The In Situ Polymerization - Carbothermic Reduction Synthesis Of Lithium Iron Phosphate And Its Electrochemical Properties

Olivine-structured LiFePO4 is considered as a promising cathode material for lithium-ion batteries, due to its advantages such as relatively high capacity, abundant raw materials, environmentally benign, inexpensive price and high safety. However, its power performance is greatly limited by slow diffusion of lithium ions across the two-phase boundary and its low electronic conductivity. To improve the electrochemical performances of LiFePO4 material, the synthesis condition, carbon coating, metal ion doping, electrochemical behavior and kinetics of lithium insertion-extraction process of LiFePO4 as cathode materials were studied in detail in this dissertation. The main points can be summarized as follows.The FePO4/polyaniline(PANI) precusor was prepared by in-situ polymerization reaction and characterized by FTIR, TG-DTA, SEM. The LiFePO4/C composite cathode materials were synthesized by carbothermal reduction method using as-prepared FePO4/ANI, LiAC·2H2O and glucose as raw materials. The reaction mechanism and the effects of conditions on the structure, morphology and electrochemical performance of materials were also investigated.The products were characterized by FTIR, XRD, SEM, TEM, and the electrochemical performance of the samples was measured. The results show that the composite synthesized at 700℃for 10 h has a simple pure olive-type phase with 3.80 wt% carbon content, and particle sizes of around 100nm. Electrochemical test results show that the sample exhibits the favorable electrochemical performance with initial discharge capacities of 151,136,131,126,113 mAh·g-1 at 0.1 C,0.2 C,0.5 C,1 C,2 C, respectively. It retains discharge capacity of 122 mAh·g-1 at 1C after 50 cycles, with only 0.78‰capacity faded.The effect of doping Na+ in place of Li-site of LiFePO4 on the electrochemical properties was studied in detail. It showed that the doping of Na+ improved the electrochemical performance apparently, and the Li0.97Na0.03FePO4/C sample exhibited the best performance with initial discharge capacity of 158,151,142,134, and 105 mAhg-1 as it was discharged with 0.1 C,0.5 C,1 C,2 C, and 3 C rates, respectively.The electrode process kinetics of LiFePO4/C was studied by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). CV analysis indicated that LiFePO4/C was quasi reversible and Li-ion diffusion coefficients in oxidation, reduction process were 3.85×10-10, 2.15×10-10cm2·s-1. The EIS revealed that the charge transfer resistance (Rct) had a minimum at 3.4V. The Li-ion diffusion coefficients calculated by EIS range from 1.12×10-13 to 7.78×10-13cm2·s-1.