Phase Separations in LiFe_(1-x)Mn_xPO4: A Random Stack Model for Efficient Cathode Materials

摘要

Olivine-type LiFe_(1-x)Mn_xPO4 compounds were systematically synthesized by a solvothermal method. Synchrotron radiation X-ray absorption spectroscopy combined vrith first-principles calculations and energy-dispersive X- ray spectroscopy measurements show that the as-prepared LiFe_(1-x)Mn_xPO4 samples contain two different phases: LiFePO4 and LiMnPO4. Actually, due to the crystal field effect, these two structures are randomly stacked and characterized by a pronounced structural distortion of the MOg (M: Fe or Mn) octahedra. Moreover, increasing the Mn doping concentration, the distortion of the MOg octahedra increases. Considering the size of LiMnPO4 stacks and the distortion of MO6 octahedra, the best performance occurs at the optimal Mn doping concentration. Among the different Fe/Mn ratios, the electrochemical tests show that the as-prepared LiFe_(0.75)Mn_(0.25)PO4 sample exliibits the best electrochemical performance. Moreover, in order to optimize electrochemical performances, data point out that the doping procedure may control the size of the LiMnPO4 stacks, but it is also necessary to control the interplay between the increase of the working voltage and the electrical conductivity of LiMnPO4 stacks.