Using Siloxane-Based Liquid Electrolytes With High Stability For Fluoride Shuttle Batteries

To benefit from the excellent safety as well as the thermal and electrochemical properties of siloxane-based liquid electrolytes, 2,2,4,4-tetramethyl-3,8,11,14,17-pentaoxa-2,4-disilaoctadecane (Sx) of the silsesquioxane family was used as an electrolyte for the first time in a fluoride shuttle battery (FSB) system. To overcome the solubility problem of a typical fluorine-based ionic salt (CsF) in Sx solvent, Sx was used in conjunction with an anion acceptor (triphenyl boroxine; TPhBX). The possible effects of Sx on the solubility of BiF3 in the electrolyte and on the electrochemical compatibility of the BiF3/C electrode for FSBs were fully investigated through cyclic voltammetry (CV) and charge-discharge tests at room temperature (RT) as well as at 55 degrees C. The related discharge and charge reactions were confirmed by X-ray diffraction analysis. The reaction potential of BiF3/Bi-0 was investigated first in this study and was found to be -0.225 V vs. a standard hydrogen electrode (SHE) in Sx/TPhBX. A temperature increase from RT to 55 degrees C resulted in a lower IR drop and a more-efficient redox reaction at more positive potentials. Achieving a wider electrochemical stability window and reasonable ionic conductivity was speculated to have improved the FSB performance at 55 degrees C. In addition, Raman spectroscopy and atomic absorption spectroscopy (AAS) confirmed that BiF3 did not dissolve in the Sx/TPhBX system. The stable thermal and electrochemical properties of siloxane-based liquid electrolytes render them promising candidates as electrolytes that can be used at elevated temperatures.