Li₃BO₃ and Li₃BN₂: Computational study of structural and electrolyte properties of pure and doped crystals

Structural and electrolyte properties of crystalline Li₃BO₃ and Li₃BN₂ were investigated using first-principles modeling techniques. Literature reports conclude that both materials have measurable ionic conductivity in their monoclinic crystalline forms (P2₁/c). From molecular dynamics and nudged elastic band simulations, Li-ion migration was found to most likely proceed via vacancy mechanisms. To enhance the ionic conductivity by increasing vacancy concentrations, we computationally substituted F for O in Li₃BO₃ and B for C in β-Li₃BN₂, finding encouraging results in both cases. For Li₃BN₂, additional considerations were identified due to the existence of multiple crystalline forms and instability. The reported tetragonal phase of α-Li₃BN₂ was found to be unstable as evidenced by imaginary phonon modes near the M point of its Brillouin zone. Our simulations suggested that the real α phase has an orthorhombic structure formed with twice as many formula units and very small adjustments of the fractional coordinates compared with the original analysis. Quasiharmonic phonon analysis of the orthorhombic structure at elevated temperature is in good agreement with experimental room temperature X-ray patterns reported in the literature.