Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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T~IS INVENTION relates to an electrochemical cell.
In particular, it relates to a rechargeable electrochemical
cell suitable for secondary use.
According to the invention, an electrochemical cell
comprises a sodium anode which is molten at the operating
temperature of the cell, a sodium aluminium halide molten salt
electrolyte which is also molten at the operating temperature
of the cell, a cathode which i5 impregnated by the electrolyte
and which comprises, as the electrochemically active cathode
substance of the cell, a transition metal chloride selected
from the group consisting in FeC12, NiC12, CoC12 and CrC12
dispersed in an electrolyte-permeable matrix which is
electronically conductive, and, between the anode and the
electrolyte and isolating the anode from the electrolyte, a
solid conductor of sodium ions or a micromolecular sieve which
contains sodium sorbed therein, the proportions of sodium ions
~ and aluminium ions in the electrolyte being selected so that
; ~ ~ the solubility of the active cathode substance in the molten
~ eleckrolyte is at or near its minimum.
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By 'isolating' is meant that any ionic sodium or
metallic sodium moving from the anode to the electrolyte or
vice versa, has to pass through the internal crystal structure
of the solid conductor or through the microporous interior of
the carrier, as the case may be.
The electrolyte is conveniently a sodium aluminium
chloride molten salt electrolyte, which can, depending on the
proportions of sodium and aluminiumr have a melting point of
the order of 15QC or less, and wherein, also depending on its
composition, the active cathode substance can be virtually
insoluble. This electrolyte may contain a minor proportion of
up to, say, lO~ by mass and usually less, o~ a dopant such as
an alkali metal halide other than sodium chloride, by means of
which its melting point is reduced. The dopant may thus
comprise an alkali metal fluoride, but the proportions of the
constituents o~ the electrolyte should be selected such that
the solubility of the active cathode substance in the electrolyte
~ is kept to a minimum.
: ~:
The Applicant has found that the minimum solubility
o~ the active cathode substances in the sodium aluminium
chloride electrolytes (which may be doped as described above),
occurs when the molar ratio of the alkali metal halide to the
aluminium halide is about l:l. In other words, the relative
quantities of said alkali metal ions, aluminium ions and halide
` ~ ~72~2
ions should conform substantially with the stoichiometric
product:
M Al X4
wherein
M represents alkali metal cations; and
X represents halide anions.
Such electrolytes are among those described in the Applicant's
United States Patent 4 287 271.
In this way, the proportions of the constituents can
be selected so that the melting point of the electrolyte at
atmospheric pressure is below 140C. Minor proportions of
dopants may be tolerated in the electrolyte, e.g. substances
which will ionize in the molten electrolyte to provide ions
which affect the electrolytic action of the electrolyte or, as
mentioned above, substances which reduce its melting point,
but their nature and quantity should be insu~ficient to alter
~; the essential character of the electrolyte as a sodium
aluminium chloride electrolyte, wherein the M Al X4 product is
maintained~ ~
~:
~o ~hen the cell contains a solid conductor of sodium
ions, said solid conductor may be beta-alumina or nasicon.
:::
Instead, when the cell contains a micromolecular
sieve this carrier can be regarded as a conductor of sodium
metal and/or sodium ions, depending on the mechanism whereby
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sodium is transported therethrough.
By 'micromolecular sieve is meant a molecular sieve
having interconnected cavities and/or channels in its interior
and windows and/or pores in its surface leading to said
5 cavities and channels, the windows, pores, cavities and/or
channels having a size of not more than 50 Angstroms and
preferably less than 20 Angstroms.
Suitable micromolecular sieves are mineral micromole~
cular sieves, ie inorganic lattice or framework structures such
as tectosilicates, eg the zeolites 13X, 3A, 4~ or the like,
although certain essentially organic micromolecular sieves such
as clathrates may, in certain circumstances, be suitable.
The active cathode substance should preferably be
evenly dispersed throughout the matrix; and it may be in finely
divided particulate form and/or it may adhere as fine particles
ar a thin layer to the matrix, preferably so that there are no
large particles or thick layers of active cathode substance
present, and preferably so that none of the active cathode
substance is spaced physically from the material of the matrix,
which acts as a current collector, by an excessive spacing, eg
in large cavities in the matrix. In other words, the active
cathode substance preEerably should be close to or adherent to
the material of the matrix, and should be as thinly spread as
possible, consistent with the porosity of the matrix and the
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quantity of cathode substance required to be present. Large
particles or thick layers of active cathode substance will not
prevent the cell from working, but will merely be inefficient,
a proportion of the active cathode substance remote from the
cathode material amounting merely to dead weight.
As the electrochemically active cathode substance,
FeCl (ferrous chloride) is attractive, for reasons of
availability and cost, and because it can be substantially
insoluble in a sodium aluminium chloride electrolyte in which
the molar ratio of sodium chloride to aluminium chloride is
The matrix of the ~athode in turn can be any suitable
electronically conductive subs-tance capable of oroviding access
to the cathode substance of the sodium ions of the electrolyte.
Carbon in the form of graphite may ~e used, or a porous matrix
of the transition metal itself can be used. Suitable solid
artifacts for use as cathodes can ~e made from graphite or the
metal, for use in the cathodes, as described hereunder.
The transition metal chlorides of the cathodes of the
present invention can be obtained from the metals in question
or from compounds of the metals in question which can be
treated to yield the desired chloride, eg refractory compounds
of the transition metal, or other chlorides -thereof. In each
case, the oxida-tion state of the metal in the metal chloride in
the cathode should be as low as possible, and the presence of
higher chlorides of the metal should be avoided, so that
solubility of cathode material in -the electrolyte melt is
avoided as far as is practicable.
Thus, a sintered artifact can be made o~ the
transition metal in question, in a manner similar to that used
for the construction of porous iron electrodes. This can then
be chlorinated electrochemically, or chemically by reaction
with chlorine gas, or with chlorine gas diluted by a suitable
diluent.
When electrochemical chlorination is being employed,
: the cathode so formed can be removed to the cell where it is to
be used, or .i~ it is chlorinated in situ, the original
composition o~ the electrolyte should be selected, or the
electrolyte should be modiEied a~ter chlorination, so that the
electrochemically active cathode substance is substantially
insoluble therein~
:
If chemical chlorination has been used, subsequent
heating under vacuum can be employed to sublime off unwanted
volatiles, such as any FeC13 obtained in making an FeC12/Fe
cathode artifact. According to this method of manufacture, the
resultant cathode is the desired transition metal chloride in
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question, finely dispersed through a porous matrix of the
transition metal, which is an electronic conductor and can be
electrochemically active, depending on the cell environment,
and can thus ~urther enhance cell capacity.
Instead, a refractory compound of the transition
metal in question, such as a carbide thereof, can be mixed with
a small quantity of a carbon-forming binder, eg phenol
formaldehyde resin. The resulting mix is then pressed into
electrode shape and the resin can be cracked in a vacuum at
temperatures in excess o~ 600C, the temperature being selected
to pyrolyse the binder to conductive carbon and to degrade the
carbide to the metal and graphite. Thus, in the case of iron,
Fe3C can be degraded to alpha iron and graphite. The resulting
electrode is a ~ine dispersion of alpha iron and carbon which
can be chlorinated by the method described above, the matrix
comprising any conductive iron or graphite remaining after the
chlorination.
Still further, the chloride itself can be finely
divided and mixed with a suitable conducting medium for the
matrix, such as graphite, and the cathode pressed as an
artifact from the mixture.
In each case, prior to assembling the cell, the
cathode must be loaded with the electrolyte with which it is to
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be used, and this can be effected by vacuum impregnation
followed by pressurization, to promote complete penetration o
the electrolyte into the artifact.
Test cells were made in accordance with the inuen-
tion, assembled under an argon atmosphere. In each case,beta alumina separated the sodi~m anode from the electrolyte,
and to ensure good wetting in use of the beta alumina by the
molten sodium, the beta alumina and sodium were preheated to
400C and cooled under argon. The cathode was then placed in
position and suficient molten electrolyte was added under
argon, the electrolyte comprising an equimolar mix of sodium
chloride and aluminium chloride. The anode and cathode were
arranged to have suitable current collectors in contact
therewith, and the beta alumina was arranged so that it form-
ed a continuous barrier between the electrolyte and sodium.
Test cells of this nature were used in the follow-
ing examples as illustrated in the accompanying drawings,
in which:
Figure 1 shows a Tafel Plot at the start of discharge
of a cell for Example 1, voltage versus current being shown,
and the current being shown logarithmically;
Figure 2 shows, for the cell of Example 1, a plot of
voltage against capacity for the 12th charge/discharge cycle
of the cell of Example 1 whose Tafel Plot is shown in Figure l;
and
Figure 3 shows a plot, similar to Figure 2, of
voltage against capacity for the cell described in Example 2.
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EXAMPLE 1
A 5 g disc-shaped sintered iron electrode having a diameter
of 30 mm and a thickness of 3 mm was chlorinated chemically by
reaction with chlorine gas and heated under vacuum to sublime
off volatile FeC13. From the uptake of chlorine, the discharge
capacity was calculated ~o be approximately O,6 Amp hr. It
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should be noted that experimental capacity was found to be in
good agreement with calculated capacity.
It was found that the charge-dtscharge process could represented
as fallows:
2Na+ + (Fe2~Cl-) conductin~ +2e discharge, (Fe~ conducting ~2Na+C1~
charge -2e matrix
In the electrolyte melt Na+ is the charge-carrying species.
Reduction and oxidation of the iron takes place at the
conducting matrix, with which the iron makes electronic
contact. Charge transfer was found to be rapid and the cathode
was found to tolerate high current densities, in excess of
150 mAcm 2, with little cell polarisation. Figure 1 shows a
Tafel Plot at the start of discharge of voltage vs current for
the cell, current being shown logarithmically. The ohmic
internal resistance-free plot shows the absence of polarisation
up to current densities in excess of 150 mAcm 2, at a
tempeFature of 180C.
Figure 2 shows the twelfth charge-discharge cycle, ie:
Charge: 2,38 V - 2,60 V, 5 hour rate
Discharge: 2,28 V - 1,96 V, 5 hour rate
; 20 Capacity: 740 J g 1 ~excluding electrolyte)
Coulombic efficiency: 100%
Temperature: 230C
Current Density: 50 mA/cm
Open Cîrcuit Vol-tage (O.C.V.~: 2,35 V.
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EXAMPL~ 2
5 g of Fe3C (having a 325 mesh average particle size obtained
from Cerac Inc.) together with 0,5 g phenol formaldehyde binder
(obtained from Polyresin Products (Pty) Limited) were intimately
mixed and then pressed in a uniaxial press (about 34 500 kPa)
into a pellet which was heated under argon for a period and at
a temperature (eg about 3 hours at 1000C) sufficient to
effect breakdown of the Fe3C to alpha iron and graphite. The
only identifia~le crystalline products found were indeed alpha
iron and graphite. The artifact was then chlorinated, as
described with reference to Example 1, and the chlorine uptake
gave an estimated capacity of 0,5 Amp hr, based on the
calculated quantity of 1ron chloride present. As in Example 1,
the experimental capacity was found to ~e in good agreement
~ith the calculated capacity.
. .
: Figure 3 shows the eleventh charge-discharge cycle of this
cell, i.e.:
; ~'
Charge: 2,64 - 2,90 V, 5 hour rate
Discharge: 2,05 - 1,70 V, 5 hour rate
~;~ 20 Capacity: 540 J g 1 (excluding electrolyte)
Coulombic efficienc~: 100%
Temperature: 230C
Current Density: 50 mA/cm
Open Circuit Voltage (O.C.V.): 2,35 V.
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The present invention shows striking advantages,
particularly as regards current density and the absence of any
high internal resistance caused by polarization at high current
densities, when compared with similar cells where the electro-
chemically active cathode material is soluble in the electrolyte.In the case of the latter, concentration polarization takes
place and high internal resistances are encountered, so that
only low curren-t densities can be tolerated, rendering these
cells unsuitable for high power applications such as automotive
propulsion.
