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The purpose of this program is to develop an ambient temperature Li/S secondary battery. The proposed configuration is Li/Organic Electrolyte, Dissolved S/catalytic electrode or current collector In a typical practical arrangement, this battery will have an energy density of 100 Whr/lb, including the weight of cell hardware, if a 5M S solution is discharged with 50% efficiency at 2.0V. Investigations of S solubility and electrochemistry have been undertaken in polar, aprotic nonaqueous solvents, chosen for their stability toward Li. Highest S solubility was achieved if S were dissolved in the form of Li polysulfides, Li2Sn. Both electrochemical and chemical techniques were surveyed as methods for preparing Li2Sn electrolytic solutions. Of the solvents tested, highest S solubilities were achieved in dimethyl sulfoxide (DMSO) and tetrahydrofuran (THE), where 9-lOM S solutions were obtained as Li2S9_3^Q. The solutions of highest S concentrations were most readily prepared through the direct reaction of Ss with Li2S in the presence of the solvent. The discharge and charge capacities of such solutions, 1-2M in S, were measured galvanostatically (1 mA/cm2) between preset limits of 1 and 3.7V (vs. Li) in a specially designed coulometry cell. The working electrode was C cloth. The capacities measured from the it curves were checked in the case of DMSO by chemical analysis of total S and S~2 before and after cycling. In this solvent, discharge of a 2M S solution prepared as Li2S24 yielded a Li2S5 solution i'^0.33 e~/S). Two plateaus were observed, at 2.7V and at 2.2V. Discharge capacities of approximately 0.25 e~/S were observed in THE and in methyl acetate (MA). Recharge generally occurred between 2.5 and 3.0V. Solutions were subjected to >10 cycles. Some loss of capacity was noted for each cycle, but much of this was Inherent in the design of our coulometry cell. The reasons for limitations of charge and discharge capacities were sought in spectroelectrochemical investigations of dilute polysulfide solutions. In DMSO, such experiments on individual polysulfides showed that S8 or S8~2 could not be reduced on C below S/^~^, positive of l.OV vs. Li+ZLi. However, S-2 and S4-2 could be readily oxidized on C up to ^>10» ^thout the formation of (insoluble) SQ. Only Ss"^ gave S3 precipitation upon oxidation. i It is therefore concluded that the c/cling capacity of the dissolved Li2S cathode has been limited so f by the reduction of S~^A on C. During the next report period, a sea.. for an appropriate heterogeneous or homogeneous catalyst for this reduct:. i will be carried out. Work on the Li electrode in these polysulfide solutions is being undertaken on another program.'^" Preliminary results Indicate that 10 coul/cm2 of Li may be repeatedly cycled on a Li substrate with an 85% efficiency in TKF, 5M S (as Li2SjQ). This efficiency is comparable to that in the S-free solvent, and indicates that rechargeability of the negative will not be a special problem.

Die Bildung von LiZSn in aprotischen Lösungsmitteln wie Dimethylacetamid, DMF, DMSO, Propy1encarbonat, THF, Butyrolacton, Sulfolan (interessant im Zusammenhang mit Li/S‐Batterien) kann entweder durch elektrochemische Reduktion von S8 oder in situ durch direkte Reaktion von S8 mit Li oder Li,Serreicht werden.

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 Sg, 0.1-1M L1C104 solutions in dimethyl sulfoxide (DMSO), dimethly formamide (DMF), tetrahydrofuran (THF), propylene carbonate (PC), y-butyrolactone (BL), sulfolane (SL), methyl acetate (MA) and pyridine was investigated using cyclic voltammetry. Ss and Sg~ reductions occur at more positive potentials and the redox reactions are more reversible in DMSO and DMF than in the other solvents. Raising the temperature generally enhances reversibility and reaction rates. Some evidence exists for the precipitation of Li2S x species on Au and Pt working electrodes (but not on C) leading to their passivation, especially at elevated temperatures .

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