Discovery may lead to longer-lasting, longer-range EV batteries

Lithium-ion batteries power everything from smart phones and laptops to electric cars and large-scale energy storage facilities. Batteries lose capacity over time even when they are not in use, and older cellphones run out of power more quickly. This common phenomenon, however, is not completely understood.

Now, an international team of researchers, led by the University of Colorado-Boulder, SLAC National Accelerator Laboratory, and Stanford University has revealed an underlying mechanism behind such battery degradation. Their discovery could help scientists develop better batteries, which would allow electric vehicles to run farther and last longer, while also advancing energy storage technologies that would accelerate the transition to clean energy.

The findings were published Sept. 12 in the journal Science.

“We are helping to advance lithium-ion batteries by figuring out the molecular level processes involved in their degradation,” said Michael Toney, a senior author of the study and a professor of chemical and biological engineering at the University of Colorado.

“Having a better battery is very important in shifting our energy infrastructure away from fossil fuels to more renewable energy sources,” said Toney, who was a senior staff scientist at SLAC when most of this study’s experiments were done.

Transportation is the single largest source of greenhouse gases generated in the U.S, accounting for 28% of the country’s emissions in 2021. In an effort to reduce emissions, many automakers have committed to moving away from developing gasoline cars to produce more EVs instead. But EV manufacturers face a host of challenges, including limited driving range, higher production costs and shorter battery lifespan than conventional vehicles. In the U.S. market, a typical all-electric car can run about 250 miles in a single charge, about 60% that of a gasoline car. The new study has the potential to address all these issues, Toney said.

Another school of thought

Rechargeable batteries lose stored energy when they’re not being used because an idle battery undergoes internal chemical reactions that slowly drain its energy. This “self-discharge” process can eventually consume active ingredients in the cathode, where the electron-spent lithium ions collect while the device is in use. This shortens a battery’s life expectancy.

For decades, researchers have assumed that self-discharge in lithium-ion batteries is caused by the movement of lithium ions, but the new research finds compelling evidence that hydrogen, not lithium, is the true culprit. Using facilities from U.S. Department of Energy’s Argonne National Laboratory and Pacific Northwest National Laboratory, this team discovered that hydrogen atoms from the battery’s electrolyte would move to cathode and the protons will take some of the spots that lithium ions normally bind to. The cathode is also the conduit for electrons while charging the battery, but not so much if many hydrogen atoms – the tiniest of all atoms – are occupying lithium-ion parking spots.

“Numerous electrolyte degradation pathways have been predicted by various theoretical modeling,” said Kang Xu, a senior author of this study and the chief scientist at SES AI, a producer of lithium metal batteries. “Our study provides one of the first direct and solid experimental evidence on the hydrogen transfer that led to the electrolyte degradation.”

The scientists discovered that the more lithium is pulled out of the cathode during charging, the more hydrogen atoms accumulate on the surface.

“Also, removing lithium from the charged cathode generates a lot of openings on the surface that allow hydrogen atoms to go deeper inside,” said Gang Wan, lead author of the study and a research scientist in Stanford’s School of Engineering. “This process induces self-discharge and causes mechanical stress that can cause cracks in the cathode and accelerate degradation.”

Potential solutions

Focusing on hydrogen instead of lithium could lead to new solutions that address the underlying cause of self-discharge and improve the performance of a wide range of rechargeable batteries, the researchers write. In this study, they provide three possible solutions. For the electrolyte, special molecular structures that do not contain these unstable hydrogen sources should be designed. For cathodes, a coating layer that blocks the direct contact between the electrolyte and the cathodes should be provided. Last, over the past decades, there has been a driving force to increase the voltage of cathodes. However, this study shows that the ability to maintain that high voltage would likely trigger the depletion of the electrolyte and mechanical problems on the cathode. Not charging the battery to its full capacity could improve its lifetime.

The study’s other senior author is Oleg Borodin, staff scientist at DEVCOM Army Research Laboratory and co-principal investigator at the Stanford/SLAC-led Aqueous Battery Consortium. Key contributors are Chia-Chin Chen at National Taiwan University, Travis P. Pollard and Marshall A. Schroeder at DEVCOM Army Research Laboratory, Zihua Zhu and Yingge Du at Pacific Northwest National Laboratory, Ye Zhang at University of Houston. Other Stanford authors are Harry Thaman, PhD student of materials science and engineering; Arturus Vailionis, senior research scientist; Haoyuan Li, a mechanical engineering postdoctoral scholar; and Christopher Tassone, a lead scientist at SLAC. Additional contributing authors are at Argonne, Lawrence Berkeley National Laboratory, and the National Renewable Energy Laboratory. Funding was provided by the U.S. Department of Energy and the National Science Foundation.