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Using EPR for Analysis of Metallic Lithium Microstructure

To meet the growing demand for energy storage, substantial scientific efforts are undertaken to improve secondary battery performance.1 Lithium-ion batteries are still state-of-the-art for commercial applications in the near future, but researchers are extensively investigating material improvements as well as alternative battery designs. Metallic lithium would be an ideal anode material due to its high specific capacity and low electric potential.2 However, upon charging of a battery the formation of microstructured Li, e.g. in the form of dendrites or mossy species, when deposited on Li-metal surfaces prevented the use as metallic anodes in commercial products so far. It can cause severe safety problems due to internal cell shortening and reduces cell life due to increased electrolyte consumption. Measuring the build-up of these structures during cell operation is a challenging task but crucial for analyzing the cell behavior in operation mode.

Recently, magnetic resonance techniques have been introduced as appropriate non-invasive in operando methods to study the evolution of metallic Li species during battery cycling.3,4 Beside NMR, EPR is particularly suited to detecting the desired structural change with a simple setup, high sensitivity, and, in comparison to NMR, a higher surface selectivity due to the low penetration depth of microwaves into the bulk. Therefore, a good differentiation between bulk and fine structured ‘mossy’ lithium can be made.

In this report, we demonstrate the applicability of the Bruker EMXnano benchtop setup for Li-microstructure detection during the charging process of a battery cell with metallic lithium as anode and LiCoO2 as cathode material. We consider the affordable and easy-to-use benchtop instrumentation as a step closer to fast and batchwise analysis of dendritic/mossy lithium in batteries and elsewhere. In addition to Li spectra analysis, crucial issues to set up an in operando measurement are presented herein, such as considerations of necessary hardware, internal referencing, and handling of metals and other conducting materials inside the EPR microwave cavity.