Sealed Lithium-Reducible Metal Salt Cell

This invention relates to sealed cells and, more particularly, to such cells employing a lithium anode, a reducible metal salt cathode, and a solid lithium-sodium aluminate electrolyte.

Cross-reference is made to copending patent applications Ser. Nos. 517,511, 517,512, and 517,513 filed Oct. 24, 1974 in the names of Gregory C. Farrington and Walter L. Roth and entitled "Sealed Lithium-Bromine Cell", "Sealed Lithium-Iodine Cell", and "Sealed Lithium-Chlorine Cell", respectively. Cross-reference is made to copending patent applications Ser. Nos. 557,583 and 557,584 filed Mar. 12, 1975 in the names of Walter L. Roth and Gregory C. Farrington and entitled "Sealed Lithium-Reducible Phosphorous Oxyhalide Cell" and "Sealed Lithium-Reducible Sulfur Oxyhalide Cell", respectively. Cross-reference is made to copending patent applications Ser. Nos. 559,901 and 559,990 filed Mar. 19, 1975 in the names of Walter L. Roth and Gregory C. Farrington and entitled "Sealed Lithium-Reducible Gas Cell" and "Sealed Lithium-Sulfur Monochloride Cell", respectively. Cross-reference is made to copending patent applications Ser. Nos. 571,500 and 571,556 filed Apr. 25, 1975 in the names of Gregory C. Farrington and Walter L. Roth and entitled "Sealed Lithium-Phosphorous Cell" and "Sealed Lithium-Sulfur Cell", respectively. All of the above copending applications are assigned to the same assignee as the present application.

Reducible metal salts have been extensively investigated as possible cathodes in lithium/nonaqueous cells. Jasinski reviews this previous work in detail (R. Jasinski, High-Energy Batteries, Plenum Press (New York), 1967, pp 149-156).

The typical previous application uses an inorganic oxidant mixed with an electronic conductor such as C pressed onto a metal screen to form a cathode structure. The cathode is then combined with a Li metal anode and the two electrodes separated by porous paper or plastic. When the electrode package is immersed in a nonaqueous electrolyte, an electrochemical cell is created.

Cells made in this manner generally have suffered from high self-discharge rates arising from cathode solubility in the nonaqueous electrolytes. For example, CuF.sub.2, a particularly appealing oxidant, dissolves in acetonitrile to the extent of 0.252 wt. %. When in solution, Cu.sup.+ migrates to the anode and there plates out, discharging Li.

In Liang U.S. Pat. No. 3,713,897, there are described electrolyte materials for high voltage solid electrolyte battery systems. This patent describes a solid ion-conductive electrolyte material containing lithium iodide, lithium hydroxide and aluminum oxide. This patent does not describe or teach a solid ceramic electrolyte. Our electrolyte is not prone to destruction in nonaqueous solvents as opposed to the Liang patent material.

In "Progress in Solid State Chemistry", No. 7, edited by A. Reiss and J. O. McCaldin, published by Pergamon Press in 1972, there is a Section 5 entitled ".beta.-Alumina Electrolytes" comprising pages 141-175. This Section 5 was authored by J. T. Kummer of the Ford Motor Company. Of particular interest in Section 5 are pages 141-151. On page 149, FIG. 7, equilibria are shown between .beta.-alumina and various binary nitrate melts containing NaNO.sub.3 and another metal nitrate at 300.degree.-350.degree. C. It will be noted from FIG. 7 that the equilibration of sodium beta-alumina with molten LiNO.sub.3 results in the partial replacement of 50 percent of the sodium ion content by lithium ions. On page 151 it is discussed in lines 1-5 that the equilibration of sodium beta-alumina with molten LiNO.sub.3 does not produce a complete replacement of sodium ions by lithium ions. However, it is further pointed out that complete lithium ion replacement of sodium ions in sodium beta-alumina can be effected by first exchanging sodium ions by silver ions and then silver ions by lithium ions in a melt of LiNO.sub.3 -LiCl. Throughout the above Kummer publication there is no recognition that the lithium-sodium .beta.-alumina produced by equilibration of sodium beta-alumina with molten LiNO.sub.3 is a unique and stable compound which can readily transport lithium ions without substantially altering its content of sodium ions.

Our present invention is directed to a sealed lithium-reducible metal salt cell with positive separation of the anode and cathode by a solid lithium-sodium aluminate electrolyte which is lithium ion-conductive.

The primary object of our invention is to provide a sealed lithium-reducible metal salt cell which has high cell voltage, high energy density, a near-zero self discharge rate, exceptionally long storage life, and stability at ambient temperatures.

In accordance with one aspect of our invention, a sealed lithium-reducible metal salt cell employs a lithium anode, a reducible metal salt cathode, and a solid lithium-sodium aluminate electrolyte which is a lithium-ion conductor therebetween.

These and various other objects, features and advantages of the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a sectional view of lithium-reducible metal salt cell made in accordance with our invention;

FIG. 2 is a sectional view of a modified lithium-reducible metal salt cell made in accordance with our invention; and

FIGS. 3-7 are sets of polarization curves showing cell performances of the cells shown in FIGS. 1 and 2 employing various reducible metal salt cathodes.

In FIG. 1 of the drawing, there is shown generally at 10 a lithium-reducible metal salt cell embodying our invention. While we tested this open cell for operability, the cell for general use is sealed. The cell has a two part Teflon polymer casing 11 including an anode portion 12 and a cathode portion 13. Anode portion 12 defines a chamber 14 therein with an upper opening 15. An opening 16 is provided in one side wall. An anode 17 comprises a lithium metal foil 18 in a nonaqueous electrolyte or solvent 19 within chamber 14. Cathode portion 13 defines a chamber 20 therein with an upper opening 21. An opening 22 is provided in one side wall, which opening 22 is shown with a first portion 23 and a recessed portion 24. A cathode 25 comprises a reducible metal salt 26 in a nonaqueous solvent 27 with an ionic conductivity enhancing material. Reducible metal salt 26 as anhydrous powder is shown pressed onto a nickel screen 28 which is welded to a current collector lead 29. Two part casing 11 has its anode portion 12 and cathode portion 13 positioned adjacent to one another and in communication with one another in a leak-proof manner by aligning openings 16 and 22 and positioning between the two portions a washer 30, for example, of silicone rubber. A solid lithium-sodium aluminate electrolyte 31 in the form of a disc is positioned against the outer surface of washer 30 and the outer surface of a similar silicone washer 32 fitted within recess 24 of opening 22. The two part casing 11 is held together tightly and in a leak-proof fashion by employing a pair of threaded fasteners 33 which extend through an appropriate opening 34 through both parts of casing 11. A washer 35 and a nut 36 are provided for the threaded end of each fastener to position the structure together. The above assembly results in a lithium-reducible metal salt cell which can be employed as a primary cell.

In FIG. 2 of the drawing there is shown generally at 40 a modified sealed lithium-reducible metal salt cell embodying our invention. An outer casing 41 comprising a lower casing portion 42 of glass and an upper casing portion 43 of polyethylene affixed tightly to the upper open end of the lower casing portion 42 thereby provides a chamber 44 for a cathode 25 of a reducible metal salt 26 in a nonaqueous solvent 27 with an ionic conductivity enhancing material. Reducible metal salt 26 is shown pressed onto a nickel screen 28 which is welded to a current collector lead 29. Lead 29 extends to the exterior of cell 40 through the junction of the lower and upper casing portions 42 and 43. An inner casing 45 in the form of a tube of solid lithium-sodium aluminate electrolyte is positioned within outer casing 41 and immersed partially in nonaqueous solvent 27. An opening 46 is provided in the top of upper casing portion 43 into which tube 45 fits tightly. An anode 47 of lithium metal in the form of a lithium ribbon pressed onto a nickel mesh 48 which is folded together and attached to the end of a nickel electrical lead 49. An anolyte 50 partially fills tube 45 and is in contact with lithium anode 47. An electrically insulating closure 51 with a hole 52 therethrough is provided at the upper end of tube 45 to seal the initially open end of the tube. Lead 49 extends through hole 52 in closure 51 to the exterior of cell 40.

In FIGS. 3-7, performances of the cells shown in FIGS. 1 and 2, respectively, which employ various reducible metal salt cathodes, are provided by polarization curves which were each produced at a temperature of 26.degree. C. In each of these Figures, cell voltage in volts is plotted against current density in microamperes per square centimeter.

We found that we could form a sealed lithium-reducible metal salt cell with a lithium ion-conductive electrolyte by employing a casing having a cathode portion and an anode portion. These two portions are separated by a solid lithium-sodium aluminate electrolyte in disc or tube form which will be further described below. Such a casing may be provided in various configurations such as, for example, shown in FIGS. 1 and 2. For purposes of showing the operability of our cell, we used first an anode portion and a cathode portion each of which had top and side openings as shown in FIG. 1. Since it was not necessary, the top openings were not sealed during assembly and testing. The casing material chosen was Teflon polymer. A silicone rubber washer was positioned in the recessed opening of the cathode portion and a solid lithium-sodium electrolyte was positioned adjacent the washer within the recessed opening. A silicone rubber washer was positioned between the casing portions. The side openings of the casing portions and the washers were aligned to provide for contact of the cathode with one surface of the solid electrolyte and for contact of the anode with the other surface of the electrolyte. We employed threaded fasteners to hold the casing portions together in a unitary cell structure. It will, of course, be appreciated that various other cell configurations can be employed, for example, as shown in FIG. 2. In addition to the Teflon polymer casing material various metals and nonmetals can be used. Other materials can be substituted for the silicone washers. If desired, in the configuration, the solid electrolyte disc could be sealed by glass seals to the casing to separate the cathode from the anode as shown, for example, in the above-mentioned U.S. Pat. No. 3,817,790.

We found further that we could form various modified sealed lithium-reducible metal salt cells embodying our invention. One such modified cell employs an outer casing comprising a lower casing portion of glass and an upper casing portion of a plastic such as polyethylene affixed tightly to the upper open end of the lower casing portion thereby providing a chamber for a cathode of phosphorous in a nonaqueous solvent with an ionic conductivity enhancing material. The reducible metal salt in anhydrous powder form can be pressed onto a nickel mesh which is immersed in the nonaqueous solvent with ionic conductivity enhancing material. A current collector lead which is welded to the screen extends to the exterior of the cell through the junction of the lower and upper casing portions. An inner casing in the form of a tube of solid lithium-sodium aluminate electrolyte is positioned within the outer casing and immersed partially in the nonaqueous solvent with ionic conductivity enhancing material. An opening is provided in the top of the upper casing portion into which the tube fits tightly. An anode of lithium metal in the form such as lithium ribbon pressed onto a nickel mesh is folded together and attached to the end of a nickel electrical lead. An anolyte partially fills the tube and is in contact with the lithium anode. An electrically insulating closure with a hole therethrough is provided at the upper end of the tube to seal the initially open end of the tube. The lead extends through the hole in the closure to the exterior of the cell.

For the anode we employ lithium, lithium as an amalgam or lithium in a nonaqueous electrolyte, such as propylene carbonate. Ionic conductivity enhancing materials can be added to the nonaqueous solvent.

We employ a solid lithium-sodium aluminate electrolyte between the cathode and anode to provide a solid barrier preventing contact between the electrodes and to provide lithium ion conductivity. The solid lithium-sodium ion-conductive electrolyte has an approximate composition of LiNaO.sup.. 9Al.sub.2 O.sub.3 of which 1.3 to 85 percent of the total alkali content is lithium. As it was discussed above in "Progress in Solid State Chemistry", J. T. Kummer, in Section 5 is described a lithium-sodium, .beta.-alumina material, particularly on pages 149-151. Further, it is described in the article how to manufacture such material. As it will be particularly noted throughout the above Kummer publication, there is no recognition that the lithium-sodium .beta.-alumina produced by equilibration of sodium beta-alumina with molten LiNO.sub.3 is a unique and stable compound which can readily transport lithium ions. We used such material containing 50 percent lithium ions as a solid electrolyte in our initial cells as shown in FIG. 1 and described above. The results of performance of these cells are shown in FIGS. 3-7. Such electrolyte material containing 50 percent lithium ions appears to be the optimum amount of lithium ions in the material. A range of 40 to 60 percent lithium ions in the electrolyte material with the remainder sodium ions provides the desirable conductivity for the operation of our cells. We found unexpectedly that we could obtain the desirable conductivity necessary for the operation of our cells by employing a broader range of 1.3 to 85 percent lithium ions in the electrolyte material with the remainder sodium ions. Tubes made of solid lithium-sodium aluminate electrolyte containing, respectively, 1.34 and 84.7 percent sodium ion substitution by lithium ions were used in later cells of the type shown in FIG. 2, described above, and the results of which are shown in FIGS. 3-7.

For the cathode, we can employ a reducible metal salt in a nonaqueous solvent with an ionic conductivity enhancing material. Various suitable salts include the transition metal fluorides, chlorides, iodides, sulfides and oxides.

The reducible metal salt in anhydrous powder is pressed onto a screen such as a nickel screen and is immersed in a nonaqueous solvent which solvent does not react with the salt. While various nonaqueous solvents are suitable, we have found propylene carbonate to be a quite satisfactory solvent for the reducible metal salts. A satisfactory manner of suspending the reducible metal salt on the nickel screen in the solvent is by means of an electric current conductor in the form of a wire which is welded to the screen. The opposite end of the wire can then be extended through the battery casing to the exterior to provide a suitable electrical lead to the cathode. Various ionic conductivity enhancing materials, which are suitable, include chemically stable salts such as tetraalkylammonium perchlorates, tetrafluoroborates and lithium perchlorate.

Lithium is the lightest practical solid battery anode material and is also the most reducing. The lithium ion is a small and strongly polarizing ion. The salts of the lithium ion are generally more soluble in nonaqueous solvents than their sodium ion counterparts. Such high solubility helps eliminate salt precipitation on the faces of a solid electrolyte. Various other nonaqueous electrolytes which are suitable with lithium include butyrolactone, tetrahydrofuran, acetonitrile, thionyl chloride, phosphorous oxychloride with a wide variety of conductivity salts such as lithium perchlorate, lithium and tetraalkylammonium chlorides, perchlorates, cyanides, thiocyanates, tetrafluoroborates, and hexafluorophosphates.

Examples of lithium-reducible metal salt cells, which can be readily sealed or are sealed, made in accordance with our invention are set forth below:

A cell casing No. 1 was assembled as generally described above and shown in FIG. 1 of the drawing. For the cell, a lithium-sodium aluminate electrolyte disc was made by first preparing a cylinder of .beta.-alumina by firing Na.sub.2 O+Al.sub.2 O.sub.3 plus 1 percent MgO at 1750.degree. C. The density of the .beta.-alumina cylinder was 3.224 g/cm.sup.3 corresponding to less than 1 percent void volume. A disc of 1 mm thickness was slided from the cylinder and converted to a lithium-sodium aluminate electrolyte by immersion in molten LiNO.sub.3 at 400.degree. C for 24 hours. The exchange of the sodium ions for the lithium ions was accompanied by a 1.91 percent decrease in weight corresponding to approximately 50 percent sodium ion substitution by lithium ions and the final density was 3.148 g/cm.sup.3. X-ray diffraction showed that the electrolyte disc has a hexagonal crystal structure with lattice parameters a = 5.603 .+-. 0.001 A and c = 22.648 .+-. 0.003 A.

For the cell a two part Teflon polymer casing which included an anode portion and a cathode portion was employed to assemble the cell. Each portion had a chamber with an upper opening and a side opening. The side opening in one portion, the cathode portion, was further recessed. A silicone washer was positioned in the side opening of the cathode portion. The above prepared lithium-sodium aluminate electrolyte disc was positioned against the washer and within the recessed opening in the cathode portion. A silicone washer was positioned between the casing portions and the openings in the washer and in the casing portions were aligned. A pair of threaded fasteners were then employed to hold the casing portions together and tightened at one end by nuts. For each of the 10 examples, Nos. 1-10, the chamber of the anode portion for the cell was provided each time with an anode consisting of an electrolyte of propylene carbonate with dissolved lithium perchlorate and tetrabutylammonium tetrafluoroborate and a lithium foil anode inserted therein and held in position in the chamber and in contact with the electrolyte. For each of the 10 examples, a nonaqueous solvent of propylene carbonate containing lithium perchlorate and tetrabutylammonium tetrafluoroborate was placed each time in the cathode chamber of the cell. For each example, a different reducible metal salt was pressed onto a nickel screen which was immersed in the solvent in the cathode chamber resulting in ten different cathodes. Each cathode structure was supported by a current collector lead. Each resulting device was a lithium-reducible metal salt cell made in accordance with our invention which cell could be readily sealed. The example number, the cathode class, the specific reducible metal salt, the open circuit potential of the cell, the cell polarization curve number, and the Figure of the drawing for the polarization curve is set forth below in Table I.

The above polarization curves were produced at a temperature of 26.degree. C. In FIGS. 3-7, the cell voltage in volts is plotted against current in microamperes per square centimeter for each cell. No attempts were made to minimize interfacial polarization at the lithium-sodium aluminate ion-conductive electrolyte interfaces.

Two cell casings Nos. 2 and 3 were assembled as generally described above and as shown in FIG. 2 of the drawing. Each of the cell casings was constructed in the same manner with the exception that cell casing No. 2 had a lithium-sodium aluminate tube with a 84.7 percent lithium ion content while cell casing No. 3 had a lithium-sodium aluminate tube with a 1.34 percent lithium ion content. The remaining alkali ion content of each tube was sodium ions.

The tube for cell casing No. 2 was formed from a tube of sodium .beta.-alumina approximately 6.2 cm. long, 1.1 cm. OD, and 0.15 cm. wall thickness. The tube was baked out overnight at 1175.degree. C prior to lithium ion exchange. The lithium ion exchange was made by immersion in lithium nitrate at 600.degree. C for 13 hours. A resulting 3.12 percent weight decrease corresponded to 84.7 percent sodium substitution by lithium ions.

The tube for cell casing No. 3 was formed from an identical sodium .beta.-alumina tube which was baked out in the same manner. The lithium ion exchange was made by immersion in 20 mole percent lithium nitrate and 80 mole percent sodium nitrate at 400.degree. C for 72 hours. A resulting 0.049 percent weight decrease corresponded to 1.34 percent sodium substitution by lithium ions.

For each cell casing, an outer casing was formed of a lower casing portion of glass and an upper casing portion of polyethylene affixed tightly to the upper open end of the lower casing portion thereby providing a chamber for each cathode which consisted of a reducible metal salt immersed in a nonaqueous solvent of propylene carbonate containing lithium perchlorate and tetrabutylammonium tetrafluoroborate. The reducible metal salt in anhydrous form was pressed onto a nickel screen. An electrical current collector lead was welded to the screen and extended to the exterior of the cell through the junction of the lower and upper casing portions. An inner casing in the form of a tube of solid lithium-sodium aluminate electrolyte was positioned within each outer casing and immersed partially in the nonaqueous solvent. The tube for cell casing No. 2 contained 84.7 percent lithium ion content while the tube for cell casing No. 3 contained 1.34 percent lithium ion content. An opening was provided in the top of each upper casing portion into which the respective tube fitted tightly. An anode of lithium metal in the form of a lithium metal ribbon pressed onto a nickel mesh was folded together and attached to the end of a nickel electrical lead. An anolyte of 0.1M tetrabutylammonium tetrafluoroborate in propylene carbonate saturated with LiClO.sub.4 partially filled each tube and was in contact with the lithium anode. An electrically insulating closure with a hole therethrough was provided at the upper end of each tube to seal the initially open end of the tube. The lead extended through the hole in the closure to the exterior of the cell.

Five cells, Examples 11-15, employed casing No. 2 while five cells, Examples 16-20, employed casing No. 3. The example number, the specific reducible metal salt, the open circuit potential of the cell, the cell polarization curve number, and the figure of the drawing for the polarization curve is set forth below in Table II.

The above polarization curves were produced at a temperature of 26.degree. C. The cell voltage in volts is plotted against current in microamperes per square centimeter for each cell. No attempts were made to minimize interfacial polarization at the lithium-sodium aluminate ion-conductive electrolyte interfaces.

While other modifications of the invention and variations thereof which may be employed within the scope of the invention have not been described, the invention is intended to include such as may be embraced within the following claims: