JPS5843866B2 - Battery separator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a separator used to prevent short circuits between the positive and negative electrodes of negative and secondary batteries.

Conventionally, plastic microporous films or nonwoven fabrics have been used as separators for batteries, but these porous separators tend to allow the active material that makes up the electrodes to pass through the pores and diffuse to the counter electrode. Batteries using silver oxide or zinc oxide as an active material have the drawback that the degree of diffusion is large, resulting in large self-discharge and short battery life.

U.S. Patent No. 3.427, which improves this drawback,
No. 206 discloses a battery separator obtained by graft copolymerizing a polyethylene film with an unsaturated carboxylic acid.

The battery separator shown in this U.S. patent has almost no problems with active material diffusion as described above, and also has no problems with low electrical resistance and electrolyte resistance among the various performances required for battery separators. However, satisfactory results have not been obtained regarding oxidation resistance, which is one of the important properties.

In other words, the discharge phenomenon of a battery is due to the progress of oxidation-reduction within the battery, and since battery separators are placed under extremely severe oxidation conditions, they must have excellent oxidation resistance. The battery separator described in the above-mentioned US patent does not have sufficient oxidation resistance to withstand such oxidation in the battery, so batteries using this separator also have the disadvantage of short life. there were.

The present inventors conducted various studies to overcome the drawbacks of conventional separators, and as a result, we developed a separator based on a plastic film containing acrylic acid, methacrylic acid, and their hydroxyalkyl esters. It was discovered that by graft copolymerization, oxidation resistance can be significantly improved compared to graft copolymerization of only acrylic acid and methacrylic acid, without adversely affecting various performances such as electrical resistance and electrolyte resistance. , completed this invention.

That is, the battery separator according to the present invention is a graft copolymer obtained by graft copolymerizing one or more monomers selected from acrylic acid and methacrylic acid and one or more of these hydroxyalkyl ester monomers onto a plastic film base. It consists of a combined film.

Plastic films in this invention include polyethylene, polypropylene, polyester, polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene, 47-fluoroethylene-67-fluoropropylene copolymer, polyacrylonitrile, polycarbonate, polystyrene. Films made of various polymers such as polyamide, polyvinyl alcohol, polydimethylsiloxane, and ethylene-propylene polymer are used, among which fluorine-containing films such as polyethylene film, polypropylene film, polyvinyl fluoride film, and polytetrafluoroethylene film are used. Films are preferably used.

These plastic films typically have a thickness of about 2 to 200 microns.

If this thickness is too thin, the mechanical strength will be low and it will not be practical as a separate base, while if it is too thick, the electrical resistance will increase, which is not desirable.

In addition, due to the balance between mechanical strength and electrical resistance, approximately 5~
Preferably, it is 100 microns.

In this invention, the plastic film as a separator base is grafted with one or more monomers selected from acrylic acid and methacrylic acid and one or more monomers selected from acrylic acid hydroxyalkyl ester and methacrylic acid hydroxyalkyl ester. Copolymerized.

The hydroxyalkyl ester used here is
Hydroxy(C,-) of acrylic acid or methacrylic acid
C,) alkyl esters are preferably used, and specific examples thereof include hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxymimethyl methacrylate, and hydroxymethyl acrylate. Examples include oxyethyl, hydroxypropyl methacrylate, and hydroxybutyl methacrylate.

What is important in this invention is to copolymerize acrylic acid, methacrylic acid, and one or more of each of these hydroxyalkyl esters as monomers to be graft copolymerized; However, the purpose of this invention is not achieved.

At this time, the composition ratio of both (after graft copolymerization) is such that the hydroxyalkyl ester unit is 100 parts by weight or less, preferably 5 to 30 parts by weight, per 100 parts by weight of acrylic acid and methacrylic acid units. egoism,
If the amount of hydroxyalkyl ester units is too large, the resulting separator may become brittle and may not be suitable for practical use.

A method for graft copolymerizing the above acid and hydroxyalkyl ester onto a plastic film is as follows:
(1) A method in which a plastic film is irradiated with energy rays such as radiation or ultraviolet rays in advance, and then brought into contact with a monomer to be grafted and copolymerized. (2) A method in which a plastic film is brought into contact with a monomer to be grafted and irradiated with energy rays. (3) A method in which a radical generator is pre-contained in a plastic film, and graft copolymerization is carried out by bringing the plastic film into contact with a monomer and heating.
Method 1) or (2) is used.

The grafting ratio in the graft copolymer film thus obtained is usually about 10 to 200%, preferably 30%.
It is preferable to set it to 100%.

Note that this grafting rate is calculated using the following formula.

Graft Kyodo q) - to - Sufi (A')! t) F/l/A (7) My Weight of base film x 100 This graft copolymer film can be used as it is as a battery separator, but two or more of these films may be laminated. Alternatively, it may be used by laminating it with cellophane or polyvinyl alcohol film.

In addition, in order to incorporate the battery separator obtained as described above into a battery, it is preferable to react it with electrolyte components of the battery such as potassium hydroxide, sodium hydroxide, etc. to form a salt in advance. It may be incorporated as is and reacted with an electrolyte to form a salt.

The thus obtained battery separator of the present invention has low electrical resistance and excellent electrolyte resistance, and has significantly improved oxidation resistance compared to conventional separators. Batteries in which this battery separator is glued have a significantly longer lifespan than conventional products.

Furthermore, the battery separator of the present invention has the advantage that it is easier to handle because it has less self-fusing properties than a battery separator made by graft copolymerizing only acrylic acid and methacrylic acid. There is.

Next, the present invention will be explained in more detail with reference to Examples.
A graft copolymer film was obtained by soaking a 0 micron high-pressure polyethylene film (DFD 0111 manufactured by Nippon Unicar Co., Ltd.) and irradiating it with cobalt-60 gamma rays at a dose rate of 5 megarads at a dose rate of 5 x 104 rads per hour. .

Next, this graft copolymer film was immersed in hot water at 80° C. for 1 hour to remove the homopolymer of acrylic acid or hydroxyethyl methacrylate, and then dried.

The graft copolymer film thus obtained had a graft ratio of 79.

Next, this graft copolymer film was immersed for 20 minutes in a 10% potassium hydroxide aqueous solution heated to 50° C. to form a potassium salt.

Comparative Example 1 A grafting ratio of 7 was carried out in the same manner as in Example 1 except that hydroxyethyl methacrylate of Example 1 was not used.
A 1% graft copolymer film was obtained.

Example 2 The same method as in Example 1 was applied to a 20 micron thick polyvinyl fluoride film (Tetra manufactured by DuPont) in the coexistence of a solution consisting of 17 parts by weight of methacrylic acid, 3 parts by weight of hydroxyethyl methacrylate and 80 parts by weight of toluene. Graft copolymerization was performed by irradiation with gamma rays, and the same operation as in Example 1 was then performed to remove the homopolymer, and then potassium salt was obtained.

The grafting rate of the obtained graft copolymer film was 53
He was in charge.

Comparative Example 2 A graft copolymer film with a graft ratio of 49 was obtained in the same manner as in Example 2 except that hydroxyethyl methacrylate was not used.

In order to investigate the performance of the battery separators obtained in each of the above examples and ratios, the following measurements and tests were conducted, and the results shown in the table below were obtained.

(4) Conductive dust measurement A 40 weight polypotassium hydroxide aqueous solution was used as the electrolyte, nickel was used as the positive electrode, and cadmium was used as the negative electrode.
The conductive dust of each separator was measured according to 2313.

(B) Oxidation resistance test Each separator is immersed in a 10% weight hydrochloric acid aqueous solution for 10 minutes to return the potassium salt to the acid form, and then dry and weighed (the weight at this time is referred to as A).

Next, after being oxidized by immersion in a solution consisting of 10.5 parts by weight of potassium permanganate, 12.5 parts by weight of potassium hydroxide, and 240 parts by weight of water at 50°C, the manganese dioxide was removed. 15 parts by weight of oxalic acid (H2C204/2H20), 2.7 parts by weight of concentrated sulfuric acid
It was immersed in a solution consisting of parts by weight and 300 parts by weight of water at 65°C for 5 minutes and weighed dry (the weight at this time is designated as B), and the weight loss before and after the oxidation treatment was shown as multi-B (T X100). .

(C) Alkali resistance test Each separator was placed in a 40 weight polypotassium hydroxide aqueous solution.
The weight before and after immersion at 0° C. for 2 hours was measured, and the weight loss was indicated.

(D) Mounting test A mixture of silver oxide and graphite powder was used as the positive electrode, amalgamated zinc powder was used as the negative electrode, and zinc oxide was saturated.
Using a zero weight potassium hydroxide aqueous solution as an electrolytic solution, a separator was installed between the positive and negative electrodes to produce 100 silver-zinc oxide primary batteries for each separator.

This was left at 60° C. for 60 days, and the percentage of batteries that did not maintain the initial capacity was expressed as a percentage.

(E) Charge/discharge cycle number test Silver oxide with a capacity of 1 ampere/hour using ferric oxide as the positive electrode, zinc as the negative electrode, potassium hydroxide aqueous solution as the electrolyte, and a separator installed between the positive and negative electrodes. - Ten zinc secondary batteries were made for each separator.

For each battery thus obtained, charging and discharging were repeated with each cycle consisting of 20 minutes of discharging and 40 seconds of charging, and the number of charge/discharge cycles was determined, with the life span defined as the time when the original capacity could not be recovered.

As is clear from the above results, the separator of the example has significantly improved oxidation resistance without impairing conductivity and alkali resistance compared to the comparative example, and the mounting test and test results are also excellent. battery separators have been found to be very useful.

Claims (1)

[Claims] 1 By graft copolymerizing a plastic film as a base with one or more monomers selected from acrylic acid and methacrylic acid and one or more monomers selected from acrylic acid hydroxyalkyl ester and methacrylic acid hydroxyalkyl ester. A battery separator made of the obtained graft copolymer film.