Abstract
Fractionation of stable (32)S/(34)S isotopes occurs via a 'lithium polysulfide (LiPS) shuttle' process upon (dis)charging a Li-S battery, with lighter (32)S 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 (32)S species from the cathode to the electrolyte (and finally to the anode) accounts for enrichment of (34)S species at the cathode during battery cycling. Based on isotope mass conservation, the separation factors of (32)S and (34)S 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.