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Magnesium Mystery in Rechargeable Battery Performance Solved

19 October 2017

Magnesium-based rechargeable batteries have the potential to extend electric vehicle range by packing more energy into smaller batteries. But unforeseen chemical roadblocks have slowed this process.

The places where solid meets liquid — where the oppositely charged battery electrodes interact with the surrounding chemical electrolyte — are the problem.

This is a photo illustration showing rechargeable batteries in the shape of an automobile. Source: iStock/GettyThis is a photo illustration showing rechargeable batteries in the shape of an automobile. Source: iStock/Getty

A research team at the U.S. Department of Energy’s Joint Center for Energy Storage Research, led by scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered a set of chemical reactions involving magnesium that degrades battery performance before the battery can be fully charged.

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

The battery tested by the team used magnesium as its anode, in contact with an electrolyte composed of a liquid solvent known as diglyme, and dissolved salt, Mg(TFSI)2.

The materials the research team used were thought to be compatible and nonreactive in the battery’s resting state. But experiments at Berkeley Lab’s Advanced Light Source (ALS) uncovered that is not the case.

"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," said David Prendergast, who directs the Theory of Nanostructured Materials Facility at the Molecular Foundry and served as one of the study's leaders.

"At that point, it got very interesting," he said. "What could possibly cause these reactions between substances that are supposed to be stable under these conditions?"

Molecular Foundry researchers developed detailed simulations of where the electrode and electrolyte meet, indicating that no spontaneous chemical reactions should occur under ideal conditions. But the simulations did not account for all of the chemical details.

"Prior to our investigations," said Ethan Crumlin, an ALS scientist who coordinated the X-ray experiments and co-led the study with Prendergast, "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."

Commercially popular lithium-ion batteries, which power many portable electronic devices, shuttle lithium ions back and forth between the two battery electrodes. The electrode materials are porous at the atomic scale and are alternatively loaded up or emptied of lithium ions as the battery is charged or discharged.

In this type of battery, the negative electrode is typically composed of carbon, which has a more limited capacity for storing these lithium ions than other materials would.

Increasing the density of stored lithium by using another material would make lighter, smaller and more powerful batteries. Using lithium metal in the electrode can pack in more lithium ions in the same space. But it is a highly reactive substance that burns when exposed to air and requires further research on how to best package and protect it for long-term stability.

Magnesium metal has a higher energy density than lithium metal. It could potentially store more energy in a battery of the same size if magnesium is used rather than lithium.

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. Researchers are looking for ways to avoid this.

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

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 pictures of these materials and how they interact.

The research team determined 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 is a problem," Prendergast said. "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."

A further round of simulations 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 that dries out the battery over time. The products of these reactions foul the anodes’ surface impairing the battery’s function.

It took several iterations to develop a model consistent with X-ray measurements. The efforts were supported by millions of hours of computing power at the lab’s National Energy Research Scientific Computing Center.

It is important to have access to X-ray techniques, nanoscale expertise and computing resources all in the same lab.

The results could be relevant to other types of battery materials, including prototypes based on lithium or aluminum metal.

Prendergast said, "This could be a more general phenomenon defining electrolyte stability."

Crumlin added, "We've already started running new simulations that could show us how to modify the electrolyte to reduce the instability of these reactions." Likewise, he said, 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," he added. "Right now it leads to uncontrollable events."

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