JP2005071978A - Separator for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve battery performance such as a load characteristic of a nonaqueous electrolyte secondary battery using a separator comprising a thermoplastic resin porous membrane containing a filler. <P>SOLUTION: This separator for the nonaqueous electrolyte secondary battery comprises the porous membrane containing the filler in a thermoplastic resin. In the separator, a ratio of an average hole diameter (μm) to a maximum hole diameter (μm) defined by ASTM F316-86 is 0.6 or above. The nonaqueous electrolyte secondary battery uses the separator. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a separator for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

  Specifically, the present invention relates to a separator for a non-aqueous electrolyte secondary battery that realizes a secondary battery excellent in battery performance, in which discharge efficiency is extremely low even during high-load discharge, and non-aqueous electrolysis using this separator. The present invention relates to a liquid secondary battery.

Lithium secondary batteries, which are non-aqueous electrolyte secondary batteries having high energy density and light weight with the reduction in weight and size of electrical products, are used in a wide range of fields. A lithium secondary battery has a positive electrode in which an active material layer containing a positive electrode active material such as a lithium compound typified by lithium cobaltate is formed on a current collector, and occlusion / release of lithium typified by graphite. A non-aqueous solution in which an active material layer containing a negative electrode active material such as a carbon material is formed on a current collector and an electrolyte such as a lithium salt such as LiPF ₆ dissolved in a normal aprotic non-aqueous solvent It is mainly composed of an electrolytic solution and a separator made of a porous film.

  Separators used in lithium secondary batteries are required to satisfy requirements such as not impeding ion conduction between both electrodes, being able to hold an electrolytic solution, and having resistance to an electrolytic solution. A porous film made of a thermoplastic resin such as polypropylene or polypropylene is used.

Conventionally, as a method for producing these porous membranes, for example, the following methods are known as known techniques.
(1) An extraction method in which a plasticizer that can be easily extracted and removed in a post-process is added to a polymer material and then molded, and then the plasticizer is removed with a suitable solvent to make it porous.
(2) A stretching method in which after forming a crystalline polymer material, a structurally weak amorphous portion is selectively stretched to form micropores.
(3) An interfacial exfoliation method in which a filler is added to a polymer material, molding is performed, and the interface between the polymer material and the filler is exfoliated by a subsequent stretching operation to form micropores.

  As a method for obtaining a porous membrane made of thermoplastic resin, the extraction method (1) described above requires treatment of a large amount of waste liquid, and has problems in both environmental and economic aspects. Further, it is difficult to obtain a uniform film due to the contraction of the film generated in the extraction process, and there is a problem in productivity such as yield. The stretching method (2) requires heat treatment for a long time in order to control the pore size distribution by controlling the structure of the crystalline phase and the amorphous phase before stretching, which is problematic in terms of productivity.

  In addition, as an improved technique of (1), JP-A-6-240036 discloses a solution of a polyolefin containing an ultrahigh molecular weight component and having a large molecular weight distribution, and extruding it to form a sheet, By subjecting the gel-like sheet obtained by quenching to stretching and solvent removal at a specific temperature, a polyolefin porous membrane having a sharp pore size distribution with a maximum pore size / average through pore size value of 1.5 or less is obtained. It is disclosed.

  However, in this method, in order to form uniform holes and expand them to a practical size, it is essential to perform two stretching operations at different temperatures. Compared with the extraction method, the process is complicated and there is a problem in terms of productivity. In addition, since there are two stretching steps, problems such as stretching unevenness in each step may be considered. Furthermore, since this method is essentially an extraction method, it is necessary to treat a large amount of waste liquid as described above, which is problematic in terms of environment and economy. In addition, it is difficult to obtain a uniform film due to the shrinkage of the film generated in the extraction process, and there is a problem in productivity such as yield.

On the other hand, the interfacial peeling method (3) has no waste liquid and is excellent in both environmental and economic aspects. In addition, since the interface between the polymer material and the filler can be easily peeled off by a stretching operation, a porous film can be obtained without the need for a pretreatment such as heat treatment, which is excellent in terms of productivity. It is a technique. As a porous film by the interfacial peeling method, for example, JP 2002-201298 A discloses a porous film composed of a thermoplastic resin and a filler, the thickness of which is Y (μm), the Gurley value. (Gurley air permeability) of the _{T GUR} (sec / 100 cc), when the average pore diameter and d ([mu] _m), a porous to the _{X R} defined by _{X R = 25 × T GUR ×} d 2 ÷ Y less than 5 A membrane is disclosed.
JP-A-6-240036 JP 2002-201298 A

  However, in the conventional porous membrane by the interfacial peeling method, attention is paid to the average particle diameter of the filler to be blended for porosity as described in JP-A-2002-201298. However, there is no example of interest in the particle size distribution, and as a result, the use of a filler with a large ratio of large particle size particles reduces pore connectivity for the reasons described later, and desirable performance. It was not possible to obtain a separator having For example, as shown in Example 1 of Japanese Patent Application Laid-Open No. 2002-201298, the load characteristics of the battery are a discharge capacity of about 4C and a discharge capacity of about 40% at most with respect to the discharge capacity at the discharge rate C / 3. It was not obtained.

  Therefore, the present invention is a case where a separator composed of a porous film made of a thermoplastic resin containing a filler obtained by an interface peeling method excellent in environment, economy and productivity is applied to a non-aqueous electrolyte secondary battery. Even so, an object of the present invention is to realize a non-aqueous electrolyte secondary battery excellent in battery performance such as load characteristics.

The separator for a non-aqueous electrolyte secondary battery according to the present invention is a separator for a non-aqueous electrolyte secondary battery comprising a porous film containing a filler in a thermoplastic resin, and is defined by ASTM F316-86. The ratio d _ave / d _max between the average pore diameter d _ave (μm) and the maximum pore diameter d _max (μm) is 0.6 or more.

  A non-aqueous electrolyte secondary battery of the present invention includes a positive electrode capable of inserting and extracting lithium ions, a negative electrode capable of inserting and extracting lithium ions, an electrolytic solution containing an electrolyte in a non-aqueous solvent, and a non-aqueous system having a separator In the electrolyte secondary battery, the separator according to the present invention is used as the separator.

  As a result of intensive studies on the properties of the filler and the control of the film physical properties of the thermoplastic resin porous membrane containing the filler, the present inventors have not been industrially used as a filler. By using a filler with a specific particle size distribution, the porous membrane made of thermoplastic resin obtained by the interfacial peeling method has extremely uniform pore size, and extremely excellent battery performance as a separator for non-aqueous electrolyte secondary batteries, In particular, the present invention has been completed by finding that the load characteristics can be improved.

  The reason why a nonaqueous electrolyte secondary battery excellent in load characteristics can be obtained by using the separator for nonaqueous electrolyte secondary batteries of the present invention is considered as follows.

  That is, in the interfacial exfoliation method, the particle size distribution of the filler has a very large influence on the film structure because the interface between the base resin and the filler is exfoliated by a stretching operation to form a porous structure. For example, even when fillers having the same average particle size are used, particles having a large particle size distribution are mixed with particles having a large particle size, so that the total number of particles is smaller than that having a narrow particle size distribution. This means that the number of starting points of the opening in the stretching operation is reduced, and it is considered that the ion passage resistance is increased due to a decrease in hole connectivity. The control of the particle size distribution of the filler can be managed by selecting the shape of the filler in the interfacial peeling method, so that it becomes easy for those skilled in the art to control the porous state very delicately for the battery performance. Industrially very advantageous.

  In the present invention, for example, a separator having a uniform pore diameter is realized by controlling the particle size distribution of the filler, thereby preventing an increase in ion passage resistance due to a lack of the number of opening start points, and a decrease in ion passage resistance. Uniformity is improved, and a non-aqueous electrolyte secondary battery excellent in load characteristics is realized in which the discharge capacity at a discharge rate of 6C is 60% or more with respect to the discharge capacity at a discharge rate of C / 3.

  As is clear from the results of Examples and Comparative Examples described later, according to the present invention, a separator for a non-aqueous electrolyte secondary battery having a uniform pore diameter, which is made of a porous film made of a thermoplastic resin containing a filler. Thus, a non-aqueous electrolyte secondary battery having excellent battery performance, particularly load characteristics, and stable performance is provided.

Hereinafter, embodiments of the present invention will be described in detail.
[Pore diameter of the separator of the present invention]
The separator for a non-aqueous electrolyte secondary battery of the present invention comprises a porous film containing a filler in a thermoplastic resin, and has an average pore diameter d _ave (μm) and a maximum pore diameter d _max (defined by ASTM F316-86). The ratio d _ave / d _max with respect to μm) is 0.6 or more. The average pore size and the maximum pore size according to the present invention are those defined in ASTM F316-86.

The average pore diameter of the separator of the present invention, that is, the lower limit of the average pore diameter d _ave of the porous membrane constituting the separator is usually 0.03 μm or more, preferably 0.05 μm or more, more preferably 0.1 μm or more, particularly preferably. The upper limit is usually 5 μm or less, preferably 3 μm or less, and more preferably 2 μm or less. If this average pore diameter d _ave is less than 0.03 μm, it becomes difficult to obtain a connection between pores formed by interfacial peeling, or clogging due to reaction by-products in the battery tends to occur, resulting in electrical resistance. Increases, and the load characteristics of the obtained secondary battery tend to be lowered. When the average pore diameter d _ave exceeds 5 μm, the by-product of the reaction inside the battery tends to move, and the electrode active material is promoted to deteriorate, so that the cycle characteristics of the obtained secondary battery tend to be lowered.

The ratio of the average pore diameter to the maximum pore diameter of the separator of the present invention, that is, the value of the average pore diameter d _ave / maximum pore diameter d _max of the porous membrane constituting the separator of the present invention is 0.6 or more. This ratio d _ave / d _max is preferably 0.65 or more, more preferably 0.7 or more. When d _ave / d _max is less than 0.6, the variation in the pore diameter of the separator becomes large, and there is a problem that the battery performance such as load characteristics is deteriorated.

As d _ave / d _max is higher, the variation in the pore diameter of the separator is smaller and the variation toward the larger side is particularly small. However, an upper limit of d _ave / d _max is preferably about 0.95. is there.

[Constituent Components and Physical Properties of the Separator of the Present Invention]
The thermoplastic resin that is the base resin of the porous film constituting the separator of the present invention is not particularly limited as long as the filler can be uniformly dispersed. For example, polyolefin resin, fluororesin Styrene resin such as polystyrene, ABS resin, vinyl chloride resin, vinyl acetate resin, acrylic resin, polyamide resin, acetal resin, polycarbonate resin and the like. Among these, polyolefin resin is particularly preferable because of its excellent balance of heat resistance, solvent resistance, and flexibility. Examples of the polyolefin resin include monoolefin polymers such as ethylene, propylene, 1-butene, 1-hexene, 1-octene or 1-decene, ethylene, propylene, 1-butene, 1-hexene, 1-octene or The main component is a copolymer of 1-decene and another monomer such as 4-methyl-1-pentene or vinyl acetate. Specifically, low-density polyethylene, linear low-density polyethylene, Examples thereof include high-density polyethylene, polypropylene, crystalline ethylene-propylene block copolymer, polybutene, and ethylene-vinyl acetate copolymer. In the present invention, it is preferable to use high density polyethylene or polypropylene among the polyolefin resins. The above thermoplastic resins such as polyolefin resins may be used alone or in combination of two or more.

  The weight average molecular weight of such a thermoplastic resin is such that the lower limit is usually 50,000 or more, especially 100,000 or more, and the upper limit is usually 500,000 or less, preferably 400,000 or less, more preferably 300,000 or less, especially 200,000 or less. I just need it. When this upper limit is exceeded, melt molding becomes difficult because the melt viscosity of the resin increases in addition to the decrease in fluidity due to the addition of filler. Further, even when a molded product is obtained, the filler is not uniformly dispersed in the resin, and pore formation due to interfacial peeling becomes non-uniform, which is not preferable. Below this lower limit, the mechanical strength decreases, which is not preferable.

  The filler contained in the porous membrane according to the present invention is important as a factor affecting the pore size distribution of the separator of the present invention, and it is important to manage the particle size distribution as will be described later. Any filler that meets the conditions may be used, and one kind of filler may be used alone, or two or more kinds may be mixed and used.

  The type of the filler is not particularly limited, but it is preferable to use an inorganic filler in that it hardly reacts with the electrolytic solution and is not easily oxidized or reduced. Among them, those having the property of not decomposing the carbonate-based organic electrolyte used in the lithium secondary battery are preferable. Examples of such a filler include sparingly water-soluble sulfates and alumina. Barium sulfate and alumina are preferably used, and barium sulfate is particularly preferably used. “Slightly water-soluble” as used herein means that the solubility in water at 25 ° C. is 5 mg / l or less.

In general, carbonates such as calcium carbonate, titanium oxide, silica, and the like that are often used as fillers are not preferable because they cause decomposition of the non-aqueous electrolyte components of the lithium secondary battery, as will be described later. Here, decomposition of the organic electrolyte component is a ratio of 0.5 g of filler per 1 ml of electrolyte in an electrolyte solution composed of a mixed nonaqueous solvent solution of EC / EMC = 3: 7 (volume ratio) of 1M LiPF ₆ It is defined that the concentration of lithium ions in the electrolytic solution after being added at 85 ° C. and held for 72 hours is reduced to 0.75 mmol / g or less. The amount of lithium ions is measured by ion chromatography. In addition, it is necessary to put electrolyte solution in an airtight container so that it may not contact external air during 72 hours holding | maintenance. This is because decomposition of the electrolyte component proceeds by reacting with moisture in the air.

The table below shows the results of holding various fillers added to the electrolyte solution (1M LiPF ₆ / (EC + EMC) (3: 7, volume ratio)) under the above conditions. Compared with the ionic composition of the electrolytic solution to which no filler was added, barium sulfate and alumina showed almost no change in composition, indicating that they are suitable as the filler in the present invention. On the other hand, carbonates such as calcium carbonate and lithium carbonate, or silica and titanium oxide show a significant decrease in lithium ions and an increase in fluorine ions due to the generation of hydrofluoric acid, indicating that they are not preferred as fillers in the present invention.

  As the particle size of the filler, the lower limit of the number-based average particle size is usually 0.01 μm or more, preferably 0.1 μm or more, especially 0.2 μm or more, and the upper limit is usually 2 μm or less, preferably 1.5 μm. In particular, the thickness is preferably 1 μm or less. When the number-based average particle diameter of the filler exceeds 2 μm, the diameter of the holes formed by stretching becomes too large, which tends to cause stretching breakage and a decrease in film strength. In addition, if the number average particle size is smaller than 0.01 μm, the filler is likely to aggregate, so that it is difficult to uniformly disperse the filler in the base resin.

  In the present invention, as long as the inorganic filler meets the above conditions, one kind can be used alone, or two or more kinds can be mixed and used.

  The blending amount of the filler in the porous membrane according to the present invention is such that the lower limit is usually 40 parts by weight or more, preferably 50 parts by weight or more, more preferably 60 parts by weight or more, more preferably 100 parts by weight of the thermoplastic resin. The upper limit is usually 300 parts by weight or less, preferably 200 parts by weight or less, more preferably 150 parts by weight or less with respect to 100 parts by weight of the thermoplastic resin. If the blending amount of the filler with respect to 100 parts by weight of the thermoplastic resin in the porous film is less than 40 parts by weight, it is difficult to form the communication holes and it is difficult to exhibit the function as a separator. On the other hand, if it exceeds 300 parts by weight, not only the viscosity at the time of film forming becomes high and the processability is inferior, but also the film breaks during stretching for making it porous, which is not preferable. In the present invention, since the filler blended in the production of the porous film remains in the substantially formed porous film, the blending amount range of the filler is the same as the filling in the porous film. It becomes the agent content range.

The number of fillers affects the number of pores formed in the porous membrane. The number of fillers is the number of fillers per 1 cm ³ of resin volume, and the lower limit is usually 1 × 10 ^11. Above, preferably 3 × 10 ¹¹ or more, more preferably 5 × 10 ¹¹ or more, and the upper limit is usually 1 × 10 ¹⁴ or less, preferably 7 × 10 ¹³ or less. When the blended number exceeds the above upper limit, too many vacancies are formed and the by-products of the reaction inside the battery are likely to move, and the deterioration of the electrode active material is promoted. There is a tendency for characteristics to decrease. On the other hand, when the value is lower than the lower limit, it becomes difficult to obtain the connection between the formed holes, and as a result, the electric resistance increases and the load characteristics of the obtained secondary battery tend to be lowered.

  In addition, as a filler, in order to improve the dispersibility to a thermoplastic resin, what is surface-treated with the surface treating agent can also be used. As the surface treatment, when the thermoplastic resin is a polyolefin resin, for example, a treatment with a fatty acid such as stearic acid or a metal salt thereof, polysiloxane, or a silane coupling agent can be given.

  When molding the porous film according to the present invention, a low molecular weight compound having compatibility with the thermoplastic resin may be added. This low molecular weight compound enters between the molecules of the thermoplastic resin, lowers the interaction between molecules and inhibits crystallization, and as a result, improves the stretchability of the resin composition during sheet molding. In addition, the low molecular weight compound moderately increases the interfacial adhesive force between the thermoplastic resin and the filler, thereby preventing the pores from becoming coarse due to stretching, and also increases the interfacial adhesive force between the thermoplastic resin and the filler. This has the effect of preventing the filler from falling off the film.

  As this low molecular weight compound, those having a molecular weight of 200 to 3000 are suitably used, and those having a molecular weight of 200 to 1000 are more preferably used. When the molecular weight of the low molecular weight compound exceeds 3000, the low molecular weight compound is difficult to enter between the molecules of the thermoplastic resin, so that the effect of improving stretchability is insufficient. On the other hand, when the molecular weight is less than 200, compatibility is improved, but so-called blooming, in which a low molecular weight compound is precipitated on the surface of the porous film, is likely to occur, and film properties are deteriorated and blocking is not preferable.

  As the low molecular weight compound, when the thermoplastic resin is a polyolefin resin, an aliphatic hydrocarbon or glyceride is preferably used. In particular, when the polyolefin resin is polyethylene, liquid paraffin or low melting point wax is preferably used.

  In the resin composition as the film forming material of the porous film according to the present invention, the lower molecular weight compound is added at a lower limit of usually 1 part by weight or more, preferably 5 parts by weight or more with respect to 100 parts by weight of the thermoplastic resin. The upper limit is usually 20 parts by weight or less, preferably 15 parts by weight or less, relative to 100 parts by weight of the thermoplastic resin. When the blending amount of the low molecular weight compound is less than 1 part by weight with respect to 100 parts by weight of the thermoplastic resin, the above effect by blending the low molecular weight compound cannot be sufficiently obtained, and when the blending amount exceeds 20 parts by weight, thermoplasticity is obtained. The interaction between the molecules of the resin is reduced too much and sufficient strength cannot be obtained. In addition, smoke generation occurs during sheet forming, and slipping occurs at the screw portion, making it difficult to form a stable sheet.

  If necessary, other additives such as a heat stabilizer can be added to the resin composition as the film forming material for the porous film according to the present invention. The additive is not particularly limited as long as it is a known additive. The compounding quantity of these additives is 0.05-1 weight% normally with respect to the whole quantity of a resin composition.

  The porosity of the porous membrane according to the present invention is usually 30% or more, preferably 40% or more, more preferably 50% or more as the lower limit of the porosity of the porous membrane, and usually 80% or less as the upper limit, preferably Is 70% or less, more preferably 65% or less, and particularly preferably 60% or less. If the porosity is less than 30%, the ion permeability is not sufficient, and the function as a separator cannot be achieved. On the other hand, if the porosity exceeds 80%, the actual strength of the film is lowered, which is not preferable because breakage or short-circuiting due to the active material and short-circuiting occur during battery production.

The porosity of the porous film is a value calculated by the following calculation formula.
Porosity Pv (%) = 100 × (1−w / [ρ · S · t])
S: Area of porous membrane t: Thickness of porous membrane w: Weight of porous membrane ρ: True specific gravity of porous membrane

When the blend weight of the component i (resin, filler, etc.) constituting the polymer porous membrane is Wi and the specific gravity is ρi, the true specific gravity ρ can be obtained by the following equation. In the formula, Σ represents the sum of all components.
True specific gravity of porous membrane ρ = ΣWi / Σ (Wi / ρi)

  The upper limit value of the thickness of the porous membrane according to the present invention is usually 100 μm or less, especially 50 μm or less, preferably 40 μm or less, and the lower limit value is usually 5 μm or more, preferably 10 μm or more. If the thickness is less than 5 μm, the actual strength is low, and therefore, it is not preferable because breakage at the time of producing the battery, punch-through due to the active material, and short circuit occur. Moreover, since the electrical resistance of a separator will become high when thickness exceeds 100 micrometers, the capacity | capacitance of a battery falls and it is not preferable. By setting the thickness in the range of 5 to 100 μm, a separator having good ion permeability can be obtained.

  In the porous membrane according to the present invention, the lower limit value of the Gurley air permeability is 20 seconds / 100 cc or more, particularly 100 seconds / 100 cc or more, and the upper limit value is 500 seconds / 100 cc or less, particularly 300 seconds / 100 cc or less. It is preferable. When the Gurley permeability is below this lower limit, the porosity is often too high or the thickness is too thin, and as described above, the actual strength of the film is low, and breakage during battery creation and penetration by active material A short circuit occurs, which is not preferable. When the upper limit is exceeded, the ion permeability is not sufficient, and the function as a separator cannot be achieved, which is not preferable. The Gurley air permeability is measured according to JIS P8117, and indicates the number of seconds that 100 cc of air passes through the membrane at a pressure of 1.22 kPa.

[Manufacturing method of separator]
Next, a method for producing the separator of the present invention will be described. Prior to that, a general method for producing a separator made of a porous film made of a thermoplastic resin containing a filler will be described.

<General separator manufacturing method>
The method for producing a porous film made of a thermoplastic resin containing a filler is not particularly limited, and includes the following extraction method (1), stretching method (2), and interfacial peeling method (3), but is particularly preferable. This is an interfacial peeling method.
(1) Extraction method: Melting a resin composition that is a mixture of a polymer material, a filler, and a plasticizer that can be removed by solvent extraction in the subsequent process, and then forming this into a film by a molding method such as extrusion molding. Then, it is made porous by treating it with a solvent to remove the plasticizer.
(2) Stretching method: A resin composition obtained by mixing a filler with a crystalline polymer material is melted, formed into a film by a molding method such as extrusion, and then structurally stretched. Micropores are formed by cutting weak amorphous parts.
(3) Interfacial peeling method: A resin composition obtained by mixing a filler with a polymer material is melted, formed into a film by a molding method such as extrusion molding, and then stretched to obtain a polymer material. Fine pores are formed by peeling the interface with the filler.

  Among the above production methods, in the extraction method, it is difficult to selectively contain a filler on the polymer material side during molding, and the filler contained in the plasticizer portion is removed together with the plasticizer during extraction. Therefore, it is not efficient as compared with the interface peeling method. In addition, in the stretching method, when a filler is contained in the polymer material, pores are formed by stretching at the interface between the polymer material and the filler in addition to the amorphous portion, so that it is essentially not different from the interface peeling method. Therefore, in the present invention, it is preferable to employ the interfacial peeling method.

  More specifically, the production of the porous membrane is performed by the following method.

  First, a resin composition is prepared by blending a predetermined amount of a filler, a thermoplastic resin, and additives such as a low molecular weight compound and an antioxidant that are added as necessary, and melt-kneading. Here, the resin composition may be preliminarily mixed with a Henschel mixer or the like, and thereafter prepared using a commonly used single screw extruder, twin screw extruder, mixing roll, twin screw kneader, or the like, or The pre-kneading may be omitted and the resin composition may be directly prepared with the above extruder or the like.

  Next, the resin composition is formed into a sheet. Sheet forming can be performed by a T-die method using a commonly used T-die or an inflation method using a circular die.

  Next, the formed sheet is stretched. For the stretching, longitudinal uniaxial stretching for stretching in the sheet take-up direction (MD), transverse uniaxial stretching for stretching in the transverse direction (TD) with a tenter stretching machine, etc., uniaxial stretching to MD, followed by TD with a tenter stretching machine, etc. There is a sequential biaxial stretching method in which stretching is performed, or a simultaneous biaxial stretching method in which the longitudinal direction and the transverse direction are simultaneously stretched. The uniaxial stretching can be performed by roll stretching. The stretching can be performed at any temperature at which the resin composition constituting the sheet can be easily stretched to a predetermined stretching ratio, and the resin composition does not melt and block the pores to lose the connectivity. However, it is preferably stretched in the temperature range of the melting point of the resin—70 ° C. to the melting point of the resin—5 ° C. The stretching ratio is arbitrarily set according to the required pore diameter and strength, but preferably, stretching is performed at least 1.2 times in a uniaxial direction.

<The manufacturing method of the separator of this invention>
Next, a method for producing the separator of the present invention in which the ratio d _ave / d _max between the average pore diameter d _ave (μm) and the maximum pore diameter d _max (μm) determined by ASTM F316-86 is 0.6 or more will be described. To do.

The method for producing the porous membrane constituting the separator of the present invention may be any method as long as a porous membrane having d _ave / d _max of 0.6 or more can be obtained, and the molding material and production method are not particularly limited. . The separator of the present invention is manufactured by the same method as the conventional general separator manufacturing method described above. However, in the present invention, d _ave / d _max of the manufactured separator is set to 0.6 or more. In addition, [1] strictly control the particle size distribution of the filler, [2] control the mixing and stretching conditions of the resin and the filler, or adopt both of them.

[1] Control of particle size distribution of filler: Control of skewness of number-based particle size distribution of filler In the present invention, the filler compounded in the porous membrane has a skewness of 0. It is preferably 5 or more. The particle size distribution is evaluated by a laser diffraction / scattering method. The particle size distribution of the filler may be measured with the filler in a state before kneading into the resin, or may be measured by pulverizing the ash collected by baking the porous membrane. The skewness of the particle size distribution of the filler is derived from the particle size distribution using, for example, the formula described in “Statistical Engineering Handbook” edited by Tokyo Institute of Technology Statistical Engineering Study Group, pages 194-195. When the skewness of the particle size distribution is 0 or more, it indicates that the particle size distribution is biased toward the low particle size side, but when the skewness is close to 0, the bias of the particle size distribution toward the low particle size side is sufficient. Therefore, it is difficult to obtain a porous film having a d _ave / d _max of 0.6 or more, and the degree of distortion is 0.5 or more. It is preferable for reducing the contribution and obtaining a porous membrane having d _ave / d _max of 0.6 or more. Further, when the skewness is smaller than 0, the particle size distribution is biased toward the large particle size side, and the contribution of the large particle size particle is increased, so that a porous film having d _ave / d _max of 0.6 or more can be obtained. difficult.

  Accordingly, the filler used in the present invention preferably has a skewness derived from the number-based particle size distribution of 0.5 or more, more preferably a skewness of 2 or more, more preferably a skewness of 2. 5 or more.

  In order to prepare the filler having such a particle size distribution, it is possible to adjust the particle size using a classifier such as a sieve. This particle size adjustment can be repeated a plurality of times as necessary.

[2] Mixing of resin and filler and control of stretching conditions Mixing and stirring conditions are set such that the filler is sufficiently uniformly dispersed in the thermoplastic resin. For example, the temperature and time of the melt kneading conditions to be used are strictly controlled.

In the stretching operation, it is necessary to appropriately set the temperature and the stretching speed so that the entire film is stretched uniformly. If the conditions are not set appropriately, stretching unevenness occurs and the pore size distribution of the formed pores is widened, making it difficult to obtain a porous membrane having a d _ave / d _max of 0.6 or more as intended by the present invention.

[Non-aqueous electrolyte secondary battery]
Next, the non-aqueous electrolyte secondary battery of the present invention using the above-described separator for a non-aqueous electrolyte secondary battery of the present invention will be described. The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode capable of inserting and extracting lithium ions, a negative electrode capable of inserting and extracting lithium ions, an electrolytic solution containing an electrolyte in a non-aqueous solvent, and a separator.

  As the non-aqueous solvent of the electrolyte used in the non-aqueous electrolyte secondary battery of the present invention, any known solvent can be used as the solvent for the non-aqueous electrolyte secondary battery. For example, alkylene carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate; dialkyl carbonates such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, and ethyl methyl carbonate (the alkyl group of the dialkyl carbonate is an alkyl having 1 to 4 carbon atoms) A cyclic ether such as tetrahydrofuran and 2-methyltetrahydrofuran; a chain ether such as dimethoxyethane and dimethoxymethane; a cyclic carboxylic acid ester such as γ-butyrolactone and γ-valerolactone; methyl acetate, methyl propionate and propion Examples thereof include chain carboxylic acid esters such as ethyl acid. These may be used alone or in combination of two or more.

Arbitrary things can be used as lithium salt which is a solute of nonaqueous system electrolyte. For example, inorganic lithium salts such as LiClO ₄ , LiPF ₆ and LiBF ₄ ; LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C _{4 F 9 SO 2), LiC} (CF 3 SO 2) 3, LiPF 4 (CF 3) 2, LiPF 4 (C 2 F 5) 2, LiPF 4 (CF 3 SO 2) 2, LiPF 4 (C 2 F _{5 SO 2) 2, LiBF 2} (CF 3) 2, LiBF 2 (C 2 F 5) 2, LiBF 2 (CF 3 SO 2) 2 and _{LiBF 2 (C 2 F 5 SO} 2) 2 , etc. the fluorine-containing organic Examples include lithium salts. Of these, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ or LiN (C ₂ F ₅ SO ₂ ) ₂ , particularly LiPF ₆ or LiBF ₄ are preferred. In addition, about lithium salt, 1 type may be used independently and 2 or more types may be used together.

  The lower limit of the concentration of these lithium salts in the non-aqueous electrolyte is usually 0.5 mol / l or more, especially 0.75 mol / l or more, and the upper limit is usually 2 mol / l or less, especially 1.5 mol / l. l or less. When the concentration of the lithium salt exceeds this upper limit value, the viscosity of the nonaqueous electrolytic solution increases and the electrical conductivity also decreases. Moreover, since electrical conductivity will become low if less than a lower limit, it is preferable to prepare a non-aqueous electrolyte within the said concentration range.

  The non-aqueous electrolyte solution according to the present invention has other useful components as required, for example, conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents, capacity maintenance characteristics and cycle characteristics after high-temperature storage. You may contain various additives, such as an auxiliary agent for improving.

  Auxiliaries for improving capacity maintenance characteristics and cycle characteristics after high temperature storage include carbonate compounds such as vinylene carbonate, fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate and erythritan carbonate; succinic anhydride, anhydrous glutar Carboxylic acid anhydrides such as acid, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride; Sulfur containing ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methyl methanesulfonate, busulfan, sulfolane, sulfolene, dimethyl sulfone, tetramethylthiuram monosulfide, etc. Compound; Nitrogen-containing compound such as 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide; Examples thereof include hydrocarbon compounds such as heptane, octane, and cycloheptane. When the non-aqueous electrolyte contains these auxiliaries, the concentration is usually 0.1 to 5% by weight.

  As the positive electrode, a material in which an active material layer containing a positive electrode active material and a binder is usually formed on a current collector is used.

  Examples of the positive electrode active material include materials capable of inserting and extracting lithium, such as lithium transition metal composite oxide materials such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide. These may be used individually by 1 type, or may use multiple types together.

  The binder is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), Examples thereof include fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, and nitrocellulose. These may be used individually by 1 type, or may use multiple types together.

  As for the ratio of the binder in the positive electrode active material layer, the lower limit is usually 0.1% by weight or more, preferably 1% by weight or more, more preferably 5% by weight or more, and the upper limit is usually 80% by weight or less, preferably 60% by weight or less, more preferably 40% by weight or less, and still more preferably 10% by weight or less. If the proportion of the binder is small, the active material cannot be sufficiently retained, so that the mechanical strength of the positive electrode is insufficient, and the battery performance such as cycle characteristics may be deteriorated. On the contrary, if the amount is too large, the battery capacity and conductivity are lowered. It will be.

  The positive electrode active material layer usually contains a conductive agent in order to increase conductivity. Examples of the conductive agent include carbonaceous materials such as graphite fine particles such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon fine particles such as needle coke. These may be used individually by 1 type, or may use multiple types together. The ratio of the conductive agent in the positive electrode active material layer is such that the lower limit is usually 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 1% by weight or more, and the upper limit is usually 50% by weight or less. , Preferably 30% by weight or less, more preferably 15% by weight or less. If the proportion of the conductive agent is small, the conductivity may be insufficient, and conversely if too large, the battery capacity may be reduced.

  In addition, the positive electrode active material layer can contain additives for a normal active material layer such as a thickener.

  The thickener is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein. These may be used individually by 1 type, or may use multiple types together.

  Aluminum, stainless steel, nickel-plated steel or the like is used for the current collector of the positive electrode.

  The positive electrode can be formed by applying a slurry obtained by slurrying the above-described positive electrode active material, a binder, a conductive agent, and other additives added as necessary with a solvent onto a current collector, and drying the positive electrode active material. . As the solvent used for slurrying, an organic solvent that dissolves the binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran and the like are used, but not limited thereto. These may be used individually by 1 type, or may use multiple types together. Moreover, a dispersing agent, a thickener, etc. can be added to water, and an active material can also be slurried with latex, such as SBR.

  Thus, the thickness of the positive electrode active material layer formed is about 10-200 micrometers normally. The active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the active material.

  As the negative electrode, a material in which an active material layer containing a negative electrode active material and a binder is usually formed on a current collector is used.

  As a negative electrode active material, a carbonaceous material capable of occluding / releasing lithium such as organic pyrolysate, artificial graphite and natural graphite under various pyrolysis conditions; capable of occluding / releasing lithium such as tin oxide and silicon oxide Metal oxide material; lithium metal; various lithium alloys can be used. These negative electrode active materials may be used individually by 1 type, and may mix and use 2 or more types.

  The binder is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, styrene / butadiene rubber, isoprene rubber, and butadiene rubber. These may be used individually by 1 type, or may use multiple types together.

  The ratio of the binder in the negative electrode active material layer is such that the lower limit is usually 0.1% by weight or more, preferably 1% by weight or more, more preferably 5% by weight or more, and the upper limit is usually 80% by weight or less. Preferably it is 60 weight% or less, More preferably, it is 40 weight% or less, More preferably, it is 10 weight% or less. If the ratio of the binder is small, the active material cannot be sufficiently retained, so that the negative electrode mechanical strength may be insufficient, and the battery performance such as cycle characteristics may be deteriorated. become.

  The negative electrode active material layer usually contains a conductive agent in order to increase conductivity. Examples of the conductive agent include carbonaceous materials such as graphite fine particles such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon fine particles such as needle coke. These may be used individually by 1 type, or may use multiple types together. As for the ratio of the conductive agent in the negative electrode active material layer, the lower limit is usually 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 1% by weight or more, and the upper limit is usually 50% by weight or less. , Preferably 30% by weight or less, more preferably 15% by weight or less. If the proportion of the conductive agent is small, the conductivity may be insufficient, and conversely if too large, the battery capacity may be reduced.

  In addition, the negative electrode active material layer may contain additives for a normal active material layer such as a thickener.

  The thickener is not particularly limited as long as it is a material that is stable with respect to the solvent and electrolyte used during electrode production and other materials used during battery use. Specific examples thereof include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, and casein. These may be used individually by 1 type, or may use multiple types together.

  Copper, nickel, stainless steel, nickel-plated steel, or the like is used for the negative electrode current collector.

  The negative electrode can be formed by applying a slurry obtained by slurrying the above-described negative electrode active material, a binder, a conductive agent, and other additives added as necessary with a solvent onto a current collector, and then drying the negative electrode active material. .

  As the solvent for forming a slurry, an organic solvent that dissolves the binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran and the like are used, but not limited thereto. These may be used individually by 1 type, or may use multiple types together. Moreover, a dispersing agent, a thickener, etc. can be added to water, and an active material can also be slurried with latex, such as SBR.

  Thus, the thickness of the negative electrode active material layer formed is about 10-200 micrometers normally. The active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the active material.

  The lithium secondary battery of the present invention is manufactured by assembling the positive electrode, the negative electrode, the non-aqueous electrolyte, and the separator of the present invention into an appropriate shape. Furthermore, other components such as an outer case can be used as necessary.

  The battery shape is not particularly limited, and can be appropriately selected from various commonly used shapes according to the application. Examples of commonly used shapes include a cylinder type with a sheet electrode and separator spiral, an inside-out cylinder type with a combination of pellet electrode and separator, a coin type with a stack of pellet electrode and separator, and a sheet Examples include a laminate type in which electrodes and separators are laminated. The method for assembling the battery is not particularly limited, and can be appropriately selected from various commonly used methods according to the shape of the target battery.

  The general embodiment of the lithium secondary battery of the present invention has been described above. However, the lithium secondary battery of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist thereof. Can be implemented.

  The use of the lithium secondary battery of the present invention is not particularly limited, and can be used for various known uses. Specific examples include notebook computers, pen input computers, mobile computers, electronic book players, mobile phones, mobile faxes, mobile copy, mobile printers, headphone stereos, video movies, LCD TVs, handy cleaners, portable CDs, minidiscs, and transceivers. , Electronic notebooks, calculators, memory cards, portable tape recorders, radios, backup power supplies, motors, lighting equipment, toys, gaming devices, small devices such as watches, strobes and cameras, and large devices such as electric vehicles and hybrid vehicles Can be mentioned.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples as long as the gist thereof is not exceeded.

In the following, the air permeability and pore diameter of the separator were evaluated by the following methods.
Air permeability: B-type Gurley Denso meter (Toyo Seiki) according to JIS P8117
Measurement was carried out using a machine manufacturer).
Pore diameter: Measurement was performed according to ASTM F316-86 using a Coulter Porometer.

[Example 1]
<Manufacture of porous membrane>
25.9 parts by weight of commercially available polypropylene 1 (homotype, “FY6C” manufactured by Nippon Polychem (MFR: 2.4 g / 10 min)) and polyprompyrene 2 (copolymer, “INSPiRE” manufactured by Dow Chemical Co., Ltd. (MFR: 0.5 g) / 10 min)) 6.5 parts by weight, hardened castor oil (“HY-CASTOR OIL” molecular weight 938 manufactured by Toyokuni Seiyaku Co., Ltd.) 2.6 parts by weight, commercially available barium sulfate as a filler [number-based average particle diameter 0.17 μm, strain Degree 2.91] The resin composition obtained by blending 65 parts by weight was melt-molded at a temperature of 250 ° C. to obtain an original sheet. The thickness of the original fabric sheet was 50 μm on average, and the number of fillers blended was as shown in Table 2. Next, the obtained raw sheet was stretched 4.5 times in the sheet longitudinal direction (MD) at 80 ° C., and the film thickness, porosity, Gurley permeability, and pore size shown in Table 2 were porous. A membrane was obtained.

<Preparation of electrolyte>
Under a dry argon atmosphere, a sufficiently dried LiPF ₆ was dissolved in a mixture of ethylene carbonate and ethyl methyl carbonate (volume ratio 3: 7) so as to have a ratio of 1.0 mol / liter to obtain an electrolytic solution.

<Preparation of positive electrode>
LiCoO ₂ was used as the positive electrode active material, and 6 parts by weight of carbon black and 9 parts by weight of polyvinylidene fluoride (trade name “KF-1000” manufactured by Kureha Chemical Co., Ltd.) were added to 85 parts by weight of LiCoO ₂ and mixed. Disperse with 2-pyrrolidone to form a slurry. This was uniformly applied to one side of a 20 μm-thick aluminum foil as a positive electrode current collector, dried, and then pressed by a press machine so that the density of the positive electrode active material layer was 1.9 g / cm ^3. It was.

<Production of negative electrode>
Using natural graphite powder as a negative electrode active material, 94 parts by weight of natural graphite powder was mixed with 6 parts by weight of polyvinylidene fluoride, and dispersed with N-methyl-2-pyrrolidone to form a slurry. This was uniformly applied to one side of a 18 μm-thick copper foil as a negative electrode current collector, dried, and then pressed by a press machine so that the density of the negative electrode active material layer was 1.3 g / cm ^3. did.

<Battery assembly>
A 2032 type coin cell was produced using the porous membrane as a separator together with the electrolyte, positive electrode and negative electrode. That is, a stainless steel can that also serves as a positive electrode conductor is accommodated with a positive electrode impregnated with an electrolyte by punching into a disk shape having a diameter of 12.5 mm, and a separator having a diameter of 18.8 mm impregnated with the electrolyte A negative electrode impregnated with an electrolyte by punching into a disk shape having a diameter of 12.5 mm was placed. The can body and a sealing plate serving also as a negative electrode conductor were caulked and sealed via an insulating gasket to produce a coin-type battery. Here, the impregnation of the battery member with the electrolytic solution was performed by immersing each member in the electrolytic solution for 2 minutes.

<Battery evaluation>
After the initial charge / discharge of the produced coin-type battery, C / 3 (the rated capacity due to the discharge capacity at a one-hour rate is set to 1C, and the same applies hereinafter) and 4C and 6C discharges The discharge capacity was measured at a speed, the ratio of the discharge capacity based on the C / 3 discharge capacity was determined, and the results are shown in Table 2.

[Example 2]
30.8 parts by weight of polypropylene 1, 1.6 parts by weight of polypropylene 2 and 2.6 parts by weight of hardened castor oil, barium sulfate [number-based average particle size 0.17 μm, skewness 2.91] Was used in the same manner as in Example 1 except that 65 parts by weight was used and the stretching in the longitudinal direction (MD) of the raw sheet was made 3.5 times.

  A coin-type battery was assembled in the same manner as in Example 1 except that this porous membrane was used as a separator, and evaluation was performed in the same manner. The results are shown in Table 2.

Example 3
46.3 parts by weight of commercially available polypropylene 1 used in Example 1, 3.7 parts by weight of hardened castor oil used in Example 1, and commercially available barium sulfate as a filler [number-based average particle size of 0.18 μm , Degree of distortion 3.57] was compounded using 50 parts by weight, and the resin composition was melt-molded at a temperature of 250 ° C. and an average thickness of 180 μm. A sheet was obtained. The obtained raw fabric sheet was stretched 4.5 times in the sheet longitudinal direction (MD) at 70 ° C., and then stretched 4.4 times in the sheet width direction (TD) at 120 ° C. A porous film having the physical properties shown was obtained.

  A coin-type battery was assembled in the same manner as in Example 1 except that this porous membrane was used as a separator, and evaluation was performed in the same manner. The results are shown in Table 2.

Example 4
46.3 parts by weight of the commercially available polypropylene 1 used in Example 1, 3.7 parts by weight of hardened castor oil used in Example 1, and commercially available barium sulfate as a filler [number-based average particle diameter of 0.46 μm, The resin composition was blended using 50 parts by weight of a degree of distortion of 1.15], and the resin composition was melt-molded at a temperature of 250 ° C. to obtain an average thickness of 170 μm and the number of filler sheets shown in Table 2 as the blending number of fillers. Obtained. The obtained raw fabric sheet was stretched 4.0 times in the sheet longitudinal direction (MD) at 70 ° C., and then stretched 3.5 times in the sheet width direction (TD) at 120 ° C. A porous film having the physical properties shown was obtained.

  A coin-type battery was assembled in the same manner as in Example 1 except that this porous membrane was used as a separator, and evaluation was performed in the same manner. The results are shown in Table 2.

[Comparative Example 1]
46.3 parts by weight of the commercially available polypropylene 1 used in Example 1, 3.7 parts by weight of the hardened castor oil used in Example 1, and commercially available barium sulfate as a filler [number-based average particle diameter of 1.07 μm , Skewness -0.67] was blended using 50 parts by weight, and the resin composition was melt-molded at a temperature of 250 ° C. and an average thickness of 180 μm. A sheet was obtained. The obtained raw sheet is stretched 3.5 times in the sheet longitudinal direction (MD) at 70 ° C., and then stretched 3.0 times in the sheet width direction (TD) at 120 ° C. and shown in Table 2. A porous film having physical properties was obtained.

  A coin-type battery was assembled in the same manner as in Example 1 except that this porous membrane was used as a separator, and evaluation was performed in the same manner. The results are shown in Table 2.

  As is clear from Table 2, the discharge capacity of the coin-type battery using the porous membranes obtained in Examples 1 to 4 as a separator was 60% of the case of C / 3 at both discharge rates of 4C and 6C. It was good in the above. In contrast, the discharge capacity of the coin-type battery using the porous membrane of Comparative Example 1 as a separator is less than 50% of the case of C / 3 at any discharge speed of 4C and 6C, and the battery performance is degraded. Admitted.

That is, commercially available barium sulfate also has various physical properties, and even if barium sulfate is simply used as a filler, d _ave / d _max specified in the present invention cannot be achieved. By managing this physical property, battery performance can be improved.

  The present invention is useful for improving the performance of a non-aqueous electrolyte secondary battery, in particular, load characteristics and stability thereof.

Claims (7)

A separator for a non-aqueous electrolyte secondary battery comprising a porous film containing a filler in a thermoplastic resin, and having an average pore diameter d _ave (μm) and a maximum pore diameter d _max (μm) defined by ASTM F316-86 The non-aqueous electrolyte secondary battery separator is characterized in that the ratio d _ave / d _{max to)} is 0.6 or more. The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the average pore diameter d _ave is 0.03 to 5 µm.   The separator for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the porosity is 30 to 70%.   The separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, wherein the filler has a number-based average particle diameter of 0.01 to 2 µm and a skewness of 0.5 or more. A separator for a non-aqueous electrolyte secondary battery.   5. The non-aqueous electrolyte secondary battery separator according to claim 1, wherein the filler is selected from the group consisting of barium sulfate and alumina. Battery separator.   A non-aqueous electrolyte secondary battery having a positive electrode capable of inserting and extracting lithium ions, a negative electrode capable of inserting and extracting lithium ions, an electrolytic solution containing an electrolyte in a non-aqueous solvent, and a separator, as a separator. A non-aqueous electrolyte secondary battery using the separator according to any one of 1 to 5.   The non-aqueous electrolyte secondary battery according to claim 6, wherein the discharge capacity at a discharge rate of 6C is 60% or more with respect to the discharge capacity at a discharge rate of C / 3. Next battery.