Anionic Sublattices in Halide Solid Electrolytes: A Case Study with the High‐Pressure Phase of Li3ScCl6

Abstract The Li 3 MX 6 compounds ( M =Sc, Y, In; X =Cl, Br) are known as promising ionic conductors due to their compatibility with typical metal oxide cathode materials. In this study, we have successfully synthesized γ‐Li 3 ScCl 6 using high pressure for the first time in this family. Structural analysis revealed that the high‐pressure polymorph crystallizes in the polar and chiral space group P 6 3 mc with hexagonal close‐packing (hcp) of anions, unlike the ambient‐pressure α‐Li 3 ScCl 6 and its spinel analog with cubic closed packing (ccp) of anions. Investigation of the known Li 3 MX 6 family further revealed that the cation/anion radius ratio, r M / r X , is the factor that determines which anion sublattice is formed and that in γ‐Li 3 ScCl 6 , the difference in compressibility between Sc and Cl exceeds the ccp r M / r X threshold under pressure, enabling the ccp‐to‐hcp conversion. Electrochemical tests of γ‐Li 3 ScCl 6 demonstrate improved electrochemical reduction stability. These findings open up new avenues and design principles for lithium solid electrolytes, enabling routes for materials exploration and tuning electrochemical stability without compositional changes or the use of coatings.