Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Patent
1994-01-05
1999-07-13
Maples, John S.
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
427400, 4273935, 442171, 429254, B05D 306, H01M 216
Patent
active
059224177
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a polymeric sheet and to a method of making a polymeric sheet. The sheet is wettable by aqueous solvents, and is suitable for use in applications where ionic conduction is required, for example as an electrode separator in an electrochemical device.
Separators for use in electrochemical devices such as electrochemical cells are required to permit migration of ionic species for electrochemical reaction on electrode surfaces, and in some cases gases, while presenting a barrier to migration of electrode material between electrodes of a device. Separators can be substantially non-porous for certain applications, when ion migration takes place by an ion exchange mechanism. For certain applications, electrode separators are porous, so that ion migration can take place in liquid electrolyte which impregnates the pores of the separator.
An electrode separator should be capable of withstanding significant levels of physical maltreatment, including being subjected to physical shock such as when a device incorporating the separator is subjected to vibration or is dropped. A separator will sometimes be required to be capable of allowing passage of gas through it. The material of the separator should also be chemically stable towards materials encountered in the device, whether present for electrochemical reaction, or as the product of such a reaction.
In order that the electrochemical characteristics of a device remain substantially constant over a period of time, it is preferred that the ability of ions to migrate through the separator of a device should remain substantially constant. In the case of a porous separator, the material of the separator should be wettable by the device electrolyte, and the wettability of the separator material by the electrolyte should remain substantially constant with time.
It would appear that certain polyolefin materials have certain physical properties which would make them suitable for use in the manufacture of electrode separators. A difficulty arises, however, in that such materials are not inherently wettable by aqueous electrolytes. Such electrolytes are therefore unable to penetrate the pores of an electrode separator formed from such a material, so that ion migration through the pores in solution is not possible. This problem is sometimes overcome by treating the polyolefin material with a surfactant which allows an aqueous electrolyte to wet the material. However, such surfactant can be removed from the surfaces of the polyolefin material when electrolyte is lost from the device, for example during charging and discharging cycles, and it is not subsequently replaced on the material when the electrolyte is replenished.
This problem has been addressed by modifying the surface properties of polyolefin materials used to form polymeric sheets, for example for use as electrode separators, by graft-copolymerising to those surfaces a monomeric substance which, after co-polymerisation, confers hydrophilic properties and, in some cases ion exchange properties, on the polyolefin substrate. This technique has been found to be practicable when the porous substrate is formed from polyethylene, which has been found to lend itself well to a graft-copolymerisation reaction of this kind. However, it has been found that, when such a reaction is attempted using polyolefin materials other than polyethylene, the rate of the grafting reaction is reduced significantly.
JP-A-82.141862 (Yuasa Battery Company Limited) discloses a separator for a battery, comprising a vinyl monomer which is graft-copolymerised on a non-woven fabric of fibres, the fibres comprising polypropylene resin with a coating of polyethylene resin on their surfaces. Such fibres are formed by co-extrusion. The use of composite fibres of this type allows a separator to be formed which has physical properties which are characteristic to some extent of a separator formed from polypropylene. However, the disclosed separator has the significant disadvantage that manufacture of the co-extruded polyethylen
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Cook John Anthony
Gargan Kenneth
Singleton Raymond William
Maples John S.
Scimat Limited
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