Method of preparing iron disulfide electrodes

#This invention relates to iron disulfide (FeS.sub.2) electrodes for high temperature, molten salt batteries such as lithium/alkali metal halide/iron disulfide batteries (e.g., Li/LiCl-KCl/FeS.sub.2). The overall reaction for that battery is essentially:

#4Li + FeS.sub.2 .revreaction. Fe + 2Li.sub.2 S

#wherein the Li and the FeS.sub.2 are respectively the negative and positive electrodes which, during discharge, combine electrochemically (after a number of intermediate steps) to form iron and lithium sulfide.

#This and other objects of the invention will become more readily apparent from the following detailed description thereof in which:

#FIG. 1 is a partially sectioned, side elevational view through the diameter of cylindrical Li-FeS.sub.2 cell having a laminated positive electrode made with FeS.sub.2 immobilized in accordance with the present invention; and

#FIG. 2 is an enlarged view of the positive FeS.sub.2 electrode of FIG. 1.

#This invention comprehends: slurrying iron disulfide granules with molten sulfur; integrating the slurry with a current collector; and volatizing away the sulfur to provide a relatively uniform distribution of FeS.sub.2 and current collector throughout the electrode. In a particular embodiment, the FeS.sub.2 -S slurry is sheeted as by spreading or rolling. The sheet is solidified and laminated together with porous current collector sheets to form the finished electrode structure. Preferably, the slurry is heated to at least about 250.degree. C and then rapidly quenched to impart flexibility to the sheet, which flexibility is not present for any extended period of time when quenching from lower temperatures. The finished electrode is then heated at an appropriate temperature and pressure to volatize and substantially remove the sulfur from the electrode leaving the FeS.sub.2 contained in the electrode by the current collectors.

#Thin FeS.sub.2 -S sheets made from molten slurries solidified from just above the melting point of S to about 250.degree. C become quite brittle and friable within minutes after quenching, and hence require rather delicate handling thereafter. These sheets can be rendered more flexible, however, by heating the slurry to at least about 250.degree. C and preferably to approximately 300.degree.-325.degree. C followed by rapid quenching to fix the amorphism achieved at these temperatures. The amorphism of FeS.sub.2 -S wafers so fixed has been retained for as long as about 18 months by storing at approximately -18.degree. C and is sustained at room temperature for many hours to permit less delicate handling procedures. It has been observed that maximum flexibility with minimum sulfur loss is achieved in the 300.degree.-325.degree. C temperature range. In this regard, higher temperatures (i.e., 350.degree. C) improve flexibility, but care must be taken to prevent burning and loss of sulfur from the slurry. Sulfur ignition and loss may be reduced by minimizing the air available to the sulfur and/or heating in a substantially closed environment.

#The FeS.sub.2 -S can be integrated with appropriate current collectors in a number of ways including, but not limited to, co-extruding with the collector, sheeting and laminating with the collector, sheeting onto the collector, etc. Likewise, the shape that the electrode takes is dictated only by the overall cell design rather than any limitations of the FeS.sub.2 electrode itself, and the FeS.sub.2 -S slurry technique of this invention lends itself to fabrication to a variety of shapes, e.g., tubes, planar laminates, spiral laminates (i.e., jellyroll-like FeS.sub.2 -S-collector coils), etc. A presently preferred technique for integrating the FeS.sub.2 -S with the electrode's current collectors is to sheet the FeS.sub.2 -S into self-supporting wafer-like sheets and sandwich the sheets twixt planar laminae of porous current collector sheets (e.g., graphite cloth). Alternatively, the FeS.sub.2 -S is sheeted directly onto sheets of the porous current collector and these composites laminated together to form the basic electrode structure.