1. Field of the Invention
The invention relates to a nickel hydrogen rechargeable battery.
2. Description of the Related Art
Nickel hydrogen rechargeable batteries have been used for various purposes and applied to a large variety of mobile electronic devices and the like, because of their higher capacity and their superiority in environmental safety, as compared to nickel-cadmium rechargeable batteries.
A non-sintered positive electrode is well known as a positive electrode used for the nickel hydrogen rechargeable batteries. The non-sintered positive electrode is fabricated, for example, in the following manner. First, positive paste is produced by kneading nickel hydroxide particles functioning as a positive-electrode active material, a binding agent, and water. A substrate having a 3D network-like framework structure and made of a nickel foam sheet is filled with the positive paste. The paste is subjected to a drying process, whereas the substrate undergoes a rolling process for the purpose of densifying positive mixture, thereby forming an intermediate product of a positive electrode. The intermediate product is then cut into predetermined size. This is how a non-sintered positive electrode is fabricated. An advantage of the non-sintered positive electrode is that it can be filled with the positive-electrode active material more densely than a sintered positive electrode.
On the other hand, although conventional non-sintered positive electrodes can be filled with active material, the utilization rate of the active material in the conventional non-sintered positive electrodes is low because nickel hydroxide particles functioning as active material have a relatively low conductivity. The low utilization rate of the active material causes the problem that cell reaction at charge and discharge is unlikely to smoothly progress.
A well-known method for increasing the utilization rate of the active material in a non-sintered positive electrode is to add a cobalt compound such as cobalt hydroxide powder to the positive mixture to make the cobalt compound function as a conducting agent (see Unexamined Japanese Patent Application Publication No. 62-237667, for example). If the positive electrode containing the nickel hydroxide as a positive-electrode active material and the cobalt compound as a conducting agent in the positive mixture is installed in a nickel hydrogen rechargeable battery, the cobalt compound dissolves in alkaline electrolyte in the form of cobalt-ion (Co2+) and uniformly disperses on the surface of the nickel hydroxide. When a battery is first charged, the cobalt-ion (Co2+) is oxidized into highly-conductive cobalt oxyhydroxide and forms a conductive network that bonds active materials together and also bonds the active materials and the substrate. This increases conductivity among the active materials and between the active materials and the substrate, and thus improves the utilization rate of the active materials.
In recent years, the above-described mobile electric devices are getting more and more popular and used in various ways by a wide range of users. It is anticipated that some users might forget to turn the devices off. If a device is left on, and a nickel hydrogen rechargeable battery is kept connected to load for long periods of time, the battery is discharged until voltage falls in a usable voltage range (for example, 0.8 V or higher) or below. If left standing in such a discharge state for long periods of time, the battery comes into a so-called deep discharge state.
When the battery in which the conductive network is formed inside comes into the deep discharge state, the potential of the positive electrode becomes equal to or less than a reductive potential of cobalt oxyhydroxide, so that the cobalt oxyhydroxide forming the conductive network is reduced and eluted. The reduction and elution of cobalt oxyhydroxide partially destroy the conductive network. As a result, the conductivity of the positive electrode is decreased, which deteriorates charge acceptance and lowers the utilization rate of the positive-electrode active material. For this reason, even if the battery is recharged, it is difficult to recover a charge capacity to an initial capacity value. Assuming that a recovery level of the charge capacity after deep discharge is a capacity recovery property, it can be said that the higher the capacity recovery property is, the closer to an original capacity the charge capacity gets at the time of charge after deep discharge.
Since the nickel hydrogen rechargeable battery has expanded in application, it is demanded that the capacity recovery property should be improved so that a predetermined capacity is recovered by recharging the battery even if the battery comes into the deep discharge state from being overused. Batteries designed so that the capacity recovery property is improved include an alkali storage battery disclosed in Japanese Patent No. 3191751.
In the alkali storage battery disclosed in Japanese Patent No. 3191751, a lithium-cobalt composite oxide functioning as a conducting agent is added into a positive electrode, so that a conductive network is formed by the composite oxide. Having a relatively high stability against reductive reaction, the composite oxide is unlikely to be resolved or eluted when the battery is brought into a deep discharge state.
Along with further expansion of use, nickel hydrogen rechargeable batteries are expected to be used under tough conditions. In such situations, the capacity recovery property of a conventional battery like the one disclosed in the U.S. Pat. No. 3,191,751 is not sufficient, and further improvement of the capacity recovery property has been demanded. Especially, under tough conditions, such as when the battery is repeatedly kept in the deep discharge state, the stability of cobalt against the reductive reaction is drastically decreased. As a result, the conductive network is gradually destroyed as the deep discharge is repeated, and the destroyed area grows larger. Along with this, the conductivity of the active material is decreased, and the utilization rate of the active material is also lowered. The battery repeatedly kept in the deep discharge state is therefore difficult to recover the original charge capacity even if recharged.