Enhancing the oxidation of Li₂O₂ in Li-O₂ batteries : mechanistic and chemical efficacy probing

Abstract

The wide consensus regarding anthropogenic climate change, the positive correlation between economic growth and greenhouse gas emissions, and the humanitarian need for further global growth urges the decoupling of energy usage and emissions. To power portable electronics, enable electrification of transport, level the load on the current fossil-fuel powered grid, and provide storage for clean but intermittent wind and solar, low-cost and high energy density battery chemistries such as lithium-oxygen (Li-O₂) are being vigorously pursued beyond Li-Ion. The present thesis reports on efforts to devise and understand reaction promoters to enhance the kinetics of charging of Li-0₂ cells for the purpose of boosting round-trip efficiency, one of the most severe issues in the system. Investigating trends in electrochemical current output during charge in electrodes containing transition metal nanoparticles and metal oxides, we revealed a strong correlation between the conversion enthalpy of the promoter with Li₂O₂ towards formation of a corresponding lithium-rich metal oxide. Experimental evidence of formation of Li₂CrO₄, and Li₂MoO₄ is provided. Ru nanoparticles showed the formation of a surface phase in contact with Li₂O₂ which is assigned to Li₂RuO₃. We postulate solid-state promoters activate the oxidation of Li₂O₂ by enabling the formation of a lithium-rich metal oxide intermediate which proceeds to delithiate with enhanced kinetics compared to the direct decomposition of Li₂O₂. A microkinetics analysis successfully explains the excellent Li₂O₂ oxidation activity of metal nanoparticles such as Cr, Mo, and Ru as well as the relative inactivity at 3.9 VU of Mn, Co and other derivative oxides. Using differential electrochemical mass spectrometry (DEMS), the same conversion mechanism appears to result in sub-stoichiometric evolution of oxygen on charging as conversion enthalpy increase. In the second and last part of this thesis, cobalt bis(terpyridine) metal complex (Co(Terp)2) is demonstrated as redox mediator of the electron transfer to the insulating Li₂O₂. However, chemical probing using DEMS revealed a parasitic Co II to Co l reduction during discharge using the metal complex while the ideal 2.0 e-/O₂ formation of Li₂O₂ is observed with benchmark mediator tetrathiafulvalene. On charge substoichiometric O₂ regeneration is observed for both mediators; however, improved oxygen regeneration is seen using TTF.