ZEBRA cell sodium

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. 

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

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. 

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... 

Overcharge reaction. The charge capacity of the ZEBRA cell is determined by the quantity of salt (NaCl) available in the cathode. In case a cell is fully charged and the charge voltage continues to be applied to the cell for whatever reasons, the liquid salt NaAlCU supplies a sodium reserve following the reversible 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. 

Electrical Networking. During the lifetime of a battery, individual cells may fail. Such an occurrence will result only in the reduction of the open circuit voltage by 2.58 V per cell failure because the failure mode of ZEBRA cells is an internal short (due to the reaction of the secondary liquid electrolyte with sodium forming a solid aluminum shunt). Because of this characteristic, long series chains with 216 cells and 557 V can be built. Intercell connections or voltage taps are not necessary. 

The ZEBRA battery is a high-energy battery based on a cell with electrodes of sodium and metal chloride. The ZEBRA system was first described by Coetzer in 1986 12J. 

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

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. 

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. 

One innovative aspect of the ZEBRA technology is that the cell is constructed with the reaction products , metallic nickel and sodium chloride in situ that is the cell is in the discharged state, greatly facilitating fabrication. The liquid sodium and nickel chloride are formed by charging the cell. 

Also in Germany, Mercedes-Benz has been developing electric vehicles for over 30 years. Many of the later vehicles utilized advanced sodium—nickel chloride (ZEBRA) batteries. By 1997, over 1 million km of road testing of these batteries had been accumulated, much of it in the electric version of the Mercedes-Benz A-Class car. In 1994, the company commenced its research on fuel cell vehicles - the New Electric Car (NECAR) programme. Following the evaluation of a series of prototypes, the NECAR 5 was launched in November 2000. This, too, was based on the A-Class car. Power was supplied by a 75 kW fuel-cell system that was fed by an on-board methanol reformer. The car featured a cold-start facility to remove the need for the reformer to... 

ZEBRA battery is actually a Z E B R A (Zeolite Battery Research Africa) battery, that is, sodium-nickel chloride cell. This battery consists of a liquid Na negative electrode and NiCU separated by S-alumina solid electrolyte with Na" conduction. Total cell reaction is as follows ... 

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. 

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