Na Vacancy-Driven\nPhase Transformation and Fast Ion\nConduction in W‑Doped Na<sub>3</sub>SbS<sub>4</sub> from Machine\nLearning Force Fields

Solid-state sodium batteries require effective electrolytes\nthat\nconduct at room temperature. The Na<sub>3</sub>PnCh<sub>4</sub> (Pn\n= P, Sb; Ch = S, Se) family has been studied for their high Na ion\nconductivity. The population of Na vacancies, which mediate ion diffusion\nin these materials, can be enhanced through aliovalent doping on the\npnictogen site. To probe the microscopic role of extrinsic doping\nand its impact on diffusion and phase stability, we trained a machine\nlearning force field for Na<sub>3–<i>x</i></sub>W<sub><i>x</i></sub>Sb<sub>1–<i>x</i></sub>S<sub>4</sub> based on an equivariant graph neural network. Analysis of\nlarge-scale molecular dynamics trajectories shows that an increased\nNa vacancy population stabilizes the global cubic phase at lower temperatures\nwith enhanced Na ion diffusion and that the explicit role of the substitutional\nW dopants is limited. In the global cubic phase, we observe large\nand long-lived deviations of atoms from the averaged symmetry, echoing\nrecent experimental suggestions. Evidence of correlated Na ion diffusion\nis also presented that underpins the suggested superionic nature of\nthese materials.