Characterization of NaSICON Solid Electrolytes Exposed to Thermal and Electrochemical Cycling in Molten Sodium Environment

Concerns over grid reliability, resiliency, and sustainability have driven active research into long-lasting, safe, and low-cost energy storage solutions. Sodium-based batteries, such as molten sodium-metal halide batteries, have emerged as a promising technological solution to these long-term energy storage questions. Particularly, batteries that operate near the melting point of sodium provide an opportunity for improved safety and reduced capital and operating costs compared to conventional sodium batteries. The sodium super ionic conductor (NaSICON) solid electrolyte is a critical component in these batteries due to its high conductivity near their operating temperature and chemical inactivity with electrode materials. However, environmental conditions, such as high current densities coupled with temperature changes at the micro- and macro-scale, can affect the structural, mechanical, and electrochemical properties of NaSICON. Understanding these properties is essential in evaluating the long-term performance of molten sodium batteries. In this study, NaSICON solid electrolytes are exposed to critical current densities in molten sodium cells to examine their failure mechanisms and subjected to thermal cycling to explore the changes in electrolyte properties. Microstructural characterization by x-ray diffraction and electron microscopy is coupled with mechanical and electrochemical techniques, such as nanoindentation and impedance spectroscopy, to reveal the material changes that alter the performance of these solid electrolytes. These measurements have revealed localized damages and residual stress that affect the performance and durability of NaSICON. A fundamental understanding of structure, property, and performance of NaSICON under thermal and electrochemical cycling will guide further development of solid-state ionic conductors for molten sodium batteries.<br/> <br/> <br/>Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & 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.