Na_4-Sn_2-SbGe₅O₁₆, an Air-Stable Solid-State Na-Ion Conductor

The structure of a Na₄Sn₂Ge₅O₁₆ phase was established via single-crystal X-ray diffraction. Unusually large displacement parameters of Na atoms suggested the possibility of Na⁺ ionic conductivity. To create Na deficiencies and thus increase the Na⁺ mobility in Na₄Sn₂Ge₅O₁₆, Sn⁴⁺ cations were partially substituted with Sb⁵⁺. A series of Na_4-Sn_2-SbGe₅O₁₆ samples (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, or 0.35) were prepared by solid-state reactions and characterized with electrical impedance spectroscopy in the range of 25-200 °C. The highest ionic conductivity value was achieved in the Na_3.8Sn_1.8Sb_0.2Ge₅O₁₆ sample (1.6 mS cm^-1 at 200 °C). Na⁺ migration pathways were calculated using the bond-valence energy landscape approach, and two-dimensional conductivity channels with low energy barriers (≈0.4 eV) were found in the structure. Three-dimensional conductivity can also be achieved in the structure; however, it has a much higher energy barrier. The pristine phase and Na_3.8Sn_1.8Sb_0.2Ge₅O₁₆ sample were studied via ²³Na and ¹¹⁹Sn solid-state nuclear magnetic resonance. A faster exchange between the Na sites was observed in the doped sample.