Note: Descriptions are shown in the official language in which they were submitted.
t32B~
THIS INVENTION relates to an electrochemical
cell. In particular, it relates to an electro~
chemical cell of the type having two or more
reactive components at least one of which is liquid
at the operating temperature of the cell, the
components being separated from one another and being
capable of reacting together with potentially un-
desirable consequences; and the invention relates
also to a method of reducing the potential hazard
constituted thereby.
Accordlng to one aspect there is provided an
electrochemical cell which comprises at least two
reactive components which are liquid in the operating
temperature of the cell, the components being sepa-
rated from one another and being capable of reactingtogether with potentially undesirable consequences
to form at least one reaction product which is solid
at the operating temperature of the cell and to which
they are inert, at least one of the said reactive
components being separa-ted from the cathode of the
cell and being impregnated into a macroporous
material in at least one zone where contact between
the reactive components can take place, thereby to
enhance the ability of any such reaction product
formed in said zone to separate said components from
one another.
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The potential hazard which the invention
is intended to guard against would arise from
failure and particularly damage to a cell, which can
arise for example from an accident, such as a motor
accident when the cell forms part of the propulsion
system of a motor vehicle~ Reactive components in
an electochemical cell, when they come into contact
with one another, form reaction products to which
the components are inherently inert. Such reaction
products will tend to separate the reactive
components from one another, and the purpose of
providing the macroporous material in the cell is to
strengthen and/or trap said reaction products, in
order, as mentioned above, to enhance the ability of
the reaction products to separate the reactive
components from one another, to reduce the
potentially undesirable consequences arising from
the reaction of the reactive components with one
another, as the result of failure or accidental
damage to the cell.
The porous materiàl may be particulate, ie
fibrous or granular, eg so as to provide a body,
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layer or bed of particles which is macroporous.
Macroporous in this context means that the material
will be impregnated by a liquid cell component in
the sense of impregnation by capillary action or the
like, as contrasted with microporous materials such
as atomic sieves provided by zeolites or the like,
which are porous at the atomic or molecular level,
and are impregnated by mechanisms other than
ordinary capillary reaction.
The particles may be loose or unconnected
to one another, and capable of movement relative to
one anothex. In this case the porous material may
have a different density from that of the reactive
component with which it is in contact, occupying
part of the space occupied by said component, so
that the porous material forms a layer in the
component at the operating temperature of the cell.
Thus, for example, if contact between the components
is anticipated at a high elevation or level in one
of the components, the particles of the porous
component may have a lower density than said
component so that they form a floating bed in it at
said high level. On the othex hand, if contact is
anticipated at a low elevation or level, the porous
material may have particles of a greater density
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than that of the component, so that they settle to
form a bed or layer at the bottom of the component.
On the other hand, the particles, while
separate from one another, may be held in contact
with one another, by being forced together in a
compacted mass which is held together by compression
and is impregnated by said component. In this case
the porous material may occupy substantially the
whole of the space occupied by the reactive cell
component with which it is impregnated.
Instead~ the porous material may be in the
form of an artifact, eg an artifact which comprises
woven or felted fibrous material.Instead, th~ porous
artifact may be one having a sponge-like structure,
such as one made by a sintered bed of particles.
The artifact may thus be a sintered artifact.
As mentioned above, the component of the
cell which is impregnated into or surrounds and
saturates the porous material will be a liquid, at
least at the operating temperature of the cell. The
porosity of the porous material is thus preferably
such as to provide for unrestricted diffusion
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therethrough of the component with which it is in
contact at the operating temperature of the cell.
In general, while frequently the entire
body or mass of the component in question will be
impregnated into the porous material so as to
surround and/or saturate it, the invention will
often be applied by providing the porous material
only in a zone or 7.ones in the cell, where contact
between reactive components is expected or
anticipated, in the event of certain anticipated
types of damage to the cell.
.
When reactive cell components come into
contact with each other in a gentle or orderly
fashion, it can happen that a layer of reaction
products is created between them, effectively
sepaxating them from each other and preventing or
reducing any ~urther reaction between the
components. However, in a more or less catastrophic
or violent accident, causing substantial damage to
the cell, the components can become more or less
thoroughly intermixed, particularly when they are
both liquid, thus leading to a violent,
uncontrollable and possible catastrophic reaction.
In such case, even if a layer of reaction products
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between the components has been created, the layer
can be destroyed, possibly repeatedly, if the
accident or damage to the cell is sufficiently
violent, making possible further contact between the
components, and aggravating the undesirable
consequences.
~ owever, in accordance with the present
invention, the presence of particles such as fibres
in a zone where contact between the components takes
place and reaction products are formed, can act to
reinforce the layer of reaction products and render
it less liable to subsequent mechanical failure; or
the porous material can entrap and immobilise the
reaction products (as with granular particles or
porous sponge-like artifacts) thereby enhancing the
ability of the reaction products to separate the
components. Depending on the porous material used,
such reinorcement a~nd entrapment can both be
present.
Two of the reactive cell components may be
liquid at the operating temperature of the cell,
being separated from one another by a separator wlth
which they are both in contact, a layer of the
porous material being provided on at least one side
of the separator, at the interface between the
. . .
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separator and the component on that side of the
separato.r.
The separator may be a solid electrolyte,
the components on opposite sides thereof comprising
anode material and liquid electrolyte material
respectively. The layer may be provided at the
interface between the liquid electrolyte and solid
electrolyte, and the anode material-may comprise an
alkali metal and/or an alkaline earth metal, the
liquid electrolyte being a molten salt electrolyte
comprising one or more alkali metal and/or alkaline
earth metal halide salts, and the layer of porous
material comprising non-conductive fibrous material.
As examples of this type of cell to which
the invention can be applied, there are cells having
an anode and a cathode, a solid electrolyte
separating the anode from the cathode, and a liquid
electrolyte in the cathode department which acts as
ionic conductor without otherwise taking part in the
cell reaction but which is capable of reacting
violently with the active anode material. In such a
case, the active anode material may for example be
liquid sodium, the solid electrolyte being beta
alumina (beta-A1203), and the electrolyte being
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molten sodium aluminium chloride (NaAlC14),
surrounding the cathode material which may be in the
form of a solid, porous, active matrix. The beta
alumina electrolyte acts effectively as a
separator which separates the sodium aluminium
chloride from the sodium anode material, this
liquid electrolyte/anode combination comprising two
substances which are extremely reactive with each
other, and which, upon accidental contact in the
event of damage, can react dangerously with each
other, depending on the degree of intermixing
therebetween.
In accordance with the invention, a layer
of porous material as described above, for example
in the form of a fibrous structure may be provided
in zones in the cell, where the liquid electrolyte
is able or is anticipated, in the event of damage,
to react with the anode material, for example in the
event of fracture of the solid electrolyte. The
inert reaction products formed by such reaction
would tend to be reinforced by the fibrous
structure, to form a fibre-reinforced barrier
resistant to penetration by either the liquid
electrolyte or the active anode material, one or
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both of which will be liquid at the operating
temperature of the cell.
Although the fibrous structure can be
disposed at the interface between the solid
electrolyte and the liquid electrolyte as described
above, it can equally easily be disposed throughout
the space in the cathode compartment which contains
the liquid electrolyte. Further such fibrous
material may also be provided in the anode
compartment, eg throughout the anode compartment or
at the solid electrolyte/anode material interface.
For cells of this type, the fibrous
structure is preferably non-conductive, as described
above, and may comprise ceramic material, alumina,
glass fibres, or the like.
For cells which have a cathode which is
intended to be of the type comprising an artifact,
eg a sintered artifact, which is porous to liquid
electrolyte, fibrous material can also in accordance
with the invention be incorporated into the cathode
material prior to formation of the artifact, thereby
aiding the sintering process, and binding of the
cathode material.
,.
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A method of reducing the potential hazard
constituted by contact between at least two reactive
components in an electrochemical cell which are liquid
at the operating temperature of the cell, the compo-
nents being separated from one another and beingcapable of reacting together with potentially un-
desirable consequences to form at least one reaction
product which is solid at the operating temperature
of the cell and to which they are inert, the method
comprising providing in at least one zone in the cell
where contact between the reactive components can take
place ~ macroporous material which is impregnated
with at least one of the reactive components which is
separated from the cathode of the cell, thereby to
enchance the ability of the reaction product to
separate said components from one another.
This invention will now be described, by
way of example, with reference to the accompanying
diagrammatic drawings, in which
Figure 1 shows a sectional slde elevation of a
vessel containing cell components tested by the
Applicant;
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Figure 2 shows a sectional side elevation of a
cell in accordance with the present invention; and
Figure 3 shows a sectional side elevation of a
further cell in accordance with the present
invention.
In Figure l, reference numeral lO
generally designates a suitable heated vessel into
which liquid sodium, designated by reference numeral
12 was charged at 250C, and after and on top of
which liquid sodium aluminium chloride, designated
14, was also charged at 250C. These components
were added at a gentle and controlled rate, and
separated into layers, as shown in Figure l.
The addition was carried out under argon,
with the vessel 10 held at a temperature o~ 250C.
The sodium and sodium aluminium chloride were found
to react rapidly at this temperature/ and
exothermically, to glve solid reaction products
forming the layer 16, in accordance with the
reaction:
NaAlC14 ~ 3Na ~ ~ 4 NaCl + Al.
With gentle addition of the components, the layer 16
was found to form in a fashion such that it
effectively separated the components from one
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another, without an unacceptable temperature
increase. The solid barrier formed by the reaction
product served to slow down reaction between the
components rapidly, and the temperature increase was
found to be limited to less than 50C.
It was further found that when the barrier
16 was badly ruptured, eg by stirring, a violent
reaction took place with a rapid rise in temperature
of up to 1000C, demonstrating the danger inherent
in violent damage to cells containing the sodium and
sodium aluminium chloride, in a fashion that
extensive and rapid mixing between these components
takes place.
With reference to Figure 2, re~erence
numeral 18 generally designates an electrochemical
cell in accordance with the invention. The cell
comprises a housing 20 within which is located
molten sodium 22. A cup-shaped beta-alumina solid
electrolyte separator 24 is partially immersed in
the sodium 22, within which is located a solid
active cathode matrix 26 in turn immersed in the cup
2~ in molten sodium aluminium chloride liquid
electrolyte 28. The housing 20 acts as an anode
current collector, and a cathode current collector
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13
30 is provided, embedded in and extending upwardly
away from the matrix 26, and out of the electrolyte
28 and housing 20, from which it is insulated at 32.
In accordance with the invention, graphite
felt (not shown) has been used to line the inner
surface of the cup 24, where it is saturated by the
liquid electrolyte 28. In the event of rupture or
breakage of the cup 24, sodium 22 penetrating into
the cathode compartment constituted by said cup 24,
reacts with the liquid electrolyte in the felt
lining, according to the reaction given above, to
provide sodium chloride and aluminium as reaction
products in a layer which is reinforced by the
fibres of the carbon felt. It has been found that
this carbon fibre reinforced layer of reaction
products is much more robust and difficult to
rupture than a corresponding layer of unreinforced
reaction products without the carbon fibres, eg
during subsequent vibration or impacts to which the
cell may be subjected.
The Applicant further contemplates, in
addition, the possibility of entirely filling the
space of the cakhode compartment occupied by the
liquid electrolyte- with such felt. However, if it
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14
is found that there is a danger of premature
over-discharge, particularly in the zone in the
liquid electrolyte 28 above the cathode matrix 26,
it would be preferable to use fibres which are
electrically insulating, such as ceramic fibres,
alumina fibres, or glass fibres, the fibres
naturally being selected to be stable with regard to
reaction with the liquid electrolyte. If a mat or
wool of such fibres is employed, they can act
partially to immobilize the melt 28, thereby
enhancing safety, although the Applicant believes
that, from this point of view, the use of particles
instead of fibres, as in a powder saturated by the
liquid electrolyte, may be a better immobilizer.
Similarly, a layer of suitable fibrous
material, inert to sodium at the temperature in
question, can be used to line the outer suxface of
the cup 24, to guard against penetration of liquid
electrolyte 28 into the sodium 22 of the anode, by
forming a fibre reinforced layer of reaction
products on the outside of the cup; or such fibrous
material can fill the whole of the anode space.
In Figure 3, a similar cell is shown, and
unless otherwise specified, the same reference
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numerals refer to the same parts. In this case, the
matrix 26 is omitted, and the cathode compartment is
filled with molten sodium aluminium chloride with,
as active species dissolved therein, SbCl5. In this
case, too, the invention can be applied analogously,
in the same fashion as described above for Figure 2.
A further aspect contemplated by the
invention is reduction of the danger of an
undesirable reaction (with xeference particularly to
Figure 2) between the liquid sodium anode material
and liquid electrolyte, such as sodium aluminium
chloride, when the electrolyte is impregnated into
the porous cathode matrix. The degree of
impregnation of the liquid electrolyte into the
matrix presently attained is insufficient to lead to
a violent reaction between the liquid electrolyte
impregnated into the matrix material and the sodium,
but if increased available porosity is developed for
the matrix, the sodium aluminium chloride
impregnated therein may increase to a level where
reaction thereof with the sodium, in the event of
breakage to the matrix and contact thereof with the
sodium, could become a problem. To guard against
this, similar fibres could be incorporated in the
pores of the matrix to reduce the danger. Such
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16
fibres would be added during the manufacturing
stage, being mixed for example with powders from
which the matrix is formed, prior to fabrication and
sintering. The presence of such fibres can also aid
in the control of the sintering, and help bind the
cathode matrix together during subsequent operation.
Furthermore, if a porous artifact such as
a mineral sponge impregnated by capillary action, or
a bed (possibly sintered) of particles, is used in a
fashion similar to the fibres described above, e.g.
as an inner and/or outer lining to the cup 24, or
filling the whole of the anode or cathode
compartments, then should rupture of the cup lead to
penetration of the anode compartment by liquid
electrolyte or penetration of the cathode
compartment by anode material, reaction products
would be trapped in a layer in the artifact or bed,
at the interface with-the cup. This will serve to
immobilize and strengthen the layer of reaction
products formed, enhacing its ability to separate
the liquid electrolyte and anode material, and if
the artifact or bed contains fibres, fibre
reinforcement of the layer can be obtained as well.
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17
Although electrochemical cells according
to the invention have been described with reference
to cells of cylindrical shape in the drawings, it
will naturally be appreciated that cells of other
shapes, eg flat cells, can easily be made in
accordance with the invention.
