Thermal Drying in the Recycling Process of Lithium-Ion Batteries—Kinetics and Selectivity Aspects for the Evaporation of Electrolyte–Solvent Mixtures

The removal of the electrolyte solvents in an early-stage thermal drying step is crucial for safe and efficient recycling processes for end-of-life lithium-ion batteries. A comprehensive understanding of the governing influences on the solvent volatilization during the drying step enables optimized processes. The initial phase of this process is of particular interest because, due to the high spatial accessibility of the solvent, drying is determined by the mass transport in the surrounding gas phase, which can be precisely controlled through the process boundary conditions. In this study, the evaporation of representative binary and ternary electrolyte–solvent mixtures containing linear and cyclic organic carbonates is investigated under defined boundary conditions. The evaporation kinetics and selectivity are assessed by time-discrete measurement of the amount of solvent and its composition during the evaporation experiments. At the conditions applied, the vapor pressure of the solvents governs the evaporation selectivity, with the evaporation kinetics dictated by the mass transport of the solvent vapor in the gas phase. Hence, the evaporation of highly mobile but low volatile solvents, such as ethylene carbonate (EC), is the constraining aspect within this process. Moreover, molecular interactions within mixtures can further hinder the volatilization of EC. The developed simulation model describes the evaporation behavior with high accuracy and thus allows the prediction of minimum drying times. It establishes a solid foundation for designing and scaling the drying processes of end-of-life batteries, which involve complex material interactions.