December 2021 - Solid-state sodium (Na) batteries have received extensive attention as a promising alternative to room-temperature liquid electrolyte Na-ion batteries and high-temperature liquid electrode Na–S batteries because of safety concerns. However, the major issues for solid-state Na batteries are a high interfacial resistance between solid electrolytes and electrodes, and Na dendrite growth. Here we report that a yttria-stabilized zirconia (YSZ)-enhanced beta-alumina solid electrolyte (YSZ@BASE) has an extremely low interface impedance of 3.6 Ω cm2 with the Na metal anode at 80 °C, and also exhibits an extremely high critical current density of ~7.0 mA cm–2 compared with those of other Li- and Na-ion solid electrolytes reported so far. With a trace amount of eutectic NaFSI–KFSI molten salt at the electrolyte/cathode interface, a quasi-solid-state Na/YSZ@BASE/NaNi0.45Cu0.05Mn0.4Ti0.1O2 full cell achieves a high capacity of 110 mAh g–1 with a Coulombic efficiency >99.99% and retains 73% of the cell capacity over 500 cycles at 4C and 80 °C. Extensive characterizations and theoretical calculations prove that the stable β-NaAlO2-rich solid–electrolyte interphase and strong YSZ support matrix play a critical role in suppressing the Na dendrite as they maintain robust interfacial contacts, lower electronic conduction and prevent the continual reduction of BASE through oxygen-ion compensation.