High Rate Metal-Sulfuryl Chloride Batteries

An electrochemical cell based on lithium-sulfuryl chloride (Li/SO.sub.2 Cl.sub.2) electrochemical system offers a potentially higher energy performance than the lithium thionyl chloride (Li/SOCl.sub.2) system. Poor electrochemical discharge and relatively narrow operating temperature range have, however, hindered a practical cell development. One of the factors believed responsible for poor discharge performance is cathode substrate. High surface area Teflon (trademark of E. I. du Pont DeNemours)-bonded carbon cathodes, where SO.sub.2 Cl.sub.2 undergoes reduction, and irreversibility of the cathode reaction contribute excessive cathode overpotential. Another factor contributing to premature cell failure, particularly at high discharge rates, is the presence of reactive intermediate reduction species.

The present invention relates to improved electrochemical current producing cells based on a lithium-sulfuryl chloride system. More particularly, the present invention relates to a catalyzed porous cathode for the reduction of sulfuryl chloride. The catalysts are macrocyclic complexes of transition metals such as polymeric and monomeric cobalt and iron phthalocyanines.

It has been discovered that the rate of SO.sub.2 Cl.sub.2 reduction can be increased substantially by doping the carbon cathodes with CoPc, FePc and (CoPc) catalysts. Both monomeric CoPc and FePc are soluble in acid (AlCl.sub.3 /SO.sub.2 Cl.sub.2) as well as neutral (LiAlCl.sub.4 /SO.sub.2 Cl.sub.2) electrolytes. However, the heat treated (500.degree. to 600.degree. C.) polymeric (CoPc).sub.n, which is synthesized by heating a mixture of 3, 3, 4, 4-benzophenone tetracarboxylic dianhydride (BTDA), CoCl.sub.2 and urea at 200.degree. C. for one hour, does not dissolve and is stable in both acid and neutral electrolytes.

A cell according to the present invention comprises a lithium anode, a sulfuryl chloride depolarizer and an electrolyte salt, and a porous carbon cathode doped with a macrocyclic complex of a transition metal, such as FePc, CoPc, or (CoPc).sub.n. A preferred cathode material is Shawinigan carbon black. A binder, such as Teflon (trademark of E. I. du Pont DeNemours) may also be included.

A number of cells were constructed employing a lithium anode, a sulfuryl chloride depolarizer and lithium tetrachloroaluminate electrolyte salt, and carbon cathodes doped with a macrocyclic complex of a transition metal. Experiments, as described below, were performed on these various cells having cathodes catalyed with FePc, CoPc, or (CoPc).sub.n and compared to a cell with uncatalyzed cathode. Catalyst doping of cathodes can be achieved by mixing the catalyst with carbon before cathode fabrication or, in the case of FePc and CoPc, by dissolving the catalyst in the electrolyte.

Each of the cells for which data is given in Table 1 below comprised a lithium anode, an electrolyte of 1.5 mole solution of lithium tetrachloroaluminate in sulfuryl chloride and a porous carbon cathode.

In the cells with FePc and CoPc, the catalyst doping of the cathodes was accomplished by adding catalyst to the electrolyte. Approximately 3 milligrams of catalyst per cc of electrolyte were used. It has been found, however, that much smaller amount of the catalyst, i.e. less than 0.5 milligrams per cc of electrolyte, produce a significant improvement in the cell operation. On the other hand, much larger amounts of the catalyst will, beyond a certain point, not produce any further improvement.

Cathode doping with (CoPc).sub.n was accomplished by adding it to the cathode during the manufacture of the cathode. The cathode was formed by mixing (CoPc).sub.n with Shawinigan carbon black in approximate ratio of 5% to 95% and heat treating the mixture at 550.degree. C. or higher for two or more hours. Upon cooling, a slurry was formed by adding to the mixture a nonaqueous solvent and a binder, such as Teflon. The slurry was then shaped into the desired cathode shape and allowed to dry.

Although in the specific example given here, the ratio of (CoPc).sub.n to carbon was almost 5% to 95%, it has been found that the amount of (CoPc).sub.n could vary from 0.5% to 20% by weight, based on the weight of carbon. Preferrably, from 2% to 10% by weight, based on the weight of carbon, is employed.

As can be seen from Table 1 above, the cells doped with FePc, CoPc, and (CoPc).sub.n produced significantly higher voltages. The greatest improvement throughout the entire current rate discharge range was shown by the cell having its cathode catalyzed by (CoPc).sub.n.

Table 2 below comparative polarization characteristics of cells similar to those represented by Table 1, but utilizing 2.3 M AlCl.sub.3 /SOCl.sub.2 as the electrolyte.