Solid-State Lithium-Sulfur Batteries: Neutrons unveil sluggish charge transport

The change in neutron attenuation in the cathode, shows where lithium accumulates: at the top during discharging, at the bottom during charging. d0 is the boundary to the solid electrolyte, dmax is the boundary between the cathode and the current collector.

The change in neutron attenuation in the cathode, shows where lithium accumulates: at the top during discharging, at the bottom during charging. d0 is the boundary to the solid electrolyte, dmax is the boundary between the cathode and the current collector. © HZB

A schematic illustration of the solid state battery. The anode is Li/In, the solid electrolyte separator Li<sub>6</sub>PS<sub>5</sub>Cl and the cathode composite is S/C/Li<sub>6</sub>PS<sub>5</sub>Cl.

A schematic illustration of the solid state battery. The anode is Li/In, the solid electrolyte separator Li6PS5Cl and the cathode composite is S/C/Li6PS5Cl. © HZB

3-D tomography images of the discharged state (top) and the recharged state (middle), as well as the difference between the two (bottom), showing where the mobile lithium ions (green) are located.

3-D tomography images of the discharged state (top) and the recharged state (middle), as well as the difference between the two (bottom), showing where the mobile lithium ions (green) are located. © HZB

Solid-state Lithium-Sulfur batteries offer the potential for much 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. Now, a new study from HZB shows 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. In conjunction with Dr. Nikolay Kardjilov, HZB, they 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.

 Lithium ions observed directly

"We now have  much better idea what is limiting the battery performance," says Bradbury: "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." Additionally, the 3D neutron tomography images show trapped lithium concentrated near the current collector during recharging. "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" says Bradbury.

Bottleneck identified

These results unveil a previously overlooked development bottleneck for solid-state batteries, showing that limitations exist in the cathode composites due to the slow ionic transport. The challenge now is to enable faster ion delivery within the cathode composite. "Without direct visualization of the reaction front inside the cathode composite this effect might have gone unnoticed, despite its importance for solid-state battery development," Bradbury says.

 

Footnote 1: The experiments took place at the end of 2019, before the neutron source BER II was shut down. The work will be continued in the future as part of the joint research group "NI-Matters" between HZB, the Institut Laue-Langevin (France) and the University of Grenoble (France).

 

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