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In situ electrochemical cells were assembled with an amorphous germanium (a-Ge) film as working electrode and sodium foil as reference and counter electrode. The stresses generated in a-Ge electrodes due to electrochemical reaction with sodium were measured in real-time during the galvanostatic cycling. A specially designed patterned a-Ge electrode was cycled against sodium and the corresponding volume changes were measured using an AFM; it was observed that sodiation/desodiation of a-Ge results in more than 300% volume change, consistent with literature. The potential and stress response showed that the a-Ge film undergoes irreversible changes during the first sodiation process, but the subsequent desodiation/sodiation cycles are reversible. The stress response of the film reached steady-state after the initial sodiation and is qualitatively similar to the response of Ge during lithiation, i.e., initial linear elastic response followed by extensive plastic deformation of the film to accommodate large volume changes. However, despite being bigger ion, sodiation of Ge generated lower stress levels compared to lithiation. Consequently, the mechanical dissipation losses associated with plastic deformation are lower during sodiation process than it is for lithiation.

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Sputter deposited germanium thin films were assembled in a half-cell configuration with lithium foil as counter/reference electrode and 1M LiPF 6 in EC, DEC, DMC solution (1:1:1, wt%) as electrolyte. The Ge films were subjected to potentiostatic intermittent titration technique (PITT) and galvanostatic intermittent technique (GITT) conditions while simultaneously measuring the stress evolution in the electrodes. It was observed that the electrode stresses varied significantly in a single titration step during a GITT experiment, which violates the assumptions of simple Fickian transport model where the electrode stresses are usually neglected. Therefore, only the PITT data was analyzed to obtain the chemical diffusion coefficient D of Li in Ge. As expected, the diffusion coefficient value increased considerably with Li concentration; however, the D values obtained during delithiation are at least two times greater than those obtained during lithiation at any given Li concentration, with the difference becoming significantly higher at higher Li concentration. This difference is attributed to the stress state, i.e., tensile stress during delithiation leads to higher D values compared to the compressive stresses during lithiation. The data and observations presented here will be helpful in developing and using electrochemomechanical models in producing optimized electrode microstructures.