated, but the ratio of other components in the insulating layer may be reduced, and the effects of these other components may be reduced. Therefore, the amount is preferably 5 parts by mass or less with respect to 100 parts by mass of the heat-resistant fine particles.

  In addition, an additive can be appropriately added to the insulating layer forming slurry for the purpose of controlling the interfacial tension. Examples of the additive include alcohol (methanol, ethanol, isopropanol, ethylene glycol, etc.) and the like.

  In the insulating layer forming slurry of the present invention, the solids content including heat-resistant fine particles, thickener, and a binder used as necessary, heat-meltable fine particles, swellable fine particles described later, It is preferable to set it as 80 mass%.

  The viscosity of the insulating layer forming slurry is preferably 5 mPa · s or more, more preferably 10 mPa · s or more, from the viewpoint of satisfactorily suppressing the precipitation of the heat-resistant fine particles and improving the dispersion stability. More preferably, it is 20 mPa · s or more. In addition, when the viscosity of the insulating layer forming slurry is too high, it becomes difficult to uniformly apply to the required thickness. Therefore, the viscosity is preferably 500 mPa · s or less, and 300 mPa · s or less. It is more preferable, and it is still more preferable that it is 200 mPa * s or less. In addition, the viscosity of the slurry for insulating layer formation in this specification is a value measured at 25 degreeC using a vibration viscometer.

  Various methods conventionally known can be used for the preparation of the insulating layer forming slurry. For example, the various materials (heat-resistant fine particles, thickeners, and binders used as necessary, Insulating layer forming slurry by dispersing preservatives, bactericides, antifoaming agents, dispersing agents, and heat melting fine particles, swellable fine particles, etc. described later) in the medium using various commercially available dispersing devices. (Some materials may be dissolved in the medium). If the dispersion device generates a strong share and affects the properties of polymer substances such as thickeners and binders, heat-resistant fine particles and media, and if necessary, dispersants are added. First, heat-resistant fine particles are dispersed in a medium using a high-share dispersing device, and then a thickener or a binder is added, and a slurry is prepared using a device with a weak share (such as a propeller type stirring device). desirable.

  Specific examples of the dispersing device include media-type dispersers such as a bead mill, a ball mill, a planetary ball mill, and a sand mill; and media-less dispersers such as a jet mill, a rod mill, a nanomizer, and a homogenizer.

  The separator for a lithium ion secondary battery of the present invention comprises at least the slurry for forming an insulating layer of the present invention selected from the group consisting of a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a porous substrate. It has a porous insulating layer formed through a process of applying to one type of substrate and drying.

  When a porous base material is used as a base material on which the insulating layer forming slurry is applied, the insulating layer and the porous base material constitute separate layers, and a separator having a structure in which these are integrated, or an insulating material A separator having a structure in which the component of the slurry for layer formation and the porous substrate are integrated to form an insulating layer can be obtained. Moreover, the separator may be comprised by the layer which the said insulating layer and the porous base material integrated, and another porous layer. In this case, the layer in which the insulating layer and the porous base material are integrated and the porous layer may be integrated, and the porous layer is the layer in which the insulating layer and the porous base material are integrated. These are independent membranes, and these may be stacked in a lithium ion secondary battery to constitute a separator.

  On the other hand, when a positive electrode for a lithium ion secondary battery or a negative electrode for a lithium ion secondary battery is used as a substrate on which the slurry for forming an insulating layer is applied, the insulating layer and the positive electrode for a lithium ion secondary battery and / or lithium ion The negative electrode for the secondary battery is integrated. In this case, the separator may be constituted only by the insulating layer, or the separator may be constituted by such an insulating layer and another porous layer. When the separator is composed of an insulating layer and another porous layer, the insulating layer and the porous layer may be integrated, and the porous layer is an independent film separate from the insulating layer, The separator may be formed by being overlapped with the insulating layer in the lithium ion secondary battery.

  Examples of the positive electrode for lithium ion secondary battery and the negative electrode for lithium ion secondary battery include a positive electrode and a negative electrode for constituting the lithium ion secondary battery of the present invention described later.

  The porous substrate may be any material as long as the material is electrically insulating, stable to the electrochemical reaction inside the lithium ion secondary battery, and stable to the organic electrolyte solution. Or it is preferable that it is a microporous film. Examples of the microporous membrane include those having the same structure as the microporous membrane (microporous film) used in separators such as ordinary lithium ion secondary batteries.

  Specific examples of the constituent material of the porous substrate include cellulose, cellulose modified products (such as carboxymethyl cellulose), polyolefin [polypropylene (PP), polyethylene (PE), and co-polymers containing 85 mol% or more of structural units derived from ethylene. Polymerized polyolefin, etc.], polyester [polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), etc.], resins such as polyacrylonitrile (PAN), aramid, polyamideimide, polyimide; glass, alumina, silica And inorganic materials (inorganic oxides). Examples of the copolymer polyolefin include an ethylene-vinyl monomer copolymer, more specifically, an ethylene-vinyl acetate copolymer (EVA), an ethylene-methyl acrylate copolymer, or an ethylene-ethyl acrylate copolymer. it can. The porous substrate may contain one kind of these constituent materials or may contain two or more kinds. Moreover, the porous base material may contain various known additives (for example, an antioxidant in the case of a resin) as a constituent component in addition to the constituent materials described above. I do not care.

  In constructing a separator using a porous substrate, in the case of placing more importance on the heat resistance of the separator, among the materials exemplified above as the constituent material of the porous substrate, those having high heat resistance (polyester, It is preferable to use a heat-resistant resin such as aramid, polyamideimide, polyimide, or an inorganic material.

  On the other hand, when the lithium ion secondary battery becomes high temperature, the pores of the separator are closed by melting the porous base material, so that ion conduction is inhibited and the safety of the lithium ion secondary battery is ensured, so-called When providing the shutdown function, it is preferable to use a porous substrate made of a material that melts and softens at a predetermined temperature. The temperature at which the constituent resin of the porous base material melts and softens is preferably 80 ° C. or higher, more preferably 100 ° C. or higher, preferably 150 ° C. or lower, more preferably 140 ° C. or lower. The melting and softening temperature of the constituent resin of the porous substrate can be determined by the melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of JIS K 7121 (the heat melting property described later). The same applies to fine particles.) As such a constituent material, polyolefin is preferable, and PE is more preferable. In order to obtain a better shutdown function, it is particularly preferable to configure the separator using a PE microporous film.

  The porous substrate may be subjected to a surface treatment such as a corona treatment or a surfactant treatment in order to enhance the adhesion with the heat-resistant fine particles.

  When a woven fabric or a nonwoven fabric is used as the porous substrate, the constituent material is fibrous, but the fiber diameter may be equal to or less than the thickness of the separator. Specifically, the diameter of the fiber is preferably 0.01 to 5 μm, for example. If the diameter is too large, the entanglement of the fibers will be insufficient, and the strength of the porous substrate composed of these, and consequently the strength of the separator, may be reduced, making handling difficult. On the other hand, if the diameter is too small, the pores of the separator become too small and the ion permeability tends to decrease, which may reduce the load characteristics of the lithium ion secondary battery.

  The woven or non-woven fabric as the porous substrate may use a binder as appropriate in order to bind the fibers together. Examples of the binder include various binders exemplified above as binders contained in the insulating layer (binders contained in the slurry for forming the insulating layer).

  Further, the porous substrate is composed of fibers such as a woven fabric or a non-woven fabric, and especially when the opening diameter of the pores is relatively large (for example, the opening diameter of the pores is 5 μm or more). In this case, this tends to cause a short circuit of the lithium ion secondary battery. Therefore, in this case, it is preferable to have a structure in which part or all of the heat-resistant fine particles are present in the pores of the substrate. Further, it is more preferable that a part or all of fine particles other than the heat-resistant fine particles (such as heat-meltable fine particles and swellable fine particles described later) exist in the pores of the porous substrate. By adopting such a structure, the effect of using fine particles other than the heat-resistant fine particles (shutdown function in the case of hot-melt fine particles or swellable fine particles) is more effectively exhibited. In order to allow heat-resistant fine particles, heat-meltable fine particles, swellable fine particles, etc. to exist in the pores of the porous base material, for example, after impregnating the porous base material with the slurry for forming the insulating layer, What is necessary is just to pass through processes, such as removing an excess slurry and drying after that.

  As the porous substrate used in the separator of the present invention, a nonwoven fabric or a microporous membrane is more preferable in that the thickness can be reduced. As the nonwoven fabric and the microporous membrane, those produced by a conventionally known method can be used. More specifically, a nonwoven fabric produced by a method such as spunbonding, meltblowing, wet, dry, or electrospinning can be used. Moreover, about a microporous film, what was manufactured by well-known various methods, such as a foaming method, a solvent extraction method, a dry-type extending | stretching method, and a wet extending | stretching method, can be used.

  In addition, the material for ensuring a shutdown function can be contained in an insulating layer, or the layer for ensuring a shutdown function can be provided in a separator, and a shutdown function can also be provided to a separator. In particular, in order to ensure the shutdown function of the separator when using a porous substrate composed of a resin with high heat resistance or when configuring a separator without using a porous substrate, Recommended.

  In addition, the layer for imparting a shutdown function to the separator is a separator composed of the above-described layer integrated with the insulating layer and the porous substrate and another porous layer, or a porous material different from the insulating layer. This corresponds to “another porous layer” in a separator composed of a porous layer. Examples of such “another porous layer” include a porous substrate (more preferably a microporous film) made of the material that melts and softens at a predetermined temperature.

  Note that the layer for providing a shutdown function (particularly a microporous film) is likely to undergo thermal shrinkage at high temperatures. However, in the separator of the present invention, the heat of the entire separator is obtained by the action of the insulating layer containing heat-resistant fine particles. Shrinkage is suppressed and the dimensional stability of the separator at high temperatures can be increased. Further, even in a separator having a configuration in which a layer for providing a shutdown function is not separately provided, thermal contraction of the separator is suppressed by the action of the heat-resistant fine particles related to the insulating layer. Therefore, in the lithium ion secondary battery using the separator of the present invention, the occurrence of an internal short circuit due to the shrinkage of the separator at a high temperature can be suppressed, and the reliability and safety can be improved.

  Examples of the material for adding a material for ensuring a shutdown function to the insulating layer include hot-melt fine particles that melt and soften when the temperature inside the lithium ion secondary battery becomes high, and swelling in an organic electrolyte. In addition, swellable fine particles whose degree of swelling increases with an increase in temperature can be mentioned.

  The temperature at which the heat-meltable fine particles are melted and softened is preferably 80 ° C. or higher, more preferably 100 ° C. or higher, preferably 150 ° C. or lower, more preferably 140 ° C. or lower. Specific examples of the constituent material of such heat-meltable fine particles include PE, a copolymerized polyolefin having a structural unit derived from ethylene of 85 mol% or more, PP, or a polyolefin derivative (chlorinated polyethylene, chlorinated polypropylene, etc.), polyolefin Examples thereof include wax, petroleum wax, carnauba wax and the like. Examples of the copolymer polyolefin include an ethylene-vinyl monomer copolymer, more specifically, EVA, an ethylene-methyl acrylate copolymer, or an ethylene-ethyl acrylate copolymer. Moreover, polycycloolefin etc. can also be used. The heat-meltable fine particles may have only one kind of these constituent materials, or may have two or more kinds. Among these, PE, polyolefin wax, or EVA having a structural unit derived from ethylene of 85 mol% or more is preferable. The heat-meltable fine particles may contain various known additives (for example, antioxidants) added to the resin as necessary in addition to the constituent materials described above. I do not care.

  The particle diameter of the heat-meltable fine particles is a number average particle diameter measured by the same measurement method as that of the heat-resistant fine particles, and is preferably 0.001 μm or more, more preferably 0.1 μm or more, and preferably 15 μm. Below, more preferably 1 μm or less.

  When the separator has swellable fine particles that can swell in the organic electrolyte and increase in degree of swelling as the temperature rises, the swelling of the swellable fine particles when exposed to high temperatures in a lithium ion secondary battery By absorbing the organic electrolyte and expanding greatly (hereinafter, the function of increasing the degree of swelling with increasing temperature in the swellable fine particles is referred to as “thermal swelling”), the conductivity of Li ions in the separator is increased. Since the internal resistance of the lithium ion secondary battery is increased, the shutdown function can be reliably ensured. As the swellable fine particles, those having a temperature of 75 to 125 ° C. exhibiting the above heat swellability are preferable.

  Examples of such swellable fine particles having thermal swellability include crosslinked polystyrene (PS), crosslinked acrylic resin [for example, crosslinked polymethyl methacrylate (PMMA)], and crosslinked fluororesin [for example, crosslinked polyvinylidene fluoride (PVDF). ] Is preferred, and cross-linked PMMA is particularly preferred.

  The particle size of the swellable fine particles is a number average particle size measured by dispersing the fine particles in a non-swelling medium (for example, water) using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA). It is preferable that it is 1-20 micrometers.

  Examples of commercially available swellable fine particles include cross-linked PMMA “Gantz Pearl (product name)” manufactured by Gantz Kasei Co., Ltd. and cross-linked PMMA “RSP1079 (product name)” manufactured by Toyo Ink Co., Ltd. are available.

  Note that the shutdown function related to the separator can be evaluated by, for example, an increase in resistance due to the temperature of the model cell. That is, a model cell including a positive electrode, a negative electrode, a separator, and an organic electrolyte is prepared, and the model cell is held in a high-temperature bath, and the internal resistance value of the model cell is increased while the temperature is increased at a rate of 5 ° C./min. By measuring and measuring the temperature at which the measured internal resistance value is at least five times that before heating (resistance value measured at room temperature), this temperature can be evaluated as the shutdown temperature of the separator.

  The separator may have a plurality of layers made of an insulating layer or a porous substrate. For example, a separator may be formed by disposing a microporous film on both surfaces of the insulating layer, or a separator by disposing an insulating layer on both surfaces of the microporous film. However, if the separator has too many layers, it is not preferable because the thickness of the separator is increased, which may increase the internal resistance of the lithium ion secondary battery or decrease the energy density. The number of layers in the separator is 5 It is preferable that it is below a layer.

  The content of the heat-resistant fine particles in the separator of the present invention is preferably 10% by volume or more in the total volume of the constituent components of the separator, from the viewpoint of more effectively exerting the action by using the heat-resistant fine particles, It is more preferably 30% by volume or more, and further preferably 40% by volume or more.

  For example, when an insulating layer is formed on the electrode surface without using a porous base material, the heat-resistant fine particles in the separator are used when the above-mentioned heat-meltable fine particles and swellable fine particles are contained to provide a shutdown function. The upper limit of the volume ratio is preferably 80% by volume, for example. On the other hand, when a separator that does not use a porous base material and does not have a shutdown function, the volume ratio of the heat-resistant fine particles in the separator may be a higher ratio, for example, 95% by volume or less. .

  In addition, when the separator is configured using a porous substrate, the insulating layer contains the heat-fusible fine particles or the swellable fine particles, so that a separator having a shutdown function can be obtained. When a separator is composed of a porous substrate (for example, a microporous film) composed of the above resin and an insulating layer, the upper limit of the volume ratio of the heat-resistant fine particles in the separator is, for example, 60% by volume. preferable. On the other hand, when a porous substrate is used and the separator does not have a shutdown function, the volume ratio of the heat-resistant fine particles in the separator may be a higher ratio, for example, 80% by volume or less.

  In addition, from the viewpoint of ensuring a good shutdown function in the separator, the separator contains a heat-meltable resin (a porous base material or heat-meltable fine particles composed of a heat-meltable resin) and / or swellable fine particles. The amount is preferably 5 to 70% by volume in the total volume of the constituent components of the separator. If the content of the heat-meltable resin or swellable fine particles in the separator is too small, the shutdown effect due to the inclusion thereof may be reduced, and if it is too large, the content of the heat-resistant fine particles in the separator is reduced. Therefore, the effect secured by these may be reduced.

  Therefore, in the slurry for forming an insulating layer, it is preferable that these constituent components are blended in such an amount that the content of each constituent component in the insulating layer after the formation can satisfy the preferable value.

  From the viewpoint of further improving the short-circuit prevention effect of the lithium ion secondary battery, ensuring the strength of the separator, and improving the handleability, the thickness of the separator is preferably 3 μm or more, for example, and is 5 μm or more. It is more preferable. On the other hand, from the viewpoint of further increasing the energy density of the lithium ion secondary battery, the thickness of the separator is preferably 50 μm or less, and more preferably 30 μm or less. For example, when the separator is constituted only by the insulating layer, the thickness of the insulating layer may satisfy the preferable thickness of the separator.

  Further, for example, in the case of a separator having an insulating layer and a porous substrate, which form separate layers (especially when the porous substrate is a microporous film), the thickness of the insulating layer Is X (μm), and the thickness of the porous substrate is Y (μm), the ratio Y / X between X and Y is set to 1 to 10 so that the thickness of the entire separator satisfies the above-mentioned preferable value. It is preferable to do. If Y / X is too large, the insulating layer becomes too thin. For example, when the dimensional stability of the porous substrate at high temperatures is poor, the effect of suppressing the thermal shrinkage may be reduced. On the other hand, if Y / X is too small, the insulating layer becomes too thick, increasing the thickness of the entire separator, which may cause deterioration in characteristics of the lithium ion secondary battery such as load characteristics. When the separator has a plurality of insulating layers, the thickness X is the total thickness, and when the separator has a plurality of porous substrates, the thickness Y is the total thickness.

  In terms of specific values, the thickness of the porous substrate (when the separator has a plurality of porous substrates, the total thickness) is preferably 5 μm or more, and 30 μm or less. preferable. And the thickness of the insulating layer (when the separator has a plurality of insulating layers, the total thickness) is preferably 1 μm or more, more preferably 2 μm or more, further preferably 4 μm or more, Moreover, it is preferable that it is 20 micrometers or less, It is more preferable that it is 10 micrometers or less, It is more preferable that it is 6 micrometers or less.

In addition, the porosity of the separator is preferably 20% or more, and preferably 30% or more in the dried state in order to secure the amount of the organic electrolyte retained and to improve the ion permeability. Is more preferable. On the other hand, from the viewpoint of ensuring the strength of the separator and preventing internal short circuit, the porosity of the separator is preferably 70% or less, and more preferably 60% or less, in a dry state. The porosity of the separator: P (%) can be calculated by obtaining the sum for each component i from the thickness of the separator, the mass per area, and the density of the constituent components using the following equation.
P = Σa _i ρ _i / (m / t)
Here, in the above formula, a _i : ratio of component i expressed by mass%, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of separator (g / cm ² ), t: thickness of separator (cm).

The air permeability of the separator is measured by a method according to JIS P 8117, and is 10 to 300 sec as a Gurley value indicated by the number of seconds that 100 ml of air passes through the membrane under a pressure of 0.879 g / mm ^2. It is desirable to be. If the air permeability is too high, the ion permeability is reduced, and if it is too low, the strength of the separator may be reduced. The air permeability can be ensured by using the separator having the configuration described above.

  The separator of the present invention is formed, for example, on the production method of the present invention, that is, an insulating layer is formed on at least one substrate selected from a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a porous substrate. After the slurry is applied, it can be manufactured by a method of removing the medium by drying at a predetermined temperature.

  When the insulating layer forming slurry is applied to the substrate surface, a conventionally known coating machine such as a die coater, gravure coater, reverse roll coater, squeeze roll coater, curtain coater, blade coater, knife coater, etc. A machine tool can be used.

  Further, when plate-like particles are used as the heat-resistant fine particles in the separator, in order to increase the orientation, when the insulating layer forming slurry is applied to the substrate, the insulating layer forming slurry must be shared. Good. In order to apply a share to the insulating layer forming slurry, for example, in the case of manufacturing a separator using a porous base material, after impregnating the insulating layer forming slurry into the porous base material, an extra portion is passed through a certain gap. What is necessary is just to pass through processes, such as removing a slurry and drying after that.

  In addition, in order to further enhance the orientation of the plate-like heat-resistant fine particles in the separator, in addition to the above-described method of applying a shear, a slurry for forming an insulating layer having a high solid content concentration (for example, 50 to 80% by mass) is used. Method used: Disperse heat-resistant fine particles in a solvent using various mixing / stirring devices such as dispersers, agitators, homogenizers, ball mills, attritors, jet mills, dispersion devices, etc., and a binder (further , A method of using a slurry for forming an insulating layer prepared by adding and mixing hot-melting fine particles, swellable fine particles, etc., if necessary); dispersibility of oils, surfactants, silane coupling agents, etc. on the surface Using a slurry for forming an insulating layer prepared using heat-resistant fine particles whose surface has been modified by the action of an agent; heat resistance with different shapes, diameters or aspect ratios Method of using slurry for forming an insulating layer prepared by using particles together; Method of controlling drying conditions after applying or impregnating a slurry for forming an insulating layer on a porous substrate or coating the surface of a substrate A method in which the separator is pressed or heated and pressed; a method in which the insulating layer forming slurry is applied and impregnated on the base material, or applied to the surface of the base material and then a magnetic field is applied before drying; These methods may be carried out alone or in combination of two or more methods.

  In the production method of the present invention, the insulating layer forming slurry of the present invention capable of maintaining a good dispersion state of the heat-resistant fine particles over a long period of time is used for forming the insulating layer constituting the separator. Therefore, according to the manufacturing method of the present invention, for example, a separator having an insulating layer with high uniformity can be manufactured even if a slurry for forming an insulating layer stored for a long time is used. In addition, according to the manufacturing method of the present invention, even when a long separator is continuously manufactured, it is possible to obtain a separator having a highly uniform insulating layer at the initial stage of manufacturing and at the end of manufacturing and having a stable quality. it can.

  The lithium ion secondary battery of the present invention has the separator of the present invention with good heat resistance, and can suppress the occurrence of internal short circuit due to the thermal contraction of the separator even when the inside of the battery becomes high temperature. Since the separator can also prevent the occurrence of a micro short circuit due to the precipitation of lithium dendrite, it is excellent in safety and reliability.

  As long as the lithium ion secondary battery of the present invention uses the lithium ion secondary battery separator of the present invention, the other configurations and structures are not particularly limited, and conventionally known lithium ion secondary batteries can be used. Various configurations and structures can be employed.

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

The positive electrode is not particularly limited as long as it is a positive electrode used in a conventionally known lithium ion secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing Li ions. For example, as an active material, lithium-containing transition metal oxide represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, etc.); lithium manganese such as LiMn ₂ O ₄ Oxide; LiMn _x M _(1-x) O _{2 in} which part of Mn of LiMn ₂ O ₄ is substituted with another element; olivine type LiMPO ₄ (M: Co, Ni, Mn, Fe); LiMn _0.5 Ni _0.5 O ₂ ; Li _{(1 + a)} Mn _x Ni _y Co _(1-xy) O ₂ (−0.1 <a <0.1, 0 <x <0.5, 0 <y <0. 5); can be applied, and a known conductive additive (carbon material such as carbon black) or a binder such as polyvinylidene fluoride (PVDF) is appropriately added to these positive electrode active materials. The positive electrode mixture is formed from a current collector as a core material (ie, Such as those finished electrode mixture layer) may be used.

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

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

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

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

  As with the lead portion on the negative electrode side, the negative electrode layer (including a layer having a negative electrode active material and a negative electrode mixture layer) is usually formed on a part of the current collector during the preparation of the negative electrode. Without leaving the exposed portion of the current collector, it is provided as a lead portion. However, the lead portion on the negative electrode side is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

  The electrode can be used in the form of an electrode group having a laminated structure in which the positive electrode and the negative electrode are laminated via the separator of the present invention, or a wound structure electrode group in which the electrode is wound.

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

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

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

  Also, instead of the organic solvent, melting at room temperature such as ethyl-methylimidazolium trifluoromethylsulfonium imide, heptyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide A salt can also be used.

  Further, a polymer material that contains the organic electrolyte and gels may be added, and the organic electrolyte may be made into a gel to be used for a battery. As a polymer material for making the organic electrolyte into a gel, PVDF, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), PAN, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer, Examples of the host polymer capable of forming a known gel electrolyte, such as a crosslinked polymer having an ethylene oxide chain in the main chain or side chain, and a crosslinked poly (meth) acrylate.

  The lithium ion secondary battery of the present invention can be applied to the same uses as various uses in which conventionally known lithium ion secondary batteries are used.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention. The average particle size and aspect ratio of various fine particles shown below are values measured by the above method.

<Preparation of slurry for insulating layer formation>
Example 1
The distilled water was sterilized with an ultraviolet lamp having a wavelength of 365 nm for 30 minutes to obtain purified water. About the said purified water, when the number of fungi and the number of viable bacteria were confirmed by the said method, all were 10 or less in 1 mL.

  Plate-like boehmite (average particle size: 1 μm, aspect ratio: 10), which is heat-resistant fine particles, was washed with water and vacuum-dried at 120 ° C.

  To 1000 g of the plate-like boehmite washed with water, 1000 g of the purified water and 1 part by mass of ammonium polyacrylate (dispersing agent) with respect to 100 parts by mass of the plate-like boehmite are added, and this is added for 6 days using a table-top ball mill. Dispersion was performed to obtain a dispersion. The pH of this dispersion was measured with a zeta potential measurement device [“Zetaprobe ZP-11 (trade name)” manufactured by Nippon Bell Co., Ltd.] and found to be 10.

  A self-crosslinking acrylic resin emulsion as a binder (3 parts by mass with respect to 100 parts by mass of plate boehmite) is added to the dispersion, and 2 g of xanthan gum is further added as a thickener. The mixture was stirred and dispersed for 1 hour to obtain a uniform slurry for forming an insulating layer. The viscosity of this insulating layer forming slurry was measured at 25 ° C. using a vibration viscometer [“SV-10 (trade name)” manufactured by A & D Co., Ltd.] and found to be 30 mPa · s. Moreover, it was 9.5 when pH of this slurry for insulating layer formation was measured using the said zeta potential measuring apparatus.

  Furthermore, the stability of the slurry for forming an insulating layer was evaluated by measuring the sedimentation height in a sample tube using a Turbid scan [“MA-2000 (trade name)” manufactured by Eihiro Seiki Co., Ltd.]. . The slurry for forming the insulating layer was injected into the sample tube at a height of 60 mm, the backscattered light intensity was measured, the height up to the point where the scattered light intensity became 1 or more was measured, and the settling height was obtained. The sedimentation height in one week after the start of measurement was 54 mm.

  Moreover, when the viscosity after storing for 3 months from the preparation of the slurry for forming the insulating layer was measured at 25 ° C. in the same manner as described above, it was 30 mPa · s, and the viscosity change with the passage of the storage period was I was not able to admit. Hereinafter, among the above-mentioned slurry for forming an insulating layer, the slurry immediately after the preparation is referred to as the slurry for forming an insulating layer of Example 1-1, and the slurry stored for 3 months from the preparation is used for forming the insulating layer of Example 1-2. This is called slurry.

Example 2
In a dispersion containing plate-like boehmite prepared in the same manner as in Example 1 and ammonium polyacrylate, an emulsion of a self-crosslinking acrylic resin as a binder (3 parts by mass with respect to 100 parts by mass of plate-type boehmite). Further, 6 g of xanthan gum and purified water (purified water subjected to the same treatment as that used in the preparation of the insulating layer forming slurry in Example 1) were added as a thickener, and 1 by three-one motor. The mixture was stirred for time and dispersed to obtain a uniform slurry for forming an insulating layer. The purified water introduced into the dispersion together with xanthan gum was confirmed to have a fungal count and viable count by the sterility test method described in the Japanese Pharmacopoeia General Test Method, and both were 50 or less in 50 mL.

  About the obtained slurry for forming an insulating layer, the pH measured in the same manner as in Example 1 was 7.5, and the sedimentation height after one week measured in the same manner as in Example 1 was 56 mm. The viscosity after preparation and the viscosity after storage for 3 months were 15 mPa · s and 15 mPa · s, respectively. Hereinafter, among the slurry for forming an insulating layer, the slurry immediately after the preparation is referred to as the slurry for forming an insulating layer of Example 2-1, and the slurry stored for 3 months from the preparation is used for forming the insulating layer of Example 2-2. This is called slurry.

Example 3
An insulating layer forming slurry was prepared in the same manner as in Example 1 except that the thickener was changed to carboxymethylcellulose: 6 g. About the obtained slurry for forming an insulating layer, the pH measured in the same manner as in Example 1 was 8.7, and the sedimentation height after one week measured in the same manner as in Example 1 was 55 mm. The viscosity after preparation and the viscosity after storage for 3 months were 70 mPa · s and 70 mPa · s, respectively. Hereinafter, among the above-mentioned slurry for forming an insulating layer, the slurry immediately after the preparation is referred to as the slurry for forming an insulating layer of Example 3-1, and the slurry stored for 3 months from the preparation is used for forming the insulating layer of Example 3-2. This is called slurry.

Example 4
To the insulating layer forming slurry prepared in the same manner as in Example 1, 0.1 g of a polyether-based antifoaming agent was further added to prepare an insulating layer forming slurry. For this insulating layer forming slurry, the pH, the sedimentation height after 1 week, the viscosity after preparation, and the viscosity after storage for 3 months from the preparation were measured in the same manner as in Example 1. It was the same as the prepared slurry for insulating layer formation. Hereinafter, among the slurry for forming an insulating layer, the slurry immediately after the preparation is referred to as the slurry for forming an insulating layer of Example 4-1, and the slurry stored for 3 months from the preparation is used for forming the insulating layer of Example 4-2. This is called slurry.

Comparative Example 1
Slurry for forming an insulating layer in the same manner as in Example 1 except that granular boehmite (average particle size 0.6 μm) is used instead of plate-like boehmite, and ion-exchanged water is used instead of sterilized purified water. Was prepared. In addition, when the said ion-exchange water confirmed the number of fungi and viable bacteria by the said method, they were 80 and 90, respectively, in 1 mL.

  About the obtained insulating layer forming slurry, the pH measured in the same manner as in Example 1 was 7.0, and the sedimentation height after one week measured in the same manner as in Example 1 was 56 mm. Moreover, about the said slurry for insulating layer formation, the viscosity after the preparation measured in the same manner as in Example 1, the viscosity after one month from the preparation and after storage for three months from the preparation are 30 mPa · s and 30 mPa · s, respectively. s, 25 mPa · s, and the viscosity decreased by about 17% by storage for 3 months. Hereinafter, among the slurry for forming an insulating layer, the slurry immediately after the preparation is referred to as the slurry for forming an insulating layer of Comparative Example 1-1, and the slurry stored for 3 months from the preparation is used for forming the insulating layer of Comparative Example 1-2. This is called slurry.

Comparative Example 2
A slurry for forming an insulating layer was prepared in the same manner as in Example 1 except that granular silica (average particle diameter 1.3 μm) was used instead of plate-like boehmite. About the obtained insulating layer forming slurry, the pH measured in the same manner as in Example 1 was 6.7, and the sedimentation height after one week measured in the same manner as in Example 1 was 56 mm. Moreover, about the said slurry for insulating layer formation, the viscosity after the preparation measured in the same manner as in Example 1, the viscosity after one month from the preparation and after storage for three months from the preparation are 40 mPa · s and 35 mPa · s, respectively. s, 30 mPa · s, and the viscosity decreased by about 25% by storage for 3 months. Hereinafter, among the slurry for forming an insulating layer, the slurry immediately after preparation is referred to as the slurry for forming an insulating layer of Comparative Example 2-1, and the slurry stored for 3 months from the preparation is used for forming the insulating layer of Comparative Example 2-2. This is called slurry.

Comparative Example 3
A slurry for forming an insulating layer was prepared in the same manner as in Example 1 except that the plate boehmite was used without being washed with water. The pH measured in the same manner as in Example 1 was 11.5, and the viscosity after preparation measured in the same manner as in Example 1 was 40 mPa · s. However, the viscosity gradually increased, and one day after the preparation, The viscosity was 200 mPa · s.

<Manufacture of separator>
Examples 5-12 and Comparative Examples 4-8
Insulating layer forming slurries prepared in Examples 1 to 4 and Comparative Example 1 (immediately after preparation and those stored for 3 months after preparation) were uniformly stirred and degassed, and then each insulating layer was formed. A PET nonwoven fabric having a thickness of 15 μm was passed through the slurry, and the slurry was applied by pulling up and then passing through a gap having a predetermined interval, followed by drying to produce a separator having a thickness of 20 μm. Table 1 shows the insulating layer forming slurry used for the production of each separator. As described above, the slurry for forming the insulating layer prepared in Comparative Example 3 is greatly thickened after 1 day from the preparation, and the PET nonwoven fabric is excellent in the same conditions as the other slurry for forming the insulating layer. Since it could not be applied, only the product immediately after preparation was used for the production of the separator. Moreover, in the case where the insulating layer forming slurry of Examples 4-1 and 4-2 containing an antifoaming agent was used, it was not necessary to perform a defoaming treatment, and thus this treatment was omitted to produce a separator.

  The insulating layer forming slurries of Examples 1 to 4 are highly stable and can be applied uniformly and continuously, either immediately after preparation or stored for 3 months from preparation. The separators of Examples 5 to 12 each had an insulating layer with high uniformity.

  On the other hand, the insulating layer forming slurries of Comparative Example 1 and Comparative Example 2 were stored immediately after preparation when stored for 3 months from the preparation (insulating layer forming slurries of Comparative Example 1-2 and Comparative Example 2-2). Since the viscosity change from is large, it could not be satisfactorily applied to the PET nonwoven fabric (Comparative Examples 5 and 7).

Examples 13-20
The insulating layer forming slurries of Examples 1 to 4 (immediately after preparation and those stored for 3 months from the preparation) were separately applied to one side of another PE microporous membrane (long and 16 μm thick). A separator having a thickness of 20 μm was manufactured by continuously applying and drying using a ter. Table 2 shows the insulating layer forming slurry used for the production of each separator.

  The slurry for forming an insulating layer of Examples 1 to 4 can be applied uniformly and continuously, either immediately after preparation or stored for 3 months from preparation, and Example 13 produced using these slurries. Each of the? -20 separators had a highly uniform insulating layer.

<Manufacture of lithium ion secondary batteries>
Example 21
85 parts by mass of LiCoO _{2 as a} positive electrode active material, 10 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of PVDF as a binder are uniformly mixed with N-methyl-2-pyrrolidone (NMP) as a solvent. It mixed so that positive electrode mixture containing paste might be prepared. This paste is intermittently applied on both sides of a 15 μm thick aluminum foil serving as a current collector so that the coating length is 320 mm on the front surface and 250 mm on the back surface, dried, and then calendered to a total thickness of 150 μm. Thus, the thickness of the positive electrode material mixture layer was adjusted and cut so as to have a width of 43 mm to produce a positive electrode having a length of 340 mm and a width of 43 mm. Further, the exposed portion of the aluminum foil of the positive electrode was tabbed.

  Also, a negative electrode mixture-containing paste was prepared by mixing 90 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVDF as a binder so as to be uniform using NMP as a solvent. This negative electrode mixture-containing paste is intermittently applied on both sides of a 10 μm-thick current collector made of copper foil so that the coating length is 200 mm on the front surface and 260 mm on the back surface, dried, and then calendered to obtain a total thickness. The thickness of the negative electrode mixture layer was adjusted to 142 μm and cut to a width of 45 mm to produce a negative electrode having a length of 330 mm and a width of 45 mm. Further, a tab was attached to the exposed portion of the copper foil of the negative electrode.

The positive electrode and the negative electrode obtained as described above were overlapped with each other through the separator of Example 5 and wound in a spiral shape to form a wound electrode group. This electrode group was inserted into a cylindrical battery case having a diameter of 14 mm and a height of 50 mm, and an electrolyte (LiPF ₆ was added to a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2 at a ratio of 1.2 mol / liter. 1) was injected, and the opening of the battery case was sealed according to a conventional method to produce a lithium ion secondary battery.

  In the pre-charging after battery assembly (charging at the time of chemical formation), constant current charging up to 4.2V at 150mA and then constant voltage at 4.2V so that the amount of electricity is 20% of the rated capacity of 750mAh. Voltage charging was performed for a total of 6 hours, and then a constant current discharge was performed up to 3 V at 150 mA.

Examples 22-36
A lithium ion secondary battery was produced in the same manner as in Example 21 except that the separator was changed to that shown in Table 3.

About the lithium ion secondary battery of Examples 21-36, after carrying out the constant current charge to 4.2V and the constant voltage charge at 4.2V with the current 150mA, it discharges to 3.0V with 150mA, and is charged. When the discharge characteristics were evaluated, it was found that the battery had good charge / discharge characteristics and good reliability.

Claims (9)

A slurry for forming an insulating layer having ion permeability and heat resistance, applicable to a separator for a lithium ion secondary battery,
Contains at least heat-resistant fine particles, thickener and medium,
The medium is mainly composed of 50 or less water each per 1 mL of fungi and viable bacteria determined by the sterility test method described in the Japanese Pharmacopoeia General Test Methods,
A slurry for forming an insulating layer, having a pH of 7 to 11.   The slurry for forming an insulating layer according to claim 1, wherein the medium is sterilized by gamma rays, ethylene oxide gas or ultraviolet rays.   The insulating layer-forming slurry according to claim 1 or 2, comprising at least one kind of fine particles selected from the group consisting of metal oxides, metal hydroxides, and metal hydroxide oxides as heat-resistant fine particles.   The slurry for forming an insulating layer according to any one of claims 1 to 3, wherein the thickener is a natural polysaccharide.   The slurry for forming an insulating layer according to any one of claims 1 to 4, having a viscosity of 5 to 500 mPa · s.   For forming an insulating layer according to any one of claims 1 to 5, wherein at least one substrate selected from the group consisting of a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a porous substrate is used. A separator for a lithium ion secondary battery, comprising a porous insulating layer formed through a step of applying a slurry.   The separator for a lithium ion secondary battery according to claim 6, wherein the porous substrate is a woven fabric, a nonwoven fabric or a microporous membrane.   For forming an insulating layer according to any one of claims 1 to 5, wherein at least one substrate selected from the group consisting of a positive electrode for a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery, and a porous substrate is used. A method for producing a separator for a lithium ion secondary battery, comprising: applying a slurry continuously by a coater and drying to form a porous insulating layer integrated with the substrate.   A lithium ion secondary battery comprising at least a positive electrode, a negative electrode, and a separator, wherein the separator is a separator for a lithium ion secondary battery according to claim 6 or 7.