Visualizing Lithium Ion Transport in Solid State Li S Batteries Using 6Li Contrast Enhanced Neutron Imaging

The elucidation of lithium ion transport pathways through a solid electrolyte separator is a vital step toward development of reliable, functional all solid state batteries. Here, advantage has been taken of the significantly higher neutron attenuation coefficient of one of the most abundant stable isotopes of lithium, 6Li, with respect to that of naturally occurring lithium isotope mixture, to perform neutron imaging on a purpose built all solid state lithium sulfur battery. Increasing the 6Li content in the anode while using natural lithium in the solid electrolyte separator and the cathode enhances the contrast such that it is possible to differentiate, during the initial discharge, between the mobile lithium ions diffusing through the cell from the anode and those that are initially located in the solid electrolyte. The sensitivity of neutrons to the different lithium isotopes means that operando neutron radiography allows demonstration of the lithium ion diffusion through the cell while in situ neutron tomography has permitted presentation, in three dimensions, of the distribution of the trapped lithium ions inside the cell in charged and discharged state

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 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 materia