70
PVDF–HFP–PEGDME copolymer [52], PMMA [53, 54], and PAN [55]. However,
there is no active research into the use of gel-polymer electrolytes in molten sodium
batteries due to the high reactivity of molten sodium with the majority of organic
and aqueous liquid electrolytes used in these types of separators. As polymer-based
separators continue to be developed for emerging solid-state batteries, opportunities
may arise for translation of these materials to molten sodium battery systems.
Metal Borohydrides and Higher Borates
Metal borohydrides and their related compounds have attracted attention due to the
discovery of some compounds exhibiting good ionic conductivities in high temperature polymorphs. Metal borohydrides have further sparked research interest due
their apparent stability with solid Li and Na metal, though their stability with molten sodium has not yet been determined. To date metal borohydrides are relatively
unexplored, and current work has largely focused on Li
+
-ion conductors as solid
electrolytes in all-solid-state batteries, but there has been some work on Na
+
-ion
conductors as well. Metal borohydrides achieve their good conductivities by what
has been dubbed the “paddle wheel” mechanism, in which the borohydride anion,
(BH 4 )
−
, is orientationally disordered within the crystal lattice. This allows the anion
complex to rotate to accommodate fast alkali cation (M
+
) ion conduction [56].
Closo-polyborate salts were identified as possible solid electrolytes with the discovery of superionic Na
+
conductivity in sodium dodecahydro-closo-dodecaborate
(Na 2 B 12 H 12 ) by Udovic et al. in 2014 [57]. At room temperature Na 2 B 12 H 12 forms a
monoclinic phase with B 12 H 12
2−
polyanions with ordered orientation. They reported
the material to undergo a phase transition from ordered monoclinic to disordered
bcc phase, shown in Fig. 6, enabling the rotation of the polyanion complexes in a
manner similar to the “paddle wheel” mechanism of ion conduction [58]. As such
Fig. 6 Crystal structure of disordered cubic Na 2 B 12 H 12 (cations omitted). Adapted with permission
from Ref. [58] Copyright 2017 American Chemical Society
E. D. Spoerke et al.
PVDF–HFP–PEGDME copolymer [52], PMMA [53, 54], and PAN [55]. However,
there is no active research into the use of gel-polymer electrolytes in molten sodium
batteries due to the high reactivity of molten sodium with the majority of organic
and aqueous liquid electrolytes used in these types of separators. As polymer-based
separators continue to be developed for emerging solid-state batteries, opportunities
may arise for translation of these materials to molten sodium battery systems.
Metal Borohydrides and Higher Borates
Metal borohydrides and their related compounds have attracted attention due to the
discovery of some compounds exhibiting good ionic conductivities in high temperature polymorphs. Metal borohydrides have further sparked research interest due
their apparent stability with solid Li and Na metal, though their stability with molten sodium has not yet been determined. To date metal borohydrides are relatively
unexplored, and current work has largely focused on Li
+
-ion conductors as solid
electrolytes in all-solid-state batteries, but there has been some work on Na
+
-ion
conductors as well. Metal borohydrides achieve their good conductivities by what
has been dubbed the “paddle wheel” mechanism, in which the borohydride anion,
(BH 4 )
−
, is orientationally disordered within the crystal lattice. This allows the anion
complex to rotate to accommodate fast alkali cation (M
+
) ion conduction [56].
Closo-polyborate salts were identified as possible solid electrolytes with the discovery of superionic Na
+
conductivity in sodium dodecahydro-closo-dodecaborate
(Na 2 B 12 H 12 ) by Udovic et al. in 2014 [57]. At room temperature Na 2 B 12 H 12 forms a
monoclinic phase with B 12 H 12
2−
polyanions with ordered orientation. They reported
the material to undergo a phase transition from ordered monoclinic to disordered
bcc phase, shown in Fig. 6, enabling the rotation of the polyanion complexes in a
manner similar to the “paddle wheel” mechanism of ion conduction [58]. As such
Fig. 6 Crystal structure of disordered cubic Na 2 B 12 H 12 (cations omitted). Adapted with permission
from Ref. [58] Copyright 2017 American Chemical Society
E. D. Spoerke et al.