Correlating Ionic Conductivity, Oxygen Transport and ORR with Structure of Dialkylacetamide-Based Electrolytes for Lithium-Air Batteries

Abstract

Chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc), two high Donor Number organic solvents, have been studied. In LiX salt solutions (where X = PF6−, CF3SO3−, ClO4− and NO3−), solvation occurs when the Li+ bonds with the solvent's carbonyl group forming Li+[O=C(CH3)N(CH3)2]nX− ion pairs. Infrared and 13C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF6−, ClO4− or NO3−, and a contact ion pair in the case of CF3SO3−. Chemical interactions between TBA+ and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBA+ and Li+ electrolytes were measured and correlated with their viscosities. In 0.1M TBAPF6/DMAc, the O2 solubility and diffusion coefficient (3.09 × 10−6 mol/cm−3 and 5.09 × 10−5 cm2s−1, respectively) measured using microelectrode technique are typical of values measured in several TBA+ solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory.