JP5657856B2 - Porous membrane, battery separator and lithium secondary battery - Google Patents

Description

  The present invention relates to an inexpensive separator excellent in dimensional stability at high temperature, and a safe lithium secondary battery excellent in reliability using the separator.

  A lithium secondary battery, which is a type of nonaqueous electrolyte battery, is widely used as a power source for portable devices such as mobile phones and notebook personal computers because of its high energy density. As the performance of portable devices increases, the capacity of lithium secondary batteries tends to increase further, and it is important to ensure safety.

  In current lithium secondary batteries, a polyolefin-based 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 film separator is used, if the battery temperature reaches the shutdown temperature in the case 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 battery temperature easily rises to the contraction temperature of the separator, and there is a risk of ignition due to 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, it has a porous substrate with good heat resistance, filler particles, and a resin component for ensuring a shutdown function It has been proposed to configure an electrochemical element with a separator (Patent Document 1).

International Publication No. 2006/62153

  According to the technique disclosed in Patent Document 1, it is possible to provide a battery with excellent safety in which thermal runaway hardly occurs even when abnormal overheating occurs.

  By the way, recently, the downsizing and thinning of the devices to which the lithium secondary battery is applied are progressing, and in response to this, there is a demand for further downsizing and thinning of the lithium secondary battery. . As a thin lithium secondary battery, for example, a sheet-like positive electrode and a sheet-like negative electrode are overlapped via a separator and wound into a spiral shape to form a wound electrode group. After being crushed into pieces, a thin prismatic outer can or a laminate film outer package is used.

  Therefore, in order to more closely fill the outer package with the electrode group in order to increase the capacity while achieving further thinning of the lithium secondary battery, the innermost curved portion of the wound electrode group However, it is easy to cause a crack in the separator in such a curved portion. When such a crack occurs in the separator, for example, a minute short circuit may occur due to lithium dendrite precipitation, and battery characteristics may be impaired.

  The separator disclosed in Patent Document 1 is relatively less likely to cause cracks or the like in a curved portion even as a flat wound electrode group applied to a thin lithium secondary battery conventionally used. The battery characteristics can be kept high to some extent. However, since future lithium secondary batteries are expected to require further improvement in characteristics at the same time as further miniaturization and thinning, they have characteristics superior to the separator disclosed in Patent Document 1, Even if applied to an electrode group with a flat winding structure, it is necessary to develop a separator that can constitute a more reliable lithium secondary battery by suppressing the micro short circuit caused by lithium dendrite at a high level. It is done.

  The present invention has been made in view of the above circumstances, and the object thereof is a separator that can constitute a lithium secondary battery having excellent reliability, a porous film that can constitute the separator, and lithium having the separator. It is to provide a secondary battery.

  The porous membrane of the present invention that can achieve the above object is composed of at least insulating fine particles that are stable with respect to the organic electrolyte and a binder, and the tensile strength is at least 10 MPa as the binder, and It has a binder (A) having a tensile modulus of 100 MPa or more or a binder (B) having a breaking elongation of 300% or more.

  The battery separator of the present invention is characterized by having the porous film of the present invention.

  Furthermore, the lithium secondary battery of the present invention includes a positive electrode, a negative electrode, an organic electrolyte, and a separator, and has the battery separator of the present invention as the separator.

  The tensile strength and tensile modulus of the binder (A) and the elongation at break of the binder (B) as measured in the present invention were measured by the method described in JIS K 7113. Specifically, the orientation Using a “Tensilon UCT-30T type” manufactured by Ku-ku, etc., a No. 2 type test piece (thickness 1 to 3 mm) of binder (A) or binder (B) is subjected to a tensile test at a tensile speed of 5 mm / min. Is the tensile strength and tensile elastic modulus [binder (A)], and the elongation at break [binder (B)].

  ADVANTAGE OF THE INVENTION According to this invention, the lithium secondary battery which has the outstanding reliability, the battery separator which can comprise this lithium secondary battery, and the porous film | membrane which can comprise this battery separator can be provided.

  The porous membrane of the present invention is suitably used for a battery separator, and is composed of insulating fine particles and a binder that are stable with respect to an organic electrolyte (hereinafter sometimes referred to as “electrolyte”). The binder is a binder (A) having a tensile strength of 10 MPa or more and a tensile elastic modulus of 100 MPa or more, or a binder (B) having a breaking elongation of 300% or more.

  The separator of the present invention obtained using the porous film of the present invention is composed of, for example, only the porous film of the present invention, or composed of the porous film and a resin microporous film, A porous membrane and the microporous film may be integrated. In the separator of the present invention, the porous film may be integrated with an electrode (positive electrode or negative electrode). In the separator of the present invention, the constituents in the porous film [insulating fine particles, preferably used fibrous materials (details will be described later), the insulating fine particles and the fibrous material, depending on the binder in the porous film. In the case where the porous film and the electrode are integrated, the insulating fine particles and the electrode are bonded.

  Here, for example, in a battery using a wound electrode group (particularly a flat electrode group) configured using a separator, the insulating fine particles are separated from each other at a curved portion having a small winding diameter. In addition, separation between the fibrous material and the insulating fine particles and separation between the fibrous materials are likely to occur. Due to the occurrence of such a peeled portion, the lithium ion permeability in the separator becomes uneven, a minute lithium dendrite precipitates and penetrates the separator, a weak short circuit occurs, and the reliability of the battery is impaired, The charge / discharge cycle characteristics may deteriorate.

  In the case of a separator composed of a porous film and a microporous film, for example, when the resin microporous film has a complicated pore shape with a large flexibility, it has a function of preventing dendrite. Therefore, there is little risk of internal short circuit due to dendrite. However, in the porous film having insulating fine particles, when the separation of the insulating fine particles as described above occurs, even if the separator also has a microporous film, because dendrite precipitation can occur, There is a possibility that the battery characteristics are deteriorated, the safety of the battery is deteriorated during overheating, and the heat resistance improving action by using the heat-resistant insulating fine particles may not be sufficiently exhibited.

  However, when the binder of the separator (porous membrane) is the binder (A) or binder (B) having the above characteristics, even in a curved portion with a small winding diameter in the electrode group having a winding structure, Separation between the components of the porous film as described above can be suppressed, so that lithium dendrite precipitation and the occurrence of minute short circuits due to dendrite can be suppressed, and a separator composed of a porous film and a microporous film In such cases, the effect of improving the heat resistance by using the heat-resistant insulating fine particles is sufficiently exhibited.

  The binder (A) has characteristics such as a tensile strength of 10 MPa or more and a tensile elastic modulus of 100 MPa or more, and is very rigid. Therefore, the binder (A) is bent at a curved portion with a small winding diameter in an electrode group having a winding structure. Even when stress is applied, deformation of the binder hardly occurs, and it is presumed that this suppresses separation between constituent components of the porous film. In addition, it is preferable that the tensile strength of a binder (A) is 50 Mpa or less. And it is preferable that the tensile elasticity modulus of a binder (A) is 1000 Mpa or less.

  The binder (A) satisfies the above-mentioned tensile strength and tensile elastic modulus, can satisfactorily bond the constituent components of the separator, is electrochemically stable, and is stable with respect to the organic electrolyte. If there is no particular limitation, in addition to high tensile strength and tensile modulus, adhesion to fibrous materials and insulating fine particles will be good, for example, olefin having a carboxyl group in the molecule A binder containing a polymer is preferred, and a binder containing an olefin polymer having an amino group in the molecule is more preferred. In this case, the carboxyl group and the amino group in the olefin polymer according to the binder (A) may be contained in different molecules, or may be contained in the same molecule. Good. Preferably, the state in which the carboxyl group and the amino group are contained in different molecules, that is, the binder (A) is an olefin polymer having a carboxyl group in the molecule and an olefin polymer having an amino group in the molecule. It is in a state of being a mixture with coalescence.

  The molecular weight of the olefin polymer (the olefin polymer having a carboxyl group in the molecule and the olefin polymer having an amino group in the molecule) according to the binder (A) is measured using gel permeation chromatography. The number average molecular weight (polystyrene conversion value) is preferably 1000 or more, more preferably 4000 or more, preferably 10,000 or less, more preferably 7000 or less, and still more preferably 5000 or less.

  The binder (B) has an elongation at break of 300% or more, and is flexible enough to follow the bending even when a bending stress is applied to a curved portion having a small winding diameter in the electrode group having a winding structure. Since it has, peeling between the structural components of a porous membrane is suppressed. The breaking elongation of the binder (B) is preferably 3000% or less.

  The binder (B) satisfies the above elongation at break, and is particularly limited as long as it can adhere the constituent components of the separator well, is electrochemically stable, and is stable with respect to the organic electrolyte. There is no. Specifically, for example, ethylene-vinyl acetate copolymer (EVA, vinyl acetate-derived structural unit is 20 to 35 mol%), (meth) acrylate polymer ["(meth) acrylate" is acrylate And methacrylate. same as below. ], Fluorinated rubber, styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), polyurethane, and the like, and some of these resins are organic. What introduced the crosslinked structure in order to prevent melt | dissolution to electrolyte solution can also be used. These binders (B) may be used individually by 1 type, and may use 2 or more types together.

  Among these, a (meth) acrylate copolymer having a crosslinked structure is more preferable, and a (meth) acrylate copolymer having a crosslinked structure and having a glass transition temperature (Tg) of −20 ° C. or lower is further preferable. . Specific examples of those having Tg are preferably those in which the terminal of the side chain ester group is an alkyl group having 2 to 10 carbon atoms, and more specifically, an n-propyl group, an iso-propyl group, The copolymer mainly composed of n-butyl group, sec-butyl group and n-hexyl group is more preferable. If the number of carbon atoms in the terminal alkyl group of the side chain ester group is too small, the Tg of the binder (B) becomes higher and the flexibility is lowered. Moreover, when there are too many carbon atoms of the terminal alkyl group of a side chain ester group, side chains will crystallize and the softness | flexibility of a binder (B) will fall on the contrary.

  Moreover, it is preferable that the crosslinked structure in the (meth) acrylate copolymer having a crosslinked structure is formed by self-crosslinking of a self-crosslinking type (meth) acrylate copolymer. As will be described in detail later, the porous membrane of the present invention comprises a step of applying a composition in which constituents of the porous membrane are dispersed or dissolved in a solvent (dispersion medium) to a substrate and the like, and removing the solvent by drying. If a porous film is formed by adding a self-crosslinking type (meth) acrylate copolymer to the composition, the copolymer self-crosslinks at the drying stage to form a crosslinked structure. . Therefore, while ensuring the adhesion between the insulating fine particles by the binder (B) and the insulating fine particles and other constituent components (fibrous matter, microporous film, etc.), ensuring good electrolytic solution resistance, The stability of the composition for forming a porous film can be increased.

  That is, when a binder (B) having no cross-linked structure is used, the electrolytic solution resistance may be inferior. When a binder (B) having a cross-linked structure is originally used, dispersibility such as an emulsion is particularly good. When forming a porous layer using a composition for forming a porous layer, the composition may be dried in a state in which the shape of the binder (B) particles is maintained when the composition is dried. There is a possibility that the contact area between the object to be bonded such as an object or a microporous film and the binder (B) is reduced, and the unattached property is lowered. In addition, when using a so-called two-component binder (B) in which a crosslinking agent is added to the main material (resin) to form a crosslinked structure, the electrolytic solution resistance and adhesion to the adherend are used. However, the stability of the composition for forming a porous layer is lowered, and there is a possibility that the usable time is shortened.

  In order to obtain a self-crosslinking type (meth) acrylate copolymer, a (meth) acrylate copolymer is synthesized by a conventionally known method, for example, using a monomer capable of imparting self-crosslinking properties according to a conventional method. That's fine. Examples of the monomer capable of imparting self-crosslinkability include, for example, an unsaturated monomer having a self-crosslinkable functional group; a monomer having a functional group such as a hydroxyl group or a carboxyl group [including (meth) acrylates And a functional monomer having a functional group that reacts with a hydroxyl group or a carboxyl group.

  Examples of unsaturated monomers having a self-crosslinkable functional group include methylol group-containing monomers such as N-methylol methacrylamide, N-methylol acrylamide, N-butyrol methacrylamide, and N-butyrol acrylamide; Epoxy group-containing monomers such as allyl glycidyl ether and glycidyl (meth) acrylate; alkoxyalkyl groups such as methoxymethyl (meth) acrylate, ethoxymethyl (meth) acrylate, methoxyethyl (meth) acrylate, and butoxyethyl (meth) acrylate Containing monomer; acetoacetoxyalkyl (meth) acrylate such as 2-acetoacetoxyethyl (meth) acrylate, 3-acetoacetoxypropyl (meth) acrylate, 4-acetoacetoxybutyl (meth) acrylate Conductor; vinyl monomer having hydrolyzed silyl group such as vinyltrimethoxysilane, vinylmethoxydimethylsilane, vinyltrichlorosilane, allyltrichlorosilane; aziridinyl group-containing single monomer such as 2- (1-aziridinyl) ethyl (meth) acrylate And the like.

  Examples of the monomer having a functional group such as a hydroxyl group or a carboxyl group include (meth) acrylic acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, fumaric acid, β-hydroxyethyl (meth) acrylate, β -Hydroxypropyl (meth) acrylate, (beta) -hydroxyethyl vinyl ether, etc. are mentioned. Examples of the functional monomer having a functional group that reacts with a hydroxyl group or a carboxyl group include N-methylol group-containing monomers such as N-methylolacrylamide and N-butoxymethylacrylamide; dimethylaminoethyl (meth) acrylate And amino group-containing monomers such as dimethylaminopropylacrylamide; epoxy group-containing monomers such as glycidyl (meth) acrylate;

  In addition to the specific examples of the binder (A), a functional group that improves adhesion such as a carboxyl group is introduced into a known resin, an acrylic ester or a methacrylic ester is mixed, a glass transition temperature. What improved the tensile strength and the tensile elasticity modulus by raising (Tg) etc. can be used for a binder (A). In addition to the specific examples of the binder (B), a known resin may be mixed with an amine compound or a polyacrylic acid resin to increase flexibility, reduce Tg, or use a known plasticizer (phthalate ester). Etc.), and those having improved elongation at break can be used for the binder (B). Moreover, the adhesiveness of a binder (A) or a binder (B) can also be improved by introduce | transducing a carboxyl group. In order to increase the Tg of the resin, various known methods such as introducing a cross-linked structure to increase its cross-link density or introducing a rigid molecular structure (such as an aryl group) can be employed. In order to lower the value, various known methods such as introducing a crosslinked structure having a low crosslinking density or introducing a long side chain can be employed.

  In the porous film of the present invention, the binder (A) and the binder (B) may be used together as appropriate, or may be partially unevenly distributed.

  Insulating fine particles act as a main component in the porous film of the present invention or fill gaps formed between fibrous materials to be described later, thereby suppressing the occurrence of short circuits caused by lithium dendrite. have. The insulating fine particles have electrical insulating properties, are electrochemically stable, and are stable to an electrolyte solution described later and a solvent used for a porous film forming composition (a composition containing a solvent). As long as it is stable at high temperature and has high heat resistance, there is no particular limitation.

  The term “insulating fine particles that are stable with respect to an organic electrolyte” as used herein refers to deformation and chemical composition change in an organic electrolyte (an organic electrolyte used as an electrolyte for a lithium secondary battery). Insulating fine particles that do not occur. In addition, the “high temperature state” in the present specification is specifically a temperature of 150 ° C. or higher, and is a stable particle that does not undergo deformation and chemical composition change in an electrolyte solution at such a temperature. Good. Further, “electrochemically stable” as used in the present specification means that no chemical change occurs during charging / discharging of the battery.

Specific examples of such insulating fine particles include, for example, oxide fine particles such as iron oxide, SiO ₂ , Al ₂ O ₃ , TiO ₂ , BaTiO ₂ , and ZrO; nitride fine particles such as aluminum nitride and silicon nitride; Slightly soluble ionic crystal particles such as calcium fluoride, barium fluoride and barium sulfate; Covalent crystal particles such as silicon and diamond; Clay particles such as talc and montmorillonite; Boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine , Sericite, bentonite and other mineral resource-derived substances or their artificial products; Further, the surface of conductive fine particles such as metal fine particles; oxide fine particles such as SnO ₂ and tin-indium oxide (ITO); carbon fine particles such as carbon black and graphite; It may be fine particles that have been made electrically insulating by surface treatment with the above-mentioned materials constituting the electrically insulating fine particles. These insulating fine particles may be used alone or in combination of two or more.

The form of the insulating fine particles may be any form such as a spherical shape, a particle shape, or a plate shape, but a plate shape is preferable. Examples of the plate-like particles include various commercially available products, such as “Sun Lovely” (SiO ₂ ) manufactured by Asahi Glass S-Tech, pulverized product (TiO ₂ ) of “NST-B1” manufactured by Ishihara Sangyo Co., Ltd., Sakai Chemical Industry Co., Ltd. Plate-like barium sulfate “H series”, “HL series”, Hayashi Kasei “micron white” (talc), Hayashi Kasei “bengel” (bentonite), Kawai lime “BMM” and “BMT” (Boehmite), “Cerasure BMT-B” [Alumina (Al ₂ O ₃ )] manufactured by Kawai Lime Co., Ltd., “Seraph” (alumina) manufactured by Kinsei Matec Co., Ltd. ) Etc. are available. In addition, SiO ₂ , Al ₂ O ₃ , ZrO, and CeO ₂ can be produced by the method disclosed in Japanese Patent Laid-Open No. 2003-206475.

  When the insulating fine particles are plate-like, it is preferable to orient the insulating fine particles in the porous film so that the flat plate surface is substantially parallel to the surface of the porous film. By using a separator having a film, the occurrence of a short circuit of the battery can be suppressed more favorably. This is because the insulating fine particles are oriented as described above so that the insulating fine particles are arranged so as to overlap each other on a part of the flat plate surface. It is considered that the through-hole) is formed in a bent shape instead of a straight line (that is, the curvature is increased), and this can prevent the lithium dendrite from penetrating the separator. It is presumed to be well suppressed.

  As a form in the case where the insulating fine particles are plate-like particles, for example, the aspect ratio (ratio of the maximum length in the plate-like particles to the thickness of the plate-like particles) is 5 or more, more preferably 10 or more. , 100 or less, more preferably 50 or less. Further, the average value of the ratio of the major axis direction length to the minor axis direction length of the flat plate surface of the grains is preferably 0.3 or more, more preferably 0.5 or more (ie, the major axis length). And the length in the minor axis direction may be the same). When the plate-like insulating fine particles have an aspect ratio as described above or an average value of the ratio of the long axis direction length to the short axis direction of the flat plate surface, the above-mentioned short circuit prevention effect is more effectively exhibited. The

  The average value of the ratio of the long axis direction length to the short axis direction length of the flat plate surface when the insulating fine particles are plate-like is, for example, image analysis of an image taken with a scanning electron microscope (SEM). Can be obtained. Further, the aspect ratio in the case where the insulating fine particles are plate-like can also be obtained by image analysis of an image taken by SEM.

  The insulating fine particles may be particles containing two or more materials constituting the various insulating fine particles exemplified above.

  The average particle size of the insulating fine particles is preferably 0.01 μm or more, more preferably 0.1 μm or more, preferably 15 μm or less, more preferably 5 μm or less. In addition, the average particle diameter of the insulating fine particles referred to in this specification is obtained by, for example, using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA) to dissolve the insulating fine particles, It can be defined as the number average particle diameter measured by dispersing insulating fine particles in a medium that does not swell.

  Moreover, the porous film of this invention can suppress the thermal contraction at the time of high temperature, and can improve dimensional stability by the effect | action, when the insulating fine particles with high heat resistance as mentioned above are used. In addition, when the porous membrane of the present invention containing such highly heat-resistant insulating fine particles is integrated with an electrode (positive electrode or negative electrode), the dimensional stability of the entire separator at a high temperature is further improved. Can do. Furthermore, when the separator is composed of the porous film of the present invention containing insulating fine particles having high heat resistance and the microporous film, the microporous film is inferior in dimensional stability at high temperatures. However, since it is integrated with a porous membrane with good dimensional stability at high temperatures by the action of insulating fine particles, thermal shrinkage of the microporous film is suppressed, and the overall dimensional stability of the separator at high temperatures is reduced. improves. Therefore, in the separator of the present invention using the porous membrane of the present invention, for example, it is possible to prevent the occurrence of a short circuit due to thermal shrinkage that has occurred in a separator composed only of a conventional PE microporous film. The reliability and safety when the inside is abnormally overheated can be further improved.

  As described above, according to the present invention, prevention of a short circuit due to thermal contraction of the separator at a high temperature can be achieved by, for example, a configuration other than increasing the thickness of the separator. Therefore, in the separator of the present invention, the thickness is made relatively thin. It is possible to reduce the energy density of a battery using the same as much as possible.

  The porous membrane may contain a fibrous material having a heat resistant temperature of 150 ° C. or higher. For example, in the case where the separator is constituted only by the porous membrane and the porous membrane is not integrated with the electrode, the fibrous material is contained from the viewpoint of reinforcing the porous membrane and enhancing its handleability. It is preferable that the fibrous material is the main component of the separator. In addition, when a separator is composed of a porous membrane and a resin microporous film, or when the porous membrane is integrated with an electrode, a fibrous material having a heat-resistant temperature of 150 ° C. or higher is used for reinforcement. Can be contained.

  In particular, the porous membrane contains a material that can be melted at a temperature of 140 ° C. or less to block the pores of the porous membrane (separator) and provide a function of blocking the movement of ions in the separator (so-called shutdown function). In this case (details will be described later), a fibrous material having a heat-resistant temperature of 150 ° C. or higher is also contained in the porous membrane, so that a shutdown occurs due to heat generation in the battery, and then the separator is further 20 ° C. or higher. Even if the temperature rises, the shape can be kept more stable. On the other hand, even when the shutdown function is not provided, the separator configured using the porous film using the fibrous material having a heat resistant temperature of 150 ° C. or higher substantially does not deform even at a temperature of 150 ° C. Can be eliminated.

  As used herein, “heat-resistant temperature of 150 ° C. or higher” means that there is substantially no deformation at 150 ° C., more specifically, when a fibrous material or insulating fine particles heated to 150 ° C. are visually observed. This means that heat shrinkage cannot be confirmed.

  The fibrous material has a heat-resistant temperature of 150 ° C. or higher, is electrically insulating, is electrochemically stable, and further contains an electrolyte solution, insulating fine particles, and the like described in detail below. There is no particular limitation as long as the solvent used in the composition for forming a film is stable. The “fibrous material” in the present specification means that having an aspect ratio [length in the longitudinal direction / width (diameter) in a direction perpendicular to the longitudinal direction] of 4 or more. The aspect ratio of the fibrous material is preferably 10 or more.

  Specific constituent materials of the fibrous material include, for example, cellulose, modified cellulose (such as carboxymethyl cellulose), polypropylene (PP), polyester [polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT). And the like], resins such as polyacrylonitrile (PAN), aramid, polyamideimide, and polyimide; inorganic materials (inorganic oxides) such as glass, alumina, and silica; and the like. The fibrous material may contain one kind of these constituent materials, or may contain two or more kinds. In addition to the above-described constituent materials, the fibrous material may contain various known additives (for example, an antioxidant in the case of a resin) as necessary. Absent.

  The fibrous material may be subjected to a surface treatment such as a corona treatment or a surfactant treatment in order to improve the adhesion with the insulating fine particles.

  Although the diameter of a fibrous material should just be below the thickness of a porous membrane, it is preferable that it is 0.01-5 micrometers, for example. If the diameter is too large, the entanglement between the fibrous materials may be insufficient, and the strength of the sheet-like material constituted by these, and consequently the strength of the porous film may be reduced, making it difficult to handle. On the other hand, if the diameter is too small, the pores of the porous membrane become too small, and the ion permeability tends to decrease, which may reduce the load characteristics of the battery.

  The state of the fibrous material in the porous membrane (sheet-like material) is, for example, preferably that the angle of the long axis (axis in the long direction) with respect to the separator surface is 30 ° or less on average, 20 ° The following is more preferable.

  The content of the insulating fine particles in the porous film may be 10% by volume or more in the total volume of the constituent components of the porous film from the viewpoint of more effectively exerting the action by using the insulating fine particles. Preferably, it is 30% by volume or more, more preferably 40% by volume or more.

  In the case where the porous film does not contain the fibrous material and includes the heat-meltable fine particles and the swellable fine particles described later and has a shutdown function, the upper limit of the volume ratio of the insulating fine particles is For example, it is preferably 80% by volume. On the other hand, when a porous film having no shutdown function is used, the volume ratio of the insulating fine particles may be a higher ratio.

  On the other hand, in the case of the porous film containing the fibrous material described above and containing the heat-meltable fine particles and the swellable fine particles described later and also having a shutdown function, the upper limit of the volume ratio of the insulating fine particles is For example, it is preferable that it is 70 volume%. In the case of a porous film having no shutdown function, the volume ratio of the insulating fine particles may be a higher ratio, for example, no problem as long as it is 80% by volume or less.

  In addition, the content of the fibrous material in the porous membrane when the porous membrane contains the fibrous material is the configuration of the porous membrane from the viewpoint of more effectively exerting the action due to the use of the fibrous material. The total volume of the components is preferably 10% by volume or more, and more preferably 30% by volume or more. On the other hand, in the porous membrane containing the fibrous material, if the content of the fibrous material is too large, the content of other components (insulating fine particles, etc.) decreases, and the action of these other components is exerted. Since the content may decrease, the content of the fibrous material is preferably 90% by volume or less, and more preferably 70% by volume or less, in the total volume of the constituent components of the porous membrane.

  Furthermore, the content of the binder (A) or the binder (B) in the porous film is 0.5% by volume or more in the total volume of the constituent components of the porous film from the viewpoint of satisfactorily exerting the action of these binders. Preferably, it is more preferably 2% by volume or more. On the other hand, if the content of the binder (A) or the binder (B) is too large, the content of other components (insulating fine particles, etc.) decreases, and the action of these other components may decrease. The content of the binder (A) or binder (B) in the porous film is preferably 15% by volume or less, more preferably 10% by volume or less, in the total volume of the constituent components of the porous film.

  The separator of the present invention having the porous membrane of the present invention can be given a shutdown function. In order to obtain a separator having a shutdown function, for example, a heat-fusible fine particle that melts at 80 to 140 ° C. or a swelling property that swells in an electrolyte and increases in degree of swelling as the temperature rises. A method of containing fine particles; a method of forming a separator with the porous film and a microporous film made of a thermoplastic resin that melts at 80 to 140 ° C. It should be noted that both the hot-melt fine particles and the swellable fine particles may be added to the porous film, or a composite of these may be added.

  The shutdown function in the separator can be evaluated by, for example, an increase in resistance due to the temperature of the model cell. That is, a model cell including a positive electrode, a negative electrode, a separator, and an electrolytic solution is prepared, and the model cell is held in a high-temperature bath, and the internal resistance value of the model cell is measured while the temperature is increased at a rate of 5 ° C./min. Then, by measuring the temperature at which the measured internal resistance value is 5 times or more that before heating (resistance value measured at room temperature), this temperature can be evaluated as the shutdown temperature of the separator.

  Hot-melting fine particles that melt at 80 to 140 ° C., that is, a porous material having a melting temperature of 80 to 140 ° C. measured using a differential scanning calorimeter (DSC) according to JIS K 7121 In a separator constituted by using a membrane, when the separator is exposed to 80 to 140 ° C. (or higher temperature), the hot-melt fine particles melt and the pores of the separator are closed. Is inhibited, and a rapid discharge reaction at a high temperature is suppressed. Therefore, in this case, the shutdown temperature of the separator evaluated by the increase in internal resistance is not less than the melting point of the heat-fusible fine particles and not more than 140 ° C. The melting point of the heat-meltable fine particles (the melting temperature) is more preferably 130 ° C. or lower.

  Specific examples of the constituent material of the heat-meltable fine particles include polyethylene (PE), copolymerized polyolefin, polypropylene, or polyolefin derivative (chlorinated polyethylene, chlorinated polypropylene, etc.) having a structural unit derived from ethylene of 85 mol% or more, polyolefin Examples thereof include wax, petroleum wax, carnauba wax and the like. Examples of the copolymer polyolefin include an ethylene-vinyl monomer copolymer, more specifically, an ethylene-vinyl acetate copolymer (EVA), an ethylene-methyl acrylate copolymer, or an ethylene-ethyl acrylate copolymer. it can. Moreover, polycycloolefin etc. can also be used. The heat-meltable fine particles may have only one kind of these constituent materials, or may have two or more kinds. Among these, PE, polyolefin wax, or EVA having a structural unit derived from ethylene of 85 mol% or more is preferable. Further, the heat-meltable fine particles may contain various known additives (for example, antioxidants) added to the resin as necessary, in addition to the above-described constituent materials. Absent.

  The particle diameter of the heat-meltable fine particles is, for example, 0.001 μm or more, more preferably 0.1 μm or more and 15 μm or less, more preferably a number average particle diameter measured by the same measurement method as that for the insulating fine particles. Is recommended to be 1 μm or less.

  In a separator configured using a porous membrane having swellable fine particles that can swell in the electrolyte and increase in degree of swelling as the temperature rises, the swellable fine particles swell when exposed to high temperatures in the battery. It absorbs the electrolyte and expands greatly (hereinafter, the function of increasing the degree of swelling with increasing temperature in the swellable fine particles is called “thermal swelling”), thereby significantly reducing the conductivity of Li ions in the separator. Therefore, the internal resistance of the battery is increased, and the shutdown function can be reliably ensured. As the swellable fine particles, those having a temperature of 75 to 125 ° C. exhibiting the above heat swellability are preferable.

  Examples of such swellable fine particles having thermal swellability include crosslinked polystyrene (PS), crosslinked acrylic resin [for example, crosslinked polymethyl methacrylate (PMMA)], and crosslinked fluororesin [for example, crosslinked polyvinylidene fluoride (PVDF). ] Is preferred, and cross-linked PMMA is particularly preferred.

  The particle size of the swellable fine particles is a number average particle size measured by dispersing the fine particles in a non-swelling medium (for example, water) using a laser scattering particle size distribution meter (for example, “LA-920” manufactured by HORIBA). It is preferable that it is 1-20 micrometers.

  Examples of commercially available swellable fine particles include cross-linked PMMA “Gantz Pearl (product name)” manufactured by Gantz Kasei Co., Ltd. and cross-linked PMMA “RSP1079 (product name)” manufactured by Toyo Ink Co., Ltd. are available.

  In addition, when the separator has a shutdown function by containing the heat-meltable fine particles and the swellable fine particles in the porous film, the heat-meltable fine particles in the porous film and / or Alternatively, the content of the swellable fine particles is preferably 5 to 70% by volume in the total volume of the constituent components of the porous membrane. If the content of these fine particles is too small, the shutdown effect due to the inclusion of these may be reduced, and if too large, the content of insulating fine particles and fibrous materials in the porous film will decrease. Therefore, the effect secured by these may be reduced.

  When using a microporous film made of a resin together with a porous membrane, the separator is formed by integrating the porous membrane and the microporous film, and when the shutdown function is given to the separator by the microporous film, As the constituent resin, the same resin (thermoplastic resin) as the resin constituting the thermomeltable fine particles can be used. The shutdown temperature of the microporous film is desirably set in the range of 80 ° C. to 140 ° C. Therefore, it is desirable to use a thermoplastic resin having a melting point of 80 ° C. to 140 ° C.

  On the other hand, when the heat resistance of the separator is emphasized and the shutdown function is not given, a heat resistant microporous film can be used. As a specific constituent material of such a microporous film, the heat-resistant temperature is 150 ° C. or more, it is stable with respect to the electrolyte used in the battery, and further stable with respect to the oxidation-reduction reaction inside the battery. Any resin may be used. More specifically, heat-resistant resins such as polyimide, polyamideimide, aramid, polytetrafluoroethylene, polysulfone, polyurethane, PAN, polyester (PET, PBT, PEN, etc.) can be mentioned.

As the microporous film, any of those made of the above-mentioned thermoplastic resin and those made of the above-mentioned heat-resistant resin can be produced by a conventionally known method. For example, an ion-permeable porous film produced by a solvent extraction method, a dry or wet stretching (uniaxial or biaxial stretching) method, or the like can be used. In addition, a film microporous by a foaming method using a drug, supercritical CO ₂ or the like can be used.

  More specific embodiments of the separator of the present invention include, for example, the following embodiments (a), (b), (c) and (d).

  The separator according to the embodiment of (a) uses a material in which a large number of fibrous materials are gathered to form a sheet-like material, for example, a woven fabric, a nonwoven fabric (including paper), Insulating fine particles and other fine particles as needed are contained in this sheet-like material, and are constituted by a porous film constituted by binding fibrous materials or insulating fine particles related to the sheet-like material with a binder. Is.

  The separator according to the embodiment (b) is a sheet (porous) in which insulating fine particles and fibrous materials (and other fine particles if necessary) are uniformly dispersed and these are bound by a binder. Material).

  The separator according to the embodiment (c) is composed of a sheet (porous film) formed by binding insulating fine particles with a binder, and this sheet is integrated with at least one of the positive electrode and the negative electrode. .

  The separator according to the embodiment (d) is formed by integrating a sheet (porous film) formed by binding insulating fine particles with a binder and a resin microporous film. .

  Moreover, in the aspect which combined the aspect of (a) and the aspect of (b), ie, the independent sheet-like thing comprised with a fibrous material, insulating fine particles and a fibrous material (further, as needed) Other fine particles) may be dispersed and bound by a binder. Furthermore, the aspect which combined the aspect of (a) or (b), and the aspect of (c), ie, the porous film which concerns on the separator of the aspect of (a) or (b) is integrated with at least one of a positive electrode and a negative electrode It may be a simplified form.

  Further, the aspect in which the aspect of (a) or (b) and the aspect of (d) are combined, that is, the porous film integrated with the resin-made microporous film is the aspect of (a) or (b). A porous film according to the separator may be used. Furthermore, the aspect which combined the aspect of (a) or (b), the aspect of (c), and the aspect of (d), ie, the porous membrane integrated with the resin-made microporous film, ( A porous film according to the separator of the aspect of a) or (b), and the porous film may be integrated with at least one of the positive electrode and the negative electrode.

  In the separator having the porous membrane and the microporous film and integrated with each other, the porous membrane and the microporous film do not need to be one each, You may comprise the separator with the sheet | seat. For example, it is good also as a structure which has arrange | positioned the microporous film on both surfaces of the porous film, or the structure which has arrange | positioned the porous film on both surfaces of the microporous film. However, if the total number of the porous film and the microporous film is excessively increased, the thickness of the separator is increased, which may increase the internal resistance of the battery and decrease the energy density. The total number of the membrane and the microporous film is preferably 5 or less.

  The thickness of the separator is preferably 3 μm or more, and more preferably 5 μm or more, from the viewpoint of further enhancing the effect of preventing short-circuiting of the battery, ensuring the strength of the separator, and improving its handleability. . On the other hand, from the viewpoint of further increasing the energy density of the battery, the thickness of the separator is preferably 50 μm or less, and more preferably 30 μm or less. In the case where the separator is constituted only by the porous membrane, the thickness of the porous membrane may be set so as to satisfy the preferable thickness of the separator.

  On the other hand, in the case of a separator having the porous film and the microporous film, when the thickness of the porous film is X (μm) and the thickness of the microporous film is Y (μm), X and Y It is preferable that the thickness of the entire separator satisfies the above-mentioned preferred value while the ratio Y / X is 1 to 10. When Y / X is too large, the porous film becomes too thin, and for example, when the dimensional stability of the microporous film at high temperatures is poor, the effect of suppressing the thermal shrinkage may be reduced. On the other hand, if Y / X is too small, the porous membrane becomes too thick, increasing the thickness of the entire separator, and possibly causing deterioration of battery characteristics such as load characteristics. When the separator has a plurality of porous films, the thickness X is the total thickness, and when the separator has a plurality of microporous films, the thickness Y is the total thickness.

  In terms of specific values, the thickness of the microporous film (when the separator has a plurality of microporous films, the total thickness) is preferably 5 μm or more, and 30 μm or less. Is preferred. The thickness of the porous film (when the separator has a plurality of porous films, the total thickness) is preferably 1 μm or more, more preferably 2 μm or more, and more preferably 4 μm or more. More preferably, it is preferably 20 μm or less, more preferably 10 μm or less, and even more preferably 6 μm or less.

In addition, the porosity of the separator is preferably 20% or more, and preferably 30% or more in a dried state in order to ensure the amount of electrolyte retained and improve ion permeability. More preferred. On the other hand, from the viewpoint of ensuring the strength of the separator and preventing internal short circuit, the porosity of the separator is preferably 70% or less, more preferably 60% 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 = 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: thickness of separator (cm).

In the case of a separator having the porous film and the microporous film, in the formula (1), m is the mass per unit area of the porous film (g / cm ² ), and t is porous. By setting the thickness (cm) of the porous film, the porosity: P (%) of the porous film can also be obtained using the equation (1). The porosity of the porous membrane obtained by this method is preferably 20% or more (more preferably 30% or more) and 70% or less (more preferably 60% or less).

Further, in the formula (1), m is the mass per unit area (g / cm ² ) of the microporous film, and t is the thickness (cm) of the microporous film. Can also be used to determine the porosity of the microporous film: P (%). It is preferable that the porosity of the microporous film calculated | required by this method is 30 to 70%.

  Further, the strength of the separator is desirably 50 g or more in terms of piercing strength using a needle having a diameter of 1 mm. If the piercing strength is too low, a short circuit may occur due to the piercing of the separator when lithium dendrite crystals are generated.

The air permeability of the separator is measured by a method according to JIS P 8117, and is 10 to 300 sec as a Gurley value 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 to be. If the air permeability is too high, the ion permeability is reduced, and if it is too low, the strength of the separator may be reduced.

  The piercing strength and air permeability can be ensured by using the separator having the configuration described so far.

  As a method for producing the separator of the present invention, for example, the following methods (I) and (II) can be employed. In the method (I), after applying or impregnating a composition for forming a porous film (slurry or the like) containing insulating fine particles and an organic binder on an ion-permeable sheet-like material that does not substantially deform at 150 ° C. The manufacturing method is to dry at a predetermined temperature.

  That is, the “sheet-like material” referred to in the method (I) corresponds to a sheet-like material composed of fibrous materials (various woven fabrics, nonwoven fabrics, etc.). Specifically, a woven fabric composed of at least one kind of fibrous material containing the above-mentioned exemplified materials as constituent components, a porous sheet such as a nonwoven fabric having a structure in which these fibrous materials are entangled with each other, and the like. It is done. More specifically, non-woven fabrics such as paper, PP non-woven fabric, polyester non-woven fabric (PET non-woven fabric, PEN non-woven fabric, PBT non-woven fabric, etc.) and PAN non-woven fabric can be exemplified.

  The composition for forming a porous film contains insulating fine particles and a binder, and further contains heat-meltable fine particles, swellable fine particles, etc., if necessary, in a solvent (including a dispersion medium, the same applies hereinafter). (The binder may be dissolved in a solvent). The solvent used in the composition for forming a porous film may be any solvent that can uniformly disperse insulating fine particles, heat-meltable fine particles, and swellable fine particles, and can uniformly dissolve or disperse the organic binder. Organic solvents such as aromatic hydrocarbons such as toluene, furans such as tetrahydrofuran; ketones such as methyl ethyl ketone and methyl isobutyl ketone; In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In addition, when the binder is water-soluble or used as an emulsion, water may be used as a solvent. In this case, an alcohol (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) is added as appropriate to the interface. The tension can also be controlled.

  In the composition for forming a porous film, the solid content including insulating fine particles, binder, hot-melt fine particles, swellable fine particles and the like is preferably set to 10 to 80% by mass, for example.

  When the sheet-like material is composed of a fibrous material such as paper, PP nonwoven fabric, polyester nonwoven fabric or the like, and the opening diameter of the gap is relatively large (for example, the opening diameter of the gap). This is likely to cause a short circuit of the battery. Therefore, in this case, it is preferable to have a structure in which some or all of the insulating fine particles are present in the voids of the sheet-like material. Further, it is more preferable that a part or all of fine particles (such as heat-meltable fine particles and swellable fine particles) other than the insulating fine particles exist in the voids of the sheet-like material. By adopting such a structure, the effect of using fine particles other than insulating fine particles (shutdown function in the case of hot-melt fine particles or swellable fine particles) is more effectively exhibited. In order to make insulating fine particles, heat-meltable fine particles, swellable fine particles, etc. exist in the voids of the sheet-like material, for example, after impregnating the porous film-forming composition into the sheet-like material, a certain gap The excess porous film-forming composition may be removed and then dried.

  In the separator (II) production method of the present invention, the above-mentioned composition for forming a porous film further contains a fibrous material as necessary, and this is applied to a substrate such as a film or a metal foil on an electrode ( It is a method of forming a porous film by coating on a positive electrode or a negative electrode) or on a microporous film and drying at a predetermined temperature. In addition, when a porous film is formed on the base material, the porous film is peeled from the base material as necessary.

  The composition for forming a porous film used in the method (II) is the same as the composition for forming a porous film used in the method (I) except that a fibrous material is contained as necessary. Further, in the separator obtained by the method (II), when the fibrous material is contained, a structure in which part or all of the insulating fine particles are present in the voids of the sheet material formed of the fibrous material and It is desirable to do.

  In the case where plate-like particles are used as insulating fine particles in the separator (porous film), in order to increase the orientation, the composition in the sheet-like material impregnated with the porous film-forming composition is used. You just need to share things. For example, in the production method (I), after impregnating the porous film-forming composition described above as a method for causing insulating fine particles or the like to exist in the voids of the sheet-like material, a certain gap is formed. By passing the composition, it is possible to share the composition for forming a porous film, and the orientation of the insulating fine particles can be improved.

  In addition, in order to further enhance the orientation of the plate-like insulating fine particles in the separator, a composition for forming a porous film having a high solid content concentration (for example, 50 to 80% by mass) other than the above-described method of applying a share. Dispersing insulating fine particles in a solvent using various mixing / stirring devices such as dispersers, agitators, homogenizers, ball mills, attritors, jet mills, dispersing devices, etc., and binding to the resulting dispersion (Furthermore, a method of using a composition for forming a porous film prepared by adding and mixing fibrous materials, heat-meltable fine particles, swellable fine particles, etc., if necessary; oils and fats on the surface, surfactants, A method of using a porous film-forming composition prepared using insulating fine particles whose surface has been modified by the action of a dispersing agent such as a silane coupling agent; different shape, diameter or aspect ratio Using a composition for forming a porous film prepared by using in combination with insulating fine particles; after applying or impregnating a composition for forming a porous film on a sheet or coating on a substrate A method for controlling drying conditions; a method for pressurizing or heating and pressing a separator; a composition for forming a porous film is applied and impregnated on a sheet or coated on a substrate, and then dried. The method of applying a magnetic field; etc. can be employed, and these methods can be carried out independently or in combination of two or more methods.

  The lithium secondary battery of the present invention is not particularly limited as long as it has the separator of the present invention, and conventionally known configurations and structures can be adopted for other configurations and structures.

  Examples of the form of the lithium secondary battery 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 positive electrode is not particularly limited as long as it is a positive electrode used in a conventionally known lithium secondary battery, that is, a positive electrode containing an active material capable of occluding and releasing Li ions. For example, as an active material, lithium-containing transition metal oxide represented by Li _{1 + x} MO ₂ (−0.1 <x <0.1, M: Co, Ni, Mn, etc.); lithium manganese such as LiMn ₂ O ₄ Oxide; LiMn _x M _(1-x) O _{2 in} which 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, and a known conductive additive (carbon material such as carbon black) or a binder such as polyvinylidene fluoride (PVDF) is appropriately added to these positive electrode active materials. The positive electrode mixture is formed from a current collector as a core material (ie, Such as those finished electrode mixture layer) may 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.

  The negative electrode is not particularly limited as long as it is a negative electrode used in a conventionally known lithium secondary battery, that is, a negative electrode containing an active material capable of occluding and releasing Li ions. For example, carbon that can occlude and release lithium, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers as active materials One type or a mixture of two or more types of system materials is used. In addition, elements such as Si, Sn, Ge, Bi, Sb, In and their alloys, 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. A negative electrode mixture obtained by appropriately adding a conductive additive (carbon material such as carbon black) or a binder such as PVDF to these negative electrode active materials, and a molded body (negative electrode mixture layer) using a current collector as a core material And those having a negative electrode agent layer such as those made of the above-mentioned various alloys and lithium metal foils alone or 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.

  As with the lead portion on the negative electrode side, the negative electrode layer (including a layer having a negative electrode active material and a negative electrode mixture layer) is usually formed on a part of the current collector during the preparation of the negative electrode. Without leaving the exposed portion of the current collector, it is provided as a lead portion. However, the lead portion on the negative electrode side is not necessarily integrated with the current collector from the beginning, and may be provided by connecting a copper foil or the like to the current collector later.

  The electrode can be used in the form of an electrode group having a laminated structure in which the positive electrode and the negative electrode are laminated via the separator of the present invention, or a wound structure electrode group in which the electrode is wound.

As the electrolytic solution (organic electrolytic solution), as described above, a solution in which a lithium salt is dissolved in an organic solvent is used. The lithium salt is not particularly limited as long as it dissociates in a solvent to form Li ⁺ ions and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, inorganic lithium salts such as 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 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]; it can.

  The organic solvent used in the electrolytic solution is not particularly limited as long as it dissolves the lithium salt and does not cause side reactions such as decomposition in the voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; Chain ethers such as ethane, diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; nitriles such as acetonitrile, propionitrile and methoxypropionitrile; Sulfites such as ethylene glycol sulfite; and the like, and these may be used alone. , It may be used in combination of two or more thereof. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate. In addition, vinylene carbonates, 1,3-propane sultone, diphenyl disulfide, cyclohexane, biphenyl, fluorobenzene, t- for the purpose of improving safety, charge / discharge cycleability, and high-temperature storage properties of these electrolytes. Additives such as butylbenzene can also be added as appropriate.

  The concentration of the lithium salt in the electrolytic solution is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  Also, instead of the organic solvent, melting at room temperature such as ethyl-methylimidazolium trifluoromethylsulfonium imide, heptyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide A salt can also be used.

  Furthermore, a polymer material that contains the electrolytic solution and gels may be added, and the electrolytic solution may be gelled and used for the battery. Polymer materials for making the electrolyte into a gel include PVDF, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), PAN, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer, main Examples of the host polymer capable of forming a known gel electrolyte such as a cross-linked polymer having an ethylene oxide chain in a chain or a side chain, and a cross-linked poly (meth) acrylic ester.

  The lithium secondary battery of the present invention can be applied to the same uses as various uses in which conventionally known lithium secondary batteries are used.

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

  The tensile strength, tensile modulus, and elongation at break of the binders used in each of the examples and comparative examples described below are as follows. The binder in the form of an emulsion is placed in a dumbbell type for preparing No. 2 type test pieces defined in JIS K 7113. After drying at 25 ° C. for 170 hours, the solvent is removed stepwise by drying at 50 ° C. for 12 hours, 80 ° C. for 2 hours, and 120 ° C. for 15 hours to obtain a test piece having a thickness of 1 to 3 mm. This is a value obtained by measurement under the condition of a tensile speed of 5 mm / min using “Tensilon UCT-5T type” manufactured by Orientec.

Example 1
<Preparation of separator>
An olefin having a structure in which a slurry of 1000 g of plate-like boehmite (average particle size 1 μm, aspect ratio 10), which is an insulating fine particle, and water 1000 g is dispersed for 6 days in a table-top ball mill, and a carboxyl group is added as a binder. Emulsion of a mixture of a polymer (number average molecular weight 5000) and an olefin polymer having a structure to which an amino group is added (number average molecular weight 4000) (as solid content, 3 mass per 100 mass parts of insulating fine particles) Part) and stirred and dispersed with a three-one motor for 1 hour to obtain a uniform slurry (a composition for forming a porous film). The binder used in Example 1 has a tensile strength of 21 MPa, a tensile elastic modulus of 170 MPa, and a breaking elongation of 290%, and corresponds to the binder (A).

  A non-woven fabric made of PET having a thickness of 15 μm is passed through the slurry, and after applying the slurry by pulling up, the slurry is passed through a gap having a predetermined interval, and then dried under reduced pressure at 60 ° C. for 15 hours to have a thickness of 20 μm. A separator was obtained.

For the separator of Example 1, the volume of the insulating fine particles calculated with the specific gravity of the insulating fine particles being 3 g / cm ³ , the specific gravity of the binder being 1 g / cm ³ , and the specific gravity of the PET relating to the PET nonwoven fabric being 1.38 g / cm ³ The content is 43.5%.

<Preparation of positive electrode>
85 parts by mass of LiCoO _{2 as a} positive electrode active material, 10 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of PVDF as a binder are uniformly mixed with N-methyl-2-pyrrolidone (NMP) as a solvent. It mixed so that positive electrode mixture containing paste might be prepared. This paste was intermittently applied to both sides of a 15 μm thick aluminum foil serving as a current collector so that the active material application length was 320 mm on the front surface and 250 m on the back surface, dried, and then subjected to a calendering treatment to obtain a total thickness. The thickness of the positive electrode mixture layer was adjusted to 150 μm, and the positive electrode mixture layer was cut to a width of 43 mm to produce a positive electrode having a length of 340 mm and a width of 43 mm. Further, the exposed portion of the aluminum foil of the positive electrode was tabbed.

<Production of negative electrode>
A negative electrode mixture-containing paste was prepared by mixing 90 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVDF as a binder so as to be uniform using NMP as a solvent. This negative electrode mixture-containing paste is intermittently applied on both sides of a 10 μm thick collector made of copper foil so that the active material application length is 320 mm on the front surface and 260 mm on the back surface, dried, and then calendered. The thickness of the negative electrode mixture layer was adjusted so that the total thickness was 142 μm, and the negative electrode mixture layer was cut to have a width of 45 mm to produce a negative electrode having a length of 330 mm and a width of 45 mm. Further, a tab was attached to the exposed portion of the copper foil of the negative electrode.

<Battery assembly>
The positive electrode and the negative electrode obtained as described above were overlapped via the separator and wound in a spiral shape to obtain a wound electrode group. This electrode group is crushed into a flat shape, inserted into a rectangular battery case having a thickness of 4.2 mm and a width of 34 mm, and an electrolyte solution (in a solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 1: 2, A solution in which LiPF ₆ was dissolved at a concentration of 1.2 mol / l) was injected, and the opening of the battery case was sealed in accordance with a standard method to produce a lithium secondary battery. In pre-charging (charging at the time of chemical conversion) after battery assembly, constant current charging up to 4.2 V at 150 mA and then constant voltage at 4.2 V are performed so that the amount of electricity is 20% of the rated capacity of 750 mAh. Voltage charging was performed for a total of 6 hours, and then a constant current discharge was performed up to 3 V at 150 mA.

Example 2
Example 1 except that the binder was changed to an acrylic acid group-containing polyacrylate crosslinked product (acrylate copolymer having a crosslinked structure) having a tensile strength of 1.4 MPa, a tensile elastic modulus of 0.4 MPa, and a breaking elongation of 440%. Thus, a separator was produced. The binder used in Example 2 was obtained by adding a self-crosslinking acrylate copolymer to a slurry for forming a porous layer and drying to obtain an acrylate copolymer having a crosslinked structure. Binder (B) It corresponds to.

  A lithium secondary battery was produced in the same manner as in Example 1 except that the separator was used.

Example 3
A negative electrode mixture-containing paste was prepared by mixing 90 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVDF as a binder so as to be uniform using NMP as a solvent. This negative electrode mixture-containing paste was applied to both sides of a 10 μm-thick current collector made of copper foil, dried, and then calendered to adjust the thickness of the negative electrode mixture layer so that the total thickness was 142 μm. . On the obtained negative electrode, the same slurry (the composition for forming a porous film) as that prepared in Example 1 was applied with an applicator, and then dried under reduced pressure at 60 ° C. for 15 hours. A 20 μm negative electrode integrated separator was obtained.

  The separator surface of the negative electrode integrated separator and the positive electrode produced in the same manner as in Example 1 were superposed and wound into a spiral shape to form a wound electrode group, which was crushed flat. A lithium secondary battery was produced in the same manner as in Example 1 except that this flat wound electrode group was used.

Example 4
The same slurry (composition for forming a porous film) as that prepared in Example 1 was applied onto a microporous film made of PE (thickness 16 μm, porosity 40%) using a micro gravure coater, and the thickness was 20 μm. A separator was obtained.

  A lithium secondary battery was produced in the same manner as in Example 1 except that the separator was used.

Comparative Example 1
A separator was produced in the same manner as in Example 1 except that the binder was changed to SBR having a tensile strength of 4 MPa, a tensile elastic modulus of 25 MPa, and a breaking elongation of 260%.

  A lithium secondary battery was produced in the same manner as in Example 1 except that the separator was used.

  The lithium secondary batteries of Examples 1 to 4 and Comparative Example 1 were subjected to the following discharge capacity ratio measurement. The results are shown in Table 1 together with the tensile properties of the separator used in the battery.

<Discharge capacity ratio>
About the batteries of Examples 1 to 4 and Comparative Example 1, constant current charging at 0.2 C (up to 4.2 V) and charging by constant voltage charging at 4.2 V (total time of constant current charging and constant voltage charging 15 Thereafter, discharging was performed at 0.2 C up to 3.0 V, and the ratio of the obtained discharge capacity to the charge capacity, that is, the discharge capacity ratio (%) was determined. The discharge capacity ratio was determined as an average value of 10 batteries.

  As can be seen from Table 1, the lithium secondary battery of Comparative Example 1 including a separator made of a porous film formed using a binder having a tensile property that does not correspond to the binder (A) or the binder (B) is discharged. The capacity ratio is 91.7%, and it has good reliability and has sufficient practicality. However, Example 1, Example 3 comprising a porous film formed using a binder (A) having a tensile strength of 10 MPa or more and a tensile elastic modulus of 100 MPa or more, or provided with a separator having the porous film. In the lithium secondary battery of Example 4 and the lithium secondary battery of Example 2 including a separator made of a porous film formed using a binder (B) having a breaking elongation of 300% or more, the discharge capacity ratio is It exceeds 99% and has a very good reliability over the battery of Comparative Example 1.

  That is, in a battery (Example 1, Example 3 and Example 4) comprising a porous film using a binder (A) or provided with a separator having the porous film, the fibrous material and the insulating fine particles And adhesion between insulating fine particles are improved, and cracking of the separator and peeling of the insulating fine particles at the innermost peripheral portion of the electrode group having a wound structure are suppressed, and minute densification due to lithium dendrite precipitation is suppressed. Since the short circuit is highly suppressed, the discharge capacity ratio is improved. Further, in the battery (Example 2) provided with the separator made of the porous film using the binder (B), the flexibility of the separator is improved, and the crack of the separator in the innermost peripheral portion of the electrode group having the wound structure is achieved. In addition, since the exfoliation of the insulating fine particles is suppressed and a minute short circuit due to lithium dendrite precipitation is highly suppressed, the discharge capacity ratio is improved.

Claims (14)

At least composed of insulating fine particles and binder that are stable with respect to the organic electrolyte,
As the binder, a (meth) acrylate copolymer having a crosslinked structure, the elongation at break of 300% or more of the binder (B), 0.5% by volume or more, and characterized in that it has a proportion of 15 vol% or less Porous membrane.   The porous film according to claim 1, wherein the binder (B) has a binder formed by self-crosslinking of a self-crosslinking type (meth) acrylate copolymer.   The porous film according to claim 2, wherein the copolymer is synthesized using a monomer having a hydroxyl group or a carboxyl group.   The monomers are (meth) acrylic acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, fumaric acid, β-hydroxyethyl (meth) acrylate, β-hydroxypropyl (meth) acrylate and β-hydroxyethyl. The porous membrane according to claim 3, which is at least one selected from vinyl ethers. As the insulating fine particles, porous membrane according to any one of claims 1 to 4 containing plate-like particles. The porous membrane according to any one of claims 1 to 5 , further comprising a fibrous material having a heat resistant temperature of 150 ° C or higher. Battery separator characterized by having a porous membrane according to any one of claims 1-6. And the porous membrane according to any one of claims 1 to 6 and a microporous film made of resin, according to claim 7, said porous membrane and said microporous film are integrated Battery separator. The battery separator according to claim 8 , wherein the thickness is 5 μm or more and 30 μm or less. 10. The battery according to claim 9 , wherein the ratio Y / X of X to Y is 1 to 10 where the thickness of the porous film is X (μm) and the thickness of the microporous film is Y (μm). Separator. The thickness of the porous membrane, battery separator according to any one of claims 8-1 0 is 1μm or more 10μm or less. The battery separator according to any one of claims 8 to 11, wherein the microporous film contains a thermoplastic resin having a melting point of 80 to 140 ° C. A positive electrode, a negative electrode, and includes an organic electrolyte and a separator, as the separator, a lithium secondary battery, characterized by having a battery separator according to any one of claims 7-1 2. The porous membrane, a lithium secondary battery according to claim 1 3 which is integral with at least one of the positive electrode and the negative electrode in the battery separator.