Computational Study Of Li3bo3 And Li3bn2 Ii: Stability Analysis Of Pure Phases And Of Model Interfaces With Li Anodes

Both Li3BO3 and Li3BN2 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 Li3BO3 and three reported phases of Li3BN2 are investigated using first-principles modeling techniques. For alpha-Li3BN2, the tetragonal P4(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 a 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 alpha-Li3BN2 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 Li3BO3 and Li3BN2 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 Li3BO3 and beta-Li3BN2 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.