Characterization of LiFePO₄ Nanostructures Synthesized by Solvothermal Method

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Journal of Science & Technology 118 (2017) 045-050

Characterization of LiFePO₄Nanostructures Synthesized

by Solvothermal Method

Nguyen Thi My Anh^1*, Doan Luong Vu¹, Nguyen Thai Hoa¹, Le My Loan Phung¹,

Nguyen Ba Tai¹, La Thi Hang², Nguyen Ngoc Trung³, Nguyen Nhi Tru¹

¹Ho Chi Minh City University Of Technology - 268 Ly Thuong Kiet Str., District 10, Ho Chi Minh City

²Graduate University of Science & Technology – VAST

³Hanoi University of Science and Technology – No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam

Received: September 16, 2016; accepted: June 9, 2017

Abstract

In this work, we have synthesized LiFePO₄particles with the size around 200 nm by solvothermal method.

The crystalline LiFePO₄was synthesized from LiOH.H₂O, FeSO₄.7H₂O and H₃PO₄precursors, using

ethylene glycol and water as solvents. Ascorbic acid was added to the solution to prevent oxidation of Fe²⁺

to Fe³⁺. The structure was characterized by X-ray diffraction (XRD) and scanning electron microscope

(SEM). Carbon black is determined to reduce the phase structure of Fe³⁺remaining in the LiFePO₄

composition into Fe²⁺after the annealing step. The LiFePO₄particles were also mixed with EC 600JD

carbon black and followed by the annealing at 550 ^oC for composite formation. Their electrochemical

properties were determined by cyclic voltammetry (CV) and galvanostatic cycling with potential limit

(GCPL).

Keywords: LiFePO₄, Li-ion battery, Solvothermal.

1. Introduction

LiFePO₄(LFP) is^*a promising cathode material

for lithium ion batteries because of its remarkable

electrochemical properties, such as its high

theoretical specific capacity (170 Ah/kg), long life

cycles, safety with environment, and low cost (Padhi

A.K. et al., 1997).

the length elongated of [010] prominent for the

lithium ion diffusion, that being of great advantage to

improve the rate capability performance.

In this study, we report the synthesis of LiFePO₄

crystalline size around 200nm by solvothermal

method, using LiOH.H₂O, FeSO₄.7H₂O, H₃PO₄

precursors in a mixture of ethylene glycol (EG) and

deionized water (DI water). Moreover, we also used

ascorbic acid as an agent for preventing the oxidation

of Fe²⁺to Fe³⁺.

However, LiFePO₄has very low electronic and

ionic conductivity at room temperature (~10^-9S/cm

and 10^-5S/cm, respectively) and has only one channel

of direction [010], which restricts its rate capability.

So, to make LiFePO₄a suitable cathode material for

lithium ion batteries, its electronic/ionic conductivity

must be increased and its length of [010] channels

must be also controlled shorter for facilitation of

lithium ion diffusion. To improve the electrochemical

properties, various methods have been used by the

researchers, ranging from the LiFePO₄particle size

control to carbon coating through LiFePO₄composite

synthesis or doping with cations superlative to Li⁺

(Nan et al., 2013; Padhi et al., 1997; Safronova et al.,

2012; Wang et al, 2012; Wu et al., 2012; Yang et al.,

2013; Zhou et al., 2012).

2. Materials and methodes

2.1. Synthesis procedure

The LiFePO₄nanocrystals were prepared by

solvothermal method in a Teflon-lined autoclave at

180^oC for 10 hours.

At first, solutions of FeSO₄-EG-DI and LiOH-

EG-DI was prepared respectively, by adding 0.004 M

FeSO₄.7H₂O and 0.01 M LiOH.H₂O in a mixture of

EG and DI water (3:2 volume ratio) under ultrasonic

dispersion for 20 minutes. Further, the FeSO₄-EG-DI

solution was added with 0.02 g ascorbic acid and

0.004 M H₃PO₄. Then the solution FeSO₄-EG-DI

with ascorbic acid, H₃PO₄and LiOH-EG-DI were

mixed in an autoclave.

The obtained LiFePO₄nanocrystals appeared

under different shapes, including: spindle, rod,

urchin, small-particle, cuboid and flower (Nan et al.,

2013). The LiFePO₄nanocrystals with rod shape have

The grayish green precipitates formed after

solvothermal treatment (Fig.1) were filtered and

repeatedly washed with DI water and ethanol for

several times. The product was then dried in a

vacuum furnace at 80^oC for 2hours before being

^*Corresponding author: Tel.: (+84) 938. 920.815

Email: myanhnguyen@hcmut.edu.vn

Journal of Science & Technology 118 (2017) 045-050

heated to 550^oC for 5 hours in argon atmosphere with

2.2. Characterization

the heating rate of 4^oC/min.

The LiFePO₄crystalline structure, phase purity

and size of the particles were characterized by using a

Rigaku/max 2500Pc X-ray diffractometer (XRD)

with Cu-Kα radiation (λ=1.5418 Å)._.The temperature

and time of the crystallization of this compound were

revealed by thermal analysis - TG/TDA curves. The

morphology of the sample was determined by a LEO

-1530 field - emission scanning electron microscopy

(SEM).

(a)

(b)

For electrochemical characterization, the cyclic

voltammetry measurements (CVs) were first

performed for the synthesized electrode material at

Fig. 1. Images of the compound before (a) and after

solvothermal treatment (b).

room temperature with

Potentio/Galvanostat

Autolab 30 (MetrOhm AG) using a three electrode

system. The LiFePO₄powder was mixed with

acetylene black and polytetrafluoroethylene (PTFE)

(weight ratio 80:10:10), pasted on the aluminium foil

and cut into pellets. The cell consisted of a working

electrode (WE), a Pt wire as a counter electrode (CE)

and a reference electrode (RE). The reference

electrode consists of a silver wire immersed in 0.1

mol.l^-1tetrabutyl ammonium perchlorate (TBAP)

solution dissolved in acetonitrile within 10 mmoll^-1

AgNO₃. The potential of this reference electrode is

0.548 V versus a standard hydrogen electrode (SHE).

The electrolyte is a 1 M solution of lithium

hexafluorophosphate (LiPF₆) in a solvent mixture of

ethylene carbonate and dimethyl carbonate (EC-

DMC) (50:50 volume ratio). The measurement was

carried out in a potential window of 2.5 – 4.5 V with

a scan rate of about 50 μV.s^-1.

(a)

A charge/discharge cycling test for Swagelok-

type battery was carried out in liquid electrolyte at

room temperature. Cathodic paste was prepared by

mixing the LiFePO₄powder with carbon black and

PTFE emulsion in the weight ratio of 80:10:10. This

paste was then rolled down to 0.1 mm thickness, cut

into pellets of 10 mm diameter and dried 130^oC under

a vacuum. Typical active material masses used were

15 – 20 mg.cm^-2. The electrolyte was a 1 M solution

of LiPF₆in EC-DMC 1:2 (Merk Co.), negative

electrodes were 200 µm thick lithium foil (Metel

Ges., Germany). Cells were assembled in a glove box

under argon atmosphere with <2 ppm H₂O.

Electrochemical studies were carried out using a

MacPile Controller (Bio-Logic, France) in the

potential window 2.8 – 4.2 V versus Li/Li⁺in the

galvanostatic mode at the C/10 regime.

(b)

3. Results and discussion

Fig. 2. XRD patterns of olivine LiFePO₄prepared

using ascorbic acid (a) and without ascorbic (b)

3.1. Effect of ascorbic acid on the LiFePO₄

crystalline structure

Figure 2 shows X-ray diffraction patterns of two

LiFePO₄samples after solvothermal treatment with

Journal of Science & Technology 118 (2017) 045-050

and without using ascorbic acid. The comparison with

3.2. Effect of carbon on the LiFePO₄crystalline

structure

published spectra of Li, Fe, P and O reveal that both

XRD patterns of the as-prepared LiFePO₄samples are

indexed to be the orthorhombic olivine-type LiFePO₄

(space group Pnma, JCPDS 96-400-1849) with its

characteristic main peaks at the diffraction angles 2θ

= 36^o, 30^o, 26^o, 15^ocorrespondent with the crystal

planes of {311};{211},{202};{111}{200}.

In order to determine the electrochemical

properties of materials, we mixed LiFePO₄

synthesized with using ascorbic acid with carbon

black. The carbon black using here is the type of

Ketjen EC 600 JD (K-carbon) due to its high

conductivity. The LiFePO₄/K-carbon composite was

annealed at the temperature of 550^oC for 5 hours in

argon atmosphere to form a connection between the

carbon and LiFePO₄particles.

However, the LiFePO₄sample without ascorbic

acid contains not much olivine material LiFePO₄

phase due to the very low intensities of the

characteristic peaks. In the XRD pattern,

Fe₃(PO₄)₂.3H₂O phase was clearly observed at the

remarkable position peaks at 17^o, 27^oand 28^o. There

is also the presence of Fe₂O₃phase at 2θ = 24^o, 33^o,

35^oand FeO₂phase at the characterictic peak of 21^o.

In this case, the sample is partially oxidized. The

LiFePO₄sample with ascorbic acid is clearly

identified and the typicalolivine structure of LiFePO₄

is indicated by the strong and intense peaks.

Meanwhile, the Fe₃(PO₄)₂.3H₂O phase was not

observed in this pattern. The presence of FeO₂and

Fe₂O₃structures were not clearly observed due to very

weak intense peaks. The grain size (D) of single

phase LiFePO₄was calculated by the Scherrer

formula with β cosθ = kλ/D, where β is the full-width-

at-half-maximum length of the diffraction peak on a

2θ scale and k is a constant here close to unit. The

calculated mean value of D was approximately 200

nm.

The XRD diagrams of the obtained samples

with and without K-carbon were shown in Fig. 3: the

one without K-carbon before heating and the other

with K-carbon followed a heating at 550^oC. The

XRD results indicate that the LiFePO₄sample with

K-carbon after the heating gives the only crystalline

phase of an olivine structure (JCPDS 96-210-0917);

meanwhile, the sample without K-carbon before the

heating beside a main olivine structure, shows a

signal of two crystalline phases of Fe₂O₃with the

main peaks at 24^o, 29^o, 33^o, 35^oand FeO₂at 21^o, 37⁰

with weak intensity despite of their unclear presence.

3.3. Effect of acid ascorbic reduce Fe³⁺

2Fe³⁺+ C₆H₈O₆=> 2Fe²⁺+2H⁺+C₆H₆O₆(Eq.1)

From Fig. 2 and (Eq.1) analysis show that using

acid ascorbic as an agent in solvents to remove

impurity phases. Moveover, it supported control pH

between 3.0 and 5.0 which oxidation stage from Fe²⁺

to Fe³⁺was significantly reduced. Actually, by

volumetric titration method detetimined the

percentage of Fe²⁺compare to without acid ascorbic.

The rults displayed that only 3-8% of Fe³⁺contents in

solvents before solvothermal.

3.4. Effect of Thermal on the LiFePO₄phase

Thermal gravimetric analysis (TGA) was used to

determine thermal stability of LiFePO₄phase. TGA

plot illustrated that Less than 20 % of weight loss was

observed in temperature range of 80 – 220^oC. The

first weight loss (10%) started at 70^oC related to the

residual water molecules in the composite structure.

At around 200^oC, most residual water was totally

released. The second weight loss (~7%) started at

190^oC could be

phosphonate excess

a decomposition of organo-

Between 550 and 800^oC, a negligible weight

loss was observed, which thereby indicated the small

amount of carbon free non-bonded to the LiFePO₄

particle. At 860^oC, the oxygen loss from [PO₄] group

can be occurred with negligible amount. Hence,

LiFePO₄composite can be annealed at 500 – 750^oC

to obtain well – crystallized phase(Fig. 4).

Fig. 3. XRD patterns of olivine LiFePO₄prepared

without carbon black Ketjen EC 600 JD (K-carbon)

by solvothermal method and with K-carbon followed

an annealing of 550^oC

Journal of Science & Technology 118 (2017) 045-050

The free carbon of ogarnic agent content in the

composite material was difined aproximately 2-3%(

over 900^oC)

3.5. SEM image analysis

The morphology, size and shape of the LiFePO₄

particles after solvothermal treatment were examined

by SEM as shown in Fig. 5. The image reveals that

the crystalline sizes have an elongated rod like shape

with a size about 200 nm.

3.6. Electrochemical characterization

The electrochemical properties for the

synthesized electrode materials were characterized by

cyclic voltammetry measurements (CVs).

The CV characterization was performed in

nonaqueous 1M LiPF₆/EC:DMC (2:1) solution in the

range 2.5 – 4.2 V. Fig. 6 shows two oxidation-

reduction peaks symmetric at 3.4 – 3.5 V (vs Li⁺/Li)

that confirms the reversible intercalation of Li⁺ions

into the host LiFePO₄, corresponding to the redox

reaction Fe(II)  Fe(III). The redox reaction

Fe³⁺/Fe²⁺releases Li⁺ion from LiFePO₄by the

folowing reaction:

Fig. 4. Thermal analysis of LiFePO₄composite

conducted at heating rate of 5 ^oC/min.

LiFe(II)Fe(III)PO₄ Li⁺+ e ^-+ Fe(III)PO₄.

It was observed from Figure 5 that the first cycle

of CV curve is irreversible, which presents two

oxidation-reduction peaks unsymmetrical. This seems

to be due to the unstable system for first second of

measurement

1^st

2^nd

10^th

50^th

-20

-40

-60

2.5

3.5

E (V) vs Li⁺/Li

Fig. 6. Cyclic voltammetry measurements (CVs) of

LiFePO₄/K-carbon composite for 50 cycles at scan

rate 50 μV.s^-1.

The charge/discharge characteristics of the

cathode material LiFePO₄/K-carbon were determined

by cycling test in the potential range 2.5 – 4.2 V

versus Li/Li⁺and in galvanostatic mode in the C/10

regime. The discharge specific capacity Q_sin Fig. 6

was estimated by the formula:

Fig. 5. SEM images of crystalline LiFePO₄obtained

(a) after solvothermal treatment and (b) by heating

with K-carbon at 550^oC for 5 hours in argon

atmosphere.

Journal of Science & Technology 118 (2017) 045-050

explained that the prepared LiFePO₄sample is not a

1000

26802

[2]

Q_S

i.dt  



totally crystalline structure and a partially amorphous

phase is still occurred. In addition, the conductivity of

LiFePO₄/K-carbon material owing to a connection

between LiFePO₄and K-carbon is also affected on

the electrode material capacity. The SEM image in

Figure 4 clearly showed the weak connection

between LiFePO₄material and K-carbon. In fact, the

electrochemical properties of the electrode depend on

several factors, such as the specific surface area, the

material conductivity and the adsorption capacity,

which determined the electron transfer process.

where M is the molar mass of LiFePO₄(157.75 g.mol^-

) , x is the number of intercalated Li⁺ions per

formula (number of transferred electrons per

intercalated ion) and 26802 is the Faraday number in

mAh.

4. Conclusion

By solvothermal method, we have successfully

synthesized the LiFePO₄crystal phase in the form of

elongated rod like shape with the crystalline size

around 200 nm, using ascorbic acid and K-carbon

like reduction agents preventing the oxidation of Fe²⁺

to Fe³⁺.

Cathode materials based on the LiFePO₄/K-

carbon composite was used for rechargeable cell

assembly. This LiFePO₄/K-carbon material exhibits

good cyclability, alongside with low discharge

capacity, which is due to insufficient crystallinity of

LiFePO₄phase and poor connection between

LiFePO₄and K-carbon, limiting its conductivity.

Acknowledgements

This work was financially supported by Ho Chi

Minh City University of Technology and Vietnam

National University – Ho Chi Minh through the

Science and Technology Funds granted for T-CNVL-

2015-08 and C2015-20-25 projects respectively.

References

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Qing, Li Yadong, 2013. Size and shape control of

LiFePO₄nanocrystals for better lithium ion battery

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Fig. 7. (a) Initial fifty cycles of charge/discharge

performance and (b) Energy storage performance of

LiFePO₄/K- Carbon at C/10 rate between 2.5 and 4.2

V (vs. Li⁺/Li).

[2] Padhi A.K., Nanjundaswamy K.S., Goodenough J.B.,

1997. Phospho-olivines

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For the first cycle of charge-discharge performance,

the measurement system is not yet stable related to

the asymmetric charge-discharge curve. For the

second cycle, this material could intercalate 0.30 ion

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(for one mole of the material), corresponding to the

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[4] Wang Yan, Feng Zhesheng, Chen Jinju, Zhang

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