Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1998-07-17
2001-09-11
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S223000, C429S224000
Reexamination Certificate
active
06287724
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to metal hydride cells and more particularly, to nickel metal hydride cells for use in high rate applications in low temperature environments.
BACKGROUND OF THE INVENTION
Nickel cadmium cells have long been the standard for high discharge applications such as power tools. However, increasing emphasis is being placed on nickel metal hydride electrochemical cells as replacements for nickel cadmium cells. Nickel metal hydride cells avoid environmental concerns relating to cadmium and have improved energy density. Nickel metal hydride cells are mainly used with low rate discharge devices such as cellular phones and portable computers due to problems in nickel metal hydride power performance when used with high rate discharge devices.
The shortfalls in power performance characteristics generally occur at high discharge rates and at low temperatures during initial use of existing nickel metal hydride cells. When discharged at a high pulsed rate, for example 18 C (a current in amperes which is 18 times the value of the rated capacity of the cell) as in most power tools, nickel metal hydride batteries known in the art exhibit a voltage polarization during the initial operational discharge. At low temperatures, for example, at or below 0° C., this voltage polarization is even more pronounced and severely limits the power capabilities of the battery. Cycling of the cell or battery that occurs during subsequent charging and discharging may reduce the effects of this polarization, but obviously has no effect on the disabling voltage polarization encountered initially. Consequently, there is a need for enhancing the performance of metal hydride electrochemical cells during the initial operational discharge of the cells.
BRIEF SUMMARY OF THE INVENTION
It is one object of the present invention to provide a metal hydride cell designed for the demands of power tools, household appliances, electric vehicles and other battery-powered devices.
It is a further object of the present invention to provide a metal hydride cell which provides required power at high discharge rates and at low temperatures during the initial operational discharge.
It is a further object of the present invention to provide a rechargeable power source for high drain applications at low temperatures with no added cadmium.
It is a further object of the present invention to provide a method of manufacturing a metal hydride cell which provides required power at high discharge rates and low temperatures during the initial operational discharge, where the method avoids unnecessary processing of the hydrogen storage metal alloy material prior to incorporating the material into the negative electrode.
To achieve the foregoing and other objects and advantages and in accordance with a purpose of the present invention, as embodied and broadly described herein, a metal hydride cell is provided which discharges during a test discharge at a cell temperature of 0° C. and a discharge rate of 8 C to 18 C with the voltage remaining above 0.2 volts during the first half of the discharge curve. The metal hydride cell is comprised of a positive electrode preferably utilizing nickel hydroxide as the active material, a negative electrode utilizing a hydrogen storage metal alloy material as the active material, and an alkaline electrolyte. These components are arranged in a sealed container in a manner effective to provide electrochemical energy.
In another aspect of the within invention, the negative electrode of the cell of the present invention comprises a metal alloy with a particle size volume distribution whose d(50) value prior to the initial operational discharge of the cell does not exceed the required maximum particle size. The required maximum particle size=2(2D
h
t)
½
, where D
h
is the hydrogen diffusion coefficient for the alloy at 0° C. and t is a time from 200 to 450 seconds, corresponding to a discharge rate from 18 C to 8 C.
In yet another aspect of the within invention, the metal hydride cell of the present invention is prepared by subjecting the cell to at least one formation cycle, heat treating said cell and cycling said cell, where said steps are performed prior to the initial operational discharge of the cell. In a preferred embodiment, this method comprises a formation cycle at about a C/8 to a C/6 charge for approximately 11 to 16 hours followed by a discharge at 2 C to 0.9 volts. The heat treatment preferably comprises subjecting the cell to a temperature of 30° C. to 120° C. for approximately 20 to 24 hours followed by a hold period sufficient to enable the cell to return to room temperature. The cycling preferably comprises 1 to 10 charge/discharge cycles.
The cells of this invention provide a rechargeable power source for high drain applications at low temperatures and are substantially free of cadmium. The cells do not experience disabling voltage polarization during the initial operational discharge of the cell, and can be manufactured without processing the hydrogen storage metal alloy material prior to incorporating the material into the negative electrode.
REFERENCES:
patent: 0 284 333 A2 (1988-09-01), None
patent: 0 577 991 A2 (1994-01-01), None
patent: 0 586 718 A1 (1994-03-01), None
patent: 0 696 825 A1 (1996-02-01), None
patent: 4-065067 (1992-03-01), None
patent: 5-275082 (1993-10-01), None
DeMoully Thomas R.
Klein Michael R.
Prakash Jai
Puglisi Vince
Singh Deepika B.
Eveready Battery Company Inc.
Gebauer Linda
Weiner Laura
Welsh Robert W.
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