sustainability

Article A Sustainable Process for the Recovery of Anode and Cathode Materials Derived from Spent -Ion Batteries

Guangwen Zhang 1, Zhongxing Du 1, Yaqun He 1,2,*, Haifeng Wang 1, Weining Xie 2,3 and Tao Zhang 4 1 School of Chemical Engineering and Technology, China University of and Technology, No.1 Daxue Road, Xuzhou 221116, China; [email protected] (G.Z.); [email protected] (Z.D.); [email protected] (H.W.) 2 Advanced Analysis and Computation Center, China University of Mining and Technology, No.1 Daxue Road, Xuzhou 221116, China; [email protected] 3 Jiangsu Huahong Technology Stock Limited Company, Wuxi 214400, China 4 Research Institute of Tsinghua University in Shenzhen, Shenzhen 518057, China; [email protected] * Correspondence: [email protected]; Tel.: +86-516-8359-2928; Fax: +86-516-8399-5026

 Received: 15 March 2019; Accepted: 15 April 2019; Published: 20 April 2019 

Abstract: The recovery of cathode and anode materials plays an important role in the process of spent lithium-ion batteries (LIBs). Organic binders reduce the liberation efficiency and flotation efficiency of electrode materials derived from spent LIBs. In this study, pyrolysis technology is used to improve the recovery of cathode and anode materials from spent LIBs by removing organic binders. Pyrolysis characteristics of organics in electrode materials are investigated, and on this basis, the effects of pyrolysis parameters on the liberation efficiency of electrode materials are studied. Afterwards, flotation technology is used to separate cathode material from anode material. The results indicate that the optimum liberation efficiency of electrode materials is obtained at a pyrolysis temperature of 500 ◦C, a pyrolysis time of 15 min and a pyrolysis heating rate of 10 ◦C/min. At this time, the liberation efficiency of cathode materials is 98.23% and the liberation efficiency of anode materials is 98.89%. Phase characteristics of electrode materials cannot be changed under these pyrolysis conditions. Ultrasonic cleaning was used to remove pyrolytic residues to further improve the flotation efficiency of electrode materials. The cathode material grade was up to 93.89% with a recovery of 96.88% in the flotation process.

Keywords: electrode materials; spent lithium-ion battery; pyrolysis; liberation; flotation; recovery

1. Introduction Lithium-ion batteries (LIBs) are widely used as an energy-storage component in various electrical and electronic products, including mobile phones, cameras, laptop computers, and new-energy vehicles, due to their excellent characteristics, such as high energy density, safe handling, and low self-discharge [1,2]. The demand for LIBs is presenting a rapid growth trend [3]; meanwhile, the growth rate is increasing gradually, which means that large amounts of spent LIBs need to be treated because of their limited lifetime [4]. Spent LIBs mainly comprise a metallic shell, membrane separator, cathode materials (LiCoO2, LiMn2O4, LiFePO4, as well as other lithium metal oxides), aluminum foil, anode materials (graphite), foil, and organic electrolytes [5]. The abundant components of spent LIBs indicate that the recycling of valuable metals from spent LIBs is a significant process. From the viewpoint of environmental protection and , the recycling of spent LIBs has attracted more and more attention, and some sophisticated processes including ,

Sustainability 2019, 11, 2363; doi:10.3390/su11082363 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 2363 2 of 11 , and bio-metallurgy have been proposed [6–8]. It is worth noting that current metallurgy technologies always focus on the recovery of high economic value metals derived from cathode materials, especially Co, Li, Mn, and Ni [9]. Therefore, how to obtain pure cathode materials is the core process that connects the pretreatment process with metallurgy. Manual dismantling is extensively used to obtain cathode materials for the metallurgy process. Each component is obtained using the dismantling process and then cathode is selected as a research target. N-methyl-2-pyrrolidone or other chemical solutions are usually used to remove the organic binder, and then cathode materials are liberated from the aluminum foils [10,11]. By sieving, pure cathode materials are used in subsequent metallurgy processes. However, manual dismantling is harmful to people’s health and limits the industrial recovery efficiency of spent LIBs. At the same time, using a chemical solution to remove the organic binder has some problems, including low liberation efficiency and secondary . In recent years, with the production increase of spent LIBs, mechanical-physical methods have been applied in the pretreatment process for spent LIBs. Multistage crushing can liberate electrode particles from foils, while screening processes are utilized to achieve the separation of electrode materials and foils [12–14]. Flotation technology is then utilized to realize the separation of cathode and anode materials due to their obvious hydrophilic/hydrophobic differences [15–17]. This flowchart is significant for large scale industrial application and the preparation of pure cathode materials for the metallurgy process. Two main problems need to be solved in this process: (a) Electrode particles are attached to foils by organic binder and they are difficult to liberate under only the action of mechanical crushing force, which results in some electrode materials remaining on the foils and decreases the recovery efficiency; (b) electrode materials from this flowchart are still enveloped by electrolyte and organic binder, which will decrease their flotation efficiency because the hydrophilic/hydrophobic differences between cathode materials and anode materials are decreased. Therefore, the removal of organic binder and electrolyte is an essential process in order to obtain high purity cathode materials [18,19]. Some chemical agents, including N-methyl-2-pyrrolidone, dimethylformamide, dimethyl acetamide, and dimethyl sulfoxide, have been utilized to dissolve the organic binder [20,21]. However, chemical dissolution methods characterized by high cost, low efficiency, and potential pollution limit its wide application. A Fenton high-order oxidation process has been used to remove organic binder and residual electrolytes to improve the flotation efficiency of electrode materials. However, Fe2+ was introduced into this reaction, and the Fe element remained on the surface of the electrode material, complicating the subsequent metallurgical process [15,22]. Yu et al. adopted a grinding process to improve the flotation efficiency of electrode materials. Although grinding can improve the flotation efficiency, the organic binders and electrolytes still remain on the particles, resulting in a low cathode material recovery [23]. A roasting method can remove the organic binder and electrolytes adequately [24], but the serious secondary pollution limits its application. Pyrolysis is an effective method that can not only remove organics from electrode materials but also reduce the environmental pollution. A sustainable process for the recovery of cathode and anode materials derived from spent lithium-ion batteries is proposed in this study. The main purpose of this research work is to obtain high purity cathode material to lay the foundation for metallurgy. This research focuses on the following aspects: (a) The pyrolysis characteristics of cathode and anode materials; (b) the facilitation effect of pyrolysis on the liberation of electrode materials; (c) the effects of pyrolysis on the phase properties of electrode materials; and (d) the enhancement of electrode materials’ flotation behavior by pyrolysis.

2. Experimental Section

2.1. Experimental Methods Spent LIBs derived from mobile phones were collected, comprising various types of batteries. The main cathode material was LiCoO2, but there was a small amount of Mn and Ni elements in the cathode material [15]. The anode material was graphite. In this study, the spent LIBs were treated Sustainability 2019, 11, 2363 3 of 11 Sustainability 2019, 11, x FOR PEER REVIEW 3 of 11 following the flowchart shown in Figure1. They were firstly discharged with 5 wt% NaCl solution for were treated following the flowchart shown in Figure 1. They were firstly discharged with 5 wt% 48 h and then naturally air-dried. The discharged spent LIBs were firstly cut up by a shredder. As a NaCl solution for 48 h and then naturally air-dried. The discharged spent LIBs were firstly cut up by result, the metallic shells were shredded while the membrane separator, electrode sheets, and other a shredder. As a result, the metallic shells were shredded while the membrane separator, electrode components were liberated from each other. After this shredding process, the metallic shell, membrane sheets, and other components were liberated from each other. After this shredding process, the separator, and electrode sheets presented flake shapes, and pneumatic separation was then used metallic shell, membrane separator, and electrode sheets presented flake shapes, and pneumatic to remove the membrane separator and other . Magnetic (for shell) or eddy current separation was then used to remove the membrane separator and other plastics. Magnetic (for steel (for aluminum shell) separations were applied to remove metallic shells. The mixture of cathode shell) or eddy current (for aluminum shell) separations were applied to remove metallic shells. The and anode were obtained after the pretreatment. Part of the cathode and anode scraps was mixture of cathode and anode scraps were obtained after the pretreatment. Part of the cathode and directly crushed and sieved. Other electrode mixtures were treated using pyrolysis combined with anode scraps was directly crushed and sieved. Other electrode mixtures were treated using pyrolysis a mechanical-physical process. Afterwards, a flotation process was used to achieve the separation combined with a mechanical-physical process. Afterwards, a flotation process was used to achieve of the cathode and anode materials. The effects of pyrolysis on the liberation efficiency and flotation the separation of the cathode and anode materials. The effects of pyrolysis on the liberation efficiency efficiency of electrode materials were then investigated. and flotation efficiency of electrode materials were then investigated.

Figure 1. ExperimentalExperimental flowchart flowchart of this study. 2.2. Pyrolysis Experiment 2.2. Pyrolysis Experiment A portion of the cathode and anode scraps were hand-sorted to analyze their pyrolysis A portion of the cathode and anode scraps were hand-sorted to analyze their pyrolysis characteristics. Pyrolysis experiments of cathode and anode mixtures were conducted in a characteristics. Pyrolysis experiments of cathode and anode mixtures were conducted in a controlled- controlled-atmosphere tube furnace (MXG1200-80, Shanghai Micro-X furnace Co., Ltd., Shanghai, atmosphere tube furnace (MXG1200-80, Shanghai Micro-X furnace Co., Ltd., Shanghai, China), using China), using the pyrolysis system shown in Figure2. The mixed cathode and anode scraps were the pyrolysis system shown in Figure 2. The mixed cathode and anode scraps were placed into the placed into the tube furnace. Afterwards, the corun-dum tube was filled with nitrogen, with the tube furnace. Afterwards, the corun-dum tube was filled with nitrogen, with the nitrogen flow nitrogen flow remaining at 100 mL/min to carry pyrolysis products away. Pyrolysis parameters were remaining at 100 mL/min to carry pyrolysis products away. Pyrolysis parameters were set using a set using a temperature control system. Pyrolysis products from the organics in electrode materials temperature control system. Pyrolysis products from the organics in electrode materials were taken were taken out of the tube by nitrogen flow, and pyrolysis oils were then collected by condensation, out of the tube by nitrogen flow, and pyrolysis oils were then collected by condensation, while while organic gas was collected and treated by a gas collector system. Deionized and an NaOH organic gas was collected and treated by a gas collector system. Deionized water and an NaOH solution were used to remove the acidic gas. solution were used to remove the acidic gas. Sustainability 2019, 11, 2363 4 of 11 Sustainability 2019, 11, x FOR PEER REVIEW 4 of 11

Figure 2. SchematicSchematic diagram diagram of of the the laboratory laboratory pyrolysis pyrolysis device: device: (A) (A) temperature temperature control control system, system, (B) (B)tubular tubular furnace, furnace, (C) (C)condenser condenser pipe pipe,, (D) (D)condensation condensation products products collec collector,tor, (E) (E)deionized deionized water water 1, (F) 1, (F)deionized deionized water water 2, (G) 2, (G) NaOH NaOH solution, solution, (H) (H) tailing tailing gas gas drying drying device. device. 2.3. Flotation Experiment 2.3. Flotation Experiment Flotation technology was used to separate cathode material from anode material, because cathode Flotation technology was used to separate cathode material from anode material, because material is hydrophilic, while anode material is hydrophobic. After flotation, anode material will attach cathode material is hydrophilic, while anode material is hydrophobic. After flotation, anode material to bubbles and be collected in the froth product. Flotation experiments were conducted on electrode will attach to bubbles and be collected in the froth product. Flotation experiments were conducted on materials before and after pyrolysis. Before the flotation experiments, the pyrolytic electrode materials electrode materials before and after pyrolysis. Before the flotation experiments, the pyrolytic were ultrasonically-cleaned to remove the pyrolysis residues. Flotation experiments were conducted electrode materials were ultrasonically-cleaned to remove the pyrolysis residues. Flotation using an XFD-63 flotation machine (Jianfeng Mining Machinery Manufacturing co. LTD, Nanchang, experiments were conducted using an XFD-63 flotation machine (Jianfeng Mining Machinery China) at room temperature with an impeller speed of 1800 rpm, an aeration quantity of 2.0 L/min, Manufacturing co. LTD, Nanchang, China) at room temperature with an impeller speed of 1800 rpm, and a pulp density of 40 g/L. A dilute pulp concentration was used in this study because of the fine size an aeration quantity of 2.0 L/min, and a pulp density of 40 g/L. A dilute pulp concentration was used of the electrode materials. n-Dodecane was used as a collector with a dosage of 300 g/t, while methyl in this study because of the fine size of the electrode materials. n-Dodecane was used as a collector isobutyl carbinol (MIBC) was used as frother with a dosage of 150 g/t. Cathode material concentrate with a dosage of 300 g/t, while methyl isobutyl carbinol (MIBC) was used as frother with a dosage of and anode material concentrate were obtained by filtration and drying. Afterwards, cathode material 150 g/t. Cathode material concentrate and anode material concentrate were obtained by filtration and grade in flotation products was tested by ICP. drying. Afterwards, cathode material grade in flotation products was tested by ICP. 2.4. Analytical Methods 2.4. Analytical Methods The pyrolysis characteristics of cathode and anode materials were analyzed by a The pyrolysis characteristics of cathode and anode materials were analyzed by a Thermogravimetric Analyzer (TGA) (NETZSCH, STA 449-F5, Selb, , testing condition: Thermogravimetric Analyzer (TGA) (NETZSCH, STA 449-F5, Selb, Germany, testing condition: temperature rose from 20 to 700 ◦C with a heating rate of 10 ◦C/min; N2 was used as a shielding gas temperature rose from 20 to 700 °C with a heating rate of 10 °C/min; N2 was used as a shielding gas with an airflow velocity of 20 mL/min). An Electro-probe microanalyzer (EPMA) (EPMA-8050G, Kyoto, with an airflow velocity of 20 mL/min). An Electro-probe microanalyzer (EPMA) (EPMA-8050G, Japan) was used to investigate the pyrolysis-enhanced liberation mechanism of electrode materials. Kyoto, Japan) was used to investigate the pyrolysis-enhanced liberation mechanism of electrode Surface chemical states of electrode materials were analyzed using an X-ray photoelectron spectroscopy materials. Surface chemical states of electrode materials were analyzed using an X-ray photoelectron (XPS), (ESCALAB 250Xi, Thermo Fisher Scientific Inc., Waltham, MA, USA). A scanning electron spectroscopy (XPS), (ESCALAB 250Xi, Thermo Fisher Scientific Inc., Waltham, MA, USA). A scanning microscope (SEM), (FEI quanta 250, Hillsboro, Oregon, USA) was used to determine the morphology electron microscope (SEM), (FEI quanta 250, Hillsboro, Oregon, USA) was used to determine the of the electrode material particles before and after pyrolysis. The cathode material grades in flotation morphology of the electrode material particles before and after pyrolysis. The cathode material products were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) (NWR 213-7900 grades in flotation products were analyzed by inductively coupled plasma-mass spectrometry (ICP- ICP-MS, Palo Alto, California, USA). X-ray powder diffractometry (XRD) (Bruker D8 advance, Bremen, MS) (NWR 213-7900 ICP-MS, Palo Alto, California, USA). X-ray powder diffractometry (XRD) Germany, database: Inorganic Crystal Structural Database and Powder Diffraction File) was used to (Bruker D8 advance, Bremen, Germany, database: Inorganic Crystal Structural Database and Powder analyze the effects of pyrolysis on the mineral phases. Diffraction File) was used to analyze the effects of pyrolysis on the mineral phases. 3. Results and Discussion 3. Results and Discussion 3.1. TG Analysis of Cathode and Anode Materials 3.1. TG Analysis of Cathode and Anode Materials TG analysis played an important role in guiding the selection of the pyrolysis parameters. Cathode and anodeTG analysis materials played were an obtained important by manual role in processing.guiding the Afterwards, selection of TG the analyses pyrolysis of cathodeparameters. and anodeCathode materials and anode were materials conducted were respectively, obtained by with ma thenual results processing. shown Afterwards, in Figure3. TG The analyses first main of cathode and anode materials were conducted respectively, with the results shown in Figure 3. The first main weight-loss stage appeared at about 100 °C, while the second main weight-loss stage Sustainability 2019, 11, 2363 5 of 11

Sustainability 2019, 11, x FOR PEER REVIEW 5 of 11 weight-loss stage appeared at about 100 ◦C, while the second main weight-loss stage presented at aboutpresented 500 ◦atC. about The evaporation 500 °C. The of electrolyteevaporation caused of elec thetrolyte first weight-loss caused the stage. first Theweight-loss second weight-lossstage. The wassecond from weight-loss the decomposition was from of the organic decomposition binders that of areorganic wrapped binders on thethat surface are wrapped of electrode on the materials. surface Onof electrode the basis materials. of the TG analysis,On the basis cathode of the and TG anode analysis, materials cathode have and similar anode pyrolysis materials characteristics, have similar andpyrolysis the mixed characteristics, pyrolysis ofand cathode the mixed and anodepyrolysi materialss of cathode is feasible. and anode materials is feasible.

Figure 3. TG curves of cathode and anode ma materialsterials derived from spent LIBs.

3.2. Crushing Characteristics of Electrode Scraps Derived from Spent LIBs Cathode and anode anode scraps scraps were were manually manually sorted sorted and and then then crushed crushed using using an animpact impact crusher. crusher. A Asieving sieving process process was was conducted conducted to obtain to obtain five fivediffer dientfferent sized sized fractions, fractions, and afterwards, and afterwards, ICP was ICP used was usedto analyze to analyze the element the element distributions distributions in each in eachfraction fraction size, size,the results the results being being given given in Table in Table 1. Co,1. Co,Ni, Ni,Mn, Mn, and and Li, Li,derived derived from from cathode cathode materials, materials, and and C, C, derived derived from from anode anode materials, materials, were were mainly concentrated in in the the −0.20.2 mm mm size size fraction, fraction, while while Al Aland and Cu Cufoils foils were were mainly mainly concentrated concentrated in the in +1.4 the − +mm1.4 mmsize sizefraction, fraction, which which demonstrates demonstrates that that electr electrodeode scraps scraps have have selective selective crushing crushing properties. However, somesome elements elements from from electrode electrode materials materials were were distributed distributed in + in1.4 +1.4 mm mm and and0.5 −+0.50.2 + mm0.2 mm size − fractions,size fractions, which which indicates indicates that some that some electrode electrode materials materials still attach still toattach foils to under foils theunder action the of action organic of binders.organic binders. It is also It worth is also noting worth that noting some that electrode some electrode materials materials that were that liberated werefrom liberated foils werefrom hardfoils towere separate hard to from separate foils usingfrom foils the sieving using the process, sieving because process, they because still attached they still to attached each other to each under other the actionunder ofthe organic action binders.of organic Based binders. on this Based element on distributionthis element analysis, distribution the removal analysis, of organicthe removal binders of isorganic a necessary binders process is a necessary to improve process the recycling to improve effi theciency recycling of electrode efficiency materials. of electrode materials.

Table1.Table 1.Element Element distribution distribution in in di ffdifferenterent sized sized fractions fraction of crushings of crushing products products of the of raw the electrode raw electrode scraps. scraps. Cathode Anode Size Fraction (mm) Co NiCathode Mn Li AlAnode Cu C Size Fraction (mm) +1.4 5.79Co 5.80Ni 5.72Mn 5.55Li Al80.45 Cu 84.01 C 2.62 1.4 + 0.71+1.4 1.06 5.79 0.99 5.80 1.035.72 1.015.55 80.45 16.17 84.01 13.20 2.62 0.26 − 0.71 + 0.5 0.29 0.31 0.26 0.28 0.60 0.61 0.16 − −1.4 + 0.71 1.06 0.99 1.03 1.01 16.17 13.20 0.26 0.5 + 0.2 2.29 2.09 2.63 1.95 2.09 1.53 6.70 − −0.71 + 0.5 0.29 0.31 0.26 0.28 0.60 0.61 0.16 0.2 90.57 90.81 90.36 91.21 0.69 0.65 90.26 − −0.5 + 0.2 2.29 2.09 2.63 1.95 2.09 1.53 6.70 −0.2 90.57 90.81 90.36 91.21 0.69 0.65 90.26 3.3. Facilitation Effect of Pyrolysis on the Liberation of Electrode Materials 3.3. FacilitationCathode materials Effect of Pyrolysis were adopted on the toLiberation analyze of the Electrode facilitation Materials effect of pyrolysis on the liberation of electrodeCathode materials. materials Before were and adopted after pyrolysis, to analyze electrode the facilitation materials effect were of first pyrolysis embedded on the in self-curing liberation of electrode materials. Before and after pyrolysis, electrode materials were first embedded in self- curing resin. After polishing, a smooth section of electrode materials was exposed. The EPMA images are shown in Figure 4. As we can see from Figures 4a–c, the electrode particles were tightly adhered Sustainability 2019, 11, 2363 6 of 11

Sustainability 2019, 11, x FOR PEER REVIEW 6 of 11 resin. After polishing, a smooth section of electrode materials was exposed. The EPMA images are shownto eachin other Figure by4 .the As organic we can seebinder. from The Figure gaps4a–c, between the electrode electrode particles materials were were tightly filled adhered with toorganic each otherbinders. by Electrode the organic materials binder. Theare hard gaps to between liberate electrode using only materials mechanical were filledcrushing. with Figure organic 4d binders. shows Electrodethat organic materials binders are were hard decomposed to liberate using after onlypyrolysi mechanicals treatment. crushing. Pyrolysis Figure residues4d shows are that porous organic and binderseasy crush were by decomposed mechanical after crushing, pyrolysis which treatment. can im Pyrolysisprove the residues liberation are porous efficiency and easyof electrode crush by mechanicalmaterials. crushing, which can improve the liberation efficiency of electrode materials.

Figure 4. EPMA analysis of cathode materials: ( a–c)) before before pyrolysis, ( d) after pyrolysis.

A pyrolysis treatment combined with mechanical crushing was applied to achieve the liberation between electrode materials and foils. After crushing crushing,, a sieving process was used to obtain electrode materials with a size of 0.2 mm. The effects of the pyrolysis parameters on the liberation efficiency of materials with a size of− −0.2 mm. The effects of the pyrolysis parameters on the liberation efficiency theof the cathode cathode and and anode anode materials materials were were investigated. investigated. The The crushing crushing parametersparameters remainedremained unchanged in everyevery crushingcrushing experiment.experiment. Th Thee liberation efficiencies efficiencies of the cathodecathode and anode materials with didifferentfferent pyrolysispyrolysis parametersparameters areare givengiven inin FigureFigure5 5.. The pyrolysispyrolysis temperature temperature played played the the most most important important role inrole the in liberation the liberation process ofprocess the electrode of the materials.electrode materials. The effect The of the effect pyrolysis of the pyrolysis temperature temperature on the liberation on the liberation efficiency efficiency of the electrode of the electrode materials wasmaterials evaluated was evaluated at a pyrolysis at a pyrolysis time of 15time min of 15 and min a pyrolysisand a pyrolysis heating heating rate of rate 10 of◦C 10/min. °C/min. As theAs pyrolysisthe pyrolysis temperature temperature rose, therose, liberation the liberation efficiency efficiency first increased first increased and then and decreased. then decreased. The optimum The pyrolysisoptimum temperaturepyrolysis temperature of the cathode of the material cathode was materi 500 al◦C was and 500 550 °C◦C and for 550 the °C anode for the material. anode However,material. theHowever, pyrolysis the temperaturepyrolysis temperature had little had effect little on theeffect liberation on the liberation of the anode of the material anode material at 450 ◦ C.at 450 Under °C. theUnder higher the higher pyrolysis pyrolysis temperature, temperature, the liberation the libera oftion cathode of cathode material material declined declined sharply sharply because because of the sinteringof the sintering of organic of organic binders. binders. Differences Differences in organic in binders organic resulted binders in resulted differing in liberation differing e ffiliberationciencies betweenefficiencies the between cathode materialthe cathode and thematerial anode material.and the Theanode optimum material. pyrolysis The optimum temperature pyrolysis of the mixedtemperature electrode of the materials mixed electrode was intended materials to be was 500 ◦intendedC. to be 500 °C. The eeffectffect of of the the pyrolysis pyrolysis time time on the on liberation the liberation efficiency efficiency of electrode of materialselectrode was materials investigated was atinvestigated a pyrolysis at temperature a pyrolysis of temperature 500 ◦C with aof pyrolysis 500 °C heatingwith a pyrolysis rate of 10 ◦heatingC/min. Therate liberationof 10 °C/min. efficiency The ofliberation cathode efficiency materials of changed cathode materials significantly changed as the significantly pyrolysis heating as the pyrolysis rate changed. heating The rate optimumchanged. pyrolysisThe optimum time ofpyrolysis cathode time materials of cathode was 15 materials min. At thewa sames 15 min. time, At the the pyrolysis same time, time the had pyrolysis little effect time on thehad liberationlittle effect of anodeon the materials.liberation Theof anode longer materials. pyrolysis timeThe aggravatedlonger pyrolysis the sintering time aggravated phenomenon the sintering phenomenon of the organic binder, decreasing the liberation efficiency of the electrode materials. The optimum pyrolysis time of the mixed electrode materials was 15 min. Sustainability 2019, 11, 2363 7 of 11 of the organic binder, decreasing the liberation efficiency of the electrode materials. The optimum pyrolysis time of the mixed electrode materials was 15 min. Sustainability 2019, 11, x FOR PEER REVIEW 7 of 11

FigureFigure 5. 5.E ffEffectsects of of pyrolysis pyrolysis parameters parameters on on the the liberation liberation e efficiencyfficiency ofof electrodeelectrode materials:materials: ((aa)) eeffectffect of pyrolysisof pyrolysis temperature temperature on on the the liberation liberation of of cathode cathode materials; materials; (b) effect effect of of pyrolysis pyrolysis temperature temperature on on thethe liberation liberation of of anodeanode materials; (c (c) )effect effect of of pyrolysis pyrolysis time time on onthe theliberation liberation of cathode of cathode materials; materials; (d) (deffect) effect of of pyrolysis pyrolysis time time on on the the liberation liberation of ofanode anode materials; materials; (e) ( eeffect) effect of ofpyrolysis pyrolysis heating heating rate rate on on the the liberationliberation of of cathode cathode materials; materials; ((ff)) eeffectffect ofof pyrolysis heating rate rate on on the the liberation liberation of of anode anode materials. materials.

Pyrolysis-crushingPyrolysis-crushing experiments experiments were were conducted conducted to to evaluate evaluate the the eff effectect of of the the pyrolysis pyrolysis heating heating rate rate on the liberation efficiency of electrode materials at a pyrolysis temperature of 500 °C and a on the liberation efficiency of electrode materials at a pyrolysis temperature of 500 ◦C and a pyrolysis timepyrolysis of 15 min. time Theof 15 liberation min. The e liberationfficiency of efficiency electrode of materials electrode decreased materials asdecreased the pyrolysis as the heatingpyrolysis rate rose.heating The fastrate heatingrose. The rate fast resulted heating in rate the resulted inadequate in the decomposition inadequate decomposition of the organic binder,of the organic and then binder, and then decreased the liberation efficiency. The effect of the pyrolysis heating rate on the decreased the liberation efficiency. The effect of the pyrolysis heating rate on the liberation efficiency liberation efficiency of electrode materials was not obvious. There was no obvious change in the of electrode materials was not obvious. There was no obvious change in the liberation efficiency of liberation efficiency of electrode materials when the pyrolysis heating rate improved from 5 °C/min electrode materials when the pyrolysis heating rate improved from 5 C/min to 10 C/min, but the time to 10 °C/min, but the time was saved by half. From the viewpoint ◦of energy saving,◦ the optimum was saved by half. From the viewpoint of energy saving, the optimum pyrolysis heating rate was set at pyrolysis heating rate was set at 10 °C/min. 10 C/min. ◦ The optimum pyrolysis parameters for the liberation of electrode materials were determined to be a pyrolysis temperature of 500 °C, a pyrolysis time of 15 min, and a pyrolysis heating rate of 10 °C/min. Electrode scraps were pyrolysis-treated at the optimum parameters and were mechanically crushed. At this time, the liberation efficiency of cathode materials was 98.23% and the liberation Sustainability 2019, 11, 2363 8 of 11

The optimum pyrolysis parameters for the liberation of electrode materials were determined to be a pyrolysis temperature of 500 ◦C, a pyrolysis time of 15 min, and a pyrolysis heating rate of 10 ◦C/min. Electrode scraps were pyrolysis-treated at the optimum parameters and were mechanically crushed. AtSustainability this time, 2019 the, 11 liberation, x FOR PEER e ffiREVIEWciency of cathode materials was 98.23% and the liberation e8ffi ofciency 11 of anode materials was 98.89%. Afterwards, the element distribution of different sized fractions of crushingefficiency products of anode of thematerials pyrolytic was electrode 98.89%. scrapsAfterwar wasds, analyzed, the element and distribution the results areof different shown in sized Table 2. fractions of crushing products of the pyrolytic electrode scraps was analyzed, and the results are From the results, we can see that the contents of Co, Ni, Mn, Li, and C in coarse fractions decreased shown in Table 2. From the results, we can see that the contents of Co, Ni, Mn, Li, and C in coarse after pyrolysis treatment, which indicates that the electrode materials had been adequately liberated fractions decreased after pyrolysis treatment, which indicates that the electrode materials had been from the foils and that the liberation between electrode materials was more adequate. adequately liberated from the foils and that the liberation between electrode materials was more adequate. Table 2. Element distribution in different sized fractions of crushing products from the pyrolytic electrode scraps. Table 2. Element distribution in different sized fractions of crushing products from the pyrolytic electrode scraps. Cathode Anode Size Fraction (mm) Co NiCathode Mn Li AlAnode Cu C Size Fraction (mm) +1.4 0.63Co 0.73Ni 0.59Mn 0.55Li Al 83.76 Cu 88.63 C 0.67 1.4 + 0.71+1.4 0.36 0.63 0.37 0.73 0.310.59 0.55 0.39 83.76 11.21 88.63 8.930.67 0.03 − 0.71 +−0.51.4 + 0.71 0.09 0.36 0.11 0.37 0.090.31 0.39 0.08 11.21 1.03 8.93 0.550.03 0.06 − 0.5 + 0.2 0.79 0.42 0.62 0.95 2.79 1.06 0.53 − −0.71 + 0.5 0.09 0.11 0.09 0.08 1.03 0.55 0.06 0.2 98.13 98.37 98.39 98.03 1.21 0.83 98.71 − −0.5 + 0.2 0.79 0.42 0.62 0.95 2.79 1.06 0.53 −0.2 98.13 98.37 98.39 98.03 1.21 0.83 98.71 3.4. Effects of Pyrolysis on the Phase Properties of Electrode Materials 3.4. Effects of Pyrolysis on the Phase Properties of Electrode Materials XRD was used to evaluate the effects of pyrolysis on the phase properties of electrode materials. BeforeXRD and was after used pyrolysis to evaluate at 500 the◦C, effects electrode of pyrolysis materials on the were phase tested properties by XRD, of electrode and their materials. results are Before and after pyrolysis at 500 °C, electrode materials were tested by XRD, and their results are shown in Figure6. The raw electrode material phase indicated that the cathode material was LiCoO 2, shown in Figure 6. The raw electrode material phase indicated that the cathode material was LiCoO2, and the anode material was graphite. After pyrolysis treatment at 500 ◦C, no new material peaks were detectedand the byanode XRD material in electrode was graphite. materials, After which pyrolysis indicated treatment that phaseat 500 °C, properties no new material of electrode peaks materials were detected by XRD in electrode materials, which indicated that phase properties of electrode materials were not changed at the pyrolysis temperature of 500 ◦C. The XRD results demonstrate that 500 ◦C were not changed at the pyrolysis temperature of 500 °C. The XRD results demonstrate that 500 °C is is an optimum pyrolysis temperature and it not only can achieve adequate liberation of electrode an optimum pyrolysis temperature and it not only can achieve adequate liberation of electrode materials, but also does not change their phase properties. materials, but also does not change their phase properties.

FigureFigure 6. 6.XRD XRD resultsresults of electrode materials materials be beforefore and and after after pyrolysis pyrolysis treatment. treatment.

3.5. Enhancement Flotation Behavior of Electrode Materials by Pyrolysis XPS was used to analyze the surface properties of the pyrolytic electrode materials, and the results are given in Figure 7a and Table 3. C–CF and C–CF groups that were from the pyrolysis products (fluorobenzene) were detected on the surface of the pyrolytic electrode materials, and the Sustainability 2019, 11, 2363 9 of 11

3.5. Enhancement Flotation Behavior of Electrode Materials by Pyrolysis XPS was used to analyze the surface properties of the pyrolytic electrode materials, and the results areSustainability given in 2019 Figure, 11, x 7FORa and PEER Table REVIEW3. C–CF and C–CF groups that were from the pyrolysis products9 of 11 (fluorobenzene) were detected on the surface of the pyrolytic electrode materials, and the contents of Ccontents–CF and of C– CC–CFF groups and C– wereCF groups 10.89%. were In addition, 10.89%. pyrolyticIn addition, pyrolytic was also carbon detected was onalso the detected surface on of the pyrolyticsurface of electrode the pyrolytic materials, electrode the relativematerials, content the relative of pyrolytic content carbon of pyrolytic was 7.98%. carbon The was pyrolysis 7.98%. residuesThe pyrolysis were hydrophobic,residues were making hydrophobic, some cathode making materials some cathode easily attachmaterials to bubbles easily attach and collect to bubbles in the frothand collect product, in the which froth reduced product, its which flotation reduced efficiency. its flotation efficiency. In order to improve the separation eefficiencyfficiency of cathode and anode material, ultrasonic cleaning was used to remove the pyrolysis residues. Figure 7 7bb andand TableTable3 3show show the the XPS XPS analysis analysis of of pyrolytic pyrolytic electrode materialsmaterials after after ultrasonic ultrasonic cleaning. cleaning. The The contents contents of C of–CF C–CF and C–andC FC– groupsCF groups decreased decreased to 2.18%, to while2.18%, the while content the ofcontent pyrolytic of pyrolytic carbon decreased carbon todecre 4.59%.ased After to 4.59%. ultrasonic After cleaning, ultrasonic some cleaning, hydrophobic some pyrolysishydrophobic residues pyrolysis were residues removed. were The removed. hydrophily The of hydrophily cathode material of cathode improved, material enhancing improved, the flotationenhancing behavior the flotation of cathode behavior and of anode cathode materials. and anode materials.

Figure 7.7. C 1s XPSXPS spectrumsspectrums ofof electrodeelectrode materialsmaterials beforebefore andand afterafter pyrolysis:pyrolysis: (a) beforebefore ultrasonicultrasonic cleaning, (b)) afterafter ultrasonicultrasonic cleaning.cleaning.

Table 3. Chemical states of carbon in the surface of electrode materialsmaterials before andand afterafter pyrolysis.pyrolysis.

Pyrolytic Electrode Materials Pyrolysis-UltrasonicPyrolysis-Ultrasonic Cleaning Cleaning Electrode Electrode Materials Pyrolytic Electrode Materials Components BE (eV) FWHM (eV) Atomic (%) Components BE (eV)Materials FWHM (eV) Area Pyrolytic Pyrolytic 283.97BE FWHM 0.87 Atomic 2058.53 283.97BE 0.95FWHM 2206.17 Componentscarbon Componentscarbon Area Graphite 284.30(eV) 0.57(eV) 8442.1(%) Graphite 284.30(eV) 27,161.01(eV) 0.55 PyrolyticC–C 284.80 0.93 5574.71Pyrolytic C–C 284.80 7791.765 0.75 283.97 0.87 2058.53 283.97 0.95 2206.17 carbonC–H 285.71 1.00 350.76 C–Hcarbon 285.71 3607.48 1.12 C–CF 286.00 1.21 1803.41 C–CF 286.00 589.84 0.93 GraphiteO–C–O 287.14284.30 0.990.57 8442.1 734.85 O–C–O Graphite 287.14284.30 1491.1527,161.01 1.090.55 C–CFC–C 288.10284.80 1.010.93 5574.71 489.02 C–CF C–C 288.10284.80 938.7917791.765 1.150.75 C–HCF 289.40285.71 1.251.00 350.76 619.15 CF C–H 289.40285.71 1230.9623607.48 1.231.12 C–F 291.02 1.85 975.02 C–F 291.02 3001.55 1.88 C–CF 286.00 1.21 1803.41 C–CF 286.00 589.84 0.93 O–C–O 287.14 0.99 734.85 O–C–O 287.14 1491.15 1.09 FlotationC–CF technology288.10 was used1.01 to separate489.02 anode material C–CF from cathode288.10 material938.791 and the1.15 results were shownCF in Table289.404. The cathode1.25 material619.15 grade in the cathode CF material289.40 concentrate 1230.962 was increased1.23 to 93.89%,C–F while it291.02 was only 2.09%1.85 in anode975.02 material concentrate. C–F The291.02 cathode 3001.55 material recovery1.88 increased to 96.88%, which indicates a better flotation efficiency was obtained than in the previous studyFlotation [23]. Therefore, technology high was separation used to eseparatefficiency anode is beneficial material for from the subsequentcathode material purification. and the results wereIn shown this recycling in Table process,4. The catho organicsde material in electrode grade materials in the cathode were removed material usingconcentrate pyrolysis was technology. increased Atto 93.89%, the same while time, it the was pyrolysis only 2.09% products in anode were collectedmaterial toconcentrate. reduce the The environmental cathode material pollution recovery of the process.increased After to 96.88%, pyrolysis, which the indicates liberation a ebetterfficiency flotation and flotation efficiency effi wasciency obtained of electrode than in materials the previous were study [23]. Therefore, high separation efficiency is beneficial for the subsequent purification. In this recycling process, organics in electrode materials were removed using pyrolysis technology. At the same time, the pyrolysis products were collected to reduce the environmental pollution of the process. After pyrolysis, the liberation efficiency and flotation efficiency of electrode materials were improved, which is advantageous to the subsequent treating process. This research may provide an alternative flowchart for recycling spent LIBs. Sustainability 2019, 11, 2363 10 of 11 improved, which is advantageous to the subsequent treating process. This research may provide an alternative flowchart for recycling spent LIBs.

Table 4. Flotation results of pyrolysis-ultrasonic cleaning electrode materials.

Flotation Products Cathode Material Grade (%) Yield (%) Anode material concentrate 2.09 30.87 Cathode material concentrate 93.89 69.13 Cathode material recovery 96.88

4. Conclusions Pyrolysis technology was used to improve the recovery of cathode and anode materials from spent LIBs. Pyrolysis can remove organic binders that are wrapped on the surface of electrode materials, improving their liberation efficiency and flotation efficiency. The optimum pyrolysis parameters for the liberation of electrode materials were determined to be a pyrolysis temperature of 500 ◦C, a pyrolysis time of 15 min, and a pyrolysis heating rate of 10 ◦C/min. At this time, the liberation efficiency of cathode materials was 98.23% and the liberation efficiency of anode materials was 98.89%. Pyrolysis residues were further removed using ultrasonic cleaning. After this treatment, the flotation efficiency of electrode materials was clearly improved. The cathode material grade increased to 93.89% with a recovery of 96.88%. This research work provides an alternative method for recycling cathode and anode materials from spent LIBs.

Author Contributions: In this research, G.Z. is responsible for data processing and writing while Y.H. is in charge of directing the whole research work. Z.D. and T.Z. are in charge of carrying out the experiments. H.W. and W.X. are responsible for test analysis. Funding: This research was funded by the National Natural Science Foundation of China (No. 51574234 and No. 51674257), the National College Students’ innovative and entrepreneurial training program (No.201710290029) and Jiangsu Planned Projects for Postdoctoral Research Funds (1701038C). Guangwen Zhang also appreciates the support of Shanghai Tongji Gao Tingyao Environmental Science and Technology Development Foundation. Conflicts of Interest: The authors declare no conflict of interest.

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