MICROSTRUCTURE AND CONDUCTIVITY OF THE SODIUM NICKEL CHLORIDE (ZEBRA) BATTERY CATHODE

Abstract

<p>The microstructure of the 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.</p> <p>The presence of isolated nickel particles within the cathode was confirmed by SEM and FIB techniques. Furthermore, the conductivity of the NaAlCl₄ electrolyte was measured at high temperatures and various additives were used to make the electrolyte a mixed ionic-electronic conductor.</p> <p>A special cell was designed to measure the conductivity of hygroscopic and volatile electrolyte at high temperatures. The best conductivity was obtained when using 0.2 mole fraction Bi as an additive to the NbCl₅+NaAlCl₄ mixture (Nb:Na=0.3, Bi:Nb=0.2). The conductivity values were doubled between 190 and 500˚C. The DC measurements confirm the presence of electronic conductivity in Bi+NbCl₅+NaAlCl₄ mixtures. In addition, the effect of NaF and Na₂S on the conductivity of the NaAlCl₄ electrolyte was measured.</p>