Impact of Catholyte Lewis Acidity at the Molten Salt–NaSICON Interface in Low-Temperature Molten Sodium Batteries

Abstract

Low-temperature molten sodium batteries comprising molten sodium anodes, a NaSICON solid-state separator, and molten halide salt catholytes offer promise as low-cost, earth-abundant energy storage technologies. The emergence of a specific, high-voltage, sodium iodide (NaI)-based catholyte chemistry has prompted the evaluation of chemical and electrochemical properties of the molten salts, particularly at critical interfaces with high-performance NaSICON separators. Herein, batteries operated at 110 °C with NaI-AlCl3-based catholytes of differing Lewis acidities were evaluated. Batteries with >50 mol % AlCl3 (acidic catholytes) experienced a linear decline in energy efficiency during cycling, whereas batteries with >50 mol % NaI (basic catholytes) maintained >95% energy efficiency for 50 cycles (>80 days) at 2.5 mA cm–2. A three-electrode cell was developed, enabling identification of the NaSICON–catholyte interface as the source of increased battery impedance. Complementary physical and chemical characterization of the NaSICON exposed to acidic and basic catholytes showed no changes in crystallinity, bulk morphology, or bulk chemical composition, but surface sensitive X-ray photoelectron spectroscopy (XPS), however, revealed subtle changes in local NaSICON surface chemistry. In addition, Raman spectroscopy indicated that stably performing basic catholytes lack the dimer species Al2Cl6I– present in acidic catholytes. Select thermodynamic and formal charge assessments suggest that preferential interactions between these acidic dimeric species and the NaSICON surface may be responsible for the observed increases in electrochemical impedance and degraded battery performance. These results indicate that maintaining a Lewis basic catholyte avoids such potentially deleterious interactions, enabling efficient and stable battery cycling.

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

“…The cell still exhibited the gradually increasing overpotential observed in the uncoated cell, and again, this was likely due to an interfacial reaction. However, in this case, the reaction converted the Sn coating to a NaSn intermetallic that has proven to be an effective coating in previous works at lower current densities (up to 50 mA cm –2 ). − The intermetallic mostly formed during the low-current conditioning steps, and any Sn remaining after conditioning was likely converted to NaSn when the high current density was applied. The NaSn is slightly more resistive than the Sn alone and, coupled with some NaSICON reduction near coating defects, caused the slight overpotential increase throughout the test .…”

“…The size of each point is scaled, corresponding to the half-cycle capacity of the cells tested in each study. Specific values for each point are provided in Table S5. ,,,,,− …”

“…NaSICON (Na Super Ion CONductor), a solid electrolyte discovered by Hong and Goodenough in 1976, however, has a high conductivity below 200 °C, making it a viable electrolyte for lower-temperature molten Na batteries. , Recently, NaSICON has been used as the electrolyte in next-generation molten Na batteries operating at 110 °C. These batteries take advantage of low-melting-point Na (as the negative electrode) and molten NaI salt mixtures (positive electrode) and can achieve a high current density of 50 mA cm –2 . ,,− However, this class of batteries still faces a number of challenges. Although the conductivity of NaSICON is high at 110 °C, Na metal does not wet the surface of NaSICON particularly well at this temperature .…”

“…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. − Similar approaches have been taken for solid-state and molten Na batteries, where the coating is applied to the electrolyte. ,,,− Landmann et al applied a porous carbon coating to BASE to enable impressive current densities (up to 2600 mA cm –2 ) at 250 °C. Chang et al used a Pb microparticle coating to improve the performance of Na–NiCl 2 batteries at 190 °C.…”

Electrode Blocking Due to Redox Reactions in Aluminum Chloride-Sodium Iodide Molten Salts

“…20 For lowtemperature MNaBs with NaI/AlCl 3 electrolytes in particular, the species and phases present depend on the electrolyte composition, which varies with the battery's state of charge (SoC) via the ratio of I − to higher order polyiodides such as I 3 − . 28,29 Understanding the complex and interrelated physicochemical behavior of these systems requires careful analysis of what processes limit the cycling rate at temperatures and electrolyte compositions relevant to practical battery operation.…”