Closo-borates and closo-carborates recently emerged as promising class of solid electrolytes for all-solid-state batteries, but are too expensive. Here we introduce NaB₁₁H₁₄nido-borates as a new building block for solid electrolytes that can be synthesized via a cost-effective low-temperature method using cheap NaBH₄ as 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-NaB₁₁H₁₄ with closo-Na₂B₁₂H₁₂ into cubic unit cells. Importantly, Na₂B₁₂H₁₂ can also be synthesized cost-effectively from NaBH₄. Crystal structures are solved and refined by synchrotron X-ray diffraction and density functional theory. Cyclic voltammetry reveals an 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 μA/cm². We also discuss how our results can be translated to render lithium solid electrolytes economically viable.