Finding the Right Blend: Interplay Between Structure and Sodium Ion Conductivity in the System Na₅AlS₄-Na₄SiS₄

The rational design of high performance sodium solid electrolytes is one of the key challenges in modern battery research. In this work, we identify new sodium ion conductors in the substitution series Na_5- Al_1- Si S4 (0 ≤ x ≤ 1), which are entirely based on earth-abundant elements. These compounds exhibit conductivities ranging from 1.64 · 10^-7 for Na₄SiS₄ to 2.04 · 10^-5 for Na_8.5(AlS₄)_0.5(SiS₄)_1.5 (x = 0.75). We determined the crystal structures of the Na⁺-ion conductors Na₄SiS₄ as well as hitherto unknown Na₅AlS₄ and Na₉(AlS₄)(SiS₄). Na⁺-ion conduction pathways were calculated by bond valence energy landscape (BVEL) calculations for all new structures highlighting the influence of the local coordination symmetry of sodium ions on the energy landscape within this family. Our findings show that the interplay of charge carrier concentration and low site symmetry of sodium ions can enhance the conductivity by several orders of magnitude.