Fundamental Research On Liberation And Flotation Of Electrode Materials Derived From Spent Lithium-ion Battery Based On Pyrolysis

As an excellent storage matter of electric energy,the lithium-ion batteries?LIBs?have been widely used in electronics and new energy vehicles.At the same time,the output of LIBs will present a rapid growth trend.However,large amounts of spent LIBs are producing because of the limited lifetime of LIBs.From the viewpoints of resource recovery and environmental protection,recycling of spent LIBs is a significant process and it has been attracted more attention in recent years.Liberation of electrode materials and the separation of cathode materials and anode materials play an important role in spent LIBs recycling process.In this research,spent LIBs derived from waste mobile phones were used as the sample.Pyrolysis technology was proposed to remove organic binders and electrolyte for enhancing the liberation between electrode materials and foils and between electrode material particles and particles.At the same time,pyrolysis technology also can improve the flotation behavior of cathode and anode material.Firstly,thermogravimetry gas chromatograph-mass spectroscopy?TG-GC-MS?was used to investigate the pyrolysis characteristics and kinetics of organics in electrode materials derived from spent LIBs.At the same time,combined utilization of X-ray diffractometer?XRD?,X-ray photoelectron spectroscopy?XPS?,and transmission electron microscope?TEM?was applied to evaluate the effects of pyrolysis on the phase properities of electrode materials.The testing results lay the foundation for the subsequent pyrolysis research.Results demonstrate that the main organics in electrode materials are organic binders and electrolyte.Organic binder in cathode material is polyvinylidene fluoride?PVDF?while the main organic binder in anode material is styrene butadiene rubber?SBR?and little organic binder PVDF.The electrolytes in spent LIBs are ester compounds.Three weight-loss stages are presented in the electrode materials pyrolysis process:the first weight-loss stage is caused by the volatilization of electrolyte that remain on the surface of electrode materials;the second weight-loss stage is from the volatilization of electrolyte that remain in the electrode particles and the decomposition of part of electrolyte;and the decomposition of organic binder attributes to the third weight-loss stage.There is a similar weight-loss behavior between cathode materials and anode materials,which indicates that it is feasible for the mixed pyrolysis of cathode materials and anode materials.Crystal structure of graphite cannot be changed when the pyrolysis temperature is within the range of 600°C.However,LiCoO₂ is decomposed to CoO,Li₂CO₃,LiF,and Li₂O under the reduction of carbon black and organic binders.Crushing characteristics of the electrode materials from spent LIBs were investigated.Afterwards,high solution 3D X-ray microanalyser?3D-XRM?,scanning electron microscope-energy dispersive spectrum?SEM-EDS?,and electro-probe microanalyzer?EPMA?were combinedly used to investigate the pyrolysis-enhanced liberation mechanism of electrode materials.Organic binders result in the hard liberation of electrode materials from foils only using mechanical crushing.In addition,electrode materials are still in the aggregate form with large size fraction under the action of residual organic binders resulting in the difficult separation between electrode materials and current collectors by sieving process.After pyrolysis,orgainic binders have been romoved and only some pyrolysis residues remain in the gaps between electrode particles.The pores between electrode particles become larger as well as the adhesion force between electrode particles is weakened.Electrode materials are easy to liberate from foils while electrode particle can be fully liberated each other under a weak mechanical crushing.The optimum liberation efficiency of electrode materials was obtained at a pyrolysis temperature of 500°C,pyrolysis time of 15 min,pyrolysis heating rate of 10°C/min.At the same time,the liberation efficiency of cathode materials is up to 98.23%and the liberation efficiency of anode materials is up to 98.89%.Electrode materials with-0.045 mm size account for more than 70%,which demonstrates that there is a sufficient liberation among electrode particles.On the basis of fully understanding the pyrolysis characteristics of organics in electrode materials,microscopic tests?SEM,XPS,and TEM?combined with macroscopic tests?contact angle,introduction time,and surface wettability?were used to analyze the effects of pyrolysis on surface characteristics of electrode materials derived from spent LIBs.By these research results,pyrolysis-enhanced flotation mechanism of electrode materials was evaluated.Testing results demonstrate that electrode materials are wrapped by organic binders and residual electrolyte.Residual organic binders decrease the flotation efficiency of electrode materials by influencing the surface wettability of electrode materials,the interaction between electrode materials and reagents,and the liberation of electrode materials.The hydrophilic electrolyte on particle surface reduces the adsorption of flotation collectors.In addition,residual electrolyte acts as a foaming agent in flotation slurry,which worsens the flotation efficiency of electrode materials.Surface chemical properities of electrode materials have been changed after pyrolysis.The surface hydrophilicity index of anode materials decreases,which results in the increase of hydrophobicity and collector adsorption capacity of anode materials.At the same time,the surface hydrophilicity index of cathode materials increases,which results in the hydrophilcity of the cathode material increases.Hydrophilcity and hydrophobicity differences between cathode materials and anode materials are improved by pyrolysis treatment.From the fully understanding the pyrolysis-enhanced flotation mechanism of electrode materials,advanced anaslysis technology of SEM and XPS combined with flotation experiments were used to investigate the effects of pyrolysis parameters on flotation efficiency of electrode materials.The results demonstrate that lower pyrolysis temperature,shorter pyrolysis time,and faster pyrolysis heating rate result in the inadequate decomposition of organic binders and the inadequate removal of organic pyrolysis products,which cause the strong hydrophobicity of cathode materials.But higher pyrolysis temperature and longer pyrolysis time result in some pyrolytic carbon attaches on the surface of electrode materials,which also enhances the hydrophobicity of the pyrolytic cathode materials.The optimum flotation efficiency was obtained at pyrolysis temperature of 550°C,pyrolysis time of 15 min,pyrolysis heating rate of 10°C/min.The cathode material grade is up to 94.72%with the recovery of 83.75%.The low cathode material recovery is caused by the residual pyrolytic carbon and the entrainment of the fine particles.Multi-stage flotation processes can improve the grade of cathode material,which is up to 98.00%by two-stage flotation.Wet-ball grinding and ultrasonic cleaning can remove the residual pyrolytic carbon on the surface of electrode materials and improve the flotation efficiency of electrode materials.Wet-ball grinding can improve the flotation efficiency from 83.75%to 91.35%.After ultrasonic cleaning,the cathode material grade is up to 93.39%with the recovery of 96.88%from one-stage flotation.The optimum pyrolysis temperature for the liberation and flotation efficiency of electrode materials is 550°C.This dissertation contains 105 figures,31 tables and 143 references.