Hongshuai Zheng,Jiaqi Huang,2,Tao Dong,,Yifan Sha,Haitao Zhang,2,Jie Gao,Suojiang Zhang,

1 CAS Key Laboratory of Green Process and Engineering,Beijing Key Laboratory of Ionic Liquids Clean Process,State Key Laboratory of Multiphase Complex Systems,Innovation Academy for Green Manufacture,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2 Henan Institute of Advanced Technology,Zhengzhou University,Zhengzhou 450002,China

3 Ningbo Institute of Materials Technology &Engineering,Chinese Academy of Sciences,Ningbo 315201,China

Keywords:Ionic liquid Lithium Selective extraction Spent lithium batteries Recovery

ABSTRACT In light of the increasing demand for environmental protection and energy conservation,the recovery of highly valuable metals,such as Li,Co,and Ni,from spent lithium-ion batteries(LIBs)has attracted widespread attention.Most conventional recycling strategies,however,suffer from a lack of lithium recycling,although they display high efficiency in the recovery of Co and Ni.In this work,we report an efficient extraction process of lithium from the spent LIBs by using a functional imidazolium ionic liquid.The extraction efficiency can be reached to 92.5% after a three-stage extraction,while the extraction efficiency of Ni-Co-Mn is less than 4.0%.The new process shows a high selectivity of lithium ion.FTIR spectroscopy and ultraviolet are utilized to characterize the variations in the functional groups during extraction to reveal that the possible extraction mechanism is cation exchange.The results of this work provide an effective and sustainable strategy of lithium recycling from spent LIBs.

1.Introduction

Lithium-ion batteries(LIBs)are extensively employed in mobile power supplies,other portable electronic equipment and electric vehicles on account of the advantages of light weight,high energy density and long service life[1–6].However,as the thriving application of LIBs,a large number of spent batteries are generated with the presence of harmful metals and flammable electrolytes which may threaten the environment and human health [7].Meanwhile,the cathode material contains several kinds of valuable metals(e.g.,Li,Co and Ni)[8].Hence,considering the serious environmental issues and the scarcity of resources,the recycling of valuable metals from spent LIBs is urgently requiring significant attention.However,nowadays,up to 3%of LIBs are working on the recovery of valuable metals,while the recycling rate of lithium is quite few[9–11].

Currently,the methodology for spent cathode material recycling consists of hydrometallurgy and pyrometallurgy,among which the hydrometallurgy as a kind of well-established process enjoys advantages in higher purity,lower energy loss and lower gas release[12,13].In particular,solvent extraction with high separation effect plays an important role in hydrometallurgy and is widely applied to industrialization[14].But the application of traditional extraction agents needs saponification and using organic diluents,which generally cause extraction agents loss and a large amount of spent liquor produced [15].Many efforts have been made to emerge a potential alternative recently [16–19].

Ionic liquids (ILs) are composed of ions over a wide range of temperatures and have the characteristic of low volatility,nonflammability and environmental friendliness,which is greatly applicable for the solvent extraction process in separation metals[20,21].Moreover,the properties of ILs could be transformed by choosing different cations and anions.During the solvent extraction process,the change of the ILs’ cations and anions and their structure leads to some diversities in the behavior of the metals,so that influences the efficiency and mechanism of extraction[22,23].

Torkaman et al.[24] demonstrated that the extraction system containing bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex301) and tri-octyl amine (TOA) can effectively recover higher than 90% Co from spent LIBs.In addition,Dhima and Gupta [25]reported that toluene diluted trihexyl(tetradecyl)phosphonium bromide(Cyphos IL-102)had excellent recovery ability of Co from spent LIBs,and the recovery of Co,Mg and Li as oxides or carbonate are 98.6%,99.9% and 99.6% respectively.

Lithium is well known as a rare metal with high economic value,and is seen as a vital resource by many countries [26–28].Lithium has important application value in lithium-ion batteries and other fields.Considering the current global lithium production and the high demand,it is very necessary to recover and recycle lithium.Over the extraction of lithium in the spent lithium-ion batteries,there are currently few studies.But many researchers have done a lot of research on the recovery of lithium from salt brine using ILs.Shi et al.[29] take advantage of ILs [C2mim][NTf2],[C4mim][NTf2] and functional ILs tetrabutylammonium bis(2-ethylhexyl) phosphate esters ([N4444][DEHP]) to study the extraction of Li and Mg from lake brine.The recovery of Li could reach over 91% by utilizing an extraction system containing a task-specific ILs [N4444][DEHP].Wang et al.[30] used methyltrioctylammonium ion ([A336]+) as the cation and four deprotonated nordione as the anion,synthesized an ILs and extracted more than 91% lithium from a lithium-containing solution system.However,these extraction systems are not suitable for the sulfuric acid leaching solution of spent LIBs because the extraction and separation are more difficult due to the high content of metals in the leachate and the wide variety.The current recovery of lithium is faced with great challenges.The solvent extraction process to extract Mn,Ni or Co will lead to co-extraction of lithium,resulting in a decline in lithium recovery.

To address this issue,a preferential selective extraction of lithium was proposed and a solvent extraction system containing a novel ILs with Tributyl phosphate(TBP)extractant that was used to recover Li was prepared.The effects of the aqueous acidity,temperature and extractant concentration were also evaluated in the aspects of their influence on the extraction behavior of lithium.Fourier transform infrared (FTIR) spectroscopy was used to examine the reactions between the ligands and the lithium ions.The thermodynamic parameters of the extraction reaction were determined.An effective and sustainable strategy of Li recycling from spent LIBs using ionic liquid as extraction agent was proposed.

2.Experimental

2.1.Materials

The 1-carboxymethyl-3-methylimidazolium bis(trifluorome thylsulfonyl)imide([HO2CMMIm][NTf2])(99%(mass))was supplied by Linzhou Keneng Material Technology Co.Ltd.,China and used without further purification.Tributyl phosphate (TBP) was purchased from Macklin Co.Ltd.,China.Sulfuric acid (99.9%(mass)high purity) and H2O2(30%(mass) high purity) were purchased from Aladdin Co.,China.Sodium hydroxide (95.0%(mass),Macklin Co.,China,analytical grade) was used for pH adjustment and all aqueous solutions were prepared from high-purity Milli-Q water(conductivity ＜0.1 μS·cm-1,resistivity ＞18.2 MΩ·cm).

2.2.Procedure and method

The leachate was obtained from sulfuric acid leaching liquid of cathode materials (LiNi0.5Mn0.3Co0.2O2) in spent LIBs under conditions as follows:H2O2(2.0%(vol))+H2SO4(2.0 mol·L-1),the reaction temperature of 80 °C,the solid/liquid ratio of 50.0 g L-1,and the reaction time of 120 minute which the concentrations of different metals were listed in Table 1.The obtained leachate was adjustedby NaOH solutions to be an aqueous phase.The organic phase was prepared by adding a required amount of ionic liquid into TBP.At a constant temperature,the mixture of organic phase and water phase was shaken with a powerful shaker(HJ-4A,Guohua,China).Then the organic phase and the water phase were separated using a centrifuge (TG16-WS,Cence,China) (6000 r·min-1,9 min).After phase separation,the metal concentration in the aqueous solution was measured by inductively coupled plasma atomic emission spectrometry (ICPE-9000,Japan).And calculate the concentration in the organic phase according to the mass balance during the extraction reaction.The multistage cross-flow extraction was a repetition of single stage extraction,but the aqueous phase was the separation of the previous stage extraction.The concentration of [HO2CMMIm]+in the raffinate was measured by an ultraviolet–visible spectrophotometer (UV-2550,Shimadzu,Japan).The detailed process was described as following:An acetonitrile solution of 0.1 mol·L-1ionic liquid was used as standard and then diluted to different multiple with deionized water to draw a standard curve by UV spectrum.The raffinate was diluted to a certain multiple and determined quantitatively to calculate the concentration of imidazole cation in the raffinate.

Table 1 Compositions and concentrations of metals in the leachate.

The extraction efficiency (E) and separation factor (β) can be determined by combining the data obtained in the extraction experiment and the following formula:

Among them,the equilibrium concentrations of metal ions in the organic phase in organic or aqueous phase is Coor Ce.The volumes of the organic phase and the water phase after equilibrium are Voand Vaqrespectively.The initial concentration of metal ions in the water phase is denoted by Ci,and the initial volume is denoted by Vi.

3.Results and Discussion

The compositions and concentrations of the valuable element in the leachate obtained from spent LiNi0.5Mn0.3Co0.2O2was listed in Table 1 and the contents of lithium,cobalt,nickel and manganese in the cathode powder were around 33.50%,7.75%,10.13% and 10.23%,respectively.

3.1.Effect of volume percentage of ILs

The effect of the concentration([HO2CMMIm][NTf2])on lithium extraction was investigated under the conditions of extraction conditions:O/A=2:1,extraction time of 30 minutes,and initial pH of 5.0 and at room temperature.The extraction efficiencies of Li,Co,Ni and Mn with the concentrations of [HO2CMMIm][NTf2] were shown in Fig.1.Obviously,when TBP was used alone as the extractant,the extraction efficiency of lithium was 24.3%.But the extraction efficiency was significantly improved after adding ILs.And it was clear that the extraction efficiency increased sharply with the increase [HO2CMMIm][NTf2] from 0%(vol) to 20%(vol),and then decreased slowly with the further increase of [HO2CMMIm][NTf2]to 40%(vol).The extraction efficiencies of other kinds of ions had a similar trend,but far lower than lithium ion,so[HO2CMMIm][NTf2] had a good selectivity for lithium.With the increase of[HO2CMMIm][NTf2]from zero to 20%(vol),the extraction efficiency of lithium increased to 82.1%.Subsequently,the lithium extraction efficiency decreased to 71.3% by further increasing [HO2CMMIm][NTf2] to 40%(vol).It might be that as the concentration of the ILs increases,the viscosity of the extractant increases gradually,which will affect the diffusion of lithium ions from the aqueous phase to the organic phase,thereby reducing the extraction efficiency.In addition,due to the increase in the proportion of ILs in the organic phase,the concentration of TBP decreases,thereby may be reduce the feasibility of the reaction between lithium ions and TBP.From the results obtained above,the 20%(vol) volume concentration of ILs was chosen as one of the optimum conditions for further study.

Fig.1.Effect of [HO2CMMIm][NTf2] concentration on lithium extraction at an O/A ratio of 2:1,298.15 K and pH of 5.0.

3.2.Effect of initial pH

It is necessary to study the impact of pH value of leachate on the extraction of lithium ion using in[HO2CMMIm][NTf2]and 80%(vol)TBP at an O/A=2/1,extraction time of 30 min and room temperature.The pH value of leachate was adjusted by sodium hydroxide varied from 1.0 to 5.0 as shown in Fig.2,It was observed that the extraction efficiencies of lithium ion increased with an increase in pH value.The extraction efficiency of lithium was increased from 31.2% to 82.7% as the range of pH=1.0–3.0.This trend was similar to the characteristics of other metal ions in the leachate.The results may indicate a relatively strong competition between hydrogen and metal in the leachate during the extraction reaction.And excessive hydrogen ion concentration inhibits the extraction efficiency of metal ions.

3.3.Effect of O/A on the extraction efficiency

The O/A phase ratio has a substantial impact on extraction efficiency for the solvent extraction process.The effect of O/A ratio on the extraction was studied in the range of 0.5–3.0 with the conditions:20%(vol) [HO2CMMIm][NTf2] and 80%(vol) TBP,extraction time of 30 min,room temperature and pH value of 5.0.The results were listed in Fig.3.It was clear that the extraction efficiency of lithium exhibited an increased trend with increased O/A ratio.The extraction efficiency of lithium was 48.3% when the O/A ratio was 1/2,while the O/A ratio up to 3/1,the extraction efficiency of lithium increased to 83.7%,which was a small increase compared with the O/A=2/1.In the view of economics,the O/A=2/1 was chosen for further study.

As mentioned above,lithium can be selectively extracted from the sulfuric acid leachate of spent LIBs under optimal conditions:20.0%(vol) ILs,80.0%(vol) TBP,extraction time of 30 minutes,an initial pH of 5.0 and an O/A=2/1 in room temperature.The extraction efficiency of lithium reaches to 83.7%,and the extraction efficiencies of Co,Mn,Ni are less than 3.0%,which can realize the preferential and selective recovery of lithium.

Fig.2.Effect of leachate pH on the extraction of lithium using the extraction system consisting of 20% [HO2CMMIm][NTf2] and 80%(vol) TBP at an O/A ratio of 2:1,298.15 K.

Fig.3.Effect of leachate O/A on the extraction of lithium at 298.15 K and pH of 5.0.

3.4.Extraction mechanism

Generally,the cation ion exchange mechanism often occurs in the extraction process when imidazole ionic liquids are diluents.[31,32].In order to further examine the potential mechanisms underlying the extraction processes using [HO2CMMIm][NTf2]combined with TBP as extractant,extraction experiments were performed using optimized parameters.The concentration of[HO2CMMIm]+in the raffinate is analyzed with an ultraviolet–visible spectrophotometer after extraction equilibrium and calculated according to the standard curve.It was clearly observed from Fig.4:the raffinate exhibited single ultraviolet absorption peak(λmax=211 nm),which belonged to [HO2CMMIm]+cation.Obviously,when the leachate was adopted as aqueous phase,the spectral absorbance was significantly enhanced after extraction,which indicated that the cations in the ILs were transported into the aqueous phase by ion exchange during the extraction.

Fig.4.(a) UV–vis spectra of the raffinate after extraction;(b) The standard curve of different concentration of [HO2CMMIm]+.

To further verify the above mechanism,Fig.5 shows the FT-IR of the organic phase before and after extraction.The peak at 2961 cm-1and 2876 cm-1can be attributed to the stretching vibration of -CH3group with no change after extraction.In addition,the shift of P＝O stretching vibration from 1264 cm-1to 1254 cm-1revealed that the lithium ion might be coordinated to the oxygen of P＝O.

Combined the analysis of UV and FTIR spectra,the reaction is elucidated as Eq.(3) [33]:

Where n is the number of TBP molecules involved in the reaction,and the aqueous phase and the organic phase are denoted by aq and org,respectively.

From Eq.(3),Kexcould be calculated by Eq.(4):

The distribution ratio of Li+was calculated by Eq.(5):

Fig.5.FT-IR transmittance spectra of organic phase before and after extraction.

Therefore,the relationship between DLiand Kexwas shown in the Eq.(6).

Kexof the reaction is invariable at a certain temperature.The concentration of TBP is similar to the initial value which was higher than that of lithium,while Fig.6 described the detailed function ofon TBP concentration.The slope was calculated as 1.1023 according to decay analysis of the extracted data,indicating that one molecule of TBP formed a complex with lithium ions during the reaction as shown in Eq.(7).

3.5.The thermodynamic parameters of extraction

The thermodynamic parameters of the reaction were determined through previous extraction experiment data and Vanter Hoff equation.It was clear from Fig.7 that the extraction efficiency decreases with the temperature increase varied from 293.13 K to 333.13 K.Inferred by the van’t Hoff and equations in extraction reaction,the enthalpy change (ΔH) was obtained by Eq.(8):

Fig.6.Plot of lg DLi + as a function of lg [TBP].

Fig.7.Effect of temperature on extraction at an A/O ratio of 1:2,initial pH of 5 and standard atmospheric pressure.

Where C0represent the integration constant.Combine the Eq.(7)and other equation,Eq.(9) can be obtained:

The integral constant(C)is constant if the temperature does not change.From the Fig.8,the slope of a plot of lgDLi+versus 1000/T,the enthalpy change(ΔH) was calculated.Therefore,ΔH is a negative value (-6.72 ± 1.76 kJ·mol-1),indicating that it is more conducive to lowtemperature extraction.In order to get the thermodynamics of the reaction,Gibbs free energy (ΔG) and entropy change (ΔS) are expressed by the following formula:

It is found that the ΔG is negative (-7.62 ± 2.43 kJ·mol-1),which means the lithium extraction process is spontaneous at room temperature.The ΔS is positive (2.85 ± 2.71 J·mol-1·K-1),indicating that the degree of disorder of the extraction system increases during the reaction.

Fig.8.Plot of lg DLi+ as a function of 1000/T.

Fig.9.Effect of No.of extraction stages on extraction efficiency at a temperature of 298.15 K,initial pH of 5.0,A/O ratio of 1:2 and standard atmospheric pressure.

3.6.Multistage extraction

The extraction efficiency of a single stage is limited.For further improving the recovery of lithium,the experiments of multistage cross-flow extraction were carried out under the optimized conditions to facilitate the re-exchange equilibrium of lithium ion in the two phases and achieving the aim of enrichment of lithium ion.The results showed that the extraction efficiency of lithium was up to 92.5%,while the extraction efficiencies of cobalt,nickel and manganese are 2.1%,3.9% and 3.2%,respectively after the threestage extraction,which indicated that the efficient and selective extraction and separation of lithium ions can basically be realized,as shown in Fig.9.In summary,multi-stage extraction can better achieve efficient lithium recovery than single-stage extraction.

3.7.Scrubbing and stripping process

In order to obtain the high purity lithium products,the studies of scrubbing and stripping were investigated.The solution contains 0.50 mol·L-1H2SO4and 0.50 mol·L-1Li2SO4to remove other metal ions in the organic phase,which can improve the purity of lithium ion recovery effectively.And then H2SO4was selected to be used as a stripping agent suitable for eluting from the loaded organic phase Li+.The effect of different stripping times of sulfuric acid on the reverse extraction of Li+under the condition of phase volume ratio of 1:1 was investigated shown in Fig.10.It can be seen that 96.53%of lithium ions can be recovered after three times of stripping with 1.00 mol·L-1H2SO4.Finally,a lithium sulfate solution with higher purity can be obtained through the above methods.

Fig.10.Effect of the stripping stages on stripping efficiency using 1.0 mol·L-1 H2SO4 at a temperature of 298.15 K,A/O ratio of 1:1 and standard atmospheric pressure.

4.Conclusions

A large number of discarded lithium-ion batteries have become an important lithium-containing resource because of the high lithium content and purity.A facile,economically and environmentally efficient approach was proposed to extract lithium from the sulfuric acid extract of spent LIBs by novel ILs.It was found that the lithium ion was easy to recover by an organic phase composed of 20.0%(vol) [HO2CMMIm][NTf2] and 80.0%(vol) TBP with an O/A=2:1,room temperature,initial pH of 5.0.The extraction efficiency of Li can be reached to 92.5% after the three-stage extraction,while the extraction efficiencies of Ni,Co,Mn were less than 4.0%.The findings of the slope method,UV–visible spectrometer and FTIR spectroscopy provided an insight into the mechanisms of extracting lithium ions,which can be obtained from a 1:1 complex formed between lithium ions and TBP molecule,indicating an ion-exchange reaction.The calculated thermodynamic function of the system represents the spontaneous and exothermic of the extraction process.Thus,it is hoped that the approach given in this article will be presented with great significance to recycling and this ILs extractant is feasible for separating lithium from complex multi-metal systems,and provides a theoretical basis for recycling waste lithium-ion batteries and reducing resource waste.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Science Fund for Major Program of National Natural Science Foundation of China(21890762) and Innovation Academy for Green Manufacture,Chinese Academy of Sciences (IAGM-2020-C28).