JP2020161473A - Separator for lithium-ion battery - Google Patents

Abstract

To provide a separator capable of improving cycle life and productivity of a battery.SOLUTION: The separator for a lithium-ion battery is a non-woven fabric base material on which inorganic particles are supported. The thickness elastic modulus at the measured surface pressure of 189 kPa is 6.3 MPa or less based on the measured surface pressure of 63 kPa.SELECTED DRAWING: None

Description

The present invention relates to a separator for a lithium ion battery.

Lithium-ion batteries (hereinafter, sometimes abbreviated as "batteries") have been widely used as a power source for portable electric devices such as mobile phones, portable music players, and notebook personal computers. In recent years, its use in large equipment such as electric vehicles has been rapidly expanding. With the development of devices using lithium-ion batteries, the guaranteed life required for batteries has increased more than five times from the conventional two years to ten years or more.

Conventionally, as a separator for a lithium ion battery (hereinafter, may be abbreviated as "separator"), a polyolefin porous film having penetrating fine pores has been used. However, the polyolefin porous film has a problem that the charge / discharge capacity decreases as the charge / discharge is repeated. To solve such a problem, a separator has been proposed that enhances the deformability of the separator and enhances the followability of the electrode to expansion and contraction during charging and discharging (see, for example, Patent Document 1). However, in Patent Document 1, a high pressure of 50 kg / cm ^{2 is} applied as a deformation load, and it remains doubtful whether the deformation can be quickly followed by the low pressure due to the expansion and contraction of the electrodes.

Further, in batteries for automobile applications, a cylindrical type in which electrodes and separators are laminated and then wound in a roll shape, and a flat type in which both flat and curved ends are formed by crushing the cylindrical type, 90. A ninety-nine-fold type in which an electrode cut out in a single leaf is inserted between the separators cut into nine folds, a single-leaf laminated type in which a separator cut out in a single leaf and an electrode cut out in a single leaf are laminated. The wound body or laminated body is housed in a can or laminated battery cell. Since multiple battery cells are stored in a standard outer can / pack, the size of the battery cells is strictly determined. If the thickness of the wound or laminated body of the electrode and separator exceeds the design tolerance (tolerance), it will not fit in the outer can / pack of the battery, so strict thickness control is required for the separator and electrode. This is one of the factors that reduce the productivity of separators.

Japanese Unexamined Patent Publication No. 2000-21322

An object of the present invention is to provide a separator for a lithium ion battery, which has an excellent battery cycle life and is expected to improve productivity.

As a result of diligent research, the present inventors have invented a separator for a lithium ion battery that can solve the problem.

(1) A separator for a lithium ion battery, wherein the thickness elastic modulus at a measurement surface pressure of 189 kPa is 6.3 MPa or less based on a measurement surface pressure of 63 kPa.
(2) The separator for a lithium ion battery according to (1) above, wherein the thickness elastic modulus at a measured surface pressure of 316 kPa is 7.0 MPa or less based on a measured surface pressure of 63 kPa.
(3) The separator for a lithium ion battery according to (1) or (2) above, wherein the separator for a lithium ion battery is a non-woven fabric base material on which inorganic particles are supported.
(4) The separator for a lithium ion battery according to (3) above, wherein inorganic particles are supported on only one side of a non-woven fabric base material.

According to the present invention, in a separator for a lithium ion battery, a separator for a lithium ion battery having high deformability due to pressure can be obtained, and an effect that a lithium ion battery using the separator for a lithium ion battery has an excellent cycle life can be obtained. .. In addition, a separator for a lithium ion battery, which can be expected to improve productivity, can be obtained.

The separator for a lithium ion battery of the present invention has a thickness elastic modulus at a measurement surface pressure of 189 kPa of 6.3 MPa or less, more preferably 5.3 MPa or less, still more preferably 5.0 MPa based on a measurement surface pressure of 63 kPa. It is as follows.

Further, the thickness elastic modulus of the separator at a measurement surface pressure of 316 kPa is preferably 7.0 MPa or less, more preferably 6.5 MPa or less, and 6.0 MPa or less based on the measurement surface pressure of 63 kPa. Is even more preferable.

In the present invention, the thickness elastic modulus is defined as follows. That is, 10 separators are stacked, and the thickness of the separator laminate is measured using an outer micrometer whose measurement pressure can be arbitrarily adjusted. The thickness of the laminate at the measurement surface pressure of 63 kPa is Ta, the thickness of the laminate at the measurement surface pressure of 189 kPa is Tb, the thickness of the laminate at the measurement surface pressure of 316 kPa is Tc, and the thickness change at each measurement surface pressure from the thickness at 63 kPa. The amount is calculated by the following formula, and the thickness elastic modulus at each measurement surface pressure is calculated by dividing each measurement surface pressure by the thickness change amount. The thickness elastic modulus is calculated by the following formula.

(Equation 1)
Amount of change in thickness at measured surface pressure of 189 kPa = (Ta-Tb) / Ta
(Equation 2)
Amount of change in thickness at measured surface pressure of 316 kPa = (Ta-Tc) / Ta

(Equation 3)
Thickness elastic modulus at measurement surface pressure of 189 kPa = 189 / {(Ta-Tb) / Ta}
(Equation 4)
Thickness elastic modulus at measurement surface pressure of 316 kPa = 316 / {(Ta-Tc) / Ta}

In the present invention, as a method for adjusting each thickness elastic modulus, for example, a separator in which inorganic particles are supported on a non-woven fabric base material is used, and the type, size, etc. of the non-woven fabric base material and the inorganic particles, and / or the manufacturing process are adjusted. The method can be mentioned.

Compared to the polyolefin porous film produced by melt-stretching an olefin resin, the non-woven fabric base material, which is a laminated structure of fibers, has high responsiveness to pressure and easily deforms even at a relatively weak pressure, so that the electrode expands and contracts. Excellent followability to. In addition, due to the three-dimensional porous structure with excellent liquid absorption and retention properties, the electrolytic solution quickly follows when the separator is compressed and released, and local electrolyte shortage is unlikely to occur. Therefore, electrode deterioration due to non-uniform current density is prevented. It can be suppressed. On the other hand, the non-woven fabric base material generally has a large pore diameter, and it is difficult to prevent a short circuit due to lidendrite, so that it is difficult to function as a separator by itself. In order to improve the safety of a battery by adjusting the pore size of the non-woven fabric base material and improving the heat resistance of the separator, a separator in which inorganic particles are supported on the non-woven fabric base material is known. However, by supporting the inorganic particles having a high elastic modulus, the elastic modulus of the entire separator is also increased, and the followability of the electrode to expansion and contraction is impaired. According to the separator of the present invention, the battery using the separator of the present invention has an effect of excellent cycle life because it has a followability to expansion and contraction of the electrode and is highly deformable by pressure. In addition, a separator that can be expected to improve productivity can be obtained.

A method of adjusting the thickness elastic modulus of a separator in which inorganic particles are supported on a non-woven fabric base material will be described.

The first method is to adjust the fiber diameter of the non-woven fabric base material. By reducing the fiber diameter, the voids in the non-woven fabric base material can be easily distributed three-dimensionally, so that it is possible to suppress the occurrence of a portion having a locally high elastic modulus. In addition, by reducing the fiber diameter, the pore size of the non-woven fabric base material becomes small, and it becomes difficult for inorganic particles to enter the non-woven fabric base material. Therefore, voids in the non-woven fabric base material are maintained, and high responsiveness to pressure is maintained. Is possible. It is also effective to increase the number of voids by reducing the binder component in the non-woven fabric base material.

The second method is to adjust the non-woven fabric base material / inorganic particle ratio. By setting the ratio of the non-woven fabric base material having high pressure response to a certain level or more, it is possible to reduce the thickness elastic modulus of the entire separator.

The third method is to adjust the particle size of the inorganic particles. By increasing the particle size of the inorganic particles, it becomes difficult for the inorganic particles to enter the voids of the non-woven fabric base material, and the voids in the non-woven fabric base material are maintained. Further, the elastic modulus of the inorganic particles can be adjusted and the thickness elastic modulus of the separator can be adjusted by the primary structure and the secondary structure of the particles.

The fourth method is to adjust the method of supporting the inorganic particles. By suppressing the penetration of the inorganic particles into the non-woven fabric substrate and supporting the inorganic particles, it is possible to reduce the thickness elastic modulus of the separator. Further, it is possible to reduce the thickness elastic modulus of the separator by not performing a thickness adjusting process such as a calendar or a hot press on the separator. Inorganic particles are supported on only one side of the non-woven fabric base material, and only the side surface on which the inorganic particles are supported has a dense structure in order to suppress the penetration of the inorganic particles into the non-woven fabric base material, and the surface on the opposite side of the non-woven fabric base material. Then, in order to maintain the high responsiveness of the non-woven fabric base material, it is also effective to have a structure having many voids and to make the non-woven fabric base material an inclined structure.

By appropriately combining these methods, it is possible to adjust the thickness elastic modulus of the separator.

A separator in which inorganic particles are supported on a non-woven fabric substrate according to the present invention will be described. The constituent materials of the non-woven substrate include polyethylene terephthalate, polybutylene terephthalate and their derivatives, polyesters such as semi-aromatic polyester and total aromatic polyester, polyolefins, acrylics, polyacetals, polycarbonates, aliphatic polyketones, aromatic polyketones and fats. Group polyamide, semi-aromatic polyamide, polyimide, polyamideimide, polyphenylene sulfide, polybenzoimidazole, polyether ether ketone, polyether sulfone, poly (paraphenylene benzobisthiazole), poly (paraphenylene-2,6-benzobisoxazole) ), Resins such as polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl alcohol, polyurethane and polyvinyl chloride; cellulose and the like can be mentioned. These fibers may be used alone or in combination of two or more. Polyethylene terephthalate, which has excellent heat resistance, is particularly preferable. The ratio of the binder fiber to the constituent fiber is preferably 50% by mass or less from the viewpoint of the elastic modulus of the non-woven fabric base material.

Binder fiber is a process of raising the temperature above the softening point or melting temperature (melting point) of the binder fiber into the manufacturing process of the base material, so that the binder fiber is softened or melted and the mechanical strength of the non-woven fabric base material is improved. It is a fiber to make. For example, the non-woven fabric base material can be produced by a wet papermaking method, and the binder fibers can be softened or melted in a subsequent drying step. Further, the binder fiber can be softened or melted by subjecting the non-woven fabric base material to a thermal calendar treatment. In this step of raising the temperature, fibers that do not soften or melt, or are difficult to soften or melt, form the skeleton of the non-woven fabric base material, and maintain the shape of the non-woven fabric base material are called main fibers. The main fiber and the binder fiber can be appropriately selected from the constituent materials of the above-mentioned non-woven fabric base material in consideration of the temperature in the step of raising the temperature. Examples of the combination of the main fiber and the binder fiber include a combination of a drawn polyethylene terephthalate (PET) fiber and an undrawn PET fiber, a combination of a PET fiber and a polyolefin fiber, and the like.

As the non-woven fabric base material, for example, a non-woven fabric base material produced by a method such as a spunbond method, a melt blow method, a dry method, a wet method, or an electrospinning method can be used. In particular, a wet non-woven fabric produced by a wet papermaking method is preferable. In the wet papermaking method, fibers are dispersed in water to form a uniform papermaking slurry, and the papermaking slurry is made by a paper machine to produce a wet non-woven fabric.

As the papermaking method, for example, a long net, a circular net, an inclined wire type or the like can be used. A paper machine having one type of papermaking method selected from these papermaking methods may be used, or a combination paper machine in which two or more types of papermaking methods of the same type or different types are installed online may be used. .. Further, when the non-woven fabric base material has a multi-layer structure of two or more layers, it can be manufactured by a laminating method, a court method, or the like. The papermaking method is a method of laminating wet papers made by each papermaking method. The court method is a method in which one layer is formed and then a slurry in which fibers are dispersed is spread on the layer. In the casting method, the previously formed layer may be in a wet paper state or in a dry state. In addition, two or more layers in a dry state can be heat-sealed to form a multi-layer structure.

In addition to the fibers, a dispersant, a thickener, an antifoaming agent, etc. can be appropriately added to the papermaking slurry, and the papermaking slurry has a solid content concentration of about 0.001 to 5% by mass. To tune. This papermaking slurry is further diluted to a predetermined concentration, papermaking is performed, and the paper is dried. In the step of manufacturing the wet non-woven fabric, water flow entanglement treatment may be performed if necessary.

The wet non-woven fabric is subjected to calendar treatment, thermal calendar treatment, heat treatment and the like, if necessary. By melting the binder fibers on the surface layer of the non-woven fabric base material to partially form a film, it is possible to suppress the penetration of inorganic particles into the non-woven fabric base material. Therefore, the non-woven fabric base material is subjected to thermal calendar treatment. Is preferable. Further, in this case, the thermal calendar temperature is preferably 170 ° C. or higher from the viewpoint of the degree of melting of the binder fiber. Further, the thermal calendar temperature is preferably 230 ° C. or lower in order to suppress an increase in the internal resistance of the separator due to the film formation more than necessary.

The fiber diameter of the constituent fibers of the non-woven fabric base material is preferably 0.6 dtex or less, more preferably 0.3 dtex or less, and further preferably 0.2 dtex or less. When the fiber diameter is small, the voids in the non-woven fabric base material are easily distributed three-dimensionally, and the thickness elastic modulus of the separator is easily reduced. On the other hand, from the viewpoint of productivity of the non-woven fabric base material, the fiber diameter is preferably 0.02 dtex or more.

The basis weight of the non-woven fabric base material is preferably 4 to 30 g / m ² , and more preferably 5 to 20 g / m ² . When the basis weight is 4 g / m ² or more, it becomes easy to obtain uniformity as a non-woven fabric base material, and when it is 30 g / m ² or less, the thickness is suitable for a separator for a lithium ion battery. The basis weight means the basis weight based on the method specified in JIS P 8124: 2011.

Inorganic particles include kaolin, calcined kaolin, heavy calcium carbonate, light calcium carbonate, magnesium carbonate, zinc oxide, alumina, boehmite, aluminum hydroxide, magnesium hydroxide, titanium dioxide, barium sulfate, zinc sulfate, amorphous silica. , Calcium silicate and the like. These may be used alone or in combination of two or more. Of these, alumina, boehmite or magnesium hydroxide is preferably used from the viewpoint of thermal stability. Only one kind of inorganic particles may be used, or two or more kinds may be used in combination. Further, in order to reduce the thickness elastic modulus of the separator, the amount of inorganic particles contained in the separator is preferably 70% by mass or less, and more preferably 60% by mass or less in the total solid content of the separator. From the viewpoint of short-circuit resistance of the separator, it is preferably 30% by mass or more.

The particle size of the inorganic particles is preferably 0.1 to 10 μm, more preferably 0.3 to 7.5 μm. When the particle size is 0.1 μm or more, it becomes difficult for the inorganic particles to penetrate into the non-woven fabric base material when the inorganic particles are supported on the non-woven fabric base material, and when the particle size is 10 μm or less, the thickness is suitable as a separator. It is possible to maintain. The particle size referred to here refers to the median diameter (D50) measured by the laser diffraction / scattering method.

A binder may be used when the inorganic particles are supported on the non-woven fabric substrate. As the binder, an organic polymer is preferably used. Specific examples include, for example, styrene / butadiene copolymer, acrylonitrile / butadiene copolymer, methyl acrylate / butadiene copolymer, acrylonitrile / butadiene / styrene ternary copolymer, polyvinyl acetate, vinyl acetate / acrylic acid ester. Examples thereof include, but are not limited to, copolymers, ethylene / vinyl acetate copolymers, polyacrylic acid esters, styrene / acrylic acid ester copolymers, and organic polymers such as polyurethane. These organic polymers are preferably in the form of latex. In the present invention, the amount of the binder with respect to the total amount of the inorganic particles and the binder is preferably 2 to 15% by mass based on the solid content, from the viewpoint of the high rate characteristics of the separator and the supportability of the inorganic particles.

Various additives such as a dispersant, a wetting agent, and a thickener can be used when the inorganic particles are supported on the non-woven fabric base material as long as the effects of the present invention are not impaired.

There is no particular limitation on the method in which the inorganic particles are supported on the non-woven fabric substrate. For example, a coating liquid containing inorganic particles by an air doctor coater, a blade coater, a knife coater, a rod coater, a squeeze coater, an impregnation coater, a gravure coater, a kiss roll coater, a die coater, a reverse roll coater, a transfer roll coater, a spray coater, or the like. Inorganic particles can be supported on the non-woven fabric base material by coating and drying.

The coating amount (absolute dry coating amount) of the inorganic particles and the binder is preferably 5 to 30 g / m ² , and more preferably 10 to 20 g / m ² . When the coating amount is 5 g / m ² or more, it becomes easy to sufficiently cover the surface of the non-woven fabric base material, and it becomes easy to prevent minute short circuits. Further, when the coating amount is 30 g / m ² or less, it becomes easy to suppress an increase in the thickness of the separator.

In the separator for a lithium ion battery of the present invention, the basis weight of the separator is preferably 10 to 50 g / m ² , and more preferably 17 to 40 g / m ² . The thickness of the separator is preferably 10 to 50 μm, more preferably 15 to 40 μm. The density of the separator is preferably 0.4 to 1.2 g / cm ³ , and more preferably 0.5 to 1.0 g / cm ³ . The basis weight means the basis weight based on the method specified in JIS P 8124: 2011. The density is a value obtained by dividing the basis weight by the thickness. The thickness in this paragraph means a value measured by an outer micrometer specified in JIS B 7502: 2016, and is a thickness measured at a measurement surface pressure of 189 kPa.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples. In the examples,% and parts are all based on mass unless otherwise specified.

[Preparation of papermaking slurry]
The fibers were mixed, dissociated in water with a pulper and stirred with an agitator to prepare a uniform papermaking slurry with a concentration of 1%.

Papermaking slurry a
Fineness 0.3 dtex (average fiber diameter 5.4 μm), fiber length 3 mm drawn PET short fiber 70 parts Fineness 0.2 dtex (average fiber diameter 4.3 μm), fiber length 3 mm PET type short for single component type binder 30 parts of fiber (unstretched fiber, softening point 120 ° C, melting point 230 ° C)

Papermaking slurry b
Fineness 0.06 dtex (average fiber diameter 2.4 μm), fiber length 3 mm drawn PET short fiber 60 parts Fineness 0.2 dtex (average fiber diameter 4.3 μm), fiber length 3 mm PET type short for single component type binder 40 parts of fiber (undrawn fiber, softening point 120 ° C, melting point 230 ° C)

Papermaking slurry c
Fineness 0.06 dtex (average fiber diameter 2.4 μm), fiber length 3 mm drawn polyethylene terephthalate (PET) type short fiber 50 parts Fineness 0.2 dtex (average fiber diameter 4.3 μm), fiber length 3 mm single component type binder 50 parts of PET-based short fibers (undrawn fibers, softening point 120 ° C, melting point 230 ° C)

[Manufacturing of non-woven fabric base material A]
Using an inclined wire type paper machine, this slurry a for papermaking is made by a wet papermaking method, and PET-based short fibers for binder are adhered by a cylinder dryer at 130 ° C. to develop the strength of the non-woven fabric, and the grain size is 10 g / m ^2. It was made into a non-woven fabric. Further, using a 1-nip type thermal calendar composed of a dielectric heating jacket roll (metal thermal roll) and an elastic roll on this non-woven fabric, the conditions are that the thermal roll temperature is 200 ° C., the linear pressure is 100 kN / m, and the processing speed is 60 m / min. The non-woven fabric base material A having a thickness of 18 μm was produced by subjecting it to a thermal calendar treatment.

[Manufacturing of non-woven fabric base material B]
The slurry b for making was made using an inclined wire type paper machine, and PET-based short fibers for binder were adhered with a cylinder dryer at 130 ° C. to develop the strength of the non-woven fabric to obtain a non-woven fabric having a mesh size of 10 g / m ² . Further, using a 1-nip type thermal calendar composed of a dielectric heating jacket roll (metal thermal roll) and an elastic roll on this non-woven fabric, the conditions are that the thermal roll temperature is 200 ° C., the linear pressure is 100 kN / m, and the processing speed is 60 m / min. The non-woven fabric base material B having a thickness of 15 μm was produced by subjecting it to a thermal calendar treatment.

[Manufacturing of non-woven fabric base material C]
Using a combination machine of inclined wire and circular net, the slurry b for papermaking is made by the inclined wire method, and the slurry a for making is made by the circular net method, and the wet papers made at 5 g / m ² in each layer are made together. The non-woven fabric was laminated by the method, and PET-based short fibers for binder were adhered with a cylinder dryer at 130 ° C. to develop the strength of the non-woven fabric to obtain a non-woven fabric having a grain size of 10 g / m ² . Further, using a 1-nip type thermal calendar composed of a dielectric heating jacket roll (metal thermal roll) and an elastic roll on this non-woven fabric, the conditions are that the thermal roll temperature is 200 ° C., the linear pressure is 100 kN / m, and the processing speed is 60 m / min. The non-woven fabric base material C having a thickness of 17 μm was produced by subjecting it to a thermal calendar treatment.

[Manufacturing of non-woven fabric base material D]
Using a combination machine of inclined wire and circular net, the slurry c for papermaking is made by the inclined wire method, and the slurry b for making is made by the circular net method, and the wet papers made at 5 g / m ² in each layer are made together. The non-woven fabric was laminated by the method, and PET-based short fibers for binder were adhered with a cylinder dryer at 130 ° C. to develop the strength of the non-woven fabric to obtain a non-woven fabric having a grain size of 10 g / m ² . Further, using a 1-nip type thermal calendar composed of a dielectric heating jacket roll (metal thermal roll) and an elastic roll on this non-woven fabric, the conditions are that the thermal roll temperature is 200 ° C., the linear pressure is 100 kN / m, and the processing speed is 60 m / min. A non-woven fabric base material D having a thickness of 14 μm was produced by subjecting it to a thermal calendar treatment.

Preparation of coating liquid α As inorganic particles, 100 parts of magnesium hydroxide having a particle size of 0.7 μm was dispersed in 120 parts of a 0.3% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. of the 1% aqueous solution. , Stir well to prepare a magnesium hydroxide dispersion. Next, 3000 parts of a 0.5% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 7,000 mPa · s at 25 ° C. of the 1% aqueous solution was mixed and stirred, and a latex polymer of a 45% styrene / butadiene copolymer as a binder was further mixed and stirred. Twelve parts were mixed and stirred to prepare a coating solution α.

Preparation of coating liquid β The coating is applied in the same manner as the coating liquid α, except that 100 parts of magnesium hydroxide having a particle size of 3.0 μm is used instead of 100 parts of magnesium hydroxide having a particle size of 0.7 μm as the inorganic particles. Liquid β was prepared.

Preparation of coating liquid γ As inorganic particles, instead of 100 parts of magnesium hydroxide having a particle size of 0.7 μm, secondary particles of boehmite having an average particle size of 2.3 μm formed by aggregating primary particles having a particle size of 0.7 μm are used. A coating liquid γ was prepared in the same manner as the coating liquid α except that it was used.

Preparation of Coating Solution δ A coating solution δ was prepared in the same manner as the coating solution α, except that 60 parts of a 0.5% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 7,000 mPa · s at 25 ° C. of the 1% aqueous solution was used. ..

[Manufacturing of separator]
Example 1
A separator was produced by coating and drying the coating liquid α on the non-woven fabric base material A using a gravure coater so that the absolute dry coating amount was 12 g / m ² . The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 6.3 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 9.6 MPa.

Example 2
A coating liquid α is applied onto a PET film having a thickness of 25 μm using a gravure coater so that the absolute dry coating amount is 12 g / m ^2, and then the non-woven fabric base material A is attached to the coated surface and dried as it is. After drying, the PET film was peeled off to produce a separator. The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 5.9 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 7.5 MPa.

Example 3
A separator was produced by coating and drying the coating liquid α on the layer side of the non-woven fabric base material C produced from the papermaking slurry b using a gravure coater so that the absolute dry coating amount was 12 g / m ² . .. The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 5.4 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 6.9 MPa.

Example 4
On the non-woven fabric base material B, the coating liquid β is applied by using a gravure coater so that the amount of the coating liquid β is 5 g / m ^2, and after drying, the coating liquid α is further applied by absolute dry coating on the same coated surface. A separator was produced by coating and drying with a gravure coater so that the amount was 7 g / m ² . The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 4.7 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 5.3 MPa.

Example 5
The coating liquid γ is applied on the non-woven fabric base material B using a gravure coater so that the amount of the coating liquid γ is 5 g / m ^2, and after drying, the coating liquid γ is further applied on the same coating surface by absolute dry coating. A separator was produced by coating and drying with a gravure coater so that the amount was 7 g / m ² . The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 4.2 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 4.2 MPa.

Example 6
A coating liquid γ is applied to the layer side of the non-woven fabric base material D produced from the papermaking slurry c using a gravure coater so that the absolute dry coating amount is 5 g / m ^2, and after drying, the same coated surface is further applied. The coating liquid γ was applied and dried using a gravure coater so that the absolute dry coating amount was 7 g / m ² to produce a separator. The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 4.0 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 4.2 MPa.

Comparative Example 1
40 parts of high-density polyethylene and 60 parts of liquid paraffin having a density of 0.96 g / cm ³ were put into a twin-screw extruder and melt-kneaded at 200 ° C. After the melt was cooled and solidified, it was sandwiched between metal plates and compressed with a hot press at 200 ° C. to obtain a sheet having a thickness of 1 mm. A sheet obtained 7 × 7 times at 120 ° C. was stretched using a biaxial stretching machine, then immersed in methylene chloride to remove liquid paraffin, and then dried to produce a separator made of a polyolefin porous film F. did. The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 9.4 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 14.4 MPa.

Comparative Example 2
A separator was produced by applying the coating liquid α on the polyolefin porous film F so that the absolute dry coating amount was 3.5 g / m ² and drying. The thickness elastic modulus of the separator at the measured surface pressure of 189 kPa was 7.6 MPa, and the thickness elastic modulus at the measured surface pressure of 316 kPa was 10.5 MPa.

Comparative Example 3
A separator was produced by coating and drying the coating liquid δ on the non-woven fabric base material A using an impregnating coater so that the absolute dry coating amount was 12 g / m ² . The thickness elastic modulus of the separator at the measurement surface pressure of 189 kPa was 7.0 MPa, and the thickness elasticity at the measurement surface pressure of 316 kPa was 10.2 MPa.

<Evaluation>
[Change in thickness of electrode foil / separator laminate]
Ten samples of 50 mm × 50 mm were cut out from each separator and laminated alternately with 11 copper foils of the same size to prepare a laminated body. The thickness of the laminate was measured with a micrometer. The thickness at a measurement surface pressure of 63 kPa is Td, the thickness at a measurement surface pressure of 189 kPa is Te, the thickness at a measurement surface pressure of 316 kPa is Tf, and the thickness change rate at each measurement surface pressure from the thickness at a measurement surface pressure of 63 kPa is calculated by the following formula. did. The laminated body imitates a laminated body of an electrode and a separator, and schematically shows a change in thickness when pressure is applied to the laminated body of the electrode and the separator before inserting the battery exterior material.

(Equation 5)
Rate of change in thickness of laminated body at measurement surface pressure of 189 kPa = (1-Te / Td) x 100 [%]
(Equation 6)
Rate of change in thickness of laminated body at measurement surface pressure of 316 kPa = (1-Tf / Td) x 100 [%]

[Cycle life]
Using each separator, the positive electrode active material is LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , the negative electrode active material is artificial graphite, and the electrolytic solution is lithium hexafluorophosphate (LiPF ₆ ) ethylene carbonate and diethyl carbonate. A laminated lithium-ion battery having a design capacity of 100 mAh, which is a 1/1/1 (volume ratio) mixed solvent solution (1 mol / L) of dimethyl carbonate, was prepared.

After that, for each of the manufactured lithium-ion batteries, under room temperature conditions, 100 mA constant current charge → 4.2 V constant voltage charge → 100 mA constant current discharge when the charge current reaches 10 mA → the next cycle sequence when the charge current reaches 2.8 V. The charge and discharge were performed for 500 cycles, and the capacity reduction rate was determined as [1- (discharge capacity at the 500th cycle / discharge capacity at the 4th cycle)] × 100 (%). The evaluation was as follows.

◎ (Excellent): Capacity reduction rate is less than 10%.
◯ (Good): The capacity reduction rate is 10% or more and less than 15%.
Δ (Average): The volume reduction rate is 15% or more and less than 25%. There is a concern that the capacity will decrease during long-term use.
X (Poor): The capacity reduction rate is 25% or more. There is concern that the capacity will decrease in a relatively short period of time.

As is clear from Table 1, the separators of Examples 1 to 6 have a larger rate of change in thickness depending on the pressure when laminated with the copper foil, as compared with the separators of Comparative Examples 1 to 3. The larger the rate of change in thickness, the easier it is to adjust the thickness of the laminate of electrodes and separators depending on the pressure. Depending on the pressure adjustment during manufacturing of the laminate, it is easier to fit the laminate in the outer can / pack of the battery. Can be expected to be. As a result, the degree of freedom (allowable range) of thickness control is widened when the separator is manufactured, and productivity improvement can be expected. Further, the lithium ion batteries using the separators of Examples 1 to 6 were excellent in cycle life.

From the comparison of Examples 1 to 6, when the thickness elastic modulus at the measurement surface pressure of 316 kPa is 7.0 MPa or less based on the measurement surface pressure of 63 kPa, the thickness change rate is larger, which is preferable, and further, 6.0 MPa or less. In the case of, the thickness change rate is further large, and the cycle life is also excellent.

The separator for a lithium ion battery of the present invention can be used not only for lithium ion battery applications but also for lithium ion polymer batteries, lithium ion capacitors and the like, and further, metal ion batteries and metal ion polymer batteries using metals other than lithium. It can also be used for metal ion capacitors and the like.

Claims (4)

A separator for a lithium ion battery, wherein the thickness elastic modulus at a measurement surface pressure of 189 kPa is 6.3 MPa or less based on a measurement surface pressure of 63 kPa. The separator for a lithium ion battery according to claim 1, wherein the thickness elastic modulus at a measurement surface pressure of 316 kPa is 7.0 MPa or less based on a measurement surface pressure of 63 kPa. The separator for a lithium ion battery according to claim 1 or 2, wherein the separator for a lithium ion battery is a non-woven fabric base material on which inorganic particles are supported. The separator for a lithium ion battery according to claim 3, wherein the inorganic particles are supported on only one side of the non-woven fabric base material.