JP2007115479A - Separator for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a nonaqueous electrolyte secondary battery excellent in mechanical physical properties like strength against piercing or the like, free from a problem of short-circuit between electrodes, and a nonaqueous electrolyte secondary battery using the above separator excellent in safety. <P>SOLUTION: The separator for a nonaqueous electrolyte secondary battery, composed of thermoplastic resin composition containing thermoplastic resin and a filler, has a strength against piercing of 2.5 N per a thickness of 25μm, to which an extension treatment is applied at least one time at a temperature of a melting point or lower of the thermoplastic resin, after applying extension treatment at least one time at a temperature of the melting point or higher. The nonaqueous electrolyte secondary battery provided with the above separator is provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a separator for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the separator, and more specifically, a separator for a non-aqueous secondary battery excellent in piercing strength, the separator, and lithium. The present invention relates to a non-aqueous electrolyte secondary battery comprising a negative electrode and a positive electrode that can be occluded / released, and a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt.

  2. Description of the Related Art Lithium secondary batteries, which are light nonaqueous electrolyte secondary batteries that have a high energy density with the reduction in weight and size of electrical products, are used in a wide range of fields.

Lithium secondary batteries usually have 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 lithium occlusion / representative typified by graphite. A negative electrode in which an active material layer containing a negative electrode active material such as a carbon material that can be released is formed on a current collector, and an electrolyte such as a lithium salt such as LiPF ₆ are dissolved in a normal aprotic non-aqueous solvent. It is mainly composed of a non-aqueous electrolyte and a separator made of a polymer porous membrane.

  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 polymer membrane made of a thermoplastic resin such as polypropylene or polypropylene is used.

  By the way, the lithium secondary battery has a high energy density, so when a short circuit between the electrodes occurs, a current flows at a time and a large amount of heat is generated, causing decomposition and ignition of the electrolyte, which is very dangerous. . This short-circuit between the two electrodes is caused by the active material breaking through the separator due to tightening during winding of the electrode and separator, pressure due to expansion / contraction of the electrode during charging / discharging, or impact when the battery is dropped. There are many cases. Therefore, in order to prevent such an abnormal situation, the separator is required to have higher piercing strength.

Conventionally, as a method for producing these polymer 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, followed by molding, and then the plasticizer is removed with a suitable solvent to make it porous (Patent Document 1).
(2) A stretching method in which after forming a crystalline polymer material, a structurally weak amorphous portion is selectively stretched to form micropores (Patent Document 2).
(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 (Patent Document 3).

  However, the extraction method (1) 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 a long heat treatment in order to control the pore size distribution by controlling the crystal phase / amorphous phase structure before stretching, which is problematic in terms of productivity.

  On the other hand, the interfacial peeling method (3) does not generate waste liquid and is an excellent method 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.

  However, the interfacial debonding method can easily delaminate the interface between the polymer material and the filler by stretching operation. However, once the holes are opened, the holes are easily expanded by the subsequent stretching, and the porosity is excessive. As a result, mechanical properties such as piercing strength tend to decrease, and sufficient safety can be ensured in the case of winding tightening, winding expansion / contraction during charging / discharging, and battery can dropping. It was hard to say.

In addition, as a separator containing a filler in a thermoplastic resin, known techniques are disclosed in Patent Documents 4 and 5. However, since all of the examples disclosed in Patent Document 4 are resins having a molecular weight exceeding 1 million and difficult to mold, a large amount of mineral oil must be added as a plasticizer and molding must be performed. Because of the holes, it is necessary to extract and remove the plasticizer after molding, and as described above, there are problems in both environmental and economic aspects. Even in Patent Document 5, since the molecular weight of the base resin is high, it is essential to add a low molecular weight resin having a molecular weight of 20000 or less in order to ensure moldability. However, these low molecular weight resins have a low melting point, so It has the disadvantage that it melts at the operating temperature of the battery and clogs the pores so that it does not function as a separator.
JP 7-029563 A JP-A-7-304110 JP 2002-201298 A JP-A-11-260338 JP 2002-69221 A

  The present invention has been made in view of the above-described conventional situation, and is excellent in mechanical properties such as piercing strength and has no problem of short circuit between both electrodes, and a separator for a non-aqueous electrolyte secondary battery, and this non-aqueous electrolysis It aims at providing the non-aqueous electrolyte secondary battery excellent in safety | security using the separator for liquid secondary batteries.

As a result of diligent research, the present inventors have conducted a stretching operation under a specific temperature condition on a thermoplastic resin composition comprising a normal molecular weight thermoplastic resin and a filler that can be thermoformed without requiring a low molecular weight component. The present inventors have found that the puncture strength of the separator obtained by application is remarkably improved, and the present invention has been completed.
That is, the gist of the present invention is as follows.

[1] A separator for a non-aqueous electrolyte secondary battery comprising a thermoplastic resin composition containing a thermoplastic resin and a filler, and having a puncture strength per thickness of 25 μm of 2.5 N or more.

[2] The separator for a non-aqueous electrolyte secondary battery according to [1], wherein the thermoplastic resin is a polyolefin resin having a melt flow rate based on JISK7210 of 0.01 to 0.3 g / 10 min.

[3] The separator for a non-aqueous electrolyte secondary battery according to [2], wherein the polyolefin resin is polyethylene and / or polypropylene.

[4] The separator for a nonaqueous electrolyte secondary battery according to [2] or [3], wherein the polyolefin resin is high-density polyethylene.

[5] The separator for a non-aqueous electrolyte secondary battery according to any one of [1] to [4], wherein the filler is an inorganic substance.

[6] The nonaqueous electrolyte secondary battery separator according to any one of [1] to [5], wherein the filler content in the thermoplastic resin composition is 10 to 50% by volume.

[7] The separator for a nonaqueous electrolyte secondary battery according to any one of [1] to [6], wherein the filler is a sulfate.

[8] The separator for a non-aqueous electrolyte secondary battery according to [7], wherein the sulfate is barium sulfate.

[9] The separator for a non-aqueous electrolyte secondary battery according to any one of [1] to [6], wherein the filler is a metal oxide.

[10] The separator for a nonaqueous electrolyte secondary battery according to [9], wherein the metal oxide is alumina.

[11] A separator for a non-aqueous electrolyte secondary battery comprising a thermoplastic resin composition containing a thermoplastic resin and a filler, wherein the separator is at least once at a temperature equal to or higher than the melting point of the thermoplastic resin. A non-aqueous electrolyte secondary battery that has been subjected to a stretching treatment and is subjected to at least one stretching treatment at a temperature lower than the melting point in the same direction as the stretching treatment at or above the melting point. Separator for use.

[12] A non-aqueous electrolyte secondary having a positive electrode capable of inserting and extracting lithium ions, a negative electrode capable of inserting and extracting lithium ions, a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, and a separator A non-aqueous electrolyte secondary battery using the separator according to any one of [1] to [11] as a separator.

  According to the present invention, the active material does not break through the separator due to the tightening during winding of the electrode and the separator, the pressure due to the expansion / contraction of the electrode during charging / discharging, or the impact when the battery is dropped, A separator for a non-aqueous electrolyte secondary battery excellent in mechanical properties such as piercing strength that does not cause a short circuit between both electrodes, a positive electrode and a negative electrode capable of inserting and extracting lithium ions, a non-aqueous solvent, and A highly safe non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte containing a lithium salt is provided.

  Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention.

[Separator for non-aqueous electrolyte secondary battery]
First, the separator for a non-aqueous electrolyte secondary battery of the present invention will be described.
The separator for a non-aqueous electrolyte secondary battery of the present invention is composed of a thermoplastic resin composition containing a thermoplastic resin and a filler, and has a puncture strength of 2.5 N or more per 25 μm thickness.

{Thermoplastic resin composition}
<Thermoplastic resin>
The thermoplastic resin as the base resin of the thermoplastic resin composition constituting the separator of the present invention is not particularly limited as long as the filler described later can be uniformly dispersed. For example, a polyolefin resin And styrene resins such as fluorine resin and 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.

  Such a thermoplastic resin preferably has a melt flow rate based on JISK7210 of 0.01 to 0.3 g / 10 min, more preferably 0.02 to 0.2 g / 10 min, still more preferably 0.03 to 0.1 g / 10 min. When the melt flow exceeds this upper limit, the molecular weight of the resin becomes low, and it tends to be difficult to obtain sufficient mechanical strength. Also, below this lower limit, the resin molecular weight becomes too high and normal thermoforming becomes difficult, the addition of a low molecular weight component is essential, the extraction of the low molecular weight component is necessary, or while using the battery, When the battery temperature rises, the separator may be blocked.

<Filler>
As a filler contained in the thermoplastic resin composition constituting the separator for a non-aqueous electrolyte secondary battery of the present invention, generally, an inorganic filler having a property of not decomposing a carbonate-based organic electrolyte used in a lithium secondary battery The agent is selected. Examples of such a filler include sparingly water-soluble sulfates and alumina, but barium sulfate is particularly preferably used. The poorly water-soluble 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 may cause decomposition of non-aqueous electrolyte components.

  As the particle size of the filler, the lower limit of the 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 10 μm or less, preferably 5 μm or less, more preferably 3 μm. In particular, the thickness is preferably 1 μm or less. When the average particle diameter of the filler exceeds 10 μm, the diameter of the holes formed by stretching becomes too large, and it tends to cause stretching breakage and a decrease in film strength. Further, if the 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, one kind of filler can be used alone, or two or more kinds can be mixed and used as long as the filler satisfies the above conditions.

  The blending amount of the filler in the thermoplastic resin composition according to the present invention is preferably 10 to 50% by volume in the thermoplastic resin composition containing the thermoplastic resin and the filler, and more preferably 15 to 40%. % By volume, particularly preferably 20 to 30% by volume. If the blending amount of the filler is less than 10% by volume, it is difficult to form a communication hole, and it becomes difficult to exhibit a function as a separator. Moreover, when it exceeds 50 volume%, the viscosity at the time of film formation becomes high and it is inferior to workability, and there exists a possibility of producing a film fracture | rupture at the time of extending | stretching for porosity.

  As the filler, one that has been surface-treated with a surface treatment agent in order to enhance dispersibility in the thermoplastic resin 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.

<Other additives>
A small amount of a low molecular weight compound having compatibility with the thermoplastic resin may be added to the thermoplastic resin composition according to the present invention. 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 thermoplastic 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 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, if the molecular weight is less than 200, the compatibility increases, but so-called blooming, in which a low molecular weight compound precipitates on the surface of the polymer porous membrane constituting the separator, is likely to occur, and the film properties are likely to deteriorate or block.

  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.

  The blending amount of the low molecular weight compound is preferably 1 to 10 parts by weight with respect to 100 parts by weight of the total of the thermoplastic resin and the filler. If the blending amount of the low molecular weight compound is less than 1 part by weight, the above-mentioned effect by blending the low molecular weight compound cannot be sufficiently obtained, and if the blending amount exceeds 10 parts by weight, the interaction between the molecules of the thermoplastic resin is caused. If it is lowered too much, 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. Furthermore, if the extraction of a low molecular weight compound becomes necessary, or the battery temperature rises while the battery is in use, the separator may be clogged.

  The thermoplastic resin composition according to the present invention may further contain other additives such as a heat stabilizer as necessary. The additive is not particularly limited as long as it is a known additive. The compounding amount of these additives is usually 0.05 to 1 part by weight with respect to 100 parts by weight of the total of the thermoplastic resin and the filler.

{Production method}
The method for producing the separator for a non-aqueous electrolyte secondary battery of the present invention is preferably an interfacial peeling method.
More specifically, the separator for a non-aqueous electrolyte secondary battery of the present invention is manufactured by the following method.

  First, a thermoplastic 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 premixed by a Henschel mixer or the like, and then prepared using a commonly used single screw extruder, twin screw extruder, mixing roll or twin screw kneader, etc. The pre-kneading may be omitted and the resin composition may be directly prepared with the above extruder or the like.

  Next, the thermoplastic resin composition is formed into a sheet. Sheet forming can be performed by a commonly used T-die method using a 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 treatment is preferably performed in at least one direction, and the stretching treatment includes a high-temperature stretching step in which stretching is performed at least once at a temperature equal to or higher than the melting point of the thermoplastic resin, and high-temperature stretching after the high-temperature stretching. It consists of two processes, a low-temperature stretching process in which stretching is performed at least once at a temperature below the melting point in the same direction. Here, the melting point of the thermoplastic resin is determined based on JISK7121.

  The high temperature stretching is performed at a temperature of the melting point of the thermoplastic resin to the melting point + 10 ° C., preferably at a temperature of the melting point to the melting point + 5 ° C. The high-temperature stretching is performed in order to improve the mechanical strength by orienting the thermoplastic resin composition without causing an opening portion, and it is necessary to perform the stretching at a temperature at which interface peeling does not occur. Is done. When high-temperature stretching is performed at a temperature lower than the melting point, interfacial delamination occurs between the resin and the filler in the initial stage of stretching, resulting in opening of holes, and subsequent stretching only improves the mechanical strength by expanding the opening. it dose not connect. Further, when the high temperature stretching exceeds the melting point + 10 ° C., the fluidity of the resin becomes too high, and the high temperature stretching does not lead to the orientation of the resin composition, and it is difficult to improve the sufficient mechanical strength.

  The low temperature stretching is performed in the same direction as the high temperature stretching after the high temperature stretching. The low temperature stretching is preferably performed at a temperature of the melting point of −30 ° C. to the melting point of −2 ° C., more preferably at a temperature of the melting point of −20 ° C. to the melting point of −5 ° C. If the low-temperature stretching is performed at a temperature lower than the melting point of −30 ° C., the thermoplastic resin composition is not sufficiently softened, so that the stretching breakage is likely to occur, which is not preferable. Further, when the low-temperature stretching is performed at a temperature higher than the melting point of −2 ° C., interfacial peeling between the resin and the filler is difficult to occur, so that the holes are not sufficiently formed and it becomes difficult to function as a separator.

  In the present invention, it is important to perform low-temperature stretching after high-temperature stretching, and the order of high-temperature stretching and low-temperature stretching cannot be interchanged. That is, if this order is changed, the holes opened by the low-temperature stretching are crushed by the high-temperature stretching and become a non-porous film, so that it does not function as a separator.

  Moreover, it is necessary to perform high temperature stretching and low temperature stretching in the same direction. If the directions of the high temperature drawing and the low temperature drawing are different, the orientation of the resin composition by the high temperature drawing is relaxed by the low temperature drawing applied later, so that sufficient mechanical strength is not exhibited.

  The draw ratio is arbitrarily set according to the required pore diameter and strength, but since the piercing strength per thickness of 25 μm is 2.5 N or more, the product of the draw ratio of high temperature drawing and low temperature drawing is at least uniaxial. In addition, it is preferably 4 times or more and 8 times or less. When the product of the uniaxial stretching ratio is less than 4 times, the strength is not sufficiently improved when the hot stretching ratio is too low, and it becomes difficult to increase the piercing strength per 25 μm thickness to 2.5 N or more. When the magnification is too low, there is no sufficient opening due to interfacial peeling, making it difficult to function as a separator. In order to sufficiently perform the piercing strength and the opening, it is preferable that the high-temperature stretching ratio and the low-temperature stretching ratio are each 2 times or more. On the other hand, if the product of the draw ratios in the uniaxial direction exceeds 8 times, the draw ratio tends to be too high and stretch breakage tends to occur.

  In the production of the separator of the present invention, as described above, the stretching treatment is not limited as long as it is subjected to high-temperature stretching in at least one direction and then low-temperature stretching in the same direction. Both the high-temperature stretching and the low-temperature stretching are uniaxial stretching. Or biaxial stretching, one of high-temperature stretching and low-temperature stretching may be biaxial stretching, and the other may be uniaxial stretching. Furthermore, it is preferable to perform high-temperature stretching and low-temperature stretching co-biaxial stretching because the piercing strength is easily increased and the porous structure is easily formed.

{Puncture strength}
The separator for a non-aqueous electrolyte secondary battery of the present invention is characterized in that the puncture strength per 25 μm thickness is 2.5 N or more. If the piercing strength is less than 2.5, the object of the present invention cannot be achieved. The piercing strength per 25 μm thickness is preferably 3.0 N or more. The puncture strength is preferably as high as possible, and the upper limit is not particularly defined, but is usually about 5.0 N or less.
The piercing strength per 25 μm thickness is measured by the piercing strength measuring method described in the example section described later.

{Other physical properties}
The porosity of the separator for a non-aqueous electrolyte secondary battery of the present invention is usually 30% or more, preferably 40% or more, more preferably 50%, as the lower limit of the porosity of the porous film constituting the separator of the present invention. The upper limit is usually 80% or less, preferably 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 may not be achieved. Further, if the porosity exceeds 80%, the actual strength of the film is lowered, and therefore there is a possibility that breakage or short-circuiting due to the active material and short-circuiting may occur at the time of battery preparation.

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 the porous membrane t: Thickness of the porous membrane w: Weight of the porous membrane ρ: True specific gravity of the porous membrane In addition, the component i (resin, filler, etc.) constituting the polymeric porous membrane When the blend weight is Wi and the specific gravity is ρi, the true specific gravity ρ is 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)

  Moreover, the upper limit of the thickness of the separator of the present invention is usually 100 μm or less, particularly 50 μm or less, preferably 40 μm or less, and the lower limit 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 there is a risk that breakage or short-circuiting due to the active material and short-circuiting may occur during battery production. On the other hand, when the thickness exceeds 100 μm, the electric resistance of the separator increases, and the battery capacity may be reduced. By setting the thickness in the range of 5 to 100 μm, a separator having good ion permeability can be obtained.

[Non-aqueous electrolyte secondary battery]
Next, the 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 occluding and releasing lithium ions, a negative electrode capable of occluding and releasing lithium ions, a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, and And a separator for a non-aqueous electrolyte secondary battery according to the present invention.

{Non-aqueous electrolyte solution}
<Non-aqueous solvent>
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, cyclic carbonate such as alkylene carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate (preferably alkylene carbonate having 3 to 5 carbon atoms); dialkyl such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate Chain carbonates such as carbonates (preferably dialkyl carbonates having an alkyl group having 1 to 4 carbon atoms); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; chain ethers such as dimethoxyethane and dimethoxymethane; γ-butyrolactone, γ -Cyclic carboxylic acid esters such as valerolactone; and chain carboxylic acid esters such as methyl acetate, methyl propionate, and ethyl propionate. These may be used alone or in combination of two or more.

  Among the above exemplified solvents, a mixed non-aqueous solvent in which a cyclic carbonate and a chain carbonate are mixed is preferable from the viewpoint of improving overall battery performance such as charge / discharge characteristics and battery life. The mixed non-aqueous solvent contains cyclic carbonate and chain carbonate in an amount of 15% by volume or more of the whole non-aqueous solvent, and the total of these volumes is 70% by volume or more of the whole non-aqueous solvent. It is preferable to do.

  As the cyclic carbonate used in the mixed non-aqueous solvent in which the cyclic carbonate and the chain carbonate are mixed, an alkylene carbonate having 2 to 4 carbon atoms in the alkylene group is preferable. Specific examples thereof include ethylene carbonate, propylene carbonate, butylene carbonate and the like. Of these, ethylene carbonate and propylene carbonate are preferable.

  Moreover, as the chain carbonate used for the mixed non-aqueous solvent in which the cyclic carbonate and the chain carbonate are mixed, a dialkyl carbonate having an alkyl group having 1 to 4 carbon atoms is preferable. Specific examples thereof include dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, ethyl methyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate and the like. Of these, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate are preferable.

  Each of these cyclic carbonates and chain carbonates may be used independently, or a plurality of types may be used in any combination and ratio.

  The ratio of the cyclic carbonate in the mixed non-aqueous solvent is 15% by volume or more, particularly 20 to 50% by volume, and the ratio of the chain carbonate is 30% by volume or more, particularly 40 to 80% by volume. The content ratio is preferably cyclic carbonate: chain carbonate = 1: 1 to 4 (volume ratio).

  Furthermore, the mixed non-aqueous solvent may contain a solvent other than the cyclic carbonate and the chain carbonate as long as the battery performance of the manufactured lithium battery is not deteriorated. The ratio of the solvent other than the cyclic carbonate and the chain carbonate in the mixed non-aqueous solvent is usually 30% by volume or less, preferably 10% by volume or less.

<Lithium salt>
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 , 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 F9SO 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, LiBF 2 (C 2 F 5 SO 2) 2 fluorine-containing organic lithium salt such as Etc. Of these, fluorine-containing organic lithium salts such as LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (C ₂ F ₅ SO ₂ ) ₂ , particularly LiPF ₆ and LiBF ₄ are preferable. . 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.

<Film-forming agent>
The nonaqueous electrolytic solution according to the present invention may contain a film forming agent capable of forming a resistive film on the negative electrode surface. As the film forming agent used in the present invention, carbonate compounds having ethylenically unsaturated bonds such as vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate, erythritan carbonate, succinic anhydride, Carboxylic anhydride such as glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride Thing etc. are mentioned. In particular, vinylene carbonate, vinyl ethylene carbonate, and succinic anhydride are preferable as the film forming agent from the viewpoint of good cycle characteristic improvement effect and temperature dependency of film resistance, and vinylene can be formed because particularly good quality film can be formed. More preferably, carbonate is used. In addition, these film formation agents may be used individually by 1 type, and may mix and use 2 or more types.

  In the present invention, the content of the film forming agent in the nonaqueous electrolytic solution is 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 0.3% by weight or more, and 10% by weight or less. , Preferably 8% by weight or less, more preferably 7% by weight or less. If the content of the film forming agent is below the lower limit of the above range, it is difficult to obtain the effect of improving the cycle characteristics of the battery. On the other hand, if the content exceeds the upper limit, the rate characteristics at low temperatures may be lowered.

  In addition to the non-aqueous solvent, lithium salt, and film forming agent, the non-aqueous electrolyte solution according to the present invention includes other useful components as required, for example, conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents. You may contain various additives, such as an agent and a positive electrode protective agent.

{Positive electrode}
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.

  The positive electrode active material is not particularly limited as long as it can electrochemically occlude and release lithium ions. Preferable examples include lithium transition metal composite oxides.

Specific examples of the lithium-transition metal composite oxide, lithium cobalt complex oxides such as LiCoO _2, lithium-nickel composite oxide such as LiNiO _2, include lithium-manganese composite oxides such as LiMnO _2. In these lithium transition metal composite oxides, some of the main transition metal atoms are Al, Ti, V, Cr, Mn, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, Si, etc. Replacing with other metals is preferable because it can be stabilized. Any one of these positive electrode active materials may be used alone, or two or more thereof may be used in any combination and ratio.

  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, N, N-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.

{Negative electrode}
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, butadiene rubber and the like. 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.

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, and other additives added as necessary with a solvent, and then drying the current collector. 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, N, N-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.

{Battery configuration}
The non-aqueous electrolyte secondary battery of the present invention is manufactured by assembling the above-described positive electrode, negative electrode, non-aqueous electrolyte, and separator 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 non-aqueous electrolyte secondary battery of the present invention has been described above, but the non-aqueous electrolyte secondary battery of the present invention is not limited to the above-described embodiment and does not exceed the gist thereof. As long as it is possible, various modifications can be made.

  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 examples and comparative examples, the piercing strength of the separator was measured by the following method.
<Method of measuring piercing strength of separator>
A hole made by a metal (SUS440C) needle with a diameter of 1 mm and a tip curvature radius of 0.5 mm is pierced at a speed of 300 mm / min in the thickness direction into a sample (measurement part: a circle with a diameter of 10 mm) fixed with a holder. Measure the maximum load. Assuming that the piercing strength is proportional to the thickness, the piercing strength per 25 μm thickness is obtained.

[Example 1]
50 parts by weight (82.2% by volume) of high-density polyethylene ("Hi-Zex 7000F" manufactured by Prime Polymer Co., Ltd., melt flow rate of 0.04 g / 10 min based on JISK7210, melting point 131 ° C based on JISK7121) [Average particle size 0.66 μm] 50 parts by weight (17.8% by volume) were blended and melt-kneaded at 180 ° C., and the resulting resin composition was hot-pressed at 180 ° C. to obtain a raw sheet. . The thickness of the original fabric sheet was 450 μm on average.
Next, the obtained raw sheet was subjected to 2 × 2 simultaneous biaxial stretching at 135 ° C. Next, 3 × 3 simultaneous biaxial stretching was performed at 125 ° C.
The obtained film had a thickness of 24 μm, a porosity of 47%, and a puncture strength per 25 μm of 350 g (3.43 N).

[Example 2]
50 parts by weight (82.2% by volume) of high-density polyethylene ("Hi-Zex 7000F" manufactured by Prime Polymer Co., Ltd., melt flow rate based on JISK7210 is 0.04 g / 10min, melting point is 131 ° C based on JISK7121), and barium sulfate as a filler [Average particle size 0.66 μm] 50 parts by weight (17.8% by volume) were blended and melt-kneaded at 180 ° C., and the resulting resin composition was hot-pressed at 180 ° C. to obtain a raw sheet. . The thickness of the raw sheet was an average of 360 μm.
Next, the obtained raw sheet was subjected to 2 × 2 simultaneous biaxial stretching at 135 ° C. Next, 3 × 3 simultaneous biaxial stretching was performed at 120 ° C.
The obtained film had a film thickness of 16 μm, a porosity of 36%, and a puncture strength per 25 μm of 440 g (4.31 N).

[Comparative Example 1]
50 parts by weight (82.2% by volume) of high-density polyethylene ("Hi-Zex 7000F" manufactured by Prime Polymer Co., Ltd., melt flow rate of 0.04 g / 10 min based on JISK7210, melting point 131 ° C based on JISK7121) [Average particle size 0.66 μm] 50 parts by weight (17.8% by volume) were blended and melt-kneaded at 180 ° C., and the resulting resin composition was hot-pressed at 180 ° C. to obtain a raw sheet. . The thickness of the original fabric sheet was 200 μm on average.
Next, the obtained original sheet was subjected to 3 × 3 simultaneous biaxial stretching at 125 ° C.
The obtained film had a film thickness of 62 μm, a porosity of 64%, and a piercing strength per 25 μm of 90 g (0.88 N).

Claims (12)

  A separator for a non-aqueous electrolyte secondary battery comprising a thermoplastic resin composition containing a thermoplastic resin and a filler, and having a puncture strength of 2.5 N or more per 25 μm thickness.   The separator for a non-aqueous electrolyte secondary battery according to claim 1, wherein the thermoplastic resin is a polyolefin resin having a melt flow rate based on JISK7210 of 0.01 to 0.3 g / 10 min.   The separator for a non-aqueous electrolyte secondary battery according to claim 2, wherein the polyolefin resin is polyethylene and / or polypropylene.   The separator for a non-aqueous electrolyte secondary battery according to claim 2 or 3, wherein the polyolefin resin is high-density polyethylene.   The separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the filler is an inorganic substance.   The separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the filler content in the thermoplastic resin composition is 10 to 50% by volume.   The separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the filler is a sulfate.   The separator for a non-aqueous electrolyte secondary battery according to claim 7, wherein the sulfate is barium sulfate.   The separator for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, wherein the filler is a metal oxide.   The separator for a non-aqueous electrolyte secondary battery according to claim 9, wherein the metal oxide is alumina.   A separator for a non-aqueous electrolyte secondary battery comprising a thermoplastic resin composition containing a thermoplastic resin and a filler, wherein the separator is subjected to at least one stretching treatment at a temperature equal to or higher than the melting point of the thermoplastic resin. A separator for a non-aqueous electrolyte secondary battery that has been subjected to at least one stretching treatment at a temperature below the melting point in the same direction as the stretching treatment at or above the melting point.   In a non-aqueous electrolyte secondary battery comprising a positive electrode capable of inserting and extracting lithium ions, a negative electrode capable of inserting and extracting lithium ions, a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, and a separator, A non-aqueous electrolyte secondary battery using the separator according to claim 1 as the separator.