Electric vehicles (EVs) have the potential to revolutionize transportation and protect the environment at the same time. EVs, however, are not quite perfected.

One issue that must be improved is the energy content stored in the battery, which limits the driving range of the EV. Another obstacle is that current lithium-ion batteries use cobalt, a mineral with a mining process that studies have shown increases violence, substance abuse, food and water insecurity, and physical and mental health challenges in the areas where it is found. 

There are battery materials known that overcome these limitations; however, they come with their own significant drawback: they lose a little bit of voltage every time they are recharged.

Conscious of those problems, a research team led by Northwestern Engineering’s Chris Wolverton is working toward a solution. 

In a recently published paper, Wolverton and his colleagues demonstrated the potential effectiveness of a cobalt-free battery cathode using lithium transition-metal oxides and various stacked components composed of lithium, manganese, and oxygen. This alignment strengthened the structure of the cathode, reducing the loss of oxygen and subsequently the voltage fade, which results in a longer lasting and more durable battery. 

The work could have implications with EVs, and products such as smart phones and laptop computers.

“One of the main things everybody wants is more capacity in their batteries. Increased capacities would mean cell phones that lasted longer, laptops that stay charged longer, or EVs that didn’t require as frequent recharges,” Wolverton said. “Getting more energy content out of these batteries is a game everybody is very interested in playing. The current battery materials, the ones that are actually used now in your cell phone, are limited in how much capacity you can get out of them, and how much you can cycle reversibly.”

Wolverton is a professor of materials science and engineering at the McCormick School of Engineering. The findings were presented in the paper “A Li-rich Layered Oxide Cathode with Negligible Voltage Decay,” published in July in Nature Energy. Contributors included Yi Xia, an assistant professor of mechanical and materials engineering at Portland State University and a former postdoc at Northwestern; and researchers from Argonne National Laboratory; Lanzhou University (China); Chinese Academy of Sciences; Tsinghua University (China); and the City University of Hong Kong.

Researchers from the City University of Hong Kong worked with Wolverton to explain the behavior of the material combination. Wolverton and his colleagues found that the atomic structure of the lithium transition-metal oxides, combined with the lithium, manganese, and oxygen, was unusual and had a structure where nickel would go instead of lithium. That arrangement stabilizes the structure, prevents oxygen from being released, and doesn’t suffer from voltage fade, allowing the cathode to be safer, stronger, and store more energy.

The playground is open. We can search for new materials that have this magic arrangement of nickel and lithium and see if we can find even better chemistries that have this stabilizing atomic structure. Chris Wolverton

Using a synthesis method for sodium, the researchers in Hong Kong exchanged the sodium for lithium. The sodium layers were offset and shuffled the atomic planes, forcing the unusual nickel structure to develop.

“We did calculations of that particular structure, and we found that it enhances the stability of the compound in terms of the oxygen release being less favored and the voltage decay is disfavored as well,” Wolverton said.

Though this material is not yet ready for wide use, the research provides a jumping-off point for Wolverton and other researchers. Wolverton and his group perform atomic-scale computational modeling, placing atoms in certain arrangements and calculating the arrangement’s properties.

“The playground is open,” Wolverton said. “We can search for new materials that have this magic arrangement of nickel and lithium and see if we can find even better chemistries that have this stabilizing atomic structure.”