Characterization of reaction products in sodium-oxygen batteries : An electrolyte concentration study

• Main Author: Hedman, Jonas
• Format: Others
• Language: English
• Published: Uppsala universitet, Strukturkemi 2017
• Subjects: Sodium-oxygen batteries; Sodium superoxide; Sodium peroxide; Energy storage; Scanning electron microscopy; X-Ray diffraction; Carbon cathodes; Electrochemistry; Material characterization; Sodium triflate; Diglyme; Chemical Engineering; Kemiteknik
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-317969

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.