All-solid-state sodium batteries are potentially game-changer devices in the transition towards a fossil-fuel-free society. They combine better sustainability, switching from lithium, to the largely available sodium with enhanced safety, replacing the flammable liquid electrolyte by a more thermally stable solid material. Nonetheless, several challenges plague the development of highly performing cells, such as the electrochemical stability of the solid electrolyte vs. both sodium and positive electrode, and the fragile interfacial contact between the components of the cell. On the basis of a Na+ conductor recently developed by our group, Na4(CB11H12)2(B12H12), herein we report its superior performance as solid electrolyte in room-temperature-working batteries. This carbacloso-closohydroborate-based material shows stable Na plating/stripping with limited polarization. Galvanostatic tests in Na||NaCrO2 cells exhibit good electrochemical performance, owing to a specific capacity around 100 mAh g−1 up to a delivered current density of 24 mA g−1. These findings represent a step further towards viable Na-based solid-state batteries operating safely while keeping elevate performance even at room temperature.