Closo-borates and carba-closo-borates recently emerged as a promising class of solid electrolytes for all-solid-state batteries, but are too expensive. Here we introduce NaB 11 H 14 nido-borates as a new building block for solid electrolytes that can be synthesized via a costeffective low-temperature method using cheap NaBH 4 as a chemical feedstock. We employ 2D nuclear magnetic resonance techniques to analyze the structure of this new building block. High ionic conductivity of up to 4 mS/cm at 20 C is achieved by combining nido-NaB 11 H 14 with closo-Na 2 B 12 H 12 into cubic unit cells. Importantly, Na 2 B 12 H 12 can also be synthesized costeffectively from NaBH 4. Crystal structures are solved and refined by synchrotron X-ray diffraction and density functional theory. Cyclic voltammetry reveals electrochemical stability from 0.15 to 2.6 V vs Na/Na +. Galvanostatic cycling in symmetrical cells with sodium metal electrodes shows only a small overpotential increase from 2 mV to 2.5 mV after 400 cycles at 50 A/cm 2. We also discuss how our results can be translated to render lithium solid electrolytes economically viable.