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Controllable preparation, surface structure characterization and electrochemical study of carbon coated lithium iron phosphate as cathode material for lithium ion battery

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  • Save International Journal of M aterials and Chemistry 2012, 2(5): 218-224 DOI: 10.5923/j.ijmc.20120205.06 Controlled Preparation and Surface Structure Characterization of Carbon-Coated Lithium Iron Phosphate and Electrochemical Studies as Cathode Materials for Lithium Ion Battery Xiangcheng Sun1,*, Kai Sun2, Caiyun Chen3, Haiping Sun2 , Bo Cui1 1Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Canada 2Department of M aterials Science and Engineering, University of M ichigan, Ann Arbor, M ichigan 48109 US 3Department of M aterials Science and Engineering, Shandong University of Technology, Zibo, PR China Abstract Amo rphous carbon-coated lithiu m iron phosphate (C-LiFePO4) particles have been mass synthesized at the commercial scale by a controlled solid-state reaction method. Particles morphologies, o livine-type phase structures and the carbon coating features were investigated in details by various techniques as X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) imaging, and transmission electron microscopy (TEM, HRTEM ) imag ing, selected-area electron diffraction (SAED), X-ray energy dispersive spectroscopy microanalysis (XEDS), and X-ray photoelectron spectroscopy (XPS). Single-crystal nature of the olivine type LiFePO4 structures was revealed by XRD and SA ED analyses. TEM imaging showed rough nanoparticles spherical features with average size range of 50-200 n m. Ho mogenous coating features of carbon layers on the particles surface and olivine-LiFePO4 phase are clea rly observed in HR-TEM imaging and confirmed by the corresponding SAED pattern. Elemental b inding energy fro m XPS analysis also confirmed that an amo rphous sp2 carbon coating layer and olivine type LiFePO4 structures. It is indicated that the characteristics of sp2 type conducting-coating layer on the particles surfaces gave rise to improved electrical conductivity by reducing the diffusion path of the electron and lithiu m ions, as directly evidenced fro m our charge-discharge cycling testing as the cathode in the Lithiu m ion battery cell. Keywords Lithiu m Iron Phosphate, Olivine-Type Phase, X-Ray Diffraction, Transmission Electron Microscopy, X-Ray Photoelectron Spectroscopy, Electrochemical Capacity 1. Introduction Extensive attention has been devoted to the development and characterizations of the phospho-olivine type lithiu m iron phosphate (LiFePO4) as an attractive cathode candidate in the lithiu m ion battery market, wh ich was first reported by John Goodenough in 1997 as a pro mising cathode electrode for the rechargeable lithiu m-ion batteries[1]. Olivine-type LiFePO4 exh ib its various unique advantages such as low toxicity, lo w cost, high thermal and chemical stability, and good electrochemical performance in the fully charged state, and it also has a higher theoretical specific capacity (170 mAh g−1) and a flat charge–discharge profile at intermediate voltage (3.45 V vs Li/ Li+), and reasonable cycle life[2-3]. However, LiFePO4 has inherently lo w electrical conductivity, wh ich results in its poor rate capab ility due to the poor * Corresponding author: (Xiangcheng Sun) Published online at Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved kinetics of lithiu m intercalat ion/deintercalation process [2-15]. Th is causes a bigger challenge for power-demanding applications such as hybrid electric vehicles and electric vehicles[2-6]. Nowadays many approaches have been successfully adopted to improve its electrical conductivity, such as metal cation doping and carbon-coating or addition[7-12], decreasing the part icle size and p roducing n ano part icles [13-18]. It has b een repo rted th at carboncoating would increase the surface electrical conductivity and smaller size of LiFePO4 could shorten the diffusion length of Li-ion during lith iumintercalation/deinte rcalation process[3, 6, 20]. Part icularly, it is apparent that carbon coating has been reported to be very effective in the enhancement of capacity and rate capability on LiFePO4 cathode[6-20, 24-33]. In the present work, we report large-scale co mmercial synthesis and structural morphologies of carbon coated lithiu m iron phosphate (C-LiFePO4) particles prepared using home-made amorphous micro -FePO4 as the iron source and conducting black as carbon source. Furthermore, we examined the electrochemical performance of the 219 International Journal of M aterials and Chemistry 2012, 2(5): 218-224 C-LiFePO4 particles as a cathode for the lithiu m-ion battery. 2. Experiments 2.1. Sample Preparation First of all, in order to produce the homogenous carbon-coated LiFePO4 products, the choice of suitable chemical precursors and starting reagents is very essential. Meanwhile, the appropriate temperature and pressure must be carefully selected to prevent the undesirable particle growth and the presence of impurity phase. Otherwise, undesired impurities, such as Fe2O3 and Fe2P, can be contained in the final products. Here, micro-FePO4 powders were firstly synthesized via the solid-state reaction (i.e. pre-calcination), wh ich was typically conducted by heating the mixture of the relat ively cheap precursors of FeSO4 and NH4H2PO4 (e.g. FeSO4:NH4H2PO4=1:2) at up to 300oC. Consequently, the appropriate stoichiometric amount (e.g. 2:1) of amo rphous micro -FePO4 powders and Li2CO3 powders were deeply calcinated under the high-temperature carbonizat ion with presence of the conductive carbon source (i.e. acetylene black) in an industrial-type furnace. A ll the reagents used in the experiment are of analytica l purity. Usually, the high temperature solid state calcination was systemically carried out under controlled temperature ranges (700o C-800oC) and certain pressure for over 5-10 h. By controlling the calcination temperature and atmosphere pressures, the LiFePO4 powders products could be produced maximu m at 5kg/day. The yield purity of the as-prepared LiFePO4 powders was up to 95% that collected in the bag filter regardless of the conducting carbon sources. 2.2. Sample Characterizati on The phase purity and microstructure were characterized and recorded by XRD using a D/ max−2000 Rigaku diffractometry with Cu Kα rad iation (λ = 0.15406 n m) operated at 40 kV and 30 mA with in the scanning angle (2θ) ranging fro m 20° to 70°, and a step of 0.01°. The mo rphologies of as-prepared particles were examined using a scanning electron microscopy (Carl Zeiss, InLens, WD = 9.6, 15Kv). The d ispersed particles supported on a 200 mesh holy carbon film coated Cu grids were characterized at nanoscale by a conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM), and selected area electron diffraction (SA ED) techniques using a JEOL 3010 microscopy equipped with XEDX systemat 300 kV. Surface chemical elements characterizations of particles were determined and recorded by X-ray photoelectron spectrometer (XPS) using a Kratos ULTRA DLD XPS with a mono-chromated Al source that gives an energy resolution better than 0.5 eV at a pass energy smaller than 20eV. Both survey scans (with a pass energy of 160 eV and a scan step of 1eV) and core scans (with a pass energy of 20 eV and a scan step of 0.1eV) were collected. The spectra were calibrated by setting the C 1s peak as 285 e V. Electrochemical properties were measured on electrodes prepared by using the mixtures of co mprising 85 wt% active materials, 10 wt% acetylene black, and 5 wt% polyvinylidene fluorides (PVDF) binder. The LiFePO4 electrode films were fabricated by doctor blade technique on aluminum foil. The cells consisted of the electrode, a lithiu m metal counter electrode and the electrolyte of a 1 M solution of LiPF6 in ethylene carbonate/dimethyl carbonate (EC/ DM C, 1:1). The cells were assembled and handled in an Ar-filled glove bo x and were evaluated using coin-type cells. Galvanostatic discharge/charge tests were carried out using a program-controlled LAND Celltest-2001A system between 2.5 V and 4.2 V versus the Lithiu m counter electrode at room temperature. 3. Results and Discussion The powder X-ray diffraction pattern (XRD) taken fro m the as-prepared particles is shown in Fig.1. It clearly revealed that the C-Li FePO4 particles were inde xed we ll to a pure orthorhombic system of oli vine-type structure (Pnma, JCPDS: 40-1499). No any impurities phase such as transition meta l co mpounds Li3PO4 and FePO4 we re detected by XRD. Carbon phase was not found apparently fro m the XRD pattern, which indicates it may exist in amorphous form. 10000 LiFePO4 (Pnma) 9000 JCPDS 40-1499 8000 Intensity(a.u.) 7000 6000 5000 4000 3000 2000 1000 0 20 30 40 50 60 70 Two Theta (2θ) Powder XRD pattern of the C-LiFePO4 particles Figure 1. Powder XRD pattern Fro m the SEM images shown in Fig.2a and Fig.2b, it is observed that the as-prepared LiFePO4 particle assemblies exhibit ing spherical or elliptical morphology with nano-aggregated surface features having a size distribution fro m 1–3 μm. The typical bright-field magnified TEM images in Fig.3a shows that the LiFePO4 particles assemblies are co mposed of densely aggregated nanoparticles showing rough spherical shapes with sizes in the range of 50-200 n m regardless of the types of carbon sources used. Further details are evidenced from the selected area electron diffraction (SA ED) pattern showed in the Fig. 3b. The appearance of distinct and diffuse SAED rings Xiangcheng Sun et al.: Controlled Preparation and Surface Structure Characterization of Carbon-Coated Lithium 220 Iron Phosphate and Electrochemical Studies as Cathode M aterials for Lithium Ion Battery confirms the co-existence of an amo rphous phase together with LiFe PO4 crystal phase. Fig.3c is a HR-TEM image that gives further insight of the morphology of C-LiFePO4 particle, such a HR-TEM image v iew clearly indicated that an amorphous carbon layer overed the surface o f the LiFePO4 part icles with a thickness of 3-4 nm layer, and its corresponding SAED pattern shown in Fig.3d can be indexed as the[100] direct ion of the olivine phase crystal. It has been reported that the formation of amo rphous carbon in the surfaces of the particles owes to the nature of the carbon source during solid-reactions, and was indeed generated by carbonization of the conductive carbon source precursor[19, 20]. Meanwhile, it is observed that the coating layer was uniform and the thickness of carbon on the particle was in the range of several nanometers, suggesting that the carbon precursor played an important role in reducing the LiFePO4 particle size during the high te mperature ca lcinations process. This result is obviously attributable to the amorphous carbon coating that inhibits grain growth as reported by several groups[19-21]. The well-resolved lattice fringes demonstrate the high crystalline and single-crystal features of the core-LiFePO4 structures. The d-spacing of 0.47 n m as marked in Fig.3c is consistent with spacing of the (001) plane of the orthorhombic LiFePO4 phase. The corresponding SEAD pattern also clearly confirms the single-crystal and olivine-phase nature of the LiFePO4 particle. The index of the electron diffraction spots corresponding to the (020) and (002) planes of LiFePO4 crystals are labelled in Fig.3d, further revealing the single-crystalline nature of the LiFePO4 particles during solid state reaction under higher temperature and pressures. The corresponding selected area XEDS spectrum collected fro m the part icles shown in Fig.3a is given in Fig.4. It shows that the as-prepared particles consist of all the elements of the crystal phase and the amo rphous carbon ele ment. SEM images of the C-LiFePO4 part icles assemblies Figure 2(a,b). SEM images 221 International Journal of M aterials and Chemistry 2012, 2(5): 218-224 TEM and HRTEM images and SAED patterns of the C-LiFePO4 particles Figure 3(a,b,c,d). TEM and HRTEM images and SAED pattern XEDS spectrum of the C-LiFePO4 particles Figure 4. The XEDS spectrum Fro m the above analysis, it convincingly shows that the as-prepared C-LiFePO4 particle assemblies possessed unique core-shell structure and carbon interconnection networks. This is ideal for producing the cathode with enhanced electric conductivity and excellent rate capabilit ies. As mentioned previously that the nano-sized C-LiFePO4 particles reduce the solid-state diffusion path[13-18], which further expedite the electron and ion transport during charge-discharging processing as the cathode. These improved characteristics meet all the prerequisites for generating an excellent rate capability and a high volu metric discharge capacity in the development of the higher energy density lithiu m ion batteries in terms of stability, safety and e xcellent electroche mica l properties[21-23]. The typical XPS spectrum survey profile and the core-scan spectrum of Fe 2p are acquired and shown in the Fig.5a and Fig.5b, respectively. The main b inding energy (BEs) o f Fe 2p3/2, O 1s, and C1s peaks are determined to be 710 eV, 531 eV, and 285 eV, respectively, the appearance of the Ti and N peaks was fro m the sample support. It is also clearly seen fro m the Fig.5b that the Fe 2p spectrum split into 2p1/2 and 2p3/2 due to the spin-orbit coupling. Each part consists of a main peak and a corresponding satellite peak at BEs of 710.9 eV and 724.4 eV for Fe 2p3/2 and Fe 2p1/2, respectively. In fact, the appearance of satellite peaks or shoulder peaks is a characteristic feature of transition metal ions with partially filled d-orbits[24, 26], which indicated that two distinct peaks at binding energies of 710.9 eV and 724.4 eV were observed in the core-scan spectrum of Fe 2p. This means that the chemical o xidation state of Fe is +2. Xiangcheng Sun et al.: Controlled Preparation and Surface Structure Characterization of Carbon-Coated Lithium 222 Iron Phosphate and Electrochemical Studies as Cathode M aterials for Lithium Ion Battery Indeed, the two peaks could be ascribed to Fe 2p3/2 and Fe 2p1/2, which is characteristic of the valence of Fe 2+ state in the olivine-type LiFePO4 products[25-29]. The binding energy of C 1s spectrum in 285 eV shown at Fig.5a is clearly assigned amorphous carbon with sp2 C−C bonds (284.7 eV)[25]. Meanwhile, the O1s peak in the binding energy of 531 eV rep resents the PO43− group oxide ions in LiFePO4. It is believed that the O 1s spectra and P 2p spectra exhib it features at 531.6 and 133.5 eV due to the phosphate structure[25-29]. As a result, the XPS analysis further confirms the co-existence of the olivine phase and amorphous carbon in the as-prepared C-LiFePO4 cathode samples, which is in good agreement with the above both XRD and HR-TEM analysis. Galvanostatic discharge-charge testing was electrochemically performed and with the data showed in Fig.6. It is very clear that as-prepared C-LiFePO4 part icles delivered and achieved an initial discharge capacity of nearly160 mAh/g at a 0.2 C rate as the cathode in a Li-ion cell. The peaks of capacity profiles are rather symmet ric, indicating good cycle reversibility. Moreover, it also showed an excellent capacity retention ratio of 97% after the 50th charg ing/discharging cycles, which also was indicating high rate power perfo rmance as well. Obviously, such a fu ll 3-4 n m carbon coating layer (i.e. carbon interconnection networks) can effectively improve particles surface electronic conductivity during discharge-charge processing. In particular, it has been believed from many reports that, the crucial role played by the surface carbon coating on the LiFePO4 particles has been responsible for the imp roved electrochemical performance due to the increasing of both electron and ionic transport of the LiFePO4 cathode composite particles[26-34]. 140000 120000 (a) 100000 O1s Intensity 80000 60000 C1s 40000 20000 Li1s P2p P2s Ti N 0 0 100 200 300 400 500 Binding Energy (eV) 4200 4000 710 eV 3800 3600 724 eV 3400 3200 3000 2800 2600 2400 2200 Fe2p 600 700 (b) Intensity 705 710 715 720 725 730 735 Binding Energy (eV) The typical XPS spectrum and (b) the core-XPS spectra of Fe 2p of the C-LiFePO4 particles Figure 5. (a) The typical XPS surve y spe ctrum and (b) the core-scan spectra of Fe 2p V 4.2 4.1 4 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3 2.9 2.8 2.7 2.6 2.5 0 0.2C 0.5C 1C 3C 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 mAh/g The discharge and charge profiles of C-LiFePO4 nanoparticles at different current rates Figure 6. The discharge and charge profiles 223 International Journal of M aterials and Chemistry 2012, 2(5): 218-224 4. Conclusions 1), Carbon coated LiFePO4 (C-LiFePO4) powders products with the o liv ine-type structure has been successfully synthesized at commercial scale. XRD, SEM, SAED, HR-TEM image, and XPS analyses clearly provide a comprehensive view of the structure-correlated performance of the C-LiFePO4 products. It demonstrates that each LiFePO4 part icle has a sp2 amo rphous carbon-coating shell and a phospho-olivine type crystal core. 2), The unique o liv ine-type C-LiFePO4, co mb ined with its full coating of conductive carbon, effectively enhances its electrochemical performance due to the presence of the carbon coating networks for both electronic and ionic transport increasing during the electrochemical testing. 3), The present mass production procedure is very valuable to optimize the process for producing carbon coated LiFePO4 cathode materials, this method is likely to be easy to scale up for industrial production. ACKNOWLEDGMENTS This works are financially support fro m the President’s Award of Un iversity of Waterloo and Postgraduate Scholarship of the Natural Sciences and Engineering Research Council of Canada (NSERC). Thanks to Jiangsu Fangzhou New Energy Co mpany, China for offering me the collaborations for the samples preparations. 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