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Non-aqueous metal-air (Li-O2 and Na-O2) batteries have been emerging as one of the most promising high-energy storage systems to meet the requirements for demanding applications due to their high theoretical specific energy. In the present thesis work, advanced characterization techniques are demonstrated for the exploration of metal-O2 batteries. Prominently, the electrochemical reactions occurring within the Li-O2 and Na-O2 batteries upon cycling are studied by in operando powder X-ray diffraction (XRD). In the first part, a new in operando cell with a combined form of coin cell and pouch cell is designed. In operando synchrotron radiation powder X-ray diffraction (SR-PXD) is applied to investigate the evolution of Li2O2 inside the Li-O2 cells with carbon and Ru-TiC cathodes. By quantitatively tracking the Li2O2 evolution, a two-step process during growth and oxidation is observed. This newly developed analysis technique is further applied to the Na-O2 battery system. The formation of NaO2 and the influence of the electrolyte salt are followed quantitatively by in operando SR-PXD. The results indicate that the discharge capacity of Na-O2 cells containing a weak solvating ether solvent depends heavily on the choice of the conducting salt anion, which also has impact on the growth of NaO2 particles. In addition, the stability of the discharge product in Na-O2 cells is studied. Using both ex situ and in operando XRD, the influence of sodium anode, solvent, salt and oxygen on the stability of NaO2 are quantitatively identified. These findings bring new insights into the understanding of conflicting observations of different discharge products in previous studies. In the last part, a binder-free graphene based cathode concept is developed for Li-O2 cells. The formation of discharge products and their decomposition upon charge, as well as different morphologies of the discharge products on the electrode, are demonstrated. Moreover, considering the instability of carbon based cathode materials, a new type of titanium carbide on carbon cloth cathode is designed and fabricated. With a surface modification by loading Ru nanoparticles, the titanium carbide shows enhanced oxygen reduction/evolution activity and stability. Compared with the carbon based cathode materials, titanium carbide demonstrated a higher discharge and charge efficiency.

In this thesis, the discharge products formed at the cathode and the performance and cell chemistry of sodium-oxygen batteries have been studied. This was carried out using different NaOTf salt concentrations. The influence of different salt concentrations on sodium-oxygen batteries was investigated since it has been shown that increasing the salt concentration beyond conventional concentrations could result in advantages such as increased stability of the electrolytes towards decomposition, higher thermal stability and lower volatility. An increase in salt concentration has also been shown to influence the electrochemical potential window. The solubility of NaOTf was investigated in two different solvents, DME and diglyme. NaOTf was found to be more soluble in DME compared to diglyme but due to the volatile nature of DME, three different concentrations of NaOTf were prepared with diglyme as solvent. Experimentation involved discharging the batteries to either maximum or limited capacity. The discharge products were examined and characterized using XRD and SEM. The main discharge product was identified as sodium superoxide although sodium peroxide dihydrate was also identified in one battery. A trend of higher capacity and voltage plateaus with higher salt concentration was also found. The influence of trace amounts of water was suggested as one explanation as it works as a catalyst, promoting sodium superoxide cube growth due to improved transportation of superoxide. Another or contributing explanation could be a possible change in donor number with increased salt concentration, resulting in higher solubility and longer lifetime of superoxide, promoting the growth of sodium superoxide cubes.