JPH0277108A - Electrolyte separator - Google Patents

Abstract

PURPOSE:To immediately close and seal a hole formed by short circuit even when the short circuit occurs by alternately piling up 1st polyolefinic porous layers having a specific melting point and 2nd polyolefinic porous layers having another specific melting point and specifying the overall mean porosity and mean diameter of the circular openings of all layers. CONSTITUTION:This electrolyte separator is constituted by alternately piling 1st polyolefinic porous layers having a melting point >=158 deg.C and 2nd polyolefinic porous layers having another melting point of 110-150 deg.C. It is preferable to unite layers A and B or two layers B and one layer A, with the layer A being between the layers B, to one body. When the mean porosity of the separator is <50%, the electrolyte holding quantity of the separator is small and the separator may dries up. When the porosity exceeds 85%, on the contrary, the mechanical strength of the separator drops. When the overall mean diameter of the layers is 0.01mum, the equivalent serial resistance becomes larger and, when the mean diameter exceeds 5mum, conductive substances can easily pass through the pores.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrolyte separator used in electrolytic capacitors, lithium batteries, batteries, etc.

[Prior Art] Electrolytic papers such as kraft paper and manila paper have been used for a long time as electrolyte separators for electrolytic capacitors, lithium batteries, batteries, etc., but recently, for example, JP-A-51-18851, JP-A-61-13614,
Utility Model Publication No. 59-140429 and Japanese Patent Application Publication No. 62-2007
It has been proposed to use porous polyolefin membranes, as described in No. 16. Porous polyolefin membranes are stronger than paper and have a lower incidence of short circuits.

[Problems to be Solved by the Invention] The electrolyte separators made of porous polyolefin membranes that have been proposed so far are all made of a single layer of porous polyolefin membrane, and have a lower short circuit occurrence rate than electrolytic paper. The short circuit occurrence rate is still not satisfactory.

An object of the present invention is to provide an electrolyte separator that has a significantly lower short circuit occurrence rate than conventional electrolyte separators, and has excellent equivalent series resistance, heat resistance, and mechanical strength as an electrolyte separator.

[Means for Solving the Problems] As a result of intensive research, the inventors of the present application found that the first porous polyolefin layer has a melting point of 158°C or higher and a melting point of 110°C.
When used as an electrolyte separator by laminating a second porous polyolefin layer at 150°C to The second polyolefin porous layer having a relatively low melting point is melted by the heat generated, and the pores formed by the short circuit are automatically closed and sealed.
The inventors discovered that the probability of complete failure of the separator could be reduced and completed this invention.

That is, the present invention includes a first polyolefin porous layer having a melting point of 158°C or higher, and a layer laminated on the first polyolefin porous layer having a melting point of 11O'C to 150'C.
C second polyolefin porous layer, the average porosity of all layers is 50% to 85%, and the average pore diameter of all layers is o.
, oiμm to 5μm, and an electrolyte separator made of a porous polyolefin-based laminate.

As mentioned above, in the electrolyte separator of the present invention, the first polyolefin porous layer having a melting point of 158°C or higher (
A second porous polyolefin layer (hereinafter referred to as layer B) having a melting point of 110° C. to 150° C. is laminated.

The melting point of layer A is 158°C or higher, preferably 160'
C or higher. If the melting point of layer A is less than 158°C, the ability to support layer B at high temperatures will be reduced, and the heat resistance of the separator will be reduced. In addition, the glass transition point of layer A is 1
The temperature is preferably 0'C or less, more preferably 0C or less.

When the glass transition point exceeds 10°C, it becomes brittle and cold resistance deteriorates. As the polyolefin constituting layer A, polypropylene, poly4-methylpentene-1, poly3-methylbutene-1, and copolymers or blends thereof are preferable, and among these, polypropylene, poly4-methylpentene-1, poly3-methylbutene-1, and copolymers or blends thereof are preferable. Polypropylene is particularly preferred, and it is preferable that the melt crystallization temperature of the polypropylene is 106° C. or higher, preferably 108° C. or higher, and most preferably 110° C. or higher, since the short-circuit guarantee function is further enhanced.

The melting point of layer B is 110°C to 150°C, preferably 120'C to 140°C. Melting point of layer B is 110
If the temperature is less than 150°C, the heat resistance will decrease and there is a risk of melting during normal operation, while if the temperature exceeds 150°C, the formed hole will be difficult to close and seal even if a short circuit occurs. The polyolefin constituting layer B is preferably polyethylene, polybutene, ethylene propylene copolymer, etc., and polyethylene, ethylene propylene copolymer, or a blend of the two is particularly preferred. Like the A layer, the glass transition point of the B layer is preferably 10° C. or lower, and more preferably O'C or lower. When the glass transition point exceeds 10°C, it becomes brittle and cold resistance deteriorates.

The layer structure of the separator is preferably A layer/B layer or B layer/A layer/B layer with layer A sandwiched in the center. Although the A layer and the B layer may simply be laminated, it is preferable that they are laminated in a mutually interlocking manner. The All and B layers can be laminated together by a coextrusion method or a point fusion method widely known in this field.

The average porosity of all layers of the separator is 50% to 85%, preferably 55% to 75%. The average porosity of all layers is 5
When the average porosity is less than 0%, the amount of electrolyte retained is small and the separator may dry up, and when the average porosity exceeds 85%, the mechanical strength of the separator deteriorates. In addition, the average pore diameter of the entire layer is 0.01 μm to 5 μm, preferably 0.1 μm.
It is from μm to 3 μm. The average pore diameter of the whole layer is o, oiμ
If it is less than m, the equivalent series resistance will increase, and if it exceeds 5 μm, the conductive material will easily pass through, resulting in a high short-circuit occurrence rate.

The thickness of the entire separator layer is 10 μm to 70 μm from the viewpoint of mechanical strength, show 1 to occurrence rate, and compactness of the device.
m is preferable, and 25 μm to 50 μm is particularly preferable.

The ratio of the thickness of layer B to the total thickness of the separator is 20% to 60% from the viewpoint of short circuit prevention effect and heat resistance.
is preferable, particularly preferably 30% to 50%.

Layer A and layer B constituting the electrolyte separator of this invention are made by adding vinyl chloride such as dicyclohexyl phthalate (DCHP) or triphenyl phosphate (TPP) to 100 parts by weight of the polyolefin resin constituting the layers. 80 to 240 parts by weight, preferably 100 parts to 200 parts by weight, of an organic solid such as phthalic acid or phosphoric acid ester used as a plasticizer is blended and melt-extruded, followed by trichloromethane or trichloroethane. ,acetone,
It can be produced by extracting 95% or more, preferably 98% or more of the added amount of the organic solid using a good organic solid solvent such as methyl ethyl ketone, ethyl acetate, methanol, toluene, or xylene. Here, it is preferable to set the extraction temperature to above the melting point of the organic solid -25°C, preferably above the melting point of the organic solid -15°C, since this increases the melt crystallization temperature of the polypropylene and improves its properties.

When laminating All and B layers in an integrated manner, it is also possible to manufacture A and B layers as described in section 2 and then laminate them together using a point fusion method.
The lamination can also be achieved by co-extruding the polyolefin and then carrying out the extrusion described in Section 7.
It is preferable to stretch the film by 5 to 6 times by 1° at a temperature of 10'C or below, since both mechanical properties and electrical properties will be good. Furthermore, as with ordinary polyolefin films, it is preferable to heat-set the film at a temperature that is higher than the melt crystallization temperature of the polyolefin and lower than the melting point of -5°C.

The separator is preferably subjected to hydrophilic treatment in order to improve its affinity with the electrolytic solution. The hydrophilization treatment can be performed by coating with a nonionic surfactant, an anionic or cationic surfactant, corona or plasma treatment, grafting treatment, ultraviolet treatment, or a combination thereof.

The polyolefin porous layers (layer A and layer B) constituting the electrolyte separator of the present invention are each preferably made of only polyolefin, but the melting point, average porosity, and average pore diameter of As long as it falls within the scope of the invention, there is no problem even if it contains a trace amount of impurity, and for example, it may contain a trace amount of additives such as heat stabilizers, antioxidants, slip agents, antistatic agents, and other substances other than olefins. There is no problem even if they are combined. In the claims, the term "polyolefin porous layer" is used to include polyolefin porous layers containing such impurities.

[Effects of the Invention] In the electrolyte separator of the present invention, even if a short circuit occurs, the pores formed at that time are immediately closed and sealed. The short circuit occurrence rate is significantly lower. Further, since the electrolyte separator of the present invention has an optimized average porosity and average pore diameter, it has excellent equivalent series resistance and mechanical strength. Further, when the A layer is made of polypropylene and the BIN is made of polyethylene and/or ethylene propylene copolymer, the temperature range from the glass transition point to the melting point is wide and the reliability is high.

[Method for Measuring Characteristics and Evaluating Effects 1] Next, the measuring methods and evaluation methods related to the present invention will be summarized.

(1) Melting point, melt crystallization temperature and glass transition temperature of polyolefin Scanning calorimeter DSC-2 model (manufactured by Perkin-Elmer)
5 mg of sample was heated at a heating rate of 20°C/under nitrogen flow using
The melting point is determined by the endothermic peak temperature accompanying melting. The peak of latent heat associated with crystallization of the polyolefin when the temperature was further increased to 280'C, held for 5 minutes, and then cooled at a rate of decline of 20'C/min was defined as the melt crystallization temperature.

Similarly, the temperature is raised above the liquid nitrogen temperature, and the change in specific heat accompanying the glass transition (secondary transition) of the polyolefin is read and taken as the glass transition temperature.

(2) Average pore diameter The long axis and short axis of the pore diameter are measured by scanning electron microscopy (SEM) observation of the surface of the sample, and the geometric mean of the average long axis and the average short axis is defined as the average pore diameter.

(3) Porosity (Pr) Sample (10cm x 10cm>) was placed in liquid paraffin for 24 hours.
After immersing the sample for an hour and thoroughly wiping off the liquid paraffin on the surface, measure the weight of the sample (W2), and calculate the pore volume (Vo) from the weight (Wl) of the sample before immersion and the density (ρ) of the liquid paraffin. It is calculated using the following formula.

Vo = (W2-Wl)/.

The porosity (Pr) is calculated by the following formula from the apparent volume (value calculated from thickness and dimensions) and the pore volume Vo.

Pr=Vo/Vx 100(%) (4> Equivalent series resistance (E'SR) JP-A-1983-
Based on the method described in No. 187221, triethylamine and 7-talic acid were dissolved in γ-butyrolactone, and 3.1
An electrolytic solution of ms/cm was prepared. The DC resistance component at Ilz of the microporous film in this electrolytic solution is expressed as FSR (Ω
).

Here, as a comparison sample, the value of electrolytic capacitor paper (Manila paper MER50) (2, OΩ) 2.5 is used as the standard, 1.7Ω or less is ○, 1.8 to 2.20 is Δ
, 2.3Ω or more was defined as X.

Note that the measurement conditions were as follows.

(a) Electrode: Platinum electrode (25 mm square) Measuring load: 240 g (b) Impedance measuring device: AG-4311LCRMETER (manufactured by Ando Electric) Measuring conditions: 1 KH 2,5 V range (5) Use electrolytic capacitor test (forced life test) Ten through holes were formed in each separator in advance using a stainless steel needle with a diameter Q and 2 mm.
Fifty electrolytic capacitor elements of 20 μF and 35 wv were manufactured and subjected to a life test.

The number of defective pieces immediately after manufacture and the number of defective pieces after 1500 hours at 85°C were evaluated.

[Example] Next, Examples and Comparative Examples of this invention will be shown, and the effects of this invention will be explained more specifically.

Example 1 Polypropylene homopolymer (PP,
A blend of 100 parts by weight of melting point 161°C) and 11 parts of dicyclohexyl phthalate (DCHP) and 100 parts by weight of ethylene propylene copolymer (EPC1 melting point 145°C) as B layer resin and DCHPl.
The blends with 50 parts by weight were each melt-extruded from separate extruders, laminated in a T-die to form a molten sheet consisting of layers A and B, and cooled in a water bath.

Subsequently, this sheet was extracted in a 1-1-1-trichloroethane solvent bath at 50°C, dried, and stretched 3.5 times at 130'C to obtain a microporous film.

The properties of the 1q film are summarized below. From the table, it can be seen that in the forced life test, the number of broken pieces was small and stable characteristics were exhibited.

Comparative Example 1 A 23 μm thick microporous PP film was produced in the same manner as in Example 1, and two layers of this film were used as a separator to produce an electrolytic capacitor.

The characteristics are shown in the table.

From the table, 30% after aging in forced life test (15 pieces)
It can be seen that there is almost no security functionality.

Comparative Example 2 Using high-density polyethylene (melting point 130°C), Example 1
A microporous film was produced in a similar manner. The characteristics and forced life test results are shown in Table 1, and 20% (10 pieces) were destroyed after time.

Example 2 A microporous polyethylene film with a thickness of 20 μm was produced using high-density polyethylene (melting point 130'C) in the same manner as in Example 1, and this was mixed with the microporous polypropylene film prepared in Comparative Example 1. An electrolytic capacitor was fabricated by winding it together and tested.

The results are shown in Table 1. Comparison with the results of Comparative Example 1 shows that the two-layer structure of the F layer and the B layer significantly reduces the number of broken pieces and improves reliability.

Claims (3)

[Claims] (1) A first porous polyolefin layer with a melting point of 158°C or higher, and a second porous polyolefin layer with a melting point of 110°C to 150°C laminated on the first porous polyolefin layer. An electrolyte separator made of a porous polyolefin laminate having an average porosity of 50% to 85% in all layers and an average pore diameter of 0.01 μm to 5 μm in all layers. (2) The first porous polyolefin layer is made of polypropylene, and the second porous polyolefin layer is made of polyethylene and/or a copolymer of ethylene and propylene. Electrolyte separator. (3) The electrolyte separator according to claim 2, wherein the polypropylene has a melt crystallization temperature of 106° C. or higher.