Visualizing Sulfur Conversion in Li–S Batteries with Operando SAXS/WAXS

Real-time structural insights across multiple length scales in solvating and sparingly solvating electrolytes

Lithium–sulfur (Li–S) batteries are among the most promising next-generation energy storage technologies, offering exceptionally high theoretical energy density, low cost, and abundant raw materials. Yet, despite decades of research, their performance remains hindered by poorly understood sulfur-to-sulfide conversion mechanisms especially under lean electrolyte conditions. This results in problems controlling parasitic polysulfide shuttling, active material loss, and capacity fading.

Three primary sulfur-to-sulfide conversion pathways have been identified in liquid-electrolyte Li-S systems:

• the classical solid-liquid-solid (dissolution-precipitation) mechanism in standard solvating ether-based electrolytes,
• solid-state conversion in microporous carbons
• quasi-solid-state conversion using sparingly solvating electrolytes.

Sparingly solvating electrolytes suppress polysulfide solubility and, in principle, enable stable cycling at ultralow electrolyte-to-sulfur (E/S) ratios. Yet, despite their theoretical promise, the structural evolution during quasi-solid-state sulfur-to-sulfide conversion remains insufficiently understood, and the associated performance benefits have not been fully realized in practical devices.

In this application note, we demonstrate how operando small- and wide-angle X-ray scattering (SAXS/WAXS) can be used to monitor the nanoscale structural evolution in real-time during electrochemical cycling in solvating and sparingly solvating electrolytes. The structural interpretation of the SAXS/WAXS data is consistent with ex-situ cryogenic transmission electron microscopy (cryo-TEM) and electron energy loss spectroscopy (EELS).

Acknowledgements

Operando SAXS/WAXS data reproduced from [2] with the kind permission of Dr. Christian Prehal.