Visualization of Solid-State Synthesis for Chalcogenide Na Superionic Conductors by in-situ Neutron Diffraction

Chalcogenide superionic sodium (Na) conductors have great potential as solid electrolytes (SEs) in all-solid-state Na batteries with advantages of high energy density, safety, and cost effectiveness. The crystal structures and ionically conductive properties of solid Na-ion conductors are strongly influenced by synthetic approaches and processing parameters. Thus, understanding the synthesis process is essential to control the structures and phases and to obtain Na-ion conductors with desirable properties. Thanks to the high-flux and deep-penetrating time-of-flight neutron diffraction (ND), in-situ experiments were able to track real-time structural changes of two chalcogenide SEs (Na₃ SbS₄ and Na₃ SbS_3.5 Se_0.5 ) during the solid-state synthesis. For these two conductors, the ND results revealed a fast one-step reaction for the synthesis and the molten process when heating up, and the recrystallization as well as the cubic-to-tetragonal phase transition up on cooling. Moreover, Se-doping was found to influence the reaction temperatures, lattice parameter, and structure stability based on neutron experimental observations and theoretical simulation. This work presents a detailed structural study using in-situ ND technology for the solid synthesis process of chalcogenide Na-ion conductors, beneficial for the design and synthesis of new solid-state conductors.