A_<i>m</i>V₂O₅ with Binary Phases as High-Performance Cathode Materials for Zinc-Ion Batteries: Effect of the Pre-Intercalated Cations A and Reversible Transformation of Coordination Polyhedra

Abstract

In this work, five vanadium oxide materials with a series of pre-intercalated cations A (A_<i>m</i>V₂O₅), including Zn²⁺, Mg²⁺, NH₄⁺, Li⁺, and Ag⁺, have been successfully prepared by a two-step method. All of them possess binary monoclinic and orthorhombic V₂O₅ phases with an open layered structure that allows the ionic storage and diffusion of hydrated cations. The interlayer space for the monoclinic V₂O₅ phase is strongly dependent on the radii of hydrated cations A, while the one for the orthorhombic V₂O₅ phase remains the same regardless of the radii of cations A. Among them, A_<i>m</i>V₂O₅ with pre-intercalated Zn²⁺ (ZVO) has the best storage ability of Zn²⁺ with a reversible capacity close to 400 mAh g^-1, and A_<i>m</i>V₂O₅ with pre-intercalated Ag⁺ shows the highest rate capacity with a nearly 40% capacity retention at a current of 20 A g^-1 (≈25 C). Kinetic studies have clearly shown that pseudocapacitive behavior dominates the electrochemical reaction on ZVO. During the Zn²⁺ (de)intercalation reaction, a highly reversible transformation of binary monoclinic or orthorhombic V₂O₅ phases into a single triclinic Zn_<i>x</i>V₂O₅·<i>n</i>H₂O phase is demonstrated on ZVO. Vanadium atoms are identified as the redox centers that undergo the mutual transition among the chemical states of V³⁺, V⁴⁺, and V⁵⁺. They together with oxygen atoms constitute reasonable V-O coordination polyhedra to generate a layered structure with a suitable interlayer space for the insertion or removal of zinc ions. Actually, the intrinsic coordination chemistry changes between VO₅ square pyramids and VO₆ octahedra account for the phase transformation during the Zn²⁺-(de)intercalation reaction.