Facile synthesis and electrochemical properties of Na-rich anti-perovskite solid electrolytes

Abstract

All-solid-state sodium batteries are promising candidates in energy storage applications due to their high safety and low cost. A suitable solid electrolyte is a key component for high-performance all-solid-state sodium battery. Current inorganic solid electrolytes mainly include oxide- and sulfide-based electrolytes. However, the oxide-based electrolytes require to be sinetred above 1000 ℃ for high ionic conductivity, and most sulfide-based electrolytes can react with H<sub>2</sub>O torelease toxic H<sub>2</sub>S gas. These features will hinder the practical application of all-solid-state sodium batteries. In recent years, novel sodium ionic conductors have appeared successively. Among them, anti-perovskite type of Li/Na ionic conductor has received a lot of attention because of its high ionic conductivity and flexible structure design. Nevertheless, the synthesis of Na-rich anti-perovskite Na<sub>3</sub>OBr<i><sub>x</sub></i>I<sub>1–<i>x</i> </sub>(0 < <i>x</i> < 1) is complex, the ionic conductivity at room temperature is relatively low, and its electrochemical properties remain unknown. Here in this work, the phase-pure Na-rich anti-perovskite Na<sub>3</sub>OBr<i><sub>x</sub></i>I<sub>1–<i>x</i></sub> is synthesized by a facile synthesis way. The X-ray diffraction patterns show that the anti-perovskite structure without any impurity phase is obtained. Alternating-current (AC) impedance spectrum is used for measuring ionic conductivity of electrolyte pellets after thermally being treated at around 100 ℃. The Na<sub>3</sub>OBr<sub>0.3</sub>I<sub>0.7</sub> exhibits an ionic conductivity of 1.47 × 10<sup>–3</sup> S/cm at 100 ℃. Unfortunately, the ionic conductivity experiences a sharp drop with the decrease of temperature, which may be related to the change of structural symmetry and Na sites in the structure revealed by solid state <sup>23</sup>Na NMR. In particular, the ionic conductivities of Na<sub>3</sub>OBr<i><sub>x</sub></i>I<sub>1–<i>x</i></sub> demonstrate the potential applications at medium temperature (40－80 ℃ in which the ionic conductivity of Na<sub>3</sub>OBr<i><sub>x</sub></i>I<sub>1–<i>x</i></sub> is close to or higher than 10<sup>–4</sup> S/cm) for all-solid-state sodium battery. Therefore, the compatibility against Na metal and the electrochemical performance in all-solid-state batteries have been evaluated. Since Na<sub>3</sub>OBr<i><sub>x</sub></i>I<sub>1–<i>x</i></sub> is not “Na-philic”, the resistance in impedance of the Na/Na<sub>3</sub>OBr<sub>0.5</sub>I<sub>0.5</sub>/Na is very high. However, after modifying the interface by ionic liquid, the Na<sub>3</sub>OBr<sub>0.5</sub>I<sub>0.5</sub> exhibits good compatibility against Na metal and tiny ionic liquid also leads to high initial discharge specific capacity of 190 mAh/g and excellent cycling stability (around 127 mAh/g after 10 cycles) in the TiS<sub>2</sub>/Na<sub>3</sub>OBr<sub>0.5</sub>I<sub>0.5</sub>/Na-Sn solid-state battery. The capacity decay maybe results from the inferior interfacial contact between the solid electrolyte and the electrode materials because the electrode materials in this system experience large volume change during cycling. The successful operation in solid-state sodium batteries indicates that the Na<sub>3</sub>OBr<sub><i>x</i></sub>I<sub>1–<i>x</i></sub> is feasible to be used as a sodium solid electrolyte, which is of great importance for practical application of Na-rich anti-perovskite solid electrolytes.