Anhydrous N,N-Dimethyl-N,N-Dineopentylammonium Fluoride Electrolyte for Fluoride Ion Batteries

Abstract

Fluoride ion batteries (FIB) with quaternary ammonium fluoride electrolytes have recently shown notable room-temperature performance. As the voltage window is known to be limited by the water content, effective synthetic routes for anhydrous quaternary ammonium fluorides are critical for the further development of FIBs. This work describes the high-yield synthesis of an anhydrous quaternary ammonium fluoride salt, N,N-dimethyl-N,N-dineopentylammonium fluoride (Np 2 F), via an anion exchange reaction. Karl Fischer titrations determined the water content of the solid anhydrous product to be 71 ppm (1.7 M in MeOH)�an exceptionally low value. The anhydrous Np 2 F/ bis(2,2,2-trifluoroethyl) ether (BTFE) electrolyte also shows a wide electrochemical window of 3.5 V. Quantifying the relationship between the electrolyte performance and water content, we found that the voltage window decreases from 3.5 to 3.1 V as the water content increases from 25 to 135 ppm.