JPS6019624B2 - Battery separator and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a battery separator and a method for manufacturing the same, and provides a separator with excellent dimensional stability.

In recent years, with the demand for higher performance and longer life of batteries, highly durable separators, especially graft membranes made of polyethylene film as a base material and graft polymerized with monomers having ion exchange groups, have been developed. It is being considered as a battery separator.

In these graft membranes, monomers having ion exchange groups in the polyethylene main chain form branch polymers, thereby absorbing electrolyte into the membrane and providing ion conductivity. The ionic conductivity increases as the monomer grafting rate increases. On the other hand, the membrane swells to contain the electrolyte in the pus.
Its dimensions change in length, width, thickness, and direction. The dimensional change due to this swelling droplet increases as the grafting ratio increases. The dimensional change of a membrane having ionic conductivity (electrical resistance) that can be used as a separator in a normal battery in an electrolytic solution is 15 to 20%, and reaches as much as 25%. Such a significant dimensional change, particularly in the vertical and horizontal directions, makes battery assembly extremely difficult in a small battery with limited dimensions and volume. Furthermore, even in large batteries where dimensional changes are relatively permissible, the separator sandwiched between the electrodes swells after electrolyte injection, creating uneven voids between the electrodes and causing gas retention, which impairs battery performance. There were drawbacks such as lowering the Conventionally, these grafted membranes have been developed to eliminate such drawbacks (in the pre-mechanically stretched state) and to provide the battery electrolyte with other separators that have dimensional stability (particularly in the longitudinal and transverse directions), such as cellophane, synthetic By pasting it with a porous film made of resin, swelling changes caused by the electrolyte were prevented. For this reason, the graft membrane has not been used alone, but in combination with other separators. Therefore, despite the excellent performance of graft membranes, graft membranes have not been effective in improving the performance of batteries, particularly in improving volumetric efficiency.
The present invention eliminates the above-mentioned drawbacks of the conventional art and provides a separator with particularly excellent dimensional stability.

That is, when graft polymerizing a monomer having an ion exchange group using a polyethylene film as a backbone polymer,
A part with a low grafting ratio or no grafting at all (hereinafter referred to as a non-grafting part) is continuously formed around a part with a high grafting ratio, and the part with a high grafting ratio is made discontinuous. It separates it from the part with a high grafting rate. In a graft membrane with discontinuous areas with a high graft ratio, even if only that area is wetted by the electrolyte, the surrounding non-grafted area does not swell or has very low swelling, so the separator as a whole does not swell. The dimensional change will be extremely small. That is, since the non-grafted portion is arranged around the portion with a high grafting ratio, swelling of the membrane occurs only in the portion with a high grafting ratio, and does not affect the dimensions when used as a separator. The electrical resistance of this separator is lower than that of a graft film that is uniformly graft-polymerized over the entire surface of the film and that has a low graft ratio of 4 to suppress the swelling ratio. In a grafted film, the relationship between electrical resistance and grafting ratio is generally logarithmically proportional, and a slight decrease in the grafting ratio results in a significant increase in electrical resistance. Since the electrical resistance of the separator of the present invention depends on the ratio of the non-grafted portion to the highly graphed portion, the electrical resistance will not increase excessively even if the average grafting rate decreases. A suitable method for forming the non-grafted portion and the highly graphed portion can be selected depending on the graft polymerization method. Graft polymerization methods include: ■ Graft polymerization by irradiating ionizing radiation in the coexistence of the base film and monomer (simultaneous irradiation method); ■ After irradiating the base film with ionizing radiation in advance, There is a graft polymerization method (pre-irradiation method) in which the monomer is brought into contact with the monomer, and a graft polymerization method (chemical method) in which a polymerization initiator is used in the coexistence of the monomer and the Motomura film. In other words, in the pre-irradiation method, when a polyethylene film is irradiated with ionizing radiation, a material that shields the radiation, such as a metallic mesh, is applied to the irradiated film. Irradiation is carried out by contacting the surface of the metal mesh.Since the area shielded by the metal mesh is not irradiated with radiation, no starting points (mainly radicals) for graft polymerization are generated, so the monomers cannot be brought into contact with each other. Another method is to apply a substance that inhibits graft polymerization between polyethylene and the monomer, such as oil-based ink when the monomer solution is an aqueous solution, to the surface of the polyethylene film. It is possible to form a non-graphed area by printing or coating it in a lattice pattern on top to prevent contact between this area and the monomer.
In the simultaneous irradiation method and the chemical method, the non-graph portion can be formed by the same method as described above, taking into consideration the type of monomer, solvent, etc. as appropriate. The shape and size of the high graph area and the non-graph area can be appropriately selected depending on the battery used.

For example, in the case of a sealed small battery, where the separator size is around 1 mm in diameter, there is a grid-like non-grafted part with a shape and size equivalent to 10 mm or less, and a height surrounded by the non-grafted part. Graft membranes having graph sections are suitable as separators. On the other hand, open type Ni-
Like CD batteries, the separator dimensions are 12cm long and 10cm wide.
For a slightly larger battery used by the enemy, a lattice pattern equivalent to 2 mesh may be used. Furthermore, the shapes of the graph section and the non-graph section may be any shape such as polka dot, hexagonal, or lattice, but at least the non-graph section is continuous, and the graph section is separated from each other and surrounded by the non-graph section. It is necessary that the Next, the present invention will be specifically explained with reference to Examples, but the present invention is not limited by such explanation.

Example 1 Oil-based paint was printed on a low-density polyethylene film with a waist thickness of 25 mm in a grid pattern equivalent to 15 esh sheets, so that the film consisted of independent square non-printed areas and continuous printed areas.

This polyethylene film was coated with an electron beam (acceleration voltage MeV,
An accelerating current &hA) of 30 Mrad was applied. Then, remove dissolved oxygen in advance with nitrogen gas (less than 0.1 leg)
50 parts of acrylic acid, 50 parts of water, 0.25% Mohr's salt
This irradiated film was added to a monomer aqueous solution consisting of
0 for 5 hours. The graft membrane thus obtained was washed with water, immersed in a 2.5% KOH aqueous solution, and heated at 90 oo for 5 minutes to form a potassium salt. The average grafting rate of the obtained separator was 65%, and 250% in 40% KOH aqueous solution.
The electrical resistance measured by the loo0 cross-current method at 0 is 85○
・I met you. In addition, 100 m x 10 in 40% KOH
The elongation of the separator soaked with the dimension of 0 skin is 0.5 x 0
．． It was 3 skin. On the other hand, a graft film obtained in the same manner as in Example 1 using a regular polyethylene film that is not printed with oil-based paint has a grafting rate of 75% and an electrical resistance of 70%.
○・Lump, elongation rate in 40% KOH is 15 both vertically and horizontally.
%Met. Example 2 Five esh stainless steel nets were placed on the surface of a high-density polyethylene film with a film thickness of 25 mm, and the net was irradiated with an electron beam for 2 times on the ground while cooling with liquid nitrogen.

Claims (1)

[Scope of Claims] 1. It consists of a graft membrane in which a monomer having an ion exchange group is graft-polymerized onto a polyethylene film, and on the surface of the graft membrane, there is a part that is not grafted at all or a part with a low grafting rate. 1. A battery separator characterized in that a portion with a high grafting rate is discontinuously divided into continuous layers. 2. A substance that inhibits the grafting reaction of monomers is coated on the surface of a polyethylene film by printing, and then a monomer having an ion exchange group is grafted onto the surface of the polyethylene film. 1. A method for producing a battery separator in which a portion with a high grafting rate is discontinuously divided by a continuous layer consisting of a portion with no grafting or a portion with a low grafting rate. 3. A mesh metal porous plate is placed on the surface of a polyethylene film, the film is irradiated with ionizing radiation, and then the irradiated film is brought into contact with a monomer having an ion exchange group to carry out graft polymerization. A method for producing a battery separator in which a portion with a high grafting rate is discontinuously divided into continuous layers consisting of a portion with no grafting or a portion with a low grafting rate.