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Article

Selective Recovery of Li and Fe from Spent Lithium-ion Batteriesby an Environmentally Friendly Mechanochemical Approach

Ersha Fan, Li Li, Xiaoxiao Zhang, Yifan Bian, Qing Xue, Jiawei Wu, Feng Wu, and Renjie ChenACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI:

10.1021/acssuschemeng.8b02503 • Publication Date (Web): 17 Jul 2018

Downloaded from http://pubs.acs.org on July 19, 2018

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Selective Recovery of Li and Fe from Spent

Lithium-ion Batteries by an Environmentally

Friendly Mechanochemical Approach

Ersha Fan†, Li Li†,‡, Xiaoxiao Zhang†, Yifan Bian†, Qing Xue†, Jiawei Wu†, Feng Wu†,‡,

and Renjie Chen*,†,‡

† 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

‡ Collaborative Innovation Center of Electric Vehicles in Beijing, No. 5 South

Zhongguancun Street, Beijing 100081, China

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

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ABSTRACT: Recycling of spent LiFePO4 batteries has drawn recent attention

relating to recovering their high contents of rare elements and negating potential

negative environmental effects of their disposal. However, the stable crystal structure

of LiFePO4 materials has prevented the development of a recycling process with high

selectivity and extraction efficiency. We report the selective extraction of Fe and Li

from spent LiFePO4 batteries via an environmentally friendly mechanochemical

process with oxalic acid. With the use of a mechanochemical treatment and water

leaching, the Li extraction efficiency can be improved to 99%. Furthermore, 94% of

Fe can be simultaneously recovered as FeC2O4·2H2O. To understand the reaction

mechanism and determine the optimum reaction conditions, we investigated various

parameters, including the LiFePO4 to oxalic acid mass ratio, rotation speed, milling

time, and ball-to-powder mass ratio. Moreover, metal ions from the water leaching

process were recovered by chemical precipitation. This study provides an efficient

and selective process for recovery of valuable metals from spent LiFePO4 materials.

KEYWORDS: Spent LiFePO4 batteries, Mechanochemical method, Oxalic acid,

Water leaching

� INTRODUCTION

Lithium iron phosphate (LiFePO4, LFP), first reported in the 1980s, is considered to

be an excellent cathode material for applications to electric vehicles (EVs), hybrid

electric vehicles (HEVs), and large-scale energy storage facilities.1 LiFePO4 has low

toxicity and its raw materials are widely abundant. When LiFePO4 is applied in

batteries the devices show superior thermal safety, long cycle lifetimes, and good

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reversibility.2, 3 Owing to the rapid development of EVs, consumption of LiFePO4

batteries has increased markedly. In 2015, the amount of LiFePO4 shipped to China,

the largest producer and consumer of LiFePO4, reached 32,400 tons.4 Although

LiFePO4 is considered to be relatively environmentally friendly, improper disposal of

spent LiFePO4 batteries represents a waste of valuable resources, such as lithium, and

can cause environmental problems owing to the use of toxic organic electrolytes.

Therefore, there is a need to develop an efficient and cost-effective route to recycling

spent LiFePO4 batteries.

Recycling technologies for spent LiFePO4 batteries can be classified into two

categories: direct regeneration and hydrometallurgy, as shown in Table 1. Direct

regeneration refers to high-temperature repair or regeneration of the material without

destroying its structure. For example, Chen et al.5 developed a small-scale model line

for regenerating cathode powder from spent LiFePO4 batteries. The repaired cathode

powder subjected to a heat treatment at 650 °C under an Ar/H2 flow, showed almost

the same discharge capacities and specific energy densities as those of the unused

cathode material. Li et al.6 designed a new direct regeneration process by adding

Li2CO3 to a recycled cathode material mixture from scrapped LiFePO4 batteries. The

cathode material mixture, regenerated at 650 °C under an Ar/H2 flow, exhibited

excellent physical, chemical, and electrochemical performances; thus, meeting the

reuse requirements for mid-end lithium-ion batteries. The hydrometallurgical process

involves leaching of cathode materials and selective separation of different metal ions

from the leaching solution to obtain the final products. In the reported

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hydrometallurgical processes for recycling of spent LiFePO4 batteries, the mineral

acids HCl,7, 8

H2SO4,4, 9

and H3PO410

have been used to recover metals ions from the

cathode materials. For spent LiFePO4 batteries, direct regeneration is not appropriate

for recycling cathode materials containing a large amount of impurities. Owing to the

stable crystal structure of LiFePO4 materials, effective extraction of Li and Fe is

currently achieved with strong acid treatments and long extraction times. Recently, a

mechanochemical method has been proposed to destroy the material structure and

promote the leaching process.

Table 1. Summary the Reported Methods for the Recycling of Spent LiFePO4

Batteries.

recycling method agent/process Products ref

direct regeneration

heat-treatment cathode powders at 650 °C for

1 h under an Ar/H2 flow LiFePO4 5

regenerated with Li2CO3 at 650 °C for 1 h

under Ar/H2 flow LiFePO4 6

heat-treatment with doping of new LiFePO4

at the ratio of 3:7 at 700 °C for 8 h under N2

flow

LiFePO4 11

hydrometallurgical

method

0.3 M H2SO4, H2O2/Li molar ratio 2.07,

H2SO4/Li molar ratio 0.57, 60 °C, and 120

min

FePO4·2H2O and Li3PO4 4

H3PO4 as leaching agent FePO4·2H2O and LiH2PO4 10

2.5 M H2SO4 as leaching agent, L/S ratio (10

ml g-1), 60 °C, 4h; NH3·H2O and Na2CO3 as

precipitants

FePO4·2H2O and Li2CO3 9

Mechanochemical activation time (2 h), mass

ratio of cathode powder to EDTA-2Na = 3:1;

leaching conditions: 0.6 M H3PO4, S/L ratio

(50 g/L), 20 min;

FePO4·2H2O and Li3PO4 12

The mechanochemical method has been widely used in recycling spent lithium-ion

batteries and is known to induce physical and chemical changes, including phase

transitions, structural defects, strains, amorphization, and even direct reactions under

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normal temperature and pressure.13 This method enables the recovery of metals at

room temperature with a high extraction efficiency. For example, Yang used a

mechanochemical activation pretreatment with EDTA-2Na and diluted H3PO4

leaching to recover Fe and Li.12 Wang proposed that EDTA can be used as a

co-grinding reagent to chelate Co and Li from LiCoO2 powder, allowing Co and Li to

be easily recovered by a water leaching procedure.14 These mechanical approaches are

typically performed with metal-ion chelates such as EDTA,14

EDTA-2Na,12

PVC15, 16

,

and the solid reducing agent Fe,15, 17. This step is followed by a leaching process using

water or mineral acid. Because of their natural origin and ready degradable features,

organic acids have been used as leaching agents,11, 18-20 reductants,21, 22

precipitants,23-25 and chelating agents11, 26, 27 in the recovery processes of spent LIBs.

Herein, we developed a new mechanochemical process to recycle spent lithium-ion

batteries with the use of a natural organic acid at room temperature. We aimed to

improve the metal extraction efficiency and selectively recover individual metals

while reducing acid consumption and secondary pollution. We examined the effects of

operating parameters on the recycling process, including activation time, cathode

powder to additive mass ratio, acidic concentration, the liquid-to-solid ratio and

leaching time.

� EXPERIMENTAL SECTION

Materials. The spent LiFePO4 batteries were collected from the Shenzhen BAK

Battery Co., Ltd. (Shenzhen, China). The spent LiFePO4 batteries were first

completely discharged in saturated saline solution (NaCl) for safety reasons. After

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dismantling, an alkaline solution was used to dissolve the Al foil. The residue was

dried and then calcined at 700 °C for 5 h to remove organic impurities. As shown in

Figure 1, the obtained powder was used as the raw material for investigation. All

chemical reagents used were analytical-grade.

Figure 1. A mechanochemical activation process for spent LiFePO4 batteries

recycling.

Mechanochemical process. The obtained cathode powder was mixed with oxalic

acid as a co-grinding aid in a range of mass ratios from 5:1 to 0.5:1, and mechanically

activated with a planetary ball mill (XQM-1, Changsha Tianchuang Powder

Technology Co., Ltd., China). Approximate 0.5 g of cathode powder, a known amount

of oxalic acid powder, and 1 mL of deionized water were mixed with zirconia beads.

After milling, the activated materials and zirconia balls adherent with powders were

washed with 50 ml of deionized water and soaked for 30 min at room temperature.

During the activation process, an interval of 10 min was set between grinding

clockwise for 20 min followed by a counterclockwise for 20 min. The following

variables during the mechanochemical process were investigated: mass ratio of

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LiFePO4 to oxalic acid (5:1–0.5:1), rotation speed (200–500 rpm), milling time (0.08–

4 h), and ball-to-powder mass ratio (10:1–40:1).

Materials Recovery. Precipitation experiments were conducted to recover Li. The

obtained leachate in a beaker was stirred at 90 °C to evaporate water until the

concentration of Li+ was greater than 5 g L–1. Trace amounts of Fe2+ in the leachate

can be removed by adjusting pH to 4 with 1 mol L–1 sodium hydroxide solution.28

Once the pH is 4, the mixed solution was stirred well at least 2h until the

concentration of Fe ions was less than 4 mg L–1, obtaining a high purity product. After

filtering, the purified lithium solution was transferred to a three-bottomed flask and

stirred at 90 °C for 2 h. The precipitates were collected after filtering and drying at

60 °C for 24 h.

Characterization. Concentrations of metal ions in the solutions were determined

by inductively coupled plasma optical emission spectroscopy (ICP-OES; Optima

8300, Perkin Elmer, USA). The crystal structure and morphology of samples were

characterized by X-ray diffractometry (XRD; Rigaku, Japan) and scanning electron

microscopy (SEM; S-4800, Hitachi, Japan), respectively. The E-pH diagrams were

constructed with the HSC 6.0 chemistry software. The extraction efficiencies (EE)

and precipitation efficiencies (PE) of different metal ions were calculated by

equations 1 and 2:

EE =��

��%× 100%, (1)

PE =��

��× 100%, (2)

where M, C0, V0 are the molecular mass, concentration of metal, and volume of

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leachate, respectively; m and w% are the mass and mass fraction of metals in raw

materials, respectively; C1 and C2 are concentrations of metal ions in the solution

before and after precipitation, respectively; V1 and V2 are volumes of the solutions

before and after filtration, respectively. The purities of the obtained products (P) were

calculated according to equation 3.

P = ��

��× 100%, (3)

where C and V are the molar concentration of metal and volume of the solution,

respectively; M and m0 are the molar mass and mass of the corresponding precipitate,

respectively.

� RESULTS AND DISCUSSION

Performance evaluation of the mechanochemical process. To evaluate the

effects of the mechanochemical activation on the extraction efficiencies of Li and Fe,

we tested powders, which were pretreated by the different mechanochemical

processes. The results are shown in Figure 2, approximately 53% of Li and less than 6%

of Fe were leached out with direct oxalic acid leaching. Even when the spent LiFePO4

was pretreated by milling, only 57.21% of the Li and 6.14% of Fe were extracted by

oxalic acid leaching. By comparison, the Li extraction of both the activated sample

(LiFePO4 and oxalic acid with a mass ratio of 1:1 pretreated by milling) and the

mechanochemical sample (LiFePO4 and oxalic acid with the mass ratio of 1:1, and 1

mL water pretreated by milling) increased markedly to 85.27% and 99.34%,

respectively, after water leaching for 30 min. This behavior could be explained by

mechanical forces, such as shearing, impaction, and squeezing, exerted during ball

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milling transmitting energy to the powder, reducing the particle size, destroying

crystal structures, and accelerating the reaction processes. 29, 30 Compared with dry

milling, a high Li extraction efficiency was achieved by the wet milling treatment in

the presence of oxalic acid. This result might be attributed to there being no dead

angle during the wet milling reaction.

Figure 2. Extraction of Li and Fe from different samples (direct acid leaching sample,

milling pretreatment and direct acid leaching sample; dry mechanochemical sample,

wet mechanochemical sample). Conditions: grinding time = 2 h, rotation speed = 500

rpm, mass ratio of LiFePO4 to oxalic acid = 1:1, ball to powder mass ratio = 20:1.

Effects of various parameters on the extraction efficiencies of Li and Fe. The

mass ratio of LiFePO4 to oxalic acid has a considerable influence on the chelation

efficiency of metal ions. As shown in Figure 3a, the extraction of Li increased as the

mass of oxalic acid was increased, and 99.34% Li was extracted at a LiFePO4 to

oxalic acid mass ratio of 1:1. However, the Fe extraction did not increase notably.

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Figure 3b shows the ex-situ XRD patterns of the leaching residue obtained at different

mass ratios of LiFePO4 to oxalic acid from 5:1 to 0.5:1. The XRD pattern intensity of

the LiFePO4 decreased as the mass of oxalic acid was increased, the peaks of

FeC2O4·2H2O appeared at a mass ratio of 5:1, and peaks of LiFePO4 disappeared at a

mass ratio of 1:1. The low extraction efficiency of Fe was related to the generation of

FeC2O4·2H2O. Therefore, the optimal mass ratio of LiFePO4 to oxalic acid was 1:1.

Figure 3. Effect of mass ratio of LiFePO4 to oxalic acid on Li and Fe leaching (a) and

XRD pattern change (b). Conditions: grinding time = 2 h, rotation speed = 500 rpm,

ball-to-powder mass ratio = 20:1.

The effects of the rotation speed (200–500 rpm) on the extraction of Fe and Li were

examined under the conditions of a LiFePO4 to oxalic acid mass ratio of 1:1, a

grinding time of 2 h, and a ball-to-powder mass ratio of 20:1. In the milling process

conducted by planetary ball milling, different rotation speeds provided different

energies, which could markedly affect the reaction rate of the mechanochemical

reaction. Figure 4a shows that the extraction efficiencies increased from 94.16% to

99.43% for Li and from 10.37% to 11.13% for Fe as the rotation speed was increased

from 200 to 500 rpm. These results indicate that the faster rotation speed not only

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promoted extraction of Li but also destroyed the structure of the ferric oxalate

precipitate. Therefore, 500 rpm was selected as the optimized rotation speed to ensure

highly efficient recovery of lithium.

The milling time is another key parameter for extraction of Li and Fe. As shown in

Figure 4b, the extraction efficiencies of Li and Fe increased as the milling time was

increased, namely, 99.34% of Li and 11.13% of Fe were extracted after 2 h milling,

compared with 75.99% of Li and 2.15% of Fe after 5 min milling. This result can be

attributed to the small particles having a large specific surface area and tending to

dissolve more easily than large particles.31 As the milling time was increased from 2

to 4 h, the extraction efficiency of the Li changed little; however, the Fe extraction

efficiency increased to 13.42%. This behavior could be explained by the structure of

the ferric oxalate precipitate becoming damaged at long milling times, resulting in

more Fe ions becoming dissolved in the solution. Hence, the optimum milling time

was determined to be 2 h.

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Figure 4. Effect on extraction efficiency of (a) rotation speed, (b) milling time, (c)

ball-to-powder mass ratio.

The effect of the ball-to-powder mass ratio on the extraction efficiency was

investigated for ratios in the range from 10:1 to 40:1 under the following conditions:

LiFePO4 to oxalic acid mass ratio of 1:1, rotation speed of 500 rpm, and grinding time

of 2 h (Figure 4c). As the mass ratio increased from 10:1 to 20:1, the extraction

efficiency of Li increased from 93.91% to 99.34%, while that of Fe increased from

6.71% to 7.12%. As the mass ratio of the ball to powder was increased further to 40:1,

the extraction efficiencies of Fe and Li remained almost unchanged, indicating that

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the ball-to-powder mass ratio had little effect on the extraction efficiency. Therefore,

20:1 was selected as the optimal ball-to-powder mass ratio.

Mechanochemical reaction mechanism. To better understand the reaction

mechanism of the mechanochemical process, physicochemical changes were also

investigated. On the basis of the XRD results, as shown in Figure 5A, the intensity of

the characteristic diffraction peaks of LiFePO4 decreased as milling time was

increased, while those of FeC2O4·2H2O increased. When the rotation speed and

milling time were 500 rpm and 2 h respectively, the peaks of LiFePO4 disappeared.

We observed the morphologies of the raw material, the mechanically activated sample,

and the mechanochemical sample by SEM (Figure 5B). The SEM patterns indicated

that the structure of LiFePO4 was destroyed during the mechanochemical process.

Compared with the raw LiFePO4 materials, which had a relatively large particle size,

we observed a considerable reduction of the particle size of the activated and

mechanochemical samples after grinding for 2 h. Moreover, after grinding with oxalic

acid, the typical rod-like morphology of oxalates appeared in Figure 5B (c) and 5B (d),

indicating the formation of FeC2O4·2H2O. Thus, we observed that the size of the

mechanochemical samples decreased as milling time was increased. The EDX results

are shown in Figure S1 (Supporting Information). According to the results, P is found

in the powders before and after mechanochemical process decreased after water

leaching, indicating the mass of P is in the form of PO43- into the solution. The Fe/O

ratio of the powders before and after mechanochemical process increased from 1:4.09

to 1:4.71, which is caused by the addition of oxalic acid. While the Fe/O ratio of the

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residue after water leaching was 1:4, which is consistent with the product FeC2O4, and

both the elements were uniformly distributed in the particles.

Figure 5. XRD pattern (A) and SEM images (B) of different samples: (a) raw

materials, (b) activated samples (grinding time = 2 h, rotation speed = 500 rpm), (c)

wet mechanochemical samples (grinding time = 5 min, rotation speed = 500 rpm,

LiFePO4/oxalic acid mass ratio = 1:1), (d) wet mechanochemical samples (grinding

time = 2 h, rotation speed = 500 rpm, LiFePO4/oxalic acid mass ratio = 1:1)

Figure 6. Possible products and mechanism in the mechanochemical process.

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A reaction mechanism for the mechanochemical process is illustrated in Figure 6,

which can be described in three steps: reduction of particle size, breaking of chemical

bonds, and generation of new chemical bonds. Under the mechanical forces, the size

of the particles decreased and the local temperature increased, which facilitated

chemical reaction processes. The reactions of the mechanochemical process may be

represented as follows:

��(�) +�

�� (�)

!�"#$%

�

�� (�) + ��&('() +

�

��&('() +

��)�('() + �&('() (4)

Herein, thermodynamics analysis was conducted to explore the reaction trend for

selective leaching of Li by this mechanochemical process in the presence of H2O,

which act as reaction medium. According to the thermodynamic data presented in

Table S1 and the equation S1 (see Supporting Information), the values of ∆rGmθ from

equation (4) is -112.545 kJ mol–1, indicating that the reaction is spontaneous at room

temperature.

Recovery of iron and lithium. After the mechanochemical process, the

concentration of Li+ and Fe2+ are 0.45 g L–1 and 0.21 g L–1 respectively, and the pH of

the leachate is about 1.7. A 94% content of Fe was recovered as FeC2O4·2H2O. The

E-pH diagrams of the Fe-H2O system and Li-P-H2O system are shown in Figure 7.

The Fe(OH)3 phase is thermodynamically stable under acidic and alkaline conditions;

however, the Li3PO4 phase is thermodynamically stable under neutral or alkaline

conditions. Hence, trace amounts of Fe2+ in the leachate can be oxidized into Fe3+ and

easily removed by adjusting the pH to 4 with 1 M sodium hydroxide solution. After

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the experiment of Fe2+ removal, the concentration of Fe2+ is less than 4 mg L–1 by

ICP-OES testing and analysis. The residual Li+ and PO4

3– in the solution can be

recovered as Li3PO4 by adjusting the pH to 8 with 1 M sodium hydroxide solution at

90 °C. The XRD patterns and SEM images of the obtained Li3PO4 are shown in

Figure 8. Notably, the XRD patterns of the recovered Li3PO4 agreed well with the

standard pattern peaks. The precipitated Li3PO4 presented as agglomerates of

numerous rhombic sheets. On the basis of the ICP-OES analytical results, the

recovery rate and purity of the obtained Li3PO4 product were 93% and 98%,

respectively. Therefore, the global recovery rate of Li is calculated to be 92.38%. It

was found that 0.66% Li was lost in the mechanochemical process and 6% Li

remained in the final solutions. The recovered Li3PO4 could be used for the synthesis

of the LiFePO4 cathode material, catalyst, and dental material. As reported in our

previous work, the solution with phosphate ions after recovery of Li can be effectively

removed by the Mg-enriched engineered carbon with a 95% removal rate and the

Mg(OH)2-modified mesocarbon microbeads (MCMB) carbon with a high adsorption

capacity of 588.4 mg/g, which can be prepared from the spent LIB anode materials.32,

33

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Figure 7. E-pH diagrams of Fe-H2O system at 25 °C and Li-P-H2O system at 90 °C.

(Soluble species concentration (except H+) =1.0 M at 1 atm pressure)

Figure 8. XRD pattern and SEM image of the recovered lithium phosphate.

Preliminary economic analysis. In order to investigate the cost and the profit of

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the mechanochemical recycling process, an economic assessment was carried out and

the detailed analysis was given in the Supporting Information. The mechanochemical

recycling process in this study can be divided into two parts: mechanochemical

process and metal recovery process. The equipment parameters used in the calculation

referred to the existing equipment in our laboratory. It was assumed that 1 kg of spent

LiFePO4 powder was treated by our process. For this recycling process, the energy

consumption is mainly the metal recovery process, which need energy to increase the

temperature. Due to the ability to handle large quantities of powders and no

requirement for temperature, the cost of the mechanochemical process is relatively

low. The cost of this recycling process was calculated as $43.14, including the

depreciation cost of equipment, cost of equipment maintenance, electricity

consumption, water consumption, and chemical cost. The revenues of products were

calculated as $100.75. Therefore, the profit of the recycling process for 1kg spent

LiFePO4 powders was about $57.61.

� CONCLUSIONS

An innovative method for selective leaching of Li and Fe from spent LiFePO4 is

developed, which combines a mechanochemical treatment with oxalic acid and water

leaching. Approximately 99% of Li and 94% of Fe were recovered under the optimal

conditions of rotation speed of 500 rpm, milling time of 2 h, mass ratio of LiFePO4 to

oxalic acid of 1:1, ball-to-powder mass ratio of 20:1, and water leaching time of 30

min. On the basis of our characterization conducted by XRD and SEM, a

mechanochemical mechanism is proposed. The reduction of average particle size,

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breaking of chemical bonds, and generation of new chemical bonds by mechanical

activation led to a considerable enhancement of the selective leaching efficiency of Li.

We calculated thermodynamic diagrams of Fe-H2O system and Li-P-H2O system. As

a result, approximately 94% of Fe and 93% of Li could be recovered as FeC2O4·2H2O

and Li3PO4, respectively. This study provides a simple and environmentally friendly

technology for selective recycling of valuable metals from spent LiFePO4 batteries.

� ASSOCIATED CONTENT

Supporting Information

The EDX results of the powders before and after mechanochemical process, and the

residue after water leaching; Thermodynamic data of relevant chemical reaction

during mechanochemical process; Economic Assessments; The chemical cost for the

recycling process and the revenues of products by this recycling process.

� ACKNOWLEDGEMENTS

The experimental work of this study was supported by National Key R&D Program of

China (2017YFB0102104), the Joint Funds of the National Natural Science

Foundation of China (U1564206), the National Natural Science Foundation of China

(51772030) and the Major Achievements Transformation Project for Central

University in Beijing.

� REFERENCES

(1) Manthiram, A.; Goodenough, J. B. Lithium insertion into Fe2(MO4)3 frameworks:

Comparison of M = W with M = Mo. J. Solid State Chem. 1987, 71, 349-360.

(2) Yuan, L.-X.; Wang, Z.-H.; Zhang, W.-X.; Hu, X.-L.; Chen, J.-T.; Huang, Y.-H.;

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Table of Contents

Synopsis

An efficient and eco-friendly mechanochemical process has been successfully

designed to selectively recover Li and Fe from spent LiFePO4 batteries.

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