Vitreous Solid Lithium Cation Conductive Electrolyte

1. Technical Field

The invention relates to a solid state electrolyte based on vitreous lithium cation conductor of the composition:

where

x is from 0.8 to 1.5,

y is from 0 to about 2 and

wherein said composition has a conductivity of at least 0.75.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

2. Background of the Art

Ionic conductivity is usually associated with the flow of ions through a liquid solution of salts. In the vast majority of practical uses of ionic conductors, i.e., as electrolytes for dry cell batteries, the liquid solution is immobilized in the form of a paste or gelled matrix or is absorbed in a separator to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the use of a large volume of immobilizing material has hindered the aims of miniaturization.

In addition, improved microelectronic circuit designs have generally decreased the current requirements for electronic devices. This in turn has enhanced the applicability of solid electrolyte power sources which usually can deliver currents only in the microampere range. These solid electrolyte systems have the inherent advantages of being free of electrolyte leakage and internal gassing problems due to the absence of a liquid phase. In addition, they also have a much longer shelf life than the conventional liquid electrolyte power sources.

In attempting to avoid the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds seeking to find compounds which are solid at room temperature and have specific conductances approaching those exhibited by the commonly used liquid systems. Solid electrolytes must be essentially electronic insulators so as not to internally short the cell while at the same time they must allow ionic migration in the crystal lattice for the cell to operate. It has been discovered that certain metallic salts which are solids at room temperature have specific conductances sufficiently high to permit their use in practical battery applications. For example, U.S. Pat. No. 3,723,185 discloses solid state electrolytes of compounds conforming to the general formula AgI-MCN-AgCn or modifications thereof where M is potassium, rubidium, cesium or mixtures thereof.

U.S. Pat. No. 4,331,750 discloses a cation conductive vitreous composition suitable for use as a solid electrolyte and having the general formula:

where:

X represents chlorine, bromine or iodine;

c is greater than or equal to 0;

the ratio b/(a+b) lies between 0.61 and 0.70; and

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.

This reference also discloses that the use of various vitreous compositions that conduct cations has been recommended and that these compositions are included in the general formula:

where:

A represents boron, silicon, germanium or phosphorus;

Y represents oxygen or sulfur;

m represents lithium or sodium; and

X represents chlorine, bromine or iodine and c can be equal to or greater than zero.

However, this reference further states that the conductivity at 25.degree. C. of all compositions of the above type which were known was less than 10.sup.-5 ohm.sup.-1 cm.sup.-1 and that the P.sub.2 S.sub.5 -based compositions had a higher conductivity at ordinary temperature.

However, even though P.sub.2 S.sub.5 -based compositions are recited as forming good conductivity, their synthesis must be performed in sealed pressure vessels due to the volatility of P.sub.2 S.sub.5. The high pressures generated at the fusion temperature of about 950.degree. C. for P.sub.2 S.sub.5 -based materials coupled with the extreme reactivity of molten sulfides with common pressure vessel materials is a formidable technological obstacle to producing this solid electrolyte on a commercial basis for solid state cell systems.

It is an object of the present invention to provide a SiS.sub.2 -based vitreous lithium cation conductor which can be produced, at atmospheric pressure, which has a conductivity of at least 0.75.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C. and which is suitable for use as a solid electrolyte in solid state cell systems.

Another object of the present invention is to provide a solid electrolyte comprising a vitreous lithium cation conductor of the composition:

where

x is from 0.8 to 1.5,

y is from 0 to about 2 and said composition having a conductivity of at least 0.75.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

Another object of the present invention is to provide a solid electrolyte comprising a vitreous lithium cation conductor of the composition SiS.sub.2, Li.sub.2 S, LiI and having a conductivity of at least 10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

The foregoing and additional objects will become more fully apparent from the following description.

The invention relates to a solid electrolyte comprising a vitreous lithium cation conductor of the composition:

where

x is from 0.8 to 1.5,

y is from 0 to about 2 and said vitreous lithium cation conductor having a conductivity at 25.degree. C. of at least 0.75.times.10.sup.-4 ohm.sup.-1 cm.sup.-1, preferably at least 10.sup.-4 ohm.sup.-1 cm.sup.-1.

Preferably, x should be about 1 to about 1.2 and y should be about 0.75 to about 1.2. As used herein, vitreous shall mean a composition in a glassy (non-crystalline) state and shall also include materials that have been cooled so rapidly from the molten condition that crystal formation is prevented.

The conductivity of SiS.sub.2, Li.sub.2 S, LiI (LiI.Li.sub.2 SiS.sub.3) vitreous material according to this invention was found to be at least 6.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C. The conductivity of Li.sub.2 SiS.sub.3 vitreous material according to this invention can be at least 0.75.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C., preferably at least 0.9.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C. Accordingly, the conductivity of the vitreous materials of this invention having a "y" value above 0 will be greater than 0.75.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

Vitreous Li.sub.2 SiS.sub.3 can be prepared at 1 atmosphere pressure by first mixing silicon disulfide (SiS.sub.2) and lithium sulfide (Li.sub.2 S) in 1:1 mole ratio in an inert gas filled dry box. The mixture is then placed in a vitreous carbon crucible which in turn is placed in a inert gas reaction chamber. The SiS.sub.2 and Li.sub.2 S mixture is heated at an elevated temperature for a time period sufficient for the SiS.sub.2 to react with the Li.sub.2 S to form Li.sub.2 SiS.sub.3. Generally, the SiS.sub.2 and Li.sub.2 S mixture can be heated at about 950.degree. C. for about 6 hours. The Li.sub.2 SiS.sub.3 is then quenched to ambient temperature (about 20.degree. C.) to form the vitreous solid. If desired, lithium iodide (LiI) can be added by grinding together the vitreous Li.sub.2 SiS.sub.3 and LiI, placing the mixture in a vitreous carbon crucible and then heating the mixture at an elevated temperature for a time period sufficient to form a solid solution of LiI in the Li.sub.2 SiS.sub.3 material. The material is then quenched to room temperature. Generally the mixture can be heated at about 950.degree. C. for about 6 hours and then quenched to room temperature.

Anode materials suitable for use with the solid electrolyte of this invention include lithium, silver, sodium, potassium, and rubidium. The preferred anode material is lithium.

Cathodes suitable for use with the solid electrolyte of this invention include poly(N-vinylpyrrolidone) (PVP)+iodine, PVP+iodine+TiS.sub.2, FeS.sub.2, Sb.sub.2 S.sub.3, TiS.sub.2, organic charge transfer complexes with halogens, and MnO.sub.2.

Vitreous Li.sub.2 SiS.sub.3 was prepared by mixing 67 grams SiS.sub.2 and 33 grams Li.sub.2 S in 1:1 mole ratio in a helium filled dry box. The mixture was placed in a vitreous carbon crucible which in turn was placed in a vitreous silica reaction tube. The tube was closed and was equipped with a conventional exhaust vent and a small conventional intake feed tube adapted for feeding helium into the reaction tube. The SiS.sub.2 and Li.sub.2 S mixture, under a 1 atmosphere pressure of helium, was heated at 950.degree. C. for 6 hours and then the reaction tube was quenched to room temperature (20.degree. C.) by submerging the reaction tube in cold water. The vitreous Li.sub.2 SiS.sub.3 solid electrolyte produced was then ground, pelletized and tested in the conventional manner. It was found to have a conductivity of about 0.73.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

Specifically the conductivity was measured by pelletizing (via the application of uniaxial pressure) the powdered material between titanium disulfide electrodes at 13,000 psi in a conventional steel mold with steel rams. The disc of solid electrolyte with TiS.sub.2 electrodes was ejected from the mold and then heat sealed in a polyethylene bag. The sample sealed in the bag was then placed in an alcohol filled polytetrafluoroethylene cylinder with a tight fitting lid which was in turn placed in a large steel mold fitted with steel rams. The alcohol filled polytetrafluoroethylene cylinder containing sample was then compressed to 54,000 psi which results in an isostatic compression of the glass sample disc and its associated electrodes. The TiS.sub.2 /solid electrolyte/TiS.sub.2 sample was placed in a spring-loaded holder fitted with gold contacts. The conductivity of the sample was measured using the complex plane technique first applied to solid electrolytes by J. E. Bauerle, J. Phys. Chem. Solids, 30, 2657 (1969). The complex plane technique is at present almost universally applied for the measurement of conductivity of solid electrolytes.

A vitreous Li.sub.2 SiS.sub.3 was produced as in Example 1 except that it was produced in a 10 gram batch. The conductivity was found to be 0.97.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

51 grams of the vitreous Li.sub.2 SiS.sub.3 produced as in Example 1 was combined with 49 grams of LiI. The mixture was ground, placed in a vitreous carbon crucible as in Example 1 and then placed in a reaction tube. Under 1 atmosphere helium pressure, the Li.sub.2 SiS.sub.3 and LiI mixture was heated at 950.degree. C. for 6 hours and then the reaction tube was quenched to room temperature (20.degree. C.) by submerging the reaction tube in cold water. The vitreous LiI.Li.sub.2 SiS.sub.3 solid electrolyte produced was ground, pelletized and tested as described in Example 1. It was found to have a conductivity of 6.4.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

Vitreous 1.18 LiI+Li.sub.2 SiS.sub.3 was produced as generally described in Example 3 and tested as described in Example 2 and found to have a conductivity of 4.1.times.10.sup.-4 ohm.sup.-1 cm.sup.-1 at 25.degree. C.

A 0.787 inch diameter by 0.063 inch high coin cell was constructed as follows. A cathode composed of poly(N-vinylpyrrolidone), iodine and TiS.sub.2 was prepared by ball milling these three components together with a solid electrolyte to insure homogeneity and then pelletizing the mixture. The PVP+I.sub.2 +TiS.sub.2 -containing cathode along with a separate layer of 0.75 LiI.Li.sub.2 SiS.sub.3 electrolyte as prepared in the same manner as in Example 3 and a lithium anode were isotatically compressed at 25,000 psi as described in copending application Ser. No. 509,133 by J. R. Akridge and H. Vourlis and then assembled into the cell housing. The cell was continuously discharged across a 50 K-ohm load to a 1.0 volt cutoff. The voltage readings observed with time are shown in the Table.

A cell was constructed as in Example 5 except the cathode was Sb.sub.2 S.sub.3. The cell was continuously discharged across a 50 K-ohm load to a 1.0 volt cutoff and delivered 9.6 mAh.

The data presented in Examples 4 and 5 clearly demonstrate the suitability of the SiS.sub.2 -based vitreous lithium cation conductor of this invention as a solid electrolyte for solid state cell systems.

It should be understood that the foregoing disclosure relates to preferred embodiments of the invention and it is intended to cover all changes and modifications of the invention which do not depart from the spirit and scope of the appended claims.