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June 16, 2022

Sodium Sulfur Battery FAQ

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Sodium/Sulfur Cells

Anode: Molten sodium

Cathode: Molten sulfur

Electrolyte: Solid ceramic beta alumina (ß"-Al2O3)

Applications: Electric vehicles, aerospace (satellites)

This cell have been studied extensively for electric vehicles because of its inexpensive materials, high cycle life, and high specific energy and power. Specific energies have reached levels of 150 W-h/kg and specific powers of 200 W/kg. The half-reactions are:

half-reaction V vs SHE
2Na —> 2Na+ + 2e-
3S + 2e- —> S32-

2Na + 3S —> Na2S3 2.076 V

Despite these advantages there are couple of disadvantages serious enough that other alternatives, such as lithium-ion, nickel-metal hydride, and lithium polymer, have emerged as the most promising solutions to electric vehicle power. One is that the power output is very small at room temperature. The temperature must be kept at around 350 ºC to keep the sulfur in liquid form and to be effective in motive power applications. This is achieved through insulation or heating through the cells own power. This lowers the energy density.

The second problem has to do with electrolyte breakdown, which is one of the principal causes of sodium sulfur cell failure. The electrolyte, ceramic beta"-alumina, has several attractive characteristics. It has all the benefits of a solid electrolyte with the added qualities of a high ionic conductivity with a small electronic transfer, all with the added benefit of being a solid. However, ceramic beta"-alumina also is brittle and develops microfissures. Thus the liquid sodium and sulfur come in contact—with explosively violent results.

Recently, some research efforts have focussed on replacing the molten sulfur cathode with a poly(disulfide) such as poly(ethylenedisulfide), (SSCH2CH2)n. These cells can be discharged just above the melting temperature of Na (90 °C). The net cell reaction becomes:

2 Na + (SSR)n=Na2SSR

where the discharge reaction involves scission of the S-S disulfide linkage in the polymer backbone, and charge involves repolymerization of the resulting dithiolate salt.

One of these is the sodium/metal chloride, which in addition to beta"-alumina has a secondary electrolyte (NaAlCl4) to conduct ions from the first electrolyte to the cathode. This is necessary because the metal chloride is a solid.

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