Fluorine substances are no strangers to the lithium (Li)-ion battery architecture – they are found across the whole battery, from the binders within the electrodes in the form of polyvinylidene fluoride (PVDF), to the salts within the electrolyte as lithium hexafluorophosphate (LiPF6). This is notwithstanding the fluorinated species that form during the device’s lifetime, nor the ones that can also present within the separator.
These substances have been carefully selected for their inherent properties – from the binder’s robust strength and stability to the electrolyte’s key decomposition products, forming the critical passivation and protective layer on the graphite. This is known as a solid electrolyte interphase (SEI) formed in the early cycles of a battery.
However, while these properties are essential for Li-ion battery manufacture and operation, they present challenges in the later stages of its life – nominally in the safe recycling and reclamation of valuable critical materials, such as Li, cobalt (Co) and nickel (Ni).
There are two dominant industrial recycling methods – pyrometallurgy and hydrometallurgy. The former uses high-temperature smelting at typically more than 1,000°C to form a metallic alloy – with little discrimination given to cell type and chemistry. Meanwhile, hydrometallurgy processing involves wet chemistry routes. Acidic solutions are used to dissolve the active materials, which then allow for selective recovery of the metal ions from the system.
The end product is a metal salt formed with strong acids and a series of purification steps that result in a significant volume of waste.
According to research on Fluorinated substances in lithium-ion batteries and solid state batteries: Recycling challenges and environmental impacts, in the Journal of Power Sources, during these processes, the fluorine-containing materials (PVDF and LiPF6) combust and decompose at the higher temperatures. They produce a plethora of gaseous fluorocarbons and gaseous hydrogen fluoride (HF). Hydrogen fluoride is a highly corrosive and toxic compound that introduces significant health hazards and requires an additional off-gas treatment system.
LiPF6 readily hydrolyses to HF and other phosphoric acid species in the presence of moisture, which can cause issues during disassembly or pretreatment. The fluoride ions in solution complicate precipitation and can contaminate the product due to the difficulty in separation.
Meanwhile, PVDF is chemically insoluble in most aqueous and mild organic solvents, which makes it difficult to remove from the electrode material in the current collector, reducing the purity of the recovered (cathode) material.
A third recycling method, known as direct recycling, is available – a nascent technology that seeks to retain the material in a high-value state with regeneration of the original framework – nominally replacing the missing Li inventory within the materials.
The challenge in uptake of this method is the need for a high-purity feedstock. Part of direct recycling requires binder negation. This is where, after disassembly and separation of the electrodes stack, the coating is removed from the current collector in a delamination process. This process step can involve either a thermal or solvent treatment. However, in the former, there is a risk of the binder fluorinating the active material, which can influence structural and performance properties, in addition to HF release.
Moreover, the stability of the fluorinated compounds exploited in the Li-ion battery system is also the same property that makes them environmentally persistent. Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a class of fluoropolymers, such as PVDF and polytetrafluoroethylene (PTFE), that are highly resistant to chemical and biological degradation, leading to their accumulation in the environment.
These substances have been detected in water sources, soils and biological tissue globally. Their ‘forever chemical’ nature and wide persistence in the environment is a significant cause for concern.
The Organisation for Economic Co-operation and Development defines PFAS as fluorinated substances that contain at least one fully fluorinated methyl or methylene carbon atom without any hydrogen/chlorine/bromine/iodine atom attached to it. With a few noted exceptions, any chemical with at least a perfluorinated methyl group (?CF3) or a perfluorinated methylene group (?CF2?) is a PFAS.
A recent study examining Short-chain PFAS predominate in large-scale lithium battery industrial parks in Eastern China, published in the Journal of Hazardous Materials, found nearly a third of the total concentrations to be the result of the Li-ion battery industry.
On the other hand, the European Chemicals Agency (ECHA) has imposed restrictions on the use of PFAS across a range of industrial applications, namely textiles, food packaging and cosmetics. This has been conducted through REACH, the EU’s chemical legislation body, which stands for registration, evaluation, authorisation and restriction of chemicals. Will this impact Li-ion batteries?
Specific exemptions have yet to be determined, but the use and reliance on fluoropolymers are under scrutiny and will be subject to material restrictions.
A recent publication by ECHA on PFAS in the energy sector for recycling activities identifies the potential for human occupational exposure, more so with hydrometallurgy routes. The lower temperatures do not enable PFAS mineralisation and result in impurity residuals in the feedstocks.
There are opportunities to recover the fluorinated binders and electrolytes. Green solvents can be used to dissolve the PVDF component out – this is in place of a teratogenic solvent (NMP – N-Methyl-2-pyrrolidone), which is used in the manufacturing of electrodes to disperse PVDF.
For electrolytes, the application of solvent extraction can support reclamation of LiPF6 salt – however, achieving high purity can be challenging.
To mitigate against the problematic nature of fluorine, fluorine-free options are of growing interest, and with that, ensuring a design-for-recycling approach exists from the offset of (novel) battery production. The challenge, however, will be ensuring performance is matched to fluorine-containing batteries, in particular the electrolyte that is optimised for SEI formation.
Ultimately, the use of fluorine substances within batteries will be guided by regulation, but the prospective bans will not remove pre-existing PFAS in current commercial cells.
And while we can ban these substances and move towards fluorine-free solutions, we need to devise strategies to prevent contamination from these ‘forever chemicals’ and ensure recycling is as safe as possible.
Original article online at: https://www.iom3.org/resource/the-fluorine-problem-in-battery-recycling.html