Chemistry: electrical current producing apparatus – product – and – Means externally releasing internal gas pressure from closed...
Reexamination Certificate
1998-12-18
2001-04-03
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Means externally releasing internal gas pressure from closed...
C429S054000, C429S056000, C429S059000, C429S061000, C429S072000, C429S007000
Reexamination Certificate
active
06210824
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to electrochemical cells and more particularly to current interrupt and vent apparatus used for such cells responsive to various 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 by various abusive treatment, which, in turn, can cause damage to the device which is powered by the cell or to the surrounding environment. In the case of rechargeable cells self-heating occurs as such cells are charged. Charging at too rapid a rate or overcharging can lead to an excessive increase in temperature. When the temperature exceeds a certain point, which varies depending upon the chemistry and structure of the cell, an undesirable and uncontrollable thermal runaway condition begins. In addition, because of the overheating and/or chemical reaction, internal pressure builds up, and electrolyte may suddenly be expelled from the cell.
Conventional cell designs employ an end cap fitting which is inserted into an open ended cylindrical casing after the cell's active material, appropriate separator material and electrolyte have been inserted therein. The end cap is in electrical contact with one of the anode or cathode material and the exposed portion of the cap forms one of the cell terminals. A portion of the cell casing forms the other terminal. Typically, various loose components are received in the end cap to provide over-pressure, short circuit and/or over-temperature protection. The typical header includes one or more seals to prevent leakage of the electrolyte through the header. However, these seals tend to leak over time due to temperature and pressure conditions and the like. Conventional cells frequently employ polymer PTC (positive temperature coefficient of resistivity) components for over-current protection which are susceptible to deformation when the header is crimp sealed to the electrochemical cell during manufacture. Deformation of the PTC component adversely effects its performance. Further, polymer PTC inherently results in relatively low ampere capacity and high resistance.
Copending U.S. Pat. No. (Ser. No. 08/720,585- A40743), which is incorporated herein by this reference, shows a current interrupt device responsive to over-temperature and over-pressure conditions which is incorporated into an end cap assembly which, in turn, is welded into the open end of the casing of an electrochemical cell to form a hermetic seal. This end cap assembly has particular application to rechargeable (secondary) cells, such as lithium-ion, nickel metal hydride, nickel cadmium and the like, to overcome the danger of the cell over-heating and pressure building up in the cell during exposure to high temperatures, excessive or improper charging, or shorting of the cell. While the current interrupt device of the patent is very effective, it would be desirable to provide an end cap assembly which has fewer component parts and which can be crimped into the casing of the electrochemical cell to lower the cost of the device while forming an effective hermetic seal. Additionally, it would be desirable to integrate an additional safety feature relating to low rate overcharge and overdischarge protection.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide end cap apparatus for an electrochemical cell which includes improved safety features which is an assembled unit to which an electrode tab or strap can be easily welded to effect electrical connection and, as a single calibrated unit, can be deposited in the open end of a cell casing and crimped thereto in hermetically sealed relation to the casing without the danger of upsetting or changing the preset calibration of the safety features. Another object of the invention is the provision of end cap apparatus for such a cell incorporating a thermally responsive member adapted to interrupt current flow in response to over-temperature due to heat received through a heat transfer path from the interior of the cell to the thermally responsive member as well as heat generated by l
2
r heating of components within the end cap apparatus. Yet another object is the provision of end cap apparatus for such a cell having a pressure responsive diaphragm disposed in pressure receiving communication with the atmosphere inside the cell in which welding of the cell's electrode tab to the end cap apparatus is facilitated. Yet another object is the provision of end cap apparatus which has improved electrolyte leak protection as well as providing venting in response to selected over-pressure conditions. Another object is the provision of electrochemical cell end cap apparatus which is easily handled and installed and which is small in size and mass. Yet another object of the invention is to provide an end cap apparatus which has increased ampere capacity over a broad ambient temperature range, e.g., −20° to 80° C., to serve high power applications and allow fast charging utilizing higher energy density cell chemistries.
Briefly, in accordance with the invention, end cap apparatus for electrochemical cells particularly useful with lithium-ion cell chemistry, comprises a header which allows the cell manufacturer to attach the end cap apparatus utilizing existing crimp technology to produce an electrolyte seal with the header also serving as a vehicle to mount several condition sensing mechanisms for a single action, safety protection device. End cap apparatus made in accordance with the invention provides the capability of opening the electrical charging circuit across a broad range of incremental cell pressure, without venting, as well as venting upon a high predetermined pressure level. The safety protection features include high rate overcharge protection (over-temperature and/or low pressure switch), low rate overcharge/overdischarge protection (low pressure switch), extended short circuit protection (thermally sensitive switch) and explosion protection (high pressure vent). The apparatus can be modified to include all or selective ones of the safety protective features, as desired.
The end cap apparatus in the preferred embodiments is of positive polarity and is electrically isolated from the negative polarity of the cell's casing by means of a polymer gasket which is compressed between the inside diameter of the cell casing and the outside diameter of the end cap apparatus. An electrical strap, typically aluminum, from the cell plates is welded to a portion of the bottom surface of the header forming the positive current path. The header is formed with a rigid, circumferential flange extending about the periphery thereof which allows the end cap apparatus to be crimped to the cell as well as cell pressurization to occur, both without changing the calibration of the safety protection features. In selected embodiments, the header is suitably formed of aluminum by machining, forging or the like to form a centrally disposed recess with a thin diaphragm portion, e.g., 0.008 inches thick extending across the recess, the diaphragm having a frangible portion which ruptures at a predetermined pressure level to prevent cell explosion. Suitable vent holes are formed through the header and ancillary components to allow the pressurized fluid, i.e., electrolyte, to escape upon venting. In certain other embodiments a diaphragm cap member, typically aluminum, is attached to the circumferential flange and extends over the lower half of the header, i.e., over the portion which will be exposed to electrolyte in the cell forming a leak proof seal. A corresponding diaphragm portion and frangible portion is formed in the diaphragm cup member.
According to a feature of the invention, a motion transfer member of electrically insulative material is movably mounted in the recess and is adapted to transfer motion from the diaphragm portion to a movable contact normally biased into engagement with a stationary contact portion to force the movable conta
Dexter Mark G.
French Kevin R.
Sullivan Steven K.
Baumann Russell E.
Kalafut Stephen
Martin Angela J.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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