Na-based hydroborates, such as NaCB9H10, Na2B10H10, NaCB11H12, and Na2B12H12, are known for their high ionic conductivity above a phase transition, usually far from room temperature. Here, a systematic investigation to suppress this phase transition and thus to stabilize novel crystal structures is conducted by means of anion mixing. The obtained compounds crystallize either with hexagonal and cubic close packed (hcp and ccp) or body-centered cubic (bcc) arrangement of anions without polymorphic evolution in the temperature range 100 < T < 700 K. The high symmetry and the configurational ionic disorder, which characterize all crystals, give room-temperature Na+ conductivity close or above 1 mS cm−1. Finally, high oxidative electrochemical stability between 3 and 4 V versus Na+/Na results from the aromatic nature of the boron cage, making Na-based hydroborates potentially suitable as efficient solid electrolytes for next-generation, high-voltage, Na-ion batteries.