Study On Doping, Coating, Controlling Morphology And Performance Of Lithium Iron Phosphate As Li-ion Battery Cathodes

Olivine structured LiFePO4is one of the most promising cathode materials fornext-generation lithium-ion power batteries due to its high theoretical rate capacity,environmental benignity, low-cost, and good thermal stability. However, the high-rate andlow-temperature performances of LiFePO4were hindered by its poor electronicconductivity and sluggish ionic diffusion. Though this material has been commercialized,the requirement for high power battery has hardly been satisfied. The high rate and lowtemperature performances of LiFePO4material still need to be enhanced. In addition, boththe tap density and the cost of this material need to be further improved. In this paper, theLiFePO4was prepared and modified by different methods, and the exploratory researchwas carried out for practical application. The microstructure of the samples werecharacterized using XRD, SEM and TEM. The electrochemical performances of thesamples were investigated by constant current charge-discharge and AC impedancespectroscopy technology.Transition metal elements Cu2+, Mo6+and Nb5+doped LiFePO4/C materials on Fesite or Li site were synthesized by two-step ball-milling solid state method. The preparedmaterials had high tap density. The effects of different doping contents on themicrostructure of the LiFePO4/C were investigated. The reasons of the improvements inthe electrochemical performance by doping transition metal elements were also analyzed.It was found that appropriate amounts of transition metal elements could widen thediffusion channels of Li+along [010] direction. Therefore, the electrochemicalperformance of the LiFePO4/C was improved.The LiFePO4cathode material coated with ionic conductor and carbon were preparedvia liquid-phase precipitation reaction combined with carbon thermal reduction methodusing CePO4, SmPO4and Li3V2(PO4)3as Li+ionic conductor. The microstructure andelectrochemical performance of the LiFePO4/C modified with ionic conductor wereinvestigated, and the contribution mechanism of the ionic conductor coating wereanalyzed. An appropriate content of ionic conductor and carbon hybrid coating with goodionic and electronic conductivity enhanced the ionic and electronic transport in the LiFePO4electrode, facilitating the mass and charge transfer and improving the reversiblecapacity of LiFePO4electrode.LiFePO4nanoplates possessing large (010) plane was prepared by facile glycol-basedsolvothermal synthesis for the fist time. The formation mechanism of the LiFePO4nanoplates was investigated by changing the solvothermal reaction time. In addition,olivine LiFePO4nanoparticles with different morphologies with short b-axis weresuccessfully synthesized by solvothermal in glycerol and water system. The growth andtransformation mechanism of the LiFePO4with different morphologies were researched.After coating carbon, the prepared LiFePO4@C nanoparticles with short b-axis exhibitedsuperior electrochemical properties.LiFePO4nanoplates with large (010) plane was synthesized by solvothermal method.High-graphitized carbon coated LiFePO4nanoplates was prepared in-situ using ferroceneas catalyst during carbon coating. The influences of graphitization carbon onelectrochemical properties of LiFePO4nanoplates, especially the high rate and lowtemperature performances, were investigated. In comparison with the amorphous carbon,the graphitic carbon was more ideal due to its excellent electronic conductivity. Therefore,the high-graphitized carbon coated LiFePO4nanoplates exhibited superior electrochemicalperformance. In addition, to cut down the costs of LiFePO4, LiFePO4/CNT wassynthesized by directly low temperature (300oC) solvothermal method without hightemperature thermal treatment. The effects of the CNT content on the electrochemicalperformances of LiFePO4were systematically studied.