78
battery has the effect of raising the operating costs of the battery. The combination
of the high self-discharge rate and low voltage in the case of the Na–Bi couple
(0.74 V) has led research in liquid metal batteries to other molten anode materials
beyond sodium.
3 Battery Design
Although the focus of this chapter is on the materials chemistry of molten sodium
batteries, it is worth briefly discussing the design of these systems, as design can
have substantial impact on the materials chemistry and battery performance overall.
There are multiple configurations for these battery systems, each appropriate for
different battery chemistries, scales, and applications of interest. Figure 7 shows
examples of both a concentric tubular configuration and a planar stack configuration
for these batteries. Although the basic electrochemistry of the system is not generally expected to be significantly affected by the configuration, it is important to
acknowledge that the form of the battery and the material behaviors are connected.
One of the principle differences between these two primary designs relates to how
the molten constituents remain in contact with the solid electrolyte during charge
and discharge cycles. The tubular system relies on gravity and wicking of the molten materials along the sides of the tubular separator, and changes in volume of
sodium or catholyte are accommodated by the free volume inside and outside the
separator. In contrast, the planar design often requires the planar components be
capable of “flexing” under compression to allow for intimate contact of molten species with the separator while accommodating changes in volume during charge and
discharge. Depending on the specific materials chemistry, these design parameters
can significantly influence battery performance. It was demonstrated that for Na–
NiCl 2 batteries, changing from a tubular to a planar design allowed for lower operating temperatures, improved energy density, and extended battery cycle life [90].
Form has an impact on materials chemistry through the battery cost as well.
Expensive configurations, in turn, require less expensive cathodes and separators to
balance the cost of the system.
Additional components of the battery assembly must also be carefully selected,
ranging from the battery housing to the battery seals. High temperature operation
places restraints on the types of materials that can be used to house and seal the battery, as does the use of highly corrosive materials or volatile species [19, 91].
Finally, it must be mentioned that there are numerous practical considerations for
how the system will be assembled and sealed when developing molten sodium battery chemistries. For example, it is possible to assemble cells in the fully discharged
state, meaning that sodium metal need not be handled in large quantities during
assembly; sodium is generated during an initial charge state. While this is a desirable processing approach, it does introduce complications of providing free volume
for the sodium generated on charge and maintaining suitable electrical contact
E. D. Spoerke et al.
battery has the effect of raising the operating costs of the battery. The combination
of the high self-discharge rate and low voltage in the case of the Na–Bi couple
(0.74 V) has led research in liquid metal batteries to other molten anode materials
beyond sodium.
3 Battery Design
Although the focus of this chapter is on the materials chemistry of molten sodium
batteries, it is worth briefly discussing the design of these systems, as design can
have substantial impact on the materials chemistry and battery performance overall.
There are multiple configurations for these battery systems, each appropriate for
different battery chemistries, scales, and applications of interest. Figure 7 shows
examples of both a concentric tubular configuration and a planar stack configuration
for these batteries. Although the basic electrochemistry of the system is not generally expected to be significantly affected by the configuration, it is important to
acknowledge that the form of the battery and the material behaviors are connected.
One of the principle differences between these two primary designs relates to how
the molten constituents remain in contact with the solid electrolyte during charge
and discharge cycles. The tubular system relies on gravity and wicking of the molten materials along the sides of the tubular separator, and changes in volume of
sodium or catholyte are accommodated by the free volume inside and outside the
separator. In contrast, the planar design often requires the planar components be
capable of “flexing” under compression to allow for intimate contact of molten species with the separator while accommodating changes in volume during charge and
discharge. Depending on the specific materials chemistry, these design parameters
can significantly influence battery performance. It was demonstrated that for Na–
NiCl 2 batteries, changing from a tubular to a planar design allowed for lower operating temperatures, improved energy density, and extended battery cycle life [90].
Form has an impact on materials chemistry through the battery cost as well.
Expensive configurations, in turn, require less expensive cathodes and separators to
balance the cost of the system.
Additional components of the battery assembly must also be carefully selected,
ranging from the battery housing to the battery seals. High temperature operation
places restraints on the types of materials that can be used to house and seal the battery, as does the use of highly corrosive materials or volatile species [19, 91].
Finally, it must be mentioned that there are numerous practical considerations for
how the system will be assembled and sealed when developing molten sodium battery chemistries. For example, it is possible to assemble cells in the fully discharged
state, meaning that sodium metal need not be handled in large quantities during
assembly; sodium is generated during an initial charge state. While this is a desirable processing approach, it does introduce complications of providing free volume
for the sodium generated on charge and maintaining suitable electrical contact
E. D. Spoerke et al.