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NaS

Battery

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Cathode: molten sulfur

Anode: molten sodium

Electrolyte: solid beta-alumina.

Beta-alumina solid electrolyte

(BASE) is a fast ion conductor

material used as a membrane in

several types of molten salt

electrochemical cell. Currently

there is no known substitute

available.

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During the discharge phase,• Molten elemental sodium at the core serves as the anode,

meaning that the Na donates electrons to the external circuit.

• The sodium is separated by a beta-alumina solid electrolyte (BASE)cylinder from the container of molten sulfur, which is fabricated

from an inert metal serving as the cathode. The sulfur is absorbed

in a carbon sponge.

•

BASE is a good conductor of sodium ions, but a poor conductor ofelectrons, and thus avoids self-discharge.

• When sodium gives off an electron, the Na+ ion migrates to the

sulfur container. The electron drives an electric current through

the molten sodium to the contact, through the electrical load and

back to the sulfur container. Here, another electron reacts with

sulfur to form Sn2−, sodium polysulfide.

• The discharge process can be represented as follows:

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As the cell discharges, the sodium level drops. During the

charging phase the reverse process takes place. Once

running, the heat produced by charging and discharging

cycles is sufficient to maintain operating temperatures and

usually no external source is required.

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Safety in NaS

Pure sodium presents a hazard, because it spontaneously

burns in contact with air and moisture, thus the systemmust be protected from water and oxidizing

atmospheres.

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2011 Tsukuba Plant fire incident

On September 21, 2011, NGK-manufactured NaS

batteries for storing electricity installed at the Tsukuba,Japan, plant caught fire. Following the incident, NGK

temporarily suspended production of NaS batteries.

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Challenges

1. Corrosion of the insulators was found to be a problemin the harsh chemical environment as they gradually

 became conductive and the self-discharge rate

increased.

2. Dendritic-sodium growth can also be a problem.3. High operating temperature (250-300 oC

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What we Know about NaS

Refer to technical data of EES

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Case Study:

See Project file on Classroom.