Catalyst and Electrolyte Design for Metal-Oxygen Batteries and Beyond

Abstract

Metal-oxygen batteries recently emerge as one of the most promising post-Li-ion energy storage technologies. The key feature of this technology lies in the conversion reactions of O2 at the cathode. Such a chemistry promises the highest theoretical energy densities due to the contribution from the cathode reactions. However, the conversion between various oxygen-based species suffer severe kinetic penalties, resulting in poor energy efficiencies and low rate capabilities. To promote these reactions, catalysts with desired functionality and stability are needed. On the other hand, the O2-based chemistry incurs severe parasitic chemical reactions against various cell components, including the anode, the cathode and the electrolyte. Consequently, the reported cyclabilities of metal-oxygen batteries remain much worse than required. While stable cathode and anode candidates have been developed, further advance of this technology still hinges on developing stable electrolyte and efficient catalyst to ensure prolonged and stable cell operations. In the first part of this thesis, two distinct strategies were exploited as proof-of-concept demonstrations on the catalyst design for metal-oxygen batteries. For one, using Li-O2 batteries as a study platform, we show that the stability of catalyst can be heavily dependent on the synthesis history. A novel approach, namely carbothermal shock method, was found to enable superior chemical and structural stability of the catalyst compared to those of the catalyst prepared by conventional methods. For another, using Mg-O2 batteries as prototypical system, we demonstrate a strategy using two redox mediators that concertedly operate for discharge and recharge. As a result, a total overpotential reduction by ca. 600 mV can be achieved through manipulating the charge transfer mechanism. To meet the need of a stable electrolyte for metal-oxygen batteries, in the second part of this thesis, we analyzed the decomposition pathways of the electrolyte in the presence of reactive oxygen species. Using Li-O2 battery as a model system, we address this issue by employing a water-in-salt (WiS) electrolyte that eliminates organic solvents all together. WiS was found stable under Li-O2 battery operation conditions. When carbon was used as a cathode, much longer cycling numbers (>70) can be achieved in WiS than in organic ones. When carbon was replaced with a carbon-free cathode (TiSi2 nanonets decorated with Ru catalyst), over 300 reversible cycles was measured. The unique feature of WiS also enables other opportunities beyond O2 chemistry in metal-oxygen batteries. Toward the end of this thesis, we employ WiS for electrochemical CO2 reduction reactions. By controlling the concentration of H2O in WiS, the rate determining step on Au catalyst was found to be the first electron transfer from the electrode to CO2. Moreover, the reduced H2O activity by WiS significantly suppressed hydrogen evolution reactions, through which high selectivity toward CO can be measured. Our study provides important knowledge base on the design of electrolyte for future optimizations.