SEPARATOR FOR ALKALI ZINC BATTERY

Abstract: A separator for alkali zinc batteries consisting of an alkali-resistant microporous film, part of the porous film having a hydrophilic property of high persistency, and the other part having water repellency. In the hydrophilic part, the hydrophilic property is so highly persistent that the deposition of ZnO can be suppressed enough. Accordingly, any dendrite short in the battery can be prevented enough. Also, in the water-repellent part, any O₂ gas is permeable favorably, so that the capacity of the battery can be suppressed from reducing.

Description: This invention relates to a separator for an alkali-zinc battery for use in a power source for portable equipment, a portable power source, an electric vehicle power source, etc. The alkali-zinc battery has a high energy density and a high power characteristic. Because of the high solubility of zinc, however, zinc acid ions existing in electrolyte during charging become deposited on an anode in dendrite form or spongy form, a short-circuiting piercing through the separator can take place, or a shape change may occur to minimize an utilization factor; so that the cycle life of battery is shortened. A micro-porous membrane comprising polypropylene, polyethylene etc. subjected only to surface active agent treatment has been used for a conventional separator for an alkali-zinc battery. Such a separator has the desirable property for use in a sealed-type battery in which O 2 gas is absorbed by a zinc electrode of O 2 gas being permeable through it. However, since it is subjected only to the surface active agent treatment, its hydrophilic property is weakened while it is immersed in electrolyte. Further, the mechanism wherein ZnO is gradually precipitated in micro-pores, bringing about short-circuiting, (i.e. dendrite short-circuiting) can not be avoided entirely so that it is not suitable for an application requiring long-term storage or a charge/discharge cycle life of a long period. A separator of this general type is disclosed in JP-2-216757. In order to improve the above disadvantages, there has been used a separator fabricated by a semi-permeable membrane and a micro-porous membrane placed on upon another. However, the permeability of O 2 gas cannot be achieved in these membranes. An object of this invention is to provide a separator for alkali-zinc battery, which has both properties of a semi-permeable membrane and a micro-porous membrane so that dendrite short-circuiting can be avoided sufficiently and decease in battery capacity can be controlled by good permeability to O 2 gas. This invention provides a separator for an alkali-zinc battery comprising a micro-porous membrane being O 2 gas permeable, having a plurality of pores and having alkali resistance, characterised by a part of said porous membrane being hydrophilic and the remaining part being hydrophobic. According to this invention, the precipitation of ZnO is sufficiently controlled because the hydrophilic part is persistent. Consequently, the dendrite short-circuiting in battery can be avoided sufficiently. Since O 2 gas is highly permeable in the hydrophobic part, the decrease in battery capacity can be controlled. Brief Description of Drawings Fig. 1 is a vertical sectional partial view showing a separator of embodiment 1 of this invention. Fig. 2 is a diagram showing cycle life characteristics of batteries using separators of embodiments 1 and 2 of this invention and a battery using a conventional separator. Fig. 3 is a diagram showing a change in a charge/discharge cycle number in case when a ratio of pores filled with ion permeable resin to the entire pores is changed. Fig. 4 is a vertical sectional partial view showing the separator of the embodiment 2 of this invention. Fig. 5 is a vertical sectional partial view showing another example of the separator of the embodiment 2. Fig.6 is a vertical sectional view showing yet another separator of the embodiment 2 of this invention. (Embodiment 1) Fig. 1 is a vertical sectional view showing the separator of embodiment 1 of this invention. In Fig. 1, 1 denotes a micro-porous membrane, and a membrane having a trade name of "CELGARD #3401" (made by DAICEL CHEMICAL INDUSTRIES Ltd.) is used therefor in this figure. This is a porous membrane made of poly-propylene having a thickness of 25µm (25 microns), a porosity of 38%, and pore diameters of 0.05-0.125µm (0.05 to 0.125 microns). This porous membrane is subjected to surface active agent treatment. 2 denotes cellulose which is an ion permeable resin filled in about 90% pores of the porous membrane 1. 1a denotes pores in which the cellulose is filled, and 1b denotes about 10% remaining pores in which the cellulose is not filled. The pores 1b are uniformly distributed over a surface fronting on a plate surface. The pore 1b is hydrophobic by removing the surface active agent. In order to fill the cellulose 2, viscose is applied on an about 90% area of surface of the porous membrane 1 and impregnated under reduced pressure to be solidified. As described above, the pores 1a are filled with the cellulose and the pores 1b are hydrophobic so that a separator 10 is composed. A property of the separator 10 thus constructed was investigated as follows. A battery A using the separator 10, a battery B using a separator comprising a simple micro-porous membrane made of polypropylene, and a battery C using a separator comprising a simple micro-porous membrane made of polypropylene and a cellophane forming a semi-permeable membrane, were prepared each two cells, respectively. Electrodes, containers and electrolyte composing the batteries were the same for all of them. Namely, a zinc electrode forming an anode was fabricated in such a manner that a punched copper current collector on which a number of pores were made was used as a core metal, and zinc active material sheets were pressure crimped on both faces of the core metal. A nickel electrode forming a cathode was fabricated in such a manner that an active material having a principal component of nickel hydroxide was filled in a sintered nickel plaque by means of a chemical impregnation method. Non woven fabric made of polypropylene was used for the container. The electrolyte was KOH solution having a specific gravity of 1.35. A nominal capacity of battery was 10Ah. Changes in capacities of the above batters A, B and C were investigated by repeating charge and discharge operations under the following conditions. The discharge was conducted at 2A until a voltage per cell reached 1V. The charge was conducted at 1A up to a charge capacity corresponding to 105% of the discharge capacity. Open circuit voltages of the above batteries A, B and C were 1.70V, voltages at 50% discharge were 1.65V. Results are shown in Fig. 2. As seen from Fig. 2, the decrease in discharge capacity is small in battery A even when the cycle is repeated. In battery B, the gas absorption is executed well and the decrease in capacity is approximately equal to that of the battery A because it consists of the polypropylene micro-porous membrane. However, the capacity abruptly decreases caused by short-circuiting due to the dendrite of zinc because it is porous. In battery C, the gas absorption is poor, so causing a decrease in capacity because the cellophane is used. In case of the separator 10 as described above, the cellulose is introduced into the pores 1a, so that it has an excellent persistency in ion permeability as compared with a separator subjected only to the surface active agent treatment, ZnO is prevented from precipitating in the pores 1a so as to avoid the dendrite short-circuiting sufficiently. Since the pores 1b are hydrophobic, O 2 gas is highly permeable so as to control the decrease in battery capacity. Further, the pores 1b are uniformly distributed over the surface fronting on the plate surface, so that O2 gas is absorbed uniformly and a change in shape of anode is minimized. Accordingly, a battery having an excellent cycle life can be provided by the separator 10. The percentage of the pores la filled with cellulose to the total pores is about 90% in the separator used for the battery A, however, this percentage is not limited to this value. Fig. 3 is the diagram showing the change in charge/discharge cycle number in case when a ratio of pores la to the total pores in separator 10 is changed. The axis of abscissa denotes the percentage of pores 1a and the axis of ordinate denotes the charge/discharge cycle number. According to this diagram, even when the percentage of pores 1a is extremely small, the cycle number increases if any pores filled with cellulose exist. However, in the event when the pores 1a occupy approximately the entire part i.e. the percentage exceeds a point X (about 98%), the cycle life decreases because O 2 gas absorption becomes insufficient due to an excessively small quantity of pore 1b. Consequently, it is enough to set the percentage of pores 1a to a value larger than zero and smaller than the point X, so that the percentage is not limited to about 90%. However, the percentage is preferably set to about 90% because the peak of cycle life exists at about 90%. In place of the membrane made of polypropylene, those made of polyethylene and nylon etc. may be used for the micro-porous membrane. Further, in place of cellulose, poval etc. may be used for the ion permeable resin. As described above, the present embodiment is able to provide the separator for alkali-zinc battery which can prevent the dendrite short-circuiting satisfactorily and can control the decrease in battery capacity by allowing O 2 gas to be permeable well. Therefore, the separator of this embodiment is able to provide the alkali-zinc battery having an excellent charge/discharge cycle life so that its industrial value is extremely large. (Embodiment 2) Fig. 4 is the vertical sectional partial view showing the separator of embodiment 2 of this invention. The separator of this embodiment is formed by using a micro-porous membrane having trade name of "CELGARD #2400" (made by DAISEL Chemical Co.). The "CELGARD #2400" is a porous membrane made of polypropylene and not subjected to the surface active agent treatment, and has a thickness of 25µm (25 microns), a porosity of 38% and a max. pore diameter of 0.05 x 0.125µm (0.05 x 0.125 microns). In Fig. 8, 21 denotes a hydrophilic portion and 22 denotes a hydrophobic portion, and the separator of this embodiment consists of the two portions 21 and 22. The hydrophilic portion 21 is uniformly distributed so as to occupy about 90% area of a surface of the above porous membrane i.e. a surface fronting on the plate surface, and the hydrophobic portion 22 also consists of a plurality of banded portions 22a uniformly distributed corresponding to the hydrophilic portion 21. The percentage of area occupied by the hydrophilic portion 21 is not limited to about 90% so far as the percentage of area occupied by the hydrophobic portion 22 is under 20% incl. An area of one banded portion 22a of the hydrophobic portion 22 is set to under 20 mm 2 incl. The hydrophilic portion 21 is formed by radiation graft polymerizing a monomer acrylic acid after being subjected to heat treatment, so that it becomes thinner than an original membrane thickness X due to the heat treatment. Pores of the hydrophilic portion 21 are filled up due to the heat treatment, and its hydrophilic property is based on a hydrophilic group of acrylic acid. The monomer used may be any monomer so far as it has a hydrophilic group, and may be methacrylic or styrene sulfonic acid. The hydrophobic portion 22 is formed by leaving the above porous membrane as it is. Namely, it is formed by creating the hydrophilic portion 21 on the above porous membrane surface under a state where a lead mask covering an area of the hydrophobic portion 22 is attached to the surface. Micro pores are left on the hydrophobic portion 22 as they are. The shape of the hydrophobic portion 22 is determined by the shape of the portion covered by the mask, and the shape may be circular, polygonal or slit for instance. A property of the separator 30 thus constructed was investigated in the same way as the embodiment 1. That is a battery A constructed in the same way as the embodiment 1 using the separator 30, and a battery B and battery C similar to those used in the embodiment 1 were prepared, charge/discharge operations were repeated in the same conditions as the embodiment 1, thus the changes in capacity were investigated. The results were the same as Fig. 2. Namely, the decrease in capacity was small in case of the battery A even when the cycle was repeated. As described above, since the separator 30 includes the hydrophilic portion 21 formed by radiation graft polymerizing the monomer acrylic acid, the persistency of ion permeability is excellent as compared with that subjected only to the surface active agent treatment. Accordingly, the precipitation of ZnO is controlled in a part of the hydrophilic portion 21. Further, since micro-pores of the hydrophilic portion 21 are completely filled up by the heat treatment, the precipitation of ZnO can be avoided positively. Therefore, the dendrite short-circuiting in battery can be avoided satisfactorily. Moreover, the separator 30 includes the hydrophobic portion 22, and O 2 gas is highly permeable through the hydrophobic portion 22. For this reason, the decrease in capacity of battery can be controlled. In addition, O 2 gas is absorbed uniformly and the shape change of the anode is minimized because the hydrophobic portion 22 is uniformly distributed on the surface fronting on the plate surface. Accordingly a battery having an excellent cycle life can be provided by using the separator 30. In the event when an area of one banded portion 22a of the hydrophobic portion 22 is larger than 22 mm 2 and/or when a percentage of an area occupied by the hydrophobic portion 22 to that fronting on the plate surface is larger than 20%, an effective area of the zinc electrode is minimized so that these cases are not preferable. A micro-porous membrane made of polyethylene or nylon may be used in place of the micro-porous membrane made of polypropylene. The entire part of the above hydrophilic portion 21 of the separator 30 thus constructed is subjected to heat treatment. However, the heat treatment is completely negligible as shown by Fig. 5 or some part of it may be subjected to heat treatment as shown by Fig. 6. In Fig. 5, a thickness of the hydrophilic portion 21 is not reduced as compared with an original thickness X. And in Fig. 6, the hydrophilic portion 21 is composed of a portion 21b formed by radiation graft polymerizing the monomer acrylic acid after being subjected to the heat treatment, and a portion 21c formed by radiation graft polymerizing the monomer acrylic acid without being subjected to the heat treatment. The portion 21b is formed on a boundary between the hydrophilic portion 21 and the hydrophobic portion 22. The portion 21c is not subjected to heat treatment so that it maintains the original thickness X. According to the example of Fig. 6, occurrence of the dendrite short-circuiting on the boundary between the hydrophilic portion 21 and the hydrophobic portion 22 can be controlled as compared with that not subjected to the heat treatment. As described above, the present embodiment is able to provide the separator for alkali-zinc batter which can prevent the dendrite short-circuiting satisfactorily and can control the decrease in battery capacity by allowing O 2 gas to by highly permeable. Therefore, the separator of this embodiment can provide the alkali-zinc battery having an excellent charge/discharge cycle life so that its industrial value is extremely large.

Claims: A separator (10) for an alkali-zinc battery comprising a micro-porous membrane (1) being O 2 gas permeable, having a plurality of pores (1a,1b) and having alkali resistance, characterised by a part of said porous membrane being hydrophilic and the remaining part being hydrophobic. A separator (10) for an alkali-zinc battery comprising a micro-porous membrane (1) being O 2 gas permeable, having a plurality of pores, and having alkali resistance, characterised by the pores (1a) of a part of said porous membrane comprise a surface active agent and are filled with ion permeable resin, and all the remaining pores (1b) are hydrophobic due to the absence of that surface active agent and ion permeable resin in the said remaining pores. A separator (10) for an alkali-zinc battery as set forth in claim 2, in which said remaining pores (1b) are distributed uniformly over a surface of the separator intended to face a surface of a battery plate. A separator (30) for an alkali-zinc battery comprising a micro-porous membrane being O 2 gas permeable, having a plurality of pores and having alkali resistance, in which a part (21) of said porous membrane is hydrophilic due to radiation graft polymerizing of a monomer having a hydrophilic group, and the remaining part (22) being hydrophobic. A separator (30) for an alkali-zinc battery as set forth in claim 4, in which the pores of the hydrophilic part are closed by heat treatment. A separator for an alkali-zinc battery as set forth in claim 5, in which said heat treated part (21b) is disposed at least on a boundary between the hydrophilic part and said hydrophobic part. A separator for an alkali-zinc battery as set forth in claim 4, in which approximately all of the hydrophobic part is composed of uniformly distributed plural banded portions (22a), one banded portion has an area of under 20 mm 2 , and the hydrophobic part occupies an area of under 20% incl. of a surface fronting on a plate surface. A separator for an alkali-zinc battery as set forth in claim 7, in which each of said banded portions has a circular, polygonal or slit shape. A separator for an alkali-zinc battery as set forth in claim 4, in which said monomer is acrylic acid, methacrylic acid or styrene sulfonic acid. A separator for an alkali-zinc battery as set forth in claim 1, 2 or 4, in which said micro-porous membrane is made of polyethylene, polypropylene or nylon. An alkali-zinc battery comprising at least one separator according to any preceding claim.

Inventors: BOGAUCHI TAKEHITO, EGUCHI YOSHIHIRO, KISHIMOTO TOMONORI YUASA BATTE, YAMANE MITSUO, BOGAUCHI, TAKEHITO, EGUCHI, YOSHIHIRO, KISHIMOTO, TOMONORI, YAMANE, MITSUO, KISHIMOTO, TOMONORI, YUASA BATTERY CO., LTD.
Applicants: YUASA BATTERY CO LTD, YUASA CORPORATION
Application year: 1991
Main classification: H01M2
IPC codes: H01M2