GB2062939A - Na/S cells discharge control - Google Patents

Abstract

A limiting means for controlling the discharge through a cell e.g., Na-S cell, which may be connected in series with a chain of similar cells to form a battery, comprises a control cell having a housing containing a tube of solid electrolyte material, e.g. beta-alumina, which is conductive for ions of alkali metal such as sodium. Liquid sodium is provided outside the tube whilst, inside the tube, there is liquid sodium together with a second electronically conductive material, such as lead or tin which is not ionically transportable through the electrolyte and which maintain an electrical path between the conductor and a current collector contained in the liquid metal electrode. The additive may also be a powdered metal or a carbon felt.

Description

SPECIFICATION Cells with a solid electrolyte and a liquid electrode material and batteries employing such cells This invention relates to cells with a solid electrolyte and a liquid electrode material and to batteries employing such cells.

A sodium sulphur cell is an example of a cell with a solid electrolyte, for example, a sodium betaalumina which will permit the passage of sodium ions, and a liquid electrode material, namely sodium. The sodium constitutes not only an electrode material which is transported, as ions, through the solid electrolyte but also provides an electronic conduction path for one terminal of the cell. In such a cell, if the sodium continues to be ionically transported through the electrolyte, a condition will be reached when the sodium is exhausted and no more current can flow. If the cell is allowed to get to this state however it now cannot be recharged by a reverse current flow because of the lack of the electronic conductive path which had been provided through the liquid metal. For this reason, the discharge of sodium sulphur cells has to be limited.

When a battery is formed of a number of cells connected together in series to form a chain of cells and, in particular, when several of these chains are connected together in parallel, it becomes difficult to control the depth of discharge of the individual chains and cells without some form of external monitoring and control.

It is an object of the present invention to provide a means of limiting or controlling the amount of discharge through a cell and, as will be explained more fully later, permitting appropriate control of the discharge of cells in a battery.

According to one aspect of the present invention, there is provided a cell comprising a solid electrolyte through which can pass ions of a first metal and, on one side of the electrolyte, a first electrode comprising said first metal in liquid form, said electrode also containing a second electronically-conductive material which is not ionically transportable through the electrolyte but which extends between the surface of the electrolyte and a current collector. The second material is conveniently a liquid metal, for example lead or tin if the first metal is sodium.

Other forms of electronic conductor however may be employed to provide an electronically-conductive path between the current collector and the surface of the electrolyte, preferably to a plurality of points on or to a substantial area of the electrolyte surface. For example a powdered electronically conductive material such as a powdered metal may be employed or a resiliently deformable material such as carbon felt.

The operation of the cell of the present invention may most readily be explained by considering an example. Assuming that the electrolyte is a material, such as sodium beta-alumina through which sodium ions can pass, there may be provided on one side of the electrolyte a mixture of molten sodium and molten lead. If the sodium in this mixture is transported, as ions, through the electrolyte, the electric current flow will cease when all the sodium has passed through the electrolyte. This liquid lead however will remain to form an electronic conductive path through the surface of the electrolyte and hence the cell is instantaneously reversible if the current direction is altered.

A simple form of limiter to give a predetermined quantity of an electric discharge before stopping the current may have the ionically-transportable material together with the second electronicallyconductive material, for example the sodium and lead, on one face of the electrolyte and have solely the ionically-transportable material, on the opposite face of the electrolyte. Thus for example a sodium/ sodium cell with a sodium ion conductive electrolyte may be used as a control cell by using a liquid mixture of lead and sodium adjacent one face of the electrolyte and having liquid sodium adjacent the other face, the cell being used to control a quantity of current going through the cell in the direction so that the sodium ions can pass through the electrolyte from said one face to said other face.The cell may obviously be used for controlling the amount of flow in both directions by utilising the mixed material, e.g. the sodium lead mixture on both faces of the electrolyte.

Such a cell which serves to pass a predetermined quantity of electricity may be connected in series with a chain of sodium sulphur cells so as to limit the current discharge through the chain. In a battery comprising a number of such chains, such a cell, which may be referred to as a control cell or clock cell, may be utilised in each of the chains so that, separately for each chain, over-discharge of any cell in the chain is limited.

For many purposes, it is preferred however to utilise the mixed material as one electrode of an electro-chemical cell, for example a sodium sulphur cell.

Thus the information furthermore includes within its scope an electro-chemical cell having a solid electrolyte material through which can pass ions of a first metal and having, on one face of the electrolyte material, a mixture of said first material and a second electronically-conductive material, the ions of which will not pass through the electrolyte, said cell furthermore having on the opposite face a reactant which combines with the transported ions from the first material and which is in contact with a current collector.

The invention furthermore includes within its scope a sodium sulphur cell having a solid electrolyte through which sodium ions can pass, the electrolyte separating the cathodic region containing sulphur/sodium polysulphides from an anodic region containing liquid sodium mixed with a second electronically-conductive liquid material which cannot pass in ionic form through the electrolyte. As indicated above the second material conveniently is lead or tin.

A battery may be formed of a plurality of cells, each of which is constructed as described above, to have a limited discharge, the second electronicallyconductive material permitting subsequent recharging. However such a cell may be used, as described above, as a control or clock cell in combination with cells of conventional construction. For example in a battery formed of a plurality of cells connected in series to form a chain with a plurality of chains in parallel, there may be provided one limited discharge cell as described above, in each chain, the discharge cell being constructed to limit the discharge to a level sufficient to protect the remaining cells in the chain against becoming overdischarged to a state where they are not immediately reversible at the start of recharge.

The invention thus includes within its scope a sodium-sulphur battery comprising a plurality of similar sodium-sulphur cells, the cells being seriesconnected in at least one chain, wherein each chain contains a series-connected discharge limiter, the discharge limiter being a cell with a solid electrolyte through which ions can pass, which electrolyte separates two liquid electrodes, each with an associated current collector one of which electrodes contains liquid sodium together with a further electronically-conductive material extending between the associated current collector and the surface of the electrolyte which is exposed to said one electrode, said discharge limiter having in said one electrode, when the battery is charged, a limited quantity of sodium such that all this sodium will pass through the solid electrolyte before any of the cells in the chain containing the discharge limiter is overdischarged.

As indicated above, the second liquid electrode in the discharge limiter may be sodium but, very conveniently the discharge limiter is a sodium sulphur cell.

In the following description reference will be made to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of a cell for controlling the total quantity of currentflow; Figure 2 is a graphical diagram illustrating the changes in current and voltage of a cell such as is illustrated in Figure 1; Figure 3 illustrates a battery utilising cells of the kind shown in Figure 1; and Figure 4 illustrates a preferred form of cell for use in a battery.

Referring to Figure 1 there is illustrated diagrammatically a cell for controlling the total quantity of current discharge. This cell comprises an outer container 10 and an inner tubular separator 11 comprising a tube, closed at one end and formed of sodium beta-alumina ceramic electrolyte material.

The region 12 between the container and the outer surface of the tube 11 contains molten sodium into which extends an electrical current collector illustrated diagrammatically at 13. This current collector might be constituted by the container 10. Within the tube 11 is a molten mixture 14 of sodium and lead into which extends a second electrical current collector 15. On application of suitable voltages to the two current collectors 13,15, sodium can be ionically transported into or out of the beta-alumina tube 11. If sodium is transported out of the tube, eventually the sodium within the tube will be exhausted and no more current will flow.However the lead remaining inside the tube will maintain a good electrical contact with the inner surface of the electrolyte tube, such that on reversal of the applied voltage, sodium can immediately begin to be transported back into the beta-alumina tube at whatever rate may be desired.

It will be seen that the total amount of electrical charge which can be transported out of the tube depends on the quantity of sodium that is within the tube initially. The device thus forms a limiter for the total quantity of current flow.

The material within the beta-alumina tube comprises liquid sodium together with a second material which should not be readily capable of being ionically-transported through the sodium betaalumina. It should permit the passage of the required current without an unacceptably high voltage drop.

It should not be corrosive or otherwise detrimental to the materials involved and particularly the sodium beta-alumina. It must not react with the sodium in such a manner that the ionic transport of the sodium out of the beta-alumina tube requires an unacceptably high voltage. There should be sufficient material there to ensure a low electrical resistance at the start of filling and to ensure that too high a current density is not reached when the sodium is nearly exhausted. It must remain in physical contact with the beta-alumina tube and with the current collector despite the varying amounts of sodium and despite possible temperature variations. Although lead is a convenient material, other metals may be employed for example molten tin.Alternatively it is possible to use a powder, e.g. a powdered metal or a resilient deformable material such as an electronicallyconductive felt, e.g. a carbon felt.

If lead is used, typically the amount of lead is such that, when molten, it covers about 40% of the inside surface of the electrolyte tube 11. If the operating temperature of the cell is 350"C, solids will be formed if the amount of sodium is increased above about 8% by weight. At about 10% by weight of sodium, the solid intermetallic compound NaPb is formed. The quantity of sodium is determined by the required total discharge limit. The quantity of lead is such that only a small proportion, if any, of solids are formed.

Using sodium and lead as described above, the current collectors are conveniently formed of stainless steel.

Figure 2 is a graphical diagram illustrating the voltage (solid line) and current (dashed line) plotted against time as a lead/sodium cell, such as that of Figure 1 is charged and discharged.

From A to B, under a constant current drive, sodium is transported into the beta-alumina tube. As the area of the tube surface, which is covered, increases the voltage required for a constant current drive falls slightly. At B the voltage is reversed and sodium flows out of the tube and the voltage rises slowly. At C the sodium is nearly exhausted and the voltage rises to try to maintain the current (constant current power supply) which soon falls as the maximum voltage of the supply is reached. At D the current has nearly stopped flowing; the cell has thus acted as a discharge limiter. In the diagram, the graphs illustrate the conditions if the voltage is now reversed. The required current begins to flow immediately.Experimental work has shown that this type of cell will withstand in excess of 45V before breakdown although to ensure a long life, repeated operation to voltages in excess of several volts should be avoided.

Figure 3 shows a battery with six cells 20 in each of five chains. The five chains are in parallel; each chain has one clock or control cell 21 in series. In this way the total discharge through the cells of any chain will be limited by the clock cell 21. This will result in a voltage being applied across the clock cell. The magnitude of this voltage will be the difference between the open circuit voltage of the switched off chain minus the voltage of the working chains. For a battery of twelve sodium sulphur cells per chain this would typically be 5 to 10 volts.

If more clock cells switch off then the voltage across these cells rises until the whole battery voltage is seen across these cells when all have cut off. Clearly this would be disadvantageous for high voltage batteries, but a sensing circuit (not shown) may be attached to each clock cell and arranged to turn off the battery after a proportion of the clock cells has switched, thereby acting as a simple battery limit device and coulometer.

In the above, there has been described more specifically the use of a control cell designed to act solely as a control cell. In a battery, this is wasteful of space and it may be preferred therefore to use, for the discharge limiter in each chain, a sodium sulphur cell constructed to act also as a discharge limiter.

Such a cell is shown in Figure 4. In this cell, a beta-alumina ceramic electrolyte tube 30, closed at one end, acts as a separator in a cathodic compartment 31 inside the tube and an anodic compartment 32 in the region between the tube 30 and an outer stainless steel container 33. The cathodic compartment 32 contains sulphur/sodium polysulphides impregnated in a graphite felt matrix between the inner surface of the electrolyte tube and a cathode current collector 34 which is secured on a seal 35 across the top of the electrolyte tube.The anodic compartment 32 contains liquid metal which is pressurised by gas trapped in a region 36 within a pressure can 37, the gas pressure forcing the liquid metal downwardly through the open lower end of the pressure can and upwardly around the pressure can into the annular region between the tube 30 and container 33 so as to keep the outer surface of the electrolyte tube substantially covered with the liquid metal.

The cell of Figure 5, as thus far described, is of known type. In a conventional sodium sulphur cell however the liquid metal consists solely of sodium.

To form a discharge limiting cell, this liquid metal in the embodiment shown in Figure 4 comprises a mixture of sodium and lead, the lead being in a sufficient quantity, as previously explained, to ensure that no solids or only a small quantity of solids are formed by interaction between the lead and sodium.

With the construction of Figure 4, the sodium sulphur cell cannot be overdischarged past a predetermined limit. When all the sodium is used up the annulus between the tube 30 and container 33 will be full of lead and the gas bubble in the pressure can will be retained. Insertion of such a cell into a chain in a battery, as in Figure 3, would have all the advantages of a clock cell but would not reduce the volumetric energy density of the battery as a whole.

The weight saving on reduction of the excess sodium in all the other pressure cans would typically offset a substantial part of the additional weight of lead.

Claims (20)

1. A cell comprising a solid electrolyte through which can pass ions of a first metal and, on one side of the electrolyte, a first electrode comprising said first metal in liquid form, said electrode also containing a second electronically-conductive material which is not ionically transportable through the electrolyte but which extends between the surface of the electrolyte and a current collector.

2. A cell as claimed in claim 1 wherein the second material is a liquid metal.

3. A cell as claimed in claim 2 wherein the first metal is sodium and the second material is lead.

4. A cell as claimed in claim 2 wherein the first metal is sodium and the second material is tin.

5. A cell as claimed in claim 1 wherein said second material is a powdered electronic conductor.

6. A cell as claimed in claim 1 wherein said second material is a resilient deformable electronically-conductive material.

7. A cell as claimed in claim 6 wherein said second material is a carbon felt.

8. A cell as claimed in any of the preceding claims and having, on the opposite side of the electrolyte to said first electrode, a second electrode and second current collector, which second electrode comprises said first metal in liquid form

9. A cell as claimed in any of claims 1 to 7 and having, on the opposite side of the electrolyte to said first electrode, a second electrode and second current collector, said second electrode comprising a material reacting electro-chemically with the first material so that said cell is a rechargeable currentgenerating cell.

10. A cell as claimed in claim 9 and having sodium as said first metal and having sulphur/ sodium polysulphides as said second electrode.

11. An electro-chemical cell having a solid electrolyte material through which can pass ions of a first metal and having, on one face of the electrolyte material, a mixture of said first material and a second electronically-conductive material, the ions of which will not pass through the electrolyte, said cell furthermore having on the opposite face a reactant which combines with the transported ions from the first material and which is in contact with a current collector.

12. A sodium sulphur cell having a solid electrolyte through which sodium ions can pass, the electrolyte separating the cathodic region containing sulphur/sodium polysulphides from an anodic region containing liquid sodium mixed with a second electronically-conductive liquid material which cannot pass in ionic form through the electrolyte.

13. A cell as claimed in claim 12 wherein the second material is lead or tin.

14. A sodium sulphur cell having a solid electrolyte through which sodium ions can pass, the electrolyte separating the cathodic region containing sulphur/sodium polysulphides from an anodic region containing liquid sodium mixed with a powdered electronically-conductive material which cannot pass in ionic form through the electrolyte.

15. A sodium sulphur battery comprising a plurality of similar sodium sulphur cells, the cells being series-connected in at least one chain, wherein each chain contains a series-connected discharge limiter, the discharge limiter being a cell with a solid electrolyte through which sodium ions can pass, which electrolyte separates two liquid electrodes, each with an associated current collector one of which electrodes contains liquid sodium together with a further electronically-conductive material extending between the associated current collector and the surface of the electrolyte which is exposed to said one electrode, said discharge limiter having in said one electrode, when the battery is charged, a limited quantity of sodium such that all this sodium will pass through the solid electrolyte before any of the cells in the chain containing the discharge limiter is discharged.

16. A battery as claimed in claim 15 wherein the second liquid electrode in the discharge limiter is sodium.

17. A battery as claimed in claim 15 wherein the discharge limiter is a sodium sulphur cell.

18. A battery as claimed in claim 15 wherein the discharge limiter is a cell as claimed in any of claims 1 to 14.

19. A discharge limiting cell substantially as hereinbefore described with reference to Figures 1 and 2 or Figure 4 of the accompanying drawings.

20. A sodium sulphur battery substantially as hereinbefore described with reference to the accompanying drawings.