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铁杆木虫 (职业作家)

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[交流] 【讨论】溶胶-凝胶技术——这样居然也可以

刚刚在JMC上看到的一篇关于溶胶凝胶的论文

居然还发了【没有任何歧视的含义】
~_~

http://www.rsc.org/Publishing/Jo ... asp?Type=AdvArticle
First published on the web: 27 October 2009

Up-scalable synthesis, structure and charge storage properties of porous microspheres of LiFePO4@C nanocomposites

Feng Yu, Jing-Jie Zhang, Yan-Feng Yang and Guang-Zhi Song, J. Mater. Chem., 2009
DOI: 10.1039/b916938e

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fishwater

铁杆木虫 (职业作家)

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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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