Alkaline cation conductive vitreous composition and a method of preparing such a composition

A cation conductive vitreous alkaline composition and a method of preparing such a composition. The composition corresponds to a formula of aP.sub.2 S.sub.5, bLi.sub.2 S, cLiX, where: PA1 X represents chlorine, bromine or iodine; PA1 c is greater than or equal to 0; PA1 the ratio b/(a+b) lies between 0.61 and 0.70; and PA1 the ratio c/(a+b+c) is less than or equal to a limit which corresponds to solubility in the vitreous phase of LiX in the composition aP.sub.2 S.sub.5, bLi.sub.2 S.

FIELD OF THE INVENTION 
The invention relates to a vitreous composition which conducts alkaline 
cations, said compound being used as a solid cation electrolyte; the 
invention also relates to a method of preparing such a composition. 
BACKGROUND OF THE INVENTION 
Solid electrolytes are used in various electrochemical units such as 
electric cells and display devices. Their advantage over liquid 
electrolytes is that they simplify technological problems which arise in 
use, and in particular problems concerning sealing. Unfortunately known 
solid electrolytes which conduct alkaline cations have relatively low 
conductivity at temperatures lower than 100.degree. C. so that the 
performance of electric cells which contain such electrolytes is very much 
lower than that of electric cells with a liquid electrolyte. 
The use of various vitreous compositions that conduct cations has already 
been recommended, these compositions being included in the general formula 
aA.sub.m Y.sub.n, bM.sub.2 Y, cMX, where: 
A represents boron, silicon, germanium or phosphorus; 
Y represents oxygen or sulphur; 
M represents lithium or sodium; and 
X represents chlorine, bromine or iodine and c can be equal to or greater 
than zero. 
The conductivity at 25.degree. C. of all compositions of this type known up 
to now is less than 10.sup.-5 .OMEGA..sup.-1 cm.sup.-1. 
The present invention aims to provide a cation-conductive vitreous 
composition which is more conductive at ordinary temperatures. 
SUMMARY OF THE INVENTION 
The invention provides a cation conductive vitreous composition of general 
formula aP.sub.2 S.sub.5, bLi.sub.2 S, cLiX where X is an element chosen 
from the group consisting of chlorine, bromine and iodine and where a, b 
and c are numbers chosen so that the ratio b/(a+b) lies in the range 0.61 
to 0.70 and the ratio c/(a+b+c) is less than or equal to a limit value 
corresponding to the maximum solubility in the vitreous phase of LiX in 
the composition aP.sub.2 S.sub.5, bLi.sub.2 S. 
Therefore, the composition in accordance with the invention differs from 
previous ones in particular by the following details: 
simultaneously, we have Y=S and M=Li; and 
the ratio b:a is close to 2 where A=P 
Unexpectedly, conductivity at 25.degree. C. is then about 10.sup.-4 
.OMEGA..sup.-1 cm.sup.-1 where c=0 and even higher where c&gt;0. If X is 
bromine, said limit value of the ratio c/(a+b+c) is about 0.20. For 
iodine, it reaches about 0.50. 
The invention also provides a method of preparing the above composition, 
the method comprising melting together the compounds P.sub.2 S.sub.5, 
Li.sub.2 S and, if need be, LiX at a temperature which lies between 
700.degree. and 950.degree. C., followed by tempering and then by 
annealing at a temperature of about 100.degree.. 
This method can include incorporating a quantity of LiX in the melt 
exceeding its solubility in the glass so as to obtain an intimate mixture 
of glass saturated with LiX and of crystallized LiX.

DESCRIPTION OF PREFERRED EMBODIMENTS 
EXAMPLE 1 
1 part of P.sub.2 S.sub.5 and 2 parts of Li.sub.2 S in powder form are 
mixed together in a silica phial under a dehydrated argon atmosphere. The 
parts are measured in moles. The phial is sealed under a vacuum of 
1.33.times.10.sup.-5 bars and is heated to about 900.degree. C. for 10 
minutes. The melted mixture is slowly cooled to 700.degree. C. then 
tempered at ambient temperature and then annealed at 100.degree. C. Glass 
with a formula of P.sub.2 S.sub.5,2Li.sub.2 S, i.e. a ratio of 
b/(a+b)=(2/3) is thereby obtained. 
The variation in the conductivity of this glass as a function of 
temperature is shown by curve A in the FIGURE, which curve represents the 
decimal logarithm of the conductivity .GAMMA. in .OMEGA..sup.-1 cm.sup.-1 
as a function of 1000/T; where T is the temperature in degrees Kelvin. 
Conductivity at 25.degree. C. is 10.sup.-4 .OMEGA..sup.-1 cm.sup.-1. 
An electric cell was prepared, with a 1 mm thick disk of this glass between 
a negative lithium electrode and a positive electrode which included 50% 
of PbI.sub.2 and 50% of lead by weight. The voltage of this cell is 
2.1-volts. 
EXAMPLES 2 TO 5 
The process of example 1 is repeated with mixtures of powders which always 
have a molar ratio of Li.sub.2 S/P.sub.2 S.sub.5 equal to 2, 
(b/[a+b]=2/3), but further containing LiI at molar fractions (c/[a+b+c]) 
of 0.15, 0.25, 0.40 and 0.45 respectively for examples 2 to 5. The 
conductivities of glass obtained are given by curves B, C, D and E in the 
FIGURE. In particular, the conductivity at 25.degree. of glass containing 
45% LiI is 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1, i.e. more than 100 times 
higher than that of known cation-conductive glasses. 
EXAMPLE 6 
The same process is applied using, for 1 mole of P.sub.2 S.sub.5, 2 moles 
of Li.sub.2 S and 0.75 moles of LiBr, i.e. a molar ratio of 0.2 for LiBr. 
The conductivity at 25.degree. C. of this glass is 1.4.times.10.sup.-4 
.OMEGA..sup.-1 cm.sup.-1. 
The molar fraction 0.2 corresponds to the maximum solubility of LiBr in the 
vitreous phase, whereas this limiting ratio is substantially equal to 0.5 
for LiI. If these ratios are exceeded, a mixture is obtained of glass 
saturated with LiX and of crystallized LiX, in which mixture the 
crystallized LiX performs the function of a binder and facilitates the use 
of solid electrolyte. Further, the glass contained in the mixture has 
optimum conductivity since, as shown by the curves in the FIGURE, 
conductivity is an increasing function of the rate of LiX. 
As for the molar ratio of Li.sub.2 S/P.sub.2 S.sub.5, it may be different 
from the value 2 without loss of homogeneity, and vary between 1.56 and 
2.33 corresponding respectively to 0.61 and 0.70 for b/(a+b) in the 
formula aP.sub.2 S.sub.5, bLi.sub.2 S, cLiX. 
The ingredients can be melted at any temperature between 700.degree. C. and 
950.degree. C. However, glass obtained at a relatively high temperature, 
e.g. 900.degree. C., is clearer than that obtained at a lower temperature. 
Slow cooling down to 700.degree. C. makes it possible to combine all of 
the P.sub.2 S.sub.5 whose vapour pressure is relatively high at 
900.degree. C. 
The glasses in accordance with the invention can be used as solid 
electrolytes in lithium cells whose active material is, for example, 
MoO.sub.3, WO.sub.3, TiS.sub.2, NiPS.sub.3 or a bismuthate of copper or 
lead. 
Because of their fragility, the glasses are preferably used in the form of 
plates sintered from a compressed powder obtained by crushing the glass. 
However, the relatively low softening temperature of the glasses in 
accordance with the invention (about 140.degree. C. for glasses rich in 
LiI) also makes it possible to envisage putting a continuous film of 
softened glass into use by hot compression. Another way of applying the 
invention is to associate the powdered glass with a binder which promotes 
compacting and cohesion of the mixture. Halides of lithium, for example, 
are included among the binders which can be used, which halides have the 
advantage of being themselves conductors of lithium ions, although to a 
lesser extent than glasses in accordance with the invention. As seen 
above, the glass-crystallized lithium halide mixture can advantageously be 
obtained by inserting an excess of halide when the glass is being prepared 
.