Probing lithium germanide phase evolution and structural change in a germanium-in-carbon nanotube energy storage system.

Lithium alloys of group IV elements such as silicon and germanium are attractive candidates for use as anodes in high-energy-density lithium-ion batteries. However, the poor capacity retention arising from volume swing during lithium cycling restricts their widespread application. Herein, we report high reversible capacity and superior rate capability from core-shell structure consisting of germanium nanorods embedded in multiwall carbon nanotubes. To understand how the core-shell structure helps to mitigate volume swings and buffer against mechanical instability, transmission electron microscopy, X-ray diffraction, and in situ (7)Li nuclear magnetic resonance were used to probe the structural rearrangements and phase evolution of various Li-Ge alloy phases during (de)alloying reactions with lithium. The results provide insights into amorphous-to-crystalline transition and lithium germanide alloy phase transformation, which are important reactions controlling performance in this system.