Studies On Synthesis, Characterization And Electrochemical Performance Of LiFePO₄/C

LiFePCU was synthesized by solid-state method. It was shown that this material had stable discharge plateau near 3.4V and good cycle performance. The sample synthesized at 600℃ could deliver 100mAh/g at the 10th cycle under the current density of 0.05mA/cm2.LiFePCVC composite was synthesized using lithium oleate (OA-Li) , lithium cinnamate (CA-Li) and lithium polyacrylate (PAA-Li) that were synthesized by ourselves as both lithium and carbon source; the discharge capacity of the samples synthesized from CA-Li and OA-Li was apparently lower than that of the sample synthesized from PAA-Li; with the molar ratio of LiOH to PAA-Li was 1/3 in the raw materials, the sample synthesized at 625℃ could deliver 161mAh/g at the 10th cycle under the current density of 0.04mA/cm2. LiFePCVC composite was also synthesized using surfactant p-octyl polyethylene glycol phenylether (OP) as carbon sources; the sample synthesized at 625℃ with 22.5% OP (wt%) in the raw materials could deliver 160mAh/g at the 10th cycle under the current density of 0.05mA/cm2. These two synthesizing ways have not been reported by far.It was proved that the main crystalline phase of all the samples were LiFeP04 by XRD. Impurity was found in the samples synthesized at 550癈 or the temperature lower than that, while the crystalline size of the samples synthesized at 700℃ was too big, both two factors caused the capacity loss. Some samples were also characterized by FT-IR and SEM.With the data of electrochemical test and XRD, it was found that the carbon formed from the raw materials had different structure than the carbon black used as additive when the cathode was prepared; and they played different roles to the cathodic performance. It was discovered that the ratio of Fe to P in the randomly selected areas of a sample with the best discharge capacity were inhomogeneous and the average values deviated from 1:1 significantly, the probable reason causing the phenomenon and the possible result of it were discussed. Brook's theory was used to semi-quantitatively explain the phenomenon that the increase of the content of OP in the raw materials restrained the growth of LiFePO4 crystal. It was also found that the kinetics of the solid-state reaction was influenced by OP, but further research is needed to reveal the particular mechanism.