A high energy density cell operating at 250°C has been developed employing a liquid sodium anode, a beta alumina electrolyte, and a novel solid‐state cathode. The cathode consists of a partially chlorinated porous iron matrix, impregnated with a sodium ion conducting molten salt
NaAlCl4
which acts as a pure electrolyte. The reversibility of the cell reaction has been proven and considerable progress has been made towards optimization of the cathode, demonstrating high energy and power density, long life (>1000 cycles), and a high degree of safety.
Medium‐temperature liquid sodium (ZEBRA) battery cells, employing solid transition metal dichloride cathodes, are under active development. This paper details key aspects of the development of a Ni,
NiCl2
positive electrode, operating in
NaAlCl4
molten salt electrolyte. Cells using these materials have achieved a high degree of reversibility and long cell life (>2000 cycles) and operate over the temperature range 200°–350°C. The cell has an open‐circuit voltage of 2.58V at 300°C and is an attractive contender for use in high‐energy and power density advanced batteries. Mention is made to the related Fe‐based electrode.
The stability of beta alumina in fully developed Zebra
normalNa/FeCl2
cells under long‐term cycling has been proven. In the early stages of development of the
normalNa/FeCl2
cell, some problems in the form of increased resistances were encountered under certain extreme conditions of temperature and voltage. While these difficulties have now been overcome in practical cells, the scientific investigation into the causes of the anomalous resistance behavior showed it to be related to the interaction of the beta alumina with constitutents of the positive electrode compartment. In particular, iron has been shown to enter beta alumina in the presence of an electric field when current is passing, if the cell is deliberately overheated. However, it was found that only the β (but not the β″) phase was modified by the incursion of iron. The resistance of the iron‐doped regions of the ceramic was very high. Chloroaluminate melts also irreversibly damage the beta alumina in the absence of an electric field, when
FeCl3
or excess
AlCl3
is present, by abstraction of Na+ ions, forming γ alumina.
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