Comparative Study of different Recycling Methods for the Electrolyte Recovery from spent Li-Ion Batteries

Licentiate thesis, 2023

The state-of-the-art Li-Ion battery (LiB) recycling strategies focus mainly on recycling of the electrode materials, i.e., Li, Co, Mn, Ni, Al, and Cu. Thereby, the electrolyte, which is a multicomponent system consisting of a conductive salt dissolved in a mixture of organic carbonate solvents and additives, either evaporates and/or decomposes (uncontrollably). This causes a risk of immeasurable toxic and environmental emissions. Moreover, the remaining electrolyte in the produced black mass is problematic due to the presence of the organic solvents. Secondary streams coming from the recycling plants are then considered hazardous and represent a technical and financial burden for recycling companies.

As traditional methods fail to recycle the electrolyte from spent LiBs, it is evident that a new approach must be implemented to fill the gap. This work investigated two promising approaches to separate the electrolyte from spent LiBs; low temperature thermal treatment and sub-supercritical carbon dioxide (scCO2) extraction. Thereby, the analysis emphasized the best possible process conditions to separate the electrolyte from spent LiBs, the composition and purity of the recovered products, and the composition of the exhaust gas emissions during the processes.

The results showed that the low temperature thermal treatment approach enables full separation and collection of the electrolyte solvents dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) after 80 minutes at 130ºC. However, the conductive salt LiPF6 decomposed during the process leading to the generation of toxic exhaust gas emissions containing hydrogen fluoride (HF) and phosphorous oxyfluoride (POF3). Sub-scCO2 extraction with CO2 densities between 600-900 kg/m3 was suitable for selectively separating the non-polar electrolyte solvents DMC and EMC from the spent LiB in 30 minutes. The exhaust gas analysis and elemental analysis of the extracted product indicated that LiPF6 did not decompose during the process but remained intact in the spent LiB. Thus, the sub-scCO2 electrolyte extraction can be a promising toxic-emission-free approach for the selective extraction of the non-polar electrolyte solvents from spent LiBs. However, further investigation is required to extract the polar electrolyte solvents and LiPF6.