Measurement of Volume Changes and Associated Stresses in Ge Electrodes Due to Na/Na$^+$ Redox Reactions

TL;DR

This study measures real-time volume changes and stresses in amorphous germanium electrodes during sodium ion redox reactions, revealing significant volume expansion, stress evolution, and differences from lithiation processes.

Contribution

It provides the first in situ measurements of volume and stress changes in Ge electrodes during Na/Na+ reactions, highlighting differences from lithiation.

Findings

Over 300% volume change during sodiation/desodiation.

Initial irreversible stress and volume changes, followed by reversible cycles.

Lower stress levels during sodiation compared to lithiation.

Abstract

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.