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Modification on LiFePO4and Li2FeSiO4Cathode Materials for Lithium Ion Batteries

Author: PengGang
Tutor: YangXueLin
School: Three Gorges University
Course: Mechanical Manufacturing and Automation
Keywords: Lithium ion battery Cathode material Litium ion phosphate Lithium ion silicate Carbon coating Metal oxide coating
CLC: TM912
Type: Master's thesis
Year: 2013
Downloads: 55
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Abstract


In order to overcome the defects of low electronic conductivity and low lithium iondiffiusion coefficient of LiFePO4and Li2FeSiO4cathode materials, different approacheshave been employed, such as carbon coating, metal oxide coating, and so on. The resultsare as follows:LiFePO4/C composites have been successfully prepared by high-temperature solid-statereaction route using citric acid as carbon source. The experimental results indicate thatcarbon content of LiFePO4/C composite decreased with increasing temperature, anddetrimental effects on electrochemical performance of LiFePO4/C sample have caused bylarger particle size under higher sintering temperature. The sample synthesized at700°Cshows the best electrochemical performance with moderate carbon content and uniformparticle distribution. The LiFePO4/C electrode could deliver discharge capacity of110.9mAh g-1and108.8mAh g-1at10C of the first and50thtime, respectively, with theretention ratios of98.1%; Under20C, The discharge capacity is up to78.4mAh g-1, andthe capacity retention ratio is92.7%after50cycles. LiFePO4-C, LiFePO4-G andLiFePO4-（C+G） composites have been successfully synthesised by carbonthermalreduction method by using glucose, activated graphite and glucose-activated graphite ascarbon source, respectively. Among the three samples, LiFePO4-（C+G） has the best highrate performance, which results from the increased degree of graphitization of residualcarbon, the decreased charge transfer resistance and the reduced particles size. A novelCdO and carbon co-coated LiFePO4cathode material has been successfully synthesized.Combined with the results of XRD, XPS, XAS and TEM, it reveals that CdO-incorporatedLFP particles are coated with a mixed amorphous carbon and CdO layer with a thicknessof23nm. Compared to the LiFePO4/C electrode, the capacity retention ratio and highC-rate performance for LiFePO4/（C+CdO） have been effectively improved, which is due tothe enhanced structural stability, the reduced charge-transfer resistance, and the increasedelectronic conductivity. Therefore, the CdO and carbon co-coating is an effective surfacemodification technique.Compared with the electrochemical performance of Li2FeSiO4/C composites preparedby solid stated reaction, the Li2FeSiO4/C composite synthesized by sol-gel method hasdisplayed a better electrochemical performance. The results indicate that the samplesprepared by the two methods contain Li2SiO3and Fe3O4impurties and the sample prepared by sol-gel sample exhibit a better performance because of the smaller particle size, itexhibits a discharge capacity as high as227.8mAh g-1,219mAh g-1at the first and secondcycle at0.1C, respectively, and achieves more than1mol Li+intercalation/deintercalation.At1.0C and3.0C the sample delivers a reversible discharge capacity as high as151.7mAh g-1,130.4mAh g-1after50cycles. But it has lower degree of crystallinity andstronger polarization effect in the process of the charge-discharge.Nano-Li2FeSiO4/C/MWCNTs composite with enhanced rate performance and cyclestability has been successfully synthesized via sol-gel method. SEM images show thedecreased particle size, and EIS results confirm the decreased charge-transfer resistanceand improved Li+diffusion coefficient after MWCNTs-incorporation. Therefore, theLi2FeSiO4/C/MWCNTs electrode exhibits a discharge capacity as high as206.8mAh g-1at2nd cycle, corresponding to1.24mol Li+intercalation and deintercalation during thecharge/discharge process. Furthermore, the Li2FeSiO4/C/MWCNTs displays excellenthigh-rate capacity and cycling stability, i.e., it delivers a reversible discharge capacity ashigh as82mAh g-1at20C after500cycles, which is attributed to the high conductivenetwork between the MWCNTs and the Li2FeSiO4/C particles, and the reducing particlesize. Our results suggest that the nano-Li2FeSiO4/C/MWCNTs composite is a promisingcathode material for lithium ion batteries.

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CLC: > Industrial Technology > Electrotechnical > Independent power supply technology (direct power) > Battery
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