Lithium isotopes as a “fingerprint” of battery ageing: BAM measured the separation of lithium-6 and lithium-7 in electrodes by depth-resolved mass spectrometry
On 9 September 2026 Germany’s Federal Institute for Materials Research and Testing (BAM) described a method for diagnosing the ageing of lithium-ion cells from the distribution of the isotopes lithium-6 and lithium-7 in the electrodes. In cycled cells lithium-6 accumulates at the anode while the cathode becomes enriched in lithium-7; the isotopic traces are linked to the loss of capacity.
- The cells studied had a lithium nickel manganese cobalt oxide (NMC) cathode and a graphite anode. According to BAM, lithium-6 began to accumulate at the anode after the very first charging cycles; after a further 280 cycles the effect became stronger, the cathode showed a clearly increased lithium-7 content and the capacity of the cells decreased.
- The measurement was made possible by high-resolution mass spectrometry that determined the isotope distribution layer by layer through the full depth of the electrodes. According to the abstract of the ChemRxiv preprint, this was depth-resolved glow discharge mass spectrometry (GD-MS) of the ⁷Li/⁶Li ratio in NMC111||graphite coin cell electrodes at defined life-cycle stages.
- According to that abstract, pristine electrodes have a uniform isotope ratio, while in cycled electrodes the degree of separation depended on the charging rate; the isotopic signatures correlated with capacity fade. The authors see lithium isotope mapping as a sensitive tool for tracking electrode degradation and the evolution of electrode–electrolyte interphases.
- The work was carried out with Nu Instruments Ltd. (United Kingdom) and the Leibniz Institute IFW Dresden and was published in ACS Energy Letters (vol. 11, pp. 2851–2857, 2026). According to BAM, ageing could in future be tracked earlier, before it shows up as a loss of capacity.