Hydroborate‐based solid electrolytes have recently been successfully employed in high voltage, room temperature all‐solid‐state sodium batteries. The transfer to analogous lithium systems has failed up to now due to the lower conductivity of the corresponding lithium compounds and their high cost. Here LiB11H14 nido‐hydroborate as a cost‐effective building block and its high‐purity synthesis is introduced. The crystal structures of anhydrous LiB11H14 as well as of LiB11H14‐based mixed‐anion solid electrolytes are solved and high ionic conductivities of 1.1 × 10−4 S cm−1 for Li2(B11H14)(CB11H12) and 1.1 × 10−3 S cm−1 for Li3(B11H14)(CB9H10)2 are obtained, respectively. LiB11H14 exhibits an oxidative stability limit of 2.6 V versus Li+/Li and the proposed decomposition products are discussed based on density functional theory calculations. Strategies are discussed to improve the stability of these compounds by modifying the chemical structure of the nido‐hydroborate cage. Galvanostatic cycling in symmetric cells with two lithium metal electrodes shows a small overpotential increase from 22.5 to 30 mV after 620 h (up to 0.5 mAh cm−2), demonstrating that the electrolyte is compatible with metallic anodes. Finally, the Li2(B11H14)(CB11H12) electrolyte is employed in a proof‐of‐concept half cell with a TiS2 cathode with a capacity retention of 82% after 150 cycles at C/5.