Neutrons reveal slow charge transport for solid-state Li-ion batteries

Solid-state lithium-sulfur batteries offer the potential for higher energy densities and increased safety, compared to conventional lithium-ion batteries. However, the performance of solid-state batteries is currently lacking, with slow charging and discharging being one of the primary causes. A new study from Helmholtz Zentrum Berlin (HZB) has shown that sluggish lithium-ion transport within a composite cathode is the cause of this slow charging and discharging.

The scientists designed a special cell in order to observe the transport of lithium ions between the anode and the cathode in a solid-state lithium-sulfur battery. Since lithium can hardly be detected with X-ray methods, HZB physicists Dr Robert Bradbury and Dr Ingo Manke examined the sample cell with neutrons, which are extremely sensitive to lithium. The researchers used neutron radiography and neutron tomography methods on the CONRAD2 instrument at the Berlin neutron source BER II1. Groups from Giessen (JLU), Braunschweig (TUBS) and Jülich (FZJ) were also involved in the work.

Bradbury said that the researchers now have a better idea of what is limiting the battery performance. “We see from the operando neutron radiography data that there is a reaction front of lithium ions propagating through the composite cathode confirming the negative influence of a low effective ionic conductivity,” Bradbury said.

Additionally, the 3D neutron tomography images showed trapped lithium concentrated near the current collector during recharging; according to Bradbury, this results in a diminished capacity because only some of the lithium is transported back when the battery is charged. The observed lithium distribution was an excellent fit to a model based on the theory of porous electrodes. “What we observe here in the neutron imaging data correlates well with the relevant electronic and ionic conductivity conditions from the model,” Bradbury said.

The results revealed a previously overlooked development bottleneck for solid-state batteries, showing that limitations exist in the cathode composites due to the slow ionic transport. Researchers will not try to enable faster ion delivery within the cathode composite. “Without direct visualisation of the reaction front inside the cathode composite this effect might have gone unnoticed, despite its importance for solid-state battery development,” Bradbury said.

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