carbon composite cathode materials synthesized with organophosphorus source

Electrochimica Acta 167 (2015) 278–286

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Vanadium-doping of LiFePO4/carbon composite cathode materials synthesized with organophosphorus source Ming Chen a,1, Leng-Leng Shao a,1, Hua-Bin Yang a , Tie-Zhen Ren b , Gaohui Du c , Zhong-Yong Yuan a, * a Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin 300071, China b School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China c Key Laboratory of the Ministry of Education for Advanced Catalytic Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 January 2015 Received in revised form 27 February 2015 Accepted 25 March 2015 Available online 28 March 2015

A series of V-doped LiFePO4/C composites are synthesized through a quasi-sol–gel method using organophosphorus source. Amino tris(methylene phosphonic acid) is utilized as phosphorus and carbon co-precursor, sucrose as assisted carbon source, and NH4VO3 as dopant. The effect of vanadium doping on the structural property and electrochemical performance of LiFePO4/C cathode material is systematically investigated. Carbon is coated uniformly on the LiFePO4 nanoparticles and vanadium (+4) is incorporated into LiFePO4/C without altering the olivine structure. The appropriate V-doping refined LiFePO4 particle
O bonds, leading to the enhancement of Li+ size, induced the lattice distortion and weakened the Li
diffusion and electronic conductivity, and hence resulting in the improvement of the electrochemical performance of LiFePO4/C composites. A high rate discharge capacity of 140.2 mAh g
1 at 5 C, 120.4 mAh g
1 at 10 C, and 105.8 mAh g
1 at 20 C is obtained for LiFe0.97V0.03PO4/C. It is demonstrated that the simple and green synthesis process of the quasi-sol–gel route with organophosphonic acid as precursor is efficient to fabricate V-doped LiFePO4/C composite cathode materials for high-power lithium-ion batteries. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Lithium iron phosphate Vanadium doping Carbon coating Organophosphonic acid Lithium-ion batteries

1. Introduction Since the successful utilization of lithium ion batteries (LIBs) as the green and sustainable power sources for electric vehicles and hybrid electric vehicles, it proposes higher requirements for the cathode materials in LIBs, such as capacity, cycle life, safety and cost [1]. Olivine-type LiFePO4 is regarded as one of the most promising cathode materials for LIBs, because of its environmental benignity, low cost, good thermal stability, long life span and high power capability [2,3]. However, the poor electronic (10
9–10
10 S cm
1) and ionic (10
14 and 10
16 cm2 s
1) conductivity seriously undermine its high rate performance [4,5]. Carbon coating and particle size controlling are commonly used ways to overcome these drawbacks [6–9], in which the carbon coating enhanced the electrical contact of particles, and the particle size

* Corresponding author. E-mail address: [email protected] (Z.-Y. Yuan). These authors have made an equal contribution to this work.

1

http://dx.doi.org/10.1016/j.electacta.2015.03.185 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

controlling shortened the diffusion pathway of Li+. Nevertheless, the volumetric energy density and tap density would simultaneously decrease with the increase of the carbon content and the reduction of the particle size. Therefore, in view of practical application, it needs a more effective method for further improving the kinetics of Li+ and the rate performance of LiFePO4/C. Chemical doping of LiFePO4/C with multivalent cations (V3+, Al3 + , Ti4+, Ni2+, Co2+, Mg2+) could improve the internal electronic and ionic conductivity by tuning the LiFePO4 crystalline structure such as inducing lattice distortion, and refining particle size [4,10–12]. Theoretically, doping cations may occur on Li or Fe sites [11,13,14], but many previous studies have been devoted to Fe-site doping, since doping at Li site will hinder the Li+ diffusion [15]. Among the various Fe-site doping elements, vanadium was demonstrated to be the extraordinary one in enhancing the overall electrochemical performance of LiFePO4 [16–18]. Much effort has thus recently been taken on the physicochemical and electrochemical properties of V-doped LiFePO4/C. For example, Zhang et al. [17] synthesized V-doped LiFePO4/C via a two-step solid-state reaction route, which achieved a specific capacity of 151.4 mAh g
1 at 0.1 C due to the

M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

inorganic phosphorus source (H3PO4 and NH4H2PO4), was developed for the preparation of the LiFePO4/C composites, and the effect of V-doping on the structural property and electrochemical performance of LiFePO4/C composites was systematically investigated. The higher Li+ diffusion and better rate capability to those of the pristine LiFePO4/C were achieved based on the optimized crystal lattice and the weakened Li-O bonds that induced by V doping, making the V-doped LiFePO4/C as the potential cathode materials for the high-power LIBs.

x= 0.05 x = 0.03 x = 0.01

Intensity (a.u.)

x =0 /0

279

x = 0.05 x = 0.03

2. Experimental

x = 0.01

2.1. Preparation of LiFe1-xVxPO4/C composites

x=0

20

30

40

50

60

70

80

2 / Fig. 1. XRD patterns of LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, and 0.05) and the inset is the magnification of LiFe1
xVxPO4/C in the 2u range of 20–35 .

The V-doped LiFePO4/C composites were synthesized by a quasi-sol–gel route with the use of amino tri(methylene phosphonic acid) (ATMP) as raw material, and all the used starting materials were of analytical grade and used as received without further purification. In the typical synthesis, 7.2 g of FeC2O4  2H2O, 1.7 g of LiOH  H2O, 4.2 g of ATMP and 0.6 g of sucrose, together with an appropriate amount of NH4VO3 (0, 0.046, 0.14, or 0.23 g), were

Table 1 Lattice parameters obtained by Rietveld refinement for the LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, 0.05) samples. Sample

LiFePO4/C LiFe0.99V0.01PO4/C LiFe0.97V0.03PO4/C LiFe0.95V0.05PO4/C

Lattice (Pnma)

Interatomic distances (Å)

Reliability factors (%)

a (Å)

b (Å)

c (Å)

Volume (Å )

Li–O1

Li–O2

Li–O3

S Li–O

Rwp

Rp

x2

10.3282 10.3255 10.3247 10.3221

6.0074 6.0065 6.0062 6.0058

4.6943 4.6945 4.6949 4.6960

291.27 291.16 291.14 291.11

2.1248 2.1304 2.1762 2.0966

2.1045 2.1222 2.1246 2.1180

2.1476 2.1547 2.1753 2.2418

2.1256 2.1358 2.1587 2.1522

4.54 4.48 4.59 4.68

5.7 5.69 5.73 5.88

5.64 5.67 5.68 5.78

3

increased electronic conductivity of 7.48  10
3 to ca. 10
6 S cm
1 of the pristine LiFePO4/C. Graetz et al. [10] further found that the substitution of V into the Fe sites reduced the bottleneck of Li+ migration by enlarging the Li+ cross-sectional area in the LiO6 octahedral face and contributed to the improved cycling rate performance of LiFePO4/C. Therefore, the appropriate substitution of V for Fe is beneficial to keeping high electron-ion conductivity in LiFePO4 and hence enhancing its high rate performance. In addition, it is very noticeable that the high-level V doping into the LiFePO4 often resulted in the mixed phases of LiFePO4 and Li3V2(PO4)3 [16,19–21]. Although it is demonstrated that the Li3V2(PO4)3 improved the electrical conductivity and the rate capability of the LiFePO4-based cathode, the enhancement is lower than that provided by cation substitution into the LiFePO4 crystal structure [10,13]. Also, the lithium-ion transport in the LiFePO4 was particularly susceptible to blockage by the impurities [16,22]. For the high-performance cathode materials, it is highly desirable to obtain the low content vanadium-doped LiFePO4 crystal structure. In this work, a simple quasi-sol-gel route using organophosphonic acid as precursor, instead of conventional-used

mixed in 6 ml of deionized water, manually grinded to form the yellow quasi-sol-gel solution, and continually stirred under magnetic stirring (300 rpm) until the evaporation of water. The obtained yellow paste was dried at room temperature overnight and transferred to a quartz boat. Precalcination was carried out at 350  C for 3 h in a tubular furnace under nitrogen protection. After cooling to room temperature, the resulting powders were further grinded and calcinated at 700  C for 3 h. The obtained products were correspondingly denoted as LiFe1-xVxPO4/C (x = 0, 0.01, 0.03 or 0.05). 2.2. Characterization X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus diffractometer with Cu Ka radiation (l = 1.5418 Å) operated at 40 kV and 40 mA. The diffraction data were collected over the 2u range of 10 to 80 at a scan rate of 0.5 min
1. Rietveld refinement was performed by using Rietica software. A Pnma space group was used as the model. X-Ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos Axis Ultra DLD

Table 2 The content of V, Fe (ICP results) and carbon (TG results), and electrochemical parameters of LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, 0.05) samples. Sample

LiFePO4/C LiFe0.99V0.01PO4/C LiFe0.97V0.03PO4/C LiFe0.95V0.05PO4/C

ICP

TG

V (wt.%)

Fe (wt.%)

Practice x values

C (wt.%)

0 0.29 0.87 1.47

34.33 33.98 33.49 32.70

0 0.0092 0.0275 0.0465

2.54 2.53 2.49 2.43

Electronic conductivity (S cm
1)

1.58  10
5 8.41 10
5 1.05  10
4 1.48  10
4

EIS

Rs (V)

Rct (V)

DLi+ (cm2 s
1)

3.5 3.5 3.4 3.3

34.6 31.1 27.7 42.3

2.61 10
13 3.16  10
13 1.48  10
12 2.63  10
13

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M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

(delay line detector) spectrometer equipped with a monochromatic Al Ka X-ray source (1486.6 eV). The XPS survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra with a pass energy of 40 eV. Binding energies were calibrated by using the contaminant carbon (C1s 284.6 eV). Scanning electron microscopy (SEM) was carried out on a JEOL JSM-7500F microscope equipped with an energy dispersive X-ray detector (EDX) at

5 kV. Transmission electron microscopy (TEM) was performed on a Jeol JEM 2010F microscope at 200 kV. Thermogravimetric analysis (TGA) of the samples were conducted on a TA SDT Q600 analyzer in air atmosphere with a heating rate of 5  C min
1. The content of V and Fe were investigated by inductive coupled plasma (ICP) emission spectroscopy on a Thermo Jarrell-Ash ICP-9000 (N + M) spectrometer.

Fig. 2. SEM images of LiFe1
xVxPO4/C composites (x = 0, 0.01, 0.03, and 0.05 corresponding to A, B, C and D, respectively), the elemental mappings of V and Fe for the LiFe0.97V0.03PO4/C sample (E, F), and the EDX spectrum of LiFe0.97V0.03PO4/C sample (G).

M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

281

Fig. 3. TEM images of LiFePO4/C (A) and LiFe0.97V0.03PO4/C (B, D) composites, and SAED patterns of LiFe0.97V0.03PO4/C composite bulk region (C) and particle boundary (inset in (D)).

Galvanostatic charge–discharge tests were performed by using a 2032-type coin cell. The working electrode was fabricated by cathode composite, carbon black (Super-P) and polyvinylidene fluoride in a weight ratio of 80:10:10. The counter electrode was a disk of lithium metal foil (13 mm in diameter, 0.5 mm thick). The electrolyte was a 1 M solution of LiPF6 in 1:1 (v/v) ethylene carbonate/dimethyl carbonate. Celgard 2400 membrane was used as a separator. The cells were galvanostatically charged and discharged between 2.5 V and 4.2 V versus Li/Li+ at room temperature (25  C) on a LAND Electronic CT2001A electrochemical test instrument. The current densities corresponding to 0.1, 5, 10, and 20 C rates (1 C corresponds to 170 mAh g
1) were tested. The CV and EIS measurments were evaluated in a three-electrode system with lithium metal as the counter electrode and reference electrodes, LiFe1
xVxPO4/C composites as the working electrode. Cyclic voltammetry (CV) was conducted by using an IM6 instrument at a scanning rate of 0.05 to 1 mV s
1 between 2.5 and 4.4 V. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a discharge state with a sinusoidal signal of 5 mV over a frequency range from 100 kHz to 100 mHz. The conductivity of the composite was measured by a four-probe meter detector (RTS-8, with a probe station of S-2A, and four probes of FT-201) at room temperature (relative humidity ca. 25%). The test samples were prepared by uniaxially pressing LiFe1xVxPO4/C composites into pellets with 13 mm in diameter, and 0.20–0.30 mm in thickness at 10 MPa, then Ag paste was coated on the both sides of the pellets. 3. Results and discussion 3.1. Structural characterization In the traditional synthesis of LiFePO4, H3PO4 and ammonium phosphate have been the dominant phosphorus sources. The use of these conventional phosphorus sources pose major risks to the practical production and local environment, in which the liquid

acids (H3PO4) may cause the corrosion of equipment and ammonium phosphate would easily decompose into toxic ammonia. Thus, the more desirable alternative phosphorus source should be developed in order to meet the demand of economic synthesis, clean environment and high performance. In this work, organophosphonic acid ATMP was used as a new phosphorus source for the synthesis of high-performance LiFePO4/C and LiFe1xVxPO4/C composite cathode materials by a simple and effective quasi-sol–gel route (Fig. S1, Supporting Information). Indeed, ATMP is one of the industrially used water clean compounds, which is low-cost and environmental-friendly, demonstrating recently its potential in the fabrication of new inorganic-organic hybrid multifunctional materials such as porous metal phosphonates [23,24]. In consideration of appropriate phosphorus atoms in the organic framework, the strong interaction with metal ions and the characteristics of solid acid, the organophosphonic acid can be a potential phosphorus source for LiFePO4. In the designed synthesis route, the terminal RPO3 groups in the ATMP can in situ chelate Li+ and subsequently bind with FeC2O4 in aqueous solution, forming the molecule-scale homogeneous precursor, and the organic carbon contained in ATMP with the assistance of sucrose can form highly uniformly distributed conductive carbon networks among LiFePO4 particles after calcination, resulting in the LiFePO4/C composites with enhanced electrochemical performance. Moreover, the small amount of alien metal ion V can be efficiently doped in the LiFePO4/C due to the presence of ATMP and can be well distributed, showing a more powerful and efficient approach than the conventionally solid-state reactions involving in the H3PO4 or ammonium phosphate. Fig. 1 shows the XRD profiles of the ATMP-synthesized LiFe1xVxPO4/C (x = 0, 0.01, 0.03, 0.05) samples. The diffraction patterns of all the samples can be indexed as an olivine orthorhombic LiFePO4 structure (Fig. S2, Supporting Information) with the space group of Pnma. The crystallographic evidence indicates that the samples with vanadium molar ratio less than 0.05 are well-crystallized. No impurity phases such as V5+ related crystalline substances could be

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M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

observed, but it still cannot determine whether all the samples are single-phase due to the low doping concentration. Nevertheless, the corresponding diffraction peaks of V-doped samples monotonically shifted to higher 2u with the increment of V doping, suggesting the change of lattice parameters and the shrinkage of the cell volume [10]. Rietveld refinement was performed to further confirm the lattice parameter change, extract the atomic occupation, and identify the doping position of the V [25,26]. Fig. S3 in Supporting

LiFe0.97V0.03PO4/C-V2p

Intensity (a.u.)

2p3/2 517 eV

2p1/2 524 eV Fig. 5. TGA profiles of the LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, 0.05) composites under air atmosphere with a heating rate of 5  C/min.

Information displays the Rietveld refinement plots of LiFePO4/C and LiFe0.97V0.03PO4/C composites, and the refined lattice parameters of all the samples were listed in Table 1. As Rp,Rwp and x2

Binding Energy (eV)

Intensity (a.u.)

LiFe0.97V0.03PO4/C

LiFePO4/C

Binding Energy (eV)

Intensity (a.u.)

LiFe0.97V0.03PO4/C

Binding Energy (eV) Fig. 4. XPS (a) V 2p3/2, (b) Fe 2p3/2 and (c) Li 1s spectra of LiFePO4/C and LiFe0.97V0.03PO4/C.

Fig. 6. (a) The first discharge curves of LiFePO4/C and LiFe0.97V0.03PO4/C composites at different C-rates, and (b) the cyclic performance of LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, 0.05) composites at different discharge rates.

M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

values are less than 6%, the Rietveld refinement results are believable. Noticeably, the lattice parameters of a and b reduced while c parameter enlarged with the increase of V doping content, which finally resulted in an overall decreased cell volume. The variation of these crucial parameters may imply that the alien vanadium was incorporated into the lattice of LiFePO4/C to substitute Fe2+. The smaller radius of the doped V4+ (0.72 Å) than that of Fe2+ (0.78 Å) is responsible for the shifts of lattice parameters [13]. What is more, V doping also causes the Li

O bonds stretched. As shown in the Table S1 (Supporting Information), the calculated occupancy of V at Fe site is about 0.026 for LiFe0.97V0.03PO4/C, which is in good accordance with the ICP results (Table 2). Fig. 2 shows the SEM images of LiFe1-xVxPO4/C (x = 0, 0.01, 0.03, 0.05) samples. The SEM images of V-doped LiFePO4/C show the particle size centered at ca. 450 nm, and is smaller than ca. 600 nm of LiFePO4/C. Furthermore, the particle sizes of LiFe1-xVxPO4/C get more homogeneous with the increase of V doping content. The result suggests that V doping optimized the particle size on the basis of carbon coated LiFePO4 [16]. In the TEM images of LiFePO4/C and LiFe0.97V0.03PO4/C composites (Fig. 3), both of the samples present a typical core–shell structure with the thickness of ca. 2 nm carbon layers wrapping on the 50–60 nm LiFePO4 particles. The

283

LiFe0.97V0.03PO4/C lattice fringe with the interplanar spacing of 0.44 and 0.52 nm corresponding to the (0 11) and (0 2 0) lattice planes, respectively, and the interfacial angle of (0 11) and (0 2 0) is about 90 , as shown in Fig. 3D. Moreover, the SAED pattern of the carbon layer in the LiFe0.97V0.03PO4/C (Fig. 3D (inset)) with a hollow ring pattern indicates its amorphous phase, and the one with bright spots of well-defined diffraction pattern (Fig. 3C) of olivine phase suggests that the good crystalline LiFePO4 structure was still maintained after V doping for LiFe0.97V0.03PO4/C [27,28]. To identify the V elements and its distribution in the LiFe1-xVxPO4/C, the EDX and EDS were carried out and shown in Fig. 2. In the case of LiFe0.97V0.03PO4/C, a small characteristic peak of V element in the EDX further confirmed the existence of V in the surface of the composite and the V element is uniformly distributed. XPS spectrum was utilized to further characterize the valence of V in the LiFe0.97V0.03PO4/C sample. As shown in Fig. 4a, the typical two-peak structure of V 2p3/2 at 517 eV and V 2p1/2 at 524 eV was observed, and the binding energy position consists well with that appeared in VO2 [16,29], suggesting the oxidation state of V in the doped sample is +4. The Fe 2p3/2 XPS (Fig. 4b) of LiFePO4/C and LiFe0.97V0.03PO4/C fits to a single peak with a binding energy of ca. 711.1 eV, demonstrating that the oxidation state of Fe is +2. As the oxidation state of V is higher than that of Fe, the supervalent doping occurred. Moreover, the binding energy of Li 1s (Fig. 4c) shifts down ca. 0.2 eV from LiFePO4/C to LiFe0.97V0.03PO4/C, which can be explained that the Li

O interaction is weakened by V doping in LiFePO4/C [11]. This agrees well with the analysis from the Rietveld refinement. Fig. 5 shows the TG profiles of the LiFe1-xVxPO4/C (x = 0, 0.01, 0.03, 0.05). It could be observed that the main weight gain occurs in the range of 300  C and 600  C, which is due to the oxidation of Fe2+ ions and carbon (12LiFePO4 + 3O2 ! 4Li3Fe2(PO4)3 + 2Fe2O3, C + O2 ! CO2). The actual carbon content calculated from the TG results is listed in Table 2. For LiFe1-xVxPO4/C (x = 0, 0.01, 0.03, 0.05), the carbon content is similar due to the same addition of ATMP and sucrose. It is expected that the carbon in the samples could render the LiFe1-xVxPO4/C with high electronic conductivity. It is measured that the electronic conductivities of LiFePO4/C, LiFe0.99V0.01PO4/C, LiFe0.97V0.03PO4/C and LiFe0.95V0.05PO4/C are 1.58  10
5, 8.41 10
5, 1.05  10
4 and 1.48  10
4 S cm
1, respectively. Interestingly, the electronic conductivity increased gradually with the increasing content of V doping. Since the carbon content in the composites are approximately equal (ca. 2.5 wt%), the enhanced electronic conductivity arises from V doping. The positive effect of V doping on the electronic conductivity can be explained on the basis of the electroneutrality principle that after higher valence V doping at Fe sites, a certain amount of Li vacancies were produced correspondingly in order to balance the difference valence between Fe2+ and V4+ [19]. The new appeared vacancies improved the P type conductivity of the LiFe1-xVxPO4 [4,30]. 3.2. Electrochemical performance

1/2 Fig. 7. (a) The Nyquist plots and equivalent circuit of LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, 0.05) composites, and (b) the relationship between Z0 and v
1/2 of LiFe1
xVxPO4/C (x = 0, 0.01, 0.03, 0.05) composites in the low-frequency region.

Fig. 6a shows the specific capacities versus the voltage curves of the LiFePO4/C and LiFe0.97V0.03PO4/C samples at different C-rates. Both the LiFePO4/C and LiFe0.97V0.03PO4/C samples showed flat voltage platforms around 3.4 V during discharge at 0.1C. When C rates increased from 0.1C to 20C, the discharge voltage platforms of both samples were decreased. Nevertheless, the LiFe0.97V0.03PO4/C displays a higher discharge voltage platform than that of LiFePO4/C at each rate. It is indicated that the polarization degree for both samples is strengthened with the C-rates increasing, but LiFe0.97V0.03PO4/C possesses a superior Li+ diffusibility to that of LiFePO4/C. Fig. 6b displays the specific capacities for LiFe1-xVxPO4/C (x = 0, 0.01, 0.03, 0.05) electrodes at 0.1, 5, 10 and 20 C-rates vs cycle numbers. At 0.1 C, LiFe0.97V0.03PO4 showed a specific capacity of

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M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

5 4 3 Current (mA)

2 1 0

-2 -3 -4 -5

0.05 mv/s 0.1 mv/s 0.2 mv/s 0.3 mv/s 0.4 mv/s 0.5 mv/s 1 mv/s

0.05 mv/s 0.1 mv/s 0.2 mv/s 0.3 mv/s 0.4 mv/s 0.5 mv/s 1 mv/s

-1

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Ip (mA)

Voltage (V)

(

)

(

)

Fig. 8. CV curves of LiFePO4/C (a) and LiFe0.97V0.03PO4/C (b) measured at different scan rates, and the variation of redox peak current (ip) to square root of scan rate (n 1/2) of LiFePO4/C (c) and LiFe0.97V0.03PO4/C (d).

150.4 mAh g
1, a little lower than 156.6 mAh g
1 of LiFePO4/C, suggesting that V doping has no advantage at low C-rate. However, when the current density increased, the enhancement on electrochemcal performance originated from V doping became distinctly larger. For LiFe0.97V0.03PO4/C, the specific capacities delivered at C-rates of 5, 10 and 20 C were 140.2, 120.4 and 105.8 mAh g
1, respectively, much higher than 126.0, 110.7 and 81.8 mAh g
1 of LiFePO4/C, respectively. The excellent electrochemical performance can be illustrated as following: As it had been proved that the Li+ migration path occurred in a curve type between adjacent Li sites (Fig. S4, Supporting Information) [31], the triangular LiO6 octahedral faces would provide space for the mobile Li+ to migrate. However, for the pristine LiFePO4 crystal structure, the Li+ diffuses extremely slowly along b axis under the confinement of nearly hexagonal close-packed oxygen (Fig. S5, Supporting Information) [32]. After introduction of V, the six closest Li

O bonds are weakened (Fig. S6, Supporting Information), lowering the activation energy for Li+ to effectively move and cross the LiO6 octahedral faces (Fig. S7, Supporting Information), and thus resulting in the higher ionic conductivity [33,34]. In addition, the Li vacancies, derived from V4+ doping at Fe2+ sites, would like to benefit the diffusion of Li+ by decreasing the space resistance [35]. Obviously, the three crucial factors including the carbon coating, nanosized particles, and V doping all contributed to the high electrical conductivity and fast lithium ion diffusion. Moreover, the V doping exhibited a crucial function in further enhancing electrical conductivity, refining particle size, and particularly improving

lithium-ion diffusion. Therefore, the rate performance of LiFe1electrode could be dramatically improved.

xVxPO4

3.3. Lithium ion diffusion coefficient EIS is conducted to evaluate the lithium ion diffusion coefficient and investigate the electrochemical behaviors during the charge/ discharge process. For the stable SEI film formation and the percolation of electrolyte through electrode particles, all cells were performed galvano statical cycles for three times before impedance measurements. Fig. 7a shows the Nyquist plots and the equivalent circuit of LiFe1-xVxPO4/C (x = 0, 0.01, 0.03, 0.05) tested at full depth of discharge. The curves of LiFe1-xVxPO4/C exhibit a semicircle in the high-frequency region and a beeline in the lowfrequency range, which represent charge-transfer resistance at the electrode/electrolyte interface and the diffusion resistance of lithium ions in the bulk electrode materials, respectively [36]. The corresponding equivalent circuit model (Fig. 7a (inset)) is composed of a system resistance (Rs), a constant phase element (CPE), a charge-transfer resistance (Rct), and a Warburg impedance (Zw). The detailed fitting data are summarized in Table 2. It can be seen that the Rs of LiFe1-xVxPO4 is similar owing to the same addition of the conductive carbon black (Super-P) and the reaction in the same electrolyte. However, the Rct is getting smaller with the increasing amount of V from the LiFePO4/C to LiFe0.97V0.03PO4/C electrode, and the LiFe0.97V0.03PO4/C with the optimal V content achieved the smallest Rct, which can be attributed to the unique

M. Chen et al. / Electrochimica Acta 167 (2015) 278–286

lattice structure of LiFe0.97V0.03PO4/C with the weakest Li

O bond resulting in the fastest lithium ion diffusion. However, excessive V doping in the LiFe0.95V0.05PO4/C may bring in some unfavorable impurity phases such as Li3V2(PO4)3 and block the Li+ diffusion channels, which has an adverse effect on the electrochemical performance [16,17]. To further study the lithium ion diffusion effect of the LiFe1xVxPO4/C (x = 0, 0.01, 0.03, 0.05), the lithium ion diffusion coefficients were calculated according to the Eq. (1) [37,38]: D¼

R2 T 2

(1)

2A2 n4 F 4 C 2 s 2

where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode (ca. 16 cm2), n is the number of electrons per molecule during oxidization, F is the Faraday constant, C is the concentration of lithium ion (7.69  10
3 mol cm
3 in this study), s is the Warburg factor associated with Z0 . Z 0 ¼ RD þ RL þ sv
1=2

(2)

Fig. 7b shows the relationship between Z0 and reciprocal square root of angular frequency (v
1/2) in the low-frequency region, and the slope of the beeline is the Warburg factor s. Based on Eqs. (1) and (2), the calculated diffusion coefficients are 2.61 10
13, 3.16  10
13, 1.48  10
12 and 2.63  10
13 cm2 s
1 for LiFePO4/C, LiFe0.99V0.01PO4/C, LiFe0.97V0.03PO4/C and LiFe0.95V0.05PO4/C, respectively. It is indicated that the diffusion velocity of lithium-ion, which is considered to be the rate-determining step in the redox process [39], has been drastically promoted in the V-doped composites. The decreased charge-transfer resistance and the increased lithium-ion diffusion velocity in the V-doped LiFePO4/C account for the superior rate performance.Fig. 8 presents the CV curves of LiFe1-xVxPO4/C (x = 0, 0.03), which are tested at different scanning rates. A good linear relationship of the peak current versus the square root of the sweeping rate is observed, indicating a diffusion-controlled electrode reaction [40]. For a reversible reaction in the LiFePO4, the relationship between peak current (ip) and lithium ion diffusion coefficient DLi+ under linear voltage scan rates can be described as the Eq. (3) [41]:  ip ¼ 0:4463F

F RT

1=2

1=2

n3=2 Cv1=2 ADLiþ

(3)

where ip is the peak current in amperes, n is scan rate in V s
1. The obtained DLi+ for charge/discharge processes of LiFePO4/C and LiFe0.97V0.03PO4/C are 2.46  10
13/2.28  10
13 and 1.55  10
12/ 1.36  10
12 cm2 s
1, respectively, which are similar to the values calculated from EIS results. The higher Li+ ion diffusion coefficient implies that Li+ is more mobile after V doping, well consistent with the Rietveld refinement and XPS results that the Li

O bond is weakened. All the results from CV and EIS suggested that V-doped LiFePO4/C composites exhibited higher lithium ion diffusivity than that of pristine LiFePO4/C composite. The significant improvement in chemical diffusion coefficient suggests that V doping increased DLi+ by optimizing the crystal structure of the LiFe1-xVxPO4/C. On the whole, the V doping in LiFePO4/C is an efficient way to enhance both the Li-ions and electrons conductivity, thus leading to higher reaction kinetics of lithiation and delithiation and a lower chargetransfer resistance. 4. Conclusions The LiFe1-xVxPO4/C (x = 0, 0.01, 0.03, 0.05) composites were synthesized for the first time by a quasi-sol–gel method using organophosphonic acid ATMP as precursor. The XRD, Rietveld refinement, EDX, and XPS demonstrated that the V ions were

285

sufficiently doped in LiFe1-xVxPO4/C and did not alter its crystal structure. Importantly, V doping in LiFe1-xVxPO4/C is favorable to the distortion of the lattice, the refinement of the particle size, and the increase in the ion-electron conductivity, thus greatly improving the electrochemical performance. The superior highrate performance was achieved for the LiFe0.97V0.03PO4/C, with the specific capacities of 140.2, 120.4 and 105.8 mAh g
1 at C-rates of 5, 10 and 20 C, respectively. As the synthesis route is simple and the reactants are green and cheap, this novel quasi-sol–gel method is very promising to produce high-power LiFePO4 for lithium ion secondary batteries. Acknowledgements This work was supported by the National Natural Science Foundation of China (21421001), the Natural Science Foundation of Tianjin (13JCZDJC32000), the Program for Innovative Research Team in University (IRT13022), the 111 project (B12015), and the Key Laboratory of Advanced Catalytic Materials in Zhejiang Normal University (ZJHX201301). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.03.185. References [1] F. Cheng, W. Wan, Z. Tan, Y. Huang, H. Zhou, J. Chen, X. Zhang, Electrochim. Acta 56 (2011) 2999–3005. [2] W. Kim, W. Ryu, D. Han, S. Lim, J. Eom, H. Kwon, ACS Appl. Mater. Interfaces 6 (2014) 4731–4736. [3] J. Zhang, J. Lu, D. Bian, Z. Yang, Q. Wu, W. Zhang, Ind. Eng. Chem. Res. 53 (2014) 12209–12215. [4] S.-Y. Chung, J.T. Bloking, Y.-M. Chiang, Nat. Mater. 1 (2002) 123–128. [5] P.P. Prosini, M. Lisi, D. Zane, M. Pasquali, Solid State Ionics 148 (2002) 45–51. [6] L.-B. Kong, P. Zhang, M.-C. Liu, H. Liu, Y.-C. Luo, L. Kang, Electrochim. Acta 70 (2012) 19–24. [7] Y. Wang, Y. Wang, E. Hosono, K. Wang, H. Zhou, Angew. Chem. -Int. Edit. 47 (2008) 7461–7465. [8] A. Kuwahara, S. Suzuki, M. Miyayama, J. Electroceram. 24 (2008) 69–75. [9] S. Ferrari, R.L. Lavall, D. Capsoni, E. Quartarone, A. Magistris, P. Mustarelli, P. Canton, J. Phys. Chem. C 114 (2010) 12598–12603. [10] J. Hong, X.-L. Wang, Q. Wang, F. Omenya, N.A. Chernova, M.S. Whittingham, J. Graetz, J. Phys. Chem. C 116 (2012) 20787–20793. [11] D. Wang, H. Li, S. Shi, X. Huang, L. Chen, Electrochim. Acta 50 (2005) 2955–2958. [12] C. -l. Fan, C. -r. Lin, S. -c. Han, J. Chen, L. -f. Li, Y. -m. Bai, K. -h. Zhang, X. Zhang, New J. Chem. 38 (2014) 795–801. [13] C.-Y. Chiang, H.-C. Su, P.-J. Wu, H.-J. Liu, C.-W. Hu, N. Sharma, V.K. Peterson, H.W. Hsieh, Y.-F. Lin, W.-C. Chou, C.-H. Lee, J.-F. Lee, B.-Y. Shew, J. Phys. Chem. C 116 (2012) 24424–24429. [14] M.-R. Yang, W.-H. Ke, J. Electrochem. Soc. 155 (2008) A729–A732. [15] M. Wagemaker, B.L. Ellis, D. Lützenkirchen-Hecht, F.M. Mulder, L.F. Nazar, Chem. Mater. 20 (2008) 6313–6315. [16] J. Ma, B. Li, H. Du, C. Xu, F. Kang, J. Electrochem. Soc. 158 (2011) A26–A32. [17] L.-L. Zhang, G. Liang, A. Ignatov, M.C. Croft, X.-Q. Xiong, I.M. Hung, Y.-H. Huang, X.-L. Hu, W.-X. Zhang, Y.-L. Peng, J. Phys. Chem. C. 115 (2011) 13520–13527. [18] C.S. Sun, Z. Zhou, Z.G. Xu, D.G. Wang, J.P. Wei, X.K. Bian, J. Yan, J. Power Sources 193 (2009) 841–845. [19] A. Moretti, G. Giuli, F. Nobili, A. Trapananti, G. Aquilanti, R. Tossici, R. Marassi, J. Electrochem. Soc. 160 (2013) A940–A949. [20] Y. Guo, Y. Huang, D. Jia, X. Wang, N. Sharma, Z. Guo, X. Tang, J. Power Sources 246 (2014) 912–917. [21] X. -d. Guo, B. -h. Zhong, H. Liu, Y. Song, J. -j. Wen, Y. Tang, T. Nonferr, Metal Soc. 21 (2011) 1761–1766. [22] D. Morgan, A. Van der Ven, G. Ceder, Electrochem. Solid. ST. 7 (2004) A30–A32. [23] T.-Y. Ma, X.-J. Zhang, Z.Y. Yuan, J. Phys. Chem. C 113 (2009) 12854–12862. [24] T.-Y. Ma, Z.-Y. Yuan, Eur. J. Inorg. Chem. 2010 (2010) 2941–2948. [25] J. Hong, C.S. Wang, X. Chen, S. Upreti, M.S. Whittingham, Electrochem. Solid. ST. 12 (2009) A33–A38. [26] M. Vujkovi c, D. Jugovi c, M. Mitri c, I. Stojkovic, N. Cvjeti canin, S. Mentus, Electrochim. Acta 109 (2013) 835–842. [27] Y. Liu, C. Cao, J. Li, Electrochim. Acta 55 (2010) 3921–3926. [28] D. Rangappa, K. Sone, T. Kudo, I. Honma, J. Power Sources 195 (2010) 6167–6171.

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