JP5920441B2 - Laminated film and non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a laminated film and a nonaqueous electrolyte secondary battery. In particular, the present invention relates to a laminated film useful as a separator and a nonaqueous electrolyte secondary battery using the same.

  The separator is a film having fine pores, and is disposed between the positive electrode and the negative electrode of a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery or a lithium polymer secondary battery. A nonaqueous electrolyte secondary battery is manufactured by impregnating a nonaqueous electrolyte solution after an electrode group obtained by stacking and winding a positive electrode sheet, a separator, a negative electrode sheet, and a separator in order is housed in a battery case.

  In the non-aqueous electrolyte secondary battery, the separator has a function of interrupting an excessive current by blocking the current when an abnormal current flows in the battery due to a short circuit between the positive electrode and the negative electrode, that is, The separator is required to have a shutdown function, and the separator shuts down by closing the fine holes when the normal use temperature of the battery is exceeded. And even after shutdown, the temperature inside the battery may rise, and even if the temperature inside the battery rises to a certain high temperature, the separator maintains the shutdown state without being broken by that temperature. In other words, high heat resistance is also being demanded.

  As a conventional separator, Patent Document 1 discloses a laminated film obtained by laminating a heat-resistant porous layer containing an inorganic filler on at least one side of a polyethylene porous film having a shutdown function, specifically, , Using polyvinyl alcohol as a binder, applying a dispersion of inorganic filler and polyvinyl alcohol in water to the surface of the porous film, removing the water by drying, and laminating the heat-resistant porous layer doing.

JP 2009-143060 A

  However, in the production of a secondary battery using the laminated film obtained as described above as a separator, since the frictional force between the sheets during winding is large, the positive electrode sheet, the separator and the negative electrode sheet are brought into close contact with each other. Winding is difficult, and as a result, the battery characteristics such as the cycle of the discharge capacity of the battery finally obtained are lowered. An object of the present invention is to provide a laminated film extremely useful as a separator having a shutdown function, excellent in heat resistance, and excellent in battery characteristics and providing a nonaqueous electrolyte secondary battery.

As a result of various studies in view of the above circumstances, the present inventors have reached the present invention. That is, the present invention provides the following inventions.
<1> A laminated film comprising a porous film having a shutdown function, a heat-resistant porous layer containing plate-like inorganic particles and a binder, and a protective porous layer, which are sequentially in contact with each other. .
<2> The laminate according to <1>, wherein in the heat resistant porous layer, the volume ratio of the plate-like inorganic particles to the total volume of the plate-like inorganic particles and the binder is in the range of 50% by volume or more and less than 100% by volume. the film.
<3> The laminated film according to <1> or <2>, wherein the aspect ratio of the plate-like inorganic particles is in the range of 10 to 100.
<4> The laminated film according to any one of <1> to <3>, wherein the protective porous layer is a layer made of particles.
<5> The laminated film according to <4>, wherein the average particle size of the particles in the protective porous layer is in the range of 0.01 μm to 3 μm.
<6> The laminated film according to any one of <1> to <5>, wherein the porosity in the protective porous layer is in the range of 30% by volume to 80% by volume.
<7> The laminated film according to any one of <1> to <6>, wherein the porous film has a thickness in a range of 13 μm to 17 μm.
<8> The laminated film according to any one of <1> to <7>, wherein the heat-resistant porous layer has a thickness in the range of 1 μm to 10 μm.
<9> The laminated film according to any one of <1> to <8>, wherein the protective porous layer has a thickness in a range of 0.02 μm to 5 μm.
<10> The laminated film according to any one of <1> to <9>, which is a separator.
<11> A nonaqueous electrolyte secondary battery having a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the separator is any one of <1> to <9> A nonaqueous electrolyte secondary battery, which is a laminated film according to claim 1.
<12> The nonaqueous electrolyte secondary battery according to <11>, wherein the protective porous layer of the laminated film which is a separator is disposed on the positive electrode side.

  If the laminated film of the present invention is used as a separator for a non-aqueous electrolyte secondary battery, in the production of a secondary battery, a positive electrode sheet, a separator, a negative electrode sheet, and a separator are sequentially stacked and wound between sheets when the electrode group is produced. Thus, an electrode group in which the positive electrode sheet, the separator, and the negative electrode sheet are more closely attached can be obtained. As a result, a secondary battery having excellent cycle performance can be obtained. Therefore, the nonaqueous electrolyte secondary battery having the laminated film of the present invention as a separator has a shutdown function and is excellent not only in heat resistance but also in battery characteristics such as cycleability. In addition, since the laminated film of the present invention is less likely to wear apparatus members such as a winding roll at the time of producing the laminated film, generation of metal powder, resin powder, etc. and mixing into the laminated film can be suppressed, and moisture can be reduced. Since it is difficult to adsorb, it is possible to suppress a decrease in electrical insulation due to moisture absorption, and furthermore, since it is difficult to be charged, it is possible to suppress adsorption of foreign substances in the atmosphere, and it is a laminated film that is extremely excellent in handling. Rich in practicality.

  The laminated film of the present invention is characterized in that a porous film having a shutdown function, a heat-resistant porous layer containing plate-like inorganic particles and a binder, and a protective porous layer are sequentially laminated. And

<Porous film>
In the present invention, the porous film has a shutdown function. In order to have a shutdown function in the nonaqueous electrolyte secondary battery, the material of the porous film is preferably a material that softens at 80 ° C. to 180 ° C. As a material of the porous film, polyolefin exemplified by polyethylene, polypropylene and the like is preferable. Polyethylene is more preferable in terms of softening and shutting down at a lower temperature. Specific examples of polyethylene include polyethylene such as low density polyethylene, high density polyethylene, and linear polyethylene, and ultra high molecular weight polyethylene having a molecular weight of 1,000,000 or more. In order to further increase the puncture strength of the porous film, the thermoplastic resin preferably contains at least ultra high molecular weight polyethylene. In addition, in terms of production of the porous film, the thermoplastic resin may preferably contain a wax made of polyolefin having a low molecular weight (weight average molecular weight of 10,000 or less).

  The porous film has fine pores, and the pore size is usually 3 μm or less, preferably 1 μm or less. The porosity of the porous film is usually in the range of 30% by volume to 80% by volume, and preferably in the range of 40% by volume to 70% by volume. In a non-aqueous electrolyte secondary battery, when the normal use temperature is exceeded, the porous film can close the micropores by softening the material constituting the porous film.

In addition, the porosity of a porous film can be calculated | required by the following formula | equation (1).
Pv ₁ (%) = {(Va ₁ −Vt ₁ ) / Va ₁ } × 100 (1)

Pv ₁ (%): porosity of porous film (volume%)
Va ₁ : Apparent volume of porous film Vt ₁ : Theoretical volume of porous film

Here, Va ₁ can be calculated from the values of the vertical, horizontal, and thickness of the porous film, and Vt ₁ is calculated from the weight of the porous film, the weight ratio of the constituent materials, and the true specific gravity values of the constituent materials. can do.

  The thickness of the porous film is usually in the range of 3 μm to 30 μm, preferably in the range of 3 μm to 25 μm, more preferably in the range of 13 μm to 17 μm. By setting the thickness within the range of 13 μm or more and 17 μm or less, it is possible to reduce the film thickness without particularly impairing the strength of the porous film.

<Method for producing porous film>
In the present invention, the method for producing the porous film is not particularly limited. For example, as described in JP-A-7-29563, a method of removing a plasticizer with an appropriate solvent after adding a plasticizer to a thermoplastic resin to form a film, or JP-A-7-304110 As described, there is a method of using a film made of a thermoplastic resin produced by a known method and selectively stretching a structurally weak amorphous portion of the film to form micropores.

Further, when the porous film is formed from a polyolefin resin containing ultra-high molecular weight polyethylene and low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, it is produced by the following method from the viewpoint of production cost. It is preferable to do. That is,
(1) A step of kneading 100 parts by weight of ultrahigh molecular weight polyethylene, 5 to 200 parts by weight of a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, and 100 to 400 parts by weight of an inorganic filler to obtain a polyolefin resin composition (2) Step of forming a sheet using the polyolefin resin composition (3) Step of removing inorganic filler from the sheet obtained in step (2) (4) Sheet obtained in step (3) Or (1) 100 parts by weight of ultrahigh molecular weight polyethylene, 5 to 200 parts by weight of a low molecular weight polyolefin having a weight average molecular weight of 10,000 or less, and inorganic fillers 100 to 400 Step (2) of obtaining a polyolefin resin composition by kneading with parts by weight (2) Step of molding a sheet using the polyolefin resin composition (3) Step (2) From the obtained step (4) for stretching the sheet process (3) in the obtained stretched sheet was removed inorganic filler (C) is a method comprising the step of the porous film. From the viewpoint of lowering the shutdown temperature of the resulting laminated film, the former method, that is, the method of stretching after removing the inorganic filler in the sheet is preferred.

From the viewpoint of the strength of the porous film and the lithium ion permeability, the inorganic filler used preferably has an average particle size of 0.5 μm or less, and more preferably 0.2 μm or less. Here, the average particle diameter of the inorganic filler is a volume-based D ₅₀ value obtained using a laser diffraction particle size distribution analyzer.

  Inorganic fillers include calcium carbonate, magnesium carbonate, barium carbonate, zinc oxide, calcium oxide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, calcium sulfate, silicic acid, zinc oxide, calcium chloride, sodium chloride, magnesium sulfate, etc. Is mentioned. These inorganic fillers can be removed from the sheet or film with an acid or alkaline solution. In the present invention, it is preferable to use calcium carbonate because it is easy to obtain a fine particle size.

  The method for producing the polyolefin resin composition is not particularly limited, but the material constituting the polyolefin resin composition such as a polyolefin resin or an inorganic filler is mixed with an apparatus such as a roll, a Banbury mixer, a single screw extruder, a twin screw extruder. Mix using a machine to obtain a polyolefin resin composition. When mixing the materials, additives such as fatty acid esters, stabilizers, antioxidants, ultraviolet absorbers and flame retardants may be added as necessary.

  The manufacturing method of the sheet | seat which consists of the said polyolefin resin composition is not specifically limited, It can manufacture by sheet | seat shaping | molding methods, such as an inflation process, a calendar process, T-die extrusion process, and a Skyf method. Since a sheet with higher film thickness accuracy can be obtained, it is preferable to produce the sheet by the following method.

  A preferred method for producing a sheet comprising a polyolefin resin composition is a polyolefin resin composition using a pair of rotational molding tools adjusted to a surface temperature higher than the melting point of the polyolefin resin contained in the polyolefin resin composition. This is a method of rolling a product. The surface temperature of the rotary forming tool is preferably (melting point + 5) ° C. or higher. The upper limit of the surface temperature is preferably (melting point + 30) ° C. or less, and more preferably (melting point + 20) ° C. or less. Examples of the pair of rotary forming tools include a roll and a belt. The peripheral speeds of the two rotary forming tools do not necessarily have to be exactly the same peripheral speed, and the difference between them may be about ± 5% or less. By producing a porous film using a sheet obtained by such a method, a porous film excellent in strength, lithium ion permeability, air permeability and the like can be obtained. Moreover, you may use what laminated | stacked the sheet | seat of the single layer obtained by the above methods for manufacture of a porous film.

  When a polyolefin resin composition is rolled with a pair of rotary molding tools, the polyolefin resin composition discharged in a strand form from an extruder may be directly introduced between the pair of rotary molding tools, and once pelletized. The polyolefin resin composition may be used.

  When stretching a sheet made of a polyolefin resin composition or a sheet from which the inorganic filler has been removed, a tenter, a roll, an autograph or the like can be used. From the air permeable side, the draw ratio is preferably 2 to 12 times, more preferably 4 to 10 times. The stretching temperature is usually carried out at a temperature not lower than the softening point and not higher than the melting point of the polyolefin resin, and is preferably performed at 80 to 115 ° C. If the stretching temperature is too low, film breakage tends to occur during stretching, and if it is too high, the air permeability and lithium ion permeability of the resulting film may be lowered. Moreover, it is preferable to heat set after extending | stretching. The heat set temperature is preferably a temperature below the melting point of the polyolefin resin.

<Heat resistant porous layer>
In the present invention, the heat-resistant porous layer contains plate-like inorganic particles and a binder. The heat-resistant porous layer is in contact with the porous film. In non-aqueous electrolyte secondary batteries, the heat-resistant porous layer is porous even if the temperature inside the battery further rises to a certain high temperature after the porous film shuts down. It has a function of maintaining a shut-down state without breaking the quality film. The heat resistant porous layer may be laminated in contact with one surface of the porous film, or may be laminated in contact with both surfaces.

  In the heat-resistant porous layer, examples of the plate-like inorganic particles include oxide ceramics such as alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, iron oxide; silicon nitride, titanium nitride, boron nitride, etc. Nitride ceramics: silicon carbide, light calcium carbonate, heavy calcium carbonate, aluminum sulfate, aluminum hydroxide, potassium titanate, talc, kaolin clay, kaolinite, halloysite, pyrophyllite, montmorillonite, sericite, mica, Examples include acerite, bentonite, asbestos, zeolite, ceramics such as calcium silicate, magnesium silicate, diatomaceous earth, and silica sand; among materials such as glass fibers, the constituent particles are plate-like. In any case, the melting point exceeds 200 ° C. These may be used singly or in combination of two or more. The material is preferably alumina, silica, boehmite, titania, kaolin clay, light calcium carbonate, or magnesia, and these may be used in combination of two or more. By using such plate-like inorganic particles, it is possible to increase the degree of opening and further increase the lithium ion permeability while maintaining the heat resistance of the heat resistant porous layer. The heat-resistant porous layer may further contain particles other than the plate-like inorganic particles, for example, spherical particles, but from the viewpoint of maintaining strength in the thickness direction, the heat-resistant porous layer contains the plate-like inorganic particles and It preferably consists essentially of a binder.

The average particle size of the plate-like inorganic particles is appropriately selected in consideration of the ease of forming the heat-resistant porous layer and the ease of controlling the layer thickness. The average particle size of the plate-like inorganic particles is preferably in the range of 0.01 μm to 2 μm, more preferably in the range of 0.01 μm to 0.5 μm. By setting the average particle diameter of the inorganic filler as described above, the heat-resistant porous layer can be efficiently formed with a more uniform layer thickness. Here, in the present invention, the average particle size of the plate-like inorganic particles is a volume-based D ₅₀ value obtained using a laser diffraction particle size distribution measuring device.

  In the present invention, the aspect ratio of the plate-like inorganic particles is preferably in the range of 10 to 100, and more preferably in the range of 30 to 90. By setting the aspect ratio of the plate-like inorganic particles in such a range, the strength in the thickness direction can be maintained. In the present invention, the aspect ratio means a value obtained by dividing the longest length on the surface of the plate-like inorganic particles by the thickness, and can be determined by SEM observation. In SEM observation, 50 plate-like inorganic particles are arbitrarily extracted, the aspect ratio is obtained for each, and the average value of these is used.

  The heat resistant porous layer contains a binder in addition to the plate-like inorganic particles. The binder can bind the plate-like inorganic particles to the porous film, and can also bind the particles constituting the plate-like inorganic particles. In addition, when the protective porous layer described later is composed of particles, the protective porous layer may be bound to the heat resistant porous layer.

  The binder is preferably one that is insoluble in the electrolyte solution in the non-aqueous electrolyte secondary battery. Preferred binders include styrene-butadiene copolymers; cellulose compounds such as carboxymethyl cellulose; ethylene-vinyl acetate. Copolymers; Fluorine-containing resins such as polyvinylidene fluoride (hereinafter also referred to as PVdF) and polytetrafluoroethylene; polyvinyl alcohol and the like.

  In the heat resistant porous layer, the volume ratio of the plate-like inorganic particles to the total volume of the plate-like inorganic particles and the binder is preferably in the range of 20% by volume or more and less than 100% by volume, more preferably 50% by volume or more. It is less than 100% by volume, more preferably 80% by volume or more and less than 100% by volume. By setting the volume ratio of the plate-like inorganic particles to 80% by volume or more and less than 100% by volume, it is possible to particularly improve the lithium ion permeability.

  The thickness of the heat resistant porous layer is preferably in the range of 1 μm or more and 10 μm or less, more preferably 1 μm or more and 8 μm or less, and further preferably 1 μm or more and 5 μm or less from the viewpoint of the balance between heat resistance and lithium ion permeability. is there.

The porosity of the heat-resistant porous layer can be appropriately set in consideration of heat resistance, mechanical strength, lithium ion permeability, etc., and is usually preferably in the range of 30% by volume to 85% by volume. More preferably, it is the range of 40 volume% or more and 85 volume% or less. The porosity of the heat resistant porous layer can be obtained by the following formula (2).
Pv ₂ (%) = {(Va ₂ −Vt ₂ ) / Va ₂ } × 100 (2)

Pv ₂ (%): porosity of heat-resistant porous layer (volume%)
Va ₂ : Apparent volume of heat-resistant porous layer Vt ₂ : Theoretical volume of heat-resistant porous layer

Here, Va ₂ can be calculated from the vertical, horizontal, and thickness values of the heat resistant porous layer, and Vt ₂ is the value of the weight of the heat resistant porous layer, the weight ratio of the constituent materials, and the true specific gravity of each constituent material. Can be calculated.

<Method for forming heat-resistant porous layer>
The heat resistant porous layer can be formed by coating plate-like inorganic particles and a binder on at least one surface of the porous film. Before coating, the plate-like inorganic particles and the binder may be dispersed or dissolved in a solvent to form a coating solution. When using a coating liquid, after applying a coating liquid to at least one surface of a porous film, a solvent can be removed by drying etc. and a heat resistant porous layer can be obtained.

  Examples of the solvent used in the coating liquid include N-methylpyrrolidone (hereinafter also referred to as NMP), N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, hot xylene, hexane, and the like. It is done. Various additions such as surfactants, dispersants, thickeners, wetting agents, antifoaming agents, PH preparations containing acids and alkalis are added to the coating solution in order to stabilize dispersion and improve coating properties. An agent or the like may be added. These additives are preferably those that can be removed when the solvent is removed, but are electrochemically stable in the range of use of the nonaqueous electrolyte secondary battery, do not inhibit the battery reaction, and are stable up to about 200 ° C. Any material may remain in the heat-resistant porous layer. Even when the heat-resistant porous layer is substantially composed of plate-like inorganic particles and a binder, other components such as residual solvent and additives contained in the binder are contained. Also good.

  The manufacturing method of the coating liquid, that is, the method of dissolving or dispersing the plate-like inorganic particles and the binder in the solvent includes ball mill, bead mill, planetary ball mill, vibration ball mill, sand mill, colloid mill, attritor, roll mill, high speed Examples thereof include impeller dispersion, disperser, homogenizer, high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade. Examples of the method for applying the coating liquid to the porous film include a bar coater method, a gravure coater method, a small diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, and a knife coater method. , Air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, spray coating method and the like.

  If the surface treatment is performed on the surface of the porous film before coating, the coating liquid is easily applied, and the adhesiveness between the heat-resistant porous layer after coating and the porous film may be improved. Examples of the surface treatment method include a corona discharge treatment method, a mechanical surface roughening method, a solvent treatment method, an acid treatment method, and an ultraviolet oxidation method.

  Examples of the method for removing the solvent from the coating film after coating include a method of drying at a temperature below the melting point of the porous film, a method of drying under reduced pressure, and the like.

<Protective porous layer>
In the present invention, the protective porous layer is in contact with the heat resistant porous layer and has a role of protecting the heat resistant porous layer. In particular, suppression of wear of equipment materials such as a winding roll during film production, suppression of moisture adsorption induced by the binder in the heat resistant porous layer, suppression of charging property of the plate-like inorganic particles in the heat resistant porous layer The heat-resistant porous layer can be protected by suppressing adhesion of dust and the like due to.

  The protective porous layer is preferably a layer made of particles, and can further reduce the frictional force between the sheets when manufacturing the electrode group. Here, the particles do not necessarily completely cover the surface of the heat resistant porous layer, and the particles do not have to be closely adjacent to each other. The average particle size of the particles in the protective porous layer is preferably in the range of 0.01 μm to 3 μm, more preferably in the range of 0.01 μm to 0.5 μm. By setting it as such an average particle diameter, it becomes possible to improve lithium ion permeability | transmittance, taking on the role of a protective porous layer.

  The material of the protective porous layer may be any material that is electrochemically stable. The substance is a substance that does not change even when the battery is molded into a porous film and used as a separator for a lithium ion secondary battery, and the battery is held at a charged state of 4.2 to 4.5 V for several hours. is there. Examples of such materials include polyolefins exemplified as polyethylene and polypropylene; fluorine-containing polymers exemplified as polytetrafluoroethylene and tetrafluoroethylene-hexafluoropropylene copolymers; carboxymethyl cellulose and the like. Water-soluble cellulose; polyolefin-based copolymer exemplified as an ethylene-propylene copolymer; and aromatic polyester exemplified as polyethylene terephthalate. Among these, polyolefin and fluorine-containing polymer are preferable.

The porosity in the protective porous layer is preferably in the range of 30% by volume to 80% by volume, and more preferably in the range of 50% by volume to 80% by volume. By setting it as such a porosity, it becomes possible to improve lithium ion permeability | transmittance, taking on the role of a protective porous layer. The porosity of the protective porous layer can be obtained from the following formula (3).
Pv ₃ (%) = {(Va ₃ −Vt ₃ ) / Va ₃ } × 100 (3)

Pv ₃ (%): porosity of heat-resistant porous layer (volume%)
Va ₃ : Apparent volume of heat-resistant porous layer Vt ₃ : Theoretical volume of heat-resistant porous layer

Here, Va ₃ can be calculated from the vertical, horizontal, and thickness values of the heat resistant porous layer, and Vt ₃ is the value of the weight of the heat resistant porous layer, the weight ratio of the constituent materials, and the true specific gravity of each constituent material. Can be calculated.

  The thickness of the protective porous layer is preferably in the range of 0.02 μm to 5 μm, more preferably in the range of 0.02 μm to 3 μm. By setting it as such thickness, it becomes possible to improve lithium ion permeability | transmittance, taking on the role of a protective porous layer.

<Method for forming protective porous layer>
The protective porous layer can be formed by coating the surface of the heat-resistant porous layer with a coating liquid in which particles constituting the protective porous layer are dispersed in a solvent, and then removing the solvent by drying or the like. .

  Examples of the solvent used for the coating liquid include N-methylpyrrolidone, N, N-dimethylformamide, N, N-dimethylacetamide, water, ethanol, toluene, hot xylene, hexane, and the like. Various additions such as surfactants, dispersants, thickeners, wetting agents, antifoaming agents, PH preparations containing acids and alkalis are added to the coating solution in order to stabilize dispersion and improve coating properties. An agent or the like may be added. These additives are preferably those that can be removed when the solvent is removed, but are electrochemically stable in the range of use of the nonaqueous electrolyte secondary battery, do not inhibit the battery reaction, and are stable up to about 200 ° C. If it is, it may remain in the protective porous layer.

  As a method for producing a coating liquid, that is, a method of dispersing particles constituting the protective porous layer in a solvent, a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, a high-speed impeller dispersion, Examples thereof include a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and mechanical stirring using a stirring blade. Examples of the method for coating the coating liquid on the surface of the heat resistant porous layer include a bar coater method, a gravure coater method, a small diameter gravure coater method, a reverse roll coater method, a transfer roll coater method, a kiss coater method, a dip coater method, and a knife coater. Method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, spray coating method and the like.

  Examples of the method for removing the solvent from the coating film after coating include a method of drying at a temperature below the melting point of the porous film, a method of drying under reduced pressure, and the like.

<Separator>
The laminated film of the present invention described above has almost no deterioration in strength up to about 200 ° C., is a film that maintains its form up to about 300 ° C. and is extremely excellent in heat resistance, and in a non-aqueous electrolyte secondary battery. Is particularly useful as a separator for non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries and lithium polymer secondary batteries because it can improve battery characteristics such as cycle capacity, It can also be used satisfactorily as a separator for non-aqueous electrolyte primary batteries and capacitors.

<Nonaqueous electrolyte secondary battery>
The non-aqueous electrolyte secondary battery of the present invention is a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte. It is a laminated film of the present invention. Next, the nonaqueous electrolyte secondary battery of the present invention will be described with reference to an example of a lithium ion secondary battery.

  A known technique may be used for manufacturing the lithium ion secondary battery. That is, for example, a positive electrode sheet, a separator, a negative electrode sheet, and a separator are sequentially laminated, or an electrode group obtained by stacking and winding is housed in a battery case such as a battery can, and the electrode group is impregnated with an electrolytic solution. Can be manufactured. Here, the laminated film of the present invention may be used as the separator. Further, when the positive electrode sheet, the separator, the negative electrode sheet, and the separator are laminated, the protective porous layer of the laminated film as a separator is laminated so that the protective porous layer in the laminated film of the present invention is disposed on the positive electrode sheet side. A nonaqueous electrolyte secondary battery in which the layer is disposed on the positive electrode side is obtained. By disposing the protective porous layer on the positive electrode side, the electrochemical stability of the battery can be further enhanced.

  As the shape of the electrode group, for example, a shape in which the cross section when the electrode group is cut in a direction perpendicular to the winding axis is a circle, an ellipse, a rectangle, a rectangle with rounded corners, etc. Can be mentioned. In addition, examples of the shape of the battery include a paper shape, a coin shape, a cylindrical shape, and a square shape.

<Positive electrode>
As the positive electrode sheet, one obtained by applying a positive electrode mixture containing a positive electrode active material, a conductive agent and a binder to a positive electrode current collector is usually used. The positive electrode mixture preferably includes a material capable of doping and dedoping lithium ions as a positive electrode active material, a carbon material as a conductive agent, and a thermoplastic resin as a binder.

Specifically, the positive electrode active material is a composite metal containing at least one transition metal element selected from V, Mn, Fe, Co, Ni, Cr and Ti, and an alkali metal element such as Li or Na. Oxides, preferably composite oxides based on α-NaFeO ₂ type structures, and more preferably lithium cobaltate, lithium nickelate, nickel nickelate in terms of high average discharge potential A lithium composite metal oxide in which a part of is substituted with other elements such as Mn and Co can be given. In addition, a composite oxide having a base of a spinel structure such as lithium manganese spinel can be given.

  Examples of the binder include thermoplastic resins. Specifically, polyvinylidene fluoride, vinylidene fluoride copolymer, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, ethylene-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, thermoplastic polyimide, carboxymethylcellulose, polyethylene, polypropylene, etc. Can be mentioned.

  Examples of the conductive agent include carbon materials, and specific examples include carbon black such as natural graphite, artificial graphite, coke, acetylene black, ketjen black, and the like. You may mix and use.

  Examples of the positive electrode current collector include Al and stainless steel, and Al is preferable from the viewpoints of light weight, low cost, and ease of processing. As a method of applying the positive electrode mixture to the positive electrode current collector, a method of pressure molding, pasting the positive electrode mixture using a solvent, etc., applying it on the positive electrode collector, drying and pressing Examples of the method include pressure bonding.

<Negative electrode>
The negative electrode sheet only needs to use a negative electrode material capable of doping and dedoping lithium ions at a lower potential than the positive electrode, and a negative electrode mixture containing the negative electrode material is supported on the negative electrode current collector. And those composed of the negative electrode material alone. Examples of the negative electrode material include carbon materials, chalcogen compounds (oxides, sulfides, and the like), nitrides, metals, and alloys that can be doped / undoped with lithium ions at a lower potential than the positive electrode. Moreover, you may use these negative electrode materials in mixture.

The negative electrode material is exemplified below. Specific examples of the carbon material include graphite such as natural graphite and artificial graphite, cokes, carbon black, pyrolytic carbons, carbon fiber, and fired organic polymer compound. As the oxide, specifically, an oxide of silicon represented by the formula SiO _x (where x is a positive real number) such as SiO ₂ and SiO, a formula TiO _x such as TiO ₂ and TiO (where x is x) Is a positive oxide of titanium, V ₂ O ₅ , VO _2, etc., and VO _x (where x is a positive real number), vanadium oxide, Fe ₃ O ₄ , Fe ₂ O ₃ , FeO and the like FeO _x (where x is a positive real number) Iron oxide, SnO ₂ , SnO and the like SnO _x (where x is a positive real number) Tin oxide, WO ₃ , WO _{2 and} other tungsten oxides represented by the general formula WO _x (where x is a positive real number), Li ₄ Ti ₅ O ₁₂ , LiVO ₂ (for example Li _1.1 V _0.9 O and the like containing mixed metal oxide for the ₂₎ and lithium such as titanium and / or vanadium Door can be. Specific examples of the sulfide include titanium sulfides represented by the formula TiS _x (where x is a positive real number) such as Ti ₂ S ₃ , TiS ₂ , and TiS, V ₃ S ₄ , VS _2, VS and other formulas VS _x (where x is a positive real number), vanadium sulfide, Fe ₃ S ₄ , FeS ₂ , FeS and other formulas FeS _x (where x is a positive real number) Iron sulfide, Mo ₂ S ₃ , MoS _{2 and the} like MoS _x (where x is a positive real number) molybdenum sulfide, SnS _2, SnS and other formula SnS _x (where x is Tin sulfide represented by positive real number), WS _{2 and} other formulas WS _x (where x is a positive real number), tungsten sulfide represented by SbS _x such as Sb ₂ S ₃ (where, x is antimony represented by a positive real _{number), Se 5 S 3, SeS} 2, SeS formula SeS _x (wherein such, Mention may be made of such sulfide selenium represented by a positive real number). Specific examples of the nitride include lithium-containing nitrides such as Li ₃ N, Li _3-x A _x N (where A is Ni and / or Co, and 0 <x <3). Can be mentioned. These carbon materials, oxides, sulfides, and nitrides may be used in combination, and may be crystalline or amorphous. Further, these carbon materials, oxides, sulfides and nitrides are mainly used by being supported on a negative electrode current collector.

Specific examples of the metal include lithium metal, silicon metal, and tin metal. Examples of the alloy include lithium alloys such as Li—Al, Li—Ni, and Li—Si, silicon alloys such as Si—Zn, Sn—Mn, Sn—Co, Sn—Ni, Sn—Cu, and Sn—La. In addition to tin alloys such as Cu ₂ Sb, La ₃ Ni ₂ Sn _{7 and the} like can also be mentioned. These metals and alloys are mainly used alone (for example, used in a sheet form).

  Among the negative electrode materials, a carbon material mainly composed of graphite such as natural graphite or artificial graphite is preferably used from the viewpoints of high potential flatness, low average discharge potential, and good cycleability. The shape of the carbon material may be any of a flake shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or an aggregate of fine powder.

  The negative electrode mixture may contain a binder as necessary. Examples of the binder include thermoplastic resins, and specific examples include PVdF, thermoplastic polyimide, carboxymethyl cellulose, polyethylene, and polypropylene.

  Examples of the negative electrode current collector include Cu, Ni, and stainless steel. Cu may be used because it is difficult to form an alloy with lithium and it is easy to process into a thin film. The method of supporting the negative electrode mixture on the negative electrode current collector is the same as in the case of the positive electrode. The method is a method of pressure molding, pasted using a solvent, etc., coated on the negative electrode current collector, dried, pressed and pressed. And the like.

<Electrolyte>
The electrolytic solution usually contains an electrolyte and an organic solvent. Examples of the electrolyte include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LIBF ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF ₃ ) (COCF ₃ ), Li (C ₄ F ₉ SO ₃ ), LiC (SO ₂ CF ₃ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , LiBOB (where BOB is bis (oxalato) borate). ), Lithium salts such as lower aliphatic carboxylic acid lithium salts and LiAlCl _4, and a mixture of two or more of these may be used. The lithium salt is usually selected from the group consisting of LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ and LiC (SO ₂ CF ₃ ) ₃ containing fluorine among these. Those containing at least one selected are used.

In the electrolytic solution, examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di ( Carbonates such as methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyl Ethers such as tetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N-dimethylacetate Amides such as amides; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone, or those obtained by further introducing a fluorine substituent into the above organic solvent Usually, two or more of these are mixed and used. Among these, a mixed solvent containing carbonates is preferable, and a mixed solvent of cyclic carbonate and acyclic carbonate or cyclic carbonate and ether is more preferable. The mixed solvent of cyclic carbonate and non-cyclic carbonate has a wide operating temperature range, excellent load characteristics, and is hardly decomposable even when a graphite material such as natural graphite or artificial graphite is used as the negative electrode active material. In addition, a mixed solvent containing ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is preferable. Further, in view of particularly excellent safety improving effect is obtained, it is preferable to use an electrolytic solution containing an organic solvent having a lithium salt and a fluorine substituent containing fluorine such as LiPF _6. A mixed solvent containing ethers having fluorine substituents such as pentafluoropropyl methyl ether and 2,2,3,3-tetrafluoropropyl difluoromethyl ether and dimethyl carbonate has excellent high-current discharge characteristics, preferable.

  EXAMPLES Next, although an Example demonstrates this invention further more concretely, this invention is not limited to these Examples.

Production Example 1 (Preparation of positive electrode sheet)
A lithium composite metal oxide represented by LiCoO ₂ was used as the positive electrode active material. Acetylene black was used as a conductive agent. Polytetrafluoroethylene and carboxymethylcellulose were used as the binder. Water was used as the solvent. Al foil was used as a current collector (positive electrode current collector). A positive electrode active material, a conductive agent, a binder and a solvent were mixed to obtain a positive electrode mixture. The weight ratio of positive electrode active material: conductive agent: binder: solvent in the positive electrode mixture was 92: 3: 5: 45. The weight ratio of polytetrafluoroethylene: carboxymethylcellulose in the binder is 9: 1.

  This positive electrode mixture was applied to both sides of an Al foil and dried, and after thickening with a roll press, an aluminum lead was welded to obtain a positive electrode sheet.

Production Example 2 (Preparation of negative electrode sheet)
Natural graphite was used as the negative electrode material. Carboxymethylcellulose was used as a binder. Water was used as the solvent. Copper foil was used as a current collector (negative electrode current collector). A negative electrode material, a binder, and a solvent were mixed to obtain a negative electrode mixture. The weight ratio of negative electrode material: binder: solvent in the negative electrode mixture was 98: 2: 110.

  This negative electrode mixture was applied to both sides of the copper foil and dried, and after thickening with a roll press machine, a copper lead was welded to obtain a negative electrode sheet.

Production Example 3 (Preparation of electrolyte)
As an electrolytic solution, a 16:10:74 (volume ratio) mixed organic solvent of ethylene carbonate (hereinafter also referred to as EC), dimethyl carbonate (hereinafter also referred to as DMC) and ethyl methyl carbonate (hereinafter also referred to as EMC) is used. Then, a solution of LiPF ₆ as an electrolyte dissolved to 1.3 mol / liter (hereinafter also referred to as LiPF ₆ / EC + DMC + EMC) was prepared.

Production Example 4 (Production and Evaluation of Nonaqueous Electrolyte Secondary Battery)
A positive electrode sheet, a separator, a negative electrode sheet of Production Example 2, and a separator are laminated in order, and an electrode group obtained by winding is placed in a battery can, and then impregnated with the electrolytic solution of Production Example 3 to obtain lithium. An ion secondary battery was manufactured. The secondary battery was subjected to a charge / discharge test and a cycle test under the following conditions.
<Charge / discharge test>
Test temperature 20 ° C
Charging maximum voltage 4.2V, charging time 4 hours, charging current 1mA / cm ²
Discharge minimum voltage 3.0V, constant current discharge, charging current 1mA / cm ²
<Cycle test>
Test temperature 20 ° C
Charging maximum voltage 4.2V, charging time 4 hours, charging current 15mA / cm ²
Discharge minimum voltage 3.0V, constant current discharge, charging current 10mA / cm ²
Number of cycles 50 times Discharge capacity maintenance rate (%) = 50th cycle discharge capacity / 1st cycle discharge capacity × 100

Comparative Example 1
As the porous film having a shutdown function, a polyethylene porous film was used (thickness 15 μm, porosity 50%). Α-alumina (average particle size 0.06 μm, aspect ratio 70) was used as the plate-like inorganic particles. Acrylic resin was used as a binder. Water was used as the solvent. Plate-like inorganic particles, a binder and a solvent were mixed to prepare a coating liquid (1). The weight ratio of plate-like inorganic particles: binder: solvent in the coating liquid (1) was 18: 2: 80. This coating liquid (1) was applied to one side of a porous film and dried at 70 ° C. to form a heat-resistant porous layer, whereby a comparative film 1 was obtained. A bar coater was used as the coating apparatus.
The thickness of the heat-resistant porous layer in Comparative Film 1 was 4.8 μm, and the porosity was 79% by volume. Moreover, the volume ratio of the plate-like inorganic particles to the total volume of the plate-like inorganic particles and the binder in the heat-resistant porous layer was 80% by volume.

A comparative secondary battery was produced according to Production Example 4 using the comparative film 1 as a separator.
The heat resistant porous layer in the comparative film 1 was set to the positive electrode sheet side. The comparative secondary battery was subjected to a charge / discharge test. The obtained discharge capacity was taken as 100. In addition, a cycle test was performed on the comparative secondary battery. The obtained discharge capacity retention rate was taken as 100. In addition, after performing the cycle test, the battery was disassembled and the winding state of the electrode group was confirmed, and loosening was observed.

Example 1
As the porous film having a shutdown function, a polyethylene porous film was used (thickness 15 μm, porosity 50%). Α-alumina (average particle size 0.06 μm, aspect ratio 70) was used as the plate-like inorganic particles. Acrylic resin was used as a binder. Water was used as the solvent. Plate-like inorganic particles, a binder and a solvent were mixed to prepare a coating liquid (1). The weight ratio of plate-like inorganic particles: binder: solvent in the coating liquid (1) was 18: 2: 80. This coating liquid (1) was applied to one side of a porous film and dried at 70 ° C. to form a heat resistant porous layer. A bar coater was used as the coating apparatus. The heat-resistant porous layer had a thickness of 4.6 μm and a porosity of 78% by volume. Moreover, the volume ratio of the plate-like inorganic particles to the total volume of the plate-like inorganic particles and the binder in the heat-resistant porous layer was 80% by volume.

  As the particles constituting the protective porous layer, polytetrafluoroethylene particles (average particle size 0.3 μm) were used. Water was used as the solvent. The polytetrafluoroethylene particles and the solvent were mixed and dispersed to prepare a coating liquid (2). The weight ratio of particles: solvent in this coating liquid (2) was 5:95. This coating liquid (2) was applied to the surface of the heat resistant porous layer and dried at 70 ° C. to form a protective porous layer, whereby a laminated film 1 was obtained. A bar coater was used as the coating apparatus. The thickness of the protective porous layer was 0.5 μm, and the porosity was 70% by volume.

  A lithium ion secondary battery 1 was produced in accordance with Production Example 4 using the laminated film 1 as a separator. The protective porous layer in the laminated film 1 was on the positive electrode sheet side. The lithium ion secondary battery 1 was subjected to a charge / discharge test. The obtained discharge capacity was almost 100 with respect to 100 which is that of the comparative secondary battery, and no difference in capacity was recognized. In addition, a cycle test was performed on the lithium ion secondary battery 1. The obtained discharge capacity maintenance ratio was 103 with respect to 100 which is that of the comparative secondary battery, and an improvement in the discharge capacity maintenance ratio was recognized. Further, after performing the cycle test, the battery was disassembled and the winding condition of the electrode group was confirmed, and no looseness was observed.

Example 2
A laminated film 2 was obtained in the same manner as in Example 1 except that polyethylene particles (average particle size 0.6 μm) were used as particles constituting the protective porous layer. The thickness of the protective porous layer was 0.6 μm, and the porosity was 68% by volume.

  A lithium ion secondary battery 2 was produced according to Production Example 4 using the laminated film 2 as a separator. The protective porous layer in the laminated film 2 was arranged on the positive electrode sheet side. The lithium ion secondary battery 2 was subjected to a charge / discharge test. The obtained discharge capacity was almost 100 with respect to 100 which is that of the comparative secondary battery, and no difference in capacity was recognized. In addition, a cycle test was performed on the lithium ion secondary battery 2. The obtained discharge capacity retention ratio was 104 with respect to 100, which is that of the comparative secondary battery, and an improvement in the discharge capacity retention ratio was recognized. Further, after performing the cycle test, the battery was disassembled and the winding condition of the electrode group was confirmed, and no looseness was observed.

Claims (12)

A porous film having a shutdown function;
A heat resistant porous layer containing plate-like inorganic particles and a binder, wherein the volume ratio of the plate-like inorganic particles to the total volume of the plate-like inorganic particles and the binder is 50% by volume or more and less than 100% by volume. When,
A protective porous layer whose material is polyolefin (excluding polypropylene), fluorine-containing polymer, water-soluble cellulose, polyolefin copolymer (excluding polypropylene copolymer), or aromatic polyester,
A laminated film characterized by being laminated in contact with each other. The laminated film according to claim 1, wherein the porous film having a shutdown function contains polyethylene. The laminated film according to claim 1 or 2 , wherein the material of the protective porous layer is polyethylene, a fluorine-containing polymer, water-soluble cellulose, or aromatic polyester. The laminated film according to claim 1 or 2 , wherein the material of the protective porous layer is a fluorine-containing polymer. The laminated film according to any one of claims 1 to 4 , wherein the aspect ratio of the plate-like inorganic particles is in the range of 10 or more and 100 or less. The laminated film according to any one of claims 1 to 5 , wherein a porosity of the protective porous layer is in a range of 30% by volume to 80% by volume. The laminated film according to any one of claims 1 to 6 , wherein a thickness of the porous film is in a range of 13 µm to 17 µm. The laminated film according to any one of claims 1 to 7 , wherein the heat-resistant porous layer has a thickness in a range of 1 µm to 10 µm. The laminated film according to any one of claims 1 to 8 , wherein the protective porous layer has a thickness in a range of 0.02 µm to 5 µm. The laminated film according to any one of claims 1 to 9 which is a separator. It is a non-aqueous electrolyte secondary battery which has a positive electrode, a negative electrode, the separator arrange | positioned between this positive electrode and this negative electrode, and electrolyte, Comprising: This separator is a laminated | multilayer film in any one of Claims 1-10. A non-aqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to claim 11 , wherein the protective porous layer of the laminated film as a separator is disposed on the positive electrode side.