Sodium-Nickel Chloride Batteries

The sodium-nickel chloride battery, commonly referred to as the Zebra battery, had its origins in South Africa in the late 1970s. The early research on these batteries capitalized on the chemistries and designs of two high-temperature battery systems, sodium-sulfur and lithium-iron sulfide, that had been made public several years before. The first generation of Zebra cells used an iron dichloride electrode with the configuration,

Na/b-alumina/FeCl2,

in which the sodium electrode and b-alumina electrolyte components mimic the sodium-sulfur cell and the iron dichloride electrode acts as a substitute for the FeSx electrode in the LiAl/KCl, LiCl/FeSx cell. Because FeCl2 is a solid, a molten salt electrolyte must be added to the positive electrode compartment of Na/b-alumina/FeCl2 cells. NaAlCl4 is an ideal candidate for this purpose because it has a low melting point (152°C) and because it dissociates into two stable ionic species, Na+ and AlCl4, thereby providing good Na + ion conductivity at elevated temperatures. Moreover, it is probable that the chloride ions of the molten electrolyte play a significant role in the dechlorination and rechlorina­tion reactions that occur at the iron electrode during discharge and charge.

The electrochemical reaction of a Na/b-alumina/ FeCl2 cell occurs in two distinct steps:

6Na + 4FeCl2 — Na6FeCl8 + 3Fe 2Na + NagFeCh — NaCl + 4Fe.

The overall reaction is

8Na + 4FeCl2 — NaCl + 4Fe.

The initial reaction provides an open-circuit voltage of 2.35 V; the second reaction occurs only a few tens of millivolts lower. The intermediate phase that is formed, Na6FeCl8, has a defect rock salt structure in which one iron divalent atom replaces two Na atoms in the NaCl structure. Moreover, despite a volume increase, the closely packed arrangement of the chloride atoms of the FeCl2 structure remains essen­tially unaltered during the transitions to Na6FeCl8 and NaCl. Crystallographically, the reaction at the positive electrode is therefore a relatively simple one. Moreover, all the compounds formed during charge and discharge are essentially insoluble in the NaAlCl4 electrolyte, provided that the electrolyte is kept basic [i. e., slightly NaCl rich (or AlCl3 deficient)]. This prevents ion-exchange reactions from occurring between the Fe2 + in the electrode and Na+ in the b-alumina structure, which would severely degrade the conductivity of the solid electrolyte membrane. The simplicity of the electro­chemical reaction and the insolubility of the metal chloride electrode in the molten salt electrolyte are key factors that contribute to the excellent cycle life of Zebra cells.

The second generation of Zebra cells used a nickel chloride electrode because it provided a slightly higher cell voltage (2.58 V), resulting in cells with a higher power capability. Unlike FeCl2 electrodes, NiCl2 electrodes do not form any intermediate compositions but react directly with sodium to form NaCl:

2Na + NiCl2 —2NaCl + Ni.

Zebra cells are usually designed with the positive electrode housed inside the b-alumina tube. There­fore, during charge, sodium is electrochemically extracted from the NaCl/Ni composite electrode to form NiCl2; the metallic sodium is deposited on the outside of the tube and fills the void between the tube and inner wall of the cylindrical metal container. Although the power capability of the early Zebra cells was limited to approximately 100W/kg, sig­nificantly improved performance was achieved by changing the shape of the cylindrical b-alumina tube to a fluted-tube design, thereby increasing the surface area of the solid electrolyte considerably. Fully developed Zebra batteries now have a specific energy of approximately 100Wh/kg and a specific power of 170 W/kg.

Zebra cells have several major advantages over sodium-sulfur cells: [8]

small proportion of iron, which significantly improves the power capability of the cells toward the end of discharge.)

3. From a safety standpoint, if the b-alumina tubes fracture, the molten NaAlCl4 electrolyte is reduced according to the reaction

NaAlCl4 — 4NaCl + Al,

resulting in solid products; the deposited aluminum metal also provides an internal short circuit that allows for failed cells to be electrically bypassed.

4. Zebra cells can be safely overdischarged by electrochemical reduction of the electrolyte at 1.58 V (the reaction is identical to that given for point 3), provided that the reaction does not go to completion when the electrode solidifies completely.

5. There is less corrosion in Zebra cells.

6. Zebra cells offer a wider operating temperature range (220-450°C) compared with sodium – sulfur cells (290-390°C).

7. Zebra cells are more tolerant to freeze-thaw cycles.

Zebra batteries have been tested extensively in EVs by Daimler-Benz in Europe, for example, in a Mercedes A Class sedan. These batteries have met all the USABC mid-term performance goals listed in Table III, except for operating temperature. Although development work on Zebra batteries is ongoing, the market for pure EVs for which Zebra batteries are best suited is lacking, particularly because of the greater attention that is currently being paid to HEVs. Also, high-temperature batteries have been criticized because they require sophisticated thermal management systems for both heating and cooling. Nonetheless, because they are temperature con­trolled, these systems have been shown to work exceptionally well in both extremely hot and extremely cold climates.

Updated: September 26, 2015 — 10:03 am