GB2226441A - High temperature cell with switch - Google Patents

Description

1 Electrochemical Cell THIS INVENTION relates to an electrochemical cell.

More particularly the invention relates to a high temperature rechargeable electrochemical power storage cell having a molten alkali metal anode.

According to the invention there is provided a high temperature rechargeable electrochemical power storage cell having a molten alkali metal anode, a cathode and a separator which separates the anode from the cathode and which is a conductor of the alkali metal of the anode, the cell having a housing divided by the separator into an anode compartment containing the anode and a cathode compartment containing the cathode, the alkali metal of the anode being located in an enclosure comprising the anode compartment, which enclosure has a wall or walls comprising two electronically conducting portions separated by an electronically insulating zone and the cell. having an operative attitude in which it is uprig'.',.t and in which the portions are vertically spaced from each other by the zone, the upper portion being electronically connected to the cathode by an external circuit and said zone acting as a switch which automatically switches the cell during operation thereof, in response to changes in level of the alkali metal in the enclosure past said zone.

Said switch may be arranged to prevent excessive charging of the cell. Instead, said switch may be arranged to prevent excessive discharging of the cell. Conveniently the wall or walls of the enclosure comprise three electronically conducting portions separated by two electronically insulating zones, namely an uppermost portion, a central portion and a lowernost portion, vertically spaced in series from one another respectively by an upper insulating zone and a lower insulating zone, the uppermost and central portions being connected to the cathode by external 2 circuits and the upper and lower zones respectively acting as switches which automatically switch the cell during operation thereof, in response to changes in level of the alkali metal in the enclosure past said zones, so as to prevent both excessive charging and discharging of the cell.

In a particular embodiment of the cell having an upper portion and a lower portion as described above, the switch may be arranged to prevent excessive charging of the cell, the upper portion being located, in the operative attitude of the cell, above the level of the alkali metal in the enclosure in the fully charged state of the cell, the external circuit whereby the upper portion is connected to the cathode being a safety circuit and the lower portion being connectable to the cathode by an external cell operating circuit, so that, upon a sufficient increase in the level of the alkali metal in the enclosure in response to overcharging of the cell, the level of the alkali metal rises past the zone into contact with the upper portion to short-circuit the anode to the cathode to prevent further charging of the cell through the operating circuit. By the external cell operating circuit is meant the external circuit whereby the cell in use is normally charged and discharged. The upper portion will thus be located above the level of the alkali metal in the enclosure in normal use and during normal operation employing the operating circuit, the alkali metal level fluctuates but remains below the upper portion, out of contact therewith. In the event of excessive charging by means of the operating circuit, the alkali metal level rises past the zone and into contact with the upper portion. This short-circuits the anode to the cathode and prevents further charging of the cell, an optional resistance in the safety circuit, which may be high,preventing excessive shortcircuit currents and the cell discharging via the safety circuit until the alkali metal drops out of contact with the upper portion. Unless the charging via the operating circuit is discontinued, the cell will then resume charging and the above cycle of events will be cyclically 3 repeated, with the alkali metal level cycling up and down, into and out of the cathode with the upper portion.

is In a different embodiment of the cell having an upper portion and a lower portion as described above, the switch may be arranged to resist excessive discharging of the cell, the zone being located, in the operative attitude of the cell, below the level of the alkali metal in the enclosure in the fully discharged state of the cell, the external circuit whereby the upper portion is connected to the cathode being a cell operating circuit, so that, upon a sufficient drop in the level of the alkali metal in the enclosure in response toexcessive discharging of the cell, the level of the alkali metal drops out of contact with the upper portion, to interrupt further discharging of the cell through said operating circuit. Thus, the zone will be located below the level of the alkali metal in the enclosure in normal use and the upper portion will be connected by the cell external operating circuit to the cathode. In this case during normal operation via the operating circuit, the alkali metal level fluctuates but remains above the zone, in contact with the upper portion. In the event of excessive discharging, the alkali metal level drops out of contact with the upper portion, thereby interrupting the external operating circuit and resisting further discharge of the cell. If desired, the upper portion and lower portion may be connected via a circuit containing a resistance, which may be high, so that, instead of discharging being discontinued, it is permitted to continue at a very low rate. This feature permits subsequent recharging of the cell, which would otherwise be prevented.

In the embodiment mentioned above having three portions and two zones, the wall or walls of the enclosure will have three electronically conducting portions, the portions being, in the operative attitude of the cell, an uppermost portion, a central portion and a lowermost portion, and two electronically insulating zones, the zones, in the operative attitude of the cell, being an upper zone between said uppermost and central 4 portions and a lower zone between said central and lowermost portions, the uppermost portion being connected to the cathode by an external safety circuit and the central portion being connected to the cathode by an external cell operating circuit, the uppermost portion being located, in the operative attitude of the cell, above the level of alkali metal in the enclosure in the fully charged state of the cell and the lower zone being located, in said operative attitude, below the,level of alkali metal in the enclosure in the fully discharged state of the cell, so that, upon a sufficient increase in the level of alkali metal in the enclosure in response to overcharging of the cell, the level of alkali metal rises past the upper zone into contact with the uppermost portion to short-circuit the anode to the cathode to prevent further charging of the cell through the operating circuit, and, upon a sufficient drop in the level of the alkali metal in the enclosure in response to excessive discharging of the cell, the level of alkali metal drops out of contact with the central portion, to interrupt further discharging of the cell through said operating circuit. Accordingly, said portions and zones will be arranged to prevent both excessive charging and excessive discharging, the uppermost portion and the lower zone being arranged to be respectively above and below the level of the alkali metal in normal use, the uppermost portion being connected by the external safety circuit, optionally containing a high resistance, to the cathode, the central portion being connected by the external cell operating circuit to the cathode, and the central and lowermost portions optionally being connected together via a circuit containing a high resistance. In this case the alkali metal level will, in normal operation, fluctuate up and down below the uppermost portion and above the lower zone, out of contact with the uppermost portion and in contact with the central portion. This cell combines the features of automatica]-ly preventing excessive charging and resisting excessive discharging.

As indicated above, in each case when it is used, the external safety circuit may contain a resistance whereby any current flowing from the anode to the cathode along the safety circuit is limited to a predetermined maximum value; and in each case where excessive discharging is resisted the lowermost portion and the portion immediately above it may be interconnected by a trickle circuit, separate from the operating circuit, containing a resistance whereby the interruption of the operating circuit is such as to reduce the discharge current flowing through the operating circuit to a trickle.

In a particular embodiment of the cell, the enclosure may include an extension which, in the operative attitude of the cell, extends upwardly from the anode chamber and forms a reservoir for said alkali metal in communication with the anode chamber, the reservoir being located above the level of the anode chamber, and each insulating zone being provided in the extension. In this case the reservoir may be connected to the anode chamber by a pipe,of relatively small horizontal cross section, relative to the horizontal cross-sections respectively of the anode chamber and reservoir and, instead or in addition, the reservoir may have a stand pipe projecting upwardly therefrom, which may be provided with a non-return relief valve.

In other words, the reservoir may thus be connected to the anode chamber by a pipe having a smaller horizontal cross-section, with the cell in said operative attitude, than those of the reservoir and anode chamber; and the reservoir may have a stand pipe which projects upwardly therefrom when the cell is in its operative attitude, the stand pipe being provided with a non-return pressure relief valve. In this construction, in normal use, the chamber will be filled with alkali metal at all times, and the level of the alkali metal will fluctuate up and down, in the reservoir. When the enclosure is arranged only to prevent excessive charging, the reservoir and the pipe connecting it to the anode chamber will form the lower portion, the zone being provided in the stand pipe and the upper portion being provided by the stand pipe above the zone. When the enclosure is arranged only to prevent excessive discharging, the stand pipe may be omitted, the reservoir forming the upper portion and the zone 6 being provided in the pipe connecting the reservoir to the chamber, the lower portion being provided by said pipe below the zone. When the enclosure is arranged to prevent both excessive charging and discharging, the reservoir will form the central portion, the upper zone being in the stand pipe and the lower zone being in the pipe connecting the reservoir to the chamber, the stand pipe above the upper zone providing the uppermost portion and the lower part of the pipe connecting the reservoir to the chamber, below the lower zone, providing the lowermost 10 portion.

The cell may be of composite construction, the anode being divided into a plurality of anode portions, each in its own said enclosure, and the cathode being divided into a plurality of cathode portions, the anode portions being spaced from one another by the cathode portions and the cathode portions being spaced from one another by the anode portions, the anode portions and cathode portions being located in a common said housing which is divided by a plurality of said separators into a plurality of said anode chambers and a plurality of said cathode chambers, wherein the anode portions and cathode portions are respectively contained, the anode portions being electronically connected in parallel with one another and the cathode portions being electronically connected in parallel with one another, the upper or uppermost portions of the enclosures being electronically connected to the cathode portions by a common said external circuit; and in this case the enclosures may be in communication with each other so that they are, above the alkali metal therein when the cell is in its operative attitude, at the same pressure.

Usually the alkali metal will be sodium and the separator may be a solid electrolyte conductor of alkali metal ions, eg a sodium ion conductor such as beta-alumina, particularly betallalumina, or nasicon. When the anode is sodium and the separator is beta-alumina, the cathode may be sulphur/sodium sulphide/polysulphide. Instead, in a particular construction -1 7 when the anode is sodium and the separator is beta-alumina, the cathode may comprise an electronically conductive electrolytepermeable porous matrix impregnated with a molten salt sodium aluminium halide molten salt electrolyte, the matrix having, dispersed therein, an electrochemically active cathode substance which in its charged state is a member of the group consisting of FeC12, K'C12, COC12, CrCl2 and MnC12, the proportion of sodium ions and aluminium ions in the molten salt electrolyte being selected so that, in its charged state, the active cathode 10 substance is substantially insoluble therein. This may easily be effected by ensuring that the Al:Na mole ratio is at all times no greater than 1:1, and this in turn may be easily achieved by ensuring that the electrolyte is at all times saturated with respect to NaCl by having solid NaCl in contact with the is electrolyte in the charged state of the cell.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows a schematic representation of one embodiment of a cell in accordance with the invention; Figure 2 shows a similar view of another embodiment of a cell according to the invention; Figure 3 shows a similar view of another embodiment of a cell according to the invention; and Figure 4 shows a similar view of a composite cell according to the invention.

In Figure 1 of the drawings, reference numeral 10 generally designates a high temperature rechargeable electrochemical cell in accordance with the invention. The cell 10 comprises a housing 12 divided by a beta"-alumina separator 14 into a pair of electrode compartments, namely an anode compartment containing an anode 16 and a cathode compartment containing a cathode 18. In the schematic example shown the housing 12 is of steel and comprises two portions defining the electrode compartments and electronically insulated from each other at 20.

8 The anode 16 is molten sodium and the cathode 18 is a porous iron matrix saturated with molten NaAlC14 molten salt electrolyte and having FeC12 dispersed in its porous interior in the charged state of the cell, together with a small amount of NaCl. A current collector 22 is shown projecting from the cathode in the form of a steel rod embedded in the matrix, and a steel pipe 24 is shown projecting from the anode compartment. The pipe 24 leads upwardly to a steel reservoir 26 having a steel stand pipe 26 projecting upwardly therefrom.

The pipe 24 and reservoir 26 are shown containing molten sodium. The reservoir 26, pipe 24 and the sodium therein act as an anode current collector. The stand pipe 28 is also of steel except for part 30 thereof which is of an electronically insulating material, namely alpha-alumina. The stand pipe 28 has a non-return relief valve 32 at its upper end.

"I lector The reservoir 26 is shown connected to the current co.L 22 by an external cell operating circuit 34. The stand pipe 28, above the part 30, is connected to the current collector 22 by an external safety circuit 36 which has a high resistance 38 therein.

It will be appreciated that the anode compartment, pipe 24, reservoir 26 and stand pipe 28 together form an enclosure which contains the molten sodium anode material. This enclosure has two electronically conducting portions separated by an electronically insulating zone formed by the part 30 of the pipe, namely an upper portion formed by the steel part of the pipe 28 above the alpha-alumina part 30, and a lower portion formed by the steel part of the pipe 28 below the alpha-alumina part 30 and by the reservoir 26, pipe 24 and anode compartment.

In use, as the cell 10 is charged and discharged via the external operating circuit 34, the level 40 of sodium in the reservoir respectively rises and falls in the reservoir 26. The amount of sodium employed is selected so that, if the cell 9 becomes excessively charged, the sodium in the reservoir 26 rises into the pipe 28. If the level of sodium rises in the pipe 28 past the part 30 and comes into contact with the steel part of pipe 28 above the part 30, the cell becomes short-circuited by the safety circuit 36, automatically preventing any further charging of the cell 10. The resistance 38 will be selected to avoid excessive current through the circuit 36, but to permit the cell 10 to be discharged through the circuit 36 faster than it is charged via the circuit 34. The sodium level in the pipe 28 will thus drop until it is out of contact with the steel part of the pipe 28 above the alumina part 30. At this stage, unless charging has been discontinued, further charging will then take place, causing sodium again to rise in the pipe 28, this cycle of events being repeated cyclically until charging via the circuit 34 is discontinued.

In Figure 2 the same reference numerals refer to the same parts as in Figure 1, unless otherwise specified. Differences between Figures 1 and 2 are that Figure 2 omits the pipe 28 and circuit 36 of Figure 1; and that Figure 2 has, in the pipe 24, a part 42 of alpha-alumina insulating material, the steel parts of the pipe 24 above and below the alumina part 42 being interconnected by a circuit 44 which has a resistance 46 therein.

In use the cell of Figure 2 similarly has the sodium level in the reservoir 26 rise with charging and drop with discharging. In the event of excessive discharging, the sodium level drops into the pipe 24. If this level drops so that the sodium moves out of contact with the steel part of the pipe 24 above the alumina part 42, the cell operating circuit 34 is broken and further discharging is automatically prevented except for trickle discharging at a slow rate through the circuit 44.

It will be appreciated that, if such trickle discharging is unacceptable, the circuit 44 may be provided with a switch [not shown] whereby the circuit 44 may be broken, this switch being closed to close the circuit 44 [and hence the operating circuit 34] when it is required to again charge the cell. Such charging k will be trickle charging until the sodium in the pipe 24 rises into contact with the steel part of the pipe 24 above the alumina part 42, after which charging can continue at the normal rate.

It will be appreciated that in Figure 2 the anode compartment, pipe 24 and reservoir 26 together form an enclosure which contains the molten sodium anode material. This enclosure has two electronically conducting portions separated by an electronically insulating zone formed by the part 42 of the pipe 24, namely an upper portion formed by the reservoir 26 and the steel part of the pipe 24 above the part 42, and a lower portion formed by the steel part of the pipe 24 below the part 42 and the anode compartment.

In Figure 3, once again, the same reference numerals refer to the same parts as in Figures 1 and 2, unless otherwise specified. Excessive charging is automatically prevented in the same fashion as in Figure 1; and excessive discharging is automatically prevented in the same fashion as in Figure 2.

In Figure 3 it will be appreciated that an enclosure which contains the sodium anode material is, as in Figure 1, forned by the stand pipe 28, reservoir 26, pipe 24 and anode compartment.

In the case of Figure 3, however, the enclosure has three electronically conducting portions vertically separated in series by two electronically insulating zones. Thus, there is an uppermost portion formed by the steel part of the stand pipe 28 above the part 30 which is an upper insulating zone, a central portion f ormed by the steel part of the pipe 28 below the insulating part 30, the reservoir 26 apd the steel part of the pipe 24 above the insulating part 42 which forms a lower insulating zone, and a lowermost portion formed by the steel part of the pipe 24 below the panel 42 and by the anode chamber.

In Figure 4 a composite cell according to the invention ILs shown, the same reference numerals referring to the same parts as in Figure 3. The composite cell is designated 48 and comprises 11 three spaced anode portions 16 arranged in a row, in a common housing 12 and connected in parallel.

Each anode portion 16 has a pair of separators 14 on opposite sides thereof, and a pair of cathode portions 18 on opposite sides thereof. The cathode portions 18 between the anode portions 16 are full-size cathode portions which are shared by the anoe portions 16 on opposite sides thereof, and the cathode portions 18 at the ends of the row are half-size cathode portions, each acting only on the adjacent anode portions 16 to one side thereof. The separators 14 divide the housing into three anode chambers and four cathode chambers, respectively containing said anode portions 16 and cathode portions 18.

The reservoirs 26 of the three anode portions are three separate chambers in a common steel box and their stand pipes 28 is are connected together by a manifold 50 leading to a single relief valve 32 which is an upward extension of the stand pipe 28 of the central reservoir and is shared by the three reservoirs 26. Each cathode portion 18 has its own cathode current collector 22, which collectors 22 are similarly arranged to those in Figures 1 to 3 but for ease of illustration in Figure 4 are shown projecting downwardly from their respective cathode portions 18. The current collectors 22 are shown connected in parallel to the circuit 34 which leads to the box housing the reservoirs 26, the anode portions 16 sharing the circuit 34. Likewise, the anode portions 16 share a common safety circuit 36 with resistance 38.

The cells of the present invention may be loaded in a overdischarged state, as described in the Applicant's Published British Patent Application 2 191 332A. Initial charging can then take place at a trickle rate via the resistances 46 until sodium in the anode compartments rises into electronic contact with the reservoirs 26, followed by at charging the full rate. Naturally, the cell of Figure I can be charged at the full rate immediately. To start charging with the anode compartment empty, a gauze or mesh-type current collector [not shown], eg of steel, can be k 12 used in each anode compartment, held up against the associated separator or separators 14, at a multiplicity of positions, the gauze being connected to the associated pipe 24.

Z It is an advantage of the cells of the present invention that they provide for automatic prevention of excessive charging and/or discharging or, if desired, automatic reduction of discharge rate to a trickle if discharging becomes excessive. A further advantage is safety in the event of separator failure, in that all the sodium in the associated anode and the associated reservoir can be consumed byreacting with NaAlCl4 electrolyte, to provide solid reaction products, namely Al and NaCl. A yet further advantage is that cell failure, arising from separator failure, is automatically to open circuit and not to short circuit. This is desirable for para 1 lel - connected anode portions and cathode portions of the type shown in Figure 4.

11 13

Claims (11)

Claims:

1. A high temperature rechargeable electrochemical power storage cell having a molten alkali metal anode, a cathode and a separator which separates the anode from the cathode and which is a conductor of the alkali metal of the anode, the cell having a housing divided by the separator into an anode chamber containing the anode and a cathode chamber containing the cathode, the alkali metal of the anode being located in an enclosure comprising the anode chamber, which enclosure has a wall or walls comprising two electronically conducting portions separated by an electronically insulating zone and the cell having an operative attitude in which it is upright and in which the portions are vertically spaced from each other by the zone, the upper portion being electronically connected to the cathode by an external circuit and said zone acting as a switch which automatically switches the cell during operation thereof, in response to changes in level of the alkali metal in the enclosure past said zone.

2. A cell as claimed in claim 1, in which the switch is arranged to prevent excessive charging of the cell, the upper portion being located, in the operative attitude of the cell, above the level of the alkali metal in the enclosure in the fully charged state of the cell, the external circuit whereby the upper portion is connected to the cathode being a safety circuit and the lower portion being connectable to the cathode by an external cell operating circuit, so that, upon a sufficient increase in the level of the alkali metal in the enclosure in response to overcharging of the cell, the level of the alkali metal rises past the zone into contact with the upper portion to short-circuit the anode to the cathode to prevent further charging of the cell through the operating circuit.

3. A cell as claimed in claim 1, in which the switch is arranged to resist excessive discharging of the cell, the zone being located, in the operative attitude of the cell, below the 14 level of the alkali metal in the enclosure in the fully discharged state of the cell, the external circuit whereby the upper portion connected to the cathode being a cell operating circuit, so that, upon a sufficient drop in the level of Ithe alkali metal in the enclosure in response to excessive discharging of the cell, the level of the alkali metal drops out of contact with the upper portion, to interrupt further discharging of the cell through said operating circuit.

4. A cell as claimed in any one of claims 1 to 3 inclusive, in which the wall or walls of the enclosure have three electronically conducting portions, the portions being, in the operative attitude of the cell, an uppermost portion, a central portion and a lowermost portion, and two electronically insulating zones, the zones, in the operative attitude of the cell, being an upper zone between said uppermost and central portions and a lower zone between said central and lowermost portions, the uppermost portion being connected to the cathode by an external safety circuit and the central portion being connected to the cathode by an external cell operating circuit, the uppermost portion being located, in the operative attitude of the cell, above the level of alkali metal in the enclosure in the fully charged state of the cell and the lower zone being located, in said operative attitude, below the level of alkali metal in the enclosure in the fully discharged state of the cell, so that, upon a sufficient increase in the level of alkali metal in the enclosure in response to overcharging of the cell, the level of alkali metal rises past the upper zone into contact with the uppermost portion to short-circuit the anode to the cathode to prevent further charging of the cell through the operating circuit, and, upon a sufficient drop in the level of the alkali metal in the enclosure in response to excessive discharging of the cell, the level of alkali metal drops out of contact with the central portion, to interrupt further discharging of the cell through said operating circuit.

5. A cell as claimed in claim 2 or claim 4, in which the saf ety circuit contains a resistance whereby any current flowing from the anode to the cathode along the safety circuit is limited to a predetermined maximum value.

6. A cell as claimed in claim 3 or claim 4, in which the lowermost portion and the portion immediately above it are interconnected by a trickle circuit, separate from the operating circuit, containing a resistance whereby the interruption of the operating circuit is such as to reduce the discharge current flowing through the operating circuit to a trickle.

7. A cell as claimed in any one of the preceding claims, in which the enclosure includes an extension, which, in the operative attitude of the cell, extends upwardly from the anode chamber and forms a reservoir for said alkali metal in communication with the anode chamber, the reservoir being located above the level of the anode chamber, and each insulating zone being provided in the extension.

8. A cell as claimed in claim 7, in which the reservoir is connected to the anode chamber by a pipe having a smaller horizontal cross-section, with the cell in said operative attitude, than those of the reservoir and anode chamber.

9. A cell as claimed in claim 7 or claim 8, in which the reservoir has a stand pipe which projects upwardly therefrom when the cell is in its operative attitude, the stand pipe being provided with a non-return pressure relief valve.

10. A cell as claimed in any one of the preceding claims, which is of composite construction, the anode being divided into a plurality of anode portions, each in its own said enclosure, and the cathode being divided into a plurality of cathode portions, the anode portions being spaced from one another by the cathode portions and the cathode portions being spaced from one another by the anode portions, the anode portions and cathode portions 16 being located in a common said housing which is divided by a plurality of said separators into a plurality of said anode chambers and a plurality of said cathode chambers, wherein the anode portions and cathode portions are respectively contained, the anode portions being electronically connected in parallel with one another and the cathode portions being electronically connected in parallel with one another, the upper or uppermost portions of the enclosure being electronically connected to the cathode portions by a common said external circuit.

11. A cell as claimed in claim 10, in which the enclosures are in communication with each other so that they are, above the alkali metal therein when the cell is in its operative attitude, at the same pressure.

12 A high temperature rechargeable electrochemical power storage cell, substantially as described herein with reference to and as illustrated in the accompanying drawings.

Published 1990atThePatent Office. StateHouse.66 71 High Holborn- London WC1R 4TP Purther copies maybe obtained from The Patent.Offlicc Sales Branch. St Maiy Cray. Orpinglon. Kent BR5 3RD Plinted by techriques ltd. S, Ma-y Cray- Kent Ccr. 187