Defect-Mediated Conductivity Enhancements in Na 3– x Pn 1– x W x S ₄ (Pn = P, Sb) Using Aliovalent Substitutions

The sodium-ion conducting family of Na 3 PnS 4, with Pn = P, Sb, has 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 Na 3 PnS 4, the solid solutions with WS 4 2– introduction are explored. The influence of the substitution with WS 4 2– for PS 4 3– and SbS 4 3– 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 2.9 P 0.9 W 0.1 S 4 and 41 ± 8 mS·cm –1 for Na 2.9 Sb 0.9 W 0.1 S 4 can be observed. This work acts as a stepping-stone toward further engineering of ionic conductors using vacancy injection via aliovalent substituents.