C composite cathode material for lithium ion batteries prepared by modified solid state method

Accepted Manuscript Electrochemical performance of Co-doped LiVPO4F/C composite cathode material for lithium ion batteries prepared by modified solid state method Zhongdong Peng, Zhanggen Gan, Ke Du, Yanbing Cao, Xiaoming Xie, Yong Wang, Yuanjun Li, Guorong Hu PII:

S0925-8388(17)33395-9

DOI:

10.1016/j.jallcom.2017.09.333

Reference:

JALCOM 43389

To appear in:

Journal of Alloys and Compounds

Received Date: 18 July 2017 Revised Date:

29 September 2017

Accepted Date: 30 September 2017

Please cite this article as: Z. Peng, Z. Gan, K. Du, Y. Cao, X. Xie, Y. Wang, Y. Li, G. Hu, Electrochemical performance of Co-doped LiVPO4F/C composite cathode material for lithium ion batteries prepared by modified solid state method, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.333. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Electrochemical performance of Co-doped LiVPO4F/C composite cathode material for lithium ion batteries prepared by modified solid state method

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Zhongdong Peng, Zhanggen Gan, Ke Du, Yanbing Cao, Xiaoming Xie, Yong Wang, Yuanjun Li and Guorong Hu*

School of Metallurgy and Environment, Central South University, Changsha 410083, P. R. China

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Abstract The LiV1-xCoxPO4F/C (x = 0, 0.01, 0.03 and 0.05) composite cathode

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material for lithium ion batteries has been successfully prepared through the modified solid state method assisted by sonication and mechanical activation. Here, the influence of cobalt doping on the physical and electrochemical performances are investigated. In this paper, we adopted the X-ray diffraction (XRD), X-ray

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photoelectron spectroscopy (XPS), scanning electron microscope (SEM)/energy dispersive spectrometer (EDS), cycle voltammetry (CV), electrochemical impedance spectrum (EIS) and charge-discharge test to investigate the influence of cobalt doping

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on the different aspects of the materials, including the physical and electrochemical

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properties. Compared with the original simple, the cycle and rate performance of cobalt doped samples has increased to some extent. We found that the LiV0.97Co0.03PO4F/C sample showed the best electrochemical performance. The lithium ions diffusion constant (DLi+) of the LiV0.97Co0.03PO4F/C (1.86321×10-13 cm2 s-1) is much higher than that of LiVPO4F/C (2.06453×10-14 cm2 s-1). Keywords: cobalt doping; lithium-ion batteries; lithium vanadium fluorophosphate; cathode material

ACCEPTED MANUSCRIPT 1. Introduction Recently, countries all over the world have paid great attention to electric vehicles (EV), especially in hybrid electric vehicles (HEVs) and plug-in hybrid electric

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vehicles (PHEVs). Most of the commercial lithium ion batteries[1-4] are made of layered lithium cobalt oxide[5-7] as cathode material. However, due to lack of raw material resources, as well as quite a few problems such as impoverished thermal

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power battery and renewable energy storage[8-10].

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stability, which restricts the application of lithium-ion batteries in the fields of high

Recently, the study and application of lithium iron phosphate[11-13], as the representative of the polyanion materials[12, 14-16], has gradually become a mainstream alternative to layered lithium cobalt material[4]. Unfortunately, the

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electronic conductivity of lithium iron phosphate[13] is low, and the working voltage is below3.5 V, which still cannot meet the demand of high energy density cathode materials[17]. In 2000, Barker et al.[18, 19] successfully synthesized the lithium

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vanadium fluorophosphate (LiVPO4F) material that has a similar structure with the

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naturally-occurring mineral LiFePO4OH and LiAlPO4F[20, 21], belonging to the triclinic crystal system adjoint space group P1. In the crystal structure of LiVPO4F cell[22-24], as shown in Scheme 1, the PO4 tetrahedron shares an oxygen vertex, and the VO4F2 octahedral is connected with each other by F atom, which forms a three-dimensional structural frame. It is a potential cathode material with high voltage, high specific energy and high safety[25, 26]. It not only inherits the advantages of the thermodynamic stability of the phosphate system[27-29], but also combines the strong

ACCEPTED MANUSCRIPT electronegativity of fluorine ions, which leads to a higher lithium intercalation potential. There are two charging platforms at 4.24 V and 4.28 V in the charge-discharge curve of lithium vanadium fluorophosphate (LiVPO4F), but only

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one discharging platform at 4.2 V, which is 0.8 V higher than the olivine lithium iron phosphate (LiFePO4). Generally speaking, the main traditional synthesis methods are solid state method[30], hydrothermal method[31] and sol-gel process[32]. In this

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sonication and mechanical activation.

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paper, we prepared the samples through the modified solid state method assisted by

However, it has to be pointed out that the electronic and ionic conductivity of lithium vanadium fluorophosphate (LiVPO4F) without any modification are low, which are the main obstacle to hinder its industrial production. So far, many

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researchers have done a lot of scientific research on how to improve its electronic conductivity. A large number of cation doping researches, such as Y3+[33], Al3+[34], Ti4+[35] and Mn3+[36] doping, have proved that its disadvantages can be impressed to

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a certain degree. In addition, Gao et al[37]. found that cobalt doping can not only

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improve the electronic conductivity of lithium iron phosphate (LiFePO4) to some extent but also increase its ionic conductivity. As far as we know, there is no study on cobalt doping in lithium vanadium fluorophosphate (LiVPO4F). In this paper, we successfully prepared the LiV1-xCoxPO4F/C (x = 0, 0.01, 0.03 and 0.05) cathode material and studied the influence of cobalt doping on physical and electrochemical performances of lithium vanadium fluorophosphate (LiVPO4F).

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Experimental

2.1. Material preparation The LiV1-xCoxPO4F/C (x = 0, 0.01, 0.03 and 0.05) cathode material have been

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successfully synthesized through the modified solid state method assisted by sonication and mechanical activation. The synthesis steps are as follows: (1) Stoichiometric ratio of vanadium pentoxide (V2O5, 99%), cobalt oxide (Co3O4,

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99.5%), lithium fluoride (LiF, 99%), ammonium dihydrogen phosphate (NH4H2PO4,

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99.5%), and 60% excess oxalic acid dehydrate (H2C2O4-2H2O, 99%) were completely dispersed in dehydrated alcohol assisted by sonication and agitation; (2) The above homogeneous slurry was ball-milled (QXQM-4, 0.75KW) in a simoloyer mill with a revolving speed of 320 r/min for 6 h; (3) The acquired material was then dried in a

heated at 300

for 6 h; (4) The obtained dry precursor powder was

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blast drying furnace at 90

for 2 h, and then sintered at 650

in an argon atmosphere for 6 h to

obtain LiV1-xCoxPO4F/C. Schematic illustration of the synthesis process for the

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LiV1-xCoxPO4F/C composites is shown in Scheme 1. Here, in order to facilitate the

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description of different samples, the obtained LiV1-xCoxPO4F/C (x=0, 0.01, 0.03, 0.05) simples were labeled as Co-0, Co-1, Co-3, and Co-5. 2.2. Material characterization The powder X-ray diffraction (XRD) equipped by Cu Kα diffraction with a

scan speed of 2°/min in the range of 10°–80° was applied in the analysis of the purity and crystal structure of the composite phase. Besides, the surface profile and grain size of the synthesized samples were obtained through the Scanning electron

ACCEPTED MANUSCRIPT microscopy (SEM, JEOL JSM-6360LV) and the transmission electron microscope (TEM). Beyond that, we adopt the Energy Dispersive x-ray Spectroscopy (EDS) to observe the elements distribution (Co, O and F) in the sample surface. Moreover, X

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ray photoelectron spectroscopy (XPS) was made to analyze the species and valence in the samples. 2.3. Electrochemical measurements

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CR2025 coin-type cells were made to estimate its electrochemical property. The

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positive electrode consists of cathode material, polyvinylidene fluoride, acetylene black, with the weight ratios 8:1:1 with lithium sheet as the corresponding anode material. Beyond that, 1 mol L-1 LiPF6 dissolves in EC/DMC/EMC solution (1: 1: 1 in volume ratios) as the electrolyte. Finally, all of compositions were packed in an Ar

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filled glove box. After that, the charging-discharging performance of the battery was carried out on the test platform (Land system) within the voltage of 3.0 V–4.5 V (vs. Li/Li+) at 25

. Furthermore, CHI660d electrochemical work station (Shanghai Chen

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Hua) was used to measure the cyclic voltammetry (CV) with a 0.1 mV s-1 scan rate

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(from 3.0 V to 4.6 V). Then, we use it to test the AC impedance with the frequency of the alternating voltage range of application 0.005 V amplitude and within the frequency 10−3–105 Hz. 3. Results and discussion The purity and crystal structure of LiV1-xCoxPO4F/C (x =0, 0.01, 0.03 and 0.05) were characterized by the powder X-ray diffraction. Here, all the samples were analyzed by using the Rietveld method. As displayed in Fig.1, all the X-ray diffraction

ACCEPTED MANUSCRIPT spectrograms may be identified as triclinic crystal system adjoint space group P1. The main characteristic peaks detected are consistent with the main characteristic peaks of lithium vanadium fluorophosphate (LiVPO4F), which means that the addition of a

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small amount of cobalt does not significantly affect the triclinic crystal structure of LiVPO4F/C. However, due to the loss of fluorine compounds more or less in the preparation process, making the synthesis of pure phase lithium vanadium

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fluorophosphate (LiVPO4F) material particularly difficult. In the Co-3 sample, we

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found weak heterogeneous peaks (Li3V2(PO4)3), which located at 20.7°and 24.4°, respectively. After quantitative analysis by Topas software, the content of Li3V2(PO4)3 phase was only 2.52 %. Moreover, the diffraction peaks of carbon are not observed in the XRD spectra of each sample, which shows that the rest of the carbon is formless.

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The calculated lattice parameters, corresponding to composite phase, were listed in Table 1. As can be seen from Table 1, lattice parameters a, b, and c of the corresponding composite phase all decreases in different degree with the increase of

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the amount of cobalt doping, while the unit cell volume

decreases in different

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degree with the increase of the amount of cobalt doping. This implies that the doped cobalt replaces some of the vanadium sites and enters the lattice of lithium vanadium fluorophosphates (LiVPO4F), which forms a LiV1-xCoxPO4F/C (x =0, 0.01, 0.03 and 0.05) composite phase. The decrease in the unit cell volume of the corresponding composite phase hints that with the increase of cobalt doping, the diffusion path of lithium ion will be shortened. The change of lattice parameters of the corresponding composite phase is caused by the smaller size of Co3+ (0.0545nm) than that of V3+

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(0.064nm)[38].

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Scheme 1 Schematic illustration of the synthesis process for the LiV1-xCoxPO4F/C composites

10

20

30

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Intersity/counts

Ydif | Bragg Position

0

40

50

60

70

80

90

0

10

20

30

Yobs Ycal

20

30

40

50

2θ/°

60

70

50

60

80

90

70

80

90

Yobs Ycal Ydif | Bragg Position

Intersity/counts

Intersity/counts

10

40

(d)

Ydif | Bragg Position

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Ydif | Bragg Position

2θ/°

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2θ/°

(c)

Yobs Ycal

(b)

Intersity/counts

Yobs Ycal

(a)

0

10

20

30

40

50

60

70

80

90

2θ/°

Fig.1 XRD patterns of the LiV1-xCoxPO4F/C (x = 0, 0.01, 0.03 and 0.05) samples fitted by Rietveld refinement (a) Co-0, (b) Co-1, (c) Co-3,(d) Co-5 Table 1 The calculated lattice parameters of the corresponding composite phase

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b

c

β

v

LiVPO4F/C

5.1817

5.3091

7.2656

107.936

174.802

10.24

LiV0.99Co0.01PO4F/C

5.1814

5.3085

7.2651

107.922

174.780

10.88

LiV0.97Co0.03PO4F/C

5.1763

5.3086

7.2658

107.970

174.602

11.21

LiV0.95Co0.05PO4F/C

5.1768

5.3070

7.2634

107.953

WRp(%)

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sample

174.527

12.32

The SEM images of the LiV1-xCoxPO4F/C (x = 0, 0.01, 0.03 and 0.05) samples

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are displayed in Fig.2(a-h) (Fig.2 (b), (d), (f) and (h) show the enlarged regions of Fig.

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2(a), (c), (e) and (g), respectively). Combined with the results of TEM (as displayed in Fig.2(i) and (j)), it is obvious that the Co-3 sample (the evaluated particle size is approximately 0.1-1 microns in diameter) have smaller scale particles than the Co-0 simple (approximately 0.5-2 microns in diameter). Additionally, it is worth noting that

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the Co doping particles loosely agglomerate with each other in different degree, which makes them have a porous structure. These results indicate that the addition of cobalt may affect the microstructure of the samples and bring better contact with the

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electrolyte. This fluffy structure facilitates the absorption of more electrolytes and

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improves the electrochemical properties of the materials. From Fig.2(e) and (f), we can see clearly that the amorphous carbon is coated on the lithium vanadium fluorophosphate (LiVPO4F) material, which facilitates the formation of a fluffy structure that coincides with the above discussion. The EDS mapping analysis was carried out to study the distribution of Co, O and F in the cobalt doped sample (Co-3). The results are shown in Fig.3. Obviously, Co, O and F elements homogeneously distribute in the Co-3 sample particles. Further combined with the analysis of the

ACCEPTED MANUSCRIPT previous XRD, we can draw the conclusion that the majority of the doped cobalt enter the lattice of lithium vanadium fluorophosphate (LiVPO4F), rather than simply

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attached to the surface of particles.

Amorphous carbon

Co-3

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Co-0

0.295nm

Amorphous carbon

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0.322nm

Fig.2 SEM images of LiV1-xCoxPO4F/C with different Co contents: (a), (b) x=0; (c),

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(d) x=0.01; (e), (f) x=0.03; (g), (h) x=0.05. TEM images of Co-0 (i) and Co-3 (j) (after half an hour of ultrasonic treatment, ultra-thin copper foil as the carrier)

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Fig.3 EDS mappings of (a) Co-3, (b) OK, (c) FK, (d) CoK in Co-3 sample

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X-ray photoelectron spectroscopy was performed to confirm vanadium and cobalt oxidation state in LiVPO4F/C (Co-0) and LiV0.97Co0.03PO4F/C (Co-3). Each element consists of a main peak and a satellite peak, and the satellite peak was caused by the

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spin orbit coupling. Furthermore, special attention should be paid to Fig.4(a) and

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Fig.4(b), in which the Co-3 sample has a characteristic peak of cobalt, which does not appear in Co-0 sample. Similarly, we can see that the highest peak of the cobalt can be assigned to Co 2p3/2 and the signals at 780.1 eV and 781.3 eV corresponds to Co2+ and Co3+[38, 39], respectively. Here, the fitting process is completed by the XPS-PEAK analysis software. For convenience, we do not fit the V

2p1/2

spectra and the Co

2p1/2

spectra, thus, which will not discussed further. As displayed in Fig.4(c) and Fig.4(d), the fitting curves overlap the experimental results very well. And the fitting results are

ACCEPTED MANUSCRIPT shown in Table 2, it can be easily found that the major peak of vanadium can be assigned to V

2p3/2

and the signals at 516.8 eV and 516.3 eV corresponds to V3+ and

V4+[40, 41], respectively. Besides, from the point of view of their distribution area

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aspect, we can basically conclude that there is very low content of V4+ ion in the Co-3 sample. Table 2 clearly shows that there is not only Co2+, but also Co3+ in the sample. Moreover, because of the presence of a part of Co2+, a certain amount of electron hole

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was formed in the structure, which improves the electronic conductivity of the

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materials, and it is also beneficial to the improvement of electrochemical properties of materials.

500000

250000

x=0.03 x=0

(a)

(b)

Co

230000

400000

x=0.03 x=0

240000

Counts/s

300000

Co

200000

100000

0

0

200

400

600

800

1000

1200

210000 200000 190000 180000

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Counts / s

220000

170000 160000 150000 760

1400

770

780

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(c) 25000

Counts/s

AC C

20000

28000

Raw Intensity Peak Sum Background 3+ V 2p3/2 4+ V 2p3/2

15000

515

520

820

26500 26000 25500 25000

5000

510

810

Raw Intensity Peak Sum Background 3+ Co 2p3/2 2+ Co 2p3/2

(d)

27000

10000

0 505

800

27500

Counts/s

30000

790

Binding Energy(eV)

Binding Energy (eV)

24500 525

530

770

Binding Energy(eV)

780

790

800

810

820

Binding Energy(eV)

Fig.4 X-ray photoelectron spectroscopy(XPS) of (a) Co-0 and Co-3, (b) an enlarged version of the special region in (a), (c) V 2p3/2 and (d) Co 2p3/2 in LiV0.97Co0.03(PO4)F/C sample

ACCEPTED MANUSCRIPT Table 2 the fitting parameters of the Co 2p3/2 and V 2p3/2 signals in Co-3 Element

desired line(s)

BE(eV)

FWHM(eV)

symbol

Area(P) CPS.eV

516.8

2.22

V4+

2p3/2

516.3

2.01

Co2+

2p3/2

780.1

7.68

Co3+

2p3/2

781.3

3.25

49104.8

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2p3/2

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V3+

1746.3

3581.5

11048.2

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Table 3 shows the discharge capacity of all the samples at different rates, from which we can see that the cobalt doped samples, compared with the undoped one, have a different degree of improvement in the rate performance. Especially at high charging and discharging rates, the specific capacity of discharge is greatly improved.

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The Co-0 sample displays a discharge capacity about 123.9 mAh g-1, 125.2 mAh g-1, 112.1 mAh g-1, 98.9 mAh g-1, 79.8 mAh g-1, 70.1 mAh g-1 and 52.8 mAh g-1 at the discharge rates of 0.2 C, 0.5 C, 1 C, 3 C, 5 C, 7 C and 10 C, respectively.

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Correspondingly, the Co-3 sample displays a discharge capacity approximately 125.0

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mAh g-1, 128.3 mAh g-1, 122.5 mAh g-1, 111.0 mAh g-1, 102.6 mAh g-1, 94.8 mAh g-1 and 84.0 mAh g-1 at the discharge rates of 0.2 C, 0.5 C, 1 C, 3 C, 5 C, 7 C and 10 C, respectively. Fig.5(a) presents the discharge capacity of Co-0 sample at different rates. The discharge capacity of Co-1, Co-3 and Co-5 samples at different rates are shown in Fig.5(b), Fig.5(c) and Fig.5(d), respectively. By contrast, we can see that the cobalt doped samples, compared with the original sample, have a different degree of improvement in the rate performance. Here, we find that the discharge capacity of 0.2

ACCEPTED MANUSCRIPT C is smaller than that of 0.5 C, which may be interpreted as the appearance of activation or formation of solid electrolyte interface (SEI) film[42] on electrode surface. Besides, it is worth mentioning that four samples all have only one discharge

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platform at 4.2 V, which is consistent with the reported data[43].

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Table 3 The discharge capacity of different samples at different rates

1C

LiVPO4F/C

112.1

LiV0.99Co0.01PO4F/C

117.2

LiV0.97Co0.03PO4F/C

122.5

(a)

4.2

115.6

Voltage (V)

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4.0 3.8 3.6 3.4 3.2 3.0

2.8 -20

7C

10 C

98.9

79.8

70.1

52.8

105.3

96.6

82.5

72.2

111.0

102.6

94.8

84

91.9

77.0

62.4

104.9

0.2C 0.5C 1C 3C 5C 7C 10C

0

20

40

60

80

4.4

(b)

4.2 4.0

Voltage (V)

4.4

5C

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LiV0.95Co0.05PO4F/C

3C

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sample

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The discharge capacity (mAh g-1)

3.8 3.6 3.4 3.2 3.0

100

120

Discharge capicity (mAh g-1)

140

2.8 -20

0.2C 0.5C 1C 3C 5C 7C 10C 0

20

40

60

80

100

120

Discharge capicity (mAh g-1)

140

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4.6

(c)

4.4

(d)

4.2

4.2

4.0 3.8 3.6 3.4 3.2 3.0 2.8 -20

0.2C 0.5C 1C 3C 5C 7C 10C 0

3.8 3.6

0.2C 0.5C 1C 3C 5C 7C 10C

3.4 3.2 3.0 2.8 2.6

20

40

60

80

100

120

140

0

Discharge capicity (mAh g-1)

20

40

60

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Voltage (V)

Voltage (V)

4.0

80

100

120

140

Discharge capicity (mAh g-1)

at 25

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Fig.5 (a) The discharge curves of Co-0; (b) Co-1; (c) Co-3; (d) Co-5 at different rates

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The cycle performance of LiV1-xCoxPO4F/C(x=0, 0.01, 0.03 and 0.05) materials (range from 3.0 V to 4.5 V) at 1 C rate are displayed in Fig.6. In Fig.6(a), all the cobalt doped samples (Co-1, Co-3 and Co-5) exhibit better cycling performances than the pristine sample (Co-0). Especially, sample (Co-3) has the highest discharge

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capacity and preferable cycle performance among all the samples, and after 50 cycles at 1C rate, their capacity retentions were 89.1%, 91.2%, 94.9% and 91.8%, respectively. In order to be more intuitive, a comparison between simple (Co-3) and

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sample (Co-0) were carried out at different rates. As displayed in Fig.6(b), we can see

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that the simple (Co-3), compared with the sample (Co-0), have a different degree of improvement in the cycle performance at different rates. Typically, after 50 cycles of charge-discharge at 3C rate, the discharge specific capacity of sample (Co-3) decreased from 111.0 mAh g-1 to 107.1 mAh g-1, while the specific discharge capacity of sample (Co-0) decreased from 98.9 mAh g-1 to 90.7 mAh g-1, and the capacity retention rate was 96.4% and 91.7%, respectively. So it can be concluded that a small amount of cobalt doping improves not only the rate performance, but also the cycle

ACCEPTED MANUSCRIPT performance of lithium vanadium fluorophosphate (LiVPO4F). But too much cobalt introduction will deteriorate the electrochemical performance.

140

110 100

Co-0 Co-1 Co-3 Co-5

80 70

(b)

1C

120

5C

100

80

60

Co-0

40 10

20

30

40

50

0

Cycle number (n)

20

40

60

80

100

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0

Co-3 3C

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120

90

0.1 C

140

Dischage Capacity(mAh/g)

Discharge capicity (mAh g-1)

(a) 130

120

140

160

Cycle Number (n)

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Fig.6 (a) Cycle performance of LiV1-xCoxPO4F/C(x=0, 0.01, 0.03, 0.05) at the 1C rate. (b) Cycle performance of Co-0 and Co-3 at different rates

In order to explore the electrochemical mechanism of increasing the rate and

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cycle performance of the Co-doped LiV1-xCoxPO4F/C (x =0.01, 0.03 and 0.05) samples, the cyclic voltammetry tests were taken with a 0.1 mV s-1 scan rate (from 3.0 V to 4.6 V) on the pristine and the (Co-3) composites without any charge–discharge

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process. The cyclic voltammogram curves of the LiV1-xCoxPO4F/C (x = 0 and 0.03)

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samples are shown in Fig.7. For the cyclic voltammogram curve of LiVPO4F/C (Co-0), there are two oxidation peaks, but only one reduction peak. The process of charge-discharge shows a two-phase reaction mechanism, which is induced by the intermediate phase Li0.67VPO4F[44]. Besides, Table 4 shows that the redox potential difference of Co-3 (0.320 V) is less than the Co-0 (0.377 V), which indicates the decrease of electrode polarization. The reversibility of electrode reaction has also been improved, i.e. the capacity retention of the (Co-3) is better than that of the

ACCEPTED MANUSCRIPT pristine, and the cyclic reversibility is also improved. In summary, the results of the CV test echoes with the electrochemical properties listed above.

4.393V

(b)

4.421V

Co-3

Current

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Co-0

Current

(a)

∆V=0.320V

∆V=0.377V

4.044V

4.073V

2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

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2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

Voltage (V)

Voltage (V)

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Fig.7 The cyclic voltammogram curves of the LiV1-xCoxPO4F/C simple (a) x = 0 and (b) x = 0.03

Table 4 The redox potential values on cyclic voltammetry curves Eoxidation(V)

Ereduction(V)

∆E(V)

Co-0

4.421

4.044

0.377

4.393

4.073

0.320

Co-3

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sample

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Furthermore, the electrochemical impedance spectroscopy (EIS) was used to investigate the electrochemical kinetics parameters of the electrode process. Fig.11

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shows the Nyquist diagrams of the original and Cobalt doping samples (all the button batteries were prepared in the same way, and before the EIS measurement, all the batteries were tested by the cyclic voltammetry for three cycles). Here, we used the Zview software to fit the AC impedance spectrum. As displayed in Fig.8(a), the insert picture in Fig.8(a) shows the equivalent circuit model for fitting the AC impedance spectrum. In addition, the kinetic parameters obtained from the equivalent circuit model of ZView software were showed in Table 5, which demonstrate that the

ACCEPTED MANUSCRIPT change-transfer resistance (Rct) of cobalt doped sample Co-3 is less than that of the pristine Co-0. This indicates that the sample Co-3 has a better performance than the sample Co-0. And the results are consistent with the electrochemical properties

obtained according to the equation listed below DLi+= R 2T 2 2 A2 n 4 F 4C 2σ 2

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discussed earlier. In addition, the lithium ions diffusion constant (DLi+)[45] was

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where R is a constant value of gas, A is the surface area of the cathode, T represents

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the absolute temperature, C is the concentration of lithium ions in the material (herein C is about 1.9 × 10−2mol/cm3[46]), n is the charge-transfer number, σ is the Warburg factor which have relationship with Z’ and w -1/2, F is the Faraday constant. The calculated diffusion coefficients of lithium ions (DLi+) are also showed in Table 5,

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which demonstrate that the diffusion constant of Co doped sample (The lithium ions diffusion constant (DLi+) = 1.86321×10-13 cm2 s-1) was much larger than that of original simple (2.06453×10-14 cm2 s-1). Moreover, it can be said that doping a small

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amount of cobalt can not only increase the diffusion coefficient of lithium ions (DLi+),

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but also reduce the change-transfer resistance (Rct), which is beneficial to the improvement of electrochemical performance.

Fig.8 (a) Nyquist plots of the Co-0 and Co-3 alone with their fitting curves, (b)

ACCEPTED MANUSCRIPT the relationship between Z’ and w -1/2 in low frequency according to (a). Table 5 The kinetic parameters obtained from the equivalent circuit model of ZView software. Rs (Ω)

Rct (Ω)

σ (Ω cm2 s1/2 )

Co-0

2.23

334

72.55

Co-3

5.934

210.1

24.15

2.06453×10-14

1.86321×10-13

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4. Conclusions

DLi+ (cm2/s)

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sample

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The LiV1-xCoxPO4F/C (x = 0, 0.01, 0.03 and 0.05) samples were successfully prepared through the modified solid state method assisted with sonication and mechanical activation. The XRD analysis shows that the addition of a small amount of cobalt does not significantly affect the triclinic crystal structure of LiVPO4F/C.

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The Co doping particles loosely agglomerate with a porous morphology. And the doped cobalt elements disperse homogeneously in the samples. Based on the results of rate and cycle performance tests, it can be found that the electrochemical

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performance of LiV0.97Co0.03PO4F/C sample is the best in all samples, which exhibits

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a discharge capacity of 125.0 mAh g-1, 128.3 mAh g-1, 122.5 mAh g-1, 111.0 mAh g-1, 102.6 mAh g-1, 94.8 mAh g-1 and 84.0 mAh g-1 at the discharge rates of 0.2 C, 0.5 C, 1 C, 3 C, 5 C, 7 C and 10 C, respectively. Especially at high charge-discharge rates, the specific capacity of discharge has a greater improvement. And after 50 cycles of charge-discharge at 3 C rate, the discharge specific capacity of LiV0.97Co0.03PO4F/C decreases from 111.0 to 107.1 mAh g-1, while the specific discharge capacity of LiVPO4F/C decreases from 98.9 to 90.7 mAh g-1, the capacity retention rate is 96.4

ACCEPTED MANUSCRIPT and 91.7 %, respectively. EIS results reveals that doping a small amount of cobalt can not only increase the lithium ions diffusion constant (DLi+), but also reduce the change-transfer resistance (Rct), which are beneficial to the improvement of

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electrochemical property. Accordingly, we conclude that doping a small amount of cobalt improves not only the rate performance, but also the cycle performance of lithium vanadium fluorophosphate (LiVPO4F). The improvement of electrochemical

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and reduce of the change-transfer resistance (Rct).

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performance can be explained by the increase of lithium ions diffusion constant (DLi+)

Acknowledgments

We gratefully acknowledge the Nature Science Foundation of China (Grant No.51602352 ), the Nature Science Foundation of Hunan province (Grant

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HIGHLIGHTS
Successfully

synthesized

lithium

vanadium

fluorophosphate

materials.

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Adopt a modified solid state method assisted by sonication and mechanical activation.


The electrochemical properties of LiV0.97Co0.03PO4F/C sample is

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improved extremely compared with that of the original sample.