Study On The Structure,Morphology Controland Modification Of Lithium Iron Phosphate As Cathode Material For Lithium Ion Batteries

As a cathode material for Li-ion batteries,Olivine-type lithium iron phosphate?LiFePO₄?has become a research focus in the filed of battery because of its environmental compatibility,low cost,thermal stability and superior capacity retention.However,the major problems of LiFePO₄ originate from its low lithium ion diffusivity and poor electric conductivity,which lead to its undesirable high-rate performances,retarding its wide applications in energy storage systems.In this paper,various methods were made to tackle the problems of LiFePO₄,providing basic exploration to accelerate the industrialization process of LiFePO₄.The microstructure of as-prepared LiFePO₄ composites were characterized by SEM,TEM and XRD.The electrochemical performance was performed by galvanostatic charge/discharge test,AC impedance and cyclic voltammetry technique.Pristine LiFePO₄/C and Co²⁺ doped LiFePO₄/C nanoplates were prepared by a facile solvothermal method followed by high temperature calcination process.The effect of different doping concentration on the structure and electrochemical properties of the materials were studied.Electrochemical results show that the Co²⁺ doping with appropriate concentration can decrease the transfer resistance and increase the diffusion coefficient of the lithium ion,and thus improving the electrochemical properties of the materials.Among all the doped samples,LiFe0.99Co0.01PO₄/C exhibited the best rate capability and cycling stability,with a high discharge capacity of 154.5 mAh g^-1 at 0.5 C.Monodispersed LiFePO₄ nanocrystals were successfully synthesized via a mild solvothermal approach with a mixture of ethylene glycol and oleic acid as solvent.Morphology evolution of Li FePO₄ nanoparticles from nanoplates to nanorods can be simply realized by varying the volume ratio of oleic acid to ethylene glycol.The crystal growth orientation of LiFePO₄ with different morphologies were fully characterized,and the in-depth study was made to investigate the transformation mechanism of LiFePO₄ morphology.The results show that the prepared nanomaterials have short distance in b axis,and nanorod morphology with the one-dimensional structure can shorten the distanceof electron transfer and ion diffusion,which can facilitate the diffusion of lithium ion and improve the electrochemical properties of the materials.The heating process of the solvothermal method is studied.It is found that the slow step heating method is helpful to reduce the antisite defect concentration of the LiFePO₄ samples,and it is also beneficial to the good connection between LiFePO₄ and CNT.The LiFePO₄/CNT composites with less antisite defect concentration were prepared by this modified solvothermal method,and the electronic conductivity and the lithium ion diffusion rate of the composites were improved.After coating carbon,The C/LiFePO₄/CNT composite could deliver a discharge capacity of 96 mAh g^-1 at high rate of 20 C,with the capacity retention rate of 95 % after 100 cycles,and the columbic efficiency of 100 %.The hierarchical porous C/LiFePO₄/Bio-C composites with different carbon content were fabricated by a sol-gel method.As a natural waste,the artemia cyst shells were used as biological carbon sources.The hierarchical porous structure including macro-and meso-pores can retain pathways for the rapid and massive transport of electrolyte ions.The bio-carbon was used as the conductive skeleton in this designed electrode,which can enhance the electronic contact for the interconnected LiFePO₄ paticles,and thus the excellent high-rate performance was achieved.The as-prepared composites show a discharge capacity of 69.1 m Ah g^-1even under a high rate of 40 C.In addition,the uniform highly-graphtic carbon coating layer was prepared by in-situ carbonization of perylene-3,4,9,10-tetracarboxylic dianhydride.Owing its poly bezenyl rings struvture,the Perylene-3,4,9,10-tetracarboxylic dianhydride can be easily converted to highly graphitic carbon during thermal treatment without any catalyst.In favor of the high electronic conductivity and short lithium ion diffusion distance,the LiFePO₄/graphitic carbon composites exhibit an excellent cycling stability at high current rates at room temperature and superior performance at low temperature?-20 °C?.