All‐solid‐state batteries (ASSBs) promise higher power and energy density than batteries based on liquid electrolytes. Recently, a stable 3 V ASSB based on the super ionic conductor (1 mS cm−1 near room temperature) Na4(B12H12)(B10H10) has demonstrated excellent cycling stability. This study concerns the development of a five‐step, scalable, and solution‐based synthesis of Na4(B12H12)(B10H10). The use of a wet chemistry approach allows solution processing with high throughput and addresses the main drawbacks for this technology, specifically, the limited electrode–electrolyte contact and high cost. Moreover, a cost‐efficient synthesis of the expensive precursors Na2B10H10 and Na2B12H12 is also achieved through the same process. The mechanism of the reactions is investigated and two key parameters to tune the kinetics and selectivity are highlighted: the choice of counter cation (tetraethylammonium) and solvent.