JP4586359B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and in particular, includes a negative electrode and a positive electrode capable of inserting and extracting lithium, a separator, and a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt. The present invention relates to a non-aqueous electrolyte secondary battery having excellent cycle characteristics and excellent rate characteristics in a low-temperature environment.

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.

As non-aqueous electrolytes for lithium secondary batteries, there are usually cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate (dialkyl carbonate), γ-butyrolactone, and γ-valero. Cyclic esters such as lactone, chain esters such as methyl acetate and methyl propionate, cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran and tetrahydropyran, chain ethers such as dimethoxyethane and dimethoxymethane, and sulfolane, and a non-aqueous solvent such as sulfur-containing organic solvents such as diethyl _{sulfone, LiPF 6, LiBF 4, LiClO} 4, LiCF 3 SO 3, LiAsF 6, LiN (CF 3 SO 2) 2, LiCF _{(CF 2)} a non-aqueous electrolyte solution containing solutes such as 3 SO ₃ is used.

  In such a secondary battery using a non-aqueous electrolyte, the reactivity varies depending on the composition of the non-aqueous electrolyte, so that the battery characteristics vary greatly depending on the non-aqueous electrolyte used. In particular, the decomposition and side reaction of the electrolytic solution affect the cycle characteristics and storage characteristics of the secondary battery, and therefore, degradation of the cycle characteristics and storage characteristics of the secondary battery due to the decomposition of the electrolytic solution and side reactions becomes a problem.

  In contrast, conventionally, attempts have been made to improve the cycle characteristics and storage characteristics of secondary batteries by mixing various additives with a non-aqueous electrolyte. For example, Patent Document 1 contains vinylene carbonate. A non-aqueous electrolyte is used. In this non-aqueous electrolytic solution, it is considered that a passivation layer is formed on the negative electrode surface by reduction of vinylene carbonate, and this layer constitutes a physical barrier that prevents decomposition of the electrolytic solution. The effect of vinylene carbonate is to improve the charge / discharge efficiency and cycle characteristics of the battery.

  However, as shown in Non-Patent Document 1, in the technique of Patent Document 1, the discharge capacity (hereinafter referred to as “rate characteristic”) when large current discharge is performed in a low temperature environment is reduced. There was a point. This is proved in Comparative Examples 1 and 4 described later. This is presumably because the internal resistance of the battery increases remarkably at low temperatures because of the passive film formed on the negative electrode surface.

  For this reason, development of a technique for improving the cycle characteristics of a secondary battery that is not accompanied by a decrease in rate characteristics under a low temperature environment is demanded.

In addition, the separator used in the lithium secondary battery is required to satisfy requirements such as not impeding ion conduction between both electrodes, being able to hold the electrolytic solution, having resistance to the electrolytic solution, A polymer porous film mainly made of a thermoplastic resin such as polyethylene or polypropylene is used. 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 to perform molding, and then the plasticizer is removed with a suitable solvent to make it porous (Patent Document 2).
(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 3).
(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 4).

  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.
JP-A-8-45545 JP 7-029563 A JP-A-7-304110 JP 2002-201298 A J. Power Sources 119-121, (2003), 943

  As described above, a lithium secondary battery in which a film-forming agent is added to a non-aqueous electrolyte for improving cycle characteristics has a problem that rate characteristics in a low temperature environment deteriorate.

  Therefore, an object of the present invention is to provide a technique capable of improving the cycle characteristics of a lithium secondary battery without deteriorating the rate characteristics in a low temperature environment. That is, an object of the present invention is to provide a non-aqueous electrolyte secondary battery excellent in cycle characteristics and rate characteristics under a low temperature environment.

  As a result of diligent research, the present inventor has made a non-aqueous electrolyte contain a film-forming agent and use a porous film made of a thermoplastic resin containing an inorganic filler as a separator in a low-temperature environment. The present inventors have found that the cycle characteristics of a lithium secondary battery can be improved without lowering the rate characteristics of the present invention.

That is, the gist of the present invention is a non-aqueous electrolyte secondary comprising a negative electrode and a positive electrode capable of inserting and extracting lithium, a separator, and a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt. In the battery, the separator has a porous film made of a thermoplastic resin containing an inorganic filler, and the blending amount of the inorganic filler in the porous film is 40 parts by weight with respect to 100 parts by weight of the thermoplastic resin. More than 300 parts by weight and less than 300 parts by weight, and the inorganic filler is added to an electrolyte solution composed of a mixed nonaqueous solvent solution of EC / EMC = 3: 7 (volume ratio) of 1M LiPF ₆ per 1 ml of the electrolyte solution. The concentration of lithium ions in the electrolytic solution after adding the agent at a ratio of 0.5 g and maintaining at 85 ° C. for 72 hours does not decrease to 0.75 mmol / g or less, and the nonaqueous electrolytic solution is a film forming agent. Characterized by containing It resides in a nonaqueous electrolyte secondary battery (claim 1).

  In the present invention, the details of the mechanism of action by combining a specific separator and a non-aqueous electrolyte containing a film-forming agent are not clear, but are estimated as follows.

  That is, as described above, by adding a film-forming agent to the non-aqueous electrolyte, a passivating film is formed on the surface of the negative electrode to prevent decomposition of the non-aqueous electrolyte, thereby improving the cycle characteristics of the secondary battery. Although it can be improved, the resistance on the inside of the battery is remarkably increased at a low temperature due to the film on the negative electrode surface formed by this film forming agent. In the present invention, after using a film-forming agent effective for improving the cycle characteristics of the secondary battery, a specific separator is combined, and the effect of reducing the battery internal resistance of the separator is reduced by the battery-forming agent. The increase in internal resistance is offset and rate characteristics are maintained in a low temperature environment.

  The reason why the separator used in the present invention is effective in reducing the internal resistance of the battery is as follows. That is, among the methods (1) to (3) described above as the method for producing the separator, the extraction method (1) needs to select a plasticizer that has good compatibility with the polymer material, or is extracted thereafter. However, there is a drawback that a sufficiently large hole cannot be formed even if the film is stretched, and the electrical resistance of the resulting separator is increased. Moreover, a method for enlarging the pores by adding a plasticizer having poor compatibility is also known as a known technique. However, in this method, it is difficult to obtain a film having good properties because of unstable molding. In any method, it is necessary to treat a large amount of waste liquid in the extraction step, and this is not an economically preferable method. In addition, since the stretching method (2) selectively stretches only the amorphous part between the crystal domains, it is difficult to stretch at a high magnification. Therefore, it is difficult to increase the hole diameter, and there is a drawback that the electric resistance of the obtained separator is increased. In addition, a long-time heat treatment is necessary for controlling the crystal phase, the productivity is low, and this is not an economically preferable method.

  On the other hand, a separator made of a porous film made of a thermoplastic resin containing an inorganic filler used in the present invention is easily manufactured as a porous film having a relatively large pore diameter by, for example, an interfacial peeling method as described later. can do. Thus, a separator having a large hole diameter has high hole communication and is effective in reducing the internal resistance of the battery.

  In the present invention, the film forming agent is preferably at least one selected from the group consisting of carbonates having an ethylenically unsaturated bond and carboxylic acid anhydrides (Claim 2).

  Moreover, it is preferable that content of this film formation agent in a non-aqueous electrolyte is 0.01 to 10 weight% (Claim 3).

  The separator has a thickness of 5 μm to 100 μm, a porosity of 30% to 80%, an average pore size defined by ASTM F316-86 of 0.05 μm to 10 μm, and a Gurley permeability determined by JIS P8117. Is preferably 20 seconds / 100 cc or more and 700 seconds / 100 cc or less (claim 4).

  Further, the non-aqueous electrolyte is preferably one using a mixed non-aqueous solvent containing a fluorine-containing lithium salt as a lithium salt and containing at least both a cyclic carbonate and a chain carbonate as a non-aqueous solvent (claim). Item 5).

  The film-forming agent is particularly preferably vinylene carbonate (claim 6).

  The negative electrode active material is preferably a carbon material.

  The positive electrode active material is preferably a lithium transition metal composite oxide.

  According to the present invention, it is possible to improve the cycle characteristics of a non-aqueous electrolyte secondary battery without deteriorating rate characteristics in a low temperature environment.

  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.

  The non-aqueous electrolyte secondary battery of the present invention includes a separator as described in detail below, a non-aqueous electrolyte, and a positive electrode and a negative electrode capable of inserting and extracting lithium.

[Separator]
<Constituent components and physical properties of separator>
The thermoplastic resin as the base resin of the porous film constituting the separator of the present invention is not particularly limited as long as the inorganic filler described later can be uniformly dispersed. For example, a polyolefin resin, Examples thereof include fluorine resins, styrene resins such as polystyrene, ABS resins, vinyl chloride resins, vinyl acetate resins, acrylic resins, polyamide resins, acetal resins, and polycarbonate resins. 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 evenly dispersed in the resin, and pore formation due to interfacial peeling becomes nonuniform, which is not preferable. Below this lower limit, the mechanical strength decreases, which is not preferable.

  As the inorganic filler contained in the porous polymer membrane according to the present invention, those having the property of not decomposing the carbonate-based organic electrolyte used in the lithium secondary battery are selected. Examples of such a filler include sparingly water-soluble sulfates and alumina, but 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 inorganic 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 10 μm or less, preferably 5 μm or less, More preferably, it is 3 μm or less, and particularly preferably 1 μm or less. If the number-based average particle diameter of the inorganic 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. 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, one kind of inorganic filler can be used alone, or two or more kinds can be mixed and used as long as they satisfy the above conditions.

  The blending amount of the inorganic filler in the polymer 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, especially 60 parts by weight or more, relative to 100 parts by weight of the thermoplastic resin. More preferably, it is 100 parts by weight or more, and 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. When the blending amount of the inorganic filler with respect to 100 parts by weight of the thermoplastic resin in the polymer porous membrane is less than 40 parts by weight, it is difficult to form a communication hole, and it becomes difficult to exhibit a 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 membrane remains in the substantially molded porous membrane, the blending amount range of the filler is within the range of the porous membrane. It becomes a filler content range.

  As the inorganic 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.

  At the time of molding the porous polymer membrane 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 enhances the interfacial adhesive force between the thermoplastic resin and the inorganic filler and prevents the pores from becoming coarse due to stretching, and the interfacial adhesive force between the thermoplastic resin and the inorganic filler. The effect of preventing the inorganic filler from falling off from the film is obtained.

  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, when the molecular weight is less than 200, the compatibility is improved, but so-called blooming, in which a low molecular weight compound is precipitated on the surface of the polymer porous membrane, is likely to occur, and the membrane properties are easily 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 a film forming material for the polymer porous membrane 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 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 a film forming material for the polymer porous membrane 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 polymer 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 polymer porous membrane, and usually 80 as the upper limit. % Or less, preferably 70% or less, and more preferably 65% 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 polymer porous membrane is a value calculated by the following calculation formula.
Porosity Pv (%) = 100 × (1−w / [ρ · S · t])
S: Area of polymer porous membrane t: Thickness of polymer porous membrane w: Weight of polymer porous membrane ρ: True specific gravity of polymer porous membrane

In addition, 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 formula (where Σ is all components) Represents the sum of
True specific gravity ρ = ΣWi / Σ (Wi / ρi)

  The upper limit of the thickness of the polymer porous membrane according to the present invention is usually 100 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, 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 unpreferable. On the other hand, when the thickness exceeds 100 μm, the amount of the active material put into the battery is reduced, so that the capacity of the whole battery is also lowered, which is not preferable. By setting the thickness of the separator to a range of 5 to 100 μm, a separator having good ion permeability can be obtained.

  The lower limit of the average pore diameter of the polymer porous membrane according to the present invention is 0.05 μm or more, preferably 0.1 μm or more, and more preferably 0.2 μm or more. The upper limit is 10 μm or less, preferably 5 μm or less, more preferably 3 μm or less, and particularly preferably 1 μm or less. If the average pore size is smaller than 0.05 μm, it is difficult to ensure sufficient connectivity of the porous membrane, and if it is larger than 10 μm, it is difficult to ensure practical film strength. As will be described later, the pore diameter of the polymer porous membrane can be arbitrarily changed by selecting the properties (particle size, etc.) and the filling amount of the inorganic filler as necessary. Here, the average pore diameter of the polymer porous membrane is determined by ASTM F316-86.

  The polymer porous membrane according to the present invention has a lower limit value of Gurley air permeability of 20 seconds / 100 cc or more, particularly 100 seconds / 100 cc or more, and an upper limit value of 700 seconds / 100 cc or less, particularly 300 seconds / 100 cc or less. It is preferable that 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, 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>
The method for producing the porous polymer membrane containing the inorganic filler is not particularly limited, and includes the following extraction method (1), stretching method (2), and interfacial peeling method (3), but particularly preferred. Is an interfacial peeling method.
(1) Extraction method: Melting a resin composition obtained by mixing a polymer material, an inorganic filler, and a plasticizer that can be removed by solvent extraction in a later step, and forming this by a molding method such as extrusion molding After forming into a shape, it is made porous by treating it with a solvent to remove the plasticizer.
(2) Stretching method: A resin composition obtained by mixing a crystalline polymer material with an inorganic filler is melted, formed into a film by a molding method such as extrusion molding, and then stretched to obtain a structural Micropores are formed by stretching an amorphous portion that is weak to the surface.
(3) Interfacial peeling method: A polymer composition obtained by mixing a polymer material with an inorganic filler is melted, formed into a film by a molding method such as extrusion molding, and then stretched to obtain a polymer material. The fine pores are formed by peeling the interface between the inorganic filler and the inorganic filler.

  Among the above production methods, in the extraction method, it is difficult to selectively contain an inorganic filler on the polymer material side during molding, and the inorganic filler contained in the plasticizer part is removed together with the plasticizer during extraction. Therefore, it is not efficient as compared with the interfacial peeling method. In addition, in the stretching method, when an inorganic filler is contained in the polymer material, pores are formed by stretching at the interface between the polymer material and the inorganic 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. In the case of the interfacial peeling method, the pore diameter of the obtained polymer porous membrane can be freely changed by adjusting the particle size and filling amount of the inorganic filler. That is, in order to reduce the internal resistance of the polymer porous membrane which is the subject of the present invention, a separator having a large pore diameter is required. However, as described above, in the methods (1) and (2), the pore diameter is reduced. It is difficult to form large holes. On the other hand, the interfacial peeling method can easily realize a separator having a large pore diameter and effectively reduce the internal resistance.

  In addition to the above methods (1) to (3), as a method for forming a porous polymer membrane containing an inorganic filler, a polymer material, a plasticizer, and an inorganic filler are mixed in an organic solvent to obtain a paste. A method of extracting and evaporating a plasticizer after preparing and forming a film by casting or the like is also known as a known technique. However, this method is the same as the above (1) in that an extra step is added for removing the plasticizer, and is inferior in simplicity compared with the interface peeling method.

  More specifically, the polymer porous membrane according to the present invention is produced by the following method.

  First, a resin composition is prepared by blending a predetermined amount of an additive such as an inorganic filler, a thermoplastic resin, and a low molecular weight compound or an antioxidant that is added as necessary, and 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. Alternatively, the preliminary kneading may be omitted and the resin composition may be directly prepared using the 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. In addition, although there is no restriction | limiting in particular about the upper limit of this draw ratio, Usually, it is 7 times or less in a uniaxial direction. If the stretching exceeds this upper limit, the porosity of the obtained porous film becomes too high, the strength is lowered, and there is a possibility that it cannot be put into practical use.

[Non-aqueous electrolyte]
<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 carbonates such as alkylene carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate (preferably alkylene carbonates having 3 to 5 carbon atoms); dialkyls such as dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, and 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 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 , LiBF ₂ (C ₂ F ₅ SO ₂ ) _2, etc. Examples include lithium salts. Of these, fluorine-containing 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 this lower limit, it is preferable to prepare a non-aqueous electrolyte within the said concentration range.

<Film-forming agent>
The non-aqueous electrolyte solution of the present invention contains a film forming agent. The film-forming agent used in the present invention is a film-forming agent that forms a resistive film on the surface of the negative electrode and has a moderately large temperature dependency as described above. The combination with the specific separator used in the present invention is particularly preferable in that the effect of preventing the deterioration of rate characteristics under a low temperature environment is effectively obtained.

  From the viewpoint of such required characteristics, the film forming agent used in the present invention includes ethylenically unsaturated bonds such as vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, trifluoropropylene carbonate, phenylethylene carbonate and erythritan carbonate. Carbonate compounds, succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, diglycolic anhydride, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenyl Examples thereof include carboxylic acid anhydrides such as succinic anhydride. 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 non-aqueous electrolyte 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.

  As the positive electrode active material, the type of the positive electrode active material is not 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.

  In particular, among the above, when a carbonaceous material is used, since the resistance of the film formed by the film forming agent in the non-aqueous electrolyte and its temperature dependence are appropriate, the effects of the present invention are more effective. It becomes easy to obtain and is preferable.

  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.

  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 lithium 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 in a spiral shape, 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.

  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.

[Example 1]
<Manufacture of separator>
High density polyethylene [“HI-ZEX7000FP” manufactured by Mitsui Chemicals, Inc., weight average molecular weight: 200,000, density: 0.956 g / cm ³ , melt flow rate: 0.04 g / 10 min], 100 parts by weight, soft polypropylene [Idemitsu Petrochemical "PER R110E", weight average molecular weight: 330,000] 8.8 parts by weight, hydrogenated castor oil ["HY-CASTOR" manufactured by Toyokuni Oil Co., Ltd.
OIL ", molecular weight 938] 8.8 parts by weight, barium sulfate [number-based average particle size 0.18 μm] 176.5 parts by weight as an inorganic filler were blended and melt-kneaded, and the resulting resin composition was heated to 210 ° C. Inflation molding was performed at 0 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was 105 μm on average. Next, the obtained raw sheet was successively stretched at 90 ° C. in the sheet longitudinal direction (MD) 4 times, and then at 120 ° C. in the width direction (TD) 2.9 times to obtain a film thickness of 26 μm and porosity. A porous polymer membrane having a molecular weight of 64%, an average pore diameter (average pore diameter determined by ASTM F316-86) of 0.27 μm, and a Gurley air permeability (Gurley air permeability determined by JIS P8117) of 44 seconds / 100 cc was obtained. This polymer porous membrane is called separator A. During the stretching process, the inorganic filler was not removed from the porous polymer membrane.

<Preparation of non-aqueous electrolyte>
Purified ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 in a dry argon atmosphere to prepare a mixed solvent. In this mixed solvent, sufficiently dried LiPF ₆ is dissolved so as to have a ratio of 1.0 mol / l, and then vinylene carbonate (VC) so as to have a ratio of 2% by weight as a concentration in the nonaqueous electrolytic solution. Were mixed to obtain a non-aqueous electrolyte 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 3.0 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 pressing machine so that the density of the negative electrode active material layer was 1.5 g / cm ^3. did.

<Battery assembly>
A 2032 type coin cell was prepared using the separator A, the non-aqueous electrolyte, the positive electrode, and the 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 with 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>
1) Initial charge / discharge:
At a constant current corresponding to 0.2 C at 25 ° C. (the rated capacity due to the discharge capacity at 1 hour rate is 1 C, and the same applies hereinafter), the charge end voltage is 4.2 V and the discharge end voltage is 3 V. Charge and discharge are stabilized by 3 cycles, and the 4th cycle is charged to a charge end voltage of 4.2 V with a current corresponding to 0.5 C, and charged until the charge current value reaches a current value corresponding to 0.05 C. After 4.2V-constant current constant voltage charge (CCCV charge) (0.05C cut), 3V discharge was performed at a constant current value corresponding to 0.2C.

2) Cycle test The cycle test was conducted by charging the battery that had been subjected to the above-mentioned 1) initial charge / discharge up to the charge upper limit voltage of 4.2V by the constant current constant voltage method of 2C, and then the charge of 2C until the discharge end voltage of 3.0V. A charge / discharge cycle for discharging at a constant current was defined as one cycle, and this cycle was repeated 100 cycles. The cycle test is performed at 25 ° C. The ratio of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle in this cycle test is shown in Table 2 as the cycle retention rate.

3) Low-temperature rate measurement 2) The battery subjected to the cycle test is charged with a current corresponding to 0.5 C to a charge end voltage of 4.2 V, and charged until the charging current value reaches a current value corresponding to 0.05 C. After performing 4.2V-constant current constant voltage charge (CCCV charge) (0.05C cut), 3V discharge was performed at a constant current value corresponding to 0.2C. Based on the discharge capacity at this time, Table 2 shows the ratio of the discharge capacity when a similarly charged battery is discharged at 3 V at a constant current value corresponding to 2 C in a low temperature environment of 0 ° C. as discharge efficiency. It was.

[Example 2]
High density polyethylene [“HI-ZEX7000FP” manufactured by Mitsui Chemicals, Inc., weight average molecular weight: 200,000, density: 0.956 g / cm ³ , melt flow rate: 0.04 g / 10 min], 100 parts by weight, soft polypropylene [Idemitsu Petrochemical "PER R110E", weight average molecular weight: 330,000] 8.8 parts by weight, hydrogenated castor oil [HY-CASTOR OIL, Toyokuni Seiyaku Co., Ltd., molecular weight 938] 8.8 parts by weight, barium sulfate as an inorganic filler [ Number reference average particle size 0.17 μm] 117.6 parts by weight were blended and melt-kneaded, and the resulting resin composition was subjected to inflation molding at a temperature of 210 ° C. to obtain a raw sheet. The thickness of the original fabric sheet was 110 μm on average. Next, the obtained raw sheet was successively stretched at 90 ° C. in the sheet longitudinal direction (MD) 4 times, and then at 120 ° C. in the width direction (TD) 2.9 times to obtain a film thickness of 25 μm and porosity. A porous polymer membrane having 61%, an average pore size of 0.19 μm, and a Gurley air permeability of 85 seconds / 100 cc was obtained. This polymer porous membrane is called separator B. During the stretching process, the inorganic filler was not removed from the porous polymer membrane.

  A coin-type battery (lithium secondary battery of Example 2) was prepared in the same procedure as in Example 1 except that the separator B was used. The evaluation was performed in the same manner, and the results are shown in Table 2.

Example 3
When preparing the non-aqueous electrolyte, the same procedure as in Example 1 was performed except that vinylethylene carbonate (VEC) was mixed in the non-aqueous electrolyte so that the concentration was 2% by weight instead of vinylene carbonate. A coin-type battery (lithium secondary battery of Example 3) was prepared and evaluated in the same manner, and the results are shown in Table 2.

[Comparative Example 1]
A mixture film of 25 parts by weight of polyethylene having a viscosity average molecular weight of 1,000,000 and 75 parts by weight of paraffin wax (average molecular weight 389) was extruded using a 40 mmφ twin screw extruder at an extrusion temperature of 170 ° C. to prepare a raw film. The obtained raw film was immersed in isopropanol at 60 ° C. to extract and remove paraffin wax. The obtained film was longitudinally stretched 2.0 times at a temperature of 90 ° C. using a roll stretching machine, and then stretched 6.0 times at a temperature of 100 ° C. using a tenter stretching machine, and the film thickness was 22 μm and pores were obtained. A porous membrane having a rate of 50%, an average pore size of 0.04 μm, and a Gurley air permeability of 440 seconds / 100 cc was obtained. This polymer porous membrane is called separator C.

  A coin-type battery (lithium secondary battery of Comparative Example 1) was prepared in the same procedure as in Example 1 except that the separator C was used. The evaluation was performed in the same manner, and the results are shown in Table 2.

  Since the separator C of Comparative Example 1 has the same area stretch ratio (about 12 times) as compared with the separators A and B of Examples 1 and 2, the effect of the hole diameter on the resistance is compared. It can be carried out.

[Comparative Example 2]
A coin-type battery (lithium secondary battery of Comparative Example 2) was prepared in the same procedure as in Example 1 except that vinylene carbonate was not mixed during the preparation of the electrolyte, and the evaluation was performed in the same manner. Are shown in Table 2.

[Comparative Example 3]
A coin-type battery (lithium secondary battery of Comparative Example 3) was prepared in the same procedure as in Example 2 except that vinylene carbonate was not mixed during the preparation of the electrolyte, and the evaluation was performed in the same manner. Are shown in Table 2.

[Comparative Example 4]
As a separator, a coin-type battery (lithium secondary battery of Comparative Example 4) was prepared in the same procedure as in Example 3 except that the separator C manufactured in Comparative Example 1 was used, and the evaluation was similarly performed. The results are shown in Table 2.

  As apparent from Table 2 above, the lithium secondary batteries of Examples 1 to 3 provided with a non-aqueous electrolytic solution containing a film forming agent and a separator containing an inorganic filler in a thermoplastic resin were formed as a film. Compared to Comparative Examples 1 and 4 which show superior cycle characteristics than Comparative Examples 2 and 3 that do not contain an agent, and use a separator having a small pore diameter that does not contain an inorganic filler obtained by an extraction method. It showed high low temperature rate characteristics.

  That is, in Comparative Examples 2 and 3 in which the non-aqueous electrolyte does not contain a film forming agent, good cycle characteristics with the film forming agent cannot be obtained. Even in the case of using a non-aqueous electrolyte containing a film-forming agent, in Comparative Examples 1 and 4 using a separator having a small pore diameter obtained by an extraction method as a separator, there is a problem of a decrease in low-temperature rate characteristics due to the film-forming agent. is there.

  As described above, the separator C by the extraction method used in Comparative Examples 1 and 4 is equivalent to the separators A and B manufactured by the interfacial peeling method in Examples 1 and 2 and has the same area as the original sheet. Although it was obtained by stretching at a draw ratio (about 12 times), the average pore diameters thereof were greatly different, and separators A and B had average pore diameters of 0.27 μm and 0.19 μm, whereas separator C was 0.04 μm. Thus, in Examples 1-3 using separators A and B with large pore diameters, the increase in resistance due to the film forming agent is offset by the effect of reducing the electrical resistance by the separator, and the low-temperature rate characteristics are improved. maintain. On the other hand, in Comparative Examples 1 and 4 using the separator C having a small hole diameter, such an effect cannot be obtained, and the low-temperature rate characteristics are deteriorated.

  In addition, in the film formation by the extraction method, even if the stretching ratio is increased, densification (shrinkage in the thickness direction due to stretching) occurs. Therefore, the pore diameter of the obtained porous film is not so large, and on the contrary, the pore diameter is small. Therefore, it is difficult to realize a separator effective for reducing the internal resistance of the battery.

  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, game machines, small devices such as watches, strobes, cameras, and large devices such as electric cars and hybrid cars Can be mentioned. In particular, the lithium secondary battery of the present invention is excellent in cycle characteristics and has a high low temperature rate characteristic, so that particularly excellent effects can be obtained in applications that are used repeatedly in an environment where the temperature difference is high.

Claims (8)

In a non-aqueous electrolyte secondary battery comprising a negative electrode and a positive electrode capable of inserting and extracting lithium, a separator, and a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt,
The separator has a porous film made of a thermoplastic resin containing an inorganic filler, and
The blending amount of the inorganic filler in the porous membrane is 40 parts by weight or more and 300 parts by weight or less with respect to 100 parts by weight of the thermoplastic resin,
The inorganic filler is added at a ratio of 0.5 g per 1 ml of the electrolyte solution to an electrolyte solution composed of a mixed nonaqueous solvent solution of EC / EMC = 3: 7 (volume ratio) of 1M LiPF _6. The concentration of lithium ions in the electrolyte after being held at 85 ° C. for 72 hours is not reduced to 0.75 mmol / g or less,
The non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte contains a film forming agent.   2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the film forming agent is at least one selected from the group consisting of a carbonate having an ethylenically unsaturated bond and a carboxylic acid anhydride. Water-based electrolyte secondary battery.   The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of the film forming agent in the non-aqueous electrolyte is 0.01 to 10% by weight. Next battery.   4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the separator has a thickness of 5 μm to 100 μm, a porosity of 30% to 80%, and is determined by ASTM F316-86. A non-aqueous electrolyte secondary battery having an average pore diameter of 0.05 μm or more and 10 μm or less and a Gurley permeability determined by JIS P8117 of 20 seconds / 100 cc or more and 700 seconds / 100 cc or less.   5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the lithium salt is a fluorine-containing lithium salt, and the non-aqueous solvent contains a cyclic carbonate and a chain carbonate. A non-aqueous electrolyte secondary battery.   6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the film forming agent is vinylene carbonate.   7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode uses a carbon material as an active material. 8.   8. The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode uses a lithium transition metal composite oxide as an active material. 9.