Exploring the Influence of Substitution on the Structure and Transport Properties in the Sodium Superionic Conductor Na11+xSn2+x(Sb1−yPy)1−xS12

Sulfidic sodium ion conductors are currently investigated for the possible use in all-solid-state sodium ion batteries. The design of high performing electrolytes in terms of temperature-dependent ionic transport is based upon the fundamental understanding of structure – transport relationships within the given structural phase boundaries inherent to the investigated materials class. In this work, the Na⁺ superionic structural family of Na₁₁Sn₂PS₁₂ is explored by using the systematic antimony substitution with phosphorous in Na_11+xSn_2+x(Sb_1-yP_y)_1-xS₁₂. A combination of Rietveld refinements against X-ray synchrotron diffraction data with electrochemical impedance spectroscopy is used to monitor the changes in the anionic framework, the Na⁺ substructure and the ionic transport. A new simplified descriptor for the average Na⁺ diffusion pathways, the average Na⁺ polyhedral volume is introduced, which is used to correlate the contraction of the overall lattice and the found activation barriers in the system. This study exemplifies how substitution affects diffusion pathways in ionic conductors and widens the knowledge about the related structural motifs and their influence on the ionic transport in this novel class of ionic conductors.