Structure and Defect Chemistry of Low- and High-Temperature Phases of LiBH₄

Density functional theory simulations were performed to resolve the recent controversy regarding the crystal structure of the low-temperature and high-temperature phases of the hydrogen storage and Li ion electrolyte material LiBH 4 . It is demonstrated that, independent of hybrid free exchange-correlation functional, the experimentally proposed orthorhombic Pnma structure is energetically the most favorable low- T structure while the experimentally proposed hexagonal P 6 3 mc structure is the most favorable high- T structure. Furthermore, we discover a new monoclinic C 2/ c structure for the high-temperature phase which is competitive in stability with the P 6 3 mc structure. Both high-temperature P 6 3 mc and C 2/ c structures are stable against dehydrogenation and formation of Li vacancies. For all high-temperature structures, the formation energy of the Li Frenkel defect is low at 0.50–0.90 eV, with major implications for Li ion transport properties.