It's difficult to see the details of atomic and topographical changes that lead to battery failure. However a team of researchers developed a method to see reactions leading to a layer that smothers the electrode in energy-dense-but short-lived-lithium-sulfur batteries.

The Forest, the Trees and Parasitic Reactions in Batteries. Researchers built a new stage and created a designer electrolyte to obtain both detailed and broad overviews of a troubling layer that causes promising lithium-sulfur batteries to fail. (Image courtesy Nathan Johnson, PNNL.)

This research is thanks in part to a new device that let the team track the progression of sulfur in a vacuum inside a powerful scientific instrument and to the ability to model the reaction using advanced software and computing resources.

The results from this fundamental study benefit energy storage in two ways. First, to do the work, the team created a new “stage.” This device let scientists determine the atomic composition and electronic and chemical states of the atoms on the electrode while the battery was running.

The second benefit of this study is the potential to solve the fading issue in lithium-sulfur batteries. Since sulfur is significantly cheaper than current cathode materials in lithium-ion batteries, the total cost of a lithium-sulfur battery will be low. Simultaneously, the increased energy density will be an advantage — approximately five times more than lithium-ion batteries.

The team achieved the results thanks to a combination of scientific innovation and serendipity. The innovation came in building a unique stage for their X-ray photoelectron spectroscopy (XPS) instrument. They needed to track the sulfur in the battery, but sulfur volatilizes in a vacuum. All samples in an XPS are studied under vacuum. Combining the newly designed stage and ionic liquids as electrolyte media let the team operate the battery inside the XPS and monitor the growth of sulfur-based compounds to see the parasitic reactions.

The electrolyte's composition is crucial, as it must survive the vacuum used by XPS. The team tested different compositions to see how well each electrolyte performed. They chose an electrolyte that contained 20 percent of the traditional solvent (DOL/DME) combined with an ionic solvent.

Using the XPS in analysis or spectroscopy mode, the team obtained the atomic information, including the atoms present and the chemical bonds between them. Switching over to an imaging or microscopic mode, the researchers acquired topological views of the solid-electrolyte interphase (SEI) layer forming. This view enabled them to see where the elements were on the surface. The combination of views let them obtain critical information over a wide range of spatial resolutions, spanning from angstroms to micrometers as the battery drained and charged.

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