JP2011065850A - Separator for battery and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery excellent in safety at an abnormal over-heating and load characteristics, and to provide a separator capable of composing the lithium secondary battery. <P>SOLUTION: The separator for a battery has a porous membrane A containing mainly massive inorganic particulates A on one face of a polyolefin fine porous membrane, and a porous membrane B containing mainly plate-shape inorganic particulates B on the other face thereof. The lithium secondary battery is provided with the separator. <P>COPYRIGHT: (C)2011,JPO&INPIT

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

  Currently, the range of use of lithium secondary batteries tends to be further widened, and accordingly, higher capacity and higher performance are required. Although the separator using the plate-like particles also plays a role in improving the performance of the battery, for example, in the case of a lithium secondary battery used for power tools, the charge / discharge characteristics under a high current ( That is, the load characteristic is important, and there is still room for improvement in the above technique in this respect.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a lithium secondary battery excellent in safety and load characteristics at the time of abnormal overheating, and a separator that can constitute the lithium secondary battery. There is to do.

  The battery separator of the present invention that has achieved the above object has a porous membrane A mainly composed of massive inorganic fine particles A on one side of a polyolefin microporous membrane, and the polyolefin microporous membrane. It has a porous film B mainly containing plate-like inorganic fine particles B on the other surface.

  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. .

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery excellent in the safety | security and load characteristic at the time of abnormal overheating, and the separator excellent in the heat resistance which can comprise the said lithium secondary battery can be provided.

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 film 2 (porous film A) mainly containing massive inorganic fine particles A on one surface of a polyolefin microporous film 1, and the other surface of the microporous film. In addition, a porous film 3 (porous film B) mainly containing plate-like inorganic fine particles B is provided.

  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 and the porous membrane B in the separator play a role of imparting heat resistance to the separator. That is, when the inside of the battery becomes high due to malfunction or the like, the presence of the porous film A and the porous film B suppresses thermal contraction of the entire separator and suppresses short circuit due to contact between the positive and negative electrodes.

  Among these, the porous membrane B mainly contains the plate-like inorganic fine particles B and has high heat shrinkage resistance. However, in a porous film containing plate-like inorganic fine particles, the inorganic fine particles overlap in the film, and the path length (so-called curvature) of the pores in the porous film increases, so the ions in the porous film Permeation is inhibited. In order to improve the heat resistance of the separator satisfactorily, it is preferable to thicken the porous film containing the plate-like inorganic fine particles to some extent, but in this way, the path length of the pores in the porous film is very long. Thus, the ion permeability in the porous membrane is greatly impaired, and for example, the load characteristics of the battery are reduced.

  Therefore, in the separator of the present invention, a porous membrane B mainly containing plate-like inorganic fine particles B is provided on one side of a polyolefin microporous membrane serving as a substrate, and massive inorganic fine particles A are provided on the other side. Is provided as a main component. The porous membrane A contains massive inorganic fine particles A, so that the heat shrinkage is inferior to that of the porous membrane B. On the other hand, the path length of the pores is shorter than that of the porous membrane B. . Therefore, in the separator of the present invention, even if the porous membrane B is made thin to some extent and the path length of the pores is shortened as much as possible to increase the ion permeability, the heat shrinkage that decreases with that Can be compensated for by the porous membrane A having a shorter pore path length and good ion permeability, so that both heat resistance and ion permeability can be achieved at a high level.

  Therefore, by using such a separator of the present invention, a lithium secondary battery having excellent safety during abnormal overheating and good load characteristics can be configured.

  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 inorganic fine particles A mainly composed of the porous membrane A are in a lump shape, for example, substantially spherical (including true sphere). And a polyhedron shape excluding a plate shape (for example, an aspect ratio which is a ratio of the maximum length to the minimum length in the particles is less than 5, preferably 3 or less).

  The average particle diameter of the inorganic fine particles A is preferably 1 μm or less from the viewpoint of making the pore path length of the porous membrane A shorter and improving the ion permeability in the porous membrane A. However, if the inorganic fine particles A are too small, the surface area thereof becomes large, and the dispersibility in the porous membrane A may be reduced, or the amount of water adhering to the particle surface may increase, which may adversely affect the battery characteristics. Therefore, the average particle diameter of the inorganic fine particles A is preferably 0.2 μm or more. 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 film A is too thin, the effect of suppressing the thermal shrinkage of the separator may be reduced. Therefore, the thickness is preferably 1 μm or more. If the thickness is too thick, the total thickness of the separator increases. It is preferable that the thickness is 5 μm or less.

Of the porous membranes formed on both surfaces of the polyolefin microporous membrane according to the separator, the porous membrane B mainly contains plate-like inorganic fine particles B, and the inorganic fine particles B have a specific surface area by the BET method. It is preferable that it is 7-10 m < ² > / g, and it is preferable that it is a primary particle body. By using such inorganic fine particles B, it is possible to form a porous film B having more excellent heat shrinkage resistance.

  Furthermore, the inorganic fine particles related to the porous membrane B have a difference of ± 15% between the theoretical specific surface area calculated from the size and true density of the primary particles having a geometric shape as the primary particle shape and the specific surface area by the BET method. In this case, the filling property of the inorganic fine particles in the porous membrane B is further improved, and a separator having higher heat shrinkage resistance can be obtained. In addition, the inorganic fine particles A related to the porous film A and the inorganic fine particles B related to the porous film B may be subjected to a crushing treatment of the secondary aggregate as necessary, and the average particle diameter and specific surface area thereof may be set. It can be adjusted and can be a primary particle body preferable as the inorganic fine particle B. The difference between the theoretical specific surface area and the specific surface area by the BET method is the formation of the primary particle body by this crushing treatment. It can be a measure of the degree.

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, as for the said inorganic fine particle B which concerns on the porous membrane B, it is preferable that the maximum length in the particle | grains calculated | required by the said method is 0.5-3 micrometers, and the thickness calculated | required by the said method is as follows. It is preferable that it is 0.05-0.3 micrometer. Furthermore, the aspect ratio represented by the ratio between the maximum length and the thickness in the inorganic fine particles B 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 inorganic fine particles B and the inorganic fine particles A can be obtained by image analysis of images taken 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 the thickness is too thick, the path length in the membrane becomes long, and the effect of improving the load characteristics is increased. Is preferably 5 μm or less.

Specific examples of the inorganic fine particles A contained in the porous film A and the inorganic fine particles B contained in the porous film B include, for example, iron oxide, Al ₂ O ₃ (alumina), SiO ₂ (silica), TiO ₂ , BaTiO 3. ₃ , inorganic oxides such as ZrO ₂ ; inorganic nitrides such as aluminum nitride and silicon nitride; poorly soluble ionic crystals such as calcium fluoride, barium fluoride and barium sulfate; covalently bonded crystals such as silicon and diamond; montmorillonite Such as clay; 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, various commercial products are mentioned as an inorganic fine particle whose primary particle is a plate-like, 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), Hayashi Kasei's “Bengels (trade name)” (bentonite), Kawai Lime's “BMM (trade name)” and “BMT (trade name)” (boehmite), Kawai Lime's “Cerasur BMT” -B (trade name) "(alumina), Kinsei Matec" Seraph (trade name) "(alumina), Yodogawa Mining Co., Ltd." Yodogawa Mica Z-20 (trade name) "(sericite), etc. are available. is there.

  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, a solvent (water, etc.) and a 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 massive inorganic fine particles A, and the term “comprising mainly” here includes 50% by volume or more of the inorganic fine particles A in the total volume of the constituent components of the porous membrane A. Means. The amount of the inorganic fine particles A 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 inorganic fine particles A have a high content in the porous layer A as described above, the thermal contraction of the entire separator can be satisfactorily suppressed. The porous layer A preferably contains a binder in order to bind the inorganic fine particles A or bind the inorganic fine particles A related to the porous layer A and the polyolefin microporous film. From such a viewpoint, the preferable upper limit value of the amount of the inorganic fine particles A 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 A in the porous film A is less than 70% by volume, for example, the amount of the binder in the porous film A needs to be increased. For example, the function as a separator may be reduced due to being filled with a binder, and when pores are formed using a pore opening agent or the like, the interval between the inorganic fine particles A becomes too large and heat shrinkage occurs. There is a possibility that the effect of suppressing the decrease.

  Further, the porous layer B mainly contains the plate-like inorganic fine particles B, and the term “mainly containing” as used herein means that the inorganic fine particles B are 50% by volume in the total volume of the constituent components of the porous film B. It is meant to include the above. The amount of the inorganic fine particles B 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 inorganic fine particles B have a high content as described above in the porous layer B, the thermal contraction of the entire separator can be satisfactorily suppressed. The porous layer B preferably contains a binder in order to bind the inorganic fine particles B or bind the inorganic fine particles B related to the porous layer B and the polyolefin microporous film. From such a viewpoint, the preferable upper limit of the amount of the inorganic fine particles B 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 B 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 it is made porous by using a pore opening agent or the like, the interval between the inorganic fine particles B becomes too large and heat shrinkage occurs. 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 include “Evaflex series (EVA, trade name)” manufactured by Mitsui DuPont Polychemical Co., Ltd., EVA manufactured by Nihon Unicar Co., Ltd., and “Evaflex-EEA Series (ethylene-acrylic) manufactured by Mitsui DuPont Polychemical 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 the volume ratio of the inorganic fine particles A contained in the porous film A in terms of volume ratio from the viewpoint of satisfactorily binding the inorganic fine particles between the inorganic fine particles and the polyolefin microporous film. When 100 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 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 A contained in the porous membrane A is 100. More preferred.

  The content of the binder in the porous membrane B is such that the inorganic fine particles B contained in the porous membrane B are in a volume ratio from the viewpoint of satisfactorily binding the inorganic fine particles between the inorganic fine particles and the polyolefin microporous membrane. 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, and preferably 20 or less when the volume ratio of the inorganic fine particles B contained in the porous membrane B is 100. More preferred.

  The separator of the present invention includes, for example, a slurry for forming a porous film A prepared by dispersing massive inorganic fine particles A in a medium such as water and adding a binder, a thickener, an antifoaming agent, or the like as necessary. And a slurry for forming a porous membrane B prepared by dispersing plate-like inorganic fine particles B in a medium such as water and adding a binder, a thickener, an antifoaming agent, etc. as necessary. Apply the slurry for forming the porous membrane A on one side of the microporous membrane and dry it to form the porous membrane A, and apply the slurry for forming the porous membrane B on the other side of the polyolefin microporous membrane. It can be manufactured by drying to form the porous membrane B.

  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 is only required to have the separator of the present invention, and there are no particular restrictions on other configurations and structures, and various configurations employed in conventionally known lithium secondary batteries. The structure can be applied.

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.

For the non-aqueous electrolyte, for example, 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, and the like) are preferably contained, thereby improving the safety of the lithium secondary battery during overcharging. Cyclohexylbenzene and its derivatives are polymerized to form a film on the surface of the positive electrode when the potential exceeds 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.

  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.

  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, it is preferable to arrange the separator so that the porous film A faces the positive electrode and the porous film B faces the negative electrode.

  For example, when non-aqueous electrolytes containing cyclohexylbenzene or its derivatives are used, they polymerize slightly even in the normal operating potential range of the battery, and the formed polymer enters the separator airport and becomes clogged. Battery characteristics may be deteriorated. However, when the porous film A of the separator faces the positive electrode, clogging of the separator pores due to the polymer of cyclohexylbenzene or its derivative can be suppressed.

  Further, when a wound electrode body is formed using a negative electrode in which graphite whose surface is coated with a low crystalline carbon material having an R value of 0.1 or more and 0.5 or less is used as a negative electrode active 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, since the separator of the present invention whose strength is increased by having the porous film A and the porous film B is used, the above-described decrease in reliability can be suppressed, and the strength is increased. By arranging the separator so that the porous membrane B faces the negative electrode, the effect of suppressing the above-described reliability reduction can be further enhanced.

  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.

  Since the lithium secondary battery of the present invention is excellent in safety and load characteristics at the time of abnormal overheating, the lithium secondary battery, which has been conventionally known, is used for power tools for power tools. It can be preferably applied to the same application as the 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 ion-exchanged water and 0.5 kg of an aqueous dispersant (containing 40% by mass of polycarboxylic acid ammonium salt) are added to 5 kg of roughly spherical boehmite. Dispersion was prepared by crushing for 1 hour with a ball mill having a product volume of 20 L and a rotation rate of 40 times / min. A part of the obtained dispersion was collected, and the average particle diameter (D50%) of boehmite therein was measured using a laser scattering type particle size distribution meter (“LA-920” manufactured by Horiba, Ltd.). When measured as 65, it was 0.6 μm. Further, when a part of the dispersion after the pulverization treatment was vacuum-dried at 120 ° C. and observed by SEM, the shape of the boehmite particles was almost spherical.

  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). In addition, the volume content of the inorganic fine particles A (boehmite) in the porous film A in the separator (the volume content in the total volume of the constituent components of the porous film A. The volume content of the inorganic fine particles A in the porous film A. The same applies hereinafter)) is 90% by volume, and the volume content of inorganic fine particles B (boehmite) in the porous membrane B (volume content in the total volume of the constituent components of the porous membrane B) is 90 volumes. %Met.

(Example 2)
A slurry A was prepared in the same manner as in Example 1 except that spherical alumina having an average particle size of 0.2 μm was used for the inorganic fine particles A. A slurry B was prepared in the same manner as in Example 1 except that the slurry B was prepared with a ball mill crushing time of 9 hours. And the separator was produced like Example 1 except having used these slurry A and slurry B. The volume content of the inorganic fine particles A in the porous film A in the separator was 90% by volume.

(Example 3)
A slurry A was prepared in the same manner as in Example 1 except that spherical boehmite having an average particle diameter of 1.0 μm was used for the inorganic fine particles A. A slurry B was prepared in the same manner as in Example 1 except that the slurry B was prepared with a ball mill crushing time of 14 hours. And the separator was produced like Example 1 except having used these slurry A and slurry B. The volume content of the inorganic fine particles A in the porous film A in the separator was 90% by volume.

Example 4
A slurry A was prepared in the same manner as in Example 1 except that spherical alumina having an average particle size of 2.0 μm was used for the inorganic fine particles A, and a separator was prepared in the same manner as in Example 1 except that this slurry A was used. Was made.

(Example 5)
A separator was prepared in the same manner as in Example 1 except that the slurry B was prepared with a ball mill crushing time of 5 hours. A part of the crushed dispersion used for the preparation of slurry B was vacuum-dried at 120 ° C. and observed by SEM. As a result, the plate-like particles of the boehmite particles were partially agglomerated.

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

About the separator of Examples 1-5 and Comparative Examples 1 and 2, 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.)

  Further, the same heat shrinkage rate measurement as that of Comparative Example 3 was performed for the PE microporous membrane used for manufacturing the separator in each of the Examples and Comparative Examples. 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 5 have a low shrinkage 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 3 in which no porous film is formed has a high shrinkage rate. The separator of Comparative Example 1 also had a thermal contraction rate lower than that of the separator of Comparative Example 5 which is a normal separator, but did not reach that of the separator of the example.

  Next, lithium secondary batteries were produced using the separators of Examples 1 to 3 and Comparative Examples 1 to 3.

(Example 6)
(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 was 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 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.

(Examples 7 and 8 and Comparative Examples 4 to 6)
A lithium secondary battery was produced in the same manner as in Example 6 except that the separator was changed to that shown in Table 2.

Example 9
A negative electrode was produced in the same manner as in Example 6 except that only the graphite B having the same R value of 0.05 as that used in Example 6 was used as the negative electrode active material, and Example 6 was used except that this negative electrode was used. In the same manner, a lithium secondary battery was produced.

  About the lithium secondary battery of Examples 6-9 and Comparative Examples 4-6, 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 6 to 9 and Comparative Examples 4 to 6 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, but it is preferably 50V or more, more preferably 100V or more, and particularly preferably 300V or more in order to further improve the reliability. 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.

<Load characteristic test>
The lithium secondary batteries of Examples 6 to 9 and Comparative Examples 4 to 6 were charged at a constant current of 1 C and a constant voltage of 4.2 V (charging time was regulated for 3 hours), and then at a constant current of 0.2 C It discharged until the battery voltage became 2.75V, and the discharge capacity in 0.2C was obtained. Next, each battery was charged under the same conditions as described above, and then discharged at a constant current of 2C until the battery voltage reached 2.75V to obtain a discharge capacity at 2C. The ratio of the discharge capacity at 2C to the discharge capacity at 0.2C was expressed as a percentage (%), and the load characteristics of each battery were evaluated (in Table 2, "2C / 0.2C capacity ratio" is described. )

<Overcharge test>
About the lithium secondary battery of Examples 6-9 and Comparative Examples 4-6, 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 6 to 9 and Comparative Examples 4 to 6 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 9 is 100.

  As shown in Table 2, the lithium secondary batteries of Examples 6 to 9 have a large 2C / 0.2C capacity ratio and good load characteristics. Further, the separator of Comparative Example 1 in which the porous film containing the massive inorganic fine particles A was formed on both surfaces of the PE microporous film without forming the porous film B containing the plate-like inorganic fine particles B. The battery of Comparative Example 4 using the separator and the battery of Comparative Example 6 using the separator of Comparative Example 3 which is a normal separator have the same 2C / 0.2C capacity ratio as the batteries of Examples 6-9. As described above, since the separator used has a large thermal shrinkage rate, the safety at high temperatures is inferior to that of the batteries of Examples 6-9.

  On the other hand, in the battery of Comparative Example 5 using the separator of Comparative Example 2 in which the porous film containing the plate-like inorganic fine particles B was formed on both surfaces of the PE microporous film, the capacity ratio was 2C / 0.2C. The load characteristics are inferior.

  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.

Furthermore, the batteries of Examples 6 to 9 use a non-aqueous electrolyte solution containing cyclohexylbenzene, and are not ignited during an overcharge test, so that the safety during overcharge is good. In addition, the batteries of Examples 6 to 8 using graphite having an R value and d ₀₀₂ suitable for the negative electrode active material were charged at −5 ° C. and 10% charging than Example 9 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 (7)

  One side of the polyolefin microporous membrane has a porous membrane A mainly containing massive inorganic fine particles A, and the other side of the polyolefin microporous membrane mainly contains plate-like inorganic fine particles B. A battery separator comprising a porous membrane B.   The battery separator according to claim 1, wherein the inorganic fine particles A contained in the porous membrane A have an average particle diameter of 0.2 to 1 μm. 3. The battery separator according to claim 1, wherein the inorganic fine particles B contained in the porous membrane B have a specific surface area of 7 to 10 m ² / g by BET method. A lithium secondary battery having a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator,
The lithium secondary battery, wherein the separator is the battery separator according to any one of claims 1 to 3.   The lithium secondary battery according to claim 4, wherein the porous film A of the separator faces the positive electrode, and the porous film B faces the negative electrode.   The lithium secondary battery according to claim 4 or 5, wherein the nonaqueous electrolytic solution 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 any one of claims 4 to 6, comprising graphite as a negative electrode active material.

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