BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to an electrical connector for use in a high temperature battery and, more partic- ularly, to a connector to join the electrode and the terminal in a lithium/metal sulfide battery.

2. Description of the Prior Art Lithium/metal sulfide batteries have become increasingly desirable based on their ability to provide high power performance at generally low cost. In building such an electrochemical cell, a common problem is faced in making a good electrical connection between the individual electrodes and the cell terminal. The simplest form of such a connection is a bolted mechanical joint. However, this is not always possible due to severe space limitations within the cell case. Moreover, such bolted joints are prone to loosening caused by the higher thermal expansion coefficient of the bolt compared to the electrodes and the terminal. Alternative approaches are to weld or braze the components together. However, these alternative methods are not always feasible because of incompatibilities between the different alloys to be joined and poor corro¬ sion resistance of the brazing alloys in the harsh cell environment. These cell conditions become even a greater concern when the operating temperature of the battery is at elevated temperatures of 400| to 500| C. as during battery operation.

A rechargeable high temperature battery, for example a lithium-alloy/metal disulfide battery, imposes additional stress on connectors in that the battery will be thermally cycled a number of times between ambient and operating temperature during its life. During these abusive conditions, the connection between the electrodes and terminal must remain intact and must maintain low electrical resistance. Accordingly, there is a need for an improved electrical connection between the individual electrodes and the cell terminal which can withstand the harsh conditions imposed by a lithium alloy/metal disulfide battery.

SUMMARY OF THE INVENTION An electrical connector is provided which over- comes the problems of making a good electrical connection in the harsh environment of the lithium/metal sulfide electrochemical cell. The proposed connector is a mechani¬ cal connection in which the two current carrying components are physically forced into intimate contact with each other by a central core of high expansivity material. An axially extending opening is provided in the terminal and an extension member provided on the electrode is adapted to mate within the opening. A core provided within the extension member is designed to expand at a greater rate than either the extension member or the opening, thereby mechanically forcing the extension member into electrical contact with the terminal opening upon heating. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic isometric exploded view of a first presently preferred embodiment of the electrical connector of the present invention.

Figure 2 is a schematic view of a second preferred embodiment of the electrical connector of the present invention. Figure 3 is a schematic view of a third embodiment of the electrical connector of the present invention.

Figure 4 is a schematic view of an electrochemical cell into which an electrical connector of the present invention has been provided.

Figure 5 is a graph showing the electrical resistance of the electrical connector of the present invention over time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in Figure 1, an electrical connector system 10 is provided having a terminal 12 through which a hey-hole shaped opening 14 is provided. A tab 16 having an end portion adapted to mate within key-hole opening 14 is provided on one end of the electrode. Pin 18 is inserted into tab 16 and forms a core within tab 16. Tab 16 and terminal 12 are physically forced into intimate contact with each other by pin 18.

In a preferred embodiment of the present design, positive terminal 12 is made from low carbon steel, the electrode current collector tab 16 is made from pure molybdenum, and the central core pin 18 is made from stainless steel. These materials were chosen for their particular physical and chemical properties in relation to the connector system 10 and the environment within the lithium-alloy/iron disulfide cell. The properties of each of these three materials are presented in Table 1 below. TABLE 1

Expansion Electrical Electrical

Coefficient Resistivity Resistivity

Material uc /°C uΩcm ® 20°C uΩcm ® 450°C

Molybdenum 5.1 5.7 16.5 Low Carbon Steel 12.1 10.1 49

Stainless Steel 18 71 100

As can be seen from Table 1 above, stainless steel pin 18 provided at the center of connector system 10 will expand significantly more than the molybdenum tab 16 and the key-hole opening 14 in positive terminal 12 of the cell. This insures good contact between the molybdenum tab 16 and the steel terminal 12. Because of the electrical

resistivities, the main current path is through molybdenum tab 16 and into low carbon steel terminal 12.

The data in Table 1 shows that molybdenum and low carbon steel have the lower resistance at the battery operating temperature of 450 °C. The positive electrode active material, iron disulfide, is extremely corrosive at the battery operating temperature, and molybdenum is one of the few metals that can survive such an aggressive environ¬ ment. Hence, molybdenum has been selected for the positive current collector tab 16 which is embedded in the main body of the carbon current collector is appropriate.

Several different key-hole shapes can be used in the base of the terminal to make the high-temperature connector. Figures 2 and 3 show such alternative configu- rations. In Figure 2, the electrical connector system 20 includes a terminal 22 having a triangular dovetail key-way 24 provided therein. A mating triangular dovetail tab 26 and pin 28 can be used in place of the proposed circular configuration of Figure 1. It should be noted that it is not essential that pin 28 have the same geomet¬ ric shape as key-way 24. Rather, it is sufficient that the configuration of pin 28 be such that upon thermal expan¬ sion, pin 28 will force tab- 26 into physical contact with terminal 22. Figure 3 shows another alternative arrangement 30 in which tab 36 is a continuous loop around the inside of the key-hole opening 34 of terminal 32 rather than the hook arrangement shown in Figure 1. The advantage of using a continuous loop with the tab 36 is that it provides a higher conductivity in the electrode tab 36 which is typically a relatively high resistance component of the current collection system in a cell. The continuous loop tab can be implemented in all the different key-hole designs. Although pin 38 is shown to conform to the configuration of key-hole opening 34, it is only necessary that pin 38 supply sufficient physical force on tab 36 to force tab 36 into contact with terminal 32 in order for electrical connector system 30 to be operational.

The pin used in the electrical connector systems described above can be made from a solid material or can be made from a memory alloy. A memory alloy is a material that exhibits different configurations at different temperatures. A pin made from a memory alloy can be shaped to fit within the opening in the tab of the electrode at ambient temperature. Upon heating, such a pin reverts to its initial shape or size whereupon it applies pressure to the tab and physically forces the tab against the key hole opening to make good contact with the base of the terminal.

Figure 4 is a schematic view of an electrochemical cell 40 having two terminals 42 each having an electrode 44 in contact therewith. Key-holes 46 are provided in termi¬ nals 42 and tabs 48 are provided on electrodes 44. Pin 50 fits within tab 48 and, upon thermal expansion, forces tab 48 into contact with terminal 42.

Thermal cycling tests have been performed on the positive collector tab and terminal assembly of the present invention. Figure 5 shows the results of that testing performed on an electrochemical cell similar to cell 40 shown in Figure 4. Those results indicate that there is improved contact between the molybdenum tab and the steel terminal when the assembly is heated from room temperature to the battery operating temperature. The results also show that there is a minimal increase in the electrical resistance of the assembly after it has been thermally cycled several times. This is a vast improvement over the prior art connectors which generally cannot survive such cycling. The electrical connector of the present invention is compact in design, easy to assemble, and of generally low cost. The present system eliminates all the problems of bulkiness and difficulty of assembly associated with a conventional bolt-type assembly. The proposed connector does not require excessive heat to make the joint as in the case of the welded and brazed connections. The use of a high expansivity core material ensures that good contact is maintained with increasing temperature and that the resis-

tivity of the joint is essentially unaffected by thermal cycling.

In the foregoing specification certain preferred practices and embodiments of this invention have been set out, however, it will be understood that the invention may be otherwise embodied within the scope of the following claims.