Crystal structure, stability and Li superionic conductivity of pyrochlore-type solid electrolyte Li2−xLa(1+x)/3Nb2O6F: a first-principles calculation study

Abstract

Recently, a new oxide-type solid electrolyte (SE) for all-solid-state Li ion batteries, pyrochlore-type Li_2−xLa_(1+x)/3Nb₂O₆F (LLNOF), was reported to exhibit a Li⁺ superionic conductivity of 3.9 × 10⁻³ S cm⁻¹ at room temperature. To gain understanding of this novel SE, in this work, thermodynamic analysis and ab initio molecular dynamics (AIMD) calculations based on the density functional theory (DFT) framework were performed to clarify the material's phase stability, electrochemical stability and Li⁺ ion transport properties. LLNOF is predicted to be a metastable phase, and thus, its electrochemical window is likely to be determined by its decomposition phases. The Li transport pathway is revealed to be defined by the large hexagonal tunnels formed by corner-sharing NbO₆ units. These tunnels can either be La-free or La-blocked depending on the Li/La/vacancy configuration. The Li⁺ ion conduction mechanism proceeds in a concerted migration manner in the 16d sites via an intermediate site. Analysis of phonon density of states shows that F atoms have a lower phonon-energy band center position than O atoms; this is correlated with the characteristically low phonon-energy band center position of Li atoms and the observed Li⁺ superionic conductivity of LLNOF.