Engineering strategies based on “nanostructuring” and “active/inactive composites” are commonly used separately to improve the performance of alkali‐ion battery electrodes. Here, these two strategies are merged to further enhance the performance of alloy‐type alkali‐ion battery anodes. Specifically, macroporous antimony (Sb)/magnesium fluoride (MgF2) active/inactive composite material is used as a high‐performance Na‐ion battery anode. The porous Sb phase with pore size in the sub‐micrometer range acts as the electrochemically active component and the electrochemically inactive dense MgF2 phase acts as a mechanical buffer. Na‐ion battery anodes made of porous Sb/MgF2 active/inactive composites are reversibly sodiated for over 300 cycles, delivering a capacity of ≈551 mAh g−1 after 300 cycles at a C‐rate of C/2. This performance is remarkable because the porous Sb/MgF2 composite is not made of mesoporous structures. Furthermore, the cycling longevity of this porous Sb/MgF2 composite outperforms the common nanostructured Sb‐based Na‐ion battery anode materials. This good performance is attributed to the “porous active/inactive” configuration, where the dense inactive mechanical buffer phase absorbs part of the phase transformation‐induced stresses, while porosity in the active phase helps to accommodate the phase transformation induced volume expansions and electrolyte transfer into the bulk of this composite.

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.

A large amount of literature exists on the electrochemical characterization of various high capacity sodium-ion battery electrodes; however, their mechanical behavior has not been characterized before. In this study, evolution of stress in sodium-ion battery electrode has been measured in real-time using an optical-based substrate curvature measurement apparatus. A 100 nm Ge thin film, as working electrode, was cycled against sodium reference/counter electrode with 1 M sodium perchlorate (NaClO4) in propylene carbonate (PC) as electrolyte. Electrochemical and stress response of the electrodes to the prescribed electrochemical cycling conditions were measured simultaneously. Further, the sodiated Ge samples were analyzed using SEM, XRD, and AFM techniques to understand the microstructural changes. It was observed that sodiation of Ge films resulted in distinctly different mechanics and electrochemical phenomenon compared to that of lithiated Ge films. These observations and implications will be further discussed in the talk.