High-utilization sodium–metal chloride cells with a mixed-metal Fe, Zn, NaCl cathode enable efficient high-temperature cycling (300°C) while reducing cost and environmental impact. Near-theoretical capacity and high efficiency are achieved in single cells. Aging, driven by resistance growth and cathode restructuring, is mitigated by voltage limitation at module level, highlighting a scalable, Ni-free pathway for durable, cost-effective energy storage.

ABSTRACT

Cycling performance and ageing mechanisms of high-temperature sodium–metal chloride cells were investigated using a modified Na–NiCl₂ 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–NiCl₂ 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.