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

Abstract

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.

Electrochemical characterization of hexamethylguanidinium bis(fluorosulfonyl)imide [HMG][FSI] based electrolyte and its application in sodium metal batteries

“…The long-term cycling stability of 25 mol% NaFSI in [HMG][FSI] was also investigated at high current density at 0.5 mA cm −2 with 1 h polarization steps in a Na symmetrical coin cell at 50 • C. As can be seen from figure 3, the electrolyte shows outstanding stability over 160 plating/stripping cycles (320 h), without short circuit while maintaining a low polarization potential of ≈45 mV throughout the cycling period, indicative of efficient and stable Na + transport within the electrolyte and SEI layer. The recorded overpotential is significantly lower than what was obtained for other previously published IL/NaFSI systems, such as dry/wet 42 mol% NaFSI in [P 111i4 ][FSI] (131 mV 81 mV −1 ) or 50 mol% NaFSI in C 3 mpyrFSI IL (139 mV 83 mV −1 ) when cycled at 1 mA cm −2 at 50 • C [43].…”

Task-Specific Phosphonium Iongels by Fast UV-Photopolymerization for Solid-State Sodium Metal Batteries

“…While previous studies focused on pyrrolidinium ionic liquids, only recently has there been interest in phosphonium-based ionic liquids. For instance, Hilder et al described an electrolyte based on 42%mol NaFSI in trimethyl iso-butyl phosphonium bis(fluorosulfonyl)imide (42% mol NaFSI in P1114iFSI) electrolytes for long lasting and stable sodium metal batteries [ 9 , 10 ]. Despite these advantages, IL electrolytes require a porous separator and are limited by the risks of leakage [ 11 ].…”

Polar Organic Cations at Electrified Metal with Superconcentrated Ionic Liquid Electrolyte and Implications for Sodium Metal Batteries

“…This observation may explain the improved Na stripping behavior and the formation of stable SEI for the [C3mpyr] + containing electrolyte compared to the other two pyrrolidinium-based ILs (Figure a–c); i.e. the IL cations are more readily reduced and contribute to the formation of unfavorable organic-rich interphase for [C2mpyr] + and [C2O1mpyr] + systems during the negative potential scan, whereas the [C3mpyr] + system will favor an inorganic NaF based SEI as we have previously shown …”

“…the IL cations are more readily reduced and contribute to the formation of unfavorable organic-rich interphase for [C2mpyr] + and [C2O1mpyr] + systems during the negative potential scan, whereas the [C3mpyr] + system will favor an inorganic NaF based SEI as we have previously shown. 33 The orientation of the pyrrolidinium ring near the Au (111) surface at different applied surface charges was examined through the angular-distribution analysis shown in Figure 4. The probability of angle (Θ) represents an orientation of the pyrrolidinium ring perpendicular and parallel to Au (111) surface at 90°and 0°/ 180°, respectively.…”