An Electrochemical Impedance Study of the Capacity Limitations in Na-O2 Cells
Authors: Knudsen Kristian Bastholm , Nichols Jessica E. , Vegge Tejs , Luntz Alan C. , McCloskey Bryan D , Hjelm Johan 
Published: February 2020

Metal-O2 batteries have in recent history received a great deal of attention since the Li-O2 cell was first reported in 19961. The operating principle of the Na-O2 battery is similar to the Li-O2 battery, with Na metal acting as the anode and inexpensive carbon acting as the cathode material upon which the discharge product is deposited. The interest in metal-O2 batteries is a result of the relatively high theoretical energy density of 1105 Wh/kg and 3456 Wh/kg for Na-O2 and Li-O2, respectively2. However both technologies suffer from many technical limitations, such as the cell potential suddenly droping well before their theoretical energy densities are obtained3,4. In this and previous studies, cubic NaO2 crystals were formed upon discharge in a glyme electrolyte5. The cubic NaO2 crystals have been suggested to form through a solution mediated mechanism6, which allows the Na-O2 cell to retain its capacity relatively longer than the anhydrous Li-O2cell.
However the reason for the sudden drop in potential at limited discharge capacity is still not completely clear. In this study porous electrode theory was used to interpret the electrochemical impedance of Na-O2 cells to characterize the surface processes related to the redox of oxygen. The model accounted for charge transfer resistance, and ionic conductivity within the cathode pores that functions as a proxy for pore clogging resistance. This allowed for simultaneous in situ monitoring of the relative extent of either pore clogging or buildup of NaO2 at the cathode/electrolyte interface to determine the main cause of the potential drop/rise on discharge and charge, respectively. The study relates the size of the NaO2 crystal to a range of current densities and the eventual sudden drop or rise in potential on discharge and charge, respectively. A sketch of the impedance model at the cathode/electrolyte interface is shown in Figure a, while experimentally measured EIS spectra are shown in Figure b. The study also contains information about the stability of the SEI on the Na anode upon operation, which eventually caused cell failure upon cycling.

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Figure 1