Positive-electrode active material for alkaline secondary...

Details Positive-electrode active material for alkaline secondary... Positive-electrode active material for alkaline secondary...

1999-03-15

2001-01-16

Weiner, Laura (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S223000, C429S224000, C429S229000

Reexamination Certificate

active

06174624

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to a positive-electrode active material for alkaline secondary battery and an alkaline secondary battery. In particular, the invention is directed to an improved positive-electrode active material for alkaline secondary battery employing a nickel hydroxide material such that the occurrence of leakage of gas, an electrolyte solution and the like is prevented in a case where the discharged alkaline secondary battery is stored under high temperature conditions and thereafter subject to a charge/discharge process.
DESCRIPTION OF THE RELATED ART
In past, various types of alkaline secondary batteries have been used whereas manganese dioxide or nickel hydroxide has been generally used as the positive-electrode active material for use in such alkaline secondary batteries.
In the case of the positive-electrode active material employing manganese dioxide, the charge/discharge reaction process suffers low reversibility, which results in an alkaline secondary battery failing to present a satisfactory charge/discharge cycle performance.
On this account, the use of nickel hydroxide as the positive-electrode active material for alkaline secondary battery has spread. On the other hand, various studies have been made on the improvement of the positive-electrode active materials employing nickel hydroxide.
More recently, Japanese Unexamined Patent Publication No. 5(1993)-21064 has proposed an alkaline secondary battery improved in the charge/discharge cycle performance by using a positive-electrode active material including nickel hydroxide with manganese, cadmium or the like added thereto. Alternatively, Japanese Unexamined Patent Publication No. 7(1995)-335214 has proposed a positive-electrode active material which includes nickel hydroxide incorporating therein trivalent manganese for achieving improved capacity and cycle stability thereof.
Unfortunately, a problem exists with the alkaline secondary batteries using the positive-electrode materials proposed by such official gazettes. If such an alkaline secondary battery, in a discharged state, is stored under high temperature conditions for an extended period of time and then is charged, the positive electrode produces oxygen gas to cause leakage of gas, electrolyte solution or the like. This results in a reduced battery capacity. Particularly, in an alkaline secondary battery of an inside-out type construction wherein a battery case contains increased amounts of positive-electrode active material and negative-electrode active material for achieving high battery capacity, frequent occurrences of the leakage of gas or electrolyte solution are responsible for a notable decline in the battery capacity.
SUMMARY OF THE INVENTION
In view of the foregoing, one objective of the invention is a positive-electrode active material for use in an alkaline secondary battery, which material comprises nickel hydroxide and is less susceptible to crystal structure transformation when the discharged alkaline secondary battery is stored under high temperature conditions.
Another objective of the invention is an alkaline secondary battery employing nickel hydroxide as the positive-electrode active material, which battery does not suffer the decline in the battery capacity by virtue of the prevention of occurrence of the leakage of gas or electrolyte solution when the battery is subjected to the charge/discharge process after having been stored in the discharged state and under high temperature conditions.
The positive-electrode active material for alkaline secondary battery in accordance with the invention has an &agr;-Ni(OH)
2
crystal structure incorporating therein manganese and a trivalent metal other than manganese.
In the positive-electrode active material for alkaline secondary battery according to the invention, manganese and the trivalent metal other than manganese incorporated in &agr;-Ni(OH)
2
are normally substituted with nickel contained in &agr;-Ni(OH)
2
.
If manganese and the trivalent metal element other than manganese are incorporated in &agr;-Ni(OH)
2
as suggested by the positive-electrode active material for alkaline secondary battery according to the invention, sulfate ions in this &agr;-Ni(OH)
2
crystal structure are less liable to escape therefrom. Therefore, when stored under high temperature conditions, the positive-electrode active material can retain the &agr;-Ni(OH)
2
crystal structure and hence, is less susceptible to transformation into a &bgr;-Ni(OH)
2
crystal structure. Thus, when charged, the positive-electrode active material is transformed into &ggr;-NiOOH producing less oxygen gas.
In the positive-electrode active material for alkaline secondary battery according to the invention, the trivalent metal other than manganese is comprised of at least one metal element selected from the group consisting of, for example, scandium Sc, yttrium Y, lanthanide, aluminum Al and bismuth Bi. It is particularly preferred to use at least one metal element selected from the group consisting of erbium Er, yttrium Y and aluminum Al. It is more preferred to use, in combination, two or more types selected from the group consisting of erbium, yttrium and aluminum.
If, in the positive-electrode active material for alkaline secondary battery according to the invention, an excessive amount of manganese and the trivalent metal other than manganese is incorporated in &agr;-Ni(OH)
2
, the positive-electrode active material contains an insufficient amount of Ni. This leads to a failure to achieve a sufficient battery capacity. If, on the other hand, the content of manganese and the trivalent metal other than manganese is insufficient, the &agr;-Ni(OH)
2
crystal structure is more susceptible to the transformation when stored under high temperature conditions. Thus, the preservability of the battery is lowered under high temperature conditions. Accordingly, manganese is preferably present in a proportion of between 8 and 50 mol % based on total metal elements of the positive-electrode active material, and more preferably between 8 and 30 mol %. The trivalent metal other than manganese is preferably present in a proportion of between 0.3 and 10 mol % based on total metal elements of the positive-electrode active material and more preferably between 1 and 5 mol %.
Where the alkaline secondary battery, employing the aforesaid positive-electrode active material for alkaline secondary battery, is discharged and then stored for an extended period of time under high temperature conditions, the positive-electrode active material retains the &agr;-Ni(OH)
2
crystal structure and hence, is less susceptible to the transformation into the &bgr;-Ni(OH)
2
crystal structure, as described above. When this alkaline secondary battery is subsequently subject to the charge/discharge process, the charged positive-electrode active material goes through the transformation into &ggr;-NiOOH which, in turn, suppresses the oxygen gas production. Thus, the alkaline secondary battery does not suffer the lowered battery capacity resulting from the leakage of gas or electrolyte solution.
Where increased amounts of positive-electrode active material and negative-electrode active material are loaded in the battery case for achieving a high battery capacity, or particularly in the case of the alkaline secondary battery of inside-out type construction wherein the positive-electrode and negative-electrode active materials constitute not less than 70% in total by volume of the battery case, the battery suffers less leakage of gas or electrolyte solution if the discharged battery is stored under high temperature conditions and thereafter is subject to the charge/discharge process. Thus, the battery presents the high battery capacity even after the storage in the discharged state and under high temperature conditions.
In the alkaline secondary battery according to the invention, the negative-electrode active material for negative electrode is not particularly limited and zinc, hydrogen-absorbing alloys and the like, which are

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