72
xS + 2Na ↔ Na 2 S x (3 ≤ x ≤ 5)
E cell ~ 2.08 V at 350 °C
In theory the battery can be discharged to Na 2 S, but in practice both Na 2 S 2 and
Na 2 S are solid at the battery operating temperatures of 350 °C, and the formation of
these solid products dramatically increases the resistance to the battery so as to
prohibit further discharge. This effectively limits the capacity of the battery to about
half its theoretical value.
Since its initial discovery, the majority of research on this system has been toward
its practical development. The molten sodium, sulfur and polysulfides, and their
vapors are highly corrosive, and great effort has been made toward the development
of housing materials, their coatings, and sealing materials that are resistant to such
corrosion. In the case of catastrophic battery failure, a violent reaction between the
molten sodium and the sulfur/polysulfide cathode takes place, further requiring
adequate materials and engineering controls. This risk of catastrophic failure has
restricted Na–S batteries to utility applications despite the original intent in the
1960s to develop the Na–S battery for vehicles.
A further restriction on the Na–S battery applications is the ionic and electronic
resistance of the sulfur catholyte. The material’s poor electrical conductivity restricts
the battery to low-power applications such as load-leveling and emergency power
distribution [63, 64]. Despite these limitations, the Na–S battery has seen over
560 MW (4000 MWh) of storage deployed worldwide, and some of these installations have demonstrated over a decade of reliable cycling [19, 65]. However, recent
fires demonstrate the risk of this technology, despite many decades of development
[64]. Current research of the Na–S battery has focused on intermediate and room
temperature operation to improve battery capacity and safety [66, 67].
Sodium Metal Halides
ZEBRA/Na–NiCl 2
Closely related to the Na–S system is the sodium metal halide battery, which uses
solid or semisolid metal halides as the active cathode material. The most common
form of the sodium metal halide battery is the Na–NiCl 2 battery, often referred to as
a ZEBRA battery (for Zeolite Battery Research Africa, or Zero-Emission Battery
Research Activities), as an acknowledgement of the South African origins of its
development. This system was originally conceived as a high temperature battery
that shared the same type of BASE ion conductor used by the Na–S batteries. In
Na–NiCl 2 batteries, sodium ions are transported through the oxide membrane from
the anode to the cathode during discharge, reducing NiCl 2 to Ni via migration of
sodium ions in a NaAlCl 4 molten salt electrolyte, as shown in the reaction:
NiCl 2 + 2Na ↔ 2NaCl + Ni(s)
E cell ~ 2.58 V at 300 °C
E. D. Spoerke et al.
xS + 2Na ↔ Na 2 S x (3 ≤ x ≤ 5)
E cell ~ 2.08 V at 350 °C
In theory the battery can be discharged to Na 2 S, but in practice both Na 2 S 2 and
Na 2 S are solid at the battery operating temperatures of 350 °C, and the formation of
these solid products dramatically increases the resistance to the battery so as to
prohibit further discharge. This effectively limits the capacity of the battery to about
half its theoretical value.
Since its initial discovery, the majority of research on this system has been toward
its practical development. The molten sodium, sulfur and polysulfides, and their
vapors are highly corrosive, and great effort has been made toward the development
of housing materials, their coatings, and sealing materials that are resistant to such
corrosion. In the case of catastrophic battery failure, a violent reaction between the
molten sodium and the sulfur/polysulfide cathode takes place, further requiring
adequate materials and engineering controls. This risk of catastrophic failure has
restricted Na–S batteries to utility applications despite the original intent in the
1960s to develop the Na–S battery for vehicles.
A further restriction on the Na–S battery applications is the ionic and electronic
resistance of the sulfur catholyte. The material’s poor electrical conductivity restricts
the battery to low-power applications such as load-leveling and emergency power
distribution [63, 64]. Despite these limitations, the Na–S battery has seen over
560 MW (4000 MWh) of storage deployed worldwide, and some of these installations have demonstrated over a decade of reliable cycling [19, 65]. However, recent
fires demonstrate the risk of this technology, despite many decades of development
[64]. Current research of the Na–S battery has focused on intermediate and room
temperature operation to improve battery capacity and safety [66, 67].
Sodium Metal Halides
ZEBRA/Na–NiCl 2
Closely related to the Na–S system is the sodium metal halide battery, which uses
solid or semisolid metal halides as the active cathode material. The most common
form of the sodium metal halide battery is the Na–NiCl 2 battery, often referred to as
a ZEBRA battery (for Zeolite Battery Research Africa, or Zero-Emission Battery
Research Activities), as an acknowledgement of the South African origins of its
development. This system was originally conceived as a high temperature battery
that shared the same type of BASE ion conductor used by the Na–S batteries. In
Na–NiCl 2 batteries, sodium ions are transported through the oxide membrane from
the anode to the cathode during discharge, reducing NiCl 2 to Ni via migration of
sodium ions in a NaAlCl 4 molten salt electrolyte, as shown in the reaction:
NiCl 2 + 2Na ↔ 2NaCl + Ni(s)
E cell ~ 2.58 V at 300 °C
E. D. Spoerke et al.