Study On The Synthesis And Modification Of Cathode Material LiFePO₄ Of Li-Ion Battery

Globular LiFePO4, the cathode material of Li-ion battery, was synthesized by co-precipitation method in this thesis. The precursor was prepared with H3PO4, FeSO4·7H2O, LiOH·H2O as raw materials and citric acid as complexing agent, and LiFePO4 powder was obtained after heat-treatment. The main preparative processing parameters and the effects of carbon-doping and ion-doping on characteristics of material were systematically investigated by physico-chemical analysis, XRD, SEM, EDS, grain size analysis and electrochemical test.The experiment results showed that the globular LiFePO4 powder, average size 0.5μm, was synthesized. The molar ratio was Fe∶P∶L i∶c itric acid= 1∶1 .4∶3 .4∶0 .1; reaction concentration of precursor was 0.1mol/L, Sol pH=9; the reaction solution was heated in 50℃water bath, and stirred at high speed for 30min. After aging, filtering and dying, the precursor was put into tube furnace, pretreated at 350℃for 8h and then sintered at 600℃for 6h under N2 atmosphere. The globular LiFePO4 thus produced had a rounded pattern, the surface of particles was smooth.The conductivity of LiFePO4 was obviously improved by carbon-doping. The conductivity of the carbon-doped LiFePO4 was increased from 10-7 to 10-2 magnitude. And the addition of carbon had no effect on the crystalline structure of LiFePO4.Because of small grain size, the carbon was coated on the surface of globular particles uniformly. Different particles were agglomerated by carbon. Moreover, the doped carbon could avoid the sintering of particles and maintain their integrity. The first specific discharge capacity was 69.6mAh/g at the rate of 1C (1C=100mAh/g), and the charge-discharge efficiency was 92.5%. The specific capacity after 15 times cycling was 91.5% of the first cycle.The tap density and electrochemical performance of LiFePO4 were improved by ion-doping. The ion-doped LiFePO4 had much more smaller-size particles (< 0.5μm), and the tap density was increased obviously from 0.9-1.0 to 1.3-1.4g/cm3. The first specific discharge capacities of ion-doped samples at the rate of 1C were about 80mAh/g. The capacitiy was much more stable in cycling, and there was only micro-attenuation on capacity after 15 times cycling.The reasons of improvement of electrochemical performance,one was excess electron conduction resulted from crystal defects caused by ion-doping,the other was hole conduction generated from the ion substitution for Li or Fe position.