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• J. Mater. Chem., 2009
• DOI: 10.1039/b916938e
• Paper

Up-scalable synthesis, structure and charge storage properties of porous microspheres of LiFePO4@C nanocomposites
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Feng Yu ab, Jing-Jie Zhang *a, Yan-Feng Yang a and Guang-Zhi Song a
aTechnical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China. E-mail: jjzhang@mail.ipc.ac.cn, yufeng05@mail.ipc.ac.cn.; Fax: +86-10-82543691; Tel: +86-10-82543691
bGraduate University of Chinese Academy of Sciences, Beijing 100049, P.R. China
Received 17th August 2009 , Accepted 30th September 2009
First published on the web 27th October 2009
________________________________________
Novel porous micro-spherical aggregates of LiFePO4@C nanocomposites have been synthesized in large quantities via an improved sol–gel method combined with spray drying technology (sol–gel-SD method), which required no surfactants or templates. With this new procedure, a precursor was prepared through the process of sol–gel and subsequent spray drying. A series of analyses, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and a combined system focused ion beam (FIB)/SEM were performed after the as-obtained LiFePO4@C was heat-treated at 700 °C for 12 h. The as-obtained LiFePO4@C had a large specific surface area (20.2 m2g−1), with an average nano-size of 32 nm and a main pore diameter of 45 nm. Contact with electrolyte occurred easily, which facilitated the electrical and lithium ion diffusion. In comparison with nano-sized LiFePO4@C particles prepared by a sol–gel method, the current product presented a high coulombic efficiency of 97.2%, a large reversible capacity of 133.8 mAh/g and an excellent capacity retention rate close to 100% after 50 cycles. The sol–gel-SD method provides an additional strategy to easily deal with gelatin and shows potential for use in the preparation of similar superstructures of other composites.
________________________________________
1.0 Introduction
Since Goodenough and co-workers1 reported reversible lithium intercalation in the phosphate polyanionic compound of LiFePO4 as a positive electrode material in 1997, interest has grown quickly amongst researchers in the field of rechargeable lithium ion batteries. Compared with the typical commercial layered LiCoO2 material, LiFePO4 materials with olivine structures possess excellent structural flexibility and stability.2–4 It exhibits superior performance including high reversible capacity, acceptable operating voltage, long cycle life, low cost, superior safety and it is environmentally benign.5,6 However, further advancements in promising applications are still somewhat bottlenecked by limitations associated with the following obstacles: (a) the poor electronic conductivity and the low Li+ diffusion, which lead to initial capacity loss and poor charge/discharge capacity7,8 and (b) the low tap-density, which leads to low volumetric energy density.9,10 These limitations are still to be overcome.
Modifications of LiFePO4 particles by minimizing particle size11,12 and coating them with an electronically conductive carbon agent13,14 are considered to be effective in surmounting electronic and ionic transport limitations. By combining both of these methods, many researchers have been able to prepare nano-sized LiFePO4/C composite materials. Conventional synthesis methods, such as traditional solid state methods,15 hydrothermal methods16 and sol–gel methods17,18 have all been employed to obtain nano-sized LiFePO4/C composite materials. However, the interfacial energy of nano-sized LiFePO4/C particles is very large and the particles tend to aggregate easily. Consequently, the synthesized powders are typically composed of irregular aggregates, which seriously impacts the electrochemical performance of LiFePO4, making morphology an important physical property that must be considered.
As has been reported in previous publications, morphology has a great influence on the properties and practical applications of LiFePO4. Morphology control is also believed to be crucial for the determination of size/structure-dependent properties and for the development of new pathways for materials synthesis. Among various materials with different morphologies, spherical morphology shows a great advantage that is superior to other morphologies. Compared with irregular aggregation, the spherical particles have lower interfacial energy, high volumetric energy density and better fluidity characteristics. When the spherical particles vibrate, they can easily move and occupy the available vacancies, so that they closely pack readily.19–22
Recently, much effort has been devoted to the realization of spherical LiFePO4 products. Ying et al.23 successfully prepared solid micro-spherical Li0.97Cr0.01FePO4/C powders by a co-precipitation method. However, this material often had low discharge capacity and unsatisfactory rate capability due to large amounts of inert LiFePO4 at the heart of the solid LiFePO4 microsphere, which contacted poorly with electrolytes (Fig. 1a). Otherwise, although hollow micro-spherical LiFePO4 powders can usually be synthesized by a spray pyrolysis method,24 the hollow part without active LiFePO4 at the heart impacts the volumetric energy density of LiFePO4 (Fig. 1a). Therefore, it is still a challenge for researchers to seek out an efficient route for the synthesis of LiFePO4 with well-defined microspheres and nano-sized particles.

Fig. 1 (a) Degrees of active LiFePO4 at different interfaces of electrolyte and various LiFePO4 microspheres: a solid microsphere with inert LiFePO4 (left), a hollow microsphere with a hollow part without LiFePO4 (middle) and a porous microsphere with sufficient interface with electrolyte (right). (b) Advantages of the designed LiFePO4@C porous microspheres: outstanding morphology with nano-sized, porous and spherical distribution for achieving good electrochemical performance. (c) Synthesis of the porous macro-spherical aggregates of LiFePO4@C nanocomposites using the sol–gel-SD method.

In the present study, we developed an improved sol–gel method combined with spray-drying technology (sol–gel-SD method) to design and synthesize porous micro-spherical aggregates of LiFePO4@C nanocomposites without employing surfactants or templates (Fig. 1a). This designed LiFePO4@C microsphere possesses outstanding morphology with nano-sized, porous and spherical distribution for achieving good electrochemical performances (Fig. 1b). Recently, increasing attention has been given to the risks of nanotechnologies which have possible impacts on human health and the environment.25–27 It is worth mentioning that the LiFePO4@C microspheres prepared by our procedure reduce the potential toxicity of pure nano-structured particles. Furthermore, the large porous particles are in principle easier to bind and to bring into electrical contact than purely nano-sized particles.28,29
2.0 Experimental
Porous micro-spherical aggregates of LiFePO4@C nanocomposites have been synthesized by a facile template-free process using an improved sol–gel method (sol–gel-SD method). Amounts of Li2CO3 (AR), Fe(NO3)3•9H2O (AR) and H3PO4 (AR) were dissolved in distilled water in the stoichiometric ratio nLi : nFe : nP = 1 : 1 : 1. The desired amount of tartaric acid was added and the aqueous solution formed a homogeneous gelatin. The gelatin was then dissolved in distilled water and made into a liquid suspension without surfactants or templates. The obtained suspension liquid was spray-dried in a spray dryer unit at a rate of 15 mL min−1 with inlet and outlet temperatures maintained at 200 °C and 130 °C. Carbothermal reduction of the obtained spray-dried precursor was performed in a tube furnace using a graphite crucible, heated at 10 °C min−1 in a flowing argon atmosphere (100 mL min−1) until it reached 700 °C, it was then held at this temperature for 12 h to obtain the LiFePO4@C composite sample.
Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectra were obtained using a Hitachi S-4300 microscope and EMAX Horiba, respectively. A Nova 200 Nanolab Dual-beam FIB/SEM instrument was used to gather information on the inside of the obtained LiFePO4@C composite. This instrument is a combination of SEM and focused ion beam (FIB) with two focused beams in the same location. The FIB instrument is equipped with 30 kV Ga+ ions with a beam current from 1 pÅ to 20 nÅ. The minimum ion beam spot size is 7 nm at 1 pÅ beam current. A scanning probe microscope in tapping mode was employed to obtain the topographic features. X-Ray diffraction (XRD) analysis was carried out on a Rigaku D/max2200PC diffractometer with Cu K radiation ( = 1.5406 Å

. X-Ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al K radiation. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images and the selected area electron diffraction (SAED) were performed with a Philips TECNAI-20 transmission electron microscope. The specific surface area and pore structure were examined with a Quantachrome NOVA 4000 BET apparatus.
For fabrication of the working electrodes, the obtained LiFePO4@C composite was mixed with acetylene black and polyvinylidene fluoride (PVDF) in a weight ratio of 80 : 15 : 5 in N-methyl-2 pyrrolidinone (NMP). The obtained slurry was coated on to Al foil and dried at 80 °C for 4 h. The dried tape was punched into round discs with a diameter of 10.0 mm as the cathode electrodes. The electrodes were dried again at 120 °C for 5 h in a vacuum prior to use. Finally, the prepared cathodes and Celgard2400 separator (diameter of 16.0 mm) were placed into an argon filled glove box (H2O and O2 < 1 ppm) and assembled into a coin cell (CR2032) with lithium anode, electrolyte of 1 M LiPF6 in EC-DEC-DMC (1 : 1 : 1 vol.%) and the other components of the coin-type cell. The cells were examined with capacity retention studies performed with various rates between 2.0 and 4.3 V. The cells were retained for ten minutes at 4.3 V in charging.
3.0 Results and discussion
3.1 Schematic illustration of the sol–gel-SD method
In order to obtain the designed porous micro-spherical aggregates of LiFePO4@C nanocomposites, a sol–gel method combined with spray drying technology (sol–gel-SD method) was employed. Fig. 1c demonstrates the formation mechanism of this designed product. The main processes were as follows: (1) the sol–gel method was employed to produce a gelatin, in which multiple reactants were homogeneously mixed at the molecular level, resulting in good particle size control.30 (2) In the spray-drying step, the obtained gelatin was dissolved in distilled water and made into a liquid suspension without surfactants or templates. Subsequently, the obtained suspension was spray-dried with a spray dryer unit. The evaporation of the gelatin was more rapid and yielded finer homogeneous and micro-spherical aggregates of precursor particles compared to the routine evaporation technique. Spray-drying technology is considered an attractive route that exhibits superior performance including energy savings, cost effectiveness, continuous preparation and environmental safety.22,31 (3) In the carbothermal reduction step, the carbon from degradation and carbonization of tartaric acid provides a special environment that is favourable for the reduction of Fe(III) and for the formation of the nanocrystalline composite LiFePO4@C powders.
3.2 Structural characterization
The crystallinity and the phase information for the as-obtained product, have been confirmed with the X-ray diffraction (XRD) method, as shown in Fig. 2. All of the reflections can be attributed to the orthorhombic phase LiFePO4 (triphylite) (JCPDS no. 40-1499) that lacks any impurity phase. The profiles of the reflection peaks are quite narrow and symmetric, indicating the high crystallinity of the LiFePO4 sample. Although the result of the energy dispersive X-ray (EDX) analysis confirms the presence of carbon (Fig. 3c), this carbon is not detected in the XRD pattern since residual carbon is amorphous.

Fig. 2 An XRD pattern of the as-obtained porous microspheres of LiFePO4@C nanocomposites prepared by the sol–gel-SD method.

Fig. 3 (a) A TEM image of the as-obtained porous microspheres of LiFePO4@C nanocomposites. (b) An enlarged TEM image of the surface of one individual LiFePO4@C microsphere and the corresponding SAED pattern (inset in panel a). (c) An EDX spectrum of as-obtained porous microspheres of LiFePO4@C nanocomposites. (d) A HRTEM image of A area (indicated by a rectangle in panel b). (e) A HRTEM image of B area (indicated by a rectangle in panel b).

A space group of Pnma was selected as the refinement model. The absolute lattice parameters for the as-obtained product were as follows: a = 6.0320 nm, b = 10.3588 nm, c = 4.7063 nm and the cell volume was V = 294.07 nm3, similar to the previous reports.1,10 The estimated crystallite size, as deduced using the Debye-Scherrer equation and five crystal lattice indexes of (011), (111), (121), (031) and (131), is 32 nm, which is consistent with the SEM observations. Therefore, a pure, homogeneous and well-crystallized LiFePO4@C composite is indicated.
X-Ray photoelectron spectroscopy (XPS) analysis of the as-obtained product has been carried out to examine the oxidation state of Fe in the sample (Fig. 4). Two distinct peaks at binding energies of 710.9 eV and 724.4 eV were observed in the high resolution spectrum of Fe2p. The two peaks could be ascribed to Fe2p3/2 and Fe2p1/2, which is characteristic of Fe2+ in LiFePO4. Otherwise, the binding energy of Li 1s, P 2p, and O 1s are determined to be 55.8 eV, 133.6 eV and 531.8 eV respectively, with the reference binding energy at 284.8 eV for the C 1s peak. Thus, the XPS analysis further confirms the purity of the LiFePO4 sample.

Fig. 4 An XPS pattern of the as-obtained porous microspheres of LiFePO4@C nanocomposites and a high-resolution XPS Fe2p spectrum (insert).

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3.3 Morphological characterization
The scanning electron microscopy (SEM) images at different magnifications of the spray-dried precursor microspheres are shown in Fig. 5a and b. In Fig. 5a, a panoramic image of the precursor microspheres without any dispersion treatment, shows the micro-spherical morphology of the precursor with an average centre diameter of 10 m and good uniformity of morphology. The SEM image of a single precursor microsphere clearly demonstrates that the micro-spherical aggregates are proposed of nano-sized precursor particles (Fig. 5b). A close observation of the precursor reveals the nano-sized particles as the building units possess a mean diameter of 30 nm. (Fig. 5b, insert). Then, the as-obtained title product was prepared by carbothermal reduction of the spray-dried precursor at 700 °C for 12 h. A combined system focused ion beam (FIB)/SEM provided further insight into the morphology and the structural state of the as-obtained title product, which had a spherical agglomerate structure. From the FIB image shown in Fig. 5c, the 3D cross section of the as-obtained title product can be seen to have a porous structure both inside and on the surface. As shown in Fig. 5d and e, there are obviously inter-connected pores and surface-open pores in the as-obtained title product, a scheme of which is shown in Fig. 5f. The specific BET surface of the as-obtained title product is 20.2 m2 g−1 and the pore diameter is essentially 45 nm. When filled with electrolyte, the pores greatly aid the solid-state diffusion kinetics and are responsible for the immediate supply of lithium ions.

Fig. 5 (a and b) SEM images of the micro-spherical aggregates of precursor particles at different magnifications. (c) FIB images showing 3D information of the as-obtained porous micro-spherical aggregates of LiFePO4@C nanocomposites. (d) A SEM image of area A (indicated by a rectangle in panel c). (e) SEM image of area B (indicated by a rectangle in panel c). (f) A scheme showing the structure of LiFePO4@C nanocomposites in porous microspheres.

The morphology and microstructure details of the as-obtained title product superstructures have been further examined by transmission electron microscopy (TEM) accompanied by selected area electron diffraction (SEAD). As shown in Fig. 3a, the TEM image of two LiFePO4@C microspheres indicated that both were spherical in morphology, which agrees with the SEM result. A closer TEM observation of the surface of a single microsphere, as shown in Fig. 3b, further confirms that the as-obtained LiFePO4@C sample consists of large-scale nano-sized microstructures with sizes ranging from 20–40 nm. The corresponding SEAD pattern taken from the individual particle clearly suggests a single-crystal nature of the as-obtained title product (inset of Fig. 3a). It is worth mentioning that these microstructures are adequately stable; consequently, they cannot be disrupted into fragments or dispersed into nano-sized particles even after ultrasonic treatment.
In order to further reveal the fine structure of the LiFePO4@C superstructures, high-resolution TEM (HRTEM) analysis was also carried out. In Fig. 3d and e, the corresponding HRTEM images of the different areas marked by rectangles (A and B), are shown. The clear lattice image demonstrates the high crystallinity and single-crystal features of the LiFePO4@C superstructures, which is in good agreement with the XRD and the SEAD results. The typical d-spacing of 0.30 nm and 0.35 nm are consistent with the (200) and (021) planes of orthorhombic phase LiFePO4, respectively. In addition, it also can be clearly seen from the HRTEM images that an amorphous carbon layer in the interstitial particle/boundary region covered the surface of LiFePO4. This amorphous carbon layer, with a thickness of 2–3 nm, was generated by carbonization of the tartaric acid precursor.
3.4 Electrochemical characterization
Fig. 6 shows initial charge-discharge profiles and the corresponding cyclic performance of LiFePO4@C samples prepared by the sol–gel method and the sol–gel-SD method. Compared with the nano-sized LiFePO4@C powders (sample LFPCb) prepared by traditional sol–gel method, the as-obtained title product (sample LFPCa) prepared by the sol–gel-SD method had a high reversible discharge capacity, high coulombic efficiency and excellent capacity retention rate at close to 100% cycled at a current rate of 0.1 C in the voltage range of 2.0–4.3 V at room temperature. The initial charge–discharge profiles of both samples had the perfect plateau voltage of 3.4 V (versus Li+/Li), indicating a typical two-phase reaction between LiFePO4 and FePO4.1 However, the polarization between the charge and discharge plateaus of sample LFPCa was less than that of sample LFPCb, due to its excellent Li+ diffusion rate across the two-phase interface. For this reason, the sample LFPCa showed a higher specific discharge capacity and a lower polarization. The initial specific discharge capacities of sample LFPCa and sample LFPCb were 106.7 mAh g−1 and 137.5 mAh g−1 respectively. The sample LFPCa electrodes delivered a charge capacity of 141.5 mAh g−1, and calculation showed that the coulombic efficiency during the first cycle of sample LFPCa was about 97.2%, which was higher than the 91.5% obtained for sample LFPCb. There was no capacity fading and a slight increase in capacity for sample LFPCa in the first 10 cycles (inset of Fig. 6), which demonstrated the excellent cycling stability of the as-obtained title product.

Fig. 6 Initial charge-discharge profiles of LiFePO4@C samples prepared by the sol–gel method ( ) and the sol–gel-SD method ( ) cycled at a current rate of 0.1 C and the corresponding cyclic performance (insert). Galvanostatic tests were carried out in the voltage range of 2.0–4.3 V at room temperature.

4.0 Conclusions
In summary, the sol–gel-SD method was a novel and facile route for preparing porous micro-spherical aggregates of LiFePO4@C nanocomposites without employing surfactants or templates. The as-obtained LiFePO4@C possessed outstanding morphology with nano-sized, porous and spherical distribution for achieving good electrochemical performance. In addition, this work provides a novel strategy for solving the evaporation problem of gelatin, especially in large quantities. Compared with the traditional sol–gel method, this novel and facile route is a tempting prospect, as it enables quick evaporation of the gelatin and exhibits superior performance, including energy savings, cost effectiveness, continuous preparation and environmental safety. We believe that this synthesis route illuminates a new way to prepare the title product without employing surfactants or templates and holds the potential to be extended for the preparation of similar superstructures of many other composites.
Acknowledgements
The authors thank Dr Kai-Fu Peng (National Center for Nanoscience and Technology, China) for his kind help with the FIB/SEM measurements.
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