| As a cathode material in the new generation of lithium-ion batteries, lithium ironphosphate(LiFePO4) has attracted significant attention because of its high safety,environmental benignity, stable cycle performance and low cost. But it had low electronicconductivity and the reversible capacity loss at higher current density.LiFePO4 was synthesized by one-step solid-state reaction used Li3PO4, FeC2O4.2H2O,(NH4)2HPO4 as raw materials. The capcity and rate capability were improved by carboncoating and cation doping. The structure, morphology, particle size and electrochemicalperformance were characterized by means of XRD, SEM, laser particle sizer andelectrochemical techniques.Results indicated that single phase LiFePO4 could be prepared by one-step solid-statesintering. Ball milling time, synthesis temperature and reaction time are important factorsaffecting the performance of the samples. When the materials were ground at 6h and sinteredat 600℃for 24h, the outcome showed excellent performance.The effects of different carbon source and lithium source, such as acetylene black, sugar,Li2CO3 and Li3PO4, on the performance of samples were investigated. Carbon coatedLiFePO4 did not affect the structure of the material but descreased the grain size and increasedthe electrical conductivity of sample. LiFePO4/C composite obtained with sugar as carboncoated possessed excellent electrochemiacal performace, whose first specific dischargecapacities and ture densities were 130.1 mAh/g and 1.23 g/cm3, respectively. Their electronicconductivities were calculated to be 1.56×10-3 S/cm, which increased the conductivites ofpure LiFePO4 by a factor of～106.Low concentration Mg2+, Cr3+, Ti4+ dopant can increase the capacity and cylceperformace of LiFePO4 because they cause the shrinkage of crystal cell and the reduction ofgrain radium. If the concentration of doping ion is too high, lithium ion diminished in quantitywould descrease the performance of LiFePO4. It was found that Li0.98Mg0.02FePO4,Li0.98Cr(0.01FePO4, Li0.98Ti0.01FePO4 had a higher rate capability and cycle performace, whosefirst discharge specific capability were 123.6, 114.3 and 123.5 mAh/g, respectively. After 20cycles, the capacity of Li0.99Ti0.01FePO4 was 128.3 Ah/g, with its decline rate of 1.36%compared to the highest capacity.The sample doped with rare earth elements was observed that particle size distributiondescrease with the increase of doping levels. Single phase olivine crystal can be obstarinedwith y3+, Sm3</sup> and Pr3+ dopant at low concentration. However, at higher doping levels,impurity phases appeared in XRD patterns. The results revealed the first discharge capacity ofthe samples doped 0.5% Y3+, 1% Sm3+ and 1% Pr3+ were 118.6, 128.5 and 117.9 mAh/g,respectively. |
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