Fast-ionic conductivity of Li + in LiBH 4

High Li${}^{+}$ conductivity in the high-temperature (hexagonal) phase of LiBH${}_{4}$ is revealed through a first-principles molecular dynamics simulation of 1200 atoms using periodic boundary conditions. The high ionic conductivity originates from the generation of a Li${}^{+}$ metastable state located at an interstitial site surrounded by three Li${}^{+}$ ions and three BH${}_{4}^{\ensuremath{-}}$ ions in the $a$-$b$ plane. A defect is created by Li${}^{+}$ ions hopping from their original sites to this interstitial site. The defect then diffuses through a path connecting the nearby Li sites separated in the $a$ and $c$ directions. Coupling of these movements is observed. The double splitting of the Li occupation in the original Li site plays an important role in the creation of the metastable state and migration through the connection path. The activation energy and diffusion coefficient are estimated, and are within one order of magnitude of experimentally retrieved values.