Sodium-ion batteries have emerged
as a promising alternative
to
Li-ion batteries due to the abundance of sodium. However, anodes in
Na-ion batteries face challenges such as dendrite formation and an
unstable solid electrolyte interface layer. To address these challenges,
NaK liquid metal alloy anodes have been proposed as an alternative
because they do not form dendrites. In our study, we demonstrate that
the NaK alloy anode interacts with the commonly used ethylene carbonate
and dimethyl carbonate electrolyte, leading to a continuously growing
unstable SEI layer, evidenced by cycling failures under 100 cycles
and an increasing charge transfer resistance in electrochemical impedance
spectroscopy studies. In situ surface-enhanced Raman spectroscopy
and X-ray photoelectron spectroscopy reveal that over the course of
cycling the surface of the NaK anode becomes increasingly sodium-rich.
After 30 cycles, XPS analysis detects only trace amounts of potassium
on the NaK anode surface. When the electrolyte is analyzed postcycling
using inductively coupled plasma optical emission spectroscopy, there
is a noticeable increase in potassium levels, suggesting that potassium
metal dissolves into the electrolyte. The introduction of a 10 wt
% fluoroethylene carbonate additive can mitigate this problem to some
extent, enabling an enhanced cycling performance of up to 800 cycles
at 1C. Nevertheless, the dissolution of K metal is still evident in
the XPS results, albeit to a lesser degree. These discoveries provide
valuable insights for designing a more robust SEI layer for the NaK
anode.