Lowering Interfacial Resistance at the NaSICON Interface for Use in Molten Sodium Batteries

Abstract Molten Na batteries have typical operating temperatures of 300-350°C, requiring complex thermal management and specialized seals. Lowering the operating temperature to less than 150°C would improve the economic feasibility of large-scale molten Na energy storage. The use of Hg metal as a positive electrode enables an operating temperature below 150°C when paired with a Na superionic conductor (NaSICON) solid-state electrolyte. However, poor wetting of NaSICON by liquid Hg increases interfacial resistance and limits achievable current density. In this work, Pt, Cu, and Ag coatings were investigated as a route to improve Hg wetting of NaSICON and reduce the interfacial resistance. The Pt coating lowered the Hg contact angle from 114° (bare NaSICON) to 100°, reduced symmetric-cell impedance nearly seven-fold (18.15 vs. 2.64 Ω cm² at low-frequency), and decreased overpotential nearly four-fold at 0.5 mA cm⁻² (-0.08 V vs. -0.02 V), while forming a stable, coherent film after cycling. Cu and Ag coatings instead worsened the contact angle (135–139°), increased impedance and overpotential relative to bare NaSICON, and were largely stripped from the electrode during testing. Based on these results, a Pt coating on the NaSICON is recommended for improving Hg wetting and lowering interfacial resistance in molten Na-Hg batteries.