Low temperature lithium/sulfur secondary battery. Semiannual progress report, December 1, 1974--June 1, 1975. [Development of dissolved sulfur cathode operable at ambient temperatures

Abstract

The objective of this program is to develop a dissolved sulfur cathode which can be repeatably charged and discharged with minimal degradation and which is compatible with a Li anode. The redox behavior of dilute S/sub 8/, 0.1--1M LiClO/sub 4/ solutions in dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), tetrahydrofuran (THF), propylene carbonate (PC), ..gamma..-butyrolactone (BL), sulfolane (SL), methyl acetate (MA) and pyridine was investigated. S/sub 8/ and S/sub 8//sup - -/ reductions occur at more positive potentials and the redox reactions are more reversible in DMSO and DMF than in the other solvents. Some evidence exists for the precipitation of Li/sub 2/S/sub x/ species on Au and Pt working electrodes (but not on C). The electrochemical behavior of Li/sub 2/S/sub x/ solutions was examined. Cyclic voltammograms of polysulfide solutions in DMSO showed generally reductions at more negative potentials and oxidations at more positive potentials than those for S/sub 8/ solutions. The polysulfide and S/sub 8/ voltammograms in THF were more similar. Steady-state current potential curves of DMSO--Li/sub 2/S/sub x/ solutions generally revealed no sizeable reduction currents positive of approximately 1.5 V vs. Li. Galvanostatic discharge of Li/sub 2/S/sub x/ solutions occurs between 1.0 and 2.0 V. Recharge (with a 4.0more » V limit) was possible in THF, BL and DMSO. The fact that steady-state discharge in these solutions, andparticularly in DMSO--Li/sub 2/S/sub x/ solutions, occurs at more negative potentials than indicated by cyclic voltammograms suggests a degradation of active species in solution following initial reduction. If the species S/sub 3//sup -/ is more difficult to reduce than S/sup - -//sub x/, then solvents or electrolyte additives which stabilize S/sup - -//sub x/ in preference to S/sub 3//sup -/ would be most favorable for use in a practical battery. 23 figures, 8 tables.« less