Catalytic Behavior of Lithium Nitrate in Li-O₂ Cells

(Formula Presented) The development of a successful Li-O₂ battery depends to a large extent on the discovery of electrolyte solutions that remain chemically stable through the reduction and oxidation reactions that occur during cell operations. The influence of the electrolyte anions on the behavior of Li-O₂ cells was thought to be negligible. However, it has recently been suggested that specific anions can have a dramatic effect on the chemistry of a Li-O₂ cell. In the present paper, we describe how LiNO₃ in polyether solvents can improve both oxygen reduction (ORR) and oxygen evolution (OER) reactions. In particular, the nitrate anion can enhance the ORR by enabling a mechanism that involves solubilized species like superoxide radicals, which allows for the formation of submicronic Li₂O₂ particles. Such phenomena were also observed in Li-O₂ cells with high donor number solvents, such as dimethyl sulfoxide dimethylformamide (DMF) and dimethylacetamide (DMA). Nevertheless, their instability toward oxygen reduction, lithium metals, and high oxidation potentials renders them less suitable than polyether solvents. In turn, using catalysts like LiI to reduce the OER overpotential might enhance parasitic reactions. We show herein that LiNO₃ can serve as an electrolyte and useful redox mediator. NO₂^- ions are formed by the reduction of nitrate ions on the anode. Their oxidation forms NO₂, which readily oxidizes to Li₂O₂. The latter process moves the OER overpotentials down into a potential window suitable for polyether solvent-based cells. Advanced analytical tools, including in situ electrochemical quartz microbalance (EQCM) and ESR plus XPS, HR-SEM, and impedance spectroscopy, were used for the studies reported herein.