LiBH_4 NH_3BH_3: A new lithium borohydride ammonia borane compound with a novel structure and favorable hydrogen storage properties

摘要

Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio results in the formation of a new complex, LiBH_4NH_3BH_3. Its structure was successfully determined using combined X-ray diffraction and first-principles calculations. UBH_4NH_3BH_3 was carefully studied in terms of its decomposition behavior and reversible dehydrogenation property, particularly in comparison with the component phases. In parallel to the property examination, X-ray diffraction and Fourier transformation infrared spectroscopy techniques were employed to monitor the phase evolution and bonding structure changes in the reaction process. Our study found that LiBH_4-NH_3BH_3 first disproportionates into (LiBH_4)_2NH_3BH_3 and NH_3BH_3, and the resulting mixture exhibits a three-step decomposition behavior upon heating to 450 ℃, totally yielding ～15.7 wt% hydrogen. Interestingly, it was found that h-BN was formed at such a moderate temperature. And owing to the in situ formation of h-BN, LiBH_4NH_3BH_3 exhibits significantly improved reversible dehydrogenation properties in comparison with the LiBH_4 phase.