(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 7Li 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 Li7Ge3 and Li7Ge2 are formed successively, followed by the appearance of crystalline Li15Ge4 (c-Li15Ge4) 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.

Nano-Architectured Composite Anode Enabling Long-Term Cycling Stability for High-Capacity Lithium-Ion Batteries.

Failure mechanisms associated with silicon-based anodes are limiting the implementation of high-capacity lithium-ion batteries. Understanding the aging mechanism that deteriorates the anode performance and introducing novel-architectured composites offer new possibilities for improving the functionality of the electrodes. Here, the characterization of nano-architectured composite anode composed of active amorphous silicon domains (a-Si, 20 nm) and crystalline iron disilicide (c-FeSi2 , 5-15 nm) alloyed particles dispersed in a graphite matrix is reported. This unique hierarchical architecture yields long-term mechanical, structural, and cycling stability. Using advanced electron microscopy techniques, the nanoscale morphology and chemical evolution of the active particles upon lithiation/delithiation are investigated. Due to the volumetric variations of Si during lithiation/delithiation, the morphology of the a-Si/c-FeSi2 alloy evolves from a core-shell to a tree-branch type structure, wherein the continuous network of the active a-Si remains intact yielding capacity retention of 70% after 700 cycles. The root cause of electrode polarization, initial capacity fading, and electrode swelling is discussed and has profound implications for the development of stable lithium-ion batteries.