High-Performance Cycling of Na Metal Anodes in Phosphonium and Pyrrolidinium Fluoro(sulfonyl)imide Based Ionic Liquid Electrolytes.

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 (P 111i4 FSI) in comparison to its pyrrolidinium-based counterpart (C 3 mpyrFSI) 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 P 111i4 FSI system having a lower viscosity, higher conductivity, and higher ionicity in comparison to the C 3 mpyrFSI-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 C 3 mpyrFSI rapidly failed (at 1 mA cm -2 /4 mAh cm -2 ). The SEI was characterized ex situ using solid-state 23 Na NMR, XPS, and SEM and showed that the presence of a Na complex, identified in our previous work on C 3 mpyrFSI 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 P 111i4 FSI 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.