Li2O2 quantification in non-aqueous Li-O2 batteries with binder-free cathodes

Sammanfattning: The non-aqueous Li-air (Li-O2) battery has been emerging as one of the most promising high-energy storage systems to meet the requirements for electric vehicle applications due to its high theoretical energy density. In order to uncover the underlying electrochemistry and enable an informed battery design, it is crucial to gain a detailed understanding of the cell´s chemical components as well as its behavior during cycling.    These two fundamental tasks are reflected in this thesis’ structure: First, advanced characterization techniques are demonstrated in the search for a novel cathode material for Li-O2 batteries. Second, the electrochemical reactions occurring within the battery upon cycling are studied by in operando powder X-ray diffraction.    In the first part, a novel free-standing oxygen cathode was prepared by a facile and efficient solution-process followed by a low-temperature exfoliation, which displayed a 3-D structure arrangement of graphene foam (GF) derived from a graphene oxide (GO) gel on an aluminum substrate (GF@Al). The as prepared GF@Al was directly used as cathode in Li-O2 batteries without any binder and catalyst, delivering a high capacity about 9×104 mA h·g-1 (based on the weight of graphene) or about 60 mAh·g-1 (based on the weight of the whole electrode) at the first discharge with a current density of 100 mA·ggraphene-1. Furthermore, electrodes have been investigated by X-ray diffraction (XRD), Fourier-transform infrared reflection (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-Vis spectroscopy titration. The formation of a discharge product and its decomposition upon charge as well as different morphologies of discharge products on the electrode were observed by SEM and TEM.    In the second part, the evolution of Li2O2 was investigated by synchrotron radiation powder X-ray diffraction (SR-PXD). By quantitatively tracking Li2O2 under the actual electrochemical conditions, a two-step process during growth and oxidation is observed for Li2O2. This is due to different evolution steps during the two stages of both oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). By analyzing the anisotropic broadening of Li2O2 X-ray diffraction peaks, anisotropic disc-like Li2O2 grains were found to be formed rapidly in the first step of discharge, followed by a nucleation and growth of toroidal Li2O2 particles with a LiO2-like surface. During the charging process, Li2O2 was oxidized from the surface first, followed by an oxidation process with a higher decomposition rate for the bulk. This new analysis technique brings additional information on the evolution of Li2O2 in Li-O2 batteries.