Thermodynamic Analysis of Reaction Products Observed in ZEBRA Cell Cathodes

The microstructure of ZEBRA cells was examined at different cycle lifetimes. Various methods of sample preparation were used to remove the NaAlCl4 electrolyte and expose the cathode microstructure. Features such as layered NiCl2 crystals, large NaCl grains and needle-like FeCl2 phases were observed by SEM. The results indicate that nickel particles grow in size with age of the cell. Moreover, the presence of both Na6FeCl8 and NiAl2Cl8 phases was confirmed by XRD. Thermodynamic modeling was used to predict the phases expected when a cell has undergone overcharge or overdischarge during cycling. It is postulated that some phases observed in the cathode at room temperature may be artifacts due to transformations that occur during cooling and do not exist at the operating temperature.