AVO 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.

In this work, five vanadium oxide materials with a series of pre-intercalated cations A (AVO), including Zn, Mg, NH, Li, and Ag, have been successfully prepared by a two-step method. All of them possess binary monoclinic and orthorhombic VO phases with an open layered structure that allows the ionic storage and diffusion of hydrated cations. The interlayer space for the monoclinic VO phase is strongly dependent on the radii of hydrated cations A, while the one for the orthorhombic VO phase remains the same regardless of the radii of cations A. Among them, AVO with pre-intercalated Zn (ZVO) has the best storage ability of Zn with a reversible capacity close to 400 mAh g, and AVO with pre-intercalated Ag shows the highest rate capacity with a nearly 40% capacity retention at a current of 20 A g (≈25 C). Kinetic studies have clearly shown that pseudocapacitive behavior dominates the electrochemical reaction on ZVO. During the Zn (de)intercalation reaction, a highly reversible transformation of binary monoclinic or orthorhombic VO phases into a single triclinic ZnVO·HO phase is demonstrated on ZVO. Vanadium atoms are identified as the redox centers that undergo the mutual transition among the chemical states of V, V, and V. 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 VO square pyramids and VO octahedra account for the phase transformation during the Zn-(de)intercalation reaction.