69
Glass Ceramics
The conductivities of sulfide glasses exceed that of their oxide counterparts, but
remain low compared to ceramic electrolytes. This drawback cannot fully overcome
their superiority in terms of manufacturability and, for this reason, more recent
work has explored glass-ceramic composites which combine some of the ease of
glass production with the increased conductivity expected from a ceramic. To this
end, work has largely focused on the development of the cubic Na 3 PS 4 phase within
a glass matrix owing to its high ionic conductivity of ~2 × 10
−4
S cm
−1
at room temperature [42]. Glass ceramics are typically produced by the heat treatment of glasses,
such as the ones discussed previously, to precipitate a ceramic phase. Cubic Na 3 PS 4
is generally considered to be a high temperature phase, while its lower conductivity
tetragonal phase is considered to be the low-temperature phase. However, it has
been found that the applications of moderate temperatures can precipitate cubic
Na 3 PS 4 within a glassy matrix with minimal formation of the tetragonal phase. It
has been shown that care must be taken, as treatment at higher temperatures can still
result in the precipitation of the low-temperature tetragonal Na 3 PS 4 [42–44]. Study
of the Na 3 PS 4 ceramic alone has indicated that halogen doping may help to further
improve the conductivity of the Na 3 PS 4 [45]. Other work has also shown that substitution of the S for Se, and substitution of Sb or As for P can also raise the conductivity of the ceramic phase [5, 46]. Work on lithium-based analogues has indicated
good stability with Li metal, but further work is necessary to determine the long
term stability of the sodium glass ceramics with molten sodium anodes [19].
Polymers
Polymer-based separators are another area of interest due to the low cost of materials and ease of manufacturing. The use of polymer separators for molten sodium
batteries is almost completely unexplored as traditional battery chemistries require
operating temperatures in excess of 300 °C, which exceeds the functional operation
temperature of most polymers. However, with the development of new cathode
chemistries with lower operating temperatures, polymer separators are becoming
attractive candidates for these batteries. Poly(ethylene oxide) (PEO) has been studied as a Na
+
-conducting polymer electrolyte in batteries since at least 1985, when it
was mixed with NaI salt and employed as the separator for an all-solid-state Na–
MoS 3 battery at 70–98 °C [47]. A drawback of PEO is its relatively low conductivity
(1 × 10
−5
–1 × 10
−6
S cm
−1
at room temperature) [45]. PEO has been studied with a
variety of sodium salts, such as NaClO 4 , NaI, NaCF 3 COO, NaPF 6 , NaCF 3 SO 3 , and
NaC 2 F 6 NOS 2 in an effort to boost its conductivity [47–51]. It has received widespread attention due to its relative stability with solid Na, though it is not recognized
as stable against molten sodium because 1) its melting temperature is well below
that of sodium, and 2) both the PEO and its additives are reactive with molten
sodium. Other candidate polymers studied for use in sodium batteries have primarily been studied as gel-polymer electrolytes. These include, for example,
Molten Sodium Batteries
Glass Ceramics
The conductivities of sulfide glasses exceed that of their oxide counterparts, but
remain low compared to ceramic electrolytes. This drawback cannot fully overcome
their superiority in terms of manufacturability and, for this reason, more recent
work has explored glass-ceramic composites which combine some of the ease of
glass production with the increased conductivity expected from a ceramic. To this
end, work has largely focused on the development of the cubic Na 3 PS 4 phase within
a glass matrix owing to its high ionic conductivity of ~2 × 10
−4
S cm
−1
at room temperature [42]. Glass ceramics are typically produced by the heat treatment of glasses,
such as the ones discussed previously, to precipitate a ceramic phase. Cubic Na 3 PS 4
is generally considered to be a high temperature phase, while its lower conductivity
tetragonal phase is considered to be the low-temperature phase. However, it has
been found that the applications of moderate temperatures can precipitate cubic
Na 3 PS 4 within a glassy matrix with minimal formation of the tetragonal phase. It
has been shown that care must be taken, as treatment at higher temperatures can still
result in the precipitation of the low-temperature tetragonal Na 3 PS 4 [42–44]. Study
of the Na 3 PS 4 ceramic alone has indicated that halogen doping may help to further
improve the conductivity of the Na 3 PS 4 [45]. Other work has also shown that substitution of the S for Se, and substitution of Sb or As for P can also raise the conductivity of the ceramic phase [5, 46]. Work on lithium-based analogues has indicated
good stability with Li metal, but further work is necessary to determine the long
term stability of the sodium glass ceramics with molten sodium anodes [19].
Polymers
Polymer-based separators are another area of interest due to the low cost of materials and ease of manufacturing. The use of polymer separators for molten sodium
batteries is almost completely unexplored as traditional battery chemistries require
operating temperatures in excess of 300 °C, which exceeds the functional operation
temperature of most polymers. However, with the development of new cathode
chemistries with lower operating temperatures, polymer separators are becoming
attractive candidates for these batteries. Poly(ethylene oxide) (PEO) has been studied as a Na
+
-conducting polymer electrolyte in batteries since at least 1985, when it
was mixed with NaI salt and employed as the separator for an all-solid-state Na–
MoS 3 battery at 70–98 °C [47]. A drawback of PEO is its relatively low conductivity
(1 × 10
−5
–1 × 10
−6
S cm
−1
at room temperature) [45]. PEO has been studied with a
variety of sodium salts, such as NaClO 4 , NaI, NaCF 3 COO, NaPF 6 , NaCF 3 SO 3 , and
NaC 2 F 6 NOS 2 in an effort to boost its conductivity [47–51]. It has received widespread attention due to its relative stability with solid Na, though it is not recognized
as stable against molten sodium because 1) its melting temperature is well below
that of sodium, and 2) both the PEO and its additives are reactive with molten
sodium. Other candidate polymers studied for use in sodium batteries have primarily been studied as gel-polymer electrolytes. These include, for example,
Molten Sodium Batteries