Metal dodecaborates, particularly lithium dodecaborate (Li₂B₁₂H₁₂), are promising ionic conductors, but their broader application is hindered by complex synthesis. Here, we report a facile solvothermal synthesis of Li₂B₁₂H₁₂ via the reaction of lithium borohydride (LiBH₄) with borane dimethyl sulfide complex (DMS·BH₃) in glyme solvents. This synthesis can be conveniently performed either in a Schlenk flask (with or without reflux) or in an autoclave, demonstrating high yields (up to 96%) and excellent purity. The enclosed system provided by the autoclave was shown to be more favorable for the synthesis of the B₁₂H₁₂^2- anion. A detailed mechanistic investigation utilizing ¹¹B nuclear magnetic resonance (¹¹B NMR) spectroscopy revealed the stepwise formation of B₂H₇^-, B₃H₈^-, B₉H₁₄^-, B₁₁H₁₄^-, and B₁₁H₁₃^2- intermediates. This synthetic strategy was successfully extended to other alkali metal dodecaborates (Na, K), and their glyme-coordinated complexes were characterized by single-crystal X-ray diffraction. Furthermore, we introduce a solvent-exchange approach using weakly coordinating solvents such as dimethyl sulfoxide (DMSO) or water, enabling simple and efficient desolvation, thereby offering a practical new approach to obtain anhydrous metal dodecaborates.