(De)Lithiation and Strain Mechanism in Crystalline Ge Nanoparticles

Area of Science:

• Materials Science
• Electrochemistry
• Nanotechnology

Background:

• Germanium (Ge) is a promising anode material for high energy density lithium-ion batteries.
• Understanding the (de)lithiation mechanism of Ge requires advanced characterization techniques.
• Ge offers good Li-ion conduction and mechanical properties, potentially overcoming silicon's limitations.

Purpose of the Study:

• To investigate the structural and chemical evolution of crystalline germanium nanoparticles (c-Ge Nps) during lithium-ion battery cycling.
• To correlate structural changes with electrochemical performance.
• To compare the mechanical behavior of Ge anodes with silicon anodes.

Main Methods:

• Combined *operando* X-ray diffraction (XRD) and *ex situ* ⁷Li solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Electrochemical cycling of c-Ge Nps based anodes against lithium metal at a C/10 rate.
• High-resolution XRD data acquisition during partial and complete cycling.
• *Ex situ* NMR analysis of lithiated phases.

Main Results:

• XRD revealed the reversible formation of crystalline core-amorphous shell structures and reversible lattice strain in c-Ge Nps.
• Amorphous Li₇Ge₃ and Li₇Ge₂ phases formed successively during lithiation.
• Crystalline Li₁₅Ge₄ (c-Li₁₅Ge₄) was identified at the end of the lithiation cycle.
• The observed reversible strain in Ge contrasts with the pulverization issues in silicon nanoparticles.

Conclusions:

• Germanium anodes exhibit enhanced mechanical stability and reversible strain accommodation during cycling.
• The identified lithiated phases provide insights into the germanium (de)lithiation mechanism.
• These findings support the development of high-performance, stable germanium-based anodes for next-generation lithium-ion batteries.