Key Factors Controlling the Performance of Na–In Alloy Electrode in Sodium Solid-State Batteries with Pressure, Impurities, and Surface Roughness

Room-temperature solid-state sodium batteries are considered a promising technology for high-energy-density energy storage. β-alumina ceramic solid electrolytes provide good mechanical strength and high ionic conductivity at room temperature, which help to suppress dendrite growth; however, their performance is limited by high-impedance charge transfer at the electrode/electrolyte interfaces. This work investigates the effects of applied pressure, thermal etching, and electrolyte surface roughness in a model β-alumina solid electrolyte coupled with an Na₉₈In₂ alloy anode in a symmetrical cell setup. A combination of microscopic, spectroscopic, and electrochemical techniques was used to gain deeper insight into the mechanisms governing the relationships between these factors and the interfacial resistance and critical current density. To better understand the processes occurring in this system, a distribution of relaxation times (DRT) analysis was employed to study the impedance spectra. We found that even a low indium content in the sodium alloy significantly improves adhesion to the solid electrolyte without the need for advanced surface modifications. Increased pressure and reduced electrolyte roughness were found to play complementary roles by promoting close contact between the electrode and electrolyte, thereby lowering the true microscopic current density, reducing the interfacial impedance, and increasing the critical current density. Thermal etching, performed by annealing in an inert atmosphere in the 900–1200 °C range, demonstrated an effective cleaning effect by removing secondary phases, mainly sodium carbonate. The optimized interface exhibited a low interfacial resistance of 4.6 Ω·cm² and a significantly improved critical current density of 1.65 mA·cm^–2.