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

Helm, Bianca; Strotmann, Kyra; Böger, Thorben; Banik, Ananya; Lange, Martin Alexander; Li, Yuheng; Li, Cheng; Canepa, Pieremanuele; Zeier, Wolfgang G.

Abstract

The search for highly conducting Li+ solid electro-lytes focuses on sulfide- and halide-based materials, where typicallythe strongly atomic disordered materials are the most promising.The atomic disorder corresponds to a flattened energy landscapehaving similar relative site energies for different Li+ positionsfacilitating motion. In addition, the highly disordered Li+conductors have negligible defect formation energy as movingcharges are readily available. This work investigates the isovalentLi3Sb1−xPxS4 (0 ≤ x ≤ 0.5) and the aliovalent Li3+xSb1−xSnxS4 (0 ≤x ≤ 0.2) substitution series of thio-LISICON materials by using X-ray diffraction, high-resolution neutron diffraction, impedancespectroscopy, and defect calculations. The starting compositionLi3SbS4 has a low ionic conductivity of ∼10−11 S·cm−1 and bothsubstituents improve the ionic conductivity strongly by up to 4 orders of magnitude. On the one hand, in substituted Li3SbS4structures, 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 defectcalculations, displaying significantly reduced defect formation energy upon substitution. Here, we show within two differentsubstitution series that the defect formation energy plays a major role for ionic motion in this class of thio-LISICON materials.