Reducing the Defect Formation Energy by Aliovalent Sn(+IV) and Isovalent P(+V) Substitution in Li₃SbS₄ Promotes Li⁺ Transport

The search for highly conducting Li⁺ solid electrolytes focuses on sulfide- and halide-based materials, where typically the strongly atomic disordered materials are the most promising. The atomic disorder corresponds to a flattened energy landscape having similar relative site energies for different Li⁺ positions facilitating motion. In addition, the highly disordered Li⁺ conductors have negligible defect formation energy as moving charges are readily available. This work investigates the isovalent Li₃Sb_1-xP_xS₄ (0 ≤ x ≤ 0.5) and the aliovalent Li_3+xSb_1-xSn_xS₄ (0 ≤ x ≤ 0.2) substitution series of thio-LISICON materials by using X-ray diffraction, high-resolution neutron diffraction, impedance spectroscopy, and defect calculations. The starting composition Li₃SbS₄ has a low ionic conductivity of ∼10^-11 S·cm^-1 and both substituents improve the ionic conductivity strongly by up to 4 orders of magnitude. On the one hand, in substituted Li₃SbS₄ structures, only minor structural changes are observed which cannot sufficiently explain the significant impact on the Li⁺ conductivity. On the other hand, the Li⁺ carrier density reveals a correlation to the activation energy and first-principles defect calculations, displaying significantly reduced defect formation energy upon substitution. Here, we show within two different substitution series that the defect formation energy plays a major role for ionic motion in this class of thio-LISICON materials.