Manufacturing Bits: Feb. 27

Magnesium-ion batteries
Texas A&M University and others have discovered a new metal-oxide magnesium battery cathode material—a technology that promises to deliver a higher density of energy storage than today’s traditional lithium-ion (Li-ion) cells.

Magnesium-ion battery technology is promising. A battery consists of an anode (negative), cathode (positive), electrolytes and a separator. In a simple operation, ions are transported from the anode to the cathode and back.

In lithium-ion batteries, a carbon-based material makes up the anode. A compound based on lithium-cobalt-oxide is used for the cathode in mobile phones, laptops and other products. In addition, electric vehicles use lithium-ion batteries. Graphite is used for the anode. The cathode is based on lithium with other metals. For example, Tesla uses a battery based on lithium with a nickel-cobalt-aluminum (NCA) mix.

The problem? The safety and supply of these materials present some challenges.

Looking for a new battery material, researchers from Texas A&M and others redesigned an old form of a Li-ion cathode material, dubbed vanadium pentoxide. Researchers reconfigured the atoms in these materials. This, in turn, provides a new and different path for magnesium ions to travel in the cell.

Vanadium pentoxide also prevents the magnesium ions from getting trapped within the material, enabling an optimal charge-storing capacity with negligible degradation.

A redesigned metastable phase of vanadium pentoxide (V2O5) shows extraordinary performance as a cathode material for magnesium batteries. (Credit: Justin Andrews.)

Magnesium-ion battery technology has several advantages. “Apart from being much safer for consumer applications, magnesium-ion technology is appealing fundamentally because each magnesium ion gives up two electrons per ion — twice the charge, whereas each lithium ion gives up only one,” said Texas A&M chemistry graduate student and NASA Space Technology Research Fellow Justin Andrews, on the university’s Web site. “This means that, all other considerations aside, if you can store as much magnesium in a material as you can store lithium, you immediately almost double the capacity of the battery.

“While this research has provided a great deal of insight, there are still several other fundamental problems to overcome before magnesium batteries become a reality,” Andrews said. “Nevertheless, this work moves magnesium batteries one step closer to reality — namely, a reality where batteries would be less expensive, lighter and safer for allowing for easier adoption to large-area formats necessary for electric vehicles and to store energy generated by solar and wind sources.”

Watching lithium ions
Using an X-ray synchrotron source, the Department of Energy’s SLAC National Accelerator Laboratory has examined how lithium ions navigate in a battery cell.

In the lab, researchers have examined what happens within a few nanometers of the electrode in a battery. Generally, the electrolyte molecules organize themselves into layers. These layers stand directly in the path of the lithium ions. The ions move through the electrolytes, which determine how fast the battery can charge.

In the lab, researchers used a metal-oxide material to represent the electrode. Then, using SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), researchers focused an X-ray beam on the surface of the electrode. They analyzed the sample. Researchers found that as the concentration of lithium ions in the electrolyte increased, the arrangement of the layers became more orderly. The layers were also farther apart.

The results were surprising. “Our hypothesis is that if you want to improve lithium ion transport, you want to decrease the amount of order in the layers, and that means decreasing the lithium ion concentration rather than increasing it,” said Hans-Georg Steinrück, a postdoctoral researcher, on SLAC’s Web site.

“That process of the ions finding their way into the electrode is very important in terms of how fast you can charge the battery and how long the battery lasts,” said Michael Toney, a distinguished staff scientist at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL). “Understanding the nanoscale details of how this works could suggest ways to increase charging speed and efficiency.”

An illustration of electrolyte molecules arranging themselves into layers within a few nanometers of a battery electrode. (Credit: Greg Stewart/SLAC National Accelerator Laboratory)

Chemical reactions
The Max-Planck-Institut has taken a step closer to understanding the atomic structure of the interface between the electrode and the electrolyte.

This, in turn, would open up new possibilities in chemical reactions in batteries and other products. Previous studies from others had the same findings. They show that the structure of the surface hardly changes in contact with a liquid electrolyte.

Within the framework of RESOLV, a joint research initiative of seven research institutions in Germany, the Max-Planck-Institut devised a semiconducting surface. The surface was brought into contact with an electrolyte. “We were completely surprised to see the formation of structures, which are unstable in the absence of water and are also not observed,” said Mira Todorova, head of the Electrochemistry and Corrosion group at the research organization.

Jörg Neugebauer, a professor in the group, added: “Our simulation methods allowed us not only to find a completely new and unexpected phenomenon, but also to identify the underlying mechanisms. This opens up totally new possibilities to shape and design surfaces with atomic precision.”