Prediction and analysis of a sodium ion electrolyte: Li₂Na₂P₂S₆

Recent experimental results of Hood et al.¹ show that Na₄P₂S₆, which crystallizes in the based-centered monoclinic structure C2/m (#12), has significant Na ion conductivity (3 × 10^-6 S/cm) at room temperature. Using density functional theory and density functional perturbation theory within the harmonic phonon approximation, we predict that the Na ion conductivity can be enhanced by alloying this electrolyte with Li to form Li₂Na₂P₂S₆ having the same C2/m structure with compacted a and b lattice constants. The calculation of Helmholz free energies suggests that the alloy material is stable for a range of temperatures at and above room temperature in terms of the energy difference F(Li₂Na₂P₂S₆ + 2Na) − (F(Na₄P₂S₆ + 2Li) ≤ −0.35 eV. Molecular dynamics simulations indicate that Li₂Na₂P₂S₆ has larger Na ion conductivity than does Na₄P₂S₆ and therefore is promising as a possible solid-state electrolyte for all-solid-state Na ion batteries.
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¹Z. D. Hood et al. to be published.