77
couple restricted the battery to low current densities, but achieved reasonable
cyclability for a first demonstration. Future work optimizing the conductivity of the
solid electrolyte separator could enhance the power density and lifetime of
the system.
Molten Metals
Due to the restrictions that the solid electrolyte places on cost, manufacturability,
and achievable power density (due to its low conductivity), there has been substantial effort to try to eliminate the use of a separator in molten sodium batteries. Work
has recently been undertaken in the area of liquid metal batteries, in which a molten
sodium anode is separated from a molten metal cathode by differences in density
and immiscibility between the anode, molten salt electrolyte, and cathode, without
the need for a solid electrolyte. A comprehensive history of the liquid metal battery
can be found elsewhere [85]. Many different molten anodes and cathodes have been
tested within this system, but with molten sodium anodes, cathodes to date have
been restricted to Bi, eutectic Pb–Bi alloys, and Zn. These cathodes have been tested
using eutectic NaF–NaCl–NaI, NaI–NaOH or NaI–NaOH–NaNH 2 , and NaCl–
CaCl 2 molten salt electrolytes [85–88]. Na–Hg and Na–Sb systems have also been
proposed [85]. A common disadvantage of sodium metal liquid batteries is the high
solubility of sodium in the molten salts at typical operating temperatures, though
this can be partially mitigated by the choice of molten salt electrolyte [89]. This
high solubility results in high self-discharge of the battery, which must be counteracted by high current density operation. Overall the high self-discharge of the
Fig. 7 (a) Tubular Na–S battery cell, adapted from image by NASA John Glenn Research Center/
Public domain. (b) Planar Na–NiCl 2 battery cell, expanded view. Reprinted with permission from
Ref. [90]
Molten Sodium Batteries
couple restricted the battery to low current densities, but achieved reasonable
cyclability for a first demonstration. Future work optimizing the conductivity of the
solid electrolyte separator could enhance the power density and lifetime of
the system.
Molten Metals
Due to the restrictions that the solid electrolyte places on cost, manufacturability,
and achievable power density (due to its low conductivity), there has been substantial effort to try to eliminate the use of a separator in molten sodium batteries. Work
has recently been undertaken in the area of liquid metal batteries, in which a molten
sodium anode is separated from a molten metal cathode by differences in density
and immiscibility between the anode, molten salt electrolyte, and cathode, without
the need for a solid electrolyte. A comprehensive history of the liquid metal battery
can be found elsewhere [85]. Many different molten anodes and cathodes have been
tested within this system, but with molten sodium anodes, cathodes to date have
been restricted to Bi, eutectic Pb–Bi alloys, and Zn. These cathodes have been tested
using eutectic NaF–NaCl–NaI, NaI–NaOH or NaI–NaOH–NaNH 2 , and NaCl–
CaCl 2 molten salt electrolytes [85–88]. Na–Hg and Na–Sb systems have also been
proposed [85]. A common disadvantage of sodium metal liquid batteries is the high
solubility of sodium in the molten salts at typical operating temperatures, though
this can be partially mitigated by the choice of molten salt electrolyte [89]. This
high solubility results in high self-discharge of the battery, which must be counteracted by high current density operation. Overall the high self-discharge of the
Fig. 7 (a) Tubular Na–S battery cell, adapted from image by NASA John Glenn Research Center/
Public domain. (b) Planar Na–NiCl 2 battery cell, expanded view. Reprinted with permission from
Ref. [90]
Molten Sodium Batteries