Lithium/sulfur dioxide cell

Abstract

The lithium/carbon/SO₂ energy cell is disclosed wherein the electrolyte includes lithium tetrachloroaluminate. In order to increase the electrical capacity of the cell, the concentration of lithium tetrachloroaluminate in the electrolyte is increased to and calcium tetrachloroaluminate and is added to the solution to aid the mass transport as well as the to lower the freezing point of the electrolyte. In order to eliminate any corrosive effect of the calcium added additive to on the lithium anode, this disclosure provides a teaching of the use of an electrolyte solution and that is essentially free of any moist moisture content.

Description

anY any harmful effect and improves the capacity of the cell and lowers the fading characteristic as the cycling progresses.

The ability to use the mixture of lithium/calcium tetrachloroaluminates in the sulfur dioxide solvent without encountering any harmful corrosive effect, resides in the knowledge that, in so far as it is possible, it is necessary to exclude the presence of moisture from the electrolyte. When the cell is being assembled, a substantially moisture free mixture of lithium/calcium tetrachloroaluminate is dissolved in reagent grade sulfur dioxide, wherein the calcium tetrachloroaluminate is used to lower the melting point of the solution and to add aluminum to the electrolyte. The addition of the calcium salt also has also been found to aid the mass transport in the reaction and decrease electrode polarization. The lithium anode is protected from direct reaction with the solvent and calcium component of the electrolyte by an insoluble thin film of lithium chloride salt which is formed thereon when the electrolyte is poured in the cell, which film allows passage of lithium ions, and therefore electrochemical discharging, and charging or plating of lithium. Such films are believed to be common in cells and batteries with lithium negative electrodes, as explained for example by E. Peled in "Lithium Batteries" (J. P. Gabano, ed., Acedemic Press, London, 1983), and are identified as "solid electrolyte interphases."

The lithium electrode in a LiAlCl₄ -3SO₂ electrolyte held at 70° F. for 3 weeks was found to be coated by crystalline lithium chloride but no aluminum was found either in the salt film or in the lithium metal under the film (Schlaikjer et al, Electrchemical Society Fall Meeting, 1986, Abstract #7). It is likely that the calcium ion in solution in the electrolyte of the current invention is likewise forced to remain in the solution by the lithium chloride salt film on the lithium surface, and does not plate out on the lithium anode as a metallic film of calcium. The presence of moisture in this instance would be detrimental because it would cause corrosion of the cell components, particularly of the lithium negative electrode. For this reason, the concentration of moisture and of hydrolysis products must be kept as low as possible. Moisture can be inadvertently introduced into the cell through the component salts, lithium chloride, calcium chloride, or aluminum chloride, or it may be also present in the sulfur dioxide.

State of the art electrolytes containing aluminum chloride and alkali chloride, with or without alkaline earth chlorides, and sulfur dioxide, are made by mixing the salts together in a vented container and exposing the mixture to a large excess of sulfur dioxide gas by bubbling it through the mixture. Most of the moisture present in the sulfur dioxide gas is absorbed by the highly hygroscopic salt mixture, even if the initial concentration of moisture in the sulfur dioxide gas is low.

In the present invention, the moisture content of such mixtures is minimized first by heating the alkali chloride, in this case lithium chloride and the alkaline earth chloride, in this case calcium chloride in vacuo to at least 200°C for at least 16 hours, following by cooling and storage while protected from the atmosphere. The aluminum chloride is purified by distillation at atmospheric pressure from molten lithium tetrachloroaluminate, in the presence of calcium turnings. The sulfur dioxide is added to the purified salts in an air tight container. To minimize the moisture introduced by the sulfur dioxide, the amount of this material added is only that which is sufficient to produce the required molar ratio of aluminum to sulfur. The essentially moisture free electrolyte including the lithium tetrachloroaluminate, calcium tetrachloroaluminate, and sulfur dioxide must then be filled into a moisture free cell that contains the lithium anode and carbon cathode wound in a spiral in the conventional manner and separated one from the other with nonwoven Pyrex fabric separators, while minimizing exposure of the electrolyte solution to the atmosphere. As soon as the cell is filled it is essential that it be sealed with the usual welded on cap while the assembly is held in a moisture free environment.

One example of a preferred cell made in accordance with this teaching is an AA size battery that includes a stainless steel case 1/2" in diameter by 17/8" having a volume of 6.03 cubic centimeters. The steel case and its cover are adapted to be welded together after the known form of wound lithium anode and carbon cathode assembly have been placed in the case, it being essential that such assembly be accomplished in a manner to minimize the exposure of any of the parts to moisture either when outside or inside the case.

The pure lithium anode that measures 3"×1.5"×0.024" is supported in the known manner by being tab welded to the case and the porous carbon cathode that measures 3"×1.5"×0.016" and is made of Ketjenblack carbon with a 3% Teflon binder which is carried on a nickel screen that is connected to the positive terminal of the battery. Crane glass non-woven Pyrex fabric separators 0.005" to 0.007" thick were wound between the anode and cathode as the core of the battery was being assembled prior to its insertion in the case. The substantially moisture free electrolyte including the lithium/calcium tetrachloroaluminates dissolved in sulfur dioxide was filled into the moisture free and sealed assembly through the usual glass/metal compression seal in the cover without any exposure to the atmosphere.

In this preferred form of the AA battery, the lithium/calcium tetrachloroaluminates were dissolved in the solvent in the molar ratio of 2 to 1 to 12, that is the LiAlCl₄ to Ca(AlCl₄)₂ to SO₂.

The battery made as above described was tested with discharge/charge time limits of 20 hrs/20 hrs discharge/charge potential limits of 2 V/4 V(100% depth of discharge); discharge/charge constant current of 50 mA, for a maximum total of 1 ampere hour, at operating temperatures within a range of from 19°C to 31°C FIG. 1 shows the start-up profiles of the potential versus time for the cell and FIG. 2 shows the profiles for cycles 19, 20, and 21.

The electrolyte composition may be varied from that described above for use in the preferred cell. The molar ratio of lithium tetrachloroaluminate to calcium tetrachloroaluminate and to sulfur dioxide may be varied within a range of lithium/calcium/sulfur dioxide of from 1/1/9 to 2/1/12. Other alkaline earth tetrachloroaluminates may be present singly or mixed in any combination, it being required only that a substantially moisture free solution be placed in a dry cell to realize the desired results of this invention.

While the above sets forth the preferred form of this invention, it is possible that modifications thereof may occur to those skilled in the art that will fall within the scope of the following claims:

Claims

1. A substantially moisture free electrochemical cell comprising a lithium anode, an inorganic electrolyte including sulfur dioxide that provides a solvent for lithium tetrachloroaluminate and at least one alkaline earth tetrachloroaluminate and that functions in conjunction with dissolved aluminum and lithium to produce a discharge product and in which solution there is a mole equivalent ratio of lithium to alkaline earth to sulfur dioxide within the range of from 1 to 1 to 9, to lb 2 2 to 1 to 12, said cell having a cathode supported on a current collector, and said cathode consisting of a porous carbon for supporting the discharge product.

2. A substantially moisture free electrochemical cell as in claim 1 wherein said at least one tetrachloroaluminate is calcium tetrachloroaluminate.

3. A substantially moisture free electrochemical cell as in claim 2 wherein said lithium tetrachloroaluminate, said calcium tetrachloroaluminate, and said sulfur dioxide are present in a mole equivalent ratio of 2 to 1 to 12.

4. A substantially moisture free electrochemical cell as in claim 1 wherein said at least one tetrachloroaluminate is magnesium tetrachloroaluminate.

5. A substantially moisture free electrochemical cell as in claim 1 wherein said at least one tetrachloroaluminate is strontium tetrachloroaluminate.

6. A substantially moisture free electrochemical cell as in claim 1 wherein said at least one tetrachloroaluminate is barium tetrachloroaluminate.