The electrochemistry of stable sulfur isotopes versus lithium

Abstract

Sulfur in nature consists of two abundant stable isotopes, with two more neutrons in the heavy one ( 34 S) than in the light one ( 32 S). The two isotopes show similar physicochemical properties and are usually considered an integral system for chemical research in various fields. In this work, a model study based on a Li–S battery was performed to reveal the variation between the electrochemical properties of the two S isotopes. Provided with the same octatomic ring structure, the cyclo - 34 S 8 molecules form stronger S–S bonds than cyclo - 32 S 8 and are more prone to react with Li. The soluble Li polysulfides generated by the Li– 34 S conversion reaction show a stronger cation–solvent interaction yet a weaker cation–anion interaction than the 32 S-based counterparts, which facilitates quick solvation of polysulfides yet hinders their migration from the cathode to the anode. Consequently, the Li– 34 S cell shows improved cathode reaction kinetics at the solid–liquid interface and inhibited shuttle of polysulfides through the electrolyte so that it demonstrates better cycling performance than the Li– 32 S cell. Based on the varied shuttle kinetics of the isotopic-S-based polysulfides, an electrochemical separation method for 34 S/ 32 S isotope is proposed, which enables a notably higher separation factor than the conventional separation methods via chemical exchange or distillation and brings opportunities to low-cost manufacture, utilization, and research of heavy chalcogen isotopes.