Alkaline Cation Conductive Vitreous Composition and a Method of Preparing Such a Composition

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.

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.

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>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.

The invention will be better understood from the description of embodiments given hereinafter by way of nonlimiting illustration with reference to the accompanying drawing in which the sole FIGURE shows conductivity curves of compositions in accordance with the invention as a function of temperature.

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.

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.

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 .