In-situ pyrolysis based on alkaline medium removes fluorine-containing contaminants from spent lithium-ion batteries

Abstract

Pyrolysis is an effective method for removing organic contaminants (e.g. electrolytes, solid electrolyte interface (SEI), and polyvinylidene fluoride (PVDF) binders) from spent lithium-ion batteries (LIBs). However, during pyrolysis, the metal oxides in black mass (BM) readily react with fluorine-containing contaminants, resulting in a high content of dissociable fluorine in pyrolyzed BM and fluorine-containing wastewater in subsequent hydrometallurgical processes. Herein, an in-situ pyrolysis process is proposed to control the transition pathway of fluorine species in BM using Ca(OH)2-based materials. Results show that the designed fluorine removal additives (FRA@Ca(OH)2) can effectively scavenge SEI components (LixPOFy) and PVDF binders from BM. During the in-situ pyrolysis, potential fluorine species (e.g. HF, PF5, and POF3) are adsorbed and converted to CaF2 on the surface of FRA@Ca(OH)2 additives, thereby inhibiting the fluorination reaction with electrode materials. Under the optimal experimental conditions (temperature = 400 °C, BM: FRA@Ca(OH)2 = 1: 4, holding time = 1.0 h), the dissociable fluorine content in BM was reduced from 3.84 wt% to 2.54 wt%. The inherent metal fluorides in BM feedstock hinder the further removal of fluorine with pyrolysis treatment. This study provides a potential strategy for source control of fluorine-containing contaminants in the recycling process of spent LIBs.