Isovalent substitution of vanadium in LiFePO4: Evolution of monoclinic α-Li3Fe2(PO4)3 phase

Abstract

We report the synthesis and characterization of isovalent substitution of vanadium at phosphorus site in orthorhombic lithium iron phosphate (LiFePO4) nanoparticles by non-aqueous sol–gel method. Detailed analysis of X-ray diffraction patterns reveals the evolution of monoclinic α-Li3Fe2(PO4)3 phase and its formation is favored for the vanadium content higher than 20%. A continuous increase in unit cell volume of LiFePO4 is observed with increase in vanadium content from x = 0.05 to 0.20, followed by rapid decrease above x = 0.20, due to the creation of a greater number of Li-ion vacancies. FTIR studies demonstrate that the wavenumbers of various absorption maxima and the covalency strength parameter do not show a variation with x, thus confirm the substitution of vanadium in the form of VO4. Our results through Mössbauer and electron paramagnetic resonance spectroscopy unambiguously demonstrate the enhancement in the formation of α-Li3Fe2(PO4)3 phase for vanadium content higher than 20%.