Lifepo₄ As Cathode Material In Li-ion Batteries

LiFePO₄ has a theoretical capacity of 170mAh/g and a discharge potential of 3.5V versus Li/Li+. In addition, because of the low cost, excellent thermal stability, preferable safety and environmentally friendly, LiFePO₄ is a promising cathode material for high power lithium-ion batteries.LiFePO₄ has a very long and flat charge/discharge potential plateau by which it is difficult to ascertain the State of Charge (SOC). We find a novel way to confirm the SOC by means of dopding some different transition metal cations with different potentials to generate sevrial potential plateau.The XRD analysis and charge/discharge test indicate that a new phase of LiMnPO₄ are formed at a charge potential plateau of 4.1V by doping with Mn²⁺; Ti oxides generate a lower potential plateau at 2.6V by doping with Ti⁴⁺ and the length of plateau will get longer when doping amounts increase.Besides, It was found that increasing the doping amounts will not decrease the capacity of samples. This characteristic can be used to confirm the SOC in order to prevent overcharge and overdischarge. The results indicate that Mn²⁺ ion dopant can improve the cycle performance and high rate performance of LiFePO₄. Constant current charge/discharge tests display that the sample has a good precision. LiFePO₄ doped with the Ti⁴⁺ ion shows the good electrochemical properties. The first charge/discharge capacity was 150mAh/g with a current density of 0.1C and the capacity can maintain without evident decay after 30 cyclings.LiFePO₄/C by doping Ti⁴⁺ shows good cycle properties at low/high current and the capacity can maintain without evident decay after 30 cyclings. The samples'performance at high rate was improved by a handful of Zr⁴⁺ ion dopant. The discharge capacity of LiFe0.95Zr0.05PO₄/C is 147mAh/g at 0.1C, even at 8C; a discharge capacity of 83.1mAh/g is obtained which occupy 56.5% of the capacity at 0.1C.In this paper, the morphology and the particle size of the LiFePO₄/C samples (0.5μm and 1μm) were controlled by high temperature solid technique and spry drying. The study suggests that the sample with fine particle and low polarizationon exhibit excellent cycle life and good high rate abilities.