Na-S or Sodium-Sulfur Battery

NaS or Sodium-Sulfur battery is a kind of molten metal battery used in non mobile applications like grid energy storage. Sodium-Sulfur battery is made up of Sodium and Sulphur and has very high energy density and very high charge/ discharge efficiency compared to other batteries. The Sodium-Sulfur battery is composed of a solid electrolyte membrane between its anode and cathode. Due to very high energy efficiency, Sodium-Sulphur battery finds applications in grid energy storage and space explorations.

In structure, the Sodium – Sulfur battery is cylindrical in shape and is enclosed in a steel case coated with Chromium and Molybdenum to prevent corrosion by the chemicals. The liquid sodium filled in the case is the Negative electrode and the Outer case is the Positive electrode. The case is sealed an air tight lid formed of Alumina. The battery has a permeable membrane called BASE membrane which selectively conducts the Sodium ions. BASE is the short form of Beta Alumina Solid Electrolyte.

Structure of NaS battery

The Positive electrode is the Sulphur while the Sodium forms the Negative electrode. Both the electrodes are insulated by the BASE membrane which is selectively permeable to Sodium ions. The active materials of both the electrodes are kept as liquid by very high temperature of around 300 degree. The electrolyte is kept as solid. The high temperature keeps the active materials of the electrodes reactive and hence the efficiency increases.

NaS Cell

The Module battery has many cells enclosed in a thermal case with an electric heater to maintain temperature. The single cells are packed inside the enclosure and connected together by metal bars.

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During the discharge state, the molten sodium acts as the anode which provides electrons. The container contains molten sulphur which is separated from the sodium by the BASE membrane. The container serves as the cathode. When the Sodium gives out electrons, they migrate into the sulphur container. Due to this movement, the electrons drive a current through the molten sulphur to the contact and then to the load. The current returns back to the sulphur container. In the container, another electron reacts with the sulphur to form Sodium poly sulphide. So during the discharge phase, level of Sodium drops considerably.

The chemical reaction in the discharge phase is

2 Na + 4 S …….. Na2S4 E cell 2 V

Charge – Discharge cycle

During the charging phase, the chemical reaction reverses and the elements restore their chemistry.

The Sodium-Sulfur battery needs careful attention since it uses pure sodium which may explode when makes contact with water. Sodium present in the battery also creates sodium dendrites which will reduce the performance. Moreover, sodium is corrosive in nature so that the battery may show conduction and self discharge.