DE3727059A1 - ELECTROCHEMICAL CELL - Google Patents

Abstract

The invention provides a secondary high temperature electrochemical power storage cell (10) which has a molten sodium anode (20) in contact with a beta- alumina cell separator (14). The sodium anode has magnesium dissolved therein to form a saturated solution for gettering impurities in the sodium. The invention also extends to an anode sub-assembly for such cell which comprises said sodium and said separator, the sodium comprising said magnesium getter dissolved therein to form a saturated solution; and the invention further provides a method of combatting progressive internal resistance rise in said call by dissolving magnesium in the sodium to form a saturated solution. <IMAGE>

Description

The invention relates to high-temperature electrochemicals memory cells. It particularly affects cells whose active anode substance essentially molten sodium which is of the active cathode substance and / or the Electrolytes through a beta alumina separator Solid electrolyte is separated. The invention relates further anode subassemblies for such cells and a Process, the increasing increase in internal resistance to counteract such cells with the anode and / or the anode / separator interface is connected.

According to one aspect of the invention, an electrochemical High-temperature secondary energy storage cell provided, the sodium as active anode substance, which in the Operating temperature of the cell has melted, contains and which is electrochemical with an active cathode substance connected and of it by a beta alumina Solid electrolyte separator is separated, the melted Sodium anode magnesium to form a saturated Solution contains to act as a getter for impurities in the solution To act sodium.

Since the magnesium acts as a getter, for example for Impurities such as oxygen in any form, water, Hydronium ions or the like, which during the Cell loading / unloading cycles on the sodium / separator the sodium can dissolve the interface Magnesium consumed while using the cell will. The sodium of the anode is thus preferably in Contact with solid magnesium. The excess solids of the Gettermetals provides substitute for by dissolving in sodium the proportion of the magnesium dissolved in sodium, which Getters of the above-mentioned contaminants during the Cell cycles was consumed. This gettering leads to Formation of compounds that can be insoluble and that  are inert with respect to gettering, so that in the absence of excess getter metal the concentration of the im Sodium getter metal undesirably below the Saturation limit can decrease. It is assumed that that the impurities mentioned the anode / separator can affect unfavorably, especially at the anode / separator interface, and possibly through Reaction with the sodium to form reaction products, that accumulate at the interface.

Since the solid magnesium is designed to be a saturated one It should maintain magnesium solution in sodium have a relatively large specific surface, i.e. a relatively large surface area per unit mass, and thus usually in particulate form, such as finely divided Powder form with, for example, a maximum particle size of up to 250 µm.

The particulate magnesium also acts as a getter, because it provides the additional magnesium that is needed for Maintaining the saturation of dissolved magnesium in the Sodium is necessary.

The proportion of necessary excess getter metal is determined by routine experimentation, so that the Saturation solution of magnesium in the sodium of the anode during the life of the cell is maintained or at least for a period of time that of the assumed Operating time (service life) of the cell corresponds to the Time in which the cell should be in operation and especially the expected number of loading and Discharge cycles to which the cell is exposed at this time intends to suspend.  

The application of the dissolved magnesium getter metal in the The manner described above essentially acts on one increasing increase in the internal resistance of the in question conflicting cell type, which with the anode and / or the anode / separator interface and which results from the repeated cycles. Accordingly concerns the invention mainly the subassembly resulting from the Anode and the separator exists, and is independent of the Kind of active cathode substance, any used liquid electrolyte on the side facing away from the anode of the separator, or indeed any of the Cell components through the separator from the anode are separated. Numerous cathode arrangements that from each other essentially both in structure and electrochemically deviate, are effective for the person skilled in the art known to electrochemically in with a sodium smelting anode a cell through a beta alumina separator to be connected. The present invention is basically applicable to each of these cells.

For this anode subassembly for electrochemical Connection with an active cathode substance for formation an electrochemical high temperature secondary energy element protection is claimed because the Content of dissolved in the molten sodium of the anode Magnesium increases in many cathode arrays Internal resistance of the cell significantly inhibits. It is therefore a preferred means of performing the method for Inhibition of the increase in the internal resistance of electrochemical secondary high temperature energy storage cher cells and thus a preferred component of such Cells themselves.

Thus, the active cathode substance can be any suitable cathode substance, such as molten sulfur / sodium polysulfide, or it can be a transition metal / transition metal chloride such as Fe / FeCl ₂ , Ni / NiCl ₂ , Co / CoCl ₂ , Cr / CrCl ₂ and / or Mn / MnCl ₂ , immersed in a liquid molten salt electrolyte in contact with a separator, such as, for example, stoichiometrically accurate NaAlCl ₄ , which was formed from an equimolar mixture of NaCl and AlCl ₃ and is in contact with solid NaCl, the active cathode substance preferably being in one Macroscopically porous electrically conductive matrix is distributed, which serves as a current conductor and is permeable to the liquid electrolyte in liquid form.

The anode sodium experiences a substantial amount Reduction in volume after discharge of the cell, such as it is possible in the case of sodium / sulfur cells problems from excessive deposition of magnesium resulting from the sodium. To reduce these problems, it may be desirable to have excess in the anode Apply sodium, for example at least 40 wt .-% more than is necessary to completely discharge the cell. It is also desirable to keep the cell within a operate relatively low temperature range, so that Minimum and maximum operating temperature not more than 50 ° C apart, i.e. not more than 25 ° C from the mean or nominal operating temperature.

Since the invention is essentially of a particular Is independent of the electrode type, the invention extends accordingly also to an anode subassembly of a cell of the type described above for electrochemical connection with an active cathode substance to form a high temperature electrochemical secondary energy storage cell, its sub-arrangement as an active anode substance Sodium, which is at the operating temperature of this cell is melted, and a beta alumina solid electrode  trolyte separator in contact with sodium to separate the Sodium in a cell from the active cathode substance, contains, the molten sodium of the anode magnesium dissolved to form a saturated solution.

The more precise structure and composition of the subordinate system can be done as above, taking into account the cell described according to the invention, look, and Separator is typically in the form of a beta aluminum oxide tube, which is open at one of its ends and is locked on the other. The anode can be in the Inside this tube, then sodium and Magnesium is simply contained inside the tube, or the anode is on the outside of the tube, with the Sub-assembly in this case comprises a cell in the Inside the tube is placed, and the sodium of the anode and the magnesium in the housing outside of this tube are provided.

Both the cell and the subassembly can if they are cooled below their operating temperatures to one Temperature drop at which the sodium of the anode solidified. The invention extends to such Cells and subassemblies in which the sodium is solid for example at room temperature.

The invention further relates to a method, the increasing increase in internal resistance Counteracting high-temperature secondary energy storage cell ken, the molten sodium as active anode substance, electrochemically linked to an active cathode substance contains and of it by a beta alumina fixed elec trolyte separator is separated, this method by Magnesium dissolved in the sodium anode material for formation a saturated solution thereof in sodium.  This increase in internal resistance depends on the anode and / or the anode / separator interface and occurs during the cell's charge and discharge cycles.

The procedure can be the maintenance of saturated Include solution during cell cyclic operation, by allowing more magnesium to Sodium dissolves and as a getter material during operation is consumed, leaving all the impurities in the sodium occur during use, for example by the Separator during the cyclization. The The process can also include the addition of solid magnesium to this saturated solution in particulate form, preferably with a high specific surface, like in a fine one crushed powder to dissolve it in sodium favor, include.

To counteract magnesium deposition, the Cell are filled with sufficient sodium so that they are in loaded state at least 140 wt .-% of the discharge the active cathode substance of the cell electrochemically contains necessary sodium.

In line with an essential point of view In this invention, it is desirable that the magnesium preferably at an elevated temperature of for example 120 ° C or more up to the operating temperature the cell is dissolved to a saturated solution in sodium to get before the sodium into the cell or into the Anode / separator subassembly for cell use is filled. The excess, solid getter metal can to the sodium of the anode easily by adding it along with the sodium into the cell or the Subassembly is filled.  

The process thus involves dissolving the magnesium in the Sodium to form a saturated solution before that Sodium is filled into the cell, and the dissolving process can take place at a temperature of at least 120 ° C.

As a result, the cell can operate at a temperature be at most 25 ° C from the average Operating temperature deviates, i.e. the process also includes operation of the cell within a temperature range of at least 50 ° C.

The invention is hereinafter taking into account the Drawings and the examples explained in more detail:

Fig. 1 shows a schematic longitudinal section of a cell according to the invention,

FIGS. 2 to 5 show various inventive cells and control cells were used for comparison purposes, the curve of the minimum internal resistance of the cell stands in ohms against cell life in cycles, and

Fig. 6 shows the course of the cell voltage versus the discharge capacity of selected cell cycles for a further inventive cell.

In Fig. 1 of the drawing, reference numeral 10 relates in each case to an electrochemical high-temperature secondary energy storage cell. The cell with the covering 12 is divided into an anode space 16 and a cathode space 18 by a beta-aluminum oxide separator 14 . In the anode compartment there is a saturated solution of the magnesium in the molten sodium 20 , the active anode substance, and in the cathode compartment molten sulfur / sodium polysulfide 22 as the active cathode substance. Solid powdered magnesium with a particle size of less than 250 µm is shown in the anode space under 24 . The anode space 16 and the cathode space 18 are each shown with the current conductors (terminals) ( 26 , 28 ).

In certain experiments, sodium / sulfur cells were used manufactured and tested. In any case, they passed Separators are open at one end and at the other closed beta alumina tubes. Sulfur was in filled the inside of the tubes and concentrically the tubes arranged in the steel case, with sodium in the rooms inserted between the housing envelopes and the tubes has been. The tubes and housings were appropriately designed sealed and the sulfur cathodes and the Sodium anodes with suitable lead to the cell clamps Current collectors equipped.

There were beta alumina tubes made of alpha aluminum oxide were used. The anodes contained when loading 140% of the amount of sodium required for unloading the cathode is necessary.

In some cases the sodium of the anodes was invented treated appropriately and in other cases (controls) found one such treatment does not take place or the beta alumina tubes were known at the tubes / sodium interface Treated way to increase the resistance counteract. All of these cells were in one Operating temperature of 360 ° C, and the temperature was kept in the range of 335 to 395 ° C. These tests are in Examples 1 to 4 described below.

example 1

22 beta alumina tubes were made using Reynolds RC-HPS-DBM alpha alumina (Reynolds Chemicals, Malakoff, Texas, USA) as a starting material. The  Tubes were 160 mm long and had an outer one Diameter of 33 mm with a wall thickness of 1.7 mm. The Tubes were made in sodium / sulfur cells with sulfur electrons Treads of 90 mm length and capacities of 38 Ah installed. The sulfur was on the inside of the tubes and that Sodium attached to the outside of the tubes. 100 g Sodium was filled into each cell, which was 260% of that used Discharge of the cathode corresponded to the necessary amount.

Five of these cells were assembled without one Treatment, the rise of the anode / separator internal resistance to counteract this. Five of these cells were with 1.5 g of magnesium powder with a particle size of less prepared as 250 µm in the anode room. Six cells were with 1.5 g of this added to the anode compartments Magnesium, and in addition the anode sides of their tubes covered with lead acetate. The last six Cells were made with lead acetate on the anode side coated tubes, with aluminum chips to the molten sodium anodes were added.

All cells were subjected to successive charging and discharging cycles at a cycle speed of 8 cycles per day with a current density of 50 mA / cm ² during loading and 250 mA / cm ² during discharging.

The cells with lead acetate coatings on their tubes showed Susceptibility to early failure. So the cells failed the lead acetate coatings on their tubes and magnesium powder in their anode rooms, each after 7 cycles, 15 Cycles (two cells), 22 cycles and 71 cycles: one cell failed after 111 cycles, the cycle being interrupted here has been. Of the cells that have a lead acetate coating and Aluminum chips in their anode rooms occurred  Fail after 40 cycles, 76 cycles and 134 cycles, respectively on. Three cells worked 292 cycles, here the Operation was interrupted.

On the cells with no lead acetate coatings on their tubes no early failure discovered. Of those who have none Treatment of an anode / separator resistance increase to counteract this, a tube failed after 457 Cycles and with four cells, their operation became after 389 cycles canceled. The cells according to the invention, their treatment to reduce the increase in resistance to adding Magnesium was limited to the anode space without running Impairment and its operation was completed after 259 cycles canceled.

Fig. 2 shows the course of the mean minimum internal resistance for the cells that had not undergone any treatment (neither magnesium nor tubes coated with lead acetate in order to counteract the increase in internal resistance) and the cells whose treatment was based on magnesium in the anode. Limited space (no lead acetate coating on the tube) against the cell cycles. From Fig. 2 it is clear that the cells according to the invention have a largely constant internal resistance between about 18 and about 22 mOhm from one cycle to 260 cycles. In contrast, the untreated cells showed an internal resistance which increased progressively with the number of cycles and reached 24.5 mOhm after 320 cycles, after a maximum internal resistance of about 25.4 mOhm was obtained after 288 cycles.

This example shows that adding magnesium no adverse effect on the life of the Has beta alumina tubes and an increasing increase the internal resistance associated with the anode and the separator  seems to counteract. In this regard it should be noted that Reynolds HPS is alpha aluminum oxide a relatively pure starting material for the production of Is beta alumina tubes.

Example 2

Ten cells were made and filled in the same manner as described for Example 1 above. Five of these cells were provided with 1.5 g of the aforementioned magnesium powder in their sodium anodes and the remaining five were mounted without getter magnesium. In this case, the cells were operated with a charge current density of 300 mA / cm ² and a discharge current density of 500 mA / cm ² with 16 cycles per day.

A failure of the cell tubes in the cells without getter took place after 300 cycles (two cells), 423 cycles (two cells) and after 452 cycles. Of the cells whose Anodes treated with magnesium failed three Cell tubes after 2810 cycles, 2907 cycles and 2923, respectively Cycles. The operation of a cell was stopped after 2165 cycles interrupted because it had a seal leak, but that was not due to tube damage. The The remaining cells were loaded / unloaded interrupted after 3109 cycles, taking until then had worked flawlessly.

The results of Example 2 clearly show that Magnesium actually has a beneficial effect on that Tube life of the cells in question in the current densities used.

Fig. 3 shows, similarly as Fig. 2 shows the course of the mean minimum cell resistance in milliohms versus the cell cycles for two groups of cells from the present example. The cells treated according to the invention show a slowly increasing increase in internal resistance during their service life, whereas the untreated cells, at least before their failure, have a significantly higher internal resistance.

Example 3

Four other cells were largely matched produced with the method of Example 1, but with except that the beta alumina tubes are one length of 300 mm instead of 160 mm and that the Sulfur electrodes lengths of 220 mm instead of 90 mm and Had capacities of 88 Ah instead of 38 Ah.

Two of these cells were treated with 2 g of the above Magnesium powder created in the anode rooms and the two other cells were also in with 2 g of magnesium the anode spaces and additionally contained that Sodium with which the cell was loaded before dissolved magnesium until saturation at one temperature of 120 ° C.

150 g of sodium was filled into each cell, at least 170% of the amount necessary to discharge the cathode corresponded.

These cells were cycled at about 8 cycles per day at a charge current of 150 mA / cm ² and a discharge current of 250 mA / cm ² .

Of the two cells that only have magnesium powder in their Anode rooms contained one cell failed after 1784 Cycles, and the operation of the other cell was stopped because of one  Permeability in the seal (no tube shortage) 2374 cycles interrupted.

Of the two cells, which are both magnesium powder as well contained magnesium pre-dissolved in sodium, one failed Cell after 2206 cycles, and the operation of the other cell was interrupted after 2635 cycles, going up there worked flawlessly.

FIG. 4 shows, similar to Fig. 2 and 3, the profile of the mean minimum cell resistance of two groups of cells from the cell cycles, ie on the one hand, the cells with magnesium powder and on the other hand, the cells that were previously in sodium contain dissolved magnesium. In both cases, the internal resistance remained largely constant between about 7.5 mOhm and about 9.8 mOhm over a large number of cycles, the cells with pre-dissolved magnesium having a lower internal resistance.

The cells that have both pre-dissolved magnesium as well Contained magnesium powder essentially had one lower average internal resistance and actually one more stable internal resistance.

Example 4

Twelve cells were made as described in Example 1, except that a different alpha alumina was used for the tubes, Alcoa A16 SG alpha alumina, available from Alcoa (UK) Ltd., Droitwich, UK. This alpha alumina each had higher CaO and SiO ₂ contents than the Reynolds HPS alpha alumina, which was used in Examples 1 to 3.

Five cells were mixed with 1.5 g of magnesium powder Made anode spaces, and the sodium of the anodes contained previously dissolved magnesium until saturated at 120 ° C.

Each cell was loaded with 100 g sodium, which was 260% of the corresponds to the amount necessary for discharging the cathode.

Two cells were produced without getter magnesium and the remaining five cells were treated with 1.5 g of this Getter magnesium, which is powdered in the anode spaces was put in, assembled using their tubes with Lead acetate were coated on the anode side.

The cells were subjected to charge / discharge cycles at a charge current of 300 mA / cm ² and a discharge current of 500 mA / cm ² at 16 cycles per day.

From the cells that coated lead acetate on their tubes contained, three cell tubes failed after about 125 cycles, one cell tube after about 300 cycles and one cell failed immediately, but not because of tube failure, but because of a short circuit in their connections.

The cells without getter magnesium both failed after about 70 up to 100 cycles.

Of the remaining five cells, i.e. the cells that both magnesium and getter powder in their anode spaces and contained magnesium pre-dissolved in the sodium of their anodes, a cell tube failed after about 150 cycles, and the Operation of the remaining 4 cells was completed after approximately 1425 cycles interrupted without a failure.  

These results again show that lead acetate coatings relatively high current densities for loading and unloading adverse effect on the life of beta aluminum effect oxide tubes in a sodium / sulfur cell, whereby however, the cells that have no treatment whatsoever have equally short lifetimes.

The Fig. 5 is a graphical representation, similar to Fig. 2 to 4, the mean minimum cell internal resistance stands for the group of five cells, the magnesium powders in their anode spaces and pre-dissolved in sodium their anodes magnesium. Figure 5 shows that there is no increasing increase in cell internal resistance at least during the first 1000 cycles. When the charge / discharge cycles were interrupted, the internal resistance of the cell was only slightly higher than when it started operating.

Example 5

Four cells were produced in largely the same way as in Examples 1 to 4, but with a different cathode material, namely Ni / NiCl ₂ . Ni / NiCl ₂ was present in a cathode, which consisted of an electrically conductive, electrolyte-permeable, porous nickel cathode matrix in which NiCl _{2 was distributed} together with solid NaCl ₂ when the cell was fully loaded. The matrix was also saturated with stoichiometrically exact NaAlCl ₄ , ie a molten salt electrolyte from NaCl and AlCl ₃ in a 50:50 molar ratio. When the cells are discharged, such cathodes react with the conversion of NiCl ₂ to Ni to form NaCl.

The cells had inside their beta aluminum oxide tubes arranged cathodes, the tubes from one the quality of the Alcoa alpha aluminum oxide from Example 4  corresponding raw material (similar levels of Verunreini gung) and a length of 160 mm, an outer diameter of 33 mm and a wall thickness of 1.6 mm. The anode sodium was on the outside of the tubes arranged.

In each case the sodium anodes were 1.5 g Magnesium powder of that used in Examples 1 to 4 Type endowed. Before the cell was assembled, it contained sodium Magnesium already dissolved up to the solubility limit, i.e. until saturation at 150 ° C. These cells were Operating temperature of 270 ° C, i.e. 245 to 295 ° C and contained about 100 g of sodium in their loaded state Anodes. The amount of sodium in the fully loaded state was 240% the amount necessary to discharge the cathode.

The cells were then operated at 16 cycles per day at a charge current density of 300 mA / cm ² and a discharge current density of 400 mA / cm ² . All cells survived 575 charge / discharge cycles and for comparison purposes the cell internal resistance was measured at a low current density, ie at about 15 mA / cm ² (1.5 A) charge current and at about 50 mA / cm ² (5 A) discharge current during the 10th and 575th charge / discharge cycle. These measurements, which show the course of the cell voltage in volts versus the discharge capacity in Ah for the said cell cycles, are shown in FIG. 6.

It is clear from Figure 6 that there is no significant increase in cell internal resistance between the 10th and 575th cycles and no early cell failure.

This example further shows that the use of Magnesium the increase in internal resistance at the Prevents sodium / beta alumina interface and also,  that no adverse effect on the life of the Beta alumina at a lower operating temperature is present. In this case would be lead acetate as a coating for the Beta alumina would have been used for life of the beta alumina typically less than 300 Cycles and, as in Example 2, would not be magnesium would have been used in the sodium of the electrode Resistance typically up to 50% after 300 cycles increased. In example 5, however, is not significant Resistance rise seen after 575 cycles.

So the examples show that the use of magnesium in sodium anodes no adverse effect on the Has lifetime of the beta alumina tubes used and on the contrary, the tube life through use of magnesium pre-dissolved in the sodium of the anodes increases too seems to be. It is also more advantageous to use magnesium Solve the sodium of the anode before this into the Cell is filled when the mere addition of the Leaving magnesium powder to thereby ensure that from the start of operation a saturated one Magnesium solution in sodium is present.

Before loading / unloading cycles were started the cell at operating temperature for about 12 hours kept (equilibrated). This measure can lead to a by prolonging the life of the cell Magnesium already enables the getter effect To commence operation of the cell.

Accordingly, preferred embodiments of the invention are Process, the increasing increase in internal resistance a high temperature electrochemical secondary energy supply counteract the cell as an active anode substance molten sodium, which is electrochemically treated with a  active cathode substance is coupled, and by it a beta alumina electrolyte separator is separated, by forming magnesium in the sodium anode material a, preferably saturated, solution thereof in sodium solves as well as a means of performing this procedure an anode subassembly for electrochemical coupling with an active cathode substance to form a high temperature electrochemical secondary energy storage cell whose sub-arrangement is sodium as active Anode substance that is at the operating temperature of this cell is melted, and a beta alumina solid electrode trolyte separator in contact with the sodium to separate the Sodium in the cell from this active cathode substance contains, again the molten sodium of the anode Magnesium dissolved to form one, preferably saturated solution contains as well as another means for Carrying out the process an electrochemical Containing high temperature secondary energy storage cell Sodium as the active anode substance, which is used in the plant temperature of the cell is melted and electrochemically an active cathode substance and coupled by separated a beta alumina solid electrolyte separator is again the molten sodium of the anode Magnesium contains dissolved to a, preferably saturated, Provide solution in sodium that acts as a getter for the Impurities in the sodium are effective.

Claims (13)

1. Electrochemical high-temperature secondary energy storage cell ( 10 ), containing sodium ( 20 ) as the active anode substance, which has melted at the operating temperature of the cell and is electrochemically connected to an active cathode substance ( 22 ) and thereof by a beta-alumina solid electrolyte separator ( 14 ) separately, characterized in that the molten sodium of the anode contains magnesium dissolved to provide a saturated solution in the sodium which acts as a getter for impurities in the sodium. 2. Cell according to claim 1, characterized in that the sodium of the anode is in contact with solid magnesium ( 24 ). 3. Cell according to claim 2, characterized in that the solid magnesium a finely divided powder one maximum particle size of up to 250 microns. 4. Cell according to one of the preceding claims, characterized characterized in that the amount of sodium in the cathode in full loaded state at least 140 wt .-% of the discharge contains the necessary amount of sodium in the cathode. 5. Anode subassembly for the electrochemical connection of an active cathode substance to form an electrochemical high-temperature secondary energy storage cell ( 10 ), the subassembly of sodium as the active anode substance ( 20 ), which is melted at the operating temperature of this cell, and a beta-aluminum oxide solid electrolyte separator ( 14 ) in contact with the sodium to separate the sodium in a cell from this active cathode substance, characterized in that the molten sodium of the anode contains magnesium dissolved to form a saturated solution. 6. Subassembly according to claim 5, characterized in that the sodium is in contact with magnesium ( 24 ) in finely divided powder form with a maximum particle size of up to 250 microns. 7. The method to counteract the increasing increase in the internal resistance of an electrochemical high-temperature secondary energy storage cell ( 10 ) according to claim 1, which contains molten sodium as the active anode substance ( 20 ), which is electrochemically connected to an active cathode substance ( 22 ) and thereof by a beta -Aluminium oxide electrolyte separator ( 14 ) is separated, characterized in that magnesium is dissolved in the sodium anode material to form a saturated solution thereof in the sodium. 8. The method according to claim 7, characterized in that solid magnesium ( 24 ) is added to this saturated solution. 9. The method according to claim 8, characterized in that magnesium in finely divided powder form with a maximum particle size of up to 250 microns used. 10. The method according to any one of claims 7 to 9, characterized characterized in that the cell with sufficient Fills sodium so that it is in its fully loaded State at least 140% by weight of the electrochemically Discharge of the active cathode substance of the cell contains necessary sodium. 11. The method according to any one of claims 7 to 10, characterized characterized in that magnesium in sodium for Formation of a saturated solution resolves before that Sodium is filled into the cell. 12. The method according to claim 11, characterized in that one magnesium in sodium to form a saturated Solution dissolves at a temperature of at least 120 ° C. 13. The method according to any one of claims 7 to 12, characterized characterized in that the cell within a Operating temperature range up to a maximum of 50 ° C.