Electrochemical cell

A method of making an electrochemical cell comprises loading into the cathode compartment of a cell housing comprising an anode compartment separated from a cathode compartment by a separator which is a solid conductor of ions of alkali metal M or is a micromolecular sieve which contains alkali metal M sorbed therein; an alkali metal aluminium halide molten salt electrolyte having the formula MAlHal.sub.4 wherein M is the alkali metal of the anode and Hal is a halide; Cu as an active cathode substance; and an alkali metal halide MHal wherein M and Hal are respectively an alkali metal and a halide; thereby to make an electrochemical cell precursor. The precursor is charged at a temperature at which the molten salt electrolyte and alkali metal M are molten, thereby to halogenate the Cu, with alkali metal being produced and passing through the separator into the anode compartment, the proportions of alkali metal halide MHal, Cu and molten salt electrolyte loaded into the cathode compartment being selected so that when the cell is fully charged and all the available active cathode substance has been halogenated, the proportion of alkali metal ions and aluminium ions is such that the solubility of the active cathode substance in the molten electrolyte is at or near its minimum.

THIS INVENTION relates to an electrochemical cell. It relates also to a 
method of making an electrochemical cell. 
According to a first aspect of the invention, there is provided a method of 
making an electrochemical cell of the type comprising an anode compartment 
containing at the operating temperature of the cell and when the cell is 
in its charged state, a molten alkali metal anode, and a cathode 
compartment containing, at said operating temperature and when the cell is 
in its discharged state, an alkali metal aluminium halide molten salt 
electrolyte which is also molten at the operating temperature of the cell, 
and having the formula MAlHal.sub.4, wherein M is the alkali metal of the 
anode and Hal is a halide, the cathode compartment containing also a 
cathode which comprises an electronically conductive electrolyte-permeable 
matrix which has dispersed therein Cu as an active cathode substance, the 
matrix being impregnated with said electrolyte, and, separating the anode 
compartment from the cathode compartment, a separator which comprises a 
solid conductor of the ions of the alkali metal of the anode or a 
micromolecular sieve which contains said alkali metal sorbed therein, the 
method comprising 
loading into the cathode compartment of a cell housing comprising an anode 
compartment separated from a cathode compartment by a separator which is a 
solid conductor of ions of alkali metal M or is a micromolecular sieve 
which contains alkali metal M sorbed therein, 
an alkali metal aluminium halide molten salt electrolyte having the formula 
MAlHal.sub.4 wherein M is the alkali metal of the anode and Hal is a 
halide; 
Cu as an active cathode substance; and 
an alkali metal halide MHal wherein M and Hal are respectively an alkali 
metal and a halide, thereby to make an electrochemical cell precursor; and 
charging the precursor at a temperature at which the molten salt 
electrolyte and alkali metal M are molten, thereby to halogenate the Cu, 
with alkali metal being produced and passing through the separator into 
the anode compartment, the proportions of alkali metal halide MHal, Cu and 
molten salt electrolyte loaded into the cathode compartment being selected 
so that when the cell is fully charged and all the available active 
cathode substance has been halogenated, the proportion of alkali metal 
ions and aluminium ions is such that the solubility of the active cathode 
substance in the molten electrolyte is at or near its minimum. 
The molar proportion or ratio of alkali metal ions and aluminium ions in 
the electrolyte is preferably not less than one, to obtain said minimum 
solubility. 
While M and Hal of the alkali metal halide MHal will normally be the same 
as the M and Hal respectively of the electrolyte MAlHal.sub.4, they can, 
however, instead be different. 
The alkali metal, M, may be sodium, with the separator being beta-alumina. 
However, in other embodiments of the invention, the alkali metal may 
instead be potassium or lithium, with the separator then being selected to 
be compatible therewith. The halide may be chlorine, so that the alkali 
metal halide, MHal, is sodium chloride. The electrolyte may then be in the 
form of a sodium chloride-aluminium chloride mixture or in the form of the 
double salt, ie NaAlCl.sub.4. However, the halide may instead be bromine 
or iodine, so that the alkali metal halide is then NaBr or NaI, with the 
electrolyte being NaAlBr.sub.4 or NaAlI.sub.4 respectively. 
It is believed that when the chloride salts are used, the initial reaction 
on charging the cell is 
EQU Cu+NaCl.fwdarw.CuCl+Na (1) 
Further electrochemical oxidation should result in the following reaction 
that would occur at a higher voltage 
EQU CuCl+NaCl.fwdarw.CuCl.sub.2 +Na (2) 
The overall reaction would therefore be 
EQU Cu+2NaCl.fwdarw.2Na+CuCl.sub.2 ( 3) 
However, it is believed that reaction (2) may be prevented from occurring, 
at least partially, if an excess of Cu is present in the electrode which 
will result in reduction of CuCl.sub.2 to CuCl according to the reaction 
EQU CuCl.sub.2 +Cu.fwdarw.2CuCl (4) 
Other halide ions may also be used, and in certain instances to the 
advantage of the cell reaction. For example, CuBr and NaAlBr.sub.4 may be 
used. Although the Na/CuBr open circuit voltage is lower than the Na/CuCl 
open circuit voltage, it is believed that CuBr (melting point 492.degree. 
C.) is less soluble in NaAlHal.sub.4, and specifically NaAlCl.sub.4, than 
CuCl (melting point 430.degree. C.). In this case the method thus 
involves, on charging of the cell, the following reactions taking place in 
the cathode compartment: 
EQU Cu+NaBr.fwdarw.CuBr+Na (5) 
EQU CuBr+NaBr.fwdarw.CuBr.sub.2 +Na (6) 
EQU Overall: Cu+2NaBr.fwdarw.2Na+CuBr (7) 
with the Na migrating to the anode compartment via the separator. 
The method may include initially loading also some aluminium into the 
cathode compartment, with no sodium being present in the anode 
compartment. On subjecting the cell precursor to an initial charging, the 
aluminium reacts with the alkali metal halide MHal to produce further 
molten salt electrolyte and to form alkali metal M which passes through 
the separator into the anode compartment. Sufficient aluminium is then 
provided so that the initial reaction with the aluminium on charging 
provides the initial starting up amount of sodium in the anode 
compartment, with normal charge and discharge reactions of the cell 
between its fully charged and discharged states, in accordance with 
reaction (1) thereafter taking place. 
The copper may initially be in the form of copper powder or filings. 
When a NaAlBr.sub.4 electrolyte is used, it may be doped with NaAlCl.sub.4 
to facilitate initial handling of the electrolyte. Since the charging 
plateau of Cu/copper bromide//Na is lower than that of Cu/copper 
chloride//Na, copper bromide will form preferentially in the cathode 
compartment. It is believed that in such doped systems the solid solution 
which forms between copper bromide and copper chloride will result in the 
lowering of the solubility of the copper salt in the electrolyte, as well 
as lowering of the open cell voltage. 
The method may also include adding a fluoro containing salt to the 
electrolyte, ie doping the electrolyte with a fluoro containing salt, to 
combat progressively increasing internal resistance of the cell associated 
with cyclic charging and discharging thereof and to act as a copper ion 
scavenger. The fluoro containing salt may be NaF, and the proportion of 
NaF in the liquid electrolyte may be between 2% and 25% on a molar basis, 
preferably between 10% and 20% on a molar basis, and typically 10-5%. 
When the copper is in metallic form, it may be in the form of copper powder 
or filings, with the aluminium, when present, being in electrical contact 
with the copper, eg being present in the form of a surface coating, alloy, 
or the like. 
The copper may instead initially be loaded into the cathode compartment in 
the form of an electronically conducting backbone, with carbon dispersed 
therein. Carbon has the advantage of being lighter and cheaper than 
copper, and results in a less dense matrix. 
The copper and the alkali metal halide may be mixed together in particulate 
form, eg granules, to form a mixture, the mixture sintered to form an 
electrolyte-permeable matrix, and the matrix impregnated with the molten 
salt electrolyte prior to loading thereof into the cathode compartment. 
The matrix may hence contain sufficient copper to fulfill the function as 
active cathode substance as well as to act as current collector. 
The method may include adding at least one transition metal to the cathode 
compartment in a minor proportion, to prevent or retard grain growth. The 
transition metal may then be in the form of a compound with the copper. 
The transition metal may be mixed or alloyed with the copper. The 
transition metal may constitute less than 30%, typically less than 10% and 
preferably less than 5%, of the alloy or mixture. 
When the matrix has been impregnated with, for example, said NaAlCl.sub.4 
electrolyte and the NaCl incorporated therein, it is a cathode precursor 
which is coupled via the separator with the anode compartment and which 
becomes a cathode after it has been subjected to at least one charge 
cycle. 
More specifically, forming the matrix may comprise sintering particles such 
as powders or fibres of the copper, optionally with carbon particles 
dispersed therein, in a reducing atmosphere. 
Incorporating the NaCl into the matrix may be effected simultaneously with 
the formation of the matrix, the NaCl in finely divided particulate form 
being dispersed into the particulate material, eg granules, from which the 
matrix is formed, prior to formation of the matrix. Such granules can have 
an extremely high density. Hence, the incorporation into the granules of 
the NaCl or an additive such as aluminium results in additional porosity 
when the cell precursor is subjected to a first charging cycle. 
Granulation also promotes homogeneity of chemical species within the 
cathode compartment. 
Instead, the NaCl may be incorporated into the matrix by melting the 
electrolyte and suspending particulate NaCl in finely divided form in the 
molten electrolyte, prior to impregnating the electrolyte into the matrix. 
It is hence apparent from the foregoing that the NaCl may be incorporated 
into the matrix in any one of a number of suitable different ways. 
In another more specific version of the invention, the NaCl and the copper 
may be combined by forming a mixture in particulate form of the copper and 
NaCl. Still more particularly, the formation of the mixture may comprise 
mixing together NaCl and Cu powders. 
Impregnating the molten salt electrolyte into the matrix may be by means of 
vacuum impregnation with the electrolyte in the molten state. 
The powder mixture may then be impregnated, eg saturated, with the 
electrolyte, which may be effected simply by wetting the mixture with the 
electrolyte in molten liquid form, for example after the powder mixture 
has been packed, for example by tamping, into the cathode compartment. 
This will, in effect, provide a cathode precursor in a discharged state, 
which can then be activated by charging. 
In the charging reaction of this precursor, the metal component of the 
mixture is halogenated, sodium being produced during the halogenation, 
which sodium moves through the separator in ionic form, into the anode 
compartment where it exists in the charged or partially charged cell as 
molten sodium metal, electrons passing during the charging along the 
external circuit from the cathode compartment to the anode compartment. 
Although the powder mixture may be charged in powder form after saturation 
with liquid electrolyte, the method may include as mentioned hereinbefore 
the additional step of sintering the powder mix to form a macroporous 
electrolyte permeable matrix prior to saturation with electrolyte, and 
activation by taking it through one or more charge cycles as a cathode to 
halogenate it. 
The invention also extends to an electrochemical cell, when made according 
to a method as hereinbefore described. 
The upper working or operating temperature of the cell will be such that 
the solubility of the active cathode substance in the molten electrolyte 
is low. Thus, the upper operating temperature may be below the halogenated 
copper/sodium chloride eutectic temperature in the presence of the 
MAlHal.sub.4 electrolyte and excess copper and excess sodium chloride. 
According to a second aspect of the invention, there is provided a 
precursor for a high temperature electrochemical cell which comprises a 
cell housing having an anode compartment and a cathode compartment 
separating from each other by a separator which comprises a solid 
conductor of ions of an alkali metal M or a micromolecular sieve which 
contains alkali metal M sorbed therein, the cathode compartment 
containing: 
an alkali metal aluminium halide molten salt electrolyte having the formula 
MAlHal.sub.4 wherein M is the alkali metal of the separator and Hal is a 
halide; 
a cathode which comprises copper as an active cathode substance; and 
an alkali metal halide M wherein M and Hal are respectively an alkali metal 
and a halide, with the precursor being chargeable at a temperature at 
which the molten salt electrolyte and alkali metal M are molten to cause 
the active cathode substance to be halogenated, with alkali metal M being 
produced and passing through the separator into the anode compartment, the 
proportions of alkali metal halide MHal, copper and molten salt 
electrolyte being selected so that when the cell is fully charged and all 
the available active cathode substance has been halogenated, the 
proportion of alkali metal ions and aluminium ions in the electrolyte is 
such that the solubility of the active cathode substance in the molten 
electrolyte is at or near its minimum. 
As mentioned hereinbefore, the active cathode substance as well as the 
alkali metal halide may be in particulate form, eg granules, with the 
alkali metal halide being mixed with the active cathode substance, and the 
mixture being impregnated with the molten salt electrolyte. The copper may 
be particulate and form part of said mixture, or form part of the 
granules. As also mentioned hereinbefore, the M and Hal of the alkali 
metal halide MHal will usually be the same as the M and Hal respectively 
of the electrolyte. 
In another embodiment of the invention, the cathode may comprise an 
electronically conductive electrolyte-permeable matrix in which the active 
cathode substance and alkali metal halide are dispersed, the matrix being 
impregnated with the molten salt electrolyte. 
It will be appreciated that minor quantities of impurities may be tolerated 
in the electrolyte, ie substances which will ionize in the molten 
electrolyte to provide ions which affect the electrochemical action of the 
electrolyte, but the quantity of such impurities should be insufficient to 
alter the essential character of the electrolyte as an MAlHal.sub.4 system 
as defined. 
When the separator is a micromolecular sieve, it may be a tectosilicate, eg 
a felspar, felspathoid or zeolite. When it is a zeolite, the zeolite may 
be a synthetic zeolite such as zeolite 3A, 4A, 13X, or the like. 
Preferably, however, the separator is a solid conductor of sodium ions 
such as beta-alumina or nasicon. For example, a beta-alumina tube can be 
used. The interior of the tube may be used as the anode compartment, with 
the tube being located in a cell housing which defines a cathode 
compartment outside the tube, in the interior of the housing, and with an 
anode compartment current collector being in intimate electrical contact 
with substantially the entire separator. The tube will be sealed and may 
be evacuated prior to charging to resist undesirable pressure build-up 
therein as sodium moves into the anode compartment during charging, 
through the tube wall. In this specification, beta-alumina is used broadly 
to include all phases of sodium-ion conducting beta-alumina, such as 
beta"-alumina. 
To spread the sodium over the inside of the tube wall and to effect said 
intimate contact of the anode compartment current collector with the 
separator, suitable wicking material, electrically connected to the 
current collector and containing finely divided electrically conductive 
particles, may be spread over the wall surface. The material may, for 
example, be iron mesh, optionally tinned. This mesh hence acts as a part 
of an anode current collector, and may be attached to an evacuation pipe 
of the same metal, used to evacuate the tube interior prior to charging 
and projecting out of the cell to form the remainder of the anode current 
collector. 
The main current collector of the cathode of the cell of the present 
invention will usually be the copper current collecting backbone of the 
matrix together with the housing. The housing may also be of copper. To 
improve the initial activation or charging characteristics of the cell, 
the cathode current collector may include a metal mesh or gauze connected, 
for example by welding, to the housing. 
For close packing in batteries, the cell may have an elongate rectangular 
housing along the interior of which the tube extends in a more or less 
central position. To facilitate wicking in the anode compartment, the cell 
may be used horizontally, but this can lead to voids in the cathode 
compartment formed upon charging as the sodium moves into the anode 
compartment. For this reason, the cell may incorporate an electrolyte 
reservoir, more or less separate from but in communication with the 
cathode compartment, from which the electrolytes can pass, for example by 
draining under gravity, into the cathode compartment to keep it flooded 
with liquid electrolytes at all times. Naturally, for close packing, cells 
of similar construction but having an hexagonal cross-section can be 
employed instead. 
The electrolyte may include a minor proportion of sodium fluoride as 
dopant. This combats potential progressive internal resistance rise on 
cell cycling and the invention accordingly contemplates incorporating a 
small proportion of sodium fluoride in the powder mix from which the 
cathode is formed. This sodium fluoride dissolves in the liquid 
electrolyte, in use. The electrolyte should be selected so that, at all 
states of charge, the solubility therein of the copper salt is at a 
minimum. This is achieved when the electrolyte comprises a mixture of 
sodium halide and aluminium halide in a 1:1 mole ratio, with the molten 
salt being in the presence of at least some solid sodium halide at all 
stages of charge. The only alkali metal present should be those which do 
not adversely affect the beta-alumina separator, and, although pure sodium 
aluminium halide can be used, said minor proportion of up to 10% on a 
molar basis or more of the electrolyte may be made up of sodium fluoride. 
This sodium fluoride could replace the equivalent proportion of sodium 
halide, so that said 1:1 mole ratio is retained. The proportion of sodium 
fluoride will, however, be sufficiently low for the electrolyte to retain 
its essential character as a sodium aluminium halide electrolyte. There 
must thus be enough sodium halide, as mentioned above, for some solid 
sodium halide to remain in the cathode compartment when the cell is fully 
charged, ie to maintain minimum solubility. 
The liquid electrolyte and/or active cathode substance may contain a minor 
proportion of a suitable chalcogen dispersed therein for resisting a 
progressive drop in the capacity of the cathode with repeated 
charge/discharge cycling of the cell. 
The chalcogen may comprise one or more species, such as selenium or 
sulphur, or compounds containing sulphur such as a transition metal 
sulphide. The chalcogen is preferably in extremely finely divided form, 
and it or reaction products between it and components of the liquid 
electrolyte may even be dissolved in the electrolyte. 
The invention will now be described, by way of example, with reference to 
the accompanying drawings.

Referring to FIG. 1, reference numeral 10 generally indicates an 
electrochemical cell in accordance with the invention. 
The cell 10 includes an outer cylindrical casing 12 having a side wall 22 
connected to a circular floor 24; a beta-alumina tube 14 located 
concentrically within the casing 12, the tube 14 being closed at its one 
end 16 and open at its other end 18; and a collar assembly 20 around the 
end 18 of the tube 14. The collar assembly 20 comprises a circular 
insulating ring 26 of alpha-alumina, with the end 18 of the tube 14 
mounted to the ring 26 by being sealingly located in an annular groove 28 
in the ring. Two concentric truncated cylinders of nickel, designated 30, 
32, are bonded fluid tightly to the outer and inner curved surfaces 
respectively of the ring 26. An annular closure disc 34 closes off the 
open end 18 of the tube 14, the disc 34 being secured to the truncated 
cylinder or ring 32 at 36. An annular disc 40 also closes off the end of 
the casing 12 remote from the floor 24, the disc 40 being secured, eg 
welded, to the casing at 42 and to the ring 30 at 44. A steel rod current 
collector 46 projects into the tube 14, and a steel rod current collector 
50 protrudes from the disc 40 at 52. The current collector 46 is 
electrically connected to a porous wicking layer 47 lining the inside of 
the separator tube 14, ie in intimate contact with the tube 14, with 
finely divided electrically conductive particles, eg Ni and/or Fe 
particles incorporated in the layer. 
An anode compartment 56 is hence provided inside the tube 14, with a 
cathode compartment 58 being provided around the outside of the tube 14, 
within the casing 12, the beta-alumina tube 14 hence constituting a 
separator between the anode and cathode compartments. 
Into the cathode compartment 58 there is placed an electrolyte permeable 
matrix 60 of Cu, with sodium chloride powder incorporated therein in 
dispersed form. Sufficient molten NaAlCl.sub.4 electrolyte is then added 
to the cathode compartment so that the matrix is impregnated with the 
electrolyte and the electrolyte wets the separator or tube 14. The 
beta-alumina tube 14 hence forms a continuous barrier between the 
electrolyte containing cathode compartment 58 and the anode compartment 
56, within the housing 12. Initially, the layer 47 ensures the required 
electrical contact between the collector 46 and the separator 16. However, 
on the first sodium passing through the separator it `wicks` along the 
layer 47 thereby providing further electrical contact between the anode 
compartment and the separator. 
On charging the cell 10, the following reactions take place in the cathode 
compartment: 
EQU NaCl+Cu.fwdarw.Na+CuCl (1) 
EQU CuCl+NaCl.fwdarw.Na+CuCl.sub.2 (2) 
EQU Overall: 2NaCl+Cu.fwdarw.2Na+CuCl.sub.2 (3) 
The Na generated by the reactions passes through the beta-alumina into the 
anode compartment. A small amount of starting Na can be provided in the 
anode compartment, in molten form, to connect the current collector 46 to 
the separator 14. 
A series of tests were conducted with cells in accordance with the 
invention and being similar to that of FIG. 1: 
EXAMPLE 1 
A copper cathode comprising 60 g Cu (Merck &lt;63 um) admixed intimately with 
40 g NaCl (&lt;53 um) was sintered around a Cu metal strip current collector 
in hydrogen gas to form an electrode. The electrode was then loaded into a 
beta alumina tube, impregnated with the electrolyte sodium aluminium 
chloride and finally a small amount of sodium added to the negative 
electrode. The assembled cell was warmed to 220.degree. C. and cycled at 
1/2A charge (5 mA cm.sup.-2) and 1A discharge (10mA cm.sup.-2). FIG. 2 
shows the charge/discharge curve of cycle 2 of this cell, demonstrating 
the reversibility of the Na/Cu-chloride couple in a basic NaAlCl.sub.4 
melt. The cell was capable of maintaining capacity, as shown in FIG. 3. 
EXAMPLE 2 
A copper cathode comprising 108 g Cu (15% bronze (BDH)+85% Cu (Merck) 
intimately admixed with 83 g NaBr (&lt;53 um) was sintered around a Cu metal 
strip current collector in hydrogen gas to form an electrode. The 
electrode was then loaded into a beta alumina tube, impregnated with the 
electrolyte sodium aluminium chloride and finally a small amount of sodium 
added to the negative electrode. The assembled cell was warmed to 
250.degree. C. and cycled at 1/2A charge (2.5 mA cm.sup.-2) and 1A 
discharge (5 mA cm.sup.-2). FIG. 4 shows the charge/discharge curves of 
cycle 15 of this cell, demonstrating the reversibility of the 
Na/Cu-bromide couple in a basic NaAlCl.sub.4 melt. The two stages of the 
charging reactions are evident in FIG. 4, being identified as zones (a) 
and (b)respectively. On discharge only a fraction of the electrochemical 
reaction is accounted for by the reduction of CuBr.sub.2 (zone (c)); the 
major portion is as a result of the reduction of CuBr to Cu and NaBr (zone 
(d)). FIG. 5 shows the capacity retention with cycling for 16 cycles. 
EXAMPLE 3 
A mixed Cu plus Ni cathode (ie a Ni cell having a Cu backbone) comprising 
30 g Ni (Inco 255) intimately admixed with, 30 g Cu (&lt;63 um) and 40 g NaCl 
(&lt;53 um) wax sintered in hydrogen gas around a Cu metal strip current 
collector, to form an electrode. The cell assembly was identical to 
EXAMPLE 1. The cell was operated at 250.degree. C. and charged at 5 mA 
cm.sup.-2 and discharged at 10 mA cm.sup.-2. FIG. 6 illustrates the 
charge/discharge curves of cycle 10 of this cell, in which zone (a) 
represent the reduction of copper chloride and zone (b) the reduction of 
nickel chloride, during discharge. The charge curve shows that both 
reactions are reversible. FIG. 7 shows the capacity retention for 30 
cycles. 
EXAMPLE 4 
As for EXAMPLE 3 except that a Ni current collector was used. FIG. 8 is the 
charge/discharge curve for cycle no. 20 of this cell, in which zone (a) 
represent the reduction of copper chloride and zone (b) the reduction of 
nickel chloride during discharge. The charge curve shows that both 
reactions are reversible. FIG. 9 shows an improved capacity retention with 
cycling. The cell retained 90% of its theoretical capacity once it had 
been run in.