Study On Synthesis And Doping Modification Of LiFePO₄ Cathode Materials

Olivine structured lithium iron phosphate, LiFePO₄, is one of the most promising cathode materials for power lithium ion batteries used in power tools or battery electric vehicles due to its long cycling life, high safty, environmental benignity, as well as low-cost raw materials. However, the low electronic conductivity and low ion diffusion kinetics limit the electrochemical properties and application of pure LiFePO₄. In this work, LiFePO₄/C composites were synthesized by different solid-state reaction synthesis processes. The micro-structures and morphologies of the composites were investigated by XRD, TEM and SEM. The electrochemical performances have been evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The effects of the synthesis processes on the physico-electrochemical properties of LiFePO₄/C composites were investigated. The electrochemical properties of LiFePPO₄/C composites doped with anion of F^- and co-doped with F- and Nb⁵⁺ were also investigated.LiFePPO₄/C composite was synthesized by two-step solid-state reaction using FePO₄ as iron source and polypropylene as conductive carbon source. FePO₄ and LiOH were ball-milled for 10h and then were mixed with 40g polypropylene per mole FePO₄. The mixture was pre-sintered at 500℃for 2h and then fired at 700℃for 10h. This is the optimum synthetic process. The particle size of LiFePO₄/C composite is small (100-300nm) and homogeneous. Large discharge capacity and good cycle stability are observed. The initial discharge capacities are 134mAh/g, 127mAh/g, 116mAh/g at the charge and discharge rates of 0.5C, 1C, 2C, respectively and the capacity preserves after 100 cycles at 1C. The carbon content of LiFePO₄/C composite is low (3.47%), which facilitates higher tap density.It was found that the synthesis atomsphere plays also an important role on the electrochemical properties of LiFePO₄/C. The initial discharge capacitie of LiFePO₄/C composite synthesizd in tube furnace is higher than the one synthesizd in vertical furnace, especially at a high charge/discharge rate (2C). LiFePO₄/C composite synthesized in vertical furnace remains 100.1% of the initial capacity after 100 cycles at 1C, which is higher than 98.4% of LiFePO₄/C composite synthesized in tube furnace.Li_0.99Nb_0.01FePO₄/C,Li_0.99Nb_0.01FePO_3.98F_0.02/C, LiFePO_4-xF-x/C(x=0, 0.01, 0.02, 0.03, 0.04), Li_0.99Nb_0.01FePO_3.98F_0.02/C were synthesized to investigate the influence of F^- doping and F^- and Nb⁵⁺ co-doping on the electrochemical properties of LiFePO₄/C composite. It is found that all prepared materials show the single olivine structure. The particle size is small (50nm-200nm) and homogeneous. The partial surface of the particles is coated with carbon layers, while parts of carbon connect the LiFePO₄ particles.The results of electrochemical measurement of LiFePO_4-xF_x/C(x=0, 0.01, 0.02, 0.03,0.04) show that F^- doping improves the rate capability, electrochemical reversibility, structural stability, and decreases the polarization of LiFePO₄ electrode effectively. The optimized F doping content is x=2. The discharge capacities of LiFePO_3.98F_0.02/C cathode material are 146mAh/g, 137mAh/g, 122mAh/g at the charge and discharge rates of 1C, 2C, 3C, respectively, which show 20mAh/g higher capacities than LiFePO₄/C. Cycling efficiency of 99.3% can be obtained after 55 cycles at 1C.Compared with LiFePO₄/C , LiFePO_3.98F_0.02/C , Li_0.99Nb_0.01FePO₄/C, Li_0.99Nb_0.01FePO_3.98F_0.02/C shows best electrochemical performances. The discharge capacities of Li_0.99Nb_0.01FePO_3.98F_0.02/C cathode material are 146mAh/g, 137mAh/g, 122mAh/g at the charge and discharge rates of 1C, 2C, 3C, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) show that F^- and Nb⁵⁺ co-doping decreases the polarization and electrochemical impedance of LiFePO₄ electrode more effectively. The highest cycling efficiency of 99.1% can be obtained after 100 cycles at 2C.