Additive to Improve Voltage Regulation in a Lithium-Copper Chloride Rechargeable Cell

The present invention involves an electrochemical cell employing lithium as an anode, lithium tetrachloroaluminate as an electrolyte, sulfur dioxide as a solvent, and a cathode of high surface area carbon black containing cupric chloride. It has been found that voltage regulation of such cells can be enhanced by the addition of a lithium halide to the cathode.

Over the past several years, a class of cells commonly known as liquid cathode cells has emerged as the best candidate to provide substantially increased performance over the age-old zinc-carbon, alkaline and silver oxide cells. The liquid cathode cells are distinguished from more conventional cells in that the active cathode depolarizer is a liquid. The basic elements of the cell are an anode, typically consisting of an alkaline or alkaline earth metal, a current collector consisting of a high surface area material that is catalytically active in the reduction of the liquid cathode, a suitable separator located between the current collector and the anode and mechanically separating the two, and the electrolyte, which includes the liquid cathode as well as an ionically conductive solute dissolved in a liquid solvent. In certain type cells, the solvent performs the additional function of the active cathode depolarizer.

The liquid cathode cells that are most commonly discussed in the literature as having the best performance characteristics are those using lithium metal anodes and active cathode depolarizers that are either oxyhalides, such as thionyl chloride or sulfur dioxide. Sulfur dioxide has also been employed as the cell solvent, particularly in secondary or rechargeable cells of the type which are made the subject of the present invention.

Cells for contemplation herein employ a lithium anode and a solution of a highly soluble lithium salt as the electrolyte, such as lithium tetrachloroaluminate. Sulfur dioxide is used as the solvent, while the cathode is a high surface area carbon black containing cupric or copper II chloride.

Under ideal conditions, the cupric chloride contained within the porous carbon cathode discharges, displaying a single voltage plateau at approximately 3.4V. There is, however, a tendency for the cupric chloride to decompose to cuprous chloride via the reaction:

If impurity levels of copper I chloride are high, the cell displays additional voltage plateaus on charge and discharge. Typically, the above-referenced decomposition reaction results in between 2 and 10% cuprous chloride in the cathode, having the effect outlined above.

It is thus an object of the present invention to provide a rechargeable Li-SO.sub.2 cell having a cathode of porous carbon containing CuCl.sub.2 while minimizing the effects of CuCl contamination in the cathode.

This and further objects will be more fully appreciated when considering the following disclosure and appended drawings, wherein

FIGS. 1 and 2 represent graphs of cell voltages during charge and discharge for the prior art as well as for cells produced according to the present invention.

The present invention is based upon the discovery that a lithium based non-aqueous cell employing liquid sulfur dioxide as the solvent and cupric chloride contained in a porous carbon body as the cathode will display a single voltage plateau if the impurity which normally occurs in the cathode as a result of the reduction of Cu.sup.+2 is minimized or eliminated completely. This impurity results from the formation of cuprous chloride and chlorine from the cupric chloride cathode additive. The present invention is carried out by adding a lithium halide such as LiBr and LiCl to the cell electrolyte. Ideally, these additives are employed in amounts between approximately 1% to 15% by weight based upon the weight of cupric chloride in the positive electrode.

During the preparation of carbon-cupric chloride mixtures for use as the positive electrode of a secondary cell, some of the cupric chloride decomposes via the reaction

This results in a significant percentage of copper I chloride in the cathode generally measured to be between approximately 2 to 10% by weight as measured by titration. The effect of this contamination is to introduce an additional voltage plateau during the recharging process as shown in FIG. 1.

FIG. 1 represents the charge-discharge voltage curve of an AA cell which consisted of a 6 mil lithium anode separated from a 23 mil cathode comprised initially of copper II chloride and carbon. The separator was a microporous Tefzel material capable of inhibiting the dendritic growth of lithium. Tefzel is a trademark of an ethylene-tetrafluoroethylene copolymer membrane available from Raychem Corporation. Prior to winding the components, the positive electrode was heated to 120.degree. C. under vacuum to remove water. The cell was filled with an electrolyte consisting of lithium tetrachloroaluminate dissolved in liquid sulfur dioxide.

FIG. 2 represents the charge-discharge voltage curve of a similar AA size cell which contained additional lithium chloride added directly to the cathode. The lithium chloride weight was approximately 4% of that of the copper II chloride initially present. This cathode (or positive electrode) was also subjected to the 120.degree. C. drying procedure.

Turing to FIG. 1, the typical EMF of an "AA" cell is shown during charge and discharge. At A, charging begins and a first plateau is reached at B. Without contamination, the voltage of the cell would remain constant until a peak at D is reached. Instead, a secondary plateau is established at C which is indicative of CuCl contamination in the cathode. Discharge then takes place which should result in the establishment of a secondary plateau at F. Instead, an elbow appears at E which again is indicative of cuprous chloride contamination.

These plateaus are associated with the following reaction:

It has been discovered that a most convenient way to minimize or eliminate CuCl contamination is to add a lithium halide to the cathode. Ideal lithium halides for this purpose are lithium chloride and lithium bromide.

Typically, 1% to 15% by weight of the lithium halide is added to the cathode based upon the weight of the cupric chloride with the results shown in FIG. 2. In this case, a cathode containing 2.8 gm of copper II chloride was doped with 0.14 gm of lithium chloride. As noted in FIG. 2, which is again a plot of EMF voltage of an "AA" cell, during the charge-discharge cycle, a first plateau is established during the charging of the cell at G. Once fully charged, a spike is achieved at H, at which time discharge occurs at I. Clearly, two distinct plateaus, one at charge and one at discharge, are evidenced. No secondary plateaus, such as shown in FIG. 1, are evident in producing a cell pursuant to the present invention.

Although not intending to be bound by any particular scientific theory in explaining the mechanism for the present invention, it is noted that the addition of a lithium halide does affect the acidity of the cell. The lithium halide reacts with the aluminum chloride (AlCl.sub.3) of equation (2), above, thus forcing the reaction to the right and consuming the cuprous chloride impurity. Aluminum chloride is liberated in the cell by reaction (2), above, in the first recharge of the cell such that subsequent cycles occur in a neutral or slightly basic electrolyte.

The consumption of aluminum chloride and the consequent reduction in acidity of the cell enhances the cell's ability to generate chlorine on overcharge. Chlorine generation is oftentimes viewed as beneficial since it is transported to the lithium electrode and reacts with any surface film formed thereon to regenerate lithium tetrachloroaluminate and sulfur dioxide, thus preventing excessive film buildup and lithium isolation. Under circumstances where the acidity of the cell is variable, the degree of lithium film erosion is also variable. In situations where several cells are configured in series or in parallel, this can result in variable degrees of overcharge and thus lithium film removal, which can in turn result in mismatched capacities and early battery failure.