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 (³⁴S) than in the light one (³²S). 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 <i>cyclo</i>-³⁴S₈ molecules form stronger S-S bonds than <i>cyclo</i>-³²S₈ and are more prone to react with Li. The soluble Li polysulfides generated by the Li-³⁴S conversion reaction show a stronger cation-solvent interaction yet a weaker cation-anion interaction than the ³²S-based counterparts, which facilitates quick solvation of polysulfides yet hinders their migration from the cathode to the anode. Consequently, the Li-³⁴S 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-³²S cell. Based on the varied shuttle kinetics of the isotopic-S-based polysulfides, an electrochemical separation method for ³⁴S/³²S 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.