Investigating Aliovalent Sn(+IV) and Isovalent P(+V) Substitution Series in Li₃SbS₄

An in-depth understanding of Li⁺ migration within solid electrolytes is fundamentally needed to develop new material classes and to enhance the existing ones. The thio-LISICON family exhibits three distinct ordering types of the non-lithium polyhedra leading to three different polymorphs, namely <i>α</i>-, <i>β</i>- and <i>γ</i>-Li₃PS₄.^1,2 These show completely different Li⁺ substructures and migration pathways. Depending on the non-lithium cation either the <i>β</i>- or <i>γ</i>-polymorph is preferred synthetically. Li₃SbS₄ crystallizes in the <i>γ</i>-polymorph, which has the most unpropitious Li⁺ substructure for fast Li⁺ motion in comparison to the <i>α</i>-, <i>β</i>- and <i>γ</i>-polymorph. Nevertheless, the aliovalent Li_3+<i>x</i>Sb_1−<i>x</i>Sn<i>_x</i>S₄ (0 ≤ <i>x</i> ≤ 0.2) and isovalent Li₃Sb_1−<i>x</i>P<i>_x</i>S₄(0 ≤ <i>x</i> ≤ 0.5) substitution series were prepared, and the ionic conductivity increased from 9.2×10⁻¹¹ S×cm⁻¹ to 2.9×10⁻⁶ S×cm⁻¹ and 4.9×10⁻⁷ S×cm⁻¹ for the highest substitution degree, respectively. Remarkably, the conductivity is three times faster improved for the Sn(+IV) compared to the P(+V) substitution implying that more Li⁺ are beneficial for faster ionic transport within this system. However, as these materials are bad ionic conductors at low substitution degree, the defect formation energy might be the dominating factor for easier ionic transport.<br/><br/>References.<br/>(1) Homma, K.; Yonemura, M.; Kobayashi, T.; Nagao, M.; Hirayama, M.; Kanno, R. Crystal Structure and Phase Transitions of the Lithium Ionic Conductor Li₃PS₄. <i>Solid State Ionics</i> <b>2011</b>, <i>182</i> (1), 53–58.<br/>(2) Forrester, F. N.; Quirk, J. A.; Famprikis, T.; Dawson, J. A. Disentangling Cation and Anion Dynamics in Li₃PS₄ Solid Electrolytes. <i>Chem. Mater.</i> <b>2022</b>, <i>34</i> (23), 10561–10571.