Research On The Synthesis And Electrochemical Performances Of Rare Earth Ions Doped LiFePO₄/C Used As Cathode Materials

Owing to the advantages of abundant raw material resources, cheap price, good thermal stability, nontoxicity, good thermal stability, high theoretical capacity (170mAh/g) and good cyclic performance, LiFePO₄ has become a new kind of lithium ion battery cathodes of great developmental and applicable potential. However, the electronic conductivity of LiFePO₄ is too low, and the ionic diffusion coefficient of Li-ions in the microcrystallites is too small, therefore its capacities and the rate capabilities in the charge-discharge process are limited. Adding carbon, doping with metal ions were adopted by many researchers in order to improve its electrochemical performances. Adding carbon can avoid the oxidation of Fe2+ in the synthetic process, prevent the microcrystallites of LiFePO₄ from overgrowing and increasing the electronic conductivities in the electrode. Doping with metal ions can change the lattice constants and volume of the unit cell, which can improve the electronic conductivity and the ionic diffusion coefficient of Li-ion in the microcrystallites.LiFePO₄/C composites doped with rare earth ions were synthesized by solid-state mixing process. The influences of the kinds and the content of the rare earth ions (R=La³⁺, Nd³⁺, Y³⁺), the kinds and the content of adding carbon sources (acetylene black, glucose, sucrose and phenolic resin) on the crystalline structures of LiFePO₄ and the electrochemical performances of the composites were investigated. The chemical composition of the LiFePO₄/C doped with rare earth ions was optimized.XRD experiments show that the LiFePO₄ microcrystallites in the with rare earth ions composite materials still remain the olivine structure, but the lattice constants and volumes of them increase a little bit.The galvanostatic experiments show that both doping rare earth ions and adding carbon can improve the electrochemical performances of LiFePO₄ composites effectively. When the mole fraction of the atomic number of Y/the atomic number of Y and Li is 1%, the theoretical mass percentage of carbon which phenolic resin was used as carbon source is 5%, the electrochemical performances of LiFePO₄/C composite doped with rare earth ions are the best. When the current density is 0.1C, its dicharge capacity in the third cycle is 142.09mAh/g, and the corresponding charge-dicharge efficiency is 99.02%. When the current density is 0.5C and 1C, its corresponding dicharge capacities are 133.38 mAh/g and 116.92mAh/g. Doping with too much Y³⁺ ions may obstruct the diffusion of Li ion in the crystallites, thus decrease the charge and discharge capacities. Among the above four carbon resources, the effect of adding with phenolic resin is the best.