75
Ionic Liquids
Ionic liquids are starting to attract attention for a wide variety of battery applications, including molten sodium batteries. There are, it appears, two approaches to
using ionic liquids. The first is use of the ionic liquid as a traditional electrolyte, as
was demonstrated by Peled et al. in a Na-air battery with a molten sodium anode,
glass-fiber, and Celgard separators soaked in ionic liquid electrolyte (0.5 M sodium
triflate (NaTf) + 0.1 M Na 2 SO 4 in 1-butyl-1-methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide (PYR 14 TFSI)), and dry air as the cathode. The
battery was operated at 105 °C, just above the melting point of sodium. Though an
interesting demonstration, the battery was not able to be charged at low current
densities owing to a high rate of self-discharge [78].
Another approach is to use the ionic liquid as a catholyte, in which the redoxactive species are fully dissolved in the ionic liquid, and to protect the molten
sodium anode with a solid electrolyte separator. This was demonstrated by Xue
et al. [79] in which FeCl 3 and NaAlCl 4 were dissolved in ionic liquid ethylmethylimidazolium chloride (EMICl) to form EMIFeCl 4 –NaAlCl 4 in which the redoxactive species were Fe
2+
/Fe
3+
per the reaction:
E cell ~ 3.25 V at 180 °C
EMICl suffers from alkali cation trapping and poor ionic conductivity at temperatures below 100 °C. This issue was overcome by operation of the battery at
180 °C, and the use of NaSICON as a separator for good ionic conduction at the
intermediate temperature. One primary disadvantage of the demonstrated system
was the precipitation of solid NaCl on discharge. This problem, however, can be
overcome by the choice of different redox-active species such as, for example, the
Fe 2 Cl 7
−
anion.
Aqueous Cathodes
An interesting area of research is aqueous cathodes and catholytes. As the melting
point of sodium is only a few degrees below the boiling point of water, batteries
with aqueous cathodes rely on modification of the anode to lower the melting point
of sodium substantially below water’s boiling point. Work at the Pacific Northwest
National Laboratory demonstrated that the sodium metal anode can be successfully
alloyed with cesium which, depending on the alloy composition, can create a fully
liquid sodium anode at room temperature [10]. This novel anode alloy was utilized
Na EMI
FeCl
EMI
FeCl
NaCl
+
↔
+
+
−
+
−
4
3
Molten Sodium Batteries
Ionic Liquids
Ionic liquids are starting to attract attention for a wide variety of battery applications, including molten sodium batteries. There are, it appears, two approaches to
using ionic liquids. The first is use of the ionic liquid as a traditional electrolyte, as
was demonstrated by Peled et al. in a Na-air battery with a molten sodium anode,
glass-fiber, and Celgard separators soaked in ionic liquid electrolyte (0.5 M sodium
triflate (NaTf) + 0.1 M Na 2 SO 4 in 1-butyl-1-methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide (PYR 14 TFSI)), and dry air as the cathode. The
battery was operated at 105 °C, just above the melting point of sodium. Though an
interesting demonstration, the battery was not able to be charged at low current
densities owing to a high rate of self-discharge [78].
Another approach is to use the ionic liquid as a catholyte, in which the redoxactive species are fully dissolved in the ionic liquid, and to protect the molten
sodium anode with a solid electrolyte separator. This was demonstrated by Xue
et al. [79] in which FeCl 3 and NaAlCl 4 were dissolved in ionic liquid ethylmethylimidazolium chloride (EMICl) to form EMIFeCl 4 –NaAlCl 4 in which the redoxactive species were Fe
2+
/Fe
3+
per the reaction:
E cell ~ 3.25 V at 180 °C
EMICl suffers from alkali cation trapping and poor ionic conductivity at temperatures below 100 °C. This issue was overcome by operation of the battery at
180 °C, and the use of NaSICON as a separator for good ionic conduction at the
intermediate temperature. One primary disadvantage of the demonstrated system
was the precipitation of solid NaCl on discharge. This problem, however, can be
overcome by the choice of different redox-active species such as, for example, the
Fe 2 Cl 7
−
anion.
Aqueous Cathodes
An interesting area of research is aqueous cathodes and catholytes. As the melting
point of sodium is only a few degrees below the boiling point of water, batteries
with aqueous cathodes rely on modification of the anode to lower the melting point
of sodium substantially below water’s boiling point. Work at the Pacific Northwest
National Laboratory demonstrated that the sodium metal anode can be successfully
alloyed with cesium which, depending on the alloy composition, can create a fully
liquid sodium anode at room temperature [10]. This novel anode alloy was utilized
Na EMI
FeCl
EMI
FeCl
NaCl
+
↔
+
+
−
+
−
4
3
Molten Sodium Batteries