Solvent-free and low temperature synthesis of chalcogenide Na superionic conductors for solid-state batteries

Abstract

Sodium chalcogenide ionic conductors are attractive candidates as solid electrolytes (SEs) in solid-state Na metal batteries. They show the advantages of high ionic conductivity of 10−4–10−2 S cm−1 at room temperature and great chemical stability in air. However, simple and efficient approaches for the scalable synthesis of chalcogenide solid electrolytes (SEs) are required. In this work, we report a solvent-free mixing to form dry intermediate products, which are subjected to different treatments (electron-beam assisted method or low temperature heating (≤150 °C)) to produce pure phase of Na3SbS4-ySey (0 ≤ y ≤ 2) chalcogenides. Heavy Se-doping in Na3SbS4 results in the tetragonal-to-cubic phase transition as well as a significant change of Sb-S bonding in Raman spectra. Among all chalcogenide SEs, Na3SbS3Se showed the highest ionic conductivity of 3.75 × 10−4 S cm−1 at room temperature, 47% higher than that of pristine Na3SbS4. Moreover, the Se-dopant also enhanced the electrochemical stability towards Na metal in solid-state batteries. The solid-state Na||FeS2 battery with Na3SbS3Se SE displayed long-term cycling ability up to 1,000 cycles within the voltage window of 1.0–2.7 V and retained a specific discharge capacity of 105 mAh g−1 after 600 cycles. This technique promotes the practical applications of chalcogenide SEs in solid-state Na batteries.