|
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. |
