Rechargeable battery electrode testing device

Details Rechargeable battery electrode testing device Rechargeable battery electrode testing device

2000-01-21

2001-01-23

Wong, Peter S. (Department: 2838)

Electricity: measuring and testing

Electrolyte properties

C429S090000

Reexamination Certificate

active

06177799

ABSTRACT:

FIELD OF INVENTION
This invention concerns a testing device which can measure in real-time minute changes in electrode dimensions, in particular electrode thickness, due to repeated recharge and discharge cycles in either a rechargeable battery cell, or in a test cell used for testing either a single electrode, or a set of electrodes assembled into an electrode stack. More particularly, this invention concerns a testing device for use with NiCd rechargeable batteries.
DESCRIPTION OF THE PRIOR ART
There has been a recent increase in activity in the design and construction of rechargeable batteries. Both wet cell and dry cell batteries are being studied as a consequence of the increasing need for clean air vehicles such as electric cars. It is well known that in rechargeable batteries, particularly of the NiCd type, redox cycling of the electrode material, coupled with an exchange of certain chemical moieties with the electrolyte, induces dimensional changes of the electrode active mass. This causes reversible mechanical strain and irreversible mechanical degeneration of the electrode material. By studying the behaviour of the electrode through repeated cycles of charge and discharge, one can optimize the chemical formulation and design of electrodes. In a related area, the results of such studies can lead to optimized battery charge/discharge/recharge procedures. Such procedures can minimize the electrode swelling and thereby maximize the cycle life of a battery. While there are no known devices which can measure the swelling of electrodes during charge/discharge/recharge cycles, devices have been described which take advantage of the electrode swelling phenomenon to indicate the condition of a battery.
CA 2,139,855 discloses an apparatus for testing the internal pressure of sealed rechargeable electrochemical cells during operation. However, this apparatus only tests for the pressure within the cell and not the electrode thickness within the cell.
U.S. Pat. No. 4,076,906 issued to Jensen, takes advantage of the electrode swelling phenomenon to determine the condition of a battery cell. An indicator is described with a generally triangular cross-section, which is captive between one side of an electrode and a transparent wall of the battery. As the battery loses charge, the electrode swells, thereby pushing the indicator against the wall. The more the battery loses charge, the larger the degree of swelling and thus, the further the indicator is pressed against the wall. Each time the indicator is pressed to the wall, it leaves a mark visible from the outside of the battery. The marks indicate the condition of the battery: the greater the number of marks, the lesser the charge level of the battery.
While the above devices are useful, neither of them provide a means to test and record, for example by means of a suitable digital recording means, the dimensional changes, typically the thickness, of an electrode as it is repeatedly cycled through charge/discharge/recharge cycles. Also, the above devices test and indicate an extrinsic quality of the battery, namely its charge. What is required is a device that tests an intrinsic quality of an electrode, namely its response to repeated charge/discharge/recharge cycles.
SUMMARY OF THE INVENTION
The present invention seeks to provide a testing device that can measure minute changes in at least one dimension of an electrode, or in at least one dimension of a plurality of electrodes assembled into an electrode stack, while the electrode, or electrode stack, is being cycled. The electrode, or plurality of electrodes assembled into an electrode stack, can be part of either a commercial battery or an experimental battery. In a preferred embodiment, the testing device measures thickness changes, either averaged over the whole surface of an electrode, or at at least one selected locus on the electrode surface. The testing device also functions while the battery is in use, and does not interfere with the normal charge/discharge/recharge cycle.
Thus in a preferred embodiment, the present invention seeks to provide a testing device for measuring changes in the thickness of either an electrode, or an electrode stack, for a battery undergoing repeated charge/discharge/recharge cycling comprising:
a container having a cavity containing the electrode, or the electrode stack, and a suitable electrolyte,
a first and a second pressure plate, each in contact with and supporting at least the periphery of each side of the electrode, or of the electrode stack,
a moveable wall member adjacent to and maintained in contact with at least a portion of one surface of the electrode, or of the electrode stack, and supported by the first pressure plate,
a temperature sensing means for sensing the temperature of the device,
a wall member displacement detection means connected to the moveable wall, and
a data processing means connected to both the detection means and the temperature sensing means.
In an alternative embodiment, the present invention seeks to provide a testing device for measuring dimensional changes in at least one fixed electrode within a rechargeable battery cell having a casing, the testing device comprising:
a moveable wall member adjacent to and maintained in contact with at least a portion of a surface of the at least one fixed electrode,
a wall member displacement detection means connected to the moveable wall,
a temperature sensing means for sensing the temperature of the device, and
a displacement measurement means outside the battery cell connected to the detection means.
Preferably, the battery cell is of the NiCd type.
Preferably, the fixed electrode is carried by an interior surface of the battery cell casing.
Preferably, the data processing means includes a suitable computer based device constructed and arranged to process the data provided by the detection means and the temperature sensing means. More preferably, the data processing means processes the data provided by the temperature sensing means to provide a thermal expansion correction value to the data provided by the detection means.
Preferably, the temperature sensing means is a thermocouple.
Conveniently, the detection means is magnetic. Conveniently, a magnetic detection means comprises a linear voltage differential transformer.
Alternatively, the detection means is optical. Conveniently, an optical detection means comprises a laser.
Alternatively, the detection means is electrical. Conveniently, an electrical detection means comprises a variable capacitor associated with the moveable wall.
Preferably, the moveable wall is a flexible membrane. Alternatively, the moveable wall is a bellows structure.
Preferably the testing device includes means to monitor an electrolyte level within the battery.
Preferably, the detection means is supported by the container including the cavity. More preferably, the supporting means together with the container comprises a monolithic block of material of suitable shape. Most preferably, the monolithic block of material is composed of polytetrafluoroethylene.
The advantages of the present invention are numerous. The testing device allows for the evaluation of electrodes either in a testing device, or in the battery itself, during repeated charge/discharge/recharge cycles. Although the testing device is theoretically capable of monitoring any of the dimensions of an electrode, in practise it is more meaningful to monitor electrode thickness. For a relatively small area electrode, the measurements will generally be taken over more or less the full surface area. For a larger electrode it is contemplated that the measurements will be taken at at least one location, and if desired at several locations, on the electrode surface. It is also contemplated that the testing device can be used to monitor dimensional changes, for example over-all thickness changes often loosely called “swelling”, of a plurality of electrodes assembled into a stack of electrodes in a multi-electrode stacked battery. When assessing an electrode stack within an assembled battery i

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