Low temperature lithium/sulfur secondary battery. Annual progress report, December 1, 1974--December 1, 1975. [Ambient-temperature battery with dissolved S]

Abstract

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.

Lithium Sulfur Battery: Oxidation/Reduction Mechanisms of Polysulfides in THF Solutions

“…There is good agreement between the experimental results and those calculated from numbers based on the above overall mechanisms (Eq. [6], [7]).…”

“…This has been attributed to blockage of the cathode by reaction products and to slow reduction kinetics of low order polysulfides (PS), the latter being solvent dependent (5). Studies of the redox behavior of sulfur and PS in various aprotic solvents showed the overall reaction to be a multielectron process with many equilibrium reactions between sulfur and PS (6,7). It was further found that reduction of low order PS was more pronounced in THF than in DMSO (8).…”

Conductivity of Block Copolymer Electrolytes Containing Lithium Polysulfides

“…There is considerable interest in rechargeable lithium–sulfur batteries because their theoretical specific energy, 2600 Wh/kg, is 5 times greater than that of current lithium-ion batteries. − Elemental sulfur is abundant, nontoxic, and inexpensive compared to cobalt- and iron-based cathodes in conventional lithium-ion batteries . There are, however, many challenges that must be addressed before lithium–sulfur batteries become a commercial reality .…”

Effect of Lithium Polysulfides on the Morphology of Block Copolymer Electrolytes

“…Until this point we have provided no insight on the distribution of values of x present in the system. UV–vis spectroscopy has been used to distinguish the Li 2 S x speciation in organic solvents such as DMSO, ,− DMF, − THF, ,,,, dimethylacetamide, liquid ammonia, , and acetonitrile . Detailed spectroelectrochemical experiments are necessary for unambiguous determination of speciation. ,,, These studies show that the distributions of species present depend crucially on the solvent.…”