Exhaust Gas Cleaning in the Recycling of Li-Ion-Batteries

Abstract

E-Mobility will gain increasing importance in the future thus not only leading to reduced emissions in individual mobility but also new challenges and opportunities in the field of Li-ion battery recycling. While the recycling of low voltage batteries is a common process, high voltage batteries introduce new challenges. [1] Li-ion batteries consist of metal-based anodes and cathodes (Cu, Al), separated by a polymeric separator membrane as well as active material (graphene) and the liquid electrolyte. The latter contains the conducting salt dissolved in a mixture of low boiling, such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and high boiling solvents, e.g. ethylene carbonate (EC). Within the collaborative research project “LithoRec II” an integrated recycling process for spent Li-ion batteries was investigated. The overall process included the steps of discharging, dismantling, shredding, extraction, drying and separation of the value components from a spent battery. These steps were addressed in a comprehensive approach to increase the overall recycling quota and sustainability of Li-ion batteries. After the shredding process step, the conducting salt and high boiling electrolyte components are washed out of the material through extraction with DMC. This leaves the solid fraction wetted with DMC after the extraction process. In the following drying step, DMC is purged from the extracted shredder material with nitrogen. Nitrogen together with the volatile electrolyte components then forms an exhaust gas that exceeds emission limits. Therefore the released exhaust gas has to be cleaned, e.g. by a combination of condensation and adsorption. Condensation recovers a high percentage of electrolyte, however, sufficient cleaning by condensation required a high energy demand at low temperatures. Hence, final cleaning is preferably achieved through another process such as adsorption. Applying a flowsheet simulation of the process sequence extraction, drying, condensation the influence of the crucial process parameters of these steps, such as solvent-to-raffinate ratio and condensation temperature, on the required drying temperature and resulting exhaust gas is shown. This quantifies the influences on the composition and overall loading of the exhaust gas as well as the gaseous adsorption of DMC and EMC onto activated carbon. Based on this information, the adsorption equilibria of the pure components DMC and EMC and their binary mixtures at the relevant concentration and temperatures ranges are determined. Commonly used isotherm models, like Tóth for pure components or the Ideal Adsorbed Solution Theory for mixtures, are applied and show a satisfactory match with experimental data. Additionally, DMC and EMC show a decomposition into Methanol and Ethanol. This effect increases when the electrolyte contacts activated carbon which serves as a catalyst. Therefore the influence of the decomposition on the adsorption equilibria has been investigated. This decomposition may have advantages in monitoring the adsorption process in an industrial plant. Hence, the adsorption of DMC in a fixed-bed was investigated. Breakthrough curves at different bed temperatures, gas loadings and overall gas flows have been determined and will be presented. Monitoring parameters like temperature profile and concentration level of DMC and methanol have been evaluated and determined to be well suited. Finally, the possibility of recycling the carbon by desorption was investigated. Desorption at different temperatures and pressures in a fixed bed was possible. A regeneration of activated carbon was achieved even with moderate process parameters of 60°C and 0.5 bars. [1] C. Hanisch, J. Diekmann, A. Stieger, W. Haselrieder and A. Kwade. Recycling of Lithium-Ion Batteries in Handbook of Clean Energy Systems - Volume 5 Energy Storage, J. Yan Editor, John Wiley & Sons, Ltd. (2015). ISBN: 978-1-118-38858-7