Exercises
235
Figure 97 – Activation energy
of ionic conductivity as a function
of ionic radius of charge carrier
(from Whittingham, 1973).
B – Sodium-sulfur battery
The electrolyte for the sodium-sulfur battery is β ′′ alumina, which has the
formula Na 1.67 Mg 0.67 Al 10.33 O 17 .
1. Write the formula in the form (Na 2 O) α (MgO) β (Al 2 O 3 ) γ .
2. Starting with NaNO 3 , Al(NO 3 ) 3 , and Mg(NO 3 ) 2 , determine the quantities
of substances required to make a 300-μm-thick, 20-cm-diameter plate of β ′′
alumina.
Data
Element
N
O
Na
Mg
Al
Molar mass [g mol
−1
]
14
16
23
24.3
27
3. Sodium is liquid at the operating temperature of the battery. Give the electrochemical chain that describes the Na-S battery.
4. Write the electrode reactions and the overall operating reaction of the battery.
5. The cathodic reaction occurs in two stages: the first gives polysulfide S
2−
5 ,
and the second gives non-stoichiometric polysulfide S
2−
5−x . Considering only
the first stage of reduction, determine the theoretical specific energy W th of
the Na-S battery in W h kg
−1
, given that the potential difference between its
terminals is 2.08 V at 300 °C.
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