Synthesis, Characterization, and Electrochemical Performance of Ce

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Synthesis, Characterization, and Electrochemical Performance of CeDoped Ordered Macroporous Li3V2(PO4)3/C Cathode Materials for Lithium Ion Batteries Senlin Wang,† Zhengxi Zhang,*,† Aniruddha Deb,‡ Li Yang,*,†,§ and Shin-ichi Hirano§ †

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States § Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ‡

ABSTRACT: A series of Ce3+-doped ordered macroporous Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples have been fabricated via a colloidal crystal template method. The X-ray powder diffraction and scanning electron microscopy analysis demonstrate that the Ce element doping does not affect the original monoclinic structure and macroporous morphology of the pristine Li3V2(PO4)3/C sample. Electrochemical measurement results prove that the Li3V1.97Ce0.03(PO4)3/C sample presents the best electrochemical performance as the cathode material for lithium ion batteries among the as-prepared samples in the potential ranges of both 3−4.3 V and 3−4.8 V. The substitution of V3+ with an appropriate amount of Ce3+ increases the Li+ diffusion coefficient based on the electrochemical impedance spectroscopy results, which is mainly responsible for the excellent electrochemical performance.

1. INTRODUCTION Lithium ion batteries have attracted extensive attention as promising energy storage systems of electric vehicles (EVs) and smart grids due to their long service life and being environmental friendly.1 Cathode materials, as one of the most important components of lithium ion batteries, play a critical role in determining the performance of lithium ion batteries. LiCoO2 is the most used cathode material, but poor thermal stability and safety problems prohibit its application in large-scale power devices. Recently, lithium transition metal phosphates, such as LiMnPO4, LiCoPO4, LiFePO4, and Li3V2(PO4)3 have received much attention for their outstanding safety merits. Among the above-mentioned phosphate cathode materials, the monoclinic Li3V2(PO4)3 has the highest theoretical capacity (197 mAh g−1) and operating voltage (up to 4.8 V) of those employed as a potential cathode candidate for lithium ion batteries.2−4 However, the inherent low electronic conductivity and lithium ion diffusivity of the pristine Li3V2(PO4)3 limits its commercial utilization in lithium ion batteries. So far, various methods have been presented to improve its electrochemical properties, such as coating the Li3V2(PO4)3 surface with a conductive layer5−10 and scaling down the Li3V2(PO4)3 particle size.3,11−13 Moreover, doping the pristine Li3V2(PO4)3 with an alien ion at Li+, V3+, or PO43− sites is another efficient way to enhance the lithium ion diffusivity. Up to now, lots of elements have been extensively investigated, such as alkali metals (Na,14−16 K17), alkaline-earth metals (Mg18−20), transition metals (Ti,21 Cr,22,23 Mn,21,24 Fe,10,21,25 Co,26 Ni,27 Zr,28 Nb,29 Mo30), main group element metals (Al,31,32 Sn,33,34 Ge35), and halogens (Cl,36 F37). Relative to them, there are few works involving the study of the substitution behavior of rare earth elements (Sc,17 Y38 and La39) in the vanadium site in Li3V2(PO4)3 systems. Y-doped Li3V2−xYx(PO4)3 samples were © 2014 American Chemical Society

prepared by a carbothermal reduction (CTR) process using Y 2 O 3 as the Y source. 3 8 It was found that the Li3V1.97Y0.03(PO4)3 sample exhibited a discharge capacity of 124 mAh g−1 at 0.5 C rate in the range of 3.0−4.2 V. After 30 cycles, it delivered a capacity of 117 mAh g−1 with 94.4% capacity retention. Jiang et al.39 synthesized the La-doped Li3V2−xLax(PO4)3 samples using lanthanum oxide (La2O3) as the La source. The Li3V2−xLax(PO4)3 (x = 0.02) sample showed the highest discharge capacity of 153.28 mAh g−1 at 0.2 C rate in the potential range of 3.0−4.8 V among assynthesized samples. In addition, it is known that cerium (Ce) is the most abundant rare earth element in the earth, making up about 0.0046% of the earth’s crust by weight. More importantly, cerium has a variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only a small amount of energy is required to change the relative occupancy of these electronic levels. These inspiring characteristics make Ce an ideal doping element for cathode materials. Recently, Yao et al.40 prepared the Ce-doped Li3V2−xCex(PO4)3/C (x = 0, 0.03, 0.05, 0.07, 0.10) composites by a sol−gel method, and investigated their electrochemical performances between 3.0 and 4.3 V. The results showed that the Li3V2−xCex(PO4)3/C (x = 0.05) composite exhibited the best rate and cycle performances. Subsequently, Dang et al.41 also synthesized the Ce-doped Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.02, 0.03, 0.05, 0.10) composites by a microwave assisted sol−gel process, and the electrochemical performance measurements exhibited that the Li3V1.98Ce0.02(PO4)3/C composite can still deliver discharge Received: Revised: Accepted: Published: 19525

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capacities of up to ∼140 mAh g−1 and up to ∼120 mAh g−1 at 5 and 10 C rates between 3.0 and 4.8 V, respectively. These works revealed that doping ions in the crystal lattice of the Li3V2(PO4)3/C samples is able to improve the electrochemical performance (e.g., discharge capacity and rate capability), which is mainly attributed to that the doped Li3V2(PO4)3/C samples have smaller particle size, higher structure stability, and lower charge transfer resistance compared to the pristine Li3V2(PO4)3/C samples. In our previous study, it was evidenced that the Li3V2(PO4)3/C materials with ordered macroporous structure is beneficial to the electrochemical performance in the potential range of 3.0−4.8 V.42 To further improve the electrochemical performance (e.g., cycling performance), a series of ordered macroporous Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples with a pore size of 210 nm have been prepared by a template method employing PMMA colloidal crystal beads as templates in this work. The samples all exhibit the same order macroporous morphology and pore size. To fully evaluate the as-prepared samples, electrochemical properties of samples are systematically investigated in the potential ranges of both 3−4.3 V and 3−4.8 V. By comparison to the pristine Li3V2(PO4)3/C sample, the Li3V1.97Ce0.03(PO4)3/C sample exhibits much improved rate capability and cycle performance.

2.3. Electrochemical Measurements. The galvanostatic charge−discharge tests of as-prepared samples were conducted in CR2016 coin-type cell with a Land CT2001 battery tester at different C rates in the potential range of 3.0−4.3 V (1 C = 133 mA g−1) and 3.0−4.8 V (1 C = 197 mA g−1). The same charge and discharge rates were used during every cycle. The cathode electrodes of the cells were fabricated by blending as-prepared materials (Li3V2−xCex(PO4)3/C) with carbon black (CA) and polyvinylidene fluoride (PVDF) binder at a weight ratio of 80:10:10 in an appropriate amount of N-methyl-2-pyrrolidone and stirring until a slurry was obtained. The slurry was pasted onto the aluminum current collectors and the electrodes were dried at 120 °C in vacuum for 12 h. The weight of active material (Li3V2(PO4)3) was calculated by subtracting the weight of the residual carbon from the total weight of the asprepared samples. The active material (Li3V2(PO4)3) loading was in the range of 1−2 mg cm−2. The coin-type cells were assembled in a argon-filled glovebox (M. Braun Co., [O2] < 1 ppm, [H2O] < 1 ppm) with as-prepared cathode electrode, Li metal as the anode, 1 M LiPF6 in EC/DMC (1/1 vol %) as the electrolyte, and glass fiber filter (Whatman, GF/A) as the separator. Cyclic voltammetry dates were carried out with a CHI604b electrochemical workstation at a scan rate of 0.2 mV s−1 in the range of 3.0−4.3 V. Electrochemical impedance spectroscopy (EIS) measurements were obtained with a CHI604b electrochemical workstation in the frequency range of 100 kHz to 10 mHz with an excitation voltage of 5 mV.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ordered Macroporous Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples. The ordered macroporous Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples were prepared via a template method. The templates were the PMMA latex sphere with a diameter of 250 nm, which were synthesized by emulsion polymerization of methyl methacrylate (MMA) according to procedures reported previously.43,44 All the reagents used in the experiment were analytical grade and without further purification. First, stoichiometric amounts of vanadium pentoxide (V2O5), cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O), and oxalic acid dehydrate (C2H2O4·2H2O) were dissolved in deionized water with magnetic stirring at 50 °C. After a clear blue solution was obtained, lithium acetate (CH3COOLi) and phosphoric acid (H3PO4, ≥ 85 wt % in H2O) were added to the above solution and stirring continued for 1 h. Then the blue solution was added dropwise to the PMMA template in a dish and dried in an oven at 30 °C overnight to get the precursor. The precursor was decomposed at 220 °C in air for 3 h to remove the excess water, and then sintered at 850 °C for 8 h in an Ar atmosphere. After that, the macroporous Ce 3+ doped Li3V2−xCex(PO4)3/C samples were obtained. In this experiment, PMMA acts as not only a template but also a carbon source. 2.2. Samples Characterization. The content of Li, V, Ce, and P in the Li3V2−xCex(PO4)3/C samples were analyzed by inductive coupled plasma emission spectrometer (ICP). Structure and morphology of the as-prepared samples were characterized by X-ray powder diffraction (XRD, Rigaku, D/ max-RBusing Cu Kα radiation with λ = 1.5418 Å) and field emitting scanning electron microscopy (FE-SEM, Hitachi Co., Ltd., Japan) measurements. The carbon contents in the asprepared samples were determined by a C−S 00 infrared carbon−sulfur analyzer. Raman scattering spectra were recorded with a confocal Raman spectrometer (Senterra R200-L, Bruker Co., Germany), using the 532 nm line of a He−Ne laser as the excitation source.

3. RESULTS AND DISCUSSION From the ICP composition analysis, as shown in Table 1, it can be seen that the molar ratios among Li, V, Ce, and P for Li3V2−xCex(PO4)3/C samples prepared in this work are close to those of the expected values. Table 1. Compositions of the As-Prepared Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) Samples. Note That P Is Referenced to 3 for Comparison Purpose atomic ratio samples

Li

V

Ce

P

Li3V2(PO4)3/C Li3V1.99Ce0.01(PO4)3/C Li3V1.97Ce0.03(PO4)3/C Li3V1.95Ce0.05(PO4)3/C

3.02 2.99 3.00 3.01

2 1.99 1.97 1.95

0.010 0.029 0.049

3 3 3 3

The X-ray powder diffraction (XRD) is used to examine the structure change of the Li3V2(PO4)3/C samples after doping various amounts of Ce3+ (0, 0.01, 0.03, 0.05), and the results are shown in Figure 1. In all cases, the as-prepared samples have strong and sharp reflection peaks and are in good agreement with the monoclinic Li3V2(PO4)3 with a space group of P21/n without other impurity phases. This result indicates that Ce3+ ions are successfully inserted into the crystal lattice of the Li3V2(PO4)3/C and does not affect the original monoclinic structure of the Li3V2(PO4)3/C. We expected that Ce3+ ions may occupy the V3+ sites in the lattice to form a stable solidsolution.10,41,45,46 Table 2 reports the unit cell lattice parameters of all the experimental Li3V2−xCex(PO4)3/C phase. It clearly appears that the unit cell volume of Li3V2−xCex(PO4)3/C samples increases after Ce3+ doping, suggesting that Ce3+ doping could cause the lattice expansion, which may be attributed to that the radius of the Ce3+ ion is 19526

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Figure 2. SEM images of the Li3V2−xCex(PO4)3/C samples, (a) x = 0, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05.

Figure 1. XRD patterns of the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples.

ties,48 and it is worth mentioning that such a porous structure is advantageous to the penetration of the electrolyte into the Li3V2(PO4)3 active material.49−51 EDS spectrum and EDS dot maping measurments are adopted to further clarify the elemental composition of asprepared samples. Figure 3a reveals the EDS spectrum of the

Table 2. Lattice Parameters of the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) Samples x

a (Å)

b (Å)

c (Å)

β(deg)

V (Å3)

0 0.01 0.03 0.05

8.6045 8.6215 8.6202 8.6377

8.5987 8.6081 8.6041 8.6172

12.0902 12.0789 12.0926 12.0824

89.8943 89.9936 89.9383 90.2216

894.53 896.43 896.90 899.33

much bigger than that of the V3+ ion. This observation is consistent with the result of the previously reported Nb-doped Li3V2(PO4)3 materials.29 Such expansion in the crystal lattice could provide more space for lithium ion diffusion, which is beneficial to electrochemical performance of Li3V2(PO4)3 materials.21,47 The formation steps of ordered macroporous samples are as follows. First, the initial Li3V2(PO4)3 precursor solution infiltrates into the PMMA templates, and then solidifies in the void space of the PMMA template. After dried and sintered at 850 °C under the Ar atmosphere, the PMMA beads were decomposed and converted into carbon in situ, and the ordered macroporous Ce3+ doped Li3V2−xCex(PO4)3/C samples were obtained. The carbon content of all as-prepared samples is about 4.1%, which was determined by carbon−sulfur analyzer. Figure 2 reveals the morphology of the Ce3+-doped macroporous Li3V2−xCex(PO4)3/C samples. As it is seen in the SEM images, Ce3+-doped Li3V2−xCex(PO4)3/C samples with different Ce3+ content all exhibit the honeycomb-type structure and long-range order morphology, as observed in the pristine sample. Also interesting to find is that the lighter regions represent the macroporous walls, while the darker regions delineate the macroporous windows once occupied by the PMMA template. The pores left in the Ce 3+ -doped Li3V2−xCex(PO4)3/C samples still show interconnected networks and well-defined close-packed structure. The average pore size is 214 nm for the pristine Li3V2(PO4)3 sample, 209 nm for the Li3V1.99Ce0.01(PO4)3/C sample, 206 nm for the Li 3 V 1.97 Ce 0.03 (PO 4 ) 3 /C sample, and 208 nm for the Li3V1.95Ce0.05(PO4)3/C sample, respectively. Such phenomenon proves that the doping of Ce element does not affect the formation of ordered macroporous structure. The previous report demonstrates that the morphology of the Li3V2(PO4)3 material has a notable effect on the electrochemical proper-

Figure 3. (a) EDS spectrum of the Li3V1.99Ce0.01(PO4)3/C sample, and elemental mapping for the Ce element, (b) Ce (x = 0.01), (c) Ce (x = 0.03), (d) Ce (x = 0.05).

Li3V1.99Ce0.01(PO4)3/C sample. The existence of carbon, oxygen, phosphorus, and vanadium elements can be clearly confirmed. However, the peak assigned to Ce element cannot be clearly found because of the small doping amount. A similar observation is also reported for the nickel element in the Li2.5Na0.5V(2−2x/3)Nix(PO4)3/C composite.52 Figure 3 panels b−d display the EDX dot mapping, which is effective to prove the existence of the Ce element in the Li3V1.99Ce0.01(PO4)3/C sample. Also, it can be qualitatively found that the Ce amount increases with enhancing the x value. The steady-state cyclic voltammetry (CV) measurements are conducted to investigate the effect of Ce3+ doping on the reversibility of the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples with a scanning rate of 0.2 mV s−1, and the corresponding results are shown in Figure 4. The CV curves 19527

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same rates. The Li3V1.97Ce0.03(PO4)3/C sample delivers an initial discharge capacity of 131.3 mAh g−1 at 0.2 C, which is very close to the theoretical capacity (133 mAh g−1) between 3.0 and 4.3 V. Although raising the current rate results in a drop in the discharge capacity, satisfactory initial discharge capacities of 129.6, 127.8, 124.7, and 119.1 mAh g−1 can still be obtained at the rates of 0.5 C, 1 C, 2 C, and 5 C, respectively. At the same time, in the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.05) system, the initial discharge capacities are 129.3, 125.3, 124.5, 120.7, and 117.6 mAh g−1 for the pristine Li3V2(PO4)3/C sample, 129.4, 125.5, 124.4, 122.6, and 118.0 mAh g−1 for Li3V1.99Ce0.01(PO4)3/C sample, and 129.3, 127.8, 125.2, 120.8, and 113.4 mAh g−1 for Li3V1.95Ce0.05(PO4)3/C sample at the rates of 0.2 C, 0.5 C, 1 C, 2 C, and 5 C, respectively. Figure 6 shows the cycle performance of the Li3V2−xCex(PO4)3 samples in the range of 3.0−4.3 V. At the low rate of 0.2 C, discharge capacities of 127.8, 126.1, 130.5, and 128.2 mAh g−1 with capacity retention of 98.7%, 97.4%, 99.4% and 99.1% can be retained after 50 cycles when x = 0, 0.01, 0.03, 0.05, respectively. Even at the high rate of 5 C, the Li3V1.97Ce0.03(PO4)3/C sample can still present a discharge capacity of 96.5 mAh g−1 with 81% capacity retention after 100 cycles, while the discharge capacities fade to 69.9, 90.0, and 53.2 mAh g−1 with capacity retention of 59.4%, 76.3%, and 46.9% when x = 0, 0.01, and 0.05, respectively. It is obvious that compared with the other three samples, the Li3V1.97Ce0.03(PO4)3/C sample presents higher discharge capacity and capacity retention, and such difference enhances with increasing charge/discharge rate, indicating that substitution of V3+ by Ce3+ with suitable amount (x = 0.03) is beneficial to improving rate capability and cycle stability. This finding may be mainly attributed to that the larger radius Ce3+ substitution could enlarge the cell volume of the pristine sample, which is confirmed by the XRD result, and thus lead to larger channels for Li+ rapid diffusion in active material.14 In addition, it should be noted that the Li3V1.95Ce0.05(PO4)3/C sample has lower discharge capacities and poorer cycle performance at high rates (2 and 5 C) than the pristine sample, indicating that excess Ce 3+ substitution in Li3V2(PO4)3/C lattice may have negative impact on the NASICON structure. To study the cycle stability of as-obtained samples at high operation voltage, the cycle performance of the Li3V1.97Ce0.03(PO4)3/C sample is examined in the potential range of 3.0−4.8 V at the rates of 1 and 5 C, and the result is displayed in Figure 7. It can be seen that the sample has capacity fade on cycling, because of the electrolyte instability at the high voltage of 4.8 V,54,55 which is similar to the investigation reported in the literature.56 Even so, such a sample exhibits acceptable cycle stability. It can achieve initial discharge capacities of 169 and 148.4 mAh g−1 at 1 and 5 C, respectively. After 100 cycles, discharge capacities of 147.9 mAh g−1 with 87.5% capacity retention and 124.7 mAh g−1 with 84.0% capacity retention can be reached at 1 and 5 C, respectively. In comparison, according to our previous report,42 the pristine sample presents discharge capacities of 118.3 mAh g−1 and 112.5 mAh g−1 at 1 and 5 C after 100 cycles, respectively. It is evident that compared with the pristine sample, the Li3V1.97Ce0.03(PO4)3/C sample shows obviously improved rate ability and cycle performance between 3.0 and 4.8 V. Recently, Dang et al.41 synthesized Ce-doped Li3V2(PO4)3/C samples by a microwave assisted sol−gel process and investigated their electrochemical performances

Figure 4. Cyclic voltammograms of the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples with a scan rate of 0.2 mV s−1 in the potential range of 3.0−4.3 V.

have a similar shape, indicating that the doping with Ce element does not obviously affect the electrochemical reaction mechanism of Li3V2(PO4)3. All four samples exhibit three oxidation peaks, labeled as OP1, OP2, and OP3, as well as three corresponding reduction peaks, denoted as RP1, RP2, and RP3, respectively. These oxidation/reduction peaks correspond to the lithium ion extraction from the lattice or insertion into the lattice during the phase transition processes: Li3V2(PO4)3 ↔ Li2.5V2(PO4)3 ↔ Li2V2(PO4)3 ↔ LiV2(PO4)3.53 Also, as evidenced in Figure 4, the Li3V1.97Ce0.03(PO4)3/C sample exhibits the largest peak height among these four samples, indicating the highest lithium ion diffusion coefficient of the Li3V1.97Ce0.03(PO4)3/C sample. In addition, the potential differences of the Li2V2−xCex(PO4)3/C samples between reduction and oxidation peaks, which reflect the polarization degree of samples, are shown in Figure 4. It can be observed that compared with the other three samples, oxidation peaks of the Li3V1.97Ce0.03(PO4)3/C sample move to a more negative direction, and reduction peaks move to a more positive direction. This means that the Li3V1.97Ce0.03(PO4)3/C sample shows the smallest potential differences between the oxidation peaks and reduction peaks, from which it can be deduced that the Li3V1.97Ce0.03(PO4)3/C sample has the lowest electrochemical polarization, and thus leads to the best reversibility during the charge/discharge process. Figure 5 panels a−d depict the initial charge−discharge curves of the as-prepared Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples at various current rates between 3.0 and 4.3 V. It can be seen that all above-mentioned samples exhibit three charge plateaus and three corresponding discharge plateaus, corresponding to the phase transition process in LiyV2(PO4)3 (y = 3.0, 2.5, 2.0, 1.0) during electrochemical reactions between 3.0 and 4.3 V, which is in good agreement with the CV result. In the case of Li3V2−xCex(PO4)3/C (x = 0.01, 0.03) samples, the charge/discharge plateaus are still maintained at high rates of 2 and 5 C, however, the plateaus for the Li3V2−xCex(PO4)3/ C (x = 0, 0.05) samples are not clearly visible. Figure 5e illustrates the relationship between the specific discharge capacity and x value at various C rates for all samples. It is found that the Li3V1.97Ce0.03(PO4)3/C sample exhibits a bit higher initial discharge capacities than the pristine sample at the 19528

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Figure 5. Initial charge−discharge curves of the Li3V2−xCex(PO4)3/C samples (a) x = 0, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05 at the current rates of 0.2 C, 0.5 C, 1 C, 2 C, and 5 C in the potential range of 3.0−4.3 V. (e) The relationship between the specific discharge capacity and x value at various C rates for all samples.

istic of as-obtained samples, and the Nyquist plots are described in Figure 9a. The EIS curves of all four samples are almost the same. The intercept in the high frequency corresponds to the solution resistance (RS); the semicircle in the medium frequency is related to the charge transfer resistance (Rct) and double layer capacitance (Cdl); and the sloping line in the low frequency is assigned to the diffusion of the lithium ions in the electrode. The Rct value of the Li3V1.97Ce0.03(PO4)3/C sample (166.3 Ω) is much smaller than that of the pristine sample (196.1 Ω), the Li3V1.99Ce0.01(PO4)3/C sample (182.4 Ω), and the Li3V1.95Ce0.05(PO4)3/C sample (211.2 Ω), as presented in the inset of Figure 9a, which demonstrates that the electrons are capable of transferring more quickly than other three samples. Figure 9b shows the linear fittings of Z′ vs ω−1/2 in the low frequency region, where ω is the angle frequency. A linear trend can be observed for all four samples, and the slope of the lines corresponds to the Warburg factor that is inversely proportional to the Li+ diffusion coefficient.30,58,59 It can be found that the Li3V1.97Ce0.03(PO4)3/C sample exhibits the

between 3.0 and 4.8 V. It can be found that the discharge capacity of the Li3V1.97Ce0.03(PO4)3/C sample obtained in this work is comparable to that of Ce-doped Li3V2(PO4)3/C samples prepared by Dang et al. The results prove that doping with Ce3+ at an appropriate amount could also improve the electrochemical performance in the wide potential range of 3.0−4.8 V. Figure 8 exhibits Raman spectra of Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples. In all cases, the as-prepared samples show similar Raman spectra including two intense broad bands at ∼1350 and 1590 cm−1, which are associated with the D band (disorder carbon, sp3) and G band (graphite, sp 2 ) of residual carbon in the samples, respectively. Furthermore, no obvious difference in the intensity ratio of ID/IG can be observed, which further reveals that the improvement in electrochemical performance mainly depends on Ce3+ substitution rather than the carbon coating.9,57 Electrochemical impedance spectroscopy (EIS) tests are conducted to investigate the electrochemical kinetic character19529

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Figure 6. Cycle performances of the Li3V2−xCex(PO4)3/C samples (a) x = 0, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05 at the rates from 0.2 to 5 C in the potential range of 3.0−4.3 V.

Figure 7. Cycle performance of the Li3V1.97Ce0.03(PO4)3/C sample in the potential range of 3.0−4.8 V at 1 and 5 C.

Figure 8. Raman spectra of the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples.

smallest slope among the four samples, indicating the highest lithium ion diffusion coefficient of the Li3V1.97Ce0.03(PO4)3/C sample. This result clearly reveals that the Li+ diffusion coefficients of Li3V2−xCex(PO4)3/C samples can be effectively improved by a suitable amount of Ce3+ substitution, which leads to better electrochemical performance.

which is similar to the pristine one. At the same time, an increase in the Ce3+ doping amount leads to the enlargement of lattice volume in Ce3+-doped samples. The Li3V1.97Ce0.03(PO4)3/C sample exhibits the best electrochemical performance among the four samples. It delivers initial discharge capacities of 131.3, 129.6, 127.8, 124.7, and 119.1 mAh g−1 at 0.2 C, 0.5 C, 1 C, 2 C, and 5 C rates in the potential range of 3.0−4.3 V, respectively. At the high rate of 5 C, it can present a discharge capacity of 96.5 mAh g−1 with 81% capacity retention after 100 cycles. Moreover, between 3.0 and 4.8 V, discharge capacities of 147.9 mAh g−1 with 87.5% capacity retention and 124.7 mAh g−1 with 84.0% capacity retention can still be reached at 1 and 5 C after 100 cycles, respectively. The comparative investigation regarding the Li+ diffusion coef-

4. CONCLUSIONS In this study, a series of Ce3+-doped Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) cathode materials have been synthesized by a facile template method using the colloidal crystal PMMA as a template. It is found that the Ce3+-doped Li3V2(PO4)3/C samples all reveal the single monocline phase with the space group P21/n, as well as ordered macroporous morphology, 19530

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Figure 9. (a) Nyquist plots of the Li3V2−xCex(PO4)3/C (x = 0, 0.01, 0.03, 0.05) samples, (b) the linear fitting of the Z′ vs square root of frequency (ω−1/2) relationship.

ficients of Li3V2−xCex(PO4)3/C samples suggests that substitution of V3+ with an appropriate amount of Ce3+ could enhance the diffusion rate of Li+, and thus lead to the impressive rate capability and cycle performance.

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(9) Zhang, L. L.; Liang, G.; Peng, G.; Zou, F.; Huang, Y. H.; Croft, M. C.; Ignatov, A. Significantly improved electrochemical performance in Li3V2(PO4)3/C promoted by SiO2 coating for lithium-ion batteries. J. Phys. Chem. C 2012, 116, 12401−12408. (10) Vujković, M.; Jugović, D.; Mitrić, M.; Stojkovic, I.; Cvjetićanin, N.; Mentus, S. The LiFe(1−x)VxPO4/C composite synthesized by gelcombustion method, with improved rate capability and cycle life in aerated aqueous solutions. Electrochim. Acta 2013, 109, 835−842. (11) Duan, W.; Hu, Z.; Zhang, K.; Cheng, F.; Tao, Z.; Chen, J. Li3V2(PO4)3@C core-shell nanocomposite as a superior cathode material for lithium-ion batteries. Nanoscale 2013, 5, 6485−6490. (12) Pan, A.; Choi, D.; Zhang, J.-G.; Liang, S.; Cao, G.; Nie, Z.; Arey, B. W.; Liu, J. High-rate cathodes based on Li3V2(PO4)3 nanobelts prepared via surfactant-assisted fabrication. J. Power Sources 2011, 196, 3646−3649. (13) Rui, X.; Sim, D.; Wong, K.; Zhu, J.; Liu, W.; Xu, C.; Tan, H.; Xiao, N.; Hng, H. H.; Lim, T. M.; Yan, Q. Li3V2(PO4)3 nanocrystals embedded in a nanoporous carbon matrix supported on reduced graphene oxide sheets: Binder-free and high rate cathode material for lithium-ion batteries. J. Power Sources 2012, 214, 171−177. (14) Chen, Q.; Qiao, X.; Wang, Y.; Zhang, T.; Peng, C.; Yin, W.; Liu, L. Electrochemical performance of Li3‑xNaxV2(PO4)3/C composite cathode materials for lithium ion batteries. J. Power Sources 2012, 201, 267−273. (15) Kuang, Q.; Zhao, Y.; Liang, Z. Synthesis and electrochemical properties of Na-doped Li3V2(PO4)3 cathode materials for Li-ion batteries. J. Power Sources 2011, 196, 10169−10175. (16) Wang, R.; Xiao, S.; Li, X.; Wang, J.; Guo, H. Structural and electrochemical performance of Na-doped Li3V2(PO4)3/C cathode materials for lithium-ion batteries via rheological phase reaction. J. Alloys Compd. 2013, 575, 268−272. (17) Mateyshina, Y. G.; Uvarov, N. F. Electrochemical behavior of Li3‑xM′xV2‑yM″y(PO4)3 (M′ = K, M″ = Sc, Mg + Ti)/C composite cathode material for lithium-ion batteries. J. Power Sources 2011, 196, 1494−1497. (18) Huang, J. S.; Yang, L.; Liu, K. Y.; Tang, Y. F. Synthesis and characterization of Li3V(2−2x/3)Mgx(PO4)3/C cathode material for lithium-ion batteries. J. Power Sources 2010, 195, 5013−5018. (19) Chen, Z.; Yuan, G.; Dai, C.; Hu, X.; Luo, X. Electrochemical behavior of Mg-doped 7LiFePO4-Li3V2(PO4)3 composite cathode material for lithium-ion batteries. Ionics 2013, 1−8. (20) Dai, C.; Chen, Z.; Jin, H.; Hu, X. Synthesis and performance of Li3(V1‑xMgx)2(PO4)3 cathode materials. J. Power Sources 2010, 195, 5775−5779. (21) Zhang, S.; Wu, Q.; Deng, C.; Liu, F. L.; Zhang, M.; Meng, F. L.; Gao, H. Synthesis and characterization of Ti-Mn and Ti-Fe codoped Li3V2(PO4)3 as cathode material for lithium ion batteries. J. Power Sources 2012, 218, 56−64. (22) Chen, Y.; Zhao, Y.; An, X.; Liu, J.; Dong, Y.; Chen, L. Preparation and electrochemical performance studies on Cr-doped Li3V2(PO4)3 as cathode materials for lithium-ion batteries. Electrochim. Acta 2009, 54, 5844−5850.

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 21 54748917. Fax: +86 21 54741297. E-mail: [email protected] (Z.X. Zhang) *E-mail: [email protected] (L. Yang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to the National Natural Science Foundation of China (Grants No. 21373136) and University of Michigan−Shanghai Jiao Tong University 2012 Collaborative Research Project. We thank the Instrumental Analysis Center of Shanghai Jiao Tong University for Materials Characterization.

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