Lithium-sulfur battery diagnostics through distribution of relaxation times analysis

Abstract

• Distribution of relaxation times (DRT) is a powerful tool for deconvolution relaxation processes occurring in the overlapping frequency region in the EIS. • DRT peaks are correlated to the specific electrode process through analysis of relaxation processes in symmetric cells and evolution of the DRT profile of a fresh cell at different state-of-charge. • Impedance features of a pristine cell are dominated by the Li metal negative electrode and by the positive electrode after the initial charge-discharge. • Flooded electrolyte improves electrode kinetics and ion diffusion while lean electrolyte conditions cause high polarization throughout the discharge. Electrochemical impedance spectroscopy (EIS) is widely used in battery analysis as it is simple to implement and non-destructive. However, the data provided is a global representation of all electrochemical processes within the cell and much useful information is ambiguous or inaccessible when using traditional analysis techniques. This is a major challenge when EIS is used to analyse systems with complex cell chemistries, like lithium-sulfur (Li-S), one of the strongest candidates to supersede conventional Li-ion batteries. Here we demonstrate the application of distribution of relaxation times (DRT) analysis for quantitative deconvolution of EIS spectra from Li-S batteries, revealing the contributions of (eight) distinct electrode processes to the total cell polarisation. The DRT profile is shown to be strongly dependent on cell state-of-charge, offering a route to automated and on-board analysis of Li-S cells.