Chemically Understanding the Liquid-Phase Synthesis of Argyrodite Solid Electrolyte Li6PS5Cl with the Highest Ionic Conductivity for All-Solid-State Batteries

Solid electrolytes (SEs), which essentially act as both the electron separator and ion conductor, play an important role in all-solid-state lithium-ion batteries. Liquid-phase synthesis is one of the promising methods as the synthesis of SEs is easily scalable and consumes lower energy. However, due to the complexity of the SEs prepared by liquid-phase synthesis, many problems such as impurities arise, making the liquid electrolytes irreplaceable. This study examines and solves the question why Li3PO4 is produced as an impurity upon preparing Li6PS5Cl argyrodite by approaching the chemical factors. This is accomplished by replacing the hydroxide-based solvent with a thiol-based solvent through liquid-phase synthesis. As a result, the absence of Li3PO4 from the Li6PS5Cl SEs in this study resulted in Li6PS5Cl attaining the highest ionic conductivity value (>2 mS·cm–1) ever obtained through liquid-phase synthesis. Furthermore, the absence of Li3PO4 in the argyrodite SE could magnificently increase the cell’s capacity with remarkable stability.