Sodium/Nickel Chloride (Zebra) Batteries


The negative electrode and the electrolyte of this battery are those of the Na/S battery, but a metal chloride, preferentially NiCl2, is used instead of sulphur as a positive electrode. As the initial research was carried out in South Africa for EV applications, the acronym Zebra (zero emission battery research activity) is often used.

During normal discharge, Ni and NaCl are formed. The voltage, 2.58 V, is higher than that of Na/S, 2.06–1.78 V. Other differences are given below. Liquid Na is placed in the outer part of the cell, while in practical Na/S cells it is in the central part. Furthermore, Na is not initially included in the cell assembly, but produced in situ during charge, as the cell is assembled in the discharged state. The positive electrode is solid and this would pose problems, as the solid-solid interface with the electrolyte would not allow high currents. Therefore, a second electrolyte, NaAlCl4, which is liquid at the batteryoperating temperatures, is added.

The tolerance to overcharge and overdischarge is outstanding by virtue of the reversible reactions occurring in these conditions. On overcharge, NaAlCl4 further chlorinates the Ni matrix, while on overdischarge excess Na reacts with the liquid electrolyte. This feature has a practical implication: it is possible to connect in series several cells, without parallel connections, as cell imbalances are levelled out by the above reactions.

Another advantage of this battery, with respect to the Na/S system, is represented by its enhanced safety. As a further advantage, the Zebra cell can work in a wider temperature range (i.e. 220–450C), although the practical range has to be restricted to 270–350C.

The Zebra system is specifically intended for EV applications, where such factors as safety, easiness in construction, cyclability, and low corrosion, are appreciated.

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This article was sent to us by: Giani Rimeollo at 06272010

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