JP5234662B2 - Battery separator and lithium secondary battery - Google Patents

Description

  The present invention relates to a battery separator excellent in heat resistance and a lithium secondary battery using the same.

  Electrochemical elements such as lithium secondary batteries are widely used as power sources for portable devices such as mobile phones and notebook personal computers because of their high energy density. For example, lithium secondary batteries tend to have higher capacities as mobile devices become more sophisticated, and ensuring safety is important.

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

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

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

  As a technique for preventing such a short circuit due to thermal contraction of the separator and improving the reliability of the battery, for example, a first separator layer mainly including a resin for ensuring a shutdown function, and a filler having a heat resistant temperature of 150 ° C. or more It has been proposed to form an electrochemical element by using a porous separator having a second separator layer containing mainly as a main component (Patent Document 1).

  According to the technique of Patent Document 1, it is possible to provide an electrochemical element such as a lithium secondary battery that is less likely to cause thermal runaway even when abnormally overheated and has excellent safety.

  In addition, for the purpose of further improving the heat shrinkability of the separator, proposals have been made to use plate-like particles as a filler having a heat resistant temperature of 150 ° C. or more (Patent Documents 2 and 3).

International Publication No. 2007/66768 JP 2007-157723 A JP 2008-4439 A

  By the way, in the lithium secondary battery, for example, an additive such as cyclohexylbenzene or a derivative thereof may be added to the non-aqueous electrolyte in order to ensure safety during overcharge. Cyclohexylbenzene and its derivatives are polymerized to form a film on the surface of the positive electrode when the potential becomes higher than a predetermined potential. For this reason, the reaction between the positive electrode and the non-aqueous electrolyte during overcharging is suppressed by this coating, so that the safety of the lithium secondary battery is improved.

  However, since cyclohexylbenzene and its derivatives cause a polymerization reaction in a small amount even in the normal use potential region of a lithium secondary battery, in a lithium secondary battery using a non-aqueous electrolyte containing such additives, cyclohexyl A phenomenon in which a polymer such as benzene enters into the pores of the separator occurs.

  When a lithium secondary battery is configured using a multilayer separator having a heat-resistant layer containing a filler as described in Patent Documents 1 to 3 and a non-aqueous electrolyte containing an additive such as cyclohexylbenzene In addition, a polymer such as cyclohexylbenzene may enter into the pores of the separator and close the pores, impeding ion mobility and adversely affecting battery characteristics such as charge / discharge cycle characteristics. In the above-mentioned laminated separator, for example, in order to improve the heat shrinkage resistance, the filler filling property in the heat resistant layer is often increased. In such a case, battery characteristics due to a polymer such as cyclohexylbenzene are improved. The potential for adverse effects is particularly great.

  In a lithium secondary battery using the above-described laminated separator, as a method for avoiding the problem of battery characteristic deterioration due to the polymer of cyclohexylbenzene, for example, it is conceivable to lower the filler density in the heat-resistant layer, In such a separator, there is a possibility that the heat shrinkage may be impaired.

  The present invention has been made in view of the above circumstances, and its purpose is to provide a lithium secondary battery excellent in safety during abnormal overheating and overcharge, and charge / discharge cycle characteristics, and the lithium secondary battery. It is in providing the separator which can be comprised.

The battery separator of the present invention that has achieved the above object has a porous membrane A mainly comprising inorganic fine particles having a specific surface area of 4 to 6 m ² / g by BET method on one side of a polyolefin microporous membrane. In addition, a porous membrane B mainly containing inorganic fine particles having a specific surface area of 7 to 10 m ² / g by a BET method is provided on the other surface of the polyolefin microporous membrane.

  The lithium secondary battery of the present invention is a lithium secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, wherein the separator is the battery separator of the present invention, and the porous membrane A is a positive electrode. Oppositely, the porous membrane B faces the negative electrode, and the non-aqueous electrolyte contains cyclohexylbenzene or a derivative thereof.

  According to the present invention, there are provided a lithium secondary battery excellent in safety during abnormal overheating and overcharge, and charge / discharge cycle characteristics, and a separator excellent in heat resistance that can constitute the lithium secondary battery. be able to.

It is sectional drawing which represents typically an example of the battery separator of this invention.

FIG. 1 is a cross-sectional view schematically showing an example of a battery separator (hereinafter abbreviated as “separator”) of the present invention. The separator of the present invention has a porous membrane 2 (porous membrane A) mainly containing inorganic fine particles having a specific surface area of 4 to 6 m ² / g by BET method on one side of a polyolefin microporous membrane 1. In addition, a porous film 3 (porous film B) mainly containing inorganic fine particles having a specific surface area of 7 to 10 m ² / g by BET method is provided on the other surface of the microporous film.

  The polyolefin microporous membrane in the separator has the original function of permeating ions while preventing a short circuit between the positive electrode and the negative electrode in a battery using the separator of the present invention. In addition, the polyolefin microporous membrane can ensure a shutdown function that melts the polyolefin constituting the microporous membrane and closes the pores of the separator when the temperature of the battery becomes high due to malfunction or the like.

The porous membrane A in the separator is relatively porous because it mainly contains inorganic fine particles having a specific surface area of 4 to 6 m ² / g by the BET method. Therefore, in the lithium secondary battery, when the separator is disposed so that the porous film A faces the positive electrode, clogging of the separator pores by the polymer of cyclohexylbenzene or its derivative polymerized by the positive electrode potential can be suppressed. In addition, deterioration of battery characteristics (particularly deterioration of charge / discharge cycle characteristics) due to long-term use or storage can be suppressed.

Further, the porous membrane B in the separator mainly contains inorganic fine particles having a specific surface area of 7 to 10 m ² / g as measured by the BET method, and has higher heat shrinkage resistance than the porous membrane A. Therefore, in particular, when the inside of the battery becomes high temperature due to malfunction or the like, the porous film B can suppress the thermal contraction of the entire separator and suppress a short circuit due to the contact between the positive and negative electrodes.

  The polyolefin microporous membrane used for the separator is a microporous film made of polyolefin [polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, etc.] conventionally used as a separator for lithium ion secondary batteries. A porous membrane (an ion-permeable microporous membrane having a large number of pores formed by a solvent extraction method, a dry method or a wet stretching method) can be used. From the viewpoint of ensuring a better shutdown function, the polyolefin constituting the microporous membrane has a melting point of 80 ° C. or higher and 150 ° C. or lower (more preferably 100 ° C. or higher), such as PE, It is preferable that the melting temperature measured using a differential scanning calorimeter (DSC) is 80 ° C. or higher and 150 ° C. or lower (more preferably 100 ° C. or higher) in accordance with JIS K 7121.

  The thickness of the polyolefin microporous membrane is preferably 10 μm or more from the viewpoint of securing a certain degree of strength and preventing breakage, and from the viewpoint of suppressing an increase in the internal resistance of the battery, it is 30 μm or less. Is preferred.

Among the porous membranes formed on both surfaces of the polyolefin microporous membrane according to the separator, the porous membrane A mainly contains inorganic fine particles having a specific surface area of 4 to 6 m ² / g by the BET method. It is preferable that the shape is substantially plate-like, and it is preferable that a part of the secondary aggregate is included. As will be described later, the inorganic fine particles related to the porous film A can be obtained by adjusting the specific surface area by crushing the secondary aggregate as necessary, In addition, when the secondary aggregates are crushed to such an extent that they partially remain, it is easy to adjust the specific surface area of the inorganic fine particles by the BET method to 4 to 6 m ² / g. In addition, since the inorganic fine particles include secondary aggregates in a part thereof, the porous membrane A is more porous. Therefore, in the lithium secondary battery, the separator is arranged so that the porous membrane A faces the positive electrode. When the is disposed, clogging of the separator due to the polymer derived from the components in the nonaqueous electrolytic solution can be more effectively suppressed.

  The average particle diameter of the inorganic fine particles in the porous film A is preferably 3 μm or less, and preferably 0.5 μm or more, since it is difficult to control the thickness of the porous film A if it is too large. . In addition, the average particle diameter of the inorganic fine particles (including inorganic fine particles possessed by the porous film B described later) as used in this specification is a laser scattering type particle size distribution meter (for example, “LA-920” manufactured by Horiba, Ltd.). D50%, which is the value of the particle diameter at a volume-based integrated fraction of 50%, measured by dispersing inorganic fine particles in a medium that does not dissolve.

  If the thickness of the porous membrane A is too thin, the effect of suppressing clogging by the polymer may be reduced. Therefore, the thickness is preferably 1 μm or more. If the thickness is too thick, the total thickness of the separator is large. Accordingly, the impedance of the battery may be excessively increased, so that the thickness is preferably 5 μm or less.

Among the porous membranes formed on both surfaces of the polyolefin microporous membrane according to the separator, the porous membrane B mainly contains inorganic fine particles having a specific surface area of 7 to 10 m ² / g by the BET method. The shape is preferably approximately plate-like, and is preferably a primary particle body.

  Furthermore, the inorganic fine particles related to the porous membrane B have a difference between the theoretical specific surface area calculated from the size and density of the primary particles having a geometric shape as the primary particle shape and the specific surface area by the BET method within ± 15%. In this case, the filling property of the inorganic fine particles in the porous film B is further improved, and a separator having higher heat shrinkage resistance can be obtained. As with the inorganic fine particles related to the porous membrane A, the inorganic fine particles related to the porous membrane B may be adjusted to have a specific surface area by, for example, crushing the secondary aggregate. However, the difference between the theoretical specific surface area and the specific surface area obtained by the BET method can be used as a measure of the degree of formation of primary particles by this crushing treatment.

The theoretical specific surface area of the inorganic fine particles referred to in this specification can be calculated from the surface area, volume, and density according to the following formula, assuming that the primary particles of the inorganic fine particles are geometric shapes.
Theoretical specific surface area = surface area / (volume x density)
(Here, if the unit of theoretical specific surface area is m ² / g, the unit of surface area is m ² , the unit of volume is m ³ , and the unit of density is g / m ³ )

The density of the inorganic fine particles is a value measured by a liquid phase substitution method (pycnometer method), specifically, for example, “MAT-7000” manufactured by Seishin Enterprise Co., Ltd., and ethanol as a substitution medium. A value measured at a measurement temperature of 25 ± 5 ° C. or a value measured by a constant volume expansion method, specifically, for example, a dry automatic densimeter “Acpic 1330-01” manufactured by Shimadzu Micromeritic Co. He was used, the measurement temperature was 25 ° C., and the sample charge volume was apparently 10 cm ³ .

  When geometrically calculating the surface area and volume of the inorganic fine particles, information on the particle diameter of the inorganic fine particles is necessary. In the case of spherical inorganic fine particles, the particle diameter can also be determined by an average particle diameter (D50%) obtained from a general particle size distribution measuring apparatus such as a laser scattering method. However, in the case of plate-like particles having a high aspect ratio (described later) (for example, 5 or more), information on both the maximum length and thickness in the plate-like particles is necessary. In this case, since only one of the information can be obtained only by the average particle size obtained from a normal particle size distribution measuring apparatus, for example, the inorganic fine particles are observed with a scanning electron microscope (SEM), and the size of an arbitrary particle is measured. A method of measuring the value with a scale or the like is preferably employed. In particular, if any 100 or more particles are measured, more accurate information can be obtained.

  In addition, when the said inorganic fine particle which concerns on the porous membrane B is plate-shaped particle | grains, it is preferable that the maximum length in the particle | grains calculated | required by the said method is 0.5-3 micrometers, and calculated | required by the said method. The thickness to be obtained is preferably 0.05 to 0.3 μm. Furthermore, the aspect ratio represented by the ratio between the maximum length and the thickness in the plate-like particles is preferably 5 or more, more preferably 10 or more, and preferably 100 or less, The following is more preferable. The aspect ratio of the plate-like particles is obtained by image analysis of an image photographed by SEM.

  If the thickness of the porous membrane B is too thin, the heat shrinkage may be reduced. Therefore, the thickness is preferably 1 μm or more. If it is too thick, the total thickness of the separator becomes too large, and the impedance of the battery becomes excessive. Since there exists a possibility of raising, it is preferable that it is 5 micrometers or less.

Specific examples of the inorganic fine particles contained in the porous film A and the porous film B include inorganic materials such as iron oxide, Al ₂ O ₃ (alumina), SiO ₂ (silica), TiO ₂ , BaTiO ₃ , and ZrO _2. Oxides; inorganic nitrides such as aluminum nitride and silicon nitride; poorly soluble ionic crystals such as calcium fluoride, barium fluoride and barium sulfate; covalent bonds such as silicon and diamond; clays such as montmorillonite; It is done. Here, the inorganic oxide may be boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, or other mineral resource-derived substances or artificial products thereof. In addition, the surface of a conductive material exemplified by metal, SnO ₂ , conductive oxide such as tin-indium oxide (ITO), carbonaceous material such as carbon black, graphite, etc., is a material having electrical insulation ( For example, the particle | grains which gave the electrical insulation property by coat | covering with the said inorganic oxide etc. may be sufficient. From the viewpoint of further improving the oxidation resistance, the above-mentioned inorganic oxide particles (fine particles) are preferable, among which alumina, silica and boehmite are preferable, and boehmite is particularly preferable.

  In addition, as described above, the inorganic fine particles contained in the porous film A and the porous film B are more preferably plate-like in the shape of the primary particles. In this case, the specific surface area and particle diameter are controlled to predetermined values. It becomes easy to do.

Examples of the inorganic fine particles in which the primary particles are plate-like include various commercially available products. For example, “Sun Lovely (trade name)” (silica) manufactured by Asahi Glass S-Tech Co., Ltd. “NST-B1 (trade name)” manufactured by Ishihara Sangyo Co., Ltd. Crushed product (TiO ₂ ), Sakai Chemical Industry's plate-like barium sulfate “H series (trade name)”, “HL series (trade name)”, Hayashi Kasei Co., Ltd. “micron white (trade name)” (talc ), “Bengel (trade name)” (bentonite) manufactured by Hayashi Kasei Co., Ltd., “BMM (trade name)” or “BMT (trade name)” (boehmite) manufactured by Kawai Lime Co., “Cerasur BMT-B manufactured by Kawai Lime Co., Ltd. (Product name) ”(alumina),“ Seraph (product name) ”(alumina) manufactured by Kinsei Matec Co., Ltd.,“ Yodogawa Mica Z-20 (product name) ”(sericite) manufactured by Yodogawa Mining Co., Ltd., etc. are available.

  As described above, when the inorganic fine particles are secondary aggregates, the pulverization treatment can be performed by a dry method or a wet method for the purpose of forming primary particles or adjusting the specific surface area and particle diameter. For example, alumina, silica, boehmite, etc., which form a secondary aggregate having an average particle diameter of 4-5 μm by agglomeration of plate-like particles, solvent (water, etc.) and dispersant (polycarboxylic acid ammonium salt, etc.) At the same time, the crushing process can be performed by loading the crusher into the crusher.

  Examples of the crusher include a medialess pulverizer such as a jet mill, a high-pressure homogenizer, and a hybridizer, and a disperser that uses media such as a ball mill, a bead mill, a sand mill, and a vibration mill. In particular, in order to increase the crushing efficiency with low energy, a disperser using media is preferable to a pulverizer that uses the collision force of members. As the medium, an ordinary ceramic material such as zirconia or alumina having a diameter of about 0.1 to 10 mm is preferably used.

The porous layer A mainly contains inorganic fine particles having a specific surface area of 4 to 6 m ² / g according to the BET method. The term “comprising mainly” means that the inorganic fine particles are components of the porous film A. It means that 50 volume% or more is included in the total volume. The amount of the inorganic fine particles in the porous layer A is preferably 70% by volume or more, more preferably 80% by volume or more, and 90% by volume or more in the total volume of the constituent components of the porous film A. More preferably it is. By making the said inorganic fine particle into high content in the porous layer A as mentioned above, the thermal contraction of the whole separator can be suppressed favorably. The porous layer A preferably contains a binder in order to bind the inorganic fine particles or to bind the inorganic fine particles related to the porous layer A and the polyolefin microporous film. From such a viewpoint, the preferable upper limit of the amount of the inorganic fine particles in the porous layer A is, for example, 99% by volume in the total volume of the constituent components of the porous film A. If the amount of the inorganic fine particles in the porous film A is less than 70% by volume, for example, it is necessary to increase the amount of the binder in the porous film A. For example, the function as a separator may be reduced due to being filled with a binder, and when porous using a pore-opening agent or the like, the interval between the inorganic fine particles becomes too large, causing heat shrinkage. There is a possibility that the effect of suppressing the decrease.

The porous layer B mainly contains inorganic fine particles having a specific surface area of 7 to 10 m ² / g according to the BET method. The term “mainly containing” as used herein means that the inorganic fine particles are contained in the porous film B. It means that it contains 50 volume% or more in the whole volume of a component. The amount of the inorganic fine particles in the porous layer B is preferably 70% by volume or more, more preferably 80% by volume or more, and 90% by volume or more in the total volume of the constituent components of the porous film B. More preferably it is. By making the said inorganic fine particle into high content in the porous layer B as mentioned above, the thermal contraction of the whole separator can be suppressed favorably. The porous layer B preferably contains a binder in order to bind the inorganic fine particles or to bind the inorganic fine particles related to the porous layer B and the polyolefin microporous film. From such a viewpoint, the preferable upper limit value of the amount of the inorganic fine particles in the porous layer B is, for example, 99% by volume in the total volume of the constituent components of the porous film B. If the amount of the inorganic fine particles in the porous film B is less than 70% by volume, for example, the amount of the binder in the porous film B needs to be increased. For example, the function as a separator may be reduced due to being filled with a binder, and when porous using a pore-opening agent or the like, the interval between the inorganic fine particles becomes too large, causing heat shrinkage. There is a possibility that the effect of suppressing the decrease.

  The porous film A and the porous film B may contain a binder for the purpose of binding between the inorganic fine particles or between the inorganic fine particles and the polyolefin microporous film. The binder is not limited as long as it is electrochemically stable and stable with respect to the electrolytic solution, and can be satisfactorily bonded. For example, an ethylene-vinyl acetate copolymer (EVA, vinyl acetate-derived structural unit is 20 to 35 mol%), ethylene-acrylic acid copolymer such as ethylene-ethyl acrylate copolymer, fluorine rubber, styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), poly N-vinylacetamide, crosslinked acrylic resin, polyurethane, epoxy resin and the like are used. These binders may be used individually by 1 type, and may use 2 or more types together.

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

  The content of the binder in the porous film A is determined by the volume ratio of the inorganic fine particles contained in the porous film A from the viewpoint of satisfactorily binding between the inorganic fine particles and between the inorganic fine particles and the polyolefin microporous film. When 100, it is preferably 1 or more, more preferably 5 or more, and still more preferably 10 or more. However, if the amount of the binder in the porous membrane A is too large, the pores of the porous membrane A are filled with the binder, which may deteriorate the ion permeability and adversely affect the battery characteristics. Therefore, the binder content in the porous membrane A is preferably 30 or less, more preferably 20 or less, when the volume ratio of the inorganic fine particles contained in the porous membrane A is 100. preferable.

  The content of the binder in the porous film B is such that the inorganic fine particles contained in the porous film B are in a volume ratio from the viewpoint of satisfactorily binding between the inorganic fine particles and between the inorganic fine particles and the polyolefin microporous film. When the volume is 100, it is preferably 1 or more, more preferably 5 or more, and still more preferably 10 or more. However, if the amount of the binder in the porous membrane B is too large, the pores of the porous membrane B are filled with the binder, and the ion permeability is deteriorated, which may adversely affect the battery characteristics. Therefore, the binder content in the porous membrane B is preferably 30 or less, more preferably 20 or less, when the volume ratio of the inorganic fine particles contained in the porous membrane B is 100. preferable.

The separator of the present invention is prepared, for example, by dispersing inorganic fine particles having a specific surface area of 4 to 6 m ² / g by a BET method in a medium such as water, and adding a binder, a thickener, an antifoaming agent, or the like as necessary. The slurry for forming the porous film A and inorganic fine particles having a specific surface area of 7 to 10 m ² / g by BET method are dispersed in a medium such as water, and a binder, a thickener, an antifoaming agent, etc. are added as necessary. In addition, a slurry for forming the porous membrane B prepared is prepared, and the slurry for forming the porous membrane A is applied to one side of the polyolefin microporous membrane and dried to form the porous membrane A. It can be produced by forming a porous film B by applying a slurry for forming the porous film B on the other surface of the microporous film and drying it.

  By adding a thickener to the slurry for forming the porous membrane A and the slurry for forming the porous membrane B, during the storage of these slurries and during the continuous production of the separator, Since sedimentation of inorganic fine particles can be suppressed, the storage property of the slurry can be improved, and the separator can be stably manufactured. Specific examples of the thickener include, for example, synthetic polymers such as polyethylene glycol, polyacrylic acid, polyvinyl alcohol, vinyl methyl ether-maleic anhydride copolymer; xanthan gum, welan gum, gellan gum, guar gum, carrageenan, cellulose derivative ( Carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, etc.), natural polysaccharides such as starches such as dextrin and pregelatinized starch; clay minerals such as montmorillonite and hectorite; inorganics such as fumed silica, fumed alumina, and fumed titania Oxides; and the like. These may be used alone or in combination of two or more. Among these, a natural polysaccharide is more preferable in terms of high solubility in water suitable as a medium for the slurry for forming the porous membrane A and the slurry for forming the porous membrane B, and a high thickening effect in a small amount. Xanthan gum, welan gum, and gellan gum are more preferable, and xanthan gum is particularly preferable. In addition, when adding thixotropy to the slurry for forming the porous film A and the slurry for forming the porous film B, inorganic oxides such as fumed silica, fumed alumina, and fumed titania should be added. Is preferred.

  The content of the thickener in the slurry for forming the porous membrane A and the slurry for forming the porous membrane B is suitable for the viscosity of the slurry to suppress sedimentation of inorganic fine particles and to maintain a stable dispersion state. In addition, it is preferable that the amount be adjusted to a viscosity range in which good coating properties can be obtained when coating using a coating machine.

  The viscosity of the slurry for forming the porous film A and the slurry for forming the porous film B is preferably 5 mPa · s or more, more preferably 10 mPa · s or more, and 20 mPa · s or more. Is more preferably 500 mPa · s or less, more preferably 300 mPa · s or less, and still more preferably 100 mPa · s or less. If the viscosity of the slurry is too low, it may be difficult to suppress sedimentation of inorganic fine particles, and it may be difficult to ensure the stability of the slurry. If the viscosity is too high, the slurry should be uniformly applied to the required thickness. Becomes difficult. The viscosities of the slurry for forming the porous membrane A and the slurry for forming the porous membrane B can be measured with a vibration viscometer or an E-type viscometer.

  However, when a thickening agent that does not volatilize in the drying step after slurry application is used, the thickening agent remains in the porous film A and the porous film B. Absent. Therefore, the content of the thickener in the slurry for forming the porous membrane A and the slurry for forming the porous membrane B is, when the total volume of all solids (all components other than the medium) in the slurry is 100, It is preferably 10 or less, more preferably 5 or less, and still more preferably 1 or less.

  When applying the slurry for forming the porous membrane A or the slurry for forming the porous membrane B to the polyolefin microporous membrane, a blade coater, a roll coater, a die coater, a micro gravure coater, or the like can be used. .

  In addition, the coating property of the slurry for forming the porous film A and the slurry for forming the porous film B to the polyolefin microporous film is improved, or the porous film A or the porous film B and the polyolefin microporous For the purpose of enhancing the adhesion to the membrane, the polyolefin microporous membrane may be subjected to surface modification. Examples of the surface modification method for the polyolefin microporous membrane include corona discharge treatment, plasma discharge treatment, and ultraviolet irradiation treatment. In addition, it is desirable that the medium of the slurry for forming the porous film A and the slurry for forming the porous film B is water due to environmental problems, and in the case of using water as the medium, in order to ensure affinity. The surface modification treatment is particularly effective.

  The separator of the present invention produced as described above preferably has a thickness of 12 to 40 μm.

  The lithium secondary battery of the present invention has a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The separator is the separator of the present invention, and the porous film A is opposed to the positive electrode, and the porous film B is Opposite to the negative electrode, the non-aqueous electrolyte contains cyclohexylbenzene or a derivative thereof.

As the non-aqueous electrolyte, a solution in which a lithium salt is dissolved in an organic solvent is used. Examples of the organic solvent related to the non-aqueous electrolyte include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propionate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, 1,2-dimethoxy. An organic solvent composed of only one kind such as ethane, 1,3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, diethyl ether or a mixed solvent of two or more kinds can be used. Further, the lithium salt, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN (CF 3 One or _{two of} SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group] The above can be used. The concentration of the lithium salt in the non-aqueous electrolyte is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  In addition, cyclohexylbenzene or a derivative thereof (1-fluoro-2-cyclohexylbenzene, 1-fluoro-3-cyclohexylbenzene, 1-fluoro-4-cyclohexylbenzene, 1-chloro-4-cyclohexylbenzene, 1-bromo-4-cyclohexylbenzene, 1-iodo-4-cyclohexylbenzene, etc.) can be included to increase the safety of the lithium secondary battery during overcharging.

  The amount of cyclohexylbenzene or a derivative thereof in the non-aqueous electrolyte used for the production of the lithium secondary battery is 0 with respect to 100 parts by mass of the solvent of the non-aqueous electrolyte from the viewpoint of better securing the effect of its use. It is preferable to be 5 parts by mass or more. However, if the amount of cyclohexylbenzene or its derivative in the non-aqueous electrolyte is too large, the battery characteristics may be deteriorated. Therefore, the amount of cyclohexylbenzene or a derivative thereof in the non-aqueous electrolyte used for manufacturing the lithium secondary battery is preferably 3 parts by mass or less with respect to 100 parts by mass of the solvent of the non-aqueous electrolyte.

  In addition, non-aqueous electrolytes include vinylene carbonates, cyclic sulfur compounds (1,3-propane sultone, 1, 4 for the purpose of further improving characteristics such as battery safety, charge / discharge cycle characteristics, and high-temperature storage characteristics. -Butane sultone, 3-phenyl-1,3-propane sultone, 4-phenyl-1,4-butane sultone, etc.), diphenyl disulfide, biphenyl, vinyl ethyl carbonate, fluorobenzene and the like may be added as appropriate.

As the positive electrode according to the lithium secondary battery of the present invention, a positive electrode used in a conventionally known lithium secondary battery, that is, a positive electrode containing a positive electrode active material capable of occluding and releasing lithium ions is used. Can do. For example, for the positive electrode active material, lithium-containing transition metal oxide represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, etc.); LiMn ₂ O _4, etc. LiMn _x M _(1-x) O _{2 in} which a part of Mn of LiMn ₂ O ₄ is substituted with another element; olivine-type LiMPO ₄ (M: Co, Ni, Mn, Fe); LiMn _{0 .5} Ni _0.5 O ₂ ; Li _{(1 + a)} Mn _x Ni _y Co _(1-xy) O ₂ (−0.1 <a <0.1, 0 <x <0.5, 0 <y <0.5); can be applied to these positive electrode active materials, and known conductive additives (carbon materials such as carbon black) and binders such as polyvinylidene fluoride (PVDF). An appropriately added positive electrode mixture is formed using a current collector as a core material (positive electrode). What was finished in the mixture layer) can be used.

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

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

  As the negative electrode for the lithium secondary battery of the present invention, a negative electrode used in a conventionally known lithium secondary battery, that is, a negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions is used. Can do. For example, the anode active material can occlude and release lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers. One kind or a mixture of two or more kinds of carbon-based materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb and In and alloys thereof, lithium-containing nitrides, oxides and other compounds that can be charged and discharged at a low voltage close to lithium metal, or lithium metals and lithium / aluminum alloys Can also be used as a negative electrode active material.

The negative electrode active material, R value is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum _(I 1360 _/ I ₁₅₈₀₎ is 0.1 to 0.5, 002 It is more preferable to use graphite having a face spacing d ₀₀₂ of 0.338 nm or less. By using a negative electrode containing such a negative electrode active material, a lithium secondary battery that can maintain excellent charging characteristics even at low temperatures can be obtained.

Examples of the graphite whose R value and d ₀₀₂ satisfy the above values include graphite whose surface is coated with a low crystalline carbon material. Such graphite is obtained by using natural graphite or artificial graphite having a d ₀₀₂ of 0.338 nm or less in a spherical shape as a base material, covering the surface with an organic compound, and firing at 800 to 1500 ° C. It can be obtained by crushing and sizing through a sieve. The organic compound covering the base material includes aromatic hydrocarbons; tars or pitches obtained by polycondensation of aromatic hydrocarbons under heat and pressure; tars mainly composed of a mixture of aromatic hydrocarbons. , Pitch or asphalt; In order to coat the base material with the organic compound, a method of impregnating and kneading the base material into the organic compound can be employed. Also, the R value and d ₀₀₂ satisfy the above values by a vapor phase method in which hydrocarbon gas such as propane or acetylene is carbonized by pyrolysis and deposited on the surface of graphite having d ₀₀₂ of 0.338 nm or less. Graphite can be produced.

  In the battery, a negative electrode mixture in which a conductive additive (carbon material such as carbon black) or a binder such as PVDF is appropriately added to these negative electrode active materials, and a molded body (negative electrode) using the current collector as a core material In addition to the negative electrode finished in the mixture layer), the above-mentioned various alloys and lithium metal foils may be used alone or a negative electrode formed on a current collector.

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

  Similarly to the lead portion on the positive electrode side, the negative electrode lead portion is usually exposed to the current collector without forming a negative electrode agent layer (a layer having a negative electrode active material) on a part of the current collector during negative electrode fabrication. It is provided by leaving a part and using it as a lead part. However, the lead portion on the negative electrode side is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

  The electrode can be used in the form of a laminated electrode body in which the positive electrode and the negative electrode are laminated via the separator of the present invention, or a wound electrode body in which this is wound. When forming the laminated electrode body or the wound electrode body, as described above, the separator is disposed so that the porous film A faces the positive electrode and the porous film B faces the negative electrode. Thereby, clogging of the separator pores due to the polymer of cyclohexylbenzene or its derivative in the non-aqueous electrolyte can be suppressed.

  When a wound electrode body is formed using a negative electrode using, as a negative electrode active material, graphite whose surface has an R value of 0.1 or more and 0.5 or less and coated with a low crystalline carbon material, When the separator is compressed depending on the strength of the separator, the distance between the electrodes is reduced even if there is no short circuit, the capacity is reduced during the charge / discharge cycle, and the reliability is impaired There is also. However, in the battery of the present invention, the separator of the present invention, which has increased strength by having the porous film A and the porous film B, is used, and the stronger porous film B faces the negative electrode. Since it arrange | positions in this way, the said reliability fall can be suppressed.

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

  The lithium secondary battery of the present invention can be applied to the same use as a conventionally known lithium secondary battery.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
(1) Preparation of slurry for forming porous membrane A (slurry A) 5 kg of boehmite (density: 3.04 g / cm ³ ) having an average slab shape and an average particle diameter of 4 μm, and 5 kg of ion-exchanged water and an aqueous dispersant (polycarbon) 0.5 kg of acid ammonium salt was added), and the mixture was crushed for 5 hours with a ball mill with an internal volume of 20 L and a rotation number of 40 times / min to prepare a dispersion. The average particle size (D50%) of boehmite in the obtained dispersion was measured using a laser scattering particle size distribution meter (“LA-920” manufactured by Horiba, Ltd.) with a refractive index of 1.65.

  Also, a part of the dispersion was vacuum-dried at 120 ° C., 0.3 g of the dried sample (boehmite) was taken out, and the specific surface area (specific surface area by BET method) using “Bell Soap Mini” manufactured by Bell Japan ) Was measured. Moreover, when the boehmite after drying was observed by SEM, those shapes were substantially plate-shaped.

  Further, 17 g of an aqueous binder (a dispersion containing 45% by mass of modified polybutyl acrylate) was added to 500 g of the dispersion, and the mixture was stirred for 3 hours with a three-one motor to obtain slurry A.

(2) Preparation of slurry for forming porous membrane B (slurry B) 5 kg of boehmite (density: 3.04 g / cm ³ ) having an average slab shape and an average particle size of 4 μm, and 5 kg of ion-exchanged water and an aqueous dispersant (polycarbon) 0.5 kg of acid ammonium salt was added), and the mixture was crushed for 10 hours with a ball mill with an internal volume of 20 L and a rotation number of 40 times / min to prepare a dispersion. When a part of the treated dispersion was vacuum dried at 120 ° C. and observed by SEM, the shape of the boehmite particles was almost plate-like. Among them, the size of 100 particles was measured to calculate the theoretical specific surface area. The maximum length in the particles was defined as the particle diameter (I), and the thickness was defined as the particle diameter (II).

  Moreover, 0.3 g of particles after drying was taken out, and the specific surface area (specific surface area by BET method) was measured using “Bell Soap Mini” manufactured by Bell Japan.

  Furthermore, 17 g of an aqueous binder (a dispersion containing 45% by mass of modified polybutyl acrylate) was added to 500 g of the dispersion, and the mixture was stirred for 3 hours with a three-one motor to obtain slurry B.

(3) Production of separator Corona discharge treatment was applied to both sides of a PE microporous membrane (width 300 mm, thickness 16 μm) used in a lithium ion battery separator, slurry A on one side and slurry on the other side. Each of B was applied using a die coater and dried to prepare a separator having a porous film A (thickness 2 μm) and a porous film B (thickness 2 μm). Note that the volume content of inorganic fine particles (boehmite) in the porous membrane A in the separator (volume content in the total volume of the constituent components of the porous membrane A) is 90% by volume. The volume content of inorganic fine particles (boehmite) (the volume content in the total volume of the constituent components of the porous membrane B) was 90% by volume.

(Example 2)
A separator was prepared in the same manner as in Example 1 except that slurry A was prepared with a ball mill crushing time of 3 hours, and slurry B was prepared with a ball mill crushing time of 9 hours.

(Example 3)
A separator was prepared in the same manner as in Example 1 except that slurry A was prepared with a ball mill crushing time of 6 hours, and slurry B was prepared with a crushing time of 14 hours.

(Comparative Example 1)
A separator was produced in the same manner as in Example 1 except that the slurry A prepared in Example 3 was used instead of the slurry B.

(Comparative Example 2)
A separator was produced in the same manner as in Example 1 except that the thickness of the porous membrane A was 4 μm and the porous membrane B was not formed.

(Comparative Example 3)
A separator was produced in the same manner as in Example 1 except that the porous membrane A was not formed and the thickness of the porous membrane B was 4 μm.

About the separator of Examples 1-3 and Comparative Examples 1-3, the thermal contraction rate was measured with the following method. Each separator was cut out to 10 × 10 cm to prepare test pieces, and these test pieces were placed in a mail envelope and left in a thermostat adjusted to 150 ° C. for 1 hour. Then, measure both the vertical and horizontal dimensions of each test piece, calculate the heat shrinkage rate based on the following formula, and choose the larger value of the vertical heat shrinkage rate and the horizontal heat shrinkage rate of the test piece. Was defined as the thermal contraction rate of the separator.
Thermal contraction rate (%) = 100 × (ab) / b
(In the above formula, a is the length in the vertical direction or the horizontal direction of the test piece after being left in the thermostat, and b is the length in the vertical or horizontal direction of the test piece before being put in the thermostatic bath.)

  In addition, the same heat shrinkage ratio measurement as Comparative Example 4 was performed for the PE microporous membrane used in the production of the separator in each Example and Comparative Example. These results are shown in Table 1 together with the form of the inorganic fine particles contained in the porous layer A and the porous layer B.

  As shown in Table 1, the separators of Examples 1 to 3 have a low shrinkage rate and excellent heat resistance even when exposed to a high temperature of 150 ° C. Therefore, when a lithium secondary battery is configured using these separators, even if the temperature in the battery becomes high, a short circuit between the positive and negative electrodes due to the shrinkage of the separator can be suppressed, and thus high safety is achieved. It becomes a battery. On the other hand, the separator (comparative separator) of Comparative Example 4 in which no porous film is formed has a high shrinkage rate. The separators of Comparative Examples 1 and 2 also had a lower thermal shrinkage than the separator of Comparative Example 4 which is a normal separator, but did not reach the separator of the example. This is because in the separators of Comparative Examples 1 and 2, the porous film B containing the inorganic fine particles that were sufficiently crushed and increased in specific surface area by the BET method was not formed, and the porous fine film was filled with the inorganic fine particles. This is considered to be because the adhesion between the inorganic fine particles and the adhesion between the porous membrane and the polyolefin microporous membrane were not sufficient. Although the porous membrane A is not formed, the separator of Comparative Example 3 having the porous membrane B containing inorganic fine particles that are sufficiently crushed and have a specific surface area increased by the BET method has a low thermal shrinkage rate. This is thought to support the above assumption.

  Next, lithium secondary batteries were produced using the separators of Examples 1 to 3 and Comparative Example 3 that had a low thermal shrinkage rate, and the separator of Comparative Example 4 that was a normal separator.

Example 4
(1) Production of positive electrode LiCoO _{2 as} positive electrode active material: 70 parts by mass, LiNi _0.8 Co _0.2 O ₂ : 15 parts by mass, acetylene black as a conductive additive: 10 parts by mass, and PVDF as a binder : 5 parts by mass was mixed so that N-methyl-2-pyrrolidone (NMP) was used as a solvent to prepare a positive electrode mixture-containing paste. This paste is intermittently applied to both sides of an aluminum foil having a thickness of 15 μm to be a current collector, dried, and then subjected to a calendar treatment to adjust the thickness of the positive electrode mixture layer so that the total thickness becomes 150 μm. The positive electrode was produced by cutting to a width of 43 mm. Further, a tab was welded to the exposed portion of the aluminum foil of the positive electrode to form a lead portion.

(2) the negative electrode fabricated average particle size D50% of 18 [mu] _{m, d 002} is 0.338 nm, and the graphite A having a specific surface area of 3.2 m ² / g, an average particle diameter D50% is 16 [mu] _{m, d 002} is 0.336nm R value was obtained by Raman spectroscopy using graphite B. A target in which a very small amount of graphite A or graphite B was attached to a frosted glass was used as a target. Each sample folder is placed in a laser Raman measuring device (“U-1000” manufactured by Jovan Yvon), irradiated with an argon ion laser (wavelength 514.5 nm, output 250 mW) focused on 1 μm, and Raman scattering is performed by backscattering. was condensed and integration time 1 sec, thereby integrating three times the range of the 1200～1800Cm ^-1 at a feed step ^{1 cm -1} to obtain a Raman spectrum. The R value calculated from the obtained Raman spectrum was 0.18 for graphite A and 0.05 for graphite B.

  A mixture of graphite A and graphite B mixed at a mass ratio of 85:15: 98 parts by mass, carboxymethylcellulose: 1 part by mass, and SBR: 1 part by mass in the presence of water to form a slurry-like negative electrode A mixture-containing paste was prepared. This negative electrode mixture-containing paste is intermittently applied on both sides of a current collector made of copper foil with a thickness of 10 μm, dried, and then subjected to calendering so that the total thickness of the negative electrode mixture layer is 142 μm. It was adjusted. The arithmetic mean roughness (Ra) of the negative electrode mixture layer surface of the negative electrode obtained using a confocal laser microscope was 0.75 μm. Then, this was cut | disconnected so that it might become 45 mm in width, and the negative electrode was obtained. Further, a tab was welded to the exposed portion of the copper foil of the negative electrode to form a lead portion.

(3) Battery assembly The positive electrode, the negative electrode, and the separator of Example 1 were overlapped with the porous film A facing the positive electrode and the porous film B facing the negative electrode, and wound in a spiral shape. A wound electrode body was obtained. The wound electrode body was crushed into a flat shape and loaded into a rectangular outer can. Further, LiPF ₆ was dissolved at a concentration of 1.2 mol / l in a non-aqueous electrolyte (a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 2), and cyclohexylbenzene was added to 1 part by mass of 100 parts by mass of the solvent. (Added at a rate of 0.0 part by mass) was injected and sealed to prepare a lithium secondary battery. This battery is provided with a cleavage vent for lowering the pressure when the internal pressure rises at the top of the can.

(Example 5)
A lithium secondary battery was produced in the same manner as in Example 4 except that the separator was changed to the separator of Example 2.

(Example 6)
A lithium secondary battery was produced in the same manner as in Example 4 except that the separator was changed to the separator of Example 3.

(Example 7)
A negative electrode was produced in the same manner as in Example 4 except that only the graphite B having the same R value of 0.05 as that used in Example 4 was used as the negative electrode active material, and Example 4 except that this negative electrode was used. In the same manner, a lithium secondary battery was produced.

(Comparative Example 5)
A spirally wound electrode body was prepared in the same manner as in Example 4 except that the separator porous film A was opposed to the negative electrode and the porous film B was opposed to the positive electrode. A lithium secondary battery was produced in the same manner as in Example 4.

(Comparative Example 6)
A lithium secondary battery was produced in the same manner as in Example 4 except that the separator was changed to the separator of Comparative Example 3.

(Comparative Example 7)
A lithium secondary battery was produced in the same manner as in Example 4 except that the separator was changed to the separator of Comparative Example 4.

  About the lithium secondary battery of Examples 4-7 and Comparative Examples 5-7, the following characteristic tests were done. These results are shown in Table 2.

<Withstand voltage test>
A voltage of 500 V (AC 60 Hz) was applied to each of the 20 lithium secondary batteries of Examples 4 to 7 and Comparative Examples 5 to 7 before injection of the non-aqueous electrolyte, and a battery in which a current of 5 mA or more flowed was defective. And the number of occurrences was examined.

  In order to know how much charge / discharge cycle reliability can be secured, the withstand voltage test reduces the distance between the electrodes even if they are not short-circuited. It is a test means. If the dielectric breakdown does not occur for a certain withstand voltage, it means that the distance between the electrodes is maintained above the reference. In order to clarify the difference here, a higher value is tested.

  The test voltage of the withstand voltage may be 1V if it is a positive voltage if only a normal short circuit check is performed. However, in order to further improve the reliability, it is preferably 50V or more, more preferably 100V or more, and particularly preferably 300V or more. Moreover, although the test voltage has never exceeded a high level, 2000V or lower is desirable because of excessive quality. Furthermore, since the current as a capacitor flows, the determination current value is preferably 2 G (mA) or more when the discharge capacity of the battery at 0.2 C in terms of Ah value is G (Ah), and is preferably 4 G (mA). The above is more desirable. In addition, since the detection rate decreases if the determination current value is set too high, it is preferably 20 G (mA) or less, and more preferably 10 G (mA) or less. Since the discharge capacity G of the batteries of this example and the comparative example is 1.2 (Ah), 5 mA is 4 G (mA) or more and 10 G (mA) or less. By incorporating this withstand voltage test into the battery manufacturing process, the reliability of the manufactured battery is further increased, which is desirable.

<Charge / discharge cycle test>
Regarding the lithium secondary batteries of Examples 4 to 7 and Comparative Examples 5 to 7, charging at a constant current of 1 C and a constant voltage of 4.2 V (charging time is restricted to 3 hours) and a battery voltage of 2 at a constant current of 1 C The discharge until .75V was taken as one cycle, and this was repeated 500 cycles. The ratio of the discharge capacity at the 500th cycle with respect to the discharge capacity at the first cycle was expressed as a percentage to obtain the capacity retention rate. The test was carried out five times for each example and comparative example, and Table 2 shows the average values.

<Overcharge test>
About the lithium secondary battery of Examples 4-7 and Comparative Examples 5-7, it charged until it became 15V with the constant current of 1C, and the presence or absence of ignition was investigated.

<Measurement of charge current at -5 ° C and 10% charge>
The lithium secondary batteries of Examples 4 to 7 and Comparative Examples 5 to 7 were allowed to stand in a thermostatic bath at −5 ° C. for 5 hours, and then, for each battery, a constant current of 1200 mA (1.0 C) to 4.2 V. After reaching 4.2V, constant voltage charging was performed at 4.2V, and the current value was measured when the charging depth (the ratio of the actually charged capacity to the standard capacity) reached 10%. . In Table 2, the current value of each battery is shown as a relative value when the value of the battery of Example 7 is 100.

  From the results shown in Table 2, in the lithium secondary batteries of Examples 4 to 7 configured such that the porous film A according to the separator was opposed to the positive electrode and the porous film B was opposed to the negative electrode, the lithium secondary batteries of Examples 4 to 7 were subjected to the charge / discharge cycle test. It can be seen that the capacity retention rate is high and it has excellent charge / discharge cycle characteristics. That is, in the batteries of Examples 4 to 7, clogging of the pores of the separator due to the polymer of cyclohexylbenzene formed during the charge / discharge cycle is satisfactorily suppressed by the presence of the porous film A related to the separator. Thus, it is speculated that high charge / discharge performance is maintained well. In addition, the batteries of Examples 4 to 7 are not ignited during the overcharge test, and are excellent in safety during overcharge.

  On the other hand, as in the battery of Example 4, the separator of Example 1 was used, the battery of Comparative Example 5 in which the porous film A was arranged on the negative electrode side and the porous film B was arranged on the positive electrode side, the ratio by the BET method In the battery of Comparative Example 6 using a separator that does not have a porous film A containing inorganic fine particles with a small surface area, and the battery of Comparative Example 7 using a normal PE microporous film as a separator, the charge / discharge cycle test The capacity retention rate at the time is inferior to the batteries of Examples 4-7.

  Moreover, regardless of the examples and comparative examples, in the battery using the separator formed with the porous film, no defect was observed in the withstand voltage test.

In addition, the batteries of Examples 4 to 6 using graphite having an R value and d ₀₀₂ suitable for the negative electrode active material were charged at −5 ° C. and 10% charge than Example 7 using graphite having a small R value. The current value is large and the charging characteristics at low temperature are excellent.

1 Polyolefin microporous membrane 2 Porous membrane A
3 Porous membrane B

Claims (5)

On one side of the polyolefin microporous membrane, there is a porous membrane A mainly comprising inorganic fine particles having a specific surface area of 4 to 6 m ² / g by the BET method, and on the other side of the polyolefin microporous membrane, A battery separator comprising a porous film B mainly containing inorganic fine particles having a specific surface area of 7 to 10 m ² / g by BET method.   The battery separator according to claim 1, wherein the inorganic fine particles contained in the porous membrane A and / or the inorganic fine particles contained in the porous membrane B are plate-shaped.   The battery separator according to claim 1 or 2, wherein the inorganic fine particles contained in the porous membrane A and / or the inorganic fine particles contained in the porous membrane B are boehmite. A lithium secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator,
The separator is a battery separator according to any one of claims 1 to 3, wherein the porous film A faces the positive electrode, the porous film B faces the negative electrode,
The lithium secondary battery, wherein the non-aqueous electrolyte contains cyclohexylbenzene or a derivative thereof. The negative electrode is in R value 0.1 to 0.5, which is the peak intensity ratio of 1360 cm ^-1 to the peak intensity of 1580 cm ^-1 in the argon ion laser Raman spectrum, the surface spacing _{d 002} of (002) lattice planes of the following 0.338nm The lithium secondary battery according to claim 4, comprising graphite as a negative electrode active material.

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