(De)Lithiation and Strain Mechanism in Crystalline Ge Nanoparticles.

Germanium is a promising active material for high energy density anodes in Li-ion batteries thanks to its good Li-ion conduction and mechanical properties. However, a deep understanding of the (de)lithiation mechanism of Ge requires advanced characterizations to correlate structural and chemical evolution during charge and discharge. Here we report a combined operando X-ray diffraction (XRD) and ex situ 7 Li solid-state NMR investigation performed on crystalline germanium nanoparticles (c-Ge Nps) based anodes during partial and complete cycling at C/10 versus Li metal. High-resolution XRD data, acquired along three successive partial cycles, revealed the formation process of crystalline core-amorphous shell particles and their associated strain behavior, demonstrating the reversibility of the c-Ge lattice strain, unlike what is observed in the crystalline silicon nanoparticles. Moreover, the crystalline and amorphous lithiated phases formed during a complete lithiation cycle are identified. Amorphous Li 7 Ge 3 and Li 7 Ge 2 are formed successively, followed by the appearance of crystalline Li 15 Ge 4 (c-Li 15 Ge 4 ) at the end of lithiation. These results highlight the enhanced mechanical properties of germanium compared to silicon, which can mitigate pulverization and increase structural stability, in the perspective for developing high-performance anodes.