Handbook of Energy Storage for Transmission or ... - W2agz.com

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After all of the free sulfur phase is consumed, the Na 2
S 5
is progressively converted into singlephase
sodium polysulfides with progressively higher sulfur content (Na 2
S 5-x
.). Cells undergo
exothermic and ohmic heating during discharge. During charge, these chemical reactions are
reversed. Half-cell and overall-cell reactions are as follow:
Negative electrode:
Positive electrode:
2
Ch arg e
xS
Disch arg e
NA ⎯⎯
⎯⎯ →←⎯⎯⎯
2NA
+ +
− Disch arg e
−2
+ 2e
⎯⎯ ⎯⎯ →←⎯⎯⎯
S
Ch arg e x
2e
−
Overall cell:
2 ⎯
Disch arg e
NA + xS ⎯⎯ ⎯⎯ →←⎯ ⎯ Na
Ch arg e 2
S
x
(x = 5 to 3), E ocv
= 2.076 to 1.78 V
Although the actual electrical
characteristics of sodium-sulfur cells are
design dependent, voltage behavior
follows that predicted by
thermodynamics. A typical cell
response is shown in Figure 1-1. This
figure is a plot of equilibrium potential
(or open circuit voltage (OCV)) during
charge and discharge as a function of
depth of discharge. The OCV is a
Voltage /V
2.3
2.2
2.1
2.0
1.9
1.8
1.7
Discharge Voltage
Charge Voltage
Open Circuit Voltage (OCV)
0 100 200 300 400 500 600 700
Depth of Discharge /Ah
Figure 1-1. NAS Cell Voltage Characteristics
constant 2.076V over 60 to 75% of discharge while a two-phase mixture of sulfur and Na 2
S 5
is
present. The voltage then linearly decreases while discharged within the single-phase Na 2
S x
regime to the selected end-of-discharge, usually about 1.8 V. Greater depths of discharge cause
the formation of Na 2
S x
species with progressively higher internal resistance and greater
corrosivity (Ref. 1-3 and 1-4).
1.2.2 NAS Cell Design
The NAS cell design developed by NGK is illustrated in Figure 1-2.
The negative sodium electrode in the center is surrounded by the beta
alumina solid electrolyte tube, which in turn is surrounded by the
positive sulfur electrode. In a charged state, liquid elemental sodium
fills the central reservoir. As the cell is discharged, the liquid sodium
is channeled through a narrow annulus between the inner surface of the
beta alumina solid electrolyte and the safety tube. The safety tube is a
design feature to control the amount of sodium and sulfur that can
potentially combine in the unlikely event that the beta alumina tube
fails. The volume of potential reactants is limited to that contained in
the narrow annulus between the electrolyte tube and the safety tube,
preventing the generation of sufficient heat to rupture the cell.
Figure 1-2. NAS Cell
1-2