New X-ray discovery could lead to the holy grail of long-lasting EV batteries

An international team of researchers led by Michael F. Toney, a Chemical and Biological Engineering professor at the University of Colorado Boulder, has determined the root cause of battery capacity loss over time, also known as self-discharge, using powerful X-rays.

It is common knowledge that batteries will lose capacity over time. Whether in a mobile phone or an electric vehicle, rechargeable batteries eventually lose their ability to hold charge and must be replaced.

Even after decades of experience designing and manufacturing different battery types, people have yet to learn what caused this phenomenon. “Having a better battery is very important in shifting our energy infrastructure away from fossil fuels to more renewable energy sources,” said Toney in a statement.

As a fellow of the Renewable and Sustainable Energy Institute, Toney led a research group that examined this problem.

What is self-discharge?

In a lithium-ion battery , the most extensively used battery worldwide, lithium ions move from the anode, the positive terminal, to the cathode, the negative terminal, through an electrolyte solution. This forms a current that powers devices from phones to laptops and even electric vehicles.

Lithium ions are returned to the anode during the charging process, and the cycle can repeat. However, it was believed that not all lithium ions are returned to the anode, which leads to a drop in capacity over time or self-discharge.

Since using rare Earth mineral cobalt in batteries has raised concerns about human rights and environmental costs, researchers have been working on developing batteries that do not use cobalt but nickel and magnesium. However, these batteries are even more prone to self-discharge

Toney’s team used the Advanced Photon Source (APS), a powerful X-ray machine at the Argonne National Laboratories in Illinois, to determine the cause of the self-charge. “The APS is a powerful and large (1.1 km circumference) outside of Chicago and provides very intense X-ray beams,” Toney told Interesting Engineering in an email.

The root cause of self-discharge

The team found hydrogen atoms are formed due to a reaction between the electrolyte and the cathode after the battery is fully charged. These atoms bind to the cathode where lithium would otherwise bind during discharge.

This prevents lithium from reaching the cathode, thereby reducing the electric current generated and the battery’s overall capacity, a press release said. Now that researchers understand what causes self-discharge, they can direct efforts to prevent it.

Coating the cathode with a special material that blocks only hydrogen atoms could be an option. “Typically protective oxides such as alumina (Al2O3) can be used to coat the cathode,” added Toney in the email. “These are not expensive and there are processes to do the coating but we are not sure how long the coating survives.”

Nevertheless, the research opens up new frontiers and could help develop better batteries. As the world looks to phase out fossil fuels, batteries that do not lose charging capacity over their lifespan will be crucial.

It will advance renewable energy storage solutions and reduce the cost of storing excess energy. For electric vehicles, this would mean no decrease in range or a requirement to replace the battery pack even after years of car usage, thereby increasing their adoption.

The research findings were published in the journal Science today.