BAM identifies lithium isotopes as diagnostic tool for battery ageing
Researchers at the Federal Institute for Materials Research and Testing have discovered that the distribution of two lithium isotopes in the cells is directly linked to degradation processes, like a fingerprint. This insight is intended to help better understand the ageing mechanisms of batteries.
Symbolic image: A typical high-voltage battery for electric cars.
Image: Stellantis
The service life of lithium-ion batteries is limited by complex ageing processes, the causes of which are often difficult to distinguish. Researchers at BAM have demonstrated that examining the lithium isotopes within the cells can provide valuable insights, 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 within a cell as a diagnostic tool for ageing processes. The key finding: “Their characteristic traces and patterns provide a fingerprint of the underlying degradation mechanisms,” states a BAM press release.
The background is that multiple complex processes inside the battery contribute to degradation. “To date, the challenge of clearly identifying these aging mechanisms and distinguishing them from one another remains unresolved. While conventional diagnostic methods show that a battery is aging, they often provide only limited information about when, where, and by what mechanism degradation is triggered,” the Federal Institute for Materials Research and Testing (BAM) explained.
The newly developed method leverages the fact that natural lithium consists of two isotopes: lithium-6 and the slightly heavier lithium-7. These are variants of the same chemical element with a different number of neutrons, as the research team explains. In new batteries, both lithium isotopes are distributed in the same ratio as in all natural occurrences of the alkali metal, i.e., 2.4 per cent to 97.6 per cent.
The researchers specifically examined a lithium battery with a cathode made of lithium-nickel-manganese-cobalt oxide and a graphite anode. According to the scientists, lithium-6 began to accumulate preferentially at the anode after just the first few charge cycles. After a further 280 cycles, this effect intensified, while the cathode exhibited a significantly increased lithium-7 content. At the same time, the cells’ capacity decreased.
According to BAM, the detection was made possible by a high-resolution form of mass spectrometry, which allowed the distribution of lithium isotopes to be determined layer by layer across the entire depth of the electrodes. The investigations were carried out 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 thus provides direct information about aging processes in lithium-ion batteries. The isotopes act as natural markers. Their distribution provides insight into the mechanisms that controls lithium movement during battery operation and where changes occur,” said Beatrice Battistella. The researchers were also able to demonstrate that these traces are closely linked to the batteries’ capacity loss.
For BAM, the significance of the results lies primarily in their potential applications for precise measurements: instead of detecting ageing solely through capacity loss, it could in the future be tracked much earlier based on 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 in a more targeted manner and predicting their service life more accurately,” concludes the institute.