Visualizing reaction fronts and transport limitations in solid-state Li-S batteries via operando neutron imaging

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 towards a sustainable future. Yet, the exact rate-limiting step, bottlenecking further development of solid-state lithium-sulfur batteries, has not been determined. Here, we directly visualize the spatial distribution of lithium via neutron imaging during operation and show that sluggish macroscopic ion transport within the composite cathode is rate-limiting. Observing a reaction front propagating from the separator side towards the current collector confirms detrimental influences 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 will be a cornerstone for future research on solid-state batteries, irrespective of the type of active material.