71
the high temperature phase of NaB 12 H 12 exhibits very high conductivities of
1 × 10
−1
S cm
−1
at 540–573 K (267–300 °C).
Na 2 B 12 H 12 displayed hysteretic behavior in its conductivity on heating and cooling, leading researchers to focus on increasing the conductivity by exploration of
other closo-polyborate salts and stabilizing the high conductivity disordered phase
at lower temperatures [57]. High conductivities of 1 × 10
−1
S cm
−1
at 400 K (127 °C)
and 3 × 10
−2
S cm
−1
at 297 K (24 °C) have been reported in, respectively, NaCB 11 H 12
and NaCB 9 H 10 [59]. Similar to glass-ceramic materials, mechanical milling was
found to stabilize the high temperature, high conductivity phase at lower temperatures [60]. Only recently has work moved beyond fundamental improvement of the
conductivity of metal borohydride and closo-polyborate salt electrolytes into real
battery applications. Several potential drawbacks of these salt electrolytes have
been identified, however, limiting their application to all-solid-state batteries.
NaBH 4 has been identified as reacting violently with molten sulfur at temperatures
as low as 190 °C to form pure H 2 gas [56]. Additionally, metal borohydrides and
higher borates are extremely hygroscopic. Some of the precursor materials are also
air-sensitive, placing stringent restrictions on manufacturing of these materials.
Lastly, it was found that Na 2 (B 12 H 12 ) 0.5 (B 10 H 10 ) 0.5 electrolyte decomposed above
3.2 V vs Na/Na
+
, which may indicate that these types of electrolytes cannot be used
with a variety of battery cathodes for high voltage battery systems [61, 62].
2.3 Cathodes
Cathode chemistries vary significantly among molten sodium batteries, and tailoring these materials provides excellent opportunities to innovate new sodium battery
technologies. In addition to providing desired battery voltages and usable energy
storage capacities, any new cathode chemistry must be compatible with the solidstate separators, should be cost-effective, and should minimize or eliminate any
hazardous reactions, especially in the event of a separator failure (exposure to molten sodium). One of the most desirable traits of these new chemistries is to enable
operation at temperatures considerably lower than the 270–350 °C common to traditional molten sodium batteries.
Sulfur
The Na–S battery was first patented by Ford Motor Company in 1968 and represents one of the first battery systems to use the molten Na anode. The Na–S battery
is simple in its construction, consisting of a molten Na anode, a BASE separator,
and a molten sulfur cathode. During discharge, the Na is oxidized to Na
+
which
migrates through the ionically conductive separator to react with the sulfur being
reduced at the cathode to form molten polysulfide Na 2 S 5 , which can be further
reduced to lower order polysulfides as shown in the reaction:
Molten Sodium Batteries
the high temperature phase of NaB 12 H 12 exhibits very high conductivities of
1 × 10
−1
S cm
−1
at 540–573 K (267–300 °C).
Na 2 B 12 H 12 displayed hysteretic behavior in its conductivity on heating and cooling, leading researchers to focus on increasing the conductivity by exploration of
other closo-polyborate salts and stabilizing the high conductivity disordered phase
at lower temperatures [57]. High conductivities of 1 × 10
−1
S cm
−1
at 400 K (127 °C)
and 3 × 10
−2
S cm
−1
at 297 K (24 °C) have been reported in, respectively, NaCB 11 H 12
and NaCB 9 H 10 [59]. Similar to glass-ceramic materials, mechanical milling was
found to stabilize the high temperature, high conductivity phase at lower temperatures [60]. Only recently has work moved beyond fundamental improvement of the
conductivity of metal borohydride and closo-polyborate salt electrolytes into real
battery applications. Several potential drawbacks of these salt electrolytes have
been identified, however, limiting their application to all-solid-state batteries.
NaBH 4 has been identified as reacting violently with molten sulfur at temperatures
as low as 190 °C to form pure H 2 gas [56]. Additionally, metal borohydrides and
higher borates are extremely hygroscopic. Some of the precursor materials are also
air-sensitive, placing stringent restrictions on manufacturing of these materials.
Lastly, it was found that Na 2 (B 12 H 12 ) 0.5 (B 10 H 10 ) 0.5 electrolyte decomposed above
3.2 V vs Na/Na
+
, which may indicate that these types of electrolytes cannot be used
with a variety of battery cathodes for high voltage battery systems [61, 62].
2.3 Cathodes
Cathode chemistries vary significantly among molten sodium batteries, and tailoring these materials provides excellent opportunities to innovate new sodium battery
technologies. In addition to providing desired battery voltages and usable energy
storage capacities, any new cathode chemistry must be compatible with the solidstate separators, should be cost-effective, and should minimize or eliminate any
hazardous reactions, especially in the event of a separator failure (exposure to molten sodium). One of the most desirable traits of these new chemistries is to enable
operation at temperatures considerably lower than the 270–350 °C common to traditional molten sodium batteries.
Sulfur
The Na–S battery was first patented by Ford Motor Company in 1968 and represents one of the first battery systems to use the molten Na anode. The Na–S battery
is simple in its construction, consisting of a molten Na anode, a BASE separator,
and a molten sulfur cathode. During discharge, the Na is oxidized to Na
+
which
migrates through the ionically conductive separator to react with the sulfur being
reduced at the cathode to form molten polysulfide Na 2 S 5 , which can be further
reduced to lower order polysulfides as shown in the reaction:
Molten Sodium Batteries