GB2193837A - Sodium-sulphur cell containing magnesium gettering agent - Google Patents

Description

GB2193837A 1 SPECIFICATION the extra magnesium required to maintain sa

turation of the magnesium. dissolved in the so Electrochemical power storage cells dium.

The proportion of excess solid getter metal THIS INVENTION relates to high temperature 70 required will be determined by routine experi secondary electrochemical power storage mentation, so that a saturated solution of cells. More particularly the invention relates to magnesium in the sodium of the anode is such cells wherein the active anode substance maintained throughout the life of the cell, or at is essentially molten sodium and is separated least for a period equal to the anticipated ser from the active cathode substance and/or 75 vice life of the cell, bearing in mind both the electrolyte by means of a beta-alumina separa- period of time which the cell is intended to be tor/solid electrolyte. The invention relates also in service, and particularly the expected num to anode sub-assemblies for such cells and to ber of charge/discharge cycles through which a method of combatting progressive internal it is expected to be put during this period.

resistance rise in such cells associated with 80 The employment of the dissolved- magne- the anode and/or anode/separator interface. sium getter metal as described above func- According to one aspect of the invention tions essentially to combat a progressive rise there is provided a high temperature secon- in internal resistance of cells of the type in dary electrochemical power storage cell which question which is associated with the anode comprises sodium as its active anode sub- 85 and/or with the anode/separator interface, and stance which is molten at the operating tem- which results from repeated cycling thereof.

perature of the cell and which is coupled elec- The invention accordingly relates principally to trochemically with an active cathode sub- the sub-assembly constituted by the anode stance and separated therefrom by a beta-alu- and separator and is indifferent to the nature mina solid electrolyte separator, the molten 90 of the active cathode substance, to the nature sodium of the anode having magnesium dis- of any liquid electrolyte employed on the side solved therein to form a saturated solution in of the separator remote from the anode, or the sodium, and to act as a getter for impuri- indeed to any cell components separated by ties in the sodium. the separator from the anode. Numerous cath- As the magnesium will function as a getter, 95 ode assemblies, which vary substantially from eg for impurities such as any oxygen, water, one another, both constructionally and electro hydronium ions or the like released during chemically, are known in the art as capable of charge/discharge cycling of the cell at the so- being coupled electrochemically with a molten dium/separator interface, magnesium dissolved sodium anode in a cell via a beta-alumina sep in the sodium can be consumed during use/ 100 arator, and the present invention is in principle cycling of the cell. The sodium of the anode capable of application to all such cells.

is thus preferably in contact with magnesium Thus, for example, the active cathode sub- in solid form. This solid excess getter metal is stance may be any suitable cathode sub provided to replace, by dissolving in the sostance, such as molten sulphur/sodium poly dium, such proportion of the magnesium dis- 105 sulphide; or it may be a transition solved in the sodium as is consumed by its metal /transition metal chloride such as Fe/ gettering the aforesaid impurities during cell FeCl, Ni/NiCl, C0/C0C12, Cr/CrC12 and/or cycling. This gettering leads to the formation Mn/MnC121 immersed in a molten salt liquid of compounds which may be insoluble, and electrolyte in contact with the separator, such which are inert from. a gettering point of view, 110 as stoichiometrically exact NaAIC14 formed so that, in the absence of solid excess getter from an equimolar mix of NaCl and AIC13 and metal, the concentration of the getter metal in contact with solid NaCl, the active cathode dissolved in the sodium can drop below satu- substance preferably being dispersed in a ma ration, which is undesirable. In this regard it is croscopically porous electronically conductive believed that the aforesaid impurities can ad- 115 matrix which acts as a current collector and versely affect the anode/separator internal re- which is permeable to the liquid electrolyte in sistance, particularly at the anode separator liquid form.

interface, possibly by reacting with the sodium When the sodium of the anode undergoes a to form reaction products which accumulate at substantial reduction in volume upon discharge said interface. 120 of the cell, as can be the case with sodium/ As the solid magnesium is intended to sulphur cells, problems can arise from exces- maintain a saturated solution thereof in the sive plating out of the magnesium from the sodium, it should have a relatively large spe- sodium. To alleviate these problems it may be cific surface, ie a relatively large surface ar- desirable to employ excess sodium in the an ea/unit mass, and is thus conveniently in par- 125 ode, eg at least 40% by mass more than is ticulate form, such as finely divided powder required for full discharge of the cell, It is also form, having eg a maximum particle size of at desirable to operate the cell within a relatively most 250 microns. narrow temperature range, so that its mini- The magnesium in such particulate form will mum and maximum operating temperatures also act as a solid getter as well as provide 130 are no more than 50'C apart, ie no more than 2 GB2193837A 2 25% from the nominal or average operating finely divided powder, to promote dissolution temperature. thereof in the sodium.

As the invention is essentially indifferent to As indicated above, it may be desirable, in the exact nature of the cathode, the invention order to resist plating out of the magnesium, accordingly extends also to an anode sub-as- 70 to load the cell with sufficient sodium so that sembly for a cell of the type described above, in its charged state it has at least 140% by for coupling electrochemically with an active mass of the sodium necessary electrochemi cathode substance to form a high temperature cally to discharge the active cathode sub secondary electrochemical power storage cell, stance of the cell.

which sub-assembly comprises a sodium ac- 75 In accordance with an important optional tive anode substance which is molten at the feature of the invention, it is desirable for the operating temperature of said cell, and a beta- magnesium to be dissolved, preferably at an alumina solid electrolyte separator in contact elevated temperature of eg 120C, or more, with the sodium for separating the sodium in up to the operating temperature of the cell, to a cell from said active cathode substance, the 80 obtain a saturated solution thereof in the so molten sodium of the anode having magne- dium, before the sodium is loaded into the cell sium dissolved therein to form a saturated so- or into an anode/separator sub-assembly for lution. use in the cell. The excess solid getter metal The more detailed construction and compo- can easily be added to the sodium of the sition of the sub-assembly may be as de- 85 anode, by loading it into the cell or sub-as scribed above with reference to the cell of the sembly together with the sodium.

present invention, and the: separator will typi- Accordingly, in other words, the method cally be in the form of a beta-alumina tube may comprise dissolving the magnesium in the which is open at one end thereof and closed sodium to form the saturated solution, before at the other. The anode may be inside the 90 the sodium is loaded into the cell; and this tube, in which case the sodium and magne- dissolving may take place at a temperature of sium are simply contained inside the tube; or at least 1200C.

the anode may be outside the tube, in which In accordance with the method, the cell may case the sub-assembly may include a cell be operated at a temperature at most 25C housing within the interior of which the tube 95 different from its average operating tempera is located, the sodium of the anode and the ture, ie the method may comprise operating magnesium being provided in the housing out- the cell within a temperature range of at least side the tube. 50C.

Naturally, both the cell and sub-assembly, if The invention will now be described, with they are cooled below their operating temperreference to the accompanying drawings, and atures, can drop to a temperature at which with reference to the following non-limiting il the sodium of the anode solidifies, and the lustrative examples:

invention extends also to the cell and sub- Figure 1 shows a schematic sectional side assembly in which the sodium is solid, eg at elevation of a cell in accordance with the pre ambient temperatures. 105 sent invention; The invention extends further to a method Figures 2 to 5 show, for various cells in of combatting progressive internal resistance accordance with the present invention, and for rise in a high temperature secondary electrocontrol cells employed for comparative pur chemical power storage cell comprising a mol- poses, plots of minimum cell internal resis ten sodium active anode substance coupled 110 tance in ohms against cell life in cycles; and electrochemically with an active cathode sub- Figure 6 shows, for a further cell in accor- stance and separated therefrom by a beta-aludance with the invention, a plot of cell voltage mina solid electrolyte separator, the method against discharge capacity for selected cell comprising dissolving magnesium in the so- cycles.

dium anode material to form a saturated solu- 115 In Fig. 1 of the drawings, reference numeral tion thereof in the sodium. 10 generally designates a high temperature The internal resistance rise in question is secondary electrochemical power storage cell associated with the anode and/or the anode/ according to the invention. The cell is shown separator interface, and arises during char- having a housing 12 divided by a beta-alumina ge/discharge cycling of the cell. 120 separator 14 into an anode compartment 16 The method may include maintaining the and a cathode compartment 18. In the anode saturated solution during cell operation/cycling compartment a saturated solution of magne by causing or allowing further magnesium to sium in molten sodium is shown at 20 as the dissolve in the sodium, as getter material is active anode substance; and in the cathode consumed in use, by gettering any impurities 125 compartment molten sulphur/sodium polysul arising in the sodium during use, eg derived phide is shown at 22 as the active cathode from the separator during cycling. The method substance. Solid powdered magnesium having may thus include also adding solid magnesium a particle size of less than 250 microns is to said saturated solution, in particulate form shown at 24 in the anode compartment; and preferably with a high specific surface, as in a 130 the anode compartment 16 and cathode com- 3 GB2193837A 3 partment 18 are respectively shown provided on their tubes were found to be prone to with current collectors/cell terminals 26, 28. early tube failures. Thus, of the cells which In certain tests conducted by the Applicant, had lead acetate coatings on their tubes and sodium/sulphur cells were made and tested. In magnesium powder in their anode compart each case the separators were beta-alumina 70 ments, tube failures occurred respectively after tubes open at one end and closed at the 7 cycles, 15 cycles (two cells), 22 cycles and other, sulphur was loaded into the interiors of 71 cycles, one cell remaining after 111 cycles, the tubes and the tubes were located concen- at which stage its cycling was discontinued.

trically in steel casings or housings, with so- Of the cells having a lead acetate coating and dium loaded into the housings in the spaces 75 aluminium flake in their anode compartments, between the housings and the tubes. The tube failures occurred respectively after 40 tubes and housings were suitably sealed and cycles, 76 cycles and 134 cycles, three cells the sulphur cathodes and sodium anodes were remaining after 292 cycles, at which stage provided with suitable current collectors lead- their cycling was discontinued.

ing to cell terminals. 80 Of the cells with no lead acetate coatings Beta-alumina tubes manufactured from alpha- on their tubes, no early tube failures were en- alumina were used, and the anodes, when countered. Of those which had no treatment charged, comprised an amount of sodium whatsoever to counteract anode/separator re which was at least 140% of that required to sistance rise, one tube failed after 457 cycles, discharge the cathodes. 85 and four cells had their cycling discontinued In certain cases, the sodium of the anodes after 389 cycles. The cells in accordance with was treated in accordance with the invention, the invention, whose treatment to resist inter and in other cases (controls) there was no nal resistance rise was confined to addition of such treatment or the beta-alumina tubes were magnesium to the anode compartment were treated at the tube/sodium interface in known 90 all functioning, and had their cycling discontin fashion to combat resistance rise. All these ued, after 259 cycles.

cells were operated at a nominal temperature Fig. 2 shows a plot of the average minimum of 360'C, the temperature being kept in the internal resistance respectively for the group range 335-395'C. These tests are described of cells which had no treatment (no magne in Examples 1-4 hereunder. 95 sium or lead acetate tube coating to resist the i crease in internal resistance) and the group EXAMPLE 1 of cells whose treatment was confined to 22 beta-alumina tubes were manufactured magnesium in the anode compartment (no using Reynolds RC-HPS-DBM alpha-alumina as lead acetate tube coating), against cell cycles.

the starting material, supplied by Reynolds 100 From Fig. 2 it is apparent that the cells in Chemicals, Malakoff, Texas, U.S.A. The tubes accordance with the invention, for a cycle life were 160 mm long and had a 33 mm external of up to 260 cycles, had a broadly constant diameter with a wall thickness of 1,7 mm. internal resistance, between about 18 and The tubes were assembled into sodium/sul- about 22 mOhms. The untreated cells, in con phu. r cells having sulphur electrodes of 90 mm 105 trast, showed an internal resistance which in length and had capacities of 38 Ah. The sulcreased progressively with the number of phur was located inside the tubes and the cycles, and after 320 cycles had an internal sodium outside the tubes. 100 g of sodium resistance of 24,5 mOhms, after reaching a was loaded into eac h cell which was 260% of maximum internal resistance of about 25.4 that required to discharge the cathode. 110 mOhms after 288 cycles.

Five of these cells were assembled without This Example demonstrates that adding the any treatment to resist anode/separator inter- magnesium had no adverse effect on the life nal resistance rise; five of the cells were asof the beta-alumina tubes, and appeared to sembled with 1,5g of magnesium powder hav- counteract the progressive internal resistance ing a particle size of less than 250 microns 115 rise of the cells, which is associated with the added to the anode compartments thereof; six anode and separator. In this regard it is to be cells were assembled with 1,5g of said magnoted that Reynolds HPS alpha- alumina is a nesium added to their anode compartments, relatively pure starting material for the manu and, in addition, theanode sides of their facture of the beta-alumina tubes.

tubes were coated with a layer of lead ace- 120 tate; and the final six cells were assembled EXAMPLE 2 with the anode sides of their tubes coated Ten further cells were constructed and with lead acetate and flake aluminium was loaded in the same fashion as described added to the molten sodium anodes thereof. above for Example 1. Five of these cells were All the cells were put through successive 125 assembled with 1,5g of the aforesaid magne- charge/discharge cycles, at a cycle rate of sium powder in their sodium anodes, and the about 8 cycles a day, with a current density remaining five were assembled without such of 50 mA/CM2 on charge and 250 mA/CM2 getter magnesium. In this case the cells were on discharge. cycled with a 300 mA/CM2 charging current Those cells which had lead acetate coatings 130 density and a 500 mA/CM2 discharge current 4 GB2193837A 4 density, at 16 cycles a day. mOhms against cell cycles, for the two groups Cell tube failures occurred in the cells having of cells, ie the group which had magnesium no getter respectively after 300 cycles (two powder added on the one hand and the group cells), 423 cycles (two cells) and 452 cycles. which had magnesium powder added and Of those cells whose anodes were treated 70 magnesium pre-dissolved in the sodium, on with magnesium, three cell tubes failed re- the other. In both cases cell internal resistance spectively after 2810 cycles, 2907 cycles and remained broadly constant over large numbers 2923 cycles. Cycling of one cell was discon- of cycles, being between about 7,5 mOhms tinued because of a seal leak (unassociated and about 9,8 mOhms, the cells with pre- with tube failure) after 2165 cycles, and the 75 dissolved magnesium having a lower internal remaining cell had its charge/discharge cycling resistance.

discontinued, while functioning and intact, The cells with both predissolved magne- after 3109 cycles. sium and magnesium powder had a substan- The results of Example 2 clearly demon- tially lower average internal resistance, and, strate that the magnesium indeed has a beindeed, a more stable internal resistance.

neficial effect on tube life in the cells in ques tion, and at the current densities used. EXAMPLE 4 Fig. 3 shows a plot, similar to Fig. 2, for 12 cells were manufactured as described the two groups of cells of the present above for Example 1, except that the alpha Example, of average minimum cell internal re- 85 alumina used for the tubes was different, and sistance in mOhms plotted against cell cycles. was Alcoa A16 SG alphaalumina, available The cells treated in accordance with the in- from Alcoa (Great Britain) Limited, Droitwich, vention displayed a slow progressive internal Great Britain. This alpha- alumina had higher resistance rise over their lifetimes, and the un- CaO and Si02 contents respectively than the treated cells, at least before failure thereof, 90 Reynolds HPS alphaalumina used for demonstrated a substantially higher internal re- Examples 1-3.

sistance. Five cells were assembled with 1,5g of the aforesaid magnesium powder added to the an EXAMPLE 3 ode compartments, and the sodium of the an- Four further cells were manufactured broadly 95 odes had previously had magnesium dissolved in accordance with the procedure described therein up to saturation at 120'C.

for Example 1, except that the beta-alumina Each cell was loaded with 100 g of sodium tubes had a length of 300 mm instead of 160 which was 260% of that required to discharge mm, and the sulphur electrodes had lengths the cathode.

and capacities of 220 mm and 88 Ah respec- 100 Two cells were assembled with no getter tively, instead of 90 mm and 38 Ah respec- magnesium whatsoever, and the remaining five tively. cells were assembled with 1,5g of said mag- Two of these cells were assembled with 2g nesium getter added in powder form to the of the aforesaid magnesium powder in the an- anode compartments, and had their tubes ode compartments, and the other two cells 105 coated with lead acetate on the anode sides were assembled with the aforesaid 2g of of their tubes.

magnesium in the anode compartment and, in These cells were subjected to charge/dis- addition, the sodium loaded into the cell had charge cycling at a charging current of 300 previously had magnesium dissolved therein mA/CM2 and a discharge current of 500 up to saturation at a temperature of 120'C. 110 mA/CM2, for a cycle rate of 16 cycles a day.

g of sodium was loaded into each cell Of those cells which had lead acetate coat- which was at least 170% of that required to ings on their tubes, three cell tubes failed discharge the cathode. - after about 125 cycles, one cell tube failed These cells were cycled at about 8 cycles a after about 300 cycles, and one cell failed day, with a 150 mA/CM2 charge rate and a 115 immediately for reasons unrelated to tube fail 250 mA/CM2 discharge rate. ure, in that its connections short circuited.

Of the two cells which only had magnesium Of the cells which had no getter magnesium powder added to their anode compartments, added, both had tube failures after about one cell failed after 1784 cycles, and cycling 70-100 cycles.

of the other cell was discontinued because of 120 Of the remaining five cells, ie those which a seal leak (unrelated to tube failure) after had both magnesium getter powder in their 2374 cycles. anode compartments and pre-dissolved mag- Of the two cells which contained both mag- nesium in the sodium of their anodes, one cell nesium powder and magnesium pre-dissolved tube failed after about 1050 cycles, and the in the sodium, one cell failed after 2206 125 remaining four cells had their cycling discontin cycles, and the other cell had its cycling dis- ued, intact and without failure, after about continued, while functioning and intact, after 1425 cycles.

2635 cycles. Once again these results demonstrate that, Fig. 4 shows a plot, similar to Figs. 2 and at the relatively high current densities em3, of average cell minimum resistance in 130 ployed for charging and discharging, the lead GB2193837A 5 acetate coating has a deleterious effect on the discharge capacity in Ampere hours for the life of the beta-alumina tubes in the sodium/ cell cycles in question.

sulphur cell environment, and indeed those From Fig. 6 it is apparent that no significant cells with no treatment whatsoever showed increase in internal resistance of the cells took equally bad tube lives. 70 place, between the 10th and 575th cycles, Fig. 5 is a plot, similar to Figs. 2 to 4, of and no early cell failure took place.

average minimum internal cell resistance, for This example demonstrates that the use of the batch of five cells having magnesium pow- magnesium inhibits resistance rise at the sodi der in their anode compartments and predis- um/beta-alumina interface and also that there solved magnesium in the sodium of their an- 75 is no detrimental effect on the life of the beta odes, and Fig. 5 shows that, at least for the alumina at a lower operating temperature. In first 1000 cycles, there is no progressive rise this case if lead acetate had been used to in internal resistance, the internal resistance of coat the beta-alumina, then the life of the the cells, when charge/discharge cycling was beta-alumina would typically have been less discontinued, being only marginally higher than 80 than 300 cycles; and, as in Example 2, if no the internal resistance at the beginning of cycl- magnesium had been used in the sodium elec ing. trode, then resistance would typically have risen by 50% after 300 cycles. In example 5 EXAMPLE 5 there is however no significant increase in re- Four cells were assembled in a broadly simi- 85 sistance after 575 cycles.

lar fashion to Examples 1-4, but with a differ- In general, the Examples show that the use ent cathode material, namely Ni/NiC12. This of magnesium in the sodium anodes had no Ni/NiCl, was provided by a cathode structure adverse affect on the lives of the beta-alumina comprising an electronically conductive electro- tubes used, and indeed, on the contrary, tube lyte-permeable porous nickel cathode matrix in 90 life appears to be increased by the use of which, in the fully charged state of the cell, predissolved magnesium in the sodium of the N'Cl2 was dispersed together with solid NaCl. anodes. In this regard it is to be noted that The matrix was also saturated with stoichiom- general reagent grade sodium was used. Fur etrically exact NaAICI, ie a molten salt elec- thermore, it is advantageous to pre-dissolve trolyte comprising NaCl and AIC13 in a 50:50 95 magnesium in the sodium of the anodes, mole ratio. Such cathodes, upon discharge of rather than to rely merely upon addition of the cells, react by having the N'C12 converted magnesium powder thereto, thereby to ensure to Ni, with the production of NaCl. that a saturated solution of magnesium in the The cells had their cathodes located in the sodium is present from the beginning.

interiors of their beta-alumina tubes, which 100 It is further noted that the cells were al- tubes were made from raw material of similar lowed to soak at the cell operating tempera impurity levels to the Alcoa alpha-alumina re- ture for about 12 hours before charge/dis- ferred to in Example 4 and had dimensions of charge cycling was started, and this feature mrn length, 33 mm external diameter and may contribute to extended cell life, by allow 1,6 mm wall thickness. The sodium anodes 105 ing the magnesium to perform a gettering were located outside the tubes. function before cell cycling starts, In each case, the sodium anodes were

Claims (1)

1. doped with 1,5 g magnesium powder of the CLAIMS

   I type used in Examples 1-4, and, prior to cell 1. A high temperature secondary electro- assembly, the sodium had magnesium dis- 110 chemical power storage cell which comprises solved therein up to its solubility limit, ie satu- sodium as its active anode substance which is ration, at 150'C. These cells were operated at molten at the operating temperature of the cell a nominal temperature of 270'C, ie and is coupled electrochemically with an active 245-295'C, and in the charged state concathode substance and separated therefrom tained about 1009 sodium in their anodes. 115 by a beta-alumina solid electrolyte separator, The amount of sodium in the fully charged the molten sodium of the anode having mag- state was 240% of that required to discharge nesium dissolved therein to form a saturated the cathode. solution in the sodium and to act as a getter The cells were then operated at a rate of for impurities in the sodium.

   16 cycles a day, with a charging current den- 120 2. A cell as claimed in Claim 1, in which sity of 300mA/CM2 and a discharge current the sodium of the anode is in contact with density of 400 mA/CM2. All the cells survived magnesium in solid form.

   575 charge/discharge cycles and, for compar- 3. A cell as claimed in Claim 2, in which ative purposes, cell internal resistance was the solid magnesium is in finely divided pow measured at a low current density, ie about 125 der form having a maximum particle size of at mA/CM2 (1,5 A) charge current and about most 250 microns.

   mA/CM2 (5 A) discharge current during the 4. A cell as claimed in any one of the 10th and 575th charge/discharge cycles. preceding claims, in which the amount of so These measurements are illustrated in Fig. 6, dium in the cathode in its fully charged state which is a plot of cell voltage in volts against 130 comprises at least 140% by mass of the so- 6 GB2193837A 6 dium required to discharge the cathode.

   5. An anode sub-assembly for coupling Published 1988 at The PatentOffice, State House, 66/71 High Holborn, London WCI R 4TP. Further copies may be. obtained from electrochemically with an active cathode sub- The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD.

   stance to form a high temperature secondary Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.

   electrochemical power storage cell, which sub assembly comprises a sodium active anode substance which is molten at the operating temperature of said cell, and a beta-alumina solid electrolyte separator in contact with the sodium for separating the sodium in a cell from said active cathode substance, the mol ten sodium of the anode having magnesium dissolved therein to form a saturated solution.

   6. A sub-assembly as claimed in Claim 5, z 14 in which the sodium is in contact with magne sium in finely divided powder form having a maximum particle size of at most 250 mi crons.

   7. A method of combatting progressive in- ternal resistance rise in a high temperature secondary electrochemical power storage cell comprising a molten sodium active anode sub stance coupled electrochemically with an ac tive cathode substance and separated there from by a beta-alumina electrolyte separator, the method comprising dissolving magnesium in the sodium anode material to form a satu rated solution thereof in the sodium.

   8. A method as claimed in Claim 7, which includes the step of adding solid magnesium to said saturated solution.

   9. A method as claimed in Claim 8, in which the solid magnesium is in finely divided powder form having a maximum particle size of at most 250 microns.

   10. A method as claimed in any one of Claims 7 to 9 inclusive, which comprises load ing the cell with sufficient sodium so that in its charged state it has at least 140% by mass of the sodium necessary electrochemi cally to discharge the active cathode sub stance of the cell.

   11. A method as claimed in any one of Claims 7 to 10 inclusive, which comprises dis solving the magnesium in the sodium to form the saturated solution, before the sodium is loaded into the cell.

   12. A method as claimed in Claim 12, in which the magnesium is dissolved in the so dium at a temperature of at least 120T to form the saturated solution.

   13. A method as claimed in any one of Claims 7 to 12 inclusive, which comprises op erating the cell within a temperature range of at most 50'C.

   14. A cell as claimed in claim 1, substan- tially as described and as illustrated herein.

   15. A sub-assembly as claimed in claim 5, substantially as described and as illustrated herein.

   16. A method as claimed in claim 7, sub- stantially as described herein.