JP2006286311A - Composite porous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composite porous film being a separator used for a nonaqueous solvent battery, allowing compatibility of high heat resistance with appropriate SD characteristics and having high practicality. <P>SOLUTION: In this composite porous film, the base material of the film is formed of a porous material capable of keeping its porous structure 140°C or higher, and having an average pore diameter of 0.2-10 μm; a porous layer formed of a resin particle aggregate exist, from the surface of the base material to the inside of the porous structure; and pores of the base material are closed, by softening or melting of the resin particles constituting the porous layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a separator used for a nonaqueous solvent battery.

  Non-aqueous solvent batteries, such as negative electrodes that use light metals such as lithium and sodium as active materials and batteries that use active materials such as metal oxides or halides as positive electrodes and lithium ion batteries, use electrolytes that are organic solvents. In addition, there is a problem that the safety against heat generation of the battery is inferior to that of the aqueous solvent of the aqueous battery. Therefore, conventionally, as a technique for improving the safety of a non-aqueous solvent battery, particularly a lithium ion battery having a large energy density, a separator using a microporous porous film of an olefin-based material mainly composed of polyethylene has been used. I have been. Polyethylene is mainly used in addition to the fact that polyethylene can be used in an organic solvent. In addition, when a battery abnormally generates heat due to a short circuit or the like, the polyethylene melts at an appropriate temperature (around 130 ° C.), resulting in a porous structure. This is because the safety can be maintained by blocking (abbreviated as “shutdown” or “SD”).

  However, for the high temperature test of the battery, the separator using polyethylene has a problem that it is inferior in heat resistance, for example, shrinkage easily occurs at a temperature of 140 ° C. or less, and heat is generated due to a short circuit between the electrodes. The use of a polymer having higher heat resistance than polyethylene such as polypropylene has the advantage that shrinkage at a temperature of 140 ° C. or lower can be reduced, but in that case, the property of SD at an appropriate temperature cannot be expressed, It was restricted.

  On the other hand, a microporous membrane composite is characterized in that a blocking material is attached to one side of the microporous membrane, and the blocking material is heat-meltable and heat-melted to cover the surface of the microporous membrane (for example, Patent Document 1). Has been proposed. In Patent Document 1, the plugging material is attached on the surface of the microporous membrane, and the plugging material becomes a film by heating and melting in order to completely block the pores of the microporous membrane and covers the surface of the microporous membrane. It is said. For this reason, it is necessary for the plugging material to be present in a layered manner on the surface of the microporous membrane, and the microporous membrane composite is usually a multilayer film. Such a multilayer film is not preferable because it has a problem of increasing the film thickness.

  Moreover, the separator (for example, patent document 2) which made the polyethylene powder particle adhere to the surface of a polypropylene nonwoven fabric is proposed. In this case, since the base material is a non-woven fabric composed of fibers and usually has a large average pore diameter of about several tens of μm, it takes time until the resin melts and closes the pores. Although it is not perfect, it is not preferable.

  Furthermore, a separator (for example, Patent Document 3) has been proposed in which a synthetic resin microporous film is used as a base material and at least one surface thereof is coated with a resin porous powder aggregate. SD characteristics important for maintaining safety are manifested by complete blockage of the porous structure. In Patent Document 3, the resin porous powder is present only on the surface of the base material. In such a case, a complete non-porous film is not formed only by softening and melting of the resin porous powder, and the expression of SD characteristics is not achieved. Have difficulty. On the other hand, in Patent Document 3, the synthetic resin microporous film itself of the base material has SD characteristics. The problem of improving safety is solved by further improving the SD characteristics of the base material by coating with the resin porous powder aggregate.

  In addition, since the resin porous powder is supported only on the surface of the base material without the resin particles being inserted into the holes of the base material, there is a limit to the adhesive strength of the resin porous powder to the base material. Therefore, there has been a problem in practicality such that the resin particles carried during the battery assembly process and the charge / discharge of the battery are repeated. For the adhesive strength, a separator (for example, Patent Document 4) is proposed in which resin particles obtained by adding adhesive resin particles in addition to SD resin particles are applied to the surface of a porous resin film.

However, in this separator, in order to develop the adhesive strength of the resin particle layer, it is necessary to melt only the adhesive particles once, so that the productivity is deteriorated and the porosity of the surface particle layer is lost and the electric resistance is likely to be increased. There were problems such as.
Japanese Patent No. 1828177 Japanese Patent No. 1869019 Japanese Patent No. 2955323 JP-A-9-219185

  An object of the present invention is to provide a separator that can achieve both high heat resistance and appropriate SD characteristics and has high practicality.

As a result of intensive studies to achieve the above object, the present inventors have reached the present invention.
That is, the present invention is as follows.
(1) The membrane substrate is made of a porous material having an average pore diameter of 0.2 μm or more and 10 μm or less capable of maintaining a porous structure at 140 ° C. or higher, and a porous layer formed of resin particle aggregates is formed on the surface of the substrate and inside the porous structure. A composite porous membrane having a temperature at which pores of a base material are blocked by softening or melting of resin particles that are present and constituting the porous layer is 140 ° C. or lower.
(2) The composite porous membrane according to (1), wherein a porous layer made of a resin particle aggregate is formed on one side of the substrate surface.
(3) The composite porous membrane according to (1) or (2), which is used for a nonaqueous solvent battery separator.

  According to the present invention, it is possible to provide a separator for a non-aqueous solvent battery that has high safety and high practicality by achieving both high heat resistance and appropriate SD characteristics.

The membrane substrate in the present invention is a thin-film porous material having communication holes, and examples thereof include a resin porous membrane, paper, nonwoven fabric, and inorganic porous membrane.
The porous structure is a network structure in which voids and skeletons are continuously present. The maintainable temperature is a temperature at which the pore diameter and voids (porosity) of the porous structure can be substantially maintained, and is 140 ° C. or higher and preferably 150 ° C. or higher from the viewpoint of high heat resistance. Furthermore, it is more preferable if the porous structure can be maintained at a high temperature, but practically, it is preferably 300 ° C. or less in view of cost and heat resistance of other battery constituent materials.

  The average pore diameter of the porous material of the base material needs to be 0.2 μm or more, and preferably 0.4 μm or more so that the resin particles can be sufficiently present inside the pores of the base material. Moreover, it is necessary to be 10 μm or less, and preferably 8 μm or less so that it does not take time for the pores to be closed due to softening or melting of the resin particle aggregate, and the pores are completely closed and appropriate SD characteristics are obtained. Although the film thickness of a base material is not specifically limited, From the point of ensuring reliability as a separator, it is preferable that it is 1 micrometer or more, and it is still more preferable that it is 5 micrometers or more. Moreover, it is preferable that it is 500 micrometers or less from the point of the high energy density of a battery, and it is more preferable that it is 200 micrometers or less.

The porosity of the substrate is preferably 90% or less, more preferably 85% or less, from the viewpoint of film strength and withstand voltage reliability. Moreover, 30% or more is preferable from the point of low electrical resistance, and 35% or more is more preferable. The porous layer of the resin particle aggregate in the present invention is a layer of an aggregate composed of a continuous body in which resin particles are in contact with each other and having voids between the particles. The porous layer of the resin particle aggregate can be provided on both surfaces of the substrate surface, but it is preferably formed on one surface from the viewpoint of productivity and thinning.
The average particle diameter of the resin particles is preferably 0.1 μm or more, more preferably 0.2 μm or more in order to suppress an increase in electrical resistance. Further, it is preferably 20 μm or less, more preferably 15 μm or less in order not to deteriorate the SD characteristics.

The presence of a porous layer composed of resin particle aggregates from the surface of the base material to the inside of the porous structure means that the resin particles enter from the surface of the base material into the pores of the base material, and the resin particles are continuously connected. In this state, a porous layer is formed. The presence of resin particles from the surface of the base material to the inside of the pores of the base material makes it easier to soften and melt the resin particles than when the resin particles exist only on the base material surface. The pores can be made non-uniform and effective. As a result, appropriate SD characteristics can be obtained.
In the present invention, the appropriate SD characteristic means a characteristic that when the battery temperature rises to a certain temperature or higher, the electrical resistance of the separator in the battery rapidly increases with the temperature rise to 1000 Ωcm ² or more.

  The temperature at which the pores of the base material are closed by the softening or melting of the resin particles means that the resin particles are softened or melted and the porous layer of the resin particles is made nonporous, so that the pores of the base material are closed by the resin. Temperature. The electrical resistance of the substrate is greatly increased by the blockage of the holes. From this, the hole closing temperature can be measured by electric resistance measurement. The hole closing temperature is preferably 140 ° C. or lower in order to ensure battery safety. Furthermore, 138 degrees C or less is more preferable. Moreover, 80 degreeC or more is preferable from a heat resistant point, and 90 degreeC or more is more preferable.

  The resin particle material is not particularly limited as long as it is a resin capable of bringing the closing temperature of the pores of the base material into a range of 80 to 140 ° C. by softening or melting, and an example thereof is given. And polyolefin resins such as low density polyethylene, linear low density polyethylene, high density polyethylene, polypropylene and copolymers thereof, polystyrene resins such as polystyrene and styrene / acrylonitrile copolymers, polymethyl methacrylate, polymethacrylic acid Examples thereof include polyacrylic resins such as ethyl. Of these, resins mainly composed of low density polyethylene, linear low density polyethylene, and high density polyethylene are preferred.

  The film thickness on the substrate surface of the resin particle porous layer is the difference between the film thickness of the substrate where the resin particle aggregate does not exist and the film thickness of the entire substrate where the resin particle aggregate exists on the surface. The thickness is not particularly limited, but is preferably 0.1 μm or more, more preferably 1 μm or more from the viewpoint of reliability of SD characteristics. From the viewpoint of low electrical resistance, 50 μm or less is preferable, and 30 μm or less is more preferable.

  Although the manufacturing method of the composite porous membrane of this invention is not specifically limited, If the example is given, the method of apply | coating on a base material by various coating systems using the aqueous dispersion liquid or oil dispersion of a resin particle will be mentioned. After application, the resin particles and the substrate can be dried at a temperature at which the resin particles and the substrate are not greatly deformed. In some cases, partial heat fusion between the resin particles can be performed under the condition that the porosity is not lost.

The air permeability of the composite porous membrane of the present invention is not particularly limited, but is preferably 1000 seconds or less, more preferably 600 seconds or less. The lower the air permeability, the better the permeation performance such as electrical resistance. However, if it is too low, a problem is likely to occur as an interelectrode separator (separator) in the battery, and therefore it is preferably 5 seconds or more, more preferably 10 More than a second. Further, although not electrical resistance also particularly limited, but is preferably 10 .OMEGA.cm ² or less, more preferably 5Omucm ² or less. From the aspect of preventing a short circuit between the electrodes, it is preferably 0.2 Ωcm ² or more, more preferably 0.5 Ωcm ² or more.

  Since the composite porous membrane of the present invention has high safety characteristics due to both high heat resistance and appropriate SD characteristics, a negative electrode using a light metal such as lithium and sodium as an active material, a metal oxide or It can be used as a separator used in a nonaqueous solvent battery having a positive electrode made of an active material such as a halide or a nonaqueous solvent battery such as a lithium ion battery.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further in detail, this invention is not specifically limited to an Example. In addition, the measuring method and evaluation method in an Example are as follows.
(1) Particle size distribution measurement The particle size distribution was measured using a particle size distribution measuring device (SALD-3000) manufactured by Shimadzu Corporation.
(2) Film thickness (μm)
Measurement was performed using a micro thickness gauge (type KBN, terminal diameter 5 mm) manufactured by Toyo Seiki.

(3) Porosity (%)
A sample of Xcm × Ycm was cut out and calculated from the following formula.
Porosity = {1- (M / ρ) / (X × Y × T)} × 100
T: Sample thickness (cm)
M: sample weight (g)
ρ: Sample skeleton density

(4) Average pore diameter (μm)
Using a mercury intruder (manufactured by Shimadzu Corp.) (auto pore type 9220), pore distribution was measured under conditions of an initial pressure of 20 kPa, and an average pore diameter was measured.
(5) Air permeability (sec / 100cc)
It was measured using a digital type Oken air permeability tester (EG01 type) manufactured by Asahi Seiko.

(6) Electric resistance (Ωcm ² )
It measured at 25 degreeC on the following conditions using the Ando Electric LCR meter (AG-4311).
Electrolytic solution: 50% by volume of propylene carbonate
Dimethoxyethane 50% by volume
Lithium perchlorate 1 mol / dm ³
Electrode: Platinum black electrode, electrode plate distance 3 mm
Electrode area: 0.785 cm ²
AC: 1kHz
Assembly drawing: described in Fig. 1

(7) Hole closing temperature (° C)
2A to 2C are schematic views of a hole closing temperature measuring device. FIG. 2A is a configuration diagram of the measuring apparatus. 9 is a measurement sample, 2A and 2B are Ni foils having a thickness of 10 μm, and 3A and 3B are glass plates. 4 is an electrical resistance measuring device (Ando Electric LCR meter AG4311), which is connected to Ni foils (2A, 2B). A thermocouple 5 is connected to the thermometer 6. A data collector 7 is connected to the electrical resistance measuring device 4 and the thermometer 6. Reference numeral 8 denotes an oven for heating the sample.

  More specifically, the sample 9 is impregnated with a prescribed electrolyte, and as shown in FIG. 2 (B), only the MD is fixed on the Ni foil 2A in a form stopped with Teflon (registered trademark) tape. ing. The Ni foil 2B is masked with Teflon (registered trademark) tape leaving a 15 mm × 10 mm portion as shown in FIG. The Ni foil 2A and the Ni foil 2B are overlapped so as to sandwich the sample 9, and two Ni foils are sandwiched by the glass plates 3A and 3B from both sides thereof. The two glass plates are fixed by sandwiching them with commercially available clips. Using the apparatus shown in FIG. 2A, temperature and electric resistance are continuously measured. The temperature is raised at a rate of 2 ° C./min, and the electrical resistance value is measured with an alternating current of 1 kHz. The hole closing temperature is defined as the temperature at which the electrical resistance value of sample 9 reaches 1000Ω.

The prescribed electrolyte is as follows.
Electrolytic solution: mixed organic solvent of propylene carbonate / ethylene carbonate / γ-butyllactone = 25/25/50% by volume containing 1 mol / L lithium borofluoride (LiBF ₄ ) and 0.5% by weight of trioctyl phosphate.

[Example 1]
A cellulose separator (TF40 manufactured by Nippon Kogyo Paper Co., Ltd.) was used as the substrate. A 20% by weight solution of Chemipearl W401 (Mitsui Chemicals, average particle size of 1 μm), which is a dispersion of polyethylene particles, was applied to one side of this substrate as No. 1 Coating was performed by a bar coater method using 11 wire bars. After coating, hot air drying was performed at 80 ° C. for 1 hour. The composition of the sample is shown in Table 1, and the physical properties of the coated sample after coating are shown in Table 2.

[Example 2]
As a base material, a Tencel thin film prepared by beating paper after tencel fiber (manufactured by Tencel Japan) was used. A 20% by weight solution of Chemipearl W400 (Mitsui Chemicals, average particle size of 4 μm), which is a dispersion of polyethylene particles, was applied to one side of this substrate. Coating was performed by a bar coater method using 11 wire bars. After coating, hot air drying was performed at 80 ° C. for 1 hour.
The composition of the sample is shown in Table 1, and the physical properties of the coated sample after coating are shown in Table 2.

[Comparative Example 1]
The same operation as in Example 1 was performed except that a polypropylene microporous membrane (Celgard No. 2400) was used as the substrate. The composition of the sample is shown in Table 1, and the physical properties of the coated sample after coating are shown in Table 2.

[Comparative Example 2]
The same operation as in Example 1 was carried out except that polypropylene non-woven fabric (ELTAS manufactured by Asahi Kasei Fibers Co., Ltd.) was used and Chemipearl W410 (Mitsui Chemicals, average particle size 10 μm) was used as the dispersion of polyethylene particles. The composition of the sample is shown in Table 1, and the physical properties of the coated sample after coating are shown in Table 2.

[Comparative Example 3]
The same operation as in Example 1 was performed except that cellulose Japanese paper (AH manufactured by Asahi Kasei Chemicals Corporation) was used as the substrate and Chemipearl W410 (Mitsui Chemicals, average particle size 10 μm) was used as the dispersion of polyethylene particles. The composition of the sample is shown in Table 1, and the physical properties of the coated sample after coating are shown in Table 2.

[Comparative Example 4]
The same operation as in Example 1 was carried out except that a base material in which fine fibrous cellulose (Cerish manufactured by Daicel Corp.) was applied to Japanese paper made of cellulose (AH manufactured by Asahi Kasei Chemicals Co., Ltd.) and the surface pore diameter was reduced was used. . The composition of the sample is shown in Table 1, and the physical properties of the coated sample after coating are shown in Table 2.

  The composite porous membrane of the present invention has high safety characteristics by achieving both high heat resistance and appropriate SD characteristics, and includes a negative electrode using a light metal such as lithium or sodium as an active material, a metal oxide or a halogen. It is useful as a separator used in a nonaqueous solvent battery using an active material such as a chemical compound as a positive electrode or a nonaqueous solvent battery such as a lithium ion battery.

The schematic of the cell for measuring the electrical resistance of the composite porous membrane of this invention. The schematic of the apparatus for measuring the hole blockage temperature of the composite porous membrane of this invention.

Explanation of symbols

1: Electrode
2: Packing (outer diameter 2 cm, inner diameter 1 cm, thickness 1 mm)
3: Measurement sample 2A, 2B: Ni foil 3A having a thickness of 10 μm, 3B: Glass plate 4: Electrical resistance measuring device 5: Thermocouple 6: Thermometer 7: Data collector 8: Oven 9: Measurement sample

Claims (3)

  The membrane substrate is made of a porous material having an average pore diameter of 0.2 μm or more and 10 μm or less capable of maintaining a porous structure at 140 ° C. or higher, and there is a porous layer of resin particle aggregates from the surface of the substrate to the inside of the porous structure, A composite porous membrane characterized in that the temperature at which the pores of the base material are closed by softening or melting of the resin particles constituting the porous layer is 140 ° C. or lower.   The composite porous membrane according to claim 1, wherein a porous layer made of a resin particle aggregate is formed on one side of the substrate surface.   The composite porous membrane according to claim 1, which is used for a nonaqueous solvent battery separator.

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