Multidimensional Operando Analysis of Li/S Batteries with Neutrons and Photons

Abstract

Lithium/sulfur (Li/S) batteries have a fivefold higher theoretical gravimetric energy density (2680 Wh/kg) than state-of-the-art lithium-ion batteries [1]. Furthermore, the abundance and environmental friendliness of sulfur qualify this system as a promising candidate for the post-lithium-ion era. However, the strong capacity fading with increasing cycle number is still a major obstacle to a broad technical utilization despite decades of research. Operando techniques [2] are very suitable tools to gain a mechanistic understanding of degradation processes. Especially the simultaneous combination of several independent measurements (multidimensional) while the Li/S cell is in operation allows deep insights into the degradation mechanisms and provides a further mechanistic understanding of the complex Li/S chemistry.Here we present results where up to five independent measurements are simultaneously performed with one setup (see image). Electrochemical impedance spectroscopy (EIS), UV-vis spectroscopy, temperature measurement, and X-ray imaging [3,4] or neutron small-angle scattering [5] were performed over several cycles while the cell was galvanostatically or potentiostatically charged and discharged. Structural changes on the macroscopic and microscopic scale can be correlated to characteristic signals in the EIS, charge-discharge curve, and UV-vis spectroscopy.The first part of the presentation focuses on X-ray imaging studies [3,4] that allow for valuable insights into operating Li/S cells. This method is a suitable tool for the investigation of the formation/dissolution processes of the charge product sulfur. The correlation of image analysis and impedance spectra enables a better mechanistic understanding of this system. Major observations are the formation of macroscopic alpha and beta-sulfur crystals, the formation of non-wetted cathode areas, and voltage-dependent electrolyte movement within the Li/S cell.The second part of the presentation deals with the formation and dissolution process of the discharge product, lithium sulfide. Here operando small-angle neutron scattering (SANS) is the weapon of choice to monitor nanoscopic lithium sulfide particles [5]. The SANS curves are correlated to the distribution of relaxation times calculated from impedance spectra. Furthermore, SANS gives information about the position of sulfur aggregation. In an ether-based electrolyte, neither lithium sulfide nor sulfur precipitate in the micropores.The last part of this presentation will give an outlook over the next experiments and discusses open questions in this field.