Better degradation understanding to extend EV battery lifetimes

A team of researchers has published new findings into the underlying mechanism behind battery degradation. Their research aims to help scientists to develop better batteries, to allow EVs to run farther and last longer, while also advancing energy storage technologies.

“We are helping to advance lithium-ion batteries by figuring out the molecular level processes involved in their degradation,” says Michael Toney, co-corresponding author of a paper on the findings, published in the journal Science this month, and a professor in the University of Colorado Boulder’s Department of Chemical and Biological Engineering.

“Having a better battery is very important in shifting our energy infrastructure away from fossil fuels to more renewable energy sources.”

Non-cobalt lithium-ion batteries, with nickel or magnesium as a replacement, exacerbate the degradation problem, as these alternatives have even higher rates of self-discharge — which is when the battery’s internal chemical reactions reduce stored energy and degrade its capacity over time. Because of self-discharge, most EV batteries have a lifespan of 7-10 years before they need to be replaced, the research team says.

Toney, who is also a fellow of the Renewable and Sustainable Energy Institute, a joint research institute between CU Boulder and the National Renewable Energy Laboratory, and his team set out to investigate 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 an 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 the charged ions and returns them to the anode. Previously, scientists thought batteries self-discharge because not all lithium ions return to the anode when charging, reducing the number of charged ions available to form the current and provide power.

Using a powerful X-ray machine at the U.S. Department of Energy’s Argonne National Laboratory in Illinois, the research team discovered that hydrogen molecules from the battery’s electrolyte would move to the cathode and take the spots to which lithium ions normally bind. As a result, lithium ions have fewer places to bind to on the cathode, weakening the electric current and decreasing the battery’s capacity.

“We discovered that the more lithium you pull out of the cathode during charging, the more hydrogen atoms accumulate on the surface,” said Gang Wan, the study’s first author at Stanford University.” This process induces self-discharge and causes mechanical stress that can cause cracks in the cathode and accelerate degradation.”

And the researchers are hopeful that their findings can have a practical application in the EV space. “All consumers want cars with a large driving range. Some of these low cobalt-containing batteries can potentially provide a higher driving range, but we also need to make sure they do not fall apart in a short period of time,” says Toney.

With a better understanding of the self-discharge mechanism, engineers can explore a few 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 is causing batteries to degrade, we can inform the battery chemistry community on what needs to be improved when designing in batteries,” Toney continues.