Molten salts, which are ionic compounds, are excellent conductors of electricity. This ability to carry an electrical current stems from their unique atomic structure in the liquid state. The mechanism of conduction in these high-temperature liquids is entirely different from the electron flow seen in metals, relying instead on the physical movement of charged atoms.
The Difference Between Solid and Liquid Salt
The ability of a salt to conduct electricity is dependent on its physical state. In its solid form, an ionic compound like sodium chloride is arranged in a rigid crystal lattice. Within this structure, positively charged cations and negatively charged anions are held firmly in fixed positions by strong electrostatic forces. Because these ions are immobile, they cannot migrate to carry a charge, which is why solid salt does not conduct electricity.
Melting the salt requires applying significant thermal energy to overcome the powerful forces binding the crystal lattice. When the salt reaches its high melting point—sodium chloride melts around 801°C—the ordered structure breaks down. The resulting liquid, or molten salt, is a highly energized medium where the ions are no longer fixed but are free to move and flow. This liberation of charge carriers is the prerequisite for electrical conduction.
How Molten Salts Conduct Electricity
Molten salts conduct electricity through the physical relocation of their constituent charged ions, not through electrons. This process, known as ionic conduction, is distinct from metallic conduction where delocalized electrons carry the current. When an external voltage is applied across the molten salt, it creates an electric field that guides the movement of the free-floating ions.
The positively charged ions (cations) are drawn toward the negatively charged electrode (the cathode). Concurrently, the negatively charged ions (anions) move toward the positively charged electrode (the anode). This coordinated, directional movement of charged particles constitutes the electrical current flow. The process often involves an electrochemical reaction at the electrodes, where ions gain or lose electrons, leading to the creation of new chemical species.
Where Molten Salt Conductivity Matters
The high ionic conductivity of molten salts, coupled with their ability to dissolve other materials and withstand extreme temperatures, makes them invaluable in industrial and advanced energy applications. One long-standing use is in the production of reactive metals that cannot be extracted using water-based solutions. The Hall-Héroult process, the primary method for producing aluminum, relies entirely on the electrolysis of aluminum oxide dissolved in molten cryolite (sodium aluminum fluoride) at temperatures near 960°C.
Molten salts are also gaining traction in advanced energy systems, where their conductive properties are harnessed for large-scale storage. Molten salt batteries, such as those employing sodium and sulfur, use a molten salt electrolyte, typically operating between 200°C and 600°C. The high temperature ensures the salt remains liquid and maintains high ionic conductivity, enabling the rapid movement of ions during charging and discharging cycles.
Molten Salt Reactors (MSRs) utilize these conductive liquids as a fluid fuel carrier or a coolant in advanced nuclear designs. Their low vapor pressure at high temperatures allows MSRs to operate safely at near-atmospheric pressure. The salt’s conductive nature is fundamental to the reactor’s unique chemistry and safety systems.