Microwave-Assisted Solvothermal Synthesis and Characterization of Metastable LiFe1-x(VO)xPO4 Cathodes

Vanadyl ion substituted LiFePO4 cathodes of the form LiFe1-x(VO)(x)PO4 for 0 <= x <= 0.25 have been synthesized by a rapid microwave-solvothermal process at <300 degrees C within 10 min. Clear evidence of vanadyl ion substitution is demonstrated, despite a large size difference between Fe2+ and (VO)(2+), by characterizing the products structurally, spectroscopically, and electrochemically. The vanadyl ion substitution is accompanied by the formation of iron vacancies in the lattice and Fe3O4 impurity phase, which increases with increasing (VO)(2+) substitution for Fe2+ and could be removed with a magnetic stir bar. The formation of iron vacancies, along with the oxidation of some Fe2+ to Fe3+ to maintain charge neutrality, results in a decrease in the unit cell volume with increasing x despite the substitution of larger (VO)(2+) for Fe2+. Charge-discharge data of the vanadyl ion substituted samples suggest suppression of the two-phase plateau behavior that is characteristic of LiFePO4. Electrochemical data collected without any carbon coating reveal that the capacity and rate capability decreases, but the capacity retention improves with (VO)(2+) substitution.