Note: Descriptions are shown in the official language in which they were submitted.
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ELECT~OCHEMICAL CELL H~VING A
ALKALI METAL NITRATE ELECTRODE
The invention relates to high temperature, secondary
electrochemical cells for producing power that includes 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 gases for instance, 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
~0 under consideration for secondary sodium cells is sulfur.
At the high operating temperature of these cells a very
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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 tetrachloro-
aluminate 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 30OWh/kg.
It is an ob~ect of the present invention to provide
an improved secondary electrochemical power producing
cell that can employ molten alkali meta] 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 electro-
chemical 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, there is
provided a secondary electrochemical storage cell comprising
a negative electrode containing elemental alkali metal
as active material, a positive electrode containing a
salt including metal ions of the alkali metal as active
material, the salt including a nitrate, a mixture of
nitrates or a mixture of a nitrate and a nitrite; a solid
electrolyte including and being capable of conducting ions
of the alkali metal between the positive and negative
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electrodes; discharge means for conducting an electrical
current through an electrical load connected in series
between the positive and negative electrodes as alkali
metal oxidizes to alkali metal ions in the negative
electrode and alkali metal nitrate reduces to alkali
metal nitrite in the positive electrode; recharge means
for applying a source of electrical potential between
the positive and negative electrodes sufEicient to
oxidize alkali metal nitrite to alkali metal nitrate in
the positive electrode and reduce alkali metal ions to
alkali metal in the negative electrode; and sealing
means for retaining the salt of alkali metal in the
positive electrode as the cell discharges through the
electrical load and as the cell is recharged for storing
electrical energy.
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 ~ aluminasO These compositions
permit the conduction of sodium ions from the negative
to the positive electrodes during discharge of the cell.
In one other aspect of the invention the nitrate salt
is selected from a mixture oE 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
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for a particular selec-tion 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 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 qlass
fibers extending between the positive and negative electrodes
and in contact with the molten alkali metal and the molten
metal nitrate salts. The glass Eibers consist essentially
of an alkali metal ion conducting material including such
as sodium oxide and boron oxide.
Further in accordance with the present invention there
is provided a rechargeable electrochemical cell in at least
a partially uncharged state comprising a negative electrode
chamber containing elemental alkali metal as negative
electrode material and means for collecting electronic current
in contact with the alkali metal; a sealed positive electrode
chamber containing a salt of alkali metal including nitrites
and oxides as positive electrode material and means for
collecting electronic current in contact with the alkali
metal salt; a solid electrolyte wall forming at least a portion
of the sealed positive electrode chamber and being in
contact at one surface with the elemental alkali metal
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and its opposite surface with the alkali metal salt, the
solid electrolyte wall being capable of conducting ions
of the alkali metal between the active materials of the
positive and negative electrodes; and electrical recharge
means or connecting a source of electrical potential across
the current collecting means of the negative and the positive
electrodes to oxidize alkali metal nitrite to alkali metal
nitrate in the positive electrode.
The present invention is illustrated in the
accompanying drawin~s wherein:
Fig. 1 is a schematic elevation view o~ an electro-
chemical cell including a molten alkali metal and a molten
nitrate salt as active materials.
Fig. 2 is a graph of volts vs. capacity over two
charge and discharge cycles of a sodium-sodium nitrate
secondary electrochemical cell.
Fig. 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 contem-
plated 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. ~olten sodium metal is of principle
interest as the negative electrode material 13. However
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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 ~ aluminas of the type commonly
used in sodium-sulfur cells. The ~ aluminas are poly-
crystalline composition of sodium oxide and alumina
having typically 8-20 mole percent Na2O 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 ~ alumina (nominally Na2O:5A12O3) stabilizers with
up to about one weight percent of Li2O.
Various ionic cond~ctive 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, Na2O:2~2O3:~.2SiO2 and
Na2O:2B2O3:0.2SiO2:0.16NaCl as well as other sodium
oxide modified glasses of boron oxide and silicon oxide.
The positive electrode material 17 within container
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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
NaN2~KN3' NaN3~KN3~ Mg(N3)2, NaNO3-LiNO3, LiNO3-
KNO3 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
included 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 inexpensive current collector material
available for use.
A closure 21, with an electrical feedthrou~h, is
illustrated sealing the positive electrode compartment
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.
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The electrochemical cell of Fig. 1 is provided with
electrical conductors 23 and 25 connected to the current
collectors of the positive and negative electrodes. These
conductors are illustrated coupled to an electrical load
27 and a recharger means 29 employed in cycling the Fig. 1
electrochemical cell through charge and discharge cycles.
The electrochemical cell described herein has been
found to be a reversible secondary electrochemical cell
operating at about 1.7 volts. The cell reaction is
thought to be:
2Na + NaNO3 = Na2O + NaNO2
However, under certain circumstances, the sodium oxide and
sodium nitrite may combine to form the species Na3NO3
within the molten salt, positive electrode material.
The following e~ample is presented merely as an
illustration of the electrochemical cell of this invention.
A sodium ~ alumina tube of about 8 cm length and
about 1.5 cm diameter were assembled within a stainless
steel cup to form an annulus for the 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 ~ alumina
tube. Electrical conductors were connected to the stainless
steel cup as a negative electrode current collector and
a spool of type 304 stainless steel screen employed within
the ~ alumina tube as the positive electrode current collector
material. The cell was operated in a helium blanketed
furnace at about 325-335C for 14 cycles over a period
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of a~out 500 hours.
Fig. 2 illustrates charge and discharge curves from
two cycles operated at 10 and ~ hour rates that is at
S0 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.
