In-Operando Electron Paramagnetic Resonance Spectroscopy - Formation of Mossy Lithium on Lithium Anodes and Lithium Plating on Graphite Anodes during Charge/Discharge Cycling
In this work we propose to use in-operandoelectron paramagnetic resonance (EPR) spectroscopy as a new analytical technique for the time resolved and quantitative determination of lithium microstructures both on lithium metal and graphite anodes. We present a novel cell design which can be cycled directly in the cavity of the EPR spectrometer and which has been successfully benchmarked against a standard cell design.
Firstly, we investigate the morphological changes of a lithium metal anode during cycling of a Li/LiFePO4 (LFP) cell as case study to demonstrate the capabilities of electrochemical in-operando EPR spectroscopy. Figure 1 shows the voltage profiles and the evolution of the EPR resonance of the lithium anodes using a standard electrolyte with or without fluoroethylene carbonate (FEC) additive, which is known to reduce lithium dendrite formation.In comparison to the standard electrolyte, the FEC additive significantly increases the reversibility of the lithium stripping/plating process. These results are also confirmed by ex-situ SEM images of cycled electrodes.
Secondly, we investigate lithium plating on graphite electrodes during the cycling of graphite/LiFePO4 (LFP) cells. Since plated lithium (due to low temperatures or high C-rates) can chemically intercalate into the underlying graphite at open circuit conditions, a dedicated in-operando technique like EPR has to be used for a thorough study of lithium plating on graphite.
Figure 1: Overview of cycling of in-operando EPR cells containing electrolyte without additive (black) and with 10 wt-% FEC (red). Top and bottom panel: Voltage profiles according to cycling procedure shown above top panel; blue 'SEM1’ and ‘SEM2’ markers indicate positions where ex-situ SEM images of lithium anodes were recorded. Central panel:Normalized intensity of EPR signal obtained by double integration of recorded first derivate Li spectrum.
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Experimental support by Hans Kungl, Magnus Graf, Johannes Landesfeind and Yi-Chun Lu as well as financial support by BMW AG, by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology (EEBatt project; TUM) and by the German Federal Ministry of Education and Research (BMBF-project DESIREE, grant number 03SF0477A; IEK-9) are gratefully acknowledged.