Large scale electrical energy storage is a key piece of the next generation electrical grid infrastructure. Sodium batteries offer great promise as safe, low cost and high energy density storage technologies that can be utilized to meet the growing demand for these types of grid scale storage systems. We are exploring a fully molten sodium battery system that operates at low to intermediate temperatures and provides inherent safety for a large scale battery application. Here we characterize the electrochemical properties of new molten salt catholyte chemistries that enable high performance cycling behavior at lower the operating temperature. We present electrochemical analysis of candidate NaI-AlCl₃ molten salt catholytes at various temperatures and compositions. We find concentration dependencies on the iodide oxidation potentials as well as the observed current densities. Insight from these studies reveal catholyte compositions that can support high current densities and good cycle life that would be critical to reliable battery performance. Understanding the phase behavior of these salt compositions is key to optimizing the catholyte performance and a preliminary phase diagram is used help explain the promising electrochemical observations. For example, solids present in the catholyte at very high or very low NaI concentrations hinder mass and charge transport to electrode surfaces. Successful understanding and manipulation of this phase space will enable increases in catholyte performance, both in terms of lower operating temperatures and also increased current densities for improved molten sodium batteries for grid scale energy storage.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.