Germanium as a Sodium Ion Battery Material: <i>In Situ</i> TEM Reveals Fast Sodiation Kinetics with High Capacity

A significant amount of research is taking place to create energy storage concepts beyond the lithium ion battery and to utilize alternative ions, such as Na, Ca, or Mg, to name a few. This has been a challenge, as materials that work well to store lithium do not necessarily function for other ions. Crystalline germanium (Ge) represents such an example: Li can be readily inserted and extracted but not Na. However, by amorphizing the crystalline Ge nanowires with an initial lithiation step, Ge can be readily and reversibly sodiated. Here, we examine the sodiation and desodiation processes that occur in Ge nanowires using real-time <i>in situ</i> transmission electron microscopy (TEM). Amorphous germanium (<i>a</i>-Ge) nanowires exhibit a 300% expansion in volume upon sodiation, which corresponds approximately to Na<sub>1.6</sub>Ge, which indicates a higher than expected capacity to store Na, i.e., compared to NaGe. When the nanowires desodiate they form pores. The pores disappear when the nanowire is again sodiated. The nanowires retain their structural integrity over the course of several cycles. These results show that the potential of <i>a</i>-Ge for Na-ion battery applications may have been previously underestimated, and, more generally, electrode materials that might appear to be inert for one type of ion storage might be enabled by preinsertion of other active ions.