Low temperature alkali metal-sulfur batteries. Final report, December 1, 1974-November 30, 1978

Abstract

This report describes work carried out on the development of rechargeable, ambient temperature Li/sulfur and Li/metal sulfide batteries. The Li/S system has the cathode material dissolved in the electrolyte, as Li2S n. Tetrahydrofuran, 1M LiAsF6, is one of the more attractive electrolytes discovered for this cell, since it can dissolve up to ^10M S as Li2S n. Despite the oxidative nature of the electrolyte, Li is stable in it and can be electrodeposited from it on battery charge. Passivation of the Li due to insoluble Li +-permeable films is thought to be responsible for this stability. Cells of the configuration Li 5M S(as Li2S n), THF, 1M LiAsFs/carbon can be discharged at 50°C with a utilization of nearly 1.5e"/S at the C/3 rate. This corresponds to our rate-capacity goal for this battery (>50% utilization at C/3) in its proposed vehicular or load-leveling applications. Further improvements in rate are possible with Lewis acidic • catalysts such as <f>3B or Ba + 2. Rechargeability of 135 cycles of 0.1e"/S and ^45 cycles of 0.5e~/S have been demonstrated for this cell. The self-discharge reaction keeps the Li electrode free of electrically isolated dendrites. Ultimate failure on cycling is due to cathode depletion via precipitation of Li2S on the anode in a form which is insoluble in the electrolyte. To date, attempts to solubilize the Li2S by the internal generation of an oxidizing "scavenger" (e.g., Br2) or by addition of Lewis acids have met only with limited success. Cells of configuration Li/THF, 1M LiAsF6/insoluble metal sulfide were investigated, using the following cathodes: CuS, NiS, SiS2, MnS2, FeS and Bi2S3. Of these, the most promising new material in terms of energy density and rechargeability is CuS. Well over 100 cycles for Li/CuS cells with moderate cathode loadings have been demonstrated, giving cathode efficiencies of >0.8 eq/mole. CuS compares favorably with TiS2 in terms of energy density and rechargeability and is superior in terms of economics.