Effect of Precipitator and Quantity on the Formation of Fe_3(PO_4)_2

Abstract

The effect of the precipitator (NaOH, <TEX>$NH_4OH$</TEX>) and the amount of the precipitator (150, 200, 250, 300 ml) on the formation of <TEX>$Fe_3(PO_4)_2$</TEX>, which is the precursor used for cathode material <TEX>$LiFePO_4$</TEX> in Li-ion rechargeable batteries was investigated by the co-precipitation method. A pure precursor of olivine <TEX>$LiFePO_4$</TEX> was successfully prepared with coprecipitation from an aqueous solution containing trivalent iron ions. The acid solution was prepared by mixing 150 ml <TEX>$FeSO_4$</TEX>(1M) and 100 ml <TEX>$H_3PO_4$</TEX>(1M). The concentration of the NaOH and <TEX>$NH_4OH$</TEX> solution was 1 M. The reaction temperature (25<TEX>$^{\circ}C$</TEX>) and reaction time (30 min) were fixed. Nitrogen gas (500 ml/min) was flowed during the reaction to prevent oxidation of <TEX>$Fe^{2+}$</TEX>. Single phase <TEX>$Fe_3(PO_4)_2$</TEX> was formed when 150, 200, 250 and 300 ml NaOH solutions were added and 150, 200 ml <TEX>$NH_4OH$</TEX> solutions were added. However, <TEX>$Fe_3(PO_4)_2$</TEX> and <TEX>$NH_4FePO_4$</TEX> were formed when 250 and 300 ml <TEX>$NH_4OH$</TEX> was added. The morphology of the <TEX>$Fe_3(PO_4)_2$</TEX> changed according to the pH. Plate-like lenticular shaped <TEX>$Fe_3(PO_4)_2$</TEX> formed in the acidic solution below pH 5 and plate-like rhombus shaped <TEX>$Fe_3(PO_4)_2$</TEX> formed around pH 9. For the <TEX>$NH_4OH$</TEX>, the pH value after 30 min reaction was higher with the same amount of additions of NaOH and <TEX>$NH_4OH$</TEX>. It is believed that the formation mechanism of <TEX>$Fe_3(PO_4)_2$</TEX> is quite different between NaOH and <TEX>$NH_4OH$</TEX>. Further investigation on this mechanism is needed. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the pH value was measured by pH-Meter.