Abnormally Low Activation Energy in Cubic Na₃SbS₄ Superionic Conductors

Inorganic Na-ion superionic conductors play a vital role in all-solid-state Na batteries that operate at room temperature. Sodium thioantimonate (Na3SbS4), a popular sulfide-based solid electrolyte, has attracted serious attention due to its advantages of high ionic conductivity at room temperature and impressive chemical stability under ambient conditions. Much research detailing Na3SbS4 focused on its synthetic approaches and interfacial stability against Na metal, yet, there is limited information elucidating a fundamental understanding of the Na-ion diffusion mechanisms in Na3SbS4 with different crystal structures (e.g., tetragonal and cubic). Herein, we combine real-time electrochemical impedance measurements with theoretical simulations based on density functional theory and in situ quasi-elastic neutron scattering to study the Na-ion conductive properties of Na3SbS4 during its phase transition from a tetragonal to cubic structure. Although there is a slight change in the lattice parameters, the energy barrier for Na-ion diffusion in the tetragonal structure was determined to be much larger (5–10 times) than that in the cubic structure from both theoretical and experimental perspectives. The high degree of symmetry in cubic Na3SbS4 leads to less interatomic correlations between Na and S(Sb) atoms, a shorter jump distance (2.85 Å), and a larger diffusion coefficient. This research provides insight into understanding the Na-ion diffusion in solid electrolytes with phase transitions and provides fundamental guidance for designing novel solid-state Na-ion conductors.