Reactivity of the anion acceptor in electrolyte: An important factor in achieving high electrochemical performance of a lead (II) fluoride electrode in a fluoride shuttle battery

Abstract

The electrochemical reaction in a fluoride shuttle battery (FSB) requires an organic solvent containing a supporting electrolyte salt. To dissolve an insoluble inorganic fluoride compound (e.g., cesium fluoride (CsF)) in an organic solvent (e.g., bis[2-(2-methoxyethoxy)ethyl]ether [tetraglyme, G4]), an anion acceptor (AA) (e.g., triphenylborane (TPhB) or triphenylboroxine (TPhBX)) is typically added. This study investigates the effect of AA type and CsF/AA molar ratio on the electrochemical performance of a lead (II) fluoride (PbF2) FSB electrode. The PbF2 electrodes were cycled in an FSB with G4 containing CsF (0.45 mol dm−3) and TPhB (0.5 mol dm−3) as the electrolyte. The capacity was high during the first cycle, but drastically decreased during subsequent cycling. X-ray diffraction (XRD) and atomic absorption spectrometry (AAS) revealed Pb formation during the discharging process, but PbF2 formation and Pb dissolution during the charging process. The loss of active material through Pb dissolution caused the capacity degradation. Changing the AA type from TPhB to TPhBX promoted Pb dissolution, thereby aggravating the capacity retention problem. In contrast, increasing the CsF concentration from 0.45 mol dm−3 to its saturation concentration effectively preserved the capacity during cycling by suppressing the Pb dissolution, as confirmed in the AAS results. However, increasing the CsF concentration deteriorated the discharge and charge capacities of the PbF2 electrode. Changing the AA type from TPhB to TPhBX in the electrolyte containing saturated CsF increased the discharge and charge capacities during the first cycle, and maintained a high capacity in the subsequent discharging and charging processes, as supported in the XRD results. Overall, the AA type and CsF concentration in the electrolyte strongly affected the electrochemical performance of the PbF2 electrodes. Thus, an FSB with high electrochemical performance requires judicious selection of both AA type and CsF/AA ratio.