Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1999-12-21
2001-12-11
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C420S900000
Reexamination Certificate
active
06329100
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to hydrogen absorbing alloy electrodes for use as negative electrodes in nickel-hydrogen cells or batteries.
BACKGROUND OF THE INVENTION
Nickel-hydrogen cells are available which comprise a negative electrode prepared from a hydrogen absorbing alloy which reversibly absorbs or desorbs hydrogen. Nickel-hydrogen cells are known as cells having a high capacity, giving a high output and also high in energy density per unit volume and per unit weight.
It is required that nickel-hydrogen cells be given a still higher capacity. As a primary factor impeding increases in capacity, there is the problem that during charging, especially during rapid charging or overcharging, a side reaction of the charge reaction occurs at the hydrogen absorbing alloy electrode to produce hydrogen gas, increasing the internal pressure of the cell.
To solve this problem, there is a need to cause the electrode to efficiently absorb the hydrogen in its interior. Accordingly, a hydrogen absorbing alloy electrode is known which is prepared from a hydrogen absorbing alloy powder subjected to a water-repellent treatment and thereby held out of direct contact with the electrolyte within the cell to effect a promoted gas-solid reaction between the hydrogen and the alloy powder.
When the alloy is treated for water repellency, the reaction between the hydrogen (gas phase) and the hydrogen absorbing alloy (solid phase) is promoted, but there arises the problem of impeding the cell reaction of the alloy through the electrolyte (liquid phase).
JP-A No. 162353/1992 therefore discloses a hydrogen absorbing alloy electrode which is prepared from two kinds of hydrogen absorbing alloy powders only one of which is subjected to a water-repellent treatment, whereby the electrode is given improved hydrogen gas absorbing ability.
The disclosed electrode nevertheless has the problem of being unsatisfactory in discharge characteristics at low temperatures since the two kinds of hydrogen absorbing alloys have the same stoichiometric ratio represented by AB
5
(in atomic ratio).
An object of the present invention is to provide a hydrogen absorbing alloy electrode which is given further improved hydrogen gas absorbing ability and improved low-temperature discharge characteristics.
SUMMARY OF THE INVENTION
To fulfill the above object, the present invention provides a hydrogen absorbing alloy electrode containing a hydrogen absorbing alloy having a crystal structure of the CaCu5 type and represented by the stoichiometric ratio ABx, the hydrogen absorbing alloy being represented by MmNiaCobAlcMd wherein Mm is a misch metal, M is Mn and/or Cu, the atomic ratios a, b, c and d are in the respective ranges of 3.0≦a≦5.2, 0≦b≦1.2, 0.1≦c≦0.9, 0.1≦d≦0.8, and the sum of the atomic ratios a, b, c, and d, i.e., X=a+b+c+d, is in the range of 4.4≦X≦5.4, the hydrogen absorbing alloy electrode containing a powder obtained by mixing a powder of a hydrogen absorbing alloy wherein X is at least 5.0 with a powder of a hydrogen absorbing alloy wherein X is less than 5.0.
It is desired that the hydrogen absorbing alloy powder wherein X is less than 5.0 be subjected to a water-repellent treatment over the surface thereof.
It is desired that the hydrogen absorbing alloy powder wherein X is less than 5.0 be subjected to a surface treatment (acid treatment) with an aqueous acid solution or to a reduction treatment with hydrogen. When the hydrogen absorbing alloy powder less than 5.0 in X is to be subjected to both the water-repellent treatment and the acid treatment or hydrogen reduction treatment, the acid treatment or reduction treatment is conducted before the water-repellent treatment.
The hydrogen absorbing alloy powder wherein X is at least 5.0 contains the component Mm in a smaller amount relative to the component B than the other powder, so that the amount of the hydroxide of Mm formed which is thought detrimental to the electrochemical reaction on the alloy surface is lesser to result in higher discharge characteristics at low temperatures for advantageous charge-discharge reactions. The hydrogen absorbing alloy powder wherein X is less than 5.0 is capable of absorbing hydrogen in a more stabilized state and has high ability to absorb the hydrogen gas to be evolved in the cell in an overcharged state. Accordingly, the hydrogen absorbing alloy electrode prepared from a powder obtained by mixing the two hydrogen absorbing alloy powders is excellent in both the low-temperature discharge characteristics and cell internal pressure characteristics. An improvement in the cell internal pressure characteristics gives the cell a higher capacity.
The lower limit of the value X is 4.4, and the upper limit thereof is 5.4 so as to increase the discharge capacity of the hydrogen absorbing alloy electrode prepared.
The water-repellent treatment conducted for the hydrogen absorbing alloy powder wherein X is less than 5.0 forms a film on the surface of the alloy powder, rendering the powder less likely to come into direct contact with the electrolyte to efficiently absorb the hydrogen to be produced especially during overcharging or rapid charging.
The acid treatment or hydrogen reduction treatment conducted for the hydrogen absorbing alloy powder less than 5.0 in X makes the surface of the powder activated and rich in the component B, rendering the powder surface more readily wettable with the electrolyte to achieve a higher discharge efficiency. When the hydrogen absorbing alloy powder less than 5.0 in X is subjected to the acid treatment or hydrogen reduction treatment and thereafter to the water-repellent treatment, a film is formed by the water-repellent treatment on the power as made rich in the component B by the acid treatment or reduction treatment in advance, whereby the powder is rendered less likely to contact the electrolyte, attaining a further improved hydrogen absorption efficiency and achieving improvements in both low-temperature discharge characteristics and internal pressure characteristics.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Processes are not limited specifically for preparing a hydrogen absorbing alloy powder represented by MmNiaCobAlcMd (wherein Mm is a misch metal, i.e., a mixture of rare-earth metals such as La, Ce, Pr and Nd, M is Mn and/or Cu, the atomic ratios a, b, c and d are in the respective ranges of 3.0≦a≦5.2, 0≦b≦1.2, 0.1≦c≦0.9, 0.1≦d≦0.8, and the sum of the atomic ratios a, b, c, and d, i.e., X=a+b+c+d, is in the range of 4.4≦X≦5.4) and at least 5.0 in X, and a hydrogen absorbing alloy powder represented by the same formula as above and less than 5.0 in X. Such a powder can be prepared, for example, by weighing out specified amounts of metals, melting the metals in an arc furnace, casting the molten mixture into an ingot, and pulverizing the ingot. The hydrogen absorbing alloy powder prepared is adjusted in particle size as by suitable screening.
The water-repellent treatment for the hydrogen absorbing alloy powder which is less than 5.0 in X is conducted by stirring and kneading the powder in a suspension of PTFE and drying the resulting mixture in a vacuum for the removal of the solvent. This treatment forms a water-repellent film on the surface of the hydrogen absorbing alloy powder for holding the powder out of direct contact with the electrolyte. The film therefore ensures a smooth gas-solid reaction between hydrogen and the hydrogen absorbing alloy powder.
The acid treatment for the hydrogen absorbing alloy powder wherein X is less than 5.0 can be conducted by placing the hydrogen absorbing alloy powder into an aqueous acid solution having a pH of about 1.0, stirring the mixture and thereafter drying the mixture in a vacuum. The acid treatment conducted activates the surface of the hydrogen absorbing alloy powder, making the surface rich in the component B. The component B on the surface of the powder, especially Ni and Co thereon, provide sites of reaction with hydro
Fujitani Shin
Higashiyama Nobuyuki
Hirota Yohei
Imoto Teruhiko
Kato Kikuko
Armstrong Westerman Hattori McLeland & Naughton LLP
Brouillette Gabrielle
Sanyo Electric Co,. Ltd.
Wills M.
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