Sodium–sulfur battery

A sodium -sulfur battery, abbreviated NASA, a rechargeable battery, a so-called secondary cells. Compared to other battery types, a solid electrolyte and electrodes are used instead of a liquid electrolyte and the liquid for the operation of high operating temperatures in the range of 270 to 350 ° C are required. It thus belongs to the group of thermal batteries. Although this type of battery was developed in the late 1970s, obtained sodium-sulfur batteries, primarily due to the complex temperature management, except for special areas not far-reaching economic significance.

General

The positive electrode is made of molten sodium, the negative electrode is made of a cloth soaked with liquid sulfur graphite fabric. As the electrolyte is a sodium -containing alumina is used. Since sodium is a violently reactive with water, alkali metal, the accumulator must be well protected against environmental influences. Have sodium-sulfur batteries, besides the advantage that the essential basic materials such as sodium, sulfur and aluminum are readily available, a relatively high storage density in the range just above 200 Wh / kg.

Experimental treatments were in the 1980s until the mid- 1990s drive system for electric cars and energy storage in communication satellites. Current applications are small to medium sized stationary battery storage power plants in Japan, which are used to supply peak load and grid stabilization in the public power grid. In Germany since 2010 runs the Berlin company Younicos together with Vattenfall a 1 MW NaS battery as a pilot project to compensate for volatile renewable energies. In the same year an even greater NaS battery to increase the security of supply of an entire town has been installed in Texas. In applications such as electric cars and also as power systems in space applications NaS cells have been replaced by other, each more suitable energy storage systems.

As a manufacturer of NaS batteries are only Japanese producers of worldwide importance. As of 2010 the largest and most dominant manufacturers in this segment is NGK Insulators, which together with the energy network operators Tōkyō Denryoku ( Tokyo Electric Power, Tokyo Electric Power Company ) is used in the context of smaller, stationary battery storage power plants since the beginning of the 1990s NaS cells. Other manufacturers are the Japanese companies Hitachi and GS Yuasa. Former manufacturer of NaS cells for mobile applications have included Asea Brown Boveri ( electric car ), Silent Power Ltd.. in England ( electric car ) and Ford Aerospace in the United States.

Operation

NaS cell is a high temperature secondary cell. In contrast to many other accumulator, it has a very low electrochemical self-discharge between the efficiency of charge and discharge is in the range about 70 to 85%. The virtually non-existent electrochemical self-discharge is, however, qualified by the fact that the cell must be kept to maintain the operability in a high temperature range of about 300 to 350 ° C, which in addition to an appropriate thermal insulation for the cooler environment requires additional heating systems. This is necessary for the operation of heating the self-discharge or the heating power attributed to the removed battery system is small systems because of the high specific surface area, a high overall self discharge before.

As with all systems in which the thermal losses are to be reduced, the following applies:

  • Isolated super or very large NaS cells can minimize the extent that the efficiency is greatly improved, but because of the inverse proportionality between the surface area to volume, and thus the theoretical small thermal losses.

The number of charge and discharge cycles is as compared to other battery types great, but, as shown in the adjacent figure, heavily dependent on the depth of discharge. If the battery is discharged in each cycle only very low, corresponding to a significant reduction of the effective capacity, some 10,000 charging cycles are possible. If, however, before a new charge always a discharge up to 10% done, the number is reduced to some 1000 cycles to failure. Wear this delicate against deep discharge batteries is a result of thermal processes in the cell, this includes in particular at deep discharge the thermal runaway.

The high temperature is necessary because sulfur and sodium have to be in liquid form. The respective freezing temperatures must be greatly exceeded, so that an adequate flow of energy between the electrodes can come about. While the electrodes are present at a high temperature in liquid form of the electrolyte is always present in solid form in NaS cells. It consists of a sodium ion-conducting ceramic, which is the same for electrons, an insulator. An essential component of the ceramic is sodium - β - aluminate ( NaAl11O17 ) in which starting from a temperature of 270 ° C, the sodium ions are movable so that a sufficient conductivity. Other possible materials include sodium oxide or magnesium oxide.

The U.S. company Ceramatec developed (2009) in Utah a version that works well at lower temperatures. When using a new NASICON membrane, the accumulator can be operated at 90 ° C. In this case, all components remain fixed.

Electrochemistry

During discharge oxidized sodium on the sodium - β - aluminate and forms positively charged sodium ions. These ions migrate through the electrolyte and reduce the sulfur at the positive electrode to sodium pentasulfide ( Na2S5 ):

At the negative electrode, liquid sodium is oxidized:

When charge the processes run in the opposite direction. The overall reaction is then:

The electrochemical reaction depends on factors such as cell design and temperature, the internal resistance is approximately 35 milliohms, and is virtually independent of the state of charge of the cell. The open circuit voltage of a charged NaS cell is 2.076 V, and this voltage up to approximately 65 % discharge remains constant in most existing sodium pentasulfide ( Na2S5 ).

After the consumption of sulfur is reduced in the area of ​​deep discharge a portion of the sodium pentasulfide to various forms of sodium polysulfide ( Na2S5 -x }).

Then, with increasing formation of the various sodium polysulfides, the cell voltage drops to the discharge voltage of 1.78 and 1.9 V approximately linearly. For a discharge voltage of 1.9 V is primarily Na2S4 ago, at 1.78 V is Na2S3 ago. Upon further, harmful for the battery discharge depth, is formed in the cell Na2S2, which is undesirable because it leads to a high internal resistance, and thus large thermal losses in the cell. The thermal stress can cause damage to the cell.

Specifications

The technical data of some on the Market NaS cells from Japan are summarized in the following table. The design is only an elongated cylindrical shape.

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