Modeling of reaction kinetics and transport in the positive porous electrode in a sodium–iron chloride battery

A one-dimensional mathematical model of the positive electrode of a sodium–iron chloride battery for an isothermal, constant-current discharge–charge cycle is presented. Macroscopic theory of porous electrodes and concentrated solution theory are used in the model to describe the transport processes. The change in the solubility of FeCl2 with position and time within the cell is included in the model by defining an equilibrium constant that is a function of the NaCl:NaAlCl4 molar ratio. The concentrated solution theory for a three-ion system with common cation is extended to account for a diffusive flux of a sparingly soluble ferrous complex. It is seen that this flux is important, especially at moderate depths of discharge. The effect of the assumed solubility constant Ksp,FeCl on the battery performance is characterized. When Ksp,FeCl is higher than 106, its variation does not change the short-time behavior of the system appreciably. Simulations suggest that the iron accumulates near the sodium tetrachloroaluminate reservoir during discharge. When charging, the net movement is reversed. As a result of continuous cycling, simulations predict that iron is depleted at this boundary. For instance, at the end of the fifth cycle, the iron amount decreases by ∼1% near the reservoir.

► The variable solubility of FeCl2 as a function of NaCl:NaAlCl4 molar ratio.
► The flux of the sparingly soluble ferrous complex to describe transport.
► The assumed solubility constant Ksp,FeCl to describe battery performance.
► The relocation of iron within the cell can be predicted.
► With continuous cycling, iron is depleted near the NaAlCl4 reservoir.