Influence of the Ge content on the lithiation process of crystalline Si_1-xGe_x nanoparticle-based anodes for Li-ion batteries

In the quest for high-capacity Li-ion batteries, moving from classical intercalation reactions such as those occurring at graphite-based electrodes to alloying reactions is a promising alternative. Among active materials which form alloys upon lithiation, silicon is a good candidate thanks to its high theoretical capacity, although it shows limited cyclability due to significant aging effects. In comparison, germanium presents improved Li-ion conduction and mechanical properties. Mixing silicon and germanium, as in Si1-xGex alloys, is an attractive strategy for combining the best advantages of both elements. In this study, we report a combined operando X-ray diffraction (XRD) and electrochemical investigation of the influence of the germanium content on the (de)lithiation processes in crystalline Si1-xGex nanoparticle-based anodes during the first charge/discharge cycle. The alloyed particles, which show pronounced heterogeneities in composition, evidence a sequential amorphization of the different c-Si1-xGex phases depending on their Ge content, where the lithiation potential decreases upon increasing the silicon content, following Vegard's law-type of behavior. Operando XRD and galvanostatic cycling investigation of the highly lithiated crystalline phase Li-15(Si1-xGex)(4) evidence a narrow domain of existence with a composition close to x = 1. This study brings essential knowledge on the (de)lithiation mechanisms at play in Si1-xGex alloys, which is critical for mastering these promising materials that combine the best properties of silicon and germanium, with the possibility to tune their composition to tailor (de)lithiation properties and trade off performance and cycle life.