Method for forming solid electrolyte composite

A solid electrolyte composite is provided which is suitable for use as the separating member between a sodium reservoir source and a sulfur reservoir source in a sodium-sulfur battery; the solid electrolyte composite is manufactured by providing a membrane of a crystalline ionic conductive sodium polyaluminate on a portion of a porous, anhydrous crystalline supporting body consisting of alpha-Al.sub.2 O.sub.3. The sodium polyaluminate is initially applied in the form of a precursor which is then subsequently heated to form the ionic conductive crystalline sodium polyaluminate membrane.

THE INVENTION 
The present invention is related to solid electrolytes and, more 
particularly, to methods for forming such solid electrolyte structures. 
Still more particularly the present invention is directed to sodium 
polyaluminate electrolyte structures finding utility in sodium sulfur 
batteries. 
Sodium polyaluminate solid electrolytes for use, for example, in 
sodium-sulfur batteries are well known in the art. For example in U.S. 
Pat. No. 3,901,733 a thin film solid electrolyte is described of a thin 
film beta-alumina on a porous beta-alumina substrate. As is well known in 
the art beta-aluminas are ionic conductive sodium polyaluminates which may 
generally be represented by the formula Na.sub.2 O.XAl.sub.2 O.sub.3 
wherein X generally ranges from about 5 to about 11; generally included in 
the terminology beta-aluminas, or ionic conductive sodium polyaluminates, 
are compositions of the formula ZMO.sub.y.Na.sub.2 O.XAl.sub.2 O.sub.3, 
the latter sometimes being more commonly referred to as beta double prime 
aluminas, i.e. B"Al.sub.2 O.sub.3. The MO.sub.y refers to effective 
stabilizing amounts of metal oxides such as for example lithium oxide, 
magnesium oxide, and calcium oxide and Z generally corresponds to a molar 
amount equivalent to from zero, or about 0.5%, by weight up to about 5% by 
weight of the total composition. X generally ranges from about 5 to about 
11 with Y, of course, being an integer corresponding to the valence of M. 
In contrast to the ionic conductive sodium polyaluminates anhydrous, 
crystalline, alpha-Al.sub.2 O.sub.3 is non-conductive. U.S. Pat. No. 
3,900,381 and a related article entitled "The Electrophoretic Forming of 
Beta-Alumina Ceramic" by R. W. Powers, Journal of the Electrochemical 
Society, April, 1975, describes electrophoretic methods of forming 
beta-alumina articles which are useful as solid electrolytes in, for 
example, sodium-sulfur batteries. That patent, and article, describes the 
electrophoretic deposition of beta-alumina by first employing a grinding 
process which involves the use of alpha-alumina as a grinding media and 
describes the sintering of green beta-alumina ware in alpha-alumina 
saggers. U.S. Pat. No. 3,499,796 discloses an energy storage device which 
is a sandwich type structure having a central polycrystalline member; 
Example 2 of this patent for example discloses a central member which is 
formed of fusion cast beta-alumina/alpha-alumina eutectic. U.S. Pat. No. 
3,404,036 describes an energy conversion device having a solid crystalline 
electrolyte separator of beta-alumina. The American Ceramic Society 
Bulletin Vol. 50, No. 7, p.615 discloses contacting beta-alumina 
structures with beta-alumina. An abstract appearing in America Ceramic 
Society Bulletin, April, 1975, Vol. 54, No. 4, at page 453 indicates the 
formation of beta-aluminas from alkoxides at temperatures in excess of 
about 950.degree. C. None of the foregoing patents or articles are 
believed to describe or recognize the present inventive contribution. 
In accordance with the present invention an improved method, and composite 
produced by that method, is provided for forming a solid electrolyte 
composite which comprises contacting a surface of a porous, anhydrous, 
crystalline body consisting of alpha-Al.sub.2 O.sub.3 with a precursor of 
an ionic conductive crystalline sodium polyaluminate and heating the 
contacted surface, which of course will have thereon the precursor, at a 
temperature and for a time sufficient to convert the precursor to a 
crystalline ionic conductive membrane of sodium polyaluminate. As used 
herein the term sodium polyaluminate comprehends a composition of the 
formula ZMO.sub.y.Na.sub.2 O.XAl.sub.2 O.sub.3 wherein MO.sub.y is a metal 
oxide which is present in an amount sufficient to thermally stabilize the 
poly-aluminate composition, Y is an integer dependent upon the valence of 
M, X is an integer varying between about 5 and 11 and Z is as indicated 
above. An exemplary sodium polyaluminate is represented by the formula 
Na.sub.2 O.11Al.sub.2 O.sub.3 with another exemplary polyaluminate being a 
stabilized specie represented by the formula .24LiO.sub.2.1Na.sub.2 
O.6.3Al.sub.2 O.sub.3. Other suitable stabilizing metal oxides include for 
example calcium oxide and magnesium oxide. On a weight basis, based on the 
total aluminate composition, the amount of lithium oxide or magnesium 
oxide or calcium oxide will be from about 0% up to about 5% by weight and 
more typically, when employed, these stabilizing oxides will be present in 
an amount between about 0.5% by weight to about 5% by weight. As 
indicated, the precursor once applied to the surface of the porous, 
anhydrous, crystalline alpha-Al.sub.2 O.sub.3 body will be heated at a 
temperature and for a time sufficient to convert the precursor to a 
crystalline ionic conductive sodium polyaluminate and to sinter the sodium 
polyaluminate so as to form a barrier layer, or membrane portion, on the 
alpha-Al.sub.2 O.sub.3 body which, for example, is impermeable to the 
passage of molten sulfur. 
The precursor of the ionic conductive crystalline sodium polyaluminate in 
one embodiment may be the hydrolysis product, with water, of hydrolyzable 
compounds of the formula Na(OR) and Al(OR).sub.3 and, when it is desired 
to employ effective stabilizing amounts of other metal oxides, for example 
stabilizing amounts of lithium oxide or calcium oxide or magnesium oxide, 
the precursor will optionally also be the hydrolysis product of 
hydrolyzable compounds of the formula Li(OR), or Ca(OR).sub.2, or 
Mg(OR).sub.2, or CaAl(OR).sub.8 or MgAl(OR).sub.8 wherein R is an alkyl 
group of 1 to 5, preferably 2-5, carbon atoms, for example the ethyl 
radical, or R is a radical of the formula --CH.sub.2 --CH.sub.2 O--R' 
wherein R' is an alkyl group of 1 to 4 carbon atoms, for example methyl, 
ethyl, propyl, and butyl. These compounds are conveniently prepared for 
example by the reaction of the metal with an alcohol to form an alkoxide, 
or alcoholate, or by the reaction of the metals with the organic solvents 
generally available in the art under the Cellosolve designation to form 
alcoholates which may be viewed as cellosolvates. In the preferred 
practice, the hydrolysis product of the hydrolyzable compounds indicated 
above will be formed by hydrolyzing these compounds, with water, in the 
presence of an organic solvent diluent such as for example alkyl alcohols, 
e.g. of up to 5 carbon atoms, or the methyl, ethyl, propyl, or butyl 
Cellosolves with the amount of water generally being at least 1 mole per 
OR group. Thus, for example if Na(OR) and Al(OR).sub.3 are employed, three 
moles of water will typically be employed per mole of the aluminum 
compound and 1 mole of water will typically be employed per mole of the 
sodium compound. Other suitable organic solvent diluents, for example 
benzene, will be routinely selected by those skilled in the art. In 
another embodiment the precursor of the crystalline ionic conductive 
sodium polyaluminate will be obtained by hydrolyzing the hydrolyzable 
compounds of the type indicated above and then drying the hydrolysis 
product to produce an amorphous, particulate, precursor which upon further 
heating converts to the crystalline, ionic conductive sodium 
polyaluminate. In an especially suitable technique, the surface of the 
porous, anhydrous, crystalline body consisting of alpha-Al.sub.2 O.sub.3 
will be contacted with a precursor of an ionic conductive crystalline 
sodium polyaluminate in which the precursor represents a combination of 
hydrolyzable compounds indicated above with the precursor being applied in 
a substantially anhydrous organic solvent solution. In this embodiment, 
after contacting the surface, the solution will be heated to allow solvent 
evaporation which, in all likelihood will be somewhat coupled with partial 
hydrolysis because of air in the environment, and then further heated to 
convert the precursor to a crystalline ionically conductive sodium 
polyaluminate membrane which will be impermeable to, for example, molten 
sulfur. 
The anhydrous crystalline porous body consisting of alpha-Al.sub.2 O.sub.3 
is preferably a body having a porosity in the range of about 25% to about 
60% and an average pore size diameter in the range of about 0.03 to about 
2 microns. Preferably the anhydrous crystalline porous body consisting of 
alpha-alumina will have a porosity between about 35 to about 60%. The 
porous alumina body may be in tubular form but it can likewise be in other 
suitable form such as for example a sheet. Exemplary modes of forming the 
anhydrous crystalline porous body consisting of alpha-alumina is described 
in copending application U.S. Ser. No. 605,499 filed Aug. 18, 1975, now 
abandoned, which is hereby incorporated by reference, said application 
being a continuation of application U.S. Ser. No. 474,693 filed on May 30, 
1974 now abandoned. According to that application crystalline anhydrous 
bodies of the type contemplated are formed by extrusion of an alumina 
paste followed by heating, including sintering, to form an anhydrous, 
crystalline, porous body consisting of alpha-alumina. The paste comprises 
alumina, a binder, and water and the binder can also include suitable 
wetting agents. After extrusion, the sheet or tubes are then dried and 
then heated at an elevated temperature to effect sintering and the 
formation of the anhydrous crystalline porous body.

As will be seen the porous crystalline alpha-Al.sub.2 O.sub.3 body carries 
an ionically conductive crystalline sodium polyaluminate membrane, or 
layer, with a portion of the membrane being generally diffused into the 
porous alpha-Al.sub.2 O.sub.3 body per se and, in exaggerated fashion, 
includes a thin dense layer of the crystalline sodium polyaluminate at 
what may be considered the surface of the porous alpha-Al.sub.2 O.sub.3. 
While the temperatures and time of heating the precursor and then further 
heating to effect sintering in the formation of the membrane will vary, 
generally, it will be found quite suitable to effect the formation of the 
ionically conductive crystalline sodium polyaluminate from the precursor 
by heating at temperatures in excess of about 1000.degree. C., quite 
suitably temperatures in the range of about 1200.degree. C. to about 
1650.degree. C., for times which may vary from several minutes, for 
example 5 minutes, to as much as 24 hours. Additionally the heating may be 
effected in air or it may be effected in a controlled isolated environment 
such as for example by encapsulating with platinum. 
While the above is believed to set forth the invention with sufficient 
particularity to enable those skilled in the art to make and use same, 
nonetheless, there follows a few detailed examples which are included only 
for purposes of further exemplification. In these examples an anhydrous, 
crystalline, porous tubular body consisting of alpha-Al.sub.2 O.sub.3 and 
having a porosity of about 50% is employed and is made in accordance with 
the teachings of U.S. Ser. No. 605,499. 
EXAMPLE 1 
A precursor of a 1Na.sub.2 O.11Al.sub.2 O.sub.3, sodium polyaluminate, was 
prepared by first reacting about 2.3 grams of sodium metal with about 50 
milliliters of ethylene glycol monomethylether (methyl Cellosolve) to form 
a sodium cellosolvate. To this solution there was then added about 271 
grams of aluminum tri (secondary-butoxide) and about 1500 ml of benzene. 
With vigorous stirring approximately 50 grams of water was then added to 
hydrolyze the hydrolyzable aluminum and sodium compound and this resulted 
in the formation of a precipitate. The precipitate was filtered and dried 
by heating at a temperature of about 500.degree. C. for about 8 hrs. to 
produce a particulate, amorphous precursor of a crystalline ionic 
conductive sodium polyaluminate. 
The dried hydrolyzed precursor was then heated at a temperature of about 
1200.degree. C. in air for about 24 hours and X-ray analysis showed the 
presence of beta-alumina and alpha-Al.sub.2 O.sub.3 with the beta-alumina 
being around 70% and the alpha-alumina around 30%. Similar results were 
obtained when heating in air at about 1450.degree. C. for about 30 
minutes. When heated for about 20 minutes, while being encapsulated in 
platinum, at about 1585.degree. C. the dried hydrolyzed precursor showed 
the presence of around 80% of beta-Al.sub.2 O.sub.3 and around 20% of 
alpha-Al.sub.2 O.sub.3. 
A crystalline, porous, tubular body consisting of alpha-Al.sub.2 O.sub.3, 
having a porosity of about 50%, is then employed to form a composite solid 
electrolyte as contemplated herein. The dried hydrolyzed precursor is 
contacted, or applied, to one of the surfaces of the tube, and then the 
contacted surface is heated, for example in the range of about 
1500.degree. C. to about 1650.degree. C. for about 15 minutes, to form a 
layer, or membrane, of a crystalline ionic conductive sodium 
polyaluminate. This structure is ideally suited for use in sodium-sulfur 
batteries. 
EXAMPLE 2 
A precursor of a sodium polyaluminate in an organic solvent solution was 
prepared by reacting about 47.8 grams of aluminum metal with about 500 
grams of methyl Cellosolve at reflux for about 2 to 3 hours followed by 
cooling to about room temperature. To this there was then added about 6.45 
grams of sodium metal and about 0.46 grams of lithium metal to produce a 
solution of a hydrolyzable cellosolvate which is a precursor to a lithia 
stabilized sodium polyaluminate. This solution of the combined respective 
LiO.sub.2, Na.sub.2 O and Al.sub.2 O.sub.3 providing precursor was then 
made up to about 800 grams with an additional quantity of methyl 
Cellosolve and, on a theoretical dry metal oxide basis, the solution 
contained about 90.3% by weight Al.sub.2 O.sub.3, about 8.7% by weight 
Na.sub.2 O, and about 1.0% by weight of Li.sub.2 O, and generally 
corresponds to a precursor of a sodium polyaluminate of the formula 0.24 
Li.sub.2 O.1Na.sub.2 O.6.3Al.sub.2 O.sub.3. This solution of the 
hydrolyzable precursor was then heated and the table hereinbelow 
summarizes the heating schedule along with the formed crystalline specie. 
______________________________________ 
Crystalline Specie 
Temperature 
Time Atmosphere Formed 
______________________________________ 
1200.degree. C. 
24 hr air beta-Al.sub.2 O.sub.3 
1200.degree. C. 
5 min air beta-Al.sub.2 O.sub.3 
1200.degree. C. 
5 min platinum 
encapsulated 
beta-Al.sub.2 O.sub.3 
1450.degree. C 
30 min air beta-Al.sub.2 O.sub.3 
1450.degree. C. 
5 min air beta-double prime 
B"Al.sub.2 O.sub.3 
1585.degree. C. 
20 min. platinum 
encapsulated 
of beta-Al.sub.2 O.sub.3 
______________________________________ 
A surface of another portion of a tube, like that of Example 1, is 
contacted with the organic solvent solution of the hydrolyzable precursor 
indicated immediately above and the contacted surface is then heated to 
evaporate the solvent and is then further heated at a temperature of about 
1500.degree. C. to about 1650.degree. C. for about 15 minutes so as to 
produce a crystalline ionically conductive membrane of a sodium 
polyaluminate which is well adapted for use in a sodium-sulfur battery. 
The embodiment of employing the hydrolyzable precursor in an organic 
solvent solution will be found to be especially suitable and convenient 
for utilization. 
It may be stated that some of the advantages of the present inventive 
contribution reside in ease of forming various high strength structural 
configurations, e.g. sheets, squares, tubes, etc., economy of manufacture, 
lower processing times and temperatures and the provision of a membrane 
allowing for a high sodium flux, making the composite ideally suited for 
sodium-sulfur batteries, and a membrane of high homogeneity. 
While the above describes the present invention it will, of course, be 
apparent that modifications are possible which pursuant to the patent 
statute and laws do not depart from the spirit and scope of the invention.