Recently, sodium-metal halide (Na-MH, or also called ZEBRA) batteries have been extensively investigated due to its potential application for large scale energy storage systems. Particularly, compared to the conventional high temperature (> 280^oC) ZEBRA batteries, a Na-MH battery operated at intermediate temperature (IT< 200^oC) presents several advantages such as longer cycle life, lower battery manufacturing cost, and easier thermal management, etc. b″-alumina (Na₂O×5.33 Al₂O₃) is typically used for IT Na-MH batteries as Na ion conducting solid-state electrolytes. Thus, the fabricating a thinner b″-alumina solid-state electrolytes (BASE) that results in a high Na ion conductivity is critical to improve performances of IT Na-MH batteries. In here, we will present a new fabrication process for the BASE with a double-layer structure that can drastically reduce the thickness of the BASE. We have examined the conductivity and morphology of new BASEs using electrochemical impedance spectroscopy and SEM. The IT Na-MH batteries assembled using new BASEs are also tested up to 300 cycles, and show excellent energy density (~380 Wh/kg) and around trip efficiency (~95%).