Fluoride removal from NMC black mass leachates during Lithium-Ion battery recycling via aluminum sulfate precipitation.

Abstract

Introduction

Because of significant surge in the production of EVs, by 2030, the global electric vehicle fleet is projected to reach 350 million units (Azimi and Chan, 2024). Lithium-ion secondary batteries (LIBs) are the preferred energy storage solution for EVs due to their high energy density, elevated operating voltage, lightweight design, and compact structure (Chan et al., 2021). With LIB systems having a projected lifespan of 10–15 years, a substantial wave of end-of-life LIBs is expected in the coming decades (Latini et al., 2022). By 2030, the global fleet of electric vehicles will generate an estimated 11 million tons of spent batteries (IEA, 2022). Without repurposing for second-life energy storage applications, these batteries face premature disposal, contributing to a growing volume of hazardous waste. This waste stream consists of flammable organic solvents, polymeric layers, graphite, metallic foils, and transition metal oxides, such as nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), and lithium (Li), posing environmental and safety risks (Chan et al., 2021).

Despite the challenges associated with managing this influx of spent LIBs, it also presents a significant opportunity. Effective recycling strategies can transform these decommissioned batteries into a valuable source of critical materials, mitigating resource scarcity while reducing the environmental impact of LIB waste (Latini et al., 2022).

Recycling of lithium-ion batteries is accelerating because of growing end-of-life volumes and the need to recover critical materials; however, fluorine-bearing components create specific processing risks and quality losses that are often overlooked. In particular, the cathode binder polyvinylidene fluoride, PVDF, and the LiPF₆-based electrolyte are primary sources of fluoride species that can corrode equipment, degrade extractants, and contaminate products, which motivates explicit defluorination steps in recycling flowsheets (Wang et al., 2023).

Work focused on removing PVDF shows that both solvent routes and low-temperature thermal strategies can separate the active material from Al foil while limiting hazardous fluorinated emissions. A frequently cited approach uses CaO as a reaction medium during low-temperature decomposition of PVDF to capture fluoride as CaF₂, thereby mitigating HF release and enabling efficient delamination of cathode coatings. Complementary studies survey PVDF removal by solution chemistry and “green” solvent systems, as well as pyrolysis variants, and highlight that PVDF handling directly affects downstream fluorine inventories (Wang et al., 2019).

Electrolyte recovery is equally important. LiPF₆ is thermally and hydrolytically unstable, producing PF₅, POF₃, and HF, and recent reviews synthesize practical routes for electrolyte collection and regeneration, including distillation or freezing, solvent extraction, and supercritical CO₂, while detailing safety and environmental considerations (Niu et al., 2023). Bringing PVDF removal and electrolyte recycling into the foreground clarifies why fluoride control is central to battery-grade product quality and motivates the Al-based precipitation strategy evaluated in this work.

Cathode materials from spent lithium-ion batteries utilizing nickel-manganese-cobalt (NMC) chemistry are predominantly processed through hydrometallurgical methods (Azimi and Chan, 2024). These processes typically employ inorganic and organic acid reagents to extract valuable metals from the cathode’s active materials (Chan et al., 2021). However, during the pretreatment stage, residual organic binders, primarily polyvinylidene fluoride (PVDF), and lithium hexafluorophosphate (LiPF₆) in the electrolyte are not entirely separated from the battery system. As a result, fluoride compounds migrate into the acidic leach solution along with the waste anode material

In addition to hydrometallurgical leaching, pyrolysis pretreatment of spent LIBs has been shown to generate fluoride-containing byproducts. During pyrolysis, decomposition of PVDF binders and LiPF₆ electrolytes leads to the release of hydrogen fluoride (HF) and other fluorinated gases, which may subsequently form inorganic fluorides in the solid residues or volatilize into the off-gas stream (Lombardo et al., 2020, Xu et al., 2021). These fluorinated emissions not only pose operational and environmental challenges but also complicate downstream material recovery by introducing fluoride impurities into the recycling process. Recognizing fluoride formation during pyrolysis highlights the importance of effective defluorination strategies across both thermal and hydrometallurgical treatment pathways.

The presence of fluoride ions in the leach solution presents significant processing challenges. During the production of battery-grade lithium carbonate (Li₂CO₃) and the organic extraction of Ni and Co, fluoride contamination leads to operational inefficiencies, including reactor corrosion, excessive decomposition of organic extractants, and a decline in Li₂CO₃ product quality. This is particularly an issue for sensitive process equipment such as Crystallizers (Demirel et al., 2022). Additionally, industrial wastewater and solid waste containing fluoride pose severe environmental risks, exacerbating pollution concerns (Ghosh et al., 2022).

To ensure the sustainable development of the renewable energy sector, effective fluoride ion separation from Ni- and Co-rich leach solutions is essential. Implementing advanced purification techniques can enhance process efficiency, mitigate environmental hazards, and improve the overall quality of recovered materials.

Currently, research on the defluorination of pregnant leach solutions (PLS) derived from the leaching of waste Cathode material remains limited and no previous study has investigated this subject for PLS obtained from the black mass of waste LIBs. Most existing studies have primarily focused on the removal of fluoride ions from drinking water and other industrial wastewater streams. Various treatment technologies, including membrane filtration technologies (Plattner et al., 2017), adsorption techniques (Yang et al., 2020), ion-exchange methods (Pan et al., 2013), and chemical precipitation (Liu and Liu, 2016), have been employed for fluoride removal in aqueous systems.

Among these methods, chemical precipitation using calcium or magnesium salts, as well as aluminum- or iron-based compounds, is widely implemented in industrial wastewater treatment due to its ability to achieve low residual metal ion concentrations in treated water (Dubey et al., 2018, Prabhu et al., 2016). While adsorption and ion-exchange technologies offer high selectivity for target ions, their efficacy diminishes in complex multi-anion solutions due to competitive adsorption, limiting their applicability in industrial fluoride separation processes (Alhassan et al., 2021). Given the technical and economic constraints of membrane-based treatments, such as high operational costs and the low salt content of untreated wastewater, advanced membrane technologies, including nanofiltration, reverse osmosis, and electrodialysis (Chan et al., 2022), are more suitable for fluoride removal and drinking water purification (Nunes-Pereira et al., 2018). However, the adaptation of these techniques for the purification of nickel sulfate solutions in battery recycling processes remains an area requiring further investigation.

In the treatment of high-fluoride wastewater, the precipitation method demonstrates a fluoride removal efficiency exceeding 90 %. Compared with alternative techniques, it offers a cost-effective solution with rapid defluorination achieved within a short processing time. Among various precipitants, aluminum salts have proven particularly effective, facilitating faster solid–liquid separation through coagulation and adsorption (Wan et al., 2021, Xu et al., 2023).

During the production of battery-grade Ni and related salts from waste battery materials and other Ni-Co feedstocks, solvent extraction is commonly employed to separate metal ions at elevated pH levels (pH to be adjusted). Under these conditions, Fe³⁺ and Al³⁺ concentrations in the solution remain negligible due to the hydrolysis of these metal ions. Building on these principles, in this study, aluminum sulfate was used to precipitate the majority of fluoride ions in the PLS of NMC black mass of a waste LIB, leveraging the low solubility of AlF₃ in solution (K_sp = 5.32 10^-4 at 25 °C)(O’Neil, 2001). In this process, minor degree of co-precipitation of Ni, Co, Mn, and Li was observed. To optimize the proposed fluoride purification process, key factors influencing fluoride precipitation from the black mass PLS were systematically examined using a single-factor experimental approach. The variables assessed included the molar ratio of aluminum sulfate to fluoride (1.3–3.9), solution pH (4–6), and temperature (25–75 °C). An empirical model was used to optimize the fluoride removal process with the objective function of achieving more than 97 % fluoride precipitation with less than 10 % target metals co-precipitation. Based on these investigations, the optimal process conditions were identified to achieve the objective function. The selected values allow the fluoride impurity to be removed upstream of the desired products.

This work contributes a waste-management oriented solution by quantifying fluoride removal from real black-mass pregnant leach solution using commodity aluminum sulfate and by defining an operating window that maximizes defluorination while minimizing Ni, Co, Mn, and Li losses. Unlike most prior reports that rely on simplified matrices, we use industrially representative PLS and demonstrate high fluoride removal at ambient temperature with low reagent intensity, then close the loop by showing how trace co-precipitated metals are recoverable via a mild sulfate wash. We further benchmark equilibrium predictions against measurements to delineate where surface and mixing phenomena, common in multicomponent waste streams, drive departures from bulk thermodynamics. This positions the method as a scalable, low-energy, and compliance-oriented unit operation that reduces corrosion and extractant degradation, improves product quality, and lowers fluoride burdens in downstream wastewater treatment.

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ABSTRACT ONLINE AT https://www.sciencedirect.com/science/article/abs/pii/S0956053X25005471