Solvent Key Parameters for the Wet Chemical Synthesis of the Li3PS4 Solid Electrolyte

Solid sulfide electrolytes hold great promise for making all-solid state batteries a reality. Therefore, their wet chemical synthesis has become increasingly important in recent years due to various benefits such as scalability and versatility. The Li3PS4 electrolyte has a relatively high ionic conductivity (≈10–4 S cm–1 at 25 °C) and can be synthesized in various organic solvents by reacting Li2S and P2S5. It is well-known that the choice of the solvent is critical for the synthesis. Herein, we investigated several new organic polar solvents such as sulfur-based solvents (propanethiol, thiolane, and thiophene), amines, and carbonyls (esters, carbonates, ketones) for the synthesis of Li3PS4. The relationship between the ability to perform the reaction, the ionic conductivity, and the solvents’ characteristics revealed that the electronic density of the polar function plays a key role. The results of this study can be used as a guideline to select new synthesis solvents. We also identify a particular solvent, isobutyl isobutyrate (IBIB), which allows the decomposition of the corresponding Li3PS4 solvato-complex below the boiling point of the solvent with one of the highest reported ionic conductivities for the wet chemical method (2.1 × 10–4 S cm–1 at room temperature).