Recovering valuable metals from LiNixCoyMn1-x-yO2 cathode materials of spent lithium ion batteries via a combination of reduction roasting and stepwise leaching

Accepted Manuscript Recovering valuable metals from LiNixCoyMn1-x-yO2 cathode materials of spent lithium ion batteries via a combination of reduction roasting and stepwise leaching Pengcheng Liu, Li Xiao, Yifeng Chen, Yiwei Tang, Jian Wu, Han Chen PII:

S0925-8388(18)34782-0

DOI:

https://doi.org/10.1016/j.jallcom.2018.12.226

Reference:

JALCOM 48867

To appear in:

Journal of Alloys and Compounds

Received Date: 3 September 2018 Revised Date:

23 November 2018

Accepted Date: 17 December 2018

Please cite this article as: P. Liu, L. Xiao, Y. Chen, Y. Tang, J. Wu, H. Chen, Recovering valuable metals from LiNixCoyMn1-x-yO2 cathode materials of spent lithium ion batteries via a combination of reduction roasting and stepwise leaching, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2018.12.226. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Recovering valuable metals from LiNixCoyMn1-x-yO2 cathode materials of spent lithium ion batteries via a combination of reduction roasting and stepwise leaching

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Pengcheng Liu1, Li Xiao1,⃰, Yifeng Chen1, Yiwei Tang2,⃰ Jian Wu3, Han Chen1

1. School of Metallurgy and Material Engineering, Hunan University of Technology,

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ZhuZhou 412007, China；

China

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2. Guangdong JiaNa Energy Technology company, Qing yuan, Guangdong, 513000,

3. School of Metallurgy and Environment, Central South of University, Changsha 410012, China author:

Li

Xiao

([email protected])

and

Yiwei

Tang

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⃰Corresponding

([email protected])

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Abstract

This work focuses on the recovery of valuable metal from the cathode materials

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of spent lithium ion batteries to ensure resource recycling and environmental protection. An environmentally friendly process involving reduction roasting and stepwise leaching is proposed to recover Li and Ni, Co, and Mn from spent LiNixCoyMn1-x-yO2 materials. Suitable leaching conditions are obtained from thermodynamic analysis (Eh-pH diagram). The effects of several factors, such as acid concentration, temperature, leaching time and liquid‒solid ratio, on the leaching efficiency of valuable metals are investigated. Under optimum conditions, the

ACCEPTED MANUSCRIPT leaching efficiency of Li, Ni, Co, and Mn are as high as 93.68%, 99.56%, 99.87%, and 99.9%, respectively. The kinetic aspect of the acid leaching process is analyzed by shrinking the core model, which suggests a residue layer diffusion process. The

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reaction activation energies of Ni, Co, and Mn are 29.35 kJ mol−1, 24.00 kJ mol−1, and 23.29 kJ mol−1, respectively. This metallurgic method contributes to environmentally friendly and economical recovery of valuable metals from spent lithium-ion batteries.

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Key words: Spent Li-ion battery; Recycling; LiNixCoyMn1-x-yO2; Kinetics; Liquid

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solid reactions

1. Introduction

Lithium-ion batteries (LIBs), which are known for their advantages, such as high energy density, no-memory effect, and low self-discharge rate, have been widely used

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in portable electronic devices, electric vehicles (EVs), and hybrid electric vehicles [1]. Navigant Research estimates that over 5 million public EVs will be on roads globally;

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by the end of 2018 and may reach 10 times this figure by 2027 [2]. With their rapidly growing consumption and relatively short average service lifetime of 1–3 years, the

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amount of discarded electrode scrap has increased dramatically [3]. By 2020, the weight of spent LIBs in the European Union alone is expected to reach 13828 tons [4]. In China, the total quantity and weight of exhausted LIBs exceeds 25 billion and may even reach 500,000 metric tons by 2020 [5]. These spent LIBs not only contain a large amount of toxic materials and organic chemicals, but also substantial valuable metals, such as lithium, nickel, cobalt, and manganese [6, 7]. These metals have higher grades than those found in natural raw ores. Therefore, proper treatment and sustainable

ACCEPTED MANUSCRIPT recovery of valuable metals from spent LIBs are beneficial to environmental protection and resource recycling. In general, LIB comprises an anode, a cathode, an electrolyte, a collector,

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separators, and a metallic shell. Given the demand to reduce costs and increase energy density, graphite has been used as anode materials. LiNixCoyMn1-x-yO2 materials, which exhibit high energy density and outstanding thermal stability, are widely used

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as cathodes in commercial LIBs [8-10]. Therefore, the main valuable metals, such as

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lithium, nickel, cobalt, and manganese, are enriched in cathode. However, strong chemical bonds of Me-O (Me = Ni, Co and Mn) are formed during the high-temperature preparation of LiNixCoyMn1-x-yO2. In addition, high temperature sintering in prepared process of LiNixCoyMn1-x-yO2 increases the valence states of

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transition metals [11, 12]. In view of Eh-pH diagrams, whether in acid leaching or ammonia leaching systems, low-valence (+2) transition metals have high tendencies

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to be easily leached in thermodynamics [13-17]. Based on this principle, reduction leaching and reduction roasting procedures are commonly used to treat spent

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LiNixCoyMn1-x-yO2 materials [16, 18-20]. Many studies have reported that hydrogen peroxide as the reducing agent can remarkable improve the leaching efficiency of valuable metals [17, 21, 22]. Na2SO3 [15], NaHSO3 [23], (NH4)2SO3 [16], glucose [20], and sucrose and cellulose [18], among others, are also used as reductants in the leaching process. However, due to H2O2 easily decomposed in leaching process, the consumption of H2O2 inevitably exceeds the theoretical amount [24, 25]. Therefore, it significantly increases the cost of the recovery process. In addition, sulfur reducing

ACCEPTED MANUSCRIPT regent usually releases SOx in reaction, organic reducing agent is expensive. A summary of the reduction leaching systems used for LiNixCoyMn1-x-yO2 materials is shown in Table 1. By contrast, the oxidizing roasting step for the preparation process

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of LiNixCoyMn1-x-yO2 enlighten us that the reduction roasting pretreatment for LiNixCoyMn1-x-yO2 materials can be efficiently broken Me-O chemical bond and reduce the valence of transition metal elements. In view of our previous work [26], a

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quasi-reversible process theoretically consumes the least amount of energy if the

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reduction roasting of the cathode active materials occurs during the reversible process of formation. Therefore, in terms of thermodynamics, reduction roasting pretreatment can reduce the energy consumption and avoid the use of reductant in the leaching process.

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Kinetics is another important aspect of improving the leaching efficiency of valuable metals in the leaching process, which is commonly described by using the shrinking core model. Meshram [27] showed that the leaching process of cathode

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materials with sulfuric acid and sodium bisulfite is controlled by the diffusion of

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leaching agents on the surface of cathode materials. Li [22] argued that the reaction of LiNi1/3Co1/3Mn1/3O2 with organic acids (acetic acid or maleic acid) is controlled by diffusion. Li[28] also studied the leaching of LiCoO2 in a succinic acid system. Kinetics analysis showed that the initial leaching process is controlled initially by chemical reactions and then by diffusion control after 20 min. Similarly, Tacakova et al.[29] studied the leaching kinetics of LiCoO2 by using sulfuric acid and hydrochloric acid, and the results of active energy calculations indicate that chemical

ACCEPTED MANUSCRIPT reaction controls the reaction rate at first period (0–90 min) while diffusion dominates the reaction rate at the second period (90–180 min). The leaching of LiNi1/3Co1/3Mn1/3O2 has also been studied by using acetic acid and hydrogen peroxide

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as the leaching systems, and kinetics analysis showed that the addition of hydrogen peroxide as reductant can alter the rate-controlling step from diffusion to surface chemical reaction[12]. The abovementioned works obtained different results on the

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leaching kinetics of the cathode materials of spent LIBs. Therefore, an analysis of the

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kinetics aspect of the leaching process of reduced products is necessary. In this study, low-cost carbon black, which plays the role of reductant, is roasted with spent cathode materials of LiNixCoyMn1-x-yO2 to reduce the valences of transition metals.

In

consideration

materials,

their

practical

including

industrial

applications,

LiNi1/3Co1/3Mn1/3O2

spent

(NCM111),

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LiNixCoyMn1-x-yO2

of

LiNi0.5Co0.2Mn0.3O2 (NCM523), and LiNi0.6Co0.2Mn0.2O2 (NCM622), were treated together for large-scale recovery of spent LIBs. Subsequently, pure water and sulfuric

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acid were applied to leach Li and Ni, Co and Mn in the roasted products. Various

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factors, such as leaching temperature, liquid–solid ratio, and acid concentration, that may influence the leaching efficiency of metals in the leaching process were studied. The leaching kinetics of Ni, Co and Mn from the water leaching residue is characterized by the shrinking core model, and the reaction activation energies of sulfuric acid with transition metals were calculated.

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Samples

Leaching conditions

Leaching efficiency

References

H2SO4

LiNi1/3Co1/3Mn1/3O2

4 M H2SO4 + 2 times theoretical dosage of H2O2

98% Ni, 99% Co, 84% Mn

[24]

LiNixCoyMnzO2

1.25 M H2SO4 + 0.1 M Na2S2O5

about 90% of Li, Ni, Co, Mn

[23]

LiNixCoyMnzO2

1 M H2SO4 + 0.075 M NaHSO3

96.7% Li, 96.4% Ni, 91.6% Co, 87.9% Mn

[30]

HCl

LiNi0.8Co0.15Al0.05O2

4 M HCl

All of valuable metals are leached out

[31]

HNO3

LiCoO2

1 M HNO3 + 1.7 vol.% H2O2

Over 95% of Li, Co

[32]

Acetic acid

LiNi1/3Co1/3Mn1/3O2

1 M acetic acid + 3 ml H2O2

98.39% Li, 97.27% Ni, 97.72% Co, 97.07% Mn

[22]

DL-malic acid

LiNi1/3Co1/3Mn1/3O2

1.2 M DL-malic acid + 1.5 vol.% H2O2

98.9% Li, 95.1% Ni, 94.3% Co, and 96.4% Mn

[33]

Citric acid

Mixed-type

0.5 M Citric acid + 1.5 vol.% H2O2

Exceed 95% of Li, Ni, Co, Mn

[21]

Tartaric acid

Mixed-type

2 M L-tartaric acid + 4 vol.% H2O2

99.07% Li, 99.31% Ni, 98.64% Co, 99.31% Mn

[34]

Lactic Acid

LiNi1/3Co1/3Mn1/3O2

1.5 M lactic acid + 0.5 vol.% H2O2

More than 98% of Li, Ni, Co, Mn

[35]

Formic acid

LiNi1/3Co1/3Mn1/3O2

2 M formic acid + 6 vol.% H2O2

98.22% Li, 99.96% Ni, 99.96% Co, 99.95% Mn

[36]

Ammonia

LiNi1/3Co1/3Mn1/3O2

4 M NH3 + 1.5 M (NH4)2SO4 + 0.5 M Na2SO3

95.3% Li, 89.8% Ni, 80.7% Co, 4.3% Mn

[15]

10.5 M NH3·H2O + 1.8 M NH4HCO3 + 1.86 M H2O2

81.2% Li, 96.4% Ni, 96.3% Co

[14]

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LiNi0.5Co0.2Mn0.3O2

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Leaching agent

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Table 1 A summary of leaching systems for spent cathode materials

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2. Experimental 2.1 Materials and reagents The spent cathode materials of NCM111, NCM523 and NCM622 were provided

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by Jiana Energy Technology Company. The main components of the powder are shown in Table 2.

Li

Ni

NCM111

7.16

20.02

NCM523

7.75

NCM622

7.47

Co

Mn

20.46

19.35

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Cathode materials

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Table 2 Main components of waste NCM materials /wt%

31.211

11.85

18.04

34.46

11.39

11.30

Carbon black (C.wt% ≥ 85%) was used as reductant. During the leaching

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process of the roasted products, distilled water was utilized to dissolve Li2CO3, and sulfuric acid was used to leach the other valuable metals.

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2.2 Measurement and characterization

The reduction roasting process were analyzed using thermogravimetry–

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differential scanning calorimetry (TG/DSC, NETZSCH STA 449C) measurement. The structures of the spent NCM and roasted products were characterized by X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Kα radiation. Data were recorded from 5° to 90° with a scanning speed of 5° min−1. The valences of Ni, Co, and Mn present in the roasted products were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab 250XI). The Li, Ni, Co, and Mn in the leaching solution and the spent LIBs were measured by an inductively coupled plasma optical emission

ACCEPTED MANUSCRIPT spectrometer (ICP-OES, PerkinElmer Optima 8000). The morphology and components of the spent NCM and those of the roasted products before and after leaching were analyzed with scanning electron microscopy (SEM, Fei Quanta 400)

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and energy-dispersive X-ray spectroscopy (EDS, Vuestl 2X). 2.3 Experimental procedure

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2.3.1 Reduction roasting procedure

The uniform mixture containing 90 g of LiNixCoyMn1-x-yO2 and 10 g of carbon

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black was mixed by a planetary ball mill for 1 h and stirred at 200 rpm. After completing the mixing process, the mixture was roasted at 550  for 0.5 h in a tube furnace with argon atmosphere (0.05 L min−1).

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2.3.2 Water leaching procedure of roasted products

The roasted products were leached in pure water at 25  with a magnetic stirrer

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(stirring speed = 300 rpm) to selectively recover Li2CO3. The effects of different liquid–solid ratios (10, 20, 30, 50, 70, and 100 ml g−1) and leaching time of the

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process (from 1 to 180 min) on the leaching efficiency of Li were investigated. After completing the water leaching, the slurry was filtered. The residue was dried in an oven at 60  for 12 h.

2.3.3 Acid leaching procedure of water leaching residue An evaluation of the optimal acid leaching parameters based on the leaching efficiency of Ni, Co, and Mn was conducted. Sulfuric acid concentration (0.5, 1.0, 2.0,

ACCEPTED MANUSCRIPT 3.0, and 4.0 mol L−1), liquid–solid ratio (5, 7, 10, 12, and 15 ml g−1), temperature (25 , 40 , 55 , 70 , and 90 ) and time (0–180 min) were considered in determining the influence on the leaching efficiencies of Ni, Co and Mn. The leaching

Ei =

ci V mi wi

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efficiency of metals is calculated as follows: ×100%

(1)

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where Ei is the leaching efficiency of element i, while ci, V, mi, and wi refer to the concentration of metal ion in the solution (g L−1), leachate volume (L), mass of spent

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NCM materials (g), and metal contents in the spent NCM materials, respectively. 2.3.4 Kinetics and reaction mechanism of acid leaching

To understand the reaction mechanism of the acid leaching process, the shrinking

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core model was used to determine the kinetics. The leaching efficiency of Ni, Co, and Mn versus leaching time were recorded from 1 min to 180 min at different

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temperatures.

3. Results and discussion

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3.1. Analysis of roasting procedure The Gibbs free energy is a state function of the reaction system when system at

constant temperature, pressure and non-volume work is zero. The negative change in Gibbs free energy of reaction before and after is an indispensable precondition for the spontaneity of chemical reaction at constant pressure and temperature [37]. Due to the lack of thermodynamic parameters of LiNixCoyMn1-x-yO2 and LiNixCoyMn1-x-yO2 can be decomposed above 500 [10, 26]. HSC Chemistry 6.0 was used to calculate the

ACCEPTED MANUSCRIPT change of Gibbs free energy of feasible chemical reaction of transition metal oxide in roasting process corresponding to temperature (Fig. S1). As shown in Fig. S1, MnO, Ni, NiO, Co, CoO, and Li2CO3 are more stable in almost 550. The TG/DSC

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measurement results of spent cathode materials LiNixCoyMn1-x-yO2 with carbon black are presented in Fig. S2. As is vividly shown in Fig. S2, spent NCM materials were drastic reacting with carbon black in the temperature range of 500 - 900. The first

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exothermic reaction peak appeared in the temperature range of 550 to 600, at the

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same time, the mass of the mixture decreased slightly. With the temperature increase, three very obvious reaction peaks appear, and the mass loss increases rapidly. According to above mentioned discussion, carbon black can be effectively performing as reductant for roasting process.

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Fig. 1 presents the XRD patterns of the spent cathode material and its roasted products. All the diffraction peaks of the spent cathode materials can be indexed to LiNixCoyMn1-x-yO2 (Figs. 1a, 1b, 1c). After reduction roasting, all of spent NCMs

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were completely transformed into Li2CO3, Ni, Co, NiO, and MnO (Figs. 1d, 1e, 1f).

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Hence, subsequent procedures were conducted to uniformly recover valuable metals from the mixed roasted products of NCM111, NCM523, and NCM622. The water leaching procedure can be used to selectively exact Li, and the sulfuric acid can leach Ni, Co, and Mn in residue without a reductant.

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♥—Ni ♣—Co ∇—NiO ♠—MnO ♦—Li2CO3 ∆—LiNixCoyMn1-x-yO2

♦

♣ ♥ ∇ ♦♦♦ ∇ ♠

♣ ♥

♣ ♥

♠

(f) (e)

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Intensity (a.u.)

(d) (c)

∆ 20

30

∆ ∆

40

50

∆ 60

∆∆ ∆

70

∆ ∆ 80

(a)

90

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10

∆∆

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(b)

2θ (degree)

Fig. 1. XRD patterns of (a) NCM111, (b) NCM523, (c) NCM622, (d) roasted products of NCM111, (e) roasted products of NCM523, and (f) roasted products of NCM622

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According to the literature, the valences of Ni, Co, and Mn in the layered material of LiNixCoyMn1-x-yO2 are 2+/3+, 3+, and 4+, respectively [38-40]. To further identify the chemical states of transition metal in the roasted products, XPS has been

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performed to determine the valences of Ni, Co, and Mn. The XPS spectra of Ni 2p,

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Co 2p, and Mn 2p for the reduction products and spent NCM are shown in Fig. 2. As shown in Fig. 2a, for the spent NCM, the Ni 2p spectra are characteristic of Ni2+ (854.5 ev). However, the Ni 2p3/2 and Ni 2p1/2 binding energies of roasted products are approximately at (854.8 eV, 852.4 eV) and (870.8 eV, 873.3 eV), respectively, and consistent with Ni2+ and Ni in previous reports. Moreover, the dominating Ni 2p3/2 peak that appears at 854.8 eV corresponds well with Ni2+, which suggests that nickel mainly exists in the form of Ni2+ [41]. The Co 2p3/2 spectra of the spent NCM and

ACCEPTED MANUSCRIPT roasted products (Fig. 2b) show binding energies of 779.9 ev and 777.8 ev, which correspond to valences of +3 and 0, respectively. Therefore it can be assumed that Co3+ has been completely reduced to Co [42]. Fig. 2c showed the Mn 2p spectra of

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spent NCM and roasted products. As seen in Fig. 1c. In spent NCM materials, the Mn 2p3/2 peak is at 653.5 ev, and the Mn 2p1/2 is at 641.6 ev, which suggest that Mn ion are present as Mn4+. For the roasted products. Mn 2p3/2 and Mn 2p1/2 binding energy

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peaks of roasted products appear at 652.9 eV and 640.8 eV, respectively, which are

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related to Mn2+ [43]. The above results shows that the layered structures of LiNixCoyMn1-x-yO2 have been destroyed and the oxidation of transition metals are partially reduced, which are consistent with XRD results. (a) Ni2p

2p3/2

Ni2+ Ni

2p1/2

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Ni

Ni2+

Roasted products

2p3/2

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Ni2+

845

850

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(b) Co2p Co

855

860

2p1/2 Ni2+

865

870

Spent NCM

875

880

Binding energy (ev)

2p3/2

2p1/2 Co Roasted products

Co3+

2p3/2 2p1/2 Co3+ Spent NCM

775

780

785

790

795

Bindigng energy (ev)

800

805

810

ACCEPTED MANUSCRIPT (c) Mn2p

2p3/2 Mn2+ Mn2+

2p1/2 Roasted products

2p3/2 Mn4+ 2p1/2 Mn4+

635

640

645

Binding energy (ev)

650

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Spent NCM

655

660

Fig. 2. XPS spectra of (a) Ni2p, (b) Co2p, and (c) Mn2p for roasted products and spent NCM

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3.2. Selective leaching of Li

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According to the reduction roast results, water-soluble Li2CO3 can be selectively extracted by water leaching. However, the solubility of Li2CO3 decreases as the temperature increases [19]. Therefore, in this study, we selected 25  as the suitable temperature for water leaching. In addition, the key factors affecting the leaching

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process are liquid‒solid ratio and leaching time. 3.2.1. Effect of liquid–solid ratio

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The solubility of Li2CO3 was only 13.3 g L−1 at 20  [19], which can limit the

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leaching efficiency of Li in roasted products. Therefore, a series of gradually increasing liquid‒solid ratios (10–100 ml g−1) were performed for water leaching to maximize Li recovery. The other water leaching conditions were as follows: stirring speed of 300 rpm at 25 for 180 min.

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80

70

60

50

40 0

10

20

30

40

50

60

70

80

Liquid−solid ratio (ml g−1)

90

100

110

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Leaching Efficiency of Li (%)

90

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Fig. 3. Effect of liquid‒solid ratio on leaching efficiency of Li

As presented in Fig. 3, liquid‒solid ratio has a significant impact on the leaching

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efficiency of Li2CO3. With an increase in the liquid‒solid ratio from 10 to 30 ml g−1, the leaching efficiency of Li sharply increased. In the conditions of liquid‒solid ratio of 10 ml g−1 and 20 ml g−1, the leaching efficiencies of lithium were only 38% and 69%, respectively; moreover, due to the limit of Li2CO3’s solubility, the

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concentrations of Li ion were only 2.4 g L−1 and 2.2 g L−1, respectively. When the liquid–solid ratio exceeded 30 ml g−1, the leaching efficiency of lithium was higher

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than 90%. However, an extremely high liquid‒solid ratio will cause a decrease in Li ion concentration in the leaching solution. Therefore, the appropriate liquid‒solid

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ratio of 30 ml g−1 was selected. 3.2.2. Effect of leaching time

The influence of leaching time on the process was investigated given the

conditions of liquid‒solid ratio of 30 ml g−1 and stirring speed of 300 rpm. The leaching time of the process varied from 1 to 180 min. The results are presented in Fig. 4.

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80

70

60

50 0

20

40

60

80

100

120

Leaching Time (min)

140

160

180

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Leaching effiency of Li (%)

90

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Fig. 4. Effect of leaching time on leaching efficiency of Li

As shown in Fig. 4, the leaching efficiency of lithium increases rapidly from 1

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min to 5 min, and more than 70% of the lithium has been leached after 5 min. From 5 min to 90 min, the increase of leaching efficiency became slower. At the leaching time of 90 min, the leaching efficiency reached 93%, and the concentration of Li ion was high at 2.02 g L−1. After 90 min, the increase of leaching efficiency was negligible.

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The XRD patterns of water leaching residue (Fig. S3) also indicates that nearly no Li2CO3 is present in the residue. Considering the energy consumption and recovery

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efficiencies, 90 min is recommended as the optimum leaching time.

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3.2.3. Recovery of Li from water leaching solution After water leaching, about 93.68% Li extracted from roasted products to

leaching solution, the concentration of Li in solution was 2.02 g/L. Due to the characteristic of Li2CO3 solubility in water which showed a decrease in solubility from 15.4 g/L at 0 °C to 7.20 g/L at 100 °C [44]. In addition, Li2CO3 could be separated from leaching solution extract using chemical precipitation with K2CO3 or Na2CO3. According to the above mention, we propose to first heated the leaching

ACCEPTED MANUSCRIPT solution to 80 to evaporation crystallization part Li2CO3. Afterward, stoichiometric sodium carbonate was then introduced for the precipitation and recovery of Li2CO3. The reaction equation can be presented as follows: 2 Li+ + Na2 CO3 = Li2 CO3 ↓+ 2 Na+

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(2)

The precipitation reaction was kept at 80 for 5 min with a stirring speed of 300 rpm. The precipitated Li2CO3 was filtrated and then washed by 80 pure water to

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redissolve the precipitated Na2CO3. To avoid possible Li loss in filtrate, the filtrate

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can be reused to water leaching process. 3.3. Leaching process of Ni, Co, and Mn

After the water leaching process, the residue of water leaching was leached again with sulfuric acid to recycle Ni, Co, and Mn. To determine the suitable conditions of

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the acid leaching process, HSC Chemistry 6.0 was used to calculate the Eh-pH diagrams of Ni-H2O system, Co-H2O system, and Mn-H2O system. The results are

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shown in Fig. S4.

When the pH value of the solution was lower than 4.9 (Fig. S4), Ni, Co, and Mn

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all existed in the form of Me2+ in Me-H2O systems (Me = Ni, Co, Mn). Therefore, the residue can be easily leached by sulfuric acid. Furthermore, with the decrease of pH value, the leaching process can be realized even much more easily. The main equations of the process are listed as follows: Ni+H2SO4=NiSO4+H2↑

NiO+H2SO4=NiSO4+H2O

Co+H2SO4=CoSO4+H2↑

MnO+H2SO4=MnSO4+H2O

ACCEPTED MANUSCRIPT 3.3.1. Effect of acid concentration on the leaching of metals To investigate the effect of acid concentration (0.5–4 M) on the leaching efficiencies of metals, a series of acid leaching experiments was conducted in the

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following conditions: liquid‒solid ratio of 10 ml g−1, stirring speed of 300 rpm, and leaching at 25  for 180 min. 100

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60

Ni Co Mn

40

20

0 0.0

0.5

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Leaching effciency (%)

80

1.0

1.5

2.0

2.5

3.0

Acid concentration (mol L−1)

3.5

4.0

4.5

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Fig. 6. Effect of acid concentration on leaching efficiency of metals

As shown in Fig. 6, the leaching rates of Ni, Co, and Mn clearly increase with rising sulfuric acid concentrations, and Mn is more easily leached out than Ni and Co.

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This phenomena can be attributed to the Mn in the residue, which exists in the form of

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MnO that can be dissolved easily in acid, whereas the Ni and Co not only exist at low valence state oxides but also in the form of simple substances that are hard to dissolve. When the concentration of the acid increased from 0.5 M to 4 M, the leaching rates of Ni, Co, and Mn also increased from 19.03% to 96.14%, from 25.4% to 96.56%, and from 46.56% to 95.9%, respectively. Therefore, 4 M is selected as the optimal acid concentration for subsequent experiments. 3.3.2. Effect of liquid‒solid ratio

ACCEPTED MANUSCRIPT The influence of liquid‒solid ratio (5–15 ml g−1) on the leaching efficiency of valuable metals was investigated. The results are shown in Fig. 7. The process was conducted in the following conditions: temperature of 25 , stirring speed of 300 rpm,

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H2SO4 concentration of 4 M, and leaching time of 180 min. 100

92

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Leaching Efficiency (%)

96

88

Ni Co Mn

80 4

6

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84

8

10

12

Liquid−Solid ratio (ml g−1)

14

16

Fig. 7. Effect of liquid‒solid ratio on leaching efficiencies of metals

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As shown in Fig. 7, when the liquid‒solid ratio is lower than 10 ml g−1, it has a positive effect on the leaching rates of Ni, Co, and Mn. When the liquid‒solid ratio is between 10 ml g−1 and 15 ml g−1, the leaching rates of metals are relatively high. To

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ensure sufficient metal leaching and low acid consumption, the optimal liquid‒solid

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ratio can be set to 10 ml g−1.

3.3.3. Effect of reaction time To clarify the effect of reaction time on the leaching efficiencies of metals, the

relationship of leaching efficiency versus time at different temperatures is determined (Fig. 8). The leaching temperatures were 25 , 40 , 55 , 70 , and 90 . The other leaching conditions were as follows: liquid‒solid ratio of 10 ml g−1, stirring speed of 300 rpm, and acid concentration of 4 M.

ACCEPTED MANUSCRIPT 100

100 95

Leaching effciency (%)

80

70

Ni 25 °C 40 °C 55 °C 70 °C 90 °C

60

50

90 85

Co 25 °C 40 °C 55 °C 70 °C 90 °C

80 75 70 65 60

40 0

20

40

60

80

100

120

140

160

180

0

20

40

60

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100

120

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180

Time (min)

Time (min)

100 95

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90 85

Mn 25 °C 40 °C 55 °C 70 °C 90 °C

80 75 70 65 60 0

20

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Leaching effciency (%)

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Leaching effciency (%)

90

40

60

80

100

120

140

160

180

Time (min)

Fig. 8. Effect of time on leaching efficiency of Ni, Co, and Mn with different temperatures

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As presented in Fig. 8, the leaching efficiency of Mn increases significantly with the increase of time from 1 min to 60 min. Then, the leaching process reaches equilibrium and the corresponding leaching efficiency achieves the maximum.

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However, the leaching of nickel and cobalt requires 150 min to reach equilibrium, and this phenomenon can be attributed to the existence of metallic nickel and cobalt. The

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XRD patterns of acid leaching residue (Fig. S5) also shows that there are only difficult-dissolved metallic Co and Ni in the residue. As temperature increases, the time required for metal leaching reaction to reach the equilibrium becomes shorter. Therefore, the optimal leaching temperature can be selected as 90 , and the required reaction time is 30 min only. 3.4. Kinetics and reaction mechanism analysis for acid leaching process The spent cathode materials, roast products, and water leaching residue were

ACCEPTED MANUSCRIPT characterized by SEM images and mapping of elements. The results are presented in Fig. 9. Fig. 9a shows a typical morphological structure and the elemental distribution of LiNixCoyMn1-x-yO2 materials, in which spent LiNixCoyMn1-x-yO2 materials are the

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agglomerating primary particles that become bound tightly together, while Ni, Co, and Mn have become evenly distributed on the surface of spent cathode materials. Fig. 9b shows that the roasted products are agglomerated with a large number of smaller

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particles, which clearly suggests that the structure of the active materials has been

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destroyed during the reduction roast. This finding is also consistent with the XRD results in Fig. 1. The decrease in particle size and the increase of surface have contributed in the improvement of the reaction speed of acid with valuable metals. Fig. 9c shows the rough surface of the water leaching residue, an indication of a relatively

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large reaction area with sulfuric acid.

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Fig. 9. SEM images and mapping of elements: initial cathode material (a), roasted products (b), residue of water leaching (c)

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From the SEM results shown in Fig. 9(c), the residue is a spherical particle with

a diameter of 10 µm. Furthermore, the whole acid leaching process is a solid– liquid-gas heterogeneous reaction [12, 36], which suggests that the shrinking core model can be used to analyze the kinetics of the acid leaching process [21, 22]. The reaction mechanism of the recycling process and the shrinking core model of the leaching process are illustrated in Fig. 10.

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Fig. 10. Concept of reaction mechanism for recycling process

The dissolution behavior of water leaching residue in sulfuric acid includes

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following steps: (1) mass transfer of sulfuric cross the liquid film, (2) diffusion from residue-film interface to the reaction interface through the residue layer, (3) chemical reaction at the particle interface of water leaching residue, (4) reaction products diffuse through the residue layer, (5) reaction products across the liquid film to the

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sulfuric acid solution. The kinetic model can be simplified to be liquid boundary layer mass transfer (Eq.3), surface chemical reaction (Eq.4) or residue layer diffusion (Eq.5)

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[12, 22, 36]:

x=
1 t,

(3) 

1 − (1 − x)3 =
 t, 2

(4)

2

1 − 3 x − (1 − x)3 =k3 t,

(5)

Where, the k1, k2, and k3 are the slopes of fitted lines, t is the reaction time. As show in Fig. S6-8 and Table S1, the reaction kinetic fitted with model equation 3, 4, and 5, respectively. The equation 5 (residue layer diffusion) exhibits the best fitting relevance among the three model.

ACCEPTED MANUSCRIPT Fig. 11 shows the best fitted straight line (residue layer diffusion) among the three model with different temperatures (Fig. S6-S8, Table S1). 0.25

0.25

0.15

Ni 25 °C 40 °C 55 °C 70 °C 90 °C

0.10

0.05

0.15

20

40

60

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Co 25 °C 40 °C 55 °C 70 °C 90 °C

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Time (min)

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1-2/3x-(1-x)2/3

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0.20

1-2/3x-(1-x)2/3

1-2/3x-(1-x)2/3

0.20

Mn 25 °C 40 °C 55 °C 70 °C 90 °C

0.15

0.10

0.05 0

5

10

15

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25

30

35

40

45

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Time (min)

Fig. 11. Fitted kinetics analysis under residue layer diffusion model

Furthermore, the relationship of the reaction rate constants of Ni, Co, and Mn with



lnk = ln A − RTa ,

(5)

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different temperatures accords with the Arrhenius equation [30],

where k is reaction rate constant, A is pre-exponential factor, Ea is apparent activation energy, R is gas constant, and T is reaction temperature. Table 3 summarizes the reaction activation energies of the acid leaching process

and the corresponding linear equations that represent the relationship between leaching coefficient and reaction temperature of valuable metals.

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Table 3 Arrhenius equation for the leaching of Ni, Co, and Mn Linear equation

R2

Ea/kJ mol−1

Ni

ln k=-3.53048/T+5.37344

0.99454

29.35

Co

ln k=-2.88703/T+3.63767

0.99246

24.00

Mn

ln k=-2.80233/T+3.84834

0.99296

23.29

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Elements

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From Fig. 10 and Table 3, the kinetics of the acid leaching process is controlled by the residue layer diffusion, in which the activation energies are 29.35, 24.00, and

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23.29 kJ·mol−1 for Ni, Co, and Mn, respectively. Therefore, increasing temperature and increasing stirring speed are beneficial in the efficient leaching of valuable metals.

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4. Conclusions

In this work, a metallurgical method was applied to recover valuable metals in a cathode material from spent LIBs. On the basis of this method, the following

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conclusions are derived:

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(1) Spent cathode materials (NCM111, NCM523, and NCM622) were transformed into Li2CO3, MnO, Ni, NiO, and Co through reduction roasting with carbon black.

(2) In optimized water leaching conditions (liquid‒solid ratio = 30 ml g−1,

leaching time = 90 min, leaching temperature = 25 , stirring speed = 300 rpm), approximately 93.68% of Li can be selectively extracted from roasted products by pure water.

ACCEPTED MANUSCRIPT (3) The water leaching residue was leached again with sulfuric acid to recycle Ni, Co, and Mn. In optimal conditions (liquid‒solid ratio = 10 ml g−1, leaching time = 30 min, leaching temperature = 90 , stirring speed = 300 rpm, and acid concentration =

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4 M), nearly 99.56% of Ni, 99.87% of Co, and 99.9% of Mn can be leached out from the water leaching residue.

(4) Kinetic analysis based on the shrinking core model showed that the acid

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leaching process was controlled by residue layer diffusion. The reaction–activation

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energies of Ni, Co, and Mn were calculated as 29.35 kJ mol−1, 24.00 kJ mol−1, and 23.29 kJ mol−1, respectively.

Acknowledgements

This work was financially supported by the National Natural Science Foundation

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of China (No. 51774127, No. 51604105), and Natural Science Foundation of Hunan Province (No. 2018jj2091).

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ACCEPTED MANUSCRIPT Highlights (1) A novel metallurgical method is proposed to recycling spent LiNixCoyMn1-x-yO2. (2) The leaching conditions are comprehensively investigated and optimized.

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(3) The kinetics and the controlled step of acid leaching process are analyzed.

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(4) The active energies of Ni, Co and Mn reacting with H2SO4 are calculated.