Sodium Penetration in Nasicon Solid Electrolytes Under High Current

Abstract

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 (<200 °C) operation, combined with increased discharge times for long duration storage (>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 low levels of electronic conductivity leading to ion-electron recombination (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 1-hour intervals at 110 °C. While galvanostatic testing shows a relatively constant, yet increasingly noisy voltage profile, electrochemical impedance spectroscopy (EIS) between intervals reveals a significant decrease in cell impedance which can be correlated with sodium metal penetration, as observed in scanning electron microscopy (SEM). Significant sodium 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), an important parameter for long duration storage, may be a more critical parameter than current density (mA cm-2) 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.