Abstract
Abstract
The rapid expansion of renewable energy technologies requires the development of efficient and reliable stationary energy storage systems. Among emerging alternatives to lithium-ion batteries, molten sodium batteries represent a promising solution due to the abundance of sodium resources, intrinsic safety, and favorable electrochemical characteristics. However, conventional sodium–sulfur and sodium–metal halide batteries operate at elevated temperatures (typically above 270 °C), which increases system complexity, materials degradation, and thermal management requirements. In this work, we investigate the use of NaI–AlCl
3
eutectic mixtures as redox-active catholytes for molten sodium batteries operating at intermediate temperatures. A composition of 60:40 mol % NaI–AlCl
3
was synthesized and integrated into planar laboratory cells employing NASICON solid electrolytes. The electrochemical behavior of the system was evaluated at 150 °C using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge testing. The selected eutectic composition exhibited stable molten behavior and reversible electrochemical activity associated with the iodide/triiodide redox couple. Initial impedance measurements revealed a progressive decrease in interfacial resistance during the first hours of operation, attributed to improved wetting of the catholyte within the carbon felt current collector. Galvanostatic cycling performed at a current density of 1.4 mA cm showed stable voltage profiles with an average cell voltage of approximately 3.24 V. The cell maintained an energy efficiency above 95% over 50 cycles, corresponding to more than 1200 h of operation, with negligible voltage drift. These results demonstrate that NaI–AlCl
−2
3
molten catholytes enable stable sodium battery operation at significantly reduced temperatures compared to conventional molten salt systems. The combination of halide-based eutectic catholytes and NASICON solid electrolytes represents a promising pathway toward safer and more cost-effective stationary energy storage technologies.