We
have investigated the sodium electrochemistry and the evolution
and chemistry of the solid–electrolyte interphase (SEI) upon
cycling Na metal electrodes in two ionic liquid (IL) electrolytes.
The effect of the IL cation chemistry was determined by examining
the behavior of a phosphonium IL (P111i4FSI) in comparison
to its pyrrolidinium-based counterpart (C3mpyrFSI) at near-saturated
NaFSI salt concentrations (superconcentrated ILs) in their dry state
and with water additive. The differences in their physical properties
are reported, with the P111i4FSI system having a lower
viscosity, higher conductivity, and higher ionicity in comparison
to the C3mpyrFSI-based electrolyte, although the addition
of 1000 ppm (0.1 wt %) of water had a more dramatic effect on these
properties in the latter case. Despite these differences, there was
little effect in the ability to sustain stable cycling at moderate
current densities and capacities (being nearly identical at 1 mA cm–2 and 1 mAh cm–2). However, the IL
based on the phosphonium cation is shown to support more demanding
cycling with high stability (up to 4 mAh cm–2 at
1, 2, and 4 mA cm–2 current density), whereas C3mpyrFSI rapidly failed (at 1 mA cm–2 /4
mAh cm–2). The SEI was characterized ex
situ using solid-state 23Na NMR, XPS, and SEM
and showed that the presence of a Na complex, identified in our previous
work on C3mpyrFSI to correlate with stable, dendrite-free
Na metal cycling, was also more prominent and coexisted with a NaF-rich
surface. The results here represent a significant breakthrough in
the development of high-capacity Na metal anodes, clearly demonstrating
the superior performance and stability of the P111i4FSI
electrolyte, even after the addition of water (up to 1000 ppm (0.1
wt %)), and show great promise to enable future higher-temperature
(50 °C) Na-metal-based batteries.