Na wetting in a sunny-side-up shape is observed on a β′′-alumina surface decorated with micron-sized lead spherical particles.

a molten Na anode and β″-alumina solidstate electrolyte (BASE). Despite their similarities, sodium-sulfur (Na-S) and sodium-metal halide (Na-MH) batteries, which are the two main analogs of NBBs, are quite different in their chemistries and the cell operating conditions. Typically, a Na-S battery contains molten sulfur/ polysulfide cathodes that require an operating temperature range of 300 to 350 °C to retain the molten state of the electrode materials to facilitate the electrochemical reactions. [3] On the other hand, a conventional Na-MH battery (ZEBRA) contains metal halide based cathodes (e.g., NiCl 2 , FeCl 2 , etc.), a secondary electrolyte (NaAlCl 4 ), and operates at a temperature near 280 °C. [4,5] Intrinsically, the safer cathode chemistries in Na-MH batteries are achieved from using secondary electrolytes along with the inherent higher open-circuit voltage, the lower operating temperature, and easier cell assembly at the discharge state. [6] In fact, the sodium-nickel chloride (Na-NiCl 2 ) chemistry has been extensively investigated over the past few decades. [7][8][9][10][11] The overall redox reaction of a Na-NiCl 2 battery can be described as follows [7,12]