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Sustainable recovery of cathode materials from spent lithium-ion batteries using lactic acid leaching system Li Li, Ersha Fan, Yibiao Guan, Xiaoxiao Zhang, Qing Xue, Lei Wei, Feng Wu, and Renjie Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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ACS Sustainable Chemistry & Engineering

Sustainable recovery of cathode materials from spent lithium-ion batteries using lactic acid leaching system Li Lia,b, Ersha Fana, Yibiao Guanc, Xiaoxiao Zhanga, Qing Xuea, Lei Weia, Feng Wua,b, Renjie Chena,b* a. Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, No. 5 South Zhongguancun Street, Beijing 100081, China. b. Collaborative Innovation Center of Electric Vehicles in Beijing, No. 5 South Zhongguancun Street, Beijing 100081, China. c. State Key Laboratory of Operation and Control of Renewable Energy ＆ Storage Systems, China Electric Power Research Institute, Haidian District, Beijing 100192, China.

Corresponding author: Renjie Chen, E-mail address: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT An environmentally friendly leaching process for recycling valuable metals from spent lithium-ion batteries is developed. A sol–gel method is utilized to resynthesize LiNi1/3Co1/3Mn1/3O2 from the leachate. Lactic acid is chosen as a leaching and chelating agent. The leaching efficiency is investigated by determining the contents of metal elements such as Li, Ni, Co, and Mn in the leachate using inductively coupled plasma optical emission spectroscopy. The spent cathode materials for the pretreatment process and the regenerated and freshly synthesized materials are examined using X-ray diffraction and scanning electronic microscopy. The results show that the leaching efficiencies of Li, Ni, Co, and Mn reached 97.7%, 98.2%, 98.9%, and 98.4%, respectively. The optimum conditions are lactic acid concentration 1.5 mol L−1, solid/liquid ratio 20 g L−1, leaching temperature 70 °C, H2O2 content 0.5 vol%, and reaction time 20 min. The leaching kinetics of cathode scrap in lactic acid fit the Avrami equation well. Electrochemical analysis indicate that the regenerated LiNi1/3Co1/3Mn1/3O2 cathode materials deliver the highly reversible discharge capacity, 138.2 mA h g−1, at 0.5 C after 100 cycles, with a capacity retention of 96%, comparable to those of freshly synthesized LiNi1/3Co1/3Mn1/3O2 cathodes. KEYWORDS: Spent lithium-ion batteries; Lactic acid; Recovery; Kinetic analysis INTRODUCTION Lithium-ion batteries (LIBs), which were first produced commercially by Sony in 1991, are widely used as electrochemical power sources in electronic equipment and electric vehicles due to their high energy density, long cycling life, and low self-discharge rate.1−4 Worldwide 2


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production of LIBs reached about 500 million cells in 2000, and annual production increased by 800% between 2000 and 2010.5 Recently, the development of the electric vehicle industry has stimulated growth in LIB use. It is estimated that the quantity and weight of discarded LIBs in 2020 will exceed 25 billion units and 500 thousand tones, respectively.5 Recycling of the valuable metals in spent LIBs is therefore imperative for at least two important reasons: (1) governments will enforce recycling of spent LIBs for environmental protection; and (2) the recovery of valuable materials, particularly those that have limited sources, is necessary.6−8 Recently, much research has focused on the use of alternatives to LiCoO2 as the cathode material because of its high cost and poor safety. New cathode materials such as LiFePO4, LiNi1/3Co1/3Mn1/3O2, LiNi0.8Co0.15Al0.05O2, and LiMn2O4 have been developed, which use cheap transition metals such as Ni, Mn, and Fe as complete or partial substitutes for Co.9 A survey of the battery market in 2012 showed that of the cathode materials used commercially LiCoO2 accounted for 37.20%, LiNi1/3Co1/3Mn1/3O2 for 29.00%, LiMn2O4 for 21.40%, LiNiO2 for 7.20%, and LiFePO4 for 5.20%.10 Because of the rapid development of ternary materials, the market share of LiNi1/3Co1/3Mn1/3O2 is increasing annually. Much research has been performed on recycling waste LIBs with LiCoO2 as the cathode material. We have developed a novel approach to reusing valuable metals from spent LiNi1/3Co1/3Mn1/3O2. LIBs

are

currently

recycled

using

methods

based

on

pyro-metallurgy,11,

12

hydro-metallurgy,13 and bio-metallurgy.14, 15 Pyro-metallurgical processes are often associated with hazardous gas emissions.16 Because of the long treatment period and difficulty of growing bacteria, bio-hydrometallurgical processes have gradually been replaced by 3



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hydrometallurgical methods.14 Hydrometallurgical processes are widely used in industry to separate and recycle metal ions because of their benefits such as high efficiency and less toxic emissions to the atmosphere.17 Among the hydrometallurgical techniques for recycling spent LIBs, acid leaching is the most cost effective, simple, and environmentally friendly.18 The leaching of cathode materials from spent LIBs is usually performed using inorganic acids such as HCl,19−21 HNO3,22, 23 H2SO4,10, 24−26 H3PO427 and organic acids such as citric,13, 28, 29 DL-malic,

30

ascorbic,4, 13 succinic,18 oxalic acids,31, 32 L-tartaric acid33 and formic acid34 as the

leaching agents. The use of the inorganic strong acids results in release of Cl2, SO3, and NOx during leaching, and these are a threat to the environment and human health. In contrast to inorganic acids, organic acids are almost natural and easily degradable, and some organic acids can be used as reductant,4, 13 precipitant,31, 32, 35 or chelating agent36 during the recovery process of the cathode materials. As for the leachate, most of the research are focused on the separation of different metals, 31, 32, 34, 35 and a few reports investigated the regeneration of new materials. In this work, we developed a new hydrometallurgical process that uses a natural organic acid for recycling spent LIBs. Lactic acid, widely distributed in nature and miscible with water, is chosen to be the leaching acid. Meanwhile, the industrial production of lactic acid is also a green process, mainly through fermentation. As a green platform compounds, lactic acid aroused widespread concern in research.37, 38 For example, J.A. Tang and M. Valix38 studied the use of lactic acid as a reagent to recover nickel and cobalt from limonite and nontronite ores. Here we established an acid leaching system using lactic acid and determined the optimum reaction conditions for recycling spent LiNi1/3Co1/3Mn1/3O2 cathode materials. 4


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In contrast to conventional leachate treatments, the cathode material is resynthesized using a sol–gel method, without separation of metal ions. EXPERIMENTAL SECTION Materials and reagents. Spent LIBs were collected from the Shenzhen BAK Battery Co., Ltd (Shenzhen, China). All reagents used were analytical grade and all solutions were prepared or diluted with distilled water to a specified concentration. A low-concentration NaOH solution was used to dissolve Al foil to enable its separation from the cathode material. HNO3 was used to leach the spent cathode material completely so that the total contents of Ni, Co, Mn, and Li could be determined to calculate the leaching efficiency. Experimental procedures. The spent LIBs were completely discharged by electrolyzing with saturated saline solution to avoid the potential dangers of short circuiting or self-ignition.39 A flowsheet of the process used for separation and regeneration of the cathode material is shown in Fig. 1. The main procedures were as follows. (1) Dismantling and anode/cathode separation: The spent LIBs were manually dismantled to remove and recycle the plastic and steel cases. The anodes and cathodes were then separated and the cathode foils were cut into small pieces of dimensions 1 cm × 1 cm. The copper foil in anode can be recycled after removing the spent carbon materials. (2) Alkaline leaching and thermal treatment: The cathode foils were immersed in NaOH solution at room temperature with simple stirring. The process can be expressed as follows: 2Al + 2NaOH +2H2O = 2NaAlO2 + 3H2

(1)

5



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After washing with distilled water and filtration, the cathode active material and Al foil were effectively separated. Al ions in the filtrate were recycled and reused in the next study, and the residue was dried at 60 °C for 24 h. Impurities such as carbon and poly(vinylidene fluoride) (PVdF) binder, were burnt off by calcining the cathode material at 610 °C for 5 h in a muffle furnace and cooling to room temperature. The waste gas can be collected and purified in a system, which consists of a cooler, a condensation chamber, activated carbon filters, and bag filters.28 After roasting and cooling, the obtained powder was ground using a planetary ball mill for 30 min to obtain smaller particles with larger surface areas to increase the dissolution rate and leaching efficiency. (3) Lactic acid leaching: All the metal-leaching tests were performed in a 100 mL three-necked, round-bottomed, thermostatic Pyrex reactor. The reactor was placed in a water bath to maintain a constant reaction temperature. The reactor was equipped with a stirrer to accelerate the reaction and a condenser pipe to reduce water loss. A measured amount of powder was added to the reactor. A solution of lactic acid and H2O2 of known concentration was poured into the reactor and the mixture was stirred at 300 rpm. The following variables in the leaching process were investigated: the lactic acid concentration (0.25–2 mol L−1), the solid/liquid (S/L) ratio (10–40 g L−1), temperature (40–90 °C), H2O2 volume percentage (0–3 vol%), and reaction time (10–60 min). (4) Regeneration and fresh synthesis of LiNi1/3Co1/3Mn1/3O2: The stoichiometric amount of Li+, Ni2+, Co2+, and Mn2+ in the leachate were obtained together after acid leaching, and their molar ratios were adjusted to 3.05:1 :1:1 by adding CH3COOLi·2H2O, (CH3COO)2Ni·4H2O, (CH3COO)2Co·4H2O, and (CH3COO)2Mn·4H2O. The solution was 6


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stirred at room temperature for 0.5 h, and the pH was adjusted to 7 with NH3·H2O solution. The solution was heated at 80 °C with magnetic stirring until a transparent sol formed. The obtained gel was dried at 100 °C for 24 h, followed by heating at 450 °C for 5 h in a muffle furnace to remove the organic contents. The decomposed precursor were ground for 0.5 h and then calcined in air at 900 °C for 12 h. The final product, denoted by R-NCM, was obtained by grinding. For comparison, LiNi1/3Co1/3Mn1/3O2 was synthesized by the same method using stoichiometric amounts of CH3COOLi·2H2O, (CH3COO)2Ni·4H2O, (CH3COO)2Co·4H2O, and (CH3COO)2Mn·4H2O. Lactic acid was used as a chelating agent. This product was denoted by F-NCM.

Fig. 1. The flowsheet of recycling and regeneration process. Characterization. After leaching with lactic acid, the amounts of Li, Ni, Co, and Mn metals in the pink filtrate were determined using inductively coupled plasma optical emission

7



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spectroscopy (ICP-OES). The leaching efficiency is defined as the ratio of the amount of a component in the leachate to the total amount of that species in the original cathode powder.4 Thermogravimetric analysis and differential scanning calorimetry (TG/DSC; Netzsch STA 449F3) were utilized to determine the optimum temperature for calcination of the spent cathode material after alkaline leaching. The measurements were performed in an air flow at a heating rate of 10 °C min−1. Crystal structures were identified using X-ray diffraction (XRD; Rigaku Ultima IV-185). The surface configuration and particle size were examined using field-emission scanning electron microscopy (FESEM; FEI, Quanta 200f). The LIB electrode consisted of 80 wt% active materials, 10 wt% acetylene black, and 10 wt% PVdF. A slurry was smoothly cast on Al foil. The electrode was dried at 80 °C in a vacuum oven. The electrolyte was a 1 mol L−1 solution of LiPF6 in a mixture of equal volumes of ethyl carbonate, dimethyl carbonate, and diethyl carbonate. A 2025 coin cell was assembled in an Ar-filled glove box, with Li metal as the anode. The electrochemical performance was examined using a Land battery tester (LAND-CT2001A) with a voltage window of 2.8−4.3 V at selected rates from 0.2 to 5 C (1 C = 150 mA g−1). Electrochemical impedance spectroscopy (EIS) tests were performed on a CHI660d workstation in the frequency range from 0.1 MHz to 0.01 Hz. RESULTS AND DISCUSSION Composition of the cathode active material and its characterization. The Ni, Co, Mn, and Li contents of the cathode material were determined as 17.58%, 17.68%, 16.46%, and 6.79%, respectively. Fig. 2 and the data in Table 1 show the thermal behavior of the cathode 8


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material after alkaline leaching. There are three main weight-loss regions at 27-320, 320-610 and 610-1000 °C, respectively. The first weight-loss region, i.e. between 27 and 320 °C, arises from the loss of bound water (ca. 2.77 wt%). In the second weight-loss region, i.e., between 320 and 610 °C (ca. 5.62 wt%), there are two exothermic DSC peaks at 456 and 609 °C, which likely correspond to the pyrolysis of PVdF and burning of acetylene black, respectively. The optimum calcination temperature is therefore 610 °C. A phase change of LiNi1/3Co1/3Mn1/3O2 and loss of Li occur in the temperature range of 610–1000 °C (ca. 6.10 wt%).

Fig. 2. TG/DSC curves of the spent cathode materials. Table 1. The weight loss rate of cathode materials. Temperature No

range/°C

Weight loss

Thermal decomposition

rate (%)

1

27-320 °C

2.77

2

320-610 °C

5.62

process Loss of bound water Burning of acetylene black and decomposition of PVDF

9



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Phase change of 3

610-1000 °C

6.10

LiNi1/3Co1/3Mn1/3O2 and the loss of lithium

Fig. 3 shows the XRD patterns and SEM images of the spent cathode material (a) after alkaline leaching and (b) after heat treatment. The XRD patterns clearly show the characteristic peaks of a layered structure. The differences between Fig. 3a and 3b arise from impurities. The peak from conductive carbon cannot be seen in Fig. 3b, indicating that the carbon was burnt off during calcination.28 MnO, which can be identified in Fig. 3a, mainly comes from decomposition of the active materials when batteries are charged/discharged. Co3O4 and MnO2 are present in the calcined dust. This indicates that during heat treatment the PVdF binder decomposes to HF, which leads to the decomposition of LiNi1/3Co1/3Mn1/3O2 and generation of impurities.40 The morphology of the spent cathode material was examined using SEM (Fig. 3). The images show that small particles agglomerate and form larger active material particles because of the PVdF binder; acetylene black is also present in the unheated scrap materials (Fig. 3a). In Fig. 3b, the agglomeration degree is clearly lower and the particle surfaces are smooth. The difference between Fig. 3a and 3b is caused by the presence in the former case of floccules and acetylene black; this shows that most of the PVdF and acetylene black was burned off during calcination.

10


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Fig. 3. XRD patterns and SEM micrographs of (a) the cathode materials after treatment with NaOH from spent LIBs; (b) materials after heat treatment. Acid leaching Order of parameter effects on leaching process. Orthogonal experiments were performed to determine the order of parameter effects on the leaching process. The results and analysis are shown in Tables 2 and 3. The presence of Ni, Co, Mn, and Li in the solution indicates that LiNi1/3Co1/3Mn1/3O2 reacts with lactic acid (Table 2). The concentrations of these metals in solution frequently change depending on the conditions; this indicates that the five conditions jointly influence the leaching efficiency. The obtained results (Table 3) show that the conditions affect the leaching process in the order lactic acid concentration, solid/liquid ratio, temperature, amount of H2O2, and leaching time. Table 2. Experiments results of the orthogonal design. No

Influencing factor

Leaching rate (%)

11



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Temperature ◦

Reaction

Concentration −1

H2O2

Solid/liquid

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Li

Ni

Co

Mn

10

70.62

71.42

70.19

71.25

20

82.37

72.45

70.77

78.64

( C)

time (min)

(mol L )

(vol. %)

ratio (g L−1)

1

60

30

0.5

0.5

2

60

40

1

1

3

60

50

1.5

1.5

30

87.77

75.97

76.06

80.21

4

60

60

2

2

40

75.21

72.19

72.89

75.54

5

70

30

1

1.5

40

47.12

42.88

43.91

43.93

6

70

40

0.5

2

30

24.89

28.66

29.49

29.95

7

70

50

2

0.5

20

73.54

75.09

73.71

78.24

8

70

60

1.5

1

10

76.15

75.37

76.07

75.81

9

80

30

1.5

2

20

77.98

73.99

72.75

77.18

10

80

40

2

1.5

10

100

92.8

93.37

94.38

11

80

50

0.5

1

40

26.94

23.72

23.91

24.39

12

80

60

1

0.5

30

78.25

62.65

63.01

66.25

13

90

30

2

1

30

86.92

76.78

77.43

81.43

14

90

40

1.5

0.5

40

82.32

69.66

70.53

73.59

15

90

50

1

2

10

82.14

84.57

85.17

81.81

16

90

60

0.5

1.5

20

50.23

48.17

47.09

48.88

Table 3. Analysis results of orthogonal experiments. Effect factor

Li

Tempe rature

Time

Acid

H2O2

S/L ratio

Effect factor

rature

Time

Acid

H2O2

S/L ratio

K1

78.992

70.660

43.170

76.183

82.228

K1

72.477

66.070

42.670

69.360

81.200

K2

55.425

72.395

72.470

68.095

71.030

K2

55.795

66.040

65.715

62.045

66.080

K3

70.793

67.597

81.055

71.280

69.457

K3

63.260

64.712

73.852

65.108

61.498

K4

75.403

69.960

83.918

65.055

57.897

K4

70.055

64.765

79.350

65.075

52.810

16.682

1.358

36.680

7.315

28.390

Extreme

Co 23.567

4.798

40.748

11.128

24.331

deviation

Ni

Tempe

Extreme deviation

Priority

Lactic acid concentration > solid/liquid ratio >

Priority


order

Temperature > Amount of H2O2 > Time

order


K1

73.007

66.268

42.992

69.705

81.040

K1

76.410

68.448

43.617

72.333

80.813

K2

55.500

65.892

65.638

62.080

67.425

K2

56.983

69.140

67.657

65.067

70.735

K3

63.290

64.838

73.748

64.955

61.015

K3

65.550

66.162

76.697

66.850

64.460

K4

69.795

64.595

79.215

64.852

52.112

K4

71.428

66.620

82.398

66.120

54.363

19.427

2.978

38.781

7.266

26.450

Extreme

Mn 17.507

1.673

36.223

7.625

28.928

deviation

Extreme deviation

Priority


Priority


order


order


Use of single variable to explore optimum conditions for leaching process 12


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Effect of lactic acid concentration on acid leaching. The orthogonal experimental results show that the concentration of lactic acid significantly affects the leaching process. The concentration of lactic acid was varied from 0.25 to 2.0 mol L−1 at 60 °C, with an S/L ratio of 15 g L−1, a reductant concentration of 0.5 vol% H2O2, and a leaching time of 30 min. Fig. 4a shows that the leaching efficiencies of Ni, Co, Mn, and Li are 38.81%, 39.13%, 36.51%, and 37.16%, respectively at a low lactic acid concentration, i.e., 0.25 mol L−1. The leaching efficiencies of the four metals increase rapidly with increasing lactic acid concentration from 0.25 to 1.0 mol L−1. Almost 98% of Ni, Co, and Mn, and 93% of Li are leached at a lactic acid concentration of 1.0 mol L−1. As the concentration increases from 1.0 to 1.5 mol L−1, the Li-leaching efficiency increases to 96.73%, but those of the other metals change little. When the concentration is greater than 1.5 mol L−1, there is a small improvement in the efficiency. The optimum initial lactic acid concentration is therefore 1.5 mol L−1. Effect of S/L ratio on acid leaching. Fig. 4b shows the effect of the S/L ratio on the leaching efficiency under the conditions lactic acid concentration 1.5 mol L−1, 0.5 vol% H2O2, temperature 60 °C, and leaching time 30 min. The leaching efficiencies of the four metals decrease slightly as S/L increases from 10 to 20 g L−1. However, the leaching efficiencies of Ni, Co, Mn, and Li decrease to 88.52%, 88.4%, 88.69%, and 91.81%, respectively, at an S/L ratio of 40 g L−1. In terms of a high leaching efficiency, the best S/L ratios is 20 g L−1. Meanwhile, when the S/L ratio increases to 40 g L−1, the leaching efficiencies of metals are still higher than 80%, which is acceptable in the scale-up industry applications.

13



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Effect of temperature on acid leaching. We also investigated the effect of temperature on the efficiency of waste LiNi1/3Co1/3Mn1/3O2 leaching. The temperature varied from 40 to 90 °C; the other conditions were lactic acid concentration 1.5 mol L−1, H2O2 0.5 vol%, S/L ratio 20 g L−1, and leaching time 30 min. Fig. 4c shows that almost 67% of Ni, Co, Mn, and 69.6% of Li can be leached at 40 °C. The leaching efficiencies increase rapidly to 90% at 50 °C. More than 96.7% of Li, Ni, Co, and Mn is leached at 70 °C. A plateau is observed above 70 °C. A temperature of 70 °C is therefore suitable for the leaching process. Effect of amount of H2O2 on acid leaching. Fig. 4d shows the effect of the volume concentration of H2O2 (vol%) on leaching for a leaching time of 30 min at 70 °C. Only about 33.22% of Ni, 33.63% of Co, 30.72% of Mn, and 59.34% of Li are leached without the addition of H2O2. The leaching efficiencies of Ni, Co, Mn, and Li increase to 96.27%, 96.73%, 96.32%, and 96.84%, respectively, with increasing H2O2 concentration from 0 to 0.5 vol%. However, the leaching efficiencies of the metals are almost constant at H2O2 concentrations from 0.5 to 3 vol%. This confirms that a low volume concentration of H2O2 (0.5 vol%) is sufficient for the leaching process. Effect of leaching time on acid leaching. The effect of the reaction time on the leaching efficiency was investigated from 10 to 60 min under the conditions lactic acid concentration 1.5 mol L−1, S/L ratio 20 g L−1, 0.5 vol.% H2O2, and temperature 70 °C. The leaching efficiencies of the different metals rapidly reach more than 95% at 10 min (Fig. 4e). At 20 min, the total efficiencies are about 98%. Extending the time from 20 to 60 min does not improve the efficiencies, which indicates that acid leaching reaches its reaction equilibrium within 20 min. 14


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Fig. 4. Effect on leaching efficiency of (a) initial acid concentration, (b) solid/liquid ratio; (c) temperature (d) amount of H2O2, (e) reaction time; (f) metal leaching efficiencies under optimized conditions. Kinetic analysis of LiNi1/3Co1/3Mn1/3O2 leaching with lactic acid. Multi-metal leaching in acid solution was investigated by performing kinetic studies for different leaching times (0–30 min) and temperatures (40–80°C) under the following conditions: lactic acid concentration 1.5 mol L−1, S/L ratio 20 g L−1, and H2O2 concentration 0.5 vol%. The results 15



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are shown in Fig. 5. The optimum leaching conditions identified in the kinetic studies are in agreement with those already obtained (Fig. 4).

Fig. 5. Effects of reaction temperature and time on the leaching rates of Li (a), Ni (b), Co (c), and Mn (d) (1.5 mol L−1, 0.5 vol. % H2O2, S/L ratio of 20 g L−1). Metal leaching from the cathode materials is a solid–liquid heterogeneous reaction, and is considered to occur on the outer surfaces of unreacted particles. Multi-metal-leaching kinetics has been successfully interpreted based on shrinking-core models and the Avrami equation. 41−43

However, the metal extraction data shown in Fig. 5 do not fit various shrinking-core

models. The Avrami equation is therefore used to explain the leaching kinetics: ln( − ln(1 − x ) = ln k + n ln t

(2)

where x is the fraction reacted (i.e., leaching rate), k is the reaction rate constant (min−1), n is a suitable parameter, and t is the reaction time (min). Fitting plots of ln[−ln(1 − x)] vs lnt at 16


Page 17 of 31

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different temperatures for different metals, based on the data in Fig. 5, are shown in Fig. 6. The plots show good linear relationships for Li, Ni, Co, and Mn with R2 values all greater than 0.96 (Table 4). This shows that the Avrami equation well describes the leaching process under the given leaching conditions. The values of n are calculated to be 0.5−1. This indicates that the initial reaction rate is high and continually decreases with time until the reaction reaches equilibrium.42

Fig. 6. Plots of ln(-ln(1-x)) vs. ln t at different reaction temperatures: Li (a), Ni(b), Co(c) and Mn (d).

Table 4. Parameters of the kinetics model for Li, Ni, Co and Mn at different leaching temperatures. T(oC) 40

Li

Ni 2

n

lnk

R

0.8094

-2.6843

0.9746

Co 2

n

lnk

R

0.8710

-2.9053

0.9903

Mn 2

n

lnk

R

0.8559

-2.8480

0.9887

17


n

lnk

R2

0.9464

-3.1152

0.9955


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 18 of 31

50

0.8486

-2.1250

0.9918

0.8785

-2.2392

0.9872

0.8742

-2.2178

0.9881

0.9138

-2.3130

0.9920

60

0.6319

-0.8647

0.9876

0.6307

-0.9188

0.9649

0.6356

-0.9254

0.9701

0.6136

-0.8226

0.9873

70

0.6251

-0.4704

0.9671

0.6683

-0.5934

0.9687

0.6362

-0.5452

0.9761

0.6666

-0.5419

0.9693

80

0.7740

-0.1087

0.9606

0.7648

-0.2698

0.9893

0.7593

-0.2873

0.9702

0.7145

-0.1857

0.9609

The Arrhenius equation is used to describe the relationship between reaction rate constant and the reaction temperature.

(3) where, k is the reaction rate constant (min−1), A is the pre-exponential factor, Ea is the apparent activation energy (kJ/mol), R is the universal gas constant (8.314 J/K/mol), and T is the absolute temperature (K). The activation energies of the leaching reactions are usually calculated by the linear form of the Arrhenius equation.

(4) Plots of lnk (see in Table 4) vs. 1/T are shown in Figure 7, and the activation energies for the leaching of Li, Ni, Co and Mn are, 62.81 kJ/mol, 63.96 kJ/mol, 62.83 kJ/mol and 70.62 kJ/mol, respectively, indicating that the rate-controlling step of this leaching process is the surface chemical reactions.

18


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Fig. 7. Arrhenius plots for the leaching of Li, Ni, Co and Mn in the temperature range 313.15–353.15K.

Mechanism of acid leaching. Lactic acid (C3H6O3) is an α-hydroxy acid and contains a carboxyl group adjacent to a hydroxyl group. The presence of an α-hydroxyl group increases the acidity compared with that of an average monobasic acid. The pKa of lactic acid is 3.86. The possible products and mechanism of the reaction of LiNi1/3Co1/3Mn1/3O2 with lactic acid are shown in Fig. 8. This reaction occurs at the liquid–solid interfaces, and the reaction rate is influenced by the acid concentration, S/L ratio, amount of reductant, reaction temperature, and reaction time. This leaching reaction is mainly controlled by the chemical reaction. And the leaching reaction between LiNi1/3Co1/3Mn1/3O2 and lactic acid is a multiphase reaction, which can be explained in two steps: (1) conversion of high valence Ni, Co, and Mn to Ni2+, Co2+, and Mn2+ in the presence of H2O2 and subsequent dissolution of spent

19



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Page 20 of 31

LiNi1/3Co1/3Mn1/3O2 in the acid solution; (2) the chelation of Ni2+, Co2+, Mn2+, and Li+ with lactate. The leaching reaction can be represented as 3LiNi1/3Co1/3Mn1/3O2 (s) + 9C3H6O3 (aq.) + 1/2H2O2 = 3C3H5O3Li (aq.) + (C3H5O3)2Ni (aq.) + (C3H5O3)2Co (aq.) + (C3H5O3)2Mn (aq.) + 5H2O (l) + O2 (g)

Fig.8. The possible products and mechanism in the lactic acid leaching process.

Fig. 9. XRD and SEM patterns of R-NCM (a) and F-NCM (b) samples.

20


(5)

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Characterization

and

electrochemical

performance

of

regenerated

LiNi1/3Mn1/3Co1/3O2. Fig. 9 shows XRD patterns and SEM images of the regenerated LiNi1/3Co1/3Mn1/3O2 (R-NCM) and freshly synthesized LiNi1/3Co1/3Mn1/3O2 (F-NCM) samples. All peaks can be indexed to the hexagonal α-NaFeO2 structure, with the space group R3̅m. The distinct split of (006)/(012), (008)/(110) peaks indicates the formation of a well-layered structure. The ratio of the intensities of the (003) and (104) peaks is greater than 1.2 and c/a is high, indicating low cation mixing and good ordering of the transition-metal ions in the metal layer. The SEM images clearly show that both samples have a uniform particle size (100–300 nm). Generally, small particles make Li insertion/extraction easy because of decreased Li+ diffusion. The surface area of LiNi1/3Co1/3Mn1/3O2 synthesized using a sol−gel method is large, which improves the electrochemical performance. The regenerated material disperses well without addition of a chelating agent. The cycling performances of R-NCM and F-NCM at 0.2, 0.5, and 1 C are shown in Fig. 10a, 10b, and 10c. The reversible discharge capacity of R-NCM is 142.6 mA h g−1 at 0.2 C after 70 cycles, 138.2 mA h g−1 at 0.5 C after 100 cycles, and 128.3 mA h g−1 at 1 C after 100 cycles. F-NCM achieves 136.8 mA h g−1 at 0.2 C after 70 cycles, 125.4 mA h g−1 at 0.5 C after 100 cycles, and 109.8 mA h g−1 at 1 C after 100 cycles. The regenerated LiNi1/3Mn1/3Co1/3O2 clearly gives a higher discharge capacity and has the highest capacity retention (95.2% at 0.2 C, 96.0% at 0.5 C, and 94.5% at 1 C) under the same conditions. The rate capacities of the samples were investigated by charging the cells at 0.2 C and discharging them at various current rates (0.2−5 C), as shown in Fig. 10d. The discharge capacities of R-NCM are 151.6, 145.7, 138.3, 129.7, and 120.6 mA h g−1 at 0.2, 0.5, 1, 2, and 5 C, 21



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Page 22 of 31

respectively. After cycling at 5 C, the discharge capacity recovers to 148 mA h g−1, indicating that the regenerated material has a superior rate capability. The electrochemical reactions of the samples were further investigated based on the charge/discharge profiles for different cycles and EIS tests (Fig. 10e and 10f). The plateaus of the charge/discharge curves occur at around 3.8 V. Long charge/discharge curve plateaus improve the electrode stability and reversibility. Fig. 10f shows that the charge-transfer resistance of R-NCM (Rct = 58.78 Ω) is lower

than

that

of

F-NCM

(Rct

=

70.02

Ω),

which

indicates

better

Li

intercalation/deintercalation kinetic properties. It is because the ratio of the intensities of the (003) and (104) peaks of R-NCM (I003/I104=1.469) is greater than that of F-NCM (I003/I104=1.415), indicating low cation mixing and good ordering of the transition-metal ions in the metal layer in R-NCM. Meanwhile, Al or Mg compound are usually doped or coated in commercial NCM cathode.

22


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Fig. 10. Electrochemical performances of R-NCM and F-NCM samples: (a, b, c) cycling performances at 0.2C,0.5C and 1C, (d) rate performances at different currents, (e) charge/discharge profiles at 0.2C, (f) Nyquist plots in the frequency range from 100 kHz to 0.01 Hz.

CONCLUSION A hydrometallurgical method was developed for recycling LiNi1/3Co1/3Mn1/3O2 cathode active materials in spent LIBs. The results show that the concentration of lactic acid, S/L ratio, temperature, amount of H2O2, and leaching time have important effects on the leaching 23



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Page 24 of 31

efficiencies of the four metals. More than 98% of Ni, Co, Mn, and Li can be leached from spent LiNi1/3Co1/3Mn1/3O2 under the optimum leaching conditions: 1.5 mol L−1 lactic acid, 0.5 vol% H2O2, a relatively low temperature of 70 °C, a S/L ratio of 20 g L−1, and a short time of 20 min. The leaching kinetics of Li, Ni, Co, and Mn can be explained by the Avrami equation. The leaching mechanism was investigated in terms of the material structure. The regenerated cathode material has a high specific capacity, good rate capability, and excellent

cycling

performance

compared

with

those

of

freshly

synthesized

LiNi1/3Co1/3Mn1/3O2. More importantly, lactic acid has been proved to be an effective chelating agent for the sol–gel method. This closed-loop recycling process for regenerating spent LIB cathode material for LIBs will enable conservation and recycling of resources.

ACKNOWLEDGMENT The experimental work of this study was supported by the Chinese National 973 Program (2015CB251106), the Joint Funds of the National Natural Science Foundation of China (U1564206), and the Major achievements Transformation Project for Central University in Beijing and Beijing Science and Technology Project (D151100003015001).

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Table of Contents

Synopsis: LiNi1/3Co1/3Mn1/3O2 electrode materials from spent LIBs were regenerated as new cathodes using a sustainable method, which includes lactic acid leaching and sol-gel synthesis.

31


Sustainable Recovery of Cathode Materials from ... - ACS Publications

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