Electrochemical and electrical properties of Nb- and/or C-containing LiFePO4 composites

A study of LiFCPO4-based electrodes prepared through various synthesis conditions is presented. From X-Ray diffraction, high resolution transmission electron microscopy, electrochemical Li+ extraction/insertion and electrical conductivity data we conclude that the use of starting precursors such as Li2O3, FeC(2)O(4)(.)2H(2)O and/or Nb(OC6H5)(5) produces LiFePO4-based composites containing significant amounts of carbon. We never succeeded in doping LiFePO4 with Nb to yield Li1-xNbxFePO4 but produced, instead, crystalline beta-NbOPO4 and/or an amorphous (Nb, Fe, C, O, P) '' cobweb '' around LiFePO4 particles which is responsible for superior electrochemical activity. AC-conductivity measurements conclude to a total electrical conductivity of similar to 10(-9) S cm(-1) at 25 degrees C with an activation energy of ca. 0.65 eV for pure LiFePO4 and LiFePO4/ beta-NbOPO4 composites. C-containing LiFePO4 samples, including those that were tentatively but unsuccessfully doped with Nb, are much more conductive (up to 1.6(.)10(-1) S cm(-1)) with an activation energy Delta E similar to 0.08 eV (c) 2005 Elsevier B.V. All rights reserved.