Study On Synthesis And Electrochemistry Performance Of LiFePO₄ As Cathode Materials For Lithium-ion Batteries

Along with the increasing popularization of the portable electronic products, battery as a portable power source receives high attention in the world. LiFePO₄ is emerging as a promising cathode material for lithium-ion batteries because of low cost and environmental compatibility. In addition, LiFePO₄ has a large theoretical capacity of 170mAh·g⁽-1_, a flat discharge potential of 3.4V versus Li/Li+, the good cycle stability, and the excellent thermal stability.In this paper, LiFePO₄, LiFePO₄/C and Li_1-x+δTi_xFe_1-yMn_yPO₄/C cathode materials were synthesized by solid-state reaction. The effects of thermal processing conditions, carbon coating, ion-doping and particle size on the phase-purity, particle size, morphology and electrochemical performance of the materials are investigated by X-ray, SEM, CV, EIS. The dependence of kinetics and lithium ion diffusion coefficient of LiFePO₄/C electrode is also evaluated by means of the EIS.The results indicate that the synthesis temperature and carbon-coating affect the morphology, the crystallinity, tap density and discharge performance of the products, and the best synthesis temperature is 650℃. The most optimized carbon-coating content should be controlled in the 3-5 wt%.The tap density of LiFePO₄/C samples synthesized by three-step solid-state method is higher than that by two-step solid-state method, reaching to 1.35 g·cm^-3. The discharge capacity of LiFePO₄/C is 135.8mAh·g^-1 at the C/5 rate, and the capacity remained 99.4% after 20 cycles. Even at the 5C rate, a discharge capacity of 129.8mAh·g?1 is obtained with only a capacity fading of 0.03% per cycle.In order to improve the performance of LiFePO₄ cathode materials, composites Li_1-x+δTi_xFe_1-yMn_yPO₄/C with Ti⁴⁺ and Mn4+ dopant were synthesized. The XRD analysis shows that the samples by doping Ti⁴⁺ and Mn4+ are pure well-ordered olivine phase with no impurities. And the Ti⁴⁺ and Mn4+ ion dopant considerably improves the electrochemical performances of Li_1-x+δTi_xFe_1-yMn_yPO₄/C.To control the particle size of the LiFePO₄/C samples is the key to improve the Li+ diffusion. The samples LiFePO₄ dealt with ball-milling for 12h or airflow milling display uniform particle size and exhibit excellent cycle life.