Molten Sodium Penetration in NaSICON Electrolytes at 0.1 A cm^–2

Abstract

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.

Can a Coating Mitigate Molten Na Dendrite Growth in NaSICON Under High Current Density?

“…Recent studies have shown that Mode II penetration due to small levels of electronic conductivity may lead to significant metal penetration in solid electrolytes. 45,65 Since the cells in this work only had current applied in one direction, distinct stripping and plating interfaces could be observed, and these two penetration modes could be distinguished. In Figure 3b, large dark regions could be seen at the top electrolyte interface (plating interface�Mode I driven) and a few smaller dark regions were observed disconnected from either interface (Mode II driven).…”

“…A previous study found that both degradation mechanisms became relevant at these high current densities, and this study confirmed that observation. 45 In regard to Figure 3c, it is important to note that the ridge observed near the stripping interface in Figure 3b blocked the X-rays for EDS and made elemental mapping of the region containing Mode II penetration challenging.…”

“…Building on an experimental setup described in our previous work, unidirectional galvanostatic testing at 100, 500, and 1000 mA cm −2 is performed on symmetric cells containing uncoated and Sn-coated NaSICON to simulate high-current long duration (dis)charge half-cycles. 45 Electrochemical impedance spectroscopy (EIS) is performed after each interval to monitor the interfacial resistance as a function of discharge time. After electrochemical tests, the NaSICON pellets are fractured in a homemade bending device to expose regions weakened by dendrite formation for ex situ scanning electron microscope (SEM) observations.…”

“…43,44 We have previously reported that unmodified NaSICON was susceptible to both Mode I and Mode II Na penetration at 100 mA cm −2 and 110 °C, particularly if high-capacity discharges were used. 45 To combat high interfacial impedance in ASSBs, numerous coatings have been applied to solid electrolytes or alkali metals to facilitate intimate contact and reduce current concentration. 46−51 Similar approaches have been taken for solid-state and molten Na batteries, where the coating is applied to the electrolyte.…”

“…Similar cells have been used in previous studies, and a schematic is included in Figure S2. 34,45 The Na|SnNaSICONSn|Na cells were first conditioned by using 24 low-current cycles at 0.75 mA cm −2 . The capacity for each half-cycle was 2 mA h. The conditioning cycles established the low-resistance Na|SnNaSICON interfaces.…”