First-Principles Calculation Study of Na+ Superionic Conduction Mechanism in W- and Mo-Doped Na3SbS4 Solid Electrolytes

Abstract The guiding principle for the design of inorganic compounds with high ionic conductivity has been extensively sought to realize next-generation all-solid-state batteries (ASSBs). Recently, a sulfide-type Na+ ion conductor, cubic Na3SbS4 with W doping (Na2.88Sb0.88W0.12S4), was reported with unprecedentedly high ionic conductivity of 3.2 × 10–2 S cm–1, making it now a champion solid electrolyte for Na-ASSB (A. Hayashi et al., Nat. Commun. 2019, 10, 5266). Herein, density functional theory molecular dynamics (DFT-MD) calculations were performed for pristine, W-doped, and Mo-doped Na3SbS4 to examine the ionic conduction mechanism (Boltzmann factor vs prefactor) and the aliovalent cation dopant effects in Na3SbS4. We showed that Na vacancies induced by cation doping play crucial roles in superionic conductivity, while the diffusion process is rather characterized by the concerted motion of Na+ ions. A comparison between the two dopants, Mo6+ and W6+, revealed that the conductivity enhancement can be primarily explained by a decrease of Na+ ion activation energy, which is found to be strongly correlated to the enlargement of Na Wyckoff site cages brought upon by the smaller WS4/MoS4 tetrahedral volume relative to the host SbS4 volume. This descriptor of the pathway free volume can suggest a general guiding principle for superionic conduction that can be applied to other cations, in addition to explaining the superior performance of W-doped Na3SbS4.