Molten Sodium Penetration Through Solid-State NaSICON Electrolytes Under High Current

High conductivity solid electrolytes, such as NaSICON, are poised to play an increasingly important role in safe, reliable battery-based energy storage, enabling a new class of sodium-based batteries. Coupled demands of high current densities (≥0.1 A cm^-2) and low temperature (&lt;200 °C) operation, combined with increased discharge times for long duration storage (&gt;12 h), challenge the limitations of solid electrolytes. Here, we explore the penetration of sodium into NaSICON at 0.1 A cm^-2 in a symmetric molten sodium cell. Previous studies of β’’-alumina proposed that Poiseuille pressure-driven cracking (Mode I) and recombination of ions and electrons within the solid electrolyte (Mode II) can cause metal accumulation within solid electrolytes, but a comprehensive study at high current density is necessary. To understand and differentiate these modes in NaSICON, this work employs unidirectional galvanostatic testing of Na|NaSICON|Na symmetric cells at 0.1 A cm^-2 over 23 hours at 110 °C. While galvanostatic testing shows a relatively constant, yet increasingly noisy voltage profile, electrochemical impedance spectroscopy (EIS) reveals a significant decrease in cell impedance which can be correlated with significant sodium penetration, as observed in scanning electron microscopy (SEM). Metal accumulation from the stripping-side electrode suggests that Mode II failure may be far more prevalent than previously considered. Further, these findings suggest that total charge transported per unit area (mAh cm^-2), as opposed to current density (mA cm^-2), may be a more critical parameter when examining solid electrolyte failure. Together, these results provide a better understanding of the limitations of NaSICON solid electrolytes under high current density and can guide the design of coatings to improve electrode-electrolyte interfaces.<br/><br/><br/>This work was done in collaboration with Sandia National Laboratories and was supported through the Energy Storage Program, managed by Dr. Imre Gyuk, within the U.S. Department of Energy’s Office of Electricity. 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.