64
and conform to variable geometries required in battery design. Importantly, this
means that during electrochemical discharge, gaps at the sodium-separator interface
(a potential challenge for solid-state metal batteries) can be avoided. Batteries that
employ solid-state sodium anodes represent an area of growing interest, though the
relatively high reactivity of sodium against organic catholytes has made solid
sodium anodes impractical for consideration in many sodium-ion systems, and the
comparatively low energy density (relative to solid state Li, for example), has limited research efforts into solid-state sodium batteries. Future efforts may see these
technologies realize greater opportunities with time. The present work is focused on
molten sodium batteries, which are more mature technologies today.
The molten metal anode does, however, introduce a number of challenges. First,
the batteries must be operated at temperatures above the 97.8 °C melting temperature of sodium. Historically, molten sodium batteries have been operated closer to
300 °C to accommodate other material requirements of the batteries, which has
made the molten character of the battery a minor concern. A more common concern
is that metallic sodium is strongly reactive with water, which requires that the anode
must be handled in a dry or inert atmosphere until the battery is hermetically sealed.
By assembling batteries in a discharged state where the sodium is tied up in the
cathode chemistry, it is possible to significantly reduce (or potentially eliminate) the
amount of sodium metal that must be handled during battery assembly [8]. As a
liquid anode, there can also be concerns with wetting of the anode at the anodeseparator interface. This has been a well-documented problem with traditional separator materials, such as β”-alumina solid electrolyte (BASE), though a number of
reports have described methods to address this issue through chemical modification
of either the BASE surface or the metallic anode composition [9–15]. As this issue
of sodium wetting is not unique to BASE [16], it must be considered for sodium
battery development for a variety of systems, particularly for lower temperature
applications.
2.2 Separators
The sodium-ion conducting membrane that separates the anode and cathode remains
one of the most important aspects of sodium batteries and presents some of the
greatest opportunities to advance molten sodium batteries. Not only must this serve
as a robust electrical and physical barrier between the two electrodes, it must also
provide high ionic conductivity while maintaining good chemical and thermal stability in contact with both molten sodium and various, often chemically aggressive
cathodic chemistries. Here we give an overview of common separators that have
been traditionally used in molten sodium batteries and brief discussions of promising new separator materials.
E. D. Spoerke et al.
and conform to variable geometries required in battery design. Importantly, this
means that during electrochemical discharge, gaps at the sodium-separator interface
(a potential challenge for solid-state metal batteries) can be avoided. Batteries that
employ solid-state sodium anodes represent an area of growing interest, though the
relatively high reactivity of sodium against organic catholytes has made solid
sodium anodes impractical for consideration in many sodium-ion systems, and the
comparatively low energy density (relative to solid state Li, for example), has limited research efforts into solid-state sodium batteries. Future efforts may see these
technologies realize greater opportunities with time. The present work is focused on
molten sodium batteries, which are more mature technologies today.
The molten metal anode does, however, introduce a number of challenges. First,
the batteries must be operated at temperatures above the 97.8 °C melting temperature of sodium. Historically, molten sodium batteries have been operated closer to
300 °C to accommodate other material requirements of the batteries, which has
made the molten character of the battery a minor concern. A more common concern
is that metallic sodium is strongly reactive with water, which requires that the anode
must be handled in a dry or inert atmosphere until the battery is hermetically sealed.
By assembling batteries in a discharged state where the sodium is tied up in the
cathode chemistry, it is possible to significantly reduce (or potentially eliminate) the
amount of sodium metal that must be handled during battery assembly [8]. As a
liquid anode, there can also be concerns with wetting of the anode at the anodeseparator interface. This has been a well-documented problem with traditional separator materials, such as β”-alumina solid electrolyte (BASE), though a number of
reports have described methods to address this issue through chemical modification
of either the BASE surface or the metallic anode composition [9–15]. As this issue
of sodium wetting is not unique to BASE [16], it must be considered for sodium
battery development for a variety of systems, particularly for lower temperature
applications.
2.2 Separators
The sodium-ion conducting membrane that separates the anode and cathode remains
one of the most important aspects of sodium batteries and presents some of the
greatest opportunities to advance molten sodium batteries. Not only must this serve
as a robust electrical and physical barrier between the two electrodes, it must also
provide high ionic conductivity while maintaining good chemical and thermal stability in contact with both molten sodium and various, often chemically aggressive
cathodic chemistries. Here we give an overview of common separators that have
been traditionally used in molten sodium batteries and brief discussions of promising new separator materials.
E. D. Spoerke et al.