JP2014175075A - Metal ion secondary battery separator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal ion secondary battery separator ensuring excellent micro short circuit prevention and high rate characteristics, when manufacturing a metal ion secondary battery separator having especially excellent heat resistance.SOLUTION: In a metal ion secondary battery separator where an inorganic pigment is carried on a nonwoven fiber base material, melting point of the constituent fiber of the nonwoven fiber base material is 200°C or more, dehydration temperature of the inorganic pigment is 250°C or more, and the ratio s/n of the maximum pore diameter s of the separator for the maximum pore diameter n of the nonwoven fiber base is 1/5-1/20.

Description

  The present invention relates to a metal ion secondary battery separator.

  A metal ion secondary battery, which is one of electrochemical elements, has a feature of high energy density. For example, a lithium ion secondary battery, which is one of them, is a mobile phone, a portable music player, a notebook personal Widely used as a power source for portable electric devices such as computers. In addition, the movement to use lithium ion secondary batteries is spreading in large equipment such as electric bicycles, hybrid cars, and electric cars. In addition, other metal ion secondary batteries such as sodium ion secondary batteries have attracted attention. For this reason, metal ion secondary batteries are required to have high-rate discharge characteristics (high rate characteristics) and repetitive characteristics (cycle characteristics), but metal ion secondary batteries are generally non-aqueous batteries. It is known that there is a higher risk of smoke, ignition, rupture, etc., and there is a demand for improved safety.

  In a metal ion secondary battery, the risk of smoke generation increases due to temperature rise due to external heat, overcharge, internal short circuit, external short circuit, and the like. These can be prevented to some extent by an external protection circuit. In addition, the polyolefin resin porous film used as a metal ion secondary battery separator melts at around 120 ° C, and the pores are blocked to block current and ion flow, thereby suppressing battery temperature rise. Is done. This is called a shutdown function. However, when the temperature rises due to external heat or when a chemical reaction occurs inside the battery due to the temperature rise, the battery temperature further rises even if the shutdown function works, and when the battery temperature reaches 150 ° C or higher, The porous film contracted, causing an internal short circuit, which could cause ignition.

  As described above, it is difficult to suppress the ignition of the battery by the shutdown function of the separator. Therefore, by increasing the heat shrinkage temperature more than the polyolefin resin porous film, it is difficult to cause an internal short circuit and suppresses the ignition of the battery. Nonwoven fabric separators containing aramid fibers, which are heat resistant fibers, have been proposed (see, for example, Patent Documents 1 to 3). However, although these nonwoven fabric separators are excellent in heat shrinkability, the pore diameter is large, and internal short circuit due to contact of the bipolar active material or micro short circuit due to dendrite generated on the negative electrode is likely to occur, and it was difficult to say that it is practical. In order to suppress these short circuits and further improve the heat resistance, examples in which a pigment or resin is supported on a substrate such as a nonwoven fabric or a woven fabric are disclosed (for example, Patent Documents 4 to 4). 6). However, even if a pigment or resin is applied, if the hole in the substrate is large, it is easy to cause a back-through of the coating liquid or a coating defect called pinhole, and the effect of preventing a micro short circuit becomes insufficient. There was a case. In addition, by applying a thick pigment or resin in order to prevent a micro short circuit, there is a problem that the metal ion permeability is lowered, and the high rate characteristic which is an advantage of the nonwoven fabric separator is impaired.

JP 2003-123728 A JP 2007-317675 A JP 2006-19191 A JP 2005-536857 A JP 2007-157723 A International Publication No. 2010/029994 Pamphlet

  An object of the present invention is to provide a metal ion secondary battery separator that is excellent in prevention of minute short-circuiting and high-rate characteristics in manufacturing a metal ion secondary battery battery separator that is particularly excellent in heat resistance.

  As a result of intensive studies, the present inventors have invented a metal ion secondary battery separator that can solve the problem. That is, in a metal ion secondary battery separator in which an inorganic pigment is supported on a nonwoven fabric substrate, the melting point of the constituent fibers of the nonwoven fabric substrate is 200 ° C. or higher, and the dehydration temperature of the crystal water or structural water of the inorganic pigment is The metal ion secondary battery having a temperature ratio of 250 ° C. or higher and a ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate is 1/5 to 1/20 It is a separator.

  In the metal ion secondary battery separator in which an inorganic pigment is supported on a nonwoven fabric substrate, the melting point of the constituent fibers of the nonwoven fabric substrate is 200 ° C. or higher, and the dehydration temperature of the inorganic pigment is 250 ° C. or higher. When the ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate is 1/5 to 1/20, it is particularly excellent in heat resistance, excellent in short-circuit suppression and high-rate characteristics. A metal ion secondary battery separator can be manufactured.

  Melting | fusing point of the fiber which comprises the nonwoven fabric base material which concerns on this invention is 200 degreeC or more. When the melting point is 200 ° C. or more, when local heat generation occurs inside the battery, it is possible to suppress the shrinkage of the separator due to the melting of the fibers and to prevent the ignition due to the internal short circuit. . In addition, the melting point of the fiber in the present invention refers to a melting point peak temperature measured based on the method defined in JIS K7121.

  Examples of the fiber having a melting point of 200 ° C. or higher constituting the nonwoven fabric substrate according to the present invention include polyesters such as polyethylene terephthalate, polyethylene isophthalate, and polyethylene naphthalate, acrylics such as polyacrylonitrile, 6,6 nylon, and 6 nylon. Examples include various synthetic fibers such as polyamide, and various cellulose pulps such as wood pulp, hemp pulp, and cotton pulp. Polyester is preferably used especially for reasons such as heat resistance and low hygroscopicity.

  The inorganic pigment according to the present invention has a dehydration temperature of 250 ° C. or higher. Alternatively, it is possible to use an inorganic pigment having no crystal water or structural water. When the dehydration temperature is 250 ° C. or higher, the separator is less likely to be deformed due to the structural change of the inorganic pigment crystal when abnormal heat generation occurs inside the battery. By suppressing both the heat shrinkage of the nonwoven fabric substrate and the structural change of the inorganic pigment layer, a metal ion secondary battery separator having particularly excellent heat resistance is obtained. A preferable dehydration temperature is 300 ° C. or higher, and more preferably 350 ° C. or higher. In the present invention, the dehydration temperature of the inorganic pigment is measured by differential scanning calorimetry (DSC). The measurement conditions are a temperature increase rate of 10 ° C./min and a temperature range of 30 to 900 ° C. in a nitrogen gas atmosphere.

  Examples of the inorganic pigment include alumina such as α-alumina, β-alumina, and γ-alumina, alumina hydrate such as boehmite, magnesium oxide, and calcium oxide. In particular, α-alumina or boehmite is preferably used from the viewpoint of stability.

  In the separator carrying the inorganic pigment according to the present invention, the ratio s / n of the separator's maximum pore diameter s to the maximum pore diameter n of the nonwoven fabric substrate is 1/5 to 1/20. Although it is preferable that the maximum pore diameter of the nonwoven fabric base material is large for high-rate characteristics, by increasing the maximum pore diameter, a micro short circuit is likely to occur. By carrying the inorganic pigment, the ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate is set to 1/5 to 1/20, so that the high rate characteristics are not impaired, and a short circuit is achieved. Can be suppressed. More preferably, the ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate is 1/6 to 1/18. The maximum pore diameter in the present invention is a value measured based on a method defined in JIS K3832.

  A method for adjusting the ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate to 1/5 to 1/20 is arbitrarily selected, and examples thereof include the following methods. . As the first method, for example, there is a method of adjusting the fibers constituting the nonwoven fabric substrate. In this method, the maximum pore diameter of the nonwoven fabric substrate can be adjusted by selecting the fiber diameter. If the fiber diameter is increased, the maximum pore diameter of the nonwoven fabric substrate is increased, and if the fiber diameter is decreased, the maximum pore size of the nonwoven fabric substrate is increased. It is possible to reduce the pore diameter.

  As the second method, there is a method of adjusting the particle diameter and particle structure of the inorganic pigment. In this method, the maximum pore diameter of the separator can be increased by increasing the particle diameter of the inorganic pigment, and the maximum pore diameter of the separator can be decreased by decreasing the particle diameter of the inorganic pigment. is there. Also, the maximum pore diameter of the separator can be adjusted by the primary structure and secondary structure of the particles. In addition, the particle diameter in this invention refers to the average particle diameter (D50) measured by a laser diffraction scattering method.

  As a third method, there is a method of adjusting the number of times of coating. In this method, even if the final coating amount is the same, it is possible to reduce the maximum pore diameter of the separator by coating in multiple times, that is, by increasing the number of times of coating. The reason why the maximum pore diameter of the separator is reduced by increasing the number of coatings is not clear, but a single coating causes a back-through and there are large pores in the nonwoven fabric base that cannot be covered. In this case, it seems that pores are gradually reduced in diameter by covering the coating layers, so that the coating becomes possible.

  As a fourth method, there is a method in which a nonwoven fabric substrate or a separator is calendered. The density of the separator is increased by the calendar process, and the pore diameter of the nonwoven fabric substrate or the separator can be reduced.

  By appropriately combining these methods, the maximum pore diameter of the nonwoven fabric substrate and the separator can be adjusted, and the ratio s / n of the maximum pore diameter n of the nonwoven fabric substrate and the maximum pore diameter s of the separator is 1/5. It becomes possible to make 1/20.

  What was manufactured by the conventionally well-known method can be used for the nonwoven fabric base material which concerns on this invention. For example, what was manufactured by methods, such as the spun bond method, the melt blow method, the dry method, the wet method, and the electrospinning method, can be used.

  In the present invention, calendering or thermal calendering may be performed for the purpose of controlling the surface of the nonwoven fabric substrate and controlling the maximum pore diameter and density.

As a nonwoven fabric base material which concerns on this invention, it is preferable that a fabric weight is 5-30 g / m < ² >, More preferably, it is 7-20 g / m < ² >. By setting the basis weight to 5 g / m ² or more, it becomes easy to obtain uniformity as a nonwoven fabric, and by setting it to 30 g / m ² or less, the thickness is suitable for a metal ion secondary battery separator. The basis weight means the basis weight based on the method defined in JIS P 8124. The density is a value obtained by dividing the basis weight by the thickness.

  The maximum pore diameter n of the nonwoven fabric substrate according to the present invention is preferably 5 to 30 μm, more preferably 7 to 25 μm. By setting the maximum pore diameter n to 5 μm or more, rate characteristics are easily obtained, and by setting the maximum pore diameter n to 30 μm or less, the back-through of the coating liquid during coating is easily suppressed, and the occurrence of pinholes is easily suppressed.

  The particle diameter of the inorganic pigment according to the present invention is preferably 0.1 to 10.0 μm, more preferably 0.2 to 7.5 μm, and still more preferably 0.3 to 5.0 μm. By setting the particle size to 0.1 μm or more, the stability of the coating liquid tends to be high, and by setting the particle size to 10.0 μm or less, a flat coating surface can be easily obtained. From the viewpoint of thermal stability, the inorganic pigment contained in the separator of the present invention is preferably 30 to 70% by mass in the total solid content of the separator.

  In the present invention, an adhesive may be used when the inorganic pigment is supported on the nonwoven fabric substrate. As the adhesive, latex polymer is preferably used. Specific examples include, for example, styrene / butadiene copolymers, acrylonitrile / butadiene copolymers, methyl acrylate / butadiene copolymers, acrylonitrile / butadiene / styrene terpolymers, polyvinyl acetate, vinyl acetate / acrylate esters. Examples include, but are not limited to, latex polymers such as copolymers, ethylene / vinyl acetate copolymers, polyacrylates, styrene / acrylate copolymers, and polyurethanes. In the present invention, from the viewpoint of the high rate characteristics of the separator and the strength of the coating layer, the amount of the adhesive in the coating layer is preferably 2 to 15% by mass in the solid content.

  In the present invention, various additives such as a dispersant, a wetting agent, and a thickener can be used as long as the effects of the invention are not impaired.

  In the present invention, the method for supporting the inorganic pigment on the nonwoven fabric substrate is not particularly limited, and a known method can be used. For example, an air doctor coater, a blade coater, a knife coater, a rod coater, a squeeze coater, and an impregnation coater. The coating liquid can be applied with a gravure coater, kiss roll coater, die coater, reverse roll coater, transfer roll coater, spray coater, etc. and supported by drying.

In this invention, as a coating amount of the coating layer containing an inorganic pigment, 5-30 g / m < ² > is preferable, More preferably, it is 10-20 g / m < ² >. When the coating amount is 5 g / m ² or more, it becomes easy to sufficiently cover the nonwoven fabric surface, and it is easy to prevent a micro short circuit. Moreover, it becomes easy to suppress the thickness increase of a separator by setting it as the coating amount of 30 g / m < ² > or less.

In the metal ion secondary battery separator 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 . Moreover, 10-50 micrometers is preferable and, as for the thickness of a separator, More preferably, it is 15-40 micrometers. The density of the separator is preferably 0.4 to 1.2 g / cm ³ , more preferably 0.5 to 1.0 g / cm ³ .

  In the metal ion secondary battery separator of the present invention, the maximum pore diameter s of the separator is preferably 0.5 to 5.0 μm, more preferably 1.0 to 4.0 μm. By setting the maximum pore diameter s to 0.5 μm or more, rate characteristics can be easily obtained, and by setting the maximum pore diameter s to 5.0 μm or less, an internal short circuit is easily suppressed.

  In the present invention, after coating and drying, the metal ion secondary battery separator may be smoothed by calendering for the purpose of controlling the surface of the coating layer and controlling the thickness and maximum pore diameter.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In Examples,% and parts are based on mass unless otherwise specified. The coating amount is an absolutely dry coating amount.

Production of Non-woven Fabric Base A Fineness 0.06 dtex (average fiber diameter 2.4 μm), oriented crystallized polyethylene terephthalate (PET) short fiber 40 mass parts with fiber length 3 mm and fineness 0.1 dtex (average fiber diameter 3.0 μm) , 20 mass parts of oriented crystallized PET short fibers with a fiber length of 3 mm, fineness of 0.2 dtex (average fiber diameter 4.3 μm), single component binder PET short fibers with a fiber length of 3 mm (softening point 120 ° C., melting point) (230 ° C.) and 40 parts by mass were mixed together, disaggregated in water by a pulper, and a uniform papermaking slurry having a concentration of 1% by mass was prepared under stirring by an agitator. Using an inclined type paper machine, raising the paper making the sheet-forming slurry by a wet method, the cylinder dryer of 130 ° C., and to adhere the PET-based short fibers binders were expressed nonwoven fabric strength, and basis weight 12 g / m ² nonwoven did. Furthermore, this nonwoven fabric was used under the conditions of a heat roll temperature of 205 ° C., a linear pressure of 100 kN / m, and a processing speed of 40 m / min, using a 1-nip heat calender composed of a dielectric heating jacket roll (metal heat roll) and an elastic roll. A heat calender treatment was performed to prepare a nonwoven fabric substrate having a thickness of 17 μm.

Fabrication of Non-woven Fabric Base B 60 μ parts of oriented crystallized polyethylene terephthalate (PET) short fibers having a fineness of 0.06 dtex (average fiber diameter of 2.4 μm) and a fiber length of 3 mm, and a fineness of 0.1 dtex (average fiber diameter of 3. 0 μm), a nonwoven fabric substrate B having a thickness of 16 μm was prepared in the same manner as the nonwoven fabric substrate A, except that the oriented crystallized PET short fibers having a fiber length of 3 mm were not used.

Fabrication of Non-woven Fabric Base C Fineness of 0.8 dtex instead of PET short fibers for single component binder (softening point 120 ° C., melting point 230 ° C.) having a fineness of 0.2 dtex (average fiber diameter 4.3 μm) and a fiber length of 3 mm PP / PE short fibers for core-sheath binders (average fiber diameter 10.4 μm) and fiber length 5 mm (core melting point 165 ° C., sheath melting point 135 ° C.), linear pressure 200 kN / instead of thermal calendering A nonwoven fabric substrate C having a thickness of 20 μm was produced in the same manner as the nonwoven fabric substrate A except that the calendar treatment was performed with m.

Preparation of Coating Liquid A As an inorganic pigment, 100 parts of boehmite having an average particle diameter of 2.3 μm and a dehydration temperature of 500 ° C., a 1% by weight aqueous solution of a 0.3% aqueous solution of carboxymethyl cellulose sodium salt having a viscosity of 200 mPa · s at 25 ° C. Dispersed in 120 parts and stirred well to prepare a boehmite dispersion. Subsequently, 200 parts of a 0.5% aqueous solution of carboxymethylcellulose sodium salt having a viscosity of 7000 mPa · s at 25 ° C. in a 1% by mass aqueous solution was mixed and stirred, and a 45% carboxy-modified styrene / butadiene copolymer was used as an adhesive. 10 parts of the latex polymer was mixed and stirred to prepare a coating solution.

Preparation of Coating Liquid B Coating liquid A was used except that 50 parts of boehmite having an average particle size of 2.3 μm and a dehydration temperature of 500 ° C. and 50 parts of boehmite having an average particle diameter of 0.4 μm and a dehydration temperature of 500 ° C. were used as inorganic pigments. A coating solution B was prepared in the same manner as described above.

Preparation of coating liquid C Coating liquid C was prepared in the same manner as coating liquid A, except that 100 parts of boehmite having an average particle size of 0.4 μm and a dehydration temperature of 500 ° C. was used as the inorganic pigment.

Preparation of coating liquid D Coating liquid D was prepared in the same manner as in coating liquid A, except that 100 parts of aluminum hydroxide having an average particle size of 2.5 μm and a dehydration temperature of 200 ° C. was used as the inorganic pigment.

Production of Separator A On the surface of the nonwoven fabric substrate A that is in contact with the cylinder dryer side, the coating liquid A is applied and dried so that the dry coating amount is 8 g / m ^2, and then again on the same coated surface. The separator A was produced by coating and drying the coating liquid A so that the dry coating amount was 8 g / m ² .

Production of Separator B On the surface of the nonwoven fabric substrate A in contact with the cylinder dryer, the coating liquid B was applied and dried so that the dry coating amount was 16 g / m ² to produce a separator B.

Preparation of Separator C On the surface of the nonwoven fabric substrate A that is in contact with the cylinder dryer side, the coating liquid A is applied and dried so that the dry coating amount is 8 g / m ^2, and then applied to the same coated surface. The separator C was produced by coating and drying the liquid C so that the completely dry coating amount was 8 g / m ² .

Production of Separator D A separator D was produced in the same manner as the separator A except that the nonwoven fabric substrate B was used instead of the nonwoven fabric substrate A.

Production of Separator E On the surface of the nonwoven fabric substrate A in contact with the cylinder dryer side, the coating liquid A was applied and dried so that the dry coating amount was 16 g / m ² , thereby producing a separator E.

Preparation of Separator F On the surface of the nonwoven fabric substrate A in contact with the cylinder dryer side, the coating liquid C was applied and dried so that the dry coating amount was 8 g / m ^2, and then applied to the same coated surface. Liquid C was applied and dried so that the dry coating amount was 8 g / m ² to prepare separator F.

Preparation of Separator G On the surface of the nonwoven fabric substrate A in contact with the cylinder dryer side, the coating liquid D was applied and dried so that the dry coating amount was 8 g / m ^2, and then applied to the same coated surface. The separator D was produced by coating and drying the liquid D so that the absolute dry coating amount was 8 g / m ² .

Production of Separator H On the surface of the nonwoven fabric substrate C in contact with the cylinder dryer side, coating liquid A was applied and dried so that the dry coating amount was 8 g / m ^2, and then applied to the same coated surface. The separator A was prepared by coating and drying the liquid A so that the dry coating amount was 8 g / m ² .

<Evaluation>

[Maximum pore diameter]
About each nonwoven fabric base material and each separator, the maximum pore diameter was measured using the palm porometer CFP-1500A by PMI. The results are shown in Table 1.

[Heat-resistant]
A sheet sample of 50 mm × 50 mm is cut out from each separator produced, and the CD (cross direction, lateral direction) side of the sheet sample is fixed with a clip and sandwiched between heat resistant glass plates, and 1 in a thermostatic chamber at 150 ° C. and 180 ° C. After holding for a period of time, the sample was taken out, the width of the sample was measured, and the shrinkage ratio before and after heating was calculated. The evaluation was as follows.
A: The shrinkage rate is less than 2% and almost no shrinkage is observed.
○: The shrinkage rate is 2 to 5%, which is a practically acceptable level.
(Triangle | delta): A shrinkage rate is 5 to 8%, and there is some concern about shrinkage by local overheating.
X: The shrinkage rate is 8% or more, and there is a concern about shrinkage during local overheating.

[Coulomb efficiency during initial charge / discharge]
Using each separator, the positive electrode active material is lithium manganate, the negative electrode active material is artificial graphite, the electrolyte is a lithium hexafluorophosphate ethylene carbonate / diethyl carbonate / dimethyl carbonate 1/1/1 (volume ratio) mixed solvent solution A laminate type lithium ion secondary battery having a design capacity of 100 mAh (1 mol / L) was produced.

  After that, for each battery manufactured, 100 mA constant current charge → 4.4 V constant voltage charge → charge current 10 mA, 100 mA, constant current discharge to 2.8 V, charge capacity and discharge capacity are measured, (Coulomb efficiency) ) = (Discharge capacity) / (charge capacity). Those with low Coulomb efficiency are considered to have a micro short circuit.

[High-rate battery characteristics]
For each battery produced, 100 mA constant current charge → 4.2 V constant voltage charge → charge current 10 mA, 100 mA constant current discharge up to 2.8 V → 100 mA constant current charge → 4.2 V constant voltage charge → charge current 10 mA Then, constant current discharge was performed at 3000 mA up to 2.8 V, and the discharge capacity ratio was determined as [(discharge capacity at 300 mA) / (discharge capacity at 100 mA)] × 100 (%) to obtain high rate characteristics.

  As is clear from Table 1, the melting point of the constituent fibers of the nonwoven fabric base material is 200 ° C. or higher, the dehydration temperature of the inorganic pigment is 250 ° C. or higher, and the separator with respect to the maximum pore diameter n of the nonwoven fabric base material The separator of the present invention in which the ratio s / n of the maximum pore diameter s is 1/5 to 1/20 is particularly excellent in heat resistance, and is excellent in Coulomb efficiency and high rate characteristics at the first charge / discharge.

  On the other hand, in the separator E in which the ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate is more than 1/5, the Coulomb efficiency is low, and the ratio is less than 1/20. In some separators F, the high rate characteristics were low. Further, the separator G in which the dehydration temperature of the inorganic pigment was less than 250 ° C. had low heat resistance, and the separator H in which the melting point of the constituent fibers of the nonwoven fabric base material was less than 200 ° C. also had low heat resistance.

  The metal ion secondary battery separator of the present invention can be used for metal ion polymer batteries, metal ion capacitors and the like in addition to metal ion secondary battery applications.

Claims (1)

  In the metal ion secondary battery separator in which an inorganic pigment is supported on a nonwoven fabric substrate, the melting point of the constituent fibers of the nonwoven fabric substrate is 200 ° C. or higher, and the dehydration temperature of the inorganic pigment is 250 ° C. or higher. A metal ion secondary battery separator, wherein a ratio s / n of the maximum pore diameter s of the separator to the maximum pore diameter n of the nonwoven fabric substrate is 1/5 to 1/20.