JPH07122257A - Separator for battery - Google Patents

Abstract

PURPOSE:To improve ion permeability, alkali resistance, oxidation resistance, heat resistance, liquid holdability and gas permeability as a separator for a battery where an alkali water solution serves as an electrolyte, in a film stabilizing a hydrophilic high polymer in porous space of a hydrphobic porous film consisting of fluororesin or the like. CONSTITUTION:A hydropholic high polymer of polyvinyl alcohol, polyethylene imine, etc., is stabilized in a hydrophobic porous film consisting of fluororesin or the like, to obtain insolubility. In the case of forming insolubility in water, first the hydrophobic resin porous film is impregnated with a solution of water soluble high polymer, to introduce the high polymer into porous space. Next for insolubility of the water soluble high polymer, a bridging process is performed. In order to uniformly distribute the water soluble high polymer in the inside of the porous space, impregnation and drying are repeated over several times. In order to give oxygen permeability to an obtained separator, a hydrophilic process is not completely performed, to leave a hydrophobic part. This separator is used in a battery with an alkali water solution serving as the electrolyte.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator, and more particularly to a battery separator such as a nickel-cadmium battery or a nickel-hydrogen battery which uses an alkaline aqueous solution as an electrolytic solution.

[0002]

2. Description of the Related Art In an alkaline storage battery, for example, a nickel-cadmium battery, a negative electrode containing metal cadmium as an active material and a positive electrode containing nickel oxyhydroxide as an active material are spirally wound with a separator interposed therebetween.
This is stored in a battery case and immersed in a strong alkaline solution.

As a separator for such an alkaline storage battery, a woven or non-woven fabric of polyamide fiber or a combination of these with a cellophane or polyvinyl alcohol film has been adopted. In addition, recently, a woven fabric or a non-woven fabric made of polyolefin such as polypropylene has been partially used particularly in view of alkali resistance and oxidation resistance.

Further, in JP-A-4-286863, a fluororesin having a porous structure, such as polytetrafluoroethylene, which is surface-treated with plasma to be hydrophilic, is used as a separator for an alkaline storage battery. It is disclosed.

[0005]

The separator has good ion passage, resistance to alkalis, resistance to oxidation, resistance to heat, sufficient retention of electrolyte, and oxygen generated from the positive electrode during overcharge. Since the gas is reduced at the negative electrode, it is required that the gas can permeate quickly.

Woven or non-woven fabric of polyamide fiber, or a combination of these with cellophane or polyvinyl alcohol has problems in alkali resistance and oxidation resistance. For example, nylon non-woven fabrics may undergo degradation in batteries. Such decomposition may generate carbon dioxide gas or ammonia, increase the internal pressure of the battery, and adversely affect the characteristics of the battery. Woven and non-woven fabrics made of polyolefin such as polypropylene have poor wettability and poor electrolyte retention. Further, the non-woven fabric has a large pore size. Therefore, in some cases, in order to prevent a short circuit between the positive electrode and the negative electrode, it is necessary to reduce the porosity to sacrifice the ion permeability to some extent.

Therefore, in order to improve the liquid-retaining property, the hydrophilic group can be introduced by treating the polyolefin with fuming sulfuric acid or concentrated sulfuric acid. However, this treatment causes a problem of deterioration of the polyolefin. In addition, since the number of hydrophilic groups such as sulfonic acid groups that can be introduced is limited, it is difficult to obtain sufficient liquid retention.

In addition to these problems, the heat resistance of the separator due to the temperature rise during the operation of the battery has become a problem with the charging and discharging at high current density and the increase in size of the battery. During rapid charging, the temperature inside the battery rises significantly, and heat resistance is insufficient only with polyamide or polyolefin materials, which affects the charge / discharge characteristics and life of the battery.

Japanese Patent Laid-Open No. 4-286863 attempts to use a fluororesin having high heat resistance as a separator. However, there is a limit to the number of hydrophilic groups that can be introduced into the fluororesin by the plasma treatment as disclosed in the publication, and in this case too, sufficient liquid retaining properties have not been obtained. Further, it has been desired to further improve the gas permeability of the fluororesin separator.

An object of the present invention is to provide a separator which is excellent in ion permeability, alkali resistance, oxidation resistance, heat resistance and liquid retention, and is also excellent in gas permeability.

[0011]

Means for Solving the Problems The battery separator of the present invention comprises a membrane in which one or more hydrophilic polymers are immobilized in the porous space of a porous membrane made of a hydrophobic resin. Characterize.

In the present invention, a polyolefin such as polyethylene or polypropylene, or a fluororesin is preferably used as the hydrophobic resin forming the porous film. The porous film can be formed of a woven fabric, a non-woven fabric, a stretched film or the like.

In the present invention, the fluororesin includes polymers such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride and polychlorotrifluoroethylene, and copolymers of tetrafluoroethylene and vinylidene fluoride, and perfluoroethylene. A copolymer such as a fluoroethylene propylene copolymer or a copolymer of perfluoroalkyl ether and tetrafluoroethylene can be used. In the present invention, the hydrophobic resin porous film is intended to have a space in which the hydrophobic resin serves as a matrix and has a space inside, and the space penetrates from the front surface to the back surface of the film. Membranes with only are excluded.

Since the vinylidene fluoride polymer and its copolymer can be dissolved in a specific solvent, by combining it with a third non-solvent, a porosity of 60% or more can be obtained by a method similar to the method for producing a cellulose ester membrane. You can get one.
For polytetrafluoroethylene, for example, Japanese Patent Publication No.
By stretching at least uniaxially, preferably uniaxially and biaxially at a temperature equal to or lower than the crystalline melting point of the resin, as in -13560 and Japanese Patent Publication No. 51-18991, and then heating to a crystalline melting point or higher in the stretched state, Porosity 6 having a three-dimensional network structure in which fibrous bodies are integrally connected to each other 6
A porous body of 0% or more, and further 90% or more can be obtained.

In the present invention, the porosity of the porous film such as fluororesin is preferably 70% or more in order to ensure sufficient liquid retention and gas permeability. In order to prevent a short circuit between the positive electrode and the negative electrode, the average pore diameter of the porous film is preferably 50 μm or less, more preferably 20 μm or less, and 10 μm.
The following is even more preferable. It is considered that the lower limit of the pore size is mainly determined by the amount of oxygen generated from the positive electrode. However, this amount of oxygen depends on the amount of current during overcharge, and the amount of current is affected by the operating temperature, battery size, performance of the active material, etc., so the lower limit of the pore diameter is not specified. The average pore size is set so that the particles forming the electrodes in the battery do not pass through the porous membrane.

In the present invention, a water-soluble polymer is preferably used as the hydrophilic polymer. Examples of the water-soluble polymer include polymers (polymers or copolymers) containing a hydroxyl group such as polyvinyl alcohol, polyethylene glycol, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, polyacrylic acid, polymethacrylic acid, etc. Polymers containing carboxyl groups (polymers or copolymers), polyacrylamide, polyvinylpyrrolidone, polyvinylamine, polyethyleneimine and other nitrogen-containing polymers (polymers or copolymers), polystyrene sulfonic acid, Nafion (registered) A polymer (polymer or copolymer) containing a sulfonic acid group such as a trademark or a combination thereof can be used. For example, as polyvinyl alcohol and polyethyleneimine, commercially available products that are stable in quality in terms of the degree of polymerization and the like can be used as they are. The water-soluble polymer is
As a substance for suppressing the water repellency of the hydrophobic resin porous film and imparting hydrophilicity, it is insolubilized in water in the porous space of the film.

For water insolubilization, first, an aqueous solution of a water-soluble polymer is prepared, and the aqueous solution is impregnated into a hydrophobic resin porous membrane to introduce the polymer into the porous space. Next, a cross-linking treatment is performed to make the water-soluble polymer insoluble in water.
Due to this insolubilization in water, heat treatment, acetalization treatment, esterification treatment, chemical reaction with potassium dichromate, crosslinking reaction with ionizing radiation, or the like can be performed. Since the water-soluble polymer is uniformly distributed inside the porous space, the concentration of the water-soluble polymer in the inner space can be increased, for example, by impregnating and drying the dilute aqueous solution several times. Also, when the pore size of the porous membrane is small, first dip the porous membrane in ethanol, methanol, acetone, or an aqueous solution of a surfactant, and then soak it in water so that water is retained in the porous space. Perform processing. Then, the polymer can be efficiently introduced into the porous space by immersing the porous film in an aqueous solution of the water-soluble polymer.

The water-insolubilized polymer is not only chemically bonded to the surface of the hydrophobic resin porous membrane, but also can be cross-linked to increase the molecular weight of the polymer to make it insoluble in water. The water-insoluble polymer is entangled with the hydrophobic resin porous membrane and covers the surface of the porous structure.

Further, the hydrophilic resin is dissolved in a solvent in which the hydrophobic resin is insoluble and the hydrophilic resin is soluble, and the porous membrane of the hydrophobic resin is immersed in this solvent and then dried to obtain a porous film. A hydrophilic resin can be enclosed in the flexible film.

On the other hand, in the sealed alkaline storage battery, oxygen is generated at the positive electrode during rapid charging, for example. Therefore, in order to prevent the internal pressure from rising due to oxygen, the separator is required to have an oxygen gas permeable capacity. According to the research conducted by the present inventors, the oxygen gas permeability of the separator is increased by partially not providing the hydrophilic polymer, that is, by leaving the hydrophobic portion without completely performing the hydrophilic treatment. It was revealed.

That is, in a preferred embodiment of the present invention, the distribution of the hydrophilic polymer in the porous membrane is non-uniform, so that the porous space has a hydrophilic first structure.
And a second portion that is less hydrophilic or more hydrophobic than the first portion. The first part is the second
The hydrophilic polymer may be contained in a larger amount than that of the portion. Further, the second portion can be a portion that does not contain the hydrophilic polymer. The property of the second part is strongly influenced by the hydrophobic resin constituting the porous membrane, and the property of the first part is strongly influenced by the hydrophilic polymer.

FIG. 1 shows a concrete example of a separator having such a portion. In the separator shown in FIG. 1A, a portion 1 (shown by diagonal lines in the figure) having low hydrophilicity or hydrophobicity is formed in a stripe shape. The other portion 2 is given sufficient hydrophilicity by the hydrophilic polymer.

In the separator shown in FIG. 1 (b), the portion 11 having low hydrophilicity or hydrophobicity (shown by diagonal lines)
Are provided in the form of spots or polka dots. The other portion 12 is given sufficient hydrophilicity.

In order to form the pattern as shown above, for example, when a porous film is impregnated with a hydrophilic polymer solution and irradiated with an electron beam or the like for crosslinking and immobilization, a mask having a desired pattern is used. It suffices to cover the porous film and irradiate it with an electron beam or the like. According to this method, the hydrophilic polymer can be selectively immobilized on the portion irradiated with the electron beam according to the pattern. The part of the polymer that has not been irradiated may be dissolved.

Further, as shown in FIG. 1 (c), it is also possible to provide a portion 21 (shown by diagonal lines) in which only one of a pair of opposing main surfaces of the separator is hydrophobic. Part 21
Can be a portion having less hydrophilic polymer content than the other portion 22 or containing no such polymer. Portion 21 is less hydrophilic or hydrophobic than portion 22. When such a separator is used in a Ni-Cd battery, a Ni-MH battery, or the like, it is preferable that the hydrophobic surface is in contact with the Ni electrode.

When it is difficult to make the distribution of the hydrophilic polymer non-uniform in one porous membrane, a plurality of separate porous membranes are prepared, and the degree of hydrophilicity of some membranes,
The degree of hydrophobicity may be changed. For example, different contents of hydrophilic polymer can be immobilized on each membrane and the membranes can be superposed. It is also possible to superimpose a membrane having sufficient hydrophilicity and a membrane that is less hydrophilic or hydrophobic than this membrane. The latter membrane can be a membrane containing a small amount of hydrophilic groups or no hydrophilic groups at all. FIG. 1D shows an example of a separator formed by superposing a hydrophobic film 31 and a hydrophilic film 32. The composition of such a film can be as described above. Such superposition of the films can bring about the same effect as that in which one surface is made hydrophobic in one film as shown in FIG. 1 (c).

The present invention is applied to a storage battery using an alkaline aqueous solution as an electrolytic solution, and particularly preferably to an alkaline secondary battery such as a nickel-cadmium battery or a nickel-hydrogen battery, but is not limited to these, for example, It can also be applied to iron-nickel batteries, zinc-nickel batteries, and the like.

[0028]

In the separator of the present invention, the porous film made of the hydrophobic resin has the functions of strength, chemical resistance and heat resistance. By immobilizing a hydrophilic polymer on this porous membrane, a larger amount of hydrophilic groups can be introduced as compared with the treatment with concentrated sulfuric acid or the like of polyolefin or the treatment with plasma of fluororesin. Therefore, the present invention is superior to the conventional separator in the liquid retaining power. In the battery using the separator of the present invention, the electrolyte solution in the separator is changed to Cd of the Ni-Cd battery by repeating the charge / discharge reaction.
It is suppressed that the electrode is absorbed by the hydrogen storage alloy electrode of the Ni-hydrogen battery, and the life of the battery is extended. The present invention is based on the excellent mechanical strength, chemical resistance and heat resistance of the hydrophobic resin film and imparts sufficient hydrophilicity thereto.

Particularly, the fluororesin is excellent in chemical stability. For example, fluororesins are excellent in alkali resistance and oxidation resistance, and do not show thermoplasticity up to 240 ° C. and also have excellent heat resistance. With respect to the fluororesin, a porous film having a good morphology can be obtained by using the process described above. If the maximum pore diameter of the porous film is smaller than that of the particles forming the electrodes, there is no risk of short circuit between the positive and negative electrodes.

By immobilizing a water-soluble polymer in the porous space of such a fluororesin porous membrane, the membrane is rendered hydrophilic. In this case, hydrophilization is performed without impairing the chemical stability and heat resistance of the fluororesin itself. on the other hand,
When the fluororesin is made hydrophilic by plasma treatment or corona discharge according to the conventional technique, its chemical stability and heat resistance are deteriorated.

It should also be noted that, as shown in the following examples, the separator used in the present invention is superior in gas permeability to the conventional separator hydrophilized by plasma treatment. Although the reason why the gas permeability is improved is not clear, the battery having the separator hydrophilized according to the present invention can more effectively suppress the internal pressure increase due to the gas generation at the end of charge than the battery having the conventional separator. .

Further, in the separator used in the present invention, the concentration of the water-soluble polymer retained in the porous space can be accurately adjusted, and by this adjustment, the hydrophilicity and the liquid retention property can be adjusted according to the battery conditions. Can be controlled. further,
The characteristics of the separator, such as liquid retention, can be controlled according to the conditions of the battery, depending on the type of water-soluble polymer and the degree of insolubilization. As described above, according to the present invention, it is possible to provide a battery including a separator having appropriate characteristics depending on the positive electrode active material, the negative electrode active material, the electrolytic solution and the like of the battery.

[0033]

【Example】

Example 1 Pore Freon (registered trademark) (PT) manufactured by Sumitomo Electric Industries, Ltd.
Polyethylene alcohol (Kuraray Poval PVA-217, average degree of polymerization 1750, degree of saponification 88 mol%, manufactured by Kuraray Co., Ltd.) 0.5% It was immersed in an aqueous solution of. Then, the porous membrane impregnated with the aqueous solution of polyvinyl alcohol was irradiated with an electron beam of 6 Mrad to insolubilize the polyvinyl alcohol with water to form a hydrophilic resin layer. The membrane thus obtained was used as a battery separator.

A known foaming nickel electrode filled with nickel hydroxide powder, nickel powder and cobalt powder was used for the positive electrode of the battery.

For the negative electrode, MmNi _3.8 Mn _0.4 Al _0.3
Using a hydrogen storage alloy of Co _0.5 (Mm represents misch metal), this alloy powder was mixed with a 2% aqueous solution of polyvinyl alcohol to form a paste, and a foamed nickel porous body (Celmet (registered trademark of Sumitomo Electric Industries, Ltd.) )) And was roller-pressed.

A separator is sandwiched between the positive electrode and the negative electrode, which are manufactured as described above, and they are spirally wound to have a diameter of 22.5.
mm, height 49.2 mm, and stored in a potassium hydroxide aqueous solution with a specific gravity of 1.20.
An electrolytic solution in which g / l was dissolved was injected and then sealed to produce a sealed battery.

Example 2 Sumitomo Electric Industries, Ltd. Pore Freon (registered trademark) having a porosity of 80%, an average pore diameter of 10 μm and a film thickness of 100 μm was diluted with a commercially available 30% polyethyleneimine aqueous solution to a 7% solution with isopropyl alcohol. The sample was dipped in the above solution for 10 minutes, air-dried, and then dipped in a 5% glyoxal aqueous solution for 1 minute for crosslinking treatment to form a hydrophilic resin layer.
Next, this membrane was washed with water and used as a separator. A sealed battery was produced in the same manner as in Example 1 except that this separator was used.

Example 3 An ethylene / vinyl alcohol random copolymer (trade name, Eval, manufactured by Kuraray Co., Ltd.) was dissolved in a solution of isopropyl alcohol: water = 7: 1, and the same Poreflon as used in Example 1 was dissolved therein. The membrane was immersed. Then, it was pulled up and immersed in water to form a hydrophilic resin layer made of the copolymer in the film. The obtained film is used as a separator,
A sealed battery was produced in the same manner as in Example 1.

Example 4 A hydrophilic polymer layer was formed in the same manner as in Example 1 except that a polyethylene non-woven fabric was used in place of the Poreflon membrane to prepare a separator. A sealed battery similar to that in Example 1 was manufactured using the obtained separator.

Example 5 In the same manner as in Example 1, the polyvinyl alcohol aqueous solution was impregnated into the Poreflon membrane. Next, lead rods having a width of 0.5 mm are arranged at intervals of 10 mm, and 6 Mrad is applied to the film through the lead rods.
Was irradiated with an electron beam. The immobilization of the impregnated polyvinyl alcohol proceeded in the part of the film irradiated with the electron beam, while the immobilization was suppressed in the part shielded from the irradiation by the lead rod. As a result, it was possible to leave a hydrophobic portion in the non-irradiated portion. Using the separator thus obtained, a sealed battery was produced in the same manner as in Example 1.

Example 6 After preparing a separator in the same manner as in Example 3, one surface of the separator was wiped with a cloth containing isopropyl alcohol.
The surface was made hydrophobic. This membrane was immersed in water in which a surfactant was dissolved, dried, and then used as a separator to prepare a sealed battery similar to that of Example 1.

Example 7 A hydrophilic treatment was carried out in the same manner as in Example 1 using a 50 μm thick Poreflon membrane. The obtained film was laminated with a 50 μm-thick Poreflon film coated with a surfactant to prepare a separator. A sealed battery similar to that of Example 1 was manufactured using this separator.

Example 8 Two pieces of 25 μm-thick Poreflon membranes were hydrophilized in the same manner as in Example 1. The two films obtained have a thickness of 50
A sulfonated polypropylene nonwoven fabric (porosity 60%) having a thickness of μm was sandwiched to obtain a separator as a laminate.
A sealed battery similar to that of Example 1 was manufactured using this separator.

Example 9 A 25 μm-thick Poreflon membrane was subjected to hydrophilic treatment in the same manner as in Example 1. The obtained membrane was laminated with a sulfonated polypropylene nonwoven fabric (porosity 60%) having a thickness of 75 μm to obtain a separator. A sealed battery similar to that of Example 1 was manufactured using this separator.

Comparative Example 1 Example 1 was repeated except that a polypropylene non-woven fabric soaked in fuming sulfuric acid, which was conventionally used, was used as the separator.
A sealed battery was manufactured in the same manner as in.

Comparative Example 2 Poreflon (registered trademark) manufactured by Sumitomo Electric Industries, Ltd. having a porosity of 80%, an average pore diameter of 10 μm and a film thickness of 100 μm was hydrophilized by plasma treatment in 0.5 Torr nitrogen with a discharge power of 10 W. Used as a separator. A sealed battery was produced in the same manner as in Example 1 except that this separator was used.

The above 11 kinds of batteries are operated at 0.5 A at 7
A cycle test of discharging with time charge of −0.5 A to 0.9 V was performed. The discharge capacity at the 5th cycle was 2.
It was 5 Ah. This value was used as the capacity of each battery.
Next, the 11 kinds of batteries were tested by repeating a rapid charging / discharging cycle of charging at 2.5 A for 1.5 hours and discharging at 2.5 A to 0.9 V for a long period of time. The results are shown in Table 1. As shown in Table 1, it can be seen that the batteries of Examples 1 to 9 according to the present invention have a long life, and even in the rapid charge / discharge cycle, oxygen gas is successfully permeated through the separator and the internal pressure is kept low. Further, it can be seen that by providing a hydrophobic portion or a portion having low hydrophilicity in the separator, gas permeation is promoted and the internal pressure of the battery can be suppressed low.

[0048]

[Table 1]

[0049]

As described above, according to the present invention,
By using a separator that is excellent in ion permeability, can sufficiently retain an electrolyte solution, and is excellent in oxygen gas permeability at the time of charging, in a battery using an alkaline aqueous solution as an electrolyte solution, it is possible to improve the characteristics and life of the battery. it can.

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing a specific example of the present invention.

[Explanation of symbols]

1,11,21 parts with low hydrophilicity or hydrophobicity 2,12,22 parts with sufficient hydrophilicity 31 hydrophobic membranes 32 hydrophilic membranes

Claims (7)

[Claims] 1. A battery separator, comprising a porous membrane made of a hydrophobic resin and a membrane having one or more hydrophilic polymers immobilized in the porous space. 2. The battery separator according to claim 1, wherein the hydrophobic resin is a fluororesin and is used in a battery using an alkaline aqueous solution as an electrolytic solution. 3. The hydrophilic polymer is one or two selected from the group consisting of a polymer containing a hydroxyl group, a polymer containing a carboxyl group, a polymer containing an imine, and a polymer containing a sulfonic acid group. The battery separator according to claim 1, wherein the battery separator is one or more kinds of polymers. 4. The distribution of the hydrophilic polymer in the porous membrane is non-uniform, whereby the porous space has a first portion having hydrophilicity due to the hydrophilic polymer,
A second part having a lower hydrophilicity or a hydrophobicity than the first part is provided.
The battery separator described. 5. The battery separator according to claim 4, wherein the second portion is formed in a spot shape or a stripe shape on the porous film. 6. The battery separator according to claim 4, wherein the second portion is formed on one of a pair of opposing main surfaces of the porous membrane. 7. The porous membrane is formed by stacking a plurality of separate membranes, wherein the plurality of membranes are a hydrophilic first membrane and a hydrophilic membrane more hydrophilic than the first membrane. 5. The battery separator according to claim 4, comprising a second film having low property or having a hydrophobic property.