A m V 2 O 5 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 (A m V 2 O 5 ), including Zn 2+ , Mg 2+ , NH 4 + , Li + , and Ag + , have been successfully prepared by a two-step method. All of them possess binary monoclinic and orthorhombic V 2 O 5 phases with an open layered structure that allows the ionic storage and diffusion of hydrated cations. The interlayer space for the monoclinic V 2 O 5 phase is strongly dependent on the radii of hydrated cations A, while the one for the orthorhombic V 2 O 5 phase remains the same regardless of the radii of cations A. Among them, A m V 2 O 5 with pre-intercalated Zn 2+ (ZVO) has the best storage ability of Zn 2+ with a reversible capacity close to 400 mAh g -1 , and A m V 2 O 5 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 Zn 2+ (de)intercalation reaction, a highly reversible transformation of binary monoclinic or orthorhombic V 2 O 5 phases into a single triclinic Zn x V 2 O 5 · n H 2 O phase is demonstrated on ZVO. Vanadium atoms are identified as the redox centers that undergo the mutual transition among the chemical states of V 3+ , V 4+ , and V 5+ . 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 5 square pyramids and VO 6 octahedra account for the phase transformation during the Zn 2+ -(de)intercalation reaction.