Entropy-Driven Disorder and Aliovalent Substitution Induce Defects in Na₃PnS₄ (Pn = P, As, Sb) Solid-State Electrolytes: A Sluice Gate for Sodium Ions

Sodium-containing chalcogenides are attractive candidates for use as solid-state electrolytes; however, their ionic conductivities remain a challenge. Simultaneously applying isovalent and aliovalent substitution can enhance ionic conductivity by generating substantial site disorder and high vacancy concentrations. To elucidate the mechanism that facilitates sodium ion conduction, a series of mixed-pnicogen solid solutions were prepared from the parent ternary sulfides Na 3 Pn S 4 ( Pn = P, As, Sb) by high-temperature reactions, including an entropy-driven W-substituted phase, Na 3−δ P 0.32 As 0.32 Sb 0.32 W 0.04 S 4 (N-PASS-W). N-PASS-W exhibits a very high ionic conductivity of 10 mS cm –1 and a low activation energy of 0.15 eV. Using PXRD and NMR spectroscopy, an atomic-level model for N-PASS-W was proposed, in which ion hopping occurs over two Na sites within a tetragonal structure ( P 4̅2 1 c ). Relationships were also established between the structure and ionic conductivities of the other members to evaluate the influence of crystalline phase, cation size, and site disorder.