62
ceramic membrane) and ion-conductive electrolytes (ceramic or other solid-state
electrolytes often also act as the separator). Current collectors, interfaced with a
battery’s anode and cathode, facilitate electron transfer in and out of the battery. In
a molten sodium battery, the anode is sodium metal, and the battery must be operated above its melting temperature (97.8 °C). This molten sodium anode wets a
sodium-ion conducting ceramic separator that isolates the anode physically and
electronically from various cathode chemistries that will be discussed in greater
detail later in this chapter. As the battery discharges, an oxidation reaction at the
anode extracts electrons from the metallic sodium, creating Na
+
ion. The extracted
electrons are collected by the current collectors and provide power as they are shuttled through an electrical circuit to the current collector at the cathode where they
participate in electrochemical reductions, dependent on the specific cathode chemistry. For example, in a sodium-sulfur battery, molten sulfur is reduced to form
molten polysulfides, while in a ZEBRA battery, Ni
2+
ions are reduced to metallic
nickel. Meanwhile, the oxidized sodium ions (Na
+
) must cross through the ion conducting separator and/or electrolytes to the cathodic side of the battery, where they
participate in charge-balancing electrochemical reactions. Together the external
transport of the electron from anode to cathode and the internal transport of the
oxidized ion complete the electrochemical circuit of the discharging battery. In the
case of rechargeable (also known as secondary) batteries, this process is reversed to
recharge the battery. One complete charge/discharge process is known as a “cycle.”
The energy density of a battery is the amount of energy in a given system, per
unit mass (gravimetric energy density, also called specific energy) or per unit volume (volumetric energy density). This energy density is the product of its capacity
(how much charge the battery can effectively charge and discharge) and its voltage.
The maximum ideal voltage the battery is capable of achieving is determined by the
free energies of the electrochemical reactions at the anode and cathode. The voltage,
and therefore the energy density of the battery, is strongly dependent on the chemistry within the battery. The power of a battery is, effectively, how quickly the battery can release its energy. This power can also be normalized by weight or size to
determine the power density of the system. It is worth noting that a “high-power”
system may not necessarily have a high energy density; a small amount of energy
delivered very rapidly would be considered a high-power system.
One of the common goals of battery researchers in general is to create a system
that combines high-power and high energy density, but not all battery applications
require these capabilities. Figure 2 relates different grid-scale electrical energy storage technologies’ power ratings and discharge times, highlighting the application
space where these technologies may provide meaningful utility. On this plot, molten
sodium batteries are represented by the NaS and the Na-NiCl 2 batteries, which provide sufficient energy storage to discharge on the order of hours (often 4–6) with
reasonable power capabilities for applications such grid support and load shifting,
but they are clearly not ideal for all applications. It should be noted that the application categories are not absolutely defined by the boundaries shown; significant overlap in these applications is common and dependent on the specific needs of a system
E. D. Spoerke et al.
ceramic membrane) and ion-conductive electrolytes (ceramic or other solid-state
electrolytes often also act as the separator). Current collectors, interfaced with a
battery’s anode and cathode, facilitate electron transfer in and out of the battery. In
a molten sodium battery, the anode is sodium metal, and the battery must be operated above its melting temperature (97.8 °C). This molten sodium anode wets a
sodium-ion conducting ceramic separator that isolates the anode physically and
electronically from various cathode chemistries that will be discussed in greater
detail later in this chapter. As the battery discharges, an oxidation reaction at the
anode extracts electrons from the metallic sodium, creating Na
+
ion. The extracted
electrons are collected by the current collectors and provide power as they are shuttled through an electrical circuit to the current collector at the cathode where they
participate in electrochemical reductions, dependent on the specific cathode chemistry. For example, in a sodium-sulfur battery, molten sulfur is reduced to form
molten polysulfides, while in a ZEBRA battery, Ni
2+
ions are reduced to metallic
nickel. Meanwhile, the oxidized sodium ions (Na
+
) must cross through the ion conducting separator and/or electrolytes to the cathodic side of the battery, where they
participate in charge-balancing electrochemical reactions. Together the external
transport of the electron from anode to cathode and the internal transport of the
oxidized ion complete the electrochemical circuit of the discharging battery. In the
case of rechargeable (also known as secondary) batteries, this process is reversed to
recharge the battery. One complete charge/discharge process is known as a “cycle.”
The energy density of a battery is the amount of energy in a given system, per
unit mass (gravimetric energy density, also called specific energy) or per unit volume (volumetric energy density). This energy density is the product of its capacity
(how much charge the battery can effectively charge and discharge) and its voltage.
The maximum ideal voltage the battery is capable of achieving is determined by the
free energies of the electrochemical reactions at the anode and cathode. The voltage,
and therefore the energy density of the battery, is strongly dependent on the chemistry within the battery. The power of a battery is, effectively, how quickly the battery can release its energy. This power can also be normalized by weight or size to
determine the power density of the system. It is worth noting that a “high-power”
system may not necessarily have a high energy density; a small amount of energy
delivered very rapidly would be considered a high-power system.
One of the common goals of battery researchers in general is to create a system
that combines high-power and high energy density, but not all battery applications
require these capabilities. Figure 2 relates different grid-scale electrical energy storage technologies’ power ratings and discharge times, highlighting the application
space where these technologies may provide meaningful utility. On this plot, molten
sodium batteries are represented by the NaS and the Na-NiCl 2 batteries, which provide sufficient energy storage to discharge on the order of hours (often 4–6) with
reasonable power capabilities for applications such grid support and load shifting,
but they are clearly not ideal for all applications. It should be noted that the application categories are not absolutely defined by the boundaries shown; significant overlap in these applications is common and dependent on the specific needs of a system
E. D. Spoerke et al.