Molten ZEBRA

The molten salt, sodium aluminum chloride, fulfills two other tasks in the cell system. The ceramic electrolyte "-alumina is sensitive to high-current spots. The inner surface of the ceramic electrolyte tube is completely covered with molten salt, leading to uniform current distribution over the ceramic surface. This uniform current flow is one reason for the excellent cycle life of ZEBRA batteries. 

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 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 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. 

Currently interest has now been directed toward a similar high temperature system, the ZEBRA Battery, which also uses P-alumina as a Na ion conductor. The sulfur electrode is replaced by nickel chloride or by a mixture of ferrous and nickel chlorides. Contact between the NiCl2 electrode and the solid electrolyte is poor as they are both solids, and current flow is improved by adding a second liquid electrolyte (molten NaAlCb) between this electrode and the P-alumina. The overall cell reaction is now ... 

Such a battery was proposed for the first time by Yu et al. [475] and by Gray et al. [470], These batteries utilize the same pair of electrodes as the Zebra battery, Na/FeCl2, but the electrolyte is a room temperature molten salt, A1C13-MEIC-NaCl. Yu et al. [475] used this electrolyte with the following composition 47 45 8 mole%. Gray et al. [470] proposed the addition of HC1 to the electrolyte. The electrode reactions during discharge are... 

The last type of nickel based battery here considered is the so-called sodium-nickel chloride or Zebra battery, firstly developed in 80s in Pretoria, South Africa (Zebra stands for ZEolite Battery Research Africa). The anode is made of liquid sodium, the electrolyte is based on sodium ion conducting -alumina and the cathode is constituted by nickel chloride. This is flooded with liquid NaAlCU which acts as a secondary electrolyte, i.e., its function is to enhance the transport of sodium ions from the solid nickel chloride to and from the alumina electrolyte [19]. They work at high temperature (157°C is the temperature necessary to have sodium in its molten state, but the better performance is obtained in the range 250-350°C) and operate with the following discharge semi- reactions at the anode ... 

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... 

Finally, high-temperature molten salt electrolyte batteries (NaS, Zebra) require completely inorganic separators capable of withstanding liquid metal temperature and chemical attack, effectively acidic conditions at temperatures >200 °C. Beta-AlaOs has been significantly engineered to serve this role [10]. 

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