| Olivine LiFePO4 was considered as state-of-the-art lithium-ion batteries to substitute the toxic and expensive LiCoO2 material due to its nontoxicity, low cost, high safety, superior thermal stability, and environmental benignancy. In addition, it has a large theoretical capacity (about 170 mAh/g), better cycling stability and flat voltage profile of about 3.4 V versus Li/Li+. However, two major impediments for the commercial application of LiFePO4 are the low electronic conductivity which led to its poor rate capability and slow lithium ion diffusion across the LiFePO4/FePO4 boundaries. In order to overcome these disadvantages, a series of materials were synthesized and doped by supervalent metals or conductive agent by solid-state method.Physical-chemical characterizations of the as-synthesized samples were investigated by the measurements of X-ray diffraction (XRD), scanning electronic microscope (SEM) and transmission electron microscope (TEM). Besides, electrochemical behaviors of the cathode materials were tested via cycling voltammentry, impedance spectroscopy, and galvanostatic charge/discharge cycling studies.(1) LiFePO4 material was synthesized by two-step solid state route using stoichiometric amounts of Li2CO3, FeC2O4·2H2O and (NH4)2HPO4 as raw materials. After milling, the synthesis temperature was confirmed by the thermogravimetric analysis. The XRD results proved that LiFePO4 was pure, matching with the standard JCPDS completely. But the performance of pure LiFePO4 was dissatisfactory for its low intrinsic conductivity.(2) The effects of carbon coating on the properties of materials with various carbon sources including sucrose, glucose, oxalic acid and polyethylene alcohol were discussed in detail. Among these samples, the sample coating with oxalic acid presented better electrochemistry properties. In addition, the effects of oxalic acid contents were investigated. The test results show that 15% oxalic acid content is best. What's more exciting are the properties of LiFePO4 coating with oxalic acid and citric acid. The LiFePO4/C particles exhibit small size distribution and excellent cycling. The discharge capacity at 1 C has no evidence fading up to 400 cycles.(3) The properties of LiFePO4 doped by small amount of Nd3+ and coating with 1% citric acid were studied. First, the influence of Nd3+ substituted Li site and Fe site were discussed. XRD results proved that doped Nd3+ had no evident effect on the structure of LiFePO4. However, the Lattice parameters occurred to change due to Nd3+ enters into the position of Li or Fe. And the electrochemical results show that the sample doped with Nd3+ substituted Li site has better electrochemical performance than that of Fe site. Second, the electrochemical performance of Li0.99Nd0.01FePO4 calcinated at 600℃, 700℃and 800℃were investigated. Charge and discharge tests show that the sample synthesized at 700℃has better electrochemical property. Besides, the performance of Li0.99Nd0.01FePO4/C was further investigated. All results exhibit that Li0.99Nd0.01FePO4/C has high discharge capacity and good cycling performance. |
