GB2048557A

GB2048557A - Sodium-sulphur cell

Info

Publication number: GB2048557A
Application number: GB7914051A
Authority: GB
Original assignee: Chloride Silent Power Ltd
Current assignee: Chloride Silent Power Ltd
Priority date: 1979-04-23
Filing date: 1979-04-23
Publication date: 1980-12-10

Abstract

In a sodium sulphur cell, a high charge acceptance at high current densities is obtained by forming blind holes 20 (Figure 1) in the cathode matrix 18 (typically of carbon felt), the holes extending from the surface of matrix adjacent the cathode current collector 14 through the matrix towards the electrolyte 11 and being filled with a material such as finely powdered activated carbon having a substantially higher surface area per unit mass than the matrix material. <IMAGE>

Description

SPECIFICATION Improvements in or relating to sodium sulphur cells This invention relates to sodium sulphur cells and is concerned more particularly with the cathode electrode structure in such cells.

In a sodium sulphur cell, a cation permeable membrane, usually a polycrystalline ceramic material, for example beta-alumina, constitutes a solid electrolyte separating an anodic region containing sodium, which is liquid under cell operating conditions, from a cathodic region containing the cathodic reactant. The cathodic region contains a porous electronically conductive matrix, commonly a graphite felt, which extends between the surface of the electrolyte and a cathode current collector, this matrix being impregnated with the cathodic reactant comprising sulphur and sodium polysulphides. This cathode reactant is a liquid at the cell operating temperature.

On discharge of the cell, sodium ions pass through the electrolyte and, in the cathodic region, give up their charge and combine with the sulphur to form sodium polysulphides. On recharge, the reverse action takes place. It is found however that, with a uniformly distributed homogeneous graphite felt in the cathodic region, it is not possible to recharge cells to the full capacity corresponding to 100% conversion of the polysulphides back to sulphur.On discharge, the cathodic reactant is converted to Na2S3 but, on recharge, this material is converted initially to a higher polysulphide, more particularly Na2Ss. Further charging leads to the formation of a two-phase mixture of Na2s5 and sulphur.The sulphur has a high electrical resistance and, if it accumulates around the electrolyte surface, it causes overall cell resistance, on initial discharge of the cell, to be very high. The extent of passivation of the cell by sulphur depends on many factors and results in the charge acceptance of sulphur electrodes varying widely. Up to 60% of the theoretical charge capacity of the cell can be lost this way.In order to improve the charge acceptance, it has been proposed to add materials such a selenium or boron trisulphide to the sulphur melt, to reduce the sulphur viscosity and so improve mass transport (Kleinschmager et ai 'Performance of Differenttypes of Sodium/Sulphur Cells' Paper 45 presented at 10th International Conference on Power Sources, Brighton, U.K. September 1976).It has been proposed to add a thin insulating layer between the electrolyte and the conductive porous matrix to move the electrochemical reaction away from the surface of the electrolyte (Fally et al, J.Electrochem.Soc.,120(1973)1292.). In addition, felt free channels in the vicinity of the electrolyte surface have been shown to improve the utilisation of Na2S5 on charge and hence to improve rechargeability (United States Patent Specification 3993503). Forcing the electrochemical reaction away from the electrolyte towards the cathode current collector however increases the problems of corrosion of the current collector, particularly at high current densities.

The present invention is directed to enabling high charge acceptance to be obtained in a sodium sulphur cell at high current densities.

According to this invention, in a sodium sulphur cell having a solid cation permeable membrane separating an anodic region containing sodium from a cathodic region containing a cathodic reactant, the cathodic region lying between a surface of the electrolyte membrane and a surface of a cathode current collector, the cathode region including a porous electronically-conductive matrix impregnated with the cathodic reactant, the porous matrix is formed of two electronically-conductive porous materials, one of which has a substantially higher surface area per unit mass than the other and is in a plurality of discrete channels extending from the surface of the matrix adjacent the cathode current collector through the matrix towards the electrolyte membrane and the other of which materials extends between said surfaces.Preferably the channels extend only part of the way through the matrix from the surface thereof adjacent the current collector.

The provision, in discrete channels of the material with the higher surface area per unit mass results in discrete pockets of high electrochemical activity. The sulphur, which is a passivating species in the cathodic reactant, is preferentially formed at these zones on charging the cell, thereby leaving the other regions relatively free of sulphur.

Preferably the channels containing the material with the higher surface area per unit mass are distributed in regular discrete zones in the other material. These channels are preferably distributed in discrete regions which, in a plane parallel to the surface of the electrolyte, are substantially uniformly distributed. The material with the higher surface area per unit mass is preferably a micro-porous material, for example activated charcoal. The material with the lower surface area per unit mass is preferably a carbon material for example a fibrous carbon material such as a felt, or a foamed carbon material or a particulate material, e.g. particles of graphite or carbon black.

It is found that, by introducing the material of higher surface area per unit mass, the charge acceptance can be greatly improved compared with a sulphur electrode formed solely from the material of lower surface area per unit mass. In a preferred form of the invention, the sulphur electrode comprises a carbon felt matrix having a plurality of spaced cavities, e.g. blind bores, which cavities are filled with activated charcoal. The cavities are preferably evenly distributed, considered in a plane parallel to the surface of the electrolyte. The carbon felt matrix provides a slightly resilient body ensuring good electrical contact with the cathode current collector and acts as the absorbantforthe cathodic reactant which is liquid at the operating temperature of the cell.The high surface area of the activated charcoal however gives the improved charge acceptance and it has been found possible to construct cells in which the recharge capacity corresponds to about 95% conversion of the polysulphides back to sulphur. The high charge acceptance can be obtained at high current density and it thus becomes possible to recharge the cell more quickly than if the activated charcoal was not present.

It is preferred to use relatively large particles of activated charcoal, for example in the range of 0.2 to 0.5 mm; such large particles are easier to handle and make it easier to avoid contamination of the surface of the felt to come in contact with the current collector. More particularly, with larger particles, there is less particle to particle contact and hence higher conduction of the electronic pathway from the activated charcoal particles to the surrounding felt.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a longitudinal section through a sodium sulphur cell; Figure 2 is a diagrammatic cr6ss-sebtion through a sulphur electrode structure for the cell of Figure 1; and Figure 3 is a graphical diagram showing the effect of incorporating various different carbon materials into a carbon felt eletrode structure for a sodium sulphur cell.

Referring to Figure 1 of the drawings, there is shown a sodium sulphur cell of cylindrical form comprising a mild steel case 10 containing an electrolyte tube 11 of beta alumina which tube is closed at the lower end. The region 12 between the tube 11 and casing 10 forms a capillary region communicating with a gas-pressurised sodium reservoir 13 in the bottom ofthe casing so that sodium is drawn up to wet the outer surface of the tube 11.

Located centrally within the electrolyte tube 11 is a cathode current collector 14 which, in this embodiment is of tubular form. The top end of the current collector is electrically connected to a cathode terminal 15 situated above a sealing system 16 closing the top of the cell. An anode terminal 17 is connected to the outer casing 10. Between the current collector and the electrolyte tube 11 is a carbon felt 18 which is impregnated with sulphur/ sodium polysulphides. This felt extends from the surface of the current collector to the surface of the electrolyte. The felt 18 has a plurality of blind holes 20 regularly distributed and extending inwardly in radial directions from the surface in contact with the current collector 14. These blind holes are filled with activated charcoal 21. Typically the holes 20 extend inwardly through about half the thickness of the felt 18.In this particular embodiment these holes 20 are 2 mm in diameter and 5 mm apart. The purpose of these holes is to enable the activated charcoal to be distributed regularly throughout the body of the carbon felt 18 but with the activated charcoal packed with particles in electrical contact with one another.

It is desirable to ensure that the activated charcoal does not form a layer between the carbon felt and the beta-alumina electrolyte since this would cause early passivation of the cell and low charge acceptance. Similarly, a uniform layer of the activated charcoal between the current collector and the carbon felt would also cause early passivation and low charge acceptance. The distribution of the activated charcoal in pockets within the body of the matrix formed by the carbon felt gives the improved charge acceptance. The pockets have to be big enough to accommodate particles which can be packed to ensure electronic conductivity through particle to particle contact.

As is seen from Figure 1 the holes 20 containing the activated charcoal are distributed along the length of the cathode electrode and, as seen from Figure 2, they are distributed evenly around the current collector 14.

Figure 3 is a diagram illustrating the effect of adding various carbon materials into a carbon felt structure formed of RVC 4000 carbon felt and with the particles packed into holes as described above.

Each of the curves in Figure 3 is a graph illustrating the relationship between cell voltage and the percentage discharge with 100% Na2S3 being regarded as fully discharged. As previously explained, on recharging, the Na2S3 initially is converted into a higher polysulphide, more particularly Na2S5. This continues until the cell reaches a condition of approximately 58% discharge. Further recharging leads to the formation of a two-phase mixture of Na2S5 and sulphur. In general, in sodium sulphur cells, it is not possible to recharge the cells to a condition corresponding to zero percentage discharge on the graph. Up to 60% of the theoretical charge capacity can be lost.

The curve marked 1,2,3 and 4 in Figure 3 shows the relationship between cell voltage and percentage discharge on recharging a cell with a constant current charge of 100 ma per square cm and with a voltage limit of 5 volts using four different packings for the holes 20.

Example 1 The bores in the graphite felt were, in this example, packed with 300 mesh activated charcoal.

This particular embodiment used charcoal supplied by Hopkin & Williams, Grade SX1, which has an active surface area of 850 sq.m per gram. As shown in curve 1 of Figure 3, the recharging continued well into the two-phase region and the cell could be recharged to a condition of about only 20% discharge.

Example 2 Curve 2 of Figure 3 illustrates the performance of a cell, of the same construction as that used in Example 1 but in which the bores in the graphite felt were packing with a material formed of equal proportions of SX1 300 mesh activated charcoal and 240 mesh graphite powder. The graphite powder in this example was type Foliac 1371 supplied by Rocol Limited. The graphite powder improved the conductivity of the packing and it is believed that the flatter curve during the recharging is due to the improved conductivity of the material in the bores.

Example 3 Curve 3 shows the performance of a test cell similar to that of Examples 1 and 2 but in which the bores in the graphite felt were packed solely with graphite powder (Foliac 1371 240 mesh). This is not an example of the invention. It wiil be noted that the charge acceptance has been very considerably de creased and there is very little recharging in the two-phase region. This graph is generally similar to that obtained with an unmodified cell in which the elctrode structure consists solely of the graphite felt.

Curve 3 shows that the charge acceptance is not a function of the conductivity of the packing but depended on the presence of the activated charcoal with the higher surface area per unit mass.

Example 4 Curve 4 illustrates the effect of using activated charcoal in larger particles. The cell tested was similar to that used for Examples 1,2 and 3 but the bores were packed with Hopkin & Williams' activated charcoal having a particle size of 0.50 to 0.21 mm. This material had a surface area per unit mass of 1200 sq. m per gram. It will be noted that curve 4 shows a marked further improvement in the charge acceptance and an even flatter part of the curve during the recharging in the two-phase region.

It is believed that, when the high surface area material constituted by the activated charcoal is introduced into the low surface area material, namely the carbon felt, in a regular zone, the charging, when in the two-phase region, is carried out preferentially at the zones. The surrounding low surface area material which would normally passivate simply acts as a belt absorbant and an electronic conductor to remove the sodium ions and electrons.

Tests have been carried out with the high surface area material used in Example 4 above at much higher current densities. It has been found that the improved performance can be obtained even with current densities as high as 400 ma. per sq. cm.

Claims

1. A sodium sulphur cell having a solid cation permeable membrane separating an anodic region containing sodium from a cathodic region containing a cathodic reactant, the cathodic region lying between a surface of the electrolyte membrane and a surface of a cathode current collector, the cathode region including a porous electronically-conductive matrix impregnated with the cathodic reactant wherein the porous matrix is formed of two electronically-conductive porous materials, one of which has a substantially higher surface area per unit mass than the other and is in a plurality of discrete channels extending from the surface of the matrix adjacent the cathode current collector through the matrix towards the electrolyte membrane and the other of which porous materials extends between said surfaces.

2. A sodium sulphur cell as claimed in claim 1 wherein the channels containing the material with the higher surface area per unit mass are distributed in regular discrete zones in the other material.

3. A sodium sulphur cell as claimed in claim 2 wherein the channels are distributed in discrete regions which, in a plane parallel to the surface of the electrolyte, are substantially uniformly distributed.

4. A sodium sulphur cell as claimed in any of the preceding claims wherein the channels extend part of the way through the matrix from the surface thereof adjacent the current collector.

5. A sodium sulphur cell as claimed in claim 4 wherein the cell is cylindrical and said channels extend in a radial direction from the surface of the matrix adjacent the current collector for part of the way through the matric towards the electrolyte tube.

6. A sodium sulphur cell as claimed in any of the preceding claims wherein the material with the higher surface area per unit mass is a micro-porous material.

7. A sodium sulphur cell as claimed in any of the preceding claims wherein the material with the higher surface area per unit mass is activated charcoal.

8. A sodium sulphur cell as claimed in any of the preceding claims wherein the material with the lower surface area per unit mass is a carbon material.

9. A sodium sulphur cell as claimed in any of the preceding claims wherein the material with the lower surface area per unit mass is a fibrous carbon material.

10. A sodium sulphur cell as claimed in claim 9 wherein the material with the lower surface area per unit mass is a carbon felt.

11. A sodium sulphur cell as claimed in any of claims 1 to 8 wherein the material with the lower surface area per unit mass is a foamed carbon material.

12. A sodium sulphur cell as claimed in any of claims 1 to 8 wherein the material with the lower surface area per unit mass is a particulate material.

13. A sodium sulphur cell as claimed in claim 12 wherein said particulate material comprises particles of graphite.

14. A sodium sulphur cell as claimed in claim 12 wherein said particulate material comprises carbon black.

15. A sodium sulphur cell having a solid cationpermeable membrane separating an anodic region containing sodium from a cathodic region containing a cathodic reactant, the cathodic region lying between a surface of the electroyte membrance and a surface of a cathode current collector wherein the cathode region includes a carbon felt matrix extending between the current collector and the electrolyte membrane and having a plurality of spaced channels extending part of the way through the matrix from the surface of the matrix adjacent the current collector towards the electrolyte, which channels are filled with activated charcoal.

16. A sodium sulphur cell as claimed in claim 15 wherein the cavities are evenly distributed, considered in a plane parallel to the surface of the electrolyte.

17. A sodium sulphur cell as claimed in any of claim 14 to 16 wherein the cavities are blind holes.

18. A sodium sulphur cell as claimed in claim 17 wherein the cell is cylindrical and wherein the blind holes extend into the felt in a radial direction.

19. A sodium sulphur cell as claimed in claim 7 or any of claims 15 to 18 wherein the activated charcoal comprises particles in the range of 0.2 to 0.5 mm.

20. A sodium sulphur cell substantially as herein-before described with reference to the accompanying drawings.