Temperature dependence of aliovalent-vanadium doping in LiFePO₄ cathodes

Vanadium-doped olivine LiFePO₄ cathode materials have been synthesized by a low-temperature microwave-assisted solvothermal (MW-ST) method at ≤300 C. The samples have been extensively characterized by neutron/X-ray powder diffraction, infrared and Raman spectroscopy, elemental analysis, electron microscopy, and electrochemical techniques. The compositions of the as-synthesized materials were found to be LiFe_1-3x/2V _x□_x/2PO₄ (0 ≤ x ≤ 0.2) with the presence of a small number of lithium vacancies (□) charge-compensated by V⁴⁺, not Fe³⁺, leading to an average oxidation state of ∼3.2+ for vanadium. The vacancies on the Fe site likely provide an additional conduction pathway for Li⁺ ions to transfer between neighboring 1D conduction channels along the crystallographic b axis. Heating the pristine 15% V-doped sample in inert or reducing atmospheres led to a loss of vanadium from the olivine lattice with the concomitant formation of a Li ₃V₂(PO₄)₃ impurity phase; after phase segregation, a partially V-doped olivine phase remained. For comparison, V-doped samples were also synthesized by conventional ball milling and heating, but only ∼10% V could be accommodated in the olivine lattice in agreement with previous studies. The higher degree of doping realized with the MW-ST samples demonstrates the temperature dependence of the aliovalent-vanadium doping in LiFePO₄.