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Germanium as a Sodium Ion Battery Material: In Situ TEM Reveals Fast Sodiation Kinetics with High Capacity
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posted on 2016-01-29, 00:00 authored by Xiaotang Lu, Emily R. Adkins, Yang He, Li Zhong, Langli Luo, Scott
X. Mao, Chong-Min Wang, Brian A. KorgelA 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 in situ transmission electron microscopy (TEM). Amorphous
germanium (a-Ge) nanowires exhibit a 300% expansion
in volume upon sodiation, which corresponds approximately to Na1.6Ge, 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 a-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.