A Study of the Influence of Lithium Salt Anions on Oxygen Reduction Reactions in Li-Air Batteries

The influence of lithium salts on O-2 reduction reactions (ORR) in I. 2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) has been investigated. Microelectrode studies in a series of tetrabutylammonium salt (TBA salt)/DME-based electrolytes showed that O-2 solubility and diffusion coefficient are not significantly affected by the electrolyte anion. The ORR voltammograms on microelectrodes in these electrolytes exhibited steady-state limiting current behavior. In contrast, peak-shaped voltammograms were observed in Li+-conducting electrolytes suggesting a reduction of the effective electrode area by passivating ORR products as well as migration-diffusion control of the reactants at the microelectrode. FT-IR spectra have revealed that Li+ ions are solvated to form solvent separated ion pairs of the type Li+(DME)(n)PF6- and Li-(TEGDME)PF6- in LiPF6-based electrolytes. On the other hand, the contact ion pairs (DME),Li+(CF3S03) and(TEGDNIE)Li+(CF3SO3-) appear to form in LiSO3CF3-containing electrolytes. In the LiSO3CF3 based electrolytes the initial ORR product, superoxide (O-2). is stabilized in solution by forming I(DME)(m-1) (O-2(-))[Li+(CF3SO3-) and RTEGDME)(O-2(-))1Li(+)(CF3SO3-) complexes. These soluble superoxide complexes are able to diffuse away from the electrode surface reaction sites to the bulk electrolyte in the electrode pores where they decompose to form Li2O2. This explains the higher capacity obtained in Li/02 cells utilizing LiCF3SO3-/TEGDME electrolytes. (C) 2015 The Electrochemical Society. All rights reserved.