Direct Solution-Based Synthesis of Na₄ (B₁₂ H₁₂ )(B₁₀ H₁₀ ) Solid Electrolyte

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 (B₁₂ H₁₂ )(B₁₀ H₁₀ ) has demonstrated excellent cycling stability. This study concerns the development of a five-step, scalable, and solution-based synthesis of Na₄ (B₁₂ H₁₂ )(B₁₀ H₁₀ ). 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 Na₂ B₁₀ H₁₀ and Na₂ B₁₂ H₁₂ 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.