C nanoplates synthesized by solvothermal route

Solid State Ionics 253 (2013) 39–46

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The effect of doping Co on the electrochemical properties of LiFePO4/C nanoplates synthesized by solvothermal route Jianjun Song a, Guangjie Shao a,b,⁎, Meiwu Shi c, Zhipeng Ma a, Wei Song a, Caixia Wang a, Shuang Liu a a b c

Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China State key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China The Equipment Research Institute of the General Logistic Department of the Chinese People's Liberation Army, Beijing 100010, China

a r t i c l e

i n f o

Article history: Received 15 June 2013 Received in revised form 11 August 2013 Accepted 12 August 2013 Available online 19 September 2013 Keywords: Lithium iron phosphate Solvothermal synthesis Cobalt doping High rate Diffusion energy barriers

a b s t r a c t A series of olivine LiFe1 − xCoxPO4/C (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025) nanoplates were synthesized by a facile solvothermal synthesis combined with esterification reaction. The structure, morphology and electrochemical performance of the samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), galvanostatic intermittent titration technique (GITT), galvanostatic charge/discharge tests and electrochemical impedance spectroscopy (EIS). Based on the first-principle density functional theory (DFT), the diffusion energy barriers of Li ions for LiFe1 − xCoxPO4 (x = 0–0.025) were also calculated to further investigate the influence of doping Co on LiFePO4/C cathode material. The results showed that the prepared nanoplates with a very thin thickness along b-axis grow preferentially along the [001] direction of (101) lattice planes, which can minish the distance of Li+ ion diffusion along the [010] direction. The calculated results suggested that the LiFe0.99Co0.01PO4/C had a lowest lithium ion diffusion energy barrier, accordingly possessing a highest lithium ion diffusion coefficient. The electrochemical performance was improved by doping an appropriate amount of Co, and it might be attributed to the fact that the doped Co ion can enhance exchange current density and lithium ion diffusion coefficient. Among all the doped samples, LiFe0.99Co0.01PO4/C exhibited the best rate capability and cycling stability, with the initial discharge capacity of 154.5 mAh g−1 at 0.5 C. Remarkably, it still showed a high discharge capacity of over 96.9 mAh g−1 and good cycle retention even at a high rate of 10 C. © 2013 Elsevier B.V. All rights reserved.

1. Introduction As a cathode material for Li-ion batteries, olivine-type lithium iron phosphate (LiFePO4) proposed by Padhi et al. [1] has attracted increasing attention due to its low cost, environmental compatibility, superior capacity retention, thermal stability and safety [1–3]. The main obstacles for applicable electrochemical performances are its low electronic conductivity and lithium ion diffusivity [4]. Progressive efforts, including coating LiFePO4 particles with electrically conductive materials like carbon [5,6], metal and metal oxides [6–8], minimizing the particle size [9,10], and doping with metal ion [11–13], have been made to tackle the problems. Among these approaches, metal ion doping of LiFePO4 is a convenient and effective way to improve the electrochemical performance of LiFePO4. The previous research has proved that doping metal ions at Li-site can hinder the diffusion of Li ions [14–17], which led to poor electrochemical performance, so considerable effort is paid to the Fe-site doping. The doping at Fe-site can effectively stabilize the ⁎ Corresponding author at: Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China. Tel./fax: +86 335 8061569. E-mail address: [email protected] (G. Shao). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.08.019

crystal structure and also improve the transportation of electrons and probably Li ion [18–22]. First-principle calculation shows that metal cations doping at Fe-site of LiFePO4 facilitate the diffusion of Li ions, and thus the electronic and ionic conductivity can be improved [21]. It is worth mentioning that first-principle method has been demonstrated crucial in the study of electrode materials for practical lithium ion battery [14,23–29]. Cobalt element is a good doping candidate of LiFePO4 owing to the similar ionic radii and properties between Fe and Co. Wang et al. [30] have compared the structure, electrochemical performances and the chemical environments around the O and P atoms of LiFePO4, LiCoPO4 and LiFe1 − xCoxPO4/C (0 ≤ x ≤ 1) solid solutions. And they reported that the formation of a solid solution lowers the oxidation potential of the Co2+ ions and makes the Co2+ → Co3+ reaction complete at a lower voltage, which makes more contribution of capacity in the solid solution than in LiCoPO4. Shanmukaraj et al. [31] synthesized the LiFe1 − xCoxPO4/C (x = 0, 0.02, 0.04, 0.08, and 0.1) composite by citrate gel technique. They found that cobalt doping does not have a favorable effect on the electrochemical performance of lithium iron phosphate cathode materials. However, doping with Co can markedly improve the material performance as reported by Zhao et al. [32]. They have synthesized LiFe1 − xCoxPO4/C via a hydrothermal route. The doped

J. Song et al. / Solid State Ionics 253 (2013) 39–46

samples exhibit good discharge capacity especially for LiFe0.75Co0.25 PO4/C, which delivers a capacity of ∼ 170 mAh/g at 0.1 C, much higher than 150 mAh/g of pure LiFePO4/C, but they did not show the high-rate performance of the samples. It is well known that to improve electrochemical performance, LiFePO4/C at high charge– discharge rate has become a unanimous goal to researchers all over the world. Recently, Gao et al. [33] investigated the Co-doped LiFe1 − xCoxPO4/C cathode materials by oxalic acid-assisted sol–gel method, the rate performance and cyclic stability were effectively enhanced and LiFe0.99Co0.01PO4/C exhibited the best electrochemical performance. Different from other groups, a facile glycol-based solvothermal synthesis combined with esterification reaction is reported to prepare a series of olivine LiFe1 − xCoxPO4/C (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) nanoplates in this paper. We explored the crystal orientations and the formation mechanism of the as-prepared nanoplates, and the electrochemical properties of LiFe1 − xCoxPO4/C (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) composites were also investigated. In addition, the diffusion energy barriers of Li ions for LiFe1 − xCoxPO4 (x = 0–0.025) were calculated based on the first-principle density functional theory (DFT) to further study the influence of doping Co on LiFePO4/C cathode material. The relation between the diffusion energy barriers and electrochemical properties will be discussed. 2. Experimental 2.1. Preparation of electrode materials All the chemicals are analytical grade and used without further purification. The LiFePO4/C composites were prepared as follows. FeSO4· 7H2O, H3PO4, and LiOH·H2O were used as starting materials in a molar ratio of 1:1:3 and ethylene glycol (EG) was applied as solvent. H3PO4 (0.015 mol) was slowly added to the LiOH (0.045 mol) solution under stirring. When a white suspension formed, FeSO4 (0.015 mol) solution was slowly introduced into the suspension under stirring for 30 min. The precursor was transferred into a Teflon-lined autoclave and heated to 180 °C for 18 h and then cooled down to room temperature. The obtained precipitate was washed with distilled water and ethanol. To achieve carbon coating, LiFePO4 and proper ethylene glycol were

O Li P

x=0.025 x=0.02

Intensity / a.u.

40

x=0.015

x=0.01

x=0.005

x=0

20

30

40

50

2 Theta / degree Fig. 2. XRD patterns of LiFe1 − xCoxPO4/C (0 ≤ x ≤ 0.025) samples.

added to 10 mL citric acid (5 wt.%) solution under stirring, and then the solution was evaporated to obtain a dark blue gel at 80 °C in water bath. Then the gel was carbonized for 6 h at 700 °C in nitrogen atmosphere. For the synthesis of LiFe1 − xCoxPO4/C (x = 0.005, 0.01, 0.015, 0.02, and 0.025) composites, the procedure was identical except that FeSO4 was replaced as the mixture of stoichiometric amounts of FeSO4 and CoSO4. 2.2. Characterization The crystal structures of the samples were characterized by X-ray diffraction (XRD) using a Rigakud/MAX-2500/pc X-ray diffractometer with Cu Kα radiation. The morphology and microstructure of the powder was examined with the Hitachi Model S-4800 field-emission scanning electron microscope (FE-SEM) at 15 KV and high-resolution transmission electron microscope (HRTEM) with model JEM2010. 2.3. Computational details Calculations of the diffusion energy barriers in the present work are performed using Cambridge serial total energy package (CASTEP) program. All results are calculated within a conventional orthorhombic unit cell which contains 4 LiFePO4 formulas depicted in Fig. 1. The ground state of the electronic structure is described within density functional theory (DFT) and the generalized gradient approximation (GGA) proposed by Perdew et al. [34]. And the electron–ion interaction was described by the optimized ultrasoft pseudopotentials (USPP) introduced by Vanderbilt [26,35]. The Perdew–Wang exchange-correlation functional (PW91) was used for the calculation of the electron exchange correlations energy. The Monkhorst–Pack scheme with a set of 6 × 3 × 4 k-points is used for the integration in the irreducible Brillouin zone. Energy cut-off for the plane waves is chosen to be 300 eV. The total energy of the systems converges to within 1.0 × 10−5 eV per atom for

Fe Table 1 Lattice parameters of LiFe1 − xCoxPO4/C.

Fig. 1. The crystal structure of a LiFePO4 unicell.

Samples

a(Å)

b(Å)

c(Å)

V(Å3)

LiFePO4/C LiFe0.995Co0.005PO4/C LiFe0.99Co0.01PO4/C LiFe0.985Co0.015PO4/C LiFe0.98Co0.02PO4/C LiFe0.975Co0.025PO4/C

10.36081 10.35427 10.34413 10.3437 10.33953 10.35036

6.03934 6.01447 6.01532 6.00873 6.01083 6.01339

4.73414 4.70355 4.69783 4.69738 4.70155 4.70246

296.23 292.92 292.31 291.95 292.2 292.68

J. Song et al. / Solid State Ionics 253 (2013) 39–46

a

b

c

d

41

Fig. 3. FESEM images of LiFePO4/C (a), LiFe0.99Co0.01PO4/C (c); TEM images of LiFePO4/C (b), LiFe0.99Co0.01PO4/C (d).

all the calculations. In this work, we defined Fe1 − xCox (x =0–0.025) as the mixed atoms based on the Mixture Atom Editor of CASTEP program. The method of mixed atoms has larger superiority for the less computer time than that of the supercell approach. 2.4. Electrochemical measurements The electrochemical performances of the samples were measured in simulative cells, which consisted of a working electrode and a lithium

a

(001)

[010]

foil electrode separated by a Celgard 2400 microporous membrane. The working electrode was prepared by dispersing 80 wt.% active materials, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) solvent to form uniform slurry. The slurry was coated on Al foil and dried in vacuum at 120 °C for 12 h. The electrolyte was 1 M LiPF6/EC + DEC (1:1, v/v). The cell was assembled in an argon-filled glove box and galvanostatically cycled in the voltage ranges of 2.4–4.2 V. In the galvanostatic intermittent titration technique (GITT) experiment, the cell was charged at C/20 rate for

b

(200)

Carbon

Fig. 4. TEM images of the LiFe0.99Co0.01PO4/C. (a) Low-magnification TEM image of one nanoplate and the corresponding selected area electron diffraction (SAED) (inset). (b) HRTEM image taken from the edge of the single nanoplate as marked by a small white pane in (a).

42

J. Song et al. / Solid State Ionics 253 (2013) 39–46

4.40

Table 2 Optimized lattice parameters of LiFe1 − xCoxPO4. a(Å)

b(Å)

c(Å)

V(Å3)

LiFePO4 LiFe0.995Co0.005PO4 LiFe0.99Co0.01PO4 LiFe0.985Co0.015PO4 LiFe0.98Co0.02PO4 LiFe0.975Co0.025PO4

10.012 10.018 9.993 9.973 9.975 9.967

5.977 5.972 5.935 5.933 5.929 5.934

4.779 4.773 4.768 4.773 4.771 4.773

285.55 285.54 282.83 282.42 282.19 282.15

2 h followed by open circuit relaxation for 4 h. This was repeated until a cut-off voltage of 4.2 V was reached followed by a discharge to 2.4 V under the same setting conditions. When recording the EIS spectra, the frequency range from 1 MHz to 1 MHz. EIS were tested on CHI660E electrochemical workstation (Chenhua, Shanghai China). Galvanostatical charge–discharge experiment was conducted using computer controlled cycling equipment (NEWARE, Shenzhen China).

4.35

Energy barrier / eV

Samples

4.30

4.25

4.20

4.15

4.10 0.000

0.005

0.010

0.015

0.020

0.025

Doping concentration Fig. 6. The energy barriers of LiFe1 − xCoxPO4 (x = 0–0.025).

3. Results and discussion 3.1. Structural and morphology analysis XRD patterns of LiFe1 − xCoxPO4/C samples with different Co-doped contents (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) are shown in Fig. 2. The crystal phases of all the samples are in full accord with the ordered olivine structure indexed orthorhombic Pnma (JCPDS Card No. 832092), and no extra reflection peak from impurity is observed, indicating that a small amount of doped Co does not destruct the lattice structure of LiFePO4. Table 1 shows the corresponding structural parameters and unit cell volume of the samples. The lattice parameters and unit cell volume slightly decrease after doping Co. This result is attributed to the smaller ionic radii of Co2+ (0.74 Å) compared with that of Fe2+ (0.76 Å) [36]. Fig. 3 depicts the FESEM and TEM images of LiFePO4/C and LiFe0.99 Co0.01PO4/C. As shown in Fig. 3a and c, both the LiFePO4/C and LiFe0.99 Co0.01PO4/C samples achieved by the solvothermal synthesis are composed of uniform nanoplates, indicating no influence on morphology with the Co doping. The nanoplates have a very thin thickness of about 40 nm, a length in the range of 100–200 nm and a width of about 100 nm, which is confirmed by the TEM image shown in Fig. 3b and d. The short length along b-axis can minish the distance of Li+ ion

LiFePO4

4

LiFe0.995Co0.005PO4 LiFe0.99Co0.01PO4

Energy difference / eV

LiFe0.985Co0.015PO4 LiFe0.98Co0.02PO4 LiFe0.975Co0.025PO4

3

2

diffusion along the [010] direction, which are favorable for fast Li+ ion insertion/extraction. Further insight into the morphology and crystal orientations of the LiFe0.99Co0.01PO4/C nanoplates has been gained by high-resolution TEM. The inserted selected area electron diffraction (SAED) patterns shown in Fig. 4a and the HRTEM image in Fig. 4b (corresponding to the area marked by a pane in Fig. 4a) indicate that the large plane of the LiFe0.99Co0.01PO4/C nanoplate lies in the ac plane, viewed along the b direction. The interplanar spacings of 0.519 nm and 0.479 nm correspond to the (200) and (001) lattice planes, respectively, demonstrating that the LiFe0.99Co0.01PO4/C nanoplates grow preferentially along the [001] direction of (101) lattice planes. Fig. 4b shows uniform carbon coating layer with a thickness of ~5 nm on the surface of LiFe0.99Co0.01 PO4/C nanoplates in the crystalline fringe region, which results from the esterification reaction between citric acid and ethylene glycol. The formation of the polyester network entirely wrapped the LiFe1 − xCox PO4/C composites. After high temperature calcination, the polyester covered on the surface of LiFe1 − xCoxPO4/C composites transformed into the uniform carbon layer, which benefits the electronic and ionic conductivity of the materials. The approximate amount of carbon in the LiFePO4/C and LiFe0.99Co0.01PO4/C samples was calculated to be 4.78 wt.% and 4.82 wt.%, respectively. The method we adopted to measure the carbon content is dissolving the samples into hydrochloric acid solution. After filtrated and dried we get the residual carbon. The formation of nanoplates could be explained by a classic crystallization theory. The crystal growth is mainly controlled by the nuclei surface energy. Nuclei surfaces with high surface energy have faster growth rate along the vertical directions of nuclei surfaces. Saravanan et al. [37] have reported that ethylene glycol is a favorable solvent for the growth of nanoparticles to nanoplates. Hence, there is a great possibility that ethylene glycol solvent could dramatically decrease the surface energy of the (010) facet, which causes the growth of the nanoplates with the (010) plane. 3.2. Diffusion energy barriers

1

0 0

b/4

b/2

Diffusion distance Fig. 5. The relationship of diffusion energy difference of LiFe1 − xCoxPO4 (x = 0–0.025) and different diffusion distance of Li ions.

It has been validated that Li ion diffusion in olivine LiFePO4 is one dimensional along b axis [27,38]. Thus the diffusion energy barriers for Li ions along the b-axis diffusion pathway have been calculated for pure LiFePO4 and doped LiFe1 − xCoxPO4 (x = 0.005–0.025). The optimized lattice parameters of LiFe1 − xCoxPO4 (x = 0–0.025) are listed in Table 2. It is apparent that the optimized results are in good agreement with our experimental values and the relative error is less than 4%. In the present study, a frozen molecular structure has been adopted for

J. Song et al. / Solid State Ionics 253 (2013) 39–46

3.8

a

3.65

b

3.60

LiFePO4/C LiFe0.99Co0.01PO4/C

3.55 3.50

Voltage / V

3.6

43

3.45 3.40 3.35 3.4

3.30 80

90

100

110

3.2 0

20

40

60

80

100

120

140

160

Capacity / mAh g-1 3.8

c LiFePO4/C LiFe0.99Co0.01PO4/C

Voltage / V

3.6

3.4

3.2 0.0

0.2

0.4

0.6

0.8

1.0

x in LixFePO4 and LixFe0.99Co0.01PO4 Fig. 7. GITT charge–discharge curves of LiFePO4/C and LiFe0.99Co0.01PO4/C, a current corresponding to C/20 was applied to the cells for 2 h before a 4 h relaxation: (a) compares the overvoltage of LiFePO4/C and LiFe0.99Co0.01PO4/C, (b) the magnified dotted region of (a), (c) the electrode potential as a function of lithium content x for LiFePO4/C and LiFe0.99Co0.01PO4/C.

4.4 4.2

4.4

1C

4.2 4.0

3.8

Potential vs. ( Li/Li+ ) / V

Potential vs. ( Li/Li+ ) / V

4.0

3.6 3.4 3.2

x=0 x=0.005 x=0.01 x=0.015 x=0.02 x=0.025

3.0 2.8 2.6

3.6 3.4 3.2 3.0 2.8 2.6

2.4 2.2

3.8

2.4 0

20

40

60

80

100

120

140

160

Specific capacity / mAh g-1 Fig. 8. Initial charge and discharge curves of LiFe1 − xCoxPO4/C (0 ≤ x ≤ 0.025) electrodes at 1 C.

10C

5C 2C 1C 0.5C

2.2 0

20

40

60

80

100

120

140

160

Specific capacity / mAh g-1 Fig. 9. Initial discharge curves of LiFe0.99Co0.01PO4/C electrodes at various rates.

44

J. Song et al. / Solid State Ionics 253 (2013) 39–46

1200

0.5C

160

(a) CPE

2C

140

Rs

120

Wo

Rct

5C 800

10C

100

-Z'' / ohm

Specific discharge capacity / (mAh/g)

1C

80 60

400

40

LiFePO4/C

LiFePO4/C

LiFe0.99Co0.01PO4/C

20

LiFe0.99Co0.01PO4/C

0

0 0

10

20

30

40

0

50

400

800

1200

1600

Z' / ohm

Cycle number Fig. 10. Cycle performance of LiFe0.99Co0.01PO4/C and LiFePO4/C electrodes at various rates.

(b)

1800 1600

LiFePO4/C

1400

LiFe0.99Co0.01PO4/C

1200

Z' / ohm

the energy profile scan. In particular, the Li atom is moved along the b axis, while all other atoms are fixed in the space. The diffusion energy difference is obtained through values of the cell's energy during the movement of Li ions along the diffusion pathway minus the cell's energy before Li ions migrate, and then draw the curve of these values, which are shown in Fig. 5, so the peak value is equal to diffusion energy barrier for Li ions. Fig. 6 gives the energy barriers of pure LiFePO4 and doped LiFe1 − x CoxPO4 (x = 0.005–0.025), respectively. It can be found that the energy barrier decreases at first, reaching its minimum at x = 0.01, and then becomes higher gradually with the increasing concentration of the Codoping, indicating that lithium ions can diffuse more easily in LiFe0.99 Co0.01PO4 than pure LiFePO4, which favors fast Li+ ion insertion/ extraction further to improve the electrochemical performance of LiFe0.99Co0.01PO4. Obviously, the lower diffusion barrier may be attributed to the strengthened P\O bands and the stretched Li\O bands caused by doping an appropriate amount of Co, as Gao et al. [33] reported.

1000 800 600 400 200 0 0

2

4

6

8

10

ω-1/2 / Hz-1/2 Fig. 11. EIS spectra and equivalent circuit used for fitting the experimental EIS data (inset) of LiFePO4/C and LiFe0.99Co0.01PO4/C cathodes (a), the relationship between Z′ and square root of frequency (ω−1/2) in the low-frequency region (b).

3.3. Electrochemical characterization In order to study the influence of Co doping on the kinetic reaction of LiFePO4 electrode, GITT results of the LiFePO4/C and LiFe0.99Co0.01PO4/C shown in Fig. 7 are performed to provide kinetic information of the electrode materials. Even though it is designed for studying the chemical diffusion coefficient in solid solution systems, it can provide comparative information of the electrode kinetics of a two-phase system through the voltage relaxation time to equilibrium potential and through the magnitude of overpotential. Fig. 7a and b shows that the LiFe0.99Co0.01PO4/C electrode was characterized by a lower polarization and flatter equilibrium potential than LiFePO4/C. Fig. 7b describes the magnified dotted region in Fig. 7a, further demonstrating the favorable effect of the Co doping on electrochemical performance of LiFePO4.

Fig. 7c shows the electrode potential as a function of lithium content x for LiFePO4/C and LiFe0.99Co0.01PO4/C. The graph shows that LiFe0.99 Co0.01PO4/C electrode has overpotential of about 25 mV, much lower than 100 mV of LiFePO4/C. The LiFe0.99Co0.01PO4/C also exhibits a lower polarization than LiFePO4/C of similar particle sizes, 0.07 vs 0.2 V. Therefore, much faster kinetics and better reversibility are achieved by the LiFe0.99Co0.01PO4/C electrode in comparison with the pure LiFePO4. The initial charge/discharge curves of the LiFe1 − xCoxPO4/C samples at 1 C rate between cutoff voltage 2.4 and 4.2 V are shown in Fig. 8. Each of the charge/discharge curves consists of a charge voltage plateau and a corresponding discharge voltage plateau at around 3.5 V and 3.4 V, which are the result of a two-phase reaction based on the redox couple of Fe2+/Fe3+ during lithium-ion extraction and insertion process [39].

Table 3 The rate capacity retention ratios of LiFePO4/C and LiFe0.99Co0.01PO4/C at different rates compared to 0.5 C. Samples

0.5 C (mAh g−1)

LiFePO4/C LiFe0.99Co0.01PO4/C

142.4 154.5

a

Ra(%) 0.5 C

1C

2C

5C

10 C

100.0 100.0

85.8 91.7

62.4 86.0

40.2 75.6

15.4 62.7

Rate capacity retention ratios of LiFePO4/C and LiFe0.99Co0.01PO4/C compared to 0.5 C.

Table 4 Results of electrochemical impedance and exchange current density. Sample

Rs (Ω)

Rct (Ω)

I0 (mAh g−1)

LiFePO4/C LiFe0.99Co0.01PO4/C

13.61 6.66

579.3 91.2

13.8 61.8

J. Song et al. / Solid State Ionics 253 (2013) 39–46

The initial special discharge capacities for LiFe1 − xCoxPO4/C samples with x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025 are 122.2, 133.2, 143.8, 141.4, 137.5, and 135.3 mAh g−1, respectively. The specific capacities of LiFe1 − xCoxPO4/C (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) increase gradually with the increase of Co-doping amount until Codoping amounts up to 0.01, and then decreases. Particularly, the LiFe0.99Co0.01PO4/C sample exhibits the best electrochemical performance among the prepared powders, consistent with its lowest diffusion energy barrier. Fig. 9 presents the initial charge/discharge capability of LiFe0.99Co0.01PO4/C electrodes at various rates. The LiFe0.99 Co0.01PO4/C sample exhibited discharge capacity of 154.5, 141.7, 132.9, 116.8, and 96.9 mAh g−1 as it is discharged at 0.5, 1, 2, 5, and 10 C, respectively. The electrode exhibits very flat voltage plateaus at 0.5 C and 1 C rate. Even at a 10 C rate, the electrode can keep a shorter discharge voltage plateau, and the discharge capacity reaches up to 96.9 mAh g−1 . Fig. 10 shows the rate and cycling performances of LiFePO4/C and LiFe0.99Co0.01PO4/C electrodes. It can be clearly seen that the LiFe0.99 Co0.01PO4/C has a much better stability than LiFePO4/C. Table 3 displays rate capacity retention ratios of the LiFePO4/C and LiFe0.99Co0.01PO4/C compared to 0.5 C. Fig. 10 and Table 3 demonstrate that Co-doping can evidently enhance the rate capability and cycling stability of LiFePO4/C, which could be attributed to the decrease of the diffusion energy barrier and the improvement of electronic conductivity and reversibility by doping Co. The results match well with the diffusion energy barriers analysis and GITT analysis. The electrochemical impedance spectra (EIS) are used to further analyze the effect of doping Co2+ ion on the electrode reaction impedance. Before EIS tests, the cells are cycled for two cycles at 0.1 C rate. The electrodes were conducted at 50% of discharge or charge state to avoid the huge potential change brought by small current alternation during EIS measurements, and the results were fitted using Zview-Impedance 2.80 software. Fig. 11a shows the impedance spectra and simplified equivalent circuit model (inset) of LiFePO4/C and LiFe0.99Co0.01PO4/C samples. Usually, an intercept in the high frequency region of the Z′ real axis corresponds to the ohmic resistance (Rs), which represents the resistance of the electrolyte and electrode material. The semicircle in the middle frequency range indicates the charge transfer resistance (Rct). The inclined line in the low frequency represents the Warburg impedance (Zw), which is associated with lithium-ion diffusion in the LiFePO4 particles. The constant phase element (CPE) represented for the double layer capacitance was used. The parameters in the equivalent circuit are calculated and tabulated in Table 4. The LiFe0.99Co0.01 PO4/C exhibits a smaller Rct, 91.2 vs 579.3Ω, than pure LiFePO4/C, which is convenient for rapid electrochemical reaction. The exchange current density (I0) is a very important parameter of kinetics for an electrochemical reaction, and can measure the catalytic activity of electrodes. It is calculated using the following formula: I0 ¼

RT

. nRct  F

ð1Þ

where R is the gas constant (8.314 J mol−1 K−1), T is the temperature (298 K), n is the charge transfer number per molecule during the intercalation, and F is the Faraday's constant (96,500 C mol−1). The results are listed in Table 4, and the higher I0 of LiFe0.99Co0.01PO4/C implies the better catalytic activity and reversibility than the pristine LiFePO4/C, which is in prefect agreement with its excellent electrochemical performance. The lithium ion diffusion coefficients (D) of LiFePO4/C and LiFe0.99 Co0.01PO4/C are calculated according to the following equation: 2 2

D¼

R T 2A n F 4 C 2 σ 2 2 4

ð2Þ

where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule

45

during oxidization, F is the Faraday constant, C is the concentration of lithium ion, and σ is the Warburg factor which has relationship with Z′: ′

Z ¼ RD þ RC þ σω

−12

:

ð3Þ

Fig. 11b shows the relationship between Z′ and square root of frequency (ω−1/2) in the low-frequency region. The diffusion coefficient of lithium ion is calculated based on Eqs. (2) and (3) [40–44]. The calculated lithium ion diffusion coefficients of LiFePO4/C and LiFe0.99Co0.01 PO4/C are 8.43 × 10−14 and 9.91 × 10−13 cm2 s−1, respectively. Apparently, the DLi of LiFe0.99Co0.01PO4/C shows an increase, which probably is owing to the lower diffusion energy barriers of LiFe0.99Co0.01PO4/C, suggesting that the LiFe0.99Co0.01PO4/C is more mobile for diffusion of lithium ion and thus has better electrochemical performance than pure LiFePO4/C. The EIS results make it clear that the increase of the exchange current density and the lithium ion diffusion coefficient which resulted from an appropriate amount of Co ion doping leads to the elevated electrochemical activity of LiFePO4/C. 4. Conclusions A facile solvothermal synthesis combined with esterification reaction has been developed to prepare LiFe1 − xCoxPO4/C (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025) nanoplates with the large (010) plane. The thin thickness of prepared nanoplates along b-axis favors the fast Li+ ion insertion/extraction. The theoretical calculation validates that the LiFe0.99Co0.01PO4/C has a lowest lithium ion diffusion energy barrier, which is in well accord with the experimental results. The doped Co ion can enhance exchange current density and lithium ion diffusion coefficient, thus markedly improved the capacity performance and cyclic stability of the LiFePO4/C. The LiFe0.99Co0.01PO4/C exhibited the best electrochemical performance among all the prepared samples, with the initial discharge capacity of 154.5 mAh g−1 at 0.5 C. Observably, it still showed a high discharge capacity of over 96.9 mAh g−1 and good cycle retention even at a high rate of 10 C. Therefore, Co doping is an effective way to improve the performance of LiFePO4 as cathode material for rechargeable lithium ion batteries. Acknowledgments We are grateful for the financial support from the Natural Science Research Keystone Program of Universities in Hebei Province China (No. ZH2011228) and the Natural Science Foundation in Hebei Province China (No. B2012203069). References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (4) (1997) 1188. [2] J. Wang, X. Sun, Energy Environ. Sci. 5 (2012) 5163. [3] S. Nishimura, G. Kobayashi, K. Ohoyama, R. Kanno, M. Yashima, A. Yamada, Nat. Mater. 7 (2008) 707. [4] L.X. Yuan, Z.H. Wang, W.X. Zhang, X.L. Hu, J.T. Chen, Y.H. Huang, J.B. Goodenough, Energy Environ. Sci. 4 (2011) 269. [5] C.R. Sides, F. Croce, V.Y. Young, C.R. Martin, B. Scrosati, Electrochem. Solid-State Lett. 8 (9) (2005) A484. [6] Y.S. Hu, Y.G. Guo, R. Dominko, M. Gaberscek, J. Jamnik, J. Maier, Adv. Mater. 19 (15) (2007) 1963. [7] C.H. Mi, Y.X. Cao, X.G. Zhang, X.B. Zhao, H.L. Li, Powder Technol. 181 (3) (2008) 301. [8] K.S. Park, J.T. Son, H.T. Chung, S.J. Kim, C.H. Lee, K.T. Kang, H.G. Kim, Solid State Commun. 129 (5) (2004) 311. [9] D. Choi, P.N. Kumta, J. Power Sources 163 (2) (2007) 1064. [10] C. Delacourt, P. Poizot, S. Levasseur, C. Masquelier, Electrochem. Solid-State Lett. 9 (7) (2006) A352. [11] I. Bilecka, A. Hintennach, M.D. Rossell, D. Xie, P. Novak, M. Niederberger, J. Mater. Chem. 21 (2011) 5881. [12] Y. Ge, X. Yan, J. Liu, X. Zhang, J. Wang, X. He, R. Wang, H. Xie, Electrochim. Acta 55 (20) (2010) 5886. [13] Z.L. Wang, S.R. Sun, D.G. Xia, J. Phys. Chem. C 112 (44) (2008) 17450. [14] K. Hoang, M. Johannes, Chem. Mater. 23 (11) (2011) 3003.

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