73
The use of solid or semisolid cathodes makes Na–NiCl 2 batteries intrinsically
safer and less corrosive than Na–S batteries. Furthermore, the system chemistry
allows for assembly of the battery in the discharge state which allows manufacturers
to avoid handling sodium metal. Often Na-NiCl 2 batteries contain additions of FeCl 2
which undergoes the same electrochemical reactions as NiCl 2 at the lower voltage
of 2.35 V. This increases the power response of the battery. Pure FeCl 2 is rarely
used, as FeCl 3 can form during overcharge, and reaction between the soluble Fe
3+
and the BASE can cause degradation of the solid electrolyte [12]. The molten salt
electrolyte, NaAlCl 4 , also provides some degree of protection from overdischarge in
the battery by the formation of Al metal, and serves to protect the battery in the case
of catastrophic failure. Contact between the electrolyte and the molten salt result,
not in a violent reaction like in the case of Na–S batteries, but instead in the formation of harmless products NaCl and Al. The high voltage of Na–NiCl 2 batteries
compared to Na–S batteries further helps boost the system energy density.
Nevertheless, the Na–NiCl 2 battery has yet to achieve widespread use as there
remain a number of challenges in its development. Further improvement in power,
reliability, and cost are necessary.
Particular focus has been placed on reducing the amount of Ni in the battery,
which is expensive and weighty, and on reducing the operating temperature to
extend the system lifetime. Excess Ni is used in the battery cathode to ensure a good
electrical pathway as well as a high surface area for faster reaction kinetics to cycle
NiCl 2 , and thus improve the power performance of the cathode [68]. Serendipitously,
it has been found that in reduced Ni batteries, a lower operating temperature is actually required to improve the cyclability of the battery [68]. Na–NiCl 2 batteries have
thus been demonstrated with Ni contents reduced by as much as 40% with stable
cycling up to 150 cycles at 190 °C [8]. Research into reducing the operating temperature of the Na–NiCl 2 battery has also allowed the replacement of BASE with
higher conductivity NaSICON [69, 70]. Research and development of the Na–NiCl 2
will continue to optimize individual parameters to make the system more robust and
less expensive to operate and maintain.
Hybrid S/NiCl 2
Other areas of research on Na–NiCl 2 batteries include the use of hybrid, or mixed
catholytes to boost the energy density of the system. One example is a hybrid Na–S/
NiCl 2 battery which added Na 2 S to the traditional cathode materials and cycled
under the normal operating conditions of a Na–NiCl 2 battery. Unlike in a Na–S battery in which the formation of solid low-order polysulfides (Na 2 S x , 1 ≤ x ≤ 3) prevents further discharge of the battery, in the presence of molten NaAlCl 4 the lower
order polysulfides are able to be cycled, boosting the energy density of the system.
Despite an initial drop in capacity, likely due to the reaction between Na 2 S and Ni
to form nickel sulfides, the battery exhibited good cyclability in retaining 95% of its
capacity. The hybrid Na–S/NiCl 2 battery is still in its infancy, having demonstrated
only 60 cycles, but the good performance shows promise for this line of work [71].
Molten Sodium Batteries
The use of solid or semisolid cathodes makes Na–NiCl 2 batteries intrinsically
safer and less corrosive than Na–S batteries. Furthermore, the system chemistry
allows for assembly of the battery in the discharge state which allows manufacturers
to avoid handling sodium metal. Often Na-NiCl 2 batteries contain additions of FeCl 2
which undergoes the same electrochemical reactions as NiCl 2 at the lower voltage
of 2.35 V. This increases the power response of the battery. Pure FeCl 2 is rarely
used, as FeCl 3 can form during overcharge, and reaction between the soluble Fe
3+
and the BASE can cause degradation of the solid electrolyte [12]. The molten salt
electrolyte, NaAlCl 4 , also provides some degree of protection from overdischarge in
the battery by the formation of Al metal, and serves to protect the battery in the case
of catastrophic failure. Contact between the electrolyte and the molten salt result,
not in a violent reaction like in the case of Na–S batteries, but instead in the formation of harmless products NaCl and Al. The high voltage of Na–NiCl 2 batteries
compared to Na–S batteries further helps boost the system energy density.
Nevertheless, the Na–NiCl 2 battery has yet to achieve widespread use as there
remain a number of challenges in its development. Further improvement in power,
reliability, and cost are necessary.
Particular focus has been placed on reducing the amount of Ni in the battery,
which is expensive and weighty, and on reducing the operating temperature to
extend the system lifetime. Excess Ni is used in the battery cathode to ensure a good
electrical pathway as well as a high surface area for faster reaction kinetics to cycle
NiCl 2 , and thus improve the power performance of the cathode [68]. Serendipitously,
it has been found that in reduced Ni batteries, a lower operating temperature is actually required to improve the cyclability of the battery [68]. Na–NiCl 2 batteries have
thus been demonstrated with Ni contents reduced by as much as 40% with stable
cycling up to 150 cycles at 190 °C [8]. Research into reducing the operating temperature of the Na–NiCl 2 battery has also allowed the replacement of BASE with
higher conductivity NaSICON [69, 70]. Research and development of the Na–NiCl 2
will continue to optimize individual parameters to make the system more robust and
less expensive to operate and maintain.
Hybrid S/NiCl 2
Other areas of research on Na–NiCl 2 batteries include the use of hybrid, or mixed
catholytes to boost the energy density of the system. One example is a hybrid Na–S/
NiCl 2 battery which added Na 2 S to the traditional cathode materials and cycled
under the normal operating conditions of a Na–NiCl 2 battery. Unlike in a Na–S battery in which the formation of solid low-order polysulfides (Na 2 S x , 1 ≤ x ≤ 3) prevents further discharge of the battery, in the presence of molten NaAlCl 4 the lower
order polysulfides are able to be cycled, boosting the energy density of the system.
Despite an initial drop in capacity, likely due to the reaction between Na 2 S and Ni
to form nickel sulfides, the battery exhibited good cyclability in retaining 95% of its
capacity. The hybrid Na–S/NiCl 2 battery is still in its infancy, having demonstrated
only 60 cycles, but the good performance shows promise for this line of work [71].
Molten Sodium Batteries