Li_3Fe_2(PO_4)_3/C正极材料的电化学性能及其反应机理研究

Abstract

采用溶胶凝胶及高能球磨制得lI3fE2(PO4)3/C材料,利用多种物理及其电化学技术观察材料形貌,表征材料结构及电化学性能,用电化学原位XAfS等初步研究lI3fE2(PO4)3/C超理论容量电化学反应机理.结果显示,lI3fE2(PO4)3/C的结构为单斜晶系,空间群P21/n.2.0~4.0 V电位区间,10 M AH·g-1电流密度,lI3fE2(PO4)3/C电极的首周期放电比容量为129 M AH·g-1,达到其理论容量.若电位区间拓宽至2.0~4.95 V,其首周期放电比容量高达165 M AH·g-1,超出理论的“额外“容量30%.电化学原位XAfS测试未观察到明显的fE3+/fE4+氧化还原对参与电化学反应,初步推测“额外“容量可能来自于该复合材料的高浓度表面缺陷.

The cathode material of carbon coated Li3Fe2(PO4)3(Li3Fe2(PO4)3/C) is successfully prepared by sol-gel method and ball-milling carbon coating technique.The cathode material is characterized by XRD, SEM, TEM, and in-situ XAFS techniques.XRD results indicate that the as prepared Li3Fe2(PO4)3is of high purity and single phase, and has monoclinic structure with space group of P 21/n.Electrochemical measurements show that the carbon coated material is able to deliver a high initial discharge specific capacity of 128.6 m Ah·g-1in the voltage range 2.0 ~ 4.0 V at 10 m A·g-1.While in the voltage range 2.0 ~ 4.95 V, its initial discharge specific capacities can reach 165 m Ah·g-1, which is 1.3 times higher than the theoretical capacity.In-situ XAFS study show that no obvious evidence of Fe3+/Fe4+redox couple is involved in the electrochemical reaction, indicating that the capacities which beyond the theoretical capacity may come from the high concentration of surface defects introduced by ball milling process.