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

The influence of lithium salts on O2 reduction reactions (ORR) in 1, 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 O₂ 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)nPF6- and Li⁺(TEGDME)PF₆^- in LiPF₆-based electrolytes. On the other hand, the contact ion pairs (DME)_mLi⁺(CF₃SO₃^-) and(TEGDME) Li⁺(CF₃SO₃^-) appear to form in LiS0₃CF₃- containing electrolytes. In the LiS0₃CF₃-based electrolytes the initial ORR product, superoxide (O₂^-), is stabilized in solution by forming [(DME)_m-1( O₂^-) Li⁺(CF₃SO₃^-) and [(TEGDME)(O₂^- ) Li⁺(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/O₂ cells utilizing LiCF₃SO₃/TEGDME electrolytes. [ABSTRACT FROM AUTHOR]