Borolan-2-yl involving anion acceptors for organic liquid electrolyte-based fluoride shuttle batteries

Abstract

Boron-based anion acceptors (AAs) have been used to dissociate MF salt (CsF) in organic liquid electrolyte-based fluoride shuttle batteries (FSBs). Here, two boron-based compounds, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (DiOB-Py) and N,N-diethyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline) (DiOB-An), were examined as electrolyte additives for G4 containing CsF. DiOB-Py and DiOB-An are structurally similar except that the former is functionalized with pyridine whereas the latter with aniline. Since pyridine in DiOB-Py withdraws an electron pair on the phenyl ring via the resonance effect, the Lewis acidity of borate increases. On the other hand, since lone pair electrons of aniline are donated to the phenyl ring through the resonance effect, the acidity of borate in DiOB-An decreases. AAs are known to have a moderately strong fluoride affinity to provide sufficient dissolution of CsF in the electrolyte solvent. Thus, the effects of the acidity strength of the borates on the electrochemical compatibility of BiF3 were investigated. DiOB-Py imparted relatively high fluoride ion conductivity and CsF solubility, thereby allowing enhanced FSB performance.