Defect-Mediated Conductivity Enhancements in Na3-xPn1-xWxS4 (Pn = P, Sb) using Aliovalent Substitutions

The sodium-ion conducting family of Na₃PnS₄, with Pn = P, Sb, have gained interest for the use in solid-state batteries due to their high ionic conductivity. However, significant improvements to the conductivity have been hampered by the lack of aliovalent dopants that can introduce vacancies into the structure. Inspired by the need for vacancy introduction into Na₃PnS₄, the solid solutions with WS₄^2- introduction are explored. The influence of the substitution with WS₄^2- for PS₄^3- and SbS₄^3-, respectively, is monitored using a combination of X-ray diffraction, Raman and impedance spectroscopy. With increasing vacancy concentration improvements resulting in a very high ionic conductivity of 13 ± 3 mS·cm^-1 for Na_2.9P_0.9W_0.1S₄ and 41 ± 8 mS·cm^-1 for Na_2.9Sb_0.9W_0.1S₄ can be observed. This work acts as a stepping-stone towards further engineering of ionic conductors using vacancy-injection via aliovalent substituents.