Cathode structure for sodium sulphur cells

In a sodium sulphur cell having a cathodic region containing sulphur/polysulphides forming a cathodic reactant between a solid electrolyte and a cathode current collector, the electronically conductive matrix containing the cathode reactant is formed of a porous metal structure, preferably metal fibres, coated with graphite or carbon to form an electronically conductive protective coating.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a cathode electrode structure for a sodium 
sulphur cell. 
In a sodium sulphur cell, a solid electrolyte material separates molten 
sodium, forming the anode, from a sulphur/polysulphide cathodic reactant. 
In such a cell the solid electrolyte is a material, such as betaalumina, 
which conducts sodium ions. On discharge of the cell, the sodium gives up 
electrons at the anodic interface of the solid electrolyte and the sodium 
ions pass through the solid electrolyte into the cathode adjacent the 
electrolyte. In the cathodic region, these sodium ions have to combine 
with sulphide ions to form a sodium polysulphide. The electrons pass 
through the sodium to the anode current collector and thence around an 
external circuit to a cathode current collector, e.g. a carbon or graphite 
tube or rod, in the cathodic reactant. The electrons must pass from this 
cathode current collector to the region of the cathode adjacent the 
surface of the solid electrolyte where they react with the sulphur to form 
sulphide ions. The sulphide ions and sodium ions form a polysulphide. The 
electronic conductivity of molten sulphur is low and hence it is the 
practice to pack the cathodic region with a fibrous carbon or graphite 
material which provides the required electronic conductivity. 
2. Prior Art 
Carbon or graphite has been used for this packing because of the highly 
corrosive nature of the cathodic reactant comprising molten sulphur and 
sodium polysulphides, which reactant, when the cell is in operation, is 
typically at a temperature of the order of 350.degree.. Metals such as 
stainless steel are corroded in this environment. The carbon or graphite 
may be in the form of loose fibres or the fibres may be in the form of a 
felt or a woven cloth; another form of carbon which has been employed is 
reticulated vitreous carbon. The carbon or graphite material has to form a 
matrix, through which the liquid cathodic reactant can move. The 
polysulphides formed by the electrochemical reaction have to be 
transferred away from the neighbourhood of the electrolyte on discharge of 
the cell and have to be transferred to this region on charging of the 
cell. The matrix however must constitute an electronic conductor to 
transfer electrons from the reaction zone to the cathode current collector 
when charging the cell and to provide the required electronic current path 
between the cathode current collector and the regions near the surface of 
the electrolyte where the sulphide ions have to be formed on discharge of 
the cell. 
Examples of the use of graphite or vitreous carbon as a conductive matrix 
material in the cathode electrode of a sodium sulphur cell are shown in 
U.S. Pat. Nos. 3,966,492, 3,980,496, 3,985,575 and 4,002,807. As is 
explained in U.S. Pat. No. 3,993,503, certain advantages can be obtained 
by utilising two different materials, disposed in different parts of the 
cathodic region. One, for use in charging the cell, is preferentially 
wetted by the sulphur and the other, for use on discharge, is 
preferentially wetted by the polysulphides. For the former material, it is 
proposed to use graphite felt or foam on porous graphite or vitreous 
carbon foam or pyrolytic graphite felt or foam or other unspecified 
materials covered or coated with such felt or foam. The material to be 
preferentially wetted by the polysulphides must essentially have surface 
properties differing from those of carbon or graphite; they may for 
example be oxide materials. Such materials, in general, are even less 
conductive than carbon or graphite and one of the problems with the use of 
two different materials, one effective during charging and the other 
effective during discharge of the cell, is the reduction in overall 
conductance in the cathodic region. 
SUMMARY OF THE INVENTION 
It is one of the objects of the present invention to provide an improved 
form of cathode structure in a sodium sulphur cell. More particularly, it 
is an object to provide a much more highly conductive matrix material 
which will withstand the corrosive conditions in the cathodic region of a 
sodium sulphur cell. 
According to this invention, in a sodium sulphur cell having a cathodic 
region containing sulphur/polysulphides forming the cathodic reactant 
between a solid electrolyte and a cathode current collector, there is 
provided a porous packing to form an electronic conductor which packing is 
constituted at least partly of a porous metal structure, e.g. a metal 
fibre material or a porous metal material coated with graphite or carbon 
forming an electronically conductive protective coating. 
Metal with a carbon or graphite coating has a very much better electronic 
conductivity than the graphite or carbon felt or fiber material used on 
its own such as has been the common practice in sodium sulphur cells. One 
of the major advantages of sodium sulphur cells, compared with batteries 
in common use at the present day, is the ability to provide a very large 
output current per unit volume of cell. The conductance of the carbon 
matrix material in the cathodic region constitutes one of the limitations 
on the performance of sodium sulphur cells. The use of a carbon or 
graphite coated metal matrix in a cathodic region enables a substantially 
higher electronic conductivity to be obtained compared with the use of 
graphite or carbon fibres or felt yet, by the use of this coated material, 
the matrix is still able chemically to withstand the highly corrosive 
condition in the cathodic region of the cell. 
The matrix material may comprise, for example, steel wool coated with 
graphite or carbon. A preferable form of construction is to have a 
substrate of chromium alloy, for example a nickel-chromium alloy such as 
that known under the Trade Mark "Nichrome" or a nickel-chromium iron alloy 
such as that known under the Trade Mark "Inconel". Such a material has 
quite a high resistance to corrosion in the cathodic reactant if there 
should be any flaws in the carbon or graphite coating. Obviously, however, 
other metals may be employed. A higher conductance may be obtained using 
aluminium fibres as the substrate. Composite constructions may be 
employed, for example aluminium or copper fibres with an interface layer, 
conveniently of nickel-chromium alloy, between the substrate and the 
carbon or graphite coating to give further protection for the substrate. 
In another form of construction, a porous metal structure, for example a 
porous nickel element, is coated with carbon or graphite. Such a porous 
nickel element may be similar to the porous electrodes used in fuel cells 
where a gaseous reactant has to be fed into the cell at the electrodes 
surface. Dual or multiple-porosity structures, as known in the fuel cell 
art, and coated with carbon as described above, may be employed. A rigid 
metal matrix with a carbon coating may be combined with carbon or graphite 
felt to provide flexibility to assist assembly or to provide a less 
active, more resistive layer close to the electrolyte surface. 
The carbon or graphite coating might be applied to loose fibres which may 
then be employed as the matrix in the sodium sulphur cell. The fibres may 
be randomly oriented or they may be oriented in a direction of preferred 
conductance, e.g. axially or radially in a cylindrical cell. The fibres, 
after coating with carbon or graphite may be needled to form a felt 
similar to the known types of carbon or graphite felt. Sodium sulphur 
cells are commonly of cylindrical form with the cathodic reactant in an 
annular region either inside or outside the electrolyte tube. A felt as 
described above may be inserted into such an annular region in the form of 
washers or it may be bent into a cylinder or cylinders or wrapped 
helically within or around the electrolyte tube or laid in the form of a 
strip extending longitudinally along the surface of the electrolyte tube. 
The coating of the substrate with carbon may be effected using known carbon 
deposition techniques, for example a chemical vapour deposition technique. 
It may be preferred in some cases to deposit two or more superposed layers 
of carbon on the substrate. In some cases, one or more intermediate layers 
of other material or materials may be deposited on the substrate before 
applying an outer carbon layer or layers. 
Such an intermediate layer may be used, for example, for ease of 
fabrication or to obtain better adherence despite differences in 
coefficients of thermal expansion of the carbon and the substrate. If the 
metal fibres are of a good conductor, such as aluminium or copper, which 
is readily attacked chemically by the cathodic reactant, an intermediate 
layer of a material having greater resistance to corrosion, e.g. an 
iron-nickel-cobalt alloy may be put over the aluminium or copper. The 
carbon or graphite layer and the intermediate layer are, in this case, of 
such thickness as to form an impermeable protective coating. 
The above-described carbon or graphite coated material may in some cases be 
mixed with an electrically non-conductive material, for example in the 
manner described in the specification of co-pending U.S. application Ser. 
No. 768,929, filed Feb. 15, 1977, to form a composite matrix.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates diagrammatically a sodium sulphur cell having a solid 
electrolyte tube 10, formed of betaalumina, which tube is closed at one 
end and supported in a metal case 11 constituting the anode current 
collector. Sodium 12 fills the annular space around the electrolyte tube 
10. Inside this tube is a graphite rod 13 forming the cathode current 
collector. The space between the rod 13 and electrolyte tube 10 is packed 
with a porous structure 14 forming an electronically conductive matrix 
which is filled initially with sulphur. The cell is operated at a 
temperature, typically 300.degree. C. to 400.degree. C., at which the 
sodium and sulphur are molten. 
The present invention is concerned more particularly with the porous 
structure 14. In this embodiment, this structure is formed by packing into 
the annular region fibres of graphite-coated metal fibres. The metal is 
preferably a nickel-chromium alloy such as that sold under the Trade Mark 
"Nichrome" but other materials may be used, e.g. a nickel-chromium-iron 
alloy such as that sold under the Trade Mark "Inconel". Aluminium or 
copper may be used with an interface of nickel chromium alloy between the 
substrate and the carbon or graphite. Staple fibres are used of a length 
of a few centimeters and preferably between 10 and 100 microns diameter. 
The fibres may be packed into the cell as randomly oriented loose fibres 
but preferably they are needled to form a felt similar in form to the 
carbon and graphite felts already known for use in sodium sulphur cells. 
In another arrangement, the fibres are woven into a cloth which is stacked 
or folded to the required thickness. If it is formed into a felt having a 
porosity of about 80 to 95% which is that of conventional carbon or 
graphite felts, this material will have substantially greater bulk 
conductance than conventional carbon or graphite felts. Using 
nickel-chromium or nickel-chromium-iron fibres the improvement in 
conductance is about tenfold, with aluminium fibres, the conductance is 
about one hundred times that of carbon fibres. 
The felt may be inserted into an annular cathodic region of a sodium 
sulphur cell in the known way, for example in the form of washers, or a 
sheet bent into a cylinder or wrapped helically within or around the 
electrolyte tube or in the form of strips, preferably of trapezoidal 
section, laid over the surface of the electrolyte tube parallel to the 
axis thereof. The matrix material may be impregnated with molten sulphur 
after insertion in the cell or an electrode assembly may be formed of the 
matrix material impregnated with sulphur, which assembly is then inserted 
in the cell. 
The felt density or fibre thickness may be graded across the electrode 
region to control the distribution of the reaction rate. Metallic felt may 
be used in combination with carbon or graphite felt to produce more 
pronounced gradations in conductance or activity. 
If loose fibres are used, they may be packed into the cell before sulphur 
impregnation. The loose fibres may however be moulded with molten sulphur 
into the required shape for incorporation into the electrode and cooled to 
solidify the assembly before putting this assembly into the cell. 
If loose fibres are used, it is convenient to sandwich these fibres between 
layers of a thin cloth, conveniently a woven cloth. FIG. 2 illustrates 
such an assembly with loose fibres 20 sandwiched between thin sheets of a 
woven cloth 21. The sandwich typically is 1 to 10 mm thick. This 
facilitates handling of the matrix assembly and, in particular, 
facilitates compression of the fibres when packing the assembly into a 
cell. 
By sandwiching the loose fibres between cloth, the structural integrity of 
a needled felt can be obtained. In some cases, however, it may be 
preferred to needle the coated fibres. Conveniently as shown in FIGS. 2 
and 3, this composite is formed into a number of elongate elements, such 
as elements 23, 24, joined along their length by the layers of cloth 21 so 
that the assembly can be formed into an annular unit to fit within the 
cell. The assembly may be impregnated with sulphur before or after putting 
it in the cell. 
The woven cloth 21 may be formed from carbon fibres or metallic fibres or 
of an insulating material such as the alumina material sold under the 
Trade Mark "Saffil" or it may be formed of a mixture of materials. 
By using nickel-chromium alloy fibres having a bulk resistivity of 0.1 ohm 
core, compared with about 1 ohm cm of a graphite felt, in a typical 
cylindrical electrode, 3 cm in diameter and 7 mm in annular width, a 
threefold increase in conductance of the composite electrode is obtained.