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 P1̅ 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.