Self-Propagating Catalysis: On the Comparison of ORR/Oer Mechanism in Li-O2 Battery with Fuel Cell

Abstract

Li-O2 battery has attracted a great deal of attentions recently due to its high theoretical energy density.[1] Many studies were reported on the investigation of catalysts and cathode structures to the battery performance.[2] They revealed that the actual redox mechanism in Li-O2 battery could be significantly more complicated than originally thought. During the discharge-charge cycling, cathodic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occur in Li-O2 battery. Various catalysts were disclosed in attempts to promote ORR and OER processes at lower overpotentials therefore higher overall round-trip efficiency. However, little information is available on the catalytic mechanism. For example, it is commonly accepted that lithium peroxide, Li2O2, serves as a redox intermediate during cycling reactions. Li2O2 is nearly insoluble in the organic electrolytes and precipitates on the surface of the cathode once formed. How a solid-state catalyst could promote not only Li2O2 formation during ORR, but also its dissipation during OER is completely unclear. Such catalytic reactions would require a solid-solid interaction with limited or no mass-transport. How the catalytic transition intermediates can be formed under such condition remains a mystery. Without a better understanding of the catalytic mechanism, it is difficult to rationally design and improve the next generation Li-O2battery. At Argonne National Laboratory, our team has explored a low-cost, high surface area Fe/N/C composite as the cathode catalyst for Li-O2 battery application, inspired by the non-precious metal catalyst study for the proton exchange membrane fuel cell.[3] The catalyst demonstrated reduced ORR and OER overpotentials in the discharge-charge cycle and minimized the electrolyte decomposition. We also developed a holistic approach in studying electrochemical processes in Li-O2 battery using various characterization tools, such as SEM, TEM, FTIR, XRD, etc..[4] Particularly, we introduced a microfocused synchrotron X-ray diffraction (m-XRD) and a micro-tomographic techniques (µ-CT) for the spatiotemporal study on the phase and structural change in Li-O2 battery under actual cell cycling condition.[5] The m-XRD has a spatial resolution at micron level with the complete penetration and sampling from cell’s radial direction, rendering it possible to probe battery’s composition layer-by-layer under in situconditions. The data collection at any given position usually takes a few seconds, making the method particularly suitable to study the dynamic change inside the battery in real time. In this presentation, we will focus on our recent operando, spatiotemporal investigation on the catalytic processes at the Li-O2 battery cathode using the m-XRD technique. We prepared an in situ Li-O2 battery using the representative material and cell design and studied formation, dissipation and distribution of the lithium redox intermediate under the multiple discharge-charge cycling condition in the entire cathode region. We demonstrated, for the first time, the evolutions of the grain size, concentration and spatial distribution of lithium peroxide as the functions of ORR/OER potentials and capacity. A new “self-propagating” catalytic mechanism was derived from the experimental data and compared to the similar electrocatalytic processes in fuel cell. The new finding could clarify some of the on-going controversies on the redox mechanism. More importantly, it shed lights on the new structural and material research directions for better cycloability and durability of the Li-O2battery. Acknowledgement: The work performed at Argonne is supported by DOE under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.