Emulsion drying synthesis of olivine LiFePO4/C composite and its electrochemical properties as lithium intercalation material

Abstract The electroactive LiFePO 4 /C nano-composite has been synthesized by an emulsion drying method. During burning-out the oily emulsion precipitates in an air-limited atmosphere at 300 °C, amorphous or low crystalline carbon was generated along with releasing carbon oxide gases, and trivalent iron as a cheap starting material was immediately reduced to the divalent one at this stage as confirmed by X-ray photoelectron spectroscopy, leading to a low crystalline LiFePO 4 /C composite. Heat-treatment of the low crystalline LiFePO 4 /C in an Ar atmosphere resulted in a well-ordered olivine structure, as refined by Rietveld refinement of the X-ray diffraction pattern. From secondary electron microscopic and scanning transmission electron microscopic observations with the corresponding elemental mapping images of iron and phosphorous, it was found that the LiFePO 4 powders are modified by fine carbon. The in situ formation of the nano-sized carbon during crystallization of LiFePO 4 brought about two advantages: (i) an optimized particle size of LiFePO 4 , and (ii) a uniform distribution of fine carbon in the product. These effects of the fine carbon on LiFePO 4 /C composite led to high capacity retention upon cycling at 25 and 50 °C and high rate capability, resulting from a great enhancement of electric conductivity as high as 10 −4  S cm −1 . That is, the obtained capacity was higher than 90 mAh (g-phosphate) −1 by applying a higher current density of about 1000 mA g −1 (11 C) at 50 °C.

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