1. Field of the Invention
The present invention relates to a surface treatment method of a hydrogen absorbing alloy, in particular, to an activation treatment method of a hydrogen absorbing alloy used for an active material of a battery.
2. Description of the Related Art
A hydrogen absorbing alloy absorbs hydrogen at a higher density than a hydrogen-storing cylinder or a liquid hydrogen. In addition, the hydrogen absorbing alloy can reversibly repeat a cycle of absorbing/releasing hydrogen. With this characteristic, the hydrogen absorbing alloy has been used for a heat engine that uses hydrogen as fuel, a chemical heat pump that uses heat generation/absorption corresponding to the absorption/release of hydrogen, a nickel-hydrogen battery using electrochemical hydrogen absorption/release, and so forth.
As hydrogen absorbing alloys that have been used or that will be used, the LaNi.sub.5 type, Ti--Fe type, and Zr alloy Laves phase type, that absorb/release hydrogen at normal temperature and at normal pressure or in the vicinity thereof, are known. In particular, the equilibrium pressure at room temperature of the AB.sub.5 type hydrogen absorbing alloy represented by LaNi.sub.5 and MmNi.sub.5 (Mm: misch metal that is a mixture of rare earth group elements such as lanthanum and cerium) or AB.sub.2 type hydrogen absorbing alloy represented by TiZrVni type Laves phase alloy such as ZrV.sub.0.4 Ni.sub.1.6, is approximately one atmosphere. Thus, these hydrogen absorbing alloys reversibly absorb and release hydrogen at normal temperature and at normal pressure. In addition, these hydrogen absorbing alloys have relatively good corrosive resistance against alkali solutions. Consequently, the hydrogen absorbing alloys can be used as an active material of a negative electrode of a secondary battery that repeats the charging and discharging operations expressed by the following equation. ##STR1##
However, when the surface of the above-described hydrogen absorbing alloy is exposed to air, an oxide layer is easily formed on the surface. The oxide layer prevents the hydrogen absorbing alloy from absorbing/releasing hydrogen. In particular, when an oxide layer is formed on the surface of the hydrogen absorbing alloy used for the negative electrode of a nickel-metal hydride battery, since an Ni catalyst layer that dissolves and activates hydrogen is not present, the hydrogen absorbing alloy does not easily absorb/release hydrogen. Thus, in the initial stage of the battery, it does not have an enough discharging capacity. In other words, when the oxide film is formed on the surface of the hydrogen absorbing alloy, the initial activating characteristic is low.
To solve such a problem, in Tokkaihei 5-13077 and 4-137361, powder of hydrogen absorbing alloy is soaked in alkali solution so as to remove the oxide layer on the surface of the hydrogen absorbing alloy. In addition, mish metal, Co, Al, and Mn are dissolved from the activation surface. Thus, with only Ni, an Ni catalyst layer is formed. However, in this method, the dissolved Co.sup.2+, Mn.sup.2+, and so forth become oxides, thereby contaminating the surface of the alloy.
When the alloy powder is soaked in an acidic solution, the similar effect can be obtained. However, in this method, an Ni film that hardly permeates hydrogen may be formed. When a hard Ni film is formed on the surface of the hydrogen absorbing alloy, when it is used for a secondary battery, the initial discharging characteristic deteriorates.
When alkali solution or an acidic solution is used for the surface treatment of the hydrogen absorbing alloy, the resultant solution contains heavy metals (Co, Al, Mn, mish metal, and so forth). Thus, a troublesome waste treatment is required.
Besides the above-described methods, Tokkaihei 3-289047 shows a method for improving the initial activating characteristic of the negative electrode of a battery. In Tokkaihei 3-289047, an electrode composed of the hydrogen absorbing alloy is treated with hydrogen gas so as to be the charging state. The electrode is treated with steam that contains SO.sub.2, CO, CO.sub.2, and alkali mist so as to deactivate the surface of the electrode and thereby maintain the charging state. However, in this method, since the surface of the electrode is deactivated, the initial discharging capacity is not sufficiently improved.