Investigation of Unusual Conductivity Behavior and Ion Dynamics in Hexamethylguanidinium Bis(fluorosulfonyl)imide-Based Electrolytes for Sodium Batteries

The development of nonflammable, chemically and thermally stable electrolytes that can replace current organic electrolytes will support improved and safer energy storage technologies. Organic ionic plastic crystals (OIPCs) and their salt mixtures are promising solid-state candidates for battery applications. In this work, the hexamethylguanidinium bis(fluorosulfonyl)imide ([HMG][FSI]) OIPC is investigated in the context of sodium batteries, where sodium bis(fluorosulfonyl)imide (NaFSI) salt is mixed with the OIPC to enhance the ionic conductivity. The thermal behavior of the neat OIPC and the effect of sodium salt addition were investigated by differential scanning calorimetry (DSC). Broadband dielectric spectroscopy (BDS) experiments, along with electrochemical impedance spectroscopy (EIS) used to study ion conductivity, showed this system to have unusual temperature-dependent conductivity behavior in comparison to other OIPC systems. The conductivity measured in phase II was higher during the heating cycle compared to that measured upon cooling. Solid-state nuclear magnetic resonance (NMR) spectroscopy combined with XRD and modeling was used to investigate the molecular origin of this behavior. This behavior was also observed in the OIPC containing 5 mol % NaFSI. Pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR) combined with line width analysis was used to examine the ion dynamics. The [HMG]+cation has almost a 2 orders of magnitude lower diffusion coefficient relative to the [FSI]−anion, and combined with the very narrow23Na line width, it appears that the dynamics of the two latter ions are decoupled from the larger [HMG]+cation, suggesting the possibility of high Na+transport in this electrolyte. Our study contributes to the fundamental understanding of dynamics in OIPC-based solid electrolytes for sodium batteries and highlights the complexity and importance of external parameters such as the thermal history of the material.