Electrochemical properties of LiFe 0.9Mg 0.1PO 4 / carbon cathode materials prepared by ultrasonic spray pyrolysis

Abstract

The LiFe 0.9Mg 0.1PO 4/C powder of pure olivine phase can be prepared with the duplex process of spray pyrolysis synthesis (at 450 ∘C) and subsequent heat treatment (at 700 ∘C for 2, 4 and 8 h). From scanning electron microscopy observation with corresponding elemental mapping images of iron, phosphorous and magnesium, it could be found that the LiFe 0.9Mg 0.1PO 4 powders are covered with fine pyrolyzed carbon. Raman spectra indicate that the phase of carbon with higher electronic conductive phase is predominant when prolonged subsequent heat treatment is carried out. The carbon coatings on the LiFe 0.9Mg 0.1PO 4 surface can improve the conductivity of the LiFe 0.9Mg 0.1PO 4 powder (3.8×10 −5 S cm −1) to about a factor of ∼10 4 higher than the conductivity of LiFePO 4. The stability and cycle life of a charge/discharge cycle test of lithium ion secondary batteries are also enhanced. The results indicate that the LiFe 0.9Mg 0.1PO 4 powder, prepared at a pyrolysis temperature of 450 ∘C and with post-heat-treatment at 700 ∘C for 8 h, exhibits a specific initial discharge capacity of about 132 mA h g −1 at C/10 rate, 105 mA h g −1 at 1C, and 87 mA h g −1 at 5C.