The ZEBRA Cell

The -alumina tube inserted in a steel cell case which forms the negative terminal. The space between the ceramic tube and the cell case is the sodium compartment. 

Nickel chloride is preferred and ZEBRA batteries are based today on nickel chloride and sodium. According to the very simple cell reaction 

This cell reaction necessitates a so-dium-ion-conductive electrolyte. At present, the best and most stable sodium ion conductor is -alumina. This electrolyte has sufficient high sodium ion conductivity at temperatures of about 300 °C. The fi -alumina electrolyte is normally designed as a tube closed at one end with a negative 

MoHen salt electrolyte Sodium ahiminium chloride 

The cell is hermetically sealed. This is performed by a glass connection from the -alumina ceramic tube to a ring of a-alumina. Metal parts are connected to the a-alumina by thermocompression bonding and the metal parts are either con- 

Mollen salt eleclrolyte Sodium aluminium chloride 

This cell reaction necessitates a sodium-ion-conductive electrolyte. At present, the best and most stable sodium ion conductor is j8 -alumina. This electrolyte has sufficiently high sodium ion conductivity at temperatures of about 300 °C. The S -alumina electrolyte is normally designed as a tube, closed at one end, with a negative electrode on the outside and the positive electrode on the inside of the tube. The positive electrode, which consists of nickel chloride, nickel, and salt, throughout the reaction is incorporated in a nickel powder matrix and has a nickel current collector. The positive electrode is solid, as is the ceramic electrolyte therefore the soHd electrode must be made to work together with the soHd electrolyte, so that cell reactions can proceed. This is achieved by using a second electrolyte, sodium aluminum chloride, which is molten at the operating 

---

Figure 2. Open-circuit voltage of the Zebra cell versus temperature.

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. 

The ZEBRA cells are connected in series to obtain the demanded voltages, and chains of these cells are connected in parallel to obtain the capacity which is requested. At present there is a request from the car companies to obtain battery voltages close to 300 V, which means that about 110-120 cells will be connected in series in one string. The capacity of one... 

The properties of the molten electrolyte sodium aluminum chloride influence the performance and the behavior of the ZEBRA cell. 

The dissolution of A1C13 in the NaAlCl4 melt makes the melt acidic. The acidic-base concept has been discussed by Blander et al. [40]. An acidic melt influences the solubility of the nickel chloride in the ZEBRA cell the solubility of the nickel chloride increases. 

Figure 18. Phase diagram of the NaCl - NiCI2 system. The stipled area is the operating region of the ZEBRA cell.

Strong contenders for automotive power are the sodium/sulphur and sodium/ nickel chloride batteries, the latter known as the ZEBRA cell. ZEBRA was originally (c. 1979) an acronym devised for commercial security reasons but now it stands for the very apt Zero Emissions Batteries Research Activity . Several European car manufacturers including BMW and Mercedes have incorporated the ZEBRA cell into prototype cars, vans and buses. The performance of the battery far outstrips that of the lead/acid counterpart, as is evident from Fig. 

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 ... 

The zebra cell is a related cell that possesses much less damage potential in this respect [695]. The solid electrolyte is the same as in the Na-S battery, and the operating temperature is similar. However, the cell reaction comprises the reaction of Na with NiCl2 to Ni and NaCl, which is contained in a NaAlCU melt (open circuit voltage per cell 2.6V). If the electrolyte ceramic breaks, Na reacts with NaAlC to yield Al. The short circuit that occurs ensmres that the cell chain maintains its function even if 5% of the cell has been destroyed in this manner. 

---