Na Nonstoichiometric Modifications Unraveling the Sodium Ion Mobility and Transport Mechanism in Sodium Solid Electrolyte NaxZn2TeO6
Sodium-ion conducting materials in sodium-ion battery have drawn widespread attention in energy storage technologies due to the advantages of low cost, high performance, and efficient environmental adaptability. Herein, bond valence site energy (BVSE) calculations were used to predict the sodium ion electrical performances by the Na nonstoichiometric modifications, and we have carried out fine experiments to modulate the sodium ion conductivity of Na x Zn 2 TeO 6 guided by BVSE calculations. The optimized composition Na 2.1 Zn 2 TeO 6 shows the superior sodium ionic conductivity of 5.3×10 −3 S/cm at 190 °C, with a low activation energy of 0.28 eV. The excess Na preferentially occupies the Na1 site with tetrahedral voids, which has a higher capacity for sodium ion migration, as revealed by the combined neutron powder diffraction technique with the 1D and 2D 23 Na solid-state NMR technique, which is responsible for the variations in sodium ion conductivity. In addition, it is worth noting that the resulting Na 2.1 Zn 2 TeO 6 material maintains superior thermal and phase stability, as well as approximately the same thermal expansion coefficient values even during the temperature rise and fall cycles in the temperature range of 25–800 °C. Furthermore, molecular dynamics simulations revealed that the sodium ions exhibit longrange anisotropic migration within the Na + interlayers of Na 2.1 Zn 2 TeO 6 .
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