Ceramic 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 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 Na/S and ZEBRA batteries, which incorporate ceramic electrolytes, will be discussed in detail below but first the elementary basic science is summarized. 

The prospects of development of sodium ion batteries are very uncertain. The developers of such batteries remember the numerous efforts directed at the commercialization of batteries with a sodium negative electrode and ceramic electrolyte of P-alumina. Intensive development of batteries with the system of sodium-sulfur has been carried out since 1966 (for almost half a century ) and development of batteries with the system of sodium-nickel chloride (ZEBRA batteries) has been performed since 1978. It was assumed that these high-temperature batteries would form a basis for electric transport, but these systems are still referred to in the future tense. 

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. 

There is no side reaction in the cell chemistry of ZEBRA battery, and therefore, the charge and discharge cycle has almost 100 % efficiency. This is due to the ceramic electrolyte. Figure 2 shows a first charge and discharge curves of Na/NiCl2 ceU assembled in a discharged state [4]. This ceU contains no additives like FeS. 

Africa)io is a variant of sodium-sulfur technology where sulfur is replaced with a metal chloride such as NiCl2 (nickel chloride) or FeCU. It was specifically developed for applications in electric vehicles, freight transport and public transport the ZEBRA battery is more particularly intended to serve buses and utility vehicles. As with the Na-S battery, the vibrations felt in a vehicle may cause premature aging of the ceramic/metal interface. Today, such batteries are also being considered for stationary applications. 

The ZEBRA battery comprises a NiCU positive electrode in a central compartment with NaCl salt, impregnated with NaAlCls, which is a liquid mixture of NaCl and AICI3 (considered to be a secondary electrolyte). The negative electrode is liquid sodium confined in a second, outer compartment. The wall separating the two compartments is made of a P alumina ceramic (or P-AI2O3), conductive of sodium ions, considered to be the primary electrolyte. The element is sealed hermetically and functions at temperatures equal to or higher than 300°C so that the active components remain in the liquid state. 

ZEBRA batteries use Ni power and plain salt for the electrode material the electrolyte and separator is jS"-Al203-ceramic, which is conductive for Na" ions but an insulator for electrons [4]. 

This same reaction of the liquid salt and liquid sodium is relevant for the high safety standard of ZEBRA batteries In case of mechanical damage of the ceramic separator due to a crash of the car the two liquids react in the same way, and the salt and aluminum passivates the NiCb cathode. The energy released is reduced by about 1/3 compared to the normal discharge reaction of sodium with nickel chloride. 

Nowadays any product that is introduced to the market has to be recycled at the end of its usage. ZEBRA batteries are dismantled. The box material is stainless steel and Si02, both of which are recycled by established processes. The cells contain Ni, Fe, salt, and ceramic. For recycling they are simply added to the steel melting process of the stainless steel production. Nickel and iron are contributed to the material production and the ceramic and salt is welcome to form the slag. The recycling is certihcated and cost effective. 

Battery Recycling. The recycling options for ZEBRA batteries have been comprehensively studied The result is that very simple process has been developed the batteries are disassembled and then the cells are cut and put into a standard pyrometallurgical stainless-steel production process. The steel and nickel goes into the metal melt and the salt, aluminum chloride, and ceramic partition into the slag. 

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. 

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