Recovering metal values from spent lithium ion battery via a

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Recovering metal values from spent lithium ion battery via a combination of reduction thermal treatment and facile acid leaching Yue Yang, Wei Sun, Yongjie Bu, Chenyang Zhang, Shaole Song, and Yuehua Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01805 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Sustainable Chemistry & Engineering

Recovering metal values from spent lithium ion battery via a combination of reduction thermal treatment and facile acid leaching Yang Yue†, Sun Wei†,*, Bu Yongjie†, Zhang Chenyang†,*, Song Shaole†, Hu Yuehua†,*

†

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083,

PR China *E-mail:

[email protected]

(W.

Sun);

[email protected]

(C.

Zhang);

[email protected] (Y. Hu).

ABSTRACT Traditional acid leaching process for leaching metal values from spent lithium-ion batteries (LIBs) is low efficiency and inevitably consumes large amounts of reductants. In this study, a novel process, based on reduction thermal treatment and reductant-free acid leaching, for recycling valuable metals from spent LIBs has been developed. Firstly, thermodynamics calculation was performed to judge whether the reducing reaction between LiCoO2 and graphite can occur or not. And then, reduction thermal treatment experiments were conducted. The process was tested by TG-DTA method and reaction products were measured by X-ray powder diffraction and X-ray photoelectron spectroscopy. The experimental results agree well with thermodynamics analysis, and desired CoO and Li2CO3 were obtained under the optimum processing conditions of 600 oC, 120 min and molar ratio of LiCoO2 to graphite=2:1. Finally, almost 100% Li and Co were easily leached from reaction product under the conditions of 2.25 M H2SO4, 80 oC, 30 min and S/L=100 g·L-1, and Co and Li in leaching liquor were further separated with 35% PC88 at the ratio of aqueous to organic (A:O) equaling 0.5, 25 oC and pH=5.5. The proposed approach can not only make full utilization of waste anode graphite, but also benefit leaching valuable metals in absence of reductant, which significantly improves the economy and recovery performance of recycling spent LIBs.

Keywords Spent lithium ion battery, Valuable metal, Recycling, Reduction thermal treatment, Acid leaching

INTRODUCTION

The rapid development of lithium ion battery (LIBs) is jeopardized by two serious problems. The first challenge is spent LIBs. The number of spent LIBs has been constantly increasing as a result

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of LIBs consumption soaring. A large quantity of toxic metal elements (Co, Ni, Li, etc) and organic matter contained in spent LIBs pollute soil, air, and even ground water, resulting in serious environmental problems 1-3. The second issue is the shortage of resources, especially of lithium and cobalt. It is predicated that the demand of Li2CO3 in 2025 will reach 498 kt, so lithium will be in short supply in the next five years at the present mining rate 4, 5. Cobalt has been widely used in batteries and other industries. According to the quantitative analysis by Mudd et al., global cobalt resource currently is about 26.8 Mt 6. China is the largest refined cobalt producer around the world. It is predicated that China’s cobalt demand in 2030 will exceed 100 kt 7. The high criticality of lithium and cobalt causes their prices soaring and imposes significant pressure on battery industry. Therefore, recovering valuable metals from spent LIBs can not only eliminate environmental pollution, but also alleviate resource shortage.

Hydrometallurgy process has been widely used to recover metal values from spent LIBs due to the advantages of high metal recovery and low energy consumption 8. The whole hydrometallurgical process includes the following steps 9-11, i.e. discharging, dismantling, crushing, cathode material enrichment, leaching, purification, separation and product preparation. Among these steps, leaching enjoys great significance. Currently, the spent cathode materials, including LiCoO2 12-16, LiNixCoyMn1-x-yO2 11, 17, LiNixCoyAl1-x-yO2

18

and the mixture of two or more of them 19, 20, can be

effectively extracted by acid leaching. Some of transition metals are present in spent cathode materials at high valence(>+2), for example, Co3+ in LiCoO2, Ni2+, Co3+ and Mn4+ in LiNi0.33Co0.33Mn0.33O2, and Co3+ and Mn4+ in LiNi0.5Co0.2Mn0.3O2

21, 22

. But nickel, cobalt or

manganese is stable at the valence of +2 in aqueous solution, so reducing agent is necessary for leaching valuable metals from spent cathode materials to reduce transition metal elements from high valence states to a low valence state (+2). Generally, the reducing agent used in leaching process is Na2SO3 19, H2O2

12, 13

or organic reductant (ascorbic acid) 23. Among these reductants,

organic reductant is expensive, while Na2SO3 is easy to release pollutants of SOx 24. H2O2 is an environmentally friendly reductant, but it decomposes during the leaching process as Eq.(1) 4. 2 H 2 O 2 → 2 H 2 O + O 2↑

(1)

Therefore, to ensure sufficient reducing agent participating in the main reaction process (Eq.(2)) 25, the actual amount of H2O2 added must exceed the theoretical dosage 4, 12, 13, 16, which significantly increases the cost of leaching. 2LiMO 2 + H 2 O 2 + 6H + → 4H 2 O + 2Li + + 2M 2+ + O 2↑

(2)

In our previous work 26, we found that the structure of spent cathode material can be changed by


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the graphite contained in current collector of spent LIBs when the current collector is heated under the protection of nitrogen. Because some of valuable metals are reduced by thermal treatment, the obtained products can be easily leached with smaller H2O2 dosage. Inspired by this, a more effective and cost-efficient process for recovering metal values from spent cathode materials may be established. First of all, the spent negative graphite can be used as reducing agent, and the high valence state metal elements (such as Co3+) contained in spent cathode material are reduced to a low valence state (+2) with spent negative graphite by thermal reduction process. And then, the reduction product suffers acid leaching in absence of reductant. If the above process can be achieved, it will bring at least two advantages. On one hand, the mixture of spent cathode material and anode graphite enters the reducing system together, so it is unnecessary to separate them in the cathode material enrichment step, and the waste graphite can be fully utilized. On the other hand, when the high-valence metal elements are reduced to a low valence state (+2) with help of graphite, it is not necessary to use massive reductant in leaching valuable metals and the recovery cost reduces.

In this study, we attempt to develop a novel pyrometallurgical-hydrometallurgical combination method of thermal reduction followed by reductant-free leaching for recovering metal values from spent cathode materials. Spent cathode material LiCoO2 is chosen as a research object. LiCoO2 is relatively simple in structure but representative. It is of hexagonal α-NaFeO2 structure of

R 3m

group, similar to that of LiNixCoyMn1-x-yO2 or LiNixCoyAl1-x-yO2 cathode materials

27-29

.

Consequently, as the research object, LiCoO2 is not only convenient to study, but also provides significant

reference

for

treatment

of

other

materials,

such

as

LiNixCoyMn1-x-yO2,

LiNixCoyAl1-x-yO2, et al. The thermodynamic analysis of spent cathode material LiCoO2 reacting with graphite was performed. The thermal treatment process was characterized by TG-DTA. The factors impacting heating treatment performance, such as temperature, time and dosage of graphite, were investigated. The reaction products were measured by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Furthermore, the leaching and experiments of spent cathode material LiCoO2 after thermal reduction were carried out to check the effect of reductive thermal treatment. Finally, solvent extraction has been conducted to separate lithium and cobalt from leaching liquor.

EXPERIMENTAL Recycling process



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Spent lithium ion battery was provided by Brunp Recycling Technology Co., Ltd of China. Fig.1 shows the flow sheet for recovering valuable metals from spent lithium ion battery with proposed pyrometallurgical-hydrometallurgical combination method. After discharging, crushing and pneumatic separation, the plastic diaphragm was removed from current collectors and steel shell. The steel shell contained in the mixture can be recovered through magnetic method. The separated spent cathode and anode current collectors were preheated at 450 oC for 1 h to recover copper and aluminum foils and to enrich cathode and anode materials. The molar ratio of spent cathode material and graphite was firstly adjusted, and then the mixture of their powders was further sintered at a higher temperature to complete reduction process. Finally, the reduction product was leached with sulfuric acid. The leaching residue (left graphite) can return to thermal reduction process. The lithium and cobalt in leaching liquor were separated by solvent extraction method. A small amount of acid mist generating in leaching process can be effectively collected by a gas collecting system and washed with water before being discharged10. The acid washing water can return to leaching procedure for sulfuric solution preparation. Volatile kerosene is firstly collected with a gas collecting system, and then adsorbed with activated carbon. Moreover, the gas emitted from thermal treatment also needs treatment. The gas was firstly collected, and then the gas passes through a cyclone dust collector to remove dust. Finally, HF can be effectively adsorbed with NaOH solution10. After above disposal, the pollution in this process can be effectively controlled.

Fig. 1 The flow sheet for recovering valuable metals from spent lithium ion battery

Thermal reduction Thermodynamics calculation plays a significant role in the analysis of the studied reaction processing, which could predict the possible products for a thermodynamic system at a constant temperature and pressure. The Gibbs energy (also remarked as G) is the thermodynamic potential that is minimized when a system reaches chemical equilibrium at constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system vanishes at the equilibrium point. As such, a reduction in G is a necessary condition for the spontaneity of processes at constant pressure and temperature. It should be mentioned that all the reactions concerning heat changes of Gibbs free energy at different temperatures when the pressure being °

fixed at 1 atm are approximatively calculated from the standard formation enthalpy ( ∆ f H ) and °

formation entropy ( ∆ f S ) of the involved species at 298 K by the following equation: ∆G ( T ) =

∑ν P

P

∆ f H ° ( P , 298 K ) - T ∑ν P ∆ f S ° ( P, 298 K )

(3)

P

Where, P represents reactant or product;

ν P is the stoichiometric number of P with negative sign


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for product or positive sign for reactant; ∆ f H ° ( P, 298K ) and ∆ f S ° ( P, 298K ) are the corresponding standard formation enthalpy and entropy of P at 298 K, respectively. The involved standard formation enthalpy and entropy are extracted from Metallurgical Thermochemistry, which is written by Kubaschewski O and Alcock C B 30 and Refs 31 and 32 31, 32. Gibbs free energy can be thought of as a "dynamic" quantity, in that it is a representative measure of the competing effects of the enthalpic and entropic driving forces involved in a thermodynamic process. Usually, for a given reaction, G is the most crucial parameter to judge whether a given reaction can occur or not.

Thermal reduction reaction was carried out in a tube furnace (YQL-1400-80). 10 g mixture of spent cathode materials and waste graphite was ground in a mortar. Then, the mixture was heated to the experimental temperature at the rate of 8 °C·min−1. The thermal treatment was performed under the protection of high-purity argon (>99.999%) with the flow rate of 60 cm3·min-1.

Leaching All reagents used were of analytical grade. Sulfuric acid was purchased from Xilong Chemical Reagent Limited Corporation of China. Spent cathode material LiCoO2 after thermal reduction was leached with sulfuric acid solution. 10 g material was added in a 500 ml, three-necked and round-bottomed flask containing sulfuric acid solution. The pulp in the flask was magnetic stirred (200 rpm) in a water bath (Chao Yue, DF-101S). A condenser was installed at the bottom of the flask to prevent water loss. The leaching conditions, e.g. liquid-solid ratio (L/S), temperature (T), time (t) and sulfuric acid concentration, were investigated.

Solvent extraction Co and Li were recovered from leaching liquor by solvent extraction method. 2-ethylhexyl hydrogen-2-ethylhexyl phosphonate (PC88A) was used as extractant, and kerosene as diluent. Both PC88A and kerosene were purchased from Luoyang Zhongda Chemical Co. Ltd. Sulfuric acid and sodium hydroxide were applied to adjust pH values of the aqueous solution during extraction. The solvent extraction process lasted 8 min and the equilibrium pH value was measured by a pH meter of model 545.

Measurement and Characterization Crystal phases of spent cathode material before and after thermal reduction were measured through a powder X-ray diffraction instrument (XRD, Rigaku-2000) with help of a Cu Kα



radiation at a scan rate of 2°·min-1 with a step size of 0.02°. The thermal reduction process of spent cathode material and graphite was measured by TG-DTA analyzer (NETZSCH, STA449F3) at the heating rate of 10 °C·min−1. The TG-DTA test was divided into two steps, (1) the experiment conducted under the protection of high-purity argon when the temperature is less than 600 oC. (2) CO2 was injected into reaction system when the temperature when the temperature kept around 650 oC. The valence states of cobalt in spent cathode material and thermal treatment products were tested by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The concentrations of Li and Co in the leaching liquor were measured with an inductively coupled plasma atomic emission spectrometry (VISTA-MAPX CCD, VARIAN, ICP-AES). The leaching efficiency was calculated according to material balance. The morphology and composition of leaching residue were tested by a scanning electron microscope (KYKY-3900, SEM) equipped with an energy dispersive X-ray spectroscopy system (Thermo, EDXS).

RESULTS AND DISCUSSION Thermal reduction Table 1 displays the possible chemical reactions of thermal decomposition of LiCoO2 in presence of graphite and the corresponding reaction heat change of Gibbs free energy. It is obvious that reaction (4) and (5) are thermodynamically favorable, whose reaction heat changes of Gibbs free energy (∆G) are -289.0 kJ/mol and -173.0 kJ/mol at 25 oC (ambient temperature), respectively. Although ∆G(4) and ∆G(5) are negative at 25 oC, it is well known that LiCoO2 can not react with graphite at room temperature, which means that the happen of reaction 4 and reaction 5 need to overcome the barrier. Moreover, the ∆G(4) and ∆G(5) both become more negative as the increase of temperature, which means that the heating is beneficial to the reaction (4) and (5). Thus, the products should have Li2O, CoO, Co3O4 and CO2 based on the thermodynamics calculation of reaction (4) and (5). However, the reaction (6) heat change of Gibbs free energy is below -300 kJ/mol after 500°C, which shows that Co3O4 favors to react with graphite to form CoO and CO2. Thus, the intermediate Co3O4 continues to react with graphite to form the final product CoO. As shown by reaction (7) concerning heat changes of Gibbs free energy at different temperatures, ∆G turns positive when temperature exceeds 500°C, indicating that CoCO3 does not form over 500°C. In addition, in reaction (8), heat changes of Gibbs free energy are negative at all temperatures, suggesting that Li2O prefers to react with CO2 to form Li2CO3 in the temperature range of 500-900 °C. However, based on the variation tendency of ∆G, it can be predicted that Li2CO3 will be unstable when temperature exceeds 900 °C. Reaction (9) concerning heat changes of Gibbs free


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energy at different temperatures shows that when temperature is higher than 600 °C, CoO will react with graphite to form Co. To better demonstrate the possible products of the reaction between active graphite and LiCoO2 at different temperatures, the possible reaction products with respect to temperature has been given in Fig. 2.

Table 1 The possible chemical reactions of thermal decomposition of LiCoO2 in presence of graphite and the corresponding reaction heat change of Gibbs free energy

Fig. 2 Possible reaction intermediate and final products with respect to temperature

As shown in Fig.2, Co will be observed in products when the temperature is higher than 600 °C. For metal recovery of spent cathode material in this case, Co is undesirable for two reasons. On one hand, Co is difficult to be completely separated from Li2CO3 and graphite from agglomeration products of sintering by physical method. On the other hand, H2, an inflammable and explosive dangerous gas, will be easily released if the sintering product is leached with acid. Compared with Co metal, CoO is more suitable for leaching. The valence state of Co in CoO is +2, the same as in water, so Co2+ can be leached from sintering product conveniently without any reducing agent if the controlled roasting product only contains CoO. Therefore, the temperature of reduction thermal treatment should be kept at around 600 °C to avoid the generation of Co metal.

Fig. 3 TG-DTA curve of the mixture of LiCoO2 and graphite with the molar ratio of 1:1

Fig 3 shows the TG-DTA curve of the mixture of LiCoO2 and graphite below 700 °C. The weight of the mixture almost remains unchanged when going through thermal treatment with the temperature rising from 20 oC to 600 oC under the protection of argon, which indicates that no gas release from the reaction systerm. Based on the thermodynamic calculation, the possible products of reaction of graphite and LiCoO2 are CoO, CO2 and Li2O when temperature reaches 600 oC. However, CO2 can not react with CoO for the ∆G(7) is positive, so there is only one possibility that the product CO2 fully react with Li2O to form Li2CO3 as soon as CO2 formation (reaction (5)). For Li2O is excessive according to reactions (5) and (8), Li2O can be left after CO2 produced by the reaction (5) completely being consumed. To verify the accuracy of the above analysis, CO2 was injected into reaction system when the temperature between 600 oC and 700 oC. It can be clearly seen that the total weight of the substances increases after CO2 injection. The increase in weight is about 0.1%, which consists with the weight growth caused by CO2 reacting with left



Li2O to form Li2CO3 according to reactions (5) and (8). Therefore, when the heat treatment temperature is 600 oC, the whole reaction process can be expresses by reactions (5) and (8) and desired products CoO and Li2CO3 should be obtained.

Fig.4 The products of reduction thermal treatment of spent cathode material LiCoO2 and graphite at different temperatures, molar ratios of LiCoO2 to graphite=1:1, 60 min.

The thermodynamic analysis was examined by practical experiments. Fig.4 shows the products of reduction thermal treatment of spent cathode material LiCoO2 and graphite under different temperatures. As seen from Fig.4, LiCoO2 does not react with graphite when temperature keeps at 500 oC for 60 min, while Co3+ in LiCoO2 can be reduced to CoO (600 oC) and mixture of CoO and Co can be observed when the temperature increases to 700 oC and 800 oC. Meanwhile, Li is dissociated from LiCoO2 as Li2CO3 when reductive reaction happens. The experimental results agree very well with the thermodynamic calculation. Fig.5 shows the effects of sintering time on reductive process when the temperature is kept at 600 oC. It can be seen that the characteristic peak of LiCoO2 gradually disappears with prolonging processing time. When the time gets to 120 min, the products only consist of CoO, Li2CO3 and left graphite. It indicates that Co metal can be effectively avoided generating and CoO can be obtained through controlling the temperature and reaction time. Fig.6 shows the influence of molar ratios of LiCoO2 to graphite on the products. It proves that graphite is definitely going to be left and LiCoO2 can not be completely reduced when the molar ratio of LiCoO2 to graphite is higher than 2. It also can be seen from Fig.6 that the excess graphite does not affect the reaction product. Therefore, the target products of CoO and Li2CO3 can be obtained when the reaction conditions are controlled at 600 oC, 120 min and molar ratios of LiCoO2 to graphite is 2:1.

Fig.5 Influence of sintering time on reductive process: temperature kept at 600 oC, molar ratio of LiCoO2 to graphite=1:1.

Fig.6 Influence of molar ratios of LiCoO2 to graphite on reductive process: temperature kept at 600 oC, 120 min.

Fig.7 shows the XPS spectra of spent cathode materials (LiCoO2) before and after reduction thermal treatment under the optimum conditions of 600 oC, 120 min and molar ratios of LiCoO2 to graphite is 2:1. The Co 2p spectra of spent LiCoO2 before reduction thermal treatment are


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characteristic of Co3+ (780 eV) and the satellite structure (marked with S, 790 eV and 805 eV) is quite week 33, 34 (Fig.7 (a)). However, the Co 2p3/2 binding energy peaks of product obtained after reductive treatment at 781.2 eV and 797.3 eV are attributed to Co2+ in CoO

35, 36

(Fig.7 (b)).

Additionally, the characteristically intense satellite peaks (786 eV and 804 eV) of CoO are also investigated

35, 37

(Fig.7 (b)), which further demonstrates that Co3+ has been reduced to Co2+ and

CoO is obtained.

Fig.7 XPS spectra of spent cathode materials (LiCoO2) (a) before and (b) after reduction thermal treatment under the optimum conditions of 600 oC, 120 min and molar ratio of LiCoO2 to graphite=2:1.

Leaching As mentioned above, reductive process can transform LiCoO2 into CoO and Li2CO3. Compared with Co3+ contained in LiCoO2, Co2+ in CoO is obviously easier to be leached under more moderate conditions. Co and Li elements recovered from the products of reduction thermal treatment with acid can be expressed as Eq.(11) and (12). CoO + 2H + → Co 2 + + H 2 O

(11)

Li 2 CO 3 + 2H + → H 2 O + 2Li + + CO 2 ↑

(12)

As seen from Eq.(11) and (12), the valuable metals in spent cathode material LiCoO2 after reduction thermal process may be leached in absence of reductants (Na2SO3 or H2O2). To investigate the enhancement of leaching effect caused by the thermal reduction, the leaching experiments have been carried out. Fig. 8 shows the leaching behavior of powders after being treated at optimum reductive condition. It can be seen from Fig. 8 that the leaching efficiencies of Li and Co can almost reach 100% under the optimum leaching conditions of 2.25 M H2SO4, 80 oC, 30 min and S/L=100 g·L-1. To further investigate the leaching performance, the SEM image and corresponding dot maps of various elements in leaching residue under the optimum leaching conditions are provided. The results in Fig.9 indicate that the leaching residue only contains graphite, which corroborates that Co and Li are completely leached. Compared to the reported results of directly leaching valuable metals from spent cathode materials without reductive treatment (Table 2), this process enjoys advantages in greater throughput (S/L), being free of reductant, less acid dosage or shorter reaction time, which means a much lower cost and better metal recovery can be achieved with the help of reductive process.

Fig. 8 The leaching behavior of powders after reductive treatment under the conditions of 600 oC,



Page 10 of 28

120 min and molar ratio of LiCoO2 to graphite=2:1: (a) effect of H2SO4 concentration under leaching conditions of 80 oC, S/L=20 g·L-1 and 60 min; (b) effect of temperature under leaching conditions of 2.25 M H2SO4, S/L=20 g·L-1 and 60 min; (c) effect of time under leaching conditions of 2.25 M H2SO4, S/L=10 g·L-1 and 80 oC; (d) effect of S/L under leaching conditions of 2.25 M H2SO4, 30 min and 80 oC

Table 2 Operating conditions for leaching valuable metals from spent LiCoO2

Fig.9 SEM image and corresponding dot maps of various elements in leaching residue after leaching at 2.25 M H2SO4, 30 min, S/L=100 g·L-1 and 80 oC

Solvent extraction The cobalt and lithium in leaching liquor is separated by solvent extraction method. Fig. 10 shows the effect of pH on the separation of lithium and cobalt when the solvent extraction was conducted under the conditions of volume content of PC88A 35%, ratio of aqueous:organic (A:O)=0.5 and 25 oC. The extraction efficiency of cobalt increases with pH increasing. The extraction reaction can be expressed as Eq.(13) 38, nHA(org)+Mn+=MAn(org)+nH+

(13)

Where HA represents the extractant PC88A, M represents metal ion, n represents the valence state of metal ion. According to Eq.(13), the distribution ratio can be expressed as Eq.(14)38, logD=logK+nlog[HA](org)+npH

(14)

where D is distribution ratio (the ratio of the concentration of metal ion in the organic phase to the aqueous phase), K represents the equilibrium constant of Eq.(13). As seen from Eq.(14), distribution ratio D increases with pH increasing. Meanwhile, the extraction ability is related to the valence state of metal ion. Metal ion is much easier to be extracted by PC88A when the value of n is higher. Therefore, the extraction efficiency of Co2+ is higher than that of Li+. Moreover, when the pH goes up by 1 unit, the distribution ratio goes up 10n times, which means pH increase favors the extraction of Co2+ than Li+. Therefore, almost 100% cobalt can be extracted from leaching liquor when the solvent extraction was conducted at pH=5.5 with negligible extraction rate of lithium, which means the cobalt and lithium can be successfully separated from each other with PC88A through a simple single-stage extraction. Moreover, the extracted cobalt can be recovered as CoSO4 by stripping and evaporative crystallization method as described in Ref 39 39. And the lithium left in liquid phase can be further directly recycled by carbonate precipitation method 40, 41.


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Fig. 10 The effect of pH values on the separation of lithium and cobalt by solvent extraction under the conditions of volume content of PC88A 35%, A:O=0.5 and 25 oC

CONCLUSION

In this study, a more effective process with the combination of reduction thermal treatment and facile acid leaching has been developed for recycling valuable metals from spent LIBs. The thermodynamics calculation and practical thermal treatment experiments indicate that LiCoO2 can be reduced to CoO and Li2CO3 by graphite at 600 oC based on the following reaction mechanism, C+4LiCoO2 → 2Li2O+4CoO+CO2 Li2O+CO2→Li2CO3 The optimum conditions for reduction of LiCoO2 is 600 oC, 120 min and molar ratio of LiCoO2 to graphite=2:1. The reduction process is greatly beneficial to subsequent acid leaching, which makes the valuable metals easier to be leached. The leaching results indicate that almost 100% Li and Co can be leached from reaction products through a reductant-free leaching process under the optimum conditions of 2.25 M H2SO4, 80 oC, 30 min and S/L=100 g·L-1. Finally, Co and Li contained in leaching liquor is separated with solvent extraction method under the conditions of volume content of PC88A 35%, A:O=0.5, 25 oC and pH=5.5. The whole process can make full use of waste graphite and enhance the leaching efficiency of valuable metals, which enjoys great significance in spent LIBs recycling.

Supporting Information The effect of A:O and content of PC88A on the extraction of lithium and cobalt under the condition of pH=5.5 at 25 oC.

CORRESPONDING AUTHOR *E-mail: [email protected]; [email protected]; [email protected].

ACKNOWLEDGEMENT This work was supported by the scientific research starting foundation of central south university (202045006).

NOTES



Page 12 of 28

The authors declare no competing financial interest.

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Graphical abstract Reductive thermal treatment Graphite+LiCoO2

Li + Co 2+

Co 2+

Co 2+

Co 2+

Co 2+

Li +

Li +

+graphite

2 Co

2+

Li +

Co

Li +

+

Acid leaching CoO Li2CO3 LiCoO2

Cobalt product Lithium product

Li C O

Oil phase

Co

Co 2+

Co 2+ Co 2+

Co 2+

Co 2+

Co 2 +

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

Co 2+

Aqueous phase Li +

Li + Li +

Li

+

Co2+ Li

+

Li +

Li +

Solvent extraction

A more economic and effective approach for recycling metal values from spent LIBs was developed with utilization of waste graphite


Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 1 The flow sheet for recovering valuable metals from spent lithium ion battery



Fig. 2 Possible reaction intermediate and final products with respect to temperature


Page 18 of 28

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Fig. 3 TG-DTA curve of the mixture of LiCoO2 and graphite with the molar ratio of 1:1



Fig.4 The products of reduction thermal treatment of spent cathode material LiCoO2 and graphite at different temperatures, molar ratio of LiCoO2 to graphite=1:1., 60 min.


Page 20 of 28

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Fig.5 Influence of sintering time on reductive process: temperature kept at 600 oC, molar ratio of LiCoO2 to graphite=1:1.



Fig.6 Influence of molar ratios of LiCoO2 to graphite on reductive process: temperature kept at 600 oC, 120 min.


Page 22 of 28

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co 2p 2+

2p3/2

Co

Co

S

Co

3+

2+

2p1/2

(b)

2p3/2 Co

3+

2p1/2

S

S

780

S

790

800

(a)

810

Binding energy/(eV) Fig.7 XPS spectra of spent cathode materials (LiCoO2) (a) before and (b) after reduction thermal treatment under the optimum conditions of 600 oC, 120 min and molar ratio of LiCoO2 to graphite=2:1.



(b)

(a)

100

96

Leaching efficiency/(%)


100

Li Co

92

88

95

Li Co

90 85 80 75

0.5

1.0

1.5

2.0

2.5

H2SO4/(M)

(c)

70

3.0

50

60

70

80

90

Temperature/(oC)

(d)

100

100

90



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

Li Co

80

70

98

Li Co

96

94

60 0

20

40

60

80

100

0

0

20

40

60

80

.

100

120

140

160

S/L/(g L-1)

Time/(min)

Fig. 8 The leaching behavior of powders after reductive treatment under the conditions of 600 oC, 120 min and molar ratio of LiCoO2 to graphite=2:1: (a) effect of H2SO4 concentration under leaching conditions of 80 oC, S/L=20 g·L-1 and 60 min; (b) effect of temperature under leaching conditions of 2.25 M H2SO4, S/L=20 g·L-1 and 60 min; (c) effect of time under leaching conditions of 2.25 M H2SO4, S/L=10 g·L-1 and 80 oC; (d) effect of S/L under leaching conditions of 2.25 M H2SO4, 30 min and 80 oC


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Fig.9 SEM image and corresponding dot maps of various elements in leaching residue after leaching at 2.25 M H2SO4, 30 min, S/L=100 g·L-1 and 80 oC



100

Extraction efficiency/(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Lithium Cobalt

80

60

40

20

0 3.5

4.0

4.5

5.0

5.5

6.0

6.5

pH value

Fig. 10 The effect of pH values on the separation of lithium and cobalt by solvent extraction under the conditions of volume content of PC88A 35%, A:O=0.5 and 25 oC


Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 The possible chemical reactions of thermal decomposition of LiCoO2 in presence of graphite and the corresponding reaction heat change of Gibbs free energy ∆G

Reactions

o

(4). C+12LiCoO2→6Li2O+4Co3O4+CO2 (5). C+4LiCoO2 → 2Li2O+4CoO+CO2 (6). 2Co3O4+C→6CoO+CO2 (7). CoO+CO2→CoCO3 (8). Li2O+CO2→Li2CO3 (9). 2CoO+C→2Co+CO2 (10). CO2+C→2CO

o

o

25 C

500 C

600 C

700 oC

800 oC

900 oC

1000 oC

-289.0

-391.4

-413.0

-434.5

-456.1

-477.7

-499.3

-173.0

-311.4

-340.5

-369.6

-398.8

-427.9

-457.0

-181.7

-334.0

-366.0

-398.1

-430.1

-462.2

-494.2

-25.8

58.7

76.5

94.2

113.0

129.8

149.6

-175.8

-99.2

-83.1

-66.9

-50.8

-34.7

-0.1

83.8

6.8

-9.4

-25.7

-41.9

-58.1

-74.3

120.1

36.6

19.0

1.5

-16.1

-33.7

-51.3



Page 28 of 28

Table 2 Operating conditions for leaching valuable metals from spent LiCoO2 Material

Leaching agent

Conditions/Temp., time and pulp density

LiCoO2

2 M H2SO4 + 5 vol % H2O2

75 oC, 1 h, 100 g·L-1

Li~70 %, Co~99.1 %

12

LiCoO2

1 M HNO3 + 1.7 vol% H2O2

75 oC, 1 h, 20 g·L-1

Li~95, Co~95

13

LiCoO2

4 M HCl

80 oC, 1 h, 20 g·L-1

Li>99%, Co>99%

14

LiCoO2

1.5 M H2C2O4

80 oC, 2 h, 50 g·L-1

Li>98%, Co>98%

15

LiCoO2

1.5 M succinic acid + 4 vol% H2O2

70 oC, 0.67 h, 15 g·L-1

Li>96, Co∼100

16

LiCoO2

2.25 M H2SO4

80 oC, 0.5 h, 100 g·L-1

Co~100 %, Li ~100 %


Recovery efficiency

Ref

This work

Recovering metal values from spent lithium ion battery via a

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