JP2002324538A - Separator for battery and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a battery, which is excellent in self discharging property. SOLUTION: Ultrahigh-molecular polyethylene(UHPE) powders and particle powders such as zirconia particle powders or silica gel particle powders or the like which have ammonia trapping function are mixed. A sintered body is prepared by heating and sintering the mixture at a higher temperature than the melting point of the UHPE. A porous film is obtained by cutting the sintered body with a cutting lathe. The porous film is used as the separator for the battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a battery separator capable of suppressing self-discharge of a battery.

[0002]

2. Description of the Related Art Alkaline secondary batteries such as nickel-cadmium (nickel) batteries and nickel-metal hydride batteries
It is widely used as a small battery for electronic devices, but is also expected to be used as a power source for driving (driving) electric vehicles.

As a battery separator used for an alkaline secondary battery, a hydrophilic non-woven fabric using nylon fibers or the like and a non-woven fabric using polyolefin fibers subjected to hydrophilic treatment (for example, JP-A-4-167355, JP-A-4-167355) 1
No. 1-238496) has been conventionally used. As the hydrophilic treatment, a surfactant impregnation treatment,
Examples include plasma treatment, graft polymerization treatment of a hydrophilic monomer, and sulfonation treatment.

Since the nylon fiber nonwoven fabric has an amide bond, the battery characteristics such as self-discharge characteristics are inferior to those of the polyolefin fiber nonwoven fabric. However, the polyolefin fiber nonwoven fabric also has room for improvement in self-discharge characteristics.

It is considered that the self-discharge phenomenon is caused by nitrate ions, which are impurities contained in a trace amount in nickel hydroxide or the like, which is a positive electrode material of a battery, and its mechanism is speculated as follows ( Japanese Patent No. 2799389). That is, first, the nitrate ion present in the positive electrode is
It diffuses in the electrolyte and reaches the negative electrode. Thereafter, the nitrate ions are electrochemically reduced to nitrite ions.
This reduction step reduces the charge stored on the negative electrode. Thereafter, the nitrite ions reach the positive electrode via the electrolytic solution and are oxidized to nitrate ions again.

A mechanism has been proposed in which nitrate ions and nitrite ions are further reduced and exist as ammonium ions. It is considered that this ammonium ion exists in the electrolyte as a part of ammonia gas in a nickel-cadmium battery and a nickel-cadmium battery using a high-concentration alkaline electrolyte.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a battery separator having excellent self-discharge characteristics.

[0008]

Means for Solving the Problems To achieve the above object, a battery separator of the present invention is a battery separator including a porous film, wherein the porous film is
It is configured to contain particles having an ammonia capturing function (hereinafter, referred to as “ammonia capturing particles”).

The present inventors have conducted intensive studies on the above-described self-discharge mechanism. As a result, when a porous film containing ammonia-trapping particles is used as a battery separator, the self-discharge characteristics are superior to those of a conventional battery separator. The inventors have found that the present invention has been achieved. The battery separator of the present invention is preferably used for an alkaline battery.

The ammonia-capturing particles are preferably inorganic particles such as activated clay, silica gel, zirconia, activated bauxite, activated alumina, natural zeolite and activated magnesium oxide, and organic particles such as charcoal, bone charcoal and activated carbon.
These may be used alone or in combination of two or more.

The porous film is preferably a porous sintered body film obtained by sintering a polyolefin powder.

Next, in the method for producing a battery separator according to the present invention, a polyolefin powder and an ammonia-trapping particle powder are mixed and sintered at a temperature equal to or higher than the melting point of the polyolefin. Is cut into a film. The battery of the present invention is an alkaline secondary battery using the battery separator of the present invention.

[0013]

DETAILED DESCRIPTION OF THE INVENTION The raw material for the porous film is not particularly limited, but a polyolefin resin and a fluororesin are preferred from the viewpoint of electrochemical stability and the like. As the polyolefin resin, for example, ethylene, propylene, 1-
Crystalline homopolymers and copolymers obtained by polymerizing butene, 4-methyl-1-pentene, 1-hexene, and the like, and blends thereof, and the like can be given. Of these, ultra high molecular weight polyethylene (UHPE) is preferred. The viscosity average molecular weight of UHPE is, for example, in the range of 500,000 to 16,000,000, and preferably in the range of 2,000,000 to 10,000,000. Examples of the fluorine resin include polytetrafluoroethylene and polyvinylidene fluoride.

[0014] The porous film is not particularly limited. However, when the battery separator of the present invention is used for an alkaline secondary battery, it is necessary to consume oxygen gas and hydrogen gas generated at the electrode during charge and discharge at the counter electrode, and these gases serve as the battery separator. Must be able to penetrate. In this case, the pore size of the porous film is, for example,
It is in the range of 1 to 100 μm, preferably 10 to 50 μm. Further, the form of the porous film is not particularly limited,
For example, there are a nonwoven fabric form, a porous sintered body film form, and the like. As the nonwoven fabric, a nonwoven fabric produced by a method such as a wet method, a spun bond method, and a melt blow method is preferable.
The porous sintered body film can be produced, for example, by heating and sintering an ultra-high molecular weight polyolefin powder such as UHPE at a temperature not lower than its melting point and cutting the obtained sintered body. The thickness of the porous film is not particularly limited, and is, for example, in the range of 20 to 250 μm, preferably 50 to 20 μm.
The range is 0 μm. The porosity of the porous film is in the range of 30 to 70% by volume, preferably in the range of 50 to 70% by volume. The porosity is determined based on the area S (cm ² ), thickness d (cm), and mass m (g) of one side of the porous film.
And the specific gravity r (g / cm ³ ) of the forming material, it can be calculated by the following equation (Equation 1). The permeability of the porous film by the soap film flow method is, for example, 1 × 1
0 ^-4 to 1 × 10 ^-2 cm ³ · cm / cm ² · Pa · sec, preferably 5 × 10 ^-4 to 1 × 10 ^-2 cm ³ · cm
/ Cm ² · Pa · sec. Breathable by the soap film flow method, under pressure conditions of 12.7mmH ₂ O, the permeation amount of oxygen passed through the porous membrane, is measured by the effective area 1.23Cm ² of film.

(Equation 1) Porosity (volume%) = [1-((m / r) / (S ×
d))] x 100

As the ammonia trapping particles, those described above can be used. Among them, preferred are zirconia particles, silica gel particles, and natural zeolites. The diameter of the ammonia trapping particles is, for example, in the range of 0.01 to 10 μm, and preferably in the range of 0.1 to 1 μm. The ratio of the ammonia trapping particles is, for example, in the range of 0.01 to 5% by weight, and preferably in the range of 0.01 to 1% by weight, based on the entire porous film.

Next, the porous film according to the present invention can be produced, for example, as follows. First, ammonia capture particles and ultra-high molecular weight polyolefin powder such as UHPE are mixed, and the mixture is put into a shape retainer (mold). The shape retainer preferably has a hole into which steam can be introduced. The shape retainer is placed in a pressure-resistant container, and after evacuating air, steam heated to a temperature equal to or higher than the melting point of the polyolefin is introduced, followed by heat sintering. Thereafter, the resultant is cooled, the sintered body is taken out, and cut to a predetermined thickness, whereby a porous sintered body film containing ammonia-trapped particles can be obtained. This film can be used as it is as a battery separator. In addition, when the porous film is used for an alkaline battery (a primary battery and a secondary battery), it is preferable to perform a hydrophilic treatment if the porous film is hydrophobic. The method of the hydrophilic treatment is not particularly limited, and examples thereof include a surfactant impregnation treatment, a plasma treatment, a graft polymerization treatment of a hydrophilic monomer, and a sulfonation treatment.

[0018]

Next, examples of the present invention will be described together with comparative examples.

Example 1 10 g of zirconia particle powder (trade name: Kesmon NS-10, manufactured by Toa Gosei Chemical Co., Ltd.)
And UHPE powder (viscosity average molecular weight 4.5 million, melting point 13
5 ° C, average particle size 106 µm, mesh classification product) 2000
g., and the mixture was placed in a shape retainer. In this shape retainer, a cylindrical wire mesh basket (outer diameter 4 cm) smaller than the cylindrical wire mesh basket (outer diameter 4 cm) is arranged in a centered state, and a donut-like shape is placed between the two baskets. A space is formed, and a polytetrafluoroethylene porous film is adhered to a surface forming the space. The shape retainer was placed in a metal heat-resistant and pressure-resistant container (provided with a steam introduction pipe and an opening / closing valve), and the atmospheric pressure in the container was set to 1.3 kPa with a vacuum pump. Then, stop the pump and leave it for 30 minutes, then open the valve, introduce steam, and raise the temperature to 120 ° C. in 10 minutes.
After maintaining at the same temperature for 30 minutes, the water vapor pressure was increased to 0.4.
Up to 145 ° C, and at the same temperature
It was heated and sintered for 3 hours. Thereafter, the valve was closed and allowed to cool naturally to obtain a cylindrical porous body. The obtained porous body was cut to a thickness of 200 μm by a cutting lathe to obtain a porous film having a porosity of 50% by volume.

(Example 2) 5 g of silica gel particle powder (trade name: Aerosil 200, manufactured by Nippon Aerosil Co., Ltd.) and 2000 g of the UHPE powder were mixed, and this was mixed with Example 1
Under the same conditions and operation as in the above, and the obtained sintered body was cut to obtain a porous film having a thickness of 200 μm (porosity of 50 μm).
% By volume).

(Comparative Example 1) Only the UHPE powder was heated and sintered under the same conditions and operation as in Example 1, and the obtained sintered body was cut to form a porous film having a thickness of 200 μm (porosity of 43 volumes). %).

Each of the porous films of Examples 1 and 2 and Comparative Example 1 thus obtained was used as a battery separator, and the self-discharge characteristics were examined by the following method.

(Evaluation of Self-Discharge Characteristics) The porous film was previously immersed in an electrolytic solution to perform vacuum impregnation, and the electrolytic solution was introduced into the pores. Then, using the porous film as a battery separator, a 2032 size (diameter: 20 mm, height: 3.2 mm) coin-type nickel-metal hydride battery (positive electrode active material: Ni hydroxide, negative electrode active material: Hydrogen storage alloy, electrolytic solution: aqueous potassium hydroxide solution) were prepared. After subjecting the battery to a chemical conversion treatment, the discharge capacity was measured. After the battery was fully charged, it was stored in an atmosphere at 45 ° C., and the self-discharge was measured one week after the full charge. The results are shown in Table 1 below. The discharge rate was 0.2 C ₅ A, and the self-discharge rate (capacity retention rate) was determined by the following equation.

Capacity retention (%) = [a / ((b + c) /
2)] × 100 a: Discharge capacity after self-discharge (Ah) b: Discharge capacity before self-discharge (Ah) c: Discharge capacity after self-discharge and full recharge (Ah)

(Table 1) Self-discharge rate (capacity retention rate) Example 1 85% Example 2 78% Comparative example 1 44%

From the above results, it is understood that the battery using the porous film containing the ammonia trapping particles has excellent self-discharge characteristics.

[0027]

As described above, the use of the battery separator of the present invention improves the self-discharge characteristics of the battery.

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Claims (6)

[Claims] 1. A battery separator including a porous film, wherein the porous film contains particles having an ammonia trapping function. 2. Particles having an ammonia trapping function are activated clay, silica gel, zirconia, activated bauxite,
The battery separator according to claim 1, wherein the separator is at least one selected from the group consisting of activated alumina, natural zeolite, activated magnesium oxide, charcoal, bone charcoal and activated carbon. 3. The battery separator according to claim 1, wherein the porous film is a porous sintered body film obtained by sintering a polyolefin powder. 4. The battery separator according to claim 1, which is used for an alkaline battery. 5. A battery for mixing a polyolefin powder and a particle powder having an ammonia trapping function, sintering the mixed powder at a temperature equal to or higher than the melting point of the polyolefin, and cutting the obtained sintered body into a film. Manufacturing method of separator. 6. An alkaline secondary battery using the battery separator according to any one of claims 1 to 3.