Kinetic Study of Lithium Sulfide Formation from Lithium Sulfate via Hydrogen Reduction: Autocatalytic Reaction Behavior and Nucleation and Growth Mechanism

Abstract Lithium sulfide (Li 2 S) is a key precursor for sulfide-based solid electrolytes used in all-solid-state batteries. Hydrogen (H 2 ) reduction of lithium sulfate (Li 2 SO 4 ) has attracted attention as an economic process for Li 2 S production. However, a kinetic study of the H 2 reduction of Li 2 SO 4 has not yet been reported. In this study, the H 2 reduction of Li 2 SO 4 was investigated at 1023–1123 K using 10% H 2 –argon (Ar) mixed gas. During the reaction, the Li 2 SO 4 –Li 2 S mixtures formed a molten phase, indicating a phase transition from solid to liquid. Microstructural observations revealed the formation of sulfur-rich nuclei and their growth into clusters during the reaction. The reaction kinetics were analyzed based on the shapes of the conversion–time curves and interpreted using both the autocatalytic reaction and nucleation and growth models. The kinetic behavior exhibited strong temperature dependence at 1023–1123 K. At 1123 K, a significant acceleration in the reaction rate was observed as the product accumulated, indicating strong autocatalytic behavior. The initial addition of Li 2 S seeds further increased the reaction rate. In contrast, the nucleation and growth mechanism dominated at 1023 K. These results indicate that the dominant reaction mechanism transitions from nucleation and growth to autocatalytic behavior with increasing reaction temperature.