High-conductivity solid electrolytes,
such as the Na
superionic
conductor, NaSICON, are poised to play an increasingly important role
in safe, reliable battery-based energy storage, enabling advanced
sodium-based batteries. Coupled demands of increased current density
(≥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 molten sodium into NaSICON at
high current densities. 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) are the
two main mechanisms for Na penetration, but a comprehensive study
of Na penetration in NaSICON is necessary, particularly at high current
density. To further understand these modes, this work employs unidirectional
galvanostatic testing of Na|NaSICON|Na symmetric cells at 0.1 A cm–2 over 23 h 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 correlated with significant sodium penetration,
as observed in scanning electron microscopy (SEM). Further SEM analysis
of sodium accumulation within NaSICON suggests that mode II failure
may be far more prevalent than previously considered. Further, these
findings suggest that total (dis)charge density (mAh cm–2), as opposed to current density (mA cm–2), may
be a more critical parameter when examining solid electrolyte failure,
highlighting the challenge of achieving long discharge times in batteries
using solid electrolytes. Together, these results provide a better
understanding of the limitations of NaSICON solid electrolytes under
high current and emphasize the need for improved electrode–electrolyte
interfaces.