The next breakthrough in battery technology might come from a thin sheet of copper no thicker than a human hair.
Scientists at Ruhr University Bochum in Germany have discovered why lithium batteries gradually lose their ability to hold a charge. The culprit? Lithium particles getting trapped inside the copper foil that helps direct electricity in the battery. The finding, published in the journal Advanced Energy Materials, opens a new path toward longer-lasting, more powerful batteries.
Professor Tong Li led the research team at the university's Institute for Physical Chemistry. Working with collaborators at the Helmholtz Institute Ulm and the Karlsruhe Institute of Technology, the group used a powerful imaging technique called atom probe tomography to watch what happens inside batteries during charging. This tool can map individual elements in three dimensions with incredible precision—far smaller than what regular microscopes can see.
When the researchers looked at the copper current collector—the thin copper sheet that carries electrons away from the battery's negative terminal—they found something surprising. After just one charge-and-discharge cycle, tiny amounts of lithium had already worked their way into the copper, lodging at the boundaries where metal grains meet. After three cycles, the copper surface began changing at the tiniest scale, becoming rough and oxidized. These changes created more spots where lithium could get stuck, accelerating the loss.
This process had gone undetected for years. "The loss of lithium to the copper collector has been largely overlooked until now in discussions about the performance degradation of lithium-metal batteries," Li explained.
Understanding the problem is the first step toward solving it. Lithium-metal batteries, and especially a newer type called zero-excess or anode-free batteries, could store far more energy than today's common lithium-ion batteries. They could make electric vehicles go farther and smartphones last longer between charges. But for these next-generation batteries to become reality, engineers need to figure out why they lose power over time.
"This information is important for understanding how the current collector influences the performance of lithium batteries of the future," Li said. The team hopes their detailed maps of where and how lithium gets trapped will help battery designers create materials that prevent this loss—bringing better batteries one step closer to reality.