Verifying Safety in Pre-Lithium Extraction (PLE) Process for Battery Recycling: HF Gas Suppression and Fluorine Compound Behavior Analysis

Abstract

The recycling of lithium-ion batteries (LIBs) is a critical challenge to address both environmental sustainability and resource scarcity. This study investigates the chemical discharge process for recovering fluorine-based compounds from spent LIBs while minimizing gaseous emissions and environmental impact. Gas-phase fluorine compounds, representing approximately 0.005% of the initial fluorine content in the battery, were effectively trapped in a 0.1 M NaOH solution, and their fluoride ion concentration was quantified using UV-Vis spectroscopy. Over 99% of the fluorine was retained in the liquid phase, demonstrating the efficiency of the chemical discharge process. The combustion ion chromatography (C-IC) and ion chromatography (IC) analyses revealed that the retained fluorine content in the solution was approximately 390 mg/L, of which 363 mg/L was dissolved, and the remainder was likely precipitated as solid compounds. Subsequent analyses of leached powders using X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD) confirmed that lithium carbonate (Li₂CO₃) and lithium fluoride (LiF) were the dominant components, highlighting the stabilization of lithium in recoverable forms. Further investigation utilizing quantitative nuclear magnetic resonance (qNMR) spectroscopy elucidated the molar distribution of fluorine compounds in the chemically discharged solution. The dissolved fluorine, determined to be 19.107 mmol/L by IC, was primarily composed of PF₆⁻ and LiF, with minor contributions from PO₃F₂⁻, PO₂F₂⁻, and POF₃. Notably, no HF peaks were observed in the ¹⁹F NMR spectrum, indicating its absence in the solution. A thermal stability test conducted at 70°C over 30 days revealed that the concentration of dissolved fluorine decreased to 17.528 mmol/L due to further decomposition of PF₆⁻ and subsequent precipitation. Remarkably, the growth of the LiF peak in ¹⁹F NMR was 1.7 times greater than expected based on the reduction of PF₆⁻, suggesting that HF generated during PF₆⁻ decomposition reacted with Li⁺ to form LiF. These findings collectively highlight the efficiency and chemical dynamics of the discharge process, offering a comprehensive approach to recovering fluorine compounds in a stable and environmentally friendly manner.