JP2625798B2 - Electrolyte separator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrolytic solution separator used for electrolytic capacitors, lithium batteries, batteries and the like.

[Prior art] Kraft paper, an electrolyte separator used for electrolytic capacitors, lithium batteries, batteries, etc., has long been used.
Electrolytic papers such as manila paper have been used, but recently, for example, JP-A-51-18851, JP-A-61-13614, and JP-A-59-14
As described in No. 0429 and JP-A-62-200716, use of a porous polyolefin membrane has been proposed. Porous polyolefin membrane has higher strength than paper,
The occurrence rate of short circuit (short between poles) is also low.

[Problems to be Solved by the Invention] Each of the conventionally proposed electrolyte separators composed of a porous polyolefin membrane is composed of a single layer of a porous polyolefin membrane, and the occurrence rate of short circuits is lower than that of electrolytic paper. The short circuit rate is not yet satisfactory.

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

[Means for Solving the Problems] As a result of intensive studies, the present inventors have found that a first polyolefin porous layer having a melting point of 158 ° C. or more and a melting point of 110 ° C. to 1 ° C.
When a second polyolefin porous layer at 50 ° C. is laminated and used as an electrolyte separator, even if a fine conductive material on the electrode breaks through the separator and moves to the opposite pole to cause a short circuit, the short circuit occurs. The present invention has been found that the second polyolefin porous layer having a relatively low melting point is melted by heat, and the holes formed by short-circuiting are automatically closed and sealed, thereby reducing the probability of the separator being completely destroyed. Was completed.

That is, the present invention relates to a first polyolefin-based porous layer having a melting point of 158 ° C. or higher, and a second polyolefin-based porous layer having a melting point of 110 ° C. to 150 ° C. laminated on the first polyolefin-based porous layer. And the average porosity of all the layers is
50% to 85%, average pore size of all layers is 0.01μm to 5μm
An electrolytic solution separator comprising a porous polyolefin-based laminate is provided.

As described above, in the electrolytic solution separator of the present invention, the first polyolefin-based porous layer having a melting point of 158 ° C. or higher (hereinafter referred to as “A layer”) and the second polyolefin-based porous layer having a melting point of 110 ° C. to 150 ° C. (Hereinafter referred to as layer B).

The melting point of the layer A is 158 ° C. or higher, preferably 160 ° C. or higher. When the melting point of the layer A is less than 158 ° C., the function of supporting the layer B at high temperature is reduced, and the heat resistance of the separator is reduced. The glass transition point of the layer A is preferably 10 ° C. or lower, more preferably 0 ° C. or lower. Glass transition point is 10
It becomes brittle above ℃, deteriorating cold resistance. The polyolefin constituting the layer A is polypropylene, poly 4
-Methylpentene 1, poly-3-methylbutene-1, and copolymers or blends thereof are preferable, and among them, polypropylene is particularly preferable because heat resistance and cold resistance are balanced, and the melt crystallization temperature of the polypropylene is preferable. When the temperature is 106 ° C. or higher, preferably 108 ° C. or higher, more preferably 110 ° C. or higher, the short-circuit guarantee function is further enhanced.

The melting point of the B layer is 110 ° C. to 150 ° C., preferably 120 ° C.
C to 140C. If the melting point of the B layer is lower than 110 ° C, the heat resistance is reduced and there is a risk of melting during normal operation. On the other hand, if the temperature exceeds 150 ° C, the formed hole is closed and sealed even if a short circuit occurs. It is hard to be done. As the polyolefin constituting the layer B, polyethylene, polybutene, ethylene propylene copolymer and the like are preferable, and polyethylene, ethylene propylene copolymer and a blend of both are particularly preferable. The glass transition point of the B layer is the same as that of the A layer,
The temperature is preferably 10 ° C or lower, more preferably 0 ° C or lower. The glass transition point becomes brittle above 10 ° C, and the cold resistance deteriorates.

The layer configuration of the separator is preferably A layer / B layer or B layer / A layer / B layer sandwiching the A layer at the center. The A layer and the B layer may be simply laminated, but are preferably integrated and laminated. The lamination and integration of the layer A and the layer B can be performed 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%. If the average porosity of all layers is less than 50%, the amount of the retained electrolyte is small, and the separator may dry up. If the average porosity exceeds 85%, the mechanical strength of the separator deteriorates. The average pore diameter of all layers is
It is 0.01 μm to 5 μm, preferably 0.1 μm to 3 μm. If the average pore diameter of all layers is less than 0.01 μm, the equivalent series resistance increases, and if it exceeds 5 μm, the conductive material easily passes through, and the short-circuit occurrence rate increases.

The thickness of all layers of the separator is preferably from 10 μm to 70 μm, particularly preferably from 25 μm to 50 μm, from the viewpoint of mechanical strength, occurrence rate of short circuit and compactness of the element.

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

The A layer and the B layer constituting the electrolyte separator of the present invention are formed by adding a plasticizer such as vinyl chloride such as dicyclohexyl phthalate (DCHP) or triphenyl phosphate (TPP) to 100 parts by weight of the polyolefin resin constituting the layer. 80 parts by weight to 240 parts by weight of an organic solid such as phthalic acid or a phosphoric acid ester used are mixed, preferably from 100 parts by weight to 200 parts by weight, and after melt extrusion, trichloromethane, trichloroethane, acetone, methyl ethyl ketone, Using a good solvent of an organic solid such as ethyl acetate, methanol, toluene, xylene, etc.,
%, Preferably 98% or more. Here, the extraction temperature is determined as the melting point of the organic solid−
It is preferable to keep the melting point of the organic solid at -15 ° C. or higher because the melting crystallization temperature of the polypropylene is increased and the properties are improved. When the A layer and the B layer are integrated and laminated, the A layer and the B layer can be laminated and integrated by the point fusion method after manufacturing the respective layers as described above, or the A layer and the B layer can be integrated. Can be also co-extruded, and then the above extraction is performed to perform lamination and integration. Before or after lamination, it is preferable to stretch 1.5 to 6 times at a temperature not lower than the glass transition point of the polyolefin and not higher than the melting point of −10 ° C., since both the mechanical properties and the electrical properties are improved. Furthermore, similarly to a normal polyolefin film, the melting point of the polyolefin is higher than the melting crystallization temperature, and the melting point is -5 ° C
It is preferable to heat set in the following temperature range.

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

The polyolefin-based porous layers (layer A and layer B) constituting the electrolytic solution separator of the present invention are preferably each composed of only polyolefin, but the melting point, average porosity, and average pore diameter described above are different from those of the present invention. If it falls within the range, it may be possible to contain a trace amount of impurities, and for example, a small amount of additives such as heat stabilizers, antioxidants, slip agents, antistatic agents and monomers other than olefins No problem. The term "polyolefin-based porous layer" as used in the claims includes a polyolefin-based porous layer containing such impurities and the like.

[Effect 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, so that the electrolyte separator is compared with a conventional electrolyte separator composed of a single polyolefin porous layer. And the incidence of short circuits is significantly lower. Also,
Since the electrolyte separator of the present invention has an optimized average porosity and average pore size, it has excellent equivalent series resistance and mechanical strength. When the layer A is made of polypropylene and the layer B is made of polyethylene and / or an 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 Method for Evaluating Effect] Next, the measuring method and the evaluation method according to the present invention will be summarized.

(1) Melting point, melt crystallization temperature and glass transition temperature of polyolefin Scanning calorimeter DSC-2 (Perkin-Elmer)
5 mg of a sample is measured from room temperature at a heating rate of 20 ° C./min under a nitrogen stream, and an endothermic peak temperature accompanying melting is defined as a melting point. Further, when the temperature is raised to 280 ° C. and held for 5 minutes and then cooled at a rate of 20 ° C./min, the peak of the latent heat accompanying the crystallization of the polyolefin is defined as the melt crystallization temperature.

Similarly, the temperature is raised from the temperature of liquid nitrogen, and the specific heat change accompanying the glass transition (secondary transition) of the polyolefin is read, and this is defined as the glass transition temperature.

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

(3) Porosity (Pr) A sample (10 cm × 10 cm) was immersed in liquid paraffin for 24 hours, and the weight (W ₂ ) after sufficiently wiping the surface liquid paraffin was measured. From the weight (W ₁ ) and the density of liquid paraffin (ρ), the pore volume (V ₀ ) is determined by the following equation.

V ₀ = (W ₂ −W ₁ ) / ρ The porosity (Pr) is calculated from the apparent volume (value calculated from the thickness and dimensions) V and the pore volume V _{0 according} to the following equation.

Pr = V ₀ / V × 100 (%) (4) Equivalent series resistance (ESR) Based on the method described in JP-A-61-187221, triethylamine and phthalic acid are dissolved in γ-butyrolactone,
An electrolyte of 3.1 mS / cm was prepared. The DC resistance component of the microporous film at 1 KHz in this electrolytic solution was defined as ESR (Ω).

Here, as a comparative sample, a reference value (2.0Omu) of the electrolytic capacitor paper (manila MER _2.5 50), the following 1.7 ohm ○, △ and 1.8～2.2Omu, was × or more 2.3Omu.

 The measurement conditions were as follows.

(A) Electrode: Platinum electrode (25 mm square) Measurement load 240 g (b) Impedance measurement machine: AG-4131 LCR METER (manufactured by Ando Electric Co., Ltd.) Measurement conditions: 1 KHz, 5 V range (5) Electrolytic capacitor test (forced life) Test) 10 through-holes were formed in each separator in advance using a stainless needle with a diameter of 0.2 mm for each separator.
Fifty-five μF, 35 WV electrolytic capacitor elements were manufactured and subjected to a life test. The number of defects immediately after the production and the number of defects after a lapse of 500 hours at 85 ° C. were evaluated.

[Examples] Next, examples of the present invention and comparative examples will be shown, and effects of the present invention will be described more specifically.

Example 1 100 parts by weight of a polypropylene homopolymer (PP, melting point 161 ° C.) and dicyclohexyl phthalate (DC
HP) A blend of 150 parts by weight and a blend of 100 parts by weight of ethylene propylene copolymer (EPC, melting point 145 ° C.) and 150 parts by weight of DCHP as a layer B resin are melt-extruded from different extruders, and are then extruded in a T-die. To form a molten sheet composed of layer A / layer B, and cooled in a water bath.

Subsequently, the sheet was extracted in a 1-1-1 trichloroethane solvent bath at 50 ° C, dried, and then dried at 130 ° C for 3.5 hours.
The film was stretched twice to obtain a microporous film.

The properties of the film thus obtained are summarized in the following table. It can be seen from the table that in the forced life test, the number of broken pieces is small and stable characteristics are exhibited.

Comparative Example 1 A microporous film having a thickness of 23 μm was produced from PP in the same manner as in Example 1, and an electrolytic capacitor was produced using the two layers of the film as a separator. The characteristics are shown in the table.

From the table, it can be seen that 30% (15 pieces) were destroyed after a lapse of time in the forced life test, and there was almost no security functionality.

Comparative Example 2 Example 1 using high-density polyethylene (melting point 130 ° C.)
A microporous film was produced in the same manner as described above. Table 1 shows the characteristics and the results of the forced life test.

Example 2 Example 1 using high-density polyethylene (melting point 130 ° C.)
A microporous polyethylene film having a thickness of 20 μm was produced in the same manner as described above, and this was wound together with the microporous polypropylene film obtained in Comparative Example 1 to produce an electrolytic capacitor.
The test was performed.

Table 1 shows the results. Comparing with the result of Comparative Example 1, it can be seen that the two-layer structure of the A layer and the B layer significantly reduces the number of breaks and improves the reliability.

Claims (3)

(57) [Claims] 1. A first polyolefin-based porous layer having a melting point of 158 ° C. or higher, and a second polyolefin-based porous layer having a melting point of 110 ° C. to 150 ° C. laminated on the first polyolefin-based porous layer. And the average porosity of all layers is from 50% to 85%.
%, Wherein the average pore diameter of all layers is 0.01 μm to 5 μm. 2. The method according to claim 1, wherein said first polyolefin porous layer is made of polypropylene, and said second polyolefin porous layer is made of polyethylene and / or a copolymer of ethylene and propylene. The electrolyte separator according to any one of the preceding claims. 3. The melt crystallization temperature of the polypropylene is 106 ° C.
The electrolyte separator according to claim 2, which is as described above.