1. Field of the Invention
The present invention relates to an alkaline secondary battery such as a nickel-metal hydride battery, a separator for an alkaline secondary battery to be interposed between a positive electrode and a negative electrode in the alkaline secondary battery and a method for preparing the above-mentioned separator for an alkaline secondary battery, and is characterized in that storage characteristic of the alkaline secondary battery is improved by improving the separator for an alkaline secondary battery so as to prevent self-discharge in the alkaline secondary battery.
2. Description of the Related Art
A nickel-metal hydride battery, a nickel-cadmium battery and the like have been heretofore used as an alkaline secondary battery. In the alkaline secondary battery as described above, polyolefin materials excellent in resistance to alkalinity and residence to oxidization have been used for a separator to be interposed between a positive electrode and a negative electrode.
However, the polyolefin materials are not hydrophilic in themselves, hence, in the separator comprising the polyolefin materials, it is difficult to maintain an alkaline electrolyte solution of the alkaline secondary battery.
Therefore, conventionally, as proposed in the Japanese-Laid Open No. Sho 63-34849, there has been proposed an alkaline secondary battery whose alkaline electrolyte solution is set to be maintained in the separator upon providing the polyolefin materials for separator with an OH-group so as to increase hydrophilicity by way of coating a surface of the above-mentioned polyolefin materials with an ethylene-vinyl alcohol copolymer resin, for example.
Further, in the alkaline secondary battery as described above, the positive electrode, for example, contains impurity ions such as ammonium ions. Such impurity ions transfer at discharge/charge, and thereby, self-discharge generates in the alkaline secondary battery due to the impurity ions at a middle charge depth of about 50 to 80%. Consequently, a problem exists that storage characteristic is degraded.
In this connection, in recent years, there have been proposed a separator wherein an acrylic resin monomer such as acrylic acid or the like is graft-polymerized with the polyolefin materials and then nitrogen impurity ions are captured by way of a carboxyl group contained in a resultant polymer, and a separator wherein the polyolefin materials are treated with concentrated sulfuric acid or fuming sulfuric acid so as to interpose a sulfone group into the polyolefin materials and then impurity ions are captured by the sulfone group interposed in the polyolefin materials, have been proposed.
However, in the case of the separator wherein the acrylic resin monomer such as acrylic acid or the like is graft-polymerized with the polyolefin materials, resistance to heat and resistance to oxidization are generally weak, and thereby, the separator cannot stand being used for a long time. Hence, there exists another problem that life span of the alkaline secondary battery is short.
Further, in the above-mentioned case of the separator wherein the polyolefin materials are treated with concentrated sulfuric acid or fuming sulfuric acid so as to interpose the sulfone group into the polyolefin materials, the separator is degraded when being treated with concentrated sulfuric acid or fuming sulfuric acid. Therefore, there exists a further problem that a short-circuit easily occurs between a positive electrode and a negative electrode.
In addition, when the carboxyl group or the sulfone group is interposed into the separator as described above, the alkaline electrolyte solution is consumed due to such groups having ions-exchange ability. Therefore, there also exists an additional problem that characteristics of the alkaline secondary battery are degraded.
An object of the present invention is to improve a separator for an alkaline secondary battery to be interposed between a positive electrode and a negative electrode in an alkaline secondary battery such as a nickel-metal hydride battery and the like.
Another object of the present invention is to attain an alkaline secondary battery excellent in storage characteristic upon improving the separator for an alkaline secondary battery as described above so as to prevent self-discharge due to impurity ions such as ammonium ions contained in the positive electrode.
In the present invention, the separator for an alkaline secondary battery to be interposed between the positive electrode and the negative electrode has carbon-carbon double bonds. An average amount of carbon-carbon double bonds in the separator is in the range of 10 xcexcmol/g to 200 xcexcmol/g, and an average amount of increased nitrogen in the separator after the separator is immersed in an alkaline aqueous solution having ammonium salt dissolved therein is not less than 140 xcexcmol/g.
Further, when the separator for an alkaline secondary battery according to the present invention is used for an alkaline secondary battery, impurity ions such as ammonium ions and the like are captured at the part of the above-mentioned carbon-carbon double bonds existing in the separator for an alkaline secondary battery, whereby preventing self-discharge of the alkaline secondary battery due to the impurity ions. Therefore, storage characteristic of the alkaline secondary battery is improved.
In the separator for an alkaline secondary battery according to the present invention, when an amount of carbon-carbon double bonds in the separator is small, it is impossible to capture sufficiently impurity ions such as ammonium ions and the like, hence self-discharge can not be sufficiently prevented. On the other hand, when the amount is too large, said carbon reacts with oxygen which generates at the positive electrode, and thereby, the separator is oxidized, which makes characteristics thereof degraded. On this account, in the separator for an alkaline secondary battery according to the present invention, an average amount of carbon-carbon double bonds in the separator is set in the range of 10 xcexcmol/g to 200 xcexcmol/g.
Additionally, in the separator for an alkaline secondary battery according to the present invention, an average amount of increased nitrogen in the separator after the separator is immersed in an alkaline aqueous solution having ammonium salt dissolved therein is set not less than 140 xcexcg/g so that impurity ions such as ammonium ions should be sufficiently captured.
In forming carbon-carbon double bonds in the separator for an alkaline secondary battery according to the present invention, a method of dehydrating polyolefin fiber provided with an OH-group, for example, exists.
In forming carbon-carbon double bonds using the method of dehydrating the polyolefin fiber provided with an OH-group, acid of every type can be used. However, when concentrated sulfuric acid or fuming sulfuric acid or the like is used, the polyolefin fiber is degraded, and thereby, a short-circuit easily occurs between the positive electrode and the negative electrode. Consequently, it is preferable that an acid selected from phosphoric acid, hydrochloric acid, and nitric acid is used for dehydration.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.