Cycling Performance of Sodium‐Metal Chloride Battery Cells and Modules Based on Iron and Zinc

ABSTRACT Cycling performance and ageing mechanisms of high‐temperature sodium–metal chloride cells were investigated using a modified Na–NiCl ₂ chemistry, where Fe and Zn replace Ni in the cathode. These abundant metals reduce cost and environmental impact while maintaining theoretical capacity. Metal utilization was increased from 30% to 39% compared with state‐of‐the‐art Na–NiCl ₂ cells. A single cell and a five‐cell module were operated for three months at 300°C, demonstrating stable cycling and high energy efficiency. The single cell achieved near‐complete utilization of its theoretical capacity with excellent coulombic and voltage efficiencies even under elevated current densities. Aging manifested as a gradual decline in discharge energy, linked to rising Zn ^{2 +} ‐reduction overpotentials at low states‐of‐charge. Post‐mortem analysis revealed NaCl‐rich layers at the cathode–electrolyte interface and gas‐induced porosity, indicating active‐material redistribution and side reactions during extended cycling. These reactions were mitigated in a five‐cell module by lowering the upper cut‐off voltage by 0.1 V (2.55 V→2.45 V). The module exhibited similar initial behavior but required current adjustments to counter capacity fade caused by rising cycle resistance, ultimately stabilizing at a substantial fraction of theoretical capacity. These findings highlight the potential of Fe, Zn‐based sodium–metal chloride cells as cost‐effective alternatives to Ni‐based cell chemistries.