Promotion mechanisms of LiBH4 dehydrogenation dominated by charge redistribution

Abstract

LiBH₄ is a promising solid-state hydrogen storage material, but its complex dehydrogenation reaction mechanism severely hinders the regulation of dehydrogenation barriers and reversibility. To elucidate the micro-mechanism of its dehydrogenation reaction, the potential of mean force method, ab initio molecular dynamics simulations, and the electronic structure through density functional theory with Grimme D3 dispersion corrections have been conducted for different surfaces of LiBH₄. The dehydrogenation barriers of various surfaces typically fall within the range of 110.84–122.96 kJ mol⁻¹. The dehydrogenation barrier can be remarkably reduced from 110.84 to 70.6 kJ mol⁻¹, as the concentration of Li vacancies varies from 0 to 12.5%. Besides, doping transition metals (TMs) can effectively reduce the dehydrogenation barrier of LiBH₄ (74.28–104.06 kJ mol⁻¹). The existence of Li vacancies results in the loss of electrons in the shared electron pairs of B–H bonds and weakens the strength between B–H covalent bonds. The TMs doping makes the B–H bonds gain electrons, which occupy the antibonding orbitals and result in weakening the B–H bond strength. Consequently, the dehydrogenation barriers of LiBH₄ are significantly reduced. This work reveals two different promotion mechanisms for Li vacancies and TMs doping, and provides a new perspective for lowering its dehydrogenation temperature in future experiments.