A discovery that could lead to more durable electric vehicle batteries and accelerate the energy transition

Batteries lose capacity over time, which is why older cell phones drain more quickly. This common phenomenon, however, is not fully understood.

An international team of researchers, led by a University of Colorado Boulder engineer, has revealed the mechanism underlying battery degradation. Their discovery could help scientists develop better batteries that would allow electric vehicles to go further and last longer, while also advancing energy storage technologies that would accelerate the transition to clean energy.

The results were published on September 12 in the journal Science.

“We are contributing to the improvement of lithium-ion batteries by uncovering the molecular processes involved in their degradation,” said Michael Toney, corresponding author of the study and professor in the Department of Chemical and Biological Engineering. “Having a better battery is very important to the evolution of our energy infrastructure, moving from fossil fuels to more renewable energy sources.”

Engineers have been working for years to design lithium-ion batteries (the most common type of rechargeable battery) without cobalt. Cobalt is a rare and expensive mineral, and its extraction is linked to serious environmental and human rights concerns. In the Democratic Republic of Congo, which supplies more than half of the world’s cobalt, many miners are children.

So far, scientists have tried using other elements like nickel and magnesium to replace cobalt in lithium-ion batteries. But these batteries have even higher self-discharge rates, which is when the battery’s internal chemical reactions reduce the stored energy and degrade its capacity over time. Because of self-discharge, most electric vehicle batteries last seven to 10 years before needing to be replaced.

Toney, who is also a fellow at the Renewable and Sustainable Energy Institute, and his team set out to find the cause of self-discharge. In a typical lithium-ion battery, lithium ions, which carry charges, move from one side of the battery, called the anode, to the other side, called the cathode, through a medium called the electrolyte.

During this process, the flow of these charged ions forms an electric current that powers electronic devices. Charging the battery reverses the flow of charged ions and returns them to the anode.

Previously, scientists thought that batteries self-discharged because not all of the lithium ions returned to the anode when charged, reducing the number of charged ions available to form current and provide energy.

Using the Advanced Photon Source, a powerful X-ray machine at the U.S. Department of Energy’s Argonne National Laboratory in Illinois, the research team found that hydrogen molecules in the battery’s electrolyte were moving toward the cathode and occupying the spots where lithium ions normally bind. As a result, lithium ions have fewer places to bind on the cathode, weakening the electric current and decreasing the battery’s capacity.

Transportation is the largest source of greenhouse gases generated in the United States, accounting for 28% of the country’s emissions in 2021. In an effort to reduce emissions, many automakers have pledged to shift away from developing gasoline-powered cars and toward producing more electric vehicles.

But electric vehicle makers face a host of challenges, including limited range, higher production costs and shorter battery life than conventional vehicles. In the U.S. market, a typical electric car can travel about 250 miles on a single charge, about 60 percent of the range of a gasoline-powered car. The new study has the potential to address all of those issues, Toney said.

“All consumers want cars with a long range. Some of these low-cobalt batteries have the potential to offer longer range, but we also need to make sure they don’t fail in a short period of time,” he said, noting that reducing cobalt can also reduce costs and address human rights and energy justice concerns.

With a better understanding of the self-discharge mechanism, engineers can explore several ways to prevent the process, such as coating the cathode with a special material to block hydrogen molecules or using a different electrolyte.

“Now that we understand what causes batteries to degrade, we can inform the battery chemist community about what needs to be improved when designing batteries,” Toney said.