Doping Supervalent Rare Earth Ion In LiFePO₄/C Through Hydrothermal Method

Hydrothermal synthesized LiFePO₄and LiFe_0.95M_0.05PO₄/C （M=Nd, Sm, Eu, Yb） arestudied first time. The synthesized temperature is determined according to TG-DTAcurves of precursor and related literature. The samples are characterized by X-raydiffraction （XRD）, scanning electron microscope （SEM）, energy dispersive spectroscopy（EDS）, thermogravimetric analysis. The electrochemical behavior of the samples areinvestigated by galvanostatic charge/discharge, electrochemical impedance spectroscopy（EIS） and cyclic voltammetry （CV）.According to TG-DTA curves of precursor, hydrothermal synthesis temperature isdetermined by variable experiment. The sample with best electrochemical performancehas been gotten under hydrothermal reacting for6h and heating treatment for3h. Caborncoating from glucose can improve electric conductivity.The microstrcture and electrochemical performance of LiFe_0.95M_0.05PO₄/C （M=Nd, Sm,Eu, Yb） composite are investigated in this paper. The results show that hydrothermalsynthesized particles are nano-sized and have preferable orientation. Doping rare earthelement can not reduce the size of particle. However, doping rare earth ion affected theintensity ratio of I（200）/I（020）. The intensity of （020） plane seemly becomes weaker withdoping rare earth ion. The vacancy caused by supervalent ions leads to lattice defect whichcan significantly improve the electrochemical performance. The electrochemicalperformance is apparently improved due to doping supervalent rare earth ion in LiFePO₄.The results show that the first discharge capacity of the as-prepared LiFePO₄/C andLiFe_0.95M_0.05PO₄/C （M=Nd, Sm, Eu, Yb） are109.4mAh/g,148.6mAh/g,153.6mAh/g,144.0mAh/g and160.7mAh/g, respectively. The capacity remains90.1%,96.2%,98.1%and94.9%after20cycles, respectively. The rate discharge capacity ofLiFe_0.95Yb_0.05PO₄/C composite rises to143.2mAh/g,131.0mAh/g,108.8mAh/g,99.7mAh/g at0.5C,1C,2C and5C, respectively.The specific surface of LiFePO₄/C andLiFe_0.95M_0.05PO₄/C （M=Nd, Sm, Eu, Yb） rises from18.3m²/g to21.5m²/g,25.2m²/g,21.5m²/g and25.4m²/g, respectively. The charge transfer resistance of LiFePO₄/C andLiFe_0.95M_0.05PO₄/C （M=Nd, Sm, Eu, Yb） reduces from1327.8to236.2,137.3, 185.1and119.1, respectively. The result of cyclic voltammetry indicates that thereversibility of electrochemical reaction for doped sample is enhanced.