JP5451426B2 - Battery separator and lithium ion secondary battery using the same - Google Patents

Description

  The present invention relates to a battery separator excellent in heat resistance, and further relates to a lithium ion secondary battery excellent in load characteristics and cycle characteristics by using it as a separator.

  A lithium ion secondary battery, which is one type of electrochemical element, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density. In addition, due to consideration of environmental issues, the importance of rechargeable secondary batteries is increasing, and in addition to portable devices, there is a need for automobiles, electric tools, electric wheelchairs, and household and commercial power storage systems. Application is under consideration.

  As described above, there are a wide variety of characteristics required for batteries, and various measures are required depending on applications. For example, higher energy density for longer usage time, higher energy density when using at high current, and shorter charging time (ie, further improvement of input / output characteristics for adapting to high load equipment) ) Is required. Furthermore, characteristics that can withstand repeated charge / discharge cycles are also required. In order to ensure the safety of a battery that can withstand these severe use conditions, various studies have been made on various members such as electrodes, separators, and electrolytic solutions.

  By the way, in the current lithium secondary battery, as a separator interposed between the positive electrode and the negative electrode, for example, a polyolefin microporous film having a thickness of about 20 to 30 μm is used. 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.

  For such a separator, for example, a uniaxially stretched film or a biaxially stretched film is used to increase the porosity and 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 using a polyolefin-based microporous membrane separator, if the battery temperature reaches the shutdown temperature in the event of abnormal charging, 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).

International Publication No. 2007/66768

  However, in the separator described in Patent Document 1, although heat shrinkage is ensured by the effect of the heat resistant filler, it does not necessarily provide battery characteristics such as load characteristics and cycle characteristics.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a battery separator excellent in heat resistance and a lithium ion secondary battery excellent in load characteristics and cycle characteristics. is there.

  The battery separator of the present invention capable of achieving the above object is mainly composed of a filler having a heat resistant temperature of 150 ° C. or more on at least one surface of the porous layer (I) comprising a microporous film mainly composed of a thermoplastic resin. In the separator having the porous layer (II) included, the filler contained in the porous layer (II) is composed of an oxidation-resistant filler A and a filler B made of a carbonaceous material. Further, the lithium ion secondary battery of the present invention is characterized by using the battery separator.

The conventionally known heat-resistant layer (corresponding to the porous layer (II) referred to in the present invention) was almost an insulator, but as in the present invention, the conventional metal oxide or metal hydroxide was used as the oxidation resistant filler A. The porous layer (II) exhibits conductivity by using an inorganic filler mainly composed of a material and further adding a conductive filler B such as a carbonaceous material to form the porous layer (II). . It has been found that the load characteristics and cycle characteristics of a battery using the separator of the present invention are improved by making this conductive layer face the positive electrode and the negative electrode.
When applying the slurry for forming the porous layer (II) to the porous layer (I) composed of a microporous film mainly composed of a plastic resin, especially when the slurry uses water as a solvent, In many cases, hydrophilic treatment such as corona discharge is performed to ensure the property. When corona discharge is performed, static electricity is easily charged, and many can be removed by static elimination. However, the degree often depends on the coating environment. For example, when the humidity is high (the amount of water vapor in the air is large), it tends to be relatively easy to remove electricity, but it is difficult to remove electricity in a low humidity atmosphere. Sometimes static electricity is charged.

  When a wound body electrode is produced by winding together with the electrode in a charged state, defective withstand voltages may occur frequently, resulting in an increase in defective products and a decrease in production yield. However, as in the present invention, by imparting conductivity to the porous layer (II), the conductivity of the separator surface is ensured, and the charged static electricity is easily discharged spontaneously. I found out that it can be restricted.

  Except for the porous substrate described later, “heat-resistant temperature is 150 ° C. or higher” in this specification means that deformation such as softening is not observed at least at 150 ° C.

  In the present specification, the term “mainly comprising a thermoplastic resin” in the porous layer (I) means a solid content ratio in the porous layer (I), and the thermoplastic resin is 50% by volume or more. Means. Furthermore, in the porous layer (II) in the present specification, “mainly containing a filler having a heat resistant temperature of 150 ° C. or higher” means the solid content ratio in the layer (however, in the case of having a porous substrate described later) The solid content ratio excluding the porous substrate) means that the filler having a heat resistant temperature of 150 ° C. or higher is 50% by volume or higher.

  The porous layer (II) referred to in the present specification is not limited as long as the filler A and the filler B are contained. For example, the porous layer (II) is in a single layer state in which the filler A and the filler B are mixed. ), Or the filler A and the filler B may be independent, that is, the porous layer (II) may be formed by laminating two layers of the filler A layer and the filler B layer. Alternatively, the porous layer (II) may be formed by laminating the filler A or (and) the filler B layer in one layer in which the filler A and the filler B are mixed.

  ADVANTAGE OF THE INVENTION According to this invention, while providing the battery separator excellent in heat resistance, the lithium ion secondary battery excellent in load characteristics and cycling characteristics can be provided.

It is a figure which shows typically an example of the lithium ion secondary battery of this invention, (a) is the top view, (b) is the fragmentary longitudinal cross-sectional view. It is a perspective view of the lithium ion secondary battery shown in FIG.

  The battery separator of the present invention comprises a porous layer (II) mainly comprising a filler having a heat resistant temperature of 150 ° C. or higher on at least one surface of a porous layer (I) comprising a microporous film mainly comprising a thermoplastic resin. And the filler contained in the porous layer (II) is composed of an oxidation-resistant filler A and a filler B made of a carbonaceous material.

  The porous layer (I) related to the separator is mainly for ensuring a shutdown function. When the temperature of the battery of the present invention exceeds the melting point of the thermoplastic resin [hereinafter referred to as resin (A)], which is the main component of the porous layer (I), the resin related to the porous layer (I) (A) melts and closes the pores of the separator, thereby causing a shutdown that suppresses the progress of the electrochemical reaction.

  Further, the porous layer (II) according to the separator has a function of preventing a short circuit due to direct contact between the positive electrode and the negative electrode even when the internal temperature of the battery rises, and the heat resistant temperature is 150 ° C. or higher. Its function is secured by the filler. That is, when the battery becomes hot, even if the porous layer (I) shrinks, the porous layer (II) that does not easily shrink can cause the positive and negative electrodes directly when the separator is thermally contracted. It is possible to prevent a short circuit due to the contact. In the case where the porous layer (I) and the porous layer (II) are integrated as will be described later, the heat-resistant porous layer (II) acts as a skeleton of the separator, and is porous. The thermal contraction of the layer (I), that is, the thermal contraction of the entire separator is suppressed.

  The resin (A) relating to the porous layer (I) of the separator has an electrical insulation property, is electrochemically stable, and further includes a non-aqueous electrolyte (hereinafter referred to as “electrolysis”) of the battery described in detail later. There is no particular limitation as long as it is a thermoplastic resin that is stable to the solvent used in the production of the separator (details will be described later), but polyethylene (PE), polypropylene (PP), Polyolefins such as ethylene-propylene copolymers; polyesters such as polyethylene terephthalate and copolymerized polyesters; and the like are preferable.

  In addition, it is preferable that a separator has the property (namely, shutdown function) which the hole obstruct | occludes in 80 degreeC or more and 150 degrees C or less (more preferably 100 degreeC or more). Therefore, the porous membrane (I) has a melting point, that is, a melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of JIS K 7121 of 80 ° C. or higher and 150 ° C. (more preferably 100 ° C. More preferably, it is a single layer microporous film having PE as a main component, or a laminated microporous film in which 2 to 5 layers of PE and PP are laminated. And the like.

  When the porous layer (I) is constituted by using a thermoplastic resin having a melting point of 80 ° C. or more and 150 ° C. or less like PE and a thermoplastic resin having a melting point exceeding 150 ° C. like PP, for example, A microporous film formed by mixing PE and a resin having a higher melting point than PE such as PP is used as the porous layer (I), or a resin having a higher melting point than PE such as the PE layer and the PP layer. When the laminated microporous membrane constituted by laminating the layers constituted by the above is used as the porous layer (I), the melting point of the resin (A) constituting the porous layer (I) is 80 ° C. The resin (for example, PE) having a temperature of 150 ° C. or less is preferably 30% by mass or more, and more preferably 50% by mass or more.

  As the microporous membrane as described above, for example, a microporous membrane composed of the above-mentioned exemplified thermoplastic resin used in a conventionally known lithium ion secondary battery or the like, that is, a solvent extraction method, dry or wet stretching An ion-permeable microporous membrane produced by a method or the like can be used.

  Further, the porous layer (I) may contain a filler or the like in order to improve the strength and the like within a range that does not impair the action of imparting the shutdown function to the separator. Examples of the filler that can be used for the porous layer (I) include the same fillers that can be used for the porous layer (II) described later.

  The particle diameter of the filler is an average particle diameter, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 10 μm or less, more preferably 1 μm or less. The average particle diameter of the filler referred to in the present specification is measured by, for example, using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA), dispersing these fine particles in a medium in which the filler is not dissolved. The same applies to the filler according to the porous layer (II) described later. ].

  By providing the porous layer (I) having the above-described configuration, it becomes easy to provide a shutdown function to the separator, and it is possible to easily ensure safety when the internal temperature of the battery rises.

  The content of the resin (A) in the porous layer (I) is preferably as follows, for example, in order to more easily obtain the shutdown effect. The volume of the resin (A) as a main component in all the constituent components of the porous layer (I) is 50% by volume or more, more preferably 70% by volume or more, and may be 100% by volume. Furthermore, the porosity of the porous layer (II) obtained by the method described later is 20 to 60%, and the volume of the resin (A) is 50% or more of the pore volume of the porous layer (II). Preferably there is.

  The filler A related to the porous layer (II) has an heat-resistant temperature of 150 ° C. or higher, is stable with respect to the electrolyte solution of the lithium ion secondary battery, and is electrochemically resistant to redox in the battery operating voltage range. Organic particles or inorganic particles may be used as long as they are stable. However, fine particles are preferable from the viewpoint of dispersion and the like, and inorganic fine particles are more preferably used from the viewpoint of stability (particularly oxidation resistance).

  The filler A is preferably plate-shaped. The porous layer (II) contains a plate-like filler, so that even if the surface of the mixture layer is combined with an electrode having a rough surface, the decrease in battery productivity can be better suppressed, and the porous layer (II) Even when it is integrated with the porous layer (I), the force that the porous membrane (I) contracts due to the collision between the plate-like fillers can be more effectively suppressed. Further, the use of the plate-like filler increases the path between the positive electrode and the negative electrode in the separator, that is, the so-called curvature. Therefore, even when dendrite is generated, it becomes difficult for the dendrite to reach the positive electrode from the negative electrode, and the reliability against a dendrite short can be improved.

Examples of the plate-like filler include various commercially available products, such as “Sun Lovely (trade name)” (SiO ₂ ) manufactured by Asahi Glass S-Tech Co., Ltd. and “NST-B1 (trade name)” manufactured by Ishihara Sangyo Co., Ltd. (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., Ltd. Name) ”[Alumina (Al ₂ O ₃ )],“ Seraph (trade name) ”(alumina) manufactured by Kinsei Matech Co., Ltd.,“ Yodogawa Mica Z-20 (trade name) ”(sericite), etc. Is possible. In addition, SiO ₂ , Al ₂ O ₃ , ZrO ₂ , and CeO ₂ can be produced by the method disclosed in Japanese Patent Laid-Open No. 2003-206475. Among these, boehmite, alumina, and silica (SiO ₂ ) are preferable.

  As the form of the plate-like filler, the aspect ratio (ratio between the maximum length in the plate-like filler and the thickness of the plate-like filler) is preferably 5 or more, more preferably 10 or more, and preferably 100 or less. More preferably, it is 50 or less. The aspect ratio of the plate-like filler can be obtained by, for example, image analysis of an image taken with a scanning electron microscope (SEM).

  In addition, since the plate-like filler has a problem of being easily cracked by impact when the plate thickness is thin, the average thickness is preferably 0.02 μm or more, and more preferably 0.05 μm or more. . However, if the thickness of the plate-like filler is too large, the thickness of the separator is increased, the discharge capacity is reduced, and the porous layer (II) is liable to break during the production of the electrochemical element. The thickness is preferably 0.7 μm or less, and more preferably 0.5 μm or less.

  The average thickness of the plate-like filler can be determined as an average value (number average value) of the thickness of 100 fillers by observing the cross section of the separator with an SEM.

  The filler A may contain a filler having a shape other than the plate shape (for example, a filler such as a sphere or a substantially sphere) together with the plate-like filler. The filler having a shape other than the plate shape preferably has a heat resistant temperature of 150 ° C. or higher as in the case of the plate filler, and examples thereof include inorganic particles or organic particles having such heat resistant temperature.

Specific examples of the constituent material of the inorganic particles include inorganic oxides such as iron oxide, Al ₂ O ₃ (alumina), SiO ₂ (silica), TiO ₂ , BaTiO ₃ , ZrO ₂ ; aluminum nitride, silicon nitride, etc. Inorganic nitrides of the above; poorly soluble ionic crystals such as calcium fluoride, barium fluoride and barium sulfate; covalent bonds such as silicon and diamond; clays such as montmorillonite; 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. Among them, the inorganic oxide particles (fine particles) are preferable, and alumina, silica and boehmite are particularly preferable.

  Organic particles (organic powder) include crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine-formaldehyde condensate, etc. Examples thereof include various crosslinked polymer particles and heat-resistant polymer particles such as polysulfone, polyacrylonitrile, aramid, polyacetal, and thermoplastic polyimide. The organic resin (polymer) constituting these organic particles is a mixture, modified body, derivative, or copolymer (random copolymer, alternating copolymer, block copolymer, graft copolymer) of the materials exemplified above. Polymer) or a crosslinked product (in the case of the above-mentioned heat-resistant polymer).

The filler A may contain fine particles having a secondary particle structure in which primary particles are aggregated together with the plate-like filler. The filler having the secondary particle structure also has the same heat shrinkage suppressing action as that of the plate-like filler and the action of suppressing dendrite shorts. Examples of the filler having the secondary particle structure include “Boehmite C06 (trade name)”, “Boehmite C20 (trade name)” (boehmite) manufactured by Daimei Chemical Co., Ltd., “ED-1 (trade name) manufactured by Yonesho Lime Industry Co., Ltd. ) ”(CaCO ₃ ), J. et al. M.M. Examples include “Zeolex 94HP (trade name)” (clay) manufactured by Huber.

  In addition, when the filler A contains a filler having a shape other than the plate shape together with the plate-like filler, the porous layer is used from the viewpoint of better ensuring the above-described effect due to the use of the plate-like filler. The total amount of filler A contained in (II) is preferably 80% by volume or more, more preferably 90% by volume or more.

  If the average particle diameter of filler A having a heat resistant temperature of 150 ° C. or higher related to the porous layer (II) (average particle diameter of plate-like filler and other-shaped filler. The same shall apply hereinafter) is too small, ion permeability decreases. Therefore, it is preferably 0.3 μm or more, more preferably 0.5 μm or more. Moreover, since an electrical characteristic will deteriorate easily when too large, the average particle diameter of the filler whose heat-resistant temperature is 150 degreeC or more becomes like this. Preferably it is 5 micrometers or less, More preferably, it is 2 micrometers or less.

  The amount of filler A having a heat resistant temperature of 150 ° C. or higher in the porous layer (II) [when fillers having a shape other than the plate shape are used together with the plate-like filler, the total amount thereof. The same applies hereinafter for the amount of filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (II). ] In the total volume of the constituent components of the porous layer (II) [However, in the case of using the porous substrate described later, in the total volume of the constituent components excluding the porous substrate. The same applies to the content of each component of the porous layer (II). ], 50% by volume or more, and more preferably 70% by volume or more. By making the filler A in the porous layer (II) high as described above, the occurrence of a short circuit due to direct contact between the positive electrode and the negative electrode when the battery becomes high temperature is better suppressed. In particular, in the case of a separator having a configuration in which the porous layer (I) and the porous layer (II) are integrated, the thermal contraction of the entire separator can be satisfactorily suppressed.

  Further, the presence form of the plate-like particles in the porous layer (II) of the plate-like filler is preferably such that the flat plate surface is substantially parallel to the surface of the separator. The plate-like filler in the vicinity of the surface preferably has an average angle between the flat plate surface and the separator surface of 30 ° or less [most preferably, the average angle is 0 °, that is, a plate shape in the vicinity of the separator surface. The flat plate surface is parallel to the separator surface]. Here, “near the surface” refers to a range of about 10% from the surface of the separator to the entire thickness. By increasing the orientation of the plate-like filler so that the form of the plate-like filler is in the state as described above, it becomes possible to more strongly exert the heat shrinkage suppressing action of the porous layer (II). In addition, internal short-circuiting that may be caused by lithium dendrite deposited on the electrode surface or protrusions of the active material on the electrode surface can be more effectively prevented. In addition, the presence form of the said plate-like filler in porous layer (II) can be grasped | ascertained by observing the cross section of a separator by SEM.

  Furthermore, the porous layer (II) in the separator of the present invention is characterized by containing a filler B made of a carbonaceous material in addition to the filler A. As described above, by adding a highly conductive carbonaceous material, the porous layer (II) is also provided with conductivity, and the porous layer (II) faces the positive electrode or the negative electrode to form a battery. Thus, various characteristics of the battery such as load characteristics and cycle characteristics can be improved while maintaining the conventional high heat resistance.

  The reason for improving the various characteristics is not clear, but the lithium ion can quickly enter and exit the crystal lattice of the active material of the positive electrode or negative electrode by facing the conductive porous layer (II) to the electrode. Inferred.

The carbonaceous material used as the filler B is thermal black, acetylene black, channel black, gas furnace black, for the purpose of ensuring higher conductivity and for the purpose of being highly dispersed and present in the porous layer (II). Various carbon blacks such as oil furnace black are particularly preferable.

If the particle size of the filler B is too small, the dispersibility is deteriorated. If the particle size is too large, it may hinder the formation of the porous layer (II). Therefore, the primary particle size is preferably 10 to 100 nm. In particular, 30 to 60 nm is preferable. The specific surface area (by the BET method) is usually preferably 500 m ² / g or less, and particularly preferably 100 m ² / g or less in order to suppress water absorption into the porous layer (II).

  As long as the porous layer (II) of the present invention contains the filler A and the filler B, the structure is not limited. For example, the porous layer (II) is formed in a single layer in which the filler A and the filler B are mixed. Alternatively, the filler A and the filler B may be independent, that is, the porous layer (II) may be configured by laminating two layers of the filler A layer and the filler B layer. Alternatively, the porous layer (II) may be formed by laminating the filler A or (and) the filler B layer in one layer in which the filler A and the filler B are mixed.

  When the porous layer (II) is formed in a single layer state in which the filler A and the filler B are mixed, the amount of the filler B in the porous layer (II) is the total volume of the constituent components of the porous layer (II). Among them, it is 5% by volume or more, more preferably 10% by volume or more, and preferably 20% by volume or less. By setting the filler B in the porous layer (II) to 5% by volume or more, the porous layer (II) can be provided with more conductivity than necessary, and by setting it to 20% by volume or less, the porous layer The mass per unit area of (II) can be sufficiently secured, and the thermal shrinkage of the entire separator can be satisfactorily suppressed.

  The porous layer (II) preferably contains an organic binder in order to ensure the shape stability of the separator and to integrate the porous layer (II) and the porous layer (I). Examples of organic binders include ethylene-vinyl acetate copolymers (EVA, those having a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-acrylic acid copolymers such as ethylene-ethyl acrylate copolymers, and fluorine-based binders. Rubber, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), cross-linked acrylic resin, polyurethane, epoxy resin, etc. In particular, a heat-resistant binder having a heat-resistant temperature of 150 ° C. or higher is preferably used. As the organic binder, those exemplified above may be used singly or in combination of two or more.

  Among the organic binders exemplified above, highly flexible binders such as EVA, ethylene-acrylic acid copolymer, fluorine-based rubber, and SBR are preferable. Specific examples of such highly flexible organic binders include “Evaflex Series (EVA)” from Mitsui DuPont Polychemical, EVA from Nihon Unicar, and “Evaflex-EEA Series (Ethylene) from Mitsui DuPont Polychemical. -Acrylic acid copolymer) ", Nippon Unicar EEA, Daikin Industries" DAI-EL Latex Series (Fluororubber) ", JSR" TRD-2001 (SBR) ", Nippon Zeon" BM-400B " (SBR) ".

  When the organic binder is used for the porous layer (II), it can be used in the form of an emulsion dissolved or dispersed in the solvent for the composition for forming the porous layer (II) described later. Good.

  In order to ensure the shape stability and flexibility of the separator, a fibrous material or the like may be mixed with the filler in the porous layer (II). The fibrous material has a heat-resistant temperature of 150 ° C. or higher, has an electrical insulation property, is electrochemically stable, and further uses an electrolyte solution described in detail below and a solvent used in manufacturing a separator. If it is stable, the material is not particularly limited. The “fibrous material” in the present specification means an aspect ratio [length in the longitudinal direction / width in the direction perpendicular to the longitudinal direction (diameter)] of 4 or more. The ratio is preferably 10 or more.

  Specific examples of the constituent material of the fibrous material include, for example, cellulose and modified products thereof [carboxymethylcellulose (CMC), hydroxypropylcellulose (HPC), etc.], polyolefin [polypropylene (PP), propylene copolymer, etc.], Polyester [polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), etc.], polyacrylonitrile (PAN), polyaramid, polyamideimide, polyimide, and other resins; glass, alumina, zirconia, silica, and other inorganic materials An oxide; etc. can be mentioned, and two or more of these constituent materials may be used in combination to form a fibrous material. The fibrous material may contain various known additives (for example, an antioxidant in the case of a resin) as necessary.

  In addition, the separator according to the present invention improves the handleability particularly when the porous layer (II) is used as an independent film without integrating the porous layer (I) and the porous layer (II). Therefore, a porous substrate can be used for the porous layer (II). The porous substrate has a heat resistant temperature of 150 ° C. or more formed by forming a sheet-like material such as a woven fabric or a nonwoven fabric (including paper), and a commercially available nonwoven fabric or the like is used as the substrate. Can do. In the separator of this aspect, the filler having a heat resistant temperature of 150 ° C. or higher is preferably contained in the voids of the porous substrate, but the organic binder is used to bind the porous substrate and the filler. You can also

  The “heat resistance” of the porous substrate means that a substantial dimensional change due to softening or the like does not occur, and the change in the length of the object, that is, the porous substrate with respect to the length at room temperature. The heat resistance is evaluated based on whether or not the upper limit temperature (heat resistance temperature) at which the shrinkage ratio (shrinkage ratio) can be maintained at 5% or less is sufficiently higher than the shutdown temperature of the separator. In order to increase the safety of the electrochemical device after shutdown, the porous substrate preferably has a heat resistant temperature that is 20 ° C. or more higher than the shutdown temperature. More specifically, the heat resistant temperature of the porous substrate is 150 ° C. It is preferable that the temperature is higher than or equal to ° C, and more preferably higher than or equal to 180 ° C.

  In the separator according to the present invention, the thickness of the porous layer (II) [when the separator has a plurality of porous layers (II), the total thickness] From the viewpoint of exhibiting effectively, it is preferably 3 μm or more. However, if the porous layer (II) is too thick, the energy density of the battery may be lowered. Therefore, the thickness of the porous layer (II) is preferably 10 μm or less.

  Further, the thickness of the porous layer (I) [when the separator has a plurality of porous layers (I), the total thickness thereof. same as below. ] Is preferably 6 μm or more, and more preferably 10 μm or more, from the viewpoint of more effectively exerting the above-described action (particularly shutdown action) by using the porous layer (I). However, if the porous layer (I) is too thick, there is a possibility that the energy density of the battery may be lowered. In addition, the force that the porous layer (I) tends to shrink by heat increases. In the configuration in which the layer (I) and the porous layer (II) are integrated, the action of suppressing the thermal contraction of the entire separator may be reduced. Therefore, the thickness of the porous layer (I) is preferably 25 μm or less, and more preferably 20 μm or less.

  When the thickness of the porous layer (I) constituting the separator is A (μm) and the thickness of the porous layer (II) is B (μm), the ratio A / B between A and B is 10 or less. Preferably, it is 5 or less, more preferably 1 or more, and more preferably 2 or more. In the separator according to the present invention, even when the thickness ratio of the porous layer (I) is increased and the porous layer (II) is thinned, the occurrence of a short circuit due to the thermal contraction of the separator is ensured while ensuring a good shutdown function. Can be suppressed. In the separator, when there are a plurality of porous layers (I), the thickness A is the total thickness, and when there are a plurality of porous layers (II), the thickness B is the total thickness. .

  The porosity of the separator as a whole is preferably 30% or more in a dried state in order to secure the amount of electrolyte solution retained and to improve ion permeability. On the other hand, from the viewpoint of securing separator strength and preventing internal short circuit, the separator porosity is preferably 70% or less in a dry state. The porosity of the separator: P (%) can be calculated by obtaining the sum of each component i from the thickness of the separator, the mass per area, and the density of the constituent components using the following equation (1).

P = 100− (Σa _i / ρ _i ) × (m / t) (1)
Here, in the above formula, a _i : ratio of component i expressed by mass%, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of separator (g / cm ² ), t: The thickness (cm) of the separator.

In the formula (1), m is the mass per unit area (g / cm ² ) of the porous layer (I), and t is the thickness (cm) of the porous layer (I). The porosity: P (%) of the porous layer (I) can also be obtained using the equation (1). The porosity of the porous layer (I) determined by this method is preferably 30 to 70%.

Furthermore, in the formula (1), m is the mass per unit area (g / cm ² ) of the porous layer (II), and t is the thickness (cm) of the porous layer (II), The porosity: P (%) of the porous layer (II) can also be obtained using the formula (1). The porosity of the porous layer (II) obtained by this method is preferably 20 to 60%.

The separator according to the present invention is measured by a method according to JIS P 8117, and has a Gurley value of 10 to 300 sec indicated by the number of seconds that 100 ml of air passes through the membrane under a pressure of 0.879 g / mm ^2. It is desirable that If the air permeability is too high, the ion permeability is reduced, whereas if it is too low, the strength of the separator may be reduced. By employ | adopting the said structure, it can be set as the separator which has the said air permeability.

  Furthermore, the separator according to the present invention preferably has a penetration strength required by the following method of 3.0 N or more. If it is a separator which has such penetration strength, the fall of productivity by comprising a battery in combination with an electrode with a rough surface as mentioned above can be suppressed more favorably. In addition, it can be set as the separator which has the said penetration strength by employ | adopting the said structure.

  The penetration strength of the separator is determined by the following method. The separator is fixed on a plate having a hole with a diameter of 2 inches so as not to be wrinkled or bent, and a semicircular metal pin having a tip diameter of 1.0 mm is lowered to the separator at a speed of 120 mm / min. Measure the force when the separator is perforated 5 times. And an average value is calculated | required about the measurement of 3 times except the maximum value and the minimum value among the said 5 times of measured values, and this is made into the penetration strength of a separator.

  The average pore size of the separator is preferably 0.01 μm or more, more preferably 0.05 μm or more, preferably 1 μm or less, more preferably 0.5 μm or less. The average pore size of the porous layer (I) is preferably 0.01 to 0.5 μm, and the average pore size of the porous layer (II) is preferably 0.05 to 1 μm.

  The shutdown characteristic of the battery of the present invention having the separator having the above-described configuration can be obtained, for example, by the temperature change of the internal resistance of the battery. Specifically, the measurement can be performed by installing the battery in a thermostatic chamber, increasing the temperature from room temperature at a rate of 1 ° C. per minute, and determining the temperature at which the internal resistance of the battery increases. In this case, the internal resistance of the battery at 150 ° C. is preferably 5 times or more of room temperature, more preferably 10 times or more, and such a characteristic is ensured by using the separator having the above-described configuration. be able to.

  The separator according to the present invention preferably has a heat shrinkage rate at 150 ° C. of 5% or less. If the separator has such characteristics, even when the inside of the battery reaches about 150 ° C., the separator hardly contracts, so that a short circuit due to contact between the positive and negative electrodes can be prevented more reliably, and the battery at high temperature The safety of the can be further increased. By employ | adopting the said structure, it can be set as the separator which has the above thermal contraction rates.

  When the porous layer (I) and the porous layer (II) are integrated, the heat shrinkage here refers to the shrinkage rate of the integrated separator as a whole, and the porous layer (I) and the porous layer (I) When the layer (II) is independent, the value of the smaller shrinkage rate is indicated. As will be described later, the porous layer (I) and / or the porous layer (II) can be integrated with the electrode. In that case, the measurement was performed in an integrated state with the electrode. Refers to heat shrinkage.

  The “150 ° C. heat shrinkage rate” means that the separator or the porous layer (I) and the porous layer (II) (when integrated with the electrode, in an integrated state with the electrode) , The temperature is raised to 150 ° C. and left for 3 hours, then taken out, and the dimensions required by comparing with the dimensions of the separator or porous layer (I) and porous layer (II) before being put in the thermostatic bath The percentage of decrease is expressed as a percentage.

  The separator of the present invention is produced, for example, through a step of using a porous layer (I) as a base material, applying a composition (slurry or the like) for forming the porous layer (II) on the surface, and drying the composition. be able to.

  The composition for forming a porous layer (II) contains, in addition to fillers A and B, a binder and the like as required, and these are dispersed in a medium. The binder can be dissolved in the medium. The medium used for the composition for forming the porous layer (II) may be any medium that can uniformly disperse the filler and the like, and can dissolve or disperse the binder uniformly. For example, an aromatic hydrocarbon such as toluene. Common organic solvents such as francs such as tetrahydrofuran, ketones such as methyl ethyl ketone and methyl isobutyl ketone are preferably used. In addition, for the purpose of controlling the interfacial tension, alcohol (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these media. In addition, when the binder is water-soluble or used as an emulsion, water may be used as a medium. In this case, alcohols (methanol, ethanol, isopropanol, ethylene glycol, etc.) are appropriately added to control the interfacial tension. You can also

  Among the above-mentioned examples, it is preferable to use a medium containing water as a main component in consideration of ease of medium recovery after coating and drying and environmental problems. In addition, “having water as a main component” means that 70% by mass or more of water is contained among the components in the medium. Examples of the other medium containing water as a main component include the alcohols added for controlling the interfacial tension of the composition for forming the porous layer (II). From the viewpoint of environmental protection, it is particularly preferable to use a medium containing 100% by mass of water.

  The water used for the medium is preferably ion-exchanged water obtained by ion-exchange of well water, tap water, etc .; purified water obtained by subjecting these to distillation; the ion-exchanged water or purified water may be gamma rays, ethylene oxide gas or ultraviolet light. Water that has been sterilized by, for example, is more preferable, and the water that has been sterilized by the purified water is particularly preferable. As will be described later, in the composition for forming the porous layer (II), in order to stabilize the dispersion state of the heat-resistant fine particles, it is preferable to add a thickener to increase the viscosity of the composition. However, when the composition for forming the porous layer (II) is stored for a long period of time, the thickener may be decomposed by bacteria in the composition during the storage. Even if it is a composition for forming a porous layer (II) in which heat-resistant fine particles are well dispersed immediately after preparation, if the thickener is decomposed during storage, heat-resistant fine particles may be precipitated. There is. However, by using the sterilized water as the medium for forming the porous layer (II), for example, even when natural polysaccharides that are more easily decomposed are used as thickeners, the composition Since the decomposition of the thickener during the storage period of the product can be suppressed to prevent the precipitation of the heat-resistant fine particles, the composition for forming the porous layer (II) excellent in long-term storage can be obtained.

  In addition, when performing the sterilization process to the water used for a medium, what is necessary is just to judge the degree of sterilization by the number of fungi and a living microbe contained in water. Specifically, sterilization is preferably performed until the number of fungi and viable bacteria determined by the sterility test method described in the general test method of the Japanese Pharmacopoeia is 50 or less in 50 mL of water.

  Furthermore, in order to ensure the storage stability of the slurry, an antiseptic and a bactericidal agent may be added as appropriate to suppress the decomposition of the thickener. Examples of these include benzoic acid, parahydroxybenzoic acid esters, alcohols such as ethanol and methanol, chlorines such as sodium hypochlorite, acids such as hydrogen peroxide, boric acid and acetic acid, sodium hydroxide, water Examples include alkalis such as potassium oxide and nitrogen-containing organic sulfur compounds (for example, “Nopcoside (trade name) series” manufactured by San Nopco).

Moreover, when a slurry is easy to foam and affects applicability | paintability, an antifoamer can be used suitably. As the antifoaming agent, various defoaming agents of mineral oil type, silicone type, acrylic type and polyether type can be used. Specific examples of antifoaming agents include “Formrex (trade name)” manufactured by Nikka Chemical Co., Ltd., “Surfinol (trade name) series” manufactured by Nissin Chemical Co., Ltd., and “Awazero” (trade name) manufactured by Sugawara Engineering Co., Ltd. "Series", "SN deformer (trade name) series" manufactured by San Nopco, etc. can be used.
In the slurry, a dispersant can be appropriately used for the purpose of preventing aggregation of fillers. Specific examples of the dispersant include various anionic, cationic, and nonionic surfactants, and polymer dispersants such as polyacrylic acid and polyacrylate. More specifically, ADEKA "Adekator (product name) series", "Adekanol (product name) series", Sannopco "SN Dispersant (product name) series", Lion Corporation "Politi (product name) "Series", "Armin (trade name) series", "Duomin (trade name) series", "Homogenol (trade name) series", "Leodoll (trade name) series", "Amate (trade name) series" , “Falback (trade name) series”, “Ceramisole (trade name) series”, “Polystar (trade name) series” manufactured by NOF Corporation, “Ajisper (trade name) series” manufactured by Ajinomoto Fine Techno Co., Ltd., Toagosei Co., Ltd. There are "Aron Dispersant (trade name) series" made by the company.
In addition, an additive can be appropriately added to the slurry for the purpose of controlling the interfacial tension. As the additive, when the medium is an organic solvent, alcohol (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate can be used. When the medium is water, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.), modified silicones, hydrophobic silicas (eg “SN wet (trade name) series, SN deformer (manufactured by San Nopco) Interfacial tension can also be controlled using a "trade name" series.

  A thickener can be added to the composition for forming the porous layer (II), for example, in order to stabilize the dispersion state of the heat-resistant fine particles. Specific examples of the thickener include, for example, synthetic polymers such as polyethylene glycol, urethane-modified polyether, polyacrylic acid, polyvinyl alcohol, vinyl methyl ether-maleic anhydride copolymer (for example, “SN thickener (manufactured by San Nopco) Product name) Series ”); Cellulose derivatives such as carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose; Natural polysaccharides such as xanthan gum, welan gum, gellan gum, guar gum, carrageenan; Dextrin; Starches such as pregelatinized starch; Clay minerals such as; inorganic oxides such as fumed silica, fumed alumina, fumed titania; and the like. These may be used alone or in combination of two or more. In the case of the above clay minerals and inorganic oxides, it is preferable to use a primary particle having a particle size smaller than that of the heat-resistant fine particles (for example, about several nm to several tens of nm). Those having a structure structure in which a large number of are connected (such as fumed silica) are preferred.

  Among the thickeners exemplified above, natural polysaccharides are more preferable in that they are highly soluble in water, which is a suitable medium for the composition for forming the porous layer (II), and have a high thickening effect in a small amount. Xanthan gum, welan gum, and gellan gum are more preferable, and xanthan gum is particularly preferable. Moreover, when providing thixotropy to the composition for forming the porous layer (II), it is preferable to add inorganic oxides such as fumed silica, fumed alumina, fumed titania and the like.

  The content of the thickener in the composition for forming the porous layer (II) is, for example, 0.1 to 10% by volume in the total volume of the solid content (components excluding the medium) in the composition. Is preferred.

  The composition for forming the porous layer (II) preferably has a solid content including the fillers A and B and the binder of, for example, 10 to 80% by mass.

  Examples of the coating machine used for applying the composition for forming the porous layer (II) include conventionally known die coaters, gravure coaters, reverse roll coaters, squeeze roll coaters, curtain coaters, blade coaters and knife coaters. Various coating machines that are used are listed.

  In addition, when the filler in the porous layer (II) is a plate-like particle, in order to increase the orientation, when the composition for forming the porous layer (II) is applied to the surface of the porous layer (I) In addition, a share should be given to the composition for forming the porous layer (II). In order to share the composition for forming the porous layer (II), for example, after applying the composition for forming the porous layer (II) to the surface of the porous layer (I), excess slurry is passed through a certain gap. May be removed and then dried.

  Further, in order to further enhance the orientation of the plate-like filler in the porous layer (II), in addition to the above-described method of applying the share, the porous layer (II having a high solid content concentration (for example, 50 to 80 mass%) ) Method of using the forming composition; a plate-like inorganic filler is dispersed in a solvent using various mixing / stirring devices such as a disper, an agitator, a homogenizer, a ball mill, an attritor, a jet mill, a dispersing device, etc. A method of using a composition for forming a porous layer (II) prepared by adding and mixing a binder to the obtained dispersion; a dispersant such as fats and oils, a surfactant, and a silane coupling agent is allowed to act on the surface Using a porous layer (II) forming composition prepared using a plate-like inorganic filler whose surface has been modified; prepared using a plate-like inorganic filler having a different shape, diameter or aspect ratio Perforated Method of using the composition for forming the layer (II); Method of controlling the drying conditions after applying the composition for forming the porous layer (II) to the porous layer (I); A method of pressing; a method of applying a magnetic field before drying after applying the porous layer (II) forming composition to the porous layer (I); and the like. Alternatively, two or more methods may be combined.

  The lithium ion secondary battery of the present invention only needs to have the separator of the present invention, and there are no particular restrictions on the other configuration and structure, and it is adopted in conventionally known lithium ion secondary batteries. Various configurations and structures 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 assistants (carbon materials such as carbon black), polyvinylidene fluoride (PVDF), SBR, fluororubber, etc. Positive electrode mixture with appropriate addition of binders and thickeners It can be used such as those finished molded body (positive electrode mixture layer) a current collector as a core material.

  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, coating 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, non-aqueous electrolytes include acid anhydrides, sulfonic acid esters, dinitriles, vinylene carbonates, cyclic sulfur compounds (1, 2) for the purpose of further improving battery safety, charge / discharge cycle characteristics, and high-temperature storage characteristics. 3-propane sultone, 1,4-butane sultone, 3-phenyl-1,3-propane sultone, 4-phenyl-1,4-butane sultone, etc.), diphenyl disulfide, biphenyl, vinyl ethylene carbonate, fluorobenzene, cyclohexyl benzene , Additives such as t-butylbenzene (including these derivatives) can also 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.

  In forming a laminated electrode body or a wound electrode body, when the porous layer (II) faces the positive electrode, an effect of improving load characteristics and cycle characteristics is observed. Moreover, it is more effective when the porous layer (II) faces both the positive electrode and the negative electrode.

  When non-aqueous electrolytes containing cyclohexylbenzene or its derivatives are used that suppress the battery reaction by polymerizing into a film when the battery is overcharged, these are the normal uses of the battery. Even in the electric potential region, it is slightly polymerized, and the formed polymer may enter the pores of the separator and cause clogging, which may deteriorate the battery characteristics. However, when the porous layer (II) of the separator faces the positive electrode, clogging of the separator holes due to the polymer of cyclohexylbenzene or its derivative 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 battery separator of the present invention is excellent in heat resistance, and the lithium ion secondary battery using the separator of the present invention is not only excellent in safety during heating, but also excellent in important battery characteristics such as load characteristics and cycle characteristics. Therefore, taking advantage of these characteristics, it can be preferably applied to mobile devices such as mobile phones and power tools for power tools, as well as to the same applications as hitherto known lithium ion secondary batteries. it can.

Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.
Example 1
<Production of negative electrode>
The average particle diameter D50% is 18 [mu] _{m, d 002} is at 0.338 nm, a R value is 0.18, and the graphite A specific surface area by BET method is 3.2 m ² / g, an average particle diameter D50% is 16μm , D ₀₀₂ of 0.336 nm, R value of 0.05, and graphite B mixed at a mass ratio of 85:15: 98 parts by mass, viscosity adjusted to a range of 1500 to 5000 mPa · s CMC aqueous solution having a concentration of mass%: 1.0 part by mass and SBR: 1.0 part by mass using ion-exchanged water having a specific conductivity of 2.0 × 10 ⁵ Ω / cm or more as a solvent, A negative electrode mixture-containing paste was prepared.

The negative electrode mixture-containing paste is intermittently applied to both sides of a 10 μm-thick current collector made of copper foil, dried, and then calendered to give a total thickness of 142 μm. Was adjusted to obtain a negative electrode. The arithmetic average roughness Ra of the negative electrode mixture layer surface of this negative electrode was measured using a confocal laser microscope. The negative electrode was cut to a width of 45 mm, and a tab was welded to the exposed portion of the copper foil to form a lead portion.
<Preparation of positive electrode>
LiCoO _{2 as a} positive electrode active material: 70 parts by mass and LiNi _0.8 Co _0.2 O ₂ : 15 parts by mass, acetylene black as a conductive auxiliary agent: 10 parts by mass, and PVDF as a binder: 5 parts by mass, NMP was mixed uniformly as a solvent to prepare a positive electrode mixture-containing paste. This paste is intermittently applied to both sides of a 15 μm-thick aluminum foil serving as a current collector, dried, and then calendered to adjust the thickness of the positive electrode mixture layer so that the total thickness becomes 150 μm. Then, the positive electrode was fabricated 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.
<Preparation of separator>
Add 5 kg of ion-exchanged water and 0.5 kg of a dispersant (aqueous polycarboxylic acid ammonium salt, solid content concentration 40%) to 5 kg of secondary aggregate boehmite, and use a ball mill for 20 hours with an internal volume of 20 L and a rotation rate of 40 times / minute for 8 hours. Crushing was performed, 0.5 kg of carbon black (specific surface area by BET method was 50 m ^{2 /} g) was added, and dispersion treatment was further performed for 2 hours. The treated dispersion was vacuum-dried at 120 ° C. and observed by SEM. As a result, the shape of boehmite was almost plate-like. Moreover, when the average particle diameter (D50%) of boehmite was measured as refractive index 1.65 using the laser scattering particle size distribution analyzer ("LA-920" by HORIBA), it was 1.0 micrometer.

  To 500 g of the above dispersion, 0.5 g of xanthan gum as a thickener and 17 g of a resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) as a binder are added and stirred with a three-one motor for 3 hours to form a uniform slurry. [Slurry a for forming porous layer (II), solid content ratio 50 mass%] was prepared.

PE microporous separator for lithium ion secondary battery [Porous layer (I): thickness 16 μm, porosity 40%, average pore size 0.08 μm, PE melting point 135 ° C.] on one side corona discharge treatment (discharge amount) 40 W · min / m ² ) was applied, and the slurry a for forming the porous layer (II) was applied to the treated surface with a microgravure coater and dried to form a porous layer (II) to obtain a separator.

The thickness of the porous layer (II) in the obtained separator was 4 μm, and the mass per unit area was 6.1 g / m ² . Further, the penetration strength of the separator measured by the above-described method (the penetration strength of the separator was also measured by the same method in each of Examples and Comparative Examples described later) was 3.9 N, and the volume content of the filler was 88. The porosity of the porous layer (II) was 55%.
<Battery assembly>
The positive electrode, the negative electrode, and the separator obtained as described above were overlapped with the porous layer (II) facing the positive electrode, and wound in a spiral shape to produce a wound electrode body. The obtained wound electrode body is crushed into a flat shape, put into an aluminum outer can having a thickness of 6 mm, a height of 50 mm, and a width of 34 mm, and an electrolytic solution (ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 1: 2). In the solvent, LiPF ₆ was dissolved at a concentration of 1.2 mol / l and further 3% by weight of vinylene carbonate was added) and sealed, and the structure shown in FIG. 1 had the appearance shown in FIG. A lithium ion secondary battery was produced. This battery is provided with a cleavage vent for lowering the pressure when the internal pressure rises at the top of the can.

  Here, the battery shown in FIGS. 1 and 2 will be described. FIG. 1A is a plan view, and FIG. 1B is a partial cross-sectional view thereof. As shown in FIG. 2 is spirally wound through the separator 3 as described above, and then pressed so as to be flattened and accommodated in a rectangular tube-shaped outer can 4 together with the electrolyte as a flat wound electrode body 6 Has been. However, in FIG. 1, in order to avoid complication, a metal foil, an electrolytic solution, and the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 are not illustrated. Also, the separator layers are not shown separately.

  The outer can 6 is made of an aluminum alloy and constitutes an outer casing of the battery. The outer can 4 also serves as a positive electrode terminal. And the insulator 5 which consists of PE sheets is arrange | positioned at the bottom part of the armored can 4, and it connects to each one end of the positive electrode 1 and the negative electrode 2 from the flat wound electrode body 6 which consists of the positive electrode 1, the negative electrode 2, and the separator 3. The positive electrode lead body 7 and the negative electrode lead body 8 thus drawn are drawn out. A stainless steel terminal 11 is attached to a sealing lid plate 9 made of aluminum alloy for sealing the opening of the outer can 4 through a PP insulating packing 10, and an insulator 12 is attached to the terminal 11. A stainless steel lead plate 13 is attached.

  And this cover plate 9 is inserted in the opening part of the armored can 4, and the opening part of the armored can 4 is sealed by welding the junction part of both, and the inside of a battery is sealed. Further, in the battery of FIG. 1, a non-aqueous electrolyte inlet 14 is provided in the cover plate 9, and a sealing member is inserted into the non-aqueous electrolyte inlet 14, for example, laser welding or the like. (See FIG. 1 and FIG. 2, in practice, the non-aqueous electrolyte inlet 14 is actually sealed with the non-aqueous electrolyte inlet.) Although it is a member, for ease of explanation, it is shown as a non-aqueous electrolyte inlet 14). Further, the lid plate 9 is provided with a cleavage vent 15 as a mechanism for discharging the internal gas to the outside when the temperature of the battery rises.

  In the battery of this Example 1, the outer can 5 and the cover plate 9 function as a positive electrode terminal by directly welding the positive electrode lead body 7 to the lid plate 9, and the negative electrode lead body 8 is welded to the lead plate 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead plate 13, but depending on the material of the outer can 4, the sign may be reversed. There is also.

FIG. 2 is a perspective view schematically showing the external appearance of the battery shown in FIG. 1. FIG. 2 is shown for the purpose of showing that the battery is a square battery. FIG. 1 schematically shows a battery, and only specific members of the battery are shown. Also in FIG. 1, the inner peripheral portion of the electrode group is not cross-sectional.
Example 2
PE microporous separator for a lithium ion secondary battery [Porous layer (I): thickness 12 μm, porosity 45%, average pore diameter 0.06 μm, PE melting point 135 ° C.] was subjected to corona discharge treatment, The slurry a for forming the porous layer (II) used in Example 1 was applied to both surfaces by a micro gravure coater and dried to form a porous layer (II) to obtain a separator.

The thickness of the porous layer (II) in the obtained separator was 4 μm on both sides, and the mass per unit area was 6.1 g / m ² . Further, the penetration strength of the separator measured by the above-described method (the penetration strength of the separator was also measured by the same method in each of Examples and Comparative Examples described later) was 3.9 N, and the volume content of the filler was 88. The porosity of the porous layer (II) was 55%.
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the separator having the porous layer (II) formed on both sides was used.
Example 3
A porous layer (II) forming slurry b was prepared in the same manner as the porous layer (II) forming slurry a except that secondary aggregate alumina was used instead of the secondary aggregate boehmite. In addition, when the SEM observation was performed similarly to Example 1 about the alumina dispersion liquid formed in the middle of preparation of the slurry b for porous layer (II) formation, the shape of the alumina was plate shape. Further, with respect to the alumina dispersion, the average particle diameter D50% of alumina measured in the same manner as in Example 1 was 1 μm. Thereafter, in the same manner as in Example 1, the slurry b for forming the porous layer (II) was applied by a micro gravure coater and dried to form the porous layer (II) to obtain a separator.

The thickness of the porous layer (II) in the obtained separator was 4 μm, and the mass per unit area was 7.1 g / m ² . Further, the penetration strength of the separator measured by the above-described method (the penetration strength of the separator was also measured by the same method in each of Examples and Comparative Examples described later) was 3.9 N, and the volume content of the filler was 88. The porosity of the porous layer (II) was 55%. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.
Example 4
A porous layer (II) forming slurry c was prepared in the same manner as the porous layer (II) forming slurry a, except that secondary aggregate silica was used instead of the secondary aggregate boehmite. The silica dispersion formed during the preparation of the porous layer (II) slurry c was subjected to SEM observation in the same manner as in Example 1. As a result, the shape of the silica was plate-like. Further, with respect to the silica dispersion, the average particle diameter D50% of silica measured in the same manner as in Example 1 was 1 μm. Hereinafter, in the same manner as in Example 1, the slurry c for forming the porous layer (II) was applied by a micro gravure coater and dried to form the porous layer (II) to obtain a separator.

The thickness of the porous layer (II) in the obtained separator was 4 μm, and the mass per unit area was 4.0 g / m ² . Further, the penetration strength of the separator measured by the above-described method (the penetration strength of the separator was also measured by the same method in each of Examples and Comparative Examples described later) was 3.9 N, and the volume content of the filler was 88. The porosity of the porous layer (II) was 55%. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.
Example 5
Add 5 kg of ion-exchanged water and 0.5 kg of a dispersant (aqueous polycarboxylic acid ammonium salt, solid content concentration 40%) to 5 kg of secondary aggregate boehmite, and use a ball mill for 20 hours with an internal volume of 20 L and a rotation speed of 40 times / minute for 10 hours. A dispersion was prepared by crushing treatment. The treated dispersion was vacuum-dried at 120 ° C. and observed by SEM. As a result, the shape of boehmite was almost plate-like. Moreover, when the average particle diameter (D50%) of boehmite was measured as refractive index 1.65 using the laser scattering particle size distribution analyzer ("LA-920" by HORIBA), it was 1.0 micrometer.
A porous layer (II) forming slurry d was prepared in the same manner as the porous layer (II) forming slurry a except that this dispersion was used.
1 kg of ion-exchanged water and 0.1 kg of a dispersant (aqueous polycarboxylic acid ammonium salt, solid content concentration 40%) are added to 1 kg of carbon black similar to that used in Example 1, and the internal volume is 20 L and the number of turns is 40 times. Dispersion was prepared by crushing for 2 hours with a ball mill per minute.
To 500 g of the above dispersion, 0.1 g of xanthan gum as a thickener and 2 g of resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) as a binder are added and stirred for 3 hours with a three-one motor to obtain a uniform slurry. [Porous layer (II) forming slurry e, solid content ratio 50 mass%] was prepared.

PE microporous separator for lithium ion secondary battery used in Example 2 [Porous layer (I): thickness 12 μm, porosity 45%, average pore diameter 0.06 μm, PE melting point 135 ° C.] A corona discharge treatment (discharge amount 40 W · min / m ² ) is applied, a porous layer (II) forming slurry d is applied to the treated surface, and a slurry e is further applied thereon by a die coater, dried and porous. A separator was obtained by forming a quality layer (II).

The thickness of the porous layer (II) in the obtained separator is 6 μm (filler A layer coated with slurry d; 4 μm, filler B layer coated with slurry e; 2 μm), and the mass per unit area is 7. It was 0 g / m ² . Further, the penetration strength of the separator measured by the above-described method (the penetration strength of the separator was also measured by the same method in each of Examples and Comparative Examples described later) was 3.9 N, and the volume content of the filler was 88. The porosity of the porous layer (II) was 55%. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.
Comparative Example 1
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the same PE microporous membrane separator as used in Example 1 was used as the separator without forming the porous layer (II). .
Comparative Example 2
A porous layer (II) forming slurry d was prepared in the same manner as in Example 1 except that the porous layer (II) forming slurry d prepared in Example 5 was used. Thereafter, as in Example 1, the slurry d for forming the porous layer (II) was applied by a micro gravure coater and dried to form the porous layer (II) to obtain a separator.
The thickness of the porous layer (II) in the obtained separator was 4 μm, and the mass per unit area was 6.5 g / m ² . Further, the penetration strength of the separator measured by the above-described method (the penetration strength of the separator was also measured by the same method in each of Examples and Comparative Examples described later) was 3.9 N, and the volume content of the filler was 88. The porosity of the porous layer (II) was 55%. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this separator was used.

About the lithium ion secondary battery of Examples 1-4 and Comparative Examples 1-2, the following load characteristic test, the cycle test, and the heating test were done. These results are shown in Table 1.
<Load characteristic test>
The prepared lithium ion secondary battery was charged at a constant current of 1 C (1.5 A) and a constant voltage of 4.2 V at normal temperature (25 ° C.) (the charging time was regulated for 3 hours), and then 0.2 C The battery was discharged at a constant current until the battery voltage reached 2.75 V, and a discharge capacity at 0.2 C 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 (%) to evaluate the load characteristics of the battery.
<Cycle test>
About the produced lithium ion secondary battery, after performing constant current-constant voltage charge (total charge time: 2.5 hours) by the constant current of 1C and the constant voltage of 4.2V at normal temperature (25 degreeC), A constant current discharge (discharge end voltage: 2.5 V) was performed at 1C. With this as one cycle, 500 cycles of charge and discharge were repeated under the above conditions, and the capacity retention rate (= 500th cycle capacity / 1st cycle capacity × 100%) was calculated.
<Heating test>
About the lithium ion secondary battery (each 10 pieces) of Examples 1-4 and Comparative Examples 1-2, constant current charge was performed until the battery voltage became 4.25 V at a current value of 1 C, and then at 4.25 V The constant current-constant voltage charging was performed. The total charging time until the end of charging was 2.5 hours. Each battery charged under the above conditions was placed in a constant temperature bath, heated to 150 ° C. at a rate of 5 ° C. per minute, and then allowed to stand at 150 ° C. for 3 hours to measure the surface temperature of the battery. Table 1 lists the number of batteries (overheated temperature number) in which the battery surface temperature rose to 160 ° C. or higher.
<Withstand voltage test>
A battery in which a voltage of 500 V (AC 60 Hz) was applied to each of 50,000 lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 and 2 before injection of the non-aqueous electrolyte and a current of 7 mA or more flowed. The number of occurrences was examined.

  The load characteristics (2C / 0.2C capacity ratio) all showed good values, but the batteries of Examples 1 to 5 had a high ratio and exhibited higher load characteristics. Further, the cycle characteristics of the battery of Comparative Example 1 in which the porous layer (II) was not provided were relatively low. By repeating the charge / discharge cycle, it is considered that cyclohexylbenzene in the electrolytic solution was polymerized and the separator was clogged, leading to capacity deterioration. In the heating test, all the batteries other than Comparative Example 1 provided with the porous layer (II) were good. In addition, the batteries of Examples 1 to 5 did not generate defective pieces in the withstand voltage test, and it was confirmed that productivity was improved.

1 Positive electrode 2 Negative electrode 3 Separator

Claims (2)

In a separator having a porous layer (II) mainly comprising a filler having a heat resistant temperature of 150 ° C. or higher on at least one surface of the porous layer (I) composed of a microporous film mainly comprising a thermoplastic resin,
The filler contained in the porous layer (II) is composed of an oxidation-resistant filler A and a filler B made of a carbonaceous material,
The filler A contains at least one plate-like particle selected from the group consisting of alumina, silica and boehmite,
The filler B contains at least one carbon black selected from the group consisting of thermal black, acetylene black, channel black, gas furnace black, and oil furnace black . A lithium ion secondary battery having a positive electrode, a negative electrode, an organic electrolyte, and a separator,
The lithium ion secondary battery, wherein the separator is the battery separator according to claim 1.

Images