Conductivity and electrochemical behavior of Li1-x Fe1-2x (MIIMIII) x PO4 with olivine structure

Lithium iron phosphates with olivine structure doped by trivalent cations and co-doped by divalent and trivalent cations of the compositions Li1-xFe1-xMIIIxPO4 (MIII = Al, Cr, Ga, Y, In) and Li1-xFe1-2x(NiMIII)xPO4 (MIII = Al, In) were prepared by the sol-gel method. Structure, morphology, and conductivity of the prepared materials were studied by the X-ray diffraction analysis, scanning electron microscopy, and impedance spectroscopy. The solubility of M3+ cations in LiFePO4 with olivine structure was estimated. Single-phase samples were shown to be preserved at x ≤ 0.05 for Cr3+ and Al3+ cations, at x ≤ 0.02 for In3+, and at x ≤ 0.005 for Ga3+ and Y3+ cations. The materials doped with M3+ and co-doped with both M2+ and M3+ cations were characterized by a higher ionic conductivity and by a lower activation energy of ionic conductivity. Carbon-coated materials Li0.995Fe0.99(NiM)0.005PO4/C (MIII = Al, In) were prepared, and their electrochemical behavior was studied using the charge/discharge tests. Due to LiFePO4 co-doping with M2+ and M3+ cations, capacity slightly decreases but the charge/discharge rate increases.