Crystal Structures and Energy Storage Properties of Ammine Sodium Decahydro-closo-decaboranes (Na₂B₁₀H₁₀·nNH₃, n = 1, 2)

Metal closo-boranes show remarkable thermal and chemical stabilities, making them appealing candidates for a wide range of applications, such as electrolytes in electrochemical batteries and ammonia storage for indirect hydrogen storage. Furthermore, owing to the large size and nonspherical geometry of the anion (e.g., B₁₀H₁₀^2– or B₁₂H₁₂^2–), metal closo-boranes display a rich structural diversity and thermal polymorphism. Here, we present the synthesis, characterization, and structural determination of the ammoniated metal closo-boranes, Na₂B₁₀H₁₀·nNH₃ (n = 1, 2). The thermal decomposition of Na₂B₁₀H₁₀·2NH₃ was investigated with synchrotron radiation in situ powder X-ray diffraction and simultaneous thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry, revealing a reversible ammonia storage capacity of 15 wt % below 150 °C. Additionally, ionic conductivities of 2.7 × 10^–8 (RT) and 4.7 × 10^–8 S/cm (30 °C) for Na₂B₁₀H₁₀·2NH₃ and Na₂B₁₀H₁₀·NH₃, respectively, were measured with electrochemical impedance spectroscopy. A lower Na⁺ conductivity compared to the parent compound, Na₂B₁₀H₁₀, is explained by an anchoring effect of ammonia in the rigid framework of the B₁₀H₁₀^2–-anions.