Lithium superoxide (LiO₂) has long been known to be fairly unstable in the literature. It has only been isolated below 10 K with matrix isolation technique and characterized with use of electron paramagnetic resonance (EPR) spectroscopy as isolated molecules. Recently theoretical calculations suggested that LiO₂ may be stabilized over the surface layer of lithium peroxide (Li₂O₂). Since Li₂O₂ is a commonly recognized discharge product from a Li-O₂ battery through a two-electron oxygen reduction reaction (ORR), we are motivated to study the Li-O₂ battery discharge process in details. Initial characterization of commercial Li₂O₂ powders with EPR technique indicates that a weak signal appears at temperature below 100 K. At 4 K the signal becomes fairly strong and notable. Since Li₂O₂ contains no unpaired electron and will not give any EPR signal, we attribute the low temperature EPR signal to a LiO₂-like species. (ChemSusChem 2013 6 1196) Following the finding, when activated carbon is used as the cathode for a Li-O₂ battery with tetra(ethylene) glycol dimethyl ether–lithium triflate (TEGDME/LiCF₃SO₃) as the electrolyte, a two-plateaus voltage profile during charging cycle at ~3.3 V and 4.2 V vs Li/Li⁺ is observed. To identify the uncommon low charge plateau, Raman spectroscopy of the discharge product reveals that a not yet reported peak at 1125 cm^-1. Comparing to well characterized Li-discharge product Li₂O₂ (790 cm^-1) and neutral oxygen (O₂, 1538 cm^-1), the newly identified Raman peak fits nearly in the middle. (Phys. Chem. Chem. Phys., 2013, 15, 3764) In addition, when other reported superoxides such as NaO₂, KO₂, NBu₄O₂ (in CH₃CN), and K(18-crown-6)O₂ (in DMSO) are plotted together with the LiO₂-like species, the O-O Raman peaks (O₂^2-, O₂^-, and O₂) form a nearly straight line (Figure 1). The O-O vibration peak of the LiO₂-like species is in good agreement with other reported superoxides.

To better understand the nature of the two-plateaus charging voltage profile at 3.3 and 4.2 V (vs Li/Li⁺), we analyze the charging voltage curve with the following assumption. Assuming we can carry out an one-electron ORR reaction, we will then generate exclusively LiO₂ as the only Li-O₂ battery discharge product. We carry out four parallel Li-O₂ cells that are discharged at a constant current to four different capacities which are equivalent to four different time. Based on the quantitative amount of the low voltage plateau that is equivalent to the amount of remaining LiO₂ from Li-discharge, a linear correlation between the natural log value of the disproportionated LiO₂ vs discharge capacity (time) is observed. We reach the conclusion that LiO₂ undergoes disproportionation with first order kinetics to generate Li₂O₂ and O₂ (2LiO₂ → Li₂O₂ + O₂). (J. Am. Chem. Soc. 2013, 135, 15364) With micro-Raman technique, we further demonstrate that LiO₂ is indeed coated over an inner core of Li₂O₂. In addition to the Raman O-O peak of LiO₂ at 1125 cm^-1, we also identify another Raman peak at 1505 cm^-1 due to the interaction of LiO₂ with carbon substrate. Therefore, the characteristic 1125 and 1505 cm^-1 pair serves as the signature of the presence of LiO₂ over carbon cathode. (J. Phys. Chem. Lett. 2014, 5, 2705) Another strong evidence for the presence of LiO₂ in the Li-O₂ battery discharge product under ambient temperature is that selected area electron diffraction pattern of it is in good consistency with the calculated LiO₂ crystal structure based on density function theory. (Nano Lett. 2015, 15, 1041) Very recently, with use of Ir catalyst over reduced graphene oxide as cathode, our new Li-O₂ cell demonstrates low charge voltage plateau, characteristic LiO₂ Raman signature, one electron ORR reaction based on differential electrochemical mass spectroscopy (DEMS), and a new EPR signal at low temperature. (Nature 2016 asap on-line letter) This presentation will focus on the characterization of LiO₂ from the Li-O₂battery discharge product(s).

This work was supported by the US Department of Energy under contract DE-AC02-06CH11357 from the Vehicle Technologies Office, Department of Energy, Office of Energy Efficiency and Renewable Energy.