The aim of this master thesis was to establish if sodium is trapped in aluminium current collectors, which in turn could affect the capacity fade in sodium-ion battery systems. In the case of lithium-ion batteries, previous studies have shown that a trapping mechanism, where lithium diffuses through the active material and current collectors, can explain the capacity fade observed for several systems. However, no such reports have been published in the sodium case, motivating this pioneering investigation. Contact samples of sodium and aluminium current collector material confirmed the uptake of sodium as shown by ICP-AES analyses. The uptake of sodium in the aluminium was equivalent to a charge of 0.4 µAh after 70 days of contact at 55°C. The main characterisation method was galvanostatic plating and stripping of sodium on aluminium in a pouch-cell configuration. When using a bare aluminium working electrode with a metallic sodium counter electrode in a 1 M NaPF6/diglyme electrolyte, the galvanostatic cycling showed coulombic efficiency instabilities. It was concluded that a more stable, high efficient plating-stripping would be needed to quantify the effects of sodium trapping with the employed electrochemical methods. Coulombic efficiency values that exceeded 100 % were attributed to the oxidation of disconnected (detached) sodium from previous plating cycles. On consecutive cycles some of the disconnected sodium got reconnected, resulting in coulombic efficiency values well over 100 %.

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.