Fluoride shuttle batteries: On the performance of the BiF3 electrode in organic liquid electrolytes containing a mixture of lithium bis(oxalato)borate and triphenylboroxin

Abstract

In a typical organic liquid electrolyte-based fluoride shuttle battery (FSB), a high concentration of a boron-based anion acceptor (AA) capable of binding specific anions is required to provide a sufficient amount of dissolved fluoride salt. In this study, and a mixture of lithium bis(oxalato)borate (LiBOB) and an AA, triphenylboroxin (TPhBX), was used as an organic liquid electrolyte. The tetraglyme (G4)-based electrolyte system (LiBOB0.25/TPhBX0.25/sat_CsF/G4) containing equal concentrations of LiBOB, TPhBX, and saturated cesium fluoride (CsF) was prepared. The potential effects of reducing the amount of the AA and using a mixture of LiBOB and TPhBX on the electrochemical compatibility of the BiF3 electrode were investigated through cyclic voltammetry, charge–discharge tests, and alternating current impedance measurements. The potential advantages of using the LiBOB/TPhBX mixture as an electrolyte additive include the fact that it increases ionic conductivity, widens the cathodic and anodic stability window, and enhances the electrochemical performance of the BiF3 positive electrode. Moreover, according to Raman microscopy, the direct insertion mechanism was found to be predominant for the FSB reaction mechanism of BiF3 microparticles in LiBOB0.25/TPhBX 0.25/sat_CsF/G4. These improvements can be attributed to the increase in fluorine anion mobility, which occurs when the cesium cation mobility is reduced; this, in turn, is a result of the stabilization of the cesium cation due to the interaction between LiBOB and TPhBX. Therefore, mixing equal concentrations of LiBOB and TPhBX can be a promising alternative method to ensure electrolyte stability and prevent the potential loss of active materials during the redox reactions.