C cathode material

Solid State Ionics 181 (2010) 1757–1763

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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i

Investigation on the microwave-derived LiFePO4/C cathode material Xiangfeng Guo, Yuting Zhang, Hui Zhan ⁎, Yunhong Zhou Department of Chemistry, Wuhan University, Wuhan 430072, China

a r t i c l e

i n f o

Article history: Received 22 March 2010 Received in revised form 11 August 2010 Accepted 7 October 2010 Keywords: Lithium iron phosphate Microwave method Cyclic voltammetry Ferric impurity Calcination

a b s t r a c t A facile preparation of the LiFePO4/C composite is achieved through the microwave irradiation method in several minutes. Electrochemical measurements were conducted to examine the evolution of the impurity phase during the microwave heating. It is found that a short microwave heating usually leads to the existence of an amorphous, ferric intermediate, while microwave firing longer than 3 min favors the formation of the Li3Fe2(PO4)3 impurity. A further calcination treatment on the microwave-derived sample has been proved to be an effective way to eliminate the ferric impurity and thus-obtained, phase-pure sample can release a capacity of 160 mAhg− 1. © 2010 Elsevier B.V. All rights reserved.

1. Introduction LiFePO4 has attracted much attention as a potential cathode material for next-generation lithium-ion batteries since being proposed in 1997 because of its low cost, environmental benign, and superior thermal stability [1–3]. Although the intrinsic low electronic conductivity and slow Li+ diffusion ever seem to be great obstacles for its application [4,5], efforts aiming to improve the conductivity, such as: carbon coating, or creation of a conductive network lead to a significant increase of its electrochemical performance [2,3,6-8]. Besides these, many “soft” preparation methods, including sol–gel synthesis [9,10], template technology [11], have been developed to synthesize the ultra fine LiFePO4/C powder, but all the strategies require a heating at a high temperature in an inert or reductive atmosphere for several hours. Microwave processing has also been applied in the preparation of many functional materials [12,13], and there are many successful examples to obtain LiFePO4/C by microwave heating [14–18]. During this procedure, LiFePO4 can be obtained by microwave heating in just a few minutes without the inert gas flow, which means that the energy consumption can be greatly reduced and the whole preparation can be significantly facilitated. However, the purity of the final products cannot be well guaranteed through this facile way. Some Fe3+-containing species always coexist with LiFePO4 [14,19–21], and its amorphous state or low concentration make it difficult to be detected by X-ray diffraction (XRD) or other traditional characterization methods. Although the existence of the impurity does not badly affect the capacity release of LiFePO4 when a lower cutoff voltage of 2 V is adopted, it really results in the

⁎ Corresponding author. Tel.: + 86 27 68756931; fax: + 86 27 68754067. E-mail address: [email protected] (H. Zhan). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.10.008

decrease in the amount of Li+ that can be de-inserted in the first charging in a practical LiFePO4/graphite cell and thus the capacity shrinks accordingly. Thus, more insight of the microwave preparation is necessary in order to achieve the optimum performance. In this paper, we prepared LiFePO4/C composites by the microwave method from FePO4∙4H2O. The evolution of the ferric impurities and the variation of the electrochemical behavior with the microwave heating time are characterized by a simple electrochemical method, and a postcalcination treatment is proposed to eliminate the Fe3+ impurities. 2. Experiment Preparation of the LiFePO4/C composite was started from the ballmilled mixture of FePO4·4H2O (CP, Shanghai Chemical Reagent Company, SCRC), LiOH·H2O (AR, SCRC) and glucose (AR, SCRC) which acted as both the carbon source and the reducing agent (the amount of glucose slightly exceeds that is required for a complete reduction of Fe3+, and the weight analysis indicates that the carbon content in the final LiFePO4/C sample varies from 4.9% to 5.1% according to different heating time). In each microwave heating run, about 5.0 g mixture was put into the self-designed container which was surrounded by the graphite microwave-absorber, and then microwave-heated for n (n = 2–7) minutes in a household microwave oven (rated power output: 700 W, frequency: 2.45 GHz, rotation rate: 5 rpm). In the following, the sample will be named as “mvn” according to the heating time. A further calcination at 650 °C for 1 h in Ar/H2 was conducted on the “mv3” and “mv5” samples and the resulting materials were named as “mv3 + 650” and “mv5 + 650”, respectively. XRD patterns were collected on a Shimadzu XRD6000 diffractometer with Cu Kα radiation (Ni-filter and graphite monochrometer). Particle morphologies were observed on a scanning electron microscope

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(Quanta 200). Fourier transform infrared (FTIR) absorption spectrum was recorded with a Nicolet AVATAR360 spectrometer in a wavenumber range of 400–1400 cm− 1 at a spectral resolution of 4 cm− 1. The samples were ground into fine powder and then dispersed onto a KBr pellet in a proportion of 1:200. The cycling test was performed on 2016 coin cells at room temperature. The cathode was made by mixing 75 wt.% active powder, 20 wt.% acetylene black and 5 wt.% PTFE binder, then pressing the mixture onto an Al screen. Metallic lithium was used as the anode and the electrolyte was 1 M LiClO4 dissolved in a 1:1 volume ratio solution of ethylene carbonate (EC) and dimethyl

carbonate (DMC). The cells were tested on the LAND cycler (Wuhan Kingnuo Electronic Company, China). The cyclic voltammetry measurements were conducted using an Autolab PGSTAT302 electrochemical workstation on the three-electrode cell, in which lithium foil worked as both the counter electrode and the reference electrode. 3. Results and discussion SEM images are shown in Fig. 1. The micrographs of the products clearly indicate that longer microwave heating leads to a larger particle size. When the irradiation time is prolonged from 2 to 7 min,

Fig. 1. SEM images of the LiFePO4/C composite synthesized by microwave heating.

X. Guo et al. / Solid State Ionics 181 (2010) 1757–1763

1759

160

mv7

mv6 120

mv5 100

mv2 mv3 mv4 mv5 mv6 mv7

Cutoff Voltage:2.5-4.0V Current: 17mA/g

80

Intensity

Capacity/mAhg-1

140

60

mv4

mv3

40 5

0

10

15

20

25

30

35

40

45

mv2

50

Cycle Number 15

Fig. 2. Cycleability of the microwave-derived samples at 0.1C between 2.5 and 4.0 V vs. Li+/Li.

20

25

30

35

40

45

50

2 Theta Degrees Fig. 4. XRD patterns of “mvn”(n = 2–7 min) samples.

the average particle size increases from 200–400 nm to more than 1.2 um. In addition, it is also observed that even after 7 min irradiation, there are still a few fine crumbs with a diameter around 400–600 nm adhering to the surface of large particles. Obviously, besides the growth of the particle size, longer microwave heating will result to the more nonuniform particle size distribution, and both factors have a negative impact on the electrochemical performance of the LiFePO4/C material. The cycling tests are done at the C/10 rate(17 mAg− 1) in a potential range of 2.5–4.0 V at room temperature. Fig. 2 shows the cycleability of the resulting samples. We find that they all present an excellent capacity retentivity within 50 cycles. Among the samples studied, a stable discharge capacity about 150 mAhg–1 can be delivered by the samples of “mv2”–“mv5”. Longer heating time does not greatly affect the cycling stability, whereas the available capacity dramatically decreases to 130 mAhg–1 for “mv6” and 110 mAhg–1 for “mv7”. The difference in the capacity performance is reasonable if the particle morphology is considered. SEM images shown in Fig. 1 clearly indicate that longer microwave heating leads to particle agglomeration and it becomes quite serious in the “mv6” and “mv7” samples. Fig. 3 shows the rate performance of the resulting samples. A tendency can be clearly revealed as that longer microwave heating leads to poorer rate performance. If we use C2c/Cc/2 (C = discharging

160 140

Capacity/mAhg-1

120 100 80

mv2 mv3 mv4 mv5 mv6 mv7

60 40

Voltage: 2.5-4.0V -1

1C=170mAhg

20 0

0.1C

0.5C

1.0C

Cycle Number Fig. 3. Rate performance of the resulting samples.

2.0C

capacity) as the index of the rate capability, it decreases from 85.7% for the mv3 sample to 78.7% for the mv7 sample. This tendency can be mainly explained by the variation in the particle size, However, the mv2 sample is an exception, it has the smallest particle size but the poorest rate performance. In our previous research [22], it has been pointed out that although microwave processing greatly shortens the preparation time comparing with the traditional solid-state route, it also leads to a smaller amount of sp2 carbon, which in turn negatively influences the electro-conductibility. Accordingly, the poor rate performance of the mv2 sample can be rationalized. Fig. 4 shows the XRD patterns of our LiFePO4/C samples. No obvious proof for the existence of impurities can be found. All the XRD patterns show a main phase with an olivine structure indexed to the orthorhombic Pnma space group (JCPDS card no. 83-2092). The lattice parameters of the samples are determined by the Rietveld method, and the results are listed in Table 1. The cell volumes are almost same for all the resulting samples, and no definite relationship between microwave heating time and cell volume can be found. But considering the amorphous state or the low concentration of the impurity phase, the absence of an extra diffraction peak doesn't necessarily mean that all of our samples are impurity free. In fact, as being pointed out by S.W. Song [23] and M. Maccario [19], some Fe3+-containing compounds are really difficult to be detected by XRD because of their amorphous structure. To get more information about our product, FTIR measurements were conducted and the result is shown in Fig. 5. Comparing with previous reports [24–26], no extra band other than the vibrational mode of LiFePO4 can be found. Also, this fact cannot guarantee the obtaining of phase-pure LiFePO4/C, because the low concentration of the impurity phase may put difficulties in its identification. Traditional structural characterization seems inadequate to identify the impurity, however electrochemical measurement may be helpful as many ferric species such as FePO4 or Li3Fe2(PO4)3 really show some reactivity of

Table 1 The lattice parameters of the LiFePO4/C composite synthesized by microwave heating. Sample

a(Å)

b(Å)

c(Å)

V(Å3)

mv2 mv3 mv4 mv5 mv6 mv7

10.315(3) 10.325(3) 10.321(3) 10.320(4) 10.330(2) 10.326(4)

6.002(2) 6.002(2) 6.005(1) 6.004(2) 5.999(3) 6.002(2)

4.698(1) 4.696(1) 4.693(1) 4.691(2) 4.694(1) 4.694(1)

290.84 290.98 290.87 290.66 290.90 290.89

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1.2

1.0

0.8

Absorbance

Table 2 The voltage hysteresis between 50% SOC and 50% DOD.

mv2 mv5 mv7 Standard LiFePO4

1058 1096 964

Sample

mv2

mv3

mv4

mv5

mv6

mv7

Voltage hysteresis between 50%SOC and 50%DOD (V)

0.0347

0.0344

0.0338

0.0350

0.0407

0.0475

946 1138

0.6

0.4

638

549

469 503 579

648

0.2

0.0 400

500

600

700

800

900

1000

1100

1200

1300

Wavenumber/cm-1 Fig. 5. FTIR absorption spectra of mv2, mv5 and mv7 samples, and the blue line is the reference FTIR spectrum for the LiFePO4 sample obtained by the solid-state method. The peak intensities are normalized according to a peak intensity of 1058 cm− 1.

Li+ insertion. Therefore, a more detailed discussion will focus on the investigation of electrochemical behavior of the microwave-prepared samples and we try to reveal the evolution of the ferric impurity during the microwave heating. The charge/discharge curves of LiFePO4/C are shown in Fig. 6. All samples show a long and flat voltage plateau around 3.4 V, which represents a typical two-phase Li+ extraction/insertion reaction. With the longer microwave heating, the voltage hysteresis between charge and discharge curves becomes more serious. To better evaluate the voltage polarization, we use the voltage hysteresis between 50% SOC (state of charge) and 50% DOD (depth of discharge) to index the voltage polarization and the results are listed in Table 2. It is found that for the samples “mv2”, “mv3”, “mv4” and “mv5”, the hysteresis are all around 0.034 V While for the sample “mv6” and “mv7”, this value significantly increases to 0.041 V even 0.048 V, suggesting a great increase in the voltage polarization. For LiFePO4, particle size and the carbon coating are two of the determining factors for the electrochemical polarization. Usually, the bigger the particle is, the larger the voltage polarization will be. The data in Table 2 is in agreement with this rule. In addition to the dominant plateau around 3.4 V, another pair of charging/discharging plateaus around 2.8/2.65 V can be observed, though they are much shorter. According to the location of this pair of

plateaus, they should be associated with the redox reaction of the ferric impurity. Similar results have been reported previously [14,18,22,27,28]. K.S. Park [14] ascribed the charge/discharge plateau below 3.0 V to the Li3Fe2(PO4)3 phase which was formed as a byproduct; while K. Shiraishi [27] attributed the plateau at 2.5 V to the presence of α-Fe2O3, and X. Xia [28] argued that both FePO4 and α-Fe2O3 impurity contributed to the appearance of these plateaus. So, in order to clarify the origin of the plateau in this potential range, we particularly examine the electrochemical behavior of our microwave-derived LiFePO4/C samples below 3.0 V. Fig. 7 shows the cyclic voltammetry (CV) curves. The voltammograms of “mv2”, “mv4” and“mv7” samples are quite alike except a slight variation in the redox potential and a small change in the peak width. But it is noticed that these three samples all show weak though discernable redox peaks below 3 V, and the existence of these peaks is more clearly observed in the inset picture. To better understand the redox reaction occurring in the lower potential range, the CV tests were further conducted between 2.0 and 3.2 V for all the microwave-prepared samples. In this potential range, neither LiFePO4 nor Fe2O3 shows any electrochemical reactivity, because the typical redox reaction of the former LiFePO4 phase occurs near 3.4 V and the latter one cannot be electrochemically reduced above 2 V [29]. Fig. 8 compares the CV curves of our resulting samples. There are two possible ferric impurities FePO4 and Li3Fe2(PO4)3 combined with the derived samples. As to FePO4, only one pair of redox peaks can be observed at 2.85/3.2 V according to Hong's results [30]; while for the Li3Fe2(PO4)3 electrode, its voltammetry usually shows two pairs of redox peaks at 2.65/2.85 V and 2.83/3.05 V as being reported by Morcrette et al. [31]. In the CV curve of the sample “mv2” (Fig. 8.a), only one pair of broad though symmetrical redox peaks are found around 2.6 V, and their location matches neither that for FePO4 nor that for Li3Fe2(PO4)3. Carefully observing Fig. 8a–f, a tendency can be revealed that the redox peaks around 2.6 V tend to weaken with longer microwave-firing time. This tendency implies that the redox peaks in Fig. 8a may be caused by the reaction of a ferric intermediate and thus longer heating possibly leads to its transformation and its decreasing content. In fact, similar result has been mentioned in reference [20] and it was then explained by some -400 -10

4.0

-300

Current/mAg-1

Capacity/mAhg-1

-200 3.5

mv2 mv3 mv4 mv5 mv6 mv7

3.0

2.5

10 20

-100

30 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

0 100

mv2 mv4 mv7

200

Scan rate:0.1mv/s

300

-1

2.0

0

Current:17mAg 0

20

40

60

80

100

120

140

160

180

Voltage/V Fig. 6. 10th charge/discharge curves of microwave synthesized samples at 0.1C (17 mAg− 1) between 2.0 and 4.0 V vs. Li+/Li.

400

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Potential/V Fig. 7. Cyclic voltammograms of mv2 (black), mv4 (green) and mv7 (magenta) samples between 2.3 and 4.2 V vs. Li+/Li at 0.1 mVs− 1.

mv2 2.0 2.2 2.4 2.6 2.8 3.0 3.2

-10 -8 -6 -4 -2 0 2 4 6 8 10

b Current/mAg-1

a

mv3 2.0 2.2 2.4 2.6 2.8 3.0 3.2

-10 -8 -6 -4 -2 0 2 4 6 8 10

1761

-10 -8 -6 -4 -2 0 2 4 6 8 10

c

mv4 2.0 2.2 2.4 2.6 2.8 3.0 3.2

Potential/V

d

mv5

Current/mAg-1

Current/mAg-1

Potential/V

2.0 2.2 2.4 2.6 2.8 3.0 3.2

-10 -8 -6 -4 -2 0 2 4 6 8 10

Potential/V

e

mv6 2.0 2.2 2.4 2.6 2.8 3.0 3.2

Potential/V

Current/mAg-1

-10 -8 -6 -4 -2 0 2 4 6 8 10

Current/mAg-1

Current/mAg-1

X. Guo et al. / Solid State Ionics 181 (2010) 1757–1763

-10 -8 -6 -4 -2 0 2 4 6 8 10

f

mv7 2.0 2.2 2.4 2.6 2.8 3.0 3.2

Potential/V

Potential/V

Fig. 8. Cyclic voltammograms of microwave synthesized samples between 2.0 and 3.2 V vs. Li+/Li at 0.1 mVs− 1.

amorphous intermediate. From Fig. 8b to f, another tendency is noticed that, besides the change in the redox peak located near 2.6 V, another pair of redox peaks around 2.7/2.9 V grows gradually; even in Fig. 8d–f, the growths of these new peaks are so prominent that they become dominant in the CV profile. Basing on the location of these new peaks and the fact that a high temperature often favors the formation of Li3Fe2(PO4)3, we think that the newly-emerging redox peaks near 2.7/2.9 V should be assigned to the Li3Fe2(PO4)3 phase. Therefore, the evolution of the ferric impurity during the microwave heating can be summarized as follows: when a short microwave heating is adopted, such as that for samples “mv2”, “mv3” and “mv4”, a ferric intermediate is the major component of the impurity; with Table 3 The first charging/discharging efficiencies of the LiFePO4/C composite synthesized by microwave heating. Sample

mv2

mv3

mv4

mv5

mv6

mv7

Efficiency (%)

108.5

109.8

106.6

109.8

104.3

102.8

longer microwave heating, the temperature increases and another side reaction engendering the Li3Fe2(PO4)3 impurity occurs. Although either ferric intermediate or Li3Fe2(PO4)3 shows some discharging activity during a normal charge/discharge test between 2 and 4 V, its existence still should be avoided especially when the LiFePO4 cathode co-works with a non-lithium anode, such as graphite. In a practical LiFePO4/graphite cell, all the recyclable Li+ comes from the cathode. If the LiFePO4/C cathode contains some ferric impurity phase, the capacity of the whole cell will shrinks. In Table 3, the efficiencies in the first charging/discharging of all the resulting samples are compared. It is noticed that all the microwave-derived samples show an efficiency above 100%, so when they are used in combination with the graphite anode, capacity degradation is unavoidable because only the LiFePO4 phase can contribute to the capacity. In view of this consideration, we try to eliminate the impurities through a post-treatment. Sample “mv3” was further calcined at 650 °C for 1 h in H2/Ar flow and its SEM image is compared with that for “mv3” in Fig. 9. The post-treatment leads to a slight growth of the particle size, but the particle size of the post-treated LiFePO4/C still remains in the submicron range. In addition to the growth

Fig. 9. Comparison of the SEM images of “mv3” and “mv3 + 650” samples.

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mv3 mv3+650

4.0

-1

Current:17mAg

4.0

3.0

3.5 Voltage/V

Voltage/V

3.5

2.5

3.0 2.5 2.0 0

20

2.0

40

60

80

100 120 140 160 180

Capacity/mAhg

0

20

40

60

80

100

120

140

160

Capacity/mAhg-1 Fig. 10. The 10th charge/discharge curves of “mv3” samples before and after the posttreatment. The inset picture shows the 1st charge and discharge plots.

of the particle size, post-treatment also results to the change in electrochemical performance. Fig. 10 compares the charge/discharge profiles for the LiFePO4/C samples before and after that post-treatment, and some variation can be observed. First, the voltage hysteresis between charge and discharge becomes a little more significant with the adoption of post-heating, and this should be associated with the growth in the particle size. For the LiFePO4 phase, bigger particle means a longer

a

-8

-400 -4

Current/mAg-1

0

-200

4 8 2 .0 2 .2 2 .4 2 .6 2 .8 3 .0 3 .2

Li+ diffusion path, and it usually further causes the increased polarization. Secondly, in Fig. 10, it is found that, the plateaus below 3.0 V disappear after the post-treatment. Additionally, the first charge/ discharge plots of the “mv3” sample and the “mv3+ 650” sample are compared in the inset picture. We can see that the first charging capacity increases from 142 mAhg–1 for the “mv3” sample to about 170 mAhg–1 for the “mv3 + 650” sample and the efficiency in the first cycle reduces from 110% to 96% accordingly, while the discharging capacity of the “mv3 + 650” sample is well maintained at 164 mAhg–1. This result means that if the “mv3 + 650” sample instead of the “mv3” sample is used in a LiFePO4/graphite cell, the practical capacity can be increased by more than 10% (from less than 142 mAhg–1 to 164 mAhg–1), regardless of the irreversible capacity of the graphite anode. The effect of posttreatment is more clearly reflected in Fig. 11a. Unlike the “mv3” sample, in the CV curves of the post-treated “mv3 + 650” sample, no obvious current can be observed below 3 V. To test the effectiveness of this posttreatment in eliminating the Li3Fe2(PO4)3 impurity, we also recalcined the “mv5” sample under the same condition. The CV curve shown in Fig. 11b indicates that the ferric impurity such as Li3Fe2(PO4)3 can also be effectively removed from the “mv5” sample through the post-heating treatment. 4. Conclusion A facile preparation of the LiFePO4/C composite is achieved through the microwave irradiation method in several minutes, but the existence of the ferric impurity seems unavoidable even extending the firing time. The low concentration or the amorphous state of the impurity phase put difficulties in its identification. In order to solve this problem, the electrochemical characterization was conducted to examine the impurity evolution with the microwave firing. It is found that a short microwave heating usually leads to the existence of an amorphous, ferric intermediate, while longer microwave heating favors the formation of the Li3Fe2(PO4)3 impurity. A post-calcining strategy is proved to be an effective way to eliminate these impurities, and the thus-obtained sample shows a capacity of 160 mAhg− 1 at a current rate of 17 mAg− 1.

0

Acknowledgements 200

400

Authors would express their sincere thanks to the Natural Science Foundation of China (No. 20873094) for the financial support.

mv3 mv3+650

Scan rate:0.1mV/s

References 2.0

2.4

2.8

3.2

3.6

4.0

Potential/V -400

b

-8

-300

Current/mAg-1

-200

-4 0 4

-100

8 2 .0 2 .2 2 .4 2 .6 2 .8 3 .0 3. 2

0 100 200

mv5 mv5+650

Scan rate:0.1mv/s

300 2.0

2.4

2.8

3.2

3.6

4.0

Potential / V Fig. 11. CV curves of mv3 (a) and mv5 (b) samples before and after the post-treatment.

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