68
Glasses
Glass separators have been studied for molten sodium batteries almost since the
development of the sodium-sulfur battery [32]. Glasses offer advantages over
ceramic electrolytes in that they exhibit isotropic properties, they do not have grain
boundaries, and they are easy and inexpensive to produce in a variety of different
form factors [33]. Early work looked primarily at oxide glasses, such as sodium
borate glasses and NaSICON derivative NASIGLAS [33, 34]. However, research in
this area stalled due to the low conductivities of the glass materials produced
(~1 × 10
−3
S cm
−1
at 300 °C), as well as a lower corrosion resistance compared to
BASE [33]. Interest in glass electrolytes has been revived in part due to desire on the
part of researchers to improve the room temperature conductivity of the solid electrolyte, with an eye on both all-solid-state batteries as well as low and intermediate
temperature molten sodium batteries. Recently, new glass electrolytes have been
demonstrated with nominal composition of A 2.99 M 0.005 OCl 1-x (OH) x where A = Li or
Na and M = Ba or Ca [35, 36]. These glasses have high conductivities at room temperature which were demonstrated as being >1 × 10
−2
S cm
−1
[35, 36]. These new
types of ion conducting glasses have been successfully incorporated into lab scale
solid-state batteries [36], but have yet to be shown stable against molten sodium
metal. Sulfide glasses have shown promise of high ionic conductivities at room
temperature compared to oxide glasses, owing to the weaker bonding between
sodium and sulfur ions than oxygen in the glass [37]. One disadvantage of sulfidebased glasses is the need to handle them in dry atmosphere owing to their highly
hygroscopic, reactive nature [38].
Glasses can be produced both by melt quenching and by mechanical alloying. In
mechanical alloying, precursor materials are mechanically mixed until molecular
bonding of the materials occurs by a mechanochemical reaction to form a powder.
The powder is subsequently pressed to the desired form [37, 38]. The predominant
sulfide glasses studied for use in batteries consist of Na 2 S–GeS 2 , Na 2 S–P 2 S 5 , and
mixed Na 2 S–GeS 2 –P 2 S 5 which have room temperature conductivities ranging from
~1 × 10
−5
to 1 × 10
−6
S cm
−1
depending on the exact composition of the glass [39].
Glasses containing GeS 2 typically are composed of GeS 4 tetrahedral units, while
those containing P 2 S 5 are thought to consist of a mixture of PS 4 tetrahedra, P 2 S 7 with
bridging sulfur, and P 2 S 6 with P–P bonds [38, 40, 41]. The mechanical and electronic properties of the glasses are highly dependent on both composition as well as
the synthesis and forming methods [38–40]. Research in this field has so far largely
focused on material conductivity and mechanical properties, as well as application
to room temperature all-solid-state batteries. More work is needed to determine the
long-term chemical and electrochemical stability of these glasses with molten
sodium and many of the cathode chemistries discussed below across a range of
operating temperatures.
E. D. Spoerke et al.
Glasses
Glass separators have been studied for molten sodium batteries almost since the
development of the sodium-sulfur battery [32]. Glasses offer advantages over
ceramic electrolytes in that they exhibit isotropic properties, they do not have grain
boundaries, and they are easy and inexpensive to produce in a variety of different
form factors [33]. Early work looked primarily at oxide glasses, such as sodium
borate glasses and NaSICON derivative NASIGLAS [33, 34]. However, research in
this area stalled due to the low conductivities of the glass materials produced
(~1 × 10
−3
S cm
−1
at 300 °C), as well as a lower corrosion resistance compared to
BASE [33]. Interest in glass electrolytes has been revived in part due to desire on the
part of researchers to improve the room temperature conductivity of the solid electrolyte, with an eye on both all-solid-state batteries as well as low and intermediate
temperature molten sodium batteries. Recently, new glass electrolytes have been
demonstrated with nominal composition of A 2.99 M 0.005 OCl 1-x (OH) x where A = Li or
Na and M = Ba or Ca [35, 36]. These glasses have high conductivities at room temperature which were demonstrated as being >1 × 10
−2
S cm
−1
[35, 36]. These new
types of ion conducting glasses have been successfully incorporated into lab scale
solid-state batteries [36], but have yet to be shown stable against molten sodium
metal. Sulfide glasses have shown promise of high ionic conductivities at room
temperature compared to oxide glasses, owing to the weaker bonding between
sodium and sulfur ions than oxygen in the glass [37]. One disadvantage of sulfidebased glasses is the need to handle them in dry atmosphere owing to their highly
hygroscopic, reactive nature [38].
Glasses can be produced both by melt quenching and by mechanical alloying. In
mechanical alloying, precursor materials are mechanically mixed until molecular
bonding of the materials occurs by a mechanochemical reaction to form a powder.
The powder is subsequently pressed to the desired form [37, 38]. The predominant
sulfide glasses studied for use in batteries consist of Na 2 S–GeS 2 , Na 2 S–P 2 S 5 , and
mixed Na 2 S–GeS 2 –P 2 S 5 which have room temperature conductivities ranging from
~1 × 10
−5
to 1 × 10
−6
S cm
−1
depending on the exact composition of the glass [39].
Glasses containing GeS 2 typically are composed of GeS 4 tetrahedral units, while
those containing P 2 S 5 are thought to consist of a mixture of PS 4 tetrahedra, P 2 S 7 with
bridging sulfur, and P 2 S 6 with P–P bonds [38, 40, 41]. The mechanical and electronic properties of the glasses are highly dependent on both composition as well as
the synthesis and forming methods [38–40]. Research in this field has so far largely
focused on material conductivity and mechanical properties, as well as application
to room temperature all-solid-state batteries. More work is needed to determine the
long-term chemical and electrochemical stability of these glasses with molten
sodium and many of the cathode chemistries discussed below across a range of
operating temperatures.
E. D. Spoerke et al.