A BATTERY OF HIGH TEMPERATURE CELLS

The present invention relates to a battery comprising a plurality of high temperature cells and, in' particular, to the electrically conductive structural members used in such a battery.

High temperature cells are cells which operate at an elevated temperature above ambient temperature, typically above 100°C. One example of a high temperature cell is a sodium/sulphur cell whose operating temperature is between 300°C and 400°C, typically 350°C.

Many forms of battery are known, from a single cell to arrangements comprising a plurality of interconnected cells as necessary to give a required energy storage capacity.

Secondary batteries have been developed which are rechargeable when exhausted and thus have a relative long life, such batteries being use for starting automobiles, powering fork lift trucks and electric vehicles and providing standby power for buildings and telephone exchange equipment. Recently, secondary batteries of sodium/sulphur type have become known, such batteries having the advantages of light weight, high storage capacity and relative quick recharging time. Further, such batteries use sodium and sulphur, both of which are cheap and abundant materials.

Unlike conventional lead acid batteries in which a liquid electrolyte - dilute sulphuric acid - separates two solid electrodes, in a sodium/sulphur cell, a solid electrolyte - beta alumina - separates two liquid electrodes, namely liquid sulphur and sodium electrodes.

Such a sodium/sulphur cell is shown in Figure 1 of the drawings, which is a perspective view of the cell with part broken away. As shown, the cell comprises a case l of, for example steel, in the form of a right circular cylinder and containing a solid electrolyte cup 2 of beta alumina, the cup 2 containing a sodium electrode 3, while a space between the case 1 and the cup 2 contains a sulphur electrode 4. For use, the cell is maintained at a temperature of between 300°C and 400°C such that the sodium and sulphur electrodes 3 and 4 are in liquid form.

The open end of the cup 2 is closed by an insulating disc 5 of alpha alumina, while the case 1 is closed by an annular sealing steel disc 6.

The case 1 serves as a terminal for the sulphur electrode 4, while the sodium electrode 3 contains an elongate metal current collector 8 which extends axially of the case 1 out through the disc 5 where it is connected to a centre terminal disc 7 mounted on the disc 5, the necessary connections being made by welding.

As sulphur is essentially non-conducting, a means of making an electrical connection between the case 1 and the cup 2 has to be provided. This is generally achieved by forming the sulphur electrode 4 as a carbon fibre mat impregnated with sulphur.

It will be appreciated that with such a cell, the sodium and sulphur electrodes 3 and 4 can have their locations reversed.

To provide a battery capable of powering a vehicle, it may be necessary for about 3,000 cells, as described above to be assembled together in array of series-connected arrangements of cells, the series arrangement in each array being connected in parallel and the arrays of arrangements being connected in series.

WO89/00344 (Chloride Silent Power Limited) discloses various constructions of battery, each comprising one or more arrays of single cells or series arrangements of cell", arranged between bus plates. Figure 2 of the present specification is based on that document and shows a plurality of series arrangements of three cells 10, as described above with reference to Figure 1. Adjacent cells in each arrangement are separated by spacer members 11 and connected by conductive metal strips 12. The series arrangements of cells are arranged in parallel between a pair of mild steel bus plates 13.

Each bus plate 13 is formed with a matrix of holes 14 aligned with the central axes of the series arrangements of cells 10. The case 15 of the lower (as shown) cell 10 in each arrangement, and the centre terminal disc 16 of the upper (as shown) cell 10 in each arrangement are electrically connected to the respective adjacent bus plate 13. This electrical connection is made by means of a conductive metal strip 17 welded at its centre to the case 15 or terminal disc 16. The free ends of the metal strip 17 pass through the aligned hole 14 and are welded to the side of the bus plate 13 remote from the cells 10.

Thus the cells 10 are connected in series within each series arrangement while the series arrangements are connected in parallel by the bus plates 13. As shown, the cases 15 of corresponding cells 10 in the series arrangements are interconnected by electrically conductive members 18 arranged between them. Accordingly equalisation of voltage is achieved at each cell level in the battery.

Figure 3 of the present specification, also based on the aforementioned publication, shows a complete battery comprising two arrays of series arrangements of cells 20 arranged between respective pairs of bus plates 21 as described above. The lower bus plates 21 of the arrays rest on a common aluminium heat sink 22 having an ' electrical heating element 23 sandwiched therein. An electrical insulating layer 24 is provided between the bus plates 21 and the heat sink 22. As shown in Figure 3, the upper bus plate 21 of the one array of cells is connected to the lower bus plate 21 of the other array of cells, thereby to connect the two arrays in series. This connection is provided by means of a number (only one shown) of rods 25. The rods 25 can be of aluminium but with stainless steel end portions. Thus, the end portions of the rods 25 can be readily welded to the mild steel bus plates. Welded connections are desirable in order to keep the internal resistance of the battery as low as possible. The stainless steel end portions can readily be friction welded to the aluminium rods, whereas aluminium cannot be welded to mild steel.

Otherwise, a single plate can be bent to serve as the upper bus plate of one array and the lower bus plate of the adjacent array. The intermediate portion of the single plate extends between the arrays and in effect replaces the rods 25.

It is an object of the present invention to provide an improved battery.

According to the present invention there is provided a battery comprising a plurality of high temperature cells and at least one electrically conductive structural member made of a composite material, the composite material comprising a layer of copper sandwiched between a first and a second layer of steel.

The inventor has appreciated that the composite material used in the present invention will have a relatively low resistivity because of the layer of copper. In comparison with prior art electrically conductive structural members of a required low resistance, structural members made of a composite material as defined in the present invention have a reduced thickness because of the relatively low resistivity of the composite material. The presence of a layer of a steel on either side of the copper layer provides strength at the operating temperature of the cells. It was found that electrically conductive structural members made of a composite material as defined in the present invention were reduced in weight by 59% as compared with prior art electrically conductive structural members of the same strength and resistance. This reduction in weight of the electrically conductive structural members represents a significant weight saving per battery which is particularly significant for secondary batteries used in vehicles or required to be portable for other reasons.

The use of steel as the outer layer in the composite material enables the structural member to be easily welded to other materials as steel is more weldable than copper. Those skilled in the art will appreciate that the weldability of steel is at least in part determined by the surface of the steel to which a weld is to be made. In certain types of steel, an oxide layer may be formed, either due to the manufacturing process or due to exposure of the steel to the atmosphere. This oxide layer may reduce the weldability of the steel and so may need to be removed prior to welding.

Advantageously, the steel is not easily oxidised and is preferably resistant to sulphur attack. As the steel is not easily oxidised, the problem of removing an oxide layer before the steel can be welded is alleviated. Furthermore, the structural member is more resistant to attack during operation of the battery, particularly if a cell is damaged during operation, allowing sulphur to escape therefrom. The steel accordingly provides protection for the copper from sulphur attack.

Advantageously, the steel has a chromium content of less than 13% by weight and preferably less than 5% by weight. Steel having a relatively high proportion of chromium, such as stainless steel, is oxidation resistant but is also relatively expensive. Preferably, the chromium content of the steel is 1.25% by weight.

The copper content of the steel is advantageously greater than 0.1% by weight, preferably in the range of from 0.25% to 0.55%. The addition of copper in small amounts to steel can aid in the formation of a artensititic structure, providing strength to the steel. Advantageously, the steel has a carbon content in the range of from 0.075% to 0.2% by weight, preferably about 0.1%. The addition of a sufficient amount of carbon provides strength to the steel but too much carbon will cause the steel to be hard but too brittle to be formable.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings of which:

Figure 1 shows a perspective view of a sodium/sulphur cell;

Figure 2 illustrates a battery comprising a plurality of series arrangements of cells arranged between a pair of bus plates;

Figure 3 shows a perspective view of a prior art battery comprising two arrays of series arrangements of cells;

Figure 4 shows schematically a battery comprising a plurality of arrays of series arrangements of sodium/sulphur cells; and Figure 5 shows schematically a composite material according to the present invention.

Figure 4 shows schematically part of a battery comprising a plurality of arrays (two designated as 30A and 3OB) of series arrangements of sodium/sulphur cells 32. The mechanical construction of the array is similar to that described with reference to Figure 2 and so will not be described in detail here. The positive terminal of each series arrangement of cells is connected to a first bus plate 34 while the negative terminal of each series arrangement of cells in the array is connected to a second bus plate 36 (in each of the arrays 30A, 3OB only one series arrangement of cells is shown for clarity) .

As described previously, a battery of sodium/sulphur cells will consist of several arrays of series arrangements of cells. To enable the orientation of each cell in the battery to be the same, the arrays 30A, 3OB in the battery are orientated so that all the bus plates connected to terminals of the same plurality are on the same side of the battery. In the example shown, the bus plates 34, connected to the positive terminals of the series arrangements of cells, are uppermost whereas the bus plates 36, connected to the negative terminals of the series arrangements of cells, are lowermost. Inter-array connectors 38, shaped substantially as a parallelogram, connect the arrays 30A, 3OB in series. As well as providing electrical connection, the inter-array connectors 38 also provide a structural connection.

Components of the battery which need to provide both electrical connection and structural connection, such as the bus plates 30A, 3OB and the inter-array connectors 38 are made of a composite material 40, shown schematically in Figure 5. The material 40 is formed in sheets with a layer of copper 42 sandwiched between two layers of steel 44, 46. The steel used in a specific embodiment is a formable high stength low alloy weldable steel, manufactured under the name Cor-Ten "A", which possesses a resistance to atmospheric corrosion estimated to be over three times that of mild steel. The approximate composition of the Cor-Ten "A" by percentage weight is as follows:

Element Percentage By Weight

Carbon Maximum of 0.12

Silicon 0.25 to 0.75

Manganese 0.20 to 0.25

Phosphorous Maximum of 0.04

Sulphur Maximum of 0.050

Chromium 0.30 to 1.25

Nickel Maximum of 0.65

Copper 0.25 to 0.55

Iron Remainder

The copper used was electrical grade or general purpose copper.

The composite material is typically manufactured by high pressure cold rolling. The materials to be combined are cleaned and fed through rollers. The very high pressures, and the cleanliness of the materials, cause the

materials to cold weld. Further rolling reduces the material to the correct overall thickness. The finished ratio of material thicknesses can be determined prior to rolling by the selection of appropriate starting I thicknesses of the materials to be combined, as known to those skilled in the art.

The inventor has found that the composite material described has the properties necessary for providing electrically conductive structural members in a battery of sodium/sulphur cells. The copper layer 42 has a low resistivity to keep the internal resistance of the battery as low as possible. The layers 44, 46 of Cor-Ten "A" protect the copper 42 from sulphur attack. In contrast to copper, Cor-Ten "A" is more weldable and so electrical connections can easily be made thereto. Furthermore, Cor-Ten "A" is sufficiently strong at the operating temperatures of the cells (in contrast to copper) . As the Cor-Ten "A" itself is resistant to sulphur attack, it reduces the risk of a reduction in plate cross sectional area due to sulphur attack, resulting in an increase in resistance and a physical loss of contact between components.

A prior art electrical component made of low carbon plate (mild steel) of thickness 2mm had a resistance at 350°C of 0.84Ω. in contrast, an electrical component having a resistance of 0.83Ω was made of composite material, as described, with a copper layer 42 of thickness 0.2mm and two layers 44, 46 of Cor-Ten "A" of thickness of 0.3mm and hence a total thickness of 0.8mm. . As indicated previously, comparison of the component densities and sectional thicknesses of the prior art low carbon plate and the composite material used in the present invention show that use of the composite material will give a reduction in component weight of 59%, representing a significant weight saving per battery.

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The reduction in overall cross sectional thickness of the components is made possible by the low resistivity of the copper. The strength of the Cor-Ten "A" material enables the components to be made of this reduced thickness, although strengthing ribs can be provided along the length of the components to increase the second moment of area. It is envisaged that, if necessary, the strengthening ribs will take the form of pressed flutes or contours running the length and width of the structural member. The depth of the flutes will depend on the level of stiffness required and the available space. Electrically conductive structural members made of a composite material having a thickness up to 3mm or 4mm is also contemplated.

The present invention has been described with reference to a composite material including Cor-Ten "A". The use Cor-Ten "A" has a number of advantages: in particular, a lower cost than stainless steel and sufficient resistance to sulphur attack. Those skilled in the art will appreciate the effect of each of the alloying elements on the properties of the steel. It will be appreciated that composite materials using other types of steel, with the required properties, may also be used.

Modifications to the embodiments described within the scope of the present invention will be apparent to those skilled in the art.