DFT-based ab initio study of electronic band structure and elastic properties of Li2B3O4F3 and Li2B6O9F2 crystals Original Research Article

An ab initio DFT-based study of the electronic band structure and elastic properties of the new lithium ion conductors Li2B3O4F3 and Li2B6O9F2 has been performed. The hybrid functional B3LYP was employed with the CRYSTAL code, and the local density approximation (LDA) with the VASP code. Starting from the experimentally determined structures, both crystals were computationally optimized. The relaxed structures were found to be very close to the initial, experimental, ones. The band structure shows wide band gaps Eg, near 9–10 eV (B3LYP) or 6–7 eV (LDA), depending on the level of theory applied. The top valence bands of both crystals mainly derive from oxygen and fluorine, and the bottom conduction bands are mainly associated with boron and oxygen. An analysis of the elastic properties of the two compounds has been done, and all coefficients of the elastic stiffness tensor cij and the bulk modulus B are calculated. The interatomic distances, Mulliken charges and overlap populations are discussed from the viewpoint of lithium ion conductivity.