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

A study of LiFePO4-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 Li2CO3, FeC2O4·2H2O 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 β-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 ∼10− 9 S cm− 1 at 25 °C with an activation energy of ca. 0.65 eV for pure LiFePO4 and LiFePO4/β-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 ΔE∼0.08 eV.