Researchers: Batteries lose lithium to the current collector

Image: Ruhr-Universität Bochum | Tong Li

According to Ruhr-Universität Bochum, scientific observations support the hypothesis that lithium ions are lost from lithium batteries at the copper current collector. This could limit their performance and lifespan.

Every loss of lithium results in a decrease in the capacity of lithium batteries and shortens their lifespan. After recent research suggested that lithium is lost to the current collector during charging, researchers from Ruhr-Universität Bochum led by Tong Li, along with those from Helmholtz Institute Ulm and Karlsruhe Institute of Technology led by Dominic Bresser, carefully tested this hypothesis.

Using atomic force microscopy, the scientists were able to prove that lithium ions do indeed get incorporated into the copper current collector. This loss increases with the number of charge cycles, even in state-of-the-art battery types. “Understanding these processes is crucial for future anode-free batteries,” says Tong Li.

The copper current collector is a thin layer of copper that serves as a support layer for the battery’s anode and collects electrons to guide them to the negative pole. In lithium-metal batteries, lithium can diffuse into the copper current collector during charging and discharging cycles. “However, it was previously unclear exactly where the lithium ions accumulate,” explains Li. “Lithium in copper is difficult to detect due to a lack of analytical methods capable of tracking highly reactive and light lithium.”

The research team used atomic probe tomography, which enables the creation of precise three-dimensional maps of any element with a resolution in the subnanometer range. It was found that lithium becomes bound to grain boundaries and interfaces on the copper foil after just one charge/discharge cycle. After three cycles, the surface of the copper foil becomes nanocrystalline and oxidized. The defects resulting from this further bind lithium and oxygen beneath the surface, leading to the degradation of the copper current collector.

"All of this information is important to understand how the current collector affects the performance of future lithium batteries," says Li. This is because lithium metal batteries (LMBs) and, more recently, "zero-excess" or "anode-free" LMBs are seen as the next major steps toward even higher energy densities. They are expected to far surpass the currently dominant lithium-ion technology. "In zero-excess LMBs, it is generally assumed that lithium does not diffuse into or interact with the copper current collector," explains Li. "In discussions about performance losses in LMBs, lithium loss to the copper collector has so far been largely overlooked."

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Thomas Langenbucher is an expert in electromobility with professional experience in the automotive industry and finance sector. Since 2011, he has been covering electric vehicles, sustainable technologies, and mobility solutions for ecomento.de. Learn more.

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