Understanding Phase Transformation in Crystalline Ge Anodes for Li-Ion Batteries

Lithium-ion batteries using germanium as the anode material are attracting attention because of their high-capacity, higher conductivity, and lithium-ion diffusivity relative to silicon. Despite recent studies on Ge electrode reactions, there is still limited understanding of the reaction mechanisms governing crystalline Ge and the transformations into intermediate amorphous phases that form during the electrochemical charge and discharge process. In this work, we carry out in operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies on Ge anodes during the initial cycles to better understand these processes. These two probes track both crystalline (XRD) and amorphous (XAS) phase transformations with potential, which allows detailed information on the Ge anode to be obtained. We find that crystalline Ge lithiates inhomogeneously, first forming amorphous Li9Ge4 during the beginning stage of lithiation, followed by the conversion of the remaining crystalline Ge to amorphous Ge. The lithiation of amorphous Ge then forms amorphous LixGe, which are then further lithiated to form crystalline Li15Ge4. During delithiation, crystalline Li15Ge4 transforms directly into a heterogeneous mix of amorphous LixGe, which eventually form amorphous Ge, and interestingly, no amorphous Li9Ge4 are detected. Both our in operando XRD and XAS results present new insights into the reaction mechanism of Ge as anodes in LIBs, and demonstrate the importance of correlating electrochemical results with in operando studies.