Disproportionation in Li–O₂ Batteries Based on a Large Surface Area Carbon Cathode

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li–O₂ cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li₂O₂ component with superoxide-like character to a Li₂O₂ component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2–3.5 V) than the Li₂O₂ component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li–O₂ cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li–O₂ cell

Size- and Support-Dependent Evolution of the Oxidation State and Structure by Oxidation of Subnanometer Cobalt Clusters

Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamond (UNCD) surfaces were oxidized after exposure to ambient air. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) were used to characterize the clusters revealed a strong dependency of the oxidation state and structure of the clusters on the surface. A dominant Co²⁺ phase was identified in all samples. However, XANES analysis of cobalt clusters on UNCD showed that ∼10% fraction of a Co⁰ phase was identified for all three cluster sizes and about 30 and 12% fraction of a Co³⁺ phase in 4, 7, and 27 atom clusters, respectively. In the alumina-supported clusters, the dominating Co²⁺ component was attributed to a cobalt aluminate, indicative of a very strong binding to the support. NEXAFS showed that in addition to strong binding of the clusters to alumina, their structure to a great extent follows the tetrahedral morphology of the support. All supported clusters were found to be resistant to agglomeration when exposed to reactive gases at elevated temperatures and atmospheric pressure

Interfacial Effects on Lithium Superoxide Disproportionation in Li-O₂ Batteries

During the cycling of Li-O₂ batteries the discharge process gives rise to dynamically evolving agglomerates composed of lithium–oxygen nanostructures; however, little is known about their composition. In this paper, we present results for a Li-O₂ battery based on an activated carbon cathode that indicate interfacial effects can suppress disproportionation of a LiO₂ component in the discharge product. High-intensity X-ray diffraction and transmission electron microscopy measurements are first used to show that there is a LiO₂ component along with Li₂O₂ in the discharge product. The stability of the discharge product was then probed by investigating the dependence of the charge potential and Raman intensity of the superoxide peak with time. The results indicate that the LiO₂ component can be stable for possibly up to days when an electrolyte is left on the surface of the discharged cathode. Density functional calculations on amorphous LiO₂ reveal that the disproportionation process will be slower at an electrolyte/LiO₂ interface compared to a vacuum/LiO₂ interface. The combined experimental and theoretical results provide new insight into how interfacial effects can stabilize LiO₂ and suggest that these interfacial effects may play an important role in the charge and discharge chemistries of a Li–O₂ battery

Raman Evidence for Late Stage Disproportionation in a Li–O₂ Battery

Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li–O₂ cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO₂-like and inner Li₂O₂ regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO₂-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm^–1, which appears only when the LiO₂ peak at 1123 cm^–1 is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO₂-like component based on density functional calculations. These new results on the LiO₂-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li–O₂ batteries