JP2011192529A - Polyolefin microporous film, separator for nonaqueous secondary battery, and nonaqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyolefin microporous film capable of giving excellent ion conductivity in addition to heat resistance and shutdown characteristics when compounded with a heat-resistant porous layer. <P>SOLUTION: The polyolefin microporous film has a fibril diameter of 30-80 nm, a hole diameter of 30-70 nm, and a rate of surface aperture of 15-25%. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a polyolefin microporous membrane, and particularly to a technique for improving the safety and battery characteristics of a non-aqueous secondary battery.

  In general, a lithium ion secondary battery uses a lithium-containing transition metal oxide typified by lithium cobaltate as a positive electrode, and uses a carbon material capable of doping and dedoping lithium as a negative electrode. Such a non-aqueous secondary battery typified by a lithium ion secondary battery is important as a power source for portable electronic devices typified by mobile phones because of its high energy density. With the rapid spread of, the demand is increasing. In addition, many automobiles that are environmentally conscious, such as hybrid cars, have been developed, but lithium ion secondary batteries having a high energy density have attracted a great deal of attention as one of the power sources to be mounted.

  Many lithium ion secondary batteries are composed of a laminate of a positive electrode, a separator containing an electrolytic solution, and a negative electrode. The separator is responsible for preventing a short circuit between the positive electrode and the negative electrode as a main function, but other required properties include lithium ion mobility, strength, and durability.

  At present, many types of polyolefin microporous membranes have been proposed as films suitable for lithium ion secondary battery separator applications. Among polyolefin microporous membranes, polyethylene microporous membranes meet the above-mentioned required characteristics, and have a function of preventing thermal runaway by shutting off current from hole blockage due to high temperatures, a so-called shutdown function, as a safety function at high temperatures. It is widely used as a separator for lithium ion secondary batteries.

  However, even if the pores of the microporous membrane are blocked due to the temperature rise and the current is interrupted once, if the battery temperature exceeds the melting point of polyethylene constituting the microporous membrane and exceeds the heat resistance limit of polyethylene, the microporous membrane The membrane itself melts and the shutdown function is lost. As a result, a short circuit between the electrodes causes a thermal runaway of the battery, which may cause destruction of a device incorporating the lithium ion secondary battery or occurrence of an accident due to ignition. For this reason, in order to ensure further safety, there is a demand for a separator that can maintain a shutdown function even at high temperatures.

  Therefore, Patent Document 1 proposes a separator for a nonaqueous secondary battery in which the surface of a polyethylene microporous membrane is covered with a heat resistant porous layer made of a heat resistant polymer such as wholly aromatic polyamide. Patent Document 2 discloses a configuration in which inorganic fine particles such as alumina are included in a heat-resistant porous layer to improve heat resistance in addition to a shutdown function. Patent Document 3 shows a structure in which metal hydroxide particles such as aluminum hydroxide are included in the heat-resistant porous layer to improve the flame retardancy in addition to the shutdown function and heat resistance. ing. Any of these configurations can be expected to have an excellent effect in terms of battery safety in terms of both a shutdown function and heat resistance.

  However, the above-mentioned separator for non-aqueous secondary batteries has a structure in which a polyolefin microporous membrane is coated with a heat-resistant porous layer, so that the polyolefin microporous membrane alone is excellent for keeping the separator's ion mobility good. Ionic conductivity was sought. However, if the polyolefin microporous membrane is thinned to improve the ionic conductivity, the mechanical strength of the polyolefin microporous membrane is lowered, resulting in a problem that the mechanical strength of the separator for non-aqueous secondary batteries is lowered.

JP 2005-209570 A International Publication No. 2008/062727 Pamphlet International Publication No. 2008/156033 Pamphlet

  An object of the present invention is to provide a polyolefin microporous membrane capable of imparting excellent mechanical strength and ionic conductivity in addition to heat resistance and shutdown characteristics when combined with a heat resistant porous layer.

  As a result of intensive studies to provide a polyolefin microporous membrane capable of imparting excellent mechanical strength in addition to heat resistance and shutdown characteristics when combined with a heat-resistant porous layer, the inventors have obtained fibril diameter, pore diameter. The inventors have found that this is possible with a polyolefin microporous membrane with a controlled surface porosity, and have reached the present invention.

That is, the present invention is a polyolefin microporous membrane characterized in that the fibril diameter is 30 to 80 nm, the pore diameter is 30 to 70 nm, and the surface porosity is 15 to 25%.
The present invention also provides a separator for a non-aqueous secondary battery in which the polyolefin microporous membrane is coated with a heat resistant porous layer. In addition, the present invention is a non-aqueous secondary battery that obtains an electromotive force by doping or dedoping lithium, wherein the non-aqueous secondary battery separator is used.

  In the present invention, it is possible to provide a polyolefin microporous membrane capable of imparting excellent mechanical strength and ionic conductivity in addition to heat resistance and shutdown characteristics when combined with a heat resistant porous layer. The polyolefin microporous membrane of the present invention can provide a separator for a non-aqueous secondary battery and a non-aqueous secondary battery that can improve safety.

  Hereinafter, embodiments of the present invention will be sequentially described. In addition, these description and Examples illustrate this invention, and do not restrict | limit the scope of the present invention.

[Polyolefin microporous membrane]
In the polyolefin microporous membrane of the present invention, the microporous membrane has a structure in which a large number of micropores are connected to each other, and these micropores are connected to each other. Refers to a membrane that can pass through.

  Examples of the raw material for the polyolefin microporous membrane used in the present invention include polyolefins, for example, polyethylene, polypropylene, polymethylpentene and copolymers thereof. Among these, polyethylene is preferable, and high-density polyethylene and a mixture of high-density polyethylene and ultrahigh molecular weight polyethylene are more preferable from the viewpoints of strength, heat resistance, and the like. The polyolefin microporous membrane used in the present invention is not limited as long as 90% by weight or more is made of polyolefin, and may contain 10% by weight or less of other components that do not affect the battery characteristics.

  The polyolefin microporous membrane of the present invention is characterized in that the fibril diameter is 30 to 80 nm, the pore diameter is 30 to 70 nm, and the surface porosity is 15 to 25%. When the fibril diameter, the pore diameter, and the surface open area ratio are in this range, the ionic conductivity and mechanical strength of the separator for nonaqueous secondary batteries using the polyolefin microporous membrane of the present invention are excellent.

  When the fibril diameter is less than 30 μm, the mechanical strength of one fibril is low, and as a result, the mechanical strength of the polyolefin microporous membrane is lowered, which is not preferable. On the other hand, when the fibril diameter exceeds 80 μm, the microporous diameter of the polyolefin microporous membrane simultaneously increases, and the mechanical strength tends to decrease, which is not preferable.

  When the pore diameter is less than 30 μm, the mobility of the electrolytic solution is lowered, and the battery characteristics tend to be lowered, which is not preferable. Conversely, when the pore diameter exceeds 70 μm, the mechanical strength of the polyolefin microporous membrane tends to decrease, which is not preferable.

  The surface porosity of the polyolefin microporous membrane of the present invention is preferably 15 to 25%. There are several methods for calculating the surface open area ratio. Specifically, it can be calculated using the fibril diameter and the pore diameter of the polyolefin microporous membrane. When the surface area ratio is less than 15%, the cushioning property due to the network structure is lowered and the mechanical strength tends to be lowered, which is not preferable. On the contrary, when the surface open area ratio exceeds 25%, the mechanical strength of the polyolefin microporous membrane tends to decrease, which is not preferable.

There is no particular limitation on the method for controlling the fibril diameter, the pore diameter, and the surface open area ratio, and examples thereof include a method for controlling the raw material charge ratio of the polyolefin microporous film, stretching conditions, and annealing conditions.
Regarding the raw material conditions, the surface porosity increases and the pore diameter increases as the polyolefin component decreases. For example, when obtaining a film having a surface porosity of 15 to 25%, the polyolefin component in the raw material is preferably 10 to 30%.

  As for the stretching conditions, the more tensioned stretching conditions are used, the larger the surface opening rate, the smaller the fibril diameter, and the larger the pore diameter. For example, when it is intended to obtain a film having a surface open area ratio of 15 to 25%, the stretching in the machine direction is carried out at a melting point of the polyolefin minus 25 to 50 ° C. at a stretching ratio of 6 to 10 times, or in the machine vertical direction. It is preferable to carry out the draw ratio at a draw ratio of 6 to 15 times at a melting point of the polyolefin minus 25 to 50 ° C.

  Regarding annealing conditions, the higher the annealing temperature, the larger the surface open area ratio, the smaller the fibril diameter, and the larger the pore diameter. Is preferred.

  The film thickness of the polyolefin microporous membrane of the present invention is preferably 5 to 25 μm. When the film thickness is less than 5 μm, the mechanical strength is insufficient and the handling property may be deteriorated. If it exceeds 25 μm, the ionic conductivity is lowered, and it may be difficult to achieve sufficient load characteristics, which is not preferable.

  The porosity of the polyolefin microporous membrane of the present invention is preferably 30 to 60%. More preferably, it is 40% to 60%. When the porosity is less than 30%, the permeability is lowered and the mobility of lithium ions is lowered, which is not preferable. On the other hand, when the porosity exceeds 60%, the mechanical strength is insufficient and the handling property may be deteriorated.

  The Gurley value (JIS P8117) of the polyolefin microporous membrane of the present invention is preferably 50 to 500 sec / 100 cc. When the Gurley value is in this range, the separator mechanical strength and the membrane resistance are balanced. If it is less than 50 sec / 100 cc, the mechanical strength of the separator tends to decrease, which is not preferable. When it exceeds 500 sec / 100 cc, the membrane resistance of the polyolefin microporous membrane tends to decrease, which is not preferable.

The membrane resistance of the microporous polyolefin membrane of the present invention is preferably 0.5 to ⁵ ohm · cm ² . Since the membrane resistance of the polyolefin microporous membrane affects the load characteristics of the non-aqueous secondary battery separator and non-aqueous secondary battery obtained by processing the polyolefin microporous membrane, a smaller one is preferable.

  The puncture strength of the polyolefin microporous membrane of the present invention is preferably 300 g or more. When it is less than 300 g, when a non-aqueous secondary battery is produced, pinholes or the like are generated in the separator due to the unevenness of the electrode or impact, and the possibility that the non-aqueous secondary battery is short-circuited increases. When it is 300 g or more, it means that it has sufficient strength as a battery separator, and indicates that handling and durability at the time of manufacturing and processing are high.

  The tensile strength of the polyolefin microporous membrane of the present invention is preferably 10 N or more. When less than 10N, when winding a separator when producing a non-aqueous secondary battery, possibility that a separator will be damaged becomes high. 10N or more means that the battery has sufficient strength as a battery separator, and indicates that the handleability is high when manufacturing and processing.

The shutdown temperature of the polyolefin microporous membrane of the present invention is preferably 130 to 150 ° C. The shutdown temperature is the temperature at which the resistance value is 10 ³ ohm · cm ² . When the shutdown temperature is lower than 130 ° C., the phenomenon called meltdown, in which the polyolefin microporous membrane completely melts and the short circuit phenomenon occurs, is generated at a low temperature. Absent. Further, when the shutdown temperature is higher than 150 ° C., a sufficient safety function at a high temperature cannot be expected, which is not preferable. Preferably it is 135-145 degreeC.

  The heat shrinkage rate at 105 ° C. of the polyolefin microporous membrane of the present invention is preferably 5 to 30% or less. When the thermal shrinkage is in this range, the shape stability and shutdown characteristics of the separator for a non-aqueous secondary battery obtained by processing the polyolefin microporous membrane are balanced. When the heat shrinkage rate is less than 5%, it means that the polyolefin has poor fluidity, and the shutdown characteristics are lowered, which is not preferable. When the thermal shrinkage rate exceeds 30%, the shape stability at high temperature is deteriorated, which is not preferable.

[Production method of microporous polyolefin membrane]
Although there is no restriction | limiting in particular in the manufacturing method of the polyolefin microporous membrane of this invention, Specifically, manufacturing through the following (1)-(6) processes is preferable. The polyolefin used as a raw material is as described above.

(1) Preparation of polyolefin solution A solution in which polyolefin is dissolved in a solvent is prepared. At this time, a solution may be prepared by mixing solvents. Examples of the solvent include paraffin, liquid paraffin, paraffin oil, mineral oil, castor oil, tetralin, ethylene glycol, glycerin, decalin, toluene, xylene, diethyltriamine, ethyldiamine, dimethyl sulfoxide, hexane, and the like. The concentration of the polyolefin solution is preferably 1 to 35% by weight, more preferably 10 to 30% by weight. When the concentration of the polyolefin solution is less than 1% by weight, the gel-like molded product obtained by cooling and gelation is highly swollen with a solvent, so that it is easily deformed, which may hinder handling. On the other hand, if it exceeds 35% by weight, the pressure at the time of extrusion increases, so the discharge amount may decrease and the productivity may not be increased. Further, the orientation in the extrusion process proceeds, and stretchability and uniformity cannot be ensured. There is a case.

(2) Extrusion of polyolefin solution The adjusted solution is kneaded with a single screw extruder or a twin screw extruder, and extruded with a T die or an I die at a temperature not lower than the melting point and not higher than the melting point + 60 ° C. A twin screw extruder is preferably used. Then, the extruded solution is passed through a chill roll or a cooling bath to form a gel composition. At this time, it is preferable to rapidly cool below the gelation temperature to cause gelation.

(3) Desolvation treatment Next, the solvent is removed from the gel composition. When a volatile solvent is used, the solvent can also be removed from the gel composition by evaporating by heating or the like, which also serves as a preheating step. In the case of a non-volatile solvent, the solvent can be removed by squeezing out under pressure. The solvent need not be completely removed.

(4) Stretching of the gel-like composition After the solvent removal treatment, the gel-like composition is stretched. Here, a relaxation treatment may be performed before the stretching treatment. In the stretching treatment, the gel-like molded product is heated and biaxially stretched at a predetermined magnification by a normal tenter method, roll method, rolling method, or a combination of these methods. Biaxial stretching may be simultaneous or sequential. Moreover, it can also be set as longitudinal multistage extending | stretching, 3 or 4 steps extending | stretching.
The stretching temperature is preferably 90 ° C. to less than the melting point of the polyolefin, more preferably 100 to 120 ° C. When the heating temperature exceeds the melting point, it cannot be stretched because the gel-like molded product is dissolved. On the other hand, when the heating temperature is less than 90 ° C., the gel-like molded product is not sufficiently softened, and the film is likely to be broken during stretching, making it difficult to stretch at a high magnification.
Moreover, although a draw ratio changes with thickness of an original fabric, it carries out by at least 2 times or more in a uniaxial direction, Preferably it is 4-20 times.
After stretching, heat setting is performed as necessary to provide thermal dimensional stability.

(5) Extraction and removal of solvent The stretched gel composition is immersed in an extraction solvent to extract the solvent. Examples of the extraction solvent include hydrocarbons such as pentane, hexane, heptane, cyclohexane, decalin, and tetralin, chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride, and methylene chloride, fluorinated hydrocarbons such as ethane trifluoride, diethyl, and the like. Easily volatile compounds such as ethers and ethers such as dioxane can be used. These solvents are appropriately selected according to the solvent used for dissolving the polyolefin composition, and can be used alone or in combination. Solvent extraction removes the solvent in the microporous membrane to less than 1% by weight.

(6) Annealing of microporous film The microporous film is heat-set by annealing. Annealing is performed at 80 to 150 ° C.

[Separator for non-aqueous secondary battery]
A separator for a non-aqueous secondary battery according to the present invention includes the above-described polyolefin microporous membrane and a heat-resistant porous layer formed by including a heat-resistant resin and laminated on one or both sides of the polyolefin microporous membrane. It is a separator for non-aqueous secondary batteries.

  According to such a separator for a non-aqueous secondary battery of the present invention, a polyolefin microporous membrane as a base material can provide a shutdown function, and the heat-resistant porous layer retains the polyolefin even at a temperature higher than the shutdown temperature. Therefore, safety at high temperatures can be secured. Therefore, according to the separator of the present invention, a non-aqueous secondary battery excellent in safety can be obtained.

  The separator for a non-aqueous secondary battery of the present invention preferably has a total film thickness of 30 μm or less. When a separator exceeds 30 micrometers, there exists a tendency for the energy density of a non-aqueous secondary battery to fall, and it is unpreferable.

  The porosity of the nonaqueous secondary battery separator of the present invention is preferably 30 to 70%. More preferably, it is 40% to 60%. When the porosity is less than 30%, the permeability is lowered and the mobility of lithium ions is lowered, which is not preferable. On the other hand, when the porosity exceeds 70%, the mechanical strength becomes insufficient and the handling property is lowered, which is not preferable.

  The Gurley value (JIS P8117) of the separator for a non-aqueous secondary battery of the present invention is preferably 100 to 500 sec / 100 cc. When the Gurley value is in this range, the separator mechanical strength and the membrane resistance are balanced. If it is less than 100 sec / 100 cc, the mechanical strength of the separator tends to decrease, which is not preferable. When it exceeds 500 sec / 100 cc, the membrane resistance of the separator for non-aqueous secondary batteries tends to decrease, which is not preferable.

The membrane resistance of the non-aqueous secondary battery separator of the present invention is preferably 1.5 to 10 ohm · cm ² . Since the membrane resistance of the non-aqueous secondary battery separator affects the load characteristics of the non-aqueous secondary battery, a smaller one is preferable.

  The puncture strength of the nonaqueous secondary battery separator of the present invention is preferably 300 g or more. When the puncture strength is less than 300 g, when a non-aqueous secondary battery is produced, pinholes or the like may be generated in the separator due to electrode irregularities or impacts, which may cause a short circuit of the non-aqueous secondary battery. 300 g or more means having sufficient strength as a separator for a non-aqueous secondary battery, and indicates that handling and durability at the time of manufacturing and processing are high.

  The tensile strength of the nonaqueous secondary battery separator of the present invention is preferably 10 N or more. If it is less than 10N, the separator is likely to be damaged when the separator is wound when a non-aqueous secondary battery is produced. 10N or more means that it has sufficient strength as a battery separator for non-aqueous secondary batteries, and indicates that it is easy to handle and process.

The shutdown temperature of the separator for a non-aqueous secondary battery of the present invention is preferably 130 to 155 ° C. The shutdown temperature is the temperature at which the resistance value is 10 ³ ohm · cm ² . When the shutdown temperature is less than 130 ° C., a phenomenon called meltdown, in which the polyolefin microporous film is completely melted and a short-circuit phenomenon occurs at the same time, is not preferable for safety. Moreover, when the shutdown temperature is higher than 155 ° C., a sufficient safety function at a high temperature cannot be expected, which is not preferable. Preferably it is 135-150 degreeC.

  The thermal contraction rate at 105 ° C. of the separator for a non-aqueous secondary battery of the present invention is preferably 0.5 to 10%. When the heat shrinkage rate is within this range, the shape stability and shutdown characteristics of the non-aqueous secondary battery separator are balanced. When it is 10% or more, the shape stability at high temperature is deteriorated, which is not preferable. Preferably it is 0.5 to 5%.

[Heat-resistant porous layer]
In the present invention, examples of the heat-resistant porous layer include microporous film-like, non-woven fabric, paper-like, and other layers having a three-dimensional network-like porous structure. From the viewpoint of being obtained, the layer is preferably a microporous film. Here, the microporous film-like layer has a large number of micropores inside, and has a structure in which these micropores are connected. Gas or liquid can pass from one surface to the other. It says the layer that became.

  As the heat-resistant resin used in the present invention, a polymer having a melting point of 200 ° C. or higher or a polymer having no melting point but having a decomposition temperature of 200 ° C. or higher is suitable, preferably a wholly aromatic polyamide, polyimide, polyamideimide, polysulfone, It is at least one resin selected from the group consisting of polyketone, polyetherketone, polyetherimide and cellulose. In particular, wholly aromatic polyamides are preferable from the viewpoint of durability, and polymetaphenylene isophthalamide, which is a meta-type wholly aromatic polyamide, is more preferable from the viewpoint of easy formation of a porous layer and excellent redox resistance. is there.

  In the present invention, the heat-resistant porous layer may be formed on both surfaces or one surface of the polyolefin microporous membrane. However, from the viewpoint of handling properties, durability, and the effect of suppressing heat shrinkage, it is preferable to form the heat-resistant porous layer on both surfaces of the substrate. preferable. In order to fix the heat-resistant porous layer on the substrate, a method in which the heat-resistant porous layer is directly formed on the substrate by a coating method is preferable. A technique of adhering the quality layer sheet on the base material using an adhesive or the like, a technique such as heat fusion or pressure bonding can also be employed.

  In the present invention, regarding the thickness of the heat resistant porous layer, when the heat resistant porous layer is formed on both surfaces of the substrate, the total thickness of the heat resistant porous layer may be 3 μm or more and 12 μm or less. Preferably, when the heat resistant porous layer is formed only on one side of the substrate, the thickness of the heat resistant porous layer is preferably 3 μm or more and 12 μm or less. The porosity of the heat resistant porous layer is preferably in the range of 60 to 90%.

[Inorganic filler]
In the present invention, the heat resistant porous layer preferably contains an inorganic filler. The inorganic filler is not particularly limited, but specifically, metal oxides such as alumina, titania, silica, zirconia, metal carbonates such as calcium carbonate, metal phosphates such as calcium phosphate, aluminum hydroxide, hydroxide A metal hydroxide such as magnesium is preferably used. Such an inorganic filler is preferably highly crystalline from the viewpoints of impurity elution and durability.

  Especially, as an inorganic filler, what produces an endothermic reaction in 200-400 degreeC is preferable. Although it does not specifically limit as an inorganic filler which has such a characteristic, It is an inorganic filler which consists of a metal hydroxide, a boron salt compound, or a clay mineral, Comprising: What produces an endothermic reaction in 200-400 degreeC is mentioned. Specific examples include aluminum hydroxide, magnesium hydroxide, calcium aluminate, dosonite, zinc borate and the like, and these can be used alone or in combination of two or more. In addition, these flame retardant inorganic fillers are appropriately mixed with other inorganic fillers such as metal oxides such as alumina, zirconia, silica, magnesia, and titania, metal nitrides, metal carbides, and metal carbonates. You can also.

  Here, in the non-aqueous secondary battery, heat generation accompanying the decomposition of the positive electrode is considered to be the most dangerous, and this decomposition occurs in the vicinity of 300 ° C. For this reason, if the generation | occurrence | production temperature of endothermic reaction is the range of 200 to 400 degreeC, it is effective in preventing the heat_generation | fever of a non-aqueous secondary battery. It should be noted that at 200 ° C. or higher, since the negative electrode almost loses its activity, it does not react with water generated from the metal hydroxide to cause heat generation and is safe. Moreover, when the endothermic reaction temperature of an inorganic filler exceeds 400 degreeC, since there exists a possibility that the heat_generation | fever of a non-aqueous secondary battery cannot be prevented suitably, it is unpreferable. For example, aluminum hydroxide, dawsonite, and calcium aluminate undergo a dehydration reaction in the range of 200 to 300 ° C, and magnesium hydroxide and zinc borate undergo a dehydration reaction in the range of 300 to 400 ° C. It is preferable to use at least one of them.

  In particular, in the present invention, the inorganic filler is preferably made of a metal hydroxide. Since the metal hydroxide undergoes a dehydration reaction accompanied by a large endotherm by heating, an effect of improving flame retardancy due to both the release of water and endotherm can be obtained. The release of water is effective in diluting the flammable electrolyte and making the battery itself incombustible. In addition, since metal hydroxide is a softer material than metal oxides such as alumina, the parts used in each process at the time of manufacturing are worn by the inorganic filler contained in the separator. There is no problem. In addition, when a metal hydroxide such as aluminum hydroxide or magnesium hydroxide is added to the heat-resistant porous layer, the charged charge decays quickly, so that the charge can be kept at a low level and handling properties are improved. Is improved. Furthermore, aluminum hydroxide and magnesium hydroxide have the function of adsorbing and co-precipitating hydrofluoric acid, so it is possible to maintain the hydrofluoric acid concentration in the electrolyte at a low level, and the durability of non-aqueous secondary batteries Can be improved. Therefore, the inorganic filler is preferably a metal hydroxide, and particularly preferably aluminum hydroxide or magnesium hydroxide.

  In the present invention, the average particle size of the inorganic filler is preferably in the range of 0.1 to 2 μm. When the average particle diameter of the inorganic filler exceeds 2 μm, the short circuit resistance at high temperature of the heat resistant porous layer is lowered, which is not preferable. Further, there is a problem that it hinders the formation of the heat-resistant porous layer with an appropriate thickness. In addition, when the average particle size of the inorganic filler is less than 0.1 μm, not only does the coating film strength decrease and the problem of powder falling occurs, but using such a small one is substantially from the viewpoint of cost. Have difficulty.

  In this invention, it is preferable that content of the inorganic filler in a heat resistant porous layer is 50 to 95 weight%. If the content of the inorganic filler is less than 50% by weight, the effect of improving heat resistance by the inorganic filler may not be sufficiently obtained, which is not preferable. On the other hand, if the content of the inorganic filler exceeds 95% by weight, the heat-resistant porous layer may be excessively densified and ion permeability may be reduced, or the heat-resistant porous layer may become brittle and handling properties may be reduced. This is not preferable.

  In addition, the inorganic filler in the heat resistant porous layer is present in a state of being trapped by the heat resistant resin when the heat resistant porous layer is in the form of a microporous film, and the heat resistant porous layer is a nonwoven fabric or the like. In such a case, it may be present in the constituent fibers or fixed to the nonwoven fabric surface or the like with a binder such as a resin.

[Method for producing heat-resistant porous layer]
In the present invention, the method for producing a separator for a non-aqueous secondary battery is not particularly limited as long as the separator of the present invention having the above-described configuration can be produced. For example, it can be produced through the following steps (1) to (5). It is.

(1) Preparation of coating slurry A heat-resistant resin is dissolved in a solvent to prepare a coating slurry. The solvent is not particularly limited as long as it dissolves the heat-resistant resin, but specifically, a polar solvent is preferable, and examples thereof include N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and dimethylsulfoxide. Moreover, the said solvent can add the solvent used as a poor solvent with respect to heat resistant resin in addition to these polar solvents. By applying such a poor solvent, a microphase separation structure is induced and the formation of a heat-resistant porous layer is facilitated. As the poor solvent, alcohols are preferable, and polyhydric alcohols such as glycol are particularly preferable. The concentration of the heat resistant resin in the coating slurry is preferably 4 to 9% by weight. Moreover, an inorganic filler is disperse | distributed to this as needed, and it is set as the slurry for coating. In dispersing the inorganic filler in the coating slurry, when the dispersibility of the inorganic filler is not preferable, a method of improving the dispersibility by surface-treating the inorganic filler with a silane coupling agent or the like is also applicable.

(2) Coating of slurry The slurry is coated on at least one surface of the polyolefin microporous membrane. When forming a heat-resistant porous layer on both surfaces of a polyolefin microporous film, it is preferable from a viewpoint of shortening of a process to apply simultaneously on both surfaces of a base material. Examples of the method for coating the slurry for coating include a knife coater method, a gravure coater method, a screen printing method, a Meyer bar method, a die coater method, a reverse roll coater method, an ink jet method, a spray method, and a roll coater method. . Among these, the reverse roll coater method is preferable from the viewpoint of uniformly forming a coating film. When applying simultaneously to both sides of the base material, for example, by passing the base material between a pair of Meyer bars, an excess coating slurry is applied to both sides of the base material, and this is applied to a pair of reverse roll coaters. There is a method of precise measurement by scraping off excess slurry in between.

(3) Solidification of slurry The base material on which the slurry is applied is treated with a coagulating liquid capable of coagulating the heat-resistant resin. Heat-resistant porous material with inorganic filler bound by coagulating the heat-resistant resin by treating the substrate coated with the coating slurry with a coagulating liquid capable of coagulating the heat-resistant resin. Form a layer. As a method of treating with the coagulating liquid, a method of spraying the coagulating liquid on the substrate coated with the coating slurry, or a method of immersing the substrate in a bath (coagulating bath) containing the coagulating liquid. Etc. Here, when installing a coagulation bath, it is preferable to install it below the coating apparatus. The coagulation liquid is not particularly limited as long as it can coagulate the heat-resistant resin, but water or a solution obtained by mixing an appropriate amount of water with the solvent used in the slurry is preferable. Here, the mixing amount of water is preferably 40 to 80% by weight with respect to the coagulation liquid. When the amount of water is less than 40% by weight, there arises a problem that the time required for solidifying the heat-resistant resin becomes long or the solidification becomes insufficient. On the other hand, when the amount of water is more than 80% by weight, there arises a problem that the cost for solvent recovery becomes high, or the surface that comes into contact with the coagulation liquid is solidified too quickly and the surface is not sufficiently porous.

(4) Removal of coagulating liquid The coagulating liquid is removed by washing with water.

(5) Drying Dry and remove water from the sheet. Although there is no particular limitation on the drying method, the drying temperature is preferably 50 to 80 ° C. When a high drying temperature is applied, a method of contacting with a roll to prevent dimensional change due to heat shrinkage. It is preferable to apply.

[Non-aqueous secondary battery]
The non-aqueous secondary battery of the present invention is a non-aqueous secondary battery that obtains an electromotive force by doping or dedoping lithium, and is characterized by using the separator for a non-aqueous secondary battery having the above-described configuration. Or the non-aqueous secondary battery of this invention may use the polyolefin microporous film | membrane surface-treated with the ozone water mentioned above as a separator. The non-aqueous secondary battery has a structure in which a battery element in which a negative electrode and a positive electrode face each other with a separator interposed therebetween is impregnated with an electrolytic solution, and this is enclosed in an exterior.

  The negative electrode has a structure in which a negative electrode mixture composed of a negative electrode active material, a conductive additive and a binder is formed on a current collector. Examples of the negative electrode active material include materials capable of electrochemically doping lithium, and examples thereof include carbon materials, silicon, aluminum, tin, and wood alloys. Examples of the conductive assistant include carbon materials such as acetylene black and ketjen black. The binder is made of an organic polymer, and examples thereof include polyvinylidene fluoride and carboxymethyl cellulose. As the current collector, a copper foil, a stainless steel foil, a nickel foil, or the like can be used.

The positive electrode has a structure in which a positive electrode mixture composed of a positive electrode active material, a conductive additive and a binder is formed on a current collector. Examples of the positive electrode active material include lithium-containing transition metal oxides. Specifically, LiCoO ₂ , LiNiO ₂ , LiMn _0.5 Ni _0.5 O ₂ , LiCo _1/3 Ni _1/3 Mn _{1/3 O 2, LiMn 2 O 4,} LiFePO 4 , and the like. Examples of the conductive assistant include carbon materials such as acetylene black and ketjen black. The binder is made of an organic polymer, and examples thereof include polyvinylidene fluoride. As the current collector, aluminum foil, stainless steel foil, titanium foil, or the like can be used.

The electrolytic solution has a structure in which a lithium salt is dissolved in a non-aqueous solvent. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO _{4 and the} like. Examples of the non-aqueous solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, vinylene carbonate, and the like. These may be used alone or in combination.

  Examples of the exterior material include a metal can or an aluminum laminate pack. The shape of the battery includes a square shape, a cylindrical shape, a coin shape, and the like, but the separator of the present invention can be suitably applied to any shape.

Examples are shown below, but the present invention is not limited thereto.
[Measuring method]
Each value in this example was determined according to the following method.
(1) The film thickness of the polyolefin microporous membrane and the separator for a non-aqueous secondary battery was determined by measuring 20 points with a contact-type film thickness meter (manufactured by Mitutoyo Corporation) and averaging them. Here, the contact terminal used was a cylindrical one having a bottom surface of 0.5 cm in diameter.

(2) The basis weight of the polyolefin microporous membrane and the separator for a non-aqueous secondary battery is obtained by cutting a sample into 10 cm × 10 cm and measuring the weight. By dividing this weight by the area, the basis weight which is the weight per 1 m ² was obtained.

(3) The porosity of the polyolefin microporous membrane and the non-aqueous secondary battery separator is
It calculated | required from (epsilon) = {1-Ws / (ds * t)} * 100.
Here, ε: porosity (%), Ws: basis weight (g / m ² ), ds: true density (g / cm ³ ), and t: film thickness (μm).

(4) The fibril diameter, pore diameter, and surface area ratio of the polyolefin microporous membrane were determined by the following methods.
The BET specific surface area of the polyolefin microporous membrane was measured according to JIS K 8830. Specifically, NOVA-1200 (manufactured by Yuasa Ionics Co., Ltd.) was used for analysis and determination by a nitrogen gas adsorption method. The sample weight during measurement was 0.1 to 0.15 g. The analysis was carried out by a three-point method, and the specific surface area Ss (m ² / g) was determined from the BET plot.
The pore diameter is Vs ₁ , the total pore volume is Vs ₂ , the fibril diameter is Rs ₁ , the pore diameter is Rs ₂ , the fibril total length Ls ₁ , and the cylindrical hole total length is Ls _2. Formulas (i) to (v) hold.
Ss · Ws = πRs ₁ · Ls ₁ = πRs ₂ · Ls ₂ (i)
^{_{Vs 1 = π (Rs 1/}} 2) 2 · Ls 1 ··· (ii)
^{_{Vs 2 = π (Rs 2/}} 2) 2 · Ls 2 ··· (iii)
Vs ₂ = ε · (Vs ₁ + Vs ₂ ) (iv)
Vs ₁ = Ws / ds (v)
Here, Ss: specific surface area (m ² / g), Ws: basis weight (g / m ² ), ε: porosity (%), ds: specific gravity (g / cm ³ ).
Rs ₁ and Rs ₂ can be obtained from the equations (i) to (v).
Using the fibril diameter and the hole diameter obtained here, the surface opening ratio D was determined using the following formula.
D = [Rs ₂ / (Rs ₁ + Rs ₂ )] ² × 100

(5) The Gurley values of the polyolefin microporous membrane and the non-aqueous secondary battery separator were determined according to JIS P8117.

(6) The membrane resistance of the polyolefin microporous membrane and the non-aqueous secondary battery separator was determined by the following method.
First, a sample is cut out to a size of 2.6 cm × 2.0 cm. A sample cut out in a methanol solution (methanol: manufactured by Wako Pure Chemical Industries, Ltd.) in which 3% by weight of a nonionic surfactant (Emalgen 210P manufactured by Kao Corporation) is dissolved is immersed and air-dried. A 20 μm thick aluminum foil is cut into 2.0 cm × 1.4 cm and a lead tab is attached. Two aluminum foils are prepared, and a sample cut between the aluminum foils is sandwiched so that the aluminum foils are not short-circuited. The sample is impregnated with 1M LiBF ₄ propylene carbonate / ethylene carbonate (1/1 weight ratio) as an electrolyte. This is sealed under reduced pressure in an aluminum laminate pack so that the tab comes out of the aluminum pack. Such cells are prepared so that there are one, two, and three separators in the aluminum foil, respectively. The cell is placed in a constant temperature bath at 20 ° C., and the resistance of the cell is measured by an AC impedance method at an amplitude of 10 mV and a frequency of 100 kHz. The measured resistance value of the cell is plotted against the number of separators, and the plot is linearly approximated to obtain the slope. The film resistance (ohm · cm ² ) per separator was determined by multiplying the inclination by 2.0 cm × 1.4 cm which is the electrode area.

(7) The piercing strength of the polyolefin microporous membrane and the separator for the non-aqueous secondary battery was measured using a KES-G5 handy compression tester manufactured by Kato Tech Co., with a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec. The puncture test was performed under the conditions, and the maximum puncture load was defined as the puncture strength. Here, the sample was fixed by sandwiching a silicon rubber packing in a metal frame (sample holder) having a hole of Φ11.3 mm.

(8) Tensile strength of polyolefin microporous membrane and non-aqueous secondary battery separator was adjusted to 10 × 100 mm using a tensile tester (A & D, RTC-1225A), load cell load 5 kgf, distance between chucks Measurement was performed under the condition of 50 mm.

(9) The shutdown temperature of the polyolefin microporous membrane and the non-aqueous secondary battery separator was determined by the following method.
First, the sample was punched into Φ19 mm, and the sample cut out in a methanol solution (methanol: Wako Pure Chemical Industries, Ltd.) in which 3% by weight of a nonionic surfactant (Emulgen 210P manufactured by Kao Corporation) was dissolved was immersed and air-dried. The sample was sandwiched between SUS plates with a diameter of 15.5 mm. The sample was impregnated with 1M LiBF ₄ propylene carbonate / ethylene carbonate (1/1 weight ratio) (manufactured by Kishida Chemical Co., Ltd.) as an electrolyte. This was enclosed in a 2032 type coin cell. I took the lead from the coin cell, put a thermocouple, and put it in the oven. The temperature of the cell was increased at a temperature increase rate of 1.6 ° C./min, and the resistance of the cell was measured at the same time by the AC impedance method at an amplitude of 10 mV and a frequency of 100 kHz. The temperature at which the resistance value reached 10 ³ ohm · cm ² or more was taken as the shutdown temperature.

(10) The thermal shrinkage rate of the polyolefin microporous membrane and the non-aqueous secondary battery separator was measured by heating the sample at 105 ° C. for 1 hour.

(11) The discharge property evaluation of the non-aqueous secondary battery was performed by the following method.
10 cycles of constant current / constant voltage charging at 1.6 mA, 4.2 V for 8 hours, and constant current discharge at 1.6 mA, 2.75 V were carried out 10 cycles, and the discharge capacity obtained at the 10th cycle was determined for this battery. Discharge capacity. Next, constant current / constant voltage charging was performed at 1.6 mA and 4.2 V for 8 hours, and constant current discharging was performed at 16 mA and 2.75 V. The capacity obtained at this time was divided by the discharge capacity of the battery at the 10th cycle, and the obtained value was used as an index of load characteristics.

[Example 1]
Ticona's GUR2126 (weight average molecular weight 4150,000, melting point 141 ° C.) and GURX143 (weight average molecular weight 560,000, melting point 135 ° C.) were used as polyethylene powder. Liquid paraffin (Smoyl P-350, boiling point 480 ° C. by Matsumura Oil Research Co., Ltd.) and decalin (Wako Pure) so that GUR2126 and GURX143 become 3: 7 (weight ratio) and the polyethylene concentration becomes 30% by weight. A polyethylene solution was prepared by dissolving in a mixed solvent having a boiling point of 193 ° C. manufactured by Yakuhin Co., Ltd. The composition of the polyethylene solution is polyethylene: liquid paraffin: decalin = 30: 60: 10 (weight ratio).
This polyethylene solution was extruded from a die at 148 ° C. and cooled in a water bath to prepare a gel tape (base tape). The base tape was dried at 60 ° C. for 8 minutes and at 95 ° C. for 15 minutes, and the base tape was stretched by biaxial stretching in which longitudinal stretching and lateral stretching were sequentially performed. Here, the longitudinal stretching was 5.5 times, the stretching temperature was 90 ° C., the transverse stretching was 11 times the stretching ratio, and the stretching temperature was 105 ° C. After transverse stretching, heat setting was performed at 118 ° C. Next, this was immersed in a methylene chloride bath to extract liquid paraffin and decalin. Then, the polyolefin microporous film was obtained by drying at 50 degreeC and annealing at 120 degreeC. The resulting polyolefin microporous membrane had a structure in which fibrillar polyolefins were entangled in a network and constituted pores.
Measurement of properties (thickness, basis weight, Gurley value, porosity, membrane resistance, puncture strength, tensile strength, shutdown temperature, thermal shrinkage, fibril diameter, pore diameter, surface porosity) of the obtained polyolefin microporous film The results are shown in Table 1. The following Comparative Examples 1 and 2 are similarly shown in Table 1.

[Comparative Example 1]
A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that the composition of the polyethylene solution was polyethylene: liquid paraffin: decalin = 20: 80: 0 (weight ratio) and the heat setting temperature was 130 ° C.

[Comparative Example 2]
A polyolefin microporous membrane was obtained in the same manner as in Example 1 except that the composition of the polyethylene solution was polyethylene: liquid paraffin: decalin = 20: 20: 60 (weight ratio) and the heat setting temperature was 130 ° C.

[Example 2]
Using the polyolefin microporous membrane obtained in Example 1, a heat-resistant porous layer made of a heat-resistant resin was laminated on both sides to produce a separator for a non-aqueous secondary battery of the present invention.
Specifically, polymetaphenylene isophthalamide (manufactured by Teijin Techno Products, Conex) was used as the heat resistant resin. This heat-resistant resin was dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) in a weight ratio of 50:50 to prepare a coating slurry. The concentration of polymetaphenylene isophthalamide in the coating slurry was adjusted to 5.5% by weight. Then, two Meyer bars were opposed to each other, and an appropriate amount of coating liquid was put between them. Thereafter, the polyolefin microporous film was passed between Mayer bars on which the coating liquid was placed, and the coating liquid was applied to the front and back surfaces of the polyolefin microporous film. Here, the clearance between the Meyer bars was set to 20 μm, and # 6 was used for both the Mayer bars. This was immersed in a coagulating liquid having a weight ratio of water: DMAc: TPG = 50: 25: 25 and 40 ° C., followed by washing and drying. This obtained the separator for non-aqueous secondary batteries in which the heat resistant porous layer was formed in the both surfaces of the polyolefin microporous film.
Table 2 shows the measurement results of the characteristics (film thickness, basis weight, Gurley value, porosity, membrane resistance, puncture strength, tensile strength, shutdown temperature, heat shrinkage rate) of the obtained nonaqueous secondary battery separator. The following Examples 3 and 4 and Comparative Examples 3 and 4 are also shown in Table 2.

[Example 3]
The polyolefin microporous membrane obtained in Example 1 was used, and a heat-resistant porous layer made of a heat-resistant resin and an inorganic filler was laminated thereon to produce the non-aqueous secondary battery separator of the present invention.
Specifically, polymetaphenylene isophthalamide (manufactured by Teijin Techno Products, Conex) was used as the heat resistant resin. This heat resistant resin was dissolved in a mixed solvent of dimethylacetamide (DMAc) and tripropylene glycol (TPG) in a weight ratio of 50:50. In this polymer solution, α-alumina (SA-1, manufactured by Iwatani Chemical Industry Co., Ltd., average particle size 0.8 μm) as an inorganic filler was dispersed to prepare a slurry for coating. The concentration of polymetaphenylene isophthalamide in the coating slurry was adjusted to 5.5% by weight, and the weight ratio of polymetaphenylene isophthalamide to inorganic filler was adjusted to 25:75. Then, two Meyer bars were opposed to each other, and an appropriate amount of coating liquid was put between them. Thereafter, the polyolefin microporous film was passed between Mayer bars on which the coating liquid was placed, and the coating liquid was applied to the front and back surfaces of the polyolefin microporous film. Here, the clearance between the Meyer bars was set to 20 μm, and # 6 was used for both the Mayer bars. This was immersed in a coagulating liquid having a weight ratio of water: DMAc: TPG = 50: 25: 25 and 40 ° C., followed by washing and drying. This obtained the separator for non-aqueous secondary batteries in which the heat resistant porous layer was formed in the both surfaces of the polyolefin microporous film.

[Example 4]
A separator for a non-aqueous secondary battery was obtained in the same manner as in Example 3 except that magnesium hydroxide (Kyowa Chemical Industry, Kisuma-5P, average particle size 1.0 μm) was used as the inorganic filler.

[Comparative Example 3]
A non-aqueous secondary battery separator was obtained in the same manner as in Example 4 except that the polyolefin microporous membrane obtained in Comparative Example 1 was used and the Mayer bar clearance was 22 μm.

[Comparative Example 4]
A non-aqueous secondary battery separator was obtained in the same manner as in Example 4 except that the polyolefin microporous film obtained in Comparative Example 2 was used and the Mayer bar clearance was 17 μm.

[Example 5]
Lithium cobalt oxide (LiCoO ₂ : manufactured by Nippon Chemical Industry Co., Ltd.) 89.5 parts by weight, acetylene black (Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd.) 4.5 parts by weight, polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) 6 parts by weight Thus, these were kneaded using N-methyl-pyrrolidone to prepare a slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed to obtain a positive electrode having a thickness of 100 μm.
87 parts by weight of mesophase carbon micro beads (MCMB: manufactured by Osaka Gas Chemical Co., Ltd.), 3 parts by weight of acetylene black (trade name Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) and 10 parts by weight of polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) These were kneaded using N-methyl-2pyrrolidone to prepare a slurry. The obtained slurry was applied onto a copper foil having a thickness of 18 μm, dried and pressed to obtain a negative electrode having a thickness of 90 μm.
The positive electrode and the negative electrode were opposed to each other through the non-aqueous secondary battery separator produced in Example 2. This was impregnated with an electrolytic solution and sealed in an exterior made of an aluminum laminate film to produce a non-aqueous secondary battery. Here, 1M LiPF ₆ ethylene carbonate / ethyl methyl carbonate (3/7 weight ratio) (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolytic solution.
Here, this prototype battery has a positive electrode area of 2 × 1.4 cm ² , a negative electrode area of 2.2 × 1.6 cm ² , and a set capacity of 8 mAh (in the range of 4.2V-2.75V).
Table 3 shows the measurement results of the characteristics (discharge capacity, load characteristics) of the obtained nonaqueous secondary battery. The following Examples 6 and 7 and Comparative Examples 5 and 6 are similarly shown in Table 3.

[Example 6]
A nonaqueous secondary battery was obtained in the same manner as in Example 5 except that the separator obtained in Example 3 was used as the separator for the nonaqueous secondary battery.

[Example 7]
A nonaqueous secondary battery was obtained in the same manner as in Example 5 except that the separator obtained in Example 4 was used as the separator for the nonaqueous secondary battery.

[Comparative Example 5]
A non-aqueous secondary battery was obtained in the same manner as in Example 5 except that the separator obtained in Comparative Example 3 was used as the separator for the non-aqueous secondary battery.

[Comparative Example 6]
A non-aqueous secondary battery was obtained in the same manner as in Example 5 except that the separator obtained in Comparative Example 4 was used as the separator for the non-aqueous secondary battery.

  The polyolefin microporous membrane of the present invention controls the fibril diameter, the pore diameter, and the surface porosity, thereby shutting down the nonaqueous secondary battery separator combined with the heat resistant porous layer made of a heat resistant resin, In addition to heat resistance, mechanical strength is excellent. This ensures the safety and battery characteristics of the non-aqueous secondary battery using the non-aqueous secondary battery separator.

Claims (7)

  A polyolefin microporous membrane having a fibril diameter of 30 to 80 nm, a pore diameter of 30 to 70 nm, and a surface porosity of 15 to 25%.   A non-aqueous secondary battery comprising the polyolefin microporous membrane according to claim 1 and a heat-resistant porous layer formed by containing a heat-resistant resin and laminated on one or both sides of the polyolefin microporous membrane. Separator for use.   The heat-resistant resin is at least one resin selected from the group consisting of wholly aromatic polyamide, polyimide, polyamideimide, polysulfone, polyketone, polyetherketone, polyetherimide, and cellulose. A separator for a non-aqueous secondary battery according to 1.   The separator for a nonaqueous secondary battery according to any one of claims 2 to 3, wherein the heat resistant porous layer contains an inorganic filler.   The separator for a non-aqueous secondary battery according to claim 4, wherein the inorganic filler is an inorganic filler that generates an endothermic reaction at 200 to 400 ° C.   The separator for a non-aqueous secondary battery according to claim 5, wherein the inorganic filler is aluminum hydroxide or magnesium hydroxide.   A non-aqueous secondary battery that obtains an electromotive force by doping and dedoping of lithium, wherein the non-aqueous secondary battery separator according to claim 2 is used. Secondary battery.