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Teknikimobil.com – In the previous section we discussed nickel-based batteries for electric vehicles , this time we will discuss sodium-based batteries for electric vehicles. How is it different from nickel batteries ? Let’s take a look at what sodium-based batteries actually are.
Parameters for Electric Vehicles 2 ”), six times that of lead acid cells, but in experimental batteries, the mass of the enclosure usually halves this potential increase.
The negative electrode in the cell consists of liquid sodium, and the positive electrode consists of liquid sulfur polysulfir. The electrolyte is a solid beta alumina ceramic, which conducts sodium ions and also separates the two electrodes. The actual cells are kept small, and they are joined together and placed in an evacuated chamber to reduce heat loss. The design of the container needs to be considered carefully because it can double the mass of the battery. Before batteries can be used they must be heated slowly to their working temperature. When used, the cell basically experiences self-heating because the electric current passes through the internal resistance of the battery. If not used for more than one day the battery interior must be kept hot by using an electric heater. Electrical energy is obtained from combining sodium with sulfur to form sodium sulfide.
The basic chemical formula used in this reaction is
2Na+ xS ↔ Na2Sx
Overall battery characteristics are shown in the following table.
Due to battery requirements good thermal insulation is impractical. Heating and cooling batteries require careful design and management. Although sodium sulfur batteries had much promise, concerns about the safety of two reactive materials separated by a fragile ceramic tube largely resulted in the battery not appearing on the commercial market. This fear was driven by spontaneous actions involving test vehicles during testing.
Sodium Metal Chloride Battery or Sodium Metal Chloride (ZEBRA)
Sodium metal chloride or Zebra batteries are in many ways similar to sodium sulfur batteries and have many of the advantages of these batteries. However, with this system most (and some would say all) of the safety concerns associated with sodium sulfur batteries have been addressed. The main reason for the greater safety of Zebra cells is the use of a solid positive electrolyte that is separated from the liquid sodium metal by solid and liquid electrolytes. It is certainly the case that the prototype Zebra battery has passed qualification tests for the European region, including rigorous tests such as crashing the cell at 50 kph into a steel pole. This battery has a lot of promise and can be obtained commercially.
Zebra cells use solid nickel chloride for the positive electrode and liquid sodium for the negative electrode. Two electrodes are used, a beta ceramic electrode surrounding the sodium and a secondary electrolyte, sodium-aluminum chloride, used in the positive electrode chamber. Chlorine ions are mobile ions in the electrolyte. The electrical energy in the discharge is obtained by combining sodium with nickel chloride to produce nickel and sodium chloride. The overall chemical reaction that occurs in the Zebra battery is as follows:
2Na+ NiCl2 ↔ Ni + 2NaCl
The following images show the reactions at each electrode during the middle and early parts of cell discharge. This reaction produces an open circuit voltage of about 2.5 V per cell. In later stages of discharge the reaction becomes more complex, involving aluminum ions from the electrolyte, and produces lower voltages.
Reactions at each sodium metal chloride battery electrode
Indeed an unfortunate feature of this type of cell is the way the cell voltage drops upon discharge, from about 2.5 V to about 1.6 V. The internal resistance of the cell also increases, which in turn affects the output voltage. However, as can be seen from the data in the following table, the specific energy is very high, even with this effect.
The overall characteristics of this battery are taken from a 17.8kWh peak power unit (~ 280V, 64Ah, 180kg, 32kW) which is produced by MESDEA from Switzerland.
The main problem with the Zebra battery is that it needs to operate at a temperature of around 320 ◦C, similar to a sodium sulfur battery. Thermal insulation is maintained using a double-skinned stainless steel box, with 2-3 cm of insulation between the two skins. All air is removed from the insulation, and a vacuum is maintained for several years. However, except for very short periods of a few hours, these batteries must remain connected to the mains supply when not in use. This is to keep the battery hot, and is a major limitation of its application. For example, the battery shown below, which fits neatly under the battery seat of an electric car, has an impressive 17.8 kWh of energy.
Commercial Zebra battery neatly installed under the battery seat of an experimental
electric vehicle by MES-DEA. The battery stores about 18 kWh of electrical energy
[Also read: Electric and Hybrid car news ]
However, if not used, it consumes about 100 W of temperature resistance. So, in a 24 hour period heating will require 0.1 × 24 = 2.4 kWh of energy, corresponding to about 13% of the stored energy. In energy terms, this is in line with the self-discharge of other types of batteries, and this is a fairly high figure.[]