Electrochemically Induced Phase Transition in V₃ O₇·H₂O Nanobelts/Reduced Graphene Oxide Composites for Aqueous Zinc-Ion Batteries

V₃O₇·H₂O nanobelts/reduced graphene oxide (rGO) composites (weight ratio: 86%/14%) are synthesized by a microwave approach with a high yield (85%) through controlling pH with acids. The growth mechanisms of the highly crystalline nanobelts (average diameter: 25 nm; length: ≈20 µm; oriented along the [101] direction) have been thoroughly investigated, with the governing role of the acid upon the morphology and oxidation state of vanadium disclosed. When used as the ZIB cathode, the composite can deliver a high specific capacity of 410.7 and 385.7 mAh g^-1 at the current density of 0.5 and 4 A g^-1 , respectively, with a high retention of the capacity of 93%. The capacity of the composite is greater than those of V₃O₇· H₂O, V₂O₅ nanobelts, and V₅O₁₂ · 6H₂O film. Zinc ion storage in V₃O₇·H₂O/rGO is mainly a pseudocapacitive behavior rather than ion diffusion. The presence of rGO enables outstanding cycling stability of up to 1000 cycles with a capacity retention of 99.6%. Extended cycling shows a gradual phase transition, that is, from the original orthorhombic V₃O₇· H2 O to a stable hexagonal Zn₃(VO₄)₂(H₂O)_2.93 phase, which is a new electrochemical route found in V₃O₇ materials. This phase transition process provides new insight into the reactions of aqueous ZIBs.