First-principles simulations of Li boracites Li4B7O12Cl and Li5B7O12.5Cl

Abstract

Lithium boracite crystals have been identified as promising ion conductors for possible use in all-solid-state batteries. With the help of first-principles modeling techniques, we are able to show that these materials have structures with natural interstitial sites which play important roles in Li ion migration processes. The arrangements of these natural interstitial sites follow from group theory analyses of the computed and experimentally analyzed structures. Specifically, the low-temperature $\ensuremath{\alpha}$ phase of ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ is computed to have the face-centered rhombohedral $R3c$ structure which is closely related to an ideal face-centered cubic $F\overline{4}3c$ structure with 24 natural interstitial sites per conventional unit cell. Li ion diffusion in this material is found to proceed largely by a concerted motion involving these interstitial sites and two neighboring host lattice sites, consistent with the large measured ionic conductivity of this material. Adding one addition ${\mathrm{Li}}_{2}\mathrm{O}$ cluster per formula unit to ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ forms ${\mathrm{Li}}_{5}{\mathrm{B}}_{7}{\mathrm{O}}_{12.5}\mathrm{Cl}$, which crystallizes in the face-centered cubic $F23$ structure, a subgroup of the $F\overline{4}3c$ structure. While these $F23$ crystals have 16 natural interstitial sites per conventional unit cell, their distribution is such that they do not participate in Li ion diffusion mechanisms, as is consistent with the negligible ionic conductivity measured for this material. Phonon spectral analyses of ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ in its $R3c$ structure and ${\mathrm{Li}}_{5}{\mathrm{B}}_{7}{\mathrm{O}}_{12.5}\mathrm{Cl}$ in its $F23$ structure show that both crystals are dynamically stable. Chemical stability of these materials is indicated by convex hull analysis of ${\mathrm{Li}}_{4}{\mathrm{B}}_{7}{\mathrm{O}}_{12}\mathrm{Cl}$ and ${\mathrm{Li}}_{5}{\mathrm{B}}_{7}{\mathrm{O}}_{12.5}\mathrm{Cl}$ with respect to their building blocks of LiCl, ${\mathrm{Li}}_{2}\mathrm{O}$, and ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$.