C cathode materials for Li-ion batteries

C cathode materials for Li-ion batteries

Journal of Alloys and Compounds 532 (2012) 49–54 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage...

939KB Sizes 0 Downloads 12 Views

Journal of Alloys and Compounds 532 (2012) 49–54

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Synthesis and properties of Li3 V2−x Cex (PO4 )3 /C cathode materials for Li-ion batteries Jinhan Yao a,∗ , Shuoshuo Wei a , Pinjie Zhang a , Chaoqi Shen a , Kondo-Francois Aguey-Zinsou b , Lianbang Wang a,∗ a State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou, Zhejiang, PR China b School of Chemical Engineering, The University of New South Wales, Sydney, Australia

a r t i c l e

i n f o

Article history: Received 1 March 2012 Received in revised form 4 April 2012 Accepted 5 April 2012 Available online 12 April 2012 Keywords: Sol–gel process Li3 V2 (PO4 )3 Cathode material Rare earth metal Doping

a b s t r a c t Ce-doped compounds Li3 V2−x Cex (PO4 )3 /C (x = 0, 0.03, 0.05, 0.07, 0.10) were successfully prepared by a sol–gel process with CH3 COOLi·2H2 O, NH4 VO3 , NH4 H2 PO4 , citric acid and Ce(CH3 COO)3 ·5H2 O as raw materials. X-ray diffraction analysis suggests that single-phase Li3 V2−x Cex (PO4 )3 /C with monoclinic structure can be obtained for x ≤ 0.10. The result of Rietveld refinement analysis indicates that cell volume increases with the increasing of the Ce content. As compared with the pure Li3 V2 (PO4 )3 , although there is a small decrease in the initial specific capacity at low rate (0.2 C), a proper amount of Ce3+ doping, e.g. x = 0.05, could improve its cycle and rate performances. When discharge at 0.2 C and 10 C, the initial specific discharge capacities of Li3 V1.95 Ce0.05 (PO4 )3 were 120 and 88.6 mAh g−1 respectively. Furthermore, the capacity retention at the 100th cycle was 96.8% and 94.3% for the respective discharge capacities, which displayed excellent cycling properties. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

In this respect, 3d-metal ions such as Mg2+ , Co2+ , Fe3+ , Al3+ , Ti4+ , Zr4+ [19–22,12,23–27] have been employed to dope Li3 V2 (PO4 )3 at vanadium sites or lithium sites. And some positive effects have been reported. Although there are lots of 3d-metal ion doping strategies to improve the physical and electrochemical performance of Li3 V2 (PO4 )3 , exploration of a new doping strategy for lithium vanadium phosphate may be necessary to further improve its electrochemical properties. Another option is to use rare-earth metals. Rare earth (Re) elements have many outstanding features such as high electric charge, large radius and high self-polarization ability [28], so the research of their usage in the lithium ion batteries has tremendous application significance. Wei et al. [29] reported that LiCoO2 cathode material with Re-doping (La,Y) possessed improved charge–discharge capacity and potential plateau. Similarly, Peng et al. [30] have proved that partial substitution of Re ions at Mn site in LiMn2 O4 (Re = Nd,Ce) improved cycling performances of the cathode material. However, the capacity faded with the increase of doping content. Research of Luo et al. [31] displayed that doping LiFePO4 with Re (La) ions could enhance its cycle capacity value and longterm stability. Wu and Zhou [32] have found that doping LiV3 O8 with Ce was effective in suppressing the capacity fading. To the best of our knowledge, there are few studies about the substitution of V with Re ions for Li3 V2 (PO4 )3 . The effects of partial substitution of Re ions in V site are still unclear and worth exploring (Here Re represents rare earths). So far, no studies relating to Ce Cr3+ ,

Lithium-ion batteries are receiving an increasing amount of attention because of the rapid development of portable electronic [1]. Phosphate polyanion frame materials are being considered as favorable replacements for conventional oxide-based materials as the cathode materials in lithium-ion batteries. In this regard, LiMPO4 (M = Fe, Co, Ni, Mn) [2–7], and Li3 V2 (PO4 )3 [8,9] have attracted significant interests due to their high reversible capacity, high operating voltage, good ionic mobility and good thermal stability [10]. They include both mobile Li ions and redox-active metals within a rigid phosphate network, displaying remarkable electrochemical and stability. Among the above phosphates, monoclinic Li3 V2 (PO4 )3 has the highest theoretical capacity (197 mAh g−1 ) and thus is a excellent candidate for commercial lithium-ion batteries. However, the main drawback of pristine Li3 V2 (PO4 )3 is its intrinsically low electronic conductivity (2.4 × 10−7 S cm−1 ) as LiFePO4 (10−10 to 10−9 S cm−1 ) [11] which greatly limits its practical applications. Thereby, various methods have been attempted to improve its electronic conductivity. So far, the most successful approaches revolves around metal-ion doping [12–16], particle size reduction [17] and carbon coating at the surface of Li3 V2 (PO4 )3 particles [18].

∗ Corresponding authors. Tel.: +86 571 88320611; fax: +86 571 88320803. E-mail addresses: [email protected] (J. Yao), [email protected] (L. Wang). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.014

2. Experimental 2.1. Synthesis of Li3 V2−x Ce(PO4 )3 /C The Li3 V2−x Cex (PO4 )3 /C (x = 0, 0.03, 0.05, 0.07, 0.10) compounds were prepared by a sol–gel method employing CH3 COOLi·2H2 O, NH4 VO3 , NH4 H2 PO4 and Ce(CH3 COO)3 ·5H2 O as raw materials in the molar ratio of 3.1:(2−x):x:3. Citric acid was used here not only as a chelating reagent but also as a source of carbon to coat the materials with carbon layer. The mol ratio of citric acid: (V + Ce) = 2:1 was selected. A over-dosed usage of Li salt is to compensate any Li loss during the high temperature synthesis [20]. A typical synthetic procedure consisted of first dissolving citric acid and NH4 VO3 in deionized water with magnetic stirring at 80 ◦ C. After a clear blue solution formed, a mixture of NH4 H2 PO4 , CH3 COOLi 2H2 O and Ce(CH3 COO)3 ·5H2 O was added to the solution while stirring for 5 h. After evaporating the water at 80 ◦ C, a blue sol became a gel was obtained. The resulting gel was put into a vacuum drying oven at 120 ◦ C for 10 h. The material obtained was then grinded and heated at 350 ◦ C in a tube furnace under a flowing N2 atmosphere for 4 h to expel H2 O and NH3 . The resulting powder was reground and sintered at 750 ◦ C for 8 h under N2 flow, followed by natural cooling to room temperature to yield the final products.

3. Results and discussion XRD patterns of Li3 V2−x Cex (PO4 )3 /C (0 ≤ x ≤ 0.10) are shown in Fig. 1. All the materials displayed a pure single phase with a monoclinic structure (space group P21 /n) corresponding to Li3 V2 (PO4 )3 . No other phases were detected by XRD. This indicates that Ce3+ are completely doped into the crystal lattice of Li3 V2 (PO4 )3 and the doping of Ce3+ would not change the basic Li3 V2 (PO4 )3 crystal structure. Ce3+ may be located at V3+ sites in the crystal lattice. It is noteworthy that no traces of crystalline carbon observed by XRD even if an slightly excess of the citric acid was used for the preparation of the Li3 V2−x Cex (PO4 )3 /C samples. In order to further confirm any structure change a Rietveld refinement was conducted for the Li3 V2−x Cex (PO4 )3 /C compounds with x = 0, 0.03, 0.05, 0.07, 0.10. Shown in Fig. 2 is the representative refinement result for Li3 V1.9 Ce0.1 (PO4 )3 /C. The calculated pattern matches well with the observed one, Li3 V1.9 Ce0.1 (PO4 )3 /C sample is found to be a single phase (monoclinic symmetry, space group P21 /n) with no detection of impurities or unwanted structures. These results indicated that Ce was successfully substituted for

x=0.10

x=0.07 x=0.05 x=0.03 x=0

10

20

30

40

50

60

2θ/ degree Fig. 1. XRD patterns of Li3 V2−x Cex (PO4 )3 /C samples (x = 0, 0.03, 0.05, 0.07, 0.10).

Observed Calculated Difference Bragg Position

Intensity / a.u.

2.2. Characterization method Structural and crystallographic analyses were performed by X-ray Diffraction (XRD) using PANalytical X’Pert Pro with Cu K␣ radiation equipped with Cu K␣ radiation ( = 0.15418 nm). The diffraction data were recorded in the 2 range of 10–60◦ with a step of 0.02◦ . The morphologies of samples were characterized by Scanning Electron Microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, FEI Tec-nai G2-F30), respectively. For electrochemical measurements, the Li3 V2−x Cex (PO4 )3 /C composite cathode electrodes were fabricated by mixing of 80 wt.% active materials, 10 wt.% acetylene black and 10 wt.% polylvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). The obtained slurry was uniformly pasted onto the aluminum current collectors and the electrodes were dried at 120 ◦ C in vacuum for 12 h. (The weight of active material was ∼4 mg/cm2 .) Subsequently, CR2032 coin-type cells were assembled in an argon-filled glove box using a Celgard 2400 membrane as a separator, 1 M LiPF6 in ethylene carbonate, diethyl carbonate and ethylmethyl carbonate (EC/DMC/EMC, 1:1:1 vol.) as electrolyte. A lithium foil was used as the counter and reference electrodes. The coin cells were galvanostatically charged and discharged over a voltage range of 3.0–4.3 V (vs. Li/Li+ ) at room temperature on a Land® (Wuhan, China) battery tester. Electrochemical impedance spectroscopy (EIS) was collected using a Solartron Impedance/gain phase analyzer (model SI 1260) coupled to a potentiostat (SI 1287) at room temperature. The frequency range was 10−2 Hz to 105 Hz on the open circuit voltage.

(-152)

(420) (-403)

(-233)

( -204)

(121) (-113) (-122) (220) ( -221) (-301) (310) (311)

(120) (-201)

(-111)

doping in Li3 V2 (PO4 )3 have yet been reported. Compared with LiV3 O8 , Li3 V2 (PO4 )3 has a more complicated crystal structure, and the doping effect should be more complex and inspiring. Accordingly, in this work, a series of Ce3+ -doped Li3 V2−x Cex (PO4 )3 /C (x = 0, 0.03, 0.05, 0.07, 0.10) cathode materials were synthesized via a sol–gel route. The effects of Ce3+ doping on the electrochemical properties of Li3 V2 (PO4 )3 were discussed in detail.

(020)

J. Yao et al. / Journal of Alloys and Compounds 532 (2012) 49–54

Intensity/ a.u.

50

10

20

30

40

50

60

2θ/degree Fig. 2. Refinement result of the Li3 V1.9 Ce0.1 (PO4 )3 /C sample with the observed, calculated, and difference profiles.

V, leading to formation of Li3 V2−x Cex (PO4 )3 /C solid solution phase with x ≤ 0.1. Further analysis at different x values is summarized in Table 1. In particular, it was found that cell unit increases after the Ce3+ doping. This further indicates that Ce doping would cause the lattice expansion and this result is consistent with the fact that the radius of Ce3+ is larger than V3+ at the VO6 octahedra. Fig. 3(a)–(e) shows the typical SEM images of the synthesized Li3 V2−x Cex (PO4 )3 /C samples. As shown in Fig. 3, irregular particles with a broad particle size distribution of 0.2–6.0 ␮m and a Table 1 Lattice parameters of Li3 V2−x Cex (PO4 )3 /C (x = 0, 0.03, 0.05, 0.07, 0.10) derived from refinement of corresponding XRD patterns. The reasonably small Rwp factors revealed that a single phase was obtained in our experimental process. Li3 V2−x Cex (PO4 )3

x = 0.00 x = 0.03 x = 0.05 x = 0.07 x = 0.10

Lattice parameters

R value

˚ a (A)

˚ b (A)

˚ c (A)

ˇ (◦ )

V (A˚ 3 )

Rwp (%)

Rp (%)

8.605 8.611 8.613 8.616 8.617

8.591 8.603 8.602 8.608 8.614

12.034 12.041 12.047 12.051 12.059

90.72 90.58 90.64 90.43 90.35

891.6 895.3 896.7 897.4 899.3

6.21 6.17 5.95 5.91 5.93

5.09 4.75 4.93 4.57 4.63

J. Yao et al. / Journal of Alloys and Compounds 532 (2012) 49–54

51

Fig. 3. (a)–(e) SEM images of Li3 V2−x Cex (PO4 )3 /C samples with different Ce contents: (a) x = 0, (b) x = 0.03, (c) x = 0.05, (d) x = 0.07, (e) x = 0.1; (f) TEM images of Li3 V1.95 Ce0.05 (PO4 )3 /C samples.

severely aggregated structure were observed for Li3 V2 (PO4 )3 /C sample in Fig. 3(a). However, for the materials doped with Ce3+ doping, smaller particles size and narrower particles size distribution were obtained (Fig. 3(b)–(e)). The reduced particle size observed for Li3 V2−x Cex (PO4 )3 /C might be related to the decomposition of Ce(CH3 COO)3 ·5H2 O in the precursor mixtures, which prevent the agglomeration of final products during high temperature sintering. It has been reported that the morphology and surface area of obtained particles have a notable effect on the electrochemical performance of Li3 V2 (PO4 )3 /C [33,34]. A reduction of particle size would shorten the diffusion paths of lithium ion as well as increase the area for electrode reaction. This would benefit electrochemical performance. In order to confirm the presence of a carbon layer on the Li3 V2−x Cex (PO4 )3 /C surfaces, the TEM investigation was conducted. As shown in Fig. 3(f) a carbon layer with a thickness of about 1–2 nm was found to evenly coated on surfaces

of Li3 V1.95 Ce0.05 (PO4 )3 /C particles. Besides, the Li3 V2−x Cex (PO4 )3 /C particles are connected together by the carbon layers, which will make a contribution to the electrical conductivity in the interstitial particle/boundary region. Fig. 4(a) depicts the first-cycle EVS voltage profiles and corresponding differential capacity response for Li3 V2 (PO4 )3 /C and Li3 V1.95 Ce0.05 (PO4 )3 /C electrode. It was found that the both undoped and doped samples demonstrated oxidation peaks at ca. 3.61, 3.70 and 4.1 V, which are associated with the extraction of the Li ions from the energetically in equivalent sites within the NASICON structure [25–29]. However, during the anodic process, the reduction peaks (Li insertion) for the undoped Li3 V2 (PO4 )3 /C are rough. By contrast, the doped Li3 V1.95 Ce0.05 (PO4 )3 /C demonstrated much sharper reduction peaks at ca. 4.05, 3.65 and 3.57 V, indicating a better redox behavior. Fig. 4 (b) shows the initial charge and discharge curves of Li3 V2−x Cex (PO4 )3 /C cell (x = 0, 0.01, 0.03,

52

J. Yao et al. / Journal of Alloys and Compounds 532 (2012) 49–54

0.5C

120

1C

3000

2C 5C

100

0 -3000

Li3V2(PO4)3 /C

6000 3000 0 -3000

Capacity / mAh g -1

Differential capacity, dQ/dV [C/V]

6000

Li3V1.95Ce0.05(PO4)3/C 3.4

3.6

3.8

4.0

4.2

10C

80 60 40

x=0.00 x=0.03 x=0.05 x=0.07 x=0.10

20

4.4

0

Potential / V ( vs. Li+/Li)

0

20

40

60

80

100

Cycle number

b)

4 3

x = 0.1

3

x = 0.07

4 3

x = 0.05

4

3

x = 0.03

4 3

x = 0.00 0

30

60

90

Capacity / mAh g

120 -1

Fig. 4. (a) EVS differential capacity data for a range of Li/Li3 V2 (PO4 )3 /C and Li/Li3 V1.95 Ce0.05 (PO4 )3 /C cell; (b) initial charge–discharge curves of the Li3 V2−x Cex (PO4 )3 samples at a current density of 0.2 C in the voltage range of 3.0–4.3 V.

0.05, 0.07, and 0.10) at the 0.2 C rate in the voltage range between 3.0 and 4.3 V. (Here, 0.2 C rate correspond to a current density of 26.6 mA g−1). All the electrodes show similar charge–discharge behaviors. The voltage profiles exhibit three charge plateaus around 3.61, 3.70 and 4.10 V and three discharge plateaus around 3.56, 3.63 and 4.05 V. These plateaus correspond to the two-phase transition processes during the electrochemical reactions [35]. These regions are related to a sequence of transitions process between two adjacent single phases of Lix V2 (PO4 )3 :x = 3, 2.5, 2 and 1, respectively. Furthermore, a decrease of discharge capacity was observed with Ce-doping. Such a decrease could be related to the initial extraction of the two lithium ions that associated with the V3+ /V4+ redox couple. With Ce3+ doping, the amount of active V3+ decreases gradually which will lessen the active spots for charge transfer and hinder the transfer of lithium ion. It is worthy noting that the substitution of 10 mol.% of cerium(III) for vanadium(III) leading to a more than 10% loss of capacity of original Li3 V2 (PO4 )3 /C. The observed larger loss of capacity can be attributed to the effect of over-dosed Ce doping, which may affect the local structure of the pristine Li3 V2 (PO4 )3 /C

by some Ce ions possible occupancy at Li sites [20]. And thus, the Li ions in the structure of 10% Ce doped Li3 V2−x Cex (PO4 )3 /C cannot fully be extracted during the (de)lithiation process, causing more than 10% capacity loss. Fig. 5 compares the rate cycling performance of Li3 V2−x Cex (PO4 )3 /C (x = 0.00, 0.03, 0.05, 0.07, 0.10) samples. All the samples were cycled at five current densities (0.5, 1, 2, 5 and 10 C) between 3.0 and 4.3 V. Each current density was applied for 20 cycles. For each sample, the discharge capacity decreases with increasing current density. Among the doped samples, x = 0.05 displays good rate performances. Furthermore, when the discharge rate is low (0.5 C), the discharge capacity monotonously decreases with increasing x. When the current density is higher (≥1 C), the highest discharge capacity is obtained for x = 0.05. In other words, Ce-doping was found to improve the rate performance at the expense of the first discharge capacity. In Fig. 6, the relative long-term cycling performance of Li3 V2−x Cex (PO4 )3 /C (x = 0.00 and 0.05) compounds at the current density of 0.2 C and 10 C are compared. The specific capacity was held at 90.0% and 96.8% for x = 0.00 and 0.05 respectively after 100 cycles at 0.2 C. It should be noted that 140 (b)

120

(a)

Capacity / mAh g -1

4

Voltage / V ( Vs. Li+/Li)

Fig. 5. Discharge capacities of the Li3 V2−x Cex (PO4 )3 /C (x = 0, 0.03, 0.05, 0.07, 0.10) samples at current densities of 0.5 C, 1 C, 2 C, 5 C and 10 C in the voltage range of 3.0–4.3 V.

100 80

(d)

60

(c)

40 x = 0.00 x = 0.05

20 0 0

20

40

60

80

100

Cycle number Fig. 6. Long-term cycle performances of Li3 V2 (PO4 )3 /C and Li3 V1.95 Ce0.05 (PO4 )3 /C at different C-rates: (a) Li3 V2 (PO4 )3 /C at 0.2 C rate, (b) Li3 V1.95 Ce0.05 (PO4 )3 /C at 0.2 C rate, (c) Li3 V2 (PO4 )3 /C at 10 C rate, (d) Li3 V1.95 Ce0.05 (PO4 )3 /C at 10 C rate.

J. Yao et al. / Journal of Alloys and Compounds 532 (2012) 49–54

53

4. Conclusions

300

(a)

Li3 V2 ( PO4 ) 3

200

-Z'' / Ohm

Li3 V1.95 Ce0.05 ( PO4 ) 3

100

0 0

100

200

300

400

Z' / Ohm

A series of Ce3+ -doped Li3 V2−x Cex (PO4 )3 /C (x = 0, 0.03, 0.05, 0.07, 0.10) cathode materials have been successfully synthesized by a sol–gel method. XRD and the Rietveld refinement results indicate that single-phase Li3 V2−x Cex (PO4 )3 /C (x ≤ 0.1) materials with monoclinic structure can be obtained. SEM and TEM images illustrate that the samples are covered with an amorphous carbon layer and have a smaller particle after Ce3+ doping which are beneficial to improve electronic conductivity of Li3 V2 (PO4 )3 . The electrochemical measurement results show that a proper amount of Ce3+ doping (x = 0.05) exhibits best rate discharge performance and cycle stability which is also indicated by its minimum charge transfer resistance. The optimal composition of Li3 V1.95 Ce0.05 (PO4 )3 /C displays an initial discharge of 88.6 mAh g−1 and 94.3% retention rate after 100 cycles at 10 C rate, much better than the undoped one and other ions-doped systems. These inspiring results can be attributed to the existence of Ce3+ in the crystal lattice which could lessen the distortion of the crystal lattice.

CPE

(b)

Acknowledgements

Rs Rct

Zw

This work was supported by the Natural Science Foundation of China (Grant No. 20506024) and the State Key Development Program for Basic Research of China (Grant No. 2007CB216409).

Fig. 7. (a) Nyquist plots of the Li3 V2−x Cex (PO4 )3 /C (x = 0 and 0.05); (b) equivalent circuit for the Nyquist plots of the Li3 V2−x Cex (PO4 )3 /C (x = 0 and 0.05) samples.

References at 10 C, x = 0.05 exhibited a higher initial discharge capacity than that of undoped Li3 V2 (PO4 )3 /C. The Li3 V2 (PO4 )3 /C and Li3 V1.95 Ce0.05 (PO4 )3 /C present an initial discharge capacity of 82.1 mAh g−1 and 88.6 mAh g−1 . After 100 cycles, the capacity retention was 75.5% and 94.3%, respectively. Therefore, a proper amount of Ce3+ doping was found to increase both cycle stability and discharge capacity at a high current density. By analyzing the above results, the improved rate capability and cycle stability of the Li3 V1.95 Ce0.05 (PO4 )3 (x = 0.05) sample would be due to the structural stabilization effect induced by Ce3+ doping. The oxidation/reduction of V and the simultaneous de-/intercalation of the Li+ can cause the crystal lattice of Li3 V2 (PO4 )3 /C shrink/expand. Because Ce3+ itself is not involved in the oxidation/reduction reaction, the presence of Ce3+ in the crystal lattice can lessen the distortion of the crystal lattice during charge and discharge process and thus improve the cycle stability and rate performance [36]. To further understand the electrochemical dynamic behavior of electrodes, electrochemical impedance spectroscopy (EIS) measurements were performed. The typical Nyquist plots of EIS are presented in Fig. 7(a) for Li3 V2−x Cex (PO4 )3 /C with x = 0 and 0.05. Similar EIS patterns are observed for both samples. The semicircle in the high-frequency region represents charge-transfer resistance. The straight line with an angle of about 45◦ in the low-frequency region is attributed to the diffusion of the lithium ions into the bulk active mass. Fig. 7(b) shows an equivalent circuit model for analyzing impedance spectra. These impedance spectra can be explained with solution resistance (Rs ), a constant phase element (CPE) associated with the particle-to-particle resistance (Rct ), and Warburg impedance (Zw ) which is attributed to the diffusion of Li-ions in the bulk materials [37]. The Li3 V1.95 Ce0.05 (PO4 )3 /C sample exhibited smaller Rct which means that Ce3+ doping would effectively enhance the on charge transfer. This observation could be Ce3+ doping enhances the electrochemical activity of Li3 V2 (PO4 )3 and facilitates the electron diffusion in the Li+ insertion and extraction process which are both favorable for improving the cycle performance of positive lithium vanadium phosphate materials.

[1] Y.F. Liu, Y.H. Cao, L. Huang, M.X. Gao, H.G. Pan, Journal of Alloys and Compounds 509 (2011) 675. [2] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Journal of the Electrochemical Society 144 (1997) 1188. [3] X.F. Ouyang, M. Lei, S.Q. Shi, C.L. Luo, D.S. Liu, D.Y. Jiang, Z.Q. Ye, M.S. Lei, Journal of Alloys and Compounds 476 (2009) 462. [4] A.Y. Shenouda, H.K. Liu, Journal of Alloys and Compounds 477 (2009) 498. [5] J.J. Chen, S.J. Wang, M.S. Whittingham, Journal of Power Sources 174 (2007) 442. [6] H.H. Li, J. Jin, J.P. Wei, Z. Zhou, J. Yan, Journal of Electrochemistry Communications 11 (2009) 95. [7] F. Zhou, M. Cococcioni, K. Kang, G. Ceder, Journal of Electrochemistry Communications 6 (2004) 1144. [8] D.Y. Wang, H. Buqa, M. Crouzet, G. Deghenghi, T. Drezen, I. Exnar, N.-H. Kwon, J.H. Miners, L. Poletto, M. Grätzel, Journal of Power Sources 189 (2009) 624. [9] M.M. Ren, Z. Zhou, X.P. Gao, W.X. Peng, J.P. Wei, Journal of Physical Chemistry C 112 (2008) 5689. [10] S.Q. Liu, S.C. Li, K.L. Huang, B.L. Gong, G. Zhang, Journal of Alloys and Compounds 450 (2008) 499. [11] Y.Q. Qiao, X.L. Wang, Y.J. Mai, J.Y. Xiang, D. Zhang, C.D. Gu, J.P. Tu, Journal of Power Sources 196 (2011) 8706. [12] M.M. Ren, Z. Zhou, Y.Z. Li, X.P. Gao, J. Yan, Journal of Power Sources 162 (2006) 1357. [13] H.G. Pan, R. Li, M.X. Gao, Y.F. Liu, Q.D. Wang, Journal of Alloys and Compounds 404 (2005) 669. [14] H.G. Pan, Y.J. Yue, M.X. Gao, X.F. Wu, N. Chen, Y.Q. Lei, Q.D. Wang, Journal of Alloys and Compounds 397 (2005) 269. [15] H.G. Pan, Q.W. Jin, M.X. Gao, Y.F. Liu, R. Li, Y.Q. Lei, Journal of Alloys and Compounds 379 (2004) 237. [16] H.G. Pan, R. Li, Y.F. Liu, M.X. Gao, H. Miao, Y.Q. Lei, Q.D. Wang, Journal of Alloys and Compounds 463 (2008) 189. [17] Y.Z. Li, X. Liu, J. Yan, Journal of Electrochimica Acta 53 (2007) 473. [18] J. Barker, M.Y. Saidi, R.K.B. Gover, P. Burns, A. Bryan, Journal of Power Sources 17 (2007) 927. [19] Y.Z. Dong, Y.M. Zhao, H. Duan, Journal of Electroanalytical Chemistry 660 (2011) 14. [20] C.S. Dai, Z.Y. Chen, H.Z. Jin, X.G. Hu, Journal of Power Sources 195 (2010) 5775. [21] J.S. Huang, L. Yang, K.Y. Liu, Y.F. Tang, Journal of Power Sources 195 (2010) 5013. [22] Q. Kuang, Y.M. Zhao, X.N. An, J.M. Liu, Y.Z. Dong, L. Chen, Electrochimica Acta 55 (2010) 1575. [23] J. Barker, R.K.B. Gover, P. Burns, A. Bryan, Journal of the Electrochemical Society 154 (2007) A307. [24] D.J. Ai, K.Y. Liu, Z.G. Lu, M.M. Zou, D.Q. Zeng, J. Ma, Electrochimica Acta 56 (2011) 2823. [25] Y.H. Chen, Y.M. Zhao, X.N. An, J.M. Liu, Y.Z. Dong, L. Chen, Electrochimica Acta 54 (2009) 5844. [26] S.Q. Liu, S.C. Li, K.L. Huang, Z.H. Chen, Acta Physico-Chimica Sinica 23 (2007) 537.

54

J. Yao et al. / Journal of Alloys and Compounds 532 (2012) 49–54

[27] M. Sato, H. Ohkawa, K. Yoshida, M. Saito, K. Uematsu, K. Toda, Solid State Ionics 135 (2000) 137. [28] B.Q. Jiang, S.F. Hu, M.W. Wang, X.P. OuYang, Z.Y. Gong, Rare Metals 30 (2011) 115. [29] J.P. Wei, X.Y. Cao, G.L. Pang, Journal of Rare Earths 21 (2003) 466. [30] Z.S. Peng, Y. Jiang, Z.Y. Jin, Journal of Rare Earths 18 (2000) 115. [31] S.H. Luo, Y. Tian, H. Li, K.J. Shi, Z.L. Tang, Z.T. Zhang, Journal of Rare Earths 28 (2010) 439. [32] Z.J. Wu, Y. Zhou, Journal of Power Sources 199 (2012) 300.

[33] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Electrochemical and Solid-State Letters 5 (2002) A149. [34] Y. Li, Z. Zhou, M. Ren, X. Gao, J. Yan, Electrochimica Acta 51 (2006) 6498. [35] Y.Q. Qiao, X.L. Wang, Y. Zhou, J.Y. Xiang, D. Zhang, S.J. Shi, J.P. Tu, Electrochimica Acta 56 (2010) 513. [36] C. Deng, S. Zhang, S.Y. Yang, Y. Gao, B. Wu, L. Ma, B.L. Fu, Q. Wu, F.L. Liu, Journal of Physical Chemistry C 115 (2011) 15053. [37] Y.M. Choi, Y.H. Pyun, Solid State Ionics 99 (1999) 173.