Computational study of Li 3 BO 3 and Li 3 BN 2 II: Stability analysis of pure phases and of model interfaces with Li anodes

Both ${\mathrm{Li}}_{3}{\mathrm{BO}}_{3}$ and ${\mathrm{Li}}_{3}{\mathrm{BN}}_{2}$ materials have promising properties for use in all solid-state batteries and other technologies dependent on electrolytes with significant ionic conductivity. As the second of a two-part study, the structural properties of ${\mathrm{Li}}_{3}{\mathrm{BO}}_{3}$ and three reported phases of ${\mathrm{Li}}_{3}{\mathrm{BN}}_{2}$ are investigated using first-principles modeling techniques. For $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{BN}}_{2}$, the tetragonal $P{4}_{2}/mnm$ structure reported in the literature is found to be unstable as evidenced by imaginary phonon modes near the M point of its Brillouin zone. Our simulations within the harmonic approximation suggest that the real $\ensuremath{\alpha}$ phase has the orthorhombic space group symmetry $Pmmn$ formed with twice as many formula units and tiny adjustments of the equivalent lattice parameters and fractional coordinates. Extending the analysis of the $Pmmn\phantom{\rule{4pt}{0ex}}\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{BN}}_{2}$ structure to the quasiharmonic approximation improves the agreement between the room-temperature x-ray pattern reported in the literature and the corresponding simulation results. In anticipation of the use of the monoclinic phases of ${\mathrm{Li}}_{3}{\mathrm{BO}}_{3}$ and ${\mathrm{Li}}_{3}{\mathrm{BN}}_{2}$ in Li ion conducting applications, chemical stability is investigated in terms of free-energy differences of possible decomposition and Li reaction processes, finding encouraging results. As further investigations of ${\mathrm{Li}}_{3}{\mathrm{BO}}_{3}$ and $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{BN}}_{2}$ as electrolyte or coating materials, particularly for use with Li metal anodes, idealized electrolyte/Li interfaces were investigated in terms of their geometric, energetic, and electronic properties. The results find the electrolyte/Li interfaces to be quite favorable, perhaps comparable to the pioneering LiPON/Li system.