All-solid-state sodium-ion batteries utilizing a Na superionic conductor solid electrolyte (SE) are promising candidates for cheap and safe large-scale energy storage. Among the most promising family of Na SEs are the Na₃PnS₄ (Pn = P, Sb, As) family^1–5 and the recently discovered Na₁₁Sn₂PnS₁₂ (Pn=P, Sb). ^6,7 In this talk, we will present a comprehensive investigation of cation mixing in the Na₄TtS₄-Na₃PnS₄ (Tt = Ge, Si, Sn) composition space using first principles calculations. This composition space, which includes Na₃PnS₄ and Na₁₁Sn₂PnS₁₂ as a subset, provides several levers to tune Na concentration and cation types to simultaneously achieve the twin goals of high ionic conductivity and good electrochemical and chemical stability. We will demonstrate that isovalent-mixed Na₃Pn’Pn’’S₄ compositions have low enthalpies of mixing, indicating that they are likely to be highly miscible. In the aliovalent mixed Na_4-xSn_1-xP_xS₄ system, we will show that the experimentally-reported Na₁₁Sn₂PS₁₂ prototype with space group I4₁/acd leads to not only lower formation energies, but also larger 3D diffusion channels. The predicted ionic conductivities from ab initio molecular dynamics simulations are also in excellent agreement with experiments. Finally, we will propose new mixed compositions with potentially higher conductivities and better chemical stability than those already reported experimentally.

Reference:

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