Structure and defect chemistry of low- and high-temperature phases of LiBH<inf>4</inf>

Abstract

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 LiBH4. 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 P63mc structure is the most favorable high-T structure. Furthermore, we discover a new monoclinic C2/c structure for the high-temperature phase which is competitive in stability with the P63mc structure. Both high-temperature P63mc and C2/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. © 2012 American Chemical Society.