Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
1999-05-13
2001-03-20
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S147000, C429S053000
Reexamination Certificate
active
06204635
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to electrochemical cells and more particularly to current interrupt apparatus for use with prismatic electrochemical cells responsive to selected adverse conditions.
BACKGROUND OF THE INVENTION
Electrochemical cells, especially high energy density cells such as those in which lithium is an active material, are subject to leakage or rupture as a result of various abusive treatment, which in turn, can cause damage to a device which is powered by the cell or to the surrounding environment. Such cells are sealed to prevent egress of electrolyte solvent and ingress of moisture from the exterior environment. When a cell is overcharged, self-heating occurs during the charging procedure. Examples of abusive treatment include charging at too rapid a rate, overcharging above a specified maximum voltage or discharging below a specified minimum cell voltage which can lead to an increase in temperature and/or pressure. In the case of cells utilizing lithium, should the temperature exceed a certain point an exothermic reaction begins which leads to overheating and causes a build-up in pressure which may result in expulsion of electrolyte from the cell.
It is known for commercially available cylindrical lithium ion electrochemical cells to incorporate a pressure driven current interrupt device to terminate current flow under adverse charge or discharge conditions. Typically, a thin metallic diaphragm, e.g., formed of aluminum, mounted in a header of the cell and exposed to pressure generated in the cell is attached to a pressure plate at the central axis, as by laser or ultrasonic welding, with an electrical insulator sandwiched therebetween. As pressure increases stress is created at the welded interface with the weld joint fracturing at a selected pressure range allowing the diaphragm to translate away from the plate to permanently open the electric circuit and terminate current flow. In some designs, the diaphragm is formed with a reduced thickness portion, e.g., a C-shaped groove extending along the peripheral edge of the diaphragm so that the diaphragm ruptures at a calibrated predetermined pressure to allow controlled venting of pressurized electrolyte. A current interrupt device of this type is shown in U.S. Pat. No. 5,691,073.
The above described end cap and current interrupt device assembly can be used conveniently in cylindrical electrochemical cells typically having diameters of 17 mm or more. However, as cell size is reduced the available membrane surface used to drive the pressure current interrupt device is reduced so that with prismatic cells having a thickness of only 10 mm, 8 mm or 6 mm, for example, the diameter of the working portion of the diaphragm is too small to provide sufficient translation to fracture the welded joint with the pressure plate so that a different operational mechanism is needed to provide the current interrupt function. As a result of the size limitations of such prismatic cells, the prevalent header related safety feature currently in commercial use is a pressure vent which allows controlled release of pressurized electrolyte in order to prevent cell explosion.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a current interrupt device particularly suited for prismatic electrochemical cells having a relatively narrow thickness free of the above noted limitations. Another object of the invention is the provision of a current interrupt device which is adaptable to a wide range of cell sizes, but particularly to cells having a small thickness, as small as 6 mm or even less. Yet another object is the provision of a current interrupt device which may also include other safety features such as short circuit and over current protection as well as a pressure vent mechanism.
Briefly stated, in accordance with one preferred embodiment of the invention, an annular header, adapted to be attached in fluid sealing relation to a cover of an electrochemical cell, mounts a generally cup-shaped member having a generally flat, relatively stiff end wall and a cylindrical side wall extending in a direction from the end wall toward the exterior of the cell with a free end of the side wall attached to the header and with the flat stiff end wall exposed to the pressurized fluid in the cell. The cup-shaped member is formed of electrically conductive material and in this embodiment and several others forms part of the current path between one of the plates of the core of the cell through a strap welded to the cup-shaped member and a top cap mounted on the header. In a lithium ion cell where the top cap serves as the positive terminal of the cell, the cup-shaped member may advantageously be formed of aluminum with side walls formed to catastrophically collapse/buckle with a sudden or snap action movement at a specific pressure level due to axial and moment stress created by internal cell pressure. According to an optional feature of the invention, a reduced thickness groove, preferably generally circular in shape, and a transition section connecting the side wall and the groove is used to aid in achieving low pressure actuation protection and/or to provide calibration of the pressure level at which the side wall collapse occurs by controlling the effective length of the moment arm of the applied stress to the side wall to offset material geometry and variations in material properties. In a modified embodiment, calibration for variations in material geometry and properties can be provided by changing the internal radius between the side and end walls to effectively change the moment arm. The instantaneous, catastrophic collapsing motion is used in one preferred embodiment to fracture a welded joint between the end wall and a pressure plate to interrupt the current path. The collapsing motion is used to move an electrical insulator to permanently open a spring loaded electrical contact interface in one embodiment and to fracture welded electrical contact interface disposed externally of the casing of the cell in another embodiment. According to a feature of a modified embodiment, single action short circuit protection can be provided in certain embodiments by using spring material having thermal memory, such as Nitinol, or thermostat metal which revert back to a calibrated, open contact configuration upon sufficient I
2
r heating. Yet another modified embodiment employs a positive temperature coefficient of resistivity element for short circuit protection.
Several embodiments utilize a current interrupt device which is crimped to a cover assembly which in turn is welded to the side wall of the electrochemical cell. Another embodiment includes a composite cover comprising a membrane or sheet layer which is essentially co-extensive with a support layer in which both a pressure vent and a current interrupt device is incorporated with the composite cover and membrane crimped to the side wall of the cell. Still other embodiments show a composite cover provided with both a pressure vent and current interrupt device in which the composite is welded to the side wall of the cell. In the latter embodiments a separate terminal is mounted through an aperture formed in and electrically isolated from the composite.
REFERENCES:
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patent: 5750277 (1998-05-01), Vu et al.
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patent: WO 9738455 (1997-10-01), None
Baumann Russell E.
Telecky Frederick J.
Texas Instruments Incorporated
Tso Edward H.
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