Lithium-Ion (De)Insertion Reaction of Germanium Thin-Film Electrodes: An Electrochemical and In Situ XRD Study

Germanium is a promising negative electrode candidate for lithium-ion thin-film batteries because of its very high theoretical storage capacity. When assuming full conversion of the material into the room-temperature equilibrium lithium saturated germanium phase Li 22 Ge 5 , a theoretical capacity of 1625 mAh g -1 or 8643 mAh cm -3 of germanium starting material is expected. However, the lithium-ion deinsertion reaction of pure germanium thin films and the resulting electrochemical thermodynamic and kinetic properties are not yet fully understood. To address some of these questions, a combined electrochemical and in situ X-ray diffraction XRD study is presented. Results on the crystallographic phase transitions, occurring upon Li-deinsertion of evaporated and sputtered germanium thin films are discussed. Moreover, the difference in reaction between evaporated and sputtered films is addressed. In addition, a detailed electrochemical investigation cyclic voltammetry, galvanostatic intermittent titration technique, and electrochemical impedance spectroscopy of evaporated germanium is conducted. The results reveal that evaporated and sputtered germanium crystallizes into Li 15 Ge 4 when fully inserted with Li ions. This composition corresponds to a maximum storage capacity of 1385 mAh g -1 or 7366 mAh cm -3 of germanium starting material.