A comprehensive study on the cell chemistry of the sodium superoxide (NaO2) battery

Abstract

This work reports on the cell chemistry of a room temperature sodium–oxygen battery using an electrolyte of diethylene glycol dimethyl ether (diglyme) and sodium trifluoromethanesulfonate (NaSO₃CF₃, sodium triflate). Different from lithium–oxygen cells, where lithium peroxide is found as the discharge product, sodium superoxide (NaO₂) is formed in the present cell, with overpotentials as low as 100 mV during charging. Several analytical methods are used to follow the cell reaction during discharge and charge. Changes in structure and morphology are studied by SEM and XRD. It is found that NaO₂ grows as cubic particles with feed sizes in the range of 10–50 μm; upon recharge the particles consecutively decompose. Pressure monitoring during galvanostatic cycling shows that the coulombic efficiency (e⁻/O₂) for discharge and charge is approx. 1.0, the expected value for NaO₂ formation. Also optical spectroscopy is identified as a convenient and useful tool to follow the discharge–charge process. The maximum discharge capacity is found to be limited by oxygen transport within the electrolyte soaked carbon fiber cathode and pore blocking near the oxygen interface is observed. Finally electrolyte decomposition and sodium dendrite growth are identified as possible reasons for the limited capacity retention of the cell. The occurrence of undesired side reactions is analyzed by DEMS measurements during cycling as well as by post mortem XPS investigations.