BAM identifies lithium isotopes as a diagnostic tool for battery aging
Researchers at the Federal Institute for Materials Research and Testing have discovered that the distribution of two lithium isotopes in cells acts like a fingerprint, directly linking to degradation processes. This insight is intended to help better understand the aging mechanisms of batteries.
Illustration: A typical high-voltage battery for electric vehicles.
Image: Stellantis
The lifespan of lithium-ion batteries is limited by complex aging processes, the causes of which are often difficult to distinguish from one another. Researchers at BAM have shown that it is worthwhile to take a closer look at the lithium isotopes in the cells, as they explain in an article published in the journal ACS Energy Letters.
Specifically, the team led by Carlos Abad and Beatrice Battistella used the distribution of two lithium isotopes in a cell as a diagnostic tool for aging processes. The key finding: “Their characteristic traces and patterns provide a fingerprint of the underlying degradation mechanisms,” according to a BAM statement.
The background is that several complex processes are responsible for degradation inside the battery. “To date, the challenge of clearly identifying these aging mechanisms and distinguishing them from one another remains unsolved. Although conventional diagnostic methods show that a battery is aging, they often provide only limited information about when, where, and through which mechanism degradation occurs,” the federal agency describes the current situation.
The newly developed method takes advantage of the fact that natural lithium consists of the two isotopes lithium-6 and slightly heavier lithium-7. These are variants of the same chemical element with different numbers of neutrons, according to the research team. In new batteries, both lithium isotopes are distributed as they are in all natural occurrences of this alkali metal, namely in a ratio of 2.4 percent to 97.6 percent.
Specifically, the researchers examined a lithium battery with a cathode made of lithium-nickel-manganese-cobalt oxide (NMC) and a graphite anode. Even after the first charging cycles, lithium-6 accumulated preferentially at the anode, according to the scientists. After another 280 cycles, this effect intensified, while the cathode showed significantly higher levels of lithium-7. At the same time, the capacity of the cells decreased.
According to BAM, this detection was made possible through a high-resolution form of mass spectrometry, which allowed for the determination of lithium isotope distribution layer by layer throughout the entire depth of the electrodes. The studies were conducted in collaboration with Nu Instruments Ltd. in the UK and the Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden).
"The distribution of lithium isotopes provides direct information on aging processes in lithium-ion batteries. The isotopes act like natural markers. Their distribution shows us how lithium moves during battery operation and where changes occur," summarizes Beatrice Battistella. The researchers were also able to demonstrate that these traces are closely related to battery capacity loss.
For BAM, the significance of these results lies primarily in their potential for accurate measurements: instead of detecting aging only through capacity loss, it may now be possible to track it much earlier by analyzing the traces left by lithium isotopes in the battery cell. “This enables a more precise understanding of aging mechanisms and is ultimately crucial for improving batteries more effectively and predicting their lifespan with greater accuracy,” concludes the report.