Cubic Na₃PS₄ (c-Na₃PS₄) is a highly promising sodium superionic conductor solid electrolyte that can enable safer and more energy dense all-solid-state sodium batteries. First reported by Hayashi et al.,¹ Na⁺ conductivities as high as 0.74 mS/cm have been achieved through improvements in synthesis and the addition of suitable dopants. In this work, we report a first-principles investigation of doping strategies to further enhance the Na⁺ conductivity of c-Na₃PS₄. We show that pristine c-Na₃PS₄ is an extremely poor Na ionic conductor, and the introduction of Na⁺ excess is the key to achieving reasonable Na⁺ conductivities observed in experiments. Using ab initio molecular dynamics simulations (AIMD), we predict that 6.25% Si-doped c-Na₃PS₄ has the Na⁺ conductivity of 1.66 mS/cm, in excellent agreement with previous experiment results (0.74 mS/cm). Remarkably, we find that Sn doping at the same concentration yields a much higher predicted Na⁺ conductivity of 10.7 mS/cm, though with a slightly higher dopant formation energy. We will also discuss the effect of other dopants and different doping concentrations on the Na⁺ conductivity. Finally, we will also discuss the role of topology and Na disorder in enabling fast ionic conduction in this material.

References:

[1] Hayashi, A.; Noi, K.; Sakuda, A.; Tatsumisago, M. Superionic Glass-Ceramic Elec- trolytes for Room-temperature Rechargeable Sodium Batteries. Nat. Commun. 2012, 3, 856.