Role of Na+ Interstitials and Dopants in Enhancing the Na+ Conductivity of the Cubic Na3PS4 Superionic Conductor

In this work, we performed a first-principles investigation of the phase stability, dopant formation energy and Na4+ conductivity of pristine and doped cubic Na3PS4 (c-Na3PS4). We show that pristine c-Na3PS4 is an extremely poor Na ionic conductor, and the introduction of Na4+ excess is the key to achieving reasonable Na+ conductivities. We studied the effect of aliovalent doping of M4+ for P5+ in c-Na3PS4, yielding Na3+xMxP1-xS4 (M = Si, Ge, and Sn with x = 0.0625; M = Si with x = 0.125). The formation energies in all the doped structures with dopant concentration of x = 0.0625 are found to be relatively low. Using ab initio molecular dynamics simulations, we predict that 6.25% Si-doped c-Na3PS4 has a Na+ conductivity of 1.66 mS/cm, in excellent agreement with previous experimental results. Remarkably, we find that Sn4+ doping at the same concentration yields a much higher predicted Na+ conductivity of 10.7 mS/cm, though with a higher dopant formation energy. A higher Si4+ doping concentration of x = 0.125 also yields a significant increase in Na+ conductivity with an even higher dopant formation energy. Finally, topological and van Hove correlation function analyses suggest that the channel volume and correlation in Na+ motions may play important roles in enhancing Na+ conductivity in this structure.