Zebra cells

ZEBRA cells have a high open-circuit voltage (OCV) of 2.58 V at 300 °C. The temperature dependency of the OCV is shown in Fig. 2. 

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

The thermoneutral potential is 2.72 V this means that during charging at voltage below this value ZEBRA cells will be cooled thermodynamically. 

The molten salt electrolyte also contributes to the safety behavior of ZEBRA cells. The large amount of energy stored in a 700 g cell, which means about 30 kWh in a 300 kg battery, is not released suddenly as heat as be expected in a system with liquid electrodes such as the sodium sulfur cell. In the case of accidental destruction of ZEBRA cells, the sodium will react mainly with the molten salt, forming A1 sponge and NaCl. -The diffusion of the NaAICI ... 

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. 

Figure 4. Qminbuuun.v to the internal resistance of a ZEBRA cell.

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 investigation of the stability of P -alumina in ZEBRA cells, which always contain some iron, showed an increase of resistance under certain extreme conditions of temperature (370 °C) and of voltage. This is related to the interaction of the P alumina with iron and it was shown that iron enters / -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 P -phase but not the P"-phase was modified by the incursion of iron. The resistance of the iron-doped regions was high. It was shown that the addition of NaF inhibits access of the iron to the / " -alumina ceramic. By doping practical cells these difficulties have now been overcome and lifetime experiments show that the stability of / "-alumina electrolytes are excellent in ZEBRA cells. 

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

For the calculation of free volume inside the cell, which is essential for the design of a ZEBRA cell to keep internal pressure low for safety reasons, the density of molten NaAlCl4 over the full temperature range between 160 and 600 °C should be known. Berg et al. [33] have compared these values with the literature. The densities are compiled in Table 9. 

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.

The high ionic conductivity of sodium (3"-alumina suggested that it would form a suitable electrolyte for a battery using sodium as one component. Two such cells have been extensively studied, the sodium-sulfur cell and the sodium-nickel chloride (ZEBRA) cell. The principle of the sodium-sulfur battery is simple (Fig. 6.13a). The (3"-alumina electrolyte, made in the form of a large test tube, separates an anode of molten sodium from a cathode of molten sulfur, which is contained in a porous carbon felt. The operating temperature of the cell is about 300°C. 

Figure 6.13 Batteries using p"-alumina electrolyte, schematic (a) the sodium-sulfur cell and (b) the sodium-nickel chloride (ZEBRA) cell.

The standard Gibbs energy of formation of NaCl is — 384 kJ mol 1 and that of NiCl2 is — 62kJmol 1. Calculate the ideal voltage of a ZEBRA cell. 

The related Zebra cell 205 uses the same solid electrolyte as in the Na-S battery, and operates at similar temperatures, but the cell reaction comprises the reaction of Na with NiCl2 to Ni and NaCl,... 

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. 

A further possible and important application for the Na/S and ZEBRA cells is in load levelling . By this is meant the storage of electrical energy from power supply generators when demand is relatively low and its release into the supply network (the grid ) at peak demand times. In this way the generators can be run continuously at their optimal efficiency speeds. 

Na/NiCl2 ( ZEBRA cell) To illustrate the principle a schematic of one design of cell is shown in Fig. 4.35. Cell design and technology have evolved very considerably and the reader is referred for details to [7] and to the paper by R.C. Galloway and S. Haslam [14]. 

Fig. 4.35 Schematic of the Na/NiCl2 ZEBRA cell (charged state).

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... 

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