Electrochemically Induced Phase Transition in V3 O7 · H2 O Nanobelts/Reduced Graphene Oxide Composites for Aqueous Zinc-Ion Batteries.

V3 O7 ·H2 O 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 V3 O7  · H2 O, V2 O5 nanobelts, and V5 O12  · 6H2 O film. Zinc ion storage in V3 O7 ·H2 O/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 V3 O7  · H2 O to a stable hexagonal Zn3 (VO4 )2 (H2 O)2.93 phase, which is a new electrochemical route found in V3 O7 materials. This phase transition process provides new insight into the reactions of aqueous ZIBs.