铁源对溶胶-凝胶法制备LiFePO4／C的结构和电化学性能的影响

Effects of iron sources on structure and electrochemical properties of LiFePO_4/C prepared by sol-gel method

以二价和三价铁为铁源，乙二醇为络合剂及碳源，水为溶剂，采用溶胶-凝胶一步烧结法制备LiFePO4／C正极材料。采用x射线衍射仪、扫描电子显微镜和元素分析仪等分别对样品的相结构、形貌和碳含量进行表征。研究电极在不同放电倍率下的充放电特性，采用电化学交流阻抗谱和循环伏安法研究LiFePO4／C电极反应的动力学性能。结果表明：以水为溶剂，采用三价铁源获得产物中未发现除LiFeP04以外的相，而采用二价铁源制备的产物中除LiFePO4主相外还存在微量的FeP相；微量FeP相的存在提高LiFePO4／C的综合电化学性能；以二价铁源制备的LiFePO4／C在0．1C和1C（1C=170mA／g，2．5-4．2V）下的放电比容量分别为142和112mA·h／g，高于采用三价铁源制备的LiFePO4／C。

Ferrous and ferric iron precursors were used to synthesize LiFePO4/C by an aqueous sol-gel method, using ethylene glycol as chelating agent and carbon source. The phase structure, morphology and carbon content of LiFePO4/C products were characterized by XRD, SEM and elemental analysis, respectively. Electrochemical impedance spectroscopy and cyclic voltammetry were applied to study the reaction kinetics of the LiFePO4/C cathode material. The results show that considerable pure LiFePO4 phase is obtained when iron source of Fe（Ⅲ） is used, but when the iron precursor of Fe（ Ⅱ ） is used, besides the main phase of LiFePO4, a little amount of FeP forms. The existence of FeP phase favors the overall electrochemical properties of LiFePO4/C. The LiFePO4/C prepared from ferrous iron exhibits discharge capacities of 142 and 112 mA.h/g at rates of 0.1C and 1C （ 1C= 170 mA/g, voltage between 2.5 V and 4.2 V）, respectively, which are higher than those of LiFePO4/C prepared from ferric iron.