Environmental Engineering Reference
In-Depth Information
battery-powered electric vehicles in high volumes. Their shorter ranges,
lower power, and especially their higher cost compared with conventional
internal combustion-powered vehicles were believed to ultimately make
them unacceptable.
The basic cell structure and associated electrochemistry of the NaS
technologies are depicted in Figure 6.1. NaS cells use solid sodium ion-
conducting β-Al
2
O
3
electrolytes. Cells must be operated at a sufficiently
high temperature (270 to 350°C) to keep all the active electrode materi-
als in a molten state and ensure adequate ionic conductivity through the
β-Al
2
O
3
electrolyte. During discharge, the sodium (negative electrode) is
oxidized at the sodium β-Al
2
O
3
interface, forming Na
+
ions that migrate
through the electrolyte and combine with the sulfur being reduced in
the positive electrode compartment to form sodium pentasulfide (Na
2
S
5
).
The Na
2
S
5
is immiscible with the remaining sulfur, thus forming a two-
phase liquid mixture. After all the free sulfur phase is consumed, the
Na
2
S
5
is progressively converted into single-phase sodium polysulfides
with progressively higher sulfur content (Na
2
S
5-
x
). During charge, these
chemical reactions are reversed. The half cell reactions in discharge are
as follows:
Positive: xS + 2e
-
→ S
x
-2
Negative: 2Na → 2Na
+
+ 2e
-
The overall discharge reaction is
2Na + xS → Na
2
S
x
(x = 5 - 3) E
OCV
= 2.076 - 1.78 V
Although the actual electrical characteristics of sodium-sulfur cells are
design-dependent, the general voltage behavior follows that predicted by
thermodynamics. A typical cell response is shown in Figure 6.2. The figure
is a plot of the equilibrium potential or open-circuit voltage and the working
voltages (charge and discharge) as a function of the depth of discharge. The
open-circuit voltage is constant (at 2.076 V) during the 60 to 75% of the dis-
charge when the two-phase mixture of sulfur and Na
2
S
5
is present. The volt-
age then linearly decreases in the single-phase Na
2
S
x
region to the selected
end-of-discharge point.
End of discharge is normally defined at open-circuit voltages of 1.78 to
1.9 V. The approximate sodium polysulfide composition corresponding to 1.9
V per cell is Na
2
S
4
; for 1.78 V per cell, it is Na
2
S
3
. Many developers choose to
limit the discharge to less than 100% of theoretical (e.g., 1.9 V) for two rea-
sons: (1) the corrosivity of Na
2
S
x
increases as
x
decreases and (2) to prevent
local cell over-discharge due to possible non-uniformities within the battery
