A_mV₂O₅ 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 (AmV2O5), 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, AmV2O5 with pre-intercalated Zn2+ (ZVO) has the best storage ability of Zn2+ with a reversible capacity close to 400 mAh g–1, and AmV2O5 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 ZnxV2O5·nH2O 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.