GB2121597A - Sodium - sodium nitrate cell - Google Patents

Abstract

A power producing secondary electrochemical cell includes a molten alkali metal as the negative electrode material and a molten nitrate salt as the positive electrode material. The molten material in the respective electrodes are separated by a solid barrier of alkali metal ion conducting material. A typical cell includes active materials of molten sodium separated from molten sodium nitrate and other nitrates in mixture by a layer of sodium beta '' alumina.

Description

SPECIFICATION Electrochemical cell having a akali metal nitrate electrode The invention relates to high temperature, secondary electrochemical cells for producing power that include a molten alkali metal and nitrate salts. The invention particularly relates to the use of molten sodium metal and salts including sodium nitrate as the active materials.

It was previously believed that electrochemical cells involving molten alkali metals and their nitrate salts were only of interest as unrechargable primary cells or as cells for the electrolytic production of alkali metal. Electrochemical reactions such as for the production of molten sodium metal from sodium nitrate have involved the release of reactant gases for intance, nitrogen dioxide and oxygen gas which impose substantial difficulties in incorporating such a reaction in a secondary rechargable electrochemical cell. Furthermore, the nitrates within molten nitrate salts were thought to be decomposed to nitrite plus oxygen gas thus preventing the recharge of the cell to this original state.

The principal positive electrode material presently under consideration for secondary sodium cells is sulfur. At the high operating temperature of these cells a very corrosive environment is developed that requires expensive current collector materials such as titanium oxide or chrome plated steel. Other positive electrodes for molten sodium cells that have been considered include sodium tetrachloroaluminate solvent containing sulfur species or metal chlorides. These systems also exhibit severe corrosion problems and have relatively low theoretical specific energies on the order of 300Wh/kg.

It is an object of the present invention to provide an improved secondary electrochemical power producing cell that can employ molten alkali metal as the negative electrode material.

It is a further object to provide a new positive electrode for use in combination with an alkali metal negative electrode within a secondary rechargable electrochemical cell.

It is also an object to provide a high temperature secondary power producing electrochemical cell with a reactive alkali metal as negative electrode material with reduced corrosion problems in the positive electrode.

In accordance with the present invention, a secondary power producing electrochemical cell is provided. The cell includes a negative electrode containing an alkali metal as active material, a positive electrode containing a nitrate salt including ions of the alkali metal as active material and solid oxide means disposed between the two electrodes for conducting ions therebetween.

In more specific aspects of the invention the negative electrode contains molten sodium and the positive electrode contains a molten salt including sodium nitrate. In such a cell the electrolyte separating the electrodes can be of a sodium oxide and alumina composition, for example one of the well known sodium t3 aluminas. These compositions permit the conduction of sodium ions from the negative to the positive electrode during discharge of the cell.

In one other aspect of the invention the nitrate salt is selected from a mixture of nitrate salts that permit reduced melting points below that of sodium nitrate salt. Mixtures of alkali metal nitrates, alkaline earth metal nitrates and transition metal nitrates are contemplated with eutectic compositions generally providing low melting points for a particular selection of molten salts. In one other aspect of the invention, at least the positive electrode is contained within a sealed chamber to prevent incidental venting of nitrogen dioxide or of oxygen gases that may arise from the decomposition of the various nitrate salts.

In one other characterization of the invention a partially charged secondary electrochemical cell includes a molten alkali metal in the negative electrode chamber and a molten salt containing alkali metal nitrate, alkali metal nitrite and alkali metal oxide within the positive electrode chamber.

In a further embodiment an alkali metal is included in the negative electrode and nitrate salts in the positive electrode each in communication with a plurality of glass fibers extending between the positive and negative electrodes and in contact with the molten alkali metal and the molten metal nitrate salts. The glass fibers consist essentially of an alkali metal ion conducting material including such as sodium oxide and boron oxide.

One further aspect of the electrochemical cell is electrical means for conducting electrical current through an external load between the positive and negative electrodes and for subsequently connecting a source of electrical potential to recharge the electrochemical cell.

The present invention is illustrated in the accompanying drawings wherein: Figure 1 is a schematic elevation view of an electrochemical cell including a molten alkali metal and a molten nitrate salt as active materials.

Figure 2 is a graph of volts vs. capacity over two charge and discharge cycles of a sodium-sodium nitrate secondary electrochemical cell.

Figure 1 shows a laboratory style electrochemical cell used in demonstrating the electrochemical cell of this invention. It will be understood that this cell is presented merely by way of example and that various forms and constructions of cells more appropriate for commercial and industrial applications are also contemplated within the scope of the present invention.

A cell container or housing 11 of corrosion resistant material, such as stainless steel, is illustrated as both the current collector and container for the negative electrode material 13. Molten sodium metal is of principle interest as the negative electrode material 13. However other alkali metals such as potassium and lithium may be appropriate in molten mixture with sodium or as a separate electrode material.

A container 15 for the positive electrode material 17 is shown with its outer surfaces partially immersed in the molten alkali metal 13. Container 15 can be provided with some or all of its walls of a solid electrolyte material to establish means for ionic conduction between the positive 17 and negative electrode materials during cell operation.

The electrolyte material is advantageously selected from one of the sodium ss aluminas of the type commonly used in sodium-sulfur cells. The ss aluminas are polycrystalline composition of sodium oxide and alumina having typically 8-20 mole percent Na20 and the balance alumina. Small amounts of lithia, magnesia and other constituents also may be included as stabilizer or to attribute other properties. A preferred form is that of ssD alumina (nominally Na2O 5A1203) stabilizers with up to about one weight percent to Li2O.

Various ionic conductive glasses such as those formed of boron oxide with a sodium oxide or other alkali metal oxide modifiers also are contemplated for use. Such glasses may include 94-96% by weight boron oxide modified by 4-6% sodium oxide, Na20:2B203:0.2SiO2 and Na20:2B20 3:0.2SiO2:0.1 6NaCl as well as other sodium oxide modified glasses of boron oxide and silicon oxide.

The positive electrode material 17 within container 15 includes molten alkali metal salts, particularly the alkali metal nitrate of the negative electrode material. Sodium nitrate or various mixtures of alkali metal nitrates and in some cases transition metal nitrates including NaNO2-KNO3, NaNO3-K NO3-Mg(NO3)2, NaNO3-LiNO3, LiNO3-NaNO3-K NO3 are contemplated as positive electrode material.

Mixtures with melting points less than 350 C advantageously can be selected. During operation of the cell through charge and discharge cycles, the positive electrode also is expected to contain alkali metal nitrites and alkali metal oxides that occur in the partially charged and uncharged states. Accordingly nitrites and oxides can be includes in the initial positive electrode formulation.

Positive electrode 17 is illustrated as having a suitable current collector 19 in the shape of a screen or grid that may be of stainless steel, nickel or other inert metal. Certain nitrate compositions such as NaNO2-KNO3 have been found to be compatable with mild steel containment thus making this inex pensive current collector material available for use.

A closure 21,with an electrical feedthrough, is illustrated sealing the positive electrode compart ment to prevent escape of any gases such as NO2 and 2 that may incidentally be evolved during cycling of the cell. The cell chemistry does not contemplate evolution of these gases, but should it occur as a result of undesirable side reactions, closure 21 or other means can advantageously be employed to restrict loss of constituents.

The electrochemical cell of Figure 1 is provided with electrical conductors 23 and 25 connected to the current collectors of the positive and negative elec trodes. These conductors are illustrated coupled to an electrical load 27 and a recharger means 29 employed in cycling the Figure 1 electrochemical cell through charge and discharge cycles.

The electrochemial cell described herein has been found to be a reversible secondary electrochemical cell operating at about 1.7 volts. The cell reaction is thoughtto be: 2Na + NaNO3 = Na20 + NaNO2 However, under certin circumstances, the sodium oxide and sodium nitrite may combine to form the species Na3NO3 within the molten salt, positive electrode material.

The following example is presented merely as an illustration of the electrochemical cell of this invention.

A sodium p alumina tube of about 8 cm length and about 1.5 cm diameter were assembled within a stainless steel cup to form an annulusforthe negative electrode material. About 10 gm of molten sodium was filled into the annulus and about 2 gm of sodium nitrate was used as the positive electrode active material within the p alumina tube. Electrical conductors where connected to the stainless steel cup as a negative electrode current collector and a spool of type 304 stainless steel screen employed within the p alumina tube as the positive electrode current collector material. The cell was operated in a helium blanketed furnace at about 325-335 C for 14 cycles over a period of about 500 hours.

Figure 2 illustrates charge and discharge curves from two cycles operated at 10 and 4 hour rates that is at 50 and 80 milliamps respectfully. From these cycles the voltage is estimated to be at about 1.75 V and theoretical specific energy at this EMF is about 720Wh/kg .

It is therefore seen that the present invention provides a new improved secondary electrochemical power producing cell that can employ molten alkali metal in the negative electrode opposite a molten salt containing alkali metal nitrate in the positive electrode. The cell has potential for reduced corrosion problems over that of the traditional high temperature sodium-sulfur cell and can employ various known sodium-ion-conductive electrolytes.

Although the present invention is described in terms of specific embodiments it would be clear to one skilled in the art that various modifications in the structures, materials and procedures can be made within the scope of the following claims.

Claims (15)

1. In a power producing secondary electroche mical cell comprising: a negative electrode containing alkali metal as active material, a positive electrode containing a nitrate salt in cluding metal ions of said alkali metal as active material solid oxide means for conduction of ions between said positive and negative electrodes; and means for passing an electical current in series with an electrical load between said positive and negative electrodes.

2. The electrochemical cell of claim 1 wherein said alkali metal in the negative electrode and the nitrate salt in the positive electrode are in molten state.

3. The electrochemical cell of claim 1 wherein the alkali metal is sodium.

4. Tbe electrochemical cell of claim 1 wherein the solid oxide means comprises a barrier of sodium oxide and alumina intermediate the positive and negative electrodes in communication with the active material in each of said electrodes.

5. The electrochemical cell of claim 1 wherein the nitrate salt is selected from the group of nitrate salts consisting of alkali metal nitrates, alkaline earth metal nitrates, transition metal nitrates and mixtures thereof.

6. The electrochemical cell of claim 5 wherein the nitrate salt comprises a mixture of salts having a melting point less than 350 C.

7. The electrochemical cell of claim 1 wherein the nitrate salt is selected from the group of nitrate salts consisting of NaNO3, NaNO2-KNO3, NaNO3-K NO3-Mg(NO3)2, NaNO3-LiNO3, LlNO3BNaNO3-K NO3 and mixtures thereof.

8. The electrochemical cell of claim 1 wherein the solid oxide means is a solid media communicating with both the positive electrode active material and the negative electrode active material, the solid media is selected from the group of alkali metal conducting materials consisting of sodium ss" alumina and a glass including alkali metal oxide.

9. The electrochemical cell of claim 8 wherein said glass comprises Na2O and B2O3.

10. The electrochemical cell of claim 8 wherein said glass including alkali metal oxide comprises a plurality of glass fibers between the positive and negative elelectrodes said glass fibers selected from the group of glass material consisting of 94-96% by weight B203-4-6% Na2O by weight, Na20:2B203:0.2SiO2 and Na20:2 B2O3:0.2SiO2:0.16NaCl.

11. The electrochemical cell of claim 1 wherein means are provided for connecting an electrical load between the positive and negative electrodes whereby electric current is provided through the load, sodium is depleted from the negative electrode and alkali metal oxide and alkali metal nitrite is formed in the positive electrode.

12. The electrochemical cell of claim 1 wherein the positive electrode is contained within a sealed chamber adequate to restrict venting of NO2 or gases.

13. The electrochemical cell of claim 1 wherein the positive electrode in the partially charged state includes a molten mixture of alkali metal nitrates, alkali metal nitrites, and alkali metal oxides.

14. A power producing, high-temperature, secondary electrochemical cell comprising: a positive electrode chamber containing a molten salt including alkali metal nitrates in contact with means for collecting electronic current from said molten salt; a negative electrode chamber containing a molten alkali metal and means for collecting electronic current in contact with said molten metal; a solid oxide electrolyte separating and in communication with both said molten alkali metal and said molten alkali metal nitrate salt; electrical means for conducting electronic current between said negative electrode current collecting means and positive electrode current collecting means through an electrical load during discharge of the cell and for imposing a source of electrial potential sufficient to reverse said electronic current during charge of the cell.

15. The electrochemical cell of claim 14 wherein means are provided to restrict escape of NO2 and 2 gases from the positive electrode chamber.