C via Ni doping

Journal of Physics and Chemistry of Solids 71 (2010) 1196–1200

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Synthesis of spherical LiFePO4/C via Ni doping Wenkui Zhang a,n, Yilan Hu a, Xinyong Tao a,n, Hui Huang a, Yongping Gan a, Chuntao Wang b a b

College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, China College of Electromechnical Engineering, Zhejiang Ocean University, Zhoushan 316000, China

a r t i c l e in f o

a b s t r a c t

Article history: Received 22 December 2009 Received in revised form 1 March 2010 Accepted 9 April 2010

Spherical LiFePO4/C powders were synthesized by the conventional solid-state reaction method via Ni doping. Low-cost asphalt was used as both the reduction agent and the carbon source. An Ni-doped spherical LiFePO4/C composite exhibited better electrochemical performances compared to an undoped one. It presented an initial discharge capacity of 161 mAhg  1 at 0.1 C rate (the theoretical capacity of LiFePO4 with 5 wt% carbon is about 161 mAhg  1). After 50 cycles at 0.5 C rate, its capacity remained 137 mAhg  1 (100% of the initial capacity) compared to 115 mAhg  1 (92% of the initial capacity) for an un-doped one. The electrochemical impedance spectroscopy analysis and cyclic voltammograms results revealed that Ni doping could decrease the resistance of LiFePO4/C composite electrode drastically and improve its reversibility. & 2010 Elsevier Ltd. All rights reserved.

Keywords: A. LiFePO4 A. Asphalt A. Lithium battery B. Ni doping D. Spherical

1. Introduction In the past few years, LiFePO4 has been extensively studied and developed as a potential cathode candidate for the next generation of second lithium batteries [1]. It drew so much attention because it presented many significant advantages, such as high capacity, good safety attribute, low cost and low toxicity [2,3]. It has a specific capacity of 170 mAhg  1 and a flat charge/discharge potential at 3.45 V vs. Li + /Li [4,5]. It is well known that the morphology, particle size and distribution have tremendous effect on the electrochemical performance of the cathode material. Recent results show that the powders that composed of spherical particles have lower interfacial energy, better fluidity characteristics and higher volumetric energy density than irregular particles [6–10]. So lots of effort have been devoted to the realization of spherical LiFePO4 products. Yang et al. [11] obtained LiFePO4 spherical product with well particle size distribution by ultrasonic spray pyrolysis. Ying et al. [12] and Xie et al. [9] got spherical LiFePO4 through a two-step reaction route, which prepared spherical FePO4  xH2O precursors via a controlled crystallization method. Besides, molten salt [10] and hydrothermal method [13] also have been introduced to get spherical LiFePO4. Most of these methods need complex synthetic procedures. Synthesis of spherical LiFePO4 via a facile cost-effective method, such as solid-state reaction is a great challenge.

n

Corresponding authors: Tel./fax: + 86 571 88320394. E-mail addresses: [email protected] (W. Zhang), [email protected] (X. Tao).

0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.04.015

The main drawback of LiFePO4 was its low electronic conductivity. Carbon coating of LiFePO4 is an efficient strategy to overcome this problem. Over 20 kinds of carbon sources have been reported which include glucose [14], graphite [15], acetylene black [16], ethylene glycol [17], citric acid [18], glycolic acid [19], carbon gel [20], polyvinyl alcohol [21] and carbon black [11], etc. All the traditional carbon sources suffer from their high cost. Many researchers are aiming to find new carbon sources. In this work, we proved that spherical LiFePO4/C composite can be produced by the conventional solid-state reaction method via Ni doping. A new carbon source, the low-cost asphalt, was selected as both reduction agent and carbon source. The effects of Ni doping on the morphology and electrochemical properties of LiFePO4 were both investigated. The results proved that Ni doping improved the electrochemical performances of LiFePO4/C effectively and contributed to the formation of the spherical LiFePO4/C grain.

2. Experimental LiFePO4/C was prepared by the solid-state reaction method. A general procedure of the experiment is described as follows. LiOH  H2O, FePO4  4 H2O, asphalt were mixed and ground by ball milling for 6 h. Asphalt powder as carbon source (80 mm in diameter) was added in proportion of 5% with respect to the total weight. The mixture was then sintered in a cube furnace under an inert atmosphere by two steps: 380 1C for 1 h and 650 1C for 8 h. An Ni-doped LiFePO4/C was synthesized through the same procedure as above. (CH3COO)2Ni  4H2O was used as the nickel

W. Zhang et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1196–1200

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Electrochemical performances were evaluated with CR2032 coin cells. The lithium foil was used as the counter electrode. The electrolyte was 1 M LiPF6 in blended ethylene carbonate (EC) and diethylene carbonate (DEC) solution with a volume ratio of 1:1. The cathode was prepared by mixing 80 wt% active materials with 15 wt% conductive carbon and 5 wt% polyvinylidene fluoride (PVDF) in an n-methyl-2-pyrrolidone (NMP) solution. The resulting slurry was coated on an aluminum foil current collector (25 mm in thickness) and dried at 80 1C for 4 h. After pressed under 16 MPa pressure, the cathodes were dried at 120 1C for another 12 h. The cells were assembled in an argon-filled glove box. The cells were charged/discharged in the voltage range 2.5–4.2 V at room temperature. Cyclic voltammetry (CV) and AC impedance (EIS) were conducted by a CHI650b electrochemical work station. CV tests were carried out in the voltage range 2.5–4.2 V at a scan rate of 0.1 mVs  1, and EIS tests were carried out in the frequency range 0.1–106 Hz.

(040)

(222) (402) (412) (610) (331) (430)

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(311) (121) (410) (102) (221) (401) (112) (321) (212)

(211) (020) (301)

(210) (011)

(200)

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(111) (201)

source. LiOH  H2O, FePO4  4H2O, and (CH3COO)2Ni  4H2O were mixed in a molar ratio of 1.01:0.99:0.01. The crystalline structure was analyzed by the X-ray diffractometer (XRD) with a Cu Ka radiation. The microstructure was studied by transmission electron microscopy (TEM, FEI Tecnai G2 F30) and high resolution TEM (HRTEM).

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2θ (degree) Fig. 1. XRD patterns of (a) LiFePO4/C and (b) Ni-doped LiFePO4/C.

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3. Results and discussion In this work, asphalt was used as both the reduction agent and the carbon source for the formation of LiFePO4/C. Asphalt composed of aromatic hydrocarbons, saturates, resins and asphaltenes. It contains about 85 wt% carbon and 10 wt% hydrogen. According to Gong et al.’s research, under the high temperatures (  250 1C to 425 1C and  425 1C to 530 1C), the asphalt undergoes a fast cracking and the chemical bonds are broken, so that the large hydrocarbon molecules decompose into

Fig. 2. (a) Representative TEM image of the spherical LiFePO4 crystals; (b) [1 2 1] zone axis HRTEM image of one carbon coated spherical LiFePO4 grain; (c) the magnification of the square zone in (b), the arrow indicates the carbon nano-interconnects; (d) the FFT image corresponds to (b).

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W. Zhang et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1196–1200

4.6 4.4 4.2 4.0 Volatge (V)

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Specific capacity (mAhg-1)

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Fig. 3. (a) First charge/discharge curves of LiFePO4/C and Ni-doped LiFePO4/C at 0.1 C; (b) cycling performances of LiFePO4/C and Ni-doped LiFePO4/C at 0.5 C rate.

small molecules in the gas phase [22]. And hydrogen and hydrocarbon gas produced via gasification [23], which can provide a reduction atmosphere for the formation of LiFePO4. Fig. 1 shows the XRD patterns of the two prepared samples. The diffraction peaks of the two samples can be assigned to an ordered olivine structure indexed by orthorhombic Pnma (JCPDS Card no. 832092). The sharp peaks indicate that the LiFePO4/C and Ni-doped LiFePO4/C composites are well crystalline without any detectable impurity phases. No carbon related diffraction peak is detected, indicating that the residual carbon is amorphous. Ni doping does not destroy the lattice structure of LiFePO4 because of the low doping concentration. The crystal cell parameters calculated from the XRD ˚ b¼6.011 A, ˚ c¼4.698 A, ˚ V¼292.00 A˚ 3 for patterns are a¼10.336 A, ˚ ˚ ˚ LiFePO4/C phase and a¼10.335 A, b¼6.014 A, c¼4.700 A, V¼292.15 A˚ 3 for an Ni-doped LiFePO4/C phase. The slight change in cell size indicates that the Fe2+ should be partly substituted by Ni2 + inducing the expansion of the crystal cell. The TEM images of an Ni-doped LiFePO4/C sample are shown in Fig. 2. Fig. 2(a) is a representative TEM image of the spherical LiFePO4 crystals, which are connected together through carbon nanointerconnect structures. Large amount of spherical LiFePO4 particles can be found. Fig. 2(b) shows a representative HRTEM image of a spherical LiFePO4 particle. Fig. 2(c) is the local magnification of the square area of Fig. 2(b). Only disordered graphite layers with

interlayer spacing of 0.34 nm can be observed in the carbon shell (Fig. 2(c)), indicating that the LiFePO4 grain is coated with an amorphous carbon. The big arrow in Fig. 2(c) indicates that the carbon interconnect structure between the LiFePO4 particles. As Wang et al. [24] have reported, this kind of carbon interconnect lead to electronic inter-particle connection, but did not block the direct contact between the active particles and the penetrated electrolyte. This kind of carbon structure can also prevent the LiFePO4 particles from subsequently growing up. The [1 2 1] zone axis HRTEM image (Fig. 2(b)) and the corresponding fast Fourier transform (FFT) image (Fig. 2(d)) revealed that the LiFePO4 particles were single crystalline. According to Zhao’s result [15], the spherical structure will supply a big specific surface area for the subsequent reactions, which can improve the electrochemical performance of LiFePO4 [25]. TEM results also proved that LiFePO4/C without Ni doping exhibited irregular morphology. It was believed that the formation of the spherical LiFePO4/C grain resulted from the Ni doping. Ni2+ can occupy the site of Fe2 + via doping, which induces the lattice distortion of LiFePO4 crystal. This lattice distortion could reduce the surface energy of LiFePO4 crystal and then impede the growth of LiFePO4 crystal. So LiFePO4 crystal can be controlled in a small size and regular shape. Fig. 3(a) shows the first charge/discharge curves of an undoped and doped LiFePO4/C at 0.1 C. The initial discharge capacity of LiFePO4/C was about 150 mAhg  1, which of an Ni-doped LiFePO4/C was improved to 161 mAhg  1 (the theoretical capacity of LiFePO4 with 5 wt% carbon is about 161 mAhg  1). The discharge plateau was extended about 10 mAhg  1 via Ni doping. The calculation showed that the coulombic efficiency during the first cycle of an Ni-doped LiFePO4/C was about 94.7%. The irreversible capacity loss (IRCL) in the first cycle was about 9.1 mAhg  1, which was less than the 24.3 mAhg  1 of an undoped material. The low lithium-ion diffusion rate and low electronic conductivity resulted in the capacity loss in the first cycle [26]. It was believed that Ni doping can improve the electronic conductivity of LiFePO4 [27], so that the IRCL can be decreased after Ni doping. The cycling performance of LiFePO4/C and an Ni-doped LiFePO4/C at 0.5 C rate was presented in Fig. 3(b). In the first cycle at the 0.5 C rate, an Ni-doped LiFePO4/C showed a specific capacity of about 135 mAhg  1. The first discharge capacity of LiFePO4/C at 0.5 C rate was about 125 mAhg  1, which was less than that of an Ni-doped one. In the following cycling process, the capacity declined gradually. After 50 cycles, the discharge capacity of LiFePO4/C was about 115 mAhg  1. The fading rate of the capacity was about 8%. However, the discharge capacity of an Ni-doped LiFePO4/C remained 137 mAhg  1 after 50 cycles. The capacity retention was 100%. This kind of Ni-doped spherical LiFePO4/C exhibited higher rate capability than Cr doped spherical LiFePO4/C prepared via two-step reaction method [12] and Ni-doped LiFePO4/C synthesized via solid-state reaction method using glucose as carbon source [28]. Based on our experiments, we think that Ni doping and spherical morphology do contribute to the improved electrochemical performances. According to Wang’s [29] research, the capacity fading was related to the cracking during cycling. Therefore, LiFePO4 with stable structure will present good cyclic performance. Lu et al. [28] considered that nickel doping could stabilize the structure of LiFePO4 through enhancing the P–O bond. To provide more information about the electrochemical property, CV and EIS were performed. Fig. 4(a) shows the CV curves of an Ni-doped and un-doped LiFePO4/C composites at a scan rate of 0.1 mV/s in the voltage range 2.5–4.2 V vs. Li/Li + . The LiFePO4/C sample had a couple of oxidation and reduction peaks at 3.58 and 3.31 V. For an Ni-doped LiFePO4/C, the oxidation peak and the corresponding reduction peak were observed at a potential of 3.56 and 3.32 V, respectively. It can be found that

W. Zhang et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1196–1200

1.0x10-3

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Current (A)

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the Warburg Impedance (Zw), which is attributed to the diffusion of Li + in the LiFePO4 particles [30]. A simplified equivalent circuit model (Fig. 4(c)) was constructed to analyze the impedance spectra in Fig. 4(b). Significant decrease of Rct from 442.2 O in LiFePO4/C to 75.9 O in an Ni-doped LiFePO4/C composite can be found. This implied that the charge-transfer reaction of the LiFePO4/C was enhanced by Ni doping. Thus, the transportation of electrons was improved. According to the Wang’s results [27], Li–O interaction was weakened by Ni doping, which could improve the electronic conductivity of LiFePO4. They also found that Ni doping do contribute to the enhanced mobility of Li + ions [27]. Similar phenomena have been found by Yang et al. [28]. It was proved that Ni doping enhanced the P–O bond and stabilized the structure of LiFePO4, so that the charge-transfer resistance was decreased [28]. We believe that the low Rct of an Ni-doped LiFePO4/C results from both carbon coating and Ni doping.

4. Conclusions

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In this work, we proved that spherical LiFePO4/C can be prepared by a new cost-effective method via Ni doping using lowcost asphalt as both carbon source and reduction agent. The XRD results analysis showed that the material had sharp peaks and fine framework. Ni doping did not substantially influence the structure of LiFePO4. TEM results revealed that an Ni-doped LiFePO4/C particles were spherical, which were connected together through carbon nano-interconnect structures. An Nidoped LiFePO4/C showed a discharge capacity of 161 mAhg  1 at 0.1 C rate, which was higher than that of an un-doped one. The cyclic performance was also improved by an Ni doping. The data of CV and EIS demonstrated that Ni doping enhanced the electronic conductivity of LiFePO4/C. This work offers a costeffective method for the preparation of spherical LiFePO4/C with high electrochemical performance.

Acknowledgements

Re

Rct

Zw

Fig. 4. (a) CV curves of LiFePO4/C and Ni-doped LiFePO4/C; (b) AC impedance spectra of LiFePO4/C and Ni-doped LiFePO4/C after three cycles at 0.1 C rate; (c) equivalent circuit used for fitting the experimental EIS data.

the voltage difference between oxidation and reduction peaks of LiFePO4/C is larger than that of an Ni-doped one. The smaller potential difference between the anodic and cathodic peaks implies good reversibility during the charge–discharge cycling. An Ni-doped LiFePO4/C also had higher current peak intensity, indicating that the lithium intercalation and deintercalation reaction were stronger. The well-defined and symmetrical CV peaks demonstrated that the reversibility of the electrode reaction of LiFePO4/C was improved by the Ni doping. The typical Nyquist plots of LiFePO4/C and Ni-doped LiFePO4/C composites electrodes are shown in Fig. 4(b). Both profiles exhibited a semicircle in the high frequency region and a straight inclined line in the low frequency region. The intercept at the Z ’ axis in high frequency corresponds to the ohimic resistance (Re), which represents the resistance of the electrolyte. The semicircle in the middle frequency range represents the charge-transfer resistance (Rct). The inclined line in the low frequency indicates

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