The DSC measurement may be performed on the porous substrate on which the layer such as the porous layer is not provided, or may be performed on the porous substrate after the porous layer or the like of the porous substrate on which the layer such as the porous layer is provided is removed.

Examples of the method of removing the layer on the porous substrate include a method in which a separator is washed with a good solvent of a resin. In this case, the operation of washing the separator with the good solvent of the resin, drying the separator, and measuring a weight is repeated a plurality of times, and the time point when the change in the weight of the separator disappears can be regarded as the time point when the removal of the layer is completed.

It can be confirmed that the difference in temperature between the endothermic peaks in the DSC measurement is 1.50° C. or higher by the following method.

The porous substrate not provided with the porous layer is cut out so as to be 5 mg±1 mg, and the measurement is performed using the differential scanning calorimeter. As the differential scanning calorimeter, for example, the measurement can be performed using Q20 manufactured by TA Instruments. Specifically, the DSC measurement is performed by raising the temperature from 30° C. to 200° C. at a temperature raising rate of 5° C./min (temperature raising process 1) in the nitrogen atmosphere, lowering the temperature to 30° C. at 5° C./min, and further raising the temperature to 200° C. at 5° C./min (temperature raising process 2). The first and second measurements are performed continuously, and the measurement is performed without providing a pause time between the temperature rise and the temperature drop. Then, the temperature of the endothermic peak (top peak) between 120° C. and 145° C., is determined from the DSC chart obtained by the measurement. The difference between the temperature of the endothermic peak (top peak) of the first time at from 120° C. to 145° C. and the temperature of the endothermic peak (top peak) of the second time at from 120° C. to 145° C., is calculated.

As the porous substrate, an aspect in which a microporous film containing polyethylene (in the present specification, the polyethylene microporous film is referred to as a “polyethylene microporous film”) is used is preferable. The polyethylene microporous film can be selected from the polyethylene microporous films applied to the conventional separators for non-aqueous secondary batteries, and the polyethylene microporous film having preferable mechanical properties and ion permeability are preferable.

The porous substrate may be a substrate made of the polyethylene microporous film from the viewpoint of the mechanical properties and shutdown characteristics.

As the porous substrate, a microporous film containing polyethylene and polyolefin other than polyethylene can also be used. The microporous film containing polyethylene and polyolefin other than the polyethylene is preferably a film in which the content of polyethylene in the resin component in the film is 95% by mass or more. The microporous film containing polyethylene and a polyolefin other than the polyethylene may be, for example, a microporous film containing polyethylene and polypropylene, or a microporous film containing polyethylene and polypropylene at a ratio of 95:5 (=polyethylene: polypropylene [mass ratio]).

When a microporous film containing, for example, polyethylene and polypropylene is used as the porous substrate, the difference in temperature between the endothermic peaks in the DSC measurement can be determined by determining the temperature of the endothermic peak derived from polyethylene, that is, an endothermic peak (top peak) between 120° C. and 145° C., in the DSC chart obtained by the DSC measurement. The same applies to a microporous film containing a combination of two types other than a combination of polyethylene and polypropylene.

The polyethylene contained in the polyethylene microporous film preferably has a weight-average molecular weight in the range of from 100,000 to 5,000,000. When the weight-average molecular weight is 100,000 or more, preferable mechanical properties can be secured. On the other hand, when the weight-average molecular weight is 5,000,000 or less, it is easy to form a film.

The polyethylene microporous film can be produced, for example, by the following method. That is, the method is a method of producing a microporous film by extruding a molten polyethylene resin from a T-die to form a sheet, performing a crystallization treatment on the sheet and then stretching the sheet, and performing a heat treatment on the sheet, or a method of producing a microporous member by extruding a polyethylene resin melted together with a plasticizer such as liquid paraffin from a T-die into a sheet shape, cooling and stretching the extruded resin, and then extracting a plasticizer and performing a heat treatment.

The average pore size of the porous substrate is preferably in a range of from 20 nm to 100 nm. When the average pore size of the porous substrate is 20 nm or more, ions are likely to move, and preferable battery performance is likely to be obtained. From such a viewpoint, the average pore size of the porous substrate is more preferably 30 nm or more, and still more preferably 40 nm or more. On the other hand, when the average pore size of the porous substrate is 100 nm or less, the peel strength between the porous substrate and the porous layer is improved. From such a viewpoint, the average pore size of the porous substrate is more preferably 90 nm or less, and still more preferably 80 nm or less.

Note that the average pore size of the porous substrate is a value measured using a palm porometer, and can be measured using a palm porometer (CFP-1500-A manufactured by PMI) in accordance with ASTM E1294-89, for example.

The thickness of the porous substrate is preferably in a range of from 3 μm to 25 μm from the viewpoint of obtaining preferable mechanical properties and internal resistance. In particular, the thickness of the porous substrate is more preferably in the range of from 5 μm to 20 μm.

The Gurley value (JIS P8117:2009) of the porous substrate is preferably in the range of from 50 seconds/100 ml to 400 seconds/100 ml from the viewpoint of obtaining the ion permeability:

The porosity of the porous substrate is preferably in a range of from 20% to 60% from the viewpoint of obtaining an appropriate film resistance.

The puncture strength of the porous substrate is preferably 200 g or more from the viewpoint of improving the production yield.

The porous substrate is preferably subjected to various surface treatments. By performing the surface treatment, wettability with a coating liquid for forming the porous layer described later can be improved. Specific examples of the surface treatment include a corona treatment, a plasma treatment, a flame treatment, an ultraviolet irradiation treatment, and the like, and the treatment can be performed in the range in which the characteristics of the porous substrate are not impaired.

(Porous Layer)

The porous layer in the present disclosure contains a resin (hereinafter, also referred to as a specific resin) having at least one bonding group selected from the group consisting of an amide bond, an imide bond, and a sulfonyl bond only on one side of the porous substrate. The porous layer in the present disclosure preferably contains inorganic particles, and may further contain other components such as resins other than the specific resin and additives as necessary.

The porous layer has a structure in which a plurality of pores are provided inside the layer and the plurality of pores are connected to each other, and is configured to pass a gas or a liquid from one side to the other side of the layer.

-Resin

The porous layer contains at least one type of resin (specific resin) having at least one bonding group selected from the group consisting of the amide bond, the imide bond, and the sulfonyl bond. Since the resin has the bonding group selected from the amide bond, the imide bond, and the sulfonyl bond, the affinity for the porous substrate (in particular, a polyethylene film) is improved, and the affinity with the electrolytic solution is also high, so permeability of the electrolytic solution into the porous layer is improved. As a result, in the separator for a non-aqueous secondary battery of the present disclosure, the permeation of the electrolytic solution easily proceeds rapidly, and the separator that is well impregnated with the electrolytic solution is easily obtained. As a result, this contributes to improvement of battery characteristics when the secondary battery is produced.

The specific resin in the present disclosure is a polymer having at least one bonding group selected from the group consisting of the amide bond, the imide bond, and the sulfonyl bond, and may have any structure as long as it is a polymer having the above bonding group. Note that each of the resins in the present disclosure may be contained singly or in combination of two or more kinds thereof.

Examples of the resin having the amide bond include nylon, wholly aromatic polyamide, polyamideimide, and the like.

Examples of the resin having the imide bond include polyamideimide, polyimide, and the like.

Examples of the resin having a sulfonyl bond include polysulfone, polyethersulfone, and the like.

Among the specific resins, the wholly aromatic polyamides are preferable from the viewpoint of having good affinity for a porous substrate (particularly, a polyethylene film) and an electrolytic solution, and also from the viewpoint of having excellent heat resistance. The wholly aromatic polyamide can be dissolved in a polar organic solvent represented by an amide-based solvent at an appropriate concentration. Therefore, a solution (coating liquid) in which the wholly aromatic polyamide is dissolved in an organic solvent is applied onto a porous substrate including a polyethylene microporous film, and the coating film is solidified, washed with water, and dried, so the porous layer can be easily formed. In addition, it is easy to control the porous structure. Furthermore, since the coating liquid easily enters the pores of the porous substrate, the electrolytic solution impregnation property of the porous substrate can also be enhanced.

In addition, since the melting point of the wholly aromatic polyamide is 200° C. or higher, the heat resistance of the separator is enhanced, and the safety of the secondary battery is improved.

The wholly aromatic polyamide includes meta-polyamide (in the present specification, also referred to as meta-aramid) and para-polyamide. Among these, the meta-polyamide is more suitable than the para-polyamide in that the porous layer is easily formed from the viewpoint of crystallinity:

Examples of the meta-polyamide include polymethaphenylene isophthalamide and the like. In addition, examples of the para-polyamide include copolyparaphenylene 3.4′oxy dipheny lene terephthalamide, polyparapheny lene terephthalamide, and the like.

As the wholly aromatic polyamide, a commercially available product on the market may be used. Examples of commercially available products include CONEX (registered trademark: meta type) manufactured by TEIJIN CORPORATION, TECHNORA (registered trademark: para-type), Twaron (registered trademark: para type), and the like.

The content of the resin (preferably the wholly aromatic polyamide) in the porous layer is preferably 10% by mass to 40% by mass, and more preferably 15% by mass to 35% by mass with respect to the total solid content of the porous layer.

-Particles

The porous layer preferably contains particles, and the particles include both inorganic particles and organic particles. The porous layer preferably contains inorganic particles. Note that the particles in the porous layer may be contained singly or in mixture of two or more kinds thereof.

Inorganic particles contained in the porous layer can improve heat resistance, reduce film resistance (improvement in easiness of impregnation of electrolytic solution and easiness of formation of pores), and reduce a friction coefficient.

Examples of the inorganic particles include metal oxides such as alumina, zirconia, yttria, ceria, magnesia, titania, and silica: metal nitrides such as boron nitride and aluminum nitride: metal salts such as calcium carbonate and barium sulfate: metal hydroxides such as magnesium hydroxide; and the like.

Among these, as the inorganic particles, divalent metal-containing particles are preferable, and divalent metal sulfate particles or divalent metal hydroxide particles are more preferably from the viewpoint of improving the heat resistance, reducing the film resistance (improvement in easiness of impregnation of electrolytic solution and easiness of formation of pores), and reducing the friction coefficient. For example, magnesium-containing particles or barium-containing particles are preferred.

As the magnesium-containing particles, particles of magnesium sulfate, magnesium hydroxide, magnesium oxide, and the like are preferable, and particles of magnesium hydroxide are more preferable.

As the barium-containing particles, particles of barium sulfate are preferable.

The average primary particle diameter of the inorganic particles is preferably from 0.01 μm to 2.0 μm. When the average primary particle diameter is 0.01 μm or more, a porous structure is easily formed during the manufacturing the separator. In addition, when the average primary particle diameter is 2.0 μm or less, it is advantageous for thinning the porous layer, and the packing density of the inorganic particles and the resin in the heat-resistant porous layer is increased.

The average primary particle diameter of the inorganic particles is more preferably from 0.02 μm to 1.5 μm, and still more preferably from 0.03 μm to 0.9 μm.

The average primary particle diameter is obtained by measuring major diameters of 100 inorganic particles randomly selected in observation with a scanning electron microscope (SEM), and averaging the major diameters of 100 inorganic particles. The sample to be subjected to the observation of the SEM is inorganic particles as a material of the heat-resistant porous layer or inorganic particles taken out from the separator. The method of taking out the inorganic particles from the separator is not limited, and examples thereof include a method in which the separator is heated to about 800° C. to remove the binder resin and take out the inorganic particles, and a method in which the separator is immersed in an organic solvent to dissolve the binder resin with the organic solvent and take out the inorganic particles.

When the average primary particle diameter of the inorganic particles is small, or when the aggregation of the inorganic particles is remarkable and the major axis of the inorganic particles cannot be measured, the specific surface area of the inorganic particles is measured by the BET method, and assuming that the inorganic particles are true spheres, the particle diameter is calculated from the specific gravity and the specific surface area of the inorganic particles according to the following formula.

Average primary particle diameter (μm)=6+ [specific gravity (g/cm³)× BET specific surface area (m²/g)]

Note that in the measurement of the specific surface area by the BET method, an inert gas is used as an adsorbent, and the surface of the inorganic particles is adsorbed at a boiling point temperature (−196° C.) of liquid nitrogen. The amount of gas adsorbed to the sample is measured as a function of the pressure of the adsorbent, and the specific surface area of the sample is determined from the adsorption amount.

The shape of the inorganic particles is not limited, and may be a sphere, a shape close to a sphere, a plate shape, or a fibrous shape.

Other Components

The porous layer in the present disclosure can contain other components such as resins other than the specific resin and additives as necessary, in addition to the above components. Any of the other components can be used singly or in mixture of two or more kinds thereof.

The resin other than the specific resin may be appropriately selected from any known resins according to the purpose or the case as long as the effect of the present disclosure is not significantly impaired.

Examples of the additive include a dispersant such as a surfactant, a wetting agent, an antifoaming agent, a pH adjusting agent, and the like.

The porous layer can be formed by preparing a coating liquid for forming the porous layer and applying the coating liquid to the porous substrate. The coating can be performed by a coating method of a method of pressing a roll or the like against the porous substrate for coating, such as a method using a reverse coater, from the viewpoint of adjusting the contact angle to the above range.

-Characteristics of Porous Layer

[Thickness]

The thickness of the porous layer provided on one side of the porous substrate is preferably from 0.3 μm to 5.0 μm. When the thickness of the porous layer is 0.3 μm or more, a smooth and homogeneous layer is obtained, and the cycle characteristics of the battery are further improved. From the same viewpoint, the thickness of one side of the porous layer is more preferably 1.5 μm or more.

On the other hand, when the thickness of one side of the porous layer is 5.0 μm or less, the ion permeability is further improved, and the load characteristics of the battery are more excellent. From the same viewpoint, the thickness of one side of the porous layer is more preferably 4.0 μm or less, still more preferably 3.0 μm or less, and particularly preferably 2.5 μm or less.

[Porosity]

The porosity of the porous layer is preferably in a range of from 30% to 80%. When the porosity is 80% or less, it is easy to secure mechanical properties, the surface opening ratio does not become too high, and it is suitable for securing adhesive force. On the other hand, when the porosity is 30% or more, the ion permeability is further improved.

The porosity(s) is a value obtained from the following formula.

ε={1-Ws/(ds·t)}×100

In the formula, ε represents porosity (%), Ws represents basis weight (g/m²), ds represents true density (g/cm³), and t represents film thickness (μm).

[Average Pore Size]

The average pore size of the porous layer is preferably in a range of from 10 nm to 300 nm. When the average pore size is 300 nm or less, nonuniformity of pores is suppressed, adhesion points are relatively uniformly scattered, and adhesiveness is further improved. In addition, when the average pore size is 300 nm or less, uniformity of movement of ions is high, and cycle characteristics and load characteristics are further improved. On the other hand, if the average pore size is 10 nm or more, when the porous layer is impregnated with the electrolytic solution, the phenomenon that the resin constituting the porous layer swells and closes the pores, so that ion permeability is hindered is unlikely to occur.

The average pore size (diameter, unit: nm) of the porous layer is calculated from the following formula using the pore surface area S of the porous layer composed of the wholly aromatic polyamide calculated from the nitrogen gas adsorption amount and the pore volume V of the porous layer calculated from the porosity, assuming that all pores are cylindrical.

d=4·V/S

In the formula, d represents an average pore size (nm) of the porous layer, V represents a pore volume per 1 m²of the porous layer, and S represents a pore surface area per 1 m²of the porous layer.

In addition, the pore surface area S per 1 m²of the porous layer is determined by the following method.

The specific surface area (m²/g) of the porous substrate and the specific surface area (m²/g) of the composite film obtained by laminating the porous substrate and the porous layer are measured by applying the BET equation by a nitrogen gas adsorption method. Each specific surface area is multiplied by each basis weight (g/m²) to calculate each pore surface area per 1 m². Subsequently, the pore surface area per 1 m²of the porous substrate is subtracted from the pore surface area per 1 m²of the separator to calculate the pore surface area S per 1 m²of the porous layer.

-Characteristics of Separator

[Macmillan Number (Mn)]

From the viewpoint of ion permeability, the separator for a non-aqueous secondary battery of the present disclosure preferably has a Macmillan number (Mn) of 20 or less, which is obtained by the following formula.

The Macmillan number is an index of ion permeability, and is a value obtained by dividing the conductivity of only the electrolytic solution by the conductivity when the separator is impregnated with the electrolytic solution. That is, when Mn is too large, the ion permeability becomes insufficient.

Mn is more preferably 15 or less, still more preferably 12 or less, and still more preferably 10 or less. The lower limit value of Mn can be 1 or more, and is preferably 4 or more.

Mn=(σe)/(σs)

σs=t/Rm

σe represents a conductivity (S/m) of an electrolytic solution at 20° C., in which 1 mol/l of LiPF₆is dissolved in a mixed solvent of ethylene carbonate and propylene carbonate (mixing ratio 1:1 [mass ratio]). σe is a value measured by an electric conductivity meter CM-41X and an electric conductivity cell CT-5810B manufactured by DKK-TOA CORPORATION.

σs represents a conductivity (S/m) of the separator impregnated with the electrolytic solution at 20° C. os is a value measured by dividing the film thickness by the film resistance.

τ represents a film thickness (m). The film thickness is a value obtained by a method similar to the following [thickness].

Rm represents the film resistance (ohm·cm²) of the separator. Rm is a value obtained by a method similar to the following “[Film resistance (ion permeability)]”.

[Thickness]

The separator for a non-aqueous secondary battery of the present disclosure preferably has a thickness of from 7.5 μm to 20 μm.

When the thickness of the separator is 7.5 μm or more, it is easy to maintain sufficient strength capable of handling the separator. In addition, when the thickness of the separator is 20 μm or less, the ion permeability can be favorably maintained, the dischargeability and low-temperature characteristics of the battery can be easily maintained, and the energy density of the battery can be favorably maintained.

Among them, the thickness of the separator is more preferably 7.6 μm to 14 μm for the same reason.

The thickness is a value measured with a cylindrical measurement terminal having a diameter of 5 mm using a contact type thickness meter (LITEMATIC manufactured by Mitutoyo Corporation). During the measurement, a load of 0.01N is adjusted to be applied, arbitrary 20 points within 10 cm×10 cm are measured, and an average value thereof is calculated.

[Film Resistance (Ion Permeability)]

The film resistance of the separator is preferably in the range of from 1 ohm·cm²to 10 ohm·cm²from the viewpoint of securing the load characteristics of the battery.

The film resistance refers to a resistance value in a state where the separator is impregnated with an electrolytic solution, and is a value measured by an alternating current method. The film resistance is measured at 20° C. using the electrolytic solution in which 1 mol/l of LiPF₆is dissolved in a mixed solvent of ethylene carbonate and propylene carbonate (mixing ratio 1:1 [mass ratio]). A detailed measurement method will be described in Examples.

It is preferable that in the porous substrate, a resin is provided inside a pore of the porous substrate and on a surface of the porous substrate on a side opposite to a side having the porous layer. As a result, the entire separator can be impregnated with the electrolytic

Solution

The phrase “resin is provided inside a pore of the porous substrate” refers to a state in which a part or all of the inside of the pores of the porous substrate is impregnated with the resin.

The phrase “resin is provided on a surface of the porous substrate on a side opposite to a side having the porous layer” refers to a state in which, when a coating liquid for forming a porous layer is applied to one side of the porous substrate, the resin in the coating liquid penetrates from one side of the porous substrate and passes through the inside of pores in the porous substrate, and the resin reaches the other side, so the presence of the resin can be confirmed. The presence of the resin can be determined by the fact that the contact angle after applying the coating liquid for forming the porous layer is lower than the contact angle of the surface of the porous substrate before applying the coating liquid for forming the porous layer.

Whether or not the resin adheres to the inner surfaces of the pores of the porous substrate can be confirmed by energy dispersive X-ray spectroscopy (Energy Dispersive X-ray Spectroscopy (SEM-EDX)), secondary ion mass spectrometry (Secondary Ion Mass Spectrometry (SIMS)), or X-ray photoelectron spectroscopy (Electron Spectroscopy for Chemical Analysis (ESCA)), observation by staining a resin, and the like.

<Non-aqueous Secondary Battery>

The non-aqueous secondary battery according to the present disclosure includes: a positive electrode: a negative electrode: the separator for a non-aqueous secondary battery described above disposed between the positive electrode and the negative electrode; and an electrolytic solution in which a lithium salt is dissolved in a solvent containing a cyclic carbonate in an amount of 90% by mass or more based on a total mass of the solvent, in which an electromotive force is obtained by doping and dedoping lithium.

The dope means occlusion, support, adsorption, or insertion, and means a phenomenon in which lithium ions enter an active material of an electrode such as a positive electrode.

The non-aqueous secondary battery according to the present disclosure is preferably a lithium ion secondary battery having a structure in which a separator is disposed between a positive electrode and a negative electrode, and includes (1) a secondary battery in which battery elements such as a positive electrode, a negative electrode, and a separator are enclosed in an exterior material together with an electrolytic solution, and (2) an all-resin battery in which a resin is used for all battery elements including an electrode.

For the secondary battery of the above (1), the material described in paragraphs from 0056 to 0061 of WO 2008/062727 A and the like can be referred to.

The all-resin battery of the above (2) is a secondary battery in which an electrode active material covered with a gel-like polymer that has absorbed an electrolytic solution, a conductive auxiliary agent, conductive fibers, and the like are mixed to form a mixture, a mixture for the positive electrode and a mixture for the negative electrode are stacked with a separator interposed therebetween, and a current collector is more preferably provided on surfaces of the mixtures.

The positive electrode may be a positive electrode layer obtained by molding a mixture obtained by mixing a positive electrode active material, a gel-like electrolyte covering the positive electrode active material, and a conductive fiber. The positive electrode layer may further contain a conductive auxiliary agent.

The negative electrode may be a negative electrode layer obtained by molding a mixture obtained by mixing a negative electrode active material, a gel-like electrolyte covering the negative electrode active material, and a conductive fiber. The negative electrode layer may further contain a conductive auxiliary agent.

Examples of the positive electrode active material include lithium-containing transition metal oxides or the like, and examples thereof include LiCoO₂, LiNiO₂, LiMn_1/2Ni_1/202, LiCo_1/3Mn_1/3Ni_1/3O₂, LiMn₂₀₄, LiFePO₄, LiCo_1/2Ni_1/2O₂, LiAl_1/4Ni_3/4O₂, and the like.

Examples of the negative electrode active material include hard carbon (non-graphitizable carbon).

Examples of the gel-like electrolyte include a gel-like polymer (polymer gel) in which an electrolytic solution is absorbed.

The electrolytic solution is a solution in which a lithium salt is dissolved in a non-aqueous solvent.

Examples of the lithium salt include LiPF₆, LiBF₄, LiClO₄, and the like.

Examples of the non-aqueous solvent include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, and the like. The non-aqueous solvent may be used singly or in mixture of two or more kinds thereof. As the non-aqueous solvent, a non-aqueous solvent having a high relative permittivity and a low melting point is preferable, a cyclic carbonate is more preferable, and a mixed solvent containing a cyclic carbonate in an amount of 90% by mass or more with respect to the total mass of the non-aqueous solvent is more preferable.

The cyclic carbonate-based non-aqueous solvent such as EC and PC is a solvent having a relatively high viscosity.

An electrolytic solution containing a cyclic carbonate-based non-aqueous solvent such as EC and PC is preferable from the viewpoint of easily retaining a large amount of electrolyte and excellent electrochemical stability. Conventionally, when the electrolytic solution does not necessarily have sufficient permeability into the separator, the electrolytic solution to be impregnated becomes insufficient, and therefore a high-concentration electrolytic solution containing a large amount of electrolyte is generally used after being diluted. In this respect, since the non-aqueous secondary battery of the present disclosure includes the separator for a non-aqueous secondary battery of the present disclosure having excellent electrolyte permeability as described above, a cyclic carbonate having a relatively high viscosity can be used as the solvent. In addition, a high-concentration electrolytic solution using a cyclic carbonate can be used for the non-aqueous secondary battery, and the impregnation property of the electrolytic solution is excellent, so the battery characteristics are excellent.

EXAMPLES

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples unless it goes beyond the gist of the present invention. Note that unless otherwise specified, “parts” are on a mass basis.

(Measurement)

A porous substrate and a separator produced in Examples and Comparative Examples described later were subjected to the following measurement.

[Film Thickness]

The thicknesses (μm) of the porous substrate and the separator were determined by measuring 20 points with a contact type thickness meter (Mitutoyo Corporation, LITEMATIC VL-50) and averaging the measured values. The measurement terminal used was a cylindrical terminal with a diameter of 5 mm, and was adjusted so that a load of 0.01 N was applied during the measurement.

[Film Thickness of Porous Layer]

The difference between the thickness of the separator and the thickness of the porous substrate was defined as the film thickness of the porous layer.

[Film Resistance]

The produced separator is cut into a sample piece having a size of 2.6 cm×2.0 cm. An aluminum foil having a thickness of 20 μm is cut into a size of 2.0 cm×1.4 cm, and a lead tab is attached thereto. Two aluminum foils are prepared, and the cut sample piece is sandwiched between the aluminum foils so that the aluminum foil is not short-circuited. The sample piece is impregnated with an electrolytic solution in which 1 mol/l of LiPF₆is dissolved in a mixed solvent of ethylene carbonate and propylene carbonate (mixing ratio 1:1 [mass ratio]) as an electrolytic solution. This is sealed in an aluminum laminate pack under reduced pressure so that the tab comes out of the aluminum pack to form a cell. Such cells are prepared so that one, two, and three sample pieces (separators) are provided in an aluminum foil. This cell is placed in a thermostatic bath at 20° C., and the resistance of the cell is measured at an amplitude of 10 mV and a frequency of 100 kHz by an AC impedance method. The measured resistance value of the cell is plotted with respect to the number of separators, and the plot is linearly approximated to obtain a gradient. The gradient was multiplied by the electrode area of 2.0 cm×1.4 cm to determine the film resistance (ohm·cm²) per separator.

[Macmillan Number (Mn)]

The Macmillan number (Mn) was calculated from the following formula.

Mn=(σe)/(σs)

σs=t/Rm

In the formula, σe represents a conductivity (S/m) of an electrolytic solution at 20° C. obtained by dissolving 1 mol/l of LiPF₆in a mixed solvent (mixing ratio 1:1 [mass ratio]) of ethylene carbonate and propylene carbonate, os represents a conductivity (S/m) of the separator impregnated with the electrolytic solution at 20° C., t represents a film thickness (m), and Rm represents a film resistance (ohm·cm²) of the separator. σe was measured by an electric conductivity meter CM-41X and an electric conductivity cell CT-58101B manufactured by DKK-TOA CORPORATION, and os was measured by dividing the film thickness by the film resistance. τ and Rm were measured in the same manner as in the above [Film thickness of porous layer] and [Film resistance], respectively.

[DSC Measurement]

The porous substrate before coating was cut out so as to be 5 mg+1 mg, and the DSC measurement was performed using a differential scanning calorimeter (manufactured by TA Instruments, Q20).

The DSC measurement was performed by raising the temperature from 30° C. to 200° C. at a temperature raising rate of 5° C./min in a nitrogen atmosphere (temperature raising process 1), then lowering the temperature to 30° C. at 5° C./min, and further raising the temperature to 200° C. at 5° C./min (temperature raising process 2). The first measurement and the second measurement were continuously performed, and the measurement was performed without providing a pause time between the temperature rise and the temperature drop. Then, the temperature of the endothermic peak (top peak) at from 120° C. to 145° C. was obtained from the obtained DSC chart. The difference between the top peak temperatures at from 120° C. to 145° C. (difference in temperature between endothermic peaks) was calculated from the DSC chart of the first temperature increase and the DSC chart of the second temperature increase.

[Test Production of Non-Aqueous Secondary Battery]

lithium cobalt oxide (LiCoO₂: manufactured by Nippon Chemical Industrial Co., Ltd.) powder, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.: trade name Denka Black), and polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) are kneaded using a N-methyl-2 pyrrolidone solvent so that the lithium cobalt oxide powder is 94 parts by mass, acetylene black is 3 parts by mass, and polyvinylidene fluoride is 3 parts by mass, thereby preparing a slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried, and then pressed to obtain a positive electrode having a thickness of 100 μm.

Hard carbon (Belfine LN-0001: AT Electrode) powder, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.: trade name Denka Black), and polyvinylidene fluoride (manufactured by Kureha Chemical Co., Ltd.) are kneaded using a N-methyl-2 pyrrolidone solvent so that the hard carbon powder is 87 parts by mass, the acetylene black is 3 parts by mass, and the polyvinylidene fluoride is 10 parts by mass, thereby preparing a slurry. The obtained slurry was applied onto a copper foil having a thickness of 18 μm, dried, and then pressed to obtain a negative electrode having a thickness of 90 μm.

The positive electrode and the negative electrode were opposed to each other with a separator interposed therebetween. This was impregnated with an electrolytic solution and sealed in an exterior made of an aluminum laminate film to prepare a non-aqueous secondary battery: Here, as the electrolytic solution, a solution obtained by dissolving 1 mol/l of LiPF₆in a mixed solvent of ethylene carbonate and propylene carbonate (mixing ratio 1:1 [mass ratio]) was used as it was.

Here, this prototype battery has a positive electrode area of 5.0× 3.0 cm², a negative electrode area of 5.2× 3.2 cm², and a set capacity of 10 mAh (range of from 4.2 V to 2.5 V).

(Evaluation)

[Dischargeability]

Dischargeability evaluation was performed using the battery produced by the method as described above. A charge/discharge cycle of constant current/constant voltage charge at 1 mA and 4.2 V for 15 hours and constant current discharge at 1 mA and 2.5 V was performed for 5 cycles, and a discharge capacity obtained at a second cycle was divided by a discharge capacity of a battery at a fifth cycle, the obtained numerical value was used as an index of the dischargeability, and evaluation was performed according to the following evaluation criteria.

- A: 85% or more
- B: 70% or more but less than 85%
- C: less than 70%
  
  [Heat Resistance]

The produced separator is cut out by 18 cm (machine direction (MD))×6 cm (transverse direction (TD)). Locations (point A and point B) of 2 cm and 17 cm from the top are marked on a line that divides TD into two equal parts. Further, marks are made at locations (point C and point D) of 1 cm and 5 cm from the left on a line that divides MD into two equal parts. A clip is attached thereto (the location where the clip is attached is within 2 cm from the upper portion of the MD), hung in an oven adjusted to 120° C., and heat-treated for 60 minutes under no tension. Lengths between the two points AB and CD were measured before and after the heat treatment, and the thermal shrinkage ratio was obtained from the following formula.

MD thermal shrinkage ratio (%)={(length of AB before heat treatment-length of AB after heat treatment)/length of AB before heat treatment}×100

TD thermal shrinkage ratio (%)={(length of CD before heat treatment-length of CD after heat treatment)/length of CD before heat treatment}×100

<Evaluation Criteria>

- A: MD thermal shrinkage ratio and TD thermal shrinkage ratio are both less than 5%.
- B: Either MD thermal shrinkage or TD thermal shrinkage is 5% or more.

Example 1

Meta-wholly aromatic polyamide (meta-aramid) was dissolved in dimethylacetamide (DMAc) so as to have a concentration of 4.5% by mass, and barium sulfate particles (average primary particle diameter: 0.05 μm) were further mixed while stirring to obtain a coating liquid A. Subsequently, the coating liquid A was applied to one side of a polyethylene microporous film A (thickness: 7 μm) with a reverse roll coater to form a coating film. A difference in temperature between endothermic peaks of the polyethylene microporous film A measured by DSC was 1.77° C.

Then, the coating film was immersed in a coagulation liquid (DMAc:water=50:50 [mass ratio], liquid temperature 40° C.) to solidify a coating layer. Next, the coating layer was washed in a water washing tank having a water temperature of 40° C. and dried. As a result of evaluation, a ratio of meta-aramid: barium sulfate in a porous layer was 20:80 (mass ratio). In this way, a separator in which a porous layer having a thickness of 2.2 μm was formed on one side of a polyethylene microporous film was prepared.

Example 2

A separator was produced in the same manner as in Example 1, except that the barium sulfate particles were changed to magnesium hydroxide particles (average primary particle diameter: 0.88 μm) in Example 1.

Example 3

In Example 1, a separator was prepared in a manner similar to Example 1 except that the polyethylene microporous film A was changed to a polyethylene microporous film B (thickness 7 μm, and temperature in difference between endothermic peaks by DSC measurement: 3.53° C.).

Example 4

In Example 1, a separator was prepared in a manner similar to Example 1 except that the polyethylene microporous film A was changed to a polyethylene microporous film C (thickness 7 μm, and temperature in difference between endothermic peaks by DSC measurement: 2.57° C.).

Example 5

In Example 1, a separator was prepared in the same manner as in Example 1 except that the coating liquid was prepared without adding barium sulfate particles, and a separator having a porous layer having a film thickness of 0.6 μm formed on one side of the polyethylene microporous film was prepared.

Comparative Example 1

In Example 1, a separator was prepared in the same manner as in Example 1 except that the coating liquid A was applied to both sides of the polyethylene microporous film A to form a coating film on the front and back of the polyethylene microporous film A.

Comparative Example 2

A polyvinylidene fluoride type resin (VDF-HFP copolymer, VDF:HFP (molar ratio)=97.6:2.4, weight-average molecular weight 1,130,000) was dissolved in a mixed solvent (DMAc:TPG=80:20 [mass ratio]) of dimethylacetamide (DMAc) and tripropylene glycol (TPG) so as to have a concentration of 4% by mass, and barium sulfate particles (average primary particle diameter: 0.05 μm) were further stirred and mixed to obtain a coating liquid P.

The coating liquid P was applied to one side of the polyethylene microporous film B with a reverse roll coater to form a coating film. The coating film was immersed in a coagulation liquid (DMAc:TPG:water=30:8:62 [mass ratio], liquid temperature 40° C.) to solidify the coating film. Subsequently, the coating film was washed with a water washing tank having a water temperature of 40° C. and dried. In this way, the separator in which a porous layer was formed on one side of the polyethylene microporous film was obtained.

Comparative Example 3

In example 1, a separator was prepared in a manner similar to Example 1 except that the polyethylene microporous film A was changed to a polyethylene microporous film D (thickness 7 μm, and temperature in difference between endothermic peaks by DSC measurement: 1.25° C.).

Comparative Example 4

In example 1, a separator was prepared in a manner similar to Example 1 except that the polyethylene microporous film A was changed to a polyethylene microporous film E (thickness 10.4 μm, and temperature in difference between endothermic peaks by DSC measurement: 1.45° C.).

Comparative Example 5

In example 1, a separator was prepared in a manner similar to Example 1 except that the polyethylene microporous film A was changed to a polyethylene microporous film F (thickness 7 μm, and temperature in difference between endothermic peaks by DSC measurement: 0.89° C.).