Enhancing sodium ionic conductivity in tetragonal-Na3PS4 by halogen doping: a first principles investigation

Abstract

Tetragonal Na₃PS₄ (t-Na₃PS₄) has been demonstrated as a very promising candidate for a solid-state sodium-ion electrolyte with high Na ionic conductivity at ambient temperature. In this paper, we systematically investigated the Na ionic conductivity in pristine and halogen (F, Cl, Br, and I) doped tetragonal-Na₃PS₄ superionic conductors using first-principles calculations. The Na ionic conductivity of pristine t-Na₃PS₄ is calculated to be about 0.01 mS cm⁻¹, while much higher Na ionic conductivities could be achieved by introducing Na ion vacancies via a halogen doping strategy. The calculated Na ionic conductivity of t-Na₃PS₄ doped with 1.56% Cl is 1.07 mS cm⁻¹ at ambient temperature. Among different halogen-doped t-Na₃PS₄, Br-doped t-Na₃PS₄ shows the lowest activation energy and the highest Na ionic conductivity, which reaches 2.37 mS cm⁻¹ at 300 K. The low activation energy and high Na ionic conductivity in Br-doped t-Na₃PS₄ are due to a relatively lower defect binding energy of the defect pair of halogen substitution and a Na ion vacancy. Our results suggest Br-doped t-Na₃PS₄ may serve as a very promising Na-ion solid-state superionic conductor.