Amorphous Sulfide Solid Electrolytes Based on Na<sub>3</sub>PS<sub>4</sub>–Na<sub><i>x</i></sub>MO<sub><i>y</i></sub> (M = P and S) for All-Solid-State Sodium Batteries
Yushi Fujita, Rina Yamanaka, Daiki Suehiro, Keitaro Imai, Kenta Koga, Taichi Asakura, Kota Motohashi, Atsushi Sakuda, Akitoshi Hayashi
Abstract
Na-ion batteries are low-cost energy-storage systems that are expected to be one of the next-generation energy devices. All-solid-state Na batteries using a solid electrolyte and a Na metal negative electrode are promising as energy-storage devices because of their safety and high energy density. Among the solid electrolytes, sulfide solid electrolytes are attractive because of their high ionic conductivity and formability. However, most of them undergo undesirable side reactions with Na metal to form reductive decomposition layers, leading to poor cell cycling. Therefore, sulfide electrolytes with high tolerance to electrochemical reduction need to be developed. In this study, to improve the reduction tolerance and Na plating/stripping performance of Na 3 PS 4 electrolyte in an all-solid-state Na-ion battery, a series of Na 3 PS 4 electrolytes added with a sodium oxysalt (e.g., Na 3 PO 4, Na 2 SO 4 and Na 2 SO 3 ) was mechanochemically prepared. The addition of a small amount of each sodium oxysalt led to the amorphization of Na 3 PS 4 . Although the addition of sodium oxysalt decreased the ionic conductivity of the Na 3 PS 4 crystal to approximately 1 × 10 –5 S cm –1 at room temperature, its formability was enhanced and the relative density of the compact powder increased to ∼90%. The Na plating/stripping performance of the all-solid-state Na symmetric cell improved after the incorporation of the sodium oxysalt into the Na 3 PS 4 because of the formation of a uniform interface with the Na metal. In particular, the increase in the cell overpotential during Na plating/stripping cycling was considerably suppressed with the addition of Na 3 PO 4 with a high reduction tolerance to Na 3 PS 4 . This study demonstrates that the addition of a sodium oxysalt and the amorphization of the sulfide electrolyte are promising for improving the performance of all-solid-state Na batteries with Na metal.