Revealing the Dissolution Mechanism of Polyvinylidene Fluoride of Spent Lithium-Ion Batteries in Waste Oil-Based Methyl Ester Solvent

A widely sourced and low-cost waste oil-based fatty acid methyl esters (FAMEs) solvent was prepared from waste cooking oil, as an alternative to separate cathode active materials (CAMs) and Al foil of spent lithium-ion batteries (LIBs). For Stage I (synthesis of methyl ester solvent), the waste oil was successfully converted into FAMEs solvent by transesterification with methanol using sodium hydroxide as a catalyst. The results of Stage II (CAMs and Al foil separation) show that 99.1 wt % of CAMs in spent LIBs can be stripped from Al foil after a heating treatment in FAMEs solvents at 190 °C for 20.0 min. The characterization results confirmed that the PVDF had been dissolved after heating treatment in FAMEs solvents, marked by the disappearance of fluorine from the cathode electrodes. And after the reaction was completed, the PVDF did not precipitate again from the FAMEs solvent. FAMEs solvent is considered as a transition state solvent capable of dissolving PVDF under heating conditions. By the cross-linking of hydrogen bonds, PVDF can be dissolved well, thereby causing the exfoliation of Al foils. The successful use of FAMEs solvents may replace that of organic solvents and become a new alternative for spent LIB cathode materials recovery.