The team used X-ray experiments, theoretical modeling, and supercomputer simulations to develop a full understanding of the chemical breakdown of a liquid electrolyte occurring within tens of nanometers of an electrode surface that degrades battery performance.

The battery they were testing featured magnesium metal as its anode in contact with an electrolyte composed of a liquid (a type of solvent known as diglyme) and a dissolved salt, Mg(TFSI)₂.

While the combination of materials they used were believed to be compatible and nonreactive in the battery’s resting state, experiments at Berkeley Lab’s Advanced Light Source (ALS), an X-ray source called a synchrotron, uncovered that this is not the case and led the study in new directions.

People had thought the problems with these materials occurred during the battery’s charging, but instead the experiments indicated that there was already some activity. At that point it got very interesting. What could possibly cause these reactions between substances that are supposed to be stable under these conditions?

—David Prendergast, who directs the Theory of Nanostructured Materials Facility at the Molecular Foundry and served as one of the study’s leaders

Molecular Foundry researchers developed detailed simulations of the interface between the electrode and electrolyte, indicating that no spontaneous chemical reactions should occur under ideal conditions, either. The simulations, though, did not account for all of the chemical details.

Prior to our investigations, there were suspicions about the behavior of these materials and possible connections to poor battery performance, but they hadn’t been confirmed in a working battery.

—Ethan Crumlin, an ALS scientist who coordinated the X-ray experiments and co-led the study with Prendergast

Magnesium’s surface forms a self-protecting “oxidized” layer as it reacts with moisture and oxygen in the air. But within a battery, this oxidized layer is believed to reduce efficiency and shorten battery life, so researchers are looking for ways to avoid its formation.

To explore the formation of this layer in more detail, the team employed a unique X-ray technique developed recently at the ALS, called APXPS (ambient pressure X-ray photoelectron spectroscopy). This new technique is sensitive to the chemistry occurring at the interface of a solid and liquid, which makes it an ideal tool to explore battery chemistry at the surface of the electrode, where it meets the liquid electrolyte.

Simulations show the weakening of a bond in a liquid solvent due to the presence of free-floating hydroxide ions that contain a single oxygen atom bound to a hydrogen atom. In this illustration, atoms are color-coded, as shown at the bottom: hydrogen (white), oxygen (red), carbon (light blue), magnesium (green), nitrogen (dark blue), sulfur (yellow), fluorine (brown). (Credit: Berkeley Lab) Click to enlarge.

Even before a current was fed into the test battery, the X-ray results showed signs of chemical decomposition of the electrolyte, specifically at the interface of the magnesium electrode. The findings forced researchers to rethink their molecular-scale picture of these materials and how they interact.

What they determined is that the self-stabilizing, thin oxide surface layer that forms on the magnesium has defects and impurities that drive unwanted reactions.

It’s not the metal itself, or its oxides, that are a problem. It’s the fact you can have imperfections in the oxidized surface. These little disparities become sites for reactions. It feeds itself in this way.

—David Prendergast

A further round of simulations, which proposed possible defects in the oxidized magnesium surface, showed that defects in the oxidized surface layer of the anode can expose magnesium ions that then act as traps for the electrolyte’s molecules.

If free-floating hydroxide ions—molecules containing a single oxygen atom bound to a hydrogen atom that can be formed as trace amounts of water react with the magnesium metal—meet these surface-bound molecules, they will react. This wastes electrolyte, drying out the battery over time. And the products of these reactions foul the anode’s surface, impairing the battery’s function.

It took several iterations back and forth, between the experimental and theoretical members of the team, to develop a model consistent with the X-ray measurements. The efforts were supported by millions of hours’ worth of computing power at the Lab’s National Energy Research Scientific Computing Center.

The results could be relevant to other types of battery materials, too, including prototypes based on lithium or aluminum metal. This could be a more general phenomenon defining electrolyte stability, Prendergast noted.

The team is already running new simulations that could show how to modify the electrolyte to reduce the instability of these reactions. Further, suggested Crumlin, it may be possible to tailor the surface of the magnesium to reduce or eliminate some of the unwanted chemical reactivity.

Rather than allowing it to create its own interface, you could construct it yourself to control and stabilize the interface chemistry. Right now it leads to uncontrollable events.Ethan Crumlin

Berkeley Lab’s Advanced Light Source, Molecular Foundry, and National Energy Research Scientific Computing Center are DOE Office of Science User Facilities that are open to visiting researchers from around the nation and world.

The team included researchers from the Joint Center for Energy Storage Research at Berkeley Lab and Sandia National Laboratories in New Mexico, together with scientists from the University of Maryland, and from the Shanghai Institute of Microsystem and Information Technology in China. Funding for the project was provided by the Joint Center for Energy Storage Research, a Department of Energy Innovation Hub, and the research was also supported by the USDepartment of Energy’s Office of Basic Energy Sciences.

Resources

• Yi Yu, Artem Baskin, Carlos Valero-Vidal, Nathan T. Hahn, Qiang Liu, Kevin R. Zavadil, Bryan W. Eichhorn, David Prendergast, and Ethan J. Crumlin (2017) “Instability at the Electrode/Electrolyte Interface Induced by Hard Cation Chelation and Nucleophilic Attack” Chemistry of Materials 29 (19), 8504-8512 doi: 10.1021/acs.chemmater.7b03404