Note: Descriptions are shown in the official language in which they were submitted.
li3~S26
--2--
ELECTROCHEMICAL CELL AND THE PROTEC-
TION OF AN ELECTROCHEMICAL CELL
THIS INVENTION relates to an electrochemical
cell and to the protection of an electrochemical cell.
Electrochemical cells within a battery thereof
are capable of failure and have been known to fail. In
high temperature electrochemical cells in particular,
such failure can cause catastrophic safety hazards.
The larger the electrochemical cell or battery thereof,
the worse this problem becomes.
For battery driven cars, to drive an average car
for about 300 Km, a battery storing in the region of
50 KW hrs is required. Should all this energy or a
substantial proportion Ihereof, suddenly be released,
then the battery would be subject to catastrophic and
explosive failure. Such failure can easily become a
reality if some of the presently availahle high tempera-
ture batteries are installed in vehicles as is intended,
and such vehicles crash.
' - 11385Z~
--3--
Leading contenders for commercial vehicle batteries
are sodium/sulphur batteries and lithium/sulphide
batteries. In the medium term, alkaline batteries
typified by zinc/nickel oxide batteries, are also
contenders.
~ or such batteries, a very high energy density is
re~uired, which can only be met by using combinations
of the most reactive chemical substances, such as the
extremely electropositive elements lithium or sodium,
together with extremely electronegative elements such
as sulphur, chlorine or the like. The mere use of such
materials creates intrinsic safety problems, which
safety problems exist in the exploitation of such
batteries from the research stage, through development
and to commercial production and use thereof, for
example in vehicles.
In such batteries, the chemical reaction between
the reactive species in the battery is controlled to
provide useful electrical energy. However, in the
event of such reaction's becoming uncontrolled, catastrophic
and dangerous safety hazards can arise. This can occur
for example in a motor vehicle crash, which forces the
reactive materials of the battery electrochemical cell
~138S~
anodes and cathodes together. Such failures can also
occur from corrosion of the cell housing which can
occur over a long period of in service use or storage,
and thermal cycling which can cause stresses and strains
which can crack brittle seals and other brittle components,
such as solid electrolytes in sodium/sulphur cells.
Said corrosion or thermal cycling will generally
cause a particular cell to undergo exothermic destructive
failure. When the cell is used in a battery, it is
thus necessary to prevent such individual cell failures
from spreading to neighbouring cells and hence causing
the catastrophic runaway cascade failure of the
entire battery.
Means which have been proposed of overcoming the
problem of catastrophic cell/battery failure all involve
significant disadvantages in terms of additional undesired
weight or volume requirements. Thus vermiculite and
similar minerals have been used to surround such batteries
and cells, acting merely as low density, non-imflammable
thermal insulation, in a physical fashion on a macro-
scopic level, rather than by dealing with cell contents
on a molecular or microscopic level or in a chemical
fashion.
1138526
--5--
Conse~uently, unless the problem of catastro-
phic battery failure can be solved, commercial end use of
many of presently proposed batteries under development, may
be restricted to technically competent end-users, and
safety hazards can prevent them from entering the important
mass market of vehicle production.
According to one aspect of this invèntion there
is provided a method of reducing the potential hazard
presented by e~ectrochemical cell contents escaping from an
electrochemical cell, which method includes the step of
associating a sorbent micromolecular sieve material with
an electrochemical cell to sorb the electrochemical cell
contents escaping from the cell, the micromolecular sieve
material being sealed off from said cell contents and the
cavities in the micromolecular sieve material being sub-
stantially free of said cell contents, and said material
having an average window size of less than 1 micron lead-
ing into its internal cavities the window size being
selected such as to permit rapid sorption of the cell
contents into said cavities, effectively to reduce the
potential hazard.
The cell contents are potentially dangerous and
hazardous electrochemical reactants and~or reaction products.
The molecular sieve material may conveniently be
provided in at least one sealed layer which is sealed and
which surrounds, or partially surrounds, the cell casing.
1~38S26
In an embodiment of the invention, the
molecular sieve material may be provided in a sealed
layer between tAe cell casing and an outer sealing wall
which seals the matertal~
In an alternative embodiment of the invention,
the molecular sieve material may be provided in a
sealed holder which is shaped to receive or removably
to receive a cell casing, the holder comprising sealingly
connected inner and outer sealing walls between which
the molecular sieve material is sealed.
The sealing walls may conveniently be of
fracture resistant material, such as a flexible or
resiliently flexible material, or a deformable or
resiliently deformable material to resist fracture
under impact.
The sealed molecular sieve material may, if
it contains sorbed water as in zeolites, conveniently
be at least partially dehydrated to improve its capacity
to sorb electrochemical cell contents.
1138526
In an embodiment of the invention, the sealed
molecular sieve material may be evacuated so as to improve
its thermal insulation properties.
In an alternative embodiment of the invention,
the sealed molecular sieve material may include an inert
gas which is inert in relation to the electrochemical cell
contents.
The inert gas may thus, for example, be a noble
gas, nitrogen, carbon dioxide or the like.
According to another aspect of this invention
there is provided an electrochemical cell which has a
sorbent micromolecular sieve material associated there-
with for sorbing electrochemical cell contents escaping
from the cell thereby reducing the potential hazard presented
by such escaping contents, the micromolecular sieve material
being sealed off from said cell contents and the cavities in
the micromolecular sieve material being substantially free of
said cell contents, and said material having an average
window size of less than 1 micron leading into its internal
cavities the window size being selected such as to permit
rapid sorption of the cell contents into said cavities,
effectively to reduce the potential hazard.
--, ..
",, .
1~385Z6
While this invention can have application in
regard to various types of electrochemical cells
including electrochemical cells in the form of fuel
cells, it can have particular application in regard to
electrochemical cells which utilize electrochemical
substances or electrolytes, or which produce reaction
products which are hazardous upon escaping from a cell,
or which are potentially hazardous if they react with
one another or the air outside a cell environment.
Thus, for example, the invention can have
particular application in regard to high temperature
cells, in regard to molten electrolyte cells, in
regard to cells employing molten or potentially hazardous
electronegative and/or electropositive substances, or
the li~e.
The micromolecular sieve material of this
invention may be any material which is capable of
rapidly sorbing electrochemical cell contents which
escape from a cell to reduce the potential hazards
presented by such contents by retaining them in a
dispersed form to allow them to cool or to reduce the
possibility or extent or rate of their reaction with
air or with one another.
il38S26
g
Micromolecular sieve materials are materials
which have molecular cavities in the form of cages,
pores or channels, with the cavities having windows
leading to them, the windows, cages, pores and channels
having an average size of not more than 1 Micron,
preferably less than 100 Angstroms and typically less
than 20 Angstroms.
The window sizes of the material should
therefore be sufficiently large to permit ready entry
of the electrochemical cell contents to be sorbed by
the material, and the cavities should preferably be
such that the cell contents can be held sufficiently
captive therein for a sufficient period to reduce the
potential hazards presented by such contents.
Furthermore, the micromolecular sieve material
may be chosen so that it has a pore size which allows
only one of the species of cell contents to be sorbed
therein, such sorbed species thus being isolated from
the other cell contents.
Various types of natural and synthetic
molecular sieve materials are known and they are
widely used in industry for purification, scavenging
1138526
--10--
and separation. Furthermore, because of the demand
for these materials, they are being thoroughly
investigated and new molecular sieve materials are
being developed and manufactured throughout the world.
The micromolecular sieve material may be
present in any convenient form, e.g. powder, pellets,
porous artifacts or porous bodies.
By taking into consideration factors such as
window size, cavity size, the ability rapidly to sorb
electrochemical cell contents, and the ability to hold
such contents captive sufficiently dispersed to reduce
the potential hazards presented by such contents, a
rough guide will be provided for the selection of
appropriate molecular sieve materials.
Further factors which can serve as a guide,
can be the degree of porosity, the density, the
availability, the cost, and the stability of the molecular
sieve materials.
1138SZ6
--ll--
The exact type of molecular sieve chosen will
depend on the exact chemistry of the cell in question.
Thus the molecular sieve should be:
(a) stable at temperatures above the working
temperature range of the cell;
(b) the micromolecular sieve material may be
stable against reaction with any of the
contents of the cell which may be emitted
therefrom. However, if the sieve material
does react with such contents, it should
preferably do so endothermically; and
(c) the sieve material should be low in density,
inexpensive and preferably a good thermal
insulator.
On the basis of factors such as these,
molecular sieve materials such as clathrates, carbon
molecular sieves, composite carbon molecular sieves,
and certain sorbent natural or synthetic mineral
substances (i.e. mineral molecular sieves) such as
tectosilicates, modified tectosilicates, and tecto-
silicate-like substances may be considered.
li38S26
-12-
Tectosilicates are particularly fayQured f~r
use with sodium/sulphur high temperature cells, as
many tectosilicates readily sorbed sodium, sulphur and
sodium polysulphides. Zeolitic tectosilicates are
particularly preferred in this respect.
Suitable mineral micromolecular sieve carriers
thus may be selected from the group of substances which
make up the tectosilicates, i.e. the class of substances
also known as "framework silicates" which may be
natural or synthetic, crystalline or non-crystalline/amorphous,
and which include:
(a) silicates such as silica gel
(b) zeolites
(c) felspars and
(d) felspathoids
(b), (c) and (d) being silicates of a structural type
in which all four oxygen atoms of the silicate tetrahedra
are shared with neighbouring tetrahedra. The framework
of the tectosilicate is made up of silicon with in some
cases aluminium atoms, together with other atoms.
Mineral micromolecular sieve carriers include also
mixtures o~ or analogues of tectosilicates in which the
silicon and~or aluminium atoms of the framework may be
substituted by atoms of one or more of:
1138S~6
iron
beryllium
boron
phosphorous
carbon
germanium and
gallium
in minor or major proportions and wherein the micro-
molecular sieving characteristics and properties are
maintained.
Many tectosilicates are cheap and abundant and are
essentially non-imflammable. In use according to the
invention they may, if desired, be mixed with substances
which are not molecular sieves, e.g. glass fibre or
vermiculite, which act as fillers to provide additional
volume and thermal insulation properties.
In a specific embodiment of the invention, the
molecular sieve material may comprise natural or
synthetic zeolites, or modified zeolites which have
been physically or chemically modified but still
possess appropriate molecular cavities for sorbing
electrochemical cell contents.
1138SZ6
Zeolites contain water molecules which may be
removed, usually reversibly, by heat and/or evacuation.
Some modified zeolites may be particularly suitable,
particularly those known as "superstable" or "ultrastable"
zeolites. One such group of ultrastable or superstable
zeolites is formed by a decationisation process which
results in the formation of a zeolite-like molecular
sieve material that is stable up to 1000C (see Donald
W. Breck - "Zeolite Molecular Sieves" - Wiley Interscience,
1973).
Where zeolites (or other hydrated tecto-
silicates) are therefore used as the molecular sieve
materi~l in this invention, they may conveniently be
dehydrated or at least partially dehydrated to improve
their ability to sorb electrochemical cell contents.
An advantage in using dehydrated zeolites or
either dehydrated tectosilicates, is that they readily
sorb water and hence in use according to the invention
will act to prevent rusting or similar water-aided
corrosion of the cell container. This water can get
~138526
-15-
in, for example in the eyent of a failure of the outer
battery casing.
Without wishing to limit the scope of this
application/ it may be noted that "zeolites" are
usually identified as the class of crystalline or
amorphous natural or synthetic materials which contain
aluminium and silicon in fairly definite proportions,
and their analogues. For a more detailed discussion
of zeolites reference can be made to the January 1975
publication of the International Union of Pure and
Applied Chemistry entitled: "Chemical Nomenclature, and
Formulation of Compositions, of Synthetic and Natural
Zeolites".
In a specific embodiment of the invention,
the molecular sieve material may be in the form of a
typical zeolite such as zeolite 3A, zeolite 4A, zeolite
13X, or the like.
In an alternative embodiment of the inyention,
for example/ the zeolite may be in the form of naturally
occurring zeolite crystals selected, for example, from
t~e group comprising erionite and faujasite crystals.
1138S~6
-16-
It will be appreciated that this invention
can be applied to electrochemical cells as well as to
batteries of such cells.
Where the invention is applled to a battery
of cells, molecular sieve material layers may be
associated with each of the cells and/or with the
battery itself.
In accordance with the invention associating
the sorbent micromolecular sieve material with the
electrochemical cell may be by having means (activated,
for example automatically, by cell failure) arranged to
apply the micromolecular sieve material to the cell or
its vicinity. Thus, a container similar to a fire
extinguisher provided with propellent and micromolecular
sorbent sieve material could be associated with the
cell, and connected to the cell so that cell failure
causes such dispenser to spray the micromolecular sieve
material over and around the cell when failure occurs.
Embodiments of the invention are now described
and illustrated by way of example with reference to
accompanying drawings, in which:- .
11385~6
Figure 1 shows a fragmentary, diagrammatic
representation of a typical electrochemical
cell which has been protected in accordance with this
invention to reduce the potential hazards presented by
electrochemical cell contents escaping from the cell;
Figure 2 shows a diagrammatic sectional side
elevation of an experimental electrochemical cell
protected according to the invention; and
Figure 3 shows a sectional end elevation of
the cell of Figure 1, in the direction of line III -
III in Figure 2.
With reference to the drawing, reference
numeral 10 refers generally to a typical electrochemical
cell.
In the embodiments illustrated, the cells 10
are each in the form of a high temperature cell comprising
a molten sulphur/polysulphide cathode 12, a molten
sodium anode 14, and a solid ~ -alumina or nasicon
electrolyte 16 which separates the anode 14 from the
cathode 12.
-18-
The cell 10 includes a casing 18 wherein the
cell contents are sealingly housed.
The cell 10 includes a conducting terminal 20
for the cathode 12, and a conducting terminal 22 for
the anode 14.
It will be appreciated that, in accordance
with this invention, the typical cell 10 may instead,
for example, be in the form of a cell which has a
molten or liquid electrolyte, and which has an anode or
cathode which is solid or liquid.
The cell 10 is completely surrounded by a
layer 24 of dehydrated zeolite 13X crystals which are
maintained in a sealed condition by an outer sealing
wall 26.
The outer sealing wall 26 is preferably of a
fracture resistant, lightweight material which will
resist fracture particularly under impact.
In use, if any of the electrochemical
reactants of the cell 10 escape from the cell 10
either as a result of corrosion of the casing 18 or
1138526
--19--
as a result of an impact, electrochemical cell contents
escaping from the cell 10 will be effectively sorbed by
the zeolite layer 24, and should be held captive
sufficiently to allow them to cool down or to combat a
violent reaction upon contact with each other, or upon
exposure to the air, thereby reducing the potential hazard
presented by the escape of such electrochemical contents.
The embodiments of the invention as illustrated
in the drawings can provide the advantage that they can
reduce the potential hazard presented by electrochemical
contents escaping out of an electrochemical cell either
as a result of corrosion or as a result of an impact
which inverts the cell or fractures the cell components.
The embodiments of the invention can therefore
reduce the potential health and fire hazards resulting
from a vehicle to which such cells are fitted, being
involved in an accident.
The embodiments of the invention can provide
the further advantage that they can reduce the risk of
a cascade effect as a result of cell failure in a
1138526
-20-
multi-celled battery. Thus, for example, where one
cell of a battery fractures as a result of an impact or
as a result of corrosion, corrosive cell contents can
be captured, thereby reducing the risk of a reaction
which can damagingly heat adjacent cells or reducing
the risk of the corrosive materials corroding adjacent
cells.
The embodiments as illustrated in the drawing
can provide the further advantage that the zeolite
layer can also act as light and effective thermal
insulator for the cell. This is particularly advan-
tageous for high temperature cells.
The embodiments as illustrated in the drawing
can provide the further advantage that zeolite is
relatively cheap and plentiful. In addition, while
such a layer and its sealing wall will add additional
masq to a cell, the mass should be relatively insignificant
in relation to the advantages which can be provided.
Although not shown in the drawings it will be
appreciated that instead of the constructions shown, the
layer 24 can be provided in a separate holder. This holder
will have the layer 24 sandwiched between an outer sealing
wall such as the wall 26 shown in the drawings, and an inner
sealing wall (not shown~ of similar construction. The
) 1138526
-20(a)-
holder will have a hollow interior, defined by the inner
sealing wall, for receiving the cell, and access for the
cell to the hollow interior will be provided, optionally
so that the cell is later removable.
, . ~,.............................. . .
