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.
Commercial battery electrodes are composites made of active particles bonded to current collected by polymer binders. Binder not only keeps all the particles together but also provides the electrical network necessary for battery operation. Several studies showed that the mechanical behavior of the polymer binder is very important, and it governs the cyclic performance of batteries. To develop advanced degradation models of battery electrodes, the mechanical behavior of binders needs to be characterized. Recently, some studies developed experimental methods to characterize the mechanical behavior of polymer binders, which showed that the most widely used binder, polyvinylidene fluoride (PVdF), behaves as an elastic-viscoplastic material under the typical charge/discharge rates of battery electrodes. However, the time dependent behavior of PVdF is not the only characteristic behavior to keep in mind. It should be noted that the electrolyte interacts with the binder and potentially changes its mechanical behavior which has not been characterized before. Most of the existing studies were carried out under dry conditions (no electrolyte present), and no study exists on the understanding of how electrolyte solvents and salts affect the mechanical behavior of polymer binders. In this talk, we will present the effect of electrolyte on the stress-strain behavior PVdF binder.
Li-ion battery has been widely used due to its high energy and power density. However, due to scarcity of Li reserves there has been increased focus on Li-ion alternatives. One of the promising alternatives is Na-ion battery (NIB). However, NIB also suffers from similar degradation phenomena’s as Li-ion when cycled with large volume expansion anodes like Ge. Electrodes undergoing large volume expansion are subjected to significant amount of stress during electrochemical cycling process which causes mechanical degradation and capacity fade. Here, we measured volume expansion and the associated stresses in Ge thin film electrodes during sodiation/desoadiaiton cycling. In spite of the bigger size, Na-ion generated lower stresses in Ge during cycling as compared to Li-ion. The mechanical dissipation losses were observed to be comparable to other polarization losses in the Na-ion cell.
Na-ion batteries (NIB) are been developed as an alternative to Li-ion batteries due to cost and scarcity of Li metal, but one of the major issues is capacity fade due to stress generation in electrodes during sodiation/desodiation process. Stress generation occurs due to volume expansion of Na when it alloys and dealloys with the anode. Here, we chose Ge as a representative anode material and measured its stress response during sodiation/desodiation cycling. The mechanical behavior of Ge due to sodiaiton/desodiation is compared and contrasted with that due to lithation/delithation. In this talk the interesting observations from this in-situ measurements and their implications will be presented.
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