Design of Non-aqueous Liquid Electrolytes for Rechargeable Li-O2 Batteries

Liquid electrolytes for rechargeable Li-air batteries were chosen from viewpoints of the electrochemical stability against O-2 radical, O-2(-). Mulliken atomic charges of electrolyte solvents and their reversibility of O-2/O-2(-) redox couple were first examined. In the carbonate-based electrolytes, the localization of positive charge in the molecules was confirmed, resulting in low reversibility of O-2 radical. The electrolytes must be decomposed by the nucleophilic O-2 radical. On the other hand, the nitrile-based and piperidinium-based electrolytes provided high O-2 radical reversibility because all of the atomic charges in molecules and cations were either negative or almost zero. It was found that the electronic distribution of electrolyte solvents affected their electrochemical stability against O-2 radical. Considering the electrochemical and chemical stability against Li metal, the piperidinium-based ionic liquid was adopted as an electrolyte solvent in this study. The cell with piperidinium-based electrolyte achieved considerably low charging voltage of around 3.2 V and low voltage gap of about 0.75 V in the discharge-charge profiles, compared to conventional cells with carbonate-based electrolyte. It was, thus, concluded that the charging performances strongly influenced on the O-2 radical stability of electrolyte solvent.