Thermodynamic Analysis and Application to the Wet Chemical Process

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C: Energy Conversion and Storage; Energy and Charge Transport 2

E-pH Diagrams for the Li-Fe-P-HO System from 298 K to 473 K: Thermodynamic Analysis and Application to the Wet Chemical Processes of LiFePO Cathode Material 4

Qiankun Jing, Jialiang Zhang, Yubo Liu, Cheng Yang, Baozhong Ma, Yongqiang Chen, and Chengyan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02074 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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The Journal of Physical Chemistry

E-pH Diagrams for the Li-Fe-P-H2O System from 298 K to 473 K: Thermodynamic Analysis and Application to the Wet Chemical Processes of LiFePO4 Cathode Material Qiankun Jinga, Jialiang Zhanga,b,*, Yubo Liua, Cheng Yanga, Baozhong Maa,b, Yongqiang Chena,b, Chengyan Wanga,b,** a

School of Metallurgical and Ecological Engineering, University of Science and

Technology Beijing, Beijing 100083, PR China b

Beijing Key Laboratory of Rare and Precious Metals Green Recycling and

Extraction, University of Science and Technology Beijing, Beijing 100083, PR China *Corresponding author. **Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (C. Wang)

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Abstract The wet chemical processes of LiFePO4, hydrothermal synthesis and hydrometallurgical recovery, are of great importance during the life cycle of LiFePO4. To analyze these two processes, E-pH diagrams for the Li-Fe-P-H2O system are plotted from 298 K to 473 K in this study. The E-pH diagrams can well explain practical operating conditions of hydrothermal synthesis and hydrometallurgical recovery, and provide thermodynamic basis for them. Besides, suitable conditions for hydrothermal synthesizing LiFePO4 are suggested from E-pH diagrams, including high temperature, low redox potential, optimum pH 7.8-8.4 and excess stoichiometric lithium. As found in E-pH diagrams, LiFePO4 will change to ferric phosphate by promoting the redox potential while lithium will be extracted to the aqueous solution. Based on the above, a method is proposed for selective leaching of lithium from spent LiFePO4, which is successfully verified by leaching experiments. In condition of room temperature (298 K), neutral pH (7.0) and low liquid-solid ratio (5:1), 95.4% of lithium can be extracted using 2.7 M H2O2 as the oxidant, while iron remains in the residue. This method shows promising commercial value as it can realize selective extraction of valuable lithium from spent LiFePO4 and avoid using large amount of acid and alkaline.

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1. INTRODUCTION Lithium ion batteries (LIBs) have been broadly applied as the most successful secondary batteries in the market of energy conversion and storage.1-2 In recent years, the output of LIBs worldwide has been growing due to the rapidly increasing market demands.3 Olivine-type lithium iron phosphate (LiFePO4, LFP) is considered as one of the most common used commercial cathode materials for LIBs, occupying huge market share in the fields of energy storage, electronic equipment and electric vehicles with the advantages of low costs, abundant resources, long life span and excellent electrochemical performance.4-6 Many methods of synthesizing LFP have been developed, such as solid-state reaction, sol-gel, microwave heating, hydrothermal, carbothermal reduction and spray pyrolysis.7 The hydrothermal method has been recognized as a promising ways to synthesize LFP because of its wide material sources, short processing time and good performance of the products.8-11 During a typical hydrothermal synthesis, the aqueous solutions of raw material (i.e. FeSO4, LiOH, H3PO4) are simply mixed under reducing atmosphere, and the mixture subsequently undergoes the hydrothermal process at a high temperature (>100 oC) in an autoclave for a few hours. Eventually, LFP can be obtained as a solid product. On the other hand, spent LIBs have been inevitably generated along with the deactivation of LIBs, including considerable amount of spent LFP batteries. For economic and safety reasons,12 spent LFP batteries have been highly concerned as well as other types of spent LIBs. They contain numerous metals (e.g., Li, Fe, Al, and 3



Cu, etc.) and bring potential environmental risk because of the hazardous electrolyte and heavy metals.13-15 The cathode of LFP batteries consists of current collector (aluminum foil), binder, active material (LFP) and conductive agent. In general recovery process,14 physical and chemical methods are used to separate aluminum foil, and hydrometallurgical method is the major way to deal with the rest LFP cathode powder. Spent LFP can be decomposed to aqueous soluble species by strong acid leaching reagents, e.g., H2SO4, HCl, HNO3 and H3PO4. Although lithium is more valuable than iron, the leaching by strong acid can hardly be selective. Li, Fe and P in leachate must be precipitated successively from the leachate to form their respective products.16 Obviously, hydrothermal synthesis and hydrometallurgical recovery (leaching and precipitation) of LFP are both wet chemistry methods and processed in aqueous medium. Factors (e.g., temperature, pH, redox potential and concentration of species) can affect the decomposition and synthesis of LFP, whereas the thermodynamic equilibrium of Li-Fe-P-H2O system has been rarely researched,17 especially at high temperature aqueous. Accordingly, the wet chemical process of LFP is almost “black box” in theory, and sometimes it is blind to explore the feasible routines and corresponding operative conditions for both synthesis and recovery of LFP. Thermodynamic analysis of Li-Fe-P-H2O system could predict the transformation of existing species under different operating conditions, which is instructive to LFP’s hydrometallurgical recovery and hydrothermal synthesis. As a vital means for aqueous thermodynamic analysis, E-pH diagram, also 4


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known as Pourbaix diagram, has been widely used in the fields of hydrometallurgy, material corrosion and synthesis,18-19 and extended in related fields of cathode materials. For similar system, E-pH diagrams of Li-Co-H2O20 system had been employed to guide the leaching and wet chemical synthesis of LiCoO2. Lin et al. 21 plotted E-pH diagrams of Li-Ti-H2O system from 333.15 K to 453.15 K, and found the predominant areas of Li4Ti5O12 and Li2TiO3 in the E-pH diagrams. Accordingly, they verified the feasibility of synthesizing Li4Ti5O12 and Li2TiO3 at high temperature aqueous, and successfully synthesized Li4Ti5O12 and Li2TiO3 by using the hydrothermal method. In terms of the Li-Fe-P-H2O system, Zhao22 plotted E-pH diagrams of Li-Fe-P-H2O system at 298.15 K, under the premise of estimating the Gibbs free energy of formation of LiFePO4 at 298.15 K by the congeneric linearity rule. However, E-pH diagrams of Li-Fe-P-H2O at high temperatures and their application in hydrothermal synthesis have not been researched. Through the in-depth researches on LFP of recent years, thermodynamic parameters of LFP (e.g., heat capacity, entropy and Gibbs free energy of formation) have been obtained23-24. In this work, a series of E-pH diagrams for Li-Fe-P-H2O system were plotted at room and high temperatures (from 298.15 K to 473.15 K). These E-pH diagrams provide a new way to explain current processes of LFP’s hydrothermal synthesis and hydrometallurgical recovery. Furthermore, according to the E-pH diagrams, suitable operating conditions for hydrothermal synthesis were obtained, and a novel method was proposed to selectively extract lithium from spent LFP. 5



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2. METHODS The plotting process of E-pH diagrams is accorded to the general procedure25, and the principles and method are shown in the Supporting information. Besides, some reasonable assumptions have been proposed to simplify the complex multi-element system in the plotting process. Some unstable and uncommon complex ions of iron hydroxyl, e.g., Fe(OH)4-, were ignored; and it was presumed that phosphorus existed in orthophosphate group, hydrogen phosphate or phosphoric acid form; various iron phosphates and ramifications were uniformly represented by the most common and stable forms, namely FePO4·2H2O and Fe3(PO4)2·nH2O. 2.1. Data Sources of the standard Gibbs free energy of formation at T,  f GTo

 f GTo of each species is the precondition to plot E-pH diagram at temperature T. According to J. W. Cobble26, the volume change of species  f GTo in aqueous can be o ignored below 573 K. Based on this, the relationship between  f GTo and  f G298K

is expressed as: 27 o Δ f GTo  Δ f G298 K 

T

298K

C p dT  T 

T

298K

o C p d ln T  (T  298 K ) S 298K

(1)

o Where Cp denotes the heat capacity, and S 298K is the standard entropy.

o o For solid compounds, Cp and S 298K are generally known so that the  f GT can o be calculated according to Equation (1). For aqueous ions, S 298K and Cp can be

inferred according to the entropy correspondence principle of ions28 and the liner ionic heat capacity approximation27, respectively. By using the same thermodynamic basis with Eq. (1), thermodynamic software, e.g., HSC Chemistry®29 and Factsage®30, have already collected and calculated massive thermodynamic data from room to high 6


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temperatures. For a specific species, there may be slight difference in the value of

 f GTo from HSC Chemistry and Factsage. To select more reliable data, we take o Lange’s handbook of chemistry31 as reference to compare  f G298K , and thus adopt o the closer data source.  f GT of each involved species at 298.15 K, 363.15 K, 423.15

K, and 473.15 K is listed in Table 1, respectively. Besides, the  f GTo of LiFePO4 is calculated by Equation (1). To be specific, the o o values of  f G298K (-1480.97 kJ·mol-1) and S 298K (136.75 J·mol-1·K-1) are from the

ref. 17, and Cp(T) is from the ref. 18, respectively. For the Fe3(PO4)2·nH2O, the n values are determined according to the thermal decomposition behavior of Fe3(PO4)2·nH2O (n=8 when T=298.15 K, 363.15 K; n=2.5 when T=423.15 K; n=1.5 when T=473.15 K)32. Iglesia33 reported a reliable approach to calculate  f GTo of phosphate by using constituent units of oxides and hydroxides, and the  f GTo of Fe3(PO4)2·nH2O can be obtained by adding constituent units of P2O5 (-1726.84 + 0.30T) kJ·mol-1, FeO (-319.16 + 0.17T) kJ·mol-1, H2O (-299.22 + 0.20T) kJ·mol-1 according to its chemical formula. Accordingly,

 f GTo

of Fe3(PO4)2·8H2O is

(-5078.08 + 2.41T) kJ·mol-1,  f GTo of Fe3(PO4)2·2.5H2O is (-2878.04 + 1.31T) kJ·mol-1, and  f GTo of Fe3(PO4)2·1.5H2O is (-2607.95 + 1.11T) kJ·mol-1.

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Table 1.  f GTo of involved species at different temperatures.

 f GTo (kJ·mol-1) Species

Resource 298.15 K

363.15 K

423.15 K

473.15 K

Li+

-293.69

-297.26

-301.02

-304.50

FactSage

Fe2+

-78.87

-76.85

-75.22

-73.96

FactSage

Fe3+

-4.61

4.79

13.32

20.45

FactSage

Fe(OH)2

-492.05

-474.27

-458.01

-444.57

FactSage

Fe(OH)3

-705.58

-677.83

-652.19

-630.85

FactSage

H3PO4

-1118.92

-1085.98

-1056.56

-1032.38

HSC

H2PO4-

-1137.15

-1100.00

-1064.02

-1032.71

HSC

HPO42-

-1089.13

-1042.30

-994.80

-952.31

FactSage

PO43-

-1018.77

-958.89

-897.79

-842.65

FactSage

Li3PO4

-1965.90

-1937.49

-1911.02

-1888.40

HSC

FePO4·2H2O

-1657.45

-1606.99

-1560.35

-1521.54

HSC

Fe3(PO4)2·nH2O

-4359.07,

-4202.89,

-2878.04,

-2607.95,

Calculated

n=8

n=8

n=2.5

n=1.5

LiFePO4

-1480.97

-1490.71

-1501.13

-1510.79

Calculated

H2O

-237.14

-226.74

-217.47

-209.97

HSC

2.2. E-pH diagrams Possible reactions among the involved species and their corresponding E-pH relationships are listed in Table S1. According to the E-pH relationships, the E-pH diagrams from 298.15 K to 473.15 K were plotted as shown in Fig. 1. The stable area 8


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of water lies between the short dash lines marked with a and b as shown in Fig. 1, which correspond to reaction (a) and (b) in Table S1, respectively. The predominant area of each species is divided by those lines representing the reactions with adjacent species.

Figure 1. E-pH diagrams for Li-Fe-P-H2O system at 298.15 K (a), 363.15 K (b), 423.15 K (c) and 473.15 K (d). 3. DISCUSSION 3.1. Discussion on hydrothermal synthesis of LFP A complete hydrothermal synthesis process commonly contains three steps: first, mix the raw materials in aqueous solution at room temperature, then heat up the mixture to high temperature and maintain a few hours, and finally obtain LFP solid products after cooling down. This entire process involves E-pH diagrams from room 9



to high temperatures. (1) Thermodynamic feasibility of synthesizing LiFePO4 in aqueous solution According to Fig. 1, predominant area of LFP occupies a large area especially at high temperatures. In other words, under certain pH and redox condition, LFP is the ultimately and only thermodynamic stable condensed phase when appropriate amount of Li source, Fe source and PO43- source coexist. Early in 2001, Yang et al.34 reported the hydrothermal synthesis of LFP for the first time by using raw materials of LiOH, FeSO4 and H3PO4, and the hydrothermal synthesis underwent a 5-hour heating preservation of 120 oC. Subsequently, diverse sources of Li, Fe and PO43- have been utilized to synthesize LFP at different pH and temperatures. Table 2 summarizes operating conditions for hydrothermal synthesis of LFP.

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Table 2. Summary of operating conditions for hydrothermal synthesis of LFP Molar

Raw Materials

pH

Ratio Additives

Li

Fe

PO43-

source

source

source

LiOH

FeSO4·7H2O

H3PO4

Hydrothermal Temperature &

Ref.

Li:Fe:P

Initial

Final

Time

3:1:1

7.56

6.91

120 oC, 5 h

34

9.85

5.67

175-220 oC, 5 h

35

Sugar, LiOH

FeSO4·7H2O

H3PO4

3:1:1

Ascorbic acid, Hydrazine

LiOH

FeSO4·7H2O

H3PO4

1:1:1

Hydrazine

6

—

190 oC, 6 h

36

LiOH

FeSO4·7H2O

H3PO4

3:1:1

Ascorbic acid

7

—

190 oC, 12 h

37

LiOH

FeSO4·7H2O

H3PO4

3:1:1

8.2

—

180 oC, 6 h

38

LiOH

FeSO4

H3PO4

~3:1:1

—

180 oC, 10 h

39

LiOH

FeSO4·7H2O

H3PO4

3:1:1

—

180 oC, 3 h

40

FeSO4·7H2O

KH2PO4

3:1:1

EG

200 oC, 16 h

7

(NH4)2Fe(SO4)2

(NH4)2HPO

·6H2O

1:1:1

Ascorbic acid

7

—

140-200 oC, 4-20 h

41

4

FeSO4·7H2O

(NH4)3PO4

2:1:1

Sucrose

—

—

220 °C, 18 h

42

FeCl2·7H2O

H3PO4

3:1:1

Ascorbic acid

—

—

160 oC, 12 h

43

x:1:1

LiOH

—

240 oC, 2 h

44

5.0

—

200 oC, 6 h

45

5.0

—

100 oC, 5 h

46

220 oC, 24 h

47

200 oC, 20 min

48

Li2SO4· H2O Li2CO3 LiOH LiOOC CH3 LiC6H5 O7·4H2

FeSO4·7H2O

(NH4)2HPO

Pyrogallic acid Sucrose

Li2CO3

Fe powder

Li3PO4

FeSO4·7H2O

Li2CO3

FeSO4·7H2O

(NH4)3PO4

5:1:1

Li3PO4,

3.6:1:1.

H3PO4

5

(NH4)2HPO

3:1:1

0 6.5~8. 1

4

O

6.3~9.

Na2EDTA

6.0-9. 5

2~8

4

LiH2PO4

FeC6H5O7

LiH2PO4

1:0.92:1

3

—

(2) Suitable operation conditions for hydrothermal synthesizing LFP 11



For the hydrothermal synthesis of LFP, the locations and proportions of LFP’s predominant area in the E-pH diagrams correspond to the operating conditions and formation tendency, respectively. Therefore, current operating conditions of hydrothermal synthesis could be clarified by the E-pH diagrams. On the other hand, more suitable operating conditions can be obtained according to the analysis of the variation of LFP’s predominant area in E-pH diagrams under different conditions. Temperature. Fig. 1 shows the variation of predominant area from 298.15 K to 473.15 K. The proportion of LFP’s predominant area is relatively small at 298 K. At high temperatures, shown in Fig. 1b to 1d, E-pH diagrams basically share same profile with Fig. 1a, but LFP occupies larger predominant area, and predominant areas of Li+, Fe2+, Li3PO4, Fe(OH)2, Fe(OH)3, etc. decrease in varying degrees. This suggests that the formation tendency of LFP is much greater at higher temperatures. Therefore, the high temperature contributes to the formation of LFP, and as shown in Table 2, the synthesizing processes are commonly conducted at high temperatures (> 100 oC). pH. In the Li-Fe-P-H2O system, condensed compounds show different forms according to Fig. 1, i.e. Fe3(PO4)2·nH2O, FePO4·2H2O, LFP, Li3PO4, Fe(OH)2 and Fe(OH)3. There exists equilibrium between the condensed compounds and soluble species. The equilibrium is affected by solution pH, and it can be described by the boundary lines with different concentrations (10-1 M, 10-3 M, 10-5 M) of predominant areas in E-pH diagrams. Specifically in Fig. 1, the relationship between pH and solubility of FePO4·2H2O is corresponding to line ② and ③, Fe3(PO4)2·8H2O to line 12


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④ , LiFePO4 to line ⑥ and ⑦ , Li3PO4 to line ⑨ , respectively. The E-pH diagrams

describe the effect of pH on the solubility of condensed compounds by reflecting the balanced concentrations of soluble species at different pH. Fig. 1 indicates a tendency that the lower the pH, the higher the balanced concentration of soluble species. Hydrothermal synthesis is the opposite of LFP’s decomposition, and it can be analyzed according to the relationships between pH and compounds’ solubilities. As for actual hydrothermal synthesis, the raw materials concentrations will decrease with the hydrothermal reaction proceeding, and the predominant area of LFP will move to higher pH, as shown in Fig. 1 with red and blue dashed line. Thus, in order to obtain higher conversion rate of LFP and avoid impurities, the suitable pH range should locate at the coincident part of LFP’s predominant areas at high and low species concentrations so that LFP can maintain at the condition of the predominant area. To be specific, the suitable pH ranges are 7.8~8.4 at 298.15 K, 3.5~10.1 at 363.15 K, 2.2~11.4 at 423.15 K, and 1.3~12.4 at 473.15 K. These pH ranges are well consistent with the operating pH conditions (overall from 2 to 9.5) as shown in Table 2. The suitable pH ranges are obviously broader at higher temperatures. Given that the system of hydrothermal synthesis usually undergoes room temperature before and after the hydrothermal synthesis, so the optimum operating pH should be 7.8~8.4 in order to avoid impurities which might generate at room temperature. Besides, Lee49 and Dokko50 also found that acid or basic aqueous will result in LFP’s decomposition, thereby decreasing the yield of LFP and formatting impurities, e.g., Fe3(PO4)2·8H2O, Li3PO4, Fe(OH)2, and Fe(OH)3. 13



Redox potential. As shown in Fig. 1, LFP’s predominant area is located at low electric potential position. The low electric potential is corresponding to low redox potential, suggesting that the hydrothermal synthesis requires reducing or inert atmosphere. Accordingly, some reductants (shown in Table 2), e.g., sugar, ascorbic acid, hydrazine or pyrogallic, are introduced to maintain low redox potential and avoid oxidation of Fe(II). Dosage of raw materials. If the dosage of raw materials depends on the stoichiometry of LFP, i.e. mole ratio Li: Fe: P =1:1:1, the concentrations of raw materials will decrease with the hydrothermal synthesis proceeding. Therefore, LFP’s predominant area will narrow down (as shown in Fig. 1, and the operating condition is assumed at neutral pH). This reveals that the formation tendency of LFP will drop down with the reaction proceeding, especially at the final stage. Assuming the final residue [Fe]=[P]=10-5 M, Fig 2 shows the variation of LFP’s predominant area at different equilibrium lithium concentrations. It shows a significant enlargement with the increase in lithium concentration at neutral pH range (10-3 M with red dosh line and 10-1 M with blue dosh line in Fig 2). To maintain high formation tendency of LFP, excess lithium is usually added in most hydrothermal synthesis processes, as shown in Table 2. However, excess iron or phosphate will not enlarge LFP’s formation tendency, as shown in Fig S1 and S2 since they do not affect LFP’s predominant area at neutral pH.

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Figure 2. E-pH diagram for Li-Fe-P-H2O system at 423K with different soluble lithium concentrations, 10-1, 10-3, and 10-5 M. 3.2. Discussion on hydrometallurgical recovery of spent LFP As shown in Fig. 1a, when pH is low, species are all soluble in aqueous solution. Depending on the electric potential, iron exists in form of Fe2+ or Fe3+, while lithium and phosphate exist in the Li+ and H3PO4 form, respectively. With the increase in pH, Fe3(PO4)2·8H2O, FePO4·2H2O and LiFePO4 become the main species under different electric potential conditions. When pH is further increased, the stable condensed phases change to Li3PO4, Fe(OH)3 or Fe(OH)2. For spent LFP’s hydrometallurgical leaching, positions of aqueous soluble species (Li+, Fe2+, Fe3+, and H3PO4) correspond to the operating conditions (pH and redox potential). Accordingly, the E-pH diagram can analyze the current hydrometallurgical routes for leaching and recovery of lithium and iron from spent LFP. (1) Current recovery routes The existing forms of lithium, iron and phosphate at different pH values and redox potential conditions are shown in Fig. 3. To be specific, Li+ and Fe2+/3+ are all 15



soluble and co-existed at the blue colored areas, and the areas correspond to the leaching of lithium and iron under strong acid condition.

Figure 3. E-pH diagram for Li-Fe-P-H2O system at 298.15 K and the hydrometallurgical recovery routes (I to III) of spent LFP. Route I: under low pH and reductive condition, LFP will decompose to Li+, Fe2+, and H3PO4, as represented by the yellow line in Fig. 3. Strong acids (e.g., H2SO4, HCl, HNO3, and H3PO4, etc.) and reductants (e.g., ascorbic acid, hydrazine, and Na2SO3, etc.) can leach lithium and iron from spent LFP. After leaching, FePO4·2H2O can be precipitated from the leachate by promoting the pH and redox potential, namely adding some alkali and oxidant. Furthermore, lithium can be precipitated as Li3PO4 or Li2CO3 by promoting pH to alkaline and adding precipitants. As a demonstration, Yang et al.51 have reported a recycling method of spent LFP, which is well consistent with Route I. Route II: under strong acid and oxidative condition, LFP will decompose to Li+, Fe3+ and H3PO4, as represented by the blue line in Fig. 3. Unlike Route I, the oxidation of Fe(II) to Fe(III) occurs at the leaching stage, and FePO4·2H2O can be 16


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precipitated from the leachate by only promoting the pH. As an instance, Cai et al.52 used H2SO4 and H2O2 to dissolve LFP, then promoted the pH and precipitated FePO4·2H2O and Li3PO4 from the leachate successively. Likewise, Huang et al.53 adopted a leaching approach using HCl and H2O2. In general, route I and route II can simultaneously realize the leaching of lithium, iron and phosphate. These elements can be successively precipitated as respective products of FePO4·2H2O and Li2CO3 from the leachate. However, as shown in Fig.3, these two routes require consuming massive strong acid to decompose LFP at the leaching stage, then they consume more alkali to promote the pH back at the precipitating stage, which causes massive resource consumption and environment risk. (2) A novel method for selective leaching of lithium from spent LFP The leachate of noted route I and II contains lithium, iron and phosphate. As we know, lithium is much more valuable than iron because of its resource reserves and extraction cost.54 Therefore, iron and phosphate will cause a burden of cost if they are leached out. It would be meaningful if lithium could be selectively extracted from spent LFP rather than simultaneously being leached out with iron and phosphate. According to the E-pH diagrams, the decomposition products of LFP are not all soluble. As the white color areas shown in Fig. 3, lithium is in soluble Li+ form while iron is in insoluble FePO4·2H2O or Fe3(PO4)2·8H2O form, which means lithium could be selectively leached out by controlling the pH or redox potential condition. Therefore, we suggested a novel route for selective leaching of lithium. 17



Route III: under mediate pH and oxidative condition, the area upper LFP belongs to FePO4·2H2O and Li+. Thus, lithium can be selectively leached out while iron is in the residue in form of ferric phosphate. The horizontal boundary line with LFP corresponds to Reaction (S8) in Table S1, which is irrelevant to pH but associated with electric potential. It means that selective leaching of lithium can be approached at neutral pH condition by simply promoting the redox potential of the leaching environment, namely adding oxidant. To verify route III, leaching experiments have been carried out by using NaClO, O3, O2, and H2O2 as oxidants, and the details are shown in the supporting information.

Figure 4.

Leaching efficiency of metals from spent LFP by using oxidants NaClO,

O3, O2, and H2O2. Fig. 4 shows the leaching efficiency of metals from spent LFP by using various oxidants. Obviously, most lithium in spent LFP could be effectively extracted into leachate under the action of NaClO, O3, O2, and H2O2, while most iron remained in the residue. In particular, 95.4% of lithium and only 0.05% of iron were leached out in 2.7 M H2O2 aqueous after a 4-hour leaching process, and the liquid-solid ratio was as low as 5:1. H2O2 is cheap and easy to get, with its reduction productions (H2O and 18


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O2) having no harm to the environment. Also, the operating conditions were mild, because it was carried out at room temperature and the pH was controlled at 7.0. The entire leaching process is simple, practicable, effective and energy saving. In contrast to route I and route II, the route III has a shorter and more practicable process for the exaction of lithium. It not only avoids the consumption of acid and neutralizing alkali, but also will lighten the throughput of high salinity waste water after precipitating lithium. For the subsequent recovery, lithium in the leachate can be precipitated in Li2CO3 form by adding Na2CO3. (3) Mechanism for the selective leaching of lithium To further investigate the mechanism for the selective leaching process of lithium, XRD, XPS, and FT-IR tests have been performed to characterize the solid phase of the leaching process (spent LFP, the intermediate, and the final leaching residue). The transitions of phase, crystal structure, and oxidation state of iron during selective leaching of lithium are shown in Fig. 5. According to the XRD results in Fig. 5(a-c), the spent LFP (JCPDS 40-1499, space group Pnmb) and the final leaching residue (heterosite FePO4, JCPDS 37-0478, space group Pnmb) both belong to olivine structure55; in the intermediate phase, Li0.5FePO4 becomes mixture of LFP and heterosite FePO4. In other words, the solid phase maintains the olivine framework in the leaching process, and the extraction of lithium does not affect the style of crystal structure. Besides, the Fe 2p spectrum, as shown in Fig. 5(d-f), exhibited peak position shift to higher bonding energy along with the lithium extraction. Dedryvere et al.56 found the same phenomenon of peak 19



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position shift when LFP battery was charging. According to the fitting result, the main peak of Fe 2p3/2 at 710.6 eV characterized by Fe2+ gradually shifts to 712.0 eV (Fe3+). Thus, the oxidation state of iron was changed from Fe(II) to Fe(III) along with the lithium extraction, which corroborated the phase transition from LFP to heterosite FePO4. It has been already confirmed that heterosite FePO4 is the very phase after LFP being charged as cathode57. Furthermore, the intermediate phase consists of heterosite FePO4 and LFP, corresponding to the two-phase process when LFP battery is charging58-59. Therefore, the selective leaching of lithium is equivalent to charging process of LFP battery. LFP loses electron and transforms to FePO4, while lithium de-intercalates from the crystal into the aqueous solution, and the reaction should be: oxidant LiFePO 4 (olivine)  e   Li  (aqueous )  FePO 4 (olivine)

Figure 5.

Reaction (1)

Transition of phase, crystal structure and iron oxidation state during

selective leaching of lithium: (a, d) the raw material of spent LFP, (b, e) the residue 20


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when leaching efficiency of lithium is 51%, (c, f) the final leaching residue The FT-IR spectrum was also consistent with reported charging process of LFP60, as shown in Fig. 6, which exhibited some obvious change during the leaching process. With the leaching of lithium, new absorption peaks at wavenumbers of 1237, 683, 656 and 531 cm-1 emerged, while they disappeared at 647, 636, 502, 470 cm-1. Absorption peaks in the range of 400-600 cm-1 correspond to v2 and v4 bending modes of PO4, 600-700 cm-1 are assigned to FeO6 vibration modes, and 700-1400 cm-1 correspond to v1 and v3 stretching modes of PO461-62. Octahedral FeO6 and tetrahedral PO4 constitute the framework of olivine structure which will not be destroyed during the leaching process. However, the extraction of lithium and oxidation of iron will certainly affect electronic configuration of FeO6 and PO4 and result in electron density redistribution and dipole moments change. As a result, the vibration modes of FeO6 and PO4 were affected, and the FT-IR spectrum showed the change of absorption peaks61. The results of XRD, XPS and FTIR were verified and compensated by with each other, indicating that the selective leaching of lithium is equivalent to the LFP charging process. Given the discharging process and the reverse process of Reaction 1, the leaching residue heterosite FePO4 can serve as ionic sieve to extract Li+ in the aqueous63-65 or re-synthesize LiFePO4 as cathode materials.

21



Figure 6.

FT-IR characterization of the solid phase during selective leaching of

lithium: (a) spent LFP, (b) the residue at leaching efficiency of lithium of 51%, (c) the final leaching residue 4. CONCLUSIONS In this work, E-pH diagrams for Li-Fe-P-H2O system from 298K to 473K are plotted, which provide a novel way to analyze the equilibrium compositions of Li-Fe-P-H2O system. The E-pH diagrams can well explain the reported operating conditions of LFP’s hydrothermal synthesis and hydrometallurgical recovery in theory. Furthermore, in terms of LFP’s hydrothermal synthesis, suitable operating conditions of high temperature, low redox potential, neutral pH (optimum 7.8~8.4), and excess stoichiometric lithium are suggested to maintain high formation tendency and avoid impurities. For the hydrometallurgical recovery of LFP, current routes are analyzed, and a method is proposed for selective leaching of lithium. The E-pH diagrams reveal that LFP will lose electron and Li+ at high redox potential aqueous. The selective leaching method of lithium is also verified by experiments: 95.4% of 22


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lithium can be successfully leached out in 2.7 M H2O2 while iron remains in the residue. The XRD results of solid phases indicate that, the lithium de-intercalation of lithium does not break LFP’s olive-type crystal structure during the leaching process. Thus, the selective leaching of lithium is equivalent to charging process of LFP battery. Also, the equivalency is further supported by XPS and FTIR analysis. Supporting Information Principles and method of plotting E-pH diagrams; table of possible reactions and E-pH relationships; experimental details for selective leaching of lithium; E-pH diagrams with different soluble iron and phosphate concentration. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51834008, 51874040, U1802253), and the Fundamental Research Funds for the Central Universities (No. FRF-TP-18-020A3, 230201606500078, FRF-BD-18-010A). References 1.

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Thermodynamic Analysis and Application to the Wet Chemical Process

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