INVESTIGATION OF THE OXYGEN REDUCTION REACTION AT THE LITHIUM-OXYGEN CELL CATHODE
MetadataShow full item record
Development of Li-O2 cells, which could possibly provide ~3 times the capacity of conventional Li-ion cells, depends on a fundamental understanding the oxygen reduction reaction (ORR) at the cathode. This abstract summarizes the application of ab initio density functional theory (DFT) calculations and electrochemical measurements in conjunction with kinetic modeling to elucidate the mechanism of the ORR in the Li-O2 cell. First principles, density functional theory (DFT) modeling of the ORR on noble metal (Pt, Au, Pd), Pt3M (M=Fe, Co, Ni, Cu) and Pd3M (M=Fe, Co, Ni, Cu) alloy surfaces, was carried out. Periodic models of close-packed (111) surfaces were constructed, their geometry was optimized and the most stable geometric surface configuration was identified. The correlation between the intermediate species binding energy and the favored reaction pathway from amongst 1e-, 2e-, and 4e- mechanisms was studied by calculating the binding energies of a 1/4 monolayer of O, O2, LiO, LiO2, Li2O2, and Li2O on various sites and orientations. The reaction free energies (ΔGrxn) were calculated and used to compute the catalytic activity of the surfaces using molecular kinetics theory. Plots of the catalytic activity vs. Oxygen binding energy (EBinding (O)) showed a typical “volcano” profile. The insights gained from this study can be used to guide the choice of cathode catalysts in Li-O2 cells. The mechanism and kinetics of the ORR was investigated in 0.1M LiTFSI/DME on a glassy carbon (GC) electrode in an oxygen saturated solution of 0.1M Lithium bis-trifluoromethanesulfonimidate (LiTFSI) in Dimethoxyethane (DME) using cyclic voltammetery (CV) and the rotating ring-disk electrode (RRDE) technique. A comprehensive reaction scheme considering disproportionation both on the cathode surface and the electrolyte bulk to form Li2O2 was proposed and the data from the RRDE measurements was used along with an electrochemical kinetics model to evaluate the corresponding rate constants. The surface disproportionation reaction was found to dominate the kinetics of the ORR and the model was found to be able to explain experimental observations regarding the cell discharge products. Further, the widely reported anomalous Tafel behavior was observed over the course of these studies. Highly accurate, potentiostatic, point-by-point measurements of the kinetic current were carried out and a scan rate independent evaluation of the corresponding transfer coefficient from a calculated, dimensionless CV was made. The measured transfer coefficient was explained invoking the Marcus-Hush-Levich quadratic model rather than the linearized Butler-Volmer empirical law. Thus, these studies provides a comprehensive account of the ORR mechanism, direct evidence of the surface disproportionation reaction being dominant and explain the widely reported (and previously unexplained) anomalous Tafel behavior in Li-O2 cells.