Electrochemical fractionation of stable sulfur isotopes in a rechargeable lithium–sulfur battery: a revisit from the law of mass conservation

Abstract

Fractionation of stable ³²S/³⁴S isotopes occurs via a ‘lithium polysulfide (LiPS) shuttle’ process upon (dis)charging a Li–S battery, with lighter ³²S isotope species with a larger diffusion coefficient enriched at the anode side. However, the global distribution of S isotopes within the battery and its dependence on the state of charge (SoC) of battery remain unclear. In this work, we quantitatively measured the S isotope distribution in the cathode, anode, and electrolyte of a Li–S battery at different SoCs using triple-quadrupole inductively coupled plasma mass spectrometry (TQ-ICP-MS). By establishing a unified sample pretreatment protocol for all S species, we demonstrate, for the first time, mass conservation of S isotopes within a cycled Li–S battery. Loss of active ³²S species from the cathode to the electrolyte (and finally to the anode) accounts for enrichment of ³⁴S species at the cathode during battery cycling. Based on isotope mass conservation, the separation factors of ³²S and ³⁴S were found to be positively correlated, while trade-offs were found both between the single-stage yields of the two isotopes and between the separation factor and yield of a given isotope. Our findings help define the boundary conditions for key parameters (e.g., separation factors, yields and stages) for electrochemical cascade separation of stable S isotopes.