Using Howardevansite Framework Adaptivity to Explore the Li₂O–Fe₂O₃–V₂O₅ Phase Diagram

An iron vanadate in the Li₂O-Fe₂O₃-V₂O₅ ternary phase diagram was successfully synthesized by using β-Cu₃Fe₄(VO₄)₆ framework adaptivity. The three Cu²⁺ ions in Howardevansite β-Cu₃Fe₄(VO₄)₆ were substituted by 1.5 Li⁺ and 1.5 Fe³⁺ to form Li_1.5Fe_5.5(VO₄)₆. The structure was determined based on single-crystal X-ray diffraction data, which showed that it crystallizes in the triclinic <i>P</i>1̅ space group. The structure consists of zigzag chains formed of iron octahedra and distorted trigonal bipyramids running along the [10-1] direction and connected into a 3D framework by the VO₄ tetrahedra. The framework can accommodate Li⁺ ions. Magnetic susceptibility revealed the presence of high spin Fe³⁺ ions in the material. Li_1.5Fe_5.5(VO₄)₆ was tested in electrochemistry and can reversibly insert up to 5.5 Li⁺ ions, with a theoretical capacity of 146 mA h/g, making it a suitable insertion positive electrode material. During galvanostatic discharge, a maximum capacity of 128 mA h/g and a reversible capacity of 78 mA h/g were obtained after 100 cycles at C/10.