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Exploring a green route for recycling spent lithium-ion batteries: Revealing and solving deep screening problem
Journal Pre-proof Exploring a green route for recycling spent lithium-ion batteries: Revealing and solving deep screening problem Jiadong Yu, Quanyin Tan, Jinhui Li PII:
S0959-6526(20)30316-4
DOI:
https://doi.org/10.1016/j.jclepro.2020.120269
Reference:
JCLP 120269
To appear in:
Journal of Cleaner Production
Received Date: 11 November 2019 Revised Date:
31 December 2019
Accepted Date: 25 January 2020
Please cite this article as: Yu J, Tan Q, Li J, Exploring a green route for recycling spent lithium-ion batteries: Revealing and solving deep screening problem, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120269. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Credit author statement Jiadong Yu: Conceptualization, Methodology, Visualization, Writing Original draft preparation, revising; Quanyin Tan: Formal analysis, co- supervision, co-project administration; Jinhui
Li:
Conceptualization,
administration, funding acquisition.
Resources,
supervision,
project
Graphical abstract
1
Exploring a green route for recycling spent Lithium-ion batteries:
2
revealing and solving deep screening problem
3
Jiadong Yu a, Quanyin Tan *,a, Jinhui Li *,a
4
a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of
5
Environment, Tsinghua University, Beijing 100084, China
6 7
Abstract:
8
The conventional recycling methods of spent lithium-ion batteries (LIBs) regard Al/Cu foils as
9
impurities, which are usually removed by deep screening or alkali leaching. Deep screening refers to
10
the process of over-screening the electrode materials by passing the crushed materials of spent LIBs
11
through a screen with a pore size of only 0.075 mm. However, the serious hole-blocking
12
phenomenon of deep screening and the environmental hazards of alkali leaching restrict their
13
practical application. Herein, a combination of mild screening, reduction leaching and selective
14
purification is proposed to achieve green and sustainable recycling of spent LIBs. Specifically, spent
15
LIBs are subjected to discharge, crushing and mild screening to obtain fine-mixture materials under
16
4-mm sieving, leaving large pieces of Cu/Al foils on the screen mesh, which can be separated as
17
crude products by color sorting. Then, dilute H2SO4 is employed to dissolve the valuable elements in
18
underflow fractions, where residual Cu/Al foils serve as reductants to assist leaching. Eventually,
19
through multi-step precipitates and hydrothermal treatment, a series of promising products (CuO,
20
NaAlCO3(OH)2, precursor and Li2CO3) are obtained, at high quality. This route provides a clever
21
strategy for fine management and cascade recovery of the valuable components from spent LIBs.
22 1
23
Key words: Spent Lithium-ion battery, Deep screening, Reductive leaching, Ternary precursor,
24
NaAlCO3(OH)2.
25 26
1. Information
27
With the representative codes of high energy density, high battery voltage, long life span and no
28
memory effect, lithium-ion batteries (LIBs) have dominated the global market for smart devices and
29
electric vehicles (Golmohammadzadeh, R., et al. 2018). According to the latest research report, the
30
global market value of LIBs in 2017 was US$ 25 billion, and it is predicted to reach US$ 47 billion
31
by 2023, exhibiting a CAGR of around 11% during 2018-2023(Wood, L., 2018). The flourishing
32
development of LIBs stems from the robust demand for safer and stronger rechargeable batteries
33
(Hannan, M.A., et al. 2017), but this popularity will also lead to a surge in the amount of LIBs
34
entering the waste stream, in the near future. It is estimated that in the European Union alone, there
35
will be 13,828 tons of spent LIBs in need of efficient recycling by 2020 (Träger, T., et al. 2015).
36
Spent LIBs contain many heavy metals, such as Mn, Ni, Co and Cu, and poor recycling treatments
37
for these wastes will result in irreparable damage to the environment (Yu, J., et al. 2017). Conversely,
38
responsible recycling of end-of-life LIBs can not only facilitate the development of environmental
39
protection and resource sustainability, but also achieve considerable economic benefits.
40
Typical spent LIBs usually include electrode materials, Cu/Al foils, shells, separators and
41
electrolytes (Yu, J., et al. 2018; Wang, M., et al. 2018). The recycling methods for spent LIBs focus
42
on the valuable cathode materials, and can be primarily divided into hydrometallurgy (Nayaka, G.P.,
43
et al. 2016; Vieceli, N., et al. 2018) and pyrometallurgy (Li, J., et al. 2016; Xiao, J., et al. 2017). 2
44
High-temperature smelting is a simple and efficient approach, with the advantages of less
45
consumption of chemical reagents and short recycling route (Dos Santos, C.S., et al. 2019), but it
46
could produce some toxic sludge containing lithium, aluminum and other heavy metals (Li, L., et al.
47
2017; Roshanfar, M., et al. 2019). On the other hand, based on the special functions in precise
48
separation, effective recovery and selective preparation of high-value products, hydrometallurgy has
49
attracted more attention (Joulié, M., et al. 2014). Although the two companies, Sungeel Hi-Metal Co.
50
(South Korea) and Umicore Co. (Belgium), have claimed to master the key technology for
51
commercial application of spent LIBs, it is difficult for developing countries to learn and transform
52
locally due to technical barriers and equipment manufacturing defects. Therefore, the urgent vision is
53
to explore facile technologies that can be replicated and promoted worldwide. Furthermore, cathode
54
materials are often composed of trivalent cobalt and tetravalent manganese, and these high-valent
55
transition metal oxides are poorly soluble in hydrometallurgy (Sattar, R., et al. 2019). In order to
56
alleviate this problem, an additional reductant (Chen, X., et al. 2015; Hu, J., et al. 2017) or a
57
reducing organic acid (Gao, W., et al. 2017; Li, L., et al. 2017) should be employed, to enhance the
58
leaching effect of cathode materials. Nevertheless, the purchase, transportation and storage of
59
reducing agents will increase operating costs (Ma, X., et al. 2018). And since most acid leaching
60
methods regard Al as an impurity element, deep screening (the passing of powders through a screen
61
with the pore size less than 75 microns (Zhang, X., et al. 2018; Zheng, X., et al. 2018)) was studied
62
to forcibly remove the residual Al foils from the cathode electrode powders after efficient crushing,
63
and even an alkali leaching step (Gratz, E., et al. 2014; Ferreira, D.A., et al. 2009) could be added to
64
fully remove Al element. However, deep screening is almost impossible to realize in industrial 3
65
application, and the sudden addition of the alkali leaching will significantly increase the acid
66
consumption in subsequent processes. In addition, some scholars used deep eutectic solvent (Wang,
67
M., et al. 2019) or toxic NMP solvent (Contestabile, M., et al. 2001) to separate the cathode powder
68
from the Al foils completely by removing the organic binder, so as to avoid the Al pollution. But this
69
may increase the complexity and cost of the recycling process. Therefore, reusing the Al foils in
70
spent LIBs as a reductant to assist in the leaching of cathode materials is a promising strategy.
71
Joulié (Joulié, M., et al. 2017) first put forward the idea that Cu and Al could be utilized as
72
reducing agents to dissolve NCM cathode materials by acid leaching, and gave a simple dynamic
73
explanation. Peng (Peng, C., et al. 2019) also found that Cu, Al and Fe in spent LIBs could
74
significantly improve the leaching effect of Li and Co up to almost 100%. Unfortunately, they did
75
not consider the effective amount of Cu and Al foils, nor did they specify the corresponding
76
purification methods. It is precisely these two key factors that limit the practical application of Cu/Al
77
foils as reductants in waste battery recovery.
78
In this study, we are committed to consuming the least amount of Cu/Al foils as reductants to
79
assist in leaching and recovering the most valuable metal elements. This meticulous management
80
strategy is applied to cleverly solve the deep-screening bottleneck problem in the industrial
81
application of LIB recycling. Compared with traditional methods, this technology will bring the
82
following benefits: (1) This technology refuses to use caustic alkali or toxic solvents in the recycling
83
process; (2) It is unnecessary to add additional reducing agents, such as H2O2 and Na2S2O5, which
84
reduces the consumption of chemical reagents and greenhouse gas emissions; (3) This technology
85
realizes the recovery of all components of valuable materials in spent LIBs. The general framework 4
86
of this research is as follows: the feasibility of Cu/Al powder as reductants was firstly verified in
87
simulation experiments. Secondly, thermodynamic analysis gave the reduction mechanism and
88
determined the theoretical demand of Cu and Al foils. In the third section, fine management of Cu/Al
89
foils was conducted under the actual accumulative effect of mild screening. Then, the systematic
90
exploratory experiment of actual screening products revealed the optimum leaching conditions. Next,
91
through progressive precipitation and hydrothermal process, some high value products were prepared,
92
and their crystallographic and microscopic morphologies were discussed in detail. Finally, a green
93
and fully mechanized process was distilled, removing an important obstacle to industrial recycling of
94
spent LIBs.
95 96
2. Experimental materials and methods
97
2.1 Materials and reagents
98
Spent ternary lithium-ion batteries from electric vehicle battery packs were donated by
99
TES-AMM, an environmental protection company in Beijing. As shown in Figure 1, the diffraction
100
analysis results of electrode materials from discarded LIBs gave that the cathode material was a
101
high-nickel ternary material with LiNiO2 crystal phase. Furthermore, the element analysis showed
102
that the electrode materials were composed of 5.19 wt.% lithium, 30.77 wt.% nickel, 12.23 wt.%
103
cobalt and 17.21 wt.% manganese and 36.4 wt.% graphite. Therefore, the waste cathode material in
104
this experiment was Li(Ni0.5Co0.2Mn0.3)O2, namely, 523 ternary cathode materials. The chemical
105
reagents used in the experiments, including Cu, Al, H2SO4, H2O2, Na2CO3, NaOH, and
106
diaminomethanal, were of analytical grade. Deionized water was used as the experimental water. 5
107 108
2.2 Mild screening experiments
109
In order to avoid the fire caused by the short circuit in the crushing process, three (600g)
110
lithium-ion batteries were first immersed in 5% sodium chloride solution for 48 hours to discharge
111
completely. Besides, this could also allow the residual Li ions on the anode surface to return to the
112
cathode materials (Liu, C., et al. 2016), improving the recovery rate of Li element in the whole
113
recovery process. All batteries were thoroughly crushed in the fume hood by the hand-held universal
114
crusher. The crushed products were screened in turn by mild screening with sieve hole sizes of 4 mm,
115
2 mm, 1.45 mm, 0.9 mm and 0.5 mm. Specifically, mild screening means that only 5 minutes of
116
screen vibration is needed to make materials permeable, without water pressure or other external
117
force to assist in the sieving.
118 119
2.3 Leaching and purification experiments
120
The leaching experiments were completed in a 500-ml three-necked flask with a reflux
121
condenser. In addition, it was necessary to use a heating bath equipped with magnetic stirring to
122
assist in material agitation and temperature control. During the experiments, a certain amount of raw
123
materials was added to 100 ml dilute sulfuric acid. The leaching experiments in this study were
124
divided into simulation experiments and conditional optimization experiment. In order to illustrate
125
the feasibility of using Cu and Al as reducing agents, simulation experiments were first performed.
126
Different combinations of leaching agents (H2SO4 alone, H2SO4 + Cu/Al, H2SO4 + 6 vol % H2O2)
127
with analytically pure grade were employed to leach 523 high-nickel cathode materials. Because the 6
128
mass ratio of NCM / Cu and NCM / Al in spent LIBs were 1:1.2 and 1:0.7 respectively (Doberdò et
129
al., 2014; Loeffler et al., 2014), the proportion of NCM / Cu / Al was simplified to 2:1:1 in the
130
simulation experiment. The operating parameters of simulation experiments were 2.5 mol/L H2SO4,
131
200 mL/g, 60℃ and 6h of reaction time. For conditional optimization experiment, the raw materials
132
were the mixture obtained under 4 mm mild screening. Different from the simulation experiments,
133
the raw materials and reducing agents were the electrode materials and Cu/Al foils inherent in the
134
spent LIBs. In this section, the temperature was controlled at 0℃, 30℃, 45℃, 60℃ and 75℃, the
135
liquid-solid ratio was adjusted within the range of 20 mL/g, 50 mL/g, 100 mL/g, 200 mL/g and 400
136
mL/g, and the sulfuric acid concentration range was 0.5- 4.0 mol / L. When leaching experiments
137
were finished, the residue and leachate were separated using a filter paper with a mesh opening of
138
0.02 µm.
139
Depending on the difference of solubility product (Ksp) and pH range of precipitation, Cu2+ and
140
Al3+ ions in leachate can be precipitated thoroughly. As shown in Table S1, the pH values of Cu2+ and
141
Al3+ ions reaching the minimum concentration were 4.49 and 6.65, respectively. In order to prevent
142
the simultaneous precipitation of Ni element at low pH, the key is to dilute the concentration of Ni2+
143
to less than 1 M (Lain M., et al. 2001; Zou, H., et al. 2013). In this experiment, the concentrations of
144
all the metal ions (Al, Cu, Ni, Co and Mn) were first adjusted to below 0.5 M after acid leaching, and
145
then the pH value could increase slowly to 6.65 by adding 1 M of NaOH. The Cu(OH)2 and Al(OH)3,
146
separated through filtration, were stirred in other NaOH solution at pH=12 for 2 h. After re-filtration
147
and heating, Cu(OH)2 residue and NaAlCO3(OH)2 filtrate could be translated into CuO and
148
NaAlCO3(OH)2 powders, respectively. 7
149
The leachate retrieved after removing Cu2+ and Al3+ is a neutral solution for hydrothermal
150
reaction. The specific steps were that 50 ml leachate and 10 g diaminomethanal were placed in a
151
100-ml hydrothermal reactor and reacted for 12 hours at 160℃. The NCM precursors could be
152
obtained by vacuum filtration with large surface morphology. The pH of residual solution after
153
hydrothermal reaction was adjusted to 12, to remove all the heavy metal ions in the solution. The
154
Li2CO3 precipitate was obtained with the adding of Na2CO3 and washed with boiling ultra-pure
155
water.
156 157
2.4 Analytical methods
158
The concentration of metal elements in solution were measured with an inductively coupled
159
plasma-optical emission spectrometer (ICP-OES, OPTIMA 2000, PerkinElmer, USA). In addition,
160
the leaching rate of each element was calculated with the following equation:
161
W wt % =
× 100%
162
where
163
the solid material before acid leaching.
(1)
is the final content of metals in solution, and
denotes the original content of metals in
164
The crystal structures and phase analysis of the solid powders were characterized by X-ray
165
diffraction (XRD, Philips PW 1700, USA) using Cu Kα radiation, and the data were analyzed by
166
MDI Jade 6.0 software. The microscopic morphology and content distribution of surface elements
167
were characterized by field emission scanning electron microscopy, which contained an energy
168
dispersive spectrometer (FE-SEM and EDX, Carl Zeiss MERLIN Compact, Germany).
169 8
170
3. Results and discussion
171
3.1 Feasibility analysis
172
High-nickel batteries are distinguished from conventional NCM batteries in that they have better
173
electrochemical performance and structural stability, but this also increases the difficulty of recycling.
174
The use of Cu and Al as reducing agents for leaching Li(Ni1/3Co1/3Mn1/3)O2 (namely 111 ternary
175
cathode materials) with sulfuric acid has proved to be effective, but no similar research has been
176
done on the feasibility of recovering high-nickel ternary materials. The combination of H2SO4 and
177
H2O2, though, is generally considered to have a good leaching effect. In this simulation experiment,
178
mass ratio of cathode material to Cu and Al was 2:1:1. The final leaching results were shown in
179
Figure 2, and suggested that only 40% of the ternary materials could be dissolved by pure dilute
180
H2SO4; its effect was poor. The addition of H2O2 could help dissolve 90% of the cathode materials.
181
More gratifying was that the addition of Cu and Al could recover almost 100% of all the metal
182
elements, by leaching, mainly due to the reduction of insoluble Mn4+ and Co3+ to soluble Mn2+ and
183
Co2+. However, determining the reason Al and Cu powders have a better reduction effect requires an
184
in-depth analysis of their Gibbs free energy in leaching process.
185 186
3.2 Thermodynamic analysis
187
The good electrochemical performance of the NCM battery derives from its stable layered
188
structure, which mainly includes transition metal oxides with embedded lithium elements. It can be
189
determined through assimilation that the NCM ternary material is composed of a stack of transition
190
metal oxides—namely NiO, MnO2 and Co2O3—with inserted lithium (Joulié, M., et al. 2017). In 9
191
particular, bivalent nickel is easily soluble in dilute H2SO4 and does not participate in reduction
192
reaction. Therefore, the reaction of the NCM electrode material with the above reducing agents (Cu,
193
Al and H2O2) can be approximated by the reaction of MnO2 and Co2O3. In the case of Co2O3, three
194
different redox reactions may occur, and their standard Gibbs free energies are listed below (Wang L.,
195
2013). 3Co2O3 + 2Al + 9H2SO4 → 6CoSO4 + Al2(SO4)3 + 9H2O = −477.636 − 0.02684 ,
0~1000!
Co2O3 + Cu + 3H2SO4 → 2CoSO4 + CuSO4 + 3H2O = −69.497 − 0.01245 ,
0~1000!
Co2O3 + H2O2(l) + 2H2SO4 → 2CoSO4 + 3H2O + O2(g) = −37.262 − 0.04583 ,
0~1000!
(1) (2) (3) (4) (5) (6)
196
Similarly, trivalent manganese ions also need to be reduced to divalent transition metal ions; the
197
corresponding standard Gibbs free energies were calculated by HSC Chemistry 6.0 as shown in
198
follows: 3MnO2 + 2Al + 6H2SO4 → 3MnSO4 + Al2(SO4)3 + 6H2O = −491.349 + 0.20149 ,
0~1000!
MnO2 + Cu + 2H2SO4 → MnSO4 + CuSO4 + 2H2O = −74.024 + 0.0635 ,
0~1000!
MnO2 + H2O2(l) + H2SO4 → MnSO4 + 2H2O + O2(g) = −26.677 − 0.03372 , 199
0~1000!
(7) (8) (9) (10) (11) (12)
Figure 3 shows the relationship between standard Gibbs free energy and temperature in different 10
200
reactions (1-12). In Figure 3, the reactions of Al, Cu and H2O2 as reductants are marked in green,
201
blue and black, respectively. It is well known that if the Gibbs free energy is lower than zero and no
202
other potential energy is generated, the reaction can proceed spontaneously; otherwise the reaction
203
cannot occur. According to the curve trend in Figure 3, only the standard Gibbs free energy of
204
reaction 9 may be higher than 0, within 1000 K. Therefore, based on Eq. 13, the reaction (9) was
205
subjected to further quantitative calculation to determine its Gibbs free energy at specific
206
temperatures. G =
+ R ln
)*+ )
(13)
207
where R is the molar gas constant, 8.314 J/(mol· K); T is the thermodynamic temperature, degrees
208
Kelvin; )*+ is the partial pressure of the gas product; and )
209
When the reaction is carried out in an air atmosphere without applied pressure, this
210
approximately equal to 0.21, and Eq. 10 should change to: G = −74.024 + 0.0508 ,
is the standard atmospheric pressure.
0~1000!
, -+ ,.
value is
(14)
G < 0, 4-valent manganese ions may theoretically be reduced to
211
On the basis of Eq.14, when
212
2-valent manganese ions by copper, within 1457.165 K (1184℃).
213
The leaching reaction of NCM materials occurs in water-based solution, so the range of reaction
214
temperature is usually 273.15-373.15 K (0-100℃). The variation of Gibbs free energy for each
215
reaction in this temperature range is presented in Figure 3 with a champagne background. It indicates
216
that the absolute value order of Gibbs free energy for each reaction within the temperature range of 0
217
-100 ℃ is (1) > (7) > (3) > (9) ≈ (5) > (11). In other words, the order of reducibility is Al > Cu >
218
H2O2, and the reaction rate is Al > Cu > H2O2. Since there is a large gap between Al and Cu both in 11
219
the Gibbs free energy and the reductivity sequence of the metals (Al > hydrogen (H) > Cu), we can
220
conjecture that the cathode materials preferentially react with Al foils. Only after the Al foils are
221
completely consumed, the cathode materials begin to react with the Cu foils in dilute sulfuric acid.
222
On the whole, the reduction of high-valent transition metal ions in NCM materials by Al and Cu is
223
efficient, and the reduction effect is better than that of H2O2, in theory, a result consistent with those
224
of previous experiments (Joulié, M., et al. 2017).
225
More importantly, according to the reaction equations (1) and (7), 2 moles of Al can react with 3
226
moles of Co2O3 or MnO2, implying that 1 mole of Al can induce at least 1.5 moles of NCM cathode
227
materials to be completely dissolved in 3 moles of H2SO4 by the redox reaction: in other words, 1
228
gram of Al can catalyze about 5.36 grams of ternary cathode material in 10.89 grams of H2SO4.
229
Similarly, 1 gram of Cu can catalyze about 1.52 grams of NCM powders in 3.08 grams of H2SO4.
230
For simplicity of calculation, it can be considered that 1 g Al is equivalent to 5 g NCM material and
231
10 g H2SO4, while 1 g Cu is equivalent to 1.5 g NCM material and 3 g H2SO4. It should be noted that
232
in this calculation, the total content of Co2O3 and MnO2 in the NCM material is approximately 100%,
233
but their actual content is only 30-70% in 111, 523, 532, 622, 811 cathode material. Therefore, this
234
result can be applicable to all kinds of NCM cathode materials.
235 236
3.3 Fine management of Cu/Al foils through mild screening
237
As the simplest method of separating Cu/Al foils and electrode materials, mechanical crushing
238
has the most practical application prospects. In this section, we hoped to find the best proportion of
239
Cu/Al foils to cathode materials by adjusting the sieving size of the mild screening, based on the 12
240
above thermodynamic analysis.
241
After efficient crushing and mild screening of spent LIBs, the morphology and element
242
distribution of each component are shown in Figure 4. It indicates that 60.51 wt.% of the crushed
243
materials were enriched in the -0.5 mm particle size, while the material contents of +0.5-0.9 mm,
244
+0.9-1.45 mm, +1.45-2 mm, +2-4 mm and +4 mm were 12.24%, 4.89%, 3.61%, 5.68% and 5.88%,
245
respectively. Besides, the metal content in -0.5 mm fine materials was 57.24%, and the remaining
246
42.76% was non-metallic graphite. It should be noted that -0.5mm particles mean that the particle
247
size is less than 0.5mm, while +0.5-0.9 mm particles mean that the particle size is greater than
248
0.5mm and less than 0.9mm. Furthermore, the experimental results show that the content of electrode
249
materials in the -0.5 mm particles is as high as 98.94%, and the content of Cu/Al foils in the +4 mm
250
crude material could reach 94.19%. This remarkable selective crushing behavior is consistent with
251
other scholars’ research (Zhang T., et al. 2013). In order to determine the optimum screening size, the
252
Al and Cu contents in Figure 4 were converted into NCM cathode material equivalents: the materials
253
were then compared by sieving the cumulative specimens. The results are shown in Figure 5 and
254
Table S2, and it illustrates that the cumulative content of actual NCM materials increased slightly,
255
from 33.99% in -0.5mm level to 41.29% in total. Nevertheless, with the increasing accumulative
256
amount of Cu/Al foils, the NCM equivalent content (initially 2.28%) gradually caught up with the
257
actual content of NCM, finally reaching 73.36%. More importantly, when mild screening was carried
258
out with 4 mm sieve holes, the contents of Al/Cu foils in the screening product were 6.30 wt.% and
259
9.98 wt.% respectively, converted into NCM equivalents of 31.49 wt.% and 14.97 wt.% respectively.
260
In fact, only 41.13 wt.% of the products under mild screening were NCM cathode materials, while 13
261
the total content of NCM equivalent from Al/Cu foils was 46.46 wt.%, which was 5.33 wt.% higher
262
than the actual content. Therefore, 4 mm sieve holes are the optimal screening conditions.
263
Furthermore, if 100g of this mild screening products need to process, containing 6.3 g Al, 9.98 g Cu,
264
and 41.13g NCM materials, the demand for H2SO4 is 92.94 g. Consequently, the mass ratio of
265
sulfuric acid to sieving product is about 1:1, while the specific mass ratio of H2SO4 to Cu, Al and
266
NCM materials is about 2:3:15:30.
267
Large pore size not only reduces the difficulty of materials’ passing through the mesh, but also
268
controls the proportion of reductant for subsequent leaching reactions. This fine management
269
strategy of Cu/Al foils subtly solve the industrialization restriction brought by 0.075 mm deep
270
screening, and paves the way for the cascade recycling of copper and aluminum elements.
271 272
3.4 Optimum leaching condition
273
In this section, the effects of temperature, acid concentration and liquid-solid ratio on leaching
274
results were considered comprehensively, as shown in Figure 6. In order to explore the optimal
275
conditions of liquid-solid ratio, the experimental temperature was controlled at 60°C and the
276
concentration of sulfuric acid at 3 mol/L. As shown in Figure 6A, the liquid-solid ratio of 20 mL/g
277
could only dissolve about 25% of the valuable metal elements. In contrast, a higher liquid-solid ratio,
278
such as 200 mL/g, could dissolve more than 90% of the cathode materials. When the liquid-solid
279
ratio reached 400 mL/g, the NCM cathode materials could be completely dissolved. In terms of total
280
component recovery and economic value, the best liquid to solid ratio was 200 mL/g. When
281
exploring the optimum temperature conditions, the liquid-solid ratio of the reaction was kept at 200 14
282
mL/g, and the acid concentration was 3 mol/L. Temperature is an important condition for reducing
283
the dissolution energy barrier. In this experiment, the reaction temperatures were set to 0℃, 30℃,
284
45℃, 60℃, 75℃ and 90℃, and the results are shown in Figure 6B. Due to the increased diffusivity
285
and decreased viscosity of liquid, the leaching rate of each metal increased with the increase of
286
temperature, and reached a maximum at 60℃. As the temperature continued to increase, the leaching
287
rate showed a small fluctuation, but remained stable on the whole. Accordingly, considering the
288
economics and security of the heating process, the optimal temperature was 60℃.
289
The optimal concentration of dilute sulfuric acid was explored at 60℃ and 200 mL/g. As seen in
290
Figure 6C, with the increase of dilute sulfuric acid concentration, the leaching rates of Li, Ni, Co, Mn
291
first increased and then decreased. The main reason may be that there is a side reaction here and the
292
reaction equation is as follows. 4Al + 3O3 + 6H3 SO6 → 2Al3 SO6 = −885.442 + 0.17961 ,
+ 6H3 O
(15)
0~1000!
(16)
8
293
When the H2SO4 concentration is low (0.5-2.5 mol/L), most Al foils play a role of reducing agent to
294
promote the leaching of cathode materials. Nevertheless, NCM cathode material is a kind of
295
spherical secondary particles composed of Li2O, NiO, Co3O4 and MnO2 micro-particles. The
296
leaching and release of these metal ions into liquid phase are relatively slow. When the H2SO4
297
concentration is high (2.5-4.0 mol/L), a large number of hydrogen ions will consume some Al foils,
298
resulting in the decrease of reductant in the system. Thus, the leaching of metal ions from ternary
299
materials is insufficient at the same time unit. Figure 6C shows that the optimum leaching rate
300
appeared at 2.0-2.5 mol/L H2SO4. From the viewpoint of cost savings and environmental load 15
301
reduction, dilute sulfuric acid of 2.0 mol/L could be the optimum leaching condition.
302
Therefore, the optimum conditions for leaching 523 NCM cathode materials with Cu/Al foils
303
inherent in spent LIBs are: 4 mm mild screening, liquid-solid ratio of 200 mL/g, reaction temperature
304
of 60℃ and dilute sulfuric acid concentration of 2 mol/L.
305 306
3.5 Purification and characterization
307
H+, Li+, Ni2+, Co2+, Mn2+, Al3+, Cu2+ and SO42- were all present in the leaching solution. The
308
interactions of these six metal cations made the solution system more chaotic, resulting in a great
309
challenge of multi-metal separation and purification. Fortunately, there is a distinct pH gap between
310
Al3+, Cu2+ and Ni2+, Co2+, Mn2+ during precipitation, Namely, Al3+ and Cu2+ could be pre-removed
311
first, at pH less than 6.5, and then Ni2+, Co2+, Mn2+ could be co-precipitated completely when the pH
312
was more than 12. For the mixture of Al(OH)3 and Cu(OH)2, Al(OH)3 precipitation can be converted
313
into NaAlCO3(OH)2 solution by adding NaOH solution, so that copper-rich solid and aluminum-rich
314
solution can be separated by filtration. The possible dissolution reaction equations are as follows. Al OH
8
+ 9:3 + ;<:= → ;<>?9:8 :=
= −36.768 + 0.03472 ,
3
+ =3 :
0~1000!
(17) (18)
315
For further surface morphology and phase composition analysis, the blue-violet liquid
316
(NaAlCO3(OH)2 solution) was dried and crystallized, and then subjected to SEM and XRD tests,
317
whose results were shown in Figures 7B and C. The SEM image indicates that the basic carbonates
318
are mainly flocculent and flaky, with some rod-like crystals. The XRD pattern suggests that only the
319
diffraction peaks of the NaAlCO3(OH)2 match those of the materials exactly, and the standard card 16
320
number is 45-1359. Since the detection limit of XRD is 5% and no other impurity peaks occurred,
321
the purity of the NaAlCO3(OH)2 can be considered to be higher than 95%. In this recovery system,
322
the Al foil powder can be recovered in the form of crude Al sheets after mild screening and
323
high-quality NaAlCO3(OH)2 after reductive leaching, which realizes the cascade recovery to some
324
extent.
325
After application of the co-precipitation and the hydrothermal method, the ternary precursor
326
was produced. In order to analyze the quality of the product, the precursors were characterized by
327
FE-SEM, EDX and XRD in detail, and the results are shown in Figure 7. Due to changes in the
328
contents of nickel-cobalt-manganese elements and crystalline hydrate, there was no unified chemical
329
structure for the precursor, and no single XRD phase standard card corresponding to it. From the
330
XRD pattern in Figure 7D, three diffraction characteristic peaks of MnO·OH, CoO and Ni(OH)2
331
existed in the precursor phase, and analyzed under the standard cards of 18-0804, 42-1300 and
332
14-0117, respectively. In general, these ternary precursor materials belong to a mixture of metal
333
oxides or metal hydroxides, which can be understood as a form of NiO·MnO·CoO·(OH)x. Figure 7A,
334
E and F show the microscopic surface topography of the precursor. These indicate that the precursor
335
prepared by the diaminomethanal-hydrothermal method is a type of chrysanthemum-cluster spherical
336
particle. The obvious wrinkles and clear stripes on the surface of the particles prove that the
337
precursor will have a good layered structure after calcination and hold a good electrochemical
338
performance potential, as shown in S1 (He, Q., et al. 2017; Zhang, F., et al. 2017; Chen, M., et al.
339
2019). In order to observe the element distribution on the surface of particles more intuitively, EDX
340
energy spectrum analysis was carried out, as shown in Figure 7G-L. The distributions of nickel, 17
341
cobalt, manganese and oxygen elements on the surface of the particles are relatively uniform,
342
consistent with the XRD results. Relative quantitative analysis of the elements shows that the
343
contents of nickel, cobalt, manganese and oxygen were 25.9 wt.%, 17.2 wt.%, 13.8 wt.% and 43.1
344
wt.%, respectively. All these characterizations indicate that the precursors have good performance
345
potential.
346 347
3.6 A green recycling route of spent ternary lithium-ion batteries for industrial application
348
With the preparation of the key products NaAlCO3(OH)2 and NCM precursors, we were able to
349
introduce a simple technical flowchart for manufacturing advanced materials from spent LIBs.
350
However, the main factors determining if a technical route can be industrialized are whether it can
351
recover as much valuable product as possible, whether the input cost of equipment is appropriate,
352
and whether labor costs can be contained. Taking the above factors into consideration, this study
353
optimized the main technology of mild screening and multi-step precipitation, and obtained a new
354
green recycling route, as shown in Figure 8.
355
The mild screening technology breaks through the bottleneck of LIB recycling, and is one of the
356
core technical points of the entire route. The process developed in this study was as follows: the
357
crushed products of spent LIBs were screened slightly by a 4-mm sieve; the coarse materials with
358
particle size larger than 4 mm were spherical-like or sheet-shaped Al and Cu foils, which could be
359
separated by color sorting (Zhong, X., et al. 2019). The fine powder of Cu/Al foils, under mild
360
screening, entered the leaching step, along with the electrode material, and were eventually
361
recovered in the form of CuO and NaAlCO3(OH)2, respectively. It is here that practice the concepts 18
362
of fine management and cascade recycling of Cu/Al foils. For the leaching process, all the metal
363
elements entered into an aqueous solution; only the graphite from the anode electrode remained as
364
solid residue, and this could be separated out by filtering. The literature (Guo, Y., et al. 2016) shows
365
that acid leaching can modify the layered structure of graphite, so that it can also be sold as a single
366
product. In addition, graphite can also be regard as adsorbent to purify organic electrolyte impurities
367
in waste liquids produced by this technology and to promote the recycling of alkali liquor in the
368
system (Rodrigues T.M., et al. 2016; Natarajan, S., et al. 2015). Therefore, the technological route
369
can realize the recovery of Cu and Al foils, graphite, CuO, NaAlCO3(OH)2, NCM precursor and
370
lithium carbonate from spent LIBs. It is noteworthy that the entire technology given in Figure 8
371
requires no manual assistance; the entire route can be designed as a fully mechanized production line
372
or facility. Since it does not involve intense chemical reactions and the main processes are
373
conventional technologies such as crushing, sieving, room-temperature leaching and precipitation,
374
equipment investment keeps at a very low level.
375
Last but not least, the global recovery rates of main metals from the spent LIBs have been listed
376
in Table 1. In this research, two precipitation processes were involved, namely, precipitate Ⅰ of
377
Cu-Al at pH 6.5 and precipitate Ⅰ of Ni-Co-Mn at a pH of 12. As a matter of fact, it is difficult to
378
completely separate Ni2+ from Cu2+ based on solubility differences. It suggests that the total removal
379
of Cu ions in solution requires an increase in pH to 6.65, while Ni ions begin to precipitate when pH
380
reaches 5.156. This shows that if we want to recover more Ni metal, the pH of precipitate Ⅰ only
381
needs to be controlled at about 5.0. And if we want to recover high-quality Ni metal and avoid the Cu
382
impurities, the pH of precipitate Ⅰ needs to be controlled at about 6.5, but this will inevitably lead to 19
383
the loss of some Ni metal. From the economic point of view, we have chosen the latter scheme. It
384
needs to be stated that if the pH of precipitate Ⅰ is controlled at about 6.5, 0.05 wt.% of Cu will still
385
be allowed to enter precipitate Ⅰ and then co-precipitate with the precursor. Fortunately, (Yang, L., et
386
al. 2018) showed that Cu doping can optimize the electrochemical performance of NCM cathode
387
materials. Therefore, although the recovery rate of Ni element in this process is relatively low and
388
the major loss is in precipitate Ⅰ, the recovery rates of main metal in spent LIBs have remained
389
above 95% from the overall perspective. It proves that this green sustainable technology also meets
390
the requirement of economic feasibility.
391 392
4. Conclusion
393
Fine management and cascade recovery of Cu/Al materials are proposed for the first time to
394
assist in high-quality recycling of full valuable materials from spent ternary LIBs. The key problem
395
of deep screening has been revealed and solved to avoid alkali leaching or any other toxic solution
396
(NMP) to remove Cu and Al impurities before acid leaching, practicing the concept of green and
397
sustainable development. Compared with the inadequate management and recycling of Cu/Al foils in
398
the past, this study takes full advantages of their selective crushing behavior and excellent reduction
399
performance, involving direct recovery of coarse globular-like particles and refinement extraction of
400
fine powders. Using thermodynamic theories and experimental verification, the strategy of leaching
401
NCM electrode materials assisted by fine Cu/Al foils is superior to using traditional additive
402
reductants. Almost 100% of the valuable metal elements in the fine crushed powders are dissolved
403
into aqueous solution and eventually converted into CuO, NaAlCO3(OH)2, NCM precursor, and 20
404
Li2CO3 products. In addition, further characterization of the novel products indicates that the purity
405
of NaAlCO3(OH)2 was more than 95%, and that the surface of the NCM precursor was evenly folded,
406
indicating a promising potential to form a good layered structure. This strategy breaks through the
407
long-standing gap between theoretical research and practical application, and may help reduce the
408
consumption of chemicals in the waste battery recycling industry. The optimal sintering conditions of
409
ternary precursors and the final electrochemical properties need further study.
410 411 412 413
Acknowledgments This research is supported by financial supports by “National Natural Science Foundation of China” (71804085) and “National Key Technology R&D Program of China” (2018YFC1900101).
414 415
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Table 1. Global recovery rates of main metals from the spent LIBs Element
Al
Cu
Li
Ni
Co
Mn
Recovery rate / %
97.85
99.42
98.23
95.45
98.37
98.64
Products
Al foil/ NaAlCO3(OH)2
Cu foil/ CuO
Li2CO3
Precursor
Figure 1. The XRD pattern of the electrode materials of spent Lithium-ion battery
Figure 2. Leaching rate of transition metals using different methods (Especially, Al and Cu powder were analytical reagent)
Figure 3. The relationship between Gibbs free energy and temperature in different reactions
Figure 4. The morphology and element distribution of each component of spent LIBs after efficient crushing and mild screening
Figure 5. Actual content and equivalent content of NCM cathode material under different screening sizes
Figure 6. Effect of different experimental conditions on leaching efficiency (A. Liquid to solid ratio; B. Temperature; C. Acid concentration;
Especially, the -4 mm crushing powders of spent LIBs were the raw materials.)
Figure 7. Crystal structure, surface morphology and element distribution analysis of novel products (For NCM precursors: A.E.F. Surface morphology by FE-SEM; D. Crystal structure by XRD; G-L. Element distribution and quantitative analysis by EDX; For NaAlCO3(OH)2: B. Surface morphology by FE-SEM; C. Crystal structure by XRD)
Figure 8. A green recycling route of spent ternary lithium batteries for industrial application
1. The fine management and cascade recovery of Cu/Al foils are first proposed. 2. Thermodynamic analysis of Cu/Al foils as reducing agent was carried out. 3. Multi-step precipitation and hydrothermal method were used for purification. 4. The deep screening problem has been solved in a green and sustainable way.
Declaration of interest statement The publication of this work has been approved by all authors. This paper has not been published/submitted or being submitted to another journal. We believe that this manuscript will be of exceptional interest to the specialist in related research field. If accepted, it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the Publisher. We confirm that this research is supported by financial supports by “National Natural Science Foundation of China” (71804085) and “National Key Technology R&D Program of China” (2018YFC1900101). All financial support for this project comes from legal channels. All sources of funds do not involve other enterprises, research institutions and other units, and do not involve property rights disputes.
S0959-6526(20)30316-4
DOI:
https://doi.org/10.1016/j.jclepro.2020.120269
Reference:
JCLP 120269
To appear in:
Journal of Cleaner Production
Received Date: 11 November 2019 Revised Date:
31 December 2019
Accepted Date: 25 January 2020
Please cite this article as: Yu J, Tan Q, Li J, Exploring a green route for recycling spent lithium-ion batteries: Revealing and solving deep screening problem, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120269. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier Ltd.
Credit author statement Jiadong Yu: Conceptualization, Methodology, Visualization, Writing Original draft preparation, revising; Quanyin Tan: Formal analysis, co- supervision, co-project administration; Jinhui
Li:
Conceptualization,
administration, funding acquisition.
Resources,
supervision,
project
Graphical abstract
1
Exploring a green route for recycling spent Lithium-ion batteries:
2
revealing and solving deep screening problem
3
Jiadong Yu a, Quanyin Tan *,a, Jinhui Li *,a
4
a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of
5
Environment, Tsinghua University, Beijing 100084, China
6 7
Abstract:
8
The conventional recycling methods of spent lithium-ion batteries (LIBs) regard Al/Cu foils as
9
impurities, which are usually removed by deep screening or alkali leaching. Deep screening refers to
10
the process of over-screening the electrode materials by passing the crushed materials of spent LIBs
11
through a screen with a pore size of only 0.075 mm. However, the serious hole-blocking
12
phenomenon of deep screening and the environmental hazards of alkali leaching restrict their
13
practical application. Herein, a combination of mild screening, reduction leaching and selective
14
purification is proposed to achieve green and sustainable recycling of spent LIBs. Specifically, spent
15
LIBs are subjected to discharge, crushing and mild screening to obtain fine-mixture materials under
16
4-mm sieving, leaving large pieces of Cu/Al foils on the screen mesh, which can be separated as
17
crude products by color sorting. Then, dilute H2SO4 is employed to dissolve the valuable elements in
18
underflow fractions, where residual Cu/Al foils serve as reductants to assist leaching. Eventually,
19
through multi-step precipitates and hydrothermal treatment, a series of promising products (CuO,
20
NaAlCO3(OH)2, precursor and Li2CO3) are obtained, at high quality. This route provides a clever
21
strategy for fine management and cascade recovery of the valuable components from spent LIBs.
22 1
23
Key words: Spent Lithium-ion battery, Deep screening, Reductive leaching, Ternary precursor,
24
NaAlCO3(OH)2.
25 26
1. Information
27
With the representative codes of high energy density, high battery voltage, long life span and no
28
memory effect, lithium-ion batteries (LIBs) have dominated the global market for smart devices and
29
electric vehicles (Golmohammadzadeh, R., et al. 2018). According to the latest research report, the
30
global market value of LIBs in 2017 was US$ 25 billion, and it is predicted to reach US$ 47 billion
31
by 2023, exhibiting a CAGR of around 11% during 2018-2023(Wood, L., 2018). The flourishing
32
development of LIBs stems from the robust demand for safer and stronger rechargeable batteries
33
(Hannan, M.A., et al. 2017), but this popularity will also lead to a surge in the amount of LIBs
34
entering the waste stream, in the near future. It is estimated that in the European Union alone, there
35
will be 13,828 tons of spent LIBs in need of efficient recycling by 2020 (Träger, T., et al. 2015).
36
Spent LIBs contain many heavy metals, such as Mn, Ni, Co and Cu, and poor recycling treatments
37
for these wastes will result in irreparable damage to the environment (Yu, J., et al. 2017). Conversely,
38
responsible recycling of end-of-life LIBs can not only facilitate the development of environmental
39
protection and resource sustainability, but also achieve considerable economic benefits.
40
Typical spent LIBs usually include electrode materials, Cu/Al foils, shells, separators and
41
electrolytes (Yu, J., et al. 2018; Wang, M., et al. 2018). The recycling methods for spent LIBs focus
42
on the valuable cathode materials, and can be primarily divided into hydrometallurgy (Nayaka, G.P.,
43
et al. 2016; Vieceli, N., et al. 2018) and pyrometallurgy (Li, J., et al. 2016; Xiao, J., et al. 2017). 2
44
High-temperature smelting is a simple and efficient approach, with the advantages of less
45
consumption of chemical reagents and short recycling route (Dos Santos, C.S., et al. 2019), but it
46
could produce some toxic sludge containing lithium, aluminum and other heavy metals (Li, L., et al.
47
2017; Roshanfar, M., et al. 2019). On the other hand, based on the special functions in precise
48
separation, effective recovery and selective preparation of high-value products, hydrometallurgy has
49
attracted more attention (Joulié, M., et al. 2014). Although the two companies, Sungeel Hi-Metal Co.
50
(South Korea) and Umicore Co. (Belgium), have claimed to master the key technology for
51
commercial application of spent LIBs, it is difficult for developing countries to learn and transform
52
locally due to technical barriers and equipment manufacturing defects. Therefore, the urgent vision is
53
to explore facile technologies that can be replicated and promoted worldwide. Furthermore, cathode
54
materials are often composed of trivalent cobalt and tetravalent manganese, and these high-valent
55
transition metal oxides are poorly soluble in hydrometallurgy (Sattar, R., et al. 2019). In order to
56
alleviate this problem, an additional reductant (Chen, X., et al. 2015; Hu, J., et al. 2017) or a
57
reducing organic acid (Gao, W., et al. 2017; Li, L., et al. 2017) should be employed, to enhance the
58
leaching effect of cathode materials. Nevertheless, the purchase, transportation and storage of
59
reducing agents will increase operating costs (Ma, X., et al. 2018). And since most acid leaching
60
methods regard Al as an impurity element, deep screening (the passing of powders through a screen
61
with the pore size less than 75 microns (Zhang, X., et al. 2018; Zheng, X., et al. 2018)) was studied
62
to forcibly remove the residual Al foils from the cathode electrode powders after efficient crushing,
63
and even an alkali leaching step (Gratz, E., et al. 2014; Ferreira, D.A., et al. 2009) could be added to
64
fully remove Al element. However, deep screening is almost impossible to realize in industrial 3
65
application, and the sudden addition of the alkali leaching will significantly increase the acid
66
consumption in subsequent processes. In addition, some scholars used deep eutectic solvent (Wang,
67
M., et al. 2019) or toxic NMP solvent (Contestabile, M., et al. 2001) to separate the cathode powder
68
from the Al foils completely by removing the organic binder, so as to avoid the Al pollution. But this
69
may increase the complexity and cost of the recycling process. Therefore, reusing the Al foils in
70
spent LIBs as a reductant to assist in the leaching of cathode materials is a promising strategy.
71
Joulié (Joulié, M., et al. 2017) first put forward the idea that Cu and Al could be utilized as
72
reducing agents to dissolve NCM cathode materials by acid leaching, and gave a simple dynamic
73
explanation. Peng (Peng, C., et al. 2019) also found that Cu, Al and Fe in spent LIBs could
74
significantly improve the leaching effect of Li and Co up to almost 100%. Unfortunately, they did
75
not consider the effective amount of Cu and Al foils, nor did they specify the corresponding
76
purification methods. It is precisely these two key factors that limit the practical application of Cu/Al
77
foils as reductants in waste battery recovery.
78
In this study, we are committed to consuming the least amount of Cu/Al foils as reductants to
79
assist in leaching and recovering the most valuable metal elements. This meticulous management
80
strategy is applied to cleverly solve the deep-screening bottleneck problem in the industrial
81
application of LIB recycling. Compared with traditional methods, this technology will bring the
82
following benefits: (1) This technology refuses to use caustic alkali or toxic solvents in the recycling
83
process; (2) It is unnecessary to add additional reducing agents, such as H2O2 and Na2S2O5, which
84
reduces the consumption of chemical reagents and greenhouse gas emissions; (3) This technology
85
realizes the recovery of all components of valuable materials in spent LIBs. The general framework 4
86
of this research is as follows: the feasibility of Cu/Al powder as reductants was firstly verified in
87
simulation experiments. Secondly, thermodynamic analysis gave the reduction mechanism and
88
determined the theoretical demand of Cu and Al foils. In the third section, fine management of Cu/Al
89
foils was conducted under the actual accumulative effect of mild screening. Then, the systematic
90
exploratory experiment of actual screening products revealed the optimum leaching conditions. Next,
91
through progressive precipitation and hydrothermal process, some high value products were prepared,
92
and their crystallographic and microscopic morphologies were discussed in detail. Finally, a green
93
and fully mechanized process was distilled, removing an important obstacle to industrial recycling of
94
spent LIBs.
95 96
2. Experimental materials and methods
97
2.1 Materials and reagents
98
Spent ternary lithium-ion batteries from electric vehicle battery packs were donated by
99
TES-AMM, an environmental protection company in Beijing. As shown in Figure 1, the diffraction
100
analysis results of electrode materials from discarded LIBs gave that the cathode material was a
101
high-nickel ternary material with LiNiO2 crystal phase. Furthermore, the element analysis showed
102
that the electrode materials were composed of 5.19 wt.% lithium, 30.77 wt.% nickel, 12.23 wt.%
103
cobalt and 17.21 wt.% manganese and 36.4 wt.% graphite. Therefore, the waste cathode material in
104
this experiment was Li(Ni0.5Co0.2Mn0.3)O2, namely, 523 ternary cathode materials. The chemical
105
reagents used in the experiments, including Cu, Al, H2SO4, H2O2, Na2CO3, NaOH, and
106
diaminomethanal, were of analytical grade. Deionized water was used as the experimental water. 5
107 108
2.2 Mild screening experiments
109
In order to avoid the fire caused by the short circuit in the crushing process, three (600g)
110
lithium-ion batteries were first immersed in 5% sodium chloride solution for 48 hours to discharge
111
completely. Besides, this could also allow the residual Li ions on the anode surface to return to the
112
cathode materials (Liu, C., et al. 2016), improving the recovery rate of Li element in the whole
113
recovery process. All batteries were thoroughly crushed in the fume hood by the hand-held universal
114
crusher. The crushed products were screened in turn by mild screening with sieve hole sizes of 4 mm,
115
2 mm, 1.45 mm, 0.9 mm and 0.5 mm. Specifically, mild screening means that only 5 minutes of
116
screen vibration is needed to make materials permeable, without water pressure or other external
117
force to assist in the sieving.
118 119
2.3 Leaching and purification experiments
120
The leaching experiments were completed in a 500-ml three-necked flask with a reflux
121
condenser. In addition, it was necessary to use a heating bath equipped with magnetic stirring to
122
assist in material agitation and temperature control. During the experiments, a certain amount of raw
123
materials was added to 100 ml dilute sulfuric acid. The leaching experiments in this study were
124
divided into simulation experiments and conditional optimization experiment. In order to illustrate
125
the feasibility of using Cu and Al as reducing agents, simulation experiments were first performed.
126
Different combinations of leaching agents (H2SO4 alone, H2SO4 + Cu/Al, H2SO4 + 6 vol % H2O2)
127
with analytically pure grade were employed to leach 523 high-nickel cathode materials. Because the 6
128
mass ratio of NCM / Cu and NCM / Al in spent LIBs were 1:1.2 and 1:0.7 respectively (Doberdò et
129
al., 2014; Loeffler et al., 2014), the proportion of NCM / Cu / Al was simplified to 2:1:1 in the
130
simulation experiment. The operating parameters of simulation experiments were 2.5 mol/L H2SO4,
131
200 mL/g, 60℃ and 6h of reaction time. For conditional optimization experiment, the raw materials
132
were the mixture obtained under 4 mm mild screening. Different from the simulation experiments,
133
the raw materials and reducing agents were the electrode materials and Cu/Al foils inherent in the
134
spent LIBs. In this section, the temperature was controlled at 0℃, 30℃, 45℃, 60℃ and 75℃, the
135
liquid-solid ratio was adjusted within the range of 20 mL/g, 50 mL/g, 100 mL/g, 200 mL/g and 400
136
mL/g, and the sulfuric acid concentration range was 0.5- 4.0 mol / L. When leaching experiments
137
were finished, the residue and leachate were separated using a filter paper with a mesh opening of
138
0.02 µm.
139
Depending on the difference of solubility product (Ksp) and pH range of precipitation, Cu2+ and
140
Al3+ ions in leachate can be precipitated thoroughly. As shown in Table S1, the pH values of Cu2+ and
141
Al3+ ions reaching the minimum concentration were 4.49 and 6.65, respectively. In order to prevent
142
the simultaneous precipitation of Ni element at low pH, the key is to dilute the concentration of Ni2+
143
to less than 1 M (Lain M., et al. 2001; Zou, H., et al. 2013). In this experiment, the concentrations of
144
all the metal ions (Al, Cu, Ni, Co and Mn) were first adjusted to below 0.5 M after acid leaching, and
145
then the pH value could increase slowly to 6.65 by adding 1 M of NaOH. The Cu(OH)2 and Al(OH)3,
146
separated through filtration, were stirred in other NaOH solution at pH=12 for 2 h. After re-filtration
147
and heating, Cu(OH)2 residue and NaAlCO3(OH)2 filtrate could be translated into CuO and
148
NaAlCO3(OH)2 powders, respectively. 7
149
The leachate retrieved after removing Cu2+ and Al3+ is a neutral solution for hydrothermal
150
reaction. The specific steps were that 50 ml leachate and 10 g diaminomethanal were placed in a
151
100-ml hydrothermal reactor and reacted for 12 hours at 160℃. The NCM precursors could be
152
obtained by vacuum filtration with large surface morphology. The pH of residual solution after
153
hydrothermal reaction was adjusted to 12, to remove all the heavy metal ions in the solution. The
154
Li2CO3 precipitate was obtained with the adding of Na2CO3 and washed with boiling ultra-pure
155
water.
156 157
2.4 Analytical methods
158
The concentration of metal elements in solution were measured with an inductively coupled
159
plasma-optical emission spectrometer (ICP-OES, OPTIMA 2000, PerkinElmer, USA). In addition,
160
the leaching rate of each element was calculated with the following equation:
161
W wt % =
× 100%
162
where
163
the solid material before acid leaching.
(1)
is the final content of metals in solution, and
denotes the original content of metals in
164
The crystal structures and phase analysis of the solid powders were characterized by X-ray
165
diffraction (XRD, Philips PW 1700, USA) using Cu Kα radiation, and the data were analyzed by
166
MDI Jade 6.0 software. The microscopic morphology and content distribution of surface elements
167
were characterized by field emission scanning electron microscopy, which contained an energy
168
dispersive spectrometer (FE-SEM and EDX, Carl Zeiss MERLIN Compact, Germany).
169 8
170
3. Results and discussion
171
3.1 Feasibility analysis
172
High-nickel batteries are distinguished from conventional NCM batteries in that they have better
173
electrochemical performance and structural stability, but this also increases the difficulty of recycling.
174
The use of Cu and Al as reducing agents for leaching Li(Ni1/3Co1/3Mn1/3)O2 (namely 111 ternary
175
cathode materials) with sulfuric acid has proved to be effective, but no similar research has been
176
done on the feasibility of recovering high-nickel ternary materials. The combination of H2SO4 and
177
H2O2, though, is generally considered to have a good leaching effect. In this simulation experiment,
178
mass ratio of cathode material to Cu and Al was 2:1:1. The final leaching results were shown in
179
Figure 2, and suggested that only 40% of the ternary materials could be dissolved by pure dilute
180
H2SO4; its effect was poor. The addition of H2O2 could help dissolve 90% of the cathode materials.
181
More gratifying was that the addition of Cu and Al could recover almost 100% of all the metal
182
elements, by leaching, mainly due to the reduction of insoluble Mn4+ and Co3+ to soluble Mn2+ and
183
Co2+. However, determining the reason Al and Cu powders have a better reduction effect requires an
184
in-depth analysis of their Gibbs free energy in leaching process.
185 186
3.2 Thermodynamic analysis
187
The good electrochemical performance of the NCM battery derives from its stable layered
188
structure, which mainly includes transition metal oxides with embedded lithium elements. It can be
189
determined through assimilation that the NCM ternary material is composed of a stack of transition
190
metal oxides—namely NiO, MnO2 and Co2O3—with inserted lithium (Joulié, M., et al. 2017). In 9
191
particular, bivalent nickel is easily soluble in dilute H2SO4 and does not participate in reduction
192
reaction. Therefore, the reaction of the NCM electrode material with the above reducing agents (Cu,
193
Al and H2O2) can be approximated by the reaction of MnO2 and Co2O3. In the case of Co2O3, three
194
different redox reactions may occur, and their standard Gibbs free energies are listed below (Wang L.,
195
2013). 3Co2O3 + 2Al + 9H2SO4 → 6CoSO4 + Al2(SO4)3 + 9H2O = −477.636 − 0.02684 ,
0~1000!
Co2O3 + Cu + 3H2SO4 → 2CoSO4 + CuSO4 + 3H2O = −69.497 − 0.01245 ,
0~1000!
Co2O3 + H2O2(l) + 2H2SO4 → 2CoSO4 + 3H2O + O2(g) = −37.262 − 0.04583 ,
0~1000!
(1) (2) (3) (4) (5) (6)
196
Similarly, trivalent manganese ions also need to be reduced to divalent transition metal ions; the
197
corresponding standard Gibbs free energies were calculated by HSC Chemistry 6.0 as shown in
198
follows: 3MnO2 + 2Al + 6H2SO4 → 3MnSO4 + Al2(SO4)3 + 6H2O = −491.349 + 0.20149 ,
0~1000!
MnO2 + Cu + 2H2SO4 → MnSO4 + CuSO4 + 2H2O = −74.024 + 0.0635 ,
0~1000!
MnO2 + H2O2(l) + H2SO4 → MnSO4 + 2H2O + O2(g) = −26.677 − 0.03372 , 199
0~1000!
(7) (8) (9) (10) (11) (12)
Figure 3 shows the relationship between standard Gibbs free energy and temperature in different 10
200
reactions (1-12). In Figure 3, the reactions of Al, Cu and H2O2 as reductants are marked in green,
201
blue and black, respectively. It is well known that if the Gibbs free energy is lower than zero and no
202
other potential energy is generated, the reaction can proceed spontaneously; otherwise the reaction
203
cannot occur. According to the curve trend in Figure 3, only the standard Gibbs free energy of
204
reaction 9 may be higher than 0, within 1000 K. Therefore, based on Eq. 13, the reaction (9) was
205
subjected to further quantitative calculation to determine its Gibbs free energy at specific
206
temperatures. G =
+ R ln
)*+ )
(13)
207
where R is the molar gas constant, 8.314 J/(mol· K); T is the thermodynamic temperature, degrees
208
Kelvin; )*+ is the partial pressure of the gas product; and )
209
When the reaction is carried out in an air atmosphere without applied pressure, this
210
approximately equal to 0.21, and Eq. 10 should change to: G = −74.024 + 0.0508 ,
is the standard atmospheric pressure.
0~1000!
, -+ ,.
value is
(14)
G < 0, 4-valent manganese ions may theoretically be reduced to
211
On the basis of Eq.14, when
212
2-valent manganese ions by copper, within 1457.165 K (1184℃).
213
The leaching reaction of NCM materials occurs in water-based solution, so the range of reaction
214
temperature is usually 273.15-373.15 K (0-100℃). The variation of Gibbs free energy for each
215
reaction in this temperature range is presented in Figure 3 with a champagne background. It indicates
216
that the absolute value order of Gibbs free energy for each reaction within the temperature range of 0
217
-100 ℃ is (1) > (7) > (3) > (9) ≈ (5) > (11). In other words, the order of reducibility is Al > Cu >
218
H2O2, and the reaction rate is Al > Cu > H2O2. Since there is a large gap between Al and Cu both in 11
219
the Gibbs free energy and the reductivity sequence of the metals (Al > hydrogen (H) > Cu), we can
220
conjecture that the cathode materials preferentially react with Al foils. Only after the Al foils are
221
completely consumed, the cathode materials begin to react with the Cu foils in dilute sulfuric acid.
222
On the whole, the reduction of high-valent transition metal ions in NCM materials by Al and Cu is
223
efficient, and the reduction effect is better than that of H2O2, in theory, a result consistent with those
224
of previous experiments (Joulié, M., et al. 2017).
225
More importantly, according to the reaction equations (1) and (7), 2 moles of Al can react with 3
226
moles of Co2O3 or MnO2, implying that 1 mole of Al can induce at least 1.5 moles of NCM cathode
227
materials to be completely dissolved in 3 moles of H2SO4 by the redox reaction: in other words, 1
228
gram of Al can catalyze about 5.36 grams of ternary cathode material in 10.89 grams of H2SO4.
229
Similarly, 1 gram of Cu can catalyze about 1.52 grams of NCM powders in 3.08 grams of H2SO4.
230
For simplicity of calculation, it can be considered that 1 g Al is equivalent to 5 g NCM material and
231
10 g H2SO4, while 1 g Cu is equivalent to 1.5 g NCM material and 3 g H2SO4. It should be noted that
232
in this calculation, the total content of Co2O3 and MnO2 in the NCM material is approximately 100%,
233
but their actual content is only 30-70% in 111, 523, 532, 622, 811 cathode material. Therefore, this
234
result can be applicable to all kinds of NCM cathode materials.
235 236
3.3 Fine management of Cu/Al foils through mild screening
237
As the simplest method of separating Cu/Al foils and electrode materials, mechanical crushing
238
has the most practical application prospects. In this section, we hoped to find the best proportion of
239
Cu/Al foils to cathode materials by adjusting the sieving size of the mild screening, based on the 12
240
above thermodynamic analysis.
241
After efficient crushing and mild screening of spent LIBs, the morphology and element
242
distribution of each component are shown in Figure 4. It indicates that 60.51 wt.% of the crushed
243
materials were enriched in the -0.5 mm particle size, while the material contents of +0.5-0.9 mm,
244
+0.9-1.45 mm, +1.45-2 mm, +2-4 mm and +4 mm were 12.24%, 4.89%, 3.61%, 5.68% and 5.88%,
245
respectively. Besides, the metal content in -0.5 mm fine materials was 57.24%, and the remaining
246
42.76% was non-metallic graphite. It should be noted that -0.5mm particles mean that the particle
247
size is less than 0.5mm, while +0.5-0.9 mm particles mean that the particle size is greater than
248
0.5mm and less than 0.9mm. Furthermore, the experimental results show that the content of electrode
249
materials in the -0.5 mm particles is as high as 98.94%, and the content of Cu/Al foils in the +4 mm
250
crude material could reach 94.19%. This remarkable selective crushing behavior is consistent with
251
other scholars’ research (Zhang T., et al. 2013). In order to determine the optimum screening size, the
252
Al and Cu contents in Figure 4 were converted into NCM cathode material equivalents: the materials
253
were then compared by sieving the cumulative specimens. The results are shown in Figure 5 and
254
Table S2, and it illustrates that the cumulative content of actual NCM materials increased slightly,
255
from 33.99% in -0.5mm level to 41.29% in total. Nevertheless, with the increasing accumulative
256
amount of Cu/Al foils, the NCM equivalent content (initially 2.28%) gradually caught up with the
257
actual content of NCM, finally reaching 73.36%. More importantly, when mild screening was carried
258
out with 4 mm sieve holes, the contents of Al/Cu foils in the screening product were 6.30 wt.% and
259
9.98 wt.% respectively, converted into NCM equivalents of 31.49 wt.% and 14.97 wt.% respectively.
260
In fact, only 41.13 wt.% of the products under mild screening were NCM cathode materials, while 13
261
the total content of NCM equivalent from Al/Cu foils was 46.46 wt.%, which was 5.33 wt.% higher
262
than the actual content. Therefore, 4 mm sieve holes are the optimal screening conditions.
263
Furthermore, if 100g of this mild screening products need to process, containing 6.3 g Al, 9.98 g Cu,
264
and 41.13g NCM materials, the demand for H2SO4 is 92.94 g. Consequently, the mass ratio of
265
sulfuric acid to sieving product is about 1:1, while the specific mass ratio of H2SO4 to Cu, Al and
266
NCM materials is about 2:3:15:30.
267
Large pore size not only reduces the difficulty of materials’ passing through the mesh, but also
268
controls the proportion of reductant for subsequent leaching reactions. This fine management
269
strategy of Cu/Al foils subtly solve the industrialization restriction brought by 0.075 mm deep
270
screening, and paves the way for the cascade recycling of copper and aluminum elements.
271 272
3.4 Optimum leaching condition
273
In this section, the effects of temperature, acid concentration and liquid-solid ratio on leaching
274
results were considered comprehensively, as shown in Figure 6. In order to explore the optimal
275
conditions of liquid-solid ratio, the experimental temperature was controlled at 60°C and the
276
concentration of sulfuric acid at 3 mol/L. As shown in Figure 6A, the liquid-solid ratio of 20 mL/g
277
could only dissolve about 25% of the valuable metal elements. In contrast, a higher liquid-solid ratio,
278
such as 200 mL/g, could dissolve more than 90% of the cathode materials. When the liquid-solid
279
ratio reached 400 mL/g, the NCM cathode materials could be completely dissolved. In terms of total
280
component recovery and economic value, the best liquid to solid ratio was 200 mL/g. When
281
exploring the optimum temperature conditions, the liquid-solid ratio of the reaction was kept at 200 14
282
mL/g, and the acid concentration was 3 mol/L. Temperature is an important condition for reducing
283
the dissolution energy barrier. In this experiment, the reaction temperatures were set to 0℃, 30℃,
284
45℃, 60℃, 75℃ and 90℃, and the results are shown in Figure 6B. Due to the increased diffusivity
285
and decreased viscosity of liquid, the leaching rate of each metal increased with the increase of
286
temperature, and reached a maximum at 60℃. As the temperature continued to increase, the leaching
287
rate showed a small fluctuation, but remained stable on the whole. Accordingly, considering the
288
economics and security of the heating process, the optimal temperature was 60℃.
289
The optimal concentration of dilute sulfuric acid was explored at 60℃ and 200 mL/g. As seen in
290
Figure 6C, with the increase of dilute sulfuric acid concentration, the leaching rates of Li, Ni, Co, Mn
291
first increased and then decreased. The main reason may be that there is a side reaction here and the
292
reaction equation is as follows. 4Al + 3O3 + 6H3 SO6 → 2Al3 SO6 = −885.442 + 0.17961 ,
+ 6H3 O
(15)
0~1000!
(16)
8
293
When the H2SO4 concentration is low (0.5-2.5 mol/L), most Al foils play a role of reducing agent to
294
promote the leaching of cathode materials. Nevertheless, NCM cathode material is a kind of
295
spherical secondary particles composed of Li2O, NiO, Co3O4 and MnO2 micro-particles. The
296
leaching and release of these metal ions into liquid phase are relatively slow. When the H2SO4
297
concentration is high (2.5-4.0 mol/L), a large number of hydrogen ions will consume some Al foils,
298
resulting in the decrease of reductant in the system. Thus, the leaching of metal ions from ternary
299
materials is insufficient at the same time unit. Figure 6C shows that the optimum leaching rate
300
appeared at 2.0-2.5 mol/L H2SO4. From the viewpoint of cost savings and environmental load 15
301
reduction, dilute sulfuric acid of 2.0 mol/L could be the optimum leaching condition.
302
Therefore, the optimum conditions for leaching 523 NCM cathode materials with Cu/Al foils
303
inherent in spent LIBs are: 4 mm mild screening, liquid-solid ratio of 200 mL/g, reaction temperature
304
of 60℃ and dilute sulfuric acid concentration of 2 mol/L.
305 306
3.5 Purification and characterization
307
H+, Li+, Ni2+, Co2+, Mn2+, Al3+, Cu2+ and SO42- were all present in the leaching solution. The
308
interactions of these six metal cations made the solution system more chaotic, resulting in a great
309
challenge of multi-metal separation and purification. Fortunately, there is a distinct pH gap between
310
Al3+, Cu2+ and Ni2+, Co2+, Mn2+ during precipitation, Namely, Al3+ and Cu2+ could be pre-removed
311
first, at pH less than 6.5, and then Ni2+, Co2+, Mn2+ could be co-precipitated completely when the pH
312
was more than 12. For the mixture of Al(OH)3 and Cu(OH)2, Al(OH)3 precipitation can be converted
313
into NaAlCO3(OH)2 solution by adding NaOH solution, so that copper-rich solid and aluminum-rich
314
solution can be separated by filtration. The possible dissolution reaction equations are as follows. Al OH
8
+ 9:3 + ;<:= → ;<>?9:8 :=
= −36.768 + 0.03472 ,
3
+ =3 :
0~1000!
(17) (18)
315
For further surface morphology and phase composition analysis, the blue-violet liquid
316
(NaAlCO3(OH)2 solution) was dried and crystallized, and then subjected to SEM and XRD tests,
317
whose results were shown in Figures 7B and C. The SEM image indicates that the basic carbonates
318
are mainly flocculent and flaky, with some rod-like crystals. The XRD pattern suggests that only the
319
diffraction peaks of the NaAlCO3(OH)2 match those of the materials exactly, and the standard card 16
320
number is 45-1359. Since the detection limit of XRD is 5% and no other impurity peaks occurred,
321
the purity of the NaAlCO3(OH)2 can be considered to be higher than 95%. In this recovery system,
322
the Al foil powder can be recovered in the form of crude Al sheets after mild screening and
323
high-quality NaAlCO3(OH)2 after reductive leaching, which realizes the cascade recovery to some
324
extent.
325
After application of the co-precipitation and the hydrothermal method, the ternary precursor
326
was produced. In order to analyze the quality of the product, the precursors were characterized by
327
FE-SEM, EDX and XRD in detail, and the results are shown in Figure 7. Due to changes in the
328
contents of nickel-cobalt-manganese elements and crystalline hydrate, there was no unified chemical
329
structure for the precursor, and no single XRD phase standard card corresponding to it. From the
330
XRD pattern in Figure 7D, three diffraction characteristic peaks of MnO·OH, CoO and Ni(OH)2
331
existed in the precursor phase, and analyzed under the standard cards of 18-0804, 42-1300 and
332
14-0117, respectively. In general, these ternary precursor materials belong to a mixture of metal
333
oxides or metal hydroxides, which can be understood as a form of NiO·MnO·CoO·(OH)x. Figure 7A,
334
E and F show the microscopic surface topography of the precursor. These indicate that the precursor
335
prepared by the diaminomethanal-hydrothermal method is a type of chrysanthemum-cluster spherical
336
particle. The obvious wrinkles and clear stripes on the surface of the particles prove that the
337
precursor will have a good layered structure after calcination and hold a good electrochemical
338
performance potential, as shown in S1 (He, Q., et al. 2017; Zhang, F., et al. 2017; Chen, M., et al.
339
2019). In order to observe the element distribution on the surface of particles more intuitively, EDX
340
energy spectrum analysis was carried out, as shown in Figure 7G-L. The distributions of nickel, 17
341
cobalt, manganese and oxygen elements on the surface of the particles are relatively uniform,
342
consistent with the XRD results. Relative quantitative analysis of the elements shows that the
343
contents of nickel, cobalt, manganese and oxygen were 25.9 wt.%, 17.2 wt.%, 13.8 wt.% and 43.1
344
wt.%, respectively. All these characterizations indicate that the precursors have good performance
345
potential.
346 347
3.6 A green recycling route of spent ternary lithium-ion batteries for industrial application
348
With the preparation of the key products NaAlCO3(OH)2 and NCM precursors, we were able to
349
introduce a simple technical flowchart for manufacturing advanced materials from spent LIBs.
350
However, the main factors determining if a technical route can be industrialized are whether it can
351
recover as much valuable product as possible, whether the input cost of equipment is appropriate,
352
and whether labor costs can be contained. Taking the above factors into consideration, this study
353
optimized the main technology of mild screening and multi-step precipitation, and obtained a new
354
green recycling route, as shown in Figure 8.
355
The mild screening technology breaks through the bottleneck of LIB recycling, and is one of the
356
core technical points of the entire route. The process developed in this study was as follows: the
357
crushed products of spent LIBs were screened slightly by a 4-mm sieve; the coarse materials with
358
particle size larger than 4 mm were spherical-like or sheet-shaped Al and Cu foils, which could be
359
separated by color sorting (Zhong, X., et al. 2019). The fine powder of Cu/Al foils, under mild
360
screening, entered the leaching step, along with the electrode material, and were eventually
361
recovered in the form of CuO and NaAlCO3(OH)2, respectively. It is here that practice the concepts 18
362
of fine management and cascade recycling of Cu/Al foils. For the leaching process, all the metal
363
elements entered into an aqueous solution; only the graphite from the anode electrode remained as
364
solid residue, and this could be separated out by filtering. The literature (Guo, Y., et al. 2016) shows
365
that acid leaching can modify the layered structure of graphite, so that it can also be sold as a single
366
product. In addition, graphite can also be regard as adsorbent to purify organic electrolyte impurities
367
in waste liquids produced by this technology and to promote the recycling of alkali liquor in the
368
system (Rodrigues T.M., et al. 2016; Natarajan, S., et al. 2015). Therefore, the technological route
369
can realize the recovery of Cu and Al foils, graphite, CuO, NaAlCO3(OH)2, NCM precursor and
370
lithium carbonate from spent LIBs. It is noteworthy that the entire technology given in Figure 8
371
requires no manual assistance; the entire route can be designed as a fully mechanized production line
372
or facility. Since it does not involve intense chemical reactions and the main processes are
373
conventional technologies such as crushing, sieving, room-temperature leaching and precipitation,
374
equipment investment keeps at a very low level.
375
Last but not least, the global recovery rates of main metals from the spent LIBs have been listed
376
in Table 1. In this research, two precipitation processes were involved, namely, precipitate Ⅰ of
377
Cu-Al at pH 6.5 and precipitate Ⅰ of Ni-Co-Mn at a pH of 12. As a matter of fact, it is difficult to
378
completely separate Ni2+ from Cu2+ based on solubility differences. It suggests that the total removal
379
of Cu ions in solution requires an increase in pH to 6.65, while Ni ions begin to precipitate when pH
380
reaches 5.156. This shows that if we want to recover more Ni metal, the pH of precipitate Ⅰ only
381
needs to be controlled at about 5.0. And if we want to recover high-quality Ni metal and avoid the Cu
382
impurities, the pH of precipitate Ⅰ needs to be controlled at about 6.5, but this will inevitably lead to 19
383
the loss of some Ni metal. From the economic point of view, we have chosen the latter scheme. It
384
needs to be stated that if the pH of precipitate Ⅰ is controlled at about 6.5, 0.05 wt.% of Cu will still
385
be allowed to enter precipitate Ⅰ and then co-precipitate with the precursor. Fortunately, (Yang, L., et
386
al. 2018) showed that Cu doping can optimize the electrochemical performance of NCM cathode
387
materials. Therefore, although the recovery rate of Ni element in this process is relatively low and
388
the major loss is in precipitate Ⅰ, the recovery rates of main metal in spent LIBs have remained
389
above 95% from the overall perspective. It proves that this green sustainable technology also meets
390
the requirement of economic feasibility.
391 392
4. Conclusion
393
Fine management and cascade recovery of Cu/Al materials are proposed for the first time to
394
assist in high-quality recycling of full valuable materials from spent ternary LIBs. The key problem
395
of deep screening has been revealed and solved to avoid alkali leaching or any other toxic solution
396
(NMP) to remove Cu and Al impurities before acid leaching, practicing the concept of green and
397
sustainable development. Compared with the inadequate management and recycling of Cu/Al foils in
398
the past, this study takes full advantages of their selective crushing behavior and excellent reduction
399
performance, involving direct recovery of coarse globular-like particles and refinement extraction of
400
fine powders. Using thermodynamic theories and experimental verification, the strategy of leaching
401
NCM electrode materials assisted by fine Cu/Al foils is superior to using traditional additive
402
reductants. Almost 100% of the valuable metal elements in the fine crushed powders are dissolved
403
into aqueous solution and eventually converted into CuO, NaAlCO3(OH)2, NCM precursor, and 20
404
Li2CO3 products. In addition, further characterization of the novel products indicates that the purity
405
of NaAlCO3(OH)2 was more than 95%, and that the surface of the NCM precursor was evenly folded,
406
indicating a promising potential to form a good layered structure. This strategy breaks through the
407
long-standing gap between theoretical research and practical application, and may help reduce the
408
consumption of chemicals in the waste battery recycling industry. The optimal sintering conditions of
409
ternary precursors and the final electrochemical properties need further study.
410 411 412 413
Acknowledgments This research is supported by financial supports by “National Natural Science Foundation of China” (71804085) and “National Key Technology R&D Program of China” (2018YFC1900101).
414 415
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Table 1. Global recovery rates of main metals from the spent LIBs Element
Al
Cu
Li
Ni
Co
Mn
Recovery rate / %
97.85
99.42
98.23
95.45
98.37
98.64
Products
Al foil/ NaAlCO3(OH)2
Cu foil/ CuO
Li2CO3
Precursor
Figure 1. The XRD pattern of the electrode materials of spent Lithium-ion battery
Figure 2. Leaching rate of transition metals using different methods (Especially, Al and Cu powder were analytical reagent)
Figure 3. The relationship between Gibbs free energy and temperature in different reactions
Figure 4. The morphology and element distribution of each component of spent LIBs after efficient crushing and mild screening
Figure 5. Actual content and equivalent content of NCM cathode material under different screening sizes
Figure 6. Effect of different experimental conditions on leaching efficiency (A. Liquid to solid ratio; B. Temperature; C. Acid concentration;
Especially, the -4 mm crushing powders of spent LIBs were the raw materials.)
Figure 7. Crystal structure, surface morphology and element distribution analysis of novel products (For NCM precursors: A.E.F. Surface morphology by FE-SEM; D. Crystal structure by XRD; G-L. Element distribution and quantitative analysis by EDX; For NaAlCO3(OH)2: B. Surface morphology by FE-SEM; C. Crystal structure by XRD)
Figure 8. A green recycling route of spent ternary lithium batteries for industrial application
1. The fine management and cascade recovery of Cu/Al foils are first proposed. 2. Thermodynamic analysis of Cu/Al foils as reducing agent was carried out. 3. Multi-step precipitation and hydrothermal method were used for purification. 4. The deep screening problem has been solved in a green and sustainable way.
Declaration of interest statement The publication of this work has been approved by all authors. This paper has not been published/submitted or being submitted to another journal. We believe that this manuscript will be of exceptional interest to the specialist in related research field. If accepted, it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the Publisher. We confirm that this research is supported by financial supports by “National Natural Science Foundation of China” (71804085) and “National Key Technology R&D Program of China” (2018YFC1900101). All financial support for this project comes from legal channels. All sources of funds do not involve other enterprises, research institutions and other units, and do not involve property rights disputes.