Visualizing Reaction Fronts and Transport Limitations in Solid‐State Li–S Batteries via Operando Neutron Imaging

Abstract The exploitation of high‐capacity conversion‐type materials such as sulfur in solid‐state secondary batteries is a dream combination for achieving improved battery safety and high energy density in the push toward a sustainable future. However, the exact reason behind the low rate‐capability, bottlenecking further development of solid‐state lithium–sulfur batteries, has not yet been determined. Here, using neutron imaging, the spatial distribution of lithium during cell operation is directly visualized and it is shown that sluggish macroscopic ion transport within the composite cathode is rate‐limiting. Observing a reaction front propagating from the separator side toward the current collector confirms the detrimental influence of a low effective ionic conductivity. Furthermore, irreversibly concentrated lithium in the vicinity of the current collector, revealed via state‐of‐charge‐dependent tomography, highlights a hitherto‐overlooked loss mechanism triggered by sluggish effective ionic transport within a composite cathode. This discovery can be a cornerstone for future research on solid‐state batteries, irrespective of the type of active material.