Microwave-assisted solvothermal synthesis and characterization of metastable LiFe(1-x)(VO)(x)PO4 cathodes

Vanadyl ion substituted LiFePO(4) cathodes of the form LiFe(1-x)(VO)(x)PO(4) for 0 ≤ x ≤ 0.25 have been synthesized by a rapid microwave-solvothermal process at <300 °C within 10 min. Clear evidence of vanadyl ion substitution is demonstrated, despite a large size difference between Fe(2+) 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 Fe(3)O(4) impurity phase, which increases with increasing (VO)(2+) substitution for Fe(2+) and could be removed with a magnetic stir bar. The formation of iron vacancies, along with the oxidation of some Fe(2+) to Fe(3+) to maintain charge neutrality, results in a decrease in the unit cell volume with increasing x despite the substitution of larger (VO)(2+) for Fe(2+). Charge-discharge data of the vanadyl ion substituted samples suggest suppression of the two-phase plateau behavior that is characteristic of LiFePO(4). Electrochemical data collected without any carbon coating reveal that the capacity and rate capability decreases, but the capacity retention improves with (VO)(2+) substitution.