66
sodium volatility. Poor control over the microstructure can significantly affect the
mechanical properties of the BASE, which is important for several reasons. First,
the BASE is required not only to separate the anode and cathode electrically, but it
must also serve as a robust physical barrier between the two sides of the battery.
Allowing physical contact between the highly reactive molten sodium and the
catholyte leads to failure of the cell and, for example, with NaS batteries, can lead
to violent and hazardous reactions. To ensure the mechanical integrity of these
materials they must be made relatively thick and quite uniform. The uniformity
represents a processing challenge, while the thickness impacts the overall conductance of the membrane. Introducing resistive elements, such as thick BASE membranes decreases overall cell performance and requires undesirable operation at
relatively higher temperatures (≥300 °C) where membrane conductivity is sufficiently high. The introduction of zirconia into the BASE has shown promise in
increasing the mechanical properties of the membranes, potentially allowing for
higher performing batteries or lower temperature operations, though the introduction of this secondary phase may impact ionic conductivity, particularly at higher
concentrations [17, 19, 21].
NaSICON
NaSICON (Na Super Ion CONductor) ceramics have been considered as attractive
candidates for Na-based batteries, particularly for lower temperature applications.
Although some reports have described the use of BASE for reduced temperature
batteries, the conductivity of NaSICON below 150 °C is generally considered to be
higher than that of traditional BASE separators [10, 11], (Fig. 4). This difference
can vary as a function of the quality and composition of each ceramic.
The typical sodium-conducting NaSICON is described as Na 1+x Zr 2 P 3-x Si x O 12
(0 ≤ x ≤ 3), and forms a rigid hexagonal framework of zirconia (ZrO 6 ) octahedra
corner-linked to silica and phosphate ((Si,P)O 4 ) tetrahedra, and contain the so-called
2
NaSICON
Ln (
s T ) / S cm
-1
K
-1
b"-Al 2 O 3
-6
-4
-2
2
4
0
3
2.5
1000/T / K
-1
3.5
Fig. 4 Comparison of the
ionic conductivities of
NaSICON and
BASE. Arrhenius plot of
Na
+ conductivity vs.
inverse temperature for
both. For reference, room
temperature is near a value
of 3.4 and 150 °C is near a
value of 2.1 on the x-axis.
Reprinted from Ref. [22],
Copyright 2017, with
permission from Elsevier
E. D. Spoerke et al.
sodium volatility. Poor control over the microstructure can significantly affect the
mechanical properties of the BASE, which is important for several reasons. First,
the BASE is required not only to separate the anode and cathode electrically, but it
must also serve as a robust physical barrier between the two sides of the battery.
Allowing physical contact between the highly reactive molten sodium and the
catholyte leads to failure of the cell and, for example, with NaS batteries, can lead
to violent and hazardous reactions. To ensure the mechanical integrity of these
materials they must be made relatively thick and quite uniform. The uniformity
represents a processing challenge, while the thickness impacts the overall conductance of the membrane. Introducing resistive elements, such as thick BASE membranes decreases overall cell performance and requires undesirable operation at
relatively higher temperatures (≥300 °C) where membrane conductivity is sufficiently high. The introduction of zirconia into the BASE has shown promise in
increasing the mechanical properties of the membranes, potentially allowing for
higher performing batteries or lower temperature operations, though the introduction of this secondary phase may impact ionic conductivity, particularly at higher
concentrations [17, 19, 21].
NaSICON
NaSICON (Na Super Ion CONductor) ceramics have been considered as attractive
candidates for Na-based batteries, particularly for lower temperature applications.
Although some reports have described the use of BASE for reduced temperature
batteries, the conductivity of NaSICON below 150 °C is generally considered to be
higher than that of traditional BASE separators [10, 11], (Fig. 4). This difference
can vary as a function of the quality and composition of each ceramic.
The typical sodium-conducting NaSICON is described as Na 1+x Zr 2 P 3-x Si x O 12
(0 ≤ x ≤ 3), and forms a rigid hexagonal framework of zirconia (ZrO 6 ) octahedra
corner-linked to silica and phosphate ((Si,P)O 4 ) tetrahedra, and contain the so-called
2
NaSICON
Ln (
s T ) / S cm
-1
K
-1
b"-Al 2 O 3
-6
-4
-2
2
4
0
3
2.5
1000/T / K
-1
3.5
Fig. 4 Comparison of the
ionic conductivities of
NaSICON and
BASE. Arrhenius plot of
Na
+ conductivity vs.
inverse temperature for
both. For reference, room
temperature is near a value
of 3.4 and 150 °C is near a
value of 2.1 on the x-axis.
Reprinted from Ref. [22],
Copyright 2017, with
permission from Elsevier
E. D. Spoerke et al.