Positive ZEBRA cell

The power of the ZEBRA cell depends on the resistance of the cell during discharge. The resistance of the ZEBRA cell rises with increasing depth of discharge (DOD). There is a contribution to the resistance from the fixed values of the solid metal components and of the/ "-alumina solid electrolyte. The variable parts of the resistance arc the sodium electrode and the positive electrode. The increase in internal resistance during discharge is almost entirely due to the positive electrode, as can be seen from Fig. 4. 

The ZEBRA cell shows the similar behavior during the charging reaction, in which the nickel is converted to nickel chloride within the reaction front. During the charging reaction the reaction front also moves from the / " -alumina into the positive electrode. 

Another type of battery is the so-called Zebra cell, obtained for the first time in South Africa by Coetzer [425] at Zebra Power Systems (Pty) Limited. The development of this battery is being actively pursued in the United Kingdom for high energy density applications such as electric vehicles, load leveling and spacecraft. This type of battery has sodium as the negative electrode, and the positive electrode is made from Fe/FeCl2 or Ni/NiCU. 

The ZEBRA cell, which is under development by the General Electric Co., uses a molten-sodium anode and a solid p,p"-alumina solid electrolyte as in the sodium-sulfur cell, but the positive electrode is large-surface-area nickel rather than molten sulfur with a large-surface-area current collector. The electrolyte on the cathode side of the ZEBRA solid electrolyte is an aqueous NaAlCLt containing NaCl and Nal as well as a little FeS. The FeS and Nal are added to limit growth of the Ni particles and to aid the overall cathode reaction, which is... 

The charge capacity of the ZEBRA cell is governed by the quantity of NaCl available in the positive electrode. During an operation, the liquid salt NaAlCU is a Na reserve as shown in the following reaction ... 

ZEBRA cells are produced in the discharged state so that no metallic sodium can be handled. All the required sodium is inserted as salt. Figure 10.6 shows the cell design. The positive pole is connected to the current collector, which is a hair-needle shaped wire with an inside copper core for low resistivity and an outside nickel plating so that all material in contact with the cathode is consistent with the cell chemistry. 

One of the problems encountered with the Werth cell was an increase in resistance with cycling. This may have been caused in part by the /3-alumina reacting with the acidic sodium chloroaluminate melt. Coetzer had the idea of using transition metal chlorides as a positive electrode and chose a basic sodium chloroaluminate melt as the liquid electrolyte. This is compatible with /3-alumina, and a new class of secondary cells based upon the reaction between sodium metal and transition metal chloride has resulted from this work. Collectively, the term Zebra battery is used to describe this new class of cell. 

At a charged state, positive and negative electrode could be NiQ2 and Na metal. It is quite difficult to handle these materials in a production scale, and the cell is usually built with NaQ and Ni metal as discharged products. In order to maintain the sufficient utilization of Na electrode, A1 powder is incorporated in the positive electrode side to form NaAlCU that can be an electrolyte when the electrode materials are melted. Operation temperature is at around 270-350 °C. Sodium ion conductivity has a practical value (>0.2 - 1 cm ) at 260 °C and is temperature dependent with a positive gradient [3]. Thus, the operational temperature of ZEBRA batteries has been chosen for the range above. Current collector for positive electrode is Cu wire with Ni plating. 

ZEBRA Batteries, Fig. 1 Basic cell structure of ZEBRA battery. 1 Cell can for the Na negative electrode, 2 Na negative electrode, 3 ceramic electrode tube made of P-alumina, 4 mixture for the positive electrode (NiCl2 + NaAlCl4), 5 current collector for positive electrode, 6 thermal compression bond... 

Small cracks in the ceramic electrolyte can be closed by formed salt and Al. When crack is larger, formed Al short-circuits between positive and negative electrode. This cell loses voltages, but still the whole system can be operated as long as failed cell was within 5-10 % of the total cells. The battery controller detects this and adjusts all operative parameters. In this meaning, ZEBRA battery is failure tolerant to some extent. 

The ZEBRA-Battery developed by AEG Co. in Germary is an advanced high-energy battery based on nickel chloride as the positive electrode and sodittm as the negative electrode. The cell reaction... 

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