Conductivity and electrochemical behavior of Li_1-xFe_1-2x(M^IIM^III) _xPO₄ with olivine structure

Lithium iron phosphates with olivine structure doped by trivalent cations and co-doped by divalent and trivalent cations of the compositions Li_1-xFe_1-xM^III _xPO₄ (M^III = Al, Cr, Ga, Y, In) and Li_1-xFe_1-2x(NiM^III)_xPO₄ (M^III = 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 Li_0.995Fe_0.99(NiM)_0.005PO₄/C (M^III = 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.