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An environmental-friendly synthesis of LiFePO4 using Fe-P waste slag and greenhouse gas CO2 Qian Cui, Chunhui Luo, Gen Li, Guixin Wang, and Kang-Ping Yan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00023 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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Industrial & Engineering Chemistry Research

An environmental-friendly synthesis of LiFePO4 using Fe-P waste slag and greenhouse gas CO2 Qian Cui, Chunhui Luo*, Gen Li, Guixin Wang, Kangping Yan*

College of Chemical Engineering, Sichuan University, Chengdu 610065, China

Abstract: An environmental-friendly and feasible route was creatively put forward to synthesize LiFePO4 using two industrial wastes Fe-P and CO2 from yellow phosphorus manufacture. LiFePO4 was obtained via a one-step synthesis, in which the mixture of Fe1.5P, Li2CO3 and H3PO4 in an appropriate molar ratio was calcined at 800°C for 6h in CO2. The chemical reaction mechanism was investigated by TG/DSC and XRD measurements; thereby the appropriate synthesis conditions were determined. The electrochemical performances of the synthesized LiFePO4 coated with 5.3wt% carbon were characterized by charge/discharge tests, cyclic voltammetric experiments and electrochemical impedance spectroscopic measurements, which exhibit high capacity, good cyclic capability and low impedance. Fe-P waste slag and the greenhouse gas CO2 are successfully converted into the energy materials in this work, which improves the traditional two-step synthesis route, and provides a new perspective of LiFePO4 production. Key words: LiFePO4, CO2, Fe-P waste slag, yellow phosphorus, synthesis. 1. Introduction With the rapid development of the automobile industry, the energy crisis and the environmental pollution are the issues that need to be solved urgently. Olivine-type LiFePO4, a kind of clean and alternative energy material, is widely used in electric and hybrid electric vehicles with the advantages of environmental compatibility, low cost and safety performance, and now is becoming a promising cathode material of lithium-ion batteries1-4. Generally, LiFePO4 is synthesized by using iron oxide such as FeC2O4·2H2O, Fe2O3 and even the expensive FePO4 as the raw materials5-7. Using the by-product Fe-P alloy obtained from the commercial manufacture of yellow phosphorus to produce 1

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LiFePO4 is a novel and facile synthesis technique8, 9, which can reduce the cost greatly in comparison to that of the conventional preparation methods mentioned above. However, the present synthesis route of LiFePO4 with Fe-P alloy reported early by our research group seems theoretically unreasonable, where Fe and P in the precursor were totally oxidized to the highest valence in the first step, and then reduced to the appropriate valence of LiFePO4 by heat treatment using pre-mixed carbon or hydrogen as the reduction agent under an inert atmosphere in the second6, 10. In the synthesis process large amount of oxygen and reductive materials such as C, H2, Ar, CO and N2 were consumed, that is actually unfavorable for the industrial production, which increases the manufacture cost to a great extent. Therefore, it is necessary to simplify the two-step synthesis route to reduce the consumption of the resources, which makes great sense to the practical applications. It is known that the chemical bond strength of P-O in PO43− is stronger than that of C-O in CO210, 11. Theoretically, it is reasonable to expect that P in Fe-P alloy could be oxidized into PO43− by CO2. Meanwhile, CO generated from CO2 reduction is able to prevent Fe2+ oxidation into Fe3+. Consequently, LiFePO4 could be synthesized conveniently by extra adding lithium and phosphorus sources in the reactant in an appropriate molar ratio with CO2 participation. Furthermore, CO2 can be easily obtained, for example, by combusting the off-gas of yellow phosphorus manufacture, of which the main ingredient is CO12. In this work, a novel and facile synthesis route was investigated in detail, in which LiFePO4 was obtained by one-step reaction using Fe-P waste slag and CO2 as the starting materials. The synthesis method of LiFePO4 proposed in this paper could not only simply the production process, and further decrease the manufacture cost, but also reduce greenhouse gas emissions, which is of great significance for industrial applications. 2. Experimental Section 2.1 Preparation. The atomic ratio of Fe and P in Fe-P alloy was previously determined as Fe1.5P. In order to meet the molar element ratio of LiFePO4, phosphoric acid (~85%) and 2


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Li2CO3 (~99.6%) were used as the Li and P sources in this work, which were well mixed with Fe1.5P in the molar ratio of Li: Fe: P = (0.9~1.1):1:1. The mixture was thoroughly ground with a dispersive solution of ethanol to form a rheological phase. After being dried, the substance was calcined at 650∼850 °C for 4~10h in a quartz tube furnace flushed by CO2, and LiFePO4 sample was thereby obtained. The as-synthesized LiFePO4 sample was further well-mixed with glucose in ethanol, and then calcined at 800°C for 2h in a quartz tube furnace flushed by Ar to prepare LiFePO4/C composite. The synthesis route was illustrated in Figure 1.

Fe1.5P alloy Ball milling H3PO4

Calcination LiFePO4

Glucose

Ethanol

Ball milling

CO2

Li2CO3

Rheological Phase Precursor precursor

Ethanol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Calcined in Ar LiFePO4/C

Rheological Phase

Figure 1. Synthesis route of LiFePO4/C using Fe−P waste slag as the starting material.

2.2 Materials Characterization. Thermogravimetric and differential scanning calorimetric analysis (TG/DSC) were carried out on NETZSCH STA 449 C instrument, where the samples were analyzed by heating from ambient to 850°C with a heating rate of 10K/min in flowing CO2. The phase structures were examined by X-ray diffraction (XRD, Philips X’Pert Pro, Holland) with a step of 0.04°/s within the range of 10 −70° using Cu Kα radiation at the power of 35 kV ×25mA. The morphology and the particle size were observed by using scanning electron microscope (HITACHI S-4800, Japan). The carbon content was determined on a carbon-sulfur analyzer (CS-902, China). 2.3 Electrochemical Measurements. The electrochemical performances of the as-prepared LiFePO4/C were investigated as 3



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the cathode in 2032 type coin cells by galvanostatic charge/discharge measurements, electrochemical

impedance

spectroscopic

characterization

(EIS)

and

cyclic

voltammetric (CV) experiments. LiFePO4/C powders were well mixed and ground with 10wt% of conductive acetylene black and 7wt% of commercial LA-132 binder (Chengdu Indigo Power Sources Co. Ltd., China) to form rheological phase slurry, which was coated on a cleaned aluminum foil current collector with amount of about 1.04 ± 0.32 mg/cm2. After being dried at 100 °C under vacuum for 10h, the foil was cut into about 1.54cm2 round wafers as working electrodes. Metal lithium was applied as both the counter electrode and the reference electrode. Celgard 2300 microporous film was used as the separator. The electrolyte was 1.0M LiPF6 dissolved in a solution of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) (1:1:1 in vol., Shenzhen Capchem Chemicals Co. Ltd., China). The 2032 type coin cells were assembled in a glove box filled with argon (≥99.99%). Galvanostatic charge-discharge measurements were performed on a Neware battery-testing instrument (Shenzhen Neware Technology Ltd., China) in the voltage range of 2.4−4.2V versus Li+/Li at room temperature. EIS and CV measurements were carried out on an electrochemical workstation controlled by the Powersuit software (Princeton Applied Research, USA). 3. Results and Discussion 3.1 Thermal Reaction Analysis. TG/DSC curves of Fe1.5P powder heated in CO2 from ambient to 850°C were shown in Figure 2(a). The slight weight loss (less than ~1wt %) below 600°C is attributed to the evaporation of the adsorbed water and removal of the impurities in Fe1.5P. Thereafter, the sample weight increases gradually, and the increase rate is relative high especially above 700°C, indicating the occurrence of the combination reactions between Fe1.5P and CO2. The amplified pattern of TG/DSC curves between 700 and 800°C was given in Figure 2(b), which exhibits an exothermic peak at ~722°C and an endothermic peak at ~754°C. Therefore, it is reasonable to suggest that the chemical reactions take place between 700 and 800°C.

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104

0.1

(a)

TG DSC

exo

-0.1

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-0.2

-0.3

101

DSC(mW/mg)

72 ℃ TG(%)

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DSC

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655 ℃

TG

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Temperature ( ℃ )

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-0.21

(b)

TG DSC

721℃

103

-0.24

-0.27

754℃

102

-0.30 101

DSC(mW/mg)

exo TG(%)

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DSC -0.33

TG

100

-0.36 99 700

720

740

760

780

800

Temperature( ℃ )

Figure 2. Thermal analysis of raw Fe1.5P powder heated in CO2: (a) from ambient to 850°C, (b) from 700°C to 800°C.

The TG/DSC measurements of Li2CO3, H3PO4 and Fe1.5P mixture prepared according to the description in experimental were also performed, which were shown in Figure 3.

5



0.2

TG DSC

100

DSC 610℃

0.0

exo

98

-0.4

96

78℃

- 8% -0.6

TG

94

809℃

DSC(mW/mg)

-0.2

786 ℃ TG(%)

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-0.8 92

836 ℃

-1.0

-1.2

90 0

100

200

300

400

500

600

700

800

Temperature( ℃ )

Figure 3. Thermal analysis of Fe1.5P, Li2CO3 and H3PO4 mixture heated in CO2.

The weight loss of the mixture below 600°C is nearly 8wt% as labeled in TG curve, which is much more than that of raw Fe1.5P heated at the same temperature range. It is deduced that such a large amount of weight loss is attributed not only to evaporation of the residual water and removal of the impurities, but also to the chemical reaction between phosphoric acid and Li2CO3, in which Li3PO4 is formed with release of CO2 and H2O. That is further discussed according to XRD analysis later in this paper. The sample weight drops continuously with the increase of temperature up to about 610°C, at which an exothermic peak appears and the weight loss rate slows down, implying a chemical reaction occurs at the temperature. When heated over 700°C the sample weight begins to increase gradually with the appearance of a small exothermic peak at ~786°C in DSC curve, and thereafter exhibits an accelerated growth, that is accordant with the TG curve of raw Fe1.5P mentioned above, indicating the chemical reaction of the sample with CO2. The existence of two remarkable exothermic peaks at ~809°C and ~836°C in the DSC curve implies that two chemical reactions occur at the temperatures, which result in the continuous increase of the sample weight as well. Based on the above discussion, it can be deduced that the reactant composed of Fe1.5P, Li2CO3 and H3PO4 do react 6


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with CO2. 3.2 Phase Structure Analysis. Raw Fe1.5P powder (sample A), Fe1.5P calcined at 700oC for 6h in CO2 (sample B), and Fe1.5P calcined at 800oC for 10h in CO2 (sample C) were respectively analyzed by XRD, which patterns were presented in Figure 4. By Rietveld refinement of XRD profiles sample B was determined as FeP (JPCDS card number 65-2595) and Fe2P (JPCDS card number 51-0943), which exhibits the similar diffraction as that of sample A, indicating that no obvious reactions occur below 700oC, corresponding to the slight weight loss in TG curve without DSC peaks. The XRD pattern of sample C exhibits typical diffraction peaks of Fe3(PO4)2 (JPCDS card number 49-1087) with minor impurity of FeP and Fe2P, which were indexed on curve (c) in Figure 4. Apparently, the formation of Fe3(PO4)2 implies that P in Fe1.5P is oxidized from valence -3 to +5 between 700°C and 800°C, while Fe maintains valence +2, which agrees well with the results of TG/DSC measurements discussed above. Furthermore, the only possible oxidizing agent in the reactant is CO2, which could be reduced to CO due to the stronger chemical bond strength of P-O. Therefore, the synthesis reaction between 700°C and 800°C could be described as follows:

4Fe1.5 P+8CO 2   → Fe 3 (PO 4 ) 2 +FeP+Fe 2 P+8CO ↑

(1)

If not taking the small amount of FeP and Fe2P in the resultant into account, the reaction could be written as:

2Fe1.5 P+8CO 2   → Fe 3 (PO 4 ) 2 +8CO ↑

(2)

in which P in Fe-P alloy is oxidized into PO43- by CO2, suggesting the feasibility to synthesize PO43- by calcinating Fe1.5P in CO2, meanwhile, Fe maintains the reduced valence state of +2.

7



Fe2P

(a) sample A

FeP

( -343) ( 361 )

( -413 )

( 161 ) ( -323 )

( 061 )

( 410 )

( 340 ) ( 151 ) ( -251 ) ( -402 )

( 041 ) ( -321 )

(c) sample C ( 311 )

( 221 ) (-102 ) ( 131)

(-311 )

(b) sample B

( 130 )

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JPCDS card No. 49-1087 20

30

40

50

60

2 Theta/ °

Figure 4. XRD patterns of (a) Fe1.5P powder, (b) Fe1.5P powder calcined at 700oC for 6h in CO2, (c) Fe1.5P powder calcined at 800oC for 10h in CO2.

The XRD patterns of Li2CO3, H3PO4 and Fe1.5P mixture calcined for 6 hours in CO2 at different temperatures were shown in Figure 5. For simplification, the as-prepared mixture without calcination is denoted as sample”1”, and the mixture calcined at 650°C, 700°C, 750°C, 800°C and 850°C were labeled as sample “2” to “6”, respectively.

8


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FeP

Fe2P

Li2CO3

Li3PO4

sample "1"

011

111

121

011

111

121

131

sample "2"

131

sample "3"

113 260

340

222

sample "5" 142 160 331

202 311 151

112

231 122 240

131 211 140 211 041

121 031

120 010

011

111

sample "4"

020

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sample "6" JPCDS card No. 40-1499 10

20

30

40

50

60

2 Theta/ °

Figure 5. XRD patterns of Fe1.5P, Li2CO3 and H3PO4 mixture calcined in CO2 at different temperatures.

The diffraction peaks of sample “1” agree well with that of FeP, Fe2P, Li2CO3 and Li3PO4, indicating the occurrence of the chemical reaction between H3PO4 and Li2CO3 at room temperature, which is accordant with the deduction proposed above by TG/DSC analysis. The XRD pattern of sample “2” exhibits LiFePO4 diffraction peaks, implying the formation of LiFePO4 below 650°C. Referring to the TG/DSC measurement, it is reasonable to suggest that the synthesis temperature of LiFePO4 is 610°C, at which an exothermic peak exists on DSC curve. With calcination temperature increasing, the peak intensities of LiFePO4 increase obviously, and that of the initial reactants decrease. At 800°C the diffraction peaks of the starting materials disappear completely, and only pure LiFePO4 phase remained, which four main peaks centered at 2θ≈20.7°、25.5°、29.7°and 35.5° are attributed to (011), (111), (121) and 9



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(131) planes of LiFePO4 (JPCDS card number 40-1499), respectively13. The XRD pattern of sample “6” exhibits two extra peaks as circled on the curve, which are not part of LiFePO4 diffractogram. Apparently, the impurities are generated over 800°C, which are also confirmed by DSC measurement mentioned above. The formation temperatures of the impurities correspond to the two exothermic peaks at 809°C and 836°C as shown in Figure 3. Consequently, it is reasonable to conclude that pure LiFePO4 could be synthesized in one step by calcinating Fe1.5P, Li2CO3 and H3PO4 mixture at 800°C in CO2. On the basis of XRD and TG/DSC analysis discussed above, the reaction mechanism was summarized as follows.

According to the chemical thermodynamic principle, the bond energies of C-O bond in CO, P-O bond in PO43−and C-O bond in CO2 are 1075.0, 596.6, 532.2 kJ/mol10, 12, respectively, indicating that the bond combining capacity of P-O in PO43− is stronger than that of C-O bond in CO2, but far less than that of C-O bond in CO. Therefore, C-O bond in CO2 could be broken to provide one oxygen atom for oxidizing P in Fe1.5P into PO43−. Meanwhile, carbon atom and the remained oxygen atom form the thermodynamically stable CO, which offers the reductive gas atmosphere to prevent Fe2+ from being oxidized into Fe3+. Consequently, Fe1.5P is oxidized by CO2 and further reacts with Li2CO3 and H3PO4 to form LiFePO4. If considering all FeP and Fe2P are consumed completely in the synthesis process of LiFePO4, the entire reaction could be described as:

4Fe1.5P+3Li2CO3 +2H3PO4 +13CO2  →6LiFePO4 +3H2O ↑ +16CO ↑

(3)

According to the results obtained above, the reaction kinetics was further investigated by calcinating the as-prepared Fe1.5P, Li2CO3 and H3PO4 mixture at 800°C in CO2 for 4, 6, 8 and 10 hours, respectively. The X-ray diffraction (XRD) patterns of the obtained products labeled correspondingly as sample “a”, “b”, “c” and “d” were collected as given in Figure 6.

10


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131 011

111

121

111

121

Fe2P

FeP

sample "a"

131

sample "b"

011

131 111

121

sample "c"

011

131 111

011

121

sample "d"

JPCDS card No. 40-1499 10

20

30

40

50

60

2 Theta/ °

Figure 6. XRD patterns of the as-prepared mixture calcined at 800°C in CO2 for different time.

Obviously, Fe2P and FeP coexist with LiFePO4 after the as-prepared mixture being calcined for 4h, and disappear completely when the calcination time increases to 6h, indicating that enough reaction time is necessary to obtain pure and well-crystallized LiFePO4. Furthermore, no obvious impurity diffraction peaks appear on the patterns of samples “c” and “d”, implying that LiFePO4 is thermodynamically stable at 800°C in CO2. As a result, it could be concluded that at least 6h calcination time is required in terms of the XRD patterns obtained in this work. 3.3 Morphological Analysis. The FESEM images of Fe1.5P, Li2CO3 and H3PO4 mixture calcined at 800°C in CO2 for 6h (sample “b”) and 10h (sample “d”) were shown in Figure 7(a) and 7(b), respectively. The particle sizes of sample “b” distribute more uniformly than that of sample “d”. With the calcination time increasing, the particle size increases and the aggregation forms, that results in the decrease of the specific surface area and the increase of Li+ diffusion path, which has negative effects on the electrochemical 11



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performance of the electrode3, 14-17. (a)

(b)

(c)

(d)

Figure 7. SEM images of as-synthesized LiFePO4: (a) calcined at 800°C for 6h in CO2, (b) calcined at 800°C for 10h in CO2, (c) high-magnification SEM image of the sample calcined at 800°C for 6h in CO2, (d) high-magnification SEM image of LiFePO4/C composite.

The high-magnification SEM image of sample “b” was shown in Figure 7(c). The particle size of LiFePO4 crystallites with polygonal form ranges from 500 to 800nm. It is known that LiFePO4 exhibits the defects of low electronic conductivity and low diffusivity of lithium ions, which could be improved by coating LiFePO4 with carbon18-23. In order to observe the morphology of the as-synthesized LiFePO4 coated with carbon, sample “b” was further mixed with glucose to prepare LiFePO4/C composite as described in experimental, which carbon content was determined as 5.3wt%. The high-magnification SEM image of the as-prepared LiFePO4/C was shown in Figure 7(d), which exhibits two different forms, the regular spherical particles with smooth surface and the cotton-like substances. The spherical particles correspond to LiFePO4 grains, which are obviously smaller (300 to 600 nm) than that of the uncoated samples, because the coated carbon can form a layer of carbon 12


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network on the surface of LiFePO4 particles, which limits the growth of the LiFePO4 crystallites9. The cotton-like substances on the surface of or among the spherical particles were carbon6,

10

, which are beneficial for improving conductivity and

enhancing the electrochemical performances of LiFePO4 by increasing the active sites on the surface. 3.4 Electrochemical Performance. The galvanostatic charge/discharge curve and cycle performance of the as-prepared LiFePO4/C with 5.3wt% carbon in the voltage range of 2.4–4.2V at 0.1C current rate were shown in Figure 8, which exhibits a steady charge plateau at ~3.45V and a discharge plateau at ~3.43V, corresponding to a two-phase reaction of lithium ions extraction and insertion between LiFePO4/FePO4 redox couple, respectively. The voltage difference between the charge and discharge plateau was 0.02V, indicating that the resistance is relative small, which is further confirmed by EIS analysis later. The initial discharge capacity of the synthesized LiFePO4/C is 150.6mAh/g, while the Coulombic efficiency expressed by the ratio of discharge specific capacity to charge specific capacity was 98.7%. After 15 cycles, the discharge capacity still remained 146.6mAh/g, about 97.3% of the initial capacity, which exhibits high charge and discharge capacity and good electrochemical reaction reversibility.

4.4 4.2

(a)

4.0 3.8

Voltage(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 0

20

40

60

80

100

120

140

Capacity(mAh/g)

13


160


(b)

200

Discharge capacity (mAh/g)

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150

100

50 2

4

6

8

10

12

14

Cycle number

Figure 8. Galvanostatic charge/discharge properties of as-prepared LiFePO4/C composite at 0.1 C current rate (a) first charge/discharge cycle (b) cycle performance.

The cyclic voltammogram of the as-prepared LiFePO4/C with 5.3wt% carbon recorded with a scan rate of 0.1 mV/s was shown in Figure 9, where a single pair of redox peaks exist, indicating the two-phase reaction between LiFePO4/FePO4, and the high purity of the LiFePO4/C sample. The oxidization and reduction peaks are corresponding to the lithium ion extraction from LiFePO4 and insertion into FePO4, which potentials are ~3.53V and ~3.38V, respectively. The separation of the redox peak potentials is 0.15V, indicating the small polarization level in the electrode reaction process. Furthermore, the redox peaks are nearly symmetric, implying the good cyclic reversibility of the as-prepared LiFePO4/C.

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-4

2.0x10

3.53V -4

1.5x10

-4

1.0x10

-5

5.0x10

Current/A

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0.0 -5

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-4

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3.38V -4

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2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Potential/ V

Figure 9. Cyclic voltammogram of as-prepared LiFePO4/C.

EIS measurement was carried out under the open circuit potential of 3.4V during charging to investigate the electrode reaction process of the as-prepared LiFePO4/C composite with 5.3wt% carbon. The Nyquist plot of the LiFePO4/C composite with the equivalent circuit applied for EIS data modeling were presented in Figure 10. The curve is consisted of a depressed semicircle in high-to-medium frequency region and an oblique line in low frequency region, which denote the electronic and the ionic transport process, respectively. The depressed semicircle is generally considered as the electrolyte film and charge transfer impedances, and the intercept on the abscissa axis in the high-frequency region is associated with the ohmic resistance of the electrolyte. The oblique line in low frequency region corresponds to Warburg impedance, which is related to the semi-infinite diffusion of lithium ions in electrode. The EIS curve was well fitted by the R(Q(RW))(QR) equivalent circuit model using ZSimpWin software. Considering the difference between the electric double layer impedance behavior of the solid electrode and the equivalent capacitance impedance behavior, the constant phase element Q is adopted to substitute for capacitance in consideration of the nonhomogeneity such as porosity, roughness, and geometry in the system. In the equivalent circuit, Rs, Rct, W, Rf, Qd and Qf denote the solution 15



resistance, the charge transfer resistance, the Warburg impedance, the solid electrolyte film resistance, the constant phase element of the electrolyte film/electrode interface and the constant phase element of the ﬁlm, respectively. The simulated results are as follows: Rs= 1.658Ω, Rct= 197.2Ω, W= 0.002745Ω, Rf = 61.57Ω, Qd= 0.00389µF/cm2, Qf= 8.841×10-6µF/cm2 ， indicating that the as-prepared LiFePO4/C composite possesses a relatively low impedance.

Qd

400

Qf

350

Rs

300

W

Rct

Rf

250

R(Q(RW))(QR) -Z''/Ω

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200 150 100

Experimental Fitting curve

50 0 0

100

200

300

400

500

Z'/Ω

Figure 10. EIS plot and corresponding equivalent circuit model of the LiFePO4/C composite.

4. Conclusions LiFePO4 was successfully synthesized by using Fe-P waste slag as the starting material via a one-step synthesis route, in which the mixture of Fe1.5P, Li2CO3 and H3PO4 in appropriate molar ratio was calcined at 800°C for 6h in CO2. The mechanism of the chemical reaction was investigated by TG/DSC and XRD analysis. It was confirmed that CO2 is able to oxidize P of valence -3 in Fe1.5P into P of valence +5 in PO43-. At the same time, CO2 is reduced to CO, which provides the reductive gas atmosphere to prevent Fe2+ from being oxidized into Fe3+. The as-synthesized LiFePO4 was further carbon coated with glucose to prepare LiFePO4/C composite, which was applied as the cathode materials in a coin cell to investigate the electrochemical performances by charge-discharge tests, cyclic voltammetric experiments and electrochemical impedance spectroscopic measurements. The 16


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as-prepared LiFePO4/C composite with 5.3wt% carbon content exhibits high charge-discharge capacity, low impedance and good cyclic characteristics. A novel and environmental-friendly synthesis route is put forward in this work, in which the industrial waste slag Fe-P alloy and the greenhouse gas CO2 are used to produce the energy materials LiFePO4. Furthermore, not only the synthesis route is simplified, but also the oxygen consumption in the traditional oxidation synthesis method is avoided. Consequently, the manufacture cost is greatly reduced, which provide a new perspective of LiFePO4 preparation for the industrial applications. Acknowledgements This work is financially supported by the National Science Foundation of China (Grant No. 21206099). Help from the Analytical & Testing Center of Sichuan University is greatly acknowledged. References [1] Tarascon, J. –M.; Armand, M. Issues and challenges facing rechargeable lithium batteries Nature. 2001, 414, 359. [2] Armand, M.; Tarascon, J. –M. Building better batteries. Nature 2008, 451, 652. [3] Zhang, C. H.; Liang, Y. Z.; Yao, L.; Qiu, Y. P. Effect of thermal treatment on the properties of electrospun LiFePO4–carbon nanofiber composite cathode materials for lithium-ion batteries. J. Alloys Compd. 2015, 627, 91. [4] Whittingham, M. S. Lithium batteries and cathode materials, Chem. Rev. 2004, 104, 4271. [5] Hassoun, J.; Croce, F.; Hong, I.; Scrosati, B. Lithium-ion battery: Fe2O3 anode versus LiFePO4 cathode. Electrochem. Commun. 2011, 13, 228. [6] Mi, C. H.; Cao, G. S.; Zhao, X. B. Low-cost, one-step process for synthesis of carbon-coated LiFePO4 cathode. Mater. Lett. 2005, 59, 27. [7] Wu, Z. J.; Jiang, B. F.; Liu, W. M.; Cao, F. B.; Wu, X. R.; Li, L. S. Selective Recovery of Valuable Components from Converter Steel Slag for Preparing. Multidoped FePO4. Ind. Eng. Chem. Res. 2011, 50, 13778. [8] Su, Y.; Li, G. B.; Xia, J. P. Kinetic Study of Fe Removal from Precipitated Silica Prepared from Yellow Phosphorus Slag. Can. J. Chem. Eng. 2009, 87, 610. 17



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[9] Kang, H. C.; Wang, G. X.; Guo, H. Y.; Chen, M.; Luo, C. H.; Yan, K. P. Facile synthesis and electrochemical performance of LiFePO4/C composites using Fe−P waste slag. Ind. Eng. Chem. Res. 2012, 51, 7923. [10] Wang, G. X.; Liu, R.; Chen, M.; Kang, H. C.; Li, X. L.; Yan, K. P. A novel synthesis of spherical LiFePO4/C composite using Fe1.5P and mixed lithium salts via oxygen permeation. Korean J. Chem. Eng. 2012, 29, 1094. [11] Dean, John. A. Lange's Handbook of Chemistry (15th Ed.); McGraw-Hill: New York, 1999. [12] Wang, Z. G.; Jiang, M.; Ning, P.; Xie G. Thermodynamic Modeling and Gaseous Pollution Prediction of the Yellow Phosphorus Production. Ind. Eng. Chem. Res. 2011, 50, 12194. [13] Kang, C. S.; Kim, C.; Kim, J. E.; Lim, J. H.; Son, J. T. New observation of morphology of Li[Fe1-xMnx]PO4 nano-fibers (x =0, 0.1, 0.3) as a cathode for lithium secondary batteries by electrospinning process. J. Phys. Chem. Solids 2013, 74, 536. [14] Liu, S. X.; Wang, H. B.; Yin, H. B.; Wang, H.; He, J. C. Effect of carbon source on the morphology and electrochemical performances of LiFePO4/C nanocomposites. J. Nanosci. Nanotechnol. 2014, 14, 2408. [15] Gibot, P.; Casas-Cabanas, M.; Laffont, L.; Levasseur, S.; Carlach, P.; Hamelet, S.; Tarascon,

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insertion/extraction in nanoscale LixFePO4. Nat Mater. 2008, 7, 741. [16] Kang, S. H.; Kim, B. R.; Kim, C.; Park, T. J.; Son, J. T. Electrochemical and morphologic studies of spherical LiFePO4/nanostructured LiFePO4 fibers composite by solid-state blending. Ceram. Int. 2015, 41, 1963. [17] Cheng, F.; Li, D.; Lu, A. H.; Li, W. C. Controllable synthesis of high loading LiFePO4/C nanocomposites using bimodal mesoporous carbon as support for high power Li-ion battery cathodes. J. Energ. Chem. 2013, 22, 907. [18] Fathollahi, F.; Javanbakht, M.; Omidvar, H.; Ghaemi, M. Improved electrochemical properties of LiFePO4/graphene cathode nanocomposite prepared by one-step hydrothermal method. J. Alloys Compd. 2015, 627, 146. [19] Delacourt, C.; Laffont, L.; Bouchet, R.; Wurm, C.; Leriche, J. B.; Morcrette, M.; 18


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Tarascon, J. M.; Masquelier, C. Toward understanding of electrical limitations (electronic, ionic) in LiMPO4 (M = Fe, Mn) electrode materials. J. Electrochem. Soc. 2005, 152, A913. [20] Oh, S. W.; Myung, S. T.; Oh, S. M.; Yoon, C. S.; Amine, K.; Sun, Y. K. Polyvinylpyrrolidone-assisted synthesis of microscale C-LiFePO4 with high tap density as positive electrode materials for lithium batteries. Electrochim. Acta 2010, 55, 1193. [21] Kim, J. K.; Choi, J. W.; Cheruvally, G.; Kim, J. U.; Ahn, J. H.; Cho, G. B.; Kim, K. W.; Ahn, H. J. A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries. Mater. Lett. 2007, 61, 3822. [22] Li, Gen.; Wu, Pengcheng.; Luo, Chunhui.; Cui, Qian.; Wang, Guixin.; Yan Kangping. Mass production of LiFePO4/C energy materials using Fe–P waste slag. J. Energ. Chem. 2015, 21, 375. [23] Wang, G. X.; Yang, L.; Bewlay, S. L.; Chen, Y.; Liu, H. K.; Ahn, J. H. Electrochemical properties of carbon coated LiFePO4 cathode materials. J. Power Sources 2005, 146, 521.

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