On the origin of enhanced electrochemical kinetics in guest-ions pre-intercalated layered vanadium oxides: Interlayer spacing vs lattice distortion

Cathode materials of highly stable structure and high capacity are essentially required for aqueous rechargeable zinc ion batteries (ARZIBs), where layered vanadium oxide hydration (VOH) is one of the most promising cathode materials for reversible zinc ion intercalation/deintercalation. Previous studies have shown that both the interlayer spacing and the Zn-ion storage performance can be altered by the pre-intercalation of certain guest ions between the layers of VOH. In addition to previous discussions on the increase in effective Zn2+ storage as a result of the widening in interlayer spacing after guest ion pre-intercalation, there is a variation in the exposed active O sites, which are shown to be equally important. In the metal ion pre-intercalated layered vanadium oxides, there is a delicate balance between the interlayer spacing and lattice distortion, depending on the type and amount of pre-intercalated metal ions. In particular, those pre-intercalated guest ions with high electronegativity can steadily shift towards the VO layers and break the V-O bonds leading to exposed active O sites. The high electronegativity endows the combination of guest ions with newly exposed O sites. These active O sites give rise to an inhomogeneous distribution of the charge density, leading to an improvement in electrochemical performance. The present study demonstrates the complicated interplays of more than one parameter in the guest ion pre-intercalated layered vanadium oxides, going much beyond the simple widening in inter-layer spacing.