Electrochemical Lithiation of Ge: New Insights by Operando Spectroscopy and Diffraction

The relatively high cost of metallic germanium (Ge) as a lithium-ion battery negative electrode material is more than counterbalanced by its high capacity, high lithium diffusivity, and electronic conductivity. Using a unique and highly complementary set of operando characterization techniques, we propose a complete mechanism for the reversible lithiation of Ge. The electrochemical mechanism is found to be determined by the process of discharge/charge: (i) independent of the charge/discharge rate amorphous a-LiGe is proposed as the first intermediate during the lithiation of c-Ge, followed by Li7Ge3, and (ii) at low potential Li15Ge4 is observed, but only for moderate rates and never at low rates, where indeed an "overlithiated" phase is preferred. The complementarity of the data obtained from XAS, Raman spectroscopies, and XRD, all in operando mode, was crucial in order to understand the complex mechanism based on reversible formation of the various crystalline and amorphous phases.